Adding upstream version 1.20.14.upstream/1.20.14upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

749 files changed, 66676 insertions, 0 deletions

diff --git a/src/internal/abi/abi.go b/src/internal/abi/abi.go
new file mode 100644
index 0000000..11acac3
--- /dev/null
+++ b/src/internal/abi/abi.go
@@ -0,0 +1,126 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi
+
+import (
+	"internal/goarch"
+	"unsafe"
+)
+
+// RegArgs is a struct that has space for each argument
+// and return value register on the current architecture.
+//
+// Assembly code knows the layout of the first two fields
+// of RegArgs.
+//
+// RegArgs also contains additional space to hold pointers
+// when it may not be safe to keep them only in the integer
+// register space otherwise.
+type RegArgs struct {
+	// Values in these slots should be precisely the bit-by-bit
+	// representation of how they would appear in a register.
+	//
+	// This means that on big endian arches, integer values should
+	// be in the top bits of the slot. Floats are usually just
+	// directly represented, but some architectures treat narrow
+	// width floating point values specially (e.g. they're promoted
+	// first, or they need to be NaN-boxed).
+	Ints   [IntArgRegs]uintptr  // untyped integer registers
+	Floats [FloatArgRegs]uint64 // untyped float registers
+
+	// Fields above this point are known to assembly.
+
+	// Ptrs is a space that duplicates Ints but with pointer type,
+	// used to make pointers passed or returned  in registers
+	// visible to the GC by making the type unsafe.Pointer.
+	Ptrs [IntArgRegs]unsafe.Pointer
+
+	// ReturnIsPtr is a bitmap that indicates which registers
+	// contain or will contain pointers on the return path from
+	// a reflectcall. The i'th bit indicates whether the i'th
+	// register contains or will contain a valid Go pointer.
+	ReturnIsPtr IntArgRegBitmap
+}
+
+func (r *RegArgs) Dump() {
+	print("Ints:")
+	for _, x := range r.Ints {
+		print(" ", x)
+	}
+	println()
+	print("Floats:")
+	for _, x := range r.Floats {
+		print(" ", x)
+	}
+	println()
+	print("Ptrs:")
+	for _, x := range r.Ptrs {
+		print(" ", x)
+	}
+	println()
+}
+
+// IntRegArgAddr returns a pointer inside of r.Ints[reg] that is appropriately
+// offset for an argument of size argSize.
+//
+// argSize must be non-zero, fit in a register, and a power-of-two.
+//
+// This method is a helper for dealing with the endianness of different CPU
+// architectures, since sub-word-sized arguments in big endian architectures
+// need to be "aligned" to the upper edge of the register to be interpreted
+// by the CPU correctly.
+func (r *RegArgs) IntRegArgAddr(reg int, argSize uintptr) unsafe.Pointer {
+	if argSize > goarch.PtrSize || argSize == 0 || argSize&(argSize-1) != 0 {
+		panic("invalid argSize")
+	}
+	offset := uintptr(0)
+	if goarch.BigEndian {
+		offset = goarch.PtrSize - argSize
+	}
+	return unsafe.Pointer(uintptr(unsafe.Pointer(&r.Ints[reg])) + offset)
+}
+
+// IntArgRegBitmap is a bitmap large enough to hold one bit per
+// integer argument/return register.
+type IntArgRegBitmap [(IntArgRegs + 7) / 8]uint8
+
+// Set sets the i'th bit of the bitmap to 1.
+func (b *IntArgRegBitmap) Set(i int) {
+	b[i/8] |= uint8(1) << (i % 8)
+}
+
+// Get returns whether the i'th bit of the bitmap is set.
+//
+// nosplit because it's called in extremely sensitive contexts, like
+// on the reflectcall return path.
+//
+//go:nosplit
+func (b *IntArgRegBitmap) Get(i int) bool {
+	return b[i/8]&(uint8(1)<<(i%8)) != 0
+}
+
+// FuncPC* intrinsics.
+//
+// CAREFUL: In programs with plugins, FuncPC* can return different values
+// for the same function (because there are actually multiple copies of
+// the same function in the address space). To be safe, don't use the
+// results of this function in any == expression. It is only safe to
+// use the result as an address at which to start executing code.
+
+// FuncPCABI0 returns the entry PC of the function f, which must be a
+// direct reference of a function defined as ABI0. Otherwise it is a
+// compile-time error.
+//
+// Implemented as a compile intrinsic.
+func FuncPCABI0(f any) uintptr
+
+// FuncPCABIInternal returns the entry PC of the function f. If f is a
+// direct reference of a function, it must be defined as ABIInternal.
+// Otherwise it is a compile-time error. If f is not a direct reference
+// of a defined function, it assumes that f is a func value. Otherwise
+// the behavior is undefined.
+//
+// Implemented as a compile intrinsic.
+func FuncPCABIInternal(f any) uintptr
diff --git a/src/internal/abi/abi_amd64.go b/src/internal/abi/abi_amd64.go
new file mode 100644
index 0000000..d3c5678
--- /dev/null
+++ b/src/internal/abi/abi_amd64.go
@@ -0,0 +1,18 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// RAX, RBX, RCX, RDI, RSI, R8, R9, R10, R11.
+	IntArgRegs = 9
+
+	// X0 -> X14.
+	FloatArgRegs = 15
+
+	// We use SSE2 registers which support 64-bit float operations.
+	EffectiveFloatRegSize = 8
+)
diff --git a/src/internal/abi/abi_arm64.go b/src/internal/abi/abi_arm64.go
new file mode 100644
index 0000000..4dc5143
--- /dev/null
+++ b/src/internal/abi/abi_arm64.go
@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// R0 - R15.
+	IntArgRegs = 16
+
+	// F0 - F15.
+	FloatArgRegs = 16
+
+	EffectiveFloatRegSize = 8
+)
diff --git a/src/internal/abi/abi_generic.go b/src/internal/abi/abi_generic.go
new file mode 100644
index 0000000..76ef2e2
--- /dev/null
+++ b/src/internal/abi/abi_generic.go
@@ -0,0 +1,38 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !goexperiment.regabiargs && !amd64 && !arm64 && !ppc64 && !ppc64le && !riscv64
+
+package abi
+
+const (
+	// ABI-related constants.
+	//
+	// In the generic case, these are all zero
+	// which lets them gracefully degrade to ABI0.
+
+	// IntArgRegs is the number of registers dedicated
+	// to passing integer argument values. Result registers are identical
+	// to argument registers, so this number is used for those too.
+	IntArgRegs = 0
+
+	// FloatArgRegs is the number of registers dedicated
+	// to passing floating-point argument values. Result registers are
+	// identical to argument registers, so this number is used for
+	// those too.
+	FloatArgRegs = 0
+
+	// EffectiveFloatRegSize describes the width of floating point
+	// registers on the current platform from the ABI's perspective.
+	//
+	// Since Go only supports 32-bit and 64-bit floating point primitives,
+	// this number should be either 0, 4, or 8. 0 indicates no floating
+	// point registers for the ABI or that floating point values will be
+	// passed via the softfloat ABI.
+	//
+	// For platforms that support larger floating point register widths,
+	// such as x87's 80-bit "registers" (not that we support x87 currently),
+	// use 8.
+	EffectiveFloatRegSize = 0
+)
diff --git a/src/internal/abi/abi_ppc64x.go b/src/internal/abi/abi_ppc64x.go
new file mode 100644
index 0000000..73416d7
--- /dev/null
+++ b/src/internal/abi/abi_ppc64x.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// R3 - R10, R14 - R17.
+	IntArgRegs = 12
+
+	// F1 - F12.
+	FloatArgRegs = 12
+
+	EffectiveFloatRegSize = 8
+)
diff --git a/src/internal/abi/abi_riscv64.go b/src/internal/abi/abi_riscv64.go
new file mode 100644
index 0000000..2bcd9d6
--- /dev/null
+++ b/src/internal/abi/abi_riscv64.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi
+
+const (
+	// See abi_generic.go.
+
+	// X8 - X23
+	IntArgRegs = 16
+
+	// F8 - F23.
+	FloatArgRegs = 16
+
+	EffectiveFloatRegSize = 8
+)
diff --git a/src/internal/abi/abi_test.go b/src/internal/abi/abi_test.go
new file mode 100644
index 0000000..f0d8dce
--- /dev/null
+++ b/src/internal/abi/abi_test.go
@@ -0,0 +1,79 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi_test
+
+import (
+	"internal/abi"
+	"internal/testenv"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestFuncPC(t *testing.T) {
+	// Test that FuncPC* can get correct function PC.
+	pcFromAsm := abi.FuncPCTestFnAddr
+
+	// Test FuncPC for locally defined function
+	pcFromGo := abi.FuncPCTest()
+	if pcFromGo != pcFromAsm {
+		t.Errorf("FuncPC returns wrong PC, want %x, got %x", pcFromAsm, pcFromGo)
+	}
+
+	// Test FuncPC for imported function
+	pcFromGo = abi.FuncPCABI0(abi.FuncPCTestFn)
+	if pcFromGo != pcFromAsm {
+		t.Errorf("FuncPC returns wrong PC, want %x, got %x", pcFromAsm, pcFromGo)
+	}
+}
+
+func TestFuncPCCompileError(t *testing.T) {
+	// Test that FuncPC* on a function of a mismatched ABI is rejected.
+	testenv.MustHaveGoBuild(t)
+
+	// We want to test internal package, which we cannot normally import.
+	// Run the assembler and compiler manually.
+	tmpdir := t.TempDir()
+	asmSrc := filepath.Join("testdata", "x.s")
+	goSrc := filepath.Join("testdata", "x.go")
+	symabi := filepath.Join(tmpdir, "symabi")
+	obj := filepath.Join(tmpdir, "x.o")
+
+	importcfgfile := filepath.Join(tmpdir, "hello.importcfg")
+	testenv.WriteImportcfg(t, importcfgfile, nil)
+
+	// parse assembly code for symabi.
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "asm", "-gensymabis", "-o", symabi, asmSrc)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go tool asm -gensymabis failed: %v\n%s", err, out)
+	}
+
+	// compile go code.
+	cmd = exec.Command(testenv.GoToolPath(t), "tool", "compile", "-importcfg="+importcfgfile, "-p=p", "-symabis", symabi, "-o", obj, goSrc)
+	out, err = cmd.CombinedOutput()
+	if err == nil {
+		t.Fatalf("go tool compile did not fail")
+	}
+
+	// Expect errors in line 17, 18, 20, no errors on other lines.
+	want := []string{"x.go:17", "x.go:18", "x.go:20"}
+	got := strings.Split(string(out), "\n")
+	if got[len(got)-1] == "" {
+		got = got[:len(got)-1] // remove last empty line
+	}
+	for i, s := range got {
+		if !strings.Contains(s, want[i]) {
+			t.Errorf("did not error on line %s", want[i])
+		}
+	}
+	if len(got) != len(want) {
+		t.Errorf("unexpected number of errors, want %d, got %d", len(want), len(got))
+	}
+	if t.Failed() {
+		t.Logf("output:\n%s", string(out))
+	}
+}
diff --git a/src/internal/abi/abi_test.s b/src/internal/abi/abi_test.s
new file mode 100644
index 0000000..93ace3e
--- /dev/null
+++ b/src/internal/abi/abi_test.s
@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+#ifdef GOARCH_386
+#define PTRSIZE 4
+#endif
+#ifdef GOARCH_arm
+#define PTRSIZE 4
+#endif
+#ifdef GOARCH_mips
+#define PTRSIZE 4
+#endif
+#ifdef GOARCH_mipsle
+#define PTRSIZE 4
+#endif
+#ifndef PTRSIZE
+#define PTRSIZE 8
+#endif
+
+TEXT	internal∕abi·FuncPCTestFn(SB),NOSPLIT,$0-0
+	RET
+
+GLOBL	internal∕abi·FuncPCTestFnAddr(SB), NOPTR, $PTRSIZE
+DATA	internal∕abi·FuncPCTestFnAddr(SB)/PTRSIZE, $internal∕abi·FuncPCTestFn(SB)
diff --git a/src/internal/abi/export_test.go b/src/internal/abi/export_test.go
new file mode 100644
index 0000000..2a87e9d
--- /dev/null
+++ b/src/internal/abi/export_test.go
@@ -0,0 +1,14 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package abi
+
+func FuncPCTestFn()
+
+var FuncPCTestFnAddr uintptr // address of FuncPCTestFn, directly retrieved from assembly
+
+//go:noinline
+func FuncPCTest() uintptr {
+	return FuncPCABI0(FuncPCTestFn)
+}
diff --git a/src/internal/abi/testdata/x.go b/src/internal/abi/testdata/x.go
new file mode 100644
index 0000000..cae103d
--- /dev/null
+++ b/src/internal/abi/testdata/x.go
@@ -0,0 +1,22 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package x
+
+import "internal/abi"
+
+func Fn0() // defined in assembly
+
+func Fn1() {}
+
+var FnExpr func()
+
+func test() {
+	_ = abi.FuncPCABI0(Fn0)           // line 16, no error
+	_ = abi.FuncPCABIInternal(Fn0)    // line 17, error
+	_ = abi.FuncPCABI0(Fn1)           // line 18, error
+	_ = abi.FuncPCABIInternal(Fn1)    // line 19, no error
+	_ = abi.FuncPCABI0(FnExpr)        // line 20, error
+	_ = abi.FuncPCABIInternal(FnExpr) // line 21, no error
+}
diff --git a/src/internal/abi/testdata/x.s b/src/internal/abi/testdata/x.s
new file mode 100644
index 0000000..63c1385
--- /dev/null
+++ b/src/internal/abi/testdata/x.s
@@ -0,0 +1,6 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+TEXT	·Fn0(SB), 0, $0-0
+	RET
diff --git a/src/internal/buildcfg/cfg.go b/src/internal/buildcfg/cfg.go
new file mode 100644
index 0000000..a0736aa
--- /dev/null
+++ b/src/internal/buildcfg/cfg.go
@@ -0,0 +1,234 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package buildcfg provides access to the build configuration
+// described by the current environment. It is for use by build tools
+// such as cmd/go or cmd/compile and for setting up go/build's Default context.
+//
+// Note that it does NOT provide access to the build configuration used to
+// build the currently-running binary. For that, use runtime.GOOS etc
+// as well as internal/goexperiment.
+package buildcfg
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+var (
+	GOROOT   = runtime.GOROOT() // cached for efficiency
+	GOARCH   = envOr("GOARCH", defaultGOARCH)
+	GOOS     = envOr("GOOS", defaultGOOS)
+	GO386    = envOr("GO386", defaultGO386)
+	GOAMD64  = goamd64()
+	GOARM    = goarm()
+	GOMIPS   = gomips()
+	GOMIPS64 = gomips64()
+	GOPPC64  = goppc64()
+	GOWASM   = gowasm()
+	ToolTags = toolTags()
+	GO_LDSO  = defaultGO_LDSO
+	Version  = version
+)
+
+// Error is one of the errors found (if any) in the build configuration.
+var Error error
+
+// Check exits the program with a fatal error if Error is non-nil.
+func Check() {
+	if Error != nil {
+		fmt.Fprintf(os.Stderr, "%s: %v\n", filepath.Base(os.Args[0]), Error)
+		os.Exit(2)
+	}
+}
+
+func envOr(key, value string) string {
+	if x := os.Getenv(key); x != "" {
+		return x
+	}
+	return value
+}
+
+func goamd64() int {
+	switch v := envOr("GOAMD64", defaultGOAMD64); v {
+	case "v1":
+		return 1
+	case "v2":
+		return 2
+	case "v3":
+		return 3
+	case "v4":
+		return 4
+	}
+	Error = fmt.Errorf("invalid GOAMD64: must be v1, v2, v3, v4")
+	return int(defaultGOAMD64[len("v")] - '0')
+}
+
+func goarm() int {
+	def := defaultGOARM
+	if GOOS == "android" && GOARCH == "arm" {
+		// Android arm devices always support GOARM=7.
+		def = "7"
+	}
+	switch v := envOr("GOARM", def); v {
+	case "5":
+		return 5
+	case "6":
+		return 6
+	case "7":
+		return 7
+	}
+	Error = fmt.Errorf("invalid GOARM: must be 5, 6, 7")
+	return int(def[0] - '0')
+}
+
+func gomips() string {
+	switch v := envOr("GOMIPS", defaultGOMIPS); v {
+	case "hardfloat", "softfloat":
+		return v
+	}
+	Error = fmt.Errorf("invalid GOMIPS: must be hardfloat, softfloat")
+	return defaultGOMIPS
+}
+
+func gomips64() string {
+	switch v := envOr("GOMIPS64", defaultGOMIPS64); v {
+	case "hardfloat", "softfloat":
+		return v
+	}
+	Error = fmt.Errorf("invalid GOMIPS64: must be hardfloat, softfloat")
+	return defaultGOMIPS64
+}
+
+func goppc64() int {
+	switch v := envOr("GOPPC64", defaultGOPPC64); v {
+	case "power8":
+		return 8
+	case "power9":
+		return 9
+	case "power10":
+		return 10
+	}
+	Error = fmt.Errorf("invalid GOPPC64: must be power8, power9, power10")
+	return int(defaultGOPPC64[len("power")] - '0')
+}
+
+type gowasmFeatures struct {
+	SatConv bool
+	SignExt bool
+}
+
+func (f gowasmFeatures) String() string {
+	var flags []string
+	if f.SatConv {
+		flags = append(flags, "satconv")
+	}
+	if f.SignExt {
+		flags = append(flags, "signext")
+	}
+	return strings.Join(flags, ",")
+}
+
+func gowasm() (f gowasmFeatures) {
+	for _, opt := range strings.Split(envOr("GOWASM", ""), ",") {
+		switch opt {
+		case "satconv":
+			f.SatConv = true
+		case "signext":
+			f.SignExt = true
+		case "":
+			// ignore
+		default:
+			Error = fmt.Errorf("invalid GOWASM: no such feature %q", opt)
+		}
+	}
+	return
+}
+
+func Getgoextlinkenabled() string {
+	return envOr("GO_EXTLINK_ENABLED", defaultGO_EXTLINK_ENABLED)
+}
+
+func toolTags() []string {
+	tags := experimentTags()
+	tags = append(tags, gogoarchTags()...)
+	return tags
+}
+
+func experimentTags() []string {
+	var list []string
+	// For each experiment that has been enabled in the toolchain, define a
+	// build tag with the same name but prefixed by "goexperiment." which can be
+	// used for compiling alternative files for the experiment. This allows
+	// changes for the experiment, like extra struct fields in the runtime,
+	// without affecting the base non-experiment code at all.
+	for _, exp := range Experiment.Enabled() {
+		list = append(list, "goexperiment."+exp)
+	}
+	return list
+}
+
+// GOGOARCH returns the name and value of the GO$GOARCH setting.
+// For example, if GOARCH is "amd64" it might return "GOAMD64", "v2".
+func GOGOARCH() (name, value string) {
+	switch GOARCH {
+	case "386":
+		return "GO386", GO386
+	case "amd64":
+		return "GOAMD64", fmt.Sprintf("v%d", GOAMD64)
+	case "arm":
+		return "GOARM", fmt.Sprintf("%d", GOARM)
+	case "mips", "mipsle":
+		return "GOMIPS", GOMIPS
+	case "mips64", "mips64le":
+		return "GOMIPS64", GOMIPS64
+	case "ppc64", "ppc64le":
+		return "GOPPC64", fmt.Sprintf("power%d", GOPPC64)
+	case "wasm":
+		return "GOWASM", GOWASM.String()
+	}
+	return "", ""
+}
+
+func gogoarchTags() []string {
+	switch GOARCH {
+	case "386":
+		return []string{GOARCH + "." + GO386}
+	case "amd64":
+		var list []string
+		for i := 1; i <= GOAMD64; i++ {
+			list = append(list, fmt.Sprintf("%s.v%d", GOARCH, i))
+		}
+		return list
+	case "arm":
+		var list []string
+		for i := 5; i <= GOARM; i++ {
+			list = append(list, fmt.Sprintf("%s.%d", GOARCH, i))
+		}
+		return list
+	case "mips", "mipsle":
+		return []string{GOARCH + "." + GOMIPS}
+	case "mips64", "mips64le":
+		return []string{GOARCH + "." + GOMIPS64}
+	case "ppc64", "ppc64le":
+		var list []string
+		for i := 8; i <= GOPPC64; i++ {
+			list = append(list, fmt.Sprintf("%s.power%d", GOARCH, i))
+		}
+		return list
+	case "wasm":
+		var list []string
+		if GOWASM.SatConv {
+			list = append(list, GOARCH+".satconv")
+		}
+		if GOWASM.SignExt {
+			list = append(list, GOARCH+".signext")
+		}
+		return list
+	}
+	return nil
+}
diff --git a/src/internal/buildcfg/cfg_test.go b/src/internal/buildcfg/cfg_test.go
new file mode 100644
index 0000000..0123593
--- /dev/null
+++ b/src/internal/buildcfg/cfg_test.go
@@ -0,0 +1,26 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buildcfg
+
+import (
+	"os"
+	"testing"
+)
+
+func TestConfigFlags(t *testing.T) {
+	os.Setenv("GOAMD64", "v1")
+	if goamd64() != 1 {
+		t.Errorf("Wrong parsing of GOAMD64=v1")
+	}
+	os.Setenv("GOAMD64", "v4")
+	if goamd64() != 4 {
+		t.Errorf("Wrong parsing of GOAMD64=v4")
+	}
+	Error = nil
+	os.Setenv("GOAMD64", "1")
+	if goamd64(); Error == nil {
+		t.Errorf("Wrong parsing of GOAMD64=1")
+	}
+}
diff --git a/src/internal/buildcfg/exp.go b/src/internal/buildcfg/exp.go
new file mode 100644
index 0000000..71f8f56
--- /dev/null
+++ b/src/internal/buildcfg/exp.go
@@ -0,0 +1,191 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buildcfg
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+
+	"internal/goexperiment"
+)
+
+// ExperimentFlags represents a set of GOEXPERIMENT flags relative to a baseline
+// (platform-default) experiment configuration.
+type ExperimentFlags struct {
+	goexperiment.Flags
+	baseline goexperiment.Flags
+}
+
+// Experiment contains the toolchain experiments enabled for the
+// current build.
+//
+// (This is not necessarily the set of experiments the compiler itself
+// was built with.)
+//
+// experimentBaseline specifies the experiment flags that are enabled by
+// default in the current toolchain. This is, in effect, the "control"
+// configuration and any variation from this is an experiment.
+var Experiment ExperimentFlags = func() ExperimentFlags {
+	flags, err := ParseGOEXPERIMENT(GOOS, GOARCH, envOr("GOEXPERIMENT", defaultGOEXPERIMENT))
+	if err != nil {
+		Error = err
+		return ExperimentFlags{}
+	}
+	return *flags
+}()
+
+// DefaultGOEXPERIMENT is the embedded default GOEXPERIMENT string.
+// It is not guaranteed to be canonical.
+const DefaultGOEXPERIMENT = defaultGOEXPERIMENT
+
+// FramePointerEnabled enables the use of platform conventions for
+// saving frame pointers.
+//
+// This used to be an experiment, but now it's always enabled on
+// platforms that support it.
+//
+// Note: must agree with runtime.framepointer_enabled.
+var FramePointerEnabled = GOARCH == "amd64" || GOARCH == "arm64"
+
+// ParseGOEXPERIMENT parses a (GOOS, GOARCH, GOEXPERIMENT)
+// configuration tuple and returns the enabled and baseline experiment
+// flag sets.
+//
+// TODO(mdempsky): Move to internal/goexperiment.
+func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
+	// regabiSupported is set to true on platforms where register ABI is
+	// supported and enabled by default.
+	// regabiAlwaysOn is set to true on platforms where register ABI is
+	// always on.
+	var regabiSupported, regabiAlwaysOn bool
+	switch goarch {
+	case "amd64", "arm64", "ppc64le", "ppc64", "riscv64":
+		regabiAlwaysOn = true
+		regabiSupported = true
+	}
+
+	baseline := goexperiment.Flags{
+		RegabiWrappers:   regabiSupported,
+		RegabiArgs:       regabiSupported,
+		Unified:          true,
+		CoverageRedesign: true,
+	}
+
+	// Start with the statically enabled set of experiments.
+	flags := &ExperimentFlags{
+		Flags:    baseline,
+		baseline: baseline,
+	}
+
+	// Pick up any changes to the baseline configuration from the
+	// GOEXPERIMENT environment. This can be set at make.bash time
+	// and overridden at build time.
+	if goexp != "" {
+		// Create a map of known experiment names.
+		names := make(map[string]func(bool))
+		rv := reflect.ValueOf(&flags.Flags).Elem()
+		rt := rv.Type()
+		for i := 0; i < rt.NumField(); i++ {
+			field := rv.Field(i)
+			names[strings.ToLower(rt.Field(i).Name)] = field.SetBool
+		}
+
+		// "regabi" is an alias for all working regabi
+		// subexperiments, and not an experiment itself. Doing
+		// this as an alias make both "regabi" and "noregabi"
+		// do the right thing.
+		names["regabi"] = func(v bool) {
+			flags.RegabiWrappers = v
+			flags.RegabiArgs = v
+		}
+
+		// Parse names.
+		for _, f := range strings.Split(goexp, ",") {
+			if f == "" {
+				continue
+			}
+			if f == "none" {
+				// GOEXPERIMENT=none disables all experiment flags.
+				// This is used by cmd/dist, which doesn't know how
+				// to build with any experiment flags.
+				flags.Flags = goexperiment.Flags{}
+				continue
+			}
+			val := true
+			if strings.HasPrefix(f, "no") {
+				f, val = f[2:], false
+			}
+			set, ok := names[f]
+			if !ok {
+				return nil, fmt.Errorf("unknown GOEXPERIMENT %s", f)
+			}
+			set(val)
+		}
+	}
+
+	if regabiAlwaysOn {
+		flags.RegabiWrappers = true
+		flags.RegabiArgs = true
+	}
+	// regabi is only supported on amd64, arm64, riscv64, ppc64 and ppc64le.
+	if !regabiSupported {
+		flags.RegabiWrappers = false
+		flags.RegabiArgs = false
+	}
+	// Check regabi dependencies.
+	if flags.RegabiArgs && !flags.RegabiWrappers {
+		return nil, fmt.Errorf("GOEXPERIMENT regabiargs requires regabiwrappers")
+	}
+	return flags, nil
+}
+
+// String returns the canonical GOEXPERIMENT string to enable this experiment
+// configuration. (Experiments in the same state as in the baseline are elided.)
+func (exp *ExperimentFlags) String() string {
+	return strings.Join(expList(&exp.Flags, &exp.baseline, false), ",")
+}
+
+// expList returns the list of lower-cased experiment names for
+// experiments that differ from base. base may be nil to indicate no
+// experiments. If all is true, then include all experiment flags,
+// regardless of base.
+func expList(exp, base *goexperiment.Flags, all bool) []string {
+	var list []string
+	rv := reflect.ValueOf(exp).Elem()
+	var rBase reflect.Value
+	if base != nil {
+		rBase = reflect.ValueOf(base).Elem()
+	}
+	rt := rv.Type()
+	for i := 0; i < rt.NumField(); i++ {
+		name := strings.ToLower(rt.Field(i).Name)
+		val := rv.Field(i).Bool()
+		baseVal := false
+		if base != nil {
+			baseVal = rBase.Field(i).Bool()
+		}
+		if all || val != baseVal {
+			if val {
+				list = append(list, name)
+			} else {
+				list = append(list, "no"+name)
+			}
+		}
+	}
+	return list
+}
+
+// Enabled returns a list of enabled experiments, as
+// lower-cased experiment names.
+func (exp *ExperimentFlags) Enabled() []string {
+	return expList(&exp.Flags, nil, false)
+}
+
+// All returns a list of all experiment settings.
+// Disabled experiments appear in the list prefixed by "no".
+func (exp *ExperimentFlags) All() []string {
+	return expList(&exp.Flags, nil, true)
+}
diff --git a/src/internal/bytealg/bytealg.go b/src/internal/bytealg/bytealg.go
new file mode 100644
index 0000000..ebebce7
--- /dev/null
+++ b/src/internal/bytealg/bytealg.go
@@ -0,0 +1,150 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+import (
+	"internal/cpu"
+	"unsafe"
+)
+
+// Offsets into internal/cpu records for use in assembly.
+const (
+	offsetX86HasSSE42  = unsafe.Offsetof(cpu.X86.HasSSE42)
+	offsetX86HasAVX2   = unsafe.Offsetof(cpu.X86.HasAVX2)
+	offsetX86HasPOPCNT = unsafe.Offsetof(cpu.X86.HasPOPCNT)
+
+	offsetS390xHasVX = unsafe.Offsetof(cpu.S390X.HasVX)
+
+	offsetPPC64HasPOWER9 = unsafe.Offsetof(cpu.PPC64.IsPOWER9)
+)
+
+// MaxLen is the maximum length of the string to be searched for (argument b) in Index.
+// If MaxLen is not 0, make sure MaxLen >= 4.
+var MaxLen int
+
+// FIXME: the logic of HashStrBytes, HashStrRevBytes, IndexRabinKarpBytes and HashStr, HashStrRev,
+// IndexRabinKarp are exactly the same, except that the types are different. Can we eliminate
+// three of them without causing allocation?
+
+// PrimeRK is the prime base used in Rabin-Karp algorithm.
+const PrimeRK = 16777619
+
+// HashStrBytes returns the hash and the appropriate multiplicative
+// factor for use in Rabin-Karp algorithm.
+func HashStrBytes(sep []byte) (uint32, uint32) {
+	hash := uint32(0)
+	for i := 0; i < len(sep); i++ {
+		hash = hash*PrimeRK + uint32(sep[i])
+	}
+	var pow, sq uint32 = 1, PrimeRK
+	for i := len(sep); i > 0; i >>= 1 {
+		if i&1 != 0 {
+			pow *= sq
+		}
+		sq *= sq
+	}
+	return hash, pow
+}
+
+// HashStr returns the hash and the appropriate multiplicative
+// factor for use in Rabin-Karp algorithm.
+func HashStr(sep string) (uint32, uint32) {
+	hash := uint32(0)
+	for i := 0; i < len(sep); i++ {
+		hash = hash*PrimeRK + uint32(sep[i])
+	}
+	var pow, sq uint32 = 1, PrimeRK
+	for i := len(sep); i > 0; i >>= 1 {
+		if i&1 != 0 {
+			pow *= sq
+		}
+		sq *= sq
+	}
+	return hash, pow
+}
+
+// HashStrRevBytes returns the hash of the reverse of sep and the
+// appropriate multiplicative factor for use in Rabin-Karp algorithm.
+func HashStrRevBytes(sep []byte) (uint32, uint32) {
+	hash := uint32(0)
+	for i := len(sep) - 1; i >= 0; i-- {
+		hash = hash*PrimeRK + uint32(sep[i])
+	}
+	var pow, sq uint32 = 1, PrimeRK
+	for i := len(sep); i > 0; i >>= 1 {
+		if i&1 != 0 {
+			pow *= sq
+		}
+		sq *= sq
+	}
+	return hash, pow
+}
+
+// HashStrRev returns the hash of the reverse of sep and the
+// appropriate multiplicative factor for use in Rabin-Karp algorithm.
+func HashStrRev(sep string) (uint32, uint32) {
+	hash := uint32(0)
+	for i := len(sep) - 1; i >= 0; i-- {
+		hash = hash*PrimeRK + uint32(sep[i])
+	}
+	var pow, sq uint32 = 1, PrimeRK
+	for i := len(sep); i > 0; i >>= 1 {
+		if i&1 != 0 {
+			pow *= sq
+		}
+		sq *= sq
+	}
+	return hash, pow
+}
+
+// IndexRabinKarpBytes uses the Rabin-Karp search algorithm to return the index of the
+// first occurrence of substr in s, or -1 if not present.
+func IndexRabinKarpBytes(s, sep []byte) int {
+	// Rabin-Karp search
+	hashsep, pow := HashStrBytes(sep)
+	n := len(sep)
+	var h uint32
+	for i := 0; i < n; i++ {
+		h = h*PrimeRK + uint32(s[i])
+	}
+	if h == hashsep && Equal(s[:n], sep) {
+		return 0
+	}
+	for i := n; i < len(s); {
+		h *= PrimeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i-n])
+		i++
+		if h == hashsep && Equal(s[i-n:i], sep) {
+			return i - n
+		}
+	}
+	return -1
+}
+
+// IndexRabinKarp uses the Rabin-Karp search algorithm to return the index of the
+// first occurrence of substr in s, or -1 if not present.
+func IndexRabinKarp(s, substr string) int {
+	// Rabin-Karp search
+	hashss, pow := HashStr(substr)
+	n := len(substr)
+	var h uint32
+	for i := 0; i < n; i++ {
+		h = h*PrimeRK + uint32(s[i])
+	}
+	if h == hashss && s[:n] == substr {
+		return 0
+	}
+	for i := n; i < len(s); {
+		h *= PrimeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i-n])
+		i++
+		if h == hashss && s[i-n:i] == substr {
+			return i - n
+		}
+	}
+	return -1
+}
diff --git a/src/internal/bytealg/compare_386.s b/src/internal/bytealg/compare_386.s
new file mode 100644
index 0000000..27b660c
--- /dev/null
+++ b/src/internal/bytealg/compare_386.s
@@ -0,0 +1,144 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT,$0-28
+	MOVL	a_base+0(FP), SI
+	MOVL	a_len+4(FP), BX
+	MOVL	b_base+12(FP), DI
+	MOVL	b_len+16(FP), DX
+	LEAL	ret+24(FP), AX
+	JMP	cmpbody<>(SB)
+
+TEXT runtime·cmpstring(SB),NOSPLIT,$0-20
+	MOVL	a_base+0(FP), SI
+	MOVL	a_len+4(FP), BX
+	MOVL	b_base+8(FP), DI
+	MOVL	b_len+12(FP), DX
+	LEAL	ret+16(FP), AX
+	JMP	cmpbody<>(SB)
+
+// input:
+//   SI = a
+//   DI = b
+//   BX = alen
+//   DX = blen
+//   AX = address of return word (set to 1/0/-1)
+TEXT cmpbody<>(SB),NOSPLIT,$0-0
+	MOVL	DX, BP
+	SUBL	BX, DX // DX = blen-alen
+	JLE	2(PC)
+	MOVL	BX, BP // BP = min(alen, blen)
+	CMPL	SI, DI
+	JEQ	allsame
+	CMPL	BP, $4
+	JB	small
+#ifdef GO386_softfloat
+	JMP	mediumloop
+#endif
+largeloop:
+	CMPL	BP, $16
+	JB	mediumloop
+	MOVOU	(SI), X0
+	MOVOU	(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, BX
+	XORL	$0xffff, BX	// convert EQ to NE
+	JNE	diff16	// branch if at least one byte is not equal
+	ADDL	$16, SI
+	ADDL	$16, DI
+	SUBL	$16, BP
+	JMP	largeloop
+
+diff16:
+	BSFL	BX, BX	// index of first byte that differs
+	XORL	DX, DX
+	MOVB	(SI)(BX*1), CX
+	CMPB	CX, (DI)(BX*1)
+	SETHI	DX
+	LEAL	-1(DX*2), DX	// convert 1/0 to +1/-1
+	MOVL	DX, (AX)
+	RET
+
+mediumloop:
+	CMPL	BP, $4
+	JBE	_0through4
+	MOVL	(SI), BX
+	MOVL	(DI), CX
+	CMPL	BX, CX
+	JNE	diff4
+	ADDL	$4, SI
+	ADDL	$4, DI
+	SUBL	$4, BP
+	JMP	mediumloop
+
+_0through4:
+	MOVL	-4(SI)(BP*1), BX
+	MOVL	-4(DI)(BP*1), CX
+	CMPL	BX, CX
+	JEQ	allsame
+
+diff4:
+	BSWAPL	BX	// reverse order of bytes
+	BSWAPL	CX
+	XORL	BX, CX	// find bit differences
+	BSRL	CX, CX	// index of highest bit difference
+	SHRL	CX, BX	// move a's bit to bottom
+	ANDL	$1, BX	// mask bit
+	LEAL	-1(BX*2), BX // 1/0 => +1/-1
+	MOVL	BX, (AX)
+	RET
+
+	// 0-3 bytes in common
+small:
+	LEAL	(BP*8), CX
+	NEGL	CX
+	JEQ	allsame
+
+	// load si
+	CMPB	SI, $0xfc
+	JA	si_high
+	MOVL	(SI), SI
+	JMP	si_finish
+si_high:
+	MOVL	-4(SI)(BP*1), SI
+	SHRL	CX, SI
+si_finish:
+	SHLL	CX, SI
+
+	// same for di
+	CMPB	DI, $0xfc
+	JA	di_high
+	MOVL	(DI), DI
+	JMP	di_finish
+di_high:
+	MOVL	-4(DI)(BP*1), DI
+	SHRL	CX, DI
+di_finish:
+	SHLL	CX, DI
+
+	BSWAPL	SI	// reverse order of bytes
+	BSWAPL	DI
+	XORL	SI, DI	// find bit differences
+	JEQ	allsame
+	BSRL	DI, CX	// index of highest bit difference
+	SHRL	CX, SI	// move a's bit to bottom
+	ANDL	$1, SI	// mask bit
+	LEAL	-1(SI*2), BX // 1/0 => +1/-1
+	MOVL	BX, (AX)
+	RET
+
+	// all the bytes in common are the same, so we just need
+	// to compare the lengths.
+allsame:
+	XORL	BX, BX
+	XORL	CX, CX
+	TESTL	DX, DX
+	SETLT	BX	// 1 if alen > blen
+	SETEQ	CX	// 1 if alen == blen
+	LEAL	-1(CX)(BX*2), BX	// 1,0,-1 result
+	MOVL	BX, (AX)
+	RET
diff --git a/src/internal/bytealg/compare_amd64.s b/src/internal/bytealg/compare_amd64.s
new file mode 100644
index 0000000..fdd015f
--- /dev/null
+++ b/src/internal/bytealg/compare_amd64.s
@@ -0,0 +1,237 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "asm_amd64.h"
+#include "textflag.h"
+
+TEXT ·Compare<ABIInternal>(SB),NOSPLIT,$0-56
+	// AX = a_base (want in SI)
+	// BX = a_len  (want in BX)
+	// CX = a_cap  (unused)
+	// DI = b_base (want in DI)
+	// SI = b_len  (want in DX)
+	// R8 = b_cap  (unused)
+	MOVQ	SI, DX
+	MOVQ	AX, SI
+	JMP	cmpbody<>(SB)
+
+TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT,$0-40
+	// AX = a_base (want in SI)
+	// BX = a_len  (want in BX)
+	// CX = b_base (want in DI)
+	// DI = b_len  (want in DX)
+	MOVQ	AX, SI
+	MOVQ	DI, DX
+	MOVQ	CX, DI
+	JMP	cmpbody<>(SB)
+
+// input:
+//   SI = a
+//   DI = b
+//   BX = alen
+//   DX = blen
+// output:
+//   AX = output (-1/0/1)
+TEXT cmpbody<>(SB),NOSPLIT,$0-0
+	CMPQ	SI, DI
+	JEQ	allsame
+	CMPQ	BX, DX
+	MOVQ	DX, R8
+	CMOVQLT	BX, R8 // R8 = min(alen, blen) = # of bytes to compare
+	CMPQ	R8, $8
+	JB	small
+
+	CMPQ	R8, $63
+	JBE	loop
+#ifndef hasAVX2
+	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
+	JEQ     big_loop_avx2
+	JMP	big_loop
+#else
+	JMP	big_loop_avx2
+#endif
+loop:
+	CMPQ	R8, $16
+	JBE	_0through16
+	MOVOU	(SI), X0
+	MOVOU	(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, AX
+	XORQ	$0xffff, AX	// convert EQ to NE
+	JNE	diff16	// branch if at least one byte is not equal
+	ADDQ	$16, SI
+	ADDQ	$16, DI
+	SUBQ	$16, R8
+	JMP	loop
+
+diff64:
+	ADDQ	$48, SI
+	ADDQ	$48, DI
+	JMP	diff16
+diff48:
+	ADDQ	$32, SI
+	ADDQ	$32, DI
+	JMP	diff16
+diff32:
+	ADDQ	$16, SI
+	ADDQ	$16, DI
+	// AX = bit mask of differences
+diff16:
+	BSFQ	AX, BX	// index of first byte that differs
+	XORQ	AX, AX
+	MOVB	(SI)(BX*1), CX
+	CMPB	CX, (DI)(BX*1)
+	SETHI	AX
+	LEAQ	-1(AX*2), AX	// convert 1/0 to +1/-1
+	RET
+
+	// 0 through 16 bytes left, alen>=8, blen>=8
+_0through16:
+	CMPQ	R8, $8
+	JBE	_0through8
+	MOVQ	(SI), AX
+	MOVQ	(DI), CX
+	CMPQ	AX, CX
+	JNE	diff8
+_0through8:
+	MOVQ	-8(SI)(R8*1), AX
+	MOVQ	-8(DI)(R8*1), CX
+	CMPQ	AX, CX
+	JEQ	allsame
+
+	// AX and CX contain parts of a and b that differ.
+diff8:
+	BSWAPQ	AX	// reverse order of bytes
+	BSWAPQ	CX
+	XORQ	AX, CX
+	BSRQ	CX, CX	// index of highest bit difference
+	SHRQ	CX, AX	// move a's bit to bottom
+	ANDQ	$1, AX	// mask bit
+	LEAQ	-1(AX*2), AX // 1/0 => +1/-1
+	RET
+
+	// 0-7 bytes in common
+small:
+	LEAQ	(R8*8), CX	// bytes left -> bits left
+	NEGQ	CX		//  - bits lift (== 64 - bits left mod 64)
+	JEQ	allsame
+
+	// load bytes of a into high bytes of AX
+	CMPB	SI, $0xf8
+	JA	si_high
+	MOVQ	(SI), SI
+	JMP	si_finish
+si_high:
+	MOVQ	-8(SI)(R8*1), SI
+	SHRQ	CX, SI
+si_finish:
+	SHLQ	CX, SI
+
+	// load bytes of b in to high bytes of BX
+	CMPB	DI, $0xf8
+	JA	di_high
+	MOVQ	(DI), DI
+	JMP	di_finish
+di_high:
+	MOVQ	-8(DI)(R8*1), DI
+	SHRQ	CX, DI
+di_finish:
+	SHLQ	CX, DI
+
+	BSWAPQ	SI	// reverse order of bytes
+	BSWAPQ	DI
+	XORQ	SI, DI	// find bit differences
+	JEQ	allsame
+	BSRQ	DI, CX	// index of highest bit difference
+	SHRQ	CX, SI	// move a's bit to bottom
+	ANDQ	$1, SI	// mask bit
+	LEAQ	-1(SI*2), AX // 1/0 => +1/-1
+	RET
+
+allsame:
+	XORQ	AX, AX
+	XORQ	CX, CX
+	CMPQ	BX, DX
+	SETGT	AX	// 1 if alen > blen
+	SETEQ	CX	// 1 if alen == blen
+	LEAQ	-1(CX)(AX*2), AX	// 1,0,-1 result
+	RET
+
+	// this works for >= 64 bytes of data.
+#ifndef hasAVX2
+big_loop:
+	MOVOU	(SI), X0
+	MOVOU	(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, AX
+	XORQ	$0xffff, AX
+	JNE	diff16
+
+	MOVOU	16(SI), X0
+	MOVOU	16(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, AX
+	XORQ	$0xffff, AX
+	JNE	diff32
+
+	MOVOU	32(SI), X0
+	MOVOU	32(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, AX
+	XORQ	$0xffff, AX
+	JNE	diff48
+
+	MOVOU	48(SI), X0
+	MOVOU	48(DI), X1
+	PCMPEQB X0, X1
+	PMOVMSKB X1, AX
+	XORQ	$0xffff, AX
+	JNE	diff64
+
+	ADDQ	$64, SI
+	ADDQ	$64, DI
+	SUBQ	$64, R8
+	CMPQ	R8, $64
+	JBE	loop
+	JMP	big_loop
+#endif
+
+	// Compare 64-bytes per loop iteration.
+	// Loop is unrolled and uses AVX2.
+big_loop_avx2:
+	VMOVDQU	(SI), Y2
+	VMOVDQU	(DI), Y3
+	VMOVDQU	32(SI), Y4
+	VMOVDQU	32(DI), Y5
+	VPCMPEQB Y2, Y3, Y0
+	VPMOVMSKB Y0, AX
+	XORL	$0xffffffff, AX
+	JNE	diff32_avx2
+	VPCMPEQB Y4, Y5, Y6
+	VPMOVMSKB Y6, AX
+	XORL	$0xffffffff, AX
+	JNE	diff64_avx2
+
+	ADDQ	$64, SI
+	ADDQ	$64, DI
+	SUBQ	$64, R8
+	CMPQ	R8, $64
+	JB	big_loop_avx2_exit
+	JMP	big_loop_avx2
+
+	// Avoid AVX->SSE transition penalty and search first 32 bytes of 64 byte chunk.
+diff32_avx2:
+	VZEROUPPER
+	JMP diff16
+
+	// Same as diff32_avx2, but for last 32 bytes.
+diff64_avx2:
+	VZEROUPPER
+	JMP diff48
+
+	// For <64 bytes remainder jump to normal loop.
+big_loop_avx2_exit:
+	VZEROUPPER
+	JMP loop
diff --git a/src/internal/bytealg/compare_arm.s b/src/internal/bytealg/compare_arm.s
new file mode 100644
index 0000000..80d01a2
--- /dev/null
+++ b/src/internal/bytealg/compare_arm.s
@@ -0,0 +1,86 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT|NOFRAME,$0-28
+	MOVW	a_base+0(FP), R2
+	MOVW	a_len+4(FP), R0
+	MOVW	b_base+12(FP), R3
+	MOVW	b_len+16(FP), R1
+	ADD	$28, R13, R7
+	B	cmpbody<>(SB)
+
+TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-20
+	MOVW	a_base+0(FP), R2
+	MOVW	a_len+4(FP), R0
+	MOVW	b_base+8(FP), R3
+	MOVW	b_len+12(FP), R1
+	ADD	$20, R13, R7
+	B	cmpbody<>(SB)
+
+// On entry:
+// R0 is the length of a
+// R1 is the length of b
+// R2 points to the start of a
+// R3 points to the start of b
+// R7 points to return value (-1/0/1 will be written here)
+//
+// On exit:
+// R4, R5, R6 and R8 are clobbered
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	CMP	R2, R3
+	BEQ	samebytes
+	CMP 	R0, R1
+	MOVW 	R0, R6
+	MOVW.LT	R1, R6		// R6 is min(R0, R1)
+
+	CMP	$0, R6
+	BEQ	samebytes
+	CMP	$4, R6
+	ADD	R2, R6		// R2 is current byte in a, R6 is the end of the range to compare
+	BLT	byte_loop	// length < 4
+	AND	$3, R2, R8
+	CMP	$0, R8
+	BNE	byte_loop	// unaligned a, use byte-wise compare (TODO: try to align a)
+aligned_a:
+	AND	$3, R3, R8
+	CMP	$0, R8
+	BNE	byte_loop	// unaligned b, use byte-wise compare
+	AND	$0xfffffffc, R6, R8
+	// length >= 4
+chunk4_loop:
+	MOVW.P	4(R2), R4
+	MOVW.P	4(R3), R5
+	CMP	R4, R5
+	BNE	cmp
+	CMP	R2, R8
+	BNE	chunk4_loop
+	CMP	R2, R6
+	BEQ	samebytes	// all compared bytes were the same; compare lengths
+byte_loop:
+	MOVBU.P	1(R2), R4
+	MOVBU.P	1(R3), R5
+	CMP	R4, R5
+	BNE	ret
+	CMP	R2, R6
+	BNE	byte_loop
+samebytes:
+	CMP	R0, R1
+	MOVW.LT	$1, R0
+	MOVW.GT	$-1, R0
+	MOVW.EQ	$0, R0
+	MOVW	R0, (R7)
+	RET
+ret:
+	// bytes differed
+	MOVW.LT	$1, R0
+	MOVW.GT	$-1, R0
+	MOVW	R0, (R7)
+	RET
+cmp:
+	SUB	$4, R2, R2
+	SUB	$4, R3, R3
+	B	byte_loop
diff --git a/src/internal/bytealg/compare_arm64.s b/src/internal/bytealg/compare_arm64.s
new file mode 100644
index 0000000..cc02c46
--- /dev/null
+++ b/src/internal/bytealg/compare_arm64.s
@@ -0,0 +1,125 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-56
+	// R0 = a_base (want in R0)
+	// R1 = a_len  (want in R1)
+	// R2 = a_cap  (unused)
+	// R3 = b_base (want in R2)
+	// R4 = b_len  (want in R3)
+	// R5 = b_cap  (unused)
+	MOVD	R3, R2
+	MOVD	R4, R3
+	B	cmpbody<>(SB)
+
+TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// R0 = a_base
+	// R1 = a_len
+	// R2 = b_base
+	// R3 = b_len
+	B	cmpbody<>(SB)
+
+// On entry:
+// R0 points to the start of a
+// R1 is the length of a
+// R2 points to the start of b
+// R3 is the length of b
+//
+// On exit:
+// R0 is the result
+// R4, R5, R6, R8, R9 and R10 are clobbered
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	CMP	R0, R2
+	BEQ	samebytes         // same starting pointers; compare lengths
+	CMP	R1, R3
+	CSEL	LT, R3, R1, R6    // R6 is min(R1, R3)
+
+	CBZ	R6, samebytes
+	BIC	$0xf, R6, R10
+	CBZ	R10, small        // length < 16
+	ADD	R0, R10           // end of chunk16
+	// length >= 16
+chunk16_loop:
+	LDP.P	16(R0), (R4, R8)
+	LDP.P	16(R2), (R5, R9)
+	CMP	R4, R5
+	BNE	cmp
+	CMP	R8, R9
+	BNE	cmpnext
+	CMP	R10, R0
+	BNE	chunk16_loop
+	AND	$0xf, R6, R6
+	CBZ	R6, samebytes
+	SUBS	$8, R6
+	BLT	tail
+	// the length of tail > 8 bytes
+	MOVD.P	8(R0), R4
+	MOVD.P	8(R2), R5
+	CMP	R4, R5
+	BNE	cmp
+	SUB	$8, R6
+	// compare last 8 bytes
+tail:
+	MOVD	(R0)(R6), R4
+	MOVD	(R2)(R6), R5
+	CMP	R4, R5
+	BEQ	samebytes
+cmp:
+	REV	R4, R4
+	REV	R5, R5
+	CMP	R4, R5
+ret:
+	MOVD	$1, R0
+	CNEG	HI, R0, R0
+	RET
+small:
+	TBZ	$3, R6, lt_8
+	MOVD	(R0), R4
+	MOVD	(R2), R5
+	CMP	R4, R5
+	BNE	cmp
+	SUBS	$8, R6
+	BEQ	samebytes
+	ADD	$8, R0
+	ADD	$8, R2
+	SUB	$8, R6
+	B	tail
+lt_8:
+	TBZ	$2, R6, lt_4
+	MOVWU	(R0), R4
+	MOVWU	(R2), R5
+	CMPW	R4, R5
+	BNE	cmp
+	SUBS	$4, R6
+	BEQ	samebytes
+	ADD	$4, R0
+	ADD	$4, R2
+lt_4:
+	TBZ	$1, R6, lt_2
+	MOVHU	(R0), R4
+	MOVHU	(R2), R5
+	CMPW	R4, R5
+	BNE	cmp
+	ADD	$2, R0
+	ADD	$2, R2
+lt_2:
+	TBZ	$0, R6, samebytes
+one:
+	MOVBU	(R0), R4
+	MOVBU	(R2), R5
+	CMPW	R4, R5
+	BNE	ret
+samebytes:
+	CMP	R3, R1
+	CSET	NE, R0
+	CNEG	LO, R0, R0
+	RET
+cmpnext:
+	REV	R8, R4
+	REV	R9, R5
+	CMP	R4, R5
+	B	ret
diff --git a/src/internal/bytealg/compare_generic.go b/src/internal/bytealg/compare_generic.go
new file mode 100644
index 0000000..b04e275
--- /dev/null
+++ b/src/internal/bytealg/compare_generic.go
@@ -0,0 +1,60 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !386 && !amd64 && !s390x && !arm && !arm64 && !loong64 && !ppc64 && !ppc64le && !mips && !mipsle && !wasm && !mips64 && !mips64le && !riscv64
+
+package bytealg
+
+import _ "unsafe" // for go:linkname
+
+func Compare(a, b []byte) int {
+	l := len(a)
+	if len(b) < l {
+		l = len(b)
+	}
+	if l == 0 || &a[0] == &b[0] {
+		goto samebytes
+	}
+	for i := 0; i < l; i++ {
+		c1, c2 := a[i], b[i]
+		if c1 < c2 {
+			return -1
+		}
+		if c1 > c2 {
+			return +1
+		}
+	}
+samebytes:
+	if len(a) < len(b) {
+		return -1
+	}
+	if len(a) > len(b) {
+		return +1
+	}
+	return 0
+}
+
+//go:linkname runtime_cmpstring runtime.cmpstring
+func runtime_cmpstring(a, b string) int {
+	l := len(a)
+	if len(b) < l {
+		l = len(b)
+	}
+	for i := 0; i < l; i++ {
+		c1, c2 := a[i], b[i]
+		if c1 < c2 {
+			return -1
+		}
+		if c1 > c2 {
+			return +1
+		}
+	}
+	if len(a) < len(b) {
+		return -1
+	}
+	if len(a) > len(b) {
+		return +1
+	}
+	return 0
+}
diff --git a/src/internal/bytealg/compare_loong64.s b/src/internal/bytealg/compare_loong64.s
new file mode 100644
index 0000000..54c2dab
--- /dev/null
+++ b/src/internal/bytealg/compare_loong64.s
@@ -0,0 +1,86 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT,$0-56
+	MOVV	a_base+0(FP), R6
+	MOVV	b_base+24(FP), R7
+	MOVV	a_len+8(FP), R4
+	MOVV	b_len+32(FP), R5
+	MOVV	$ret+48(FP), R13
+	JMP	cmpbody<>(SB)
+
+TEXT runtime·cmpstring(SB),NOSPLIT,$0-40
+	MOVV	a_base+0(FP), R6
+	MOVV	b_base+16(FP), R7
+	MOVV	a_len+8(FP), R4
+	MOVV	b_len+24(FP), R5
+	MOVV	$ret+32(FP), R13
+	JMP	cmpbody<>(SB)
+
+// On entry:
+// R4 length of a
+// R5 length of b
+// R6 points to the start of a
+// R7 points to the start of b
+// R13 points to the return value (-1/0/1)
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0
+	BEQ	R6, R7, samebytes // same start of a and b
+
+	SGTU	R4, R5, R9
+	BNE	R0, R9, r2_lt_r1
+	MOVV	R4, R14
+	JMP	entry
+r2_lt_r1:
+	MOVV	R5, R14	// R14 is min(R4, R5)
+entry:
+	ADDV	R6, R14, R12	// R6 start of a, R14 end of a
+	BEQ	R6, R12, samebytes // length is 0
+
+	SRLV	$4, R14		// R14 is number of chunks
+	BEQ	R0, R14, byte_loop
+
+	// make sure both a and b are aligned.
+	OR	R6, R7, R15
+	AND	$7, R15
+	BNE	R0, R15, byte_loop
+
+chunk16_loop:
+	BEQ	R0, R14, byte_loop
+	MOVV	(R6), R8
+	MOVV	(R7), R9
+	BNE	R8, R9, byte_loop
+	MOVV	8(R6), R16
+	MOVV	8(R7), R17
+	ADDV	$16, R6
+	ADDV	$16, R7
+	SUBVU	$1, R14
+	BEQ	R16, R17, chunk16_loop
+	SUBV	$8, R6
+	SUBV	$8, R7
+
+byte_loop:
+	BEQ	R6, R12, samebytes
+	MOVBU	(R6), R8
+	ADDVU	$1, R6
+	MOVBU	(R7), R9
+	ADDVU	$1, R7
+	BEQ	R8, R9, byte_loop
+
+byte_cmp:
+	SGTU	R8, R9, R12 // R12 = 1 if (R8 > R9)
+	BNE	R0, R12, ret
+	MOVV	$-1, R12
+	JMP	ret
+
+samebytes:
+	SGTU	R4, R5, R8
+	SGTU	R5, R4, R9
+	SUBV	R9, R8, R12
+
+ret:
+	MOVV	R12, (R13)
+	RET
diff --git a/src/internal/bytealg/compare_mips64x.s b/src/internal/bytealg/compare_mips64x.s
new file mode 100644
index 0000000..117a9ef
--- /dev/null
+++ b/src/internal/bytealg/compare_mips64x.s
@@ -0,0 +1,88 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips64 || mips64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT,$0-56
+	MOVV	a_base+0(FP), R3
+	MOVV	b_base+24(FP), R4
+	MOVV	a_len+8(FP), R1
+	MOVV	b_len+32(FP), R2
+	MOVV	$ret+48(FP), R9
+	JMP	cmpbody<>(SB)
+
+TEXT runtime·cmpstring(SB),NOSPLIT,$0-40
+	MOVV	a_base+0(FP), R3
+	MOVV	b_base+16(FP), R4
+	MOVV	a_len+8(FP), R1
+	MOVV	b_len+24(FP), R2
+	MOVV	$ret+32(FP), R9
+	JMP	cmpbody<>(SB)
+
+// On entry:
+// R1 length of a
+// R2 length of b
+// R3 points to the start of a
+// R4 points to the start of b
+// R9 points to the return value (-1/0/1)
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0
+	BEQ	R3, R4, samebytes // same start of a and b
+
+	SGTU	R1, R2, R7
+	BNE	R0, R7, r2_lt_r1
+	MOVV	R1, R10
+	JMP	entry
+r2_lt_r1:
+	MOVV	R2, R10	// R10 is min(R1, R2)
+entry:
+	ADDV	R3, R10, R8	// R3 start of a, R8 end of a
+	BEQ	R3, R8, samebytes // length is 0
+
+	SRLV	$4, R10		// R10 is number of chunks
+	BEQ	R0, R10, byte_loop
+
+	// make sure both a and b are aligned.
+	OR	R3, R4, R11
+	AND	$7, R11
+	BNE	R0, R11, byte_loop
+
+chunk16_loop:
+	BEQ	R0, R10, byte_loop
+	MOVV	(R3), R6
+	MOVV	(R4), R7
+	BNE	R6, R7, byte_loop
+	MOVV	8(R3), R13
+	MOVV	8(R4), R14
+	ADDV	$16, R3
+	ADDV	$16, R4
+	SUBVU	$1, R10
+	BEQ	R13, R14, chunk16_loop
+	SUBV	$8, R3
+	SUBV	$8, R4
+
+byte_loop:
+	BEQ	R3, R8, samebytes
+	MOVBU	(R3), R6
+	ADDVU	$1, R3
+	MOVBU	(R4), R7
+	ADDVU	$1, R4
+	BEQ	R6, R7, byte_loop
+
+byte_cmp:
+	SGTU	R6, R7, R8 // R8 = 1 if (R6 > R7)
+	BNE	R0, R8, ret
+	MOVV	$-1, R8
+	JMP	ret
+
+samebytes:
+	SGTU	R1, R2, R6
+	SGTU	R2, R1, R7
+	SUBV	R7, R6, R8
+
+ret:
+	MOVV	R8, (R9)
+	RET
diff --git a/src/internal/bytealg/compare_mipsx.s b/src/internal/bytealg/compare_mipsx.s
new file mode 100644
index 0000000..857ac13
--- /dev/null
+++ b/src/internal/bytealg/compare_mipsx.s
@@ -0,0 +1,72 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips || mipsle
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT,$0-28
+	MOVW	a_base+0(FP), R3
+	MOVW	b_base+12(FP), R4
+	MOVW	a_len+4(FP), R1
+	MOVW	b_len+16(FP), R2
+	BEQ	R3, R4, samebytes
+	SGTU	R1, R2, R7
+	MOVW	R1, R8
+	CMOVN	R7, R2, R8	// R8 is min(R1, R2)
+
+	ADDU	R3, R8	// R3 is current byte in a, R8 is last byte in a to compare
+loop:
+	BEQ	R3, R8, samebytes
+
+	MOVBU	(R3), R6
+	ADDU	$1, R3
+	MOVBU	(R4), R7
+	ADDU	$1, R4
+	BEQ	R6, R7 , loop
+
+	SGTU	R6, R7, R8
+	MOVW	$-1, R6
+	CMOVZ	R8, R6, R8
+	JMP	cmp_ret
+samebytes:
+	SGTU	R1, R2, R6
+	SGTU	R2, R1, R7
+	SUBU	R7, R6, R8
+cmp_ret:
+	MOVW	R8, ret+24(FP)
+	RET
+
+TEXT runtime·cmpstring(SB),NOSPLIT,$0-20
+	MOVW	a_base+0(FP), R3
+	MOVW	a_len+4(FP), R1
+	MOVW	b_base+8(FP), R4
+	MOVW	b_len+12(FP), R2
+	BEQ	R3, R4, samebytes
+	SGTU	R1, R2, R7
+	MOVW	R1, R8
+	CMOVN	R7, R2, R8	// R8 is min(R1, R2)
+
+	ADDU	R3, R8	// R3 is current byte in a, R8 is last byte in a to compare
+loop:
+	BEQ	R3, R8, samebytes	// all compared bytes were the same; compare lengths
+
+	MOVBU	(R3), R6
+	ADDU	$1, R3
+	MOVBU	(R4), R7
+	ADDU	$1, R4
+	BEQ	R6, R7 , loop
+	// bytes differed
+	SGTU	R6, R7, R8
+	MOVW	$-1, R6
+	CMOVZ	R8, R6, R8
+	JMP	cmp_ret
+samebytes:
+	SGTU	R1, R2, R6
+	SGTU	R2, R1, R7
+	SUBU	R7, R6, R8
+cmp_ret:
+	MOVW	R8, ret+16(FP)
+	RET
diff --git a/src/internal/bytealg/compare_native.go b/src/internal/bytealg/compare_native.go
new file mode 100644
index 0000000..34964e2
--- /dev/null
+++ b/src/internal/bytealg/compare_native.go
@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64 || s390x || arm || arm64 || loong64 || ppc64 || ppc64le || mips || mipsle || wasm || mips64 || mips64le || riscv64
+
+package bytealg
+
+import _ "unsafe" // For go:linkname
+
+//go:noescape
+func Compare(a, b []byte) int
+
+// The declaration below generates ABI wrappers for functions
+// implemented in assembly in this package but declared in another
+// package.
+
+//go:linkname abigen_runtime_cmpstring runtime.cmpstring
+func abigen_runtime_cmpstring(a, b string) int
diff --git a/src/internal/bytealg/compare_ppc64x.s b/src/internal/bytealg/compare_ppc64x.s
new file mode 100644
index 0000000..cbe0525
--- /dev/null
+++ b/src/internal/bytealg/compare_ppc64x.s
@@ -0,0 +1,502 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-56
+	// incoming:
+	// R3 a addr -> R5
+	// R4 a len  -> R3
+	// R5 a cap unused
+	// R6 b addr -> R6
+	// R7 b len  -> R4
+	// R8 b cap unused
+	MOVD	R3, R5
+	MOVD	R4, R3
+	MOVD	R7, R4
+	CMP     R5,R6,CR7
+	CMP	R3,R4,CR6
+	BEQ	CR7,equal
+	MOVBZ	internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R16
+	CMP	R16,$1
+	BNE	power8
+	BR	cmpbodyp9<>(SB)
+power8:
+	BR	cmpbody<>(SB)
+equal:
+	BEQ	CR6,done
+	MOVD	$1, R8
+	BGT	CR6,greater
+	NEG	R8
+greater:
+	MOVD	R8, R3
+	RET
+done:
+	MOVD	$0, R3
+	RET
+
+TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// incoming:
+	// R3 a addr -> R5
+	// R4 a len  -> R3
+	// R5 b addr -> R6
+	// R6 b len  -> R4
+	MOVD	R6, R7
+	MOVD	R5, R6
+	MOVD	R3, R5
+	MOVD	R4, R3
+	MOVD	R7, R4
+	CMP     R5,R6,CR7
+	CMP	R3,R4,CR6
+	BEQ	CR7,equal
+	MOVBZ	internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R16
+	CMP	R16,$1
+	BNE	power8
+	BR	cmpbodyp9<>(SB)
+power8:
+	BR	cmpbody<>(SB)
+equal:
+	BEQ	CR6,done
+	MOVD	$1, R8
+	BGT	CR6,greater
+	NEG	R8
+greater:
+	MOVD	R8, R3
+	RET
+
+done:
+	MOVD	$0, R3
+	RET
+
+#ifdef GOARCH_ppc64le
+DATA byteswap<>+0(SB)/8, $0x0706050403020100
+DATA byteswap<>+8(SB)/8, $0x0f0e0d0c0b0a0908
+GLOBL byteswap<>+0(SB), RODATA, $16
+#define SWAP V21
+#endif
+
+// Do an efficient memcmp for ppc64le/ppc64/POWER8
+// R3 = a len
+// R4 = b len
+// R5 = a addr
+// R6 = b addr
+// On exit:
+// R3 = return value
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	MOVD	R3,R8		// set up length
+	CMP	R3,R4,CR2	// unequal?
+	BLT	CR2,setuplen	// BLT CR2
+	MOVD	R4,R8		// use R4 for comparison len
+setuplen:
+	CMP	R8,$32		// optimize >= 32
+	MOVD	R8,R9
+	BLT	setup8a		// optimize < 32
+	MOVD	$16,R10		// set offsets to load into vectors
+	CMP	R8,$64
+	BLT	cmp32		// process size 32-63
+
+	DCBT	(R5)		// optimize >= 64
+	DCBT	(R6)		// cache hint
+	MOVD	$32,R11		// set offsets to load into vector
+	MOVD	$48,R12		// set offsets to load into vector
+
+loop64a:// process size 64 and greater
+	LXVD2X	(R5)(R0),V3	// load bytes of A at offset 0 into vector
+	LXVD2X	(R6)(R0),V4	// load bytes of B at offset 0 into vector
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different	// jump out if its different
+
+	LXVD2X	(R5)(R10),V3	// load bytes of A at offset 16 into vector
+	LXVD2X	(R6)(R10),V4	// load bytes of B at offset 16 into vector
+
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	LXVD2X	(R5)(R11),V3	// load bytes of A at offset 32 into vector
+	LXVD2X	(R6)(R11),V4	// load bytes of B at offset 32 into vector
+
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	LXVD2X	(R5)(R12),V3	// load bytes of A at offset 64 into vector
+	LXVD2X	(R6)(R12),V4	// load bytes of B at offset 64 into vector
+
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	ADD	$-64,R9,R9	// reduce remaining size by 64
+	ADD	$64,R5,R5	// increment to next 64 bytes of A
+	ADD	$64,R6,R6	// increment to next 64 bytes of B
+	CMPU	R9,$64
+	BGE	loop64a		// loop back to loop64a only if there are >= 64 bytes remaining
+	
+	CMPU	R9,$32
+	BGE	cmp32		// loop to cmp32 if there are 32-64 bytes remaining
+	CMPU	R9,$0
+	BNE	rem		// loop to rem if the remainder is not 0
+
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+cmp32:
+	LXVD2X	(R5)(R0),V3	// load bytes of A at offset 0 into vector
+	LXVD2X	(R6)(R0),V4	// load bytes of B at offset 0 into vector
+
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	LXVD2X	(R5)(R10),V3	// load bytes of A at offset 16 into vector
+	LXVD2X	(R6)(R10),V4	// load bytes of B at offset 16 into vector
+
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	ADD	$-32,R9,R9	// reduce remaining size by 32
+	ADD	$32,R5,R5	// increment to next 32 bytes of A
+	ADD	$32,R6,R6	// increment to next 32 bytes of B
+	CMPU	R9,$0
+	BNE	rem		// loop to rem if the remainder is not 0
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+rem:
+	MOVD	R9,R8
+	ANDCC	$24,R8,R9	// Any 8 byte chunks?
+	BEQ	leftover	// and result is 0
+	BR	setup8a
+
+different:
+#ifdef	GOARCH_ppc64le
+	MOVD	$byteswap<>+00(SB), R16
+	LXVD2X	(R16)(R0),SWAP	// Set up swap string
+
+	VPERM	V3,V3,SWAP,V3
+	VPERM	V4,V4,SWAP,V4
+#endif
+	MFVSRD	VS35,R16	// move upper doublwords of A and B into GPR for comparison
+	MFVSRD	VS36,R10
+
+	CMPU	R16,R10
+	BEQ	lower
+	BGT	greater
+	MOVD	$-1,R3		// return value if A < B
+	RET
+lower:
+	VSLDOI	$8,V3,V3,V3	// move lower doublwords of A and B into GPR for comparison
+	MFVSRD	VS35,R16
+	VSLDOI	$8,V4,V4,V4
+	MFVSRD	VS36,R10
+
+	CMPU	R16,R10
+	BGT	greater
+	MOVD	$-1,R3		// return value if A < B
+	RET
+setup8a:
+	SRADCC	$3,R8,R9	// get the 8 byte count
+	BEQ	leftover	// shifted value is 0
+	CMPU	R8,$8		// optimize 8byte move
+	BEQ	size8
+	CMPU	R8,$16
+	BEQ	size16
+	MOVD	R9,CTR		// loop count for doublewords
+loop8:
+#ifdef  GOARCH_ppc64le
+	MOVDBR	(R5+R0),R16	// doublewords to compare
+	MOVDBR	(R6+R0),R10	// LE compare order
+#else
+	MOVD	(R5+R0),R16	// doublewords to compare
+	MOVD	(R6+R0),R10	// BE compare order
+#endif
+	ADD	$8,R5
+	ADD	$8,R6
+	CMPU	R16,R10		// match?
+	BC	8,2,loop8	// bt ctr <> 0 && cr
+	BGT	greater
+	BLT	less
+leftover:
+	ANDCC	$7,R8,R9	// check for leftover bytes
+	BEQ	zeroremainder
+simplecheck:
+	MOVD	R0,R14
+	CMP	R9,$4		// process 4 bytes
+	BLT	halfword
+#ifdef  GOARCH_ppc64le
+	MOVWBR	(R5)(R14),R10
+	MOVWBR	(R6)(R14),R11
+#else
+	MOVWZ	(R5)(R14),R10
+	MOVWZ	(R6)(R14),R11
+#endif
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	ADD	$-4,R9
+	ADD	$4,R14
+	PCALIGN	$16
+
+halfword:
+	CMP	R9,$2		// process 2 bytes
+	BLT	byte
+#ifdef  GOARCH_ppc64le
+	MOVHBR	(R5)(R14),R10
+	MOVHBR	(R6)(R14),R11
+#else
+	MOVHZ	(R5)(R14),R10
+	MOVHZ	(R6)(R14),R11
+#endif
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	ADD	$-2,R9
+	ADD	$2,R14
+	PCALIGN	$16
+byte:
+	CMP	R9,$0		// process 1 byte
+	BEQ	skip
+	MOVBZ	(R5)(R14),R10
+	MOVBZ	(R6)(R14),R11
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	PCALIGN	$16
+skip:
+	BEQ	CR2,equal
+	BGT	CR2,greater
+
+less:	MOVD	$-1,R3		// return value if A < B
+	RET
+size16:
+	LXVD2X	(R5)(R0),V3	// load bytes of A at offset 0 into vector
+	LXVD2X	(R6)(R0),V4	// load bytes of B at offset 0 into vector
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+zeroremainder:
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+size8:
+#ifdef  GOARCH_ppc64le
+	MOVDBR	(R5+R0),R16	// doublewords to compare
+	MOVDBR	(R6+R0),R10	// LE compare order
+#else
+	MOVD	(R5+R0),R16	// doublewords to compare
+	MOVD	(R6+R0),R10	// BE compare order
+#endif
+	CMPU	R16,R10		// match?
+	BGT	greater
+	BLT	less
+	BGT	CR2,greater	// 2nd len > 1st len
+	BLT	CR2,less	// 2nd len < 1st len
+equal:
+	MOVD	$0, R3		// return value if A == B
+	RET
+greater:
+	MOVD	$1,R3		// return value if A > B
+	RET
+
+// Do an efficient memcmp for ppc64le/ppc64/POWER9
+// R3 = a len
+// R4 = b len
+// R5 = a addr
+// R6 = b addr
+// On exit:
+// R3 = return value
+TEXT cmpbodyp9<>(SB),NOSPLIT|NOFRAME,$0-0
+	MOVD	R3,R8		// set up length
+	CMP	R3,R4,CR2	// unequal?
+	BLT	CR2,setuplen	// BLT CR2
+	MOVD	R4,R8		// use R4 for comparison len
+setuplen:
+	CMP	R8,$16		// optimize for size<16
+	MOVD	R8,R9
+	BLT	simplecheck
+	MOVD	$16,R10		// set offsets to load into vectors
+	CMP	R8,$32		// optimize for size 16-31
+	BLT	cmp16
+	CMP	R8,$64
+	BLT	cmp32		// optimize for size 32-63
+	DCBT	(R5)		// optimize for size>=64
+	DCBT	(R6)		// cache hint
+
+	MOVD	$32,R11		// set offsets to load into vector
+	MOVD	$48,R12		// set offsets to load into vector
+
+loop64a:// process size 64 and greater
+	LXVB16X	(R0)(R5),V3	// load bytes of A at offset 0 into vector
+	LXVB16X	(R0)(R6),V4	// load bytes of B at offset 0 into vector
+	VCMPNEBCC	V3,V4,V1	// record comparison into V1
+	BNE	CR6,different	// jump out if its different
+
+	LXVB16X	(R10)(R5),V3	// load bytes of A at offset 16 into vector
+	LXVB16X	(R10)(R6),V4	// load bytes of B at offset 16 into vector
+	VCMPNEBCC	V3,V4,V1
+	BNE	CR6,different
+
+	LXVB16X	(R11)(R5),V3	// load bytes of A at offset 32 into vector
+	LXVB16X	(R11)(R6),V4	// load bytes of B at offset 32 into vector
+	VCMPNEBCC	V3,V4,V1
+	BNE	CR6,different
+
+	LXVB16X	(R12)(R5),V3	// load bytes of A at offset 48 into vector
+	LXVB16X	(R12)(R6),V4	// load bytes of B at offset 48 into vector
+	VCMPNEBCC	V3,V4,V1
+	BNE	CR6,different
+
+	ADD	$-64,R9,R9	// reduce remaining size by 64
+	ADD	$64,R5,R5	// increment to next 64 bytes of A
+	ADD	$64,R6,R6	// increment to next 64 bytes of B
+	CMPU	R9,$64
+	BGE	loop64a		// loop back to loop64a only if there are >= 64 bytes remaining
+
+	CMPU	R9,$32
+	BGE	cmp32		// loop to cmp32 if there are 32-64 bytes remaining
+	CMPU	R9,$16
+	BGE	cmp16		// loop to cmp16 if there are 16-31 bytes left
+	CMPU	R9,$0
+	BNE	simplecheck	// loop to simplecheck for remaining bytes
+
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+cmp32:
+	LXVB16X	(R0)(R5),V3	// load bytes of A at offset 0 into vector
+	LXVB16X	(R0)(R6),V4	// load bytes of B at offset 0 into vector
+
+	VCMPNEBCC	V3,V4,V1	// record comparison into V1
+	BNE	CR6,different	// jump out if its different
+
+	LXVB16X	(R10)(R5),V3	// load bytes of A at offset 16 into vector
+	LXVB16X	(R10)(R6),V4	// load bytes of B at offset 16 into vector
+	VCMPNEBCC	V3,V4,V1
+	BNE	CR6,different
+
+	ADD	$-32,R9,R9	// reduce remaining size by 32
+	ADD	$32,R5,R5	// increment to next 32 bytes of A
+	ADD	$32,R6,R6	// increment to next 32 bytes of B
+	CMPU	R9,$16		// loop to cmp16 if there are 16-31 bytes left
+	BGE	cmp16
+	CMPU	R9,$0
+	BNE	simplecheck	// loop to simplecheck for remainder bytes
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+different:
+
+	MFVSRD	VS35,R16	// move upper doublwords of A and B into GPR for comparison
+	MFVSRD	VS36,R10
+
+	CMPU	R16,R10
+	BEQ	lower
+	BGT	greater
+	MOVD	$-1,R3		// return value if A < B
+	RET
+lower:
+	MFVSRLD	VS35,R16	// next move lower doublewords of A and B into GPR for comparison
+	MFVSRLD	VS36,R10
+
+	CMPU	R16,R10
+	BGT	greater
+	MOVD	$-1,R3		// return value if A < B
+	RET
+
+greater:
+	MOVD	$1,R3		// return value if A > B
+	RET
+cmp16:
+	ANDCC	$16,R9,R31
+	BEQ	tail
+
+	LXVB16X	(R0)(R5),V3	// load bytes of A at offset 16 into vector
+	LXVB16X	(R0)(R6),V4	// load bytes of B at offset 16 into vector
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+
+	ADD	$16,R5
+	ADD	$16,R6
+tail:
+	ANDCC	$15,R9		// Load the last 16 bytes (we know there are at least 32b)
+	BEQ	end
+
+	ADD	R9,R5
+	ADD	R9,R6
+	MOVD	$-16,R10
+
+	LXVB16X	(R10)(R5),V3	// load bytes of A at offset 16 into vector
+	LXVB16X	(R10)(R6),V4	// load bytes of B at offset 16 into vector
+	VCMPEQUDCC	V3,V4,V1
+	BGE	CR6,different
+end:
+	BEQ	CR2,equal	// remainder is zero, jump to equal if len(A)==len(B)
+	BLT	CR2,less	// jump to less if BLT CR2 that is, len(A)<len(B)
+	BR	greater		// jump to greater otherwise
+simplecheck:
+	MOVD	$0,R14		// process 8 bytes
+	CMP	R9,$8
+	BLT	word
+#ifdef  GOARCH_ppc64le
+	MOVDBR	(R5+R14),R10
+	MOVDBR	(R6+R14),R11
+#else
+	MOVD	(R5+R14),R10
+	MOVD	(R6+R14),R11
+#endif
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	ADD	$8,R14
+	ADD	$-8,R9
+	PCALIGN	$16
+word:
+	CMP	R9,$4		// process 4 bytes
+	BLT	halfword
+#ifdef  GOARCH_ppc64le
+	MOVWBR	(R5+R14),R10
+	MOVWBR	(R6+R14),R11
+#else
+	MOVWZ	(R5+R14),R10
+	MOVWZ	(R6+R14),R11
+#endif
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	ADD	$4,R14
+	ADD	$-4,R9
+	PCALIGN	$16
+halfword:
+	CMP	R9,$2		// process 2 bytes
+	BLT	byte
+#ifdef  GOARCH_ppc64le
+	MOVHBR	(R5+R14),R10
+	MOVHBR	(R6+R14),R11
+#else
+	MOVHZ	(R5+R14),R10
+	MOVHZ	(R6+R14),R11
+#endif
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	ADD	$2,R14
+	ADD	$-2,R9
+	PCALIGN	$16
+byte:
+	CMP	R9,$0		// process 1 byte
+	BEQ	skip
+	MOVBZ	(R5+R14),R10
+	MOVBZ	(R6+R14),R11
+	CMPU	R10,R11
+	BGT	greater
+	BLT	less
+	PCALIGN	$16
+skip:
+	BEQ	CR2,equal
+	BGT	CR2,greater
+less:
+	MOVD	$-1,R3		// return value if A < B
+	RET
+equal:
+	MOVD	$0, R3		// return value if A == B
+	RET
diff --git a/src/internal/bytealg/compare_riscv64.s b/src/internal/bytealg/compare_riscv64.s
new file mode 100644
index 0000000..44a743d
--- /dev/null
+++ b/src/internal/bytealg/compare_riscv64.s
@@ -0,0 +1,187 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-56
+	// X10 = a_base
+	// X11 = a_len
+	// X12 = a_cap (unused)
+	// X13 = b_base (want in X12)
+	// X14 = b_len (want in X13)
+	// X15 = b_cap (unused)
+	MOV	X13, X12
+	MOV	X14, X13
+	JMP	compare<>(SB)
+
+TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// X10 = a_base
+	// X11 = a_len
+	// X12 = b_base
+	// X13 = b_len
+	JMP	compare<>(SB)
+
+// On entry:
+// X10 points to start of a
+// X11 length of a
+// X12 points to start of b
+// X13 length of b
+// for non-regabi X14 points to the address to store the return value (-1/0/1)
+// for regabi the return value in X10
+TEXT compare<>(SB),NOSPLIT|NOFRAME,$0
+	BEQ	X10, X12, cmp_len
+
+	MOV	X11, X5
+	BGE	X13, X5, use_a_len // X5 = min(len(a), len(b))
+	MOV	X13, X5
+use_a_len:
+	BEQZ	X5, cmp_len
+
+	MOV	$32, X6
+	BLT	X5, X6, loop4_check
+
+	// Check alignment - if alignment differs we have to do one byte at a time.
+	AND	$7, X10, X7
+	AND	$7, X12, X8
+	BNE	X7, X8, loop4_check
+	BEQZ	X7, loop32_check
+
+	// Check one byte at a time until we reach 8 byte alignment.
+	SUB	X7, X5, X5
+align:
+	ADD	$-1, X7
+	MOVBU	0(X10), X8
+	MOVBU	0(X12), X9
+	BNE	X8, X9, cmp
+	ADD	$1, X10
+	ADD	$1, X12
+	BNEZ	X7, align
+
+loop32_check:
+	MOV	$32, X7
+	BLT	X5, X7, loop16_check
+loop32:
+	MOV	0(X10), X15
+	MOV	0(X12), X16
+	MOV	8(X10), X17
+	MOV	8(X12), X18
+	BEQ	X15, X16, loop32a
+	JMP	cmp8a
+loop32a:
+	BEQ	X17, X18, loop32b
+	JMP	cmp8b
+loop32b:
+	MOV	16(X10), X15
+	MOV	16(X12), X16
+	MOV	24(X10), X17
+	MOV	24(X12), X18
+	BEQ	X15, X16, loop32c
+	JMP	cmp8a
+loop32c:
+	BEQ	X17, X18, loop32d
+	JMP	cmp8b
+loop32d:
+	ADD	$32, X10
+	ADD	$32, X12
+	ADD	$-32, X5
+	BGE	X5, X7, loop32
+	BEQZ	X5, cmp_len
+
+loop16_check:
+	MOV	$16, X6
+	BLT	X5, X6, loop4_check
+loop16:
+	MOV	0(X10), X15
+	MOV	0(X12), X16
+	MOV	8(X10), X17
+	MOV	8(X12), X18
+	BEQ	X15, X16, loop16a
+	JMP	cmp8a
+loop16a:
+	BEQ	X17, X18, loop16b
+	JMP	cmp8b
+loop16b:
+	ADD	$16, X10
+	ADD	$16, X12
+	ADD	$-16, X5
+	BGE	X5, X6, loop16
+	BEQZ	X5, cmp_len
+
+loop4_check:
+	MOV	$4, X6
+	BLT	X5, X6, loop1
+loop4:
+	MOVBU	0(X10), X8
+	MOVBU	0(X12), X9
+	MOVBU	1(X10), X15
+	MOVBU	1(X12), X16
+	BEQ	X8, X9, loop4a
+	SLTU	X9, X8, X5
+	SLTU	X8, X9, X6
+	JMP	cmp_ret
+loop4a:
+	BEQ	X15, X16, loop4b
+	SLTU	X16, X15, X5
+	SLTU	X15, X16, X6
+	JMP	cmp_ret
+loop4b:
+	MOVBU	2(X10), X21
+	MOVBU	2(X12), X22
+	MOVBU	3(X10), X23
+	MOVBU	3(X12), X24
+	BEQ	X21, X22, loop4c
+	SLTU	X22, X21, X5
+	SLTU	X21, X22, X6
+	JMP	cmp_ret
+loop4c:
+	BEQ	X23, X24, loop4d
+	SLTU	X24, X23, X5
+	SLTU	X23, X24, X6
+	JMP	cmp_ret
+loop4d:
+	ADD	$4, X10
+	ADD	$4, X12
+	ADD	$-4, X5
+	BGE	X5, X6, loop4
+
+loop1:
+	BEQZ	X5, cmp_len
+	MOVBU	0(X10), X8
+	MOVBU	0(X12), X9
+	BNE	X8, X9, cmp
+	ADD	$1, X10
+	ADD	$1, X12
+	ADD	$-1, X5
+	JMP	loop1
+
+	// Compare 8 bytes of memory in X15/X16 that are known to differ.
+cmp8a:
+	MOV	$0xff, X19
+cmp8a_loop:
+	AND	X15, X19, X8
+	AND	X16, X19, X9
+	BNE	X8, X9, cmp
+	SLLI	$8, X19
+	JMP	cmp8a_loop
+
+	// Compare 8 bytes of memory in X17/X18 that are known to differ.
+cmp8b:
+	MOV	$0xff, X19
+cmp8b_loop:
+	AND	X17, X19, X8
+	AND	X18, X19, X9
+	BNE	X8, X9, cmp
+	SLLI	$8, X19
+	JMP	cmp8b_loop
+
+cmp_len:
+	MOV	X11, X8
+	MOV	X13, X9
+cmp:
+	SLTU	X9, X8, X5
+	SLTU	X8, X9, X6
+cmp_ret:
+	SUB	X5, X6, X10
+	RET
diff --git a/src/internal/bytealg/compare_s390x.s b/src/internal/bytealg/compare_s390x.s
new file mode 100644
index 0000000..5394548
--- /dev/null
+++ b/src/internal/bytealg/compare_s390x.s
@@ -0,0 +1,69 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB),NOSPLIT|NOFRAME,$0-56
+	MOVD	a_base+0(FP), R3
+	MOVD	a_len+8(FP), R4
+	MOVD	b_base+24(FP), R5
+	MOVD	b_len+32(FP), R6
+	LA	ret+48(FP), R7
+	BR	cmpbody<>(SB)
+
+TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
+	MOVD	a_base+0(FP), R3
+	MOVD	a_len+8(FP), R4
+	MOVD	b_base+16(FP), R5
+	MOVD	b_len+24(FP), R6
+	LA	ret+32(FP), R7
+	BR	cmpbody<>(SB)
+
+// input:
+//   R3 = a
+//   R4 = alen
+//   R5 = b
+//   R6 = blen
+//   R7 = address of output word (stores -1/0/1 here)
+TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	CMPBEQ	R3, R5, cmplengths
+	MOVD	R4, R8
+	CMPBLE	R4, R6, amin
+	MOVD	R6, R8
+amin:
+	CMPBEQ	R8, $0, cmplengths
+	CMP	R8, $256
+	BLE	tail
+loop:
+	CLC	$256, 0(R3), 0(R5)
+	BGT	gt
+	BLT	lt
+	SUB	$256, R8
+	MOVD	$256(R3), R3
+	MOVD	$256(R5), R5
+	CMP	R8, $256
+	BGT	loop
+tail:
+	SUB	$1, R8
+	EXRL	$cmpbodyclc<>(SB), R8
+	BGT	gt
+	BLT	lt
+cmplengths:
+	CMP	R4, R6
+	BEQ	eq
+	BLT	lt
+gt:
+	MOVD	$1, 0(R7)
+	RET
+lt:
+	MOVD	$-1, 0(R7)
+	RET
+eq:
+	MOVD	$0, 0(R7)
+	RET
+
+TEXT cmpbodyclc<>(SB),NOSPLIT|NOFRAME,$0-0
+	CLC	$1, 0(R3), 0(R5)
+	RET
diff --git a/src/internal/bytealg/compare_wasm.s b/src/internal/bytealg/compare_wasm.s
new file mode 100644
index 0000000..dc8fb33
--- /dev/null
+++ b/src/internal/bytealg/compare_wasm.s
@@ -0,0 +1,115 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Compare(SB), NOSPLIT, $0-56
+	Get SP
+	I64Load a_base+0(FP)
+	I64Load a_len+8(FP)
+	I64Load b_base+24(FP)
+	I64Load b_len+32(FP)
+	Call cmpbody<>(SB)
+	I64Store ret+48(FP)
+	RET
+
+TEXT runtime·cmpstring(SB), NOSPLIT, $0-40
+	Get SP
+	I64Load a_base+0(FP)
+	I64Load a_len+8(FP)
+	I64Load b_base+16(FP)
+	I64Load b_len+24(FP)
+	Call cmpbody<>(SB)
+	I64Store ret+32(FP)
+	RET
+
+// params: a, alen, b, blen
+// ret: -1/0/1
+TEXT cmpbody<>(SB), NOSPLIT, $0-0
+	// len = min(alen, blen)
+	Get R1
+	Get R3
+	Get R1
+	Get R3
+	I64LtU
+	Select
+	Set R4
+
+	Get R0
+	I32WrapI64
+	Get R2
+	I32WrapI64
+	Get R4
+	I32WrapI64
+	Call memcmp<>(SB)
+	I64ExtendI32S
+	Tee R5
+
+	I64Eqz
+	If
+		// check length
+		Get R1
+		Get R3
+		I64Sub
+		Set R5
+	End
+
+	I64Const $0
+	I64Const $-1
+	I64Const $1
+	Get R5
+	I64Const $0
+	I64LtS
+	Select
+	Get R5
+	I64Eqz
+	Select
+	Return
+
+// compiled with emscripten
+// params: a, b, len
+// ret: <0/0/>0
+TEXT memcmp<>(SB), NOSPLIT, $0-0
+	Get R2
+	If $1
+	Loop
+	Get R0
+	I32Load8S $0
+	Tee R3
+	Get R1
+	I32Load8S $0
+	Tee R4
+	I32Eq
+	If
+	Get R0
+	I32Const $1
+	I32Add
+	Set R0
+	Get R1
+	I32Const $1
+	I32Add
+	Set R1
+	I32Const $0
+	Get R2
+	I32Const $-1
+	I32Add
+	Tee R2
+	I32Eqz
+	BrIf $3
+	Drop
+	Br $1
+	End
+	End
+	Get R3
+	I32Const $255
+	I32And
+	Get R4
+	I32Const $255
+	I32And
+	I32Sub
+	Else
+	I32Const $0
+	End
+	Return
diff --git a/src/internal/bytealg/count_amd64.s b/src/internal/bytealg/count_amd64.s
new file mode 100644
index 0000000..efb17f8
--- /dev/null
+++ b/src/internal/bytealg/count_amd64.s
@@ -0,0 +1,208 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "asm_amd64.h"
+#include "textflag.h"
+
+TEXT ·Count(SB),NOSPLIT,$0-40
+#ifndef hasPOPCNT
+	CMPB	internal∕cpu·X86+const_offsetX86HasPOPCNT(SB), $1
+	JEQ	2(PC)
+	JMP	·countGeneric(SB)
+#endif
+	MOVQ	b_base+0(FP), SI
+	MOVQ	b_len+8(FP), BX
+	MOVB	c+24(FP), AL
+	LEAQ	ret+32(FP), R8
+	JMP	countbody<>(SB)
+
+TEXT ·CountString(SB),NOSPLIT,$0-32
+#ifndef hasPOPCNT
+	CMPB	internal∕cpu·X86+const_offsetX86HasPOPCNT(SB), $1
+	JEQ	2(PC)
+	JMP	·countGenericString(SB)
+#endif
+	MOVQ	s_base+0(FP), SI
+	MOVQ	s_len+8(FP), BX
+	MOVB	c+16(FP), AL
+	LEAQ	ret+24(FP), R8
+	JMP	countbody<>(SB)
+
+// input:
+//   SI: data
+//   BX: data len
+//   AL: byte sought
+//   R8: address to put result
+// This function requires the POPCNT instruction.
+TEXT countbody<>(SB),NOSPLIT,$0
+	// Shuffle X0 around so that each byte contains
+	// the character we're looking for.
+	MOVD AX, X0
+	PUNPCKLBW X0, X0
+	PUNPCKLBW X0, X0
+	PSHUFL $0, X0, X0
+
+	CMPQ BX, $16
+	JLT small
+
+	MOVQ $0, R12 // Accumulator
+
+	MOVQ SI, DI
+
+	CMPQ BX, $32
+	JA avx2
+sse:
+	LEAQ	-16(SI)(BX*1), AX	// AX = address of last 16 bytes
+	JMP	sseloopentry
+
+sseloop:
+	// Move the next 16-byte chunk of the data into X1.
+	MOVOU	(DI), X1
+	// Compare bytes in X0 to X1.
+	PCMPEQB	X0, X1
+	// Take the top bit of each byte in X1 and put the result in DX.
+	PMOVMSKB X1, DX
+	// Count number of matching bytes
+	POPCNTL DX, DX
+	// Accumulate into R12
+	ADDQ DX, R12
+	// Advance to next block.
+	ADDQ	$16, DI
+sseloopentry:
+	CMPQ	DI, AX
+	JBE	sseloop
+
+	// Get the number of bytes to consider in the last 16 bytes
+	ANDQ $15, BX
+	JZ end
+
+	// Create mask to ignore overlap between previous 16 byte block
+	// and the next.
+	MOVQ $16,CX
+	SUBQ BX, CX
+	MOVQ $0xFFFF, R10
+	SARQ CL, R10
+	SALQ CL, R10
+
+	// Process the last 16-byte chunk. This chunk may overlap with the
+	// chunks we've already searched so we need to mask part of it.
+	MOVOU	(AX), X1
+	PCMPEQB	X0, X1
+	PMOVMSKB X1, DX
+	// Apply mask
+	ANDQ R10, DX
+	POPCNTL DX, DX
+	ADDQ DX, R12
+end:
+	MOVQ R12, (R8)
+	RET
+
+// handle for lengths < 16
+small:
+	TESTQ	BX, BX
+	JEQ	endzero
+
+	// Check if we'll load across a page boundary.
+	LEAQ	16(SI), AX
+	TESTW	$0xff0, AX
+	JEQ	endofpage
+
+	// We must ignore high bytes as they aren't part of our slice.
+	// Create mask.
+	MOVB BX, CX
+	MOVQ $1, R10
+	SALQ CL, R10
+	SUBQ $1, R10
+
+	// Load data
+	MOVOU	(SI), X1
+	// Compare target byte with each byte in data.
+	PCMPEQB	X0, X1
+	// Move result bits to integer register.
+	PMOVMSKB X1, DX
+	// Apply mask
+	ANDQ R10, DX
+	POPCNTL DX, DX
+	// Directly return DX, we don't need to accumulate
+	// since we have <16 bytes.
+	MOVQ	DX, (R8)
+	RET
+endzero:
+	MOVQ $0, (R8)
+	RET
+
+endofpage:
+	// We must ignore low bytes as they aren't part of our slice.
+	MOVQ $16,CX
+	SUBQ BX, CX
+	MOVQ $0xFFFF, R10
+	SARQ CL, R10
+	SALQ CL, R10
+
+	// Load data into the high end of X1.
+	MOVOU	-16(SI)(BX*1), X1
+	// Compare target byte with each byte in data.
+	PCMPEQB	X0, X1
+	// Move result bits to integer register.
+	PMOVMSKB X1, DX
+	// Apply mask
+	ANDQ R10, DX
+	// Directly return DX, we don't need to accumulate
+	// since we have <16 bytes.
+	POPCNTL DX, DX
+	MOVQ	DX, (R8)
+	RET
+
+avx2:
+#ifndef hasAVX2
+	CMPB   internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
+	JNE sse
+#endif
+	MOVD AX, X0
+	LEAQ -32(SI)(BX*1), R11
+	VPBROADCASTB  X0, Y1
+avx2_loop:
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPMOVMSKB Y3, DX
+	POPCNTL DX, DX
+	ADDQ DX, R12
+	ADDQ $32, DI
+	CMPQ DI, R11
+	JLE avx2_loop
+
+	// If last block is already processed,
+	// skip to the end.
+	CMPQ DI, R11
+	JEQ endavx
+
+	// Load address of the last 32 bytes.
+	// There is an overlap with the previous block.
+	MOVQ R11, DI
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPMOVMSKB Y3, DX
+	// Exit AVX mode.
+	VZEROUPPER
+
+	// Create mask to ignore overlap between previous 32 byte block
+	// and the next.
+	ANDQ $31, BX
+	MOVQ $32,CX
+	SUBQ BX, CX
+	MOVQ $0xFFFFFFFF, R10
+	SARQ CL, R10
+	SALQ CL, R10
+	// Apply mask
+	ANDQ R10, DX
+	POPCNTL DX, DX
+	ADDQ DX, R12
+	MOVQ R12, (R8)
+	RET
+endavx:
+	// Exit AVX mode.
+	VZEROUPPER
+	MOVQ R12, (R8)
+	RET
diff --git a/src/internal/bytealg/count_arm.s b/src/internal/bytealg/count_arm.s
new file mode 100644
index 0000000..f704ea0
--- /dev/null
+++ b/src/internal/bytealg/count_arm.s
@@ -0,0 +1,43 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Count(SB),NOSPLIT,$0-20
+	MOVW	b_base+0(FP), R0
+	MOVW	b_len+4(FP), R1
+	MOVBU	c+12(FP), R2
+	MOVW	$ret+16(FP), R7
+	B	countbytebody<>(SB)
+
+TEXT ·CountString(SB),NOSPLIT,$0-16
+	MOVW	s_base+0(FP), R0
+	MOVW	s_len+4(FP), R1
+	MOVBU	c+8(FP), R2
+	MOVW	$ret+12(FP), R7
+	B	countbytebody<>(SB)
+
+// Input:
+// R0: data
+// R1: data length
+// R2: byte to find
+// R7: address to put result
+//
+// On exit:
+// R4 and R8 are clobbered
+TEXT countbytebody<>(SB),NOSPLIT,$0
+	MOVW	$0, R8	// R8 = count of byte to search
+	CMP	$0, R1
+	B.EQ	done	// short path to handle 0-byte case
+	ADD	R0, R1	// R1 is the end of the range
+byte_loop:
+	MOVBU.P	1(R0), R4
+	CMP	R4, R2
+	ADD.EQ	$1, R8
+	CMP	R0, R1
+	B.NE	byte_loop
+done:
+	MOVW	R8, (R7)
+	RET
diff --git a/src/internal/bytealg/count_arm64.s b/src/internal/bytealg/count_arm64.s
new file mode 100644
index 0000000..8cd703d
--- /dev/null
+++ b/src/internal/bytealg/count_arm64.s
@@ -0,0 +1,90 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Count(SB),NOSPLIT,$0-40
+	MOVD	b_base+0(FP), R0
+	MOVD	b_len+8(FP), R2
+	MOVBU	c+24(FP), R1
+	MOVD	$ret+32(FP), R8
+	B	countbytebody<>(SB)
+
+TEXT ·CountString(SB),NOSPLIT,$0-32
+	MOVD	s_base+0(FP), R0
+	MOVD	s_len+8(FP), R2
+	MOVBU	c+16(FP), R1
+	MOVD	$ret+24(FP), R8
+	B	countbytebody<>(SB)
+
+// input:
+//   R0: data
+//   R2: data len
+//   R1: byte to find
+//   R8: address to put result
+TEXT countbytebody<>(SB),NOSPLIT,$0
+	// R11 = count of byte to search
+	MOVD	$0, R11
+	// short path to handle 0-byte case
+	CBZ	R2, done
+	CMP	$0x20, R2
+	// jump directly to tail if length < 32
+	BLO	tail
+	ANDS	$0x1f, R0, R9
+	BEQ	chunk
+	// Work with not 32-byte aligned head
+	BIC	$0x1f, R0, R3
+	ADD	$0x20, R3
+head_loop:
+	MOVBU.P	1(R0), R5
+	CMP	R5, R1
+	CINC	EQ, R11, R11
+	SUB	$1, R2, R2
+	CMP	R0, R3
+	BNE	head_loop
+	// Work with 32-byte aligned chunks
+chunk:
+	BIC	$0x1f, R2, R9
+	// The first chunk can also be the last
+	CBZ	R9, tail
+	// R3 = end of 32-byte chunks
+	ADD	R0, R9, R3
+	MOVD	$1, R5
+	VMOV	R5, V5.B16
+	// R2 = length of tail
+	SUB	R9, R2, R2
+	// Duplicate R1 (byte to search) to 16 1-byte elements of V0
+	VMOV	R1, V0.B16
+	// Clear the low 64-bit element of V7 and V8
+	VEOR	V7.B8, V7.B8, V7.B8
+	VEOR	V8.B8, V8.B8, V8.B8
+	// Count the target byte in 32-byte chunk
+chunk_loop:
+	VLD1.P	(R0), [V1.B16, V2.B16]
+	CMP	R0, R3
+	VCMEQ	V0.B16, V1.B16, V3.B16
+	VCMEQ	V0.B16, V2.B16, V4.B16
+	// Clear the higher 7 bits
+	VAND	V5.B16, V3.B16, V3.B16
+	VAND	V5.B16, V4.B16, V4.B16
+	// Count lanes match the requested byte
+	VADDP	V4.B16, V3.B16, V6.B16 // 32B->16B
+	VUADDLV	V6.B16, V7
+	// Accumulate the count in low 64-bit element of V8 when inside the loop
+	VADD	V7, V8
+	BNE	chunk_loop
+	VMOV	V8.D[0], R6
+	ADD	R6, R11, R11
+	CBZ	R2, done
+tail:
+	// Work with tail shorter than 32 bytes
+	MOVBU.P	1(R0), R5
+	SUB	$1, R2, R2
+	CMP	R5, R1
+	CINC	EQ, R11, R11
+	CBNZ	R2, tail
+done:
+	MOVD	R11, (R8)
+	RET
diff --git a/src/internal/bytealg/count_generic.go b/src/internal/bytealg/count_generic.go
new file mode 100644
index 0000000..932a7c5
--- /dev/null
+++ b/src/internal/bytealg/count_generic.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !amd64 && !arm && !arm64 && !ppc64le && !ppc64 && !riscv64 && !s390x
+
+package bytealg
+
+func Count(b []byte, c byte) int {
+	n := 0
+	for _, x := range b {
+		if x == c {
+			n++
+		}
+	}
+	return n
+}
+
+func CountString(s string, c byte) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			n++
+		}
+	}
+	return n
+}
diff --git a/src/internal/bytealg/count_native.go b/src/internal/bytealg/count_native.go
new file mode 100644
index 0000000..90189c9
--- /dev/null
+++ b/src/internal/bytealg/count_native.go
@@ -0,0 +1,33 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64 || arm || arm64 || ppc64le || ppc64 || riscv64 || s390x
+
+package bytealg
+
+//go:noescape
+func Count(b []byte, c byte) int
+
+//go:noescape
+func CountString(s string, c byte) int
+
+// A backup implementation to use by assembly.
+func countGeneric(b []byte, c byte) int {
+	n := 0
+	for _, x := range b {
+		if x == c {
+			n++
+		}
+	}
+	return n
+}
+func countGenericString(s string, c byte) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			n++
+		}
+	}
+	return n
+}
diff --git a/src/internal/bytealg/count_ppc64x.s b/src/internal/bytealg/count_ppc64x.s
new file mode 100644
index 0000000..2d2490b
--- /dev/null
+++ b/src/internal/bytealg/count_ppc64x.s
@@ -0,0 +1,96 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64le || ppc64
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Count<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// R3 = byte array pointer 
+	// R4 = length
+	MOVBZ R6, R5              // R5 = byte
+	BR    countbytebody<>(SB)
+
+TEXT ·CountString<ABIInternal>(SB), NOSPLIT|NOFRAME, $0-32
+	// R3 = byte array pointer
+	// R4 = length
+	MOVBZ R5, R5              // R5 = byte
+	BR    countbytebody<>(SB)
+
+// R3: addr of string
+// R4: len of string
+// R5: byte to count
+// On exit:
+// R3: return value
+// endianness shouldn't matter since we are just counting and order
+// is irrelevant
+TEXT countbytebody<>(SB), NOSPLIT|NOFRAME, $0-0
+	DCBT (R3)    // Prepare cache line.
+	MOVD R0, R18 // byte count
+	MOVD R3, R19 // Save base address for calculating the index later.
+	MOVD R4, R16
+
+	MOVD   R5, R6
+	RLDIMI $8, R6, $48, R6
+	RLDIMI $16, R6, $32, R6
+	RLDIMI $32, R6, $0, R6  // fill reg with the byte to count
+
+	VSPLTISW $3, V4     // used for shift
+	MTVRD    R6, V1     // move compare byte
+	VSPLTB   $7, V1, V1 // replicate byte across V1
+
+	CMPU   R4, $32          // Check if it's a small string (<32 bytes)
+	BLT    tail             // Jump to the small string case
+	XXLXOR VS37, VS37, VS37 // clear V5 (aka VS37) to use as accumulator
+
+cmploop:
+	LXVW4X (R3), VS32 // load bytes from string
+
+	// when the bytes match, the corresponding byte contains all 1s
+	VCMPEQUB V1, V0, V2     // compare bytes
+	VPOPCNTD V2, V3         // each double word contains its count
+	VADDUDM  V3, V5, V5     // accumulate bit count in each double word
+	ADD      $16, R3, R3    // increment pointer
+	SUB      $16, R16, R16  // remaining bytes
+	CMP      R16, $16       // at least 16 remaining?
+	BGE      cmploop
+	VSRD     V5, V4, V5     // shift by 3 to convert bits to bytes
+	VSLDOI   $8, V5, V5, V6 // get the double word values from vector
+	MFVSRD   V5, R9
+	MFVSRD   V6, R10
+	ADD      R9, R10, R9
+	ADD      R9, R18, R18
+
+tail:
+	CMP R16, $8 // 8 bytes left?
+	BLT small
+
+	MOVD    (R3), R12     // load 8 bytes
+	CMPB    R12, R6, R17  // compare bytes
+	POPCNTD R17, R15      // bit count
+	SRD     $3, R15, R15  // byte count
+	ADD     R15, R18, R18 // add to byte count
+
+next1:
+	ADD $8, R3, R3
+	SUB $8, R16, R16 // remaining bytes
+	BR  tail
+
+small:
+	CMP   $0, R16   // any remaining
+	BEQ   done
+	MOVBZ (R3), R12 // check each remaining byte
+	CMP   R12, R5
+	BNE   next2
+	ADD   $1, R18
+
+next2:
+	SUB $1, R16
+	ADD $1, R3  // inc address
+	BR  small
+
+done:
+	MOVD R18, R3    // return count
+	RET
diff --git a/src/internal/bytealg/count_riscv64.s b/src/internal/bytealg/count_riscv64.s
new file mode 100644
index 0000000..d123cbd
--- /dev/null
+++ b/src/internal/bytealg/count_riscv64.s
@@ -0,0 +1,47 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Count<ABIInternal>(SB),NOSPLIT,$0-40
+	// X10 = b_base
+	// X11 = b_len
+	// X12 = b_cap (unused)
+	// X13 = byte to count (want in X12)
+	AND	$0xff, X13, X12
+	MOV	ZERO, X14	// count
+	ADD	X10, X11	// end
+
+loop:
+	BEQ	X10, X11, done
+	MOVBU	(X10), X15
+	ADD	$1, X10
+	BNE	X12, X15, loop
+	ADD	$1, X14
+	JMP	loop
+
+done:
+	MOV	X14, X10
+	RET
+
+TEXT ·CountString<ABIInternal>(SB),NOSPLIT,$0-32
+	// X10 = s_base
+	// X11 = s_len
+	// X12 = byte to count
+	AND	$0xff, X12
+	MOV	ZERO, X14	// count
+	ADD	X10, X11	// end
+
+loop:
+	BEQ	X10, X11, done
+	MOVBU	(X10), X15
+	ADD	$1, X10
+	BNE	X12, X15, loop
+	ADD	$1, X14
+	JMP	loop
+
+done:
+	MOV	X14, X10
+	RET
diff --git a/src/internal/bytealg/count_s390x.s b/src/internal/bytealg/count_s390x.s
new file mode 100644
index 0000000..2a3b5c0
--- /dev/null
+++ b/src/internal/bytealg/count_s390x.s
@@ -0,0 +1,169 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// condition code masks
+#define EQ 8
+#define NE 7
+
+// register assignments
+#define R_ZERO R0
+#define R_VAL  R1
+#define R_TMP  R2
+#define R_PTR  R3
+#define R_LEN  R4
+#define R_CHAR R5
+#define R_RET  R6
+#define R_ITER R7
+#define R_CNT  R8
+#define R_MPTR R9
+
+// vector register assignments
+#define V_ZERO V0
+#define V_CHAR V1
+#define V_MASK V2
+#define V_VAL  V3
+#define V_CNT  V4
+
+// mask for trailing bytes in vector implementation
+GLOBL countbytemask<>(SB), RODATA, $16
+DATA countbytemask<>+0(SB)/8, $0x0101010101010101
+DATA countbytemask<>+8(SB)/8, $0x0101010101010101
+
+// func Count(b []byte, c byte) int
+TEXT ·Count(SB), NOSPLIT|NOFRAME, $0-40
+	LMG   b+0(FP), R_PTR, R_LEN
+	MOVBZ c+24(FP), R_CHAR
+	MOVD  $ret+32(FP), R_RET
+	BR    countbytebody<>(SB)
+
+// func CountString(s string, c byte) int
+TEXT ·CountString(SB), NOSPLIT|NOFRAME, $0-32
+	LMG   s+0(FP), R_PTR, R_LEN
+	MOVBZ c+16(FP), R_CHAR
+	MOVD  $ret+24(FP), R_RET
+	BR    countbytebody<>(SB)
+
+// input:
+// R_PTR  = address of array of bytes
+// R_LEN  = number of bytes in array
+// R_CHAR = byte value to count zero (extended to register width)
+// R_RET  = address of return value
+TEXT countbytebody<>(SB), NOSPLIT|NOFRAME, $0-0
+	MOVD  $internal∕cpu·S390X+const_offsetS390xHasVX(SB), R_TMP
+	MOVD  $countbytemask<>(SB), R_MPTR
+	CGIJ  $EQ, R_LEN, $0, ret0 // return if length is 0.
+	SRD   $4, R_LEN, R_ITER    // R_ITER is the number of 16-byte chunks
+	MOVBZ (R_TMP), R_TMP       // load bool indicating support for vector facility
+	CGIJ  $EQ, R_TMP, $0, novx // jump to scalar code if the vector facility is not available
+
+	// Start of vector code (have vector facility).
+	//
+	// Set R_LEN to be the length mod 16 minus 1 to use as an index for
+	// vector 'load with length' (VLL). It will be in the range [-1,14].
+	// Also replicate c across a 16-byte vector and initialize V_ZERO.
+	ANDW  $0xf, R_LEN
+	VLVGB $0, R_CHAR, V_CHAR // V_CHAR = [16]byte{c, 0, ..., 0, 0}
+	VZERO V_ZERO             // V_ZERO = [1]uint128{0}
+	ADDW  $-1, R_LEN
+	VREPB $0, V_CHAR, V_CHAR // V_CHAR = [16]byte{c, c, ..., c, c}
+
+	// Jump to loop if we have more than 15 bytes to process.
+	CGIJ $NE, R_ITER, $0, vxchunks
+
+	// Load 1-15 bytes and corresponding mask.
+	// Note: only the low 32-bits of R_LEN are used for the index.
+	VLL R_LEN, (R_PTR), V_VAL
+	VLL R_LEN, (R_MPTR), V_MASK
+
+	// Compare each byte in input chunk against byte to be counted.
+	// Each byte element will be set to either 0 (no match) or 1 (match).
+	VCEQB V_CHAR, V_VAL, V_VAL // each byte will be either 0xff or 0x00
+	VN    V_MASK, V_VAL, V_VAL // mask out most significant 7 bits
+
+	// Accumulate matched byte count in 128-bit integer value.
+	VSUMB  V_VAL, V_ZERO, V_VAL // [16]byte{x0, x1, ..., x14, x15} → [4]uint32{x0+x1+x2+x3, ..., x12+x13+x14+x15}
+	VSUMQF V_VAL, V_ZERO, V_CNT // [4]uint32{x0, x1, x2, x3} → [1]uint128{x0+x1+x2+x3}
+
+	// Return rightmost (lowest) 64-bit part of accumulator.
+	VSTEG $1, V_CNT, (R_RET)
+	RET
+
+vxchunks:
+	// Load 0x01 into every byte element in the 16-byte mask vector.
+	VREPIB $1, V_MASK // V_MASK = [16]byte{1, 1, ..., 1, 1}
+	VZERO  V_CNT      // initial uint128 count of 0
+
+vxloop:
+	// Load input bytes in 16-byte chunks.
+	VL (R_PTR), V_VAL
+
+	// Compare each byte in input chunk against byte to be counted.
+	// Each byte element will be set to either 0 (no match) or 1 (match).
+	VCEQB V_CHAR, V_VAL, V_VAL // each byte will be either 0xff or 0x00
+	VN    V_MASK, V_VAL, V_VAL // mask out most significant 7 bits
+
+	// Increment input string address.
+	MOVD $16(R_PTR), R_PTR
+
+	// Accumulate matched byte count in 128-bit integer value.
+	VSUMB  V_VAL, V_ZERO, V_VAL // [16]byte{x0, x1, ..., x14, x15} → [4]uint32{x0+x1+x2+x3, ..., x12+x13+x14+x15}
+	VSUMQF V_VAL, V_ZERO, V_VAL // [4]uint32{x0, x1, x2, x3} → [1]uint128{x0+x1+x2+x3}
+	VAQ    V_VAL, V_CNT, V_CNT  // accumulate
+
+	// Repeat until all 16-byte chunks are done.
+	BRCTG R_ITER, vxloop
+
+	// Skip to end if there are no trailing bytes.
+	CIJ $EQ, R_LEN, $-1, vxret
+
+	// Load 1-15 bytes and corresponding mask.
+	// Note: only the low 32-bits of R_LEN are used for the index.
+	VLL R_LEN, (R_PTR), V_VAL
+	VLL R_LEN, (R_MPTR), V_MASK
+
+	// Compare each byte in input chunk against byte to be counted.
+	// Each byte element will be set to either 0 (no match) or 1 (match).
+	VCEQB V_CHAR, V_VAL, V_VAL
+	VN    V_MASK, V_VAL, V_VAL
+
+	// Accumulate matched byte count in 128-bit integer value.
+	VSUMB  V_VAL, V_ZERO, V_VAL // [16]byte{x0, x1, ..., x14, x15} → [4]uint32{x0+x1+x2+x3, ..., x12+x13+x14+x15}
+	VSUMQF V_VAL, V_ZERO, V_VAL // [4]uint32{x0, x1, x2, x3} → [1]uint128{x0+x1+x2+x3}
+	VAQ    V_VAL, V_CNT, V_CNT  // accumulate
+
+vxret:
+	// Return rightmost (lowest) 64-bit part of accumulator.
+	VSTEG $1, V_CNT, (R_RET)
+	RET
+
+novx:
+	// Start of non-vector code (the vector facility not available).
+	//
+	// Initialise counter and constant zero.
+	MOVD $0, R_CNT
+	MOVD $0, R_ZERO
+
+loop:
+	// Read 1-byte from input and compare.
+	// Note: avoid putting LOCGR in critical path.
+	MOVBZ (R_PTR), R_VAL
+	MOVD  $1, R_TMP
+	MOVD  $1(R_PTR), R_PTR
+	CMPW  R_VAL, R_CHAR
+	LOCGR $NE, R_ZERO, R_TMP // select 0 if no match (1 if there is a match)
+	ADD   R_TMP, R_CNT       // accumulate 64-bit result
+
+	// Repeat until all bytes have been checked.
+	BRCTG R_LEN, loop
+
+ret:
+	MOVD R_CNT, (R_RET)
+	RET
+
+ret0:
+	MOVD $0, (R_RET)
+	RET
diff --git a/src/internal/bytealg/equal_386.s b/src/internal/bytealg/equal_386.s
new file mode 100644
index 0000000..58b3cbe
--- /dev/null
+++ b/src/internal/bytealg/equal_386.s
@@ -0,0 +1,130 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT,$0-13
+	MOVL	a+0(FP), SI
+	MOVL	b+4(FP), DI
+	CMPL	SI, DI
+	JEQ	eq
+	MOVL	size+8(FP), BX
+	LEAL	ret+12(FP), AX
+	JMP	memeqbody<>(SB)
+eq:
+	MOVB    $1, ret+12(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT,$0-9
+	MOVL    a+0(FP), SI
+	MOVL    b+4(FP), DI
+	CMPL    SI, DI
+	JEQ     eq
+	MOVL    4(DX), BX    // compiler stores size at offset 4 in the closure
+	LEAL	ret+8(FP), AX
+	JMP	memeqbody<>(SB)
+eq:
+	MOVB    $1, ret+8(FP)
+	RET
+
+// a in SI
+// b in DI
+// count in BX
+// address of result byte in AX
+TEXT memeqbody<>(SB),NOSPLIT,$0-0
+	CMPL	BX, $4
+	JB	small
+
+	// 64 bytes at a time using xmm registers
+hugeloop:
+	CMPL	BX, $64
+	JB	bigloop
+#ifdef GO386_softfloat
+	JMP	bigloop
+#endif
+	MOVOU	(SI), X0
+	MOVOU	(DI), X1
+	MOVOU	16(SI), X2
+	MOVOU	16(DI), X3
+	MOVOU	32(SI), X4
+	MOVOU	32(DI), X5
+	MOVOU	48(SI), X6
+	MOVOU	48(DI), X7
+	PCMPEQB	X1, X0
+	PCMPEQB	X3, X2
+	PCMPEQB	X5, X4
+	PCMPEQB	X7, X6
+	PAND	X2, X0
+	PAND	X6, X4
+	PAND	X4, X0
+	PMOVMSKB X0, DX
+	ADDL	$64, SI
+	ADDL	$64, DI
+	SUBL	$64, BX
+	CMPL	DX, $0xffff
+	JEQ	hugeloop
+	MOVB	$0, (AX)
+	RET
+
+	// 4 bytes at a time using 32-bit register
+bigloop:
+	CMPL	BX, $4
+	JBE	leftover
+	MOVL	(SI), CX
+	MOVL	(DI), DX
+	ADDL	$4, SI
+	ADDL	$4, DI
+	SUBL	$4, BX
+	CMPL	CX, DX
+	JEQ	bigloop
+	MOVB	$0, (AX)
+	RET
+
+	// remaining 0-4 bytes
+leftover:
+	MOVL	-4(SI)(BX*1), CX
+	MOVL	-4(DI)(BX*1), DX
+	CMPL	CX, DX
+	SETEQ	(AX)
+	RET
+
+small:
+	CMPL	BX, $0
+	JEQ	equal
+
+	LEAL	0(BX*8), CX
+	NEGL	CX
+
+	MOVL	SI, DX
+	CMPB	DX, $0xfc
+	JA	si_high
+
+	// load at SI won't cross a page boundary.
+	MOVL	(SI), SI
+	JMP	si_finish
+si_high:
+	// address ends in 111111xx. Load up to bytes we want, move to correct position.
+	MOVL	-4(SI)(BX*1), SI
+	SHRL	CX, SI
+si_finish:
+
+	// same for DI.
+	MOVL	DI, DX
+	CMPB	DX, $0xfc
+	JA	di_high
+	MOVL	(DI), DI
+	JMP	di_finish
+di_high:
+	MOVL	-4(DI)(BX*1), DI
+	SHRL	CX, DI
+di_finish:
+
+	SUBL	SI, DI
+	SHLL	CX, DI
+equal:
+	SETEQ	(AX)
+	RET
diff --git a/src/internal/bytealg/equal_amd64.s b/src/internal/bytealg/equal_amd64.s
new file mode 100644
index 0000000..d178a33
--- /dev/null
+++ b/src/internal/bytealg/equal_amd64.s
@@ -0,0 +1,162 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "asm_amd64.h"
+#include "textflag.h"
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT,$0-25
+	// AX = a    (want in SI)
+	// BX = b    (want in DI)
+	// CX = size (want in BX)
+	CMPQ	AX, BX
+	JNE	neq
+	MOVQ	$1, AX	// return 1
+	RET
+neq:
+	MOVQ	AX, SI
+	MOVQ	BX, DI
+	MOVQ	CX, BX
+	JMP	memeqbody<>(SB)
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen<ABIInternal>(SB),NOSPLIT,$0-17
+	// AX = a       (want in SI)
+	// BX = b       (want in DI)
+	// 8(DX) = size (want in BX)
+	CMPQ	AX, BX
+	JNE	neq
+	MOVQ	$1, AX	// return 1
+	RET
+neq:
+	MOVQ	AX, SI
+	MOVQ	BX, DI
+	MOVQ	8(DX), BX    // compiler stores size at offset 8 in the closure
+	JMP	memeqbody<>(SB)
+
+// Input:
+//   a in SI
+//   b in DI
+//   count in BX
+// Output:
+//   result in AX
+TEXT memeqbody<>(SB),NOSPLIT,$0-0
+	CMPQ	BX, $8
+	JB	small
+	CMPQ	BX, $64
+	JB	bigloop
+#ifndef hasAVX2
+	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
+	JE	hugeloop_avx2
+
+	// 64 bytes at a time using xmm registers
+hugeloop:
+	CMPQ	BX, $64
+	JB	bigloop
+	MOVOU	(SI), X0
+	MOVOU	(DI), X1
+	MOVOU	16(SI), X2
+	MOVOU	16(DI), X3
+	MOVOU	32(SI), X4
+	MOVOU	32(DI), X5
+	MOVOU	48(SI), X6
+	MOVOU	48(DI), X7
+	PCMPEQB	X1, X0
+	PCMPEQB	X3, X2
+	PCMPEQB	X5, X4
+	PCMPEQB	X7, X6
+	PAND	X2, X0
+	PAND	X6, X4
+	PAND	X4, X0
+	PMOVMSKB X0, DX
+	ADDQ	$64, SI
+	ADDQ	$64, DI
+	SUBQ	$64, BX
+	CMPL	DX, $0xffff
+	JEQ	hugeloop
+	XORQ	AX, AX	// return 0
+	RET
+#endif
+
+	// 64 bytes at a time using ymm registers
+hugeloop_avx2:
+	CMPQ	BX, $64
+	JB	bigloop_avx2
+	VMOVDQU	(SI), Y0
+	VMOVDQU	(DI), Y1
+	VMOVDQU	32(SI), Y2
+	VMOVDQU	32(DI), Y3
+	VPCMPEQB	Y1, Y0, Y4
+	VPCMPEQB	Y2, Y3, Y5
+	VPAND	Y4, Y5, Y6
+	VPMOVMSKB Y6, DX
+	ADDQ	$64, SI
+	ADDQ	$64, DI
+	SUBQ	$64, BX
+	CMPL	DX, $0xffffffff
+	JEQ	hugeloop_avx2
+	VZEROUPPER
+	XORQ	AX, AX	// return 0
+	RET
+
+bigloop_avx2:
+	VZEROUPPER
+
+	// 8 bytes at a time using 64-bit register
+bigloop:
+	CMPQ	BX, $8
+	JBE	leftover
+	MOVQ	(SI), CX
+	MOVQ	(DI), DX
+	ADDQ	$8, SI
+	ADDQ	$8, DI
+	SUBQ	$8, BX
+	CMPQ	CX, DX
+	JEQ	bigloop
+	XORQ	AX, AX	// return 0
+	RET
+
+	// remaining 0-8 bytes
+leftover:
+	MOVQ	-8(SI)(BX*1), CX
+	MOVQ	-8(DI)(BX*1), DX
+	CMPQ	CX, DX
+	SETEQ	AX
+	RET
+
+small:
+	CMPQ	BX, $0
+	JEQ	equal
+
+	LEAQ	0(BX*8), CX
+	NEGQ	CX
+
+	CMPB	SI, $0xf8
+	JA	si_high
+
+	// load at SI won't cross a page boundary.
+	MOVQ	(SI), SI
+	JMP	si_finish
+si_high:
+	// address ends in 11111xxx. Load up to bytes we want, move to correct position.
+	MOVQ	-8(SI)(BX*1), SI
+	SHRQ	CX, SI
+si_finish:
+
+	// same for DI.
+	CMPB	DI, $0xf8
+	JA	di_high
+	MOVQ	(DI), DI
+	JMP	di_finish
+di_high:
+	MOVQ	-8(DI)(BX*1), DI
+	SHRQ	CX, DI
+di_finish:
+
+	SUBQ	SI, DI
+	SHLQ	CX, DI
+equal:
+	SETEQ	AX
+	RET
diff --git a/src/internal/bytealg/equal_arm.s b/src/internal/bytealg/equal_arm.s
new file mode 100644
index 0000000..a6c4369
--- /dev/null
+++ b/src/internal/bytealg/equal_arm.s
@@ -0,0 +1,91 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT|NOFRAME,$0-13
+	MOVW	a+0(FP), R0
+	MOVW	b+4(FP), R2
+	CMP	R0, R2
+	B.EQ	eq
+	MOVW	size+8(FP), R1
+	CMP	$0, R1
+	B.EQ	eq		// short path to handle 0-byte case
+	MOVW	$ret+12(FP), R7
+	B	memeqbody<>(SB)
+eq:
+	MOVW	$1, R0
+	MOVB	R0, ret+12(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT|NOFRAME,$0-9
+	MOVW	a+0(FP), R0
+	MOVW	b+4(FP), R2
+	CMP	R0, R2
+	B.EQ	eq
+	MOVW	4(R7), R1	// compiler stores size at offset 4 in the closure
+	CMP	$0, R1
+	B.EQ	eq		// short path to handle 0-byte case
+	MOVW	$ret+8(FP), R7
+	B	memeqbody<>(SB)
+eq:
+	MOVW	$1, R0
+	MOVB	R0, ret+8(FP)
+	RET
+
+// Input:
+// R0: data of a
+// R1: length
+// R2: data of b
+// R7: points to return value
+//
+// On exit:
+// R4, R5 and R6 are clobbered
+TEXT memeqbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	CMP	$1, R1
+	B.EQ	one		// 1-byte special case for better performance
+
+	CMP	$4, R1
+	ADD	R0, R1		// R1 is the end of the range to compare
+	B.LT	byte_loop	// length < 4
+	AND	$3, R0, R6
+	CMP	$0, R6
+	B.NE	byte_loop	// unaligned a, use byte-wise compare (TODO: try to align a)
+	AND	$3, R2, R6
+	CMP	$0, R6
+	B.NE	byte_loop	// unaligned b, use byte-wise compare
+	AND	$0xfffffffc, R1, R6
+	// length >= 4
+chunk4_loop:
+	MOVW.P	4(R0), R4
+	MOVW.P	4(R2), R5
+	CMP	R4, R5
+	B.NE	notequal
+	CMP	R0, R6
+	B.NE	chunk4_loop
+	CMP	R0, R1
+	B.EQ	equal		// reached the end
+byte_loop:
+	MOVBU.P	1(R0), R4
+	MOVBU.P	1(R2), R5
+	CMP	R4, R5
+	B.NE	notequal
+	CMP	R0, R1
+	B.NE	byte_loop
+equal:
+	MOVW	$1, R0
+	MOVB	R0, (R7)
+	RET
+one:
+	MOVBU	(R0), R4
+	MOVBU	(R2), R5
+	CMP	R4, R5
+	B.EQ	equal
+notequal:
+	MOVW	$0, R0
+	MOVB	R0, (R7)
+	RET
diff --git a/src/internal/bytealg/equal_arm64.s b/src/internal/bytealg/equal_arm64.s
new file mode 100644
index 0000000..d3aabba
--- /dev/null
+++ b/src/internal/bytealg/equal_arm64.s
@@ -0,0 +1,121 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-25
+	// short path to handle 0-byte case
+	CBZ	R2, equal
+	B	memeqbody<>(SB)
+equal:
+	MOVD	$1, R0
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen<ABIInternal>(SB),NOSPLIT,$0-17
+	CMP	R0, R1
+	BEQ	eq
+	MOVD	8(R26), R2    // compiler stores size at offset 8 in the closure
+	CBZ	R2, eq
+	B	memeqbody<>(SB)
+eq:
+	MOVD	$1, R0
+	RET
+
+// input:
+// R0: pointer a
+// R1: pointer b
+// R2: data len
+// at return: result in R0
+TEXT memeqbody<>(SB),NOSPLIT,$0
+	CMP	$1, R2
+	// handle 1-byte special case for better performance
+	BEQ	one
+	CMP	$16, R2
+	// handle specially if length < 16
+	BLO	tail
+	BIC	$0x3f, R2, R3
+	CBZ	R3, chunk16
+	// work with 64-byte chunks
+	ADD	R3, R0, R6	// end of chunks
+chunk64_loop:
+	VLD1.P	(R0), [V0.D2, V1.D2, V2.D2, V3.D2]
+	VLD1.P	(R1), [V4.D2, V5.D2, V6.D2, V7.D2]
+	VCMEQ	V0.D2, V4.D2, V8.D2
+	VCMEQ	V1.D2, V5.D2, V9.D2
+	VCMEQ	V2.D2, V6.D2, V10.D2
+	VCMEQ	V3.D2, V7.D2, V11.D2
+	VAND	V8.B16, V9.B16, V8.B16
+	VAND	V8.B16, V10.B16, V8.B16
+	VAND	V8.B16, V11.B16, V8.B16
+	CMP	R0, R6
+	VMOV	V8.D[0], R4
+	VMOV	V8.D[1], R5
+	CBZ	R4, not_equal
+	CBZ	R5, not_equal
+	BNE	chunk64_loop
+	AND	$0x3f, R2, R2
+	CBZ	R2, equal
+chunk16:
+	// work with 16-byte chunks
+	BIC	$0xf, R2, R3
+	CBZ	R3, tail
+	ADD	R3, R0, R6	// end of chunks
+chunk16_loop:
+	LDP.P	16(R0), (R4, R5)
+	LDP.P	16(R1), (R7, R9)
+	EOR	R4, R7
+	CBNZ	R7, not_equal
+	EOR	R5, R9
+	CBNZ	R9, not_equal
+	CMP	R0, R6
+	BNE	chunk16_loop
+	AND	$0xf, R2, R2
+	CBZ	R2, equal
+tail:
+	// special compare of tail with length < 16
+	TBZ	$3, R2, lt_8
+	MOVD	(R0), R4
+	MOVD	(R1), R5
+	EOR	R4, R5
+	CBNZ	R5, not_equal
+	SUB	$8, R2, R6	// offset of the last 8 bytes
+	MOVD	(R0)(R6), R4
+	MOVD	(R1)(R6), R5
+	EOR	R4, R5
+	CBNZ	R5, not_equal
+	B	equal
+lt_8:
+	TBZ	$2, R2, lt_4
+	MOVWU	(R0), R4
+	MOVWU	(R1), R5
+	EOR	R4, R5
+	CBNZ	R5, not_equal
+	SUB	$4, R2, R6	// offset of the last 4 bytes
+	MOVWU	(R0)(R6), R4
+	MOVWU	(R1)(R6), R5
+	EOR	R4, R5
+	CBNZ	R5, not_equal
+	B	equal
+lt_4:
+	TBZ	$1, R2, lt_2
+	MOVHU.P	2(R0), R4
+	MOVHU.P	2(R1), R5
+	CMP	R4, R5
+	BNE	not_equal
+lt_2:
+	TBZ	$0, R2, equal
+one:
+	MOVBU	(R0), R4
+	MOVBU	(R1), R5
+	CMP	R4, R5
+	BNE	not_equal
+equal:
+	MOVD	$1, R0
+	RET
+not_equal:
+	MOVB	ZR, R0
+	RET
diff --git a/src/internal/bytealg/equal_generic.go b/src/internal/bytealg/equal_generic.go
new file mode 100644
index 0000000..59bdf8f
--- /dev/null
+++ b/src/internal/bytealg/equal_generic.go
@@ -0,0 +1,18 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+// Equal reports whether a and b
+// are the same length and contain the same bytes.
+// A nil argument is equivalent to an empty slice.
+//
+// Equal is equivalent to bytes.Equal.
+// It is provided here for convenience,
+// because some packages cannot depend on bytes.
+func Equal(a, b []byte) bool {
+	// Neither cmd/compile nor gccgo allocates for these string conversions.
+	// There is a test for this in package bytes.
+	return string(a) == string(b)
+}
diff --git a/src/internal/bytealg/equal_loong64.s b/src/internal/bytealg/equal_loong64.s
new file mode 100644
index 0000000..dcdde89
--- /dev/null
+++ b/src/internal/bytealg/equal_loong64.s
@@ -0,0 +1,52 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+#define	REGCTXT	R29
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT|NOFRAME,$0-25
+	MOVV	a+0(FP), R4
+	MOVV	b+8(FP), R5
+	BEQ	R4, R5, eq
+	MOVV	size+16(FP), R6
+	ADDV	R4, R6, R7
+loop:
+	BNE	R4, R7, test
+	MOVV	$1, R4
+	MOVB	R4, ret+24(FP)
+	RET
+test:
+	MOVBU	(R4), R9
+	ADDV	$1, R4
+	MOVBU	(R5), R10
+	ADDV	$1, R5
+	BEQ	R9, R10, loop
+
+	MOVB	R0, ret+24(FP)
+	RET
+eq:
+	MOVV	$1, R4
+	MOVB	R4, ret+24(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT,$40-17
+	MOVV	a+0(FP), R4
+	MOVV	b+8(FP), R5
+	BEQ	R4, R5, eq
+	MOVV	8(REGCTXT), R6    // compiler stores size at offset 8 in the closure
+	MOVV	R4, 8(R3)
+	MOVV	R5, 16(R3)
+	MOVV	R6, 24(R3)
+	JAL	runtime·memequal(SB)
+	MOVBU	32(R3), R4
+	MOVB	R4, ret+16(FP)
+	RET
+eq:
+	MOVV	$1, R4
+	MOVB	R4, ret+16(FP)
+	RET
diff --git a/src/internal/bytealg/equal_mips64x.s b/src/internal/bytealg/equal_mips64x.s
new file mode 100644
index 0000000..d92f225
--- /dev/null
+++ b/src/internal/bytealg/equal_mips64x.s
@@ -0,0 +1,118 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips64 || mips64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+#define	REGCTXT	R22
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT|NOFRAME,$0-25
+	MOVV	a+0(FP), R1
+	MOVV	b+8(FP), R2
+	BEQ	R1, R2, eq
+	MOVV	size+16(FP), R3
+	ADDV	R1, R3, R4
+
+	// chunk size is 16
+	SGTU	$16, R3, R8
+	BEQ	R0, R8, chunk_entry
+
+byte_loop:
+	BNE	R1, R4, byte_test
+	MOVV	$1, R1
+	MOVB	R1, ret+24(FP)
+	RET
+byte_test:
+	MOVBU	(R1), R6
+	ADDV	$1, R1
+	MOVBU	(R2), R7
+	ADDV	$1, R2
+	BEQ	R6, R7, byte_loop
+	JMP	not_eq
+
+chunk_entry:
+	// make sure both a and b are aligned
+	OR	R1, R2, R9
+	AND	$0x7, R9
+	BNE	R0, R9, byte_loop
+	JMP	chunk_loop_1
+
+chunk_loop:
+	// chunk size is 16
+	SGTU	$16, R3, R8
+	BNE	R0, R8, chunk_tail_8
+chunk_loop_1:
+	MOVV	(R1), R6
+	MOVV	(R2), R7
+	BNE	R6, R7, not_eq
+	MOVV	8(R1), R12
+	MOVV	8(R2), R13
+	ADDV	$16, R1
+	ADDV	$16, R2
+	SUBV	$16, R3
+	BEQ	R12, R13, chunk_loop
+	JMP	not_eq
+
+chunk_tail_8:
+	AND	$8, R3, R14
+	BEQ	R0, R14, chunk_tail_4
+	MOVV	(R1), R6
+	MOVV	(R2), R7
+	BNE	R6, R7, not_eq
+	ADDV	$8, R1
+	ADDV	$8, R2
+
+chunk_tail_4:
+	AND	$4, R3, R14
+	BEQ	R0, R14, chunk_tail_2
+	MOVWU	(R1), R6
+	MOVWU	(R2), R7
+	BNE	R6, R7, not_eq
+	ADDV	$4, R1
+	ADDV	$4, R2
+
+chunk_tail_2:
+	AND	$2, R3, R14
+	BEQ	R0, R14, chunk_tail_1
+	MOVHU	(R1), R6
+	MOVHU	(R2), R7
+	BNE	R6, R7, not_eq
+	ADDV	$2, R1
+	ADDV	$2, R2
+
+chunk_tail_1:
+	AND	$1, R3, R14
+	BEQ	R0, R14, eq
+	MOVBU	(R1), R6
+	MOVBU	(R2), R7
+	BEQ	R6, R7, eq
+
+not_eq:
+	MOVB	R0, ret+24(FP)
+	RET
+eq:
+	MOVV	$1, R1
+	MOVB	R1, ret+24(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT,$40-17
+	MOVV	a+0(FP), R1
+	MOVV	b+8(FP), R2
+	BEQ	R1, R2, eq
+	MOVV	8(REGCTXT), R3    // compiler stores size at offset 8 in the closure
+	MOVV	R1, 8(R29)
+	MOVV	R2, 16(R29)
+	MOVV	R3, 24(R29)
+	JAL	runtime·memequal(SB)
+	MOVBU	32(R29), R1
+	MOVB	R1, ret+16(FP)
+	RET
+eq:
+	MOVV	$1, R1
+	MOVB	R1, ret+16(FP)
+	RET
diff --git a/src/internal/bytealg/equal_mipsx.s b/src/internal/bytealg/equal_mipsx.s
new file mode 100644
index 0000000..4c46dd4
--- /dev/null
+++ b/src/internal/bytealg/equal_mipsx.s
@@ -0,0 +1,62 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips || mipsle
+
+#include "go_asm.h"
+#include "textflag.h"
+
+#define	REGCTXT	R22
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT,$0-13
+	MOVW	a+0(FP), R1
+	MOVW	b+4(FP), R2
+	BEQ	R1, R2, eq
+	MOVW	size+8(FP), R3
+	ADDU	R1, R3, R4
+loop:
+	BNE	R1, R4, test
+	MOVW	$1, R1
+	MOVB	R1, ret+12(FP)
+	RET
+test:
+	MOVBU	(R1), R6
+	ADDU	$1, R1
+	MOVBU	(R2), R7
+	ADDU	$1, R2
+	BEQ	R6, R7, loop
+
+	MOVB	R0, ret+12(FP)
+	RET
+eq:
+	MOVW	$1, R1
+	MOVB	R1, ret+12(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT,$0-9
+	MOVW	a+0(FP), R1
+	MOVW	b+4(FP), R2
+	BEQ	R1, R2, eq
+	MOVW	4(REGCTXT), R3	// compiler stores size at offset 4 in the closure
+	ADDU	R1, R3, R4
+loop:
+	BNE	R1, R4, test
+	MOVW	$1, R1
+	MOVB	R1, ret+8(FP)
+	RET
+test:
+	MOVBU	(R1), R6
+	ADDU	$1, R1
+	MOVBU	(R2), R7
+	ADDU	$1, R2
+	BEQ	R6, R7, loop
+
+	MOVB	R0, ret+8(FP)
+	RET
+eq:
+	MOVW	$1, R1
+	MOVB	R1, ret+8(FP)
+	RET
diff --git a/src/internal/bytealg/equal_native.go b/src/internal/bytealg/equal_native.go
new file mode 100644
index 0000000..cf3a245
--- /dev/null
+++ b/src/internal/bytealg/equal_native.go
@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+import "unsafe"
+
+// The declarations below generate ABI wrappers for functions
+// implemented in assembly in this package but declared in another
+// package.
+
+// The compiler generates calls to runtime.memequal and runtime.memequal_varlen.
+// In addition, the runtime calls runtime.memequal explicitly.
+// Those functions are implemented in this package.
+
+//go:linkname abigen_runtime_memequal runtime.memequal
+func abigen_runtime_memequal(a, b unsafe.Pointer, size uintptr) bool
+
+//go:linkname abigen_runtime_memequal_varlen runtime.memequal_varlen
+func abigen_runtime_memequal_varlen(a, b unsafe.Pointer) bool
diff --git a/src/internal/bytealg/equal_ppc64x.s b/src/internal/bytealg/equal_ppc64x.s
new file mode 100644
index 0000000..f2c7cc1
--- /dev/null
+++ b/src/internal/bytealg/equal_ppc64x.s
@@ -0,0 +1,205 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// 4K (smallest case) page size offset mask for PPC64.
+#define PAGE_OFFSET 4095
+
+// TODO: At writing, ISEL and BC do not support CR bit type arguments,
+// define them here for readability.
+#define CR0LT 4*0+0
+#define CR0EQ 4*0+2
+#define CR1LT 4*1+0
+#define CR6LT 4*6+0
+
+// Likewise, the BC opcode is hard to read, and no extended
+// mnemonics are offered for these forms.
+#define BGELR_CR6 BC  4, CR6LT, (LR)
+#define BEQLR     BC 12, CR0EQ, (LR)
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-25
+	// R3 = a
+	// R4 = b
+	// R5 = size
+	BR	memeqbody<>(SB)
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-17
+	// R3 = a
+	// R4 = b
+	CMP	R3, R4
+	BEQ	eq
+	MOVD	8(R11), R5    // compiler stores size at offset 8 in the closure
+	BR	memeqbody<>(SB)
+eq:
+	MOVD	$1, R3
+	RET
+
+// Do an efficient memequal for ppc64
+// R3 = s1
+// R4 = s2
+// R5 = len
+// On exit:
+// R3 = return value
+TEXT memeqbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	MOVD	R3, R8		// Move s1 into R8
+	ADD	R5, R3, R9	// &s1[len(s1)]
+	ADD	R5, R4, R10	// &s2[len(s2)]
+	MOVD	$1, R11
+	CMP	R5, $16		// Use GPR checks for check for len <= 16
+	BLE	check0_16
+	MOVD	$0, R3		// Assume no-match in case BGELR CR6 returns
+	CMP	R5, $32		// Use overlapping VSX loads for len <= 32
+	BLE	check17_32	// Do a pair of overlapping VSR compares
+	CMP	R5, $64
+	BLE	check33_64	// Hybrid check + overlap compare.
+
+setup64:
+	SRD	$6, R5, R6	// number of 64 byte chunks to compare
+	MOVD	R6, CTR
+	MOVD	$16, R14	// index for VSX loads and stores
+	MOVD	$32, R15
+	MOVD	$48, R16
+	ANDCC	$0x3F, R5, R5	// len%64==0?
+
+	PCALIGN $32
+loop64:
+	LXVD2X	(R8+R0), V0
+	LXVD2X	(R4+R0), V1
+	VCMPEQUBCC V0, V1, V2	// compare, setting CR6
+	BGELR_CR6
+	LXVD2X	(R8+R14), V0
+	LXVD2X	(R4+R14), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	LXVD2X	(R8+R15), V0
+	LXVD2X	(R4+R15), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	LXVD2X	(R8+R16), V0
+	LXVD2X	(R4+R16), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	ADD	$64,R8		// bump up to next 64
+	ADD	$64,R4
+	BDNZ	loop64
+
+	ISEL	$CR0EQ, R11, R3, R3	// If no tail, return 1, otherwise R3 remains 0.
+	BEQLR				// return if no tail.
+
+	ADD	$-64, R9, R8
+	ADD	$-64, R10, R4
+	LXVD2X	(R8+R0), V0
+	LXVD2X	(R4+R0), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	LXVD2X	(R8+R14), V0
+	LXVD2X	(R4+R14), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	LXVD2X	(R8+R15), V0
+	LXVD2X	(R4+R15), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	LXVD2X	(R8+R16), V0
+	LXVD2X	(R4+R16), V1
+	VCMPEQUBCC	V0, V1, V2
+	ISEL	$CR6LT, R11, R0, R3
+	RET
+
+check33_64:
+	// Bytes 0-15
+	LXVD2X	(R8+R0), V0
+	LXVD2X	(R4+R0), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+	ADD	$16, R8
+	ADD	$16, R4
+
+	// Bytes 16-31
+	LXVD2X	(R8+R0), V0
+	LXVD2X	(R4+R0), V1
+	VCMPEQUBCC	V0, V1, V2
+	BGELR_CR6
+
+	// A little tricky, but point R4,R8 to &sx[len-32],
+	// and reuse check17_32 to check the next 1-31 bytes (with some overlap)
+	ADD	$-32, R9, R8
+	ADD	$-32, R10, R4
+	// Fallthrough
+
+check17_32:
+	LXVD2X	(R8+R0), V0
+	LXVD2X	(R4+R0), V1
+	VCMPEQUBCC	V0, V1, V2
+	ISEL	$CR6LT, R11, R0, R5
+
+	// Load sX[len(sX)-16:len(sX)] and compare.
+	ADD	$-16, R9
+	ADD	$-16, R10
+	LXVD2X	(R9+R0), V0
+	LXVD2X	(R10+R0), V1
+	VCMPEQUBCC	V0, V1, V2
+	ISEL	$CR6LT, R5, R0, R3
+	RET
+
+check0_16:
+	CMP	R5, $8
+	BLT	check0_7
+	// Load sX[0:7] and compare.
+	MOVD	(R8), R6
+	MOVD	(R4), R7
+	CMP	R6, R7
+	ISEL	$CR0EQ, R11, R0, R5
+	// Load sX[len(sX)-8:len(sX)] and compare.
+	MOVD	-8(R9), R6
+	MOVD	-8(R10), R7
+	CMP	R6, R7
+	ISEL	$CR0EQ, R5, R0, R3
+	RET
+
+check0_7:
+	CMP	R5,$0
+	MOVD	$1, R3
+	BEQLR		// return if len == 0
+
+	// Check < 8B loads with a single compare, but select the load address
+	// such that it cannot cross a page boundary. Load a few bytes from the
+	// lower address if that does not cross the lower page. Or, load a few
+	// extra bytes from the higher addresses. And align those values
+	// consistently in register as either address may have differing
+	// alignment requirements.
+	ANDCC	$PAGE_OFFSET, R8, R6	// &sX & PAGE_OFFSET
+	ANDCC	$PAGE_OFFSET, R4, R9
+	SUBC	R5, $8, R12		// 8-len
+	SLD	$3, R12, R14		// (8-len)*8
+	CMPU	R6, R12, CR1		// Enough bytes lower in the page to load lower?
+	CMPU	R9, R12, CR0
+	SUB	R12, R8, R6		// compute lower load address
+	SUB	R12, R4, R9
+	ISEL	$CR1LT, R8, R6, R8	// R8 = R6 < 0 ? R8 (&s1) : R6 (&s1 - (8-len))
+	ISEL	$CR0LT, R4, R9, R4	// Similar for s2
+	MOVD	(R8), R15
+	MOVD	(R4), R16
+	SLD	R14, R15, R7
+	SLD	R14, R16, R17
+	SRD	R14, R7, R7		// Clear the upper (8-len) bytes (with 2 shifts)
+	SRD	R14, R17, R17
+	SRD	R14, R15, R6		// Clear the lower (8-len) bytes
+	SRD	R14, R16, R9
+#ifdef GOARCH_ppc64le
+	ISEL	$CR1LT, R7, R6, R8      // Choose the correct len bytes to compare based on alignment
+	ISEL	$CR0LT, R17, R9, R4
+#else
+	ISEL	$CR1LT, R6, R7, R8
+	ISEL	$CR0LT, R9, R17, R4
+#endif
+	CMP	R4, R8
+	ISEL	$CR0EQ, R11, R0, R3
+	RET
diff --git a/src/internal/bytealg/equal_riscv64.s b/src/internal/bytealg/equal_riscv64.s
new file mode 100644
index 0000000..3834083
--- /dev/null
+++ b/src/internal/bytealg/equal_riscv64.s
@@ -0,0 +1,124 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+#define	CTXT	S10
+
+// func memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-25
+	// X10 = a_base
+	// X11 = b_base
+	// X12 = size
+	JMP	memequal<>(SB)
+
+// func memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-17
+	MOV	8(CTXT), X12    // compiler stores size at offset 8 in the closure
+	// X10 = a_base
+	// X11 = b_base
+	JMP	memequal<>(SB)
+
+// On entry X10 and X11 contain pointers, X12 contains length.
+// For non-regabi X13 contains address for return value.
+// For regabi return value in X10.
+TEXT memequal<>(SB),NOSPLIT|NOFRAME,$0
+	BEQ	X10, X11, eq
+
+	MOV	$32, X23
+	BLT	X12, X23, loop4_check
+
+	// Check alignment - if alignment differs we have to do one byte at a time.
+	AND	$7, X10, X9
+	AND	$7, X11, X19
+	BNE	X9, X19, loop4_check
+	BEQZ	X9, loop32_check
+
+	// Check one byte at a time until we reach 8 byte alignment.
+	SUB	X9, X12, X12
+align:
+	ADD	$-1, X9
+	MOVBU	0(X10), X19
+	MOVBU	0(X11), X20
+	BNE	X19, X20, not_eq
+	ADD	$1, X10
+	ADD	$1, X11
+	BNEZ	X9, align
+
+loop32_check:
+	MOV	$32, X9
+	BLT	X12, X9, loop16_check
+loop32:
+	MOV	0(X10), X19
+	MOV	0(X11), X20
+	MOV	8(X10), X21
+	MOV	8(X11), X22
+	BNE	X19, X20, not_eq
+	BNE	X21, X22, not_eq
+	MOV	16(X10), X14
+	MOV	16(X11), X15
+	MOV	24(X10), X16
+	MOV	24(X11), X17
+	BNE	X14, X15, not_eq
+	BNE	X16, X17, not_eq
+	ADD	$32, X10
+	ADD	$32, X11
+	ADD	$-32, X12
+	BGE	X12, X9, loop32
+	BEQZ	X12, eq
+
+loop16_check:
+	MOV	$16, X23
+	BLT	X12, X23, loop4_check
+loop16:
+	MOV	0(X10), X19
+	MOV	0(X11), X20
+	MOV	8(X10), X21
+	MOV	8(X11), X22
+	BNE	X19, X20, not_eq
+	BNE	X21, X22, not_eq
+	ADD	$16, X10
+	ADD	$16, X11
+	ADD	$-16, X12
+	BGE	X12, X23, loop16
+	BEQZ	X12, eq
+
+loop4_check:
+	MOV	$4, X23
+	BLT	X12, X23, loop1
+loop4:
+	MOVBU	0(X10), X19
+	MOVBU	0(X11), X20
+	MOVBU	1(X10), X21
+	MOVBU	1(X11), X22
+	BNE	X19, X20, not_eq
+	BNE	X21, X22, not_eq
+	MOVBU	2(X10), X14
+	MOVBU	2(X11), X15
+	MOVBU	3(X10), X16
+	MOVBU	3(X11), X17
+	BNE	X14, X15, not_eq
+	BNE	X16, X17, not_eq
+	ADD	$4, X10
+	ADD	$4, X11
+	ADD	$-4, X12
+	BGE	X12, X23, loop4
+
+loop1:
+	BEQZ	X12, eq
+	MOVBU	0(X10), X19
+	MOVBU	0(X11), X20
+	BNE	X19, X20, not_eq
+	ADD	$1, X10
+	ADD	$1, X11
+	ADD	$-1, X12
+	JMP	loop1
+
+not_eq:
+	MOVB	ZERO, X10
+	RET
+eq:
+	MOV	$1, X10
+	RET
diff --git a/src/internal/bytealg/equal_s390x.s b/src/internal/bytealg/equal_s390x.s
new file mode 100644
index 0000000..67f814d
--- /dev/null
+++ b/src/internal/bytealg/equal_s390x.s
@@ -0,0 +1,92 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// memequal(a, b unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB),NOSPLIT|NOFRAME,$0-25
+	MOVD	a+0(FP), R3
+	MOVD	b+8(FP), R5
+	MOVD	size+16(FP), R6
+	LA	ret+24(FP), R7
+	BR	memeqbody<>(SB)
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB),NOSPLIT|NOFRAME,$0-17
+	MOVD	a+0(FP), R3
+	MOVD	b+8(FP), R5
+	MOVD	8(R12), R6    // compiler stores size at offset 8 in the closure
+	LA	ret+16(FP), R7
+	BR	memeqbody<>(SB)
+
+// input:
+//   R3 = a
+//   R5 = b
+//   R6 = len
+//   R7 = address of output byte (stores 0 or 1 here)
+//   a and b have the same length
+TEXT memeqbody<>(SB),NOSPLIT|NOFRAME,$0-0
+	CMPBEQ	R3, R5, equal
+loop:
+	CMPBEQ	R6, $0, equal
+	CMPBLT	R6, $32, tiny
+	CMP	R6, $256
+	BLT	tail
+	CLC	$256, 0(R3), 0(R5)
+	BNE	notequal
+	SUB	$256, R6
+	LA	256(R3), R3
+	LA	256(R5), R5
+	BR	loop
+tail:
+	SUB	$1, R6, R8
+	EXRL	$memeqbodyclc<>(SB), R8
+	BEQ	equal
+notequal:
+	MOVB	$0, 0(R7)
+	RET
+equal:
+	MOVB	$1, 0(R7)
+	RET
+tiny:
+	MOVD	$0, R2
+	CMPBLT	R6, $16, lt16
+	MOVD	0(R3), R8
+	MOVD	0(R5), R9
+	CMPBNE	R8, R9, notequal
+	MOVD	8(R3), R8
+	MOVD	8(R5), R9
+	CMPBNE	R8, R9, notequal
+	LA	16(R2), R2
+	SUB	$16, R6
+lt16:
+	CMPBLT	R6, $8, lt8
+	MOVD	0(R3)(R2*1), R8
+	MOVD	0(R5)(R2*1), R9
+	CMPBNE	R8, R9, notequal
+	LA	8(R2), R2
+	SUB	$8, R6
+lt8:
+	CMPBLT	R6, $4, lt4
+	MOVWZ	0(R3)(R2*1), R8
+	MOVWZ	0(R5)(R2*1), R9
+	CMPBNE	R8, R9, notequal
+	LA	4(R2), R2
+	SUB	$4, R6
+lt4:
+#define CHECK(n) \
+	CMPBEQ	R6, $n, equal \
+	MOVB	n(R3)(R2*1), R8 \
+	MOVB	n(R5)(R2*1), R9 \
+	CMPBNE	R8, R9, notequal
+	CHECK(0)
+	CHECK(1)
+	CHECK(2)
+	CHECK(3)
+	BR	equal
+
+TEXT memeqbodyclc<>(SB),NOSPLIT|NOFRAME,$0-0
+	CLC	$1, 0(R3), 0(R5)
+	RET
diff --git a/src/internal/bytealg/equal_wasm.s b/src/internal/bytealg/equal_wasm.s
new file mode 100644
index 0000000..a2b76c1
--- /dev/null
+++ b/src/internal/bytealg/equal_wasm.s
@@ -0,0 +1,77 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// memequal(p, q unsafe.Pointer, size uintptr) bool
+TEXT runtime·memequal(SB), NOSPLIT, $0-25
+	Get SP
+	I64Load a+0(FP)
+	I64Load b+8(FP)
+	I64Load size+16(FP)
+	Call memeqbody<>(SB)
+	I64Store8 ret+24(FP)
+	RET
+
+// memequal_varlen(a, b unsafe.Pointer) bool
+TEXT runtime·memequal_varlen(SB), NOSPLIT, $0-17
+	Get SP
+	I64Load a+0(FP)
+	I64Load b+8(FP)
+	I64Load 8(CTXT) // compiler stores size at offset 8 in the closure
+	Call memeqbody<>(SB)
+	I64Store8 ret+16(FP)
+	RET
+
+// params: a, b, len
+// ret: 0/1
+TEXT memeqbody<>(SB), NOSPLIT, $0-0
+	Get R0
+	Get R1
+	I64Eq
+	If
+		I64Const $1
+		Return
+	End
+
+loop:
+	Loop
+		Get R2
+		I64Eqz
+		If
+			I64Const $1
+			Return
+		End
+
+		Get R0
+		I32WrapI64
+		I64Load8U $0
+		Get R1
+		I32WrapI64
+		I64Load8U $0
+		I64Ne
+		If
+			I64Const $0
+			Return
+		End
+
+		Get R0
+		I64Const $1
+		I64Add
+		Set R0
+
+		Get R1
+		I64Const $1
+		I64Add
+		Set R1
+
+		Get R2
+		I64Const $1
+		I64Sub
+		Set R2
+
+		Br loop
+	End
+	UNDEF
diff --git a/src/internal/bytealg/index_amd64.go b/src/internal/bytealg/index_amd64.go
new file mode 100644
index 0000000..c7a1941
--- /dev/null
+++ b/src/internal/bytealg/index_amd64.go
@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+import "internal/cpu"
+
+const MaxBruteForce = 64
+
+func init() {
+	if cpu.X86.HasAVX2 {
+		MaxLen = 63
+	} else {
+		MaxLen = 31
+	}
+}
+
+// Cutover reports the number of failures of IndexByte we should tolerate
+// before switching over to Index.
+// n is the number of bytes processed so far.
+// See the bytes.Index implementation for details.
+func Cutover(n int) int {
+	// 1 error per 8 characters, plus a few slop to start.
+	return (n + 16) / 8
+}
diff --git a/src/internal/bytealg/index_amd64.s b/src/internal/bytealg/index_amd64.s
new file mode 100644
index 0000000..0431491
--- /dev/null
+++ b/src/internal/bytealg/index_amd64.s
@@ -0,0 +1,276 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Index(SB),NOSPLIT,$0-56
+	MOVQ a_base+0(FP), DI
+	MOVQ a_len+8(FP), DX
+	MOVQ b_base+24(FP), R8
+	MOVQ b_len+32(FP), AX
+	MOVQ DI, R10
+	LEAQ ret+48(FP), R11
+	JMP  indexbody<>(SB)
+
+TEXT ·IndexString(SB),NOSPLIT,$0-40
+	MOVQ a_base+0(FP), DI
+	MOVQ a_len+8(FP), DX
+	MOVQ b_base+16(FP), R8
+	MOVQ b_len+24(FP), AX
+	MOVQ DI, R10
+	LEAQ ret+32(FP), R11
+	JMP  indexbody<>(SB)
+
+// AX: length of string, that we are searching for
+// DX: length of string, in which we are searching
+// DI: pointer to string, in which we are searching
+// R8: pointer to string, that we are searching for
+// R11: address, where to put return value
+// Note: We want len in DX and AX, because PCMPESTRI implicitly consumes them
+TEXT indexbody<>(SB),NOSPLIT,$0
+	CMPQ AX, DX
+	JA fail
+	CMPQ DX, $16
+	JAE sse42
+no_sse42:
+	CMPQ AX, $2
+	JA   _3_or_more
+	MOVW (R8), R8
+	LEAQ -1(DI)(DX*1), DX
+loop2:
+	MOVW (DI), SI
+	CMPW SI,R8
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop2
+	JMP fail
+_3_or_more:
+	CMPQ AX, $3
+	JA   _4_or_more
+	MOVW 1(R8), BX
+	MOVW (R8), R8
+	LEAQ -2(DI)(DX*1), DX
+loop3:
+	MOVW (DI), SI
+	CMPW SI,R8
+	JZ   partial_success3
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop3
+	JMP fail
+partial_success3:
+	MOVW 1(DI), SI
+	CMPW SI,BX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop3
+	JMP fail
+_4_or_more:
+	CMPQ AX, $4
+	JA   _5_or_more
+	MOVL (R8), R8
+	LEAQ -3(DI)(DX*1), DX
+loop4:
+	MOVL (DI), SI
+	CMPL SI,R8
+	JZ   success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop4
+	JMP fail
+_5_or_more:
+	CMPQ AX, $7
+	JA   _8_or_more
+	LEAQ 1(DI)(DX*1), DX
+	SUBQ AX, DX
+	MOVL -4(R8)(AX*1), BX
+	MOVL (R8), R8
+loop5to7:
+	MOVL (DI), SI
+	CMPL SI,R8
+	JZ   partial_success5to7
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop5to7
+	JMP fail
+partial_success5to7:
+	MOVL -4(AX)(DI*1), SI
+	CMPL SI,BX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop5to7
+	JMP fail
+_8_or_more:
+	CMPQ AX, $8
+	JA   _9_or_more
+	MOVQ (R8), R8
+	LEAQ -7(DI)(DX*1), DX
+loop8:
+	MOVQ (DI), SI
+	CMPQ SI,R8
+	JZ   success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop8
+	JMP fail
+_9_or_more:
+	CMPQ AX, $15
+	JA   _16_or_more
+	LEAQ 1(DI)(DX*1), DX
+	SUBQ AX, DX
+	MOVQ -8(R8)(AX*1), BX
+	MOVQ (R8), R8
+loop9to15:
+	MOVQ (DI), SI
+	CMPQ SI,R8
+	JZ   partial_success9to15
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop9to15
+	JMP fail
+partial_success9to15:
+	MOVQ -8(AX)(DI*1), SI
+	CMPQ SI,BX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop9to15
+	JMP fail
+_16_or_more:
+	CMPQ AX, $16
+	JA   _17_or_more
+	MOVOU (R8), X1
+	LEAQ -15(DI)(DX*1), DX
+loop16:
+	MOVOU (DI), X2
+	PCMPEQB X1, X2
+	PMOVMSKB X2, SI
+	CMPQ  SI, $0xffff
+	JE   success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop16
+	JMP fail
+_17_or_more:
+	CMPQ AX, $31
+	JA   _32_or_more
+	LEAQ 1(DI)(DX*1), DX
+	SUBQ AX, DX
+	MOVOU -16(R8)(AX*1), X0
+	MOVOU (R8), X1
+loop17to31:
+	MOVOU (DI), X2
+	PCMPEQB X1,X2
+	PMOVMSKB X2, SI
+	CMPQ  SI, $0xffff
+	JE   partial_success17to31
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop17to31
+	JMP fail
+partial_success17to31:
+	MOVOU -16(AX)(DI*1), X3
+	PCMPEQB X0, X3
+	PMOVMSKB X3, SI
+	CMPQ  SI, $0xffff
+	JE success
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop17to31
+	JMP fail
+// We can get here only when AVX2 is enabled and cutoff for indexShortStr is set to 63
+// So no need to check cpuid
+_32_or_more:
+	CMPQ AX, $32
+	JA   _33_to_63
+	VMOVDQU (R8), Y1
+	LEAQ -31(DI)(DX*1), DX
+loop32:
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPMOVMSKB Y3, SI
+	CMPL  SI, $0xffffffff
+	JE   success_avx2
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop32
+	JMP fail_avx2
+_33_to_63:
+	LEAQ 1(DI)(DX*1), DX
+	SUBQ AX, DX
+	VMOVDQU -32(R8)(AX*1), Y0
+	VMOVDQU (R8), Y1
+loop33to63:
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPMOVMSKB Y3, SI
+	CMPL  SI, $0xffffffff
+	JE   partial_success33to63
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop33to63
+	JMP fail_avx2
+partial_success33to63:
+	VMOVDQU -32(AX)(DI*1), Y3
+	VPCMPEQB Y0, Y3, Y4
+	VPMOVMSKB Y4, SI
+	CMPL  SI, $0xffffffff
+	JE success_avx2
+	ADDQ $1,DI
+	CMPQ DI,DX
+	JB loop33to63
+fail_avx2:
+	VZEROUPPER
+fail:
+	MOVQ $-1, (R11)
+	RET
+success_avx2:
+	VZEROUPPER
+	JMP success
+sse42:
+#ifndef hasSSE42
+	CMPB internal∕cpu·X86+const_offsetX86HasSSE42(SB), $1
+	JNE no_sse42
+#endif
+	CMPQ AX, $12
+	// PCMPESTRI is slower than normal compare,
+	// so using it makes sense only if we advance 4+ bytes per compare
+	// This value was determined experimentally and is the ~same
+	// on Nehalem (first with SSE42) and Haswell.
+	JAE _9_or_more
+	LEAQ 16(R8), SI
+	TESTW $0xff0, SI
+	JEQ no_sse42
+	MOVOU (R8), X1
+	LEAQ -15(DI)(DX*1), SI
+	MOVQ $16, R9
+	SUBQ AX, R9 // We advance by 16-len(sep) each iteration, so precalculate it into R9
+loop_sse42:
+	// 0x0c means: unsigned byte compare (bits 0,1 are 00)
+	// for equality (bits 2,3 are 11)
+	// result is not masked or inverted (bits 4,5 are 00)
+	// and corresponds to first matching byte (bit 6 is 0)
+	PCMPESTRI $0x0c, (DI), X1
+	// CX == 16 means no match,
+	// CX > R9 means partial match at the end of the string,
+	// otherwise sep is at offset CX from X1 start
+	CMPQ CX, R9
+	JBE sse42_success
+	ADDQ R9, DI
+	CMPQ DI, SI
+	JB loop_sse42
+	PCMPESTRI $0x0c, -1(SI), X1
+	CMPQ CX, R9
+	JA fail
+	LEAQ -1(SI), DI
+sse42_success:
+	ADDQ CX, DI
+success:
+	SUBQ R10, DI
+	MOVQ DI, (R11)
+	RET
diff --git a/src/internal/bytealg/index_arm64.go b/src/internal/bytealg/index_arm64.go
new file mode 100644
index 0000000..e87c109
--- /dev/null
+++ b/src/internal/bytealg/index_arm64.go
@@ -0,0 +1,23 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+// Empirical data shows that using Index can get better
+// performance when len(s) <= 16.
+const MaxBruteForce = 16
+
+func init() {
+	// Optimize cases where the length of the substring is less than 32 bytes
+	MaxLen = 32
+}
+
+// Cutover reports the number of failures of IndexByte we should tolerate
+// before switching over to Index.
+// n is the number of bytes processed so far.
+// See the bytes.Index implementation for details.
+func Cutover(n int) int {
+	// 1 error per 16 characters, plus a few slop to start.
+	return 4 + n>>4
+}
diff --git a/src/internal/bytealg/index_arm64.s b/src/internal/bytealg/index_arm64.s
new file mode 100644
index 0000000..3a551a7
--- /dev/null
+++ b/src/internal/bytealg/index_arm64.s
@@ -0,0 +1,206 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·Index(SB),NOSPLIT,$0-56
+	MOVD	a_base+0(FP), R0
+	MOVD	a_len+8(FP), R1
+	MOVD	b_base+24(FP), R2
+	MOVD	b_len+32(FP), R3
+	MOVD	$ret+48(FP), R9
+	B	indexbody<>(SB)
+
+TEXT ·IndexString(SB),NOSPLIT,$0-40
+	MOVD	a_base+0(FP), R0
+	MOVD	a_len+8(FP), R1
+	MOVD	b_base+16(FP), R2
+	MOVD	b_len+24(FP), R3
+	MOVD	$ret+32(FP), R9
+	B	indexbody<>(SB)
+
+// input:
+//   R0: haystack
+//   R1: length of haystack
+//   R2: needle
+//   R3: length of needle (2 <= len <= 32)
+//   R9: address to put result
+TEXT indexbody<>(SB),NOSPLIT,$0-56
+	// main idea is to load 'sep' into separate register(s)
+	// to avoid repeatedly re-load it again and again
+	// for sebsequent substring comparisons
+	SUB	R3, R1, R4
+	// R4 contains the start of last substring for comparison
+	ADD	R0, R4, R4
+	ADD	$1, R0, R8
+
+	CMP	$8, R3
+	BHI	greater_8
+	TBZ	$3, R3, len_2_7
+len_8:
+	// R5 contains 8-byte of sep
+	MOVD	(R2), R5
+loop_8:
+	// R6 contains substring for comparison
+	CMP	R4, R0
+	BHI	not_found
+	MOVD.P	1(R0), R6
+	CMP	R5, R6
+	BNE	loop_8
+	B	found
+len_2_7:
+	TBZ	$2, R3, len_2_3
+	TBZ	$1, R3, len_4_5
+	TBZ	$0, R3, len_6
+len_7:
+	// R5 and R6 contain 7-byte of sep
+	MOVWU	(R2), R5
+	// 1-byte overlap with R5
+	MOVWU	3(R2), R6
+loop_7:
+	CMP	R4, R0
+	BHI	not_found
+	MOVWU.P	1(R0), R3
+	CMP	R5, R3
+	BNE	loop_7
+	MOVWU	2(R0), R3
+	CMP	R6, R3
+	BNE	loop_7
+	B	found
+len_6:
+	// R5 and R6 contain 6-byte of sep
+	MOVWU	(R2), R5
+	MOVHU	4(R2), R6
+loop_6:
+	CMP	R4, R0
+	BHI	not_found
+	MOVWU.P	1(R0), R3
+	CMP	R5, R3
+	BNE	loop_6
+	MOVHU	3(R0), R3
+	CMP	R6, R3
+	BNE	loop_6
+	B	found
+len_4_5:
+	TBZ	$0, R3, len_4
+len_5:
+	// R5 and R7 contain 5-byte of sep
+	MOVWU	(R2), R5
+	MOVBU	4(R2), R7
+loop_5:
+	CMP	R4, R0
+	BHI	not_found
+	MOVWU.P	1(R0), R3
+	CMP	R5, R3
+	BNE	loop_5
+	MOVBU	3(R0), R3
+	CMP	R7, R3
+	BNE	loop_5
+	B	found
+len_4:
+	// R5 contains 4-byte of sep
+	MOVWU	(R2), R5
+loop_4:
+	CMP	R4, R0
+	BHI	not_found
+	MOVWU.P	1(R0), R6
+	CMP	R5, R6
+	BNE	loop_4
+	B	found
+len_2_3:
+	TBZ	$0, R3, len_2
+len_3:
+	// R6 and R7 contain 3-byte of sep
+	MOVHU	(R2), R6
+	MOVBU	2(R2), R7
+loop_3:
+	CMP	R4, R0
+	BHI	not_found
+	MOVHU.P	1(R0), R3
+	CMP	R6, R3
+	BNE	loop_3
+	MOVBU	1(R0), R3
+	CMP	R7, R3
+	BNE	loop_3
+	B	found
+len_2:
+	// R5 contains 2-byte of sep
+	MOVHU	(R2), R5
+loop_2:
+	CMP	R4, R0
+	BHI	not_found
+	MOVHU.P	1(R0), R6
+	CMP	R5, R6
+	BNE	loop_2
+found:
+	SUB	R8, R0, R0
+	MOVD	R0, (R9)
+	RET
+not_found:
+	MOVD	$-1, R0
+	MOVD	R0, (R9)
+	RET
+greater_8:
+	SUB	$9, R3, R11	// len(sep) - 9, offset of R0 for last 8 bytes
+	CMP	$16, R3
+	BHI	greater_16
+len_9_16:
+	MOVD.P	8(R2), R5	// R5 contains the first 8-byte of sep
+	SUB	$16, R3, R7	// len(sep) - 16, offset of R2 for last 8 bytes
+	MOVD	(R2)(R7), R6	// R6 contains the last 8-byte of sep
+loop_9_16:
+	// search the first 8 bytes first
+	CMP	R4, R0
+	BHI	not_found
+	MOVD.P	1(R0), R7
+	CMP	R5, R7
+	BNE	loop_9_16
+	MOVD	(R0)(R11), R7
+	CMP	R6, R7		// compare the last 8 bytes
+	BNE	loop_9_16
+	B	found
+greater_16:
+	CMP	$24, R3
+	BHI	len_25_32
+len_17_24:
+	LDP.P	16(R2), (R5, R6)	// R5 and R6 contain the first 16-byte of sep
+	SUB	$24, R3, R10		// len(sep) - 24
+	MOVD	(R2)(R10), R7		// R7 contains the last 8-byte of sep
+loop_17_24:
+	// search the first 16 bytes first
+	CMP	R4, R0
+	BHI	not_found
+	MOVD.P	1(R0), R10
+	CMP	R5, R10
+	BNE	loop_17_24
+	MOVD	7(R0), R10
+	CMP	R6, R10
+	BNE	loop_17_24
+	MOVD	(R0)(R11), R10
+	CMP	R7, R10		// compare the last 8 bytes
+	BNE	loop_17_24
+	B	found
+len_25_32:
+	LDP.P	16(R2), (R5, R6)
+	MOVD.P	8(R2), R7	// R5, R6 and R7 contain the first 24-byte of sep
+	SUB	$32, R3, R12	// len(sep) - 32
+	MOVD	(R2)(R12), R10	// R10 contains the last 8-byte of sep
+loop_25_32:
+	// search the first 24 bytes first
+	CMP	R4, R0
+	BHI	not_found
+	MOVD.P	1(R0), R12
+	CMP	R5, R12
+	BNE	loop_25_32
+	MOVD	7(R0), R12
+	CMP	R6, R12
+	BNE	loop_25_32
+	MOVD	15(R0), R12
+	CMP	R7, R12
+	BNE	loop_25_32
+	MOVD	(R0)(R11), R12
+	CMP	R10, R12	// compare the last 8 bytes
+	BNE	loop_25_32
+	B	found
diff --git a/src/internal/bytealg/index_generic.go b/src/internal/bytealg/index_generic.go
new file mode 100644
index 0000000..a59e329
--- /dev/null
+++ b/src/internal/bytealg/index_generic.go
@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !amd64 && !arm64 && !s390x && !ppc64le && !ppc64
+
+package bytealg
+
+const MaxBruteForce = 0
+
+// Index returns the index of the first instance of b in a, or -1 if b is not present in a.
+// Requires 2 <= len(b) <= MaxLen.
+func Index(a, b []byte) int {
+	panic("unimplemented")
+}
+
+// IndexString returns the index of the first instance of b in a, or -1 if b is not present in a.
+// Requires 2 <= len(b) <= MaxLen.
+func IndexString(a, b string) int {
+	panic("unimplemented")
+}
+
+// Cutover reports the number of failures of IndexByte we should tolerate
+// before switching over to Index.
+// n is the number of bytes processed so far.
+// See the bytes.Index implementation for details.
+func Cutover(n int) int {
+	panic("unimplemented")
+}
diff --git a/src/internal/bytealg/index_native.go b/src/internal/bytealg/index_native.go
new file mode 100644
index 0000000..6e4a2f3
--- /dev/null
+++ b/src/internal/bytealg/index_native.go
@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64 || arm64 || s390x || ppc64le || ppc64
+
+package bytealg
+
+//go:noescape
+
+// Index returns the index of the first instance of b in a, or -1 if b is not present in a.
+// Requires 2 <= len(b) <= MaxLen.
+func Index(a, b []byte) int
+
+//go:noescape
+
+// IndexString returns the index of the first instance of b in a, or -1 if b is not present in a.
+// Requires 2 <= len(b) <= MaxLen.
+func IndexString(a, b string) int
diff --git a/src/internal/bytealg/index_ppc64x.go b/src/internal/bytealg/index_ppc64x.go
new file mode 100644
index 0000000..ab3cbe5
--- /dev/null
+++ b/src/internal/bytealg/index_ppc64x.go
@@ -0,0 +1,26 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (aix || linux) && (ppc64 || ppc64le)
+
+package bytealg
+
+import "internal/cpu"
+
+const MaxBruteForce = 16
+
+var SupportsPower9 = cpu.PPC64.IsPOWER9
+
+func init() {
+	MaxLen = 32
+}
+
+// Cutover reports the number of failures of IndexByte we should tolerate
+// before switching over to Index.
+// n is the number of bytes processed so far.
+// See the bytes.Index implementation for details.
+func Cutover(n int) int {
+	// 1 error per 8 characters, plus a few slop to start.
+	return (n + 16) / 8
+}
diff --git a/src/internal/bytealg/index_ppc64x.s b/src/internal/bytealg/index_ppc64x.s
new file mode 100644
index 0000000..26205ce
--- /dev/null
+++ b/src/internal/bytealg/index_ppc64x.s
@@ -0,0 +1,802 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is an implementation based on the s390x
+// implementation.
+
+// Find a separator with 2 <= len <= 32 within a string.
+// Separators with lengths of 2, 3 or 4 are handled
+// specially.
+
+// This works on power8 and above. The loads and
+// compares are done in big endian order
+// since that allows the used of VCLZD, and allows
+// the same implementation to work on big and little
+// endian platforms with minimal conditional changes.
+
+// NOTE: There is a power9 implementation that
+// improves performance by 10-15% on little
+// endian for some of the benchmarks.
+// Unrolled index2to16 loop by 4 on ppc64le/power9
+// Work is still needed for a big endian
+// implementation on power9.
+
+//go:build ppc64 || ppc64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// Needed to swap LXVD2X loads to the correct
+// byte order to work on POWER8.
+
+#ifdef GOARCH_ppc64
+DATA byteswap<>+0(SB)/8, $0x0001020304050607
+DATA byteswap<>+8(SB)/8, $0x08090a0b0c0d0e0f
+#else
+DATA byteswap<>+0(SB)/8, $0x0706050403020100
+DATA byteswap<>+8(SB)/8, $0x0f0e0d0c0b0a0908
+#endif
+
+// Load bytes in big endian order. Address
+// alignment does not need checking.
+#define VLOADSWAP(base, index, vreg, vsreg) \
+	LXVD2X (base)(index), vsreg;  \
+	VPERM  vreg, vreg, SWAP, vreg
+
+GLOBL byteswap<>+0(SB), RODATA, $16
+
+TEXT ·Index<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-56
+	// R3 = byte array pointer
+	// R4 = length
+	MOVD R6, R5             // R5 = separator pointer
+	MOVD R7, R6             // R6 = separator length
+
+#ifdef GOARCH_ppc64le
+	MOVBZ internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R7
+	CMP   R7, $1
+	BNE   power8
+	BR    indexbodyp9<>(SB)
+#endif
+power8:
+	BR indexbody<>(SB)
+
+TEXT ·IndexString<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// R3 = string
+	// R4 = length
+	// R5 = separator pointer
+	// R6 = separator length
+
+#ifdef GOARCH_ppc64le
+	MOVBZ internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R7
+	CMP   R7, $1
+	BNE   power8
+	BR    indexbodyp9<>(SB)
+
+#endif
+power8:
+	BR indexbody<>(SB)
+
+	// s: string we are searching
+	// sep: string to search for
+	// R3=&s[0], R4=len(s)
+	// R5=&sep[0], R6=len(sep)
+	// R14=&ret (index where sep found)
+	// R7=working addr of string
+	// R16=index value 16
+	// R17=index value 17
+	// R18=index value 18
+	// R19=index value 1
+	// R26=LASTBYTE of string
+	// R27=LASTSTR last start byte to compare with sep
+	// R8, R9 scratch
+	// V0=sep left justified zero fill
+	// CR4=sep length >= 16
+
+#define SEPMASK V17
+#define LASTBYTE R26
+#define LASTSTR R27
+#define ONES V20
+#define SWAP V21
+#define SWAP_ VS53
+TEXT indexbody<>(SB), NOSPLIT|NOFRAME, $0
+	CMP      R6, R4                 // Compare lengths
+	BGT      notfound               // If sep len is > string, notfound
+	ADD      R4, R3, LASTBYTE       // find last byte addr
+	SUB      R6, LASTBYTE, LASTSTR  // LAST=&s[len(s)-len(sep)] (last valid start index)
+	CMP      R6, $0                 // Check sep len
+	BEQ      notfound               // sep len 0 -- not found
+	MOVD     R3, R7                 // Copy of string addr
+	MOVD     $16, R16               // Index value 16
+	MOVD     $17, R17               // Index value 17
+	MOVD     $18, R18               // Index value 18
+	MOVD     $1, R19                // Index value 1
+	MOVD     $byteswap<>+00(SB), R8
+	VSPLTISB $0xFF, ONES            // splat all 1s
+	LXVD2X   (R8)(R0), SWAP_        // Set up swap string
+
+	CMP    R6, $16, CR4        // CR4 for len(sep) >= 16
+	VOR    ONES, ONES, SEPMASK // Set up full SEPMASK
+	BGE    CR4, loadge16       // Load for len(sep) >= 16
+	SUB    R6, R16, R9         // 16-len of sep
+	SLD    $3, R9              // Set up for VSLO
+	MTVSRD R9, V9              // Set up for VSLO
+	VSLDOI $8, V9, V9, V9      // Set up for VSLO
+	VSLO   ONES, V9, SEPMASK   // Mask for separator len(sep) < 16
+
+loadge16:
+	ANDCC $15, R5, R9 // Find byte offset of sep
+	ADD   R9, R6, R10 // Add sep len
+	CMP   R10, $16    // Check if sep len+offset > 16
+	BGT   sepcross16  // Sep crosses 16 byte boundary
+
+	RLDICR $0, R5, $59, R8 // Adjust addr to 16 byte container
+	VLOADSWAP(R8, R0, V0, V0) // Load 16 bytes @R8 into V0
+	SLD    $3, R9          // Set up shift count for VSLO
+	MTVSRD R9, V8         // Set up shift count for VSLO
+	VSLDOI $8, V8, V8, V8
+	VSLO   V0, V8, V0      // Shift by start byte
+
+	VAND V0, SEPMASK, V0 // Mask separator (< 16)
+	BR   index2plus
+
+sepcross16:
+	VLOADSWAP(R5, R0, V0, V0)  // Load 16 bytes @R5 into V0
+
+	VAND V0, SEPMASK, V0 // mask out separator
+	BLE  CR4, index2to16
+	BR   index17plus     // Handle sep > 16
+
+index2plus:
+	CMP      R6, $2       // Check length of sep
+	BNE      index3plus   // If not 2, check for 3
+	ADD      $16, R7, R9  // Check if next 16 bytes past last
+	CMP      R9, LASTBYTE // compare with last
+	BGE      index2to16   // 2 <= len(string) <= 16
+	MOVD     $0xff00, R21 // Mask for later
+	MTVSRD   R21, V25     // Move to Vreg
+	VSPLTH   $3, V25, V31 // Splat mask
+	VSPLTH   $0, V0, V1   // Splat 1st 2 bytes of sep
+	VSPLTISB $0, V10      // Clear V10
+
+	// First case: 2 byte separator
+	// V1: 2 byte separator splatted
+	// V2: 16 bytes at addr
+	// V4: 16 bytes at addr+1
+	// Compare 2 byte separator at start
+	// and at start+1. Use VSEL to combine
+	// those results to find the first
+	// matching start byte, returning
+	// that value when found. Loop as
+	// long as len(string) > 16
+index2loop2:
+	VLOADSWAP(R7, R19, V3, V3) // Load 16 bytes @R7+1 into V3
+
+index2loop:
+	VLOADSWAP(R7, R0, V2, V2)  // Load 16 bytes @R7 into V2
+	VCMPEQUH V1, V2, V5        // Search for sep
+	VCMPEQUH V1, V3, V6        // Search for sep offset by 1
+	VSEL     V6, V5, V31, V7   // merge even and odd indices
+	VCLZD    V7, V18           // find index of first match
+	MFVSRD   V18, R25          // get first value
+	CMP      R25, $64          // Found if < 64
+	BLT      foundR25          // Return byte index where found
+	VSLDOI   $8, V18, V18, V18 // Adjust 2nd value
+	MFVSRD   V18, R25          // get second value
+	CMP      R25, $64          // Found if < 64
+	ADD      $64, R25          // Update byte offset
+	BLT      foundR25          // Return value
+	ADD      $16, R7           // R7+=16 Update string pointer
+	ADD      $17, R7, R9       // R9=F7+17 since loop unrolled
+	CMP      R9, LASTBYTE      // Compare addr+17 against last byte
+	BLT      index2loop2       // If < last, continue loop
+	CMP      R7, LASTBYTE      // Compare addr+16 against last byte
+	BLT      index2to16        // If < 16 handle specially
+	VLOADSWAP(R7, R0, V3, V3) // Load 16 bytes @R7 into V3
+	VSLDOI   $1, V3, V10, V3   // Shift left by 1 byte
+	BR       index2loop
+
+index3plus:
+	CMP    R6, $3       // Check if sep == 3
+	BNE    index4plus   // If not check larger
+	ADD    $19, R7, R9  // Find bytes for use in this loop
+	CMP    R9, LASTBYTE // Compare against last byte
+	BGE    index2to16   // Remaining string 2<=len<=16
+	MOVD   $0xff00, R21 // Set up mask for upcoming loop
+	MTVSRD R21, V25     // Move mask to Vreg
+	VSPLTH $3, V25, V31 // Splat mask
+	VSPLTH $0, V0, V1   // Splat 1st two bytes of sep
+	VSPLTB $2, V0, V8   // Splat 3rd byte of sep
+
+	// Loop to process 3 byte separator.
+	// string[0:16] is in V2
+	// string[2:18] is in V3
+	// sep[0:2] splatted in V1
+	// sec[3] splatted in v8
+	// Load vectors at string, string+1
+	// and string+2. Compare string, string+1
+	// against first 2 bytes of separator
+	// splatted, and string+2 against 3rd
+	// byte splatted. Merge the results with
+	// VSEL to find the first byte of a match.
+
+	// Special handling for last 16 bytes if the
+	// string fits in 16 byte multiple.
+index3loop2:
+	MOVD     $2, R21          // Set up index for 2
+	VSPLTISB $0, V10          // Clear V10
+	VLOADSWAP(R7, R21, V3, V3)// Load 16 bytes @R7+2 into V3
+	VSLDOI   $14, V3, V10, V3 // Left justify next 2 bytes
+
+index3loop:
+	VLOADSWAP(R7, R0, V2, V2)  // Load with correct order
+	VSLDOI   $1, V2, V3, V4    // string[1:17]
+	VSLDOI   $2, V2, V3, V9    // string[2:18]
+	VCMPEQUH V1, V2, V5        // compare hw even indices
+	VCMPEQUH V1, V4, V6        // compare hw odd indices
+	VCMPEQUB V8, V9, V10       // compare 3rd to last byte
+	VSEL     V6, V5, V31, V7   // Find 1st matching byte using mask
+	VAND     V7, V10, V7       // AND matched bytes with matched 3rd byte
+	VCLZD    V7, V18           // Find first nonzero indexes
+	MFVSRD   V18, R25          // Move 1st doubleword
+	CMP      R25, $64          // If < 64 found
+	BLT      foundR25          // Return matching index
+	VSLDOI   $8, V18, V18, V18 // Move value
+	MFVSRD   V18, R25          // Move 2nd doubleword
+	CMP      R25, $64          // If < 64 found
+	ADD      $64, R25          // Update byte index
+	BLT      foundR25          // Return matching index
+	ADD      $16, R7           // R7+=16 string ptr
+	ADD      $19, R7, R9       // Number of string bytes for loop
+	CMP      R9, LASTBYTE      // Compare against last byte of string
+	BLT      index3loop2       // If within, continue this loop
+	CMP      R7, LASTSTR       // Compare against last start byte
+	BLT      index2to16        // Process remainder
+	VSPLTISB $0, V3            // Special case for last 16 bytes
+	BR       index3loop        // Continue this loop
+
+	// Loop to process 4 byte separator
+	// string[0:16] in V2
+	// string[3:16] in V3
+	// sep[0:4] splatted in V1
+	// Set up vectors with strings at offsets
+	// 0, 1, 2, 3 and compare against the 4 byte
+	// separator also splatted. Use VSEL with the
+	// compare results to find the first byte where
+	// a separator match is found.
+index4plus:
+	CMP  R6, $4       // Check if 4 byte separator
+	BNE  index5plus   // If not next higher
+	ADD  $20, R7, R9  // Check string size to load
+	CMP  R9, LASTBYTE // Verify string length
+	BGE  index2to16   // If not large enough, process remaining
+	MOVD $2, R15      // Set up index
+
+	// Set up masks for use with VSEL
+	MOVD   $0xff, R21        // Set up mask 0xff000000ff000000...
+	SLD    $24, R21
+	MTVSRD R21, V10
+	VSPLTW $1, V10, V29
+	VSLDOI $2, V29, V29, V30 // Mask 0x0000ff000000ff00...
+	MOVD   $0xffff, R21
+	SLD    $16, R21
+	MTVSRD R21, V10
+	VSPLTW $1, V10, V31      // Mask 0xffff0000ffff0000...
+	VSPLTW $0, V0, V1        // Splat 1st word of separator
+
+index4loop:
+	VLOADSWAP(R7, R0, V2, V2)   // Load 16 bytes @R7 into V2
+
+next4:
+	VSPLTISB $0, V10            // Clear
+	MOVD     $3, R9             // Number of bytes beyond 16
+	VLOADSWAP(R7, R9, V3, V3)   // Load 16 bytes @R7+3 into V3
+	VSLDOI   $13, V3, V10, V3   // Shift left last 3 bytes
+	VSLDOI   $1, V2, V3, V4     // V4=(V2:V3)<<1
+	VSLDOI   $2, V2, V3, V9     // V9=(V2:V3)<<2
+	VSLDOI   $3, V2, V3, V10    // V10=(V2:v3)<<3
+	VCMPEQUW V1, V2, V5         // compare index 0, 4, ... with sep
+	VCMPEQUW V1, V4, V6         // compare index 1, 5, ... with sep
+	VCMPEQUW V1, V9, V11        // compare index 2, 6, ... with sep
+	VCMPEQUW V1, V10, V12       // compare index 3, 7, ... with sep
+	VSEL     V6, V5, V29, V13   // merge index 0, 1, 4, 5, using mask
+	VSEL     V12, V11, V30, V14 // merge index 2, 3, 6, 7, using mask
+	VSEL     V14, V13, V31, V7  // final merge
+	VCLZD    V7, V18            // Find first index for each half
+	MFVSRD   V18, R25           // Isolate value
+	CMP      R25, $64           // If < 64, found
+	BLT      foundR25           // Return found index
+	VSLDOI   $8, V18, V18, V18  // Move for MFVSRD
+	MFVSRD   V18, R25           // Isolate other value
+	CMP      R25, $64           // If < 64, found
+	ADD      $64, R25           // Update index for high doubleword
+	BLT      foundR25           // Return found index
+	ADD      $16, R7            // R7+=16 for next string
+	ADD      $20, R7, R9        // R+20 for all bytes to load
+	CMP      R9, LASTBYTE       // Past end? Maybe check for extra?
+	BLT      index4loop         // If not, continue loop
+	CMP      R7, LASTSTR        // Check remainder
+	BLE      index2to16         // Process remainder
+	BR       notfound           // Not found
+
+index5plus:
+	CMP R6, $16     // Check for sep > 16
+	BGT index17plus // Handle large sep
+
+	// Assumption is that the separator is smaller than the string at this point
+index2to16:
+	CMP R7, LASTSTR // Compare last start byte
+	BGT notfound    // last takes len(sep) into account
+
+	ADD $16, R7, R9    // Check for last byte of string
+	CMP R9, LASTBYTE
+	BGT index2to16tail
+
+	// At least 16 bytes of string left
+	// Mask the number of bytes in sep
+index2to16loop:
+	VLOADSWAP(R7, R0, V1, V1)  // Load 16 bytes @R7 into V1
+
+compare:
+	VAND       V1, SEPMASK, V2 // Mask out sep size
+	VCMPEQUBCC V0, V2, V3      // Compare masked string
+	BLT        CR6, found      // All equal
+	ADD        $1, R7          // Update ptr to next byte
+	CMP        R7, LASTSTR     // Still less than last start byte
+	BGT        notfound        // Not found
+	ADD        $16, R7, R9     // Verify remaining bytes
+	CMP        R9, LASTBYTE    // At least 16
+	BLT        index2to16loop  // Try again
+
+	// Less than 16 bytes remaining in string
+	// Separator >= 2
+index2to16tail:
+	ADD   R3, R4, R9     // End of string
+	SUB   R7, R9, R9     // Number of bytes left
+	ANDCC $15, R7, R10   // 16 byte offset
+	ADD   R10, R9, R11   // offset + len
+	CMP   R11, $16       // >= 16?
+	BLE   short          // Does not cross 16 bytes
+	VLOADSWAP(R7, R0, V1, V1) // Load 16 bytes @R7 into V1
+	BR    index2to16next // Continue on
+
+short:
+	RLDICR   $0, R7, $59, R9 // Adjust addr to 16 byte container
+	VLOADSWAP(R9, R0, V1, V1)// Load 16 bytes @R9 into V1
+	SLD      $3, R10         // Set up shift
+	MTVSRD   R10, V8         // Set up shift
+	VSLDOI   $8, V8, V8, V8
+	VSLO     V1, V8, V1      // Shift by start byte
+	VSPLTISB $0, V25         // Clear for later use
+
+index2to16next:
+	VAND       V1, SEPMASK, V2 // Just compare size of sep
+	VCMPEQUBCC V0, V2, V3      // Compare sep and partial string
+	BLT        CR6, found      // Found
+	ADD        $1, R7          // Not found, try next partial string
+	CMP        R7, LASTSTR     // Check for end of string
+	BGT        notfound        // If at end, then not found
+	VSLDOI     $1, V1, V25, V1 // Shift string left by 1 byte
+	BR         index2to16next  // Check the next partial string
+
+index17plus:
+	CMP      R6, $32      // Check if 17 < len(sep) <= 32
+	BGT      index33plus
+	SUB      $16, R6, R9  // Extra > 16
+	SLD      $56, R9, R10 // Shift to use in VSLO
+	MTVSRD   R10, V9      // Set up for VSLO
+	VLOADSWAP(R5, R9, V1, V1)// Load 16 bytes @R5+R9 into V1
+	VSLO     V1, V9, V1   // Shift left
+	VSPLTISB $0xff, V7    // Splat 1s
+	VSPLTISB $0, V27      // Splat 0
+
+index17to32loop:
+	VLOADSWAP(R7, R0, V2, V2)  // Load 16 bytes @R7 into V2
+
+next17:
+	VLOADSWAP(R7, R9, V3, V3)  // Load 16 bytes @R7+R9 into V3
+	VSLO       V3, V9, V3      // Shift left
+	VCMPEQUB   V0, V2, V4      // Compare first 16 bytes
+	VCMPEQUB   V1, V3, V5      // Compare extra over 16 bytes
+	VAND       V4, V5, V6      // Check if both equal
+	VCMPEQUBCC V6, V7, V8      // All equal?
+	BLT        CR6, found      // Yes
+	ADD        $1, R7          // On to next byte
+	CMP        R7, LASTSTR     // Check if last start byte
+	BGT        notfound        // If too high, not found
+	BR         index17to32loop // Continue
+
+notfound:
+	MOVD $-1, R3   // Return -1 if not found
+	RET
+
+index33plus:
+	MOVD $0, (R0) // Case not implemented
+	RET           // Crash before return
+
+foundR25:
+	SRD  $3, R25   // Convert from bits to bytes
+	ADD  R25, R7   // Add to current string address
+	SUB  R3, R7    // Subtract from start of string
+	MOVD R7, R3    // Return byte where found
+	RET
+
+found:
+	SUB  R3, R7    // Return byte where found
+	MOVD R7, R3
+	RET
+
+TEXT indexbodyp9<>(SB), NOSPLIT|NOFRAME, $0
+	CMP      R6, R4                // Compare lengths
+	BGT      notfound              // If sep len is > string, notfound
+	ADD      R4, R3, LASTBYTE      // find last byte addr
+	SUB      R6, LASTBYTE, LASTSTR // LAST=&s[len(s)-len(sep)] (last valid start index)
+	CMP      R6, $0                // Check sep len
+	BEQ      notfound              // sep len 0 -- not found
+	MOVD     R3, R7                // Copy of string addr
+	MOVD     $16, R16              // Index value 16
+	MOVD     $17, R17              // Index value 17
+	MOVD     $18, R18              // Index value 18
+	MOVD     $1, R19               // Index value 1
+	VSPLTISB $0xFF, ONES           // splat all 1s
+
+	CMP    R6, $16, CR4        // CR4 for len(sep) >= 16
+	VOR    ONES, ONES, SEPMASK // Set up full SEPMASK
+	BGE    CR4, loadge16       // Load for len(sep) >= 16
+	SUB    R6, R16, R9         // 16-len of sep
+	SLD    $3, R9              // Set up for VSLO
+	MTVSRD R9, V9              // Set up for VSLO
+	VSLDOI $8, V9, V9, V9      // Set up for VSLO
+	VSLO   ONES, V9, SEPMASK   // Mask for separator len(sep) < 16
+
+loadge16:
+	ANDCC $15, R5, R9 // Find byte offset of sep
+	ADD   R9, R6, R10 // Add sep len
+	CMP   R10, $16    // Check if sep len+offset > 16
+	BGT   sepcross16  // Sep crosses 16 byte boundary
+
+	RLDICR  $0, R5, $59, R8 // Adjust addr to 16 byte container
+	LXVB16X (R8)(R0), V0    // Load 16 bytes @R8 into V0
+	SLD     $3, R9          // Set up shift count for VSLO
+	MTVSRD  R9, V8          // Set up shift count for VSLO
+	VSLDOI  $8, V8, V8, V8
+	VSLO    V0, V8, V0      // Shift by start byte
+
+	VAND V0, SEPMASK, V0 // Mask separator (< 16)
+	BR   index2plus
+
+sepcross16:
+	LXVB16X (R5)(R0), V0 // Load 16 bytes @R5 into V0
+
+	VAND V0, SEPMASK, V0 // mask out separator
+	BLE  CR4, index2to16
+	BR   index17plus     // Handle sep > 16
+
+index2plus:
+	CMP      R6, $2       // Check length of sep
+	BNE      index3plus   // If not 2, check for 3
+	ADD      $16, R7, R9  // Check if next 16 bytes past last
+	CMP      R9, LASTBYTE // compare with last
+	BGE      index2to16   // 2 <= len(string) <= 16
+	MOVD     $0xff00, R21 // Mask for later
+	MTVSRD   R21, V25     // Move to Vreg
+	VSPLTH   $3, V25, V31 // Splat mask
+	VSPLTH   $0, V0, V1   // Splat 1st 2 bytes of sep
+	VSPLTISB $0, V10      // Clear V10
+
+	// First case: 2 byte separator
+	// V1: 2 byte separator splatted
+	// V2: 16 bytes at addr
+	// V4: 16 bytes at addr+1
+	// Compare 2 byte separator at start
+	// and at start+1. Use VSEL to combine
+	// those results to find the first
+	// matching start byte, returning
+	// that value when found. Loop as
+	// long as len(string) > 16
+index2loop2:
+	LXVB16X (R7)(R19), V3  // Load 16 bytes @R7+1 into V3
+
+index2loop:
+	LXVB16X  (R7)(R0), V2    // Load 16 bytes @R7 into V2
+	VCMPEQUH V1, V2, V5      // Search for sep
+	VCMPEQUH V1, V3, V6      // Search for sep offset by 1
+	VSEL     V6, V5, V31, V7 // merge even and odd indices
+	VCLZD    V7, V18         // find index of first match
+	MFVSRD   V18, R25        // get first value
+	CMP      R25, $64        // Found if < 64
+	BLT      foundR25        // Return byte index where found
+
+	MFVSRLD V18, R25        // get second value
+	CMP     R25, $64        // Found if < 64
+	ADD     $64, R25        // Update byte offset
+	BLT     foundR25        // Return value
+	ADD     $16, R7         // R7+=16 Update string pointer
+	ADD     $17, R7, R9     // R9=F7+17 since loop unrolled
+	CMP     R9, LASTBYTE    // Compare addr+17 against last byte
+	BLT     index2loop2     // If < last, continue loop
+	CMP     R7, LASTBYTE    // Compare addr+16 against last byte
+	BLT     index2to16      // If < 16 handle specially
+	LXVB16X (R7)(R0), V3    // Load 16 bytes @R7 into V3
+	VSLDOI  $1, V3, V10, V3 // Shift left by 1 byte
+	BR      index2loop
+
+index3plus:
+	CMP    R6, $3       // Check if sep == 3
+	BNE    index4plus   // If not check larger
+	ADD    $19, R7, R9  // Find bytes for use in this loop
+	CMP    R9, LASTBYTE // Compare against last byte
+	BGE    index2to16   // Remaining string 2<=len<=16
+	MOVD   $0xff00, R21 // Set up mask for upcoming loop
+	MTVSRD R21, V25     // Move mask to Vreg
+	VSPLTH $3, V25, V31 // Splat mask
+	VSPLTH $0, V0, V1   // Splat 1st two bytes of sep
+	VSPLTB $2, V0, V8   // Splat 3rd byte of sep
+
+	// Loop to process 3 byte separator.
+	// string[0:16] is in V2
+	// string[2:18] is in V3
+	// sep[0:2] splatted in V1
+	// sec[3] splatted in v8
+	// Load vectors at string, string+1
+	// and string+2. Compare string, string+1
+	// against first 2 bytes of separator
+	// splatted, and string+2 against 3rd
+	// byte splatted. Merge the results with
+	// VSEL to find the first byte of a match.
+
+	// Special handling for last 16 bytes if the
+	// string fits in 16 byte multiple.
+index3loop2:
+	MOVD     $2, R21          // Set up index for 2
+	VSPLTISB $0, V10          // Clear V10
+	LXVB16X  (R7)(R21), V3    // Load 16 bytes @R7+2 into V3
+	VSLDOI   $14, V3, V10, V3 // Left justify next 2 bytes
+
+index3loop:
+	LXVB16X  (R7)(R0), V2    // Load 16 bytes @R7
+	VSLDOI   $1, V2, V3, V4  // string[1:17]
+	VSLDOI   $2, V2, V3, V9  // string[2:18]
+	VCMPEQUH V1, V2, V5      // compare hw even indices
+	VCMPEQUH V1, V4, V6      // compare hw odd indices
+	VCMPEQUB V8, V9, V10     // compare 3rd to last byte
+	VSEL     V6, V5, V31, V7 // Find 1st matching byte using mask
+	VAND     V7, V10, V7     // AND matched bytes with matched 3rd byte
+	VCLZD    V7, V18         // Find first nonzero indexes
+	MFVSRD   V18, R25        // Move 1st doubleword
+	CMP      R25, $64        // If < 64 found
+	BLT      foundR25        // Return matching index
+
+	MFVSRLD  V18, R25     // Move 2nd doubleword
+	CMP      R25, $64     // If < 64 found
+	ADD      $64, R25     // Update byte index
+	BLT      foundR25     // Return matching index
+	ADD      $16, R7      // R7+=16 string ptr
+	ADD      $19, R7, R9  // Number of string bytes for loop
+	CMP      R9, LASTBYTE // Compare against last byte of string
+	BLT      index3loop2  // If within, continue this loop
+	CMP      R7, LASTSTR  // Compare against last start byte
+	BLT      index2to16   // Process remainder
+	VSPLTISB $0, V3       // Special case for last 16 bytes
+	BR       index3loop   // Continue this loop
+
+	// Loop to process 4 byte separator
+	// string[0:16] in V2
+	// string[3:16] in V3
+	// sep[0:4] splatted in V1
+	// Set up vectors with strings at offsets
+	// 0, 1, 2, 3 and compare against the 4 byte
+	// separator also splatted. Use VSEL with the
+	// compare results to find the first byte where
+	// a separator match is found.
+index4plus:
+	CMP  R6, $4       // Check if 4 byte separator
+	BNE  index5plus   // If not next higher
+	ADD  $20, R7, R9  // Check string size to load
+	CMP  R9, LASTBYTE // Verify string length
+	BGE  index2to16   // If not large enough, process remaining
+
+	// Set up masks for use with VSEL
+	MOVD    $0xff, R21 // Set up mask 0xff000000ff000000...
+	SLD     $24, R21
+	MTVSRWS R21, V29
+
+	VSLDOI  $2, V29, V29, V30 // Mask 0x0000ff000000ff00...
+	MOVD    $0xffff, R21
+	SLD     $16, R21
+	MTVSRWS R21, V31
+
+	VSPLTW $0, V0, V1 // Splat 1st word of separator
+
+index4loop:
+	LXVB16X (R7)(R0), V2  // Load 16 bytes @R7 into V2
+
+next4:
+	VSPLTISB $0, V10            // Clear
+	MOVD     $3, R9             // Number of bytes beyond 16
+	LXVB16X  (R7)(R9), V3       // Load 16 bytes @R7 into V3
+	VSLDOI   $13, V3, V10, V3   // Shift left last 3 bytes
+	VSLDOI   $1, V2, V3, V4     // V4=(V2:V3)<<1
+	VSLDOI   $2, V2, V3, V9     // V9=(V2:V3)<<2
+	VSLDOI   $3, V2, V3, V10    // V10=(V2:v3)<<3
+	VCMPEQUW V1, V2, V5         // compare index 0, 4, ... with sep
+	VCMPEQUW V1, V4, V6         // compare index 1, 5, ... with sep
+	VCMPEQUW V1, V9, V11        // compare index 2, 6, ... with sep
+	VCMPEQUW V1, V10, V12       // compare index 3, 7, ... with sep
+	VSEL     V6, V5, V29, V13   // merge index 0, 1, 4, 5, using mask
+	VSEL     V12, V11, V30, V14 // merge index 2, 3, 6, 7, using mask
+	VSEL     V14, V13, V31, V7  // final merge
+	VCLZD    V7, V18            // Find first index for each half
+	MFVSRD   V18, R25           // Isolate value
+	CMP      R25, $64           // If < 64, found
+	BLT      foundR25           // Return found index
+
+	MFVSRLD V18, R25     // Isolate other value
+	CMP     R25, $64     // If < 64, found
+	ADD     $64, R25     // Update index for high doubleword
+	BLT     foundR25     // Return found index
+	ADD     $16, R7      // R7+=16 for next string
+	ADD     $20, R7, R9  // R+20 for all bytes to load
+	CMP     R9, LASTBYTE // Past end? Maybe check for extra?
+	BLT     index4loop   // If not, continue loop
+	CMP     R7, LASTSTR  // Check remainder
+	BLE     index2to16   // Process remainder
+	BR      notfound     // Not found
+
+index5plus:
+	CMP R6, $16     // Check for sep > 16
+	BGT index17plus // Handle large sep
+
+	// Assumption is that the separator is smaller than the string at this point
+index2to16:
+	CMP R7, LASTSTR // Compare last start byte
+	BGT notfound    // last takes len(sep) into account
+
+	ADD $19, R7, R9    // To check 4 indices per iteration, need at least 16+3 bytes
+	CMP R9, LASTBYTE
+	// At least 16 bytes of string left
+	// Mask the number of bytes in sep
+	VSPLTISB $0, V10            // Clear
+	BGT index2to16tail
+
+	MOVD     $3, R17            // Number of bytes beyond 16
+	PCALIGN  $32
+index2to16loop:
+	LXVB16X  (R7)(R0), V1       // Load next 16 bytes of string into V1 from R7
+	LXVB16X  (R7)(R17), V5      // Load next 16 bytes of string into V5 from R7+3
+
+	VSLDOI   $13, V5, V10, V2  // Shift left last 3 bytes
+	VSLDOI  $1, V1, V2, V3     // V3=(V1:V2)<<1
+	VSLDOI  $2, V1, V2, V4     // V4=(V1:V2)<<2
+	VAND    V1, SEPMASK, V8    // Mask out sep size 0th index
+	VAND    V3, SEPMASK, V9    // Mask out sep size 1st index
+	VAND    V4, SEPMASK, V11   // Mask out sep size 2nd index
+	VAND    V5, SEPMASK, V12   // Mask out sep size 3rd index
+	VCMPEQUBCC      V0, V8, V8 // compare masked string
+	BLT     CR6, found         // All equal while comparing 0th index
+	VCMPEQUBCC      V0, V9, V9 // compare masked string
+	BLT     CR6, found2        // All equal while comparing 1st index
+	VCMPEQUBCC      V0, V11, V11    // compare masked string
+	BLT     CR6, found3        // All equal while comparing 2nd index
+	VCMPEQUBCC      V0, V12, V12    // compare masked string
+	BLT     CR6, found4        // All equal while comparing 3rd index
+
+	ADD        $4, R7          // Update ptr to next 4 bytes
+	CMP        R7, LASTSTR     // Still less than last start byte
+	BGT        notfound        // Not found
+	ADD        $19, R7, R9     // Verify remaining bytes
+	CMP        R9, LASTBYTE    // length of string at least 19
+	BLE        index2to16loop  // Try again, else do post processing and jump to index2to16next
+
+	// <19 bytes left, post process the remaining string
+index2to16tail:
+	ADD     R3, R4, R9         // End of string
+	SUB     R7, R9, R9         // Number of bytes left
+	ANDCC   $15, R7, R10       // 16 byte offset
+	ADD     R10, R9, R11       // offset + len
+	CMP     R11, $16           // >= 16?
+	BLE     short              // Does not cross 16 bytes
+	LXVB16X (R7)(R0), V1       // Load 16 bytes @R7 into V1
+	CMP     R9, $16            // Post-processing of unrolled loop
+	BLE     index2to16next     // continue to index2to16next if <= 16 bytes
+	SUB     R16, R9, R10       // R9 should be 18 or 17 hence R10 is 1 or 2
+	LXVB16X (R7)(R10), V9
+	CMP     R10, $1            // string length is 17, compare 1 more byte
+	BNE     extra2             // string length is 18, compare 2 more bytes
+	VSLDOI  $15, V9, V10, V25
+	VAND       V1, SEPMASK, V2 // Just compare size of sep
+	VCMPEQUBCC V0, V2, V3      // Compare sep and partial string
+	BLT        CR6, found      // Found
+	ADD        $1, R7          // Not found, try next partial string
+	CMP        R7, LASTSTR     // Check for end of string
+	BGT        notfound        // If at end, then not found
+	VSLDOI     $1, V1, V25, V1 // Shift string left by 1 byte
+	BR         index2to16next  // go to remainder loop
+extra2:
+	VSLDOI  $14, V9, V10, V25
+	VAND       V1, SEPMASK, V2 // Just compare size of sep
+	VCMPEQUBCC V0, V2, V3      // Compare sep and partial string
+	BLT        CR6, found      // Found
+	ADD        $1, R7          // Not found, try next partial string
+	CMP        R7, LASTSTR     // Check for end of string
+	BGT        notfound        // If at end, then not found
+	VOR        V1, V1, V4      // save remaining string
+	VSLDOI     $1, V1, V25, V1 // Shift string left by 1 byte for 17th byte
+	VAND       V1, SEPMASK, V2 // Just compare size of sep
+	VCMPEQUBCC V0, V2, V3      // Compare sep and partial string
+	BLT        CR6, found      // Found
+	ADD        $1, R7          // Not found, try next partial string
+	CMP        R7, LASTSTR     // Check for end of string
+	BGT        notfound        // If at end, then not found
+	VSLDOI     $2, V4, V25, V1 // Shift saved string left by 2 bytes for 18th byte
+	BR         index2to16next  // Check the remaining partial string in index2to16next
+
+short:
+	RLDICR   $0, R7, $59, R9   // Adjust addr to 16 byte container
+	LXVB16X  (R9)(R0), V1      // Load 16 bytes @R9 into V1
+	SLD      $3, R10           // Set up shift
+	MTVSRD   R10, V8           // Set up shift
+	VSLDOI   $8, V8, V8, V8
+	VSLO     V1, V8, V1        // Shift by start byte
+	PCALIGN  $32
+index2to16next:
+	VAND       V1, SEPMASK, V2 // Just compare size of sep
+	VCMPEQUBCC V0, V2, V3      // Compare sep and partial string
+	BLT        CR6, found      // Found
+	ADD        $1, R7          // Not found, try next partial string
+	CMP        R7, LASTSTR     // Check for end of string
+	BGT        notfound        // If at end, then not found
+	VSLDOI     $1, V1, V10, V1 // Shift string left by 1 byte
+	BR         index2to16next  // Check the next partial string
+
+index17plus:
+	CMP      R6, $32       // Check if 17 < len(sep) <= 32
+	BGT      index33plus
+	SUB      $16, R6, R9   // Extra > 16
+	SLD      $56, R9, R10  // Shift to use in VSLO
+	MTVSRD   R10, V9       // Set up for VSLO
+	LXVB16X  (R5)(R9), V1  // Load 16 bytes @R5+R9 into V1
+	VSLO     V1, V9, V1    // Shift left
+	VSPLTISB $0xff, V7     // Splat 1s
+	VSPLTISB $0, V27       // Splat 0
+
+index17to32loop:
+	LXVB16X (R7)(R0), V2  // Load 16 bytes @R7 into V2
+
+next17:
+	LXVB16X    (R7)(R9), V3    // Load 16 bytes @R7+R9 into V3
+	VSLO       V3, V9, V3      // Shift left
+	VCMPEQUB   V0, V2, V4      // Compare first 16 bytes
+	VCMPEQUB   V1, V3, V5      // Compare extra over 16 bytes
+	VAND       V4, V5, V6      // Check if both equal
+	VCMPEQUBCC V6, V7, V8      // All equal?
+	BLT        CR6, found      // Yes
+	ADD        $1, R7          // On to next byte
+	CMP        R7, LASTSTR     // Check if last start byte
+	BGT        notfound        // If too high, not found
+	BR         index17to32loop // Continue
+
+notfound:
+	MOVD $-1, R3   // Return -1 if not found
+	RET
+
+index33plus:
+	MOVD $0, (R0) // Case not implemented
+	RET           // Crash before return
+
+foundR25:
+	SRD  $3, R25   // Convert from bits to bytes
+	ADD  R25, R7   // Add to current string address
+	SUB  R3, R7    // Subtract from start of string
+	MOVD R7, R3    // Return byte where found
+	RET
+found4:
+	ADD $1, R7     // found from unrolled loop at index 3
+found3:
+	ADD $1, R7     // found from unrolled loop at index 2
+found2:
+	ADD $1, R7     // found from unrolled loop at index 1
+found:                 // found at index 0
+	SUB  R3, R7    // Return byte where found
+	MOVD R7, R3
+	RET
diff --git a/src/internal/bytealg/index_s390x.go b/src/internal/bytealg/index_s390x.go
new file mode 100644
index 0000000..9340cf1
--- /dev/null
+++ b/src/internal/bytealg/index_s390x.go
@@ -0,0 +1,31 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+import "internal/cpu"
+
+const MaxBruteForce = 64
+
+func init() {
+	// Note: we're kind of lucky that this flag is available at this point.
+	// The runtime sets HasVX when processing auxv records, and that happens
+	// to happen *before* running the init functions of packages that
+	// the runtime depends on.
+	// TODO: it would really be nicer for internal/cpu to figure out this
+	// flag by itself. Then we wouldn't need to depend on quirks of
+	// early startup initialization order.
+	if cpu.S390X.HasVX {
+		MaxLen = 64
+	}
+}
+
+// Cutover reports the number of failures of IndexByte we should tolerate
+// before switching over to Index.
+// n is the number of bytes processed so far.
+// See the bytes.Index implementation for details.
+func Cutover(n int) int {
+	// 1 error per 8 characters, plus a few slop to start.
+	return (n + 16) / 8
+}
diff --git a/src/internal/bytealg/index_s390x.s b/src/internal/bytealg/index_s390x.s
new file mode 100644
index 0000000..491d5bc
--- /dev/null
+++ b/src/internal/bytealg/index_s390x.s
@@ -0,0 +1,216 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+// Caller must confirm availability of vx facility before calling.
+TEXT ·Index(SB),NOSPLIT|NOFRAME,$0-56
+	LMG	a_base+0(FP), R1, R2  // R1=&s[0],   R2=len(s)
+	LMG	b_base+24(FP), R3, R4 // R3=&sep[0], R4=len(sep)
+	MOVD	$ret+48(FP), R5
+	BR	indexbody<>(SB)
+
+// Caller must confirm availability of vx facility before calling.
+TEXT ·IndexString(SB),NOSPLIT|NOFRAME,$0-40
+	LMG	a_base+0(FP), R1, R2  // R1=&s[0],   R2=len(s)
+	LMG	b_base+16(FP), R3, R4 // R3=&sep[0], R4=len(sep)
+	MOVD	$ret+32(FP), R5
+	BR	indexbody<>(SB)
+
+// s: string we are searching
+// sep: string to search for
+// R1=&s[0], R2=len(s)
+// R3=&sep[0], R4=len(sep)
+// R5=&ret (int)
+// Caller must confirm availability of vx facility before calling.
+TEXT indexbody<>(SB),NOSPLIT|NOFRAME,$0
+	CMPBGT	R4, R2, notfound
+	ADD	R1, R2
+	SUB	R4, R2 // R2=&s[len(s)-len(sep)] (last valid index)
+	CMPBEQ	R4, $0, notfound
+	SUB	$1, R4 // R4=len(sep)-1 for use as VLL index
+	VLL	R4, (R3), V0 // contains first 16 bytes of sep
+	MOVD	R1, R7
+index2plus:
+	CMPBNE	R4, $1, index3plus
+	MOVD	$15(R7), R9
+	CMPBGE	R9, R2, index2to16
+	VGBM	$0xaaaa, V31       // 0xff00ff00ff00ff00...
+	VONE	V16
+	VREPH	$0, V0, V1
+	CMPBGE	R9, R2, index2to16
+index2loop:
+	VL	0(R7), V2          // 16 bytes, even indices
+	VL	1(R7), V4          // 16 bytes, odd indices
+	VCEQH	V1, V2, V5         // compare even indices
+	VCEQH	V1, V4, V6         // compare odd indices
+	VSEL	V5, V6, V31, V7    // merge even and odd indices
+	VFEEBS	V16, V7, V17       // find leftmost index, set condition to 1 if found
+	BLT	foundV17
+	MOVD	$16(R7), R7        // R7+=16
+	ADD	$15, R7, R9
+	CMPBLE	R9, R2, index2loop // continue if (R7+15) <= R2 (last index to search)
+	CMPBLE	R7, R2, index2to16
+	BR	notfound
+
+index3plus:
+	CMPBNE	R4, $2, index4plus
+	ADD	$15, R7, R9
+	CMPBGE	R9, R2, index2to16
+	MOVD	$1, R0
+	VGBM	$0xaaaa, V31       // 0xff00ff00ff00ff00...
+	VONE	V16
+	VREPH	$0, V0, V1
+	VREPB	$2, V0, V8
+index3loop:
+	VL	(R7), V2           // load 16-bytes into V2
+	VLL	R0, 16(R7), V3     // load 2-bytes into V3
+	VSLDB	$1, V2, V3, V4     // V4=(V2:V3)<<1
+	VSLDB	$2, V2, V3, V9     // V9=(V2:V3)<<2
+	VCEQH	V1, V2, V5         // compare 2-byte even indices
+	VCEQH	V1, V4, V6         // compare 2-byte odd indices
+	VCEQB	V8, V9, V10        // compare last bytes
+	VSEL	V5, V6, V31, V7    // merge even and odd indices
+	VN	V7, V10, V7        // AND indices with last byte
+	VFEEBS	V16, V7, V17       // find leftmost index, set condition to 1 if found
+	BLT	foundV17
+	MOVD	$16(R7), R7        // R7+=16
+	ADD	$15, R7, R9
+	CMPBLE	R9, R2, index3loop // continue if (R7+15) <= R2 (last index to search)
+	CMPBLE	R7, R2, index2to16
+	BR	notfound
+
+index4plus:
+	CMPBNE	R4, $3, index5plus
+	ADD	$15, R7, R9
+	CMPBGE	R9, R2, index2to16
+	MOVD	$2, R0
+	VGBM	$0x8888, V29       // 0xff000000ff000000...
+	VGBM	$0x2222, V30       // 0x0000ff000000ff00...
+	VGBM	$0xcccc, V31       // 0xffff0000ffff0000...
+	VONE	V16
+	VREPF	$0, V0, V1
+index4loop:
+	VL	(R7), V2           // load 16-bytes into V2
+	VLL	R0, 16(R7), V3     // load 3-bytes into V3
+	VSLDB	$1, V2, V3, V4     // V4=(V2:V3)<<1
+	VSLDB	$2, V2, V3, V9     // V9=(V2:V3)<<1
+	VSLDB	$3, V2, V3, V10    // V10=(V2:V3)<<1
+	VCEQF	V1, V2, V5         // compare index 0, 4, ...
+	VCEQF	V1, V4, V6         // compare index 1, 5, ...
+	VCEQF	V1, V9, V11        // compare index 2, 6, ...
+	VCEQF	V1, V10, V12       // compare index 3, 7, ...
+	VSEL	V5, V6, V29, V13   // merge index 0, 1, 4, 5, ...
+	VSEL	V11, V12, V30, V14 // merge index 2, 3, 6, 7, ...
+	VSEL	V13, V14, V31, V7  // final merge
+	VFEEBS	V16, V7, V17       // find leftmost index, set condition to 1 if found
+	BLT	foundV17
+	MOVD	$16(R7), R7        // R7+=16
+	ADD	$15, R7, R9
+	CMPBLE	R9, R2, index4loop // continue if (R7+15) <= R2 (last index to search)
+	CMPBLE	R7, R2, index2to16
+	BR	notfound
+
+index5plus:
+	CMPBGT	R4, $15, index17plus
+index2to16:
+	CMPBGT	R7, R2, notfound
+	MOVD	$1(R7), R8
+	CMPBGT	R8, R2, index2to16tail
+index2to16loop:
+	// unrolled 2x
+	VLL	R4, (R7), V1
+	VLL	R4, 1(R7), V2
+	VCEQGS	V0, V1, V3
+	BEQ	found
+	MOVD	$1(R7), R7
+	VCEQGS	V0, V2, V4
+	BEQ	found
+	MOVD	$1(R7), R7
+	CMPBLT	R7, R2, index2to16loop
+	CMPBGT	R7, R2, notfound
+index2to16tail:
+	VLL	R4, (R7), V1
+	VCEQGS	V0, V1, V2
+	BEQ	found
+	BR	notfound
+
+index17plus:
+	CMPBGT	R4, $31, index33plus
+	SUB	$16, R4, R0
+	VLL	R0, 16(R3), V1
+	VONE	V7
+index17to32loop:
+	VL	(R7), V2
+	VLL	R0, 16(R7), V3
+	VCEQG	V0, V2, V4
+	VCEQG	V1, V3, V5
+	VN	V4, V5, V6
+	VCEQGS	V6, V7, V8
+	BEQ	found
+	MOVD	$1(R7), R7
+	CMPBLE  R7, R2, index17to32loop
+	BR	notfound
+
+index33plus:
+	CMPBGT	R4, $47, index49plus
+	SUB	$32, R4, R0
+	VL	16(R3), V1
+	VLL	R0, 32(R3), V2
+	VONE	V11
+index33to48loop:
+	VL	(R7), V3
+	VL	16(R7), V4
+	VLL	R0, 32(R7), V5
+	VCEQG	V0, V3, V6
+	VCEQG	V1, V4, V7
+	VCEQG	V2, V5, V8
+	VN	V6, V7, V9
+	VN	V8, V9, V10
+	VCEQGS	V10, V11, V12
+	BEQ	found
+	MOVD	$1(R7), R7
+	CMPBLE  R7, R2, index33to48loop
+	BR	notfound
+
+index49plus:
+	CMPBGT	R4, $63, index65plus
+	SUB	$48, R4, R0
+	VL	16(R3), V1
+	VL	32(R3), V2
+	VLL	R0, 48(R3), V3
+	VONE	V15
+index49to64loop:
+	VL	(R7), V4
+	VL	16(R7), V5
+	VL	32(R7), V6
+	VLL	R0, 48(R7), V7
+	VCEQG	V0, V4, V8
+	VCEQG	V1, V5, V9
+	VCEQG	V2, V6, V10
+	VCEQG	V3, V7, V11
+	VN	V8, V9, V12
+	VN	V10, V11, V13
+	VN	V12, V13, V14
+	VCEQGS	V14, V15, V16
+	BEQ	found
+	MOVD	$1(R7), R7
+	CMPBLE  R7, R2, index49to64loop
+notfound:
+	MOVD	$-1, (R5)
+	RET
+
+index65plus:
+	// not implemented
+	MOVD	$0, (R0)
+	RET
+
+foundV17: // index is in doubleword V17[0]
+	VLGVG	$0, V17, R8
+	ADD	R8, R7
+found:
+	SUB	R1, R7
+	MOVD	R7, (R5)
+	RET
diff --git a/src/internal/bytealg/indexbyte_386.s b/src/internal/bytealg/indexbyte_386.s
new file mode 100644
index 0000000..8a03054
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_386.s
@@ -0,0 +1,34 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-20
+	MOVL	b_base+0(FP), SI
+	MOVL	b_len+4(FP), CX
+	MOVB	c+12(FP), AL
+	MOVL	SI, DI
+	CLD; REPN; SCASB
+	JZ 3(PC)
+	MOVL	$-1, ret+16(FP)
+	RET
+	SUBL	SI, DI
+	SUBL	$1, DI
+	MOVL	DI, ret+16(FP)
+	RET
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-16
+	MOVL	s_base+0(FP), SI
+	MOVL	s_len+4(FP), CX
+	MOVB	c+8(FP), AL
+	MOVL	SI, DI
+	CLD; REPN; SCASB
+	JZ 3(PC)
+	MOVL	$-1, ret+12(FP)
+	RET
+	SUBL	SI, DI
+	SUBL	$1, DI
+	MOVL	DI, ret+12(FP)
+	RET
diff --git a/src/internal/bytealg/indexbyte_amd64.s b/src/internal/bytealg/indexbyte_amd64.s
new file mode 100644
index 0000000..1ca70e3
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_amd64.s
@@ -0,0 +1,149 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT	·IndexByte(SB), NOSPLIT, $0-40
+	MOVQ b_base+0(FP), SI
+	MOVQ b_len+8(FP), BX
+	MOVB c+24(FP), AL
+	LEAQ ret+32(FP), R8
+	JMP  indexbytebody<>(SB)
+
+TEXT	·IndexByteString(SB), NOSPLIT, $0-32
+	MOVQ s_base+0(FP), SI
+	MOVQ s_len+8(FP), BX
+	MOVB c+16(FP), AL
+	LEAQ ret+24(FP), R8
+	JMP  indexbytebody<>(SB)
+
+// input:
+//   SI: data
+//   BX: data len
+//   AL: byte sought
+//   R8: address to put result
+TEXT	indexbytebody<>(SB), NOSPLIT, $0
+	// Shuffle X0 around so that each byte contains
+	// the character we're looking for.
+	MOVD AX, X0
+	PUNPCKLBW X0, X0
+	PUNPCKLBW X0, X0
+	PSHUFL $0, X0, X0
+
+	CMPQ BX, $16
+	JLT small
+
+	MOVQ SI, DI
+
+	CMPQ BX, $32
+	JA avx2
+sse:
+	LEAQ	-16(SI)(BX*1), AX	// AX = address of last 16 bytes
+	JMP	sseloopentry
+
+sseloop:
+	// Move the next 16-byte chunk of the data into X1.
+	MOVOU	(DI), X1
+	// Compare bytes in X0 to X1.
+	PCMPEQB	X0, X1
+	// Take the top bit of each byte in X1 and put the result in DX.
+	PMOVMSKB X1, DX
+	// Find first set bit, if any.
+	BSFL	DX, DX
+	JNZ	ssesuccess
+	// Advance to next block.
+	ADDQ	$16, DI
+sseloopentry:
+	CMPQ	DI, AX
+	JB	sseloop
+
+	// Search the last 16-byte chunk. This chunk may overlap with the
+	// chunks we've already searched, but that's ok.
+	MOVQ	AX, DI
+	MOVOU	(AX), X1
+	PCMPEQB	X0, X1
+	PMOVMSKB X1, DX
+	BSFL	DX, DX
+	JNZ	ssesuccess
+
+failure:
+	MOVQ $-1, (R8)
+	RET
+
+// We've found a chunk containing the byte.
+// The chunk was loaded from DI.
+// The index of the matching byte in the chunk is DX.
+// The start of the data is SI.
+ssesuccess:
+	SUBQ SI, DI	// Compute offset of chunk within data.
+	ADDQ DX, DI	// Add offset of byte within chunk.
+	MOVQ DI, (R8)
+	RET
+
+// handle for lengths < 16
+small:
+	TESTQ	BX, BX
+	JEQ	failure
+
+	// Check if we'll load across a page boundary.
+	LEAQ	16(SI), AX
+	TESTW	$0xff0, AX
+	JEQ	endofpage
+
+	MOVOU	(SI), X1 // Load data
+	PCMPEQB	X0, X1	// Compare target byte with each byte in data.
+	PMOVMSKB X1, DX	// Move result bits to integer register.
+	BSFL	DX, DX	// Find first set bit.
+	JZ	failure	// No set bit, failure.
+	CMPL	DX, BX
+	JAE	failure	// Match is past end of data.
+	MOVQ	DX, (R8)
+	RET
+
+endofpage:
+	MOVOU	-16(SI)(BX*1), X1	// Load data into the high end of X1.
+	PCMPEQB	X0, X1	// Compare target byte with each byte in data.
+	PMOVMSKB X1, DX	// Move result bits to integer register.
+	MOVL	BX, CX
+	SHLL	CX, DX
+	SHRL	$16, DX	// Shift desired bits down to bottom of register.
+	BSFL	DX, DX	// Find first set bit.
+	JZ	failure	// No set bit, failure.
+	MOVQ	DX, (R8)
+	RET
+
+avx2:
+#ifndef hasAVX2
+	CMPB   internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
+	JNE sse
+#endif
+	MOVD AX, X0
+	LEAQ -32(SI)(BX*1), R11
+	VPBROADCASTB  X0, Y1
+avx2_loop:
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPTEST Y3, Y3
+	JNZ avx2success
+	ADDQ $32, DI
+	CMPQ DI, R11
+	JLT avx2_loop
+	MOVQ R11, DI
+	VMOVDQU (DI), Y2
+	VPCMPEQB Y1, Y2, Y3
+	VPTEST Y3, Y3
+	JNZ avx2success
+	VZEROUPPER
+	MOVQ $-1, (R8)
+	RET
+
+avx2success:
+	VPMOVMSKB Y3, DX
+	BSFL DX, DX
+	SUBQ SI, DI
+	ADDQ DI, DX
+	MOVQ DX, (R8)
+	VZEROUPPER
+	RET
diff --git a/src/internal/bytealg/indexbyte_arm.s b/src/internal/bytealg/indexbyte_arm.s
new file mode 100644
index 0000000..faf9797
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_arm.s
@@ -0,0 +1,46 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-20
+	MOVW	b_base+0(FP), R0
+	MOVW	b_len+4(FP), R1
+	MOVBU	c+12(FP), R2	// byte to find
+	MOVW	$ret+16(FP), R5
+	B	indexbytebody<>(SB)
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-16
+	MOVW	s_base+0(FP), R0
+	MOVW	s_len+4(FP), R1
+	MOVBU	c+8(FP), R2	// byte to find
+	MOVW	$ret+12(FP), R5
+	B	indexbytebody<>(SB)
+
+// input:
+//  R0: data
+//  R1: data length
+//  R2: byte to find
+//  R5: address to put result
+TEXT indexbytebody<>(SB),NOSPLIT,$0-0
+	MOVW	R0, R4		// store base for later
+	ADD	R0, R1		// end
+
+loop:
+	CMP	R0, R1
+	B.EQ	notfound
+	MOVBU.P	1(R0), R3
+	CMP	R2, R3
+	B.NE	loop
+
+	SUB	$1, R0		// R0 will be one beyond the position we want
+	SUB	R4, R0		// remove base
+	MOVW	R0, (R5)
+	RET
+
+notfound:
+	MOVW	$-1, R0
+	MOVW	R0, (R5)
+	RET
diff --git a/src/internal/bytealg/indexbyte_arm64.s b/src/internal/bytealg/indexbyte_arm64.s
new file mode 100644
index 0000000..40843fb
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_arm64.s
@@ -0,0 +1,126 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-40
+	MOVD	b_base+0(FP), R0
+	MOVD	b_len+8(FP), R2
+	MOVBU	c+24(FP), R1
+	MOVD	$ret+32(FP), R8
+	B	indexbytebody<>(SB)
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-32
+	MOVD	s_base+0(FP), R0
+	MOVD	s_len+8(FP), R2
+	MOVBU	c+16(FP), R1
+	MOVD	$ret+24(FP), R8
+	B	indexbytebody<>(SB)
+
+// input:
+//   R0: data
+//   R1: byte to search
+//   R2: data len
+//   R8: address to put result
+TEXT indexbytebody<>(SB),NOSPLIT,$0
+	// Core algorithm:
+	// For each 32-byte chunk we calculate a 64-bit syndrome value,
+	// with two bits per byte. For each tuple, bit 0 is set if the
+	// relevant byte matched the requested character and bit 1 is
+	// not used (faster than using a 32bit syndrome). Since the bits
+	// in the syndrome reflect exactly the order in which things occur
+	// in the original string, counting trailing zeros allows to
+	// identify exactly which byte has matched.
+
+	CBZ	R2, fail
+	MOVD	R0, R11
+	// Magic constant 0x40100401 allows us to identify
+	// which lane matches the requested byte.
+	// 0x40100401 = ((1<<0) + (4<<8) + (16<<16) + (64<<24))
+	// Different bytes have different bit masks (i.e: 1, 4, 16, 64)
+	MOVD	$0x40100401, R5
+	VMOV	R1, V0.B16
+	// Work with aligned 32-byte chunks
+	BIC	$0x1f, R0, R3
+	VMOV	R5, V5.S4
+	ANDS	$0x1f, R0, R9
+	AND	$0x1f, R2, R10
+	BEQ	loop
+
+	// Input string is not 32-byte aligned. We calculate the
+	// syndrome value for the aligned 32 bytes block containing
+	// the first bytes and mask off the irrelevant part.
+	VLD1.P	(R3), [V1.B16, V2.B16]
+	SUB	$0x20, R9, R4
+	ADDS	R4, R2, R2
+	VCMEQ	V0.B16, V1.B16, V3.B16
+	VCMEQ	V0.B16, V2.B16, V4.B16
+	VAND	V5.B16, V3.B16, V3.B16
+	VAND	V5.B16, V4.B16, V4.B16
+	VADDP	V4.B16, V3.B16, V6.B16 // 256->128
+	VADDP	V6.B16, V6.B16, V6.B16 // 128->64
+	VMOV	V6.D[0], R6
+	// Clear the irrelevant lower bits
+	LSL	$1, R9, R4
+	LSR	R4, R6, R6
+	LSL	R4, R6, R6
+	// The first block can also be the last
+	BLS	masklast
+	// Have we found something already?
+	CBNZ	R6, tail
+
+loop:
+	VLD1.P	(R3), [V1.B16, V2.B16]
+	SUBS	$0x20, R2, R2
+	VCMEQ	V0.B16, V1.B16, V3.B16
+	VCMEQ	V0.B16, V2.B16, V4.B16
+	// If we're out of data we finish regardless of the result
+	BLS	end
+	// Use a fast check for the termination condition
+	VORR	V4.B16, V3.B16, V6.B16
+	VADDP	V6.D2, V6.D2, V6.D2
+	VMOV	V6.D[0], R6
+	// We're not out of data, loop if we haven't found the character
+	CBZ	R6, loop
+
+end:
+	// Termination condition found, let's calculate the syndrome value
+	VAND	V5.B16, V3.B16, V3.B16
+	VAND	V5.B16, V4.B16, V4.B16
+	VADDP	V4.B16, V3.B16, V6.B16
+	VADDP	V6.B16, V6.B16, V6.B16
+	VMOV	V6.D[0], R6
+	// Only do the clear for the last possible block with less than 32 bytes
+	// Condition flags come from SUBS in the loop
+	BHS	tail
+
+masklast:
+	// Clear the irrelevant upper bits
+	ADD	R9, R10, R4
+	AND	$0x1f, R4, R4
+	SUB	$0x20, R4, R4
+	NEG	R4<<1, R4
+	LSL	R4, R6, R6
+	LSR	R4, R6, R6
+
+tail:
+	// Check that we have found a character
+	CBZ	R6, fail
+	// Count the trailing zeros using bit reversing
+	RBIT	R6, R6
+	// Compensate the last post-increment
+	SUB	$0x20, R3, R3
+	// And count the leading zeros
+	CLZ	R6, R6
+	// R6 is twice the offset into the fragment
+	ADD	R6>>1, R3, R0
+	// Compute the offset result
+	SUB	R11, R0, R0
+	MOVD	R0, (R8)
+	RET
+
+fail:
+	MOVD	$-1, R0
+	MOVD	R0, (R8)
+	RET
diff --git a/src/internal/bytealg/indexbyte_generic.go b/src/internal/bytealg/indexbyte_generic.go
new file mode 100644
index 0000000..b89d34f
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_generic.go
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !386 && !amd64 && !s390x && !arm && !arm64 && !loong64 && !ppc64 && !ppc64le && !mips && !mipsle && !mips64 && !mips64le && !riscv64 && !wasm
+
+package bytealg
+
+func IndexByte(b []byte, c byte) int {
+	for i, x := range b {
+		if x == c {
+			return i
+		}
+	}
+	return -1
+}
+
+func IndexByteString(s string, c byte) int {
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
diff --git a/src/internal/bytealg/indexbyte_loong64.s b/src/internal/bytealg/indexbyte_loong64.s
new file mode 100644
index 0000000..baa9c86
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_loong64.s
@@ -0,0 +1,52 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-40
+	MOVV	b_base+0(FP), R4
+	MOVV	b_len+8(FP), R5
+	MOVBU	c+24(FP), R6	// byte to find
+	MOVV	R4, R7		// store base for later
+	ADDV	R4, R5		// end
+	ADDV	$-1, R4
+
+loop:
+	ADDV	$1, R4
+	BEQ	R4, R5, notfound
+	MOVBU	(R4), R8
+	BNE	R6, R8, loop
+
+	SUBV	R7, R4		// remove base
+	MOVV	R4, ret+32(FP)
+	RET
+
+notfound:
+	MOVV	$-1, R4
+	MOVV	R4, ret+32(FP)
+	RET
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-32
+	MOVV	s_base+0(FP), R4
+	MOVV	s_len+8(FP), R5
+	MOVBU	c+16(FP), R6	// byte to find
+	MOVV	R4, R7		// store base for later
+	ADDV	R4, R5		// end
+	ADDV	$-1, R4
+
+loop:
+	ADDV	$1, R4
+	BEQ	R4, R5, notfound
+	MOVBU	(R4), R8
+	BNE	R6, R8, loop
+
+	SUBV	R7, R4		// remove base
+	MOVV	R4, ret+24(FP)
+	RET
+
+notfound:
+	MOVV	$-1, R4
+	MOVV	R4, ret+24(FP)
+	RET
diff --git a/src/internal/bytealg/indexbyte_mips64x.s b/src/internal/bytealg/indexbyte_mips64x.s
new file mode 100644
index 0000000..5689f84
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_mips64x.s
@@ -0,0 +1,54 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips64 || mips64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-40
+	MOVV	b_base+0(FP), R1
+	MOVV	b_len+8(FP), R2
+	MOVBU	c+24(FP), R3	// byte to find
+	MOVV	R1, R4		// store base for later
+	ADDV	R1, R2		// end
+	ADDV	$-1, R1
+
+loop:
+	ADDV	$1, R1
+	BEQ	R1, R2, notfound
+	MOVBU	(R1), R5
+	BNE	R3, R5, loop
+
+	SUBV	R4, R1		// remove base
+	MOVV	R1, ret+32(FP)
+	RET
+
+notfound:
+	MOVV	$-1, R1
+	MOVV	R1, ret+32(FP)
+	RET
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-32
+	MOVV	s_base+0(FP), R1
+	MOVV	s_len+8(FP), R2
+	MOVBU	c+16(FP), R3	// byte to find
+	MOVV	R1, R4		// store base for later
+	ADDV	R1, R2		// end
+	ADDV	$-1, R1
+
+loop:
+	ADDV	$1, R1
+	BEQ	R1, R2, notfound
+	MOVBU	(R1), R5
+	BNE	R3, R5, loop
+
+	SUBV	R4, R1		// remove base
+	MOVV	R1, ret+24(FP)
+	RET
+
+notfound:
+	MOVV	$-1, R1
+	MOVV	R1, ret+24(FP)
+	RET
diff --git a/src/internal/bytealg/indexbyte_mipsx.s b/src/internal/bytealg/indexbyte_mipsx.s
new file mode 100644
index 0000000..1c2b104
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_mipsx.s
@@ -0,0 +1,52 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips || mipsle
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT,$0-20
+	MOVW	b_base+0(FP), R1
+	MOVW	b_len+4(FP), R2
+	MOVBU	c+12(FP), R3	// byte to find
+	ADDU	$1, R1, R4	// store base+1 for later
+	ADDU	R1, R2	// end
+
+loop:
+	BEQ	R1, R2, notfound
+	MOVBU	(R1), R5
+	ADDU	$1, R1
+	BNE	R3, R5, loop
+
+	SUBU	R4, R1	// R1 will be one beyond the position we want so remove (base+1)
+	MOVW	R1, ret+16(FP)
+	RET
+
+notfound:
+	MOVW	$-1, R1
+	MOVW	R1, ret+16(FP)
+	RET
+
+TEXT ·IndexByteString(SB),NOSPLIT,$0-16
+	MOVW	s_base+0(FP), R1
+	MOVW	s_len+4(FP), R2
+	MOVBU	c+8(FP), R3	// byte to find
+	ADDU	$1, R1, R4	// store base+1 for later
+	ADDU	R1, R2	// end
+
+loop:
+	BEQ	R1, R2, notfound
+	MOVBU	(R1), R5
+	ADDU	$1, R1
+	BNE	R3, R5, loop
+
+	SUBU	R4, R1	// remove (base+1)
+	MOVW	R1, ret+12(FP)
+	RET
+
+notfound:
+	MOVW	$-1, R1
+	MOVW	R1, ret+12(FP)
+	RET
diff --git a/src/internal/bytealg/indexbyte_native.go b/src/internal/bytealg/indexbyte_native.go
new file mode 100644
index 0000000..c5bb2df
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_native.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64 || s390x || arm || arm64 || loong64 || ppc64 || ppc64le || mips || mipsle || mips64 || mips64le || riscv64 || wasm
+
+package bytealg
+
+//go:noescape
+func IndexByte(b []byte, c byte) int
+
+//go:noescape
+func IndexByteString(s string, c byte) int
diff --git a/src/internal/bytealg/indexbyte_ppc64x.s b/src/internal/bytealg/indexbyte_ppc64x.s
new file mode 100644
index 0000000..1a6e852
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_ppc64x.s
@@ -0,0 +1,391 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-40
+	// R3 = byte array pointer
+	// R4 = length
+	MOVD	R6, R5		// R5 = byte
+	MOVBZ	internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R16
+	BR	indexbytebody<>(SB)
+
+TEXT ·IndexByteString<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-32
+	// R3 = string
+	// R4 = length
+	// R5 = byte
+	MOVBZ	internal∕cpu·PPC64+const_offsetPPC64HasPOWER9(SB), R16
+	BR	indexbytebody<>(SB)
+
+// R3 = addr of string
+// R4 = len of string
+// R5 = byte to find
+// R16 = 1 if running on a POWER9 system, 0 otherwise
+// On exit:
+// R3 = return value
+TEXT indexbytebody<>(SB),NOSPLIT|NOFRAME,$0-0
+	MOVD	R3,R17		// Save base address for calculating the index later.
+	RLDICR	$0,R3,$60,R8	// Align address to doubleword boundary in R8.
+	RLDIMI	$8,R5,$48,R5	// Replicating the byte across the register.
+	ADD	R4,R3,R7	// Last acceptable address in R7.
+
+	RLDIMI	$16,R5,$32,R5
+	CMPU	R4,$32		// Check if it's a small string (≤32 bytes). Those will be processed differently.
+	MOVD	$-1,R9
+	RLWNM	$3,R3,$26,$28,R6	// shift amount for mask (r3&0x7)*8
+	RLDIMI	$32,R5,$0,R5
+	MOVD	R7,R10		// Save last acceptable address in R10 for later.
+	ADD	$-1,R7,R7
+#ifdef GOARCH_ppc64le
+	SLD	R6,R9,R9	// Prepare mask for Little Endian
+#else
+	SRD	R6,R9,R9	// Same for Big Endian
+#endif
+	BLT	small_string	// Jump to the small string case if it's <32 bytes.
+	CMP	R16,$1		// optimize for power8 v power9
+	BNE	power8
+	VSPLTISB	$3,V10	// Use V10 as control for VBPERMQ
+	MTVRD	R5,V1
+	LVSL	(R0+R0),V11	// set up the permute vector such that V10 has {0x78, .., 0x8, 0x0}
+	VSLB	V11,V10,V10	// to extract the first bit of match result into GPR
+	VSPLTB	$7,V1,V1	// Replicate byte across V1
+	CMP	R4,$64
+	MOVD	$16,R11
+	MOVD	R3,R8
+	BLT	cmp32
+	MOVD	$32,R12
+	MOVD	$48,R6
+
+loop64:
+	LXVB16X	(R0)(R8),V2	// scan 64 bytes at a time
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat0	// match found at R8, jump out
+
+	LXVB16X	(R8)(R11),V2
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat1	// match found at R8+16 bytes, jump out
+
+	LXVB16X	(R8)(R12),V2
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat2	// match found at R8+32 bytes, jump out
+
+	LXVB16X	(R8)(R6),V2
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat3	// match found at R8+48 bytes, jump out
+	ADD	$64,R8
+	ADD	$-64,R4
+	CMP	R4,$64		// >=64 bytes left to scan?
+	BGE	loop64
+	CMP	R4,$32
+	BLT	rem		// jump to rem if there are < 32 bytes left
+cmp32:
+	LXVB16X	(R0)(R8),V2	// 32-63 bytes left
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat0	// match found at R8
+
+	LXVB16X	(R11)(R8),V2
+	VCMPEQUBCC	V2,V1,V6
+	BNE	CR6,foundat1	// match found at R8+16
+
+	ADD	$32,R8
+	ADD	$-32,R4
+rem:
+	RLDICR	$0,R8,$60,R8	// align address to reuse code for tail end processing
+	BR	small_string
+
+foundat3:
+	ADD	$16,R8
+foundat2:
+	ADD	$16,R8
+foundat1:
+	ADD	$16,R8
+foundat0:
+	// Compress the result into a single doubleword and
+	// move it to a GPR for the final calculation.
+	VBPERMQ	V6,V10,V6
+	MFVRD	V6,R3
+	// count leading zeroes upto the match that ends up in low 16 bits
+	// in both endian modes, compute index by subtracting the number by 16
+	CNTLZW	R3,R11
+	ADD	$-16,R11
+	ADD	R8,R11,R3	// Calculate byte address
+	SUB	R17,R3
+	RET
+power8:
+	// If we are 64-byte aligned, branch to qw_align just to get the auxiliary values
+	// in V0, V1 and V10, then branch to the preloop.
+	ANDCC	$63,R3,R11
+	BEQ	CR0,qw_align
+	RLDICL	$0,R3,$61,R11
+
+	MOVD	0(R8),R12	// Load one doubleword from the aligned address in R8.
+	CMPB	R12,R5,R3	// Check for a match.
+	AND	R9,R3,R3	// Mask bytes below s_base
+	RLDICR	$0,R7,$60,R7	// Last doubleword in R7
+	CMPU	R3,$0,CR7	// If we have a match, jump to the final computation
+	BNE	CR7,done
+	ADD	$8,R8,R8
+	ADD	$-8,R4,R4
+	ADD	R4,R11,R4
+
+	// Check for quadword alignment
+	ANDCC	$15,R8,R11
+	BEQ	CR0,qw_align
+
+	// Not aligned, so handle the next doubleword
+	MOVD	0(R8),R12
+	CMPB	R12,R5,R3
+	CMPU	R3,$0,CR7
+	BNE	CR7,done
+	ADD	$8,R8,R8
+	ADD	$-8,R4,R4
+
+	// Either quadword aligned or 64-byte at this point. We can use LVX.
+qw_align:
+
+	// Set up auxiliary data for the vectorized algorithm.
+	VSPLTISB  $0,V0		// Replicate 0 across V0
+	VSPLTISB  $3,V10	// Use V10 as control for VBPERMQ
+	MTVRD	  R5,V1
+	LVSL	  (R0+R0),V11
+	VSLB	  V11,V10,V10
+	VSPLTB	  $7,V1,V1	// Replicate byte across V1
+	CMPU	  R4, $64	// If len ≤ 64, don't use the vectorized loop
+	BLE	  tail
+
+	// We will load 4 quardwords per iteration in the loop, so check for
+	// 64-byte alignment. If 64-byte aligned, then branch to the preloop.
+	ANDCC	  $63,R8,R11
+	BEQ	  CR0,preloop
+
+	// Not 64-byte aligned. Load one quadword at a time until aligned.
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6		// Check for byte in V4
+	BNE	    CR6,found_qw_align
+	ADD	    $16,R8,R8
+	ADD	    $-16,R4,R4
+
+	ANDCC	    $63,R8,R11
+	BEQ	    CR0,preloop
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6		// Check for byte in V4
+	BNE	    CR6,found_qw_align
+	ADD	    $16,R8,R8
+	ADD	    $-16,R4,R4
+
+	ANDCC	    $63,R8,R11
+	BEQ	    CR0,preloop
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6		// Check for byte in V4
+	BNE	    CR6,found_qw_align
+	ADD	    $-16,R4,R4
+	ADD	    $16,R8,R8
+
+	// 64-byte aligned. Prepare for the main loop.
+preloop:
+	CMPU	R4,$64
+	BLE	tail	      // If len ≤ 64, don't use the vectorized loop
+
+	// We are now aligned to a 64-byte boundary. We will load 4 quadwords
+	// per loop iteration. The last doubleword is in R10, so our loop counter
+	// starts at (R10-R8)/64.
+	SUB	R8,R10,R6
+	SRD	$6,R6,R9      // Loop counter in R9
+	MOVD	R9,CTR
+
+	ADD	$-64,R8,R8   // Adjust index for loop entry
+	MOVD	$16,R11      // Load offsets for the vector loads
+	MOVD	$32,R9
+	MOVD	$48,R7
+
+	// Main loop we will load 64 bytes per iteration
+loop:
+	ADD	    $64,R8,R8	      // Fuse addi+lvx for performance
+	LVX	    (R8+R0),V2	      // Load 4 16-byte vectors
+	LVX	    (R8+R11),V3
+	VCMPEQUB    V1,V2,V6	      // Look for byte in each vector
+	VCMPEQUB    V1,V3,V7
+
+	LVX	    (R8+R9),V4
+	LVX	    (R8+R7),V5
+	VCMPEQUB    V1,V4,V8
+	VCMPEQUB    V1,V5,V9
+
+	VOR	    V6,V7,V11	      // Compress the result in a single vector
+	VOR	    V8,V9,V12
+	VOR	    V11,V12,V13
+	VCMPEQUBCC  V0,V13,V14	      // Check for byte
+	BGE	    CR6,found
+	BC	    16,0,loop	      // bdnz loop
+
+	// Handle the tailing bytes or R4 ≤ 64
+	RLDICL	$0,R6,$58,R4
+	ADD	$64,R8,R8
+tail:
+	CMPU	    R4,$0
+	BEQ	    notfound
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6
+	BNE	    CR6,found_qw_align
+	ADD	    $16,R8,R8
+	CMPU	    R4,$16,CR6
+	BLE	    CR6,notfound
+	ADD	    $-16,R4,R4
+
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6
+	BNE	    CR6,found_qw_align
+	ADD	    $16,R8,R8
+	CMPU	    R4,$16,CR6
+	BLE	    CR6,notfound
+	ADD	    $-16,R4,R4
+
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6
+	BNE	    CR6,found_qw_align
+	ADD	    $16,R8,R8
+	CMPU	    R4,$16,CR6
+	BLE	    CR6,notfound
+	ADD	    $-16,R4,R4
+
+	LVX	    (R8+R0),V4
+	VCMPEQUBCC  V1,V4,V6
+	BNE	    CR6,found_qw_align
+
+notfound:
+	MOVD	$-1, R3
+	RET
+
+found:
+	// We will now compress the results into a single doubleword,
+	// so it can be moved to a GPR for the final index calculation.
+
+	// The bytes in V6-V9 are either 0x00 or 0xFF. So, permute the
+	// first bit of each byte into bits 48-63.
+	VBPERMQ	  V6,V10,V6
+	VBPERMQ	  V7,V10,V7
+	VBPERMQ	  V8,V10,V8
+	VBPERMQ	  V9,V10,V9
+
+	// Shift each 16-bit component into its correct position for
+	// merging into a single doubleword.
+#ifdef GOARCH_ppc64le
+	VSLDOI	  $2,V7,V7,V7
+	VSLDOI	  $4,V8,V8,V8
+	VSLDOI	  $6,V9,V9,V9
+#else
+	VSLDOI	  $6,V6,V6,V6
+	VSLDOI	  $4,V7,V7,V7
+	VSLDOI	  $2,V8,V8,V8
+#endif
+
+	// Merge V6-V9 into a single doubleword and move to a GPR.
+	VOR	V6,V7,V11
+	VOR	V8,V9,V4
+	VOR	V4,V11,V4
+	MFVRD	V4,R3
+
+#ifdef GOARCH_ppc64le
+	ADD	  $-1,R3,R11
+	ANDN	  R3,R11,R11
+	POPCNTD	  R11,R11	// Count trailing zeros (Little Endian).
+#else
+	CNTLZD	R3,R11		// Count leading zeros (Big Endian).
+#endif
+	ADD	R8,R11,R3	// Calculate byte address
+
+return:
+	SUB	R17, R3
+	RET
+
+found_qw_align:
+	// Use the same algorithm as above. Compress the result into
+	// a single doubleword and move it to a GPR for the final
+	// calculation.
+	VBPERMQ	  V6,V10,V6
+
+#ifdef GOARCH_ppc64le
+	MFVRD	  V6,R3
+	ADD	  $-1,R3,R11
+	ANDN	  R3,R11,R11
+	POPCNTD	  R11,R11
+#else
+	VSLDOI	  $6,V6,V6,V6
+	MFVRD	  V6,R3
+	CNTLZD	  R3,R11
+#endif
+	ADD	  R8,R11,R3
+	CMPU	  R11,R4
+	BLT	  return
+	BR	  notfound
+	PCALIGN	  $16
+
+done:
+	ADD	$-1,R10,R6
+	// Offset of last index for the final
+	// doubleword comparison
+	RLDICL	$0,R6,$61,R6
+	// At this point, R3 has 0xFF in the same position as the byte we are
+	// looking for in the doubleword. Use that to calculate the exact index
+	// of the byte.
+#ifdef GOARCH_ppc64le
+	ADD	$-1,R3,R11
+	ANDN	R3,R11,R11
+	POPCNTD	R11,R11		// Count trailing zeros (Little Endian).
+#else
+	CNTLZD	R3,R11		// Count leading zeros (Big Endian).
+#endif
+	CMPU	R8,R7		// Check if we are at the last doubleword.
+	SRD	$3,R11		// Convert trailing zeros to bytes.
+	ADD	R11,R8,R3
+	CMPU	R11,R6,CR7	// If at the last doubleword, check the byte offset.
+	BNE	return
+	BLE	CR7,return
+	BR	notfound
+
+small_string:
+	// process string of length < 32 bytes
+	// We unroll this loop for better performance.
+	CMPU	R4,$0		// Check for length=0
+	BEQ	notfound
+
+	MOVD	0(R8),R12	// Load one doubleword from the aligned address in R8.
+	CMPB	R12,R5,R3	// Check for a match.
+	AND	R9,R3,R3	// Mask bytes below s_base.
+	CMPU	R3,$0,CR7	// If we have a match, jump to the final computation.
+	RLDICR	$0,R7,$60,R7	// Last doubleword in R7.
+	CMPU	R8,R7
+	BNE	CR7,done
+	BEQ	notfound	// Hit length.
+
+	MOVDU	8(R8),R12
+	CMPB	R12,R5,R3
+	CMPU	R3,$0,CR6
+	CMPU	R8,R7
+	BNE	CR6,done
+	BEQ	notfound
+
+	MOVDU	8(R8),R12
+	CMPB	R12,R5,R3
+	CMPU	R3,$0,CR6
+	CMPU	R8,R7
+	BNE	CR6,done
+	BEQ	notfound
+
+	MOVDU	8(R8),R12
+	CMPB	R12,R5,R3
+	CMPU	R3,$0,CR6
+	CMPU	R8,R7
+	BNE	CR6,done
+	BEQ	notfound
+
+	MOVDU	8(R8),R12
+	CMPB	R12,R5,R3
+	CMPU	R3,$0,CR6
+	BNE	CR6,done
+	BR	notfound
+
diff --git a/src/internal/bytealg/indexbyte_riscv64.s b/src/internal/bytealg/indexbyte_riscv64.s
new file mode 100644
index 0000000..8be78ed
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_riscv64.s
@@ -0,0 +1,51 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte<ABIInternal>(SB),NOSPLIT,$0-40
+	// X10 = b_base
+	// X11 = b_len
+	// X12 = b_cap (unused)
+	// X13 = byte to find
+	AND	$0xff, X13
+	MOV	X10, X12		// store base for later
+	ADD	X10, X11		// end
+	ADD	$-1, X10
+
+loop:
+	ADD	$1, X10
+	BEQ	X10, X11, notfound
+	MOVBU	(X10), X14
+	BNE	X13, X14, loop
+
+	SUB	X12, X10		// remove base
+	RET
+
+notfound:
+	MOV	$-1, X10
+	RET
+
+TEXT ·IndexByteString<ABIInternal>(SB),NOSPLIT,$0-32
+	// X10 = b_base
+	// X11 = b_len
+	// X12 = byte to find
+	AND	$0xff, X12
+	MOV	X10, X13		// store base for later
+	ADD	X10, X11		// end
+	ADD	$-1, X10
+
+loop:
+	ADD	$1, X10
+	BEQ	X10, X11, notfound
+	MOVBU	(X10), X14
+	BNE	X12, X14, loop
+
+	SUB	X13, X10		// remove base
+	RET
+
+notfound:
+	MOV	$-1, X10
+	RET
diff --git a/src/internal/bytealg/indexbyte_s390x.s b/src/internal/bytealg/indexbyte_s390x.s
new file mode 100644
index 0000000..cf88d92
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_s390x.s
@@ -0,0 +1,108 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB),NOSPLIT|NOFRAME,$0-40
+	MOVD	b_base+0(FP), R3// b_base => R3
+	MOVD	b_len+8(FP), R4 // b_len => R4
+	MOVBZ	c+24(FP), R5    // c => R5
+	MOVD	$ret+32(FP), R2 // &ret => R9
+	BR	indexbytebody<>(SB)
+
+TEXT ·IndexByteString(SB),NOSPLIT|NOFRAME,$0-32
+	MOVD	s_base+0(FP), R3// s_base => R3
+	MOVD	s_len+8(FP), R4 // s_len => R4
+	MOVBZ	c+16(FP), R5    // c => R5
+	MOVD	$ret+24(FP), R2 // &ret => R9
+	BR	indexbytebody<>(SB)
+
+// input:
+// R3: s
+// R4: s_len
+// R5: c -- byte sought
+// R2: &ret -- address to put index into
+TEXT indexbytebody<>(SB),NOSPLIT|NOFRAME,$0
+	CMPBEQ	R4, $0, notfound
+	MOVD	R3, R6          // store base for later
+	ADD	R3, R4, R8      // the address after the end of the string
+	//if the length is small, use loop; otherwise, use vector or srst search
+	CMPBGE	R4, $16, large
+
+residual:
+	CMPBEQ	R3, R8, notfound
+	MOVBZ	0(R3), R7
+	LA	1(R3), R3
+	CMPBNE	R7, R5, residual
+
+found:
+	SUB	R6, R3
+	SUB	$1, R3
+	MOVD	R3, 0(R2)
+	RET
+
+notfound:
+	MOVD	$-1, 0(R2)
+	RET
+
+large:
+	MOVBZ	internal∕cpu·S390X+const_offsetS390xHasVX(SB), R1
+	CMPBNE	R1, $0, vectorimpl
+
+srstimpl:                       // no vector facility
+	MOVBZ	R5, R0          // c needs to be in R0, leave until last minute as currently R0 is expected to be 0
+srstloop:
+	WORD	$0xB25E0083     // srst %r8, %r3 (search the range [R3, R8))
+	BVS	srstloop        // interrupted - continue
+	BGT	notfoundr0
+foundr0:
+	XOR	R0, R0          // reset R0
+	SUB	R6, R8          // remove base
+	MOVD	R8, 0(R2)
+	RET
+notfoundr0:
+	XOR	R0, R0          // reset R0
+	MOVD	$-1, 0(R2)
+	RET
+
+vectorimpl:
+	//if the address is not 16byte aligned, use loop for the header
+	MOVD	R3, R8
+	AND	$15, R8
+	CMPBGT	R8, $0, notaligned
+
+aligned:
+	ADD	R6, R4, R8
+	MOVD	R8, R7
+	AND	$-16, R7
+	// replicate c across V17
+	VLVGB	$0, R5, V19
+	VREPB	$0, V19, V17
+
+vectorloop:
+	CMPBGE	R3, R7, residual
+	VL	0(R3), V16    // load string to be searched into V16
+	ADD	$16, R3
+	VFEEBS	V16, V17, V18 // search V17 in V16 and set conditional code accordingly
+	BVS	vectorloop
+
+	// when vector search found c in the string
+	VLGVB	$7, V18, R7   // load 7th element of V18 containing index into R7
+	SUB	$16, R3
+	SUB	R6, R3
+	ADD	R3, R7
+	MOVD	R7, 0(R2)
+	RET
+
+notaligned:
+	MOVD	R3, R8
+	AND	$-16, R8
+	ADD     $16, R8
+notalignedloop:
+	CMPBEQ	R3, R8, aligned
+	MOVBZ	0(R3), R7
+	LA	1(R3), R3
+	CMPBNE	R7, R5, notalignedloop
+	BR	found
diff --git a/src/internal/bytealg/indexbyte_wasm.s b/src/internal/bytealg/indexbyte_wasm.s
new file mode 100644
index 0000000..ef4bd93
--- /dev/null
+++ b/src/internal/bytealg/indexbyte_wasm.s
@@ -0,0 +1,195 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "textflag.h"
+
+TEXT ·IndexByte(SB), NOSPLIT, $0-40
+	I64Load b_base+0(FP)
+	I32WrapI64
+	I32Load8U c+24(FP)
+	I64Load b_len+8(FP)
+	I32WrapI64
+	Call memchr<>(SB)
+	I64ExtendI32S
+	Set R0
+
+	Get SP
+	I64Const $-1
+	Get R0
+	I64Load b_base+0(FP)
+	I64Sub
+	Get R0
+	I64Eqz $0
+	Select
+	I64Store ret+32(FP)
+
+	RET
+
+TEXT ·IndexByteString(SB), NOSPLIT, $0-32
+	Get SP
+	I64Load s_base+0(FP)
+	I32WrapI64
+	I32Load8U c+16(FP)
+	I64Load s_len+8(FP)
+	I32WrapI64
+	Call memchr<>(SB)
+	I64ExtendI32S
+	Set R0
+
+	I64Const $-1
+	Get R0
+	I64Load s_base+0(FP)
+	I64Sub
+	Get R0
+	I64Eqz $0
+	Select
+	I64Store ret+24(FP)
+
+	RET
+
+// initially compiled with emscripten and then modified over time.
+// params:
+//   R0: s
+//   R1: c
+//   R2: len
+// ret: index
+TEXT memchr<>(SB), NOSPLIT, $0
+	Get R1
+	Set R4
+	Block
+		Block
+			Get R2
+			I32Const $0
+			I32Ne
+			Tee R3
+			Get R0
+			I32Const $3
+			I32And
+			I32Const $0
+			I32Ne
+			I32And
+			If
+				Loop
+					Get R0
+					I32Load8U $0
+					Get R1
+					I32Eq
+					BrIf $2
+					Get R2
+					I32Const $-1
+					I32Add
+					Tee R2
+					I32Const $0
+					I32Ne
+					Tee R3
+					Get R0
+					I32Const $1
+					I32Add
+					Tee R0
+					I32Const $3
+					I32And
+					I32Const $0
+					I32Ne
+					I32And
+					BrIf $0
+				End
+			End
+			Get R3
+			BrIf $0
+			I32Const $0
+			Set R1
+			Br $1
+		End
+		Get R0
+		I32Load8U $0
+		Get R4
+		Tee R3
+		I32Eq
+		If
+			Get R2
+			Set R1
+		Else
+			Get R4
+			I32Const $16843009
+			I32Mul
+			Set R4
+			Block
+				Block
+					Get R2
+					I32Const $3
+					I32GtU
+					If
+						Get R2
+						Set R1
+						Loop
+							Get R0
+							I32Load $0
+							Get R4
+							I32Xor
+							Tee R2
+							I32Const $-2139062144
+							I32And
+							I32Const $-2139062144
+							I32Xor
+							Get R2
+							I32Const $-16843009
+							I32Add
+							I32And
+							I32Eqz
+							If
+								Get R0
+								I32Const $4
+								I32Add
+								Set R0
+								Get R1
+								I32Const $-4
+								I32Add
+								Tee R1
+								I32Const $3
+								I32GtU
+								BrIf $1
+								Br $3
+							End
+						End
+					Else
+						Get R2
+						Set R1
+						Br $1
+					End
+					Br $1
+				End
+				Get R1
+				I32Eqz
+				If
+					I32Const $0
+					Set R1
+					Br $3
+				End
+			End
+			Loop
+				Get R0
+				I32Load8U $0
+				Get R3
+				I32Eq
+				BrIf $2
+				Get R0
+				I32Const $1
+				I32Add
+				Set R0
+				Get R1
+				I32Const $-1
+				I32Add
+				Tee R1
+				BrIf $0
+				I32Const $0
+				Set R1
+			End
+		End
+	End
+	Get R0
+	I32Const $0
+	Get R1
+	Select
+	Return
diff --git a/src/internal/cfg/cfg.go b/src/internal/cfg/cfg.go
new file mode 100644
index 0000000..78664d7
--- /dev/null
+++ b/src/internal/cfg/cfg.go
@@ -0,0 +1,68 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cfg holds configuration shared by the Go command and internal/testenv.
+// Definitions that don't need to be exposed outside of cmd/go should be in
+// cmd/go/internal/cfg instead of this package.
+package cfg
+
+// KnownEnv is a list of environment variables that affect the operation
+// of the Go command.
+const KnownEnv = `
+	AR
+	CC
+	CGO_CFLAGS
+	CGO_CFLAGS_ALLOW
+	CGO_CFLAGS_DISALLOW
+	CGO_CPPFLAGS
+	CGO_CPPFLAGS_ALLOW
+	CGO_CPPFLAGS_DISALLOW
+	CGO_CXXFLAGS
+	CGO_CXXFLAGS_ALLOW
+	CGO_CXXFLAGS_DISALLOW
+	CGO_ENABLED
+	CGO_FFLAGS
+	CGO_FFLAGS_ALLOW
+	CGO_FFLAGS_DISALLOW
+	CGO_LDFLAGS
+	CGO_LDFLAGS_ALLOW
+	CGO_LDFLAGS_DISALLOW
+	CXX
+	FC
+	GCCGO
+	GO111MODULE
+	GO386
+	GOAMD64
+	GOARCH
+	GOARM
+	GOBIN
+	GOCACHE
+	GOENV
+	GOEXE
+	GOEXPERIMENT
+	GOFLAGS
+	GOGCCFLAGS
+	GOHOSTARCH
+	GOHOSTOS
+	GOINSECURE
+	GOMIPS
+	GOMIPS64
+	GOMODCACHE
+	GONOPROXY
+	GONOSUMDB
+	GOOS
+	GOPATH
+	GOPPC64
+	GOPRIVATE
+	GOPROXY
+	GOROOT
+	GOSUMDB
+	GOTMPDIR
+	GOTOOLDIR
+	GOVCS
+	GOWASM
+	GOWORK
+	GO_EXTLINK_ENABLED
+	PKG_CONFIG
+`
diff --git a/src/internal/coverage/calloc/batchcounteralloc.go b/src/internal/coverage/calloc/batchcounteralloc.go
new file mode 100644
index 0000000..2b6495d
--- /dev/null
+++ b/src/internal/coverage/calloc/batchcounteralloc.go
@@ -0,0 +1,29 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package calloc
+
+// This package contains a simple "batch" allocator for allocating
+// coverage counters (slices of uint32 basically), for working with
+// coverage data files. Collections of counter arrays tend to all be
+// live/dead over the same time period, so a good fit for batch
+// allocation.
+
+type BatchCounterAlloc struct {
+	pool []uint32
+}
+
+func (ca *BatchCounterAlloc) AllocateCounters(n int) []uint32 {
+	const chunk = 8192
+	if n > cap(ca.pool) {
+		siz := chunk
+		if n > chunk {
+			siz = n
+		}
+		ca.pool = make([]uint32, siz)
+	}
+	rv := ca.pool[:n]
+	ca.pool = ca.pool[n:]
+	return rv
+}
diff --git a/src/internal/coverage/cformat/fmt_test.go b/src/internal/coverage/cformat/fmt_test.go
new file mode 100644
index 0000000..4d6da44
--- /dev/null
+++ b/src/internal/coverage/cformat/fmt_test.go
@@ -0,0 +1,92 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cformat_test
+
+import (
+	"internal/coverage"
+	"internal/coverage/cformat"
+	"strings"
+	"testing"
+)
+
+func TestBasics(t *testing.T) {
+	fm := cformat.NewFormatter(coverage.CtrModeAtomic)
+	fm.SetPackage("my/pack")
+
+	mku := func(stl, enl, nx uint32) coverage.CoverableUnit {
+		return coverage.CoverableUnit{
+			StLine:  stl,
+			EnLine:  enl,
+			NxStmts: nx,
+		}
+	}
+	fn1units := []coverage.CoverableUnit{
+		mku(10, 11, 2),
+		mku(15, 11, 1),
+	}
+	fn2units := []coverage.CoverableUnit{
+		mku(20, 25, 3),
+		mku(30, 31, 2),
+		mku(33, 40, 7),
+	}
+	fn3units := []coverage.CoverableUnit{
+		mku(99, 100, 1),
+	}
+	for k, u := range fn1units {
+		fm.AddUnit("p.go", "f1", false, u, uint32(k))
+	}
+	for k, u := range fn2units {
+		fm.AddUnit("q.go", "f2", false, u, 0)
+		fm.AddUnit("q.go", "f2", false, u, uint32(k))
+	}
+	for _, u := range fn3units {
+		fm.AddUnit("lit.go", "f3", true, u, 0)
+	}
+
+	var b1, b2, b3 strings.Builder
+	if err := fm.EmitTextual(&b1); err != nil {
+		t.Fatalf("EmitTextual returned %v", err)
+	}
+	wantText := strings.TrimSpace(`
+mode: atomic
+lit.go:99.0,100.0 1 0
+p.go:10.0,11.0 2 0
+p.go:15.0,11.0 1 1
+q.go:20.0,25.0 3 0
+q.go:30.0,31.0 2 1
+q.go:33.0,40.0 7 2`)
+	gotText := strings.TrimSpace(b1.String())
+	if wantText != gotText {
+		t.Errorf("emit text: got:\n%s\nwant:\n%s\n", gotText, wantText)
+	}
+
+	if err := fm.EmitPercent(&b2, "", false); err != nil {
+		t.Fatalf("EmitPercent returned %v", err)
+	}
+	wantPercent := strings.TrimSpace(`
+my/pack	coverage: 62.5% of statements
+`)
+	gotPercent := strings.TrimSpace(b2.String())
+	if wantPercent != gotPercent {
+		t.Errorf("emit percent: got:\n%s\nwant:\n%s\n", gotPercent, wantPercent)
+	}
+
+	if err := fm.EmitFuncs(&b3); err != nil {
+		t.Fatalf("EmitFuncs returned %v", err)
+	}
+	wantFuncs := strings.TrimSpace(`
+p.go:10:	f1		33.3%
+q.go:20:	f2		75.0%
+total		(statements)	62.5%`)
+	gotFuncs := strings.TrimSpace(b3.String())
+	if wantFuncs != gotFuncs {
+		t.Errorf("emit funcs: got:\n%s\nwant:\n%s\n", gotFuncs, wantFuncs)
+	}
+	if false {
+		t.Logf("text is %s\n", b1.String())
+		t.Logf("perc is %s\n", b2.String())
+		t.Logf("funcs is %s\n", b3.String())
+	}
+}
diff --git a/src/internal/coverage/cformat/format.go b/src/internal/coverage/cformat/format.go
new file mode 100644
index 0000000..a8276ff
--- /dev/null
+++ b/src/internal/coverage/cformat/format.go
@@ -0,0 +1,340 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cformat
+
+// This package provides apis for producing human-readable summaries
+// of coverage data (e.g. a coverage percentage for a given package or
+// set of packages) and for writing data in the legacy test format
+// emitted by "go test -coverprofile=<outfile>".
+//
+// The model for using these apis is to create a Formatter object,
+// then make a series of calls to SetPackage and AddUnit passing in
+// data read from coverage meta-data and counter-data files. E.g.
+//
+//		myformatter := cformat.NewFormatter()
+//		...
+//		for each package P in meta-data file: {
+//			myformatter.SetPackage(P)
+//			for each function F in P: {
+//				for each coverable unit U in F: {
+//					myformatter.AddUnit(U)
+//				}
+//			}
+//		}
+//		myformatter.EmitPercent(os.Stdout, "")
+//		myformatter.EmitTextual(somefile)
+//
+// These apis are linked into tests that are built with "-cover", and
+// called at the end of test execution to produce text output or
+// emit coverage percentages.
+
+import (
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/cmerge"
+	"io"
+	"sort"
+	"text/tabwriter"
+)
+
+type Formatter struct {
+	// Maps import path to package state.
+	pm map[string]*pstate
+	// Records current package being visited.
+	pkg string
+	// Pointer to current package state.
+	p *pstate
+	// Counter mode.
+	cm coverage.CounterMode
+}
+
+// pstate records package-level coverage data state:
+// - a table of functions (file/fname/literal)
+// - a map recording the index/ID of each func encountered so far
+// - a table storing execution count for the coverable units in each func
+type pstate struct {
+	// slice of unique functions
+	funcs []fnfile
+	// maps function to index in slice above (index acts as function ID)
+	funcTable map[fnfile]uint32
+
+	// A table storing coverage counts for each coverable unit.
+	unitTable map[extcu]uint32
+}
+
+// extcu encapsulates a coverable unit within some function.
+type extcu struct {
+	fnfid uint32 // index into p.funcs slice
+	coverage.CoverableUnit
+}
+
+// fnfile is a function-name/file-name tuple.
+type fnfile struct {
+	file  string
+	fname string
+	lit   bool
+}
+
+func NewFormatter(cm coverage.CounterMode) *Formatter {
+	return &Formatter{
+		pm: make(map[string]*pstate),
+		cm: cm,
+	}
+}
+
+// SetPackage tells the formatter that we're about to visit the
+// coverage data for the package with the specified import path.
+// Note that it's OK to call SetPackage more than once with the
+// same import path; counter data values will be accumulated.
+func (fm *Formatter) SetPackage(importpath string) {
+	if importpath == fm.pkg {
+		return
+	}
+	fm.pkg = importpath
+	ps, ok := fm.pm[importpath]
+	if !ok {
+		ps = new(pstate)
+		fm.pm[importpath] = ps
+		ps.unitTable = make(map[extcu]uint32)
+		ps.funcTable = make(map[fnfile]uint32)
+	}
+	fm.p = ps
+}
+
+// AddUnit passes info on a single coverable unit (file, funcname,
+// literal flag, range of lines, and counter value) to the formatter.
+// Counter values will be accumulated where appropriate.
+func (fm *Formatter) AddUnit(file string, fname string, isfnlit bool, unit coverage.CoverableUnit, count uint32) {
+	if fm.p == nil {
+		panic("AddUnit invoked before SetPackage")
+	}
+	fkey := fnfile{file: file, fname: fname, lit: isfnlit}
+	idx, ok := fm.p.funcTable[fkey]
+	if !ok {
+		idx = uint32(len(fm.p.funcs))
+		fm.p.funcs = append(fm.p.funcs, fkey)
+		fm.p.funcTable[fkey] = idx
+	}
+	ukey := extcu{fnfid: idx, CoverableUnit: unit}
+	pcount := fm.p.unitTable[ukey]
+	var result uint32
+	if fm.cm == coverage.CtrModeSet {
+		if count != 0 || pcount != 0 {
+			result = 1
+		}
+	} else {
+		// Use saturating arithmetic.
+		result, _ = cmerge.SaturatingAdd(pcount, count)
+	}
+	fm.p.unitTable[ukey] = result
+}
+
+// sortUnits sorts a slice of extcu objects in a package according to
+// source position information (e.g. file and line). Note that we don't
+// include function name as part of the sorting criteria, the thinking
+// being that is better to provide things in the original source order.
+func (p *pstate) sortUnits(units []extcu) {
+	sort.Slice(units, func(i, j int) bool {
+		ui := units[i]
+		uj := units[j]
+		ifile := p.funcs[ui.fnfid].file
+		jfile := p.funcs[uj.fnfid].file
+		if ifile != jfile {
+			return ifile < jfile
+		}
+		// NB: not taking function literal flag into account here (no
+		// need, since other fields are guaranteed to be distinct).
+		if units[i].StLine != units[j].StLine {
+			return units[i].StLine < units[j].StLine
+		}
+		if units[i].EnLine != units[j].EnLine {
+			return units[i].EnLine < units[j].EnLine
+		}
+		if units[i].StCol != units[j].StCol {
+			return units[i].StCol < units[j].StCol
+		}
+		if units[i].EnCol != units[j].EnCol {
+			return units[i].EnCol < units[j].EnCol
+		}
+		return units[i].NxStmts < units[j].NxStmts
+	})
+}
+
+// EmitTextual writes the accumulated coverage data in the legacy
+// cmd/cover text format to the writer 'w'. We sort the data items by
+// importpath, source file, and line number before emitting (this sorting
+// is not explicitly mandated by the format, but seems like a good idea
+// for repeatable/deterministic dumps).
+func (fm *Formatter) EmitTextual(w io.Writer) error {
+	if fm.cm == coverage.CtrModeInvalid {
+		panic("internal error, counter mode unset")
+	}
+	if _, err := fmt.Fprintf(w, "mode: %s\n", fm.cm.String()); err != nil {
+		return err
+	}
+	pkgs := make([]string, 0, len(fm.pm))
+	for importpath := range fm.pm {
+		pkgs = append(pkgs, importpath)
+	}
+	sort.Strings(pkgs)
+	for _, importpath := range pkgs {
+		p := fm.pm[importpath]
+		units := make([]extcu, 0, len(p.unitTable))
+		for u := range p.unitTable {
+			units = append(units, u)
+		}
+		p.sortUnits(units)
+		for _, u := range units {
+			count := p.unitTable[u]
+			file := p.funcs[u.fnfid].file
+			if _, err := fmt.Fprintf(w, "%s:%d.%d,%d.%d %d %d\n",
+				file, u.StLine, u.StCol,
+				u.EnLine, u.EnCol, u.NxStmts, count); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// EmitPercent writes out a "percentage covered" string to the writer 'w'.
+func (fm *Formatter) EmitPercent(w io.Writer, covpkgs string, noteEmpty bool) error {
+	pkgs := make([]string, 0, len(fm.pm))
+	for importpath := range fm.pm {
+		pkgs = append(pkgs, importpath)
+	}
+	sort.Strings(pkgs)
+	seenPkg := false
+	for _, importpath := range pkgs {
+		seenPkg = true
+		p := fm.pm[importpath]
+		var totalStmts, coveredStmts uint64
+		for unit, count := range p.unitTable {
+			nx := uint64(unit.NxStmts)
+			totalStmts += nx
+			if count != 0 {
+				coveredStmts += nx
+			}
+		}
+		if _, err := fmt.Fprintf(w, "\t%s\t", importpath); err != nil {
+			return err
+		}
+		if totalStmts == 0 {
+			if _, err := fmt.Fprintf(w, "coverage: [no statements]\n"); err != nil {
+				return err
+			}
+		} else {
+			if _, err := fmt.Fprintf(w, "coverage: %.1f%% of statements%s\n", 100*float64(coveredStmts)/float64(totalStmts), covpkgs); err != nil {
+				return err
+			}
+		}
+	}
+	if noteEmpty && !seenPkg {
+		if _, err := fmt.Fprintf(w, "coverage: [no statements]\n"); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// EmitFuncs writes out a function-level summary to the writer 'w'. A
+// note on handling function literals: although we collect coverage
+// data for unnamed literals, it probably does not make sense to
+// include them in the function summary since there isn't any good way
+// to name them (this is also consistent with the legacy cmd/cover
+// implementation). We do want to include their counts in the overall
+// summary however.
+func (fm *Formatter) EmitFuncs(w io.Writer) error {
+	if fm.cm == coverage.CtrModeInvalid {
+		panic("internal error, counter mode unset")
+	}
+	perc := func(covered, total uint64) float64 {
+		if total == 0 {
+			total = 1
+		}
+		return 100.0 * float64(covered) / float64(total)
+	}
+	tabber := tabwriter.NewWriter(w, 1, 8, 1, '\t', 0)
+	defer tabber.Flush()
+	allStmts := uint64(0)
+	covStmts := uint64(0)
+
+	pkgs := make([]string, 0, len(fm.pm))
+	for importpath := range fm.pm {
+		pkgs = append(pkgs, importpath)
+	}
+	sort.Strings(pkgs)
+
+	// Emit functions for each package, sorted by import path.
+	for _, importpath := range pkgs {
+		p := fm.pm[importpath]
+		if len(p.unitTable) == 0 {
+			continue
+		}
+		units := make([]extcu, 0, len(p.unitTable))
+		for u := range p.unitTable {
+			units = append(units, u)
+		}
+
+		// Within a package, sort the units, then walk through the
+		// sorted array. Each time we hit a new function, emit the
+		// summary entry for the previous function, then make one last
+		// emit call at the end of the loop.
+		p.sortUnits(units)
+		fname := ""
+		ffile := ""
+		flit := false
+		var fline uint32
+		var cstmts, tstmts uint64
+		captureFuncStart := func(u extcu) {
+			fname = p.funcs[u.fnfid].fname
+			ffile = p.funcs[u.fnfid].file
+			flit = p.funcs[u.fnfid].lit
+			fline = u.StLine
+		}
+		emitFunc := func(u extcu) error {
+			// Don't emit entries for function literals (see discussion
+			// in function header comment above).
+			if !flit {
+				if _, err := fmt.Fprintf(tabber, "%s:%d:\t%s\t%.1f%%\n",
+					ffile, fline, fname, perc(cstmts, tstmts)); err != nil {
+					return err
+				}
+			}
+			captureFuncStart(u)
+			allStmts += tstmts
+			covStmts += cstmts
+			tstmts = 0
+			cstmts = 0
+			return nil
+		}
+		for k, u := range units {
+			if k == 0 {
+				captureFuncStart(u)
+			} else {
+				if fname != p.funcs[u.fnfid].fname {
+					// New function; emit entry for previous one.
+					if err := emitFunc(u); err != nil {
+						return err
+					}
+				}
+			}
+			tstmts += uint64(u.NxStmts)
+			count := p.unitTable[u]
+			if count != 0 {
+				cstmts += uint64(u.NxStmts)
+			}
+		}
+		if err := emitFunc(extcu{}); err != nil {
+			return err
+		}
+	}
+	if _, err := fmt.Fprintf(tabber, "%s\t%s\t%.1f%%\n",
+		"total", "(statements)", perc(covStmts, allStmts)); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/src/internal/coverage/cmddefs.go b/src/internal/coverage/cmddefs.go
new file mode 100644
index 0000000..b30c37a
--- /dev/null
+++ b/src/internal/coverage/cmddefs.go
@@ -0,0 +1,73 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package coverage
+
+// CoverPkgConfig is a bundle of information passed from the Go
+// command to the cover command during "go build -cover" runs. The
+// Go command creates and fills in a struct as below, then passes
+// file containing the encoded JSON for the struct to the "cover"
+// tool when instrumenting the source files in a Go package.
+type CoverPkgConfig struct {
+	// File into which cmd/cover should emit summary info
+	// when instrumentation is complete.
+	OutConfig string
+
+	// Import path for the package being instrumented.
+	PkgPath string
+
+	// Package name.
+	PkgName string
+
+	// Instrumentation granularity: one of "perfunc" or "perblock" (default)
+	Granularity string
+
+	// Module path for this package (empty if no go.mod in use)
+	ModulePath string
+
+	// Local mode indicates we're doing a coverage build or test of a
+	// package selected via local import path, e.g. "./..." or
+	// "./foo/bar" as opposed to a non-relative import path. See the
+	// corresponding field in cmd/go's PackageInternal struct for more
+	// info.
+	Local bool
+}
+
+// CoverFixupConfig contains annotations/notes generated by the
+// cmd/cover tool (during instrumentation) to be passed on to the
+// compiler when the instrumented code is compiled. The cmd/cover tool
+// creates a struct of this type, JSON-encodes it, and emits the
+// result to a file, which the Go command then passes to the compiler
+// when the instrumented package is built.
+type CoverFixupConfig struct {
+	// Name of the variable (created by cmd/cover) containing the
+	// encoded meta-data for the package.
+	MetaVar string
+
+	// Length of the meta-data.
+	MetaLen int
+
+	// Hash computed by cmd/cover of the meta-data.
+	MetaHash string
+
+	// Instrumentation strategy. For now this is always set to
+	// "normal", but in the future we may add new values (for example,
+	// if panic paths are instrumented, or if the instrumenter
+	// eliminates redundant counters).
+	Strategy string
+
+	// Prefix assigned to the names of counter variables generated
+	// during instrumentation by cmd/cover.
+	CounterPrefix string
+
+	// Name chosen for the package ID variable generated during
+	// instrumentation.
+	PkgIdVar string
+
+	// Counter mode (e.g. set/count/atomic)
+	CounterMode string
+
+	// Counter granularity (perblock or perfunc).
+	CounterGranularity string
+}
diff --git a/src/internal/coverage/cmerge/merge.go b/src/internal/coverage/cmerge/merge.go
new file mode 100644
index 0000000..c482b8b
--- /dev/null
+++ b/src/internal/coverage/cmerge/merge.go
@@ -0,0 +1,104 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cmerge
+
+// package cmerge provides a few small utility APIs for helping
+// with merging of counter data for a given function.
+
+import (
+	"fmt"
+	"internal/coverage"
+	"math"
+)
+
+// Merger provides state and methods to help manage the process of
+// merging together coverage counter data for a given function, for
+// tools that need to implicitly merge counter as they read multiple
+// coverage counter data files.
+type Merger struct {
+	cmode    coverage.CounterMode
+	cgran    coverage.CounterGranularity
+	overflow bool
+}
+
+// MergeCounters takes the counter values in 'src' and merges them
+// into 'dst' according to the correct counter mode.
+func (m *Merger) MergeCounters(dst, src []uint32) (error, bool) {
+	if len(src) != len(dst) {
+		return fmt.Errorf("merging counters: len(dst)=%d len(src)=%d", len(dst), len(src)), false
+	}
+	if m.cmode == coverage.CtrModeSet {
+		for i := 0; i < len(src); i++ {
+			if src[i] != 0 {
+				dst[i] = 1
+			}
+		}
+	} else {
+		for i := 0; i < len(src); i++ {
+			dst[i] = m.SaturatingAdd(dst[i], src[i])
+		}
+	}
+	ovf := m.overflow
+	m.overflow = false
+	return nil, ovf
+}
+
+// Saturating add does a saturating addition of 'dst' and 'src',
+// returning added value or math.MaxUint32 if there is an overflow.
+// Overflows are recorded in case the client needs to track them.
+func (m *Merger) SaturatingAdd(dst, src uint32) uint32 {
+	result, overflow := SaturatingAdd(dst, src)
+	if overflow {
+		m.overflow = true
+	}
+	return result
+}
+
+// Saturating add does a saturing addition of 'dst' and 'src',
+// returning added value or math.MaxUint32 plus an overflow flag.
+func SaturatingAdd(dst, src uint32) (uint32, bool) {
+	d, s := uint64(dst), uint64(src)
+	sum := d + s
+	overflow := false
+	if uint64(uint32(sum)) != sum {
+		overflow = true
+		sum = math.MaxUint32
+	}
+	return uint32(sum), overflow
+}
+
+// SetModeAndGranularity records the counter mode and granularity for
+// the current merge. In the specific case of merging across coverage
+// data files from different binaries, where we're combining data from
+// more than one meta-data file, we need to check for mode/granularity
+// clashes.
+func (cm *Merger) SetModeAndGranularity(mdf string, cmode coverage.CounterMode, cgran coverage.CounterGranularity) error {
+	// Collect counter mode and granularity so as to detect clashes.
+	if cm.cmode != coverage.CtrModeInvalid {
+		if cm.cmode != cmode {
+			return fmt.Errorf("counter mode clash while reading meta-data file %s: previous file had %s, new file has %s", mdf, cm.cmode.String(), cmode.String())
+		}
+		if cm.cgran != cgran {
+			return fmt.Errorf("counter granularity clash while reading meta-data file %s: previous file had %s, new file has %s", mdf, cm.cgran.String(), cgran.String())
+		}
+	}
+	cm.cmode = cmode
+	cm.cgran = cgran
+	return nil
+}
+
+func (cm *Merger) ResetModeAndGranularity() {
+	cm.cmode = coverage.CtrModeInvalid
+	cm.cgran = coverage.CtrGranularityInvalid
+	cm.overflow = false
+}
+
+func (cm *Merger) Mode() coverage.CounterMode {
+	return cm.cmode
+}
+
+func (cm *Merger) Granularity() coverage.CounterGranularity {
+	return cm.cgran
+}
diff --git a/src/internal/coverage/cmerge/merge_test.go b/src/internal/coverage/cmerge/merge_test.go
new file mode 100644
index 0000000..e45589f
--- /dev/null
+++ b/src/internal/coverage/cmerge/merge_test.go
@@ -0,0 +1,105 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cmerge_test
+
+import (
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/cmerge"
+	"testing"
+)
+
+func TestClash(t *testing.T) {
+	m := &cmerge.Merger{}
+	err := m.SetModeAndGranularity("mdf1.data", coverage.CtrModeSet, coverage.CtrGranularityPerBlock)
+	if err != nil {
+		t.Fatalf("unexpected clash")
+	}
+	err = m.SetModeAndGranularity("mdf1.data", coverage.CtrModeSet, coverage.CtrGranularityPerBlock)
+	if err != nil {
+		t.Fatalf("unexpected clash")
+	}
+	err = m.SetModeAndGranularity("mdf1.data", coverage.CtrModeCount, coverage.CtrGranularityPerBlock)
+	if err == nil {
+		t.Fatalf("expected mode clash, not found")
+	}
+	err = m.SetModeAndGranularity("mdf1.data", coverage.CtrModeSet, coverage.CtrGranularityPerFunc)
+	if err == nil {
+		t.Fatalf("expected granularity clash, not found")
+	}
+	m.ResetModeAndGranularity()
+	err = m.SetModeAndGranularity("mdf1.data", coverage.CtrModeCount, coverage.CtrGranularityPerFunc)
+	if err != nil {
+		t.Fatalf("unexpected clash after reset")
+	}
+}
+
+func TestBasic(t *testing.T) {
+	scenarios := []struct {
+		cmode         coverage.CounterMode
+		cgran         coverage.CounterGranularity
+		src, dst, res []uint32
+		iters         int
+		merr          bool
+		overflow      bool
+	}{
+		{
+			cmode:    coverage.CtrModeSet,
+			cgran:    coverage.CtrGranularityPerBlock,
+			src:      []uint32{1, 0, 1},
+			dst:      []uint32{1, 1, 0},
+			res:      []uint32{1, 1, 1},
+			iters:    2,
+			overflow: false,
+		},
+		{
+			cmode:    coverage.CtrModeCount,
+			cgran:    coverage.CtrGranularityPerBlock,
+			src:      []uint32{1, 0, 3},
+			dst:      []uint32{5, 7, 0},
+			res:      []uint32{6, 7, 3},
+			iters:    1,
+			overflow: false,
+		},
+		{
+			cmode:    coverage.CtrModeCount,
+			cgran:    coverage.CtrGranularityPerBlock,
+			src:      []uint32{4294967200, 0, 3},
+			dst:      []uint32{4294967001, 7, 0},
+			res:      []uint32{4294967295, 7, 3},
+			iters:    1,
+			overflow: true,
+		},
+	}
+
+	for k, scenario := range scenarios {
+		var err error
+		var ovf bool
+		m := &cmerge.Merger{}
+		mdf := fmt.Sprintf("file%d", k)
+		err = m.SetModeAndGranularity(mdf, scenario.cmode, scenario.cgran)
+		if err != nil {
+			t.Fatalf("case %d SetModeAndGranularity failed: %v", k, err)
+		}
+		for i := 0; i < scenario.iters; i++ {
+			err, ovf = m.MergeCounters(scenario.dst, scenario.src)
+			if ovf != scenario.overflow {
+				t.Fatalf("case %d overflow mismatch: got %v want %v", k, ovf, scenario.overflow)
+			}
+			if !scenario.merr && err != nil {
+				t.Fatalf("case %d unexpected err %v", k, err)
+			}
+			if scenario.merr && err == nil {
+				t.Fatalf("case %d expected err, not received", k)
+			}
+			for i := range scenario.dst {
+				if scenario.dst[i] != scenario.res[i] {
+					t.Fatalf("case %d: bad merge at %d got %d want %d",
+						k, i, scenario.dst[i], scenario.res[i])
+				}
+			}
+		}
+	}
+}
diff --git a/src/internal/coverage/decodecounter/decodecounterfile.go b/src/internal/coverage/decodecounter/decodecounterfile.go
new file mode 100644
index 0000000..fce060a
--- /dev/null
+++ b/src/internal/coverage/decodecounter/decodecounterfile.go
@@ -0,0 +1,373 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package decodecounter
+
+import (
+	"encoding/binary"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/slicereader"
+	"internal/coverage/stringtab"
+	"io"
+	"os"
+	"strconv"
+	"unsafe"
+)
+
+// This file contains helpers for reading counter data files created
+// during the executions of a coverage-instrumented binary.
+
+type CounterDataReader struct {
+	stab     *stringtab.Reader
+	args     map[string]string
+	osargs   []string
+	goarch   string // GOARCH setting from run that produced counter data
+	goos     string // GOOS setting from run that produced counter data
+	mr       io.ReadSeeker
+	hdr      coverage.CounterFileHeader
+	ftr      coverage.CounterFileFooter
+	shdr     coverage.CounterSegmentHeader
+	u32b     []byte
+	u8b      []byte
+	fcnCount uint32
+	segCount uint32
+	debug    bool
+}
+
+func NewCounterDataReader(fn string, rs io.ReadSeeker) (*CounterDataReader, error) {
+	cdr := &CounterDataReader{
+		mr:   rs,
+		u32b: make([]byte, 4),
+		u8b:  make([]byte, 1),
+	}
+	// Read header
+	if err := binary.Read(rs, binary.LittleEndian, &cdr.hdr); err != nil {
+		return nil, err
+	}
+	if cdr.debug {
+		fmt.Fprintf(os.Stderr, "=-= counter file header: %+v\n", cdr.hdr)
+	}
+	if !checkMagic(cdr.hdr.Magic) {
+		return nil, fmt.Errorf("invalid magic string: not a counter data file")
+	}
+	if cdr.hdr.Version > coverage.CounterFileVersion {
+		return nil, fmt.Errorf("version data incompatibility: reader is %d data is %d", coverage.CounterFileVersion, cdr.hdr.Version)
+	}
+
+	// Read footer.
+	if err := cdr.readFooter(); err != nil {
+		return nil, err
+	}
+	// Seek back to just past the file header.
+	hsz := int64(unsafe.Sizeof(cdr.hdr))
+	if _, err := cdr.mr.Seek(hsz, io.SeekStart); err != nil {
+		return nil, err
+	}
+	// Read preamble for first segment.
+	if err := cdr.readSegmentPreamble(); err != nil {
+		return nil, err
+	}
+	return cdr, nil
+}
+
+func checkMagic(v [4]byte) bool {
+	g := coverage.CovCounterMagic
+	return v[0] == g[0] && v[1] == g[1] && v[2] == g[2] && v[3] == g[3]
+}
+
+func (cdr *CounterDataReader) readFooter() error {
+	ftrSize := int64(unsafe.Sizeof(cdr.ftr))
+	if _, err := cdr.mr.Seek(-ftrSize, io.SeekEnd); err != nil {
+		return err
+	}
+	if err := binary.Read(cdr.mr, binary.LittleEndian, &cdr.ftr); err != nil {
+		return err
+	}
+	if !checkMagic(cdr.ftr.Magic) {
+		return fmt.Errorf("invalid magic string (not a counter data file)")
+	}
+	if cdr.ftr.NumSegments == 0 {
+		return fmt.Errorf("invalid counter data file (no segments)")
+	}
+	return nil
+}
+
+// readSegmentPreamble reads and consumes the segment header, segment string
+// table, and segment args table.
+func (cdr *CounterDataReader) readSegmentPreamble() error {
+	// Read segment header.
+	if err := binary.Read(cdr.mr, binary.LittleEndian, &cdr.shdr); err != nil {
+		return err
+	}
+	if cdr.debug {
+		fmt.Fprintf(os.Stderr, "=-= read counter segment header: %+v", cdr.shdr)
+		fmt.Fprintf(os.Stderr, " FcnEntries=0x%x StrTabLen=0x%x ArgsLen=0x%x\n",
+			cdr.shdr.FcnEntries, cdr.shdr.StrTabLen, cdr.shdr.ArgsLen)
+	}
+
+	// Read string table and args.
+	if err := cdr.readStringTable(); err != nil {
+		return err
+	}
+	if err := cdr.readArgs(); err != nil {
+		return err
+	}
+	// Seek past any padding to bring us up to a 4-byte boundary.
+	if of, err := cdr.mr.Seek(0, io.SeekCurrent); err != nil {
+		return err
+	} else {
+		rem := of % 4
+		if rem != 0 {
+			pad := 4 - rem
+			if _, err := cdr.mr.Seek(pad, io.SeekCurrent); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (cdr *CounterDataReader) readStringTable() error {
+	b := make([]byte, cdr.shdr.StrTabLen)
+	nr, err := cdr.mr.Read(b)
+	if err != nil {
+		return err
+	}
+	if nr != int(cdr.shdr.StrTabLen) {
+		return fmt.Errorf("error: short read on string table")
+	}
+	slr := slicereader.NewReader(b, false /* not readonly */)
+	cdr.stab = stringtab.NewReader(slr)
+	cdr.stab.Read()
+	return nil
+}
+
+func (cdr *CounterDataReader) readArgs() error {
+	b := make([]byte, cdr.shdr.ArgsLen)
+	nr, err := cdr.mr.Read(b)
+	if err != nil {
+		return err
+	}
+	if nr != int(cdr.shdr.ArgsLen) {
+		return fmt.Errorf("error: short read on args table")
+	}
+	slr := slicereader.NewReader(b, false /* not readonly */)
+	sget := func() (string, error) {
+		kidx := slr.ReadULEB128()
+		if int(kidx) >= cdr.stab.Entries() {
+			return "", fmt.Errorf("malformed string table ref")
+		}
+		return cdr.stab.Get(uint32(kidx)), nil
+	}
+	nents := slr.ReadULEB128()
+	cdr.args = make(map[string]string, int(nents))
+	for i := uint64(0); i < nents; i++ {
+		k, errk := sget()
+		if errk != nil {
+			return errk
+		}
+		v, errv := sget()
+		if errv != nil {
+			return errv
+		}
+		if _, ok := cdr.args[k]; ok {
+			return fmt.Errorf("malformed args table")
+		}
+		cdr.args[k] = v
+	}
+	if argcs, ok := cdr.args["argc"]; ok {
+		argc, err := strconv.Atoi(argcs)
+		if err != nil {
+			return fmt.Errorf("malformed argc in counter data file args section")
+		}
+		cdr.osargs = make([]string, 0, argc)
+		for i := 0; i < argc; i++ {
+			arg := cdr.args[fmt.Sprintf("argv%d", i)]
+			cdr.osargs = append(cdr.osargs, arg)
+		}
+	}
+	if goos, ok := cdr.args["GOOS"]; ok {
+		cdr.goos = goos
+	}
+	if goarch, ok := cdr.args["GOARCH"]; ok {
+		cdr.goarch = goarch
+	}
+	return nil
+}
+
+// OsArgs returns the program arguments (saved from os.Args during
+// the run of the instrumented binary) read from the counter
+// data file. Not all coverage data files will have os.Args values;
+// for example, if a data file is produced by merging coverage
+// data from two distinct runs, no os args will be available (an
+// empty list is returned).
+func (cdr *CounterDataReader) OsArgs() []string {
+	return cdr.osargs
+}
+
+// Goos returns the GOOS setting in effect for the "-cover" binary
+// that produced this counter data file. The GOOS value may be
+// empty in the case where the counter data file was produced
+// from a merge in which more than one GOOS value was present.
+func (cdr *CounterDataReader) Goos() string {
+	return cdr.goos
+}
+
+// Goarch returns the GOARCH setting in effect for the "-cover" binary
+// that produced this counter data file. The GOARCH value may be
+// empty in the case where the counter data file was produced
+// from a merge in which more than one GOARCH value was present.
+func (cdr *CounterDataReader) Goarch() string {
+	return cdr.goarch
+}
+
+// FuncPayload encapsulates the counter data payload for a single
+// function as read from a counter data file.
+type FuncPayload struct {
+	PkgIdx   uint32
+	FuncIdx  uint32
+	Counters []uint32
+}
+
+// NumSegments returns the number of execution segments in the file.
+func (cdr *CounterDataReader) NumSegments() uint32 {
+	return cdr.ftr.NumSegments
+}
+
+// BeginNextSegment sets up the the reader to read the next segment,
+// returning TRUE if we do have another segment to read, or FALSE
+// if we're done with all the segments (also an error if
+// something went wrong).
+func (cdr *CounterDataReader) BeginNextSegment() (bool, error) {
+	if cdr.segCount >= cdr.ftr.NumSegments {
+		return false, nil
+	}
+	cdr.segCount++
+	cdr.fcnCount = 0
+	// Seek past footer from last segment.
+	ftrSize := int64(unsafe.Sizeof(cdr.ftr))
+	if _, err := cdr.mr.Seek(ftrSize, io.SeekCurrent); err != nil {
+		return false, err
+	}
+	// Read preamble for this segment.
+	if err := cdr.readSegmentPreamble(); err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+// NumFunctionsInSegment returns the number of live functions
+// in the currently selected segment.
+func (cdr *CounterDataReader) NumFunctionsInSegment() uint32 {
+	return uint32(cdr.shdr.FcnEntries)
+}
+
+const supportDeadFunctionsInCounterData = false
+
+// NextFunc reads data for the next function in this current segment
+// into "p", returning TRUE if the read was successful or FALSE
+// if we've read all the functions already (also an error if
+// something went wrong with the read or we hit a premature
+// EOF).
+func (cdr *CounterDataReader) NextFunc(p *FuncPayload) (bool, error) {
+	if cdr.fcnCount >= uint32(cdr.shdr.FcnEntries) {
+		return false, nil
+	}
+	cdr.fcnCount++
+	var rdu32 func() (uint32, error)
+	if cdr.hdr.CFlavor == coverage.CtrULeb128 {
+		rdu32 = func() (uint32, error) {
+			var shift uint
+			var value uint64
+			for {
+				_, err := cdr.mr.Read(cdr.u8b)
+				if err != nil {
+					return 0, err
+				}
+				b := cdr.u8b[0]
+				value |= (uint64(b&0x7F) << shift)
+				if b&0x80 == 0 {
+					break
+				}
+				shift += 7
+			}
+			return uint32(value), nil
+		}
+	} else if cdr.hdr.CFlavor == coverage.CtrRaw {
+		if cdr.hdr.BigEndian {
+			rdu32 = func() (uint32, error) {
+				n, err := cdr.mr.Read(cdr.u32b)
+				if err != nil {
+					return 0, err
+				}
+				if n != 4 {
+					return 0, io.EOF
+				}
+				return binary.BigEndian.Uint32(cdr.u32b), nil
+			}
+		} else {
+			rdu32 = func() (uint32, error) {
+				n, err := cdr.mr.Read(cdr.u32b)
+				if err != nil {
+					return 0, err
+				}
+				if n != 4 {
+					return 0, io.EOF
+				}
+				return binary.LittleEndian.Uint32(cdr.u32b), nil
+			}
+		}
+	} else {
+		panic("internal error: unknown counter flavor")
+	}
+
+	// Alternative/experimental path: one way we could handling writing
+	// out counter data would be to just memcpy the counter segment
+	// out to a file, meaning that a region in the counter memory
+	// corresponding to a dead (never-executed) function would just be
+	// zeroes. The code path below handles this case.
+	var nc uint32
+	var err error
+	if supportDeadFunctionsInCounterData {
+		for {
+			nc, err = rdu32()
+			if err == io.EOF {
+				return false, io.EOF
+			} else if err != nil {
+				break
+			}
+			if nc != 0 {
+				break
+			}
+		}
+	} else {
+		nc, err = rdu32()
+	}
+	if err != nil {
+		return false, err
+	}
+
+	// Read package and func indices.
+	p.PkgIdx, err = rdu32()
+	if err != nil {
+		return false, err
+	}
+	p.FuncIdx, err = rdu32()
+	if err != nil {
+		return false, err
+	}
+	if cap(p.Counters) < 1024 {
+		p.Counters = make([]uint32, 0, 1024)
+	}
+	p.Counters = p.Counters[:0]
+	for i := uint32(0); i < nc; i++ {
+		v, err := rdu32()
+		if err != nil {
+			return false, err
+		}
+		p.Counters = append(p.Counters, v)
+	}
+	return true, nil
+}
diff --git a/src/internal/coverage/decodemeta/decode.go b/src/internal/coverage/decodemeta/decode.go
new file mode 100644
index 0000000..71f1c56
--- /dev/null
+++ b/src/internal/coverage/decodemeta/decode.go
@@ -0,0 +1,128 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package decodemeta
+
+// This package contains APIs and helpers for decoding a single package's
+// meta data "blob" emitted by the compiler when coverage instrumentation
+// is turned on.
+
+import (
+	"encoding/binary"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/slicereader"
+	"internal/coverage/stringtab"
+	"os"
+)
+
+// See comments in the encodecovmeta package for details on the format.
+
+type CoverageMetaDataDecoder struct {
+	r      *slicereader.Reader
+	hdr    coverage.MetaSymbolHeader
+	strtab *stringtab.Reader
+	tmp    []byte
+	debug  bool
+}
+
+func NewCoverageMetaDataDecoder(b []byte, readonly bool) (*CoverageMetaDataDecoder, error) {
+	slr := slicereader.NewReader(b, readonly)
+	x := &CoverageMetaDataDecoder{
+		r:   slr,
+		tmp: make([]byte, 0, 256),
+	}
+	if err := x.readHeader(); err != nil {
+		return nil, err
+	}
+	if err := x.readStringTable(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+func (d *CoverageMetaDataDecoder) readHeader() error {
+	if err := binary.Read(d.r, binary.LittleEndian, &d.hdr); err != nil {
+		return err
+	}
+	if d.debug {
+		fmt.Fprintf(os.Stderr, "=-= after readHeader: %+v\n", d.hdr)
+	}
+	return nil
+}
+
+func (d *CoverageMetaDataDecoder) readStringTable() error {
+	// Seek to the correct location to read the string table.
+	stringTableLocation := int64(coverage.CovMetaHeaderSize + 4*d.hdr.NumFuncs)
+	d.r.SeekTo(stringTableLocation)
+
+	// Read the table itself.
+	d.strtab = stringtab.NewReader(d.r)
+	d.strtab.Read()
+	return nil
+}
+
+func (d *CoverageMetaDataDecoder) PackagePath() string {
+	return d.strtab.Get(d.hdr.PkgPath)
+}
+
+func (d *CoverageMetaDataDecoder) PackageName() string {
+	return d.strtab.Get(d.hdr.PkgName)
+}
+
+func (d *CoverageMetaDataDecoder) ModulePath() string {
+	return d.strtab.Get(d.hdr.ModulePath)
+}
+
+func (d *CoverageMetaDataDecoder) NumFuncs() uint32 {
+	return d.hdr.NumFuncs
+}
+
+// ReadFunc reads the coverage meta-data for the function with index
+// 'findex', filling it into the FuncDesc pointed to by 'f'.
+func (d *CoverageMetaDataDecoder) ReadFunc(fidx uint32, f *coverage.FuncDesc) error {
+	if fidx >= d.hdr.NumFuncs {
+		return fmt.Errorf("illegal function index")
+	}
+
+	// Seek to the correct location to read the function offset and read it.
+	funcOffsetLocation := int64(coverage.CovMetaHeaderSize + 4*fidx)
+	d.r.SeekTo(funcOffsetLocation)
+	foff := d.r.ReadUint32()
+
+	// Check assumptions
+	if foff < uint32(funcOffsetLocation) || foff > d.hdr.Length {
+		return fmt.Errorf("malformed func offset %d", foff)
+	}
+
+	// Seek to the correct location to read the function.
+	d.r.SeekTo(int64(foff))
+
+	// Preamble containing number of units, file, and function.
+	numUnits := uint32(d.r.ReadULEB128())
+	fnameidx := uint32(d.r.ReadULEB128())
+	fileidx := uint32(d.r.ReadULEB128())
+
+	f.Srcfile = d.strtab.Get(fileidx)
+	f.Funcname = d.strtab.Get(fnameidx)
+
+	// Now the units
+	f.Units = f.Units[:0]
+	if cap(f.Units) < int(numUnits) {
+		f.Units = make([]coverage.CoverableUnit, 0, numUnits)
+	}
+	for k := uint32(0); k < numUnits; k++ {
+		f.Units = append(f.Units,
+			coverage.CoverableUnit{
+				StLine:  uint32(d.r.ReadULEB128()),
+				StCol:   uint32(d.r.ReadULEB128()),
+				EnLine:  uint32(d.r.ReadULEB128()),
+				EnCol:   uint32(d.r.ReadULEB128()),
+				NxStmts: uint32(d.r.ReadULEB128()),
+			})
+	}
+	lit := d.r.ReadULEB128()
+	f.Lit = lit != 0
+	return nil
+}
diff --git a/src/internal/coverage/decodemeta/decodefile.go b/src/internal/coverage/decodemeta/decodefile.go
new file mode 100644
index 0000000..6580dd5
--- /dev/null
+++ b/src/internal/coverage/decodemeta/decodefile.go
@@ -0,0 +1,223 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package decodemeta
+
+// This package contains APIs and helpers for reading and decoding
+// meta-data output files emitted by the runtime when a
+// coverage-instrumented binary executes. A meta-data file contains
+// top-level info (counter mode, number of packages) and then a
+// separate self-contained meta-data section for each Go package.
+
+import (
+	"bufio"
+	"crypto/md5"
+	"encoding/binary"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/slicereader"
+	"internal/coverage/stringtab"
+	"io"
+	"os"
+)
+
+// CoverageMetaFileReader provides state and methods for reading
+// a meta-data file from a code coverage run.
+type CoverageMetaFileReader struct {
+	f          *os.File
+	hdr        coverage.MetaFileHeader
+	tmp        []byte
+	pkgOffsets []uint64
+	pkgLengths []uint64
+	strtab     *stringtab.Reader
+	fileRdr    *bufio.Reader
+	fileView   []byte
+	debug      bool
+}
+
+// NewCoverageMetaFileReader returns a new helper object for reading
+// the coverage meta-data output file 'f'. The param 'fileView' is a
+// read-only slice containing the contents of 'f' obtained by mmap'ing
+// the file read-only; 'fileView' may be nil, in which case the helper
+// will read the contents of the file using regular file Read
+// operations.
+func NewCoverageMetaFileReader(f *os.File, fileView []byte) (*CoverageMetaFileReader, error) {
+	r := &CoverageMetaFileReader{
+		f:        f,
+		fileView: fileView,
+		tmp:      make([]byte, 256),
+	}
+
+	if err := r.readFileHeader(); err != nil {
+		return nil, err
+	}
+	return r, nil
+}
+
+func (r *CoverageMetaFileReader) readFileHeader() error {
+	var err error
+
+	r.fileRdr = bufio.NewReader(r.f)
+
+	// Read file header.
+	if err := binary.Read(r.fileRdr, binary.LittleEndian, &r.hdr); err != nil {
+		return err
+	}
+
+	// Verify magic string
+	m := r.hdr.Magic
+	g := coverage.CovMetaMagic
+	if m[0] != g[0] || m[1] != g[1] || m[2] != g[2] || m[3] != g[3] {
+		return fmt.Errorf("invalid meta-data file magic string")
+	}
+
+	// Vet the version. If this is a meta-data file from the future,
+	// we won't be able to read it.
+	if r.hdr.Version > coverage.MetaFileVersion {
+		return fmt.Errorf("meta-data file withn unknown version %d (expected %d)", r.hdr.Version, coverage.MetaFileVersion)
+	}
+
+	// Read package offsets for good measure
+	r.pkgOffsets = make([]uint64, r.hdr.Entries)
+	for i := uint64(0); i < r.hdr.Entries; i++ {
+		if r.pkgOffsets[i], err = r.rdUint64(); err != nil {
+			return err
+		}
+		if r.pkgOffsets[i] > r.hdr.TotalLength {
+			return fmt.Errorf("insane pkg offset %d: %d > totlen %d",
+				i, r.pkgOffsets[i], r.hdr.TotalLength)
+		}
+	}
+	r.pkgLengths = make([]uint64, r.hdr.Entries)
+	for i := uint64(0); i < r.hdr.Entries; i++ {
+		if r.pkgLengths[i], err = r.rdUint64(); err != nil {
+			return err
+		}
+		if r.pkgLengths[i] > r.hdr.TotalLength {
+			return fmt.Errorf("insane pkg length %d: %d > totlen %d",
+				i, r.pkgLengths[i], r.hdr.TotalLength)
+		}
+	}
+
+	// Read string table.
+	b := make([]byte, r.hdr.StrTabLength)
+	nr, err := r.fileRdr.Read(b)
+	if err != nil {
+		return err
+	}
+	if nr != int(r.hdr.StrTabLength) {
+		return fmt.Errorf("error: short read on string table")
+	}
+	slr := slicereader.NewReader(b, false /* not readonly */)
+	r.strtab = stringtab.NewReader(slr)
+	r.strtab.Read()
+
+	if r.debug {
+		fmt.Fprintf(os.Stderr, "=-= read-in header is: %+v\n", *r)
+	}
+
+	return nil
+}
+
+func (r *CoverageMetaFileReader) rdUint64() (uint64, error) {
+	r.tmp = r.tmp[:0]
+	r.tmp = append(r.tmp, make([]byte, 8)...)
+	n, err := r.fileRdr.Read(r.tmp)
+	if err != nil {
+		return 0, err
+	}
+	if n != 8 {
+		return 0, fmt.Errorf("premature end of file on read")
+	}
+	v := binary.LittleEndian.Uint64(r.tmp)
+	return v, nil
+}
+
+// NumPackages returns the number of packages for which this file
+// contains meta-data.
+func (r *CoverageMetaFileReader) NumPackages() uint64 {
+	return r.hdr.Entries
+}
+
+// CounterMode returns the counter mode (set, count, atomic) used
+// when building for coverage for the program that produce this
+// meta-data file.
+func (r *CoverageMetaFileReader) CounterMode() coverage.CounterMode {
+	return r.hdr.CMode
+}
+
+// CounterMode returns the counter granularity (single counter per
+// function, or counter per block) selected when building for coverage
+// for the program that produce this meta-data file.
+func (r *CoverageMetaFileReader) CounterGranularity() coverage.CounterGranularity {
+	return r.hdr.CGranularity
+}
+
+// FileHash returns the hash computed for all of the package meta-data
+// blobs. Coverage counter data files refer to this hash, and the
+// hash will be encoded into the meta-data file name.
+func (r *CoverageMetaFileReader) FileHash() [16]byte {
+	return r.hdr.MetaFileHash
+}
+
+// GetPackageDecoder requests a decoder object for the package within
+// the meta-data file whose index is 'pkIdx'. If the
+// CoverageMetaFileReader was set up with a read-only file view, a
+// pointer into that file view will be returned, otherwise the buffer
+// 'payloadbuf' will be written to (or if it is not of sufficient
+// size, a new buffer will be allocated). Return value is the decoder,
+// a byte slice with the encoded meta-data, and an error.
+func (r *CoverageMetaFileReader) GetPackageDecoder(pkIdx uint32, payloadbuf []byte) (*CoverageMetaDataDecoder, []byte, error) {
+	pp, err := r.GetPackagePayload(pkIdx, payloadbuf)
+	if r.debug {
+		fmt.Fprintf(os.Stderr, "=-= pkidx=%d payload length is %d hash=%s\n",
+			pkIdx, len(pp), fmt.Sprintf("%x", md5.Sum(pp)))
+	}
+	if err != nil {
+		return nil, nil, err
+	}
+	mdd, err := NewCoverageMetaDataDecoder(pp, r.fileView != nil)
+	if err != nil {
+		return nil, nil, err
+	}
+	return mdd, pp, nil
+}
+
+// GetPackagePayload returns the raw (encoded) meta-data payload for the
+// package with index 'pkIdx'. As with GetPackageDecoder, if the
+// CoverageMetaFileReader was set up with a read-only file view, a
+// pointer into that file view will be returned, otherwise the buffer
+// 'payloadbuf' will be written to (or if it is not of sufficient
+// size, a new buffer will be allocated). Return value is the decoder,
+// a byte slice with the encoded meta-data, and an error.
+func (r *CoverageMetaFileReader) GetPackagePayload(pkIdx uint32, payloadbuf []byte) ([]byte, error) {
+
+	// Determine correct offset/length.
+	if uint64(pkIdx) >= r.hdr.Entries {
+		return nil, fmt.Errorf("GetPackagePayload: illegal pkg index %d", pkIdx)
+	}
+	off := r.pkgOffsets[pkIdx]
+	len := r.pkgLengths[pkIdx]
+
+	if r.debug {
+		fmt.Fprintf(os.Stderr, "=-= for pk %d, off=%d len=%d\n", pkIdx, off, len)
+	}
+
+	if r.fileView != nil {
+		return r.fileView[off : off+len], nil
+	}
+
+	payload := payloadbuf[:0]
+	if cap(payload) < int(len) {
+		payload = make([]byte, 0, len)
+	}
+	payload = append(payload, make([]byte, len)...)
+	if _, err := r.f.Seek(int64(off), io.SeekStart); err != nil {
+		return nil, err
+	}
+	if _, err := io.ReadFull(r.f, payload); err != nil {
+		return nil, err
+	}
+	return payload, nil
+}
diff --git a/src/internal/coverage/defs.go b/src/internal/coverage/defs.go
new file mode 100644
index 0000000..4a41f57
--- /dev/null
+++ b/src/internal/coverage/defs.go
@@ -0,0 +1,374 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package coverage
+
+// Types and constants related to the output files files written
+// by code coverage tooling. When a coverage-instrumented binary
+// is run, it emits two output files: a meta-data output file, and
+// a counter data output file.
+
+//.....................................................................
+//
+// Meta-data definitions:
+//
+// The meta-data file is composed of a file header, a series of
+// meta-data blobs/sections (one per instrumented package), and an offsets
+// area storing the offsets of each section. Format of the meta-data
+// file looks like:
+//
+// --header----------
+//  | magic: [4]byte magic string
+//  | version
+//  | total length of meta-data file in bytes
+//  | numPkgs: number of package entries in file
+//  | hash: [16]byte hash of entire meta-data payload
+//  | offset to string table section
+//  | length of string table
+//  | number of entries in string table
+//  | counter mode
+//  | counter granularity
+//  --package offsets table------
+//  <offset to pkg 0>
+//  <offset to pkg 1>
+//  ...
+//  --package lengths table------
+//  <length of pkg 0>
+//  <length of pkg 1>
+//  ...
+//  --string table------
+//  <uleb128 len> 8
+//  <data> "somestring"
+//  ...
+//  --package payloads------
+//  <meta-symbol for pkg 0>
+//  <meta-symbol for pkg 1>
+//  ...
+//
+// Each package payload is a stand-alone blob emitted by the compiler,
+// and does not depend on anything else in the meta-data file. In
+// particular, each blob has it's own string table. Note that the
+// file-level string table is expected to be very short (most strings
+// will be in the meta-data blobs themselves).
+
+// CovMetaMagic holds the magic string for a meta-data file.
+var CovMetaMagic = [4]byte{'\x00', '\x63', '\x76', '\x6d'}
+
+// MetaFilePref is a prefix used when emitting meta-data files; these
+// files are of the form "covmeta.<hash>", where hash is a hash
+// computed from the hashes of all the package meta-data symbols in
+// the program.
+const MetaFilePref = "covmeta"
+
+// MetaFileVersion contains the current (most recent) meta-data file version.
+const MetaFileVersion = 1
+
+// MetaFileHeader stores file header information for a meta-data file.
+type MetaFileHeader struct {
+	Magic        [4]byte
+	Version      uint32
+	TotalLength  uint64
+	Entries      uint64
+	MetaFileHash [16]byte
+	StrTabOffset uint32
+	StrTabLength uint32
+	CMode        CounterMode
+	CGranularity CounterGranularity
+	_            [6]byte // padding
+}
+
+// MetaSymbolHeader stores header information for a single
+// meta-data blob, e.g. the coverage meta-data payload
+// computed for a given Go package.
+type MetaSymbolHeader struct {
+	Length     uint32 // size of meta-symbol payload in bytes
+	PkgName    uint32 // string table index
+	PkgPath    uint32 // string table index
+	ModulePath uint32 // string table index
+	MetaHash   [16]byte
+	_          byte    // currently unused
+	_          [3]byte // padding
+	NumFiles   uint32
+	NumFuncs   uint32
+}
+
+const CovMetaHeaderSize = 16 + 4 + 4 + 4 + 4 + 4 + 4 + 4 // keep in sync with above
+
+// As an example, consider the following Go package:
+//
+// 01: package p
+// 02:
+// 03: var v, w, z int
+// 04:
+// 05: func small(x, y int) int {
+// 06:   v++
+// 07:   // comment
+// 08:   if y == 0 {
+// 09:     return x
+// 10:   }
+// 11:   return (x << 1) ^ (9 / y)
+// 12: }
+// 13:
+// 14: func Medium(q, r int) int {
+// 15:   s1 := small(q, r)
+// 16:   z += s1
+// 17:   s2 := small(r, q)
+// 18:   w -= s2
+// 19:   return w + z
+// 20: }
+//
+// The meta-data blob for the single package above might look like the
+// following:
+//
+// -- MetaSymbolHeader header----------
+//  | size: size of this blob in bytes
+//  | packagepath: <path to p>
+//  | modulepath: <modpath for p>
+//  | nfiles: 1
+//  | nfunctions: 2
+//  --func offsets table------
+//  <offset to func 0>
+//  <offset to func 1>
+//  --string table (contains all files and functions)------
+//  | <uleb128 len> 4
+//  | <data> "p.go"
+//  | <uleb128 len> 5
+//  | <data> "small"
+//  | <uleb128 len> 6
+//  | <data> "Medium"
+//  --func 0------
+//  | <uleb128> num units: 3
+//  | <uleb128> func name: S1 (index into string table)
+//  | <uleb128> file: S0 (index into string table)
+//  | <unit 0>:  S0   L6     L8    2
+//  | <unit 1>:  S0   L9     L9    1
+//  | <unit 2>:  S0   L11    L11   1
+//  --func 1------
+//  | <uleb128> num units: 1
+//  | <uleb128> func name: S2 (index into string table)
+//  | <uleb128> file: S0 (index into string table)
+//  | <unit 0>:  S0   L15    L19   5
+//  ---end-----------
+
+// The following types and constants used by the meta-data encoder/decoder.
+
+// FuncDesc encapsulates the meta-data definitions for a single Go function.
+// This version assumes that we're looking at a function before inlining;
+// if we want to capture a post-inlining view of the world, the
+// representations of source positions would need to be a good deal more
+// complicated.
+type FuncDesc struct {
+	Funcname string
+	Srcfile  string
+	Units    []CoverableUnit
+	Lit      bool // true if this is a function literal
+}
+
+// CoverableUnit describes the source characteristics of a single
+// program unit for which we want to gather coverage info. Coverable
+// units are either "simple" or "intraline"; a "simple" coverable unit
+// corresponds to a basic block (region of straight-line code with no
+// jumps or control transfers). An "intraline" unit corresponds to a
+// logical clause nested within some other simple unit. A simple unit
+// will have a zero Parent value; for an intraline unit NxStmts will
+// be zero and and Parent will be set to 1 plus the index of the
+// containing simple statement. Example:
+//
+//	L7:   q := 1
+//	L8:   x := (y == 101 || launch() == false)
+//	L9:   r := x * 2
+//
+// For the code above we would have three simple units (one for each
+// line), then an intraline unit describing the "launch() == false"
+// clause in line 8, with Parent pointing to the index of the line 8
+// unit in the units array.
+//
+// Note: in the initial version of the coverage revamp, only simple
+// units will be in use.
+type CoverableUnit struct {
+	StLine, StCol uint32
+	EnLine, EnCol uint32
+	NxStmts       uint32
+	Parent        uint32
+}
+
+// CounterMode tracks the "flavor" of the coverage counters being
+// used in a given coverage-instrumented program.
+type CounterMode uint8
+
+const (
+	CtrModeInvalid  CounterMode = iota
+	CtrModeSet                  // "set" mode
+	CtrModeCount                // "count" mode
+	CtrModeAtomic               // "atomic" mode
+	CtrModeRegOnly              // registration-only pseudo-mode
+	CtrModeTestMain             // testmain pseudo-mode
+)
+
+func (cm CounterMode) String() string {
+	switch cm {
+	case CtrModeSet:
+		return "set"
+	case CtrModeCount:
+		return "count"
+	case CtrModeAtomic:
+		return "atomic"
+	case CtrModeRegOnly:
+		return "regonly"
+	case CtrModeTestMain:
+		return "testmain"
+	}
+	return "<invalid>"
+}
+
+func ParseCounterMode(mode string) CounterMode {
+	var cm CounterMode
+	switch mode {
+	case "set":
+		cm = CtrModeSet
+	case "count":
+		cm = CtrModeCount
+	case "atomic":
+		cm = CtrModeAtomic
+	case "regonly":
+		cm = CtrModeRegOnly
+	case "testmain":
+		cm = CtrModeTestMain
+	default:
+		cm = CtrModeInvalid
+	}
+	return cm
+}
+
+// CounterGranularity tracks the granularity of the coverage counters being
+// used in a given coverage-instrumented program.
+type CounterGranularity uint8
+
+const (
+	CtrGranularityInvalid CounterGranularity = iota
+	CtrGranularityPerBlock
+	CtrGranularityPerFunc
+)
+
+func (cm CounterGranularity) String() string {
+	switch cm {
+	case CtrGranularityPerBlock:
+		return "perblock"
+	case CtrGranularityPerFunc:
+		return "perfunc"
+	}
+	return "<invalid>"
+}
+
+//.....................................................................
+//
+// Counter data definitions:
+//
+
+// A counter data file is composed of a file header followed by one or
+// more "segments" (each segment representing a given run or partial
+// run of a give binary) followed by a footer.
+
+// CovCounterMagic holds the magic string for a coverage counter-data file.
+var CovCounterMagic = [4]byte{'\x00', '\x63', '\x77', '\x6d'}
+
+// CounterFileVersion stores the most recent counter data file version.
+const CounterFileVersion = 1
+
+// CounterFileHeader stores files header information for a counter-data file.
+type CounterFileHeader struct {
+	Magic     [4]byte
+	Version   uint32
+	MetaHash  [16]byte
+	CFlavor   CounterFlavor
+	BigEndian bool
+	_         [6]byte // padding
+}
+
+// CounterSegmentHeader encapsulates information about a specific
+// segment in a counter data file, which at the moment contains
+// counters data from a single execution of a coverage-instrumented
+// program. Following the segment header will be the string table and
+// args table, and then (possibly) padding bytes to bring the byte
+// size of the preamble up to a multiple of 4. Immediately following
+// that will be the counter payloads.
+//
+// The "args" section of a segment is used to store annotations
+// describing where the counter data came from; this section is
+// basically a series of key-value pairs (can be thought of as an
+// encoded 'map[string]string'). At the moment we only write os.Args()
+// data to this section, using pairs of the form "argc=<integer>",
+// "argv0=<os.Args[0]>", "argv1=<os.Args[1]>", and so on. In the
+// future the args table may also include things like GOOS/GOARCH
+// values, and/or tags indicating which tests were run to generate the
+// counter data.
+type CounterSegmentHeader struct {
+	FcnEntries uint64
+	StrTabLen  uint32
+	ArgsLen    uint32
+}
+
+// CounterFileFooter appears at the tail end of a counter data file,
+// and stores the number of segments it contains.
+type CounterFileFooter struct {
+	Magic       [4]byte
+	_           [4]byte // padding
+	NumSegments uint32
+	_           [4]byte // padding
+}
+
+// CounterFilePref is the file prefix used when emitting coverage data
+// output files. CounterFileTemplate describes the format of the file
+// name: prefix followed by meta-file hash followed by process ID
+// followed by emit UnixNanoTime.
+const CounterFilePref = "covcounters"
+const CounterFileTempl = "%s.%x.%d.%d"
+const CounterFileRegexp = `^%s\.(\S+)\.(\d+)\.(\d+)+$`
+
+// CounterFlavor describes how function and counters are
+// stored/represented in the counter section of the file.
+type CounterFlavor uint8
+
+const (
+	// "Raw" representation: all values (pkg ID, func ID, num counters,
+	// and counters themselves) are stored as uint32's.
+	CtrRaw CounterFlavor = iota + 1
+
+	// "ULeb" representation: all values (pkg ID, func ID, num counters,
+	// and counters themselves) are stored with ULEB128 encoding.
+	CtrULeb128
+)
+
+func Round4(x int) int {
+	return (x + 3) &^ 3
+}
+
+//.....................................................................
+//
+// Runtime counter data definitions.
+//
+
+// At runtime within a coverage-instrumented program, the "counters"
+// object we associated with instrumented function can be thought of
+// as a struct of the following form:
+//
+// struct {
+//     numCtrs uint32
+//     pkgid uint32
+//     funcid uint32
+//     counterArray [numBlocks]uint32
+// }
+//
+// where "numCtrs" is the number of blocks / coverable units within the
+// function, "pkgid" is the unique index assigned to this package by
+// the runtime, "funcid" is the index of this function within its containing
+// package, and "counterArray" stores the actual counters.
+//
+// The counter variable itself is created not as a struct but as a flat
+// array of uint32's; we then use the offsets below to index into it.
+
+const NumCtrsOffset = 0
+const PkgIdOffset = 1
+const FuncIdOffset = 2
+const FirstCtrOffset = 3
diff --git a/src/internal/coverage/encodecounter/encode.go b/src/internal/coverage/encodecounter/encode.go
new file mode 100644
index 0000000..8db4f51
--- /dev/null
+++ b/src/internal/coverage/encodecounter/encode.go
@@ -0,0 +1,284 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package encodecounter
+
+import (
+	"bufio"
+	"encoding/binary"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/slicewriter"
+	"internal/coverage/stringtab"
+	"internal/coverage/uleb128"
+	"io"
+	"os"
+	"sort"
+)
+
+// This package contains APIs and helpers for encoding initial portions
+// of the counter data files emitted at runtime when coverage instrumentation
+// is enabled.  Counter data files may contain multiple segments; the file
+// header and first segment are written via the "Write" method below, and
+// additional segments can then be added using "AddSegment".
+
+type CoverageDataWriter struct {
+	stab    *stringtab.Writer
+	w       *bufio.Writer
+	tmp     []byte
+	cflavor coverage.CounterFlavor
+	segs    uint32
+	debug   bool
+}
+
+func NewCoverageDataWriter(w io.Writer, flav coverage.CounterFlavor) *CoverageDataWriter {
+	r := &CoverageDataWriter{
+		stab: &stringtab.Writer{},
+		w:    bufio.NewWriter(w),
+
+		tmp:     make([]byte, 64),
+		cflavor: flav,
+	}
+	r.stab.InitWriter()
+	r.stab.Lookup("")
+	return r
+}
+
+// CounterVisitor describes a helper object used during counter file
+// writing; when writing counter data files, clients pass a
+// CounterVisitor to the write/emit routines. The writers will then
+// first invoke the visitor's NumFuncs() method to find out how many
+// function's worth of data to write, then it will invoke VisitFuncs.
+// The expectation is that the VisitFuncs method will then invoke the
+// callback "f" with data for each function to emit to the file.
+type CounterVisitor interface {
+	NumFuncs() (int, error)
+	VisitFuncs(f CounterVisitorFn) error
+}
+
+// CounterVisitorFn describes a callback function invoked when writing
+// coverage counter data.
+type CounterVisitorFn func(pkid uint32, funcid uint32, counters []uint32) error
+
+// Write writes the contents of the count-data file to the writer
+// previously supplied to NewCoverageDataWriter. Returns an error
+// if something went wrong somewhere with the write.
+func (cfw *CoverageDataWriter) Write(metaFileHash [16]byte, args map[string]string, visitor CounterVisitor) error {
+	if err := cfw.writeHeader(metaFileHash); err != nil {
+		return err
+	}
+	return cfw.AppendSegment(args, visitor)
+}
+
+func padToFourByteBoundary(ws *slicewriter.WriteSeeker) error {
+	sz := len(ws.BytesWritten())
+	zeros := []byte{0, 0, 0, 0}
+	rem := uint32(sz) % 4
+	if rem != 0 {
+		pad := zeros[:(4 - rem)]
+		if nw, err := ws.Write(pad); err != nil {
+			return err
+		} else if nw != len(pad) {
+			return fmt.Errorf("error: short write")
+		}
+	}
+	return nil
+}
+
+func (cfw *CoverageDataWriter) writeSegmentPreamble(args map[string]string, visitor CounterVisitor) error {
+	var csh coverage.CounterSegmentHeader
+	if nf, err := visitor.NumFuncs(); err != nil {
+		return err
+	} else {
+		csh.FcnEntries = uint64(nf)
+	}
+
+	// Write string table and args to a byte slice (since we need
+	// to capture offsets at various points), then emit the slice
+	// once we are done.
+	cfw.stab.Freeze()
+	ws := &slicewriter.WriteSeeker{}
+	if err := cfw.stab.Write(ws); err != nil {
+		return err
+	}
+	csh.StrTabLen = uint32(len(ws.BytesWritten()))
+
+	akeys := make([]string, 0, len(args))
+	for k := range args {
+		akeys = append(akeys, k)
+	}
+	sort.Strings(akeys)
+
+	wrULEB128 := func(v uint) error {
+		cfw.tmp = cfw.tmp[:0]
+		cfw.tmp = uleb128.AppendUleb128(cfw.tmp, v)
+		if _, err := ws.Write(cfw.tmp); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	// Count of arg pairs.
+	if err := wrULEB128(uint(len(args))); err != nil {
+		return err
+	}
+	// Arg pairs themselves.
+	for _, k := range akeys {
+		ki := uint(cfw.stab.Lookup(k))
+		if err := wrULEB128(ki); err != nil {
+			return err
+		}
+		v := args[k]
+		vi := uint(cfw.stab.Lookup(v))
+		if err := wrULEB128(vi); err != nil {
+			return err
+		}
+	}
+	if err := padToFourByteBoundary(ws); err != nil {
+		return err
+	}
+	csh.ArgsLen = uint32(len(ws.BytesWritten())) - csh.StrTabLen
+
+	if cfw.debug {
+		fmt.Fprintf(os.Stderr, "=-= counter segment header: %+v", csh)
+		fmt.Fprintf(os.Stderr, " FcnEntries=0x%x StrTabLen=0x%x ArgsLen=0x%x\n",
+			csh.FcnEntries, csh.StrTabLen, csh.ArgsLen)
+	}
+
+	// At this point we can now do the actual write.
+	if err := binary.Write(cfw.w, binary.LittleEndian, csh); err != nil {
+		return err
+	}
+	if err := cfw.writeBytes(ws.BytesWritten()); err != nil {
+		return err
+	}
+	return nil
+}
+
+// AppendSegment appends a new segment to a counter data, with a new
+// args section followed by a payload of counter data clauses.
+func (cfw *CoverageDataWriter) AppendSegment(args map[string]string, visitor CounterVisitor) error {
+	cfw.stab = &stringtab.Writer{}
+	cfw.stab.InitWriter()
+	cfw.stab.Lookup("")
+
+	var err error
+	for k, v := range args {
+		cfw.stab.Lookup(k)
+		cfw.stab.Lookup(v)
+	}
+
+	if err = cfw.writeSegmentPreamble(args, visitor); err != nil {
+		return err
+	}
+	if err = cfw.writeCounters(visitor); err != nil {
+		return err
+	}
+	if err = cfw.writeFooter(); err != nil {
+		return err
+	}
+	if err := cfw.w.Flush(); err != nil {
+		return fmt.Errorf("write error: %v", err)
+	}
+	cfw.stab = nil
+	return nil
+}
+
+func (cfw *CoverageDataWriter) writeHeader(metaFileHash [16]byte) error {
+	// Emit file header.
+	ch := coverage.CounterFileHeader{
+		Magic:     coverage.CovCounterMagic,
+		Version:   coverage.CounterFileVersion,
+		MetaHash:  metaFileHash,
+		CFlavor:   cfw.cflavor,
+		BigEndian: false,
+	}
+	if err := binary.Write(cfw.w, binary.LittleEndian, ch); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (cfw *CoverageDataWriter) writeBytes(b []byte) error {
+	if len(b) == 0 {
+		return nil
+	}
+	nw, err := cfw.w.Write(b)
+	if err != nil {
+		return fmt.Errorf("error writing counter data: %v", err)
+	}
+	if len(b) != nw {
+		return fmt.Errorf("error writing counter data: short write")
+	}
+	return nil
+}
+
+func (cfw *CoverageDataWriter) writeCounters(visitor CounterVisitor) error {
+	// Notes:
+	// - this version writes everything little-endian, which means
+	//   a call is needed to encode every value (expensive)
+	// - we may want to move to a model in which we just blast out
+	//   all counters, or possibly mmap the file and do the write
+	//   implicitly.
+	ctrb := make([]byte, 4)
+	wrval := func(val uint32) error {
+		var buf []byte
+		var towr int
+		if cfw.cflavor == coverage.CtrRaw {
+			binary.LittleEndian.PutUint32(ctrb, val)
+			buf = ctrb
+			towr = 4
+		} else if cfw.cflavor == coverage.CtrULeb128 {
+			cfw.tmp = cfw.tmp[:0]
+			cfw.tmp = uleb128.AppendUleb128(cfw.tmp, uint(val))
+			buf = cfw.tmp
+			towr = len(buf)
+		} else {
+			panic("internal error: bad counter flavor")
+		}
+		if sz, err := cfw.w.Write(buf); err != nil {
+			return err
+		} else if sz != towr {
+			return fmt.Errorf("writing counters: short write")
+		}
+		return nil
+	}
+
+	// Write out entries for each live function.
+	emitter := func(pkid uint32, funcid uint32, counters []uint32) error {
+		if err := wrval(uint32(len(counters))); err != nil {
+			return err
+		}
+
+		if err := wrval(pkid); err != nil {
+			return err
+		}
+
+		if err := wrval(funcid); err != nil {
+			return err
+		}
+		for _, val := range counters {
+			if err := wrval(val); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	if err := visitor.VisitFuncs(emitter); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (cfw *CoverageDataWriter) writeFooter() error {
+	cfw.segs++
+	cf := coverage.CounterFileFooter{
+		Magic:       coverage.CovCounterMagic,
+		NumSegments: cfw.segs,
+	}
+	if err := binary.Write(cfw.w, binary.LittleEndian, cf); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/src/internal/coverage/encodemeta/encode.go b/src/internal/coverage/encodemeta/encode.go
new file mode 100644
index 0000000..d211c7c
--- /dev/null
+++ b/src/internal/coverage/encodemeta/encode.go
@@ -0,0 +1,215 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package encodemeta
+
+// This package contains APIs and helpers for encoding the meta-data
+// "blob" for a single Go package, created when coverage
+// instrumentation is turned on.
+
+import (
+	"bytes"
+	"crypto/md5"
+	"encoding/binary"
+	"fmt"
+	"hash"
+	"internal/coverage"
+	"internal/coverage/stringtab"
+	"internal/coverage/uleb128"
+	"io"
+	"os"
+)
+
+type CoverageMetaDataBuilder struct {
+	stab    stringtab.Writer
+	funcs   []funcDesc
+	tmp     []byte // temp work slice
+	h       hash.Hash
+	pkgpath uint32
+	pkgname uint32
+	modpath uint32
+	debug   bool
+	werr    error
+}
+
+func NewCoverageMetaDataBuilder(pkgpath string, pkgname string, modulepath string) (*CoverageMetaDataBuilder, error) {
+	if pkgpath == "" {
+		return nil, fmt.Errorf("invalid empty package path")
+	}
+	x := &CoverageMetaDataBuilder{
+		tmp: make([]byte, 0, 256),
+		h:   md5.New(),
+	}
+	x.stab.InitWriter()
+	x.stab.Lookup("")
+	x.pkgpath = x.stab.Lookup(pkgpath)
+	x.pkgname = x.stab.Lookup(pkgname)
+	x.modpath = x.stab.Lookup(modulepath)
+	io.WriteString(x.h, pkgpath)
+	io.WriteString(x.h, pkgname)
+	io.WriteString(x.h, modulepath)
+	return x, nil
+}
+
+func h32(x uint32, h hash.Hash, tmp []byte) {
+	tmp = tmp[:0]
+	tmp = append(tmp, []byte{0, 0, 0, 0}...)
+	binary.LittleEndian.PutUint32(tmp, x)
+	h.Write(tmp)
+}
+
+type funcDesc struct {
+	encoded []byte
+}
+
+// AddFunc registers a new function with the meta data builder.
+func (b *CoverageMetaDataBuilder) AddFunc(f coverage.FuncDesc) uint {
+	hashFuncDesc(b.h, &f, b.tmp)
+	fd := funcDesc{}
+	b.tmp = b.tmp[:0]
+	b.tmp = uleb128.AppendUleb128(b.tmp, uint(len(f.Units)))
+	b.tmp = uleb128.AppendUleb128(b.tmp, uint(b.stab.Lookup(f.Funcname)))
+	b.tmp = uleb128.AppendUleb128(b.tmp, uint(b.stab.Lookup(f.Srcfile)))
+	for _, u := range f.Units {
+		b.tmp = uleb128.AppendUleb128(b.tmp, uint(u.StLine))
+		b.tmp = uleb128.AppendUleb128(b.tmp, uint(u.StCol))
+		b.tmp = uleb128.AppendUleb128(b.tmp, uint(u.EnLine))
+		b.tmp = uleb128.AppendUleb128(b.tmp, uint(u.EnCol))
+		b.tmp = uleb128.AppendUleb128(b.tmp, uint(u.NxStmts))
+	}
+	lit := uint(0)
+	if f.Lit {
+		lit = 1
+	}
+	b.tmp = uleb128.AppendUleb128(b.tmp, lit)
+	fd.encoded = bytes.Clone(b.tmp)
+	rv := uint(len(b.funcs))
+	b.funcs = append(b.funcs, fd)
+	return rv
+}
+
+func (b *CoverageMetaDataBuilder) emitFuncOffsets(w io.WriteSeeker, off int64) int64 {
+	nFuncs := len(b.funcs)
+	var foff int64 = coverage.CovMetaHeaderSize + int64(b.stab.Size()) + int64(nFuncs)*4
+	for idx := 0; idx < nFuncs; idx++ {
+		b.wrUint32(w, uint32(foff))
+		foff += int64(len(b.funcs[idx].encoded))
+	}
+	return off + (int64(len(b.funcs)) * 4)
+}
+
+func (b *CoverageMetaDataBuilder) emitFunc(w io.WriteSeeker, off int64, f funcDesc) (int64, error) {
+	ew := len(f.encoded)
+	if nw, err := w.Write(f.encoded); err != nil {
+		return 0, err
+	} else if ew != nw {
+		return 0, fmt.Errorf("short write emitting coverage meta-data")
+	}
+	return off + int64(ew), nil
+}
+
+func (b *CoverageMetaDataBuilder) reportWriteError(err error) {
+	if b.werr != nil {
+		b.werr = err
+	}
+}
+
+func (b *CoverageMetaDataBuilder) wrUint32(w io.WriteSeeker, v uint32) {
+	b.tmp = b.tmp[:0]
+	b.tmp = append(b.tmp, []byte{0, 0, 0, 0}...)
+	binary.LittleEndian.PutUint32(b.tmp, v)
+	if nw, err := w.Write(b.tmp); err != nil {
+		b.reportWriteError(err)
+	} else if nw != 4 {
+		b.reportWriteError(fmt.Errorf("short write"))
+	}
+}
+
+// Emit writes the meta-data accumulated so far in this builder to 'w'.
+// Returns a hash of the meta-data payload and an error.
+func (b *CoverageMetaDataBuilder) Emit(w io.WriteSeeker) ([16]byte, error) {
+	// Emit header.  Length will initially be zero, we'll
+	// back-patch it later.
+	var digest [16]byte
+	copy(digest[:], b.h.Sum(nil))
+	mh := coverage.MetaSymbolHeader{
+		// hash and length initially zero, will be back-patched
+		PkgPath:    uint32(b.pkgpath),
+		PkgName:    uint32(b.pkgname),
+		ModulePath: uint32(b.modpath),
+		NumFiles:   uint32(b.stab.Nentries()),
+		NumFuncs:   uint32(len(b.funcs)),
+		MetaHash:   digest,
+	}
+	if b.debug {
+		fmt.Fprintf(os.Stderr, "=-= writing header: %+v\n", mh)
+	}
+	if err := binary.Write(w, binary.LittleEndian, mh); err != nil {
+		return digest, fmt.Errorf("error writing meta-file header: %v", err)
+	}
+	off := int64(coverage.CovMetaHeaderSize)
+
+	// Write function offsets section
+	off = b.emitFuncOffsets(w, off)
+
+	// Check for any errors up to this point.
+	if b.werr != nil {
+		return digest, b.werr
+	}
+
+	// Write string table.
+	if err := b.stab.Write(w); err != nil {
+		return digest, err
+	}
+	off += int64(b.stab.Size())
+
+	// Write functions
+	for _, f := range b.funcs {
+		var err error
+		off, err = b.emitFunc(w, off, f)
+		if err != nil {
+			return digest, err
+		}
+	}
+
+	// Back-patch the length.
+	totalLength := uint32(off)
+	if _, err := w.Seek(0, io.SeekStart); err != nil {
+		return digest, err
+	}
+	b.wrUint32(w, totalLength)
+	if b.werr != nil {
+		return digest, b.werr
+	}
+	return digest, nil
+}
+
+// HashFuncDesc computes an md5 sum of a coverage.FuncDesc and returns
+// a digest for it.
+func HashFuncDesc(f *coverage.FuncDesc) [16]byte {
+	h := md5.New()
+	tmp := make([]byte, 0, 32)
+	hashFuncDesc(h, f, tmp)
+	var r [16]byte
+	copy(r[:], h.Sum(nil))
+	return r
+}
+
+// hashFuncDesc incorporates a given function 'f' into the hash 'h'.
+func hashFuncDesc(h hash.Hash, f *coverage.FuncDesc, tmp []byte) {
+	io.WriteString(h, f.Funcname)
+	io.WriteString(h, f.Srcfile)
+	for _, u := range f.Units {
+		h32(u.StLine, h, tmp)
+		h32(u.StCol, h, tmp)
+		h32(u.EnLine, h, tmp)
+		h32(u.EnCol, h, tmp)
+		h32(u.NxStmts, h, tmp)
+	}
+	lit := uint32(0)
+	if f.Lit {
+		lit = 1
+	}
+	h32(lit, h, tmp)
+}
diff --git a/src/internal/coverage/encodemeta/encodefile.go b/src/internal/coverage/encodemeta/encodefile.go
new file mode 100644
index 0000000..38ae46e
--- /dev/null
+++ b/src/internal/coverage/encodemeta/encodefile.go
@@ -0,0 +1,132 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package encodemeta
+
+import (
+	"bufio"
+	"crypto/md5"
+	"encoding/binary"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/stringtab"
+	"io"
+	"os"
+	"unsafe"
+)
+
+// This package contains APIs and helpers for writing out a meta-data
+// file (composed of a file header, offsets/lengths, and then a series of
+// meta-data blobs emitted by the compiler, one per Go package).
+
+type CoverageMetaFileWriter struct {
+	stab   stringtab.Writer
+	mfname string
+	w      *bufio.Writer
+	tmp    []byte
+	debug  bool
+}
+
+func NewCoverageMetaFileWriter(mfname string, w io.Writer) *CoverageMetaFileWriter {
+	r := &CoverageMetaFileWriter{
+		mfname: mfname,
+		w:      bufio.NewWriter(w),
+		tmp:    make([]byte, 64),
+	}
+	r.stab.InitWriter()
+	r.stab.Lookup("")
+	return r
+}
+
+func (m *CoverageMetaFileWriter) Write(finalHash [16]byte, blobs [][]byte, mode coverage.CounterMode, granularity coverage.CounterGranularity) error {
+	mhsz := uint64(unsafe.Sizeof(coverage.MetaFileHeader{}))
+	stSize := m.stab.Size()
+	stOffset := mhsz + uint64(16*len(blobs))
+	preambleLength := stOffset + uint64(stSize)
+
+	if m.debug {
+		fmt.Fprintf(os.Stderr, "=+= sizeof(MetaFileHeader)=%d\n", mhsz)
+		fmt.Fprintf(os.Stderr, "=+= preambleLength=%d stSize=%d\n", preambleLength, stSize)
+	}
+
+	// Compute total size
+	tlen := preambleLength
+	for i := 0; i < len(blobs); i++ {
+		tlen += uint64(len(blobs[i]))
+	}
+
+	// Emit header
+	mh := coverage.MetaFileHeader{
+		Magic:        coverage.CovMetaMagic,
+		Version:      coverage.MetaFileVersion,
+		TotalLength:  tlen,
+		Entries:      uint64(len(blobs)),
+		MetaFileHash: finalHash,
+		StrTabOffset: uint32(stOffset),
+		StrTabLength: stSize,
+		CMode:        mode,
+		CGranularity: granularity,
+	}
+	var err error
+	if err = binary.Write(m.w, binary.LittleEndian, mh); err != nil {
+		return fmt.Errorf("error writing %s: %v", m.mfname, err)
+	}
+
+	if m.debug {
+		fmt.Fprintf(os.Stderr, "=+= len(blobs) is %d\n", mh.Entries)
+	}
+
+	// Emit package offsets section followed by package lengths section.
+	off := preambleLength
+	off2 := mhsz
+	buf := make([]byte, 8)
+	for _, blob := range blobs {
+		binary.LittleEndian.PutUint64(buf, off)
+		if _, err = m.w.Write(buf); err != nil {
+			return fmt.Errorf("error writing %s: %v", m.mfname, err)
+		}
+		if m.debug {
+			fmt.Fprintf(os.Stderr, "=+= pkg offset %d 0x%x\n", off, off)
+		}
+		off += uint64(len(blob))
+		off2 += 8
+	}
+	for _, blob := range blobs {
+		bl := uint64(len(blob))
+		binary.LittleEndian.PutUint64(buf, bl)
+		if _, err = m.w.Write(buf); err != nil {
+			return fmt.Errorf("error writing %s: %v", m.mfname, err)
+		}
+		if m.debug {
+			fmt.Fprintf(os.Stderr, "=+= pkg len %d 0x%x\n", bl, bl)
+		}
+		off2 += 8
+	}
+
+	// Emit string table
+	if err = m.stab.Write(m.w); err != nil {
+		return err
+	}
+
+	// Now emit blobs themselves.
+	for k, blob := range blobs {
+		if m.debug {
+			fmt.Fprintf(os.Stderr, "=+= writing blob %d len %d at off=%d hash %s\n", k, len(blob), off2, fmt.Sprintf("%x", md5.Sum(blob)))
+		}
+		if _, err = m.w.Write(blob); err != nil {
+			return fmt.Errorf("error writing %s: %v", m.mfname, err)
+		}
+		if m.debug {
+			fmt.Fprintf(os.Stderr, "=+= wrote package payload of %d bytes\n",
+				len(blob))
+		}
+		off2 += uint64(len(blob))
+	}
+
+	// Flush writer, and we're done.
+	if err = m.w.Flush(); err != nil {
+		return fmt.Errorf("error writing %s: %v", m.mfname, err)
+	}
+	return nil
+}
diff --git a/src/internal/coverage/pkid.go b/src/internal/coverage/pkid.go
new file mode 100644
index 0000000..8ddd44d
--- /dev/null
+++ b/src/internal/coverage/pkid.go
@@ -0,0 +1,80 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package coverage
+
+// Building the runtime package with coverage instrumentation enabled
+// is tricky.  For all other packages, you can be guaranteed that
+// the package init function is run before any functions are executed,
+// but this invariant is not maintained for packages such as "runtime",
+// "internal/cpu", etc. To handle this, hard-code the package ID for
+// the set of packages whose functions may be running before the
+// init function of the package is complete.
+//
+// Hardcoding is unfortunate because it means that the tool that does
+// coverage instrumentation has to keep a list of runtime packages,
+// meaning that if someone makes changes to the pkg "runtime"
+// dependencies, unexpected behavior will result for coverage builds.
+// The coverage runtime will detect and report the unexpected
+// behavior; look for an error of this form:
+//
+//    internal error in coverage meta-data tracking:
+//    list of hard-coded runtime package IDs needs revising.
+//    registered list:
+//    slot: 0 path='internal/cpu'  hard-coded id: 1
+//    slot: 1 path='internal/goarch'  hard-coded id: 2
+//    slot: 2 path='runtime/internal/atomic'  hard-coded id: 3
+//    slot: 3 path='internal/goos'
+//    slot: 4 path='runtime/internal/sys'  hard-coded id: 5
+//    slot: 5 path='internal/abi'  hard-coded id: 4
+//    slot: 6 path='runtime/internal/math'  hard-coded id: 6
+//    slot: 7 path='internal/bytealg'  hard-coded id: 7
+//    slot: 8 path='internal/goexperiment'
+//    slot: 9 path='runtime/internal/syscall'  hard-coded id: 8
+//    slot: 10 path='runtime'  hard-coded id: 9
+//    fatal error: runtime.addCovMeta
+//
+// For the error above, the hard-coded list is missing "internal/goos"
+// and "internal/goexperiment" ; the developer in question will need
+// to copy the list above into "rtPkgs" below.
+//
+// Note: this strategy assumes that the list of dependencies of
+// package runtime is fixed, and doesn't vary depending on OS/arch. If
+// this were to be the case, we would need a table of some sort below
+// as opposed to a fixed list.
+
+var rtPkgs = [...]string{
+	"internal/cpu",
+	"internal/goarch",
+	"runtime/internal/atomic",
+	"internal/goos",
+	"runtime/internal/sys",
+	"internal/abi",
+	"runtime/internal/math",
+	"internal/bytealg",
+	"internal/goexperiment",
+	"runtime/internal/syscall",
+	"runtime",
+}
+
+// Scoping note: the constants and apis in this file are internal
+// only, not expected to ever be exposed outside of the runtime (unlike
+// other coverage file formats and APIs, which will likely be shared
+// at some point).
+
+// NotHardCoded is a package pseudo-ID indicating that a given package
+// is not part of the runtime and doesn't require a hard-coded ID.
+const NotHardCoded = -1
+
+// HardCodedPkgID returns the hard-coded ID for the specified package
+// path, or -1 if we don't use a hard-coded ID. Hard-coded IDs start
+// at -2 and decrease as we go down the list.
+func HardCodedPkgID(pkgpath string) int {
+	for k, p := range rtPkgs {
+		if p == pkgpath {
+			return (0 - k) - 2
+		}
+	}
+	return NotHardCoded
+}
diff --git a/src/internal/coverage/pods/pods.go b/src/internal/coverage/pods/pods.go
new file mode 100644
index 0000000..432c7b6
--- /dev/null
+++ b/src/internal/coverage/pods/pods.go
@@ -0,0 +1,194 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pods
+
+import (
+	"fmt"
+	"internal/coverage"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strconv"
+)
+
+// Pod encapsulates a set of files emitted during the executions of a
+// coverage-instrumented binary. Each pod contains a single meta-data
+// file, and then 0 or more counter data files that refer to that
+// meta-data file. Pods are intended to simplify processing of
+// coverage output files in the case where we have several coverage
+// output directories containing output files derived from more
+// than one instrumented executable. In the case where the files that
+// make up a pod are spread out across multiple directories, each
+// element of the "Origins" field below will be populated with the
+// index of the originating directory for the corresponding counter
+// data file (within the slice of input dirs handed to CollectPods).
+// The ProcessIDs field will be populated with the process ID of each
+// data file in the CounterDataFiles slice.
+type Pod struct {
+	MetaFile         string
+	CounterDataFiles []string
+	Origins          []int
+	ProcessIDs       []int
+}
+
+// CollectPods visits the files contained within the directories in
+// the list 'dirs', collects any coverage-related files, partitions
+// them into pods, and returns a list of the pods to the caller, along
+// with an error if something went wrong during directory/file
+// reading.
+//
+// CollectPods skips over any file that is not related to coverage
+// (e.g. avoids looking at things that are not meta-data files or
+// counter-data files). CollectPods also skips over 'orphaned' counter
+// data files (e.g. counter data files for which we can't find the
+// corresponding meta-data file). If "warn" is true, CollectPods will
+// issue warnings to stderr when it encounters non-fatal problems (for
+// orphans or a directory with no meta-data files).
+func CollectPods(dirs []string, warn bool) ([]Pod, error) {
+	files := []string{}
+	dirIndices := []int{}
+	for k, dir := range dirs {
+		dents, err := os.ReadDir(dir)
+		if err != nil {
+			return nil, err
+		}
+		for _, e := range dents {
+			if e.IsDir() {
+				continue
+			}
+			files = append(files, filepath.Join(dir, e.Name()))
+			dirIndices = append(dirIndices, k)
+		}
+	}
+	return collectPodsImpl(files, dirIndices, warn), nil
+}
+
+// CollectPodsFromFiles functions the same as "CollectPods" but
+// operates on an explicit list of files instead of a directory.
+func CollectPodsFromFiles(files []string, warn bool) []Pod {
+	return collectPodsImpl(files, nil, warn)
+}
+
+type fileWithAnnotations struct {
+	file   string
+	origin int
+	pid    int
+}
+
+type protoPod struct {
+	mf       string
+	elements []fileWithAnnotations
+}
+
+// collectPodsImpl examines the specified list of files and picks out
+// subsets that correspond to coverage pods. The first stage in this
+// process is collecting a set { M1, M2, ... MN } where each M_k is a
+// distinct coverage meta-data file. We then create a single pod for
+// each meta-data file M_k, then find all of the counter data files
+// that refer to that meta-data file (recall that the counter data
+// file name incorporates the meta-data hash), and add the counter
+// data file to the appropriate pod.
+//
+// This process is complicated by the fact that we need to keep track
+// of directory indices for counter data files. Here is an example to
+// motivate:
+//
+//	directory 1:
+//
+// M1   covmeta.9bbf1777f47b3fcacb05c38b035512d6
+// C1   covcounters.9bbf1777f47b3fcacb05c38b035512d6.1677673.1662138360208416486
+// C2   covcounters.9bbf1777f47b3fcacb05c38b035512d6.1677637.1662138359974441782
+//
+//	directory 2:
+//
+// M2   covmeta.9bbf1777f47b3fcacb05c38b035512d6
+// C3   covcounters.9bbf1777f47b3fcacb05c38b035512d6.1677445.1662138360208416480
+// C4   covcounters.9bbf1777f47b3fcacb05c38b035512d6.1677677.1662138359974441781
+// M3   covmeta.a723844208cea2ae80c63482c78b2245
+// C5   covcounters.a723844208cea2ae80c63482c78b2245.3677445.1662138360208416480
+// C6   covcounters.a723844208cea2ae80c63482c78b2245.1877677.1662138359974441781
+//
+// In these two directories we have three meta-data files, but only
+// two are distinct, meaning that we'll wind up with two pods. The
+// first pod (with meta-file M1) will have four counter data files
+// (C1, C2, C3, C4) and the second pod will have two counter data files
+// (C5, C6).
+func collectPodsImpl(files []string, dirIndices []int, warn bool) []Pod {
+	metaRE := regexp.MustCompile(fmt.Sprintf(`^%s\.(\S+)$`, coverage.MetaFilePref))
+	mm := make(map[string]protoPod)
+	for _, f := range files {
+		base := filepath.Base(f)
+		if m := metaRE.FindStringSubmatch(base); m != nil {
+			tag := m[1]
+			// We need to allow for the possibility of duplicate
+			// meta-data files. If we hit this case, use the
+			// first encountered as the canonical version.
+			if _, ok := mm[tag]; !ok {
+				mm[tag] = protoPod{mf: f}
+			}
+			// FIXME: should probably check file length and hash here for
+			// the duplicate.
+		}
+	}
+	counterRE := regexp.MustCompile(fmt.Sprintf(coverage.CounterFileRegexp, coverage.CounterFilePref))
+	for k, f := range files {
+		base := filepath.Base(f)
+		if m := counterRE.FindStringSubmatch(base); m != nil {
+			tag := m[1] // meta hash
+			pid, err := strconv.Atoi(m[2])
+			if err != nil {
+				continue
+			}
+			if v, ok := mm[tag]; ok {
+				idx := -1
+				if dirIndices != nil {
+					idx = dirIndices[k]
+				}
+				fo := fileWithAnnotations{file: f, origin: idx, pid: pid}
+				v.elements = append(v.elements, fo)
+				mm[tag] = v
+			} else {
+				if warn {
+					warning("skipping orphaned counter file: %s", f)
+				}
+			}
+		}
+	}
+	if len(mm) == 0 {
+		if warn {
+			warning("no coverage data files found")
+		}
+		return nil
+	}
+	pods := make([]Pod, 0, len(mm))
+	for _, p := range mm {
+		sort.Slice(p.elements, func(i, j int) bool {
+			return p.elements[i].file < p.elements[j].file
+		})
+		pod := Pod{
+			MetaFile:         p.mf,
+			CounterDataFiles: make([]string, 0, len(p.elements)),
+			Origins:          make([]int, 0, len(p.elements)),
+			ProcessIDs:       make([]int, 0, len(p.elements)),
+		}
+		for _, e := range p.elements {
+			pod.CounterDataFiles = append(pod.CounterDataFiles, e.file)
+			pod.Origins = append(pod.Origins, e.origin)
+			pod.ProcessIDs = append(pod.ProcessIDs, e.pid)
+		}
+		pods = append(pods, pod)
+	}
+	sort.Slice(pods, func(i, j int) bool {
+		return pods[i].MetaFile < pods[j].MetaFile
+	})
+	return pods
+}
+
+func warning(s string, a ...interface{}) {
+	fmt.Fprintf(os.Stderr, "warning: ")
+	fmt.Fprintf(os.Stderr, s, a...)
+	fmt.Fprintf(os.Stderr, "\n")
+}
diff --git a/src/internal/coverage/pods/pods_test.go b/src/internal/coverage/pods/pods_test.go
new file mode 100644
index 0000000..28914c5
--- /dev/null
+++ b/src/internal/coverage/pods/pods_test.go
@@ -0,0 +1,142 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pods_test
+
+import (
+	"crypto/md5"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/pods"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+func TestPodCollection(t *testing.T) {
+	//testenv.MustHaveGoBuild(t)
+
+	mkdir := func(d string, perm os.FileMode) string {
+		dp := filepath.Join(t.TempDir(), d)
+		if err := os.Mkdir(dp, perm); err != nil {
+			t.Fatal(err)
+		}
+		return dp
+	}
+
+	mkfile := func(d string, fn string) string {
+		fp := filepath.Join(d, fn)
+		if err := os.WriteFile(fp, []byte("foo"), 0666); err != nil {
+			t.Fatal(err)
+		}
+		return fp
+	}
+
+	mkmeta := func(dir string, tag string) string {
+		hash := md5.Sum([]byte(tag))
+		fn := fmt.Sprintf("%s.%x", coverage.MetaFilePref, hash)
+		return mkfile(dir, fn)
+	}
+
+	mkcounter := func(dir string, tag string, nt int) string {
+		hash := md5.Sum([]byte(tag))
+		dummyPid := int(42)
+		fn := fmt.Sprintf(coverage.CounterFileTempl, coverage.CounterFilePref, hash, dummyPid, nt)
+		return mkfile(dir, fn)
+	}
+
+	trim := func(path string) string {
+		b := filepath.Base(path)
+		d := filepath.Dir(path)
+		db := filepath.Base(d)
+		return db + "/" + b
+	}
+
+	podToString := func(p pods.Pod) string {
+		rv := trim(p.MetaFile) + " [\n"
+		for k, df := range p.CounterDataFiles {
+			rv += trim(df)
+			if p.Origins != nil {
+				rv += fmt.Sprintf(" o:%d", p.Origins[k])
+			}
+			rv += "\n"
+		}
+		return rv + "]"
+	}
+
+	// Create a couple of directories.
+	o1 := mkdir("o1", 0777)
+	o2 := mkdir("o2", 0777)
+
+	// Add some random files (not coverage related)
+	mkfile(o1, "blah.txt")
+	mkfile(o1, "something.exe")
+
+	// Add a meta-data file with two counter files to first dir.
+	mkmeta(o1, "m1")
+	mkcounter(o1, "m1", 1)
+	mkcounter(o1, "m1", 2)
+	mkcounter(o1, "m1", 2)
+
+	// Add a counter file with no associated meta file.
+	mkcounter(o1, "orphan", 9)
+
+	// Add a meta-data file with three counter files to second dir.
+	mkmeta(o2, "m2")
+	mkcounter(o2, "m2", 1)
+	mkcounter(o2, "m2", 2)
+	mkcounter(o2, "m2", 3)
+
+	// Add a duplicate of the first meta-file and a corresponding
+	// counter file to the second dir. This is intended to capture
+	// the scenario where we have two different runs of the same
+	// coverage-instrumented binary, but with the output files
+	// sent to separate directories.
+	mkmeta(o2, "m1")
+	mkcounter(o2, "m1", 11)
+
+	// Collect pods.
+	podlist, err := pods.CollectPods([]string{o1, o2}, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify pods
+	if len(podlist) != 2 {
+		t.Fatalf("expected 2 pods got %d pods", len(podlist))
+	}
+
+	for k, p := range podlist {
+		t.Logf("%d: mf=%s\n", k, p.MetaFile)
+	}
+
+	expected := []string{
+		`o1/covmeta.ae7be26cdaa742ca148068d5ac90eaca [
+o1/covcounters.ae7be26cdaa742ca148068d5ac90eaca.42.1 o:0
+o1/covcounters.ae7be26cdaa742ca148068d5ac90eaca.42.2 o:0
+o2/covcounters.ae7be26cdaa742ca148068d5ac90eaca.42.11 o:1
+]`,
+		`o2/covmeta.aaf2f89992379705dac844c0a2a1d45f [
+o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.42.1 o:1
+o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.42.2 o:1
+o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.42.3 o:1
+]`,
+	}
+	for k, exp := range expected {
+		got := podToString(podlist[k])
+		if exp != got {
+			t.Errorf("pod %d: expected:\n%s\ngot:\n%s", k, exp, got)
+		}
+	}
+
+	// Check handling of bad/unreadable dir.
+	if runtime.GOOS == "linux" {
+		dbad := "/dev/null"
+		_, err = pods.CollectPods([]string{dbad}, true)
+		if err == nil {
+			t.Errorf("exected error due to unreadable dir")
+		}
+	}
+}
diff --git a/src/internal/coverage/rtcov/rtcov.go b/src/internal/coverage/rtcov/rtcov.go
new file mode 100644
index 0000000..bbb93ac
--- /dev/null
+++ b/src/internal/coverage/rtcov/rtcov.go
@@ -0,0 +1,34 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rtcov
+
+// This package contains types whose structure is shared between
+// the runtime package and the "runtime/coverage" package.
+
+// CovMetaBlob is a container for holding the meta-data symbol (an
+// RODATA variable) for an instrumented Go package. Here "p" points to
+// the symbol itself, "len" is the length of the sym in bytes, and
+// "hash" is an md5sum for the sym computed by the compiler. When
+// the init function for a coverage-instrumented package executes, it
+// will make a call into the runtime which will create a covMetaBlob
+// object for the package and chain it onto a global list.
+type CovMetaBlob struct {
+	P                  *byte
+	Len                uint32
+	Hash               [16]byte
+	PkgPath            string
+	PkgID              int
+	CounterMode        uint8 // coverage.CounterMode
+	CounterGranularity uint8 // coverage.CounterGranularity
+}
+
+// CovCounterBlob is a container for encapsulating a counter section
+// (BSS variable) for an instrumented Go module. Here "counters"
+// points to the counter payload and "len" is the number of uint32
+// entries in the section.
+type CovCounterBlob struct {
+	Counters *uint32
+	Len      uint64
+}
diff --git a/src/internal/coverage/slicereader/slicereader.go b/src/internal/coverage/slicereader/slicereader.go
new file mode 100644
index 0000000..3d117ba
--- /dev/null
+++ b/src/internal/coverage/slicereader/slicereader.go
@@ -0,0 +1,98 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slicereader
+
+import (
+	"encoding/binary"
+	"unsafe"
+)
+
+// This file contains the helper "SliceReader", a utility for
+// reading values from a byte slice that may or may not be backed
+// by a read-only mmap'd region.
+
+type Reader struct {
+	b        []byte
+	readonly bool
+	off      int64
+}
+
+func NewReader(b []byte, readonly bool) *Reader {
+	r := Reader{
+		b:        b,
+		readonly: readonly,
+	}
+	return &r
+}
+
+func (r *Reader) Read(b []byte) (int, error) {
+	amt := len(b)
+	toread := r.b[r.off:]
+	if len(toread) < amt {
+		amt = len(toread)
+	}
+	copy(b, toread)
+	r.off += int64(amt)
+	return amt, nil
+}
+
+func (r *Reader) SeekTo(off int64) {
+	r.off = off
+}
+
+func (r *Reader) Offset() int64 {
+	return r.off
+}
+
+func (r *Reader) ReadUint8() uint8 {
+	rv := uint8(r.b[int(r.off)])
+	r.off += 1
+	return rv
+}
+
+func (r *Reader) ReadUint32() uint32 {
+	end := int(r.off) + 4
+	rv := binary.LittleEndian.Uint32(r.b[int(r.off):end:end])
+	r.off += 4
+	return rv
+}
+
+func (r *Reader) ReadUint64() uint64 {
+	end := int(r.off) + 8
+	rv := binary.LittleEndian.Uint64(r.b[int(r.off):end:end])
+	r.off += 8
+	return rv
+}
+
+func (r *Reader) ReadULEB128() (value uint64) {
+	var shift uint
+
+	for {
+		b := r.b[r.off]
+		r.off++
+		value |= (uint64(b&0x7F) << shift)
+		if b&0x80 == 0 {
+			break
+		}
+		shift += 7
+	}
+	return
+}
+
+func (r *Reader) ReadString(len int64) string {
+	b := r.b[r.off : r.off+len]
+	r.off += len
+	if r.readonly {
+		return toString(b) // backed by RO memory, ok to make unsafe string
+	}
+	return string(b)
+}
+
+func toString(b []byte) string {
+	if len(b) == 0 {
+		return ""
+	}
+	return unsafe.String(&b[0], len(b))
+}
diff --git a/src/internal/coverage/slicereader/slr_test.go b/src/internal/coverage/slicereader/slr_test.go
new file mode 100644
index 0000000..2f7cef0
--- /dev/null
+++ b/src/internal/coverage/slicereader/slr_test.go
@@ -0,0 +1,92 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slicereader
+
+import (
+	"encoding/binary"
+	"testing"
+)
+
+func TestSliceReader(t *testing.T) {
+	b := []byte{}
+
+	bt := make([]byte, 4)
+	e32 := uint32(1030507)
+	binary.LittleEndian.PutUint32(bt, e32)
+	b = append(b, bt...)
+
+	bt = make([]byte, 8)
+	e64 := uint64(907050301)
+	binary.LittleEndian.PutUint64(bt, e64)
+	b = append(b, bt...)
+
+	b = appendUleb128(b, uint(e32))
+	b = appendUleb128(b, uint(e64))
+	b = appendUleb128(b, 6)
+	s1 := "foobar"
+	s1b := []byte(s1)
+	b = append(b, s1b...)
+	b = appendUleb128(b, 9)
+	s2 := "bazbasher"
+	s2b := []byte(s2)
+	b = append(b, s2b...)
+
+	readStr := func(slr *Reader) string {
+		len := slr.ReadULEB128()
+		return slr.ReadString(int64(len))
+	}
+
+	for i := 0; i < 2; i++ {
+		slr := NewReader(b, i == 0)
+		g32 := slr.ReadUint32()
+		if g32 != e32 {
+			t.Fatalf("slr.ReadUint32() got %d want %d", g32, e32)
+		}
+		g64 := slr.ReadUint64()
+		if g64 != e64 {
+			t.Fatalf("slr.ReadUint64() got %d want %d", g64, e64)
+		}
+		g32 = uint32(slr.ReadULEB128())
+		if g32 != e32 {
+			t.Fatalf("slr.ReadULEB128() got %d want %d", g32, e32)
+		}
+		g64 = slr.ReadULEB128()
+		if g64 != e64 {
+			t.Fatalf("slr.ReadULEB128() got %d want %d", g64, e64)
+		}
+		gs1 := readStr(slr)
+		if gs1 != s1 {
+			t.Fatalf("readStr got %s want %s", gs1, s1)
+		}
+		gs2 := readStr(slr)
+		if gs2 != s2 {
+			t.Fatalf("readStr got %s want %s", gs2, s2)
+		}
+		slr.SeekTo(4)
+		off := slr.Offset()
+		if off != 4 {
+			t.Fatalf("Offset(0 returned %d wanted 4", off)
+		}
+		g64 = slr.ReadUint64()
+		if g64 != e64 {
+			t.Fatalf("post-seek slr.ReadUint64() got %d want %d", g64, e64)
+		}
+	}
+}
+
+func appendUleb128(b []byte, v uint) []byte {
+	for {
+		c := uint8(v & 0x7f)
+		v >>= 7
+		if v != 0 {
+			c |= 0x80
+		}
+		b = append(b, c)
+		if c&0x80 == 0 {
+			break
+		}
+	}
+	return b
+}
diff --git a/src/internal/coverage/slicewriter/slicewriter.go b/src/internal/coverage/slicewriter/slicewriter.go
new file mode 100644
index 0000000..3522bf5
--- /dev/null
+++ b/src/internal/coverage/slicewriter/slicewriter.go
@@ -0,0 +1,80 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slicewriter
+
+import (
+	"fmt"
+	"io"
+)
+
+// WriteSeeker is a helper object that implements the io.WriteSeeker
+// interface. Clients can create a WriteSeeker, make a series of Write
+// calls to add data to it (and possibly Seek calls to update
+// previously written portions), then finally invoke BytesWritten() to
+// get a pointer to the constructed byte slice.
+type WriteSeeker struct {
+	payload []byte
+	off     int64
+}
+
+func (sws *WriteSeeker) Write(p []byte) (n int, err error) {
+	amt := len(p)
+	towrite := sws.payload[sws.off:]
+	if len(towrite) < amt {
+		sws.payload = append(sws.payload, make([]byte, amt-len(towrite))...)
+		towrite = sws.payload[sws.off:]
+	}
+	copy(towrite, p)
+	sws.off += int64(amt)
+	return amt, nil
+}
+
+// Seek repositions the read/write position of the WriteSeeker within
+// its internally maintained slice. Note that it is not possible to
+// expand the size of the slice using SEEK_SET; trying to seek outside
+// the slice will result in an error.
+func (sws *WriteSeeker) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		if sws.off != offset && (offset < 0 || offset >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(sws.payload))
+		}
+		sws.off = offset
+		return offset, nil
+	case io.SeekCurrent:
+		newoff := sws.off + offset
+		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
+		}
+		sws.off += offset
+		return sws.off, nil
+	case io.SeekEnd:
+		newoff := int64(len(sws.payload)) + offset
+		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
+		}
+		sws.off = newoff
+		return sws.off, nil
+	}
+	// other modes not supported
+	return 0, fmt.Errorf("unsupported seek mode %d", whence)
+}
+
+// BytesWritten returns the underlying byte slice for the WriteSeeker,
+// containing the data written to it via Write/Seek calls.
+func (sws *WriteSeeker) BytesWritten() []byte {
+	return sws.payload
+}
+
+func (sws *WriteSeeker) Read(p []byte) (n int, err error) {
+	amt := len(p)
+	toread := sws.payload[sws.off:]
+	if len(toread) < amt {
+		amt = len(toread)
+	}
+	copy(p, toread)
+	sws.off += int64(amt)
+	return amt, nil
+}
diff --git a/src/internal/coverage/slicewriter/slw_test.go b/src/internal/coverage/slicewriter/slw_test.go
new file mode 100644
index 0000000..f4e75f4
--- /dev/null
+++ b/src/internal/coverage/slicewriter/slw_test.go
@@ -0,0 +1,131 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slicewriter
+
+import (
+	"io"
+	"testing"
+)
+
+func TestSliceWriter(t *testing.T) {
+
+	sleq := func(t *testing.T, got []byte, want []byte) {
+		t.Helper()
+		if len(got) != len(want) {
+			t.Fatalf("bad length got %d want %d", len(got), len(want))
+		}
+		for i := range got {
+			if got[i] != want[i] {
+				t.Fatalf("bad read at %d got %d want %d", i, got[i], want[i])
+			}
+		}
+	}
+
+	wf := func(t *testing.T, ws *WriteSeeker, p []byte) {
+		t.Helper()
+		nw, werr := ws.Write(p)
+		if werr != nil {
+			t.Fatalf("unexpected write error: %v", werr)
+		}
+		if nw != len(p) {
+			t.Fatalf("wrong amount written want %d got %d", len(p), nw)
+		}
+	}
+
+	rf := func(t *testing.T, ws *WriteSeeker, p []byte) {
+		t.Helper()
+		b := make([]byte, len(p))
+		nr, rerr := ws.Read(b)
+		if rerr != nil {
+			t.Fatalf("unexpected read error: %v", rerr)
+		}
+		if nr != len(p) {
+			t.Fatalf("wrong amount read want %d got %d", len(p), nr)
+		}
+		sleq(t, b, p)
+	}
+
+	sk := func(t *testing.T, ws *WriteSeeker, offset int64, whence int) {
+		t.Helper()
+		_, err := ws.Seek(offset, whence)
+		if err != nil {
+			t.Fatalf("unexpected seek error: %v", err)
+		}
+	}
+
+	wp1 := []byte{1, 2}
+	ws := &WriteSeeker{}
+
+	// write some stuff
+	wf(t, ws, wp1)
+	// check that BytesWritten returns what we wrote.
+	sleq(t, ws.BytesWritten(), wp1)
+	// offset is at end of slice, so reading should return zero bytes.
+	rf(t, ws, []byte{})
+
+	// write some more stuff
+	wp2 := []byte{7, 8, 9}
+	wf(t, ws, wp2)
+	// check that BytesWritten returns what we expect.
+	wpex := []byte{1, 2, 7, 8, 9}
+	sleq(t, ws.BytesWritten(), wpex)
+	rf(t, ws, []byte{})
+
+	// seeks and reads.
+	sk(t, ws, 1, io.SeekStart)
+	rf(t, ws, []byte{2, 7})
+	sk(t, ws, -2, io.SeekCurrent)
+	rf(t, ws, []byte{2, 7})
+	sk(t, ws, -4, io.SeekEnd)
+	rf(t, ws, []byte{2, 7})
+
+	// seek back and overwrite
+	sk(t, ws, 1, io.SeekStart)
+	wf(t, ws, []byte{9, 11})
+	wpex = []byte{1, 9, 11, 8, 9}
+	sleq(t, ws.BytesWritten(), wpex)
+
+	// seeks on empty writer.
+	ws2 := &WriteSeeker{}
+	sk(t, ws2, 0, io.SeekStart)
+	sk(t, ws2, 0, io.SeekCurrent)
+	sk(t, ws2, 0, io.SeekEnd)
+
+	// check for seek errors.
+	_, err := ws.Seek(-1, io.SeekStart)
+	if err == nil {
+		t.Fatalf("expected error on invalid -1 seek")
+	}
+	_, err = ws.Seek(int64(len(ws.BytesWritten())), io.SeekStart)
+	if err == nil {
+		t.Fatalf("expected error on invalid %d seek", len(ws.BytesWritten()))
+	}
+
+	ws.Seek(0, io.SeekStart)
+	_, err = ws.Seek(-1, io.SeekCurrent)
+	if err == nil {
+		t.Fatalf("expected error on invalid -1 seek")
+	}
+	_, err = ws.Seek(int64(len(ws.BytesWritten())), io.SeekCurrent)
+	if err == nil {
+		t.Fatalf("expected error on invalid %d seek", len(ws.BytesWritten()))
+	}
+
+	_, err = ws.Seek(1, io.SeekEnd)
+	if err == nil {
+		t.Fatalf("expected error on invalid 1 seek")
+	}
+	bsamt := int64(-1*len(ws.BytesWritten()) - 1)
+	_, err = ws.Seek(bsamt, io.SeekEnd)
+	if err == nil {
+		t.Fatalf("expected error on invalid %d seek", bsamt)
+	}
+
+	// bad seek mode
+	_, err = ws.Seek(-1, io.SeekStart+9)
+	if err == nil {
+		t.Fatalf("expected error on invalid seek mode")
+	}
+}
diff --git a/src/internal/coverage/stringtab/stringtab.go b/src/internal/coverage/stringtab/stringtab.go
new file mode 100644
index 0000000..156c8ad
--- /dev/null
+++ b/src/internal/coverage/stringtab/stringtab.go
@@ -0,0 +1,139 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package stringtab
+
+import (
+	"fmt"
+	"internal/coverage/slicereader"
+	"internal/coverage/uleb128"
+	"io"
+)
+
+// This package implements string table writer and reader utilities,
+// for use in emitting and reading/decoding coverage meta-data and
+// counter-data files.
+
+// Writer implements a string table writing utility.
+type Writer struct {
+	stab   map[string]uint32
+	strs   []string
+	tmp    []byte
+	frozen bool
+}
+
+// InitWriter initializes a stringtab.Writer.
+func (stw *Writer) InitWriter() {
+	stw.stab = make(map[string]uint32)
+	stw.tmp = make([]byte, 64)
+}
+
+// Nentries returns the number of strings interned so far.
+func (stw *Writer) Nentries() uint32 {
+	return uint32(len(stw.strs))
+}
+
+// Lookup looks up string 's' in the writer's table, adding
+// a new entry if need be, and returning an index into the table.
+func (stw *Writer) Lookup(s string) uint32 {
+	if idx, ok := stw.stab[s]; ok {
+		return idx
+	}
+	if stw.frozen {
+		panic("internal error: string table previously frozen")
+	}
+	idx := uint32(len(stw.strs))
+	stw.stab[s] = idx
+	stw.strs = append(stw.strs, s)
+	return idx
+}
+
+// Size computes the memory in bytes needed for the serialized
+// version of a stringtab.Writer.
+func (stw *Writer) Size() uint32 {
+	rval := uint32(0)
+	stw.tmp = stw.tmp[:0]
+	stw.tmp = uleb128.AppendUleb128(stw.tmp, uint(len(stw.strs)))
+	rval += uint32(len(stw.tmp))
+	for _, s := range stw.strs {
+		stw.tmp = stw.tmp[:0]
+		slen := uint(len(s))
+		stw.tmp = uleb128.AppendUleb128(stw.tmp, slen)
+		rval += uint32(len(stw.tmp)) + uint32(slen)
+	}
+	return rval
+}
+
+// Write writes the string table in serialized form to the specified
+// io.Writer.
+func (stw *Writer) Write(w io.Writer) error {
+	wr128 := func(v uint) error {
+		stw.tmp = stw.tmp[:0]
+		stw.tmp = uleb128.AppendUleb128(stw.tmp, v)
+		if nw, err := w.Write(stw.tmp); err != nil {
+			return fmt.Errorf("writing string table: %v", err)
+		} else if nw != len(stw.tmp) {
+			return fmt.Errorf("short write emitting stringtab uleb")
+		}
+		return nil
+	}
+	if err := wr128(uint(len(stw.strs))); err != nil {
+		return err
+	}
+	for _, s := range stw.strs {
+		if err := wr128(uint(len(s))); err != nil {
+			return err
+		}
+		if nw, err := w.Write([]byte(s)); err != nil {
+			return fmt.Errorf("writing string table: %v", err)
+		} else if nw != len([]byte(s)) {
+			return fmt.Errorf("short write emitting stringtab")
+		}
+	}
+	return nil
+}
+
+// Freeze sends a signal to the writer that no more additions are
+// allowed, only lookups of existing strings (if a lookup triggers
+// addition, a panic will result). Useful as a mechanism for
+// "finalizing" a string table prior to writing it out.
+func (stw *Writer) Freeze() {
+	stw.frozen = true
+}
+
+// Reader is a helper for reading a string table previously
+// serialized by a Writer.Write call.
+type Reader struct {
+	r    *slicereader.Reader
+	strs []string
+}
+
+// NewReader creates a stringtab.Reader to read the contents
+// of a string table from 'r'.
+func NewReader(r *slicereader.Reader) *Reader {
+	str := &Reader{
+		r: r,
+	}
+	return str
+}
+
+// Read reads/decodes a string table using the reader provided.
+func (str *Reader) Read() {
+	numEntries := int(str.r.ReadULEB128())
+	str.strs = make([]string, 0, numEntries)
+	for idx := 0; idx < numEntries; idx++ {
+		slen := str.r.ReadULEB128()
+		str.strs = append(str.strs, str.r.ReadString(int64(slen)))
+	}
+}
+
+// Entries returns the number of decoded entries in a string table.
+func (str *Reader) Entries() int {
+	return len(str.strs)
+}
+
+// Get returns string 'idx' within the string table.
+func (str *Reader) Get(idx uint32) string {
+	return str.strs[idx]
+}
diff --git a/src/internal/coverage/test/counter_test.go b/src/internal/coverage/test/counter_test.go
new file mode 100644
index 0000000..3fc111e
--- /dev/null
+++ b/src/internal/coverage/test/counter_test.go
@@ -0,0 +1,241 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/decodecounter"
+	"internal/coverage/encodecounter"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+type ctrVis struct {
+	funcs []decodecounter.FuncPayload
+}
+
+func (v *ctrVis) NumFuncs() (int, error) {
+	return len(v.funcs), nil
+}
+
+func (v *ctrVis) VisitFuncs(f encodecounter.CounterVisitorFn) error {
+	for _, fn := range v.funcs {
+		if err := f(fn.PkgIdx, fn.FuncIdx, fn.Counters); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func mkfunc(p uint32, f uint32, c []uint32) decodecounter.FuncPayload {
+	return decodecounter.FuncPayload{
+		PkgIdx:   p,
+		FuncIdx:  f,
+		Counters: c,
+	}
+}
+
+func TestCounterDataWriterReader(t *testing.T) {
+	flavors := []coverage.CounterFlavor{
+		coverage.CtrRaw,
+		coverage.CtrULeb128,
+	}
+
+	isDead := func(fp decodecounter.FuncPayload) bool {
+		for _, v := range fp.Counters {
+			if v != 0 {
+				return false
+			}
+		}
+		return true
+	}
+
+	funcs := []decodecounter.FuncPayload{
+		mkfunc(0, 0, []uint32{13, 14, 15}),
+		mkfunc(0, 1, []uint32{16, 17}),
+		mkfunc(1, 0, []uint32{18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 976543, 7}),
+	}
+	writeVisitor := &ctrVis{funcs: funcs}
+
+	for kf, flav := range flavors {
+
+		t.Logf("testing flavor %d\n", flav)
+
+		// Open a counter data file in preparation for emitting data.
+		d := t.TempDir()
+		cfpath := filepath.Join(d, fmt.Sprintf("covcounters.hash.0.%d", kf))
+		of, err := os.OpenFile(cfpath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
+		if err != nil {
+			t.Fatalf("opening covcounters: %v", err)
+		}
+
+		// Perform the encode and write.
+		cdfw := encodecounter.NewCoverageDataWriter(of, flav)
+		if cdfw == nil {
+			t.Fatalf("NewCoverageDataWriter failed")
+		}
+		finalHash := [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 0}
+		args := map[string]string{"argc": "3", "argv0": "arg0", "argv1": "arg1", "argv2": "arg_________2"}
+		if err := cdfw.Write(finalHash, args, writeVisitor); err != nil {
+			t.Fatalf("counter file Write failed: %v", err)
+		}
+		if err := of.Close(); err != nil {
+			t.Fatalf("closing covcounters: %v", err)
+		}
+		cdfw = nil
+
+		// Decode the same file.
+		var cdr *decodecounter.CounterDataReader
+		inf, err := os.Open(cfpath)
+		defer func() {
+			if err := inf.Close(); err != nil {
+				t.Fatalf("close failed with: %v", err)
+			}
+		}()
+
+		if err != nil {
+			t.Fatalf("reopening covcounters file: %v", err)
+		}
+		if cdr, err = decodecounter.NewCounterDataReader(cfpath, inf); err != nil {
+			t.Fatalf("opening covcounters for read: %v", err)
+		}
+		decodedArgs := cdr.OsArgs()
+		aWant := "[arg0 arg1 arg_________2]"
+		aGot := fmt.Sprintf("%+v", decodedArgs)
+		if aWant != aGot {
+			t.Errorf("reading decoded args, got %s want %s", aGot, aWant)
+		}
+		for i := range funcs {
+			if isDead(funcs[i]) {
+				continue
+			}
+			var fp decodecounter.FuncPayload
+			if ok, err := cdr.NextFunc(&fp); err != nil {
+				t.Fatalf("reading func %d: %v", i, err)
+			} else if !ok {
+				t.Fatalf("reading func %d: bad return", i)
+			}
+			got := fmt.Sprintf("%+v", fp)
+			want := fmt.Sprintf("%+v", funcs[i])
+			if got != want {
+				t.Errorf("cdr.NextFunc iter %d\ngot  %+v\nwant %+v", i, got, want)
+			}
+		}
+		var dummy decodecounter.FuncPayload
+		if ok, err := cdr.NextFunc(&dummy); err != nil {
+			t.Fatalf("reading func after loop: %v", err)
+		} else if ok {
+			t.Fatalf("reading func after loop: expected EOF")
+		}
+	}
+}
+
+func TestCounterDataAppendSegment(t *testing.T) {
+	d := t.TempDir()
+	cfpath := filepath.Join(d, "covcounters.hash2.0")
+	of, err := os.OpenFile(cfpath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
+	if err != nil {
+		t.Fatalf("opening covcounters: %v", err)
+	}
+
+	const numSegments = 2
+
+	// Write a counter with with multiple segments.
+	args := map[string]string{"argc": "1", "argv0": "prog.exe"}
+	allfuncs := [][]decodecounter.FuncPayload{}
+	ctrs := []uint32{}
+	q := uint32(0)
+	var cdfw *encodecounter.CoverageDataWriter
+	for idx := 0; idx < numSegments; idx++ {
+		args[fmt.Sprintf("seg%d", idx)] = "x"
+		q += 7
+		ctrs = append(ctrs, q)
+		funcs := []decodecounter.FuncPayload{}
+		for k := 0; k < idx+1; k++ {
+			c := make([]uint32, len(ctrs))
+			copy(c, ctrs)
+			funcs = append(funcs, mkfunc(uint32(idx), uint32(k), c))
+		}
+		allfuncs = append(allfuncs, funcs)
+
+		writeVisitor := &ctrVis{funcs: funcs}
+
+		if idx == 0 {
+			// Perform the encode and write.
+			cdfw = encodecounter.NewCoverageDataWriter(of, coverage.CtrRaw)
+			if cdfw == nil {
+				t.Fatalf("NewCoverageDataWriter failed")
+			}
+			finalHash := [16]byte{1, 2}
+			if err := cdfw.Write(finalHash, args, writeVisitor); err != nil {
+				t.Fatalf("counter file Write failed: %v", err)
+			}
+		} else {
+			if err := cdfw.AppendSegment(args, writeVisitor); err != nil {
+				t.Fatalf("counter file AppendSegment failed: %v", err)
+			}
+		}
+	}
+	if err := of.Close(); err != nil {
+		t.Fatalf("closing covcounters: %v", err)
+	}
+
+	// Read the result file.
+	var cdr *decodecounter.CounterDataReader
+	inf, err := os.Open(cfpath)
+	defer func() {
+		if err := inf.Close(); err != nil {
+			t.Fatalf("close failed with: %v", err)
+		}
+	}()
+
+	if err != nil {
+		t.Fatalf("reopening covcounters file: %v", err)
+	}
+	if cdr, err = decodecounter.NewCounterDataReader(cfpath, inf); err != nil {
+		t.Fatalf("opening covcounters for read: %v", err)
+	}
+	ns := cdr.NumSegments()
+	if ns != numSegments {
+		t.Fatalf("got %d segments want %d", ns, numSegments)
+	}
+	if len(allfuncs) != numSegments {
+		t.Fatalf("expected %d got %d", numSegments, len(allfuncs))
+	}
+
+	for sidx := 0; sidx < int(ns); sidx++ {
+		if off, err := inf.Seek(0, io.SeekCurrent); err != nil {
+			t.Fatalf("Seek failed: %v", err)
+		} else {
+			t.Logf("sidx=%d off=%d\n", sidx, off)
+		}
+
+		if sidx != 0 {
+			if ok, err := cdr.BeginNextSegment(); err != nil {
+				t.Fatalf("BeginNextSegment failed: %v", err)
+			} else if !ok {
+				t.Fatalf("BeginNextSegment return %v on iter %d",
+					ok, sidx)
+			}
+		}
+		funcs := allfuncs[sidx]
+		for i := range funcs {
+			var fp decodecounter.FuncPayload
+			if ok, err := cdr.NextFunc(&fp); err != nil {
+				t.Fatalf("reading func %d: %v", i, err)
+			} else if !ok {
+				t.Fatalf("reading func %d: bad return", i)
+			}
+			got := fmt.Sprintf("%+v", fp)
+			want := fmt.Sprintf("%+v", funcs[i])
+			if got != want {
+				t.Errorf("cdr.NextFunc iter %d\ngot  %+v\nwant %+v", i, got, want)
+			}
+		}
+	}
+}
diff --git a/src/internal/coverage/test/roundtrip_test.go b/src/internal/coverage/test/roundtrip_test.go
new file mode 100644
index 0000000..614f56e
--- /dev/null
+++ b/src/internal/coverage/test/roundtrip_test.go
@@ -0,0 +1,331 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/decodemeta"
+	"internal/coverage/encodemeta"
+	"internal/coverage/slicewriter"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func cmpFuncDesc(want, got coverage.FuncDesc) string {
+	swant := fmt.Sprintf("%+v", want)
+	sgot := fmt.Sprintf("%+v", got)
+	if swant == sgot {
+		return ""
+	}
+	return fmt.Sprintf("wanted %q got %q", swant, sgot)
+}
+
+func TestMetaDataEmptyPackage(t *testing.T) {
+	// Make sure that encoding/decoding works properly with packages
+	// that don't actually have any functions.
+	p := "empty/package"
+	pn := "package"
+	mp := "m"
+	b, err := encodemeta.NewCoverageMetaDataBuilder(p, pn, mp)
+	if err != nil {
+		t.Fatalf("making builder: %v", err)
+	}
+	drws := &slicewriter.WriteSeeker{}
+	b.Emit(drws)
+	drws.Seek(0, io.SeekStart)
+	dec, err := decodemeta.NewCoverageMetaDataDecoder(drws.BytesWritten(), false)
+	if err != nil {
+		t.Fatalf("making decoder: %v", err)
+	}
+	nf := dec.NumFuncs()
+	if nf != 0 {
+		t.Errorf("dec.NumFuncs(): got %d want %d", nf, 0)
+	}
+	pp := dec.PackagePath()
+	if pp != p {
+		t.Errorf("dec.PackagePath(): got %s want %s", pp, p)
+	}
+	ppn := dec.PackageName()
+	if ppn != pn {
+		t.Errorf("dec.PackageName(): got %s want %s", ppn, pn)
+	}
+	pmp := dec.ModulePath()
+	if pmp != mp {
+		t.Errorf("dec.ModulePath(): got %s want %s", pmp, mp)
+	}
+}
+
+func TestMetaDataEncoderDecoder(t *testing.T) {
+	// Test encode path.
+	pp := "foo/bar/pkg"
+	pn := "pkg"
+	mp := "barmod"
+	b, err := encodemeta.NewCoverageMetaDataBuilder(pp, pn, mp)
+	if err != nil {
+		t.Fatalf("making builder: %v", err)
+	}
+	f1 := coverage.FuncDesc{
+		Funcname: "func",
+		Srcfile:  "foo.go",
+		Units: []coverage.CoverableUnit{
+			coverage.CoverableUnit{StLine: 1, StCol: 2, EnLine: 3, EnCol: 4, NxStmts: 5},
+			coverage.CoverableUnit{StLine: 6, StCol: 7, EnLine: 8, EnCol: 9, NxStmts: 10},
+		},
+	}
+	idx := b.AddFunc(f1)
+	if idx != 0 {
+		t.Errorf("b.AddFunc(f1) got %d want %d", idx, 0)
+	}
+
+	f2 := coverage.FuncDesc{
+		Funcname: "xfunc",
+		Srcfile:  "bar.go",
+		Units: []coverage.CoverableUnit{
+			coverage.CoverableUnit{StLine: 1, StCol: 2, EnLine: 3, EnCol: 4, NxStmts: 5},
+			coverage.CoverableUnit{StLine: 6, StCol: 7, EnLine: 8, EnCol: 9, NxStmts: 10},
+			coverage.CoverableUnit{StLine: 11, StCol: 12, EnLine: 13, EnCol: 14, NxStmts: 15},
+		},
+	}
+	idx = b.AddFunc(f2)
+	if idx != 1 {
+		t.Errorf("b.AddFunc(f2) got %d want %d", idx, 0)
+	}
+
+	// Emit into a writer.
+	drws := &slicewriter.WriteSeeker{}
+	b.Emit(drws)
+
+	// Test decode path.
+	drws.Seek(0, io.SeekStart)
+	dec, err := decodemeta.NewCoverageMetaDataDecoder(drws.BytesWritten(), false)
+	if err != nil {
+		t.Fatalf("NewCoverageMetaDataDecoder error: %v", err)
+	}
+	nf := dec.NumFuncs()
+	if nf != 2 {
+		t.Errorf("dec.NumFuncs(): got %d want %d", nf, 2)
+	}
+
+	gotpp := dec.PackagePath()
+	if gotpp != pp {
+		t.Errorf("packagepath: got %s want %s", gotpp, pp)
+	}
+	gotpn := dec.PackageName()
+	if gotpn != pn {
+		t.Errorf("packagename: got %s want %s", gotpn, pn)
+	}
+
+	cases := []coverage.FuncDesc{f1, f2}
+	for i := uint32(0); i < uint32(len(cases)); i++ {
+		var fn coverage.FuncDesc
+		if err := dec.ReadFunc(i, &fn); err != nil {
+			t.Fatalf("err reading function %d: %v", i, err)
+		}
+		res := cmpFuncDesc(cases[i], fn)
+		if res != "" {
+			t.Errorf("ReadFunc(%d): %s", i, res)
+		}
+	}
+}
+
+func createFuncs(i int) []coverage.FuncDesc {
+	res := []coverage.FuncDesc{}
+	lc := uint32(1)
+	for fi := 0; fi < i+1; fi++ {
+		units := []coverage.CoverableUnit{}
+		for ui := 0; ui < (fi+1)*(i+1); ui++ {
+			units = append(units,
+				coverage.CoverableUnit{StLine: lc, StCol: lc + 1,
+					EnLine: lc + 2, EnCol: lc + 3, NxStmts: lc + 4,
+				})
+			lc += 5
+		}
+		f := coverage.FuncDesc{
+			Funcname: fmt.Sprintf("func_%d_%d", i, fi),
+			Srcfile:  fmt.Sprintf("foo_%d.go", i),
+			Units:    units,
+		}
+		res = append(res, f)
+	}
+	return res
+}
+
+func createBlob(t *testing.T, i int) []byte {
+	nomodule := ""
+	b, err := encodemeta.NewCoverageMetaDataBuilder("foo/pkg", "pkg", nomodule)
+	if err != nil {
+		t.Fatalf("making builder: %v", err)
+	}
+
+	funcs := createFuncs(i)
+	for _, f := range funcs {
+		b.AddFunc(f)
+	}
+	drws := &slicewriter.WriteSeeker{}
+	b.Emit(drws)
+	return drws.BytesWritten()
+}
+
+func createMetaDataBlobs(t *testing.T, nb int) [][]byte {
+	res := [][]byte{}
+	for i := 0; i < nb; i++ {
+		res = append(res, createBlob(t, i))
+	}
+	return res
+}
+
+func TestMetaDataWriterReader(t *testing.T) {
+	d := t.TempDir()
+
+	// Emit a meta-file...
+	mfpath := filepath.Join(d, "covmeta.hash.0")
+	of, err := os.OpenFile(mfpath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0666)
+	if err != nil {
+		t.Fatalf("opening covmeta: %v", err)
+	}
+	//t.Logf("meta-file path is %s", mfpath)
+	blobs := createMetaDataBlobs(t, 7)
+	gran := coverage.CtrGranularityPerBlock
+	mfw := encodemeta.NewCoverageMetaFileWriter(mfpath, of)
+	finalHash := [16]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}
+	err = mfw.Write(finalHash, blobs, coverage.CtrModeAtomic, gran)
+	if err != nil {
+		t.Fatalf("writing meta-file: %v", err)
+	}
+	if err = of.Close(); err != nil {
+		t.Fatalf("closing meta-file: %v", err)
+	}
+
+	// ... then read it back in, first time without setting fileView,
+	// second time setting it.
+	for k := 0; k < 2; k++ {
+		var fileView []byte
+
+		inf, err := os.Open(mfpath)
+		if err != nil {
+			t.Fatalf("open() on meta-file: %v", err)
+		}
+
+		if k != 0 {
+			// Use fileview to exercise different paths in reader.
+			fi, err := os.Stat(mfpath)
+			if err != nil {
+				t.Fatalf("stat() on meta-file: %v", err)
+			}
+			fileView = make([]byte, fi.Size())
+			if _, err := inf.Read(fileView); err != nil {
+				t.Fatalf("read() on meta-file: %v", err)
+			}
+			if _, err := inf.Seek(int64(0), io.SeekStart); err != nil {
+				t.Fatalf("seek() on meta-file: %v", err)
+			}
+		}
+
+		mfr, err := decodemeta.NewCoverageMetaFileReader(inf, fileView)
+		if err != nil {
+			t.Fatalf("k=%d NewCoverageMetaFileReader failed with: %v", k, err)
+		}
+		np := mfr.NumPackages()
+		if np != 7 {
+			t.Fatalf("k=%d wanted 7 packages got %d", k, np)
+		}
+		md := mfr.CounterMode()
+		wmd := coverage.CtrModeAtomic
+		if md != wmd {
+			t.Fatalf("k=%d wanted mode %d got %d", k, wmd, md)
+		}
+		gran := mfr.CounterGranularity()
+		wgran := coverage.CtrGranularityPerBlock
+		if gran != wgran {
+			t.Fatalf("k=%d wanted gran %d got %d", k, wgran, gran)
+		}
+
+		payload := []byte{}
+		for pi := 0; pi < int(np); pi++ {
+			var pd *decodemeta.CoverageMetaDataDecoder
+			var err error
+			pd, payload, err = mfr.GetPackageDecoder(uint32(pi), payload)
+			if err != nil {
+				t.Fatalf("GetPackageDecoder(%d) failed with: %v", pi, err)
+			}
+			efuncs := createFuncs(pi)
+			nf := pd.NumFuncs()
+			if len(efuncs) != int(nf) {
+				t.Fatalf("decoding pk %d wanted %d funcs got %d",
+					pi, len(efuncs), nf)
+			}
+			var f coverage.FuncDesc
+			for fi := 0; fi < int(nf); fi++ {
+				if err := pd.ReadFunc(uint32(fi), &f); err != nil {
+					t.Fatalf("ReadFunc(%d) pk %d got error %v",
+						fi, pi, err)
+				}
+				res := cmpFuncDesc(efuncs[fi], f)
+				if res != "" {
+					t.Errorf("ReadFunc(%d) pk %d: %s", fi, pi, res)
+				}
+			}
+		}
+		inf.Close()
+	}
+}
+
+func TestMetaDataDecodeLitFlagIssue57942(t *testing.T) {
+
+	// Encode a package with a few functions. The funcs alternate
+	// between regular functions and function literals.
+	pp := "foo/bar/pkg"
+	pn := "pkg"
+	mp := "barmod"
+	b, err := encodemeta.NewCoverageMetaDataBuilder(pp, pn, mp)
+	if err != nil {
+		t.Fatalf("making builder: %v", err)
+	}
+	const NF = 6
+	const NCU = 1
+	ln := uint32(10)
+	wantfds := []coverage.FuncDesc{}
+	for fi := uint32(0); fi < NF; fi++ {
+		fis := fmt.Sprintf("%d", fi)
+		fd := coverage.FuncDesc{
+			Funcname: "func" + fis,
+			Srcfile:  "foo" + fis + ".go",
+			Units: []coverage.CoverableUnit{
+				coverage.CoverableUnit{StLine: ln + 1, StCol: 2, EnLine: ln + 3, EnCol: 4, NxStmts: fi + 2},
+			},
+			Lit: (fi % 2) == 0,
+		}
+		wantfds = append(wantfds, fd)
+		b.AddFunc(fd)
+	}
+
+	// Emit into a writer.
+	drws := &slicewriter.WriteSeeker{}
+	b.Emit(drws)
+
+	// Decode the result.
+	drws.Seek(0, io.SeekStart)
+	dec, err := decodemeta.NewCoverageMetaDataDecoder(drws.BytesWritten(), false)
+	if err != nil {
+		t.Fatalf("making decoder: %v", err)
+	}
+	nf := dec.NumFuncs()
+	if nf != NF {
+		t.Fatalf("decoder number of functions: got %d want %d", nf, NF)
+	}
+	var fn coverage.FuncDesc
+	for i := uint32(0); i < uint32(NF); i++ {
+		if err := dec.ReadFunc(i, &fn); err != nil {
+			t.Fatalf("err reading function %d: %v", i, err)
+		}
+		res := cmpFuncDesc(wantfds[i], fn)
+		if res != "" {
+			t.Errorf("ReadFunc(%d): %s", i, res)
+		}
+	}
+}
diff --git a/src/internal/coverage/uleb128/uleb128.go b/src/internal/coverage/uleb128/uleb128.go
new file mode 100644
index 0000000..e5cd92a
--- /dev/null
+++ b/src/internal/coverage/uleb128/uleb128.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package uleb128
+
+func AppendUleb128(b []byte, v uint) []byte {
+	for {
+		c := uint8(v & 0x7f)
+		v >>= 7
+		if v != 0 {
+			c |= 0x80
+		}
+		b = append(b, c)
+		if c&0x80 == 0 {
+			break
+		}
+	}
+	return b
+}
diff --git a/src/internal/cpu/cpu.go b/src/internal/cpu/cpu.go
new file mode 100644
index 0000000..aef9fb3
--- /dev/null
+++ b/src/internal/cpu/cpu.go
@@ -0,0 +1,223 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cpu implements processor feature detection
+// used by the Go standard library.
+package cpu
+
+// DebugOptions is set to true by the runtime if the OS supports reading
+// GODEBUG early in runtime startup.
+// This should not be changed after it is initialized.
+var DebugOptions bool
+
+// CacheLinePad is used to pad structs to avoid false sharing.
+type CacheLinePad struct{ _ [CacheLinePadSize]byte }
+
+// CacheLineSize is the CPU's assumed cache line size.
+// There is currently no runtime detection of the real cache line size
+// so we use the constant per GOARCH CacheLinePadSize as an approximation.
+var CacheLineSize uintptr = CacheLinePadSize
+
+// The booleans in X86 contain the correspondingly named cpuid feature bit.
+// HasAVX and HasAVX2 are only set if the OS does support XMM and YMM registers
+// in addition to the cpuid feature bit being set.
+// The struct is padded to avoid false sharing.
+var X86 struct {
+	_            CacheLinePad
+	HasAES       bool
+	HasADX       bool
+	HasAVX       bool
+	HasAVX2      bool
+	HasBMI1      bool
+	HasBMI2      bool
+	HasERMS      bool
+	HasFMA       bool
+	HasOSXSAVE   bool
+	HasPCLMULQDQ bool
+	HasPOPCNT    bool
+	HasRDTSCP    bool
+	HasSHA       bool
+	HasSSE3      bool
+	HasSSSE3     bool
+	HasSSE41     bool
+	HasSSE42     bool
+	_            CacheLinePad
+}
+
+// The booleans in ARM contain the correspondingly named cpu feature bit.
+// The struct is padded to avoid false sharing.
+var ARM struct {
+	_        CacheLinePad
+	HasVFPv4 bool
+	HasIDIVA bool
+	_        CacheLinePad
+}
+
+// The booleans in ARM64 contain the correspondingly named cpu feature bit.
+// The struct is padded to avoid false sharing.
+var ARM64 struct {
+	_            CacheLinePad
+	HasAES       bool
+	HasPMULL     bool
+	HasSHA1      bool
+	HasSHA2      bool
+	HasSHA512    bool
+	HasCRC32     bool
+	HasATOMICS   bool
+	HasCPUID     bool
+	IsNeoverseN1 bool
+	IsNeoverseV1 bool
+	_            CacheLinePad
+}
+
+var MIPS64X struct {
+	_      CacheLinePad
+	HasMSA bool // MIPS SIMD architecture
+	_      CacheLinePad
+}
+
+// For ppc64(le), it is safe to check only for ISA level starting on ISA v3.00,
+// since there are no optional categories. There are some exceptions that also
+// require kernel support to work (darn, scv), so there are feature bits for
+// those as well. The minimum processor requirement is POWER8 (ISA 2.07).
+// The struct is padded to avoid false sharing.
+var PPC64 struct {
+	_         CacheLinePad
+	HasDARN   bool // Hardware random number generator (requires kernel enablement)
+	HasSCV    bool // Syscall vectored (requires kernel enablement)
+	IsPOWER8  bool // ISA v2.07 (POWER8)
+	IsPOWER9  bool // ISA v3.00 (POWER9)
+	IsPOWER10 bool // ISA v3.1  (POWER10)
+	_         CacheLinePad
+}
+
+var S390X struct {
+	_         CacheLinePad
+	HasZARCH  bool // z architecture mode is active [mandatory]
+	HasSTFLE  bool // store facility list extended [mandatory]
+	HasLDISP  bool // long (20-bit) displacements [mandatory]
+	HasEIMM   bool // 32-bit immediates [mandatory]
+	HasDFP    bool // decimal floating point
+	HasETF3EH bool // ETF-3 enhanced
+	HasMSA    bool // message security assist (CPACF)
+	HasAES    bool // KM-AES{128,192,256} functions
+	HasAESCBC bool // KMC-AES{128,192,256} functions
+	HasAESCTR bool // KMCTR-AES{128,192,256} functions
+	HasAESGCM bool // KMA-GCM-AES{128,192,256} functions
+	HasGHASH  bool // KIMD-GHASH function
+	HasSHA1   bool // K{I,L}MD-SHA-1 functions
+	HasSHA256 bool // K{I,L}MD-SHA-256 functions
+	HasSHA512 bool // K{I,L}MD-SHA-512 functions
+	HasSHA3   bool // K{I,L}MD-SHA3-{224,256,384,512} and K{I,L}MD-SHAKE-{128,256} functions
+	HasVX     bool // vector facility. Note: the runtime sets this when it processes auxv records.
+	HasVXE    bool // vector-enhancements facility 1
+	HasKDSA   bool // elliptic curve functions
+	HasECDSA  bool // NIST curves
+	HasEDDSA  bool // Edwards curves
+	_         CacheLinePad
+}
+
+// Initialize examines the processor and sets the relevant variables above.
+// This is called by the runtime package early in program initialization,
+// before normal init functions are run. env is set by runtime if the OS supports
+// cpu feature options in GODEBUG.
+func Initialize(env string) {
+	doinit()
+	processOptions(env)
+}
+
+// options contains the cpu debug options that can be used in GODEBUG.
+// Options are arch dependent and are added by the arch specific doinit functions.
+// Features that are mandatory for the specific GOARCH should not be added to options
+// (e.g. SSE2 on amd64).
+var options []option
+
+// Option names should be lower case. e.g. avx instead of AVX.
+type option struct {
+	Name      string
+	Feature   *bool
+	Specified bool // whether feature value was specified in GODEBUG
+	Enable    bool // whether feature should be enabled
+}
+
+// processOptions enables or disables CPU feature values based on the parsed env string.
+// The env string is expected to be of the form cpu.feature1=value1,cpu.feature2=value2...
+// where feature names is one of the architecture specific list stored in the
+// cpu packages options variable and values are either 'on' or 'off'.
+// If env contains cpu.all=off then all cpu features referenced through the options
+// variable are disabled. Other feature names and values result in warning messages.
+func processOptions(env string) {
+field:
+	for env != "" {
+		field := ""
+		i := indexByte(env, ',')
+		if i < 0 {
+			field, env = env, ""
+		} else {
+			field, env = env[:i], env[i+1:]
+		}
+		if len(field) < 4 || field[:4] != "cpu." {
+			continue
+		}
+		i = indexByte(field, '=')
+		if i < 0 {
+			print("GODEBUG: no value specified for \"", field, "\"\n")
+			continue
+		}
+		key, value := field[4:i], field[i+1:] // e.g. "SSE2", "on"
+
+		var enable bool
+		switch value {
+		case "on":
+			enable = true
+		case "off":
+			enable = false
+		default:
+			print("GODEBUG: value \"", value, "\" not supported for cpu option \"", key, "\"\n")
+			continue field
+		}
+
+		if key == "all" {
+			for i := range options {
+				options[i].Specified = true
+				options[i].Enable = enable
+			}
+			continue field
+		}
+
+		for i := range options {
+			if options[i].Name == key {
+				options[i].Specified = true
+				options[i].Enable = enable
+				continue field
+			}
+		}
+
+		print("GODEBUG: unknown cpu feature \"", key, "\"\n")
+	}
+
+	for _, o := range options {
+		if !o.Specified {
+			continue
+		}
+
+		if o.Enable && !*o.Feature {
+			print("GODEBUG: can not enable \"", o.Name, "\", missing CPU support\n")
+			continue
+		}
+
+		*o.Feature = o.Enable
+	}
+}
+
+// indexByte returns the index of the first instance of c in s,
+// or -1 if c is not present in s.
+func indexByte(s string, c byte) int {
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
diff --git a/src/internal/cpu/cpu.s b/src/internal/cpu/cpu.s
new file mode 100644
index 0000000..3c770c1
--- /dev/null
+++ b/src/internal/cpu/cpu.s
@@ -0,0 +1,6 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This assembly file exists to allow internal/cpu to call
+// non-exported runtime functions that use "go:linkname".
\ No newline at end of file
diff --git a/src/internal/cpu/cpu_arm.go b/src/internal/cpu/cpu_arm.go
new file mode 100644
index 0000000..b624526
--- /dev/null
+++ b/src/internal/cpu/cpu_arm.go
@@ -0,0 +1,34 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 32
+
+// arm doesn't have a 'cpuid' equivalent, so we rely on HWCAP/HWCAP2.
+// These are initialized by archauxv() and should not be changed after they are
+// initialized.
+var HWCap uint
+var HWCap2 uint
+
+// HWCAP/HWCAP2 bits. These are exposed by Linux and FreeBSD.
+const (
+	hwcap_VFPv4 = 1 << 16
+	hwcap_IDIVA = 1 << 17
+)
+
+func doinit() {
+	options = []option{
+		{Name: "vfpv4", Feature: &ARM.HasVFPv4},
+		{Name: "idiva", Feature: &ARM.HasIDIVA},
+	}
+
+	// HWCAP feature bits
+	ARM.HasVFPv4 = isSet(HWCap, hwcap_VFPv4)
+	ARM.HasIDIVA = isSet(HWCap, hwcap_IDIVA)
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
diff --git a/src/internal/cpu/cpu_arm64.go b/src/internal/cpu/cpu_arm64.go
new file mode 100644
index 0000000..85210aa
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64.go
@@ -0,0 +1,70 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+// CacheLinePadSize is used to prevent false sharing of cache lines.
+// We choose 128 because Apple Silicon, a.k.a. M1, has 128-byte cache line size.
+// It doesn't cost much and is much more future-proof.
+const CacheLinePadSize = 128
+
+func doinit() {
+	options = []option{
+		{Name: "aes", Feature: &ARM64.HasAES},
+		{Name: "pmull", Feature: &ARM64.HasPMULL},
+		{Name: "sha1", Feature: &ARM64.HasSHA1},
+		{Name: "sha2", Feature: &ARM64.HasSHA2},
+		{Name: "sha512", Feature: &ARM64.HasSHA512},
+		{Name: "crc32", Feature: &ARM64.HasCRC32},
+		{Name: "atomics", Feature: &ARM64.HasATOMICS},
+		{Name: "cpuid", Feature: &ARM64.HasCPUID},
+		{Name: "isNeoverseN1", Feature: &ARM64.IsNeoverseN1},
+		{Name: "isNeoverseV1", Feature: &ARM64.IsNeoverseV1},
+	}
+
+	// arm64 uses different ways to detect CPU features at runtime depending on the operating system.
+	osInit()
+}
+
+func getisar0() uint64
+
+func getMIDR() uint64
+
+func extractBits(data uint64, start, end uint) uint {
+	return (uint)(data>>start) & ((1 << (end - start + 1)) - 1)
+}
+
+func parseARM64SystemRegisters(isar0 uint64) {
+	// ID_AA64ISAR0_EL1
+	switch extractBits(isar0, 4, 7) {
+	case 1:
+		ARM64.HasAES = true
+	case 2:
+		ARM64.HasAES = true
+		ARM64.HasPMULL = true
+	}
+
+	switch extractBits(isar0, 8, 11) {
+	case 1:
+		ARM64.HasSHA1 = true
+	}
+
+	switch extractBits(isar0, 12, 15) {
+	case 1:
+		ARM64.HasSHA2 = true
+	case 2:
+		ARM64.HasSHA2 = true
+		ARM64.HasSHA512 = true
+	}
+
+	switch extractBits(isar0, 16, 19) {
+	case 1:
+		ARM64.HasCRC32 = true
+	}
+
+	switch extractBits(isar0, 20, 23) {
+	case 2:
+		ARM64.HasATOMICS = true
+	}
+}
diff --git a/src/internal/cpu/cpu_arm64.s b/src/internal/cpu/cpu_arm64.s
new file mode 100644
index 0000000..d6e7f44
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64.s
@@ -0,0 +1,18 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+// func getisar0() uint64
+TEXT ·getisar0(SB),NOSPLIT,$0
+	// get Instruction Set Attributes 0 into R0
+	MRS	ID_AA64ISAR0_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET
+
+// func getMIDR() uint64
+TEXT ·getMIDR(SB), NOSPLIT, $0-8
+	MRS	MIDR_EL1, R0
+	MOVD	R0, ret+0(FP)
+	RET
diff --git a/src/internal/cpu/cpu_arm64_android.go b/src/internal/cpu/cpu_arm64_android.go
new file mode 100644
index 0000000..fbdf7ba
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_android.go
@@ -0,0 +1,11 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64
+
+package cpu
+
+func osInit() {
+	hwcapInit("android")
+}
diff --git a/src/internal/cpu/cpu_arm64_darwin.go b/src/internal/cpu/cpu_arm64_darwin.go
new file mode 100644
index 0000000..60beadd
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_darwin.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64 && darwin && !ios
+
+package cpu
+
+func osInit() {
+	ARM64.HasATOMICS = sysctlEnabled([]byte("hw.optional.armv8_1_atomics\x00"))
+	ARM64.HasCRC32 = sysctlEnabled([]byte("hw.optional.armv8_crc32\x00"))
+	ARM64.HasSHA512 = sysctlEnabled([]byte("hw.optional.armv8_2_sha512\x00"))
+
+	// There are no hw.optional sysctl values for the below features on Mac OS 11.0
+	// to detect their supported state dynamically. Assume the CPU features that
+	// Apple Silicon M1 supports to be available as a minimal set of features
+	// to all Go programs running on darwin/arm64.
+	ARM64.HasAES = true
+	ARM64.HasPMULL = true
+	ARM64.HasSHA1 = true
+	ARM64.HasSHA2 = true
+}
+
+//go:noescape
+func getsysctlbyname(name []byte) (int32, int32)
+
+func sysctlEnabled(name []byte) bool {
+	ret, value := getsysctlbyname(name)
+	if ret < 0 {
+		return false
+	}
+	return value > 0
+}
diff --git a/src/internal/cpu/cpu_arm64_freebsd.go b/src/internal/cpu/cpu_arm64_freebsd.go
new file mode 100644
index 0000000..96ed359
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_freebsd.go
@@ -0,0 +1,14 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64
+
+package cpu
+
+func osInit() {
+	// Retrieve info from system register ID_AA64ISAR0_EL1.
+	isar0 := getisar0()
+
+	parseARM64SystemRegisters(isar0)
+}
diff --git a/src/internal/cpu/cpu_arm64_hwcap.go b/src/internal/cpu/cpu_arm64_hwcap.go
new file mode 100644
index 0000000..0fb5fb5
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_hwcap.go
@@ -0,0 +1,62 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64 && linux
+
+package cpu
+
+// HWCap may be initialized by archauxv and
+// should not be changed after it was initialized.
+var HWCap uint
+
+// HWCAP bits. These are exposed by Linux.
+const (
+	hwcap_AES     = 1 << 3
+	hwcap_PMULL   = 1 << 4
+	hwcap_SHA1    = 1 << 5
+	hwcap_SHA2    = 1 << 6
+	hwcap_CRC32   = 1 << 7
+	hwcap_ATOMICS = 1 << 8
+	hwcap_CPUID   = 1 << 11
+)
+
+func hwcapInit(os string) {
+	// HWCap was populated by the runtime from the auxiliary vector.
+	// Use HWCap information since reading aarch64 system registers
+	// is not supported in user space on older linux kernels.
+	ARM64.HasAES = isSet(HWCap, hwcap_AES)
+	ARM64.HasPMULL = isSet(HWCap, hwcap_PMULL)
+	ARM64.HasSHA1 = isSet(HWCap, hwcap_SHA1)
+	ARM64.HasSHA2 = isSet(HWCap, hwcap_SHA2)
+	ARM64.HasCRC32 = isSet(HWCap, hwcap_CRC32)
+	ARM64.HasCPUID = isSet(HWCap, hwcap_CPUID)
+
+	// The Samsung S9+ kernel reports support for atomics, but not all cores
+	// actually support them, resulting in SIGILL. See issue #28431.
+	// TODO(elias.naur): Only disable the optimization on bad chipsets on android.
+	ARM64.HasATOMICS = isSet(HWCap, hwcap_ATOMICS) && os != "android"
+
+	// Check to see if executing on a NeoverseN1 and in order to do that,
+	// check the AUXV for the CPUID bit. The getMIDR function executes an
+	// instruction which would normally be an illegal instruction, but it's
+	// trapped by the kernel, the value sanitized and then returned. Without
+	// the CPUID bit the kernel will not trap the instruction and the process
+	// will be terminated with SIGILL.
+	if ARM64.HasCPUID {
+		midr := getMIDR()
+		part_num := uint16((midr >> 4) & 0xfff)
+		implementor := byte((midr >> 24) & 0xff)
+
+		if implementor == 'A' && part_num == 0xd0c {
+			ARM64.IsNeoverseN1 = true
+		}
+		if implementor == 'A' && part_num == 0xd40 {
+			ARM64.IsNeoverseV1 = true
+		}
+	}
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
diff --git a/src/internal/cpu/cpu_arm64_linux.go b/src/internal/cpu/cpu_arm64_linux.go
new file mode 100644
index 0000000..d746bdb
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_linux.go
@@ -0,0 +1,11 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64 && linux && !android
+
+package cpu
+
+func osInit() {
+	hwcapInit("linux")
+}
diff --git a/src/internal/cpu/cpu_arm64_openbsd.go b/src/internal/cpu/cpu_arm64_openbsd.go
new file mode 100644
index 0000000..1259309
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_openbsd.go
@@ -0,0 +1,28 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64
+
+package cpu
+
+const (
+	// From OpenBSD's sys/sysctl.h.
+	_CTL_MACHDEP = 7
+
+	// From OpenBSD's machine/cpu.h.
+	_CPU_ID_AA64ISAR0 = 2
+	_CPU_ID_AA64ISAR1 = 3
+)
+
+//go:noescape
+func sysctlUint64(mib []uint32) (uint64, bool)
+
+func osInit() {
+	// Get ID_AA64ISAR0 from sysctl.
+	isar0, ok := sysctlUint64([]uint32{_CTL_MACHDEP, _CPU_ID_AA64ISAR0})
+	if !ok {
+		return
+	}
+	parseARM64SystemRegisters(isar0)
+}
diff --git a/src/internal/cpu/cpu_arm64_other.go b/src/internal/cpu/cpu_arm64_other.go
new file mode 100644
index 0000000..44592cf
--- /dev/null
+++ b/src/internal/cpu/cpu_arm64_other.go
@@ -0,0 +1,13 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64 && !linux && !freebsd && !android && (!darwin || ios) && !openbsd
+
+package cpu
+
+func osInit() {
+	// Other operating systems do not support reading HWCap from auxiliary vector,
+	// reading privileged aarch64 system registers or sysctl in user space to detect
+	// CPU features at runtime.
+}
diff --git a/src/internal/cpu/cpu_loong64.go b/src/internal/cpu/cpu_loong64.go
new file mode 100644
index 0000000..1c90c24
--- /dev/null
+++ b/src/internal/cpu/cpu_loong64.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build loong64
+
+package cpu
+
+// CacheLinePadSize is used to prevent false sharing of cache lines.
+// We choose 64 because Loongson 3A5000 the L1 Dcache is 4-way 256-line 64-byte-per-line.
+const CacheLinePadSize = 64
+
+func doinit() {}
diff --git a/src/internal/cpu/cpu_mips.go b/src/internal/cpu/cpu_mips.go
new file mode 100644
index 0000000..14a9c97
--- /dev/null
+++ b/src/internal/cpu/cpu_mips.go
@@ -0,0 +1,10 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 32
+
+func doinit() {
+}
diff --git a/src/internal/cpu/cpu_mips64x.go b/src/internal/cpu/cpu_mips64x.go
new file mode 100644
index 0000000..c452ffd
--- /dev/null
+++ b/src/internal/cpu/cpu_mips64x.go
@@ -0,0 +1,32 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips64 || mips64le
+
+package cpu
+
+const CacheLinePadSize = 32
+
+// This is initialized by archauxv and should not be changed after it is
+// initialized.
+var HWCap uint
+
+// HWCAP bits. These are exposed by the Linux kernel 5.4.
+const (
+	// CPU features
+	hwcap_MIPS_MSA = 1 << 1
+)
+
+func doinit() {
+	options = []option{
+		{Name: "msa", Feature: &MIPS64X.HasMSA},
+	}
+
+	// HWCAP feature bits
+	MIPS64X.HasMSA = isSet(HWCap, hwcap_MIPS_MSA)
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
diff --git a/src/internal/cpu/cpu_mipsle.go b/src/internal/cpu/cpu_mipsle.go
new file mode 100644
index 0000000..14a9c97
--- /dev/null
+++ b/src/internal/cpu/cpu_mipsle.go
@@ -0,0 +1,10 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 32
+
+func doinit() {
+}
diff --git a/src/internal/cpu/cpu_no_name.go b/src/internal/cpu/cpu_no_name.go
new file mode 100644
index 0000000..2adfa1b
--- /dev/null
+++ b/src/internal/cpu/cpu_no_name.go
@@ -0,0 +1,18 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !386 && !amd64 && !ppc64 && !ppc64le
+
+package cpu
+
+// Name returns the CPU name given by the vendor
+// if it can be read directly from memory or by CPU instructions.
+// If the CPU name can not be determined an empty string is returned.
+//
+// Implementations that use the Operating System (e.g. sysctl or /sys/)
+// to gather CPU information for display should be placed in internal/sysinfo.
+func Name() string {
+	// "A CPU has no name".
+	return ""
+}
diff --git a/src/internal/cpu/cpu_ppc64x.go b/src/internal/cpu/cpu_ppc64x.go
new file mode 100644
index 0000000..c4a08fe
--- /dev/null
+++ b/src/internal/cpu/cpu_ppc64x.go
@@ -0,0 +1,35 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+package cpu
+
+const CacheLinePadSize = 128
+
+func doinit() {
+	options = []option{
+		{Name: "darn", Feature: &PPC64.HasDARN},
+		{Name: "scv", Feature: &PPC64.HasSCV},
+		{Name: "power9", Feature: &PPC64.IsPOWER9},
+	}
+
+	osinit()
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
+
+func Name() string {
+	switch {
+	case PPC64.IsPOWER10:
+		return "POWER10"
+	case PPC64.IsPOWER9:
+		return "POWER9"
+	case PPC64.IsPOWER8:
+		return "POWER8"
+	}
+	return ""
+}
diff --git a/src/internal/cpu/cpu_ppc64x_aix.go b/src/internal/cpu/cpu_ppc64x_aix.go
new file mode 100644
index 0000000..f05ed6f
--- /dev/null
+++ b/src/internal/cpu/cpu_ppc64x_aix.go
@@ -0,0 +1,25 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+package cpu
+
+const (
+	// getsystemcfg constants
+	_SC_IMPL      = 2
+	_IMPL_POWER8  = 0x10000
+	_IMPL_POWER9  = 0x20000
+	_IMPL_POWER10 = 0x40000
+)
+
+func osinit() {
+	impl := getsystemcfg(_SC_IMPL)
+	PPC64.IsPOWER8 = isSet(impl, _IMPL_POWER8)
+	PPC64.IsPOWER9 = isSet(impl, _IMPL_POWER9)
+	PPC64.IsPOWER10 = isSet(impl, _IMPL_POWER10)
+}
+
+// getsystemcfg is defined in runtime/os2_aix.go
+func getsystemcfg(label uint) uint
diff --git a/src/internal/cpu/cpu_ppc64x_linux.go b/src/internal/cpu/cpu_ppc64x_linux.go
new file mode 100644
index 0000000..9df82ca
--- /dev/null
+++ b/src/internal/cpu/cpu_ppc64x_linux.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+package cpu
+
+// ppc64 doesn't have a 'cpuid' equivalent, so we rely on HWCAP/HWCAP2.
+// These are initialized by archauxv and should not be changed after they are
+// initialized.
+var HWCap uint
+var HWCap2 uint
+
+// HWCAP bits. These are exposed by Linux.
+const (
+	// ISA Level
+	hwcap2_ARCH_2_07 = 0x80000000
+	hwcap2_ARCH_3_00 = 0x00800000
+	hwcap2_ARCH_3_1  = 0x00040000
+
+	// CPU features
+	hwcap2_DARN = 0x00200000
+	hwcap2_SCV  = 0x00100000
+)
+
+func osinit() {
+	PPC64.IsPOWER8 = isSet(HWCap2, hwcap2_ARCH_2_07)
+	PPC64.IsPOWER9 = isSet(HWCap2, hwcap2_ARCH_3_00)
+	PPC64.IsPOWER10 = isSet(HWCap2, hwcap2_ARCH_3_1)
+	PPC64.HasDARN = isSet(HWCap2, hwcap2_DARN)
+	PPC64.HasSCV = isSet(HWCap2, hwcap2_SCV)
+}
diff --git a/src/internal/cpu/cpu_riscv64.go b/src/internal/cpu/cpu_riscv64.go
new file mode 100644
index 0000000..54b8c33
--- /dev/null
+++ b/src/internal/cpu/cpu_riscv64.go
@@ -0,0 +1,10 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 32
+
+func doinit() {
+}
diff --git a/src/internal/cpu/cpu_s390x.go b/src/internal/cpu/cpu_s390x.go
new file mode 100644
index 0000000..45d8ed2
--- /dev/null
+++ b/src/internal/cpu/cpu_s390x.go
@@ -0,0 +1,205 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 256
+
+var HWCap uint
+
+// bitIsSet reports whether the bit at index is set. The bit index
+// is in big endian order, so bit index 0 is the leftmost bit.
+func bitIsSet(bits []uint64, index uint) bool {
+	return bits[index/64]&((1<<63)>>(index%64)) != 0
+}
+
+// function is the function code for the named function.
+type function uint8
+
+const (
+	// KM{,A,C,CTR} function codes
+	aes128 function = 18 // AES-128
+	aes192 function = 19 // AES-192
+	aes256 function = 20 // AES-256
+
+	// K{I,L}MD function codes
+	sha1     function = 1  // SHA-1
+	sha256   function = 2  // SHA-256
+	sha512   function = 3  // SHA-512
+	sha3_224 function = 32 // SHA3-224
+	sha3_256 function = 33 // SHA3-256
+	sha3_384 function = 34 // SHA3-384
+	sha3_512 function = 35 // SHA3-512
+	shake128 function = 36 // SHAKE-128
+	shake256 function = 37 // SHAKE-256
+
+	// KLMD function codes
+	ghash function = 65 // GHASH
+)
+
+const (
+	// KDSA function codes
+	ecdsaVerifyP256    function = 1  // NIST P256
+	ecdsaVerifyP384    function = 2  // NIST P384
+	ecdsaVerifyP521    function = 3  // NIST P521
+	ecdsaSignP256      function = 9  // NIST P256
+	ecdsaSignP384      function = 10 // NIST P384
+	ecdsaSignP521      function = 11 // NIST P521
+	eddsaVerifyEd25519 function = 32 // Curve25519
+	eddsaVerifyEd448   function = 36 // Curve448
+	eddsaSignEd25519   function = 40 // Curve25519
+	eddsaSignEd448     function = 44 // Curve448
+)
+
+// queryResult contains the result of a Query function
+// call. Bits are numbered in big endian order so the
+// leftmost bit (the MSB) is at index 0.
+type queryResult struct {
+	bits [2]uint64
+}
+
+// Has reports whether the given functions are present.
+func (q *queryResult) Has(fns ...function) bool {
+	if len(fns) == 0 {
+		panic("no function codes provided")
+	}
+	for _, f := range fns {
+		if !bitIsSet(q.bits[:], uint(f)) {
+			return false
+		}
+	}
+	return true
+}
+
+// facility is a bit index for the named facility.
+type facility uint8
+
+const (
+	// mandatory facilities
+	zarch  facility = 1  // z architecture mode is active
+	stflef facility = 7  // store-facility-list-extended
+	ldisp  facility = 18 // long-displacement
+	eimm   facility = 21 // extended-immediate
+
+	// miscellaneous facilities
+	dfp    facility = 42 // decimal-floating-point
+	etf3eh facility = 30 // extended-translation 3 enhancement
+
+	// cryptography facilities
+	msa  facility = 17  // message-security-assist
+	msa3 facility = 76  // message-security-assist extension 3
+	msa4 facility = 77  // message-security-assist extension 4
+	msa5 facility = 57  // message-security-assist extension 5
+	msa8 facility = 146 // message-security-assist extension 8
+	msa9 facility = 155 // message-security-assist extension 9
+
+	// vector facilities
+	vxe facility = 135 // vector-enhancements 1
+
+	// Note: vx requires kernel support
+	// and so must be fetched from HWCAP.
+
+	hwcap_VX = 1 << 11 // vector facility
+)
+
+// facilityList contains the result of an STFLE call.
+// Bits are numbered in big endian order so the
+// leftmost bit (the MSB) is at index 0.
+type facilityList struct {
+	bits [4]uint64
+}
+
+// Has reports whether the given facilities are present.
+func (s *facilityList) Has(fs ...facility) bool {
+	if len(fs) == 0 {
+		panic("no facility bits provided")
+	}
+	for _, f := range fs {
+		if !bitIsSet(s.bits[:], uint(f)) {
+			return false
+		}
+	}
+	return true
+}
+
+// The following feature detection functions are defined in cpu_s390x.s.
+// They are likely to be expensive to call so the results should be cached.
+func stfle() facilityList
+func kmQuery() queryResult
+func kmcQuery() queryResult
+func kmctrQuery() queryResult
+func kmaQuery() queryResult
+func kimdQuery() queryResult
+func klmdQuery() queryResult
+func kdsaQuery() queryResult
+
+func doinit() {
+	options = []option{
+		{Name: "zarch", Feature: &S390X.HasZARCH},
+		{Name: "stfle", Feature: &S390X.HasSTFLE},
+		{Name: "ldisp", Feature: &S390X.HasLDISP},
+		{Name: "msa", Feature: &S390X.HasMSA},
+		{Name: "eimm", Feature: &S390X.HasEIMM},
+		{Name: "dfp", Feature: &S390X.HasDFP},
+		{Name: "etf3eh", Feature: &S390X.HasETF3EH},
+		{Name: "vx", Feature: &S390X.HasVX},
+		{Name: "vxe", Feature: &S390X.HasVXE},
+		{Name: "kdsa", Feature: &S390X.HasKDSA},
+	}
+
+	aes := []function{aes128, aes192, aes256}
+	facilities := stfle()
+
+	S390X.HasZARCH = facilities.Has(zarch)
+	S390X.HasSTFLE = facilities.Has(stflef)
+	S390X.HasLDISP = facilities.Has(ldisp)
+	S390X.HasEIMM = facilities.Has(eimm)
+	S390X.HasDFP = facilities.Has(dfp)
+	S390X.HasETF3EH = facilities.Has(etf3eh)
+	S390X.HasMSA = facilities.Has(msa)
+
+	if S390X.HasMSA {
+		// cipher message
+		km, kmc := kmQuery(), kmcQuery()
+		S390X.HasAES = km.Has(aes...)
+		S390X.HasAESCBC = kmc.Has(aes...)
+		if facilities.Has(msa4) {
+			kmctr := kmctrQuery()
+			S390X.HasAESCTR = kmctr.Has(aes...)
+		}
+		if facilities.Has(msa8) {
+			kma := kmaQuery()
+			S390X.HasAESGCM = kma.Has(aes...)
+		}
+
+		// compute message digest
+		kimd := kimdQuery() // intermediate (no padding)
+		klmd := klmdQuery() // last (padding)
+		S390X.HasSHA1 = kimd.Has(sha1) && klmd.Has(sha1)
+		S390X.HasSHA256 = kimd.Has(sha256) && klmd.Has(sha256)
+		S390X.HasSHA512 = kimd.Has(sha512) && klmd.Has(sha512)
+		S390X.HasGHASH = kimd.Has(ghash) // KLMD-GHASH does not exist
+		sha3 := []function{
+			sha3_224, sha3_256, sha3_384, sha3_512,
+			shake128, shake256,
+		}
+		S390X.HasSHA3 = kimd.Has(sha3...) && klmd.Has(sha3...)
+		S390X.HasKDSA = facilities.Has(msa9) // elliptic curves
+		if S390X.HasKDSA {
+			kdsa := kdsaQuery()
+			S390X.HasECDSA = kdsa.Has(ecdsaVerifyP256, ecdsaSignP256, ecdsaVerifyP384, ecdsaSignP384, ecdsaVerifyP521, ecdsaSignP521)
+			S390X.HasEDDSA = kdsa.Has(eddsaVerifyEd25519, eddsaSignEd25519, eddsaVerifyEd448, eddsaSignEd448)
+		}
+	}
+
+	S390X.HasVX = isSet(HWCap, hwcap_VX)
+
+	if S390X.HasVX {
+		S390X.HasVXE = facilities.Has(vxe)
+	}
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
diff --git a/src/internal/cpu/cpu_s390x.s b/src/internal/cpu/cpu_s390x.s
new file mode 100644
index 0000000..a1243aa
--- /dev/null
+++ b/src/internal/cpu/cpu_s390x.s
@@ -0,0 +1,63 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+// func stfle() facilityList
+TEXT ·stfle(SB), NOSPLIT|NOFRAME, $0-32
+	MOVD $ret+0(FP), R1
+	MOVD $3, R0          // last doubleword index to store
+	XC   $32, (R1), (R1) // clear 4 doublewords (32 bytes)
+	WORD $0xb2b01000     // store facility list extended (STFLE)
+	RET
+
+// func kmQuery() queryResult
+TEXT ·kmQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KM-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB92E0024    // cipher message (KM)
+	RET
+
+// func kmcQuery() queryResult
+TEXT ·kmcQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KMC-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB92F0024    // cipher message with chaining (KMC)
+	RET
+
+// func kmctrQuery() queryResult
+TEXT ·kmctrQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KMCTR-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB92D4024    // cipher message with counter (KMCTR)
+	RET
+
+// func kmaQuery() queryResult
+TEXT ·kmaQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KMA-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xb9296024    // cipher message with authentication (KMA)
+	RET
+
+// func kimdQuery() queryResult
+TEXT ·kimdQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KIMD-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB93E0024    // compute intermediate message digest (KIMD)
+	RET
+
+// func klmdQuery() queryResult
+TEXT ·klmdQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KLMD-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB93F0024    // compute last message digest (KLMD)
+	RET
+
+// func kdsaQuery() queryResult
+TEXT ·kdsaQuery(SB), NOSPLIT|NOFRAME, $0-16
+	MOVD $0, R0         // set function code to 0 (KLMD-Query)
+	MOVD $ret+0(FP), R1 // address of 16-byte return value
+	WORD $0xB93A0008    // compute digital signature authentication
+	RET
+
diff --git a/src/internal/cpu/cpu_s390x_test.go b/src/internal/cpu/cpu_s390x_test.go
new file mode 100644
index 0000000..ad86858
--- /dev/null
+++ b/src/internal/cpu/cpu_s390x_test.go
@@ -0,0 +1,63 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu_test
+
+import (
+	"errors"
+	. "internal/cpu"
+	"os"
+	"regexp"
+	"testing"
+)
+
+func getFeatureList() ([]string, error) {
+	cpuinfo, err := os.ReadFile("/proc/cpuinfo")
+	if err != nil {
+		return nil, err
+	}
+	r := regexp.MustCompile("features\\s*:\\s*(.*)")
+	b := r.FindSubmatch(cpuinfo)
+	if len(b) < 2 {
+		return nil, errors.New("no feature list in /proc/cpuinfo")
+	}
+	return regexp.MustCompile("\\s+").Split(string(b[1]), -1), nil
+}
+
+func TestS390XAgainstCPUInfo(t *testing.T) {
+	// mapping of linux feature strings to S390X fields
+	mapping := make(map[string]*bool)
+	for _, option := range Options {
+		mapping[option.Name] = option.Feature
+	}
+
+	// these must be true on the machines Go supports
+	mandatory := make(map[string]bool)
+	mandatory["zarch"] = false
+	mandatory["eimm"] = false
+	mandatory["ldisp"] = false
+	mandatory["stfle"] = false
+
+	features, err := getFeatureList()
+	if err != nil {
+		t.Error(err)
+	}
+	for _, feature := range features {
+		if _, ok := mandatory[feature]; ok {
+			mandatory[feature] = true
+		}
+		if flag, ok := mapping[feature]; ok {
+			if !*flag {
+				t.Errorf("feature '%v' not detected", feature)
+			}
+		} else {
+			t.Logf("no entry for '%v'", feature)
+		}
+	}
+	for k, v := range mandatory {
+		if !v {
+			t.Errorf("mandatory feature '%v' not detected", k)
+		}
+	}
+}
diff --git a/src/internal/cpu/cpu_test.go b/src/internal/cpu/cpu_test.go
new file mode 100644
index 0000000..5aa277f
--- /dev/null
+++ b/src/internal/cpu/cpu_test.go
@@ -0,0 +1,61 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu_test
+
+import (
+	. "internal/cpu"
+	"internal/godebug"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+func MustHaveDebugOptionsSupport(t *testing.T) {
+	if !DebugOptions {
+		t.Skipf("skipping test: cpu feature options not supported by OS")
+	}
+}
+
+func MustSupportFeatureDetection(t *testing.T) {
+	// TODO: add platforms that do not have CPU feature detection support.
+}
+
+func runDebugOptionsTest(t *testing.T, test string, options string) {
+	MustHaveDebugOptionsSupport(t)
+
+	testenv.MustHaveExec(t)
+
+	env := "GODEBUG=" + options
+
+	cmd := exec.Command(os.Args[0], "-test.run="+test)
+	cmd.Env = append(cmd.Env, env)
+
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%s with %s: run failed: %v output:\n%s\n",
+			test, env, err, string(output))
+	}
+}
+
+func TestDisableAllCapabilities(t *testing.T) {
+	MustSupportFeatureDetection(t)
+	runDebugOptionsTest(t, "TestAllCapabilitiesDisabled", "cpu.all=off")
+}
+
+func TestAllCapabilitiesDisabled(t *testing.T) {
+	MustHaveDebugOptionsSupport(t)
+
+	if godebug.New("cpu.all").Value() != "off" {
+		t.Skipf("skipping test: GODEBUG=cpu.all=off not set")
+	}
+
+	for _, o := range Options {
+		want := false
+		if got := *o.Feature; got != want {
+			t.Errorf("%v: expected %v, got %v", o.Name, want, got)
+		}
+	}
+}
diff --git a/src/internal/cpu/cpu_wasm.go b/src/internal/cpu/cpu_wasm.go
new file mode 100644
index 0000000..2310ad6
--- /dev/null
+++ b/src/internal/cpu/cpu_wasm.go
@@ -0,0 +1,10 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+const CacheLinePadSize = 64
+
+func doinit() {
+}
diff --git a/src/internal/cpu/cpu_x86.go b/src/internal/cpu/cpu_x86.go
new file mode 100644
index 0000000..96b8ef9
--- /dev/null
+++ b/src/internal/cpu/cpu_x86.go
@@ -0,0 +1,190 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64
+
+package cpu
+
+const CacheLinePadSize = 64
+
+// cpuid is implemented in cpu_x86.s.
+func cpuid(eaxArg, ecxArg uint32) (eax, ebx, ecx, edx uint32)
+
+// xgetbv with ecx = 0 is implemented in cpu_x86.s.
+func xgetbv() (eax, edx uint32)
+
+// getGOAMD64level is implemented in cpu_x86.s. Returns number in [1,4].
+func getGOAMD64level() int32
+
+const (
+	// edx bits
+	cpuid_SSE2 = 1 << 26
+
+	// ecx bits
+	cpuid_SSE3      = 1 << 0
+	cpuid_PCLMULQDQ = 1 << 1
+	cpuid_SSSE3     = 1 << 9
+	cpuid_FMA       = 1 << 12
+	cpuid_SSE41     = 1 << 19
+	cpuid_SSE42     = 1 << 20
+	cpuid_POPCNT    = 1 << 23
+	cpuid_AES       = 1 << 25
+	cpuid_OSXSAVE   = 1 << 27
+	cpuid_AVX       = 1 << 28
+
+	// ebx bits
+	cpuid_BMI1 = 1 << 3
+	cpuid_AVX2 = 1 << 5
+	cpuid_BMI2 = 1 << 8
+	cpuid_ERMS = 1 << 9
+	cpuid_ADX  = 1 << 19
+	cpuid_SHA  = 1 << 29
+
+	// edx bits for CPUID 0x80000001
+	cpuid_RDTSCP = 1 << 27
+)
+
+var maxExtendedFunctionInformation uint32
+
+func doinit() {
+	options = []option{
+		{Name: "adx", Feature: &X86.HasADX},
+		{Name: "aes", Feature: &X86.HasAES},
+		{Name: "erms", Feature: &X86.HasERMS},
+		{Name: "pclmulqdq", Feature: &X86.HasPCLMULQDQ},
+		{Name: "rdtscp", Feature: &X86.HasRDTSCP},
+		{Name: "sha", Feature: &X86.HasSHA},
+	}
+	level := getGOAMD64level()
+	if level < 2 {
+		// These options are required at level 2. At lower levels
+		// they can be turned off.
+		options = append(options,
+			option{Name: "popcnt", Feature: &X86.HasPOPCNT},
+			option{Name: "sse3", Feature: &X86.HasSSE3},
+			option{Name: "sse41", Feature: &X86.HasSSE41},
+			option{Name: "sse42", Feature: &X86.HasSSE42},
+			option{Name: "ssse3", Feature: &X86.HasSSSE3})
+	}
+	if level < 3 {
+		// These options are required at level 3. At lower levels
+		// they can be turned off.
+		options = append(options,
+			option{Name: "avx", Feature: &X86.HasAVX},
+			option{Name: "avx2", Feature: &X86.HasAVX2},
+			option{Name: "bmi1", Feature: &X86.HasBMI1},
+			option{Name: "bmi2", Feature: &X86.HasBMI2},
+			option{Name: "fma", Feature: &X86.HasFMA})
+	}
+
+	maxID, _, _, _ := cpuid(0, 0)
+
+	if maxID < 1 {
+		return
+	}
+
+	maxExtendedFunctionInformation, _, _, _ = cpuid(0x80000000, 0)
+
+	_, _, ecx1, _ := cpuid(1, 0)
+
+	X86.HasSSE3 = isSet(ecx1, cpuid_SSE3)
+	X86.HasPCLMULQDQ = isSet(ecx1, cpuid_PCLMULQDQ)
+	X86.HasSSSE3 = isSet(ecx1, cpuid_SSSE3)
+	X86.HasSSE41 = isSet(ecx1, cpuid_SSE41)
+	X86.HasSSE42 = isSet(ecx1, cpuid_SSE42)
+	X86.HasPOPCNT = isSet(ecx1, cpuid_POPCNT)
+	X86.HasAES = isSet(ecx1, cpuid_AES)
+
+	// OSXSAVE can be false when using older Operating Systems
+	// or when explicitly disabled on newer Operating Systems by
+	// e.g. setting the xsavedisable boot option on Windows 10.
+	X86.HasOSXSAVE = isSet(ecx1, cpuid_OSXSAVE)
+
+	// The FMA instruction set extension only has VEX prefixed instructions.
+	// VEX prefixed instructions require OSXSAVE to be enabled.
+	// See Intel 64 and IA-32 Architecture Software Developer’s Manual Volume 2
+	// Section 2.4 "AVX and SSE Instruction Exception Specification"
+	X86.HasFMA = isSet(ecx1, cpuid_FMA) && X86.HasOSXSAVE
+
+	osSupportsAVX := false
+	// For XGETBV, OSXSAVE bit is required and sufficient.
+	if X86.HasOSXSAVE {
+		eax, _ := xgetbv()
+		// Check if XMM and YMM registers have OS support.
+		osSupportsAVX = isSet(eax, 1<<1) && isSet(eax, 1<<2)
+	}
+
+	X86.HasAVX = isSet(ecx1, cpuid_AVX) && osSupportsAVX
+
+	if maxID < 7 {
+		return
+	}
+
+	_, ebx7, _, _ := cpuid(7, 0)
+	X86.HasBMI1 = isSet(ebx7, cpuid_BMI1)
+	X86.HasAVX2 = isSet(ebx7, cpuid_AVX2) && osSupportsAVX
+	X86.HasBMI2 = isSet(ebx7, cpuid_BMI2)
+	X86.HasERMS = isSet(ebx7, cpuid_ERMS)
+	X86.HasADX = isSet(ebx7, cpuid_ADX)
+	X86.HasSHA = isSet(ebx7, cpuid_SHA)
+
+	var maxExtendedInformation uint32
+	maxExtendedInformation, _, _, _ = cpuid(0x80000000, 0)
+
+	if maxExtendedInformation < 0x80000001 {
+		return
+	}
+
+	_, _, _, edxExt1 := cpuid(0x80000001, 0)
+	X86.HasRDTSCP = isSet(edxExt1, cpuid_RDTSCP)
+}
+
+func isSet(hwc uint32, value uint32) bool {
+	return hwc&value != 0
+}
+
+// Name returns the CPU name given by the vendor.
+// If the CPU name can not be determined an
+// empty string is returned.
+func Name() string {
+	if maxExtendedFunctionInformation < 0x80000004 {
+		return ""
+	}
+
+	data := make([]byte, 0, 3*4*4)
+
+	var eax, ebx, ecx, edx uint32
+	eax, ebx, ecx, edx = cpuid(0x80000002, 0)
+	data = appendBytes(data, eax, ebx, ecx, edx)
+	eax, ebx, ecx, edx = cpuid(0x80000003, 0)
+	data = appendBytes(data, eax, ebx, ecx, edx)
+	eax, ebx, ecx, edx = cpuid(0x80000004, 0)
+	data = appendBytes(data, eax, ebx, ecx, edx)
+
+	// Trim leading spaces.
+	for len(data) > 0 && data[0] == ' ' {
+		data = data[1:]
+	}
+
+	// Trim tail after and including the first null byte.
+	for i, c := range data {
+		if c == '\x00' {
+			data = data[:i]
+			break
+		}
+	}
+
+	return string(data)
+}
+
+func appendBytes(b []byte, args ...uint32) []byte {
+	for _, arg := range args {
+		b = append(b,
+			byte((arg >> 0)),
+			byte((arg >> 8)),
+			byte((arg >> 16)),
+			byte((arg >> 24)))
+	}
+	return b
+}
diff --git a/src/internal/cpu/cpu_x86.s b/src/internal/cpu/cpu_x86.s
new file mode 100644
index 0000000..2ee8eca
--- /dev/null
+++ b/src/internal/cpu/cpu_x86.s
@@ -0,0 +1,43 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64
+
+#include "textflag.h"
+
+// func cpuid(eaxArg, ecxArg uint32) (eax, ebx, ecx, edx uint32)
+TEXT ·cpuid(SB), NOSPLIT, $0-24
+	MOVL eaxArg+0(FP), AX
+	MOVL ecxArg+4(FP), CX
+	CPUID
+	MOVL AX, eax+8(FP)
+	MOVL BX, ebx+12(FP)
+	MOVL CX, ecx+16(FP)
+	MOVL DX, edx+20(FP)
+	RET
+
+// func xgetbv() (eax, edx uint32)
+TEXT ·xgetbv(SB),NOSPLIT,$0-8
+	MOVL $0, CX
+	XGETBV
+	MOVL AX, eax+0(FP)
+	MOVL DX, edx+4(FP)
+	RET
+
+// func getGOAMD64level() int32
+TEXT ·getGOAMD64level(SB),NOSPLIT,$0-4
+#ifdef GOAMD64_v4
+	MOVL $4, ret+0(FP)
+#else
+#ifdef GOAMD64_v3
+	MOVL $3, ret+0(FP)
+#else
+#ifdef GOAMD64_v2
+	MOVL $2, ret+0(FP)
+#else
+	MOVL $1, ret+0(FP)
+#endif
+#endif
+#endif
+	RET
diff --git a/src/internal/cpu/cpu_x86_test.go b/src/internal/cpu/cpu_x86_test.go
new file mode 100644
index 0000000..d7be430
--- /dev/null
+++ b/src/internal/cpu/cpu_x86_test.go
@@ -0,0 +1,39 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64
+
+package cpu_test
+
+import (
+	. "internal/cpu"
+	"internal/godebug"
+	"testing"
+)
+
+func TestX86ifAVX2hasAVX(t *testing.T) {
+	if X86.HasAVX2 && !X86.HasAVX {
+		t.Fatalf("HasAVX expected true when HasAVX2 is true, got false")
+	}
+}
+
+func TestDisableSSE3(t *testing.T) {
+	if GetGOAMD64level() > 1 {
+		t.Skip("skipping test: can't run on GOAMD64>v1 machines")
+	}
+	runDebugOptionsTest(t, "TestSSE3DebugOption", "cpu.sse3=off")
+}
+
+func TestSSE3DebugOption(t *testing.T) {
+	MustHaveDebugOptionsSupport(t)
+
+	if godebug.New("cpu.sse3").Value() != "off" {
+		t.Skipf("skipping test: GODEBUG=cpu.sse3=off not set")
+	}
+
+	want := false
+	if got := X86.HasSSE3; got != want {
+		t.Errorf("X86.HasSSE3 expected %v, got %v", want, got)
+	}
+}
diff --git a/src/internal/cpu/export_test.go b/src/internal/cpu/export_test.go
new file mode 100644
index 0000000..91bfc1b
--- /dev/null
+++ b/src/internal/cpu/export_test.go
@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+var (
+	Options = options
+)
diff --git a/src/internal/cpu/export_x86_test.go b/src/internal/cpu/export_x86_test.go
new file mode 100644
index 0000000..a12b6f2
--- /dev/null
+++ b/src/internal/cpu/export_x86_test.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build 386 || amd64
+
+package cpu
+
+var (
+	GetGOAMD64level = getGOAMD64level
+)
diff --git a/src/internal/dag/alg.go b/src/internal/dag/alg.go
new file mode 100644
index 0000000..8800279
--- /dev/null
+++ b/src/internal/dag/alg.go
@@ -0,0 +1,63 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dag
+
+// Transpose reverses all edges in g.
+func (g *Graph) Transpose() {
+	old := g.edges
+
+	g.edges = make(map[string]map[string]bool)
+	for _, n := range g.Nodes {
+		g.edges[n] = make(map[string]bool)
+	}
+
+	for from, tos := range old {
+		for to := range tos {
+			g.edges[to][from] = true
+		}
+	}
+}
+
+// Topo returns a topological sort of g. This function is deterministic.
+func (g *Graph) Topo() []string {
+	topo := make([]string, 0, len(g.Nodes))
+	marks := make(map[string]bool)
+
+	var visit func(n string)
+	visit = func(n string) {
+		if marks[n] {
+			return
+		}
+		for _, to := range g.Edges(n) {
+			visit(to)
+		}
+		marks[n] = true
+		topo = append(topo, n)
+	}
+	for _, root := range g.Nodes {
+		visit(root)
+	}
+	for i, j := 0, len(topo)-1; i < j; i, j = i+1, j-1 {
+		topo[i], topo[j] = topo[j], topo[i]
+	}
+	return topo
+}
+
+// TransitiveReduction removes edges from g that are transitively
+// reachable. g must be transitively closed.
+func (g *Graph) TransitiveReduction() {
+	// For i -> j -> k, if i -> k exists, delete it.
+	for _, i := range g.Nodes {
+		for _, j := range g.Nodes {
+			if g.HasEdge(i, j) {
+				for _, k := range g.Nodes {
+					if g.HasEdge(j, k) {
+						g.DelEdge(i, k)
+					}
+				}
+			}
+		}
+	}
+}
diff --git a/src/internal/dag/alg_test.go b/src/internal/dag/alg_test.go
new file mode 100644
index 0000000..e5ea8b6
--- /dev/null
+++ b/src/internal/dag/alg_test.go
@@ -0,0 +1,46 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dag
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestTranspose(t *testing.T) {
+	g := mustParse(t, diamond)
+	g.Transpose()
+	wantEdges(t, g, "a->b a->c a->d b->d c->d")
+}
+
+func TestTopo(t *testing.T) {
+	g := mustParse(t, diamond)
+	got := g.Topo()
+	// "d" is the root, so it's first.
+	//
+	// "c" and "b" could be in either order, but Topo is
+	// deterministic in reverse node definition order.
+	//
+	// "a" is a leaf.
+	wantNodes := strings.Fields("d c b a")
+	if !reflect.DeepEqual(wantNodes, got) {
+		t.Fatalf("want topo sort %v, got %v", wantNodes, got)
+	}
+}
+
+func TestTransitiveReduction(t *testing.T) {
+	t.Run("diamond", func(t *testing.T) {
+		g := mustParse(t, diamond)
+		g.TransitiveReduction()
+		wantEdges(t, g, "b->a c->a d->b d->c")
+	})
+	t.Run("chain", func(t *testing.T) {
+		const chain = `NONE < a < b < c < d; a, d < e;`
+		g := mustParse(t, chain)
+		g.TransitiveReduction()
+		wantEdges(t, g, "e->d d->c c->b b->a")
+	})
+}
diff --git a/src/internal/dag/parse.go b/src/internal/dag/parse.go
new file mode 100644
index 0000000..9d5b918
--- /dev/null
+++ b/src/internal/dag/parse.go
@@ -0,0 +1,314 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package dag implements a language for expressing directed acyclic
+// graphs.
+//
+// The general syntax of a rule is:
+//
+//	a, b < c, d;
+//
+// which means c and d come after a and b in the partial order
+// (that is, there are edges from c and d to a and b),
+// but doesn't provide a relative order between a vs b or c vs d.
+//
+// The rules can chain together, as in:
+//
+//	e < f, g < h;
+//
+// which is equivalent to
+//
+//	e < f, g;
+//	f, g < h;
+//
+// Except for the special bottom element "NONE", each name
+// must appear exactly once on the right-hand side of any rule.
+// That rule serves as the definition of the allowed successor
+// for that name. The definition must appear before any uses
+// of the name on the left-hand side of a rule. (That is, the
+// rules themselves must be ordered according to the partial
+// order, for easier reading by people.)
+//
+// Negative assertions double-check the partial order:
+//
+//	i !< j
+//
+// means that it must NOT be the case that i < j.
+// Negative assertions may appear anywhere in the rules,
+// even before i and j have been defined.
+//
+// Comments begin with #.
+package dag
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+type Graph struct {
+	Nodes   []string
+	byLabel map[string]int
+	edges   map[string]map[string]bool
+}
+
+func newGraph() *Graph {
+	return &Graph{byLabel: map[string]int{}, edges: map[string]map[string]bool{}}
+}
+
+func (g *Graph) addNode(label string) bool {
+	if _, ok := g.byLabel[label]; ok {
+		return false
+	}
+	g.byLabel[label] = len(g.Nodes)
+	g.Nodes = append(g.Nodes, label)
+	g.edges[label] = map[string]bool{}
+	return true
+}
+
+func (g *Graph) AddEdge(from, to string) {
+	g.edges[from][to] = true
+}
+
+func (g *Graph) DelEdge(from, to string) {
+	delete(g.edges[from], to)
+}
+
+func (g *Graph) HasEdge(from, to string) bool {
+	return g.edges[from] != nil && g.edges[from][to]
+}
+
+func (g *Graph) Edges(from string) []string {
+	edges := make([]string, 0, 16)
+	for k := range g.edges[from] {
+		edges = append(edges, k)
+	}
+	sort.Slice(edges, func(i, j int) bool { return g.byLabel[edges[i]] < g.byLabel[edges[j]] })
+	return edges
+}
+
+// Parse parses the DAG language and returns the transitive closure of
+// the described graph. In the returned graph, there is an edge from "b"
+// to "a" if b < a (or a > b) in the partial order.
+func Parse(dag string) (*Graph, error) {
+	g := newGraph()
+	disallowed := []rule{}
+
+	rules, err := parseRules(dag)
+	if err != nil {
+		return nil, err
+	}
+
+	// TODO: Add line numbers to errors.
+	var errors []string
+	errorf := func(format string, a ...any) {
+		errors = append(errors, fmt.Sprintf(format, a...))
+	}
+	for _, r := range rules {
+		if r.op == "!<" {
+			disallowed = append(disallowed, r)
+			continue
+		}
+		for _, def := range r.def {
+			if def == "NONE" {
+				errorf("NONE cannot be a predecessor")
+				continue
+			}
+			if !g.addNode(def) {
+				errorf("multiple definitions for %s", def)
+			}
+			for _, less := range r.less {
+				if less == "NONE" {
+					continue
+				}
+				if _, ok := g.byLabel[less]; !ok {
+					errorf("use of %s before its definition", less)
+				} else {
+					g.AddEdge(def, less)
+				}
+			}
+		}
+	}
+
+	// Check for missing definition.
+	for _, tos := range g.edges {
+		for to := range tos {
+			if g.edges[to] == nil {
+				errorf("missing definition for %s", to)
+			}
+		}
+	}
+
+	// Complete transitive closure.
+	for _, k := range g.Nodes {
+		for _, i := range g.Nodes {
+			for _, j := range g.Nodes {
+				if i != k && k != j && g.HasEdge(i, k) && g.HasEdge(k, j) {
+					if i == j {
+						// Can only happen along with a "use of X before deps" error above,
+						// but this error is more specific - it makes clear that reordering the
+						// rules will not be enough to fix the problem.
+						errorf("graph cycle: %s < %s < %s", j, k, i)
+					}
+					g.AddEdge(i, j)
+				}
+			}
+		}
+	}
+
+	// Check negative assertions against completed allowed graph.
+	for _, bad := range disallowed {
+		for _, less := range bad.less {
+			for _, def := range bad.def {
+				if g.HasEdge(def, less) {
+					errorf("graph edge assertion failed: %s !< %s", less, def)
+				}
+			}
+		}
+	}
+
+	if len(errors) > 0 {
+		return nil, fmt.Errorf("%s", strings.Join(errors, "\n"))
+	}
+
+	return g, nil
+}
+
+// A rule is a line in the DAG language where "less < def" or "less !< def".
+type rule struct {
+	less []string
+	op   string // Either "<" or "!<"
+	def  []string
+}
+
+type syntaxError string
+
+func (e syntaxError) Error() string {
+	return string(e)
+}
+
+// parseRules parses the rules of a DAG.
+func parseRules(rules string) (out []rule, err error) {
+	defer func() {
+		e := recover()
+		switch e := e.(type) {
+		case nil:
+			return
+		case syntaxError:
+			err = e
+		default:
+			panic(e)
+		}
+	}()
+	p := &rulesParser{lineno: 1, text: rules}
+
+	var prev []string
+	var op string
+	for {
+		list, tok := p.nextList()
+		if tok == "" {
+			if prev == nil {
+				break
+			}
+			p.syntaxError("unexpected EOF")
+		}
+		if prev != nil {
+			out = append(out, rule{prev, op, list})
+		}
+		prev = list
+		if tok == ";" {
+			prev = nil
+			op = ""
+			continue
+		}
+		if tok != "<" && tok != "!<" {
+			p.syntaxError("missing <")
+		}
+		op = tok
+	}
+
+	return out, err
+}
+
+// A rulesParser parses the depsRules syntax described above.
+type rulesParser struct {
+	lineno   int
+	lastWord string
+	text     string
+}
+
+// syntaxError reports a parsing error.
+func (p *rulesParser) syntaxError(msg string) {
+	panic(syntaxError(fmt.Sprintf("parsing graph: line %d: syntax error: %s near %s", p.lineno, msg, p.lastWord)))
+}
+
+// nextList parses and returns a comma-separated list of names.
+func (p *rulesParser) nextList() (list []string, token string) {
+	for {
+		tok := p.nextToken()
+		switch tok {
+		case "":
+			if len(list) == 0 {
+				return nil, ""
+			}
+			fallthrough
+		case ",", "<", "!<", ";":
+			p.syntaxError("bad list syntax")
+		}
+		list = append(list, tok)
+
+		tok = p.nextToken()
+		if tok != "," {
+			return list, tok
+		}
+	}
+}
+
+// nextToken returns the next token in the deps rules,
+// one of ";" "," "<" "!<" or a name.
+func (p *rulesParser) nextToken() string {
+	for {
+		if p.text == "" {
+			return ""
+		}
+		switch p.text[0] {
+		case ';', ',', '<':
+			t := p.text[:1]
+			p.text = p.text[1:]
+			return t
+
+		case '!':
+			if len(p.text) < 2 || p.text[1] != '<' {
+				p.syntaxError("unexpected token !")
+			}
+			p.text = p.text[2:]
+			return "!<"
+
+		case '#':
+			i := strings.Index(p.text, "\n")
+			if i < 0 {
+				i = len(p.text)
+			}
+			p.text = p.text[i:]
+			continue
+
+		case '\n':
+			p.lineno++
+			fallthrough
+		case ' ', '\t':
+			p.text = p.text[1:]
+			continue
+
+		default:
+			i := strings.IndexAny(p.text, "!;,<#\n \t")
+			if i < 0 {
+				i = len(p.text)
+			}
+			t := p.text[:i]
+			p.text = p.text[i:]
+			p.lastWord = t
+			return t
+		}
+	}
+}
diff --git a/src/internal/dag/parse_test.go b/src/internal/dag/parse_test.go
new file mode 100644
index 0000000..b2520c3
--- /dev/null
+++ b/src/internal/dag/parse_test.go
@@ -0,0 +1,61 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dag
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+const diamond = `
+NONE < a < b, c < d;
+`
+
+func mustParse(t *testing.T, dag string) *Graph {
+	t.Helper()
+	g, err := Parse(dag)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return g
+}
+
+func wantEdges(t *testing.T, g *Graph, edges string) {
+	t.Helper()
+
+	wantEdges := strings.Fields(edges)
+	wantEdgeMap := make(map[string]bool)
+	for _, e := range wantEdges {
+		wantEdgeMap[e] = true
+	}
+
+	for _, n1 := range g.Nodes {
+		for _, n2 := range g.Nodes {
+			got := g.HasEdge(n1, n2)
+			want := wantEdgeMap[n1+"->"+n2]
+			if got && want {
+				t.Logf("%s->%s", n1, n2)
+			} else if got && !want {
+				t.Errorf("%s->%s present but not expected", n1, n2)
+			} else if want && !got {
+				t.Errorf("%s->%s missing but expected", n1, n2)
+			}
+		}
+	}
+}
+
+func TestParse(t *testing.T) {
+	// Basic smoke test for graph parsing.
+	g := mustParse(t, diamond)
+
+	wantNodes := strings.Fields("a b c d")
+	if !reflect.DeepEqual(wantNodes, g.Nodes) {
+		t.Fatalf("want nodes %v, got %v", wantNodes, g.Nodes)
+	}
+
+	// Parse returns the transitive closure, so it adds d->a.
+	wantEdges(t, g, "b->a c->a d->a d->b d->c")
+}
diff --git a/src/internal/diff/diff.go b/src/internal/diff/diff.go
new file mode 100644
index 0000000..47b2856
--- /dev/null
+++ b/src/internal/diff/diff.go
@@ -0,0 +1,261 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package diff
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// A pair is a pair of values tracked for both the x and y side of a diff.
+// It is typically a pair of line indexes.
+type pair struct{ x, y int }
+
+// Diff returns an anchored diff of the two texts old and new
+// in the “unified diff” format. If old and new are identical,
+// Diff returns a nil slice (no output).
+//
+// Unix diff implementations typically look for a diff with
+// the smallest number of lines inserted and removed,
+// which can in the worst case take time quadratic in the
+// number of lines in the texts. As a result, many implementations
+// either can be made to run for a long time or cut off the search
+// after a predetermined amount of work.
+//
+// In contrast, this implementation looks for a diff with the
+// smallest number of “unique” lines inserted and removed,
+// where unique means a line that appears just once in both old and new.
+// We call this an “anchored diff” because the unique lines anchor
+// the chosen matching regions. An anchored diff is usually clearer
+// than a standard diff, because the algorithm does not try to
+// reuse unrelated blank lines or closing braces.
+// The algorithm also guarantees to run in O(n log n) time
+// instead of the standard O(n²) time.
+//
+// Some systems call this approach a “patience diff,” named for
+// the “patience sorting” algorithm, itself named for a solitaire card game.
+// We avoid that name for two reasons. First, the name has been used
+// for a few different variants of the algorithm, so it is imprecise.
+// Second, the name is frequently interpreted as meaning that you have
+// to wait longer (to be patient) for the diff, meaning that it is a slower algorithm,
+// when in fact the algorithm is faster than the standard one.
+func Diff(oldName string, old []byte, newName string, new []byte) []byte {
+	if bytes.Equal(old, new) {
+		return nil
+	}
+	x := lines(old)
+	y := lines(new)
+
+	// Print diff header.
+	var out bytes.Buffer
+	fmt.Fprintf(&out, "diff %s %s\n", oldName, newName)
+	fmt.Fprintf(&out, "--- %s\n", oldName)
+	fmt.Fprintf(&out, "+++ %s\n", newName)
+
+	// Loop over matches to consider,
+	// expanding each match to include surrounding lines,
+	// and then printing diff chunks.
+	// To avoid setup/teardown cases outside the loop,
+	// tgs returns a leading {0,0} and trailing {len(x), len(y)} pair
+	// in the sequence of matches.
+	var (
+		done  pair     // printed up to x[:done.x] and y[:done.y]
+		chunk pair     // start lines of current chunk
+		count pair     // number of lines from each side in current chunk
+		ctext []string // lines for current chunk
+	)
+	for _, m := range tgs(x, y) {
+		if m.x < done.x {
+			// Already handled scanning forward from earlier match.
+			continue
+		}
+
+		// Expand matching lines as far possible,
+		// establishing that x[start.x:end.x] == y[start.y:end.y].
+		// Note that on the first (or last) iteration we may (or definitey do)
+		// have an empty match: start.x==end.x and start.y==end.y.
+		start := m
+		for start.x > done.x && start.y > done.y && x[start.x-1] == y[start.y-1] {
+			start.x--
+			start.y--
+		}
+		end := m
+		for end.x < len(x) && end.y < len(y) && x[end.x] == y[end.y] {
+			end.x++
+			end.y++
+		}
+
+		// Emit the mismatched lines before start into this chunk.
+		// (No effect on first sentinel iteration, when start = {0,0}.)
+		for _, s := range x[done.x:start.x] {
+			ctext = append(ctext, "-"+s)
+			count.x++
+		}
+		for _, s := range y[done.y:start.y] {
+			ctext = append(ctext, "+"+s)
+			count.y++
+		}
+
+		// If we're not at EOF and have too few common lines,
+		// the chunk includes all the common lines and continues.
+		const C = 3 // number of context lines
+		if (end.x < len(x) || end.y < len(y)) &&
+			(end.x-start.x < C || (len(ctext) > 0 && end.x-start.x < 2*C)) {
+			for _, s := range x[start.x:end.x] {
+				ctext = append(ctext, " "+s)
+				count.x++
+				count.y++
+			}
+			done = end
+			continue
+		}
+
+		// End chunk with common lines for context.
+		if len(ctext) > 0 {
+			n := end.x - start.x
+			if n > C {
+				n = C
+			}
+			for _, s := range x[start.x : start.x+n] {
+				ctext = append(ctext, " "+s)
+				count.x++
+				count.y++
+			}
+			done = pair{start.x + n, start.y + n}
+
+			// Format and emit chunk.
+			// Convert line numbers to 1-indexed.
+			// Special case: empty file shows up as 0,0 not 1,0.
+			if count.x > 0 {
+				chunk.x++
+			}
+			if count.y > 0 {
+				chunk.y++
+			}
+			fmt.Fprintf(&out, "@@ -%d,%d +%d,%d @@\n", chunk.x, count.x, chunk.y, count.y)
+			for _, s := range ctext {
+				out.WriteString(s)
+			}
+			count.x = 0
+			count.y = 0
+			ctext = ctext[:0]
+		}
+
+		// If we reached EOF, we're done.
+		if end.x >= len(x) && end.y >= len(y) {
+			break
+		}
+
+		// Otherwise start a new chunk.
+		chunk = pair{end.x - C, end.y - C}
+		for _, s := range x[chunk.x:end.x] {
+			ctext = append(ctext, " "+s)
+			count.x++
+			count.y++
+		}
+		done = end
+	}
+
+	return out.Bytes()
+}
+
+// lines returns the lines in the file x, including newlines.
+// If the file does not end in a newline, one is supplied
+// along with a warning about the missing newline.
+func lines(x []byte) []string {
+	l := strings.SplitAfter(string(x), "\n")
+	if l[len(l)-1] == "" {
+		l = l[:len(l)-1]
+	} else {
+		// Treat last line as having a message about the missing newline attached,
+		// using the same text as BSD/GNU diff (including the leading backslash).
+		l[len(l)-1] += "\n\\ No newline at end of file\n"
+	}
+	return l
+}
+
+// tgs returns the pairs of indexes of the longest common subsequence
+// of unique lines in x and y, where a unique line is one that appears
+// once in x and once in y.
+//
+// The longest common subsequence algorithm is as described in
+// Thomas G. Szymanski, “A Special Case of the Maximal Common
+// Subsequence Problem,” Princeton TR #170 (January 1975),
+// available at https://research.swtch.com/tgs170.pdf.
+func tgs(x, y []string) []pair {
+	// Count the number of times each string appears in a and b.
+	// We only care about 0, 1, many, counted as 0, -1, -2
+	// for the x side and 0, -4, -8 for the y side.
+	// Using negative numbers now lets us distinguish positive line numbers later.
+	m := make(map[string]int)
+	for _, s := range x {
+		if c := m[s]; c > -2 {
+			m[s] = c - 1
+		}
+	}
+	for _, s := range y {
+		if c := m[s]; c > -8 {
+			m[s] = c - 4
+		}
+	}
+
+	// Now unique strings can be identified by m[s] = -1+-4.
+	//
+	// Gather the indexes of those strings in x and y, building:
+	//	xi[i] = increasing indexes of unique strings in x.
+	//	yi[i] = increasing indexes of unique strings in y.
+	//	inv[i] = index j such that x[xi[i]] = y[yi[j]].
+	var xi, yi, inv []int
+	for i, s := range y {
+		if m[s] == -1+-4 {
+			m[s] = len(yi)
+			yi = append(yi, i)
+		}
+	}
+	for i, s := range x {
+		if j, ok := m[s]; ok && j >= 0 {
+			xi = append(xi, i)
+			inv = append(inv, j)
+		}
+	}
+
+	// Apply Algorithm A from Szymanski's paper.
+	// In those terms, A = J = inv and B = [0, n).
+	// We add sentinel pairs {0,0}, and {len(x),len(y)}
+	// to the returned sequence, to help the processing loop.
+	J := inv
+	n := len(xi)
+	T := make([]int, n)
+	L := make([]int, n)
+	for i := range T {
+		T[i] = n + 1
+	}
+	for i := 0; i < n; i++ {
+		k := sort.Search(n, func(k int) bool {
+			return T[k] >= J[i]
+		})
+		T[k] = J[i]
+		L[i] = k + 1
+	}
+	k := 0
+	for _, v := range L {
+		if k < v {
+			k = v
+		}
+	}
+	seq := make([]pair, 2+k)
+	seq[1+k] = pair{len(x), len(y)} // sentinel at end
+	lastj := n
+	for i := n - 1; i >= 0; i-- {
+		if L[i] == k && J[i] < lastj {
+			seq[k] = pair{xi[i], yi[J[i]]}
+			k--
+		}
+	}
+	seq[0] = pair{0, 0} // sentinel at start
+	return seq
+}
diff --git a/src/internal/diff/diff_test.go b/src/internal/diff/diff_test.go
new file mode 100644
index 0000000..37281c5
--- /dev/null
+++ b/src/internal/diff/diff_test.go
@@ -0,0 +1,43 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package diff
+
+import (
+	"bytes"
+	"internal/txtar"
+	"path/filepath"
+	"testing"
+)
+
+func clean(text []byte) []byte {
+	text = bytes.ReplaceAll(text, []byte("$\n"), []byte("\n"))
+	text = bytes.TrimSuffix(text, []byte("^D\n"))
+	return text
+}
+
+func Test(t *testing.T) {
+	files, _ := filepath.Glob("testdata/*.txt")
+	if len(files) == 0 {
+		t.Fatalf("no testdata")
+	}
+
+	for _, file := range files {
+		t.Run(filepath.Base(file), func(t *testing.T) {
+			a, err := txtar.ParseFile(file)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(a.Files) != 3 || a.Files[2].Name != "diff" {
+				t.Fatalf("%s: want three files, third named \"diff\"", file)
+			}
+			diffs := Diff(a.Files[0].Name, clean(a.Files[0].Data), a.Files[1].Name, clean(a.Files[1].Data))
+			want := clean(a.Files[2].Data)
+			if !bytes.Equal(diffs, want) {
+				t.Fatalf("%s: have:\n%s\nwant:\n%s\n%s", file,
+					diffs, want, Diff("have", diffs, "want", want))
+			}
+		})
+	}
+}
diff --git a/src/internal/diff/testdata/allnew.txt b/src/internal/diff/testdata/allnew.txt
new file mode 100644
index 0000000..8875649
--- /dev/null
+++ b/src/internal/diff/testdata/allnew.txt
@@ -0,0 +1,13 @@
+-- old --
+-- new --
+a
+b
+c
+-- diff --
+diff old new
+--- old
++++ new
+@@ -0,0 +1,3 @@
++a
++b
++c
diff --git a/src/internal/diff/testdata/allold.txt b/src/internal/diff/testdata/allold.txt
new file mode 100644
index 0000000..bcc9ac0
--- /dev/null
+++ b/src/internal/diff/testdata/allold.txt
@@ -0,0 +1,13 @@
+-- old --
+a
+b
+c
+-- new --
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,3 +0,0 @@
+-a
+-b
+-c
diff --git a/src/internal/diff/testdata/basic.txt b/src/internal/diff/testdata/basic.txt
new file mode 100644
index 0000000..d2565b5
--- /dev/null
+++ b/src/internal/diff/testdata/basic.txt
@@ -0,0 +1,35 @@
+Example from Hunt and McIlroy, “An Algorithm for Differential File Comparison.”
+https://www.cs.dartmouth.edu/~doug/diff.pdf
+
+-- old --
+a
+b
+c
+d
+e
+f
+g
+-- new --
+w
+a
+b
+x
+y
+z
+e
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,7 +1,7 @@
++w
+ a
+ b
+-c
+-d
++x
++y
++z
+ e
+-f
+-g
diff --git a/src/internal/diff/testdata/dups.txt b/src/internal/diff/testdata/dups.txt
new file mode 100644
index 0000000..d10524d
--- /dev/null
+++ b/src/internal/diff/testdata/dups.txt
@@ -0,0 +1,40 @@
+-- old --
+a
+
+b
+
+c
+
+d
+
+e
+
+f
+-- new --
+a
+
+B
+
+C
+
+d
+
+e
+
+f
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,8 +1,8 @@
+ a
+ $
+-b
+-
+-c
++B
++
++C
+ $
+ d
+ $
diff --git a/src/internal/diff/testdata/end.txt b/src/internal/diff/testdata/end.txt
new file mode 100644
index 0000000..158637c
--- /dev/null
+++ b/src/internal/diff/testdata/end.txt
@@ -0,0 +1,38 @@
+-- old --
+1
+2
+3
+4
+5
+6
+7
+eight
+nine
+ten
+eleven
+-- new --
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+-- diff --
+diff old new
+--- old
++++ new
+@@ -5,7 +5,6 @@
+ 5
+ 6
+ 7
+-eight
+-nine
+-ten
+-eleven
++8
++9
++10
diff --git a/src/internal/diff/testdata/eof.txt b/src/internal/diff/testdata/eof.txt
new file mode 100644
index 0000000..5dc145c
--- /dev/null
+++ b/src/internal/diff/testdata/eof.txt
@@ -0,0 +1,9 @@
+-- old --
+a
+b
+c^D
+-- new --
+a
+b
+c^D
+-- diff --
diff --git a/src/internal/diff/testdata/eof1.txt b/src/internal/diff/testdata/eof1.txt
new file mode 100644
index 0000000..1ebf621
--- /dev/null
+++ b/src/internal/diff/testdata/eof1.txt
@@ -0,0 +1,18 @@
+-- old --
+a
+b
+c
+-- new --
+a
+b
+c^D
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,3 +1,3 @@
+ a
+ b
+-c
++c
+\ No newline at end of file
diff --git a/src/internal/diff/testdata/eof2.txt b/src/internal/diff/testdata/eof2.txt
new file mode 100644
index 0000000..047705e
--- /dev/null
+++ b/src/internal/diff/testdata/eof2.txt
@@ -0,0 +1,18 @@
+-- old --
+a
+b
+c^D
+-- new --
+a
+b
+c
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,3 +1,3 @@
+ a
+ b
+-c
+\ No newline at end of file
++c
diff --git a/src/internal/diff/testdata/long.txt b/src/internal/diff/testdata/long.txt
new file mode 100644
index 0000000..3fc99f7
--- /dev/null
+++ b/src/internal/diff/testdata/long.txt
@@ -0,0 +1,62 @@
+-- old --
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+14½
+15
+16
+17
+18
+19
+20
+-- new --
+1
+2
+3
+4
+5
+6
+8
+9
+10
+11
+12
+13
+14
+17
+18
+19
+20
+-- diff --
+diff old new
+--- old
++++ new
+@@ -4,7 +4,6 @@
+ 4
+ 5
+ 6
+-7
+ 8
+ 9
+ 10
+@@ -12,9 +11,6 @@
+ 12
+ 13
+ 14
+-14½
+-15
+-16
+ 17
+ 18
+ 19
diff --git a/src/internal/diff/testdata/same.txt b/src/internal/diff/testdata/same.txt
new file mode 100644
index 0000000..86b1100
--- /dev/null
+++ b/src/internal/diff/testdata/same.txt
@@ -0,0 +1,5 @@
+-- old --
+hello world
+-- new --
+hello world
+-- diff --
diff --git a/src/internal/diff/testdata/start.txt b/src/internal/diff/testdata/start.txt
new file mode 100644
index 0000000..217b2fd
--- /dev/null
+++ b/src/internal/diff/testdata/start.txt
@@ -0,0 +1,34 @@
+-- old --
+e
+pi
+4
+5
+6
+7
+8
+9
+10
+-- new --
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,5 +1,6 @@
+-e
+-pi
++1
++2
++3
+ 4
+ 5
+ 6
diff --git a/src/internal/diff/testdata/triv.txt b/src/internal/diff/testdata/triv.txt
new file mode 100644
index 0000000..ab5759f
--- /dev/null
+++ b/src/internal/diff/testdata/triv.txt
@@ -0,0 +1,40 @@
+Another example from Hunt and McIlroy,
+“An Algorithm for Differential File Comparison.”
+https://www.cs.dartmouth.edu/~doug/diff.pdf
+
+Anchored diff gives up on finding anything,
+since there are no unique lines.
+
+-- old --
+a
+b
+c
+a
+b
+b
+a
+-- new --
+c
+a
+b
+a
+b
+c
+-- diff --
+diff old new
+--- old
++++ new
+@@ -1,7 +1,6 @@
+-a
+-b
+-c
+-a
+-b
+-b
+-a
++c
++a
++b
++a
++b
++c
diff --git a/src/internal/fmtsort/export_test.go b/src/internal/fmtsort/export_test.go
new file mode 100644
index 0000000..25cbb5d
--- /dev/null
+++ b/src/internal/fmtsort/export_test.go
@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmtsort
+
+import "reflect"
+
+func Compare(a, b reflect.Value) int {
+	return compare(a, b)
+}
diff --git a/src/internal/fmtsort/sort.go b/src/internal/fmtsort/sort.go
new file mode 100644
index 0000000..278a89b
--- /dev/null
+++ b/src/internal/fmtsort/sort.go
@@ -0,0 +1,219 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fmtsort provides a general stable ordering mechanism
+// for maps, on behalf of the fmt and text/template packages.
+// It is not guaranteed to be efficient and works only for types
+// that are valid map keys.
+package fmtsort
+
+import (
+	"reflect"
+	"sort"
+)
+
+// Note: Throughout this package we avoid calling reflect.Value.Interface as
+// it is not always legal to do so and it's easier to avoid the issue than to face it.
+
+// SortedMap represents a map's keys and values. The keys and values are
+// aligned in index order: Value[i] is the value in the map corresponding to Key[i].
+type SortedMap struct {
+	Key   []reflect.Value
+	Value []reflect.Value
+}
+
+func (o *SortedMap) Len() int           { return len(o.Key) }
+func (o *SortedMap) Less(i, j int) bool { return compare(o.Key[i], o.Key[j]) < 0 }
+func (o *SortedMap) Swap(i, j int) {
+	o.Key[i], o.Key[j] = o.Key[j], o.Key[i]
+	o.Value[i], o.Value[j] = o.Value[j], o.Value[i]
+}
+
+// Sort accepts a map and returns a SortedMap that has the same keys and
+// values but in a stable sorted order according to the keys, modulo issues
+// raised by unorderable key values such as NaNs.
+//
+// The ordering rules are more general than with Go's < operator:
+//
+//   - when applicable, nil compares low
+//   - ints, floats, and strings order by <
+//   - NaN compares less than non-NaN floats
+//   - bool compares false before true
+//   - complex compares real, then imag
+//   - pointers compare by machine address
+//   - channel values compare by machine address
+//   - structs compare each field in turn
+//   - arrays compare each element in turn.
+//     Otherwise identical arrays compare by length.
+//   - interface values compare first by reflect.Type describing the concrete type
+//     and then by concrete value as described in the previous rules.
+func Sort(mapValue reflect.Value) *SortedMap {
+	if mapValue.Type().Kind() != reflect.Map {
+		return nil
+	}
+	// Note: this code is arranged to not panic even in the presence
+	// of a concurrent map update. The runtime is responsible for
+	// yelling loudly if that happens. See issue 33275.
+	n := mapValue.Len()
+	key := make([]reflect.Value, 0, n)
+	value := make([]reflect.Value, 0, n)
+	iter := mapValue.MapRange()
+	for iter.Next() {
+		key = append(key, iter.Key())
+		value = append(value, iter.Value())
+	}
+	sorted := &SortedMap{
+		Key:   key,
+		Value: value,
+	}
+	sort.Stable(sorted)
+	return sorted
+}
+
+// compare compares two values of the same type. It returns -1, 0, 1
+// according to whether a > b (1), a == b (0), or a < b (-1).
+// If the types differ, it returns -1.
+// See the comment on Sort for the comparison rules.
+func compare(aVal, bVal reflect.Value) int {
+	aType, bType := aVal.Type(), bVal.Type()
+	if aType != bType {
+		return -1 // No good answer possible, but don't return 0: they're not equal.
+	}
+	switch aVal.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		a, b := aVal.Int(), bVal.Int()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		a, b := aVal.Uint(), bVal.Uint()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.String:
+		a, b := aVal.String(), bVal.String()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Float32, reflect.Float64:
+		return floatCompare(aVal.Float(), bVal.Float())
+	case reflect.Complex64, reflect.Complex128:
+		a, b := aVal.Complex(), bVal.Complex()
+		if c := floatCompare(real(a), real(b)); c != 0 {
+			return c
+		}
+		return floatCompare(imag(a), imag(b))
+	case reflect.Bool:
+		a, b := aVal.Bool(), bVal.Bool()
+		switch {
+		case a == b:
+			return 0
+		case a:
+			return 1
+		default:
+			return -1
+		}
+	case reflect.Pointer, reflect.UnsafePointer:
+		a, b := aVal.Pointer(), bVal.Pointer()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Chan:
+		if c, ok := nilCompare(aVal, bVal); ok {
+			return c
+		}
+		ap, bp := aVal.Pointer(), bVal.Pointer()
+		switch {
+		case ap < bp:
+			return -1
+		case ap > bp:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Struct:
+		for i := 0; i < aVal.NumField(); i++ {
+			if c := compare(aVal.Field(i), bVal.Field(i)); c != 0 {
+				return c
+			}
+		}
+		return 0
+	case reflect.Array:
+		for i := 0; i < aVal.Len(); i++ {
+			if c := compare(aVal.Index(i), bVal.Index(i)); c != 0 {
+				return c
+			}
+		}
+		return 0
+	case reflect.Interface:
+		if c, ok := nilCompare(aVal, bVal); ok {
+			return c
+		}
+		c := compare(reflect.ValueOf(aVal.Elem().Type()), reflect.ValueOf(bVal.Elem().Type()))
+		if c != 0 {
+			return c
+		}
+		return compare(aVal.Elem(), bVal.Elem())
+	default:
+		// Certain types cannot appear as keys (maps, funcs, slices), but be explicit.
+		panic("bad type in compare: " + aType.String())
+	}
+}
+
+// nilCompare checks whether either value is nil. If not, the boolean is false.
+// If either value is nil, the boolean is true and the integer is the comparison
+// value. The comparison is defined to be 0 if both are nil, otherwise the one
+// nil value compares low. Both arguments must represent a chan, func,
+// interface, map, pointer, or slice.
+func nilCompare(aVal, bVal reflect.Value) (int, bool) {
+	if aVal.IsNil() {
+		if bVal.IsNil() {
+			return 0, true
+		}
+		return -1, true
+	}
+	if bVal.IsNil() {
+		return 1, true
+	}
+	return 0, false
+}
+
+// floatCompare compares two floating-point values. NaNs compare low.
+func floatCompare(a, b float64) int {
+	switch {
+	case isNaN(a):
+		return -1 // No good answer if b is a NaN so don't bother checking.
+	case isNaN(b):
+		return 1
+	case a < b:
+		return -1
+	case a > b:
+		return 1
+	}
+	return 0
+}
+
+func isNaN(a float64) bool {
+	return a != a
+}
diff --git a/src/internal/fmtsort/sort_test.go b/src/internal/fmtsort/sort_test.go
new file mode 100644
index 0000000..cddcf70
--- /dev/null
+++ b/src/internal/fmtsort/sort_test.go
@@ -0,0 +1,279 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmtsort_test
+
+import (
+	"fmt"
+	"internal/fmtsort"
+	"math"
+	"reflect"
+	"sort"
+	"strings"
+	"testing"
+	"unsafe"
+)
+
+var compareTests = [][]reflect.Value{
+	ct(reflect.TypeOf(int(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int8(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int16(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int32(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int64(0)), -1, 0, 1),
+	ct(reflect.TypeOf(uint(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint8(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint16(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint32(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint64(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uintptr(0)), 0, 1, 5),
+	ct(reflect.TypeOf(string("")), "", "a", "ab"),
+	ct(reflect.TypeOf(float32(0)), math.NaN(), math.Inf(-1), -1e10, 0, 1e10, math.Inf(1)),
+	ct(reflect.TypeOf(float64(0)), math.NaN(), math.Inf(-1), -1e10, 0, 1e10, math.Inf(1)),
+	ct(reflect.TypeOf(complex64(0+1i)), -1-1i, -1+0i, -1+1i, 0-1i, 0+0i, 0+1i, 1-1i, 1+0i, 1+1i),
+	ct(reflect.TypeOf(complex128(0+1i)), -1-1i, -1+0i, -1+1i, 0-1i, 0+0i, 0+1i, 1-1i, 1+0i, 1+1i),
+	ct(reflect.TypeOf(false), false, true),
+	ct(reflect.TypeOf(&ints[0]), &ints[0], &ints[1], &ints[2]),
+	ct(reflect.TypeOf(unsafe.Pointer(&ints[0])), unsafe.Pointer(&ints[0]), unsafe.Pointer(&ints[1]), unsafe.Pointer(&ints[2])),
+	ct(reflect.TypeOf(chans[0]), chans[0], chans[1], chans[2]),
+	ct(reflect.TypeOf(toy{}), toy{0, 1}, toy{0, 2}, toy{1, -1}, toy{1, 1}),
+	ct(reflect.TypeOf([2]int{}), [2]int{1, 1}, [2]int{1, 2}, [2]int{2, 0}),
+	ct(reflect.TypeOf(any(any(0))), iFace, 1, 2, 3),
+}
+
+var iFace any
+
+func ct(typ reflect.Type, args ...any) []reflect.Value {
+	value := make([]reflect.Value, len(args))
+	for i, v := range args {
+		x := reflect.ValueOf(v)
+		if !x.IsValid() { // Make it a typed nil.
+			x = reflect.Zero(typ)
+		} else {
+			x = x.Convert(typ)
+		}
+		value[i] = x
+	}
+	return value
+}
+
+func TestCompare(t *testing.T) {
+	for _, test := range compareTests {
+		for i, v0 := range test {
+			for j, v1 := range test {
+				c := fmtsort.Compare(v0, v1)
+				var expect int
+				switch {
+				case i == j:
+					expect = 0
+					// NaNs are tricky.
+					if typ := v0.Type(); (typ.Kind() == reflect.Float32 || typ.Kind() == reflect.Float64) && math.IsNaN(v0.Float()) {
+						expect = -1
+					}
+				case i < j:
+					expect = -1
+				case i > j:
+					expect = 1
+				}
+				if c != expect {
+					t.Errorf("%s: compare(%v,%v)=%d; expect %d", v0.Type(), v0, v1, c, expect)
+				}
+			}
+		}
+	}
+}
+
+type sortTest struct {
+	data  any    // Always a map.
+	print string // Printed result using our custom printer.
+}
+
+var sortTests = []sortTest{
+	{
+		map[int]string{7: "bar", -3: "foo"},
+		"-3:foo 7:bar",
+	},
+	{
+		map[uint8]string{7: "bar", 3: "foo"},
+		"3:foo 7:bar",
+	},
+	{
+		map[string]string{"7": "bar", "3": "foo"},
+		"3:foo 7:bar",
+	},
+	{
+		map[float64]string{7: "bar", -3: "foo", math.NaN(): "nan", math.Inf(0): "inf"},
+		"NaN:nan -3:foo 7:bar +Inf:inf",
+	},
+	{
+		map[complex128]string{7 + 2i: "bar2", 7 + 1i: "bar", -3: "foo", complex(math.NaN(), 0i): "nan", complex(math.Inf(0), 0i): "inf"},
+		"(NaN+0i):nan (-3+0i):foo (7+1i):bar (7+2i):bar2 (+Inf+0i):inf",
+	},
+	{
+		map[bool]string{true: "true", false: "false"},
+		"false:false true:true",
+	},
+	{
+		chanMap(),
+		"CHAN0:0 CHAN1:1 CHAN2:2",
+	},
+	{
+		pointerMap(),
+		"PTR0:0 PTR1:1 PTR2:2",
+	},
+	{
+		unsafePointerMap(),
+		"UNSAFEPTR0:0 UNSAFEPTR1:1 UNSAFEPTR2:2",
+	},
+	{
+		map[toy]string{{7, 2}: "72", {7, 1}: "71", {3, 4}: "34"},
+		"{3 4}:34 {7 1}:71 {7 2}:72",
+	},
+	{
+		map[[2]int]string{{7, 2}: "72", {7, 1}: "71", {3, 4}: "34"},
+		"[3 4]:34 [7 1]:71 [7 2]:72",
+	},
+}
+
+func sprint(data any) string {
+	om := fmtsort.Sort(reflect.ValueOf(data))
+	if om == nil {
+		return "nil"
+	}
+	b := new(strings.Builder)
+	for i, key := range om.Key {
+		if i > 0 {
+			b.WriteRune(' ')
+		}
+		b.WriteString(sprintKey(key))
+		b.WriteRune(':')
+		fmt.Fprint(b, om.Value[i])
+	}
+	return b.String()
+}
+
+// sprintKey formats a reflect.Value but gives reproducible values for some
+// problematic types such as pointers. Note that it only does special handling
+// for the troublesome types used in the test cases; it is not a general
+// printer.
+func sprintKey(key reflect.Value) string {
+	switch str := key.Type().String(); str {
+	case "*int":
+		ptr := key.Interface().(*int)
+		for i := range ints {
+			if ptr == &ints[i] {
+				return fmt.Sprintf("PTR%d", i)
+			}
+		}
+		return "PTR???"
+	case "unsafe.Pointer":
+		ptr := key.Interface().(unsafe.Pointer)
+		for i := range ints {
+			if ptr == unsafe.Pointer(&ints[i]) {
+				return fmt.Sprintf("UNSAFEPTR%d", i)
+			}
+		}
+		return "UNSAFEPTR???"
+	case "chan int":
+		c := key.Interface().(chan int)
+		for i := range chans {
+			if c == chans[i] {
+				return fmt.Sprintf("CHAN%d", i)
+			}
+		}
+		return "CHAN???"
+	default:
+		return fmt.Sprint(key)
+	}
+}
+
+var (
+	ints  [3]int
+	chans = makeChans()
+)
+
+func makeChans() []chan int {
+	cs := []chan int{make(chan int), make(chan int), make(chan int)}
+	// Order channels by address. See issue #49431.
+	// TODO: pin these pointers once pinning is available (#46787).
+	sort.Slice(cs, func(i, j int) bool {
+		return uintptr(reflect.ValueOf(cs[i]).UnsafePointer()) < uintptr(reflect.ValueOf(cs[j]).UnsafePointer())
+	})
+	return cs
+}
+
+func pointerMap() map[*int]string {
+	m := make(map[*int]string)
+	for i := 2; i >= 0; i-- {
+		m[&ints[i]] = fmt.Sprint(i)
+	}
+	return m
+}
+
+func unsafePointerMap() map[unsafe.Pointer]string {
+	m := make(map[unsafe.Pointer]string)
+	for i := 2; i >= 0; i-- {
+		m[unsafe.Pointer(&ints[i])] = fmt.Sprint(i)
+	}
+	return m
+}
+
+func chanMap() map[chan int]string {
+	m := make(map[chan int]string)
+	for i := 2; i >= 0; i-- {
+		m[chans[i]] = fmt.Sprint(i)
+	}
+	return m
+}
+
+type toy struct {
+	A int // Exported.
+	b int // Unexported.
+}
+
+func TestOrder(t *testing.T) {
+	for _, test := range sortTests {
+		got := sprint(test.data)
+		if got != test.print {
+			t.Errorf("%s: got %q, want %q", reflect.TypeOf(test.data), got, test.print)
+		}
+	}
+}
+
+func TestInterface(t *testing.T) {
+	// A map containing multiple concrete types should be sorted by type,
+	// then value. However, the relative ordering of types is unspecified,
+	// so test this by checking the presence of sorted subgroups.
+	m := map[any]string{
+		[2]int{1, 0}:             "",
+		[2]int{0, 1}:             "",
+		true:                     "",
+		false:                    "",
+		3.1:                      "",
+		2.1:                      "",
+		1.1:                      "",
+		math.NaN():               "",
+		3:                        "",
+		2:                        "",
+		1:                        "",
+		"c":                      "",
+		"b":                      "",
+		"a":                      "",
+		struct{ x, y int }{1, 0}: "",
+		struct{ x, y int }{0, 1}: "",
+	}
+	got := sprint(m)
+	typeGroups := []string{
+		"NaN: 1.1: 2.1: 3.1:", // float64
+		"false: true:",        // bool
+		"1: 2: 3:",            // int
+		"a: b: c:",            // string
+		"[0 1]: [1 0]:",       // [2]int
+		"{0 1}: {1 0}:",       // struct{ x int; y int }
+	}
+	for _, g := range typeGroups {
+		if !strings.Contains(got, g) {
+			t.Errorf("sorted map should contain %q", g)
+		}
+	}
+}
diff --git a/src/internal/fuzz/counters_supported.go b/src/internal/fuzz/counters_supported.go
new file mode 100644
index 0000000..79e27d2
--- /dev/null
+++ b/src/internal/fuzz/counters_supported.go
@@ -0,0 +1,21 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (darwin || linux || windows || freebsd) && (amd64 || arm64)
+
+package fuzz
+
+import (
+	"unsafe"
+)
+
+// coverage returns a []byte containing unique 8-bit counters for each edge of
+// the instrumented source code. This coverage data will only be generated if
+// `-d=libfuzzer` is set at build time. This can be used to understand the code
+// coverage of a test execution.
+func coverage() []byte {
+	addr := unsafe.Pointer(&_counters)
+	size := uintptr(unsafe.Pointer(&_ecounters)) - uintptr(addr)
+	return unsafe.Slice((*byte)(addr), int(size))
+}
diff --git a/src/internal/fuzz/counters_unsupported.go b/src/internal/fuzz/counters_unsupported.go
new file mode 100644
index 0000000..028065c
--- /dev/null
+++ b/src/internal/fuzz/counters_unsupported.go
@@ -0,0 +1,24 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// TODO: expand the set of supported platforms, with testing. Nothing about
+// the instrumentation is OS specific, but only amd64 and arm64 are
+// supported in the runtime. See src/runtime/libfuzzer*.
+//
+// If you update this constraint, also update internal/platform.FuzzInstrumeted.
+//
+//go:build !((darwin || linux || windows || freebsd) && (amd64 || arm64))
+
+package fuzz
+
+// TODO(#48504): re-enable on platforms where instrumentation works.
+// In theory, we shouldn't need this file at all: if the binary was built
+// without coverage, then _counters and _ecounters should have the same address.
+// However, this caused an init failure on aix/ppc64, so it's disabled here.
+
+// coverage returns a []byte containing unique 8-bit counters for each edge of
+// the instrumented source code. This coverage data will only be generated if
+// `-d=libfuzzer` is set at build time. This can be used to understand the code
+// coverage of a test execution.
+func coverage() []byte { return nil }
diff --git a/src/internal/fuzz/coverage.go b/src/internal/fuzz/coverage.go
new file mode 100644
index 0000000..0c5e17e
--- /dev/null
+++ b/src/internal/fuzz/coverage.go
@@ -0,0 +1,107 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"fmt"
+	"math/bits"
+)
+
+// ResetCoverage sets all of the counters for each edge of the instrumented
+// source code to 0.
+func ResetCoverage() {
+	cov := coverage()
+	for i := range cov {
+		cov[i] = 0
+	}
+}
+
+// SnapshotCoverage copies the current counter values into coverageSnapshot,
+// preserving them for later inspection. SnapshotCoverage also rounds each
+// counter down to the nearest power of two. This lets the coordinator store
+// multiple values for each counter by OR'ing them together.
+func SnapshotCoverage() {
+	cov := coverage()
+	for i, b := range cov {
+		b |= b >> 1
+		b |= b >> 2
+		b |= b >> 4
+		b -= b >> 1
+		coverageSnapshot[i] = b
+	}
+}
+
+// diffCoverage returns a set of bits set in snapshot but not in base.
+// If there are no new bits set, diffCoverage returns nil.
+func diffCoverage(base, snapshot []byte) []byte {
+	if len(base) != len(snapshot) {
+		panic(fmt.Sprintf("the number of coverage bits changed: before=%d, after=%d", len(base), len(snapshot)))
+	}
+	found := false
+	for i := range snapshot {
+		if snapshot[i]&^base[i] != 0 {
+			found = true
+			break
+		}
+	}
+	if !found {
+		return nil
+	}
+	diff := make([]byte, len(snapshot))
+	for i := range diff {
+		diff[i] = snapshot[i] &^ base[i]
+	}
+	return diff
+}
+
+// countNewCoverageBits returns the number of bits set in snapshot that are not
+// set in base.
+func countNewCoverageBits(base, snapshot []byte) int {
+	n := 0
+	for i := range snapshot {
+		n += bits.OnesCount8(snapshot[i] &^ base[i])
+	}
+	return n
+}
+
+// isCoverageSubset returns true if all the base coverage bits are set in
+// snapshot.
+func isCoverageSubset(base, snapshot []byte) bool {
+	for i, v := range base {
+		if v&snapshot[i] != v {
+			return false
+		}
+	}
+	return true
+}
+
+// hasCoverageBit returns true if snapshot has at least one bit set that is
+// also set in base.
+func hasCoverageBit(base, snapshot []byte) bool {
+	for i := range snapshot {
+		if snapshot[i]&base[i] != 0 {
+			return true
+		}
+	}
+	return false
+}
+
+func countBits(cov []byte) int {
+	n := 0
+	for _, c := range cov {
+		n += bits.OnesCount8(c)
+	}
+	return n
+}
+
+var (
+	coverageEnabled  = len(coverage()) > 0
+	coverageSnapshot = make([]byte, len(coverage()))
+
+	// _counters and _ecounters mark the start and end, respectively, of where
+	// the 8-bit coverage counters reside in memory. They're known to cmd/link,
+	// which specially assigns their addresses for this purpose.
+	_counters, _ecounters [0]byte
+)
diff --git a/src/internal/fuzz/encoding.go b/src/internal/fuzz/encoding.go
new file mode 100644
index 0000000..270ef7a
--- /dev/null
+++ b/src/internal/fuzz/encoding.go
@@ -0,0 +1,361 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"math"
+	"strconv"
+	"strings"
+	"unicode/utf8"
+)
+
+// encVersion1 will be the first line of a file with version 1 encoding.
+var encVersion1 = "go test fuzz v1"
+
+// marshalCorpusFile encodes an arbitrary number of arguments into the file format for the
+// corpus.
+func marshalCorpusFile(vals ...any) []byte {
+	if len(vals) == 0 {
+		panic("must have at least one value to marshal")
+	}
+	b := bytes.NewBuffer([]byte(encVersion1 + "\n"))
+	// TODO(katiehockman): keep uint8 and int32 encoding where applicable,
+	// instead of changing to byte and rune respectively.
+	for _, val := range vals {
+		switch t := val.(type) {
+		case int, int8, int16, int64, uint, uint16, uint32, uint64, bool:
+			fmt.Fprintf(b, "%T(%v)\n", t, t)
+		case float32:
+			if math.IsNaN(float64(t)) && math.Float32bits(t) != math.Float32bits(float32(math.NaN())) {
+				// We encode unusual NaNs as hex values, because that is how users are
+				// likely to encounter them in literature about floating-point encoding.
+				// This allows us to reproduce fuzz failures that depend on the specific
+				// NaN representation (for float32 there are about 2^24 possibilities!),
+				// not just the fact that the value is *a* NaN.
+				//
+				// Note that the specific value of float32(math.NaN()) can vary based on
+				// whether the architecture represents signaling NaNs using a low bit
+				// (as is common) or a high bit (as commonly implemented on MIPS
+				// hardware before around 2012). We believe that the increase in clarity
+				// from identifying "NaN" with math.NaN() is worth the slight ambiguity
+				// from a platform-dependent value.
+				fmt.Fprintf(b, "math.Float32frombits(0x%x)\n", math.Float32bits(t))
+			} else {
+				// We encode all other values — including the NaN value that is
+				// bitwise-identical to float32(math.Nan()) — using the default
+				// formatting, which is equivalent to strconv.FormatFloat with format
+				// 'g' and can be parsed by strconv.ParseFloat.
+				//
+				// For an ordinary floating-point number this format includes
+				// sufficiently many digits to reconstruct the exact value. For positive
+				// or negative infinity it is the string "+Inf" or "-Inf". For positive
+				// or negative zero it is "0" or "-0". For NaN, it is the string "NaN".
+				fmt.Fprintf(b, "%T(%v)\n", t, t)
+			}
+		case float64:
+			if math.IsNaN(t) && math.Float64bits(t) != math.Float64bits(math.NaN()) {
+				fmt.Fprintf(b, "math.Float64frombits(0x%x)\n", math.Float64bits(t))
+			} else {
+				fmt.Fprintf(b, "%T(%v)\n", t, t)
+			}
+		case string:
+			fmt.Fprintf(b, "string(%q)\n", t)
+		case rune: // int32
+			// Although rune and int32 are represented by the same type, only a subset
+			// of valid int32 values can be expressed as rune literals. Notably,
+			// negative numbers, surrogate halves, and values above unicode.MaxRune
+			// have no quoted representation.
+			//
+			// fmt with "%q" (and the corresponding functions in the strconv package)
+			// would quote out-of-range values to the Unicode replacement character
+			// instead of the original value (see https://go.dev/issue/51526), so
+			// they must be treated as int32 instead.
+			//
+			// We arbitrarily draw the line at UTF-8 validity, which biases toward the
+			// "rune" interpretation. (However, we accept either format as input.)
+			if utf8.ValidRune(t) {
+				fmt.Fprintf(b, "rune(%q)\n", t)
+			} else {
+				fmt.Fprintf(b, "int32(%v)\n", t)
+			}
+		case byte: // uint8
+			// For bytes, we arbitrarily prefer the character interpretation.
+			// (Every byte has a valid character encoding.)
+			fmt.Fprintf(b, "byte(%q)\n", t)
+		case []byte: // []uint8
+			fmt.Fprintf(b, "[]byte(%q)\n", t)
+		default:
+			panic(fmt.Sprintf("unsupported type: %T", t))
+		}
+	}
+	return b.Bytes()
+}
+
+// unmarshalCorpusFile decodes corpus bytes into their respective values.
+func unmarshalCorpusFile(b []byte) ([]any, error) {
+	if len(b) == 0 {
+		return nil, fmt.Errorf("cannot unmarshal empty string")
+	}
+	lines := bytes.Split(b, []byte("\n"))
+	if len(lines) < 2 {
+		return nil, fmt.Errorf("must include version and at least one value")
+	}
+	version := strings.TrimSuffix(string(lines[0]), "\r")
+	if version != encVersion1 {
+		return nil, fmt.Errorf("unknown encoding version: %s", version)
+	}
+	var vals []any
+	for _, line := range lines[1:] {
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 {
+			continue
+		}
+		v, err := parseCorpusValue(line)
+		if err != nil {
+			return nil, fmt.Errorf("malformed line %q: %v", line, err)
+		}
+		vals = append(vals, v)
+	}
+	return vals, nil
+}
+
+func parseCorpusValue(line []byte) (any, error) {
+	fs := token.NewFileSet()
+	expr, err := parser.ParseExprFrom(fs, "(test)", line, 0)
+	if err != nil {
+		return nil, err
+	}
+	call, ok := expr.(*ast.CallExpr)
+	if !ok {
+		return nil, fmt.Errorf("expected call expression")
+	}
+	if len(call.Args) != 1 {
+		return nil, fmt.Errorf("expected call expression with 1 argument; got %d", len(call.Args))
+	}
+	arg := call.Args[0]
+
+	if arrayType, ok := call.Fun.(*ast.ArrayType); ok {
+		if arrayType.Len != nil {
+			return nil, fmt.Errorf("expected []byte or primitive type")
+		}
+		elt, ok := arrayType.Elt.(*ast.Ident)
+		if !ok || elt.Name != "byte" {
+			return nil, fmt.Errorf("expected []byte")
+		}
+		lit, ok := arg.(*ast.BasicLit)
+		if !ok || lit.Kind != token.STRING {
+			return nil, fmt.Errorf("string literal required for type []byte")
+		}
+		s, err := strconv.Unquote(lit.Value)
+		if err != nil {
+			return nil, err
+		}
+		return []byte(s), nil
+	}
+
+	var idType *ast.Ident
+	if selector, ok := call.Fun.(*ast.SelectorExpr); ok {
+		xIdent, ok := selector.X.(*ast.Ident)
+		if !ok || xIdent.Name != "math" {
+			return nil, fmt.Errorf("invalid selector type")
+		}
+		switch selector.Sel.Name {
+		case "Float64frombits":
+			idType = &ast.Ident{Name: "float64-bits"}
+		case "Float32frombits":
+			idType = &ast.Ident{Name: "float32-bits"}
+		default:
+			return nil, fmt.Errorf("invalid selector type")
+		}
+	} else {
+		idType, ok = call.Fun.(*ast.Ident)
+		if !ok {
+			return nil, fmt.Errorf("expected []byte or primitive type")
+		}
+		if idType.Name == "bool" {
+			id, ok := arg.(*ast.Ident)
+			if !ok {
+				return nil, fmt.Errorf("malformed bool")
+			}
+			if id.Name == "true" {
+				return true, nil
+			} else if id.Name == "false" {
+				return false, nil
+			} else {
+				return nil, fmt.Errorf("true or false required for type bool")
+			}
+		}
+	}
+
+	var (
+		val  string
+		kind token.Token
+	)
+	if op, ok := arg.(*ast.UnaryExpr); ok {
+		switch lit := op.X.(type) {
+		case *ast.BasicLit:
+			if op.Op != token.SUB {
+				return nil, fmt.Errorf("unsupported operation on int/float: %v", op.Op)
+			}
+			// Special case for negative numbers.
+			val = op.Op.String() + lit.Value // e.g. "-" + "124"
+			kind = lit.Kind
+		case *ast.Ident:
+			if lit.Name != "Inf" {
+				return nil, fmt.Errorf("expected operation on int or float type")
+			}
+			if op.Op == token.SUB {
+				val = "-Inf"
+			} else {
+				val = "+Inf"
+			}
+			kind = token.FLOAT
+		default:
+			return nil, fmt.Errorf("expected operation on int or float type")
+		}
+	} else {
+		switch lit := arg.(type) {
+		case *ast.BasicLit:
+			val, kind = lit.Value, lit.Kind
+		case *ast.Ident:
+			if lit.Name != "NaN" {
+				return nil, fmt.Errorf("literal value required for primitive type")
+			}
+			val, kind = "NaN", token.FLOAT
+		default:
+			return nil, fmt.Errorf("literal value required for primitive type")
+		}
+	}
+
+	switch typ := idType.Name; typ {
+	case "string":
+		if kind != token.STRING {
+			return nil, fmt.Errorf("string literal value required for type string")
+		}
+		return strconv.Unquote(val)
+	case "byte", "rune":
+		if kind == token.INT {
+			switch typ {
+			case "rune":
+				return parseInt(val, typ)
+			case "byte":
+				return parseUint(val, typ)
+			}
+		}
+		if kind != token.CHAR {
+			return nil, fmt.Errorf("character literal required for byte/rune types")
+		}
+		n := len(val)
+		if n < 2 {
+			return nil, fmt.Errorf("malformed character literal, missing single quotes")
+		}
+		code, _, _, err := strconv.UnquoteChar(val[1:n-1], '\'')
+		if err != nil {
+			return nil, err
+		}
+		if typ == "rune" {
+			return code, nil
+		}
+		if code >= 256 {
+			return nil, fmt.Errorf("can only encode single byte to a byte type")
+		}
+		return byte(code), nil
+	case "int", "int8", "int16", "int32", "int64":
+		if kind != token.INT {
+			return nil, fmt.Errorf("integer literal required for int types")
+		}
+		return parseInt(val, typ)
+	case "uint", "uint8", "uint16", "uint32", "uint64":
+		if kind != token.INT {
+			return nil, fmt.Errorf("integer literal required for uint types")
+		}
+		return parseUint(val, typ)
+	case "float32":
+		if kind != token.FLOAT && kind != token.INT {
+			return nil, fmt.Errorf("float or integer literal required for float32 type")
+		}
+		v, err := strconv.ParseFloat(val, 32)
+		return float32(v), err
+	case "float64":
+		if kind != token.FLOAT && kind != token.INT {
+			return nil, fmt.Errorf("float or integer literal required for float64 type")
+		}
+		return strconv.ParseFloat(val, 64)
+	case "float32-bits":
+		if kind != token.INT {
+			return nil, fmt.Errorf("integer literal required for math.Float32frombits type")
+		}
+		bits, err := parseUint(val, "uint32")
+		if err != nil {
+			return nil, err
+		}
+		return math.Float32frombits(bits.(uint32)), nil
+	case "float64-bits":
+		if kind != token.FLOAT && kind != token.INT {
+			return nil, fmt.Errorf("integer literal required for math.Float64frombits type")
+		}
+		bits, err := parseUint(val, "uint64")
+		if err != nil {
+			return nil, err
+		}
+		return math.Float64frombits(bits.(uint64)), nil
+	default:
+		return nil, fmt.Errorf("expected []byte or primitive type")
+	}
+}
+
+// parseInt returns an integer of value val and type typ.
+func parseInt(val, typ string) (any, error) {
+	switch typ {
+	case "int":
+		// The int type may be either 32 or 64 bits. If 32, the fuzz tests in the
+		// corpus may include 64-bit values produced by fuzzing runs on 64-bit
+		// architectures. When running those tests, we implicitly wrap the values to
+		// fit in a regular int. (The test case is still “interesting”, even if the
+		// specific values of its inputs are platform-dependent.)
+		i, err := strconv.ParseInt(val, 0, 64)
+		return int(i), err
+	case "int8":
+		i, err := strconv.ParseInt(val, 0, 8)
+		return int8(i), err
+	case "int16":
+		i, err := strconv.ParseInt(val, 0, 16)
+		return int16(i), err
+	case "int32", "rune":
+		i, err := strconv.ParseInt(val, 0, 32)
+		return int32(i), err
+	case "int64":
+		return strconv.ParseInt(val, 0, 64)
+	default:
+		panic("unreachable")
+	}
+}
+
+// parseUint returns an unsigned integer of value val and type typ.
+func parseUint(val, typ string) (any, error) {
+	switch typ {
+	case "uint":
+		i, err := strconv.ParseUint(val, 0, 64)
+		return uint(i), err
+	case "uint8", "byte":
+		i, err := strconv.ParseUint(val, 0, 8)
+		return uint8(i), err
+	case "uint16":
+		i, err := strconv.ParseUint(val, 0, 16)
+		return uint16(i), err
+	case "uint32":
+		i, err := strconv.ParseUint(val, 0, 32)
+		return uint32(i), err
+	case "uint64":
+		return strconv.ParseUint(val, 0, 64)
+	default:
+		panic("unreachable")
+	}
+}
diff --git a/src/internal/fuzz/encoding_test.go b/src/internal/fuzz/encoding_test.go
new file mode 100644
index 0000000..6f6173d
--- /dev/null
+++ b/src/internal/fuzz/encoding_test.go
@@ -0,0 +1,409 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"math"
+	"strconv"
+	"testing"
+	"unicode"
+)
+
+func TestUnmarshalMarshal(t *testing.T) {
+	var tests = []struct {
+		desc   string
+		in     string
+		reject bool
+		want   string // if different from in
+	}{
+		{
+			desc:   "missing version",
+			in:     "int(1234)",
+			reject: true,
+		},
+		{
+			desc: "malformed string",
+			in: `go test fuzz v1
+string("a"bcad")`,
+			reject: true,
+		},
+		{
+			desc: "empty value",
+			in: `go test fuzz v1
+int()`,
+			reject: true,
+		},
+		{
+			desc: "negative uint",
+			in: `go test fuzz v1
+uint(-32)`,
+			reject: true,
+		},
+		{
+			desc: "int8 too large",
+			in: `go test fuzz v1
+int8(1234456)`,
+			reject: true,
+		},
+		{
+			desc: "multiplication in int value",
+			in: `go test fuzz v1
+int(20*5)`,
+			reject: true,
+		},
+		{
+			desc: "double negation",
+			in: `go test fuzz v1
+int(--5)`,
+			reject: true,
+		},
+		{
+			desc: "malformed bool",
+			in: `go test fuzz v1
+bool(0)`,
+			reject: true,
+		},
+		{
+			desc: "malformed byte",
+			in: `go test fuzz v1
+byte('aa)`,
+			reject: true,
+		},
+		{
+			desc: "byte out of range",
+			in: `go test fuzz v1
+byte('☃')`,
+			reject: true,
+		},
+		{
+			desc: "extra newline",
+			in: `go test fuzz v1
+string("has extra newline")
+`,
+			want: `go test fuzz v1
+string("has extra newline")`,
+		},
+		{
+			desc: "trailing spaces",
+			in: `go test fuzz v1
+string("extra")
+[]byte("spacing")  
+    `,
+			want: `go test fuzz v1
+string("extra")
+[]byte("spacing")`,
+		},
+		{
+			desc: "float types",
+			in: `go test fuzz v1
+float64(0)
+float32(0)`,
+		},
+		{
+			desc: "various types",
+			in: `go test fuzz v1
+int(-23)
+int8(-2)
+int64(2342425)
+uint(1)
+uint16(234)
+uint32(352342)
+uint64(123)
+rune('œ')
+byte('K')
+byte('ÿ')
+[]byte("hello¿")
+[]byte("a")
+bool(true)
+string("hello\\xbd\\xb2=\\xbc ⌘")
+float64(-12.5)
+float32(2.5)`,
+		},
+		{
+			desc: "float edge cases",
+			// The two IEEE 754 bit patterns used for the math.Float{64,32}frombits
+			// encodings are non-math.NAN quiet-NaN values. Since they are not equal
+			// to math.NaN(), they should be re-encoded to their bit patterns. They
+			// are, respectively:
+			//   * math.Float64bits(math.NaN())+1
+			//   * math.Float32bits(float32(math.NaN()))+1
+			in: `go test fuzz v1
+float32(-0)
+float64(-0)
+float32(+Inf)
+float32(-Inf)
+float32(NaN)
+float64(+Inf)
+float64(-Inf)
+float64(NaN)
+math.Float64frombits(0x7ff8000000000002)
+math.Float32frombits(0x7fc00001)`,
+		},
+		{
+			desc: "int variations",
+			// Although we arbitrarily choose default integer bases (0 or 16), we may
+			// want to change those arbitrary choices in the future and should not
+			// break the parser. Verify that integers in the opposite bases still
+			// parse correctly.
+			in: `go test fuzz v1
+int(0x0)
+int32(0x41)
+int64(0xfffffffff)
+uint32(0xcafef00d)
+uint64(0xffffffffffffffff)
+uint8(0b0000000)
+byte(0x0)
+byte('\000')
+byte('\u0000')
+byte('\'')
+math.Float64frombits(9221120237041090562)
+math.Float32frombits(2143289345)`,
+			want: `go test fuzz v1
+int(0)
+rune('A')
+int64(68719476735)
+uint32(3405705229)
+uint64(18446744073709551615)
+byte('\x00')
+byte('\x00')
+byte('\x00')
+byte('\x00')
+byte('\'')
+math.Float64frombits(0x7ff8000000000002)
+math.Float32frombits(0x7fc00001)`,
+		},
+		{
+			desc: "rune validation",
+			in: `go test fuzz v1
+rune(0)
+rune(0x41)
+rune(-1)
+rune(0xfffd)
+rune(0xd800)
+rune(0x10ffff)
+rune(0x110000)
+`,
+			want: `go test fuzz v1
+rune('\x00')
+rune('A')
+int32(-1)
+rune('�')
+int32(55296)
+rune('\U0010ffff')
+int32(1114112)`,
+		},
+		{
+			desc: "int overflow",
+			in: `go test fuzz v1
+int(0x7fffffffffffffff)
+uint(0xffffffffffffffff)`,
+			want: func() string {
+				switch strconv.IntSize {
+				case 32:
+					return `go test fuzz v1
+int(-1)
+uint(4294967295)`
+				case 64:
+					return `go test fuzz v1
+int(9223372036854775807)
+uint(18446744073709551615)`
+				default:
+					panic("unreachable")
+				}
+			}(),
+		},
+		{
+			desc: "windows new line",
+			in:   "go test fuzz v1\r\nint(0)\r\n",
+			want: "go test fuzz v1\nint(0)",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.desc, func(t *testing.T) {
+			vals, err := unmarshalCorpusFile([]byte(test.in))
+			if test.reject {
+				if err == nil {
+					t.Fatalf("unmarshal unexpected success")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unmarshal unexpected error: %v", err)
+			}
+			newB := marshalCorpusFile(vals...)
+			if err != nil {
+				t.Fatalf("marshal unexpected error: %v", err)
+			}
+			if newB[len(newB)-1] != '\n' {
+				t.Error("didn't write final newline to corpus file")
+			}
+
+			want := test.want
+			if want == "" {
+				want = test.in
+			}
+			want += "\n"
+			got := string(newB)
+			if got != want {
+				t.Errorf("unexpected marshaled value\ngot:\n%s\nwant:\n%s", got, want)
+			}
+		})
+	}
+}
+
+// BenchmarkMarshalCorpusFile measures the time it takes to serialize byte
+// slices of various sizes to a corpus file. The slice contains a repeating
+// sequence of bytes 0-255 to mix escaped and non-escaped characters.
+func BenchmarkMarshalCorpusFile(b *testing.B) {
+	buf := make([]byte, 1024*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i)
+	}
+
+	for sz := 1; sz <= len(buf); sz <<= 1 {
+		sz := sz
+		b.Run(strconv.Itoa(sz), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				b.SetBytes(int64(sz))
+				marshalCorpusFile(buf[:sz])
+			}
+		})
+	}
+}
+
+// BenchmarkUnmarshalCorpusfile measures the time it takes to deserialize
+// files encoding byte slices of various sizes. The slice contains a repeating
+// sequence of bytes 0-255 to mix escaped and non-escaped characters.
+func BenchmarkUnmarshalCorpusFile(b *testing.B) {
+	buf := make([]byte, 1024*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i)
+	}
+
+	for sz := 1; sz <= len(buf); sz <<= 1 {
+		sz := sz
+		data := marshalCorpusFile(buf[:sz])
+		b.Run(strconv.Itoa(sz), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				b.SetBytes(int64(sz))
+				unmarshalCorpusFile(data)
+			}
+		})
+	}
+}
+
+func TestByteRoundTrip(t *testing.T) {
+	for x := 0; x < 256; x++ {
+		b1 := byte(x)
+		buf := marshalCorpusFile(b1)
+		vs, err := unmarshalCorpusFile(buf)
+		if err != nil {
+			t.Fatal(err)
+		}
+		b2 := vs[0].(byte)
+		if b2 != b1 {
+			t.Fatalf("unmarshaled %v, want %v:\n%s", b2, b1, buf)
+		}
+	}
+}
+
+func TestInt8RoundTrip(t *testing.T) {
+	for x := -128; x < 128; x++ {
+		i1 := int8(x)
+		buf := marshalCorpusFile(i1)
+		vs, err := unmarshalCorpusFile(buf)
+		if err != nil {
+			t.Fatal(err)
+		}
+		i2 := vs[0].(int8)
+		if i2 != i1 {
+			t.Fatalf("unmarshaled %v, want %v:\n%s", i2, i1, buf)
+		}
+	}
+}
+
+func FuzzFloat64RoundTrip(f *testing.F) {
+	f.Add(math.Float64bits(0))
+	f.Add(math.Float64bits(math.Copysign(0, -1)))
+	f.Add(math.Float64bits(math.MaxFloat64))
+	f.Add(math.Float64bits(math.SmallestNonzeroFloat64))
+	f.Add(math.Float64bits(math.NaN()))
+	f.Add(uint64(0x7FF0000000000001)) // signaling NaN
+	f.Add(math.Float64bits(math.Inf(1)))
+	f.Add(math.Float64bits(math.Inf(-1)))
+
+	f.Fuzz(func(t *testing.T, u1 uint64) {
+		x1 := math.Float64frombits(u1)
+
+		b := marshalCorpusFile(x1)
+		t.Logf("marshaled math.Float64frombits(0x%x):\n%s", u1, b)
+
+		xs, err := unmarshalCorpusFile(b)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(xs) != 1 {
+			t.Fatalf("unmarshaled %d values", len(xs))
+		}
+		x2 := xs[0].(float64)
+		u2 := math.Float64bits(x2)
+		if u2 != u1 {
+			t.Errorf("unmarshaled %v (bits 0x%x)", x2, u2)
+		}
+	})
+}
+
+func FuzzRuneRoundTrip(f *testing.F) {
+	f.Add(rune(-1))
+	f.Add(rune(0xd800))
+	f.Add(rune(0xdfff))
+	f.Add(rune(unicode.ReplacementChar))
+	f.Add(rune(unicode.MaxASCII))
+	f.Add(rune(unicode.MaxLatin1))
+	f.Add(rune(unicode.MaxRune))
+	f.Add(rune(unicode.MaxRune + 1))
+	f.Add(rune(-0x80000000))
+	f.Add(rune(0x7fffffff))
+
+	f.Fuzz(func(t *testing.T, r1 rune) {
+		b := marshalCorpusFile(r1)
+		t.Logf("marshaled rune(0x%x):\n%s", r1, b)
+
+		rs, err := unmarshalCorpusFile(b)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(rs) != 1 {
+			t.Fatalf("unmarshaled %d values", len(rs))
+		}
+		r2 := rs[0].(rune)
+		if r2 != r1 {
+			t.Errorf("unmarshaled rune(0x%x)", r2)
+		}
+	})
+}
+
+func FuzzStringRoundTrip(f *testing.F) {
+	f.Add("")
+	f.Add("\x00")
+	f.Add(string([]rune{unicode.ReplacementChar}))
+
+	f.Fuzz(func(t *testing.T, s1 string) {
+		b := marshalCorpusFile(s1)
+		t.Logf("marshaled %q:\n%s", s1, b)
+
+		rs, err := unmarshalCorpusFile(b)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(rs) != 1 {
+			t.Fatalf("unmarshaled %d values", len(rs))
+		}
+		s2 := rs[0].(string)
+		if s2 != s1 {
+			t.Errorf("unmarshaled %q", s2)
+		}
+	})
+}
diff --git a/src/internal/fuzz/fuzz.go b/src/internal/fuzz/fuzz.go
new file mode 100644
index 0000000..7d4fe06
--- /dev/null
+++ b/src/internal/fuzz/fuzz.go
@@ -0,0 +1,1083 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fuzz provides common fuzzing functionality for tests built with
+// "go test" and for programs that use fuzzing functionality in the testing
+// package.
+package fuzz
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"internal/godebug"
+	"io"
+	"math/bits"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"time"
+)
+
+// CoordinateFuzzingOpts is a set of arguments for CoordinateFuzzing.
+// The zero value is valid for each field unless specified otherwise.
+type CoordinateFuzzingOpts struct {
+	// Log is a writer for logging progress messages and warnings.
+	// If nil, io.Discard will be used instead.
+	Log io.Writer
+
+	// Timeout is the amount of wall clock time to spend fuzzing after the corpus
+	// has loaded. If zero, there will be no time limit.
+	Timeout time.Duration
+
+	// Limit is the number of random values to generate and test. If zero,
+	// there will be no limit on the number of generated values.
+	Limit int64
+
+	// MinimizeTimeout is the amount of wall clock time to spend minimizing
+	// after discovering a crasher. If zero, there will be no time limit. If
+	// MinimizeTimeout and MinimizeLimit are both zero, then minimization will
+	// be disabled.
+	MinimizeTimeout time.Duration
+
+	// MinimizeLimit is the maximum number of calls to the fuzz function to be
+	// made while minimizing after finding a crash. If zero, there will be no
+	// limit. Calls to the fuzz function made when minimizing also count toward
+	// Limit. If MinimizeTimeout and MinimizeLimit are both zero, then
+	// minimization will be disabled.
+	MinimizeLimit int64
+
+	// parallel is the number of worker processes to run in parallel. If zero,
+	// CoordinateFuzzing will run GOMAXPROCS workers.
+	Parallel int
+
+	// Seed is a list of seed values added by the fuzz target with testing.F.Add
+	// and in testdata.
+	Seed []CorpusEntry
+
+	// Types is the list of types which make up a corpus entry.
+	// Types must be set and must match values in Seed.
+	Types []reflect.Type
+
+	// CorpusDir is a directory where files containing values that crash the
+	// code being tested may be written. CorpusDir must be set.
+	CorpusDir string
+
+	// CacheDir is a directory containing additional "interesting" values.
+	// The fuzzer may derive new values from these, and may write new values here.
+	CacheDir string
+}
+
+// CoordinateFuzzing creates several worker processes and communicates with
+// them to test random inputs that could trigger crashes and expose bugs.
+// The worker processes run the same binary in the same directory with the
+// same environment variables as the coordinator process. Workers also run
+// with the same arguments as the coordinator, except with the -test.fuzzworker
+// flag prepended to the argument list.
+//
+// If a crash occurs, the function will return an error containing information
+// about the crash, which can be reported to the user.
+func CoordinateFuzzing(ctx context.Context, opts CoordinateFuzzingOpts) (err error) {
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	if opts.Log == nil {
+		opts.Log = io.Discard
+	}
+	if opts.Parallel == 0 {
+		opts.Parallel = runtime.GOMAXPROCS(0)
+	}
+	if opts.Limit > 0 && int64(opts.Parallel) > opts.Limit {
+		// Don't start more workers than we need.
+		opts.Parallel = int(opts.Limit)
+	}
+
+	c, err := newCoordinator(opts)
+	if err != nil {
+		return err
+	}
+
+	if opts.Timeout > 0 {
+		var cancel func()
+		ctx, cancel = context.WithTimeout(ctx, opts.Timeout)
+		defer cancel()
+	}
+
+	// fuzzCtx is used to stop workers, for example, after finding a crasher.
+	fuzzCtx, cancelWorkers := context.WithCancel(ctx)
+	defer cancelWorkers()
+	doneC := ctx.Done()
+
+	// stop is called when a worker encounters a fatal error.
+	var fuzzErr error
+	stopping := false
+	stop := func(err error) {
+		if err == fuzzCtx.Err() || isInterruptError(err) {
+			// Suppress cancellation errors and terminations due to SIGINT.
+			// The messages are not helpful since either the user triggered the error
+			// (with ^C) or another more helpful message will be printed (a crasher).
+			err = nil
+		}
+		if err != nil && (fuzzErr == nil || fuzzErr == ctx.Err()) {
+			fuzzErr = err
+		}
+		if stopping {
+			return
+		}
+		stopping = true
+		cancelWorkers()
+		doneC = nil
+	}
+
+	// Ensure that any crash we find is written to the corpus, even if an error
+	// or interruption occurs while minimizing it.
+	crashWritten := false
+	defer func() {
+		if c.crashMinimizing == nil || crashWritten {
+			return
+		}
+		werr := writeToCorpus(&c.crashMinimizing.entry, opts.CorpusDir)
+		if werr != nil {
+			err = fmt.Errorf("%w\n%v", err, werr)
+			return
+		}
+		if err == nil {
+			err = &crashError{
+				path: c.crashMinimizing.entry.Path,
+				err:  errors.New(c.crashMinimizing.crasherMsg),
+			}
+		}
+	}()
+
+	// Start workers.
+	// TODO(jayconrod): do we want to support fuzzing different binaries?
+	dir := "" // same as self
+	binPath := os.Args[0]
+	args := append([]string{"-test.fuzzworker"}, os.Args[1:]...)
+	env := os.Environ() // same as self
+
+	errC := make(chan error)
+	workers := make([]*worker, opts.Parallel)
+	for i := range workers {
+		var err error
+		workers[i], err = newWorker(c, dir, binPath, args, env)
+		if err != nil {
+			return err
+		}
+	}
+	for i := range workers {
+		w := workers[i]
+		go func() {
+			err := w.coordinate(fuzzCtx)
+			if fuzzCtx.Err() != nil || isInterruptError(err) {
+				err = nil
+			}
+			cleanErr := w.cleanup()
+			if err == nil {
+				err = cleanErr
+			}
+			errC <- err
+		}()
+	}
+
+	// Main event loop.
+	// Do not return until all workers have terminated. We avoid a deadlock by
+	// receiving messages from workers even after ctx is cancelled.
+	activeWorkers := len(workers)
+	statTicker := time.NewTicker(3 * time.Second)
+	defer statTicker.Stop()
+	defer c.logStats()
+
+	c.logStats()
+	for {
+		var inputC chan fuzzInput
+		input, ok := c.peekInput()
+		if ok && c.crashMinimizing == nil && !stopping {
+			inputC = c.inputC
+		}
+
+		var minimizeC chan fuzzMinimizeInput
+		minimizeInput, ok := c.peekMinimizeInput()
+		if ok && !stopping {
+			minimizeC = c.minimizeC
+		}
+
+		select {
+		case <-doneC:
+			// Interrupted, cancelled, or timed out.
+			// stop sets doneC to nil so we don't busy wait here.
+			stop(ctx.Err())
+
+		case err := <-errC:
+			// A worker terminated, possibly after encountering a fatal error.
+			stop(err)
+			activeWorkers--
+			if activeWorkers == 0 {
+				return fuzzErr
+			}
+
+		case result := <-c.resultC:
+			// Received response from worker.
+			if stopping {
+				break
+			}
+			c.updateStats(result)
+
+			if result.crasherMsg != "" {
+				if c.warmupRun() && result.entry.IsSeed {
+					target := filepath.Base(c.opts.CorpusDir)
+					fmt.Fprintf(c.opts.Log, "failure while testing seed corpus entry: %s/%s\n", target, testName(result.entry.Parent))
+					stop(errors.New(result.crasherMsg))
+					break
+				}
+				if c.canMinimize() && result.canMinimize {
+					if c.crashMinimizing != nil {
+						// This crash is not minimized, and another crash is being minimized.
+						// Ignore this one and wait for the other one to finish.
+						break
+					}
+					// Found a crasher but haven't yet attempted to minimize it.
+					// Send it back to a worker for minimization. Disable inputC so
+					// other workers don't continue fuzzing.
+					c.crashMinimizing = &result
+					fmt.Fprintf(c.opts.Log, "fuzz: minimizing %d-byte failing input file\n", len(result.entry.Data))
+					c.queueForMinimization(result, nil)
+				} else if !crashWritten {
+					// Found a crasher that's either minimized or not minimizable.
+					// Write to corpus and stop.
+					err := writeToCorpus(&result.entry, opts.CorpusDir)
+					if err == nil {
+						crashWritten = true
+						err = &crashError{
+							path: result.entry.Path,
+							err:  errors.New(result.crasherMsg),
+						}
+					}
+					if shouldPrintDebugInfo() {
+						fmt.Fprintf(
+							c.opts.Log,
+							"DEBUG new crasher, elapsed: %s, id: %s, parent: %s, gen: %d, size: %d, exec time: %s\n",
+							c.elapsed(),
+							result.entry.Path,
+							result.entry.Parent,
+							result.entry.Generation,
+							len(result.entry.Data),
+							result.entryDuration,
+						)
+					}
+					stop(err)
+				}
+			} else if result.coverageData != nil {
+				if c.warmupRun() {
+					if shouldPrintDebugInfo() {
+						fmt.Fprintf(
+							c.opts.Log,
+							"DEBUG processed an initial input, elapsed: %s, id: %s, new bits: %d, size: %d, exec time: %s\n",
+							c.elapsed(),
+							result.entry.Parent,
+							countBits(diffCoverage(c.coverageMask, result.coverageData)),
+							len(result.entry.Data),
+							result.entryDuration,
+						)
+					}
+					c.updateCoverage(result.coverageData)
+					c.warmupInputLeft--
+					if c.warmupInputLeft == 0 {
+						fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, gathering baseline coverage: %d/%d completed, now fuzzing with %d workers\n", c.elapsed(), c.warmupInputCount, c.warmupInputCount, c.opts.Parallel)
+						if shouldPrintDebugInfo() {
+							fmt.Fprintf(
+								c.opts.Log,
+								"DEBUG finished processing input corpus, elapsed: %s, entries: %d, initial coverage bits: %d\n",
+								c.elapsed(),
+								len(c.corpus.entries),
+								countBits(c.coverageMask),
+							)
+						}
+					}
+				} else if keepCoverage := diffCoverage(c.coverageMask, result.coverageData); keepCoverage != nil {
+					// Found a value that expanded coverage.
+					// It's not a crasher, but we may want to add it to the on-disk
+					// corpus and prioritize it for future fuzzing.
+					// TODO(jayconrod, katiehockman): Prioritize fuzzing these
+					// values which expanded coverage, perhaps based on the
+					// number of new edges that this result expanded.
+					// TODO(jayconrod, katiehockman): Don't write a value that's already
+					// in the corpus.
+					if c.canMinimize() && result.canMinimize && c.crashMinimizing == nil {
+						// Send back to workers to find a smaller value that preserves
+						// at least one new coverage bit.
+						c.queueForMinimization(result, keepCoverage)
+					} else {
+						// Update the coordinator's coverage mask and save the value.
+						inputSize := len(result.entry.Data)
+						entryNew, err := c.addCorpusEntries(true, result.entry)
+						if err != nil {
+							stop(err)
+							break
+						}
+						if !entryNew {
+							continue
+						}
+						c.updateCoverage(keepCoverage)
+						c.inputQueue.enqueue(result.entry)
+						c.interestingCount++
+						if shouldPrintDebugInfo() {
+							fmt.Fprintf(
+								c.opts.Log,
+								"DEBUG new interesting input, elapsed: %s, id: %s, parent: %s, gen: %d, new bits: %d, total bits: %d, size: %d, exec time: %s\n",
+								c.elapsed(),
+								result.entry.Path,
+								result.entry.Parent,
+								result.entry.Generation,
+								countBits(keepCoverage),
+								countBits(c.coverageMask),
+								inputSize,
+								result.entryDuration,
+							)
+						}
+					}
+				} else {
+					if shouldPrintDebugInfo() {
+						fmt.Fprintf(
+							c.opts.Log,
+							"DEBUG worker reported interesting input that doesn't expand coverage, elapsed: %s, id: %s, parent: %s, canMinimize: %t\n",
+							c.elapsed(),
+							result.entry.Path,
+							result.entry.Parent,
+							result.canMinimize,
+						)
+					}
+				}
+			} else if c.warmupRun() {
+				// No error or coverage data was reported for this input during
+				// warmup, so continue processing results.
+				c.warmupInputLeft--
+				if c.warmupInputLeft == 0 {
+					fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, testing seed corpus: %d/%d completed, now fuzzing with %d workers\n", c.elapsed(), c.warmupInputCount, c.warmupInputCount, c.opts.Parallel)
+					if shouldPrintDebugInfo() {
+						fmt.Fprintf(
+							c.opts.Log,
+							"DEBUG finished testing-only phase, elapsed: %s, entries: %d\n",
+							time.Since(c.startTime),
+							len(c.corpus.entries),
+						)
+					}
+				}
+			}
+
+			// Once the result has been processed, stop the worker if we
+			// have reached the fuzzing limit.
+			if c.opts.Limit > 0 && c.count >= c.opts.Limit {
+				stop(nil)
+			}
+
+		case inputC <- input:
+			// Sent the next input to a worker.
+			c.sentInput(input)
+
+		case minimizeC <- minimizeInput:
+			// Sent the next input for minimization to a worker.
+			c.sentMinimizeInput(minimizeInput)
+
+		case <-statTicker.C:
+			c.logStats()
+		}
+	}
+
+	// TODO(jayconrod,katiehockman): if a crasher can't be written to the corpus,
+	// write to the cache instead.
+}
+
+// crashError wraps a crasher written to the seed corpus. It saves the name
+// of the file where the input causing the crasher was saved. The testing
+// framework uses this to report a command to re-run that specific input.
+type crashError struct {
+	path string
+	err  error
+}
+
+func (e *crashError) Error() string {
+	return e.err.Error()
+}
+
+func (e *crashError) Unwrap() error {
+	return e.err
+}
+
+func (e *crashError) CrashPath() string {
+	return e.path
+}
+
+type corpus struct {
+	entries []CorpusEntry
+	hashes  map[[sha256.Size]byte]bool
+}
+
+// addCorpusEntries adds entries to the corpus, and optionally writes the entries
+// to the cache directory. If an entry is already in the corpus it is skipped. If
+// all of the entries are unique, addCorpusEntries returns true and a nil error,
+// if at least one of the entries was a duplicate, it returns false and a nil error.
+func (c *coordinator) addCorpusEntries(addToCache bool, entries ...CorpusEntry) (bool, error) {
+	noDupes := true
+	for _, e := range entries {
+		data, err := corpusEntryData(e)
+		if err != nil {
+			return false, err
+		}
+		h := sha256.Sum256(data)
+		if c.corpus.hashes[h] {
+			noDupes = false
+			continue
+		}
+		if addToCache {
+			if err := writeToCorpus(&e, c.opts.CacheDir); err != nil {
+				return false, err
+			}
+			// For entries written to disk, we don't hold onto the bytes,
+			// since the corpus would consume a significant amount of
+			// memory.
+			e.Data = nil
+		}
+		c.corpus.hashes[h] = true
+		c.corpus.entries = append(c.corpus.entries, e)
+	}
+	return noDupes, nil
+}
+
+// CorpusEntry represents an individual input for fuzzing.
+//
+// We must use an equivalent type in the testing and testing/internal/testdeps
+// packages, but testing can't import this package directly, and we don't want
+// to export this type from testing. Instead, we use the same struct type and
+// use a type alias (not a defined type) for convenience.
+type CorpusEntry = struct {
+	Parent string
+
+	// Path is the path of the corpus file, if the entry was loaded from disk.
+	// For other entries, including seed values provided by f.Add, Path is the
+	// name of the test, e.g. seed#0 or its hash.
+	Path string
+
+	// Data is the raw input data. Data should only be populated for seed
+	// values. For on-disk corpus files, Data will be nil, as it will be loaded
+	// from disk using Path.
+	Data []byte
+
+	// Values is the unmarshaled values from a corpus file.
+	Values []any
+
+	Generation int
+
+	// IsSeed indicates whether this entry is part of the seed corpus.
+	IsSeed bool
+}
+
+// corpusEntryData returns the raw input bytes, either from the data struct
+// field, or from disk.
+func corpusEntryData(ce CorpusEntry) ([]byte, error) {
+	if ce.Data != nil {
+		return ce.Data, nil
+	}
+
+	return os.ReadFile(ce.Path)
+}
+
+type fuzzInput struct {
+	// entry is the value to test initially. The worker will randomly mutate
+	// values from this starting point.
+	entry CorpusEntry
+
+	// timeout is the time to spend fuzzing variations of this input,
+	// not including starting or cleaning up.
+	timeout time.Duration
+
+	// limit is the maximum number of calls to the fuzz function the worker may
+	// make. The worker may make fewer calls, for example, if it finds an
+	// error early. If limit is zero, there is no limit on calls to the
+	// fuzz function.
+	limit int64
+
+	// warmup indicates whether this is a warmup input before fuzzing begins. If
+	// true, the input should not be fuzzed.
+	warmup bool
+
+	// coverageData reflects the coordinator's current coverageMask.
+	coverageData []byte
+}
+
+type fuzzResult struct {
+	// entry is an interesting value or a crasher.
+	entry CorpusEntry
+
+	// crasherMsg is an error message from a crash. It's "" if no crash was found.
+	crasherMsg string
+
+	// canMinimize is true if the worker should attempt to minimize this result.
+	// It may be false because an attempt has already been made.
+	canMinimize bool
+
+	// coverageData is set if the worker found new coverage.
+	coverageData []byte
+
+	// limit is the number of values the coordinator asked the worker
+	// to test. 0 if there was no limit.
+	limit int64
+
+	// count is the number of values the worker actually tested.
+	count int64
+
+	// totalDuration is the time the worker spent testing inputs.
+	totalDuration time.Duration
+
+	// entryDuration is the time the worker spent execution an interesting result
+	entryDuration time.Duration
+}
+
+type fuzzMinimizeInput struct {
+	// entry is an interesting value or crasher to minimize.
+	entry CorpusEntry
+
+	// crasherMsg is an error message from a crash. It's "" if no crash was found.
+	// If set, the worker will attempt to find a smaller input that also produces
+	// an error, though not necessarily the same error.
+	crasherMsg string
+
+	// limit is the maximum number of calls to the fuzz function the worker may
+	// make. The worker may make fewer calls, for example, if it can't reproduce
+	// an error. If limit is zero, there is no limit on calls to the fuzz function.
+	limit int64
+
+	// timeout is the time to spend minimizing this input.
+	// A zero timeout means no limit.
+	timeout time.Duration
+
+	// keepCoverage is a set of coverage bits that entry found that were not in
+	// the coordinator's combined set. When minimizing, the worker should find an
+	// input that preserves at least one of these bits. keepCoverage is nil for
+	// crashing inputs.
+	keepCoverage []byte
+}
+
+// coordinator holds channels that workers can use to communicate with
+// the coordinator.
+type coordinator struct {
+	opts CoordinateFuzzingOpts
+
+	// startTime is the time we started the workers after loading the corpus.
+	// Used for logging.
+	startTime time.Time
+
+	// inputC is sent values to fuzz by the coordinator. Any worker may receive
+	// values from this channel. Workers send results to resultC.
+	inputC chan fuzzInput
+
+	// minimizeC is sent values to minimize by the coordinator. Any worker may
+	// receive values from this channel. Workers send results to resultC.
+	minimizeC chan fuzzMinimizeInput
+
+	// resultC is sent results of fuzzing by workers. The coordinator
+	// receives these. Multiple types of messages are allowed.
+	resultC chan fuzzResult
+
+	// count is the number of values fuzzed so far.
+	count int64
+
+	// countLastLog is the number of values fuzzed when the output was last
+	// logged.
+	countLastLog int64
+
+	// timeLastLog is the time at which the output was last logged.
+	timeLastLog time.Time
+
+	// interestingCount is the number of unique interesting values which have
+	// been found this execution.
+	interestingCount int
+
+	// warmupInputCount is the count of all entries in the corpus which will
+	// need to be received from workers to run once during warmup, but not fuzz.
+	// This could be for coverage data, or only for the purposes of verifying
+	// that the seed corpus doesn't have any crashers. See warmupRun.
+	warmupInputCount int
+
+	// warmupInputLeft is the number of entries in the corpus which still need
+	// to be received from workers to run once during warmup, but not fuzz.
+	// See warmupInputLeft.
+	warmupInputLeft int
+
+	// duration is the time spent fuzzing inside workers, not counting time
+	// starting up or tearing down.
+	duration time.Duration
+
+	// countWaiting is the number of fuzzing executions the coordinator is
+	// waiting on workers to complete.
+	countWaiting int64
+
+	// corpus is a set of interesting values, including the seed corpus and
+	// generated values that workers reported as interesting.
+	corpus corpus
+
+	// minimizationAllowed is true if one or more of the types of fuzz
+	// function's parameters can be minimized.
+	minimizationAllowed bool
+
+	// inputQueue is a queue of inputs that workers should try fuzzing. This is
+	// initially populated from the seed corpus and cached inputs. More inputs
+	// may be added as new coverage is discovered.
+	inputQueue queue
+
+	// minimizeQueue is a queue of inputs that caused errors or exposed new
+	// coverage. Workers should attempt to find smaller inputs that do the
+	// same thing.
+	minimizeQueue queue
+
+	// crashMinimizing is the crash that is currently being minimized.
+	crashMinimizing *fuzzResult
+
+	// coverageMask aggregates coverage that was found for all inputs in the
+	// corpus. Each byte represents a single basic execution block. Each set bit
+	// within the byte indicates that an input has triggered that block at least
+	// 1 << n times, where n is the position of the bit in the byte. For example, a
+	// value of 12 indicates that separate inputs have triggered this block
+	// between 4-7 times and 8-15 times.
+	coverageMask []byte
+}
+
+func newCoordinator(opts CoordinateFuzzingOpts) (*coordinator, error) {
+	// Make sure all of the seed corpus has marshalled data.
+	for i := range opts.Seed {
+		if opts.Seed[i].Data == nil && opts.Seed[i].Values != nil {
+			opts.Seed[i].Data = marshalCorpusFile(opts.Seed[i].Values...)
+		}
+	}
+	c := &coordinator{
+		opts:        opts,
+		startTime:   time.Now(),
+		inputC:      make(chan fuzzInput),
+		minimizeC:   make(chan fuzzMinimizeInput),
+		resultC:     make(chan fuzzResult),
+		timeLastLog: time.Now(),
+		corpus:      corpus{hashes: make(map[[sha256.Size]byte]bool)},
+	}
+	if err := c.readCache(); err != nil {
+		return nil, err
+	}
+	if opts.MinimizeLimit > 0 || opts.MinimizeTimeout > 0 {
+		for _, t := range opts.Types {
+			if isMinimizable(t) {
+				c.minimizationAllowed = true
+				break
+			}
+		}
+	}
+
+	covSize := len(coverage())
+	if covSize == 0 {
+		fmt.Fprintf(c.opts.Log, "warning: the test binary was not built with coverage instrumentation, so fuzzing will run without coverage guidance and may be inefficient\n")
+		// Even though a coverage-only run won't occur, we should still run all
+		// of the seed corpus to make sure there are no existing failures before
+		// we start fuzzing.
+		c.warmupInputCount = len(c.opts.Seed)
+		for _, e := range c.opts.Seed {
+			c.inputQueue.enqueue(e)
+		}
+	} else {
+		c.warmupInputCount = len(c.corpus.entries)
+		for _, e := range c.corpus.entries {
+			c.inputQueue.enqueue(e)
+		}
+		// Set c.coverageMask to a clean []byte full of zeros.
+		c.coverageMask = make([]byte, covSize)
+	}
+	c.warmupInputLeft = c.warmupInputCount
+
+	if len(c.corpus.entries) == 0 {
+		fmt.Fprintf(c.opts.Log, "warning: starting with empty corpus\n")
+		var vals []any
+		for _, t := range opts.Types {
+			vals = append(vals, zeroValue(t))
+		}
+		data := marshalCorpusFile(vals...)
+		h := sha256.Sum256(data)
+		name := fmt.Sprintf("%x", h[:4])
+		c.addCorpusEntries(false, CorpusEntry{Path: name, Data: data})
+	}
+
+	return c, nil
+}
+
+func (c *coordinator) updateStats(result fuzzResult) {
+	c.count += result.count
+	c.countWaiting -= result.limit
+	c.duration += result.totalDuration
+}
+
+func (c *coordinator) logStats() {
+	now := time.Now()
+	if c.warmupRun() {
+		runSoFar := c.warmupInputCount - c.warmupInputLeft
+		if coverageEnabled {
+			fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, gathering baseline coverage: %d/%d completed\n", c.elapsed(), runSoFar, c.warmupInputCount)
+		} else {
+			fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, testing seed corpus: %d/%d completed\n", c.elapsed(), runSoFar, c.warmupInputCount)
+		}
+	} else if c.crashMinimizing != nil {
+		fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, minimizing\n", c.elapsed())
+	} else {
+		rate := float64(c.count-c.countLastLog) / now.Sub(c.timeLastLog).Seconds()
+		if coverageEnabled {
+			total := c.warmupInputCount + c.interestingCount
+			fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, execs: %d (%.0f/sec), new interesting: %d (total: %d)\n", c.elapsed(), c.count, rate, c.interestingCount, total)
+		} else {
+			fmt.Fprintf(c.opts.Log, "fuzz: elapsed: %s, execs: %d (%.0f/sec)\n", c.elapsed(), c.count, rate)
+		}
+	}
+	c.countLastLog = c.count
+	c.timeLastLog = now
+}
+
+// peekInput returns the next value that should be sent to workers.
+// If the number of executions is limited, the returned value includes
+// a limit for one worker. If there are no executions left, peekInput returns
+// a zero value and false.
+//
+// peekInput doesn't actually remove the input from the queue. The caller
+// must call sentInput after sending the input.
+//
+// If the input queue is empty and the coverage/testing-only run has completed,
+// queue refills it from the corpus.
+func (c *coordinator) peekInput() (fuzzInput, bool) {
+	if c.opts.Limit > 0 && c.count+c.countWaiting >= c.opts.Limit {
+		// Already making the maximum number of calls to the fuzz function.
+		// Don't send more inputs right now.
+		return fuzzInput{}, false
+	}
+	if c.inputQueue.len == 0 {
+		if c.warmupRun() {
+			// Wait for coverage/testing-only run to finish before sending more
+			// inputs.
+			return fuzzInput{}, false
+		}
+		c.refillInputQueue()
+	}
+
+	entry, ok := c.inputQueue.peek()
+	if !ok {
+		panic("input queue empty after refill")
+	}
+	input := fuzzInput{
+		entry:   entry.(CorpusEntry),
+		timeout: workerFuzzDuration,
+		warmup:  c.warmupRun(),
+	}
+	if c.coverageMask != nil {
+		input.coverageData = bytes.Clone(c.coverageMask)
+	}
+	if input.warmup {
+		// No fuzzing will occur, but it should count toward the limit set by
+		// -fuzztime.
+		input.limit = 1
+		return input, true
+	}
+
+	if c.opts.Limit > 0 {
+		input.limit = c.opts.Limit / int64(c.opts.Parallel)
+		if c.opts.Limit%int64(c.opts.Parallel) > 0 {
+			input.limit++
+		}
+		remaining := c.opts.Limit - c.count - c.countWaiting
+		if input.limit > remaining {
+			input.limit = remaining
+		}
+	}
+	return input, true
+}
+
+// sentInput updates internal counters after an input is sent to c.inputC.
+func (c *coordinator) sentInput(input fuzzInput) {
+	c.inputQueue.dequeue()
+	c.countWaiting += input.limit
+}
+
+// refillInputQueue refills the input queue from the corpus after it becomes
+// empty.
+func (c *coordinator) refillInputQueue() {
+	for _, e := range c.corpus.entries {
+		c.inputQueue.enqueue(e)
+	}
+}
+
+// queueForMinimization creates a fuzzMinimizeInput from result and adds it
+// to the minimization queue to be sent to workers.
+func (c *coordinator) queueForMinimization(result fuzzResult, keepCoverage []byte) {
+	if result.crasherMsg != "" {
+		c.minimizeQueue.clear()
+	}
+
+	input := fuzzMinimizeInput{
+		entry:        result.entry,
+		crasherMsg:   result.crasherMsg,
+		keepCoverage: keepCoverage,
+	}
+	c.minimizeQueue.enqueue(input)
+}
+
+// peekMinimizeInput returns the next input that should be sent to workers for
+// minimization.
+func (c *coordinator) peekMinimizeInput() (fuzzMinimizeInput, bool) {
+	if !c.canMinimize() {
+		// Already making the maximum number of calls to the fuzz function.
+		// Don't send more inputs right now.
+		return fuzzMinimizeInput{}, false
+	}
+	v, ok := c.minimizeQueue.peek()
+	if !ok {
+		return fuzzMinimizeInput{}, false
+	}
+	input := v.(fuzzMinimizeInput)
+
+	if c.opts.MinimizeTimeout > 0 {
+		input.timeout = c.opts.MinimizeTimeout
+	}
+	if c.opts.MinimizeLimit > 0 {
+		input.limit = c.opts.MinimizeLimit
+	} else if c.opts.Limit > 0 {
+		if input.crasherMsg != "" {
+			input.limit = c.opts.Limit
+		} else {
+			input.limit = c.opts.Limit / int64(c.opts.Parallel)
+			if c.opts.Limit%int64(c.opts.Parallel) > 0 {
+				input.limit++
+			}
+		}
+	}
+	if c.opts.Limit > 0 {
+		remaining := c.opts.Limit - c.count - c.countWaiting
+		if input.limit > remaining {
+			input.limit = remaining
+		}
+	}
+	return input, true
+}
+
+// sentMinimizeInput removes an input from the minimization queue after it's
+// sent to minimizeC.
+func (c *coordinator) sentMinimizeInput(input fuzzMinimizeInput) {
+	c.minimizeQueue.dequeue()
+	c.countWaiting += input.limit
+}
+
+// warmupRun returns true while the coordinator is running inputs without
+// mutating them as a warmup before fuzzing. This could be to gather baseline
+// coverage data for entries in the corpus, or to test all of the seed corpus
+// for errors before fuzzing begins.
+//
+// The coordinator doesn't store coverage data in the cache with each input
+// because that data would be invalid when counter offsets in the test binary
+// change.
+//
+// When gathering coverage, the coordinator sends each entry to a worker to
+// gather coverage for that entry only, without fuzzing or minimizing. This
+// phase ends when all workers have finished, and the coordinator has a combined
+// coverage map.
+func (c *coordinator) warmupRun() bool {
+	return c.warmupInputLeft > 0
+}
+
+// updateCoverage sets bits in c.coverageMask that are set in newCoverage.
+// updateCoverage returns the number of newly set bits. See the comment on
+// coverageMask for the format.
+func (c *coordinator) updateCoverage(newCoverage []byte) int {
+	if len(newCoverage) != len(c.coverageMask) {
+		panic(fmt.Sprintf("number of coverage counters changed at runtime: %d, expected %d", len(newCoverage), len(c.coverageMask)))
+	}
+	newBitCount := 0
+	for i := range newCoverage {
+		diff := newCoverage[i] &^ c.coverageMask[i]
+		newBitCount += bits.OnesCount8(diff)
+		c.coverageMask[i] |= newCoverage[i]
+	}
+	return newBitCount
+}
+
+// canMinimize returns whether the coordinator should attempt to find smaller
+// inputs that reproduce a crash or new coverage.
+func (c *coordinator) canMinimize() bool {
+	return c.minimizationAllowed &&
+		(c.opts.Limit == 0 || c.count+c.countWaiting < c.opts.Limit)
+}
+
+func (c *coordinator) elapsed() time.Duration {
+	return time.Since(c.startTime).Round(1 * time.Second)
+}
+
+// readCache creates a combined corpus from seed values and values in the cache
+// (in GOCACHE/fuzz).
+//
+// TODO(fuzzing): need a mechanism that can remove values that
+// aren't useful anymore, for example, because they have the wrong type.
+func (c *coordinator) readCache() error {
+	if _, err := c.addCorpusEntries(false, c.opts.Seed...); err != nil {
+		return err
+	}
+	entries, err := ReadCorpus(c.opts.CacheDir, c.opts.Types)
+	if err != nil {
+		if _, ok := err.(*MalformedCorpusError); !ok {
+			// It's okay if some files in the cache directory are malformed and
+			// are not included in the corpus, but fail if it's an I/O error.
+			return err
+		}
+		// TODO(jayconrod,katiehockman): consider printing some kind of warning
+		// indicating the number of files which were skipped because they are
+		// malformed.
+	}
+	if _, err := c.addCorpusEntries(false, entries...); err != nil {
+		return err
+	}
+	return nil
+}
+
+// MalformedCorpusError is an error found while reading the corpus from the
+// filesystem. All of the errors are stored in the errs list. The testing
+// framework uses this to report malformed files in testdata.
+type MalformedCorpusError struct {
+	errs []error
+}
+
+func (e *MalformedCorpusError) Error() string {
+	var msgs []string
+	for _, s := range e.errs {
+		msgs = append(msgs, s.Error())
+	}
+	return strings.Join(msgs, "\n")
+}
+
+// ReadCorpus reads the corpus from the provided dir. The returned corpus
+// entries are guaranteed to match the given types. Any malformed files will
+// be saved in a MalformedCorpusError and returned, along with the most recent
+// error.
+func ReadCorpus(dir string, types []reflect.Type) ([]CorpusEntry, error) {
+	files, err := os.ReadDir(dir)
+	if os.IsNotExist(err) {
+		return nil, nil // No corpus to read
+	} else if err != nil {
+		return nil, fmt.Errorf("reading seed corpus from testdata: %v", err)
+	}
+	var corpus []CorpusEntry
+	var errs []error
+	for _, file := range files {
+		// TODO(jayconrod,katiehockman): determine when a file is a fuzzing input
+		// based on its name. We should only read files created by writeToCorpus.
+		// If we read ALL files, we won't be able to change the file format by
+		// changing the extension. We also won't be able to add files like
+		// README.txt explaining why the directory exists.
+		if file.IsDir() {
+			continue
+		}
+		filename := filepath.Join(dir, file.Name())
+		data, err := os.ReadFile(filename)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read corpus file: %v", err)
+		}
+		var vals []any
+		vals, err = readCorpusData(data, types)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("%q: %v", filename, err))
+			continue
+		}
+		corpus = append(corpus, CorpusEntry{Path: filename, Values: vals})
+	}
+	if len(errs) > 0 {
+		return corpus, &MalformedCorpusError{errs: errs}
+	}
+	return corpus, nil
+}
+
+func readCorpusData(data []byte, types []reflect.Type) ([]any, error) {
+	vals, err := unmarshalCorpusFile(data)
+	if err != nil {
+		return nil, fmt.Errorf("unmarshal: %v", err)
+	}
+	if err = CheckCorpus(vals, types); err != nil {
+		return nil, err
+	}
+	return vals, nil
+}
+
+// CheckCorpus verifies that the types in vals match the expected types
+// provided.
+func CheckCorpus(vals []any, types []reflect.Type) error {
+	if len(vals) != len(types) {
+		return fmt.Errorf("wrong number of values in corpus entry: %d, want %d", len(vals), len(types))
+	}
+	valsT := make([]reflect.Type, len(vals))
+	for valsI, v := range vals {
+		valsT[valsI] = reflect.TypeOf(v)
+	}
+	for i := range types {
+		if valsT[i] != types[i] {
+			return fmt.Errorf("mismatched types in corpus entry: %v, want %v", valsT, types)
+		}
+	}
+	return nil
+}
+
+// writeToCorpus atomically writes the given bytes to a new file in testdata. If
+// the directory does not exist, it will create one. If the file already exists,
+// writeToCorpus will not rewrite it. writeToCorpus sets entry.Path to the new
+// file that was just written or an error if it failed.
+func writeToCorpus(entry *CorpusEntry, dir string) (err error) {
+	sum := fmt.Sprintf("%x", sha256.Sum256(entry.Data))[:16]
+	entry.Path = filepath.Join(dir, sum)
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return err
+	}
+	if err := os.WriteFile(entry.Path, entry.Data, 0666); err != nil {
+		os.Remove(entry.Path) // remove partially written file
+		return err
+	}
+	return nil
+}
+
+func testName(path string) string {
+	return filepath.Base(path)
+}
+
+func zeroValue(t reflect.Type) any {
+	for _, v := range zeroVals {
+		if reflect.TypeOf(v) == t {
+			return v
+		}
+	}
+	panic(fmt.Sprintf("unsupported type: %v", t))
+}
+
+var zeroVals []any = []any{
+	[]byte(""),
+	string(""),
+	false,
+	byte(0),
+	rune(0),
+	float32(0),
+	float64(0),
+	int(0),
+	int8(0),
+	int16(0),
+	int32(0),
+	int64(0),
+	uint(0),
+	uint8(0),
+	uint16(0),
+	uint32(0),
+	uint64(0),
+}
+
+var debugInfo = godebug.New("fuzzdebug").Value() == "1"
+
+func shouldPrintDebugInfo() bool {
+	return debugInfo
+}
diff --git a/src/internal/fuzz/mem.go b/src/internal/fuzz/mem.go
new file mode 100644
index 0000000..4155e4e
--- /dev/null
+++ b/src/internal/fuzz/mem.go
@@ -0,0 +1,138 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"unsafe"
+)
+
+// sharedMem manages access to a region of virtual memory mapped from a file,
+// shared between multiple processes. The region includes space for a header and
+// a value of variable length.
+//
+// When fuzzing, the coordinator creates a sharedMem from a temporary file for
+// each worker. This buffer is used to pass values to fuzz between processes.
+// Care must be taken to manage access to shared memory across processes;
+// sharedMem provides no synchronization on its own. See workerComm for an
+// explanation.
+type sharedMem struct {
+	// f is the file mapped into memory.
+	f *os.File
+
+	// region is the mapped region of virtual memory for f. The content of f may
+	// be read or written through this slice.
+	region []byte
+
+	// removeOnClose is true if the file should be deleted by Close.
+	removeOnClose bool
+
+	// sys contains OS-specific information.
+	sys sharedMemSys
+}
+
+// sharedMemHeader stores metadata in shared memory.
+type sharedMemHeader struct {
+	// count is the number of times the worker has called the fuzz function.
+	// May be reset by coordinator.
+	count int64
+
+	// valueLen is the number of bytes in region which should be read.
+	valueLen int
+
+	// randState and randInc hold the state of a pseudo-random number generator.
+	randState, randInc uint64
+
+	// rawInMem is true if the region holds raw bytes, which occurs during
+	// minimization. If true after the worker fails during minimization, this
+	// indicates that an unrecoverable error occurred, and the region can be
+	// used to retrieve the raw bytes that caused the error.
+	rawInMem bool
+}
+
+// sharedMemSize returns the size needed for a shared memory buffer that can
+// contain values of the given size.
+func sharedMemSize(valueSize int) int {
+	// TODO(jayconrod): set a reasonable maximum size per platform.
+	return int(unsafe.Sizeof(sharedMemHeader{})) + valueSize
+}
+
+// sharedMemTempFile creates a new temporary file of the given size, then maps
+// it into memory. The file will be removed when the Close method is called.
+func sharedMemTempFile(size int) (m *sharedMem, err error) {
+	// Create a temporary file.
+	f, err := os.CreateTemp("", "fuzz-*")
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if err != nil {
+			f.Close()
+			os.Remove(f.Name())
+		}
+	}()
+
+	// Resize it to the correct size.
+	totalSize := sharedMemSize(size)
+	if err := f.Truncate(int64(totalSize)); err != nil {
+		return nil, err
+	}
+
+	// Map the file into memory.
+	removeOnClose := true
+	return sharedMemMapFile(f, totalSize, removeOnClose)
+}
+
+// header returns a pointer to metadata within the shared memory region.
+func (m *sharedMem) header() *sharedMemHeader {
+	return (*sharedMemHeader)(unsafe.Pointer(&m.region[0]))
+}
+
+// valueRef returns the value currently stored in shared memory. The returned
+// slice points to shared memory; it is not a copy.
+func (m *sharedMem) valueRef() []byte {
+	length := m.header().valueLen
+	valueOffset := int(unsafe.Sizeof(sharedMemHeader{}))
+	return m.region[valueOffset : valueOffset+length]
+}
+
+// valueCopy returns a copy of the value stored in shared memory.
+func (m *sharedMem) valueCopy() []byte {
+	ref := m.valueRef()
+	return bytes.Clone(ref)
+}
+
+// setValue copies the data in b into the shared memory buffer and sets
+// the length. len(b) must be less than or equal to the capacity of the buffer
+// (as returned by cap(m.value())).
+func (m *sharedMem) setValue(b []byte) {
+	v := m.valueRef()
+	if len(b) > cap(v) {
+		panic(fmt.Sprintf("value length %d larger than shared memory capacity %d", len(b), cap(v)))
+	}
+	m.header().valueLen = len(b)
+	copy(v[:cap(v)], b)
+}
+
+// setValueLen sets the length of the shared memory buffer returned by valueRef
+// to n, which may be at most the cap of that slice.
+//
+// Note that we can only store the length in the shared memory header. The full
+// slice header contains a pointer, which is likely only valid for one process,
+// since each process can map shared memory at a different virtual address.
+func (m *sharedMem) setValueLen(n int) {
+	v := m.valueRef()
+	if n > cap(v) {
+		panic(fmt.Sprintf("length %d larger than shared memory capacity %d", n, cap(v)))
+	}
+	m.header().valueLen = n
+}
+
+// TODO(jayconrod): add method to resize the buffer. We'll need that when the
+// mutator can increase input length. Only the coordinator will be able to
+// do it, since we'll need to send a message to the worker telling it to
+// remap the file.
diff --git a/src/internal/fuzz/minimize.go b/src/internal/fuzz/minimize.go
new file mode 100644
index 0000000..0e410fb
--- /dev/null
+++ b/src/internal/fuzz/minimize.go
@@ -0,0 +1,95 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"reflect"
+)
+
+func isMinimizable(t reflect.Type) bool {
+	return t == reflect.TypeOf("") || t == reflect.TypeOf([]byte(nil))
+}
+
+func minimizeBytes(v []byte, try func([]byte) bool, shouldStop func() bool) {
+	tmp := make([]byte, len(v))
+	// If minimization was successful at any point during minimizeBytes,
+	// then the vals slice in (*workerServer).minimizeInput will point to
+	// tmp. Since tmp is altered while making new candidates, we need to
+	// make sure that it is equal to the correct value, v, before exiting
+	// this function.
+	defer copy(tmp, v)
+
+	// First, try to cut the tail.
+	for n := 1024; n != 0; n /= 2 {
+		for len(v) > n {
+			if shouldStop() {
+				return
+			}
+			candidate := v[:len(v)-n]
+			if !try(candidate) {
+				break
+			}
+			// Set v to the new value to continue iterating.
+			v = candidate
+		}
+	}
+
+	// Then, try to remove each individual byte.
+	for i := 0; i < len(v)-1; i++ {
+		if shouldStop() {
+			return
+		}
+		candidate := tmp[:len(v)-1]
+		copy(candidate[:i], v[:i])
+		copy(candidate[i:], v[i+1:])
+		if !try(candidate) {
+			continue
+		}
+		// Update v to delete the value at index i.
+		copy(v[i:], v[i+1:])
+		v = v[:len(candidate)]
+		// v[i] is now different, so decrement i to redo this iteration
+		// of the loop with the new value.
+		i--
+	}
+
+	// Then, try to remove each possible subset of bytes.
+	for i := 0; i < len(v)-1; i++ {
+		copy(tmp, v[:i])
+		for j := len(v); j > i+1; j-- {
+			if shouldStop() {
+				return
+			}
+			candidate := tmp[:len(v)-j+i]
+			copy(candidate[i:], v[j:])
+			if !try(candidate) {
+				continue
+			}
+			// Update v and reset the loop with the new length.
+			copy(v[i:], v[j:])
+			v = v[:len(candidate)]
+			j = len(v)
+		}
+	}
+
+	// Then, try to make it more simplified and human-readable by trying to replace each
+	// byte with a printable character.
+	printableChars := []byte("012789ABCXYZabcxyz !\"#$%&'()*+,.")
+	for i, b := range v {
+		if shouldStop() {
+			return
+		}
+
+		for _, pc := range printableChars {
+			v[i] = pc
+			if try(v) {
+				// Successful. Move on to the next byte in v.
+				break
+			}
+			// Unsuccessful. Revert v[i] back to original value.
+			v[i] = b
+		}
+	}
+}
diff --git a/src/internal/fuzz/minimize_test.go b/src/internal/fuzz/minimize_test.go
new file mode 100644
index 0000000..2db2633
--- /dev/null
+++ b/src/internal/fuzz/minimize_test.go
@@ -0,0 +1,182 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || freebsd || linux || windows
+
+package fuzz
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"reflect"
+	"testing"
+	"time"
+	"unicode"
+	"unicode/utf8"
+)
+
+func TestMinimizeInput(t *testing.T) {
+	type testcase struct {
+		name     string
+		fn       func(CorpusEntry) error
+		input    []any
+		expected []any
+	}
+	cases := []testcase{
+		{
+			name: "ones_byte",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].([]byte)
+				ones := 0
+				for _, v := range b {
+					if v == 1 {
+						ones++
+					}
+				}
+				if ones == 3 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{[]byte{0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}},
+			expected: []any{[]byte{1, 1, 1}},
+		},
+		{
+			name: "single_bytes",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].([]byte)
+				if len(b) < 2 {
+					return nil
+				}
+				if len(b) == 2 && b[0] == 1 && b[1] == 2 {
+					return nil
+				}
+				return fmt.Errorf("bad %v", e.Values[0])
+			},
+			input:    []any{[]byte{1, 2, 3, 4, 5}},
+			expected: []any{[]byte("00")},
+		},
+		{
+			name: "set_of_bytes",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].([]byte)
+				if len(b) < 3 {
+					return nil
+				}
+				if bytes.Equal(b, []byte{0, 1, 2, 3, 4, 5}) || bytes.Equal(b, []byte{0, 4, 5}) {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{[]byte{0, 1, 2, 3, 4, 5}},
+			expected: []any{[]byte{0, 4, 5}},
+		},
+		{
+			name: "non_ascii_bytes",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].([]byte)
+				if len(b) == 3 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{[]byte("ท")}, // ท is 3 bytes
+			expected: []any{[]byte("000")},
+		},
+		{
+			name: "ones_string",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].(string)
+				ones := 0
+				for _, v := range b {
+					if v == '1' {
+						ones++
+					}
+				}
+				if ones == 3 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{"001010001000000000000000000"},
+			expected: []any{"111"},
+		},
+		{
+			name: "string_length",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].(string)
+				if len(b) == 5 {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{"zzzzz"},
+			expected: []any{"00000"},
+		},
+		{
+			name: "string_with_letter",
+			fn: func(e CorpusEntry) error {
+				b := e.Values[0].(string)
+				r, _ := utf8.DecodeRune([]byte(b))
+				if unicode.IsLetter(r) {
+					return fmt.Errorf("bad %v", e.Values[0])
+				}
+				return nil
+			},
+			input:    []any{"ZZZZZ"},
+			expected: []any{"A"},
+		},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			ws := &workerServer{
+				fuzzFn: func(e CorpusEntry) (time.Duration, error) {
+					return time.Second, tc.fn(e)
+				},
+			}
+			mem := &sharedMem{region: make([]byte, 100)} // big enough to hold value and header
+			vals := tc.input
+			success, err := ws.minimizeInput(context.Background(), vals, mem, minimizeArgs{})
+			if !success {
+				t.Errorf("minimizeInput did not succeed")
+			}
+			if err == nil {
+				t.Fatal("minimizeInput didn't provide an error")
+			}
+			if expected := fmt.Sprintf("bad %v", tc.expected[0]); err.Error() != expected {
+				t.Errorf("unexpected error: got %q, want %q", err, expected)
+			}
+			if !reflect.DeepEqual(vals, tc.expected) {
+				t.Errorf("unexpected results: got %v, want %v", vals, tc.expected)
+			}
+		})
+	}
+}
+
+// TestMinimizeFlaky checks that if we're minimizing an interesting
+// input and a flaky failure occurs, that minimization was not indicated
+// to be successful, and the error isn't returned (since it's flaky).
+func TestMinimizeFlaky(t *testing.T) {
+	ws := &workerServer{fuzzFn: func(e CorpusEntry) (time.Duration, error) {
+		return time.Second, errors.New("ohno")
+	}}
+	mem := &sharedMem{region: make([]byte, 100)} // big enough to hold value and header
+	vals := []any{[]byte(nil)}
+	args := minimizeArgs{KeepCoverage: make([]byte, len(coverageSnapshot))}
+	success, err := ws.minimizeInput(context.Background(), vals, mem, args)
+	if success {
+		t.Error("unexpected success")
+	}
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if count := mem.header().count; count != 1 {
+		t.Errorf("count: got %d, want 1", count)
+	}
+}
diff --git a/src/internal/fuzz/mutator.go b/src/internal/fuzz/mutator.go
new file mode 100644
index 0000000..bb96066
--- /dev/null
+++ b/src/internal/fuzz/mutator.go
@@ -0,0 +1,300 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"unsafe"
+)
+
+type mutator struct {
+	r       mutatorRand
+	scratch []byte // scratch slice to avoid additional allocations
+}
+
+func newMutator() *mutator {
+	return &mutator{r: newPcgRand()}
+}
+
+func (m *mutator) rand(n int) int {
+	return m.r.intn(n)
+}
+
+func (m *mutator) randByteOrder() binary.ByteOrder {
+	if m.r.bool() {
+		return binary.LittleEndian
+	}
+	return binary.BigEndian
+}
+
+// chooseLen chooses length of range mutation in range [1,n]. It gives
+// preference to shorter ranges.
+func (m *mutator) chooseLen(n int) int {
+	switch x := m.rand(100); {
+	case x < 90:
+		return m.rand(min(8, n)) + 1
+	case x < 99:
+		return m.rand(min(32, n)) + 1
+	default:
+		return m.rand(n) + 1
+	}
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+// mutate performs several mutations on the provided values.
+func (m *mutator) mutate(vals []any, maxBytes int) {
+	// TODO(katiehockman): pull some of these functions into helper methods and
+	// test that each case is working as expected.
+	// TODO(katiehockman): perform more types of mutations for []byte.
+
+	// maxPerVal will represent the maximum number of bytes that each value be
+	// allowed after mutating, giving an equal amount of capacity to each line.
+	// Allow a little wiggle room for the encoding.
+	maxPerVal := maxBytes/len(vals) - 100
+
+	// Pick a random value to mutate.
+	// TODO: consider mutating more than one value at a time.
+	i := m.rand(len(vals))
+	switch v := vals[i].(type) {
+	case int:
+		vals[i] = int(m.mutateInt(int64(v), maxInt))
+	case int8:
+		vals[i] = int8(m.mutateInt(int64(v), math.MaxInt8))
+	case int16:
+		vals[i] = int16(m.mutateInt(int64(v), math.MaxInt16))
+	case int64:
+		vals[i] = m.mutateInt(v, maxInt)
+	case uint:
+		vals[i] = uint(m.mutateUInt(uint64(v), maxUint))
+	case uint16:
+		vals[i] = uint16(m.mutateUInt(uint64(v), math.MaxUint16))
+	case uint32:
+		vals[i] = uint32(m.mutateUInt(uint64(v), math.MaxUint32))
+	case uint64:
+		vals[i] = m.mutateUInt(uint64(v), maxUint)
+	case float32:
+		vals[i] = float32(m.mutateFloat(float64(v), math.MaxFloat32))
+	case float64:
+		vals[i] = m.mutateFloat(v, math.MaxFloat64)
+	case bool:
+		if m.rand(2) == 1 {
+			vals[i] = !v // 50% chance of flipping the bool
+		}
+	case rune: // int32
+		vals[i] = rune(m.mutateInt(int64(v), math.MaxInt32))
+	case byte: // uint8
+		vals[i] = byte(m.mutateUInt(uint64(v), math.MaxUint8))
+	case string:
+		if len(v) > maxPerVal {
+			panic(fmt.Sprintf("cannot mutate bytes of length %d", len(v)))
+		}
+		if cap(m.scratch) < maxPerVal {
+			m.scratch = append(make([]byte, 0, maxPerVal), v...)
+		} else {
+			m.scratch = m.scratch[:len(v)]
+			copy(m.scratch, v)
+		}
+		m.mutateBytes(&m.scratch)
+		vals[i] = string(m.scratch)
+	case []byte:
+		if len(v) > maxPerVal {
+			panic(fmt.Sprintf("cannot mutate bytes of length %d", len(v)))
+		}
+		if cap(m.scratch) < maxPerVal {
+			m.scratch = append(make([]byte, 0, maxPerVal), v...)
+		} else {
+			m.scratch = m.scratch[:len(v)]
+			copy(m.scratch, v)
+		}
+		m.mutateBytes(&m.scratch)
+		vals[i] = m.scratch
+	default:
+		panic(fmt.Sprintf("type not supported for mutating: %T", vals[i]))
+	}
+}
+
+func (m *mutator) mutateInt(v, maxValue int64) int64 {
+	var max int64
+	for {
+		max = 100
+		switch m.rand(2) {
+		case 0:
+			// Add a random number
+			if v >= maxValue {
+				continue
+			}
+			if v > 0 && maxValue-v < max {
+				// Don't let v exceed maxValue
+				max = maxValue - v
+			}
+			v += int64(1 + m.rand(int(max)))
+			return v
+		case 1:
+			// Subtract a random number
+			if v <= -maxValue {
+				continue
+			}
+			if v < 0 && maxValue+v < max {
+				// Don't let v drop below -maxValue
+				max = maxValue + v
+			}
+			v -= int64(1 + m.rand(int(max)))
+			return v
+		}
+	}
+}
+
+func (m *mutator) mutateUInt(v, maxValue uint64) uint64 {
+	var max uint64
+	for {
+		max = 100
+		switch m.rand(2) {
+		case 0:
+			// Add a random number
+			if v >= maxValue {
+				continue
+			}
+			if v > 0 && maxValue-v < max {
+				// Don't let v exceed maxValue
+				max = maxValue - v
+			}
+
+			v += uint64(1 + m.rand(int(max)))
+			return v
+		case 1:
+			// Subtract a random number
+			if v <= 0 {
+				continue
+			}
+			if v < max {
+				// Don't let v drop below 0
+				max = v
+			}
+			v -= uint64(1 + m.rand(int(max)))
+			return v
+		}
+	}
+}
+
+func (m *mutator) mutateFloat(v, maxValue float64) float64 {
+	var max float64
+	for {
+		switch m.rand(4) {
+		case 0:
+			// Add a random number
+			if v >= maxValue {
+				continue
+			}
+			max = 100
+			if v > 0 && maxValue-v < max {
+				// Don't let v exceed maxValue
+				max = maxValue - v
+			}
+			v += float64(1 + m.rand(int(max)))
+			return v
+		case 1:
+			// Subtract a random number
+			if v <= -maxValue {
+				continue
+			}
+			max = 100
+			if v < 0 && maxValue+v < max {
+				// Don't let v drop below -maxValue
+				max = maxValue + v
+			}
+			v -= float64(1 + m.rand(int(max)))
+			return v
+		case 2:
+			// Multiply by a random number
+			absV := math.Abs(v)
+			if v == 0 || absV >= maxValue {
+				continue
+			}
+			max = 10
+			if maxValue/absV < max {
+				// Don't let v go beyond the minimum or maximum value
+				max = maxValue / absV
+			}
+			v *= float64(1 + m.rand(int(max)))
+			return v
+		case 3:
+			// Divide by a random number
+			if v == 0 {
+				continue
+			}
+			v /= float64(1 + m.rand(10))
+			return v
+		}
+	}
+}
+
+type byteSliceMutator func(*mutator, []byte) []byte
+
+var byteSliceMutators = []byteSliceMutator{
+	byteSliceRemoveBytes,
+	byteSliceInsertRandomBytes,
+	byteSliceDuplicateBytes,
+	byteSliceOverwriteBytes,
+	byteSliceBitFlip,
+	byteSliceXORByte,
+	byteSliceSwapByte,
+	byteSliceArithmeticUint8,
+	byteSliceArithmeticUint16,
+	byteSliceArithmeticUint32,
+	byteSliceArithmeticUint64,
+	byteSliceOverwriteInterestingUint8,
+	byteSliceOverwriteInterestingUint16,
+	byteSliceOverwriteInterestingUint32,
+	byteSliceInsertConstantBytes,
+	byteSliceOverwriteConstantBytes,
+	byteSliceShuffleBytes,
+	byteSliceSwapBytes,
+}
+
+func (m *mutator) mutateBytes(ptrB *[]byte) {
+	b := *ptrB
+	defer func() {
+		if unsafe.SliceData(*ptrB) != unsafe.SliceData(b) {
+			panic("data moved to new address")
+		}
+		*ptrB = b
+	}()
+
+	for {
+		mut := byteSliceMutators[m.rand(len(byteSliceMutators))]
+		if mutated := mut(m, b); mutated != nil {
+			b = mutated
+			return
+		}
+	}
+}
+
+var (
+	interesting8  = []int8{-128, -1, 0, 1, 16, 32, 64, 100, 127}
+	interesting16 = []int16{-32768, -129, 128, 255, 256, 512, 1000, 1024, 4096, 32767}
+	interesting32 = []int32{-2147483648, -100663046, -32769, 32768, 65535, 65536, 100663045, 2147483647}
+)
+
+const (
+	maxUint = uint64(^uint(0))
+	maxInt  = int64(maxUint >> 1)
+)
+
+func init() {
+	for _, v := range interesting8 {
+		interesting16 = append(interesting16, int16(v))
+	}
+	for _, v := range interesting16 {
+		interesting32 = append(interesting32, int32(v))
+	}
+}
diff --git a/src/internal/fuzz/mutator_test.go b/src/internal/fuzz/mutator_test.go
new file mode 100644
index 0000000..cea7e2e
--- /dev/null
+++ b/src/internal/fuzz/mutator_test.go
@@ -0,0 +1,117 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strconv"
+	"testing"
+)
+
+func BenchmarkMutatorBytes(b *testing.B) {
+	origEnv := os.Getenv("GODEBUG")
+	defer func() { os.Setenv("GODEBUG", origEnv) }()
+	os.Setenv("GODEBUG", fmt.Sprintf("%s,fuzzseed=123", origEnv))
+	m := newMutator()
+
+	for _, size := range []int{
+		1,
+		10,
+		100,
+		1000,
+		10000,
+		100000,
+	} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			buf := make([]byte, size)
+			b.ResetTimer()
+
+			for i := 0; i < b.N; i++ {
+				// resize buffer to the correct shape and reset the PCG
+				buf = buf[0:size]
+				m.r = newPcgRand()
+				m.mutate([]any{buf}, workerSharedMemSize)
+			}
+		})
+	}
+}
+
+func BenchmarkMutatorString(b *testing.B) {
+	origEnv := os.Getenv("GODEBUG")
+	defer func() { os.Setenv("GODEBUG", origEnv) }()
+	os.Setenv("GODEBUG", fmt.Sprintf("%s,fuzzseed=123", origEnv))
+	m := newMutator()
+
+	for _, size := range []int{
+		1,
+		10,
+		100,
+		1000,
+		10000,
+		100000,
+	} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			buf := make([]byte, size)
+			b.ResetTimer()
+
+			for i := 0; i < b.N; i++ {
+				// resize buffer to the correct shape and reset the PCG
+				buf = buf[0:size]
+				m.r = newPcgRand()
+				m.mutate([]any{string(buf)}, workerSharedMemSize)
+			}
+		})
+	}
+}
+
+func BenchmarkMutatorAllBasicTypes(b *testing.B) {
+	origEnv := os.Getenv("GODEBUG")
+	defer func() { os.Setenv("GODEBUG", origEnv) }()
+	os.Setenv("GODEBUG", fmt.Sprintf("%s,fuzzseed=123", origEnv))
+	m := newMutator()
+
+	types := []any{
+		[]byte(""),
+		string(""),
+		false,
+		float32(0),
+		float64(0),
+		int(0),
+		int8(0),
+		int16(0),
+		int32(0),
+		int64(0),
+		uint8(0),
+		uint16(0),
+		uint32(0),
+		uint64(0),
+	}
+
+	for _, t := range types {
+		b.Run(fmt.Sprintf("%T", t), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				m.r = newPcgRand()
+				m.mutate([]any{t}, workerSharedMemSize)
+			}
+		})
+	}
+}
+
+func TestStringImmutability(t *testing.T) {
+	v := []any{"hello"}
+	m := newMutator()
+	m.mutate(v, 1024)
+	original := v[0].(string)
+	originalCopy := make([]byte, len(original))
+	copy(originalCopy, []byte(original))
+	for i := 0; i < 25; i++ {
+		m.mutate(v, 1024)
+	}
+	if !bytes.Equal([]byte(original), originalCopy) {
+		t.Fatalf("string was mutated: got %x, want %x", []byte(original), originalCopy)
+	}
+}
diff --git a/src/internal/fuzz/mutators_byteslice.go b/src/internal/fuzz/mutators_byteslice.go
new file mode 100644
index 0000000..d9dab1d
--- /dev/null
+++ b/src/internal/fuzz/mutators_byteslice.go
@@ -0,0 +1,313 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+// byteSliceRemoveBytes removes a random chunk of bytes from b.
+func byteSliceRemoveBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	pos0 := m.rand(len(b))
+	pos1 := pos0 + m.chooseLen(len(b)-pos0)
+	copy(b[pos0:], b[pos1:])
+	b = b[:len(b)-(pos1-pos0)]
+	return b
+}
+
+// byteSliceInsertRandomBytes inserts a chunk of random bytes into b at a random
+// position.
+func byteSliceInsertRandomBytes(m *mutator, b []byte) []byte {
+	pos := m.rand(len(b) + 1)
+	n := m.chooseLen(1024)
+	if len(b)+n >= cap(b) {
+		return nil
+	}
+	b = b[:len(b)+n]
+	copy(b[pos+n:], b[pos:])
+	for i := 0; i < n; i++ {
+		b[pos+i] = byte(m.rand(256))
+	}
+	return b
+}
+
+// byteSliceDuplicateBytes duplicates a chunk of bytes in b and inserts it into
+// a random position.
+func byteSliceDuplicateBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	src := m.rand(len(b))
+	dst := m.rand(len(b))
+	for dst == src {
+		dst = m.rand(len(b))
+	}
+	n := m.chooseLen(len(b) - src)
+	// Use the end of the slice as scratch space to avoid doing an
+	// allocation. If the slice is too small abort and try something
+	// else.
+	if len(b)+(n*2) >= cap(b) {
+		return nil
+	}
+	end := len(b)
+	// Increase the size of b to fit the duplicated block as well as
+	// some extra working space
+	b = b[:end+(n*2)]
+	// Copy the block of bytes we want to duplicate to the end of the
+	// slice
+	copy(b[end+n:], b[src:src+n])
+	// Shift the bytes after the splice point n positions to the right
+	// to make room for the new block
+	copy(b[dst+n:end+n], b[dst:end])
+	// Insert the duplicate block into the splice point
+	copy(b[dst:], b[end+n:])
+	b = b[:end+n]
+	return b
+}
+
+// byteSliceOverwriteBytes overwrites a chunk of b with another chunk of b.
+func byteSliceOverwriteBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	src := m.rand(len(b))
+	dst := m.rand(len(b))
+	for dst == src {
+		dst = m.rand(len(b))
+	}
+	n := m.chooseLen(len(b) - src - 1)
+	copy(b[dst:], b[src:src+n])
+	return b
+}
+
+// byteSliceBitFlip flips a random bit in a random byte in b.
+func byteSliceBitFlip(m *mutator, b []byte) []byte {
+	if len(b) == 0 {
+		return nil
+	}
+	pos := m.rand(len(b))
+	b[pos] ^= 1 << uint(m.rand(8))
+	return b
+}
+
+// byteSliceXORByte XORs a random byte in b with a random value.
+func byteSliceXORByte(m *mutator, b []byte) []byte {
+	if len(b) == 0 {
+		return nil
+	}
+	pos := m.rand(len(b))
+	// In order to avoid a no-op (where the random value matches
+	// the existing value), use XOR instead of just setting to
+	// the random value.
+	b[pos] ^= byte(1 + m.rand(255))
+	return b
+}
+
+// byteSliceSwapByte swaps two random bytes in b.
+func byteSliceSwapByte(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	src := m.rand(len(b))
+	dst := m.rand(len(b))
+	for dst == src {
+		dst = m.rand(len(b))
+	}
+	b[src], b[dst] = b[dst], b[src]
+	return b
+}
+
+// byteSliceArithmeticUint8 adds/subtracts from a random byte in b.
+func byteSliceArithmeticUint8(m *mutator, b []byte) []byte {
+	if len(b) == 0 {
+		return nil
+	}
+	pos := m.rand(len(b))
+	v := byte(m.rand(35) + 1)
+	if m.r.bool() {
+		b[pos] += v
+	} else {
+		b[pos] -= v
+	}
+	return b
+}
+
+// byteSliceArithmeticUint16 adds/subtracts from a random uint16 in b.
+func byteSliceArithmeticUint16(m *mutator, b []byte) []byte {
+	if len(b) < 2 {
+		return nil
+	}
+	v := uint16(m.rand(35) + 1)
+	if m.r.bool() {
+		v = 0 - v
+	}
+	pos := m.rand(len(b) - 1)
+	enc := m.randByteOrder()
+	enc.PutUint16(b[pos:], enc.Uint16(b[pos:])+v)
+	return b
+}
+
+// byteSliceArithmeticUint32 adds/subtracts from a random uint32 in b.
+func byteSliceArithmeticUint32(m *mutator, b []byte) []byte {
+	if len(b) < 4 {
+		return nil
+	}
+	v := uint32(m.rand(35) + 1)
+	if m.r.bool() {
+		v = 0 - v
+	}
+	pos := m.rand(len(b) - 3)
+	enc := m.randByteOrder()
+	enc.PutUint32(b[pos:], enc.Uint32(b[pos:])+v)
+	return b
+}
+
+// byteSliceArithmeticUint64 adds/subtracts from a random uint64 in b.
+func byteSliceArithmeticUint64(m *mutator, b []byte) []byte {
+	if len(b) < 8 {
+		return nil
+	}
+	v := uint64(m.rand(35) + 1)
+	if m.r.bool() {
+		v = 0 - v
+	}
+	pos := m.rand(len(b) - 7)
+	enc := m.randByteOrder()
+	enc.PutUint64(b[pos:], enc.Uint64(b[pos:])+v)
+	return b
+}
+
+// byteSliceOverwriteInterestingUint8 overwrites a random byte in b with an interesting
+// value.
+func byteSliceOverwriteInterestingUint8(m *mutator, b []byte) []byte {
+	if len(b) == 0 {
+		return nil
+	}
+	pos := m.rand(len(b))
+	b[pos] = byte(interesting8[m.rand(len(interesting8))])
+	return b
+}
+
+// byteSliceOverwriteInterestingUint16 overwrites a random uint16 in b with an interesting
+// value.
+func byteSliceOverwriteInterestingUint16(m *mutator, b []byte) []byte {
+	if len(b) < 2 {
+		return nil
+	}
+	pos := m.rand(len(b) - 1)
+	v := uint16(interesting16[m.rand(len(interesting16))])
+	m.randByteOrder().PutUint16(b[pos:], v)
+	return b
+}
+
+// byteSliceOverwriteInterestingUint32 overwrites a random uint16 in b with an interesting
+// value.
+func byteSliceOverwriteInterestingUint32(m *mutator, b []byte) []byte {
+	if len(b) < 4 {
+		return nil
+	}
+	pos := m.rand(len(b) - 3)
+	v := uint32(interesting32[m.rand(len(interesting32))])
+	m.randByteOrder().PutUint32(b[pos:], v)
+	return b
+}
+
+// byteSliceInsertConstantBytes inserts a chunk of constant bytes into a random position in b.
+func byteSliceInsertConstantBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	dst := m.rand(len(b))
+	// TODO(rolandshoemaker,katiehockman): 4096 was mainly picked
+	// randomly. We may want to either pick a much larger value
+	// (AFL uses 32768, paired with a similar impl to chooseLen
+	// which biases towards smaller lengths that grow over time),
+	// or set the max based on characteristics of the corpus
+	// (libFuzzer sets a min/max based on the min/max size of
+	// entries in the corpus and then picks uniformly from
+	// that range).
+	n := m.chooseLen(4096)
+	if len(b)+n >= cap(b) {
+		return nil
+	}
+	b = b[:len(b)+n]
+	copy(b[dst+n:], b[dst:])
+	rb := byte(m.rand(256))
+	for i := dst; i < dst+n; i++ {
+		b[i] = rb
+	}
+	return b
+}
+
+// byteSliceOverwriteConstantBytes overwrites a chunk of b with constant bytes.
+func byteSliceOverwriteConstantBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	dst := m.rand(len(b))
+	n := m.chooseLen(len(b) - dst)
+	rb := byte(m.rand(256))
+	for i := dst; i < dst+n; i++ {
+		b[i] = rb
+	}
+	return b
+}
+
+// byteSliceShuffleBytes shuffles a chunk of bytes in b.
+func byteSliceShuffleBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	dst := m.rand(len(b))
+	n := m.chooseLen(len(b) - dst)
+	if n <= 2 {
+		return nil
+	}
+	// Start at the end of the range, and iterate backwards
+	// to dst, swapping each element with another element in
+	// dst:dst+n (Fisher-Yates shuffle).
+	for i := n - 1; i > 0; i-- {
+		j := m.rand(i + 1)
+		b[dst+i], b[dst+j] = b[dst+j], b[dst+i]
+	}
+	return b
+}
+
+// byteSliceSwapBytes swaps two chunks of bytes in b.
+func byteSliceSwapBytes(m *mutator, b []byte) []byte {
+	if len(b) <= 1 {
+		return nil
+	}
+	src := m.rand(len(b))
+	dst := m.rand(len(b))
+	for dst == src {
+		dst = m.rand(len(b))
+	}
+	// Choose the random length as len(b) - max(src, dst)
+	// so that we don't attempt to swap a chunk that extends
+	// beyond the end of the slice
+	max := dst
+	if src > max {
+		max = src
+	}
+	n := m.chooseLen(len(b) - max - 1)
+	// Check that neither chunk intersect, so that we don't end up
+	// duplicating parts of the input, rather than swapping them
+	if src > dst && dst+n >= src || dst > src && src+n >= dst {
+		return nil
+	}
+	// Use the end of the slice as scratch space to avoid doing an
+	// allocation. If the slice is too small abort and try something
+	// else.
+	if len(b)+n >= cap(b) {
+		return nil
+	}
+	end := len(b)
+	b = b[:end+n]
+	copy(b[end:], b[dst:dst+n])
+	copy(b[dst:], b[src:src+n])
+	copy(b[src:], b[end:])
+	b = b[:end]
+	return b
+}
diff --git a/src/internal/fuzz/mutators_byteslice_test.go b/src/internal/fuzz/mutators_byteslice_test.go
new file mode 100644
index 0000000..7886967
--- /dev/null
+++ b/src/internal/fuzz/mutators_byteslice_test.go
@@ -0,0 +1,186 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"bytes"
+	"testing"
+)
+
+type mockRand struct {
+	values  []int
+	counter int
+	b       bool
+}
+
+func (mr *mockRand) uint32() uint32 {
+	c := mr.values[mr.counter]
+	mr.counter++
+	return uint32(c)
+}
+
+func (mr *mockRand) intn(n int) int {
+	c := mr.values[mr.counter]
+	mr.counter++
+	return c % n
+}
+
+func (mr *mockRand) uint32n(n uint32) uint32 {
+	c := mr.values[mr.counter]
+	mr.counter++
+	return uint32(c) % n
+}
+
+func (mr *mockRand) exp2() int {
+	c := mr.values[mr.counter]
+	mr.counter++
+	return c
+}
+
+func (mr *mockRand) bool() bool {
+	b := mr.b
+	mr.b = !mr.b
+	return b
+}
+
+func (mr *mockRand) save(*uint64, *uint64) {
+	panic("unimplemented")
+}
+
+func (mr *mockRand) restore(uint64, uint64) {
+	panic("unimplemented")
+}
+
+func TestByteSliceMutators(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		mutator  func(*mutator, []byte) []byte
+		randVals []int
+		input    []byte
+		expected []byte
+	}{
+		{
+			name:     "byteSliceRemoveBytes",
+			mutator:  byteSliceRemoveBytes,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{4},
+		},
+		{
+			name:     "byteSliceInsertRandomBytes",
+			mutator:  byteSliceInsertRandomBytes,
+			input:    make([]byte, 4, 8),
+			expected: []byte{3, 4, 5, 0, 0, 0, 0},
+		},
+		{
+			name:     "byteSliceDuplicateBytes",
+			mutator:  byteSliceDuplicateBytes,
+			input:    append(make([]byte, 0, 13), []byte{1, 2, 3, 4}...),
+			expected: []byte{1, 1, 2, 3, 4, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceOverwriteBytes",
+			mutator:  byteSliceOverwriteBytes,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{1, 1, 3, 4},
+		},
+		{
+			name:     "byteSliceBitFlip",
+			mutator:  byteSliceBitFlip,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{3, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceXORByte",
+			mutator:  byteSliceXORByte,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{3, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceSwapByte",
+			mutator:  byteSliceSwapByte,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{2, 1, 3, 4},
+		},
+		{
+			name:     "byteSliceArithmeticUint8",
+			mutator:  byteSliceArithmeticUint8,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{255, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceArithmeticUint16",
+			mutator:  byteSliceArithmeticUint16,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{1, 3, 3, 4},
+		},
+		{
+			name:     "byteSliceArithmeticUint32",
+			mutator:  byteSliceArithmeticUint32,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{2, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceArithmeticUint64",
+			mutator:  byteSliceArithmeticUint64,
+			input:    []byte{1, 2, 3, 4, 5, 6, 7, 8},
+			expected: []byte{2, 2, 3, 4, 5, 6, 7, 8},
+		},
+		{
+			name:     "byteSliceOverwriteInterestingUint8",
+			mutator:  byteSliceOverwriteInterestingUint8,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{255, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceOverwriteInterestingUint16",
+			mutator:  byteSliceOverwriteInterestingUint16,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{255, 127, 3, 4},
+		},
+		{
+			name:     "byteSliceOverwriteInterestingUint32",
+			mutator:  byteSliceOverwriteInterestingUint32,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{250, 0, 0, 250},
+		},
+		{
+			name:     "byteSliceInsertConstantBytes",
+			mutator:  byteSliceInsertConstantBytes,
+			input:    append(make([]byte, 0, 8), []byte{1, 2, 3, 4}...),
+			expected: []byte{3, 3, 3, 1, 2, 3, 4},
+		},
+		{
+			name:     "byteSliceOverwriteConstantBytes",
+			mutator:  byteSliceOverwriteConstantBytes,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{3, 3, 3, 4},
+		},
+		{
+			name:     "byteSliceShuffleBytes",
+			mutator:  byteSliceShuffleBytes,
+			input:    []byte{1, 2, 3, 4},
+			expected: []byte{2, 3, 1, 4},
+		},
+		{
+			name:     "byteSliceSwapBytes",
+			mutator:  byteSliceSwapBytes,
+			randVals: []int{0, 2, 0, 2},
+			input:    append(make([]byte, 0, 9), []byte{1, 2, 3, 4}...),
+			expected: []byte{3, 2, 1, 4},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			r := &mockRand{values: []int{0, 1, 2, 3, 4, 5}}
+			if tc.randVals != nil {
+				r.values = tc.randVals
+			}
+			m := &mutator{r: r}
+			b := tc.mutator(m, tc.input)
+			if !bytes.Equal(b, tc.expected) {
+				t.Errorf("got %x, want %x", b, tc.expected)
+			}
+		})
+	}
+}
diff --git a/src/internal/fuzz/pcg.go b/src/internal/fuzz/pcg.go
new file mode 100644
index 0000000..c9ea0af
--- /dev/null
+++ b/src/internal/fuzz/pcg.go
@@ -0,0 +1,145 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"math/bits"
+	"os"
+	"strconv"
+	"strings"
+	"sync/atomic"
+	"time"
+)
+
+type mutatorRand interface {
+	uint32() uint32
+	intn(int) int
+	uint32n(uint32) uint32
+	exp2() int
+	bool() bool
+
+	save(randState, randInc *uint64)
+	restore(randState, randInc uint64)
+}
+
+// The functions in pcg implement a 32 bit PRNG with a 64 bit period: pcg xsh rr
+// 64 32. See https://www.pcg-random.org/ for more information. This
+// implementation is geared specifically towards the needs of fuzzing: Simple
+// creation and use, no reproducibility, no concurrency safety, just the
+// necessary methods, optimized for speed.
+
+var globalInc uint64 // PCG stream
+
+const multiplier uint64 = 6364136223846793005
+
+// pcgRand is a PRNG. It should not be copied or shared. No Rand methods are
+// concurrency safe.
+type pcgRand struct {
+	noCopy noCopy // help avoid mistakes: ask vet to ensure that we don't make a copy
+	state  uint64
+	inc    uint64
+}
+
+func godebugSeed() *int {
+	debug := strings.Split(os.Getenv("GODEBUG"), ",")
+	for _, f := range debug {
+		if strings.HasPrefix(f, "fuzzseed=") {
+			seed, err := strconv.Atoi(strings.TrimPrefix(f, "fuzzseed="))
+			if err != nil {
+				panic("malformed fuzzseed")
+			}
+			return &seed
+		}
+	}
+	return nil
+}
+
+// newPcgRand generates a new, seeded Rand, ready for use.
+func newPcgRand() *pcgRand {
+	r := new(pcgRand)
+	now := uint64(time.Now().UnixNano())
+	if seed := godebugSeed(); seed != nil {
+		now = uint64(*seed)
+	}
+	inc := atomic.AddUint64(&globalInc, 1)
+	r.state = now
+	r.inc = (inc << 1) | 1
+	r.step()
+	r.state += now
+	r.step()
+	return r
+}
+
+func (r *pcgRand) step() {
+	r.state *= multiplier
+	r.state += r.inc
+}
+
+func (r *pcgRand) save(randState, randInc *uint64) {
+	*randState = r.state
+	*randInc = r.inc
+}
+
+func (r *pcgRand) restore(randState, randInc uint64) {
+	r.state = randState
+	r.inc = randInc
+}
+
+// uint32 returns a pseudo-random uint32.
+func (r *pcgRand) uint32() uint32 {
+	x := r.state
+	r.step()
+	return bits.RotateLeft32(uint32(((x>>18)^x)>>27), -int(x>>59))
+}
+
+// intn returns a pseudo-random number in [0, n).
+// n must fit in a uint32.
+func (r *pcgRand) intn(n int) int {
+	if int(uint32(n)) != n {
+		panic("large Intn")
+	}
+	return int(r.uint32n(uint32(n)))
+}
+
+// uint32n returns a pseudo-random number in [0, n).
+//
+// For implementation details, see:
+// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction
+// https://lemire.me/blog/2016/06/30/fast-random-shuffling
+func (r *pcgRand) uint32n(n uint32) uint32 {
+	v := r.uint32()
+	prod := uint64(v) * uint64(n)
+	low := uint32(prod)
+	if low < n {
+		thresh := uint32(-int32(n)) % n
+		for low < thresh {
+			v = r.uint32()
+			prod = uint64(v) * uint64(n)
+			low = uint32(prod)
+		}
+	}
+	return uint32(prod >> 32)
+}
+
+// exp2 generates n with probability 1/2^(n+1).
+func (r *pcgRand) exp2() int {
+	return bits.TrailingZeros32(r.uint32())
+}
+
+// bool generates a random bool.
+func (r *pcgRand) bool() bool {
+	return r.uint32()&1 == 0
+}
+
+// noCopy may be embedded into structs which must not be copied
+// after the first use.
+//
+// See https://golang.org/issues/8005#issuecomment-190753527
+// for details.
+type noCopy struct{}
+
+// lock is a no-op used by -copylocks checker from `go vet`.
+func (*noCopy) lock()   {}
+func (*noCopy) unlock() {}
diff --git a/src/internal/fuzz/queue.go b/src/internal/fuzz/queue.go
new file mode 100644
index 0000000..195d6eb
--- /dev/null
+++ b/src/internal/fuzz/queue.go
@@ -0,0 +1,71 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+// queue holds a growable sequence of inputs for fuzzing and minimization.
+//
+// For now, this is a simple ring buffer
+// (https://en.wikipedia.org/wiki/Circular_buffer).
+//
+// TODO(golang.org/issue/46224): use a prioritization algorithm based on input
+// size, previous duration, coverage, and any other metrics that seem useful.
+type queue struct {
+	// elems holds a ring buffer.
+	// The queue is empty when begin = end.
+	// The queue is full (until grow is called) when end = begin + N - 1 (mod N)
+	// where N = cap(elems).
+	elems     []any
+	head, len int
+}
+
+func (q *queue) cap() int {
+	return len(q.elems)
+}
+
+func (q *queue) grow() {
+	oldCap := q.cap()
+	newCap := oldCap * 2
+	if newCap == 0 {
+		newCap = 8
+	}
+	newElems := make([]any, newCap)
+	oldLen := q.len
+	for i := 0; i < oldLen; i++ {
+		newElems[i] = q.elems[(q.head+i)%oldCap]
+	}
+	q.elems = newElems
+	q.head = 0
+}
+
+func (q *queue) enqueue(e any) {
+	if q.len+1 > q.cap() {
+		q.grow()
+	}
+	i := (q.head + q.len) % q.cap()
+	q.elems[i] = e
+	q.len++
+}
+
+func (q *queue) dequeue() (any, bool) {
+	if q.len == 0 {
+		return nil, false
+	}
+	e := q.elems[q.head]
+	q.elems[q.head] = nil
+	q.head = (q.head + 1) % q.cap()
+	q.len--
+	return e, true
+}
+
+func (q *queue) peek() (any, bool) {
+	if q.len == 0 {
+		return nil, false
+	}
+	return q.elems[q.head], true
+}
+
+func (q *queue) clear() {
+	*q = queue{}
+}
diff --git a/src/internal/fuzz/queue_test.go b/src/internal/fuzz/queue_test.go
new file mode 100644
index 0000000..3b179af
--- /dev/null
+++ b/src/internal/fuzz/queue_test.go
@@ -0,0 +1,58 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import "testing"
+
+func TestQueue(t *testing.T) {
+	// Zero valued queue should have 0 length and capacity.
+	var q queue
+	if n := q.len; n != 0 {
+		t.Fatalf("empty queue has len %d; want 0", n)
+	}
+	if n := q.cap(); n != 0 {
+		t.Fatalf("empty queue has cap %d; want 0", n)
+	}
+
+	// As we add elements, len should grow.
+	N := 32
+	for i := 0; i < N; i++ {
+		q.enqueue(i)
+		if n := q.len; n != i+1 {
+			t.Fatalf("after adding %d elements, queue has len %d", i, n)
+		}
+		if v, ok := q.peek(); !ok {
+			t.Fatalf("couldn't peek after adding %d elements", i)
+		} else if v.(int) != 0 {
+			t.Fatalf("after adding %d elements, peek is %d; want 0", i, v)
+		}
+	}
+
+	// As we remove and add elements, len should shrink and grow.
+	// We should also remove elements in the same order they were added.
+	want := 0
+	for _, r := range []int{1, 2, 3, 5, 8, 13, 21} {
+		s := make([]int, 0, r)
+		for i := 0; i < r; i++ {
+			if got, ok := q.dequeue(); !ok {
+				t.Fatalf("after removing %d of %d elements, could not dequeue", i+1, r)
+			} else if got != want {
+				t.Fatalf("after removing %d of %d elements, got %d; want %d", i+1, r, got, want)
+			} else {
+				s = append(s, got.(int))
+			}
+			want = (want + 1) % N
+			if n := q.len; n != N-i-1 {
+				t.Fatalf("after removing %d of %d elements, len is %d; want %d", i+1, r, n, N-i-1)
+			}
+		}
+		for i, v := range s {
+			q.enqueue(v)
+			if n := q.len; n != N-r+i+1 {
+				t.Fatalf("after adding back %d of %d elements, len is %d; want %d", i+1, r, n, n-r+i+1)
+			}
+		}
+	}
+}
diff --git a/src/internal/fuzz/sys_posix.go b/src/internal/fuzz/sys_posix.go
new file mode 100644
index 0000000..fec6054
--- /dev/null
+++ b/src/internal/fuzz/sys_posix.go
@@ -0,0 +1,130 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || freebsd || linux
+
+package fuzz
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+type sharedMemSys struct{}
+
+func sharedMemMapFile(f *os.File, size int, removeOnClose bool) (*sharedMem, error) {
+	prot := syscall.PROT_READ | syscall.PROT_WRITE
+	flags := syscall.MAP_FILE | syscall.MAP_SHARED
+	region, err := syscall.Mmap(int(f.Fd()), 0, size, prot, flags)
+	if err != nil {
+		return nil, err
+	}
+
+	return &sharedMem{f: f, region: region, removeOnClose: removeOnClose}, nil
+}
+
+// Close unmaps the shared memory and closes the temporary file. If this
+// sharedMem was created with sharedMemTempFile, Close also removes the file.
+func (m *sharedMem) Close() error {
+	// Attempt all operations, even if we get an error for an earlier operation.
+	// os.File.Close may fail due to I/O errors, but we still want to delete
+	// the temporary file.
+	var errs []error
+	errs = append(errs,
+		syscall.Munmap(m.region),
+		m.f.Close())
+	if m.removeOnClose {
+		errs = append(errs, os.Remove(m.f.Name()))
+	}
+	for _, err := range errs {
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// setWorkerComm configures communication channels on the cmd that will
+// run a worker process.
+func setWorkerComm(cmd *exec.Cmd, comm workerComm) {
+	mem := <-comm.memMu
+	memFile := mem.f
+	comm.memMu <- mem
+	cmd.ExtraFiles = []*os.File{comm.fuzzIn, comm.fuzzOut, memFile}
+}
+
+// getWorkerComm returns communication channels in the worker process.
+func getWorkerComm() (comm workerComm, err error) {
+	fuzzIn := os.NewFile(3, "fuzz_in")
+	fuzzOut := os.NewFile(4, "fuzz_out")
+	memFile := os.NewFile(5, "fuzz_mem")
+	fi, err := memFile.Stat()
+	if err != nil {
+		return workerComm{}, err
+	}
+	size := int(fi.Size())
+	if int64(size) != fi.Size() {
+		return workerComm{}, fmt.Errorf("fuzz temp file exceeds maximum size")
+	}
+	removeOnClose := false
+	mem, err := sharedMemMapFile(memFile, size, removeOnClose)
+	if err != nil {
+		return workerComm{}, err
+	}
+	memMu := make(chan *sharedMem, 1)
+	memMu <- mem
+	return workerComm{fuzzIn: fuzzIn, fuzzOut: fuzzOut, memMu: memMu}, nil
+}
+
+// isInterruptError returns whether an error was returned by a process that
+// was terminated by an interrupt signal (SIGINT).
+func isInterruptError(err error) bool {
+	exitErr, ok := err.(*exec.ExitError)
+	if !ok || exitErr.ExitCode() >= 0 {
+		return false
+	}
+	status := exitErr.Sys().(syscall.WaitStatus)
+	return status.Signal() == syscall.SIGINT
+}
+
+// terminationSignal checks if err is an exec.ExitError with a signal status.
+// If it is, terminationSignal returns the signal and true.
+// If not, -1 and false.
+func terminationSignal(err error) (os.Signal, bool) {
+	exitErr, ok := err.(*exec.ExitError)
+	if !ok || exitErr.ExitCode() >= 0 {
+		return syscall.Signal(-1), false
+	}
+	status := exitErr.Sys().(syscall.WaitStatus)
+	return status.Signal(), status.Signaled()
+}
+
+// isCrashSignal returns whether a signal was likely to have been caused by an
+// error in the program that received it, triggered by a fuzz input. For
+// example, SIGSEGV would be received after a nil pointer dereference.
+// Other signals like SIGKILL or SIGHUP are more likely to have been sent by
+// another process, and we shouldn't record a crasher if the worker process
+// receives one of these.
+//
+// Note that Go installs its own signal handlers on startup, so some of these
+// signals may only be received if signal handlers are changed. For example,
+// SIGSEGV is normally transformed into a panic that causes the process to exit
+// with status 2 if not recovered, which we handle as a crash.
+func isCrashSignal(signal os.Signal) bool {
+	switch signal {
+	case
+		syscall.SIGILL,  // illegal instruction
+		syscall.SIGTRAP, // breakpoint
+		syscall.SIGABRT, // abort() called
+		syscall.SIGBUS,  // invalid memory access (e.g., misaligned address)
+		syscall.SIGFPE,  // math error, e.g., integer divide by zero
+		syscall.SIGSEGV, // invalid memory access (e.g., write to read-only)
+		syscall.SIGPIPE: // sent data to closed pipe or socket
+		return true
+	default:
+		return false
+	}
+}
diff --git a/src/internal/fuzz/sys_unimplemented.go b/src/internal/fuzz/sys_unimplemented.go
new file mode 100644
index 0000000..8687c1f
--- /dev/null
+++ b/src/internal/fuzz/sys_unimplemented.go
@@ -0,0 +1,44 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// If you update this constraint, also update internal/platform.FuzzSupported.
+//
+//go:build !darwin && !freebsd && !linux && !windows
+
+package fuzz
+
+import (
+	"os"
+	"os/exec"
+)
+
+type sharedMemSys struct{}
+
+func sharedMemMapFile(f *os.File, size int, removeOnClose bool) (*sharedMem, error) {
+	panic("not implemented")
+}
+
+func (m *sharedMem) Close() error {
+	panic("not implemented")
+}
+
+func setWorkerComm(cmd *exec.Cmd, comm workerComm) {
+	panic("not implemented")
+}
+
+func getWorkerComm() (comm workerComm, err error) {
+	panic("not implemented")
+}
+
+func isInterruptError(err error) bool {
+	panic("not implemented")
+}
+
+func terminationSignal(err error) (os.Signal, bool) {
+	panic("not implemented")
+}
+
+func isCrashSignal(signal os.Signal) bool {
+	panic("not implemented")
+}
diff --git a/src/internal/fuzz/sys_windows.go b/src/internal/fuzz/sys_windows.go
new file mode 100644
index 0000000..aa85be7
--- /dev/null
+++ b/src/internal/fuzz/sys_windows.go
@@ -0,0 +1,147 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"syscall"
+	"unsafe"
+)
+
+type sharedMemSys struct {
+	mapObj syscall.Handle
+}
+
+func sharedMemMapFile(f *os.File, size int, removeOnClose bool) (mem *sharedMem, err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("mapping temporary file %s: %w", f.Name(), err)
+		}
+	}()
+
+	// Create a file mapping object. The object itself is not shared.
+	mapObj, err := syscall.CreateFileMapping(
+		syscall.Handle(f.Fd()), // fhandle
+		nil,                    // sa
+		syscall.PAGE_READWRITE, // prot
+		0,                      // maxSizeHigh
+		0,                      // maxSizeLow
+		nil,                    // name
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	// Create a view from the file mapping object.
+	access := uint32(syscall.FILE_MAP_READ | syscall.FILE_MAP_WRITE)
+	addr, err := syscall.MapViewOfFile(
+		mapObj,        // handle
+		access,        // access
+		0,             // offsetHigh
+		0,             // offsetLow
+		uintptr(size), // length
+	)
+	if err != nil {
+		syscall.CloseHandle(mapObj)
+		return nil, err
+	}
+
+	region := unsafe.Slice((*byte)(unsafe.Pointer(addr)), size)
+	return &sharedMem{
+		f:             f,
+		region:        region,
+		removeOnClose: removeOnClose,
+		sys:           sharedMemSys{mapObj: mapObj},
+	}, nil
+}
+
+// Close unmaps the shared memory and closes the temporary file. If this
+// sharedMem was created with sharedMemTempFile, Close also removes the file.
+func (m *sharedMem) Close() error {
+	// Attempt all operations, even if we get an error for an earlier operation.
+	// os.File.Close may fail due to I/O errors, but we still want to delete
+	// the temporary file.
+	var errs []error
+	errs = append(errs,
+		syscall.UnmapViewOfFile(uintptr(unsafe.Pointer(&m.region[0]))),
+		syscall.CloseHandle(m.sys.mapObj),
+		m.f.Close())
+	if m.removeOnClose {
+		errs = append(errs, os.Remove(m.f.Name()))
+	}
+	for _, err := range errs {
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// setWorkerComm configures communication channels on the cmd that will
+// run a worker process.
+func setWorkerComm(cmd *exec.Cmd, comm workerComm) {
+	mem := <-comm.memMu
+	memName := mem.f.Name()
+	comm.memMu <- mem
+	syscall.SetHandleInformation(syscall.Handle(comm.fuzzIn.Fd()), syscall.HANDLE_FLAG_INHERIT, 1)
+	syscall.SetHandleInformation(syscall.Handle(comm.fuzzOut.Fd()), syscall.HANDLE_FLAG_INHERIT, 1)
+	cmd.Env = append(cmd.Env, fmt.Sprintf("GO_TEST_FUZZ_WORKER_HANDLES=%x,%x,%q", comm.fuzzIn.Fd(), comm.fuzzOut.Fd(), memName))
+	cmd.SysProcAttr = &syscall.SysProcAttr{AdditionalInheritedHandles: []syscall.Handle{syscall.Handle(comm.fuzzIn.Fd()), syscall.Handle(comm.fuzzOut.Fd())}}
+}
+
+// getWorkerComm returns communication channels in the worker process.
+func getWorkerComm() (comm workerComm, err error) {
+	v := os.Getenv("GO_TEST_FUZZ_WORKER_HANDLES")
+	if v == "" {
+		return workerComm{}, fmt.Errorf("GO_TEST_FUZZ_WORKER_HANDLES not set")
+	}
+	var fuzzInFD, fuzzOutFD uintptr
+	var memName string
+	if _, err := fmt.Sscanf(v, "%x,%x,%q", &fuzzInFD, &fuzzOutFD, &memName); err != nil {
+		return workerComm{}, fmt.Errorf("parsing GO_TEST_FUZZ_WORKER_HANDLES=%s: %v", v, err)
+	}
+
+	fuzzIn := os.NewFile(fuzzInFD, "fuzz_in")
+	fuzzOut := os.NewFile(fuzzOutFD, "fuzz_out")
+	tmpFile, err := os.OpenFile(memName, os.O_RDWR, 0)
+	if err != nil {
+		return workerComm{}, fmt.Errorf("worker opening temp file: %w", err)
+	}
+	fi, err := tmpFile.Stat()
+	if err != nil {
+		return workerComm{}, fmt.Errorf("worker checking temp file size: %w", err)
+	}
+	size := int(fi.Size())
+	if int64(size) != fi.Size() {
+		return workerComm{}, fmt.Errorf("fuzz temp file exceeds maximum size")
+	}
+	removeOnClose := false
+	mem, err := sharedMemMapFile(tmpFile, size, removeOnClose)
+	if err != nil {
+		return workerComm{}, err
+	}
+	memMu := make(chan *sharedMem, 1)
+	memMu <- mem
+
+	return workerComm{fuzzIn: fuzzIn, fuzzOut: fuzzOut, memMu: memMu}, nil
+}
+
+func isInterruptError(err error) bool {
+	// On Windows, we can't tell whether the process was interrupted by the error
+	// returned by Wait. It looks like an ExitError with status 1.
+	return false
+}
+
+// terminationSignal returns -1 and false because Windows doesn't have signals.
+func terminationSignal(err error) (os.Signal, bool) {
+	return syscall.Signal(-1), false
+}
+
+// isCrashSignal is not implemented because Windows doesn't have signals.
+func isCrashSignal(signal os.Signal) bool {
+	panic("not implemented: no signals on windows")
+}
diff --git a/src/internal/fuzz/trace.go b/src/internal/fuzz/trace.go
new file mode 100644
index 0000000..a15c370
--- /dev/null
+++ b/src/internal/fuzz/trace.go
@@ -0,0 +1,35 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !libfuzzer
+
+package fuzz
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname libfuzzerTraceCmp1 runtime.libfuzzerTraceCmp1
+//go:linkname libfuzzerTraceCmp2 runtime.libfuzzerTraceCmp2
+//go:linkname libfuzzerTraceCmp4 runtime.libfuzzerTraceCmp4
+//go:linkname libfuzzerTraceCmp8 runtime.libfuzzerTraceCmp8
+
+//go:linkname libfuzzerTraceConstCmp1 runtime.libfuzzerTraceConstCmp1
+//go:linkname libfuzzerTraceConstCmp2 runtime.libfuzzerTraceConstCmp2
+//go:linkname libfuzzerTraceConstCmp4 runtime.libfuzzerTraceConstCmp4
+//go:linkname libfuzzerTraceConstCmp8 runtime.libfuzzerTraceConstCmp8
+
+//go:linkname libfuzzerHookStrCmp runtime.libfuzzerHookStrCmp
+//go:linkname libfuzzerHookEqualFold runtime.libfuzzerHookEqualFold
+
+func libfuzzerTraceCmp1(arg0, arg1 uint8, fakePC uint)  {}
+func libfuzzerTraceCmp2(arg0, arg1 uint16, fakePC uint) {}
+func libfuzzerTraceCmp4(arg0, arg1 uint32, fakePC uint) {}
+func libfuzzerTraceCmp8(arg0, arg1 uint64, fakePC uint) {}
+
+func libfuzzerTraceConstCmp1(arg0, arg1 uint8, fakePC uint)  {}
+func libfuzzerTraceConstCmp2(arg0, arg1 uint16, fakePC uint) {}
+func libfuzzerTraceConstCmp4(arg0, arg1 uint32, fakePC uint) {}
+func libfuzzerTraceConstCmp8(arg0, arg1 uint64, fakePC uint) {}
+
+func libfuzzerHookStrCmp(arg0, arg1 string, fakePC uint)    {}
+func libfuzzerHookEqualFold(arg0, arg1 string, fakePC uint) {}
diff --git a/src/internal/fuzz/worker.go b/src/internal/fuzz/worker.go
new file mode 100644
index 0000000..467c39b
--- /dev/null
+++ b/src/internal/fuzz/worker.go
@@ -0,0 +1,1184 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"reflect"
+	"runtime"
+	"sync"
+	"time"
+)
+
+const (
+	// workerFuzzDuration is the amount of time a worker can spend testing random
+	// variations of an input given by the coordinator.
+	workerFuzzDuration = 100 * time.Millisecond
+
+	// workerTimeoutDuration is the amount of time a worker can go without
+	// responding to the coordinator before being stopped.
+	workerTimeoutDuration = 1 * time.Second
+
+	// workerExitCode is used as an exit code by fuzz worker processes after an internal error.
+	// This distinguishes internal errors from uncontrolled panics and other crashes.
+	// Keep in sync with internal/fuzz.workerExitCode.
+	workerExitCode = 70
+
+	// workerSharedMemSize is the maximum size of the shared memory file used to
+	// communicate with workers. This limits the size of fuzz inputs.
+	workerSharedMemSize = 100 << 20 // 100 MB
+)
+
+// worker manages a worker process running a test binary. The worker object
+// exists only in the coordinator (the process started by 'go test -fuzz').
+// workerClient is used by the coordinator to send RPCs to the worker process,
+// which handles them with workerServer.
+type worker struct {
+	dir     string   // working directory, same as package directory
+	binPath string   // path to test executable
+	args    []string // arguments for test executable
+	env     []string // environment for test executable
+
+	coordinator *coordinator
+
+	memMu chan *sharedMem // mutex guarding shared memory with worker; persists across processes.
+
+	cmd         *exec.Cmd     // current worker process
+	client      *workerClient // used to communicate with worker process
+	waitErr     error         // last error returned by wait, set before termC is closed.
+	interrupted bool          // true after stop interrupts a running worker.
+	termC       chan struct{} // closed by wait when worker process terminates
+}
+
+func newWorker(c *coordinator, dir, binPath string, args, env []string) (*worker, error) {
+	mem, err := sharedMemTempFile(workerSharedMemSize)
+	if err != nil {
+		return nil, err
+	}
+	memMu := make(chan *sharedMem, 1)
+	memMu <- mem
+	return &worker{
+		dir:         dir,
+		binPath:     binPath,
+		args:        args,
+		env:         env[:len(env):len(env)], // copy on append to ensure workers don't overwrite each other.
+		coordinator: c,
+		memMu:       memMu,
+	}, nil
+}
+
+// cleanup releases persistent resources associated with the worker.
+func (w *worker) cleanup() error {
+	mem := <-w.memMu
+	if mem == nil {
+		return nil
+	}
+	close(w.memMu)
+	return mem.Close()
+}
+
+// coordinate runs the test binary to perform fuzzing.
+//
+// coordinate loops until ctx is cancelled or a fatal error is encountered.
+// If a test process terminates unexpectedly while fuzzing, coordinate will
+// attempt to restart and continue unless the termination can be attributed
+// to an interruption (from a timer or the user).
+//
+// While looping, coordinate receives inputs from the coordinator, passes
+// those inputs to the worker process, then passes the results back to
+// the coordinator.
+func (w *worker) coordinate(ctx context.Context) error {
+	// Main event loop.
+	for {
+		// Start or restart the worker if it's not running.
+		if !w.isRunning() {
+			if err := w.startAndPing(ctx); err != nil {
+				return err
+			}
+		}
+
+		select {
+		case <-ctx.Done():
+			// Worker was told to stop.
+			err := w.stop()
+			if err != nil && !w.interrupted && !isInterruptError(err) {
+				return err
+			}
+			return ctx.Err()
+
+		case <-w.termC:
+			// Worker process terminated unexpectedly while waiting for input.
+			err := w.stop()
+			if w.interrupted {
+				panic("worker interrupted after unexpected termination")
+			}
+			if err == nil || isInterruptError(err) {
+				// Worker stopped, either by exiting with status 0 or after being
+				// interrupted with a signal that was not sent by the coordinator.
+				//
+				// When the user presses ^C, on POSIX platforms, SIGINT is delivered to
+				// all processes in the group concurrently, and the worker may see it
+				// before the coordinator. The worker should exit 0 gracefully (in
+				// theory).
+				//
+				// This condition is probably intended by the user, so suppress
+				// the error.
+				return nil
+			}
+			if exitErr, ok := err.(*exec.ExitError); ok && exitErr.ExitCode() == workerExitCode {
+				// Worker exited with a code indicating F.Fuzz was not called correctly,
+				// for example, F.Fail was called first.
+				return fmt.Errorf("fuzzing process exited unexpectedly due to an internal failure: %w", err)
+			}
+			// Worker exited non-zero or was terminated by a non-interrupt
+			// signal (for example, SIGSEGV) while fuzzing.
+			return fmt.Errorf("fuzzing process hung or terminated unexpectedly: %w", err)
+			// TODO(jayconrod,katiehockman): if -keepfuzzing, restart worker.
+
+		case input := <-w.coordinator.inputC:
+			// Received input from coordinator.
+			args := fuzzArgs{
+				Limit:        input.limit,
+				Timeout:      input.timeout,
+				Warmup:       input.warmup,
+				CoverageData: input.coverageData,
+			}
+			entry, resp, isInternalError, err := w.client.fuzz(ctx, input.entry, args)
+			canMinimize := true
+			if err != nil {
+				// Error communicating with worker.
+				w.stop()
+				if ctx.Err() != nil {
+					// Timeout or interruption.
+					return ctx.Err()
+				}
+				if w.interrupted {
+					// Communication error before we stopped the worker.
+					// Report an error, but don't record a crasher.
+					return fmt.Errorf("communicating with fuzzing process: %v", err)
+				}
+				if sig, ok := terminationSignal(w.waitErr); ok && !isCrashSignal(sig) {
+					// Worker terminated by a signal that probably wasn't caused by a
+					// specific input to the fuzz function. For example, on Linux,
+					// the kernel (OOM killer) may send SIGKILL to a process using a lot
+					// of memory. Or the shell might send SIGHUP when the terminal
+					// is closed. Don't record a crasher.
+					return fmt.Errorf("fuzzing process terminated by unexpected signal; no crash will be recorded: %v", w.waitErr)
+				}
+				if isInternalError {
+					// An internal error occurred which shouldn't be considered
+					// a crash.
+					return err
+				}
+				// Unexpected termination. Set error message and fall through.
+				// We'll restart the worker on the next iteration.
+				// Don't attempt to minimize this since it crashed the worker.
+				resp.Err = fmt.Sprintf("fuzzing process hung or terminated unexpectedly: %v", w.waitErr)
+				canMinimize = false
+			}
+			result := fuzzResult{
+				limit:         input.limit,
+				count:         resp.Count,
+				totalDuration: resp.TotalDuration,
+				entryDuration: resp.InterestingDuration,
+				entry:         entry,
+				crasherMsg:    resp.Err,
+				coverageData:  resp.CoverageData,
+				canMinimize:   canMinimize,
+			}
+			w.coordinator.resultC <- result
+
+		case input := <-w.coordinator.minimizeC:
+			// Received input to minimize from coordinator.
+			result, err := w.minimize(ctx, input)
+			if err != nil {
+				// Error minimizing. Send back the original input. If it didn't cause
+				// an error before, report it as causing an error now.
+				// TODO: double-check this is handled correctly when
+				// implementing -keepfuzzing.
+				result = fuzzResult{
+					entry:       input.entry,
+					crasherMsg:  input.crasherMsg,
+					canMinimize: false,
+					limit:       input.limit,
+				}
+				if result.crasherMsg == "" {
+					result.crasherMsg = err.Error()
+				}
+			}
+			w.coordinator.resultC <- result
+		}
+	}
+}
+
+// minimize tells a worker process to attempt to find a smaller value that
+// either causes an error (if we started minimizing because we found an input
+// that causes an error) or preserves new coverage (if we started minimizing
+// because we found an input that expands coverage).
+func (w *worker) minimize(ctx context.Context, input fuzzMinimizeInput) (min fuzzResult, err error) {
+	if w.coordinator.opts.MinimizeTimeout != 0 {
+		var cancel func()
+		ctx, cancel = context.WithTimeout(ctx, w.coordinator.opts.MinimizeTimeout)
+		defer cancel()
+	}
+
+	args := minimizeArgs{
+		Limit:        input.limit,
+		Timeout:      input.timeout,
+		KeepCoverage: input.keepCoverage,
+	}
+	entry, resp, err := w.client.minimize(ctx, input.entry, args)
+	if err != nil {
+		// Error communicating with worker.
+		w.stop()
+		if ctx.Err() != nil || w.interrupted || isInterruptError(w.waitErr) {
+			// Worker was interrupted, possibly by the user pressing ^C.
+			// Normally, workers can handle interrupts and timeouts gracefully and
+			// will return without error. An error here indicates the worker
+			// may not have been in a good state, but the error won't be meaningful
+			// to the user. Just return the original crasher without logging anything.
+			return fuzzResult{
+				entry:        input.entry,
+				crasherMsg:   input.crasherMsg,
+				coverageData: input.keepCoverage,
+				canMinimize:  false,
+				limit:        input.limit,
+			}, nil
+		}
+		return fuzzResult{
+			entry:         entry,
+			crasherMsg:    fmt.Sprintf("fuzzing process hung or terminated unexpectedly while minimizing: %v", err),
+			canMinimize:   false,
+			limit:         input.limit,
+			count:         resp.Count,
+			totalDuration: resp.Duration,
+		}, nil
+	}
+
+	if input.crasherMsg != "" && resp.Err == "" {
+		return fuzzResult{}, fmt.Errorf("attempted to minimize a crash but could not reproduce")
+	}
+
+	return fuzzResult{
+		entry:         entry,
+		crasherMsg:    resp.Err,
+		coverageData:  resp.CoverageData,
+		canMinimize:   false,
+		limit:         input.limit,
+		count:         resp.Count,
+		totalDuration: resp.Duration,
+	}, nil
+}
+
+func (w *worker) isRunning() bool {
+	return w.cmd != nil
+}
+
+// startAndPing starts the worker process and sends it a message to make sure it
+// can communicate.
+//
+// startAndPing returns an error if any part of this didn't work, including if
+// the context is expired or the worker process was interrupted before it
+// responded. Errors that happen after start but before the ping response
+// likely indicate that the worker did not call F.Fuzz or called F.Fail first.
+// We don't record crashers for these errors.
+func (w *worker) startAndPing(ctx context.Context) error {
+	if ctx.Err() != nil {
+		return ctx.Err()
+	}
+	if err := w.start(); err != nil {
+		return err
+	}
+	if err := w.client.ping(ctx); err != nil {
+		w.stop()
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+		if isInterruptError(err) {
+			// User may have pressed ^C before worker responded.
+			return err
+		}
+		// TODO: record and return stderr.
+		return fmt.Errorf("fuzzing process terminated without fuzzing: %w", err)
+	}
+	return nil
+}
+
+// start runs a new worker process.
+//
+// If the process couldn't be started, start returns an error. Start won't
+// return later termination errors from the process if they occur.
+//
+// If the process starts successfully, start returns nil. stop must be called
+// once later to clean up, even if the process terminates on its own.
+//
+// When the process terminates, w.waitErr is set to the error (if any), and
+// w.termC is closed.
+func (w *worker) start() (err error) {
+	if w.isRunning() {
+		panic("worker already started")
+	}
+	w.waitErr = nil
+	w.interrupted = false
+	w.termC = nil
+
+	cmd := exec.Command(w.binPath, w.args...)
+	cmd.Dir = w.dir
+	cmd.Env = w.env[:len(w.env):len(w.env)] // copy on append to ensure workers don't overwrite each other.
+
+	// Create the "fuzz_in" and "fuzz_out" pipes so we can communicate with
+	// the worker. We don't use stdin and stdout, since the test binary may
+	// do something else with those.
+	//
+	// Each pipe has a reader and a writer. The coordinator writes to fuzzInW
+	// and reads from fuzzOutR. The worker inherits fuzzInR and fuzzOutW.
+	// The coordinator closes fuzzInR and fuzzOutW after starting the worker,
+	// since we have no further need of them.
+	fuzzInR, fuzzInW, err := os.Pipe()
+	if err != nil {
+		return err
+	}
+	defer fuzzInR.Close()
+	fuzzOutR, fuzzOutW, err := os.Pipe()
+	if err != nil {
+		fuzzInW.Close()
+		return err
+	}
+	defer fuzzOutW.Close()
+	setWorkerComm(cmd, workerComm{fuzzIn: fuzzInR, fuzzOut: fuzzOutW, memMu: w.memMu})
+
+	// Start the worker process.
+	if err := cmd.Start(); err != nil {
+		fuzzInW.Close()
+		fuzzOutR.Close()
+		return err
+	}
+
+	// Worker started successfully.
+	// After this, w.client owns fuzzInW and fuzzOutR, so w.client.Close must be
+	// called later by stop.
+	w.cmd = cmd
+	w.termC = make(chan struct{})
+	comm := workerComm{fuzzIn: fuzzInW, fuzzOut: fuzzOutR, memMu: w.memMu}
+	m := newMutator()
+	w.client = newWorkerClient(comm, m)
+
+	go func() {
+		w.waitErr = w.cmd.Wait()
+		close(w.termC)
+	}()
+
+	return nil
+}
+
+// stop tells the worker process to exit by closing w.client, then blocks until
+// it terminates. If the worker doesn't terminate after a short time, stop
+// signals it with os.Interrupt (where supported), then os.Kill.
+//
+// stop returns the error the process terminated with, if any (same as
+// w.waitErr).
+//
+// stop must be called at least once after start returns successfully, even if
+// the worker process terminates unexpectedly.
+func (w *worker) stop() error {
+	if w.termC == nil {
+		panic("worker was not started successfully")
+	}
+	select {
+	case <-w.termC:
+		// Worker already terminated.
+		if w.client == nil {
+			// stop already called.
+			return w.waitErr
+		}
+		// Possible unexpected termination.
+		w.client.Close()
+		w.cmd = nil
+		w.client = nil
+		return w.waitErr
+	default:
+		// Worker still running.
+	}
+
+	// Tell the worker to stop by closing fuzz_in. It won't actually stop until it
+	// finishes with earlier calls.
+	closeC := make(chan struct{})
+	go func() {
+		w.client.Close()
+		close(closeC)
+	}()
+
+	sig := os.Interrupt
+	if runtime.GOOS == "windows" {
+		// Per https://golang.org/pkg/os/#Signal, “Interrupt is not implemented on
+		// Windows; using it with os.Process.Signal will return an error.”
+		// Fall back to Kill instead.
+		sig = os.Kill
+	}
+
+	t := time.NewTimer(workerTimeoutDuration)
+	for {
+		select {
+		case <-w.termC:
+			// Worker terminated.
+			t.Stop()
+			<-closeC
+			w.cmd = nil
+			w.client = nil
+			return w.waitErr
+
+		case <-t.C:
+			// Timer fired before worker terminated.
+			w.interrupted = true
+			switch sig {
+			case os.Interrupt:
+				// Try to stop the worker with SIGINT and wait a little longer.
+				w.cmd.Process.Signal(sig)
+				sig = os.Kill
+				t.Reset(workerTimeoutDuration)
+
+			case os.Kill:
+				// Try to stop the worker with SIGKILL and keep waiting.
+				w.cmd.Process.Signal(sig)
+				sig = nil
+				t.Reset(workerTimeoutDuration)
+
+			case nil:
+				// Still waiting. Print a message to let the user know why.
+				fmt.Fprintf(w.coordinator.opts.Log, "waiting for fuzzing process to terminate...\n")
+			}
+		}
+	}
+}
+
+// RunFuzzWorker is called in a worker process to communicate with the
+// coordinator process in order to fuzz random inputs. RunFuzzWorker loops
+// until the coordinator tells it to stop.
+//
+// fn is a wrapper on the fuzz function. It may return an error to indicate
+// a given input "crashed". The coordinator will also record a crasher if
+// the function times out or terminates the process.
+//
+// RunFuzzWorker returns an error if it could not communicate with the
+// coordinator process.
+func RunFuzzWorker(ctx context.Context, fn func(CorpusEntry) error) error {
+	comm, err := getWorkerComm()
+	if err != nil {
+		return err
+	}
+	srv := &workerServer{
+		workerComm: comm,
+		fuzzFn: func(e CorpusEntry) (time.Duration, error) {
+			timer := time.AfterFunc(10*time.Second, func() {
+				panic("deadlocked!") // this error message won't be printed
+			})
+			defer timer.Stop()
+			start := time.Now()
+			err := fn(e)
+			return time.Since(start), err
+		},
+		m: newMutator(),
+	}
+	return srv.serve(ctx)
+}
+
+// call is serialized and sent from the coordinator on fuzz_in. It acts as
+// a minimalist RPC mechanism. Exactly one of its fields must be set to indicate
+// which method to call.
+type call struct {
+	Ping     *pingArgs
+	Fuzz     *fuzzArgs
+	Minimize *minimizeArgs
+}
+
+// minimizeArgs contains arguments to workerServer.minimize. The value to
+// minimize is already in shared memory.
+type minimizeArgs struct {
+	// Timeout is the time to spend minimizing. This may include time to start up,
+	// especially if the input causes the worker process to terminated, requiring
+	// repeated restarts.
+	Timeout time.Duration
+
+	// Limit is the maximum number of values to test, without spending more time
+	// than Duration. 0 indicates no limit.
+	Limit int64
+
+	// KeepCoverage is a set of coverage counters the worker should attempt to
+	// keep in minimized values. When provided, the worker will reject inputs that
+	// don't cause at least one of these bits to be set.
+	KeepCoverage []byte
+
+	// Index is the index of the fuzz target parameter to be minimized.
+	Index int
+}
+
+// minimizeResponse contains results from workerServer.minimize.
+type minimizeResponse struct {
+	// WroteToMem is true if the worker found a smaller input and wrote it to
+	// shared memory. If minimizeArgs.KeepCoverage was set, the minimized input
+	// preserved at least one coverage bit and did not cause an error.
+	// Otherwise, the minimized input caused some error, recorded in Err.
+	WroteToMem bool
+
+	// Err is the error string caused by the value in shared memory, if any.
+	Err string
+
+	// CoverageData is the set of coverage bits activated by the minimized value
+	// in shared memory. When set, it contains at least one bit from KeepCoverage.
+	// CoverageData will be nil if Err is set or if minimization failed.
+	CoverageData []byte
+
+	// Duration is the time spent minimizing, not including starting or cleaning up.
+	Duration time.Duration
+
+	// Count is the number of values tested.
+	Count int64
+}
+
+// fuzzArgs contains arguments to workerServer.fuzz. The value to fuzz is
+// passed in shared memory.
+type fuzzArgs struct {
+	// Timeout is the time to spend fuzzing, not including starting or
+	// cleaning up.
+	Timeout time.Duration
+
+	// Limit is the maximum number of values to test, without spending more time
+	// than Duration. 0 indicates no limit.
+	Limit int64
+
+	// Warmup indicates whether this is part of a warmup run, meaning that
+	// fuzzing should not occur. If coverageEnabled is true, then coverage data
+	// should be reported.
+	Warmup bool
+
+	// CoverageData is the coverage data. If set, the worker should update its
+	// local coverage data prior to fuzzing.
+	CoverageData []byte
+}
+
+// fuzzResponse contains results from workerServer.fuzz.
+type fuzzResponse struct {
+	// Duration is the time spent fuzzing, not including starting or cleaning up.
+	TotalDuration       time.Duration
+	InterestingDuration time.Duration
+
+	// Count is the number of values tested.
+	Count int64
+
+	// CoverageData is set if the value in shared memory expands coverage
+	// and therefore may be interesting to the coordinator.
+	CoverageData []byte
+
+	// Err is the error string caused by the value in shared memory, which is
+	// non-empty if the value in shared memory caused a crash.
+	Err string
+
+	// InternalErr is the error string caused by an internal error in the
+	// worker. This shouldn't be considered a crasher.
+	InternalErr string
+}
+
+// pingArgs contains arguments to workerServer.ping.
+type pingArgs struct{}
+
+// pingResponse contains results from workerServer.ping.
+type pingResponse struct{}
+
+// workerComm holds pipes and shared memory used for communication
+// between the coordinator process (client) and a worker process (server).
+// These values are unique to each worker; they are shared only with the
+// coordinator, not with other workers.
+//
+// Access to shared memory is synchronized implicitly over the RPC protocol
+// implemented in workerServer and workerClient. During a call, the client
+// (worker) has exclusive access to shared memory; at other times, the server
+// (coordinator) has exclusive access.
+type workerComm struct {
+	fuzzIn, fuzzOut *os.File
+	memMu           chan *sharedMem // mutex guarding shared memory
+}
+
+// workerServer is a minimalist RPC server, run by fuzz worker processes.
+// It allows the coordinator process (using workerClient) to call methods in a
+// worker process. This system allows the coordinator to run multiple worker
+// processes in parallel and to collect inputs that caused crashes from shared
+// memory after a worker process terminates unexpectedly.
+type workerServer struct {
+	workerComm
+	m *mutator
+
+	// coverageMask is the local coverage data for the worker. It is
+	// periodically updated to reflect the data in the coordinator when new
+	// coverage is found.
+	coverageMask []byte
+
+	// fuzzFn runs the worker's fuzz target on the given input and returns an
+	// error if it finds a crasher (the process may also exit or crash), and the
+	// time it took to run the input. It sets a deadline of 10 seconds, at which
+	// point it will panic with the assumption that the process is hanging or
+	// deadlocked.
+	fuzzFn func(CorpusEntry) (time.Duration, error)
+}
+
+// serve reads serialized RPC messages on fuzzIn. When serve receives a message,
+// it calls the corresponding method, then sends the serialized result back
+// on fuzzOut.
+//
+// serve handles RPC calls synchronously; it will not attempt to read a message
+// until the previous call has finished.
+//
+// serve returns errors that occurred when communicating over pipes. serve
+// does not return errors from method calls; those are passed through serialized
+// responses.
+func (ws *workerServer) serve(ctx context.Context) error {
+	enc := json.NewEncoder(ws.fuzzOut)
+	dec := json.NewDecoder(&contextReader{ctx: ctx, r: ws.fuzzIn})
+	for {
+		var c call
+		if err := dec.Decode(&c); err != nil {
+			if err == io.EOF || err == ctx.Err() {
+				return nil
+			} else {
+				return err
+			}
+		}
+
+		var resp any
+		switch {
+		case c.Fuzz != nil:
+			resp = ws.fuzz(ctx, *c.Fuzz)
+		case c.Minimize != nil:
+			resp = ws.minimize(ctx, *c.Minimize)
+		case c.Ping != nil:
+			resp = ws.ping(ctx, *c.Ping)
+		default:
+			return errors.New("no arguments provided for any call")
+		}
+
+		if err := enc.Encode(resp); err != nil {
+			return err
+		}
+	}
+}
+
+// chainedMutations is how many mutations are applied before the worker
+// resets the input to it's original state.
+// NOTE: this number was picked without much thought. It is low enough that
+// it seems to create a significant diversity in mutated inputs. We may want
+// to consider looking into this more closely once we have a proper performance
+// testing framework. Another option is to randomly pick the number of chained
+// mutations on each invocation of the workerServer.fuzz method (this appears to
+// be what libFuzzer does, although there seems to be no documentation which
+// explains why this choice was made.)
+const chainedMutations = 5
+
+// fuzz runs the test function on random variations of the input value in shared
+// memory for a limited duration or number of iterations.
+//
+// fuzz returns early if it finds an input that crashes the fuzz function (with
+// fuzzResponse.Err set) or an input that expands coverage (with
+// fuzzResponse.InterestingDuration set).
+//
+// fuzz does not modify the input in shared memory. Instead, it saves the
+// initial PRNG state in shared memory and increments a counter in shared
+// memory before each call to the test function. The caller may reconstruct
+// the crashing input with this information, since the PRNG is deterministic.
+func (ws *workerServer) fuzz(ctx context.Context, args fuzzArgs) (resp fuzzResponse) {
+	if args.CoverageData != nil {
+		if ws.coverageMask != nil && len(args.CoverageData) != len(ws.coverageMask) {
+			resp.InternalErr = fmt.Sprintf("unexpected size for CoverageData: got %d, expected %d", len(args.CoverageData), len(ws.coverageMask))
+			return resp
+		}
+		ws.coverageMask = args.CoverageData
+	}
+	start := time.Now()
+	defer func() { resp.TotalDuration = time.Since(start) }()
+
+	if args.Timeout != 0 {
+		var cancel func()
+		ctx, cancel = context.WithTimeout(ctx, args.Timeout)
+		defer cancel()
+	}
+	mem := <-ws.memMu
+	ws.m.r.save(&mem.header().randState, &mem.header().randInc)
+	defer func() {
+		resp.Count = mem.header().count
+		ws.memMu <- mem
+	}()
+	if args.Limit > 0 && mem.header().count >= args.Limit {
+		resp.InternalErr = fmt.Sprintf("mem.header().count %d already exceeds args.Limit %d", mem.header().count, args.Limit)
+		return resp
+	}
+
+	originalVals, err := unmarshalCorpusFile(mem.valueCopy())
+	if err != nil {
+		resp.InternalErr = err.Error()
+		return resp
+	}
+	vals := make([]any, len(originalVals))
+	copy(vals, originalVals)
+
+	shouldStop := func() bool {
+		return args.Limit > 0 && mem.header().count >= args.Limit
+	}
+	fuzzOnce := func(entry CorpusEntry) (dur time.Duration, cov []byte, errMsg string) {
+		mem.header().count++
+		var err error
+		dur, err = ws.fuzzFn(entry)
+		if err != nil {
+			errMsg = err.Error()
+			if errMsg == "" {
+				errMsg = "fuzz function failed with no input"
+			}
+			return dur, nil, errMsg
+		}
+		if ws.coverageMask != nil && countNewCoverageBits(ws.coverageMask, coverageSnapshot) > 0 {
+			return dur, coverageSnapshot, ""
+		}
+		return dur, nil, ""
+	}
+
+	if args.Warmup {
+		dur, _, errMsg := fuzzOnce(CorpusEntry{Values: vals})
+		if errMsg != "" {
+			resp.Err = errMsg
+			return resp
+		}
+		resp.InterestingDuration = dur
+		if coverageEnabled {
+			resp.CoverageData = coverageSnapshot
+		}
+		return resp
+	}
+
+	for {
+		select {
+		case <-ctx.Done():
+			return resp
+		default:
+			if mem.header().count%chainedMutations == 0 {
+				copy(vals, originalVals)
+				ws.m.r.save(&mem.header().randState, &mem.header().randInc)
+			}
+			ws.m.mutate(vals, cap(mem.valueRef()))
+
+			entry := CorpusEntry{Values: vals}
+			dur, cov, errMsg := fuzzOnce(entry)
+			if errMsg != "" {
+				resp.Err = errMsg
+				return resp
+			}
+			if cov != nil {
+				resp.CoverageData = cov
+				resp.InterestingDuration = dur
+				return resp
+			}
+			if shouldStop() {
+				return resp
+			}
+		}
+	}
+}
+
+func (ws *workerServer) minimize(ctx context.Context, args minimizeArgs) (resp minimizeResponse) {
+	start := time.Now()
+	defer func() { resp.Duration = time.Since(start) }()
+	mem := <-ws.memMu
+	defer func() { ws.memMu <- mem }()
+	vals, err := unmarshalCorpusFile(mem.valueCopy())
+	if err != nil {
+		panic(err)
+	}
+	inpHash := sha256.Sum256(mem.valueCopy())
+	if args.Timeout != 0 {
+		var cancel func()
+		ctx, cancel = context.WithTimeout(ctx, args.Timeout)
+		defer cancel()
+	}
+
+	// Minimize the values in vals, then write to shared memory. We only write
+	// to shared memory after completing minimization.
+	success, err := ws.minimizeInput(ctx, vals, mem, args)
+	if success {
+		writeToMem(vals, mem)
+		outHash := sha256.Sum256(mem.valueCopy())
+		mem.header().rawInMem = false
+		resp.WroteToMem = true
+		if err != nil {
+			resp.Err = err.Error()
+		} else {
+			// If the values didn't change during minimization then coverageSnapshot is likely
+			// a dirty snapshot which represents the very last step of minimization, not the
+			// coverage for the initial input. In that case just return the coverage we were
+			// given initially, since it more accurately represents the coverage map for the
+			// input we are returning.
+			if outHash != inpHash {
+				resp.CoverageData = coverageSnapshot
+			} else {
+				resp.CoverageData = args.KeepCoverage
+			}
+		}
+	}
+	return resp
+}
+
+// minimizeInput applies a series of minimizing transformations on the provided
+// vals, ensuring that each minimization still causes an error, or keeps
+// coverage, in fuzzFn. It uses the context to determine how long to run,
+// stopping once closed. It returns a bool indicating whether minimization was
+// successful and an error if one was found.
+func (ws *workerServer) minimizeInput(ctx context.Context, vals []any, mem *sharedMem, args minimizeArgs) (success bool, retErr error) {
+	keepCoverage := args.KeepCoverage
+	memBytes := mem.valueRef()
+	bPtr := &memBytes
+	count := &mem.header().count
+	shouldStop := func() bool {
+		return ctx.Err() != nil ||
+			(args.Limit > 0 && *count >= args.Limit)
+	}
+	if shouldStop() {
+		return false, nil
+	}
+
+	// Check that the original value preserves coverage or causes an error.
+	// If not, then whatever caused us to think the value was interesting may
+	// have been a flake, and we can't minimize it.
+	*count++
+	_, retErr = ws.fuzzFn(CorpusEntry{Values: vals})
+	if keepCoverage != nil {
+		if !hasCoverageBit(keepCoverage, coverageSnapshot) || retErr != nil {
+			return false, nil
+		}
+	} else if retErr == nil {
+		return false, nil
+	}
+	mem.header().rawInMem = true
+
+	// tryMinimized runs the fuzz function with candidate replacing the value
+	// at index valI. tryMinimized returns whether the input with candidate is
+	// interesting for the same reason as the original input: it returns
+	// an error if one was expected, or it preserves coverage.
+	tryMinimized := func(candidate []byte) bool {
+		prev := vals[args.Index]
+		switch prev.(type) {
+		case []byte:
+			vals[args.Index] = candidate
+		case string:
+			vals[args.Index] = string(candidate)
+		default:
+			panic("impossible")
+		}
+		copy(*bPtr, candidate)
+		*bPtr = (*bPtr)[:len(candidate)]
+		mem.setValueLen(len(candidate))
+		*count++
+		_, err := ws.fuzzFn(CorpusEntry{Values: vals})
+		if err != nil {
+			retErr = err
+			if keepCoverage != nil {
+				// Now that we've found a crash, that's more important than any
+				// minimization of interesting inputs that was being done. Clear out
+				// keepCoverage to only minimize the crash going forward.
+				keepCoverage = nil
+			}
+			return true
+		}
+		// Minimization should preserve coverage bits.
+		if keepCoverage != nil && isCoverageSubset(keepCoverage, coverageSnapshot) {
+			return true
+		}
+		vals[args.Index] = prev
+		return false
+	}
+	switch v := vals[args.Index].(type) {
+	case string:
+		minimizeBytes([]byte(v), tryMinimized, shouldStop)
+	case []byte:
+		minimizeBytes(v, tryMinimized, shouldStop)
+	default:
+		panic("impossible")
+	}
+	return true, retErr
+}
+
+func writeToMem(vals []any, mem *sharedMem) {
+	b := marshalCorpusFile(vals...)
+	mem.setValue(b)
+}
+
+// ping does nothing. The coordinator calls this method to ensure the worker
+// has called F.Fuzz and can communicate.
+func (ws *workerServer) ping(ctx context.Context, args pingArgs) pingResponse {
+	return pingResponse{}
+}
+
+// workerClient is a minimalist RPC client. The coordinator process uses a
+// workerClient to call methods in each worker process (handled by
+// workerServer).
+type workerClient struct {
+	workerComm
+	m *mutator
+
+	// mu is the mutex protecting the workerComm.fuzzIn pipe. This must be
+	// locked before making calls to the workerServer. It prevents
+	// workerClient.Close from closing fuzzIn while workerClient methods are
+	// writing to it concurrently, and prevents multiple callers from writing to
+	// fuzzIn concurrently.
+	mu sync.Mutex
+}
+
+func newWorkerClient(comm workerComm, m *mutator) *workerClient {
+	return &workerClient{workerComm: comm, m: m}
+}
+
+// Close shuts down the connection to the RPC server (the worker process) by
+// closing fuzz_in. Close drains fuzz_out (avoiding a SIGPIPE in the worker),
+// and closes it after the worker process closes the other end.
+func (wc *workerClient) Close() error {
+	wc.mu.Lock()
+	defer wc.mu.Unlock()
+
+	// Close fuzzIn. This signals to the server that there are no more calls,
+	// and it should exit.
+	if err := wc.fuzzIn.Close(); err != nil {
+		wc.fuzzOut.Close()
+		return err
+	}
+
+	// Drain fuzzOut and close it. When the server exits, the kernel will close
+	// its end of fuzzOut, and we'll get EOF.
+	if _, err := io.Copy(io.Discard, wc.fuzzOut); err != nil {
+		wc.fuzzOut.Close()
+		return err
+	}
+	return wc.fuzzOut.Close()
+}
+
+// errSharedMemClosed is returned by workerClient methods that cannot access
+// shared memory because it was closed and unmapped by another goroutine. That
+// can happen when worker.cleanup is called in the worker goroutine while a
+// workerClient.fuzz call runs concurrently.
+//
+// This error should not be reported. It indicates the operation was
+// interrupted.
+var errSharedMemClosed = errors.New("internal error: shared memory was closed and unmapped")
+
+// minimize tells the worker to call the minimize method. See
+// workerServer.minimize.
+func (wc *workerClient) minimize(ctx context.Context, entryIn CorpusEntry, args minimizeArgs) (entryOut CorpusEntry, resp minimizeResponse, retErr error) {
+	wc.mu.Lock()
+	defer wc.mu.Unlock()
+
+	mem, ok := <-wc.memMu
+	if !ok {
+		return CorpusEntry{}, minimizeResponse{}, errSharedMemClosed
+	}
+	mem.header().count = 0
+	inp, err := corpusEntryData(entryIn)
+	if err != nil {
+		return CorpusEntry{}, minimizeResponse{}, err
+	}
+	mem.setValue(inp)
+	defer func() { wc.memMu <- mem }()
+	entryOut = entryIn
+	entryOut.Values, err = unmarshalCorpusFile(inp)
+	if err != nil {
+		return CorpusEntry{}, minimizeResponse{}, fmt.Errorf("workerClient.minimize unmarshaling provided value: %v", err)
+	}
+	for i, v := range entryOut.Values {
+		if !isMinimizable(reflect.TypeOf(v)) {
+			continue
+		}
+
+		wc.memMu <- mem
+		args.Index = i
+		c := call{Minimize: &args}
+		callErr := wc.callLocked(ctx, c, &resp)
+		mem, ok = <-wc.memMu
+		if !ok {
+			return CorpusEntry{}, minimizeResponse{}, errSharedMemClosed
+		}
+
+		if callErr != nil {
+			retErr = callErr
+			if !mem.header().rawInMem {
+				// An unrecoverable error occurred before minimization began.
+				return entryIn, minimizeResponse{}, retErr
+			}
+			// An unrecoverable error occurred during minimization. mem now
+			// holds the raw, unmarshalled bytes of entryIn.Values[i] that
+			// caused the error.
+			switch entryOut.Values[i].(type) {
+			case string:
+				entryOut.Values[i] = string(mem.valueCopy())
+			case []byte:
+				entryOut.Values[i] = mem.valueCopy()
+			default:
+				panic("impossible")
+			}
+			entryOut.Data = marshalCorpusFile(entryOut.Values...)
+			// Stop minimizing; another unrecoverable error is likely to occur.
+			break
+		}
+
+		if resp.WroteToMem {
+			// Minimization succeeded, and mem holds the marshaled data.
+			entryOut.Data = mem.valueCopy()
+			entryOut.Values, err = unmarshalCorpusFile(entryOut.Data)
+			if err != nil {
+				return CorpusEntry{}, minimizeResponse{}, fmt.Errorf("workerClient.minimize unmarshaling minimized value: %v", err)
+			}
+		}
+
+		// Prepare for next iteration of the loop.
+		if args.Timeout != 0 {
+			args.Timeout -= resp.Duration
+			if args.Timeout <= 0 {
+				break
+			}
+		}
+		if args.Limit != 0 {
+			args.Limit -= mem.header().count
+			if args.Limit <= 0 {
+				break
+			}
+		}
+	}
+	resp.Count = mem.header().count
+	h := sha256.Sum256(entryOut.Data)
+	entryOut.Path = fmt.Sprintf("%x", h[:4])
+	return entryOut, resp, retErr
+}
+
+// fuzz tells the worker to call the fuzz method. See workerServer.fuzz.
+func (wc *workerClient) fuzz(ctx context.Context, entryIn CorpusEntry, args fuzzArgs) (entryOut CorpusEntry, resp fuzzResponse, isInternalError bool, err error) {
+	wc.mu.Lock()
+	defer wc.mu.Unlock()
+
+	mem, ok := <-wc.memMu
+	if !ok {
+		return CorpusEntry{}, fuzzResponse{}, true, errSharedMemClosed
+	}
+	mem.header().count = 0
+	inp, err := corpusEntryData(entryIn)
+	if err != nil {
+		return CorpusEntry{}, fuzzResponse{}, true, err
+	}
+	mem.setValue(inp)
+	wc.memMu <- mem
+
+	c := call{Fuzz: &args}
+	callErr := wc.callLocked(ctx, c, &resp)
+	if resp.InternalErr != "" {
+		return CorpusEntry{}, fuzzResponse{}, true, errors.New(resp.InternalErr)
+	}
+	mem, ok = <-wc.memMu
+	if !ok {
+		return CorpusEntry{}, fuzzResponse{}, true, errSharedMemClosed
+	}
+	defer func() { wc.memMu <- mem }()
+	resp.Count = mem.header().count
+
+	if !bytes.Equal(inp, mem.valueRef()) {
+		return CorpusEntry{}, fuzzResponse{}, true, errors.New("workerServer.fuzz modified input")
+	}
+	needEntryOut := callErr != nil || resp.Err != "" ||
+		(!args.Warmup && resp.CoverageData != nil)
+	if needEntryOut {
+		valuesOut, err := unmarshalCorpusFile(inp)
+		if err != nil {
+			return CorpusEntry{}, fuzzResponse{}, true, fmt.Errorf("unmarshaling fuzz input value after call: %v", err)
+		}
+		wc.m.r.restore(mem.header().randState, mem.header().randInc)
+		if !args.Warmup {
+			// Only mutate the valuesOut if fuzzing actually occurred.
+			numMutations := ((resp.Count - 1) % chainedMutations) + 1
+			for i := int64(0); i < numMutations; i++ {
+				wc.m.mutate(valuesOut, cap(mem.valueRef()))
+			}
+		}
+		dataOut := marshalCorpusFile(valuesOut...)
+
+		h := sha256.Sum256(dataOut)
+		name := fmt.Sprintf("%x", h[:4])
+		entryOut = CorpusEntry{
+			Parent:     entryIn.Path,
+			Path:       name,
+			Data:       dataOut,
+			Generation: entryIn.Generation + 1,
+		}
+		if args.Warmup {
+			// The bytes weren't mutated, so if entryIn was a seed corpus value,
+			// then entryOut is too.
+			entryOut.IsSeed = entryIn.IsSeed
+		}
+	}
+
+	return entryOut, resp, false, callErr
+}
+
+// ping tells the worker to call the ping method. See workerServer.ping.
+func (wc *workerClient) ping(ctx context.Context) error {
+	wc.mu.Lock()
+	defer wc.mu.Unlock()
+	c := call{Ping: &pingArgs{}}
+	var resp pingResponse
+	return wc.callLocked(ctx, c, &resp)
+}
+
+// callLocked sends an RPC from the coordinator to the worker process and waits
+// for the response. The callLocked may be cancelled with ctx.
+func (wc *workerClient) callLocked(ctx context.Context, c call, resp any) (err error) {
+	enc := json.NewEncoder(wc.fuzzIn)
+	dec := json.NewDecoder(&contextReader{ctx: ctx, r: wc.fuzzOut})
+	if err := enc.Encode(c); err != nil {
+		return err
+	}
+	return dec.Decode(resp)
+}
+
+// contextReader wraps a Reader with a Context. If the context is cancelled
+// while the underlying reader is blocked, Read returns immediately.
+//
+// This is useful for reading from a pipe. Closing a pipe file descriptor does
+// not unblock pending Reads on that file descriptor. All copies of the pipe's
+// other file descriptor (the write end) must be closed in all processes that
+// inherit it. This is difficult to do correctly in the situation we care about
+// (process group termination).
+type contextReader struct {
+	ctx context.Context
+	r   io.Reader
+}
+
+func (cr *contextReader) Read(b []byte) (int, error) {
+	if ctxErr := cr.ctx.Err(); ctxErr != nil {
+		return 0, ctxErr
+	}
+	done := make(chan struct{})
+
+	// This goroutine may stay blocked after Read returns because the underlying
+	// read is blocked.
+	var n int
+	var err error
+	go func() {
+		n, err = cr.r.Read(b)
+		close(done)
+	}()
+
+	select {
+	case <-cr.ctx.Done():
+		return 0, cr.ctx.Err()
+	case <-done:
+		return n, err
+	}
+}
diff --git a/src/internal/fuzz/worker_test.go b/src/internal/fuzz/worker_test.go
new file mode 100644
index 0000000..d0b21da
--- /dev/null
+++ b/src/internal/fuzz/worker_test.go
@@ -0,0 +1,206 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fuzz
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"internal/race"
+	"io"
+	"os"
+	"os/signal"
+	"reflect"
+	"strconv"
+	"testing"
+	"time"
+)
+
+var benchmarkWorkerFlag = flag.Bool("benchmarkworker", false, "")
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if *benchmarkWorkerFlag {
+		runBenchmarkWorker()
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func BenchmarkWorkerFuzzOverhead(b *testing.B) {
+	if race.Enabled {
+		b.Skip("TODO(48504): fix and re-enable")
+	}
+	origEnv := os.Getenv("GODEBUG")
+	defer func() { os.Setenv("GODEBUG", origEnv) }()
+	os.Setenv("GODEBUG", fmt.Sprintf("%s,fuzzseed=123", origEnv))
+
+	ws := &workerServer{
+		fuzzFn:     func(_ CorpusEntry) (time.Duration, error) { return time.Second, nil },
+		workerComm: workerComm{memMu: make(chan *sharedMem, 1)},
+	}
+
+	mem, err := sharedMemTempFile(workerSharedMemSize)
+	if err != nil {
+		b.Fatalf("failed to create temporary shared memory file: %s", err)
+	}
+	defer func() {
+		if err := mem.Close(); err != nil {
+			b.Error(err)
+		}
+	}()
+
+	initialVal := []any{make([]byte, 32)}
+	encodedVals := marshalCorpusFile(initialVal...)
+	mem.setValue(encodedVals)
+
+	ws.memMu <- mem
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		ws.m = newMutator()
+		mem.setValue(encodedVals)
+		mem.header().count = 0
+
+		ws.fuzz(context.Background(), fuzzArgs{Limit: 1})
+	}
+}
+
+// BenchmarkWorkerPing acts as the coordinator and measures the time it takes
+// a worker to respond to N pings. This is a rough measure of our RPC latency.
+func BenchmarkWorkerPing(b *testing.B) {
+	if race.Enabled {
+		b.Skip("TODO(48504): fix and re-enable")
+	}
+	b.SetParallelism(1)
+	w := newWorkerForTest(b)
+	for i := 0; i < b.N; i++ {
+		if err := w.client.ping(context.Background()); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+// BenchmarkWorkerFuzz acts as the coordinator and measures the time it takes
+// a worker to mutate a given input and call a trivial fuzz function N times.
+func BenchmarkWorkerFuzz(b *testing.B) {
+	if race.Enabled {
+		b.Skip("TODO(48504): fix and re-enable")
+	}
+	b.SetParallelism(1)
+	w := newWorkerForTest(b)
+	entry := CorpusEntry{Values: []any{[]byte(nil)}}
+	entry.Data = marshalCorpusFile(entry.Values...)
+	for i := int64(0); i < int64(b.N); {
+		args := fuzzArgs{
+			Limit:   int64(b.N) - i,
+			Timeout: workerFuzzDuration,
+		}
+		_, resp, _, err := w.client.fuzz(context.Background(), entry, args)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if resp.Err != "" {
+			b.Fatal(resp.Err)
+		}
+		if resp.Count == 0 {
+			b.Fatal("worker did not make progress")
+		}
+		i += resp.Count
+	}
+}
+
+// newWorkerForTest creates and starts a worker process for testing or
+// benchmarking. The worker process calls RunFuzzWorker, which responds to
+// RPC messages until it's stopped. The process is stopped and cleaned up
+// automatically when the test is done.
+func newWorkerForTest(tb testing.TB) *worker {
+	tb.Helper()
+	c, err := newCoordinator(CoordinateFuzzingOpts{
+		Types: []reflect.Type{reflect.TypeOf([]byte(nil))},
+		Log:   io.Discard,
+	})
+	if err != nil {
+		tb.Fatal(err)
+	}
+	dir := ""             // same as self
+	binPath := os.Args[0] // same as self
+	args := append(os.Args[1:], "-benchmarkworker")
+	env := os.Environ() // same as self
+	w, err := newWorker(c, dir, binPath, args, env)
+	if err != nil {
+		tb.Fatal(err)
+	}
+	tb.Cleanup(func() {
+		if err := w.cleanup(); err != nil {
+			tb.Error(err)
+		}
+	})
+	if err := w.startAndPing(context.Background()); err != nil {
+		tb.Fatal(err)
+	}
+	tb.Cleanup(func() {
+		if err := w.stop(); err != nil {
+			tb.Error(err)
+		}
+	})
+	return w
+}
+
+func runBenchmarkWorker() {
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
+	defer cancel()
+	fn := func(CorpusEntry) error { return nil }
+	if err := RunFuzzWorker(ctx, fn); err != nil && err != ctx.Err() {
+		panic(err)
+	}
+}
+
+func BenchmarkWorkerMinimize(b *testing.B) {
+	if race.Enabled {
+		b.Skip("TODO(48504): fix and re-enable")
+	}
+
+	ws := &workerServer{
+		workerComm: workerComm{memMu: make(chan *sharedMem, 1)},
+	}
+
+	mem, err := sharedMemTempFile(workerSharedMemSize)
+	if err != nil {
+		b.Fatalf("failed to create temporary shared memory file: %s", err)
+	}
+	defer func() {
+		if err := mem.Close(); err != nil {
+			b.Error(err)
+		}
+	}()
+	ws.memMu <- mem
+
+	bytes := make([]byte, 1024)
+	ctx := context.Background()
+	for sz := 1; sz <= len(bytes); sz <<= 1 {
+		sz := sz
+		input := []any{bytes[:sz]}
+		encodedVals := marshalCorpusFile(input...)
+		mem = <-ws.memMu
+		mem.setValue(encodedVals)
+		ws.memMu <- mem
+		b.Run(strconv.Itoa(sz), func(b *testing.B) {
+			i := 0
+			ws.fuzzFn = func(_ CorpusEntry) (time.Duration, error) {
+				if i == 0 {
+					i++
+					return time.Second, errors.New("initial failure for deflake")
+				}
+				return time.Second, nil
+			}
+			for i := 0; i < b.N; i++ {
+				b.SetBytes(int64(sz))
+				ws.minimize(ctx, minimizeArgs{})
+			}
+		})
+	}
+}
diff --git a/src/internal/goarch/gengoarch.go b/src/internal/goarch/gengoarch.go
new file mode 100644
index 0000000..0b0be5c
--- /dev/null
+++ b/src/internal/goarch/gengoarch.go
@@ -0,0 +1,60 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+)
+
+var goarches []string
+
+func main() {
+	data, err := os.ReadFile("../../go/build/syslist.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	const goarchPrefix = `var knownArch = map[string]bool{`
+	inGOARCH := false
+	for _, line := range strings.Split(string(data), "\n") {
+		if strings.HasPrefix(line, goarchPrefix) {
+			inGOARCH = true
+		} else if inGOARCH && strings.HasPrefix(line, "}") {
+			break
+		} else if inGOARCH {
+			goarch := strings.Fields(line)[0]
+			goarch = strings.TrimPrefix(goarch, `"`)
+			goarch = strings.TrimSuffix(goarch, `":`)
+			goarches = append(goarches, goarch)
+		}
+	}
+
+	for _, target := range goarches {
+		if target == "amd64p32" {
+			continue
+		}
+		var buf bytes.Buffer
+		fmt.Fprintf(&buf, "// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.\n\n")
+		fmt.Fprintf(&buf, "//go:build %s\n\n", target) // must explicitly include target for bootstrapping purposes
+		fmt.Fprintf(&buf, "package goarch\n\n")
+		fmt.Fprintf(&buf, "const GOARCH = `%s`\n\n", target)
+		for _, goarch := range goarches {
+			value := 0
+			if goarch == target {
+				value = 1
+			}
+			fmt.Fprintf(&buf, "const Is%s = %d\n", strings.Title(goarch), value)
+		}
+		err := os.WriteFile("zgoarch_"+target+".go", buf.Bytes(), 0666)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+}
diff --git a/src/internal/goarch/goarch.go b/src/internal/goarch/goarch.go
new file mode 100644
index 0000000..3dda62f
--- /dev/null
+++ b/src/internal/goarch/goarch.go
@@ -0,0 +1,60 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// package goarch contains GOARCH-specific constants.
+package goarch
+
+// The next line makes 'go generate' write the zgoarch*.go files with
+// per-arch information, including constants named $GOARCH for every
+// GOARCH. The constant is 1 on the current system, 0 otherwise; multiplying
+// by them is useful for defining GOARCH-specific constants.
+//
+//go:generate go run gengoarch.go
+
+type ArchFamilyType int
+
+const (
+	AMD64 ArchFamilyType = iota
+	ARM
+	ARM64
+	I386
+	LOONG64
+	MIPS
+	MIPS64
+	PPC64
+	RISCV64
+	S390X
+	WASM
+)
+
+// PtrSize is the size of a pointer in bytes - unsafe.Sizeof(uintptr(0)) but as an ideal constant.
+// It is also the size of the machine's native word size (that is, 4 on 32-bit systems, 8 on 64-bit).
+const PtrSize = 4 << (^uintptr(0) >> 63)
+
+// ArchFamily is the architecture family (AMD64, ARM, ...)
+const ArchFamily ArchFamilyType = _ArchFamily
+
+// BigEndian reports whether the architecture is big-endian.
+const BigEndian = IsArmbe|IsArm64be|IsMips|IsMips64|IsPpc|IsPpc64|IsS390|IsS390x|IsSparc|IsSparc64 == 1
+
+// DefaultPhysPageSize is the default physical page size.
+const DefaultPhysPageSize = _DefaultPhysPageSize
+
+// PCQuantum is the minimal unit for a program counter (1 on x86, 4 on most other systems).
+// The various PC tables record PC deltas pre-divided by PCQuantum.
+const PCQuantum = _PCQuantum
+
+// Int64Align is the required alignment for a 64-bit integer (4 on 32-bit systems, 8 on 64-bit).
+const Int64Align = PtrSize
+
+// MinFrameSize is the size of the system-reserved words at the bottom
+// of a frame (just above the architectural stack pointer).
+// It is zero on x86 and PtrSize on most non-x86 (LR-based) systems.
+// On PowerPC it is larger, to cover three more reserved words:
+// the compiler word, the link editor word, and the TOC save word.
+const MinFrameSize = _MinFrameSize
+
+// StackAlign is the required alignment of the SP register.
+// The stack must be at least word aligned, but some architectures require more.
+const StackAlign = _StackAlign
diff --git a/src/internal/goarch/goarch_386.go b/src/internal/goarch/goarch_386.go
new file mode 100644
index 0000000..c621421
--- /dev/null
+++ b/src/internal/goarch/goarch_386.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = I386
+	_DefaultPhysPageSize = 4096
+	_PCQuantum           = 1
+	_MinFrameSize        = 0
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_amd64.go b/src/internal/goarch/goarch_amd64.go
new file mode 100644
index 0000000..911e3e7
--- /dev/null
+++ b/src/internal/goarch/goarch_amd64.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = AMD64
+	_DefaultPhysPageSize = 4096
+	_PCQuantum           = 1
+	_MinFrameSize        = 0
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_arm.go b/src/internal/goarch/goarch_arm.go
new file mode 100644
index 0000000..a659171
--- /dev/null
+++ b/src/internal/goarch/goarch_arm.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = ARM
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 4
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_arm64.go b/src/internal/goarch/goarch_arm64.go
new file mode 100644
index 0000000..85d0b47
--- /dev/null
+++ b/src/internal/goarch/goarch_arm64.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = ARM64
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 8
+	_StackAlign          = 16
+)
diff --git a/src/internal/goarch/goarch_loong64.go b/src/internal/goarch/goarch_loong64.go
new file mode 100644
index 0000000..dae1f4d
--- /dev/null
+++ b/src/internal/goarch/goarch_loong64.go
@@ -0,0 +1,15 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build loong64
+
+package goarch
+
+const (
+	_ArchFamily          = LOONG64
+	_DefaultPhysPageSize = 16384
+	_PCQuantum           = 4
+	_MinFrameSize        = 8
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_mips.go b/src/internal/goarch/goarch_mips.go
new file mode 100644
index 0000000..59f3995
--- /dev/null
+++ b/src/internal/goarch/goarch_mips.go
@@ -0,0 +1,13 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = MIPS
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 4
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_mips64.go b/src/internal/goarch/goarch_mips64.go
new file mode 100644
index 0000000..9e4f827
--- /dev/null
+++ b/src/internal/goarch/goarch_mips64.go
@@ -0,0 +1,13 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = MIPS64
+	_DefaultPhysPageSize = 16384
+	_PCQuantum           = 4
+	_MinFrameSize        = 8
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_mips64le.go b/src/internal/goarch/goarch_mips64le.go
new file mode 100644
index 0000000..9e4f827
--- /dev/null
+++ b/src/internal/goarch/goarch_mips64le.go
@@ -0,0 +1,13 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = MIPS64
+	_DefaultPhysPageSize = 16384
+	_PCQuantum           = 4
+	_MinFrameSize        = 8
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_mipsle.go b/src/internal/goarch/goarch_mipsle.go
new file mode 100644
index 0000000..3e6642b
--- /dev/null
+++ b/src/internal/goarch/goarch_mipsle.go
@@ -0,0 +1,13 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = MIPS
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 4
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_ppc64.go b/src/internal/goarch/goarch_ppc64.go
new file mode 100644
index 0000000..60cc846
--- /dev/null
+++ b/src/internal/goarch/goarch_ppc64.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = PPC64
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 32
+	_StackAlign          = 16
+)
diff --git a/src/internal/goarch/goarch_ppc64le.go b/src/internal/goarch/goarch_ppc64le.go
new file mode 100644
index 0000000..60cc846
--- /dev/null
+++ b/src/internal/goarch/goarch_ppc64le.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = PPC64
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 4
+	_MinFrameSize        = 32
+	_StackAlign          = 16
+)
diff --git a/src/internal/goarch/goarch_riscv64.go b/src/internal/goarch/goarch_riscv64.go
new file mode 100644
index 0000000..3b6da1e
--- /dev/null
+++ b/src/internal/goarch/goarch_riscv64.go
@@ -0,0 +1,13 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = RISCV64
+	_DefaultPhysPageSize = 4096
+	_PCQuantum           = 4
+	_MinFrameSize        = 8
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_s390x.go b/src/internal/goarch/goarch_s390x.go
new file mode 100644
index 0000000..20c5705
--- /dev/null
+++ b/src/internal/goarch/goarch_s390x.go
@@ -0,0 +1,13 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = S390X
+	_DefaultPhysPageSize = 4096
+	_PCQuantum           = 2
+	_MinFrameSize        = 8
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/goarch_wasm.go b/src/internal/goarch/goarch_wasm.go
new file mode 100644
index 0000000..98618d6
--- /dev/null
+++ b/src/internal/goarch/goarch_wasm.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goarch
+
+const (
+	_ArchFamily          = WASM
+	_DefaultPhysPageSize = 65536
+	_PCQuantum           = 1
+	_MinFrameSize        = 0
+	_StackAlign          = PtrSize
+)
diff --git a/src/internal/goarch/zgoarch_386.go b/src/internal/goarch/zgoarch_386.go
new file mode 100644
index 0000000..4a9b0e6
--- /dev/null
+++ b/src/internal/goarch/zgoarch_386.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build 386
+
+package goarch
+
+const GOARCH = `386`
+
+const Is386 = 1
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_amd64.go b/src/internal/goarch/zgoarch_amd64.go
new file mode 100644
index 0000000..7926392
--- /dev/null
+++ b/src/internal/goarch/zgoarch_amd64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build amd64
+
+package goarch
+
+const GOARCH = `amd64`
+
+const Is386 = 0
+const IsAmd64 = 1
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_arm.go b/src/internal/goarch/zgoarch_arm.go
new file mode 100644
index 0000000..6c03b8b
--- /dev/null
+++ b/src/internal/goarch/zgoarch_arm.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build arm
+
+package goarch
+
+const GOARCH = `arm`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 1
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_arm64.go b/src/internal/goarch/zgoarch_arm64.go
new file mode 100644
index 0000000..ad342d7
--- /dev/null
+++ b/src/internal/goarch/zgoarch_arm64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build arm64
+
+package goarch
+
+const GOARCH = `arm64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 1
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_arm64be.go b/src/internal/goarch/zgoarch_arm64be.go
new file mode 100644
index 0000000..0f26003
--- /dev/null
+++ b/src/internal/goarch/zgoarch_arm64be.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build arm64be
+
+package goarch
+
+const GOARCH = `arm64be`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 1
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_armbe.go b/src/internal/goarch/zgoarch_armbe.go
new file mode 100644
index 0000000..6092fee
--- /dev/null
+++ b/src/internal/goarch/zgoarch_armbe.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build armbe
+
+package goarch
+
+const GOARCH = `armbe`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 1
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_loong64.go b/src/internal/goarch/zgoarch_loong64.go
new file mode 100644
index 0000000..21c67e1
--- /dev/null
+++ b/src/internal/goarch/zgoarch_loong64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build loong64
+
+package goarch
+
+const GOARCH = `loong64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 1
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mips.go b/src/internal/goarch/zgoarch_mips.go
new file mode 100644
index 0000000..0db1974
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mips.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mips
+
+package goarch
+
+const GOARCH = `mips`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 1
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mips64.go b/src/internal/goarch/zgoarch_mips64.go
new file mode 100644
index 0000000..738806f
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mips64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mips64
+
+package goarch
+
+const GOARCH = `mips64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 1
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mips64le.go b/src/internal/goarch/zgoarch_mips64le.go
new file mode 100644
index 0000000..8de5beb
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mips64le.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mips64le
+
+package goarch
+
+const GOARCH = `mips64le`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 1
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mips64p32.go b/src/internal/goarch/zgoarch_mips64p32.go
new file mode 100644
index 0000000..ea461be
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mips64p32.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mips64p32
+
+package goarch
+
+const GOARCH = `mips64p32`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 1
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mips64p32le.go b/src/internal/goarch/zgoarch_mips64p32le.go
new file mode 100644
index 0000000..15473ce
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mips64p32le.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mips64p32le
+
+package goarch
+
+const GOARCH = `mips64p32le`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 1
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_mipsle.go b/src/internal/goarch/zgoarch_mipsle.go
new file mode 100644
index 0000000..4955142
--- /dev/null
+++ b/src/internal/goarch/zgoarch_mipsle.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build mipsle
+
+package goarch
+
+const GOARCH = `mipsle`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 1
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_ppc.go b/src/internal/goarch/zgoarch_ppc.go
new file mode 100644
index 0000000..ec01763
--- /dev/null
+++ b/src/internal/goarch/zgoarch_ppc.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build ppc
+
+package goarch
+
+const GOARCH = `ppc`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 1
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_ppc64.go b/src/internal/goarch/zgoarch_ppc64.go
new file mode 100644
index 0000000..39be392
--- /dev/null
+++ b/src/internal/goarch/zgoarch_ppc64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build ppc64
+
+package goarch
+
+const GOARCH = `ppc64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 1
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_ppc64le.go b/src/internal/goarch/zgoarch_ppc64le.go
new file mode 100644
index 0000000..5f959e0
--- /dev/null
+++ b/src/internal/goarch/zgoarch_ppc64le.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build ppc64le
+
+package goarch
+
+const GOARCH = `ppc64le`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 1
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_riscv.go b/src/internal/goarch/zgoarch_riscv.go
new file mode 100644
index 0000000..8d81a14
--- /dev/null
+++ b/src/internal/goarch/zgoarch_riscv.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build riscv
+
+package goarch
+
+const GOARCH = `riscv`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 1
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_riscv64.go b/src/internal/goarch/zgoarch_riscv64.go
new file mode 100644
index 0000000..1df989c
--- /dev/null
+++ b/src/internal/goarch/zgoarch_riscv64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build riscv64
+
+package goarch
+
+const GOARCH = `riscv64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 1
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_s390.go b/src/internal/goarch/zgoarch_s390.go
new file mode 100644
index 0000000..56815b9
--- /dev/null
+++ b/src/internal/goarch/zgoarch_s390.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build s390
+
+package goarch
+
+const GOARCH = `s390`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 1
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_s390x.go b/src/internal/goarch/zgoarch_s390x.go
new file mode 100644
index 0000000..e61e9bd
--- /dev/null
+++ b/src/internal/goarch/zgoarch_s390x.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build s390x
+
+package goarch
+
+const GOARCH = `s390x`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 1
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_sparc.go b/src/internal/goarch/zgoarch_sparc.go
new file mode 100644
index 0000000..ee5b746
--- /dev/null
+++ b/src/internal/goarch/zgoarch_sparc.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build sparc
+
+package goarch
+
+const GOARCH = `sparc`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 1
+const IsSparc64 = 0
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_sparc64.go b/src/internal/goarch/zgoarch_sparc64.go
new file mode 100644
index 0000000..519aaa1
--- /dev/null
+++ b/src/internal/goarch/zgoarch_sparc64.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build sparc64
+
+package goarch
+
+const GOARCH = `sparc64`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 1
+const IsWasm = 0
diff --git a/src/internal/goarch/zgoarch_wasm.go b/src/internal/goarch/zgoarch_wasm.go
new file mode 100644
index 0000000..25567a1
--- /dev/null
+++ b/src/internal/goarch/zgoarch_wasm.go
@@ -0,0 +1,32 @@
+// Code generated by gengoarch.go using 'go generate'. DO NOT EDIT.
+
+//go:build wasm
+
+package goarch
+
+const GOARCH = `wasm`
+
+const Is386 = 0
+const IsAmd64 = 0
+const IsAmd64p32 = 0
+const IsArm = 0
+const IsArmbe = 0
+const IsArm64 = 0
+const IsArm64be = 0
+const IsLoong64 = 0
+const IsMips = 0
+const IsMipsle = 0
+const IsMips64 = 0
+const IsMips64le = 0
+const IsMips64p32 = 0
+const IsMips64p32le = 0
+const IsPpc = 0
+const IsPpc64 = 0
+const IsPpc64le = 0
+const IsRiscv = 0
+const IsRiscv64 = 0
+const IsS390 = 0
+const IsS390x = 0
+const IsSparc = 0
+const IsSparc64 = 0
+const IsWasm = 1
diff --git a/src/internal/godebug/godebug.go b/src/internal/godebug/godebug.go
new file mode 100644
index 0000000..dbcd980
--- /dev/null
+++ b/src/internal/godebug/godebug.go
@@ -0,0 +1,164 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package godebug makes the settings in the $GODEBUG environment variable
+// available to other packages. These settings are often used for compatibility
+// tweaks, when we need to change a default behavior but want to let users
+// opt back in to the original. For example GODEBUG=http2server=0 disables
+// HTTP/2 support in the net/http server.
+//
+// In typical usage, code should declare a Setting as a global
+// and then call Value each time the current setting value is needed:
+//
+//	var http2server = godebug.New("http2server")
+//
+//	func ServeConn(c net.Conn) {
+//		if http2server.Value() == "0" {
+//			disallow HTTP/2
+//			...
+//		}
+//		...
+//	}
+package godebug
+
+import (
+	"sync"
+	"sync/atomic"
+	_ "unsafe" // go:linkname
+)
+
+// A Setting is a single setting in the $GODEBUG environment variable.
+type Setting struct {
+	name  string
+	once  sync.Once
+	value *atomic.Pointer[string]
+}
+
+// New returns a new Setting for the $GODEBUG setting with the given name.
+func New(name string) *Setting {
+	return &Setting{name: name}
+}
+
+// Name returns the name of the setting.
+func (s *Setting) Name() string {
+	return s.name
+}
+
+// String returns a printable form for the setting: name=value.
+func (s *Setting) String() string {
+	return s.name + "=" + s.Value()
+}
+
+// cache is a cache of all the GODEBUG settings,
+// a locked map[string]*atomic.Pointer[string].
+//
+// All Settings with the same name share a single
+// *atomic.Pointer[string], so that when GODEBUG
+// changes only that single atomic string pointer
+// needs to be updated.
+//
+// A name appears in the values map either if it is the
+// name of a Setting for which Value has been called
+// at least once, or if the name has ever appeared in
+// a name=value pair in the $GODEBUG environment variable.
+// Once entered into the map, the name is never removed.
+var cache sync.Map // name string -> value *atomic.Pointer[string]
+
+var empty string
+
+// Value returns the current value for the GODEBUG setting s.
+//
+// Value maintains an internal cache that is synchronized
+// with changes to the $GODEBUG environment variable,
+// making Value efficient to call as frequently as needed.
+// Clients should therefore typically not attempt their own
+// caching of Value's result.
+func (s *Setting) Value() string {
+	s.once.Do(func() {
+		v, ok := cache.Load(s.name)
+		if !ok {
+			p := new(atomic.Pointer[string])
+			p.Store(&empty)
+			v, _ = cache.LoadOrStore(s.name, p)
+		}
+		s.value = v.(*atomic.Pointer[string])
+	})
+	return *s.value.Load()
+}
+
+// setUpdate is provided by package runtime.
+// It calls update(def, env), where def is the default GODEBUG setting
+// and env is the current value of the $GODEBUG environment variable.
+// After that first call, the runtime calls update(def, env)
+// again each time the environment variable changes
+// (due to use of os.Setenv, for example).
+//
+//go:linkname setUpdate
+func setUpdate(update func(string, string))
+
+func init() {
+	setUpdate(update)
+}
+
+var updateMu sync.Mutex
+
+// update records an updated GODEBUG setting.
+// def is the default GODEBUG setting for the running binary,
+// and env is the current value of the $GODEBUG environment variable.
+func update(def, env string) {
+	updateMu.Lock()
+	defer updateMu.Unlock()
+
+	// Update all the cached values, creating new ones as needed.
+	// We parse the environment variable first, so that any settings it has
+	// are already locked in place (did[name] = true) before we consider
+	// the defaults.
+	did := make(map[string]bool)
+	parse(did, env)
+	parse(did, def)
+
+	// Clear any cached values that are no longer present.
+	cache.Range(func(name, v any) bool {
+		if !did[name.(string)] {
+			v.(*atomic.Pointer[string]).Store(&empty)
+		}
+		return true
+	})
+}
+
+// parse parses the GODEBUG setting string s,
+// which has the form k=v,k2=v2,k3=v3.
+// Later settings override earlier ones.
+// Parse only updates settings k=v for which did[k] = false.
+// It also sets did[k] = true for settings that it updates.
+func parse(did map[string]bool, s string) {
+	// Scan the string backward so that later settings are used
+	// and earlier settings are ignored.
+	// Note that a forward scan would cause cached values
+	// to temporarily use the ignored value before being
+	// updated to the "correct" one.
+	end := len(s)
+	eq := -1
+	for i := end - 1; i >= -1; i-- {
+		if i == -1 || s[i] == ',' {
+			if eq >= 0 {
+				name, value := s[i+1:eq], s[eq+1:end]
+				if !did[name] {
+					did[name] = true
+					v, ok := cache.Load(name)
+					if !ok {
+						p := new(atomic.Pointer[string])
+						p.Store(&empty)
+						v, _ = cache.LoadOrStore(name, p)
+					}
+					v.(*atomic.Pointer[string]).Store(&value)
+				}
+			}
+			eq = -1
+			end = i
+		} else if s[i] == '=' {
+			eq = i
+		}
+	}
+}
diff --git a/src/internal/godebug/godebug_test.go b/src/internal/godebug/godebug_test.go
new file mode 100644
index 0000000..319229d
--- /dev/null
+++ b/src/internal/godebug/godebug_test.go
@@ -0,0 +1,39 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package godebug_test
+
+import (
+	. "internal/godebug"
+	"testing"
+)
+
+func TestGet(t *testing.T) {
+	foo := New("foo")
+	tests := []struct {
+		godebug string
+		setting *Setting
+		want    string
+	}{
+		{"", New(""), ""},
+		{"", foo, ""},
+		{"foo=bar", foo, "bar"},
+		{"foo=bar,after=x", foo, "bar"},
+		{"before=x,foo=bar,after=x", foo, "bar"},
+		{"before=x,foo=bar", foo, "bar"},
+		{",,,foo=bar,,,", foo, "bar"},
+		{"foodecoy=wrong,foo=bar", foo, "bar"},
+		{"foo=", foo, ""},
+		{"foo", foo, ""},
+		{",foo", foo, ""},
+		{"foo=bar,baz", New("loooooooong"), ""},
+	}
+	for _, tt := range tests {
+		t.Setenv("GODEBUG", tt.godebug)
+		got := tt.setting.Value()
+		if got != tt.want {
+			t.Errorf("get(%q, %q) = %q; want %q", tt.godebug, tt.setting.Name(), got, tt.want)
+		}
+	}
+}
diff --git a/src/internal/goexperiment/exp_arenas_off.go b/src/internal/goexperiment/exp_arenas_off.go
new file mode 100644
index 0000000..9e40ebc
--- /dev/null
+++ b/src/internal/goexperiment/exp_arenas_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.arenas
+// +build !goexperiment.arenas
+
+package goexperiment
+
+const Arenas = false
+const ArenasInt = 0
diff --git a/src/internal/goexperiment/exp_arenas_on.go b/src/internal/goexperiment/exp_arenas_on.go
new file mode 100644
index 0000000..92ef748
--- /dev/null
+++ b/src/internal/goexperiment/exp_arenas_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.arenas
+// +build goexperiment.arenas
+
+package goexperiment
+
+const Arenas = true
+const ArenasInt = 1
diff --git a/src/internal/goexperiment/exp_boringcrypto_off.go b/src/internal/goexperiment/exp_boringcrypto_off.go
new file mode 100644
index 0000000..020c75b
--- /dev/null
+++ b/src/internal/goexperiment/exp_boringcrypto_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.boringcrypto
+// +build !goexperiment.boringcrypto
+
+package goexperiment
+
+const BoringCrypto = false
+const BoringCryptoInt = 0
diff --git a/src/internal/goexperiment/exp_boringcrypto_on.go b/src/internal/goexperiment/exp_boringcrypto_on.go
new file mode 100644
index 0000000..1454329
--- /dev/null
+++ b/src/internal/goexperiment/exp_boringcrypto_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.boringcrypto
+// +build goexperiment.boringcrypto
+
+package goexperiment
+
+const BoringCrypto = true
+const BoringCryptoInt = 1
diff --git a/src/internal/goexperiment/exp_coverageredesign_off.go b/src/internal/goexperiment/exp_coverageredesign_off.go
new file mode 100644
index 0000000..95d3a6c
--- /dev/null
+++ b/src/internal/goexperiment/exp_coverageredesign_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.coverageredesign
+// +build !goexperiment.coverageredesign
+
+package goexperiment
+
+const CoverageRedesign = false
+const CoverageRedesignInt = 0
diff --git a/src/internal/goexperiment/exp_coverageredesign_on.go b/src/internal/goexperiment/exp_coverageredesign_on.go
new file mode 100644
index 0000000..330a234
--- /dev/null
+++ b/src/internal/goexperiment/exp_coverageredesign_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.coverageredesign
+// +build goexperiment.coverageredesign
+
+package goexperiment
+
+const CoverageRedesign = true
+const CoverageRedesignInt = 1
diff --git a/src/internal/goexperiment/exp_fieldtrack_off.go b/src/internal/goexperiment/exp_fieldtrack_off.go
new file mode 100644
index 0000000..e5e1326
--- /dev/null
+++ b/src/internal/goexperiment/exp_fieldtrack_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.fieldtrack
+// +build !goexperiment.fieldtrack
+
+package goexperiment
+
+const FieldTrack = false
+const FieldTrackInt = 0
diff --git a/src/internal/goexperiment/exp_fieldtrack_on.go b/src/internal/goexperiment/exp_fieldtrack_on.go
new file mode 100644
index 0000000..0d8c447
--- /dev/null
+++ b/src/internal/goexperiment/exp_fieldtrack_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.fieldtrack
+// +build goexperiment.fieldtrack
+
+package goexperiment
+
+const FieldTrack = true
+const FieldTrackInt = 1
diff --git a/src/internal/goexperiment/exp_heapminimum512kib_off.go b/src/internal/goexperiment/exp_heapminimum512kib_off.go
new file mode 100644
index 0000000..09da431
--- /dev/null
+++ b/src/internal/goexperiment/exp_heapminimum512kib_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.heapminimum512kib
+// +build !goexperiment.heapminimum512kib
+
+package goexperiment
+
+const HeapMinimum512KiB = false
+const HeapMinimum512KiBInt = 0
diff --git a/src/internal/goexperiment/exp_heapminimum512kib_on.go b/src/internal/goexperiment/exp_heapminimum512kib_on.go
new file mode 100644
index 0000000..bab684b
--- /dev/null
+++ b/src/internal/goexperiment/exp_heapminimum512kib_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.heapminimum512kib
+// +build goexperiment.heapminimum512kib
+
+package goexperiment
+
+const HeapMinimum512KiB = true
+const HeapMinimum512KiBInt = 1
diff --git a/src/internal/goexperiment/exp_pagetrace_off.go b/src/internal/goexperiment/exp_pagetrace_off.go
new file mode 100644
index 0000000..789e883
--- /dev/null
+++ b/src/internal/goexperiment/exp_pagetrace_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.pagetrace
+// +build !goexperiment.pagetrace
+
+package goexperiment
+
+const PageTrace = false
+const PageTraceInt = 0
diff --git a/src/internal/goexperiment/exp_pagetrace_on.go b/src/internal/goexperiment/exp_pagetrace_on.go
new file mode 100644
index 0000000..ea72b54
--- /dev/null
+++ b/src/internal/goexperiment/exp_pagetrace_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.pagetrace
+// +build goexperiment.pagetrace
+
+package goexperiment
+
+const PageTrace = true
+const PageTraceInt = 1
diff --git a/src/internal/goexperiment/exp_preemptibleloops_off.go b/src/internal/goexperiment/exp_preemptibleloops_off.go
new file mode 100644
index 0000000..7a26088
--- /dev/null
+++ b/src/internal/goexperiment/exp_preemptibleloops_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.preemptibleloops
+// +build !goexperiment.preemptibleloops
+
+package goexperiment
+
+const PreemptibleLoops = false
+const PreemptibleLoopsInt = 0
diff --git a/src/internal/goexperiment/exp_preemptibleloops_on.go b/src/internal/goexperiment/exp_preemptibleloops_on.go
new file mode 100644
index 0000000..a9ca28c
--- /dev/null
+++ b/src/internal/goexperiment/exp_preemptibleloops_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.preemptibleloops
+// +build goexperiment.preemptibleloops
+
+package goexperiment
+
+const PreemptibleLoops = true
+const PreemptibleLoopsInt = 1
diff --git a/src/internal/goexperiment/exp_regabiargs_off.go b/src/internal/goexperiment/exp_regabiargs_off.go
new file mode 100644
index 0000000..31a139b
--- /dev/null
+++ b/src/internal/goexperiment/exp_regabiargs_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.regabiargs
+// +build !goexperiment.regabiargs
+
+package goexperiment
+
+const RegabiArgs = false
+const RegabiArgsInt = 0
diff --git a/src/internal/goexperiment/exp_regabiargs_on.go b/src/internal/goexperiment/exp_regabiargs_on.go
new file mode 100644
index 0000000..9b26f3c
--- /dev/null
+++ b/src/internal/goexperiment/exp_regabiargs_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.regabiargs
+// +build goexperiment.regabiargs
+
+package goexperiment
+
+const RegabiArgs = true
+const RegabiArgsInt = 1
diff --git a/src/internal/goexperiment/exp_regabiwrappers_off.go b/src/internal/goexperiment/exp_regabiwrappers_off.go
new file mode 100644
index 0000000..bfa0fa3
--- /dev/null
+++ b/src/internal/goexperiment/exp_regabiwrappers_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.regabiwrappers
+// +build !goexperiment.regabiwrappers
+
+package goexperiment
+
+const RegabiWrappers = false
+const RegabiWrappersInt = 0
diff --git a/src/internal/goexperiment/exp_regabiwrappers_on.go b/src/internal/goexperiment/exp_regabiwrappers_on.go
new file mode 100644
index 0000000..11ffffb
--- /dev/null
+++ b/src/internal/goexperiment/exp_regabiwrappers_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.regabiwrappers
+// +build goexperiment.regabiwrappers
+
+package goexperiment
+
+const RegabiWrappers = true
+const RegabiWrappersInt = 1
diff --git a/src/internal/goexperiment/exp_staticlockranking_off.go b/src/internal/goexperiment/exp_staticlockranking_off.go
new file mode 100644
index 0000000..3d546c0
--- /dev/null
+++ b/src/internal/goexperiment/exp_staticlockranking_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.staticlockranking
+// +build !goexperiment.staticlockranking
+
+package goexperiment
+
+const StaticLockRanking = false
+const StaticLockRankingInt = 0
diff --git a/src/internal/goexperiment/exp_staticlockranking_on.go b/src/internal/goexperiment/exp_staticlockranking_on.go
new file mode 100644
index 0000000..78188fb
--- /dev/null
+++ b/src/internal/goexperiment/exp_staticlockranking_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.staticlockranking
+// +build goexperiment.staticlockranking
+
+package goexperiment
+
+const StaticLockRanking = true
+const StaticLockRankingInt = 1
diff --git a/src/internal/goexperiment/exp_unified_off.go b/src/internal/goexperiment/exp_unified_off.go
new file mode 100644
index 0000000..4c16fd8
--- /dev/null
+++ b/src/internal/goexperiment/exp_unified_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.unified
+// +build !goexperiment.unified
+
+package goexperiment
+
+const Unified = false
+const UnifiedInt = 0
diff --git a/src/internal/goexperiment/exp_unified_on.go b/src/internal/goexperiment/exp_unified_on.go
new file mode 100644
index 0000000..2b17ba3
--- /dev/null
+++ b/src/internal/goexperiment/exp_unified_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.unified
+// +build goexperiment.unified
+
+package goexperiment
+
+const Unified = true
+const UnifiedInt = 1
diff --git a/src/internal/goexperiment/flags.go b/src/internal/goexperiment/flags.go
new file mode 100644
index 0000000..02e7443
--- /dev/null
+++ b/src/internal/goexperiment/flags.go
@@ -0,0 +1,103 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package goexperiment implements support for toolchain experiments.
+//
+// Toolchain experiments are controlled by the GOEXPERIMENT
+// environment variable. GOEXPERIMENT is a comma-separated list of
+// experiment names. GOEXPERIMENT can be set at make.bash time, which
+// sets the default experiments for binaries built with the tool
+// chain; or it can be set at build time. GOEXPERIMENT can also be set
+// to "none", which disables any experiments that were enabled at
+// make.bash time.
+//
+// Experiments are exposed to the build in the following ways:
+//
+// - Build tag goexperiment.x is set if experiment x (lower case) is
+// enabled.
+//
+// - For each experiment x (in camel case), this package contains a
+// boolean constant x and an integer constant xInt.
+//
+// - In runtime assembly, the macro GOEXPERIMENT_x is defined if
+// experiment x (lower case) is enabled.
+//
+// In the toolchain, the set of experiments enabled for the current
+// build should be accessed via objabi.Experiment.
+//
+// The set of experiments is included in the output of runtime.Version()
+// and "go version <binary>" if it differs from the default experiments.
+//
+// For the set of experiments supported by the current toolchain, see
+// "go doc goexperiment.Flags".
+//
+// Note that this package defines the set of experiments (in Flags)
+// and records the experiments that were enabled when the package
+// was compiled (as boolean and integer constants).
+//
+// Note especially that this package does not itself change behavior
+// at run time based on the GOEXPERIMENT variable.
+// The code used in builds to interpret the GOEXPERIMENT variable
+// is in the separate package internal/buildcfg.
+package goexperiment
+
+//go:generate go run mkconsts.go
+
+// Flags is the set of experiments that can be enabled or disabled in
+// the current toolchain.
+//
+// When specified in the GOEXPERIMENT environment variable or as build
+// tags, experiments use the strings.ToLower of their field name.
+//
+// For the baseline experimental configuration, see
+// objabi.experimentBaseline.
+//
+// If you change this struct definition, run "go generate".
+type Flags struct {
+	FieldTrack        bool
+	PreemptibleLoops  bool
+	StaticLockRanking bool
+	BoringCrypto      bool
+
+	// Unified enables the compiler's unified IR construction
+	// experiment.
+	Unified bool
+
+	// Regabi is split into several sub-experiments that can be
+	// enabled individually. Not all combinations work.
+	// The "regabi" GOEXPERIMENT is an alias for all "working"
+	// subexperiments.
+
+	// RegabiWrappers enables ABI wrappers for calling between
+	// ABI0 and ABIInternal functions. Without this, the ABIs are
+	// assumed to be identical so cross-ABI calls are direct.
+	RegabiWrappers bool
+	// RegabiArgs enables register arguments/results in all
+	// compiled Go functions.
+	//
+	// Requires wrappers (to do ABI translation), and reflect (so
+	// reflection calls use registers).
+	RegabiArgs bool
+
+	// HeapMinimum512KiB reduces the minimum heap size to 512 KiB.
+	//
+	// This was originally reduced as part of PacerRedesign, but
+	// has been broken out to its own experiment that is disabled
+	// by default.
+	HeapMinimum512KiB bool
+
+	// CoverageRedesign enables the new compiler-based code coverage
+	// tooling.
+	CoverageRedesign bool
+
+	// Arenas causes the "arena" standard library package to be visible
+	// to the outside world.
+	Arenas bool
+
+	// PageTrace enables GODEBUG=pagetrace=/path/to/result. This feature
+	// is a GOEXPERIMENT due to a security risk with setuid binaries:
+	// this compels the Go runtime to write to some arbitrary file, which
+	// may be exploited.
+	PageTrace bool
+}
diff --git a/src/internal/goexperiment/mkconsts.go b/src/internal/goexperiment/mkconsts.go
new file mode 100644
index 0000000..204ca9d
--- /dev/null
+++ b/src/internal/goexperiment/mkconsts.go
@@ -0,0 +1,74 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+// +build ignore
+
+// mkconsts generates const definition files for each GOEXPERIMENT.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"internal/goexperiment"
+	"log"
+	"os"
+	"reflect"
+	"strings"
+)
+
+func main() {
+	// Delete existing experiment constant files.
+	ents, err := os.ReadDir(".")
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, ent := range ents {
+		name := ent.Name()
+		if !strings.HasPrefix(name, "exp_") {
+			continue
+		}
+		// Check that this is definitely a generated file.
+		data, err := os.ReadFile(name)
+		if err != nil {
+			log.Fatalf("reading %s: %v", name, err)
+		}
+		if !bytes.Contains(data, []byte("Code generated by mkconsts")) {
+			log.Fatalf("%s: expected generated file", name)
+		}
+		if err := os.Remove(name); err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	// Generate new experiment constant files.
+	rt := reflect.TypeOf(&goexperiment.Flags{}).Elem()
+	for i := 0; i < rt.NumField(); i++ {
+		f := rt.Field(i).Name
+		buildTag := "goexperiment." + strings.ToLower(f)
+		for _, val := range []bool{false, true} {
+			name := fmt.Sprintf("exp_%s_%s.go", strings.ToLower(f), pick(val, "off", "on"))
+			data := fmt.Sprintf(`// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build %s%s
+// +build %s%s
+
+package goexperiment
+
+const %s = %v
+const %sInt = %s
+`, pick(val, "!", ""), buildTag, pick(val, "!", ""), buildTag, f, val, f, pick(val, "0", "1"))
+			if err := os.WriteFile(name, []byte(data), 0666); err != nil {
+				log.Fatalf("writing %s: %v", name, err)
+			}
+		}
+	}
+}
+
+func pick(v bool, f, t string) string {
+	if v {
+		return t
+	}
+	return f
+}
diff --git a/src/internal/goos/gengoos.go b/src/internal/goos/gengoos.go
new file mode 100644
index 0000000..37d9706
--- /dev/null
+++ b/src/internal/goos/gengoos.go
@@ -0,0 +1,71 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+)
+
+var gooses []string
+
+func main() {
+	data, err := os.ReadFile("../../go/build/syslist.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	const goosPrefix = `var knownOS = map[string]bool{`
+	inGOOS := false
+	for _, line := range strings.Split(string(data), "\n") {
+		if strings.HasPrefix(line, goosPrefix) {
+			inGOOS = true
+		} else if inGOOS && strings.HasPrefix(line, "}") {
+			break
+		} else if inGOOS {
+			goos := strings.Fields(line)[0]
+			goos = strings.TrimPrefix(goos, `"`)
+			goos = strings.TrimSuffix(goos, `":`)
+			gooses = append(gooses, goos)
+		}
+	}
+
+	for _, target := range gooses {
+		if target == "nacl" {
+			continue
+		}
+		var tags []string
+		if target == "linux" {
+			tags = append(tags, "!android") // must explicitly exclude android for linux
+		}
+		if target == "solaris" {
+			tags = append(tags, "!illumos") // must explicitly exclude illumos for solaris
+		}
+		if target == "darwin" {
+			tags = append(tags, "!ios") // must explicitly exclude ios for darwin
+		}
+		tags = append(tags, target) // must explicitly include target for bootstrapping purposes
+		var buf bytes.Buffer
+		fmt.Fprintf(&buf, "// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.\n\n")
+		fmt.Fprintf(&buf, "//go:build %s\n\n", strings.Join(tags, " && "))
+		fmt.Fprintf(&buf, "package goos\n\n")
+		fmt.Fprintf(&buf, "const GOOS = `%s`\n\n", target)
+		for _, goos := range gooses {
+			value := 0
+			if goos == target {
+				value = 1
+			}
+			fmt.Fprintf(&buf, "const Is%s = %d\n", strings.Title(goos), value)
+		}
+		err := os.WriteFile("zgoos_"+target+".go", buf.Bytes(), 0666)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+}
diff --git a/src/internal/goos/goos.go b/src/internal/goos/goos.go
new file mode 100644
index 0000000..02dc968
--- /dev/null
+++ b/src/internal/goos/goos.go
@@ -0,0 +1,13 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// package goos contains GOOS-specific constants.
+package goos
+
+// The next line makes 'go generate' write the zgoos*.go files with
+// per-OS information, including constants named Is$GOOS for every
+// known GOOS. The constant is 1 on the current system, 0 otherwise;
+// multiplying by them is useful for defining GOOS-specific constants.
+//
+//go:generate go run gengoos.go
diff --git a/src/internal/goos/nonunix.go b/src/internal/goos/nonunix.go
new file mode 100644
index 0000000..2ba5c85
--- /dev/null
+++ b/src/internal/goos/nonunix.go
@@ -0,0 +1,9 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !unix
+
+package goos
+
+const IsUnix = false
diff --git a/src/internal/goos/unix.go b/src/internal/goos/unix.go
new file mode 100644
index 0000000..6cfd5ef
--- /dev/null
+++ b/src/internal/goos/unix.go
@@ -0,0 +1,9 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package goos
+
+const IsUnix = true
diff --git a/src/internal/goos/zgoos_aix.go b/src/internal/goos/zgoos_aix.go
new file mode 100644
index 0000000..ff86155
--- /dev/null
+++ b/src/internal/goos/zgoos_aix.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build aix
+
+package goos
+
+const GOOS = `aix`
+
+const IsAix = 1
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_android.go b/src/internal/goos/zgoos_android.go
new file mode 100644
index 0000000..e8aaa12
--- /dev/null
+++ b/src/internal/goos/zgoos_android.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build android
+
+package goos
+
+const GOOS = `android`
+
+const IsAix = 0
+const IsAndroid = 1
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_darwin.go b/src/internal/goos/zgoos_darwin.go
new file mode 100644
index 0000000..decdd49
--- /dev/null
+++ b/src/internal/goos/zgoos_darwin.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build !ios && darwin
+
+package goos
+
+const GOOS = `darwin`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 1
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_dragonfly.go b/src/internal/goos/zgoos_dragonfly.go
new file mode 100644
index 0000000..2224baa
--- /dev/null
+++ b/src/internal/goos/zgoos_dragonfly.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build dragonfly
+
+package goos
+
+const GOOS = `dragonfly`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 1
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_freebsd.go b/src/internal/goos/zgoos_freebsd.go
new file mode 100644
index 0000000..3ee5bf9
--- /dev/null
+++ b/src/internal/goos/zgoos_freebsd.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build freebsd
+
+package goos
+
+const GOOS = `freebsd`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 1
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_hurd.go b/src/internal/goos/zgoos_hurd.go
new file mode 100644
index 0000000..8a3d343
--- /dev/null
+++ b/src/internal/goos/zgoos_hurd.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build hurd
+
+package goos
+
+const GOOS = `hurd`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 1
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_illumos.go b/src/internal/goos/zgoos_illumos.go
new file mode 100644
index 0000000..fc1b9a9
--- /dev/null
+++ b/src/internal/goos/zgoos_illumos.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build illumos
+
+package goos
+
+const GOOS = `illumos`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 1
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_ios.go b/src/internal/goos/zgoos_ios.go
new file mode 100644
index 0000000..746e769
--- /dev/null
+++ b/src/internal/goos/zgoos_ios.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build ios
+
+package goos
+
+const GOOS = `ios`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 1
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_js.go b/src/internal/goos/zgoos_js.go
new file mode 100644
index 0000000..6cf2a5d
--- /dev/null
+++ b/src/internal/goos/zgoos_js.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build js
+
+package goos
+
+const GOOS = `js`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 1
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_linux.go b/src/internal/goos/zgoos_linux.go
new file mode 100644
index 0000000..cb9d6e8
--- /dev/null
+++ b/src/internal/goos/zgoos_linux.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build !android && linux
+
+package goos
+
+const GOOS = `linux`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 1
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_netbsd.go b/src/internal/goos/zgoos_netbsd.go
new file mode 100644
index 0000000..8285928
--- /dev/null
+++ b/src/internal/goos/zgoos_netbsd.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build netbsd
+
+package goos
+
+const GOOS = `netbsd`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 1
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_openbsd.go b/src/internal/goos/zgoos_openbsd.go
new file mode 100644
index 0000000..3f739a4
--- /dev/null
+++ b/src/internal/goos/zgoos_openbsd.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build openbsd
+
+package goos
+
+const GOOS = `openbsd`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 1
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_plan9.go b/src/internal/goos/zgoos_plan9.go
new file mode 100644
index 0000000..d4c1c65
--- /dev/null
+++ b/src/internal/goos/zgoos_plan9.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build plan9
+
+package goos
+
+const GOOS = `plan9`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 1
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_solaris.go b/src/internal/goos/zgoos_solaris.go
new file mode 100644
index 0000000..69e3285
--- /dev/null
+++ b/src/internal/goos/zgoos_solaris.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build !illumos && solaris
+
+package goos
+
+const GOOS = `solaris`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 1
+const IsWindows = 0
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_windows.go b/src/internal/goos/zgoos_windows.go
new file mode 100644
index 0000000..16158be
--- /dev/null
+++ b/src/internal/goos/zgoos_windows.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build windows
+
+package goos
+
+const GOOS = `windows`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 1
+const IsZos = 0
diff --git a/src/internal/goos/zgoos_zos.go b/src/internal/goos/zgoos_zos.go
new file mode 100644
index 0000000..fb6165c
--- /dev/null
+++ b/src/internal/goos/zgoos_zos.go
@@ -0,0 +1,25 @@
+// Code generated by gengoos.go using 'go generate'. DO NOT EDIT.
+
+//go:build zos
+
+package goos
+
+const GOOS = `zos`
+
+const IsAix = 0
+const IsAndroid = 0
+const IsDarwin = 0
+const IsDragonfly = 0
+const IsFreebsd = 0
+const IsHurd = 0
+const IsIllumos = 0
+const IsIos = 0
+const IsJs = 0
+const IsLinux = 0
+const IsNacl = 0
+const IsNetbsd = 0
+const IsOpenbsd = 0
+const IsPlan9 = 0
+const IsSolaris = 0
+const IsWindows = 0
+const IsZos = 1
diff --git a/src/internal/goroot/gc.go b/src/internal/goroot/gc.go
new file mode 100644
index 0000000..79403d2
--- /dev/null
+++ b/src/internal/goroot/gc.go
@@ -0,0 +1,131 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc
+
+package goroot
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+)
+
+// IsStandardPackage reports whether path is a standard package,
+// given goroot and compiler.
+func IsStandardPackage(goroot, compiler, path string) bool {
+	switch compiler {
+	case "gc":
+		dir := filepath.Join(goroot, "src", path)
+		_, err := os.Stat(dir)
+		return err == nil
+	case "gccgo":
+		return gccgoSearch.isStandard(path)
+	default:
+		panic("unknown compiler " + compiler)
+	}
+}
+
+// gccgoSearch holds the gccgo search directories.
+type gccgoDirs struct {
+	once sync.Once
+	dirs []string
+}
+
+// gccgoSearch is used to check whether a gccgo package exists in the
+// standard library.
+var gccgoSearch gccgoDirs
+
+// init finds the gccgo search directories. If this fails it leaves dirs == nil.
+func (gd *gccgoDirs) init() {
+	gccgo := os.Getenv("GCCGO")
+	if gccgo == "" {
+		gccgo = "gccgo"
+	}
+	bin, err := exec.LookPath(gccgo)
+	if err != nil {
+		return
+	}
+
+	allDirs, err := exec.Command(bin, "-print-search-dirs").Output()
+	if err != nil {
+		return
+	}
+	versionB, err := exec.Command(bin, "-dumpversion").Output()
+	if err != nil {
+		return
+	}
+	version := strings.TrimSpace(string(versionB))
+	machineB, err := exec.Command(bin, "-dumpmachine").Output()
+	if err != nil {
+		return
+	}
+	machine := strings.TrimSpace(string(machineB))
+
+	dirsEntries := strings.Split(string(allDirs), "\n")
+	const prefix = "libraries: ="
+	var dirs []string
+	for _, dirEntry := range dirsEntries {
+		if strings.HasPrefix(dirEntry, prefix) {
+			dirs = filepath.SplitList(strings.TrimPrefix(dirEntry, prefix))
+			break
+		}
+	}
+	if len(dirs) == 0 {
+		return
+	}
+
+	var lastDirs []string
+	for _, dir := range dirs {
+		goDir := filepath.Join(dir, "go", version)
+		if fi, err := os.Stat(goDir); err == nil && fi.IsDir() {
+			gd.dirs = append(gd.dirs, goDir)
+			goDir = filepath.Join(goDir, machine)
+			if fi, err = os.Stat(goDir); err == nil && fi.IsDir() {
+				gd.dirs = append(gd.dirs, goDir)
+			}
+		}
+		if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
+			lastDirs = append(lastDirs, dir)
+		}
+	}
+	gd.dirs = append(gd.dirs, lastDirs...)
+}
+
+// isStandard reports whether path is a standard library for gccgo.
+func (gd *gccgoDirs) isStandard(path string) bool {
+	// Quick check: if the first path component has a '.', it's not
+	// in the standard library. This skips most GOPATH directories.
+	i := strings.Index(path, "/")
+	if i < 0 {
+		i = len(path)
+	}
+	if strings.Contains(path[:i], ".") {
+		return false
+	}
+
+	if path == "unsafe" {
+		// Special case.
+		return true
+	}
+
+	gd.once.Do(gd.init)
+	if gd.dirs == nil {
+		// We couldn't find the gccgo search directories.
+		// Best guess, since the first component did not contain
+		// '.', is that this is a standard library package.
+		return true
+	}
+
+	for _, dir := range gd.dirs {
+		full := filepath.Join(dir, path) + ".gox"
+		if fi, err := os.Stat(full); err == nil && !fi.IsDir() {
+			return true
+		}
+	}
+
+	return false
+}
diff --git a/src/internal/goroot/gccgo.go b/src/internal/goroot/gccgo.go
new file mode 100644
index 0000000..6284122
--- /dev/null
+++ b/src/internal/goroot/gccgo.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gccgo
+
+package goroot
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// IsStandardPackage reports whether path is a standard package,
+// given goroot and compiler.
+func IsStandardPackage(goroot, compiler, path string) bool {
+	switch compiler {
+	case "gc":
+		dir := filepath.Join(goroot, "src", path)
+		_, err := os.Stat(dir)
+		return err == nil
+	case "gccgo":
+		return stdpkg[path]
+	default:
+		panic("unknown compiler " + compiler)
+	}
+}
diff --git a/src/internal/goroot/importcfg.go b/src/internal/goroot/importcfg.go
new file mode 100644
index 0000000..e324073
--- /dev/null
+++ b/src/internal/goroot/importcfg.go
@@ -0,0 +1,66 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goroot
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"strings"
+	"sync"
+)
+
+// Importcfg returns an importcfg file to be passed to the
+// Go compiler that contains the cached paths for the .a files for the
+// standard library.
+func Importcfg() (string, error) {
+	var icfg bytes.Buffer
+
+	m, err := PkgfileMap()
+	if err != nil {
+		return "", err
+	}
+	fmt.Fprintf(&icfg, "# import config")
+	for importPath, export := range m {
+		fmt.Fprintf(&icfg, "\npackagefile %s=%s", importPath, export)
+	}
+	s := icfg.String()
+	return s, nil
+}
+
+var (
+	stdlibPkgfileMap map[string]string
+	stdlibPkgfileErr error
+	once             sync.Once
+)
+
+// PkgfileMap returns a map of package paths to the location on disk
+// of the .a file for the package.
+// The caller must not modify the map.
+func PkgfileMap() (map[string]string, error) {
+	once.Do(func() {
+		m := make(map[string]string)
+		output, err := exec.Command("go", "list", "-export", "-e", "-f", "{{.ImportPath}} {{.Export}}", "std", "cmd").Output()
+		if err != nil {
+			stdlibPkgfileErr = err
+		}
+		for _, line := range strings.Split(string(output), "\n") {
+			if line == "" {
+				continue
+			}
+			sp := strings.SplitN(line, " ", 2)
+			if len(sp) != 2 {
+				stdlibPkgfileErr = fmt.Errorf("determining pkgfile map: invalid line in go list output: %q", line)
+				return
+			}
+			importPath, export := sp[0], sp[1]
+			if export != "" {
+				m[importPath] = export
+			}
+		}
+		stdlibPkgfileMap = m
+	})
+	return stdlibPkgfileMap, stdlibPkgfileErr
+}
diff --git a/src/internal/goversion/goversion.go b/src/internal/goversion/goversion.go
new file mode 100644
index 0000000..e9ecf8e
--- /dev/null
+++ b/src/internal/goversion/goversion.go
@@ -0,0 +1,12 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package goversion
+
+// Version is the Go 1.x version which is currently
+// in development and will eventually get released.
+//
+// It should be updated at the start of each development cycle to be
+// the version of the next Go 1.x release. See golang.org/issue/40705.
+const Version = 20
diff --git a/src/internal/intern/intern.go b/src/internal/intern/intern.go
new file mode 100644
index 0000000..0e6852f
--- /dev/null
+++ b/src/internal/intern/intern.go
@@ -0,0 +1,181 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package intern lets you make smaller comparable values by boxing
+// a larger comparable value (such as a 16 byte string header) down
+// into a globally unique 8 byte pointer.
+//
+// The globally unique pointers are garbage collected with weak
+// references and finalizers. This package hides that.
+package intern
+
+import (
+	"internal/godebug"
+	"runtime"
+	"sync"
+	"unsafe"
+)
+
+// A Value pointer is the handle to an underlying comparable value.
+// See func Get for how Value pointers may be used.
+type Value struct {
+	_      [0]func() // prevent people from accidentally using value type as comparable
+	cmpVal any
+	// resurrected is guarded by mu (for all instances of Value).
+	// It is set true whenever v is synthesized from a uintptr.
+	resurrected bool
+}
+
+// Get returns the comparable value passed to the Get func
+// that returned v.
+func (v *Value) Get() any { return v.cmpVal }
+
+// key is a key in our global value map.
+// It contains type-specialized fields to avoid allocations
+// when converting common types to empty interfaces.
+type key struct {
+	s      string
+	cmpVal any
+	// isString reports whether key contains a string.
+	// Without it, the zero value of key is ambiguous.
+	isString bool
+}
+
+// keyFor returns a key to use with cmpVal.
+func keyFor(cmpVal any) key {
+	if s, ok := cmpVal.(string); ok {
+		return key{s: s, isString: true}
+	}
+	return key{cmpVal: cmpVal}
+}
+
+// Value returns a *Value built from k.
+func (k key) Value() *Value {
+	if k.isString {
+		return &Value{cmpVal: k.s}
+	}
+	return &Value{cmpVal: k.cmpVal}
+}
+
+var (
+	// mu guards valMap, a weakref map of *Value by underlying value.
+	// It also guards the resurrected field of all *Values.
+	mu      sync.Mutex
+	valMap  = map[key]uintptr{} // to uintptr(*Value)
+	valSafe = safeMap()         // non-nil in safe+leaky mode
+)
+
+var intern = godebug.New("intern")
+
+// safeMap returns a non-nil map if we're in safe-but-leaky mode,
+// as controlled by GODEBUG=intern=leaky
+func safeMap() map[key]*Value {
+	if intern.Value() == "leaky" {
+		return map[key]*Value{}
+	}
+	return nil
+}
+
+// Get returns a pointer representing the comparable value cmpVal.
+//
+// The returned pointer will be the same for Get(v) and Get(v2)
+// if and only if v == v2, and can be used as a map key.
+func Get(cmpVal any) *Value {
+	return get(keyFor(cmpVal))
+}
+
+// GetByString is identical to Get, except that it is specialized for strings.
+// This avoids an allocation from putting a string into an interface{}
+// to pass as an argument to Get.
+func GetByString(s string) *Value {
+	return get(key{s: s, isString: true})
+}
+
+// We play unsafe games that violate Go's rules (and assume a non-moving
+// collector). So we quiet Go here.
+// See the comment below Get for more implementation details.
+//
+//go:nocheckptr
+func get(k key) *Value {
+	mu.Lock()
+	defer mu.Unlock()
+
+	var v *Value
+	if valSafe != nil {
+		v = valSafe[k]
+	} else if addr, ok := valMap[k]; ok {
+		v = (*Value)(unsafe.Pointer(addr))
+		v.resurrected = true
+	}
+	if v != nil {
+		return v
+	}
+	v = k.Value()
+	if valSafe != nil {
+		valSafe[k] = v
+	} else {
+		// SetFinalizer before uintptr conversion (theoretical concern;
+		// see https://github.com/go4org/intern/issues/13)
+		runtime.SetFinalizer(v, finalize)
+		valMap[k] = uintptr(unsafe.Pointer(v))
+	}
+	return v
+}
+
+func finalize(v *Value) {
+	mu.Lock()
+	defer mu.Unlock()
+	if v.resurrected {
+		// We lost the race. Somebody resurrected it while we
+		// were about to finalize it. Try again next round.
+		v.resurrected = false
+		runtime.SetFinalizer(v, finalize)
+		return
+	}
+	delete(valMap, keyFor(v.cmpVal))
+}
+
+// Interning is simple if you don't require that unused values be
+// garbage collectable. But we do require that; we don't want to be
+// DOS vector. We do this by using a uintptr to hide the pointer from
+// the garbage collector, and using a finalizer to eliminate the
+// pointer when no other code is using it.
+//
+// The obvious implementation of this is to use a
+// map[interface{}]uintptr-of-*interface{}, and set up a finalizer to
+// delete from the map. Unfortunately, this is racy. Because pointers
+// are being created in violation of Go's unsafety rules, it's
+// possible to create a pointer to a value concurrently with the GC
+// concluding that the value can be collected. There are other races
+// that break the equality invariant as well, but the use-after-free
+// will cause a runtime crash.
+//
+// To make this work, the finalizer needs to know that no references
+// have been unsafely created since the finalizer was set up. To do
+// this, values carry a "resurrected" sentinel, which gets set
+// whenever a pointer is unsafely created. If the finalizer encounters
+// the sentinel, it clears the sentinel and delays collection for one
+// additional GC cycle, by re-installing itself as finalizer. This
+// ensures that the unsafely created pointer is visible to the GC, and
+// will correctly prevent collection.
+//
+// This technique does mean that interned values that get reused take
+// at least 3 GC cycles to fully collect (1 to clear the sentinel, 1
+// to clean up the unsafe map, 1 to be actually deleted).
+//
+// @ianlancetaylor commented in
+// https://github.com/golang/go/issues/41303#issuecomment-717401656
+// that it is possible to implement weak references in terms of
+// finalizers without unsafe. Unfortunately, the approach he outlined
+// does not work here, for two reasons. First, there is no way to
+// construct a strong pointer out of a weak pointer; our map stores
+// weak pointers, but we must return strong pointers to callers.
+// Second, and more fundamentally, we must return not just _a_ strong
+// pointer to callers, but _the same_ strong pointer to callers. In
+// order to return _the same_ strong pointer to callers, we must track
+// it, which is exactly what we cannot do with strong pointers.
+//
+// See https://github.com/inetaf/netaddr/issues/53 for more
+// discussion, and https://github.com/go4org/intern/issues/2 for an
+// illustration of the subtleties at play.
diff --git a/src/internal/intern/intern_test.go b/src/internal/intern/intern_test.go
new file mode 100644
index 0000000..d1e409e
--- /dev/null
+++ b/src/internal/intern/intern_test.go
@@ -0,0 +1,199 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package intern
+
+import (
+	"fmt"
+	"runtime"
+	"testing"
+)
+
+func TestBasics(t *testing.T) {
+	clearMap()
+	foo := Get("foo")
+	bar := Get("bar")
+	empty := Get("")
+	nilEface := Get(nil)
+	i := Get(0x7777777)
+	foo2 := Get("foo")
+	bar2 := Get("bar")
+	empty2 := Get("")
+	nilEface2 := Get(nil)
+	i2 := Get(0x7777777)
+	foo3 := GetByString("foo")
+	empty3 := GetByString("")
+
+	if foo.Get() != foo2.Get() {
+		t.Error("foo/foo2 values differ")
+	}
+	if foo.Get() != foo3.Get() {
+		t.Error("foo/foo3 values differ")
+	}
+	if foo.Get() != "foo" {
+		t.Error("foo.Get not foo")
+	}
+	if foo != foo2 {
+		t.Error("foo/foo2 pointers differ")
+	}
+	if foo != foo3 {
+		t.Error("foo/foo3 pointers differ")
+	}
+
+	if bar.Get() != bar2.Get() {
+		t.Error("bar values differ")
+	}
+	if bar.Get() != "bar" {
+		t.Error("bar.Get not bar")
+	}
+	if bar != bar2 {
+		t.Error("bar pointers differ")
+	}
+
+	if i.Get() != i.Get() {
+		t.Error("i values differ")
+	}
+	if i.Get() != 0x7777777 {
+		t.Error("i.Get not 0x7777777")
+	}
+	if i != i2 {
+		t.Error("i pointers differ")
+	}
+
+	if empty.Get() != empty2.Get() {
+		t.Error("empty/empty2 values differ")
+	}
+	if empty.Get() != empty.Get() {
+		t.Error("empty/empty3 values differ")
+	}
+	if empty.Get() != "" {
+		t.Error("empty.Get not empty string")
+	}
+	if empty != empty2 {
+		t.Error("empty/empty2 pointers differ")
+	}
+	if empty != empty3 {
+		t.Error("empty/empty3 pointers differ")
+	}
+
+	if nilEface.Get() != nilEface2.Get() {
+		t.Error("nilEface values differ")
+	}
+	if nilEface.Get() != nil {
+		t.Error("nilEface.Get not nil")
+	}
+	if nilEface != nilEface2 {
+		t.Error("nilEface pointers differ")
+	}
+
+	if n := mapLen(); n != 5 {
+		t.Errorf("map len = %d; want 4", n)
+	}
+
+	wantEmpty(t)
+}
+
+func wantEmpty(t testing.TB) {
+	t.Helper()
+	const gcTries = 5000
+	for try := 0; try < gcTries; try++ {
+		runtime.GC()
+		n := mapLen()
+		if n == 0 {
+			break
+		}
+		if try == gcTries-1 {
+			t.Errorf("map len = %d after (%d GC tries); want 0, contents: %v", n, gcTries, mapKeys())
+		}
+	}
+}
+
+func TestStress(t *testing.T) {
+	iters := 10000
+	if testing.Short() {
+		iters = 1000
+	}
+	var sink []byte
+	for i := 0; i < iters; i++ {
+		_ = Get("foo")
+		sink = make([]byte, 1<<20)
+	}
+	_ = sink
+}
+
+func BenchmarkStress(b *testing.B) {
+	done := make(chan struct{})
+	defer close(done)
+	go func() {
+		for {
+			select {
+			case <-done:
+				return
+			default:
+			}
+			runtime.GC()
+		}
+	}()
+
+	clearMap()
+	v1 := Get("foo")
+	b.ReportAllocs()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			v2 := Get("foo")
+			if v1 != v2 {
+				b.Fatal("wrong value")
+			}
+			// And also a key we don't retain:
+			_ = Get("bar")
+		}
+	})
+	runtime.GC()
+	wantEmpty(b)
+}
+
+func mapLen() int {
+	mu.Lock()
+	defer mu.Unlock()
+	return len(valMap)
+}
+
+func mapKeys() (keys []string) {
+	mu.Lock()
+	defer mu.Unlock()
+	for k := range valMap {
+		keys = append(keys, fmt.Sprint(k))
+	}
+	return keys
+}
+
+func clearMap() {
+	mu.Lock()
+	defer mu.Unlock()
+	for k := range valMap {
+		delete(valMap, k)
+	}
+}
+
+var (
+	globalString = "not a constant"
+	sink         string
+)
+
+func TestGetByStringAllocs(t *testing.T) {
+	allocs := int(testing.AllocsPerRun(100, func() {
+		GetByString(globalString)
+	}))
+	if allocs != 0 {
+		t.Errorf("GetString allocated %d objects, want 0", allocs)
+	}
+}
+
+func BenchmarkGetByString(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		v := GetByString(globalString)
+		sink = v.Get().(string)
+	}
+}
diff --git a/src/internal/itoa/itoa.go b/src/internal/itoa/itoa.go
new file mode 100644
index 0000000..c6062d9
--- /dev/null
+++ b/src/internal/itoa/itoa.go
@@ -0,0 +1,33 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Simple conversions to avoid depending on strconv.
+
+package itoa
+
+// Itoa converts val to a decimal string.
+func Itoa(val int) string {
+	if val < 0 {
+		return "-" + Uitoa(uint(-val))
+	}
+	return Uitoa(uint(val))
+}
+
+// Uitoa converts val to a decimal string.
+func Uitoa(val uint) string {
+	if val == 0 { // avoid string allocation
+		return "0"
+	}
+	var buf [20]byte // big enough for 64bit value base 10
+	i := len(buf) - 1
+	for val >= 10 {
+		q := val / 10
+		buf[i] = byte('0' + val - q*10)
+		i--
+		val = q
+	}
+	// val < 10
+	buf[i] = byte('0' + val)
+	return string(buf[i:])
+}
diff --git a/src/internal/itoa/itoa_test.go b/src/internal/itoa/itoa_test.go
new file mode 100644
index 0000000..71931c1
--- /dev/null
+++ b/src/internal/itoa/itoa_test.go
@@ -0,0 +1,40 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package itoa_test
+
+import (
+	"fmt"
+	"internal/itoa"
+	"math"
+	"testing"
+)
+
+var (
+	minInt64  int64  = math.MinInt64
+	maxInt64  int64  = math.MaxInt64
+	maxUint64 uint64 = math.MaxUint64
+)
+
+func TestItoa(t *testing.T) {
+	tests := []int{int(minInt64), math.MinInt32, -999, -100, -1, 0, 1, 100, 999, math.MaxInt32, int(maxInt64)}
+	for _, tt := range tests {
+		got := itoa.Itoa(tt)
+		want := fmt.Sprint(tt)
+		if want != got {
+			t.Fatalf("Itoa(%d) = %s, want %s", tt, got, want)
+		}
+	}
+}
+
+func TestUitoa(t *testing.T) {
+	tests := []uint{0, 1, 100, 999, math.MaxUint32, uint(maxUint64)}
+	for _, tt := range tests {
+		got := itoa.Uitoa(tt)
+		want := fmt.Sprint(tt)
+		if want != got {
+			t.Fatalf("Uitoa(%d) = %s, want %s", tt, got, want)
+		}
+	}
+}
diff --git a/src/internal/lazyregexp/lazyre.go b/src/internal/lazyregexp/lazyre.go
new file mode 100644
index 0000000..2681af3
--- /dev/null
+++ b/src/internal/lazyregexp/lazyre.go
@@ -0,0 +1,78 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package lazyregexp is a thin wrapper over regexp, allowing the use of global
+// regexp variables without forcing them to be compiled at init.
+package lazyregexp
+
+import (
+	"os"
+	"regexp"
+	"strings"
+	"sync"
+)
+
+// Regexp is a wrapper around regexp.Regexp, where the underlying regexp will be
+// compiled the first time it is needed.
+type Regexp struct {
+	str  string
+	once sync.Once
+	rx   *regexp.Regexp
+}
+
+func (r *Regexp) re() *regexp.Regexp {
+	r.once.Do(r.build)
+	return r.rx
+}
+
+func (r *Regexp) build() {
+	r.rx = regexp.MustCompile(r.str)
+	r.str = ""
+}
+
+func (r *Regexp) FindSubmatch(s []byte) [][]byte {
+	return r.re().FindSubmatch(s)
+}
+
+func (r *Regexp) FindStringSubmatch(s string) []string {
+	return r.re().FindStringSubmatch(s)
+}
+
+func (r *Regexp) FindStringSubmatchIndex(s string) []int {
+	return r.re().FindStringSubmatchIndex(s)
+}
+
+func (r *Regexp) ReplaceAllString(src, repl string) string {
+	return r.re().ReplaceAllString(src, repl)
+}
+
+func (r *Regexp) FindString(s string) string {
+	return r.re().FindString(s)
+}
+
+func (r *Regexp) FindAllString(s string, n int) []string {
+	return r.re().FindAllString(s, n)
+}
+
+func (r *Regexp) MatchString(s string) bool {
+	return r.re().MatchString(s)
+}
+
+func (r *Regexp) SubexpNames() []string {
+	return r.re().SubexpNames()
+}
+
+var inTest = len(os.Args) > 0 && strings.HasSuffix(strings.TrimSuffix(os.Args[0], ".exe"), ".test")
+
+// New creates a new lazy regexp, delaying the compiling work until it is first
+// needed. If the code is being run as part of tests, the regexp compiling will
+// happen immediately.
+func New(str string) *Regexp {
+	lr := &Regexp{str: str}
+	if inTest {
+		// In tests, always compile the regexps early.
+		lr.re()
+	}
+	return lr
+}
diff --git a/src/internal/lazytemplate/lazytemplate.go b/src/internal/lazytemplate/lazytemplate.go
new file mode 100644
index 0000000..8eeed5a
--- /dev/null
+++ b/src/internal/lazytemplate/lazytemplate.go
@@ -0,0 +1,52 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package lazytemplate is a thin wrapper over text/template, allowing the use
+// of global template variables without forcing them to be parsed at init.
+package lazytemplate
+
+import (
+	"io"
+	"os"
+	"strings"
+	"sync"
+	"text/template"
+)
+
+// Template is a wrapper around text/template.Template, where the underlying
+// template will be parsed the first time it is needed.
+type Template struct {
+	name, text string
+
+	once sync.Once
+	tmpl *template.Template
+}
+
+func (r *Template) tp() *template.Template {
+	r.once.Do(r.build)
+	return r.tmpl
+}
+
+func (r *Template) build() {
+	r.tmpl = template.Must(template.New(r.name).Parse(r.text))
+	r.name, r.text = "", ""
+}
+
+func (r *Template) Execute(w io.Writer, data any) error {
+	return r.tp().Execute(w, data)
+}
+
+var inTest = len(os.Args) > 0 && strings.HasSuffix(strings.TrimSuffix(os.Args[0], ".exe"), ".test")
+
+// New creates a new lazy template, delaying the parsing work until it is first
+// needed. If the code is being run as part of tests, the template parsing will
+// happen immediately.
+func New(name, text string) *Template {
+	lt := &Template{name: name, text: text}
+	if inTest {
+		// In tests, always parse the templates early.
+		lt.tp()
+	}
+	return lt
+}
diff --git a/src/internal/nettrace/nettrace.go b/src/internal/nettrace/nettrace.go
new file mode 100644
index 0000000..0a2bf92
--- /dev/null
+++ b/src/internal/nettrace/nettrace.go
@@ -0,0 +1,46 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package nettrace contains internal hooks for tracing activity in
+// the net package. This package is purely internal for use by the
+// net/http/httptrace package and has no stable API exposed to end
+// users.
+package nettrace
+
+// TraceKey is a context.Context Value key. Its associated value should
+// be a *Trace struct.
+type TraceKey struct{}
+
+// LookupIPAltResolverKey is a context.Context Value key used by tests to
+// specify an alternate resolver func.
+// It is not exposed to outsider users. (But see issue 12503)
+// The value should be the same type as lookupIP:
+//
+//	func lookupIP(ctx context.Context, host string) ([]IPAddr, error)
+type LookupIPAltResolverKey struct{}
+
+// Trace contains a set of hooks for tracing events within
+// the net package. Any specific hook may be nil.
+type Trace struct {
+	// DNSStart is called with the hostname of a DNS lookup
+	// before it begins.
+	DNSStart func(name string)
+
+	// DNSDone is called after a DNS lookup completes (or fails).
+	// The coalesced parameter is whether singleflight de-duped
+	// the call. The addrs are of type net.IPAddr but can't
+	// actually be for circular dependency reasons.
+	DNSDone func(netIPs []any, coalesced bool, err error)
+
+	// ConnectStart is called before a Dial, excluding Dials made
+	// during DNS lookups. In the case of DualStack (Happy Eyeballs)
+	// dialing, this may be called multiple times, from multiple
+	// goroutines.
+	ConnectStart func(network, addr string)
+
+	// ConnectStart is called after a Dial with the results, excluding
+	// Dials made during DNS lookups. It may also be called multiple
+	// times, like ConnectStart.
+	ConnectDone func(network, addr string, err error)
+}
diff --git a/src/internal/obscuretestdata/obscuretestdata.go b/src/internal/obscuretestdata/obscuretestdata.go
new file mode 100644
index 0000000..d54d3f6
--- /dev/null
+++ b/src/internal/obscuretestdata/obscuretestdata.go
@@ -0,0 +1,65 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package obscuretestdata contains functionality used by tests to more easily
+// work with testdata that must be obscured primarily due to
+// golang.org/issue/34986.
+package obscuretestdata
+
+import (
+	"encoding/base64"
+	"io"
+	"os"
+)
+
+// Rot13 returns the rot13 encoding or decoding of its input.
+func Rot13(data []byte) []byte {
+	out := make([]byte, len(data))
+	copy(out, data)
+	for i, c := range out {
+		switch {
+		case 'A' <= c && c <= 'M' || 'a' <= c && c <= 'm':
+			out[i] = c + 13
+		case 'N' <= c && c <= 'Z' || 'n' <= c && c <= 'z':
+			out[i] = c - 13
+		}
+	}
+	return out
+}
+
+// DecodeToTempFile decodes the named file to a temporary location.
+// If successful, it returns the path of the decoded file.
+// The caller is responsible for ensuring that the temporary file is removed.
+func DecodeToTempFile(name string) (path string, err error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	tmp, err := os.CreateTemp("", "obscuretestdata-decoded-")
+	if err != nil {
+		return "", err
+	}
+	if _, err := io.Copy(tmp, base64.NewDecoder(base64.StdEncoding, f)); err != nil {
+		tmp.Close()
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	return tmp.Name(), nil
+}
+
+// ReadFile reads the named file and returns its decoded contents.
+func ReadFile(name string) ([]byte, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return io.ReadAll(base64.NewDecoder(base64.StdEncoding, f))
+}
diff --git a/src/internal/oserror/errors.go b/src/internal/oserror/errors.go
new file mode 100644
index 0000000..28a1ab3
--- /dev/null
+++ b/src/internal/oserror/errors.go
@@ -0,0 +1,18 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package oserror defines errors values used in the os package.
+//
+// These types are defined here to permit the syscall package to reference them.
+package oserror
+
+import "errors"
+
+var (
+	ErrInvalid    = errors.New("invalid argument")
+	ErrPermission = errors.New("permission denied")
+	ErrExist      = errors.New("file already exists")
+	ErrNotExist   = errors.New("file does not exist")
+	ErrClosed     = errors.New("file already closed")
+)
diff --git a/src/internal/pkgbits/codes.go b/src/internal/pkgbits/codes.go
new file mode 100644
index 0000000..f0cabde
--- /dev/null
+++ b/src/internal/pkgbits/codes.go
@@ -0,0 +1,77 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+// A Code is an enum value that can be encoded into bitstreams.
+//
+// Code types are preferable for enum types, because they allow
+// Decoder to detect desyncs.
+type Code interface {
+	// Marker returns the SyncMarker for the Code's dynamic type.
+	Marker() SyncMarker
+
+	// Value returns the Code's ordinal value.
+	Value() int
+}
+
+// A CodeVal distinguishes among go/constant.Value encodings.
+type CodeVal int
+
+func (c CodeVal) Marker() SyncMarker { return SyncVal }
+func (c CodeVal) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	ValBool CodeVal = iota
+	ValString
+	ValInt64
+	ValBigInt
+	ValBigRat
+	ValBigFloat
+)
+
+// A CodeType distinguishes among go/types.Type encodings.
+type CodeType int
+
+func (c CodeType) Marker() SyncMarker { return SyncType }
+func (c CodeType) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	TypeBasic CodeType = iota
+	TypeNamed
+	TypePointer
+	TypeSlice
+	TypeArray
+	TypeChan
+	TypeMap
+	TypeSignature
+	TypeStruct
+	TypeInterface
+	TypeUnion
+	TypeTypeParam
+)
+
+// A CodeObj distinguishes among go/types.Object encodings.
+type CodeObj int
+
+func (c CodeObj) Marker() SyncMarker { return SyncCodeObj }
+func (c CodeObj) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	ObjAlias CodeObj = iota
+	ObjConst
+	ObjType
+	ObjFunc
+	ObjVar
+	ObjStub
+)
diff --git a/src/internal/pkgbits/decoder.go b/src/internal/pkgbits/decoder.go
new file mode 100644
index 0000000..4fe024d
--- /dev/null
+++ b/src/internal/pkgbits/decoder.go
@@ -0,0 +1,515 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"io"
+	"math/big"
+	"os"
+	"runtime"
+	"strings"
+)
+
+// A PkgDecoder provides methods for decoding a package's Unified IR
+// export data.
+type PkgDecoder struct {
+	// version is the file format version.
+	version uint32
+
+	// sync indicates whether the file uses sync markers.
+	sync bool
+
+	// pkgPath is the package path for the package to be decoded.
+	//
+	// TODO(mdempsky): Remove; unneeded since CL 391014.
+	pkgPath string
+
+	// elemData is the full data payload of the encoded package.
+	// Elements are densely and contiguously packed together.
+	//
+	// The last 8 bytes of elemData are the package fingerprint.
+	elemData string
+
+	// elemEnds stores the byte-offset end positions of element
+	// bitstreams within elemData.
+	//
+	// For example, element I's bitstream data starts at elemEnds[I-1]
+	// (or 0, if I==0) and ends at elemEnds[I].
+	//
+	// Note: elemEnds is indexed by absolute indices, not
+	// section-relative indices.
+	elemEnds []uint32
+
+	// elemEndsEnds stores the index-offset end positions of relocation
+	// sections within elemEnds.
+	//
+	// For example, section K's end positions start at elemEndsEnds[K-1]
+	// (or 0, if K==0) and end at elemEndsEnds[K].
+	elemEndsEnds [numRelocs]uint32
+
+	scratchRelocEnt []RelocEnt
+}
+
+// PkgPath returns the package path for the package
+//
+// TODO(mdempsky): Remove; unneeded since CL 391014.
+func (pr *PkgDecoder) PkgPath() string { return pr.pkgPath }
+
+// SyncMarkers reports whether pr uses sync markers.
+func (pr *PkgDecoder) SyncMarkers() bool { return pr.sync }
+
+// NewPkgDecoder returns a PkgDecoder initialized to read the Unified
+// IR export data from input. pkgPath is the package path for the
+// compilation unit that produced the export data.
+//
+// TODO(mdempsky): Remove pkgPath parameter; unneeded since CL 391014.
+func NewPkgDecoder(pkgPath, input string) PkgDecoder {
+	pr := PkgDecoder{
+		pkgPath: pkgPath,
+	}
+
+	// TODO(mdempsky): Implement direct indexing of input string to
+	// avoid copying the position information.
+
+	r := strings.NewReader(input)
+
+	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
+
+	switch pr.version {
+	default:
+		panic(fmt.Errorf("unsupported version: %v", pr.version))
+	case 0:
+		// no flags
+	case 1:
+		var flags uint32
+		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
+		pr.sync = flags&flagSyncMarkers != 0
+	}
+
+	assert(binary.Read(r, binary.LittleEndian, pr.elemEndsEnds[:]) == nil)
+
+	pr.elemEnds = make([]uint32, pr.elemEndsEnds[len(pr.elemEndsEnds)-1])
+	assert(binary.Read(r, binary.LittleEndian, pr.elemEnds[:]) == nil)
+
+	pos, err := r.Seek(0, io.SeekCurrent)
+	assert(err == nil)
+
+	pr.elemData = input[pos:]
+	assert(len(pr.elemData)-8 == int(pr.elemEnds[len(pr.elemEnds)-1]))
+
+	return pr
+}
+
+// NumElems returns the number of elements in section k.
+func (pr *PkgDecoder) NumElems(k RelocKind) int {
+	count := int(pr.elemEndsEnds[k])
+	if k > 0 {
+		count -= int(pr.elemEndsEnds[k-1])
+	}
+	return count
+}
+
+// TotalElems returns the total number of elements across all sections.
+func (pr *PkgDecoder) TotalElems() int {
+	return len(pr.elemEnds)
+}
+
+// Fingerprint returns the package fingerprint.
+func (pr *PkgDecoder) Fingerprint() [8]byte {
+	var fp [8]byte
+	copy(fp[:], pr.elemData[len(pr.elemData)-8:])
+	return fp
+}
+
+// AbsIdx returns the absolute index for the given (section, index)
+// pair.
+func (pr *PkgDecoder) AbsIdx(k RelocKind, idx Index) int {
+	absIdx := int(idx)
+	if k > 0 {
+		absIdx += int(pr.elemEndsEnds[k-1])
+	}
+	if absIdx >= int(pr.elemEndsEnds[k]) {
+		errorf("%v:%v is out of bounds; %v", k, idx, pr.elemEndsEnds)
+	}
+	return absIdx
+}
+
+// DataIdx returns the raw element bitstream for the given (section,
+// index) pair.
+func (pr *PkgDecoder) DataIdx(k RelocKind, idx Index) string {
+	absIdx := pr.AbsIdx(k, idx)
+
+	var start uint32
+	if absIdx > 0 {
+		start = pr.elemEnds[absIdx-1]
+	}
+	end := pr.elemEnds[absIdx]
+
+	return pr.elemData[start:end]
+}
+
+// StringIdx returns the string value for the given string index.
+func (pr *PkgDecoder) StringIdx(idx Index) string {
+	return pr.DataIdx(RelocString, idx)
+}
+
+// NewDecoder returns a Decoder for the given (section, index) pair,
+// and decodes the given SyncMarker from the element bitstream.
+func (pr *PkgDecoder) NewDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
+	r := pr.NewDecoderRaw(k, idx)
+	r.Sync(marker)
+	return r
+}
+
+// TempDecoder returns a Decoder for the given (section, index) pair,
+// and decodes the given SyncMarker from the element bitstream.
+// If possible the Decoder should be RetireDecoder'd when it is no longer
+// needed, this will avoid heap allocations.
+func (pr *PkgDecoder) TempDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
+	r := pr.TempDecoderRaw(k, idx)
+	r.Sync(marker)
+	return r
+}
+
+func (pr *PkgDecoder) RetireDecoder(d *Decoder) {
+	pr.scratchRelocEnt = d.Relocs
+	d.Relocs = nil
+}
+
+// NewDecoderRaw returns a Decoder for the given (section, index) pair.
+//
+// Most callers should use NewDecoder instead.
+func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx Index) Decoder {
+	r := Decoder{
+		common: pr,
+		k:      k,
+		Idx:    idx,
+	}
+
+	r.Data.Reset(pr.DataIdx(k, idx))
+	r.Sync(SyncRelocs)
+	r.Relocs = make([]RelocEnt, r.Len())
+	for i := range r.Relocs {
+		r.Sync(SyncReloc)
+		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
+	}
+
+	return r
+}
+
+func (pr *PkgDecoder) TempDecoderRaw(k RelocKind, idx Index) Decoder {
+	r := Decoder{
+		common: pr,
+		k:      k,
+		Idx:    idx,
+	}
+
+	r.Data.Reset(pr.DataIdx(k, idx))
+	r.Sync(SyncRelocs)
+	l := r.Len()
+	if cap(pr.scratchRelocEnt) >= l {
+		r.Relocs = pr.scratchRelocEnt[:l]
+		pr.scratchRelocEnt = nil
+	} else {
+		r.Relocs = make([]RelocEnt, l)
+	}
+	for i := range r.Relocs {
+		r.Sync(SyncReloc)
+		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
+	}
+
+	return r
+}
+
+// A Decoder provides methods for decoding an individual element's
+// bitstream data.
+type Decoder struct {
+	common *PkgDecoder
+
+	Relocs []RelocEnt
+	Data   strings.Reader
+
+	k   RelocKind
+	Idx Index
+}
+
+func (r *Decoder) checkErr(err error) {
+	if err != nil {
+		errorf("unexpected decoding error: %w", err)
+	}
+}
+
+func (r *Decoder) rawUvarint() uint64 {
+	x, err := readUvarint(&r.Data)
+	r.checkErr(err)
+	return x
+}
+
+// readUvarint is a type-specialized copy of encoding/binary.ReadUvarint.
+// This avoids the interface conversion and thus has better escape properties,
+// which flows up the stack.
+func readUvarint(r *strings.Reader) (uint64, error) {
+	var x uint64
+	var s uint
+	for i := 0; i < binary.MaxVarintLen64; i++ {
+		b, err := r.ReadByte()
+		if err != nil {
+			if i > 0 && err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return x, err
+		}
+		if b < 0x80 {
+			if i == binary.MaxVarintLen64-1 && b > 1 {
+				return x, overflow
+			}
+			return x | uint64(b)<<s, nil
+		}
+		x |= uint64(b&0x7f) << s
+		s += 7
+	}
+	return x, overflow
+}
+
+var overflow = errors.New("pkgbits: readUvarint overflows a 64-bit integer")
+
+func (r *Decoder) rawVarint() int64 {
+	ux := r.rawUvarint()
+
+	// Zig-zag decode.
+	x := int64(ux >> 1)
+	if ux&1 != 0 {
+		x = ^x
+	}
+	return x
+}
+
+func (r *Decoder) rawReloc(k RelocKind, idx int) Index {
+	e := r.Relocs[idx]
+	assert(e.Kind == k)
+	return e.Idx
+}
+
+// Sync decodes a sync marker from the element bitstream and asserts
+// that it matches the expected marker.
+//
+// If EnableSync is false, then Sync is a no-op.
+func (r *Decoder) Sync(mWant SyncMarker) {
+	if !r.common.sync {
+		return
+	}
+
+	pos, _ := r.Data.Seek(0, io.SeekCurrent)
+	mHave := SyncMarker(r.rawUvarint())
+	writerPCs := make([]int, r.rawUvarint())
+	for i := range writerPCs {
+		writerPCs[i] = int(r.rawUvarint())
+	}
+
+	if mHave == mWant {
+		return
+	}
+
+	// There's some tension here between printing:
+	//
+	// (1) full file paths that tools can recognize (e.g., so emacs
+	//     hyperlinks the "file:line" text for easy navigation), or
+	//
+	// (2) short file paths that are easier for humans to read (e.g., by
+	//     omitting redundant or irrelevant details, so it's easier to
+	//     focus on the useful bits that remain).
+	//
+	// The current formatting favors the former, as it seems more
+	// helpful in practice. But perhaps the formatting could be improved
+	// to better address both concerns. For example, use relative file
+	// paths if they would be shorter, or rewrite file paths to contain
+	// "$GOROOT" (like objabi.AbsFile does) if tools can be taught how
+	// to reliably expand that again.
+
+	fmt.Printf("export data desync: package %q, section %v, index %v, offset %v\n", r.common.pkgPath, r.k, r.Idx, pos)
+
+	fmt.Printf("\nfound %v, written at:\n", mHave)
+	if len(writerPCs) == 0 {
+		fmt.Printf("\t[stack trace unavailable; recompile package %q with -d=syncframes]\n", r.common.pkgPath)
+	}
+	for _, pc := range writerPCs {
+		fmt.Printf("\t%s\n", r.common.StringIdx(r.rawReloc(RelocString, pc)))
+	}
+
+	fmt.Printf("\nexpected %v, reading at:\n", mWant)
+	var readerPCs [32]uintptr // TODO(mdempsky): Dynamically size?
+	n := runtime.Callers(2, readerPCs[:])
+	for _, pc := range fmtFrames(readerPCs[:n]...) {
+		fmt.Printf("\t%s\n", pc)
+	}
+
+	// We already printed a stack trace for the reader, so now we can
+	// simply exit. Printing a second one with panic or base.Fatalf
+	// would just be noise.
+	os.Exit(1)
+}
+
+// Bool decodes and returns a bool value from the element bitstream.
+func (r *Decoder) Bool() bool {
+	r.Sync(SyncBool)
+	x, err := r.Data.ReadByte()
+	r.checkErr(err)
+	assert(x < 2)
+	return x != 0
+}
+
+// Int64 decodes and returns an int64 value from the element bitstream.
+func (r *Decoder) Int64() int64 {
+	r.Sync(SyncInt64)
+	return r.rawVarint()
+}
+
+// Int64 decodes and returns a uint64 value from the element bitstream.
+func (r *Decoder) Uint64() uint64 {
+	r.Sync(SyncUint64)
+	return r.rawUvarint()
+}
+
+// Len decodes and returns a non-negative int value from the element bitstream.
+func (r *Decoder) Len() int { x := r.Uint64(); v := int(x); assert(uint64(v) == x); return v }
+
+// Int decodes and returns an int value from the element bitstream.
+func (r *Decoder) Int() int { x := r.Int64(); v := int(x); assert(int64(v) == x); return v }
+
+// Uint decodes and returns a uint value from the element bitstream.
+func (r *Decoder) Uint() uint { x := r.Uint64(); v := uint(x); assert(uint64(v) == x); return v }
+
+// Code decodes a Code value from the element bitstream and returns
+// its ordinal value. It's the caller's responsibility to convert the
+// result to an appropriate Code type.
+//
+// TODO(mdempsky): Ideally this method would have signature "Code[T
+// Code] T" instead, but we don't allow generic methods and the
+// compiler can't depend on generics yet anyway.
+func (r *Decoder) Code(mark SyncMarker) int {
+	r.Sync(mark)
+	return r.Len()
+}
+
+// Reloc decodes a relocation of expected section k from the element
+// bitstream and returns an index to the referenced element.
+func (r *Decoder) Reloc(k RelocKind) Index {
+	r.Sync(SyncUseReloc)
+	return r.rawReloc(k, r.Len())
+}
+
+// String decodes and returns a string value from the element
+// bitstream.
+func (r *Decoder) String() string {
+	r.Sync(SyncString)
+	return r.common.StringIdx(r.Reloc(RelocString))
+}
+
+// Strings decodes and returns a variable-length slice of strings from
+// the element bitstream.
+func (r *Decoder) Strings() []string {
+	res := make([]string, r.Len())
+	for i := range res {
+		res[i] = r.String()
+	}
+	return res
+}
+
+// Value decodes and returns a constant.Value from the element
+// bitstream.
+func (r *Decoder) Value() constant.Value {
+	r.Sync(SyncValue)
+	isComplex := r.Bool()
+	val := r.scalar()
+	if isComplex {
+		val = constant.BinaryOp(val, token.ADD, constant.MakeImag(r.scalar()))
+	}
+	return val
+}
+
+func (r *Decoder) scalar() constant.Value {
+	switch tag := CodeVal(r.Code(SyncVal)); tag {
+	default:
+		panic(fmt.Errorf("unexpected scalar tag: %v", tag))
+
+	case ValBool:
+		return constant.MakeBool(r.Bool())
+	case ValString:
+		return constant.MakeString(r.String())
+	case ValInt64:
+		return constant.MakeInt64(r.Int64())
+	case ValBigInt:
+		return constant.Make(r.bigInt())
+	case ValBigRat:
+		num := r.bigInt()
+		denom := r.bigInt()
+		return constant.Make(new(big.Rat).SetFrac(num, denom))
+	case ValBigFloat:
+		return constant.Make(r.bigFloat())
+	}
+}
+
+func (r *Decoder) bigInt() *big.Int {
+	v := new(big.Int).SetBytes([]byte(r.String()))
+	if r.Bool() {
+		v.Neg(v)
+	}
+	return v
+}
+
+func (r *Decoder) bigFloat() *big.Float {
+	v := new(big.Float).SetPrec(512)
+	assert(v.UnmarshalText([]byte(r.String())) == nil)
+	return v
+}
+
+// @@@ Helpers
+
+// TODO(mdempsky): These should probably be removed. I think they're a
+// smell that the export data format is not yet quite right.
+
+// PeekPkgPath returns the package path for the specified package
+// index.
+func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
+	var path string
+	{
+		r := pr.TempDecoder(RelocPkg, idx, SyncPkgDef)
+		path = r.String()
+		pr.RetireDecoder(&r)
+	}
+	if path == "" {
+		path = pr.pkgPath
+	}
+	return path
+}
+
+// PeekObj returns the package path, object name, and CodeObj for the
+// specified object index.
+func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
+	var ridx Index
+	var name string
+	var rcode int
+	{
+		r := pr.TempDecoder(RelocName, idx, SyncObject1)
+		r.Sync(SyncSym)
+		r.Sync(SyncPkg)
+		ridx = r.Reloc(RelocPkg)
+		name = r.String()
+		rcode = r.Code(SyncCodeObj)
+		pr.RetireDecoder(&r)
+	}
+
+	path := pr.PeekPkgPath(ridx)
+	assert(name != "")
+
+	tag := CodeObj(rcode)
+
+	return path, name, tag
+}
diff --git a/src/internal/pkgbits/doc.go b/src/internal/pkgbits/doc.go
new file mode 100644
index 0000000..4862e39
--- /dev/null
+++ b/src/internal/pkgbits/doc.go
@@ -0,0 +1,30 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package pkgbits implements low-level coding abstractions for
+// Unified IR's export data format.
+//
+// At a low-level, a package is a collection of bitstream elements.
+// Each element has a "kind" and a dense, non-negative index.
+// Elements can be randomly accessed given their kind and index.
+//
+// Individual elements are sequences of variable-length values (e.g.,
+// integers, booleans, strings, go/constant values, cross-references
+// to other elements). Package pkgbits provides APIs for encoding and
+// decoding these low-level values, but the details of mapping
+// higher-level Go constructs into elements is left to higher-level
+// abstractions.
+//
+// Elements may cross-reference each other with "relocations." For
+// example, an element representing a pointer type has a relocation
+// referring to the element type.
+//
+// Go constructs may be composed as a constellation of multiple
+// elements. For example, a declared function may have one element to
+// describe the object (e.g., its name, type, position), and a
+// separate element to describe its function body. This allows readers
+// some flexibility in efficiently seeking or re-reading data (e.g.,
+// inlining requires re-reading the function body for each inlined
+// call, without needing to re-read the object-level details).
+package pkgbits
diff --git a/src/internal/pkgbits/encoder.go b/src/internal/pkgbits/encoder.go
new file mode 100644
index 0000000..70a2cba
--- /dev/null
+++ b/src/internal/pkgbits/encoder.go
@@ -0,0 +1,394 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+import (
+	"bytes"
+	"crypto/md5"
+	"encoding/binary"
+	"go/constant"
+	"io"
+	"math/big"
+	"runtime"
+	"strings"
+)
+
+// currentVersion is the current version number.
+//
+//   - v0: initial prototype
+//
+//   - v1: adds the flags uint32 word
+//
+// TODO(mdempsky): For the next version bump:
+//   - remove the legacy "has init" bool from the public root
+//   - remove obj's "derived func instance" bool
+const currentVersion uint32 = 1
+
+// A PkgEncoder provides methods for encoding a package's Unified IR
+// export data.
+type PkgEncoder struct {
+	// elems holds the bitstream for previously encoded elements.
+	elems [numRelocs][]string
+
+	// stringsIdx maps previously encoded strings to their index within
+	// the RelocString section, to allow deduplication. That is,
+	// elems[RelocString][stringsIdx[s]] == s (if present).
+	stringsIdx map[string]Index
+
+	// syncFrames is the number of frames to write at each sync
+	// marker. A negative value means sync markers are omitted.
+	syncFrames int
+}
+
+// SyncMarkers reports whether pw uses sync markers.
+func (pw *PkgEncoder) SyncMarkers() bool { return pw.syncFrames >= 0 }
+
+// NewPkgEncoder returns an initialized PkgEncoder.
+//
+// syncFrames is the number of caller frames that should be serialized
+// at Sync points. Serializing additional frames results in larger
+// export data files, but can help diagnosing desync errors in
+// higher-level Unified IR reader/writer code. If syncFrames is
+// negative, then sync markers are omitted entirely.
+func NewPkgEncoder(syncFrames int) PkgEncoder {
+	return PkgEncoder{
+		stringsIdx: make(map[string]Index),
+		syncFrames: syncFrames,
+	}
+}
+
+// DumpTo writes the package's encoded data to out0 and returns the
+// package fingerprint.
+func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
+	h := md5.New()
+	out := io.MultiWriter(out0, h)
+
+	writeUint32 := func(x uint32) {
+		assert(binary.Write(out, binary.LittleEndian, x) == nil)
+	}
+
+	writeUint32(currentVersion)
+
+	var flags uint32
+	if pw.SyncMarkers() {
+		flags |= flagSyncMarkers
+	}
+	writeUint32(flags)
+
+	// Write elemEndsEnds.
+	var sum uint32
+	for _, elems := range &pw.elems {
+		sum += uint32(len(elems))
+		writeUint32(sum)
+	}
+
+	// Write elemEnds.
+	sum = 0
+	for _, elems := range &pw.elems {
+		for _, elem := range elems {
+			sum += uint32(len(elem))
+			writeUint32(sum)
+		}
+	}
+
+	// Write elemData.
+	for _, elems := range &pw.elems {
+		for _, elem := range elems {
+			_, err := io.WriteString(out, elem)
+			assert(err == nil)
+		}
+	}
+
+	// Write fingerprint.
+	copy(fingerprint[:], h.Sum(nil))
+	_, err := out0.Write(fingerprint[:])
+	assert(err == nil)
+
+	return
+}
+
+// StringIdx adds a string value to the strings section, if not
+// already present, and returns its index.
+func (pw *PkgEncoder) StringIdx(s string) Index {
+	if idx, ok := pw.stringsIdx[s]; ok {
+		assert(pw.elems[RelocString][idx] == s)
+		return idx
+	}
+
+	idx := Index(len(pw.elems[RelocString]))
+	pw.elems[RelocString] = append(pw.elems[RelocString], s)
+	pw.stringsIdx[s] = idx
+	return idx
+}
+
+// NewEncoder returns an Encoder for a new element within the given
+// section, and encodes the given SyncMarker as the start of the
+// element bitstream.
+func (pw *PkgEncoder) NewEncoder(k RelocKind, marker SyncMarker) Encoder {
+	e := pw.NewEncoderRaw(k)
+	e.Sync(marker)
+	return e
+}
+
+// NewEncoderRaw returns an Encoder for a new element within the given
+// section.
+//
+// Most callers should use NewEncoder instead.
+func (pw *PkgEncoder) NewEncoderRaw(k RelocKind) Encoder {
+	idx := Index(len(pw.elems[k]))
+	pw.elems[k] = append(pw.elems[k], "") // placeholder
+
+	return Encoder{
+		p:   pw,
+		k:   k,
+		Idx: idx,
+	}
+}
+
+// An Encoder provides methods for encoding an individual element's
+// bitstream data.
+type Encoder struct {
+	p *PkgEncoder
+
+	Relocs   []RelocEnt
+	RelocMap map[RelocEnt]uint32
+	Data     bytes.Buffer // accumulated element bitstream data
+
+	encodingRelocHeader bool
+
+	k   RelocKind
+	Idx Index // index within relocation section
+}
+
+// Flush finalizes the element's bitstream and returns its Index.
+func (w *Encoder) Flush() Index {
+	var sb strings.Builder
+
+	// Backup the data so we write the relocations at the front.
+	var tmp bytes.Buffer
+	io.Copy(&tmp, &w.Data)
+
+	// TODO(mdempsky): Consider writing these out separately so they're
+	// easier to strip, along with function bodies, so that we can prune
+	// down to just the data that's relevant to go/types.
+	if w.encodingRelocHeader {
+		panic("encodingRelocHeader already true; recursive flush?")
+	}
+	w.encodingRelocHeader = true
+	w.Sync(SyncRelocs)
+	w.Len(len(w.Relocs))
+	for _, rEnt := range w.Relocs {
+		w.Sync(SyncReloc)
+		w.Len(int(rEnt.Kind))
+		w.Len(int(rEnt.Idx))
+	}
+
+	io.Copy(&sb, &w.Data)
+	io.Copy(&sb, &tmp)
+	w.p.elems[w.k][w.Idx] = sb.String()
+
+	return w.Idx
+}
+
+func (w *Encoder) checkErr(err error) {
+	if err != nil {
+		errorf("unexpected encoding error: %v", err)
+	}
+}
+
+func (w *Encoder) rawUvarint(x uint64) {
+	var buf [binary.MaxVarintLen64]byte
+	n := binary.PutUvarint(buf[:], x)
+	_, err := w.Data.Write(buf[:n])
+	w.checkErr(err)
+}
+
+func (w *Encoder) rawVarint(x int64) {
+	// Zig-zag encode.
+	ux := uint64(x) << 1
+	if x < 0 {
+		ux = ^ux
+	}
+
+	w.rawUvarint(ux)
+}
+
+func (w *Encoder) rawReloc(r RelocKind, idx Index) int {
+	e := RelocEnt{r, idx}
+	if w.RelocMap != nil {
+		if i, ok := w.RelocMap[e]; ok {
+			return int(i)
+		}
+	} else {
+		w.RelocMap = make(map[RelocEnt]uint32)
+	}
+
+	i := len(w.Relocs)
+	w.RelocMap[e] = uint32(i)
+	w.Relocs = append(w.Relocs, e)
+	return i
+}
+
+func (w *Encoder) Sync(m SyncMarker) {
+	if !w.p.SyncMarkers() {
+		return
+	}
+
+	// Writing out stack frame string references requires working
+	// relocations, but writing out the relocations themselves involves
+	// sync markers. To prevent infinite recursion, we simply trim the
+	// stack frame for sync markers within the relocation header.
+	var frames []string
+	if !w.encodingRelocHeader && w.p.syncFrames > 0 {
+		pcs := make([]uintptr, w.p.syncFrames)
+		n := runtime.Callers(2, pcs)
+		frames = fmtFrames(pcs[:n]...)
+	}
+
+	// TODO(mdempsky): Save space by writing out stack frames as a
+	// linked list so we can share common stack frames.
+	w.rawUvarint(uint64(m))
+	w.rawUvarint(uint64(len(frames)))
+	for _, frame := range frames {
+		w.rawUvarint(uint64(w.rawReloc(RelocString, w.p.StringIdx(frame))))
+	}
+}
+
+// Bool encodes and writes a bool value into the element bitstream,
+// and then returns the bool value.
+//
+// For simple, 2-alternative encodings, the idiomatic way to call Bool
+// is something like:
+//
+//	if w.Bool(x != 0) {
+//		// alternative #1
+//	} else {
+//		// alternative #2
+//	}
+//
+// For multi-alternative encodings, use Code instead.
+func (w *Encoder) Bool(b bool) bool {
+	w.Sync(SyncBool)
+	var x byte
+	if b {
+		x = 1
+	}
+	err := w.Data.WriteByte(x)
+	w.checkErr(err)
+	return b
+}
+
+// Int64 encodes and writes an int64 value into the element bitstream.
+func (w *Encoder) Int64(x int64) {
+	w.Sync(SyncInt64)
+	w.rawVarint(x)
+}
+
+// Uint64 encodes and writes a uint64 value into the element bitstream.
+func (w *Encoder) Uint64(x uint64) {
+	w.Sync(SyncUint64)
+	w.rawUvarint(x)
+}
+
+// Len encodes and writes a non-negative int value into the element bitstream.
+func (w *Encoder) Len(x int) { assert(x >= 0); w.Uint64(uint64(x)) }
+
+// Int encodes and writes an int value into the element bitstream.
+func (w *Encoder) Int(x int) { w.Int64(int64(x)) }
+
+// Len encodes and writes a uint value into the element bitstream.
+func (w *Encoder) Uint(x uint) { w.Uint64(uint64(x)) }
+
+// Reloc encodes and writes a relocation for the given (section,
+// index) pair into the element bitstream.
+//
+// Note: Only the index is formally written into the element
+// bitstream, so bitstream decoders must know from context which
+// section an encoded relocation refers to.
+func (w *Encoder) Reloc(r RelocKind, idx Index) {
+	w.Sync(SyncUseReloc)
+	w.Len(w.rawReloc(r, idx))
+}
+
+// Code encodes and writes a Code value into the element bitstream.
+func (w *Encoder) Code(c Code) {
+	w.Sync(c.Marker())
+	w.Len(c.Value())
+}
+
+// String encodes and writes a string value into the element
+// bitstream.
+//
+// Internally, strings are deduplicated by adding them to the strings
+// section (if not already present), and then writing a relocation
+// into the element bitstream.
+func (w *Encoder) String(s string) {
+	w.StringRef(w.p.StringIdx(s))
+}
+
+// StringRef writes a reference to the given index, which must be a
+// previously encoded string value.
+func (w *Encoder) StringRef(idx Index) {
+	w.Sync(SyncString)
+	w.Reloc(RelocString, idx)
+}
+
+// Strings encodes and writes a variable-length slice of strings into
+// the element bitstream.
+func (w *Encoder) Strings(ss []string) {
+	w.Len(len(ss))
+	for _, s := range ss {
+		w.String(s)
+	}
+}
+
+// Value encodes and writes a constant.Value into the element
+// bitstream.
+func (w *Encoder) Value(val constant.Value) {
+	w.Sync(SyncValue)
+	if w.Bool(val.Kind() == constant.Complex) {
+		w.scalar(constant.Real(val))
+		w.scalar(constant.Imag(val))
+	} else {
+		w.scalar(val)
+	}
+}
+
+func (w *Encoder) scalar(val constant.Value) {
+	switch v := constant.Val(val).(type) {
+	default:
+		errorf("unhandled %v (%v)", val, val.Kind())
+	case bool:
+		w.Code(ValBool)
+		w.Bool(v)
+	case string:
+		w.Code(ValString)
+		w.String(v)
+	case int64:
+		w.Code(ValInt64)
+		w.Int64(v)
+	case *big.Int:
+		w.Code(ValBigInt)
+		w.bigInt(v)
+	case *big.Rat:
+		w.Code(ValBigRat)
+		w.bigInt(v.Num())
+		w.bigInt(v.Denom())
+	case *big.Float:
+		w.Code(ValBigFloat)
+		w.bigFloat(v)
+	}
+}
+
+func (w *Encoder) bigInt(v *big.Int) {
+	b := v.Bytes()
+	w.String(string(b)) // TODO: More efficient encoding.
+	w.Bool(v.Sign() < 0)
+}
+
+func (w *Encoder) bigFloat(v *big.Float) {
+	b := v.Append(nil, 'p', -1)
+	w.String(string(b)) // TODO: More efficient encoding.
+}
diff --git a/src/internal/pkgbits/flags.go b/src/internal/pkgbits/flags.go
new file mode 100644
index 0000000..6542227
--- /dev/null
+++ b/src/internal/pkgbits/flags.go
@@ -0,0 +1,9 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+const (
+	flagSyncMarkers = 1 << iota // file format contains sync markers
+)
diff --git a/src/internal/pkgbits/reloc.go b/src/internal/pkgbits/reloc.go
new file mode 100644
index 0000000..fcdfb97
--- /dev/null
+++ b/src/internal/pkgbits/reloc.go
@@ -0,0 +1,42 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+// A RelocKind indicates a particular section within a unified IR export.
+type RelocKind int32
+
+// An Index represents a bitstream element index within a particular
+// section.
+type Index int32
+
+// A relocEnt (relocation entry) is an entry in an element's local
+// reference table.
+//
+// TODO(mdempsky): Rename this too.
+type RelocEnt struct {
+	Kind RelocKind
+	Idx  Index
+}
+
+// Reserved indices within the meta relocation section.
+const (
+	PublicRootIdx  Index = 0
+	PrivateRootIdx Index = 1
+)
+
+const (
+	RelocString RelocKind = iota
+	RelocMeta
+	RelocPosBase
+	RelocPkg
+	RelocName
+	RelocType
+	RelocObj
+	RelocObjExt
+	RelocObjDict
+	RelocBody
+
+	numRelocs = iota
+)
diff --git a/src/internal/pkgbits/support.go b/src/internal/pkgbits/support.go
new file mode 100644
index 0000000..f7579df
--- /dev/null
+++ b/src/internal/pkgbits/support.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+import "fmt"
+
+func assert(b bool) {
+	if !b {
+		panic("assertion failed")
+	}
+}
+
+func errorf(format string, args ...any) {
+	panic(fmt.Errorf(format, args...))
+}
diff --git a/src/internal/pkgbits/sync.go b/src/internal/pkgbits/sync.go
new file mode 100644
index 0000000..1520b73
--- /dev/null
+++ b/src/internal/pkgbits/sync.go
@@ -0,0 +1,136 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+// fmtFrames formats a backtrace for reporting reader/writer desyncs.
+func fmtFrames(pcs ...uintptr) []string {
+	res := make([]string, 0, len(pcs))
+	walkFrames(pcs, func(file string, line int, name string, offset uintptr) {
+		// Trim package from function name. It's just redundant noise.
+		name = strings.TrimPrefix(name, "cmd/compile/internal/noder.")
+
+		res = append(res, fmt.Sprintf("%s:%v: %s +0x%v", file, line, name, offset))
+	})
+	return res
+}
+
+type frameVisitor func(file string, line int, name string, offset uintptr)
+
+// walkFrames calls visit for each call frame represented by pcs.
+//
+// pcs should be a slice of PCs, as returned by runtime.Callers.
+func walkFrames(pcs []uintptr, visit frameVisitor) {
+	if len(pcs) == 0 {
+		return
+	}
+
+	frames := runtime.CallersFrames(pcs)
+	for {
+		frame, more := frames.Next()
+		visit(frame.File, frame.Line, frame.Function, frame.PC-frame.Entry)
+		if !more {
+			return
+		}
+	}
+}
+
+// SyncMarker is an enum type that represents markers that may be
+// written to export data to ensure the reader and writer stay
+// synchronized.
+type SyncMarker int
+
+//go:generate stringer -type=SyncMarker -trimprefix=Sync
+
+const (
+	_ SyncMarker = iota
+
+	// Public markers (known to go/types importers).
+
+	// Low-level coding markers.
+	SyncEOF
+	SyncBool
+	SyncInt64
+	SyncUint64
+	SyncString
+	SyncValue
+	SyncVal
+	SyncRelocs
+	SyncReloc
+	SyncUseReloc
+
+	// Higher-level object and type markers.
+	SyncPublic
+	SyncPos
+	SyncPosBase
+	SyncObject
+	SyncObject1
+	SyncPkg
+	SyncPkgDef
+	SyncMethod
+	SyncType
+	SyncTypeIdx
+	SyncTypeParamNames
+	SyncSignature
+	SyncParams
+	SyncParam
+	SyncCodeObj
+	SyncSym
+	SyncLocalIdent
+	SyncSelector
+
+	// Private markers (only known to cmd/compile).
+	SyncPrivate
+
+	SyncFuncExt
+	SyncVarExt
+	SyncTypeExt
+	SyncPragma
+
+	SyncExprList
+	SyncExprs
+	SyncExpr
+	SyncExprType
+	SyncAssign
+	SyncOp
+	SyncFuncLit
+	SyncCompLit
+
+	SyncDecl
+	SyncFuncBody
+	SyncOpenScope
+	SyncCloseScope
+	SyncCloseAnotherScope
+	SyncDeclNames
+	SyncDeclName
+
+	SyncStmts
+	SyncBlockStmt
+	SyncIfStmt
+	SyncForStmt
+	SyncSwitchStmt
+	SyncRangeStmt
+	SyncCaseClause
+	SyncCommClause
+	SyncSelectStmt
+	SyncDecls
+	SyncLabeledStmt
+	SyncUseObjLocal
+	SyncAddLocal
+	SyncLinkname
+	SyncStmt1
+	SyncStmtsEnd
+	SyncLabel
+	SyncOptLabel
+
+	SyncMultiExpr
+	SyncRType
+	SyncConvRTTI
+)
diff --git a/src/internal/pkgbits/syncmarker_string.go b/src/internal/pkgbits/syncmarker_string.go
new file mode 100644
index 0000000..4a5b0ca
--- /dev/null
+++ b/src/internal/pkgbits/syncmarker_string.go
@@ -0,0 +1,89 @@
+// Code generated by "stringer -type=SyncMarker -trimprefix=Sync"; DO NOT EDIT.
+
+package pkgbits
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[SyncEOF-1]
+	_ = x[SyncBool-2]
+	_ = x[SyncInt64-3]
+	_ = x[SyncUint64-4]
+	_ = x[SyncString-5]
+	_ = x[SyncValue-6]
+	_ = x[SyncVal-7]
+	_ = x[SyncRelocs-8]
+	_ = x[SyncReloc-9]
+	_ = x[SyncUseReloc-10]
+	_ = x[SyncPublic-11]
+	_ = x[SyncPos-12]
+	_ = x[SyncPosBase-13]
+	_ = x[SyncObject-14]
+	_ = x[SyncObject1-15]
+	_ = x[SyncPkg-16]
+	_ = x[SyncPkgDef-17]
+	_ = x[SyncMethod-18]
+	_ = x[SyncType-19]
+	_ = x[SyncTypeIdx-20]
+	_ = x[SyncTypeParamNames-21]
+	_ = x[SyncSignature-22]
+	_ = x[SyncParams-23]
+	_ = x[SyncParam-24]
+	_ = x[SyncCodeObj-25]
+	_ = x[SyncSym-26]
+	_ = x[SyncLocalIdent-27]
+	_ = x[SyncSelector-28]
+	_ = x[SyncPrivate-29]
+	_ = x[SyncFuncExt-30]
+	_ = x[SyncVarExt-31]
+	_ = x[SyncTypeExt-32]
+	_ = x[SyncPragma-33]
+	_ = x[SyncExprList-34]
+	_ = x[SyncExprs-35]
+	_ = x[SyncExpr-36]
+	_ = x[SyncExprType-37]
+	_ = x[SyncAssign-38]
+	_ = x[SyncOp-39]
+	_ = x[SyncFuncLit-40]
+	_ = x[SyncCompLit-41]
+	_ = x[SyncDecl-42]
+	_ = x[SyncFuncBody-43]
+	_ = x[SyncOpenScope-44]
+	_ = x[SyncCloseScope-45]
+	_ = x[SyncCloseAnotherScope-46]
+	_ = x[SyncDeclNames-47]
+	_ = x[SyncDeclName-48]
+	_ = x[SyncStmts-49]
+	_ = x[SyncBlockStmt-50]
+	_ = x[SyncIfStmt-51]
+	_ = x[SyncForStmt-52]
+	_ = x[SyncSwitchStmt-53]
+	_ = x[SyncRangeStmt-54]
+	_ = x[SyncCaseClause-55]
+	_ = x[SyncCommClause-56]
+	_ = x[SyncSelectStmt-57]
+	_ = x[SyncDecls-58]
+	_ = x[SyncLabeledStmt-59]
+	_ = x[SyncUseObjLocal-60]
+	_ = x[SyncAddLocal-61]
+	_ = x[SyncLinkname-62]
+	_ = x[SyncStmt1-63]
+	_ = x[SyncStmtsEnd-64]
+	_ = x[SyncLabel-65]
+	_ = x[SyncOptLabel-66]
+}
+
+const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprExprTypeAssignOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabel"
+
+var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 226, 232, 234, 241, 248, 252, 260, 269, 279, 296, 305, 313, 318, 327, 333, 340, 350, 359, 369, 379, 389, 394, 405, 416, 424, 432, 437, 445, 450, 458}
+
+func (i SyncMarker) String() string {
+	i -= 1
+	if i < 0 || i >= SyncMarker(len(_SyncMarker_index)-1) {
+		return "SyncMarker(" + strconv.FormatInt(int64(i+1), 10) + ")"
+	}
+	return _SyncMarker_name[_SyncMarker_index[i]:_SyncMarker_index[i+1]]
+}
diff --git a/src/internal/platform/supported.go b/src/internal/platform/supported.go
new file mode 100644
index 0000000..046352f
--- /dev/null
+++ b/src/internal/platform/supported.go
@@ -0,0 +1,205 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package platform
+
+// RaceDetectorSupported reports whether goos/goarch supports the race
+// detector. There is a copy of this function in cmd/dist/test.go.
+// Race detector only supports 48-bit VMA on arm64. But it will always
+// return true for arm64, because we don't have VMA size information during
+// the compile time.
+func RaceDetectorSupported(goos, goarch string) bool {
+	switch goos {
+	case "linux":
+		return goarch == "amd64" || goarch == "ppc64le" || goarch == "arm64" || goarch == "s390x"
+	case "darwin":
+		return goarch == "amd64" || goarch == "arm64"
+	case "freebsd", "netbsd", "openbsd", "windows":
+		return goarch == "amd64"
+	default:
+		return false
+	}
+}
+
+// MSanSupported reports whether goos/goarch supports the memory
+// sanitizer option.
+// There is a copy of this function in misc/cgo/testsanitizers/cc_test.go.
+func MSanSupported(goos, goarch string) bool {
+	switch goos {
+	case "linux":
+		return goarch == "amd64" || goarch == "arm64"
+	case "freebsd":
+		return goarch == "amd64"
+	default:
+		return false
+	}
+}
+
+// ASanSupported reports whether goos/goarch supports the address
+// sanitizer option.
+// There is a copy of this function in misc/cgo/testsanitizers/cc_test.go.
+func ASanSupported(goos, goarch string) bool {
+	switch goos {
+	case "linux":
+		return goarch == "arm64" || goarch == "amd64" || goarch == "riscv64" || goarch == "ppc64le"
+	default:
+		return false
+	}
+}
+
+// FuzzSupported reports whether goos/goarch supports fuzzing
+// ('go test -fuzz=.').
+func FuzzSupported(goos, goarch string) bool {
+	switch goos {
+	case "darwin", "freebsd", "linux", "windows":
+		return true
+	default:
+		return false
+	}
+}
+
+// FuzzInstrumented reports whether fuzzing on goos/goarch uses coverage
+// instrumentation. (FuzzInstrumented implies FuzzSupported.)
+func FuzzInstrumented(goos, goarch string) bool {
+	switch goarch {
+	case "amd64", "arm64":
+		// TODO(#14565): support more architectures.
+		return FuzzSupported(goos, goarch)
+	default:
+		return false
+	}
+}
+
+// MustLinkExternal reports whether goos/goarch requires external linking.
+func MustLinkExternal(goos, goarch string) bool {
+	return MustLinkExternalGo121(goos, goarch, false)
+}
+
+// MustLinkExternalGo121 reports whether goos/goarch requires external linking,
+// with or without cgo dependencies. [This version back-ported from
+// Go 1.21 as part of a test].
+func MustLinkExternalGo121(goos, goarch string, withCgo bool) bool {
+	if withCgo {
+		switch goarch {
+		case "loong64",
+			"mips", "mipsle", "mips64", "mips64le",
+			"riscv64":
+			// Internally linking cgo is incomplete on some architectures.
+			// https://go.dev/issue/14449
+			return true
+		case "arm64":
+			if goos == "windows" {
+				// windows/arm64 internal linking is not implemented.
+				return true
+			}
+		case "ppc64":
+			// Big Endian PPC64 cgo internal linking is not implemented for aix or linux.
+			// https://go.dev/issue/8912
+			return true
+		}
+
+		switch goos {
+		case "android":
+			return true
+		case "dragonfly":
+			// It seems that on Dragonfly thread local storage is
+			// set up by the dynamic linker, so internal cgo linking
+			// doesn't work. Test case is "go test runtime/cgo".
+			return true
+		}
+	}
+
+	switch goos {
+	case "android":
+		if goarch != "arm64" {
+			return true
+		}
+	case "ios":
+		if goarch == "arm64" {
+			return true
+		}
+	}
+	return false
+}
+
+// BuildModeSupported reports whether goos/goarch supports the given build mode
+// using the given compiler.
+func BuildModeSupported(compiler, buildmode, goos, goarch string) bool {
+	if compiler == "gccgo" {
+		return true
+	}
+
+	platform := goos + "/" + goarch
+
+	switch buildmode {
+	case "archive":
+		return true
+
+	case "c-archive":
+		// TODO(bcmills): This seems dubious.
+		// Do we really support c-archive mode on js/wasm‽
+		return platform != "linux/ppc64"
+
+	case "c-shared":
+		switch platform {
+		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/ppc64le", "linux/riscv64", "linux/s390x",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"freebsd/amd64",
+			"darwin/amd64", "darwin/arm64",
+			"windows/amd64", "windows/386", "windows/arm64":
+			return true
+		}
+		return false
+
+	case "default":
+		return true
+
+	case "exe":
+		return true
+
+	case "pie":
+		switch platform {
+		case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/riscv64", "linux/s390x",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"freebsd/amd64",
+			"darwin/amd64", "darwin/arm64",
+			"ios/amd64", "ios/arm64",
+			"aix/ppc64",
+			"windows/386", "windows/amd64", "windows/arm", "windows/arm64":
+			return true
+		}
+		return false
+
+	case "shared":
+		switch platform {
+		case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x":
+			return true
+		}
+		return false
+
+	case "plugin":
+		switch platform {
+		case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x", "linux/ppc64le",
+			"android/amd64", "android/arm", "android/arm64", "android/386",
+			"darwin/amd64", "darwin/arm64",
+			"freebsd/amd64":
+			return true
+		}
+		return false
+
+	default:
+		return false
+	}
+}
+
+func InternalLinkPIESupported(goos, goarch string) bool {
+	switch goos + "/" + goarch {
+	case "darwin/amd64", "darwin/arm64",
+		"linux/amd64", "linux/arm64", "linux/ppc64le",
+		"android/arm64",
+		"windows-amd64", "windows-386", "windows-arm":
+		return true
+	}
+	return false
+}
diff --git a/src/internal/poll/copy_file_range_linux.go b/src/internal/poll/copy_file_range_linux.go
new file mode 100644
index 0000000..ba33f51
--- /dev/null
+++ b/src/internal/poll/copy_file_range_linux.go
@@ -0,0 +1,128 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"internal/syscall/unix"
+	"sync"
+	"syscall"
+)
+
+var (
+	kernelVersion53Once sync.Once
+	kernelVersion53     bool
+)
+
+const maxCopyFileRangeRound = 1 << 30
+
+// CopyFileRange copies at most remain bytes of data from src to dst, using
+// the copy_file_range system call. dst and src must refer to regular files.
+func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
+	kernelVersion53Once.Do(func() {
+		major, minor := unix.KernelVersion()
+		// copy_file_range(2) is broken in various ways on kernels older than 5.3,
+		// see issue #42400 and
+		// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS
+		if major > 5 || (major == 5 && minor >= 3) {
+			kernelVersion53 = true
+		}
+	})
+
+	if !kernelVersion53 {
+		return 0, false, nil
+	}
+
+	for remain > 0 {
+		max := remain
+		if max > maxCopyFileRangeRound {
+			max = maxCopyFileRangeRound
+		}
+		n, err := copyFileRange(dst, src, int(max))
+		switch err {
+		case syscall.ENOSYS:
+			// copy_file_range(2) was introduced in Linux 4.5.
+			// Go supports Linux >= 2.6.33, so the system call
+			// may not be present.
+			//
+			// If we see ENOSYS, we have certainly not transferred
+			// any data, so we can tell the caller that we
+			// couldn't handle the transfer and let them fall
+			// back to more generic code.
+			return 0, false, nil
+		case syscall.EXDEV, syscall.EINVAL, syscall.EIO, syscall.EOPNOTSUPP, syscall.EPERM:
+			// Prior to Linux 5.3, it was not possible to
+			// copy_file_range across file systems. Similarly to
+			// the ENOSYS case above, if we see EXDEV, we have
+			// not transferred any data, and we can let the caller
+			// fall back to generic code.
+			//
+			// As for EINVAL, that is what we see if, for example,
+			// dst or src refer to a pipe rather than a regular
+			// file. This is another case where no data has been
+			// transferred, so we consider it unhandled.
+			//
+			// If src and dst are on CIFS, we can see EIO.
+			// See issue #42334.
+			//
+			// If the file is on NFS, we can see EOPNOTSUPP.
+			// See issue #40731.
+			//
+			// If the process is running inside a Docker container,
+			// we might see EPERM instead of ENOSYS. See issue
+			// #40893. Since EPERM might also be a legitimate error,
+			// don't mark copy_file_range(2) as unsupported.
+			return 0, false, nil
+		case nil:
+			if n == 0 {
+				// If we did not read any bytes at all,
+				// then this file may be in a file system
+				// where copy_file_range silently fails.
+				// https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
+				if written == 0 {
+					return 0, false, nil
+				}
+				// Otherwise src is at EOF, which means
+				// we are done.
+				return written, true, nil
+			}
+			remain -= n
+			written += n
+		default:
+			return written, true, err
+		}
+	}
+	return written, true, nil
+}
+
+// copyFileRange performs one round of copy_file_range(2).
+func copyFileRange(dst, src *FD, max int) (written int64, err error) {
+	// The signature of copy_file_range(2) is:
+	//
+	// ssize_t copy_file_range(int fd_in, loff_t *off_in,
+	//                         int fd_out, loff_t *off_out,
+	//                         size_t len, unsigned int flags);
+	//
+	// Note that in the call to unix.CopyFileRange below, we use nil
+	// values for off_in and off_out. For the system call, this means
+	// "use and update the file offsets". That is why we must acquire
+	// locks for both file descriptors (and why this whole machinery is
+	// in the internal/poll package to begin with).
+	if err := dst.writeLock(); err != nil {
+		return 0, err
+	}
+	defer dst.writeUnlock()
+	if err := src.readLock(); err != nil {
+		return 0, err
+	}
+	defer src.readUnlock()
+	var n int
+	for {
+		n, err = unix.CopyFileRange(src.Sysfd, nil, dst.Sysfd, nil, max, 0)
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	return int64(n), err
+}
diff --git a/src/internal/poll/errno_unix.go b/src/internal/poll/errno_unix.go
new file mode 100644
index 0000000..8eed93a
--- /dev/null
+++ b/src/internal/poll/errno_unix.go
@@ -0,0 +1,33 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package poll
+
+import "syscall"
+
+// Do the interface allocations only once for common
+// Errno values.
+var (
+	errEAGAIN error = syscall.EAGAIN
+	errEINVAL error = syscall.EINVAL
+	errENOENT error = syscall.ENOENT
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case syscall.EAGAIN:
+		return errEAGAIN
+	case syscall.EINVAL:
+		return errEINVAL
+	case syscall.ENOENT:
+		return errENOENT
+	}
+	return e
+}
diff --git a/src/internal/poll/errno_windows.go b/src/internal/poll/errno_windows.go
new file mode 100644
index 0000000..6381479
--- /dev/null
+++ b/src/internal/poll/errno_windows.go
@@ -0,0 +1,31 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package poll
+
+import "syscall"
+
+// Do the interface allocations only once for common
+// Errno values.
+
+var (
+	errERROR_IO_PENDING error = syscall.Errno(syscall.ERROR_IO_PENDING)
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case syscall.ERROR_IO_PENDING:
+		return errERROR_IO_PENDING
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
diff --git a/src/internal/poll/error_linux_test.go b/src/internal/poll/error_linux_test.go
new file mode 100644
index 0000000..059fb8e
--- /dev/null
+++ b/src/internal/poll/error_linux_test.go
@@ -0,0 +1,31 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"errors"
+	"internal/poll"
+	"os"
+	"syscall"
+)
+
+func badStateFile() (*os.File, error) {
+	if os.Getuid() != 0 {
+		return nil, errors.New("must be root")
+	}
+	// Using OpenFile for a device file is an easy way to make a
+	// file attached to the runtime-integrated network poller and
+	// configured in halfway.
+	return os.OpenFile("/dev/net/tun", os.O_RDWR, 0)
+}
+
+func isBadStateFileError(err error) (string, bool) {
+	switch err {
+	case poll.ErrNotPollable, syscall.EBADFD:
+		return "", true
+	default:
+		return "not pollable or file in bad state error", false
+	}
+}
diff --git a/src/internal/poll/error_stub_test.go b/src/internal/poll/error_stub_test.go
new file mode 100644
index 0000000..48e0952
--- /dev/null
+++ b/src/internal/poll/error_stub_test.go
@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+
+package poll_test
+
+import (
+	"errors"
+	"os"
+	"runtime"
+)
+
+func badStateFile() (*os.File, error) {
+	return nil, errors.New("not supported on " + runtime.GOOS)
+}
+
+func isBadStateFileError(err error) (string, bool) {
+	return "", false
+}
diff --git a/src/internal/poll/error_test.go b/src/internal/poll/error_test.go
new file mode 100644
index 0000000..abc8b16
--- /dev/null
+++ b/src/internal/poll/error_test.go
@@ -0,0 +1,51 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"fmt"
+	"io/fs"
+	"net"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestReadError(t *testing.T) {
+	t.Run("ErrNotPollable", func(t *testing.T) {
+		f, err := badStateFile()
+		if err != nil {
+			t.Skip(err)
+		}
+		defer f.Close()
+
+		// Give scheduler a chance to have two separated
+		// goroutines: an event poller and an event waiter.
+		time.Sleep(100 * time.Millisecond)
+
+		var b [1]byte
+		_, err = f.Read(b[:])
+		if perr := parseReadError(err, isBadStateFileError); perr != nil {
+			t.Fatal(perr)
+		}
+	})
+}
+
+func parseReadError(nestedErr error, verify func(error) (string, bool)) error {
+	err := nestedErr
+	if nerr, ok := err.(*net.OpError); ok {
+		err = nerr.Err
+	}
+	if nerr, ok := err.(*fs.PathError); ok {
+		err = nerr.Err
+	}
+	if nerr, ok := err.(*os.SyscallError); ok {
+		err = nerr.Err
+	}
+	if s, ok := verify(err); !ok {
+		return fmt.Errorf("got %v; want %s", nestedErr, s)
+	}
+	return nil
+}
diff --git a/src/internal/poll/export_linux_test.go b/src/internal/poll/export_linux_test.go
new file mode 100644
index 0000000..7fba793
--- /dev/null
+++ b/src/internal/poll/export_linux_test.go
@@ -0,0 +1,22 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Export guts for testing on linux.
+// Since testing imports os and os imports internal/poll,
+// the internal/poll tests can not be in package poll.
+
+package poll
+
+var (
+	GetPipe     = getPipe
+	PutPipe     = putPipe
+	NewPipe     = newPipe
+	DestroyPipe = destroyPipe
+)
+
+func GetPipeFds(p *SplicePipe) (int, int) {
+	return p.rfd, p.wfd
+}
+
+type SplicePipe = splicePipe
diff --git a/src/internal/poll/export_posix_test.go b/src/internal/poll/export_posix_test.go
new file mode 100644
index 0000000..3415ab3
--- /dev/null
+++ b/src/internal/poll/export_posix_test.go
@@ -0,0 +1,15 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+// Export guts for testing on posix.
+// Since testing imports os and os imports internal/poll,
+// the internal/poll tests can not be in package poll.
+
+package poll
+
+func (fd *FD) EOFError(n int, err error) error {
+	return fd.eofError(n, err)
+}
diff --git a/src/internal/poll/export_test.go b/src/internal/poll/export_test.go
new file mode 100644
index 0000000..66d7c32
--- /dev/null
+++ b/src/internal/poll/export_test.go
@@ -0,0 +1,35 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Export guts for testing.
+// Since testing imports os and os imports internal/poll,
+// the internal/poll tests can not be in package poll.
+
+package poll
+
+var Consume = consume
+
+type XFDMutex struct {
+	fdMutex
+}
+
+func (mu *XFDMutex) Incref() bool {
+	return mu.incref()
+}
+
+func (mu *XFDMutex) IncrefAndClose() bool {
+	return mu.increfAndClose()
+}
+
+func (mu *XFDMutex) Decref() bool {
+	return mu.decref()
+}
+
+func (mu *XFDMutex) RWLock(read bool) bool {
+	return mu.rwlock(read)
+}
+
+func (mu *XFDMutex) RWUnlock(read bool) bool {
+	return mu.rwunlock(read)
+}
diff --git a/src/internal/poll/export_windows_test.go b/src/internal/poll/export_windows_test.go
new file mode 100644
index 0000000..88ed71a
--- /dev/null
+++ b/src/internal/poll/export_windows_test.go
@@ -0,0 +1,17 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Export guts for testing on windows.
+// Since testing imports os and os imports internal/poll,
+// the internal/poll tests can not be in package poll.
+
+package poll
+
+var (
+	LogInitFD = &logInitFD
+)
+
+func (fd *FD) IsPartOfNetpoll() bool {
+	return fd.pd.runtimeCtx != 0
+}
diff --git a/src/internal/poll/fcntl_js.go b/src/internal/poll/fcntl_js.go
new file mode 100644
index 0000000..0f42ef6
--- /dev/null
+++ b/src/internal/poll/fcntl_js.go
@@ -0,0 +1,14 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js && wasm
+
+package poll
+
+import "syscall"
+
+// fcntl not supported on js/wasm
+func fcntl(fd int, cmd int, arg int) (int, error) {
+	return 0, syscall.ENOSYS
+}
diff --git a/src/internal/poll/fcntl_libc.go b/src/internal/poll/fcntl_libc.go
new file mode 100644
index 0000000..529b8e1
--- /dev/null
+++ b/src/internal/poll/fcntl_libc.go
@@ -0,0 +1,14 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || darwin || (openbsd && !mips64) || solaris
+
+package poll
+
+import _ "unsafe" // for go:linkname
+
+// Implemented in the syscall package.
+//
+//go:linkname fcntl syscall.fcntl
+func fcntl(fd int, cmd int, arg int) (int, error)
diff --git a/src/internal/poll/fcntl_syscall.go b/src/internal/poll/fcntl_syscall.go
new file mode 100644
index 0000000..bbfc8a8
--- /dev/null
+++ b/src/internal/poll/fcntl_syscall.go
@@ -0,0 +1,20 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || (openbsd && mips64)
+
+package poll
+
+import (
+	"internal/syscall/unix"
+	"syscall"
+)
+
+func fcntl(fd int, cmd int, arg int) (int, error) {
+	r, _, e := syscall.Syscall(unix.FcntlSyscall, uintptr(fd), uintptr(cmd), uintptr(arg))
+	if e != 0 {
+		return int(r), syscall.Errno(e)
+	}
+	return int(r), nil
+}
diff --git a/src/internal/poll/fd.go b/src/internal/poll/fd.go
new file mode 100644
index 0000000..ef61d0c
--- /dev/null
+++ b/src/internal/poll/fd.go
@@ -0,0 +1,83 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package poll supports non-blocking I/O on file descriptors with polling.
+// This supports I/O operations that block only a goroutine, not a thread.
+// This is used by the net and os packages.
+// It uses a poller built into the runtime, with support from the
+// runtime scheduler.
+package poll
+
+import (
+	"errors"
+)
+
+// errNetClosing is the type of the variable ErrNetClosing.
+// This is used to implement the net.Error interface.
+type errNetClosing struct{}
+
+// Error returns the error message for ErrNetClosing.
+// Keep this string consistent because of issue #4373:
+// since historically programs have not been able to detect
+// this error, they look for the string.
+func (e errNetClosing) Error() string { return "use of closed network connection" }
+
+func (e errNetClosing) Timeout() bool   { return false }
+func (e errNetClosing) Temporary() bool { return false }
+
+// ErrNetClosing is returned when a network descriptor is used after
+// it has been closed.
+var ErrNetClosing = errNetClosing{}
+
+// ErrFileClosing is returned when a file descriptor is used after it
+// has been closed.
+var ErrFileClosing = errors.New("use of closed file")
+
+// ErrNoDeadline is returned when a request is made to set a deadline
+// on a file type that does not use the poller.
+var ErrNoDeadline = errors.New("file type does not support deadline")
+
+// Return the appropriate closing error based on isFile.
+func errClosing(isFile bool) error {
+	if isFile {
+		return ErrFileClosing
+	}
+	return ErrNetClosing
+}
+
+// ErrDeadlineExceeded is returned for an expired deadline.
+// This is exported by the os package as os.ErrDeadlineExceeded.
+var ErrDeadlineExceeded error = &DeadlineExceededError{}
+
+// DeadlineExceededError is returned for an expired deadline.
+type DeadlineExceededError struct{}
+
+// Implement the net.Error interface.
+// The string is "i/o timeout" because that is what was returned
+// by earlier Go versions. Changing it may break programs that
+// match on error strings.
+func (e *DeadlineExceededError) Error() string   { return "i/o timeout" }
+func (e *DeadlineExceededError) Timeout() bool   { return true }
+func (e *DeadlineExceededError) Temporary() bool { return true }
+
+// ErrNotPollable is returned when the file or socket is not suitable
+// for event notification.
+var ErrNotPollable = errors.New("not pollable")
+
+// consume removes data from a slice of byte slices, for writev.
+func consume(v *[][]byte, n int64) {
+	for len(*v) > 0 {
+		ln0 := int64(len((*v)[0]))
+		if ln0 > n {
+			(*v)[0] = (*v)[0][n:]
+			return
+		}
+		n -= ln0
+		(*v)[0] = nil
+		*v = (*v)[1:]
+	}
+}
+
+// TestHookDidWritev is a hook for testing writev.
+var TestHookDidWritev = func(wrote int) {}
diff --git a/src/internal/poll/fd_fsync_darwin.go b/src/internal/poll/fd_fsync_darwin.go
new file mode 100644
index 0000000..48e7596
--- /dev/null
+++ b/src/internal/poll/fd_fsync_darwin.go
@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// Fsync invokes SYS_FCNTL with SYS_FULLFSYNC because
+// on OS X, SYS_FSYNC doesn't fully flush contents to disk.
+// See Issue #26650 as well as the man page for fsync on OS X.
+func (fd *FD) Fsync() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		_, err := fcntl(fd.Sysfd, syscall.F_FULLFSYNC, 0)
+		return err
+	})
+}
diff --git a/src/internal/poll/fd_fsync_posix.go b/src/internal/poll/fd_fsync_posix.go
new file mode 100644
index 0000000..6f17019
--- /dev/null
+++ b/src/internal/poll/fd_fsync_posix.go
@@ -0,0 +1,20 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || dragonfly || freebsd || (js && wasm) || linux || netbsd || openbsd || solaris
+
+package poll
+
+import "syscall"
+
+// Fsync wraps syscall.Fsync.
+func (fd *FD) Fsync() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Fsync(fd.Sysfd)
+	})
+}
diff --git a/src/internal/poll/fd_fsync_windows.go b/src/internal/poll/fd_fsync_windows.go
new file mode 100644
index 0000000..fb12119
--- /dev/null
+++ b/src/internal/poll/fd_fsync_windows.go
@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// Fsync wraps syscall.Fsync.
+func (fd *FD) Fsync() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Fsync(fd.Sysfd)
+}
diff --git a/src/internal/poll/fd_io_plan9.go b/src/internal/poll/fd_io_plan9.go
new file mode 100644
index 0000000..3205ac8
--- /dev/null
+++ b/src/internal/poll/fd_io_plan9.go
@@ -0,0 +1,92 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"internal/itoa"
+	"runtime"
+	"sync"
+	"syscall"
+)
+
+// asyncIO implements asynchronous cancelable I/O.
+// An asyncIO represents a single asynchronous Read or Write
+// operation. The result is returned on the result channel.
+// The undergoing I/O system call can either complete or be
+// interrupted by a note.
+type asyncIO struct {
+	res chan result
+
+	// mu guards the pid field.
+	mu sync.Mutex
+
+	// pid holds the process id of
+	// the process running the IO operation.
+	pid int
+}
+
+// result is the return value of a Read or Write operation.
+type result struct {
+	n   int
+	err error
+}
+
+// newAsyncIO returns a new asyncIO that performs an I/O
+// operation by calling fn, which must do one and only one
+// interruptible system call.
+func newAsyncIO(fn func([]byte) (int, error), b []byte) *asyncIO {
+	aio := &asyncIO{
+		res: make(chan result, 0),
+	}
+	aio.mu.Lock()
+	go func() {
+		// Lock the current goroutine to its process
+		// and store the pid in io so that Cancel can
+		// interrupt it. We ignore the "hangup" signal,
+		// so the signal does not take down the entire
+		// Go runtime.
+		runtime.LockOSThread()
+		runtime_ignoreHangup()
+		aio.pid = syscall.Getpid()
+		aio.mu.Unlock()
+
+		n, err := fn(b)
+
+		aio.mu.Lock()
+		aio.pid = -1
+		runtime_unignoreHangup()
+		aio.mu.Unlock()
+
+		aio.res <- result{n, err}
+	}()
+	return aio
+}
+
+// Cancel interrupts the I/O operation, causing
+// the Wait function to return.
+func (aio *asyncIO) Cancel() {
+	aio.mu.Lock()
+	defer aio.mu.Unlock()
+	if aio.pid == -1 {
+		return
+	}
+	f, e := syscall.Open("/proc/"+itoa.Itoa(aio.pid)+"/note", syscall.O_WRONLY)
+	if e != nil {
+		return
+	}
+	syscall.Write(f, []byte("hangup"))
+	syscall.Close(f)
+}
+
+// Wait for the I/O operation to complete.
+func (aio *asyncIO) Wait() (int, error) {
+	res := <-aio.res
+	return res.n, res.err
+}
+
+// The following functions, provided by the runtime, are used to
+// ignore and unignore the "hangup" signal received by the process.
+func runtime_ignoreHangup()
+func runtime_unignoreHangup()
diff --git a/src/internal/poll/fd_mutex.go b/src/internal/poll/fd_mutex.go
new file mode 100644
index 0000000..0a8ee6f
--- /dev/null
+++ b/src/internal/poll/fd_mutex.go
@@ -0,0 +1,252 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "sync/atomic"
+
+// fdMutex is a specialized synchronization primitive that manages
+// lifetime of an fd and serializes access to Read, Write and Close
+// methods on FD.
+type fdMutex struct {
+	state uint64
+	rsema uint32
+	wsema uint32
+}
+
+// fdMutex.state is organized as follows:
+// 1 bit - whether FD is closed, if set all subsequent lock operations will fail.
+// 1 bit - lock for read operations.
+// 1 bit - lock for write operations.
+// 20 bits - total number of references (read+write+misc).
+// 20 bits - number of outstanding read waiters.
+// 20 bits - number of outstanding write waiters.
+const (
+	mutexClosed  = 1 << 0
+	mutexRLock   = 1 << 1
+	mutexWLock   = 1 << 2
+	mutexRef     = 1 << 3
+	mutexRefMask = (1<<20 - 1) << 3
+	mutexRWait   = 1 << 23
+	mutexRMask   = (1<<20 - 1) << 23
+	mutexWWait   = 1 << 43
+	mutexWMask   = (1<<20 - 1) << 43
+)
+
+const overflowMsg = "too many concurrent operations on a single file or socket (max 1048575)"
+
+// Read operations must do rwlock(true)/rwunlock(true).
+//
+// Write operations must do rwlock(false)/rwunlock(false).
+//
+// Misc operations must do incref/decref.
+// Misc operations include functions like setsockopt and setDeadline.
+// They need to use incref/decref to ensure that they operate on the
+// correct fd in presence of a concurrent close call (otherwise fd can
+// be closed under their feet).
+//
+// Close operations must do increfAndClose/decref.
+
+// incref adds a reference to mu.
+// It reports whether mu is available for reading or writing.
+func (mu *fdMutex) incref() bool {
+	for {
+		old := atomic.LoadUint64(&mu.state)
+		if old&mutexClosed != 0 {
+			return false
+		}
+		new := old + mutexRef
+		if new&mutexRefMask == 0 {
+			panic(overflowMsg)
+		}
+		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
+			return true
+		}
+	}
+}
+
+// increfAndClose sets the state of mu to closed.
+// It returns false if the file was already closed.
+func (mu *fdMutex) increfAndClose() bool {
+	for {
+		old := atomic.LoadUint64(&mu.state)
+		if old&mutexClosed != 0 {
+			return false
+		}
+		// Mark as closed and acquire a reference.
+		new := (old | mutexClosed) + mutexRef
+		if new&mutexRefMask == 0 {
+			panic(overflowMsg)
+		}
+		// Remove all read and write waiters.
+		new &^= mutexRMask | mutexWMask
+		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
+			// Wake all read and write waiters,
+			// they will observe closed flag after wakeup.
+			for old&mutexRMask != 0 {
+				old -= mutexRWait
+				runtime_Semrelease(&mu.rsema)
+			}
+			for old&mutexWMask != 0 {
+				old -= mutexWWait
+				runtime_Semrelease(&mu.wsema)
+			}
+			return true
+		}
+	}
+}
+
+// decref removes a reference from mu.
+// It reports whether there is no remaining reference.
+func (mu *fdMutex) decref() bool {
+	for {
+		old := atomic.LoadUint64(&mu.state)
+		if old&mutexRefMask == 0 {
+			panic("inconsistent poll.fdMutex")
+		}
+		new := old - mutexRef
+		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
+			return new&(mutexClosed|mutexRefMask) == mutexClosed
+		}
+	}
+}
+
+// lock adds a reference to mu and locks mu.
+// It reports whether mu is available for reading or writing.
+func (mu *fdMutex) rwlock(read bool) bool {
+	var mutexBit, mutexWait, mutexMask uint64
+	var mutexSema *uint32
+	if read {
+		mutexBit = mutexRLock
+		mutexWait = mutexRWait
+		mutexMask = mutexRMask
+		mutexSema = &mu.rsema
+	} else {
+		mutexBit = mutexWLock
+		mutexWait = mutexWWait
+		mutexMask = mutexWMask
+		mutexSema = &mu.wsema
+	}
+	for {
+		old := atomic.LoadUint64(&mu.state)
+		if old&mutexClosed != 0 {
+			return false
+		}
+		var new uint64
+		if old&mutexBit == 0 {
+			// Lock is free, acquire it.
+			new = (old | mutexBit) + mutexRef
+			if new&mutexRefMask == 0 {
+				panic(overflowMsg)
+			}
+		} else {
+			// Wait for lock.
+			new = old + mutexWait
+			if new&mutexMask == 0 {
+				panic(overflowMsg)
+			}
+		}
+		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
+			if old&mutexBit == 0 {
+				return true
+			}
+			runtime_Semacquire(mutexSema)
+			// The signaller has subtracted mutexWait.
+		}
+	}
+}
+
+// unlock removes a reference from mu and unlocks mu.
+// It reports whether there is no remaining reference.
+func (mu *fdMutex) rwunlock(read bool) bool {
+	var mutexBit, mutexWait, mutexMask uint64
+	var mutexSema *uint32
+	if read {
+		mutexBit = mutexRLock
+		mutexWait = mutexRWait
+		mutexMask = mutexRMask
+		mutexSema = &mu.rsema
+	} else {
+		mutexBit = mutexWLock
+		mutexWait = mutexWWait
+		mutexMask = mutexWMask
+		mutexSema = &mu.wsema
+	}
+	for {
+		old := atomic.LoadUint64(&mu.state)
+		if old&mutexBit == 0 || old&mutexRefMask == 0 {
+			panic("inconsistent poll.fdMutex")
+		}
+		// Drop lock, drop reference and wake read waiter if present.
+		new := (old &^ mutexBit) - mutexRef
+		if old&mutexMask != 0 {
+			new -= mutexWait
+		}
+		if atomic.CompareAndSwapUint64(&mu.state, old, new) {
+			if old&mutexMask != 0 {
+				runtime_Semrelease(mutexSema)
+			}
+			return new&(mutexClosed|mutexRefMask) == mutexClosed
+		}
+	}
+}
+
+// Implemented in runtime package.
+func runtime_Semacquire(sema *uint32)
+func runtime_Semrelease(sema *uint32)
+
+// incref adds a reference to fd.
+// It returns an error when fd cannot be used.
+func (fd *FD) incref() error {
+	if !fd.fdmu.incref() {
+		return errClosing(fd.isFile)
+	}
+	return nil
+}
+
+// decref removes a reference from fd.
+// It also closes fd when the state of fd is set to closed and there
+// is no remaining reference.
+func (fd *FD) decref() error {
+	if fd.fdmu.decref() {
+		return fd.destroy()
+	}
+	return nil
+}
+
+// readLock adds a reference to fd and locks fd for reading.
+// It returns an error when fd cannot be used for reading.
+func (fd *FD) readLock() error {
+	if !fd.fdmu.rwlock(true) {
+		return errClosing(fd.isFile)
+	}
+	return nil
+}
+
+// readUnlock removes a reference from fd and unlocks fd for reading.
+// It also closes fd when the state of fd is set to closed and there
+// is no remaining reference.
+func (fd *FD) readUnlock() {
+	if fd.fdmu.rwunlock(true) {
+		fd.destroy()
+	}
+}
+
+// writeLock adds a reference to fd and locks fd for writing.
+// It returns an error when fd cannot be used for writing.
+func (fd *FD) writeLock() error {
+	if !fd.fdmu.rwlock(false) {
+		return errClosing(fd.isFile)
+	}
+	return nil
+}
+
+// writeUnlock removes a reference from fd and unlocks fd for writing.
+// It also closes fd when the state of fd is set to closed and there
+// is no remaining reference.
+func (fd *FD) writeUnlock() {
+	if fd.fdmu.rwunlock(false) {
+		fd.destroy()
+	}
+}
diff --git a/src/internal/poll/fd_mutex_test.go b/src/internal/poll/fd_mutex_test.go
new file mode 100644
index 0000000..62f9531
--- /dev/null
+++ b/src/internal/poll/fd_mutex_test.go
@@ -0,0 +1,222 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	. "internal/poll"
+	"math/rand"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestMutexLock(t *testing.T) {
+	var mu XFDMutex
+
+	if !mu.Incref() {
+		t.Fatal("broken")
+	}
+	if mu.Decref() {
+		t.Fatal("broken")
+	}
+
+	if !mu.RWLock(true) {
+		t.Fatal("broken")
+	}
+	if mu.RWUnlock(true) {
+		t.Fatal("broken")
+	}
+
+	if !mu.RWLock(false) {
+		t.Fatal("broken")
+	}
+	if mu.RWUnlock(false) {
+		t.Fatal("broken")
+	}
+}
+
+func TestMutexClose(t *testing.T) {
+	var mu XFDMutex
+	if !mu.IncrefAndClose() {
+		t.Fatal("broken")
+	}
+
+	if mu.Incref() {
+		t.Fatal("broken")
+	}
+	if mu.RWLock(true) {
+		t.Fatal("broken")
+	}
+	if mu.RWLock(false) {
+		t.Fatal("broken")
+	}
+	if mu.IncrefAndClose() {
+		t.Fatal("broken")
+	}
+}
+
+func TestMutexCloseUnblock(t *testing.T) {
+	c := make(chan bool, 4)
+	var mu XFDMutex
+	mu.RWLock(true)
+	for i := 0; i < 4; i++ {
+		go func() {
+			if mu.RWLock(true) {
+				t.Error("broken")
+				return
+			}
+			c <- true
+		}()
+	}
+	// Concurrent goroutines must not be able to read lock the mutex.
+	time.Sleep(time.Millisecond)
+	select {
+	case <-c:
+		t.Fatal("broken")
+	default:
+	}
+	mu.IncrefAndClose() // Must unblock the readers.
+	for i := 0; i < 4; i++ {
+		select {
+		case <-c:
+		case <-time.After(10 * time.Second):
+			t.Fatal("broken")
+		}
+	}
+	if mu.Decref() {
+		t.Fatal("broken")
+	}
+	if !mu.RWUnlock(true) {
+		t.Fatal("broken")
+	}
+}
+
+func TestMutexPanic(t *testing.T) {
+	ensurePanics := func(f func()) {
+		defer func() {
+			if recover() == nil {
+				t.Fatal("does not panic")
+			}
+		}()
+		f()
+	}
+
+	var mu XFDMutex
+	ensurePanics(func() { mu.Decref() })
+	ensurePanics(func() { mu.RWUnlock(true) })
+	ensurePanics(func() { mu.RWUnlock(false) })
+
+	ensurePanics(func() { mu.Incref(); mu.Decref(); mu.Decref() })
+	ensurePanics(func() { mu.RWLock(true); mu.RWUnlock(true); mu.RWUnlock(true) })
+	ensurePanics(func() { mu.RWLock(false); mu.RWUnlock(false); mu.RWUnlock(false) })
+
+	// ensure that it's still not broken
+	mu.Incref()
+	mu.Decref()
+	mu.RWLock(true)
+	mu.RWUnlock(true)
+	mu.RWLock(false)
+	mu.RWUnlock(false)
+}
+
+func TestMutexOverflowPanic(t *testing.T) {
+	defer func() {
+		r := recover()
+		if r == nil {
+			t.Fatal("did not panic")
+		}
+		msg, ok := r.(string)
+		if !ok {
+			t.Fatalf("unexpected panic type %T", r)
+		}
+		if !strings.Contains(msg, "too many") || strings.Contains(msg, "inconsistent") {
+			t.Fatalf("wrong panic message %q", msg)
+		}
+	}()
+
+	var mu1 XFDMutex
+	for i := 0; i < 1<<21; i++ {
+		mu1.Incref()
+	}
+}
+
+func TestMutexStress(t *testing.T) {
+	P := 8
+	N := int(1e6)
+	if testing.Short() {
+		P = 4
+		N = 1e4
+	}
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P))
+	done := make(chan bool, P)
+	var mu XFDMutex
+	var readState [2]uint64
+	var writeState [2]uint64
+	for p := 0; p < P; p++ {
+		go func() {
+			defer func() {
+				done <- !t.Failed()
+			}()
+			r := rand.New(rand.NewSource(rand.Int63()))
+			for i := 0; i < N; i++ {
+				switch r.Intn(3) {
+				case 0:
+					if !mu.Incref() {
+						t.Error("broken")
+						return
+					}
+					if mu.Decref() {
+						t.Error("broken")
+						return
+					}
+				case 1:
+					if !mu.RWLock(true) {
+						t.Error("broken")
+						return
+					}
+					// Ensure that it provides mutual exclusion for readers.
+					if readState[0] != readState[1] {
+						t.Error("broken")
+						return
+					}
+					readState[0]++
+					readState[1]++
+					if mu.RWUnlock(true) {
+						t.Error("broken")
+						return
+					}
+				case 2:
+					if !mu.RWLock(false) {
+						t.Error("broken")
+						return
+					}
+					// Ensure that it provides mutual exclusion for writers.
+					if writeState[0] != writeState[1] {
+						t.Error("broken")
+						return
+					}
+					writeState[0]++
+					writeState[1]++
+					if mu.RWUnlock(false) {
+						t.Error("broken")
+						return
+					}
+				}
+			}
+		}()
+	}
+	for p := 0; p < P; p++ {
+		if !<-done {
+			t.FailNow()
+		}
+	}
+	if !mu.IncrefAndClose() {
+		t.Fatal("broken")
+	}
+	if !mu.Decref() {
+		t.Fatal("broken")
+	}
+}
diff --git a/src/internal/poll/fd_opendir_darwin.go b/src/internal/poll/fd_opendir_darwin.go
new file mode 100644
index 0000000..3ae2dc8
--- /dev/null
+++ b/src/internal/poll/fd_opendir_darwin.go
@@ -0,0 +1,39 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"syscall"
+	_ "unsafe" // for go:linkname
+)
+
+// OpenDir returns a pointer to a DIR structure suitable for
+// ReadDir. In case of an error, the name of the failed
+// syscall is returned along with a syscall.Errno.
+func (fd *FD) OpenDir() (uintptr, string, error) {
+	// fdopendir(3) takes control of the file descriptor,
+	// so use a dup.
+	fd2, call, err := fd.Dup()
+	if err != nil {
+		return 0, call, err
+	}
+	var dir uintptr
+	for {
+		dir, err = fdopendir(fd2)
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	if err != nil {
+		syscall.Close(fd2)
+		return 0, "fdopendir", err
+	}
+	return dir, "", nil
+}
+
+// Implemented in syscall/syscall_darwin.go.
+//
+//go:linkname fdopendir syscall.fdopendir
+func fdopendir(fd int) (dir uintptr, err error)
diff --git a/src/internal/poll/fd_plan9.go b/src/internal/poll/fd_plan9.go
new file mode 100644
index 0000000..0fdf4f6
--- /dev/null
+++ b/src/internal/poll/fd_plan9.go
@@ -0,0 +1,227 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"errors"
+	"io"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+type FD struct {
+	// Lock sysfd and serialize access to Read and Write methods.
+	fdmu fdMutex
+
+	Destroy func()
+
+	// deadlines
+	rmu       sync.Mutex
+	wmu       sync.Mutex
+	raio      *asyncIO
+	waio      *asyncIO
+	rtimer    *time.Timer
+	wtimer    *time.Timer
+	rtimedout atomic.Bool // set true when read deadline has been reached
+	wtimedout atomic.Bool // set true when write deadline has been reached
+
+	// Whether this is a normal file.
+	// On Plan 9 we do not use this package for ordinary files,
+	// so this is always false, but the field is present because
+	// shared code in fd_mutex.go checks it.
+	isFile bool
+}
+
+// We need this to close out a file descriptor when it is unlocked,
+// but the real implementation has to live in the net package because
+// it uses os.File's.
+func (fd *FD) destroy() error {
+	if fd.Destroy != nil {
+		fd.Destroy()
+	}
+	return nil
+}
+
+// Close handles the locking for closing an FD. The real operation
+// is in the net package.
+func (fd *FD) Close() error {
+	if !fd.fdmu.increfAndClose() {
+		return errClosing(fd.isFile)
+	}
+	return nil
+}
+
+// Read implements io.Reader.
+func (fd *FD) Read(fn func([]byte) (int, error), b []byte) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if len(b) == 0 {
+		return 0, nil
+	}
+	fd.rmu.Lock()
+	if fd.rtimedout.Load() {
+		fd.rmu.Unlock()
+		return 0, ErrDeadlineExceeded
+	}
+	fd.raio = newAsyncIO(fn, b)
+	fd.rmu.Unlock()
+	n, err := fd.raio.Wait()
+	fd.raio = nil
+	if isHangup(err) {
+		err = io.EOF
+	}
+	if isInterrupted(err) {
+		err = ErrDeadlineExceeded
+	}
+	return n, err
+}
+
+// Write implements io.Writer.
+func (fd *FD) Write(fn func([]byte) (int, error), b []byte) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	fd.wmu.Lock()
+	if fd.wtimedout.Load() {
+		fd.wmu.Unlock()
+		return 0, ErrDeadlineExceeded
+	}
+	fd.waio = newAsyncIO(fn, b)
+	fd.wmu.Unlock()
+	n, err := fd.waio.Wait()
+	fd.waio = nil
+	if isInterrupted(err) {
+		err = ErrDeadlineExceeded
+	}
+	return n, err
+}
+
+// SetDeadline sets the read and write deadlines associated with fd.
+func (fd *FD) SetDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r'+'w')
+}
+
+// SetReadDeadline sets the read deadline associated with fd.
+func (fd *FD) SetReadDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r')
+}
+
+// SetWriteDeadline sets the write deadline associated with fd.
+func (fd *FD) SetWriteDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'w')
+}
+
+func setDeadlineImpl(fd *FD, t time.Time, mode int) error {
+	d := t.Sub(time.Now())
+	if mode == 'r' || mode == 'r'+'w' {
+		fd.rmu.Lock()
+		defer fd.rmu.Unlock()
+		fd.rtimedout.Store(false)
+	}
+	if mode == 'w' || mode == 'r'+'w' {
+		fd.wmu.Lock()
+		defer fd.wmu.Unlock()
+		fd.wtimedout.Store(false)
+	}
+	if t.IsZero() || d < 0 {
+		// Stop timer
+		if mode == 'r' || mode == 'r'+'w' {
+			if fd.rtimer != nil {
+				fd.rtimer.Stop()
+			}
+			fd.rtimer = nil
+		}
+		if mode == 'w' || mode == 'r'+'w' {
+			if fd.wtimer != nil {
+				fd.wtimer.Stop()
+			}
+			fd.wtimer = nil
+		}
+	} else {
+		// Interrupt I/O operation once timer has expired
+		if mode == 'r' || mode == 'r'+'w' {
+			fd.rtimer = time.AfterFunc(d, func() {
+				fd.rmu.Lock()
+				fd.rtimedout.Store(true)
+				if fd.raio != nil {
+					fd.raio.Cancel()
+				}
+				fd.rmu.Unlock()
+			})
+		}
+		if mode == 'w' || mode == 'r'+'w' {
+			fd.wtimer = time.AfterFunc(d, func() {
+				fd.wmu.Lock()
+				fd.wtimedout.Store(true)
+				if fd.waio != nil {
+					fd.waio.Cancel()
+				}
+				fd.wmu.Unlock()
+			})
+		}
+	}
+	if !t.IsZero() && d < 0 {
+		// Interrupt current I/O operation
+		if mode == 'r' || mode == 'r'+'w' {
+			fd.rtimedout.Store(true)
+			if fd.raio != nil {
+				fd.raio.Cancel()
+			}
+		}
+		if mode == 'w' || mode == 'r'+'w' {
+			fd.wtimedout.Store(true)
+			if fd.waio != nil {
+				fd.waio.Cancel()
+			}
+		}
+	}
+	return nil
+}
+
+// On Plan 9 only, expose the locking for the net code.
+
+// ReadLock wraps FD.readLock.
+func (fd *FD) ReadLock() error {
+	return fd.readLock()
+}
+
+// ReadUnlock wraps FD.readUnlock.
+func (fd *FD) ReadUnlock() {
+	fd.readUnlock()
+}
+
+func isHangup(err error) bool {
+	return err != nil && stringsHasSuffix(err.Error(), "Hangup")
+}
+
+func isInterrupted(err error) bool {
+	return err != nil && stringsHasSuffix(err.Error(), "interrupted")
+}
+
+// IsPollDescriptor reports whether fd is the descriptor being used by the poller.
+// This is only used for testing.
+func IsPollDescriptor(fd uintptr) bool {
+	return false
+}
+
+// RawControl invokes the user-defined function f for a non-IO
+// operation.
+func (fd *FD) RawControl(f func(uintptr)) error {
+	return errors.New("not implemented")
+}
+
+// RawRead invokes the user-defined function f for a read operation.
+func (fd *FD) RawRead(f func(uintptr) bool) error {
+	return errors.New("not implemented")
+}
+
+// RawWrite invokes the user-defined function f for a write operation.
+func (fd *FD) RawWrite(f func(uintptr) bool) error {
+	return errors.New("not implemented")
+}
diff --git a/src/internal/poll/fd_poll_js.go b/src/internal/poll/fd_poll_js.go
new file mode 100644
index 0000000..84bfcae
--- /dev/null
+++ b/src/internal/poll/fd_poll_js.go
@@ -0,0 +1,99 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js && wasm
+
+package poll
+
+import (
+	"syscall"
+	"time"
+)
+
+type pollDesc struct {
+	fd      *FD
+	closing bool
+}
+
+func (pd *pollDesc) init(fd *FD) error { pd.fd = fd; return nil }
+
+func (pd *pollDesc) close() {}
+
+func (pd *pollDesc) evict() {
+	pd.closing = true
+	if pd.fd != nil {
+		syscall.StopIO(pd.fd.Sysfd)
+	}
+}
+
+func (pd *pollDesc) prepare(mode int, isFile bool) error {
+	if pd.closing {
+		return errClosing(isFile)
+	}
+	return nil
+}
+
+func (pd *pollDesc) prepareRead(isFile bool) error { return pd.prepare('r', isFile) }
+
+func (pd *pollDesc) prepareWrite(isFile bool) error { return pd.prepare('w', isFile) }
+
+func (pd *pollDesc) wait(mode int, isFile bool) error {
+	if pd.closing {
+		return errClosing(isFile)
+	}
+	if isFile { // TODO(neelance): wasm: Use callbacks from JS to block until the read/write finished.
+		return nil
+	}
+	return ErrDeadlineExceeded
+}
+
+func (pd *pollDesc) waitRead(isFile bool) error { return pd.wait('r', isFile) }
+
+func (pd *pollDesc) waitWrite(isFile bool) error { return pd.wait('w', isFile) }
+
+func (pd *pollDesc) waitCanceled(mode int) {}
+
+func (pd *pollDesc) pollable() bool { return true }
+
+// SetDeadline sets the read and write deadlines associated with fd.
+func (fd *FD) SetDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r'+'w')
+}
+
+// SetReadDeadline sets the read deadline associated with fd.
+func (fd *FD) SetReadDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r')
+}
+
+// SetWriteDeadline sets the write deadline associated with fd.
+func (fd *FD) SetWriteDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'w')
+}
+
+func setDeadlineImpl(fd *FD, t time.Time, mode int) error {
+	d := t.UnixNano()
+	if t.IsZero() {
+		d = 0
+	}
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	switch mode {
+	case 'r':
+		syscall.SetReadDeadline(fd.Sysfd, d)
+	case 'w':
+		syscall.SetWriteDeadline(fd.Sysfd, d)
+	case 'r' + 'w':
+		syscall.SetReadDeadline(fd.Sysfd, d)
+		syscall.SetWriteDeadline(fd.Sysfd, d)
+	}
+	fd.decref()
+	return nil
+}
+
+// IsPollDescriptor reports whether fd is the descriptor being used by the poller.
+// This is only used for testing.
+func IsPollDescriptor(fd uintptr) bool {
+	return false
+}
diff --git a/src/internal/poll/fd_poll_runtime.go b/src/internal/poll/fd_poll_runtime.go
new file mode 100644
index 0000000..0a2e76d
--- /dev/null
+++ b/src/internal/poll/fd_poll_runtime.go
@@ -0,0 +1,169 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package poll
+
+import (
+	"errors"
+	"sync"
+	"syscall"
+	"time"
+	_ "unsafe" // for go:linkname
+)
+
+// runtimeNano returns the current value of the runtime clock in nanoseconds.
+//
+//go:linkname runtimeNano runtime.nanotime
+func runtimeNano() int64
+
+func runtime_pollServerInit()
+func runtime_pollOpen(fd uintptr) (uintptr, int)
+func runtime_pollClose(ctx uintptr)
+func runtime_pollWait(ctx uintptr, mode int) int
+func runtime_pollWaitCanceled(ctx uintptr, mode int)
+func runtime_pollReset(ctx uintptr, mode int) int
+func runtime_pollSetDeadline(ctx uintptr, d int64, mode int)
+func runtime_pollUnblock(ctx uintptr)
+func runtime_isPollServerDescriptor(fd uintptr) bool
+
+type pollDesc struct {
+	runtimeCtx uintptr
+}
+
+var serverInit sync.Once
+
+func (pd *pollDesc) init(fd *FD) error {
+	serverInit.Do(runtime_pollServerInit)
+	ctx, errno := runtime_pollOpen(uintptr(fd.Sysfd))
+	if errno != 0 {
+		return errnoErr(syscall.Errno(errno))
+	}
+	pd.runtimeCtx = ctx
+	return nil
+}
+
+func (pd *pollDesc) close() {
+	if pd.runtimeCtx == 0 {
+		return
+	}
+	runtime_pollClose(pd.runtimeCtx)
+	pd.runtimeCtx = 0
+}
+
+// Evict evicts fd from the pending list, unblocking any I/O running on fd.
+func (pd *pollDesc) evict() {
+	if pd.runtimeCtx == 0 {
+		return
+	}
+	runtime_pollUnblock(pd.runtimeCtx)
+}
+
+func (pd *pollDesc) prepare(mode int, isFile bool) error {
+	if pd.runtimeCtx == 0 {
+		return nil
+	}
+	res := runtime_pollReset(pd.runtimeCtx, mode)
+	return convertErr(res, isFile)
+}
+
+func (pd *pollDesc) prepareRead(isFile bool) error {
+	return pd.prepare('r', isFile)
+}
+
+func (pd *pollDesc) prepareWrite(isFile bool) error {
+	return pd.prepare('w', isFile)
+}
+
+func (pd *pollDesc) wait(mode int, isFile bool) error {
+	if pd.runtimeCtx == 0 {
+		return errors.New("waiting for unsupported file type")
+	}
+	res := runtime_pollWait(pd.runtimeCtx, mode)
+	return convertErr(res, isFile)
+}
+
+func (pd *pollDesc) waitRead(isFile bool) error {
+	return pd.wait('r', isFile)
+}
+
+func (pd *pollDesc) waitWrite(isFile bool) error {
+	return pd.wait('w', isFile)
+}
+
+func (pd *pollDesc) waitCanceled(mode int) {
+	if pd.runtimeCtx == 0 {
+		return
+	}
+	runtime_pollWaitCanceled(pd.runtimeCtx, mode)
+}
+
+func (pd *pollDesc) pollable() bool {
+	return pd.runtimeCtx != 0
+}
+
+// Error values returned by runtime_pollReset and runtime_pollWait.
+// These must match the values in runtime/netpoll.go.
+const (
+	pollNoError        = 0
+	pollErrClosing     = 1
+	pollErrTimeout     = 2
+	pollErrNotPollable = 3
+)
+
+func convertErr(res int, isFile bool) error {
+	switch res {
+	case pollNoError:
+		return nil
+	case pollErrClosing:
+		return errClosing(isFile)
+	case pollErrTimeout:
+		return ErrDeadlineExceeded
+	case pollErrNotPollable:
+		return ErrNotPollable
+	}
+	println("unreachable: ", res)
+	panic("unreachable")
+}
+
+// SetDeadline sets the read and write deadlines associated with fd.
+func (fd *FD) SetDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r'+'w')
+}
+
+// SetReadDeadline sets the read deadline associated with fd.
+func (fd *FD) SetReadDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'r')
+}
+
+// SetWriteDeadline sets the write deadline associated with fd.
+func (fd *FD) SetWriteDeadline(t time.Time) error {
+	return setDeadlineImpl(fd, t, 'w')
+}
+
+func setDeadlineImpl(fd *FD, t time.Time, mode int) error {
+	var d int64
+	if !t.IsZero() {
+		d = int64(time.Until(t))
+		if d == 0 {
+			d = -1 // don't confuse deadline right now with no deadline
+		}
+	}
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	if fd.pd.runtimeCtx == 0 {
+		return ErrNoDeadline
+	}
+	runtime_pollSetDeadline(fd.pd.runtimeCtx, d, mode)
+	return nil
+}
+
+// IsPollDescriptor reports whether fd is the descriptor being used by the poller.
+// This is only used for testing.
+func IsPollDescriptor(fd uintptr) bool {
+	return runtime_isPollServerDescriptor(fd)
+}
diff --git a/src/internal/poll/fd_posix.go b/src/internal/poll/fd_posix.go
new file mode 100644
index 0000000..778fe1e
--- /dev/null
+++ b/src/internal/poll/fd_posix.go
@@ -0,0 +1,79 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || (js && wasm) || windows
+
+package poll
+
+import (
+	"io"
+	"syscall"
+)
+
+// eofError returns io.EOF when fd is available for reading end of
+// file.
+func (fd *FD) eofError(n int, err error) error {
+	if n == 0 && err == nil && fd.ZeroReadIsEOF {
+		return io.EOF
+	}
+	return err
+}
+
+// Shutdown wraps syscall.Shutdown.
+func (fd *FD) Shutdown(how int) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Shutdown(fd.Sysfd, how)
+}
+
+// Fchown wraps syscall.Fchown.
+func (fd *FD) Fchown(uid, gid int) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Fchown(fd.Sysfd, uid, gid)
+	})
+}
+
+// Ftruncate wraps syscall.Ftruncate.
+func (fd *FD) Ftruncate(size int64) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Ftruncate(fd.Sysfd, size)
+	})
+}
+
+// RawControl invokes the user-defined function f for a non-IO
+// operation.
+func (fd *FD) RawControl(f func(uintptr)) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	f(uintptr(fd.Sysfd))
+	return nil
+}
+
+// ignoringEINTR makes a function call and repeats it if it returns
+// an EINTR error. This appears to be required even though we install all
+// signal handlers with SA_RESTART: see #22838, #38033, #38836, #40846.
+// Also #20400 and #36644 are issues in which a signal handler is
+// installed without setting SA_RESTART. None of these are the common case,
+// but there are enough of them that it seems that we can't avoid
+// an EINTR loop.
+func ignoringEINTR(fn func() error) error {
+	for {
+		err := fn()
+		if err != syscall.EINTR {
+			return err
+		}
+	}
+}
diff --git a/src/internal/poll/fd_posix_test.go b/src/internal/poll/fd_posix_test.go
new file mode 100644
index 0000000..b97e465
--- /dev/null
+++ b/src/internal/poll/fd_posix_test.go
@@ -0,0 +1,43 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package poll_test
+
+import (
+	. "internal/poll"
+	"io"
+	"testing"
+)
+
+var eofErrorTests = []struct {
+	n        int
+	err      error
+	fd       *FD
+	expected error
+}{
+	{100, nil, &FD{ZeroReadIsEOF: true}, nil},
+	{100, io.EOF, &FD{ZeroReadIsEOF: true}, io.EOF},
+	{100, ErrNetClosing, &FD{ZeroReadIsEOF: true}, ErrNetClosing},
+	{0, nil, &FD{ZeroReadIsEOF: true}, io.EOF},
+	{0, io.EOF, &FD{ZeroReadIsEOF: true}, io.EOF},
+	{0, ErrNetClosing, &FD{ZeroReadIsEOF: true}, ErrNetClosing},
+
+	{100, nil, &FD{ZeroReadIsEOF: false}, nil},
+	{100, io.EOF, &FD{ZeroReadIsEOF: false}, io.EOF},
+	{100, ErrNetClosing, &FD{ZeroReadIsEOF: false}, ErrNetClosing},
+	{0, nil, &FD{ZeroReadIsEOF: false}, nil},
+	{0, io.EOF, &FD{ZeroReadIsEOF: false}, io.EOF},
+	{0, ErrNetClosing, &FD{ZeroReadIsEOF: false}, ErrNetClosing},
+}
+
+func TestEOFError(t *testing.T) {
+	for _, tt := range eofErrorTests {
+		actual := tt.fd.EOFError(tt.n, tt.err)
+		if actual != tt.expected {
+			t.Errorf("eofError(%v, %v, %v): expected %v, actual %v", tt.n, tt.err, tt.fd.ZeroReadIsEOF, tt.expected, actual)
+		}
+	}
+}
diff --git a/src/internal/poll/fd_unix.go b/src/internal/poll/fd_unix.go
new file mode 100644
index 0000000..2786064
--- /dev/null
+++ b/src/internal/poll/fd_unix.go
@@ -0,0 +1,799 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || (js && wasm)
+
+package poll
+
+import (
+	"internal/syscall/unix"
+	"io"
+	"sync/atomic"
+	"syscall"
+)
+
+// FD is a file descriptor. The net and os packages use this type as a
+// field of a larger type representing a network connection or OS file.
+type FD struct {
+	// Lock sysfd and serialize access to Read and Write methods.
+	fdmu fdMutex
+
+	// System file descriptor. Immutable until Close.
+	Sysfd int
+
+	// I/O poller.
+	pd pollDesc
+
+	// Writev cache.
+	iovecs *[]syscall.Iovec
+
+	// Semaphore signaled when file is closed.
+	csema uint32
+
+	// Non-zero if this file has been set to blocking mode.
+	isBlocking uint32
+
+	// Whether this is a streaming descriptor, as opposed to a
+	// packet-based descriptor like a UDP socket. Immutable.
+	IsStream bool
+
+	// Whether a zero byte read indicates EOF. This is false for a
+	// message based socket connection.
+	ZeroReadIsEOF bool
+
+	// Whether this is a file rather than a network socket.
+	isFile bool
+}
+
+// Init initializes the FD. The Sysfd field should already be set.
+// This can be called multiple times on a single FD.
+// The net argument is a network name from the net package (e.g., "tcp"),
+// or "file".
+// Set pollable to true if fd should be managed by runtime netpoll.
+func (fd *FD) Init(net string, pollable bool) error {
+	// We don't actually care about the various network types.
+	if net == "file" {
+		fd.isFile = true
+	}
+	if !pollable {
+		fd.isBlocking = 1
+		return nil
+	}
+	err := fd.pd.init(fd)
+	if err != nil {
+		// If we could not initialize the runtime poller,
+		// assume we are using blocking mode.
+		fd.isBlocking = 1
+	}
+	return err
+}
+
+// Destroy closes the file descriptor. This is called when there are
+// no remaining references.
+func (fd *FD) destroy() error {
+	// Poller may want to unregister fd in readiness notification mechanism,
+	// so this must be executed before CloseFunc.
+	fd.pd.close()
+
+	// We don't use ignoringEINTR here because POSIX does not define
+	// whether the descriptor is closed if close returns EINTR.
+	// If the descriptor is indeed closed, using a loop would race
+	// with some other goroutine opening a new descriptor.
+	// (The Linux kernel guarantees that it is closed on an EINTR error.)
+	err := CloseFunc(fd.Sysfd)
+
+	fd.Sysfd = -1
+	runtime_Semrelease(&fd.csema)
+	return err
+}
+
+// Close closes the FD. The underlying file descriptor is closed by the
+// destroy method when there are no remaining references.
+func (fd *FD) Close() error {
+	if !fd.fdmu.increfAndClose() {
+		return errClosing(fd.isFile)
+	}
+
+	// Unblock any I/O.  Once it all unblocks and returns,
+	// so that it cannot be referring to fd.sysfd anymore,
+	// the final decref will close fd.sysfd. This should happen
+	// fairly quickly, since all the I/O is non-blocking, and any
+	// attempts to block in the pollDesc will return errClosing(fd.isFile).
+	fd.pd.evict()
+
+	// The call to decref will call destroy if there are no other
+	// references.
+	err := fd.decref()
+
+	// Wait until the descriptor is closed. If this was the only
+	// reference, it is already closed. Only wait if the file has
+	// not been set to blocking mode, as otherwise any current I/O
+	// may be blocking, and that would block the Close.
+	// No need for an atomic read of isBlocking, increfAndClose means
+	// we have exclusive access to fd.
+	if fd.isBlocking == 0 {
+		runtime_Semacquire(&fd.csema)
+	}
+
+	return err
+}
+
+// SetBlocking puts the file into blocking mode.
+func (fd *FD) SetBlocking() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	// Atomic store so that concurrent calls to SetBlocking
+	// do not cause a race condition. isBlocking only ever goes
+	// from 0 to 1 so there is no real race here.
+	atomic.StoreUint32(&fd.isBlocking, 1)
+	return syscall.SetNonblock(fd.Sysfd, false)
+}
+
+// Darwin and FreeBSD can't read or write 2GB+ files at a time,
+// even on 64-bit systems.
+// The same is true of socket implementations on many systems.
+// See golang.org/issue/7812 and golang.org/issue/16266.
+// Use 1GB instead of, say, 2GB-1, to keep subsequent reads aligned.
+const maxRW = 1 << 30
+
+// Read implements io.Reader.
+func (fd *FD) Read(p []byte) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if len(p) == 0 {
+		// If the caller wanted a zero byte read, return immediately
+		// without trying (but after acquiring the readLock).
+		// Otherwise syscall.Read returns 0, nil which looks like
+		// io.EOF.
+		// TODO(bradfitz): make it wait for readability? (Issue 15735)
+		return 0, nil
+	}
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, err
+	}
+	if fd.IsStream && len(p) > maxRW {
+		p = p[:maxRW]
+	}
+	for {
+		n, err := ignoringEINTRIO(syscall.Read, fd.Sysfd, p)
+		if err != nil {
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, err
+	}
+}
+
+// Pread wraps the pread system call.
+func (fd *FD) Pread(p []byte, off int64) (int, error) {
+	// Call incref, not readLock, because since pread specifies the
+	// offset it is independent from other reads.
+	// Similarly, using the poller doesn't make sense for pread.
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	if fd.IsStream && len(p) > maxRW {
+		p = p[:maxRW]
+	}
+	var (
+		n   int
+		err error
+	)
+	for {
+		n, err = syscall.Pread(fd.Sysfd, p, off)
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	if err != nil {
+		n = 0
+	}
+	fd.decref()
+	err = fd.eofError(n, err)
+	return n, err
+}
+
+// ReadFrom wraps the recvfrom network call.
+func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, nil, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, nil, err
+	}
+	for {
+		n, sa, err := syscall.Recvfrom(fd.Sysfd, p, 0)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, sa, err
+	}
+}
+
+// ReadFromInet4 wraps the recvfrom network call for IPv4.
+func (fd *FD) ReadFromInet4(p []byte, from *syscall.SockaddrInet4) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := unix.RecvfromInet4(fd.Sysfd, p, 0, from)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, err
+	}
+}
+
+// ReadFromInet6 wraps the recvfrom network call for IPv6.
+func (fd *FD) ReadFromInet6(p []byte, from *syscall.SockaddrInet6) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := unix.RecvfromInet6(fd.Sysfd, p, 0, from)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, err
+	}
+}
+
+// ReadMsg wraps the recvmsg network call.
+func (fd *FD) ReadMsg(p []byte, oob []byte, flags int) (int, int, int, syscall.Sockaddr, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, nil, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, 0, 0, nil, err
+	}
+	for {
+		n, oobn, sysflags, sa, err := syscall.Recvmsg(fd.Sysfd, p, oob, flags)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			// TODO(dfc) should n and oobn be set to 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, oobn, sysflags, sa, err
+	}
+}
+
+// ReadMsgInet4 is ReadMsg, but specialized for syscall.SockaddrInet4.
+func (fd *FD) ReadMsgInet4(p []byte, oob []byte, flags int, sa4 *syscall.SockaddrInet4) (int, int, int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, 0, 0, err
+	}
+	for {
+		n, oobn, sysflags, err := unix.RecvmsgInet4(fd.Sysfd, p, oob, flags, sa4)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			// TODO(dfc) should n and oobn be set to 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, oobn, sysflags, err
+	}
+}
+
+// ReadMsgInet6 is ReadMsg, but specialized for syscall.SockaddrInet6.
+func (fd *FD) ReadMsgInet6(p []byte, oob []byte, flags int, sa6 *syscall.SockaddrInet6) (int, int, int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, 0, 0, err
+	}
+	for {
+		n, oobn, sysflags, err := unix.RecvmsgInet6(fd.Sysfd, p, oob, flags, sa6)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			// TODO(dfc) should n and oobn be set to 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, oobn, sysflags, err
+	}
+}
+
+// Write implements io.Writer.
+func (fd *FD) Write(p []byte) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, err
+	}
+	var nn int
+	for {
+		max := len(p)
+		if fd.IsStream && max-nn > maxRW {
+			max = nn + maxRW
+		}
+		n, err := ignoringEINTRIO(syscall.Write, fd.Sysfd, p[nn:max])
+		if n > 0 {
+			nn += n
+		}
+		if nn == len(p) {
+			return nn, err
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return nn, err
+		}
+		if n == 0 {
+			return nn, io.ErrUnexpectedEOF
+		}
+	}
+}
+
+// Pwrite wraps the pwrite system call.
+func (fd *FD) Pwrite(p []byte, off int64) (int, error) {
+	// Call incref, not writeLock, because since pwrite specifies the
+	// offset it is independent from other writes.
+	// Similarly, using the poller doesn't make sense for pwrite.
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	var nn int
+	for {
+		max := len(p)
+		if fd.IsStream && max-nn > maxRW {
+			max = nn + maxRW
+		}
+		n, err := syscall.Pwrite(fd.Sysfd, p[nn:max], off+int64(nn))
+		if err == syscall.EINTR {
+			continue
+		}
+		if n > 0 {
+			nn += n
+		}
+		if nn == len(p) {
+			return nn, err
+		}
+		if err != nil {
+			return nn, err
+		}
+		if n == 0 {
+			return nn, io.ErrUnexpectedEOF
+		}
+	}
+}
+
+// WriteToInet4 wraps the sendto network call for IPv4 addresses.
+func (fd *FD) WriteToInet4(p []byte, sa *syscall.SockaddrInet4) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		err := unix.SendtoInet4(fd.Sysfd, p, 0, sa)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return 0, err
+		}
+		return len(p), nil
+	}
+}
+
+// WriteToInet6 wraps the sendto network call for IPv6 addresses.
+func (fd *FD) WriteToInet6(p []byte, sa *syscall.SockaddrInet6) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		err := unix.SendtoInet6(fd.Sysfd, p, 0, sa)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return 0, err
+		}
+		return len(p), nil
+	}
+}
+
+// WriteTo wraps the sendto network call.
+func (fd *FD) WriteTo(p []byte, sa syscall.Sockaddr) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		err := syscall.Sendto(fd.Sysfd, p, 0, sa)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return 0, err
+		}
+		return len(p), nil
+	}
+}
+
+// WriteMsg wraps the sendmsg network call.
+func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, 0, err
+	}
+	for {
+		n, err := syscall.SendmsgN(fd.Sysfd, p, oob, sa, 0)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return n, 0, err
+		}
+		return n, len(oob), err
+	}
+}
+
+// WriteMsgInet4 is WriteMsg specialized for syscall.SockaddrInet4.
+func (fd *FD) WriteMsgInet4(p []byte, oob []byte, sa *syscall.SockaddrInet4) (int, int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, 0, err
+	}
+	for {
+		n, err := unix.SendmsgNInet4(fd.Sysfd, p, oob, sa, 0)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return n, 0, err
+		}
+		return n, len(oob), err
+	}
+}
+
+// WriteMsgInet6 is WriteMsg specialized for syscall.SockaddrInet6.
+func (fd *FD) WriteMsgInet6(p []byte, oob []byte, sa *syscall.SockaddrInet6) (int, int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, 0, err
+	}
+	for {
+		n, err := unix.SendmsgNInet6(fd.Sysfd, p, oob, sa, 0)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err == syscall.EAGAIN && fd.pd.pollable() {
+			if err = fd.pd.waitWrite(fd.isFile); err == nil {
+				continue
+			}
+		}
+		if err != nil {
+			return n, 0, err
+		}
+		return n, len(oob), err
+	}
+}
+
+// Accept wraps the accept network call.
+func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
+	if err := fd.readLock(); err != nil {
+		return -1, nil, "", err
+	}
+	defer fd.readUnlock()
+
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return -1, nil, "", err
+	}
+	for {
+		s, rsa, errcall, err := accept(fd.Sysfd)
+		if err == nil {
+			return s, rsa, "", err
+		}
+		switch err {
+		case syscall.EINTR:
+			continue
+		case syscall.EAGAIN:
+			if fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		case syscall.ECONNABORTED:
+			// This means that a socket on the listen
+			// queue was closed before we Accept()ed it;
+			// it's a silly error, so try again.
+			continue
+		}
+		return -1, nil, errcall, err
+	}
+}
+
+// Seek wraps syscall.Seek.
+func (fd *FD) Seek(offset int64, whence int) (int64, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	return syscall.Seek(fd.Sysfd, offset, whence)
+}
+
+// ReadDirent wraps syscall.ReadDirent.
+// We treat this like an ordinary system call rather than a call
+// that tries to fill the buffer.
+func (fd *FD) ReadDirent(buf []byte) (int, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	for {
+		n, err := ignoringEINTRIO(syscall.ReadDirent, fd.Sysfd, buf)
+		if err != nil {
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		// Do not call eofError; caller does not expect to see io.EOF.
+		return n, err
+	}
+}
+
+// Fchmod wraps syscall.Fchmod.
+func (fd *FD) Fchmod(mode uint32) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Fchmod(fd.Sysfd, mode)
+	})
+}
+
+// Fchdir wraps syscall.Fchdir.
+func (fd *FD) Fchdir() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Fchdir(fd.Sysfd)
+}
+
+// Fstat wraps syscall.Fstat
+func (fd *FD) Fstat(s *syscall.Stat_t) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Fstat(fd.Sysfd, s)
+	})
+}
+
+// tryDupCloexec indicates whether F_DUPFD_CLOEXEC should be used.
+// If the kernel doesn't support it, this is set to 0.
+var tryDupCloexec = int32(1)
+
+// DupCloseOnExec dups fd and marks it close-on-exec.
+func DupCloseOnExec(fd int) (int, string, error) {
+	if syscall.F_DUPFD_CLOEXEC != 0 && atomic.LoadInt32(&tryDupCloexec) == 1 {
+		r0, e1 := fcntl(fd, syscall.F_DUPFD_CLOEXEC, 0)
+		if e1 == nil {
+			return r0, "", nil
+		}
+		switch e1.(syscall.Errno) {
+		case syscall.EINVAL, syscall.ENOSYS:
+			// Old kernel, or js/wasm (which returns
+			// ENOSYS). Fall back to the portable way from
+			// now on.
+			atomic.StoreInt32(&tryDupCloexec, 0)
+		default:
+			return -1, "fcntl", e1
+		}
+	}
+	return dupCloseOnExecOld(fd)
+}
+
+// dupCloseOnExecOld is the traditional way to dup an fd and
+// set its O_CLOEXEC bit, using two system calls.
+func dupCloseOnExecOld(fd int) (int, string, error) {
+	syscall.ForkLock.RLock()
+	defer syscall.ForkLock.RUnlock()
+	newfd, err := syscall.Dup(fd)
+	if err != nil {
+		return -1, "dup", err
+	}
+	syscall.CloseOnExec(newfd)
+	return newfd, "", nil
+}
+
+// Dup duplicates the file descriptor.
+func (fd *FD) Dup() (int, string, error) {
+	if err := fd.incref(); err != nil {
+		return -1, "", err
+	}
+	defer fd.decref()
+	return DupCloseOnExec(fd.Sysfd)
+}
+
+// On Unix variants only, expose the IO event for the net code.
+
+// WaitWrite waits until data can be read from fd.
+func (fd *FD) WaitWrite() error {
+	return fd.pd.waitWrite(fd.isFile)
+}
+
+// WriteOnce is for testing only. It makes a single write call.
+func (fd *FD) WriteOnce(p []byte) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	return ignoringEINTRIO(syscall.Write, fd.Sysfd, p)
+}
+
+// RawRead invokes the user-defined function f for a read operation.
+func (fd *FD) RawRead(f func(uintptr) bool) error {
+	if err := fd.readLock(); err != nil {
+		return err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return err
+	}
+	for {
+		if f(uintptr(fd.Sysfd)) {
+			return nil
+		}
+		if err := fd.pd.waitRead(fd.isFile); err != nil {
+			return err
+		}
+	}
+}
+
+// RawWrite invokes the user-defined function f for a write operation.
+func (fd *FD) RawWrite(f func(uintptr) bool) error {
+	if err := fd.writeLock(); err != nil {
+		return err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return err
+	}
+	for {
+		if f(uintptr(fd.Sysfd)) {
+			return nil
+		}
+		if err := fd.pd.waitWrite(fd.isFile); err != nil {
+			return err
+		}
+	}
+}
+
+// ignoringEINTRIO is like ignoringEINTR, but just for IO calls.
+func ignoringEINTRIO(fn func(fd int, p []byte) (int, error), fd int, p []byte) (int, error) {
+	for {
+		n, err := fn(fd, p)
+		if err != syscall.EINTR {
+			return n, err
+		}
+	}
+}
diff --git a/src/internal/poll/fd_windows.go b/src/internal/poll/fd_windows.go
new file mode 100644
index 0000000..3a4a74f
--- /dev/null
+++ b/src/internal/poll/fd_windows.go
@@ -0,0 +1,1321 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"errors"
+	"internal/race"
+	"internal/syscall/windows"
+	"io"
+	"sync"
+	"syscall"
+	"unicode/utf16"
+	"unicode/utf8"
+	"unsafe"
+)
+
+var (
+	initErr error
+	ioSync  uint64
+)
+
+// This package uses the SetFileCompletionNotificationModes Windows
+// API to skip calling GetQueuedCompletionStatus if an IO operation
+// completes synchronously. There is a known bug where
+// SetFileCompletionNotificationModes crashes on some systems (see
+// https://support.microsoft.com/kb/2568167 for details).
+
+var useSetFileCompletionNotificationModes bool // determines is SetFileCompletionNotificationModes is present and safe to use
+
+// checkSetFileCompletionNotificationModes verifies that
+// SetFileCompletionNotificationModes Windows API is present
+// on the system and is safe to use.
+// See https://support.microsoft.com/kb/2568167 for details.
+func checkSetFileCompletionNotificationModes() {
+	err := syscall.LoadSetFileCompletionNotificationModes()
+	if err != nil {
+		return
+	}
+	protos := [2]int32{syscall.IPPROTO_TCP, 0}
+	var buf [32]syscall.WSAProtocolInfo
+	len := uint32(unsafe.Sizeof(buf))
+	n, err := syscall.WSAEnumProtocols(&protos[0], &buf[0], &len)
+	if err != nil {
+		return
+	}
+	for i := int32(0); i < n; i++ {
+		if buf[i].ServiceFlags1&syscall.XP1_IFS_HANDLES == 0 {
+			return
+		}
+	}
+	useSetFileCompletionNotificationModes = true
+}
+
+func init() {
+	var d syscall.WSAData
+	e := syscall.WSAStartup(uint32(0x202), &d)
+	if e != nil {
+		initErr = e
+	}
+	checkSetFileCompletionNotificationModes()
+}
+
+// operation contains superset of data necessary to perform all async IO.
+type operation struct {
+	// Used by IOCP interface, it must be first field
+	// of the struct, as our code rely on it.
+	o syscall.Overlapped
+
+	// fields used by runtime.netpoll
+	runtimeCtx uintptr
+	mode       int32
+	errno      int32
+	qty        uint32
+
+	// fields used only by net package
+	fd     *FD
+	buf    syscall.WSABuf
+	msg    windows.WSAMsg
+	sa     syscall.Sockaddr
+	rsa    *syscall.RawSockaddrAny
+	rsan   int32
+	handle syscall.Handle
+	flags  uint32
+	bufs   []syscall.WSABuf
+}
+
+func (o *operation) InitBuf(buf []byte) {
+	o.buf.Len = uint32(len(buf))
+	o.buf.Buf = nil
+	if len(buf) != 0 {
+		o.buf.Buf = &buf[0]
+	}
+}
+
+func (o *operation) InitBufs(buf *[][]byte) {
+	if o.bufs == nil {
+		o.bufs = make([]syscall.WSABuf, 0, len(*buf))
+	} else {
+		o.bufs = o.bufs[:0]
+	}
+	for _, b := range *buf {
+		if len(b) == 0 {
+			o.bufs = append(o.bufs, syscall.WSABuf{})
+			continue
+		}
+		for len(b) > maxRW {
+			o.bufs = append(o.bufs, syscall.WSABuf{Len: maxRW, Buf: &b[0]})
+			b = b[maxRW:]
+		}
+		if len(b) > 0 {
+			o.bufs = append(o.bufs, syscall.WSABuf{Len: uint32(len(b)), Buf: &b[0]})
+		}
+	}
+}
+
+// ClearBufs clears all pointers to Buffers parameter captured
+// by InitBufs, so it can be released by garbage collector.
+func (o *operation) ClearBufs() {
+	for i := range o.bufs {
+		o.bufs[i].Buf = nil
+	}
+	o.bufs = o.bufs[:0]
+}
+
+func (o *operation) InitMsg(p []byte, oob []byte) {
+	o.InitBuf(p)
+	o.msg.Buffers = &o.buf
+	o.msg.BufferCount = 1
+
+	o.msg.Name = nil
+	o.msg.Namelen = 0
+
+	o.msg.Flags = 0
+	o.msg.Control.Len = uint32(len(oob))
+	o.msg.Control.Buf = nil
+	if len(oob) != 0 {
+		o.msg.Control.Buf = &oob[0]
+	}
+}
+
+// execIO executes a single IO operation o. It submits and cancels
+// IO in the current thread for systems where Windows CancelIoEx API
+// is available. Alternatively, it passes the request onto
+// runtime netpoll and waits for completion or cancels request.
+func execIO(o *operation, submit func(o *operation) error) (int, error) {
+	if o.fd.pd.runtimeCtx == 0 {
+		return 0, errors.New("internal error: polling on unsupported descriptor type")
+	}
+
+	fd := o.fd
+	// Notify runtime netpoll about starting IO.
+	err := fd.pd.prepare(int(o.mode), fd.isFile)
+	if err != nil {
+		return 0, err
+	}
+	// Start IO.
+	err = submit(o)
+	switch err {
+	case nil:
+		// IO completed immediately
+		if o.fd.skipSyncNotif {
+			// No completion message will follow, so return immediately.
+			return int(o.qty), nil
+		}
+		// Need to get our completion message anyway.
+	case syscall.ERROR_IO_PENDING:
+		// IO started, and we have to wait for its completion.
+		err = nil
+	default:
+		return 0, err
+	}
+	// Wait for our request to complete.
+	err = fd.pd.wait(int(o.mode), fd.isFile)
+	if err == nil {
+		// All is good. Extract our IO results and return.
+		if o.errno != 0 {
+			err = syscall.Errno(o.errno)
+			// More data available. Return back the size of received data.
+			if err == syscall.ERROR_MORE_DATA || err == windows.WSAEMSGSIZE {
+				return int(o.qty), err
+			}
+			return 0, err
+		}
+		return int(o.qty), nil
+	}
+	// IO is interrupted by "close" or "timeout"
+	netpollErr := err
+	switch netpollErr {
+	case ErrNetClosing, ErrFileClosing, ErrDeadlineExceeded:
+		// will deal with those.
+	default:
+		panic("unexpected runtime.netpoll error: " + netpollErr.Error())
+	}
+	// Cancel our request.
+	err = syscall.CancelIoEx(fd.Sysfd, &o.o)
+	// Assuming ERROR_NOT_FOUND is returned, if IO is completed.
+	if err != nil && err != syscall.ERROR_NOT_FOUND {
+		// TODO(brainman): maybe do something else, but panic.
+		panic(err)
+	}
+	// Wait for cancellation to complete.
+	fd.pd.waitCanceled(int(o.mode))
+	if o.errno != 0 {
+		err = syscall.Errno(o.errno)
+		if err == syscall.ERROR_OPERATION_ABORTED { // IO Canceled
+			err = netpollErr
+		}
+		return 0, err
+	}
+	// We issued a cancellation request. But, it seems, IO operation succeeded
+	// before the cancellation request run. We need to treat the IO operation as
+	// succeeded (the bytes are actually sent/recv from network).
+	return int(o.qty), nil
+}
+
+// FD is a file descriptor. The net and os packages embed this type in
+// a larger type representing a network connection or OS file.
+type FD struct {
+	// Lock sysfd and serialize access to Read and Write methods.
+	fdmu fdMutex
+
+	// System file descriptor. Immutable until Close.
+	Sysfd syscall.Handle
+
+	// Read operation.
+	rop operation
+	// Write operation.
+	wop operation
+
+	// I/O poller.
+	pd pollDesc
+
+	// Used to implement pread/pwrite.
+	l sync.Mutex
+
+	// For console I/O.
+	lastbits       []byte   // first few bytes of the last incomplete rune in last write
+	readuint16     []uint16 // buffer to hold uint16s obtained with ReadConsole
+	readbyte       []byte   // buffer to hold decoding of readuint16 from utf16 to utf8
+	readbyteOffset int      // readbyte[readOffset:] is yet to be consumed with file.Read
+
+	// Semaphore signaled when file is closed.
+	csema uint32
+
+	skipSyncNotif bool
+
+	// Whether this is a streaming descriptor, as opposed to a
+	// packet-based descriptor like a UDP socket.
+	IsStream bool
+
+	// Whether a zero byte read indicates EOF. This is false for a
+	// message based socket connection.
+	ZeroReadIsEOF bool
+
+	// Whether this is a file rather than a network socket.
+	isFile bool
+
+	// The kind of this file.
+	kind fileKind
+}
+
+// fileKind describes the kind of file.
+type fileKind byte
+
+const (
+	kindNet fileKind = iota
+	kindFile
+	kindConsole
+	kindPipe
+)
+
+// logInitFD is set by tests to enable file descriptor initialization logging.
+var logInitFD func(net string, fd *FD, err error)
+
+// Init initializes the FD. The Sysfd field should already be set.
+// This can be called multiple times on a single FD.
+// The net argument is a network name from the net package (e.g., "tcp"),
+// or "file" or "console" or "dir".
+// Set pollable to true if fd should be managed by runtime netpoll.
+func (fd *FD) Init(net string, pollable bool) (string, error) {
+	if initErr != nil {
+		return "", initErr
+	}
+
+	switch net {
+	case "file", "dir":
+		fd.kind = kindFile
+	case "console":
+		fd.kind = kindConsole
+	case "pipe":
+		fd.kind = kindPipe
+	case "tcp", "tcp4", "tcp6",
+		"udp", "udp4", "udp6",
+		"ip", "ip4", "ip6",
+		"unix", "unixgram", "unixpacket":
+		fd.kind = kindNet
+	default:
+		return "", errors.New("internal error: unknown network type " + net)
+	}
+	fd.isFile = fd.kind != kindNet
+
+	var err error
+	if pollable {
+		// Only call init for a network socket.
+		// This means that we don't add files to the runtime poller.
+		// Adding files to the runtime poller can confuse matters
+		// if the user is doing their own overlapped I/O.
+		// See issue #21172.
+		//
+		// In general the code below avoids calling the execIO
+		// function for non-network sockets. If some method does
+		// somehow call execIO, then execIO, and therefore the
+		// calling method, will return an error, because
+		// fd.pd.runtimeCtx will be 0.
+		err = fd.pd.init(fd)
+	}
+	if logInitFD != nil {
+		logInitFD(net, fd, err)
+	}
+	if err != nil {
+		return "", err
+	}
+	if pollable && useSetFileCompletionNotificationModes {
+		// We do not use events, so we can skip them always.
+		flags := uint8(syscall.FILE_SKIP_SET_EVENT_ON_HANDLE)
+		// It's not safe to skip completion notifications for UDP:
+		// https://docs.microsoft.com/en-us/archive/blogs/winserverperformance/designing-applications-for-high-performance-part-iii
+		if net == "tcp" {
+			flags |= syscall.FILE_SKIP_COMPLETION_PORT_ON_SUCCESS
+		}
+		err := syscall.SetFileCompletionNotificationModes(fd.Sysfd, flags)
+		if err == nil && flags&syscall.FILE_SKIP_COMPLETION_PORT_ON_SUCCESS != 0 {
+			fd.skipSyncNotif = true
+		}
+	}
+	// Disable SIO_UDP_CONNRESET behavior.
+	// http://support.microsoft.com/kb/263823
+	switch net {
+	case "udp", "udp4", "udp6":
+		ret := uint32(0)
+		flag := uint32(0)
+		size := uint32(unsafe.Sizeof(flag))
+		err := syscall.WSAIoctl(fd.Sysfd, syscall.SIO_UDP_CONNRESET, (*byte)(unsafe.Pointer(&flag)), size, nil, 0, &ret, nil, 0)
+		if err != nil {
+			return "wsaioctl", err
+		}
+	}
+	fd.rop.mode = 'r'
+	fd.wop.mode = 'w'
+	fd.rop.fd = fd
+	fd.wop.fd = fd
+	fd.rop.runtimeCtx = fd.pd.runtimeCtx
+	fd.wop.runtimeCtx = fd.pd.runtimeCtx
+	return "", nil
+}
+
+func (fd *FD) destroy() error {
+	if fd.Sysfd == syscall.InvalidHandle {
+		return syscall.EINVAL
+	}
+	// Poller may want to unregister fd in readiness notification mechanism,
+	// so this must be executed before fd.CloseFunc.
+	fd.pd.close()
+	var err error
+	switch fd.kind {
+	case kindNet:
+		// The net package uses the CloseFunc variable for testing.
+		err = CloseFunc(fd.Sysfd)
+	default:
+		err = syscall.CloseHandle(fd.Sysfd)
+	}
+	fd.Sysfd = syscall.InvalidHandle
+	runtime_Semrelease(&fd.csema)
+	return err
+}
+
+// Close closes the FD. The underlying file descriptor is closed by
+// the destroy method when there are no remaining references.
+func (fd *FD) Close() error {
+	if !fd.fdmu.increfAndClose() {
+		return errClosing(fd.isFile)
+	}
+	if fd.kind == kindPipe {
+		syscall.CancelIoEx(fd.Sysfd, nil)
+	}
+	// unblock pending reader and writer
+	fd.pd.evict()
+	err := fd.decref()
+	// Wait until the descriptor is closed. If this was the only
+	// reference, it is already closed.
+	runtime_Semacquire(&fd.csema)
+	return err
+}
+
+// Windows ReadFile and WSARecv use DWORD (uint32) parameter to pass buffer length.
+// This prevents us reading blocks larger than 4GB.
+// See golang.org/issue/26923.
+const maxRW = 1 << 30 // 1GB is large enough and keeps subsequent reads aligned
+
+// Read implements io.Reader.
+func (fd *FD) Read(buf []byte) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+
+	if len(buf) > maxRW {
+		buf = buf[:maxRW]
+	}
+
+	var n int
+	var err error
+	if fd.isFile {
+		fd.l.Lock()
+		defer fd.l.Unlock()
+		switch fd.kind {
+		case kindConsole:
+			n, err = fd.readConsole(buf)
+		default:
+			n, err = syscall.Read(fd.Sysfd, buf)
+			if fd.kind == kindPipe && err == syscall.ERROR_OPERATION_ABORTED {
+				// Close uses CancelIoEx to interrupt concurrent I/O for pipes.
+				// If the fd is a pipe and the Read was interrupted by CancelIoEx,
+				// we assume it is interrupted by Close.
+				err = ErrFileClosing
+			}
+		}
+		if err != nil {
+			n = 0
+		}
+	} else {
+		o := &fd.rop
+		o.InitBuf(buf)
+		n, err = execIO(o, func(o *operation) error {
+			return syscall.WSARecv(o.fd.Sysfd, &o.buf, 1, &o.qty, &o.flags, &o.o, nil)
+		})
+		if race.Enabled {
+			race.Acquire(unsafe.Pointer(&ioSync))
+		}
+	}
+	if len(buf) != 0 {
+		err = fd.eofError(n, err)
+	}
+	return n, err
+}
+
+var ReadConsole = syscall.ReadConsole // changed for testing
+
+// readConsole reads utf16 characters from console File,
+// encodes them into utf8 and stores them in buffer b.
+// It returns the number of utf8 bytes read and an error, if any.
+func (fd *FD) readConsole(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+
+	if fd.readuint16 == nil {
+		// Note: syscall.ReadConsole fails for very large buffers.
+		// The limit is somewhere around (but not exactly) 16384.
+		// Stay well below.
+		fd.readuint16 = make([]uint16, 0, 10000)
+		fd.readbyte = make([]byte, 0, 4*cap(fd.readuint16))
+	}
+
+	for fd.readbyteOffset >= len(fd.readbyte) {
+		n := cap(fd.readuint16) - len(fd.readuint16)
+		if n > len(b) {
+			n = len(b)
+		}
+		var nw uint32
+		err := ReadConsole(fd.Sysfd, &fd.readuint16[:len(fd.readuint16)+1][len(fd.readuint16)], uint32(n), &nw, nil)
+		if err != nil {
+			return 0, err
+		}
+		uint16s := fd.readuint16[:len(fd.readuint16)+int(nw)]
+		fd.readuint16 = fd.readuint16[:0]
+		buf := fd.readbyte[:0]
+		for i := 0; i < len(uint16s); i++ {
+			r := rune(uint16s[i])
+			if utf16.IsSurrogate(r) {
+				if i+1 == len(uint16s) {
+					if nw > 0 {
+						// Save half surrogate pair for next time.
+						fd.readuint16 = fd.readuint16[:1]
+						fd.readuint16[0] = uint16(r)
+						break
+					}
+					r = utf8.RuneError
+				} else {
+					r = utf16.DecodeRune(r, rune(uint16s[i+1]))
+					if r != utf8.RuneError {
+						i++
+					}
+				}
+			}
+			buf = utf8.AppendRune(buf, r)
+		}
+		fd.readbyte = buf
+		fd.readbyteOffset = 0
+		if nw == 0 {
+			break
+		}
+	}
+
+	src := fd.readbyte[fd.readbyteOffset:]
+	var i int
+	for i = 0; i < len(src) && i < len(b); i++ {
+		x := src[i]
+		if x == 0x1A { // Ctrl-Z
+			if i == 0 {
+				fd.readbyteOffset++
+			}
+			break
+		}
+		b[i] = x
+	}
+	fd.readbyteOffset += i
+	return i, nil
+}
+
+// Pread emulates the Unix pread system call.
+func (fd *FD) Pread(b []byte, off int64) (int, error) {
+	// Call incref, not readLock, because since pread specifies the
+	// offset it is independent from other reads.
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+
+	if len(b) > maxRW {
+		b = b[:maxRW]
+	}
+
+	fd.l.Lock()
+	defer fd.l.Unlock()
+	curoffset, e := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+	if e != nil {
+		return 0, e
+	}
+	defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+	o := syscall.Overlapped{
+		OffsetHigh: uint32(off >> 32),
+		Offset:     uint32(off),
+	}
+	var done uint32
+	e = syscall.ReadFile(fd.Sysfd, b, &done, &o)
+	if e != nil {
+		done = 0
+		if e == syscall.ERROR_HANDLE_EOF {
+			e = io.EOF
+		}
+	}
+	if len(b) != 0 {
+		e = fd.eofError(int(done), e)
+	}
+	return int(done), e
+}
+
+// ReadFrom wraps the recvfrom network call.
+func (fd *FD) ReadFrom(buf []byte) (int, syscall.Sockaddr, error) {
+	if len(buf) == 0 {
+		return 0, nil, nil
+	}
+	if len(buf) > maxRW {
+		buf = buf[:maxRW]
+	}
+	if err := fd.readLock(); err != nil {
+		return 0, nil, err
+	}
+	defer fd.readUnlock()
+	o := &fd.rop
+	o.InitBuf(buf)
+	n, err := execIO(o, func(o *operation) error {
+		if o.rsa == nil {
+			o.rsa = new(syscall.RawSockaddrAny)
+		}
+		o.rsan = int32(unsafe.Sizeof(*o.rsa))
+		return syscall.WSARecvFrom(o.fd.Sysfd, &o.buf, 1, &o.qty, &o.flags, o.rsa, &o.rsan, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	if err != nil {
+		return n, nil, err
+	}
+	sa, _ := o.rsa.Sockaddr()
+	return n, sa, nil
+}
+
+// ReadFromInet4 wraps the recvfrom network call for IPv4.
+func (fd *FD) ReadFromInet4(buf []byte, sa4 *syscall.SockaddrInet4) (int, error) {
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	if len(buf) > maxRW {
+		buf = buf[:maxRW]
+	}
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	o := &fd.rop
+	o.InitBuf(buf)
+	n, err := execIO(o, func(o *operation) error {
+		if o.rsa == nil {
+			o.rsa = new(syscall.RawSockaddrAny)
+		}
+		o.rsan = int32(unsafe.Sizeof(*o.rsa))
+		return syscall.WSARecvFrom(o.fd.Sysfd, &o.buf, 1, &o.qty, &o.flags, o.rsa, &o.rsan, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	if err != nil {
+		return n, err
+	}
+	rawToSockaddrInet4(o.rsa, sa4)
+	return n, err
+}
+
+// ReadFromInet6 wraps the recvfrom network call for IPv6.
+func (fd *FD) ReadFromInet6(buf []byte, sa6 *syscall.SockaddrInet6) (int, error) {
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	if len(buf) > maxRW {
+		buf = buf[:maxRW]
+	}
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	o := &fd.rop
+	o.InitBuf(buf)
+	n, err := execIO(o, func(o *operation) error {
+		if o.rsa == nil {
+			o.rsa = new(syscall.RawSockaddrAny)
+		}
+		o.rsan = int32(unsafe.Sizeof(*o.rsa))
+		return syscall.WSARecvFrom(o.fd.Sysfd, &o.buf, 1, &o.qty, &o.flags, o.rsa, &o.rsan, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	if err != nil {
+		return n, err
+	}
+	rawToSockaddrInet6(o.rsa, sa6)
+	return n, err
+}
+
+// Write implements io.Writer.
+func (fd *FD) Write(buf []byte) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if fd.isFile {
+		fd.l.Lock()
+		defer fd.l.Unlock()
+	}
+
+	ntotal := 0
+	for len(buf) > 0 {
+		b := buf
+		if len(b) > maxRW {
+			b = b[:maxRW]
+		}
+		var n int
+		var err error
+		if fd.isFile {
+			switch fd.kind {
+			case kindConsole:
+				n, err = fd.writeConsole(b)
+			default:
+				n, err = syscall.Write(fd.Sysfd, b)
+				if fd.kind == kindPipe && err == syscall.ERROR_OPERATION_ABORTED {
+					// Close uses CancelIoEx to interrupt concurrent I/O for pipes.
+					// If the fd is a pipe and the Write was interrupted by CancelIoEx,
+					// we assume it is interrupted by Close.
+					err = ErrFileClosing
+				}
+			}
+			if err != nil {
+				n = 0
+			}
+		} else {
+			if race.Enabled {
+				race.ReleaseMerge(unsafe.Pointer(&ioSync))
+			}
+			o := &fd.wop
+			o.InitBuf(b)
+			n, err = execIO(o, func(o *operation) error {
+				return syscall.WSASend(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, &o.o, nil)
+			})
+		}
+		ntotal += n
+		if err != nil {
+			return ntotal, err
+		}
+		buf = buf[n:]
+	}
+	return ntotal, nil
+}
+
+// writeConsole writes len(b) bytes to the console File.
+// It returns the number of bytes written and an error, if any.
+func (fd *FD) writeConsole(b []byte) (int, error) {
+	n := len(b)
+	runes := make([]rune, 0, 256)
+	if len(fd.lastbits) > 0 {
+		b = append(fd.lastbits, b...)
+		fd.lastbits = nil
+
+	}
+	for len(b) >= utf8.UTFMax || utf8.FullRune(b) {
+		r, l := utf8.DecodeRune(b)
+		runes = append(runes, r)
+		b = b[l:]
+	}
+	if len(b) > 0 {
+		fd.lastbits = make([]byte, len(b))
+		copy(fd.lastbits, b)
+	}
+	// syscall.WriteConsole seems to fail, if given large buffer.
+	// So limit the buffer to 16000 characters. This number was
+	// discovered by experimenting with syscall.WriteConsole.
+	const maxWrite = 16000
+	for len(runes) > 0 {
+		m := len(runes)
+		if m > maxWrite {
+			m = maxWrite
+		}
+		chunk := runes[:m]
+		runes = runes[m:]
+		uint16s := utf16.Encode(chunk)
+		for len(uint16s) > 0 {
+			var written uint32
+			err := syscall.WriteConsole(fd.Sysfd, &uint16s[0], uint32(len(uint16s)), &written, nil)
+			if err != nil {
+				return 0, err
+			}
+			uint16s = uint16s[written:]
+		}
+	}
+	return n, nil
+}
+
+// Pwrite emulates the Unix pwrite system call.
+func (fd *FD) Pwrite(buf []byte, off int64) (int, error) {
+	// Call incref, not writeLock, because since pwrite specifies the
+	// offset it is independent from other writes.
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+
+	fd.l.Lock()
+	defer fd.l.Unlock()
+	curoffset, e := syscall.Seek(fd.Sysfd, 0, io.SeekCurrent)
+	if e != nil {
+		return 0, e
+	}
+	defer syscall.Seek(fd.Sysfd, curoffset, io.SeekStart)
+
+	ntotal := 0
+	for len(buf) > 0 {
+		b := buf
+		if len(b) > maxRW {
+			b = b[:maxRW]
+		}
+		var n uint32
+		o := syscall.Overlapped{
+			OffsetHigh: uint32(off >> 32),
+			Offset:     uint32(off),
+		}
+		e = syscall.WriteFile(fd.Sysfd, b, &n, &o)
+		ntotal += int(n)
+		if e != nil {
+			return ntotal, e
+		}
+		buf = buf[n:]
+		off += int64(n)
+	}
+	return ntotal, nil
+}
+
+// Writev emulates the Unix writev system call.
+func (fd *FD) Writev(buf *[][]byte) (int64, error) {
+	if len(*buf) == 0 {
+		return 0, nil
+	}
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if race.Enabled {
+		race.ReleaseMerge(unsafe.Pointer(&ioSync))
+	}
+	o := &fd.wop
+	o.InitBufs(buf)
+	n, err := execIO(o, func(o *operation) error {
+		return syscall.WSASend(o.fd.Sysfd, &o.bufs[0], uint32(len(o.bufs)), &o.qty, 0, &o.o, nil)
+	})
+	o.ClearBufs()
+	TestHookDidWritev(n)
+	consume(buf, int64(n))
+	return int64(n), err
+}
+
+// WriteTo wraps the sendto network call.
+func (fd *FD) WriteTo(buf []byte, sa syscall.Sockaddr) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+
+	if len(buf) == 0 {
+		// handle zero-byte payload
+		o := &fd.wop
+		o.InitBuf(buf)
+		o.sa = sa
+		n, err := execIO(o, func(o *operation) error {
+			return syscall.WSASendto(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, o.sa, &o.o, nil)
+		})
+		return n, err
+	}
+
+	ntotal := 0
+	for len(buf) > 0 {
+		b := buf
+		if len(b) > maxRW {
+			b = b[:maxRW]
+		}
+		o := &fd.wop
+		o.InitBuf(b)
+		o.sa = sa
+		n, err := execIO(o, func(o *operation) error {
+			return syscall.WSASendto(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, o.sa, &o.o, nil)
+		})
+		ntotal += int(n)
+		if err != nil {
+			return ntotal, err
+		}
+		buf = buf[n:]
+	}
+	return ntotal, nil
+}
+
+// WriteToInet4 is WriteTo, specialized for syscall.SockaddrInet4.
+func (fd *FD) WriteToInet4(buf []byte, sa4 *syscall.SockaddrInet4) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+
+	if len(buf) == 0 {
+		// handle zero-byte payload
+		o := &fd.wop
+		o.InitBuf(buf)
+		n, err := execIO(o, func(o *operation) error {
+			return windows.WSASendtoInet4(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, sa4, &o.o, nil)
+		})
+		return n, err
+	}
+
+	ntotal := 0
+	for len(buf) > 0 {
+		b := buf
+		if len(b) > maxRW {
+			b = b[:maxRW]
+		}
+		o := &fd.wop
+		o.InitBuf(b)
+		n, err := execIO(o, func(o *operation) error {
+			return windows.WSASendtoInet4(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, sa4, &o.o, nil)
+		})
+		ntotal += int(n)
+		if err != nil {
+			return ntotal, err
+		}
+		buf = buf[n:]
+	}
+	return ntotal, nil
+}
+
+// WriteToInet6 is WriteTo, specialized for syscall.SockaddrInet6.
+func (fd *FD) WriteToInet6(buf []byte, sa6 *syscall.SockaddrInet6) (int, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+
+	if len(buf) == 0 {
+		// handle zero-byte payload
+		o := &fd.wop
+		o.InitBuf(buf)
+		n, err := execIO(o, func(o *operation) error {
+			return windows.WSASendtoInet6(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, sa6, &o.o, nil)
+		})
+		return n, err
+	}
+
+	ntotal := 0
+	for len(buf) > 0 {
+		b := buf
+		if len(b) > maxRW {
+			b = b[:maxRW]
+		}
+		o := &fd.wop
+		o.InitBuf(b)
+		n, err := execIO(o, func(o *operation) error {
+			return windows.WSASendtoInet6(o.fd.Sysfd, &o.buf, 1, &o.qty, 0, sa6, &o.o, nil)
+		})
+		ntotal += int(n)
+		if err != nil {
+			return ntotal, err
+		}
+		buf = buf[n:]
+	}
+	return ntotal, nil
+}
+
+// Call ConnectEx. This doesn't need any locking, since it is only
+// called when the descriptor is first created. This is here rather
+// than in the net package so that it can use fd.wop.
+func (fd *FD) ConnectEx(ra syscall.Sockaddr) error {
+	o := &fd.wop
+	o.sa = ra
+	_, err := execIO(o, func(o *operation) error {
+		return ConnectExFunc(o.fd.Sysfd, o.sa, nil, 0, nil, &o.o)
+	})
+	return err
+}
+
+func (fd *FD) acceptOne(s syscall.Handle, rawsa []syscall.RawSockaddrAny, o *operation) (string, error) {
+	// Submit accept request.
+	o.handle = s
+	o.rsan = int32(unsafe.Sizeof(rawsa[0]))
+	_, err := execIO(o, func(o *operation) error {
+		return AcceptFunc(o.fd.Sysfd, o.handle, (*byte)(unsafe.Pointer(&rawsa[0])), 0, uint32(o.rsan), uint32(o.rsan), &o.qty, &o.o)
+	})
+	if err != nil {
+		CloseFunc(s)
+		return "acceptex", err
+	}
+
+	// Inherit properties of the listening socket.
+	err = syscall.Setsockopt(s, syscall.SOL_SOCKET, syscall.SO_UPDATE_ACCEPT_CONTEXT, (*byte)(unsafe.Pointer(&fd.Sysfd)), int32(unsafe.Sizeof(fd.Sysfd)))
+	if err != nil {
+		CloseFunc(s)
+		return "setsockopt", err
+	}
+
+	return "", nil
+}
+
+// Accept handles accepting a socket. The sysSocket parameter is used
+// to allocate the net socket.
+func (fd *FD) Accept(sysSocket func() (syscall.Handle, error)) (syscall.Handle, []syscall.RawSockaddrAny, uint32, string, error) {
+	if err := fd.readLock(); err != nil {
+		return syscall.InvalidHandle, nil, 0, "", err
+	}
+	defer fd.readUnlock()
+
+	o := &fd.rop
+	var rawsa [2]syscall.RawSockaddrAny
+	for {
+		s, err := sysSocket()
+		if err != nil {
+			return syscall.InvalidHandle, nil, 0, "", err
+		}
+
+		errcall, err := fd.acceptOne(s, rawsa[:], o)
+		if err == nil {
+			return s, rawsa[:], uint32(o.rsan), "", nil
+		}
+
+		// Sometimes we see WSAECONNRESET and ERROR_NETNAME_DELETED is
+		// returned here. These happen if connection reset is received
+		// before AcceptEx could complete. These errors relate to new
+		// connection, not to AcceptEx, so ignore broken connection and
+		// try AcceptEx again for more connections.
+		errno, ok := err.(syscall.Errno)
+		if !ok {
+			return syscall.InvalidHandle, nil, 0, errcall, err
+		}
+		switch errno {
+		case syscall.ERROR_NETNAME_DELETED, syscall.WSAECONNRESET:
+			// ignore these and try again
+		default:
+			return syscall.InvalidHandle, nil, 0, errcall, err
+		}
+	}
+}
+
+// Seek wraps syscall.Seek.
+func (fd *FD) Seek(offset int64, whence int) (int64, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+
+	fd.l.Lock()
+	defer fd.l.Unlock()
+
+	return syscall.Seek(fd.Sysfd, offset, whence)
+}
+
+// Fchmod updates syscall.ByHandleFileInformation.Fileattributes when needed.
+func (fd *FD) Fchmod(mode uint32) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+
+	var d syscall.ByHandleFileInformation
+	if err := syscall.GetFileInformationByHandle(fd.Sysfd, &d); err != nil {
+		return err
+	}
+	attrs := d.FileAttributes
+	if mode&syscall.S_IWRITE != 0 {
+		attrs &^= syscall.FILE_ATTRIBUTE_READONLY
+	} else {
+		attrs |= syscall.FILE_ATTRIBUTE_READONLY
+	}
+	if attrs == d.FileAttributes {
+		return nil
+	}
+
+	var du windows.FILE_BASIC_INFO
+	du.FileAttributes = attrs
+	l := uint32(unsafe.Sizeof(d))
+	return windows.SetFileInformationByHandle(fd.Sysfd, windows.FileBasicInfo, uintptr(unsafe.Pointer(&du)), l)
+}
+
+// Fchdir wraps syscall.Fchdir.
+func (fd *FD) Fchdir() error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Fchdir(fd.Sysfd)
+}
+
+// GetFileType wraps syscall.GetFileType.
+func (fd *FD) GetFileType() (uint32, error) {
+	if err := fd.incref(); err != nil {
+		return 0, err
+	}
+	defer fd.decref()
+	return syscall.GetFileType(fd.Sysfd)
+}
+
+// GetFileInformationByHandle wraps GetFileInformationByHandle.
+func (fd *FD) GetFileInformationByHandle(data *syscall.ByHandleFileInformation) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.GetFileInformationByHandle(fd.Sysfd, data)
+}
+
+// RawRead invokes the user-defined function f for a read operation.
+func (fd *FD) RawRead(f func(uintptr) bool) error {
+	if err := fd.readLock(); err != nil {
+		return err
+	}
+	defer fd.readUnlock()
+	for {
+		if f(uintptr(fd.Sysfd)) {
+			return nil
+		}
+
+		// Use a zero-byte read as a way to get notified when this
+		// socket is readable. h/t https://stackoverflow.com/a/42019668/332798
+		o := &fd.rop
+		o.InitBuf(nil)
+		if !fd.IsStream {
+			o.flags |= windows.MSG_PEEK
+		}
+		_, err := execIO(o, func(o *operation) error {
+			return syscall.WSARecv(o.fd.Sysfd, &o.buf, 1, &o.qty, &o.flags, &o.o, nil)
+		})
+		if err == windows.WSAEMSGSIZE {
+			// expected with a 0-byte peek, ignore.
+		} else if err != nil {
+			return err
+		}
+	}
+}
+
+// RawWrite invokes the user-defined function f for a write operation.
+func (fd *FD) RawWrite(f func(uintptr) bool) error {
+	if err := fd.writeLock(); err != nil {
+		return err
+	}
+	defer fd.writeUnlock()
+
+	if f(uintptr(fd.Sysfd)) {
+		return nil
+	}
+
+	// TODO(tmm1): find a way to detect socket writability
+	return syscall.EWINDOWS
+}
+
+func sockaddrInet4ToRaw(rsa *syscall.RawSockaddrAny, sa *syscall.SockaddrInet4) int32 {
+	*rsa = syscall.RawSockaddrAny{}
+	raw := (*syscall.RawSockaddrInet4)(unsafe.Pointer(rsa))
+	raw.Family = syscall.AF_INET
+	p := (*[2]byte)(unsafe.Pointer(&raw.Port))
+	p[0] = byte(sa.Port >> 8)
+	p[1] = byte(sa.Port)
+	raw.Addr = sa.Addr
+	return int32(unsafe.Sizeof(*raw))
+}
+
+func sockaddrInet6ToRaw(rsa *syscall.RawSockaddrAny, sa *syscall.SockaddrInet6) int32 {
+	*rsa = syscall.RawSockaddrAny{}
+	raw := (*syscall.RawSockaddrInet6)(unsafe.Pointer(rsa))
+	raw.Family = syscall.AF_INET6
+	p := (*[2]byte)(unsafe.Pointer(&raw.Port))
+	p[0] = byte(sa.Port >> 8)
+	p[1] = byte(sa.Port)
+	raw.Scope_id = sa.ZoneId
+	raw.Addr = sa.Addr
+	return int32(unsafe.Sizeof(*raw))
+}
+
+func rawToSockaddrInet4(rsa *syscall.RawSockaddrAny, sa *syscall.SockaddrInet4) {
+	pp := (*syscall.RawSockaddrInet4)(unsafe.Pointer(rsa))
+	p := (*[2]byte)(unsafe.Pointer(&pp.Port))
+	sa.Port = int(p[0])<<8 + int(p[1])
+	sa.Addr = pp.Addr
+}
+
+func rawToSockaddrInet6(rsa *syscall.RawSockaddrAny, sa *syscall.SockaddrInet6) {
+	pp := (*syscall.RawSockaddrInet6)(unsafe.Pointer(rsa))
+	p := (*[2]byte)(unsafe.Pointer(&pp.Port))
+	sa.Port = int(p[0])<<8 + int(p[1])
+	sa.ZoneId = pp.Scope_id
+	sa.Addr = pp.Addr
+}
+
+func sockaddrToRaw(rsa *syscall.RawSockaddrAny, sa syscall.Sockaddr) (int32, error) {
+	switch sa := sa.(type) {
+	case *syscall.SockaddrInet4:
+		sz := sockaddrInet4ToRaw(rsa, sa)
+		return sz, nil
+	case *syscall.SockaddrInet6:
+		sz := sockaddrInet6ToRaw(rsa, sa)
+		return sz, nil
+	default:
+		return 0, syscall.EWINDOWS
+	}
+}
+
+// ReadMsg wraps the WSARecvMsg network call.
+func (fd *FD) ReadMsg(p []byte, oob []byte, flags int) (int, int, int, syscall.Sockaddr, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, nil, err
+	}
+	defer fd.readUnlock()
+
+	if len(p) > maxRW {
+		p = p[:maxRW]
+	}
+
+	o := &fd.rop
+	o.InitMsg(p, oob)
+	if o.rsa == nil {
+		o.rsa = new(syscall.RawSockaddrAny)
+	}
+	o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+	o.msg.Namelen = int32(unsafe.Sizeof(*o.rsa))
+	o.msg.Flags = uint32(flags)
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSARecvMsg(o.fd.Sysfd, &o.msg, &o.qty, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	var sa syscall.Sockaddr
+	if err == nil {
+		sa, err = o.rsa.Sockaddr()
+	}
+	return n, int(o.msg.Control.Len), int(o.msg.Flags), sa, err
+}
+
+// ReadMsgInet4 is ReadMsg, but specialized to return a syscall.SockaddrInet4.
+func (fd *FD) ReadMsgInet4(p []byte, oob []byte, flags int, sa4 *syscall.SockaddrInet4) (int, int, int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, err
+	}
+	defer fd.readUnlock()
+
+	if len(p) > maxRW {
+		p = p[:maxRW]
+	}
+
+	o := &fd.rop
+	o.InitMsg(p, oob)
+	if o.rsa == nil {
+		o.rsa = new(syscall.RawSockaddrAny)
+	}
+	o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+	o.msg.Namelen = int32(unsafe.Sizeof(*o.rsa))
+	o.msg.Flags = uint32(flags)
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSARecvMsg(o.fd.Sysfd, &o.msg, &o.qty, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	if err == nil {
+		rawToSockaddrInet4(o.rsa, sa4)
+	}
+	return n, int(o.msg.Control.Len), int(o.msg.Flags), err
+}
+
+// ReadMsgInet6 is ReadMsg, but specialized to return a syscall.SockaddrInet6.
+func (fd *FD) ReadMsgInet6(p []byte, oob []byte, flags int, sa6 *syscall.SockaddrInet6) (int, int, int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, 0, 0, err
+	}
+	defer fd.readUnlock()
+
+	if len(p) > maxRW {
+		p = p[:maxRW]
+	}
+
+	o := &fd.rop
+	o.InitMsg(p, oob)
+	if o.rsa == nil {
+		o.rsa = new(syscall.RawSockaddrAny)
+	}
+	o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+	o.msg.Namelen = int32(unsafe.Sizeof(*o.rsa))
+	o.msg.Flags = uint32(flags)
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSARecvMsg(o.fd.Sysfd, &o.msg, &o.qty, &o.o, nil)
+	})
+	err = fd.eofError(n, err)
+	if err == nil {
+		rawToSockaddrInet6(o.rsa, sa6)
+	}
+	return n, int(o.msg.Control.Len), int(o.msg.Flags), err
+}
+
+// WriteMsg wraps the WSASendMsg network call.
+func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, error) {
+	if len(p) > maxRW {
+		return 0, 0, errors.New("packet is too large (only 1GB is allowed)")
+	}
+
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+
+	o := &fd.wop
+	o.InitMsg(p, oob)
+	if sa != nil {
+		if o.rsa == nil {
+			o.rsa = new(syscall.RawSockaddrAny)
+		}
+		len, err := sockaddrToRaw(o.rsa, sa)
+		if err != nil {
+			return 0, 0, err
+		}
+		o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+		o.msg.Namelen = len
+	}
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSASendMsg(o.fd.Sysfd, &o.msg, 0, &o.qty, &o.o, nil)
+	})
+	return n, int(o.msg.Control.Len), err
+}
+
+// WriteMsgInet4 is WriteMsg specialized for syscall.SockaddrInet4.
+func (fd *FD) WriteMsgInet4(p []byte, oob []byte, sa *syscall.SockaddrInet4) (int, int, error) {
+	if len(p) > maxRW {
+		return 0, 0, errors.New("packet is too large (only 1GB is allowed)")
+	}
+
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+
+	o := &fd.wop
+	o.InitMsg(p, oob)
+	if o.rsa == nil {
+		o.rsa = new(syscall.RawSockaddrAny)
+	}
+	len := sockaddrInet4ToRaw(o.rsa, sa)
+	o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+	o.msg.Namelen = len
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSASendMsg(o.fd.Sysfd, &o.msg, 0, &o.qty, &o.o, nil)
+	})
+	return n, int(o.msg.Control.Len), err
+}
+
+// WriteMsgInet6 is WriteMsg specialized for syscall.SockaddrInet6.
+func (fd *FD) WriteMsgInet6(p []byte, oob []byte, sa *syscall.SockaddrInet6) (int, int, error) {
+	if len(p) > maxRW {
+		return 0, 0, errors.New("packet is too large (only 1GB is allowed)")
+	}
+
+	if err := fd.writeLock(); err != nil {
+		return 0, 0, err
+	}
+	defer fd.writeUnlock()
+
+	o := &fd.wop
+	o.InitMsg(p, oob)
+	if o.rsa == nil {
+		o.rsa = new(syscall.RawSockaddrAny)
+	}
+	len := sockaddrInet6ToRaw(o.rsa, sa)
+	o.msg.Name = (syscall.Pointer)(unsafe.Pointer(o.rsa))
+	o.msg.Namelen = len
+	n, err := execIO(o, func(o *operation) error {
+		return windows.WSASendMsg(o.fd.Sysfd, &o.msg, 0, &o.qty, &o.o, nil)
+	})
+	return n, int(o.msg.Control.Len), err
+}
diff --git a/src/internal/poll/fd_windows_test.go b/src/internal/poll/fd_windows_test.go
new file mode 100644
index 0000000..e3ca0e2
--- /dev/null
+++ b/src/internal/poll/fd_windows_test.go
@@ -0,0 +1,111 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"fmt"
+	"internal/poll"
+	"os"
+	"sync"
+	"syscall"
+	"testing"
+)
+
+type loggedFD struct {
+	Net string
+	FD  *poll.FD
+	Err error
+}
+
+var (
+	logMu     sync.Mutex
+	loggedFDs map[syscall.Handle]*loggedFD
+)
+
+func logFD(net string, fd *poll.FD, err error) {
+	logMu.Lock()
+	defer logMu.Unlock()
+
+	loggedFDs[fd.Sysfd] = &loggedFD{
+		Net: net,
+		FD:  fd,
+		Err: err,
+	}
+}
+
+func init() {
+	loggedFDs = make(map[syscall.Handle]*loggedFD)
+	*poll.LogInitFD = logFD
+}
+
+func findLoggedFD(h syscall.Handle) (lfd *loggedFD, found bool) {
+	logMu.Lock()
+	defer logMu.Unlock()
+
+	lfd, found = loggedFDs[h]
+	return lfd, found
+}
+
+// checkFileIsNotPartOfNetpoll verifies that f is not managed by netpoll.
+// It returns error, if check fails.
+func checkFileIsNotPartOfNetpoll(f *os.File) error {
+	lfd, found := findLoggedFD(syscall.Handle(f.Fd()))
+	if !found {
+		return fmt.Errorf("%v fd=%v: is not found in the log", f.Name(), f.Fd())
+	}
+	if lfd.FD.IsPartOfNetpoll() {
+		return fmt.Errorf("%v fd=%v: is part of netpoll, but should not be (logged: net=%v err=%v)", f.Name(), f.Fd(), lfd.Net, lfd.Err)
+	}
+	return nil
+}
+
+func TestFileFdsAreInitialised(t *testing.T) {
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	f, err := os.Open(exe)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	err = checkFileIsNotPartOfNetpoll(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestSerialFdsAreInitialised(t *testing.T) {
+	for _, name := range []string{"COM1", "COM2", "COM3", "COM4"} {
+		t.Run(name, func(t *testing.T) {
+			h, err := syscall.CreateFile(syscall.StringToUTF16Ptr(name),
+				syscall.GENERIC_READ|syscall.GENERIC_WRITE,
+				0,
+				nil,
+				syscall.OPEN_EXISTING,
+				syscall.FILE_ATTRIBUTE_NORMAL|syscall.FILE_FLAG_OVERLAPPED,
+				0)
+			if err != nil {
+				if errno, ok := err.(syscall.Errno); ok {
+					switch errno {
+					case syscall.ERROR_FILE_NOT_FOUND,
+						syscall.ERROR_ACCESS_DENIED:
+						t.Log("Skipping: ", err)
+						return
+					}
+				}
+				t.Fatal(err)
+			}
+			f := os.NewFile(uintptr(h), name)
+			defer f.Close()
+
+			err = checkFileIsNotPartOfNetpoll(f)
+			if err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
diff --git a/src/internal/poll/fd_writev_libc.go b/src/internal/poll/fd_writev_libc.go
new file mode 100644
index 0000000..0a60473
--- /dev/null
+++ b/src/internal/poll/fd_writev_libc.go
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || darwin || (openbsd && !mips64) || solaris
+
+package poll
+
+import (
+	"syscall"
+	_ "unsafe" // for go:linkname
+)
+
+//go:linkname writev syscall.writev
+func writev(fd int, iovecs []syscall.Iovec) (uintptr, error)
diff --git a/src/internal/poll/fd_writev_unix.go b/src/internal/poll/fd_writev_unix.go
new file mode 100644
index 0000000..005638b
--- /dev/null
+++ b/src/internal/poll/fd_writev_unix.go
@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || (openbsd && mips64)
+
+package poll
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func writev(fd int, iovecs []syscall.Iovec) (uintptr, error) {
+	var (
+		r uintptr
+		e syscall.Errno
+	)
+	for {
+		r, _, e = syscall.Syscall(syscall.SYS_WRITEV, uintptr(fd), uintptr(unsafe.Pointer(&iovecs[0])), uintptr(len(iovecs)))
+		if e != syscall.EINTR {
+			break
+		}
+	}
+	if e != 0 {
+		return r, e
+	}
+	return r, nil
+}
diff --git a/src/internal/poll/file_plan9.go b/src/internal/poll/file_plan9.go
new file mode 100644
index 0000000..57dc0c6
--- /dev/null
+++ b/src/internal/poll/file_plan9.go
@@ -0,0 +1,42 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+// Expose fdMutex for use by the os package on Plan 9.
+// On Plan 9 we don't want to use async I/O for file operations,
+// but we still want the locking semantics that fdMutex provides.
+
+// FDMutex is an exported fdMutex, only for Plan 9.
+type FDMutex struct {
+	fdmu fdMutex
+}
+
+func (fdmu *FDMutex) Incref() bool {
+	return fdmu.fdmu.incref()
+}
+
+func (fdmu *FDMutex) Decref() bool {
+	return fdmu.fdmu.decref()
+}
+
+func (fdmu *FDMutex) IncrefAndClose() bool {
+	return fdmu.fdmu.increfAndClose()
+}
+
+func (fdmu *FDMutex) ReadLock() bool {
+	return fdmu.fdmu.rwlock(true)
+}
+
+func (fdmu *FDMutex) ReadUnlock() bool {
+	return fdmu.fdmu.rwunlock(true)
+}
+
+func (fdmu *FDMutex) WriteLock() bool {
+	return fdmu.fdmu.rwlock(false)
+}
+
+func (fdmu *FDMutex) WriteUnlock() bool {
+	return fdmu.fdmu.rwunlock(false)
+}
diff --git a/src/internal/poll/hook_cloexec.go b/src/internal/poll/hook_cloexec.go
new file mode 100644
index 0000000..5b3cdce
--- /dev/null
+++ b/src/internal/poll/hook_cloexec.go
@@ -0,0 +1,12 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || openbsd || solaris
+
+package poll
+
+import "syscall"
+
+// Accept4Func is used to hook the accept4 call.
+var Accept4Func func(int, int) (int, syscall.Sockaddr, error) = syscall.Accept4
diff --git a/src/internal/poll/hook_unix.go b/src/internal/poll/hook_unix.go
new file mode 100644
index 0000000..1a50356
--- /dev/null
+++ b/src/internal/poll/hook_unix.go
@@ -0,0 +1,15 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || (js && wasm)
+
+package poll
+
+import "syscall"
+
+// CloseFunc is used to hook the close call.
+var CloseFunc func(int) error = syscall.Close
+
+// AcceptFunc is used to hook the accept call.
+var AcceptFunc func(int) (int, syscall.Sockaddr, error) = syscall.Accept
diff --git a/src/internal/poll/hook_windows.go b/src/internal/poll/hook_windows.go
new file mode 100644
index 0000000..0bd950e
--- /dev/null
+++ b/src/internal/poll/hook_windows.go
@@ -0,0 +1,16 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// CloseFunc is used to hook the close call.
+var CloseFunc func(syscall.Handle) error = syscall.Closesocket
+
+// AcceptFunc is used to hook the accept call.
+var AcceptFunc func(syscall.Handle, syscall.Handle, *byte, uint32, uint32, uint32, *uint32, *syscall.Overlapped) error = syscall.AcceptEx
+
+// ConnectExFunc is used to hook the ConnectEx call.
+var ConnectExFunc func(syscall.Handle, syscall.Sockaddr, *byte, uint32, *uint32, *syscall.Overlapped) error = syscall.ConnectEx
diff --git a/src/internal/poll/iovec_solaris.go b/src/internal/poll/iovec_solaris.go
new file mode 100644
index 0000000..e68f833
--- /dev/null
+++ b/src/internal/poll/iovec_solaris.go
@@ -0,0 +1,14 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func newIovecWithBase(base *byte) syscall.Iovec {
+	return syscall.Iovec{Base: (*int8)(unsafe.Pointer(base))}
+}
diff --git a/src/internal/poll/iovec_unix.go b/src/internal/poll/iovec_unix.go
new file mode 100644
index 0000000..3f2833e
--- /dev/null
+++ b/src/internal/poll/iovec_unix.go
@@ -0,0 +1,13 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd
+
+package poll
+
+import "syscall"
+
+func newIovecWithBase(base *byte) syscall.Iovec {
+	return syscall.Iovec{Base: base}
+}
diff --git a/src/internal/poll/read_test.go b/src/internal/poll/read_test.go
new file mode 100644
index 0000000..598a52e
--- /dev/null
+++ b/src/internal/poll/read_test.go
@@ -0,0 +1,61 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"os"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestRead(t *testing.T) {
+	t.Run("SpecialFile", func(t *testing.T) {
+		var wg sync.WaitGroup
+		for _, p := range specialFiles() {
+			for i := 0; i < 4; i++ {
+				wg.Add(1)
+				go func(p string) {
+					defer wg.Done()
+					for i := 0; i < 100; i++ {
+						if _, err := os.ReadFile(p); err != nil {
+							t.Error(err)
+							return
+						}
+						time.Sleep(time.Nanosecond)
+					}
+				}(p)
+			}
+		}
+		wg.Wait()
+	})
+}
+
+func specialFiles() []string {
+	var ps []string
+	switch runtime.GOOS {
+	case "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd":
+		ps = []string{
+			"/dev/null",
+		}
+	case "linux":
+		ps = []string{
+			"/dev/null",
+			"/proc/stat",
+			"/sys/devices/system/cpu/online",
+		}
+	}
+	nps := ps[:0]
+	for _, p := range ps {
+		f, err := os.Open(p)
+		if err != nil {
+			continue
+		}
+		f.Close()
+		nps = append(nps, p)
+	}
+	return nps
+}
diff --git a/src/internal/poll/sendfile_bsd.go b/src/internal/poll/sendfile_bsd.go
new file mode 100644
index 0000000..89315a8
--- /dev/null
+++ b/src/internal/poll/sendfile_bsd.go
@@ -0,0 +1,59 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || dragonfly || freebsd
+
+package poll
+
+import "syscall"
+
+// maxSendfileSize is the largest chunk size we ask the kernel to copy
+// at a time.
+const maxSendfileSize int = 4 << 20
+
+// SendFile wraps the sendfile system call.
+func SendFile(dstFD *FD, src int, pos, remain int64) (int64, error) {
+	if err := dstFD.writeLock(); err != nil {
+		return 0, err
+	}
+	defer dstFD.writeUnlock()
+	if err := dstFD.pd.prepareWrite(dstFD.isFile); err != nil {
+		return 0, err
+	}
+
+	dst := dstFD.Sysfd
+	var written int64
+	var err error
+	for remain > 0 {
+		n := maxSendfileSize
+		if int64(n) > remain {
+			n = int(remain)
+		}
+		pos1 := pos
+		n, err1 := syscall.Sendfile(dst, src, &pos1, n)
+		if n > 0 {
+			pos += int64(n)
+			written += int64(n)
+			remain -= int64(n)
+		} else if n == 0 && err1 == nil {
+			break
+		}
+		if err1 == syscall.EINTR {
+			continue
+		}
+		if err1 == syscall.EAGAIN {
+			if err1 = dstFD.pd.waitWrite(dstFD.isFile); err1 == nil {
+				continue
+			}
+		}
+		if err1 != nil {
+			// This includes syscall.ENOSYS (no kernel
+			// support) and syscall.EINVAL (fd types which
+			// don't implement sendfile)
+			err = err1
+			break
+		}
+	}
+	return written, err
+}
diff --git a/src/internal/poll/sendfile_linux.go b/src/internal/poll/sendfile_linux.go
new file mode 100644
index 0000000..cc31969
--- /dev/null
+++ b/src/internal/poll/sendfile_linux.go
@@ -0,0 +1,59 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// maxSendfileSize is the largest chunk size we ask the kernel to copy
+// at a time.
+const maxSendfileSize int = 4 << 20
+
+// SendFile wraps the sendfile system call.
+func SendFile(dstFD *FD, src int, remain int64) (int64, error, bool) {
+	if err := dstFD.writeLock(); err != nil {
+		return 0, err, false
+	}
+	defer dstFD.writeUnlock()
+	if err := dstFD.pd.prepareWrite(dstFD.isFile); err != nil {
+		return 0, err, false
+	}
+
+	dst := dstFD.Sysfd
+	var (
+		written int64
+		err     error
+		handled = true
+	)
+	for remain > 0 {
+		n := maxSendfileSize
+		if int64(n) > remain {
+			n = int(remain)
+		}
+		n, err1 := syscall.Sendfile(dst, src, nil, n)
+		if n > 0 {
+			written += int64(n)
+			remain -= int64(n)
+		} else if n == 0 && err1 == nil {
+			break
+		}
+		if err1 == syscall.EINTR {
+			continue
+		}
+		if err1 == syscall.EAGAIN {
+			if err1 = dstFD.pd.waitWrite(dstFD.isFile); err1 == nil {
+				continue
+			}
+		}
+		if err1 != nil {
+			// This includes syscall.ENOSYS (no kernel
+			// support) and syscall.EINVAL (fd types which
+			// don't implement sendfile)
+			err = err1
+			handled = false
+			break
+		}
+	}
+	return written, err, handled
+}
diff --git a/src/internal/poll/sendfile_solaris.go b/src/internal/poll/sendfile_solaris.go
new file mode 100644
index 0000000..7ae18f4
--- /dev/null
+++ b/src/internal/poll/sendfile_solaris.go
@@ -0,0 +1,66 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// Not strictly needed, but very helpful for debugging, see issue #10221.
+//
+//go:cgo_import_dynamic _ _ "libsendfile.so"
+//go:cgo_import_dynamic _ _ "libsocket.so"
+
+// maxSendfileSize is the largest chunk size we ask the kernel to copy
+// at a time.
+const maxSendfileSize int = 4 << 20
+
+// SendFile wraps the sendfile system call.
+func SendFile(dstFD *FD, src int, pos, remain int64) (int64, error) {
+	if err := dstFD.writeLock(); err != nil {
+		return 0, err
+	}
+	defer dstFD.writeUnlock()
+	if err := dstFD.pd.prepareWrite(dstFD.isFile); err != nil {
+		return 0, err
+	}
+
+	dst := dstFD.Sysfd
+	var written int64
+	var err error
+	for remain > 0 {
+		n := maxSendfileSize
+		if int64(n) > remain {
+			n = int(remain)
+		}
+		pos1 := pos
+		n, err1 := syscall.Sendfile(dst, src, &pos1, n)
+		if err1 == syscall.EAGAIN || err1 == syscall.EINTR {
+			// partial write may have occurred
+			n = int(pos1 - pos)
+		}
+		if n > 0 {
+			pos += int64(n)
+			written += int64(n)
+			remain -= int64(n)
+		} else if n == 0 && err1 == nil {
+			break
+		}
+		if err1 == syscall.EAGAIN {
+			if err1 = dstFD.pd.waitWrite(dstFD.isFile); err1 == nil {
+				continue
+			}
+		}
+		if err1 == syscall.EINTR {
+			continue
+		}
+		if err1 != nil {
+			// This includes syscall.ENOSYS (no kernel
+			// support) and syscall.EINVAL (fd types which
+			// don't implement sendfile)
+			err = err1
+			break
+		}
+	}
+	return written, err
+}
diff --git a/src/internal/poll/sendfile_windows.go b/src/internal/poll/sendfile_windows.go
new file mode 100644
index 0000000..50c3ee8
--- /dev/null
+++ b/src/internal/poll/sendfile_windows.go
@@ -0,0 +1,81 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"io"
+	"syscall"
+)
+
+// SendFile wraps the TransmitFile call.
+func SendFile(fd *FD, src syscall.Handle, n int64) (written int64, err error) {
+	if fd.kind == kindPipe {
+		// TransmitFile does not work with pipes
+		return 0, syscall.ESPIPE
+	}
+
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+
+	o := &fd.wop
+	o.handle = src
+
+	// TODO(brainman): skip calling syscall.Seek if OS allows it
+	curpos, err := syscall.Seek(o.handle, 0, io.SeekCurrent)
+	if err != nil {
+		return 0, err
+	}
+
+	if n <= 0 { // We don't know the size of the file so infer it.
+		// Find the number of bytes offset from curpos until the end of the file.
+		n, err = syscall.Seek(o.handle, -curpos, io.SeekEnd)
+		if err != nil {
+			return
+		}
+		// Now seek back to the original position.
+		if _, err = syscall.Seek(o.handle, curpos, io.SeekStart); err != nil {
+			return
+		}
+	}
+
+	// TransmitFile can be invoked in one call with at most
+	// 2,147,483,646 bytes: the maximum value for a 32-bit integer minus 1.
+	// See https://docs.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile
+	const maxChunkSizePerCall = int64(0x7fffffff - 1)
+
+	for n > 0 {
+		chunkSize := maxChunkSizePerCall
+		if chunkSize > n {
+			chunkSize = n
+		}
+
+		o.qty = uint32(chunkSize)
+		o.o.Offset = uint32(curpos)
+		o.o.OffsetHigh = uint32(curpos >> 32)
+
+		nw, err := execIO(o, func(o *operation) error {
+			return syscall.TransmitFile(o.fd.Sysfd, o.handle, o.qty, 0, &o.o, nil, syscall.TF_WRITE_BEHIND)
+		})
+		if err != nil {
+			return written, err
+		}
+
+		curpos += int64(nw)
+
+		// Some versions of Windows (Windows 10 1803) do not set
+		// file position after TransmitFile completes.
+		// So just use Seek to set file position.
+		if _, err = syscall.Seek(o.handle, curpos, io.SeekStart); err != nil {
+			return written, err
+		}
+
+		n -= int64(nw)
+		written += int64(nw)
+	}
+
+	return
+}
diff --git a/src/internal/poll/sock_cloexec.go b/src/internal/poll/sock_cloexec.go
new file mode 100644
index 0000000..f5be2aa
--- /dev/null
+++ b/src/internal/poll/sock_cloexec.go
@@ -0,0 +1,22 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements accept for platforms that provide a fast path for
+// setting SetNonblock and CloseOnExec.
+
+//go:build dragonfly || freebsd || (linux && !arm) || netbsd || openbsd || solaris
+
+package poll
+
+import "syscall"
+
+// Wrapper around the accept system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func accept(s int) (int, syscall.Sockaddr, string, error) {
+	ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
+	if err != nil {
+		return -1, sa, "accept4", err
+	}
+	return ns, sa, "", nil
+}
diff --git a/src/internal/poll/sock_cloexec_accept.go b/src/internal/poll/sock_cloexec_accept.go
new file mode 100644
index 0000000..4b86de5
--- /dev/null
+++ b/src/internal/poll/sock_cloexec_accept.go
@@ -0,0 +1,51 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements accept for platforms that provide a fast path for
+// setting SetNonblock and CloseOnExec, but don't necessarily have accept4.
+// This is the code we used for accept in Go 1.17 and earlier.
+// On Linux the accept4 system call was introduced in 2.6.28 kernel,
+// and our minimum requirement is 2.6.32, so we simplified the function.
+// Unfortunately, on ARM accept4 wasn't added until 2.6.36, so for ARM
+// only we continue using the older code.
+
+//go:build linux && arm
+
+package poll
+
+import "syscall"
+
+// Wrapper around the accept system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func accept(s int) (int, syscall.Sockaddr, string, error) {
+	ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
+	switch err {
+	case nil:
+		return ns, sa, "", nil
+	default: // errors other than the ones listed
+		return -1, sa, "accept4", err
+	case syscall.ENOSYS: // syscall missing
+	case syscall.EINVAL: // some Linux use this instead of ENOSYS
+	case syscall.EACCES: // some Linux use this instead of ENOSYS
+	case syscall.EFAULT: // some Linux use this instead of ENOSYS
+	}
+
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	// It is probably okay to hold the lock across syscall.Accept
+	// because we have put fd.sysfd into non-blocking mode.
+	// However, a call to the File method will put it back into
+	// blocking mode. We can't take that risk, so no use of ForkLock here.
+	ns, sa, err = AcceptFunc(s)
+	if err == nil {
+		syscall.CloseOnExec(ns)
+	}
+	if err != nil {
+		return -1, nil, "accept", err
+	}
+	if err = syscall.SetNonblock(ns, true); err != nil {
+		CloseFunc(ns)
+		return -1, nil, "setnonblock", err
+	}
+	return ns, sa, "", nil
+}
diff --git a/src/internal/poll/sockopt.go b/src/internal/poll/sockopt.go
new file mode 100644
index 0000000..a7c9d11
--- /dev/null
+++ b/src/internal/poll/sockopt.go
@@ -0,0 +1,36 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package poll
+
+import "syscall"
+
+// SetsockoptInt wraps the setsockopt network call with an int argument.
+func (fd *FD) SetsockoptInt(level, name, arg int) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptInt(fd.Sysfd, level, name, arg)
+}
+
+// SetsockoptInet4Addr wraps the setsockopt network call with an IPv4 address.
+func (fd *FD) SetsockoptInet4Addr(level, name int, arg [4]byte) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptInet4Addr(fd.Sysfd, level, name, arg)
+}
+
+// SetsockoptLinger wraps the setsockopt network call with a Linger argument.
+func (fd *FD) SetsockoptLinger(level, name int, l *syscall.Linger) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptLinger(fd.Sysfd, level, name, l)
+}
diff --git a/src/internal/poll/sockopt_linux.go b/src/internal/poll/sockopt_linux.go
new file mode 100644
index 0000000..bc79c35
--- /dev/null
+++ b/src/internal/poll/sockopt_linux.go
@@ -0,0 +1,16 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// SetsockoptIPMreqn wraps the setsockopt network call with an IPMreqn argument.
+func (fd *FD) SetsockoptIPMreqn(level, name int, mreq *syscall.IPMreqn) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptIPMreqn(fd.Sysfd, level, name, mreq)
+}
diff --git a/src/internal/poll/sockopt_unix.go b/src/internal/poll/sockopt_unix.go
new file mode 100644
index 0000000..9cba44d
--- /dev/null
+++ b/src/internal/poll/sockopt_unix.go
@@ -0,0 +1,18 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package poll
+
+import "syscall"
+
+// SetsockoptByte wraps the setsockopt network call with a byte argument.
+func (fd *FD) SetsockoptByte(level, name int, arg byte) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptByte(fd.Sysfd, level, name, arg)
+}
diff --git a/src/internal/poll/sockopt_windows.go b/src/internal/poll/sockopt_windows.go
new file mode 100644
index 0000000..dd5fb70
--- /dev/null
+++ b/src/internal/poll/sockopt_windows.go
@@ -0,0 +1,25 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import "syscall"
+
+// Setsockopt wraps the setsockopt network call.
+func (fd *FD) Setsockopt(level, optname int32, optval *byte, optlen int32) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.Setsockopt(fd.Sysfd, level, optname, optval, optlen)
+}
+
+// WSAIoctl wraps the WSAIoctl network call.
+func (fd *FD) WSAIoctl(iocc uint32, inbuf *byte, cbif uint32, outbuf *byte, cbob uint32, cbbr *uint32, overlapped *syscall.Overlapped, completionRoutine uintptr) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.WSAIoctl(fd.Sysfd, iocc, inbuf, cbif, outbuf, cbob, cbbr, overlapped, completionRoutine)
+}
diff --git a/src/internal/poll/sockoptip.go b/src/internal/poll/sockoptip.go
new file mode 100644
index 0000000..41955e1
--- /dev/null
+++ b/src/internal/poll/sockoptip.go
@@ -0,0 +1,27 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package poll
+
+import "syscall"
+
+// SetsockoptIPMreq wraps the setsockopt network call with an IPMreq argument.
+func (fd *FD) SetsockoptIPMreq(level, name int, mreq *syscall.IPMreq) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptIPMreq(fd.Sysfd, level, name, mreq)
+}
+
+// SetsockoptIPv6Mreq wraps the setsockopt network call with an IPv6Mreq argument.
+func (fd *FD) SetsockoptIPv6Mreq(level, name int, mreq *syscall.IPv6Mreq) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return syscall.SetsockoptIPv6Mreq(fd.Sysfd, level, name, mreq)
+}
diff --git a/src/internal/poll/splice_linux.go b/src/internal/poll/splice_linux.go
new file mode 100644
index 0000000..96cbe4a
--- /dev/null
+++ b/src/internal/poll/splice_linux.go
@@ -0,0 +1,231 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll
+
+import (
+	"runtime"
+	"sync"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	// spliceNonblock makes calls to splice(2) non-blocking.
+	spliceNonblock = 0x2
+
+	// maxSpliceSize is the maximum amount of data Splice asks
+	// the kernel to move in a single call to splice(2).
+	// We use 1MB as Splice writes data through a pipe, and 1MB is the default maximum pipe buffer size,
+	// which is determined by /proc/sys/fs/pipe-max-size.
+	maxSpliceSize = 1 << 20
+)
+
+// Splice transfers at most remain bytes of data from src to dst, using the
+// splice system call to minimize copies of data from and to userspace.
+//
+// Splice gets a pipe buffer from the pool or creates a new one if needed, to serve as a buffer for the data transfer.
+// src and dst must both be stream-oriented sockets.
+//
+// If err != nil, sc is the system call which caused the error.
+func Splice(dst, src *FD, remain int64) (written int64, handled bool, sc string, err error) {
+	p, sc, err := getPipe()
+	if err != nil {
+		return 0, false, sc, err
+	}
+	defer putPipe(p)
+	var inPipe, n int
+	for err == nil && remain > 0 {
+		max := maxSpliceSize
+		if int64(max) > remain {
+			max = int(remain)
+		}
+		inPipe, err = spliceDrain(p.wfd, src, max)
+		// The operation is considered handled if splice returns no
+		// error, or an error other than EINVAL. An EINVAL means the
+		// kernel does not support splice for the socket type of src.
+		// The failed syscall does not consume any data so it is safe
+		// to fall back to a generic copy.
+		//
+		// spliceDrain should never return EAGAIN, so if err != nil,
+		// Splice cannot continue.
+		//
+		// If inPipe == 0 && err == nil, src is at EOF, and the
+		// transfer is complete.
+		handled = handled || (err != syscall.EINVAL)
+		if err != nil || inPipe == 0 {
+			break
+		}
+		p.data += inPipe
+
+		n, err = splicePump(dst, p.rfd, inPipe)
+		if n > 0 {
+			written += int64(n)
+			remain -= int64(n)
+			p.data -= n
+		}
+	}
+	if err != nil {
+		return written, handled, "splice", err
+	}
+	return written, true, "", nil
+}
+
+// spliceDrain moves data from a socket to a pipe.
+//
+// Invariant: when entering spliceDrain, the pipe is empty. It is either in its
+// initial state, or splicePump has emptied it previously.
+//
+// Given this, spliceDrain can reasonably assume that the pipe is ready for
+// writing, so if splice returns EAGAIN, it must be because the socket is not
+// ready for reading.
+//
+// If spliceDrain returns (0, nil), src is at EOF.
+func spliceDrain(pipefd int, sock *FD, max int) (int, error) {
+	if err := sock.readLock(); err != nil {
+		return 0, err
+	}
+	defer sock.readUnlock()
+	if err := sock.pd.prepareRead(sock.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := splice(pipefd, sock.Sysfd, max, spliceNonblock)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err != syscall.EAGAIN {
+			return n, err
+		}
+		if err := sock.pd.waitRead(sock.isFile); err != nil {
+			return n, err
+		}
+	}
+}
+
+// splicePump moves all the buffered data from a pipe to a socket.
+//
+// Invariant: when entering splicePump, there are exactly inPipe
+// bytes of data in the pipe, from a previous call to spliceDrain.
+//
+// By analogy to the condition from spliceDrain, splicePump
+// only needs to poll the socket for readiness, if splice returns
+// EAGAIN.
+//
+// If splicePump cannot move all the data in a single call to
+// splice(2), it loops over the buffered data until it has written
+// all of it to the socket. This behavior is similar to the Write
+// step of an io.Copy in userspace.
+func splicePump(sock *FD, pipefd int, inPipe int) (int, error) {
+	if err := sock.writeLock(); err != nil {
+		return 0, err
+	}
+	defer sock.writeUnlock()
+	if err := sock.pd.prepareWrite(sock.isFile); err != nil {
+		return 0, err
+	}
+	written := 0
+	for inPipe > 0 {
+		n, err := splice(sock.Sysfd, pipefd, inPipe, spliceNonblock)
+		// Here, the condition n == 0 && err == nil should never be
+		// observed, since Splice controls the write side of the pipe.
+		if n > 0 {
+			inPipe -= n
+			written += n
+			continue
+		}
+		if err != syscall.EAGAIN {
+			return written, err
+		}
+		if err := sock.pd.waitWrite(sock.isFile); err != nil {
+			return written, err
+		}
+	}
+	return written, nil
+}
+
+// splice wraps the splice system call. Since the current implementation
+// only uses splice on sockets and pipes, the offset arguments are unused.
+// splice returns int instead of int64, because callers never ask it to
+// move more data in a single call than can fit in an int32.
+func splice(out int, in int, max int, flags int) (int, error) {
+	n, err := syscall.Splice(in, nil, out, nil, max, flags)
+	return int(n), err
+}
+
+type splicePipeFields struct {
+	rfd  int
+	wfd  int
+	data int
+}
+
+type splicePipe struct {
+	splicePipeFields
+
+	// We want to use a finalizer, so ensure that the size is
+	// large enough to not use the tiny allocator.
+	_ [24 - unsafe.Sizeof(splicePipeFields{})%24]byte
+}
+
+// splicePipePool caches pipes to avoid high-frequency construction and destruction of pipe buffers.
+// The garbage collector will free all pipes in the sync.Pool periodically, thus we need to set up
+// a finalizer for each pipe to close its file descriptors before the actual GC.
+var splicePipePool = sync.Pool{New: newPoolPipe}
+
+func newPoolPipe() any {
+	// Discard the error which occurred during the creation of pipe buffer,
+	// redirecting the data transmission to the conventional way utilizing read() + write() as a fallback.
+	p := newPipe()
+	if p == nil {
+		return nil
+	}
+	runtime.SetFinalizer(p, destroyPipe)
+	return p
+}
+
+// getPipe tries to acquire a pipe buffer from the pool or create a new one with newPipe() if it gets nil from the cache.
+//
+// Note that it may fail to create a new pipe buffer by newPipe(), in which case getPipe() will return a generic error
+// and system call name splice in a string as the indication.
+func getPipe() (*splicePipe, string, error) {
+	v := splicePipePool.Get()
+	if v == nil {
+		return nil, "splice", syscall.EINVAL
+	}
+	return v.(*splicePipe), "", nil
+}
+
+func putPipe(p *splicePipe) {
+	// If there is still data left in the pipe,
+	// then close and discard it instead of putting it back into the pool.
+	if p.data != 0 {
+		runtime.SetFinalizer(p, nil)
+		destroyPipe(p)
+		return
+	}
+	splicePipePool.Put(p)
+}
+
+// newPipe sets up a pipe for a splice operation.
+func newPipe() *splicePipe {
+	var fds [2]int
+	if err := syscall.Pipe2(fds[:], syscall.O_CLOEXEC|syscall.O_NONBLOCK); err != nil {
+		return nil
+	}
+
+	// Splice will loop writing maxSpliceSize bytes from the source to the pipe,
+	// and then write those bytes from the pipe to the destination.
+	// Set the pipe buffer size to maxSpliceSize to optimize that.
+	// Ignore errors here, as a smaller buffer size will work,
+	// although it will require more system calls.
+	fcntl(fds[0], syscall.F_SETPIPE_SZ, maxSpliceSize)
+
+	return &splicePipe{splicePipeFields: splicePipeFields{rfd: fds[0], wfd: fds[1]}}
+}
+
+// destroyPipe destroys a pipe.
+func destroyPipe(p *splicePipe) {
+	CloseFunc(p.rfd)
+	CloseFunc(p.wfd)
+}
diff --git a/src/internal/poll/splice_linux_test.go b/src/internal/poll/splice_linux_test.go
new file mode 100644
index 0000000..29bcaab
--- /dev/null
+++ b/src/internal/poll/splice_linux_test.go
@@ -0,0 +1,136 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"internal/poll"
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+var closeHook atomic.Value // func(fd int)
+
+func init() {
+	closeFunc := poll.CloseFunc
+	poll.CloseFunc = func(fd int) (err error) {
+		if v := closeHook.Load(); v != nil {
+			if hook := v.(func(int)); hook != nil {
+				hook(fd)
+			}
+		}
+		return closeFunc(fd)
+	}
+}
+
+func TestSplicePipePool(t *testing.T) {
+	const N = 64
+	var (
+		p          *poll.SplicePipe
+		ps         []*poll.SplicePipe
+		allFDs     []int
+		pendingFDs sync.Map // fd → struct{}{}
+		err        error
+	)
+
+	closeHook.Store(func(fd int) { pendingFDs.Delete(fd) })
+	t.Cleanup(func() { closeHook.Store((func(int))(nil)) })
+
+	for i := 0; i < N; i++ {
+		p, _, err = poll.GetPipe()
+		if err != nil {
+			t.Skipf("failed to create pipe due to error(%v), skip this test", err)
+		}
+		_, pwfd := poll.GetPipeFds(p)
+		allFDs = append(allFDs, pwfd)
+		pendingFDs.Store(pwfd, struct{}{})
+		ps = append(ps, p)
+	}
+	for _, p = range ps {
+		poll.PutPipe(p)
+	}
+	ps = nil
+	p = nil
+
+	// Exploit the timeout of "go test" as a timer for the subsequent verification.
+	timeout := 5 * time.Minute
+	if deadline, ok := t.Deadline(); ok {
+		timeout = deadline.Sub(time.Now())
+		timeout -= timeout / 10 // Leave 10% headroom for cleanup.
+	}
+	expiredTime := time.NewTimer(timeout)
+	defer expiredTime.Stop()
+
+	// Trigger garbage collection repeatedly, waiting for all pipes in sync.Pool
+	// to either be deallocated and closed, or to time out.
+	for {
+		runtime.GC()
+		time.Sleep(10 * time.Millisecond)
+
+		// Detect whether all pipes are closed properly.
+		var leakedFDs []int
+		pendingFDs.Range(func(k, v any) bool {
+			leakedFDs = append(leakedFDs, k.(int))
+			return true
+		})
+		if len(leakedFDs) == 0 {
+			break
+		}
+
+		select {
+		case <-expiredTime.C:
+			t.Logf("all descriptors: %v", allFDs)
+			t.Fatalf("leaked descriptors: %v", leakedFDs)
+		default:
+		}
+	}
+}
+
+func BenchmarkSplicePipe(b *testing.B) {
+	b.Run("SplicePipeWithPool", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			p, _, err := poll.GetPipe()
+			if err != nil {
+				continue
+			}
+			poll.PutPipe(p)
+		}
+	})
+	b.Run("SplicePipeWithoutPool", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			p := poll.NewPipe()
+			if p == nil {
+				b.Skip("newPipe returned nil")
+			}
+			poll.DestroyPipe(p)
+		}
+	})
+}
+
+func BenchmarkSplicePipePoolParallel(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			p, _, err := poll.GetPipe()
+			if err != nil {
+				continue
+			}
+			poll.PutPipe(p)
+		}
+	})
+}
+
+func BenchmarkSplicePipeNativeParallel(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			p := poll.NewPipe()
+			if p == nil {
+				b.Skip("newPipe returned nil")
+			}
+			poll.DestroyPipe(p)
+		}
+	})
+}
diff --git a/src/internal/poll/strconv.go b/src/internal/poll/strconv.go
new file mode 100644
index 0000000..2b052fa
--- /dev/null
+++ b/src/internal/poll/strconv.go
@@ -0,0 +1,13 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build plan9
+
+package poll
+
+// stringsHasSuffix is strings.HasSuffix. It reports whether s ends in
+// suffix.
+func stringsHasSuffix(s, suffix string) bool {
+	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
+}
diff --git a/src/internal/poll/sys_cloexec.go b/src/internal/poll/sys_cloexec.go
new file mode 100644
index 0000000..7cd8001
--- /dev/null
+++ b/src/internal/poll/sys_cloexec.go
@@ -0,0 +1,36 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements accept for platforms that do not provide a fast path for
+// setting SetNonblock and CloseOnExec.
+
+//go:build aix || darwin || (js && wasm)
+
+package poll
+
+import (
+	"syscall"
+)
+
+// Wrapper around the accept system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func accept(s int) (int, syscall.Sockaddr, string, error) {
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	// It is probably okay to hold the lock across syscall.Accept
+	// because we have put fd.sysfd into non-blocking mode.
+	// However, a call to the File method will put it back into
+	// blocking mode. We can't take that risk, so no use of ForkLock here.
+	ns, sa, err := AcceptFunc(s)
+	if err == nil {
+		syscall.CloseOnExec(ns)
+	}
+	if err != nil {
+		return -1, nil, "accept", err
+	}
+	if err = syscall.SetNonblock(ns, true); err != nil {
+		CloseFunc(ns)
+		return -1, nil, "setnonblock", err
+	}
+	return ns, sa, "", nil
+}
diff --git a/src/internal/poll/writev.go b/src/internal/poll/writev.go
new file mode 100644
index 0000000..75c8b64
--- /dev/null
+++ b/src/internal/poll/writev.go
@@ -0,0 +1,92 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package poll
+
+import (
+	"io"
+	"runtime"
+	"syscall"
+)
+
+// Writev wraps the writev system call.
+func (fd *FD) Writev(v *[][]byte) (int64, error) {
+	if err := fd.writeLock(); err != nil {
+		return 0, err
+	}
+	defer fd.writeUnlock()
+	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
+		return 0, err
+	}
+
+	var iovecs []syscall.Iovec
+	if fd.iovecs != nil {
+		iovecs = *fd.iovecs
+	}
+	// TODO: read from sysconf(_SC_IOV_MAX)? The Linux default is
+	// 1024 and this seems conservative enough for now. Darwin's
+	// UIO_MAXIOV also seems to be 1024.
+	maxVec := 1024
+	if runtime.GOOS == "aix" || runtime.GOOS == "solaris" {
+		// IOV_MAX is set to XOPEN_IOV_MAX on AIX and Solaris.
+		maxVec = 16
+	}
+
+	var n int64
+	var err error
+	for len(*v) > 0 {
+		iovecs = iovecs[:0]
+		for _, chunk := range *v {
+			if len(chunk) == 0 {
+				continue
+			}
+			iovecs = append(iovecs, newIovecWithBase(&chunk[0]))
+			if fd.IsStream && len(chunk) > 1<<30 {
+				iovecs[len(iovecs)-1].SetLen(1 << 30)
+				break // continue chunk on next writev
+			}
+			iovecs[len(iovecs)-1].SetLen(len(chunk))
+			if len(iovecs) == maxVec {
+				break
+			}
+		}
+		if len(iovecs) == 0 {
+			break
+		}
+		if fd.iovecs == nil {
+			fd.iovecs = new([]syscall.Iovec)
+		}
+		*fd.iovecs = iovecs // cache
+
+		var wrote uintptr
+		wrote, err = writev(fd.Sysfd, iovecs)
+		if wrote == ^uintptr(0) {
+			wrote = 0
+		}
+		TestHookDidWritev(int(wrote))
+		n += int64(wrote)
+		consume(v, int64(wrote))
+		for i := range iovecs {
+			iovecs[i] = syscall.Iovec{}
+		}
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			if err == syscall.EAGAIN {
+				if err = fd.pd.waitWrite(fd.isFile); err == nil {
+					continue
+				}
+			}
+			break
+		}
+		if n == 0 {
+			err = io.ErrUnexpectedEOF
+			break
+		}
+	}
+	return n, err
+}
diff --git a/src/internal/poll/writev_test.go b/src/internal/poll/writev_test.go
new file mode 100644
index 0000000..b46657c
--- /dev/null
+++ b/src/internal/poll/writev_test.go
@@ -0,0 +1,62 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package poll_test
+
+import (
+	"internal/poll"
+	"reflect"
+	"testing"
+)
+
+func TestConsume(t *testing.T) {
+	tests := []struct {
+		in      [][]byte
+		consume int64
+		want    [][]byte
+	}{
+		{
+			in:      [][]byte{[]byte("foo"), []byte("bar")},
+			consume: 0,
+			want:    [][]byte{[]byte("foo"), []byte("bar")},
+		},
+		{
+			in:      [][]byte{[]byte("foo"), []byte("bar")},
+			consume: 2,
+			want:    [][]byte{[]byte("o"), []byte("bar")},
+		},
+		{
+			in:      [][]byte{[]byte("foo"), []byte("bar")},
+			consume: 3,
+			want:    [][]byte{[]byte("bar")},
+		},
+		{
+			in:      [][]byte{[]byte("foo"), []byte("bar")},
+			consume: 4,
+			want:    [][]byte{[]byte("ar")},
+		},
+		{
+			in:      [][]byte{nil, nil, nil, []byte("bar")},
+			consume: 1,
+			want:    [][]byte{[]byte("ar")},
+		},
+		{
+			in:      [][]byte{nil, nil, nil, []byte("foo")},
+			consume: 0,
+			want:    [][]byte{[]byte("foo")},
+		},
+		{
+			in:      [][]byte{nil, nil, nil},
+			consume: 0,
+			want:    [][]byte{},
+		},
+	}
+	for i, tt := range tests {
+		in := tt.in
+		poll.Consume(&in, tt.consume)
+		if !reflect.DeepEqual(in, tt.want) {
+			t.Errorf("%d. after consume(%d) = %+v, want %+v", i, tt.consume, in, tt.want)
+		}
+	}
+}
diff --git a/src/internal/profile/encode.go b/src/internal/profile/encode.go
new file mode 100644
index 0000000..77d77f1
--- /dev/null
+++ b/src/internal/profile/encode.go
@@ -0,0 +1,482 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package profile
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+)
+
+func (p *Profile) decoder() []decoder {
+	return profileDecoder
+}
+
+// preEncode populates the unexported fields to be used by encode
+// (with suffix X) from the corresponding exported fields. The
+// exported fields are cleared up to facilitate testing.
+func (p *Profile) preEncode() {
+	strings := make(map[string]int)
+	addString(strings, "")
+
+	for _, st := range p.SampleType {
+		st.typeX = addString(strings, st.Type)
+		st.unitX = addString(strings, st.Unit)
+	}
+
+	for _, s := range p.Sample {
+		s.labelX = nil
+		var keys []string
+		for k := range s.Label {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for _, k := range keys {
+			vs := s.Label[k]
+			for _, v := range vs {
+				s.labelX = append(s.labelX,
+					Label{
+						keyX: addString(strings, k),
+						strX: addString(strings, v),
+					},
+				)
+			}
+		}
+		var numKeys []string
+		for k := range s.NumLabel {
+			numKeys = append(numKeys, k)
+		}
+		sort.Strings(numKeys)
+		for _, k := range numKeys {
+			vs := s.NumLabel[k]
+			for _, v := range vs {
+				s.labelX = append(s.labelX,
+					Label{
+						keyX: addString(strings, k),
+						numX: v,
+					},
+				)
+			}
+		}
+		s.locationIDX = nil
+		for _, l := range s.Location {
+			s.locationIDX = append(s.locationIDX, l.ID)
+		}
+	}
+
+	for _, m := range p.Mapping {
+		m.fileX = addString(strings, m.File)
+		m.buildIDX = addString(strings, m.BuildID)
+	}
+
+	for _, l := range p.Location {
+		for i, ln := range l.Line {
+			if ln.Function != nil {
+				l.Line[i].functionIDX = ln.Function.ID
+			} else {
+				l.Line[i].functionIDX = 0
+			}
+		}
+		if l.Mapping != nil {
+			l.mappingIDX = l.Mapping.ID
+		} else {
+			l.mappingIDX = 0
+		}
+	}
+	for _, f := range p.Function {
+		f.nameX = addString(strings, f.Name)
+		f.systemNameX = addString(strings, f.SystemName)
+		f.filenameX = addString(strings, f.Filename)
+	}
+
+	p.dropFramesX = addString(strings, p.DropFrames)
+	p.keepFramesX = addString(strings, p.KeepFrames)
+
+	if pt := p.PeriodType; pt != nil {
+		pt.typeX = addString(strings, pt.Type)
+		pt.unitX = addString(strings, pt.Unit)
+	}
+
+	p.stringTable = make([]string, len(strings))
+	for s, i := range strings {
+		p.stringTable[i] = s
+	}
+}
+
+func (p *Profile) encode(b *buffer) {
+	for _, x := range p.SampleType {
+		encodeMessage(b, 1, x)
+	}
+	for _, x := range p.Sample {
+		encodeMessage(b, 2, x)
+	}
+	for _, x := range p.Mapping {
+		encodeMessage(b, 3, x)
+	}
+	for _, x := range p.Location {
+		encodeMessage(b, 4, x)
+	}
+	for _, x := range p.Function {
+		encodeMessage(b, 5, x)
+	}
+	encodeStrings(b, 6, p.stringTable)
+	encodeInt64Opt(b, 7, p.dropFramesX)
+	encodeInt64Opt(b, 8, p.keepFramesX)
+	encodeInt64Opt(b, 9, p.TimeNanos)
+	encodeInt64Opt(b, 10, p.DurationNanos)
+	if pt := p.PeriodType; pt != nil && (pt.typeX != 0 || pt.unitX != 0) {
+		encodeMessage(b, 11, p.PeriodType)
+	}
+	encodeInt64Opt(b, 12, p.Period)
+}
+
+var profileDecoder = []decoder{
+	nil, // 0
+	// repeated ValueType sample_type = 1
+	func(b *buffer, m message) error {
+		x := new(ValueType)
+		pp := m.(*Profile)
+		pp.SampleType = append(pp.SampleType, x)
+		return decodeMessage(b, x)
+	},
+	// repeated Sample sample = 2
+	func(b *buffer, m message) error {
+		x := new(Sample)
+		pp := m.(*Profile)
+		pp.Sample = append(pp.Sample, x)
+		return decodeMessage(b, x)
+	},
+	// repeated Mapping mapping = 3
+	func(b *buffer, m message) error {
+		x := new(Mapping)
+		pp := m.(*Profile)
+		pp.Mapping = append(pp.Mapping, x)
+		return decodeMessage(b, x)
+	},
+	// repeated Location location = 4
+	func(b *buffer, m message) error {
+		x := new(Location)
+		pp := m.(*Profile)
+		pp.Location = append(pp.Location, x)
+		return decodeMessage(b, x)
+	},
+	// repeated Function function = 5
+	func(b *buffer, m message) error {
+		x := new(Function)
+		pp := m.(*Profile)
+		pp.Function = append(pp.Function, x)
+		return decodeMessage(b, x)
+	},
+	// repeated string string_table = 6
+	func(b *buffer, m message) error {
+		err := decodeStrings(b, &m.(*Profile).stringTable)
+		if err != nil {
+			return err
+		}
+		if m.(*Profile).stringTable[0] != "" {
+			return errors.New("string_table[0] must be ''")
+		}
+		return nil
+	},
+	// repeated int64 drop_frames = 7
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).dropFramesX) },
+	// repeated int64 keep_frames = 8
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).keepFramesX) },
+	// repeated int64 time_nanos = 9
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).TimeNanos) },
+	// repeated int64 duration_nanos = 10
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).DurationNanos) },
+	// optional string period_type = 11
+	func(b *buffer, m message) error {
+		x := new(ValueType)
+		pp := m.(*Profile)
+		pp.PeriodType = x
+		return decodeMessage(b, x)
+	},
+	// repeated int64 period = 12
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).Period) },
+	// repeated int64 comment = 13
+	func(b *buffer, m message) error { return decodeInt64s(b, &m.(*Profile).commentX) },
+	// int64 defaultSampleType = 14
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Profile).defaultSampleTypeX) },
+}
+
+// postDecode takes the unexported fields populated by decode (with
+// suffix X) and populates the corresponding exported fields.
+// The unexported fields are cleared up to facilitate testing.
+func (p *Profile) postDecode() error {
+	var err error
+
+	mappings := make(map[uint64]*Mapping)
+	for _, m := range p.Mapping {
+		m.File, err = getString(p.stringTable, &m.fileX, err)
+		m.BuildID, err = getString(p.stringTable, &m.buildIDX, err)
+		mappings[m.ID] = m
+	}
+
+	functions := make(map[uint64]*Function)
+	for _, f := range p.Function {
+		f.Name, err = getString(p.stringTable, &f.nameX, err)
+		f.SystemName, err = getString(p.stringTable, &f.systemNameX, err)
+		f.Filename, err = getString(p.stringTable, &f.filenameX, err)
+		functions[f.ID] = f
+	}
+
+	locations := make(map[uint64]*Location)
+	for _, l := range p.Location {
+		l.Mapping = mappings[l.mappingIDX]
+		l.mappingIDX = 0
+		for i, ln := range l.Line {
+			if id := ln.functionIDX; id != 0 {
+				l.Line[i].Function = functions[id]
+				if l.Line[i].Function == nil {
+					return fmt.Errorf("Function ID %d not found", id)
+				}
+				l.Line[i].functionIDX = 0
+			}
+		}
+		locations[l.ID] = l
+	}
+
+	for _, st := range p.SampleType {
+		st.Type, err = getString(p.stringTable, &st.typeX, err)
+		st.Unit, err = getString(p.stringTable, &st.unitX, err)
+	}
+
+	for _, s := range p.Sample {
+		labels := make(map[string][]string)
+		numLabels := make(map[string][]int64)
+		for _, l := range s.labelX {
+			var key, value string
+			key, err = getString(p.stringTable, &l.keyX, err)
+			if l.strX != 0 {
+				value, err = getString(p.stringTable, &l.strX, err)
+				labels[key] = append(labels[key], value)
+			} else {
+				numLabels[key] = append(numLabels[key], l.numX)
+			}
+		}
+		if len(labels) > 0 {
+			s.Label = labels
+		}
+		if len(numLabels) > 0 {
+			s.NumLabel = numLabels
+		}
+		s.Location = nil
+		for _, lid := range s.locationIDX {
+			s.Location = append(s.Location, locations[lid])
+		}
+		s.locationIDX = nil
+	}
+
+	p.DropFrames, err = getString(p.stringTable, &p.dropFramesX, err)
+	p.KeepFrames, err = getString(p.stringTable, &p.keepFramesX, err)
+
+	if pt := p.PeriodType; pt == nil {
+		p.PeriodType = &ValueType{}
+	}
+
+	if pt := p.PeriodType; pt != nil {
+		pt.Type, err = getString(p.stringTable, &pt.typeX, err)
+		pt.Unit, err = getString(p.stringTable, &pt.unitX, err)
+	}
+	for _, i := range p.commentX {
+		var c string
+		c, err = getString(p.stringTable, &i, err)
+		p.Comments = append(p.Comments, c)
+	}
+
+	p.commentX = nil
+	p.DefaultSampleType, err = getString(p.stringTable, &p.defaultSampleTypeX, err)
+	p.stringTable = nil
+	return nil
+}
+
+func (p *ValueType) decoder() []decoder {
+	return valueTypeDecoder
+}
+
+func (p *ValueType) encode(b *buffer) {
+	encodeInt64Opt(b, 1, p.typeX)
+	encodeInt64Opt(b, 2, p.unitX)
+}
+
+var valueTypeDecoder = []decoder{
+	nil, // 0
+	// optional int64 type = 1
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*ValueType).typeX) },
+	// optional int64 unit = 2
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*ValueType).unitX) },
+}
+
+func (p *Sample) decoder() []decoder {
+	return sampleDecoder
+}
+
+func (p *Sample) encode(b *buffer) {
+	encodeUint64s(b, 1, p.locationIDX)
+	for _, x := range p.Value {
+		encodeInt64(b, 2, x)
+	}
+	for _, x := range p.labelX {
+		encodeMessage(b, 3, x)
+	}
+}
+
+var sampleDecoder = []decoder{
+	nil, // 0
+	// repeated uint64 location = 1
+	func(b *buffer, m message) error { return decodeUint64s(b, &m.(*Sample).locationIDX) },
+	// repeated int64 value = 2
+	func(b *buffer, m message) error { return decodeInt64s(b, &m.(*Sample).Value) },
+	// repeated Label label = 3
+	func(b *buffer, m message) error {
+		s := m.(*Sample)
+		n := len(s.labelX)
+		s.labelX = append(s.labelX, Label{})
+		return decodeMessage(b, &s.labelX[n])
+	},
+}
+
+func (p Label) decoder() []decoder {
+	return labelDecoder
+}
+
+func (p Label) encode(b *buffer) {
+	encodeInt64Opt(b, 1, p.keyX)
+	encodeInt64Opt(b, 2, p.strX)
+	encodeInt64Opt(b, 3, p.numX)
+}
+
+var labelDecoder = []decoder{
+	nil, // 0
+	// optional int64 key = 1
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Label).keyX) },
+	// optional int64 str = 2
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Label).strX) },
+	// optional int64 num = 3
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Label).numX) },
+}
+
+func (p *Mapping) decoder() []decoder {
+	return mappingDecoder
+}
+
+func (p *Mapping) encode(b *buffer) {
+	encodeUint64Opt(b, 1, p.ID)
+	encodeUint64Opt(b, 2, p.Start)
+	encodeUint64Opt(b, 3, p.Limit)
+	encodeUint64Opt(b, 4, p.Offset)
+	encodeInt64Opt(b, 5, p.fileX)
+	encodeInt64Opt(b, 6, p.buildIDX)
+	encodeBoolOpt(b, 7, p.HasFunctions)
+	encodeBoolOpt(b, 8, p.HasFilenames)
+	encodeBoolOpt(b, 9, p.HasLineNumbers)
+	encodeBoolOpt(b, 10, p.HasInlineFrames)
+}
+
+var mappingDecoder = []decoder{
+	nil, // 0
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Mapping).ID) },            // optional uint64 id = 1
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Mapping).Start) },         // optional uint64 memory_offset = 2
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Mapping).Limit) },         // optional uint64 memory_limit = 3
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Mapping).Offset) },        // optional uint64 file_offset = 4
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Mapping).fileX) },          // optional int64 filename = 5
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Mapping).buildIDX) },       // optional int64 build_id = 6
+	func(b *buffer, m message) error { return decodeBool(b, &m.(*Mapping).HasFunctions) },    // optional bool has_functions = 7
+	func(b *buffer, m message) error { return decodeBool(b, &m.(*Mapping).HasFilenames) },    // optional bool has_filenames = 8
+	func(b *buffer, m message) error { return decodeBool(b, &m.(*Mapping).HasLineNumbers) },  // optional bool has_line_numbers = 9
+	func(b *buffer, m message) error { return decodeBool(b, &m.(*Mapping).HasInlineFrames) }, // optional bool has_inline_frames = 10
+}
+
+func (p *Location) decoder() []decoder {
+	return locationDecoder
+}
+
+func (p *Location) encode(b *buffer) {
+	encodeUint64Opt(b, 1, p.ID)
+	encodeUint64Opt(b, 2, p.mappingIDX)
+	encodeUint64Opt(b, 3, p.Address)
+	for i := range p.Line {
+		encodeMessage(b, 4, &p.Line[i])
+	}
+}
+
+var locationDecoder = []decoder{
+	nil, // 0
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Location).ID) },         // optional uint64 id = 1;
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Location).mappingIDX) }, // optional uint64 mapping_id = 2;
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Location).Address) },    // optional uint64 address = 3;
+	func(b *buffer, m message) error { // repeated Line line = 4
+		pp := m.(*Location)
+		n := len(pp.Line)
+		pp.Line = append(pp.Line, Line{})
+		return decodeMessage(b, &pp.Line[n])
+	},
+}
+
+func (p *Line) decoder() []decoder {
+	return lineDecoder
+}
+
+func (p *Line) encode(b *buffer) {
+	encodeUint64Opt(b, 1, p.functionIDX)
+	encodeInt64Opt(b, 2, p.Line)
+}
+
+var lineDecoder = []decoder{
+	nil, // 0
+	// optional uint64 function_id = 1
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Line).functionIDX) },
+	// optional int64 line = 2
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Line).Line) },
+}
+
+func (p *Function) decoder() []decoder {
+	return functionDecoder
+}
+
+func (p *Function) encode(b *buffer) {
+	encodeUint64Opt(b, 1, p.ID)
+	encodeInt64Opt(b, 2, p.nameX)
+	encodeInt64Opt(b, 3, p.systemNameX)
+	encodeInt64Opt(b, 4, p.filenameX)
+	encodeInt64Opt(b, 5, p.StartLine)
+}
+
+var functionDecoder = []decoder{
+	nil, // 0
+	// optional uint64 id = 1
+	func(b *buffer, m message) error { return decodeUint64(b, &m.(*Function).ID) },
+	// optional int64 function_name = 2
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Function).nameX) },
+	// optional int64 function_system_name = 3
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Function).systemNameX) },
+	// repeated int64 filename = 4
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Function).filenameX) },
+	// optional int64 start_line = 5
+	func(b *buffer, m message) error { return decodeInt64(b, &m.(*Function).StartLine) },
+}
+
+func addString(strings map[string]int, s string) int64 {
+	i, ok := strings[s]
+	if !ok {
+		i = len(strings)
+		strings[s] = i
+	}
+	return int64(i)
+}
+
+func getString(strings []string, strng *int64, err error) (string, error) {
+	if err != nil {
+		return "", err
+	}
+	s := int(*strng)
+	if s < 0 || s >= len(strings) {
+		return "", errMalformed
+	}
+	*strng = 0
+	return strings[s], nil
+}
diff --git a/src/internal/profile/filter.go b/src/internal/profile/filter.go
new file mode 100644
index 0000000..141dd1f
--- /dev/null
+++ b/src/internal/profile/filter.go
@@ -0,0 +1,158 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Implements methods to filter samples from profiles.
+
+package profile
+
+import "regexp"
+
+// FilterSamplesByName filters the samples in a profile and only keeps
+// samples where at least one frame matches focus but none match ignore.
+// Returns true is the corresponding regexp matched at least one sample.
+func (p *Profile) FilterSamplesByName(focus, ignore, hide *regexp.Regexp) (fm, im, hm bool) {
+	focusOrIgnore := make(map[uint64]bool)
+	hidden := make(map[uint64]bool)
+	for _, l := range p.Location {
+		if ignore != nil && l.matchesName(ignore) {
+			im = true
+			focusOrIgnore[l.ID] = false
+		} else if focus == nil || l.matchesName(focus) {
+			fm = true
+			focusOrIgnore[l.ID] = true
+		}
+		if hide != nil && l.matchesName(hide) {
+			hm = true
+			l.Line = l.unmatchedLines(hide)
+			if len(l.Line) == 0 {
+				hidden[l.ID] = true
+			}
+		}
+	}
+
+	s := make([]*Sample, 0, len(p.Sample))
+	for _, sample := range p.Sample {
+		if focusedAndNotIgnored(sample.Location, focusOrIgnore) {
+			if len(hidden) > 0 {
+				var locs []*Location
+				for _, loc := range sample.Location {
+					if !hidden[loc.ID] {
+						locs = append(locs, loc)
+					}
+				}
+				if len(locs) == 0 {
+					// Remove sample with no locations (by not adding it to s).
+					continue
+				}
+				sample.Location = locs
+			}
+			s = append(s, sample)
+		}
+	}
+	p.Sample = s
+
+	return
+}
+
+// matchesName reports whether the function name or file in the
+// location matches the regular expression.
+func (loc *Location) matchesName(re *regexp.Regexp) bool {
+	for _, ln := range loc.Line {
+		if fn := ln.Function; fn != nil {
+			if re.MatchString(fn.Name) {
+				return true
+			}
+			if re.MatchString(fn.Filename) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// unmatchedLines returns the lines in the location that do not match
+// the regular expression.
+func (loc *Location) unmatchedLines(re *regexp.Regexp) []Line {
+	var lines []Line
+	for _, ln := range loc.Line {
+		if fn := ln.Function; fn != nil {
+			if re.MatchString(fn.Name) {
+				continue
+			}
+			if re.MatchString(fn.Filename) {
+				continue
+			}
+		}
+		lines = append(lines, ln)
+	}
+	return lines
+}
+
+// focusedAndNotIgnored looks up a slice of ids against a map of
+// focused/ignored locations. The map only contains locations that are
+// explicitly focused or ignored. Returns whether there is at least
+// one focused location but no ignored locations.
+func focusedAndNotIgnored(locs []*Location, m map[uint64]bool) bool {
+	var f bool
+	for _, loc := range locs {
+		if focus, focusOrIgnore := m[loc.ID]; focusOrIgnore {
+			if focus {
+				// Found focused location. Must keep searching in case there
+				// is an ignored one as well.
+				f = true
+			} else {
+				// Found ignored location. Can return false right away.
+				return false
+			}
+		}
+	}
+	return f
+}
+
+// TagMatch selects tags for filtering
+type TagMatch func(key, val string, nval int64) bool
+
+// FilterSamplesByTag removes all samples from the profile, except
+// those that match focus and do not match the ignore regular
+// expression.
+func (p *Profile) FilterSamplesByTag(focus, ignore TagMatch) (fm, im bool) {
+	samples := make([]*Sample, 0, len(p.Sample))
+	for _, s := range p.Sample {
+		focused, ignored := focusedSample(s, focus, ignore)
+		fm = fm || focused
+		im = im || ignored
+		if focused && !ignored {
+			samples = append(samples, s)
+		}
+	}
+	p.Sample = samples
+	return
+}
+
+// focusedSample checks a sample against focus and ignore regexps.
+// Returns whether the focus/ignore regexps match any tags.
+func focusedSample(s *Sample, focus, ignore TagMatch) (fm, im bool) {
+	fm = focus == nil
+	for key, vals := range s.Label {
+		for _, val := range vals {
+			if ignore != nil && ignore(key, val, 0) {
+				im = true
+			}
+			if !fm && focus(key, val, 0) {
+				fm = true
+			}
+		}
+	}
+	for key, vals := range s.NumLabel {
+		for _, val := range vals {
+			if ignore != nil && ignore(key, "", val) {
+				im = true
+			}
+			if !fm && focus(key, "", val) {
+				fm = true
+			}
+		}
+	}
+	return fm, im
+}
diff --git a/src/internal/profile/legacy_profile.go b/src/internal/profile/legacy_profile.go
new file mode 100644
index 0000000..b102c95
--- /dev/null
+++ b/src/internal/profile/legacy_profile.go
@@ -0,0 +1,1268 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements parsers to convert legacy profiles into the
+// profile.proto format.
+
+package profile
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"math"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+var (
+	countStartRE = regexp.MustCompile(`\A(\w+) profile: total \d+\n\z`)
+	countRE      = regexp.MustCompile(`\A(\d+) @(( 0x[0-9a-f]+)+)\n\z`)
+
+	heapHeaderRE = regexp.MustCompile(`heap profile: *(\d+): *(\d+) *\[ *(\d+): *(\d+) *\] *@ *(heap[_a-z0-9]*)/?(\d*)`)
+	heapSampleRE = regexp.MustCompile(`(-?\d+): *(-?\d+) *\[ *(\d+): *(\d+) *] @([ x0-9a-f]*)`)
+
+	contentionSampleRE = regexp.MustCompile(`(\d+) *(\d+) @([ x0-9a-f]*)`)
+
+	hexNumberRE = regexp.MustCompile(`0x[0-9a-f]+`)
+
+	growthHeaderRE = regexp.MustCompile(`heap profile: *(\d+): *(\d+) *\[ *(\d+): *(\d+) *\] @ growthz`)
+
+	fragmentationHeaderRE = regexp.MustCompile(`heap profile: *(\d+): *(\d+) *\[ *(\d+): *(\d+) *\] @ fragmentationz`)
+
+	threadzStartRE = regexp.MustCompile(`--- threadz \d+ ---`)
+	threadStartRE  = regexp.MustCompile(`--- Thread ([[:xdigit:]]+) \(name: (.*)/(\d+)\) stack: ---`)
+
+	procMapsRE = regexp.MustCompile(`([[:xdigit:]]+)-([[:xdigit:]]+)\s+([-rwxp]+)\s+([[:xdigit:]]+)\s+([[:xdigit:]]+):([[:xdigit:]]+)\s+([[:digit:]]+)\s*(\S+)?`)
+
+	briefMapsRE = regexp.MustCompile(`\s*([[:xdigit:]]+)-([[:xdigit:]]+):\s*(\S+)(\s.*@)?([[:xdigit:]]+)?`)
+
+	// LegacyHeapAllocated instructs the heapz parsers to use the
+	// allocated memory stats instead of the default in-use memory. Note
+	// that tcmalloc doesn't provide all allocated memory, only in-use
+	// stats.
+	LegacyHeapAllocated bool
+)
+
+func isSpaceOrComment(line string) bool {
+	trimmed := strings.TrimSpace(line)
+	return len(trimmed) == 0 || trimmed[0] == '#'
+}
+
+// parseGoCount parses a Go count profile (e.g., threadcreate or
+// goroutine) and returns a new Profile.
+func parseGoCount(b []byte) (*Profile, error) {
+	r := bytes.NewBuffer(b)
+
+	var line string
+	var err error
+	for {
+		// Skip past comments and empty lines seeking a real header.
+		line, err = r.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+		if !isSpaceOrComment(line) {
+			break
+		}
+	}
+
+	m := countStartRE.FindStringSubmatch(line)
+	if m == nil {
+		return nil, errUnrecognized
+	}
+	profileType := m[1]
+	p := &Profile{
+		PeriodType: &ValueType{Type: profileType, Unit: "count"},
+		Period:     1,
+		SampleType: []*ValueType{{Type: profileType, Unit: "count"}},
+	}
+	locations := make(map[uint64]*Location)
+	for {
+		line, err = r.ReadString('\n')
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+			return nil, err
+		}
+		if isSpaceOrComment(line) {
+			continue
+		}
+		if strings.HasPrefix(line, "---") {
+			break
+		}
+		m := countRE.FindStringSubmatch(line)
+		if m == nil {
+			return nil, errMalformed
+		}
+		n, err := strconv.ParseInt(m[1], 0, 64)
+		if err != nil {
+			return nil, errMalformed
+		}
+		fields := strings.Fields(m[2])
+		locs := make([]*Location, 0, len(fields))
+		for _, stk := range fields {
+			addr, err := strconv.ParseUint(stk, 0, 64)
+			if err != nil {
+				return nil, errMalformed
+			}
+			// Adjust all frames by -1 to land on the call instruction.
+			addr--
+			loc := locations[addr]
+			if loc == nil {
+				loc = &Location{
+					Address: addr,
+				}
+				locations[addr] = loc
+				p.Location = append(p.Location, loc)
+			}
+			locs = append(locs, loc)
+		}
+		p.Sample = append(p.Sample, &Sample{
+			Location: locs,
+			Value:    []int64{n},
+		})
+	}
+
+	if err = parseAdditionalSections(strings.TrimSpace(line), r, p); err != nil {
+		return nil, err
+	}
+	return p, nil
+}
+
+// remapLocationIDs ensures there is a location for each address
+// referenced by a sample, and remaps the samples to point to the new
+// location ids.
+func (p *Profile) remapLocationIDs() {
+	seen := make(map[*Location]bool, len(p.Location))
+	var locs []*Location
+
+	for _, s := range p.Sample {
+		for _, l := range s.Location {
+			if seen[l] {
+				continue
+			}
+			l.ID = uint64(len(locs) + 1)
+			locs = append(locs, l)
+			seen[l] = true
+		}
+	}
+	p.Location = locs
+}
+
+func (p *Profile) remapFunctionIDs() {
+	seen := make(map[*Function]bool, len(p.Function))
+	var fns []*Function
+
+	for _, l := range p.Location {
+		for _, ln := range l.Line {
+			fn := ln.Function
+			if fn == nil || seen[fn] {
+				continue
+			}
+			fn.ID = uint64(len(fns) + 1)
+			fns = append(fns, fn)
+			seen[fn] = true
+		}
+	}
+	p.Function = fns
+}
+
+// remapMappingIDs matches location addresses with existing mappings
+// and updates them appropriately. This is O(N*M), if this ever shows
+// up as a bottleneck, evaluate sorting the mappings and doing a
+// binary search, which would make it O(N*log(M)).
+func (p *Profile) remapMappingIDs() {
+	if len(p.Mapping) == 0 {
+		return
+	}
+
+	// Some profile handlers will incorrectly set regions for the main
+	// executable if its section is remapped. Fix them through heuristics.
+
+	// Remove the initial mapping if named '/anon_hugepage' and has a
+	// consecutive adjacent mapping.
+	if m := p.Mapping[0]; strings.HasPrefix(m.File, "/anon_hugepage") {
+		if len(p.Mapping) > 1 && m.Limit == p.Mapping[1].Start {
+			p.Mapping = p.Mapping[1:]
+		}
+	}
+
+	for _, l := range p.Location {
+		if a := l.Address; a != 0 {
+			for _, m := range p.Mapping {
+				if m.Start <= a && a < m.Limit {
+					l.Mapping = m
+					break
+				}
+			}
+		}
+	}
+
+	// Reset all mapping IDs.
+	for i, m := range p.Mapping {
+		m.ID = uint64(i + 1)
+	}
+}
+
+var cpuInts = []func([]byte) (uint64, []byte){
+	get32l,
+	get32b,
+	get64l,
+	get64b,
+}
+
+func get32l(b []byte) (uint64, []byte) {
+	if len(b) < 4 {
+		return 0, nil
+	}
+	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24, b[4:]
+}
+
+func get32b(b []byte) (uint64, []byte) {
+	if len(b) < 4 {
+		return 0, nil
+	}
+	return uint64(b[3]) | uint64(b[2])<<8 | uint64(b[1])<<16 | uint64(b[0])<<24, b[4:]
+}
+
+func get64l(b []byte) (uint64, []byte) {
+	if len(b) < 8 {
+		return 0, nil
+	}
+	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 | uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56, b[8:]
+}
+
+func get64b(b []byte) (uint64, []byte) {
+	if len(b) < 8 {
+		return 0, nil
+	}
+	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 | uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56, b[8:]
+}
+
+// ParseTracebacks parses a set of tracebacks and returns a newly
+// populated profile. It will accept any text file and generate a
+// Profile out of it with any hex addresses it can identify, including
+// a process map if it can recognize one. Each sample will include a
+// tag "source" with the addresses recognized in string format.
+func ParseTracebacks(b []byte) (*Profile, error) {
+	r := bytes.NewBuffer(b)
+
+	p := &Profile{
+		PeriodType: &ValueType{Type: "trace", Unit: "count"},
+		Period:     1,
+		SampleType: []*ValueType{
+			{Type: "trace", Unit: "count"},
+		},
+	}
+
+	var sources []string
+	var sloc []*Location
+
+	locs := make(map[uint64]*Location)
+	for {
+		l, err := r.ReadString('\n')
+		if err != nil {
+			if err != io.EOF {
+				return nil, err
+			}
+			if l == "" {
+				break
+			}
+		}
+		if sectionTrigger(l) == memoryMapSection {
+			break
+		}
+		if s, addrs := extractHexAddresses(l); len(s) > 0 {
+			for _, addr := range addrs {
+				// Addresses from stack traces point to the next instruction after
+				// each call. Adjust by -1 to land somewhere on the actual call.
+				addr--
+				loc := locs[addr]
+				if locs[addr] == nil {
+					loc = &Location{
+						Address: addr,
+					}
+					p.Location = append(p.Location, loc)
+					locs[addr] = loc
+				}
+				sloc = append(sloc, loc)
+			}
+
+			sources = append(sources, s...)
+		} else {
+			if len(sources) > 0 || len(sloc) > 0 {
+				addTracebackSample(sloc, sources, p)
+				sloc, sources = nil, nil
+			}
+		}
+	}
+
+	// Add final sample to save any leftover data.
+	if len(sources) > 0 || len(sloc) > 0 {
+		addTracebackSample(sloc, sources, p)
+	}
+
+	if err := p.ParseMemoryMap(r); err != nil {
+		return nil, err
+	}
+	return p, nil
+}
+
+func addTracebackSample(l []*Location, s []string, p *Profile) {
+	p.Sample = append(p.Sample,
+		&Sample{
+			Value:    []int64{1},
+			Location: l,
+			Label:    map[string][]string{"source": s},
+		})
+}
+
+// parseCPU parses a profilez legacy profile and returns a newly
+// populated Profile.
+//
+// The general format for profilez samples is a sequence of words in
+// binary format. The first words are a header with the following data:
+//
+//	1st word -- 0
+//	2nd word -- 3
+//	3rd word -- 0 if a c++ application, 1 if a java application.
+//	4th word -- Sampling period (in microseconds).
+//	5th word -- Padding.
+func parseCPU(b []byte) (*Profile, error) {
+	var parse func([]byte) (uint64, []byte)
+	var n1, n2, n3, n4, n5 uint64
+	for _, parse = range cpuInts {
+		var tmp []byte
+		n1, tmp = parse(b)
+		n2, tmp = parse(tmp)
+		n3, tmp = parse(tmp)
+		n4, tmp = parse(tmp)
+		n5, tmp = parse(tmp)
+
+		if tmp != nil && n1 == 0 && n2 == 3 && n3 == 0 && n4 > 0 && n5 == 0 {
+			b = tmp
+			return cpuProfile(b, int64(n4), parse)
+		}
+	}
+	return nil, errUnrecognized
+}
+
+// cpuProfile returns a new Profile from C++ profilez data.
+// b is the profile bytes after the header, period is the profiling
+// period, and parse is a function to parse 8-byte chunks from the
+// profile in its native endianness.
+func cpuProfile(b []byte, period int64, parse func(b []byte) (uint64, []byte)) (*Profile, error) {
+	p := &Profile{
+		Period:     period * 1000,
+		PeriodType: &ValueType{Type: "cpu", Unit: "nanoseconds"},
+		SampleType: []*ValueType{
+			{Type: "samples", Unit: "count"},
+			{Type: "cpu", Unit: "nanoseconds"},
+		},
+	}
+	var err error
+	if b, _, err = parseCPUSamples(b, parse, true, p); err != nil {
+		return nil, err
+	}
+
+	// If all samples have the same second-to-the-bottom frame, it
+	// strongly suggests that it is an uninteresting artifact of
+	// measurement -- a stack frame pushed by the signal handler. The
+	// bottom frame is always correct as it is picked up from the signal
+	// structure, not the stack. Check if this is the case and if so,
+	// remove.
+	if len(p.Sample) > 1 && len(p.Sample[0].Location) > 1 {
+		allSame := true
+		id1 := p.Sample[0].Location[1].Address
+		for _, s := range p.Sample {
+			if len(s.Location) < 2 || id1 != s.Location[1].Address {
+				allSame = false
+				break
+			}
+		}
+		if allSame {
+			for _, s := range p.Sample {
+				s.Location = append(s.Location[:1], s.Location[2:]...)
+			}
+		}
+	}
+
+	if err := p.ParseMemoryMap(bytes.NewBuffer(b)); err != nil {
+		return nil, err
+	}
+	return p, nil
+}
+
+// parseCPUSamples parses a collection of profilez samples from a
+// profile.
+//
+// profilez samples are a repeated sequence of stack frames of the
+// form:
+//
+//	1st word -- The number of times this stack was encountered.
+//	2nd word -- The size of the stack (StackSize).
+//	3rd word -- The first address on the stack.
+//	...
+//	StackSize + 2 -- The last address on the stack
+//
+// The last stack trace is of the form:
+//
+//	1st word -- 0
+//	2nd word -- 1
+//	3rd word -- 0
+//
+// Addresses from stack traces may point to the next instruction after
+// each call. Optionally adjust by -1 to land somewhere on the actual
+// call (except for the leaf, which is not a call).
+func parseCPUSamples(b []byte, parse func(b []byte) (uint64, []byte), adjust bool, p *Profile) ([]byte, map[uint64]*Location, error) {
+	locs := make(map[uint64]*Location)
+	for len(b) > 0 {
+		var count, nstk uint64
+		count, b = parse(b)
+		nstk, b = parse(b)
+		if b == nil || nstk > uint64(len(b)/4) {
+			return nil, nil, errUnrecognized
+		}
+		var sloc []*Location
+		addrs := make([]uint64, nstk)
+		for i := 0; i < int(nstk); i++ {
+			addrs[i], b = parse(b)
+		}
+
+		if count == 0 && nstk == 1 && addrs[0] == 0 {
+			// End of data marker
+			break
+		}
+		for i, addr := range addrs {
+			if adjust && i > 0 {
+				addr--
+			}
+			loc := locs[addr]
+			if loc == nil {
+				loc = &Location{
+					Address: addr,
+				}
+				locs[addr] = loc
+				p.Location = append(p.Location, loc)
+			}
+			sloc = append(sloc, loc)
+		}
+		p.Sample = append(p.Sample,
+			&Sample{
+				Value:    []int64{int64(count), int64(count) * p.Period},
+				Location: sloc,
+			})
+	}
+	// Reached the end without finding the EOD marker.
+	return b, locs, nil
+}
+
+// parseHeap parses a heapz legacy or a growthz profile and
+// returns a newly populated Profile.
+func parseHeap(b []byte) (p *Profile, err error) {
+	r := bytes.NewBuffer(b)
+	l, err := r.ReadString('\n')
+	if err != nil {
+		return nil, errUnrecognized
+	}
+
+	sampling := ""
+
+	if header := heapHeaderRE.FindStringSubmatch(l); header != nil {
+		p = &Profile{
+			SampleType: []*ValueType{
+				{Type: "objects", Unit: "count"},
+				{Type: "space", Unit: "bytes"},
+			},
+			PeriodType: &ValueType{Type: "objects", Unit: "bytes"},
+		}
+
+		var period int64
+		if len(header[6]) > 0 {
+			if period, err = strconv.ParseInt(header[6], 10, 64); err != nil {
+				return nil, errUnrecognized
+			}
+		}
+
+		switch header[5] {
+		case "heapz_v2", "heap_v2":
+			sampling, p.Period = "v2", period
+		case "heapprofile":
+			sampling, p.Period = "", 1
+		case "heap":
+			sampling, p.Period = "v2", period/2
+		default:
+			return nil, errUnrecognized
+		}
+	} else if header = growthHeaderRE.FindStringSubmatch(l); header != nil {
+		p = &Profile{
+			SampleType: []*ValueType{
+				{Type: "objects", Unit: "count"},
+				{Type: "space", Unit: "bytes"},
+			},
+			PeriodType: &ValueType{Type: "heapgrowth", Unit: "count"},
+			Period:     1,
+		}
+	} else if header = fragmentationHeaderRE.FindStringSubmatch(l); header != nil {
+		p = &Profile{
+			SampleType: []*ValueType{
+				{Type: "objects", Unit: "count"},
+				{Type: "space", Unit: "bytes"},
+			},
+			PeriodType: &ValueType{Type: "allocations", Unit: "count"},
+			Period:     1,
+		}
+	} else {
+		return nil, errUnrecognized
+	}
+
+	if LegacyHeapAllocated {
+		for _, st := range p.SampleType {
+			st.Type = "alloc_" + st.Type
+		}
+	} else {
+		for _, st := range p.SampleType {
+			st.Type = "inuse_" + st.Type
+		}
+	}
+
+	locs := make(map[uint64]*Location)
+	for {
+		l, err = r.ReadString('\n')
+		if err != nil {
+			if err != io.EOF {
+				return nil, err
+			}
+
+			if l == "" {
+				break
+			}
+		}
+
+		if isSpaceOrComment(l) {
+			continue
+		}
+		l = strings.TrimSpace(l)
+
+		if sectionTrigger(l) != unrecognizedSection {
+			break
+		}
+
+		value, blocksize, addrs, err := parseHeapSample(l, p.Period, sampling)
+		if err != nil {
+			return nil, err
+		}
+		var sloc []*Location
+		for _, addr := range addrs {
+			// Addresses from stack traces point to the next instruction after
+			// each call. Adjust by -1 to land somewhere on the actual call.
+			addr--
+			loc := locs[addr]
+			if locs[addr] == nil {
+				loc = &Location{
+					Address: addr,
+				}
+				p.Location = append(p.Location, loc)
+				locs[addr] = loc
+			}
+			sloc = append(sloc, loc)
+		}
+
+		p.Sample = append(p.Sample, &Sample{
+			Value:    value,
+			Location: sloc,
+			NumLabel: map[string][]int64{"bytes": {blocksize}},
+		})
+	}
+
+	if err = parseAdditionalSections(l, r, p); err != nil {
+		return nil, err
+	}
+	return p, nil
+}
+
+// parseHeapSample parses a single row from a heap profile into a new Sample.
+func parseHeapSample(line string, rate int64, sampling string) (value []int64, blocksize int64, addrs []uint64, err error) {
+	sampleData := heapSampleRE.FindStringSubmatch(line)
+	if len(sampleData) != 6 {
+		return value, blocksize, addrs, fmt.Errorf("unexpected number of sample values: got %d, want 6", len(sampleData))
+	}
+
+	// Use first two values by default; tcmalloc sampling generates the
+	// same value for both, only the older heap-profile collect separate
+	// stats for in-use and allocated objects.
+	valueIndex := 1
+	if LegacyHeapAllocated {
+		valueIndex = 3
+	}
+
+	var v1, v2 int64
+	if v1, err = strconv.ParseInt(sampleData[valueIndex], 10, 64); err != nil {
+		return value, blocksize, addrs, fmt.Errorf("malformed sample: %s: %v", line, err)
+	}
+	if v2, err = strconv.ParseInt(sampleData[valueIndex+1], 10, 64); err != nil {
+		return value, blocksize, addrs, fmt.Errorf("malformed sample: %s: %v", line, err)
+	}
+
+	if v1 == 0 {
+		if v2 != 0 {
+			return value, blocksize, addrs, fmt.Errorf("allocation count was 0 but allocation bytes was %d", v2)
+		}
+	} else {
+		blocksize = v2 / v1
+		if sampling == "v2" {
+			v1, v2 = scaleHeapSample(v1, v2, rate)
+		}
+	}
+
+	value = []int64{v1, v2}
+	addrs = parseHexAddresses(sampleData[5])
+
+	return value, blocksize, addrs, nil
+}
+
+// extractHexAddresses extracts hex numbers from a string and returns
+// them, together with their numeric value, in a slice.
+func extractHexAddresses(s string) ([]string, []uint64) {
+	hexStrings := hexNumberRE.FindAllString(s, -1)
+	var ids []uint64
+	for _, s := range hexStrings {
+		if id, err := strconv.ParseUint(s, 0, 64); err == nil {
+			ids = append(ids, id)
+		} else {
+			// Do not expect any parsing failures due to the regexp matching.
+			panic("failed to parse hex value:" + s)
+		}
+	}
+	return hexStrings, ids
+}
+
+// parseHexAddresses parses hex numbers from a string and returns them
+// in a slice.
+func parseHexAddresses(s string) []uint64 {
+	_, ids := extractHexAddresses(s)
+	return ids
+}
+
+// scaleHeapSample adjusts the data from a heapz Sample to
+// account for its probability of appearing in the collected
+// data. heapz profiles are a sampling of the memory allocations
+// requests in a program. We estimate the unsampled value by dividing
+// each collected sample by its probability of appearing in the
+// profile. heapz v2 profiles rely on a poisson process to determine
+// which samples to collect, based on the desired average collection
+// rate R. The probability of a sample of size S to appear in that
+// profile is 1-exp(-S/R).
+func scaleHeapSample(count, size, rate int64) (int64, int64) {
+	if count == 0 || size == 0 {
+		return 0, 0
+	}
+
+	if rate <= 1 {
+		// if rate==1 all samples were collected so no adjustment is needed.
+		// if rate<1 treat as unknown and skip scaling.
+		return count, size
+	}
+
+	avgSize := float64(size) / float64(count)
+	scale := 1 / (1 - math.Exp(-avgSize/float64(rate)))
+
+	return int64(float64(count) * scale), int64(float64(size) * scale)
+}
+
+// parseContention parses a mutex or contention profile. There are 2 cases:
+// "--- contentionz " for legacy C++ profiles (and backwards compatibility)
+// "--- mutex:" or "--- contention:" for profiles generated by the Go runtime.
+// This code converts the text output from runtime into a *Profile. (In the future
+// the runtime might write a serialized Profile directly making this unnecessary.)
+func parseContention(b []byte) (*Profile, error) {
+	r := bytes.NewBuffer(b)
+	var l string
+	var err error
+	for {
+		// Skip past comments and empty lines seeking a real header.
+		l, err = r.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+		if !isSpaceOrComment(l) {
+			break
+		}
+	}
+
+	if strings.HasPrefix(l, "--- contentionz ") {
+		return parseCppContention(r)
+	} else if strings.HasPrefix(l, "--- mutex:") {
+		return parseCppContention(r)
+	} else if strings.HasPrefix(l, "--- contention:") {
+		return parseCppContention(r)
+	}
+	return nil, errUnrecognized
+}
+
+// parseCppContention parses the output from synchronization_profiling.cc
+// for backward compatibility, and the compatible (non-debug) block profile
+// output from the Go runtime.
+func parseCppContention(r *bytes.Buffer) (*Profile, error) {
+	p := &Profile{
+		PeriodType: &ValueType{Type: "contentions", Unit: "count"},
+		Period:     1,
+		SampleType: []*ValueType{
+			{Type: "contentions", Unit: "count"},
+			{Type: "delay", Unit: "nanoseconds"},
+		},
+	}
+
+	var cpuHz int64
+	var l string
+	var err error
+	// Parse text of the form "attribute = value" before the samples.
+	const delimiter = '='
+	for {
+		l, err = r.ReadString('\n')
+		if err != nil {
+			if err != io.EOF {
+				return nil, err
+			}
+
+			if l == "" {
+				break
+			}
+		}
+		if isSpaceOrComment(l) {
+			continue
+		}
+
+		if l = strings.TrimSpace(l); l == "" {
+			continue
+		}
+
+		if strings.HasPrefix(l, "---") {
+			break
+		}
+
+		index := strings.IndexByte(l, delimiter)
+		if index < 0 {
+			break
+		}
+		key := l[:index]
+		val := l[index+1:]
+
+		key, val = strings.TrimSpace(key), strings.TrimSpace(val)
+		var err error
+		switch key {
+		case "cycles/second":
+			if cpuHz, err = strconv.ParseInt(val, 0, 64); err != nil {
+				return nil, errUnrecognized
+			}
+		case "sampling period":
+			if p.Period, err = strconv.ParseInt(val, 0, 64); err != nil {
+				return nil, errUnrecognized
+			}
+		case "ms since reset":
+			ms, err := strconv.ParseInt(val, 0, 64)
+			if err != nil {
+				return nil, errUnrecognized
+			}
+			p.DurationNanos = ms * 1000 * 1000
+		case "format":
+			// CPP contentionz profiles don't have format.
+			return nil, errUnrecognized
+		case "resolution":
+			// CPP contentionz profiles don't have resolution.
+			return nil, errUnrecognized
+		case "discarded samples":
+		default:
+			return nil, errUnrecognized
+		}
+	}
+
+	locs := make(map[uint64]*Location)
+	for {
+		if !isSpaceOrComment(l) {
+			if l = strings.TrimSpace(l); strings.HasPrefix(l, "---") {
+				break
+			}
+			value, addrs, err := parseContentionSample(l, p.Period, cpuHz)
+			if err != nil {
+				return nil, err
+			}
+			var sloc []*Location
+			for _, addr := range addrs {
+				// Addresses from stack traces point to the next instruction after
+				// each call. Adjust by -1 to land somewhere on the actual call.
+				addr--
+				loc := locs[addr]
+				if locs[addr] == nil {
+					loc = &Location{
+						Address: addr,
+					}
+					p.Location = append(p.Location, loc)
+					locs[addr] = loc
+				}
+				sloc = append(sloc, loc)
+			}
+			p.Sample = append(p.Sample, &Sample{
+				Value:    value,
+				Location: sloc,
+			})
+		}
+
+		if l, err = r.ReadString('\n'); err != nil {
+			if err != io.EOF {
+				return nil, err
+			}
+			if l == "" {
+				break
+			}
+		}
+	}
+
+	if err = parseAdditionalSections(l, r, p); err != nil {
+		return nil, err
+	}
+
+	return p, nil
+}
+
+// parseContentionSample parses a single row from a contention profile
+// into a new Sample.
+func parseContentionSample(line string, period, cpuHz int64) (value []int64, addrs []uint64, err error) {
+	sampleData := contentionSampleRE.FindStringSubmatch(line)
+	if sampleData == nil {
+		return value, addrs, errUnrecognized
+	}
+
+	v1, err := strconv.ParseInt(sampleData[1], 10, 64)
+	if err != nil {
+		return value, addrs, fmt.Errorf("malformed sample: %s: %v", line, err)
+	}
+	v2, err := strconv.ParseInt(sampleData[2], 10, 64)
+	if err != nil {
+		return value, addrs, fmt.Errorf("malformed sample: %s: %v", line, err)
+	}
+
+	// Unsample values if period and cpuHz are available.
+	// - Delays are scaled to cycles and then to nanoseconds.
+	// - Contentions are scaled to cycles.
+	if period > 0 {
+		if cpuHz > 0 {
+			cpuGHz := float64(cpuHz) / 1e9
+			v1 = int64(float64(v1) * float64(period) / cpuGHz)
+		}
+		v2 = v2 * period
+	}
+
+	value = []int64{v2, v1}
+	addrs = parseHexAddresses(sampleData[3])
+
+	return value, addrs, nil
+}
+
+// parseThread parses a Threadz profile and returns a new Profile.
+func parseThread(b []byte) (*Profile, error) {
+	r := bytes.NewBuffer(b)
+
+	var line string
+	var err error
+	for {
+		// Skip past comments and empty lines seeking a real header.
+		line, err = r.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+		if !isSpaceOrComment(line) {
+			break
+		}
+	}
+
+	if m := threadzStartRE.FindStringSubmatch(line); m != nil {
+		// Advance over initial comments until first stack trace.
+		for {
+			line, err = r.ReadString('\n')
+			if err != nil {
+				if err != io.EOF {
+					return nil, err
+				}
+
+				if line == "" {
+					break
+				}
+			}
+			if sectionTrigger(line) != unrecognizedSection || line[0] == '-' {
+				break
+			}
+		}
+	} else if t := threadStartRE.FindStringSubmatch(line); len(t) != 4 {
+		return nil, errUnrecognized
+	}
+
+	p := &Profile{
+		SampleType: []*ValueType{{Type: "thread", Unit: "count"}},
+		PeriodType: &ValueType{Type: "thread", Unit: "count"},
+		Period:     1,
+	}
+
+	locs := make(map[uint64]*Location)
+	// Recognize each thread and populate profile samples.
+	for sectionTrigger(line) == unrecognizedSection {
+		if strings.HasPrefix(line, "---- no stack trace for") {
+			line = ""
+			break
+		}
+		if t := threadStartRE.FindStringSubmatch(line); len(t) != 4 {
+			return nil, errUnrecognized
+		}
+
+		var addrs []uint64
+		line, addrs, err = parseThreadSample(r)
+		if err != nil {
+			return nil, errUnrecognized
+		}
+		if len(addrs) == 0 {
+			// We got a --same as previous threads--. Bump counters.
+			if len(p.Sample) > 0 {
+				s := p.Sample[len(p.Sample)-1]
+				s.Value[0]++
+			}
+			continue
+		}
+
+		var sloc []*Location
+		for _, addr := range addrs {
+			// Addresses from stack traces point to the next instruction after
+			// each call. Adjust by -1 to land somewhere on the actual call.
+			addr--
+			loc := locs[addr]
+			if locs[addr] == nil {
+				loc = &Location{
+					Address: addr,
+				}
+				p.Location = append(p.Location, loc)
+				locs[addr] = loc
+			}
+			sloc = append(sloc, loc)
+		}
+
+		p.Sample = append(p.Sample, &Sample{
+			Value:    []int64{1},
+			Location: sloc,
+		})
+	}
+
+	if err = parseAdditionalSections(line, r, p); err != nil {
+		return nil, err
+	}
+
+	return p, nil
+}
+
+// parseThreadSample parses a symbolized or unsymbolized stack trace.
+// Returns the first line after the traceback, the sample (or nil if
+// it hits a 'same-as-previous' marker) and an error.
+func parseThreadSample(b *bytes.Buffer) (nextl string, addrs []uint64, err error) {
+	var l string
+	sameAsPrevious := false
+	for {
+		if l, err = b.ReadString('\n'); err != nil {
+			if err != io.EOF {
+				return "", nil, err
+			}
+			if l == "" {
+				break
+			}
+		}
+		if l = strings.TrimSpace(l); l == "" {
+			continue
+		}
+
+		if strings.HasPrefix(l, "---") {
+			break
+		}
+		if strings.Contains(l, "same as previous thread") {
+			sameAsPrevious = true
+			continue
+		}
+
+		addrs = append(addrs, parseHexAddresses(l)...)
+	}
+
+	if sameAsPrevious {
+		return l, nil, nil
+	}
+	return l, addrs, nil
+}
+
+// parseAdditionalSections parses any additional sections in the
+// profile, ignoring any unrecognized sections.
+func parseAdditionalSections(l string, b *bytes.Buffer, p *Profile) (err error) {
+	for {
+		if sectionTrigger(l) == memoryMapSection {
+			break
+		}
+		// Ignore any unrecognized sections.
+		if l, err := b.ReadString('\n'); err != nil {
+			if err != io.EOF {
+				return err
+			}
+			if l == "" {
+				break
+			}
+		}
+	}
+	return p.ParseMemoryMap(b)
+}
+
+// ParseMemoryMap parses a memory map in the format of
+// /proc/self/maps, and overrides the mappings in the current profile.
+// It renumbers the samples and locations in the profile correspondingly.
+func (p *Profile) ParseMemoryMap(rd io.Reader) error {
+	b := bufio.NewReader(rd)
+
+	var attrs []string
+	var r *strings.Replacer
+	const delimiter = '='
+	for {
+		l, err := b.ReadString('\n')
+		if err != nil {
+			if err != io.EOF {
+				return err
+			}
+			if l == "" {
+				break
+			}
+		}
+		if l = strings.TrimSpace(l); l == "" {
+			continue
+		}
+
+		if r != nil {
+			l = r.Replace(l)
+		}
+		m, err := parseMappingEntry(l)
+		if err != nil {
+			if err == errUnrecognized {
+				// Recognize assignments of the form: attr=value, and replace
+				// $attr with value on subsequent mappings.
+				idx := strings.IndexByte(l, delimiter)
+				if idx >= 0 {
+					attr := l[:idx]
+					value := l[idx+1:]
+					attrs = append(attrs, "$"+strings.TrimSpace(attr), strings.TrimSpace(value))
+					r = strings.NewReplacer(attrs...)
+				}
+				// Ignore any unrecognized entries
+				continue
+			}
+			return err
+		}
+		if m == nil || (m.File == "" && len(p.Mapping) != 0) {
+			// In some cases the first entry may include the address range
+			// but not the name of the file. It should be followed by
+			// another entry with the name.
+			continue
+		}
+		if len(p.Mapping) == 1 && p.Mapping[0].File == "" {
+			// Update the name if this is the entry following that empty one.
+			p.Mapping[0].File = m.File
+			continue
+		}
+		p.Mapping = append(p.Mapping, m)
+	}
+	p.remapLocationIDs()
+	p.remapFunctionIDs()
+	p.remapMappingIDs()
+	return nil
+}
+
+func parseMappingEntry(l string) (*Mapping, error) {
+	mapping := &Mapping{}
+	var err error
+	if me := procMapsRE.FindStringSubmatch(l); len(me) == 9 {
+		if !strings.Contains(me[3], "x") {
+			// Skip non-executable entries.
+			return nil, nil
+		}
+		if mapping.Start, err = strconv.ParseUint(me[1], 16, 64); err != nil {
+			return nil, errUnrecognized
+		}
+		if mapping.Limit, err = strconv.ParseUint(me[2], 16, 64); err != nil {
+			return nil, errUnrecognized
+		}
+		if me[4] != "" {
+			if mapping.Offset, err = strconv.ParseUint(me[4], 16, 64); err != nil {
+				return nil, errUnrecognized
+			}
+		}
+		mapping.File = me[8]
+		return mapping, nil
+	}
+
+	if me := briefMapsRE.FindStringSubmatch(l); len(me) == 6 {
+		if mapping.Start, err = strconv.ParseUint(me[1], 16, 64); err != nil {
+			return nil, errUnrecognized
+		}
+		if mapping.Limit, err = strconv.ParseUint(me[2], 16, 64); err != nil {
+			return nil, errUnrecognized
+		}
+		mapping.File = me[3]
+		if me[5] != "" {
+			if mapping.Offset, err = strconv.ParseUint(me[5], 16, 64); err != nil {
+				return nil, errUnrecognized
+			}
+		}
+		return mapping, nil
+	}
+
+	return nil, errUnrecognized
+}
+
+type sectionType int
+
+const (
+	unrecognizedSection sectionType = iota
+	memoryMapSection
+)
+
+var memoryMapTriggers = []string{
+	"--- Memory map: ---",
+	"MAPPED_LIBRARIES:",
+}
+
+func sectionTrigger(line string) sectionType {
+	for _, trigger := range memoryMapTriggers {
+		if strings.Contains(line, trigger) {
+			return memoryMapSection
+		}
+	}
+	return unrecognizedSection
+}
+
+func (p *Profile) addLegacyFrameInfo() {
+	switch {
+	case isProfileType(p, heapzSampleTypes) ||
+		isProfileType(p, heapzInUseSampleTypes) ||
+		isProfileType(p, heapzAllocSampleTypes):
+		p.DropFrames, p.KeepFrames = allocRxStr, allocSkipRxStr
+	case isProfileType(p, contentionzSampleTypes):
+		p.DropFrames, p.KeepFrames = lockRxStr, ""
+	default:
+		p.DropFrames, p.KeepFrames = cpuProfilerRxStr, ""
+	}
+}
+
+var heapzSampleTypes = []string{"allocations", "size"} // early Go pprof profiles
+var heapzInUseSampleTypes = []string{"inuse_objects", "inuse_space"}
+var heapzAllocSampleTypes = []string{"alloc_objects", "alloc_space"}
+var contentionzSampleTypes = []string{"contentions", "delay"}
+
+func isProfileType(p *Profile, t []string) bool {
+	st := p.SampleType
+	if len(st) != len(t) {
+		return false
+	}
+
+	for i := range st {
+		if st[i].Type != t[i] {
+			return false
+		}
+	}
+	return true
+}
+
+var allocRxStr = strings.Join([]string{
+	// POSIX entry points.
+	`calloc`,
+	`cfree`,
+	`malloc`,
+	`free`,
+	`memalign`,
+	`do_memalign`,
+	`(__)?posix_memalign`,
+	`pvalloc`,
+	`valloc`,
+	`realloc`,
+
+	// TC malloc.
+	`tcmalloc::.*`,
+	`tc_calloc`,
+	`tc_cfree`,
+	`tc_malloc`,
+	`tc_free`,
+	`tc_memalign`,
+	`tc_posix_memalign`,
+	`tc_pvalloc`,
+	`tc_valloc`,
+	`tc_realloc`,
+	`tc_new`,
+	`tc_delete`,
+	`tc_newarray`,
+	`tc_deletearray`,
+	`tc_new_nothrow`,
+	`tc_newarray_nothrow`,
+
+	// Memory-allocation routines on OS X.
+	`malloc_zone_malloc`,
+	`malloc_zone_calloc`,
+	`malloc_zone_valloc`,
+	`malloc_zone_realloc`,
+	`malloc_zone_memalign`,
+	`malloc_zone_free`,
+
+	// Go runtime
+	`runtime\..*`,
+
+	// Other misc. memory allocation routines
+	`BaseArena::.*`,
+	`(::)?do_malloc_no_errno`,
+	`(::)?do_malloc_pages`,
+	`(::)?do_malloc`,
+	`DoSampledAllocation`,
+	`MallocedMemBlock::MallocedMemBlock`,
+	`_M_allocate`,
+	`__builtin_(vec_)?delete`,
+	`__builtin_(vec_)?new`,
+	`__gnu_cxx::new_allocator::allocate`,
+	`__libc_malloc`,
+	`__malloc_alloc_template::allocate`,
+	`allocate`,
+	`cpp_alloc`,
+	`operator new(\[\])?`,
+	`simple_alloc::allocate`,
+}, `|`)
+
+var allocSkipRxStr = strings.Join([]string{
+	// Preserve Go runtime frames that appear in the middle/bottom of
+	// the stack.
+	`runtime\.panic`,
+	`runtime\.reflectcall`,
+	`runtime\.call[0-9]*`,
+}, `|`)
+
+var cpuProfilerRxStr = strings.Join([]string{
+	`ProfileData::Add`,
+	`ProfileData::prof_handler`,
+	`CpuProfiler::prof_handler`,
+	`__pthread_sighandler`,
+	`__restore`,
+}, `|`)
+
+var lockRxStr = strings.Join([]string{
+	`RecordLockProfileData`,
+	`(base::)?RecordLockProfileData.*`,
+	`(base::)?SubmitMutexProfileData.*`,
+	`(base::)?SubmitSpinLockProfileData.*`,
+	`(Mutex::)?AwaitCommon.*`,
+	`(Mutex::)?Unlock.*`,
+	`(Mutex::)?UnlockSlow.*`,
+	`(Mutex::)?ReaderUnlock.*`,
+	`(MutexLock::)?~MutexLock.*`,
+	`(SpinLock::)?Unlock.*`,
+	`(SpinLock::)?SlowUnlock.*`,
+	`(SpinLockHolder::)?~SpinLockHolder.*`,
+}, `|`)
diff --git a/src/internal/profile/merge.go b/src/internal/profile/merge.go
new file mode 100644
index 0000000..3ea7d4c
--- /dev/null
+++ b/src/internal/profile/merge.go
@@ -0,0 +1,461 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package profile
+
+import (
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// Merge merges all the profiles in profs into a single Profile.
+// Returns a new profile independent of the input profiles. The merged
+// profile is compacted to eliminate unused samples, locations,
+// functions and mappings. Profiles must have identical profile sample
+// and period types or the merge will fail. profile.Period of the
+// resulting profile will be the maximum of all profiles, and
+// profile.TimeNanos will be the earliest nonzero one.
+func Merge(srcs []*Profile) (*Profile, error) {
+	if len(srcs) == 0 {
+		return nil, fmt.Errorf("no profiles to merge")
+	}
+	p, err := combineHeaders(srcs)
+	if err != nil {
+		return nil, err
+	}
+
+	pm := &profileMerger{
+		p:         p,
+		samples:   make(map[sampleKey]*Sample, len(srcs[0].Sample)),
+		locations: make(map[locationKey]*Location, len(srcs[0].Location)),
+		functions: make(map[functionKey]*Function, len(srcs[0].Function)),
+		mappings:  make(map[mappingKey]*Mapping, len(srcs[0].Mapping)),
+	}
+
+	for _, src := range srcs {
+		// Clear the profile-specific hash tables
+		pm.locationsByID = make(map[uint64]*Location, len(src.Location))
+		pm.functionsByID = make(map[uint64]*Function, len(src.Function))
+		pm.mappingsByID = make(map[uint64]mapInfo, len(src.Mapping))
+
+		if len(pm.mappings) == 0 && len(src.Mapping) > 0 {
+			// The Mapping list has the property that the first mapping
+			// represents the main binary. Take the first Mapping we see,
+			// otherwise the operations below will add mappings in an
+			// arbitrary order.
+			pm.mapMapping(src.Mapping[0])
+		}
+
+		for _, s := range src.Sample {
+			if !isZeroSample(s) {
+				pm.mapSample(s)
+			}
+		}
+	}
+
+	for _, s := range p.Sample {
+		if isZeroSample(s) {
+			// If there are any zero samples, re-merge the profile to GC
+			// them.
+			return Merge([]*Profile{p})
+		}
+	}
+
+	return p, nil
+}
+
+// Normalize normalizes the source profile by multiplying each value in profile by the
+// ratio of the sum of the base profile's values of that sample type to the sum of the
+// source profile's value of that sample type.
+func (p *Profile) Normalize(pb *Profile) error {
+
+	if err := p.compatible(pb); err != nil {
+		return err
+	}
+
+	baseVals := make([]int64, len(p.SampleType))
+	for _, s := range pb.Sample {
+		for i, v := range s.Value {
+			baseVals[i] += v
+		}
+	}
+
+	srcVals := make([]int64, len(p.SampleType))
+	for _, s := range p.Sample {
+		for i, v := range s.Value {
+			srcVals[i] += v
+		}
+	}
+
+	normScale := make([]float64, len(baseVals))
+	for i := range baseVals {
+		if srcVals[i] == 0 {
+			normScale[i] = 0.0
+		} else {
+			normScale[i] = float64(baseVals[i]) / float64(srcVals[i])
+		}
+	}
+	p.ScaleN(normScale)
+	return nil
+}
+
+func isZeroSample(s *Sample) bool {
+	for _, v := range s.Value {
+		if v != 0 {
+			return false
+		}
+	}
+	return true
+}
+
+type profileMerger struct {
+	p *Profile
+
+	// Memoization tables within a profile.
+	locationsByID map[uint64]*Location
+	functionsByID map[uint64]*Function
+	mappingsByID  map[uint64]mapInfo
+
+	// Memoization tables for profile entities.
+	samples   map[sampleKey]*Sample
+	locations map[locationKey]*Location
+	functions map[functionKey]*Function
+	mappings  map[mappingKey]*Mapping
+}
+
+type mapInfo struct {
+	m      *Mapping
+	offset int64
+}
+
+func (pm *profileMerger) mapSample(src *Sample) *Sample {
+	s := &Sample{
+		Location: make([]*Location, len(src.Location)),
+		Value:    make([]int64, len(src.Value)),
+		Label:    make(map[string][]string, len(src.Label)),
+		NumLabel: make(map[string][]int64, len(src.NumLabel)),
+		NumUnit:  make(map[string][]string, len(src.NumLabel)),
+	}
+	for i, l := range src.Location {
+		s.Location[i] = pm.mapLocation(l)
+	}
+	for k, v := range src.Label {
+		vv := make([]string, len(v))
+		copy(vv, v)
+		s.Label[k] = vv
+	}
+	for k, v := range src.NumLabel {
+		u := src.NumUnit[k]
+		vv := make([]int64, len(v))
+		uu := make([]string, len(u))
+		copy(vv, v)
+		copy(uu, u)
+		s.NumLabel[k] = vv
+		s.NumUnit[k] = uu
+	}
+	// Check memoization table. Must be done on the remapped location to
+	// account for the remapped mapping. Add current values to the
+	// existing sample.
+	k := s.key()
+	if ss, ok := pm.samples[k]; ok {
+		for i, v := range src.Value {
+			ss.Value[i] += v
+		}
+		return ss
+	}
+	copy(s.Value, src.Value)
+	pm.samples[k] = s
+	pm.p.Sample = append(pm.p.Sample, s)
+	return s
+}
+
+// key generates sampleKey to be used as a key for maps.
+func (sample *Sample) key() sampleKey {
+	ids := make([]string, len(sample.Location))
+	for i, l := range sample.Location {
+		ids[i] = strconv.FormatUint(l.ID, 16)
+	}
+
+	labels := make([]string, 0, len(sample.Label))
+	for k, v := range sample.Label {
+		labels = append(labels, fmt.Sprintf("%q%q", k, v))
+	}
+	sort.Strings(labels)
+
+	numlabels := make([]string, 0, len(sample.NumLabel))
+	for k, v := range sample.NumLabel {
+		numlabels = append(numlabels, fmt.Sprintf("%q%x%x", k, v, sample.NumUnit[k]))
+	}
+	sort.Strings(numlabels)
+
+	return sampleKey{
+		strings.Join(ids, "|"),
+		strings.Join(labels, ""),
+		strings.Join(numlabels, ""),
+	}
+}
+
+type sampleKey struct {
+	locations string
+	labels    string
+	numlabels string
+}
+
+func (pm *profileMerger) mapLocation(src *Location) *Location {
+	if src == nil {
+		return nil
+	}
+
+	if l, ok := pm.locationsByID[src.ID]; ok {
+		pm.locationsByID[src.ID] = l
+		return l
+	}
+
+	mi := pm.mapMapping(src.Mapping)
+	l := &Location{
+		ID:       uint64(len(pm.p.Location) + 1),
+		Mapping:  mi.m,
+		Address:  uint64(int64(src.Address) + mi.offset),
+		Line:     make([]Line, len(src.Line)),
+		IsFolded: src.IsFolded,
+	}
+	for i, ln := range src.Line {
+		l.Line[i] = pm.mapLine(ln)
+	}
+	// Check memoization table. Must be done on the remapped location to
+	// account for the remapped mapping ID.
+	k := l.key()
+	if ll, ok := pm.locations[k]; ok {
+		pm.locationsByID[src.ID] = ll
+		return ll
+	}
+	pm.locationsByID[src.ID] = l
+	pm.locations[k] = l
+	pm.p.Location = append(pm.p.Location, l)
+	return l
+}
+
+// key generates locationKey to be used as a key for maps.
+func (l *Location) key() locationKey {
+	key := locationKey{
+		addr:     l.Address,
+		isFolded: l.IsFolded,
+	}
+	if l.Mapping != nil {
+		// Normalizes address to handle address space randomization.
+		key.addr -= l.Mapping.Start
+		key.mappingID = l.Mapping.ID
+	}
+	lines := make([]string, len(l.Line)*2)
+	for i, line := range l.Line {
+		if line.Function != nil {
+			lines[i*2] = strconv.FormatUint(line.Function.ID, 16)
+		}
+		lines[i*2+1] = strconv.FormatInt(line.Line, 16)
+	}
+	key.lines = strings.Join(lines, "|")
+	return key
+}
+
+type locationKey struct {
+	addr, mappingID uint64
+	lines           string
+	isFolded        bool
+}
+
+func (pm *profileMerger) mapMapping(src *Mapping) mapInfo {
+	if src == nil {
+		return mapInfo{}
+	}
+
+	if mi, ok := pm.mappingsByID[src.ID]; ok {
+		return mi
+	}
+
+	// Check memoization tables.
+	mk := src.key()
+	if m, ok := pm.mappings[mk]; ok {
+		mi := mapInfo{m, int64(m.Start) - int64(src.Start)}
+		pm.mappingsByID[src.ID] = mi
+		return mi
+	}
+	m := &Mapping{
+		ID:              uint64(len(pm.p.Mapping) + 1),
+		Start:           src.Start,
+		Limit:           src.Limit,
+		Offset:          src.Offset,
+		File:            src.File,
+		BuildID:         src.BuildID,
+		HasFunctions:    src.HasFunctions,
+		HasFilenames:    src.HasFilenames,
+		HasLineNumbers:  src.HasLineNumbers,
+		HasInlineFrames: src.HasInlineFrames,
+	}
+	pm.p.Mapping = append(pm.p.Mapping, m)
+
+	// Update memoization tables.
+	pm.mappings[mk] = m
+	mi := mapInfo{m, 0}
+	pm.mappingsByID[src.ID] = mi
+	return mi
+}
+
+// key generates encoded strings of Mapping to be used as a key for
+// maps.
+func (m *Mapping) key() mappingKey {
+	// Normalize addresses to handle address space randomization.
+	// Round up to next 4K boundary to avoid minor discrepancies.
+	const mapsizeRounding = 0x1000
+
+	size := m.Limit - m.Start
+	size = size + mapsizeRounding - 1
+	size = size - (size % mapsizeRounding)
+	key := mappingKey{
+		size:   size,
+		offset: m.Offset,
+	}
+
+	switch {
+	case m.BuildID != "":
+		key.buildIDOrFile = m.BuildID
+	case m.File != "":
+		key.buildIDOrFile = m.File
+	default:
+		// A mapping containing neither build ID nor file name is a fake mapping. A
+		// key with empty buildIDOrFile is used for fake mappings so that they are
+		// treated as the same mapping during merging.
+	}
+	return key
+}
+
+type mappingKey struct {
+	size, offset  uint64
+	buildIDOrFile string
+}
+
+func (pm *profileMerger) mapLine(src Line) Line {
+	ln := Line{
+		Function: pm.mapFunction(src.Function),
+		Line:     src.Line,
+	}
+	return ln
+}
+
+func (pm *profileMerger) mapFunction(src *Function) *Function {
+	if src == nil {
+		return nil
+	}
+	if f, ok := pm.functionsByID[src.ID]; ok {
+		return f
+	}
+	k := src.key()
+	if f, ok := pm.functions[k]; ok {
+		pm.functionsByID[src.ID] = f
+		return f
+	}
+	f := &Function{
+		ID:         uint64(len(pm.p.Function) + 1),
+		Name:       src.Name,
+		SystemName: src.SystemName,
+		Filename:   src.Filename,
+		StartLine:  src.StartLine,
+	}
+	pm.functions[k] = f
+	pm.functionsByID[src.ID] = f
+	pm.p.Function = append(pm.p.Function, f)
+	return f
+}
+
+// key generates a struct to be used as a key for maps.
+func (f *Function) key() functionKey {
+	return functionKey{
+		f.StartLine,
+		f.Name,
+		f.SystemName,
+		f.Filename,
+	}
+}
+
+type functionKey struct {
+	startLine                  int64
+	name, systemName, fileName string
+}
+
+// combineHeaders checks that all profiles can be merged and returns
+// their combined profile.
+func combineHeaders(srcs []*Profile) (*Profile, error) {
+	for _, s := range srcs[1:] {
+		if err := srcs[0].compatible(s); err != nil {
+			return nil, err
+		}
+	}
+
+	var timeNanos, durationNanos, period int64
+	var comments []string
+	seenComments := map[string]bool{}
+	var defaultSampleType string
+	for _, s := range srcs {
+		if timeNanos == 0 || s.TimeNanos < timeNanos {
+			timeNanos = s.TimeNanos
+		}
+		durationNanos += s.DurationNanos
+		if period == 0 || period < s.Period {
+			period = s.Period
+		}
+		for _, c := range s.Comments {
+			if seen := seenComments[c]; !seen {
+				comments = append(comments, c)
+				seenComments[c] = true
+			}
+		}
+		if defaultSampleType == "" {
+			defaultSampleType = s.DefaultSampleType
+		}
+	}
+
+	p := &Profile{
+		SampleType: make([]*ValueType, len(srcs[0].SampleType)),
+
+		DropFrames: srcs[0].DropFrames,
+		KeepFrames: srcs[0].KeepFrames,
+
+		TimeNanos:     timeNanos,
+		DurationNanos: durationNanos,
+		PeriodType:    srcs[0].PeriodType,
+		Period:        period,
+
+		Comments:          comments,
+		DefaultSampleType: defaultSampleType,
+	}
+	copy(p.SampleType, srcs[0].SampleType)
+	return p, nil
+}
+
+// compatible determines if two profiles can be compared/merged.
+// returns nil if the profiles are compatible; otherwise an error with
+// details on the incompatibility.
+func (p *Profile) compatible(pb *Profile) error {
+	if !equalValueType(p.PeriodType, pb.PeriodType) {
+		return fmt.Errorf("incompatible period types %v and %v", p.PeriodType, pb.PeriodType)
+	}
+
+	if len(p.SampleType) != len(pb.SampleType) {
+		return fmt.Errorf("incompatible sample types %v and %v", p.SampleType, pb.SampleType)
+	}
+
+	for i := range p.SampleType {
+		if !equalValueType(p.SampleType[i], pb.SampleType[i]) {
+			return fmt.Errorf("incompatible sample types %v and %v", p.SampleType, pb.SampleType)
+		}
+	}
+	return nil
+}
+
+// equalValueType returns true if the two value types are semantically
+// equal. It ignores the internal fields used during encode/decode.
+func equalValueType(st1, st2 *ValueType) bool {
+	return st1.Type == st2.Type && st1.Unit == st2.Unit
+}
diff --git a/src/internal/profile/profile.go b/src/internal/profile/profile.go
new file mode 100644
index 0000000..29568aa
--- /dev/null
+++ b/src/internal/profile/profile.go
@@ -0,0 +1,613 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package profile provides a representation of
+// github.com/google/pprof/proto/profile.proto and
+// methods to encode/decode/merge profiles in this format.
+package profile
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"io"
+	"regexp"
+	"strings"
+	"time"
+)
+
+// Profile is an in-memory representation of profile.proto.
+type Profile struct {
+	SampleType        []*ValueType
+	DefaultSampleType string
+	Sample            []*Sample
+	Mapping           []*Mapping
+	Location          []*Location
+	Function          []*Function
+	Comments          []string
+
+	DropFrames string
+	KeepFrames string
+
+	TimeNanos     int64
+	DurationNanos int64
+	PeriodType    *ValueType
+	Period        int64
+
+	commentX           []int64
+	dropFramesX        int64
+	keepFramesX        int64
+	stringTable        []string
+	defaultSampleTypeX int64
+}
+
+// ValueType corresponds to Profile.ValueType
+type ValueType struct {
+	Type string // cpu, wall, inuse_space, etc
+	Unit string // seconds, nanoseconds, bytes, etc
+
+	typeX int64
+	unitX int64
+}
+
+// Sample corresponds to Profile.Sample
+type Sample struct {
+	Location []*Location
+	Value    []int64
+	Label    map[string][]string
+	NumLabel map[string][]int64
+	NumUnit  map[string][]string
+
+	locationIDX []uint64
+	labelX      []Label
+}
+
+// Label corresponds to Profile.Label
+type Label struct {
+	keyX int64
+	// Exactly one of the two following values must be set
+	strX int64
+	numX int64 // Integer value for this label
+}
+
+// Mapping corresponds to Profile.Mapping
+type Mapping struct {
+	ID              uint64
+	Start           uint64
+	Limit           uint64
+	Offset          uint64
+	File            string
+	BuildID         string
+	HasFunctions    bool
+	HasFilenames    bool
+	HasLineNumbers  bool
+	HasInlineFrames bool
+
+	fileX    int64
+	buildIDX int64
+}
+
+// Location corresponds to Profile.Location
+type Location struct {
+	ID       uint64
+	Mapping  *Mapping
+	Address  uint64
+	Line     []Line
+	IsFolded bool
+
+	mappingIDX uint64
+}
+
+// Line corresponds to Profile.Line
+type Line struct {
+	Function *Function
+	Line     int64
+
+	functionIDX uint64
+}
+
+// Function corresponds to Profile.Function
+type Function struct {
+	ID         uint64
+	Name       string
+	SystemName string
+	Filename   string
+	StartLine  int64
+
+	nameX       int64
+	systemNameX int64
+	filenameX   int64
+}
+
+// Parse parses a profile and checks for its validity. The input
+// may be a gzip-compressed encoded protobuf or one of many legacy
+// profile formats which may be unsupported in the future.
+func Parse(r io.Reader) (*Profile, error) {
+	orig, err := io.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+
+	var p *Profile
+	if len(orig) >= 2 && orig[0] == 0x1f && orig[1] == 0x8b {
+		gz, err := gzip.NewReader(bytes.NewBuffer(orig))
+		if err != nil {
+			return nil, fmt.Errorf("decompressing profile: %v", err)
+		}
+		data, err := io.ReadAll(gz)
+		if err != nil {
+			return nil, fmt.Errorf("decompressing profile: %v", err)
+		}
+		orig = data
+	}
+	if p, err = parseUncompressed(orig); err != nil {
+		if p, err = parseLegacy(orig); err != nil {
+			return nil, fmt.Errorf("parsing profile: %v", err)
+		}
+	}
+
+	if err := p.CheckValid(); err != nil {
+		return nil, fmt.Errorf("malformed profile: %v", err)
+	}
+	return p, nil
+}
+
+var errUnrecognized = fmt.Errorf("unrecognized profile format")
+var errMalformed = fmt.Errorf("malformed profile format")
+
+func parseLegacy(data []byte) (*Profile, error) {
+	parsers := []func([]byte) (*Profile, error){
+		parseCPU,
+		parseHeap,
+		parseGoCount, // goroutine, threadcreate
+		parseThread,
+		parseContention,
+	}
+
+	for _, parser := range parsers {
+		p, err := parser(data)
+		if err == nil {
+			p.setMain()
+			p.addLegacyFrameInfo()
+			return p, nil
+		}
+		if err != errUnrecognized {
+			return nil, err
+		}
+	}
+	return nil, errUnrecognized
+}
+
+func parseUncompressed(data []byte) (*Profile, error) {
+	p := &Profile{}
+	if err := unmarshal(data, p); err != nil {
+		return nil, err
+	}
+
+	if err := p.postDecode(); err != nil {
+		return nil, err
+	}
+
+	return p, nil
+}
+
+var libRx = regexp.MustCompile(`([.]so$|[.]so[._][0-9]+)`)
+
+// setMain scans Mapping entries and guesses which entry is main
+// because legacy profiles don't obey the convention of putting main
+// first.
+func (p *Profile) setMain() {
+	for i := 0; i < len(p.Mapping); i++ {
+		file := strings.TrimSpace(strings.ReplaceAll(p.Mapping[i].File, "(deleted)", ""))
+		if len(file) == 0 {
+			continue
+		}
+		if len(libRx.FindStringSubmatch(file)) > 0 {
+			continue
+		}
+		if strings.HasPrefix(file, "[") {
+			continue
+		}
+		// Swap what we guess is main to position 0.
+		p.Mapping[i], p.Mapping[0] = p.Mapping[0], p.Mapping[i]
+		break
+	}
+}
+
+// Write writes the profile as a gzip-compressed marshaled protobuf.
+func (p *Profile) Write(w io.Writer) error {
+	p.preEncode()
+	b := marshal(p)
+	zw := gzip.NewWriter(w)
+	defer zw.Close()
+	_, err := zw.Write(b)
+	return err
+}
+
+// CheckValid tests whether the profile is valid. Checks include, but are
+// not limited to:
+//   - len(Profile.Sample[n].value) == len(Profile.value_unit)
+//   - Sample.id has a corresponding Profile.Location
+func (p *Profile) CheckValid() error {
+	// Check that sample values are consistent
+	sampleLen := len(p.SampleType)
+	if sampleLen == 0 && len(p.Sample) != 0 {
+		return fmt.Errorf("missing sample type information")
+	}
+	for _, s := range p.Sample {
+		if len(s.Value) != sampleLen {
+			return fmt.Errorf("mismatch: sample has: %d values vs. %d types", len(s.Value), len(p.SampleType))
+		}
+	}
+
+	// Check that all mappings/locations/functions are in the tables
+	// Check that there are no duplicate ids
+	mappings := make(map[uint64]*Mapping, len(p.Mapping))
+	for _, m := range p.Mapping {
+		if m.ID == 0 {
+			return fmt.Errorf("found mapping with reserved ID=0")
+		}
+		if mappings[m.ID] != nil {
+			return fmt.Errorf("multiple mappings with same id: %d", m.ID)
+		}
+		mappings[m.ID] = m
+	}
+	functions := make(map[uint64]*Function, len(p.Function))
+	for _, f := range p.Function {
+		if f.ID == 0 {
+			return fmt.Errorf("found function with reserved ID=0")
+		}
+		if functions[f.ID] != nil {
+			return fmt.Errorf("multiple functions with same id: %d", f.ID)
+		}
+		functions[f.ID] = f
+	}
+	locations := make(map[uint64]*Location, len(p.Location))
+	for _, l := range p.Location {
+		if l.ID == 0 {
+			return fmt.Errorf("found location with reserved id=0")
+		}
+		if locations[l.ID] != nil {
+			return fmt.Errorf("multiple locations with same id: %d", l.ID)
+		}
+		locations[l.ID] = l
+		if m := l.Mapping; m != nil {
+			if m.ID == 0 || mappings[m.ID] != m {
+				return fmt.Errorf("inconsistent mapping %p: %d", m, m.ID)
+			}
+		}
+		for _, ln := range l.Line {
+			if f := ln.Function; f != nil {
+				if f.ID == 0 || functions[f.ID] != f {
+					return fmt.Errorf("inconsistent function %p: %d", f, f.ID)
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// Aggregate merges the locations in the profile into equivalence
+// classes preserving the request attributes. It also updates the
+// samples to point to the merged locations.
+func (p *Profile) Aggregate(inlineFrame, function, filename, linenumber, address bool) error {
+	for _, m := range p.Mapping {
+		m.HasInlineFrames = m.HasInlineFrames && inlineFrame
+		m.HasFunctions = m.HasFunctions && function
+		m.HasFilenames = m.HasFilenames && filename
+		m.HasLineNumbers = m.HasLineNumbers && linenumber
+	}
+
+	// Aggregate functions
+	if !function || !filename {
+		for _, f := range p.Function {
+			if !function {
+				f.Name = ""
+				f.SystemName = ""
+			}
+			if !filename {
+				f.Filename = ""
+			}
+		}
+	}
+
+	// Aggregate locations
+	if !inlineFrame || !address || !linenumber {
+		for _, l := range p.Location {
+			if !inlineFrame && len(l.Line) > 1 {
+				l.Line = l.Line[len(l.Line)-1:]
+			}
+			if !linenumber {
+				for i := range l.Line {
+					l.Line[i].Line = 0
+				}
+			}
+			if !address {
+				l.Address = 0
+			}
+		}
+	}
+
+	return p.CheckValid()
+}
+
+// Print dumps a text representation of a profile. Intended mainly
+// for debugging purposes.
+func (p *Profile) String() string {
+
+	ss := make([]string, 0, len(p.Sample)+len(p.Mapping)+len(p.Location))
+	if pt := p.PeriodType; pt != nil {
+		ss = append(ss, fmt.Sprintf("PeriodType: %s %s", pt.Type, pt.Unit))
+	}
+	ss = append(ss, fmt.Sprintf("Period: %d", p.Period))
+	if p.TimeNanos != 0 {
+		ss = append(ss, fmt.Sprintf("Time: %v", time.Unix(0, p.TimeNanos)))
+	}
+	if p.DurationNanos != 0 {
+		ss = append(ss, fmt.Sprintf("Duration: %v", time.Duration(p.DurationNanos)))
+	}
+
+	ss = append(ss, "Samples:")
+	var sh1 string
+	for _, s := range p.SampleType {
+		sh1 = sh1 + fmt.Sprintf("%s/%s ", s.Type, s.Unit)
+	}
+	ss = append(ss, strings.TrimSpace(sh1))
+	for _, s := range p.Sample {
+		var sv string
+		for _, v := range s.Value {
+			sv = fmt.Sprintf("%s %10d", sv, v)
+		}
+		sv = sv + ": "
+		for _, l := range s.Location {
+			sv = sv + fmt.Sprintf("%d ", l.ID)
+		}
+		ss = append(ss, sv)
+		const labelHeader = "                "
+		if len(s.Label) > 0 {
+			ls := labelHeader
+			for k, v := range s.Label {
+				ls = ls + fmt.Sprintf("%s:%v ", k, v)
+			}
+			ss = append(ss, ls)
+		}
+		if len(s.NumLabel) > 0 {
+			ls := labelHeader
+			for k, v := range s.NumLabel {
+				ls = ls + fmt.Sprintf("%s:%v ", k, v)
+			}
+			ss = append(ss, ls)
+		}
+	}
+
+	ss = append(ss, "Locations")
+	for _, l := range p.Location {
+		locStr := fmt.Sprintf("%6d: %#x ", l.ID, l.Address)
+		if m := l.Mapping; m != nil {
+			locStr = locStr + fmt.Sprintf("M=%d ", m.ID)
+		}
+		if len(l.Line) == 0 {
+			ss = append(ss, locStr)
+		}
+		for li := range l.Line {
+			lnStr := "??"
+			if fn := l.Line[li].Function; fn != nil {
+				lnStr = fmt.Sprintf("%s %s:%d s=%d",
+					fn.Name,
+					fn.Filename,
+					l.Line[li].Line,
+					fn.StartLine)
+				if fn.Name != fn.SystemName {
+					lnStr = lnStr + "(" + fn.SystemName + ")"
+				}
+			}
+			ss = append(ss, locStr+lnStr)
+			// Do not print location details past the first line
+			locStr = "             "
+		}
+	}
+
+	ss = append(ss, "Mappings")
+	for _, m := range p.Mapping {
+		bits := ""
+		if m.HasFunctions {
+			bits += "[FN]"
+		}
+		if m.HasFilenames {
+			bits += "[FL]"
+		}
+		if m.HasLineNumbers {
+			bits += "[LN]"
+		}
+		if m.HasInlineFrames {
+			bits += "[IN]"
+		}
+		ss = append(ss, fmt.Sprintf("%d: %#x/%#x/%#x %s %s %s",
+			m.ID,
+			m.Start, m.Limit, m.Offset,
+			m.File,
+			m.BuildID,
+			bits))
+	}
+
+	return strings.Join(ss, "\n") + "\n"
+}
+
+// Merge adds profile p adjusted by ratio r into profile p. Profiles
+// must be compatible (same Type and SampleType).
+// TODO(rsilvera): consider normalizing the profiles based on the
+// total samples collected.
+func (p *Profile) Merge(pb *Profile, r float64) error {
+	if err := p.Compatible(pb); err != nil {
+		return err
+	}
+
+	pb = pb.Copy()
+
+	// Keep the largest of the two periods.
+	if pb.Period > p.Period {
+		p.Period = pb.Period
+	}
+
+	p.DurationNanos += pb.DurationNanos
+
+	p.Mapping = append(p.Mapping, pb.Mapping...)
+	for i, m := range p.Mapping {
+		m.ID = uint64(i + 1)
+	}
+	p.Location = append(p.Location, pb.Location...)
+	for i, l := range p.Location {
+		l.ID = uint64(i + 1)
+	}
+	p.Function = append(p.Function, pb.Function...)
+	for i, f := range p.Function {
+		f.ID = uint64(i + 1)
+	}
+
+	if r != 1.0 {
+		for _, s := range pb.Sample {
+			for i, v := range s.Value {
+				s.Value[i] = int64((float64(v) * r))
+			}
+		}
+	}
+	p.Sample = append(p.Sample, pb.Sample...)
+	return p.CheckValid()
+}
+
+// Compatible determines if two profiles can be compared/merged.
+// returns nil if the profiles are compatible; otherwise an error with
+// details on the incompatibility.
+func (p *Profile) Compatible(pb *Profile) error {
+	if !compatibleValueTypes(p.PeriodType, pb.PeriodType) {
+		return fmt.Errorf("incompatible period types %v and %v", p.PeriodType, pb.PeriodType)
+	}
+
+	if len(p.SampleType) != len(pb.SampleType) {
+		return fmt.Errorf("incompatible sample types %v and %v", p.SampleType, pb.SampleType)
+	}
+
+	for i := range p.SampleType {
+		if !compatibleValueTypes(p.SampleType[i], pb.SampleType[i]) {
+			return fmt.Errorf("incompatible sample types %v and %v", p.SampleType, pb.SampleType)
+		}
+	}
+
+	return nil
+}
+
+// HasFunctions determines if all locations in this profile have
+// symbolized function information.
+func (p *Profile) HasFunctions() bool {
+	for _, l := range p.Location {
+		if l.Mapping == nil || !l.Mapping.HasFunctions {
+			return false
+		}
+	}
+	return true
+}
+
+// HasFileLines determines if all locations in this profile have
+// symbolized file and line number information.
+func (p *Profile) HasFileLines() bool {
+	for _, l := range p.Location {
+		if l.Mapping == nil || (!l.Mapping.HasFilenames || !l.Mapping.HasLineNumbers) {
+			return false
+		}
+	}
+	return true
+}
+
+func compatibleValueTypes(v1, v2 *ValueType) bool {
+	if v1 == nil || v2 == nil {
+		return true // No grounds to disqualify.
+	}
+	return v1.Type == v2.Type && v1.Unit == v2.Unit
+}
+
+// Copy makes a fully independent copy of a profile.
+func (p *Profile) Copy() *Profile {
+	p.preEncode()
+	b := marshal(p)
+
+	pp := &Profile{}
+	if err := unmarshal(b, pp); err != nil {
+		panic(err)
+	}
+	if err := pp.postDecode(); err != nil {
+		panic(err)
+	}
+
+	return pp
+}
+
+// Demangler maps symbol names to a human-readable form. This may
+// include C++ demangling and additional simplification. Names that
+// are not demangled may be missing from the resulting map.
+type Demangler func(name []string) (map[string]string, error)
+
+// Demangle attempts to demangle and optionally simplify any function
+// names referenced in the profile. It works on a best-effort basis:
+// it will silently preserve the original names in case of any errors.
+func (p *Profile) Demangle(d Demangler) error {
+	// Collect names to demangle.
+	var names []string
+	for _, fn := range p.Function {
+		names = append(names, fn.SystemName)
+	}
+
+	// Update profile with demangled names.
+	demangled, err := d(names)
+	if err != nil {
+		return err
+	}
+	for _, fn := range p.Function {
+		if dd, ok := demangled[fn.SystemName]; ok {
+			fn.Name = dd
+		}
+	}
+	return nil
+}
+
+// Empty reports whether the profile contains no samples.
+func (p *Profile) Empty() bool {
+	return len(p.Sample) == 0
+}
+
+// Scale multiplies all sample values in a profile by a constant.
+func (p *Profile) Scale(ratio float64) {
+	if ratio == 1 {
+		return
+	}
+	ratios := make([]float64, len(p.SampleType))
+	for i := range p.SampleType {
+		ratios[i] = ratio
+	}
+	p.ScaleN(ratios)
+}
+
+// ScaleN multiplies each sample values in a sample by a different amount.
+func (p *Profile) ScaleN(ratios []float64) error {
+	if len(p.SampleType) != len(ratios) {
+		return fmt.Errorf("mismatched scale ratios, got %d, want %d", len(ratios), len(p.SampleType))
+	}
+	allOnes := true
+	for _, r := range ratios {
+		if r != 1 {
+			allOnes = false
+			break
+		}
+	}
+	if allOnes {
+		return nil
+	}
+	for _, s := range p.Sample {
+		for i, v := range s.Value {
+			if ratios[i] != 1 {
+				s.Value[i] = int64(float64(v) * ratios[i])
+			}
+		}
+	}
+	return nil
+}
diff --git a/src/internal/profile/profile_test.go b/src/internal/profile/profile_test.go
new file mode 100644
index 0000000..e1963f3
--- /dev/null
+++ b/src/internal/profile/profile_test.go
@@ -0,0 +1,79 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package profile
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestEmptyProfile(t *testing.T) {
+	var buf bytes.Buffer
+	p, err := Parse(&buf)
+	if err != nil {
+		t.Error("Want no error, got", err)
+	}
+	if p == nil {
+		t.Fatal("Want a valid profile, got <nil>")
+	}
+	if !p.Empty() {
+		t.Errorf("Profile should be empty, got %#v", p)
+	}
+}
+
+func TestParseContention(t *testing.T) {
+	tests := []struct {
+		name    string
+		in      string
+		wantErr bool
+	}{
+		{
+			name: "valid",
+			in: `--- mutex:
+cycles/second=3491920901
+sampling period=1
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31
+34035731690 15760 @ 0x45e851 0x45f764 0x4a2b17 0x44ea31
+`,
+		},
+		{
+			name: "valid with comment",
+			in: `--- mutex:
+cycles/second=3491920901
+sampling period=1
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31
+#	0x45e850	sync.(*Mutex).Unlock+0x80	/go/src/sync/mutex.go:126
+#	0x45f763	sync.(*RWMutex).Unlock+0x83	/go/src/sync/rwmutex.go:125
+#	0x4a2be0	main.main.func3+0x70		/go/src/internal/pprof/profile/a_binary.go:58
+
+34035731690 15760 @ 0x45e851 0x45f764 0x4a2b17 0x44ea31
+#	0x45e850	sync.(*Mutex).Unlock+0x80	/go/src/sync/mutex.go:126
+#	0x45f763	sync.(*RWMutex).Unlock+0x83	/go/src/sync/rwmutex.go:125
+#	0x4a2b16	main.main.func2+0xd6		/go/src/internal/pprof/profile/a_binary.go:48
+`,
+		},
+		{
+			name:    "empty",
+			in:      `--- mutex:`,
+			wantErr: true,
+		},
+		{
+			name: "invalid header",
+			in: `--- channel:
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31`,
+			wantErr: true,
+		},
+	}
+	for _, tc := range tests {
+		_, err := parseContention([]byte(tc.in))
+		if tc.wantErr && err == nil {
+			t.Errorf("parseContention(%q) succeeded unexpectedly", tc.name)
+		}
+		if !tc.wantErr && err != nil {
+			t.Errorf("parseContention(%q) failed unexpectedly: %v", tc.name, err)
+		}
+	}
+
+}
diff --git a/src/internal/profile/proto.go b/src/internal/profile/proto.go
new file mode 100644
index 0000000..58ff0ad
--- /dev/null
+++ b/src/internal/profile/proto.go
@@ -0,0 +1,356 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a simple protocol buffer encoder and decoder.
+//
+// A protocol message must implement the message interface:
+//   decoder() []decoder
+//   encode(*buffer)
+//
+// The decode method returns a slice indexed by field number that gives the
+// function to decode that field.
+// The encode method encodes its receiver into the given buffer.
+//
+// The two methods are simple enough to be implemented by hand rather than
+// by using a protocol compiler.
+//
+// See profile.go for examples of messages implementing this interface.
+//
+// There is no support for groups, message sets, or "has" bits.
+
+package profile
+
+import (
+	"errors"
+	"fmt"
+)
+
+type buffer struct {
+	field int
+	typ   int
+	u64   uint64
+	data  []byte
+	tmp   [16]byte
+}
+
+type decoder func(*buffer, message) error
+
+type message interface {
+	decoder() []decoder
+	encode(*buffer)
+}
+
+func marshal(m message) []byte {
+	var b buffer
+	m.encode(&b)
+	return b.data
+}
+
+func encodeVarint(b *buffer, x uint64) {
+	for x >= 128 {
+		b.data = append(b.data, byte(x)|0x80)
+		x >>= 7
+	}
+	b.data = append(b.data, byte(x))
+}
+
+func encodeLength(b *buffer, tag int, len int) {
+	encodeVarint(b, uint64(tag)<<3|2)
+	encodeVarint(b, uint64(len))
+}
+
+func encodeUint64(b *buffer, tag int, x uint64) {
+	// append varint to b.data
+	encodeVarint(b, uint64(tag)<<3|0)
+	encodeVarint(b, x)
+}
+
+func encodeUint64s(b *buffer, tag int, x []uint64) {
+	if len(x) > 2 {
+		// Use packed encoding
+		n1 := len(b.data)
+		for _, u := range x {
+			encodeVarint(b, u)
+		}
+		n2 := len(b.data)
+		encodeLength(b, tag, n2-n1)
+		n3 := len(b.data)
+		copy(b.tmp[:], b.data[n2:n3])
+		copy(b.data[n1+(n3-n2):], b.data[n1:n2])
+		copy(b.data[n1:], b.tmp[:n3-n2])
+		return
+	}
+	for _, u := range x {
+		encodeUint64(b, tag, u)
+	}
+}
+
+func encodeUint64Opt(b *buffer, tag int, x uint64) {
+	if x == 0 {
+		return
+	}
+	encodeUint64(b, tag, x)
+}
+
+func encodeInt64(b *buffer, tag int, x int64) {
+	u := uint64(x)
+	encodeUint64(b, tag, u)
+}
+
+func encodeInt64Opt(b *buffer, tag int, x int64) {
+	if x == 0 {
+		return
+	}
+	encodeInt64(b, tag, x)
+}
+
+func encodeInt64s(b *buffer, tag int, x []int64) {
+	if len(x) > 2 {
+		// Use packed encoding
+		n1 := len(b.data)
+		for _, u := range x {
+			encodeVarint(b, uint64(u))
+		}
+		n2 := len(b.data)
+		encodeLength(b, tag, n2-n1)
+		n3 := len(b.data)
+		copy(b.tmp[:], b.data[n2:n3])
+		copy(b.data[n1+(n3-n2):], b.data[n1:n2])
+		copy(b.data[n1:], b.tmp[:n3-n2])
+		return
+	}
+	for _, u := range x {
+		encodeInt64(b, tag, u)
+	}
+}
+
+func encodeString(b *buffer, tag int, x string) {
+	encodeLength(b, tag, len(x))
+	b.data = append(b.data, x...)
+}
+
+func encodeStrings(b *buffer, tag int, x []string) {
+	for _, s := range x {
+		encodeString(b, tag, s)
+	}
+}
+
+func encodeBool(b *buffer, tag int, x bool) {
+	if x {
+		encodeUint64(b, tag, 1)
+	} else {
+		encodeUint64(b, tag, 0)
+	}
+}
+
+func encodeBoolOpt(b *buffer, tag int, x bool) {
+	if !x {
+		return
+	}
+	encodeBool(b, tag, x)
+}
+
+func encodeMessage(b *buffer, tag int, m message) {
+	n1 := len(b.data)
+	m.encode(b)
+	n2 := len(b.data)
+	encodeLength(b, tag, n2-n1)
+	n3 := len(b.data)
+	copy(b.tmp[:], b.data[n2:n3])
+	copy(b.data[n1+(n3-n2):], b.data[n1:n2])
+	copy(b.data[n1:], b.tmp[:n3-n2])
+}
+
+func unmarshal(data []byte, m message) (err error) {
+	b := buffer{data: data, typ: 2}
+	return decodeMessage(&b, m)
+}
+
+func le64(p []byte) uint64 {
+	return uint64(p[0]) | uint64(p[1])<<8 | uint64(p[2])<<16 | uint64(p[3])<<24 | uint64(p[4])<<32 | uint64(p[5])<<40 | uint64(p[6])<<48 | uint64(p[7])<<56
+}
+
+func le32(p []byte) uint32 {
+	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
+}
+
+func decodeVarint(data []byte) (uint64, []byte, error) {
+	var i int
+	var u uint64
+	for i = 0; ; i++ {
+		if i >= 10 || i >= len(data) {
+			return 0, nil, errors.New("bad varint")
+		}
+		u |= uint64(data[i]&0x7F) << uint(7*i)
+		if data[i]&0x80 == 0 {
+			return u, data[i+1:], nil
+		}
+	}
+}
+
+func decodeField(b *buffer, data []byte) ([]byte, error) {
+	x, data, err := decodeVarint(data)
+	if err != nil {
+		return nil, err
+	}
+	b.field = int(x >> 3)
+	b.typ = int(x & 7)
+	b.data = nil
+	b.u64 = 0
+	switch b.typ {
+	case 0:
+		b.u64, data, err = decodeVarint(data)
+		if err != nil {
+			return nil, err
+		}
+	case 1:
+		if len(data) < 8 {
+			return nil, errors.New("not enough data")
+		}
+		b.u64 = le64(data[:8])
+		data = data[8:]
+	case 2:
+		var n uint64
+		n, data, err = decodeVarint(data)
+		if err != nil {
+			return nil, err
+		}
+		if n > uint64(len(data)) {
+			return nil, errors.New("too much data")
+		}
+		b.data = data[:n]
+		data = data[n:]
+	case 5:
+		if len(data) < 4 {
+			return nil, errors.New("not enough data")
+		}
+		b.u64 = uint64(le32(data[:4]))
+		data = data[4:]
+	default:
+		return nil, fmt.Errorf("unknown wire type: %d", b.typ)
+	}
+
+	return data, nil
+}
+
+func checkType(b *buffer, typ int) error {
+	if b.typ != typ {
+		return errors.New("type mismatch")
+	}
+	return nil
+}
+
+func decodeMessage(b *buffer, m message) error {
+	if err := checkType(b, 2); err != nil {
+		return err
+	}
+	dec := m.decoder()
+	data := b.data
+	for len(data) > 0 {
+		// pull varint field# + type
+		var err error
+		data, err = decodeField(b, data)
+		if err != nil {
+			return err
+		}
+		if b.field >= len(dec) || dec[b.field] == nil {
+			continue
+		}
+		if err := dec[b.field](b, m); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func decodeInt64(b *buffer, x *int64) error {
+	if err := checkType(b, 0); err != nil {
+		return err
+	}
+	*x = int64(b.u64)
+	return nil
+}
+
+func decodeInt64s(b *buffer, x *[]int64) error {
+	if b.typ == 2 {
+		// Packed encoding
+		data := b.data
+		for len(data) > 0 {
+			var u uint64
+			var err error
+
+			if u, data, err = decodeVarint(data); err != nil {
+				return err
+			}
+			*x = append(*x, int64(u))
+		}
+		return nil
+	}
+	var i int64
+	if err := decodeInt64(b, &i); err != nil {
+		return err
+	}
+	*x = append(*x, i)
+	return nil
+}
+
+func decodeUint64(b *buffer, x *uint64) error {
+	if err := checkType(b, 0); err != nil {
+		return err
+	}
+	*x = b.u64
+	return nil
+}
+
+func decodeUint64s(b *buffer, x *[]uint64) error {
+	if b.typ == 2 {
+		data := b.data
+		// Packed encoding
+		for len(data) > 0 {
+			var u uint64
+			var err error
+
+			if u, data, err = decodeVarint(data); err != nil {
+				return err
+			}
+			*x = append(*x, u)
+		}
+		return nil
+	}
+	var u uint64
+	if err := decodeUint64(b, &u); err != nil {
+		return err
+	}
+	*x = append(*x, u)
+	return nil
+}
+
+func decodeString(b *buffer, x *string) error {
+	if err := checkType(b, 2); err != nil {
+		return err
+	}
+	*x = string(b.data)
+	return nil
+}
+
+func decodeStrings(b *buffer, x *[]string) error {
+	var s string
+	if err := decodeString(b, &s); err != nil {
+		return err
+	}
+	*x = append(*x, s)
+	return nil
+}
+
+func decodeBool(b *buffer, x *bool) error {
+	if err := checkType(b, 0); err != nil {
+		return err
+	}
+	if int64(b.u64) == 0 {
+		*x = false
+	} else {
+		*x = true
+	}
+	return nil
+}
diff --git a/src/internal/profile/proto_test.go b/src/internal/profile/proto_test.go
new file mode 100644
index 0000000..46c6d83
--- /dev/null
+++ b/src/internal/profile/proto_test.go
@@ -0,0 +1,71 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package profile
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestPackedEncoding(t *testing.T) {
+
+	type testcase struct {
+		uint64s []uint64
+		int64s  []int64
+		encoded []byte
+	}
+	for i, tc := range []testcase{
+		{
+			[]uint64{0, 1, 10, 100, 1000, 10000},
+			[]int64{1000, 0, 1000},
+			[]byte{10, 8, 0, 1, 10, 100, 232, 7, 144, 78, 18, 5, 232, 7, 0, 232, 7},
+		},
+		{
+			[]uint64{10000},
+			nil,
+			[]byte{8, 144, 78},
+		},
+		{
+			nil,
+			[]int64{-10000},
+			[]byte{16, 240, 177, 255, 255, 255, 255, 255, 255, 255, 1},
+		},
+	} {
+		source := &packedInts{tc.uint64s, tc.int64s}
+		if got, want := marshal(source), tc.encoded; !reflect.DeepEqual(got, want) {
+			t.Errorf("failed encode %d, got %v, want %v", i, got, want)
+		}
+
+		dest := new(packedInts)
+		if err := unmarshal(tc.encoded, dest); err != nil {
+			t.Errorf("failed decode %d: %v", i, err)
+			continue
+		}
+		if got, want := dest.uint64s, tc.uint64s; !reflect.DeepEqual(got, want) {
+			t.Errorf("failed decode uint64s %d, got %v, want %v", i, got, want)
+		}
+		if got, want := dest.int64s, tc.int64s; !reflect.DeepEqual(got, want) {
+			t.Errorf("failed decode int64s %d, got %v, want %v", i, got, want)
+		}
+	}
+}
+
+type packedInts struct {
+	uint64s []uint64
+	int64s  []int64
+}
+
+func (u *packedInts) decoder() []decoder {
+	return []decoder{
+		nil,
+		func(b *buffer, m message) error { return decodeUint64s(b, &m.(*packedInts).uint64s) },
+		func(b *buffer, m message) error { return decodeInt64s(b, &m.(*packedInts).int64s) },
+	}
+}
+
+func (u *packedInts) encode(b *buffer) {
+	encodeUint64s(b, 1, u.uint64s)
+	encodeInt64s(b, 2, u.int64s)
+}
diff --git a/src/internal/profile/prune.go b/src/internal/profile/prune.go
new file mode 100644
index 0000000..1924fad
--- /dev/null
+++ b/src/internal/profile/prune.go
@@ -0,0 +1,97 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Implements methods to remove frames from profiles.
+
+package profile
+
+import (
+	"fmt"
+	"regexp"
+)
+
+// Prune removes all nodes beneath a node matching dropRx, and not
+// matching keepRx. If the root node of a Sample matches, the sample
+// will have an empty stack.
+func (p *Profile) Prune(dropRx, keepRx *regexp.Regexp) {
+	prune := make(map[uint64]bool)
+	pruneBeneath := make(map[uint64]bool)
+
+	for _, loc := range p.Location {
+		var i int
+		for i = len(loc.Line) - 1; i >= 0; i-- {
+			if fn := loc.Line[i].Function; fn != nil && fn.Name != "" {
+				funcName := fn.Name
+				// Account for leading '.' on the PPC ELF v1 ABI.
+				if funcName[0] == '.' {
+					funcName = funcName[1:]
+				}
+				if dropRx.MatchString(funcName) {
+					if keepRx == nil || !keepRx.MatchString(funcName) {
+						break
+					}
+				}
+			}
+		}
+
+		if i >= 0 {
+			// Found matching entry to prune.
+			pruneBeneath[loc.ID] = true
+
+			// Remove the matching location.
+			if i == len(loc.Line)-1 {
+				// Matched the top entry: prune the whole location.
+				prune[loc.ID] = true
+			} else {
+				loc.Line = loc.Line[i+1:]
+			}
+		}
+	}
+
+	// Prune locs from each Sample
+	for _, sample := range p.Sample {
+		// Scan from the root to the leaves to find the prune location.
+		// Do not prune frames before the first user frame, to avoid
+		// pruning everything.
+		foundUser := false
+		for i := len(sample.Location) - 1; i >= 0; i-- {
+			id := sample.Location[i].ID
+			if !prune[id] && !pruneBeneath[id] {
+				foundUser = true
+				continue
+			}
+			if !foundUser {
+				continue
+			}
+			if prune[id] {
+				sample.Location = sample.Location[i+1:]
+				break
+			}
+			if pruneBeneath[id] {
+				sample.Location = sample.Location[i:]
+				break
+			}
+		}
+	}
+}
+
+// RemoveUninteresting prunes and elides profiles using built-in
+// tables of uninteresting function names.
+func (p *Profile) RemoveUninteresting() error {
+	var keep, drop *regexp.Regexp
+	var err error
+
+	if p.DropFrames != "" {
+		if drop, err = regexp.Compile("^(" + p.DropFrames + ")$"); err != nil {
+			return fmt.Errorf("failed to compile regexp %s: %v", p.DropFrames, err)
+		}
+		if p.KeepFrames != "" {
+			if keep, err = regexp.Compile("^(" + p.KeepFrames + ")$"); err != nil {
+				return fmt.Errorf("failed to compile regexp %s: %v", p.KeepFrames, err)
+			}
+		}
+		p.Prune(drop, keep)
+	}
+	return nil
+}
diff --git a/src/internal/race/doc.go b/src/internal/race/doc.go
new file mode 100644
index 0000000..8fa44ce
--- /dev/null
+++ b/src/internal/race/doc.go
@@ -0,0 +1,11 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package race contains helper functions for manually instrumenting code for the race detector.
+
+The runtime package intentionally exports these functions only in the race build;
+this package exports them unconditionally but without the "race" build tag they are no-ops.
+*/
+package race
diff --git a/src/internal/race/norace.go b/src/internal/race/norace.go
new file mode 100644
index 0000000..67b1305
--- /dev/null
+++ b/src/internal/race/norace.go
@@ -0,0 +1,43 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !race
+// +build !race
+
+package race
+
+import (
+	"unsafe"
+)
+
+const Enabled = false
+
+func Acquire(addr unsafe.Pointer) {
+}
+
+func Release(addr unsafe.Pointer) {
+}
+
+func ReleaseMerge(addr unsafe.Pointer) {
+}
+
+func Disable() {
+}
+
+func Enable() {
+}
+
+func Read(addr unsafe.Pointer) {
+}
+
+func Write(addr unsafe.Pointer) {
+}
+
+func ReadRange(addr unsafe.Pointer, len int) {
+}
+
+func WriteRange(addr unsafe.Pointer, len int) {
+}
+
+func Errors() int { return 0 }
diff --git a/src/internal/race/race.go b/src/internal/race/race.go
new file mode 100644
index 0000000..40f2c99
--- /dev/null
+++ b/src/internal/race/race.go
@@ -0,0 +1,55 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build race
+// +build race
+
+package race
+
+import (
+	"runtime"
+	"unsafe"
+)
+
+const Enabled = true
+
+func Acquire(addr unsafe.Pointer) {
+	runtime.RaceAcquire(addr)
+}
+
+func Release(addr unsafe.Pointer) {
+	runtime.RaceRelease(addr)
+}
+
+func ReleaseMerge(addr unsafe.Pointer) {
+	runtime.RaceReleaseMerge(addr)
+}
+
+func Disable() {
+	runtime.RaceDisable()
+}
+
+func Enable() {
+	runtime.RaceEnable()
+}
+
+func Read(addr unsafe.Pointer) {
+	runtime.RaceRead(addr)
+}
+
+func Write(addr unsafe.Pointer) {
+	runtime.RaceWrite(addr)
+}
+
+func ReadRange(addr unsafe.Pointer, len int) {
+	runtime.RaceReadRange(addr, len)
+}
+
+func WriteRange(addr unsafe.Pointer, len int) {
+	runtime.RaceWriteRange(addr, len)
+}
+
+func Errors() int {
+	return runtime.RaceErrors()
+}
diff --git a/src/internal/reflectlite/all_test.go b/src/internal/reflectlite/all_test.go
new file mode 100644
index 0000000..bb3cad4
--- /dev/null
+++ b/src/internal/reflectlite/all_test.go
@@ -0,0 +1,1038 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite_test
+
+import (
+	"encoding/base64"
+	"fmt"
+	. "internal/reflectlite"
+	"math"
+	"reflect"
+	"runtime"
+	"testing"
+	"unsafe"
+)
+
+func ToValue(v Value) reflect.Value {
+	return reflect.ValueOf(ToInterface(v))
+}
+
+func TypeString(t Type) string {
+	return fmt.Sprintf("%T", ToInterface(Zero(t)))
+}
+
+type integer int
+type T struct {
+	a int
+	b float64
+	c string
+	d *int
+}
+
+type pair struct {
+	i any
+	s string
+}
+
+func assert(t *testing.T, s, want string) {
+	t.Helper()
+	if s != want {
+		t.Errorf("have %#q want %#q", s, want)
+	}
+}
+
+var typeTests = []pair{
+	{struct{ x int }{}, "int"},
+	{struct{ x int8 }{}, "int8"},
+	{struct{ x int16 }{}, "int16"},
+	{struct{ x int32 }{}, "int32"},
+	{struct{ x int64 }{}, "int64"},
+	{struct{ x uint }{}, "uint"},
+	{struct{ x uint8 }{}, "uint8"},
+	{struct{ x uint16 }{}, "uint16"},
+	{struct{ x uint32 }{}, "uint32"},
+	{struct{ x uint64 }{}, "uint64"},
+	{struct{ x float32 }{}, "float32"},
+	{struct{ x float64 }{}, "float64"},
+	{struct{ x int8 }{}, "int8"},
+	{struct{ x (**int8) }{}, "**int8"},
+	{struct{ x (**integer) }{}, "**reflectlite_test.integer"},
+	{struct{ x ([32]int32) }{}, "[32]int32"},
+	{struct{ x ([]int8) }{}, "[]int8"},
+	{struct{ x (map[string]int32) }{}, "map[string]int32"},
+	{struct{ x (chan<- string) }{}, "chan<- string"},
+	{struct {
+		x struct {
+			c chan *int32
+			d float32
+		}
+	}{},
+		"struct { c chan *int32; d float32 }",
+	},
+	{struct{ x (func(a int8, b int32)) }{}, "func(int8, int32)"},
+	{struct {
+		x struct {
+			c func(chan *integer, *int8)
+		}
+	}{},
+		"struct { c func(chan *reflectlite_test.integer, *int8) }",
+	},
+	{struct {
+		x struct {
+			a int8
+			b int32
+		}
+	}{},
+		"struct { a int8; b int32 }",
+	},
+	{struct {
+		x struct {
+			a int8
+			b int8
+			c int32
+		}
+	}{},
+		"struct { a int8; b int8; c int32 }",
+	},
+	{struct {
+		x struct {
+			a int8
+			b int8
+			c int8
+			d int32
+		}
+	}{},
+		"struct { a int8; b int8; c int8; d int32 }",
+	},
+	{struct {
+		x struct {
+			a int8
+			b int8
+			c int8
+			d int8
+			e int32
+		}
+	}{},
+		"struct { a int8; b int8; c int8; d int8; e int32 }",
+	},
+	{struct {
+		x struct {
+			a int8
+			b int8
+			c int8
+			d int8
+			e int8
+			f int32
+		}
+	}{},
+		"struct { a int8; b int8; c int8; d int8; e int8; f int32 }",
+	},
+	{struct {
+		x struct {
+			a int8 `reflect:"hi there"`
+		}
+	}{},
+		`struct { a int8 "reflect:\"hi there\"" }`,
+	},
+	{struct {
+		x struct {
+			a int8 `reflect:"hi \x00there\t\n\"\\"`
+		}
+	}{},
+		`struct { a int8 "reflect:\"hi \\x00there\\t\\n\\\"\\\\\"" }`,
+	},
+	{struct {
+		x struct {
+			f func(args ...int)
+		}
+	}{},
+		"struct { f func(...int) }",
+	},
+	// {struct {
+	// 	x (interface {
+	// 		a(func(func(int) int) func(func(int)) int)
+	// 		b()
+	// 	})
+	// }{},
+	// 	"interface { reflectlite_test.a(func(func(int) int) func(func(int)) int); reflectlite_test.b() }",
+	// },
+	{struct {
+		x struct {
+			int32
+			int64
+		}
+	}{},
+		"struct { int32; int64 }",
+	},
+}
+
+var valueTests = []pair{
+	{new(int), "132"},
+	{new(int8), "8"},
+	{new(int16), "16"},
+	{new(int32), "32"},
+	{new(int64), "64"},
+	{new(uint), "132"},
+	{new(uint8), "8"},
+	{new(uint16), "16"},
+	{new(uint32), "32"},
+	{new(uint64), "64"},
+	{new(float32), "256.25"},
+	{new(float64), "512.125"},
+	{new(complex64), "532.125+10i"},
+	{new(complex128), "564.25+1i"},
+	{new(string), "stringy cheese"},
+	{new(bool), "true"},
+	{new(*int8), "*int8(0)"},
+	{new(**int8), "**int8(0)"},
+	{new([5]int32), "[5]int32{0, 0, 0, 0, 0}"},
+	{new(**integer), "**reflectlite_test.integer(0)"},
+	{new(map[string]int32), "map[string]int32{<can't iterate on maps>}"},
+	{new(chan<- string), "chan<- string"},
+	{new(func(a int8, b int32)), "func(int8, int32)(arg)"},
+	{new(struct {
+		c chan *int32
+		d float32
+	}),
+		"struct { c chan *int32; d float32 }{chan *int32, 0}",
+	},
+	{new(struct{ c func(chan *integer, *int8) }),
+		"struct { c func(chan *reflectlite_test.integer, *int8) }{func(chan *reflectlite_test.integer, *int8)(arg)}",
+	},
+	{new(struct {
+		a int8
+		b int32
+	}),
+		"struct { a int8; b int32 }{0, 0}",
+	},
+	{new(struct {
+		a int8
+		b int8
+		c int32
+	}),
+		"struct { a int8; b int8; c int32 }{0, 0, 0}",
+	},
+}
+
+func testType(t *testing.T, i int, typ Type, want string) {
+	s := TypeString(typ)
+	if s != want {
+		t.Errorf("#%d: have %#q, want %#q", i, s, want)
+	}
+}
+
+func testReflectType(t *testing.T, i int, typ Type, want string) {
+	s := TypeString(typ)
+	if s != want {
+		t.Errorf("#%d: have %#q, want %#q", i, s, want)
+	}
+}
+
+func TestTypes(t *testing.T) {
+	for i, tt := range typeTests {
+		testReflectType(t, i, Field(ValueOf(tt.i), 0).Type(), tt.s)
+	}
+}
+
+func TestSetValue(t *testing.T) {
+	for i, tt := range valueTests {
+		v := ValueOf(tt.i).Elem()
+		switch v.Kind() {
+		case Int:
+			v.Set(ValueOf(int(132)))
+		case Int8:
+			v.Set(ValueOf(int8(8)))
+		case Int16:
+			v.Set(ValueOf(int16(16)))
+		case Int32:
+			v.Set(ValueOf(int32(32)))
+		case Int64:
+			v.Set(ValueOf(int64(64)))
+		case Uint:
+			v.Set(ValueOf(uint(132)))
+		case Uint8:
+			v.Set(ValueOf(uint8(8)))
+		case Uint16:
+			v.Set(ValueOf(uint16(16)))
+		case Uint32:
+			v.Set(ValueOf(uint32(32)))
+		case Uint64:
+			v.Set(ValueOf(uint64(64)))
+		case Float32:
+			v.Set(ValueOf(float32(256.25)))
+		case Float64:
+			v.Set(ValueOf(512.125))
+		case Complex64:
+			v.Set(ValueOf(complex64(532.125 + 10i)))
+		case Complex128:
+			v.Set(ValueOf(complex128(564.25 + 1i)))
+		case String:
+			v.Set(ValueOf("stringy cheese"))
+		case Bool:
+			v.Set(ValueOf(true))
+		}
+		s := valueToString(v)
+		if s != tt.s {
+			t.Errorf("#%d: have %#q, want %#q", i, s, tt.s)
+		}
+	}
+}
+
+func TestCanSetField(t *testing.T) {
+	type embed struct{ x, X int }
+	type Embed struct{ x, X int }
+	type S1 struct {
+		embed
+		x, X int
+	}
+	type S2 struct {
+		*embed
+		x, X int
+	}
+	type S3 struct {
+		Embed
+		x, X int
+	}
+	type S4 struct {
+		*Embed
+		x, X int
+	}
+
+	type testCase struct {
+		index  []int
+		canSet bool
+	}
+	tests := []struct {
+		val   Value
+		cases []testCase
+	}{{
+		val: ValueOf(&S1{}),
+		cases: []testCase{
+			{[]int{0}, false},
+			{[]int{0, 0}, false},
+			{[]int{0, 1}, true},
+			{[]int{1}, false},
+			{[]int{2}, true},
+		},
+	}, {
+		val: ValueOf(&S2{embed: &embed{}}),
+		cases: []testCase{
+			{[]int{0}, false},
+			{[]int{0, 0}, false},
+			{[]int{0, 1}, true},
+			{[]int{1}, false},
+			{[]int{2}, true},
+		},
+	}, {
+		val: ValueOf(&S3{}),
+		cases: []testCase{
+			{[]int{0}, true},
+			{[]int{0, 0}, false},
+			{[]int{0, 1}, true},
+			{[]int{1}, false},
+			{[]int{2}, true},
+		},
+	}, {
+		val: ValueOf(&S4{Embed: &Embed{}}),
+		cases: []testCase{
+			{[]int{0}, true},
+			{[]int{0, 0}, false},
+			{[]int{0, 1}, true},
+			{[]int{1}, false},
+			{[]int{2}, true},
+		},
+	}}
+
+	for _, tt := range tests {
+		t.Run(tt.val.Type().Name(), func(t *testing.T) {
+			for _, tc := range tt.cases {
+				f := tt.val
+				for _, i := range tc.index {
+					if f.Kind() == Ptr {
+						f = f.Elem()
+					}
+					f = Field(f, i)
+				}
+				if got := f.CanSet(); got != tc.canSet {
+					t.Errorf("CanSet() = %v, want %v", got, tc.canSet)
+				}
+			}
+		})
+	}
+}
+
+var _i = 7
+
+var valueToStringTests = []pair{
+	{123, "123"},
+	{123.5, "123.5"},
+	{byte(123), "123"},
+	{"abc", "abc"},
+	{T{123, 456.75, "hello", &_i}, "reflectlite_test.T{123, 456.75, hello, *int(&7)}"},
+	{new(chan *T), "*chan *reflectlite_test.T(&chan *reflectlite_test.T)"},
+	{[10]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, "[10]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}"},
+	{&[10]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, "*[10]int(&[10]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10})"},
+	{[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, "[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}"},
+	{&[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, "*[]int(&[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10})"},
+}
+
+func TestValueToString(t *testing.T) {
+	for i, test := range valueToStringTests {
+		s := valueToString(ValueOf(test.i))
+		if s != test.s {
+			t.Errorf("#%d: have %#q, want %#q", i, s, test.s)
+		}
+	}
+}
+
+func TestPtrSetNil(t *testing.T) {
+	var i int32 = 1234
+	ip := &i
+	vip := ValueOf(&ip)
+	vip.Elem().Set(Zero(vip.Elem().Type()))
+	if ip != nil {
+		t.Errorf("got non-nil (%d), want nil", *ip)
+	}
+}
+
+func TestMapSetNil(t *testing.T) {
+	m := make(map[string]int)
+	vm := ValueOf(&m)
+	vm.Elem().Set(Zero(vm.Elem().Type()))
+	if m != nil {
+		t.Errorf("got non-nil (%p), want nil", m)
+	}
+}
+
+func TestAll(t *testing.T) {
+	testType(t, 1, TypeOf((int8)(0)), "int8")
+	testType(t, 2, TypeOf((*int8)(nil)).Elem(), "int8")
+
+	typ := TypeOf((*struct {
+		c chan *int32
+		d float32
+	})(nil))
+	testType(t, 3, typ, "*struct { c chan *int32; d float32 }")
+	etyp := typ.Elem()
+	testType(t, 4, etyp, "struct { c chan *int32; d float32 }")
+}
+
+func TestInterfaceValue(t *testing.T) {
+	var inter struct {
+		E any
+	}
+	inter.E = 123.456
+	v1 := ValueOf(&inter)
+	v2 := Field(v1.Elem(), 0)
+	// assert(t, TypeString(v2.Type()), "interface {}")
+	v3 := v2.Elem()
+	assert(t, TypeString(v3.Type()), "float64")
+
+	i3 := ToInterface(v2)
+	if _, ok := i3.(float64); !ok {
+		t.Error("v2.Interface() did not return float64, got ", TypeOf(i3))
+	}
+}
+
+func TestFunctionValue(t *testing.T) {
+	var x any = func() {}
+	v := ValueOf(x)
+	if fmt.Sprint(ToInterface(v)) != fmt.Sprint(x) {
+		t.Fatalf("TestFunction returned wrong pointer")
+	}
+	assert(t, TypeString(v.Type()), "func()")
+}
+
+var appendTests = []struct {
+	orig, extra []int
+}{
+	{make([]int, 2, 4), []int{22}},
+	{make([]int, 2, 4), []int{22, 33, 44}},
+}
+
+func sameInts(x, y []int) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, xx := range x {
+		if xx != y[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestBigUnnamedStruct(t *testing.T) {
+	b := struct{ a, b, c, d int64 }{1, 2, 3, 4}
+	v := ValueOf(b)
+	b1 := ToInterface(v).(struct {
+		a, b, c, d int64
+	})
+	if b1.a != b.a || b1.b != b.b || b1.c != b.c || b1.d != b.d {
+		t.Errorf("ValueOf(%v).Interface().(*Big) = %v", b, b1)
+	}
+}
+
+type big struct {
+	a, b, c, d, e int64
+}
+
+func TestBigStruct(t *testing.T) {
+	b := big{1, 2, 3, 4, 5}
+	v := ValueOf(b)
+	b1 := ToInterface(v).(big)
+	if b1.a != b.a || b1.b != b.b || b1.c != b.c || b1.d != b.d || b1.e != b.e {
+		t.Errorf("ValueOf(%v).Interface().(big) = %v", b, b1)
+	}
+}
+
+type Basic struct {
+	x int
+	y float32
+}
+
+type NotBasic Basic
+
+type DeepEqualTest struct {
+	a, b any
+	eq   bool
+}
+
+// Simple functions for DeepEqual tests.
+var (
+	fn1 func()             // nil.
+	fn2 func()             // nil.
+	fn3 = func() { fn1() } // Not nil.
+)
+
+type self struct{}
+
+type Loop *Loop
+type Loopy any
+
+var loop1, loop2 Loop
+var loopy1, loopy2 Loopy
+
+func init() {
+	loop1 = &loop2
+	loop2 = &loop1
+
+	loopy1 = &loopy2
+	loopy2 = &loopy1
+}
+
+var typeOfTests = []DeepEqualTest{
+	// Equalities
+	{nil, nil, true},
+	{1, 1, true},
+	{int32(1), int32(1), true},
+	{0.5, 0.5, true},
+	{float32(0.5), float32(0.5), true},
+	{"hello", "hello", true},
+	{make([]int, 10), make([]int, 10), true},
+	{&[3]int{1, 2, 3}, &[3]int{1, 2, 3}, true},
+	{Basic{1, 0.5}, Basic{1, 0.5}, true},
+	{error(nil), error(nil), true},
+	{map[int]string{1: "one", 2: "two"}, map[int]string{2: "two", 1: "one"}, true},
+	{fn1, fn2, true},
+
+	// Inequalities
+	{1, 2, false},
+	{int32(1), int32(2), false},
+	{0.5, 0.6, false},
+	{float32(0.5), float32(0.6), false},
+	{"hello", "hey", false},
+	{make([]int, 10), make([]int, 11), false},
+	{&[3]int{1, 2, 3}, &[3]int{1, 2, 4}, false},
+	{Basic{1, 0.5}, Basic{1, 0.6}, false},
+	{Basic{1, 0}, Basic{2, 0}, false},
+	{map[int]string{1: "one", 3: "two"}, map[int]string{2: "two", 1: "one"}, false},
+	{map[int]string{1: "one", 2: "txo"}, map[int]string{2: "two", 1: "one"}, false},
+	{map[int]string{1: "one"}, map[int]string{2: "two", 1: "one"}, false},
+	{map[int]string{2: "two", 1: "one"}, map[int]string{1: "one"}, false},
+	{nil, 1, false},
+	{1, nil, false},
+	{fn1, fn3, false},
+	{fn3, fn3, false},
+	{[][]int{{1}}, [][]int{{2}}, false},
+	{math.NaN(), math.NaN(), false},
+	{&[1]float64{math.NaN()}, &[1]float64{math.NaN()}, false},
+	{&[1]float64{math.NaN()}, self{}, true},
+	{[]float64{math.NaN()}, []float64{math.NaN()}, false},
+	{[]float64{math.NaN()}, self{}, true},
+	{map[float64]float64{math.NaN(): 1}, map[float64]float64{1: 2}, false},
+	{map[float64]float64{math.NaN(): 1}, self{}, true},
+
+	// Nil vs empty: not the same.
+	{[]int{}, []int(nil), false},
+	{[]int{}, []int{}, true},
+	{[]int(nil), []int(nil), true},
+	{map[int]int{}, map[int]int(nil), false},
+	{map[int]int{}, map[int]int{}, true},
+	{map[int]int(nil), map[int]int(nil), true},
+
+	// Mismatched types
+	{1, 1.0, false},
+	{int32(1), int64(1), false},
+	{0.5, "hello", false},
+	{[]int{1, 2, 3}, [3]int{1, 2, 3}, false},
+	{&[3]any{1, 2, 4}, &[3]any{1, 2, "s"}, false},
+	{Basic{1, 0.5}, NotBasic{1, 0.5}, false},
+	{map[uint]string{1: "one", 2: "two"}, map[int]string{2: "two", 1: "one"}, false},
+
+	// Possible loops.
+	{&loop1, &loop1, true},
+	{&loop1, &loop2, true},
+	{&loopy1, &loopy1, true},
+	{&loopy1, &loopy2, true},
+}
+
+func TestTypeOf(t *testing.T) {
+	// Special case for nil
+	if typ := TypeOf(nil); typ != nil {
+		t.Errorf("expected nil type for nil value; got %v", typ)
+	}
+	for _, test := range typeOfTests {
+		v := ValueOf(test.a)
+		if !v.IsValid() {
+			continue
+		}
+		typ := TypeOf(test.a)
+		if typ != v.Type() {
+			t.Errorf("TypeOf(%v) = %v, but ValueOf(%v).Type() = %v", test.a, typ, test.a, v.Type())
+		}
+	}
+}
+
+func Nil(a any, t *testing.T) {
+	n := Field(ValueOf(a), 0)
+	if !n.IsNil() {
+		t.Errorf("%v should be nil", a)
+	}
+}
+
+func NotNil(a any, t *testing.T) {
+	n := Field(ValueOf(a), 0)
+	if n.IsNil() {
+		t.Errorf("value of type %v should not be nil", TypeString(ValueOf(a).Type()))
+	}
+}
+
+func TestIsNil(t *testing.T) {
+	// These implement IsNil.
+	// Wrap in extra struct to hide interface type.
+	doNil := []any{
+		struct{ x *int }{},
+		struct{ x any }{},
+		struct{ x map[string]int }{},
+		struct{ x func() bool }{},
+		struct{ x chan int }{},
+		struct{ x []string }{},
+		struct{ x unsafe.Pointer }{},
+	}
+	for _, ts := range doNil {
+		ty := TField(TypeOf(ts), 0)
+		v := Zero(ty)
+		v.IsNil() // panics if not okay to call
+	}
+
+	// Check the implementations
+	var pi struct {
+		x *int
+	}
+	Nil(pi, t)
+	pi.x = new(int)
+	NotNil(pi, t)
+
+	var si struct {
+		x []int
+	}
+	Nil(si, t)
+	si.x = make([]int, 10)
+	NotNil(si, t)
+
+	var ci struct {
+		x chan int
+	}
+	Nil(ci, t)
+	ci.x = make(chan int)
+	NotNil(ci, t)
+
+	var mi struct {
+		x map[int]int
+	}
+	Nil(mi, t)
+	mi.x = make(map[int]int)
+	NotNil(mi, t)
+
+	var ii struct {
+		x any
+	}
+	Nil(ii, t)
+	ii.x = 2
+	NotNil(ii, t)
+
+	var fi struct {
+		x func(t *testing.T)
+	}
+	Nil(fi, t)
+	fi.x = TestIsNil
+	NotNil(fi, t)
+}
+
+// Indirect returns the value that v points to.
+// If v is a nil pointer, Indirect returns a zero Value.
+// If v is not a pointer, Indirect returns v.
+func Indirect(v Value) Value {
+	if v.Kind() != Ptr {
+		return v
+	}
+	return v.Elem()
+}
+
+func TestNilPtrValueSub(t *testing.T) {
+	var pi *int
+	if pv := ValueOf(pi); pv.Elem().IsValid() {
+		t.Error("ValueOf((*int)(nil)).Elem().IsValid()")
+	}
+}
+
+type Point struct {
+	x, y int
+}
+
+// This will be index 0.
+func (p Point) AnotherMethod(scale int) int {
+	return -1
+}
+
+// This will be index 1.
+func (p Point) Dist(scale int) int {
+	//println("Point.Dist", p.x, p.y, scale)
+	return p.x*p.x*scale + p.y*p.y*scale
+}
+
+// This will be index 2.
+func (p Point) GCMethod(k int) int {
+	runtime.GC()
+	return k + p.x
+}
+
+// This will be index 3.
+func (p Point) NoArgs() {
+	// Exercise no-argument/no-result paths.
+}
+
+// This will be index 4.
+func (p Point) TotalDist(points ...Point) int {
+	tot := 0
+	for _, q := range points {
+		dx := q.x - p.x
+		dy := q.y - p.y
+		tot += dx*dx + dy*dy // Should call Sqrt, but it's just a test.
+
+	}
+	return tot
+}
+
+type D1 struct {
+	d int
+}
+type D2 struct {
+	d int
+}
+
+func TestImportPath(t *testing.T) {
+	tests := []struct {
+		t    Type
+		path string
+	}{
+		{TypeOf(&base64.Encoding{}).Elem(), "encoding/base64"},
+		{TypeOf(int(0)), ""},
+		{TypeOf(int8(0)), ""},
+		{TypeOf(int16(0)), ""},
+		{TypeOf(int32(0)), ""},
+		{TypeOf(int64(0)), ""},
+		{TypeOf(uint(0)), ""},
+		{TypeOf(uint8(0)), ""},
+		{TypeOf(uint16(0)), ""},
+		{TypeOf(uint32(0)), ""},
+		{TypeOf(uint64(0)), ""},
+		{TypeOf(uintptr(0)), ""},
+		{TypeOf(float32(0)), ""},
+		{TypeOf(float64(0)), ""},
+		{TypeOf(complex64(0)), ""},
+		{TypeOf(complex128(0)), ""},
+		{TypeOf(byte(0)), ""},
+		{TypeOf(rune(0)), ""},
+		{TypeOf([]byte(nil)), ""},
+		{TypeOf([]rune(nil)), ""},
+		{TypeOf(string("")), ""},
+		{TypeOf((*any)(nil)).Elem(), ""},
+		{TypeOf((*byte)(nil)), ""},
+		{TypeOf((*rune)(nil)), ""},
+		{TypeOf((*int64)(nil)), ""},
+		{TypeOf(map[string]int{}), ""},
+		{TypeOf((*error)(nil)).Elem(), ""},
+		{TypeOf((*Point)(nil)), ""},
+		{TypeOf((*Point)(nil)).Elem(), "internal/reflectlite_test"},
+	}
+	for _, test := range tests {
+		if path := test.t.PkgPath(); path != test.path {
+			t.Errorf("%v.PkgPath() = %q, want %q", test.t, path, test.path)
+		}
+	}
+}
+
+func noAlloc(t *testing.T, n int, f func(int)) {
+	if testing.Short() {
+		t.Skip("skipping malloc count in short mode")
+	}
+	if runtime.GOMAXPROCS(0) > 1 {
+		t.Skip("skipping; GOMAXPROCS>1")
+	}
+	i := -1
+	allocs := testing.AllocsPerRun(n, func() {
+		f(i)
+		i++
+	})
+	if allocs > 0 {
+		t.Errorf("%d iterations: got %v mallocs, want 0", n, allocs)
+	}
+}
+
+func TestAllocations(t *testing.T) {
+	noAlloc(t, 100, func(j int) {
+		var i any
+		var v Value
+
+		// We can uncomment this when compiler escape analysis
+		// is good enough to see that the integer assigned to i
+		// does not escape and therefore need not be allocated.
+		//
+		// i = 42 + j
+		// v = ValueOf(i)
+		// if int(v.Int()) != 42+j {
+		// 	panic("wrong int")
+		// }
+
+		i = func(j int) int { return j }
+		v = ValueOf(i)
+		if ToInterface(v).(func(int) int)(j) != j {
+			panic("wrong result")
+		}
+	})
+}
+
+func TestSetPanic(t *testing.T) {
+	ok := func(f func()) { f() }
+	bad := shouldPanic
+	clear := func(v Value) { v.Set(Zero(v.Type())) }
+
+	type t0 struct {
+		W int
+	}
+
+	type t1 struct {
+		Y int
+		t0
+	}
+
+	type T2 struct {
+		Z       int
+		namedT0 t0
+	}
+
+	type T struct {
+		X int
+		t1
+		T2
+		NamedT1 t1
+		NamedT2 T2
+		namedT1 t1
+		namedT2 T2
+	}
+
+	// not addressable
+	v := ValueOf(T{})
+	bad(func() { clear(Field(v, 0)) })                     // .X
+	bad(func() { clear(Field(v, 1)) })                     // .t1
+	bad(func() { clear(Field(Field(v, 1), 0)) })           // .t1.Y
+	bad(func() { clear(Field(Field(v, 1), 1)) })           // .t1.t0
+	bad(func() { clear(Field(Field(Field(v, 1), 1), 0)) }) // .t1.t0.W
+	bad(func() { clear(Field(v, 2)) })                     // .T2
+	bad(func() { clear(Field(Field(v, 2), 0)) })           // .T2.Z
+	bad(func() { clear(Field(Field(v, 2), 1)) })           // .T2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 2), 1), 0)) }) // .T2.namedT0.W
+	bad(func() { clear(Field(v, 3)) })                     // .NamedT1
+	bad(func() { clear(Field(Field(v, 3), 0)) })           // .NamedT1.Y
+	bad(func() { clear(Field(Field(v, 3), 1)) })           // .NamedT1.t0
+	bad(func() { clear(Field(Field(Field(v, 3), 1), 0)) }) // .NamedT1.t0.W
+	bad(func() { clear(Field(v, 4)) })                     // .NamedT2
+	bad(func() { clear(Field(Field(v, 4), 0)) })           // .NamedT2.Z
+	bad(func() { clear(Field(Field(v, 4), 1)) })           // .NamedT2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 4), 1), 0)) }) // .NamedT2.namedT0.W
+	bad(func() { clear(Field(v, 5)) })                     // .namedT1
+	bad(func() { clear(Field(Field(v, 5), 0)) })           // .namedT1.Y
+	bad(func() { clear(Field(Field(v, 5), 1)) })           // .namedT1.t0
+	bad(func() { clear(Field(Field(Field(v, 5), 1), 0)) }) // .namedT1.t0.W
+	bad(func() { clear(Field(v, 6)) })                     // .namedT2
+	bad(func() { clear(Field(Field(v, 6), 0)) })           // .namedT2.Z
+	bad(func() { clear(Field(Field(v, 6), 1)) })           // .namedT2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 6), 1), 0)) }) // .namedT2.namedT0.W
+
+	// addressable
+	v = ValueOf(&T{}).Elem()
+	ok(func() { clear(Field(v, 0)) })                      // .X
+	bad(func() { clear(Field(v, 1)) })                     // .t1
+	ok(func() { clear(Field(Field(v, 1), 0)) })            // .t1.Y
+	bad(func() { clear(Field(Field(v, 1), 1)) })           // .t1.t0
+	ok(func() { clear(Field(Field(Field(v, 1), 1), 0)) })  // .t1.t0.W
+	ok(func() { clear(Field(v, 2)) })                      // .T2
+	ok(func() { clear(Field(Field(v, 2), 0)) })            // .T2.Z
+	bad(func() { clear(Field(Field(v, 2), 1)) })           // .T2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 2), 1), 0)) }) // .T2.namedT0.W
+	ok(func() { clear(Field(v, 3)) })                      // .NamedT1
+	ok(func() { clear(Field(Field(v, 3), 0)) })            // .NamedT1.Y
+	bad(func() { clear(Field(Field(v, 3), 1)) })           // .NamedT1.t0
+	ok(func() { clear(Field(Field(Field(v, 3), 1), 0)) })  // .NamedT1.t0.W
+	ok(func() { clear(Field(v, 4)) })                      // .NamedT2
+	ok(func() { clear(Field(Field(v, 4), 0)) })            // .NamedT2.Z
+	bad(func() { clear(Field(Field(v, 4), 1)) })           // .NamedT2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 4), 1), 0)) }) // .NamedT2.namedT0.W
+	bad(func() { clear(Field(v, 5)) })                     // .namedT1
+	bad(func() { clear(Field(Field(v, 5), 0)) })           // .namedT1.Y
+	bad(func() { clear(Field(Field(v, 5), 1)) })           // .namedT1.t0
+	bad(func() { clear(Field(Field(Field(v, 5), 1), 0)) }) // .namedT1.t0.W
+	bad(func() { clear(Field(v, 6)) })                     // .namedT2
+	bad(func() { clear(Field(Field(v, 6), 0)) })           // .namedT2.Z
+	bad(func() { clear(Field(Field(v, 6), 1)) })           // .namedT2.namedT0
+	bad(func() { clear(Field(Field(Field(v, 6), 1), 0)) }) // .namedT2.namedT0.W
+}
+
+func shouldPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic")
+		}
+	}()
+	f()
+}
+
+type S struct {
+	i1 int64
+	i2 int64
+}
+
+func TestBigZero(t *testing.T) {
+	const size = 1 << 10
+	var v [size]byte
+	z := ToInterface(Zero(ValueOf(v).Type())).([size]byte)
+	for i := 0; i < size; i++ {
+		if z[i] != 0 {
+			t.Fatalf("Zero object not all zero, index %d", i)
+		}
+	}
+}
+
+func TestInvalid(t *testing.T) {
+	// Used to have inconsistency between IsValid() and Kind() != Invalid.
+	type T struct{ v any }
+
+	v := Field(ValueOf(T{}), 0)
+	if v.IsValid() != true || v.Kind() != Interface {
+		t.Errorf("field: IsValid=%v, Kind=%v, want true, Interface", v.IsValid(), v.Kind())
+	}
+	v = v.Elem()
+	if v.IsValid() != false || v.Kind() != Invalid {
+		t.Errorf("field elem: IsValid=%v, Kind=%v, want false, Invalid", v.IsValid(), v.Kind())
+	}
+}
+
+type TheNameOfThisTypeIsExactly255BytesLongSoWhenTheCompilerPrependsTheReflectTestPackageNameAndExtraStarTheLinkerRuntimeAndReflectPackagesWillHaveToCorrectlyDecodeTheSecondLengthByte0123456789_0123456789_0123456789_0123456789_0123456789_012345678 int
+
+type nameTest struct {
+	v    any
+	want string
+}
+
+type A struct{}
+type B[T any] struct{}
+
+var nameTests = []nameTest{
+	{(*int32)(nil), "int32"},
+	{(*D1)(nil), "D1"},
+	{(*[]D1)(nil), ""},
+	{(*chan D1)(nil), ""},
+	{(*func() D1)(nil), ""},
+	{(*<-chan D1)(nil), ""},
+	{(*chan<- D1)(nil), ""},
+	{(*any)(nil), ""},
+	{(*interface {
+		F()
+	})(nil), ""},
+	{(*TheNameOfThisTypeIsExactly255BytesLongSoWhenTheCompilerPrependsTheReflectTestPackageNameAndExtraStarTheLinkerRuntimeAndReflectPackagesWillHaveToCorrectlyDecodeTheSecondLengthByte0123456789_0123456789_0123456789_0123456789_0123456789_012345678)(nil), "TheNameOfThisTypeIsExactly255BytesLongSoWhenTheCompilerPrependsTheReflectTestPackageNameAndExtraStarTheLinkerRuntimeAndReflectPackagesWillHaveToCorrectlyDecodeTheSecondLengthByte0123456789_0123456789_0123456789_0123456789_0123456789_012345678"},
+	{(*B[A])(nil), "B[internal/reflectlite_test.A]"},
+	{(*B[B[A]])(nil), "B[internal/reflectlite_test.B[internal/reflectlite_test.A]]"},
+}
+
+func TestNames(t *testing.T) {
+	for _, test := range nameTests {
+		typ := TypeOf(test.v).Elem()
+		if got := typ.Name(); got != test.want {
+			t.Errorf("%v Name()=%q, want %q", typ, got, test.want)
+		}
+	}
+}
+
+// TestUnaddressableField tests that the reflect package will not allow
+// a type from another package to be used as a named type with an
+// unexported field.
+//
+// This ensures that unexported fields cannot be modified by other packages.
+func TestUnaddressableField(t *testing.T) {
+	var b Buffer // type defined in reflect, a different package
+	var localBuffer struct {
+		buf []byte
+	}
+	lv := ValueOf(&localBuffer).Elem()
+	rv := ValueOf(b)
+	shouldPanic(func() {
+		lv.Set(rv)
+	})
+}
+
+type Tint int
+
+type Tint2 = Tint
+
+type Talias1 struct {
+	byte
+	uint8
+	int
+	int32
+	rune
+}
+
+type Talias2 struct {
+	Tint
+	Tint2
+}
+
+func TestAliasNames(t *testing.T) {
+	t1 := Talias1{byte: 1, uint8: 2, int: 3, int32: 4, rune: 5}
+	out := fmt.Sprintf("%#v", t1)
+	want := "reflectlite_test.Talias1{byte:0x1, uint8:0x2, int:3, int32:4, rune:5}"
+	if out != want {
+		t.Errorf("Talias1 print:\nhave: %s\nwant: %s", out, want)
+	}
+
+	t2 := Talias2{Tint: 1, Tint2: 2}
+	out = fmt.Sprintf("%#v", t2)
+	want = "reflectlite_test.Talias2{Tint:1, Tint2:2}"
+	if out != want {
+		t.Errorf("Talias2 print:\nhave: %s\nwant: %s", out, want)
+	}
+}
diff --git a/src/internal/reflectlite/asm.s b/src/internal/reflectlite/asm.s
new file mode 100644
index 0000000..a7b69b6
--- /dev/null
+++ b/src/internal/reflectlite/asm.s
@@ -0,0 +1,5 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Trigger build without complete flag.
\ No newline at end of file
diff --git a/src/internal/reflectlite/export_test.go b/src/internal/reflectlite/export_test.go
new file mode 100644
index 0000000..e9a928b
--- /dev/null
+++ b/src/internal/reflectlite/export_test.go
@@ -0,0 +1,117 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite
+
+import (
+	"unsafe"
+)
+
+// Field returns the i'th field of the struct v.
+// It panics if v's Kind is not Struct or i is out of range.
+func Field(v Value, i int) Value {
+	if v.kind() != Struct {
+		panic(&ValueError{"reflect.Value.Field", v.kind()})
+	}
+	tt := (*structType)(unsafe.Pointer(v.typ))
+	if uint(i) >= uint(len(tt.fields)) {
+		panic("reflect: Field index out of range")
+	}
+	field := &tt.fields[i]
+	typ := field.typ
+
+	// Inherit permission bits from v, but clear flagEmbedRO.
+	fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
+	// Using an unexported field forces flagRO.
+	if !field.name.isExported() {
+		if field.embedded() {
+			fl |= flagEmbedRO
+		} else {
+			fl |= flagStickyRO
+		}
+	}
+	// Either flagIndir is set and v.ptr points at struct,
+	// or flagIndir is not set and v.ptr is the actual struct data.
+	// In the former case, we want v.ptr + offset.
+	// In the latter case, we must have field.offset = 0,
+	// so v.ptr + field.offset is still the correct address.
+	ptr := add(v.ptr, field.offset, "same as non-reflect &v.field")
+	return Value{typ, ptr, fl}
+}
+
+func TField(typ Type, i int) Type {
+	t := typ.(*rtype)
+	if t.Kind() != Struct {
+		panic("reflect: Field of non-struct type")
+	}
+	tt := (*structType)(unsafe.Pointer(t))
+
+	return StructFieldType(tt, i)
+}
+
+// Field returns the i'th struct field.
+func StructFieldType(t *structType, i int) Type {
+	if i < 0 || i >= len(t.fields) {
+		panic("reflect: Field index out of bounds")
+	}
+	p := &t.fields[i]
+	return toType(p.typ)
+}
+
+// Zero returns a Value representing the zero value for the specified type.
+// The result is different from the zero value of the Value struct,
+// which represents no value at all.
+// For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
+// The returned value is neither addressable nor settable.
+func Zero(typ Type) Value {
+	if typ == nil {
+		panic("reflect: Zero(nil)")
+	}
+	t := typ.(*rtype)
+	fl := flag(t.Kind())
+	if ifaceIndir(t) {
+		return Value{t, unsafe_New(t), fl | flagIndir}
+	}
+	return Value{t, nil, fl}
+}
+
+// ToInterface returns v's current value as an interface{}.
+// It is equivalent to:
+//
+//	var i interface{} = (v's underlying value)
+//
+// It panics if the Value was obtained by accessing
+// unexported struct fields.
+func ToInterface(v Value) (i any) {
+	return valueInterface(v)
+}
+
+type EmbedWithUnexpMeth struct{}
+
+func (EmbedWithUnexpMeth) f() {}
+
+type pinUnexpMeth interface {
+	f()
+}
+
+var pinUnexpMethI = pinUnexpMeth(EmbedWithUnexpMeth{})
+
+func FirstMethodNameBytes(t Type) *byte {
+	_ = pinUnexpMethI
+
+	ut := t.uncommon()
+	if ut == nil {
+		panic("type has no methods")
+	}
+	m := ut.methods()[0]
+	mname := t.(*rtype).nameOff(m.name)
+	if *mname.data(0, "name flag field")&(1<<2) == 0 {
+		panic("method name does not have pkgPath *string")
+	}
+	return mname.bytes
+}
+
+type Buffer struct {
+	buf []byte
+}
diff --git a/src/internal/reflectlite/reflect_mirror_test.go b/src/internal/reflectlite/reflect_mirror_test.go
new file mode 100644
index 0000000..9b28b13
--- /dev/null
+++ b/src/internal/reflectlite/reflect_mirror_test.go
@@ -0,0 +1,132 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+)
+
+var typeNames = []string{
+	"rtype",
+	"uncommonType",
+	"arrayType",
+	"chanType",
+	"funcType",
+	"interfaceType",
+	"mapType",
+	"ptrType",
+	"sliceType",
+	"structType",
+}
+
+type visitor struct {
+	m map[string]map[string]bool
+}
+
+func newVisitor() visitor {
+	v := visitor{}
+	v.m = make(map[string]map[string]bool)
+
+	return v
+}
+func (v visitor) filter(name string) bool {
+	for _, typeName := range typeNames {
+		if typeName == name {
+			return true
+		}
+	}
+	return false
+}
+
+func (v visitor) Visit(n ast.Node) ast.Visitor {
+	switch x := n.(type) {
+	case *ast.TypeSpec:
+		if v.filter(x.Name.String()) {
+			if st, ok := x.Type.(*ast.StructType); ok {
+				v.m[x.Name.String()] = make(map[string]bool)
+				for _, field := range st.Fields.List {
+					k := fmt.Sprintf("%s", field.Type)
+					if len(field.Names) > 0 {
+						k = field.Names[0].Name
+					}
+					v.m[x.Name.String()][k] = true
+				}
+			}
+		}
+	}
+	return v
+}
+
+func loadTypes(path, pkgName string, v visitor) {
+	fset := token.NewFileSet()
+
+	filter := func(fi fs.FileInfo) bool {
+		return strings.HasSuffix(fi.Name(), ".go")
+	}
+	pkgs, err := parser.ParseDir(fset, path, filter, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	pkg := pkgs[pkgName]
+
+	for _, f := range pkg.Files {
+		ast.Walk(v, f)
+	}
+}
+
+func TestMirrorWithReflect(t *testing.T) {
+	reflectDir := filepath.Join(runtime.GOROOT(), "src", "reflect")
+	if _, err := os.Stat(reflectDir); os.IsNotExist(err) {
+		// On some mobile builders, the test binary executes on a machine without a
+		// complete GOROOT source tree.
+		t.Skipf("GOROOT source not present")
+	}
+
+	var wg sync.WaitGroup
+	rl, r := newVisitor(), newVisitor()
+
+	for _, tc := range []struct {
+		path, pkg string
+		v         visitor
+	}{
+		{".", "reflectlite", rl},
+		{reflectDir, "reflect", r},
+	} {
+		tc := tc
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			loadTypes(tc.path, tc.pkg, tc.v)
+		}()
+	}
+	wg.Wait()
+
+	if len(rl.m) != len(r.m) {
+		t.Fatalf("number of types mismatch, reflect: %d, reflectlite: %d", len(r.m), len(rl.m))
+	}
+
+	for typName := range r.m {
+		if len(r.m[typName]) != len(rl.m[typName]) {
+			t.Errorf("type %s number of fields mismatch, reflect: %d, reflectlite: %d", typName, len(r.m[typName]), len(rl.m[typName]))
+			continue
+		}
+		for field := range r.m[typName] {
+			if _, ok := rl.m[typName][field]; !ok {
+				t.Errorf(`Field mismatch, reflect have "%s", relectlite does not.`, field)
+			}
+		}
+	}
+}
diff --git a/src/internal/reflectlite/set_test.go b/src/internal/reflectlite/set_test.go
new file mode 100644
index 0000000..ca7ea9b
--- /dev/null
+++ b/src/internal/reflectlite/set_test.go
@@ -0,0 +1,101 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite_test
+
+import (
+	"bytes"
+	"go/ast"
+	"go/token"
+	. "internal/reflectlite"
+	"io"
+	"testing"
+)
+
+func TestImplicitSetConversion(t *testing.T) {
+	// Assume TestImplicitMapConversion covered the basics.
+	// Just make sure conversions are being applied at all.
+	var r io.Reader
+	b := new(bytes.Buffer)
+	rv := ValueOf(&r).Elem()
+	rv.Set(ValueOf(b))
+	if r != b {
+		t.Errorf("after Set: r=%T(%v)", r, r)
+	}
+}
+
+var implementsTests = []struct {
+	x any
+	t any
+	b bool
+}{
+	{new(*bytes.Buffer), new(io.Reader), true},
+	{new(bytes.Buffer), new(io.Reader), false},
+	{new(*bytes.Buffer), new(io.ReaderAt), false},
+	{new(*ast.Ident), new(ast.Expr), true},
+	{new(*notAnExpr), new(ast.Expr), false},
+	{new(*ast.Ident), new(notASTExpr), false},
+	{new(notASTExpr), new(ast.Expr), false},
+	{new(ast.Expr), new(notASTExpr), false},
+	{new(*notAnExpr), new(notASTExpr), true},
+	{new(mapError), new(error), true},
+	{new(*mapError), new(error), true},
+}
+
+type notAnExpr struct{}
+
+func (notAnExpr) Pos() token.Pos { return token.NoPos }
+func (notAnExpr) End() token.Pos { return token.NoPos }
+func (notAnExpr) exprNode()      {}
+
+type notASTExpr interface {
+	Pos() token.Pos
+	End() token.Pos
+	exprNode()
+}
+
+type mapError map[string]string
+
+func (mapError) Error() string { return "mapError" }
+
+var _ error = mapError{}
+var _ error = new(mapError)
+
+func TestImplements(t *testing.T) {
+	for _, tt := range implementsTests {
+		xv := TypeOf(tt.x).Elem()
+		xt := TypeOf(tt.t).Elem()
+		if b := xv.Implements(xt); b != tt.b {
+			t.Errorf("(%s).Implements(%s) = %v, want %v", TypeString(xv), TypeString(xt), b, tt.b)
+		}
+	}
+}
+
+var assignableTests = []struct {
+	x any
+	t any
+	b bool
+}{
+	{new(chan int), new(<-chan int), true},
+	{new(<-chan int), new(chan int), false},
+	{new(*int), new(IntPtr), true},
+	{new(IntPtr), new(*int), true},
+	{new(IntPtr), new(IntPtr1), false},
+	{new(Ch), new(<-chan any), true},
+	// test runs implementsTests too
+}
+
+type IntPtr *int
+type IntPtr1 *int
+type Ch <-chan any
+
+func TestAssignableTo(t *testing.T) {
+	for i, tt := range append(assignableTests, implementsTests...) {
+		xv := TypeOf(tt.x).Elem()
+		xt := TypeOf(tt.t).Elem()
+		if b := xv.AssignableTo(xt); b != tt.b {
+			t.Errorf("%d:AssignableTo: got %v, want %v", i, b, tt.b)
+		}
+	}
+}
diff --git a/src/internal/reflectlite/swapper.go b/src/internal/reflectlite/swapper.go
new file mode 100644
index 0000000..fc402bb
--- /dev/null
+++ b/src/internal/reflectlite/swapper.go
@@ -0,0 +1,78 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite
+
+import (
+	"internal/goarch"
+	"internal/unsafeheader"
+	"unsafe"
+)
+
+// Swapper returns a function that swaps the elements in the provided
+// slice.
+//
+// Swapper panics if the provided interface is not a slice.
+func Swapper(slice any) func(i, j int) {
+	v := ValueOf(slice)
+	if v.Kind() != Slice {
+		panic(&ValueError{Method: "Swapper", Kind: v.Kind()})
+	}
+	// Fast path for slices of size 0 and 1. Nothing to swap.
+	switch v.Len() {
+	case 0:
+		return func(i, j int) { panic("reflect: slice index out of range") }
+	case 1:
+		return func(i, j int) {
+			if i != 0 || j != 0 {
+				panic("reflect: slice index out of range")
+			}
+		}
+	}
+
+	typ := v.Type().Elem().(*rtype)
+	size := typ.Size()
+	hasPtr := typ.ptrdata != 0
+
+	// Some common & small cases, without using memmove:
+	if hasPtr {
+		if size == goarch.PtrSize {
+			ps := *(*[]unsafe.Pointer)(v.ptr)
+			return func(i, j int) { ps[i], ps[j] = ps[j], ps[i] }
+		}
+		if typ.Kind() == String {
+			ss := *(*[]string)(v.ptr)
+			return func(i, j int) { ss[i], ss[j] = ss[j], ss[i] }
+		}
+	} else {
+		switch size {
+		case 8:
+			is := *(*[]int64)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 4:
+			is := *(*[]int32)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 2:
+			is := *(*[]int16)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 1:
+			is := *(*[]int8)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		}
+	}
+
+	s := (*unsafeheader.Slice)(v.ptr)
+	tmp := unsafe_New(typ) // swap scratch space
+
+	return func(i, j int) {
+		if uint(i) >= uint(s.Len) || uint(j) >= uint(s.Len) {
+			panic("reflect: slice index out of range")
+		}
+		val1 := arrayAt(s.Data, i, size, "i < s.Len")
+		val2 := arrayAt(s.Data, j, size, "j < s.Len")
+		typedmemmove(typ, tmp, val1)
+		typedmemmove(typ, val1, val2)
+		typedmemmove(typ, val2, tmp)
+	}
+}
diff --git a/src/internal/reflectlite/tostring_test.go b/src/internal/reflectlite/tostring_test.go
new file mode 100644
index 0000000..966b0bd
--- /dev/null
+++ b/src/internal/reflectlite/tostring_test.go
@@ -0,0 +1,98 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Formatting of reflection types and values for debugging.
+// Not defined as methods so they do not need to be linked into most binaries;
+// the functions are not used by the library itself, only in tests.
+
+package reflectlite_test
+
+import (
+	. "internal/reflectlite"
+	"reflect"
+	"strconv"
+)
+
+// valueToString returns a textual representation of the reflection value val.
+// For debugging only.
+func valueToString(v Value) string {
+	return valueToStringImpl(reflect.ValueOf(ToInterface(v)))
+}
+
+func valueToStringImpl(val reflect.Value) string {
+	var str string
+	if !val.IsValid() {
+		return "<zero Value>"
+	}
+	typ := val.Type()
+	switch val.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return strconv.FormatInt(val.Int(), 10)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return strconv.FormatUint(val.Uint(), 10)
+	case reflect.Float32, reflect.Float64:
+		return strconv.FormatFloat(val.Float(), 'g', -1, 64)
+	case reflect.Complex64, reflect.Complex128:
+		c := val.Complex()
+		return strconv.FormatFloat(real(c), 'g', -1, 64) + "+" + strconv.FormatFloat(imag(c), 'g', -1, 64) + "i"
+	case reflect.String:
+		return val.String()
+	case reflect.Bool:
+		if val.Bool() {
+			return "true"
+		} else {
+			return "false"
+		}
+	case reflect.Pointer:
+		v := val
+		str = typ.String() + "("
+		if v.IsNil() {
+			str += "0"
+		} else {
+			str += "&" + valueToStringImpl(v.Elem())
+		}
+		str += ")"
+		return str
+	case reflect.Array, reflect.Slice:
+		v := val
+		str += typ.String()
+		str += "{"
+		for i := 0; i < v.Len(); i++ {
+			if i > 0 {
+				str += ", "
+			}
+			str += valueToStringImpl(v.Index(i))
+		}
+		str += "}"
+		return str
+	case reflect.Map:
+		str += typ.String()
+		str += "{"
+		str += "<can't iterate on maps>"
+		str += "}"
+		return str
+	case reflect.Chan:
+		str = typ.String()
+		return str
+	case reflect.Struct:
+		t := typ
+		v := val
+		str += t.String()
+		str += "{"
+		for i, n := 0, v.NumField(); i < n; i++ {
+			if i > 0 {
+				str += ", "
+			}
+			str += valueToStringImpl(v.Field(i))
+		}
+		str += "}"
+		return str
+	case reflect.Interface:
+		return typ.String() + "(" + valueToStringImpl(val.Elem()) + ")"
+	case reflect.Func:
+		return typ.String() + "(arg)"
+	default:
+		panic("valueToString: can't print type " + typ.String())
+	}
+}
diff --git a/src/internal/reflectlite/type.go b/src/internal/reflectlite/type.go
new file mode 100644
index 0000000..43440b1
--- /dev/null
+++ b/src/internal/reflectlite/type.go
@@ -0,0 +1,974 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package reflectlite implements lightweight version of reflect, not using
+// any package except for "runtime" and "unsafe".
+package reflectlite
+
+import "unsafe"
+
+// Type is the representation of a Go type.
+//
+// Not all methods apply to all kinds of types. Restrictions,
+// if any, are noted in the documentation for each method.
+// Use the Kind method to find out the kind of type before
+// calling kind-specific methods. Calling a method
+// inappropriate to the kind of type causes a run-time panic.
+//
+// Type values are comparable, such as with the == operator,
+// so they can be used as map keys.
+// Two Type values are equal if they represent identical types.
+type Type interface {
+	// Methods applicable to all types.
+
+	// Name returns the type's name within its package for a defined type.
+	// For other (non-defined) types it returns the empty string.
+	Name() string
+
+	// PkgPath returns a defined type's package path, that is, the import path
+	// that uniquely identifies the package, such as "encoding/base64".
+	// If the type was predeclared (string, error) or not defined (*T, struct{},
+	// []int, or A where A is an alias for a non-defined type), the package path
+	// will be the empty string.
+	PkgPath() string
+
+	// Size returns the number of bytes needed to store
+	// a value of the given type; it is analogous to unsafe.Sizeof.
+	Size() uintptr
+
+	// Kind returns the specific kind of this type.
+	Kind() Kind
+
+	// Implements reports whether the type implements the interface type u.
+	Implements(u Type) bool
+
+	// AssignableTo reports whether a value of the type is assignable to type u.
+	AssignableTo(u Type) bool
+
+	// Comparable reports whether values of this type are comparable.
+	Comparable() bool
+
+	// String returns a string representation of the type.
+	// The string representation may use shortened package names
+	// (e.g., base64 instead of "encoding/base64") and is not
+	// guaranteed to be unique among types. To test for type identity,
+	// compare the Types directly.
+	String() string
+
+	// Elem returns a type's element type.
+	// It panics if the type's Kind is not Ptr.
+	Elem() Type
+
+	common() *rtype
+	uncommon() *uncommonType
+}
+
+/*
+ * These data structures are known to the compiler (../../cmd/internal/reflectdata/reflect.go).
+ * A few are known to ../runtime/type.go to convey to debuggers.
+ * They are also known to ../runtime/type.go.
+ */
+
+// A Kind represents the specific kind of type that a Type represents.
+// The zero Kind is not a valid kind.
+type Kind uint
+
+const (
+	Invalid Kind = iota
+	Bool
+	Int
+	Int8
+	Int16
+	Int32
+	Int64
+	Uint
+	Uint8
+	Uint16
+	Uint32
+	Uint64
+	Uintptr
+	Float32
+	Float64
+	Complex64
+	Complex128
+	Array
+	Chan
+	Func
+	Interface
+	Map
+	Pointer
+	Slice
+	String
+	Struct
+	UnsafePointer
+)
+
+const Ptr = Pointer
+
+// tflag is used by an rtype to signal what extra type information is
+// available in the memory directly following the rtype value.
+//
+// tflag values must be kept in sync with copies in:
+//
+//	cmd/compile/internal/reflectdata/reflect.go
+//	cmd/link/internal/ld/decodesym.go
+//	runtime/type.go
+type tflag uint8
+
+const (
+	// tflagUncommon means that there is a pointer, *uncommonType,
+	// just beyond the outer type structure.
+	//
+	// For example, if t.Kind() == Struct and t.tflag&tflagUncommon != 0,
+	// then t has uncommonType data and it can be accessed as:
+	//
+	//	type tUncommon struct {
+	//		structType
+	//		u uncommonType
+	//	}
+	//	u := &(*tUncommon)(unsafe.Pointer(t)).u
+	tflagUncommon tflag = 1 << 0
+
+	// tflagExtraStar means the name in the str field has an
+	// extraneous '*' prefix. This is because for most types T in
+	// a program, the type *T also exists and reusing the str data
+	// saves binary size.
+	tflagExtraStar tflag = 1 << 1
+
+	// tflagNamed means the type has a name.
+	tflagNamed tflag = 1 << 2
+
+	// tflagRegularMemory means that equal and hash functions can treat
+	// this type as a single region of t.size bytes.
+	tflagRegularMemory tflag = 1 << 3
+)
+
+// rtype is the common implementation of most values.
+// It is embedded in other struct types.
+//
+// rtype must be kept in sync with ../runtime/type.go:/^type._type.
+type rtype struct {
+	size       uintptr
+	ptrdata    uintptr // number of bytes in the type that can contain pointers
+	hash       uint32  // hash of type; avoids computation in hash tables
+	tflag      tflag   // extra type information flags
+	align      uint8   // alignment of variable with this type
+	fieldAlign uint8   // alignment of struct field with this type
+	kind       uint8   // enumeration for C
+	// function for comparing objects of this type
+	// (ptr to object A, ptr to object B) -> ==?
+	equal     func(unsafe.Pointer, unsafe.Pointer) bool
+	gcdata    *byte   // garbage collection data
+	str       nameOff // string form
+	ptrToThis typeOff // type for pointer to this type, may be zero
+}
+
+// Method on non-interface type
+type method struct {
+	name nameOff // name of method
+	mtyp typeOff // method type (without receiver)
+	ifn  textOff // fn used in interface call (one-word receiver)
+	tfn  textOff // fn used for normal method call
+}
+
+// uncommonType is present only for defined types or types with methods
+// (if T is a defined type, the uncommonTypes for T and *T have methods).
+// Using a pointer to this struct reduces the overall size required
+// to describe a non-defined type with no methods.
+type uncommonType struct {
+	pkgPath nameOff // import path; empty for built-in types like int, string
+	mcount  uint16  // number of methods
+	xcount  uint16  // number of exported methods
+	moff    uint32  // offset from this uncommontype to [mcount]method
+	_       uint32  // unused
+}
+
+// chanDir represents a channel type's direction.
+type chanDir int
+
+const (
+	recvDir chanDir             = 1 << iota // <-chan
+	sendDir                                 // chan<-
+	bothDir = recvDir | sendDir             // chan
+)
+
+// arrayType represents a fixed array type.
+type arrayType struct {
+	rtype
+	elem  *rtype // array element type
+	slice *rtype // slice type
+	len   uintptr
+}
+
+// chanType represents a channel type.
+type chanType struct {
+	rtype
+	elem *rtype  // channel element type
+	dir  uintptr // channel direction (chanDir)
+}
+
+// funcType represents a function type.
+//
+// A *rtype for each in and out parameter is stored in an array that
+// directly follows the funcType (and possibly its uncommonType). So
+// a function type with one method, one input, and one output is:
+//
+//	struct {
+//		funcType
+//		uncommonType
+//		[2]*rtype    // [0] is in, [1] is out
+//	}
+type funcType struct {
+	rtype
+	inCount  uint16
+	outCount uint16 // top bit is set if last input parameter is ...
+}
+
+// imethod represents a method on an interface type
+type imethod struct {
+	name nameOff // name of method
+	typ  typeOff // .(*FuncType) underneath
+}
+
+// interfaceType represents an interface type.
+type interfaceType struct {
+	rtype
+	pkgPath name      // import path
+	methods []imethod // sorted by hash
+}
+
+// mapType represents a map type.
+type mapType struct {
+	rtype
+	key    *rtype // map key type
+	elem   *rtype // map element (value) type
+	bucket *rtype // internal bucket structure
+	// function for hashing keys (ptr to key, seed) -> hash
+	hasher     func(unsafe.Pointer, uintptr) uintptr
+	keysize    uint8  // size of key slot
+	valuesize  uint8  // size of value slot
+	bucketsize uint16 // size of bucket
+	flags      uint32
+}
+
+// ptrType represents a pointer type.
+type ptrType struct {
+	rtype
+	elem *rtype // pointer element (pointed at) type
+}
+
+// sliceType represents a slice type.
+type sliceType struct {
+	rtype
+	elem *rtype // slice element type
+}
+
+// Struct field
+type structField struct {
+	name   name    // name is always non-empty
+	typ    *rtype  // type of field
+	offset uintptr // byte offset of field
+}
+
+func (f *structField) embedded() bool {
+	return f.name.embedded()
+}
+
+// structType represents a struct type.
+type structType struct {
+	rtype
+	pkgPath name
+	fields  []structField // sorted by offset
+}
+
+// name is an encoded type name with optional extra data.
+//
+// The first byte is a bit field containing:
+//
+//	1<<0 the name is exported
+//	1<<1 tag data follows the name
+//	1<<2 pkgPath nameOff follows the name and tag
+//
+// The next two bytes are the data length:
+//
+//	l := uint16(data[1])<<8 | uint16(data[2])
+//
+// Bytes [3:3+l] are the string data.
+//
+// If tag data follows then bytes 3+l and 3+l+1 are the tag length,
+// with the data following.
+//
+// If the import path follows, then 4 bytes at the end of
+// the data form a nameOff. The import path is only set for concrete
+// methods that are defined in a different package than their type.
+//
+// If a name starts with "*", then the exported bit represents
+// whether the pointed to type is exported.
+type name struct {
+	bytes *byte
+}
+
+func (n name) data(off int, whySafe string) *byte {
+	return (*byte)(add(unsafe.Pointer(n.bytes), uintptr(off), whySafe))
+}
+
+func (n name) isExported() bool {
+	return (*n.bytes)&(1<<0) != 0
+}
+
+func (n name) hasTag() bool {
+	return (*n.bytes)&(1<<1) != 0
+}
+
+func (n name) embedded() bool {
+	return (*n.bytes)&(1<<3) != 0
+}
+
+// readVarint parses a varint as encoded by encoding/binary.
+// It returns the number of encoded bytes and the encoded value.
+func (n name) readVarint(off int) (int, int) {
+	v := 0
+	for i := 0; ; i++ {
+		x := *n.data(off+i, "read varint")
+		v += int(x&0x7f) << (7 * i)
+		if x&0x80 == 0 {
+			return i + 1, v
+		}
+	}
+}
+
+func (n name) name() string {
+	if n.bytes == nil {
+		return ""
+	}
+	i, l := n.readVarint(1)
+	return unsafe.String(n.data(1+i, "non-empty string"), l)
+}
+
+func (n name) tag() string {
+	if !n.hasTag() {
+		return ""
+	}
+	i, l := n.readVarint(1)
+	i2, l2 := n.readVarint(1 + i + l)
+	return unsafe.String(n.data(1+i+l+i2, "non-empty string"), l2)
+}
+
+func (n name) pkgPath() string {
+	if n.bytes == nil || *n.data(0, "name flag field")&(1<<2) == 0 {
+		return ""
+	}
+	i, l := n.readVarint(1)
+	off := 1 + i + l
+	if n.hasTag() {
+		i2, l2 := n.readVarint(off)
+		off += i2 + l2
+	}
+	var nameOff int32
+	// Note that this field may not be aligned in memory,
+	// so we cannot use a direct int32 assignment here.
+	copy((*[4]byte)(unsafe.Pointer(&nameOff))[:], (*[4]byte)(unsafe.Pointer(n.data(off, "name offset field")))[:])
+	pkgPathName := name{(*byte)(resolveTypeOff(unsafe.Pointer(n.bytes), nameOff))}
+	return pkgPathName.name()
+}
+
+/*
+ * The compiler knows the exact layout of all the data structures above.
+ * The compiler does not know about the data structures and methods below.
+ */
+
+const (
+	kindDirectIface = 1 << 5
+	kindGCProg      = 1 << 6 // Type.gc points to GC program
+	kindMask        = (1 << 5) - 1
+)
+
+// String returns the name of k.
+func (k Kind) String() string {
+	if int(k) < len(kindNames) {
+		return kindNames[k]
+	}
+	return kindNames[0]
+}
+
+var kindNames = []string{
+	Invalid:       "invalid",
+	Bool:          "bool",
+	Int:           "int",
+	Int8:          "int8",
+	Int16:         "int16",
+	Int32:         "int32",
+	Int64:         "int64",
+	Uint:          "uint",
+	Uint8:         "uint8",
+	Uint16:        "uint16",
+	Uint32:        "uint32",
+	Uint64:        "uint64",
+	Uintptr:       "uintptr",
+	Float32:       "float32",
+	Float64:       "float64",
+	Complex64:     "complex64",
+	Complex128:    "complex128",
+	Array:         "array",
+	Chan:          "chan",
+	Func:          "func",
+	Interface:     "interface",
+	Map:           "map",
+	Ptr:           "ptr",
+	Slice:         "slice",
+	String:        "string",
+	Struct:        "struct",
+	UnsafePointer: "unsafe.Pointer",
+}
+
+func (t *uncommonType) methods() []method {
+	if t.mcount == 0 {
+		return nil
+	}
+	return (*[1 << 16]method)(add(unsafe.Pointer(t), uintptr(t.moff), "t.mcount > 0"))[:t.mcount:t.mcount]
+}
+
+func (t *uncommonType) exportedMethods() []method {
+	if t.xcount == 0 {
+		return nil
+	}
+	return (*[1 << 16]method)(add(unsafe.Pointer(t), uintptr(t.moff), "t.xcount > 0"))[:t.xcount:t.xcount]
+}
+
+// resolveNameOff resolves a name offset from a base pointer.
+// The (*rtype).nameOff method is a convenience wrapper for this function.
+// Implemented in the runtime package.
+func resolveNameOff(ptrInModule unsafe.Pointer, off int32) unsafe.Pointer
+
+// resolveTypeOff resolves an *rtype offset from a base type.
+// The (*rtype).typeOff method is a convenience wrapper for this function.
+// Implemented in the runtime package.
+func resolveTypeOff(rtype unsafe.Pointer, off int32) unsafe.Pointer
+
+type nameOff int32 // offset to a name
+type typeOff int32 // offset to an *rtype
+type textOff int32 // offset from top of text section
+
+func (t *rtype) nameOff(off nameOff) name {
+	return name{(*byte)(resolveNameOff(unsafe.Pointer(t), int32(off)))}
+}
+
+func (t *rtype) typeOff(off typeOff) *rtype {
+	return (*rtype)(resolveTypeOff(unsafe.Pointer(t), int32(off)))
+}
+
+func (t *rtype) uncommon() *uncommonType {
+	if t.tflag&tflagUncommon == 0 {
+		return nil
+	}
+	switch t.Kind() {
+	case Struct:
+		return &(*structTypeUncommon)(unsafe.Pointer(t)).u
+	case Ptr:
+		type u struct {
+			ptrType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Func:
+		type u struct {
+			funcType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Slice:
+		type u struct {
+			sliceType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Array:
+		type u struct {
+			arrayType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Chan:
+		type u struct {
+			chanType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Map:
+		type u struct {
+			mapType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	case Interface:
+		type u struct {
+			interfaceType
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	default:
+		type u struct {
+			rtype
+			u uncommonType
+		}
+		return &(*u)(unsafe.Pointer(t)).u
+	}
+}
+
+func (t *rtype) String() string {
+	s := t.nameOff(t.str).name()
+	if t.tflag&tflagExtraStar != 0 {
+		return s[1:]
+	}
+	return s
+}
+
+func (t *rtype) Size() uintptr { return t.size }
+
+func (t *rtype) Kind() Kind { return Kind(t.kind & kindMask) }
+
+func (t *rtype) pointers() bool { return t.ptrdata != 0 }
+
+func (t *rtype) common() *rtype { return t }
+
+func (t *rtype) exportedMethods() []method {
+	ut := t.uncommon()
+	if ut == nil {
+		return nil
+	}
+	return ut.exportedMethods()
+}
+
+func (t *rtype) NumMethod() int {
+	if t.Kind() == Interface {
+		tt := (*interfaceType)(unsafe.Pointer(t))
+		return tt.NumMethod()
+	}
+	return len(t.exportedMethods())
+}
+
+func (t *rtype) PkgPath() string {
+	if t.tflag&tflagNamed == 0 {
+		return ""
+	}
+	ut := t.uncommon()
+	if ut == nil {
+		return ""
+	}
+	return t.nameOff(ut.pkgPath).name()
+}
+
+func (t *rtype) hasName() bool {
+	return t.tflag&tflagNamed != 0
+}
+
+func (t *rtype) Name() string {
+	if !t.hasName() {
+		return ""
+	}
+	s := t.String()
+	i := len(s) - 1
+	sqBrackets := 0
+	for i >= 0 && (s[i] != '.' || sqBrackets != 0) {
+		switch s[i] {
+		case ']':
+			sqBrackets++
+		case '[':
+			sqBrackets--
+		}
+		i--
+	}
+	return s[i+1:]
+}
+
+func (t *rtype) chanDir() chanDir {
+	if t.Kind() != Chan {
+		panic("reflect: chanDir of non-chan type")
+	}
+	tt := (*chanType)(unsafe.Pointer(t))
+	return chanDir(tt.dir)
+}
+
+func (t *rtype) Elem() Type {
+	switch t.Kind() {
+	case Array:
+		tt := (*arrayType)(unsafe.Pointer(t))
+		return toType(tt.elem)
+	case Chan:
+		tt := (*chanType)(unsafe.Pointer(t))
+		return toType(tt.elem)
+	case Map:
+		tt := (*mapType)(unsafe.Pointer(t))
+		return toType(tt.elem)
+	case Ptr:
+		tt := (*ptrType)(unsafe.Pointer(t))
+		return toType(tt.elem)
+	case Slice:
+		tt := (*sliceType)(unsafe.Pointer(t))
+		return toType(tt.elem)
+	}
+	panic("reflect: Elem of invalid type")
+}
+
+func (t *rtype) In(i int) Type {
+	if t.Kind() != Func {
+		panic("reflect: In of non-func type")
+	}
+	tt := (*funcType)(unsafe.Pointer(t))
+	return toType(tt.in()[i])
+}
+
+func (t *rtype) Key() Type {
+	if t.Kind() != Map {
+		panic("reflect: Key of non-map type")
+	}
+	tt := (*mapType)(unsafe.Pointer(t))
+	return toType(tt.key)
+}
+
+func (t *rtype) Len() int {
+	if t.Kind() != Array {
+		panic("reflect: Len of non-array type")
+	}
+	tt := (*arrayType)(unsafe.Pointer(t))
+	return int(tt.len)
+}
+
+func (t *rtype) NumField() int {
+	if t.Kind() != Struct {
+		panic("reflect: NumField of non-struct type")
+	}
+	tt := (*structType)(unsafe.Pointer(t))
+	return len(tt.fields)
+}
+
+func (t *rtype) NumIn() int {
+	if t.Kind() != Func {
+		panic("reflect: NumIn of non-func type")
+	}
+	tt := (*funcType)(unsafe.Pointer(t))
+	return int(tt.inCount)
+}
+
+func (t *rtype) NumOut() int {
+	if t.Kind() != Func {
+		panic("reflect: NumOut of non-func type")
+	}
+	tt := (*funcType)(unsafe.Pointer(t))
+	return len(tt.out())
+}
+
+func (t *rtype) Out(i int) Type {
+	if t.Kind() != Func {
+		panic("reflect: Out of non-func type")
+	}
+	tt := (*funcType)(unsafe.Pointer(t))
+	return toType(tt.out()[i])
+}
+
+func (t *funcType) in() []*rtype {
+	uadd := unsafe.Sizeof(*t)
+	if t.tflag&tflagUncommon != 0 {
+		uadd += unsafe.Sizeof(uncommonType{})
+	}
+	if t.inCount == 0 {
+		return nil
+	}
+	return (*[1 << 20]*rtype)(add(unsafe.Pointer(t), uadd, "t.inCount > 0"))[:t.inCount:t.inCount]
+}
+
+func (t *funcType) out() []*rtype {
+	uadd := unsafe.Sizeof(*t)
+	if t.tflag&tflagUncommon != 0 {
+		uadd += unsafe.Sizeof(uncommonType{})
+	}
+	outCount := t.outCount & (1<<15 - 1)
+	if outCount == 0 {
+		return nil
+	}
+	return (*[1 << 20]*rtype)(add(unsafe.Pointer(t), uadd, "outCount > 0"))[t.inCount : t.inCount+outCount : t.inCount+outCount]
+}
+
+// add returns p+x.
+//
+// The whySafe string is ignored, so that the function still inlines
+// as efficiently as p+x, but all call sites should use the string to
+// record why the addition is safe, which is to say why the addition
+// does not cause x to advance to the very end of p's allocation
+// and therefore point incorrectly at the next block in memory.
+func add(p unsafe.Pointer, x uintptr, whySafe string) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(p) + x)
+}
+
+// NumMethod returns the number of interface methods in the type's method set.
+func (t *interfaceType) NumMethod() int { return len(t.methods) }
+
+// TypeOf returns the reflection Type that represents the dynamic type of i.
+// If i is a nil interface value, TypeOf returns nil.
+func TypeOf(i any) Type {
+	eface := *(*emptyInterface)(unsafe.Pointer(&i))
+	return toType(eface.typ)
+}
+
+func (t *rtype) Implements(u Type) bool {
+	if u == nil {
+		panic("reflect: nil type passed to Type.Implements")
+	}
+	if u.Kind() != Interface {
+		panic("reflect: non-interface type passed to Type.Implements")
+	}
+	return implements(u.(*rtype), t)
+}
+
+func (t *rtype) AssignableTo(u Type) bool {
+	if u == nil {
+		panic("reflect: nil type passed to Type.AssignableTo")
+	}
+	uu := u.(*rtype)
+	return directlyAssignable(uu, t) || implements(uu, t)
+}
+
+func (t *rtype) Comparable() bool {
+	return t.equal != nil
+}
+
+// implements reports whether the type V implements the interface type T.
+func implements(T, V *rtype) bool {
+	if T.Kind() != Interface {
+		return false
+	}
+	t := (*interfaceType)(unsafe.Pointer(T))
+	if len(t.methods) == 0 {
+		return true
+	}
+
+	// The same algorithm applies in both cases, but the
+	// method tables for an interface type and a concrete type
+	// are different, so the code is duplicated.
+	// In both cases the algorithm is a linear scan over the two
+	// lists - T's methods and V's methods - simultaneously.
+	// Since method tables are stored in a unique sorted order
+	// (alphabetical, with no duplicate method names), the scan
+	// through V's methods must hit a match for each of T's
+	// methods along the way, or else V does not implement T.
+	// This lets us run the scan in overall linear time instead of
+	// the quadratic time  a naive search would require.
+	// See also ../runtime/iface.go.
+	if V.Kind() == Interface {
+		v := (*interfaceType)(unsafe.Pointer(V))
+		i := 0
+		for j := 0; j < len(v.methods); j++ {
+			tm := &t.methods[i]
+			tmName := t.nameOff(tm.name)
+			vm := &v.methods[j]
+			vmName := V.nameOff(vm.name)
+			if vmName.name() == tmName.name() && V.typeOff(vm.typ) == t.typeOff(tm.typ) {
+				if !tmName.isExported() {
+					tmPkgPath := tmName.pkgPath()
+					if tmPkgPath == "" {
+						tmPkgPath = t.pkgPath.name()
+					}
+					vmPkgPath := vmName.pkgPath()
+					if vmPkgPath == "" {
+						vmPkgPath = v.pkgPath.name()
+					}
+					if tmPkgPath != vmPkgPath {
+						continue
+					}
+				}
+				if i++; i >= len(t.methods) {
+					return true
+				}
+			}
+		}
+		return false
+	}
+
+	v := V.uncommon()
+	if v == nil {
+		return false
+	}
+	i := 0
+	vmethods := v.methods()
+	for j := 0; j < int(v.mcount); j++ {
+		tm := &t.methods[i]
+		tmName := t.nameOff(tm.name)
+		vm := vmethods[j]
+		vmName := V.nameOff(vm.name)
+		if vmName.name() == tmName.name() && V.typeOff(vm.mtyp) == t.typeOff(tm.typ) {
+			if !tmName.isExported() {
+				tmPkgPath := tmName.pkgPath()
+				if tmPkgPath == "" {
+					tmPkgPath = t.pkgPath.name()
+				}
+				vmPkgPath := vmName.pkgPath()
+				if vmPkgPath == "" {
+					vmPkgPath = V.nameOff(v.pkgPath).name()
+				}
+				if tmPkgPath != vmPkgPath {
+					continue
+				}
+			}
+			if i++; i >= len(t.methods) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// directlyAssignable reports whether a value x of type V can be directly
+// assigned (using memmove) to a value of type T.
+// https://golang.org/doc/go_spec.html#Assignability
+// Ignoring the interface rules (implemented elsewhere)
+// and the ideal constant rules (no ideal constants at run time).
+func directlyAssignable(T, V *rtype) bool {
+	// x's type V is identical to T?
+	if T == V {
+		return true
+	}
+
+	// Otherwise at least one of T and V must not be defined
+	// and they must have the same kind.
+	if T.hasName() && V.hasName() || T.Kind() != V.Kind() {
+		return false
+	}
+
+	// x's type T and V must  have identical underlying types.
+	return haveIdenticalUnderlyingType(T, V, true)
+}
+
+func haveIdenticalType(T, V Type, cmpTags bool) bool {
+	if cmpTags {
+		return T == V
+	}
+
+	if T.Name() != V.Name() || T.Kind() != V.Kind() {
+		return false
+	}
+
+	return haveIdenticalUnderlyingType(T.common(), V.common(), false)
+}
+
+func haveIdenticalUnderlyingType(T, V *rtype, cmpTags bool) bool {
+	if T == V {
+		return true
+	}
+
+	kind := T.Kind()
+	if kind != V.Kind() {
+		return false
+	}
+
+	// Non-composite types of equal kind have same underlying type
+	// (the predefined instance of the type).
+	if Bool <= kind && kind <= Complex128 || kind == String || kind == UnsafePointer {
+		return true
+	}
+
+	// Composite types.
+	switch kind {
+	case Array:
+		return T.Len() == V.Len() && haveIdenticalType(T.Elem(), V.Elem(), cmpTags)
+
+	case Chan:
+		// Special case:
+		// x is a bidirectional channel value, T is a channel type,
+		// and x's type V and T have identical element types.
+		if V.chanDir() == bothDir && haveIdenticalType(T.Elem(), V.Elem(), cmpTags) {
+			return true
+		}
+
+		// Otherwise continue test for identical underlying type.
+		return V.chanDir() == T.chanDir() && haveIdenticalType(T.Elem(), V.Elem(), cmpTags)
+
+	case Func:
+		t := (*funcType)(unsafe.Pointer(T))
+		v := (*funcType)(unsafe.Pointer(V))
+		if t.outCount != v.outCount || t.inCount != v.inCount {
+			return false
+		}
+		for i := 0; i < t.NumIn(); i++ {
+			if !haveIdenticalType(t.In(i), v.In(i), cmpTags) {
+				return false
+			}
+		}
+		for i := 0; i < t.NumOut(); i++ {
+			if !haveIdenticalType(t.Out(i), v.Out(i), cmpTags) {
+				return false
+			}
+		}
+		return true
+
+	case Interface:
+		t := (*interfaceType)(unsafe.Pointer(T))
+		v := (*interfaceType)(unsafe.Pointer(V))
+		if len(t.methods) == 0 && len(v.methods) == 0 {
+			return true
+		}
+		// Might have the same methods but still
+		// need a run time conversion.
+		return false
+
+	case Map:
+		return haveIdenticalType(T.Key(), V.Key(), cmpTags) && haveIdenticalType(T.Elem(), V.Elem(), cmpTags)
+
+	case Ptr, Slice:
+		return haveIdenticalType(T.Elem(), V.Elem(), cmpTags)
+
+	case Struct:
+		t := (*structType)(unsafe.Pointer(T))
+		v := (*structType)(unsafe.Pointer(V))
+		if len(t.fields) != len(v.fields) {
+			return false
+		}
+		if t.pkgPath.name() != v.pkgPath.name() {
+			return false
+		}
+		for i := range t.fields {
+			tf := &t.fields[i]
+			vf := &v.fields[i]
+			if tf.name.name() != vf.name.name() {
+				return false
+			}
+			if !haveIdenticalType(tf.typ, vf.typ, cmpTags) {
+				return false
+			}
+			if cmpTags && tf.name.tag() != vf.name.tag() {
+				return false
+			}
+			if tf.offset != vf.offset {
+				return false
+			}
+			if tf.embedded() != vf.embedded() {
+				return false
+			}
+		}
+		return true
+	}
+
+	return false
+}
+
+type structTypeUncommon struct {
+	structType
+	u uncommonType
+}
+
+// toType converts from a *rtype to a Type that can be returned
+// to the client of package reflect. In gc, the only concern is that
+// a nil *rtype must be replaced by a nil Type, but in gccgo this
+// function takes care of ensuring that multiple *rtype for the same
+// type are coalesced into a single Type.
+func toType(t *rtype) Type {
+	if t == nil {
+		return nil
+	}
+	return t
+}
+
+// ifaceIndir reports whether t is stored indirectly in an interface value.
+func ifaceIndir(t *rtype) bool {
+	return t.kind&kindDirectIface == 0
+}
diff --git a/src/internal/reflectlite/value.go b/src/internal/reflectlite/value.go
new file mode 100644
index 0000000..b9bca3a
--- /dev/null
+++ b/src/internal/reflectlite/value.go
@@ -0,0 +1,477 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflectlite
+
+import (
+	"internal/goarch"
+	"internal/unsafeheader"
+	"runtime"
+	"unsafe"
+)
+
+// Value is the reflection interface to a Go value.
+//
+// Not all methods apply to all kinds of values. Restrictions,
+// if any, are noted in the documentation for each method.
+// Use the Kind method to find out the kind of value before
+// calling kind-specific methods. Calling a method
+// inappropriate to the kind of type causes a run time panic.
+//
+// The zero Value represents no value.
+// Its IsValid method returns false, its Kind method returns Invalid,
+// its String method returns "<invalid Value>", and all other methods panic.
+// Most functions and methods never return an invalid value.
+// If one does, its documentation states the conditions explicitly.
+//
+// A Value can be used concurrently by multiple goroutines provided that
+// the underlying Go value can be used concurrently for the equivalent
+// direct operations.
+//
+// To compare two Values, compare the results of the Interface method.
+// Using == on two Values does not compare the underlying values
+// they represent.
+type Value struct {
+	// typ holds the type of the value represented by a Value.
+	typ *rtype
+
+	// Pointer-valued data or, if flagIndir is set, pointer to data.
+	// Valid when either flagIndir is set or typ.pointers() is true.
+	ptr unsafe.Pointer
+
+	// flag holds metadata about the value.
+	// The lowest bits are flag bits:
+	//	- flagStickyRO: obtained via unexported not embedded field, so read-only
+	//	- flagEmbedRO: obtained via unexported embedded field, so read-only
+	//	- flagIndir: val holds a pointer to the data
+	//	- flagAddr: v.CanAddr is true (implies flagIndir)
+	// Value cannot represent method values.
+	// The next five bits give the Kind of the value.
+	// This repeats typ.Kind() except for method values.
+	// The remaining 23+ bits give a method number for method values.
+	// If flag.kind() != Func, code can assume that flagMethod is unset.
+	// If ifaceIndir(typ), code can assume that flagIndir is set.
+	flag
+
+	// A method value represents a curried method invocation
+	// like r.Read for some receiver r. The typ+val+flag bits describe
+	// the receiver r, but the flag's Kind bits say Func (methods are
+	// functions), and the top bits of the flag give the method number
+	// in r's type's method table.
+}
+
+type flag uintptr
+
+const (
+	flagKindWidth        = 5 // there are 27 kinds
+	flagKindMask    flag = 1<<flagKindWidth - 1
+	flagStickyRO    flag = 1 << 5
+	flagEmbedRO     flag = 1 << 6
+	flagIndir       flag = 1 << 7
+	flagAddr        flag = 1 << 8
+	flagMethod      flag = 1 << 9
+	flagMethodShift      = 10
+	flagRO          flag = flagStickyRO | flagEmbedRO
+)
+
+func (f flag) kind() Kind {
+	return Kind(f & flagKindMask)
+}
+
+func (f flag) ro() flag {
+	if f&flagRO != 0 {
+		return flagStickyRO
+	}
+	return 0
+}
+
+// pointer returns the underlying pointer represented by v.
+// v.Kind() must be Pointer, Map, Chan, Func, or UnsafePointer
+func (v Value) pointer() unsafe.Pointer {
+	if v.typ.size != goarch.PtrSize || !v.typ.pointers() {
+		panic("can't call pointer on a non-pointer Value")
+	}
+	if v.flag&flagIndir != 0 {
+		return *(*unsafe.Pointer)(v.ptr)
+	}
+	return v.ptr
+}
+
+// packEface converts v to the empty interface.
+func packEface(v Value) any {
+	t := v.typ
+	var i any
+	e := (*emptyInterface)(unsafe.Pointer(&i))
+	// First, fill in the data portion of the interface.
+	switch {
+	case ifaceIndir(t):
+		if v.flag&flagIndir == 0 {
+			panic("bad indir")
+		}
+		// Value is indirect, and so is the interface we're making.
+		ptr := v.ptr
+		if v.flag&flagAddr != 0 {
+			// TODO: pass safe boolean from valueInterface so
+			// we don't need to copy if safe==true?
+			c := unsafe_New(t)
+			typedmemmove(t, c, ptr)
+			ptr = c
+		}
+		e.word = ptr
+	case v.flag&flagIndir != 0:
+		// Value is indirect, but interface is direct. We need
+		// to load the data at v.ptr into the interface data word.
+		e.word = *(*unsafe.Pointer)(v.ptr)
+	default:
+		// Value is direct, and so is the interface.
+		e.word = v.ptr
+	}
+	// Now, fill in the type portion. We're very careful here not
+	// to have any operation between the e.word and e.typ assignments
+	// that would let the garbage collector observe the partially-built
+	// interface value.
+	e.typ = t
+	return i
+}
+
+// unpackEface converts the empty interface i to a Value.
+func unpackEface(i any) Value {
+	e := (*emptyInterface)(unsafe.Pointer(&i))
+	// NOTE: don't read e.word until we know whether it is really a pointer or not.
+	t := e.typ
+	if t == nil {
+		return Value{}
+	}
+	f := flag(t.Kind())
+	if ifaceIndir(t) {
+		f |= flagIndir
+	}
+	return Value{t, e.word, f}
+}
+
+// A ValueError occurs when a Value method is invoked on
+// a Value that does not support it. Such cases are documented
+// in the description of each method.
+type ValueError struct {
+	Method string
+	Kind   Kind
+}
+
+func (e *ValueError) Error() string {
+	if e.Kind == 0 {
+		return "reflect: call of " + e.Method + " on zero Value"
+	}
+	return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
+}
+
+// methodName returns the name of the calling method,
+// assumed to be two stack frames above.
+func methodName() string {
+	pc, _, _, _ := runtime.Caller(2)
+	f := runtime.FuncForPC(pc)
+	if f == nil {
+		return "unknown method"
+	}
+	return f.Name()
+}
+
+// emptyInterface is the header for an interface{} value.
+type emptyInterface struct {
+	typ  *rtype
+	word unsafe.Pointer
+}
+
+// mustBeExported panics if f records that the value was obtained using
+// an unexported field.
+func (f flag) mustBeExported() {
+	if f == 0 {
+		panic(&ValueError{methodName(), 0})
+	}
+	if f&flagRO != 0 {
+		panic("reflect: " + methodName() + " using value obtained using unexported field")
+	}
+}
+
+// mustBeAssignable panics if f records that the value is not assignable,
+// which is to say that either it was obtained using an unexported field
+// or it is not addressable.
+func (f flag) mustBeAssignable() {
+	if f == 0 {
+		panic(&ValueError{methodName(), Invalid})
+	}
+	// Assignable if addressable and not read-only.
+	if f&flagRO != 0 {
+		panic("reflect: " + methodName() + " using value obtained using unexported field")
+	}
+	if f&flagAddr == 0 {
+		panic("reflect: " + methodName() + " using unaddressable value")
+	}
+}
+
+// CanSet reports whether the value of v can be changed.
+// A Value can be changed only if it is addressable and was not
+// obtained by the use of unexported struct fields.
+// If CanSet returns false, calling Set or any type-specific
+// setter (e.g., SetBool, SetInt) will panic.
+func (v Value) CanSet() bool {
+	return v.flag&(flagAddr|flagRO) == flagAddr
+}
+
+// Elem returns the value that the interface v contains
+// or that the pointer v points to.
+// It panics if v's Kind is not Interface or Pointer.
+// It returns the zero Value if v is nil.
+func (v Value) Elem() Value {
+	k := v.kind()
+	switch k {
+	case Interface:
+		var eface any
+		if v.typ.NumMethod() == 0 {
+			eface = *(*any)(v.ptr)
+		} else {
+			eface = (any)(*(*interface {
+				M()
+			})(v.ptr))
+		}
+		x := unpackEface(eface)
+		if x.flag != 0 {
+			x.flag |= v.flag.ro()
+		}
+		return x
+	case Pointer:
+		ptr := v.ptr
+		if v.flag&flagIndir != 0 {
+			ptr = *(*unsafe.Pointer)(ptr)
+		}
+		// The returned value's address is v's value.
+		if ptr == nil {
+			return Value{}
+		}
+		tt := (*ptrType)(unsafe.Pointer(v.typ))
+		typ := tt.elem
+		fl := v.flag&flagRO | flagIndir | flagAddr
+		fl |= flag(typ.Kind())
+		return Value{typ, ptr, fl}
+	}
+	panic(&ValueError{"reflectlite.Value.Elem", v.kind()})
+}
+
+func valueInterface(v Value) any {
+	if v.flag == 0 {
+		panic(&ValueError{"reflectlite.Value.Interface", 0})
+	}
+
+	if v.kind() == Interface {
+		// Special case: return the element inside the interface.
+		// Empty interface has one layout, all interfaces with
+		// methods have a second layout.
+		if v.numMethod() == 0 {
+			return *(*any)(v.ptr)
+		}
+		return *(*interface {
+			M()
+		})(v.ptr)
+	}
+
+	// TODO: pass safe to packEface so we don't need to copy if safe==true?
+	return packEface(v)
+}
+
+// IsNil reports whether its argument v is nil. The argument must be
+// a chan, func, interface, map, pointer, or slice value; if it is
+// not, IsNil panics. Note that IsNil is not always equivalent to a
+// regular comparison with nil in Go. For example, if v was created
+// by calling ValueOf with an uninitialized interface variable i,
+// i==nil will be true but v.IsNil will panic as v will be the zero
+// Value.
+func (v Value) IsNil() bool {
+	k := v.kind()
+	switch k {
+	case Chan, Func, Map, Pointer, UnsafePointer:
+		// if v.flag&flagMethod != 0 {
+		// 	return false
+		// }
+		ptr := v.ptr
+		if v.flag&flagIndir != 0 {
+			ptr = *(*unsafe.Pointer)(ptr)
+		}
+		return ptr == nil
+	case Interface, Slice:
+		// Both interface and slice are nil if first word is 0.
+		// Both are always bigger than a word; assume flagIndir.
+		return *(*unsafe.Pointer)(v.ptr) == nil
+	}
+	panic(&ValueError{"reflectlite.Value.IsNil", v.kind()})
+}
+
+// IsValid reports whether v represents a value.
+// It returns false if v is the zero Value.
+// If IsValid returns false, all other methods except String panic.
+// Most functions and methods never return an invalid Value.
+// If one does, its documentation states the conditions explicitly.
+func (v Value) IsValid() bool {
+	return v.flag != 0
+}
+
+// Kind returns v's Kind.
+// If v is the zero Value (IsValid returns false), Kind returns Invalid.
+func (v Value) Kind() Kind {
+	return v.kind()
+}
+
+// implemented in runtime:
+func chanlen(unsafe.Pointer) int
+func maplen(unsafe.Pointer) int
+
+// Len returns v's length.
+// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
+func (v Value) Len() int {
+	k := v.kind()
+	switch k {
+	case Array:
+		tt := (*arrayType)(unsafe.Pointer(v.typ))
+		return int(tt.len)
+	case Chan:
+		return chanlen(v.pointer())
+	case Map:
+		return maplen(v.pointer())
+	case Slice:
+		// Slice is bigger than a word; assume flagIndir.
+		return (*unsafeheader.Slice)(v.ptr).Len
+	case String:
+		// String is bigger than a word; assume flagIndir.
+		return (*unsafeheader.String)(v.ptr).Len
+	}
+	panic(&ValueError{"reflect.Value.Len", v.kind()})
+}
+
+// NumMethod returns the number of exported methods in the value's method set.
+func (v Value) numMethod() int {
+	if v.typ == nil {
+		panic(&ValueError{"reflectlite.Value.NumMethod", Invalid})
+	}
+	return v.typ.NumMethod()
+}
+
+// Set assigns x to the value v.
+// It panics if CanSet returns false.
+// As in Go, x's value must be assignable to v's type.
+func (v Value) Set(x Value) {
+	v.mustBeAssignable()
+	x.mustBeExported() // do not let unexported x leak
+	var target unsafe.Pointer
+	if v.kind() == Interface {
+		target = v.ptr
+	}
+	x = x.assignTo("reflectlite.Set", v.typ, target)
+	if x.flag&flagIndir != 0 {
+		typedmemmove(v.typ, v.ptr, x.ptr)
+	} else {
+		*(*unsafe.Pointer)(v.ptr) = x.ptr
+	}
+}
+
+// Type returns v's type.
+func (v Value) Type() Type {
+	f := v.flag
+	if f == 0 {
+		panic(&ValueError{"reflectlite.Value.Type", Invalid})
+	}
+	// Method values not supported.
+	return v.typ
+}
+
+/*
+ * constructors
+ */
+
+// implemented in package runtime
+func unsafe_New(*rtype) unsafe.Pointer
+
+// ValueOf returns a new Value initialized to the concrete value
+// stored in the interface i. ValueOf(nil) returns the zero Value.
+func ValueOf(i any) Value {
+	if i == nil {
+		return Value{}
+	}
+
+	// TODO: Maybe allow contents of a Value to live on the stack.
+	// For now we make the contents always escape to the heap. It
+	// makes life easier in a few places (see chanrecv/mapassign
+	// comment below).
+	escapes(i)
+
+	return unpackEface(i)
+}
+
+// assignTo returns a value v that can be assigned directly to typ.
+// It panics if v is not assignable to typ.
+// For a conversion to an interface type, target is a suggested scratch space to use.
+func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
+	// if v.flag&flagMethod != 0 {
+	// 	v = makeMethodValue(context, v)
+	// }
+
+	switch {
+	case directlyAssignable(dst, v.typ):
+		// Overwrite type so that they match.
+		// Same memory layout, so no harm done.
+		fl := v.flag&(flagAddr|flagIndir) | v.flag.ro()
+		fl |= flag(dst.Kind())
+		return Value{dst, v.ptr, fl}
+
+	case implements(dst, v.typ):
+		if target == nil {
+			target = unsafe_New(dst)
+		}
+		if v.Kind() == Interface && v.IsNil() {
+			// A nil ReadWriter passed to nil Reader is OK,
+			// but using ifaceE2I below will panic.
+			// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
+			return Value{dst, nil, flag(Interface)}
+		}
+		x := valueInterface(v)
+		if dst.NumMethod() == 0 {
+			*(*any)(target) = x
+		} else {
+			ifaceE2I(dst, x, target)
+		}
+		return Value{dst, target, flagIndir | flag(Interface)}
+	}
+
+	// Failed.
+	panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
+}
+
+// arrayAt returns the i-th element of p,
+// an array whose elements are eltSize bytes wide.
+// The array pointed at by p must have at least i+1 elements:
+// it is invalid (but impossible to check here) to pass i >= len,
+// because then the result will point outside the array.
+// whySafe must explain why i < len. (Passing "i < len" is fine;
+// the benefit is to surface this assumption at the call site.)
+func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
+	return add(p, uintptr(i)*eltSize, "i < len")
+}
+
+func ifaceE2I(t *rtype, src any, dst unsafe.Pointer)
+
+// typedmemmove copies a value of type t to dst from src.
+//
+//go:noescape
+func typedmemmove(t *rtype, dst, src unsafe.Pointer)
+
+// Dummy annotation marking that the value x escapes,
+// for use in cases where the reflect code is so clever that
+// the compiler cannot follow.
+func escapes(x any) {
+	if dummy.b {
+		dummy.x = x
+	}
+}
+
+var dummy struct {
+	b bool
+	x any
+}
diff --git a/src/internal/safefilepath/path.go b/src/internal/safefilepath/path.go
new file mode 100644
index 0000000..0f0a270
--- /dev/null
+++ b/src/internal/safefilepath/path.go
@@ -0,0 +1,21 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package safefilepath manipulates operating-system file paths.
+package safefilepath
+
+import (
+	"errors"
+)
+
+var errInvalidPath = errors.New("invalid path")
+
+// FromFS converts a slash-separated path into an operating-system path.
+//
+// FromFS returns an error if the path cannot be represented by the operating
+// system. For example, paths containing '\' and ':' characters are rejected
+// on Windows.
+func FromFS(path string) (string, error) {
+	return fromFS(path)
+}
diff --git a/src/internal/safefilepath/path_other.go b/src/internal/safefilepath/path_other.go
new file mode 100644
index 0000000..974e775
--- /dev/null
+++ b/src/internal/safefilepath/path_other.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package safefilepath
+
+import "runtime"
+
+func fromFS(path string) (string, error) {
+	if runtime.GOOS == "plan9" {
+		if len(path) > 0 && path[0] == '#' {
+			return "", errInvalidPath
+		}
+	}
+	for i := range path {
+		if path[i] == 0 {
+			return "", errInvalidPath
+		}
+	}
+	return path, nil
+}
diff --git a/src/internal/safefilepath/path_test.go b/src/internal/safefilepath/path_test.go
new file mode 100644
index 0000000..dc662c1
--- /dev/null
+++ b/src/internal/safefilepath/path_test.go
@@ -0,0 +1,88 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package safefilepath_test
+
+import (
+	"internal/safefilepath"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+type PathTest struct {
+	path, result string
+}
+
+const invalid = ""
+
+var fspathtests = []PathTest{
+	{".", "."},
+	{"/a/b/c", "/a/b/c"},
+	{"a\x00b", invalid},
+}
+
+var winreservedpathtests = []PathTest{
+	{`a\b`, `a\b`},
+	{`a:b`, `a:b`},
+	{`a/b:c`, `a/b:c`},
+	{`NUL`, `NUL`},
+	{`./com1`, `./com1`},
+	{`a/nul/b`, `a/nul/b`},
+}
+
+// Whether a reserved name with an extension is reserved or not varies by
+// Windows version.
+var winreservedextpathtests = []PathTest{
+	{"nul.txt", "nul.txt"},
+	{"a/nul.txt/b", "a/nul.txt/b"},
+}
+
+var plan9reservedpathtests = []PathTest{
+	{`#c`, `#c`},
+}
+
+func TestFromFS(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows":
+		if canWriteFile(t, "NUL") {
+			t.Errorf("can unexpectedly write a file named NUL on Windows")
+		}
+		if canWriteFile(t, "nul.txt") {
+			fspathtests = append(fspathtests, winreservedextpathtests...)
+		} else {
+			winreservedpathtests = append(winreservedpathtests, winreservedextpathtests...)
+		}
+		for i := range winreservedpathtests {
+			winreservedpathtests[i].result = invalid
+		}
+		for i := range fspathtests {
+			fspathtests[i].result = filepath.FromSlash(fspathtests[i].result)
+		}
+	case "plan9":
+		for i := range plan9reservedpathtests {
+			plan9reservedpathtests[i].result = invalid
+		}
+	}
+	tests := fspathtests
+	tests = append(tests, winreservedpathtests...)
+	tests = append(tests, plan9reservedpathtests...)
+	for _, test := range tests {
+		got, err := safefilepath.FromFS(test.path)
+		if (got == "") != (err != nil) {
+			t.Errorf(`FromFS(%q) = %q, %v; want "" only if err != nil`, test.path, got, err)
+		}
+		if got != test.result {
+			t.Errorf("FromFS(%q) = %q, %v; want %q", test.path, got, err, test.result)
+		}
+	}
+}
+
+func canWriteFile(t *testing.T, name string) bool {
+	path := filepath.Join(t.TempDir(), name)
+	os.WriteFile(path, []byte("ok"), 0666)
+	b, _ := os.ReadFile(path)
+	return string(b) == "ok"
+}
diff --git a/src/internal/safefilepath/path_windows.go b/src/internal/safefilepath/path_windows.go
new file mode 100644
index 0000000..7cfd6ce
--- /dev/null
+++ b/src/internal/safefilepath/path_windows.go
@@ -0,0 +1,141 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package safefilepath
+
+import (
+	"syscall"
+	"unicode/utf8"
+)
+
+func fromFS(path string) (string, error) {
+	if !utf8.ValidString(path) {
+		return "", errInvalidPath
+	}
+	for len(path) > 1 && path[0] == '/' && path[1] == '/' {
+		path = path[1:]
+	}
+	containsSlash := false
+	for p := path; p != ""; {
+		// Find the next path element.
+		i := 0
+		for i < len(p) && p[i] != '/' {
+			switch p[i] {
+			case 0, '\\', ':':
+				return "", errInvalidPath
+			}
+			i++
+		}
+		part := p[:i]
+		if i < len(p) {
+			containsSlash = true
+			p = p[i+1:]
+		} else {
+			p = ""
+		}
+		if IsReservedName(part) {
+			return "", errInvalidPath
+		}
+	}
+	if containsSlash {
+		// We can't depend on strings, so substitute \ for / manually.
+		buf := []byte(path)
+		for i, b := range buf {
+			if b == '/' {
+				buf[i] = '\\'
+			}
+		}
+		path = string(buf)
+	}
+	return path, nil
+}
+
+// IsReservedName reports if name is a Windows reserved device name.
+// It does not detect names with an extension, which are also reserved on some Windows versions.
+//
+// For details, search for PRN in
+// https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file.
+func IsReservedName(name string) bool {
+	// Device names can have arbitrary trailing characters following a dot or colon.
+	base := name
+	for i := 0; i < len(base); i++ {
+		switch base[i] {
+		case ':', '.':
+			base = base[:i]
+		}
+	}
+	// Trailing spaces in the last path element are ignored.
+	for len(base) > 0 && base[len(base)-1] == ' ' {
+		base = base[:len(base)-1]
+	}
+	if !isReservedBaseName(base) {
+		return false
+	}
+	if len(base) == len(name) {
+		return true
+	}
+	// The path element is a reserved name with an extension.
+	// Some Windows versions consider this a reserved name,
+	// while others do not. Use FullPath to see if the name is
+	// reserved.
+	if p, _ := syscall.FullPath(name); len(p) >= 4 && p[:4] == `\\.\` {
+		return true
+	}
+	return false
+}
+
+func isReservedBaseName(name string) bool {
+	if len(name) == 3 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "CON", "PRN", "AUX", "NUL":
+			return true
+		}
+	}
+	if len(name) >= 4 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "COM", "LPT":
+			if len(name) == 4 && '1' <= name[3] && name[3] <= '9' {
+				return true
+			}
+			// Superscript ¹, ², and ³ are considered numbers as well.
+			switch name[3:] {
+			case "\u00b2", "\u00b3", "\u00b9":
+				return true
+			}
+			return false
+		}
+	}
+
+	// Passing CONIN$ or CONOUT$ to CreateFile opens a console handle.
+	// https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#consoles
+	//
+	// While CONIN$ and CONOUT$ aren't documented as being files,
+	// they behave the same as CON. For example, ./CONIN$ also opens the console input.
+	if len(name) == 6 && name[5] == '$' && equalFold(name, "CONIN$") {
+		return true
+	}
+	if len(name) == 7 && name[6] == '$' && equalFold(name, "CONOUT$") {
+		return true
+	}
+	return false
+}
+
+func equalFold(a, b string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if toUpper(a[i]) != toUpper(b[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+func toUpper(c byte) byte {
+	if 'a' <= c && c <= 'z' {
+		return c - ('a' - 'A')
+	}
+	return c
+}
diff --git a/src/internal/saferio/io.go b/src/internal/saferio/io.go
new file mode 100644
index 0000000..66cc044
--- /dev/null
+++ b/src/internal/saferio/io.go
@@ -0,0 +1,135 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package saferio provides I/O functions that avoid allocating large
+// amounts of memory unnecessarily. This is intended for packages that
+// read data from an [io.Reader] where the size is part of the input
+// data but the input may be corrupt, or may be provided by an
+// untrustworthy attacker.
+package saferio
+
+import (
+	"io"
+	"reflect"
+)
+
+// chunk is an arbitrary limit on how much memory we are willing
+// to allocate without concern.
+const chunk = 10 << 20 // 10M
+
+// ReadData reads n bytes from the input stream, but avoids allocating
+// all n bytes if n is large. This avoids crashing the program by
+// allocating all n bytes in cases where n is incorrect.
+//
+// The error is io.EOF only if no bytes were read.
+// If an io.EOF happens after reading some but not all the bytes,
+// ReadData returns io.ErrUnexpectedEOF.
+func ReadData(r io.Reader, n uint64) ([]byte, error) {
+	if int64(n) < 0 || n != uint64(int(n)) {
+		// n is too large to fit in int, so we can't allocate
+		// a buffer large enough. Treat this as a read failure.
+		return nil, io.ErrUnexpectedEOF
+	}
+
+	if n < chunk {
+		buf := make([]byte, n)
+		_, err := io.ReadFull(r, buf)
+		if err != nil {
+			return nil, err
+		}
+		return buf, nil
+	}
+
+	var buf []byte
+	buf1 := make([]byte, chunk)
+	for n > 0 {
+		next := n
+		if next > chunk {
+			next = chunk
+		}
+		_, err := io.ReadFull(r, buf1[:next])
+		if err != nil {
+			if len(buf) > 0 && err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return nil, err
+		}
+		buf = append(buf, buf1[:next]...)
+		n -= next
+	}
+	return buf, nil
+}
+
+// ReadDataAt reads n bytes from the input stream at off, but avoids
+// allocating all n bytes if n is large. This avoids crashing the program
+// by allocating all n bytes in cases where n is incorrect.
+func ReadDataAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
+	if int64(n) < 0 || n != uint64(int(n)) {
+		// n is too large to fit in int, so we can't allocate
+		// a buffer large enough. Treat this as a read failure.
+		return nil, io.ErrUnexpectedEOF
+	}
+
+	if n < chunk {
+		buf := make([]byte, n)
+		_, err := r.ReadAt(buf, off)
+		if err != nil {
+			// io.SectionReader can return EOF for n == 0,
+			// but for our purposes that is a success.
+			if err != io.EOF || n > 0 {
+				return nil, err
+			}
+		}
+		return buf, nil
+	}
+
+	var buf []byte
+	buf1 := make([]byte, chunk)
+	for n > 0 {
+		next := n
+		if next > chunk {
+			next = chunk
+		}
+		_, err := r.ReadAt(buf1[:next], off)
+		if err != nil {
+			return nil, err
+		}
+		buf = append(buf, buf1[:next]...)
+		n -= next
+		off += int64(next)
+	}
+	return buf, nil
+}
+
+// SliceCap returns the capacity to use when allocating a slice.
+// After the slice is allocated with the capacity, it should be
+// built using append. This will avoid allocating too much memory
+// if the capacity is large and incorrect.
+//
+// A negative result means that the value is always too big.
+//
+// The element type is described by passing a pointer to a value of that type.
+// This would ideally use generics, but this code is built with
+// the bootstrap compiler which need not support generics.
+// We use a pointer so that we can handle slices of interface type.
+func SliceCap(v any, c uint64) int {
+	if int64(c) < 0 || c != uint64(int(c)) {
+		return -1
+	}
+	typ := reflect.TypeOf(v)
+	if typ.Kind() != reflect.Ptr {
+		panic("SliceCap called with non-pointer type")
+	}
+	size := uint64(typ.Elem().Size())
+	if size > 0 && c > (1<<64-1)/size {
+		return -1
+	}
+	if c*size > chunk {
+		c = uint64(chunk / size)
+		if c == 0 {
+			c = 1
+		}
+	}
+	return int(c)
+}
diff --git a/src/internal/saferio/io_test.go b/src/internal/saferio/io_test.go
new file mode 100644
index 0000000..356c9eb
--- /dev/null
+++ b/src/internal/saferio/io_test.go
@@ -0,0 +1,136 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package saferio
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+func TestReadData(t *testing.T) {
+	const count = 100
+	input := bytes.Repeat([]byte{'a'}, count)
+
+	t.Run("small", func(t *testing.T) {
+		got, err := ReadData(bytes.NewReader(input), count)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(got, input) {
+			t.Errorf("got %v, want %v", got, input)
+		}
+	})
+
+	t.Run("large", func(t *testing.T) {
+		_, err := ReadData(bytes.NewReader(input), 10<<30)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("maxint", func(t *testing.T) {
+		_, err := ReadData(bytes.NewReader(input), 1<<62)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("small-EOF", func(t *testing.T) {
+		_, err := ReadData(bytes.NewReader(nil), chunk-1)
+		if err != io.EOF {
+			t.Errorf("ReadData = %v, want io.EOF", err)
+		}
+	})
+
+	t.Run("large-EOF", func(t *testing.T) {
+		_, err := ReadData(bytes.NewReader(nil), chunk+1)
+		if err != io.EOF {
+			t.Errorf("ReadData = %v, want io.EOF", err)
+		}
+	})
+
+	t.Run("large-UnexpectedEOF", func(t *testing.T) {
+		_, err := ReadData(bytes.NewReader(make([]byte, chunk)), chunk+1)
+		if err != io.ErrUnexpectedEOF {
+			t.Errorf("ReadData = %v, want io.ErrUnexpectedEOF", err)
+		}
+	})
+}
+
+func TestReadDataAt(t *testing.T) {
+	const count = 100
+	input := bytes.Repeat([]byte{'a'}, count)
+
+	t.Run("small", func(t *testing.T) {
+		got, err := ReadDataAt(bytes.NewReader(input), count, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(got, input) {
+			t.Errorf("got %v, want %v", got, input)
+		}
+	})
+
+	t.Run("large", func(t *testing.T) {
+		_, err := ReadDataAt(bytes.NewReader(input), 10<<30, 0)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("maxint", func(t *testing.T) {
+		_, err := ReadDataAt(bytes.NewReader(input), 1<<62, 0)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("SectionReader", func(t *testing.T) {
+		// Reading 0 bytes from an io.SectionReader at the end
+		// of the section will return EOF, but ReadDataAt
+		// should succeed and return 0 bytes.
+		sr := io.NewSectionReader(bytes.NewReader(input), 0, 0)
+		got, err := ReadDataAt(sr, 0, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(got) > 0 {
+			t.Errorf("got %d bytes, expected 0", len(got))
+		}
+	})
+}
+
+func TestSliceCap(t *testing.T) {
+	t.Run("small", func(t *testing.T) {
+		c := SliceCap((*int)(nil), 10)
+		if c != 10 {
+			t.Errorf("got capacity %d, want %d", c, 10)
+		}
+	})
+
+	t.Run("large", func(t *testing.T) {
+		c := SliceCap((*byte)(nil), 1<<30)
+		if c < 0 {
+			t.Error("SliceCap failed unexpectedly")
+		} else if c == 1<<30 {
+			t.Errorf("got capacity %d which is too high", c)
+		}
+	})
+
+	t.Run("maxint", func(t *testing.T) {
+		c := SliceCap((*byte)(nil), 1<<63)
+		if c >= 0 {
+			t.Errorf("SliceCap returned %d, expected failure", c)
+		}
+	})
+
+	t.Run("overflow", func(t *testing.T) {
+		c := SliceCap((*int64)(nil), 1<<62)
+		if c >= 0 {
+			t.Errorf("SliceCap returned %d, expected failure", c)
+		}
+	})
+}
diff --git a/src/internal/singleflight/singleflight.go b/src/internal/singleflight/singleflight.go
new file mode 100644
index 0000000..d0e6d2f
--- /dev/null
+++ b/src/internal/singleflight/singleflight.go
@@ -0,0 +1,123 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package singleflight provides a duplicate function call suppression
+// mechanism.
+package singleflight
+
+import "sync"
+
+// call is an in-flight or completed singleflight.Do call
+type call struct {
+	wg sync.WaitGroup
+
+	// These fields are written once before the WaitGroup is done
+	// and are only read after the WaitGroup is done.
+	val any
+	err error
+
+	// These fields are read and written with the singleflight
+	// mutex held before the WaitGroup is done, and are read but
+	// not written after the WaitGroup is done.
+	dups  int
+	chans []chan<- Result
+}
+
+// Group represents a class of work and forms a namespace in
+// which units of work can be executed with duplicate suppression.
+type Group struct {
+	mu sync.Mutex       // protects m
+	m  map[string]*call // lazily initialized
+}
+
+// Result holds the results of Do, so they can be passed
+// on a channel.
+type Result struct {
+	Val    any
+	Err    error
+	Shared bool
+}
+
+// Do executes and returns the results of the given function, making
+// sure that only one execution is in-flight for a given key at a
+// time. If a duplicate comes in, the duplicate caller waits for the
+// original to complete and receives the same results.
+// The return value shared indicates whether v was given to multiple callers.
+func (g *Group) Do(key string, fn func() (any, error)) (v any, err error, shared bool) {
+	g.mu.Lock()
+	if g.m == nil {
+		g.m = make(map[string]*call)
+	}
+	if c, ok := g.m[key]; ok {
+		c.dups++
+		g.mu.Unlock()
+		c.wg.Wait()
+		return c.val, c.err, true
+	}
+	c := new(call)
+	c.wg.Add(1)
+	g.m[key] = c
+	g.mu.Unlock()
+
+	g.doCall(c, key, fn)
+	return c.val, c.err, c.dups > 0
+}
+
+// DoChan is like Do but returns a channel that will receive the
+// results when they are ready.
+func (g *Group) DoChan(key string, fn func() (any, error)) <-chan Result {
+	ch := make(chan Result, 1)
+	g.mu.Lock()
+	if g.m == nil {
+		g.m = make(map[string]*call)
+	}
+	if c, ok := g.m[key]; ok {
+		c.dups++
+		c.chans = append(c.chans, ch)
+		g.mu.Unlock()
+		return ch
+	}
+	c := &call{chans: []chan<- Result{ch}}
+	c.wg.Add(1)
+	g.m[key] = c
+	g.mu.Unlock()
+
+	go g.doCall(c, key, fn)
+
+	return ch
+}
+
+// doCall handles the single call for a key.
+func (g *Group) doCall(c *call, key string, fn func() (any, error)) {
+	c.val, c.err = fn()
+
+	g.mu.Lock()
+	c.wg.Done()
+	if g.m[key] == c {
+		delete(g.m, key)
+	}
+	for _, ch := range c.chans {
+		ch <- Result{c.val, c.err, c.dups > 0}
+	}
+	g.mu.Unlock()
+}
+
+// ForgetUnshared tells the singleflight to forget about a key if it is not
+// shared with any other goroutines. Future calls to Do for a forgotten key
+// will call the function rather than waiting for an earlier call to complete.
+// Returns whether the key was forgotten or unknown--that is, whether no
+// other goroutines are waiting for the result.
+func (g *Group) ForgetUnshared(key string) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	c, ok := g.m[key]
+	if !ok {
+		return true
+	}
+	if c.dups == 0 {
+		delete(g.m, key)
+		return true
+	}
+	return false
+}
diff --git a/src/internal/singleflight/singleflight_test.go b/src/internal/singleflight/singleflight_test.go
new file mode 100644
index 0000000..a13893d
--- /dev/null
+++ b/src/internal/singleflight/singleflight_test.go
@@ -0,0 +1,186 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package singleflight
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestDo(t *testing.T) {
+	var g Group
+	v, err, _ := g.Do("key", func() (any, error) {
+		return "bar", nil
+	})
+	if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
+		t.Errorf("Do = %v; want %v", got, want)
+	}
+	if err != nil {
+		t.Errorf("Do error = %v", err)
+	}
+}
+
+func TestDoErr(t *testing.T) {
+	var g Group
+	someErr := errors.New("some error")
+	v, err, _ := g.Do("key", func() (any, error) {
+		return nil, someErr
+	})
+	if err != someErr {
+		t.Errorf("Do error = %v; want someErr %v", err, someErr)
+	}
+	if v != nil {
+		t.Errorf("unexpected non-nil value %#v", v)
+	}
+}
+
+func TestDoDupSuppress(t *testing.T) {
+	var g Group
+	var wg1, wg2 sync.WaitGroup
+	c := make(chan string, 1)
+	var calls atomic.Int32
+	fn := func() (any, error) {
+		if calls.Add(1) == 1 {
+			// First invocation.
+			wg1.Done()
+		}
+		v := <-c
+		c <- v // pump; make available for any future calls
+
+		time.Sleep(10 * time.Millisecond) // let more goroutines enter Do
+
+		return v, nil
+	}
+
+	const n = 10
+	wg1.Add(1)
+	for i := 0; i < n; i++ {
+		wg1.Add(1)
+		wg2.Add(1)
+		go func() {
+			defer wg2.Done()
+			wg1.Done()
+			v, err, _ := g.Do("key", fn)
+			if err != nil {
+				t.Errorf("Do error: %v", err)
+				return
+			}
+			if s, _ := v.(string); s != "bar" {
+				t.Errorf("Do = %T %v; want %q", v, v, "bar")
+			}
+		}()
+	}
+	wg1.Wait()
+	// At least one goroutine is in fn now and all of them have at
+	// least reached the line before the Do.
+	c <- "bar"
+	wg2.Wait()
+	if got := calls.Load(); got <= 0 || got >= n {
+		t.Errorf("number of calls = %d; want over 0 and less than %d", got, n)
+	}
+}
+
+func TestForgetUnshared(t *testing.T) {
+	var g Group
+
+	var firstStarted, firstFinished sync.WaitGroup
+
+	firstStarted.Add(1)
+	firstFinished.Add(1)
+
+	key := "key"
+	firstCh := make(chan struct{})
+	go func() {
+		g.Do(key, func() (i interface{}, e error) {
+			firstStarted.Done()
+			<-firstCh
+			firstFinished.Done()
+			return
+		})
+	}()
+
+	firstStarted.Wait()
+	g.ForgetUnshared(key) // from this point no two function using same key should be executed concurrently
+
+	secondCh := make(chan struct{})
+	go func() {
+		g.Do(key, func() (i interface{}, e error) {
+			// Notify that we started
+			secondCh <- struct{}{}
+			<-secondCh
+			return 2, nil
+		})
+	}()
+
+	<-secondCh
+
+	resultCh := g.DoChan(key, func() (i interface{}, e error) {
+		panic("third must not be started")
+	})
+
+	if g.ForgetUnshared(key) {
+		t.Errorf("Before first goroutine finished, key %q is shared, should return false", key)
+	}
+
+	close(firstCh)
+	firstFinished.Wait()
+
+	if g.ForgetUnshared(key) {
+		t.Errorf("After first goroutine finished, key %q is still shared, should return false", key)
+	}
+
+	secondCh <- struct{}{}
+
+	if result := <-resultCh; result.Val != 2 {
+		t.Errorf("We should receive result produced by second call, expected: 2, got %d", result.Val)
+	}
+}
+
+func TestDoAndForgetUnsharedRace(t *testing.T) {
+	t.Parallel()
+
+	var g Group
+	key := "key"
+	d := time.Millisecond
+	for {
+		var calls, shared atomic.Int64
+		const n = 1000
+		var wg sync.WaitGroup
+		wg.Add(n)
+		for i := 0; i < n; i++ {
+			go func() {
+				g.Do(key, func() (interface{}, error) {
+					time.Sleep(d)
+					return calls.Add(1), nil
+				})
+				if !g.ForgetUnshared(key) {
+					shared.Add(1)
+				}
+				wg.Done()
+			}()
+		}
+		wg.Wait()
+
+		if calls.Load() != 1 {
+			// The goroutines didn't park in g.Do in time,
+			// so the key was re-added and may have been shared after the call.
+			// Try again with more time to park.
+			d *= 2
+			continue
+		}
+
+		// All of the Do calls ended up sharing the first
+		// invocation, so the key should have been unused
+		// (and therefore unshared) when they returned.
+		if shared.Load() > 0 {
+			t.Errorf("after a single shared Do, ForgetUnshared returned false %d times", shared.Load())
+		}
+		break
+	}
+}
diff --git a/src/internal/syscall/execenv/execenv_default.go b/src/internal/syscall/execenv/execenv_default.go
new file mode 100644
index 0000000..335647c
--- /dev/null
+++ b/src/internal/syscall/execenv/execenv_default.go
@@ -0,0 +1,19 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package execenv
+
+import "syscall"
+
+// Default will return the default environment
+// variables based on the process attributes
+// provided.
+//
+// Defaults to syscall.Environ() on all platforms
+// other than Windows.
+func Default(sys *syscall.SysProcAttr) ([]string, error) {
+	return syscall.Environ(), nil
+}
diff --git a/src/internal/syscall/execenv/execenv_windows.go b/src/internal/syscall/execenv/execenv_windows.go
new file mode 100644
index 0000000..a8aa1a6
--- /dev/null
+++ b/src/internal/syscall/execenv/execenv_windows.go
@@ -0,0 +1,54 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package execenv
+
+import (
+	"internal/syscall/windows"
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+// Default will return the default environment
+// variables based on the process attributes
+// provided.
+//
+// If the process attributes contain a token, then
+// the environment variables will be sourced from
+// the defaults for that user token, otherwise they
+// will be sourced from syscall.Environ().
+func Default(sys *syscall.SysProcAttr) (env []string, err error) {
+	if sys == nil || sys.Token == 0 {
+		return syscall.Environ(), nil
+	}
+	var block *uint16
+	err = windows.CreateEnvironmentBlock(&block, sys.Token, false)
+	if err != nil {
+		return nil, err
+	}
+	defer windows.DestroyEnvironmentBlock(block)
+	blockp := uintptr(unsafe.Pointer(block))
+	for {
+
+		// find NUL terminator
+		end := unsafe.Pointer(blockp)
+		for *(*uint16)(end) != 0 {
+			end = unsafe.Pointer(uintptr(end) + 2)
+		}
+
+		n := (uintptr(end) - uintptr(unsafe.Pointer(blockp))) / 2
+		if n == 0 {
+			// environment block ends with empty string
+			break
+		}
+
+		entry := (*[(1 << 30) - 1]uint16)(unsafe.Pointer(blockp))[:n:n]
+		env = append(env, string(utf16.Decode(entry)))
+		blockp += 2 * (uintptr(len(entry)) + 1)
+	}
+	return
+}
diff --git a/src/internal/syscall/unix/asm_aix_ppc64.s b/src/internal/syscall/unix/asm_aix_ppc64.s
new file mode 100644
index 0000000..9e82e3e
--- /dev/null
+++ b/src/internal/syscall/unix/asm_aix_ppc64.s
@@ -0,0 +1,12 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+//
+// System calls for aix/ppc64 are implemented in syscall/syscall_aix.go
+//
+
+TEXT ·syscall6(SB),NOSPLIT,$0
+	JMP	syscall·syscall6(SB)
diff --git a/src/internal/syscall/unix/asm_darwin.s b/src/internal/syscall/unix/asm_darwin.s
new file mode 100644
index 0000000..8662c28
--- /dev/null
+++ b/src/internal/syscall/unix/asm_darwin.s
@@ -0,0 +1,24 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+TEXT ·libc_getentropy_trampoline(SB),NOSPLIT,$0-0; JMP libc_getentropy(SB)
+TEXT ·libc_getaddrinfo_trampoline(SB),NOSPLIT,$0-0; JMP libc_getaddrinfo(SB)
+TEXT ·libc_freeaddrinfo_trampoline(SB),NOSPLIT,$0-0; JMP libc_freeaddrinfo(SB)
+TEXT ·libc_getnameinfo_trampoline(SB),NOSPLIT,$0-0; JMP libc_getnameinfo(SB)
+TEXT ·libc_gai_strerror_trampoline(SB),NOSPLIT,$0-0; JMP libc_gai_strerror(SB)
+TEXT ·libresolv_res_9_ninit_trampoline(SB),NOSPLIT,$0-0; JMP libresolv_res_9_ninit(SB)
+TEXT ·libresolv_res_9_nclose_trampoline(SB),NOSPLIT,$0-0; JMP libresolv_res_9_nclose(SB)
+TEXT ·libresolv_res_9_nsearch_trampoline(SB),NOSPLIT,$0-0; JMP libresolv_res_9_nsearch(SB)
+TEXT ·libc_grantpt_trampoline(SB),NOSPLIT,$0-0; JMP libc_grantpt(SB)
+TEXT ·libc_unlockpt_trampoline(SB),NOSPLIT,$0-0; JMP libc_unlockpt(SB)
+TEXT ·libc_ptsname_r_trampoline(SB),NOSPLIT,$0-0; JMP libc_ptsname_r(SB)
+TEXT ·libc_posix_openpt_trampoline(SB),NOSPLIT,$0-0; JMP libc_posix_openpt(SB)
+TEXT ·libc_getgrouplist_trampoline(SB),NOSPLIT,$0-0; JMP libc_getgrouplist(SB)
+TEXT ·libc_getpwnam_r_trampoline(SB),NOSPLIT,$0-0; JMP libc_getpwnam_r(SB)
+TEXT ·libc_getpwuid_r_trampoline(SB),NOSPLIT,$0-0; JMP libc_getpwuid_r(SB)
+TEXT ·libc_getgrnam_r_trampoline(SB),NOSPLIT,$0-0; JMP libc_getgrnam_r(SB)
+TEXT ·libc_getgrgid_r_trampoline(SB),NOSPLIT,$0-0; JMP libc_getgrgid_r(SB)
+TEXT ·libc_sysconf_trampoline(SB),NOSPLIT,$0-0; JMP libc_sysconf(SB)
diff --git a/src/internal/syscall/unix/asm_solaris.s b/src/internal/syscall/unix/asm_solaris.s
new file mode 100644
index 0000000..2057338
--- /dev/null
+++ b/src/internal/syscall/unix/asm_solaris.s
@@ -0,0 +1,10 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+// System calls for Solaris are implemented in runtime/syscall_solaris.go
+
+TEXT ·syscall6(SB),NOSPLIT,$0-88
+	JMP	syscall·sysvicall6(SB)
diff --git a/src/internal/syscall/unix/at.go b/src/internal/syscall/unix/at.go
new file mode 100644
index 0000000..cfb6e41
--- /dev/null
+++ b/src/internal/syscall/unix/at.go
@@ -0,0 +1,40 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || (openbsd && mips64)
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func Unlinkat(dirfd int, path string, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+
+	_, _, errno := syscall.Syscall(unlinkatTrap, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(flags))
+	if errno != 0 {
+		return errno
+	}
+
+	return nil
+}
+
+func Openat(dirfd int, path string, flags int, perm uint32) (int, error) {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+
+	fd, _, errno := syscall.Syscall6(openatTrap, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(flags), uintptr(perm), 0, 0)
+	if errno != 0 {
+		return 0, errno
+	}
+
+	return int(fd), nil
+}
diff --git a/src/internal/syscall/unix/at_aix.go b/src/internal/syscall/unix/at_aix.go
new file mode 100644
index 0000000..425df98
--- /dev/null
+++ b/src/internal/syscall/unix/at_aix.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+//go:cgo_import_dynamic libc_fstatat fstatat "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_openat openat "libc.a/shr_64.o"
+//go:cgo_import_dynamic libc_unlinkat unlinkat "libc.a/shr_64.o"
+
+const (
+	AT_REMOVEDIR        = 0x1
+	AT_SYMLINK_NOFOLLOW = 0x1
+)
diff --git a/src/internal/syscall/unix/at_fstatat.go b/src/internal/syscall/unix/at_fstatat.go
new file mode 100644
index 0000000..8f25fe9
--- /dev/null
+++ b/src/internal/syscall/unix/at_fstatat.go
@@ -0,0 +1,28 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || (linux && !loong64) || netbsd || (openbsd && mips64)
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func Fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error {
+	var p *byte
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+
+	_, _, errno := syscall.Syscall6(fstatatTrap, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if errno != 0 {
+		return errno
+	}
+
+	return nil
+
+}
diff --git a/src/internal/syscall/unix/at_fstatat2.go b/src/internal/syscall/unix/at_fstatat2.go
new file mode 100644
index 0000000..8d20e1a
--- /dev/null
+++ b/src/internal/syscall/unix/at_fstatat2.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build freebsd || (linux && loong64)
+
+package unix
+
+import "syscall"
+
+func Fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error {
+	return syscall.Fstatat(dirfd, path, stat, flags)
+}
diff --git a/src/internal/syscall/unix/at_libc.go b/src/internal/syscall/unix/at_libc.go
new file mode 100644
index 0000000..f48d379
--- /dev/null
+++ b/src/internal/syscall/unix/at_libc.go
@@ -0,0 +1,64 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || solaris
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+//go:linkname procFstatat libc_fstatat
+//go:linkname procOpenat libc_openat
+//go:linkname procUnlinkat libc_unlinkat
+
+var (
+	procFstatat,
+	procOpenat,
+	procUnlinkat uintptr
+)
+
+func Unlinkat(dirfd int, path string, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+
+	_, _, errno := syscall6(uintptr(unsafe.Pointer(&procUnlinkat)), 3, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(flags), 0, 0, 0)
+	if errno != 0 {
+		return errno
+	}
+
+	return nil
+}
+
+func Openat(dirfd int, path string, flags int, perm uint32) (int, error) {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+
+	fd, _, errno := syscall6(uintptr(unsafe.Pointer(&procOpenat)), 4, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(flags), uintptr(perm), 0, 0)
+	if errno != 0 {
+		return 0, errno
+	}
+
+	return int(fd), nil
+}
+
+func Fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+
+	_, _, errno := syscall6(uintptr(unsafe.Pointer(&procFstatat)), 4, uintptr(dirfd), uintptr(unsafe.Pointer(p)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if errno != 0 {
+		return errno
+	}
+
+	return nil
+}
diff --git a/src/internal/syscall/unix/at_libc2.go b/src/internal/syscall/unix/at_libc2.go
new file mode 100644
index 0000000..93d0cf4
--- /dev/null
+++ b/src/internal/syscall/unix/at_libc2.go
@@ -0,0 +1,33 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || (openbsd && !mips64)
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe" // for linkname
+)
+
+func Unlinkat(dirfd int, path string, flags int) error {
+	return unlinkat(dirfd, path, flags)
+}
+
+func Openat(dirfd int, path string, flags int, perm uint32) (int, error) {
+	return openat(dirfd, path, flags, perm)
+}
+
+func Fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error {
+	return fstatat(dirfd, path, stat, flags)
+}
+
+//go:linkname unlinkat syscall.unlinkat
+func unlinkat(dirfd int, path string, flags int) error
+
+//go:linkname openat syscall.openat
+func openat(dirfd int, path string, flags int, perm uint32) (int, error)
+
+//go:linkname fstatat syscall.fstatat
+func fstatat(dirfd int, path string, stat *syscall.Stat_t, flags int) error
diff --git a/src/internal/syscall/unix/at_solaris.go b/src/internal/syscall/unix/at_solaris.go
new file mode 100644
index 0000000..e917c4f
--- /dev/null
+++ b/src/internal/syscall/unix/at_solaris.go
@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+// Implemented as sysvicall6 in runtime/syscall_solaris.go.
+func syscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:cgo_import_dynamic libc_fstatat fstatat "libc.so"
+//go:cgo_import_dynamic libc_openat openat "libc.so"
+//go:cgo_import_dynamic libc_unlinkat unlinkat "libc.so"
+
+const (
+	AT_REMOVEDIR        = 0x1
+	AT_SYMLINK_NOFOLLOW = 0x1000
+)
diff --git a/src/internal/syscall/unix/at_sysnum_darwin.go b/src/internal/syscall/unix/at_sysnum_darwin.go
new file mode 100644
index 0000000..aaaaa47
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_darwin.go
@@ -0,0 +1,8 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const AT_REMOVEDIR = 0x80
+const AT_SYMLINK_NOFOLLOW = 0x0020
diff --git a/src/internal/syscall/unix/at_sysnum_dragonfly.go b/src/internal/syscall/unix/at_sysnum_dragonfly.go
new file mode 100644
index 0000000..cec9abc
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_dragonfly.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+const unlinkatTrap uintptr = syscall.SYS_UNLINKAT
+const openatTrap uintptr = syscall.SYS_OPENAT
+const fstatatTrap uintptr = syscall.SYS_FSTATAT
+
+const AT_REMOVEDIR = 0x2
+const AT_SYMLINK_NOFOLLOW = 0x1
diff --git a/src/internal/syscall/unix/at_sysnum_freebsd.go b/src/internal/syscall/unix/at_sysnum_freebsd.go
new file mode 100644
index 0000000..adfbbcb
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_freebsd.go
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+const (
+	AT_REMOVEDIR        = 0x800
+	AT_SYMLINK_NOFOLLOW = 0x200
+
+	unlinkatTrap uintptr = syscall.SYS_UNLINKAT
+	openatTrap   uintptr = syscall.SYS_OPENAT
+)
diff --git a/src/internal/syscall/unix/at_sysnum_fstatat64_linux.go b/src/internal/syscall/unix/at_sysnum_fstatat64_linux.go
new file mode 100644
index 0000000..445b0c3
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_fstatat64_linux.go
@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm || mips || mipsle || 386
+
+package unix
+
+import "syscall"
+
+const fstatatTrap uintptr = syscall.SYS_FSTATAT64
diff --git a/src/internal/syscall/unix/at_sysnum_fstatat_linux.go b/src/internal/syscall/unix/at_sysnum_fstatat_linux.go
new file mode 100644
index 0000000..73a3da5
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_fstatat_linux.go
@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build arm64 || riscv64
+
+package unix
+
+import "syscall"
+
+const fstatatTrap uintptr = syscall.SYS_FSTATAT
diff --git a/src/internal/syscall/unix/at_sysnum_linux.go b/src/internal/syscall/unix/at_sysnum_linux.go
new file mode 100644
index 0000000..b9b8495
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_linux.go
@@ -0,0 +1,17 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+const unlinkatTrap uintptr = syscall.SYS_UNLINKAT
+const openatTrap uintptr = syscall.SYS_OPENAT
+
+const (
+	AT_EACCESS          = 0x200
+	AT_FDCWD            = -0x64
+	AT_REMOVEDIR        = 0x200
+	AT_SYMLINK_NOFOLLOW = 0x100
+)
diff --git a/src/internal/syscall/unix/at_sysnum_netbsd.go b/src/internal/syscall/unix/at_sysnum_netbsd.go
new file mode 100644
index 0000000..fe45e29
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_netbsd.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+const unlinkatTrap uintptr = syscall.SYS_UNLINKAT
+const openatTrap uintptr = syscall.SYS_OPENAT
+const fstatatTrap uintptr = syscall.SYS_FSTATAT
+
+const AT_REMOVEDIR = 0x800
+const AT_SYMLINK_NOFOLLOW = 0x200
diff --git a/src/internal/syscall/unix/at_sysnum_newfstatat_linux.go b/src/internal/syscall/unix/at_sysnum_newfstatat_linux.go
new file mode 100644
index 0000000..76edf67
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_newfstatat_linux.go
@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64 || mips64 || mips64le || ppc64 || ppc64le || s390x
+
+package unix
+
+import "syscall"
+
+const fstatatTrap uintptr = syscall.SYS_NEWFSTATAT
diff --git a/src/internal/syscall/unix/at_sysnum_openbsd.go b/src/internal/syscall/unix/at_sysnum_openbsd.go
new file mode 100644
index 0000000..c2d48b9
--- /dev/null
+++ b/src/internal/syscall/unix/at_sysnum_openbsd.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+const unlinkatTrap uintptr = syscall.SYS_UNLINKAT
+const openatTrap uintptr = syscall.SYS_OPENAT
+const fstatatTrap uintptr = syscall.SYS_FSTATAT
+
+const AT_REMOVEDIR = 0x08
+const AT_SYMLINK_NOFOLLOW = 0x02
diff --git a/src/internal/syscall/unix/constants.go b/src/internal/syscall/unix/constants.go
new file mode 100644
index 0000000..e324589
--- /dev/null
+++ b/src/internal/syscall/unix/constants.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package unix
+
+const (
+	R_OK = 0x4
+	W_OK = 0x2
+	X_OK = 0x1
+)
diff --git a/src/internal/syscall/unix/copy_file_range_linux.go b/src/internal/syscall/unix/copy_file_range_linux.go
new file mode 100644
index 0000000..cf0a279
--- /dev/null
+++ b/src/internal/syscall/unix/copy_file_range_linux.go
@@ -0,0 +1,26 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func CopyFileRange(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n int, err error) {
+	r1, _, errno := syscall.Syscall6(copyFileRangeTrap,
+		uintptr(rfd),
+		uintptr(unsafe.Pointer(roff)),
+		uintptr(wfd),
+		uintptr(unsafe.Pointer(woff)),
+		uintptr(len),
+		uintptr(flags),
+	)
+	n = int(r1)
+	if errno != 0 {
+		err = errno
+	}
+	return
+}
diff --git a/src/internal/syscall/unix/eaccess_linux.go b/src/internal/syscall/unix/eaccess_linux.go
new file mode 100644
index 0000000..5695a5e
--- /dev/null
+++ b/src/internal/syscall/unix/eaccess_linux.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "syscall"
+
+func Eaccess(path string, mode uint32) error {
+	return syscall.Faccessat(AT_FDCWD, path, mode, AT_EACCESS)
+}
diff --git a/src/internal/syscall/unix/eaccess_other.go b/src/internal/syscall/unix/eaccess_other.go
new file mode 100644
index 0000000..23be118
--- /dev/null
+++ b/src/internal/syscall/unix/eaccess_other.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix && !linux
+
+package unix
+
+import "syscall"
+
+func Eaccess(path string, mode uint32) error {
+	return syscall.ENOSYS
+}
diff --git a/src/internal/syscall/unix/fcntl_linux_32bit.go b/src/internal/syscall/unix/fcntl_linux_32bit.go
new file mode 100644
index 0000000..7b39ee7
--- /dev/null
+++ b/src/internal/syscall/unix/fcntl_linux_32bit.go
@@ -0,0 +1,16 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// On 32-bit Linux systems, use SYS_FCNTL64.
+// If you change the build tags here, see syscall/flock_linux_32bit.go.
+
+//go:build (linux && 386) || (linux && arm) || (linux && mips) || (linux && mipsle)
+
+package unix
+
+import "syscall"
+
+func init() {
+	FcntlSyscall = syscall.SYS_FCNTL64
+}
diff --git a/src/internal/syscall/unix/getentropy_darwin.go b/src/internal/syscall/unix/getentropy_darwin.go
new file mode 100644
index 0000000..834099f
--- /dev/null
+++ b/src/internal/syscall/unix/getentropy_darwin.go
@@ -0,0 +1,28 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin && !ios
+
+package unix
+
+import (
+	"internal/abi"
+	"unsafe"
+)
+
+//go:cgo_import_dynamic libc_getentropy getentropy "/usr/lib/libSystem.B.dylib"
+
+func libc_getentropy_trampoline()
+
+// GetEntropy calls the macOS getentropy system call.
+func GetEntropy(p []byte) error {
+	_, _, errno := syscall_syscall(abi.FuncPCABI0(libc_getentropy_trampoline),
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/getentropy_openbsd.go b/src/internal/syscall/unix/getentropy_openbsd.go
new file mode 100644
index 0000000..ad0914d
--- /dev/null
+++ b/src/internal/syscall/unix/getentropy_openbsd.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build openbsd && !mips64
+
+package unix
+
+import _ "unsafe" // for linkname
+
+// GetEntropy calls the OpenBSD getentropy system call.
+func GetEntropy(p []byte) error {
+	return getentropy(p)
+}
+
+//go:linkname getentropy syscall.getentropy
+func getentropy(p []byte) error
diff --git a/src/internal/syscall/unix/getentropy_openbsd_mips64.go b/src/internal/syscall/unix/getentropy_openbsd_mips64.go
new file mode 100644
index 0000000..d5caa80
--- /dev/null
+++ b/src/internal/syscall/unix/getentropy_openbsd_mips64.go
@@ -0,0 +1,25 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// getentropy(2)'s syscall number, from /usr/src/sys/kern/syscalls.master
+const entropyTrap uintptr = 7
+
+// GetEntropy calls the OpenBSD getentropy system call.
+func GetEntropy(p []byte) error {
+	_, _, errno := syscall.Syscall(entropyTrap,
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/src/internal/syscall/unix/getrandom.go b/src/internal/syscall/unix/getrandom.go
new file mode 100644
index 0000000..a665933
--- /dev/null
+++ b/src/internal/syscall/unix/getrandom.go
@@ -0,0 +1,39 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux
+
+package unix
+
+import (
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+var getrandomUnsupported int32 // atomic
+
+// GetRandomFlag is a flag supported by the getrandom system call.
+type GetRandomFlag uintptr
+
+// GetRandom calls the getrandom system call.
+func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	if atomic.LoadInt32(&getrandomUnsupported) != 0 {
+		return 0, syscall.ENOSYS
+	}
+	r1, _, errno := syscall.Syscall(getrandomTrap,
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		uintptr(flags))
+	if errno != 0 {
+		if errno == syscall.ENOSYS {
+			atomic.StoreInt32(&getrandomUnsupported, 1)
+		}
+		return 0, errno
+	}
+	return int(r1), nil
+}
diff --git a/src/internal/syscall/unix/getrandom_dragonfly.go b/src/internal/syscall/unix/getrandom_dragonfly.go
new file mode 100644
index 0000000..fbf78f9
--- /dev/null
+++ b/src/internal/syscall/unix/getrandom_dragonfly.go
@@ -0,0 +1,16 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+// DragonFlyBSD getrandom system call number.
+const getrandomTrap uintptr = 550
+
+const (
+	// GRND_RANDOM is only set for portability purpose, no-op on DragonFlyBSD.
+	GRND_RANDOM GetRandomFlag = 0x0001
+
+	// GRND_NONBLOCK means return EAGAIN rather than blocking.
+	GRND_NONBLOCK GetRandomFlag = 0x0002
+)
diff --git a/src/internal/syscall/unix/getrandom_freebsd.go b/src/internal/syscall/unix/getrandom_freebsd.go
new file mode 100644
index 0000000..8c4f3df
--- /dev/null
+++ b/src/internal/syscall/unix/getrandom_freebsd.go
@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+// FreeBSD getrandom system call number.
+const getrandomTrap uintptr = 563
+
+const (
+	// GRND_NONBLOCK means return EAGAIN rather than blocking.
+	GRND_NONBLOCK GetRandomFlag = 0x0001
+
+	// GRND_RANDOM is only set for portability purpose, no-op on FreeBSD.
+	GRND_RANDOM GetRandomFlag = 0x0002
+)
diff --git a/src/internal/syscall/unix/getrandom_linux.go b/src/internal/syscall/unix/getrandom_linux.go
new file mode 100644
index 0000000..8ccd8d3
--- /dev/null
+++ b/src/internal/syscall/unix/getrandom_linux.go
@@ -0,0 +1,13 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const (
+	// GRND_NONBLOCK means return EAGAIN rather than blocking.
+	GRND_NONBLOCK GetRandomFlag = 0x0001
+
+	// GRND_RANDOM means use the /dev/random pool instead of /dev/urandom.
+	GRND_RANDOM GetRandomFlag = 0x0002
+)
diff --git a/src/internal/syscall/unix/getrandom_solaris.go b/src/internal/syscall/unix/getrandom_solaris.go
new file mode 100644
index 0000000..d86775c
--- /dev/null
+++ b/src/internal/syscall/unix/getrandom_solaris.go
@@ -0,0 +1,53 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+//go:cgo_import_dynamic libc_getrandom getrandom "libc.so"
+
+//go:linkname procGetrandom libc_getrandom
+
+var procGetrandom uintptr
+
+var getrandomUnsupported int32 // atomic
+
+// GetRandomFlag is a flag supported by the getrandom system call.
+type GetRandomFlag uintptr
+
+const (
+	// GRND_NONBLOCK means return EAGAIN rather than blocking.
+	GRND_NONBLOCK GetRandomFlag = 0x0001
+
+	// GRND_RANDOM means use the /dev/random pool instead of /dev/urandom.
+	GRND_RANDOM GetRandomFlag = 0x0002
+)
+
+// GetRandom calls the getrandom system call.
+func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	if atomic.LoadInt32(&getrandomUnsupported) != 0 {
+		return 0, syscall.ENOSYS
+	}
+	r1, _, errno := syscall6(uintptr(unsafe.Pointer(&procGetrandom)),
+		3,
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		uintptr(flags),
+		0, 0, 0)
+	if errno != 0 {
+		if errno == syscall.ENOSYS {
+			atomic.StoreInt32(&getrandomUnsupported, 1)
+		}
+		return 0, errno
+	}
+	return int(r1), nil
+}
diff --git a/src/internal/syscall/unix/ioctl_aix.go b/src/internal/syscall/unix/ioctl_aix.go
new file mode 100644
index 0000000..d361533
--- /dev/null
+++ b/src/internal/syscall/unix/ioctl_aix.go
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+//go:cgo_import_dynamic libc_ioctl ioctl "libc.a/shr_64.o"
+//go:linkname libc_ioctl libc_ioctl
+var libc_ioctl uintptr
+
+// Implemented in syscall/syscall_aix.go.
+func syscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+func Ioctl(fd int, cmd int, args unsafe.Pointer) (err error) {
+	_, _, e1 := syscall6(uintptr(unsafe.Pointer(&libc_ioctl)), 3, uintptr(fd), uintptr(cmd), uintptr(args), 0, 0, 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
diff --git a/src/internal/syscall/unix/kernel_version_linux.go b/src/internal/syscall/unix/kernel_version_linux.go
new file mode 100644
index 0000000..71e8aa4
--- /dev/null
+++ b/src/internal/syscall/unix/kernel_version_linux.go
@@ -0,0 +1,42 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"syscall"
+)
+
+// KernelVersion returns major and minor kernel version numbers, parsed from
+// the syscall.Uname's Release field, or 0, 0 if the version can't be obtained
+// or parsed.
+//
+// Currently only implemented for Linux.
+func KernelVersion() (major, minor int) {
+	var uname syscall.Utsname
+	if err := syscall.Uname(&uname); err != nil {
+		return
+	}
+
+	var (
+		values    [2]int
+		value, vi int
+	)
+	for _, c := range uname.Release {
+		if '0' <= c && c <= '9' {
+			value = (value * 10) + int(c-'0')
+		} else {
+			// Note that we're assuming N.N.N here.
+			// If we see anything else, we are likely to mis-parse it.
+			values[vi] = value
+			vi++
+			if vi >= len(values) {
+				break
+			}
+			value = 0
+		}
+	}
+
+	return values[0], values[1]
+}
diff --git a/src/internal/syscall/unix/kernel_version_other.go b/src/internal/syscall/unix/kernel_version_other.go
new file mode 100644
index 0000000..00af9f2
--- /dev/null
+++ b/src/internal/syscall/unix/kernel_version_other.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+
+package unix
+
+func KernelVersion() (major int, minor int) {
+	return 0, 0
+}
diff --git a/src/internal/syscall/unix/net.go b/src/internal/syscall/unix/net.go
new file mode 100644
index 0000000..5618d40
--- /dev/null
+++ b/src/internal/syscall/unix/net.go
@@ -0,0 +1,44 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe"
+)
+
+//go:linkname RecvfromInet4 syscall.recvfromInet4
+//go:noescape
+func RecvfromInet4(fd int, p []byte, flags int, from *syscall.SockaddrInet4) (int, error)
+
+//go:linkname RecvfromInet6 syscall.recvfromInet6
+//go:noescape
+func RecvfromInet6(fd int, p []byte, flags int, from *syscall.SockaddrInet6) (n int, err error)
+
+//go:linkname SendtoInet4 syscall.sendtoInet4
+//go:noescape
+func SendtoInet4(fd int, p []byte, flags int, to *syscall.SockaddrInet4) (err error)
+
+//go:linkname SendtoInet6 syscall.sendtoInet6
+//go:noescape
+func SendtoInet6(fd int, p []byte, flags int, to *syscall.SockaddrInet6) (err error)
+
+//go:linkname SendmsgNInet4 syscall.sendmsgNInet4
+//go:noescape
+func SendmsgNInet4(fd int, p, oob []byte, to *syscall.SockaddrInet4, flags int) (n int, err error)
+
+//go:linkname SendmsgNInet6 syscall.sendmsgNInet6
+//go:noescape
+func SendmsgNInet6(fd int, p, oob []byte, to *syscall.SockaddrInet6, flags int) (n int, err error)
+
+//go:linkname RecvmsgInet4 syscall.recvmsgInet4
+//go:noescape
+func RecvmsgInet4(fd int, p, oob []byte, flags int, from *syscall.SockaddrInet4) (n, oobn int, recvflags int, err error)
+
+//go:linkname RecvmsgInet6 syscall.recvmsgInet6
+//go:noescape
+func RecvmsgInet6(fd int, p, oob []byte, flags int, from *syscall.SockaddrInet6) (n, oobn int, recvflags int, err error)
diff --git a/src/internal/syscall/unix/net_darwin.go b/src/internal/syscall/unix/net_darwin.go
new file mode 100644
index 0000000..b9da4f1
--- /dev/null
+++ b/src/internal/syscall/unix/net_darwin.go
@@ -0,0 +1,161 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"internal/abi"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	AI_CANONNAME = 0x2
+	AI_ALL       = 0x100
+	AI_V4MAPPED  = 0x800
+	AI_MASK      = 0x1407
+
+	EAI_AGAIN    = 2
+	EAI_NONAME   = 8
+	EAI_SYSTEM   = 11
+	EAI_OVERFLOW = 14
+
+	NI_NAMEREQD = 4
+)
+
+type Addrinfo struct {
+	Flags     int32
+	Family    int32
+	Socktype  int32
+	Protocol  int32
+	Addrlen   uint32
+	Canonname *byte
+	Addr      *syscall.RawSockaddr
+	Next      *Addrinfo
+}
+
+//go:cgo_ldflag "-lresolv"
+
+//go:cgo_import_dynamic libc_getaddrinfo getaddrinfo "/usr/lib/libSystem.B.dylib"
+func libc_getaddrinfo_trampoline()
+
+func Getaddrinfo(hostname, servname *byte, hints *Addrinfo, res **Addrinfo) (int, error) {
+	gerrno, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_getaddrinfo_trampoline),
+		uintptr(unsafe.Pointer(hostname)),
+		uintptr(unsafe.Pointer(servname)),
+		uintptr(unsafe.Pointer(hints)),
+		uintptr(unsafe.Pointer(res)),
+		0,
+		0)
+	var err error
+	if errno != 0 {
+		err = errno
+	}
+	return int(gerrno), err
+}
+
+//go:cgo_import_dynamic libc_freeaddrinfo freeaddrinfo "/usr/lib/libSystem.B.dylib"
+func libc_freeaddrinfo_trampoline()
+
+func Freeaddrinfo(ai *Addrinfo) {
+	syscall_syscall6(abi.FuncPCABI0(libc_freeaddrinfo_trampoline),
+		uintptr(unsafe.Pointer(ai)),
+		0, 0, 0, 0, 0)
+}
+
+//go:cgo_import_dynamic libc_getnameinfo getnameinfo "/usr/lib/libSystem.B.dylib"
+func libc_getnameinfo_trampoline()
+
+func Getnameinfo(sa *syscall.RawSockaddr, salen int, host *byte, hostlen int, serv *byte, servlen int, flags int) (int, error) {
+	gerrno, _, errno := syscall_syscall9(abi.FuncPCABI0(libc_getnameinfo_trampoline),
+		uintptr(unsafe.Pointer(sa)),
+		uintptr(salen),
+		uintptr(unsafe.Pointer(host)),
+		uintptr(hostlen),
+		uintptr(unsafe.Pointer(serv)),
+		uintptr(servlen),
+		uintptr(flags),
+		0,
+		0)
+	var err error
+	if errno != 0 {
+		err = errno
+	}
+	return int(gerrno), err
+}
+
+//go:cgo_import_dynamic libc_gai_strerror gai_strerror "/usr/lib/libSystem.B.dylib"
+func libc_gai_strerror_trampoline()
+
+func GaiStrerror(ecode int) string {
+	r1, _, _ := syscall_syscall(abi.FuncPCABI0(libc_gai_strerror_trampoline),
+		uintptr(ecode),
+		0, 0)
+	return GoString((*byte)(unsafe.Pointer(r1)))
+}
+
+// Implemented in the runtime package.
+func gostring(*byte) string
+
+func GoString(p *byte) string {
+	return gostring(p)
+}
+
+//go:linkname syscall_syscall syscall.syscall
+func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscallPtr syscall.syscallPtr
+func syscall_syscallPtr(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall6 syscall.syscall6
+func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall6X syscall.syscall6X
+func syscall_syscall6X(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall9 syscall.syscall9
+func syscall_syscall9(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+type ResState struct {
+	unexported [70]uintptr
+}
+
+//go:cgo_import_dynamic libresolv_res_9_ninit res_9_ninit "/usr/lib/libresolv.9.dylib"
+func libresolv_res_9_ninit_trampoline()
+
+func ResNinit(state *ResState) error {
+	_, _, errno := syscall_syscall(abi.FuncPCABI0(libresolv_res_9_ninit_trampoline),
+		uintptr(unsafe.Pointer(state)),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+//go:cgo_import_dynamic libresolv_res_9_nclose res_9_nclose "/usr/lib/libresolv.9.dylib"
+func libresolv_res_9_nclose_trampoline()
+
+func ResNclose(state *ResState) {
+	syscall_syscall(abi.FuncPCABI0(libresolv_res_9_nclose_trampoline),
+		uintptr(unsafe.Pointer(state)),
+		0, 0)
+}
+
+//go:cgo_import_dynamic libresolv_res_9_nsearch res_9_nsearch "/usr/lib/libresolv.9.dylib"
+func libresolv_res_9_nsearch_trampoline()
+
+func ResNsearch(state *ResState, dname *byte, class, typ int, ans *byte, anslen int) (int, error) {
+	r1, _, errno := syscall_syscall6(abi.FuncPCABI0(libresolv_res_9_nsearch_trampoline),
+		uintptr(unsafe.Pointer(state)),
+		uintptr(unsafe.Pointer(dname)),
+		uintptr(class),
+		uintptr(typ),
+		uintptr(unsafe.Pointer(ans)),
+		uintptr(anslen))
+	if errno != 0 {
+		return 0, errno
+	}
+	return int(int32(r1)), nil
+}
diff --git a/src/internal/syscall/unix/net_js.go b/src/internal/syscall/unix/net_js.go
new file mode 100644
index 0000000..622fc8e
--- /dev/null
+++ b/src/internal/syscall/unix/net_js.go
@@ -0,0 +1,44 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe"
+)
+
+func RecvfromInet4(fd int, p []byte, flags int, from *syscall.SockaddrInet4) (int, error) {
+	return 0, syscall.ENOSYS
+}
+
+func RecvfromInet6(fd int, p []byte, flags int, from *syscall.SockaddrInet6) (n int, err error) {
+	return 0, syscall.ENOSYS
+}
+
+func SendtoInet4(fd int, p []byte, flags int, to *syscall.SockaddrInet4) (err error) {
+	return syscall.ENOSYS
+}
+
+func SendtoInet6(fd int, p []byte, flags int, to *syscall.SockaddrInet6) (err error) {
+	return syscall.ENOSYS
+}
+
+func SendmsgNInet4(fd int, p, oob []byte, to *syscall.SockaddrInet4, flags int) (n int, err error) {
+	return 0, syscall.ENOSYS
+}
+
+func SendmsgNInet6(fd int, p, oob []byte, to *syscall.SockaddrInet6, flags int) (n int, err error) {
+	return 0, syscall.ENOSYS
+}
+
+func RecvmsgInet4(fd int, p, oob []byte, flags int, from *syscall.SockaddrInet4) (n, oobn int, recvflags int, err error) {
+	return 0, 0, 0, syscall.ENOSYS
+}
+
+func RecvmsgInet6(fd int, p, oob []byte, flags int, from *syscall.SockaddrInet6) (n, oobn int, recvflags int, err error) {
+	return 0, 0, 0, syscall.ENOSYS
+}
diff --git a/src/internal/syscall/unix/nonblocking.go b/src/internal/syscall/unix/nonblocking.go
new file mode 100644
index 0000000..6c6f067
--- /dev/null
+++ b/src/internal/syscall/unix/nonblocking.go
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build dragonfly || freebsd || linux || netbsd || (openbsd && mips64)
+
+package unix
+
+import "syscall"
+
+// FcntlSyscall is the number for the fcntl system call. This is
+// usually SYS_FCNTL, but can be overridden to SYS_FCNTL64.
+var FcntlSyscall uintptr = syscall.SYS_FCNTL
+
+func IsNonblock(fd int) (nonblocking bool, err error) {
+	flag, _, e1 := syscall.Syscall(FcntlSyscall, uintptr(fd), uintptr(syscall.F_GETFL), 0)
+	if e1 != 0 {
+		return false, e1
+	}
+	return flag&syscall.O_NONBLOCK != 0, nil
+}
+
+func HasNonblockFlag(flag int) bool {
+	return flag&syscall.O_NONBLOCK != 0
+}
diff --git a/src/internal/syscall/unix/nonblocking_js.go b/src/internal/syscall/unix/nonblocking_js.go
new file mode 100644
index 0000000..cfe78c5
--- /dev/null
+++ b/src/internal/syscall/unix/nonblocking_js.go
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js && wasm
+
+package unix
+
+func IsNonblock(fd int) (nonblocking bool, err error) {
+	return false, nil
+}
+
+func HasNonblockFlag(flag int) bool {
+	return false
+}
diff --git a/src/internal/syscall/unix/nonblocking_libc.go b/src/internal/syscall/unix/nonblocking_libc.go
new file mode 100644
index 0000000..1310dbf
--- /dev/null
+++ b/src/internal/syscall/unix/nonblocking_libc.go
@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || darwin || (openbsd && !mips64) || solaris
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe" // for go:linkname
+)
+
+func IsNonblock(fd int) (nonblocking bool, err error) {
+	flag, e1 := fcntl(fd, syscall.F_GETFL, 0)
+	if e1 != nil {
+		return false, e1
+	}
+	return flag&syscall.O_NONBLOCK != 0, nil
+}
+
+func HasNonblockFlag(flag int) bool {
+	return flag&syscall.O_NONBLOCK != 0
+}
+
+// Implemented in the syscall package.
+//
+//go:linkname fcntl syscall.fcntl
+func fcntl(fd int, cmd int, arg int) (int, error)
diff --git a/src/internal/syscall/unix/pty_darwin.go b/src/internal/syscall/unix/pty_darwin.go
new file mode 100644
index 0000000..b43321a
--- /dev/null
+++ b/src/internal/syscall/unix/pty_darwin.go
@@ -0,0 +1,65 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"internal/abi"
+	"unsafe"
+)
+
+//go:cgo_import_dynamic libc_grantpt grantpt "/usr/lib/libSystem.B.dylib"
+func libc_grantpt_trampoline()
+
+func Grantpt(fd int) error {
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_grantpt_trampoline), uintptr(fd), 0, 0, 0, 0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+//go:cgo_import_dynamic libc_unlockpt unlockpt "/usr/lib/libSystem.B.dylib"
+func libc_unlockpt_trampoline()
+
+func Unlockpt(fd int) error {
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_unlockpt_trampoline), uintptr(fd), 0, 0, 0, 0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+//go:cgo_import_dynamic libc_ptsname_r ptsname_r "/usr/lib/libSystem.B.dylib"
+func libc_ptsname_r_trampoline()
+
+func Ptsname(fd int) (string, error) {
+	buf := make([]byte, 256)
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_ptsname_r_trampoline),
+		uintptr(fd),
+		uintptr(unsafe.Pointer(&buf[0])),
+		uintptr(len(buf)-1),
+		0, 0, 0)
+	if errno != 0 {
+		return "", errno
+	}
+	for i, c := range buf {
+		if c == 0 {
+			buf = buf[:i]
+			break
+		}
+	}
+	return string(buf), nil
+}
+
+//go:cgo_import_dynamic libc_posix_openpt posix_openpt "/usr/lib/libSystem.B.dylib"
+func libc_posix_openpt_trampoline()
+
+func PosixOpenpt(flag int) (fd int, err error) {
+	ufd, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_posix_openpt_trampoline), uintptr(flag), 0, 0, 0, 0, 0)
+	if errno != 0 {
+		return -1, errno
+	}
+	return int(ufd), nil
+}
diff --git a/src/internal/syscall/unix/sysnum_linux_386.go b/src/internal/syscall/unix/sysnum_linux_386.go
new file mode 100644
index 0000000..2bda08c
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_386.go
@@ -0,0 +1,10 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 355
+	copyFileRangeTrap uintptr = 377
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_amd64.go b/src/internal/syscall/unix/sysnum_linux_amd64.go
new file mode 100644
index 0000000..ae5239e
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_amd64.go
@@ -0,0 +1,10 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 318
+	copyFileRangeTrap uintptr = 326
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_arm.go b/src/internal/syscall/unix/sysnum_linux_arm.go
new file mode 100644
index 0000000..acaec05
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_arm.go
@@ -0,0 +1,10 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 384
+	copyFileRangeTrap uintptr = 391
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_generic.go b/src/internal/syscall/unix/sysnum_linux_generic.go
new file mode 100644
index 0000000..8c132c6
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_generic.go
@@ -0,0 +1,16 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux && (arm64 || loong64 || riscv64)
+
+package unix
+
+// This file is named "generic" because at a certain point Linux started
+// standardizing on system call numbers across architectures. So far this
+// means only arm64 loong64 and riscv64 use the standard numbers.
+
+const (
+	getrandomTrap     uintptr = 278
+	copyFileRangeTrap uintptr = 285
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_mips64x.go b/src/internal/syscall/unix/sysnum_linux_mips64x.go
new file mode 100644
index 0000000..bca526d
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_mips64x.go
@@ -0,0 +1,12 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips64 || mips64le
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 5313
+	copyFileRangeTrap uintptr = 5320
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_mipsx.go b/src/internal/syscall/unix/sysnum_linux_mipsx.go
new file mode 100644
index 0000000..c86195e
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_mipsx.go
@@ -0,0 +1,12 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build mips || mipsle
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 4353
+	copyFileRangeTrap uintptr = 4360
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_ppc64x.go b/src/internal/syscall/unix/sysnum_linux_ppc64x.go
new file mode 100644
index 0000000..a4dcf2b
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_ppc64x.go
@@ -0,0 +1,12 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ppc64 || ppc64le
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 359
+	copyFileRangeTrap uintptr = 379
+)
diff --git a/src/internal/syscall/unix/sysnum_linux_s390x.go b/src/internal/syscall/unix/sysnum_linux_s390x.go
new file mode 100644
index 0000000..bf2c01e
--- /dev/null
+++ b/src/internal/syscall/unix/sysnum_linux_s390x.go
@@ -0,0 +1,10 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const (
+	getrandomTrap     uintptr = 349
+	copyFileRangeTrap uintptr = 375
+)
diff --git a/src/internal/syscall/unix/user_darwin.go b/src/internal/syscall/unix/user_darwin.go
new file mode 100644
index 0000000..d05acda
--- /dev/null
+++ b/src/internal/syscall/unix/user_darwin.go
@@ -0,0 +1,121 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import (
+	"internal/abi"
+	"syscall"
+	"unsafe"
+)
+
+//go:cgo_import_dynamic libc_getgrouplist getgrouplist "/usr/lib/libSystem.B.dylib"
+func libc_getgrouplist_trampoline()
+
+func Getgrouplist(name *byte, gid uint32, gids *uint32, n *int32) error {
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_getgrouplist_trampoline),
+		uintptr(unsafe.Pointer(name)), uintptr(gid), uintptr(unsafe.Pointer(gids)),
+		uintptr(unsafe.Pointer(n)), 0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+const (
+	SC_GETGR_R_SIZE_MAX = 0x46
+	SC_GETPW_R_SIZE_MAX = 0x47
+)
+
+type Passwd struct {
+	Name   *byte
+	Passwd *byte
+	Uid    uint32 // uid_t
+	Gid    uint32 // gid_t
+	Change int64  // time_t
+	Class  *byte
+	Gecos  *byte
+	Dir    *byte
+	Shell  *byte
+	Expire int64 // time_t
+}
+
+type Group struct {
+	Name   *byte
+	Passwd *byte
+	Gid    uint32 // gid_t
+	Mem    **byte
+}
+
+//go:cgo_import_dynamic libc_getpwnam_r getpwnam_r  "/usr/lib/libSystem.B.dylib"
+func libc_getpwnam_r_trampoline()
+
+func Getpwnam(name *byte, pwd *Passwd, buf *byte, size uintptr, result **Passwd) syscall.Errno {
+	// Note: Returns an errno as its actual result, not in global errno.
+	errno, _, _ := syscall_syscall6(abi.FuncPCABI0(libc_getpwnam_r_trampoline),
+		uintptr(unsafe.Pointer(name)),
+		uintptr(unsafe.Pointer(pwd)),
+		uintptr(unsafe.Pointer(buf)),
+		size,
+		uintptr(unsafe.Pointer(result)),
+		0)
+	return syscall.Errno(errno)
+}
+
+//go:cgo_import_dynamic libc_getpwuid_r getpwuid_r  "/usr/lib/libSystem.B.dylib"
+func libc_getpwuid_r_trampoline()
+
+func Getpwuid(uid uint32, pwd *Passwd, buf *byte, size uintptr, result **Passwd) syscall.Errno {
+	// Note: Returns an errno as its actual result, not in global errno.
+	errno, _, _ := syscall_syscall6(abi.FuncPCABI0(libc_getpwuid_r_trampoline),
+		uintptr(uid),
+		uintptr(unsafe.Pointer(pwd)),
+		uintptr(unsafe.Pointer(buf)),
+		size,
+		uintptr(unsafe.Pointer(result)),
+		0)
+	return syscall.Errno(errno)
+}
+
+//go:cgo_import_dynamic libc_getgrnam_r getgrnam_r  "/usr/lib/libSystem.B.dylib"
+func libc_getgrnam_r_trampoline()
+
+func Getgrnam(name *byte, grp *Group, buf *byte, size uintptr, result **Group) syscall.Errno {
+	// Note: Returns an errno as its actual result, not in global errno.
+	errno, _, _ := syscall_syscall6(abi.FuncPCABI0(libc_getgrnam_r_trampoline),
+		uintptr(unsafe.Pointer(name)),
+		uintptr(unsafe.Pointer(grp)),
+		uintptr(unsafe.Pointer(buf)),
+		size,
+		uintptr(unsafe.Pointer(result)),
+		0)
+	return syscall.Errno(errno)
+}
+
+//go:cgo_import_dynamic libc_getgrgid_r getgrgid_r  "/usr/lib/libSystem.B.dylib"
+func libc_getgrgid_r_trampoline()
+
+func Getgrgid(gid uint32, grp *Group, buf *byte, size uintptr, result **Group) syscall.Errno {
+	// Note: Returns an errno as its actual result, not in global errno.
+	errno, _, _ := syscall_syscall6(abi.FuncPCABI0(libc_getgrgid_r_trampoline),
+		uintptr(gid),
+		uintptr(unsafe.Pointer(grp)),
+		uintptr(unsafe.Pointer(buf)),
+		size,
+		uintptr(unsafe.Pointer(result)),
+		0)
+	return syscall.Errno(errno)
+}
+
+//go:cgo_import_dynamic libc_sysconf sysconf "/usr/lib/libSystem.B.dylib"
+func libc_sysconf_trampoline()
+
+func Sysconf(key int32) int64 {
+	val, _, errno := syscall_syscall6X(abi.FuncPCABI0(libc_sysconf_trampoline),
+		uintptr(key), 0, 0, 0, 0, 0)
+	if errno != 0 {
+		return -1
+	}
+	return int64(val)
+}
diff --git a/src/internal/syscall/windows/exec_windows_test.go b/src/internal/syscall/windows/exec_windows_test.go
new file mode 100644
index 0000000..3311da5
--- /dev/null
+++ b/src/internal/syscall/windows/exec_windows_test.go
@@ -0,0 +1,139 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package windows_test
+
+import (
+	"fmt"
+	"internal/syscall/windows"
+	"os"
+	"os/exec"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+func TestRunAtLowIntegrity(t *testing.T) {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		wil, err := getProcessIntegrityLevel()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "error: %s\n", err.Error())
+			os.Exit(9)
+			return
+		}
+		fmt.Printf("%s", wil)
+		os.Exit(0)
+		return
+	}
+
+	cmd := exec.Command(os.Args[0], "-test.run=TestRunAtLowIntegrity", "--")
+	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+
+	token, err := getIntegrityLevelToken(sidWilLow)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer token.Close()
+
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Token: token,
+	}
+
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(out) != sidWilLow {
+		t.Fatalf("Child process did not run as low integrity level: %s", string(out))
+	}
+}
+
+const (
+	sidWilLow = `S-1-16-4096`
+)
+
+func getProcessIntegrityLevel() (string, error) {
+	procToken, err := syscall.OpenCurrentProcessToken()
+	if err != nil {
+		return "", err
+	}
+	defer procToken.Close()
+
+	p, err := tokenGetInfo(procToken, syscall.TokenIntegrityLevel, 64)
+	if err != nil {
+		return "", err
+	}
+
+	tml := (*windows.TOKEN_MANDATORY_LABEL)(p)
+
+	sid := (*syscall.SID)(unsafe.Pointer(tml.Label.Sid))
+
+	return sid.String()
+}
+
+func tokenGetInfo(t syscall.Token, class uint32, initSize int) (unsafe.Pointer, error) {
+	n := uint32(initSize)
+	for {
+		b := make([]byte, n)
+		e := syscall.GetTokenInformation(t, class, &b[0], uint32(len(b)), &n)
+		if e == nil {
+			return unsafe.Pointer(&b[0]), nil
+		}
+		if e != syscall.ERROR_INSUFFICIENT_BUFFER {
+			return nil, e
+		}
+		if n <= uint32(len(b)) {
+			return nil, e
+		}
+	}
+}
+
+func getIntegrityLevelToken(wns string) (syscall.Token, error) {
+	var procToken, token syscall.Token
+
+	proc, err := syscall.GetCurrentProcess()
+	if err != nil {
+		return 0, err
+	}
+	defer syscall.CloseHandle(proc)
+
+	err = syscall.OpenProcessToken(proc,
+		syscall.TOKEN_DUPLICATE|
+			syscall.TOKEN_ADJUST_DEFAULT|
+			syscall.TOKEN_QUERY|
+			syscall.TOKEN_ASSIGN_PRIMARY,
+		&procToken)
+	if err != nil {
+		return 0, err
+	}
+	defer procToken.Close()
+
+	sid, err := syscall.StringToSid(wns)
+	if err != nil {
+		return 0, err
+	}
+
+	tml := &windows.TOKEN_MANDATORY_LABEL{}
+	tml.Label.Attributes = windows.SE_GROUP_INTEGRITY
+	tml.Label.Sid = sid
+
+	err = windows.DuplicateTokenEx(procToken, 0, nil, windows.SecurityImpersonation,
+		windows.TokenPrimary, &token)
+	if err != nil {
+		return 0, err
+	}
+
+	err = windows.SetTokenInformation(token,
+		syscall.TokenIntegrityLevel,
+		uintptr(unsafe.Pointer(tml)),
+		tml.Size())
+	if err != nil {
+		token.Close()
+		return 0, err
+	}
+	return token, nil
+}
diff --git a/src/internal/syscall/windows/memory_windows.go b/src/internal/syscall/windows/memory_windows.go
new file mode 100644
index 0000000..8fb34cf
--- /dev/null
+++ b/src/internal/syscall/windows/memory_windows.go
@@ -0,0 +1,24 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+type MemoryBasicInformation struct {
+	// A pointer to the base address of the region of pages.
+	BaseAddress uintptr
+	// A pointer to the base address of a range of pages allocated by the VirtualAlloc function.
+	// The page pointed to by the BaseAddress member is contained within this allocation range.
+	AllocationBase uintptr
+	// The memory protection option when the region was initially allocated
+	AllocationProtect uint32
+	PartitionId       uint16
+	// The size of the region beginning at the base address in which all pages have identical attributes, in bytes.
+	RegionSize uintptr
+	// The state of the pages in the region.
+	State uint32
+	// The access protection of the pages in the region.
+	Protect uint32
+	// The type of pages in the region.
+	Type uint32
+}
diff --git a/src/internal/syscall/windows/mksyscall.go b/src/internal/syscall/windows/mksyscall.go
new file mode 100644
index 0000000..81f08c6
--- /dev/null
+++ b/src/internal/syscall/windows/mksyscall.go
@@ -0,0 +1,9 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build generate
+
+package windows
+
+//go:generate go run ../../../syscall/mksyscall_windows.go -output zsyscall_windows.go syscall_windows.go security_windows.go psapi_windows.go symlink_windows.go
diff --git a/src/internal/syscall/windows/net_windows.go b/src/internal/syscall/windows/net_windows.go
new file mode 100644
index 0000000..3d3df71
--- /dev/null
+++ b/src/internal/syscall/windows/net_windows.go
@@ -0,0 +1,18 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+import (
+	"syscall"
+	_ "unsafe"
+)
+
+//go:linkname WSASendtoInet4 syscall.wsaSendtoInet4
+//go:noescape
+func WSASendtoInet4(s syscall.Handle, bufs *syscall.WSABuf, bufcnt uint32, sent *uint32, flags uint32, to *syscall.SockaddrInet4, overlapped *syscall.Overlapped, croutine *byte) (err error)
+
+//go:linkname WSASendtoInet6 syscall.wsaSendtoInet6
+//go:noescape
+func WSASendtoInet6(s syscall.Handle, bufs *syscall.WSABuf, bufcnt uint32, sent *uint32, flags uint32, to *syscall.SockaddrInet6, overlapped *syscall.Overlapped, croutine *byte) (err error)
diff --git a/src/internal/syscall/windows/psapi_windows.go b/src/internal/syscall/windows/psapi_windows.go
new file mode 100644
index 0000000..b138e65
--- /dev/null
+++ b/src/internal/syscall/windows/psapi_windows.go
@@ -0,0 +1,20 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+type PROCESS_MEMORY_COUNTERS struct {
+	CB                         uint32
+	PageFaultCount             uint32
+	PeakWorkingSetSize         uintptr
+	WorkingSetSize             uintptr
+	QuotaPeakPagedPoolUsage    uintptr
+	QuotaPagedPoolUsage        uintptr
+	QuotaPeakNonPagedPoolUsage uintptr
+	QuotaNonPagedPoolUsage     uintptr
+	PagefileUsage              uintptr
+	PeakPagefileUsage          uintptr
+}
+
+//sys	GetProcessMemoryInfo(handle syscall.Handle, memCounters *PROCESS_MEMORY_COUNTERS, cb uint32) (err error) = psapi.GetProcessMemoryInfo
diff --git a/src/internal/syscall/windows/registry/export_test.go b/src/internal/syscall/windows/registry/export_test.go
new file mode 100644
index 0000000..7f1ac70
--- /dev/null
+++ b/src/internal/syscall/windows/registry/export_test.go
@@ -0,0 +1,11 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package registry
+
+func (k Key) SetValue(name string, valtype uint32, data []byte) error {
+	return k.setValue(name, valtype, data)
+}
diff --git a/src/internal/syscall/windows/registry/key.go b/src/internal/syscall/windows/registry/key.go
new file mode 100644
index 0000000..ce6397f
--- /dev/null
+++ b/src/internal/syscall/windows/registry/key.go
@@ -0,0 +1,168 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+// Package registry provides access to the Windows registry.
+//
+// Here is a simple example, opening a registry key and reading a string value from it.
+//
+//	k, err := registry.OpenKey(registry.LOCAL_MACHINE, `SOFTWARE\Microsoft\Windows NT\CurrentVersion`, registry.QUERY_VALUE)
+//	if err != nil {
+//		log.Fatal(err)
+//	}
+//	defer k.Close()
+//
+//	s, _, err := k.GetStringValue("SystemRoot")
+//	if err != nil {
+//		log.Fatal(err)
+//	}
+//	fmt.Printf("Windows system root is %q\n", s)
+//
+// NOTE: This package is a copy of golang.org/x/sys/windows/registry
+// with KeyInfo.ModTime removed to prevent dependency cycles.
+package registry
+
+import (
+	"runtime"
+	"syscall"
+)
+
+const (
+	// Registry key security and access rights.
+	// See https://msdn.microsoft.com/en-us/library/windows/desktop/ms724878.aspx
+	// for details.
+	ALL_ACCESS         = 0xf003f
+	CREATE_LINK        = 0x00020
+	CREATE_SUB_KEY     = 0x00004
+	ENUMERATE_SUB_KEYS = 0x00008
+	EXECUTE            = 0x20019
+	NOTIFY             = 0x00010
+	QUERY_VALUE        = 0x00001
+	READ               = 0x20019
+	SET_VALUE          = 0x00002
+	WOW64_32KEY        = 0x00200
+	WOW64_64KEY        = 0x00100
+	WRITE              = 0x20006
+)
+
+// Key is a handle to an open Windows registry key.
+// Keys can be obtained by calling OpenKey; there are
+// also some predefined root keys such as CURRENT_USER.
+// Keys can be used directly in the Windows API.
+type Key syscall.Handle
+
+const (
+	// Windows defines some predefined root keys that are always open.
+	// An application can use these keys as entry points to the registry.
+	// Normally these keys are used in OpenKey to open new keys,
+	// but they can also be used anywhere a Key is required.
+	CLASSES_ROOT   = Key(syscall.HKEY_CLASSES_ROOT)
+	CURRENT_USER   = Key(syscall.HKEY_CURRENT_USER)
+	LOCAL_MACHINE  = Key(syscall.HKEY_LOCAL_MACHINE)
+	USERS          = Key(syscall.HKEY_USERS)
+	CURRENT_CONFIG = Key(syscall.HKEY_CURRENT_CONFIG)
+)
+
+// Close closes open key k.
+func (k Key) Close() error {
+	return syscall.RegCloseKey(syscall.Handle(k))
+}
+
+// OpenKey opens a new key with path name relative to key k.
+// It accepts any open key, including CURRENT_USER and others,
+// and returns the new key and an error.
+// The access parameter specifies desired access rights to the
+// key to be opened.
+func OpenKey(k Key, path string, access uint32) (Key, error) {
+	p, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+	var subkey syscall.Handle
+	err = syscall.RegOpenKeyEx(syscall.Handle(k), p, 0, access, &subkey)
+	if err != nil {
+		return 0, err
+	}
+	return Key(subkey), nil
+}
+
+// ReadSubKeyNames returns the names of subkeys of key k.
+func (k Key) ReadSubKeyNames() ([]string, error) {
+	// RegEnumKeyEx must be called repeatedly and to completion.
+	// During this time, this goroutine cannot migrate away from
+	// its current thread. See #49320.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	names := make([]string, 0)
+	// Registry key size limit is 255 bytes and described there:
+	// https://msdn.microsoft.com/library/windows/desktop/ms724872.aspx
+	buf := make([]uint16, 256) //plus extra room for terminating zero byte
+loopItems:
+	for i := uint32(0); ; i++ {
+		l := uint32(len(buf))
+		for {
+			err := syscall.RegEnumKeyEx(syscall.Handle(k), i, &buf[0], &l, nil, nil, nil, nil)
+			if err == nil {
+				break
+			}
+			if err == syscall.ERROR_MORE_DATA {
+				// Double buffer size and try again.
+				l = uint32(2 * len(buf))
+				buf = make([]uint16, l)
+				continue
+			}
+			if err == _ERROR_NO_MORE_ITEMS {
+				break loopItems
+			}
+			return names, err
+		}
+		names = append(names, syscall.UTF16ToString(buf[:l]))
+	}
+	return names, nil
+}
+
+// CreateKey creates a key named path under open key k.
+// CreateKey returns the new key and a boolean flag that reports
+// whether the key already existed.
+// The access parameter specifies the access rights for the key
+// to be created.
+func CreateKey(k Key, path string, access uint32) (newk Key, openedExisting bool, err error) {
+	var h syscall.Handle
+	var d uint32
+	err = regCreateKeyEx(syscall.Handle(k), syscall.StringToUTF16Ptr(path),
+		0, nil, _REG_OPTION_NON_VOLATILE, access, nil, &h, &d)
+	if err != nil {
+		return 0, false, err
+	}
+	return Key(h), d == _REG_OPENED_EXISTING_KEY, nil
+}
+
+// DeleteKey deletes the subkey path of key k and its values.
+func DeleteKey(k Key, path string) error {
+	return regDeleteKey(syscall.Handle(k), syscall.StringToUTF16Ptr(path))
+}
+
+// A KeyInfo describes the statistics of a key. It is returned by Stat.
+type KeyInfo struct {
+	SubKeyCount     uint32
+	MaxSubKeyLen    uint32 // size of the key's subkey with the longest name, in Unicode characters, not including the terminating zero byte
+	ValueCount      uint32
+	MaxValueNameLen uint32 // size of the key's longest value name, in Unicode characters, not including the terminating zero byte
+	MaxValueLen     uint32 // longest data component among the key's values, in bytes
+	lastWriteTime   syscall.Filetime
+}
+
+// Stat retrieves information about the open key k.
+func (k Key) Stat() (*KeyInfo, error) {
+	var ki KeyInfo
+	err := syscall.RegQueryInfoKey(syscall.Handle(k), nil, nil, nil,
+		&ki.SubKeyCount, &ki.MaxSubKeyLen, nil, &ki.ValueCount,
+		&ki.MaxValueNameLen, &ki.MaxValueLen, nil, &ki.lastWriteTime)
+	if err != nil {
+		return nil, err
+	}
+	return &ki, nil
+}
diff --git a/src/internal/syscall/windows/registry/mksyscall.go b/src/internal/syscall/windows/registry/mksyscall.go
new file mode 100644
index 0000000..0e0b421
--- /dev/null
+++ b/src/internal/syscall/windows/registry/mksyscall.go
@@ -0,0 +1,9 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build generate
+
+package registry
+
+//go:generate go run ../../../../syscall/mksyscall_windows.go -output zsyscall_windows.go syscall.go
diff --git a/src/internal/syscall/windows/registry/registry_test.go b/src/internal/syscall/windows/registry/registry_test.go
new file mode 100644
index 0000000..278b0b4
--- /dev/null
+++ b/src/internal/syscall/windows/registry/registry_test.go
@@ -0,0 +1,672 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package registry_test
+
+import (
+	"bytes"
+	"crypto/rand"
+	"os"
+	"syscall"
+	"testing"
+	"unsafe"
+
+	"internal/syscall/windows/registry"
+)
+
+func randKeyName(prefix string) string {
+	const numbers = "0123456789"
+	buf := make([]byte, 10)
+	rand.Read(buf)
+	for i, b := range buf {
+		buf[i] = numbers[b%byte(len(numbers))]
+	}
+	return prefix + string(buf)
+}
+
+func TestReadSubKeyNames(t *testing.T) {
+	k, err := registry.OpenKey(registry.CLASSES_ROOT, "TypeLib", registry.ENUMERATE_SUB_KEYS)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer k.Close()
+
+	names, err := k.ReadSubKeyNames()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var foundStdOle bool
+	for _, name := range names {
+		// Every PC has "stdole 2.0 OLE Automation" library installed.
+		if name == "{00020430-0000-0000-C000-000000000046}" {
+			foundStdOle = true
+		}
+	}
+	if !foundStdOle {
+		t.Fatal("could not find stdole 2.0 OLE Automation")
+	}
+}
+
+func TestCreateOpenDeleteKey(t *testing.T) {
+	k, err := registry.OpenKey(registry.CURRENT_USER, "Software", registry.QUERY_VALUE)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer k.Close()
+
+	testKName := randKeyName("TestCreateOpenDeleteKey_")
+
+	testK, exist, err := registry.CreateKey(k, testKName, registry.CREATE_SUB_KEY)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer testK.Close()
+
+	if exist {
+		t.Fatalf("key %q already exists", testKName)
+	}
+
+	testKAgain, exist, err := registry.CreateKey(k, testKName, registry.CREATE_SUB_KEY)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer testKAgain.Close()
+
+	if !exist {
+		t.Fatalf("key %q should already exist", testKName)
+	}
+
+	testKOpened, err := registry.OpenKey(k, testKName, registry.ENUMERATE_SUB_KEYS)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer testKOpened.Close()
+
+	err = registry.DeleteKey(k, testKName)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testKOpenedAgain, err := registry.OpenKey(k, testKName, registry.ENUMERATE_SUB_KEYS)
+	if err == nil {
+		defer testKOpenedAgain.Close()
+		t.Fatalf("key %q should already been deleted", testKName)
+	}
+	if err != registry.ErrNotExist {
+		t.Fatalf(`unexpected error ("not exist" expected): %v`, err)
+	}
+}
+
+func equalStringSlice(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	if a == nil {
+		return true
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+type ValueTest struct {
+	Type     uint32
+	Name     string
+	Value    any
+	WillFail bool
+}
+
+var ValueTests = []ValueTest{
+	{Type: registry.SZ, Name: "String1", Value: ""},
+	{Type: registry.SZ, Name: "String2", Value: "\000", WillFail: true},
+	{Type: registry.SZ, Name: "String3", Value: "Hello World"},
+	{Type: registry.SZ, Name: "String4", Value: "Hello World\000", WillFail: true},
+	{Type: registry.EXPAND_SZ, Name: "ExpString1", Value: ""},
+	{Type: registry.EXPAND_SZ, Name: "ExpString2", Value: "\000", WillFail: true},
+	{Type: registry.EXPAND_SZ, Name: "ExpString3", Value: "Hello World"},
+	{Type: registry.EXPAND_SZ, Name: "ExpString4", Value: "Hello\000World", WillFail: true},
+	{Type: registry.EXPAND_SZ, Name: "ExpString5", Value: "%PATH%"},
+	{Type: registry.EXPAND_SZ, Name: "ExpString6", Value: "%NO_SUCH_VARIABLE%"},
+	{Type: registry.EXPAND_SZ, Name: "ExpString7", Value: "%PATH%;."},
+	{Type: registry.BINARY, Name: "Binary1", Value: []byte{}},
+	{Type: registry.BINARY, Name: "Binary2", Value: []byte{1, 2, 3}},
+	{Type: registry.BINARY, Name: "Binary3", Value: []byte{3, 2, 1, 0, 1, 2, 3}},
+	{Type: registry.DWORD, Name: "Dword1", Value: uint64(0)},
+	{Type: registry.DWORD, Name: "Dword2", Value: uint64(1)},
+	{Type: registry.DWORD, Name: "Dword3", Value: uint64(0xff)},
+	{Type: registry.DWORD, Name: "Dword4", Value: uint64(0xffff)},
+	{Type: registry.QWORD, Name: "Qword1", Value: uint64(0)},
+	{Type: registry.QWORD, Name: "Qword2", Value: uint64(1)},
+	{Type: registry.QWORD, Name: "Qword3", Value: uint64(0xff)},
+	{Type: registry.QWORD, Name: "Qword4", Value: uint64(0xffff)},
+	{Type: registry.QWORD, Name: "Qword5", Value: uint64(0xffffff)},
+	{Type: registry.QWORD, Name: "Qword6", Value: uint64(0xffffffff)},
+	{Type: registry.MULTI_SZ, Name: "MultiString1", Value: []string{"a", "b", "c"}},
+	{Type: registry.MULTI_SZ, Name: "MultiString2", Value: []string{"abc", "", "cba"}},
+	{Type: registry.MULTI_SZ, Name: "MultiString3", Value: []string{""}},
+	{Type: registry.MULTI_SZ, Name: "MultiString4", Value: []string{"abcdef"}},
+	{Type: registry.MULTI_SZ, Name: "MultiString5", Value: []string{"\000"}, WillFail: true},
+	{Type: registry.MULTI_SZ, Name: "MultiString6", Value: []string{"a\000b"}, WillFail: true},
+	{Type: registry.MULTI_SZ, Name: "MultiString7", Value: []string{"ab", "\000", "cd"}, WillFail: true},
+	{Type: registry.MULTI_SZ, Name: "MultiString8", Value: []string{"\000", "cd"}, WillFail: true},
+	{Type: registry.MULTI_SZ, Name: "MultiString9", Value: []string{"ab", "\000"}, WillFail: true},
+}
+
+func setValues(t *testing.T, k registry.Key) {
+	for _, test := range ValueTests {
+		var err error
+		switch test.Type {
+		case registry.SZ:
+			err = k.SetStringValue(test.Name, test.Value.(string))
+		case registry.EXPAND_SZ:
+			err = k.SetExpandStringValue(test.Name, test.Value.(string))
+		case registry.MULTI_SZ:
+			err = k.SetStringsValue(test.Name, test.Value.([]string))
+		case registry.BINARY:
+			err = k.SetBinaryValue(test.Name, test.Value.([]byte))
+		case registry.DWORD:
+			err = k.SetDWordValue(test.Name, uint32(test.Value.(uint64)))
+		case registry.QWORD:
+			err = k.SetQWordValue(test.Name, test.Value.(uint64))
+		default:
+			t.Fatalf("unsupported type %d for %s value", test.Type, test.Name)
+		}
+		if test.WillFail {
+			if err == nil {
+				t.Fatalf("setting %s value %q should fail, but succeeded", test.Name, test.Value)
+			}
+		} else {
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+	}
+}
+
+func enumerateValues(t *testing.T, k registry.Key) {
+	names, err := k.ReadValueNames()
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	haveNames := make(map[string]bool)
+	for _, n := range names {
+		haveNames[n] = false
+	}
+	for _, test := range ValueTests {
+		wantFound := !test.WillFail
+		_, haveFound := haveNames[test.Name]
+		if wantFound && !haveFound {
+			t.Errorf("value %s is not found while enumerating", test.Name)
+		}
+		if haveFound && !wantFound {
+			t.Errorf("value %s is found while enumerating, but expected to fail", test.Name)
+		}
+		if haveFound {
+			delete(haveNames, test.Name)
+		}
+	}
+	for n, v := range haveNames {
+		t.Errorf("value %s (%v) is found while enumerating, but has not been created", n, v)
+	}
+}
+
+func testErrNotExist(t *testing.T, name string, err error) {
+	if err == nil {
+		t.Errorf("%s value should not exist", name)
+		return
+	}
+	if err != registry.ErrNotExist {
+		t.Errorf("reading %s value should return 'not exist' error, but got: %s", name, err)
+		return
+	}
+}
+
+func testErrUnexpectedType(t *testing.T, test ValueTest, gottype uint32, err error) {
+	if err == nil {
+		t.Errorf("GetXValue(%q) should not succeed", test.Name)
+		return
+	}
+	if err != registry.ErrUnexpectedType {
+		t.Errorf("reading %s value should return 'unexpected key value type' error, but got: %s", test.Name, err)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+}
+
+func testGetStringValue(t *testing.T, k registry.Key, test ValueTest) {
+	got, gottype, err := k.GetStringValue(test.Name)
+	if err != nil {
+		t.Errorf("GetStringValue(%s) failed: %v", test.Name, err)
+		return
+	}
+	if got != test.Value {
+		t.Errorf("want %s value %q, got %q", test.Name, test.Value, got)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+	if gottype == registry.EXPAND_SZ {
+		_, err = registry.ExpandString(got)
+		if err != nil {
+			t.Errorf("ExpandString(%s) failed: %v", got, err)
+			return
+		}
+	}
+}
+
+func testGetIntegerValue(t *testing.T, k registry.Key, test ValueTest) {
+	got, gottype, err := k.GetIntegerValue(test.Name)
+	if err != nil {
+		t.Errorf("GetIntegerValue(%s) failed: %v", test.Name, err)
+		return
+	}
+	if got != test.Value.(uint64) {
+		t.Errorf("want %s value %v, got %v", test.Name, test.Value, got)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+}
+
+func testGetBinaryValue(t *testing.T, k registry.Key, test ValueTest) {
+	got, gottype, err := k.GetBinaryValue(test.Name)
+	if err != nil {
+		t.Errorf("GetBinaryValue(%s) failed: %v", test.Name, err)
+		return
+	}
+	if !bytes.Equal(got, test.Value.([]byte)) {
+		t.Errorf("want %s value %v, got %v", test.Name, test.Value, got)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+}
+
+func testGetStringsValue(t *testing.T, k registry.Key, test ValueTest) {
+	got, gottype, err := k.GetStringsValue(test.Name)
+	if err != nil {
+		t.Errorf("GetStringsValue(%s) failed: %v", test.Name, err)
+		return
+	}
+	if !equalStringSlice(got, test.Value.([]string)) {
+		t.Errorf("want %s value %#v, got %#v", test.Name, test.Value, got)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+}
+
+func testGetValue(t *testing.T, k registry.Key, test ValueTest, size int) {
+	if size <= 0 {
+		return
+	}
+	// read data with no buffer
+	gotsize, gottype, err := k.GetValue(test.Name, nil)
+	if err != nil {
+		t.Errorf("GetValue(%s, [%d]byte) failed: %v", test.Name, size, err)
+		return
+	}
+	if gotsize != size {
+		t.Errorf("want %s value size of %d, got %v", test.Name, size, gotsize)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+	// read data with short buffer
+	gotsize, gottype, err = k.GetValue(test.Name, make([]byte, size-1))
+	if err == nil {
+		t.Errorf("GetValue(%s, [%d]byte) should fail, but succeeded", test.Name, size-1)
+		return
+	}
+	if err != registry.ErrShortBuffer {
+		t.Errorf("reading %s value should return 'short buffer' error, but got: %s", test.Name, err)
+		return
+	}
+	if gotsize != size {
+		t.Errorf("want %s value size of %d, got %v", test.Name, size, gotsize)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+	// read full data
+	gotsize, gottype, err = k.GetValue(test.Name, make([]byte, size))
+	if err != nil {
+		t.Errorf("GetValue(%s, [%d]byte) failed: %v", test.Name, size, err)
+		return
+	}
+	if gotsize != size {
+		t.Errorf("want %s value size of %d, got %v", test.Name, size, gotsize)
+		return
+	}
+	if gottype != test.Type {
+		t.Errorf("want %s value type %v, got %v", test.Name, test.Type, gottype)
+		return
+	}
+	// check GetValue returns ErrNotExist as required
+	_, _, err = k.GetValue(test.Name+"_not_there", make([]byte, size))
+	if err == nil {
+		t.Errorf("GetValue(%q) should not succeed", test.Name)
+		return
+	}
+	if err != registry.ErrNotExist {
+		t.Errorf("GetValue(%q) should return 'not exist' error, but got: %s", test.Name, err)
+		return
+	}
+}
+
+func testValues(t *testing.T, k registry.Key) {
+	for _, test := range ValueTests {
+		switch test.Type {
+		case registry.SZ, registry.EXPAND_SZ:
+			if test.WillFail {
+				_, _, err := k.GetStringValue(test.Name)
+				testErrNotExist(t, test.Name, err)
+			} else {
+				testGetStringValue(t, k, test)
+				_, gottype, err := k.GetIntegerValue(test.Name)
+				testErrUnexpectedType(t, test, gottype, err)
+				// Size of utf16 string in bytes is not perfect,
+				// but correct for current test values.
+				// Size also includes terminating 0.
+				testGetValue(t, k, test, (len(test.Value.(string))+1)*2)
+			}
+			_, _, err := k.GetStringValue(test.Name + "_string_not_created")
+			testErrNotExist(t, test.Name+"_string_not_created", err)
+		case registry.DWORD, registry.QWORD:
+			testGetIntegerValue(t, k, test)
+			_, gottype, err := k.GetBinaryValue(test.Name)
+			testErrUnexpectedType(t, test, gottype, err)
+			_, _, err = k.GetIntegerValue(test.Name + "_int_not_created")
+			testErrNotExist(t, test.Name+"_int_not_created", err)
+			size := 8
+			if test.Type == registry.DWORD {
+				size = 4
+			}
+			testGetValue(t, k, test, size)
+		case registry.BINARY:
+			testGetBinaryValue(t, k, test)
+			_, gottype, err := k.GetStringsValue(test.Name)
+			testErrUnexpectedType(t, test, gottype, err)
+			_, _, err = k.GetBinaryValue(test.Name + "_byte_not_created")
+			testErrNotExist(t, test.Name+"_byte_not_created", err)
+			testGetValue(t, k, test, len(test.Value.([]byte)))
+		case registry.MULTI_SZ:
+			if test.WillFail {
+				_, _, err := k.GetStringsValue(test.Name)
+				testErrNotExist(t, test.Name, err)
+			} else {
+				testGetStringsValue(t, k, test)
+				_, gottype, err := k.GetStringValue(test.Name)
+				testErrUnexpectedType(t, test, gottype, err)
+				size := 0
+				for _, s := range test.Value.([]string) {
+					size += len(s) + 1 // nil terminated
+				}
+				size += 1 // extra nil at the end
+				size *= 2 // count bytes, not uint16
+				testGetValue(t, k, test, size)
+			}
+			_, _, err := k.GetStringsValue(test.Name + "_strings_not_created")
+			testErrNotExist(t, test.Name+"_strings_not_created", err)
+		default:
+			t.Errorf("unsupported type %d for %s value", test.Type, test.Name)
+			continue
+		}
+	}
+}
+
+func testStat(t *testing.T, k registry.Key) {
+	subk, _, err := registry.CreateKey(k, "subkey", registry.CREATE_SUB_KEY)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	defer subk.Close()
+
+	defer registry.DeleteKey(k, "subkey")
+
+	ki, err := k.Stat()
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	if ki.SubKeyCount != 1 {
+		t.Error("key must have 1 subkey")
+	}
+	if ki.MaxSubKeyLen != 6 {
+		t.Error("key max subkey name length must be 6")
+	}
+	if ki.ValueCount != 24 {
+		t.Errorf("key must have 24 values, but is %d", ki.ValueCount)
+	}
+	if ki.MaxValueNameLen != 12 {
+		t.Errorf("key max value name length must be 10, but is %d", ki.MaxValueNameLen)
+	}
+	if ki.MaxValueLen != 38 {
+		t.Errorf("key max value length must be 38, but is %d", ki.MaxValueLen)
+	}
+}
+
+func deleteValues(t *testing.T, k registry.Key) {
+	for _, test := range ValueTests {
+		if test.WillFail {
+			continue
+		}
+		err := k.DeleteValue(test.Name)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+	}
+	names, err := k.ReadValueNames()
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	if len(names) != 0 {
+		t.Errorf("some values remain after deletion: %v", names)
+	}
+}
+
+func TestValues(t *testing.T) {
+	softwareK, err := registry.OpenKey(registry.CURRENT_USER, "Software", registry.QUERY_VALUE)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer softwareK.Close()
+
+	testKName := randKeyName("TestValues_")
+
+	k, exist, err := registry.CreateKey(softwareK, testKName, registry.CREATE_SUB_KEY|registry.QUERY_VALUE|registry.SET_VALUE)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer k.Close()
+
+	if exist {
+		t.Fatalf("key %q already exists", testKName)
+	}
+
+	defer registry.DeleteKey(softwareK, testKName)
+
+	setValues(t, k)
+
+	enumerateValues(t, k)
+
+	testValues(t, k)
+
+	testStat(t, k)
+
+	deleteValues(t, k)
+}
+
+func TestExpandString(t *testing.T) {
+	got, err := registry.ExpandString("%PATH%")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := os.Getenv("PATH")
+	if got != want {
+		t.Errorf("want %q string expanded, got %q", want, got)
+	}
+}
+
+func TestInvalidValues(t *testing.T) {
+	softwareK, err := registry.OpenKey(registry.CURRENT_USER, "Software", registry.QUERY_VALUE)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer softwareK.Close()
+
+	testKName := randKeyName("TestInvalidValues_")
+
+	k, exist, err := registry.CreateKey(softwareK, testKName, registry.CREATE_SUB_KEY|registry.QUERY_VALUE|registry.SET_VALUE)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer k.Close()
+
+	if exist {
+		t.Fatalf("key %q already exists", testKName)
+	}
+
+	defer registry.DeleteKey(softwareK, testKName)
+
+	var tests = []struct {
+		Type uint32
+		Name string
+		Data []byte
+	}{
+		{registry.DWORD, "Dword1", nil},
+		{registry.DWORD, "Dword2", []byte{1, 2, 3}},
+		{registry.QWORD, "Qword1", nil},
+		{registry.QWORD, "Qword2", []byte{1, 2, 3}},
+		{registry.QWORD, "Qword3", []byte{1, 2, 3, 4, 5, 6, 7}},
+		{registry.MULTI_SZ, "MultiString1", nil},
+		{registry.MULTI_SZ, "MultiString2", []byte{0}},
+		{registry.MULTI_SZ, "MultiString3", []byte{'a', 'b', 0}},
+		{registry.MULTI_SZ, "MultiString4", []byte{'a', 0, 0, 'b', 0}},
+		{registry.MULTI_SZ, "MultiString5", []byte{'a', 0, 0}},
+	}
+
+	for _, test := range tests {
+		err := k.SetValue(test.Name, test.Type, test.Data)
+		if err != nil {
+			t.Fatalf("SetValue for %q failed: %v", test.Name, err)
+		}
+	}
+
+	for _, test := range tests {
+		switch test.Type {
+		case registry.DWORD, registry.QWORD:
+			value, valType, err := k.GetIntegerValue(test.Name)
+			if err == nil {
+				t.Errorf("GetIntegerValue(%q) succeeded. Returns type=%d value=%v", test.Name, valType, value)
+			}
+		case registry.MULTI_SZ:
+			value, valType, err := k.GetStringsValue(test.Name)
+			if err == nil {
+				if len(value) != 0 {
+					t.Errorf("GetStringsValue(%q) succeeded. Returns type=%d value=%v", test.Name, valType, value)
+				}
+			}
+		default:
+			t.Errorf("unsupported type %d for %s value", test.Type, test.Name)
+		}
+	}
+}
+
+func TestGetMUIStringValue(t *testing.T) {
+	if err := registry.LoadRegLoadMUIString(); err != nil {
+		t.Skip("regLoadMUIString not supported; skipping")
+	}
+	if err := procGetDynamicTimeZoneInformation.Find(); err != nil {
+		t.Skipf("%s not supported; skipping", procGetDynamicTimeZoneInformation.Name)
+	}
+	var dtzi DynamicTimezoneinformation
+	if _, err := GetDynamicTimeZoneInformation(&dtzi); err != nil {
+		t.Fatal(err)
+	}
+	tzKeyName := syscall.UTF16ToString(dtzi.TimeZoneKeyName[:])
+	timezoneK, err := registry.OpenKey(registry.LOCAL_MACHINE,
+		`SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\`+tzKeyName, registry.READ)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer timezoneK.Close()
+
+	type testType struct {
+		name string
+		want string
+	}
+	var tests = []testType{
+		{"MUI_Std", syscall.UTF16ToString(dtzi.StandardName[:])},
+	}
+	if dtzi.DynamicDaylightTimeDisabled == 0 {
+		tests = append(tests, testType{"MUI_Dlt", syscall.UTF16ToString(dtzi.DaylightName[:])})
+	}
+
+	for _, test := range tests {
+		got, err := timezoneK.GetMUIStringValue(test.name)
+		if err != nil {
+			t.Error("GetMUIStringValue:", err)
+		}
+
+		if got != test.want {
+			t.Errorf("GetMUIStringValue: %s: Got %q, want %q", test.name, got, test.want)
+		}
+	}
+}
+
+type DynamicTimezoneinformation struct {
+	Bias                        int32
+	StandardName                [32]uint16
+	StandardDate                syscall.Systemtime
+	StandardBias                int32
+	DaylightName                [32]uint16
+	DaylightDate                syscall.Systemtime
+	DaylightBias                int32
+	TimeZoneKeyName             [128]uint16
+	DynamicDaylightTimeDisabled uint8
+}
+
+var (
+	kernel32DLL = syscall.NewLazyDLL("kernel32")
+
+	procGetDynamicTimeZoneInformation = kernel32DLL.NewProc("GetDynamicTimeZoneInformation")
+)
+
+func GetDynamicTimeZoneInformation(dtzi *DynamicTimezoneinformation) (rc uint32, err error) {
+	r0, _, e1 := syscall.Syscall(procGetDynamicTimeZoneInformation.Addr(), 1, uintptr(unsafe.Pointer(dtzi)), 0, 0)
+	rc = uint32(r0)
+	if rc == 0xffffffff {
+		if e1 != 0 {
+			err = error(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}
diff --git a/src/internal/syscall/windows/registry/syscall.go b/src/internal/syscall/windows/registry/syscall.go
new file mode 100644
index 0000000..cb315ad
--- /dev/null
+++ b/src/internal/syscall/windows/registry/syscall.go
@@ -0,0 +1,31 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package registry
+
+import "syscall"
+
+const (
+	_REG_OPTION_NON_VOLATILE = 0
+
+	_REG_CREATED_NEW_KEY     = 1
+	_REG_OPENED_EXISTING_KEY = 2
+
+	_ERROR_NO_MORE_ITEMS syscall.Errno = 259
+)
+
+func LoadRegLoadMUIString() error {
+	return procRegLoadMUIStringW.Find()
+}
+
+//sys	regCreateKeyEx(key syscall.Handle, subkey *uint16, reserved uint32, class *uint16, options uint32, desired uint32, sa *syscall.SecurityAttributes, result *syscall.Handle, disposition *uint32) (regerrno error) = advapi32.RegCreateKeyExW
+//sys	regDeleteKey(key syscall.Handle, subkey *uint16) (regerrno error) = advapi32.RegDeleteKeyW
+//sys	regSetValueEx(key syscall.Handle, valueName *uint16, reserved uint32, vtype uint32, buf *byte, bufsize uint32) (regerrno error) = advapi32.RegSetValueExW
+//sys	regEnumValue(key syscall.Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, valtype *uint32, buf *byte, buflen *uint32) (regerrno error) = advapi32.RegEnumValueW
+//sys	regDeleteValue(key syscall.Handle, name *uint16) (regerrno error) = advapi32.RegDeleteValueW
+//sys   regLoadMUIString(key syscall.Handle, name *uint16, buf *uint16, buflen uint32, buflenCopied *uint32, flags uint32, dir *uint16) (regerrno error) = advapi32.RegLoadMUIStringW
+
+//sys	expandEnvironmentStrings(src *uint16, dst *uint16, size uint32) (n uint32, err error) = kernel32.ExpandEnvironmentStringsW
diff --git a/src/internal/syscall/windows/registry/value.go b/src/internal/syscall/windows/registry/value.go
new file mode 100644
index 0000000..0255740
--- /dev/null
+++ b/src/internal/syscall/windows/registry/value.go
@@ -0,0 +1,372 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package registry
+
+import (
+	"errors"
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+const (
+	// Registry value types.
+	NONE                       = 0
+	SZ                         = 1
+	EXPAND_SZ                  = 2
+	BINARY                     = 3
+	DWORD                      = 4
+	DWORD_BIG_ENDIAN           = 5
+	LINK                       = 6
+	MULTI_SZ                   = 7
+	RESOURCE_LIST              = 8
+	FULL_RESOURCE_DESCRIPTOR   = 9
+	RESOURCE_REQUIREMENTS_LIST = 10
+	QWORD                      = 11
+)
+
+var (
+	// ErrShortBuffer is returned when the buffer was too short for the operation.
+	ErrShortBuffer = syscall.ERROR_MORE_DATA
+
+	// ErrNotExist is returned when a registry key or value does not exist.
+	ErrNotExist = syscall.ERROR_FILE_NOT_FOUND
+
+	// ErrUnexpectedType is returned by Get*Value when the value's type was unexpected.
+	ErrUnexpectedType = errors.New("unexpected key value type")
+)
+
+// GetValue retrieves the type and data for the specified value associated
+// with an open key k. It fills up buffer buf and returns the retrieved
+// byte count n. If buf is too small to fit the stored value it returns
+// ErrShortBuffer error along with the required buffer size n.
+// If no buffer is provided, it returns true and actual buffer size n.
+// If no buffer is provided, GetValue returns the value's type only.
+// If the value does not exist, the error returned is ErrNotExist.
+//
+// GetValue is a low level function. If value's type is known, use the appropriate
+// Get*Value function instead.
+func (k Key) GetValue(name string, buf []byte) (n int, valtype uint32, err error) {
+	pname, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return 0, 0, err
+	}
+	var pbuf *byte
+	if len(buf) > 0 {
+		pbuf = (*byte)(unsafe.Pointer(&buf[0]))
+	}
+	l := uint32(len(buf))
+	err = syscall.RegQueryValueEx(syscall.Handle(k), pname, nil, &valtype, pbuf, &l)
+	if err != nil {
+		return int(l), valtype, err
+	}
+	return int(l), valtype, nil
+}
+
+func (k Key) getValue(name string, buf []byte) (date []byte, valtype uint32, err error) {
+	p, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return nil, 0, err
+	}
+	var t uint32
+	n := uint32(len(buf))
+	for {
+		err = syscall.RegQueryValueEx(syscall.Handle(k), p, nil, &t, (*byte)(unsafe.Pointer(&buf[0])), &n)
+		if err == nil {
+			return buf[:n], t, nil
+		}
+		if err != syscall.ERROR_MORE_DATA {
+			return nil, 0, err
+		}
+		if n <= uint32(len(buf)) {
+			return nil, 0, err
+		}
+		buf = make([]byte, n)
+	}
+}
+
+// GetStringValue retrieves the string value for the specified
+// value name associated with an open key k. It also returns the value's type.
+// If value does not exist, GetStringValue returns ErrNotExist.
+// If value is not SZ or EXPAND_SZ, it will return the correct value
+// type and ErrUnexpectedType.
+func (k Key) GetStringValue(name string) (val string, valtype uint32, err error) {
+	data, typ, err2 := k.getValue(name, make([]byte, 64))
+	if err2 != nil {
+		return "", typ, err2
+	}
+	switch typ {
+	case SZ, EXPAND_SZ:
+	default:
+		return "", typ, ErrUnexpectedType
+	}
+	if len(data) == 0 {
+		return "", typ, nil
+	}
+	u := (*[1 << 29]uint16)(unsafe.Pointer(&data[0]))[: len(data)/2 : len(data)/2]
+	return syscall.UTF16ToString(u), typ, nil
+}
+
+// GetMUIStringValue retrieves the localized string value for
+// the specified value name associated with an open key k.
+// If the value name doesn't exist or the localized string value
+// can't be resolved, GetMUIStringValue returns ErrNotExist.
+// GetMUIStringValue panics if the system doesn't support
+// regLoadMUIString; use LoadRegLoadMUIString to check if
+// regLoadMUIString is supported before calling this function.
+func (k Key) GetMUIStringValue(name string) (string, error) {
+	pname, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return "", err
+	}
+
+	buf := make([]uint16, 1024)
+	var buflen uint32
+	var pdir *uint16
+
+	err = regLoadMUIString(syscall.Handle(k), pname, &buf[0], uint32(len(buf)), &buflen, 0, pdir)
+	if err == syscall.ERROR_FILE_NOT_FOUND { // Try fallback path
+
+		// Try to resolve the string value using the system directory as
+		// a DLL search path; this assumes the string value is of the form
+		// @[path]\dllname,-strID but with no path given, e.g. @tzres.dll,-320.
+
+		// This approach works with tzres.dll but may have to be revised
+		// in the future to allow callers to provide custom search paths.
+
+		var s string
+		s, err = ExpandString("%SystemRoot%\\system32\\")
+		if err != nil {
+			return "", err
+		}
+		pdir, err = syscall.UTF16PtrFromString(s)
+		if err != nil {
+			return "", err
+		}
+
+		err = regLoadMUIString(syscall.Handle(k), pname, &buf[0], uint32(len(buf)), &buflen, 0, pdir)
+	}
+
+	for err == syscall.ERROR_MORE_DATA { // Grow buffer if needed
+		if buflen <= uint32(len(buf)) {
+			break // Buffer not growing, assume race; break
+		}
+		buf = make([]uint16, buflen)
+		err = regLoadMUIString(syscall.Handle(k), pname, &buf[0], uint32(len(buf)), &buflen, 0, pdir)
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	return syscall.UTF16ToString(buf), nil
+}
+
+// ExpandString expands environment-variable strings and replaces
+// them with the values defined for the current user.
+// Use ExpandString to expand EXPAND_SZ strings.
+func ExpandString(value string) (string, error) {
+	if value == "" {
+		return "", nil
+	}
+	p, err := syscall.UTF16PtrFromString(value)
+	if err != nil {
+		return "", err
+	}
+	r := make([]uint16, 100)
+	for {
+		n, err := expandEnvironmentStrings(p, &r[0], uint32(len(r)))
+		if err != nil {
+			return "", err
+		}
+		if n <= uint32(len(r)) {
+			return syscall.UTF16ToString(r[:n]), nil
+		}
+		r = make([]uint16, n)
+	}
+}
+
+// GetStringsValue retrieves the []string value for the specified
+// value name associated with an open key k. It also returns the value's type.
+// If value does not exist, GetStringsValue returns ErrNotExist.
+// If value is not MULTI_SZ, it will return the correct value
+// type and ErrUnexpectedType.
+func (k Key) GetStringsValue(name string) (val []string, valtype uint32, err error) {
+	data, typ, err2 := k.getValue(name, make([]byte, 64))
+	if err2 != nil {
+		return nil, typ, err2
+	}
+	if typ != MULTI_SZ {
+		return nil, typ, ErrUnexpectedType
+	}
+	if len(data) == 0 {
+		return nil, typ, nil
+	}
+	p := (*[1 << 29]uint16)(unsafe.Pointer(&data[0]))[: len(data)/2 : len(data)/2]
+	if len(p) == 0 {
+		return nil, typ, nil
+	}
+	if p[len(p)-1] == 0 {
+		p = p[:len(p)-1] // remove terminating null
+	}
+	val = make([]string, 0, 5)
+	from := 0
+	for i, c := range p {
+		if c == 0 {
+			val = append(val, string(utf16.Decode(p[from:i])))
+			from = i + 1
+		}
+	}
+	return val, typ, nil
+}
+
+// GetIntegerValue retrieves the integer value for the specified
+// value name associated with an open key k. It also returns the value's type.
+// If value does not exist, GetIntegerValue returns ErrNotExist.
+// If value is not DWORD or QWORD, it will return the correct value
+// type and ErrUnexpectedType.
+func (k Key) GetIntegerValue(name string) (val uint64, valtype uint32, err error) {
+	data, typ, err2 := k.getValue(name, make([]byte, 8))
+	if err2 != nil {
+		return 0, typ, err2
+	}
+	switch typ {
+	case DWORD:
+		if len(data) != 4 {
+			return 0, typ, errors.New("DWORD value is not 4 bytes long")
+		}
+		return uint64(*(*uint32)(unsafe.Pointer(&data[0]))), DWORD, nil
+	case QWORD:
+		if len(data) != 8 {
+			return 0, typ, errors.New("QWORD value is not 8 bytes long")
+		}
+		return uint64(*(*uint64)(unsafe.Pointer(&data[0]))), QWORD, nil
+	default:
+		return 0, typ, ErrUnexpectedType
+	}
+}
+
+// GetBinaryValue retrieves the binary value for the specified
+// value name associated with an open key k. It also returns the value's type.
+// If value does not exist, GetBinaryValue returns ErrNotExist.
+// If value is not BINARY, it will return the correct value
+// type and ErrUnexpectedType.
+func (k Key) GetBinaryValue(name string) (val []byte, valtype uint32, err error) {
+	data, typ, err2 := k.getValue(name, make([]byte, 64))
+	if err2 != nil {
+		return nil, typ, err2
+	}
+	if typ != BINARY {
+		return nil, typ, ErrUnexpectedType
+	}
+	return data, typ, nil
+}
+
+func (k Key) setValue(name string, valtype uint32, data []byte) error {
+	p, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return err
+	}
+	if len(data) == 0 {
+		return regSetValueEx(syscall.Handle(k), p, 0, valtype, nil, 0)
+	}
+	return regSetValueEx(syscall.Handle(k), p, 0, valtype, &data[0], uint32(len(data)))
+}
+
+// SetDWordValue sets the data and type of a name value
+// under key k to value and DWORD.
+func (k Key) SetDWordValue(name string, value uint32) error {
+	return k.setValue(name, DWORD, (*[4]byte)(unsafe.Pointer(&value))[:])
+}
+
+// SetQWordValue sets the data and type of a name value
+// under key k to value and QWORD.
+func (k Key) SetQWordValue(name string, value uint64) error {
+	return k.setValue(name, QWORD, (*[8]byte)(unsafe.Pointer(&value))[:])
+}
+
+func (k Key) setStringValue(name string, valtype uint32, value string) error {
+	v, err := syscall.UTF16FromString(value)
+	if err != nil {
+		return err
+	}
+	buf := (*[1 << 29]byte)(unsafe.Pointer(&v[0]))[: len(v)*2 : len(v)*2]
+	return k.setValue(name, valtype, buf)
+}
+
+// SetStringValue sets the data and type of a name value
+// under key k to value and SZ. The value must not contain a zero byte.
+func (k Key) SetStringValue(name, value string) error {
+	return k.setStringValue(name, SZ, value)
+}
+
+// SetExpandStringValue sets the data and type of a name value
+// under key k to value and EXPAND_SZ. The value must not contain a zero byte.
+func (k Key) SetExpandStringValue(name, value string) error {
+	return k.setStringValue(name, EXPAND_SZ, value)
+}
+
+// SetStringsValue sets the data and type of a name value
+// under key k to value and MULTI_SZ. The value strings
+// must not contain a zero byte.
+func (k Key) SetStringsValue(name string, value []string) error {
+	ss := ""
+	for _, s := range value {
+		for i := 0; i < len(s); i++ {
+			if s[i] == 0 {
+				return errors.New("string cannot have 0 inside")
+			}
+		}
+		ss += s + "\x00"
+	}
+	v := utf16.Encode([]rune(ss + "\x00"))
+	buf := (*[1 << 29]byte)(unsafe.Pointer(&v[0]))[: len(v)*2 : len(v)*2]
+	return k.setValue(name, MULTI_SZ, buf)
+}
+
+// SetBinaryValue sets the data and type of a name value
+// under key k to value and BINARY.
+func (k Key) SetBinaryValue(name string, value []byte) error {
+	return k.setValue(name, BINARY, value)
+}
+
+// DeleteValue removes a named value from the key k.
+func (k Key) DeleteValue(name string) error {
+	return regDeleteValue(syscall.Handle(k), syscall.StringToUTF16Ptr(name))
+}
+
+// ReadValueNames returns the value names of key k.
+func (k Key) ReadValueNames() ([]string, error) {
+	ki, err := k.Stat()
+	if err != nil {
+		return nil, err
+	}
+	names := make([]string, 0, ki.ValueCount)
+	buf := make([]uint16, ki.MaxValueNameLen+1) // extra room for terminating null character
+loopItems:
+	for i := uint32(0); ; i++ {
+		l := uint32(len(buf))
+		for {
+			err := regEnumValue(syscall.Handle(k), i, &buf[0], &l, nil, nil, nil, nil)
+			if err == nil {
+				break
+			}
+			if err == syscall.ERROR_MORE_DATA {
+				// Double buffer size and try again.
+				l = uint32(2 * len(buf))
+				buf = make([]uint16, l)
+				continue
+			}
+			if err == _ERROR_NO_MORE_ITEMS {
+				break loopItems
+			}
+			return names, err
+		}
+		names = append(names, syscall.UTF16ToString(buf[:l]))
+	}
+	return names, nil
+}
diff --git a/src/internal/syscall/windows/registry/zsyscall_windows.go b/src/internal/syscall/windows/registry/zsyscall_windows.go
new file mode 100644
index 0000000..cab1319
--- /dev/null
+++ b/src/internal/syscall/windows/registry/zsyscall_windows.go
@@ -0,0 +1,107 @@
+// Code generated by 'go generate'; DO NOT EDIT.
+
+package registry
+
+import (
+	"internal/syscall/windows/sysdll"
+	"syscall"
+	"unsafe"
+)
+
+var _ unsafe.Pointer
+
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoERROR_IO_PENDING = 997
+)
+
+var (
+	errERROR_IO_PENDING error = syscall.Errno(errnoERROR_IO_PENDING)
+	errERROR_EINVAL     error = syscall.EINVAL
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return errERROR_EINVAL
+	case errnoERROR_IO_PENDING:
+		return errERROR_IO_PENDING
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
+var (
+	modadvapi32 = syscall.NewLazyDLL(sysdll.Add("advapi32.dll"))
+	modkernel32 = syscall.NewLazyDLL(sysdll.Add("kernel32.dll"))
+
+	procRegCreateKeyExW           = modadvapi32.NewProc("RegCreateKeyExW")
+	procRegDeleteKeyW             = modadvapi32.NewProc("RegDeleteKeyW")
+	procRegDeleteValueW           = modadvapi32.NewProc("RegDeleteValueW")
+	procRegEnumValueW             = modadvapi32.NewProc("RegEnumValueW")
+	procRegLoadMUIStringW         = modadvapi32.NewProc("RegLoadMUIStringW")
+	procRegSetValueExW            = modadvapi32.NewProc("RegSetValueExW")
+	procExpandEnvironmentStringsW = modkernel32.NewProc("ExpandEnvironmentStringsW")
+)
+
+func regCreateKeyEx(key syscall.Handle, subkey *uint16, reserved uint32, class *uint16, options uint32, desired uint32, sa *syscall.SecurityAttributes, result *syscall.Handle, disposition *uint32) (regerrno error) {
+	r0, _, _ := syscall.Syscall9(procRegCreateKeyExW.Addr(), 9, uintptr(key), uintptr(unsafe.Pointer(subkey)), uintptr(reserved), uintptr(unsafe.Pointer(class)), uintptr(options), uintptr(desired), uintptr(unsafe.Pointer(sa)), uintptr(unsafe.Pointer(result)), uintptr(unsafe.Pointer(disposition)))
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func regDeleteKey(key syscall.Handle, subkey *uint16) (regerrno error) {
+	r0, _, _ := syscall.Syscall(procRegDeleteKeyW.Addr(), 2, uintptr(key), uintptr(unsafe.Pointer(subkey)), 0)
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func regDeleteValue(key syscall.Handle, name *uint16) (regerrno error) {
+	r0, _, _ := syscall.Syscall(procRegDeleteValueW.Addr(), 2, uintptr(key), uintptr(unsafe.Pointer(name)), 0)
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func regEnumValue(key syscall.Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, valtype *uint32, buf *byte, buflen *uint32) (regerrno error) {
+	r0, _, _ := syscall.Syscall9(procRegEnumValueW.Addr(), 8, uintptr(key), uintptr(index), uintptr(unsafe.Pointer(name)), uintptr(unsafe.Pointer(nameLen)), uintptr(unsafe.Pointer(reserved)), uintptr(unsafe.Pointer(valtype)), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(buflen)), 0)
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func regLoadMUIString(key syscall.Handle, name *uint16, buf *uint16, buflen uint32, buflenCopied *uint32, flags uint32, dir *uint16) (regerrno error) {
+	r0, _, _ := syscall.Syscall9(procRegLoadMUIStringW.Addr(), 7, uintptr(key), uintptr(unsafe.Pointer(name)), uintptr(unsafe.Pointer(buf)), uintptr(buflen), uintptr(unsafe.Pointer(buflenCopied)), uintptr(flags), uintptr(unsafe.Pointer(dir)), 0, 0)
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func regSetValueEx(key syscall.Handle, valueName *uint16, reserved uint32, vtype uint32, buf *byte, bufsize uint32) (regerrno error) {
+	r0, _, _ := syscall.Syscall6(procRegSetValueExW.Addr(), 6, uintptr(key), uintptr(unsafe.Pointer(valueName)), uintptr(reserved), uintptr(vtype), uintptr(unsafe.Pointer(buf)), uintptr(bufsize))
+	if r0 != 0 {
+		regerrno = syscall.Errno(r0)
+	}
+	return
+}
+
+func expandEnvironmentStrings(src *uint16, dst *uint16, size uint32) (n uint32, err error) {
+	r0, _, e1 := syscall.Syscall(procExpandEnvironmentStringsW.Addr(), 3, uintptr(unsafe.Pointer(src)), uintptr(unsafe.Pointer(dst)), uintptr(size))
+	n = uint32(r0)
+	if n == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
diff --git a/src/internal/syscall/windows/reparse_windows.go b/src/internal/syscall/windows/reparse_windows.go
new file mode 100644
index 0000000..6e11139
--- /dev/null
+++ b/src/internal/syscall/windows/reparse_windows.go
@@ -0,0 +1,90 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+const (
+	FSCTL_SET_REPARSE_POINT    = 0x000900A4
+	IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
+
+	SYMLINK_FLAG_RELATIVE = 1
+)
+
+// These structures are described
+// in https://msdn.microsoft.com/en-us/library/cc232007.aspx
+// and https://msdn.microsoft.com/en-us/library/cc232006.aspx.
+
+type REPARSE_DATA_BUFFER struct {
+	ReparseTag        uint32
+	ReparseDataLength uint16
+	Reserved          uint16
+	DUMMYUNIONNAME    byte
+}
+
+// REPARSE_DATA_BUFFER_HEADER is a common part of REPARSE_DATA_BUFFER structure.
+type REPARSE_DATA_BUFFER_HEADER struct {
+	ReparseTag uint32
+	// The size, in bytes, of the reparse data that follows
+	// the common portion of the REPARSE_DATA_BUFFER element.
+	// This value is the length of the data starting at the
+	// SubstituteNameOffset field.
+	ReparseDataLength uint16
+	Reserved          uint16
+}
+
+type SymbolicLinkReparseBuffer struct {
+	// The integer that contains the offset, in bytes,
+	// of the substitute name string in the PathBuffer array,
+	// computed as an offset from byte 0 of PathBuffer. Note that
+	// this offset must be divided by 2 to get the array index.
+	SubstituteNameOffset uint16
+	// The integer that contains the length, in bytes, of the
+	// substitute name string. If this string is null-terminated,
+	// SubstituteNameLength does not include the Unicode null character.
+	SubstituteNameLength uint16
+	// PrintNameOffset is similar to SubstituteNameOffset.
+	PrintNameOffset uint16
+	// PrintNameLength is similar to SubstituteNameLength.
+	PrintNameLength uint16
+	// Flags specifies whether the substitute name is a full path name or
+	// a path name relative to the directory containing the symbolic link.
+	Flags      uint32
+	PathBuffer [1]uint16
+}
+
+// Path returns path stored in rb.
+func (rb *SymbolicLinkReparseBuffer) Path() string {
+	n1 := rb.SubstituteNameOffset / 2
+	n2 := (rb.SubstituteNameOffset + rb.SubstituteNameLength) / 2
+	return syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))[n1:n2:n2])
+}
+
+type MountPointReparseBuffer struct {
+	// The integer that contains the offset, in bytes,
+	// of the substitute name string in the PathBuffer array,
+	// computed as an offset from byte 0 of PathBuffer. Note that
+	// this offset must be divided by 2 to get the array index.
+	SubstituteNameOffset uint16
+	// The integer that contains the length, in bytes, of the
+	// substitute name string. If this string is null-terminated,
+	// SubstituteNameLength does not include the Unicode null character.
+	SubstituteNameLength uint16
+	// PrintNameOffset is similar to SubstituteNameOffset.
+	PrintNameOffset uint16
+	// PrintNameLength is similar to SubstituteNameLength.
+	PrintNameLength uint16
+	PathBuffer      [1]uint16
+}
+
+// Path returns path stored in rb.
+func (rb *MountPointReparseBuffer) Path() string {
+	n1 := rb.SubstituteNameOffset / 2
+	n2 := (rb.SubstituteNameOffset + rb.SubstituteNameLength) / 2
+	return syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))[n1:n2:n2])
+}
diff --git a/src/internal/syscall/windows/security_windows.go b/src/internal/syscall/windows/security_windows.go
new file mode 100644
index 0000000..4a2dfc0
--- /dev/null
+++ b/src/internal/syscall/windows/security_windows.go
@@ -0,0 +1,128 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+const (
+	SecurityAnonymous      = 0
+	SecurityIdentification = 1
+	SecurityImpersonation  = 2
+	SecurityDelegation     = 3
+)
+
+//sys	ImpersonateSelf(impersonationlevel uint32) (err error) = advapi32.ImpersonateSelf
+//sys	RevertToSelf() (err error) = advapi32.RevertToSelf
+
+const (
+	TOKEN_ADJUST_PRIVILEGES = 0x0020
+	SE_PRIVILEGE_ENABLED    = 0x00000002
+)
+
+type LUID struct {
+	LowPart  uint32
+	HighPart int32
+}
+
+type LUID_AND_ATTRIBUTES struct {
+	Luid       LUID
+	Attributes uint32
+}
+
+type TOKEN_PRIVILEGES struct {
+	PrivilegeCount uint32
+	Privileges     [1]LUID_AND_ATTRIBUTES
+}
+
+//sys	OpenThreadToken(h syscall.Handle, access uint32, openasself bool, token *syscall.Token) (err error) = advapi32.OpenThreadToken
+//sys	LookupPrivilegeValue(systemname *uint16, name *uint16, luid *LUID) (err error) = advapi32.LookupPrivilegeValueW
+//sys	adjustTokenPrivileges(token syscall.Token, disableAllPrivileges bool, newstate *TOKEN_PRIVILEGES, buflen uint32, prevstate *TOKEN_PRIVILEGES, returnlen *uint32) (ret uint32, err error) [true] = advapi32.AdjustTokenPrivileges
+
+func AdjustTokenPrivileges(token syscall.Token, disableAllPrivileges bool, newstate *TOKEN_PRIVILEGES, buflen uint32, prevstate *TOKEN_PRIVILEGES, returnlen *uint32) error {
+	ret, err := adjustTokenPrivileges(token, disableAllPrivileges, newstate, buflen, prevstate, returnlen)
+	if ret == 0 {
+		// AdjustTokenPrivileges call failed
+		return err
+	}
+	// AdjustTokenPrivileges call succeeded
+	if err == syscall.EINVAL {
+		// GetLastError returned ERROR_SUCCESS
+		return nil
+	}
+	return err
+}
+
+//sys DuplicateTokenEx(hExistingToken syscall.Token, dwDesiredAccess uint32, lpTokenAttributes *syscall.SecurityAttributes, impersonationLevel uint32, tokenType TokenType, phNewToken *syscall.Token) (err error) = advapi32.DuplicateTokenEx
+//sys SetTokenInformation(tokenHandle syscall.Token, tokenInformationClass uint32, tokenInformation uintptr, tokenInformationLength uint32) (err error) = advapi32.SetTokenInformation
+
+type SID_AND_ATTRIBUTES struct {
+	Sid        *syscall.SID
+	Attributes uint32
+}
+
+type TOKEN_MANDATORY_LABEL struct {
+	Label SID_AND_ATTRIBUTES
+}
+
+func (tml *TOKEN_MANDATORY_LABEL) Size() uint32 {
+	return uint32(unsafe.Sizeof(TOKEN_MANDATORY_LABEL{})) + syscall.GetLengthSid(tml.Label.Sid)
+}
+
+const SE_GROUP_INTEGRITY = 0x00000020
+
+type TokenType uint32
+
+const (
+	TokenPrimary       TokenType = 1
+	TokenImpersonation TokenType = 2
+)
+
+//sys	GetProfilesDirectory(dir *uint16, dirLen *uint32) (err error) = userenv.GetProfilesDirectoryW
+
+const (
+	LG_INCLUDE_INDIRECT  = 0x1
+	MAX_PREFERRED_LENGTH = 0xFFFFFFFF
+)
+
+type LocalGroupUserInfo0 struct {
+	Name *uint16
+}
+
+type UserInfo4 struct {
+	Name            *uint16
+	Password        *uint16
+	PasswordAge     uint32
+	Priv            uint32
+	HomeDir         *uint16
+	Comment         *uint16
+	Flags           uint32
+	ScriptPath      *uint16
+	AuthFlags       uint32
+	FullName        *uint16
+	UsrComment      *uint16
+	Parms           *uint16
+	Workstations    *uint16
+	LastLogon       uint32
+	LastLogoff      uint32
+	AcctExpires     uint32
+	MaxStorage      uint32
+	UnitsPerWeek    uint32
+	LogonHours      *byte
+	BadPwCount      uint32
+	NumLogons       uint32
+	LogonServer     *uint16
+	CountryCode     uint32
+	CodePage        uint32
+	UserSid         *syscall.SID
+	PrimaryGroupID  uint32
+	Profile         *uint16
+	HomeDirDrive    *uint16
+	PasswordExpired uint32
+}
+
+//sys	NetUserGetLocalGroups(serverName *uint16, userName *uint16, level uint32, flags uint32, buf **byte, prefMaxLen uint32, entriesRead *uint32, totalEntries *uint32) (neterr error) = netapi32.NetUserGetLocalGroups
diff --git a/src/internal/syscall/windows/symlink_windows.go b/src/internal/syscall/windows/symlink_windows.go
new file mode 100644
index 0000000..b64d058
--- /dev/null
+++ b/src/internal/syscall/windows/symlink_windows.go
@@ -0,0 +1,39 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+import "syscall"
+
+const (
+	ERROR_INVALID_PARAMETER syscall.Errno = 87
+
+	// symlink support for CreateSymbolicLink() starting with Windows 10 (1703, v10.0.14972)
+	SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE = 0x2
+
+	// FileInformationClass values
+	FileBasicInfo                  = 0    // FILE_BASIC_INFO
+	FileStandardInfo               = 1    // FILE_STANDARD_INFO
+	FileNameInfo                   = 2    // FILE_NAME_INFO
+	FileStreamInfo                 = 7    // FILE_STREAM_INFO
+	FileCompressionInfo            = 8    // FILE_COMPRESSION_INFO
+	FileAttributeTagInfo           = 9    // FILE_ATTRIBUTE_TAG_INFO
+	FileIdBothDirectoryInfo        = 0xa  // FILE_ID_BOTH_DIR_INFO
+	FileIdBothDirectoryRestartInfo = 0xb  // FILE_ID_BOTH_DIR_INFO
+	FileRemoteProtocolInfo         = 0xd  // FILE_REMOTE_PROTOCOL_INFO
+	FileFullDirectoryInfo          = 0xe  // FILE_FULL_DIR_INFO
+	FileFullDirectoryRestartInfo   = 0xf  // FILE_FULL_DIR_INFO
+	FileStorageInfo                = 0x10 // FILE_STORAGE_INFO
+	FileAlignmentInfo              = 0x11 // FILE_ALIGNMENT_INFO
+	FileIdInfo                     = 0x12 // FILE_ID_INFO
+	FileIdExtdDirectoryInfo        = 0x13 // FILE_ID_EXTD_DIR_INFO
+	FileIdExtdDirectoryRestartInfo = 0x14 // FILE_ID_EXTD_DIR_INFO
+)
+
+type FILE_ATTRIBUTE_TAG_INFO struct {
+	FileAttributes uint32
+	ReparseTag     uint32
+}
+
+//sys	GetFileInformationByHandleEx(handle syscall.Handle, class uint32, info *byte, bufsize uint32) (err error)
diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
new file mode 100644
index 0000000..8ace2a2
--- /dev/null
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -0,0 +1,369 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package windows
+
+import (
+	"sync"
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+// UTF16PtrToString is like UTF16ToString, but takes *uint16
+// as a parameter instead of []uint16.
+func UTF16PtrToString(p *uint16) string {
+	if p == nil {
+		return ""
+	}
+	// Find NUL terminator.
+	end := unsafe.Pointer(p)
+	n := 0
+	for *(*uint16)(end) != 0 {
+		end = unsafe.Pointer(uintptr(end) + unsafe.Sizeof(*p))
+		n++
+	}
+	// Turn *uint16 into []uint16.
+	s := unsafe.Slice(p, n)
+	// Decode []uint16 into string.
+	return string(utf16.Decode(s))
+}
+
+const (
+	ERROR_BAD_LENGTH             syscall.Errno = 24
+	ERROR_SHARING_VIOLATION      syscall.Errno = 32
+	ERROR_LOCK_VIOLATION         syscall.Errno = 33
+	ERROR_NOT_SUPPORTED          syscall.Errno = 50
+	ERROR_CALL_NOT_IMPLEMENTED   syscall.Errno = 120
+	ERROR_INVALID_NAME           syscall.Errno = 123
+	ERROR_LOCK_FAILED            syscall.Errno = 167
+	ERROR_NO_UNICODE_TRANSLATION syscall.Errno = 1113
+)
+
+const GAA_FLAG_INCLUDE_PREFIX = 0x00000010
+
+const (
+	IF_TYPE_OTHER              = 1
+	IF_TYPE_ETHERNET_CSMACD    = 6
+	IF_TYPE_ISO88025_TOKENRING = 9
+	IF_TYPE_PPP                = 23
+	IF_TYPE_SOFTWARE_LOOPBACK  = 24
+	IF_TYPE_ATM                = 37
+	IF_TYPE_IEEE80211          = 71
+	IF_TYPE_TUNNEL             = 131
+	IF_TYPE_IEEE1394           = 144
+)
+
+type SocketAddress struct {
+	Sockaddr       *syscall.RawSockaddrAny
+	SockaddrLength int32
+}
+
+type IpAdapterUnicastAddress struct {
+	Length             uint32
+	Flags              uint32
+	Next               *IpAdapterUnicastAddress
+	Address            SocketAddress
+	PrefixOrigin       int32
+	SuffixOrigin       int32
+	DadState           int32
+	ValidLifetime      uint32
+	PreferredLifetime  uint32
+	LeaseLifetime      uint32
+	OnLinkPrefixLength uint8
+}
+
+type IpAdapterAnycastAddress struct {
+	Length  uint32
+	Flags   uint32
+	Next    *IpAdapterAnycastAddress
+	Address SocketAddress
+}
+
+type IpAdapterMulticastAddress struct {
+	Length  uint32
+	Flags   uint32
+	Next    *IpAdapterMulticastAddress
+	Address SocketAddress
+}
+
+type IpAdapterDnsServerAdapter struct {
+	Length   uint32
+	Reserved uint32
+	Next     *IpAdapterDnsServerAdapter
+	Address  SocketAddress
+}
+
+type IpAdapterPrefix struct {
+	Length       uint32
+	Flags        uint32
+	Next         *IpAdapterPrefix
+	Address      SocketAddress
+	PrefixLength uint32
+}
+
+type IpAdapterAddresses struct {
+	Length                uint32
+	IfIndex               uint32
+	Next                  *IpAdapterAddresses
+	AdapterName           *byte
+	FirstUnicastAddress   *IpAdapterUnicastAddress
+	FirstAnycastAddress   *IpAdapterAnycastAddress
+	FirstMulticastAddress *IpAdapterMulticastAddress
+	FirstDnsServerAddress *IpAdapterDnsServerAdapter
+	DnsSuffix             *uint16
+	Description           *uint16
+	FriendlyName          *uint16
+	PhysicalAddress       [syscall.MAX_ADAPTER_ADDRESS_LENGTH]byte
+	PhysicalAddressLength uint32
+	Flags                 uint32
+	Mtu                   uint32
+	IfType                uint32
+	OperStatus            uint32
+	Ipv6IfIndex           uint32
+	ZoneIndices           [16]uint32
+	FirstPrefix           *IpAdapterPrefix
+	/* more fields might be present here. */
+}
+
+type FILE_BASIC_INFO struct {
+	CreationTime   syscall.Filetime
+	LastAccessTime syscall.Filetime
+	LastWriteTime  syscall.Filetime
+	ChangedTime    syscall.Filetime
+	FileAttributes uint32
+}
+
+const (
+	IfOperStatusUp             = 1
+	IfOperStatusDown           = 2
+	IfOperStatusTesting        = 3
+	IfOperStatusUnknown        = 4
+	IfOperStatusDormant        = 5
+	IfOperStatusNotPresent     = 6
+	IfOperStatusLowerLayerDown = 7
+)
+
+//sys	GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapterAddresses *IpAdapterAddresses, sizePointer *uint32) (errcode error) = iphlpapi.GetAdaptersAddresses
+//sys	GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) = GetComputerNameExW
+//sys	MoveFileEx(from *uint16, to *uint16, flags uint32) (err error) = MoveFileExW
+//sys	GetModuleFileName(module syscall.Handle, fn *uint16, len uint32) (n uint32, err error) = kernel32.GetModuleFileNameW
+//sys	SetFileInformationByHandle(handle syscall.Handle, fileInformationClass uint32, buf uintptr, bufsize uint32) (err error) = kernel32.SetFileInformationByHandle
+//sys	VirtualQuery(address uintptr, buffer *MemoryBasicInformation, length uintptr) (err error) = kernel32.VirtualQuery
+
+const (
+	// flags for CreateToolhelp32Snapshot
+	TH32CS_SNAPMODULE   = 0x08
+	TH32CS_SNAPMODULE32 = 0x10
+)
+
+const MAX_MODULE_NAME32 = 255
+
+type ModuleEntry32 struct {
+	Size         uint32
+	ModuleID     uint32
+	ProcessID    uint32
+	GlblcntUsage uint32
+	ProccntUsage uint32
+	ModBaseAddr  uintptr
+	ModBaseSize  uint32
+	ModuleHandle syscall.Handle
+	Module       [MAX_MODULE_NAME32 + 1]uint16
+	ExePath      [syscall.MAX_PATH]uint16
+}
+
+const SizeofModuleEntry32 = unsafe.Sizeof(ModuleEntry32{})
+
+//sys	Module32First(snapshot syscall.Handle, moduleEntry *ModuleEntry32) (err error) = kernel32.Module32FirstW
+//sys	Module32Next(snapshot syscall.Handle, moduleEntry *ModuleEntry32) (err error) = kernel32.Module32NextW
+
+const (
+	WSA_FLAG_OVERLAPPED        = 0x01
+	WSA_FLAG_NO_HANDLE_INHERIT = 0x80
+
+	WSAEMSGSIZE syscall.Errno = 10040
+
+	MSG_PEEK   = 0x2
+	MSG_TRUNC  = 0x0100
+	MSG_CTRUNC = 0x0200
+
+	socket_error = uintptr(^uint32(0))
+)
+
+var WSAID_WSASENDMSG = syscall.GUID{
+	Data1: 0xa441e712,
+	Data2: 0x754f,
+	Data3: 0x43ca,
+	Data4: [8]byte{0x84, 0xa7, 0x0d, 0xee, 0x44, 0xcf, 0x60, 0x6d},
+}
+
+var WSAID_WSARECVMSG = syscall.GUID{
+	Data1: 0xf689d7c8,
+	Data2: 0x6f1f,
+	Data3: 0x436b,
+	Data4: [8]byte{0x8a, 0x53, 0xe5, 0x4f, 0xe3, 0x51, 0xc3, 0x22},
+}
+
+var sendRecvMsgFunc struct {
+	once     sync.Once
+	sendAddr uintptr
+	recvAddr uintptr
+	err      error
+}
+
+type WSAMsg struct {
+	Name        syscall.Pointer
+	Namelen     int32
+	Buffers     *syscall.WSABuf
+	BufferCount uint32
+	Control     syscall.WSABuf
+	Flags       uint32
+}
+
+//sys	WSASocket(af int32, typ int32, protocol int32, protinfo *syscall.WSAProtocolInfo, group uint32, flags uint32) (handle syscall.Handle, err error) [failretval==syscall.InvalidHandle] = ws2_32.WSASocketW
+
+func loadWSASendRecvMsg() error {
+	sendRecvMsgFunc.once.Do(func() {
+		var s syscall.Handle
+		s, sendRecvMsgFunc.err = syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, syscall.IPPROTO_UDP)
+		if sendRecvMsgFunc.err != nil {
+			return
+		}
+		defer syscall.CloseHandle(s)
+		var n uint32
+		sendRecvMsgFunc.err = syscall.WSAIoctl(s,
+			syscall.SIO_GET_EXTENSION_FUNCTION_POINTER,
+			(*byte)(unsafe.Pointer(&WSAID_WSARECVMSG)),
+			uint32(unsafe.Sizeof(WSAID_WSARECVMSG)),
+			(*byte)(unsafe.Pointer(&sendRecvMsgFunc.recvAddr)),
+			uint32(unsafe.Sizeof(sendRecvMsgFunc.recvAddr)),
+			&n, nil, 0)
+		if sendRecvMsgFunc.err != nil {
+			return
+		}
+		sendRecvMsgFunc.err = syscall.WSAIoctl(s,
+			syscall.SIO_GET_EXTENSION_FUNCTION_POINTER,
+			(*byte)(unsafe.Pointer(&WSAID_WSASENDMSG)),
+			uint32(unsafe.Sizeof(WSAID_WSASENDMSG)),
+			(*byte)(unsafe.Pointer(&sendRecvMsgFunc.sendAddr)),
+			uint32(unsafe.Sizeof(sendRecvMsgFunc.sendAddr)),
+			&n, nil, 0)
+	})
+	return sendRecvMsgFunc.err
+}
+
+func WSASendMsg(fd syscall.Handle, msg *WSAMsg, flags uint32, bytesSent *uint32, overlapped *syscall.Overlapped, croutine *byte) error {
+	err := loadWSASendRecvMsg()
+	if err != nil {
+		return err
+	}
+	r1, _, e1 := syscall.Syscall6(sendRecvMsgFunc.sendAddr, 6, uintptr(fd), uintptr(unsafe.Pointer(msg)), uintptr(flags), uintptr(unsafe.Pointer(bytesSent)), uintptr(unsafe.Pointer(overlapped)), uintptr(unsafe.Pointer(croutine)))
+	if r1 == socket_error {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return err
+}
+
+func WSARecvMsg(fd syscall.Handle, msg *WSAMsg, bytesReceived *uint32, overlapped *syscall.Overlapped, croutine *byte) error {
+	err := loadWSASendRecvMsg()
+	if err != nil {
+		return err
+	}
+	r1, _, e1 := syscall.Syscall6(sendRecvMsgFunc.recvAddr, 5, uintptr(fd), uintptr(unsafe.Pointer(msg)), uintptr(unsafe.Pointer(bytesReceived)), uintptr(unsafe.Pointer(overlapped)), uintptr(unsafe.Pointer(croutine)), 0)
+	if r1 == socket_error {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return err
+}
+
+const (
+	ComputerNameNetBIOS                   = 0
+	ComputerNameDnsHostname               = 1
+	ComputerNameDnsDomain                 = 2
+	ComputerNameDnsFullyQualified         = 3
+	ComputerNamePhysicalNetBIOS           = 4
+	ComputerNamePhysicalDnsHostname       = 5
+	ComputerNamePhysicalDnsDomain         = 6
+	ComputerNamePhysicalDnsFullyQualified = 7
+	ComputerNameMax                       = 8
+
+	MOVEFILE_REPLACE_EXISTING      = 0x1
+	MOVEFILE_COPY_ALLOWED          = 0x2
+	MOVEFILE_DELAY_UNTIL_REBOOT    = 0x4
+	MOVEFILE_WRITE_THROUGH         = 0x8
+	MOVEFILE_CREATE_HARDLINK       = 0x10
+	MOVEFILE_FAIL_IF_NOT_TRACKABLE = 0x20
+)
+
+func Rename(oldpath, newpath string) error {
+	from, err := syscall.UTF16PtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	to, err := syscall.UTF16PtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	return MoveFileEx(from, to, MOVEFILE_REPLACE_EXISTING)
+}
+
+//sys LockFileEx(file syscall.Handle, flags uint32, reserved uint32, bytesLow uint32, bytesHigh uint32, overlapped *syscall.Overlapped) (err error) = kernel32.LockFileEx
+//sys UnlockFileEx(file syscall.Handle, reserved uint32, bytesLow uint32, bytesHigh uint32, overlapped *syscall.Overlapped) (err error) = kernel32.UnlockFileEx
+
+const (
+	LOCKFILE_FAIL_IMMEDIATELY = 0x00000001
+	LOCKFILE_EXCLUSIVE_LOCK   = 0x00000002
+)
+
+const MB_ERR_INVALID_CHARS = 8
+
+//sys	GetACP() (acp uint32) = kernel32.GetACP
+//sys	GetConsoleCP() (ccp uint32) = kernel32.GetConsoleCP
+//sys	MultiByteToWideChar(codePage uint32, dwFlags uint32, str *byte, nstr int32, wchar *uint16, nwchar int32) (nwrite int32, err error) = kernel32.MultiByteToWideChar
+//sys	GetCurrentThread() (pseudoHandle syscall.Handle, err error) = kernel32.GetCurrentThread
+
+const STYPE_DISKTREE = 0x00
+
+type SHARE_INFO_2 struct {
+	Netname     *uint16
+	Type        uint32
+	Remark      *uint16
+	Permissions uint32
+	MaxUses     uint32
+	CurrentUses uint32
+	Path        *uint16
+	Passwd      *uint16
+}
+
+//sys  NetShareAdd(serverName *uint16, level uint32, buf *byte, parmErr *uint16) (neterr error) = netapi32.NetShareAdd
+//sys  NetShareDel(serverName *uint16, netName *uint16, reserved uint32) (neterr error) = netapi32.NetShareDel
+
+const (
+	FILE_NAME_NORMALIZED = 0x0
+	FILE_NAME_OPENED     = 0x8
+
+	VOLUME_NAME_DOS  = 0x0
+	VOLUME_NAME_GUID = 0x1
+	VOLUME_NAME_NONE = 0x4
+	VOLUME_NAME_NT   = 0x2
+)
+
+//sys	GetFinalPathNameByHandle(file syscall.Handle, filePath *uint16, filePathSize uint32, flags uint32) (n uint32, err error) = kernel32.GetFinalPathNameByHandleW
+
+func LoadGetFinalPathNameByHandle() error {
+	return procGetFinalPathNameByHandleW.Find()
+}
+
+//sys	CreateEnvironmentBlock(block **uint16, token syscall.Token, inheritExisting bool) (err error) = userenv.CreateEnvironmentBlock
+//sys	DestroyEnvironmentBlock(block *uint16) (err error) = userenv.DestroyEnvironmentBlock
+
+//sys	RtlGenRandom(buf []byte) (err error) = advapi32.SystemFunction036
diff --git a/src/internal/syscall/windows/sysdll/sysdll.go b/src/internal/syscall/windows/sysdll/sysdll.go
new file mode 100644
index 0000000..e79fd19
--- /dev/null
+++ b/src/internal/syscall/windows/sysdll/sysdll.go
@@ -0,0 +1,30 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+// Package sysdll is an internal leaf package that records and reports
+// which Windows DLL names are used by Go itself. These DLLs are then
+// only loaded from the System32 directory. See Issue 14959.
+package sysdll
+
+// IsSystemDLL reports whether the named dll key (a base name, like
+// "foo.dll") is a system DLL which should only be loaded from the
+// Windows SYSTEM32 directory.
+//
+// Filenames are case sensitive, but that doesn't matter because
+// the case registered with Add is also the same case used with
+// LoadDLL later.
+//
+// It has no associated mutex and should only be mutated serially
+// (currently: during init), and not concurrent with DLL loading.
+var IsSystemDLL = map[string]bool{}
+
+// Add notes that dll is a system32 DLL which should only be loaded
+// from the Windows SYSTEM32 directory. It returns its argument back,
+// for ease of use in generated code.
+func Add(dll string) string {
+	IsSystemDLL[dll] = true
+	return dll
+}
diff --git a/src/internal/syscall/windows/zsyscall_windows.go b/src/internal/syscall/windows/zsyscall_windows.go
new file mode 100644
index 0000000..afd64e3
--- /dev/null
+++ b/src/internal/syscall/windows/zsyscall_windows.go
@@ -0,0 +1,354 @@
+// Code generated by 'go generate'; DO NOT EDIT.
+
+package windows
+
+import (
+	"internal/syscall/windows/sysdll"
+	"syscall"
+	"unsafe"
+)
+
+var _ unsafe.Pointer
+
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoERROR_IO_PENDING = 997
+)
+
+var (
+	errERROR_IO_PENDING error = syscall.Errno(errnoERROR_IO_PENDING)
+	errERROR_EINVAL     error = syscall.EINVAL
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return errERROR_EINVAL
+	case errnoERROR_IO_PENDING:
+		return errERROR_IO_PENDING
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
+var (
+	modadvapi32 = syscall.NewLazyDLL(sysdll.Add("advapi32.dll"))
+	modiphlpapi = syscall.NewLazyDLL(sysdll.Add("iphlpapi.dll"))
+	modkernel32 = syscall.NewLazyDLL(sysdll.Add("kernel32.dll"))
+	modnetapi32 = syscall.NewLazyDLL(sysdll.Add("netapi32.dll"))
+	modpsapi    = syscall.NewLazyDLL(sysdll.Add("psapi.dll"))
+	moduserenv  = syscall.NewLazyDLL(sysdll.Add("userenv.dll"))
+	modws2_32   = syscall.NewLazyDLL(sysdll.Add("ws2_32.dll"))
+
+	procAdjustTokenPrivileges        = modadvapi32.NewProc("AdjustTokenPrivileges")
+	procDuplicateTokenEx             = modadvapi32.NewProc("DuplicateTokenEx")
+	procImpersonateSelf              = modadvapi32.NewProc("ImpersonateSelf")
+	procLookupPrivilegeValueW        = modadvapi32.NewProc("LookupPrivilegeValueW")
+	procOpenThreadToken              = modadvapi32.NewProc("OpenThreadToken")
+	procRevertToSelf                 = modadvapi32.NewProc("RevertToSelf")
+	procSetTokenInformation          = modadvapi32.NewProc("SetTokenInformation")
+	procSystemFunction036            = modadvapi32.NewProc("SystemFunction036")
+	procGetAdaptersAddresses         = modiphlpapi.NewProc("GetAdaptersAddresses")
+	procGetACP                       = modkernel32.NewProc("GetACP")
+	procGetComputerNameExW           = modkernel32.NewProc("GetComputerNameExW")
+	procGetConsoleCP                 = modkernel32.NewProc("GetConsoleCP")
+	procGetCurrentThread             = modkernel32.NewProc("GetCurrentThread")
+	procGetFileInformationByHandleEx = modkernel32.NewProc("GetFileInformationByHandleEx")
+	procGetFinalPathNameByHandleW    = modkernel32.NewProc("GetFinalPathNameByHandleW")
+	procGetModuleFileNameW           = modkernel32.NewProc("GetModuleFileNameW")
+	procLockFileEx                   = modkernel32.NewProc("LockFileEx")
+	procModule32FirstW               = modkernel32.NewProc("Module32FirstW")
+	procModule32NextW                = modkernel32.NewProc("Module32NextW")
+	procMoveFileExW                  = modkernel32.NewProc("MoveFileExW")
+	procMultiByteToWideChar          = modkernel32.NewProc("MultiByteToWideChar")
+	procSetFileInformationByHandle   = modkernel32.NewProc("SetFileInformationByHandle")
+	procUnlockFileEx                 = modkernel32.NewProc("UnlockFileEx")
+	procVirtualQuery                 = modkernel32.NewProc("VirtualQuery")
+	procNetShareAdd                  = modnetapi32.NewProc("NetShareAdd")
+	procNetShareDel                  = modnetapi32.NewProc("NetShareDel")
+	procNetUserGetLocalGroups        = modnetapi32.NewProc("NetUserGetLocalGroups")
+	procGetProcessMemoryInfo         = modpsapi.NewProc("GetProcessMemoryInfo")
+	procCreateEnvironmentBlock       = moduserenv.NewProc("CreateEnvironmentBlock")
+	procDestroyEnvironmentBlock      = moduserenv.NewProc("DestroyEnvironmentBlock")
+	procGetProfilesDirectoryW        = moduserenv.NewProc("GetProfilesDirectoryW")
+	procWSASocketW                   = modws2_32.NewProc("WSASocketW")
+)
+
+func adjustTokenPrivileges(token syscall.Token, disableAllPrivileges bool, newstate *TOKEN_PRIVILEGES, buflen uint32, prevstate *TOKEN_PRIVILEGES, returnlen *uint32) (ret uint32, err error) {
+	var _p0 uint32
+	if disableAllPrivileges {
+		_p0 = 1
+	}
+	r0, _, e1 := syscall.Syscall6(procAdjustTokenPrivileges.Addr(), 6, uintptr(token), uintptr(_p0), uintptr(unsafe.Pointer(newstate)), uintptr(buflen), uintptr(unsafe.Pointer(prevstate)), uintptr(unsafe.Pointer(returnlen)))
+	ret = uint32(r0)
+	if true {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func DuplicateTokenEx(hExistingToken syscall.Token, dwDesiredAccess uint32, lpTokenAttributes *syscall.SecurityAttributes, impersonationLevel uint32, tokenType TokenType, phNewToken *syscall.Token) (err error) {
+	r1, _, e1 := syscall.Syscall6(procDuplicateTokenEx.Addr(), 6, uintptr(hExistingToken), uintptr(dwDesiredAccess), uintptr(unsafe.Pointer(lpTokenAttributes)), uintptr(impersonationLevel), uintptr(tokenType), uintptr(unsafe.Pointer(phNewToken)))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func ImpersonateSelf(impersonationlevel uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procImpersonateSelf.Addr(), 1, uintptr(impersonationlevel), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func LookupPrivilegeValue(systemname *uint16, name *uint16, luid *LUID) (err error) {
+	r1, _, e1 := syscall.Syscall(procLookupPrivilegeValueW.Addr(), 3, uintptr(unsafe.Pointer(systemname)), uintptr(unsafe.Pointer(name)), uintptr(unsafe.Pointer(luid)))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func OpenThreadToken(h syscall.Handle, access uint32, openasself bool, token *syscall.Token) (err error) {
+	var _p0 uint32
+	if openasself {
+		_p0 = 1
+	}
+	r1, _, e1 := syscall.Syscall6(procOpenThreadToken.Addr(), 4, uintptr(h), uintptr(access), uintptr(_p0), uintptr(unsafe.Pointer(token)), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func RevertToSelf() (err error) {
+	r1, _, e1 := syscall.Syscall(procRevertToSelf.Addr(), 0, 0, 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func SetTokenInformation(tokenHandle syscall.Token, tokenInformationClass uint32, tokenInformation uintptr, tokenInformationLength uint32) (err error) {
+	r1, _, e1 := syscall.Syscall6(procSetTokenInformation.Addr(), 4, uintptr(tokenHandle), uintptr(tokenInformationClass), uintptr(tokenInformation), uintptr(tokenInformationLength), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func RtlGenRandom(buf []byte) (err error) {
+	var _p0 *byte
+	if len(buf) > 0 {
+		_p0 = &buf[0]
+	}
+	r1, _, e1 := syscall.Syscall(procSystemFunction036.Addr(), 2, uintptr(unsafe.Pointer(_p0)), uintptr(len(buf)), 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapterAddresses *IpAdapterAddresses, sizePointer *uint32) (errcode error) {
+	r0, _, _ := syscall.Syscall6(procGetAdaptersAddresses.Addr(), 5, uintptr(family), uintptr(flags), uintptr(reserved), uintptr(unsafe.Pointer(adapterAddresses)), uintptr(unsafe.Pointer(sizePointer)), 0)
+	if r0 != 0 {
+		errcode = syscall.Errno(r0)
+	}
+	return
+}
+
+func GetACP() (acp uint32) {
+	r0, _, _ := syscall.Syscall(procGetACP.Addr(), 0, 0, 0, 0)
+	acp = uint32(r0)
+	return
+}
+
+func GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procGetComputerNameExW.Addr(), 3, uintptr(nameformat), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(n)))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetConsoleCP() (ccp uint32) {
+	r0, _, _ := syscall.Syscall(procGetConsoleCP.Addr(), 0, 0, 0, 0)
+	ccp = uint32(r0)
+	return
+}
+
+func GetCurrentThread() (pseudoHandle syscall.Handle, err error) {
+	r0, _, e1 := syscall.Syscall(procGetCurrentThread.Addr(), 0, 0, 0, 0)
+	pseudoHandle = syscall.Handle(r0)
+	if pseudoHandle == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetFileInformationByHandleEx(handle syscall.Handle, class uint32, info *byte, bufsize uint32) (err error) {
+	r1, _, e1 := syscall.Syscall6(procGetFileInformationByHandleEx.Addr(), 4, uintptr(handle), uintptr(class), uintptr(unsafe.Pointer(info)), uintptr(bufsize), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetFinalPathNameByHandle(file syscall.Handle, filePath *uint16, filePathSize uint32, flags uint32) (n uint32, err error) {
+	r0, _, e1 := syscall.Syscall6(procGetFinalPathNameByHandleW.Addr(), 4, uintptr(file), uintptr(unsafe.Pointer(filePath)), uintptr(filePathSize), uintptr(flags), 0, 0)
+	n = uint32(r0)
+	if n == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetModuleFileName(module syscall.Handle, fn *uint16, len uint32) (n uint32, err error) {
+	r0, _, e1 := syscall.Syscall(procGetModuleFileNameW.Addr(), 3, uintptr(module), uintptr(unsafe.Pointer(fn)), uintptr(len))
+	n = uint32(r0)
+	if n == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func LockFileEx(file syscall.Handle, flags uint32, reserved uint32, bytesLow uint32, bytesHigh uint32, overlapped *syscall.Overlapped) (err error) {
+	r1, _, e1 := syscall.Syscall6(procLockFileEx.Addr(), 6, uintptr(file), uintptr(flags), uintptr(reserved), uintptr(bytesLow), uintptr(bytesHigh), uintptr(unsafe.Pointer(overlapped)))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func Module32First(snapshot syscall.Handle, moduleEntry *ModuleEntry32) (err error) {
+	r1, _, e1 := syscall.Syscall(procModule32FirstW.Addr(), 2, uintptr(snapshot), uintptr(unsafe.Pointer(moduleEntry)), 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func Module32Next(snapshot syscall.Handle, moduleEntry *ModuleEntry32) (err error) {
+	r1, _, e1 := syscall.Syscall(procModule32NextW.Addr(), 2, uintptr(snapshot), uintptr(unsafe.Pointer(moduleEntry)), 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func MoveFileEx(from *uint16, to *uint16, flags uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procMoveFileExW.Addr(), 3, uintptr(unsafe.Pointer(from)), uintptr(unsafe.Pointer(to)), uintptr(flags))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func MultiByteToWideChar(codePage uint32, dwFlags uint32, str *byte, nstr int32, wchar *uint16, nwchar int32) (nwrite int32, err error) {
+	r0, _, e1 := syscall.Syscall6(procMultiByteToWideChar.Addr(), 6, uintptr(codePage), uintptr(dwFlags), uintptr(unsafe.Pointer(str)), uintptr(nstr), uintptr(unsafe.Pointer(wchar)), uintptr(nwchar))
+	nwrite = int32(r0)
+	if nwrite == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func SetFileInformationByHandle(handle syscall.Handle, fileInformationClass uint32, buf uintptr, bufsize uint32) (err error) {
+	r1, _, e1 := syscall.Syscall6(procSetFileInformationByHandle.Addr(), 4, uintptr(handle), uintptr(fileInformationClass), uintptr(buf), uintptr(bufsize), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func UnlockFileEx(file syscall.Handle, reserved uint32, bytesLow uint32, bytesHigh uint32, overlapped *syscall.Overlapped) (err error) {
+	r1, _, e1 := syscall.Syscall6(procUnlockFileEx.Addr(), 5, uintptr(file), uintptr(reserved), uintptr(bytesLow), uintptr(bytesHigh), uintptr(unsafe.Pointer(overlapped)), 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func VirtualQuery(address uintptr, buffer *MemoryBasicInformation, length uintptr) (err error) {
+	r1, _, e1 := syscall.Syscall(procVirtualQuery.Addr(), 3, uintptr(address), uintptr(unsafe.Pointer(buffer)), uintptr(length))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func NetShareAdd(serverName *uint16, level uint32, buf *byte, parmErr *uint16) (neterr error) {
+	r0, _, _ := syscall.Syscall6(procNetShareAdd.Addr(), 4, uintptr(unsafe.Pointer(serverName)), uintptr(level), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(parmErr)), 0, 0)
+	if r0 != 0 {
+		neterr = syscall.Errno(r0)
+	}
+	return
+}
+
+func NetShareDel(serverName *uint16, netName *uint16, reserved uint32) (neterr error) {
+	r0, _, _ := syscall.Syscall(procNetShareDel.Addr(), 3, uintptr(unsafe.Pointer(serverName)), uintptr(unsafe.Pointer(netName)), uintptr(reserved))
+	if r0 != 0 {
+		neterr = syscall.Errno(r0)
+	}
+	return
+}
+
+func NetUserGetLocalGroups(serverName *uint16, userName *uint16, level uint32, flags uint32, buf **byte, prefMaxLen uint32, entriesRead *uint32, totalEntries *uint32) (neterr error) {
+	r0, _, _ := syscall.Syscall9(procNetUserGetLocalGroups.Addr(), 8, uintptr(unsafe.Pointer(serverName)), uintptr(unsafe.Pointer(userName)), uintptr(level), uintptr(flags), uintptr(unsafe.Pointer(buf)), uintptr(prefMaxLen), uintptr(unsafe.Pointer(entriesRead)), uintptr(unsafe.Pointer(totalEntries)), 0)
+	if r0 != 0 {
+		neterr = syscall.Errno(r0)
+	}
+	return
+}
+
+func GetProcessMemoryInfo(handle syscall.Handle, memCounters *PROCESS_MEMORY_COUNTERS, cb uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procGetProcessMemoryInfo.Addr(), 3, uintptr(handle), uintptr(unsafe.Pointer(memCounters)), uintptr(cb))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func CreateEnvironmentBlock(block **uint16, token syscall.Token, inheritExisting bool) (err error) {
+	var _p0 uint32
+	if inheritExisting {
+		_p0 = 1
+	}
+	r1, _, e1 := syscall.Syscall(procCreateEnvironmentBlock.Addr(), 3, uintptr(unsafe.Pointer(block)), uintptr(token), uintptr(_p0))
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func DestroyEnvironmentBlock(block *uint16) (err error) {
+	r1, _, e1 := syscall.Syscall(procDestroyEnvironmentBlock.Addr(), 1, uintptr(unsafe.Pointer(block)), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func GetProfilesDirectory(dir *uint16, dirLen *uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procGetProfilesDirectoryW.Addr(), 2, uintptr(unsafe.Pointer(dir)), uintptr(unsafe.Pointer(dirLen)), 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func WSASocket(af int32, typ int32, protocol int32, protinfo *syscall.WSAProtocolInfo, group uint32, flags uint32) (handle syscall.Handle, err error) {
+	r0, _, e1 := syscall.Syscall6(procWSASocketW.Addr(), 6, uintptr(af), uintptr(typ), uintptr(protocol), uintptr(unsafe.Pointer(protinfo)), uintptr(group), uintptr(flags))
+	handle = syscall.Handle(r0)
+	if handle == syscall.InvalidHandle {
+		err = errnoErr(e1)
+	}
+	return
+}
diff --git a/src/internal/sysinfo/sysinfo.go b/src/internal/sysinfo/sysinfo.go
new file mode 100644
index 0000000..961be7a
--- /dev/null
+++ b/src/internal/sysinfo/sysinfo.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sysinfo implements high level hardware information gathering
+// that can be used for debugging or information purposes.
+package sysinfo
+
+import (
+	internalcpu "internal/cpu"
+	"sync"
+)
+
+type cpuInfo struct {
+	once sync.Once
+	name string
+}
+
+var CPU cpuInfo
+
+func (cpu *cpuInfo) Name() string {
+	cpu.once.Do(func() {
+		// Try to get the information from internal/cpu.
+		if name := internalcpu.Name(); name != "" {
+			cpu.name = name
+			return
+		}
+		// TODO(martisch): use /proc/cpuinfo and /sys/devices/system/cpu/ on Linux as fallback.
+	})
+	return cpu.name
+}
diff --git a/src/internal/testenv/exec.go b/src/internal/testenv/exec.go
new file mode 100644
index 0000000..77de59c
--- /dev/null
+++ b/src/internal/testenv/exec.go
@@ -0,0 +1,175 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testenv
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// HasExec reports whether the current system can start new processes
+// using os.StartProcess or (more commonly) exec.Command.
+func HasExec() bool {
+	switch runtime.GOOS {
+	case "js", "ios":
+		return false
+	}
+	return true
+}
+
+// MustHaveExec checks that the current system can start new processes
+// using os.StartProcess or (more commonly) exec.Command.
+// If not, MustHaveExec calls t.Skip with an explanation.
+func MustHaveExec(t testing.TB) {
+	if !HasExec() {
+		t.Skipf("skipping test: cannot exec subprocess on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
+var execPaths sync.Map // path -> error
+
+// MustHaveExecPath checks that the current system can start the named executable
+// using os.StartProcess or (more commonly) exec.Command.
+// If not, MustHaveExecPath calls t.Skip with an explanation.
+func MustHaveExecPath(t testing.TB, path string) {
+	MustHaveExec(t)
+
+	err, found := execPaths.Load(path)
+	if !found {
+		_, err = exec.LookPath(path)
+		err, _ = execPaths.LoadOrStore(path, err)
+	}
+	if err != nil {
+		t.Skipf("skipping test: %s: %s", path, err)
+	}
+}
+
+// CleanCmdEnv will fill cmd.Env with the environment, excluding certain
+// variables that could modify the behavior of the Go tools such as
+// GODEBUG and GOTRACEBACK.
+func CleanCmdEnv(cmd *exec.Cmd) *exec.Cmd {
+	if cmd.Env != nil {
+		panic("environment already set")
+	}
+	for _, env := range os.Environ() {
+		// Exclude GODEBUG from the environment to prevent its output
+		// from breaking tests that are trying to parse other command output.
+		if strings.HasPrefix(env, "GODEBUG=") {
+			continue
+		}
+		// Exclude GOTRACEBACK for the same reason.
+		if strings.HasPrefix(env, "GOTRACEBACK=") {
+			continue
+		}
+		cmd.Env = append(cmd.Env, env)
+	}
+	return cmd
+}
+
+// CommandContext is like exec.CommandContext, but:
+//   - skips t if the platform does not support os/exec,
+//   - sends SIGQUIT (if supported by the platform) instead of SIGKILL
+//     in its Cancel function
+//   - if the test has a deadline, adds a Context timeout and WaitDelay
+//     for an arbitrary grace period before the test's deadline expires,
+//   - fails the test if the command does not complete before the test's deadline, and
+//   - sets a Cleanup function that verifies that the test did not leak a subprocess.
+func CommandContext(t testing.TB, ctx context.Context, name string, args ...string) *exec.Cmd {
+	t.Helper()
+	MustHaveExec(t)
+
+	var (
+		cancelCtx   context.CancelFunc
+		gracePeriod time.Duration // unlimited unless the test has a deadline (to allow for interactive debugging)
+	)
+
+	if t, ok := t.(interface {
+		testing.TB
+		Deadline() (time.Time, bool)
+	}); ok {
+		if td, ok := t.Deadline(); ok {
+			// Start with a minimum grace period, just long enough to consume the
+			// output of a reasonable program after it terminates.
+			gracePeriod = 100 * time.Millisecond
+			if s := os.Getenv("GO_TEST_TIMEOUT_SCALE"); s != "" {
+				scale, err := strconv.Atoi(s)
+				if err != nil {
+					t.Fatalf("invalid GO_TEST_TIMEOUT_SCALE: %v", err)
+				}
+				gracePeriod *= time.Duration(scale)
+			}
+
+			// If time allows, increase the termination grace period to 5% of the
+			// test's remaining time.
+			testTimeout := time.Until(td)
+			if gp := testTimeout / 20; gp > gracePeriod {
+				gracePeriod = gp
+			}
+
+			// When we run commands that execute subprocesses, we want to reserve two
+			// grace periods to clean up: one for the delay between the first
+			// termination signal being sent (via the Cancel callback when the Context
+			// expires) and the process being forcibly terminated (via the WaitDelay
+			// field), and a second one for the delay becween the process being
+			// terminated and and the test logging its output for debugging.
+			//
+			// (We want to ensure that the test process itself has enough time to
+			// log the output before it is also terminated.)
+			cmdTimeout := testTimeout - 2*gracePeriod
+
+			if cd, ok := ctx.Deadline(); !ok || time.Until(cd) > cmdTimeout {
+				// Either ctx doesn't have a deadline, or its deadline would expire
+				// after (or too close before) the test has already timed out.
+				// Add a shorter timeout so that the test will produce useful output.
+				ctx, cancelCtx = context.WithTimeout(ctx, cmdTimeout)
+			}
+		}
+	}
+
+	cmd := exec.CommandContext(ctx, name, args...)
+	cmd.Cancel = func() error {
+		if cancelCtx != nil && ctx.Err() == context.DeadlineExceeded {
+			// The command timed out due to running too close to the test's deadline.
+			// There is no way the test did that intentionally — it's too close to the
+			// wire! — so mark it as a test failure. That way, if the test expects the
+			// command to fail for some other reason, it doesn't have to distinguish
+			// between that reason and a timeout.
+			t.Errorf("test timed out while running command: %v", cmd)
+		} else {
+			// The command is being terminated due to ctx being canceled, but
+			// apparently not due to an explicit test deadline that we added.
+			// Log that information in case it is useful for diagnosing a failure,
+			// but don't actually fail the test because of it.
+			t.Logf("%v: terminating command: %v", ctx.Err(), cmd)
+		}
+		return cmd.Process.Signal(Sigquit)
+	}
+	cmd.WaitDelay = gracePeriod
+
+	t.Cleanup(func() {
+		if cancelCtx != nil {
+			cancelCtx()
+		}
+		if cmd.Process != nil && cmd.ProcessState == nil {
+			t.Errorf("command was started, but test did not wait for it to complete: %v", cmd)
+		}
+	})
+
+	return cmd
+}
+
+// Command is like exec.Command, but applies the same changes as
+// testenv.CommandContext (with a default Context).
+func Command(t testing.TB, name string, args ...string) *exec.Cmd {
+	t.Helper()
+	return CommandContext(t, context.Background(), name, args...)
+}
diff --git a/src/internal/testenv/noopt.go b/src/internal/testenv/noopt.go
new file mode 100644
index 0000000..ae2a3d0
--- /dev/null
+++ b/src/internal/testenv/noopt.go
@@ -0,0 +1,12 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build noopt
+
+package testenv
+
+// OptimizationOff reports whether optimization is disabled.
+func OptimizationOff() bool {
+	return true
+}
diff --git a/src/internal/testenv/opt.go b/src/internal/testenv/opt.go
new file mode 100644
index 0000000..1bb96f7
--- /dev/null
+++ b/src/internal/testenv/opt.go
@@ -0,0 +1,12 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !noopt
+
+package testenv
+
+// OptimizationOff reports whether optimization is disabled.
+func OptimizationOff() bool {
+	return false
+}
diff --git a/src/internal/testenv/testenv.go b/src/internal/testenv/testenv.go
new file mode 100644
index 0000000..6a28b25
--- /dev/null
+++ b/src/internal/testenv/testenv.go
@@ -0,0 +1,364 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testenv provides information about what functionality
+// is available in different testing environments run by the Go team.
+//
+// It is an internal package because these details are specific
+// to the Go team's test setup (on build.golang.org) and not
+// fundamental to tests in general.
+package testenv
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"internal/cfg"
+	"internal/goroot"
+	"internal/platform"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// Builder reports the name of the builder running this test
+// (for example, "linux-amd64" or "windows-386-gce").
+// If the test is not running on the build infrastructure,
+// Builder returns the empty string.
+func Builder() string {
+	return os.Getenv("GO_BUILDER_NAME")
+}
+
+// HasGoBuild reports whether the current system can build programs with “go build”
+// and then run them with os.StartProcess or exec.Command.
+func HasGoBuild() bool {
+	if os.Getenv("GO_GCFLAGS") != "" {
+		// It's too much work to require every caller of the go command
+		// to pass along "-gcflags="+os.Getenv("GO_GCFLAGS").
+		// For now, if $GO_GCFLAGS is set, report that we simply can't
+		// run go build.
+		return false
+	}
+	switch runtime.GOOS {
+	case "android", "js", "ios":
+		return false
+	}
+	return true
+}
+
+// MustHaveGoBuild checks that the current system can build programs with “go build”
+// and then run them with os.StartProcess or exec.Command.
+// If not, MustHaveGoBuild calls t.Skip with an explanation.
+func MustHaveGoBuild(t testing.TB) {
+	if os.Getenv("GO_GCFLAGS") != "" {
+		t.Skipf("skipping test: 'go build' not compatible with setting $GO_GCFLAGS")
+	}
+	if !HasGoBuild() {
+		t.Skipf("skipping test: 'go build' not available on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
+// HasGoRun reports whether the current system can run programs with “go run.”
+func HasGoRun() bool {
+	// For now, having go run and having go build are the same.
+	return HasGoBuild()
+}
+
+// MustHaveGoRun checks that the current system can run programs with “go run.”
+// If not, MustHaveGoRun calls t.Skip with an explanation.
+func MustHaveGoRun(t testing.TB) {
+	if !HasGoRun() {
+		t.Skipf("skipping test: 'go run' not available on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
+// GoToolPath reports the path to the Go tool.
+// It is a convenience wrapper around GoTool.
+// If the tool is unavailable GoToolPath calls t.Skip.
+// If the tool should be available and isn't, GoToolPath calls t.Fatal.
+func GoToolPath(t testing.TB) string {
+	MustHaveGoBuild(t)
+	path, err := GoTool()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Add all environment variables that affect the Go command to test metadata.
+	// Cached test results will be invalidate when these variables change.
+	// See golang.org/issue/32285.
+	for _, envVar := range strings.Fields(cfg.KnownEnv) {
+		os.Getenv(envVar)
+	}
+	return path
+}
+
+var (
+	gorootOnce sync.Once
+	gorootPath string
+	gorootErr  error
+)
+
+func findGOROOT() (string, error) {
+	gorootOnce.Do(func() {
+		gorootPath = runtime.GOROOT()
+		if gorootPath != "" {
+			// If runtime.GOROOT() is non-empty, assume that it is valid.
+			//
+			// (It might not be: for example, the user may have explicitly set GOROOT
+			// to the wrong directory, or explicitly set GOROOT_FINAL but not GOROOT
+			// and hasn't moved the tree to GOROOT_FINAL yet. But those cases are
+			// rare, and if that happens the user can fix what they broke.)
+			return
+		}
+
+		// runtime.GOROOT doesn't know where GOROOT is (perhaps because the test
+		// binary was built with -trimpath, or perhaps because GOROOT_FINAL was set
+		// without GOROOT and the tree hasn't been moved there yet).
+		//
+		// Since this is internal/testenv, we can cheat and assume that the caller
+		// is a test of some package in a subdirectory of GOROOT/src. ('go test'
+		// runs the test in the directory containing the packaged under test.) That
+		// means that if we start walking up the tree, we should eventually find
+		// GOROOT/src/go.mod, and we can report the parent directory of that.
+
+		cwd, err := os.Getwd()
+		if err != nil {
+			gorootErr = fmt.Errorf("finding GOROOT: %w", err)
+			return
+		}
+
+		dir := cwd
+		for {
+			parent := filepath.Dir(dir)
+			if parent == dir {
+				// dir is either "." or only a volume name.
+				gorootErr = fmt.Errorf("failed to locate GOROOT/src in any parent directory")
+				return
+			}
+
+			if base := filepath.Base(dir); base != "src" {
+				dir = parent
+				continue // dir cannot be GOROOT/src if it doesn't end in "src".
+			}
+
+			b, err := os.ReadFile(filepath.Join(dir, "go.mod"))
+			if err != nil {
+				if os.IsNotExist(err) {
+					dir = parent
+					continue
+				}
+				gorootErr = fmt.Errorf("finding GOROOT: %w", err)
+				return
+			}
+			goMod := string(b)
+
+			for goMod != "" {
+				var line string
+				line, goMod, _ = strings.Cut(goMod, "\n")
+				fields := strings.Fields(line)
+				if len(fields) >= 2 && fields[0] == "module" && fields[1] == "std" {
+					// Found "module std", which is the module declaration in GOROOT/src!
+					gorootPath = parent
+					return
+				}
+			}
+		}
+	})
+
+	return gorootPath, gorootErr
+}
+
+// GOROOT reports the path to the directory containing the root of the Go
+// project source tree. This is normally equivalent to runtime.GOROOT, but
+// works even if the test binary was built with -trimpath.
+//
+// If GOROOT cannot be found, GOROOT skips t if t is non-nil,
+// or panics otherwise.
+func GOROOT(t testing.TB) string {
+	path, err := findGOROOT()
+	if err != nil {
+		if t == nil {
+			panic(err)
+		}
+		t.Helper()
+		t.Skip(err)
+	}
+	return path
+}
+
+// GoTool reports the path to the Go tool.
+func GoTool() (string, error) {
+	if !HasGoBuild() {
+		return "", errors.New("platform cannot run go tool")
+	}
+	var exeSuffix string
+	if runtime.GOOS == "windows" {
+		exeSuffix = ".exe"
+	}
+	goroot, err := findGOROOT()
+	if err != nil {
+		return "", fmt.Errorf("cannot find go tool: %w", err)
+	}
+	path := filepath.Join(goroot, "bin", "go"+exeSuffix)
+	if _, err := os.Stat(path); err == nil {
+		return path, nil
+	}
+	goBin, err := exec.LookPath("go" + exeSuffix)
+	if err != nil {
+		return "", errors.New("cannot find go tool: " + err.Error())
+	}
+	return goBin, nil
+}
+
+// HasSrc reports whether the entire source tree is available under GOROOT.
+func HasSrc() bool {
+	switch runtime.GOOS {
+	case "ios":
+		return false
+	}
+	return true
+}
+
+// HasExternalNetwork reports whether the current system can use
+// external (non-localhost) networks.
+func HasExternalNetwork() bool {
+	return !testing.Short() && runtime.GOOS != "js"
+}
+
+// MustHaveExternalNetwork checks that the current system can use
+// external (non-localhost) networks.
+// If not, MustHaveExternalNetwork calls t.Skip with an explanation.
+func MustHaveExternalNetwork(t testing.TB) {
+	if runtime.GOOS == "js" {
+		t.Skipf("skipping test: no external network on %s", runtime.GOOS)
+	}
+	if testing.Short() {
+		t.Skipf("skipping test: no external network in -short mode")
+	}
+}
+
+var haveCGO bool
+
+// HasCGO reports whether the current system can use cgo.
+func HasCGO() bool {
+	return haveCGO
+}
+
+// MustHaveCGO calls t.Skip if cgo is not available.
+func MustHaveCGO(t testing.TB) {
+	if !haveCGO {
+		t.Skipf("skipping test: no cgo")
+	}
+}
+
+// CanInternalLink reports whether the current system can link programs with
+// internal linking.
+func CanInternalLink() bool {
+	return !platform.MustLinkExternal(runtime.GOOS, runtime.GOARCH)
+}
+
+// MustInternalLink checks that the current system can link programs with internal
+// linking.
+// If not, MustInternalLink calls t.Skip with an explanation.
+func MustInternalLink(t testing.TB) {
+	if !CanInternalLink() {
+		t.Skipf("skipping test: internal linking on %s/%s is not supported", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
+// HasSymlink reports whether the current system can use os.Symlink.
+func HasSymlink() bool {
+	ok, _ := hasSymlink()
+	return ok
+}
+
+// MustHaveSymlink reports whether the current system can use os.Symlink.
+// If not, MustHaveSymlink calls t.Skip with an explanation.
+func MustHaveSymlink(t testing.TB) {
+	ok, reason := hasSymlink()
+	if !ok {
+		t.Skipf("skipping test: cannot make symlinks on %s/%s%s", runtime.GOOS, runtime.GOARCH, reason)
+	}
+}
+
+// HasLink reports whether the current system can use os.Link.
+func HasLink() bool {
+	// From Android release M (Marshmallow), hard linking files is blocked
+	// and an attempt to call link() on a file will return EACCES.
+	// - https://code.google.com/p/android-developer-preview/issues/detail?id=3150
+	return runtime.GOOS != "plan9" && runtime.GOOS != "android"
+}
+
+// MustHaveLink reports whether the current system can use os.Link.
+// If not, MustHaveLink calls t.Skip with an explanation.
+func MustHaveLink(t testing.TB) {
+	if !HasLink() {
+		t.Skipf("skipping test: hardlinks are not supported on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+}
+
+var flaky = flag.Bool("flaky", false, "run known-flaky tests too")
+
+func SkipFlaky(t testing.TB, issue int) {
+	t.Helper()
+	if !*flaky {
+		t.Skipf("skipping known flaky test without the -flaky flag; see golang.org/issue/%d", issue)
+	}
+}
+
+func SkipFlakyNet(t testing.TB) {
+	t.Helper()
+	if v, _ := strconv.ParseBool(os.Getenv("GO_BUILDER_FLAKY_NET")); v {
+		t.Skip("skipping test on builder known to have frequent network failures")
+	}
+}
+
+// CPUIsSlow reports whether the CPU running the test is suspected to be slow.
+func CPUIsSlow() bool {
+	switch runtime.GOARCH {
+	case "arm", "mips", "mipsle", "mips64", "mips64le":
+		return true
+	}
+	return false
+}
+
+// SkipIfShortAndSlow skips t if -short is set and the CPU running the test is
+// suspected to be slow.
+//
+// (This is useful for CPU-intensive tests that otherwise complete quickly.)
+func SkipIfShortAndSlow(t testing.TB) {
+	if testing.Short() && CPUIsSlow() {
+		t.Helper()
+		t.Skipf("skipping test in -short mode on %s", runtime.GOARCH)
+	}
+}
+
+// SkipIfOptimizationOff skips t if optimization is disabled.
+func SkipIfOptimizationOff(t testing.TB) {
+	if OptimizationOff() {
+		t.Helper()
+		t.Skip("skipping test with optimization disabled")
+	}
+}
+
+// WriteImportcfg writes an importcfg file used by the compiler or linker to
+// dstPath containing entries for the packages in std and cmd in addition
+// to the package to package file mappings in additionalPackageFiles.
+func WriteImportcfg(t testing.TB, dstPath string, additionalPackageFiles map[string]string) {
+	importcfg, err := goroot.Importcfg()
+	for k, v := range additionalPackageFiles {
+		importcfg += fmt.Sprintf("\npackagefile %s=%s", k, v)
+	}
+	if err != nil {
+		t.Fatalf("preparing the importcfg failed: %s", err)
+	}
+	err = os.WriteFile(dstPath, []byte(importcfg), 0655)
+	if err != nil {
+		t.Fatalf("writing the importcfg failed: %s", err)
+	}
+}
diff --git a/src/internal/testenv/testenv_cgo.go b/src/internal/testenv/testenv_cgo.go
new file mode 100644
index 0000000..7426a29
--- /dev/null
+++ b/src/internal/testenv/testenv_cgo.go
@@ -0,0 +1,11 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cgo
+
+package testenv
+
+func init() {
+	haveCGO = true
+}
diff --git a/src/internal/testenv/testenv_notunix.go b/src/internal/testenv/testenv_notunix.go
new file mode 100644
index 0000000..180206b
--- /dev/null
+++ b/src/internal/testenv/testenv_notunix.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows || plan9 || (js && wasm)
+
+package testenv
+
+import "os"
+
+// Sigquit is the signal to send to kill a hanging subprocess.
+// On Unix we send SIGQUIT, but on non-Unix we only have os.Kill.
+var Sigquit = os.Kill
diff --git a/src/internal/testenv/testenv_notwin.go b/src/internal/testenv/testenv_notwin.go
new file mode 100644
index 0000000..81171fd
--- /dev/null
+++ b/src/internal/testenv/testenv_notwin.go
@@ -0,0 +1,20 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package testenv
+
+import (
+	"runtime"
+)
+
+func hasSymlink() (ok bool, reason string) {
+	switch runtime.GOOS {
+	case "android", "plan9":
+		return false, ""
+	}
+
+	return true, ""
+}
diff --git a/src/internal/testenv/testenv_test.go b/src/internal/testenv/testenv_test.go
new file mode 100644
index 0000000..ebc27f1
--- /dev/null
+++ b/src/internal/testenv/testenv_test.go
@@ -0,0 +1,53 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testenv_test
+
+import (
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+func TestGoToolLocation(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var exeSuffix string
+	if runtime.GOOS == "windows" {
+		exeSuffix = ".exe"
+	}
+
+	// Tests are defined to run within their package source directory,
+	// and this package's source directory is $GOROOT/src/internal/testenv.
+	// The 'go' command is installed at $GOROOT/bin/go, so if the environment
+	// is correct then testenv.GoTool() should be identical to ../../../bin/go.
+
+	relWant := "../../../bin/go" + exeSuffix
+	absWant, err := filepath.Abs(relWant)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wantInfo, err := os.Stat(absWant)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("found go tool at %q (%q)", relWant, absWant)
+
+	goTool, err := testenv.GoTool()
+	if err != nil {
+		t.Fatalf("testenv.GoTool(): %v", err)
+	}
+	t.Logf("testenv.GoTool() = %q", goTool)
+
+	gotInfo, err := os.Stat(goTool)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !os.SameFile(wantInfo, gotInfo) {
+		t.Fatalf("%q is not the same file as %q", absWant, goTool)
+	}
+}
diff --git a/src/internal/testenv/testenv_unix.go b/src/internal/testenv/testenv_unix.go
new file mode 100644
index 0000000..a97e88d
--- /dev/null
+++ b/src/internal/testenv/testenv_unix.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package testenv
+
+import "syscall"
+
+// Sigquit is the signal to send to kill a hanging subprocess.
+// Send SIGQUIT to get a stack trace.
+var Sigquit = syscall.SIGQUIT
diff --git a/src/internal/testenv/testenv_windows.go b/src/internal/testenv/testenv_windows.go
new file mode 100644
index 0000000..4802b13
--- /dev/null
+++ b/src/internal/testenv/testenv_windows.go
@@ -0,0 +1,47 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testenv
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+	"syscall"
+)
+
+var symlinkOnce sync.Once
+var winSymlinkErr error
+
+func initWinHasSymlink() {
+	tmpdir, err := os.MkdirTemp("", "symtest")
+	if err != nil {
+		panic("failed to create temp directory: " + err.Error())
+	}
+	defer os.RemoveAll(tmpdir)
+
+	err = os.Symlink("target", filepath.Join(tmpdir, "symlink"))
+	if err != nil {
+		err = err.(*os.LinkError).Err
+		switch err {
+		case syscall.EWINDOWS, syscall.ERROR_PRIVILEGE_NOT_HELD:
+			winSymlinkErr = err
+		}
+	}
+}
+
+func hasSymlink() (ok bool, reason string) {
+	symlinkOnce.Do(initWinHasSymlink)
+
+	switch winSymlinkErr {
+	case nil:
+		return true, ""
+	case syscall.EWINDOWS:
+		return false, ": symlinks are not supported on your version of Windows"
+	case syscall.ERROR_PRIVILEGE_NOT_HELD:
+		return false, ": you don't have enough privileges to create symlinks"
+	}
+
+	return false, ""
+}
diff --git a/src/internal/testlog/exit.go b/src/internal/testlog/exit.go
new file mode 100644
index 0000000..e15defd
--- /dev/null
+++ b/src/internal/testlog/exit.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testlog
+
+import "sync"
+
+// PanicOnExit0 reports whether to panic on a call to os.Exit(0).
+// This is in the testlog package because, like other definitions in
+// package testlog, it is a hook between the testing package and the
+// os package. This is used to ensure that an early call to os.Exit(0)
+// does not cause a test to pass.
+func PanicOnExit0() bool {
+	panicOnExit0.mu.Lock()
+	defer panicOnExit0.mu.Unlock()
+	return panicOnExit0.val
+}
+
+// panicOnExit0 is the flag used for PanicOnExit0. This uses a lock
+// because the value can be cleared via a timer call that may race
+// with calls to os.Exit
+var panicOnExit0 struct {
+	mu  sync.Mutex
+	val bool
+}
+
+// SetPanicOnExit0 sets panicOnExit0 to v.
+func SetPanicOnExit0(v bool) {
+	panicOnExit0.mu.Lock()
+	defer panicOnExit0.mu.Unlock()
+	panicOnExit0.val = v
+}
diff --git a/src/internal/testlog/log.go b/src/internal/testlog/log.go
new file mode 100644
index 0000000..3c5f780
--- /dev/null
+++ b/src/internal/testlog/log.go
@@ -0,0 +1,69 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testlog provides a back-channel communication path
+// between tests and package os, so that cmd/go can see which
+// environment variables and files a test consults.
+package testlog
+
+import "sync/atomic"
+
+// Interface is the interface required of test loggers.
+// The os package will invoke the interface's methods to indicate that
+// it is inspecting the given environment variables or files.
+// Multiple goroutines may call these methods simultaneously.
+type Interface interface {
+	Getenv(key string)
+	Stat(file string)
+	Open(file string)
+	Chdir(dir string)
+}
+
+// logger is the current logger Interface.
+// We use an atomic.Value in case test startup
+// is racing with goroutines started during init.
+// That must not cause a race detector failure,
+// although it will still result in limited visibility
+// into exactly what those goroutines do.
+var logger atomic.Value
+
+// SetLogger sets the test logger implementation for the current process.
+// It must be called only once, at process startup.
+func SetLogger(impl Interface) {
+	if logger.Load() != nil {
+		panic("testlog: SetLogger must be called only once")
+	}
+	logger.Store(&impl)
+}
+
+// Logger returns the current test logger implementation.
+// It returns nil if there is no logger.
+func Logger() Interface {
+	impl := logger.Load()
+	if impl == nil {
+		return nil
+	}
+	return *impl.(*Interface)
+}
+
+// Getenv calls Logger().Getenv, if a logger has been set.
+func Getenv(name string) {
+	if log := Logger(); log != nil {
+		log.Getenv(name)
+	}
+}
+
+// Open calls Logger().Open, if a logger has been set.
+func Open(name string) {
+	if log := Logger(); log != nil {
+		log.Open(name)
+	}
+}
+
+// Stat calls Logger().Stat, if a logger has been set.
+func Stat(name string) {
+	if log := Logger(); log != nil {
+		log.Stat(name)
+	}
+}
diff --git a/src/internal/testpty/pty.go b/src/internal/testpty/pty.go
new file mode 100644
index 0000000..f0b2a33
--- /dev/null
+++ b/src/internal/testpty/pty.go
@@ -0,0 +1,38 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testpty is a simple pseudo-terminal package for Unix systems,
+// implemented by calling C functions via cgo.
+package testpty
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+type PtyError struct {
+	FuncName    string
+	ErrorString string
+	Errno       error
+}
+
+func ptyError(name string, err error) *PtyError {
+	return &PtyError{name, err.Error(), err}
+}
+
+func (e *PtyError) Error() string {
+	return fmt.Sprintf("%s: %s", e.FuncName, e.ErrorString)
+}
+
+func (e *PtyError) Unwrap() error { return e.Errno }
+
+var ErrNotSupported = errors.New("testpty.Open not implemented on this platform")
+
+// Open returns a control pty and the name of the linked process tty.
+//
+// If Open is not implemented on this platform, it returns ErrNotSupported.
+func Open() (pty *os.File, processTTY string, err error) {
+	return open()
+}
diff --git a/src/internal/testpty/pty_cgo.go b/src/internal/testpty/pty_cgo.go
new file mode 100644
index 0000000..442fbcf
--- /dev/null
+++ b/src/internal/testpty/pty_cgo.go
@@ -0,0 +1,34 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cgo && (aix || dragonfly || freebsd || (linux && !android) || netbsd || openbsd)
+
+package testpty
+
+/*
+#define _XOPEN_SOURCE 600
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+*/
+import "C"
+
+import "os"
+
+func open() (pty *os.File, processTTY string, err error) {
+	m, err := C.posix_openpt(C.O_RDWR)
+	if m < 0 {
+		return nil, "", ptyError("posix_openpt", err)
+	}
+	if res, err := C.grantpt(m); res < 0 {
+		C.close(m)
+		return nil, "", ptyError("grantpt", err)
+	}
+	if res, err := C.unlockpt(m); res < 0 {
+		C.close(m)
+		return nil, "", ptyError("unlockpt", err)
+	}
+	processTTY = C.GoString(C.ptsname(m))
+	return os.NewFile(uintptr(m), "pty"), processTTY, nil
+}
diff --git a/src/internal/testpty/pty_darwin.go b/src/internal/testpty/pty_darwin.go
new file mode 100644
index 0000000..f29517c
--- /dev/null
+++ b/src/internal/testpty/pty_darwin.go
@@ -0,0 +1,32 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testpty
+
+import (
+	"internal/syscall/unix"
+	"os"
+	"syscall"
+)
+
+func open() (pty *os.File, processTTY string, err error) {
+	m, err := unix.PosixOpenpt(syscall.O_RDWR)
+	if err != nil {
+		return nil, "", ptyError("posix_openpt", err)
+	}
+	if err := unix.Grantpt(m); err != nil {
+		syscall.Close(m)
+		return nil, "", ptyError("grantpt", err)
+	}
+	if err := unix.Unlockpt(m); err != nil {
+		syscall.Close(m)
+		return nil, "", ptyError("unlockpt", err)
+	}
+	processTTY, err = unix.Ptsname(m)
+	if err != nil {
+		syscall.Close(m)
+		return nil, "", ptyError("ptsname", err)
+	}
+	return os.NewFile(uintptr(m), "pty"), processTTY, nil
+}
diff --git a/src/internal/testpty/pty_none.go b/src/internal/testpty/pty_none.go
new file mode 100644
index 0000000..4f9e2b7
--- /dev/null
+++ b/src/internal/testpty/pty_none.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !(cgo && (aix || dragonfly || freebsd || (linux && !android) || netbsd || openbsd)) && !darwin
+
+package testpty
+
+import "os"
+
+func open() (pty *os.File, processTTY string, err error) {
+	return nil, "", ErrNotSupported
+}
diff --git a/src/internal/trace/gc.go b/src/internal/trace/gc.go
new file mode 100644
index 0000000..c1bc862
--- /dev/null
+++ b/src/internal/trace/gc.go
@@ -0,0 +1,825 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"container/heap"
+	"math"
+	"sort"
+	"strings"
+	"time"
+)
+
+// MutatorUtil is a change in mutator utilization at a particular
+// time. Mutator utilization functions are represented as a
+// time-ordered []MutatorUtil.
+type MutatorUtil struct {
+	Time int64
+	// Util is the mean mutator utilization starting at Time. This
+	// is in the range [0, 1].
+	Util float64
+}
+
+// UtilFlags controls the behavior of MutatorUtilization.
+type UtilFlags int
+
+const (
+	// UtilSTW means utilization should account for STW events.
+	UtilSTW UtilFlags = 1 << iota
+	// UtilBackground means utilization should account for
+	// background mark workers.
+	UtilBackground
+	// UtilAssist means utilization should account for mark
+	// assists.
+	UtilAssist
+	// UtilSweep means utilization should account for sweeping.
+	UtilSweep
+
+	// UtilPerProc means each P should be given a separate
+	// utilization function. Otherwise, there is a single function
+	// and each P is given a fraction of the utilization.
+	UtilPerProc
+)
+
+// MutatorUtilization returns a set of mutator utilization functions
+// for the given trace. Each function will always end with 0
+// utilization. The bounds of each function are implicit in the first
+// and last event; outside of these bounds each function is undefined.
+//
+// If the UtilPerProc flag is not given, this always returns a single
+// utilization function. Otherwise, it returns one function per P.
+func MutatorUtilization(events []*Event, flags UtilFlags) [][]MutatorUtil {
+	if len(events) == 0 {
+		return nil
+	}
+
+	type perP struct {
+		// gc > 0 indicates that GC is active on this P.
+		gc int
+		// series the logical series number for this P. This
+		// is necessary because Ps may be removed and then
+		// re-added, and then the new P needs a new series.
+		series int
+	}
+	ps := []perP{}
+	stw := 0
+
+	out := [][]MutatorUtil{}
+	assists := map[uint64]bool{}
+	block := map[uint64]*Event{}
+	bgMark := map[uint64]bool{}
+
+	for _, ev := range events {
+		switch ev.Type {
+		case EvGomaxprocs:
+			gomaxprocs := int(ev.Args[0])
+			if len(ps) > gomaxprocs {
+				if flags&UtilPerProc != 0 {
+					// End each P's series.
+					for _, p := range ps[gomaxprocs:] {
+						out[p.series] = addUtil(out[p.series], MutatorUtil{ev.Ts, 0})
+					}
+				}
+				ps = ps[:gomaxprocs]
+			}
+			for len(ps) < gomaxprocs {
+				// Start new P's series.
+				series := 0
+				if flags&UtilPerProc != 0 || len(out) == 0 {
+					series = len(out)
+					out = append(out, []MutatorUtil{{ev.Ts, 1}})
+				}
+				ps = append(ps, perP{series: series})
+			}
+		case EvGCSTWStart:
+			if flags&UtilSTW != 0 {
+				stw++
+			}
+		case EvGCSTWDone:
+			if flags&UtilSTW != 0 {
+				stw--
+			}
+		case EvGCMarkAssistStart:
+			if flags&UtilAssist != 0 {
+				ps[ev.P].gc++
+				assists[ev.G] = true
+			}
+		case EvGCMarkAssistDone:
+			if flags&UtilAssist != 0 {
+				ps[ev.P].gc--
+				delete(assists, ev.G)
+			}
+		case EvGCSweepStart:
+			if flags&UtilSweep != 0 {
+				ps[ev.P].gc++
+			}
+		case EvGCSweepDone:
+			if flags&UtilSweep != 0 {
+				ps[ev.P].gc--
+			}
+		case EvGoStartLabel:
+			if flags&UtilBackground != 0 && strings.HasPrefix(ev.SArgs[0], "GC ") && ev.SArgs[0] != "GC (idle)" {
+				// Background mark worker.
+				//
+				// If we're in per-proc mode, we don't
+				// count dedicated workers because
+				// they kick all of the goroutines off
+				// that P, so don't directly
+				// contribute to goroutine latency.
+				if !(flags&UtilPerProc != 0 && ev.SArgs[0] == "GC (dedicated)") {
+					bgMark[ev.G] = true
+					ps[ev.P].gc++
+				}
+			}
+			fallthrough
+		case EvGoStart:
+			if assists[ev.G] {
+				// Unblocked during assist.
+				ps[ev.P].gc++
+			}
+			block[ev.G] = ev.Link
+		default:
+			if ev != block[ev.G] {
+				continue
+			}
+
+			if assists[ev.G] {
+				// Blocked during assist.
+				ps[ev.P].gc--
+			}
+			if bgMark[ev.G] {
+				// Background mark worker done.
+				ps[ev.P].gc--
+				delete(bgMark, ev.G)
+			}
+			delete(block, ev.G)
+		}
+
+		if flags&UtilPerProc == 0 {
+			// Compute the current average utilization.
+			if len(ps) == 0 {
+				continue
+			}
+			gcPs := 0
+			if stw > 0 {
+				gcPs = len(ps)
+			} else {
+				for i := range ps {
+					if ps[i].gc > 0 {
+						gcPs++
+					}
+				}
+			}
+			mu := MutatorUtil{ev.Ts, 1 - float64(gcPs)/float64(len(ps))}
+
+			// Record the utilization change. (Since
+			// len(ps) == len(out), we know len(out) > 0.)
+			out[0] = addUtil(out[0], mu)
+		} else {
+			// Check for per-P utilization changes.
+			for i := range ps {
+				p := &ps[i]
+				util := 1.0
+				if stw > 0 || p.gc > 0 {
+					util = 0.0
+				}
+				out[p.series] = addUtil(out[p.series], MutatorUtil{ev.Ts, util})
+			}
+		}
+	}
+
+	// Add final 0 utilization event to any remaining series. This
+	// is important to mark the end of the trace. The exact value
+	// shouldn't matter since no window should extend beyond this,
+	// but using 0 is symmetric with the start of the trace.
+	mu := MutatorUtil{events[len(events)-1].Ts, 0}
+	for i := range ps {
+		out[ps[i].series] = addUtil(out[ps[i].series], mu)
+	}
+	return out
+}
+
+func addUtil(util []MutatorUtil, mu MutatorUtil) []MutatorUtil {
+	if len(util) > 0 {
+		if mu.Util == util[len(util)-1].Util {
+			// No change.
+			return util
+		}
+		if mu.Time == util[len(util)-1].Time {
+			// Take the lowest utilization at a time stamp.
+			if mu.Util < util[len(util)-1].Util {
+				util[len(util)-1] = mu
+			}
+			return util
+		}
+	}
+	return append(util, mu)
+}
+
+// totalUtil is total utilization, measured in nanoseconds. This is a
+// separate type primarily to distinguish it from mean utilization,
+// which is also a float64.
+type totalUtil float64
+
+func totalUtilOf(meanUtil float64, dur int64) totalUtil {
+	return totalUtil(meanUtil * float64(dur))
+}
+
+// mean returns the mean utilization over dur.
+func (u totalUtil) mean(dur time.Duration) float64 {
+	return float64(u) / float64(dur)
+}
+
+// An MMUCurve is the minimum mutator utilization curve across
+// multiple window sizes.
+type MMUCurve struct {
+	series []mmuSeries
+}
+
+type mmuSeries struct {
+	util []MutatorUtil
+	// sums[j] is the cumulative sum of util[:j].
+	sums []totalUtil
+	// bands summarizes util in non-overlapping bands of duration
+	// bandDur.
+	bands []mmuBand
+	// bandDur is the duration of each band.
+	bandDur int64
+}
+
+type mmuBand struct {
+	// minUtil is the minimum instantaneous mutator utilization in
+	// this band.
+	minUtil float64
+	// cumUtil is the cumulative total mutator utilization between
+	// time 0 and the left edge of this band.
+	cumUtil totalUtil
+
+	// integrator is the integrator for the left edge of this
+	// band.
+	integrator integrator
+}
+
+// NewMMUCurve returns an MMU curve for the given mutator utilization
+// function.
+func NewMMUCurve(utils [][]MutatorUtil) *MMUCurve {
+	series := make([]mmuSeries, len(utils))
+	for i, util := range utils {
+		series[i] = newMMUSeries(util)
+	}
+	return &MMUCurve{series}
+}
+
+// bandsPerSeries is the number of bands to divide each series into.
+// This is only changed by tests.
+var bandsPerSeries = 1000
+
+func newMMUSeries(util []MutatorUtil) mmuSeries {
+	// Compute cumulative sum.
+	sums := make([]totalUtil, len(util))
+	var prev MutatorUtil
+	var sum totalUtil
+	for j, u := range util {
+		sum += totalUtilOf(prev.Util, u.Time-prev.Time)
+		sums[j] = sum
+		prev = u
+	}
+
+	// Divide the utilization curve up into equal size
+	// non-overlapping "bands" and compute a summary for each of
+	// these bands.
+	//
+	// Compute the duration of each band.
+	numBands := bandsPerSeries
+	if numBands > len(util) {
+		// There's no point in having lots of bands if there
+		// aren't many events.
+		numBands = len(util)
+	}
+	dur := util[len(util)-1].Time - util[0].Time
+	bandDur := (dur + int64(numBands) - 1) / int64(numBands)
+	if bandDur < 1 {
+		bandDur = 1
+	}
+	// Compute the bands. There are numBands+1 bands in order to
+	// record the final cumulative sum.
+	bands := make([]mmuBand, numBands+1)
+	s := mmuSeries{util, sums, bands, bandDur}
+	leftSum := integrator{&s, 0}
+	for i := range bands {
+		startTime, endTime := s.bandTime(i)
+		cumUtil := leftSum.advance(startTime)
+		predIdx := leftSum.pos
+		minUtil := 1.0
+		for i := predIdx; i < len(util) && util[i].Time < endTime; i++ {
+			minUtil = math.Min(minUtil, util[i].Util)
+		}
+		bands[i] = mmuBand{minUtil, cumUtil, leftSum}
+	}
+
+	return s
+}
+
+func (s *mmuSeries) bandTime(i int) (start, end int64) {
+	start = int64(i)*s.bandDur + s.util[0].Time
+	end = start + s.bandDur
+	return
+}
+
+type bandUtil struct {
+	// Utilization series index
+	series int
+	// Band index
+	i int
+	// Lower bound of mutator utilization for all windows
+	// with a left edge in this band.
+	utilBound float64
+}
+
+type bandUtilHeap []bandUtil
+
+func (h bandUtilHeap) Len() int {
+	return len(h)
+}
+
+func (h bandUtilHeap) Less(i, j int) bool {
+	return h[i].utilBound < h[j].utilBound
+}
+
+func (h bandUtilHeap) Swap(i, j int) {
+	h[i], h[j] = h[j], h[i]
+}
+
+func (h *bandUtilHeap) Push(x any) {
+	*h = append(*h, x.(bandUtil))
+}
+
+func (h *bandUtilHeap) Pop() any {
+	x := (*h)[len(*h)-1]
+	*h = (*h)[:len(*h)-1]
+	return x
+}
+
+// UtilWindow is a specific window at Time.
+type UtilWindow struct {
+	Time int64
+	// MutatorUtil is the mean mutator utilization in this window.
+	MutatorUtil float64
+}
+
+type utilHeap []UtilWindow
+
+func (h utilHeap) Len() int {
+	return len(h)
+}
+
+func (h utilHeap) Less(i, j int) bool {
+	if h[i].MutatorUtil != h[j].MutatorUtil {
+		return h[i].MutatorUtil > h[j].MutatorUtil
+	}
+	return h[i].Time > h[j].Time
+}
+
+func (h utilHeap) Swap(i, j int) {
+	h[i], h[j] = h[j], h[i]
+}
+
+func (h *utilHeap) Push(x any) {
+	*h = append(*h, x.(UtilWindow))
+}
+
+func (h *utilHeap) Pop() any {
+	x := (*h)[len(*h)-1]
+	*h = (*h)[:len(*h)-1]
+	return x
+}
+
+// An accumulator takes a windowed mutator utilization function and
+// tracks various statistics for that function.
+type accumulator struct {
+	mmu float64
+
+	// bound is the mutator utilization bound where adding any
+	// mutator utilization above this bound cannot affect the
+	// accumulated statistics.
+	bound float64
+
+	// Worst N window tracking
+	nWorst int
+	wHeap  utilHeap
+
+	// Mutator utilization distribution tracking
+	mud *mud
+	// preciseMass is the distribution mass that must be precise
+	// before accumulation is stopped.
+	preciseMass float64
+	// lastTime and lastMU are the previous point added to the
+	// windowed mutator utilization function.
+	lastTime int64
+	lastMU   float64
+}
+
+// resetTime declares a discontinuity in the windowed mutator
+// utilization function by resetting the current time.
+func (acc *accumulator) resetTime() {
+	// This only matters for distribution collection, since that's
+	// the only thing that depends on the progression of the
+	// windowed mutator utilization function.
+	acc.lastTime = math.MaxInt64
+}
+
+// addMU adds a point to the windowed mutator utilization function at
+// (time, mu). This must be called for monotonically increasing values
+// of time.
+//
+// It returns true if further calls to addMU would be pointless.
+func (acc *accumulator) addMU(time int64, mu float64, window time.Duration) bool {
+	if mu < acc.mmu {
+		acc.mmu = mu
+	}
+	acc.bound = acc.mmu
+
+	if acc.nWorst == 0 {
+		// If the minimum has reached zero, it can't go any
+		// lower, so we can stop early.
+		return mu == 0
+	}
+
+	// Consider adding this window to the n worst.
+	if len(acc.wHeap) < acc.nWorst || mu < acc.wHeap[0].MutatorUtil {
+		// This window is lower than the K'th worst window.
+		//
+		// Check if there's any overlapping window
+		// already in the heap and keep whichever is
+		// worse.
+		for i, ui := range acc.wHeap {
+			if time+int64(window) > ui.Time && ui.Time+int64(window) > time {
+				if ui.MutatorUtil <= mu {
+					// Keep the first window.
+					goto keep
+				} else {
+					// Replace it with this window.
+					heap.Remove(&acc.wHeap, i)
+					break
+				}
+			}
+		}
+
+		heap.Push(&acc.wHeap, UtilWindow{time, mu})
+		if len(acc.wHeap) > acc.nWorst {
+			heap.Pop(&acc.wHeap)
+		}
+	keep:
+	}
+
+	if len(acc.wHeap) < acc.nWorst {
+		// We don't have N windows yet, so keep accumulating.
+		acc.bound = 1.0
+	} else {
+		// Anything above the least worst window has no effect.
+		acc.bound = math.Max(acc.bound, acc.wHeap[0].MutatorUtil)
+	}
+
+	if acc.mud != nil {
+		if acc.lastTime != math.MaxInt64 {
+			// Update distribution.
+			acc.mud.add(acc.lastMU, mu, float64(time-acc.lastTime))
+		}
+		acc.lastTime, acc.lastMU = time, mu
+		if _, mudBound, ok := acc.mud.approxInvCumulativeSum(); ok {
+			acc.bound = math.Max(acc.bound, mudBound)
+		} else {
+			// We haven't accumulated enough total precise
+			// mass yet to even reach our goal, so keep
+			// accumulating.
+			acc.bound = 1
+		}
+		// It's not worth checking percentiles every time, so
+		// just keep accumulating this band.
+		return false
+	}
+
+	// If we've found enough 0 utilizations, we can stop immediately.
+	return len(acc.wHeap) == acc.nWorst && acc.wHeap[0].MutatorUtil == 0
+}
+
+// MMU returns the minimum mutator utilization for the given time
+// window. This is the minimum utilization for all windows of this
+// duration across the execution. The returned value is in the range
+// [0, 1].
+func (c *MMUCurve) MMU(window time.Duration) (mmu float64) {
+	acc := accumulator{mmu: 1.0, bound: 1.0}
+	c.mmu(window, &acc)
+	return acc.mmu
+}
+
+// Examples returns n specific examples of the lowest mutator
+// utilization for the given window size. The returned windows will be
+// disjoint (otherwise there would be a huge number of
+// mostly-overlapping windows at the single lowest point). There are
+// no guarantees on which set of disjoint windows this returns.
+func (c *MMUCurve) Examples(window time.Duration, n int) (worst []UtilWindow) {
+	acc := accumulator{mmu: 1.0, bound: 1.0, nWorst: n}
+	c.mmu(window, &acc)
+	sort.Sort(sort.Reverse(acc.wHeap))
+	return ([]UtilWindow)(acc.wHeap)
+}
+
+// MUD returns mutator utilization distribution quantiles for the
+// given window size.
+//
+// The mutator utilization distribution is the distribution of mean
+// mutator utilization across all windows of the given window size in
+// the trace.
+//
+// The minimum mutator utilization is the minimum (0th percentile) of
+// this distribution. (However, if only the minimum is desired, it's
+// more efficient to use the MMU method.)
+func (c *MMUCurve) MUD(window time.Duration, quantiles []float64) []float64 {
+	if len(quantiles) == 0 {
+		return []float64{}
+	}
+
+	// Each unrefined band contributes a known total mass to the
+	// distribution (bandDur except at the end), but in an unknown
+	// way. However, we know that all the mass it contributes must
+	// be at or above its worst-case mean mutator utilization.
+	//
+	// Hence, we refine bands until the highest desired
+	// distribution quantile is less than the next worst-case mean
+	// mutator utilization. At this point, all further
+	// contributions to the distribution must be beyond the
+	// desired quantile and hence cannot affect it.
+	//
+	// First, find the highest desired distribution quantile.
+	maxQ := quantiles[0]
+	for _, q := range quantiles {
+		if q > maxQ {
+			maxQ = q
+		}
+	}
+	// The distribution's mass is in units of time (it's not
+	// normalized because this would make it more annoying to
+	// account for future contributions of unrefined bands). The
+	// total final mass will be the duration of the trace itself
+	// minus the window size. Using this, we can compute the mass
+	// corresponding to quantile maxQ.
+	var duration int64
+	for _, s := range c.series {
+		duration1 := s.util[len(s.util)-1].Time - s.util[0].Time
+		if duration1 >= int64(window) {
+			duration += duration1 - int64(window)
+		}
+	}
+	qMass := float64(duration) * maxQ
+
+	// Accumulate the MUD until we have precise information for
+	// everything to the left of qMass.
+	acc := accumulator{mmu: 1.0, bound: 1.0, preciseMass: qMass, mud: new(mud)}
+	acc.mud.setTrackMass(qMass)
+	c.mmu(window, &acc)
+
+	// Evaluate the quantiles on the accumulated MUD.
+	out := make([]float64, len(quantiles))
+	for i := range out {
+		mu, _ := acc.mud.invCumulativeSum(float64(duration) * quantiles[i])
+		if math.IsNaN(mu) {
+			// There are a few legitimate ways this can
+			// happen:
+			//
+			// 1. If the window is the full trace
+			// duration, then the windowed MU function is
+			// only defined at a single point, so the MU
+			// distribution is not well-defined.
+			//
+			// 2. If there are no events, then the MU
+			// distribution has no mass.
+			//
+			// Either way, all of the quantiles will have
+			// converged toward the MMU at this point.
+			mu = acc.mmu
+		}
+		out[i] = mu
+	}
+	return out
+}
+
+func (c *MMUCurve) mmu(window time.Duration, acc *accumulator) {
+	if window <= 0 {
+		acc.mmu = 0
+		return
+	}
+
+	var bandU bandUtilHeap
+	windows := make([]time.Duration, len(c.series))
+	for i, s := range c.series {
+		windows[i] = window
+		if max := time.Duration(s.util[len(s.util)-1].Time - s.util[0].Time); window > max {
+			windows[i] = max
+		}
+
+		bandU1 := bandUtilHeap(s.mkBandUtil(i, windows[i]))
+		if bandU == nil {
+			bandU = bandU1
+		} else {
+			bandU = append(bandU, bandU1...)
+		}
+	}
+
+	// Process bands from lowest utilization bound to highest.
+	heap.Init(&bandU)
+
+	// Refine each band into a precise window and MMU until
+	// refining the next lowest band can no longer affect the MMU
+	// or windows.
+	for len(bandU) > 0 && bandU[0].utilBound < acc.bound {
+		i := bandU[0].series
+		c.series[i].bandMMU(bandU[0].i, windows[i], acc)
+		heap.Pop(&bandU)
+	}
+}
+
+func (c *mmuSeries) mkBandUtil(series int, window time.Duration) []bandUtil {
+	// For each band, compute the worst-possible total mutator
+	// utilization for all windows that start in that band.
+
+	// minBands is the minimum number of bands a window can span
+	// and maxBands is the maximum number of bands a window can
+	// span in any alignment.
+	minBands := int((int64(window) + c.bandDur - 1) / c.bandDur)
+	maxBands := int((int64(window) + 2*(c.bandDur-1)) / c.bandDur)
+	if window > 1 && maxBands < 2 {
+		panic("maxBands < 2")
+	}
+	tailDur := int64(window) % c.bandDur
+	nUtil := len(c.bands) - maxBands + 1
+	if nUtil < 0 {
+		nUtil = 0
+	}
+	bandU := make([]bandUtil, nUtil)
+	for i := range bandU {
+		// To compute the worst-case MU, we assume the minimum
+		// for any bands that are only partially overlapped by
+		// some window and the mean for any bands that are
+		// completely covered by all windows.
+		var util totalUtil
+
+		// Find the lowest and second lowest of the partial
+		// bands.
+		l := c.bands[i].minUtil
+		r1 := c.bands[i+minBands-1].minUtil
+		r2 := c.bands[i+maxBands-1].minUtil
+		minBand := math.Min(l, math.Min(r1, r2))
+		// Assume the worst window maximally overlaps the
+		// worst minimum and then the rest overlaps the second
+		// worst minimum.
+		if minBands == 1 {
+			util += totalUtilOf(minBand, int64(window))
+		} else {
+			util += totalUtilOf(minBand, c.bandDur)
+			midBand := 0.0
+			switch {
+			case minBand == l:
+				midBand = math.Min(r1, r2)
+			case minBand == r1:
+				midBand = math.Min(l, r2)
+			case minBand == r2:
+				midBand = math.Min(l, r1)
+			}
+			util += totalUtilOf(midBand, tailDur)
+		}
+
+		// Add the total mean MU of bands that are completely
+		// overlapped by all windows.
+		if minBands > 2 {
+			util += c.bands[i+minBands-1].cumUtil - c.bands[i+1].cumUtil
+		}
+
+		bandU[i] = bandUtil{series, i, util.mean(window)}
+	}
+
+	return bandU
+}
+
+// bandMMU computes the precise minimum mutator utilization for
+// windows with a left edge in band bandIdx.
+func (c *mmuSeries) bandMMU(bandIdx int, window time.Duration, acc *accumulator) {
+	util := c.util
+
+	// We think of the mutator utilization over time as the
+	// box-filtered utilization function, which we call the
+	// "windowed mutator utilization function". The resulting
+	// function is continuous and piecewise linear (unless
+	// window==0, which we handle elsewhere), where the boundaries
+	// between segments occur when either edge of the window
+	// encounters a change in the instantaneous mutator
+	// utilization function. Hence, the minimum of this function
+	// will always occur when one of the edges of the window
+	// aligns with a utilization change, so these are the only
+	// points we need to consider.
+	//
+	// We compute the mutator utilization function incrementally
+	// by tracking the integral from t=0 to the left edge of the
+	// window and to the right edge of the window.
+	left := c.bands[bandIdx].integrator
+	right := left
+	time, endTime := c.bandTime(bandIdx)
+	if utilEnd := util[len(util)-1].Time - int64(window); utilEnd < endTime {
+		endTime = utilEnd
+	}
+	acc.resetTime()
+	for {
+		// Advance edges to time and time+window.
+		mu := (right.advance(time+int64(window)) - left.advance(time)).mean(window)
+		if acc.addMU(time, mu, window) {
+			break
+		}
+		if time == endTime {
+			break
+		}
+
+		// The maximum slope of the windowed mutator
+		// utilization function is 1/window, so we can always
+		// advance the time by at least (mu - mmu) * window
+		// without dropping below mmu.
+		minTime := time + int64((mu-acc.bound)*float64(window))
+
+		// Advance the window to the next time where either
+		// the left or right edge of the window encounters a
+		// change in the utilization curve.
+		if t1, t2 := left.next(time), right.next(time+int64(window))-int64(window); t1 < t2 {
+			time = t1
+		} else {
+			time = t2
+		}
+		if time < minTime {
+			time = minTime
+		}
+		if time >= endTime {
+			// For MMUs we could stop here, but for MUDs
+			// it's important that we span the entire
+			// band.
+			time = endTime
+		}
+	}
+}
+
+// An integrator tracks a position in a utilization function and
+// integrates it.
+type integrator struct {
+	u *mmuSeries
+	// pos is the index in u.util of the current time's non-strict
+	// predecessor.
+	pos int
+}
+
+// advance returns the integral of the utilization function from 0 to
+// time. advance must be called on monotonically increasing values of
+// times.
+func (in *integrator) advance(time int64) totalUtil {
+	util, pos := in.u.util, in.pos
+	// Advance pos until pos+1 is time's strict successor (making
+	// pos time's non-strict predecessor).
+	//
+	// Very often, this will be nearby, so we optimize that case,
+	// but it may be arbitrarily far away, so we handled that
+	// efficiently, too.
+	const maxSeq = 8
+	if pos+maxSeq < len(util) && util[pos+maxSeq].Time > time {
+		// Nearby. Use a linear scan.
+		for pos+1 < len(util) && util[pos+1].Time <= time {
+			pos++
+		}
+	} else {
+		// Far. Binary search for time's strict successor.
+		l, r := pos, len(util)
+		for l < r {
+			h := int(uint(l+r) >> 1)
+			if util[h].Time <= time {
+				l = h + 1
+			} else {
+				r = h
+			}
+		}
+		pos = l - 1 // Non-strict predecessor.
+	}
+	in.pos = pos
+	var partial totalUtil
+	if time != util[pos].Time {
+		partial = totalUtilOf(util[pos].Util, time-util[pos].Time)
+	}
+	return in.u.sums[pos] + partial
+}
+
+// next returns the smallest time t' > time of a change in the
+// utilization function.
+func (in *integrator) next(time int64) int64 {
+	for _, u := range in.u.util[in.pos:] {
+		if u.Time > time {
+			return u.Time
+		}
+	}
+	return 1<<63 - 1
+}
diff --git a/src/internal/trace/gc_test.go b/src/internal/trace/gc_test.go
new file mode 100644
index 0000000..9b9771e
--- /dev/null
+++ b/src/internal/trace/gc_test.go
@@ -0,0 +1,202 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"bytes"
+	"math"
+	"os"
+	"testing"
+	"time"
+)
+
+// aeq returns true if x and y are equal up to 8 digits (1 part in 100
+// million).
+func aeq(x, y float64) bool {
+	if x < 0 && y < 0 {
+		x, y = -x, -y
+	}
+	const digits = 8
+	factor := 1 - math.Pow(10, -digits+1)
+	return x*factor <= y && y*factor <= x
+}
+
+func TestMMU(t *testing.T) {
+	t.Parallel()
+
+	// MU
+	// 1.0  *****   *****   *****
+	// 0.5      *   *   *   *
+	// 0.0      *****   *****
+	//      0   1   2   3   4   5
+	util := [][]MutatorUtil{{
+		{0e9, 1},
+		{1e9, 0},
+		{2e9, 1},
+		{3e9, 0},
+		{4e9, 1},
+		{5e9, 0},
+	}}
+	mmuCurve := NewMMUCurve(util)
+
+	for _, test := range []struct {
+		window time.Duration
+		want   float64
+		worst  []float64
+	}{
+		{0, 0, []float64{}},
+		{time.Millisecond, 0, []float64{0, 0}},
+		{time.Second, 0, []float64{0, 0}},
+		{2 * time.Second, 0.5, []float64{0.5, 0.5}},
+		{3 * time.Second, 1 / 3.0, []float64{1 / 3.0}},
+		{4 * time.Second, 0.5, []float64{0.5}},
+		{5 * time.Second, 3 / 5.0, []float64{3 / 5.0}},
+		{6 * time.Second, 3 / 5.0, []float64{3 / 5.0}},
+	} {
+		if got := mmuCurve.MMU(test.window); !aeq(test.want, got) {
+			t.Errorf("for %s window, want mu = %f, got %f", test.window, test.want, got)
+		}
+		worst := mmuCurve.Examples(test.window, 2)
+		// Which exact windows are returned is unspecified
+		// (and depends on the exact banding), so we just
+		// check that we got the right number with the right
+		// utilizations.
+		if len(worst) != len(test.worst) {
+			t.Errorf("for %s window, want worst %v, got %v", test.window, test.worst, worst)
+		} else {
+			for i := range worst {
+				if worst[i].MutatorUtil != test.worst[i] {
+					t.Errorf("for %s window, want worst %v, got %v", test.window, test.worst, worst)
+					break
+				}
+			}
+		}
+	}
+}
+
+func TestMMUTrace(t *testing.T) {
+	// Can't be t.Parallel() because it modifies the
+	// testingOneBand package variable.
+	if testing.Short() {
+		// test input too big for all.bash
+		t.Skip("skipping in -short mode")
+	}
+
+	data, err := os.ReadFile("testdata/stress_1_10_good")
+	if err != nil {
+		t.Fatalf("failed to read input file: %v", err)
+	}
+	_, events, err := parse(bytes.NewReader(data), "")
+	if err != nil {
+		t.Fatalf("failed to parse trace: %s", err)
+	}
+	mu := MutatorUtilization(events.Events, UtilSTW|UtilBackground|UtilAssist)
+	mmuCurve := NewMMUCurve(mu)
+
+	// Test the optimized implementation against the "obviously
+	// correct" implementation.
+	for window := time.Nanosecond; window < 10*time.Second; window *= 10 {
+		want := mmuSlow(mu[0], window)
+		got := mmuCurve.MMU(window)
+		if !aeq(want, got) {
+			t.Errorf("want %f, got %f mutator utilization in window %s", want, got, window)
+		}
+	}
+
+	// Test MUD with band optimization against MUD without band
+	// optimization. We don't have a simple testing implementation
+	// of MUDs (the simplest implementation is still quite
+	// complex), but this is still a pretty good test.
+	defer func(old int) { bandsPerSeries = old }(bandsPerSeries)
+	bandsPerSeries = 1
+	mmuCurve2 := NewMMUCurve(mu)
+	quantiles := []float64{0, 1 - .999, 1 - .99}
+	for window := time.Microsecond; window < time.Second; window *= 10 {
+		mud1 := mmuCurve.MUD(window, quantiles)
+		mud2 := mmuCurve2.MUD(window, quantiles)
+		for i := range mud1 {
+			if !aeq(mud1[i], mud2[i]) {
+				t.Errorf("for quantiles %v at window %v, want %v, got %v", quantiles, window, mud2, mud1)
+				break
+			}
+		}
+	}
+}
+
+func BenchmarkMMU(b *testing.B) {
+	data, err := os.ReadFile("testdata/stress_1_10_good")
+	if err != nil {
+		b.Fatalf("failed to read input file: %v", err)
+	}
+	_, events, err := parse(bytes.NewReader(data), "")
+	if err != nil {
+		b.Fatalf("failed to parse trace: %s", err)
+	}
+	mu := MutatorUtilization(events.Events, UtilSTW|UtilBackground|UtilAssist|UtilSweep)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		mmuCurve := NewMMUCurve(mu)
+		xMin, xMax := time.Microsecond, time.Second
+		logMin, logMax := math.Log(float64(xMin)), math.Log(float64(xMax))
+		const samples = 100
+		for i := 0; i < samples; i++ {
+			window := time.Duration(math.Exp(float64(i)/(samples-1)*(logMax-logMin) + logMin))
+			mmuCurve.MMU(window)
+		}
+	}
+}
+
+func mmuSlow(util []MutatorUtil, window time.Duration) (mmu float64) {
+	if max := time.Duration(util[len(util)-1].Time - util[0].Time); window > max {
+		window = max
+	}
+
+	mmu = 1.0
+
+	// muInWindow returns the mean mutator utilization between
+	// util[0].Time and end.
+	muInWindow := func(util []MutatorUtil, end int64) float64 {
+		total := 0.0
+		var prevU MutatorUtil
+		for _, u := range util {
+			if u.Time > end {
+				total += prevU.Util * float64(end-prevU.Time)
+				break
+			}
+			total += prevU.Util * float64(u.Time-prevU.Time)
+			prevU = u
+		}
+		return total / float64(end-util[0].Time)
+	}
+	update := func() {
+		for i, u := range util {
+			if u.Time+int64(window) > util[len(util)-1].Time {
+				break
+			}
+			mmu = math.Min(mmu, muInWindow(util[i:], u.Time+int64(window)))
+		}
+	}
+
+	// Consider all left-aligned windows.
+	update()
+	// Reverse the trace. Slightly subtle because each MutatorUtil
+	// is a *change*.
+	rutil := make([]MutatorUtil, len(util))
+	if util[len(util)-1].Util != 0 {
+		panic("irreversible trace")
+	}
+	for i, u := range util {
+		util1 := 0.0
+		if i != 0 {
+			util1 = util[i-1].Util
+		}
+		rutil[len(rutil)-i-1] = MutatorUtil{Time: -u.Time, Util: util1}
+	}
+	util = rutil
+	// Consider all right-aligned windows.
+	update()
+	return
+}
diff --git a/src/internal/trace/goroutines.go b/src/internal/trace/goroutines.go
new file mode 100644
index 0000000..4b4f13d
--- /dev/null
+++ b/src/internal/trace/goroutines.go
@@ -0,0 +1,358 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"sort"
+	"strings"
+)
+
+// GDesc contains statistics and execution details of a single goroutine.
+type GDesc struct {
+	ID           uint64
+	Name         string
+	PC           uint64
+	CreationTime int64
+	StartTime    int64
+	EndTime      int64
+
+	// List of regions in the goroutine, sorted based on the start time.
+	Regions []*UserRegionDesc
+
+	// Statistics of execution time during the goroutine execution.
+	GExecutionStat
+
+	*gdesc // private part.
+}
+
+// UserRegionDesc represents a region and goroutine execution stats
+// while the region was active.
+type UserRegionDesc struct {
+	TaskID uint64
+	Name   string
+
+	// Region start event. Normally EvUserRegion start event or nil,
+	// but can be EvGoCreate event if the region is a synthetic
+	// region representing task inheritance from the parent goroutine.
+	Start *Event
+
+	// Region end event. Normally EvUserRegion end event or nil,
+	// but can be EvGoStop or EvGoEnd event if the goroutine
+	// terminated without explicitly ending the region.
+	End *Event
+
+	GExecutionStat
+}
+
+// GExecutionStat contains statistics about a goroutine's execution
+// during a period of time.
+type GExecutionStat struct {
+	ExecTime      int64
+	SchedWaitTime int64
+	IOTime        int64
+	BlockTime     int64
+	SyscallTime   int64
+	GCTime        int64
+	SweepTime     int64
+	TotalTime     int64
+}
+
+// sub returns the stats v-s.
+func (s GExecutionStat) sub(v GExecutionStat) (r GExecutionStat) {
+	r = s
+	r.ExecTime -= v.ExecTime
+	r.SchedWaitTime -= v.SchedWaitTime
+	r.IOTime -= v.IOTime
+	r.BlockTime -= v.BlockTime
+	r.SyscallTime -= v.SyscallTime
+	r.GCTime -= v.GCTime
+	r.SweepTime -= v.SweepTime
+	r.TotalTime -= v.TotalTime
+	return r
+}
+
+// snapshotStat returns the snapshot of the goroutine execution statistics.
+// This is called as we process the ordered trace event stream. lastTs and
+// activeGCStartTime are used to process pending statistics if this is called
+// before any goroutine end event.
+func (g *GDesc) snapshotStat(lastTs, activeGCStartTime int64) (ret GExecutionStat) {
+	ret = g.GExecutionStat
+
+	if g.gdesc == nil {
+		return ret // finalized GDesc. No pending state.
+	}
+
+	if activeGCStartTime != 0 { // terminating while GC is active
+		if g.CreationTime < activeGCStartTime {
+			ret.GCTime += lastTs - activeGCStartTime
+		} else {
+			// The goroutine's lifetime completely overlaps
+			// with a GC.
+			ret.GCTime += lastTs - g.CreationTime
+		}
+	}
+
+	if g.TotalTime == 0 {
+		ret.TotalTime = lastTs - g.CreationTime
+	}
+
+	if g.lastStartTime != 0 {
+		ret.ExecTime += lastTs - g.lastStartTime
+	}
+	if g.blockNetTime != 0 {
+		ret.IOTime += lastTs - g.blockNetTime
+	}
+	if g.blockSyncTime != 0 {
+		ret.BlockTime += lastTs - g.blockSyncTime
+	}
+	if g.blockSyscallTime != 0 {
+		ret.SyscallTime += lastTs - g.blockSyscallTime
+	}
+	if g.blockSchedTime != 0 {
+		ret.SchedWaitTime += lastTs - g.blockSchedTime
+	}
+	if g.blockSweepTime != 0 {
+		ret.SweepTime += lastTs - g.blockSweepTime
+	}
+	return ret
+}
+
+// finalize is called when processing a goroutine end event or at
+// the end of trace processing. This finalizes the execution stat
+// and any active regions in the goroutine, in which case trigger is nil.
+func (g *GDesc) finalize(lastTs, activeGCStartTime int64, trigger *Event) {
+	if trigger != nil {
+		g.EndTime = trigger.Ts
+	}
+	finalStat := g.snapshotStat(lastTs, activeGCStartTime)
+
+	g.GExecutionStat = finalStat
+
+	// System goroutines are never part of regions, even though they
+	// "inherit" a task due to creation (EvGoCreate) from within a region.
+	// This may happen e.g. if the first GC is triggered within a region,
+	// starting the GC worker goroutines.
+	if !IsSystemGoroutine(g.Name) {
+		for _, s := range g.activeRegions {
+			s.End = trigger
+			s.GExecutionStat = finalStat.sub(s.GExecutionStat)
+			g.Regions = append(g.Regions, s)
+		}
+	}
+	*(g.gdesc) = gdesc{}
+}
+
+// gdesc is a private part of GDesc that is required only during analysis.
+type gdesc struct {
+	lastStartTime    int64
+	blockNetTime     int64
+	blockSyncTime    int64
+	blockSyscallTime int64
+	blockSweepTime   int64
+	blockGCTime      int64
+	blockSchedTime   int64
+
+	activeRegions []*UserRegionDesc // stack of active regions
+}
+
+// GoroutineStats generates statistics for all goroutines in the trace.
+func GoroutineStats(events []*Event) map[uint64]*GDesc {
+	gs := make(map[uint64]*GDesc)
+	var lastTs int64
+	var gcStartTime int64 // gcStartTime == 0 indicates gc is inactive.
+	for _, ev := range events {
+		lastTs = ev.Ts
+		switch ev.Type {
+		case EvGoCreate:
+			g := &GDesc{ID: ev.Args[0], CreationTime: ev.Ts, gdesc: new(gdesc)}
+			g.blockSchedTime = ev.Ts
+			// When a goroutine is newly created, inherit the task
+			// of the active region. For ease handling of this
+			// case, we create a fake region description with the
+			// task id. This isn't strictly necessary as this
+			// goroutine may not be associated with the task, but
+			// it can be convenient to see all children created
+			// during a region.
+			if creatorG := gs[ev.G]; creatorG != nil && len(creatorG.gdesc.activeRegions) > 0 {
+				regions := creatorG.gdesc.activeRegions
+				s := regions[len(regions)-1]
+				if s.TaskID != 0 {
+					g.gdesc.activeRegions = []*UserRegionDesc{
+						{TaskID: s.TaskID, Start: ev},
+					}
+				}
+			}
+			gs[g.ID] = g
+		case EvGoStart, EvGoStartLabel:
+			g := gs[ev.G]
+			if g.PC == 0 && len(ev.Stk) > 0 {
+				g.PC = ev.Stk[0].PC
+				g.Name = ev.Stk[0].Fn
+			}
+			g.lastStartTime = ev.Ts
+			if g.StartTime == 0 {
+				g.StartTime = ev.Ts
+			}
+			if g.blockSchedTime != 0 {
+				g.SchedWaitTime += ev.Ts - g.blockSchedTime
+				g.blockSchedTime = 0
+			}
+		case EvGoEnd, EvGoStop:
+			g := gs[ev.G]
+			g.finalize(ev.Ts, gcStartTime, ev)
+		case EvGoBlockSend, EvGoBlockRecv, EvGoBlockSelect,
+			EvGoBlockSync, EvGoBlockCond:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+			g.blockSyncTime = ev.Ts
+		case EvGoSched, EvGoPreempt:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+			g.blockSchedTime = ev.Ts
+		case EvGoSleep, EvGoBlock:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+		case EvGoBlockNet:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+			g.blockNetTime = ev.Ts
+		case EvGoBlockGC:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+			g.blockGCTime = ev.Ts
+		case EvGoUnblock:
+			g := gs[ev.Args[0]]
+			if g.blockNetTime != 0 {
+				g.IOTime += ev.Ts - g.blockNetTime
+				g.blockNetTime = 0
+			}
+			if g.blockSyncTime != 0 {
+				g.BlockTime += ev.Ts - g.blockSyncTime
+				g.blockSyncTime = 0
+			}
+			g.blockSchedTime = ev.Ts
+		case EvGoSysBlock:
+			g := gs[ev.G]
+			g.ExecTime += ev.Ts - g.lastStartTime
+			g.lastStartTime = 0
+			g.blockSyscallTime = ev.Ts
+		case EvGoSysExit:
+			g := gs[ev.G]
+			if g.blockSyscallTime != 0 {
+				g.SyscallTime += ev.Ts - g.blockSyscallTime
+				g.blockSyscallTime = 0
+			}
+			g.blockSchedTime = ev.Ts
+		case EvGCSweepStart:
+			g := gs[ev.G]
+			if g != nil {
+				// Sweep can happen during GC on system goroutine.
+				g.blockSweepTime = ev.Ts
+			}
+		case EvGCSweepDone:
+			g := gs[ev.G]
+			if g != nil && g.blockSweepTime != 0 {
+				g.SweepTime += ev.Ts - g.blockSweepTime
+				g.blockSweepTime = 0
+			}
+		case EvGCStart:
+			gcStartTime = ev.Ts
+		case EvGCDone:
+			for _, g := range gs {
+				if g.EndTime != 0 {
+					continue
+				}
+				if gcStartTime < g.CreationTime {
+					g.GCTime += ev.Ts - g.CreationTime
+				} else {
+					g.GCTime += ev.Ts - gcStartTime
+				}
+			}
+			gcStartTime = 0 // indicates gc is inactive.
+		case EvUserRegion:
+			g := gs[ev.G]
+			switch mode := ev.Args[1]; mode {
+			case 0: // region start
+				g.activeRegions = append(g.activeRegions, &UserRegionDesc{
+					Name:           ev.SArgs[0],
+					TaskID:         ev.Args[0],
+					Start:          ev,
+					GExecutionStat: g.snapshotStat(lastTs, gcStartTime),
+				})
+			case 1: // region end
+				var sd *UserRegionDesc
+				if regionStk := g.activeRegions; len(regionStk) > 0 {
+					n := len(regionStk)
+					sd = regionStk[n-1]
+					regionStk = regionStk[:n-1] // pop
+					g.activeRegions = regionStk
+				} else {
+					sd = &UserRegionDesc{
+						Name:   ev.SArgs[0],
+						TaskID: ev.Args[0],
+					}
+				}
+				sd.GExecutionStat = g.snapshotStat(lastTs, gcStartTime).sub(sd.GExecutionStat)
+				sd.End = ev
+				g.Regions = append(g.Regions, sd)
+			}
+		}
+	}
+
+	for _, g := range gs {
+		g.finalize(lastTs, gcStartTime, nil)
+
+		// sort based on region start time
+		sort.Slice(g.Regions, func(i, j int) bool {
+			x := g.Regions[i].Start
+			y := g.Regions[j].Start
+			if x == nil {
+				return true
+			}
+			if y == nil {
+				return false
+			}
+			return x.Ts < y.Ts
+		})
+
+		g.gdesc = nil
+	}
+
+	return gs
+}
+
+// RelatedGoroutines finds a set of goroutines related to goroutine goid.
+func RelatedGoroutines(events []*Event, goid uint64) map[uint64]bool {
+	// BFS of depth 2 over "unblock" edges
+	// (what goroutines unblock goroutine goid?).
+	gmap := make(map[uint64]bool)
+	gmap[goid] = true
+	for i := 0; i < 2; i++ {
+		gmap1 := make(map[uint64]bool)
+		for g := range gmap {
+			gmap1[g] = true
+		}
+		for _, ev := range events {
+			if ev.Type == EvGoUnblock && gmap[ev.Args[0]] {
+				gmap1[ev.G] = true
+			}
+		}
+		gmap = gmap1
+	}
+	gmap[0] = true // for GC events
+	return gmap
+}
+
+func IsSystemGoroutine(entryFn string) bool {
+	// This mimics runtime.isSystemGoroutine as closely as
+	// possible.
+	// Also, locked g in extra M (with empty entryFn) is system goroutine.
+	return entryFn == "" || entryFn != "runtime.main" && strings.HasPrefix(entryFn, "runtime.")
+}
diff --git a/src/internal/trace/mkcanned.bash b/src/internal/trace/mkcanned.bash
new file mode 100755
index 0000000..879cf1c
--- /dev/null
+++ b/src/internal/trace/mkcanned.bash
@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Copyright 2016 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# mkcanned.bash creates canned traces for the trace test suite using
+# the current Go version.
+
+set -e
+
+if [ $# != 1 ]; then
+    echo "usage: $0 <label>" >&2
+    exit 1
+fi
+
+go test -run '^$' -bench ClientServerParallel4 -benchtime 10x -trace "testdata/http_$1_good" net/http
+go test -run 'TraceStress$|TraceStressStartStop$|TestUserTaskRegion$' runtime/trace -savetraces
+mv ../../runtime/trace/TestTraceStress.trace "testdata/stress_$1_good"
+mv ../../runtime/trace/TestTraceStressStartStop.trace "testdata/stress_start_stop_$1_good"
+mv ../../runtime/trace/TestUserTaskRegion.trace "testdata/user_task_region_$1_good"
diff --git a/src/internal/trace/mud.go b/src/internal/trace/mud.go
new file mode 100644
index 0000000..8826306
--- /dev/null
+++ b/src/internal/trace/mud.go
@@ -0,0 +1,223 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"math"
+	"sort"
+)
+
+// mud is an updatable mutator utilization distribution.
+//
+// This is a continuous distribution of duration over mutator
+// utilization. For example, the integral from mutator utilization a
+// to b is the total duration during which the mutator utilization was
+// in the range [a, b].
+//
+// This distribution is *not* normalized (it is not a probability
+// distribution). This makes it easier to work with as it's being
+// updated.
+//
+// It is represented as the sum of scaled uniform distribution
+// functions and Dirac delta functions (which are treated as
+// degenerate uniform distributions).
+type mud struct {
+	sorted, unsorted []edge
+
+	// trackMass is the inverse cumulative sum to track as the
+	// distribution is updated.
+	trackMass float64
+	// trackBucket is the bucket in which trackMass falls. If the
+	// total mass of the distribution is < trackMass, this is
+	// len(hist).
+	trackBucket int
+	// trackSum is the cumulative sum of hist[:trackBucket]. Once
+	// trackSum >= trackMass, trackBucket must be recomputed.
+	trackSum float64
+
+	// hist is a hierarchical histogram of distribution mass.
+	hist [mudDegree]float64
+}
+
+const (
+	// mudDegree is the number of buckets in the MUD summary
+	// histogram.
+	mudDegree = 1024
+)
+
+type edge struct {
+	// At x, the function increases by y.
+	x, delta float64
+	// Additionally at x is a Dirac delta function with area dirac.
+	dirac float64
+}
+
+// add adds a uniform function over [l, r] scaled so the total weight
+// of the uniform is area. If l==r, this adds a Dirac delta function.
+func (d *mud) add(l, r, area float64) {
+	if area == 0 {
+		return
+	}
+
+	if r < l {
+		l, r = r, l
+	}
+
+	// Add the edges.
+	if l == r {
+		d.unsorted = append(d.unsorted, edge{l, 0, area})
+	} else {
+		delta := area / (r - l)
+		d.unsorted = append(d.unsorted, edge{l, delta, 0}, edge{r, -delta, 0})
+	}
+
+	// Update the histogram.
+	h := &d.hist
+	lbFloat, lf := math.Modf(l * mudDegree)
+	lb := int(lbFloat)
+	if lb >= mudDegree {
+		lb, lf = mudDegree-1, 1
+	}
+	if l == r {
+		h[lb] += area
+	} else {
+		rbFloat, rf := math.Modf(r * mudDegree)
+		rb := int(rbFloat)
+		if rb >= mudDegree {
+			rb, rf = mudDegree-1, 1
+		}
+		if lb == rb {
+			h[lb] += area
+		} else {
+			perBucket := area / (r - l) / mudDegree
+			h[lb] += perBucket * (1 - lf)
+			h[rb] += perBucket * rf
+			for i := lb + 1; i < rb; i++ {
+				h[i] += perBucket
+			}
+		}
+	}
+
+	// Update mass tracking.
+	if thresh := float64(d.trackBucket) / mudDegree; l < thresh {
+		if r < thresh {
+			d.trackSum += area
+		} else {
+			d.trackSum += area * (thresh - l) / (r - l)
+		}
+		if d.trackSum >= d.trackMass {
+			// The tracked mass now falls in a different
+			// bucket. Recompute the inverse cumulative sum.
+			d.setTrackMass(d.trackMass)
+		}
+	}
+}
+
+// setTrackMass sets the mass to track the inverse cumulative sum for.
+//
+// Specifically, mass is a cumulative duration, and the mutator
+// utilization bounds for this duration can be queried using
+// approxInvCumulativeSum.
+func (d *mud) setTrackMass(mass float64) {
+	d.trackMass = mass
+
+	// Find the bucket currently containing trackMass by computing
+	// the cumulative sum.
+	sum := 0.0
+	for i, val := range d.hist[:] {
+		newSum := sum + val
+		if newSum > mass {
+			// mass falls in bucket i.
+			d.trackBucket = i
+			d.trackSum = sum
+			return
+		}
+		sum = newSum
+	}
+	d.trackBucket = len(d.hist)
+	d.trackSum = sum
+}
+
+// approxInvCumulativeSum is like invCumulativeSum, but specifically
+// operates on the tracked mass and returns an upper and lower bound
+// approximation of the inverse cumulative sum.
+//
+// The true inverse cumulative sum will be in the range [lower, upper).
+func (d *mud) approxInvCumulativeSum() (float64, float64, bool) {
+	if d.trackBucket == len(d.hist) {
+		return math.NaN(), math.NaN(), false
+	}
+	return float64(d.trackBucket) / mudDegree, float64(d.trackBucket+1) / mudDegree, true
+}
+
+// invCumulativeSum returns x such that the integral of d from -∞ to x
+// is y. If the total weight of d is less than y, it returns the
+// maximum of the distribution and false.
+//
+// Specifically, y is a cumulative duration, and invCumulativeSum
+// returns the mutator utilization x such that at least y time has
+// been spent with mutator utilization <= x.
+func (d *mud) invCumulativeSum(y float64) (float64, bool) {
+	if len(d.sorted) == 0 && len(d.unsorted) == 0 {
+		return math.NaN(), false
+	}
+
+	// Sort edges.
+	edges := d.unsorted
+	sort.Slice(edges, func(i, j int) bool {
+		return edges[i].x < edges[j].x
+	})
+	// Merge with sorted edges.
+	d.unsorted = nil
+	if d.sorted == nil {
+		d.sorted = edges
+	} else {
+		oldSorted := d.sorted
+		newSorted := make([]edge, len(oldSorted)+len(edges))
+		i, j := 0, 0
+		for o := range newSorted {
+			if i >= len(oldSorted) {
+				copy(newSorted[o:], edges[j:])
+				break
+			} else if j >= len(edges) {
+				copy(newSorted[o:], oldSorted[i:])
+				break
+			} else if oldSorted[i].x < edges[j].x {
+				newSorted[o] = oldSorted[i]
+				i++
+			} else {
+				newSorted[o] = edges[j]
+				j++
+			}
+		}
+		d.sorted = newSorted
+	}
+
+	// Traverse edges in order computing a cumulative sum.
+	csum, rate, prevX := 0.0, 0.0, 0.0
+	for _, e := range d.sorted {
+		newCsum := csum + (e.x-prevX)*rate
+		if newCsum >= y {
+			// y was exceeded between the previous edge
+			// and this one.
+			if rate == 0 {
+				// Anywhere between prevX and
+				// e.x will do. We return e.x
+				// because that takes care of
+				// the y==0 case naturally.
+				return e.x, true
+			}
+			return (y-csum)/rate + prevX, true
+		}
+		newCsum += e.dirac
+		if newCsum >= y {
+			// y was exceeded by the Dirac delta at e.x.
+			return e.x, true
+		}
+		csum, prevX = newCsum, e.x
+		rate += e.delta
+	}
+	return prevX, false
+}
diff --git a/src/internal/trace/mud_test.go b/src/internal/trace/mud_test.go
new file mode 100644
index 0000000..b3d74dc
--- /dev/null
+++ b/src/internal/trace/mud_test.go
@@ -0,0 +1,87 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func TestMUD(t *testing.T) {
+	// Insert random uniforms and check histogram mass and
+	// cumulative sum approximations.
+	rnd := rand.New(rand.NewSource(42))
+	mass := 0.0
+	var mud mud
+	for i := 0; i < 100; i++ {
+		area, l, r := rnd.Float64(), rnd.Float64(), rnd.Float64()
+		if rnd.Intn(10) == 0 {
+			r = l
+		}
+		t.Log(l, r, area)
+		mud.add(l, r, area)
+		mass += area
+
+		// Check total histogram weight.
+		hmass := 0.0
+		for _, val := range mud.hist {
+			hmass += val
+		}
+		if !aeq(mass, hmass) {
+			t.Fatalf("want mass %g, got %g", mass, hmass)
+		}
+
+		// Check inverse cumulative sum approximations.
+		for j := 0.0; j < mass; j += mass * 0.099 {
+			mud.setTrackMass(j)
+			l, u, ok := mud.approxInvCumulativeSum()
+			inv, ok2 := mud.invCumulativeSum(j)
+			if !ok || !ok2 {
+				t.Fatalf("inverse cumulative sum failed: approx %v, exact %v", ok, ok2)
+			}
+			if !(l <= inv && inv < u) {
+				t.Fatalf("inverse(%g) = %g, not ∈ [%g, %g)", j, inv, l, u)
+			}
+		}
+	}
+}
+
+func TestMUDTracking(t *testing.T) {
+	// Test that the tracked mass is tracked correctly across
+	// updates.
+	rnd := rand.New(rand.NewSource(42))
+	const uniforms = 100
+	for trackMass := 0.0; trackMass < uniforms; trackMass += uniforms / 50 {
+		var mud mud
+		mass := 0.0
+		mud.setTrackMass(trackMass)
+		for i := 0; i < uniforms; i++ {
+			area, l, r := rnd.Float64(), rnd.Float64(), rnd.Float64()
+			mud.add(l, r, area)
+			mass += area
+			l, u, ok := mud.approxInvCumulativeSum()
+			inv, ok2 := mud.invCumulativeSum(trackMass)
+
+			if mass < trackMass {
+				if ok {
+					t.Errorf("approx(%g) = [%g, %g), but mass = %g", trackMass, l, u, mass)
+				}
+				if ok2 {
+					t.Errorf("exact(%g) = %g, but mass = %g", trackMass, inv, mass)
+				}
+			} else {
+				if !ok {
+					t.Errorf("approx(%g) failed, but mass = %g", trackMass, mass)
+				}
+				if !ok2 {
+					t.Errorf("exact(%g) failed, but mass = %g", trackMass, mass)
+				}
+				if ok && ok2 && !(l <= inv && inv < u) {
+					t.Errorf("inverse(%g) = %g, not ∈ [%g, %g)", trackMass, inv, l, u)
+				}
+			}
+		}
+	}
+}
diff --git a/src/internal/trace/order.go b/src/internal/trace/order.go
new file mode 100644
index 0000000..07a6e13
--- /dev/null
+++ b/src/internal/trace/order.go
@@ -0,0 +1,285 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"fmt"
+	"sort"
+)
+
+type eventBatch struct {
+	events   []*Event
+	selected bool
+}
+
+type orderEvent struct {
+	ev    *Event
+	batch int
+	g     uint64
+	init  gState
+	next  gState
+}
+
+type gStatus int
+
+type gState struct {
+	seq    uint64
+	status gStatus
+}
+
+const (
+	gDead gStatus = iota
+	gRunnable
+	gRunning
+	gWaiting
+
+	unordered = ^uint64(0)
+	garbage   = ^uint64(0) - 1
+	noseq     = ^uint64(0)
+	seqinc    = ^uint64(0) - 1
+)
+
+// order1007 merges a set of per-P event batches into a single, consistent stream.
+// The high level idea is as follows. Events within an individual batch are in
+// correct order, because they are emitted by a single P. So we need to produce
+// a correct interleaving of the batches. To do this we take first unmerged event
+// from each batch (frontier). Then choose subset that is "ready" to be merged,
+// that is, events for which all dependencies are already merged. Then we choose
+// event with the lowest timestamp from the subset, merge it and repeat.
+// This approach ensures that we form a consistent stream even if timestamps are
+// incorrect (condition observed on some machines).
+func order1007(m map[int][]*Event) (events []*Event, err error) {
+	pending := 0
+	// The ordering of CPU profile sample events in the data stream is based on
+	// when each run of the signal handler was able to acquire the spinlock,
+	// with original timestamps corresponding to when ReadTrace pulled the data
+	// off of the profBuf queue. Re-sort them by the timestamp we captured
+	// inside the signal handler.
+	sort.Stable(eventList(m[ProfileP]))
+	var batches []*eventBatch
+	for _, v := range m {
+		pending += len(v)
+		batches = append(batches, &eventBatch{v, false})
+	}
+	gs := make(map[uint64]gState)
+	var frontier []orderEvent
+	for ; pending != 0; pending-- {
+		for i, b := range batches {
+			if b.selected || len(b.events) == 0 {
+				continue
+			}
+			ev := b.events[0]
+			g, init, next := stateTransition(ev)
+			if !transitionReady(g, gs[g], init) {
+				continue
+			}
+			frontier = append(frontier, orderEvent{ev, i, g, init, next})
+			b.events = b.events[1:]
+			b.selected = true
+			// Get rid of "Local" events, they are intended merely for ordering.
+			switch ev.Type {
+			case EvGoStartLocal:
+				ev.Type = EvGoStart
+			case EvGoUnblockLocal:
+				ev.Type = EvGoUnblock
+			case EvGoSysExitLocal:
+				ev.Type = EvGoSysExit
+			}
+		}
+		if len(frontier) == 0 {
+			return nil, fmt.Errorf("no consistent ordering of events possible")
+		}
+		sort.Sort(orderEventList(frontier))
+		f := frontier[0]
+		frontier[0] = frontier[len(frontier)-1]
+		frontier = frontier[:len(frontier)-1]
+		events = append(events, f.ev)
+		transition(gs, f.g, f.init, f.next)
+		if !batches[f.batch].selected {
+			panic("frontier batch is not selected")
+		}
+		batches[f.batch].selected = false
+	}
+
+	// At this point we have a consistent stream of events.
+	// Make sure time stamps respect the ordering.
+	// The tests will skip (not fail) the test case if they see this error.
+	if !sort.IsSorted(eventList(events)) {
+		return nil, ErrTimeOrder
+	}
+
+	// The last part is giving correct timestamps to EvGoSysExit events.
+	// The problem with EvGoSysExit is that actual syscall exit timestamp (ev.Args[2])
+	// is potentially acquired long before event emission. So far we've used
+	// timestamp of event emission (ev.Ts).
+	// We could not set ev.Ts = ev.Args[2] earlier, because it would produce
+	// seemingly broken timestamps (misplaced event).
+	// We also can't simply update the timestamp and resort events, because
+	// if timestamps are broken we will misplace the event and later report
+	// logically broken trace (instead of reporting broken timestamps).
+	lastSysBlock := make(map[uint64]int64)
+	for _, ev := range events {
+		switch ev.Type {
+		case EvGoSysBlock, EvGoInSyscall:
+			lastSysBlock[ev.G] = ev.Ts
+		case EvGoSysExit:
+			ts := int64(ev.Args[2])
+			if ts == 0 {
+				continue
+			}
+			block := lastSysBlock[ev.G]
+			if block == 0 {
+				return nil, fmt.Errorf("stray syscall exit")
+			}
+			if ts < block {
+				return nil, ErrTimeOrder
+			}
+			ev.Ts = ts
+		}
+	}
+	sort.Stable(eventList(events))
+
+	return
+}
+
+// stateTransition returns goroutine state (sequence and status) when the event
+// becomes ready for merging (init) and the goroutine state after the event (next).
+func stateTransition(ev *Event) (g uint64, init, next gState) {
+	switch ev.Type {
+	case EvGoCreate:
+		g = ev.Args[0]
+		init = gState{0, gDead}
+		next = gState{1, gRunnable}
+	case EvGoWaiting, EvGoInSyscall:
+		g = ev.G
+		init = gState{1, gRunnable}
+		next = gState{2, gWaiting}
+	case EvGoStart, EvGoStartLabel:
+		g = ev.G
+		init = gState{ev.Args[1], gRunnable}
+		next = gState{ev.Args[1] + 1, gRunning}
+	case EvGoStartLocal:
+		// noseq means that this event is ready for merging as soon as
+		// frontier reaches it (EvGoStartLocal is emitted on the same P
+		// as the corresponding EvGoCreate/EvGoUnblock, and thus the latter
+		// is already merged).
+		// seqinc is a stub for cases when event increments g sequence,
+		// but since we don't know current seq we also don't know next seq.
+		g = ev.G
+		init = gState{noseq, gRunnable}
+		next = gState{seqinc, gRunning}
+	case EvGoBlock, EvGoBlockSend, EvGoBlockRecv, EvGoBlockSelect,
+		EvGoBlockSync, EvGoBlockCond, EvGoBlockNet, EvGoSleep,
+		EvGoSysBlock, EvGoBlockGC:
+		g = ev.G
+		init = gState{noseq, gRunning}
+		next = gState{noseq, gWaiting}
+	case EvGoSched, EvGoPreempt:
+		g = ev.G
+		init = gState{noseq, gRunning}
+		next = gState{noseq, gRunnable}
+	case EvGoUnblock, EvGoSysExit:
+		g = ev.Args[0]
+		init = gState{ev.Args[1], gWaiting}
+		next = gState{ev.Args[1] + 1, gRunnable}
+	case EvGoUnblockLocal, EvGoSysExitLocal:
+		g = ev.Args[0]
+		init = gState{noseq, gWaiting}
+		next = gState{seqinc, gRunnable}
+	case EvGCStart:
+		g = garbage
+		init = gState{ev.Args[0], gDead}
+		next = gState{ev.Args[0] + 1, gDead}
+	default:
+		// no ordering requirements
+		g = unordered
+	}
+	return
+}
+
+func transitionReady(g uint64, curr, init gState) bool {
+	return g == unordered || (init.seq == noseq || init.seq == curr.seq) && init.status == curr.status
+}
+
+func transition(gs map[uint64]gState, g uint64, init, next gState) {
+	if g == unordered {
+		return
+	}
+	curr := gs[g]
+	if !transitionReady(g, curr, init) {
+		panic("event sequences are broken")
+	}
+	switch next.seq {
+	case noseq:
+		next.seq = curr.seq
+	case seqinc:
+		next.seq = curr.seq + 1
+	}
+	gs[g] = next
+}
+
+// order1005 merges a set of per-P event batches into a single, consistent stream.
+func order1005(m map[int][]*Event) (events []*Event, err error) {
+	for _, batch := range m {
+		events = append(events, batch...)
+	}
+	for _, ev := range events {
+		if ev.Type == EvGoSysExit {
+			// EvGoSysExit emission is delayed until the thread has a P.
+			// Give it the real sequence number and time stamp.
+			ev.seq = int64(ev.Args[1])
+			if ev.Args[2] != 0 {
+				ev.Ts = int64(ev.Args[2])
+			}
+		}
+	}
+	sort.Sort(eventSeqList(events))
+	if !sort.IsSorted(eventList(events)) {
+		return nil, ErrTimeOrder
+	}
+	return
+}
+
+type orderEventList []orderEvent
+
+func (l orderEventList) Len() int {
+	return len(l)
+}
+
+func (l orderEventList) Less(i, j int) bool {
+	return l[i].ev.Ts < l[j].ev.Ts
+}
+
+func (l orderEventList) Swap(i, j int) {
+	l[i], l[j] = l[j], l[i]
+}
+
+type eventList []*Event
+
+func (l eventList) Len() int {
+	return len(l)
+}
+
+func (l eventList) Less(i, j int) bool {
+	return l[i].Ts < l[j].Ts
+}
+
+func (l eventList) Swap(i, j int) {
+	l[i], l[j] = l[j], l[i]
+}
+
+type eventSeqList []*Event
+
+func (l eventSeqList) Len() int {
+	return len(l)
+}
+
+func (l eventSeqList) Less(i, j int) bool {
+	return l[i].seq < l[j].seq
+}
+
+func (l eventSeqList) Swap(i, j int) {
+	l[i], l[j] = l[j], l[i]
+}
diff --git a/src/internal/trace/parser.go b/src/internal/trace/parser.go
new file mode 100644
index 0000000..b091a85
--- /dev/null
+++ b/src/internal/trace/parser.go
@@ -0,0 +1,1142 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"math/rand"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	_ "unsafe"
+)
+
+func goCmd() string {
+	var exeSuffix string
+	if runtime.GOOS == "windows" {
+		exeSuffix = ".exe"
+	}
+	path := filepath.Join(runtime.GOROOT(), "bin", "go"+exeSuffix)
+	if _, err := os.Stat(path); err == nil {
+		return path
+	}
+	return "go"
+}
+
+// Event describes one event in the trace.
+type Event struct {
+	Off   int       // offset in input file (for debugging and error reporting)
+	Type  byte      // one of Ev*
+	seq   int64     // sequence number
+	Ts    int64     // timestamp in nanoseconds
+	P     int       // P on which the event happened (can be one of TimerP, NetpollP, SyscallP)
+	G     uint64    // G on which the event happened
+	StkID uint64    // unique stack ID
+	Stk   []*Frame  // stack trace (can be empty)
+	Args  [3]uint64 // event-type-specific arguments
+	SArgs []string  // event-type-specific string args
+	// linked event (can be nil), depends on event type:
+	// for GCStart: the GCStop
+	// for GCSTWStart: the GCSTWDone
+	// for GCSweepStart: the GCSweepDone
+	// for GoCreate: first GoStart of the created goroutine
+	// for GoStart/GoStartLabel: the associated GoEnd, GoBlock or other blocking event
+	// for GoSched/GoPreempt: the next GoStart
+	// for GoBlock and other blocking events: the unblock event
+	// for GoUnblock: the associated GoStart
+	// for blocking GoSysCall: the associated GoSysExit
+	// for GoSysExit: the next GoStart
+	// for GCMarkAssistStart: the associated GCMarkAssistDone
+	// for UserTaskCreate: the UserTaskEnd
+	// for UserRegion: if the start region, the corresponding UserRegion end event
+	Link *Event
+}
+
+// Frame is a frame in stack traces.
+type Frame struct {
+	PC   uint64
+	Fn   string
+	File string
+	Line int
+}
+
+const (
+	// Special P identifiers:
+	FakeP    = 1000000 + iota
+	TimerP   // depicts timer unblocks
+	NetpollP // depicts network unblocks
+	SyscallP // depicts returns from syscalls
+	GCP      // depicts GC state
+	ProfileP // depicts recording of CPU profile samples
+)
+
+// ParseResult is the result of Parse.
+type ParseResult struct {
+	// Events is the sorted list of Events in the trace.
+	Events []*Event
+	// Stacks is the stack traces keyed by stack IDs from the trace.
+	Stacks map[uint64][]*Frame
+}
+
+// Parse parses, post-processes and verifies the trace.
+func Parse(r io.Reader, bin string) (ParseResult, error) {
+	ver, res, err := parse(r, bin)
+	if err != nil {
+		return ParseResult{}, err
+	}
+	if ver < 1007 && bin == "" {
+		return ParseResult{}, fmt.Errorf("for traces produced by go 1.6 or below, the binary argument must be provided")
+	}
+	return res, nil
+}
+
+// parse parses, post-processes and verifies the trace. It returns the
+// trace version and the list of events.
+func parse(r io.Reader, bin string) (int, ParseResult, error) {
+	ver, rawEvents, strings, err := readTrace(r)
+	if err != nil {
+		return 0, ParseResult{}, err
+	}
+	events, stacks, err := parseEvents(ver, rawEvents, strings)
+	if err != nil {
+		return 0, ParseResult{}, err
+	}
+	events = removeFutile(events)
+	err = postProcessTrace(ver, events)
+	if err != nil {
+		return 0, ParseResult{}, err
+	}
+	// Attach stack traces.
+	for _, ev := range events {
+		if ev.StkID != 0 {
+			ev.Stk = stacks[ev.StkID]
+		}
+	}
+	if ver < 1007 && bin != "" {
+		if err := symbolize(events, bin); err != nil {
+			return 0, ParseResult{}, err
+		}
+	}
+	return ver, ParseResult{Events: events, Stacks: stacks}, nil
+}
+
+// rawEvent is a helper type used during parsing.
+type rawEvent struct {
+	off   int
+	typ   byte
+	args  []uint64
+	sargs []string
+}
+
+// readTrace does wire-format parsing and verification.
+// It does not care about specific event types and argument meaning.
+func readTrace(r io.Reader) (ver int, events []rawEvent, strings map[uint64]string, err error) {
+	// Read and validate trace header.
+	var buf [16]byte
+	off, err := io.ReadFull(r, buf[:])
+	if err != nil {
+		err = fmt.Errorf("failed to read header: read %v, err %v", off, err)
+		return
+	}
+	ver, err = parseHeader(buf[:])
+	if err != nil {
+		return
+	}
+	switch ver {
+	case 1005, 1007, 1008, 1009, 1010, 1011, 1019:
+		// Note: When adding a new version, confirm that canned traces from the
+		// old version are part of the test suite. Add them using mkcanned.bash.
+		break
+	default:
+		err = fmt.Errorf("unsupported trace file version %v.%v (update Go toolchain) %v", ver/1000, ver%1000, ver)
+		return
+	}
+
+	// Read events.
+	strings = make(map[uint64]string)
+	for {
+		// Read event type and number of arguments (1 byte).
+		off0 := off
+		var n int
+		n, err = r.Read(buf[:1])
+		if err == io.EOF {
+			err = nil
+			break
+		}
+		if err != nil || n != 1 {
+			err = fmt.Errorf("failed to read trace at offset 0x%x: n=%v err=%v", off0, n, err)
+			return
+		}
+		off += n
+		typ := buf[0] << 2 >> 2
+		narg := buf[0]>>6 + 1
+		inlineArgs := byte(4)
+		if ver < 1007 {
+			narg++
+			inlineArgs++
+		}
+		if typ == EvNone || typ >= EvCount || EventDescriptions[typ].minVersion > ver {
+			err = fmt.Errorf("unknown event type %v at offset 0x%x", typ, off0)
+			return
+		}
+		if typ == EvString {
+			// String dictionary entry [ID, length, string].
+			var id uint64
+			id, off, err = readVal(r, off)
+			if err != nil {
+				return
+			}
+			if id == 0 {
+				err = fmt.Errorf("string at offset %d has invalid id 0", off)
+				return
+			}
+			if strings[id] != "" {
+				err = fmt.Errorf("string at offset %d has duplicate id %v", off, id)
+				return
+			}
+			var ln uint64
+			ln, off, err = readVal(r, off)
+			if err != nil {
+				return
+			}
+			if ln == 0 {
+				err = fmt.Errorf("string at offset %d has invalid length 0", off)
+				return
+			}
+			if ln > 1e6 {
+				err = fmt.Errorf("string at offset %d has too large length %v", off, ln)
+				return
+			}
+			buf := make([]byte, ln)
+			var n int
+			n, err = io.ReadFull(r, buf)
+			if err != nil {
+				err = fmt.Errorf("failed to read trace at offset %d: read %v, want %v, error %v", off, n, ln, err)
+				return
+			}
+			off += n
+			strings[id] = string(buf)
+			continue
+		}
+		ev := rawEvent{typ: typ, off: off0}
+		if narg < inlineArgs {
+			for i := 0; i < int(narg); i++ {
+				var v uint64
+				v, off, err = readVal(r, off)
+				if err != nil {
+					err = fmt.Errorf("failed to read event %v argument at offset %v (%v)", typ, off, err)
+					return
+				}
+				ev.args = append(ev.args, v)
+			}
+		} else {
+			// More than inlineArgs args, the first value is length of the event in bytes.
+			var v uint64
+			v, off, err = readVal(r, off)
+			if err != nil {
+				err = fmt.Errorf("failed to read event %v argument at offset %v (%v)", typ, off, err)
+				return
+			}
+			evLen := v
+			off1 := off
+			for evLen > uint64(off-off1) {
+				v, off, err = readVal(r, off)
+				if err != nil {
+					err = fmt.Errorf("failed to read event %v argument at offset %v (%v)", typ, off, err)
+					return
+				}
+				ev.args = append(ev.args, v)
+			}
+			if evLen != uint64(off-off1) {
+				err = fmt.Errorf("event has wrong length at offset 0x%x: want %v, got %v", off0, evLen, off-off1)
+				return
+			}
+		}
+		switch ev.typ {
+		case EvUserLog: // EvUserLog records are followed by a value string of length ev.args[len(ev.args)-1]
+			var s string
+			s, off, err = readStr(r, off)
+			ev.sargs = append(ev.sargs, s)
+		}
+		events = append(events, ev)
+	}
+	return
+}
+
+func readStr(r io.Reader, off0 int) (s string, off int, err error) {
+	var sz uint64
+	sz, off, err = readVal(r, off0)
+	if err != nil || sz == 0 {
+		return "", off, err
+	}
+	if sz > 1e6 {
+		return "", off, fmt.Errorf("string at offset %d is too large (len=%d)", off, sz)
+	}
+	buf := make([]byte, sz)
+	n, err := io.ReadFull(r, buf)
+	if err != nil || sz != uint64(n) {
+		return "", off + n, fmt.Errorf("failed to read trace at offset %d: read %v, want %v, error %v", off, n, sz, err)
+	}
+	return string(buf), off + n, nil
+}
+
+// parseHeader parses trace header of the form "go 1.7 trace\x00\x00\x00\x00"
+// and returns parsed version as 1007.
+func parseHeader(buf []byte) (int, error) {
+	if len(buf) != 16 {
+		return 0, fmt.Errorf("bad header length")
+	}
+	if buf[0] != 'g' || buf[1] != 'o' || buf[2] != ' ' ||
+		buf[3] < '1' || buf[3] > '9' ||
+		buf[4] != '.' ||
+		buf[5] < '1' || buf[5] > '9' {
+		return 0, fmt.Errorf("not a trace file")
+	}
+	ver := int(buf[5] - '0')
+	i := 0
+	for ; buf[6+i] >= '0' && buf[6+i] <= '9' && i < 2; i++ {
+		ver = ver*10 + int(buf[6+i]-'0')
+	}
+	ver += int(buf[3]-'0') * 1000
+	if !bytes.Equal(buf[6+i:], []byte(" trace\x00\x00\x00\x00")[:10-i]) {
+		return 0, fmt.Errorf("not a trace file")
+	}
+	return ver, nil
+}
+
+// Parse events transforms raw events into events.
+// It does analyze and verify per-event-type arguments.
+func parseEvents(ver int, rawEvents []rawEvent, strings map[uint64]string) (events []*Event, stacks map[uint64][]*Frame, err error) {
+	var ticksPerSec, lastSeq, lastTs int64
+	var lastG uint64
+	var lastP int
+	timerGoids := make(map[uint64]bool)
+	lastGs := make(map[int]uint64) // last goroutine running on P
+	stacks = make(map[uint64][]*Frame)
+	batches := make(map[int][]*Event) // events by P
+	for _, raw := range rawEvents {
+		desc := EventDescriptions[raw.typ]
+		if desc.Name == "" {
+			err = fmt.Errorf("missing description for event type %v", raw.typ)
+			return
+		}
+		narg := argNum(raw, ver)
+		if len(raw.args) != narg {
+			err = fmt.Errorf("%v has wrong number of arguments at offset 0x%x: want %v, got %v",
+				desc.Name, raw.off, narg, len(raw.args))
+			return
+		}
+		switch raw.typ {
+		case EvBatch:
+			lastGs[lastP] = lastG
+			lastP = int(raw.args[0])
+			lastG = lastGs[lastP]
+			if ver < 1007 {
+				lastSeq = int64(raw.args[1])
+				lastTs = int64(raw.args[2])
+			} else {
+				lastTs = int64(raw.args[1])
+			}
+		case EvFrequency:
+			ticksPerSec = int64(raw.args[0])
+			if ticksPerSec <= 0 {
+				// The most likely cause for this is tick skew on different CPUs.
+				// For example, solaris/amd64 seems to have wildly different
+				// ticks on different CPUs.
+				err = ErrTimeOrder
+				return
+			}
+		case EvTimerGoroutine:
+			timerGoids[raw.args[0]] = true
+		case EvStack:
+			if len(raw.args) < 2 {
+				err = fmt.Errorf("EvStack has wrong number of arguments at offset 0x%x: want at least 2, got %v",
+					raw.off, len(raw.args))
+				return
+			}
+			size := raw.args[1]
+			if size > 1000 {
+				err = fmt.Errorf("EvStack has bad number of frames at offset 0x%x: %v",
+					raw.off, size)
+				return
+			}
+			want := 2 + 4*size
+			if ver < 1007 {
+				want = 2 + size
+			}
+			if uint64(len(raw.args)) != want {
+				err = fmt.Errorf("EvStack has wrong number of arguments at offset 0x%x: want %v, got %v",
+					raw.off, want, len(raw.args))
+				return
+			}
+			id := raw.args[0]
+			if id != 0 && size > 0 {
+				stk := make([]*Frame, size)
+				for i := 0; i < int(size); i++ {
+					if ver < 1007 {
+						stk[i] = &Frame{PC: raw.args[2+i]}
+					} else {
+						pc := raw.args[2+i*4+0]
+						fn := raw.args[2+i*4+1]
+						file := raw.args[2+i*4+2]
+						line := raw.args[2+i*4+3]
+						stk[i] = &Frame{PC: pc, Fn: strings[fn], File: strings[file], Line: int(line)}
+					}
+				}
+				stacks[id] = stk
+			}
+		default:
+			e := &Event{Off: raw.off, Type: raw.typ, P: lastP, G: lastG}
+			var argOffset int
+			if ver < 1007 {
+				e.seq = lastSeq + int64(raw.args[0])
+				e.Ts = lastTs + int64(raw.args[1])
+				lastSeq = e.seq
+				argOffset = 2
+			} else {
+				e.Ts = lastTs + int64(raw.args[0])
+				argOffset = 1
+			}
+			lastTs = e.Ts
+			for i := argOffset; i < narg; i++ {
+				if i == narg-1 && desc.Stack {
+					e.StkID = raw.args[i]
+				} else {
+					e.Args[i-argOffset] = raw.args[i]
+				}
+			}
+			switch raw.typ {
+			case EvGoStart, EvGoStartLocal, EvGoStartLabel:
+				lastG = e.Args[0]
+				e.G = lastG
+				if raw.typ == EvGoStartLabel {
+					e.SArgs = []string{strings[e.Args[2]]}
+				}
+			case EvGCSTWStart:
+				e.G = 0
+				switch e.Args[0] {
+				case 0:
+					e.SArgs = []string{"mark termination"}
+				case 1:
+					e.SArgs = []string{"sweep termination"}
+				default:
+					err = fmt.Errorf("unknown STW kind %d", e.Args[0])
+					return
+				}
+			case EvGCStart, EvGCDone, EvGCSTWDone:
+				e.G = 0
+			case EvGoEnd, EvGoStop, EvGoSched, EvGoPreempt,
+				EvGoSleep, EvGoBlock, EvGoBlockSend, EvGoBlockRecv,
+				EvGoBlockSelect, EvGoBlockSync, EvGoBlockCond, EvGoBlockNet,
+				EvGoSysBlock, EvGoBlockGC:
+				lastG = 0
+			case EvGoSysExit, EvGoWaiting, EvGoInSyscall:
+				e.G = e.Args[0]
+			case EvUserTaskCreate:
+				// e.Args 0: taskID, 1:parentID, 2:nameID
+				e.SArgs = []string{strings[e.Args[2]]}
+			case EvUserRegion:
+				// e.Args 0: taskID, 1: mode, 2:nameID
+				e.SArgs = []string{strings[e.Args[2]]}
+			case EvUserLog:
+				// e.Args 0: taskID, 1:keyID, 2: stackID
+				e.SArgs = []string{strings[e.Args[1]], raw.sargs[0]}
+			case EvCPUSample:
+				e.Ts = int64(e.Args[0])
+				e.P = int(e.Args[1])
+				e.G = e.Args[2]
+				e.Args[0] = 0
+			}
+			switch raw.typ {
+			default:
+				batches[lastP] = append(batches[lastP], e)
+			case EvCPUSample:
+				// Most events are written out by the active P at the exact
+				// moment they describe. CPU profile samples are different
+				// because they're written to the tracing log after some delay,
+				// by a separate worker goroutine, into a separate buffer.
+				//
+				// We keep these in their own batch until all of the batches are
+				// merged in timestamp order. We also (right before the merge)
+				// re-sort these events by the timestamp captured in the
+				// profiling signal handler.
+				batches[ProfileP] = append(batches[ProfileP], e)
+			}
+		}
+	}
+	if len(batches) == 0 {
+		err = fmt.Errorf("trace is empty")
+		return
+	}
+	if ticksPerSec == 0 {
+		err = fmt.Errorf("no EvFrequency event")
+		return
+	}
+	if BreakTimestampsForTesting {
+		var batchArr [][]*Event
+		for _, batch := range batches {
+			batchArr = append(batchArr, batch)
+		}
+		for i := 0; i < 5; i++ {
+			batch := batchArr[rand.Intn(len(batchArr))]
+			batch[rand.Intn(len(batch))].Ts += int64(rand.Intn(2000) - 1000)
+		}
+	}
+	if ver < 1007 {
+		events, err = order1005(batches)
+	} else {
+		events, err = order1007(batches)
+	}
+	if err != nil {
+		return
+	}
+
+	// Translate cpu ticks to real time.
+	minTs := events[0].Ts
+	// Use floating point to avoid integer overflows.
+	freq := 1e9 / float64(ticksPerSec)
+	for _, ev := range events {
+		ev.Ts = int64(float64(ev.Ts-minTs) * freq)
+		// Move timers and syscalls to separate fake Ps.
+		if timerGoids[ev.G] && ev.Type == EvGoUnblock {
+			ev.P = TimerP
+		}
+		if ev.Type == EvGoSysExit {
+			ev.P = SyscallP
+		}
+	}
+
+	return
+}
+
+// removeFutile removes all constituents of futile wakeups (block, unblock, start).
+// For example, a goroutine was unblocked on a mutex, but another goroutine got
+// ahead and acquired the mutex before the first goroutine is scheduled,
+// so the first goroutine has to block again. Such wakeups happen on buffered
+// channels and sync.Mutex, but are generally not interesting for end user.
+func removeFutile(events []*Event) []*Event {
+	// Two non-trivial aspects:
+	// 1. A goroutine can be preempted during a futile wakeup and migrate to another P.
+	//	We want to remove all of that.
+	// 2. Tracing can start in the middle of a futile wakeup.
+	//	That is, we can see a futile wakeup event w/o the actual wakeup before it.
+	// postProcessTrace runs after us and ensures that we leave the trace in a consistent state.
+
+	// Phase 1: determine futile wakeup sequences.
+	type G struct {
+		futile bool
+		wakeup []*Event // wakeup sequence (subject for removal)
+	}
+	gs := make(map[uint64]G)
+	futile := make(map[*Event]bool)
+	for _, ev := range events {
+		switch ev.Type {
+		case EvGoUnblock:
+			g := gs[ev.Args[0]]
+			g.wakeup = []*Event{ev}
+			gs[ev.Args[0]] = g
+		case EvGoStart, EvGoPreempt, EvFutileWakeup:
+			g := gs[ev.G]
+			g.wakeup = append(g.wakeup, ev)
+			if ev.Type == EvFutileWakeup {
+				g.futile = true
+			}
+			gs[ev.G] = g
+		case EvGoBlock, EvGoBlockSend, EvGoBlockRecv, EvGoBlockSelect, EvGoBlockSync, EvGoBlockCond:
+			g := gs[ev.G]
+			if g.futile {
+				futile[ev] = true
+				for _, ev1 := range g.wakeup {
+					futile[ev1] = true
+				}
+			}
+			delete(gs, ev.G)
+		}
+	}
+
+	// Phase 2: remove futile wakeup sequences.
+	newEvents := events[:0] // overwrite the original slice
+	for _, ev := range events {
+		if !futile[ev] {
+			newEvents = append(newEvents, ev)
+		}
+	}
+	return newEvents
+}
+
+// ErrTimeOrder is returned by Parse when the trace contains
+// time stamps that do not respect actual event ordering.
+var ErrTimeOrder = fmt.Errorf("time stamps out of order")
+
+// postProcessTrace does inter-event verification and information restoration.
+// The resulting trace is guaranteed to be consistent
+// (for example, a P does not run two Gs at the same time, or a G is indeed
+// blocked before an unblock event).
+func postProcessTrace(ver int, events []*Event) error {
+	const (
+		gDead = iota
+		gRunnable
+		gRunning
+		gWaiting
+	)
+	type gdesc struct {
+		state        int
+		ev           *Event
+		evStart      *Event
+		evCreate     *Event
+		evMarkAssist *Event
+	}
+	type pdesc struct {
+		running bool
+		g       uint64
+		evSTW   *Event
+		evSweep *Event
+	}
+
+	gs := make(map[uint64]gdesc)
+	ps := make(map[int]pdesc)
+	tasks := make(map[uint64]*Event)           // task id to task creation events
+	activeRegions := make(map[uint64][]*Event) // goroutine id to stack of regions
+	gs[0] = gdesc{state: gRunning}
+	var evGC, evSTW *Event
+
+	checkRunning := func(p pdesc, g gdesc, ev *Event, allowG0 bool) error {
+		name := EventDescriptions[ev.Type].Name
+		if g.state != gRunning {
+			return fmt.Errorf("g %v is not running while %v (offset %v, time %v)", ev.G, name, ev.Off, ev.Ts)
+		}
+		if p.g != ev.G {
+			return fmt.Errorf("p %v is not running g %v while %v (offset %v, time %v)", ev.P, ev.G, name, ev.Off, ev.Ts)
+		}
+		if !allowG0 && ev.G == 0 {
+			return fmt.Errorf("g 0 did %v (offset %v, time %v)", EventDescriptions[ev.Type].Name, ev.Off, ev.Ts)
+		}
+		return nil
+	}
+
+	for _, ev := range events {
+		g := gs[ev.G]
+		p := ps[ev.P]
+
+		switch ev.Type {
+		case EvProcStart:
+			if p.running {
+				return fmt.Errorf("p %v is running before start (offset %v, time %v)", ev.P, ev.Off, ev.Ts)
+			}
+			p.running = true
+		case EvProcStop:
+			if !p.running {
+				return fmt.Errorf("p %v is not running before stop (offset %v, time %v)", ev.P, ev.Off, ev.Ts)
+			}
+			if p.g != 0 {
+				return fmt.Errorf("p %v is running a goroutine %v during stop (offset %v, time %v)", ev.P, p.g, ev.Off, ev.Ts)
+			}
+			p.running = false
+		case EvGCStart:
+			if evGC != nil {
+				return fmt.Errorf("previous GC is not ended before a new one (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			evGC = ev
+			// Attribute this to the global GC state.
+			ev.P = GCP
+		case EvGCDone:
+			if evGC == nil {
+				return fmt.Errorf("bogus GC end (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			evGC.Link = ev
+			evGC = nil
+		case EvGCSTWStart:
+			evp := &evSTW
+			if ver < 1010 {
+				// Before 1.10, EvGCSTWStart was per-P.
+				evp = &p.evSTW
+			}
+			if *evp != nil {
+				return fmt.Errorf("previous STW is not ended before a new one (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			*evp = ev
+		case EvGCSTWDone:
+			evp := &evSTW
+			if ver < 1010 {
+				// Before 1.10, EvGCSTWDone was per-P.
+				evp = &p.evSTW
+			}
+			if *evp == nil {
+				return fmt.Errorf("bogus STW end (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			(*evp).Link = ev
+			*evp = nil
+		case EvGCSweepStart:
+			if p.evSweep != nil {
+				return fmt.Errorf("previous sweeping is not ended before a new one (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			p.evSweep = ev
+		case EvGCMarkAssistStart:
+			if g.evMarkAssist != nil {
+				return fmt.Errorf("previous mark assist is not ended before a new one (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			g.evMarkAssist = ev
+		case EvGCMarkAssistDone:
+			// Unlike most events, mark assists can be in progress when a
+			// goroutine starts tracing, so we can't report an error here.
+			if g.evMarkAssist != nil {
+				g.evMarkAssist.Link = ev
+				g.evMarkAssist = nil
+			}
+		case EvGCSweepDone:
+			if p.evSweep == nil {
+				return fmt.Errorf("bogus sweeping end (offset %v, time %v)", ev.Off, ev.Ts)
+			}
+			p.evSweep.Link = ev
+			p.evSweep = nil
+		case EvGoWaiting:
+			if g.state != gRunnable {
+				return fmt.Errorf("g %v is not runnable before EvGoWaiting (offset %v, time %v)", ev.G, ev.Off, ev.Ts)
+			}
+			g.state = gWaiting
+			g.ev = ev
+		case EvGoInSyscall:
+			if g.state != gRunnable {
+				return fmt.Errorf("g %v is not runnable before EvGoInSyscall (offset %v, time %v)", ev.G, ev.Off, ev.Ts)
+			}
+			g.state = gWaiting
+			g.ev = ev
+		case EvGoCreate:
+			if err := checkRunning(p, g, ev, true); err != nil {
+				return err
+			}
+			if _, ok := gs[ev.Args[0]]; ok {
+				return fmt.Errorf("g %v already exists (offset %v, time %v)", ev.Args[0], ev.Off, ev.Ts)
+			}
+			gs[ev.Args[0]] = gdesc{state: gRunnable, ev: ev, evCreate: ev}
+		case EvGoStart, EvGoStartLabel:
+			if g.state != gRunnable {
+				return fmt.Errorf("g %v is not runnable before start (offset %v, time %v)", ev.G, ev.Off, ev.Ts)
+			}
+			if p.g != 0 {
+				return fmt.Errorf("p %v is already running g %v while start g %v (offset %v, time %v)", ev.P, p.g, ev.G, ev.Off, ev.Ts)
+			}
+			g.state = gRunning
+			g.evStart = ev
+			p.g = ev.G
+			if g.evCreate != nil {
+				if ver < 1007 {
+					// +1 because symbolizer expects return pc.
+					ev.Stk = []*Frame{{PC: g.evCreate.Args[1] + 1}}
+				} else {
+					ev.StkID = g.evCreate.Args[1]
+				}
+				g.evCreate = nil
+			}
+
+			if g.ev != nil {
+				g.ev.Link = ev
+				g.ev = nil
+			}
+		case EvGoEnd, EvGoStop:
+			if err := checkRunning(p, g, ev, false); err != nil {
+				return err
+			}
+			g.evStart.Link = ev
+			g.evStart = nil
+			g.state = gDead
+			p.g = 0
+
+			if ev.Type == EvGoEnd { // flush all active regions
+				regions := activeRegions[ev.G]
+				for _, s := range regions {
+					s.Link = ev
+				}
+				delete(activeRegions, ev.G)
+			}
+
+		case EvGoSched, EvGoPreempt:
+			if err := checkRunning(p, g, ev, false); err != nil {
+				return err
+			}
+			g.state = gRunnable
+			g.evStart.Link = ev
+			g.evStart = nil
+			p.g = 0
+			g.ev = ev
+		case EvGoUnblock:
+			if g.state != gRunning {
+				return fmt.Errorf("g %v is not running while unpark (offset %v, time %v)", ev.G, ev.Off, ev.Ts)
+			}
+			if ev.P != TimerP && p.g != ev.G {
+				return fmt.Errorf("p %v is not running g %v while unpark (offset %v, time %v)", ev.P, ev.G, ev.Off, ev.Ts)
+			}
+			g1 := gs[ev.Args[0]]
+			if g1.state != gWaiting {
+				return fmt.Errorf("g %v is not waiting before unpark (offset %v, time %v)", ev.Args[0], ev.Off, ev.Ts)
+			}
+			if g1.ev != nil && g1.ev.Type == EvGoBlockNet && ev.P != TimerP {
+				ev.P = NetpollP
+			}
+			if g1.ev != nil {
+				g1.ev.Link = ev
+			}
+			g1.state = gRunnable
+			g1.ev = ev
+			gs[ev.Args[0]] = g1
+		case EvGoSysCall:
+			if err := checkRunning(p, g, ev, false); err != nil {
+				return err
+			}
+			g.ev = ev
+		case EvGoSysBlock:
+			if err := checkRunning(p, g, ev, false); err != nil {
+				return err
+			}
+			g.state = gWaiting
+			g.evStart.Link = ev
+			g.evStart = nil
+			p.g = 0
+		case EvGoSysExit:
+			if g.state != gWaiting {
+				return fmt.Errorf("g %v is not waiting during syscall exit (offset %v, time %v)", ev.G, ev.Off, ev.Ts)
+			}
+			if g.ev != nil && g.ev.Type == EvGoSysCall {
+				g.ev.Link = ev
+			}
+			g.state = gRunnable
+			g.ev = ev
+		case EvGoSleep, EvGoBlock, EvGoBlockSend, EvGoBlockRecv,
+			EvGoBlockSelect, EvGoBlockSync, EvGoBlockCond, EvGoBlockNet, EvGoBlockGC:
+			if err := checkRunning(p, g, ev, false); err != nil {
+				return err
+			}
+			g.state = gWaiting
+			g.ev = ev
+			g.evStart.Link = ev
+			g.evStart = nil
+			p.g = 0
+		case EvUserTaskCreate:
+			taskid := ev.Args[0]
+			if prevEv, ok := tasks[taskid]; ok {
+				return fmt.Errorf("task id conflicts (id:%d), %q vs %q", taskid, ev, prevEv)
+			}
+			tasks[ev.Args[0]] = ev
+		case EvUserTaskEnd:
+			taskid := ev.Args[0]
+			if taskCreateEv, ok := tasks[taskid]; ok {
+				taskCreateEv.Link = ev
+				delete(tasks, taskid)
+			}
+		case EvUserRegion:
+			mode := ev.Args[1]
+			regions := activeRegions[ev.G]
+			if mode == 0 { // region start
+				activeRegions[ev.G] = append(regions, ev) // push
+			} else if mode == 1 { // region end
+				n := len(regions)
+				if n > 0 { // matching region start event is in the trace.
+					s := regions[n-1]
+					if s.Args[0] != ev.Args[0] || s.SArgs[0] != ev.SArgs[0] { // task id, region name mismatch
+						return fmt.Errorf("misuse of region in goroutine %d: span end %q when the inner-most active span start event is %q", ev.G, ev, s)
+					}
+					// Link region start event with span end event
+					s.Link = ev
+
+					if n > 1 {
+						activeRegions[ev.G] = regions[:n-1]
+					} else {
+						delete(activeRegions, ev.G)
+					}
+				}
+			} else {
+				return fmt.Errorf("invalid user region mode: %q", ev)
+			}
+		}
+
+		gs[ev.G] = g
+		ps[ev.P] = p
+	}
+
+	// TODO(dvyukov): restore stacks for EvGoStart events.
+	// TODO(dvyukov): test that all EvGoStart events has non-nil Link.
+
+	return nil
+}
+
+// symbolize attaches func/file/line info to stack traces.
+func symbolize(events []*Event, bin string) error {
+	// First, collect and dedup all pcs.
+	pcs := make(map[uint64]*Frame)
+	for _, ev := range events {
+		for _, f := range ev.Stk {
+			pcs[f.PC] = nil
+		}
+	}
+
+	// Start addr2line.
+	cmd := exec.Command(goCmd(), "tool", "addr2line", bin)
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		return fmt.Errorf("failed to pipe addr2line stdin: %v", err)
+	}
+	cmd.Stderr = os.Stderr
+	out, err := cmd.StdoutPipe()
+	if err != nil {
+		return fmt.Errorf("failed to pipe addr2line stdout: %v", err)
+	}
+	err = cmd.Start()
+	if err != nil {
+		return fmt.Errorf("failed to start addr2line: %v", err)
+	}
+	outb := bufio.NewReader(out)
+
+	// Write all pcs to addr2line.
+	// Need to copy pcs to an array, because map iteration order is non-deterministic.
+	var pcArray []uint64
+	for pc := range pcs {
+		pcArray = append(pcArray, pc)
+		_, err := fmt.Fprintf(in, "0x%x\n", pc-1)
+		if err != nil {
+			return fmt.Errorf("failed to write to addr2line: %v", err)
+		}
+	}
+	in.Close()
+
+	// Read in answers.
+	for _, pc := range pcArray {
+		fn, err := outb.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("failed to read from addr2line: %v", err)
+		}
+		file, err := outb.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("failed to read from addr2line: %v", err)
+		}
+		f := &Frame{PC: pc}
+		f.Fn = fn[:len(fn)-1]
+		f.File = file[:len(file)-1]
+		if colon := strings.LastIndex(f.File, ":"); colon != -1 {
+			ln, err := strconv.Atoi(f.File[colon+1:])
+			if err == nil {
+				f.File = f.File[:colon]
+				f.Line = ln
+			}
+		}
+		pcs[pc] = f
+	}
+	cmd.Wait()
+
+	// Replace frames in events array.
+	for _, ev := range events {
+		for i, f := range ev.Stk {
+			ev.Stk[i] = pcs[f.PC]
+		}
+	}
+
+	return nil
+}
+
+// readVal reads unsigned base-128 value from r.
+func readVal(r io.Reader, off0 int) (v uint64, off int, err error) {
+	off = off0
+	for i := 0; i < 10; i++ {
+		var buf [1]byte
+		var n int
+		n, err = r.Read(buf[:])
+		if err != nil || n != 1 {
+			return 0, 0, fmt.Errorf("failed to read trace at offset %d: read %v, error %v", off0, n, err)
+		}
+		off++
+		v |= uint64(buf[0]&0x7f) << (uint(i) * 7)
+		if buf[0]&0x80 == 0 {
+			return
+		}
+	}
+	return 0, 0, fmt.Errorf("bad value at offset 0x%x", off0)
+}
+
+// Print dumps events to stdout. For debugging.
+func Print(events []*Event) {
+	for _, ev := range events {
+		PrintEvent(ev)
+	}
+}
+
+// PrintEvent dumps the event to stdout. For debugging.
+func PrintEvent(ev *Event) {
+	fmt.Printf("%s\n", ev)
+}
+
+func (ev *Event) String() string {
+	desc := EventDescriptions[ev.Type]
+	w := new(strings.Builder)
+	fmt.Fprintf(w, "%v %v p=%v g=%v off=%v", ev.Ts, desc.Name, ev.P, ev.G, ev.Off)
+	for i, a := range desc.Args {
+		fmt.Fprintf(w, " %v=%v", a, ev.Args[i])
+	}
+	for i, a := range desc.SArgs {
+		fmt.Fprintf(w, " %v=%v", a, ev.SArgs[i])
+	}
+	return w.String()
+}
+
+// argNum returns total number of args for the event accounting for timestamps,
+// sequence numbers and differences between trace format versions.
+func argNum(raw rawEvent, ver int) int {
+	desc := EventDescriptions[raw.typ]
+	if raw.typ == EvStack {
+		return len(raw.args)
+	}
+	narg := len(desc.Args)
+	if desc.Stack {
+		narg++
+	}
+	switch raw.typ {
+	case EvBatch, EvFrequency, EvTimerGoroutine:
+		if ver < 1007 {
+			narg++ // there was an unused arg before 1.7
+		}
+		return narg
+	}
+	narg++ // timestamp
+	if ver < 1007 {
+		narg++ // sequence
+	}
+	switch raw.typ {
+	case EvGCSweepDone:
+		if ver < 1009 {
+			narg -= 2 // 1.9 added two arguments
+		}
+	case EvGCStart, EvGoStart, EvGoUnblock:
+		if ver < 1007 {
+			narg-- // 1.7 added an additional seq arg
+		}
+	case EvGCSTWStart:
+		if ver < 1010 {
+			narg-- // 1.10 added an argument
+		}
+	}
+	return narg
+}
+
+// BreakTimestampsForTesting causes the parser to randomly alter timestamps (for testing of broken cputicks).
+var BreakTimestampsForTesting bool
+
+// Event types in the trace.
+// Verbatim copy from src/runtime/trace.go with the "trace" prefix removed.
+const (
+	EvNone              = 0  // unused
+	EvBatch             = 1  // start of per-P batch of events [pid, timestamp]
+	EvFrequency         = 2  // contains tracer timer frequency [frequency (ticks per second)]
+	EvStack             = 3  // stack [stack id, number of PCs, array of {PC, func string ID, file string ID, line}]
+	EvGomaxprocs        = 4  // current value of GOMAXPROCS [timestamp, GOMAXPROCS, stack id]
+	EvProcStart         = 5  // start of P [timestamp, thread id]
+	EvProcStop          = 6  // stop of P [timestamp]
+	EvGCStart           = 7  // GC start [timestamp, seq, stack id]
+	EvGCDone            = 8  // GC done [timestamp]
+	EvGCSTWStart        = 9  // GC mark termination start [timestamp, kind]
+	EvGCSTWDone         = 10 // GC mark termination done [timestamp]
+	EvGCSweepStart      = 11 // GC sweep start [timestamp, stack id]
+	EvGCSweepDone       = 12 // GC sweep done [timestamp, swept, reclaimed]
+	EvGoCreate          = 13 // goroutine creation [timestamp, new goroutine id, new stack id, stack id]
+	EvGoStart           = 14 // goroutine starts running [timestamp, goroutine id, seq]
+	EvGoEnd             = 15 // goroutine ends [timestamp]
+	EvGoStop            = 16 // goroutine stops (like in select{}) [timestamp, stack]
+	EvGoSched           = 17 // goroutine calls Gosched [timestamp, stack]
+	EvGoPreempt         = 18 // goroutine is preempted [timestamp, stack]
+	EvGoSleep           = 19 // goroutine calls Sleep [timestamp, stack]
+	EvGoBlock           = 20 // goroutine blocks [timestamp, stack]
+	EvGoUnblock         = 21 // goroutine is unblocked [timestamp, goroutine id, seq, stack]
+	EvGoBlockSend       = 22 // goroutine blocks on chan send [timestamp, stack]
+	EvGoBlockRecv       = 23 // goroutine blocks on chan recv [timestamp, stack]
+	EvGoBlockSelect     = 24 // goroutine blocks on select [timestamp, stack]
+	EvGoBlockSync       = 25 // goroutine blocks on Mutex/RWMutex [timestamp, stack]
+	EvGoBlockCond       = 26 // goroutine blocks on Cond [timestamp, stack]
+	EvGoBlockNet        = 27 // goroutine blocks on network [timestamp, stack]
+	EvGoSysCall         = 28 // syscall enter [timestamp, stack]
+	EvGoSysExit         = 29 // syscall exit [timestamp, goroutine id, seq, real timestamp]
+	EvGoSysBlock        = 30 // syscall blocks [timestamp]
+	EvGoWaiting         = 31 // denotes that goroutine is blocked when tracing starts [timestamp, goroutine id]
+	EvGoInSyscall       = 32 // denotes that goroutine is in syscall when tracing starts [timestamp, goroutine id]
+	EvHeapAlloc         = 33 // gcController.heapLive change [timestamp, heap live bytes]
+	EvHeapGoal          = 34 // gcController.heapGoal change [timestamp, heap goal bytes]
+	EvTimerGoroutine    = 35 // denotes timer goroutine [timer goroutine id]
+	EvFutileWakeup      = 36 // denotes that the previous wakeup of this goroutine was futile [timestamp]
+	EvString            = 37 // string dictionary entry [ID, length, string]
+	EvGoStartLocal      = 38 // goroutine starts running on the same P as the last event [timestamp, goroutine id]
+	EvGoUnblockLocal    = 39 // goroutine is unblocked on the same P as the last event [timestamp, goroutine id, stack]
+	EvGoSysExitLocal    = 40 // syscall exit on the same P as the last event [timestamp, goroutine id, real timestamp]
+	EvGoStartLabel      = 41 // goroutine starts running with label [timestamp, goroutine id, seq, label string id]
+	EvGoBlockGC         = 42 // goroutine blocks on GC assist [timestamp, stack]
+	EvGCMarkAssistStart = 43 // GC mark assist start [timestamp, stack]
+	EvGCMarkAssistDone  = 44 // GC mark assist done [timestamp]
+	EvUserTaskCreate    = 45 // trace.NewContext [timestamp, internal task id, internal parent id, stack, name string]
+	EvUserTaskEnd       = 46 // end of task [timestamp, internal task id, stack]
+	EvUserRegion        = 47 // trace.WithRegion [timestamp, internal task id, mode(0:start, 1:end), stack, name string]
+	EvUserLog           = 48 // trace.Log [timestamp, internal id, key string id, stack, value string]
+	EvCPUSample         = 49 // CPU profiling sample [timestamp, stack, real timestamp, real P id (-1 when absent), goroutine id]
+	EvCount             = 50
+)
+
+var EventDescriptions = [EvCount]struct {
+	Name       string
+	minVersion int
+	Stack      bool
+	Args       []string
+	SArgs      []string // string arguments
+}{
+	EvNone:              {"None", 1005, false, []string{}, nil},
+	EvBatch:             {"Batch", 1005, false, []string{"p", "ticks"}, nil}, // in 1.5 format it was {"p", "seq", "ticks"}
+	EvFrequency:         {"Frequency", 1005, false, []string{"freq"}, nil},   // in 1.5 format it was {"freq", "unused"}
+	EvStack:             {"Stack", 1005, false, []string{"id", "siz"}, nil},
+	EvGomaxprocs:        {"Gomaxprocs", 1005, true, []string{"procs"}, nil},
+	EvProcStart:         {"ProcStart", 1005, false, []string{"thread"}, nil},
+	EvProcStop:          {"ProcStop", 1005, false, []string{}, nil},
+	EvGCStart:           {"GCStart", 1005, true, []string{"seq"}, nil}, // in 1.5 format it was {}
+	EvGCDone:            {"GCDone", 1005, false, []string{}, nil},
+	EvGCSTWStart:        {"GCSTWStart", 1005, false, []string{"kindid"}, []string{"kind"}}, // <= 1.9, args was {} (implicitly {0})
+	EvGCSTWDone:         {"GCSTWDone", 1005, false, []string{}, nil},
+	EvGCSweepStart:      {"GCSweepStart", 1005, true, []string{}, nil},
+	EvGCSweepDone:       {"GCSweepDone", 1005, false, []string{"swept", "reclaimed"}, nil}, // before 1.9, format was {}
+	EvGoCreate:          {"GoCreate", 1005, true, []string{"g", "stack"}, nil},
+	EvGoStart:           {"GoStart", 1005, false, []string{"g", "seq"}, nil}, // in 1.5 format it was {"g"}
+	EvGoEnd:             {"GoEnd", 1005, false, []string{}, nil},
+	EvGoStop:            {"GoStop", 1005, true, []string{}, nil},
+	EvGoSched:           {"GoSched", 1005, true, []string{}, nil},
+	EvGoPreempt:         {"GoPreempt", 1005, true, []string{}, nil},
+	EvGoSleep:           {"GoSleep", 1005, true, []string{}, nil},
+	EvGoBlock:           {"GoBlock", 1005, true, []string{}, nil},
+	EvGoUnblock:         {"GoUnblock", 1005, true, []string{"g", "seq"}, nil}, // in 1.5 format it was {"g"}
+	EvGoBlockSend:       {"GoBlockSend", 1005, true, []string{}, nil},
+	EvGoBlockRecv:       {"GoBlockRecv", 1005, true, []string{}, nil},
+	EvGoBlockSelect:     {"GoBlockSelect", 1005, true, []string{}, nil},
+	EvGoBlockSync:       {"GoBlockSync", 1005, true, []string{}, nil},
+	EvGoBlockCond:       {"GoBlockCond", 1005, true, []string{}, nil},
+	EvGoBlockNet:        {"GoBlockNet", 1005, true, []string{}, nil},
+	EvGoSysCall:         {"GoSysCall", 1005, true, []string{}, nil},
+	EvGoSysExit:         {"GoSysExit", 1005, false, []string{"g", "seq", "ts"}, nil},
+	EvGoSysBlock:        {"GoSysBlock", 1005, false, []string{}, nil},
+	EvGoWaiting:         {"GoWaiting", 1005, false, []string{"g"}, nil},
+	EvGoInSyscall:       {"GoInSyscall", 1005, false, []string{"g"}, nil},
+	EvHeapAlloc:         {"HeapAlloc", 1005, false, []string{"mem"}, nil},
+	EvHeapGoal:          {"HeapGoal", 1005, false, []string{"mem"}, nil},
+	EvTimerGoroutine:    {"TimerGoroutine", 1005, false, []string{"g"}, nil}, // in 1.5 format it was {"g", "unused"}
+	EvFutileWakeup:      {"FutileWakeup", 1005, false, []string{}, nil},
+	EvString:            {"String", 1007, false, []string{}, nil},
+	EvGoStartLocal:      {"GoStartLocal", 1007, false, []string{"g"}, nil},
+	EvGoUnblockLocal:    {"GoUnblockLocal", 1007, true, []string{"g"}, nil},
+	EvGoSysExitLocal:    {"GoSysExitLocal", 1007, false, []string{"g", "ts"}, nil},
+	EvGoStartLabel:      {"GoStartLabel", 1008, false, []string{"g", "seq", "labelid"}, []string{"label"}},
+	EvGoBlockGC:         {"GoBlockGC", 1008, true, []string{}, nil},
+	EvGCMarkAssistStart: {"GCMarkAssistStart", 1009, true, []string{}, nil},
+	EvGCMarkAssistDone:  {"GCMarkAssistDone", 1009, false, []string{}, nil},
+	EvUserTaskCreate:    {"UserTaskCreate", 1011, true, []string{"taskid", "pid", "typeid"}, []string{"name"}},
+	EvUserTaskEnd:       {"UserTaskEnd", 1011, true, []string{"taskid"}, nil},
+	EvUserRegion:        {"UserRegion", 1011, true, []string{"taskid", "mode", "typeid"}, []string{"name"}},
+	EvUserLog:           {"UserLog", 1011, true, []string{"id", "keyid"}, []string{"category", "message"}},
+	EvCPUSample:         {"CPUSample", 1019, true, []string{"ts", "p", "g"}, nil},
+}
diff --git a/src/internal/trace/parser_test.go b/src/internal/trace/parser_test.go
new file mode 100644
index 0000000..cdab95a
--- /dev/null
+++ b/src/internal/trace/parser_test.go
@@ -0,0 +1,110 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestCorruptedInputs(t *testing.T) {
+	// These inputs crashed parser previously.
+	tests := []string{
+		"gotrace\x00\x020",
+		"gotrace\x00Q00\x020",
+		"gotrace\x00T00\x020",
+		"gotrace\x00\xc3\x0200",
+		"go 1.5 trace\x00\x00\x00\x00\x020",
+		"go 1.5 trace\x00\x00\x00\x00Q00\x020",
+		"go 1.5 trace\x00\x00\x00\x00T00\x020",
+		"go 1.5 trace\x00\x00\x00\x00\xc3\x0200",
+	}
+	for _, data := range tests {
+		res, err := Parse(strings.NewReader(data), "")
+		if err == nil || res.Events != nil || res.Stacks != nil {
+			t.Fatalf("no error on input: %q", data)
+		}
+	}
+}
+
+func TestParseCanned(t *testing.T) {
+	files, err := os.ReadDir("./testdata")
+	if err != nil {
+		t.Fatalf("failed to read ./testdata: %v", err)
+	}
+	for _, f := range files {
+		info, err := f.Info()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if testing.Short() && info.Size() > 10000 {
+			continue
+		}
+		name := filepath.Join("./testdata", f.Name())
+		data, err := os.ReadFile(name)
+		if err != nil {
+			t.Fatal(err)
+		}
+		// Instead of Parse that requires a proper binary name for old traces,
+		// we use 'parse' that omits symbol lookup if an empty string is given.
+		_, _, err = parse(bytes.NewReader(data), "")
+		switch {
+		case strings.HasSuffix(f.Name(), "_good"):
+			if err != nil {
+				t.Errorf("failed to parse good trace %v: %v", f.Name(), err)
+			}
+		case strings.HasSuffix(f.Name(), "_unordered"):
+			if err != ErrTimeOrder {
+				t.Errorf("unordered trace is not detected %v: %v", f.Name(), err)
+			}
+		default:
+			t.Errorf("unknown input file suffix: %v", f.Name())
+		}
+	}
+}
+
+func TestParseVersion(t *testing.T) {
+	tests := map[string]int{
+		"go 1.5 trace\x00\x00\x00\x00": 1005,
+		"go 1.7 trace\x00\x00\x00\x00": 1007,
+		"go 1.10 trace\x00\x00\x00":    1010,
+		"go 1.25 trace\x00\x00\x00":    1025,
+		"go 1.234 trace\x00\x00":       1234,
+		"go 1.2345 trace\x00":          -1,
+		"go 0.0 trace\x00\x00\x00\x00": -1,
+		"go a.b trace\x00\x00\x00\x00": -1,
+	}
+	for header, ver := range tests {
+		ver1, err := parseHeader([]byte(header))
+		if ver == -1 {
+			if err == nil {
+				t.Fatalf("no error on input: %q, version %v", header, ver1)
+			}
+		} else {
+			if err != nil {
+				t.Fatalf("failed to parse: %q (%v)", header, err)
+			}
+			if ver != ver1 {
+				t.Fatalf("wrong version: %v, want %v, input: %q", ver1, ver, header)
+			}
+		}
+	}
+}
+
+func TestTimestampOverflow(t *testing.T) {
+	// Test that parser correctly handles large timestamps (long tracing).
+	w := NewWriter()
+	w.Emit(EvBatch, 0, 0)
+	w.Emit(EvFrequency, 1e9)
+	for ts := uint64(1); ts < 1e16; ts *= 2 {
+		w.Emit(EvGoCreate, ts, ts, 0, 0)
+	}
+	if _, err := Parse(w, ""); err != nil {
+		t.Fatalf("failed to parse: %v", err)
+	}
+}
diff --git a/src/internal/trace/testdata/http_1_10_good b/src/internal/trace/testdata/http_1_10_good
new file mode 100644
index 0000000..a4f2ed8
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_10_good
diff --git a/src/internal/trace/testdata/http_1_11_good b/src/internal/trace/testdata/http_1_11_good
new file mode 100644
index 0000000..0efcc6f
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_11_good
diff --git a/src/internal/trace/testdata/http_1_19_good b/src/internal/trace/testdata/http_1_19_good
new file mode 100644
index 0000000..c1d519e
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_19_good
diff --git a/src/internal/trace/testdata/http_1_5_good b/src/internal/trace/testdata/http_1_5_good
new file mode 100644
index 0000000..0736cae
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_5_good
diff --git a/src/internal/trace/testdata/http_1_7_good b/src/internal/trace/testdata/http_1_7_good
new file mode 100644
index 0000000..b0e318e
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_7_good
diff --git a/src/internal/trace/testdata/http_1_9_good b/src/internal/trace/testdata/http_1_9_good
new file mode 100644
index 0000000..ca89278
--- /dev/null
+++ b/src/internal/trace/testdata/http_1_9_good
diff --git a/src/internal/trace/testdata/stress_1_10_good b/src/internal/trace/testdata/stress_1_10_good
new file mode 100644
index 0000000..19778b0
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_10_good
diff --git a/src/internal/trace/testdata/stress_1_11_good b/src/internal/trace/testdata/stress_1_11_good
new file mode 100644
index 0000000..6468d89
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_11_good
diff --git a/src/internal/trace/testdata/stress_1_19_good b/src/internal/trace/testdata/stress_1_19_good
new file mode 100644
index 0000000..13f5926
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_19_good
diff --git a/src/internal/trace/testdata/stress_1_5_good b/src/internal/trace/testdata/stress_1_5_good
new file mode 100644
index 0000000..c5055eb
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_5_good
diff --git a/src/internal/trace/testdata/stress_1_5_unordered b/src/internal/trace/testdata/stress_1_5_unordered
new file mode 100644
index 0000000..11f7d74
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_5_unordered
diff --git a/src/internal/trace/testdata/stress_1_7_good b/src/internal/trace/testdata/stress_1_7_good
new file mode 100644
index 0000000..b4d927d
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_7_good
diff --git a/src/internal/trace/testdata/stress_1_9_good b/src/internal/trace/testdata/stress_1_9_good
new file mode 100644
index 0000000..dcf17f1
--- /dev/null
+++ b/src/internal/trace/testdata/stress_1_9_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_10_good b/src/internal/trace/testdata/stress_start_stop_1_10_good
new file mode 100644
index 0000000..b908e10
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_10_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_11_good b/src/internal/trace/testdata/stress_start_stop_1_11_good
new file mode 100644
index 0000000..457f01a
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_11_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_19_good b/src/internal/trace/testdata/stress_start_stop_1_19_good
new file mode 100644
index 0000000..92d9278
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_19_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_5_good b/src/internal/trace/testdata/stress_start_stop_1_5_good
new file mode 100644
index 0000000..72a887b
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_5_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_7_good b/src/internal/trace/testdata/stress_start_stop_1_7_good
new file mode 100644
index 0000000..c23ed7d
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_7_good
diff --git a/src/internal/trace/testdata/stress_start_stop_1_9_good b/src/internal/trace/testdata/stress_start_stop_1_9_good
new file mode 100644
index 0000000..f00f190
--- /dev/null
+++ b/src/internal/trace/testdata/stress_start_stop_1_9_good
diff --git a/src/internal/trace/testdata/user_task_region_1_11_good b/src/internal/trace/testdata/user_task_region_1_11_good
new file mode 100644
index 0000000..f4edb67
--- /dev/null
+++ b/src/internal/trace/testdata/user_task_region_1_11_good
diff --git a/src/internal/trace/testdata/user_task_region_1_19_good b/src/internal/trace/testdata/user_task_region_1_19_good
new file mode 100644
index 0000000..1daa3b2
--- /dev/null
+++ b/src/internal/trace/testdata/user_task_region_1_19_good
diff --git a/src/internal/trace/writer.go b/src/internal/trace/writer.go
new file mode 100644
index 0000000..dd0b9f1
--- /dev/null
+++ b/src/internal/trace/writer.go
@@ -0,0 +1,49 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import "bytes"
+
+// Writer is a test trace writer.
+type Writer struct {
+	bytes.Buffer
+}
+
+func NewWriter() *Writer {
+	w := new(Writer)
+	w.Write([]byte("go 1.9 trace\x00\x00\x00\x00"))
+	return w
+}
+
+// Emit writes an event record to the trace.
+// See Event types for valid types and required arguments.
+func (w *Writer) Emit(typ byte, args ...uint64) {
+	nargs := byte(len(args)) - 1
+	if nargs > 3 {
+		nargs = 3
+	}
+	buf := []byte{typ | nargs<<6}
+	if nargs == 3 {
+		buf = append(buf, 0)
+	}
+	for _, a := range args {
+		buf = appendVarint(buf, a)
+	}
+	if nargs == 3 {
+		buf[1] = byte(len(buf) - 2)
+	}
+	n, err := w.Write(buf)
+	if n != len(buf) || err != nil {
+		panic("failed to write")
+	}
+}
+
+func appendVarint(buf []byte, v uint64) []byte {
+	for ; v >= 0x80; v >>= 7 {
+		buf = append(buf, 0x80|byte(v))
+	}
+	buf = append(buf, byte(v))
+	return buf
+}
diff --git a/src/internal/txtar/archive.go b/src/internal/txtar/archive.go
new file mode 100644
index 0000000..fd95f1e
--- /dev/null
+++ b/src/internal/txtar/archive.go
@@ -0,0 +1,140 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package txtar implements a trivial text-based file archive format.
+//
+// The goals for the format are:
+//
+//   - be trivial enough to create and edit by hand.
+//   - be able to store trees of text files describing go command test cases.
+//   - diff nicely in git history and code reviews.
+//
+// Non-goals include being a completely general archive format,
+// storing binary data, storing file modes, storing special files like
+// symbolic links, and so on.
+//
+// # Txtar format
+//
+// A txtar archive is zero or more comment lines and then a sequence of file entries.
+// Each file entry begins with a file marker line of the form "-- FILENAME --"
+// and is followed by zero or more file content lines making up the file data.
+// The comment or file content ends at the next file marker line.
+// The file marker line must begin with the three-byte sequence "-- "
+// and end with the three-byte sequence " --", but the enclosed
+// file name can be surrounding by additional white space,
+// all of which is stripped.
+//
+// If the txtar file is missing a trailing newline on the final line,
+// parsers should consider a final newline to be present anyway.
+//
+// There are no possible syntax errors in a txtar archive.
+package txtar
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// An Archive is a collection of files.
+type Archive struct {
+	Comment []byte
+	Files   []File
+}
+
+// A File is a single file in an archive.
+type File struct {
+	Name string // name of file ("foo/bar.txt")
+	Data []byte // text content of file
+}
+
+// Format returns the serialized form of an Archive.
+// It is assumed that the Archive data structure is well-formed:
+// a.Comment and all a.File[i].Data contain no file marker lines,
+// and all a.File[i].Name is non-empty.
+func Format(a *Archive) []byte {
+	var buf bytes.Buffer
+	buf.Write(fixNL(a.Comment))
+	for _, f := range a.Files {
+		fmt.Fprintf(&buf, "-- %s --\n", f.Name)
+		buf.Write(fixNL(f.Data))
+	}
+	return buf.Bytes()
+}
+
+// ParseFile parses the named file as an archive.
+func ParseFile(file string) (*Archive, error) {
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return nil, err
+	}
+	return Parse(data), nil
+}
+
+// Parse parses the serialized form of an Archive.
+// The returned Archive holds slices of data.
+func Parse(data []byte) *Archive {
+	a := new(Archive)
+	var name string
+	a.Comment, name, data = findFileMarker(data)
+	for name != "" {
+		f := File{name, nil}
+		f.Data, name, data = findFileMarker(data)
+		a.Files = append(a.Files, f)
+	}
+	return a
+}
+
+var (
+	newlineMarker = []byte("\n-- ")
+	marker        = []byte("-- ")
+	markerEnd     = []byte(" --")
+)
+
+// findFileMarker finds the next file marker in data,
+// extracts the file name, and returns the data before the marker,
+// the file name, and the data after the marker.
+// If there is no next marker, findFileMarker returns before = fixNL(data), name = "", after = nil.
+func findFileMarker(data []byte) (before []byte, name string, after []byte) {
+	var i int
+	for {
+		if name, after = isMarker(data[i:]); name != "" {
+			return data[:i], name, after
+		}
+		j := bytes.Index(data[i:], newlineMarker)
+		if j < 0 {
+			return fixNL(data), "", nil
+		}
+		i += j + 1 // positioned at start of new possible marker
+	}
+}
+
+// isMarker checks whether data begins with a file marker line.
+// If so, it returns the name from the line and the data after the line.
+// Otherwise it returns name == "" with an unspecified after.
+func isMarker(data []byte) (name string, after []byte) {
+	if !bytes.HasPrefix(data, marker) {
+		return "", nil
+	}
+	if i := bytes.IndexByte(data, '\n'); i >= 0 {
+		data, after = data[:i], data[i+1:]
+	}
+	if !(bytes.HasSuffix(data, markerEnd) && len(data) >= len(marker)+len(markerEnd)) {
+		return "", nil
+	}
+	return strings.TrimSpace(string(data[len(marker) : len(data)-len(markerEnd)])), after
+}
+
+// If data is empty or ends in \n, fixNL returns data.
+// Otherwise fixNL returns a new slice consisting of data with a final \n added.
+func fixNL(data []byte) []byte {
+	if len(data) == 0 || data[len(data)-1] == '\n' {
+		return data
+	}
+	d := make([]byte, len(data)+1)
+	copy(d, data)
+	d[len(data)] = '\n'
+	return d
+}
diff --git a/src/internal/types/errors/codes.go b/src/internal/types/errors/codes.go
new file mode 100644
index 0000000..acddcbb
--- /dev/null
+++ b/src/internal/types/errors/codes.go
@@ -0,0 +1,1442 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors
+
+type Code int
+
+// This file defines the error codes that can be produced during type-checking.
+// Collectively, these codes provide an identifier that may be used to
+// implement special handling for certain types of errors.
+//
+// Error code values should not be changed: add new codes at the end.
+//
+// Error codes should be fine-grained enough that the exact nature of the error
+// can be easily determined, but coarse enough that they are not an
+// implementation detail of the type checking algorithm. As a rule-of-thumb,
+// errors should be considered equivalent if there is a theoretical refactoring
+// of the type checker in which they are emitted in exactly one place. For
+// example, the type checker emits different error messages for "too many
+// arguments" and "too few arguments", but one can imagine an alternative type
+// checker where this check instead just emits a single "wrong number of
+// arguments", so these errors should have the same code.
+//
+// Error code names should be as brief as possible while retaining accuracy and
+// distinctiveness. In most cases names should start with an adjective
+// describing the nature of the error (e.g. "invalid", "unused", "misplaced"),
+// and end with a noun identifying the relevant language object. For example,
+// "_DuplicateDecl" or "_InvalidSliceExpr". For brevity, naming follows the
+// convention that "bad" implies a problem with syntax, and "invalid" implies a
+// problem with types.
+
+const (
+	// InvalidSyntaxTree occurs if an invalid syntax tree is provided
+	// to the type checker. It should never happen.
+	InvalidSyntaxTree Code = -1
+)
+
+const (
+	// The zero Code value indicates an unset (invalid) error code.
+	_ Code = iota
+
+	// Test is reserved for errors that only apply while in self-test mode.
+	Test
+
+	// BlankPkgName occurs when a package name is the blank identifier "_".
+	//
+	// Per the spec:
+	//  "The PackageName must not be the blank identifier."
+	//
+	// Example:
+	//  package _
+	BlankPkgName
+
+	// MismatchedPkgName occurs when a file's package name doesn't match the
+	// package name already established by other files.
+	MismatchedPkgName
+
+	// InvalidPkgUse occurs when a package identifier is used outside of a
+	// selector expression.
+	//
+	// Example:
+	//  import "fmt"
+	//
+	//  var _ = fmt
+	InvalidPkgUse
+
+	// BadImportPath occurs when an import path is not valid.
+	BadImportPath
+
+	// BrokenImport occurs when importing a package fails.
+	//
+	// Example:
+	//  import "amissingpackage"
+	BrokenImport
+
+	// ImportCRenamed occurs when the special import "C" is renamed. "C" is a
+	// pseudo-package, and must not be renamed.
+	//
+	// Example:
+	//  import _ "C"
+	ImportCRenamed
+
+	// UnusedImport occurs when an import is unused.
+	//
+	// Example:
+	//  import "fmt"
+	//
+	//  func main() {}
+	UnusedImport
+
+	// InvalidInitCycle occurs when an invalid cycle is detected within the
+	// initialization graph.
+	//
+	// Example:
+	//  var x int = f()
+	//
+	//  func f() int { return x }
+	InvalidInitCycle
+
+	// DuplicateDecl occurs when an identifier is declared multiple times.
+	//
+	// Example:
+	//  var x = 1
+	//  var x = 2
+	DuplicateDecl
+
+	// InvalidDeclCycle occurs when a declaration cycle is not valid.
+	//
+	// Example:
+	//  type S struct {
+	//  	S
+	//  }
+	//
+	InvalidDeclCycle
+
+	// InvalidTypeCycle occurs when a cycle in type definitions results in a
+	// type that is not well-defined.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type T [unsafe.Sizeof(T{})]int
+	InvalidTypeCycle
+
+	// InvalidConstInit occurs when a const declaration has a non-constant
+	// initializer.
+	//
+	// Example:
+	//  var x int
+	//  const _ = x
+	InvalidConstInit
+
+	// InvalidConstVal occurs when a const value cannot be converted to its
+	// target type.
+	//
+	// TODO(findleyr): this error code and example are not very clear. Consider
+	// removing it.
+	//
+	// Example:
+	//  const _ = 1 << "hello"
+	InvalidConstVal
+
+	// InvalidConstType occurs when the underlying type in a const declaration
+	// is not a valid constant type.
+	//
+	// Example:
+	//  const c *int = 4
+	InvalidConstType
+
+	// UntypedNilUse occurs when the predeclared (untyped) value nil is used to
+	// initialize a variable declared without an explicit type.
+	//
+	// Example:
+	//  var x = nil
+	UntypedNilUse
+
+	// WrongAssignCount occurs when the number of values on the right-hand side
+	// of an assignment or initialization expression does not match the number
+	// of variables on the left-hand side.
+	//
+	// Example:
+	//  var x = 1, 2
+	WrongAssignCount
+
+	// UnassignableOperand occurs when the left-hand side of an assignment is
+	// not assignable.
+	//
+	// Example:
+	//  func f() {
+	//  	const c = 1
+	//  	c = 2
+	//  }
+	UnassignableOperand
+
+	// NoNewVar occurs when a short variable declaration (':=') does not declare
+	// new variables.
+	//
+	// Example:
+	//  func f() {
+	//  	x := 1
+	//  	x := 2
+	//  }
+	NoNewVar
+
+	// MultiValAssignOp occurs when an assignment operation (+=, *=, etc) does
+	// not have single-valued left-hand or right-hand side.
+	//
+	// Per the spec:
+	//  "In assignment operations, both the left- and right-hand expression lists
+	//  must contain exactly one single-valued expression"
+	//
+	// Example:
+	//  func f() int {
+	//  	x, y := 1, 2
+	//  	x, y += 1
+	//  	return x + y
+	//  }
+	MultiValAssignOp
+
+	// InvalidIfaceAssign occurs when a value of type T is used as an
+	// interface, but T does not implement a method of the expected interface.
+	//
+	// Example:
+	//  type I interface {
+	//  	f()
+	//  }
+	//
+	//  type T int
+	//
+	//  var x I = T(1)
+	InvalidIfaceAssign
+
+	// InvalidChanAssign occurs when a chan assignment is invalid.
+	//
+	// Per the spec, a value x is assignable to a channel type T if:
+	//  "x is a bidirectional channel value, T is a channel type, x's type V and
+	//  T have identical element types, and at least one of V or T is not a
+	//  defined type."
+	//
+	// Example:
+	//  type T1 chan int
+	//  type T2 chan int
+	//
+	//  var x T1
+	//  // Invalid assignment because both types are named
+	//  var _ T2 = x
+	InvalidChanAssign
+
+	// IncompatibleAssign occurs when the type of the right-hand side expression
+	// in an assignment cannot be assigned to the type of the variable being
+	// assigned.
+	//
+	// Example:
+	//  var x []int
+	//  var _ int = x
+	IncompatibleAssign
+
+	// UnaddressableFieldAssign occurs when trying to assign to a struct field
+	// in a map value.
+	//
+	// Example:
+	//  func f() {
+	//  	m := make(map[string]struct{i int})
+	//  	m["foo"].i = 42
+	//  }
+	UnaddressableFieldAssign
+
+	// NotAType occurs when the identifier used as the underlying type in a type
+	// declaration or the right-hand side of a type alias does not denote a type.
+	//
+	// Example:
+	//  var S = 2
+	//
+	//  type T S
+	NotAType
+
+	// InvalidArrayLen occurs when an array length is not a constant value.
+	//
+	// Example:
+	//  var n = 3
+	//  var _ = [n]int{}
+	InvalidArrayLen
+
+	// BlankIfaceMethod occurs when a method name is '_'.
+	//
+	// Per the spec:
+	//  "The name of each explicitly specified method must be unique and not
+	//  blank."
+	//
+	// Example:
+	//  type T interface {
+	//  	_(int)
+	//  }
+	BlankIfaceMethod
+
+	// IncomparableMapKey occurs when a map key type does not support the == and
+	// != operators.
+	//
+	// Per the spec:
+	//  "The comparison operators == and != must be fully defined for operands of
+	//  the key type; thus the key type must not be a function, map, or slice."
+	//
+	// Example:
+	//  var x map[T]int
+	//
+	//  type T []int
+	IncomparableMapKey
+
+	// InvalidIfaceEmbed occurs when a non-interface type is embedded in an
+	// interface (for go 1.17 or earlier).
+	_ // not used anymore
+
+	// InvalidPtrEmbed occurs when an embedded field is of the pointer form *T,
+	// and T itself is itself a pointer, an unsafe.Pointer, or an interface.
+	//
+	// Per the spec:
+	//  "An embedded field must be specified as a type name T or as a pointer to
+	//  a non-interface type name *T, and T itself may not be a pointer type."
+	//
+	// Example:
+	//  type T *int
+	//
+	//  type S struct {
+	//  	*T
+	//  }
+	InvalidPtrEmbed
+
+	// BadRecv occurs when a method declaration does not have exactly one
+	// receiver parameter.
+	//
+	// Example:
+	//  func () _() {}
+	BadRecv
+
+	// InvalidRecv occurs when a receiver type expression is not of the form T
+	// or *T, or T is a pointer type.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func (**T) m() {}
+	InvalidRecv
+
+	// DuplicateFieldAndMethod occurs when an identifier appears as both a field
+	// and method name.
+	//
+	// Example:
+	//  type T struct {
+	//  	m int
+	//  }
+	//
+	//  func (T) m() {}
+	DuplicateFieldAndMethod
+
+	// DuplicateMethod occurs when two methods on the same receiver type have
+	// the same name.
+	//
+	// Example:
+	//  type T struct {}
+	//  func (T) m() {}
+	//  func (T) m(i int) int { return i }
+	DuplicateMethod
+
+	// InvalidBlank occurs when a blank identifier is used as a value or type.
+	//
+	// Per the spec:
+	//  "The blank identifier may appear as an operand only on the left-hand side
+	//  of an assignment."
+	//
+	// Example:
+	//  var x = _
+	InvalidBlank
+
+	// InvalidIota occurs when the predeclared identifier iota is used outside
+	// of a constant declaration.
+	//
+	// Example:
+	//  var x = iota
+	InvalidIota
+
+	// MissingInitBody occurs when an init function is missing its body.
+	//
+	// Example:
+	//  func init()
+	MissingInitBody
+
+	// InvalidInitSig occurs when an init function declares parameters or
+	// results.
+	//
+	// Deprecated: no longer emitted by the type checker. _InvalidInitDecl is
+	// used instead.
+	InvalidInitSig
+
+	// InvalidInitDecl occurs when init is declared as anything other than a
+	// function.
+	//
+	// Example:
+	//  var init = 1
+	//
+	// Example:
+	//  func init() int { return 1 }
+	InvalidInitDecl
+
+	// InvalidMainDecl occurs when main is declared as anything other than a
+	// function, in a main package.
+	InvalidMainDecl
+
+	// TooManyValues occurs when a function returns too many values for the
+	// expression context in which it is used.
+	//
+	// Example:
+	//  func ReturnTwo() (int, int) {
+	//  	return 1, 2
+	//  }
+	//
+	//  var x = ReturnTwo()
+	TooManyValues
+
+	// NotAnExpr occurs when a type expression is used where a value expression
+	// is expected.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func f() {
+	//  	T
+	//  }
+	NotAnExpr
+
+	// TruncatedFloat occurs when a float constant is truncated to an integer
+	// value.
+	//
+	// Example:
+	//  var _ int = 98.6
+	TruncatedFloat
+
+	// NumericOverflow occurs when a numeric constant overflows its target type.
+	//
+	// Example:
+	//  var x int8 = 1000
+	NumericOverflow
+
+	// UndefinedOp occurs when an operator is not defined for the type(s) used
+	// in an operation.
+	//
+	// Example:
+	//  var c = "a" - "b"
+	UndefinedOp
+
+	// MismatchedTypes occurs when operand types are incompatible in a binary
+	// operation.
+	//
+	// Example:
+	//  var a = "hello"
+	//  var b = 1
+	//  var c = a - b
+	MismatchedTypes
+
+	// DivByZero occurs when a division operation is provable at compile
+	// time to be a division by zero.
+	//
+	// Example:
+	//  const divisor = 0
+	//  var x int = 1/divisor
+	DivByZero
+
+	// NonNumericIncDec occurs when an increment or decrement operator is
+	// applied to a non-numeric value.
+	//
+	// Example:
+	//  func f() {
+	//  	var c = "c"
+	//  	c++
+	//  }
+	NonNumericIncDec
+
+	// UnaddressableOperand occurs when the & operator is applied to an
+	// unaddressable expression.
+	//
+	// Example:
+	//  var x = &1
+	UnaddressableOperand
+
+	// InvalidIndirection occurs when a non-pointer value is indirected via the
+	// '*' operator.
+	//
+	// Example:
+	//  var x int
+	//  var y = *x
+	InvalidIndirection
+
+	// NonIndexableOperand occurs when an index operation is applied to a value
+	// that cannot be indexed.
+	//
+	// Example:
+	//  var x = 1
+	//  var y = x[1]
+	NonIndexableOperand
+
+	// InvalidIndex occurs when an index argument is not of integer type,
+	// negative, or out-of-bounds.
+	//
+	// Example:
+	//  var s = [...]int{1,2,3}
+	//  var x = s[5]
+	//
+	// Example:
+	//  var s = []int{1,2,3}
+	//  var _ = s[-1]
+	//
+	// Example:
+	//  var s = []int{1,2,3}
+	//  var i string
+	//  var _ = s[i]
+	InvalidIndex
+
+	// SwappedSliceIndices occurs when constant indices in a slice expression
+	// are decreasing in value.
+	//
+	// Example:
+	//  var _ = []int{1,2,3}[2:1]
+	SwappedSliceIndices
+
+	// NonSliceableOperand occurs when a slice operation is applied to a value
+	// whose type is not sliceable, or is unaddressable.
+	//
+	// Example:
+	//  var x = [...]int{1, 2, 3}[:1]
+	//
+	// Example:
+	//  var x = 1
+	//  var y = 1[:1]
+	NonSliceableOperand
+
+	// InvalidSliceExpr occurs when a three-index slice expression (a[x:y:z]) is
+	// applied to a string.
+	//
+	// Example:
+	//  var s = "hello"
+	//  var x = s[1:2:3]
+	InvalidSliceExpr
+
+	// InvalidShiftCount occurs when the right-hand side of a shift operation is
+	// either non-integer, negative, or too large.
+	//
+	// Example:
+	//  var (
+	//  	x string
+	//  	y int = 1 << x
+	//  )
+	InvalidShiftCount
+
+	// InvalidShiftOperand occurs when the shifted operand is not an integer.
+	//
+	// Example:
+	//  var s = "hello"
+	//  var x = s << 2
+	InvalidShiftOperand
+
+	// InvalidReceive occurs when there is a channel receive from a value that
+	// is either not a channel, or is a send-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x = 1
+	//  	<-x
+	//  }
+	InvalidReceive
+
+	// InvalidSend occurs when there is a channel send to a value that is not a
+	// channel, or is a receive-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x = 1
+	//  	x <- "hello!"
+	//  }
+	InvalidSend
+
+	// DuplicateLitKey occurs when an index is duplicated in a slice, array, or
+	// map literal.
+	//
+	// Example:
+	//  var _ = []int{0:1, 0:2}
+	//
+	// Example:
+	//  var _ = map[string]int{"a": 1, "a": 2}
+	DuplicateLitKey
+
+	// MissingLitKey occurs when a map literal is missing a key expression.
+	//
+	// Example:
+	//  var _ = map[string]int{1}
+	MissingLitKey
+
+	// InvalidLitIndex occurs when the key in a key-value element of a slice or
+	// array literal is not an integer constant.
+	//
+	// Example:
+	//  var i = 0
+	//  var x = []string{i: "world"}
+	InvalidLitIndex
+
+	// OversizeArrayLit occurs when an array literal exceeds its length.
+	//
+	// Example:
+	//  var _ = [2]int{1,2,3}
+	OversizeArrayLit
+
+	// MixedStructLit occurs when a struct literal contains a mix of positional
+	// and named elements.
+	//
+	// Example:
+	//  var _ = struct{i, j int}{i: 1, 2}
+	MixedStructLit
+
+	// InvalidStructLit occurs when a positional struct literal has an incorrect
+	// number of values.
+	//
+	// Example:
+	//  var _ = struct{i, j int}{1,2,3}
+	InvalidStructLit
+
+	// MissingLitField occurs when a struct literal refers to a field that does
+	// not exist on the struct type.
+	//
+	// Example:
+	//  var _ = struct{i int}{j: 2}
+	MissingLitField
+
+	// DuplicateLitField occurs when a struct literal contains duplicated
+	// fields.
+	//
+	// Example:
+	//  var _ = struct{i int}{i: 1, i: 2}
+	DuplicateLitField
+
+	// UnexportedLitField occurs when a positional struct literal implicitly
+	// assigns an unexported field of an imported type.
+	UnexportedLitField
+
+	// InvalidLitField occurs when a field name is not a valid identifier.
+	//
+	// Example:
+	//  var _ = struct{i int}{1: 1}
+	InvalidLitField
+
+	// UntypedLit occurs when a composite literal omits a required type
+	// identifier.
+	//
+	// Example:
+	//  type outer struct{
+	//  	inner struct { i int }
+	//  }
+	//
+	//  var _ = outer{inner: {1}}
+	UntypedLit
+
+	// InvalidLit occurs when a composite literal expression does not match its
+	// type.
+	//
+	// Example:
+	//  type P *struct{
+	//  	x int
+	//  }
+	//  var _ = P {}
+	InvalidLit
+
+	// AmbiguousSelector occurs when a selector is ambiguous.
+	//
+	// Example:
+	//  type E1 struct { i int }
+	//  type E2 struct { i int }
+	//  type T struct { E1; E2 }
+	//
+	//  var x T
+	//  var _ = x.i
+	AmbiguousSelector
+
+	// UndeclaredImportedName occurs when a package-qualified identifier is
+	// undeclared by the imported package.
+	//
+	// Example:
+	//  import "go/types"
+	//
+	//  var _ = types.NotAnActualIdentifier
+	UndeclaredImportedName
+
+	// UnexportedName occurs when a selector refers to an unexported identifier
+	// of an imported package.
+	//
+	// Example:
+	//  import "reflect"
+	//
+	//  type _ reflect.flag
+	UnexportedName
+
+	// UndeclaredName occurs when an identifier is not declared in the current
+	// scope.
+	//
+	// Example:
+	//  var x T
+	UndeclaredName
+
+	// MissingFieldOrMethod occurs when a selector references a field or method
+	// that does not exist.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  var x = T{}.f
+	MissingFieldOrMethod
+
+	// BadDotDotDotSyntax occurs when a "..." occurs in a context where it is
+	// not valid.
+	//
+	// Example:
+	//  var _ = map[int][...]int{0: {}}
+	BadDotDotDotSyntax
+
+	// NonVariadicDotDotDot occurs when a "..." is used on the final argument to
+	// a non-variadic function.
+	//
+	// Example:
+	//  func printArgs(s []string) {
+	//  	for _, a := range s {
+	//  		println(a)
+	//  	}
+	//  }
+	//
+	//  func f() {
+	//  	s := []string{"a", "b", "c"}
+	//  	printArgs(s...)
+	//  }
+	NonVariadicDotDotDot
+
+	// MisplacedDotDotDot occurs when a "..." is used somewhere other than the
+	// final argument in a function declaration.
+	//
+	// Example:
+	// 	func f(...int, int)
+	MisplacedDotDotDot
+
+	_ // InvalidDotDotDotOperand was removed.
+
+	// InvalidDotDotDot occurs when a "..." is used in a non-variadic built-in
+	// function.
+	//
+	// Example:
+	//  var s = []int{1, 2, 3}
+	//  var l = len(s...)
+	InvalidDotDotDot
+
+	// UncalledBuiltin occurs when a built-in function is used as a
+	// function-valued expression, instead of being called.
+	//
+	// Per the spec:
+	//  "The built-in functions do not have standard Go types, so they can only
+	//  appear in call expressions; they cannot be used as function values."
+	//
+	// Example:
+	//  var _ = copy
+	UncalledBuiltin
+
+	// InvalidAppend occurs when append is called with a first argument that is
+	// not a slice.
+	//
+	// Example:
+	//  var _ = append(1, 2)
+	InvalidAppend
+
+	// InvalidCap occurs when an argument to the cap built-in function is not of
+	// supported type.
+	//
+	// See https://golang.org/ref/spec#Length_and_capacity for information on
+	// which underlying types are supported as arguments to cap and len.
+	//
+	// Example:
+	//  var s = 2
+	//  var x = cap(s)
+	InvalidCap
+
+	// InvalidClose occurs when close(...) is called with an argument that is
+	// not of channel type, or that is a receive-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x int
+	//  	close(x)
+	//  }
+	InvalidClose
+
+	// InvalidCopy occurs when the arguments are not of slice type or do not
+	// have compatible type.
+	//
+	// See https://golang.org/ref/spec#Appending_and_copying_slices for more
+	// information on the type requirements for the copy built-in.
+	//
+	// Example:
+	//  func f() {
+	//  	var x []int
+	//  	y := []int64{1,2,3}
+	//  	copy(x, y)
+	//  }
+	InvalidCopy
+
+	// InvalidComplex occurs when the complex built-in function is called with
+	// arguments with incompatible types.
+	//
+	// Example:
+	//  var _ = complex(float32(1), float64(2))
+	InvalidComplex
+
+	// InvalidDelete occurs when the delete built-in function is called with a
+	// first argument that is not a map.
+	//
+	// Example:
+	//  func f() {
+	//  	m := "hello"
+	//  	delete(m, "e")
+	//  }
+	InvalidDelete
+
+	// InvalidImag occurs when the imag built-in function is called with an
+	// argument that does not have complex type.
+	//
+	// Example:
+	//  var _ = imag(int(1))
+	InvalidImag
+
+	// InvalidLen occurs when an argument to the len built-in function is not of
+	// supported type.
+	//
+	// See https://golang.org/ref/spec#Length_and_capacity for information on
+	// which underlying types are supported as arguments to cap and len.
+	//
+	// Example:
+	//  var s = 2
+	//  var x = len(s)
+	InvalidLen
+
+	// SwappedMakeArgs occurs when make is called with three arguments, and its
+	// length argument is larger than its capacity argument.
+	//
+	// Example:
+	//  var x = make([]int, 3, 2)
+	SwappedMakeArgs
+
+	// InvalidMake occurs when make is called with an unsupported type argument.
+	//
+	// See https://golang.org/ref/spec#Making_slices_maps_and_channels for
+	// information on the types that may be created using make.
+	//
+	// Example:
+	//  var x = make(int)
+	InvalidMake
+
+	// InvalidReal occurs when the real built-in function is called with an
+	// argument that does not have complex type.
+	//
+	// Example:
+	//  var _ = real(int(1))
+	InvalidReal
+
+	// InvalidAssert occurs when a type assertion is applied to a
+	// value that is not of interface type.
+	//
+	// Example:
+	//  var x = 1
+	//  var _ = x.(float64)
+	InvalidAssert
+
+	// ImpossibleAssert occurs for a type assertion x.(T) when the value x of
+	// interface cannot have dynamic type T, due to a missing or mismatching
+	// method on T.
+	//
+	// Example:
+	//  type T int
+	//
+	//  func (t *T) m() int { return int(*t) }
+	//
+	//  type I interface { m() int }
+	//
+	//  var x I
+	//  var _ = x.(T)
+	ImpossibleAssert
+
+	// InvalidConversion occurs when the argument type cannot be converted to the
+	// target.
+	//
+	// See https://golang.org/ref/spec#Conversions for the rules of
+	// convertibility.
+	//
+	// Example:
+	//  var x float64
+	//  var _ = string(x)
+	InvalidConversion
+
+	// InvalidUntypedConversion occurs when there is no valid implicit
+	// conversion from an untyped value satisfying the type constraints of the
+	// context in which it is used.
+	//
+	// Example:
+	//  var _ = 1 + []int{}
+	InvalidUntypedConversion
+
+	// BadOffsetofSyntax occurs when unsafe.Offsetof is called with an argument
+	// that is not a selector expression.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Offsetof(x)
+	BadOffsetofSyntax
+
+	// InvalidOffsetof occurs when unsafe.Offsetof is called with a method
+	// selector, rather than a field selector, or when the field is embedded via
+	// a pointer.
+	//
+	// Per the spec:
+	//
+	//  "If f is an embedded field, it must be reachable without pointer
+	//  indirections through fields of the struct. "
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type T struct { f int }
+	//  type S struct { *T }
+	//  var s S
+	//  var _ = unsafe.Offsetof(s.f)
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type S struct{}
+	//
+	//  func (S) m() {}
+	//
+	//  var s S
+	//  var _ = unsafe.Offsetof(s.m)
+	InvalidOffsetof
+
+	// UnusedExpr occurs when a side-effect free expression is used as a
+	// statement. Such a statement has no effect.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	i*i
+	//  }
+	UnusedExpr
+
+	// UnusedVar occurs when a variable is declared but unused.
+	//
+	// Example:
+	//  func f() {
+	//  	x := 1
+	//  }
+	UnusedVar
+
+	// MissingReturn occurs when a function with results is missing a return
+	// statement.
+	//
+	// Example:
+	//  func f() int {}
+	MissingReturn
+
+	// WrongResultCount occurs when a return statement returns an incorrect
+	// number of values.
+	//
+	// Example:
+	//  func ReturnOne() int {
+	//  	return 1, 2
+	//  }
+	WrongResultCount
+
+	// OutOfScopeResult occurs when the name of a value implicitly returned by
+	// an empty return statement is shadowed in a nested scope.
+	//
+	// Example:
+	//  func factor(n int) (i int) {
+	//  	for i := 2; i < n; i++ {
+	//  		if n%i == 0 {
+	//  			return
+	//  		}
+	//  	}
+	//  	return 0
+	//  }
+	OutOfScopeResult
+
+	// InvalidCond occurs when an if condition is not a boolean expression.
+	//
+	// Example:
+	//  func checkReturn(i int) {
+	//  	if i {
+	//  		panic("non-zero return")
+	//  	}
+	//  }
+	InvalidCond
+
+	// InvalidPostDecl occurs when there is a declaration in a for-loop post
+	// statement.
+	//
+	// Example:
+	//  func f() {
+	//  	for i := 0; i < 10; j := 0 {}
+	//  }
+	InvalidPostDecl
+
+	_ // InvalidChanRange was removed.
+
+	// InvalidIterVar occurs when two iteration variables are used while ranging
+	// over a channel.
+	//
+	// Example:
+	//  func f(c chan int) {
+	//  	for k, v := range c {
+	//  		println(k, v)
+	//  	}
+	//  }
+	InvalidIterVar
+
+	// InvalidRangeExpr occurs when the type of a range expression is not array,
+	// slice, string, map, or channel.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	for j := range i {
+	//  		println(j)
+	//  	}
+	//  }
+	InvalidRangeExpr
+
+	// MisplacedBreak occurs when a break statement is not within a for, switch,
+	// or select statement of the innermost function definition.
+	//
+	// Example:
+	//  func f() {
+	//  	break
+	//  }
+	MisplacedBreak
+
+	// MisplacedContinue occurs when a continue statement is not within a for
+	// loop of the innermost function definition.
+	//
+	// Example:
+	//  func sumeven(n int) int {
+	//  	proceed := func() {
+	//  		continue
+	//  	}
+	//  	sum := 0
+	//  	for i := 1; i <= n; i++ {
+	//  		if i % 2 != 0 {
+	//  			proceed()
+	//  		}
+	//  		sum += i
+	//  	}
+	//  	return sum
+	//  }
+	MisplacedContinue
+
+	// MisplacedFallthrough occurs when a fallthrough statement is not within an
+	// expression switch.
+	//
+	// Example:
+	//  func typename(i interface{}) string {
+	//  	switch i.(type) {
+	//  	case int64:
+	//  		fallthrough
+	//  	case int:
+	//  		return "int"
+	//  	}
+	//  	return "unsupported"
+	//  }
+	MisplacedFallthrough
+
+	// DuplicateCase occurs when a type or expression switch has duplicate
+	// cases.
+	//
+	// Example:
+	//  func printInt(i int) {
+	//  	switch i {
+	//  	case 1:
+	//  		println("one")
+	//  	case 1:
+	//  		println("One")
+	//  	}
+	//  }
+	DuplicateCase
+
+	// DuplicateDefault occurs when a type or expression switch has multiple
+	// default clauses.
+	//
+	// Example:
+	//  func printInt(i int) {
+	//  	switch i {
+	//  	case 1:
+	//  		println("one")
+	//  	default:
+	//  		println("One")
+	//  	default:
+	//  		println("1")
+	//  	}
+	//  }
+	DuplicateDefault
+
+	// BadTypeKeyword occurs when a .(type) expression is used anywhere other
+	// than a type switch.
+	//
+	// Example:
+	//  type I interface {
+	//  	m()
+	//  }
+	//  var t I
+	//  var _ = t.(type)
+	BadTypeKeyword
+
+	// InvalidTypeSwitch occurs when .(type) is used on an expression that is
+	// not of interface type.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	switch x := i.(type) {}
+	//  }
+	InvalidTypeSwitch
+
+	// InvalidExprSwitch occurs when a switch expression is not comparable.
+	//
+	// Example:
+	//  func _() {
+	//  	var a struct{ _ func() }
+	//  	switch a /* ERROR cannot switch on a */ {
+	//  	}
+	//  }
+	InvalidExprSwitch
+
+	// InvalidSelectCase occurs when a select case is not a channel send or
+	// receive.
+	//
+	// Example:
+	//  func checkChan(c <-chan int) bool {
+	//  	select {
+	//  	case c:
+	//  		return true
+	//  	default:
+	//  		return false
+	//  	}
+	//  }
+	InvalidSelectCase
+
+	// UndeclaredLabel occurs when an undeclared label is jumped to.
+	//
+	// Example:
+	//  func f() {
+	//  	goto L
+	//  }
+	UndeclaredLabel
+
+	// DuplicateLabel occurs when a label is declared more than once.
+	//
+	// Example:
+	//  func f() int {
+	//  L:
+	//  L:
+	//  	return 1
+	//  }
+	DuplicateLabel
+
+	// MisplacedLabel occurs when a break or continue label is not on a for,
+	// switch, or select statement.
+	//
+	// Example:
+	//  func f() {
+	//  L:
+	//  	a := []int{1,2,3}
+	//  	for _, e := range a {
+	//  		if e > 10 {
+	//  			break L
+	//  		}
+	//  		println(a)
+	//  	}
+	//  }
+	MisplacedLabel
+
+	// UnusedLabel occurs when a label is declared and not used.
+	//
+	// Example:
+	//  func f() {
+	//  L:
+	//  }
+	UnusedLabel
+
+	// JumpOverDecl occurs when a label jumps over a variable declaration.
+	//
+	// Example:
+	//  func f() int {
+	//  	goto L
+	//  	x := 2
+	//  L:
+	//  	x++
+	//  	return x
+	//  }
+	JumpOverDecl
+
+	// JumpIntoBlock occurs when a forward jump goes to a label inside a nested
+	// block.
+	//
+	// Example:
+	//  func f(x int) {
+	//  	goto L
+	//  	if x > 0 {
+	//  	L:
+	//  		print("inside block")
+	//  	}
+	// }
+	JumpIntoBlock
+
+	// InvalidMethodExpr occurs when a pointer method is called but the argument
+	// is not addressable.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func (*T) m() int { return 1 }
+	//
+	//  var _ = T.m(T{})
+	InvalidMethodExpr
+
+	// WrongArgCount occurs when too few or too many arguments are passed by a
+	// function call.
+	//
+	// Example:
+	//  func f(i int) {}
+	//  var x = f()
+	WrongArgCount
+
+	// InvalidCall occurs when an expression is called that is not of function
+	// type.
+	//
+	// Example:
+	//  var x = "x"
+	//  var y = x()
+	InvalidCall
+
+	// UnusedResults occurs when a restricted expression-only built-in function
+	// is suspended via go or defer. Such a suspension discards the results of
+	// these side-effect free built-in functions, and therefore is ineffectual.
+	//
+	// Example:
+	//  func f(a []int) int {
+	//  	defer len(a)
+	//  	return i
+	//  }
+	UnusedResults
+
+	// InvalidDefer occurs when a deferred expression is not a function call,
+	// for example if the expression is a type conversion.
+	//
+	// Example:
+	//  func f(i int) int {
+	//  	defer int32(i)
+	//  	return i
+	//  }
+	InvalidDefer
+
+	// InvalidGo occurs when a go expression is not a function call, for example
+	// if the expression is a type conversion.
+	//
+	// Example:
+	//  func f(i int) int {
+	//  	go int32(i)
+	//  	return i
+	//  }
+	InvalidGo
+
+	// All codes below were added in Go 1.17.
+
+	// BadDecl occurs when a declaration has invalid syntax.
+	BadDecl
+
+	// RepeatedDecl occurs when an identifier occurs more than once on the left
+	// hand side of a short variable declaration.
+	//
+	// Example:
+	//  func _() {
+	//  	x, y, y := 1, 2, 3
+	//  }
+	RepeatedDecl
+
+	// InvalidUnsafeAdd occurs when unsafe.Add is called with a
+	// length argument that is not of integer type.
+	// It also occurs if it is used in a package compiled for a
+	// language version before go1.17.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var p unsafe.Pointer
+	//  var _ = unsafe.Add(p, float64(1))
+	InvalidUnsafeAdd
+
+	// InvalidUnsafeSlice occurs when unsafe.Slice is called with a
+	// pointer argument that is not of pointer type or a length argument
+	// that is not of integer type, negative, or out of bounds.
+	// It also occurs if it is used in a package compiled for a language
+	// version before go1.17.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Slice(x, 1)
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Slice(&x, float64(1))
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Slice(&x, -1)
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Slice(&x, uint64(1) << 63)
+	InvalidUnsafeSlice
+
+	// All codes below were added in Go 1.18.
+
+	// UnsupportedFeature occurs when a language feature is used that is not
+	// supported at this Go version.
+	UnsupportedFeature
+
+	// NotAGenericType occurs when a non-generic type is used where a generic
+	// type is expected: in type or function instantiation.
+	//
+	// Example:
+	//  type T int
+	//
+	//  var _ T[int]
+	NotAGenericType
+
+	// WrongTypeArgCount occurs when a type or function is instantiated with an
+	// incorrent number of type arguments, including when a generic type or
+	// function is used without instantiation.
+	//
+	// Errors inolving failed type inference are assigned other error codes.
+	//
+	// Example:
+	//  type T[p any] int
+	//
+	//  var _ T[int, string]
+	//
+	// Example:
+	//  func f[T any]() {}
+	//
+	//  var x = f
+	WrongTypeArgCount
+
+	// CannotInferTypeArgs occurs when type or function type argument inference
+	// fails to infer all type arguments.
+	//
+	// Example:
+	//  func f[T any]() {}
+	//
+	//  func _() {
+	//  	f()
+	//  }
+	CannotInferTypeArgs
+
+	// InvalidTypeArg occurs when a type argument does not satisfy its
+	// corresponding type parameter constraints.
+	//
+	// Example:
+	//  type T[P ~int] struct{}
+	//
+	//  var _ T[string]
+	InvalidTypeArg // arguments? InferenceFailed
+
+	// InvalidInstanceCycle occurs when an invalid cycle is detected
+	// within the instantiation graph.
+	//
+	// Example:
+	//  func f[T any]() { f[*T]() }
+	InvalidInstanceCycle
+
+	// InvalidUnion occurs when an embedded union or approximation element is
+	// not valid.
+	//
+	// Example:
+	//  type _ interface {
+	//   	~int | interface{ m() }
+	//  }
+	InvalidUnion
+
+	// MisplacedConstraintIface occurs when a constraint-type interface is used
+	// outside of constraint position.
+	//
+	// Example:
+	//   type I interface { ~int }
+	//
+	//   var _ I
+	MisplacedConstraintIface
+
+	// InvalidMethodTypeParams occurs when methods have type parameters.
+	//
+	// It cannot be encountered with an AST parsed using go/parser.
+	InvalidMethodTypeParams
+
+	// MisplacedTypeParam occurs when a type parameter is used in a place where
+	// it is not permitted.
+	//
+	// Example:
+	//  type T[P any] P
+	//
+	// Example:
+	//  type T[P any] struct{ *P }
+	MisplacedTypeParam
+
+	// InvalidUnsafeSliceData occurs when unsafe.SliceData is called with
+	// an argument that is not of slice type. It also occurs if it is used
+	// in a package compiled for a language version before go1.20.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.SliceData(x)
+	InvalidUnsafeSliceData
+
+	// InvalidUnsafeString occurs when unsafe.String is called with
+	// a length argument that is not of integer type, negative, or
+	// out of bounds. It also occurs if it is used in a package
+	// compiled for a language version before go1.20.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var b [10]byte
+	//  var _ = unsafe.String(&b[0], -1)
+	InvalidUnsafeString
+
+	// InvalidUnsafeStringData occurs if it is used in a package
+	// compiled for a language version before go1.20.
+	_ // not used anymore
+
+	// InvalidClear occurs when clear is called with an argument
+	// that is not of map, slice, or pointer-to-array type.
+	//
+	// Example:
+	//  func _(x int) {
+	//  	clear(x)
+	//  }
+	InvalidClear
+)
diff --git a/src/internal/types/errors/codes_test.go b/src/internal/types/errors/codes_test.go
new file mode 100644
index 0000000..2490ade
--- /dev/null
+++ b/src/internal/types/errors/codes_test.go
@@ -0,0 +1,197 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"reflect"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+func TestErrorCodeExamples(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // go command needed to resolve std .a files for importer.Default().
+
+	walkCodes(t, func(name string, value int, spec *ast.ValueSpec) {
+		t.Run(name, func(t *testing.T) {
+			doc := spec.Doc.Text()
+			examples := strings.Split(doc, "Example:")
+			for i := 1; i < len(examples); i++ {
+				example := strings.TrimSpace(examples[i])
+				err := checkExample(t, example)
+				if err == nil {
+					t.Fatalf("no error in example #%d", i)
+				}
+				typerr, ok := err.(Error)
+				if !ok {
+					t.Fatalf("not a types.Error: %v", err)
+				}
+				if got := readCode(typerr); got != value {
+					t.Errorf("%s: example #%d returned code %d (%s), want %d", name, i, got, err, value)
+				}
+			}
+		})
+	})
+}
+
+func walkCodes(t *testing.T, f func(string, int, *ast.ValueSpec)) {
+	t.Helper()
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "codes.go", nil, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{Importer: importer.Default()}
+	info := &Info{
+		Types: make(map[ast.Expr]TypeAndValue),
+		Defs:  make(map[*ast.Ident]Object),
+		Uses:  make(map[*ast.Ident]Object),
+	}
+	_, err = conf.Check("types", fset, []*ast.File{file}, info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, decl := range file.Decls {
+		decl, ok := decl.(*ast.GenDecl)
+		if !ok || decl.Tok != token.CONST {
+			continue
+		}
+		for _, spec := range decl.Specs {
+			spec, ok := spec.(*ast.ValueSpec)
+			if !ok || len(spec.Names) == 0 {
+				continue
+			}
+			obj := info.ObjectOf(spec.Names[0])
+			if named, ok := obj.Type().(*Named); ok && named.Obj().Name() == "Code" {
+				if len(spec.Names) != 1 {
+					t.Fatalf("bad Code declaration for %q: got %d names, want exactly 1", spec.Names[0].Name, len(spec.Names))
+				}
+				codename := spec.Names[0].Name
+				value := int(constant.Val(obj.(*Const).Val()).(int64))
+				f(codename, value, spec)
+			}
+		}
+	}
+}
+
+func readCode(err Error) int {
+	v := reflect.ValueOf(err)
+	return int(v.FieldByName("go116code").Int())
+}
+
+func checkExample(t *testing.T, example string) error {
+	t.Helper()
+	fset := token.NewFileSet()
+	if !strings.HasPrefix(example, "package") {
+		example = "package p\n\n" + example
+	}
+	file, err := parser.ParseFile(fset, "example.go", example, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{
+		FakeImportC: true,
+		Importer:    importer.Default(),
+	}
+	_, err = conf.Check("example", fset, []*ast.File{file}, nil)
+	return err
+}
+
+func TestErrorCodeStyle(t *testing.T) {
+	// The set of error codes is large and intended to be self-documenting, so
+	// this test enforces some style conventions.
+	forbiddenInIdent := []string{
+		// use invalid instead
+		"illegal",
+		// words with a common short-form
+		"argument",
+		"assertion",
+		"assignment",
+		"boolean",
+		"channel",
+		"condition",
+		"declaration",
+		"expression",
+		"function",
+		"initial", // use init for initializer, initialization, etc.
+		"integer",
+		"interface",
+		"iterat", // use iter for iterator, iteration, etc.
+		"literal",
+		"operation",
+		"package",
+		"pointer",
+		"receiver",
+		"signature",
+		"statement",
+		"variable",
+	}
+	forbiddenInComment := []string{
+		// lhs and rhs should be spelled-out.
+		"lhs", "rhs",
+		// builtin should be hyphenated.
+		"builtin",
+		// Use dot-dot-dot.
+		"ellipsis",
+	}
+	nameHist := make(map[int]int)
+	longestName := ""
+	maxValue := 0
+
+	walkCodes(t, func(name string, value int, spec *ast.ValueSpec) {
+		if name == "_" {
+			return
+		}
+		nameHist[len(name)]++
+		if value > maxValue {
+			maxValue = value
+		}
+		if len(name) > len(longestName) {
+			longestName = name
+		}
+		if !token.IsExported(name) {
+			t.Errorf("%q is not exported", name)
+		}
+		lower := strings.ToLower(name)
+		for _, bad := range forbiddenInIdent {
+			if strings.Contains(lower, bad) {
+				t.Errorf("%q contains forbidden word %q", name, bad)
+			}
+		}
+		doc := spec.Doc.Text()
+		if doc == "" {
+			t.Errorf("%q is undocumented", name)
+		} else if !strings.HasPrefix(doc, name) {
+			t.Errorf("doc for %q does not start with the error code name", name)
+		}
+		lowerComment := strings.ToLower(strings.TrimPrefix(doc, name))
+		for _, bad := range forbiddenInComment {
+			if strings.Contains(lowerComment, bad) {
+				t.Errorf("doc for %q contains forbidden word %q", name, bad)
+			}
+		}
+	})
+
+	if testing.Verbose() {
+		var totChars, totCount int
+		for chars, count := range nameHist {
+			totChars += chars * count
+			totCount += count
+		}
+		avg := float64(totChars) / float64(totCount)
+		fmt.Println()
+		fmt.Printf("%d error codes\n", totCount)
+		fmt.Printf("average length: %.2f chars\n", avg)
+		fmt.Printf("max length: %d (%s)\n", len(longestName), longestName)
+	}
+}
diff --git a/src/internal/types/testdata/check/blank.go b/src/internal/types/testdata/check/blank.go
new file mode 100644
index 0000000..6a2507f
--- /dev/null
+++ b/src/internal/types/testdata/check/blank.go
@@ -0,0 +1,5 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package _ /* ERROR invalid package name */
diff --git a/src/internal/types/testdata/check/builtins0.go b/src/internal/types/testdata/check/builtins0.go
new file mode 100644
index 0000000..308f70b
--- /dev/null
+++ b/src/internal/types/testdata/check/builtins0.go
@@ -0,0 +1,967 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// builtin calls
+
+package builtins
+
+import "unsafe"
+
+func f0() {}
+
+func append1() {
+	var b byte
+	var x int
+	var s []byte
+	_ = append() // ERROR not enough arguments
+	_ = append("foo" /* ERROR must be a slice */ )
+	_ = append(nil /* ERROR must be a slice */ , s)
+	_ = append(x /* ERROR must be a slice */ , s)
+	_ = append(s)
+	_ = append(s, nil...)
+	append /* ERROR not used */ (s)
+
+	_ = append(s, b)
+	_ = append(s, x /* ERROR cannot use x */ )
+	_ = append(s, s /* ERROR cannot use s */ )
+	_ = append(s...) /* ERROR not enough arguments */
+	_ = append(s, b, s /* ERROR too many arguments */ ...)
+	_ = append(s, 1, 2, 3)
+	_ = append(s, 1, 2, 3, x /* ERROR cannot use x */ , 5, 6, 6)
+	_ = append(s, 1, 2 /* ERROR too many arguments */, s...)
+	_ = append([]interface{}(nil), 1, 2, "foo", x, 3.1425, false)
+
+	type S []byte
+	type T string
+	var t T
+	_ = append(s, "foo" /* ERROR cannot use .* in argument to append */ )
+	_ = append(s, "foo"...)
+	_ = append(S(s), "foo" /* ERROR cannot use .* in argument to append */ )
+	_ = append(S(s), "foo"...)
+	_ = append(s, t /* ERROR cannot use t */ )
+	_ = append(s, t...)
+	_ = append(s, T("foo")...)
+	_ = append(S(s), t /* ERROR cannot use t */ )
+	_ = append(S(s), t...)
+	_ = append(S(s), T("foo")...)
+	_ = append([]string{}, t /* ERROR cannot use t */ , "foo")
+	_ = append([]T{}, t, "foo")
+}
+
+// from the spec
+func append2() {
+	s0 := []int{0, 0}
+	s1 := append(s0, 2)                // append a single element     s1 == []int{0, 0, 2}
+	s2 := append(s1, 3, 5, 7)          // append multiple elements    s2 == []int{0, 0, 2, 3, 5, 7}
+	s3 := append(s2, s0...)            // append a slice              s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
+	s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
+
+	var t []interface{}
+	t = append(t, 42, 3.1415, "foo")   //                             t == []interface{}{42, 3.1415, "foo"}
+
+	var b []byte
+	b = append(b, "bar"...)            // append string contents      b == []byte{'b', 'a', 'r' }
+
+	_ = s4
+}
+
+func append3() {
+	f1 := func() (s []int) { return }
+	f2 := func() (s []int, x int) { return }
+	f3 := func() (s []int, x, y int) { return }
+	f5 := func() (s []interface{}, x int, y float32, z string, b bool) { return }
+	ff := func() (int, float32) { return 0, 0 }
+	_ = append(f0 /* ERROR used as value */ ())
+	_ = append(f1())
+	_ = append(f2())
+	_ = append(f3())
+	_ = append(f5())
+	_ = append(ff /* ERROR must be a slice */ ()) // TODO(gri) better error message
+}
+
+func cap1() {
+	var a [10]bool
+	var p *[20]int
+	var c chan string
+	_ = cap() // ERROR not enough arguments
+	_ = cap(1, 2) // ERROR too many arguments
+	_ = cap(42 /* ERROR invalid */)
+	const _3 = cap(a)
+	assert(_3 == 10)
+	const _4 = cap(p)
+	assert(_4 == 20)
+	_ = cap(c)
+	cap /* ERROR not used */ (c)
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = cap(((*T)(nil)).a)
+
+	var s [][]byte
+	_ = cap(s)
+	_ = cap(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func cap2() {
+	f1a := func() (a [10]int) { return }
+	f1s := func() (s []int) { return }
+	f2 := func() (s []int, x int) { return }
+	_ = cap(f0 /* ERROR used as value */ ())
+	_ = cap(f1a())
+	_ = cap(f1s())
+	_ = cap(f2()) // ERROR too many arguments
+}
+
+// test cases for issue 7387
+func cap3() {
+	var f = func() int { return 0 }
+	var x = f()
+	const (
+		_ = cap([4]int{})
+		_ = cap([4]int{x})
+		_ = cap /* ERROR not constant */ ([4]int{f()})
+		_ = cap /* ERROR not constant */ ([4]int{cap([]int{})})
+		_ = cap([4]int{cap([4]int{})})
+	)
+	var y float64
+	var z complex128
+	const (
+		_ = cap([4]float64{})
+		_ = cap([4]float64{y})
+		_ = cap([4]float64{real(2i)})
+		_ = cap /* ERROR not constant */ ([4]float64{real(z)})
+	)
+	var ch chan [10]int
+	const (
+		_ = cap /* ERROR not constant */ (<-ch)
+		_ = cap /* ERROR not constant */ ([4]int{(<-ch)[0]})
+	)
+}
+
+func clear1() {
+	var a [10]int
+	var m map[float64]string
+	var s []byte
+	clear(a /* ERROR cannot clear a */)
+	clear(&a)
+	clear(m)
+	clear(s)
+	clear([]int{})
+}
+
+func close1() {
+	var c chan int
+	var r <-chan int
+	close() // ERROR not enough arguments
+	close(1, 2) // ERROR too many arguments
+	close(42 /* ERROR cannot close non-channel */)
+	close(r /* ERROR receive-only channel */)
+	close(c)
+	_ = close /* ERROR used as value */ (c)
+
+	var s []chan int
+	close(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func close2() {
+	f1 := func() (ch chan int) { return }
+	f2 := func() (ch chan int, x int) { return }
+	close(f0 /* ERROR used as value */ ())
+	close(f1())
+	close(f2()) // ERROR too many arguments
+}
+
+func complex1() {
+	var i32 int32
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = complex() // ERROR not enough arguments
+	_ = complex(1) // ERROR not enough arguments
+	_ = complex(true /* ERROR mismatched types */ , 0)
+	_ = complex(i32 /* ERROR expected floating-point */ , 0)
+	_ = complex("foo" /* ERROR mismatched types */ , 0)
+	_ = complex(c64 /* ERROR expected floating-point */ , 0)
+	_ = complex(0 /* ERROR mismatched types */ , true)
+	_ = complex(0 /* ERROR expected floating-point */ , i32)
+	_ = complex(0 /* ERROR mismatched types */ , "foo")
+	_ = complex(0 /* ERROR expected floating-point */ , c64)
+	_ = complex(f32, f32)
+	_ = complex(f32, 1)
+	_ = complex(f32, 1.0)
+	_ = complex(f32, 'a')
+	_ = complex(f64, f64)
+	_ = complex(f64, 1)
+	_ = complex(f64, 1.0)
+	_ = complex(f64, 'a')
+	_ = complex(f32 /* ERROR mismatched types */ , f64)
+	_ = complex(f64 /* ERROR mismatched types */ , f32)
+	_ = complex(1, 1)
+	_ = complex(1, 1.1)
+	_ = complex(1, 'a')
+	complex /* ERROR not used */ (1, 2)
+
+	var _ complex64 = complex(f32, f32)
+	var _ complex64 = complex /* ERROR cannot use .* in variable declaration */ (f64, f64)
+
+	var _ complex128 = complex /* ERROR cannot use .* in variable declaration */ (f32, f32)
+	var _ complex128 = complex(f64, f64)
+
+	// untyped constants
+	const _ int = complex(1, 0)
+	const _ float32 = complex(1, 0)
+	const _ complex64 = complex(1, 0)
+	const _ complex128 = complex(1, 0)
+	const _ = complex(0i, 0i)
+	const _ = complex(0i, 0)
+	const _ int = 1.0 + complex(1, 0i)
+
+	const _ int = complex /* ERROR int */ (1.1, 0)
+	const _ float32 = complex /* ERROR float32 */ (1, 2)
+
+	// untyped values
+	var s uint
+	_ = complex(1 /* ERROR integer */ <<s, 0)
+	const _ = complex /* ERROR not constant */ (1 /* ERROR integer */ <<s, 0)
+	var _ int = complex /* ERROR cannot use .* in variable declaration */ (1 /* ERROR integer */ <<s, 0)
+
+	// floating-point argument types must be identical
+	type F32 float32
+	type F64 float64
+	var x32 F32
+	var x64 F64
+	c64 = complex(x32, x32)
+	_ = complex(x32 /* ERROR mismatched types */ , f32)
+	_ = complex(f32 /* ERROR mismatched types */ , x32)
+	c128 = complex(x64, x64)
+	_ = c128
+	_ = complex(x64 /* ERROR mismatched types */ , f64)
+	_ = complex(f64 /* ERROR mismatched types */ , x64)
+
+	var t []float32
+	_ = complex(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func complex2() {
+	f1 := func() (x float32) { return }
+	f2 := func() (x, y float32) { return }
+	f3 := func() (x, y, z float32) { return }
+	_ = complex(f0 /* ERROR used as value */ ())
+	_ = complex(f1()) // ERROR not enough arguments
+	_ = complex(f2())
+	_ = complex(f3()) // ERROR too many arguments
+}
+
+func copy1() {
+	copy() // ERROR not enough arguments
+	copy("foo") // ERROR not enough arguments
+	copy([ /* ERROR copy expects slice arguments */ ...]int{}, []int{})
+	copy([ /* ERROR copy expects slice arguments */ ]int{}, [...]int{})
+	copy([ /* ERROR different element types */ ]int8{}, "foo")
+
+	// spec examples
+	var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
+	var s = make([]int, 6)
+	var b = make([]byte, 5)
+	n1 := copy(s, a[0:])            // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
+	n2 := copy(s, s[2:])            // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
+	n3 := copy(b, "Hello, World!")  // n3 == 5, b == []byte("Hello")
+	_, _, _ = n1, n2, n3
+
+	var t [][]int
+	copy(t, t)
+	copy(t /* ERROR copy expects slice arguments */ , nil)
+	copy(nil /* ERROR copy expects slice arguments */ , t)
+	copy(nil /* ERROR copy expects slice arguments */ , nil)
+	copy(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func copy2() {
+	f1 := func() (a []int) { return }
+	f2 := func() (a, b []int) { return }
+	f3 := func() (a, b, c []int) { return }
+	copy(f0 /* ERROR used as value */ ())
+	copy(f1()) // ERROR not enough arguments
+	copy(f2())
+	copy(f3()) // ERROR too many arguments
+}
+
+func delete1() {
+	var m map[string]int
+	var s string
+	delete() // ERROR not enough arguments
+	delete(1) // ERROR not enough arguments
+	delete(1, 2, 3) // ERROR too many arguments
+	delete(m, 0 /* ERROR cannot use */)
+	delete(m, s)
+	_ = delete /* ERROR used as value */ (m, s)
+
+	var t []map[string]string
+	delete(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func delete2() {
+	f1 := func() (m map[string]int) { return }
+	f2 := func() (m map[string]int, k string) { return }
+	f3 := func() (m map[string]int, k string, x float32) { return }
+	delete(f0 /* ERROR used as value */ ())
+	delete(f1()) // ERROR not enough arguments
+	delete(f2())
+	delete(f3()) // ERROR too many arguments
+}
+
+func imag1() {
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = imag() // ERROR not enough arguments
+	_ = imag(1, 2) // ERROR too many arguments
+	_ = imag(10)
+	_ = imag(2.7182818)
+	_ = imag("foo" /* ERROR expected complex */)
+	_ = imag('a')
+	const _5 = imag(1 + 2i)
+	assert(_5 == 2)
+	f32 = _5
+	f64 = _5
+	const _6 = imag(0i)
+	assert(_6 == 0)
+	f32 = imag(c64)
+	f64 = imag(c128)
+	f32 = imag /* ERROR cannot use .* in assignment */ (c128)
+	f64 = imag /* ERROR cannot use .* in assignment */ (c64)
+	imag /* ERROR not used */ (c64)
+	_, _ = f32, f64
+
+	// complex type may not be predeclared
+	type C64 complex64
+	type C128 complex128
+	var x64 C64
+	var x128 C128
+	f32 = imag(x64)
+	f64 = imag(x128)
+
+	var a []complex64
+	_ = imag(a... /* ERROR invalid use of \.\.\. */ )
+
+	// if argument is untyped, result is untyped
+	const _ byte = imag(1.2 + 3i)
+	const _ complex128 = imag(1.2 + 3i)
+
+	// lhs constant shift operands are typed as complex128
+	var s uint
+	_ = imag(1 /* ERROR must be integer */ << s)
+}
+
+func imag2() {
+	f1 := func() (x complex128) { return }
+	f2 := func() (x, y complex128) { return }
+	_ = imag(f0 /* ERROR used as value */ ())
+	_ = imag(f1())
+	_ = imag(f2()) // ERROR too many arguments
+}
+
+func len1() {
+	const c = "foobar"
+	var a [10]bool
+	var p *[20]int
+	var m map[string]complex128
+	_ = len() // ERROR not enough arguments
+	_ = len(1, 2) // ERROR too many arguments
+	_ = len(42 /* ERROR invalid */)
+	const _3 = len(c)
+	assert(_3 == 6)
+	const _4 = len(a)
+	assert(_4 == 10)
+	const _5 = len(p)
+	assert(_5 == 20)
+	_ = len(m)
+	len /* ERROR not used */ (c)
+
+	// esoteric case
+	var t string
+	var hash map[interface{}][]*[10]int
+	const n = len /* ERROR not constant */ (hash[recover()][len(t)])
+	assert(n == 10) // ok because n has unknown value and no error is reported
+	var ch <-chan int
+	const nn = len /* ERROR not constant */ (hash[<-ch][len(t)])
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = len(((*T)(nil)).a)
+
+	var s [][]byte
+	_ = len(s)
+	_ = len(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func len2() {
+	f1 := func() (x []int) { return }
+	f2 := func() (x, y []int) { return }
+	_ = len(f0 /* ERROR used as value */ ())
+	_ = len(f1())
+	_ = len(f2()) // ERROR too many arguments
+}
+
+// test cases for issue 7387
+func len3() {
+	var f = func() int { return 0 }
+	var x = f()
+	const (
+		_ = len([4]int{})
+		_ = len([4]int{x})
+		_ = len /* ERROR not constant */ ([4]int{f()})
+		_ = len /* ERROR not constant */ ([4]int{len([]int{})})
+		_ = len([4]int{len([4]int{})})
+	)
+	var y float64
+	var z complex128
+	const (
+		_ = len([4]float64{})
+		_ = len([4]float64{y})
+		_ = len([4]float64{real(2i)})
+		_ = len /* ERROR not constant */ ([4]float64{real(z)})
+	)
+	var ch chan [10]int
+	const (
+		_ = len /* ERROR not constant */ (<-ch)
+		_ = len /* ERROR not constant */ ([4]int{(<-ch)[0]})
+	)
+}
+
+func make1() {
+	var n int
+	var m float32
+	var s uint
+
+	_ = make() // ERROR not enough arguments
+	_ = make(1 /* ERROR not a type */)
+	_ = make(int /* ERROR cannot make */)
+
+	// slices
+	_ = make/* ERROR arguments */ ([]int)
+	_ = make/* ERROR arguments */ ([]int, 2, 3, 4)
+	_ = make([]int, int /* ERROR not an expression */)
+	_ = make([]int, 10, float32 /* ERROR not an expression */)
+	_ = make([]int, "foo" /* ERROR cannot convert */)
+	_ = make([]int, 10, 2.3 /* ERROR truncated */)
+	_ = make([]int, 5, 10.0)
+	_ = make([]int, 0i)
+	_ = make([]int, 1.0)
+	_ = make([]int, 1.0<<s)
+	_ = make([]int, 1.1 /* ERROR int */ <<s)
+	_ = make([]int, - /* ERROR must not be negative */ 1, 10)
+	_ = make([]int, 0, - /* ERROR must not be negative */ 1)
+	_ = make([]int, - /* ERROR must not be negative */ 1, - /* ERROR must not be negative */ 1)
+	_ = make([]int, 1 /* ERROR overflows */ <<100, 1 /* ERROR overflows */ <<100)
+	_ = make([]int, 10 /* ERROR length and capacity swapped */ , 9)
+	_ = make([]int, 1 /* ERROR overflows */ <<100, 12345)
+	_ = make([]int, m /* ERROR must be integer */ )
+        _ = &make /* ERROR cannot take address */ ([]int, 0)
+
+	// maps
+	_ = make /* ERROR arguments */ (map[int]string, 10, 20)
+	_ = make(map[int]float32, int /* ERROR not an expression */)
+	_ = make(map[int]float32, "foo" /* ERROR cannot convert */)
+	_ = make(map[int]float32, 10)
+	_ = make(map[int]float32, n)
+	_ = make(map[int]float32, int64(n))
+	_ = make(map[string]bool, 10.0)
+	_ = make(map[string]bool, 10.0<<s)
+        _ = &make /* ERROR cannot take address */ (map[string]bool)
+
+	// channels
+	_ = make /* ERROR arguments */ (chan int, 10, 20)
+	_ = make(chan int, int /* ERROR not an expression */)
+	_ = make(chan<- int, "foo" /* ERROR cannot convert */)
+	_ = make(chan int, - /* ERROR must not be negative */ 10)
+	_ = make(<-chan float64, 10)
+	_ = make(chan chan int, n)
+	_ = make(chan string, int64(n))
+	_ = make(chan bool, 10.0)
+	_ = make(chan bool, 10.0<<s)
+        _ = &make /* ERROR cannot take address */ (chan bool)
+
+	make /* ERROR not used */ ([]int, 10)
+
+	var t []int
+	_ = make([]int, t[0], t[1])
+	_ = make([]int, t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func make2() {
+	f1 := func() (x []int) { return }
+	_ = make(f0 /* ERROR not a type */ ())
+	_ = make(f1 /* ERROR not a type */ ())
+}
+
+func new1() {
+	_ = new() // ERROR not enough arguments
+	_ = new(1, 2) // ERROR too many arguments
+	_ = new("foo" /* ERROR not a type */)
+	p := new(float64)
+	_ = new(struct{ x, y int })
+	q := new(*float64)
+	_ = *p == **q
+	new /* ERROR not used */ (int)
+        _ = &new /* ERROR cannot take address */ (int)
+
+	_ = new(int... /* ERROR invalid use of \.\.\. */ )
+}
+
+func new2() {
+	f1 := func() (x []int) { return }
+	_ = new(f0 /* ERROR not a type */ ())
+	_ = new(f1 /* ERROR not a type */ ())
+}
+
+func panic1() {
+	panic() // ERROR not enough arguments
+	panic(1, 2) // ERROR too many arguments
+	panic(0)
+	panic("foo")
+	panic(false)
+	panic(1<<10)
+	panic(1 << /* ERROR constant shift overflow */ 1000)
+	_ = panic /* ERROR used as value */ (0)
+
+	var s []byte
+	panic(s)
+	panic(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func panic2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	panic(f0 /* ERROR used as value */ ())
+	panic(f1())
+	panic(f2()) // ERROR too many arguments
+}
+
+func print1() {
+	print()
+	print(1)
+	print(1, 2)
+	print("foo")
+	print(2.718281828)
+	print(false)
+	print(1<<10)
+	print(1 << /* ERROR constant shift overflow */ 1000)
+	println(nil /* ERROR untyped nil */ )
+
+	var s []int
+	print(s... /* ERROR invalid use of \.\.\. */ )
+	_ = print /* ERROR used as value */ ()
+}
+
+func print2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	f3 := func() (x int, y float32, z string) { return }
+	print(f0 /* ERROR used as value */ ())
+	print(f1())
+	print(f2())
+	print(f3())
+}
+
+func println1() {
+	println()
+	println(1)
+	println(1, 2)
+	println("foo")
+	println(2.718281828)
+	println(false)
+	println(1<<10)
+	println(1 << /* ERROR constant shift overflow */ 1000)
+	println(nil /* ERROR untyped nil */ )
+
+	var s []int
+	println(s... /* ERROR invalid use of \.\.\. */ )
+	_ = println /* ERROR used as value */ ()
+}
+
+func println2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	f3 := func() (x int, y float32, z string) { return }
+	println(f0 /* ERROR used as value */ ())
+	println(f1())
+	println(f2())
+	println(f3())
+}
+
+func real1() {
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = real() // ERROR not enough arguments
+	_ = real(1, 2) // ERROR too many arguments
+	_ = real(10)
+	_ = real(2.7182818)
+	_ = real("foo" /* ERROR expected complex */)
+	const _5 = real(1 + 2i)
+	assert(_5 == 1)
+	f32 = _5
+	f64 = _5
+	const _6 = real(0i)
+	assert(_6 == 0)
+	f32 = real(c64)
+	f64 = real(c128)
+	f32 = real /* ERROR cannot use .* in assignment */ (c128)
+	f64 = real /* ERROR cannot use .* in assignment */ (c64)
+	real /* ERROR not used */ (c64)
+
+	// complex type may not be predeclared
+	type C64 complex64
+	type C128 complex128
+	var x64 C64
+	var x128 C128
+	f32 = imag(x64)
+	f64 = imag(x128)
+	_, _ = f32, f64
+
+	var a []complex64
+	_ = real(a... /* ERROR invalid use of \.\.\. */ )
+
+	// if argument is untyped, result is untyped
+	const _ byte = real(1 + 2.3i)
+	const _ complex128 = real(1 + 2.3i)
+
+	// lhs constant shift operands are typed as complex128
+	var s uint
+	_ = real(1 /* ERROR must be integer */ << s)
+}
+
+func real2() {
+	f1 := func() (x complex128) { return }
+	f2 := func() (x, y complex128) { return }
+	_ = real(f0 /* ERROR used as value */ ())
+	_ = real(f1())
+	_ = real(f2()) // ERROR too many arguments
+}
+
+func recover1() {
+	_ = recover()
+	_ = recover(10) // ERROR too many arguments
+	recover()
+
+	var s []int
+	recover(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func recover2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	_ = recover(f0 /* ERROR used as value */ ())
+	_ = recover(f1()) // ERROR too many arguments
+	_ = recover(f2()) // ERROR too many arguments
+}
+
+// assuming types.DefaultPtrSize == 8
+type S0 struct{      // offset
+	a bool       //  0
+	b rune       //  4
+	c *int       //  8
+	d bool       // 16
+	e complex128 // 24
+}                    // 40
+
+type S1 struct{   // offset
+	x float32 //  0
+	y string  //  8
+	z *S1     // 24
+	S0        // 32
+}                 // 72
+
+type S2 struct{ // offset
+	*S1     //  0
+}               //  8
+
+type S3 struct { // offset
+	a int64  //  0
+	b int32  //  8
+}                // 12
+
+type S4 struct { // offset
+	S3       //  0
+	int32    // 12
+}                // 16
+
+type S5 struct {   // offset
+	a [3]int32 //  0
+	b int32    // 12
+}                  // 16
+
+func (S2) m() {}
+
+func Alignof1() {
+	var x int
+	_ = unsafe.Alignof() // ERROR not enough arguments
+	_ = unsafe.Alignof(1, 2) // ERROR too many arguments
+	_ = unsafe.Alignof(int /* ERROR not an expression */)
+	_ = unsafe.Alignof(42)
+	_ = unsafe.Alignof(new(struct{}))
+	_ = unsafe.Alignof(1<<10)
+	_ = unsafe.Alignof(1 << /* ERROR constant shift overflow */ 1000)
+	_ = unsafe.Alignof(nil /* ERROR "untyped nil */ )
+	unsafe /* ERROR not used */ .Alignof(x)
+
+	var y S0
+	assert(unsafe.Alignof(y.a) == 1)
+	assert(unsafe.Alignof(y.b) == 4)
+	assert(unsafe.Alignof(y.c) == 8)
+	assert(unsafe.Alignof(y.d) == 1)
+	assert(unsafe.Alignof(y.e) == 8)
+
+	var s []byte
+	_ = unsafe.Alignof(s)
+	_ = unsafe.Alignof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Alignof2() {
+	f1 := func() (x int32) { return }
+	f2 := func() (x, y int32) { return }
+	_ = unsafe.Alignof(f0 /* ERROR used as value */ ())
+	assert(unsafe.Alignof(f1()) == 4)
+	_ = unsafe.Alignof(f2()) // ERROR too many arguments
+}
+
+func Offsetof1() {
+	var x struct{ f int }
+	_ = unsafe.Offsetof() // ERROR not enough arguments
+	_ = unsafe.Offsetof(1, 2) // ERROR too many arguments
+	_ = unsafe.Offsetof(int /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(x /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(nil /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(x.f)
+	_ = unsafe.Offsetof((x.f))
+	_ = unsafe.Offsetof((((((((x))).f)))))
+	unsafe /* ERROR not used */ .Offsetof(x.f)
+
+	var y0 S0
+	assert(unsafe.Offsetof(y0.a) == 0)
+	assert(unsafe.Offsetof(y0.b) == 4)
+	assert(unsafe.Offsetof(y0.c) == 8)
+	assert(unsafe.Offsetof(y0.d) == 16)
+	assert(unsafe.Offsetof(y0.e) == 24)
+
+	var y1 S1
+	assert(unsafe.Offsetof(y1.x) == 0)
+	assert(unsafe.Offsetof(y1.y) == 8)
+	assert(unsafe.Offsetof(y1.z) == 24)
+	assert(unsafe.Offsetof(y1.S0) == 32)
+
+	assert(unsafe.Offsetof(y1.S0.a) == 0) // relative to S0
+	assert(unsafe.Offsetof(y1.a) == 32)   // relative to S1
+	assert(unsafe.Offsetof(y1.b) == 36)   // relative to S1
+	assert(unsafe.Offsetof(y1.c) == 40)   // relative to S1
+	assert(unsafe.Offsetof(y1.d) == 48)   // relative to S1
+	assert(unsafe.Offsetof(y1.e) == 56)   // relative to S1
+
+	var y1p *S1
+	assert(unsafe.Offsetof(y1p.S0) == 32)
+
+	type P *S1
+	var p P = y1p
+	assert(unsafe.Offsetof(p.S0) == 32)
+
+	var y2 S2
+	assert(unsafe.Offsetof(y2.S1) == 0)
+	_ = unsafe.Offsetof(y2 /* ERROR embedded via a pointer */ .x)
+	_ = unsafe.Offsetof(y2 /* ERROR method value */ .m)
+
+	var s []byte
+	_ = unsafe.Offsetof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Offsetof2() {
+	f1 := func() (x int32) { return }
+	f2 := func() (x, y int32) { return }
+	_ = unsafe.Offsetof(f0 /* ERROR not a selector expression */ ())
+	_ = unsafe.Offsetof(f1 /* ERROR not a selector expression */ ())
+	_ = unsafe.Offsetof(f2 /* ERROR not a selector expression */ ())
+}
+
+func Sizeof1() {
+	var x int
+	_ = unsafe.Sizeof() // ERROR not enough arguments
+	_ = unsafe.Sizeof(1, 2) // ERROR too many arguments
+	_ = unsafe.Sizeof(int /* ERROR not an expression */)
+	_ = unsafe.Sizeof(42)
+	_ = unsafe.Sizeof(new(complex128))
+	_ = unsafe.Sizeof(1<<10)
+	_ = unsafe.Sizeof(1 << /* ERROR constant shift overflow */ 1000)
+	_ = unsafe.Sizeof(nil /* ERROR untyped nil */ )
+	unsafe /* ERROR not used */ .Sizeof(x)
+
+	// basic types have size guarantees
+	assert(unsafe.Sizeof(byte(0)) == 1)
+	assert(unsafe.Sizeof(uint8(0)) == 1)
+	assert(unsafe.Sizeof(int8(0)) == 1)
+	assert(unsafe.Sizeof(uint16(0)) == 2)
+	assert(unsafe.Sizeof(int16(0)) == 2)
+	assert(unsafe.Sizeof(uint32(0)) == 4)
+	assert(unsafe.Sizeof(int32(0)) == 4)
+	assert(unsafe.Sizeof(float32(0)) == 4)
+	assert(unsafe.Sizeof(uint64(0)) == 8)
+	assert(unsafe.Sizeof(int64(0)) == 8)
+	assert(unsafe.Sizeof(float64(0)) == 8)
+	assert(unsafe.Sizeof(complex64(0)) == 8)
+	assert(unsafe.Sizeof(complex128(0)) == 16)
+
+	var y0 S0
+	assert(unsafe.Sizeof(y0.a) == 1)
+	assert(unsafe.Sizeof(y0.b) == 4)
+	assert(unsafe.Sizeof(y0.c) == 8)
+	assert(unsafe.Sizeof(y0.d) == 1)
+	assert(unsafe.Sizeof(y0.e) == 16)
+	assert(unsafe.Sizeof(y0) == 40)
+
+	var y1 S1
+	assert(unsafe.Sizeof(y1) == 72)
+
+	var y2 S2
+	assert(unsafe.Sizeof(y2) == 8)
+
+	var y3 S3
+	assert(unsafe.Sizeof(y3) == 12)
+
+	var y4 S4
+	assert(unsafe.Sizeof(y4) == 16)
+
+	var y5 S5
+	assert(unsafe.Sizeof(y5) == 16)
+
+	var a3 [10]S3
+	assert(unsafe.Sizeof(a3) == 156)
+
+	// test case for issue 5670
+	type T struct {
+		a int32
+		_ int32
+		c int32
+	}
+	assert(unsafe.Sizeof(T{}) == 12)
+
+	var s []byte
+	_ = unsafe.Sizeof(s)
+	_ = unsafe.Sizeof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Sizeof2() {
+	f1 := func() (x int64) { return }
+	f2 := func() (x, y int64) { return }
+	_ = unsafe.Sizeof(f0 /* ERROR used as value */ ())
+	assert(unsafe.Sizeof(f1()) == 8)
+	_ = unsafe.Sizeof(f2()) // ERROR too many arguments
+}
+
+func Slice1() {
+	var x int
+	unsafe.Slice()        // ERROR not enough arguments
+	unsafe.Slice(1, 2, 3) // ERROR too many arguments
+	unsafe.Slice(1 /* ERROR is not a pointer */ , 2)
+	unsafe.Slice(nil /* ERROR nil is not a pointer */ , 0)
+	unsafe.Slice(&x, "foo" /* ERROR cannot convert .* to type int */ )
+	unsafe.Slice(&x, 1.2 /* ERROR truncated to int */ )
+	unsafe.Slice(&x, - /* ERROR must not be negative */ 1)
+	unsafe /* ERROR not used */ .Slice(&x, 0)
+	var _ []byte = unsafe /* ERROR value of type \[\]int */ .Slice(&x, 0)
+
+	var _ []int = unsafe.Slice(&x, 0)
+	_ = unsafe.Slice(&x, 1.0)
+	_ = unsafe.Slice((*int)(nil), 0)
+}
+
+func SliceData1() {
+	var s []int
+	unsafe.SliceData(0 /* ERROR not a slice */)
+	unsafe /* ERROR not used */ .SliceData(s)
+
+	type S []int
+	_ = unsafe.SliceData(s)
+	_ = unsafe.SliceData(S{})
+}
+
+func String1() {
+	var b byte
+	unsafe.String()        // ERROR not enough arguments
+	unsafe.String(1, 2, 3) // ERROR too many arguments
+	unsafe.String(1 /* ERROR cannot use 1 */ , 2)
+	unsafe.String(&b, "foo" /* ERROR cannot convert .* to type int */ )
+	unsafe.String(&b, 1.2 /* ERROR truncated to int */ )
+	unsafe.String(&b, - /* ERROR must not be negative */ 1)
+	unsafe /* ERROR not used */ .String(&b, 0)
+	var _ []byte = unsafe /* ERROR value of type string */ .String(&b, 0)
+
+	var _ string = unsafe.String(&b, 0)
+	_ = unsafe.String(&b, 1.0)
+	_ = unsafe.String(nil, 0) // here we allow nil as ptr argument (in contrast to unsafe.Slice)
+}
+
+func StringData1() {
+	var s string
+	type S string
+	unsafe.StringData(0 /* ERROR cannot use 0 */)
+	unsafe.StringData(S /* ERROR cannot use S */ ("foo"))
+	unsafe /* ERROR not used */ .StringData(s)
+
+	_ = unsafe.StringData(s)
+	_ = unsafe.StringData("foo")
+}
+
+// self-testing only
+func assert1() {
+	var x int
+	assert() /* ERROR not enough arguments */
+	assert(1, 2) /* ERROR too many arguments */
+	assert("foo" /* ERROR boolean constant */ )
+	assert(x /* ERROR boolean constant */)
+	assert(true)
+	assert /* ERROR failed */ (false)
+	_ = assert(true)
+
+	var s []byte
+	assert(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func assert2() {
+	f1 := func() (x bool) { return }
+	f2 := func() (x bool) { return }
+	assert(f0 /* ERROR used as value */ ())
+	assert(f1 /* ERROR boolean constant */ ())
+	assert(f2 /* ERROR boolean constant */ ())
+}
+
+// self-testing only
+func trace1() {
+	// Uncomment the code below to test trace - will produce console output
+	// _ = trace /* ERROR no value */ ()
+	// _ = trace(1)
+	// _ = trace(true, 1.2, '\'', "foo", 42i, "foo" <= "bar")
+
+	var s []byte
+	trace(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func trace2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x int, y string) { return }
+	f3 := func() (x int, y string, z []int) { return }
+	_ = f1
+	_ = f2
+	_ = f3
+	// Uncomment the code below to test trace - will produce console output
+	// trace(f0())
+	// trace(f1())
+	// trace(f2())
+	// trace(f3())
+	// trace(f0(), 1)
+	// trace(f1(), 1, 2)
+	// trace(f2(), 1, 2, 3)
+	// trace(f3(), 1, 2, 3, 4)
+}
diff --git a/src/internal/types/testdata/check/builtins1.go b/src/internal/types/testdata/check/builtins1.go
new file mode 100644
index 0000000..3348861
--- /dev/null
+++ b/src/internal/types/testdata/check/builtins1.go
@@ -0,0 +1,288 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file tests built-in calls on generic types.
+
+package builtins
+
+import "unsafe"
+
+// clear
+
+func _[T any](x T) {
+	clear(x /* ERROR cannot clear x */)
+}
+
+func _[T ~map[int]string | ~[]byte | ~*[10]int](x T) {
+	clear(x)
+}
+
+func _[T ~map[int]string | ~[]byte | ~*[10]int | string](x T) {
+	clear(x /* ERROR cannot clear x */)
+}
+
+// close
+
+type C0 interface{ int }
+type C1 interface{ chan int }
+type C2 interface{ chan int | <-chan int }
+type C3 interface{ chan int | chan float32 }
+type C4 interface{ chan int | chan<- int }
+type C5[T any] interface{ ~chan T | chan<- T }
+
+func _[T any](ch T) {
+	close(ch /* ERROR cannot close non-channel */)
+}
+
+func _[T C0](ch T) {
+	close(ch /* ERROR cannot close non-channel */)
+}
+
+func _[T C1](ch T) {
+	close(ch)
+}
+
+func _[T C2](ch T) {
+	close(ch /* ERROR cannot close receive-only channel */)
+}
+
+func _[T C3](ch T) {
+	close(ch)
+}
+
+func _[T C4](ch T) {
+	close(ch)
+}
+
+func _[T C5[X], X any](ch T) {
+	close(ch)
+}
+
+// copy
+
+func _[T any](x, y T) {
+	copy(x /* ERROR copy expects slice arguments */ , y)
+}
+
+func _[T ~[]byte](x, y T) {
+	copy(x, y)
+	copy(x, "foo")
+	copy("foo" /* ERROR expects slice arguments */ , y)
+
+	var x2 []byte
+	copy(x2, y) // element types are identical
+	copy(y, x2) // element types are identical
+
+	type myByte byte
+	var x3 []myByte
+	copy(x3 /* ERROR different element types */ , y)
+	copy(y /* ERROR different element types */ , x3)
+}
+
+func _[T ~[]E, E any](x T, y []E) {
+	copy(x, y)
+	copy(x /* ERROR different element types */ , "foo")
+}
+
+func _[T ~string](x []byte, y T) {
+	copy(x, y)
+	copy(y /* ERROR expects slice arguments */ , x)
+}
+
+func _[T ~[]byte|~string](x T, y []byte) {
+	copy(x /* ERROR expects slice arguments */ , y)
+	copy(y, x)
+}
+
+type L0 []int
+type L1 []int
+
+func _[T L0 | L1](x, y T) {
+	copy(x, y)
+}
+
+// delete
+
+type M0 interface{ int }
+type M1 interface{ map[string]int }
+type M2 interface { map[string]int | map[string]float64 }
+type M3 interface{ map[string]int | map[rune]int }
+type M4[K comparable, V any] interface{ map[K]V | map[rune]V }
+
+func _[T any](m T) {
+	delete(m /* ERROR not a map */, "foo")
+}
+
+func _[T M0](m T) {
+	delete(m /* ERROR not a map */, "foo")
+}
+
+func _[T M1](m T) {
+	delete(m, "foo")
+}
+
+func _[T M2](m T) {
+	delete(m, "foo")
+	delete(m, 0 /* ERROR cannot use .* as string */)
+}
+
+func _[T M3](m T) {
+	delete(m /* ERROR must have identical key types */, "foo")
+}
+
+func _[T M4[rune, V], V any](m T) {
+	delete(m, 'k')
+}
+
+func _[T M4[K, V], K comparable, V any](m T) {
+	delete(m /* ERROR must have identical key types */, "foo")
+}
+
+// make
+
+type myChan chan int
+
+func _[
+	S1 ~[]int,
+	S2 ~[]int | ~chan int,
+
+	M1 ~map[string]int,
+	M2 ~map[string]int | ~chan int,
+
+	C1 ~chan int,
+	C2 ~chan int | ~chan string,
+	C3 chan int | myChan, // single underlying type
+]() {
+	type S0 []int
+	_ = make([]int, 10)
+	_ = make(S0, 10)
+	_ = make(S1, 10)
+	_ = make() /* ERROR not enough arguments */
+	_ = make /* ERROR expects 2 or 3 arguments */ (S1)
+	_ = make(S1, 10, 20)
+	_ = make /* ERROR expects 2 or 3 arguments */ (S1, 10, 20, 30)
+	_ = make(S2 /* ERROR cannot make S2: no core type */ , 10)
+
+	type M0 map[string]int
+	_ = make(map[string]int)
+	_ = make(M0)
+	_ = make(M1)
+	_ = make(M1, 10)
+	_ = make/* ERROR expects 1 or 2 arguments */(M1, 10, 20)
+	_ = make(M2 /* ERROR cannot make M2: no core type */ )
+
+	type C0 chan int
+	_ = make(chan int)
+	_ = make(C0)
+	_ = make(C1)
+	_ = make(C1, 10)
+	_ = make/* ERROR expects 1 or 2 arguments */(C1, 10, 20)
+	_ = make(C2 /* ERROR cannot make C2: no core type */ )
+	_ = make(C3)
+}
+
+// unsafe.Alignof
+
+func _[T comparable]() {
+	var (
+		b int64
+		a [10]T
+		s struct{ f T }
+		p *T
+		l []T
+		f func(T)
+		i interface{ m() T }
+		c chan T
+		m map[T]T
+		t T
+	)
+
+	const bb = unsafe.Alignof(b)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Alignof(p)
+	assert(pp == 8)
+	const ll = unsafe.Alignof(l)
+	assert(ll == 8)
+	const ff = unsafe.Alignof(f)
+	assert(ff == 8)
+	const ii = unsafe.Alignof(i)
+	assert(ii == 8)
+	const cc = unsafe.Alignof(c)
+	assert(cc == 8)
+	const mm = unsafe.Alignof(m)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
+
+// unsafe.Offsetof
+
+func _[T comparable]() {
+	var (
+		b struct{ _, f int64 }
+		a struct{ _, f [10]T }
+		s struct{ _, f struct{ f T } }
+		p struct{ _, f *T }
+		l struct{ _, f []T }
+		f struct{ _, f func(T) }
+		i struct{ _, f interface{ m() T } }
+		c struct{ _, f chan T }
+		m struct{ _, f map[T]T }
+		t struct{ _, f T }
+	)
+
+	const bb = unsafe.Offsetof(b.f)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Offsetof(p.f)
+	assert(pp == 8)
+	const ll = unsafe.Offsetof(l.f)
+	assert(ll == 24)
+	const ff = unsafe.Offsetof(f.f)
+	assert(ff == 8)
+	const ii = unsafe.Offsetof(i.f)
+	assert(ii == 16)
+	const cc = unsafe.Offsetof(c.f)
+	assert(cc == 8)
+	const mm = unsafe.Offsetof(m.f)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
+
+// unsafe.Sizeof
+
+func _[T comparable]() {
+	var (
+		b int64
+		a [10]T
+		s struct{ f T }
+		p *T
+		l []T
+		f func(T)
+		i interface{ m() T }
+		c chan T
+		m map[T]T
+		t T
+	)
+
+	const bb = unsafe.Sizeof(b)
+	assert(bb == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(a)
+	const _ = unsafe /* ERROR not constant */ .Alignof(s)
+	const pp = unsafe.Sizeof(p)
+	assert(pp == 8)
+	const ll = unsafe.Sizeof(l)
+	assert(ll == 24)
+	const ff = unsafe.Sizeof(f)
+	assert(ff == 8)
+	const ii = unsafe.Sizeof(i)
+	assert(ii == 16)
+	const cc = unsafe.Sizeof(c)
+	assert(cc == 8)
+	const mm = unsafe.Sizeof(m)
+	assert(mm == 8)
+	const _ = unsafe /* ERROR not constant */ .Alignof(t)
+}
diff --git a/src/internal/types/testdata/check/chans.go b/src/internal/types/testdata/check/chans.go
new file mode 100644
index 0000000..fad2bce
--- /dev/null
+++ b/src/internal/types/testdata/check/chans.go
@@ -0,0 +1,62 @@
+package chans
+
+import "runtime"
+
+// Ranger returns a Sender and a Receiver. The Receiver provides a
+// Next method to retrieve values. The Sender provides a Send method
+// to send values and a Close method to stop sending values. The Next
+// method indicates when the Sender has been closed, and the Send
+// method indicates when the Receiver has been freed.
+//
+// This is a convenient way to exit a goroutine sending values when
+// the receiver stops reading them.
+func Ranger[T any]() (*Sender[T], *Receiver[T]) {
+	c := make(chan T)
+	d := make(chan bool)
+	s := &Sender[T]{values: c, done: d}
+	r := &Receiver[T]{values: c, done: d}
+	runtime.SetFinalizer(r, r.finalize)
+	return s, r
+}
+
+// A sender is used to send values to a Receiver.
+type Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+// Send sends a value to the receiver. It returns whether any more
+// values may be sent; if it returns false the value was not sent.
+func (s *Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+// Close tells the receiver that no more values will arrive.
+// After Close is called, the Sender may no longer be used.
+func (s *Sender[T]) Close() {
+	close(s.values)
+}
+
+// A Receiver receives values from a Sender.
+type Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+// Next returns the next value from the channel. The bool result
+// indicates whether the value is valid, or whether the Sender has
+// been closed and no more values will be received.
+func (r *Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}
+
+// finalize is a finalizer for the receiver.
+func (r *Receiver[T]) finalize() {
+	close(r.done)
+}
diff --git a/src/internal/types/testdata/check/compliterals.go b/src/internal/types/testdata/check/compliterals.go
new file mode 100644
index 0000000..60eac97
--- /dev/null
+++ b/src/internal/types/testdata/check/compliterals.go
@@ -0,0 +1,22 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Composite literals with parameterized types
+
+package comp_literals
+
+type myStruct struct {
+	f int
+}
+
+type slice[E any] []E
+
+func struct_literals[S struct{f int}|myStruct]() {
+	_ = S{}
+	_ = S{0}
+	_ = S{f: 0}
+
+        _ = slice[int]{1, 2, 3}
+        _ = slice[S]{{}, {0}, {f:0}}
+}
diff --git a/src/internal/types/testdata/check/const0.go b/src/internal/types/testdata/check/const0.go
new file mode 100644
index 0000000..402e6cf
--- /dev/null
+++ b/src/internal/types/testdata/check/const0.go
@@ -0,0 +1,382 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// constant declarations
+
+package const0
+
+import "unsafe"
+
+// constants declarations must be initialized by constants
+var x = 0
+const c0 = x /* ERROR "not constant" */
+
+// typed constants must have constant types
+const _ interface /* ERROR invalid constant type */ {} = 0
+
+func _ () {
+	const _ interface /* ERROR invalid constant type */ {} = 0
+	for i := 0; i < 10; i++ {} // don't crash with non-nil iota here
+}
+
+// untyped constants
+const (
+	// boolean values
+	ub0 = false
+	ub1 = true
+	ub2 = 2 < 1
+	ub3 = ui1 == uf1
+	ub4 = true == 0 /* ERROR "mismatched types untyped bool and untyped int" */
+
+	// integer values
+	ui0 = 0
+	ui1 = 1
+	ui2 = 42
+	ui3 = 3141592653589793238462643383279502884197169399375105820974944592307816406286
+	ui4 = -10
+
+	ui5 = ui0 + ui1
+	ui6 = ui1 - ui1
+	ui7 = ui2 * ui1
+	ui8 = ui3 / ui3
+	ui9 = ui3 % ui3
+
+	ui10 = 1 / 0 /* ERROR "division by zero" */
+	ui11 = ui1 / 0 /* ERROR "division by zero" */
+	ui12 = ui3 / ui0 /* ERROR "division by zero" */
+	ui13 = 1 % 0 /* ERROR "division by zero" */
+	ui14 = ui1 % 0 /* ERROR "division by zero" */
+	ui15 = ui3 % ui0 /* ERROR "division by zero" */
+
+	ui16 = ui2 & ui3
+	ui17 = ui2 | ui3
+	ui18 = ui2 ^ ui3
+	ui19 = 1 /* ERROR "invalid operation" */ % 1.0
+
+	// floating point values
+	uf0 = 0.
+	uf1 = 1.
+	uf2 = 4.2e1
+	uf3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286
+	uf4 = 1e-1
+
+	uf5 = uf0 + uf1
+	uf6 = uf1 - uf1
+	uf7 = uf2 * uf1
+	uf8 = uf3 / uf3
+	uf9 = uf3 /* ERROR "not defined" */ % uf3
+
+	uf10 = 1 / 0 /* ERROR "division by zero" */
+	uf11 = uf1 / 0 /* ERROR "division by zero" */
+	uf12 = uf3 / uf0 /* ERROR "division by zero" */
+
+	uf16 = uf2 /* ERROR "not defined" */ & uf3
+	uf17 = uf2 /* ERROR "not defined" */ | uf3
+	uf18 = uf2 /* ERROR "not defined" */ ^ uf3
+
+	// complex values
+	uc0 = 0.i
+	uc1 = 1.i
+	uc2 = 4.2e1i
+	uc3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286i
+	uc4 = 1e-1i
+
+	uc5 = uc0 + uc1
+	uc6 = uc1 - uc1
+	uc7 = uc2 * uc1
+	uc8 = uc3 / uc3
+	uc9 = uc3 /* ERROR "not defined" */ % uc3
+
+	uc10 = 1 / 0 /* ERROR "division by zero" */
+	uc11 = uc1 / 0 /* ERROR "division by zero" */
+	uc12 = uc3 / uc0 /* ERROR "division by zero" */
+
+	uc16 = uc2 /* ERROR "not defined" */ & uc3
+	uc17 = uc2 /* ERROR "not defined" */ | uc3
+	uc18 = uc2 /* ERROR "not defined" */ ^ uc3
+)
+
+type (
+	mybool bool
+	myint int
+	myfloat float64
+	mycomplex complex128
+)
+
+// typed constants
+const (
+	// boolean values
+	tb0 bool = false
+	tb1 bool = true
+	tb2 mybool = 2 < 1
+	tb3 mybool = ti1 == tf1 /* ERROR "mismatched types" */
+
+	// integer values
+	ti0 int8 = ui0
+	ti1 int32 = ui1
+	ti2 int64 = ui2
+	ti3 myint = ui3 /* ERROR "overflows" */
+	ti4 myint = ui4
+
+	ti5 = ti0 /* ERROR "mismatched types" */ + ti1
+	ti6 = ti1 - ti1
+	ti7 = ti2 /* ERROR "mismatched types" */ * ti1
+	ti8 = ti3 / ti3
+	ti9 = ti3 % ti3
+
+	ti10 = 1 / 0 /* ERROR "division by zero" */
+	ti11 = ti1 / 0 /* ERROR "division by zero" */
+	ti12 = ti3 /* ERROR "mismatched types" */ / ti0
+	ti13 = 1 % 0 /* ERROR "division by zero" */
+	ti14 = ti1 % 0 /* ERROR "division by zero" */
+	ti15 = ti3 /* ERROR "mismatched types" */ % ti0
+
+	ti16 = ti2 /* ERROR "mismatched types" */ & ti3
+	ti17 = ti2 /* ERROR "mismatched types" */ | ti4
+	ti18 = ti2 ^ ti5 // no mismatched types error because the type of ti5 is unknown
+
+	// floating point values
+	tf0 float32 = 0.
+	tf1 float32 = 1.
+	tf2 float64 = 4.2e1
+	tf3 myfloat = 3.141592653589793238462643383279502884197169399375105820974944592307816406286
+	tf4 myfloat = 1e-1
+
+	tf5 = tf0 + tf1
+	tf6 = tf1 - tf1
+	tf7 = tf2 /* ERROR "mismatched types" */ * tf1
+	tf8 = tf3 / tf3
+	tf9 = tf3 /* ERROR "not defined" */ % tf3
+
+	tf10 = 1 / 0 /* ERROR "division by zero" */
+	tf11 = tf1 / 0 /* ERROR "division by zero" */
+	tf12 = tf3 /* ERROR "mismatched types" */ / tf0
+
+	tf16 = tf2 /* ERROR "mismatched types" */ & tf3
+	tf17 = tf2 /* ERROR "mismatched types" */ | tf3
+	tf18 = tf2 /* ERROR "mismatched types" */ ^ tf3
+
+	// complex values
+	tc0 = 0.i
+	tc1 = 1.i
+	tc2 = 4.2e1i
+	tc3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286i
+	tc4 = 1e-1i
+
+	tc5 = tc0 + tc1
+	tc6 = tc1 - tc1
+	tc7 = tc2 * tc1
+	tc8 = tc3 / tc3
+	tc9 = tc3 /* ERROR "not defined" */ % tc3
+
+	tc10 = 1 / 0 /* ERROR "division by zero" */
+	tc11 = tc1 / 0 /* ERROR "division by zero" */
+	tc12 = tc3 / tc0 /* ERROR "division by zero" */
+
+	tc16 = tc2 /* ERROR "not defined" */ & tc3
+	tc17 = tc2 /* ERROR "not defined" */ | tc3
+	tc18 = tc2 /* ERROR "not defined" */ ^ tc3
+)
+
+// initialization cycles
+const (
+	a /* ERROR "initialization cycle" */ = a
+	b /* ERROR "initialization cycle" */ , c /* ERROR "initialization cycle" */, d, e = e, d, c, b // TODO(gri) should only have one cycle error
+	f float64 = d
+)
+
+// multiple initialization
+const (
+	a1, a2, a3 = 7, 3.1415926, "foo"
+	b1, b2, b3 = b3, b1, 42
+	c1, c2, c3  /* ERROR "missing init expr for c3" */ = 1, 2
+	d1, d2, d3 = 1, 2, 3, 4 /* ERROR "extra init expr 4" */
+	_p0 = assert(a1 == 7)
+	_p1 = assert(a2 == 3.1415926)
+	_p2 = assert(a3 == "foo")
+	_p3 = assert(b1 == 42)
+	_p4 = assert(b2 == 42)
+	_p5 = assert(b3 == 42)
+)
+
+func _() {
+	const (
+		a1, a2, a3 = 7, 3.1415926, "foo"
+		b1, b2, b3 = b3, b1, 42
+		c1, c2, c3  /* ERROR "missing init expr for c3" */ = 1, 2
+		d1, d2, d3 = 1, 2, 3, 4 /* ERROR "extra init expr 4" */
+		_p0 = assert(a1 == 7)
+		_p1 = assert(a2 == 3.1415926)
+		_p2 = assert(a3 == "foo")
+		_p3 = assert(b1 == 42)
+		_p4 = assert(b2 == 42)
+		_p5 = assert(b3 == 42)
+	)
+}
+
+// iota
+const (
+	iota0 = iota
+	iota1 = iota
+	iota2 = iota*2
+	_a0 = assert(iota0 == 0)
+	_a1 = assert(iota1 == 1)
+	_a2 = assert(iota2 == 4)
+	iota6 = iota*3
+
+	iota7
+	iota8
+	_a3 = assert(iota7 == 21)
+	_a4 = assert(iota8 == 24)
+)
+
+const (
+	_b0 = iota
+	_b1 = assert(iota + iota2 == 5)
+	_b2 = len([iota]int{}) // iota may appear in a type!
+	_b3 = assert(_b2 == 2)
+	_b4 = len(A{})
+)
+
+type A [iota /* ERROR "cannot use iota" */ ]int
+
+// constant expressions with operands across different
+// constant declarations must use the right iota values
+const (
+	_c0 = iota
+	_c1
+	_c2
+	_x = _c2 + _d1 + _e0 // 3
+)
+
+const (
+	_d0 = iota
+	_d1
+)
+
+const (
+	_e0 = iota
+)
+
+var _ = assert(_x == 3)
+
+// special cases
+const (
+	_n0 = nil /* ERROR "not constant" */
+	_n1 = [ /* ERROR "not constant" */ ]int{}
+)
+
+// iotas must not be usable in expressions outside constant declarations
+type _ [iota /* ERROR "iota outside constant decl" */ ]byte
+var _ = iota /* ERROR "iota outside constant decl" */
+func _() {
+	_ = iota /* ERROR "iota outside constant decl" */
+	const _ = iota
+	_ = iota /* ERROR "iota outside constant decl" */
+}
+
+func _() {
+	iota := 123
+	const x = iota /* ERROR "is not constant" */
+	var y = iota
+	_ = y
+}
+
+// iotas are usable inside closures in constant declarations (#22345)
+const (
+	_ = iota
+	_ = len([iota]byte{})
+	_ = unsafe.Sizeof(iota)
+	_ = unsafe.Sizeof(func() { _ = iota })
+	_ = unsafe.Sizeof(func() { var _ = iota })
+	_ = unsafe.Sizeof(func() { const _ = iota })
+	_ = unsafe.Sizeof(func() { type _ [iota]byte })
+	_ = unsafe.Sizeof(func() { func() int { return iota }() })
+)
+
+// verify inner and outer const declarations have distinct iotas
+const (
+	zero = iota
+	one  = iota
+	_    = unsafe.Sizeof(func() {
+		var x [iota]int // [2]int
+		const (
+			Zero = iota
+			One
+			Two
+			_ = unsafe.Sizeof([iota-1]int{} == x) // assert types are equal
+			_ = unsafe.Sizeof([Two]int{} == x)    // assert types are equal
+		)
+		var z [iota]int                           // [2]int
+		_ = unsafe.Sizeof([2]int{} == z)          // assert types are equal
+	})
+	three = iota // the sequence continues
+)
+var _ [three]int = [3]int{} // assert 'three' has correct value
+
+var (
+	_ = iota /* ERROR "iota outside constant decl" */
+	_ = unsafe.Sizeof(iota  /* ERROR "iota outside constant decl" */ )
+	_ = unsafe.Sizeof(func() { _ = iota /* ERROR "iota outside constant decl" */ })
+	_ = unsafe.Sizeof(func() { var _ = iota /* ERROR "iota outside constant decl" */ })
+	_ = unsafe.Sizeof(func() { type _ [iota /* ERROR "iota outside constant decl" */ ]byte })
+	_ = unsafe.Sizeof(func() { func() int { return iota /* ERROR "iota outside constant decl" */ }() })
+)
+
+// constant arithmetic precision and rounding must lead to expected (integer) results
+var _ = []int64{
+	0.0005 * 1e9,
+	0.001 * 1e9,
+	0.005 * 1e9,
+	0.01 * 1e9,
+	0.05 * 1e9,
+	0.1 * 1e9,
+	0.5 * 1e9,
+	1 * 1e9,
+	5 * 1e9,
+}
+
+const _ = unsafe.Sizeof(func() {
+	const _ = 0
+	_ = iota
+
+	const (
+	   zero = iota
+	   one
+	)
+	assert(one == 1)
+	assert(iota == 0)
+})
+
+// issue #52438
+const i1 = iota
+const i2 = iota
+const i3 = iota
+
+func _() {
+	assert(i1 == 0)
+	assert(i2 == 0)
+	assert(i3 == 0)
+
+	const i4 = iota
+	const i5 = iota
+	const i6 = iota
+
+	assert(i4 == 0)
+	assert(i5 == 0)
+	assert(i6 == 0)
+}
+
+// untyped constants must not get arbitrarily large
+const prec = 512 // internal maximum precision for integers
+const maxInt = (1<<(prec/2) - 1) * (1<<(prec/2) + 1) // == 1<<prec - 1
+
+const _ = maxInt + /* ERROR constant addition overflow */ 1
+const _ = -maxInt - /* ERROR constant subtraction overflow */ 1
+const _ = maxInt ^ /* ERROR constant bitwise XOR overflow */ -1
+const _ = maxInt * /* ERROR constant multiplication overflow */ 2
+const _ = maxInt << /* ERROR constant shift overflow */ 2
+const _ = 1 << /* ERROR constant shift overflow */ prec
+
+const _ = ^ /* ERROR constant bitwise complement overflow */ maxInt
diff --git a/src/internal/types/testdata/check/const1.go b/src/internal/types/testdata/check/const1.go
new file mode 100644
index 0000000..c912801
--- /dev/null
+++ b/src/internal/types/testdata/check/const1.go
@@ -0,0 +1,334 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// constant conversions
+
+package const1
+
+import "math"
+
+const(
+	mi = ^int(0)
+	mu = ^uint(0)
+	mp = ^uintptr(0)
+
+	logSizeofInt     = uint(mi>>8&1 + mi>>16&1 + mi>>32&1)
+	logSizeofUint    = uint(mu>>8&1 + mu>>16&1 + mu>>32&1)
+	logSizeofUintptr = uint(mp>>8&1 + mp>>16&1 + mp>>32&1)
+)
+
+const (
+	minInt8 = -1<<(8<<iota - 1)
+	minInt16
+	minInt32
+	minInt64
+	minInt = -1<<(8<<logSizeofInt - 1)
+)
+
+const (
+	maxInt8 = 1<<(8<<iota - 1) - 1
+	maxInt16
+	maxInt32
+	maxInt64
+	maxInt = 1<<(8<<logSizeofInt - 1) - 1
+)
+
+const (
+	maxUint8 = 1<<(8<<iota) - 1
+	maxUint16
+	maxUint32
+	maxUint64
+	maxUint    = 1<<(8<<logSizeofUint) - 1
+	maxUintptr = 1<<(8<<logSizeofUintptr) - 1
+)
+
+const (
+	smallestFloat32 = 1.0 / (1<<(127 - 1 + 23))
+	// TODO(gri) The compiler limits integers to 512 bit and thus
+	//           we cannot compute the value (1<<(1023 - 1 + 52))
+	//           without overflow. For now we match the compiler.
+	//           See also issue #44057.
+	// smallestFloat64 = 1.0 / (1<<(1023 - 1 + 52))
+	smallestFloat64 = math.SmallestNonzeroFloat64
+)
+
+const (
+	_ = assert(smallestFloat32 > 0)
+	_ = assert(smallestFloat64 > 0)
+)
+
+const (
+	maxFloat32 = 1<<127 * (1<<24 - 1) / (1.0<<23)
+	// TODO(gri) The compiler limits integers to 512 bit and thus
+	//           we cannot compute the value 1<<1023
+	//           without overflow. For now we match the compiler.
+	//           See also issue #44057.
+	// maxFloat64 = 1<<1023 * (1<<53 - 1) / (1.0<<52)
+	maxFloat64 = math.MaxFloat64
+)
+
+const (
+	_ int8 = minInt8 /* ERROR "overflows" */ - 1
+	_ int8 = minInt8
+	_ int8 = maxInt8
+	_ int8 = maxInt8 /* ERROR "overflows" */ + 1
+	_ int8 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int8(minInt8 /* ERROR "cannot convert" */ - 1)
+	_ = int8(minInt8)
+	_ = int8(maxInt8)
+	_ = int8(maxInt8 /* ERROR "cannot convert" */ + 1)
+	_ = int8(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int16 = minInt16 /* ERROR "overflows" */ - 1
+	_ int16 = minInt16
+	_ int16 = maxInt16
+	_ int16 = maxInt16 /* ERROR "overflows" */ + 1
+	_ int16 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int16(minInt16 /* ERROR "cannot convert" */ - 1)
+	_ = int16(minInt16)
+	_ = int16(maxInt16)
+	_ = int16(maxInt16 /* ERROR "cannot convert" */ + 1)
+	_ = int16(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int32 = minInt32 /* ERROR "overflows" */ - 1
+	_ int32 = minInt32
+	_ int32 = maxInt32
+	_ int32 = maxInt32 /* ERROR "overflows" */ + 1
+	_ int32 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int32(minInt32 /* ERROR "cannot convert" */ - 1)
+	_ = int32(minInt32)
+	_ = int32(maxInt32)
+	_ = int32(maxInt32 /* ERROR "cannot convert" */ + 1)
+	_ = int32(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int64 = minInt64 /* ERROR "overflows" */ - 1
+	_ int64 = minInt64
+	_ int64 = maxInt64
+	_ int64 = maxInt64 /* ERROR "overflows" */ + 1
+	_ int64 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int64(minInt64 /* ERROR "cannot convert" */ - 1)
+	_ = int64(minInt64)
+	_ = int64(maxInt64)
+	_ = int64(maxInt64 /* ERROR "cannot convert" */ + 1)
+	_ = int64(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int = minInt /* ERROR "overflows" */ - 1
+	_ int = minInt
+	_ int = maxInt
+	_ int = maxInt /* ERROR "overflows" */ + 1
+	_ int = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int(minInt /* ERROR "cannot convert" */ - 1)
+	_ = int(minInt)
+	_ = int(maxInt)
+	_ = int(maxInt /* ERROR "cannot convert" */ + 1)
+	_ = int(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint8 = 0 /* ERROR "overflows" */ - 1
+	_ uint8 = 0
+	_ uint8 = maxUint8
+	_ uint8 = maxUint8 /* ERROR "overflows" */ + 1
+	_ uint8 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint8(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint8(0)
+	_ = uint8(maxUint8)
+	_ = uint8(maxUint8 /* ERROR "cannot convert" */ + 1)
+	_ = uint8(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint16 = 0 /* ERROR "overflows" */ - 1
+	_ uint16 = 0
+	_ uint16 = maxUint16
+	_ uint16 = maxUint16 /* ERROR "overflows" */ + 1
+	_ uint16 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint16(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint16(0)
+	_ = uint16(maxUint16)
+	_ = uint16(maxUint16 /* ERROR "cannot convert" */ + 1)
+	_ = uint16(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint32 = 0 /* ERROR "overflows" */ - 1
+	_ uint32 = 0
+	_ uint32 = maxUint32
+	_ uint32 = maxUint32 /* ERROR "overflows" */ + 1
+	_ uint32 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint32(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint32(0)
+	_ = uint32(maxUint32)
+	_ = uint32(maxUint32 /* ERROR "cannot convert" */ + 1)
+	_ = uint32(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint64 = 0 /* ERROR "overflows" */ - 1
+	_ uint64 = 0
+	_ uint64 = maxUint64
+	_ uint64 = maxUint64 /* ERROR "overflows" */ + 1
+	_ uint64 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint64(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint64(0)
+	_ = uint64(maxUint64)
+	_ = uint64(maxUint64 /* ERROR "cannot convert" */ + 1)
+	_ = uint64(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint = 0 /* ERROR "overflows" */ - 1
+	_ uint = 0
+	_ uint = maxUint
+	_ uint = maxUint /* ERROR "overflows" */ + 1
+	_ uint = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint(0)
+	_ = uint(maxUint)
+	_ = uint(maxUint /* ERROR "cannot convert" */ + 1)
+	_ = uint(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uintptr = 0 /* ERROR "overflows" */ - 1
+	_ uintptr = 0
+	_ uintptr = maxUintptr
+	_ uintptr = maxUintptr /* ERROR "overflows" */ + 1
+	_ uintptr = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uintptr(0 /* ERROR "cannot convert" */ - 1)
+	_ = uintptr(0)
+	_ = uintptr(maxUintptr)
+	_ = uintptr(maxUintptr /* ERROR "cannot convert" */ + 1)
+	_ = uintptr(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ float32 = minInt64
+	_ float64 = minInt64
+	_ complex64 = minInt64
+	_ complex128 = minInt64
+
+	_ = float32(minInt64)
+	_ = float64(minInt64)
+	_ = complex64(minInt64)
+	_ = complex128(minInt64)
+)
+
+const (
+	_ float32 = maxUint64
+	_ float64 = maxUint64
+	_ complex64 = maxUint64
+	_ complex128 = maxUint64
+
+	_ = float32(maxUint64)
+	_ = float64(maxUint64)
+	_ = complex64(maxUint64)
+	_ = complex128(maxUint64)
+)
+
+// TODO(gri) find smaller deltas below
+
+const delta32 = maxFloat32/(1 << 23)
+
+const (
+	_ float32 = - /* ERROR "overflow" */ (maxFloat32 + delta32)
+	_ float32 = -maxFloat32
+	_ float32 = maxFloat32
+	_ float32 = maxFloat32 /* ERROR "overflow" */ + delta32
+
+	_ = float32(- /* ERROR "cannot convert" */ (maxFloat32 + delta32))
+	_ = float32(-maxFloat32)
+	_ = float32(maxFloat32)
+	_ = float32(maxFloat32 /* ERROR "cannot convert" */ + delta32)
+
+	_ = assert(float32(smallestFloat32) == smallestFloat32)
+	_ = assert(float32(smallestFloat32/2) == 0)
+	_ = assert(float32(smallestFloat64) == 0)
+	_ = assert(float32(smallestFloat64/2) == 0)
+)
+
+const delta64 = maxFloat64/(1 << 52)
+
+const (
+	_ float64 = - /* ERROR "overflow" */ (maxFloat64 + delta64)
+	_ float64 = -maxFloat64
+	_ float64 = maxFloat64
+	_ float64 = maxFloat64 /* ERROR "overflow" */ + delta64
+
+	_ = float64(- /* ERROR "cannot convert" */ (maxFloat64 + delta64))
+	_ = float64(-maxFloat64)
+	_ = float64(maxFloat64)
+	_ = float64(maxFloat64 /* ERROR "cannot convert" */ + delta64)
+
+	_ = assert(float64(smallestFloat32) == smallestFloat32)
+	_ = assert(float64(smallestFloat32/2) == smallestFloat32/2)
+	_ = assert(float64(smallestFloat64) == smallestFloat64)
+	_ = assert(float64(smallestFloat64/2) == 0)
+)
+
+const (
+	_ complex64 = - /* ERROR "overflow" */ (maxFloat32 + delta32)
+	_ complex64 = -maxFloat32
+	_ complex64 = maxFloat32
+	_ complex64 = maxFloat32 /* ERROR "overflow" */ + delta32
+
+	_ = complex64(- /* ERROR "cannot convert" */ (maxFloat32 + delta32))
+	_ = complex64(-maxFloat32)
+	_ = complex64(maxFloat32)
+	_ = complex64(maxFloat32 /* ERROR "cannot convert" */ + delta32)
+)
+
+const (
+	_ complex128 = - /* ERROR "overflow" */ (maxFloat64 + delta64)
+	_ complex128 = -maxFloat64
+	_ complex128 = maxFloat64
+	_ complex128 = maxFloat64 /* ERROR "overflow" */ + delta64
+
+	_ = complex128(- /* ERROR "cannot convert" */ (maxFloat64 + delta64))
+	_ = complex128(-maxFloat64)
+	_ = complex128(maxFloat64)
+	_ = complex128(maxFloat64 /* ERROR "cannot convert" */ + delta64)
+)
+
+// Initialization of typed constant and conversion are the same:
+const (
+	f32 = 1 + smallestFloat32
+	x32 float32 = f32
+	y32 = float32(f32)
+	_ = assert(x32 - y32 == 0)
+)
+
+const (
+	f64 = 1 + smallestFloat64
+	x64 float64 = f64
+	y64 = float64(f64)
+	_ = assert(x64 - y64 == 0)
+)
+
+const (
+	_ = int8(-1) << 7
+	_ = int8 /* ERROR "overflows" */ (-1) << 8
+
+	_ = uint32(1) << 31
+	_ = uint32 /* ERROR "overflows" */ (1) << 32
+)
diff --git a/src/internal/types/testdata/check/constdecl.go b/src/internal/types/testdata/check/constdecl.go
new file mode 100644
index 0000000..faa9b9d
--- /dev/null
+++ b/src/internal/types/testdata/check/constdecl.go
@@ -0,0 +1,160 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constdecl
+
+import "math"
+import "unsafe"
+
+var v int
+
+// Const decls must be initialized by constants.
+const _ = v /* ERROR "not constant" */
+const _ = math /* ERROR "not constant" */ .Sin(0)
+const _ = int /* ERROR "not an expression" */
+
+func _() {
+	const _ = v /* ERROR "not constant" */
+	const _ = math /* ERROR "not constant" */ .Sin(0)
+	const _ = int /* ERROR "not an expression" */
+}
+
+// Identifier and expression arity must match.
+const _ /* ERROR "missing init expr for _" */
+const _ = 1, 2 /* ERROR "extra init expr 2" */
+
+const _ /* ERROR "missing init expr for _" */ int
+const _ int = 1, 2 /* ERROR "extra init expr 2" */
+
+const (
+	_ /* ERROR "missing init expr for _" */
+	_ = 1, 2 /* ERROR "extra init expr 2" */
+
+	_ /* ERROR "missing init expr for _" */ int
+	_ int = 1, 2 /* ERROR "extra init expr 2" */
+)
+
+const (
+	_ = 1
+	_
+	_, _ /* ERROR "missing init expr for _" */
+	_
+)
+
+const (
+	_, _ = 1, 2
+	_, _
+	_ /* ERROR "extra init expr at" */
+	_, _
+	_, _, _ /* ERROR "missing init expr for _" */
+	_, _
+)
+
+func _() {
+	const _ /* ERROR "missing init expr for _" */
+	const _ = 1, 2 /* ERROR "extra init expr 2" */
+
+	const _ /* ERROR "missing init expr for _" */ int
+	const _ int = 1, 2 /* ERROR "extra init expr 2" */
+
+	const (
+		_ /* ERROR "missing init expr for _" */
+		_ = 1, 2 /* ERROR "extra init expr 2" */
+
+		_ /* ERROR "missing init expr for _" */ int
+		_ int = 1, 2 /* ERROR "extra init expr 2" */
+	)
+
+	const (
+		_ = 1
+		_
+		_, _ /* ERROR "missing init expr for _" */
+		_
+	)
+
+	const (
+		_, _ = 1, 2
+		_, _
+		_ /* ERROR "extra init expr at" */
+		_, _
+		_, _, _ /* ERROR "missing init expr for _" */
+		_, _
+	)
+}
+
+// Test case for constant with invalid initialization.
+// Caused panic because the constant value was not set up (gri - 7/8/2014).
+func _() {
+	const (
+	    x string = missing /* ERROR "undefined" */
+	    y = x + ""
+	)
+}
+
+// Test case for constants depending on function literals (see also #22992).
+const A /* ERROR initialization cycle */ = unsafe.Sizeof(func() { _ = A })
+
+func _() {
+	// The function literal below must not see a.
+	const a = unsafe.Sizeof(func() { _ = a /* ERROR "undefined" */ })
+	const b = unsafe.Sizeof(func() { _ = a })
+
+	// The function literal below must not see x, y, or z.
+	const x, y, z = 0, 1, unsafe.Sizeof(func() { _ = x /* ERROR "undefined" */ + y /* ERROR "undefined" */ + z /* ERROR "undefined" */ })
+}
+
+// Test cases for errors in inherited constant initialization expressions.
+// Errors related to inherited initialization expressions must appear at
+// the constant identifier being declared, not at the original expression
+// (issues #42991, #42992).
+const (
+	_ byte = 255 + iota
+	/* some gap */
+	_ // ERROR overflows
+	/* some gap */
+	/* some gap */ _ /* ERROR overflows */; _ /* ERROR overflows */
+	/* some gap */
+	_ = 255 + iota
+	_ = byte /* ERROR overflows */ (255) + iota
+	_ /* ERROR overflows */
+)
+
+// Test cases from issue.
+const (
+	ok = byte(iota + 253)
+	bad
+	barn
+	bard // ERROR cannot convert
+)
+
+const (
+	c = len([1 - iota]int{})
+	d
+	e // ERROR invalid array length
+	f // ERROR invalid array length
+)
+
+// Test that identifiers in implicit (omitted) RHS
+// expressions of constant declarations are resolved
+// in the correct context; see issues #49157, #53585.
+const X = 2
+
+func _() {
+	const (
+		A    = iota // 0
+		iota = iota // 1
+		B           // 1 (iota is declared locally on prev. line)
+		C           // 1
+	)
+	assert(A == 0 && B == 1 && C == 1)
+
+	const (
+		X = X + X
+		Y
+		Z = iota
+	)
+	assert(X == 4 && Y == 8 && Z == 1)
+}
+
+// TODO(gri) move extra tests from testdata/const0.src into here
diff --git a/src/internal/types/testdata/check/conversions0.go b/src/internal/types/testdata/check/conversions0.go
new file mode 100644
index 0000000..e1336c0
--- /dev/null
+++ b/src/internal/types/testdata/check/conversions0.go
@@ -0,0 +1,93 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// conversions
+
+package conversions
+
+import "unsafe"
+
+// argument count
+var (
+	_ = int() /* ERROR "missing argument" */
+	_ = int(1, 2 /* ERROR "too many arguments" */ )
+)
+
+// numeric constant conversions are in const1.src.
+
+func string_conversions() {
+	const A = string(65)
+	assert(A == "A")
+	const E = string(-1)
+	assert(E == "\uFFFD")
+	assert(E == string(1234567890))
+
+	type myint int
+	assert(A == string(myint(65)))
+
+	type mystring string
+	const _ mystring = mystring("foo")
+
+	const _ = string(true /* ERROR "cannot convert" */ )
+	const _ = string(1.2 /* ERROR "cannot convert" */ )
+	const _ = string(nil /* ERROR "cannot convert" */ )
+
+	// issues 11357, 11353: argument must be of integer type
+	_ = string(0.0 /* ERROR "cannot convert" */ )
+	_ = string(0i /* ERROR "cannot convert" */ )
+	_ = string(1 /* ERROR "cannot convert" */ + 2i)
+}
+
+func interface_conversions() {
+	type E interface{}
+
+	type I1 interface{
+		m1()
+	}
+
+	type I2 interface{
+		m1()
+		m2(x int)
+	}
+
+	type I3 interface{
+		m1()
+		m2() int
+	}
+
+	var e E
+	var i1 I1
+	var i2 I2
+	var i3 I3
+
+	_ = E(0)
+	_ = E(nil)
+	_ = E(e)
+	_ = E(i1)
+	_ = E(i2)
+
+	_ = I1(0 /* ERROR "cannot convert" */ )
+	_ = I1(nil)
+	_ = I1(i1)
+	_ = I1(e /* ERROR "cannot convert" */ )
+	_ = I1(i2)
+
+	_ = I2(nil)
+	_ = I2(i1 /* ERROR "cannot convert" */ )
+	_ = I2(i2)
+	_ = I2(i3 /* ERROR "cannot convert" */ )
+
+	_ = I3(nil)
+	_ = I3(i1 /* ERROR "cannot convert" */ )
+	_ = I3(i2 /* ERROR "cannot convert" */ )
+	_ = I3(i3)
+
+	// TODO(gri) add more tests, improve error message
+}
+
+func issue6326() {
+	type T unsafe.Pointer
+	var x T
+	_ = uintptr(x) // see issue 6326
+}
diff --git a/src/internal/types/testdata/check/conversions1.go b/src/internal/types/testdata/check/conversions1.go
new file mode 100644
index 0000000..93a5f18
--- /dev/null
+++ b/src/internal/types/testdata/check/conversions1.go
@@ -0,0 +1,313 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test various valid and invalid struct assignments and conversions.
+// Does not compile.
+
+package conversions2
+
+type I interface {
+	m()
+}
+
+// conversions between structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s S
+	var t T
+	var u struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u
+	t = T(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t /* ERROR "cannot convert" */ )
+	s = S(u /* ERROR "cannot convert" */ )
+	t = u   // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s S
+	var t T
+	var u struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u /* ERROR "cannot convert" */ )
+	t = u   // ERROR "cannot use .* in assignment"
+	t = T(u /* ERROR "cannot convert" */ )
+}
+
+// conversions between pointers to structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s *S
+	var t *T
+	var u *struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t /* ERROR "cannot convert" */ )
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u /* ERROR "cannot convert" */ )
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(*struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(*struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u /* ERROR "cannot convert" */ )
+}
diff --git a/src/internal/types/testdata/check/cycles0.go b/src/internal/types/testdata/check/cycles0.go
new file mode 100644
index 0000000..7c00c7d
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles0.go
@@ -0,0 +1,175 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cycles
+
+import "unsafe"
+
+type (
+	T0 int
+	T1 /* ERROR invalid recursive type: T1 refers to itself */ T1
+	T2 *T2
+
+	T3 /* ERROR invalid recursive type */ T4
+	T4 T5
+	T5 T3
+
+	T6 T7
+	T7 *T8
+	T8 T6
+
+	// arrays
+	A0 /* ERROR invalid recursive type */ [10]A0
+	A1 [10]*A1
+
+	A2 /* ERROR invalid recursive type */ [10]A3
+	A3 [10]A4
+	A4 A2
+
+	A5 [10]A6
+	A6 *A5
+
+	// slices
+	L0 []L0
+
+	// structs
+	S0 /* ERROR invalid recursive type: S0 refers to itself */ struct{ _ S0 }
+	S1 /* ERROR invalid recursive type: S1 refers to itself */ struct{ S1 }
+	S2 struct{ _ *S2 }
+	S3 struct{ *S3 }
+
+	S4 /* ERROR invalid recursive type */ struct{ S5 }
+	S5 struct{ S6 }
+	S6 S4
+
+	// pointers
+	P0 *P0
+	PP *struct{ PP.f /* ERROR PP.f is not a type */ }
+
+	// functions
+	F0 func(F0)
+	F1 func() F1
+	F2 func(F2) F2
+
+	// interfaces
+	I0 /* ERROR invalid recursive type: I0 refers to itself */ interface{ I0 }
+
+	I1 /* ERROR invalid recursive type */ interface{ I2 }
+	I2 interface{ I3 }
+	I3 interface{ I1 }
+
+	I4 interface{ f(I4) }
+
+	// testcase for issue 5090
+	I5 interface{ f(I6) }
+	I6 interface{ I5 }
+
+	// maps
+	M0 map[M0 /* ERROR invalid map key */ ]M0
+
+	// channels
+	C0 chan C0
+)
+
+// test case for issue #34771
+type (
+	AA /* ERROR invalid recursive type */ B
+	B C
+	C [10]D
+	D E
+	E AA
+)
+
+func _() {
+	type (
+		t1 /* ERROR invalid recursive type: t1 refers to itself */ t1
+		t2 *t2
+
+		t3 t4 /* ERROR undefined */
+		t4 t5 /* ERROR undefined */
+		t5 t3
+
+		// arrays
+		a0 /* ERROR invalid recursive type: a0 refers to itself */ [10]a0
+		a1 [10]*a1
+
+		// slices
+		l0 []l0
+
+		// structs
+		s0 /* ERROR invalid recursive type: s0 refers to itself */ struct{ _ s0 }
+		s1 /* ERROR invalid recursive type: s1 refers to itself */ struct{ s1 }
+		s2 struct{ _ *s2 }
+		s3 struct{ *s3 }
+
+		// pointers
+		p0 *p0
+
+		// functions
+		f0 func(f0)
+		f1 func() f1
+		f2 func(f2) f2
+
+		// interfaces
+		i0 /* ERROR invalid recursive type: i0 refers to itself */ interface{ i0 }
+
+		// maps
+		m0 map[m0 /* ERROR invalid map key */ ]m0
+
+		// channels
+		c0 chan c0
+	)
+}
+
+// test cases for issue 6667
+
+type A [10]map[A /* ERROR invalid map key */ ]bool
+
+type S struct {
+	m map[S /* ERROR invalid map key */ ]bool
+}
+
+// test cases for issue 7236
+// (cycle detection must not be dependent on starting point of resolution)
+
+type (
+	P1 *T9
+	T9 /* ERROR invalid recursive type: T9 refers to itself */ T9
+
+	T10 /* ERROR invalid recursive type: T10 refers to itself */ T10
+	P2 *T10
+)
+
+func (T11) m() {}
+
+type T11 /* ERROR invalid recursive type: T11 refers to itself */ struct{ T11 }
+
+type T12 /* ERROR invalid recursive type: T12 refers to itself */ struct{ T12 }
+
+func (*T12) m() {}
+
+type (
+	P3 *T13
+	T13 /* ERROR invalid recursive type */ T13
+)
+
+// test cases for issue 18643
+// (type cycle detection when non-type expressions are involved)
+type (
+	T14 [len(T14 /* ERROR invalid recursive type */ {})]int
+	T15 [][len(T15 /* ERROR invalid recursive type */ {})]int
+	T16 map[[len(T16 /* ERROR invalid recursive type */ {1:2})]int]int
+	T17 map[int][len(T17 /* ERROR invalid recursive type */ {1:2})]int
+)
+
+// Test case for types depending on function literals (see also #22992).
+type T20 chan [unsafe.Sizeof(func(ch T20){ _ = <-ch })]byte
+type T22 = chan [unsafe.Sizeof(func(ch T20){ _ = <-ch })]byte
+
+func _() {
+	type T0 func(T0)
+	type T1 /* ERROR invalid recursive type */ = func(T1)
+	type T2 chan [unsafe.Sizeof(func(ch T2){ _ = <-ch })]byte
+	type T3 /* ERROR invalid recursive type */ = chan [unsafe.Sizeof(func(ch T3){ _ = <-ch })]byte
+}
diff --git a/src/internal/types/testdata/check/cycles1.go b/src/internal/types/testdata/check/cycles1.go
new file mode 100644
index 0000000..ae2b38e
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles1.go
@@ -0,0 +1,77 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	A interface {
+		a() interface {
+			ABC1
+		}
+	}
+	B interface {
+		b() interface {
+			ABC2
+		}
+	}
+	C interface {
+		c() interface {
+			ABC3
+		}
+	}
+
+	AB interface {
+		A
+		B
+	}
+	BC interface {
+		B
+		C
+	}
+
+	ABC1 interface {
+		A
+		B
+		C
+	}
+	ABC2 interface {
+		AB
+		C
+	}
+	ABC3 interface {
+		A
+		BC
+	}
+)
+
+var (
+	x1 ABC1
+	x2 ABC2
+	x3 ABC3
+)
+
+func _() {
+	// all types have the same method set
+	x1 = x2
+	x2 = x1
+
+	x1 = x3
+	x3 = x1
+
+	x2 = x3
+	x3 = x2
+
+	// all methods return the same type again
+	x1 = x1.a()
+	x1 = x1.b()
+	x1 = x1.c()
+
+	x2 = x2.a()
+	x2 = x2.b()
+	x2 = x2.c()
+
+	x3 = x3.a()
+	x3 = x3.b()
+	x3 = x3.c()
+}
diff --git a/src/internal/types/testdata/check/cycles2.go b/src/internal/types/testdata/check/cycles2.go
new file mode 100644
index 0000000..8480b29
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles2.go
@@ -0,0 +1,98 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// Test case for issue 5090
+
+type t interface {
+	f(u)
+}
+
+type u interface {
+	t
+}
+
+func _() {
+	var t t
+	var u u
+
+	t.f(t)
+	t.f(u)
+
+	u.f(t)
+	u.f(u)
+}
+
+
+// Test case for issues #6589, #33656.
+
+type A interface {
+	a() interface {
+		AB
+	}
+}
+
+type B interface {
+	b() interface {
+		AB
+	}
+}
+
+type AB interface {
+	a() interface {
+		A
+		B
+	}
+	b() interface {
+		A
+		B
+	}
+}
+
+var x AB
+var y interface {
+	A
+	B
+}
+
+var _ = x == y
+
+
+// Test case for issue 6638.
+
+type T interface {
+	m() [T(nil).m /* ERROR undefined */ ()[0]]int
+}
+
+// Variations of this test case.
+
+type T1 /* ERROR invalid recursive type */ interface {
+	m() [x1.m()[0]]int
+}
+
+var x1 T1
+
+type T2 /* ERROR invalid recursive type */ interface {
+	m() [len(x2.m())]int
+}
+
+var x2 T2
+
+type T3 /* ERROR invalid recursive type */ interface {
+	m() [unsafe.Sizeof(x3.m)]int
+}
+
+var x3 T3
+
+type T4 /* ERROR invalid recursive type */ interface {
+	m() [unsafe.Sizeof(cast4(x4.m))]int // cast is invalid but we have a cycle, so all bets are off
+}
+
+var x4 T4
+var _ = cast4(x4.m)
+
+type cast4 func()
diff --git a/src/internal/types/testdata/check/cycles3.go b/src/internal/types/testdata/check/cycles3.go
new file mode 100644
index 0000000..4330551
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles3.go
@@ -0,0 +1,60 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+var (
+	_ A = A(nil).a().b().c().d().e().f()
+	_ A = A(nil).b().c().d().e().f()
+	_ A = A(nil).c().d().e().f()
+	_ A = A(nil).d().e().f()
+	_ A = A(nil).e().f()
+	_ A = A(nil).f()
+	_ A = A(nil)
+)
+
+type (
+	A interface {
+		a() B
+		B
+	}
+
+	B interface {
+		b() C
+		C
+	}
+
+	C interface {
+		c() D
+		D
+	}
+
+	D interface {
+		d() E
+		E
+	}
+
+	E interface {
+		e() F
+		F
+	}
+
+	F interface {
+		f() A
+	}
+)
+
+type (
+	U /* ERROR invalid recursive type */ interface {
+		V
+	}
+
+	V interface {
+		v() [unsafe.Sizeof(u)]int
+	}
+)
+
+var u U
diff --git a/src/internal/types/testdata/check/cycles4.go b/src/internal/types/testdata/check/cycles4.go
new file mode 100644
index 0000000..924aabf
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles4.go
@@ -0,0 +1,121 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// Check that all methods of T are collected before
+// determining the result type of m (which embeds
+// all methods of T).
+
+type T interface {
+	m() interface {T}
+	E
+}
+
+var _ int = T.m(nil).m().e()
+
+type E interface {
+	e() int
+}
+
+// Check that unresolved forward chains are followed
+// (see also comment in resolver.go, checker.typeDecl).
+
+var _ int = C.m(nil).m().e()
+
+type A B
+
+type B interface {
+	m() interface{C}
+	E
+}
+
+type C A
+
+// Check that interface type comparison for identity
+// does not recur endlessly.
+
+type T1 interface {
+	m() interface{T1}
+}
+
+type T2 interface {
+	m() interface{T2}
+}
+
+func _(x T1, y T2) {
+	// Checking for assignability of interfaces must check
+	// if all methods of x are present in y, and that they
+	// have identical signatures. The signatures recur via
+	// the result type, which is an interface that embeds
+	// a single method m that refers to the very interface
+	// that contains it. This requires cycle detection in
+	// identity checks for interface types.
+	x = y
+}
+
+type T3 interface {
+	m() interface{T4}
+}
+
+type T4 interface {
+	m() interface{T3}
+}
+
+func _(x T1, y T3) {
+	x = y
+}
+
+// Check that interfaces are type-checked in order of
+// (embedded interface) dependencies (was issue 7158).
+
+var x1 T5 = T7(nil)
+
+type T5 interface {
+	T6
+}
+
+type T6 interface {
+	m() T7
+}
+type T7 interface {
+	T5
+}
+
+// Actual test case from issue 7158.
+
+func wrapNode() Node {
+	return wrapElement()
+}
+
+func wrapElement() Element {
+	return nil
+}
+
+type EventTarget interface {
+	AddEventListener(Event)
+}
+
+type Node interface {
+	EventTarget
+}
+
+type Element interface {
+	Node
+}
+
+type Event interface {
+	Target() Element
+}
+
+// Check that accessing an interface method too early doesn't lead
+// to follow-on errors due to an incorrectly computed type set.
+
+type T8 interface {
+	m() [unsafe.Sizeof(T8.m /* ERROR undefined */ )]int
+}
+
+var _ = T8.m // no error expected here
diff --git a/src/internal/types/testdata/check/cycles5.go b/src/internal/types/testdata/check/cycles5.go
new file mode 100644
index 0000000..5e0d191
--- /dev/null
+++ b/src/internal/types/testdata/check/cycles5.go
@@ -0,0 +1,200 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// test case from issue #18395
+
+type (
+	A interface { B }
+	B interface { C }
+	C interface { D; F() A }
+	D interface { G() B }
+)
+
+var _ = A(nil).G // G must be found
+
+
+// test case from issue #21804
+
+type sourceBridge interface {
+	listVersions() ([]Version, error)
+}
+
+type Constraint interface {
+	copyTo(*ConstraintMsg)
+}
+
+type ConstraintMsg struct{}
+
+func (m *ConstraintMsg) asUnpairedVersion() UnpairedVersion {
+	return nil
+}
+
+type Version interface {
+	Constraint
+}
+
+type UnpairedVersion interface {
+	Version
+}
+
+var _ Constraint = UnpairedVersion(nil)
+
+
+// derived test case from issue #21804
+
+type (
+	_ interface{ m(B1) }
+	A1 interface{ a(D1) }
+	B1 interface{ A1 }
+	C1 interface{ B1 }
+	D1 interface{ C1 }
+)
+
+var _ A1 = C1(nil)
+
+
+// derived test case from issue #22701
+
+func F(x I4) interface{} {
+	return x.Method()
+}
+
+type Unused interface {
+	RefersToI1(a I1)
+}
+
+type I1 interface {
+	I2
+	I3
+}
+
+type I2 interface {
+	RefersToI4() I4
+}
+
+type I3 interface {
+	Method() interface{}
+}
+
+type I4 interface {
+	I1
+}
+
+
+// check embedding of error interface
+
+type Error interface{ error }
+
+var err Error
+var _ = err.Error()
+
+
+// more esoteric cases
+
+type (
+	T1 interface { T2 }
+	T2 /* ERROR invalid recursive type */ T2
+)
+
+type (
+	T3 interface { T4 }
+	T4 /* ERROR invalid recursive type */ T5
+	T5 = T6
+	T6 = T7
+	T7 = T4
+)
+
+
+// arbitrary code may appear inside an interface
+
+const n = unsafe.Sizeof(func(){})
+
+type I interface {
+	m([unsafe.Sizeof(func() { I.m(nil, [n]byte{}) })]byte)
+}
+
+
+// test cases for varias alias cycles
+
+type T10 /* ERROR invalid recursive type */ = *T10                 // issue #25141
+type T11 /* ERROR invalid recursive type */ = interface{ f(T11) }  // issue #23139
+
+// issue #18640
+type (
+	aa = bb
+	bb struct {
+		*aa
+	}
+)
+
+type (
+	a struct{ *b }
+	b = c
+	c struct{ *b /* ERROR invalid use of type alias */ }
+)
+
+// issue #24939
+type (
+	_ interface {
+		M(P)
+	}
+
+	M interface {
+		F() P // ERROR invalid use of type alias
+	}
+
+	P = interface {
+		I() M
+	}
+)
+
+// issue #8699
+type T12 /* ERROR invalid recursive type */ [len(a12)]int
+var a12 = makeArray()
+func makeArray() (res T12) { return }
+
+// issue #20770
+var r /* ERROR invalid cycle in declaration of r */ = newReader()
+func newReader() r
+
+// variations of the theme of #8699 and #20770
+var arr /* ERROR cycle */ = f()
+func f() [len(arr)]int
+
+// issue #25790
+func ff(ff /* ERROR not a type */ )
+func gg((gg /* ERROR not a type */ ))
+
+type T13 /* ERROR invalid recursive type T13 */ [len(b13)]int
+var b13 T13
+
+func g1() [unsafe.Sizeof(g1)]int
+func g2() [unsafe.Sizeof(x2)]int
+var x2 = g2
+
+// verify that we get the correct sizes for the functions above
+// (note: assert is statically evaluated in go/types test mode)
+func init() {
+	assert(unsafe.Sizeof(g1) == 8)
+	assert(unsafe.Sizeof(x2) == 8)
+}
+
+func h() [h /* ERROR no value */ ()[0]]int { panic(0) }
+
+var c14 /* ERROR cycle */ T14
+type T14 [uintptr(unsafe.Sizeof(&c14))]byte
+
+// issue #34333
+type T15 /* ERROR invalid recursive type T15 */ struct {
+	f func() T16
+	b T16
+}
+
+type T16 struct {
+	T15
+}
\ No newline at end of file
diff --git a/src/internal/types/testdata/check/decls0.go b/src/internal/types/testdata/check/decls0.go
new file mode 100644
index 0000000..868b318
--- /dev/null
+++ b/src/internal/types/testdata/check/decls0.go
@@ -0,0 +1,210 @@
+// -lang=go1.17
+
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// type declarations
+
+package p // don't permit non-interface elements in interfaces
+
+import "unsafe"
+
+const pi = 3.1415
+
+type (
+	N undefined /* ERROR "undefined" */
+	B bool
+	I int32
+	A [10]P
+	T struct {
+		x, y P
+	}
+	P *T
+	R (*R)
+	F func(A) I
+	Y interface {
+		f(A) I
+	}
+	S [](((P)))
+	M map[I]F
+	C chan<- I
+
+	// blank types must be typechecked
+	_ pi /* ERROR "not a type" */
+	_ struct{}
+	_ struct{ pi /* ERROR "not a type" */ }
+)
+
+
+// declarations of init
+const _, init /* ERROR "cannot declare init" */ , _ = 0, 1, 2
+type init /* ERROR "cannot declare init" */ struct{}
+var _, init /* ERROR "cannot declare init" */ int
+
+func init() {}
+func init /* ERROR "missing function body" */ ()
+
+func _() { const init = 0 }
+func _() { type init int }
+func _() { var init int; _ = init }
+
+// invalid array types
+type (
+	iA0 [... /* ERROR "invalid use of \[...\] array" */ ]byte
+	// The error message below could be better. At the moment
+	// we believe an integer that is too large is not an integer.
+	// But at least we get an error.
+	iA1 [1 /* ERROR "must be integer" */ <<100]int
+	iA2 [- /* ERROR "invalid array length" */ 1]complex128
+	iA3 ["foo" /* ERROR "must be integer" */ ]string
+	iA4 [float64 /* ERROR "must be integer" */ (0)]int
+)
+
+
+type (
+	p1 pi.foo /* ERROR "pi.foo is not a type" */
+	p2 unsafe.Pointer
+)
+
+
+type (
+	Pi pi /* ERROR "not a type" */
+
+	a /* ERROR "invalid recursive type" */ a
+	a /* ERROR "redeclared" */ int
+
+	b /* ERROR "invalid recursive type" */ c
+	c d
+	d e
+	e b
+
+	t *t
+
+	U V
+	V *W
+	W U
+
+	P1 *S2
+	P2 P1
+
+	S0 struct {
+	}
+	S1 struct {
+		a, b, c int
+		u, v, a /* ERROR "redeclared" */ float32
+	}
+	S2 struct {
+		S0 // embedded field
+		S0 /* ERROR "redeclared" */ int
+	}
+	S3 struct {
+		x S2
+	}
+	S4/* ERROR "invalid recursive type" */ struct {
+		S4
+	}
+	S5 /* ERROR "invalid recursive type" */ struct {
+		S6
+	}
+	S6 struct {
+		field S7
+	}
+	S7 struct {
+		S5
+	}
+
+	L1 []L1
+	L2 []int
+
+	A1 [10.0]int
+	A2 /* ERROR "invalid recursive type" */ [10]A2
+	A3 /* ERROR "invalid recursive type" */ [10]struct {
+		x A4
+	}
+	A4 [10]A3
+
+	F1 func()
+	F2 func(x, y, z float32)
+	F3 func(x, y, x /* ERROR "redeclared" */ float32)
+	F4 func() (x, y, x /* ERROR "redeclared" */ float32)
+	F5 func(x int) (x /* ERROR "redeclared" */ float32)
+	F6 func(x ...int)
+
+	I1 interface{}
+	I2 interface {
+		m1()
+	}
+	I3 interface {
+		m1()
+		m1 /* ERROR "duplicate method" */ ()
+	}
+	I4 interface {
+		m1(x, y, x /* ERROR "redeclared" */ float32)
+		m2() (x, y, x /* ERROR "redeclared" */ float32)
+		m3(x int) (x /* ERROR "redeclared" */ float32)
+	}
+	I5 interface {
+		m1(I5)
+	}
+	I6 interface {
+		S0 /* ERROR "non-interface type S0" */
+	}
+	I7 interface {
+		I1
+		I1
+	}
+	I8 /* ERROR "invalid recursive type" */ interface {
+		I8
+	}
+	I9 /* ERROR "invalid recursive type" */ interface {
+		I10
+	}
+	I10 interface {
+		I11
+	}
+	I11 interface {
+		I9
+	}
+
+	C1 chan int
+	C2 <-chan int
+	C3 chan<- C3
+	C4 chan C5
+	C5 chan C6
+	C6 chan C4
+
+	M1 map[Last]string
+	M2 map[string]M2
+
+	Last int
+)
+
+// cycles in function/method declarations
+// (test cases for issues #5217, #25790 and variants)
+func f1(x f1 /* ERROR "not a type" */ ) {}
+func f2(x *f2 /* ERROR "not a type" */ ) {}
+func f3() (x f3 /* ERROR "not a type" */ ) { return }
+func f4() (x *f4 /* ERROR "not a type" */ ) { return }
+// TODO(#43215) this should be detected as a cycle error
+func f5([unsafe.Sizeof(f5)]int) {}
+
+func (S0) m1 (x S0.m1 /* ERROR S0.m1 is not a type */ ) {}
+func (S0) m2 (x *S0.m2 /* ERROR S0.m2 is not a type */ ) {}
+func (S0) m3 () (x S0.m3 /* ERROR S0.m3 is not a type */ ) { return }
+func (S0) m4 () (x *S0.m4 /* ERROR S0.m4 is not a type */ ) { return }
+
+// interfaces may not have any blank methods
+type BlankI interface {
+	_ /* ERROR "methods must have a unique non-blank name" */ ()
+	_ /* ERROR "methods must have a unique non-blank name" */ (float32) int
+	m()
+}
+
+// non-interface types may have multiple blank methods
+type BlankT struct{}
+
+func (BlankT) _() {}
+func (BlankT) _(int) {}
+func (BlankT) _() int { return 0 }
+func (BlankT) _(int) int { return 0}
diff --git a/src/internal/types/testdata/check/decls1.go b/src/internal/types/testdata/check/decls1.go
new file mode 100644
index 0000000..b232fc8
--- /dev/null
+++ b/src/internal/types/testdata/check/decls1.go
@@ -0,0 +1,146 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// variable declarations
+
+package decls1
+
+import (
+	"math"
+)
+
+// Global variables without initialization
+var (
+	a, b bool
+	c byte
+	d uint8
+	r rune
+	i int
+	j, k, l int
+	x, y float32
+	xx, yy float64
+	u, v complex64
+	uu, vv complex128
+	s, t string
+	array []byte
+	iface interface{}
+
+	blank _ /* ERROR "cannot use _" */
+)
+
+// Global variables with initialization
+var (
+	s1 = i + j
+	s2 = i /* ERROR "mismatched types" */ + x
+	s3 = c + d
+	s4 = s + t
+	s5 = s /* ERROR "invalid operation" */ / t
+	s6 = array[t1]
+	s7 = array[x /* ERROR "integer" */]
+	s8 = &a
+	s10 = &42 /* ERROR "cannot take address" */
+	s11 = &v
+	s12 = -(u + *t11) / *&v
+	s13 = a /* ERROR "shifted operand" */ << d
+	s14 = i << j
+	s18 = math.Pi * 10.0
+	s19 = s1 /* ERROR "cannot call" */ ()
+ 	s20 = f0 /* ERROR "no value" */ ()
+	s21 = f6(1, s1, i)
+	s22 = f6(1, s1, uu /* ERROR "cannot use .* in argument" */ )
+
+	t1 int = i + j
+	t2 int = i /* ERROR "mismatched types" */ + x
+	t3 int = c /* ERROR "cannot use .* variable declaration" */ + d
+	t4 string = s + t
+	t5 string = s /* ERROR "invalid operation" */ / t
+	t6 byte = array[t1]
+	t7 byte = array[x /* ERROR "must be integer" */]
+	t8 *int = & /* ERROR "cannot use .* variable declaration" */ a
+	t10 *int = &42 /* ERROR "cannot take address" */
+	t11 *complex64 = &v
+	t12 complex64 = -(u + *t11) / *&v
+	t13 int = a /* ERROR "shifted operand" */ << d
+	t14 int = i << j
+	t15 math /* ERROR "not in selector" */
+	t16 math.xxx /* ERROR "undefined" */
+	t17 math /* ERROR "not a type" */ .Pi
+	t18 float64 = math.Pi * 10.0
+	t19 int = t1 /* ERROR "cannot call" */ ()
+	t20 int = f0 /* ERROR "no value" */ ()
+	t21 int = a /* ERROR "cannot use .* variable declaration" */
+)
+
+// Various more complex expressions
+var (
+	u1 = x /* ERROR "not an interface" */ .(int)
+	u2 = iface.([]int)
+	u3 = iface.(a /* ERROR "not a type" */ )
+	u4, ok = iface.(int)
+	u5, ok2, ok3 = iface /* ERROR "assignment mismatch" */ .(int)
+)
+
+// Constant expression initializations
+var (
+	v1 = 1 /* ERROR "mismatched types untyped int and untyped string" */ + "foo"
+	v2 = c + 255
+	v3 = c + 256 /* ERROR "overflows" */
+	v4 = r + 2147483647
+	v5 = r + 2147483648 /* ERROR "overflows" */
+	v6 = 42
+	v7 = v6 + 9223372036854775807
+	v8 = v6 + 9223372036854775808 /* ERROR "overflows" */
+	v9 = i + 1 << 10
+	v10 byte = 1024 /* ERROR "overflows" */
+	v11 = xx/yy*yy - xx
+	v12 = true && false
+	v13 = nil /* ERROR "use of untyped nil" */
+	v14 string = 257 // ERROR cannot use 257 .* as string value in variable declaration$
+	v15 int8 = 257 // ERROR cannot use 257 .* as int8 value in variable declaration .*overflows
+)
+
+// Multiple assignment expressions
+var (
+	m1a, m1b = 1, 2
+	m2a, m2b, m2c /* ERROR "missing init expr for m2c" */ = 1, 2
+	m3a, m3b = 1, 2, 3 /* ERROR "extra init expr 3" */
+)
+
+func _() {
+	var (
+		m1a, m1b = 1, 2
+		m2a, m2b, m2c /* ERROR "missing init expr for m2c" */ = 1, 2
+		m3a, m3b = 1, 2, 3 /* ERROR "extra init expr 3" */
+	)
+
+	_, _ = m1a, m1b
+	_, _, _ = m2a, m2b, m2c
+	_, _ = m3a, m3b
+}
+
+// Declaration of parameters and results
+func f0() {}
+func f1(a /* ERROR "not a type" */) {}
+func f2(a, b, c d /* ERROR "not a type" */) {}
+
+func f3() int { return 0 }
+func f4() a /* ERROR "not a type" */ { return 0 }
+func f5() (a, b, c d /* ERROR "not a type" */) { return }
+
+func f6(a, b, c int) complex128 { return 0 }
+
+// Declaration of receivers
+type T struct{}
+
+func (T) m0() {}
+func (*T) m1() {}
+func (x T) m2() {}
+func (x *T) m3() {}
+
+// Initialization functions
+func init() {}
+func init /* ERROR "no arguments and no return values" */ (int) {}
+func init /* ERROR "no arguments and no return values" */ () int { return 0 }
+func init /* ERROR "no arguments and no return values" */ (int) int { return 0 }
+func (T) init(int) int { return 0 }
diff --git a/src/internal/types/testdata/check/decls2/decls2a.go b/src/internal/types/testdata/check/decls2/decls2a.go
new file mode 100644
index 0000000..731405b
--- /dev/null
+++ b/src/internal/types/testdata/check/decls2/decls2a.go
@@ -0,0 +1,111 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// method declarations
+
+package decls2
+
+import "time"
+import "unsafe"
+
+// T1 declared before its methods.
+type T1 struct{
+	f int
+}
+
+func (T1) m() {}
+func (T1) m /* ERROR "already declared" */ () {}
+func (x *T1) f /* ERROR "field and method" */ () {}
+
+// Conflict between embedded field and method name,
+// with the embedded field being a basic type.
+type T1b struct {
+	int
+}
+
+func (T1b) int /* ERROR "field and method" */ () {}
+
+type T1c struct {
+	time.Time
+}
+
+func (T1c) Time /* ERROR "field and method" */ () int { return 0 }
+
+// Disabled for now: LookupFieldOrMethod will find Pointer even though
+// it's double-declared (it would cost extra in the common case to verify
+// this). But the MethodSet computation will not find it due to the name
+// collision caused by the double-declaration, leading to an internal
+// inconsistency while we are verifying one computation against the other.
+// var _ = T1c{}.Pointer
+
+// T2's method declared before the type.
+func (*T2) f /* ERROR "field and method" */ () {}
+
+type T2 struct {
+	f int
+}
+
+// Methods declared without a declared type.
+func (undefined /* ERROR "undefined" */) m() {}
+func (x *undefined /* ERROR "undefined" */) m() {}
+
+func (pi /* ERROR "not a type" */) m1() {}
+func (x pi /* ERROR "not a type" */) m2() {}
+func (x *pi /* ERROR "not a type" */ ) m3() {}
+
+// Blank types.
+type _ struct { m int }
+type _ struct { m int }
+
+func (_ /* ERROR "cannot use _" */) m() {}
+func m(_ /* ERROR "cannot use _" */) {}
+
+// Methods with receiver base type declared in another file.
+func (T3) m1() {}
+func (*T3) m2() {}
+func (x T3) m3() {}
+func (x *T3) f /* ERROR "field and method" */ () {}
+
+// Methods of non-struct type.
+type T4 func()
+
+func (self T4) m() func() { return self }
+
+// Methods associated with an interface.
+type T5 interface {
+	m() int
+}
+
+func (T5 /* ERROR "invalid receiver" */ ) m1() {}
+func (T5 /* ERROR "invalid receiver" */ ) m2() {}
+
+// Methods associated with a named pointer type.
+type ptr *int
+func (ptr /* ERROR "invalid receiver" */ ) _() {}
+func (* /* ERROR "invalid receiver" */ ptr) _() {}
+
+// Methods with zero or multiple receivers.
+func ( /* ERROR "method has no receiver" */ ) _() {}
+func (T3, * /* ERROR "method has multiple receivers" */ T3) _() {}
+func (T3, T3, T3 /* ERROR "method has multiple receivers" */ ) _() {}
+func (a, b /* ERROR "method has multiple receivers" */ T3) _() {}
+func (a, b, c /* ERROR "method has multiple receivers" */ T3) _() {}
+
+// Methods associated with non-local or unnamed types.
+func (int /* ERROR "cannot define new methods on non-local type int" */ ) m() {}
+func ([ /* ERROR "invalid receiver" */ ]int) m() {}
+func (time /* ERROR "cannot define new methods on non-local type time\.Time" */ .Time) m() {}
+func (* /* ERROR "cannot define new methods on non-local type time\.Time" */ time.Time) m() {}
+func (x /* ERROR "invalid receiver" */ interface{}) m() {}
+
+// Unsafe.Pointer is treated like a pointer when used as receiver type.
+type UP unsafe.Pointer
+func (UP /* ERROR "invalid" */ ) m1() {}
+func (* /* ERROR "invalid" */ UP) m2() {}
+
+// Double declarations across package files
+const c_double = 0
+type t_double int
+var v_double int
+func f_double() {}
diff --git a/src/internal/types/testdata/check/decls2/decls2b.go b/src/internal/types/testdata/check/decls2/decls2b.go
new file mode 100644
index 0000000..d4c5861
--- /dev/null
+++ b/src/internal/types/testdata/check/decls2/decls2b.go
@@ -0,0 +1,75 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// method declarations
+
+package decls2
+
+import "io"
+
+const pi = 3.1415
+
+func (T1) m /* ERROR "already declared" */ () {}
+func (T2) m(io.Writer) {}
+
+type T3 struct {
+	f *T3
+}
+
+type T6 struct {
+	x int
+}
+
+func (t *T6) m1() int {
+	return t.x
+}
+
+func f() {
+	var t *T6
+	t.m1()
+}
+
+// Double declarations across package files
+const c_double /* ERROR "redeclared" */ = 0
+type t_double  /* ERROR "redeclared" */ int
+var v_double /* ERROR "redeclared" */ int
+func f_double /* ERROR "redeclared" */ () {}
+
+// Blank methods need to be type-checked.
+// Verify by checking that errors are reported.
+func (T /* ERROR "undefined" */ ) _() {}
+func (T1) _(undefined /* ERROR "undefined" */ ) {}
+func (T1) _() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+// Methods with undefined receiver type can still be checked.
+// Verify by checking that errors are reported.
+func (Foo /* ERROR "undefined" */ ) m() {}
+func (Foo /* ERROR "undefined" */ ) m(undefined /* ERROR "undefined" */ ) {}
+func (Foo /* ERROR "undefined" */ ) m() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+func (Foo /* ERROR "undefined" */ ) _() {}
+func (Foo /* ERROR "undefined" */ ) _(undefined /* ERROR "undefined" */ ) {}
+func (Foo /* ERROR "undefined" */ ) _() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+// Receiver declarations are regular parameter lists;
+// receiver types may use parentheses, and the list
+// may have a trailing comma.
+type T7 struct {}
+
+func (T7) m1() {}
+func ((T7)) m2() {}
+func ((*T7)) m3() {}
+func (x *(T7),) m4() {}
+func (x (*(T7)),) m5() {}
+func (x ((*((T7)))),) m6() {}
+
+// Check that methods with parenthesized receiver are actually present (issue #23130).
+var (
+	_ = T7.m1
+	_ = T7.m2
+	_ = (*T7).m3
+	_ = (*T7).m4
+	_ = (*T7).m5
+	_ = (*T7).m6
+)
diff --git a/src/internal/types/testdata/check/decls3.go b/src/internal/types/testdata/check/decls3.go
new file mode 100644
index 0000000..01d4ffe
--- /dev/null
+++ b/src/internal/types/testdata/check/decls3.go
@@ -0,0 +1,309 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// embedded types
+
+package decls3
+
+import "unsafe"
+import "fmt"
+
+// fields with the same name at the same level cancel each other out
+
+func _() {
+	type (
+		T1 struct { X int }
+		T2 struct { X int }
+		T3 struct { T1; T2 } // X is embedded twice at the same level via T1->X, T2->X
+	)
+
+	var t T3
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func _() {
+	type (
+		T1 struct { X int }
+		T2 struct { T1 }
+		T3 struct { T1 }
+		T4 struct { T2; T3 } // X is embedded twice at the same level via T2->T1->X, T3->T1->X
+	)
+
+	var t T4
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func issue4355() {
+	type (
+	    T1 struct {X int}
+	    T2 struct {T1}
+	    T3 struct {T2}
+	    T4 struct {T2}
+	    T5 struct {T3; T4} // X is embedded twice at the same level via T3->T2->T1->X, T4->T2->T1->X
+	)
+
+	var t T5
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func _() {
+	type State int
+	type A struct{ State }
+	type B struct{ fmt.State }
+	type T struct{ A; B }
+
+	var t T
+	_ = t.State /* ERROR "ambiguous selector t.State" */
+}
+
+// Embedded fields can be predeclared types.
+
+func _() {
+	type T0 struct{
+		int
+		float32
+		f int
+	}
+	var x T0
+	_ = x.int
+	_ = x.float32
+	_ = x.f
+
+	type T1 struct{
+		T0
+	}
+	var y T1
+	_ = y.int
+	_ = y.float32
+	_ = y.f
+}
+
+// Restrictions on embedded field types.
+
+func _() {
+	type I1 interface{}
+	type I2 interface{}
+	type P1 *int
+	type P2 *int
+	type UP unsafe.Pointer
+
+	type T1 struct {
+		I1
+		* /* ERROR "cannot be a pointer to an interface" */ I2
+		* /* ERROR "cannot be a pointer to an interface" */ error
+		P1 /* ERROR "cannot be a pointer" */
+		* /* ERROR "cannot be a pointer" */ P2
+	}
+
+	// unsafe.Pointers are treated like regular pointers when embedded
+	type T2 struct {
+		unsafe /* ERROR "cannot be unsafe.Pointer" */ .Pointer
+		*/* ERROR "cannot be unsafe.Pointer" */ /* ERROR "Pointer redeclared" */ unsafe.Pointer
+		UP /* ERROR "cannot be unsafe.Pointer" */
+		* /* ERROR "cannot be unsafe.Pointer" */  /* ERROR "UP redeclared" */ UP
+	}
+}
+
+// Named types that are pointers.
+
+type S struct{ x int }
+func (*S) m() {}
+type P *S
+
+func _() {
+	var s *S
+	_ = s.x
+	_ = s.m
+
+	var p P
+	_ = p.x
+	_ = p.m /* ERROR "no field or method" */
+	_ = P.m /* ERROR "no field or method" */
+}
+
+// Borrowed from the FieldByName test cases in reflect/all_test.go.
+
+type D1 struct {
+	d int
+}
+type D2 struct {
+	d int
+}
+
+type S0 struct {
+	A, B, C int
+	D1
+	D2
+}
+
+type S1 struct {
+	B int
+	S0
+}
+
+type S2 struct {
+	A int
+	*S1
+}
+
+type S1x struct {
+	S1
+}
+
+type S1y struct {
+	S1
+}
+
+type S3 struct {
+	S1x
+	S2
+	D, E int
+	*S1y
+}
+
+type S4 struct {
+	*S4
+	A int
+}
+
+// The X in S6 and S7 annihilate, but they also block the X in S8.S9.
+type S5 struct {
+	S6
+	S7
+	S8
+}
+
+type S6 struct {
+	X int
+}
+
+type S7 S6
+
+type S8 struct {
+	S9
+}
+
+type S9 struct {
+	X int
+	Y int
+}
+
+// The X in S11.S6 and S12.S6 annihilate, but they also block the X in S13.S8.S9.
+type S10 struct {
+	S11
+	S12
+	S13
+}
+
+type S11 struct {
+	S6
+}
+
+type S12 struct {
+	S6
+}
+
+type S13 struct {
+	S8
+}
+
+func _() {
+	_ = struct{}{}.Foo /* ERROR "no field or method" */
+	_ = S0{}.A
+	_ = S0{}.D /* ERROR "no field or method" */
+	_ = S1{}.A
+	_ = S1{}.B
+	_ = S1{}.S0
+	_ = S1{}.C
+	_ = S2{}.A
+	_ = S2{}.S1
+	_ = S2{}.B
+	_ = S2{}.C
+	_ = S2{}.D /* ERROR "no field or method" */
+	_ = S3{}.S1 /* ERROR "ambiguous selector S3{}.S1" */
+	_ = S3{}.A
+	_ = S3{}.B /* ERROR "ambiguous selector" S3{}.B */
+	_ = S3{}.D
+	_ = S3{}.E
+	_ = S4{}.A
+	_ = S4{}.B /* ERROR "no field or method" */
+	_ = S5{}.X /* ERROR "ambiguous selector S5{}.X" */
+	_ = S5{}.Y
+	_ = S10{}.X /* ERROR "ambiguous selector S10{}.X" */
+	_ = S10{}.Y
+}
+
+// Borrowed from the FieldByName benchmark in reflect/all_test.go.
+
+type R0 struct {
+	*R1
+	*R2
+	*R3
+	*R4
+}
+
+type R1 struct {
+	*R5
+	*R6
+	*R7
+	*R8
+}
+
+type R2 R1
+type R3 R1
+type R4 R1
+
+type R5 struct {
+	*R9
+	*R10
+	*R11
+	*R12
+}
+
+type R6 R5
+type R7 R5
+type R8 R5
+
+type R9 struct {
+	*R13
+	*R14
+	*R15
+	*R16
+}
+
+type R10 R9
+type R11 R9
+type R12 R9
+
+type R13 struct {
+	*R17
+	*R18
+	*R19
+	*R20
+}
+
+type R14 R13
+type R15 R13
+type R16 R13
+
+type R17 struct {
+	*R21
+	*R22
+	*R23
+	*R24
+}
+
+type R18 R17
+type R19 R17
+type R20 R17
+
+type R21 struct {
+	X int
+}
+
+type R22 R21
+type R23 R21
+type R24 R21
+
+var _ = R0{}.X /* ERROR "ambiguous selector R0{}.X" */
\ No newline at end of file
diff --git a/src/internal/types/testdata/check/decls4.go b/src/internal/types/testdata/check/decls4.go
new file mode 100644
index 0000000..c1294ee
--- /dev/null
+++ b/src/internal/types/testdata/check/decls4.go
@@ -0,0 +1,199 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// type aliases
+
+package decls4
+
+type (
+	T0 [10]int
+	T1 []byte
+	T2 struct {
+		x int
+	}
+	T3 interface{
+		m() T2
+	}
+	T4 func(int, T0) chan T2
+)
+
+type (
+	Ai = int
+	A0 = T0
+	A1 = T1
+	A2 = T2
+	A3 = T3
+	A4 = T4
+
+	A10 = [10]int
+	A11 = []byte
+	A12 = struct {
+		x int
+	}
+	A13 = interface{
+		m() A2
+	}
+	A14 = func(int, A0) chan A2
+)
+
+// check assignment compatibility due to equality of types
+var (
+	xi_ int
+	ai Ai = xi_
+
+	x0 T0
+	a0 A0 = x0
+
+	x1 T1
+	a1 A1 = x1
+
+	x2 T2
+	a2 A2 = x2
+
+	x3 T3
+	a3 A3 = x3
+
+	x4 T4
+	a4 A4 = x4
+)
+
+// alias receiver types
+func (Ai /* ERROR "cannot define new methods on non-local type int" */) m1() {}
+func (T0) m1() {}
+func (A0) m1 /* ERROR already declared */ () {}
+func (A0) m2 () {}
+func (A3 /* ERROR invalid receiver */ ) m1 () {}
+func (A10 /* ERROR invalid receiver */ ) m1() {}
+
+// x0 has methods m1, m2 declared via receiver type names T0 and A0
+var _ interface{ m1(); m2() } = x0
+
+// alias receiver types (test case for issue #23042)
+type T struct{}
+
+var (
+	_ = T.m
+	_ = T{}.m
+	_ interface{m()} = T{}
+)
+
+var (
+	_ = T.n
+	_ = T{}.n
+	_ interface{m(); n()} = T{}
+)
+
+type U = T
+func (U) m() {}
+
+// alias receiver types (long type declaration chains)
+type (
+	V0 = V1
+	V1 = (V2)
+	V2 = ((V3))
+	V3 = T
+)
+
+func (V0) m /* ERROR already declared */ () {}
+func (V1) n() {}
+
+// alias receiver types (invalid due to cycles)
+type (
+	W0 /* ERROR invalid recursive type */ = W1
+	W1 = (W2)
+	W2 = ((W0))
+)
+
+func (W0) m() {} // no error expected (due to above cycle error)
+func (W1) n() {}
+
+// alias receiver types (invalid due to builtin underlying type)
+type (
+	B0 = B1
+	B1 = B2
+	B2 = int
+)
+
+func (B0 /* ERROR cannot define new methods on non-local type int */ ) m() {}
+func (B1 /* ERROR cannot define new methods on non-local type int */ ) n() {}
+
+// cycles
+type (
+	C2 /* ERROR invalid recursive type */ = C2
+	C3 /* ERROR invalid recursive type */ = C4
+	C4 = C3
+	C5 struct {
+		f *C6
+	}
+	C6 = C5
+	C7 /* ERROR invalid recursive type */  struct {
+		f C8
+	}
+	C8 = C7
+)
+
+// embedded fields
+var (
+	s0 struct { T0 }
+	s1 struct { A0 } = s0 /* ERROR cannot use */ // embedded field names are different
+)
+
+// embedding and lookup of fields and methods
+func _(s struct{A0}) { s.A0 = x0 }
+
+type eX struct{xf int}
+
+func (eX) xm()
+
+type eY = struct{eX} // field/method set of eY includes xf, xm
+
+type eZ = *struct{eX} // field/method set of eZ includes xf, xm
+
+type eA struct {
+	eX // eX contributes xf, xm to eA
+}
+
+type eA2 struct {
+	*eX // *eX contributes xf, xm to eA
+}
+
+type eB struct {
+	eY // eY contributes xf, xm to eB
+}
+
+type eB2 struct {
+	*eY // *eY contributes xf, xm to eB
+}
+
+type eC struct {
+	eZ // eZ contributes xf, xm to eC
+}
+
+var (
+	_ = eA{}.xf
+	_ = eA{}.xm
+	_ = eA2{}.xf
+	_ = eA2{}.xm
+	_ = eB{}.xf
+	_ = eB{}.xm
+	_ = eB2{}.xf
+	_ = eB2{}.xm
+	_ = eC{}.xf
+	_ = eC{}.xm
+)
+
+// ambiguous selectors due to embedding via type aliases
+type eD struct {
+	eY
+	eZ
+}
+
+var (
+	_ = eD{}.xf /* ERROR ambiguous selector eD{}.xf */
+	_ = eD{}.xm /* ERROR ambiguous selector eD{}.xm */
+)
+
+var (
+	_ interface{ xm() } = eD /* ERROR missing method xm */ {}
+)
\ No newline at end of file
diff --git a/src/internal/types/testdata/check/decls5.go b/src/internal/types/testdata/check/decls5.go
new file mode 100644
index 0000000..88d3194
--- /dev/null
+++ b/src/internal/types/testdata/check/decls5.go
@@ -0,0 +1,10 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// declarations of main
+const _, main /* ERROR "cannot declare main" */ , _ = 0, 1, 2
+type main /* ERROR "cannot declare main" */ struct{}
+var _, main /* ERROR "cannot declare main" */ int
diff --git a/src/internal/types/testdata/check/errors.go b/src/internal/types/testdata/check/errors.go
new file mode 100644
index 0000000..b3ab8af
--- /dev/null
+++ b/src/internal/types/testdata/check/errors.go
@@ -0,0 +1,66 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors
+
+// Testing precise operand formatting in error messages
+// (matching messages are regular expressions, hence the \'s).
+func f(x int, m map[string]int) {
+	// no values
+	_ = f /* ERROR f\(0, m\) \(no value\) used as value */ (0, m)
+
+	// built-ins
+	_ = println // ERROR println \(built-in\) must be called
+
+	// types
+	_ = complex128 // ERROR complex128 \(type\) is not an expression
+
+	// constants
+	const c1 = 991
+	const c2 float32 = 0.5
+	const c3 = "foo"
+	0 // ERROR 0 \(untyped int constant\) is not used
+	0.5 // ERROR 0.5 \(untyped float constant\) is not used
+	"foo" // ERROR "foo" \(untyped string constant\) is not used
+	c1 // ERROR c1 \(untyped int constant 991\) is not used
+	c2 // ERROR c2 \(constant 0.5 of type float32\) is not used
+	c1 /* ERROR c1 \+ c2 \(constant 991.5 of type float32\) is not used */ + c2
+	c3 // ERROR c3 \(untyped string constant "foo"\) is not used
+
+	// variables
+	x // ERROR x \(variable of type int\) is not used
+
+	// values
+	nil // ERROR nil is not used
+	( /* ERROR \(\*int\)\(nil\) \(value of type \*int\) is not used */ *int)(nil)
+	x /* ERROR x != x \(untyped bool value\) is not used */ != x
+	x /* ERROR x \+ x \(value of type int\) is not used */ + x
+
+	// value, ok's
+	const s = "foo"
+	m /* ERROR m\[s\] \(map index expression of type int\) is not used */ [s]
+}
+
+// Valid ERROR comments can have a variety of forms.
+func _() {
+	0 /* ERROR "0 .* is not used" */
+	0 /* ERROR 0 .* is not used */
+	0 // ERROR "0 .* is not used"
+	0 // ERROR 0 .* is not used
+}
+
+// Don't report spurious errors as a consequence of earlier errors.
+// Add more tests as needed.
+func _() {
+	if err := foo /* ERROR undefined */ (); err != nil /* no error here */ {}
+}
+
+// Use unqualified names for package-local objects.
+type T struct{}
+var _ int = T /* ERROR value of type T */ {} // use T in error message rather then errors.T
+
+// Don't report errors containing "invalid type" (issue #24182).
+func _(x *missing /* ERROR undefined: missing */ ) {
+	x.m() // there shouldn't be an error here referring to *invalid type
+}
diff --git a/src/internal/types/testdata/check/expr0.go b/src/internal/types/testdata/check/expr0.go
new file mode 100644
index 0000000..dd86eca
--- /dev/null
+++ b/src/internal/types/testdata/check/expr0.go
@@ -0,0 +1,187 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// unary expressions
+
+package expr0 
+
+type mybool bool
+
+var (
+	// bool
+	b0 = true
+	b1 bool = b0
+	b2 = !true
+	b3 = !b1
+	b4 bool = !true
+	b5 bool = !b4
+	b6 = +b0 /* ERROR "not defined" */
+	b7 = -b0 /* ERROR "not defined" */
+	b8 = ^b0 /* ERROR "not defined" */
+	b9 = *b0 /* ERROR "cannot indirect" */
+	b10 = &true /* ERROR "cannot take address" */
+	b11 = &b0
+	b12 = <-b0 /* ERROR "cannot receive" */
+	b13 = & & /* ERROR "cannot take address" */ b0
+
+	// byte
+	_ = byte(0)
+	_ = byte(- /* ERROR "cannot convert" */ 1)
+	_ = - /* ERROR "-byte\(1\) \(constant -1 of type byte\) overflows byte" */ byte(1) // test for issue 11367
+	_ = byte /* ERROR "overflows byte" */ (0) - byte(1)
+
+	// int
+	i0 = 1
+	i1 int = i0
+	i2 = +1
+	i3 = +i0
+	i4 int = +1
+	i5 int = +i4
+	i6 = -1
+	i7 = -i0
+	i8 int = -1
+	i9 int = -i4
+	i10 = !i0 /* ERROR "not defined" */
+	i11 = ^1
+	i12 = ^i0
+	i13 int = ^1
+	i14 int = ^i4
+	i15 = *i0 /* ERROR "cannot indirect" */
+	i16 = &i0
+	i17 = *i16
+	i18 = <-i16 /* ERROR "cannot receive" */
+
+	// uint
+	u0 = uint(1)
+	u1 uint = u0
+	u2 = +1
+	u3 = +u0
+	u4 uint = +1
+	u5 uint = +u4
+	u6 = -1
+	u7 = -u0
+	u8 uint = - /* ERROR "overflows" */ 1
+	u9 uint = -u4
+	u10 = !u0 /* ERROR "not defined" */
+	u11 = ^1
+	u12 = ^i0
+	u13 uint = ^ /* ERROR "overflows" */ 1
+	u14 uint = ^u4
+	u15 = *u0 /* ERROR "cannot indirect" */
+	u16 = &u0
+	u17 = *u16
+	u18 = <-u16 /* ERROR "cannot receive" */
+	u19 = ^uint(0)
+
+	// float64
+	f0 = float64(1)
+	f1 float64 = f0
+	f2 = +1
+	f3 = +f0
+	f4 float64 = +1
+	f5 float64 = +f4
+	f6 = -1
+	f7 = -f0
+	f8 float64 = -1
+	f9 float64 = -f4
+	f10 = !f0 /* ERROR "not defined" */
+	f11 = ^1
+	f12 = ^i0
+	f13 float64 = ^1
+	f14 float64 = ^f4 /* ERROR "not defined" */
+	f15 = *f0 /* ERROR "cannot indirect" */
+	f16 = &f0
+	f17 = *u16
+	f18 = <-u16 /* ERROR "cannot receive" */
+
+	// complex128
+	c0 = complex128(1)
+	c1 complex128 = c0
+	c2 = +1
+	c3 = +c0
+	c4 complex128 = +1
+	c5 complex128 = +c4
+	c6 = -1
+	c7 = -c0
+	c8 complex128 = -1
+	c9 complex128 = -c4
+	c10 = !c0 /* ERROR "not defined" */
+	c11 = ^1
+	c12 = ^i0
+	c13 complex128 = ^1
+	c14 complex128 = ^c4 /* ERROR "not defined" */
+	c15 = *c0 /* ERROR "cannot indirect" */
+	c16 = &c0
+	c17 = *u16
+	c18 = <-u16 /* ERROR "cannot receive" */
+
+	// string
+	s0 = "foo"
+	s1 = +"foo" /* ERROR "not defined" */
+	s2 = -s0 /* ERROR "not defined" */
+	s3 = !s0 /* ERROR "not defined" */
+	s4 = ^s0 /* ERROR "not defined" */
+	s5 = *s4
+	s6 = &s4
+	s7 = *s6
+	s8 = <-s7
+
+	// channel
+	ch chan int
+	rc <-chan float64
+	sc chan <- string
+	ch0 = +ch /* ERROR "not defined" */
+	ch1 = -ch /* ERROR "not defined" */
+	ch2 = !ch /* ERROR "not defined" */
+	ch3 = ^ch /* ERROR "not defined" */
+	ch4 = *ch /* ERROR "cannot indirect" */
+	ch5 = &ch
+	ch6 = *ch5
+	ch7 = <-ch
+	ch8 = <-rc
+	ch9 = <-sc /* ERROR "cannot receive" */
+	ch10, ok = <-ch
+	// ok is of type bool
+	ch11, myok = <-ch
+	_ mybool = myok /* ERROR "cannot use .* in variable declaration" */
+)
+
+// address of composite literals
+type T struct{x, y int}
+
+func f() T { return T{} }
+
+var (
+	_ = &T{1, 2}
+	_ = &[...]int{}
+	_ = &[]int{}
+	_ = &[]int{}
+	_ = &map[string]T{}
+	_ = &(T{1, 2})
+	_ = &((((T{1, 2}))))
+	_ = &f /* ERROR "cannot take address" */ ()
+)
+
+// recursive pointer types
+type P *P
+
+var (
+	p1 P = new(P)
+	p2 P = *p1
+	p3 P = &p2
+)
+
+func g() (a, b int) { return }
+
+func _() {
+	_ = -g /* ERROR multiple-value g */ ()
+	_ = <-g /* ERROR multiple-value g */ ()
+}
+
+// ~ is accepted as unary operator only permitted in interface type elements
+var (
+	_ = ~ /* ERROR cannot use ~ outside of interface or type constraint */ 0
+	_ = ~ /* ERROR cannot use ~ outside of interface or type constraint */ "foo"
+	_ = ~ /* ERROR cannot use ~ outside of interface or type constraint */ i0
+)
diff --git a/src/internal/types/testdata/check/expr1.go b/src/internal/types/testdata/check/expr1.go
new file mode 100644
index 0000000..49e8bae
--- /dev/null
+++ b/src/internal/types/testdata/check/expr1.go
@@ -0,0 +1,127 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// binary expressions
+
+package expr1
+
+type mybool bool
+
+func _(x, y bool, z mybool) {
+	x = x || y
+	x = x || true
+	x = x || false
+	x = x && y
+	x = x && true
+	x = x && false
+
+	z = z /* ERROR mismatched types */ || y
+	z = z || true
+	z = z || false
+	z = z /* ERROR mismatched types */ && y
+	z = z && true
+	z = z && false
+}
+
+type myint int
+
+func _(x, y int, z myint) {
+	x = x + 1
+	x = x + 1.0
+	x = x + 1.1 // ERROR truncated to int
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x % y
+	x = x << y
+	x = x >> y
+
+	z = z + 1
+	z = z + 1.0
+	z = z + 1.1 // ERROR truncated to int
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z << y
+	z = z >> y
+}
+
+type myuint uint
+
+func _(x, y uint, z myuint) {
+	x = x + 1
+	x = x + - /* ERROR overflows uint */ 1
+	x = x + 1.0
+	x = x + 1.1 // ERROR truncated to uint
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x % y
+	x = x << y
+	x = x >> y
+
+	z = z + 1
+	z = x + - /* ERROR overflows uint */ 1
+	z = z + 1.0
+	z = z + 1.1 // ERROR truncated to uint
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z << y
+	z = z >> y
+}
+
+type myfloat64 float64
+
+func _(x, y float64, z myfloat64) {
+	x = x + 1
+	x = x + -1
+	x = x + 1.0
+	x = x + 1.1
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x /* ERROR not defined */ % y
+	x = x /* ERROR operand x .* must be integer */ << y
+	x = x /* ERROR operand x .* must be integer */ >> y
+
+	z = z + 1
+	z = z + -1
+	z = z + 1.0
+	z = z + 1.1
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z /* ERROR operand z .* must be integer */ << y
+	z = z /* ERROR operand z .* must be integer */ >> y
+}
+
+type mystring string
+
+func _(x, y string, z mystring) {
+	x = x + "foo"
+	x = x /* ERROR not defined */ - "foo"
+	x = x /* ERROR mismatched types string and untyped int */ + 1
+	x = x + y
+	x = x /* ERROR not defined */ - y
+	x = x /* ERROR mismatched types string and untyped int */* 10
+}
+
+func f() (a, b int) { return }
+
+func _(x int) {
+	_ = f /* ERROR multiple-value f */ () + 1
+	_ = x + f /* ERROR multiple-value f */ ()
+	_ = f /* ERROR multiple-value f */ () + f
+	_ = f /* ERROR multiple-value f */ () + f /* ERROR multiple-value f */ ()
+}
diff --git a/src/internal/types/testdata/check/expr2.go b/src/internal/types/testdata/check/expr2.go
new file mode 100644
index 0000000..1929664
--- /dev/null
+++ b/src/internal/types/testdata/check/expr2.go
@@ -0,0 +1,260 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comparisons
+
+package expr2
+
+func _bool() {
+	const t = true == true
+	const f = true == false
+	_ = t /* ERROR operator .* not defined */ < f
+	_ = 0 == t /* ERROR mismatched types untyped int and untyped bool */
+	var b bool
+	var x, y float32
+	b = x < y
+	_ = b
+	_ = struct{b bool}{x < y}
+}
+
+// corner cases
+var (
+	v0 = nil == nil // ERROR operator == not defined on untyped nil
+)
+
+func arrays() {
+	// basics
+	var a, b [10]int
+	_ = a == b
+	_ = a != b
+	_ = a /* ERROR < not defined */ < b
+	_ = a == nil /* ERROR mismatched types */
+
+	type C [10]int
+	var c C
+	_ = a == c
+
+	type D [10]int
+	var d D
+	_ = c == d /* ERROR mismatched types */
+
+	var e [10]func() int
+	_ = e /* ERROR \[10\]func\(\) int cannot be compared */ == e
+}
+
+func structs() {
+	// basics
+	var s, t struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	_ = s == t
+	_ = s != t
+	_ = s /* ERROR < not defined */ < t
+	_ = s == nil /* ERROR mismatched types */
+
+	type S struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	type T struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	var ss S
+	var tt T
+	_ = s == ss
+	_ = ss == tt /* ERROR mismatched types */
+
+	var u struct {
+		x int
+		a [10]map[string]int
+	}
+	_ = u /* ERROR cannot be compared */ == u
+}
+
+func pointers() {
+	// nil
+	_ = nil == nil // ERROR operator == not defined on untyped nil
+	_ = nil != nil // ERROR operator != not defined on untyped nil
+	_ = nil /* ERROR < not defined */ < nil
+	_ = nil /* ERROR <= not defined */ <= nil
+	_ = nil /* ERROR > not defined */ > nil
+	_ = nil /* ERROR >= not defined */ >= nil
+
+	// basics
+	var p, q *int
+	_ = p == q
+	_ = p != q
+
+	_ = p == nil
+	_ = p != nil
+	_ = nil == q
+	_ = nil != q
+
+	_ = p /* ERROR < not defined */ < q
+	_ = p /* ERROR <= not defined */ <= q
+	_ = p /* ERROR > not defined */ > q
+	_ = p /* ERROR >= not defined */ >= q
+
+	// various element types
+	type (
+		S1 struct{}
+		S2 struct{}
+		P1 *S1
+		P2 *S2
+	)
+	var (
+		ps1 *S1
+		ps2 *S2
+		p1 P1
+		p2 P2
+	)
+	_ = ps1 == ps1
+	_ = ps1 == ps2 /* ERROR mismatched types */
+	_ = ps2 == ps1 /* ERROR mismatched types */
+
+	_ = p1 == p1
+	_ = p1 == p2 /* ERROR mismatched types */
+
+	_ = p1 == ps1
+}
+
+func channels() {
+	// basics
+	var c, d chan int
+	_ = c == d
+	_ = c != d
+	_ = c == nil
+	_ = c /* ERROR < not defined */ < d
+
+	// various element types (named types)
+	type (
+		C1 chan int
+		C1r <-chan int
+		C1s chan<- int
+		C2 chan float32
+	)
+	var (
+		c1 C1
+		c1r C1r
+		c1s C1s
+		c1a chan int
+		c2 C2
+	)
+	_ = c1 == c1
+	_ = c1 == c1r /* ERROR mismatched types */
+	_ = c1 == c1s /* ERROR mismatched types */
+	_ = c1r == c1s /* ERROR mismatched types */
+	_ = c1 == c1a
+	_ = c1a == c1
+	_ = c1 == c2 /* ERROR mismatched types */
+	_ = c1a == c2 /* ERROR mismatched types */
+
+	// various element types (unnamed types)
+	var (
+		d1 chan int
+		d1r <-chan int
+		d1s chan<- int
+		d1a chan<- int
+		d2 chan float32
+	)
+	_ = d1 == d1
+	_ = d1 == d1r
+	_ = d1 == d1s
+	_ = d1r == d1s /* ERROR mismatched types */
+	_ = d1 == d1a
+	_ = d1a == d1
+	_ = d1 == d2 /* ERROR mismatched types */
+	_ = d1a == d2 /* ERROR mismatched types */
+}
+
+// for interfaces test
+type S1 struct{}
+type S11 struct{}
+type S2 struct{}
+func (*S1) m() int
+func (*S11) m() int
+func (*S11) n()
+func (*S2) m() float32
+
+func interfaces() {
+	// basics
+	var i, j interface{ m() int }
+	_ = i == j
+	_ = i != j
+	_ = i == nil
+	_ = i /* ERROR < not defined */ < j
+
+	// various interfaces
+	var ii interface { m() int; n() }
+	var k interface { m() float32 }
+	_ = i == ii
+	_ = i == k /* ERROR mismatched types */
+
+	// interfaces vs values
+	var s1 S1
+	var s11 S11
+	var s2 S2
+
+	_ = i == 0 /* ERROR cannot convert */
+	_ = i == s1 /* ERROR mismatched types */
+	_ = i == &s1
+	_ = i == &s11
+
+	_ = i == s2 /* ERROR mismatched types */
+	_ = i == & /* ERROR mismatched types */ s2
+
+	// issue #28164
+	// testcase from issue
+	_ = interface{}(nil) == [ /* ERROR slice can only be compared to nil */ ]int(nil)
+
+	// related cases
+	var e interface{}
+	var s []int
+	var x int
+	_ = e == s // ERROR slice can only be compared to nil
+	_ = s /* ERROR slice can only be compared to nil */ == e
+	_ = e /* ERROR operator < not defined on interface */ < x
+	_ = x < e // ERROR operator < not defined on interface
+}
+
+func slices() {
+	// basics
+	var s []int
+	_ = s == nil
+	_ = s != nil
+	_ = s /* ERROR < not defined */ < nil
+
+	// slices are not otherwise comparable
+	_ = s /* ERROR slice can only be compared to nil */ == s
+	_ = s /* ERROR < not defined */ < s
+}
+
+func maps() {
+	// basics
+	var m map[string]int
+	_ = m == nil
+	_ = m != nil
+	_ = m /* ERROR < not defined */ < nil
+
+	// maps are not otherwise comparable
+	_ = m /* ERROR map can only be compared to nil */ == m
+	_ = m /* ERROR < not defined */ < m
+}
+
+func funcs() {
+	// basics
+	var f func(int) float32
+	_ = f == nil
+	_ = f != nil
+	_ = f /* ERROR < not defined */ < nil
+
+	// funcs are not otherwise comparable
+	_ = f /* ERROR func can only be compared to nil */ == f
+	_ = f /* ERROR < not defined */ < f
+}
diff --git a/src/internal/types/testdata/check/expr3.go b/src/internal/types/testdata/check/expr3.go
new file mode 100644
index 0000000..2da5984
--- /dev/null
+++ b/src/internal/types/testdata/check/expr3.go
@@ -0,0 +1,564 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expr3
+
+import "time"
+
+func indexes() {
+	_ = 1 /* ERROR "cannot index" */ [0]
+	_ = indexes /* ERROR "cannot index" */ [0]
+	_ = ( /* ERROR "cannot slice" */ 12 + 3)[1:2]
+
+	var a [10]int
+	_ = a[true /* ERROR "cannot convert" */ ]
+	_ = a["foo" /* ERROR "cannot convert" */ ]
+	_ = a[1.1 /* ERROR "truncated" */ ]
+	_ = a[1.0]
+	_ = a[- /* ERROR "negative" */ 1]
+	_ = a[- /* ERROR "negative" */ 1 :]
+	_ = a[: - /* ERROR "negative" */ 1]
+	_ = a[: /* ERROR "middle index required" */ : /* ERROR "final index required" */ ]
+	_ = a[0: /* ERROR "middle index required" */ : /* ERROR "final index required" */ ]
+	_ = a[0: /* ERROR "middle index required" */ :10]
+	_ = a[:10:10]
+
+	var a0 int
+	a0 = a[0]
+	_ = a0
+	var a1 int32
+	a1 = a /* ERROR "cannot use .* in assignment" */ [1]
+	_ = a1
+
+	_ = a[9]
+	_ = a[10 /* ERROR "index .* out of bounds" */ ]
+	_ = a[1 /* ERROR "overflows" */ <<100]
+	_ = a[1<< /* ERROR "constant shift overflow" */ 1000] // no out-of-bounds follow-on error
+	_ = a[10:]
+	_ = a[:10]
+	_ = a[10:10]
+	_ = a[11 /* ERROR "index .* out of bounds" */ :]
+	_ = a[: 11 /* ERROR "index .* out of bounds" */ ]
+	_ = a[: 1 /* ERROR "overflows" */ <<100]
+	_ = a[:10:10]
+	_ = a[:11 /* ERROR "index .* out of bounds" */ :10]
+	_ = a[:10:11 /* ERROR "index .* out of bounds" */ ]
+	_ = a[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = a[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = a[10:0 /* ERROR "invalid slice indices" */:0]
+	_ = &a /* ERROR "cannot take address" */ [:10]
+
+	pa := &a
+	_ = pa[9]
+	_ = pa[10 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[1 /* ERROR "overflows" */ <<100]
+	_ = pa[10:]
+	_ = pa[:10]
+	_ = pa[10:10]
+	_ = pa[11 /* ERROR "index .* out of bounds" */ :]
+	_ = pa[: 11 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[: 1 /* ERROR "overflows" */ <<100]
+	_ = pa[:10:10]
+	_ = pa[:11 /* ERROR "index .* out of bounds" */ :10]
+	_ = pa[:10:11 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = pa[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = pa[10:0 /* ERROR "invalid slice indices" */ :0]
+	_ = &pa /* ERROR "cannot take address" */ [:10]
+
+	var b [0]int
+	_ = b[0 /* ERROR "index .* out of bounds" */ ]
+	_ = b[:]
+	_ = b[0:]
+	_ = b[:0]
+	_ = b[0:0]
+	_ = b[0:0:0]
+	_ = b[1 /* ERROR "index .* out of bounds" */ :0:0]
+
+	var s []int
+	_ = s[- /* ERROR "negative" */ 1]
+	_ = s[- /* ERROR "negative" */ 1 :]
+	_ = s[: - /* ERROR "negative" */ 1]
+	_ = s[0]
+	_ = s[1:2]
+	_ = s[2:1 /* ERROR "invalid slice indices" */ ]
+	_ = s[2:]
+	_ = s[: 1 /* ERROR "overflows" */ <<100]
+	_ = s[1 /* ERROR "overflows" */ <<100 :]
+	_ = s[1 /* ERROR "overflows" */ <<100 : 1 /* ERROR "overflows" */ <<100]
+	_ = s[: /* ERROR "middle index required" */ :  /* ERROR "final index required" */ ]
+	_ = s[:10:10]
+	_ = s[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = s[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = s[10:0 /* ERROR "invalid slice indices" */ :0]
+	_ = &s /* ERROR "cannot take address" */ [:10]
+
+	var m map[string]int
+	_ = m[0 /* ERROR "cannot use .* in map index" */ ]
+	_ = m /* ERROR "cannot slice" */ ["foo" : "bar"]
+	_ = m["foo"]
+	// ok is of type bool
+	type mybool bool
+	var ok mybool
+	_, ok = m["bar"]
+	_ = ok
+	_ = m/* ERROR "mismatched types int and untyped string" */[0 /* ERROR "cannot use 0" */ ] + "foo"
+
+	var t string
+	_ = t[- /* ERROR "negative" */ 1]
+	_ = t[- /* ERROR "negative" */ 1 :]
+	_ = t[: - /* ERROR "negative" */ 1]
+	_ = t[1:2:3 /* ERROR "3-index slice of string" */ ]
+	_ = "foo"[1:2:3 /* ERROR "3-index slice of string" */ ]
+	var t0 byte
+	t0 = t[0]
+	_ = t0
+	var t1 rune
+	t1 = t /* ERROR "cannot use .* in assignment" */ [2]
+	_ = t1
+	_ = ("foo" + "bar")[5]
+	_ = ("foo" + "bar")[6 /* ERROR "index .* out of bounds" */ ]
+
+	const c = "foo"
+	_ = c[- /* ERROR "negative" */ 1]
+	_ = c[- /* ERROR "negative" */ 1 :]
+	_ = c[: - /* ERROR "negative" */ 1]
+	var c0 byte
+	c0 = c[0]
+	_ = c0
+	var c2 float32
+	c2 = c /* ERROR "cannot use .* in assignment" */ [2]
+	_ = c[3 /* ERROR "index .* out of bounds" */ ]
+	_ = ""[0 /* ERROR "index .* out of bounds" */ ]
+	_ = c2
+
+	_ = s[1<<30] // no compile-time error here
+
+	// issue 4913
+	type mystring string
+	var ss string
+	var ms mystring
+	var i, j int
+	ss = "foo"[1:2]
+	ss = "foo"[i:j]
+	ms = "foo" /* ERROR "cannot use .* in assignment" */ [1:2]
+	ms = "foo" /* ERROR "cannot use .* in assignment" */ [i:j]
+	_, _ = ss, ms
+}
+
+type T struct {
+	x int
+	y func()
+}
+
+func (*T) m() {}
+
+func method_expressions() {
+	_ = T.a /* ERROR "no field or method" */
+	_ = T.x /* ERROR "has no method" */
+	_ = T.m /* ERROR "invalid method expression T\.m \(needs pointer receiver \(\*T\)\.m\)" */
+	_ = (*T).m
+
+	var f func(*T) = T.m /* ERROR "invalid method expression T\.m \(needs pointer receiver \(\*T\)\.m\)" */
+	var g func(*T) = (*T).m
+	_, _ = f, g
+
+	_ = T.y /* ERROR "has no method" */
+	_ = (*T).y /* ERROR "has no method" */
+}
+
+func struct_literals() {
+	type T0 struct {
+		a, b, c int
+	}
+
+	type T1 struct {
+		T0
+		a, b int
+		u float64
+		s string
+	}
+
+	// keyed elements
+	_ = T1{}
+	_ = T1{a: 0, 1 /* ERROR "mixture of .* elements" */ }
+	_ = T1{aa /* ERROR "unknown field" */ : 0}
+	_ = T1{1 /* ERROR "invalid field name" */ : 0}
+	_ = T1{a: 0, s: "foo", u: 0, a /* ERROR "duplicate field" */: 10}
+	_ = T1{a: "foo" /* ERROR "cannot use .* in struct literal" */ }
+	_ = T1{c /* ERROR "unknown field" */ : 0}
+	_ = T1{T0: { /* ERROR "missing type" */ }} // struct literal element type may not be elided
+	_ = T1{T0: T0{}}
+	_ = T1{T0 /* ERROR "invalid field name" */ .a: 0}
+
+	// unkeyed elements
+	_ = T0{1, 2, 3}
+	_ = T0{1, b /* ERROR "mixture" */ : 2, 3}
+	_ = T0{1, 2} /* ERROR "too few values" */
+	_ = T0{1, 2, 3, 4  /* ERROR "too many values" */ }
+	_ = T0{1, "foo" /* ERROR "cannot use .* in struct literal" */, 3.4  /* ERROR "cannot use .*\(truncated\)" */}
+
+	// invalid type
+	type P *struct{
+		x int
+	}
+	_ = P /* ERROR "invalid composite literal type" */ {}
+
+	// unexported fields
+	_ = time.Time{}
+	_ = time.Time{sec /* ERROR "unknown field" */ : 0}
+	_ = time.Time{
+		0 /* ERROR implicit assignment to unexported field wall in struct literal */,
+		0 /* ERROR implicit assignment */ ,
+		nil /* ERROR implicit assignment */ ,
+	}
+}
+
+func array_literals() {
+	type A0 [0]int
+	_ = A0{}
+	_ = A0{0 /* ERROR "index .* out of bounds" */}
+	_ = A0{0 /* ERROR "index .* out of bounds" */ : 0}
+
+	type A1 [10]int
+	_ = A1{}
+	_ = A1{0, 1, 2}
+	_ = A1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	_ = A1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 /* ERROR "index .* out of bounds" */ }
+	_ = A1{- /* ERROR "negative" */ 1: 0}
+	_ = A1{8: 8, 9}
+	_ = A1{8: 8, 9, 10 /* ERROR "index .* out of bounds" */ }
+	_ = A1{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	_ = A1{5: 5, 6, 7, 3: 3, 4}
+	_ = A1{5: 5, 6, 7, 3: 3, 4, 5 /* ERROR "duplicate index" */ }
+	_ = A1{10 /* ERROR "index .* out of bounds" */ : 10, 10 /* ERROR "index .* out of bounds" */ : 10}
+	_ = A1{5: 5, 6, 7, 3: 3, 1 /* ERROR "overflows" */ <<100: 4, 5 /* ERROR "duplicate index" */ }
+	_ = A1{5: 5, 6, 7, 4: 4, 1 /* ERROR "overflows" */ <<100: 4}
+	_ = A1{2.0}
+	_ = A1{2.1 /* ERROR "truncated" */ }
+	_ = A1{"foo" /* ERROR "cannot use .* in array or slice literal" */ }
+
+	// indices must be integer constants
+	i := 1
+	const f = 2.1
+	const s = "foo"
+	_ = A1{i /* ERROR "index i must be integer constant" */ : 0}
+	_ = A1{f /* ERROR "truncated" */ : 0}
+	_ = A1{s /* ERROR "cannot convert" */ : 0}
+
+	a0 := [...]int{}
+	assert(len(a0) == 0)
+
+	a1 := [...]int{0, 1, 2}
+	assert(len(a1) == 3)
+	var a13 [3]int
+	var a14 [4]int
+	a13 = a1
+	a14 = a1 /* ERROR "cannot use .* in assignment" */
+	_, _ = a13, a14
+
+	a2 := [...]int{- /* ERROR "negative" */ 1: 0}
+	_ = a2
+
+	a3 := [...]int{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	assert(len(a3) == 5) // somewhat arbitrary
+
+	a4 := [...]complex128{0, 1, 2, 1<<10-2: -1i, 1i, 400: 10, 12, 14}
+	assert(len(a4) == 1024)
+
+	// composite literal element types may be elided
+	type T []int
+	_ = [10]T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	a6 := [...]T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	assert(len(a6) == 8)
+
+	// recursively so
+	_ = [10][10]T{{}, [10]T{{}}, {{1, 2, 3}}}
+
+	// from the spec
+	type Point struct { x, y float32 }
+	_ = [...]Point{Point{1.5, -3.5}, Point{0, 0}}
+	_ = [...]Point{{1.5, -3.5}, {0, 0}}
+	_ = [][]int{[]int{1, 2, 3}, []int{4, 5}}
+	_ = [][]int{{1, 2, 3}, {4, 5}}
+	_ = [...]*Point{&Point{1.5, -3.5}, &Point{0, 0}}
+	_ = [...]*Point{{1.5, -3.5}, {0, 0}}
+}
+
+func slice_literals() {
+	type S0 []int
+	_ = S0{}
+	_ = S0{0, 1, 2}
+	_ = S0{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	_ = S0{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+	_ = S0{- /* ERROR "negative" */ 1: 0}
+	_ = S0{8: 8, 9}
+	_ = S0{8: 8, 9, 10}
+	_ = S0{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	_ = S0{5: 5, 6, 7, 3: 3, 4}
+	_ = S0{5: 5, 6, 7, 3: 3, 4, 5 /* ERROR "duplicate index" */ }
+	_ = S0{10: 10, 10 /* ERROR "duplicate index" */ : 10}
+	_ = S0{5: 5, 6, 7, 3: 3, 1 /* ERROR "overflows" */ <<100: 4, 5 /* ERROR "duplicate index" */ }
+	_ = S0{5: 5, 6, 7, 4: 4, 1 /* ERROR "overflows" */ <<100: 4}
+	_ = S0{2.0}
+	_ = S0{2.1 /* ERROR "truncated" */ }
+	_ = S0{"foo" /* ERROR "cannot use .* in array or slice literal" */ }
+
+	// indices must be resolved correctly
+	const index1 = 1
+	_ = S0{index1: 1}
+	_ = S0{index2: 2}
+	_ = S0{index3 /* ERROR "undefined" */ : 3}
+
+	// indices must be integer constants
+	i := 1
+	const f = 2.1
+	const s = "foo"
+	_ = S0{i /* ERROR "index i must be integer constant" */ : 0}
+	_ = S0{f /* ERROR "truncated" */ : 0}
+	_ = S0{s /* ERROR "cannot convert" */ : 0}
+
+	// composite literal element types may be elided
+	type T []int
+	_ = []T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	_ = [][]int{{1, 2, 3}, {4, 5}}
+
+	// recursively so
+	_ = [][]T{{}, []T{{}}, {{1, 2, 3}}}
+
+	// issue 17954
+	type T0 *struct { s string }
+	_ = []T0{{}}
+	_ = []T0{{"foo"}}
+
+	type T1 *struct{ int }
+	_ = []T1{}
+	_ = []T1{{0}, {1}, {2}}
+
+	type T2 T1
+	_ = []T2{}
+	_ = []T2{{0}, {1}, {2}}
+
+	_ = map[T0]T2{}
+	_ = map[T0]T2{{}: {}}
+}
+
+const index2 int = 2
+
+type N int
+func (N) f() {}
+
+func map_literals() {
+	type M0 map[string]int
+	type M1 map[bool]int
+	type M2 map[*int]int
+
+	_ = M0{}
+	_ = M0{1 /* ERROR "missing key" */ }
+	_ = M0{1 /* ERROR "cannot use .* in map literal" */ : 2}
+	_ = M0{"foo": "bar" /* ERROR "cannot use .* in map literal" */ }
+	_ = M0{"foo": 1, "bar": 2, "foo" /* ERROR "duplicate key" */ : 3 }
+
+	_ = map[interface{}]int{2: 1, 2 /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{int(2): 1, int16(2): 1}
+	_ = map[interface{}]int{int16(2): 1, int16 /* ERROR "duplicate key" */ (2): 1}
+
+	type S string
+
+	_ = map[interface{}]int{"a": 1, "a" /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{"a": 1, S("a"): 1}
+	_ = map[interface{}]int{S("a"): 1, S /* ERROR "duplicate key" */ ("a"): 1}
+	_ = map[interface{}]int{1.0: 1, 1.0 /* ERROR "duplicate key" */: 1}
+	_ = map[interface{}]int{int64(-1): 1, int64 /* ERROR "duplicate key" */ (-1) : 1}
+	_ = map[interface{}]int{^uint64(0): 1, ^ /* ERROR "duplicate key" */ uint64(0): 1}
+	_ = map[interface{}]int{complex(1,2): 1, complex /* ERROR "duplicate key" */ (1,2) : 1}
+
+	type I interface {
+		f()
+	}
+
+	_ = map[I]int{N(0): 1, N(2): 1}
+	_ = map[I]int{N(2): 1, N /* ERROR "duplicate key" */ (2): 1}
+
+	// map keys must be resolved correctly
+	key1 := "foo"
+	_ = M0{key1: 1}
+	_ = M0{key2: 2}
+	_ = M0{key3 /* ERROR "undefined" */ : 2}
+
+	var value int
+	_ = M1{true: 1, false: 0}
+	_ = M2{nil: 0, &value: 1}
+
+	// composite literal element types may be elided
+	type T [2]int
+	_ = map[int]T{0: T{3, 4}, 1: {5, 6}}
+
+	// recursively so
+	_ = map[int][]T{0: {}, 1: {{}, T{1, 2}}}
+
+	// composite literal key types may be elided
+	_ = map[T]int{T{3, 4}: 0, {5, 6}: 1}
+
+	// recursively so
+	_ = map[[2]T]int{{}: 0, {{}}: 1, [2]T{{}}: 2, {T{1, 2}}: 3}
+
+	// composite literal element and key types may be elided
+	_ = map[T]T{{}: {}, {1, 2}: T{3, 4}, T{4, 5}: {}}
+	_ = map[T]M0{{} : {}, T{1, 2}: M0{"foo": 0}, {1, 3}: {"foo": 1}}
+
+	// recursively so
+	_ = map[[2]T][]T{{}: {}, {{}}: {{}, T{1, 2}}, [2]T{{}}: nil, {T{1, 2}}: {{}, {}}}
+
+	// from the spec
+	type Point struct { x, y float32 }
+	_ = map[string]Point{"orig": {0, 0}}
+	_ = map[*Point]string{{0, 0}: "orig"}
+
+	// issue 17954
+	type T0 *struct{ s string }
+	type T1 *struct{ int }
+	type T2 T1
+
+	_ = map[T0]T2{}
+	_ = map[T0]T2{{}: {}}
+}
+
+var key2 string = "bar"
+
+type I interface {
+	m()
+}
+
+type I2 interface {
+	m(int)
+}
+
+type T1 struct{}
+type T2 struct{}
+
+func (T2) m(int) {}
+
+type mybool bool
+
+func type_asserts() {
+	var x int
+	_ = x /* ERROR "not an interface" */ .(int)
+
+	var e interface{}
+	var ok bool
+	x, ok = e.(int)
+	_ = ok
+
+	// ok value is of type bool
+	var myok mybool
+	_, myok = e.(int)
+	_ = myok
+
+	var t I
+	_ = t /* ERROR "use of .* outside type switch" */ .(type)
+	_ = t /* ERROR "m has pointer receiver" */ .(T)
+	_ = t.(*T)
+	_ = t /* ERROR "missing method m" */ .(T1)
+	_ = t /* ERROR "wrong type for method m" */ .(T2)
+	_ = t /* STRICT "wrong type for method m" */ .(I2) // only an error in strict mode (issue 8561)
+
+	// e doesn't statically have an m, but may have one dynamically.
+	_ = e.(I2)
+}
+
+func f0() {}
+func f1(x int) {}
+func f2(u float32, s string) {}
+func fs(s []byte) {}
+func fv(x ...int) {}
+func fi(x ... interface{}) {}
+func (T) fm(x ...int)
+
+func g0() {}
+func g1() int { return 0}
+func g2() (u float32, s string) { return }
+func gs() []byte { return nil }
+
+func _calls() {
+	var x int
+	var y float32
+	var s []int
+
+	f0()
+	_ = f0 /* ERROR "used as value" */ ()
+	f0(g0 /* ERROR "too many arguments" */ )
+
+	f1(0)
+	f1(x)
+	f1(10.0)
+	f1() /* ERROR "not enough arguments in call to f1\n\thave \(\)\n\twant \(int\)" */
+	f1(x, y /* ERROR "too many arguments in call to f1\n\thave \(int, float32\)\n\twant \(int\)" */ )
+	f1(s /* ERROR "cannot use .* in argument" */ )
+	f1(x ... /* ERROR "cannot use ..." */ )
+	f1(g0 /* ERROR "used as value" */ ())
+	f1(g1())
+	f1(g2 /* ERROR "too many arguments in call to f1\n\thave \(float32, string\)\n\twant \(int\)" */ ())
+
+	f2() /* ERROR "not enough arguments in call to f2\n\thave \(\)\n\twant \(float32, string\)" */
+	f2(3.14) /* ERROR "not enough arguments in call to f2\n\thave \(number\)\n\twant \(float32, string\)" */
+	f2(3.14, "foo")
+	f2(x /* ERROR "cannot use .* in argument" */ , "foo")
+	f2(g0 /* ERROR "used as value" */ ())
+	f2(g1()) /* ERROR "not enough arguments in call to f2\n\thave \(int\)\n\twant \(float32, string\)" */
+	f2(g2())
+
+	fs() /* ERROR "not enough arguments" */
+	fs(g0 /* ERROR "used as value" */ ())
+	fs(g1 /* ERROR "cannot use .* in argument" */ ())
+	fs(g2 /* ERROR "too many arguments" */ ())
+	fs(gs())
+
+	fv()
+	fv(1, 2.0, x)
+	fv(s /* ERROR "cannot use .* in argument" */ )
+	fv(s...)
+	fv(x /* ERROR "cannot use" */ ...)
+	fv(1, s /* ERROR "too many arguments" */ ...)
+	fv(gs /* ERROR "cannot use .* in argument" */ ())
+	fv(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	var t T
+	t.fm()
+	t.fm(1, 2.0, x)
+	t.fm(s /* ERROR "cannot use .* in argument" */ )
+	t.fm(g1())
+	t.fm(1, s /* ERROR "too many arguments" */ ...)
+	t.fm(gs /* ERROR "cannot use .* in argument" */ ())
+	t.fm(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	T.fm(t, )
+	T.fm(t, 1, 2.0, x)
+	T.fm(t, s /* ERROR "cannot use .* in argument" */ )
+	T.fm(t, g1())
+	T.fm(t, 1, s /* ERROR "too many arguments" */ ...)
+	T.fm(t, gs /* ERROR "cannot use .* in argument" */ ())
+	T.fm(t, gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	var i interface{ fm(x ...int) } = t
+	i.fm()
+	i.fm(1, 2.0, x)
+	i.fm(s /* ERROR "cannot use .* in argument" */ )
+	i.fm(g1())
+	i.fm(1, s /* ERROR "too many arguments" */ ...)
+	i.fm(gs /* ERROR "cannot use .* in argument" */ ())
+	i.fm(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	fi()
+	fi(1, 2.0, x, 3.14, "foo")
+	fi(g2())
+	fi(0, g2)
+	fi(0, g2 /* ERROR "multiple-value g2" */ ())
+}
+
+func issue6344() {
+	type T []interface{}
+	var x T
+	fi(x...) // ... applies also to named slices
+}
diff --git a/src/internal/types/testdata/check/funcinference.go b/src/internal/types/testdata/check/funcinference.go
new file mode 100644
index 0000000..fedf199
--- /dev/null
+++ b/src/internal/types/testdata/check/funcinference.go
@@ -0,0 +1,104 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package funcInference
+
+import "strconv"
+
+type any interface{}
+
+func f0[A any, B interface{*C}, C interface{*D}, D interface{*A}](A, B, C, D) {}
+func _() {
+	f := f0[string]
+	f("a", nil, nil, nil)
+	f0("a", nil, nil, nil)
+}
+
+func f1[A any, B interface{*A}](A, B) {}
+func _() {
+	f := f1[int]
+	f(int(0), new(int))
+	f1(int(0), new(int))
+}
+
+func f2[A any, B interface{[]A}](A, B) {}
+func _() {
+	f := f2[byte]
+	f(byte(0), []byte{})
+	f2(byte(0), []byte{})
+}
+
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+// func f3[A any, B interface{~C}, C interface{~*A}](A, B, C)
+// func _() {
+// 	f := f3[int]
+// 	var x int
+// 	f(x, &x, &x)
+// 	f3(x, &x, &x)
+// }
+
+func f4[A any, B interface{[]C}, C interface{*A}](A, B, C) {}
+func _() {
+	f := f4[int]
+	var x int
+	f(x, []*int{}, &x)
+	f4(x, []*int{}, &x)
+}
+
+func f5[A interface{struct{b B; c C}}, B any, C interface{*B}](x B) A { panic(0) }
+func _() {
+	x := f5(1.2)
+	var _ float64 = x.b
+	var _ float64 = *x.c
+}
+
+func f6[A any, B interface{~struct{f []A}}](B) A { panic(0) }
+func _() {
+	x := f6(struct{f []string}{})
+	var _ string = x
+}
+
+func f7[A interface{*B}, B interface{~*A}]() {}
+
+// More realistic examples
+
+func Double[S interface{ ~[]E }, E interface{ ~int | ~int8 | ~int16 | ~int32 | ~int64 }](s S) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v + v
+	}
+	return r
+}
+
+type MySlice []int
+
+var _ = Double(MySlice{1})
+
+// From the draft design.
+
+type Setter[B any] interface {
+	Set(string)
+	*B
+}
+
+func FromStrings[T interface{}, PT Setter[T]](s []string) []T {
+	result := make([]T, len(s))
+	for i, v := range s {
+		// The type of &result[i] is *T which is in the type set
+		// of Setter, so we can convert it to PT.
+		p := PT(&result[i])
+		// PT has a Set method.
+		p.Set(v)
+	}
+	return result
+}
+
+type Settable int
+
+func (p *Settable) Set(s string) {
+	i, _ := strconv.Atoi(s) // real code should not ignore the error
+	*p = Settable(i)
+}
+
+var _ = FromStrings[Settable]([]string{"1", "2"})
diff --git a/src/internal/types/testdata/check/go1_12.go b/src/internal/types/testdata/check/go1_12.go
new file mode 100644
index 0000000..56c6d5a
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_12.go
@@ -0,0 +1,36 @@
+// -lang=go1.12
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check Go language version-specific errors.
+
+package p
+
+// numeric literals
+const (
+	_ = 1_000 // ERROR "underscores in numeric literals requires go1.13 or later"
+	_ = 0b111 // ERROR "binary literals requires go1.13 or later"
+	_ = 0o567 // ERROR "0o/0O-style octal literals requires go1.13 or later"
+	_ = 0xabc // ok
+	_ = 0x0p1 // ERROR "hexadecimal floating-point literals requires go1.13 or later"
+
+	_ = 0B111 // ERROR "binary"
+	_ = 0O567 // ERROR "octal"
+	_ = 0Xabc // ok
+	_ = 0X0P1 // ERROR "hexadecimal floating-point"
+
+	_ = 1_000i // ERROR "underscores"
+	_ = 0b111i // ERROR "binary"
+	_ = 0o567i // ERROR "octal"
+	_ = 0xabci // ERROR "hexadecimal floating-point"
+	_ = 0x0p1i // ERROR "hexadecimal floating-point"
+)
+
+// signed shift counts
+var (
+	s int
+	_ = 1 << s // ERROR "invalid operation: signed shift count s \(variable of type int\) requires go1.13 or later"
+	_ = 1 >> s // ERROR "signed shift count"
+)
diff --git a/src/internal/types/testdata/check/go1_13.go b/src/internal/types/testdata/check/go1_13.go
new file mode 100644
index 0000000..cc7861d
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_13.go
@@ -0,0 +1,23 @@
+// -lang=go1.13
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check Go language version-specific errors.
+
+package p
+
+// interface embedding
+
+type I interface { m() }
+
+type _ interface {
+	m()
+	I // ERROR "duplicate method m"
+}
+
+type _ interface {
+	I
+	I // ERROR "duplicate method m"
+}
diff --git a/src/internal/types/testdata/check/go1_16.go b/src/internal/types/testdata/check/go1_16.go
new file mode 100644
index 0000000..81b5290
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_16.go
@@ -0,0 +1,15 @@
+// -lang=go1.16
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check Go language version-specific errors.
+
+package p
+
+type Slice []byte
+type Array [8]byte
+
+var s Slice
+var p = (*Array)(s /* ERROR requires go1.17 or later */ )
diff --git a/src/internal/types/testdata/check/go1_19.go b/src/internal/types/testdata/check/go1_19.go
new file mode 100644
index 0000000..f899d93
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_19.go
@@ -0,0 +1,15 @@
+// -lang=go1.19
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check Go language version-specific errors.
+
+package p
+
+type Slice []byte
+type Array [8]byte
+
+var s Slice
+var p = (Array)(s /* ERROR requires go1.20 or later */)
diff --git a/src/internal/types/testdata/check/go1_8.go b/src/internal/types/testdata/check/go1_8.go
new file mode 100644
index 0000000..99f2fd4
--- /dev/null
+++ b/src/internal/types/testdata/check/go1_8.go
@@ -0,0 +1,12 @@
+// -lang=go1.8
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check Go language version-specific errors.
+
+package p
+
+// type alias declarations
+type any = /* ERROR type aliases requires go1.9 or later */ interface{}
diff --git a/src/internal/types/testdata/check/gotos.go b/src/internal/types/testdata/check/gotos.go
new file mode 100644
index 0000000..069a94b
--- /dev/null
+++ b/src/internal/types/testdata/check/gotos.go
@@ -0,0 +1,560 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a modified copy of $GOROOT/test/goto.go.
+
+package gotos
+
+var (
+	i, n int
+	x    []int
+	c    chan int
+	m    map[int]int
+	s    string
+)
+
+// goto after declaration okay
+func _() {
+	x := 1
+	goto L
+L:
+	_ = x
+}
+
+// goto before declaration okay
+func _() {
+	goto L
+L:
+	x := 1
+	_ = x
+}
+
+// goto across declaration not okay
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 36" */
+	x := 1
+	_ = x
+L:
+}
+
+// goto across declaration in inner scope okay
+func _() {
+	goto L
+	{
+		x := 1
+		_ = x
+	}
+L:
+}
+
+// goto across declaration after inner scope not okay
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 58" */
+	{
+		x := 1
+		_ = x
+	}
+	x := 1
+	_ = x
+L:
+}
+
+// goto across declaration in reverse okay
+func _() {
+L:
+	x := 1
+	_ = x
+	goto L
+}
+
+func _() {
+L: L1:
+	x := 1
+	_ = x
+	goto L
+	goto L1
+}
+
+// error shows first offending variable
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 84" */
+	x := 1
+	_ = x
+	y := 1
+	_ = y
+L:
+}
+
+// goto not okay even if code path is dead
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration" */
+	x := 1
+	_ = x
+	y := 1
+	_ = y
+	return
+L:
+}
+
+// goto into outer block okay
+func _() {
+	{
+		goto L
+	}
+L:
+}
+
+func _() {
+	{
+		goto L
+		goto L1
+	}
+L: L1:
+}
+
+// goto backward into outer block okay
+func _() {
+L:
+	{
+		goto L
+	}
+}
+
+func _() {
+L: L1:
+	{
+		goto L
+		goto L1
+	}
+}
+
+// goto into inner block not okay
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	{
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	goto L1 /* ERROR "goto L1 jumps into block" */
+	{
+	L: L1:
+	}
+}
+
+// goto backward into inner block still not okay
+func _() {
+	{
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	{
+	L: L1:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+	goto L1 /* ERROR "goto L1 jumps into block" */
+}
+
+// error shows first (outermost) offending block
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	{
+		{
+			{
+			L:
+			}
+		}
+	}
+}
+
+// error prefers block diagnostic over declaration diagnostic
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	x := 1
+	_ = x
+	{
+	L:
+	}
+}
+
+// many kinds of blocks, all invalid to jump into or among,
+// but valid to jump out of
+
+// if
+
+func _() {
+L:
+	if true {
+		goto L
+	}
+}
+
+func _() {
+L:
+	if true {
+		goto L
+	} else {
+	}
+}
+
+func _() {
+L:
+	if false {
+	} else {
+		goto L
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	L:
+	} else {
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if false {
+	L:
+	} else {
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	L:
+	} else {
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else {
+		L:
+	}
+}
+
+func _() {
+	if true {
+		L:
+	} else {
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+// for
+
+func _() {
+	for {
+		goto L
+	}
+L:
+}
+
+func _() {
+	for {
+		goto L
+	L:
+	}
+}
+
+func _() {
+	for {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for {
+		goto L
+	L1:
+	}
+L:
+	goto L1 /* ERROR "goto L1 jumps into block" */
+}
+
+func _() {
+	for i < n {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = 0; i < n; i++ {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range x {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range c {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range m {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range s {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+// switch
+
+func _() {
+L:
+	switch i {
+	case 0:
+		goto L
+	}
+}
+
+func _() {
+L:
+	switch i {
+	case 0:
+
+	default:
+		goto L
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+
+	default:
+	L:
+		goto L
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+
+	default:
+		goto L
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+		goto L
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	default:
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	case 0:
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+	L:
+		;
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+// select
+// different from switch.  the statement has no implicit block around it.
+
+func _() {
+L:
+	select {
+	case <-c:
+		goto L
+	}
+}
+
+func _() {
+L:
+	select {
+	case c <- 1:
+
+	default:
+		goto L
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+
+	default:
+	L:
+		goto L
+	}
+}
+
+func _() {
+	select {
+	case c <- 1:
+
+	default:
+		goto L
+	L:
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+		goto L
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case c <- 1:
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case c <- 1:
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case <-c:
+	default:
+	L:
+	}
+}
+
+func _() {
+	select {
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	case <-c:
+	L:
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+	L:
+		;
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
diff --git a/src/internal/types/testdata/check/importC.go b/src/internal/types/testdata/check/importC.go
new file mode 100644
index 0000000..8078021
--- /dev/null
+++ b/src/internal/types/testdata/check/importC.go
@@ -0,0 +1,56 @@
+// -fakeImportC
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importC
+
+import "C"
+import _ /* ERROR cannot rename import "C" */ "C"
+import foo /* ERROR cannot rename import "C" */ "C"
+import . /* ERROR cannot rename import "C" */ "C"
+
+// Test cases extracted from issue #22090.
+
+import "unsafe"
+
+const _ C.int = 0xff // no error due to invalid constant type
+
+type T struct {
+	Name    string
+	Ordinal int
+}
+
+func _(args []T) {
+	var s string
+	for i, v := range args {
+		cname := C.CString(v.Name)
+		args[i].Ordinal = int(C.sqlite3_bind_parameter_index(s, cname)) // no error due to i not being "used"
+		C.free(unsafe.Pointer(cname))
+	}
+}
+
+type CType C.Type
+
+const _ CType = C.X // no error due to invalid constant type
+const _ = C.X
+
+// Test cases extracted from issue #23712.
+
+func _() {
+	var a [C.ArrayLength]byte
+	_ = a[0] // no index out of bounds error here
+}
+
+// Additional tests to verify fix for #23712.
+
+func _() {
+	var a [C.ArrayLength1]byte
+	_ = 1 / len(a) // no division by zero error here and below
+	_ = 1 / cap(a)
+	_ = uint(unsafe.Sizeof(a)) // must not be negative
+
+	var b [C.ArrayLength2]byte
+	a = b // should be valid
+}
diff --git a/src/internal/types/testdata/check/importdecl0/importdecl0a.go b/src/internal/types/testdata/check/importdecl0/importdecl0a.go
new file mode 100644
index 0000000..d514ae4
--- /dev/null
+++ b/src/internal/types/testdata/check/importdecl0/importdecl0a.go
@@ -0,0 +1,53 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl0
+
+import ()
+
+import (
+	// we can have multiple blank imports (was bug)
+	_ "math"
+	_ "net/rpc"
+	init /* ERROR "cannot import package as init" */ "fmt"
+	// reflect defines a type "flag" which shows up in the gc export data
+	"reflect"
+	. /* ERROR "imported and not used" */ "reflect"
+)
+
+import "math" /* ERROR "imported and not used" */
+import m /* ERROR "imported as m and not used" */ "math"
+import _ "math"
+
+import (
+	"math/big" /* ERROR "imported and not used" */
+	b /* ERROR "imported as b and not used" */ "math/big"
+	_ "math/big"
+)
+
+import "fmt"
+import f1 "fmt"
+import f2 "fmt"
+
+// reflect.flag must not be visible in this package
+type flag int
+type _ reflect.flag /* ERROR "not exported" */
+
+// imported package name may conflict with local objects
+type reflect /* ERROR "reflect already declared" */ int
+
+// dot-imported exported objects may conflict with local objects
+type Value /* ERROR "Value already declared through dot-import of package reflect" */ struct{}
+
+var _ = fmt.Println // use "fmt"
+
+func _() {
+	f1.Println() // use "fmt"
+}
+
+func _() {
+	_ = func() {
+		f2.Println() // use "fmt"
+	}
+}
diff --git a/src/internal/types/testdata/check/importdecl0/importdecl0b.go b/src/internal/types/testdata/check/importdecl0/importdecl0b.go
new file mode 100644
index 0000000..904faff
--- /dev/null
+++ b/src/internal/types/testdata/check/importdecl0/importdecl0b.go
@@ -0,0 +1,30 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl0
+
+import "math"
+import m "math"
+
+import . "testing" // declares T in file scope
+import . /* ERROR .unsafe. imported and not used */ "unsafe"
+import . "fmt"     // declares Println in file scope
+
+import (
+	"" /* ERROR invalid import path */
+	"a!b" /* ERROR invalid import path */
+	"abc\xffdef" /* ERROR invalid import path */
+)
+
+// using "math" in this file doesn't affect its use in other files
+const Pi0 = math.Pi
+const Pi1 = m.Pi
+
+type _ T // use "testing"
+
+func _() func() interface{} {
+	return func() interface{} {
+		return Println // use "fmt"
+	}
+}
diff --git a/src/internal/types/testdata/check/importdecl1/importdecl1a.go b/src/internal/types/testdata/check/importdecl1/importdecl1a.go
new file mode 100644
index 0000000..d377c01
--- /dev/null
+++ b/src/internal/types/testdata/check/importdecl1/importdecl1a.go
@@ -0,0 +1,22 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 8969.
+
+package importdecl1
+
+import "go/ast"
+import . "unsafe"
+
+var _ Pointer // use dot-imported package unsafe
+
+// Test cases for issue 23914.
+
+type A interface {
+	// Methods m1, m2 must be type-checked in this file scope
+	// even when embedded in an interface in a different
+	// file of the same package.
+	m1() ast.Node
+	m2() Pointer
+}
diff --git a/src/internal/types/testdata/check/importdecl1/importdecl1b.go b/src/internal/types/testdata/check/importdecl1/importdecl1b.go
new file mode 100644
index 0000000..ce8b983
--- /dev/null
+++ b/src/internal/types/testdata/check/importdecl1/importdecl1b.go
@@ -0,0 +1,11 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl1
+
+import . /* ERROR .unsafe. imported and not used */ "unsafe"
+
+type B interface {
+	A
+}
diff --git a/src/internal/types/testdata/check/init0.go b/src/internal/types/testdata/check/init0.go
new file mode 100644
index 0000000..5159a17
--- /dev/null
+++ b/src/internal/types/testdata/check/init0.go
@@ -0,0 +1,106 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init0
+
+// initialization cycles (we don't know the types)
+const (
+	s0 /* ERROR initialization cycle: s0 refers to itself */ = s0
+
+	x0 /* ERROR initialization cycle for x0 */ = y0
+	y0 = x0
+
+	a0 = b0
+	b0 /* ERROR initialization cycle for b0 */ = c0
+	c0 = d0
+	d0 = b0
+)
+
+var (
+	s1 /* ERROR initialization cycle: s1 refers to itself */ = s1
+
+	x1 /* ERROR initialization cycle for x1 */ = y1
+	y1 = x1
+
+	a1 = b1
+	b1 /* ERROR initialization cycle for b1 */ = c1
+	c1 = d1
+	d1 = b1
+)
+
+// initialization cycles (we know the types)
+const (
+	s2 /* ERROR initialization cycle: s2 refers to itself */ int = s2
+
+	x2 /* ERROR initialization cycle for x2 */ int = y2
+	y2 = x2
+
+	a2 = b2
+	b2 /* ERROR initialization cycle for b2 */ int = c2
+	c2 = d2
+	d2 = b2
+)
+
+var (
+	s3 /* ERROR initialization cycle: s3 refers to itself */ int = s3
+
+	x3 /* ERROR initialization cycle for x3 */ int = y3
+	y3 = x3
+
+	a3 = b3
+	b3 /* ERROR initialization cycle for b3 */ int = c3
+	c3 = d3
+	d3 = b3
+)
+
+// cycles via struct fields
+
+type S1 struct {
+	f int
+}
+const cx3 S1 /* ERROR invalid constant type */ = S1{cx3.f}
+var vx3 /* ERROR initialization cycle: vx3 refers to itself */ S1 = S1{vx3.f}
+
+// cycles via functions
+
+var x4 = x5
+var x5 /* ERROR initialization cycle for x5 */ = f1()
+func f1() int { return x5*10 }
+
+var x6, x7 /* ERROR initialization cycle */ = f2()
+var x8 = x7
+func f2() (int, int) { return f3() + f3(), 0 }
+func f3() int { return x8 }
+
+// cycles via function literals
+
+var x9 /* ERROR initialization cycle: x9 refers to itself */ = func() int { return x9 }()
+
+var x10 /* ERROR initialization cycle for x10 */ = f4()
+
+func f4() int {
+	_ = func() {
+		_ = x10
+	}
+	return 0
+}
+
+// cycles via method expressions
+
+type T1 struct{}
+
+func (T1) m() bool { _ = x11; return false }
+
+var x11 /* ERROR initialization cycle for x11 */ = T1.m(T1{})
+
+// cycles via method values
+
+type T2 struct{}
+
+func (T2) m() bool { _ = x12; return false }
+
+var t1 T2
+var x12 /* ERROR initialization cycle for x12 */ = t1.m
diff --git a/src/internal/types/testdata/check/init1.go b/src/internal/types/testdata/check/init1.go
new file mode 100644
index 0000000..39ca314
--- /dev/null
+++ b/src/internal/types/testdata/check/init1.go
@@ -0,0 +1,97 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init1
+
+// issue 6683 (marked as WorkingAsIntended)
+
+type T0 struct{}
+
+func (T0) m() int { return y0 }
+
+var x0 = T0{}
+
+var y0 /* ERROR initialization cycle */ = x0.m()
+
+type T1 struct{}
+
+func (T1) m() int { return y1 }
+
+var x1 interface {
+	m() int
+} = T1{}
+
+var y1 = x1.m() // no cycle reported, x1 is of interface type
+
+// issue 6703 (modified)
+
+var x2 /* ERROR initialization cycle */ = T2.m
+
+var y2 = x2
+
+type T2 struct{}
+
+func (T2) m() int {
+	_ = y2
+	return 0
+}
+
+var x3 /* ERROR initialization cycle */ = T3.m(T3{}) // <<<< added (T3{})
+
+var y3 = x3
+
+type T3 struct{}
+
+func (T3) m() int {
+	_ = y3
+	return 0
+}
+
+var x4 /* ERROR initialization cycle */ = T4{}.m // <<<< added {}
+
+var y4 = x4
+
+type T4 struct{}
+
+func (T4) m() int {
+	_ = y4
+	return 0
+}
+
+var x5 /* ERROR initialization cycle */ = T5{}.m() // <<<< added ()
+
+var y5 = x5
+
+type T5 struct{}
+
+func (T5) m() int {
+	_ = y5
+	return 0
+}
+
+// issue 4847
+// simplified test case
+
+var x6 = f6
+var y6 /* ERROR initialization cycle */ = f6
+func f6() { _ = y6 }
+
+// full test case
+
+type (
+      E int
+      S int
+)
+
+type matcher func(s *S) E
+
+func matchList(s *S) E { return matcher(matchAnyFn)(s) }
+
+var foo = matcher(matchList)
+
+var matchAny /* ERROR initialization cycle */ = matcher(matchList)
+
+func matchAnyFn(s *S) (err E) { return matchAny(s) }
\ No newline at end of file
diff --git a/src/internal/types/testdata/check/init2.go b/src/internal/types/testdata/check/init2.go
new file mode 100644
index 0000000..614db6c
--- /dev/null
+++ b/src/internal/types/testdata/check/init2.go
@@ -0,0 +1,139 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init2
+
+// cycles through functions
+
+func f1() int { _ = x1; return 0 }
+var x1 /* ERROR initialization cycle */ = f1
+
+func f2() int { _ = x2; return 0 }
+var x2 /* ERROR initialization cycle */ = f2()
+
+// cycles through method expressions
+
+type T3 int
+func (T3) m() int { _ = x3; return 0 }
+var x3 /* ERROR initialization cycle */ = T3.m
+
+type T4 int
+func (T4) m() int { _ = x4; return 0 }
+var x4 /* ERROR initialization cycle */ = T4.m(0)
+
+type T3p int
+func (*T3p) m() int { _ = x3p; return 0 }
+var x3p /* ERROR initialization cycle */ = (*T3p).m
+
+type T4p int
+func (*T4p) m() int { _ = x4p; return 0 }
+var x4p /* ERROR initialization cycle */ = (*T4p).m(nil)
+
+// cycles through method expressions of embedded methods
+
+type T5 struct { E5 }
+type E5 int
+func (E5) m() int { _ = x5; return 0 }
+var x5 /* ERROR initialization cycle */ = T5.m
+
+type T6 struct { E6 }
+type E6 int
+func (E6) m() int { _ = x6; return 0 }
+var x6 /* ERROR initialization cycle */ = T6.m(T6{0})
+
+type T5p struct { E5p }
+type E5p int
+func (*E5p) m() int { _ = x5p; return 0 }
+var x5p /* ERROR initialization cycle */ = (*T5p).m
+
+type T6p struct { E6p }
+type E6p int
+func (*E6p) m() int { _ = x6p; return 0 }
+var x6p /* ERROR initialization cycle */ = (*T6p).m(nil)
+
+// cycles through method values
+
+type T7 int
+func (T7) m() int { _ = x7; return 0 }
+var x7 /* ERROR initialization cycle */ = T7(0).m
+
+type T8 int
+func (T8) m() int { _ = x8; return 0 }
+var x8 /* ERROR initialization cycle */ = T8(0).m()
+
+type T7p int
+func (*T7p) m() int { _ = x7p; return 0 }
+var x7p /* ERROR initialization cycle */ = new(T7p).m
+
+type T8p int
+func (*T8p) m() int { _ = x8p; return 0 }
+var x8p /* ERROR initialization cycle */ = new(T8p).m()
+
+type T7v int
+func (T7v) m() int { _ = x7v; return 0 }
+var x7var T7v
+var x7v /* ERROR initialization cycle */ = x7var.m
+
+type T8v int
+func (T8v) m() int { _ = x8v; return 0 }
+var x8var T8v
+var x8v /* ERROR initialization cycle */ = x8var.m()
+
+type T7pv int
+func (*T7pv) m() int { _ = x7pv; return 0 }
+var x7pvar *T7pv
+var x7pv /* ERROR initialization cycle */ = x7pvar.m
+
+type T8pv int
+func (*T8pv) m() int { _ = x8pv; return 0 }
+var x8pvar *T8pv
+var x8pv /* ERROR initialization cycle */ = x8pvar.m()
+
+// cycles through method values of embedded methods
+
+type T9 struct { E9 }
+type E9 int
+func (E9) m() int { _ = x9; return 0 }
+var x9 /* ERROR initialization cycle */ = T9{0}.m
+
+type T10 struct { E10 }
+type E10 int
+func (E10) m() int { _ = x10; return 0 }
+var x10 /* ERROR initialization cycle */ = T10{0}.m()
+
+type T9p struct { E9p }
+type E9p int
+func (*E9p) m() int { _ = x9p; return 0 }
+var x9p /* ERROR initialization cycle */ = new(T9p).m
+
+type T10p struct { E10p }
+type E10p int
+func (*E10p) m() int { _ = x10p; return 0 }
+var x10p /* ERROR initialization cycle */ = new(T10p).m()
+
+type T9v struct { E9v }
+type E9v int
+func (E9v) m() int { _ = x9v; return 0 }
+var x9var T9v
+var x9v /* ERROR initialization cycle */ = x9var.m
+
+type T10v struct { E10v }
+type E10v int
+func (E10v) m() int { _ = x10v; return 0 }
+var x10var T10v
+var x10v /* ERROR initialization cycle */ = x10var.m()
+
+type T9pv struct { E9pv }
+type E9pv int
+func (*E9pv) m() int { _ = x9pv; return 0 }
+var x9pvar *T9pv
+var x9pv /* ERROR initialization cycle */ = x9pvar.m
+
+type T10pv struct { E10pv }
+type E10pv int
+func (*E10pv) m() int { _ = x10pv; return 0 }
+var x10pvar *T10pv
+var x10pv /* ERROR initialization cycle */ = x10pvar.m()
diff --git a/src/internal/types/testdata/check/issue25008/issue25008a.go b/src/internal/types/testdata/check/issue25008/issue25008a.go
new file mode 100644
index 0000000..cf71ca1
--- /dev/null
+++ b/src/internal/types/testdata/check/issue25008/issue25008a.go
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "io"
+
+type A interface {
+        io.Reader
+}
+
+func f(a A) {
+        a.Read(nil)
+}
diff --git a/src/internal/types/testdata/check/issue25008/issue25008b.go b/src/internal/types/testdata/check/issue25008/issue25008b.go
new file mode 100644
index 0000000..f132b7f
--- /dev/null
+++ b/src/internal/types/testdata/check/issue25008/issue25008b.go
@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type B interface {
+    A
+}
diff --git a/src/internal/types/testdata/check/issues0.go b/src/internal/types/testdata/check/issues0.go
new file mode 100644
index 0000000..4a66641
--- /dev/null
+++ b/src/internal/types/testdata/check/issues0.go
@@ -0,0 +1,373 @@
+// -lang=go1.17
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p // don't permit non-interface elements in interfaces
+
+import (
+	"fmt"
+	syn "regexp/syntax"
+	t1 "text/template"
+	t2 "html/template"
+)
+
+func issue7035() {
+	type T struct{ X int }
+	_ = func() {
+		fmt.Println() // must refer to imported fmt rather than the fmt below
+	}
+	fmt := new(T)
+	_ = fmt.X
+}
+
+func issue8066() {
+	const (
+		_ = float32(340282356779733661637539395458142568447)
+		_ = float32(340282356779733661637539395458142568448 /* ERROR cannot convert */ )
+	)
+}
+
+// Check that a missing identifier doesn't lead to a spurious error cascade.
+func issue8799a() {
+	x, ok := missing /* ERROR undefined */ ()
+	_ = !ok
+	_ = x
+}
+
+func issue8799b(x int, ok bool) {
+	x, ok = missing /* ERROR undefined */ ()
+	_ = !ok
+	_ = x
+}
+
+func issue9182() {
+	type Point C /* ERROR undefined */ .Point
+	// no error for composite literal based on unknown type
+	_ = Point{x: 1, y: 2}
+}
+
+func f0() (a []int)         { return }
+func f1() (a []int, b int)  { return }
+func f2() (a, b []int)      { return }
+
+func append_([]int, ...int) {}
+
+func issue9473(a []int, b ...int) {
+	// variadic builtin function
+	_ = append(f0())
+	_ = append(f0(), f0()...)
+	_ = append(f1())
+	_ = append(f2 /* ERROR cannot use .* in argument */ ())
+	_ = append(f2()... /* ERROR cannot use ... */ )
+	_ = append(f0(), f1 /* ERROR multiple-value f1 */ ())
+	_ = append(f0(), f2 /* ERROR multiple-value f2 */ ())
+	_ = append(f0(), f1 /* ERROR multiple-value f1 */ ()...)
+	_ = append(f0(), f2 /* ERROR multiple-value f2 */ ()...)
+
+	// variadic user-defined function
+	append_(f0())
+	append_(f0(), f0()...)
+	append_(f1())
+	append_(f2 /* ERROR cannot use .* in argument */ ())
+	append_(f2()... /* ERROR cannot use ... */ )
+	append_(f0(), f1 /* ERROR multiple-value f1 */ ())
+	append_(f0(), f2 /* ERROR multiple-value f2 */ ())
+	append_(f0(), f1 /* ERROR multiple-value f1 */ ()...)
+	append_(f0(), f2 /* ERROR multiple-value f2 */ ()...)
+}
+
+// Check that embedding a non-interface type in an interface results in a good error message.
+func issue10979() {
+	type _ interface {
+		int /* ERROR non-interface type int */
+	}
+	type T struct{}
+	type _ interface {
+		T /* ERROR non-interface type T */
+	}
+	type _ interface {
+		nosuchtype /* ERROR undefined: nosuchtype */
+	}
+	type _ interface {
+		fmt.Nosuchtype /* ERROR undefined: fmt\.Nosuchtype */
+	}
+	type _ interface {
+		nosuchpkg /* ERROR undefined: nosuchpkg */ .Nosuchtype
+	}
+	type I interface {
+		I.m /* ERROR I.m is not a type */
+		m()
+	}
+}
+
+// issue11347
+// These should not crash.
+var a1, b1 /* ERROR cycle */ , c1 /* ERROR cycle */ b1 = 0 > 0<<""[""[c1]]>c1
+var a2, b2 /* ERROR cycle */ = 0 /* ERROR assignment mismatch */ /* ERROR assignment mismatch */ > 0<<""[b2]
+var a3, b3 /* ERROR cycle */ = int /* ERROR assignment mismatch */ /* ERROR assignment mismatch */ (1<<""[b3])
+
+// issue10260
+// Check that error messages explain reason for interface assignment failures.
+type (
+	I0 interface{}
+	I1 interface{ foo() }
+	I2 interface{ foo(x int) }
+	T0 struct{}
+	T1 struct{}
+	T2 struct{}
+)
+
+func (*T1) foo() {}
+func (*T2) foo(x int) {}
+
+func issue10260() {
+	var (
+		i0 I0
+		i1 I1
+		i2 I2
+		t0 *T0
+		t1 *T1
+		t2 *T2
+	)
+
+	var x I1
+	x = T1 /* ERROR cannot use T1{} .* as I1 value in assignment: T1 does not implement I1 \(method foo has pointer receiver\) */ {}
+	_ = x /* ERROR impossible type assertion: x\.\(T1\)\n\tT1 does not implement I1 \(method foo has pointer receiver\) */ .(T1)
+
+	T1{}.foo /* ERROR cannot call pointer method foo on T1 */ ()
+	x.Foo /* ERROR "x.Foo undefined \(type I1 has no field or method Foo, but does have foo\)" */ ()
+
+	_ = i2 /* ERROR impossible type assertion: i2\.\(\*T1\)\n\t\*T1 does not implement I2 \(wrong type for method foo\)\n\t\thave foo\(\)\n\t\twant foo\(int\) */ .(*T1)
+
+	i1 = i0 /* ERROR cannot use i0 .* as I1 value in assignment: I0 does not implement I1 \(missing method foo\) */
+	i1 = t0 /* ERROR .* t0 .* as I1 .*: \*T0 does not implement I1 \(missing method foo\) */
+	i1 = i2 /* ERROR .* i2 .* as I1 .*: I2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(int\)\n\t\twant foo\(\) */
+	i1 = t2 /* ERROR .* t2 .* as I1 .*: \*T2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(int\)\n\t\twant foo\(\) */
+	i2 = i1 /* ERROR .* i1 .* as I2 .*: I1 does not implement I2 \(wrong type for method foo\)\n\t\thave foo\(\)\n\t\twant foo\(int\) */
+	i2 = t1 /* ERROR .* t1 .* as I2 .*: \*T1 does not implement I2 \(wrong type for method foo\)\n\t\thave foo\(\)\n\t\twant foo\(int\) */
+
+	_ = func() I1 { return i0 /* ERROR cannot use i0 .* as I1 value in return statement: I0 does not implement I1 \(missing method foo\) */ }
+	_ = func() I1 { return t0 /* ERROR .* t0 .* as I1 .*: \*T0 does not implement I1 \(missing method foo\) */ }
+	_ = func() I1 { return i2 /* ERROR .* i2 .* as I1 .*: I2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(int\)\n\t\twant foo\(\) */ }
+	_ = func() I1 { return t2 /* ERROR .* t2 .* as I1 .*: \*T2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(int\)\n\t\twant foo\(\) */ }
+	_ = func() I2 { return i1 /* ERROR .* i1 .* as I2 .*: I1 does not implement I2 \(wrong type for method foo\)\n\t\thave foo\(\)\n\t\twant foo\(int\) */ }
+	_ = func() I2 { return t1 /* ERROR .* t1 .* as I2 .*: \*T1 does not implement I2 \(wrong type for method foo\)\n\t\thave foo\(\)\n\t\twant foo\(int\) */ }
+
+	// a few more - less exhaustive now
+
+	f := func(I1, I2){}
+	f(i0 /* ERROR missing method foo */ , i1 /* ERROR wrong type for method foo */ )
+
+	_ = [...]I1{i0 /* ERROR cannot use i0 .* as I1 value in array or slice literal: I0 does not implement I1 \(missing method foo\) */ }
+	_ = [...]I1{i2 /* ERROR cannot use i2 .* as I1 value in array or slice literal: I2 does not implement I1 \(wrong type for method foo\)\n\t\thave foo\(int\)\n\t\twant foo\(\) */ }
+	_ = []I1{i0 /* ERROR missing method foo */ }
+	_ = []I1{i2 /* ERROR wrong type for method foo */ }
+	_ = map[int]I1{0: i0 /* ERROR missing method foo */ }
+	_ = map[int]I1{0: i2 /* ERROR wrong type for method foo */ }
+
+	make(chan I1) <- i0 /* ERROR missing method foo */
+	make(chan I1) <- i2 /* ERROR wrong type for method foo */
+}
+
+// Check that constants representable as integers are in integer form
+// before being used in operations that are only defined on integers.
+func issue14229() {
+	// from the issue
+	const _ = int64(-1<<63) % 1e6
+
+	// related
+	const (
+		a int = 3
+		b = 4.0
+		_ = a / b
+		_ = a % b
+		_ = b / a
+		_ = b % a
+	)
+}
+
+// Check that in a n:1 variable declaration with type and initialization
+// expression the type is distributed to all variables of the lhs before
+// the initialization expression assignment is checked.
+func issue15755() {
+	// from issue
+	var i interface{}
+	type b bool
+	var x, y b = i.(b)
+	_ = x == y
+
+	// related: we should see an error since the result of f1 is ([]int, int)
+	var u, v []int = f1 /* ERROR cannot use f1 */ ()
+	_ = u
+	_ = v
+}
+
+// Test that we don't get "declared and not used"
+// errors in the context of invalid/C objects.
+func issue20358() {
+	var F C /* ERROR "undefined" */ .F
+	var A C /* ERROR "undefined" */ .A
+	var S C /* ERROR "undefined" */ .S
+	type T C /* ERROR "undefined" */ .T
+	type P C /* ERROR "undefined" */ .P
+
+	// these variables must be "used" even though
+	// the LHS expressions/types below in which
+	// context they are used are unknown/invalid
+	var f, a, s1, s2, s3, t, p int
+
+	_ = F(f)
+	_ = A[a]
+	_ = S[s1:s2:s3]
+	_ = T{t}
+	_ = P{f: p}
+}
+
+// Test that we don't declare lhs variables in short variable
+// declarations before we type-check function literals on the
+// rhs.
+func issue24026() {
+	f := func() int { f(0) /* must refer to outer f */; return 0 }
+	_ = f
+
+	_ = func() {
+		f := func() { _ = f() /* must refer to outer f */ }
+		_ = f
+	}
+
+	// b and c must not be visible inside function literal
+	a := 0
+	a, b, c := func() (int, int, int) {
+		return a, b /* ERROR undefined */ , c /* ERROR undefined */
+	}()
+	_, _ = b, c
+}
+
+func f(int) {} // for issue24026
+
+// Test that we don't report a "missing return statement" error
+// (due to incorrect context when type-checking interfaces).
+func issue24140(x interface{}) int {
+        switch x.(type) {
+        case interface{}:
+                return 0
+        default:
+                panic(0)
+        }
+}
+
+// Test that we don't crash when the 'if' condition is missing.
+func issue25438() {
+	if { /* ERROR missing condition */ }
+	if x := 0; /* ERROR missing condition */ { _ = x }
+	if
+	{ /* ERROR missing condition */ }
+}
+
+// Test that we can embed alias type names in interfaces.
+type issue25301 interface {
+	E
+}
+
+type E = interface {
+	m()
+}
+
+// Test case from issue.
+// cmd/compile reports a cycle as well.
+type issue25301b /* ERROR invalid recursive type */ = interface {
+	m() interface{ issue25301b }
+}
+
+type issue25301c interface {
+	notE // ERROR non-interface type struct\{\}
+}
+
+type notE = struct{}
+
+// Test that method declarations don't introduce artificial cycles
+// (issue #26124).
+const CC TT = 1
+type TT int
+func (TT) MM() [CC]TT
+
+// Reduced test case from issue #26124.
+const preloadLimit LNumber = 128
+type LNumber float64
+func (LNumber) assertFunction() *LFunction
+type LFunction struct {
+	GFunction LGFunction
+}
+type LGFunction func(*LState)
+type LState struct {
+	reg *registry
+}
+type registry struct {
+	alloc *allocator
+}
+type allocator struct {
+	_ [int(preloadLimit)]int
+}
+
+// Test that we don't crash when type-checking composite literals
+// containing errors in the type.
+var issue27346 = [][n /* ERROR undefined */ ]int{
+	0: {},
+}
+
+var issue22467 = map[int][... /* ERROR invalid use of ... */ ]int{0: {}}
+
+// Test that invalid use of ... in parameter lists is recognized
+// (issue #28281).
+func issue28281a(int, int, ...int)
+func issue28281b(a, b int, c ...int)
+func issue28281c(a, b, c ... /* ERROR can only use ... with final parameter */ int)
+func issue28281d(... /* ERROR can only use ... with final parameter */ int, int)
+func issue28281e(a, b, c  ... /* ERROR can only use ... with final parameter */ int, d int)
+func issue28281f(... /* ERROR can only use ... with final parameter */ int, ... /* ERROR can only use ... with final parameter */ int, int)
+func (... /* ERROR can only use ... with final parameter */ TT) f()
+func issue28281g() (... /* ERROR can only use ... with final parameter */ TT)
+
+// Issue #26234: Make various field/method lookup errors easier to read by matching cmd/compile's output
+func issue26234a(f *syn.Prog) {
+	// The error message below should refer to the actual package name (syntax)
+	// not the local package name (syn).
+	f.foo /* ERROR f\.foo undefined \(type \*syntax\.Prog has no field or method foo\) */
+}
+
+type T struct {
+	x int
+	E1
+	E2
+}
+
+type E1 struct{ f int }
+type E2 struct{ f int }
+
+func issue26234b(x T) {
+	_ = x.f /* ERROR ambiguous selector x.f */
+}
+
+func issue26234c() {
+	T.x /* ERROR T.x undefined \(type T has no method x\) */ ()
+}
+
+func issue35895() {
+	// T is defined in this package, don't qualify its name with the package name.
+	var _ T = 0 // ERROR cannot use 0 \(untyped int constant\) as T
+
+	// There is only one package with name syntax imported, only use the (global) package name in error messages.
+	var _ *syn.Prog = 0 // ERROR cannot use 0 \(untyped int constant\) as \*syntax.Prog
+
+	// Because both t1 and t2 have the same global package name (template),
+	// qualify packages with full path name in this case.
+	var _ t1.Template = t2 /* ERROR cannot use .* \(value of type .html/template.\.Template\) as .text/template.\.Template */ .Template{}
+}
+
+func issue42989(s uint) {
+	var m map[int]string
+	delete(m, 1<<s)
+	delete(m, 1.<<s)
+}
diff --git a/src/internal/types/testdata/check/issues1.go b/src/internal/types/testdata/check/issues1.go
new file mode 100644
index 0000000..2f3414d
--- /dev/null
+++ b/src/internal/types/testdata/check/issues1.go
@@ -0,0 +1,252 @@
+// -oldComparableSemantics
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains regression tests for bugs found.
+
+package p
+
+import "io"
+import "context"
+
+func eql[T comparable](x, y T) bool {
+	return x == y
+}
+
+func _[X comparable, Y interface{comparable; m()}]() {
+	var x X
+	var y Y
+	eql(x, y /* ERROR does not match */ ) // interfaces of different types
+	eql(x, x)
+	eql(y, y)
+	eql(y, nil /* ERROR cannot use nil as Y value in argument to eql */ )
+	eql[io /* ERROR does not satisfy comparable */ .Reader](nil, nil)
+}
+
+// If we have a receiver of pointer to type parameter type (below: *T)
+// we don't have any methods, like for interfaces.
+type C[T any] interface {
+    m()
+}
+
+// using type bound C
+func _[T C[T]](x *T) {
+	x.m /* ERROR x\.m undefined */ ()
+}
+
+// using an interface literal as bound
+func _[T interface{ m() }](x *T) {
+	x.m /* ERROR x\.m undefined */ ()
+}
+
+func f2[_ interface{ m1(); m2() }]() {}
+
+type T struct{}
+func (T) m1()
+func (*T) m2()
+
+func _() {
+	f2[T /* ERROR m2 has pointer receiver */ ]()
+	f2[*T]()
+}
+
+// When a type parameter is used as an argument to instantiate a parameterized
+// type, the type argument's type set must be a subset of the instantiated type
+// parameter's type set.
+type T1[P interface{~uint}] struct{}
+
+func _[P any]() {
+    _ = T1[P /* ERROR P does not satisfy interface{~uint} */ ]{}
+}
+
+// This is the original (simplified) program causing the same issue.
+type Unsigned interface {
+	~uint
+}
+
+type T2[U Unsigned] struct {
+    s U
+}
+
+func (u T2[U]) Add1() U {
+    return u.s + 1
+}
+
+func NewT2[U any]() T2[U /* ERROR U does not satisfy Unsigned */ ] {
+    return T2[U /* ERROR U does not satisfy Unsigned */ ]{}
+}
+
+func _() {
+    u := NewT2[string]()
+    _ = u.Add1()
+}
+
+// When we encounter an instantiated type such as Elem[T] we must
+// not "expand" the instantiation when the type to be instantiated
+// (Elem in this case) is not yet fully set up.
+type Elem[T any] struct {
+	next *Elem[T]
+	list *List[T]
+}
+
+type List[T any] struct {
+	root Elem[T]
+}
+
+func (l *List[T]) Init() {
+	l.root.next = &l.root
+}
+
+// This is the original program causing the same issue.
+type Element2[TElem any] struct {
+	next, prev *Element2[TElem]
+	list *List2[TElem]
+	Value TElem
+}
+
+type List2[TElem any] struct {
+	root Element2[TElem]
+	len  int
+}
+
+func (l *List2[TElem]) Init() *List2[TElem] {
+	l.root.next = &l.root
+	l.root.prev = &l.root
+	l.len = 0
+	return l
+}
+
+// Self-recursive instantiations must work correctly.
+type A[P any] struct { _ *A[P] }
+
+type AB[P any] struct { _ *BA[P] }
+type BA[P any] struct { _ *AB[P] }
+
+// And a variation that also caused a problem with an
+// unresolved underlying type.
+type Element3[TElem any] struct {
+	next, prev *Element3[TElem]
+	list *List3[TElem]
+	Value TElem
+}
+
+func (e *Element3[TElem]) Next() *Element3[TElem] {
+	if p := e.next; e.list != nil && p != &e.list.root {
+		return p
+	}
+	return nil
+}
+
+type List3[TElem any] struct {
+	root Element3[TElem]
+	len  int
+}
+
+// Infinite generic type declarations must lead to an error.
+type inf1[T any] struct{ _ inf1 /* ERROR invalid recursive type */ [T] }
+type inf2[T any] struct{ inf2 /* ERROR invalid recursive type */ [T] }
+
+// The implementation of conversions T(x) between integers and floating-point
+// numbers checks that both T and x have either integer or floating-point
+// type. When the type of T or x is a type parameter, the respective simple
+// predicate disjunction in the implementation was wrong because if a type set
+// contains both an integer and a floating-point type, the type parameter is
+// neither an integer or a floating-point number.
+func convert[T1, T2 interface{~int | ~uint | ~float32}](v T1) T2 {
+	return T2(v)
+}
+
+func _() {
+	convert[int, uint](5)
+}
+
+// When testing binary operators, for +, the operand types must either be
+// both numeric, or both strings. The implementation had the same problem
+// with this check as the conversion issue above (issue #39623).
+
+func issue39623[T interface{~int | ~string}](x, y T) T {
+	return x + y
+}
+
+// Simplified, from https://go2goplay.golang.org/p/efS6x6s-9NI:
+func Sum[T interface{~int | ~string}](s []T) (sum T) {
+	for _, v := range s {
+		sum += v
+	}
+	return
+}
+
+// Assignability of an unnamed pointer type to a type parameter that
+// has a matching underlying type.
+func _[T interface{}, PT interface{~*T}] (x T) PT {
+    return &x
+}
+
+// Indexing of type parameters containing type parameters in their constraint terms:
+func at[T interface{ ~[]E }, E interface{}](x T, i int) E {
+        return x[i]
+}
+
+// Conversion of a local type to a type parameter.
+func _[T interface{~int}](x T) {
+	type myint int
+	var _ int = int(x)
+	var _ T = 42
+	var _ T = T(myint(42))
+}
+
+// Indexing a type parameter with an array type bound checks length.
+// (Example by mdempsky@.)
+func _[T interface { ~[10]int }](x T) {
+	_ = x[9] // ok
+	_ = x[20 /* ERROR out of bounds */ ]
+}
+
+// Pointer indirection of a type parameter.
+func _[T interface{ ~*int }](p T) int {
+	return *p
+}
+
+// Channel sends and receives on type parameters.
+func _[T interface{ ~chan int }](ch T) int {
+	ch <- 0
+	return <- ch
+}
+
+// Calling of a generic variable.
+func _[T interface{ ~func() }](f T) {
+	f()
+	go f()
+}
+
+type F1 func()
+type F2 func()
+func _[T interface{ func()|F1|F2 }](f T) {
+	f()
+	go f()
+}
+
+// We must compare against the (possibly underlying) types of term list
+// elements when checking if a constraint is satisfied by a type.
+// The underlying type of each term must be computed after the
+// interface has been instantiated as its constraint may contain
+// a type parameter that was substituted with a defined type.
+// Test case from an (originally) failing example.
+
+type sliceOf[E any] interface{ ~[]E }
+
+func append[T interface{}, S sliceOf[T], T2 interface{}](s S, t ...T2) S { panic(0) }
+
+var f           func()
+var cancelSlice []context.CancelFunc
+var _ = append[context.CancelFunc, []context.CancelFunc, context.CancelFunc](cancelSlice, f)
+
+// A generic function must be instantiated with a type, not a value.
+
+func g[T any](T) T { panic(0) }
+
+var _ = g[int]
+var _ = g[nil /* ERROR is not a type */ ]
+var _ = g(0)
diff --git a/src/internal/types/testdata/check/labels.go b/src/internal/types/testdata/check/labels.go
new file mode 100644
index 0000000..5948952
--- /dev/null
+++ b/src/internal/types/testdata/check/labels.go
@@ -0,0 +1,207 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a modified concatenation of the files
+// $GOROOT/test/label.go and $GOROOT/test/label1.go.
+
+package labels
+
+var x int
+
+func f0() {
+L1 /* ERROR "label L1 declared and not used" */ :
+	for {
+	}
+L2 /* ERROR "label L2 declared and not used" */ :
+	select {
+	}
+L3 /* ERROR "label L3 declared and not used" */ :
+	switch {
+	}
+L4 /* ERROR "label L4 declared and not used" */ :
+	if true {
+	}
+L5 /* ERROR "label L5 declared and not used" */ :
+	f0()
+L6:
+	f0()
+L6 /* ERROR "label L6 already declared" */ :
+	f0()
+	if x == 20 {
+		goto L6
+	}
+
+L7:
+	for {
+		break L7
+		break L8 /* ERROR "invalid break label L8" */
+	}
+
+// A label must be directly associated with a switch, select, or
+// for statement; it cannot be the label of a labeled statement.
+
+L7a /* ERROR "declared and not used" */ : L7b:
+	for {
+		break L7a /* ERROR "invalid break label L7a" */
+		continue L7a /* ERROR "invalid continue label L7a" */
+		continue L7b
+	}
+
+L8:
+	for {
+		if x == 21 {
+			continue L8
+			continue L7 /* ERROR "invalid continue label L7" */
+		}
+	}
+
+L9:
+	switch {
+	case true:
+		break L9
+	defalt /* ERROR "label defalt declared and not used" */ :
+	}
+
+L10:
+	select {
+	default:
+		break L10
+		break L9 /* ERROR "invalid break label L9" */
+	}
+
+	goto L10a
+L10a: L10b:
+	select {
+	default:
+		break L10a /* ERROR "invalid break label L10a" */
+		break L10b
+		continue L10b /* ERROR "invalid continue label L10b" */
+	}
+}
+
+func f1() {
+L1:
+	for {
+		if x == 0 {
+			break L1
+		}
+		if x == 1 {
+			continue L1
+		}
+		goto L1
+	}
+
+L2:
+	select {
+	default:
+		if x == 0 {
+			break L2
+		}
+		if x == 1 {
+			continue L2 /* ERROR "invalid continue label L2" */
+		}
+		goto L2
+	}
+
+L3:
+	switch {
+	case x > 10:
+		if x == 11 {
+			break L3
+		}
+		if x == 12 {
+			continue L3 /* ERROR "invalid continue label L3" */
+		}
+		goto L3
+	}
+
+L4:
+	if true {
+		if x == 13 {
+			break L4 /* ERROR "invalid break label L4" */
+		}
+		if x == 14 {
+			continue L4 /* ERROR "invalid continue label L4" */
+		}
+		if x == 15 {
+			goto L4
+		}
+	}
+
+L5:
+	f1()
+	if x == 16 {
+		break L5 /* ERROR "invalid break label L5" */
+	}
+	if x == 17 {
+		continue L5 /* ERROR "invalid continue label L5" */
+	}
+	if x == 18 {
+		goto L5
+	}
+
+	for {
+		if x == 19 {
+			break L1 /* ERROR "invalid break label L1" */
+		}
+		if x == 20 {
+			continue L1 /* ERROR "invalid continue label L1" */
+		}
+		if x == 21 {
+			goto L1
+		}
+	}
+}
+
+// Additional tests not in the original files.
+
+func f2() {
+L1 /* ERROR "label L1 declared and not used" */ :
+	if x == 0 {
+		for {
+			continue L1 /* ERROR "invalid continue label L1" */
+		}
+	}
+}
+
+func f3() {
+L1:
+L2:
+L3:
+	for {
+		break L1 /* ERROR "invalid break label L1" */
+		break L2 /* ERROR "invalid break label L2" */
+		break L3
+		continue L1 /* ERROR "invalid continue label L1" */
+		continue L2 /* ERROR "invalid continue label L2" */
+		continue L3
+		goto L1
+		goto L2
+		goto L3
+	}
+}
+
+// Blank labels are never declared.
+
+func f4() {
+_:
+_: // multiple blank labels are ok
+	goto _ /* ERROR "label _ not declared" */
+}
+
+func f5() {
+_:
+	for {
+		break _ /* ERROR "invalid break label _" */
+		continue _ /* ERROR "invalid continue label _" */
+	}
+}
+
+func f6() {
+_:
+	switch {
+	default:
+		break _ /* ERROR "invalid break label _" */
+	}
+}
diff --git a/src/internal/types/testdata/check/linalg.go b/src/internal/types/testdata/check/linalg.go
new file mode 100644
index 0000000..f02e773
--- /dev/null
+++ b/src/internal/types/testdata/check/linalg.go
@@ -0,0 +1,82 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linalg
+
+// Numeric is type bound that matches any numeric type.
+// It would likely be in a constraints package in the standard library.
+type Numeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64 |
+		~complex64 | ~complex128
+}
+
+func DotProduct[T Numeric](s1, s2 []T) T {
+	if len(s1) != len(s2) {
+		panic("DotProduct: slices of unequal length")
+	}
+	var r T
+	for i := range s1 {
+		r += s1[i] * s2[i]
+	}
+	return r
+}
+
+// NumericAbs matches numeric types with an Abs method.
+type NumericAbs[T any] interface {
+	Numeric
+
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func AbsDifference[T NumericAbs[T]](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// OrderedNumeric is a type bound that matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64
+}
+
+// Complex is a type bound that matches the two complex types, which do not have a < operator.
+type Complex interface {
+	~complex64 | ~complex128
+}
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // OrderedAbs is a helper type that defines an Abs method for
+// // ordered numeric types.
+// type OrderedAbs[T OrderedNumeric] T
+// 
+// func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
+// 	if a < 0 {
+// 		return -a
+// 	}
+// 	return a
+// }
+// 
+// // ComplexAbs is a helper type that defines an Abs method for
+// // complex types.
+// type ComplexAbs[T Complex] T
+// 
+// func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
+// 	r := float64(real(a))
+// 	i := float64(imag(a))
+// 	d := math.Sqrt(r * r + i * i)
+// 	return ComplexAbs[T](complex(d, 0))
+// }
+// 
+// func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+// 	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
+// }
+// 
+// func ComplexAbsDifference[T Complex](a, b T) T {
+// 	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
+// }
diff --git a/src/internal/types/testdata/check/literals.go b/src/internal/types/testdata/check/literals.go
new file mode 100644
index 0000000..494a465
--- /dev/null
+++ b/src/internal/types/testdata/check/literals.go
@@ -0,0 +1,111 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file tests various representations of literals
+// and compares them with literals or constant expressions
+// of equal values.
+
+package literals
+
+func _() {
+	// 0-octals
+	assert(0_123 == 0123)
+	assert(0123_456 == 0123456)
+
+	// decimals
+	assert(1_234 == 1234)
+	assert(1_234_567 == 1234567)
+
+	// hexadecimals
+	assert(0X_0 == 0)
+	assert(0X_1234 == 0x1234)
+	assert(0X_CAFE_f00d == 0xcafef00d)
+
+	// octals
+	assert(0o0 == 0)
+	assert(0o1234 == 01234)
+	assert(0o01234567 == 01234567)
+
+	assert(0O0 == 0)
+	assert(0O1234 == 01234)
+	assert(0O01234567 == 01234567)
+
+	assert(0o_0 == 0)
+	assert(0o_1234 == 01234)
+	assert(0o0123_4567 == 01234567)
+
+	assert(0O_0 == 0)
+	assert(0O_1234 == 01234)
+	assert(0O0123_4567 == 01234567)
+
+	// binaries
+	assert(0b0 == 0)
+	assert(0b1011 == 0xb)
+	assert(0b00101101 == 0x2d)
+
+	assert(0B0 == 0)
+	assert(0B1011 == 0xb)
+	assert(0B00101101 == 0x2d)
+
+	assert(0b_0 == 0)
+	assert(0b10_11 == 0xb)
+	assert(0b_0010_1101 == 0x2d)
+
+	// decimal floats
+	assert(1_2_3. == 123.)
+	assert(0_123. == 123.)
+
+	assert(0_0e0 == 0.)
+	assert(1_2_3e0 == 123.)
+	assert(0_123e0 == 123.)
+
+	assert(0e-0_0 == 0.)
+	assert(1_2_3E+0 == 123.)
+	assert(0123E1_2_3 == 123e123)
+
+	assert(0.e+1 == 0.)
+	assert(123.E-1_0 == 123e-10)
+	assert(01_23.e123 == 123e123)
+
+	assert(.0e-1 == .0)
+	assert(.123E+10 == .123e10)
+	assert(.0123E123 == .0123e123)
+
+	assert(1_2_3.123 == 123.123)
+	assert(0123.01_23 == 123.0123)
+
+	// hexadecimal floats
+	assert(0x0.p+0 == 0.)
+	assert(0Xdeadcafe.p-10 == 0xdeadcafe/1024.0)
+	assert(0x1234.P84 == 0x1234000000000000000000000)
+
+	assert(0x.1p-0 == 1./16)
+	assert(0X.deadcafep4 == 1.0*0xdeadcafe/0x10000000)
+	assert(0x.1234P+12 == 1.0*0x1234/0x10)
+
+	assert(0x0p0 == 0.)
+	assert(0Xdeadcafep+1 == 0x1bd5b95fc)
+	assert(0x1234P-10 == 0x1234/1024.0)
+
+	assert(0x0.0p0 == 0.)
+	assert(0Xdead.cafep+1 == 1.0*0x1bd5b95fc/0x10000)
+	assert(0x12.34P-10 == 1.0*0x1234/0x40000)
+
+	assert(0Xdead_cafep+1 == 0xdeadcafep+1)
+	assert(0x_1234P-10 == 0x1234p-10)
+
+	assert(0X_dead_cafe.p-10 == 0xdeadcafe.p-10)
+	assert(0x12_34.P1_2_3 == 0x1234.p123)
+
+	assert(1_234i == 1234i)
+	assert(1_234_567i == 1234567i)
+
+	assert(0.i == 0i)
+	assert(123.i == 123i)
+	assert(0123.i == 123i)
+
+	assert(0.e+1i == 0i)
+	assert(123.E-1_0i == 123e-10i)
+	assert(01_23.e123i == 123e123i)
+}
diff --git a/src/internal/types/testdata/check/main0.go b/src/internal/types/testdata/check/main0.go
new file mode 100644
index 0000000..132a5fe
--- /dev/null
+++ b/src/internal/types/testdata/check/main0.go
@@ -0,0 +1,9 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main()
+func main /* ERROR "no arguments and no return values" */ /* ERROR redeclared */ (int)
+func main /* ERROR "no arguments and no return values" */ /* ERROR redeclared */ () int
diff --git a/src/internal/types/testdata/check/main1.go b/src/internal/types/testdata/check/main1.go
new file mode 100644
index 0000000..fb567a0
--- /dev/null
+++ b/src/internal/types/testdata/check/main1.go
@@ -0,0 +1,7 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main[T /* ERROR "func main must have no type parameters" */ any]() {}
diff --git a/src/internal/types/testdata/check/map0.go b/src/internal/types/testdata/check/map0.go
new file mode 100644
index 0000000..814d953
--- /dev/null
+++ b/src/internal/types/testdata/check/map0.go
@@ -0,0 +1,113 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+// TODO(gri) fix imports for tests
+import "chans" // ERROR could not import
+
+// Map is an ordered map.
+type Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// New returns a new map.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such a node would go.
+func (m *Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Returns true if this is a new key, false if already present.
+func (m *Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+        *pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or zero if not present.
+// The found result reports whether the key was found.
+func (m *Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans.Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &Iterator[K, V]{receiver}
+}
+
+// Iterator is used to iterate over the map.
+type Iterator[K, V any] struct {
+	r *chans.Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
diff --git a/src/internal/types/testdata/check/map1.go b/src/internal/types/testdata/check/map1.go
new file mode 100644
index 0000000..e13bf33
--- /dev/null
+++ b/src/internal/types/testdata/check/map1.go
@@ -0,0 +1,146 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is like map.go2, but instead if importing chans, it contains
+// the necessary functionality at the end of the file.
+
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+// Map is an ordered map.
+type Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// New returns a new map.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such a node would go.
+func (m *Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Returns true if this is a new key, false if already present.
+func (m *Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+	*pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or zero if not present.
+// The found result reports whether the key was found.
+func (m *Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans_Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &Iterator[K, V]{receiver}
+}
+
+// Iterator is used to iterate over the map.
+type Iterator[K, V any] struct {
+	r *chans_Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
+
+// chans
+
+func chans_Ranger[T any]() (*chans_Sender[T], *chans_Receiver[T]) { panic(0) }
+
+// A sender is used to send values to a Receiver.
+type chans_Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+func (s *chans_Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+func (s *chans_Sender[T]) Close() {
+	close(s.values)
+}
+
+type chans_Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+func (r *chans_Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}
diff --git a/src/internal/types/testdata/check/methodsets.go b/src/internal/types/testdata/check/methodsets.go
new file mode 100644
index 0000000..d145bc0
--- /dev/null
+++ b/src/internal/types/testdata/check/methodsets.go
@@ -0,0 +1,214 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package methodsets
+
+type T0 struct {}
+
+func (T0) v0() {}
+func (*T0) p0() {}
+
+type T1 struct {} // like T0 with different method names
+
+func (T1) v1() {}
+func (*T1) p1() {}
+
+type T2 interface {
+	v2()
+	p2()
+}
+
+type T3 struct {
+	T0
+	*T1
+	T2
+}
+
+// Method expressions
+func _() {
+	var (
+		_ func(T0) = T0.v0
+		_ = T0.p0 /* ERROR invalid method expression T0\.p0 \(needs pointer receiver \(\*T0\)\.p0\) */
+
+		_ func (*T0) = (*T0).v0
+		_ func (*T0) = (*T0).p0
+
+		// T1 is like T0
+
+		_ func(T2) = T2.v2
+		_ func(T2) = T2.p2
+
+		_ func(T3) = T3.v0
+		_ func(T3) = T3.p0 /* ERROR invalid method expression T3\.p0 \(needs pointer receiver \(\*T3\)\.p0\) */
+		_ func(T3) = T3.v1
+		_ func(T3) = T3.p1
+		_ func(T3) = T3.v2
+		_ func(T3) = T3.p2
+
+		_ func(*T3) = (*T3).v0
+		_ func(*T3) = (*T3).p0
+		_ func(*T3) = (*T3).v1
+		_ func(*T3) = (*T3).p1
+		_ func(*T3) = (*T3).v2
+		_ func(*T3) = (*T3).p2
+	)
+}
+
+// Method values with addressable receivers
+func _() {
+	var (
+		v0 T0
+		_ func() = v0.v0
+		_ func() = v0.p0
+	)
+
+	var (
+		p0 *T0
+		_ func() = p0.v0
+		_ func() = p0.p0
+	)
+
+	// T1 is like T0
+
+	var (
+		v2 T2
+		_ func() = v2.v2
+		_ func() = v2.p2
+	)
+
+	var (
+		v4 T3
+		_ func() = v4.v0
+		_ func() = v4.p0
+		_ func() = v4.v1
+		_ func() = v4.p1
+		_ func() = v4.v2
+		_ func() = v4.p2
+	)
+
+	var (
+		p4 *T3
+		_ func() = p4.v0
+		_ func() = p4.p0
+		_ func() = p4.v1
+		_ func() = p4.p1
+		_ func() = p4.v2
+		_ func() = p4.p2
+	)
+}
+
+// Method calls with addressable receivers
+func _() {
+	var v0 T0
+	v0.v0()
+	v0.p0()
+
+	var p0 *T0
+	p0.v0()
+	p0.p0()
+
+	// T1 is like T0
+
+	var v2 T2
+	v2.v2()
+	v2.p2()
+
+	var v4 T3
+	v4.v0()
+	v4.p0()
+	v4.v1()
+	v4.p1()
+	v4.v2()
+	v4.p2()
+
+	var p4 *T3
+	p4.v0()
+	p4.p0()
+	p4.v1()
+	p4.p1()
+	p4.v2()
+	p4.p2()
+}
+
+// Method values with value receivers
+func _() {
+	var (
+		_ func() = T0{}.v0
+		_ func() = T0{}.p0 /* ERROR "cannot call pointer method p0 on T0" */
+
+		_ func() = (&T0{}).v0
+		_ func() = (&T0{}).p0
+
+		// T1 is like T0
+
+		// no values for T2
+
+		_ func() = T3{}.v0
+		_ func() = T3{}.p0 /* ERROR "cannot call pointer method p0 on T3" */
+		_ func() = T3{}.v1
+		_ func() = T3{}.p1
+		_ func() = T3{}.v2
+		_ func() = T3{}.p2
+
+		_ func() = (&T3{}).v0
+		_ func() = (&T3{}).p0
+		_ func() = (&T3{}).v1
+		_ func() = (&T3{}).p1
+		_ func() = (&T3{}).v2
+		_ func() = (&T3{}).p2
+	)
+}
+
+// Method calls with value receivers
+func _() {
+	T0{}.v0()
+	T0{}.p0 /* ERROR "cannot call pointer method p0 on T0" */ ()
+
+	(&T0{}).v0()
+	(&T0{}).p0()
+
+	// T1 is like T0
+
+	// no values for T2
+
+	T3{}.v0()
+	T3{}.p0 /* ERROR "cannot call pointer method p0 on T3" */ ()
+	T3{}.v1()
+	T3{}.p1()
+	T3{}.v2()
+	T3{}.p2()
+
+	(&T3{}).v0()
+	(&T3{}).p0()
+	(&T3{}).v1()
+	(&T3{}).p1()
+	(&T3{}).v2()
+	(&T3{}).p2()
+}
+
+// *T has no methods if T is an interface type
+func issue5918() {
+	var (
+		err error
+		_ = err.Error()
+		_ func() string = err.Error
+		_ func(error) string = error.Error
+
+		perr = &err
+		_ = perr.Error /* ERROR "type \*error is pointer to interface, not interface" */ ()
+		_ func() string = perr.Error /* ERROR "type \*error is pointer to interface, not interface" */
+		_ func(*error) string = (*error).Error /* ERROR "type \*error is pointer to interface, not interface" */
+	)
+
+	type T *interface{ m() int }
+	var (
+		x T
+		_ = (*x).m()
+		_ = (*x).m
+
+		_ = x.m /* ERROR "type T is pointer to interface, not interface" */ ()
+		_ = x.m /* ERROR "type T is pointer to interface, not interface" */
+		_ = T.m /* ERROR "type T is pointer to interface, not interface" */
+	)
+}
diff --git a/src/internal/types/testdata/check/shifts.go b/src/internal/types/testdata/check/shifts.go
new file mode 100644
index 0000000..5cd0182
--- /dev/null
+++ b/src/internal/types/testdata/check/shifts.go
@@ -0,0 +1,399 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package shifts
+
+func shifts0() {
+	// basic constant shifts
+	const (
+		s = 10
+		_ = 0<<0
+		_ = 1<<s
+		_ = 1<<- /* ERROR "negative shift count" */ 1
+		// For the test below we may decide to convert to int
+		// rather than uint and then report a negative shift
+		// count instead, which might be a better error. The
+		// (minor) difference is that this would restrict the
+		// shift count range by half (from all uint values to
+		// the positive int values).
+		// This depends on the exact spec wording which is not
+		// done yet.
+		// TODO(gri) revisit and adjust when spec change is done
+		_ = 1<<- /* ERROR "negative shift count" */ 1.0
+		_ = 1<<1075 /* ERROR "invalid shift" */
+		_ = 2.0<<1
+		_ = 1<<1.0
+		_ = 1<<(1+0i)
+
+		_ int = 2<<s
+		_ float32 = 2<<s
+		_ complex64 = 2<<s
+
+		_ int = 2.0<<s
+		_ float32 = 2.0<<s
+		_ complex64 = 2.0<<s
+
+		_ int = 'a'<<s
+		_ float32 = 'a'<<s
+		_ complex64 = 'a'<<s
+	)
+}
+
+func shifts1() {
+	// basic non-constant shifts
+	var (
+		i int
+		u uint
+
+		_ = 1<<0
+		_ = 1<<i
+		_ = 1<<u
+		_ = 1<<"foo" /* ERROR "cannot convert" */
+		_ = i<<0
+		_ = i<<- /* ERROR "negative shift count" */ 1
+		_ = i<<1.0
+		_ = 1<<(1+0i)
+		_ = 1 /* ERROR "overflows" */ <<100
+
+		_ uint = 1 << 0
+		_ uint = 1 << u
+		_ float32 = 1 /* ERROR "must be integer" */ << u
+
+		// issue #14822
+		_ = 1<<( /* ERROR "overflows uint" */ 1<<64)
+		_ = 1<<( /* ERROR "invalid shift count" */ 1<<64-1)
+
+		// issue #43697
+		_ = u<<( /* ERROR "overflows uint" */ 1<<64)
+		_ = u<<(1<<64-1)
+	)
+}
+
+func shifts2() {
+	// from the spec
+	var (
+		s uint = 33
+		i = 1<<s           // 1 has type int
+		j int32 = 1<<s     // 1 has type int32; j == 0
+		k = uint64(1<<s)   // 1 has type uint64; k == 1<<33
+		m int = 1.0<<s     // 1.0 has type int
+		n = 1.0<<s != i    // 1.0 has type int; n == false if ints are 32bits in size
+		o = 1<<s == 2<<s   // 1 and 2 have type int; o == true if ints are 32bits in size
+		p = 1<<s == 1<<33  // illegal if ints are 32bits in size: 1 has type int, but 1<<33 overflows int
+		u = 1.0 /* ERROR "must be integer" */ <<s         // illegal: 1.0 has type float64, cannot shift
+		u1 = 1.0 /* ERROR "must be integer" */ <<s != 0   // illegal: 1.0 has type float64, cannot shift
+		u2 = 1 /* ERROR "must be integer" */ <<s != 1.0   // illegal: 1 has type float64, cannot shift
+		v float32 = 1 /* ERROR "must be integer" */ <<s   // illegal: 1 has type float32, cannot shift
+		w int64 = 1.0<<33  // 1.0<<33 is a constant shift expression
+	)
+	_, _, _, _, _, _, _, _, _, _, _, _ = i, j, k, m, n, o, p, u, u1, u2, v, w
+}
+
+func shifts3(a int16, b float32) {
+	// random tests
+	var (
+		s uint = 11
+		u = 1 /* ERROR "must be integer" */ <<s + 1.0
+		v complex128 = 1 /* ERROR "must be integer" */ << s + 1.0 /* ERROR "must be integer" */ << s + 1
+	)
+	x := 1.0 /* ERROR "must be integer" */ <<s + 1
+	shifts3(1.0 << s, 1 /* ERROR "must be integer" */ >> s)
+	_, _, _ = u, v, x
+}
+
+func shifts4() {
+	// shifts in comparisons w/ untyped operands
+	var s uint
+
+	_ = 1<<s == 1
+	_ = 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s == 1.
+
+	_ = 1<<s + 1 == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1 == 1.
+	_ = 1 /* ERROR "integer" */ <<s + 1. == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1. == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1 == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1 == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1. == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1. == 1.
+
+	_ = 1<<s == 1<<s
+	_ = 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s
+
+	_ = 1<<s + 1<<s == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1.
+
+	_ = 1<<s + 1<<s == 1<<s + 1<<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+}
+
+func shifts5() {
+	// shifts in comparisons w/ typed operands
+	var s uint
+	var x int
+
+	_ = 1<<s == x
+	_ = 1.<<s == x
+	_ = 1.1 /* ERROR "int" */ <<s == x
+
+	_ = 1<<s + x == 1
+	_ = 1<<s + x == 1.
+	_ = 1<<s + x == 1.1 /* ERROR "int" */
+	_ = 1.<<s + x == 1
+	_ = 1.<<s + x == 1.
+	_ = 1.<<s + x == 1.1 /* ERROR "int" */
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1.
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1.1
+
+	_ = 1<<s == x<<s
+	_ = 1.<<s == x<<s
+	_ = 1.1  /* ERROR "int" */ <<s == x<<s
+}
+
+func shifts6() {
+	// shifts as operands in non-arithmetic operations and as arguments
+	var a [10]int
+	var s uint
+
+	_ = a[1<<s]
+	_ = a[1.0]
+	_ = a[1.0<<s]
+
+	_ = make([]int, 1.0)
+	_ = make([]int, 1.0<<s)
+	_ = make([]int, 1.1 /* ERROR "must be integer" */ <<s)
+
+	_ = float32(1)
+	_ = float32(1 /* ERROR "must be integer" */ <<s)
+	_ = float32(1.0)
+	_ = float32(1.0 /* ERROR "must be integer" */ <<s)
+	_ = float32(1.1 /* ERROR "must be integer" */ <<s)
+
+	// TODO(gri) Re-enable these tests once types2 has the go/types fixes.
+	//           Issue #52080.
+	// _ = int32(0x80000000 /* ERROR "overflows int32" */ << s)
+	// TODO(rfindley) Eliminate the redundant error here.
+	// _ = int32(( /* ERROR "truncated to int32" */ 0x80000000 /* ERROR "truncated to int32" */ + 0i) << s)
+
+	_ = int(1+0i<<0)
+	// _ = int((1+0i)<<s)
+	// _ = int(1.0<<s)
+	// _ = int(complex(1, 0)<<s)
+	_ = int(float32/* ERROR "must be integer" */(1.0) <<s)
+	_ = int(1.1 /* ERROR must be integer */ <<s)
+	_ = int(( /* ERROR "must be integer" */ 1+1i)  <<s)
+
+	_ = complex(1 /* ERROR "must be integer" */ <<s, 0)
+
+	var b []int
+	_ = append(b, 1<<s)
+	_ = append(b, 1.0<<s)
+	_ = append(b, (1+0i)<<s)
+	_ = append(b, 1.1 /* ERROR "must be integer" */ <<s)
+	_ = append(b, (1 + 0i) <<s)
+	_ = append(b, ( /* ERROR "must be integer" */ 1 + 1i)  <<s)
+
+	_ = complex(1.0 /* ERROR "must be integer" */ <<s, 0)
+	_ = complex(1.1 /* ERROR "must be integer" */ <<s, 0)
+	_ = complex(0, 1.0 /* ERROR "must be integer" */ <<s)
+	_ = complex(0, 1.1 /* ERROR "must be integer" */ <<s)
+
+	// TODO(gri) The delete below is not type-checked correctly yet.
+	// var m1 map[int]string
+	// delete(m1, 1<<s)
+}
+
+func shifts7() {
+	// shifts of shifts
+	var s uint
+	var x int
+	_ = x
+
+	_ = 1<<(1<<s)
+	_ = 1<<(1.<<s)
+	_ = 1. /* ERROR "integer" */ <<(1<<s)
+	_ = 1. /* ERROR "integer" */ <<(1.<<s)
+
+	x = 1<<(1<<s)
+	x = 1<<(1.<<s)
+	x = 1.<<(1<<s)
+	x = 1.<<(1.<<s)
+
+	_ = (1<<s)<<(1<<s)
+	_ = (1<<s)<<(1.<<s)
+	_ = ( /* ERROR "integer" */ 1.<<s)<<(1<<s)
+	_ = ( /* ERROR "integer" */ 1.<<s)<<(1.<<s)
+
+	x = (1<<s)<<(1<<s)
+	x = (1<<s)<<(1.<<s)
+	x = ( /* ERROR "integer" */ 1.<<s)<<(1<<s)
+	x = ( /* ERROR "integer" */ 1.<<s)<<(1.<<s)
+}
+
+func shifts8() {
+	// shift examples from shift discussion: better error messages
+	var s uint
+	_ = 1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s == 1
+	_ = 1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s == 1.0
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s == 1.0
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1.0 == 1
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1.1 == 1
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1 == 1.0
+
+	// additional cases
+	_ = complex(1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s, 1)
+	_ = complex(1.0, 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s)
+
+	_ = int(1.<<s)
+	_ = int(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1. /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	// TODO(gri) the error messages for these two are incorrect - disabled for now
+	// _ = complex64(1<<s)
+	// _ = complex64(1.<<s)
+	_ = complex64(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+}
+
+func shifts9() {
+	// various originally failing snippets of code from the std library
+	// from src/compress/lzw/reader.go:90
+	{
+		var d struct {
+			bits     uint32
+			width    uint
+		}
+		_ = uint16(d.bits & (1<<d.width - 1))
+	}
+
+	// from src/debug/dwarf/buf.go:116
+	{
+		var ux uint64
+		var bits uint
+		x := int64(ux)
+		if x&(1<<(bits-1)) != 0 {}
+	}
+
+	// from src/encoding/asn1/asn1.go:160
+	{
+		var bytes []byte
+		if bytes[len(bytes)-1]&((1<<bytes[0])-1) != 0 {}
+	}
+
+	// from src/math/big/rat.go:140
+	{
+		var exp int
+		var mantissa uint64
+		shift := uint64(-1022 - (exp - 1)) // [1..53)
+		_ = mantissa & (1<<shift - 1)
+	}
+
+	// from src/net/interface.go:51
+	{
+		type Flags uint
+		var f Flags
+		var i int
+		if f&(1<<uint(i)) != 0 {}
+	}
+
+	// from src/runtime/softfloat64.go:234
+	{
+		var gm uint64
+		var shift uint
+		_ = gm & (1<<shift - 1)
+	}
+
+	// from src/strconv/atof.go:326
+	{
+		var mant uint64
+		var mantbits uint
+		if mant == 2<<mantbits {}
+	}
+
+	// from src/route_bsd.go:82
+	{
+		var Addrs int32
+		const rtaRtMask = 1
+		var i uint
+		if Addrs&rtaRtMask&(1<<i) == 0 {}
+	}
+
+	// from src/text/scanner/scanner.go:540
+	{
+		var s struct { Whitespace uint64 }
+		var ch rune
+		for s.Whitespace&(1<<uint(ch)) != 0 {}
+	}
+}
+
+func issue5895() {
+	var x = 'a' << 1 // type of x must be rune
+	var _ rune = x
+}
+
+func issue11325() {
+	var _ = 0 >> 1.1 /* ERROR "truncated to uint" */ // example from issue 11325
+	_ = 0 >> 1.1 /* ERROR "truncated to uint" */
+	_ = 0 << 1.1 /* ERROR "truncated to uint" */
+	_ = 0 >> 1.
+	_ = 1 >> 1.1 /* ERROR "truncated to uint" */
+	_ = 1 >> 1.
+	_ = 1. >> 1
+	_ = 1. >> 1.
+	_ = 1.1 /* ERROR "must be integer" */ >> 1
+}
+
+func issue11594() {
+	var _ = complex64 /* ERROR "must be integer" */ (1) << 2 // example from issue 11594
+	_ = float32 /* ERROR "must be integer" */ (0) << 1
+	_ = float64 /* ERROR "must be integer" */ (0) >> 2
+	_ = complex64 /* ERROR "must be integer" */ (0) << 3
+	_ = complex64 /* ERROR "must be integer" */ (0) >> 4
+}
+
+func issue21727() {
+	var s uint
+	var a = make([]int, 1<<s + 1.2 /* ERROR "truncated to int" */ )
+	var _ = a[1<<s - 2.3 /* ERROR "truncated to int" */ ]
+	var _ int = 1<<s + 3.4 /* ERROR "truncated to int" */
+	var _ = string(1 /* ERROR shifted operand 1 .* must be integer */ << s)
+	var _ = string(1.0 /* ERROR "cannot convert" */ << s)
+}
+
+func issue22969() {
+	var s uint
+	var a []byte
+	_ = a[0xffffffffffffffff /* ERROR "overflows int" */ <<s] // example from issue 22969
+	_ = make([]int, 0xffffffffffffffff /* ERROR "overflows int" */ << s)
+	_ = make([]int, 0, 0xffffffffffffffff /* ERROR "overflows int" */ << s)
+	var _ byte = 0x100 /* ERROR "overflows byte" */ << s
+	var _ int8 = 0xff /* ERROR "overflows int8" */ << s
+	var _ int16 = 0xffff /* ERROR "overflows int16" */ << s
+	var _ int32 = 0x80000000 /* ERROR "overflows int32" */ << s
+}
diff --git a/src/internal/types/testdata/check/slices.go b/src/internal/types/testdata/check/slices.go
new file mode 100644
index 0000000..2bacd1c
--- /dev/null
+++ b/src/internal/types/testdata/check/slices.go
@@ -0,0 +1,68 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package slices implements various slice algorithms.
+package slices
+
+// Map turns a []T1 to a []T2 using a mapping function.
+func Map[T1, T2 any](s []T1, f func(T1) T2) []T2 {
+	r := make([]T2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// Reduce reduces a []T1 to a single value using a reduction function.
+func Reduce[T1, T2 any](s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// Filter filters values from a slice using a filter function.
+func Filter[T any](s []T, f func(T) bool) []T {
+	var r []T
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
+
+// Example uses
+
+func limiter(x int) byte {
+	switch {
+	case x < 0:
+		return 0
+	default:
+		return byte(x)
+	case x > 255:
+		return 255
+	}
+}
+
+var input = []int{-4, 68954, 7, 44, 0, -555, 6945}
+var limited1 = Map[int, byte](input, limiter)
+var limited2 = Map(input, limiter) // using type inference
+
+func reducer(x float64, y int) float64 {
+	return x + float64(y)
+}
+
+var reduced1 = Reduce[int, float64](input, 0, reducer)
+var reduced2 = Reduce(input, 1i /* ERROR overflows */, reducer) // using type inference
+var reduced3 = Reduce(input, 1, reducer) // using type inference
+
+func filter(x int) bool {
+	return x&1 != 0
+}
+
+var filtered1 = Filter[int](input, filter)
+var filtered2 = Filter(input, filter) // using type inference
+
diff --git a/src/internal/types/testdata/check/stmt0.go b/src/internal/types/testdata/check/stmt0.go
new file mode 100644
index 0000000..c456aac
--- /dev/null
+++ b/src/internal/types/testdata/check/stmt0.go
@@ -0,0 +1,994 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// statements
+
+package stmt0
+
+func assignments0() (int, int) {
+	var a, b, c int
+	var ch chan int
+	f0 := func() {}
+	f1 := func() int { return 1 }
+	f2 := func() (int, int) { return 1, 2 }
+	f3 := func() (int, int, int) { return 1, 2, 3 }
+
+	a, b, c = 1, 2, 3
+	a, b, c = 1 /* ERROR "assignment mismatch: 3 variables but 2 values" */ , 2
+	a, b, c = 1 /* ERROR "assignment mismatch: 3 variables but 4 values" */ , 2, 3, 4
+	_, _, _ = a, b, c
+
+	a = f0 /* ERROR "used as value" */ ()
+	a = f1()
+	a = f2 /* ERROR "assignment mismatch: 1 variable but f2 returns 2 values" */ ()
+	a, b = f2()
+	a, b, c = f2 /* ERROR "assignment mismatch: 3 variables but f2 returns 2 values" */ ()
+	a, b, c = f3()
+	a, b = f3 /* ERROR "assignment mismatch: 2 variables but f3 returns 3 values" */ ()
+
+	a, b, c = <- /* ERROR "assignment mismatch: 3 variables but 1 value" */ ch
+
+	return /* ERROR "not enough return values\n\thave \(\)\n\twant \(int, int\)" */
+	return 1 /* ERROR "not enough return values\n\thave \(number\)\n\twant \(int, int\)" */
+	return 1, 2
+	return 1, 2, 3 /* ERROR "too many return values\n\thave \(number, number, number\)\n\twant \(int, int\)" */
+}
+
+func assignments1() {
+	b, i, f, c, s := false, 1, 1.0, 1i, "foo"
+	b = i /* ERROR "cannot use .* in assignment" */
+	i = f /* ERROR "cannot use .* in assignment" */
+	f = c /* ERROR "cannot use .* in assignment" */
+	c = s /* ERROR "cannot use .* in assignment" */
+	s = b /* ERROR "cannot use .* in assignment" */
+
+	v0, v1, v2 := 1 /* ERROR "assignment mismatch" */ , 2, 3, 4
+	_, _, _ = v0, v1, v2
+
+	b = true
+
+	i += 1
+	i /* ERROR "mismatched types int and untyped string" */+= "foo"
+
+	f -= 1
+	f /= 0
+	f = float32(0)/0 /* ERROR "division by zero" */
+	f /* ERROR "mismatched types float64 and untyped string" */-= "foo"
+
+	c *= 1
+	c /= 0
+
+	s += "bar"
+	s /* ERROR "mismatched types string and untyped int" */+= 1
+
+	var u64 uint64
+	u64 += 1<<u64
+
+	undefined /* ERROR "undefined" */ = 991
+
+	// test cases for issue 5800
+	var (
+		_ int = nil /* ERROR "cannot use nil as int value in variable declaration" */
+		_ [10]int = nil /* ERROR "cannot use nil as \[10\]int value in variable declaration" */
+		_ []byte = nil
+		_ struct{} = nil /* ERROR "cannot use nil as struct{} value in variable declaration" */
+		_ func() = nil
+		_ map[int]string = nil
+		_ chan int = nil
+	)
+
+	// test cases for issue 5500
+	_ = func() (int, bool) {
+		var m map[int]int
+		return m /* ERROR "not enough return values" */ [0]
+	}
+
+	g := func(int, bool){}
+	var m map[int]int
+	g(m[0]) /* ERROR "not enough arguments" */
+
+	// assignments to _
+	_ = nil /* ERROR "use of untyped nil" */
+	_ = 1  << /* ERROR constant shift overflow */ 1000
+	(_) = 0
+}
+
+func assignments2() {
+	type mybool bool
+	var m map[string][]bool
+	var s []bool
+	var b bool
+	var d mybool
+	_ = s
+	_ = b
+	_ = d
+
+	// assignments to map index expressions are ok
+	s, b = m["foo"]
+	_, d = m["bar"]
+	m["foo"] = nil
+	m["foo"] = nil /* ERROR assignment mismatch: 1 variable but 2 values */ , false
+	_ = append(m["foo"])
+	_ = append(m["foo"], true)
+
+	var c chan int
+	_, b = <-c
+	_, d = <-c
+	<- /* ERROR cannot assign */ c = 0
+	<-c = 0 /* ERROR assignment mismatch: 1 variable but 2 values */ , false
+
+	var x interface{}
+	_, b = x.(int)
+	x /* ERROR cannot assign */ .(int) = 0
+	x.(int) = 0 /* ERROR assignment mismatch: 1 variable but 2 values */ , false
+
+	assignments2 /* ERROR used as value */ () = nil
+	int /* ERROR not an expression */ = 0
+}
+
+func issue6487() {
+	type S struct{x int}
+	_ = &S /* ERROR "cannot take address" */ {}.x
+	_ = &( /* ERROR "cannot take address" */ S{}.x)
+	_ = (&S{}).x
+	S /* ERROR "cannot assign" */ {}.x = 0
+	(&S{}).x = 0
+
+	type M map[string]S
+	var m M
+	m /* ERROR "cannot assign to struct field" */ ["foo"].x = 0
+	_ = &( /* ERROR "cannot take address" */ m["foo"].x)
+	_ = &m /* ERROR "cannot take address" */ ["foo"].x
+}
+
+func issue6766a() {
+	a, a /* ERROR a repeated on left side of := */ := 1, 2
+	_ = a
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
+	_ = b
+	c, c /* ERROR c repeated on left side of := */, b := 1, 2, 3
+	_ = c
+	a, b := /* ERROR no new variables */ 1, 2
+}
+
+func shortVarDecls1() {
+	const c = 0
+	type d int
+	a, b, c /* ERROR "cannot assign" */ , d /* ERROR "cannot assign" */  := 1, "zwei", 3.0, 4
+	var _ int = a // a is of type int
+	var _ string = b // b is of type string
+}
+
+func incdecs() {
+	const c = 3.14
+	c /* ERROR "cannot assign" */ ++
+	s := "foo"
+	s /* ERROR "invalid operation" */ --
+	3.14 /* ERROR "cannot assign" */ ++
+	var (
+		x int
+		y float32
+		z complex128
+	)
+	x++
+	y--
+	z++
+}
+
+func sends() {
+	var ch chan int
+	var rch <-chan int
+	var x int
+	x <- /* ERROR "cannot send" */ x
+	rch <- /* ERROR "cannot send" */ x
+	ch <- "foo" /* ERROR "cannot use .* in send" */
+	ch <- x
+}
+
+func selects() {
+	select {}
+	var (
+		ch chan int
+		sc chan <- bool
+	)
+	select {
+	case <-ch:
+	case (<-ch):
+	case t := <-ch:
+		_ = t
+	case t := (<-ch):
+		_ = t
+	case t, ok := <-ch:
+		_, _ = t, ok
+	case t, ok := (<-ch):
+		_, _ = t, ok
+	case <-sc /* ERROR "cannot receive from send-only channel" */ :
+	}
+	select {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+	select {
+	case a, b := <-ch:
+		_, b = a, b
+	case x /* ERROR send or receive */ :
+	case a /* ERROR send or receive */ := ch:
+	}
+
+	// test for issue 9570: ch2 in second case falsely resolved to
+	// ch2 declared in body of first case
+	ch1 := make(chan int)
+	ch2 := make(chan int)
+	select {
+	case <-ch1:
+		var ch2 /* ERROR ch2 declared and not used */ chan bool
+	case i := <-ch2:
+		print(i + 1)
+	}
+}
+
+func gos() {
+	go 1; /* ERROR "must be function call" */
+	go int /* ERROR "go requires function call, not conversion" */ (0)
+	go ( /* ERROR expression in go must not be parenthesized */ gos())
+	go gos()
+	var c chan int
+	go close(c)
+	go len /* ERROR "go discards result" */ (c)
+}
+
+func defers() {
+	defer 1; /* ERROR "must be function call" */
+	defer int /* ERROR "defer requires function call, not conversion" */ (0)
+	defer ( /* ERROR expression in defer must not be parenthesized */ defers())
+	defer defers()
+	var c chan int
+	defer close(c)
+	defer len /* ERROR "defer discards result" */ (c)
+}
+
+func breaks() {
+	var x, y int
+
+	break /* ERROR "break" */
+	{
+		break /* ERROR "break" */
+	}
+	if x < y {
+		break /* ERROR "break" */
+	}
+
+	switch x {
+	case 0:
+		break
+	case 1:
+		if x == y {
+			break
+		}
+	default:
+		break
+		break
+	}
+
+	var z interface{}
+	switch z.(type) {
+	case int:
+		break
+	}
+
+	for {
+		break
+	}
+
+	var a []int
+	for _ = range a {
+		break
+	}
+
+	for {
+		if x == y {
+			break
+		}
+	}
+
+	var ch chan int
+	select {
+	case <-ch:
+		break
+	}
+
+	select {
+	case <-ch:
+		if x == y {
+			break
+		}
+	default:
+		break
+	}
+}
+
+func continues() {
+	var x, y int
+
+	continue /* ERROR "continue" */
+	{
+		continue /* ERROR "continue" */
+	}
+
+	if x < y {
+		continue /* ERROR "continue" */
+	}
+
+	switch x {
+	case 0:
+		continue /* ERROR "continue" */
+	}
+
+	var z interface{}
+	switch z.(type) {
+	case int:
+		continue /* ERROR "continue" */
+	}
+
+	var ch chan int
+	select {
+	case <-ch:
+		continue /* ERROR "continue" */
+	}
+
+	for i := 0; i < 10; i++ {
+		continue
+		if x < y {
+			continue
+			break
+		}
+		switch x {
+		case y:
+			continue
+		default:
+			break
+		}
+		select {
+		case <-ch:
+			continue
+		}
+	}
+
+	var a []int
+	for _ = range a {
+		continue
+		if x < y {
+			continue
+			break
+		}
+		switch x {
+		case y:
+			continue
+		default:
+			break
+		}
+		select {
+		case <-ch:
+			continue
+		}
+	}
+}
+
+func returns0() {
+	return
+	return 0 /* ERROR too many return values */
+}
+
+func returns1(x float64) (int, *float64) {
+	return 0, &x
+	return /* ERROR not enough return values */
+	return "foo" /* ERROR "cannot .* in return statement" */, x /* ERROR "cannot use .* in return statement" */
+	return 0, &x, 1 /* ERROR too many return values */
+}
+
+func returns2() (a, b int) {
+	return
+	return 1, "foo" /* ERROR cannot use .* in return statement */
+	return 1, 2, 3 /* ERROR too many return values */
+	{
+		type a int
+		return 1, 2
+		return /* ERROR a not in scope at return */
+	}
+}
+
+func returns3() (_ int) {
+	return
+	{
+		var _ int // blank (_) identifiers never shadow since they are in no scope
+		return
+	}
+}
+
+func switches0() {
+	var x int
+
+	switch x {
+	}
+
+	switch x {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+
+	switch {
+	case 1  /* ERROR "cannot convert" */ :
+	}
+
+	true := "false"
+	_ = true
+	// A tagless switch is equivalent to the bool
+        // constant true, not the identifier 'true'.
+	switch {
+	case "false" /* ERROR "cannot convert" */:
+	}
+
+	switch int32(x) {
+	case 1, 2:
+	case x /* ERROR "invalid case x in switch on int32\(x\) \(mismatched types int and int32\)" */ :
+	}
+
+	switch x {
+	case 1 /* ERROR "overflows" */ << 100:
+	}
+
+	switch x {
+	case 1:
+	case 1 /* ERROR "duplicate case" */ :
+	case ( /* ERROR "duplicate case" */ 1):
+	case 2, 3, 4:
+	case 5, 1 /* ERROR "duplicate case" */ :
+	}
+
+	switch uint64(x) {
+	case 1<<64 - 1:
+	case 1 /* ERROR duplicate case */ <<64 - 1:
+	case 2, 3, 4:
+	case 5, 1 /* ERROR duplicate case */ <<64 - 1:
+	}
+
+	var y32 float32
+	switch y32 {
+	case 1.1:
+	case 11/10: // integer division!
+	case 11. /* ERROR duplicate case */ /10:
+	case 2, 3.0, 4.1:
+	case 5.2, 1.10 /* ERROR duplicate case */ :
+	}
+
+	var y64 float64
+	switch y64 {
+	case 1.1:
+	case 11/10: // integer division!
+	case 11. /* ERROR duplicate case */ /10:
+	case 2, 3.0, 4.1:
+	case 5.2, 1.10 /* ERROR duplicate case */ :
+	}
+
+	var s string
+	switch s {
+	case "foo":
+	case "foo" /* ERROR duplicate case */ :
+	case "f" /* ERROR duplicate case */ + "oo":
+	case "abc", "def", "ghi":
+	case "jkl", "foo" /* ERROR duplicate case */ :
+	}
+
+	type T int
+	type F float64
+	type S string
+	type B bool
+	var i interface{}
+	switch i {
+	case nil:
+	case nil: // no duplicate detection
+	case (*int)(nil):
+	case (*int)(nil): // do duplicate detection
+	case 1:
+	case byte(1):
+	case int /* ERROR duplicate case */ (1):
+	case T(1):
+	case 1.0:
+	case F(1.0):
+	case F /* ERROR duplicate case */ (1.0):
+	case "hello":
+	case S("hello"):
+	case S /* ERROR duplicate case */ ("hello"):
+	case 1==1, B(false):
+	case false, B(2==2):
+	}
+
+	// switch on array
+	var a [3]int
+	switch a {
+	case [3]int{1, 2, 3}:
+	case [3]int{1, 2, 3}: // no duplicate detection
+	case [ /* ERROR "mismatched types */ 4]int{4, 5, 6}:
+	}
+
+	// switch on channel
+	var c1, c2 chan int
+	switch c1 {
+	case nil:
+	case c1:
+	case c2:
+	case c1, c2: // no duplicate detection
+	}
+}
+
+func switches1() {
+	fallthrough /* ERROR "fallthrough statement out of place" */
+
+	var x int
+	switch x {
+	case 0:
+		fallthrough /* ERROR "fallthrough statement out of place" */
+		break
+	case 1:
+		fallthrough
+	case 2:
+		fallthrough; ; ; // trailing empty statements are ok
+	case 3:
+	default:
+		fallthrough; ;
+	case 4:
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	var y interface{}
+	switch y.(type) {
+	case int:
+		fallthrough /* ERROR "cannot fallthrough in type switch" */ ; ; ;
+	default:
+	}
+
+	switch x {
+	case 0:
+		if x == 0 {
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	}
+
+	switch x {
+	case 0:
+		goto L1
+		L1: fallthrough; ;
+	case 1:
+		goto L2
+		goto L3
+		goto L4
+		L2: L3: L4: fallthrough
+	default:
+	}
+
+	switch x {
+	case 0:
+		goto L5
+		L5: fallthrough
+	default:
+		goto L6
+		goto L7
+		goto L8
+		L6: L7: L8: fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	switch x {
+	case 0:
+		fallthrough; ;
+	case 1:
+		{
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	case 2:
+		fallthrough
+	case 3:
+		fallthrough /* ERROR "fallthrough statement out of place" */
+		{ /* empty block is not an empty statement */ }; ;
+	default:
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	switch x {
+	case 0:
+		{
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	}
+}
+
+func switches2() {
+	// untyped nil is not permitted as switch expression
+	switch nil /* ERROR "use of untyped nil" */ {
+	case 1, 2, "foo": // don't report additional errors here
+	}
+
+	// untyped constants are converted to default types
+	switch 1<<63-1 {
+	}
+	switch 1 /* ERROR "cannot use .* as int value.*\(overflows\)" */ << 63 {
+	}
+	var x int
+	switch 1.0 {
+	case 1.0, 2.0, x /* ERROR "mismatched types int and float64" */ :
+	}
+	switch x {
+	case 1.0:
+	}
+
+	// untyped bools become of type bool
+	type B bool
+	var b B = true
+	switch x == x {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+	switch {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+}
+
+func issue11667() {
+	switch 9223372036854775808 /* ERROR "cannot use .* as int value.*\(overflows\)" */ {
+	}
+	switch 9223372036854775808 /* ERROR "cannot use .* as int value.*\(overflows\)" */ {
+	case 9223372036854775808:
+	}
+	var x int
+	switch x {
+	case 9223372036854775808 /* ERROR "overflows int" */ :
+	}
+	var y float64
+	switch y {
+	case 9223372036854775808:
+	}
+}
+
+func issue11687() {
+	f := func() (_, _ int) { return }
+	switch f /* ERROR "multiple-value f" */ () {
+	}
+	var x int
+	switch f /* ERROR "multiple-value f" */ () {
+	case x:
+	}
+	switch x {
+	case f /* ERROR "multiple-value f" */ ():
+	}
+}
+
+type I interface {
+	m()
+}
+
+type I2 interface {
+	m(int)
+}
+
+type T struct{}
+type T1 struct{}
+type T2 struct{}
+
+func (T) m() {}
+func (T2) m(int) {}
+
+func typeswitches() {
+	var i int
+	var x interface{}
+
+	switch x.(type) {}
+	switch (x /* ERROR "outside type switch" */ .(type)) {}
+
+	switch x.(type) {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+
+	switch x /* ERROR "declared and not used" */ := x.(type) {}
+	switch _ /* ERROR "no new variable on left side of :=" */ := x.(type) {}
+
+	switch x := x.(type) {
+	case int:
+		var y int = x
+		_ = y
+	}
+
+	switch x /* ERROR "x declared and not used" */ := i /* ERROR "not an interface" */ .(type) {}
+
+	switch t := x.(type) {
+	case nil:
+		var v bool = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	case int:
+		var v int = t
+		_ = v
+	case float32, complex64:
+		var v float32 = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	default:
+		var v float32 = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	}
+
+	var t I
+	switch t.(type) {
+	case T:
+	case T1 /* ERROR "missing method m" */ :
+	case T2 /* ERROR "wrong type for method m" */ :
+	case I2 /* STRICT "wrong type for method m" */ : // only an error in strict mode (issue 8561)
+	}
+
+
+	{
+		x := 1
+		v := 2
+		switch v /* ERROR "v [(]variable of type int[)] is not an interface" */ .(type) {
+		case int:
+			println(x)
+			println(x / 0 /* ERROR "invalid operation: division by zero" */)
+		case 1 /* ERROR "1 is not a type" */:
+		}
+	}
+}
+
+// Test that each case clause uses the correct type of the variable
+// declared by the type switch (issue 5504).
+func typeswitch0() {
+	switch y := interface{}(nil).(type) {
+	case int:
+		func() int { return y + 0 }()
+	case float32:
+		func() float32 { return y }()
+	}
+}
+
+// Test correct scope setup.
+// (no redeclaration errors expected in the type switch)
+func typeswitch1() {
+	var t I
+	switch t := t; t := t.(type) {
+	case nil:
+		var _ I = t
+	case T:
+		var _ T = t
+	default:
+		var _ I = t
+	}
+}
+
+// Test correct typeswitch against interface types.
+type A interface { a() }
+type B interface { b() }
+type C interface { a(int) }
+
+func typeswitch2() {
+	switch A(nil).(type) {
+	case A:
+	case B:
+	case C /* STRICT "cannot have dynamic type" */: // only an error in strict mode (issue 8561)
+	}
+}
+
+func typeswitch3(x interface{}) {
+	switch x.(type) {
+	case int:
+	case float64:
+	case int /* ERROR duplicate case */ :
+	}
+
+	switch x.(type) {
+	case nil:
+	case int:
+	case nil /* ERROR duplicate case */ , nil /* ERROR duplicate case */ :
+	}
+
+	type F func(int)
+	switch x.(type) {
+	case nil:
+	case int, func(int):
+	case float32, func /* ERROR duplicate case */ (x int):
+	case F:
+	}
+}
+
+func fors1() {
+	for {}
+	var i string
+	_ = i
+	for i := 0; i < 10; i++ {}
+	for i := 0; i < 10; j /* ERROR cannot declare */ := 0 {}
+}
+
+func rangeloops1() {
+	var (
+		x int
+		a [10]float32
+		b []string
+		p *[10]complex128
+		pp **[10]complex128
+		s string
+		m map[int]bool
+		c chan int
+		sc chan<- int
+		rc <-chan int
+	)
+
+	for range x /* ERROR "cannot range over" */ {}
+	for _ = range x /* ERROR "cannot range over" */ {}
+	for i := range x /* ERROR "cannot range over" */ {}
+
+	for range a {}
+	for i := range a {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range a {
+		var ii int
+		ii = i
+		_ = ii
+		var xx float64
+		xx = x /* ERROR "cannot use .* in assignment" */
+		_ = xx
+	}
+	var ii int
+	var xx float32
+	for ii, xx = range a {}
+	_, _ = ii, xx
+
+	for range b {}
+	for i := range b {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range b {
+		var ii int
+		ii = i
+		_ = ii
+		var xx string
+		xx = x
+		_ = xx
+	}
+
+	for range s {}
+	for i := range s {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range s {
+		var ii int
+		ii = i
+		_ = ii
+		var xx rune
+		xx = x
+		_ = xx
+	}
+
+	for range p {}
+	for _, x := range p {
+		var xx complex128
+		xx = x
+		_ = xx
+	}
+
+	for range pp /* ERROR "cannot range over" */ {}
+	for _, x := range pp /* ERROR "cannot range over" */ {}
+
+	for range m {}
+	for k := range m {
+		var kk int32
+		kk = k /* ERROR "cannot use .* in assignment" */
+		_ = kk
+	}
+	for k, v := range m {
+		var kk int
+		kk = k
+		_ = kk
+		if v {}
+	}
+
+	for range c {}
+	for _, _ /* ERROR "only one iteration variable" */ = range c {}
+	for e := range c {
+		var ee int
+		ee = e
+		_ = ee
+	}
+	for _ = range sc /* ERROR "cannot range over" */ {}
+	for _ = range rc {}
+
+	// constant strings
+	const cs = "foo"
+	for range cs {}
+	for range "" {}
+	for i, x := range cs { _, _ = i, x }
+	for i, x := range "" {
+		var ii int
+		ii = i
+		_ = ii
+		var xx rune
+		xx = x
+		_ = xx
+	}
+}
+
+func rangeloops2() {
+	type I int
+	type R rune
+
+	var a [10]int
+	var i I
+	_ = i
+	for i /* ERROR cannot use .* in assignment */ = range a {}
+	for i /* ERROR cannot use .* in assignment */ = range &a {}
+	for i /* ERROR cannot use .* in assignment */ = range a[:] {}
+
+	var s string
+	var r R
+	_ = r
+	for i /* ERROR cannot use .* in assignment */ = range s {}
+	for i /* ERROR cannot use .* in assignment */ = range "foo" {}
+	for _, r /* ERROR cannot use .* in assignment */ = range s {}
+	for _, r /* ERROR cannot use .* in assignment */ = range "foo" {}
+}
+
+func issue6766b() {
+	for _ := /* ERROR no new variables */ range "" {}
+	for a, a /* ERROR redeclared */ := range "" { _ = a }
+	var a int
+	_ = a
+	for a, a /* ERROR redeclared */ := range []int{1, 2, 3} { _ = a }
+}
+
+// Test that despite errors in the range clause,
+// the loop body is still type-checked (and thus
+// errors reported).
+func issue10148() {
+	for y /* ERROR declared and not used */ := range "" {
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
+	}
+	for range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
+	}
+	for y := range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
+	}
+}
+
+func labels0() {
+	goto L0
+	goto L1
+	L0:
+	L1:
+	L1 /* ERROR "already declared" */ :
+	if true {
+		goto L2
+		L2:
+		L0 /* ERROR "already declared" */ :
+	}
+	_ = func() {
+		goto L0
+		goto L1
+		goto L2
+		L0:
+		L1:
+		L2:
+	}
+}
+
+func expression_statements(ch chan int) {
+	expression_statements(ch)
+	<-ch
+	println()
+
+	0 /* ERROR "not used" */
+	1 /* ERROR "not used" */ +2
+	cap /* ERROR "not used" */ (ch)
+	println /* ERROR "must be called" */
+}
diff --git a/src/internal/types/testdata/check/stmt1.go b/src/internal/types/testdata/check/stmt1.go
new file mode 100644
index 0000000..f79f920
--- /dev/null
+++ b/src/internal/types/testdata/check/stmt1.go
@@ -0,0 +1,259 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// terminating statements
+
+package stmt1
+
+func _() {}
+
+func _() int {} /* ERROR "missing return" */
+
+func _() int { panic(0) }
+func _() int { (panic(0)) }
+
+// block statements
+func _(x, y int) (z int) {
+	{
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	{
+		return; ; ; // trailing empty statements are ok
+	}
+	; ; ;
+}
+
+func _(x, y int) (z int) {
+	{
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	{
+		; ; ;
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+// if statements
+func _(x, y int) (z int) {
+	if x < y { return }
+	return 1
+}
+
+func _(x, y int) (z int) {
+	if x < y { return; ; ; ; }
+	return 1
+}
+
+func _(x, y int) (z int) {
+	if x < y { return }
+	return 1; ;
+}
+
+func _(x, y int) (z int) {
+	if x < y { return }
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	if x < y {
+	} else { return 1
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	if x < y { return
+	} else { return
+	}
+}
+
+// for statements
+func _(x, y int) (z int) {
+	for x < y {
+		return
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	for {
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		return; ; ; ;
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		return
+		break
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	for {
+		for { break }
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		for { break }
+		return ; ;
+	}
+	;
+}
+
+func _(x, y int) (z int) {
+L:	for {
+		for { break L }
+		return
+	}
+} /* ERROR "missing return" */
+
+// switch statements
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default: return
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return;
+	default: return; ; ;
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	case 1: break
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break
+		}
+		panic(0)
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break
+		}
+		panic(0); ; ;
+	}
+	;
+}
+
+func _(x, y int) (z int) {
+L:	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break L
+		}
+		panic(0)
+	}
+} /* ERROR "missing return" */
+
+// select statements
+func _(ch chan int) (z int) {
+	select {}
+} // nice!
+
+func _(ch chan int) (z int) {
+	select {}
+	; ;
+}
+
+func _(ch chan int) (z int) {
+	select {
+	default: break
+	}
+} /* ERROR "missing return" */
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return
+	default: break
+	}
+} /* ERROR "missing return" */
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return
+	default:
+		for i := 0; i < 10; i++ {
+			break
+		}
+		return
+	}
+}
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return; ; ;
+	default:
+		for i := 0; i < 10; i++ {
+			break
+		}
+		return; ; ;
+	}
+	; ; ;
+}
+
+func _(ch chan int) (z int) {
+L:	select {
+	case <-ch: return
+	default:
+		for i := 0; i < 10; i++ {
+			break L
+		}
+		return
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+func parenPanic() int {
+	((((((panic)))(0))))
+}
+
+func issue23218a() int {
+	{
+		panic := func(interface{}){}
+		panic(0)
+	}
+} /* ERROR "missing return" */
+
+func issue23218b() int {
+	{
+		panic := func(interface{}){}
+		((((panic))))(0)
+	}
+} /* ERROR "missing return" */
diff --git a/src/internal/types/testdata/check/typeinference.go b/src/internal/types/testdata/check/typeinference.go
new file mode 100644
index 0000000..28f3e28
--- /dev/null
+++ b/src/internal/types/testdata/check/typeinference.go
@@ -0,0 +1,49 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package typeInference
+
+// As of issue #51527, type-type inference has been disabled.
+
+// basic inference
+type Tb[P ~*Q, Q any] int
+func _() {
+	var x Tb /* ERROR got 1 arguments */ [*int]
+	var y Tb[*int, int]
+	x = y /* ERROR cannot use y .* in assignment */
+	_ = x
+}
+
+// recursive inference
+type Tr[A any, B *C, C *D, D *A] int
+func _() {
+	var x Tr /* ERROR got 1 arguments */ [string]
+	var y Tr[string, ***string, **string, *string]
+	var z Tr[int, ***int, **int, *int]
+	x = y /* ERROR cannot use y .* in assignment */
+	x = z // ERROR cannot use z .* as Tr
+	_ = x
+}
+
+// other patterns of inference
+type To0[A any, B []A] int
+type To1[A any, B struct{a A}] int
+type To2[A any, B [][]A] int
+type To3[A any, B [3]*A] int
+type To4[A any, B any, C struct{a A; b B}] int
+func _() {
+	var _ To0 /* ERROR got 1 arguments */ [int]
+	var _ To1 /* ERROR got 1 arguments */ [int]
+	var _ To2 /* ERROR got 1 arguments */ [int]
+	var _ To3 /* ERROR got 1 arguments */ [int]
+	var _ To4 /* ERROR got 2 arguments */ [int, string]
+}
+
+// failed inference
+type Tf0[A, B any] int
+type Tf1[A any, B ~struct{a A; c C}, C any] int
+func _() {
+	var _ Tf0 /* ERROR got 1 arguments but 2 type parameters */ [int]
+	var _ Tf1 /* ERROR got 1 arguments but 3 type parameters */ [int]
+}
diff --git a/src/internal/types/testdata/check/typeinst0.go b/src/internal/types/testdata/check/typeinst0.go
new file mode 100644
index 0000000..c21cb53
--- /dev/null
+++ b/src/internal/types/testdata/check/typeinst0.go
@@ -0,0 +1,62 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type myInt int
+
+// Parameterized type declarations
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+type T1[P any] P // ERROR cannot use a type parameter as RHS in type declaration
+
+type T2[P any] struct {
+        f P
+        g int // int should still be in scope chain
+}
+
+type List[P any] []P
+
+// Alias type declarations cannot have type parameters.
+// Issue #46477 proposses to change that.
+type A1[P any] = /* ERROR cannot be alias */ struct{}
+
+// Pending clarification of #46477 we disallow aliases
+// of generic types.
+type A2 = List // ERROR cannot use generic type
+var _ A2[int]
+var _ A2
+
+type A3 = List[int]
+var _ A3
+
+// Parameterized type instantiations
+
+var x int
+type _ x /* ERROR not a type */ [int]
+
+type _ int /* ERROR not a generic type */ [] // ERROR expected type argument list
+type _ myInt /* ERROR not a generic type */ [] // ERROR expected type argument list
+
+// TODO(gri) better error messages
+type _ T1[] // ERROR expected type argument list
+type _ T1[x /* ERROR not a type */ ]
+type _ T1 /* ERROR got 2 arguments but 1 type parameters */ [int, float32]
+
+var _ T2[int] = T2[int]{}
+
+var _ List[int] = []int{1, 2, 3}
+var _ List[[]int] = [][]int{{1, 2, 3}}
+var _ List[List[List[int]]]
+
+// Parameterized types containing parameterized types
+
+type T3[P any] List[P]
+
+var _ T3[int] = T3[int](List[int]{1, 2, 3})
+
+// Self-recursive generic types are not permitted
+
+type self1[P any] self1 /* ERROR invalid recursive type */ [P]
+type self2[P any] *self2[P] // this is ok
diff --git a/src/internal/types/testdata/check/typeinst1.go b/src/internal/types/testdata/check/typeinst1.go
new file mode 100644
index 0000000..e7bb247
--- /dev/null
+++ b/src/internal/types/testdata/check/typeinst1.go
@@ -0,0 +1,282 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type List[E any] []E
+var _ List[List[List[int]]]
+var _ List[List[List[int]]] = []List[List[int]]{}
+
+type (
+	T1[P1 any] struct {
+		f1 T2[P1, float32]
+	}
+
+	T2[P2, P3 any] struct {
+		f2 P2
+		f3 P3
+	}
+)
+
+func _() {
+	var x1 T1[int]
+	var x2 T2[int, float32]
+
+	x1.f1.f2 = 0
+	x1.f1 = x2
+}
+
+type T3[P any] T1[T2[P, P]]
+
+func _() {
+	var x1 T3[int]
+	var x2 T2[int, int]
+	x1.f1.f2 = x2
+}
+
+func f[P any] (x P) List[P] {
+	return List[P]{x}
+}
+
+var (
+	_ []int = f(0)
+	_ []float32 = f[float32](10)
+	_ List[complex128] = f(1i)
+	_ []List[int] = f(List[int]{})
+        _ List[List[int]] = []List[int]{}
+        _ = []List[int]{}
+)
+
+// Parameterized types with methods
+
+func (l List[E]) Head() (_ E, _ bool) {
+	if len(l) > 0 {
+		return l[0], true
+	}
+	return
+}
+
+// A test case for instantiating types with other types (extracted from map.go2)
+
+type Pair[K any] struct {
+	key K
+}
+
+type Receiver[T any] struct {
+	values T
+}
+
+type Iterator[K any] struct {
+	r Receiver[Pair[K]]
+}
+
+func Values [T any] (r Receiver[T]) T {
+        return r.values
+}
+
+func (it Iterator[K]) Next() K {
+        return Values[Pair[K]](it.r).key
+}
+
+// A more complex test case testing type bounds (extracted from linalg.go2 and reduced to essence)
+
+type NumericAbs[T any] interface {
+	Abs() T
+}
+
+func AbsDifference[T NumericAbs[T]](x T) { panic(0) }
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type OrderedAbs[T any] T
+// 
+// func (a OrderedAbs[T]) Abs() OrderedAbs[T]
+// 
+// func OrderedAbsDifference[T any](x T) {
+// 	AbsDifference(OrderedAbs[T](x))
+// }
+
+// same code, reduced to essence
+
+func g[P interface{ m() P }](x P) { panic(0) }
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T4[P any] P
+// 
+// func (_ T4[P]) m() T4[P]
+// 
+// func _[Q any](x Q) {
+// 	g(T4[Q](x))
+// }
+
+// Another test case that caused  problems in the past
+
+type T5[_ interface { a() }, _ interface{}] struct{}
+
+type A[P any] struct{ x P }
+
+func (_ A[P]) a() {}
+
+var _ T5[A[int], int]
+
+// Invoking methods with parameterized receiver types uses
+// type inference to determine the actual type arguments matching
+// the receiver type parameters from the actual receiver argument.
+// Go does implicit address-taking and dereferenciation depending
+// on the actual receiver and the method's receiver type. To make
+// type inference work, the type-checker matches "pointer-ness"
+// of the actual receiver and the method's receiver type.
+// The following code tests this mechanism.
+
+type R1[A any] struct{}
+func (_ R1[A]) vm()
+func (_ *R1[A]) pm()
+
+func _[T any](r R1[T], p *R1[T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+type R2[A, B any] struct{}
+func (_ R2[A, B]) vm()
+func (_ *R2[A, B]) pm()
+
+func _[T any](r R2[T, int], p *R2[string, T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+// It is ok to have multiple embedded unions.
+type _ interface {
+	m0()
+	~int | ~string | ~bool
+	~float32 | ~float64
+	m1()
+	m2()
+	~complex64 | ~complex128
+	~rune
+}
+
+// Type sets may contain each type at most once.
+type _ interface {
+	~int|~ /* ERROR overlapping terms ~int */ int
+	~int|int /* ERROR overlapping terms int */
+	int|int /* ERROR overlapping terms int */
+}
+
+type _ interface {
+	~struct{f int} | ~struct{g int} | ~ /* ERROR overlapping terms */ struct{f int}
+}
+
+// Interface term lists can contain any type, incl. *Named types.
+// Verify that we use the underlying type(s) of the type(s) in the
+// term list when determining if an operation is permitted.
+
+type MyInt int
+func add1[T interface{MyInt}](x T) T {
+	return x + 1
+}
+
+type MyString string
+func double[T interface{MyInt|MyString}](x T) T {
+	return x + x
+}
+
+// Embedding of interfaces with term lists leads to interfaces
+// with term lists that are the intersection of the embedded
+// term lists.
+
+type E0 interface {
+	~int | ~bool | ~string
+}
+
+type E1 interface {
+	~int | ~float64 | ~string
+}
+
+type E2 interface {
+	~float64
+}
+
+type I0 interface {
+	E0
+}
+
+func f0[T I0]() {}
+var _ = f0[int]
+var _ = f0[bool]
+var _ = f0[string]
+var _ = f0[float64 /* ERROR does not satisfy I0 */ ]
+
+type I01 interface {
+	E0
+	E1
+}
+
+func f01[T I01]() {}
+var _ = f01[int]
+var _ = f01[bool /* ERROR does not satisfy I0 */ ]
+var _ = f01[string]
+var _ = f01[float64 /* ERROR does not satisfy I0 */ ]
+
+type I012 interface {
+	E0
+	E1
+	E2
+}
+
+func f012[T I012]() {}
+var _ = f012[int /* ERROR cannot satisfy I012.*empty type set */ ]
+var _ = f012[bool /* ERROR cannot satisfy I012.*empty type set */ ]
+var _ = f012[string /* ERROR cannot satisfy I012.*empty type set */ ]
+var _ = f012[float64 /* ERROR cannot satisfy I012.*empty type set */ ]
+
+type I12 interface {
+	E1
+	E2
+}
+
+func f12[T I12]() {}
+var _ = f12[int /* ERROR does not satisfy I12 */ ]
+var _ = f12[bool /* ERROR does not satisfy I12 */ ]
+var _ = f12[string /* ERROR does not satisfy I12 */ ]
+var _ = f12[float64]
+
+type I0_ interface {
+	E0
+	~int
+}
+
+func f0_[T I0_]() {}
+var _ = f0_[int]
+var _ = f0_[bool /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[string /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[float64 /* ERROR does not satisfy I0_ */ ]
+
+// Using a function instance as a type is an error.
+var _ f0 // ERROR not a type
+var _ f0 /* ERROR not a type */ [int]
+
+// Empty type sets can only be satisfied by empty type sets.
+type none interface {
+	// force an empty type set
+        int
+        string
+}
+
+func ff[T none]() {}
+func gg[T any]() {}
+func hh[T ~int]() {}
+
+func _[T none]() {
+	_ = ff[int /* ERROR cannot satisfy none \(empty type set\) */ ]
+	_ = ff[T]  // pathological but ok because T's type set is empty, too
+	_ = gg[int]
+	_ = gg[T]
+	_ = hh[int]
+	_ = hh[T]
+}
diff --git a/src/internal/types/testdata/check/typeinstcycles.go b/src/internal/types/testdata/check/typeinstcycles.go
new file mode 100644
index 0000000..74fe191
--- /dev/null
+++ b/src/internal/types/testdata/check/typeinstcycles.go
@@ -0,0 +1,11 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+func F1[T any](_ [unsafe.Sizeof(F1[int])]T) (res T)      { return }
+func F2[T any](_ T) (res [unsafe.Sizeof(F2[string])]int) { return }
+func F3[T any](_ [unsafe.Sizeof(F1[string])]int)         {}
diff --git a/src/internal/types/testdata/check/typeparams.go b/src/internal/types/testdata/check/typeparams.go
new file mode 100644
index 0000000..0e440d8
--- /dev/null
+++ b/src/internal/types/testdata/check/typeparams.go
@@ -0,0 +1,508 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// import "io" // for type assertion tests
+
+var _ any // ok to use any anywhere
+func _[_ any, _ interface{any}](any) {
+        var _ any
+}
+
+func identity[T any](x T) T { return x }
+
+func _[_ any](x int) int { panic(0) }
+func _[T any](T /* ERROR redeclared */ T)() {}
+func _[T, T /* ERROR redeclared */ any]() {}
+
+// Constraints (incl. any) may be parenthesized.
+func _[_ (any)]() {}
+func _[_ (interface{})]() {}
+
+func reverse[T any](list []T) []T {
+        rlist := make([]T, len(list))
+        i := len(list)
+        for _, x := range list {
+                i--
+                rlist[i] = x
+        }
+        return rlist
+}
+
+var _ = reverse /* ERROR cannot use generic function reverse */
+var _ = reverse[int, float32 /* ERROR got 2 type arguments */ ] ([]int{1, 2, 3})
+var _ = reverse[int]([ /* ERROR cannot use */ ]float32{1, 2, 3})
+var f = reverse[chan int]
+var _ = f(0 /* ERROR cannot use 0 .* as \[\]chan int */ )
+
+func swap[A, B any](a A, b B) (B, A) { return b, a }
+
+var _ = swap /* ERROR multiple-value */ [int, float32](1, 2)
+var f32, i = swap[int, float32](swap[float32, int](1, 2))
+var _ float32 = f32
+var _ int = i
+
+func swapswap[A, B any](a A, b B) (A, B) {
+        return swap[B, A](b, a)
+}
+
+type F[A, B any] func(A, B) (B, A)
+
+func min[T interface{ ~int }](x, y T) T {
+        if x < y {
+                return x
+        }
+        return y
+}
+
+func _[T interface{~int | ~float32}](x, y T) bool { return x < y }
+func _[T any](x, y T) bool { return x /* ERROR type parameter T is not comparable */ < y }
+func _[T interface{~int | ~float32 | ~bool}](x, y T) bool { return x /* ERROR type parameter T is not comparable */ < y }
+
+func _[T C1[T]](x, y T) bool { return x /* ERROR type parameter T is not comparable */ < y }
+func _[T C2[T]](x, y T) bool { return x < y }
+
+type C1[T any] interface{}
+type C2[T any] interface{ ~int | ~float32 }
+
+func new[T any]() *T {
+        var x T
+        return &x
+}
+
+var _ = new /* ERROR cannot use generic function new */
+var _ *int = new[int]()
+
+func _[T any](map[T /* ERROR invalid map key type T \(missing comparable constraint\) */]int) {} // w/o constraint we don't know if T is comparable
+
+func f1[T1 any](struct{T1 /* ERROR cannot be a .* type parameter */ }) int { panic(0) }
+var _ = f1[int](struct{T1}{})
+type T1 = int
+
+func f2[t1 any](struct{t1 /* ERROR cannot be a .* type parameter */ ; x float32}) int { panic(0) }
+var _ = f2[t1](struct{t1; x float32}{})
+type t1 = int
+
+
+func f3[A, B, C any](A, struct{x B}, func(A, struct{x B}, *C)) int { panic(0) }
+
+var _ = f3[int, rune, bool](1, struct{x rune}{}, nil)
+
+// indexing
+
+func _[T any] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ ~int }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ ~string }] (x T, i int) { _ = x[i] }
+func _[T interface{ ~[]int }] (x T, i int) { _ = x[i] }
+func _[T interface{ ~[10]int | ~*[20]int | ~map[int]int }] (x T, i int) { _ = x /* ERROR cannot index */ [i] } // map and non-map types
+func _[T interface{ ~string | ~[]byte }] (x T, i int) { _ = x[i] }
+func _[T interface{ ~[]int | ~[1]rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ ~string | ~[]rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+
+// indexing with various combinations of map types in type sets (see issue #42616)
+func _[T interface{ ~[]E | ~map[int]E }, E any](x T, i int) { _ = x /* ERROR cannot index */ [i] } // map and non-map types
+func _[T interface{ ~[]E }, E any](x T, i int) { _ = &x[i] }
+func _[T interface{ ~map[int]E }, E any](x T, i int) { _, _ = x[i] } // comma-ok permitted
+func _[T interface{ ~map[int]E }, E any](x T, i int) { _ = &x /* ERROR cannot take address */ [i] }
+func _[T interface{ ~map[int]E | ~map[uint]E }, E any](x T, i int) { _ = x /* ERROR cannot index */ [i] } // different map element types
+func _[T interface{ ~[]E | ~map[string]E }, E any](x T, i int) { _ = x /* ERROR cannot index */ [i] } // map and non-map types
+
+// indexing with various combinations of array and other types in type sets
+func _[T interface{ [10]int }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERROR out of bounds */ ] }
+func _[T interface{ [10]byte | string }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERROR out of bounds */ ] }
+func _[T interface{ [10]int | *[20]int | []int }](x T, i int) { _ = x[i]; _ = x[9]; _ = x[10 /* ERROR out of bounds */ ] }
+
+// indexing with strings and non-variable arrays (assignment not permitted)
+func _[T string](x T) { _ = x[0]; x /* ERROR cannot assign */ [0] = 0 }
+func _[T []byte | string](x T) { x /* ERROR cannot assign */ [0] = 0 }
+func _[T [10]byte]() { f := func() (x T) { return }; f /* ERROR cannot assign */ ()[0] = 0 }
+func _[T [10]byte]() { f := func() (x *T) { return }; f /* ERROR cannot index */ ()[0] = 0 }
+func _[T [10]byte]() { f := func() (x *T) { return }; (*f())[0] = 0 }
+func _[T *[10]byte]() { f := func() (x T) { return }; f()[0] = 0 }
+
+// slicing
+
+func _[T interface{ ~[10]E }, E any] (x T, i, j, k int) { var _ []E = x[i:j] }
+func _[T interface{ ~[10]E }, E any] (x T, i, j, k int) { var _ []E = x[i:j:k] }
+func _[T interface{ ~[]byte }] (x T, i, j, k int) { var _ T = x[i:j] }
+func _[T interface{ ~[]byte }] (x T, i, j, k int) { var _ T = x[i:j:k] }
+func _[T interface{ ~string }] (x T, i, j, k int) { var _ T = x[i:j] }
+func _[T interface{ ~string }] (x T, i, j, k int) { var _ T = x[i:j:k /* ERROR 3-index slice of string */ ] }
+
+type myByte1 []byte
+type myByte2 []byte
+func _[T interface{ []byte | myByte1 | myByte2 }] (x T, i, j, k int) { var _ T = x[i:j:k] }
+func _[T interface{ []byte | myByte1 | []int }] (x T, i, j, k int) { var _ T = x /* ERROR no core type */ [i:j:k] }
+
+func _[T interface{ []byte | myByte1 | myByte2 | string }] (x T, i, j, k int) { var _ T = x[i:j] }
+func _[T interface{ []byte | myByte1 | myByte2 | string }] (x T, i, j, k int) { var _ T = x[i:j:k /* ERROR 3-index slice of string */ ] }
+func _[T interface{ []byte | myByte1 | []int | string }] (x T, i, j, k int) { var _ T = x /* ERROR no core type */ [i:j] }
+
+// len/cap built-ins
+
+func _[T any](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~string | ~[]byte | ~int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~string }](x T) { _ = len(x) }
+func _[T interface{ ~[10]int }](x T) { _ = len(x) }
+func _[T interface{ ~[]byte }](x T) { _ = len(x) }
+func _[T interface{ ~map[int]int }](x T) { _ = len(x) }
+func _[T interface{ ~chan int }](x T) { _ = len(x) }
+func _[T interface{ ~string | ~[]byte | ~chan int }](x T) { _ = len(x) }
+
+func _[T any](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~string | ~[]byte | ~int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~string }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~[10]int }](x T) { _ = cap(x) }
+func _[T interface{ ~[]byte }](x T) { _ = cap(x) }
+func _[T interface{ ~map[int]int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ ~chan int }](x T) { _ = cap(x) }
+func _[T interface{ ~[]byte | ~chan int }](x T) { _ = cap(x) }
+
+// range iteration
+
+func _[T interface{}](x T) {
+        for range x /* ERROR cannot range */ {}
+}
+
+type myString string
+
+func _[
+        B1 interface{ string },
+        B2 interface{ string | myString },
+
+        C1 interface{ chan int },
+        C2 interface{ chan int | <-chan int },
+        C3 interface{ chan<- int },
+
+        S1 interface{ []int },
+        S2 interface{ []int | [10]int },
+
+        A1 interface{ [10]int },
+        A2 interface{ [10]int | []int },
+
+        P1 interface{ *[10]int },
+        P2 interface{ *[10]int | *[]int },
+
+        M1 interface{ map[string]int },
+        M2 interface{ map[string]int | map[string]string },
+]() {
+        var b0 string
+        for range b0 {}
+        for _ = range b0 {}
+        for _, _ = range b0 {}
+
+        var b1 B1
+        for range b1 {}
+        for _ = range b1 {}
+        for _, _ = range b1 {}
+
+        var b2 B2
+        for range b2 {}
+
+        var c0 chan int
+        for range c0 {}
+        for _ = range c0 {}
+        for _, _ /* ERROR permits only one iteration variable */ = range c0 {}
+
+        var c1 C1
+        for range c1 {}
+        for _ = range c1 {}
+        for _, _ /* ERROR permits only one iteration variable */ = range c1 {}
+
+        var c2 C2
+        for range c2 {}
+
+        var c3 C3
+        for range c3 /* ERROR receive from send-only channel */ {}
+
+        var s0 []int
+        for range s0 {}
+        for _ = range s0 {}
+        for _, _ = range s0 {}
+
+        var s1 S1
+        for range s1 {}
+        for _ = range s1 {}
+        for _, _ = range s1 {}
+
+        var s2 S2
+        for range s2 /* ERROR cannot range over s2.*no core type */ {}
+
+        var a0 []int
+        for range a0 {}
+        for _ = range a0 {}
+        for _, _ = range a0 {}
+
+        var a1 A1
+        for range a1 {}
+        for _ = range a1 {}
+        for _, _ = range a1 {}
+
+        var a2 A2
+        for range a2 /* ERROR cannot range over a2.*no core type */ {}
+
+        var p0 *[10]int
+        for range p0 {}
+        for _ = range p0 {}
+        for _, _ = range p0 {}
+
+        var p1 P1
+        for range p1 {}
+        for _ = range p1 {}
+        for _, _ = range p1 {}
+
+        var p2 P2
+        for range p2 /* ERROR cannot range over p2.*no core type */ {}
+
+        var m0 map[string]int
+        for range m0 {}
+        for _ = range m0 {}
+        for _, _ = range m0 {}
+
+        var m1 M1
+        for range m1 {}
+        for _ = range m1 {}
+        for _, _ = range m1 {}
+
+        var m2 M2
+        for range m2 /* ERROR cannot range over m2.*no core type */ {}
+}
+
+// type inference checks
+
+var _ = new /* ERROR cannot infer T */ ()
+
+func f4[A, B, C any](A, B) C { panic(0) }
+
+var _ = f4 /* ERROR cannot infer C */ (1, 2)
+var _ = f4[int, float32, complex128](1, 2)
+
+func f5[A, B, C any](A, []*B, struct{f []C}) int { panic(0) }
+
+var _ = f5[int, float32, complex128](0, nil, struct{f []complex128}{})
+var _ = f5 /* ERROR cannot infer */ (0, nil, struct{f []complex128}{})
+var _ = f5(0, []*float32{new[float32]()}, struct{f []complex128}{})
+
+func f6[A any](A, []A) int { panic(0) }
+
+var _ = f6(0, nil)
+
+func f6nil[A any](A) int { panic(0) }
+
+var _ = f6nil /* ERROR cannot infer */ (nil)
+
+// type inference with variadic functions
+
+func f7[T any](...T) T { panic(0) }
+
+var _ int = f7 /* ERROR cannot infer T */ ()
+var _ int = f7(1)
+var _ int = f7(1, 2)
+var _ int = f7([]int{}...)
+var _ int = f7 /* ERROR cannot use */ ([]float64{}...)
+var _ float64 = f7([]float64{}...)
+var _ = f7[float64](1, 2.3)
+var _ = f7(float64(1), 2.3)
+var _ = f7(1, 2.3 /* ERROR does not match */ )
+var _ = f7(1.2, 3 /* ERROR does not match */ )
+
+func f8[A, B any](A, B, ...B) int { panic(0) }
+
+var _ = f8(1) /* ERROR not enough arguments */
+var _ = f8(1, 2.3)
+var _ = f8(1, 2.3, 3.4, 4.5)
+var _ = f8(1, 2.3, 3.4, 4 /* ERROR does not match */ )
+var _ = f8[int, float64](1, 2.3, 3.4, 4)
+
+var _ = f8[int, float64](0, 0, nil...) // test case for #18268
+
+// init functions cannot have type parameters
+
+func init() {}
+func init[_ /* ERROR func init must have no type parameters */ any]() {}
+func init[P /* ERROR func init must have no type parameters */ any]() {}
+
+type T struct {}
+
+func (T) m1() {}
+func (T) m2[ /* ERROR method must have no type parameters */ _ any]() {}
+func (T) m3[ /* ERROR method must have no type parameters */ P any]() {}
+
+// type inference across parameterized types
+
+type S1[P any] struct { f P }
+
+func f9[P any](x S1[P]) {}
+
+func _() {
+        f9[int](S1[int]{42})
+	f9(S1[int]{42})
+}
+
+type S2[A, B, C any] struct{}
+
+func f10[X, Y, Z any](a S2[X, int, Z], b S2[X, Y, bool]) {}
+
+func _[P any]() {
+        f10[int, float32, string](S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[P, int, P]{}, S2[P, float32, bool]{})
+}
+
+// corner case for type inference
+// (was bug: after instanting f11, the type-checker didn't mark f11 as non-generic)
+
+func f11[T any]() {}
+
+func _() {
+	f11[int]()
+}
+
+// the previous example was extracted from
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// func f12[T interface{m() T}]() {}
+// 
+// type A[T any] T
+// 
+// func (a A[T]) m() A[T]
+// 
+// func _[T any]() {
+// 	f12[A[T]]()
+// }
+
+// method expressions
+
+func (_ S1[P]) m()
+
+func _() {
+	m := S1[int].m
+	m(struct { f int }{42})
+}
+
+func _[T any] (x T) {
+        m := S1[T].m
+        m(S1[T]{x})
+}
+
+type I1[A any] interface {
+        m1(A)
+}
+
+var _ I1[int] = r1[int]{}
+
+type r1[T any] struct{}
+
+func (_ r1[T]) m1(T)
+
+type I2[A, B any] interface {
+        m1(A)
+        m2(A) B
+}
+
+var _ I2[int, float32] = R2[int, float32]{}
+
+type R2[P, Q any] struct{}
+
+func (_ R2[X, Y]) m1(X)
+func (_ R2[X, Y]) m2(X) Y
+
+// type assertions and type switches over generic types
+// NOTE: These are currently disabled because it's unclear what the correct
+// approach is, and one can always work around by assigning the variable to
+// an interface first.
+
+// // ReadByte1 corresponds to the ReadByte example in the draft design.
+// func ReadByte1[T io.Reader](r T) (byte, error) {
+// 	if br, ok := r.(io.ByteReader); ok {
+// 		return br.ReadByte()
+// 	}
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // ReadBytes2 is like ReadByte1 but uses a type switch instead.
+// func ReadByte2[T io.Reader](r T) (byte, error) {
+//         switch br := r.(type) {
+//         case io.ByteReader:
+//                 return br.ReadByte()
+//         }
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // type assertions and type switches over generic types are strict
+// type I3 interface {
+//         m(int)
+// }
+//
+// type I4 interface {
+//         m() int // different signature from I3.m
+// }
+//
+// func _[T I3](x I3, p T) {
+//         // type assertions and type switches over interfaces are not strict
+//         _ = x.(I4)
+//         switch x.(type) {
+//         case I4:
+//         }
+//
+//         // type assertions and type switches over generic types are strict
+//         _ = p /* ERROR cannot have dynamic type I4 */.(I4)
+//         switch p.(type) {
+//         case I4 /* ERROR cannot have dynamic type I4 */ :
+//         }
+// }
+
+// type assertions and type switches over generic types lead to errors for now
+
+func _[T any](x T) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+func _[T interface{~int}](x T) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+// error messages related to type bounds mention those bounds
+type C[P any] interface{}
+
+func _[P C[P]] (x P) {
+	x.m /* ERROR x.m undefined */ ()
+}
+
+type I interface {}
+
+func _[P I] (x P) {
+	x.m /* ERROR type P has no field or method m */ ()
+}
+
+func _[P interface{}] (x P) {
+	x.m /* ERROR type P has no field or method m */ ()
+}
+
+func _[P any] (x P) {
+	x.m /* ERROR type P has no field or method m */ ()
+}
diff --git a/src/internal/types/testdata/check/unions.go b/src/internal/types/testdata/check/unions.go
new file mode 100644
index 0000000..bcd7de6
--- /dev/null
+++ b/src/internal/types/testdata/check/unions.go
@@ -0,0 +1,66 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that overlong unions don't bog down type checking.
+// Disallow them for now.
+
+package p
+
+type t int
+
+type (
+	t00 t; t01 t; t02 t; t03 t; t04 t; t05 t; t06 t; t07 t; t08 t; t09 t
+	t10 t; t11 t; t12 t; t13 t; t14 t; t15 t; t16 t; t17 t; t18 t; t19 t
+	t20 t; t21 t; t22 t; t23 t; t24 t; t25 t; t26 t; t27 t; t28 t; t29 t
+	t30 t; t31 t; t32 t; t33 t; t34 t; t35 t; t36 t; t37 t; t38 t; t39 t
+	t40 t; t41 t; t42 t; t43 t; t44 t; t45 t; t46 t; t47 t; t48 t; t49 t
+	t50 t; t51 t; t52 t; t53 t; t54 t; t55 t; t56 t; t57 t; t58 t; t59 t
+	t60 t; t61 t; t62 t; t63 t; t64 t; t65 t; t66 t; t67 t; t68 t; t69 t
+	t70 t; t71 t; t72 t; t73 t; t74 t; t75 t; t76 t; t77 t; t78 t; t79 t
+	t80 t; t81 t; t82 t; t83 t; t84 t; t85 t; t86 t; t87 t; t88 t; t89 t
+	t90 t; t91 t; t92 t; t93 t; t94 t; t95 t; t96 t; t97 t; t98 t; t99 t
+)
+
+type u99 interface {
+	t00|t01|t02|t03|t04|t05|t06|t07|t08|t09|
+	t10|t11|t12|t13|t14|t15|t16|t17|t18|t19|
+	t20|t21|t22|t23|t24|t25|t26|t27|t28|t29|
+	t30|t31|t32|t33|t34|t35|t36|t37|t38|t39|
+	t40|t41|t42|t43|t44|t45|t46|t47|t48|t49|
+	t50|t51|t52|t53|t54|t55|t56|t57|t58|t59|
+	t60|t61|t62|t63|t64|t65|t66|t67|t68|t69|
+	t70|t71|t72|t73|t74|t75|t76|t77|t78|t79|
+	t80|t81|t82|t83|t84|t85|t86|t87|t88|t89|
+	t90|t91|t92|t93|t94|t95|t96|t97|t98
+}
+
+type u100a interface {
+	u99|float32
+}
+
+type u100b interface {
+	u99|float64
+}
+
+type u101 interface {
+	t00|t01|t02|t03|t04|t05|t06|t07|t08|t09|
+	t10|t11|t12|t13|t14|t15|t16|t17|t18|t19|
+	t20|t21|t22|t23|t24|t25|t26|t27|t28|t29|
+	t30|t31|t32|t33|t34|t35|t36|t37|t38|t39|
+	t40|t41|t42|t43|t44|t45|t46|t47|t48|t49|
+	t50|t51|t52|t53|t54|t55|t56|t57|t58|t59|
+	t60|t61|t62|t63|t64|t65|t66|t67|t68|t69|
+	t70|t71|t72|t73|t74|t75|t76|t77|t78|t79|
+	t80|t81|t82|t83|t84|t85|t86|t87|t88|t89|
+	t90|t91|t92|t93|t94|t95|t96|t97|t98|t99|
+        int // ERROR cannot handle more than 100 union terms
+}
+
+type u102 interface {
+        int /* ERROR cannot handle more than 100 union terms */ |string|u100a
+}
+
+type u200 interface {
+        u100a /* ERROR cannot handle more than 100 union terms */ |u100b
+}
diff --git a/src/internal/types/testdata/check/vardecl.go b/src/internal/types/testdata/check/vardecl.go
new file mode 100644
index 0000000..5b68adb
--- /dev/null
+++ b/src/internal/types/testdata/check/vardecl.go
@@ -0,0 +1,215 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vardecl
+
+// Prerequisites.
+import "math"
+func f() {}
+func g() (x, y int) { return }
+var m map[string]int
+
+// Var decls must have a type or an initializer.
+var _ int
+var _, _ int
+
+var _; /* ERROR "expected type" */
+var _, _; /* ERROR "expected type" */
+var _, _, _; /* ERROR "expected type" */
+
+// The initializer must be an expression.
+var _ = int /* ERROR "not an expression" */
+var _ = f /* ERROR "used as value" */ ()
+
+// Identifier and expression arity must match.
+var _, _ = 1, 2
+var _ = 1, 2 /* ERROR "extra init expr 2" */
+var _, _ = 1 /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */
+var _, _, _ /* ERROR "missing init expr for _" */ = 1, 2
+
+var _ = g /* ERROR "multiple-value g" */ ()
+var _, _ = g()
+var _, _, _ = g /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */ ()
+
+var _ = m["foo"]
+var _, _ = m["foo"]
+var _, _, _ = m  /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */ ["foo"]
+
+var _, _ int = 1, 2
+var _ int = 1, 2 /* ERROR "extra init expr 2" */
+var _, _ int = 1 /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */
+var _, _, _ /* ERROR "missing init expr for _" */ int = 1, 2
+
+var (
+	_, _ = 1, 2
+	_ = 1, 2 /* ERROR "extra init expr 2" */
+	_, _ = 1 /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */
+	_, _, _ /* ERROR "missing init expr for _" */ = 1, 2
+
+	_ = g /* ERROR "multiple-value g" */ ()
+	_, _ = g()
+	_, _, _ = g /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */ ()
+
+	_ = m["foo"]
+	_, _ = m["foo"]
+	_, _, _ = m /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */ ["foo"]
+
+	_, _ int = 1, 2
+	_ int = 1, 2 /* ERROR "extra init expr 2" */
+	_, _ int = 1 /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */
+	_, _, _ /* ERROR "missing init expr for _" */ int = 1, 2
+)
+
+// Variables declared in function bodies must be 'used'.
+type T struct{}
+func (r T) _(a, b, c int) (u, v, w int) {
+	var x1 /* ERROR "declared and not used" */ int
+	var x2 /* ERROR "declared and not used" */ int
+	x1 = 1
+	(x2) = 2
+
+	y1 /* ERROR "declared and not used" */ := 1
+	y2 /* ERROR "declared and not used" */ := 2
+	y1 = 1
+	(y1) = 2
+
+	{
+		var x1 /* ERROR "declared and not used" */ int
+		var x2 /* ERROR "declared and not used" */ int
+		x1 = 1
+		(x2) = 2
+
+		y1 /* ERROR "declared and not used" */ := 1
+		y2 /* ERROR "declared and not used" */ := 2
+		y1 = 1
+		(y1) = 2
+	}
+
+	if x /* ERROR "declared and not used" */ := 0; a < b {}
+
+	switch x /* ERROR "declared and not used" */, y := 0, 1; a {
+	case 0:
+		_ = y
+	case 1:
+		x /* ERROR "declared and not used" */ := 0
+	}
+
+	var t interface{}
+	switch t /* ERROR "declared and not used" */ := t.(type) {}
+
+	switch t /* ERROR "declared and not used" */ := t.(type) {
+	case int:
+	}
+
+	switch t /* ERROR "declared and not used" */ := t.(type) {
+	case int:
+	case float32, complex64:
+		t = nil
+	}
+
+	switch t := t.(type) {
+	case int:
+	case float32, complex64:
+		_ = t
+	}
+
+	switch t := t.(type) {
+	case int:
+	case float32:
+	case string:
+		_ = func() string {
+			return t
+		}
+	}
+
+	switch t := t; t /* ERROR "declared and not used" */ := t.(type) {}
+
+	var z1 /* ERROR "declared and not used" */ int
+	var z2 int
+	_ = func(a, b, c int) (u, v, w int) {
+		z1 = a
+		(z1) = b
+		a = z2
+		return
+	}
+
+	var s []int
+	var i /* ERROR "declared and not used" */ , j int
+	for i, j = range s {
+		_ = j
+	}
+
+	for i, j /* ERROR "declared and not used" */ := range s {
+		_ = func() int {
+			return i
+		}
+	}
+	return
+}
+
+// Unused variables in function literals must lead to only one error (issue #22524).
+func _() {
+	_ = func() {
+		var x /* ERROR declared and not used */ int
+	}
+}
+
+// Invalid variable declarations must not lead to "declared and not used errors".
+// TODO(gri) enable these tests once go/types follows types2 logic for declared and not used variables
+// func _() {
+//	var a x                        // DISABLED_ERROR undefined: x
+//	var b = x                      // DISABLED_ERROR undefined: x
+//	var c int = x                  // DISABLED_ERROR undefined: x
+//	var d, e, f x                  /* DISABLED_ERROR x */ /* DISABLED_ERROR x */ /* DISABLED_ERROR x */
+//	var g, h, i = x, x, x          /* DISABLED_ERROR x */ /* DISABLED_ERROR x */ /* DISABLED_ERROR x */
+//	var j, k, l float32 = x, x, x  /* DISABLED_ERROR x */ /* DISABLED_ERROR x */ /* DISABLED_ERROR x */
+//	// but no "declared and not used" errors
+// }
+
+// Invalid (unused) expressions must not lead to spurious "declared and not used errors".
+func _() {
+	var a, b, c int
+	var x, y int
+	x, y = a /* ERROR "assignment mismatch: [1-9]+ variables but.*[1-9]+ value(s)?" */ , b, c
+	_ = x
+	_ = y
+}
+
+func _() {
+	var x int
+	return x /* ERROR too many return values */
+	return math /* ERROR too many return values */ .Sin(0)
+}
+
+func _() int {
+	var x, y int
+	return x, y /* ERROR too many return values */
+}
+
+// Short variable declarations must declare at least one new non-blank variable.
+func _() {
+	_ := /* ERROR no new variables */ 0
+	_, a := 0, 1
+	_, a := /* ERROR no new variables */ 0, 1
+	_, a, b := 0, 1, 2
+	_, _, _ := /* ERROR no new variables */ 0, 1, 2
+
+	_ = a
+	_ = b
+}
+
+// Test case for variables depending on function literals (see also #22992).
+var A /* ERROR initialization cycle */ = func() int { return A }()
+
+func _() {
+	// The function literal below must not see a.
+	var a = func() int { return a /* ERROR "undefined" */ }()
+	var _ = func() int { return a }()
+
+	// The function literal below must not see x, y, or z.
+	var x, y, z = 0, 1, func() int { return x /* ERROR "undefined" */ + y /* ERROR "undefined" */ + z /* ERROR "undefined" */ }()
+	_, _, _ = x, y, z
+}
+
+// TODO(gri) consolidate other var decl checks in this file
\ No newline at end of file
diff --git a/src/internal/types/testdata/examples/constraints.go b/src/internal/types/testdata/examples/constraints.go
new file mode 100644
index 0000000..5b14489
--- /dev/null
+++ b/src/internal/types/testdata/examples/constraints.go
@@ -0,0 +1,80 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of generic constraint interfaces.
+
+package p
+
+type MyInt int
+
+type (
+	// Arbitrary types may be embedded like interfaces.
+	_ interface{int}
+	_ interface{~int}
+
+	// Types may be combined into a union.
+	union interface{int|~string}
+
+	// Union terms must describe disjoint (non-overlapping) type sets.
+	_ interface{int|int /* ERROR overlapping terms int */ }
+	_ interface{int|~ /* ERROR overlapping terms ~int */ int }
+	_ interface{~int|~ /* ERROR overlapping terms ~int */ int }
+	_ interface{~int|MyInt /* ERROR overlapping terms p.MyInt and ~int */ }
+	_ interface{int|any}
+	_ interface{int|~string|union}
+	_ interface{int|~string|interface{int}}
+	_ interface{union|int}   // interfaces (here: union) are ignored when checking for overlap
+	_ interface{union|union} // ditto
+
+	// For now we do not permit interfaces with methods in unions.
+	_ interface{~ /* ERROR invalid use of ~ */ any}
+	_ interface{int|interface /* ERROR cannot use .* in union */ { m() }}
+)
+
+type (
+	// Tilde is not permitted on defined types or interfaces.
+	foo int
+	bar any
+	_ interface{foo}
+	_ interface{~ /* ERROR invalid use of ~ */ foo }
+	_ interface{~ /* ERROR invalid use of ~ */ bar }
+)
+
+// Stand-alone type parameters are not permitted as elements or terms in unions.
+type (
+	_[T interface{ *T } ] struct{}        // ok
+	_[T interface{ int | *T } ] struct{}  // ok
+	_[T interface{ T /* ERROR term cannot be a type parameter */ } ] struct{}
+	_[T interface{ ~T /* ERROR type in term ~T cannot be a type parameter */ } ] struct{}
+	_[T interface{ int|T /* ERROR term cannot be a type parameter */ }] struct{}
+)
+
+// Multiple embedded union elements are intersected. The order in which they
+// appear in the interface doesn't matter since intersection is a symmetric
+// operation.
+
+type myInt1 int
+type myInt2 int
+
+func _[T interface{ myInt1|myInt2; ~int }]() T { return T(0) }
+func _[T interface{ ~int; myInt1|myInt2 }]() T { return T(0) }
+
+// Here the intersections are empty - there's no type that's in the type set of T.
+func _[T interface{ myInt1|myInt2; int }]() T { return T(0 /* ERROR cannot convert */ ) }
+func _[T interface{ int; myInt1|myInt2 }]() T { return T(0 /* ERROR cannot convert */ ) }
+
+// Union elements may be interfaces as long as they don't define
+// any methods or embed comparable.
+
+type (
+	Integer interface{ ~int|~int8|~int16|~int32|~int64 }
+	Unsigned interface{ ~uint|~uint8|~uint16|~uint32|~uint64 }
+	Floats interface{ ~float32|~float64 }
+	Complex interface{ ~complex64|~complex128 }
+	Number interface{ Integer|Unsigned|Floats|Complex }
+	Ordered interface{ Integer|Unsigned|Floats|~string }
+
+	_ interface{ Number | error /* ERROR cannot use error in union */ }
+	_ interface{ Ordered | comparable /* ERROR cannot use comparable in union */ }
+)
diff --git a/src/internal/types/testdata/examples/functions.go b/src/internal/types/testdata/examples/functions.go
new file mode 100644
index 0000000..47e1c35
--- /dev/null
+++ b/src/internal/types/testdata/examples/functions.go
@@ -0,0 +1,219 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of type-parameterized functions.
+
+package p
+
+// Reverse is a generic function that takes a []T argument and
+// reverses that slice in place.
+func Reverse[T any](list []T) {
+	i := 0
+	j := len(list)-1
+	for i < j {
+		list[i], list[j] = list[j], list[i]
+		i++
+		j--
+	}
+}
+
+func _() {
+	// Reverse can be called with an explicit type argument.
+	Reverse[int](nil)
+	Reverse[string]([]string{"foo", "bar"})
+	Reverse[struct{x, y int}]([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// Since the type parameter is used for an incoming argument,
+	// it can be inferred from the provided argument's type.
+	Reverse([]string{"foo", "bar"})
+	Reverse([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// But the incoming argument must have a type, even if it's a
+	// default type. An untyped nil won't work.
+	// Reverse(nil) // this won't type-check
+
+	// A typed nil will work, though.
+	Reverse([]int(nil))
+}
+
+// Certain functions, such as the built-in `new` could be written using
+// type parameters.
+func new[T any]() *T {
+	var x T
+	return &x
+}
+
+// When calling our own `new`, we need to pass the type parameter
+// explicitly since there is no (value) argument from which the
+// result type could be inferred. We don't try to infer the
+// result type from the assignment to keep things simple and
+// easy to understand.
+var _ = new[int]()
+var _ *float64 = new[float64]() // the result type is indeed *float64
+
+// A function may have multiple type parameters, of course.
+func foo[A, B, C any](a A, b []B, c *C) B {
+	// do something here
+	return b[0]
+}
+
+// As before, we can pass type parameters explicitly.
+var s = foo[int, string, float64](1, []string{"first"}, new[float64]())
+
+// Or we can use type inference.
+var _ float64 = foo(42, []float64{1.0}, &s)
+
+// Type inference works in a straight-forward manner even
+// for variadic functions.
+func variadic[A, B any](A, B, ...B) int { panic(0) }
+
+// var _ = variadic(1) // ERROR not enough arguments
+var _ = variadic(1, 2.3)
+var _ = variadic(1, 2.3, 3.4, 4.5)
+var _ = variadic[int, float64](1, 2.3, 3.4, 4)
+
+// Type inference also works in recursive function calls where
+// the inferred type is the type parameter of the caller.
+func f1[T any](x T) {
+	f1(x)
+}
+
+func f2a[T any](x, y T) {
+	f2a(x, y)
+}
+
+func f2b[T any](x, y T) {
+	f2b(y, x)
+}
+
+func g2a[P, Q any](x P, y Q) {
+	g2a(x, y)
+}
+
+func g2b[P, Q any](x P, y Q) {
+	g2b(y, x)
+}
+
+// Here's an example of a recursive function call with variadic
+// arguments and type inference inferring the type parameter of
+// the caller (i.e., itself).
+func max[T interface{ ~int }](x ...T) T {
+	var x0 T
+	if len(x) > 0 {
+		x0 = x[0]
+	}
+	if len(x) > 1 {
+		x1 := max(x[1:]...)
+		if x1 > x0 {
+			return x1
+		}
+	}
+	return x0
+}
+
+// When inferring channel types, the channel direction is ignored
+// for the purpose of type inference. Once the type has been in-
+// fered, the usual parameter passing rules are applied.
+// Thus even if a type can be inferred successfully, the function
+// call may not be valid.
+
+func fboth[T any](chan T) {}
+func frecv[T any](<-chan T) {}
+func fsend[T any](chan<- T) {}
+
+func _() {
+	var both chan int
+	var recv <-chan int
+	var send chan<-int
+
+	fboth(both)
+	fboth(recv /* ERROR cannot use */ )
+	fboth(send /* ERROR cannot use */ )
+
+	frecv(both)
+	frecv(recv)
+	frecv(send /* ERROR cannot use */ )
+
+	fsend(both)
+	fsend(recv /* ERROR cannot use */)
+	fsend(send)
+}
+
+func ffboth[T any](func(chan T)) {}
+func ffrecv[T any](func(<-chan T)) {}
+func ffsend[T any](func(chan<- T)) {}
+
+func _() {
+	var both func(chan int)
+	var recv func(<-chan int)
+	var send func(chan<- int)
+
+	ffboth(both)
+	ffboth(recv /* ERROR cannot use */ )
+	ffboth(send /* ERROR cannot use */ )
+
+	ffrecv(both /* ERROR cannot use */ )
+	ffrecv(recv)
+	ffrecv(send /* ERROR cannot use */ )
+
+	ffsend(both /* ERROR cannot use */ )
+	ffsend(recv /* ERROR cannot use */ )
+	ffsend(send)
+}
+
+// When inferring elements of unnamed composite parameter types,
+// if the arguments are defined types, use their underlying types.
+// Even though the matching types are not exactly structurally the
+// same (one is a type literal, the other a named type), because
+// assignment is permitted, parameter passing is permitted as well,
+// so type inference should be able to handle these cases well.
+
+func g1[T any]([]T) {}
+func g2[T any]([]T, T) {}
+func g3[T any](*T, ...T) {}
+
+func _() {
+	type intSlize []int
+	g1([]int{})
+	g1(intSlize{})
+	g2(nil, 0)
+
+	type myString string
+	var s1 string
+	g3(nil, "1", myString("2"), "3")
+	g3(& /* ERROR does not match */ s1, "1", myString("2"), "3")
+	_ = s1
+
+	type myStruct struct{x int}
+	var s2 myStruct
+	g3(nil, struct{x int}{}, myStruct{})
+	g3(&s2, struct{x int}{}, myStruct{})
+	g3(nil, myStruct{}, struct{x int}{})
+	g3(&s2, myStruct{}, struct{x int}{})
+}
+
+// Here's a realistic example.
+
+func append[T any](s []T, t ...T) []T { panic(0) }
+
+func _() {
+	var f func()
+	type Funcs []func()
+	var funcs Funcs
+	_ = append(funcs, f)
+}
+
+// Generic type declarations cannot have empty type parameter lists
+// (that would indicate a slice type). Thus, generic functions cannot
+// have empty type parameter lists, either. This is a syntax error.
+
+func h[] /* ERROR empty type parameter list */ () {}
+
+func _() {
+	h /* ERROR cannot index */ [] /* ERROR operand */ ()
+}
+
+// Generic functions must have a function body.
+
+func _ /* ERROR generic function is missing function body */ [P any]()
diff --git a/src/internal/types/testdata/examples/inference.go b/src/internal/types/testdata/examples/inference.go
new file mode 100644
index 0000000..073df9c
--- /dev/null
+++ b/src/internal/types/testdata/examples/inference.go
@@ -0,0 +1,116 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of type inference.
+
+package p
+
+type Ordered interface {
+	~int | ~float64 | ~string
+}
+
+func min[T Ordered](x, y T) T { panic(0) }
+
+func _() {
+	// min can be called with explicit instantiation.
+	_ = min[int](1, 2)
+
+	// Alternatively, the type argument can be inferred from
+	// one of the arguments. Untyped arguments will be considered
+	// last.
+	var x int
+	_ = min(x, x)
+	_ = min(x, 1)
+	_ = min(x, 1.0)
+	_ = min(1, 2)
+	_ = min(1, 2.3 /* ERROR default type float64 .* does not match */)
+
+	var y float64
+	_ = min(1, y)
+	_ = min(1.2, y)
+	_ = min(1.2, 3.4)
+	_ = min(1.2, 3 /* ERROR default type int .* does not match */)
+
+	var s string
+	_ = min(s, "foo")
+	_ = min("foo", "bar")
+}
+
+func mixed[T1, T2, T3 any](T1, T2, T3) {}
+
+func _() {
+	// mixed can be called with explicit instantiation.
+	mixed[int, string, bool](0, "", false)
+
+	// Alternatively, partial type arguments may be provided
+	// (from left to right), and the other may be inferred.
+	mixed[int, string](0, "", false)
+	mixed[int](0, "", false)
+	mixed(0, "", false)
+
+	// Provided type arguments always take precedence over
+	// inferred types.
+	mixed[int, string](1.1 /* ERROR cannot use 1.1 */, "", false)
+}
+
+func related1[Slice interface{ ~[]Elem }, Elem any](s Slice, e Elem) {}
+
+func _() {
+	// related1 can be called with explicit instantiation.
+	var si []int
+	related1[[]int, int](si, 0)
+
+	// Alternatively, the 2nd type argument can be inferred
+	// from the first one through constraint type inference.
+	var ss []string
+	_ = related1[[]string]
+	related1[[]string](ss, "foo")
+
+	// A type argument inferred from another explicitly provided
+	// type argument overrides whatever value argument type is given.
+	related1[[]string](ss, 0 /* ERROR cannot use 0 */)
+
+	// A type argument may be inferred from a value argument
+	// and then help infer another type argument via constraint
+	// type inference.
+	related1(si, 0)
+	related1(si, "foo" /* ERROR cannot use "foo" */)
+}
+
+func related2[Elem any, Slice interface{ []Elem }](e Elem, s Slice) {}
+
+func _() {
+	// related2 can be called with explicit instantiation.
+	var si []int
+	related2[int, []int](0, si)
+
+	// Alternatively, the 2nd type argument can be inferred
+	// from the first one through constraint type inference.
+	var ss []string
+	_ = related2[string]
+	related2[string]("foo", ss)
+
+	// A type argument may be inferred from a value argument
+	// and then help infer another type argument via constraint
+	// type inference. Untyped arguments are always considered
+	// last.
+	related2(1.2, []float64{})
+	related2(1.0, []int{})
+	related2 /* ERROR does not satisfy */ (float64(1.0), []int{}) // TODO(gri) fix error position
+}
+
+type List[P any] []P
+
+func related3[Elem any, Slice []Elem | List[Elem]]() Slice { return nil }
+
+func _() {
+	// related3 can be instantiated explicitly
+	related3[int, []int]()
+	related3[byte, List[byte]]()
+
+	// The 2nd type argument cannot be inferred from the first
+	// one because there's two possible choices: []Elem and
+	// List[Elem].
+	related3 /* ERROR cannot infer Slice */ [int]()
+}
diff --git a/src/internal/types/testdata/examples/methods.go b/src/internal/types/testdata/examples/methods.go
new file mode 100644
index 0000000..a46f789
--- /dev/null
+++ b/src/internal/types/testdata/examples/methods.go
@@ -0,0 +1,112 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of methods on type-parameterized types.
+
+package p
+
+// Parameterized types may have methods.
+type T1[A any] struct{ a A }
+
+// When declaring a method for a parameterized type, the "instantiated"
+// receiver type acts as an implicit declaration of the type parameters
+// for the receiver type. In the example below, method m1 on type T1 has
+// the receiver type T1[A] which declares the type parameter A for use
+// with this method. That is, within the method m1, A stands for the
+// actual type argument provided to an instantiated T1.
+func (t T1[A]) m1() A { return t.a }
+
+// For instance, if T1 is instantiated with the type int, the type
+// parameter A in m1 assumes that type (int) as well and we can write
+// code like this:
+var x T1[int]
+var _ int = x.m1()
+
+// Because the type parameter provided to a parameterized receiver type
+// is declared through that receiver declaration, it must be an identifier.
+// It cannot possibly be some other type because the receiver type is not
+// instantiated with concrete types, it is standing for the parameterized
+// receiver type.
+func (t T1[[ /* ERROR must be an identifier */ ]int]) m2() {}
+
+// Note that using what looks like a predeclared identifier, say int,
+// as type parameter in this situation is deceptive and considered bad
+// style. In m3 below, int is the name of the local receiver type parameter
+// and it shadows the predeclared identifier int which then cannot be used
+// anymore as expected.
+// This is no different from locally re-declaring a predeclared identifier
+// and usually should be avoided. There are some notable exceptions; e.g.,
+// sometimes it makes sense to use the identifier "copy" which happens to
+// also be the name of a predeclared built-in function.
+func (t T1[int]) m3() { var _ int = 42 /* ERROR cannot use 42 .* as int */ }
+
+// The names of the type parameters used in a parameterized receiver
+// type don't have to match the type parameter names in the declaration
+// of the type used for the receiver. In our example, even though T1 is
+// declared with type parameter named A, methods using that receiver type
+// are free to use their own name for that type parameter. That is, the
+// name of type parameters is always local to the declaration where they
+// are introduced. In our example we can write a method m2 and use the
+// name X instead of A for the type parameter w/o any difference.
+func (t T1[X]) m4() X { return t.a }
+
+// If the receiver type is parameterized, type parameters must always be
+// provided: this simply follows from the general rule that a parameterized
+// type must be instantiated before it can be used. A method receiver
+// declaration using a parameterized receiver type is no exception. It is
+// simply that such receiver type expressions perform two tasks simultaneously:
+// they declare the (local) type parameters and then use them to instantiate
+// the receiver type. Forgetting to provide a type parameter leads to an error.
+func (t T1 /* ERROR generic type .* without instantiation */ ) m5() {}
+
+// However, sometimes we don't need the type parameter, and thus it is
+// inconvenient to have to choose a name. Since the receiver type expression
+// serves as a declaration for its type parameters, we are free to choose the
+// blank identifier:
+func (t T1[_]) m6() {}
+
+// Naturally, these rules apply to any number of type parameters on the receiver
+// type. Here are some more complex examples.
+type T2[A, B, C any] struct {
+        a A
+        b B
+        c C
+}
+
+// Naming of the type parameters is local and has no semantic impact:
+func (t T2[A, B, C]) m1() (A, B, C) { return t.a, t.b, t.c }
+func (t T2[C, B, A]) m2() (C, B, A) { return t.a, t.b, t.c }
+func (t T2[X, Y, Z]) m3() (X, Y, Z) { return t.a, t.b, t.c }
+
+// Type parameters may be left blank if they are not needed:
+func (t T2[A, _, C]) m4() (A, C) { return t.a, t.c }
+func (t T2[_, _, X]) m5() X { return t.c }
+func (t T2[_, _, _]) m6() {}
+
+// As usual, blank names may be used for any object which we don't care about
+// using later. For instance, we may write an unnamed method with a receiver
+// that cannot be accessed:
+func (_ T2[_, _, _]) _() int { return 42 }
+
+// Because a receiver parameter list is simply a parameter list, we can
+// leave the receiver argument away for receiver types.
+type T0 struct{}
+func (T0) _() {}
+func (T1[A]) _() {}
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // A generic receiver type may constrain its type parameter such
+// // that it must be a pointer type. Such receiver types are not
+// // permitted.
+// type T3a[P interface{ ~int | ~string | ~float64 }] P
+// 
+// func (T3a[_]) m() {} // this is ok
+// 
+// type T3b[P interface{ ~unsafe.Pointer }] P
+// 
+// func (T3b /* ERROR invalid receiver */ [_]) m() {}
+// 
+// type T3c[P interface{ *int | *string }] P
+// 
+// func (T3c /* ERROR invalid receiver */ [_]) m() {}
diff --git a/src/internal/types/testdata/examples/operations.go b/src/internal/types/testdata/examples/operations.go
new file mode 100644
index 0000000..18e4d60
--- /dev/null
+++ b/src/internal/types/testdata/examples/operations.go
@@ -0,0 +1,29 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// indirection
+
+func _[P any](p P) {
+        _ = *p // ERROR cannot indirect p
+}
+
+func _[P interface{ int }](p P) {
+        _ = *p // ERROR cannot indirect p
+}
+
+func _[P interface{ *int }](p P) {
+        _ = *p
+}
+
+func _[P interface{ *int | *string }](p P) {
+        _ = *p // ERROR must have identical base types
+}
+
+type intPtr *int
+
+func _[P interface{ *int | intPtr } ](p P) {
+        var _ int = *p
+}
diff --git a/src/internal/types/testdata/examples/types.go b/src/internal/types/testdata/examples/types.go
new file mode 100644
index 0000000..052d168
--- /dev/null
+++ b/src/internal/types/testdata/examples/types.go
@@ -0,0 +1,315 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of generic types.
+
+package p
+
+// List is just what it says - a slice of E elements.
+type List[E any] []E
+
+// A generic (parameterized) type must always be instantiated
+// before it can be used to designate the type of a variable
+// (including a struct field, or function parameter); though
+// for the latter cases, the provided type may be another type
+// parameter. So:
+var _ List[byte] = []byte{}
+
+// A generic binary tree might be declared as follows.
+type Tree[E any] struct {
+	left, right *Tree[E]
+	payload E
+}
+
+// A simple instantiation of Tree:
+var root1 Tree[int]
+
+// The actual type parameter provided may be a generic type itself:
+var root2 Tree[List[int]]
+
+// A couple of more complex examples.
+// We don't need extra parentheses around the element type of the slices on
+// the right (unlike when we use ()'s rather than []'s for type parameters).
+var _ List[List[int]] = []List[int]{}
+var _ List[List[List[Tree[int]]]] = []List[List[Tree[int]]]{}
+
+// Type parameters act like type aliases when used in generic types
+// in the sense that we can "emulate" a specific type instantiation
+// with type aliases.
+type T1[P any] struct {
+	f P
+}
+
+type T2[P any] struct {
+	f struct {
+		g P
+	}
+}
+
+var x1 T1[struct{ g int }]
+var x2 T2[int]
+
+func _() {
+	// This assignment is invalid because the types of x1, x2 are T1(...)
+	// and T2(...) respectively, which are two different defined types.
+	x1 = x2 // ERROR assignment
+
+	// This assignment is valid because the types of x1.f and x2.f are
+	// both struct { g int }; the type parameters act like type aliases
+	// and their actual names don't come into play here.
+	x1.f = x2.f
+}
+
+// We can verify this behavior using type aliases instead:
+type T1a struct {
+	f A1
+}
+type A1 = struct { g int }
+
+type T2a struct {
+	f struct {
+		g A2
+	}
+}
+type A2 = int
+
+var x1a T1a
+var x2a T2a
+
+func _() {
+	x1a = x2a // ERROR assignment
+	x1a.f = x2a.f
+}
+
+// Another interesting corner case are generic types that don't use
+// their type arguments. For instance:
+type T[P any] struct{}
+
+var xint T[int]
+var xbool T[bool]
+
+// Are these two variables of the same type? After all, their underlying
+// types are identical. We consider them to be different because each type
+// instantiation creates a new named type, in this case T<int> and T<bool>
+// even if their underlying types are identical. This is sensible because
+// we might still have methods that have different signatures or behave
+// differently depending on the type arguments, and thus we can't possibly
+// consider such types identical. Consequently:
+func _() {
+	xint = xbool // ERROR assignment
+}
+
+// Generic types cannot be used without instantiation.
+var _ T // ERROR cannot use generic type T
+var _ = T /* ERROR cannot use generic type T */ (0)
+
+// In type context, generic (parameterized) types cannot be parenthesized before
+// being instantiated. See also NOTES entry from 12/4/2019.
+var _ (T /* ERROR cannot use generic type T */ )[ /* ERROR unexpected \[|expected ';' */ int]
+
+// All types may be parameterized, including interfaces.
+type I1[T any] interface{
+	m1(T)
+}
+
+// There is no such thing as a variadic generic type.
+type _[T ... /* ERROR invalid use of ... */ any] struct{}
+
+// Generic interfaces may be embedded as one would expect.
+type I2 interface {
+	I1(int)     // method!
+	I1[string]  // embedded I1
+}
+
+func _() {
+	var x I2
+	x.I1(0)
+	x.m1("foo")
+}
+
+type I0 interface {
+	m0()
+}
+
+type I3 interface {
+	I0
+	I1[bool]
+	m(string)
+}
+
+func _() {
+	var x I3
+	x.m0()
+	x.m1(true)
+	x.m("foo")
+}
+
+type _ struct {
+	( /* ERROR cannot parenthesize */ int8)
+	( /* ERROR cannot parenthesize */ *int16)
+	*( /* ERROR cannot parenthesize */ int32)
+	List[int]
+
+	int8 /* ERROR int8 redeclared */
+	* /* ERROR int16 redeclared */ int16
+	List /* ERROR List redeclared */ [int]
+}
+
+// Issue #45639: We don't allow this anymore. Keep this code
+//               in case we decide to revisit this decision.
+//
+// It's possible to declare local types whose underlying types
+// are type parameters. As with ordinary type definitions, the
+// types underlying properties are "inherited" but the methods
+// are not.
+// func _[T interface{ m(); ~int }]() {
+// 	type L T
+// 	var x L
+// 
+// 	// m is not defined on L (it is not "inherited" from
+// 	// its underlying type).
+// 	x.m /* ERROR x.m undefined */ ()
+// 
+// 	// But the properties of T, such that as that it supports
+// 	// the operations of the types given by its type bound,
+// 	// are also the properties of L.
+// 	x++
+// 	_ = x - x
+// 
+// 	// On the other hand, if we define a local alias for T,
+// 	// that alias stands for T as expected.
+// 	type A = T
+// 	var y A
+// 	y.m()
+// 	_ = y < 0
+// }
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // It is not permitted to declare a local type whose underlying
+// // type is a type parameter not declared by that type declaration.
+// func _[T any]() {
+// 	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+// 	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
+// }
+
+// As a special case, an explicit type argument may be omitted
+// from a type parameter bound if the type bound expects exactly
+// one type argument. In that case, the type argument is the
+// respective type parameter to which the type bound applies.
+// Note: We may not permit this syntactic sugar at first.
+// Note: This is now disabled. All examples below are adjusted.
+type Adder[T any] interface {
+	Add(T) T
+}
+
+// We don't need to explicitly instantiate the Adder bound
+// if we have exactly one type parameter.
+func Sum[T Adder[T]](list []T) T {
+	var sum T
+	for _, x := range list {
+		sum = sum.Add(x)
+	}
+	return sum
+}
+
+// Valid and invalid variations.
+type B0 any
+type B1[_ any] any
+type B2[_, _ any] any
+
+func _[T1 B0]() {}
+func _[T1 B1[T1]]() {}
+func _[T1 B2 /* ERROR cannot use generic type .* without instantiation */ ]() {}
+
+func _[T1, T2 B0]() {}
+func _[T1 B1[T1], T2 B1[T2]]() {}
+func _[T1, T2 B2 /* ERROR cannot use generic type .* without instantiation */ ]() {}
+
+func _[T1 B0, T2 B1[T2]]() {} // here B1 applies to T2
+
+// When the type argument is left away, the type bound is
+// instantiated for each type parameter with that type
+// parameter.
+// Note: We may not permit this syntactic sugar at first.
+func _[A Adder[A], B Adder[B], C Adder[A]]() {
+	var a A // A's type bound is Adder[A]
+	a = a.Add(a)
+	var b B // B's type bound is Adder[B]
+	b = b.Add(b)
+	var c C // C's type bound is Adder[A]
+	a = c.Add(a)
+}
+
+// The type of variables (incl. parameters and return values) cannot
+// be an interface with type constraints or be/embed comparable.
+type I interface {
+	~int
+}
+
+var (
+	_ interface /* ERROR contains type constraints */ {~int}
+	_ I /* ERROR contains type constraints */
+)
+
+func _(I /* ERROR contains type constraints */ )
+func _(x, y, z I /* ERROR contains type constraints */ )
+func _() I /* ERROR contains type constraints */
+
+func _() {
+	var _ I /* ERROR contains type constraints */
+}
+
+type C interface {
+	comparable
+}
+
+var _ comparable /* ERROR comparable */
+var _ C /* ERROR comparable */
+
+func _(_ comparable /* ERROR comparable */ , _ C /* ERROR comparable */ )
+
+func _() {
+	var _ comparable /* ERROR comparable */
+	var _ C /* ERROR comparable */
+}
+
+// Type parameters are never const types, i.e., it's
+// not possible to declare a constant of type parameter type.
+// (If a type set contains just a single const type, we could
+// allow it, but such type sets don't make much sense in the
+// first place.)
+func _[T interface{~int|~float64}]() {
+	// not valid
+	const _ = T /* ERROR not constant */ (0)
+	const _ T /* ERROR invalid constant type T */ = 1
+
+	// valid
+	var _ = T(0)
+	var _ T = 1
+	_ = T(0)
+}
+
+// It is possible to create composite literals of type parameter
+// type as long as it's possible to create a composite literal
+// of the core type of the type parameter's constraint.
+func _[P interface{ ~[]int }]() P {
+	return P{}
+	return P{1, 2, 3}
+}
+
+func _[P interface{ ~[]E }, E interface{ map[string]P } ]() P {
+	x := P{}
+	return P{{}}
+	return P{E{}}
+	return P{E{"foo": x}}
+	return P{{"foo": x}, {}}
+}
+
+// This is a degenerate case with a singleton type set, but we can create
+// composite literals even if the core type is a defined type.
+type MyInts []int
+
+func _[P MyInts]() P {
+	return P{}
+}
diff --git a/src/internal/types/testdata/examples/typesets.go b/src/internal/types/testdata/examples/typesets.go
new file mode 100644
index 0000000..a50beb9
--- /dev/null
+++ b/src/internal/types/testdata/examples/typesets.go
@@ -0,0 +1,59 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of constraint literals with elided interfaces.
+// These examples are permitted if proposal issue #48424 is accepted.
+
+package p
+
+// Constraint type sets of the form T, ~T, or A|B may omit the interface.
+type (
+	_[T int]            struct{}
+	_[T ~int]           struct{}
+	_[T int | string]   struct{}
+	_[T ~int | ~string] struct{}
+)
+
+func min[T int | string](x, y T) T {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+func lookup[M ~map[K]V, K comparable, V any](m M, k K) V {
+	return m[k]
+}
+
+func deref[P ~*E, E any](p P) E {
+	return *p
+}
+
+func _() int {
+	p := new(int)
+	return deref(p)
+}
+
+func addrOfCopy[V any, P *V](v V) P {
+	return &v
+}
+
+func _() *int {
+	return addrOfCopy(0)
+}
+
+// A type parameter may not be embedded in an interface;
+// so it can also not be used as a constraint.
+func _[A any, B A /* ERROR cannot use a type parameter as constraint */]()    {}
+func _[A any, B, C A /* ERROR cannot use a type parameter as constraint */]() {}
+
+// Error messages refer to the type constraint as it appears in the source.
+// (No implicit interface should be exposed.)
+func _[T string](x T) T {
+	return x /* ERROR constrained by string */ * x
+}
+
+func _[T int | string](x T) T {
+	return x /* ERROR constrained by int|string */ * x
+}
diff --git a/src/internal/types/testdata/fixedbugs/54942.go b/src/internal/types/testdata/fixedbugs/54942.go
new file mode 100644
index 0000000..f2e733b
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/54942.go
@@ -0,0 +1,38 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import (
+	"context"
+	"database/sql"
+)
+
+type I interface {
+	m(int, int, *int, int)
+}
+
+type T struct{}
+
+func (_ *T) m(a, b, c, d int) {}
+
+var _ I = new /* ERROR have m\(int, int, int, int\)\n\t\twant m\(int, int, \*int, int\) */ (T)
+
+// (slightly modified) test case from issue
+
+type Result struct {
+	Value string
+}
+
+type Executor interface {
+	Execute(context.Context, sql.Stmt, int, []sql.NamedArg, int) (Result, error)
+}
+
+type myExecutor struct{}
+
+func (_ *myExecutor) Execute(ctx context.Context, stmt sql.Stmt, maxrows int, args []sql.NamedArg, urgency int) (*Result, error) {
+	return &Result{}, nil
+}
+
+var ex Executor = new /* ERROR have Execute\(context\.Context, sql\.Stmt, int, \[\]sql\.NamedArg, int\) \(\*Result, error\)\n\t\twant Execute\(context\.Context, sql\.Stmt, int, \[\]sql\.NamedArg, int\) \(Result, error\) */ (myExecutor)
diff --git a/src/internal/types/testdata/fixedbugs/issue20583.go b/src/internal/types/testdata/fixedbugs/issue20583.go
new file mode 100644
index 0000000..d26dbad
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue20583.go
@@ -0,0 +1,14 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20583
+
+const (
+	_ = 6e886451608 /* ERROR malformed constant */ /2
+	_ = 6e886451608i /* ERROR malformed constant */ /2
+	_ = 0 * 1e+1000000000 // ERROR malformed constant
+
+	x = 1e100000000
+	_ = x*x*x*x*x*x* /* ERROR not representable */ x
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue23203a.go b/src/internal/types/testdata/fixedbugs/issue23203a.go
new file mode 100644
index 0000000..48cb588
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue23203a.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "unsafe"
+
+type T struct{}
+
+func (T) m1()                         {}
+func (T) m2([unsafe.Sizeof(T.m1)]int) {}
+
+func main() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue23203b.go b/src/internal/types/testdata/fixedbugs/issue23203b.go
new file mode 100644
index 0000000..638ec6c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue23203b.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "unsafe"
+
+type T struct{}
+
+func (T) m2([unsafe.Sizeof(T.m1)]int) {}
+func (T) m1()                         {}
+
+func main() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue25838.go b/src/internal/types/testdata/fixedbugs/issue25838.go
new file mode 100644
index 0000000..adbd138
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue25838.go
@@ -0,0 +1,26 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// examples from the issue
+
+type (
+	e = f
+	f = g
+	g = []h
+	h i
+	i = j
+	j = e
+)
+
+type (
+	e1 = []h1
+	h1 e1
+)
+
+type (
+	P = *T
+	T P
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue26390.go b/src/internal/types/testdata/fixedbugs/issue26390.go
new file mode 100644
index 0000000..9e0101f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue26390.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// stand-alone test to ensure case is triggered
+
+package issue26390
+
+type A = T
+
+func (t *T) m() *A { return t }
+
+type T struct{}
diff --git a/src/internal/types/testdata/fixedbugs/issue28251.go b/src/internal/types/testdata/fixedbugs/issue28251.go
new file mode 100644
index 0000000..ef5e61d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue28251.go
@@ -0,0 +1,65 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for various forms of
+// method receiver declarations, per the spec clarification
+// https://golang.org/cl/142757.
+
+package issue28251
+
+// test case from issue28251
+type T struct{}
+
+type T0 = *T
+
+func (T0) m() {}
+
+func _() { (&T{}).m() }
+
+// various alternative forms
+type (
+        T1 = (((T)))
+)
+
+func ((*(T1))) m1() {}
+func _() { (T{}).m2() }
+func _() { (&T{}).m2() }
+
+type (
+        T2 = (((T3)))
+        T3 = T
+)
+
+func (T2) m2() {}
+func _() { (T{}).m2() }
+func _() { (&T{}).m2() }
+
+type (
+        T4 = ((*(T5)))
+        T5 = T
+)
+
+func (T4) m4() {}
+func _() { (T{}).m4 /* ERROR "cannot call pointer method m4 on T" */ () }
+func _() { (&T{}).m4() }
+
+type (
+        T6 = (((T7)))
+        T7 = (*(T8))
+        T8 = T
+)
+
+func (T6) m6() {}
+func _() { (T{}).m6 /* ERROR "cannot call pointer method m6 on T" */ () }
+func _() { (&T{}).m6() }
+
+type (
+        T9 = *T10
+        T10 = *T11
+        T11 = T
+)
+
+func (T9 /* ERROR invalid receiver type \*\*T */ ) m9() {}
+func _() { (T{}).m9 /* ERROR has no field or method m9 */ () }
+func _() { (&T{}).m9 /* ERROR has no field or method m9 */ () }
diff --git a/src/internal/types/testdata/fixedbugs/issue39634.go b/src/internal/types/testdata/fixedbugs/issue39634.go
new file mode 100644
index 0000000..6ee1548
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39634.go
@@ -0,0 +1,90 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Examples adjusted to match new [T any] syntax for type parameters.
+// Also, previously permitted empty type parameter lists and instantiations
+// are now syntax errors.
+
+package p
+
+// crash 1
+type nt1[_ any]interface{g /* ERROR undefined */ }
+type ph1[e nt1[e],g(d /* ERROR undefined */ )]s /* ERROR undefined */
+func(*ph1[e,e /* ERROR redeclared */ ])h(d /* ERROR undefined */ )
+
+// crash 2
+// Disabled: empty []'s are now syntax errors. This example leads to too many follow-on errors.
+// type Numeric2 interface{t2 /* ERROR not a type */ }
+// func t2[T Numeric2](s[]T){0 /* ERROR not a type */ []{s /* ERROR cannot index */ [0][0]}}
+
+// crash 3
+type t3 *interface{ t3.p /* ERROR t3.p is not a type */ }
+
+// crash 4
+type Numeric4 interface{t4 /* ERROR not a type */ }
+func t4[T Numeric4](s[]T){if( /* ERROR non-boolean */ 0){*s /* ERROR cannot indirect */ [0]}}
+
+// crash 7
+type foo7 interface { bar() }
+type x7[A any] struct{ foo7 }
+func main7() { var _ foo7 = x7[int]{} }
+
+// crash 8
+type foo8[A any] interface { ~A /* ERROR cannot be a type parameter */ }
+func bar8[A foo8[A]](a A) {}
+
+// crash 9
+type foo9[A any] interface { foo9 /* ERROR invalid recursive type */ [A] }
+func _() { var _ = new(foo9[int]) }
+
+// crash 12
+var u /* ERROR cycle */ , i [func /* ERROR used as value */ /* ERROR used as value */ (u, c /* ERROR undefined */ /* ERROR undefined */ ) {}(0, len /* ERROR must be called */ /* ERROR must be called */ )]c /* ERROR undefined */ /* ERROR undefined */
+
+// crash 15
+func y15() { var a /* ERROR declared and not used */ interface{ p() } = G15[string]{} }
+type G15[X any] s /* ERROR undefined */
+func (G15 /* ERROR generic type .* without instantiation */ ) p()
+
+// crash 16
+type Foo16[T any] r16 /* ERROR not a type */
+func r16[T any]() Foo16[Foo16[T]] { panic(0) }
+
+// crash 17
+type Y17 interface{ c() }
+type Z17 interface {
+	c() Y17
+	Y17 /* ERROR duplicate method */
+}
+func F17[T Z17](T) {}
+
+// crash 18
+type o18[T any] []func(_ o18[[]_ /* ERROR cannot use _ */ ])
+
+// crash 19
+type Z19 [][[]Z19{}[0][0]]c19 /* ERROR undefined */
+
+// crash 20
+type Z20 /* ERROR invalid recursive type */ interface{ Z20 }
+func F20[t Z20]() { F20(t /* ERROR invalid composite literal type */ {}) }
+
+// crash 21
+type Z21 /* ERROR invalid recursive type */ interface{ Z21 }
+func F21[T Z21]() { ( /* ERROR not used */ F21[Z21]) }
+
+// crash 24
+type T24[P any] P // ERROR cannot use a type parameter as RHS in type declaration
+func (r T24[P]) m() { T24 /* ERROR without instantiation */ .m() }
+
+// crash 25
+type T25[A any] int
+func (t T25[A]) m1() {}
+var x T25 /* ERROR without instantiation */ .m1
+
+// crash 26
+type T26 = interface{ F26[ /* ERROR interface method must have no type parameters */ Z any]() }
+func F26[Z any]() T26 { return F26[] /* ERROR operand */ }
+
+// crash 27
+func e27[T any]() interface{ x27 /* ERROR not a type */ } { panic(0) }
+func x27() { e27 /* ERROR cannot infer T */ () }
diff --git a/src/internal/types/testdata/fixedbugs/issue39664.go b/src/internal/types/testdata/fixedbugs/issue39664.go
new file mode 100644
index 0000000..3b3ec56
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39664.go
@@ -0,0 +1,15 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[_ any] struct {}
+
+func (T /* ERROR instantiation */ ) m()
+
+func _() {
+	var x interface { m() }
+	x = T[int]{}
+	_ = x
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39680.go b/src/internal/types/testdata/fixedbugs/issue39680.go
new file mode 100644
index 0000000..e56bc35
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39680.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+
+/*
+import "fmt"
+
+// Minimal test case.
+func _[T interface{~T}](x T) T{
+	return x
+}
+
+// Test case from issue.
+type constr[T any] interface {
+	~T
+}
+
+func Print[T constr[T]](s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func f() {
+	Print([]string{"Hello, ", "playground\n"})
+}
+*/
diff --git a/src/internal/types/testdata/fixedbugs/issue39693.go b/src/internal/types/testdata/fixedbugs/issue39693.go
new file mode 100644
index 0000000..496754d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39693.go
@@ -0,0 +1,23 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Number1 interface {
+	// embedding non-interface types is permitted
+	int
+	float64
+}
+
+func Add1[T Number1](a, b T) T {
+	return a /* ERROR not defined */ + b
+}
+
+type Number2 interface {
+	int | float64
+}
+
+func Add2[T Number2](a, b T) T {
+	return a + b
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39699.go b/src/internal/types/testdata/fixedbugs/issue39699.go
new file mode 100644
index 0000000..72f8399
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39699.go
@@ -0,0 +1,29 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T0 interface{
+}
+
+type T1 interface{
+	~int
+}
+
+type T2 interface{
+	comparable
+}
+
+type T3 interface {
+	T0
+	T1
+	T2
+}
+
+func _() {
+	_ = T0(0)
+	_ = T1 /* ERROR cannot use interface T1 in conversion */ (1)
+	_ = T2 /* ERROR cannot use interface T2 in conversion */ (2)
+	_ = T3 /* ERROR cannot use interface T3 in conversion */ (3)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39711.go b/src/internal/types/testdata/fixedbugs/issue39711.go
new file mode 100644
index 0000000..d85fa03
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39711.go
@@ -0,0 +1,13 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Do not report a duplicate type error for this term list.
+// (Check types after interfaces have been completed.)
+type _ interface {
+	// TODO(rfindley) Once we have full type sets we can enable this again.
+	// Fow now we don't permit interfaces in term lists.
+	// type interface{ Error() string }, interface{ String() string }
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39723.go b/src/internal/types/testdata/fixedbugs/issue39723.go
new file mode 100644
index 0000000..0088523
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39723.go
@@ -0,0 +1,9 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// A constraint must be an interface; it cannot
+// be a type parameter, for instance.
+func _[A interface{ ~int }, B A /* ERROR cannot use a type parameter as constraint */ ]() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue39725.go b/src/internal/types/testdata/fixedbugs/issue39725.go
new file mode 100644
index 0000000..62dc45a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39725.go
@@ -0,0 +1,16 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[T1, T2 any](T1, T2, struct{a T1; b T2}) {}
+func _() {
+	f1(42, string("foo"), struct /* ERROR does not match inferred type struct\{a int; b string\} */ {a, b int}{})
+}
+
+// simplified test case from issue
+func f2[T any](_ []T, _ func(T)) {}
+func _() {
+	f2([]string{}, func /* ERROR does not match inferred type func\(string\) */ (f []byte) {})
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39754.go b/src/internal/types/testdata/fixedbugs/issue39754.go
new file mode 100644
index 0000000..97365e2
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39754.go
@@ -0,0 +1,21 @@
+// Copyright 2020 The Go Authors. All rights reserved.

+// Use of this source code is governed by a BSD-style

+// license that can be found in the LICENSE file.

+

+package p

+

+type Optional[T any] struct {}

+

+func (_ Optional[T]) Val() (T, bool)

+

+type Box[T any] interface {

+	Val() (T, bool)

+}

+

+func f[V interface{}, A, B Box[V]]() {}

+

+func _() {

+	f[int, Optional[int], Optional[int]]()

+	_ = f[int, Optional[int], Optional /* ERROR does not satisfy Box */ [string]]

+	_ = f[int, Optional[int], Optional /* ERROR Optional.* does not satisfy Box.* */ [string]]

+}

diff --git a/src/internal/types/testdata/fixedbugs/issue39755.go b/src/internal/types/testdata/fixedbugs/issue39755.go
new file mode 100644
index 0000000..257b73a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39755.go
@@ -0,0 +1,23 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[T interface{~map[string]int}](x T) {
+	_ = x == nil
+}
+
+// simplified test case from issue
+
+type PathParamsConstraint interface {
+        ~map[string]string | ~[]struct{key, value string}
+}
+
+type PathParams[T PathParamsConstraint] struct {
+	t T
+}
+
+func (pp *PathParams[T]) IsNil() bool {
+	return pp.t == nil // this must succeed
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39768.go b/src/internal/types/testdata/fixedbugs/issue39768.go
new file mode 100644
index 0000000..696d9d9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39768.go
@@ -0,0 +1,21 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T[P any] P
+// type A = T  // ERROR cannot use generic type
+// var x A[int]
+// var _ A
+//
+// type B = T[int]
+// var y B = x
+// var _ B /* ERROR not a generic type */ [int]
+
+// test case from issue
+
+type Vector[T any] []T
+type VectorAlias = Vector // ERROR cannot use generic type
+var v Vector[int]
diff --git a/src/internal/types/testdata/fixedbugs/issue39938.go b/src/internal/types/testdata/fixedbugs/issue39938.go
new file mode 100644
index 0000000..633698d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39938.go
@@ -0,0 +1,54 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// All but E2 and E5 provide an "indirection" and break infinite expansion of a type.
+type E0[P any] []P
+type E1[P any] *P
+type E2[P any] struct{ _ P }
+type E3[P any] struct{ _ *P }
+type E5[P any] struct{ _ [10]P }
+
+type T0 struct {
+        _ E0[T0]
+}
+
+type T0_ struct {
+        E0[T0_]
+}
+
+type T1 struct {
+        _ E1[T1]
+}
+
+type T2 /* ERROR invalid recursive type */ struct {
+        _ E2[T2]
+}
+
+type T3 struct {
+        _ E3[T3]
+}
+
+type T4 /* ERROR invalid recursive type */ [10]E5[T4]
+
+type T5 struct {
+	_ E0[E2[T5]]
+}
+
+type T6 struct {
+	_ E0[E2[E0[E1[E2[[10]T6]]]]]
+}
+
+type T7 struct {
+	_ E0[[10]E2[E0[E2[E2[T7]]]]]
+}
+
+type T8 struct {
+	_ E0[[]E2[E0[E2[E2[T8]]]]]
+}
+
+type T9 /* ERROR invalid recursive type */ [10]E2[E5[E2[T9]]]
+
+type T10 [10]E2[E5[E2[func(T10)]]]
diff --git a/src/internal/types/testdata/fixedbugs/issue39948.go b/src/internal/types/testdata/fixedbugs/issue39948.go
new file mode 100644
index 0000000..c893cc0
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39948.go
@@ -0,0 +1,9 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] interface{
+	P // ERROR term cannot be a type parameter
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39976.go b/src/internal/types/testdata/fixedbugs/issue39976.go
new file mode 100644
index 0000000..d703da9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39976.go
@@ -0,0 +1,16 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type policy[K, V any] interface{}
+type LRU[K, V any] struct{}
+
+func NewCache[K, V any](p policy[K, V]) {}
+
+func _() {
+	var lru LRU[int, string]
+	NewCache[int, string](&lru)
+	NewCache(& /* ERROR does not match policy\[K, V\] \(cannot infer K and V\) */ lru)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue39982.go b/src/internal/types/testdata/fixedbugs/issue39982.go
new file mode 100644
index 0000000..9810b63
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue39982.go
@@ -0,0 +1,36 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	T[_ any] struct{}
+	S[_ any] struct {
+		data T[*T[int]]
+	}
+)
+
+func _() {
+	_ = S[int]{
+		data: T[*T[int]]{},
+	}
+}
+
+// full test case from issue
+
+type (
+	Element[TElem any] struct{}
+
+	entry[K comparable] struct{}
+
+	Cache[K comparable] struct {
+		data map[K]*Element[*entry[K]]
+	}
+)
+
+func _() {
+	_ = Cache[int]{
+		data: make(map[int](*Element[*entry[int]])),
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40038.go b/src/internal/types/testdata/fixedbugs/issue40038.go
new file mode 100644
index 0000000..5f81fcb
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40038.go
@@ -0,0 +1,15 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type A[T any] int
+
+func (A[T]) m(A[T])
+
+func f[P interface{m(P)}]() {}
+
+func _() {
+	_ = f[A[int]]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40056.go b/src/internal/types/testdata/fixedbugs/issue40056.go
new file mode 100644
index 0000000..66130c0
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40056.go
@@ -0,0 +1,15 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	NewS /* ERROR cannot infer T */ ().M()
+}
+
+type S struct {}
+
+func NewS[T any]() *S { panic(0) }
+
+func (_ *S /* ERROR S is not a generic type */ [T]) M()
diff --git a/src/internal/types/testdata/fixedbugs/issue40057.go b/src/internal/types/testdata/fixedbugs/issue40057.go
new file mode 100644
index 0000000..fdc8fb1
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40057.go
@@ -0,0 +1,17 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	var x interface{}
+	switch t := x.(type) {
+	case S /* ERROR cannot use generic type */ :
+		t.m()
+	}
+}
+
+type S[T any] struct {}
+
+func (_ S[T]) m()
diff --git a/src/internal/types/testdata/fixedbugs/issue40301.go b/src/internal/types/testdata/fixedbugs/issue40301.go
new file mode 100644
index 0000000..c78f9a1
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40301.go
@@ -0,0 +1,12 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+func _[T any](x T) {
+	_ = unsafe.Alignof(x)
+	_ = unsafe.Sizeof(x)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40350.go b/src/internal/types/testdata/fixedbugs/issue40350.go
new file mode 100644
index 0000000..08eb426
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40350.go
@@ -0,0 +1,16 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type number interface {
+	~float64 | ~int | ~int32
+	float64 | ~int32
+}
+
+func f[T number]() {}
+
+func _() {
+	_ = f[int /* ERROR int does not satisfy number \(number mentions int, but int is not in the type set of number\)*/]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40684.go b/src/internal/types/testdata/fixedbugs/issue40684.go
new file mode 100644
index 0000000..63a058d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40684.go
@@ -0,0 +1,15 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[_ any] int
+
+func f[_ any]() {}
+func g[_, _ any]() {}
+
+func _() {
+	_ = f[T /* ERROR without instantiation */ ]
+	_ = g[T /* ERROR without instantiation */ , T /* ERROR without instantiation */ ]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue40789.go b/src/internal/types/testdata/fixedbugs/issue40789.go
new file mode 100644
index 0000000..9eea4ad
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue40789.go
@@ -0,0 +1,37 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+func main() {
+	m := map[string]int{
+		"a": 6,
+		"b": 7,
+	}
+	fmt.Println(copyMap[map[string]int, string, int](m))
+}
+
+type Map[K comparable, V any] interface {
+	map[K] V
+}
+
+func copyMap[M Map[K, V], K comparable, V any](m M) M {
+	m1 := make(M)
+	for k, v := range m {
+		m1[k] = v
+	}
+	return m1
+}
+
+// simpler test case from the same issue
+
+type A[X comparable] interface {
+	[]X
+}
+
+func f[B A[X], X comparable]() B {
+	return nil
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue41124.go b/src/internal/types/testdata/fixedbugs/issue41124.go
new file mode 100644
index 0000000..80d1ff4
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue41124.go
@@ -0,0 +1,91 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Test case from issue.
+
+type Nat /* ERROR invalid recursive type */ interface {
+	Zero|Succ
+}
+
+type Zero struct{}
+type Succ struct{
+	Nat // Nat contains type constraints but is invalid, so no error
+}
+
+// Struct tests.
+
+type I1 interface {
+	comparable
+}
+
+type I2 interface {
+	~int
+}
+
+type I3 interface {
+	I1
+	I2
+}
+
+type _ struct {
+	f I1 // ERROR interface is .* comparable
+}
+
+type _ struct {
+	comparable // ERROR interface is .* comparable
+}
+
+type _ struct{
+	I1 // ERROR interface is .* comparable
+}
+
+type _ struct{
+	I2 // ERROR interface contains type constraints
+}
+
+type _ struct{
+	I3 // ERROR interface contains type constraints
+}
+
+// General composite types.
+
+type (
+	_ [10]I1 // ERROR interface is .* comparable
+	_ [10]I2 // ERROR interface contains type constraints
+
+	_ []I1 // ERROR interface is .* comparable
+	_ []I2 // ERROR interface contains type constraints
+
+	_ *I3 // ERROR interface contains type constraints
+	_ map[I1 /* ERROR interface is .* comparable */ ]I2 // ERROR interface contains type constraints
+	_ chan I3 // ERROR interface contains type constraints
+	_ func(I1 /* ERROR interface is .* comparable */ )
+	_ func() I2 // ERROR interface contains type constraints
+)
+
+// Other cases.
+
+var _ = [...]I3 /* ERROR interface contains type constraints */ {}
+
+func _(x interface{}) {
+	_ = x.(I3 /* ERROR interface contains type constraints */ )
+}
+
+type T1[_ any] struct{}
+type T3[_, _, _ any] struct{}
+var _ T1[I2 /* ERROR interface contains type constraints */ ]
+var _ T3[int, I2 /* ERROR interface contains type constraints */ , float32]
+
+func f1[_ any]() int { panic(0) }
+var _ = f1[I2 /* ERROR interface contains type constraints */ ]()
+func f3[_, _, _ any]() int { panic(0) }
+var _ = f3[int, I2 /* ERROR interface contains type constraints */ , float32]()
+
+func _(x interface{}) {
+	switch x.(type) {
+	case I2 /* ERROR interface contains type constraints */ :
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue42695.go b/src/internal/types/testdata/fixedbugs/issue42695.go
new file mode 100644
index 0000000..d0d6200
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue42695.go
@@ -0,0 +1,17 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue42695
+
+const _ = 6e5518446744 // ERROR malformed constant
+const _ uint8 = 6e5518446744 // ERROR malformed constant
+
+var _ = 6e5518446744 // ERROR malformed constant
+var _ uint8 = 6e5518446744 // ERROR malformed constant
+
+func f(x int) int {
+        return x + 6e5518446744 // ERROR malformed constant
+}
+
+var _ = f(6e5518446744 /* ERROR malformed constant */ )
diff --git a/src/internal/types/testdata/fixedbugs/issue42758.go b/src/internal/types/testdata/fixedbugs/issue42758.go
new file mode 100644
index 0000000..6d75b10
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue42758.go
@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[T any](x interface{}){
+	switch x.(type) {
+	case T: // ok to use a type parameter
+	case int:
+	}
+
+	switch x.(type) {
+	case T:
+	case T /* ERROR duplicate case */ :
+	}
+}
+
+type constraint interface {
+	~int
+}
+
+func _[T constraint](x interface{}){
+	switch x.(type) {
+	case T: // ok to use a type parameter even if type set contains int
+	case int:
+	}
+}
+
+func _(x constraint /* ERROR contains type constraints */ ) {
+	switch x.(type) { // no need to report another error
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue42881.go b/src/internal/types/testdata/fixedbugs/issue42881.go
new file mode 100644
index 0000000..7122d1c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue42881.go
@@ -0,0 +1,16 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	T1 interface{ comparable }
+	T2 interface{ int }
+)
+
+var (
+	_ comparable // ERROR cannot use type comparable outside a type constraint: interface is \(or embeds\) comparable
+	_ T1         // ERROR cannot use type T1 outside a type constraint: interface is \(or embeds\) comparable
+	_ T2         // ERROR cannot use type T2 outside a type constraint: interface contains type constraints
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue42987.go b/src/internal/types/testdata/fixedbugs/issue42987.go
new file mode 100644
index 0000000..f58c63f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue42987.go
@@ -0,0 +1,8 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that there is only one error (no follow-on errors).
+
+package p
+var _ = [ ... /* ERROR invalid use of \[...\] array */ ]byte("foo")
\ No newline at end of file
diff --git a/src/internal/types/testdata/fixedbugs/issue43056.go b/src/internal/types/testdata/fixedbugs/issue43056.go
new file mode 100644
index 0000000..8ff4e7f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43056.go
@@ -0,0 +1,31 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// simplified example
+func f[T ~func(T)](a, b T) {}
+
+type F func(F)
+
+func _() {
+	var i F
+	var j func(F)
+
+	f(i, j)
+	f(j, i)
+}
+
+// example from issue
+func g[T interface{ Equal(T) bool }](a, b T) {}
+
+type I interface{ Equal(I) bool }
+
+func _() {
+	var i I
+	var j interface{ Equal(I) bool }
+
+	g(i, j)
+	g(j, i)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue43087.go b/src/internal/types/testdata/fixedbugs/issue43087.go
new file mode 100644
index 0000000..85d4450
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43087.go
@@ -0,0 +1,43 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
+	_ = a
+	_ = b
+}
+
+func _() {
+	a, _, _ := 1, 2, 3 // multiple _'s ok
+	_ = a
+}
+
+func _() {
+	var b int
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
+	_ = a
+	_ = b
+}
+
+func _() {
+	var a []int
+	a /* ERROR non-name .* on left side of := */ [0], b := 1, 2
+	_ = a
+	_ = b
+}
+
+func _() {
+	var a int
+	a, a /* ERROR a repeated on left side of := */ := 1, 2
+	_ = a
+}
+
+func _() {
+	var a, b int
+	a, b := /* ERROR no new variables on left side of := */ 1, 2
+	_ = a
+	_ = b
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue43109.go b/src/internal/types/testdata/fixedbugs/issue43109.go
new file mode 100644
index 0000000..f242f16
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43109.go
@@ -0,0 +1,10 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Ensure there is no "imported and not used" error
+// if a package wasn't imported in the first place.
+
+package p
+
+import . "/foo" // ERROR could not import \/foo
diff --git a/src/internal/types/testdata/fixedbugs/issue43110.go b/src/internal/types/testdata/fixedbugs/issue43110.go
new file mode 100644
index 0000000..8d5c983
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43110.go
@@ -0,0 +1,43 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type P *struct{}
+
+func _() {
+	// want an error even if the switch is empty
+	var a struct{ _ func() }
+	switch a /* ERROR cannot switch on a */ {
+	}
+
+	switch a /* ERROR cannot switch on a */ {
+	case a: // no follow-on error here
+	}
+
+	// this is ok because f can be compared to nil
+	var f func()
+	switch f {
+	}
+
+	switch f {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case f /* ERROR invalid case f in switch on .* \(func can only be compared to nil\) */ :
+	}
+
+	switch nil /* ERROR use of untyped nil in switch expression */ {
+	}
+
+	// this is ok
+	switch P(nil) {
+	case P(nil):
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue43124.go b/src/internal/types/testdata/fixedbugs/issue43124.go
new file mode 100644
index 0000000..f429f74
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43124.go
@@ -0,0 +1,16 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = int(0 /* ERROR invalid use of \.\.\. in conversion to int */ ...)
+
+// test case from issue
+
+type M []string
+
+var (
+	x = []string{"a", "b"}
+	_ = M(x /* ERROR invalid use of \.\.\. in conversion to M */ ...)
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue43125.go b/src/internal/types/testdata/fixedbugs/issue43125.go
new file mode 100644
index 0000000..456888d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43125.go
@@ -0,0 +1,8 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = new(- /* ERROR not a type */ 1)
+var _ = new(1 /* ERROR not a type */ + 1)
diff --git a/src/internal/types/testdata/fixedbugs/issue43190.go b/src/internal/types/testdata/fixedbugs/issue43190.go
new file mode 100644
index 0000000..d1b46b5
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43190.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The errors below are produced by the parser, but we check
+// them here for consistency with the types2 tests.
+
+package p
+
+import ; // ERROR missing import path
+import "" // ERROR invalid import path \(empty string\)
+import
+var /* ERROR missing import path */ _ int
+import .; //  ERROR missing import path
+import 'x' // ERROR import path must be a string
+var _ int
+import /* ERROR imports must appear before other declarations */ _ "math"
+
+// Don't repeat previous error for each immediately following import ...
+import ()
+import (.) // ERROR missing import path
+import (
+	"fmt"
+	.
+) // ERROR missing import path
+
+// ... but remind with error again if we start a new import section after
+// other declarations
+var _ = fmt.Println
+import /* ERROR imports must appear before other declarations */ _ "math"
+import _ "math"
diff --git a/src/internal/types/testdata/fixedbugs/issue43527.go b/src/internal/types/testdata/fixedbugs/issue43527.go
new file mode 100644
index 0000000..b515100
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43527.go
@@ -0,0 +1,16 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+const L = 10
+
+type (
+	_        [L]struct{}
+	_        [A /* ERROR undefined array length A or missing type constraint */ ]struct{}
+	_        [B /* ERROR invalid array length B */ ]struct{}
+	_[A any] struct{}
+
+	B int
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue43671.go b/src/internal/types/testdata/fixedbugs/issue43671.go
new file mode 100644
index 0000000..6879aec
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue43671.go
@@ -0,0 +1,58 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type C0 interface{ int }
+type C1 interface{ chan int }
+type C2 interface{ chan int | <-chan int }
+type C3 interface{ chan int | chan float32 }
+type C4 interface{ chan int | chan<- int }
+type C5[T any] interface{ ~chan T | <-chan T }
+
+func _[T any](ch T) {
+	<-ch // ERROR cannot receive from ch .* \(no core type\)
+}
+
+func _[T C0](ch T) {
+	<-ch // ERROR cannot receive from non-channel ch
+}
+
+func _[T C1](ch T) {
+	<-ch
+}
+
+func _[T C2](ch T) {
+	<-ch
+}
+
+func _[T C3](ch T) {
+	<-ch // ERROR cannot receive from ch .* \(no core type\)
+}
+
+func _[T C4](ch T) {
+	<-ch // ERROR cannot receive from send-only channel
+}
+
+func _[T C5[X], X any](ch T, x X) {
+	x = <-ch
+}
+
+// test case from issue, slightly modified
+type RecvChan[T any] interface {
+	~chan T | ~<-chan T
+}
+
+func _[T any, C RecvChan[T]](ch C) T {
+	return <-ch
+}
+
+func f[T any, C interface{ chan T }](ch C) T {
+	return <-ch
+}
+
+func _(ch chan int) {
+	var x int = f(ch) // test constraint type inference for this case
+	_ = x
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue44688.go b/src/internal/types/testdata/fixedbugs/issue44688.go
new file mode 100644
index 0000000..512bfcc
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue44688.go
@@ -0,0 +1,83 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package P
+
+type A1[T any] struct{}
+
+func (*A1[T]) m1(T) {}
+
+type A2[T any] interface {
+	m2(T)
+}
+
+type B1[T any] struct {
+	filler int
+	*A1[T]
+	A2[T]
+}
+
+type B2[T any] interface {
+	A2[T]
+}
+
+type C[T any] struct {
+	filler1 int
+	filler2 int
+	B1[T]
+}
+
+type D[T any] struct {
+	filler1 int
+	filler2 int
+	filler3 int
+	C[T]
+}
+
+func _() {
+	// calling embedded methods
+	var b1 B1[string]
+
+	b1.A1.m1("")
+	b1.m1("")
+
+	b1.A2.m2("")
+	b1.m2("")
+
+	var b2 B2[string]
+	b2.m2("")
+
+	// a deeper nesting
+	var d D[string]
+	d.m1("")
+	d.m2("")
+
+	// calling method expressions
+	m1x := B1[string].m1
+	m1x(b1, "")
+	m2x := B2[string].m2
+	m2x(b2, "")
+
+	// calling method values
+	m1v := b1.m1
+	m1v("")
+	m2v := b1.m2
+	m2v("")
+	b2v := b2.m2
+	b2v("")
+}
+
+// actual test case from issue
+
+type A[T any] struct{}
+
+func (*A[T]) f(T) {}
+
+type B[T any] struct{ A[T] }
+
+func _() {
+	var b B[string]
+	b.A.f("")
+	b.f("")
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue44799.go b/src/internal/types/testdata/fixedbugs/issue44799.go
new file mode 100644
index 0000000..9e528a7
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue44799.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func Map[F, T any](s []F, f func(F) T) []T { return nil }
+
+func Reduce[Elem1, Elem2 any](s []Elem1, initializer Elem2, f func(Elem2, Elem1) Elem2) Elem2 { var x Elem2; return x }
+
+func main() {
+	var s []int
+	var f1 func(int) float64
+	var f2 func(float64, int) float64
+	_ = Map[int](s, f1)
+	_ = Map(s, f1)
+	_ = Reduce[int](s, 0, f2)
+	_ = Reduce(s, 0, f2)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue45114.go b/src/internal/types/testdata/fixedbugs/issue45114.go
new file mode 100644
index 0000000..0093660
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45114.go
@@ -0,0 +1,8 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var s uint
+var _ = string(1 /* ERROR shifted operand 1 .* must be integer */ << s)
diff --git a/src/internal/types/testdata/fixedbugs/issue45548.go b/src/internal/types/testdata/fixedbugs/issue45548.go
new file mode 100644
index 0000000..01c9672
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45548.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[F interface{*Q}, G interface{*R}, Q, R any](q Q, r R) {}
+
+func _() {
+	f[*float64, *int](1, 2)
+	f[*float64](1, 2)
+	f(1, 2)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue45550.go b/src/internal/types/testdata/fixedbugs/issue45550.go
new file mode 100644
index 0000000..498b1eb
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45550.go
@@ -0,0 +1,10 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Builder /* ERROR invalid recursive type */ [T interface{ struct{ Builder[T] } }] struct{}
+type myBuilder struct {
+	Builder[myBuilder]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue45635.go b/src/internal/types/testdata/fixedbugs/issue45635.go
new file mode 100644
index 0000000..af05ff0
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45635.go
@@ -0,0 +1,31 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+	some /* ERROR "undefined" */ [int, int]()
+}
+
+type N[T any] struct{}
+
+var _ N [] // ERROR expected type argument list
+
+type I interface {
+	~[]int
+}
+
+func _[T I](i, j int) {
+	var m map[int]int
+	_ = m[i, j /* ERROR "more than one index" */ ]
+
+	var a [3]int
+	_ = a[i, j /* ERROR "more than one index" */ ]
+
+	var s []int
+	_ = s[i, j /* ERROR "more than one index" */ ]
+
+	var t T
+	_ = t[i, j /* ERROR "more than one index" */ ]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue45639.go b/src/internal/types/testdata/fixedbugs/issue45639.go
new file mode 100644
index 0000000..80148fe
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45639.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package P
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // It is not permitted to declare a local type whose underlying
+// // type is a type parameters not declared by that type declaration.
+// func _[T any]() {
+// 	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
+// 	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
+// }
diff --git a/src/internal/types/testdata/fixedbugs/issue45920.go b/src/internal/types/testdata/fixedbugs/issue45920.go
new file mode 100644
index 0000000..0a281c5
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45920.go
@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[T any, C chan T | <-chan T](ch C) {}
+
+func _(ch chan int)   { f1(ch) }
+func _(ch <-chan int) { f1(ch) }
+func _(ch chan<- int) { f1 /* ERROR chan<- int does not satisfy chan int \| <-chan int */ (ch) }
+
+func f2[T any, C chan T | chan<- T](ch C) {}
+
+func _(ch chan int)   { f2(ch) }
+func _(ch <-chan int) { f2 /* ERROR <-chan int does not satisfy chan int \| chan<- int */ (ch) }
+func _(ch chan<- int) { f2(ch) }
diff --git a/src/internal/types/testdata/fixedbugs/issue45985.go b/src/internal/types/testdata/fixedbugs/issue45985.go
new file mode 100644
index 0000000..ae04ce2
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue45985.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue45985
+
+func app[S interface{ ~[]T }, T any](s S, e T) S {
+	return append(s, e)
+}
+
+func _() {
+	_ = app /* ERROR "int does not match" */ [int]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue46090.go b/src/internal/types/testdata/fixedbugs/issue46090.go
new file mode 100644
index 0000000..07f0101
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46090.go
@@ -0,0 +1,11 @@
+// -lang=go1.17
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The predeclared type comparable is not visible before Go 1.18.
+
+package p
+
+type _ comparable // ERROR predeclared comparable
diff --git a/src/internal/types/testdata/fixedbugs/issue46275.go b/src/internal/types/testdata/fixedbugs/issue46275.go
new file mode 100644
index 0000000..0862d5b
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46275.go
@@ -0,0 +1,26 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue46275
+
+type N[T any] struct {
+	*N[T]
+	t T
+}
+
+func (n *N[T]) Elem() T {
+	return n.t
+}
+
+type I interface {
+	Elem() string
+}
+
+func _() {
+	var n1 *N[string]
+	var _ I = n1
+	type NS N[string]
+	var n2 *NS
+	var _ I = n2
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue46403.go b/src/internal/types/testdata/fixedbugs/issue46403.go
new file mode 100644
index 0000000..9d47522
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46403.go
@@ -0,0 +1,11 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue46403
+
+func _() {
+	// a should be used, despite the parser error below.
+	var a []int
+	var _ = a[] // ERROR expected operand
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue46404.go b/src/internal/types/testdata/fixedbugs/issue46404.go
new file mode 100644
index 0000000..e3c93f6
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46404.go
@@ -0,0 +1,10 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue46404
+
+// TODO(gri) re-enable this test with matching errors
+//           between go/types and types2
+// Check that we don't type check t[_] as an instantiation.
+// type t [t /* type parameters must be named */ /* not a generic type */ [_]]_ // cannot use
diff --git a/src/internal/types/testdata/fixedbugs/issue46461.go b/src/internal/types/testdata/fixedbugs/issue46461.go
new file mode 100644
index 0000000..fce06f7
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46461.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// test case 1
+type T /* ERROR invalid recursive type */ [U interface{ M() T[U] }] int
+
+type X int
+
+func (X) M() T[X] { return 0 }
+
+// test case 2
+type A /* ERROR invalid recursive type */ [T interface{ A[T] }] interface{}
+
+// test case 3
+type A2 /* ERROR invalid recursive type */ [U interface{ A2[U] }] interface{ M() A2[U] }
+
+type I interface{ A2[I]; M() A2[I] }
diff --git a/src/internal/types/testdata/fixedbugs/issue46583.go b/src/internal/types/testdata/fixedbugs/issue46583.go
new file mode 100644
index 0000000..da1f1ff
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue46583.go
@@ -0,0 +1,28 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T1 struct{}
+func (t T1) m(int) {}
+var f1 func(T1)
+
+type T2 struct{}
+func (t T2) m(x int) {}
+var f2 func(T2)
+
+type T3 struct{}
+func (T3) m(int) {}
+var f3 func(T3)
+
+type T4 struct{}
+func (T4) m(x int) {}
+var f4 func(T4)
+
+func _() {
+	f1 = T1 /* ERROR func\(T1, int\) */ .m
+	f2 = T2 /* ERROR func\(t T2, x int\) */ .m
+	f3 = T3 /* ERROR func\(T3, int\) */ .m
+	f4 = T4 /* ERROR func\(_ T4, x int\) */ .m
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue47031.go b/src/internal/types/testdata/fixedbugs/issue47031.go
new file mode 100644
index 0000000..b184f9b
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47031.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Mer interface { M() }
+
+func F[T Mer](p *T) {
+	p.M /* ERROR p\.M undefined */ ()
+}
+
+type MyMer int
+
+func (MyMer) M() {}
+
+func _() {
+	F(new(MyMer))
+	F[Mer](nil)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue47115.go b/src/internal/types/testdata/fixedbugs/issue47115.go
new file mode 100644
index 0000000..a0bfe38
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47115.go
@@ -0,0 +1,40 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type C0 interface{ int }
+type C1 interface{ chan int }
+type C2 interface{ chan int | <-chan int }
+type C3 interface{ chan int | chan float32 }
+type C4 interface{ chan int | chan<- int }
+type C5[T any] interface{ ~chan T | chan<- T }
+
+func _[T any](ch T) {
+	ch <- /* ERROR cannot send to ch .* no core type */ 0
+}
+
+func _[T C0](ch T) {
+	ch <- /* ERROR cannot send to non-channel */ 0
+}
+
+func _[T C1](ch T) {
+	ch <- 0
+}
+
+func _[T C2](ch T) {
+	ch  <-/* ERROR cannot send to receive-only channel */ 0
+}
+
+func _[T C3](ch T) {
+	ch <- /* ERROR cannot send to ch .* no core type */ 0
+}
+
+func _[T C4](ch T) {
+	ch <- 0
+}
+
+func _[T C5[X], X any](ch T, x X) {
+	ch <- x
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue47127.go b/src/internal/types/testdata/fixedbugs/issue47127.go
new file mode 100644
index 0000000..bb4b487
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47127.go
@@ -0,0 +1,37 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Embedding of stand-alone type parameters is not permitted.
+
+package p
+
+type (
+        _[P any] interface{ *P | []P | chan P | map[string]P }
+        _[P any] interface{ P /* ERROR term cannot be a type parameter */ }
+        _[P any] interface{ ~P /* ERROR type in term ~P cannot be a type parameter */ }
+        _[P any] interface{ int | P /* ERROR term cannot be a type parameter */ }
+        _[P any] interface{ int | ~P /* ERROR type in term ~P cannot be a type parameter */ }
+)
+
+func _[P any]() {
+        type (
+                _[P any] interface{ *P | []P | chan P | map[string]P }
+                _[P any] interface{ P /* ERROR term cannot be a type parameter */ }
+                _[P any] interface{ ~P /* ERROR type in term ~P cannot be a type parameter */ }
+                _[P any] interface{ int | P /* ERROR term cannot be a type parameter */ }
+                _[P any] interface{ int | ~P /* ERROR type in term ~P cannot be a type parameter */ }
+
+                _ interface{ *P | []P | chan P | map[string]P }
+                _ interface{ P /* ERROR term cannot be a type parameter */ }
+                _ interface{ ~P /* ERROR type in term ~P cannot be a type parameter */ }
+                _ interface{ int | P /* ERROR term cannot be a type parameter */ }
+                _ interface{ int | ~P /* ERROR type in term ~P cannot be a type parameter */ }
+        )
+}
+
+func _[P any, Q interface{ *P | []P | chan P | map[string]P }]() {}
+func _[P any, Q interface{ P /* ERROR term cannot be a type parameter */ }]() {}
+func _[P any, Q interface{ ~P /* ERROR type in term ~P cannot be a type parameter */ }]() {}
+func _[P any, Q interface{ int | P /* ERROR term cannot be a type parameter */ }]() {}
+func _[P any, Q interface{ int | ~P /* ERROR type in term ~P cannot be a type parameter */ }]() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue47411.go b/src/internal/types/testdata/fixedbugs/issue47411.go
new file mode 100644
index 0000000..33b169a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47411.go
@@ -0,0 +1,26 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[_ comparable]() {}
+func g[_ interface{interface{comparable; ~int|~string}}]() {}
+
+func _[P comparable,
+        Q interface{ comparable; ~int|~string },
+        R any,                               // not comparable
+        S interface{ comparable; ~func() },  // not comparable
+]() {
+        _ = f[int]
+        _ = f[P]
+        _ = f[Q]
+        _ = f[func /* ERROR does not satisfy comparable */ ()]
+        _ = f[R /* ERROR R does not satisfy comparable */ ]
+
+        _ = g[int]
+        _ = g[P /* ERROR P does not satisfy interface{interface{comparable; ~int \| ~string} */ ]
+        _ = g[Q]
+        _ = g[func /* ERROR func\(\) does not satisfy interface{interface{comparable; ~int \| ~string}} */ ()]
+        _ = g[R /* ERROR R does not satisfy interface{interface{comparable; ~int \| ~string} */ ]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue47747.go b/src/internal/types/testdata/fixedbugs/issue47747.go
new file mode 100644
index 0000000..6f09fc2
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47747.go
@@ -0,0 +1,71 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type T1[P any] P
+// 
+// func (T1[_]) m() {}
+// 
+// func _[P any](x *T1[P]) {
+//         // x.m exists because x is of type *T1 where T1 is a defined type
+//         // (even though under(T1) is a type parameter)
+//         x.m()
+// }
+
+
+func _[P interface{ m() }](x P) {
+        x.m()
+        // (&x).m doesn't exist because &x is of type *P
+        // and pointers to type parameters don't have methods
+        (&x).m /* ERROR type \*P is pointer to type parameter, not type parameter */ ()
+}
+
+
+type T2 interface{ m() }
+
+func _(x *T2) {
+        // x.m doesn't exists because x is of type *T2
+        // and pointers to interfaces don't have methods
+        x.m /* ERROR type \*T2 is pointer to interface, not interface */()
+}
+
+// Test case 1 from issue
+
+type Fooer1[t any] interface {
+	Foo(Barer[t])
+}
+type Barer[t any] interface {
+	Bar(t)
+}
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type Foo1[t any] t
+// type Bar[t any] t
+// 
+// func (l Foo1[t]) Foo(v Barer[t]) { v.Bar(t(l)) }
+// func (b *Bar[t]) Bar(l t)        { *b = Bar[t](l) }
+// 
+// func _[t any](f Fooer1[t]) t {
+// 	var b Bar[t]
+// 	f.Foo(&b)
+// 	return t(b)
+// }
+
+// Test case 2 from issue
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type Fooer2[t any] interface {
+// 	Foo()
+// }
+// 
+// type Foo2[t any] t
+// 
+// func (f *Foo2[t]) Foo() {}
+// 
+// func _[t any](v t) {
+// 	var f = Foo2[t](v)
+// 	_ = Fooer2[t](&f)
+// }
diff --git a/src/internal/types/testdata/fixedbugs/issue47796.go b/src/internal/types/testdata/fixedbugs/issue47796.go
new file mode 100644
index 0000000..4c59106
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47796.go
@@ -0,0 +1,33 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// parameterized types with self-recursive constraints
+type (
+	T1 /* ERROR invalid recursive type */ [P T1[P]]                            interface{}
+	T2 /* ERROR invalid recursive type */ [P, Q T2[P, Q]]                      interface{}
+	T3[P T2[P, Q], Q interface{ ~string }] interface{}
+
+	T4a /* ERROR invalid recursive type */ [P T4a[P]]                                                        interface{ ~int }
+	T4b /* ERROR invalid recursive type */ [P T4b[int]]                                                      interface{ ~int }
+	T4c /* ERROR invalid recursive type */ [P T4c[string]] interface{ ~int }
+
+	// mutually recursive constraints
+	T5 /* ERROR invalid recursive type */ [P T6[P]] interface{ int }
+	T6[P T5[P]] interface{ int }
+)
+
+// verify that constraints are checked as expected
+var (
+	_ T1[int]
+	_ T2[int, string]
+	_ T3[int, string]
+)
+
+// test case from issue
+
+type Eq /* ERROR invalid recursive type */ [a Eq[a]] interface {
+	Equal(that a) bool
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue47818.go b/src/internal/types/testdata/fixedbugs/issue47818.go
new file mode 100644
index 0000000..e9b0adb
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47818.go
@@ -0,0 +1,61 @@
+// -lang=go1.17
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Parser accepts type parameters but the type checker
+// needs to report any operations that are not permitted
+// before Go 1.18.
+
+package p
+
+type T[P /* ERROR type parameter requires go1\.18 or later */ any /* ERROR predeclared any requires go1\.18 or later */] struct{}
+
+// for init (and main, but we're not in package main) we should only get one error
+func init[P /* ERROR func init must have no type parameters */ any /* ERROR predeclared any requires go1\.18 or later */]() {
+}
+func main[P /* ERROR type parameter requires go1\.18 or later */ any /* ERROR predeclared any requires go1\.18 or later */]() {
+}
+
+func f[P /* ERROR type parameter requires go1\.18 or later */ any /* ERROR predeclared any requires go1\.18 or later */](x P) {
+	var _ T[ /* ERROR type instantiation requires go1\.18 or later */ int]
+	var _ (T[ /* ERROR type instantiation requires go1\.18 or later */ int])
+	_ = T[ /* ERROR type instantiation requires go1\.18 or later */ int]{}
+	_ = T[ /* ERROR type instantiation requires go1\.18 or later */ int](struct{}{})
+}
+
+func (T[ /* ERROR type instantiation requires go1\.18 or later */ P]) g(x int) {
+	f[ /* ERROR function instantiation requires go1\.18 or later */ int](0)     // explicit instantiation
+	(f[ /* ERROR function instantiation requires go1\.18 or later */ int])(0)   // parentheses (different code path)
+	f( /* ERROR implicit function instantiation requires go1\.18 or later */ x) // implicit instantiation
+}
+
+type C1 interface {
+	comparable // ERROR predeclared comparable requires go1\.18 or later
+}
+
+type C2 interface {
+	comparable // ERROR predeclared comparable requires go1\.18 or later
+	int        // ERROR embedding non-interface type int requires go1\.18 or later
+	~ /* ERROR embedding interface element ~int requires go1\.18 or later */ int
+	int /* ERROR embedding interface element int \| ~string requires go1\.18 or later */ | ~string
+}
+
+type _ interface {
+	// errors for these were reported with their declaration
+	C1
+	C2
+}
+
+type (
+	_ comparable // ERROR predeclared comparable requires go1\.18 or later
+	// errors for these were reported with their declaration
+	_ C1
+	_ C2
+
+	_ = comparable // ERROR predeclared comparable requires go1\.18 or later
+	// errors for these were reported with their declaration
+	_ = C1
+	_ = C2
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue47887.go b/src/internal/types/testdata/fixedbugs/issue47887.go
new file mode 100644
index 0000000..4c4fc2f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47887.go
@@ -0,0 +1,28 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Fooer[t any] interface {
+	foo(Barer[t])
+}
+type Barer[t any] interface {
+	bar(Bazer[t])
+}
+type Bazer[t any] interface {
+	Fooer[t]
+	baz(t)
+}
+
+type Int int
+
+func (n Int) baz(int) {}
+func (n Int) foo(b Barer[int]) { b.bar(n) }
+
+type F[t any] interface { f(G[t]) }
+type G[t any] interface { g(H[t]) }
+type H[t any] interface { F[t] }
+
+type T struct{}
+func (n T) f(b G[T]) { b.g(n) }
diff --git a/src/internal/types/testdata/fixedbugs/issue47968.go b/src/internal/types/testdata/fixedbugs/issue47968.go
new file mode 100644
index 0000000..3dd3039
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue47968.go
@@ -0,0 +1,21 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] struct{}
+
+func (T[P]) m1()
+
+type A1 = T // ERROR cannot use generic type
+
+func (A1[P]) m2() {}
+
+type A2 = T[int]
+
+func (A2 /* ERROR cannot define new methods on instantiated type T\[int\] */) m3()   {}
+func (_ /* ERROR cannot define new methods on instantiated type T\[int\] */ A2) m4() {}
+
+func (T[int]) m5()                                     {} // int is the type parameter name, not an instantiation
+func (T[* /* ERROR must be an identifier */ int]) m6() {} // syntax error
diff --git a/src/internal/types/testdata/fixedbugs/issue48008.go b/src/internal/types/testdata/fixedbugs/issue48008.go
new file mode 100644
index 0000000..6c14c78
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48008.go
@@ -0,0 +1,60 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] struct{}
+
+func _(x interface{}) {
+	switch x.(type) {
+	case nil:
+	case int:
+
+	case T[int]:
+	case []T[int]:
+	case [10]T[int]:
+	case struct{T[int]}:
+	case *T[int]:
+	case func(T[int]):
+	case interface{m(T[int])}:
+	case map[T[int]] string:
+	case chan T[int]:
+
+	case T /* ERROR cannot use generic type T\[P any\] without instantiation */ :
+	case []T /* ERROR cannot use generic type */ :
+	case [10]T /* ERROR cannot use generic type */ :
+	case struct{T /* ERROR cannot use generic type */ }:
+	case *T /* ERROR cannot use generic type */ :
+	case func(T /* ERROR cannot use generic type */ ):
+	case interface{m(T /* ERROR cannot use generic type */ )}:
+	case map[T /* ERROR cannot use generic type */ ] string:
+	case chan T /* ERROR cannot use generic type */ :
+
+	case T /* ERROR cannot use generic type */ , *T /* ERROR cannot use generic type */ :
+	}
+}
+
+// Make sure a parenthesized nil is ok.
+
+func _(x interface{}) {
+	switch x.(type) {
+	case ((nil)), int:
+	}
+}
+
+// Make sure we look for the predeclared nil.
+
+func _(x interface{}) {
+	type nil int
+	switch x.(type) {
+	case nil: // ok - this is the type nil
+	}
+}
+
+func _(x interface{}) {
+	var nil int
+	switch x.(type) {
+	case nil /* ERROR not a type */ : // not ok - this is the variable nil
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48018.go b/src/internal/types/testdata/fixedbugs/issue48018.go
new file mode 100644
index 0000000..e6ccc6b
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48018.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type Box[A any] struct {
+	value A
+}
+
+func Nest[A /* ERROR instantiation cycle */ any](b Box[A], n int) interface{} {
+	if n == 0 {
+		return b
+	}
+	return Nest(Box[Box[A]]{b}, n-1)
+}
+
+func main() {
+	Nest(Box[int]{0}, 10)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48048.go b/src/internal/types/testdata/fixedbugs/issue48048.go
new file mode 100644
index 0000000..f401330
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48048.go
@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] struct{}
+
+func (T[_]) A() {}
+
+var _ = (T[int]).A
+var _ = (*T[int]).A
+
+var _ = (T /* ERROR cannot use generic type */).A
+var _ = (*T /* ERROR cannot use generic type */).A
diff --git a/src/internal/types/testdata/fixedbugs/issue48082.go b/src/internal/types/testdata/fixedbugs/issue48082.go
new file mode 100644
index 0000000..5395154
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48082.go
@@ -0,0 +1,7 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue48082
+
+import "init" /* ERROR init must be a func */ /* ERROR could not import init */
diff --git a/src/internal/types/testdata/fixedbugs/issue48083.go b/src/internal/types/testdata/fixedbugs/issue48083.go
new file mode 100644
index 0000000..3dae514
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48083.go
@@ -0,0 +1,9 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] struct{}
+
+type _ interface{ int | T /* ERROR cannot use generic type */ }
\ No newline at end of file
diff --git a/src/internal/types/testdata/fixedbugs/issue48136.go b/src/internal/types/testdata/fixedbugs/issue48136.go
new file mode 100644
index 0000000..b87f84a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48136.go
@@ -0,0 +1,36 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[P interface{ *P }]() {}
+func f2[P interface{ func(P) }]() {}
+func f3[P, Q interface{ func(Q) P }]() {}
+func f4[P interface{ *Q }, Q interface{ func(P) }]() {}
+func f5[P interface{ func(P) }]() {}
+func f6[P interface { *Tree[P] }, Q any ]() {}
+
+func _() {
+        f1 /* ERROR cannot infer P */ ()
+        f2 /* ERROR cannot infer P */ ()
+        f3 /* ERROR cannot infer P */ ()
+        f4 /* ERROR cannot infer P */ ()
+        f5 /* ERROR cannot infer P */ ()
+        f6 /* ERROR cannot infer P */ ()
+}
+
+type Tree[P any] struct {
+        left, right *Tree[P]
+        data P
+}
+
+// test case from issue
+
+func foo[Src interface { func() Src }]() Src {
+        return foo[Src]
+}
+
+func _() {
+        foo /* ERROR cannot infer Src */ ()
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48234.go b/src/internal/types/testdata/fixedbugs/issue48234.go
new file mode 100644
index 0000000..e069930
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48234.go
@@ -0,0 +1,10 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = interface{
+	m()
+	m /* ERROR "duplicate method" */ ()
+}(nil)
diff --git a/src/internal/types/testdata/fixedbugs/issue48312.go b/src/internal/types/testdata/fixedbugs/issue48312.go
new file mode 100644
index 0000000..2fdb7ca
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48312.go
@@ -0,0 +1,20 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T interface{ m() }
+type P *T
+
+func _(p *T) {
+	p.m /* ERROR type \*T is pointer to interface, not interface */ ()
+}
+
+func _(p P) {
+	p.m /* ERROR type P is pointer to interface, not interface */ ()
+}
+
+func _[P T](p *P) {
+	p.m /* ERROR type \*P is pointer to type parameter, not type parameter */ ()
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48472.go b/src/internal/types/testdata/fixedbugs/issue48472.go
new file mode 100644
index 0000000..2d908f4
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48472.go
@@ -0,0 +1,16 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func g() {
+	var s string
+	var i int
+	_ = s /* ERROR invalid operation: s \+ i \(mismatched types string and int\) */ + i
+}
+
+func f(i int) int {
+        i /* ERROR invalid operation: i \+= "1" \(mismatched types int and untyped string\) */ += "1"
+        return i
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48529.go b/src/internal/types/testdata/fixedbugs/issue48529.go
new file mode 100644
index 0000000..d7a70b1
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48529.go
@@ -0,0 +1,11 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T /* ERROR invalid recursive type */ [U interface{ M() T[U, int] }] int
+
+type X int
+
+func (X) M() T[X] { return 0 }
diff --git a/src/internal/types/testdata/fixedbugs/issue48582.go b/src/internal/types/testdata/fixedbugs/issue48582.go
new file mode 100644
index 0000000..9e1d526
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48582.go
@@ -0,0 +1,29 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type N /* ERROR invalid recursive type */ interface {
+	int | N
+}
+
+type A /* ERROR invalid recursive type */ interface {
+	int | B
+}
+
+type B interface {
+	int | A
+}
+
+type S /* ERROR invalid recursive type */ struct {
+	I // ERROR interface contains type constraints
+}
+
+type I interface {
+	int | S
+}
+
+type P interface {
+	*P // ERROR interface contains type constraints
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48619.go b/src/internal/types/testdata/fixedbugs/issue48619.go
new file mode 100644
index 0000000..72eea1e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48619.go
@@ -0,0 +1,22 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P any](a, _ P) {
+	var x int
+	// TODO(gri) these error messages, while correct, could be better
+	f(a, x /* ERROR type int of x does not match inferred type P for P */)
+	f(x, a /* ERROR type P of a does not match inferred type int for P */)
+}
+
+func g[P any](a, b P) {
+	g(a, b)
+	g(&a, &b)
+	g([]P{}, []P{})
+
+	// work-around: provide type argument explicitly
+	g[*P](&a, &b)
+	g[[]P]([]P{}, []P{})
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48656.go b/src/internal/types/testdata/fixedbugs/issue48656.go
new file mode 100644
index 0000000..0f60f47
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48656.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P *Q, Q any](P, Q) {
+	_ = f[P]
+}
+
+func f2[P /* ERROR instantiation cycle */ *Q, Q any](P, Q) {
+	_ = f2[*P]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48695.go b/src/internal/types/testdata/fixedbugs/issue48695.go
new file mode 100644
index 0000000..9f4a768
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48695.go
@@ -0,0 +1,14 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func g[P ~func(T) P, T any](P) {}
+
+func _() {
+	type F func(int) F
+	var f F
+	g(f)
+	_ = g[F]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48703.go b/src/internal/types/testdata/fixedbugs/issue48703.go
new file mode 100644
index 0000000..8a32c1e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48703.go
@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// The actual example from the issue.
+type List[P any] struct{}
+
+func (_ List[P]) m() (_ List[List[P]]) { return }
+
+// Other types of recursion through methods.
+type R[P any] int
+
+func (*R[R /* ERROR must be an identifier */ [int]]) m0() {}
+func (R[P]) m1(R[R[P]])                                   {}
+func (R[P]) m2(R[*P])                                     {}
+func (R[P]) m3([unsafe.Sizeof(new(R[P]))]int)             {}
+func (R[P]) m4([unsafe.Sizeof(new(R[R[P]]))]int)          {}
+
+// Mutual recursion
+type M[P any] int
+
+func (R[P]) m5(M[M[P]]) {}
+func (M[P]) m(R[R[P]])  {}
diff --git a/src/internal/types/testdata/fixedbugs/issue48712.go b/src/internal/types/testdata/fixedbugs/issue48712.go
new file mode 100644
index 0000000..63ce7bc
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48712.go
@@ -0,0 +1,41 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P comparable](x, y P) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR type parameter P is not comparable with < */ < y
+}
+
+func _[P comparable](x P, y any) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR type parameter P is not comparable with < */ < y
+}
+
+func _[P any](x, y P) {
+	_ = x /* ERROR incomparable types in type set */ == x
+	_ = x /* ERROR incomparable types in type set */ == y
+	_ = y /* ERROR incomparable types in type set */ == x
+	_ = y /* ERROR incomparable types in type set */ == y
+
+	_ = x /* ERROR type parameter P is not comparable with < */ < y
+}
+
+func _[P any](x P, y any) {
+	_ = x /* ERROR incomparable types in type set */ == x
+	_ = x /* ERROR incomparable types in type set */ == y
+	_ = y == x // ERROR incomparable types in type set
+	_ = y == y
+
+	_ = x /* ERROR type parameter P is not comparable with < */ < y
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48819.go b/src/internal/types/testdata/fixedbugs/issue48819.go
new file mode 100644
index 0000000..5d61803
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48819.go
@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type T /* ERROR invalid recursive type: T refers to itself */ struct {
+	T
+}
+
+func _(t T) {
+	_ = unsafe.Sizeof(t) // should not go into infinite recursion here
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48827.go b/src/internal/types/testdata/fixedbugs/issue48827.go
new file mode 100644
index 0000000..aa1d12a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48827.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type G[P any] int
+
+type (
+	_ G[int]
+	_ G[G /* ERROR "cannot use.*without instantiation" */]
+	_ bool /* ERROR "invalid operation: bool\[int\] \(bool is not a generic type\)" */ [int]
+	_ bool /* ERROR "invalid operation: bool\[G\] \(bool is not a generic type\)" */[G]
+)
+
+// The example from the issue.
+func _() {
+	_ = &([10]bool /* ERROR "invalid operation.*bool is not a generic type" */ [1 /* ERROR "expected type" */ ]{})
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48951.go b/src/internal/types/testdata/fixedbugs/issue48951.go
new file mode 100644
index 0000000..c94b027
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48951.go
@@ -0,0 +1,21 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+        A1[P any] [10]A1 /* ERROR invalid recursive type */ [P]
+        A2[P any] [10]A2 /* ERROR invalid recursive type */ [*P]
+        A3[P any] [10]*A3[P]
+
+        L1[P any] []L1[P]
+
+        S1[P any] struct{ f S1 /* ERROR invalid recursive type */ [P] }
+        S2[P any] struct{ f S2 /* ERROR invalid recursive type */ [*P] } // like example in issue
+        S3[P any] struct{ f *S3[P] }
+
+        I1[P any] interface{ I1 /* ERROR invalid recursive type */ [P] }
+        I2[P any] interface{ I2 /* ERROR invalid recursive type */ [*P] }
+        I3[P any] interface{ *I3 /* ERROR interface contains type constraints */ [P] }
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue48962.go b/src/internal/types/testdata/fixedbugs/issue48962.go
new file mode 100644
index 0000000..05c681d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48962.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T0[P any] struct {
+	f P
+}
+
+type T1 /* ERROR invalid recursive type */ struct {
+	_ T0[T1]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue48974.go b/src/internal/types/testdata/fixedbugs/issue48974.go
new file mode 100644
index 0000000..d8ff7c8
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue48974.go
@@ -0,0 +1,22 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Fooer interface {
+	Foo()
+}
+
+type Fooable[F /* ERROR instantiation cycle */ Fooer] struct {
+	ptr F
+}
+
+func (f *Fooable[F]) Adapter() *Fooable[*FooerImpl[F]] {
+	return &Fooable[*FooerImpl[F]]{&FooerImpl[F]{}}
+}
+
+type FooerImpl[F Fooer] struct {
+}
+
+func (fi *FooerImpl[F]) Foo() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue49003.go b/src/internal/types/testdata/fixedbugs/issue49003.go
new file mode 100644
index 0000000..ece1a27
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49003.go
@@ -0,0 +1,10 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f(s string) int {
+	for range s {
+	}
+} // ERROR missing return
diff --git a/src/internal/types/testdata/fixedbugs/issue49005.go b/src/internal/types/testdata/fixedbugs/issue49005.go
new file mode 100644
index 0000000..5f551b9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49005.go
@@ -0,0 +1,31 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T1 interface{ M() }
+
+func F1() T1
+
+var _ = F1().(*X1 /* ERROR undefined: X1 */)
+
+func _() {
+	switch F1().(type) {
+	case *X1 /* ERROR undefined: X1 */ :
+	}
+}
+
+type T2 interface{ M() }
+
+func F2() T2
+
+var _ = F2 /* ERROR impossible type assertion: F2\(\)\.\(\*X2\)\n\t\*X2 does not implement T2 \(missing method M\) */ ().(*X2)
+
+type X2 struct{}
+
+func _() {
+	switch F2().(type) {
+	case * /* ERROR impossible type switch case: \*X2\n\tF2\(\) \(value of type T2\) cannot have dynamic type \*X2 \(missing method M\) */ X2:
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49043.go b/src/internal/types/testdata/fixedbugs/issue49043.go
new file mode 100644
index 0000000..3971cf8
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49043.go
@@ -0,0 +1,24 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The example from the issue.
+type (
+	N[P any] M /* ERROR invalid recursive type */ [P]
+	M[P any] N[P]
+)
+
+// A slightly more complicated case.
+type (
+	A[P any] B /* ERROR invalid recursive type */ [P]
+	B[P any] C[P]
+	C[P any] A[P]
+)
+
+// Confusing but valid (note that `type T *T` is valid).
+type (
+	N1[P any] *M1[P]
+	M1[P any] *N1[P]
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue49112.go b/src/internal/types/testdata/fixedbugs/issue49112.go
new file mode 100644
index 0000000..dea2608
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49112.go
@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P int](P) {}
+
+func _() {
+        _ = f[int]
+        _ = f[[ /* ERROR \[\]int does not satisfy int */ ]int]
+
+        f(0)
+        f/* ERROR \[\]int does not satisfy int */ ([]int{})
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49179.go b/src/internal/types/testdata/fixedbugs/issue49179.go
new file mode 100644
index 0000000..2ddfa33
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49179.go
@@ -0,0 +1,37 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[P int | string]()            {}
+func f2[P ~int | string | float64]() {}
+func f3[P int](x P)                  {}
+
+type myInt int
+type myFloat float64
+
+func _() {
+	_ = f1[int]
+	_ = f1[myInt /* ERROR possibly missing ~ for int in int \| string */]
+	_ = f2[myInt]
+	_ = f2[myFloat /* ERROR possibly missing ~ for float64 in ~int \| string \| float64 */]
+	var x myInt
+	f3 /* ERROR myInt does not satisfy int \(possibly missing ~ for int in int\) */ (x)
+}
+
+// test case from the issue
+
+type SliceConstraint[T any] interface {
+	[]T
+}
+
+func Map[S SliceConstraint[E], E any](s S, f func(E) E) S {
+	return s
+}
+
+type MySlice []int
+
+func f(s MySlice) {
+	Map[MySlice /* ERROR MySlice does not satisfy SliceConstraint\[int\] \(possibly missing ~ for \[\]int in SliceConstraint\[int\]\) */, int](s, nil)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49242.go b/src/internal/types/testdata/fixedbugs/issue49242.go
new file mode 100644
index 0000000..524a0cb
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49242.go
@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P int](x P) int {
+	return x // ERROR cannot use x .* as int value in return statement
+}
+
+func _[P int]() int {
+	return P /* ERROR cannot use P\(1\) .* as int value in return statement */ (1)
+}
+
+func _[P int](x int) P {
+        return x // ERROR cannot use x .* as P value in return statement
+}
+
+func _[P, Q any](x P) Q {
+        return x // ERROR cannot use x .* as Q value in return statement
+}
+
+// test case from issue
+func F[G interface{ uint }]() int {
+	f := func(uint) int { return 0 }
+	return f(G /* ERROR cannot use G\(1\) .* as uint value in argument to f */ (1))
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49247.go b/src/internal/types/testdata/fixedbugs/issue49247.go
new file mode 100644
index 0000000..5be6001
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49247.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type integer interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+}
+
+func Add1024[T integer](s []T) {
+	for i, v := range s {
+		s[i] = v + 1024 // ERROR cannot convert 1024 \(untyped int constant\) to type T
+	}
+}
+
+func f[T interface{ int8 }]() {
+	println(T(1024 /* ERROR cannot convert 1024 \(untyped int value\) to type T */))
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49276.go b/src/internal/types/testdata/fixedbugs/issue49276.go
new file mode 100644
index 0000000..ab5794a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49276.go
@@ -0,0 +1,46 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type S /* ERROR invalid recursive type S */ struct {
+	_ [unsafe.Sizeof(s)]byte
+}
+
+var s S
+
+// Since f is a pointer, this case could be valid.
+// But it's pathological and not worth the expense.
+type T struct {
+	f *[unsafe.Sizeof(T /* ERROR invalid recursive type */ {})]int
+}
+
+// a mutually recursive case using unsafe.Sizeof
+type (
+	A1 struct {
+		_ [unsafe.Sizeof(B1{})]int
+	}
+
+	B1 struct {
+		_ [unsafe.Sizeof(A1 /* ERROR invalid recursive type */ {})]int
+	}
+)
+
+// a mutually recursive case using len
+type (
+	A2 struct {
+		f [len(B2{}.f)]int
+	}
+
+	B2 struct {
+		f [len(A2 /* ERROR invalid recursive type */ {}.f)]int
+	}
+)
+
+// test case from issue
+type a struct {
+	_ [42 - unsafe.Sizeof(a /* ERROR invalid recursive type */ {})]byte
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49296.go b/src/internal/types/testdata/fixedbugs/issue49296.go
new file mode 100644
index 0000000..98ad6f5
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49296.go
@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[
+        T0 any,
+        T1 []int,
+        T2 ~float64 | ~complex128 | chan int,
+]() {
+        _ = T0(nil /* ERROR cannot convert nil to type T0 */ )
+        _ = T1(1 /* ERROR cannot convert 1 .* to type T1 */ )
+        _ = T2(2 /* ERROR cannot convert 2 .* to type T2 */ )
+}
+
+// test case from issue
+func f[T interface{[]int}]() {
+	_ = T(1 /* ERROR cannot convert */ )
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49439.go b/src/internal/types/testdata/fixedbugs/issue49439.go
new file mode 100644
index 0000000..b8ad495
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49439.go
@@ -0,0 +1,26 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type T0 /* ERROR invalid recursive type */ [P T0[P]] struct{}
+
+type T1 /* ERROR invalid recursive type */ [P T2[P]] struct{}
+type T2[P T1[P]] struct{}
+
+type T3 /* ERROR invalid recursive type */ [P interface{ ~struct{ f T3[int] } }] struct{}
+
+// valid cycle in M
+type N[P M[P]] struct{}
+type M[Q any] struct { F *M[Q] }
+
+// "crazy" case
+type TC[P [unsafe.Sizeof(func() {
+        type T [P [unsafe.Sizeof(func(){})]byte] struct{}
+})]byte] struct{}
+
+// test case from issue
+type X /* ERROR invalid recursive type */ [T any, PT X[T]] interface{}
diff --git a/src/internal/types/testdata/fixedbugs/issue49482.go b/src/internal/types/testdata/fixedbugs/issue49482.go
new file mode 100644
index 0000000..2b9e748
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49482.go
@@ -0,0 +1,26 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The following is OK, per the special handling for type literals discussed in issue #49482.
+type _[P *struct{}] struct{}
+type _[P *int,] int
+type _[P (*int),] int
+
+const P = 2 // declare P to avoid noisy 'undefined' errors below.
+
+// The following parse as invalid array types due to parsing ambiguitiues.
+type _ [P *int /* ERROR "int \(type\) is not an expression" */ ]int
+type _ [P /* ERROR non-function P */ (*int)]int
+
+// Adding a trailing comma or an enclosing interface resolves the ambiguity.
+type _[P *int,] int
+type _[P (*int),] int
+type _[P interface{*int}] int
+type _[P interface{(*int)}] int
+
+// The following parse correctly as valid generic types.
+type _[P *struct{} | int] struct{}
+type _[P *struct{} | ~int] struct{}
diff --git a/src/internal/types/testdata/fixedbugs/issue49541.go b/src/internal/types/testdata/fixedbugs/issue49541.go
new file mode 100644
index 0000000..c8499c1
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49541.go
@@ -0,0 +1,45 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type S[A, B any] struct {
+	f int
+}
+
+func (S[A, B]) m() {}
+
+// TODO(gri): with type-type inference enabled we should only report one error
+// below. See issue #50588.
+
+func _[A any](s S /* ERROR got 1 arguments but 2 type parameters */ [A]) {
+	// we should see no follow-on errors below
+	s.f = 1
+	s.m()
+}
+
+// another test case from the issue
+
+func _() {
+	X(Interface[*F /* ERROR got 1 arguments but 2 type parameters */ [string]](Impl{}))
+}
+
+func X[Q Qer](fs Interface[Q]) {
+}
+
+type Impl struct{}
+
+func (Impl) M() {}
+
+type Interface[Q Qer] interface {
+	M()
+}
+
+type Qer interface {
+	Q()
+}
+
+type F[A, B any] struct{}
+
+func (f *F[A, B]) Q() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue49579.go b/src/internal/types/testdata/fixedbugs/issue49579.go
new file mode 100644
index 0000000..ee2d94a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49579.go
@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type I[F any] interface {
+	Q(*F)
+}
+
+func G[F any]() I[any] {
+	return g /* ERROR cannot use g\[F\]{} .* as I\[any\] value in return statement: g\[F\] does not implement I\[any\] \(method Q has pointer receiver\) */ [F]{}
+}
+
+type g[F any] struct{}
+
+func (*g[F]) Q(*any) {}
diff --git a/src/internal/types/testdata/fixedbugs/issue49592.go b/src/internal/types/testdata/fixedbugs/issue49592.go
new file mode 100644
index 0000000..846deaa
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49592.go
@@ -0,0 +1,11 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	var x *interface{}
+	var y interface{}
+	_ = x == y
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49602.go b/src/internal/types/testdata/fixedbugs/issue49602.go
new file mode 100644
index 0000000..208501f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49602.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type M interface {
+	m()
+}
+
+type C interface {
+	comparable
+}
+
+type _ interface {
+	int | M          // ERROR cannot use p\.M in union \(p\.M contains methods\)
+	int | comparable // ERROR cannot use comparable in union
+	int | C          // ERROR cannot use p\.C in union \(p\.C embeds comparable\)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49705.go b/src/internal/types/testdata/fixedbugs/issue49705.go
new file mode 100644
index 0000000..5b5fba2
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49705.go
@@ -0,0 +1,14 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Integer interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+}
+
+func shl[I Integer](n int) I {
+	return 1 << n
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49735.go b/src/internal/types/testdata/fixedbugs/issue49735.go
new file mode 100644
index 0000000..5087022
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49735.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P1 any, P2 ~byte](s1 P1, s2 P2) {
+        _ = append(nil /* ERROR first argument to append must be a slice; have untyped nil */ , 0)
+        _ = append(s1 /* ERROR s1 .* has no core type */ , 0)
+        _ = append(s2 /* ERROR s2 .* has core type byte */ , 0)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49739.go b/src/internal/types/testdata/fixedbugs/issue49739.go
new file mode 100644
index 0000000..7feb563
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49739.go
@@ -0,0 +1,23 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Verify that we get an empty type set (not just an error)
+// when using an invalid ~A.
+
+package p
+
+type A int
+type C interface {
+	~ /* ERROR invalid use of ~ */ A
+}
+
+func f[_ C]()              {}
+func g[_ interface{ C }]() {}
+func h[_ C | int]()        {}
+
+func _() {
+	_ = f[int /* ERROR cannot satisfy C \(empty type set\) */]
+	_ = g[int /* ERROR cannot satisfy interface{C} \(empty type set\) */]
+	_ = h[int]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue49864.go b/src/internal/types/testdata/fixedbugs/issue49864.go
new file mode 100644
index 0000000..0437e74
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue49864.go
@@ -0,0 +1,9 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P ~int, Q any](p P) {
+	_ = Q(p /* ERROR cannot convert */ )
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50259.go b/src/internal/types/testdata/fixedbugs/issue50259.go
new file mode 100644
index 0000000..6df8c64
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50259.go
@@ -0,0 +1,18 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var x T[B]
+
+type T[_ any] struct{}
+type A T[B]
+type B = T[A]
+
+// test case from issue
+
+var v Box[Step]
+type Box[T any] struct{}
+type Step = Box[StepBox]
+type StepBox Box[Step]
diff --git a/src/internal/types/testdata/fixedbugs/issue50276.go b/src/internal/types/testdata/fixedbugs/issue50276.go
new file mode 100644
index 0000000..97e477e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50276.go
@@ -0,0 +1,39 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// simplified test case
+
+type transform[T any] struct{}
+type pair[S any] struct {}
+
+var _ transform[step]
+
+type box transform[step]
+type step = pair[box]
+
+// test case from issue
+
+type Transform[T any] struct{ hold T }
+type Pair[S, T any] struct {
+	First  S
+	Second T
+}
+
+var first Transform[Step]
+
+// This line doesn't use the Step alias, and it compiles fine if you uncomment it.
+var second Transform[Pair[Box, interface{}]]
+
+type Box *Transform[Step]
+
+// This line is the same as the `first` line, but it comes after the Box declaration and
+// does not break the compile.
+var third Transform[Step]
+
+type Step = Pair[Box, interface{}]
+
+// This line also does not break the compile
+var fourth Transform[Step]
diff --git a/src/internal/types/testdata/fixedbugs/issue50281.go b/src/internal/types/testdata/fixedbugs/issue50281.go
new file mode 100644
index 0000000..f333e81
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50281.go
@@ -0,0 +1,26 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[S string | []byte](s S) {
+	var buf []byte
+	_ = append(buf, s...)
+}
+
+func _[S ~string | ~[]byte](s S) {
+	var buf []byte
+	_ = append(buf, s...)
+}
+
+// test case from issue
+
+type byteseq interface {
+	string | []byte
+}
+
+// This should allow to eliminate the two functions above.
+func AppendByteString[source byteseq](buf []byte, s source) []byte {
+	return append(buf, s[1:6]...)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50321.go b/src/internal/types/testdata/fixedbugs/issue50321.go
new file mode 100644
index 0000000..199e66e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50321.go
@@ -0,0 +1,8 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func Ln[A A /* ERROR cannot use a type parameter as constraint */ ](p A) {
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50372.go b/src/internal/types/testdata/fixedbugs/issue50372.go
new file mode 100644
index 0000000..4c9b65a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50372.go
@@ -0,0 +1,27 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _(s []int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j = range s {}
+        for i, j, k /* ERROR range clause permits at most two iteration variables|at most 2 expressions */ = range s {}
+        for i, j, k, l /* ERROR range clause permits at most two iteration variables|at most 2 expressions */ = range s {}
+}
+
+func _(s chan int) {
+        var i, j, k, l int
+        _, _, _, _ = i, j, k, l
+
+        for range s {}
+        for i = range s {}
+        for i, j /* ERROR range over .* permits only one iteration variable */ = range s {}
+        for i, j, k /* ERROR range over .* permits only one iteration variable|at most 2 expressions */ = range s {}
+        for i, j, k, l /* ERROR range over .* permits only one iteration variable|at most 2 expressions */ = range s {}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50417.go b/src/internal/types/testdata/fixedbugs/issue50417.go
new file mode 100644
index 0000000..69ebf31
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50417.go
@@ -0,0 +1,68 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
+package p
+
+type Sf struct {
+        f int
+}
+
+func f0[P Sf](p P) {
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
+}
+
+func f0t[P ~struct{f int}](p P) {
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
+}
+
+var _ = f0[Sf]
+var _ = f0t[Sf]
+
+var _ = f0[Sm /* ERROR does not satisfy */ ]
+var _ = f0t[Sm /* ERROR does not satisfy */ ]
+
+func f1[P interface{ Sf; m() }](p P) {
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
+        p.m()
+}
+
+var _ = f1[Sf /* ERROR missing method m */ ]
+var _ = f1[Sm /* ERROR does not satisfy */ ]
+
+type Sm struct {}
+
+func (Sm) m() {}
+
+type Sfm struct {
+        f int
+}
+
+func (Sfm) m() {}
+
+func f2[P interface{ Sfm; m() }](p P) {
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
+        p.m()
+}
+
+var _ = f2[Sfm]
+
+// special case: core type is a named pointer type
+
+type PSfm *Sfm
+
+func f3[P interface{ PSfm }](p P) {
+        _ = p.f // ERROR p\.f undefined
+        p.f /* ERROR p\.f undefined */ = 0
+        p.m /* ERROR type P has no field or method m */ ()
+}
+
+var _ = f3[PSfm]
diff --git a/src/internal/types/testdata/fixedbugs/issue50426.go b/src/internal/types/testdata/fixedbugs/issue50426.go
new file mode 100644
index 0000000..17ec0ce
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50426.go
@@ -0,0 +1,44 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type A1 [2]uint64
+type A2 [2]uint64
+
+func (a A1) m() A1 { return a }
+func (a A2) m() A2 { return a }
+
+func f[B any, T interface {
+	A1 | A2
+	m() T
+}](v T) {
+}
+
+func _() {
+	var v A2
+	// Use function type inference to infer type A2 for T.
+	// Don't use constraint type inference before function
+	// type inference for typed arguments, otherwise it would
+	// infer type [2]uint64 for T which doesn't have method m
+	// (was the bug).
+	f[int](v)
+}
+
+// Keep using constraint type inference before function type
+// inference for untyped arguments so we infer type float64
+// for E below, and not int (which would not work).
+func g[S ~[]E, E any](S, E) {}
+
+func _() {
+	var s []float64
+	g[[]float64](s, 0)
+}
+
+// Keep using constraint type inference after function
+// type inference for untyped arguments so we infer
+// missing type arguments for which we only have the
+// untyped arguments as starting point.
+func h[E any, R []E](v E) R { return R{v} }
+func _() []int              { return h(0) }
diff --git a/src/internal/types/testdata/fixedbugs/issue50427.go b/src/internal/types/testdata/fixedbugs/issue50427.go
new file mode 100644
index 0000000..d89d63e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50427.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The parser no longer parses type parameters for methods.
+// In the past, type checking the code below led to a crash (#50427).
+
+type T interface{ m[ /* ERROR "must have no type parameters" */ P any]() }
+
+func _(t T) {
+	var _ interface{ m[ /* ERROR "must have no type parameters" */ P any](); n() } = t /* ERROR "does not implement" */
+}
+
+type S struct{}
+
+func (S) m[ /* ERROR "must have no type parameters" */ P any]() {}
+
+func _(s S) {
+	var _ interface{ m[ /* ERROR "must have no type parameters" */ P any](); n() } = s /* ERROR "does not implement" */
+
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50450.go b/src/internal/types/testdata/fixedbugs/issue50450.go
new file mode 100644
index 0000000..bae3111
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50450.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type S struct{}
+
+func f[P S]() {}
+
+var _ = f[S]
diff --git a/src/internal/types/testdata/fixedbugs/issue50516.go b/src/internal/types/testdata/fixedbugs/issue50516.go
new file mode 100644
index 0000000..f73015e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50516.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P struct{ f int }](x P) {
+	_ = x.g // ERROR type P has no field or method g
+}
+
+func _[P struct{ f int } | struct{ g int }](x P) {
+	_ = x.g // ERROR type P has no field or method g
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50646.go b/src/internal/types/testdata/fixedbugs/issue50646.go
new file mode 100644
index 0000000..ed7261c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50646.go
@@ -0,0 +1,28 @@
+// -oldComparableSemantics
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T /* ERROR T does not satisfy comparable */ ]
+	_ = f1[any /* ERROR any does not satisfy comparable */ ]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not satisfy comparable */]
+
+	_ = f2[int]
+	_ = f2[T /* ERROR T does not satisfy comparable */ ]
+	_ = f2[any /* ERROR any does not satisfy comparable */ ]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not satisfy comparable */]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50729.go b/src/internal/types/testdata/fixedbugs/issue50729.go
new file mode 100644
index 0000000..fe19fdf
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50729.go
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// version 1
+var x1 T1[B1]
+
+type T1[_ any] struct{}
+type A1 T1[B1]
+type B1 = T1[A1]
+
+// version 2
+type T2[_ any] struct{}
+type A2 T2[B2]
+type B2 = T2[A2]
+
+var x2 T2[B2]
diff --git a/src/internal/types/testdata/fixedbugs/issue50755.go b/src/internal/types/testdata/fixedbugs/issue50755.go
new file mode 100644
index 0000000..afc7b24
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50755.go
@@ -0,0 +1,47 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The core type of M2 unifies with the type of m1
+// during function argument type inference.
+// M2's constraint is unnamed.
+func f1[K1 comparable, E1 any](m1 map[K1]E1) {}
+
+func f2[M2 map[string]int](m2 M2) {
+	f1(m2)
+}
+
+// The core type of M3 unifies with the type of m1
+// during function argument type inference.
+// M3's constraint is named.
+type Map3 map[string]int
+
+func f3[M3 Map3](m3 M3) {
+	f1(m3)
+}
+
+// The core type of M5 unifies with the core type of M4
+// during constraint type inference.
+func f4[M4 map[K4]int, K4 comparable](m4 M4) {}
+
+func f5[M5 map[K5]int, K5 comparable](m5 M5) {
+	f4(m5)
+}
+
+// test case from issue
+
+func Copy[MC ~map[KC]VC, KC comparable, VC any](dst, src MC) {
+	for k, v := range src {
+		dst[k] = v
+	}
+}
+
+func Merge[MM ~map[KM]VM, KM comparable, VM any](ms ...MM) MM {
+	result := MM{}
+	for _, m := range ms {
+		Copy(result, m)
+	}
+	return result
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50779.go b/src/internal/types/testdata/fixedbugs/issue50779.go
new file mode 100644
index 0000000..fe68c28
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50779.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type AC interface {
+	C
+}
+
+type ST []int
+
+type R[S any, P any] struct{}
+
+type SR = R[SS, ST]
+
+type SS interface {
+	NSR(any) *SR // ERROR invalid use of type alias SR in recursive type
+}
+
+type C interface {
+	NSR(any) *SR
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50782.go b/src/internal/types/testdata/fixedbugs/issue50782.go
new file mode 100644
index 0000000..0e7b712
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50782.go
@@ -0,0 +1,47 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Field accesses through type parameters are disabled
+// until we have a more thorough understanding of the
+// implications on the spec. See issue #51576.
+
+package p
+
+// The first example from the issue.
+type Numeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64
+}
+
+// numericAbs matches numeric types with an Abs method.
+type numericAbs[T Numeric] interface {
+	~struct{ Value T }
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func absDifference[T numericAbs[T /* ERROR T does not satisfy Numeric */]](a, b T) T {
+	// Field accesses are not permitted for now. Keep an error so
+	// we can find and fix this code once the situation changes.
+	return a.Value // ERROR a\.Value undefined
+	// TODO: The error below should probably be positioned on the '-'.
+	// d := a /* ERROR "invalid operation: operator - not defined" */ .Value - b.Value
+	// return d.Abs()
+}
+
+// The second example from the issue.
+type T[P int] struct{ f P }
+
+func _[P T[P /* ERROR "P does not satisfy int" */ ]]() {}
+
+// Additional tests
+func _[P T[T /* ERROR "T\[P\] does not satisfy int" */ [P /* ERROR "P does not satisfy int" */ ]]]() {}
+func _[P T[Q /* ERROR "Q does not satisfy int" */ ], Q T[P /* ERROR "P does not satisfy int" */ ]]() {}
+func _[P T[Q], Q int]() {}
+
+type C[P comparable] struct{ f P }
+func _[P C[C[P]]]() {}
+func _[P C[C /* ERROR "C\[Q\] does not satisfy comparable" */ [Q /* ERROR "Q does not satisfy comparable" */]], Q func()]() {}
+func _[P [10]C[P]]() {}
+func _[P struct{ f C[C[P]]}]() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue50816.go b/src/internal/types/testdata/fixedbugs/issue50816.go
new file mode 100644
index 0000000..e7e31d9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50816.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkg
+
+type I interface {
+	Foo()
+}
+
+type T1 struct{}
+
+func (T1) foo() {}
+
+type T2 struct{}
+
+func (T2) foo() string { return "" }
+
+func _() {
+	var i I
+	_ = i /* ERROR impossible type assertion: i\.\(T1\)\n\tT1 does not implement I \(missing method Foo\)\n\t\thave foo\(\)\n\t\twant Foo\(\) */ .(T1)
+	_ = i /* ERROR impossible type assertion: i\.\(T2\)\n\tT2 does not implement I \(missing method Foo\)\n\t\thave foo\(\) string\n\t\twant Foo\(\) */ .(T2)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50833.go b/src/internal/types/testdata/fixedbugs/issue50833.go
new file mode 100644
index 0000000..e912e4d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50833.go
@@ -0,0 +1,16 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	S  struct{ f int }
+	PS *S
+)
+
+func a() []*S { return []*S{{f: 1}} }
+func b() []PS { return []PS{{f: 1}} }
+
+func c[P *S]() []P { return []P{{f: 1}} }
+func d[P PS]() []P { return []P{{f: 1}} }
diff --git a/src/internal/types/testdata/fixedbugs/issue50912.go b/src/internal/types/testdata/fixedbugs/issue50912.go
new file mode 100644
index 0000000..f161925
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50912.go
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func Real[P ~complex128](x P) {
+	_ = real(x /* ERROR not supported */ )
+}
+
+func Imag[P ~complex128](x P) {
+	_ = imag(x /* ERROR not supported */ )
+}
+
+func Complex[P ~float64](x P) {
+	_ = complex(x /* ERROR not supported */ , 0)
+	_ = complex(0 /* ERROR not supported */ , x)
+	_ = complex(x /* ERROR not supported */ , x)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50918.go b/src/internal/types/testdata/fixedbugs/issue50918.go
new file mode 100644
index 0000000..41604b8
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50918.go
@@ -0,0 +1,21 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type thing1 struct {
+	things []string
+}
+
+type thing2 struct {
+	things []thing1
+}
+
+func _() {
+	var a1, b1 thing1
+	_ = a1 /* ERROR struct containing \[\]string cannot be compared */ == b1
+
+	var a2, b2 thing2
+	_ = a2 /* ERROR struct containing \[\]thing1 cannot be compared */ == b2
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50929.go b/src/internal/types/testdata/fixedbugs/issue50929.go
new file mode 100644
index 0000000..45e0751
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50929.go
@@ -0,0 +1,68 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is tested when running "go test -run Manual"
+// without source arguments. Use for one-off debugging.
+
+package p
+
+import "fmt"
+
+type F[A, B any] int
+
+func G[A, B any](F[A, B]) {
+}
+
+func _() {
+	// TODO(gri) only report one error below (issue #50932)
+	var x F /* ERROR got 1 arguments but 2 type parameters */ [int]
+	G(x /* ERROR does not match */)
+}
+
+// test case from issue
+// (lots of errors but doesn't crash anymore)
+
+type RC[G any, RG any] interface {
+	~[]RG
+}
+
+type RG[G any] struct{}
+
+type RSC[G any] []*RG[G]
+
+type M[Rc RC[G, RG], G any, RG any] struct {
+	Fn func(Rc)
+}
+
+type NFn[Rc RC[G, RG], G any, RG any] func(Rc)
+
+func NC[Rc RC[G, RG], G any, RG any](nFn NFn[Rc, G, RG]) {
+	var empty Rc
+	nFn(empty)
+}
+
+func NSG[G any](c RSC[G]) {
+	fmt.Println(c)
+}
+
+func MMD[Rc RC /* ERROR got 1 arguments */ [RG], RG any, G any]() M /* ERROR got 2 arguments */ [Rc, RG] {
+
+	var nFn NFn /* ERROR got 2 arguments */ [Rc, RG]
+
+	var empty Rc
+	switch any(empty).(type) {
+	case BC /* ERROR undefined: BC */ :
+
+	case RSC[G]:
+		nFn = NSG /* ERROR cannot use NSG\[G\] */ [G]
+	}
+
+	return M /* ERROR got 2 arguments */ [Rc, RG]{
+		Fn: func(rc Rc) {
+			NC(nFn /* ERROR does not match */ )
+		},
+	}
+
+	return M /* ERROR got 2 arguments */ [Rc, RG]{}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue50965.go b/src/internal/types/testdata/fixedbugs/issue50965.go
new file mode 100644
index 0000000..bf2dcc9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue50965.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _(x int, c string) {
+	switch x {
+	case c /* ERROR invalid case c in switch on x \(mismatched types string and int\) */ :
+	}
+}
+
+func _(x, c []int) {
+	switch x {
+	case c /* ERROR invalid case c in switch on x \(slice can only be compared to nil\) */ :
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51025.go b/src/internal/types/testdata/fixedbugs/issue51025.go
new file mode 100644
index 0000000..207b06e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51025.go
@@ -0,0 +1,38 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ interface{ m() } = struct /* ERROR m is a field, not a method */ {
+	m func()
+}{}
+
+var _ interface{ m() } = & /* ERROR m is a field, not a method */ struct {
+	m func()
+}{}
+
+var _ interface{ M() } = struct /* ERROR missing method M */ {
+	m func()
+}{}
+
+var _ interface{ M() } = & /* ERROR missing method M */ struct {
+	m func()
+}{}
+
+// test case from issue
+type I interface{ m() }
+type T struct{ m func() }
+type M struct{}
+
+func (M) m() {}
+
+func _() {
+	var t T
+	var m M
+	var i I
+
+	i = m
+	i = t // ERROR m is a field, not a method
+	_ = i
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51048.go b/src/internal/types/testdata/fixedbugs/issue51048.go
new file mode 100644
index 0000000..5830837
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51048.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P int]() {
+	_ = f[P]
+}
+
+func f[T int]() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue51145.go b/src/internal/types/testdata/fixedbugs/issue51145.go
new file mode 100644
index 0000000..b84391d
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51145.go
@@ -0,0 +1,18 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "fmt"
+
+type (
+	_ [fmt /* ERROR invalid array length fmt */ ]int
+	_ [float64 /* ERROR invalid array length float64 */ ]int
+	_ [f /* ERROR invalid array length f */ ]int
+	_ [nil /* ERROR invalid array length nil */ ]int
+)
+
+func f()
+
+var _ fmt.Stringer // use fmt
diff --git a/src/internal/types/testdata/fixedbugs/issue51158.go b/src/internal/types/testdata/fixedbugs/issue51158.go
new file mode 100644
index 0000000..3edc505
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51158.go
@@ -0,0 +1,18 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Type checking the following code should not cause an infinite recursion.
+func f[M map[K]int, K comparable](m M) {
+        f(m)
+}
+
+// Equivalent code using mutual recursion.
+func f1[M map[K]int, K comparable](m M) {
+        f2(m)
+}
+func f2[M map[K]int, K comparable](m M) {
+        f1(m)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51229.go b/src/internal/types/testdata/fixedbugs/issue51229.go
new file mode 100644
index 0000000..808b647
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51229.go
@@ -0,0 +1,164 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Constraint type inference should be independent of the
+// ordering of the type parameter declarations. Try all
+// permutations in the test case below.
+// Permutations produced by https://go.dev/play/p/PHcZNGJTEBZ.
+
+func f00[S1 ~[]E1, S2 ~[]E2, E1 ~byte, E2 ~byte](S1, S2) {}
+func f01[S2 ~[]E2, S1 ~[]E1, E1 ~byte, E2 ~byte](S1, S2) {}
+func f02[E1 ~byte, S1 ~[]E1, S2 ~[]E2, E2 ~byte](S1, S2) {}
+func f03[S1 ~[]E1, E1 ~byte, S2 ~[]E2, E2 ~byte](S1, S2) {}
+func f04[S2 ~[]E2, E1 ~byte, S1 ~[]E1, E2 ~byte](S1, S2) {}
+func f05[E1 ~byte, S2 ~[]E2, S1 ~[]E1, E2 ~byte](S1, S2) {}
+func f06[E2 ~byte, S2 ~[]E2, S1 ~[]E1, E1 ~byte](S1, S2) {}
+func f07[S2 ~[]E2, E2 ~byte, S1 ~[]E1, E1 ~byte](S1, S2) {}
+func f08[S1 ~[]E1, E2 ~byte, S2 ~[]E2, E1 ~byte](S1, S2) {}
+func f09[E2 ~byte, S1 ~[]E1, S2 ~[]E2, E1 ~byte](S1, S2) {}
+func f10[S2 ~[]E2, S1 ~[]E1, E2 ~byte, E1 ~byte](S1, S2) {}
+func f11[S1 ~[]E1, S2 ~[]E2, E2 ~byte, E1 ~byte](S1, S2) {}
+func f12[S1 ~[]E1, E1 ~byte, E2 ~byte, S2 ~[]E2](S1, S2) {}
+func f13[E1 ~byte, S1 ~[]E1, E2 ~byte, S2 ~[]E2](S1, S2) {}
+func f14[E2 ~byte, S1 ~[]E1, E1 ~byte, S2 ~[]E2](S1, S2) {}
+func f15[S1 ~[]E1, E2 ~byte, E1 ~byte, S2 ~[]E2](S1, S2) {}
+func f16[E1 ~byte, E2 ~byte, S1 ~[]E1, S2 ~[]E2](S1, S2) {}
+func f17[E2 ~byte, E1 ~byte, S1 ~[]E1, S2 ~[]E2](S1, S2) {}
+func f18[E2 ~byte, E1 ~byte, S2 ~[]E2, S1 ~[]E1](S1, S2) {}
+func f19[E1 ~byte, E2 ~byte, S2 ~[]E2, S1 ~[]E1](S1, S2) {}
+func f20[S2 ~[]E2, E2 ~byte, E1 ~byte, S1 ~[]E1](S1, S2) {}
+func f21[E2 ~byte, S2 ~[]E2, E1 ~byte, S1 ~[]E1](S1, S2) {}
+func f22[E1 ~byte, S2 ~[]E2, E2 ~byte, S1 ~[]E1](S1, S2) {}
+func f23[S2 ~[]E2, E1 ~byte, E2 ~byte, S1 ~[]E1](S1, S2) {}
+
+type myByte byte
+
+func _(a []byte, b []myByte) {
+	f00(a, b)
+	f01(a, b)
+	f02(a, b)
+	f03(a, b)
+	f04(a, b)
+	f05(a, b)
+	f06(a, b)
+	f07(a, b)
+	f08(a, b)
+	f09(a, b)
+	f10(a, b)
+	f11(a, b)
+	f12(a, b)
+	f13(a, b)
+	f14(a, b)
+	f15(a, b)
+	f16(a, b)
+	f17(a, b)
+	f18(a, b)
+	f19(a, b)
+	f20(a, b)
+	f21(a, b)
+	f22(a, b)
+	f23(a, b)
+}
+
+// Constraint type inference may have to iterate.
+// Again, the order of the type parameters shouldn't matter.
+
+func g0[S ~[]E, M ~map[string]S, E any](m M) {}
+func g1[M ~map[string]S, S ~[]E, E any](m M) {}
+func g2[E any, S ~[]E, M ~map[string]S](m M) {}
+func g3[S ~[]E, E any, M ~map[string]S](m M) {}
+func g4[M ~map[string]S, E any, S ~[]E](m M) {}
+func g5[E any, M ~map[string]S, S ~[]E](m M) {}
+
+func _(m map[string][]byte) {
+	g0(m)
+	g1(m)
+	g2(m)
+	g3(m)
+	g4(m)
+	g5(m)
+}
+
+// Worst-case scenario.
+// There are 10 unknown type parameters. In each iteration of
+// constraint type inference we infer one more, from right to left.
+// Each iteration looks repeatedly at all 11 type parameters,
+// requiring a total of 10*11 = 110 iterations with the current
+// implementation. Pathological case.
+
+func h[K any, J ~*K, I ~*J, H ~*I, G ~*H, F ~*G, E ~*F, D ~*E, C ~*D, B ~*C, A ~*B](x A) {}
+
+func _(x **********int) {
+	h(x)
+}
+
+// Examples with channel constraints and tilde.
+
+func ch1[P chan<- int]() (_ P)           { return } // core(P) == chan<- int   (single type, no tilde)
+func ch2[P ~chan int]()                  { return } // core(P) == ~chan<- int  (tilde)
+func ch3[P chan E, E any](E)             { return } // core(P) == chan<- E     (single type, no tilde)
+func ch4[P chan E | ~chan<- E, E any](E) { return } // core(P) == ~chan<- E    (tilde)
+func ch5[P chan int | chan<- int]()      { return } // core(P) == chan<- int   (not a single type)
+
+func _() {
+	// P can be inferred as there's a single specific type and no tilde.
+	var _ chan int = ch1 /* ERROR cannot use ch1.*value of type chan<- int */ ()
+	var _ chan<- int = ch1()
+
+	// P cannot be inferred as there's a tilde.
+	ch2 /* ERROR cannot infer P */ ()
+	type myChan chan int
+	ch2[myChan]()
+
+	// P can be inferred as there's a single specific type and no tilde.
+	var e int
+	ch3(e)
+
+	// P cannot be inferred as there's more than one specific type and a tilde.
+	ch4 /* ERROR cannot infer P */ (e)
+	_ = ch4[chan int]
+
+	// P cannot be inferred as there's more than one specific type.
+	ch5 /* ERROR cannot infer P */ ()
+	ch5[chan<- int]()
+}
+
+// test case from issue
+
+func equal[M1 ~map[K1]V1, M2 ~map[K2]V2, K1, K2 ~uint32, V1, V2 ~string](m1 M1, m2 M2) bool {
+	if len(m1) != len(m2) {
+		return false
+	}
+	for k, v1 := range m1 {
+		if v2, ok := m2[K2(k)]; !ok || V2(v1) != v2 {
+			return false
+		}
+	}
+	return true
+}
+
+func equalFixed[K1, K2 ~uint32, V1, V2 ~string](m1 map[K1]V1, m2 map[K2]V2) bool {
+	if len(m1) != len(m2) {
+		return false
+	}
+	for k, v1 := range m1 {
+		if v2, ok := m2[K2(k)]; !ok || v1 != V1(v2) {
+			return false
+		}
+	}
+	return true
+}
+
+type (
+	someNumericID uint32
+	someStringID  string
+)
+
+func _() {
+	foo := map[uint32]string{10: "bar"}
+	bar := map[someNumericID]someStringID{10: "bar"}
+	equal(foo, bar)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51232.go b/src/internal/types/testdata/fixedbugs/issue51232.go
new file mode 100644
index 0000000..3fa6a05
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51232.go
@@ -0,0 +1,30 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type RC[RG any] interface {
+	~[]RG
+}
+
+type Fn[RCT RC[RG], RG any] func(RCT)
+
+type F[RCT RC[RG], RG any] interface {
+	Fn() Fn /* ERROR got 1 arguments */ [RCT]
+}
+
+type concreteF[RCT RC[RG], RG any] struct {
+	makeFn func() Fn /* ERROR got 1 arguments */ [RCT]
+}
+
+func (c *concreteF[RCT, RG]) Fn() Fn /* ERROR got 1 arguments */ [RCT] {
+	return c.makeFn()
+}
+
+func NewConcrete[RCT RC[RG], RG any](Rc RCT) F /* ERROR got 1 arguments */ [RCT] {
+	// TODO(rfindley): eliminate the duplicate error below.
+	return & /* ERROR cannot use .* as F\[RCT\] */ concreteF /* ERROR got 1 arguments */ [RCT]{
+		makeFn: nil,
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51233.go b/src/internal/types/testdata/fixedbugs/issue51233.go
new file mode 100644
index 0000000..9c15028
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51233.go
@@ -0,0 +1,27 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// As of issue #51527, type-type inference has been disabled.
+
+type RC[RG any] interface {
+	~[]RG
+}
+
+type Fn[RCT RC[RG], RG any] func(RCT)
+
+type FFn[RCT RC[RG], RG any] func() Fn /* ERROR got 1 arguments */ [RCT]
+
+type F[RCT RC[RG], RG any] interface {
+	Fn() Fn /* ERROR got 1 arguments */ [RCT]
+}
+
+type concreteF[RCT RC[RG], RG any] struct {
+	makeFn FFn /* ERROR got 1 arguments */ [RCT]
+}
+
+func (c *concreteF[RCT, RG]) Fn() Fn /* ERROR got 1 arguments */ [RCT] {
+	return c.makeFn()
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51257.go b/src/internal/types/testdata/fixedbugs/issue51257.go
new file mode 100644
index 0000000..be4b81f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51257.go
@@ -0,0 +1,48 @@
+// -oldComparableSemantics
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[_ comparable]() {}
+
+type S1 struct{ x int }
+type S2 struct{ x any }
+type S3 struct{ x [10]interface{ m() } }
+
+func _[P1 comparable, P2 S2]() {
+	_ = f[S1]
+	_ = f[S2 /* ERROR S2 does not satisfy comparable */ ]
+	_ = f[S3 /* ERROR S3 does not satisfy comparable */ ]
+
+	type L1 struct { x P1 }
+	type L2 struct { x P2 }
+	_ = f[L1]
+	_ = f[L2 /* ERROR L2 does not satisfy comparable */ ]
+}
+
+
+// example from issue
+
+type Set[T comparable] map[T]struct{}
+
+func NewSetFromSlice[T comparable](items []T) *Set[T] {
+	s := Set[T]{}
+
+	for _, item := range items {
+		s[item] = struct{}{}
+	}
+
+	return &s
+}
+
+type T struct{ x any }
+
+func main() {
+	NewSetFromSlice /* ERROR T does not satisfy comparable */ ([]T{
+		{"foo"},
+		{5},
+	})
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51335.go b/src/internal/types/testdata/fixedbugs/issue51335.go
new file mode 100644
index 0000000..35135cd
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51335.go
@@ -0,0 +1,16 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type S1 struct{}
+type S2 struct{}
+
+func _[P *S1|*S2]() {
+	_= []P{{ /* ERROR invalid composite literal element type P \(no core type\) */ }}
+}
+
+func _[P *S1|S1]() {
+	_= []P{{ /* ERROR invalid composite literal element type P \(no core type\) */ }}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51339.go b/src/internal/types/testdata/fixedbugs/issue51339.go
new file mode 100644
index 0000000..38f8610
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51339.go
@@ -0,0 +1,18 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is tested when running "go test -run Manual"
+// without source arguments. Use for one-off debugging.
+
+package p
+
+type T[P any, B *P] struct{}
+
+func (T /* ERROR cannot use generic type */ ) m0() {}
+
+// TODO(rfindley): eliminate the duplicate errors here.
+func (/* ERROR got 1 type parameter, but receiver base type declares 2 */ T /* ERROR got 1 arguments but 2 type parameters */ [_]) m1() {}
+func (T[_, _]) m2() {}
+// TODO(gri) this error is unfortunate (issue #51343)
+func (T /* ERROR got 3 arguments but 2 type parameters */ [_, _, _]) m3() {}
diff --git a/src/internal/types/testdata/fixedbugs/issue51360.go b/src/internal/types/testdata/fixedbugs/issue51360.go
new file mode 100644
index 0000000..fe3de04
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51360.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	len.Println /* ERROR cannot select on len */
+	len.Println /* ERROR cannot select on len */ ()
+	_ = len.Println /* ERROR cannot select on len */
+	_ = len /* ERROR cannot index len */ [0]
+	_ = *len /* ERROR cannot indirect len */
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51376.go b/src/internal/types/testdata/fixedbugs/issue51376.go
new file mode 100644
index 0000000..3801d68
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51376.go
@@ -0,0 +1,24 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Map map[string]int
+
+func f[M ~map[K]V, K comparable, V any](M) {}
+func g[M map[K]V, K comparable, V any](M) {}
+
+func _[M1 ~map[K]V, M2 map[K]V, K comparable, V any]() {
+        var m1 M1
+        f(m1)
+        g /* ERROR M1 does not satisfy map\[K\]V */ (m1) // M1 has tilde
+
+        var m2 M2
+        f(m2)
+        g(m2) // M1 does not have tilde
+
+        var m3 Map
+        f(m3)
+        g /* ERROR Map does not satisfy map\[string\]int */ (m3) // M in g does not have tilde
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51386.go b/src/internal/types/testdata/fixedbugs/issue51386.go
new file mode 100644
index 0000000..ef62239
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51386.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type myString string
+
+func _[P ~string | ~[]byte | ~[]rune]() {
+	_ = P("")
+	const s myString = ""
+	_ = P(s)
+}
+
+func _[P myString]() {
+	_ = P("")
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51437.go b/src/internal/types/testdata/fixedbugs/issue51437.go
new file mode 100644
index 0000000..3762615
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51437.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T struct{}
+
+func (T) m() []int { return nil }
+
+func f(x T) {
+	for _, x := range func() []int {
+		return x.m() // x declared in parameter list of f
+	}() {
+		_ = x // x declared by range clause
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51472.go b/src/internal/types/testdata/fixedbugs/issue51472.go
new file mode 100644
index 0000000..a0f9e9c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51472.go
@@ -0,0 +1,54 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[T comparable](x T) {
+        _ = x == x
+}
+
+func _[T interface{interface{comparable}}](x T) {
+        _ = x == x
+}
+
+func _[T interface{comparable; interface{comparable}}](x T) {
+        _ = x == x
+}
+
+func _[T interface{comparable; ~int}](x T) {
+        _ = x == x
+}
+
+func _[T interface{comparable; ~[]byte}](x T) {
+        _ = x /* ERROR empty type set */ == x
+}
+
+// TODO(gri) The error message here should be better. See issue #51525.
+func _[T interface{comparable; ~int; ~string}](x T) {
+        _ = x /* ERROR empty type set */ == x
+}
+
+// TODO(gri) The error message here should be better. See issue #51525.
+func _[T interface{~int; ~string}](x T) {
+        _ = x /* ERROR empty type set */ == x
+}
+
+func _[T interface{comparable; interface{~int}; interface{int|float64}}](x T) {
+        _ = x == x
+}
+
+func _[T interface{interface{comparable; ~int}; interface{~float64; comparable; m()}}](x T) {
+        _ = x /* ERROR empty type set */ == x
+}
+
+// test case from issue
+
+func f[T interface{comparable; []byte|string}](x T) {
+        _ = x == x
+}
+
+func _(s []byte) {
+	f /* ERROR \[\]byte does not satisfy interface{comparable; \[\]byte \| string} */ (s)
+        _ = f[[ /* ERROR does not satisfy */ ]byte]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51509.go b/src/internal/types/testdata/fixedbugs/issue51509.go
new file mode 100644
index 0000000..64f5d7e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51509.go
@@ -0,0 +1,7 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T /* ERROR invalid recursive type */ T.x
diff --git a/src/internal/types/testdata/fixedbugs/issue51525.go b/src/internal/types/testdata/fixedbugs/issue51525.go
new file mode 100644
index 0000000..5856905
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51525.go
@@ -0,0 +1,20 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[T interface {
+	int
+	string
+}](x T) {
+	_ = x /* ERROR empty type set */ == x
+	_ = x /* ERROR empty type set */ + x
+	<-x /* ERROR empty type set */
+	x <- /* ERROR empty type set */ 0
+	close(x /* ERROR empty type set */)
+}
+
+func _[T interface{ int | []byte }](x T) {
+	_ = x /* ERROR incomparable types in type set */ == x
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51533.go b/src/internal/types/testdata/fixedbugs/issue51533.go
new file mode 100644
index 0000000..bf46f75
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51533.go
@@ -0,0 +1,20 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _(x any) {
+	switch x {
+	case 0:
+		fallthrough // ERROR fallthrough statement out of place
+		_ = x
+	default:
+	}
+
+	switch x.(type) {
+	case int:
+		fallthrough // ERROR cannot fallthrough in type switch
+	default:
+	}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51578.go b/src/internal/types/testdata/fixedbugs/issue51578.go
new file mode 100644
index 0000000..5c204ba
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51578.go
@@ -0,0 +1,17 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = (*interface /* ERROR interface contains type constraints */ {int})(nil)
+
+// abbreviated test case from issue
+
+type TypeSet interface{ int | string }
+
+func _() {
+	f((*TypeSet /* ERROR interface contains type constraints */)(nil))
+}
+
+func f(any) {}
\ No newline at end of file
diff --git a/src/internal/types/testdata/fixedbugs/issue51593.go b/src/internal/types/testdata/fixedbugs/issue51593.go
new file mode 100644
index 0000000..e06c39f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51593.go
@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P interface{ m(R) }, R any]() {}
+
+type T = interface { m(int) }
+
+func _() {
+	_ = f /* ERROR cannot infer R */ [T] // don't crash in type inference
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51607.go b/src/internal/types/testdata/fixedbugs/issue51607.go
new file mode 100644
index 0000000..d8df143
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51607.go
@@ -0,0 +1,65 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Interface types must be ignored during overlap test.
+
+type (
+	T1 interface{int}
+	T2 interface{~int}
+	T3 interface{T1 | bool | string}
+	T4 interface{T2 | ~bool | ~string}
+)
+
+type (
+	// overlap errors for non-interface terms
+	// (like the interface terms, but explicitly inlined)
+	_ interface{int | int /* ERROR overlapping terms int and int */ }
+	_ interface{int | ~ /* ERROR overlapping terms ~int and int */ int}
+	_ interface{~int | int /* ERROR overlapping terms int and ~int */ }
+	_ interface{~int | ~ /* ERROR overlapping terms ~int and ~int */ int}
+
+	_ interface{T1 | bool | string | T1 | bool /* ERROR overlapping terms bool and bool */ | string /* ERROR overlapping terms string and string */ }
+	_ interface{T1 | bool | string | T2 | ~ /* ERROR overlapping terms ~bool and bool */ bool | ~ /* ERROR overlapping terms ~string and string */ string}
+
+	// no errors for interface terms
+	_ interface{T1 | T1}
+	_ interface{T1 | T2}
+	_ interface{T2 | T1}
+	_ interface{T2 | T2}
+
+	_ interface{T3 | T3 | int}
+	_ interface{T3 | T4 | bool }
+	_ interface{T4 | T3 | string }
+	_ interface{T4 | T4 | float64 }
+)
+
+func _[_ T1 | bool | string | T1 | bool /* ERROR overlapping terms */ ]() {}
+func _[_ T1 | bool | string | T2 | ~ /* ERROR overlapping terms */ bool ]() {}
+func _[_ T2 | ~bool | ~string | T1 | bool /* ERROR overlapping terms */ ]() {}
+func _[_ T2 | ~bool | ~string | T2 | ~ /* ERROR overlapping terms */ bool ]() {}
+
+func _[_ T3 | T3 | int]() {}
+func _[_ T3 | T4 | bool]() {}
+func _[_ T4 | T3 | string]() {}
+func _[_ T4 | T4 | float64]() {}
+
+// test cases from issue
+
+type _ interface {
+	interface {bool | int} | interface {bool | string}
+}
+
+type _ interface {
+	interface {bool | int} ; interface {bool | string}
+}
+
+type _ interface {
+	interface {bool; int} ; interface {bool; string}
+}
+
+type _ interface {
+	interface {bool; int} | interface {bool; string}
+}
\ No newline at end of file
diff --git a/src/internal/types/testdata/fixedbugs/issue51610.go b/src/internal/types/testdata/fixedbugs/issue51610.go
new file mode 100644
index 0000000..d10c788
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51610.go
@@ -0,0 +1,9 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[P int | float64 | complex128]() {
+	_ = map[P]int{1: 1, 1.0 /* ERROR duplicate key 1 */ : 2, 1 /* ERROR duplicate key \(1 \+ 0i\) */ + 0i: 3}
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51616.go b/src/internal/types/testdata/fixedbugs/issue51616.go
new file mode 100644
index 0000000..e0efc9e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51616.go
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+        C[T any] interface{~int; M() T}
+
+        _ C[bool]
+        _ comparable
+        _ interface {~[]byte | ~string}
+
+        // Alias type declarations may refer to "constraint" types
+        // like ordinary type declarations.
+        _ = C[bool]
+        _ = comparable
+        _ = interface {~[]byte | ~string}
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue51658.go b/src/internal/types/testdata/fixedbugs/issue51658.go
new file mode 100644
index 0000000..f32051c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51658.go
@@ -0,0 +1,43 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test checks syntax errors which differ between
+// go/parser and the syntax package.
+// TODO: consolidate eventually
+
+package p
+
+type F { // ERROR expected type|type declaration
+	float64
+} // ERROR expected declaration|non-declaration statement
+
+func _[T F | int](x T) {
+	_ = x == 0 // don't crash when recording type of 0
+}
+
+// test case from issue
+
+type FloatType { // ERROR expected type|type declaration
+	float32 | float64
+} // ERROR expected declaration|non-declaration statement
+
+type IntegerType interface {
+	int8 | int16 | int32 | int64 | int |
+		uint8 | uint16 | uint32 | uint64 | uint
+}
+
+type ComplexType interface {
+	complex64 | complex128
+}
+
+type Number interface {
+	FloatType | IntegerType | ComplexType
+}
+
+func GetDefaultNumber[T Number](value, defaultValue T) T {
+	if value == 0 {
+		return defaultValue
+	}
+	return value
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue51877.go b/src/internal/types/testdata/fixedbugs/issue51877.go
new file mode 100644
index 0000000..c93242a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue51877.go
@@ -0,0 +1,18 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type S struct {
+	f1 int
+	f2 bool
+}
+
+var (
+	_ = S{0}                    /* ERROR too few values in struct literal */
+	_ = struct{ f1, f2 int }{0} /* ERROR too few values in struct literal */
+
+	_ = S{0, true, "foo" /* ERROR too many values in struct literal */}
+	_ = struct{ f1, f2 int }{0, 1, 2 /* ERROR too many values in struct literal */}
+)
diff --git a/src/internal/types/testdata/fixedbugs/issue52031.go b/src/internal/types/testdata/fixedbugs/issue52031.go
new file mode 100644
index 0000000..448a550
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue52031.go
@@ -0,0 +1,33 @@
+// -lang=go1.12
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type resultFlags uint
+
+// Example from #52031.
+//
+// The following shifts should not produce errors on Go < 1.13, as their
+// untyped constant operands are representable by type uint.
+const (
+	_ resultFlags = (1 << iota) / 2
+
+	reportEqual
+	reportUnequal
+	reportByIgnore
+	reportByMethod
+	reportByFunc
+	reportByCycle
+)
+
+// Invalid cases.
+var x int = 1
+var _ = (8 << x /* ERROR "signed shift count .* requires go1.13 or later" */)
+
+const _ = (1 << 1.2 /* ERROR "truncated to uint" */)
+
+var y float64
+var _ = (1 << y /* ERROR "must be integer" */)
diff --git a/src/internal/types/testdata/fixedbugs/issue52401.go b/src/internal/types/testdata/fixedbugs/issue52401.go
new file mode 100644
index 0000000..c7efd8c
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue52401.go
@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	const x = 0
+	x /* ERROR cannot assign to x */ += 1
+	x /* ERROR cannot assign to x */ ++
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue52529.go b/src/internal/types/testdata/fixedbugs/issue52529.go
new file mode 100644
index 0000000..de7b296
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue52529.go
@@ -0,0 +1,15 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Foo[P any] struct {
+	_ *Bar[P]
+}
+
+type Bar[Q any] Foo[Q]
+
+func (v *Bar[R]) M() {
+	_ = (*Foo[R])(v)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue52698.go b/src/internal/types/testdata/fixedbugs/issue52698.go
new file mode 100644
index 0000000..ca794f8
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue52698.go
@@ -0,0 +1,62 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// correctness check: ensure that cycles through generic instantiations are detected
+type T[P any] struct {
+	_ P
+}
+
+type S /* ERROR invalid recursive type */ struct {
+	_ T[S]
+}
+
+// simplified test 1
+
+var _ A1[A1[string]]
+
+type A1[P any] struct {
+	_ B1[P]
+}
+
+type B1[P any] struct {
+	_ P
+}
+
+// simplified test 2
+var _ B2[A2]
+
+type A2 struct {
+	_ B2[string]
+}
+
+type B2[P any] struct {
+	_ C2[P]
+}
+
+type C2[P any] struct {
+	_ P
+}
+
+// test case from issue
+type T23 interface {
+	~struct {
+		Field0 T13[T15]
+	}
+}
+
+type T1[P1 interface {
+}] struct {
+	Field2 P1
+}
+
+type T13[P2 interface {
+}] struct {
+	Field2 T1[P2]
+}
+
+type T15 struct {
+	Field0 T13[string]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue52915.go b/src/internal/types/testdata/fixedbugs/issue52915.go
new file mode 100644
index 0000000..6c43386
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue52915.go
@@ -0,0 +1,23 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type T[P any] struct {
+	T /* ERROR   invalid recursive type */ [P]
+}
+
+func _[P any]() {
+	_ = unsafe.Sizeof(T[int]{})
+	_ = unsafe.Sizeof(struct{ T[int] }{})
+
+	_ = unsafe.Sizeof(T[P]{})
+	_ = unsafe.Sizeof(struct{ T[P] }{})
+}
+
+// TODO(gri) This is a follow-on error due to T[int] being invalid.
+//           We should try to avoid it.
+const _ = unsafe /* ERROR not constant */ .Sizeof(T[int]{})
diff --git a/src/internal/types/testdata/fixedbugs/issue53358.go b/src/internal/types/testdata/fixedbugs/issue53358.go
new file mode 100644
index 0000000..774136f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue53358.go
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type A struct{}
+
+func (*A) m() int { return 0 }
+
+var _ = A.m /* ERROR invalid method expression A\.m \(needs pointer receiver \(\*A\)\.m\) */ ()
+var _ = (*A).m(nil)
+
+type B struct{ A }
+
+var _ = B.m // ERROR invalid method expression B\.m \(needs pointer receiver \(\*B\)\.m\)
+var _ = (*B).m
+
+var _ = struct{ A }.m // ERROR invalid method expression struct{A}\.m \(needs pointer receiver \(\*struct{A}\)\.m\)
diff --git a/src/internal/types/testdata/fixedbugs/issue54280.go b/src/internal/types/testdata/fixedbugs/issue54280.go
new file mode 100644
index 0000000..e83e1a1
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue54280.go
@@ -0,0 +1,7 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+const C = 912_345_678_901_234_567_890_123_456_789_012_345_678_901_234_567_890_912_345_678_901_234_567_890_123_456_789_012_345_678_901_234_567_890_912_345_678_901_234_567_890_123_456_789_012_345_678_901_234_567_890_912 // ERROR constant overflow
diff --git a/src/internal/types/testdata/fixedbugs/issue54405.go b/src/internal/types/testdata/fixedbugs/issue54405.go
new file mode 100644
index 0000000..611d830
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue54405.go
@@ -0,0 +1,16 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that we don't see spurious errors for ==
+// for values with invalid types due to prior errors.
+
+package p
+
+var x struct {
+	f *NotAType /* ERROR undefined */
+}
+var _ = x.f == nil // no error expected here
+
+var y *NotAType  /* ERROR undefined */
+var _ = y == nil // no error expected here
diff --git a/src/internal/types/testdata/fixedbugs/issue56351.go b/src/internal/types/testdata/fixedbugs/issue56351.go
new file mode 100644
index 0000000..d7d04b0
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue56351.go
@@ -0,0 +1,11 @@
+// -lang=go1.20
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _(s []int) {
+	clear /* ERROR clear requires go1\.21 or later */ (s)
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue56425.go b/src/internal/types/testdata/fixedbugs/issue56425.go
new file mode 100644
index 0000000..d85733f
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue56425.go
@@ -0,0 +1,8 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+const s float32 = 0
+var _ = 0 << s
diff --git a/src/internal/types/testdata/fixedbugs/issue56665.go b/src/internal/types/testdata/fixedbugs/issue56665.go
new file mode 100644
index 0000000..11786b9
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue56665.go
@@ -0,0 +1,30 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Example from the issue:
+type A[T any] interface {
+	*T
+}
+
+type B[T any] interface {
+	B /* ERROR invalid recursive type */ [*T]
+}
+
+type C[T any, U B[U]] interface {
+	*T
+}
+
+// Simplified reproducer:
+type X[T any] interface {
+	X /* ERROR invalid recursive type */ [*T]
+}
+
+var _ X[int]
+
+// A related example that doesn't go through interfaces.
+type A2[P any] [10]A2 /* ERROR invalid recursive type */ [*P]
+
+var _ A2[int]
diff --git a/src/internal/types/testdata/fixedbugs/issue57155.go b/src/internal/types/testdata/fixedbugs/issue57155.go
new file mode 100644
index 0000000..ec9fb2b
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57155.go
@@ -0,0 +1,14 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P *Q, Q any](p P, q Q) {
+	func() {
+		_ = f[P]
+		f(p, q)
+		f[P](p, q)
+		f[P, Q](p, q)
+	}()
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue57160.go b/src/internal/types/testdata/fixedbugs/issue57160.go
new file mode 100644
index 0000000..446d019
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57160.go
@@ -0,0 +1,10 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _(x *int) {
+	_ = 0 < x // ERROR "invalid operation"
+	_ = x < 0 // ERROR "invalid operation"
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue57486.go b/src/internal/types/testdata/fixedbugs/issue57486.go
new file mode 100644
index 0000000..43ba1b0
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57486.go
@@ -0,0 +1,29 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type C1 interface {
+	comparable
+}
+
+type C2 interface {
+	comparable
+	[2]any | int
+}
+
+func G1[T C1](t T) { _ = t == t }
+func G2[T C2](t T) { _ = t == t }
+
+func F1[V [2]any](v V) {
+	_ = G1[V /* ERROR "V does not satisfy comparable" */]
+	_ = G1[[2]any]
+	_ = G1[int]
+}
+
+func F2[V [2]any](v V) {
+	_ = G2[V /* ERROR "V does not satisfy C2" */]
+	_ = G2[[ /* ERROR "\[2\]any does not satisfy C2 \(C2 mentions \[2\]any, but \[2\]any is not in the type set of C2\)" */ 2]any]
+	_ = G2[int]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue57500.go b/src/internal/types/testdata/fixedbugs/issue57500.go
new file mode 100644
index 0000000..abdcb5e
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57500.go
@@ -0,0 +1,16 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type C interface {
+	comparable
+	[2]any | int
+}
+
+func f[T C]() {}
+
+func _() {
+	_ = f[[ /* ERROR \[2\]any does not satisfy C \(C mentions \[2\]any, but \[2\]any is not in the type set of C\) */ 2]any]
+}
diff --git a/src/internal/types/testdata/fixedbugs/issue57522.go b/src/internal/types/testdata/fixedbugs/issue57522.go
new file mode 100644
index 0000000..d83e5b2
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue57522.go
@@ -0,0 +1,24 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// A simplified version of the code in the original report.
+type S[T any] struct{}
+var V = S[any]{}
+func (fs *S[T]) M(V.M /* ERROR "V.M is not a type" */) {}
+
+// Other minimal reproducers.
+type S1[T any] V1.M /* ERROR "V1.M is not a type" */
+type V1 = S1[any]
+
+type S2[T any] struct{}
+type V2 = S2[any]
+func (fs *S2[T]) M(x V2.M /* ERROR "V2.M is not a type" */ ) {}
+
+// The following still panics, as the selector is reached from check.expr
+// rather than check.typexpr. TODO(rfindley): fix this.
+// type X[T any] int
+// func (X[T]) M(x [X[int].M]int) {}
+
diff --git a/src/internal/types/testdata/fixedbugs/issue6977.go b/src/internal/types/testdata/fixedbugs/issue6977.go
new file mode 100644
index 0000000..8f4e9ba
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue6977.go
@@ -0,0 +1,82 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "io"
+
+// Alan's initial report.
+
+type I interface { f(); String() string }
+type J interface { g(); String() string }
+
+type IJ1 = interface { I; J }
+type IJ2 = interface { f(); g(); String() string }
+
+var _ = (*IJ1)(nil) == (*IJ2)(nil) // static assert that IJ1 and IJ2 are identical types
+
+// The canonical example.
+
+type ReadWriteCloser interface { io.ReadCloser; io.WriteCloser }
+
+// Some more cases.
+
+type M interface { m() }
+type M32 interface { m() int32 }
+type M64 interface { m() int64 }
+
+type U1 interface { m() }
+type U2 interface { m(); M }
+type U3 interface { M; m() }
+type U4 interface { M; M; M }
+type U5 interface { U1; U2; U3; U4 }
+
+type U6 interface { m(); m /* ERROR duplicate method */ () }
+type U7 interface { M32 /* ERROR duplicate method */ ; m() }
+type U8 interface { m(); M32 /* ERROR duplicate method */ }
+type U9 interface { M32; M64 /* ERROR duplicate method */ }
+
+// Verify that repeated embedding of the same interface(s)
+// eliminates duplicate methods early (rather than at the
+// end) to prevent exponential memory and time use.
+// Without early elimination, computing T29 may take dozens
+// of minutes.
+type (
+        T0 interface { m() }
+        T1 interface { T0; T0 }
+        T2 interface { T1; T1 }
+        T3 interface { T2; T2 }
+        T4 interface { T3; T3 }
+        T5 interface { T4; T4 }
+        T6 interface { T5; T5 }
+        T7 interface { T6; T6 }
+        T8 interface { T7; T7 }
+        T9 interface { T8; T8 }
+
+        T10 interface { T9; T9 }
+        T11 interface { T10; T10 }
+        T12 interface { T11; T11 }
+        T13 interface { T12; T12 }
+        T14 interface { T13; T13 }
+        T15 interface { T14; T14 }
+        T16 interface { T15; T15 }
+        T17 interface { T16; T16 }
+        T18 interface { T17; T17 }
+        T19 interface { T18; T18 }
+
+        T20 interface { T19; T19 }
+        T21 interface { T20; T20 }
+        T22 interface { T21; T21 }
+        T23 interface { T22; T22 }
+        T24 interface { T23; T23 }
+        T25 interface { T24; T24 }
+        T26 interface { T25; T25 }
+        T27 interface { T26; T26 }
+        T28 interface { T27; T27 }
+        T29 interface { T28; T28 }
+)
+
+// Verify that m is present.
+var x T29
+var _ = x.m
diff --git a/src/internal/types/testdata/spec/assignability.go b/src/internal/types/testdata/spec/assignability.go
new file mode 100644
index 0000000..0ab8eb3
--- /dev/null
+++ b/src/internal/types/testdata/spec/assignability.go
@@ -0,0 +1,264 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package assignability
+
+// See the end of this package for the declarations
+// of the types and variables used in these tests.
+
+// "x's type is identical to T"
+func _[TP any](X TP) {
+	b = b
+	a = a
+	l = l
+	s = s
+	p = p
+	f = f
+	i = i
+	m = m
+	c = c
+	d = d
+
+	B = B
+	A = A
+	L = L
+	S = S
+	P = P
+	F = F
+	I = I
+	M = M
+	C = C
+	D = D
+	X = X
+}
+
+// "x's type V and T have identical underlying types
+// and at least one of V or T is not a named type."
+// (here a named type is a type with a name)
+func _[TP1, TP2 Interface](X1 TP1, X2 TP2) {
+	b = B // ERROR cannot use B .* as int value
+	a = A
+	l = L
+	s = S
+	p = P
+	f = F
+	i = I
+	m = M
+	c = C
+	d = D
+
+	B = b // ERROR cannot use b .* as Basic value
+	A = a
+	L = l
+	S = s
+	P = p
+	F = f
+	I = i
+	M = m
+	C = c
+	D = d
+	X1 = i  // ERROR cannot use i .* as TP1 value
+	X1 = X2 // ERROR cannot use X2 .* as TP1 value
+}
+
+// "T is an interface type and x implements T and T is not a type parameter"
+func _[TP Interface](X TP) {
+	i = d // ERROR missing method m
+	i = D
+	i = X
+	X = i // ERROR cannot use i .* as TP value
+}
+
+// "x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a named type"
+// (here a named type is a type with a name)
+type (
+	_SendChan = chan<- int
+	_RecvChan = <-chan int
+
+	SendChan _SendChan
+	RecvChan _RecvChan
+)
+
+func _[
+	_CC ~_Chan,
+	_SC ~_SendChan,
+	_RC ~_RecvChan,
+
+	CC Chan,
+	SC SendChan,
+	RC RecvChan,
+]() {
+	var (
+		_ _SendChan = c
+		_ _RecvChan = c
+		_ _Chan     = c
+
+		_ _SendChan = C
+		_ _RecvChan = C
+		_ _Chan     = C
+
+		_ SendChan = c
+		_ RecvChan = c
+		_ Chan     = c
+
+		_ SendChan = C // ERROR cannot use C .* as SendChan value
+		_ RecvChan = C // ERROR cannot use C .* as RecvChan value
+		_ Chan     = C
+		_ Chan     = make /* ERROR cannot use make\(chan Basic\) .* as Chan value */ (chan Basic)
+	)
+
+	var (
+		_ _CC = C // ERROR cannot use C .* as _CC value
+		_ _SC = C // ERROR cannot use C .* as _SC value
+		_ _RC = C // ERROR cannot use C .* as _RC value
+
+		_ CC = _CC /* ERROR cannot use _CC\(nil\) .* as CC value */ (nil)
+		_ SC = _CC /* ERROR cannot use _CC\(nil\) .* as SC value */ (nil)
+		_ RC = _CC /* ERROR cannot use _CC\(nil\) .* as RC value */ (nil)
+
+		_ CC = C // ERROR cannot use C .* as CC value
+		_ SC = C // ERROR cannot use C .* as SC value
+		_ RC = C // ERROR cannot use C .* as RC value
+	)
+}
+
+// "x's type V is not a named type and T is a type parameter, and x is assignable to each specific type in T's type set."
+func _[
+	TP0 any,
+	TP1 ~_Chan,
+	TP2 ~chan int | ~chan byte,
+]() {
+	var (
+		_ TP0 = c // ERROR cannot use c .* as TP0 value
+		_ TP0 = C // ERROR cannot use C .* as TP0 value
+		_ TP1 = c
+		_ TP1 = C // ERROR cannot use C .* as TP1 value
+		_ TP2 = c // ERROR .* cannot assign chan int to chan byte
+	)
+}
+
+// "x's type V is a type parameter and T is not a named type, and values x' of each specific type in V's type set are assignable to T."
+func _[
+	TP0 Interface,
+	TP1 ~_Chan,
+	TP2 ~chan int | ~chan byte,
+](X0 TP0, X1 TP1, X2 TP2) {
+	i = X0
+	I = X0
+	c = X1
+	C = X1 // ERROR cannot use X1 .* as Chan value
+	c = X2 // ERROR .* cannot assign chan byte \(in TP2\) to chan int
+}
+
+// "x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type"
+func _[TP Interface](X TP) {
+	b = nil // ERROR cannot use nil
+	a = nil // ERROR cannot use nil
+	l = nil
+	s = nil // ERROR cannot use nil
+	p = nil
+	f = nil
+	i = nil
+	m = nil
+	c = nil
+	d = nil // ERROR cannot use nil
+
+	B = nil // ERROR cannot use nil
+	A = nil // ERROR cannot use nil
+	L = nil
+	S = nil // ERROR cannot use nil
+	P = nil
+	F = nil
+	I = nil
+	M = nil
+	C = nil
+	D = nil // ERROR cannot use nil
+	X = nil // ERROR cannot use nil
+}
+
+// "x is an untyped constant representable by a value of type T"
+func _[
+	Int8 ~int8,
+	Int16 ~int16,
+	Int32 ~int32,
+	Int64 ~int64,
+	Int8_16 ~int8 | ~int16,
+](
+	i8 Int8,
+	i16 Int16,
+	i32 Int32,
+	i64 Int64,
+	i8_16 Int8_16,
+) {
+	b = 42
+	b = 42.0
+	// etc.
+
+	i8 = -1 << 7
+	i8 = 1<<7 - 1
+	i16 = -1 << 15
+	i16 = 1<<15 - 1
+	i32 = -1 << 31
+	i32 = 1<<31 - 1
+	i64 = -1 << 63
+	i64 = 1<<63 - 1
+
+	i8_16 = -1 << 7
+	i8_16 = 1<<7 - 1
+	i8_16 = - /* ERROR cannot use .* as Int8_16 */ 1 << 15
+	i8_16 = 1 /* ERROR cannot use .* as Int8_16 */ <<15 - 1
+}
+
+// proto-types for tests
+
+type (
+	_Basic     = int
+	_Array     = [10]int
+	_Slice     = []int
+	_Struct    = struct{ f int }
+	_Pointer   = *int
+	_Func      = func(x int) string
+	_Interface = interface{ m() int }
+	_Map       = map[string]int
+	_Chan      = chan int
+
+	Basic     _Basic
+	Array     _Array
+	Slice     _Slice
+	Struct    _Struct
+	Pointer   _Pointer
+	Func      _Func
+	Interface _Interface
+	Map       _Map
+	Chan      _Chan
+	Defined   _Struct
+)
+
+func (Defined) m() int
+
+// proto-variables for tests
+
+var (
+	b _Basic
+	a _Array
+	l _Slice
+	s _Struct
+	p _Pointer
+	f _Func
+	i _Interface
+	m _Map
+	c _Chan
+	d _Struct
+
+	B Basic
+	A Array
+	L Slice
+	S Struct
+	P Pointer
+	F Func
+	I Interface
+	M Map
+	C Chan
+	D Defined
+)
diff --git a/src/internal/types/testdata/spec/comparable.go b/src/internal/types/testdata/spec/comparable.go
new file mode 100644
index 0000000..f407c35
--- /dev/null
+++ b/src/internal/types/testdata/spec/comparable.go
@@ -0,0 +1,26 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T /* T does satisfy comparable */]
+	_ = f1[any /* any does satisfy comparable */]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not satisfy comparable */]
+
+	_ = f2[int]
+	_ = f2[T /* T does satisfy comparable */]
+	_ = f2[any /* any does satisfy comparable */]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not satisfy comparable */]
+}
diff --git a/src/internal/types/testdata/spec/comparable1.19.go b/src/internal/types/testdata/spec/comparable1.19.go
new file mode 100644
index 0000000..dc1c5fa
--- /dev/null
+++ b/src/internal/types/testdata/spec/comparable1.19.go
@@ -0,0 +1,28 @@
+// -lang=go1.19
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T /* ERROR T to satisfy comparable requires go1\.20 or later */]
+	_ = f1[any /* ERROR any to satisfy comparable requires go1\.20 or later */]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not satisfy comparable */]
+
+	_ = f2[int]
+	_ = f2[T /* ERROR T to satisfy comparable requires go1\.20 or later */]
+	_ = f2[any /* ERROR any to satisfy comparable requires go1\.20 or later */]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not satisfy comparable */]
+}
diff --git a/src/internal/types/testdata/spec/comparisons.go b/src/internal/types/testdata/spec/comparisons.go
new file mode 100644
index 0000000..886e78c
--- /dev/null
+++ b/src/internal/types/testdata/spec/comparisons.go
@@ -0,0 +1,120 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package comparisons
+
+type (
+	B int // basic type representative
+	A [10]func()
+	L []byte
+	S struct{ f []byte }
+	P *S
+	F func()
+	I interface{}
+	M map[string]int
+	C chan int
+)
+
+var (
+	b B
+	a A
+	l L
+	s S
+	p P
+	f F
+	i I
+	m M
+	c C
+)
+
+func _() {
+	_ = nil == nil // ERROR operator == not defined on untyped nil
+	_ = b == b
+	_ = a /* ERROR \[10\]func\(\) cannot be compared */ == a
+	_ = l /* ERROR slice can only be compared to nil */ == l
+	_ = s /* ERROR struct containing \[\]byte cannot be compared */ == s
+	_ = p == p
+	_ = f /* ERROR func can only be compared to nil */ == f
+	_ = i == i
+	_ = m /* ERROR map can only be compared to nil */ == m
+	_ = c == c
+
+	_ = b == nil /* ERROR mismatched types */
+	_ = a == nil /* ERROR mismatched types */
+	_ = l == nil
+	_ = s == nil /* ERROR mismatched types */
+	_ = p == nil
+	_ = f == nil
+	_ = i == nil
+	_ = m == nil
+	_ = c == nil
+
+	_ = nil /* ERROR operator < not defined on untyped nil */ < nil
+	_ = b < b
+	_ = a /* ERROR operator < not defined on array */ < a
+	_ = l /* ERROR operator < not defined on slice */ < l
+	_ = s /* ERROR operator < not defined on struct */ < s
+	_ = p /* ERROR operator < not defined on pointer */ < p
+	_ = f /* ERROR operator < not defined on func */ < f
+	_ = i /* ERROR operator < not defined on interface */ < i
+	_ = m /* ERROR operator < not defined on map */ < m
+	_ = c /* ERROR operator < not defined on chan */ < c
+}
+
+func _[
+	B int,
+	A [10]func(),
+	L []byte,
+	S struct{ f []byte },
+	P *S,
+	F func(),
+	I interface{},
+	J comparable,
+	M map[string]int,
+	C chan int,
+](
+	b B,
+	a A,
+	l L,
+	s S,
+	p P,
+	f F,
+	i I,
+	j J,
+	m M,
+	c C,
+) {
+	_ = b == b
+	_ = a /* ERROR incomparable types in type set */ == a
+	_ = l /* ERROR incomparable types in type set */ == l
+	_ = s /* ERROR incomparable types in type set */ == s
+	_ = p == p
+	_ = f /* ERROR incomparable types in type set */ == f
+	_ = i /* ERROR incomparable types in type set */ == i
+	_ = j == j
+	_ = m /* ERROR incomparable types in type set */ == m
+	_ = c == c
+
+	_ = b == nil /* ERROR mismatched types */
+	_ = a == nil /* ERROR mismatched types */
+	_ = l == nil
+	_ = s == nil /* ERROR mismatched types */
+	_ = p == nil
+	_ = f == nil
+	_ = i == nil /* ERROR mismatched types */
+	_ = j == nil /* ERROR mismatched types */
+	_ = m == nil
+	_ = c == nil
+
+	_ = b < b
+	_ = a /* ERROR type parameter A is not comparable with < */ < a
+	_ = l /* ERROR type parameter L is not comparable with < */ < l
+	_ = s /* ERROR type parameter S is not comparable with < */ < s
+	_ = p /* ERROR type parameter P is not comparable with < */ < p
+	_ = f /* ERROR type parameter F is not comparable with < */ < f
+	_ = i /* ERROR type parameter I is not comparable with < */ < i
+	_ = j /* ERROR type parameter J is not comparable with < */ < j
+	_ = m /* ERROR type parameter M is not comparable with < */ < m
+	_ = c /* ERROR type parameter C is not comparable with < */ < c
+}
diff --git a/src/internal/types/testdata/spec/conversions.go b/src/internal/types/testdata/spec/conversions.go
new file mode 100644
index 0000000..fc014fc
--- /dev/null
+++ b/src/internal/types/testdata/spec/conversions.go
@@ -0,0 +1,208 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package conversions
+
+import "unsafe"
+
+// constant conversions
+
+func _[T ~byte]() T { return 255 }
+func _[T ~byte]() T { return 256 /* ERROR cannot use 256 .* as T value */ }
+
+func _[T ~byte]() {
+	const _ = T /* ERROR T\(0\) .* is not constant */ (0)
+	var _ T = 255
+	var _ T = 256 // ERROR cannot use 256 .* as T value
+}
+
+func _[T ~string]() T                { return T('a') }
+func _[T ~int | ~string]() T         { return T('a') }
+func _[T ~byte | ~int | ~string]() T { return T(256 /* ERROR cannot convert 256 .* to type T */) }
+
+// implicit conversions never convert to string
+func _[T ~string]() {
+	var _ string = 0 // ERROR cannot use .* as string value
+	var _ T = 0      // ERROR cannot use .* as T value
+}
+
+// failing const conversions of constants to type parameters report a cause
+func _[
+	T1 any,
+	T2 interface{ m() },
+	T3 ~int | ~float64 | ~bool,
+	T4 ~int | ~string,
+]() {
+	_ = T1(0 /* ERROR cannot convert 0 .* to type T1: T1 does not contain specific types */)
+	_ = T2(1 /* ERROR cannot convert 1 .* to type T2: T2 does not contain specific types */)
+	_ = T3(2 /* ERROR cannot convert 2 .* to type T3: cannot convert 2 .* to type bool \(in T3\) */)
+	_ = T4(3.14 /* ERROR cannot convert 3.14 .* to type T4: cannot convert 3.14 .* to type int \(in T4\) */)
+}
+
+// "x is assignable to T"
+// - tested via assignability tests
+
+// "x's type and T have identical underlying types if tags are ignored"
+
+func _[X ~int, T ~int](x X) T { return T(x) }
+func _[X struct {
+	f int "foo"
+}, T struct {
+	f int "bar"
+}](x X) T {
+	return T(x)
+}
+
+type Foo struct {
+	f int "foo"
+}
+type Bar struct {
+	f int "bar"
+}
+type Far struct{ f float64 }
+
+func _[X Foo, T Bar](x X) T       { return T(x) }
+func _[X Foo | Bar, T Bar](x X) T { return T(x) }
+func _[X Foo, T Foo | Bar](x X) T { return T(x) }
+func _[X Foo, T Far](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by Foo\) to type T: cannot convert Foo \(in X\) to type Far \(in T\) */)
+}
+
+// "x's type and T are unnamed pointer types and their pointer base types
+// have identical underlying types if tags are ignored"
+
+func _[X ~*Foo, T ~*Bar](x X) T         { return T(x) }
+func _[X ~*Foo | ~*Bar, T ~*Bar](x X) T { return T(x) }
+func _[X ~*Foo, T ~*Foo | ~*Bar](x X) T { return T(x) }
+func _[X ~*Foo, T ~*Far](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by ~\*Foo\) to type T: cannot convert \*Foo \(in X\) to type \*Far \(in T\) */)
+}
+
+// Verify that the defined types in constraints are considered for the rule above.
+
+type (
+	B  int
+	C  int
+	X0 *B
+	T0 *C
+)
+
+func _(x X0) T0           { return T0(x /* ERROR cannot convert */) } // non-generic reference
+func _[X X0, T T0](x X) T { return T(x /* ERROR cannot convert */) }
+func _[T T0](x X0) T      { return T(x /* ERROR cannot convert */) }
+func _[X X0](x X) T0      { return T0(x /* ERROR cannot convert */) }
+
+// "x's type and T are both integer or floating point types"
+
+func _[X Integer, T Integer](x X) T  { return T(x) }
+func _[X Unsigned, T Integer](x X) T { return T(x) }
+func _[X Float, T Integer](x X) T    { return T(x) }
+
+func _[X Integer, T Unsigned](x X) T  { return T(x) }
+func _[X Unsigned, T Unsigned](x X) T { return T(x) }
+func _[X Float, T Unsigned](x X) T    { return T(x) }
+
+func _[X Integer, T Float](x X) T  { return T(x) }
+func _[X Unsigned, T Float](x X) T { return T(x) }
+func _[X Float, T Float](x X) T    { return T(x) }
+
+func _[X, T Integer | Unsigned | Float](x X) T { return T(x) }
+func _[X, T Integer | ~string](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by Integer \| ~string\) to type T: cannot convert string \(in X\) to type int \(in T\) */)
+}
+
+// "x's type and T are both complex types"
+
+func _[X, T Complex](x X) T { return T(x) }
+func _[X, T Float | Complex](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by Float \| Complex\) to type T: cannot convert float32 \(in X\) to type complex64 \(in T\) */)
+}
+
+// "x is an integer or a slice of bytes or runes and T is a string type"
+
+type myInt int
+type myString string
+
+func _[T ~string](x int) T      { return T(x) }
+func _[T ~string](x myInt) T    { return T(x) }
+func _[X Integer](x X) string   { return string(x) }
+func _[X Integer](x X) myString { return myString(x) }
+func _[X Integer](x X) *string {
+	return (*string)(x /* ERROR cannot convert x \(variable of type X constrained by Integer\) to type \*string: cannot convert int \(in X\) to type \*string */)
+}
+
+func _[T ~string](x []byte) T                           { return T(x) }
+func _[T ~string](x []rune) T                           { return T(x) }
+func _[X ~[]byte, T ~string](x X) T                     { return T(x) }
+func _[X ~[]rune, T ~string](x X) T                     { return T(x) }
+func _[X Integer | ~[]byte | ~[]rune, T ~string](x X) T { return T(x) }
+func _[X Integer | ~[]byte | ~[]rune, T ~*string](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by Integer \| ~\[\]byte \| ~\[\]rune\) to type T: cannot convert int \(in X\) to type \*string \(in T\) */)
+}
+
+// "x is a string and T is a slice of bytes or runes"
+
+func _[T ~[]byte](x string) T { return T(x) }
+func _[T ~[]rune](x string) T { return T(x) }
+func _[T ~[]rune](x *string) T {
+	return T(x /* ERROR cannot convert x \(variable of type \*string\) to type T: cannot convert \*string to type \[\]rune \(in T\) */)
+}
+
+func _[X ~string, T ~[]byte](x X) T           { return T(x) }
+func _[X ~string, T ~[]rune](x X) T           { return T(x) }
+func _[X ~string, T ~[]byte | ~[]rune](x X) T { return T(x) }
+func _[X ~*string, T ~[]byte | ~[]rune](x X) T {
+	return T(x /* ERROR cannot convert x \(variable of type X constrained by ~\*string\) to type T: cannot convert \*string \(in X\) to type \[\]byte \(in T\) */)
+}
+
+// package unsafe:
+// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
+
+type myUintptr uintptr
+
+func _[X ~uintptr](x X) unsafe.Pointer  { return unsafe.Pointer(x) }
+func _[T unsafe.Pointer](x myUintptr) T { return T(x) }
+func _[T unsafe.Pointer](x int64) T {
+	return T(x /* ERROR cannot convert x \(variable of type int64\) to type T: cannot convert int64 to type unsafe\.Pointer \(in T\) */)
+}
+
+// "and vice versa"
+
+func _[T ~uintptr](x unsafe.Pointer) T  { return T(x) }
+func _[X unsafe.Pointer](x X) uintptr   { return uintptr(x) }
+func _[X unsafe.Pointer](x X) myUintptr { return myUintptr(x) }
+func _[X unsafe.Pointer](x X) int64 {
+	return int64(x /* ERROR cannot convert x \(variable of type X constrained by unsafe\.Pointer\) to type int64: cannot convert unsafe\.Pointer \(in X\) to type int64 */)
+}
+
+// "x is a slice, T is an array or pointer-to-array type,
+// and the slice and array types have identical element types."
+
+func _[X ~[]E, T ~[10]E, E any](x X) T  { return T(x) }
+func _[X ~[]E, T ~*[10]E, E any](x X) T { return T(x) }
+
+// ----------------------------------------------------------------------------
+// The following declarations can be replaced by the exported types of the
+// constraints package once all builders support importing interfaces with
+// type constraints.
+
+type Signed interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64
+}
+
+type Unsigned interface {
+	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+}
+
+type Integer interface {
+	Signed | Unsigned
+}
+
+type Float interface {
+	~float32 | ~float64
+}
+
+type Complex interface {
+	~complex64 | ~complex128
+}
diff --git a/src/internal/types/testdata/spec/oldcomparable.go b/src/internal/types/testdata/spec/oldcomparable.go
new file mode 100644
index 0000000..081d972
--- /dev/null
+++ b/src/internal/types/testdata/spec/oldcomparable.go
@@ -0,0 +1,28 @@
+// -oldComparableSemantics
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T /* ERROR T does not satisfy comparable */]
+	_ = f1[any /* ERROR any does not satisfy comparable */]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not satisfy comparable */]
+
+	_ = f2[int]
+	_ = f2[T /* ERROR T does not satisfy comparable */]
+	_ = f2[any /* ERROR any does not satisfy comparable */]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not satisfy comparable */]
+}
diff --git a/src/internal/unsafeheader/unsafeheader.go b/src/internal/unsafeheader/unsafeheader.go
new file mode 100644
index 0000000..6d092c6
--- /dev/null
+++ b/src/internal/unsafeheader/unsafeheader.go
@@ -0,0 +1,37 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package unsafeheader contains header declarations for the Go runtime's slice
+// and string implementations.
+//
+// This package allows packages that cannot import "reflect" to use types that
+// are tested to be equivalent to reflect.SliceHeader and reflect.StringHeader.
+package unsafeheader
+
+import (
+	"unsafe"
+)
+
+// Slice is the runtime representation of a slice.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.SliceHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type Slice struct {
+	Data unsafe.Pointer
+	Len  int
+	Cap  int
+}
+
+// String is the runtime representation of a string.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.StringHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type String struct {
+	Data unsafe.Pointer
+	Len  int
+}
diff --git a/src/internal/unsafeheader/unsafeheader_test.go b/src/internal/unsafeheader/unsafeheader_test.go
new file mode 100644
index 0000000..f3d1a9b
--- /dev/null
+++ b/src/internal/unsafeheader/unsafeheader_test.go
@@ -0,0 +1,100 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unsafeheader_test
+
+import (
+	"bytes"
+	"internal/unsafeheader"
+	"reflect"
+	"testing"
+	"unsafe"
+)
+
+// TestTypeMatchesReflectType ensures that the name and layout of the
+// unsafeheader types matches the corresponding Header types in the reflect
+// package.
+func TestTypeMatchesReflectType(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.Slice{}, reflect.SliceHeader{})
+	})
+
+	t.Run("String", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.String{}, reflect.StringHeader{})
+	})
+}
+
+func testHeaderMatchesReflect(t *testing.T, header, reflectHeader any) {
+	h := reflect.TypeOf(header)
+	rh := reflect.TypeOf(reflectHeader)
+
+	for i := 0; i < h.NumField(); i++ {
+		f := h.Field(i)
+		rf, ok := rh.FieldByName(f.Name)
+		if !ok {
+			t.Errorf("Field %d of %v is named %s, but no such field exists in %v", i, h, f.Name, rh)
+			continue
+		}
+		if !typeCompatible(f.Type, rf.Type) {
+			t.Errorf("%v.%s has type %v, but %v.%s has type %v", h, f.Name, f.Type, rh, rf.Name, rf.Type)
+		}
+		if f.Offset != rf.Offset {
+			t.Errorf("%v.%s has offset %d, but %v.%s has offset %d", h, f.Name, f.Offset, rh, rf.Name, rf.Offset)
+		}
+	}
+
+	if h.NumField() != rh.NumField() {
+		t.Errorf("%v has %d fields, but %v has %d", h, h.NumField(), rh, rh.NumField())
+	}
+	if h.Align() != rh.Align() {
+		t.Errorf("%v has alignment %d, but %v has alignment %d", h, h.Align(), rh, rh.Align())
+	}
+}
+
+var (
+	unsafePointerType = reflect.TypeOf(unsafe.Pointer(nil))
+	uintptrType       = reflect.TypeOf(uintptr(0))
+)
+
+func typeCompatible(t, rt reflect.Type) bool {
+	return t == rt || (t == unsafePointerType && rt == uintptrType)
+}
+
+// TestWriteThroughHeader ensures that the headers in the unsafeheader package
+// can successfully mutate variables of the corresponding built-in types.
+//
+// This test is expected to fail under -race (which implicitly enables
+// -d=checkptr) if the runtime views the header types as incompatible with the
+// underlying built-in types.
+func TestWriteThroughHeader(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		s := []byte("Hello, checkptr!")[:5]
+
+		var alias []byte
+		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&alias))
+		hdr.Data = unsafe.Pointer(&s[0])
+		hdr.Cap = cap(s)
+		hdr.Len = len(s)
+
+		if !bytes.Equal(alias, s) {
+			t.Errorf("alias of %T(%q) constructed via Slice = %T(%q)", s, s, alias, alias)
+		}
+		if cap(alias) != cap(s) {
+			t.Errorf("alias of %T with cap %d has cap %d", s, cap(s), cap(alias))
+		}
+	})
+
+	t.Run("String", func(t *testing.T) {
+		s := "Hello, checkptr!"
+
+		var alias string
+		hdr := (*unsafeheader.String)(unsafe.Pointer(&alias))
+		hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
+		hdr.Len = len(s)
+
+		if alias != s {
+			t.Errorf("alias of %q constructed via String = %q", s, alias)
+		}
+	})
+}
diff --git a/src/internal/xcoff/ar.go b/src/internal/xcoff/ar.go
new file mode 100644
index 0000000..2b432d5
--- /dev/null
+++ b/src/internal/xcoff/ar.go
@@ -0,0 +1,228 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xcoff
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+)
+
+const (
+	SAIAMAG   = 0x8
+	AIAFMAG   = "`\n"
+	AIAMAG    = "<aiaff>\n"
+	AIAMAGBIG = "<bigaf>\n"
+
+	// Sizeof
+	FL_HSZ_BIG = 0x80
+	AR_HSZ_BIG = 0x70
+)
+
+type bigarFileHeader struct {
+	Flmagic    [SAIAMAG]byte // Archive magic string
+	Flmemoff   [20]byte      // Member table offset
+	Flgstoff   [20]byte      // 32-bits global symtab offset
+	Flgst64off [20]byte      // 64-bits global symtab offset
+	Flfstmoff  [20]byte      // First member offset
+	Fllstmoff  [20]byte      // Last member offset
+	Flfreeoff  [20]byte      // First member on free list offset
+}
+
+type bigarMemberHeader struct {
+	Arsize   [20]byte // File member size
+	Arnxtmem [20]byte // Next member pointer
+	Arprvmem [20]byte // Previous member pointer
+	Ardate   [12]byte // File member date
+	Aruid    [12]byte // File member uid
+	Argid    [12]byte // File member gid
+	Armode   [12]byte // File member mode (octal)
+	Arnamlen [4]byte  // File member name length
+	// _ar_nam is removed because it's easier to get name without it.
+}
+
+// Archive represents an open AIX big archive.
+type Archive struct {
+	ArchiveHeader
+	Members []*Member
+
+	closer io.Closer
+}
+
+// MemberHeader holds information about a big archive file header
+type ArchiveHeader struct {
+	magic string
+}
+
+// Member represents a member of an AIX big archive.
+type Member struct {
+	MemberHeader
+	sr *io.SectionReader
+}
+
+// MemberHeader holds information about a big archive member
+type MemberHeader struct {
+	Name string
+	Size uint64
+}
+
+// OpenArchive opens the named archive using os.Open and prepares it for use
+// as an AIX big archive.
+func OpenArchive(name string) (*Archive, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	arch, err := NewArchive(f)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	arch.closer = f
+	return arch, nil
+}
+
+// Close closes the Archive.
+// If the Archive was created using NewArchive directly instead of OpenArchive,
+// Close has no effect.
+func (a *Archive) Close() error {
+	var err error
+	if a.closer != nil {
+		err = a.closer.Close()
+		a.closer = nil
+	}
+	return err
+}
+
+// NewArchive creates a new Archive for accessing an AIX big archive in an underlying reader.
+func NewArchive(r io.ReaderAt) (*Archive, error) {
+	parseDecimalBytes := func(b []byte) (int64, error) {
+		return strconv.ParseInt(strings.TrimSpace(string(b)), 10, 64)
+	}
+	sr := io.NewSectionReader(r, 0, 1<<63-1)
+
+	// Read File Header
+	var magic [SAIAMAG]byte
+	if _, err := sr.ReadAt(magic[:], 0); err != nil {
+		return nil, err
+	}
+
+	arch := new(Archive)
+	switch string(magic[:]) {
+	case AIAMAGBIG:
+		arch.magic = string(magic[:])
+	case AIAMAG:
+		return nil, fmt.Errorf("small AIX archive not supported")
+	default:
+		return nil, fmt.Errorf("unrecognised archive magic: 0x%x", magic)
+	}
+
+	var fhdr bigarFileHeader
+	if _, err := sr.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	if err := binary.Read(sr, binary.BigEndian, &fhdr); err != nil {
+		return nil, err
+	}
+
+	off, err := parseDecimalBytes(fhdr.Flfstmoff[:])
+	if err != nil {
+		return nil, fmt.Errorf("error parsing offset of first member in archive header(%q); %v", fhdr, err)
+	}
+
+	if off == 0 {
+		// Occurs if the archive is empty.
+		return arch, nil
+	}
+
+	lastoff, err := parseDecimalBytes(fhdr.Fllstmoff[:])
+	if err != nil {
+		return nil, fmt.Errorf("error parsing offset of first member in archive header(%q); %v", fhdr, err)
+	}
+
+	// Read members
+	for {
+		// Read Member Header
+		// The member header is normally 2 bytes larger. But it's easier
+		// to read the name if the header is read without _ar_nam.
+		// However, AIAFMAG must be read afterward.
+		if _, err := sr.Seek(off, io.SeekStart); err != nil {
+			return nil, err
+		}
+
+		var mhdr bigarMemberHeader
+		if err := binary.Read(sr, binary.BigEndian, &mhdr); err != nil {
+			return nil, err
+		}
+
+		member := new(Member)
+		arch.Members = append(arch.Members, member)
+
+		size, err := parseDecimalBytes(mhdr.Arsize[:])
+		if err != nil {
+			return nil, fmt.Errorf("error parsing size in member header(%q); %v", mhdr, err)
+		}
+		member.Size = uint64(size)
+
+		// Read name
+		namlen, err := parseDecimalBytes(mhdr.Arnamlen[:])
+		if err != nil {
+			return nil, fmt.Errorf("error parsing name length in member header(%q); %v", mhdr, err)
+		}
+		name := make([]byte, namlen)
+		if err := binary.Read(sr, binary.BigEndian, name); err != nil {
+			return nil, err
+		}
+		member.Name = string(name)
+
+		fileoff := off + AR_HSZ_BIG + namlen
+		if fileoff&1 != 0 {
+			fileoff++
+			if _, err := sr.Seek(1, io.SeekCurrent); err != nil {
+				return nil, err
+			}
+		}
+
+		// Read AIAFMAG string
+		var fmag [2]byte
+		if err := binary.Read(sr, binary.BigEndian, &fmag); err != nil {
+			return nil, err
+		}
+		if string(fmag[:]) != AIAFMAG {
+			return nil, fmt.Errorf("AIAFMAG not found after member header")
+		}
+
+		fileoff += 2 // Add the two bytes of AIAFMAG
+		member.sr = io.NewSectionReader(sr, fileoff, size)
+
+		if off == lastoff {
+			break
+		}
+		off, err = parseDecimalBytes(mhdr.Arnxtmem[:])
+		if err != nil {
+			return nil, fmt.Errorf("error parsing offset of first member in archive header(%q); %v", fhdr, err)
+		}
+
+	}
+
+	return arch, nil
+
+}
+
+// GetFile returns the XCOFF file defined by member name.
+// FIXME: This doesn't work if an archive has two members with the same
+// name which can occur if a archive has both 32-bits and 64-bits files.
+func (arch *Archive) GetFile(name string) (*File, error) {
+	for _, mem := range arch.Members {
+		if mem.Name == name {
+			return NewFile(mem.sr)
+		}
+	}
+	return nil, fmt.Errorf("unknown member %s in archive", name)
+
+}
diff --git a/src/internal/xcoff/ar_test.go b/src/internal/xcoff/ar_test.go
new file mode 100644
index 0000000..83333d6
--- /dev/null
+++ b/src/internal/xcoff/ar_test.go
@@ -0,0 +1,79 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xcoff
+
+import (
+	"reflect"
+	"testing"
+)
+
+type archiveTest struct {
+	file              string
+	hdr               ArchiveHeader
+	members           []*MemberHeader
+	membersFileHeader []FileHeader
+}
+
+var archTest = []archiveTest{
+	{
+		"testdata/bigar-ppc64",
+		ArchiveHeader{AIAMAGBIG},
+		[]*MemberHeader{
+			{"printbye.o", 836},
+			{"printhello.o", 860},
+		},
+		[]FileHeader{
+			{U64_TOCMAGIC},
+			{U64_TOCMAGIC},
+		},
+	},
+	{
+		"testdata/bigar-empty",
+		ArchiveHeader{AIAMAGBIG},
+		[]*MemberHeader{},
+		[]FileHeader{},
+	},
+}
+
+func TestOpenArchive(t *testing.T) {
+	for i := range archTest {
+		tt := &archTest[i]
+		arch, err := OpenArchive(tt.file)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if !reflect.DeepEqual(arch.ArchiveHeader, tt.hdr) {
+			t.Errorf("open archive %s:\n\thave %#v\n\twant %#v\n", tt.file, arch.ArchiveHeader, tt.hdr)
+			continue
+		}
+
+		for i, mem := range arch.Members {
+			if i >= len(tt.members) {
+				break
+			}
+			have := &mem.MemberHeader
+			want := tt.members[i]
+			if !reflect.DeepEqual(have, want) {
+				t.Errorf("open %s, member %d:\n\thave %#v\n\twant %#v\n", tt.file, i, have, want)
+			}
+
+			f, err := arch.GetFile(mem.Name)
+			if err != nil {
+				t.Error(err)
+				continue
+			}
+			if !reflect.DeepEqual(f.FileHeader, tt.membersFileHeader[i]) {
+				t.Errorf("open %s, member file header %d:\n\thave %#v\n\twant %#v\n", tt.file, i, f.FileHeader, tt.membersFileHeader[i])
+			}
+		}
+		tn := len(tt.members)
+		an := len(arch.Members)
+		if tn != an {
+			t.Errorf("open %s: len(Members) = %d, want %d", tt.file, an, tn)
+		}
+
+	}
+}
diff --git a/src/internal/xcoff/file.go b/src/internal/xcoff/file.go
new file mode 100644
index 0000000..e859de9
--- /dev/null
+++ b/src/internal/xcoff/file.go
@@ -0,0 +1,692 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package xcoff implements access to XCOFF (Extended Common Object File Format) files.
+package xcoff
+
+import (
+	"debug/dwarf"
+	"encoding/binary"
+	"fmt"
+	"internal/saferio"
+	"io"
+	"os"
+	"strings"
+)
+
+// SectionHeader holds information about an XCOFF section header.
+type SectionHeader struct {
+	Name           string
+	VirtualAddress uint64
+	Size           uint64
+	Type           uint32
+	Relptr         uint64
+	Nreloc         uint32
+}
+
+type Section struct {
+	SectionHeader
+	Relocs []Reloc
+	io.ReaderAt
+	sr *io.SectionReader
+}
+
+// AuxiliaryCSect holds information about an XCOFF symbol in an AUX_CSECT entry.
+type AuxiliaryCSect struct {
+	Length              int64
+	StorageMappingClass int
+	SymbolType          int
+}
+
+// AuxiliaryFcn holds information about an XCOFF symbol in an AUX_FCN entry.
+type AuxiliaryFcn struct {
+	Size int64
+}
+
+type Symbol struct {
+	Name          string
+	Value         uint64
+	SectionNumber int
+	StorageClass  int
+	AuxFcn        AuxiliaryFcn
+	AuxCSect      AuxiliaryCSect
+}
+
+type Reloc struct {
+	VirtualAddress   uint64
+	Symbol           *Symbol
+	Signed           bool
+	InstructionFixed bool
+	Length           uint8
+	Type             uint8
+}
+
+// ImportedSymbol holds information about an imported XCOFF symbol.
+type ImportedSymbol struct {
+	Name    string
+	Library string
+}
+
+// FileHeader holds information about an XCOFF file header.
+type FileHeader struct {
+	TargetMachine uint16
+}
+
+// A File represents an open XCOFF file.
+type File struct {
+	FileHeader
+	Sections     []*Section
+	Symbols      []*Symbol
+	StringTable  []byte
+	LibraryPaths []string
+
+	closer io.Closer
+}
+
+// Open opens the named file using os.Open and prepares it for use as an XCOFF binary.
+func Open(name string) (*File, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	ff, err := NewFile(f)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	ff.closer = f
+	return ff, nil
+}
+
+// Close closes the File.
+// If the File was created using NewFile directly instead of Open,
+// Close has no effect.
+func (f *File) Close() error {
+	var err error
+	if f.closer != nil {
+		err = f.closer.Close()
+		f.closer = nil
+	}
+	return err
+}
+
+// Section returns the first section with the given name, or nil if no such
+// section exists.
+// Xcoff have section's name limited to 8 bytes. Some sections like .gosymtab
+// can be trunked but this method will still find them.
+func (f *File) Section(name string) *Section {
+	for _, s := range f.Sections {
+		if s.Name == name || (len(name) > 8 && s.Name == name[:8]) {
+			return s
+		}
+	}
+	return nil
+}
+
+// SectionByType returns the first section in f with the
+// given type, or nil if there is no such section.
+func (f *File) SectionByType(typ uint32) *Section {
+	for _, s := range f.Sections {
+		if s.Type == typ {
+			return s
+		}
+	}
+	return nil
+}
+
+// cstring converts ASCII byte sequence b to string.
+// It stops once it finds 0 or reaches end of b.
+func cstring(b []byte) string {
+	var i int
+	for i = 0; i < len(b) && b[i] != 0; i++ {
+	}
+	return string(b[:i])
+}
+
+// getString extracts a string from an XCOFF string table.
+func getString(st []byte, offset uint32) (string, bool) {
+	if offset < 4 || int(offset) >= len(st) {
+		return "", false
+	}
+	return cstring(st[offset:]), true
+}
+
+// NewFile creates a new File for accessing an XCOFF binary in an underlying reader.
+func NewFile(r io.ReaderAt) (*File, error) {
+	sr := io.NewSectionReader(r, 0, 1<<63-1)
+	// Read XCOFF target machine
+	var magic uint16
+	if err := binary.Read(sr, binary.BigEndian, &magic); err != nil {
+		return nil, err
+	}
+	if magic != U802TOCMAGIC && magic != U64_TOCMAGIC {
+		return nil, fmt.Errorf("unrecognised XCOFF magic: 0x%x", magic)
+	}
+
+	f := new(File)
+	f.TargetMachine = magic
+
+	// Read XCOFF file header
+	if _, err := sr.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	var nscns uint16
+	var symptr uint64
+	var nsyms int32
+	var opthdr uint16
+	var hdrsz int
+	switch f.TargetMachine {
+	case U802TOCMAGIC:
+		fhdr := new(FileHeader32)
+		if err := binary.Read(sr, binary.BigEndian, fhdr); err != nil {
+			return nil, err
+		}
+		nscns = fhdr.Fnscns
+		symptr = uint64(fhdr.Fsymptr)
+		nsyms = fhdr.Fnsyms
+		opthdr = fhdr.Fopthdr
+		hdrsz = FILHSZ_32
+	case U64_TOCMAGIC:
+		fhdr := new(FileHeader64)
+		if err := binary.Read(sr, binary.BigEndian, fhdr); err != nil {
+			return nil, err
+		}
+		nscns = fhdr.Fnscns
+		symptr = fhdr.Fsymptr
+		nsyms = fhdr.Fnsyms
+		opthdr = fhdr.Fopthdr
+		hdrsz = FILHSZ_64
+	}
+
+	if symptr == 0 || nsyms <= 0 {
+		return nil, fmt.Errorf("no symbol table")
+	}
+
+	// Read string table (located right after symbol table).
+	offset := symptr + uint64(nsyms)*SYMESZ
+	if _, err := sr.Seek(int64(offset), io.SeekStart); err != nil {
+		return nil, err
+	}
+	// The first 4 bytes contain the length (in bytes).
+	var l uint32
+	if err := binary.Read(sr, binary.BigEndian, &l); err != nil {
+		return nil, err
+	}
+	if l > 4 {
+		st, err := saferio.ReadDataAt(sr, uint64(l), int64(offset))
+		if err != nil {
+			return nil, err
+		}
+		f.StringTable = st
+	}
+
+	// Read section headers
+	if _, err := sr.Seek(int64(hdrsz)+int64(opthdr), io.SeekStart); err != nil {
+		return nil, err
+	}
+	f.Sections = make([]*Section, nscns)
+	for i := 0; i < int(nscns); i++ {
+		var scnptr uint64
+		s := new(Section)
+		switch f.TargetMachine {
+		case U802TOCMAGIC:
+			shdr := new(SectionHeader32)
+			if err := binary.Read(sr, binary.BigEndian, shdr); err != nil {
+				return nil, err
+			}
+			s.Name = cstring(shdr.Sname[:])
+			s.VirtualAddress = uint64(shdr.Svaddr)
+			s.Size = uint64(shdr.Ssize)
+			scnptr = uint64(shdr.Sscnptr)
+			s.Type = shdr.Sflags
+			s.Relptr = uint64(shdr.Srelptr)
+			s.Nreloc = uint32(shdr.Snreloc)
+		case U64_TOCMAGIC:
+			shdr := new(SectionHeader64)
+			if err := binary.Read(sr, binary.BigEndian, shdr); err != nil {
+				return nil, err
+			}
+			s.Name = cstring(shdr.Sname[:])
+			s.VirtualAddress = shdr.Svaddr
+			s.Size = shdr.Ssize
+			scnptr = shdr.Sscnptr
+			s.Type = shdr.Sflags
+			s.Relptr = shdr.Srelptr
+			s.Nreloc = shdr.Snreloc
+		}
+		r2 := r
+		if scnptr == 0 { // .bss must have all 0s
+			r2 = zeroReaderAt{}
+		}
+		s.sr = io.NewSectionReader(r2, int64(scnptr), int64(s.Size))
+		s.ReaderAt = s.sr
+		f.Sections[i] = s
+	}
+
+	// Symbol map needed by relocation
+	var idxToSym = make(map[int]*Symbol)
+
+	// Read symbol table
+	if _, err := sr.Seek(int64(symptr), io.SeekStart); err != nil {
+		return nil, err
+	}
+	f.Symbols = make([]*Symbol, 0)
+	for i := 0; i < int(nsyms); i++ {
+		var numaux int
+		var ok, needAuxFcn bool
+		sym := new(Symbol)
+		switch f.TargetMachine {
+		case U802TOCMAGIC:
+			se := new(SymEnt32)
+			if err := binary.Read(sr, binary.BigEndian, se); err != nil {
+				return nil, err
+			}
+			numaux = int(se.Nnumaux)
+			if numaux < 0 {
+				return nil, fmt.Errorf("malformed symbol table, invalid number of aux symbols")
+			}
+			sym.SectionNumber = int(se.Nscnum)
+			sym.StorageClass = int(se.Nsclass)
+			sym.Value = uint64(se.Nvalue)
+			needAuxFcn = se.Ntype&SYM_TYPE_FUNC != 0 && numaux > 1
+			zeroes := binary.BigEndian.Uint32(se.Nname[:4])
+			if zeroes != 0 {
+				sym.Name = cstring(se.Nname[:])
+			} else {
+				offset := binary.BigEndian.Uint32(se.Nname[4:])
+				sym.Name, ok = getString(f.StringTable, offset)
+				if !ok {
+					goto skip
+				}
+			}
+		case U64_TOCMAGIC:
+			se := new(SymEnt64)
+			if err := binary.Read(sr, binary.BigEndian, se); err != nil {
+				return nil, err
+			}
+			numaux = int(se.Nnumaux)
+			if numaux < 0 {
+				return nil, fmt.Errorf("malformed symbol table, invalid number of aux symbols")
+			}
+			sym.SectionNumber = int(se.Nscnum)
+			sym.StorageClass = int(se.Nsclass)
+			sym.Value = se.Nvalue
+			needAuxFcn = se.Ntype&SYM_TYPE_FUNC != 0 && numaux > 1
+			sym.Name, ok = getString(f.StringTable, se.Noffset)
+			if !ok {
+				goto skip
+			}
+		}
+		if sym.StorageClass != C_EXT && sym.StorageClass != C_WEAKEXT && sym.StorageClass != C_HIDEXT {
+			goto skip
+		}
+		// Must have at least one csect auxiliary entry.
+		if numaux < 1 || i+numaux >= int(nsyms) {
+			goto skip
+		}
+
+		if sym.SectionNumber > int(nscns) {
+			goto skip
+		}
+		if sym.SectionNumber == 0 {
+			sym.Value = 0
+		} else {
+			sym.Value -= f.Sections[sym.SectionNumber-1].VirtualAddress
+		}
+
+		idxToSym[i] = sym
+
+		// If this symbol is a function, it must retrieve its size from
+		// its AUX_FCN entry.
+		// It can happen that a function symbol doesn't have any AUX_FCN.
+		// In this case, needAuxFcn is false and their size will be set to 0.
+		if needAuxFcn {
+			switch f.TargetMachine {
+			case U802TOCMAGIC:
+				aux := new(AuxFcn32)
+				if err := binary.Read(sr, binary.BigEndian, aux); err != nil {
+					return nil, err
+				}
+				sym.AuxFcn.Size = int64(aux.Xfsize)
+			case U64_TOCMAGIC:
+				aux := new(AuxFcn64)
+				if err := binary.Read(sr, binary.BigEndian, aux); err != nil {
+					return nil, err
+				}
+				sym.AuxFcn.Size = int64(aux.Xfsize)
+			}
+		}
+
+		// Read csect auxiliary entry (by convention, it is the last).
+		if !needAuxFcn {
+			if _, err := sr.Seek(int64(numaux-1)*SYMESZ, io.SeekCurrent); err != nil {
+				return nil, err
+			}
+		}
+		i += numaux
+		numaux = 0
+		switch f.TargetMachine {
+		case U802TOCMAGIC:
+			aux := new(AuxCSect32)
+			if err := binary.Read(sr, binary.BigEndian, aux); err != nil {
+				return nil, err
+			}
+			sym.AuxCSect.SymbolType = int(aux.Xsmtyp & 0x7)
+			sym.AuxCSect.StorageMappingClass = int(aux.Xsmclas)
+			sym.AuxCSect.Length = int64(aux.Xscnlen)
+		case U64_TOCMAGIC:
+			aux := new(AuxCSect64)
+			if err := binary.Read(sr, binary.BigEndian, aux); err != nil {
+				return nil, err
+			}
+			sym.AuxCSect.SymbolType = int(aux.Xsmtyp & 0x7)
+			sym.AuxCSect.StorageMappingClass = int(aux.Xsmclas)
+			sym.AuxCSect.Length = int64(aux.Xscnlenhi)<<32 | int64(aux.Xscnlenlo)
+		}
+		f.Symbols = append(f.Symbols, sym)
+	skip:
+		i += numaux // Skip auxiliary entries
+		if _, err := sr.Seek(int64(numaux)*SYMESZ, io.SeekCurrent); err != nil {
+			return nil, err
+		}
+	}
+
+	// Read relocations
+	// Only for .data or .text section
+	for _, sect := range f.Sections {
+		if sect.Type != STYP_TEXT && sect.Type != STYP_DATA {
+			continue
+		}
+		sect.Relocs = make([]Reloc, sect.Nreloc)
+		if sect.Relptr == 0 {
+			continue
+		}
+		if _, err := sr.Seek(int64(sect.Relptr), io.SeekStart); err != nil {
+			return nil, err
+		}
+		for i := uint32(0); i < sect.Nreloc; i++ {
+			switch f.TargetMachine {
+			case U802TOCMAGIC:
+				rel := new(Reloc32)
+				if err := binary.Read(sr, binary.BigEndian, rel); err != nil {
+					return nil, err
+				}
+				sect.Relocs[i].VirtualAddress = uint64(rel.Rvaddr)
+				sect.Relocs[i].Symbol = idxToSym[int(rel.Rsymndx)]
+				sect.Relocs[i].Type = rel.Rtype
+				sect.Relocs[i].Length = rel.Rsize&0x3F + 1
+
+				if rel.Rsize&0x80 != 0 {
+					sect.Relocs[i].Signed = true
+				}
+				if rel.Rsize&0x40 != 0 {
+					sect.Relocs[i].InstructionFixed = true
+				}
+
+			case U64_TOCMAGIC:
+				rel := new(Reloc64)
+				if err := binary.Read(sr, binary.BigEndian, rel); err != nil {
+					return nil, err
+				}
+				sect.Relocs[i].VirtualAddress = rel.Rvaddr
+				sect.Relocs[i].Symbol = idxToSym[int(rel.Rsymndx)]
+				sect.Relocs[i].Type = rel.Rtype
+				sect.Relocs[i].Length = rel.Rsize&0x3F + 1
+				if rel.Rsize&0x80 != 0 {
+					sect.Relocs[i].Signed = true
+				}
+				if rel.Rsize&0x40 != 0 {
+					sect.Relocs[i].InstructionFixed = true
+				}
+			}
+		}
+	}
+
+	return f, nil
+}
+
+// zeroReaderAt is ReaderAt that reads 0s.
+type zeroReaderAt struct{}
+
+// ReadAt writes len(p) 0s into p.
+func (w zeroReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}
+
+// Data reads and returns the contents of the XCOFF section s.
+func (s *Section) Data() ([]byte, error) {
+	dat := make([]byte, s.sr.Size())
+	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
+	return dat[:n], err
+}
+
+// CSect reads and returns the contents of a csect.
+func (f *File) CSect(name string) []byte {
+	for _, sym := range f.Symbols {
+		if sym.Name == name && sym.AuxCSect.SymbolType == XTY_SD {
+			if i := sym.SectionNumber - 1; 0 <= i && i < len(f.Sections) {
+				s := f.Sections[i]
+				if sym.Value+uint64(sym.AuxCSect.Length) <= s.Size {
+					dat := make([]byte, sym.AuxCSect.Length)
+					_, err := s.sr.ReadAt(dat, int64(sym.Value))
+					if err != nil {
+						return nil
+					}
+					return dat
+				}
+			}
+			break
+		}
+	}
+	return nil
+}
+
+func (f *File) DWARF() (*dwarf.Data, error) {
+	// There are many other DWARF sections, but these
+	// are the ones the debug/dwarf package uses.
+	// Don't bother loading others.
+	var subtypes = [...]uint32{SSUBTYP_DWABREV, SSUBTYP_DWINFO, SSUBTYP_DWLINE, SSUBTYP_DWRNGES, SSUBTYP_DWSTR}
+	var dat [len(subtypes)][]byte
+	for i, subtype := range subtypes {
+		s := f.SectionByType(STYP_DWARF | subtype)
+		if s != nil {
+			b, err := s.Data()
+			if err != nil && uint64(len(b)) < s.Size {
+				return nil, err
+			}
+			dat[i] = b
+		}
+	}
+
+	abbrev, info, line, ranges, str := dat[0], dat[1], dat[2], dat[3], dat[4]
+	return dwarf.New(abbrev, nil, nil, info, line, nil, ranges, str)
+}
+
+// readImportID returns the import file IDs stored inside the .loader section.
+// Library name pattern is either path/base/member or base/member
+func (f *File) readImportIDs(s *Section) ([]string, error) {
+	// Read loader header
+	if _, err := s.sr.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	var istlen uint32
+	var nimpid int32
+	var impoff uint64
+	switch f.TargetMachine {
+	case U802TOCMAGIC:
+		lhdr := new(LoaderHeader32)
+		if err := binary.Read(s.sr, binary.BigEndian, lhdr); err != nil {
+			return nil, err
+		}
+		istlen = lhdr.Listlen
+		nimpid = lhdr.Lnimpid
+		impoff = uint64(lhdr.Limpoff)
+	case U64_TOCMAGIC:
+		lhdr := new(LoaderHeader64)
+		if err := binary.Read(s.sr, binary.BigEndian, lhdr); err != nil {
+			return nil, err
+		}
+		istlen = lhdr.Listlen
+		nimpid = lhdr.Lnimpid
+		impoff = lhdr.Limpoff
+	}
+
+	// Read loader import file ID table
+	if _, err := s.sr.Seek(int64(impoff), io.SeekStart); err != nil {
+		return nil, err
+	}
+	table := make([]byte, istlen)
+	if _, err := io.ReadFull(s.sr, table); err != nil {
+		return nil, err
+	}
+
+	offset := 0
+	// First import file ID is the default LIBPATH value
+	libpath := cstring(table[offset:])
+	f.LibraryPaths = strings.Split(libpath, ":")
+	offset += len(libpath) + 3 // 3 null bytes
+	all := make([]string, 0)
+	for i := 1; i < int(nimpid); i++ {
+		impidpath := cstring(table[offset:])
+		offset += len(impidpath) + 1
+		impidbase := cstring(table[offset:])
+		offset += len(impidbase) + 1
+		impidmem := cstring(table[offset:])
+		offset += len(impidmem) + 1
+		var path string
+		if len(impidpath) > 0 {
+			path = impidpath + "/" + impidbase + "/" + impidmem
+		} else {
+			path = impidbase + "/" + impidmem
+		}
+		all = append(all, path)
+	}
+
+	return all, nil
+}
+
+// ImportedSymbols returns the names of all symbols
+// referred to by the binary f that are expected to be
+// satisfied by other libraries at dynamic load time.
+// It does not return weak symbols.
+func (f *File) ImportedSymbols() ([]ImportedSymbol, error) {
+	s := f.SectionByType(STYP_LOADER)
+	if s == nil {
+		return nil, nil
+	}
+	// Read loader header
+	if _, err := s.sr.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	var stlen uint32
+	var stoff uint64
+	var nsyms int32
+	var symoff uint64
+	switch f.TargetMachine {
+	case U802TOCMAGIC:
+		lhdr := new(LoaderHeader32)
+		if err := binary.Read(s.sr, binary.BigEndian, lhdr); err != nil {
+			return nil, err
+		}
+		stlen = lhdr.Lstlen
+		stoff = uint64(lhdr.Lstoff)
+		nsyms = lhdr.Lnsyms
+		symoff = LDHDRSZ_32
+	case U64_TOCMAGIC:
+		lhdr := new(LoaderHeader64)
+		if err := binary.Read(s.sr, binary.BigEndian, lhdr); err != nil {
+			return nil, err
+		}
+		stlen = lhdr.Lstlen
+		stoff = lhdr.Lstoff
+		nsyms = lhdr.Lnsyms
+		symoff = lhdr.Lsymoff
+	}
+
+	// Read loader section string table
+	if _, err := s.sr.Seek(int64(stoff), io.SeekStart); err != nil {
+		return nil, err
+	}
+	st := make([]byte, stlen)
+	if _, err := io.ReadFull(s.sr, st); err != nil {
+		return nil, err
+	}
+
+	// Read imported libraries
+	libs, err := f.readImportIDs(s)
+	if err != nil {
+		return nil, err
+	}
+
+	// Read loader symbol table
+	if _, err := s.sr.Seek(int64(symoff), io.SeekStart); err != nil {
+		return nil, err
+	}
+	all := make([]ImportedSymbol, 0)
+	for i := 0; i < int(nsyms); i++ {
+		var name string
+		var ifile int32
+		var ok bool
+		switch f.TargetMachine {
+		case U802TOCMAGIC:
+			ldsym := new(LoaderSymbol32)
+			if err := binary.Read(s.sr, binary.BigEndian, ldsym); err != nil {
+				return nil, err
+			}
+			if ldsym.Lsmtype&0x40 == 0 {
+				continue // Imported symbols only
+			}
+			zeroes := binary.BigEndian.Uint32(ldsym.Lname[:4])
+			if zeroes != 0 {
+				name = cstring(ldsym.Lname[:])
+			} else {
+				offset := binary.BigEndian.Uint32(ldsym.Lname[4:])
+				name, ok = getString(st, offset)
+				if !ok {
+					continue
+				}
+			}
+			ifile = ldsym.Lifile
+		case U64_TOCMAGIC:
+			ldsym := new(LoaderSymbol64)
+			if err := binary.Read(s.sr, binary.BigEndian, ldsym); err != nil {
+				return nil, err
+			}
+			if ldsym.Lsmtype&0x40 == 0 {
+				continue // Imported symbols only
+			}
+			name, ok = getString(st, ldsym.Loffset)
+			if !ok {
+				continue
+			}
+			ifile = ldsym.Lifile
+		}
+		var sym ImportedSymbol
+		sym.Name = name
+		if ifile >= 1 && int(ifile) <= len(libs) {
+			sym.Library = libs[ifile-1]
+		}
+		all = append(all, sym)
+	}
+
+	return all, nil
+}
+
+// ImportedLibraries returns the names of all libraries
+// referred to by the binary f that are expected to be
+// linked with the binary at dynamic link time.
+func (f *File) ImportedLibraries() ([]string, error) {
+	s := f.SectionByType(STYP_LOADER)
+	if s == nil {
+		return nil, nil
+	}
+	all, err := f.readImportIDs(s)
+	return all, err
+}
diff --git a/src/internal/xcoff/file_test.go b/src/internal/xcoff/file_test.go
new file mode 100644
index 0000000..a6722e9
--- /dev/null
+++ b/src/internal/xcoff/file_test.go
@@ -0,0 +1,102 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xcoff
+
+import (
+	"reflect"
+	"testing"
+)
+
+type fileTest struct {
+	file     string
+	hdr      FileHeader
+	sections []*SectionHeader
+	needed   []string
+}
+
+var fileTests = []fileTest{
+	{
+		"testdata/gcc-ppc32-aix-dwarf2-exec",
+		FileHeader{U802TOCMAGIC},
+		[]*SectionHeader{
+			{".text", 0x10000290, 0x00000bbd, STYP_TEXT, 0x7ae6, 0x36},
+			{".data", 0x20000e4d, 0x00000437, STYP_DATA, 0x7d02, 0x2b},
+			{".bss", 0x20001284, 0x0000021c, STYP_BSS, 0, 0},
+			{".loader", 0x00000000, 0x000004b3, STYP_LOADER, 0, 0},
+			{".dwline", 0x00000000, 0x000000df, STYP_DWARF | SSUBTYP_DWLINE, 0x7eb0, 0x7},
+			{".dwinfo", 0x00000000, 0x00000314, STYP_DWARF | SSUBTYP_DWINFO, 0x7ef6, 0xa},
+			{".dwabrev", 0x00000000, 0x000000d6, STYP_DWARF | SSUBTYP_DWABREV, 0, 0},
+			{".dwarnge", 0x00000000, 0x00000020, STYP_DWARF | SSUBTYP_DWARNGE, 0x7f5a, 0x2},
+			{".dwloc", 0x00000000, 0x00000074, STYP_DWARF | SSUBTYP_DWLOC, 0, 0},
+			{".debug", 0x00000000, 0x00005e4f, STYP_DEBUG, 0, 0},
+		},
+		[]string{"libc.a/shr.o"},
+	},
+	{
+		"testdata/gcc-ppc64-aix-dwarf2-exec",
+		FileHeader{U64_TOCMAGIC},
+		[]*SectionHeader{
+			{".text", 0x10000480, 0x00000afd, STYP_TEXT, 0x8322, 0x34},
+			{".data", 0x20000f7d, 0x000002f3, STYP_DATA, 0x85fa, 0x25},
+			{".bss", 0x20001270, 0x00000428, STYP_BSS, 0, 0},
+			{".loader", 0x00000000, 0x00000535, STYP_LOADER, 0, 0},
+			{".dwline", 0x00000000, 0x000000b4, STYP_DWARF | SSUBTYP_DWLINE, 0x8800, 0x4},
+			{".dwinfo", 0x00000000, 0x0000036a, STYP_DWARF | SSUBTYP_DWINFO, 0x8838, 0x7},
+			{".dwabrev", 0x00000000, 0x000000b5, STYP_DWARF | SSUBTYP_DWABREV, 0, 0},
+			{".dwarnge", 0x00000000, 0x00000040, STYP_DWARF | SSUBTYP_DWARNGE, 0x889a, 0x2},
+			{".dwloc", 0x00000000, 0x00000062, STYP_DWARF | SSUBTYP_DWLOC, 0, 0},
+			{".debug", 0x00000000, 0x00006605, STYP_DEBUG, 0, 0},
+		},
+		[]string{"libc.a/shr_64.o"},
+	},
+}
+
+func TestOpen(t *testing.T) {
+	for i := range fileTests {
+		tt := &fileTests[i]
+
+		f, err := Open(tt.file)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if !reflect.DeepEqual(f.FileHeader, tt.hdr) {
+			t.Errorf("open %s:\n\thave %#v\n\twant %#v\n", tt.file, f.FileHeader, tt.hdr)
+			continue
+		}
+
+		for i, sh := range f.Sections {
+			if i >= len(tt.sections) {
+				break
+			}
+			have := &sh.SectionHeader
+			want := tt.sections[i]
+			if !reflect.DeepEqual(have, want) {
+				t.Errorf("open %s, section %d:\n\thave %#v\n\twant %#v\n", tt.file, i, have, want)
+			}
+		}
+		tn := len(tt.sections)
+		fn := len(f.Sections)
+		if tn != fn {
+			t.Errorf("open %s: len(Sections) = %d, want %d", tt.file, fn, tn)
+		}
+		tl := tt.needed
+		fl, err := f.ImportedLibraries()
+		if err != nil {
+			t.Error(err)
+		}
+		if !reflect.DeepEqual(tl, fl) {
+			t.Errorf("open %s: loader import = %v, want %v", tt.file, tl, fl)
+		}
+	}
+}
+
+func TestOpenFailure(t *testing.T) {
+	filename := "file.go"    // not an XCOFF object file
+	_, err := Open(filename) // don't crash
+	if err == nil {
+		t.Errorf("open %s: succeeded unexpectedly", filename)
+	}
+}
diff --git a/src/internal/xcoff/testdata/bigar-empty b/src/internal/xcoff/testdata/bigar-empty
new file mode 100644
index 0000000..851ccc5
--- /dev/null
+++ b/src/internal/xcoff/testdata/bigar-empty
@@ -0,0 +1,2 @@
+<bigaf>
+0                   0                   0                   0                   0                   0                   
\ No newline at end of file
diff --git a/src/internal/xcoff/testdata/bigar-ppc64 b/src/internal/xcoff/testdata/bigar-ppc64
new file mode 100644
index 0000000..a8d4979
--- /dev/null
+++ b/src/internal/xcoff/testdata/bigar-ppc64
diff --git a/src/internal/xcoff/testdata/gcc-ppc32-aix-dwarf2-exec b/src/internal/xcoff/testdata/gcc-ppc32-aix-dwarf2-exec
new file mode 100644
index 0000000..810e21a
--- /dev/null
+++ b/src/internal/xcoff/testdata/gcc-ppc32-aix-dwarf2-exec
diff --git a/src/internal/xcoff/testdata/gcc-ppc64-aix-dwarf2-exec b/src/internal/xcoff/testdata/gcc-ppc64-aix-dwarf2-exec
new file mode 100644
index 0000000..707d01e
--- /dev/null
+++ b/src/internal/xcoff/testdata/gcc-ppc64-aix-dwarf2-exec
diff --git a/src/internal/xcoff/testdata/hello.c b/src/internal/xcoff/testdata/hello.c
new file mode 100644
index 0000000..34d9ee7
--- /dev/null
+++ b/src/internal/xcoff/testdata/hello.c
@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+void
+main(int argc, char *argv[])
+{
+	printf("hello, world\n");
+}
diff --git a/src/internal/xcoff/testdata/printbye.c b/src/internal/xcoff/testdata/printbye.c
new file mode 100644
index 0000000..9045079
--- /dev/null
+++ b/src/internal/xcoff/testdata/printbye.c
@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+void printbye(){
+	printf("Goodbye\n");
+}
diff --git a/src/internal/xcoff/testdata/printhello.c b/src/internal/xcoff/testdata/printhello.c
new file mode 100644
index 0000000..182aa09
--- /dev/null
+++ b/src/internal/xcoff/testdata/printhello.c
@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+void printhello(){
+	printf("Helloworld\n");
+}
diff --git a/src/internal/xcoff/xcoff.go b/src/internal/xcoff/xcoff.go
new file mode 100644
index 0000000..f8465d7
--- /dev/null
+++ b/src/internal/xcoff/xcoff.go
@@ -0,0 +1,367 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xcoff
+
+// File Header.
+type FileHeader32 struct {
+	Fmagic   uint16 // Target machine
+	Fnscns   uint16 // Number of sections
+	Ftimedat int32  // Time and date of file creation
+	Fsymptr  uint32 // Byte offset to symbol table start
+	Fnsyms   int32  // Number of entries in symbol table
+	Fopthdr  uint16 // Number of bytes in optional header
+	Fflags   uint16 // Flags
+}
+
+type FileHeader64 struct {
+	Fmagic   uint16 // Target machine
+	Fnscns   uint16 // Number of sections
+	Ftimedat int32  // Time and date of file creation
+	Fsymptr  uint64 // Byte offset to symbol table start
+	Fopthdr  uint16 // Number of bytes in optional header
+	Fflags   uint16 // Flags
+	Fnsyms   int32  // Number of entries in symbol table
+}
+
+const (
+	FILHSZ_32 = 20
+	FILHSZ_64 = 24
+)
+const (
+	U802TOCMAGIC = 0737 // AIX 32-bit XCOFF
+	U64_TOCMAGIC = 0767 // AIX 64-bit XCOFF
+)
+
+// Flags that describe the type of the object file.
+const (
+	F_RELFLG    = 0x0001
+	F_EXEC      = 0x0002
+	F_LNNO      = 0x0004
+	F_FDPR_PROF = 0x0010
+	F_FDPR_OPTI = 0x0020
+	F_DSA       = 0x0040
+	F_VARPG     = 0x0100
+	F_DYNLOAD   = 0x1000
+	F_SHROBJ    = 0x2000
+	F_LOADONLY  = 0x4000
+)
+
+// Section Header.
+type SectionHeader32 struct {
+	Sname    [8]byte // Section name
+	Spaddr   uint32  // Physical address
+	Svaddr   uint32  // Virtual address
+	Ssize    uint32  // Section size
+	Sscnptr  uint32  // Offset in file to raw data for section
+	Srelptr  uint32  // Offset in file to relocation entries for section
+	Slnnoptr uint32  // Offset in file to line number entries for section
+	Snreloc  uint16  // Number of relocation entries
+	Snlnno   uint16  // Number of line number entries
+	Sflags   uint32  // Flags to define the section type
+}
+
+type SectionHeader64 struct {
+	Sname    [8]byte // Section name
+	Spaddr   uint64  // Physical address
+	Svaddr   uint64  // Virtual address
+	Ssize    uint64  // Section size
+	Sscnptr  uint64  // Offset in file to raw data for section
+	Srelptr  uint64  // Offset in file to relocation entries for section
+	Slnnoptr uint64  // Offset in file to line number entries for section
+	Snreloc  uint32  // Number of relocation entries
+	Snlnno   uint32  // Number of line number entries
+	Sflags   uint32  // Flags to define the section type
+	Spad     uint32  // Needs to be 72 bytes long
+}
+
+// Flags defining the section type.
+const (
+	STYP_DWARF  = 0x0010
+	STYP_TEXT   = 0x0020
+	STYP_DATA   = 0x0040
+	STYP_BSS    = 0x0080
+	STYP_EXCEPT = 0x0100
+	STYP_INFO   = 0x0200
+	STYP_TDATA  = 0x0400
+	STYP_TBSS   = 0x0800
+	STYP_LOADER = 0x1000
+	STYP_DEBUG  = 0x2000
+	STYP_TYPCHK = 0x4000
+	STYP_OVRFLO = 0x8000
+)
+const (
+	SSUBTYP_DWINFO  = 0x10000 // DWARF info section
+	SSUBTYP_DWLINE  = 0x20000 // DWARF line-number section
+	SSUBTYP_DWPBNMS = 0x30000 // DWARF public names section
+	SSUBTYP_DWPBTYP = 0x40000 // DWARF public types section
+	SSUBTYP_DWARNGE = 0x50000 // DWARF aranges section
+	SSUBTYP_DWABREV = 0x60000 // DWARF abbreviation section
+	SSUBTYP_DWSTR   = 0x70000 // DWARF strings section
+	SSUBTYP_DWRNGES = 0x80000 // DWARF ranges section
+	SSUBTYP_DWLOC   = 0x90000 // DWARF location lists section
+	SSUBTYP_DWFRAME = 0xA0000 // DWARF frames section
+	SSUBTYP_DWMAC   = 0xB0000 // DWARF macros section
+)
+
+// Symbol Table Entry.
+type SymEnt32 struct {
+	Nname   [8]byte // Symbol name
+	Nvalue  uint32  // Symbol value
+	Nscnum  int16   // Section number of symbol
+	Ntype   uint16  // Basic and derived type specification
+	Nsclass int8    // Storage class of symbol
+	Nnumaux int8    // Number of auxiliary entries
+}
+
+type SymEnt64 struct {
+	Nvalue  uint64 // Symbol value
+	Noffset uint32 // Offset of the name in string table or .debug section
+	Nscnum  int16  // Section number of symbol
+	Ntype   uint16 // Basic and derived type specification
+	Nsclass int8   // Storage class of symbol
+	Nnumaux int8   // Number of auxiliary entries
+}
+
+const SYMESZ = 18
+
+const (
+	// Nscnum
+	N_DEBUG = -2
+	N_ABS   = -1
+	N_UNDEF = 0
+
+	//Ntype
+	SYM_V_INTERNAL  = 0x1000
+	SYM_V_HIDDEN    = 0x2000
+	SYM_V_PROTECTED = 0x3000
+	SYM_V_EXPORTED  = 0x4000
+	SYM_TYPE_FUNC   = 0x0020 // is function
+)
+
+// Storage Class.
+const (
+	C_NULL    = 0   // Symbol table entry marked for deletion
+	C_EXT     = 2   // External symbol
+	C_STAT    = 3   // Static symbol
+	C_BLOCK   = 100 // Beginning or end of inner block
+	C_FCN     = 101 // Beginning or end of function
+	C_FILE    = 103 // Source file name and compiler information
+	C_HIDEXT  = 107 // Unnamed external symbol
+	C_BINCL   = 108 // Beginning of include file
+	C_EINCL   = 109 // End of include file
+	C_WEAKEXT = 111 // Weak external symbol
+	C_DWARF   = 112 // DWARF symbol
+	C_GSYM    = 128 // Global variable
+	C_LSYM    = 129 // Automatic variable allocated on stack
+	C_PSYM    = 130 // Argument to subroutine allocated on stack
+	C_RSYM    = 131 // Register variable
+	C_RPSYM   = 132 // Argument to function or procedure stored in register
+	C_STSYM   = 133 // Statically allocated symbol
+	C_BCOMM   = 135 // Beginning of common block
+	C_ECOML   = 136 // Local member of common block
+	C_ECOMM   = 137 // End of common block
+	C_DECL    = 140 // Declaration of object
+	C_ENTRY   = 141 // Alternate entry
+	C_FUN     = 142 // Function or procedure
+	C_BSTAT   = 143 // Beginning of static block
+	C_ESTAT   = 144 // End of static block
+	C_GTLS    = 145 // Global thread-local variable
+	C_STTLS   = 146 // Static thread-local variable
+)
+
+// File Auxiliary Entry
+type AuxFile64 struct {
+	Xfname   [8]byte // Name or offset inside string table
+	Xftype   uint8   // Source file string type
+	Xauxtype uint8   // Type of auxiliary entry
+}
+
+// Function Auxiliary Entry
+type AuxFcn32 struct {
+	Xexptr   uint32 // File offset to exception table entry
+	Xfsize   uint32 // Size of function in bytes
+	Xlnnoptr uint32 // File pointer to line number
+	Xendndx  uint32 // Symbol table index of next entry
+	Xpad     uint16 // Unused
+}
+type AuxFcn64 struct {
+	Xlnnoptr uint64 // File pointer to line number
+	Xfsize   uint32 // Size of function in bytes
+	Xendndx  uint32 // Symbol table index of next entry
+	Xpad     uint8  // Unused
+	Xauxtype uint8  // Type of auxiliary entry
+}
+
+type AuxSect64 struct {
+	Xscnlen  uint64 // section length
+	Xnreloc  uint64 // Num RLDs
+	pad      uint8
+	Xauxtype uint8 // Type of auxiliary entry
+}
+
+// csect Auxiliary Entry.
+type AuxCSect32 struct {
+	Xscnlen   int32  // Length or symbol table index
+	Xparmhash uint32 // Offset of parameter type-check string
+	Xsnhash   uint16 // .typchk section number
+	Xsmtyp    uint8  // Symbol alignment and type
+	Xsmclas   uint8  // Storage-mapping class
+	Xstab     uint32 // Reserved
+	Xsnstab   uint16 // Reserved
+}
+
+type AuxCSect64 struct {
+	Xscnlenlo uint32 // Lower 4 bytes of length or symbol table index
+	Xparmhash uint32 // Offset of parameter type-check string
+	Xsnhash   uint16 // .typchk section number
+	Xsmtyp    uint8  // Symbol alignment and type
+	Xsmclas   uint8  // Storage-mapping class
+	Xscnlenhi int32  // Upper 4 bytes of length or symbol table index
+	Xpad      uint8  // Unused
+	Xauxtype  uint8  // Type of auxiliary entry
+}
+
+// Auxiliary type
+const (
+	_AUX_EXCEPT = 255
+	_AUX_FCN    = 254
+	_AUX_SYM    = 253
+	_AUX_FILE   = 252
+	_AUX_CSECT  = 251
+	_AUX_SECT   = 250
+)
+
+// Symbol type field.
+const (
+	XTY_ER = 0 // External reference
+	XTY_SD = 1 // Section definition
+	XTY_LD = 2 // Label definition
+	XTY_CM = 3 // Common csect definition
+)
+
+// Defines for File auxiliary definitions: x_ftype field of x_file
+const (
+	XFT_FN = 0   // Source File Name
+	XFT_CT = 1   // Compile Time Stamp
+	XFT_CV = 2   // Compiler Version Number
+	XFT_CD = 128 // Compiler Defined Information
+)
+
+// Storage-mapping class.
+const (
+	XMC_PR     = 0  // Program code
+	XMC_RO     = 1  // Read-only constant
+	XMC_DB     = 2  // Debug dictionary table
+	XMC_TC     = 3  // TOC entry
+	XMC_UA     = 4  // Unclassified
+	XMC_RW     = 5  // Read/Write data
+	XMC_GL     = 6  // Global linkage
+	XMC_XO     = 7  // Extended operation
+	XMC_SV     = 8  // 32-bit supervisor call descriptor
+	XMC_BS     = 9  // BSS class
+	XMC_DS     = 10 // Function descriptor
+	XMC_UC     = 11 // Unnamed FORTRAN common
+	XMC_TC0    = 15 // TOC anchor
+	XMC_TD     = 16 // Scalar data entry in the TOC
+	XMC_SV64   = 17 // 64-bit supervisor call descriptor
+	XMC_SV3264 = 18 // Supervisor call descriptor for both 32-bit and 64-bit
+	XMC_TL     = 20 // Read/Write thread-local data
+	XMC_UL     = 21 // Read/Write thread-local data (.tbss)
+	XMC_TE     = 22 // TOC entry
+)
+
+// Loader Header.
+type LoaderHeader32 struct {
+	Lversion int32  // Loader section version number
+	Lnsyms   int32  // Number of symbol table entries
+	Lnreloc  int32  // Number of relocation table entries
+	Listlen  uint32 // Length of import file ID string table
+	Lnimpid  int32  // Number of import file IDs
+	Limpoff  uint32 // Offset to start of import file IDs
+	Lstlen   uint32 // Length of string table
+	Lstoff   uint32 // Offset to start of string table
+}
+
+type LoaderHeader64 struct {
+	Lversion int32  // Loader section version number
+	Lnsyms   int32  // Number of symbol table entries
+	Lnreloc  int32  // Number of relocation table entries
+	Listlen  uint32 // Length of import file ID string table
+	Lnimpid  int32  // Number of import file IDs
+	Lstlen   uint32 // Length of string table
+	Limpoff  uint64 // Offset to start of import file IDs
+	Lstoff   uint64 // Offset to start of string table
+	Lsymoff  uint64 // Offset to start of symbol table
+	Lrldoff  uint64 // Offset to start of relocation entries
+}
+
+const (
+	LDHDRSZ_32 = 32
+	LDHDRSZ_64 = 56
+)
+
+// Loader Symbol.
+type LoaderSymbol32 struct {
+	Lname   [8]byte // Symbol name or byte offset into string table
+	Lvalue  uint32  // Address field
+	Lscnum  int16   // Section number containing symbol
+	Lsmtype int8    // Symbol type, export, import flags
+	Lsmclas int8    // Symbol storage class
+	Lifile  int32   // Import file ID; ordinal of import file IDs
+	Lparm   uint32  // Parameter type-check field
+}
+
+type LoaderSymbol64 struct {
+	Lvalue  uint64 // Address field
+	Loffset uint32 // Byte offset into string table of symbol name
+	Lscnum  int16  // Section number containing symbol
+	Lsmtype int8   // Symbol type, export, import flags
+	Lsmclas int8   // Symbol storage class
+	Lifile  int32  // Import file ID; ordinal of import file IDs
+	Lparm   uint32 // Parameter type-check field
+}
+
+type Reloc32 struct {
+	Rvaddr  uint32 // (virtual) address of reference
+	Rsymndx uint32 // Index into symbol table
+	Rsize   uint8  // Sign and reloc bit len
+	Rtype   uint8  // Toc relocation type
+}
+
+type Reloc64 struct {
+	Rvaddr  uint64 // (virtual) address of reference
+	Rsymndx uint32 // Index into symbol table
+	Rsize   uint8  // Sign and reloc bit len
+	Rtype   uint8  // Toc relocation type
+}
+
+const (
+	R_POS = 0x00 // A(sym) Positive Relocation
+	R_NEG = 0x01 // -A(sym) Negative Relocation
+	R_REL = 0x02 // A(sym-*) Relative to self
+	R_TOC = 0x03 // A(sym-TOC) Relative to TOC
+	R_TRL = 0x12 // A(sym-TOC) TOC Relative indirect load.
+
+	R_TRLA = 0x13 // A(sym-TOC) TOC Rel load address. modifiable inst
+	R_GL   = 0x05 // A(external TOC of sym) Global Linkage
+	R_TCL  = 0x06 // A(local TOC of sym) Local object TOC address
+	R_RL   = 0x0C // A(sym) Pos indirect load. modifiable instruction
+	R_RLA  = 0x0D // A(sym) Pos Load Address. modifiable instruction
+	R_REF  = 0x0F // AL0(sym) Non relocating ref. No garbage collect
+	R_BA   = 0x08 // A(sym) Branch absolute. Cannot modify instruction
+	R_RBA  = 0x18 // A(sym) Branch absolute. modifiable instruction
+	R_BR   = 0x0A // A(sym-*) Branch rel to self. non modifiable
+	R_RBR  = 0x1A // A(sym-*) Branch rel to self. modifiable instr
+
+	R_TLS    = 0x20 // General-dynamic reference to TLS symbol
+	R_TLS_IE = 0x21 // Initial-exec reference to TLS symbol
+	R_TLS_LD = 0x22 // Local-dynamic reference to TLS symbol
+	R_TLS_LE = 0x23 // Local-exec reference to TLS symbol
+	R_TLSM   = 0x24 // Module reference to TLS symbol
+	R_TLSML  = 0x25 // Module reference to local (own) module
+
+	R_TOCU = 0x30 // Relative to TOC - high order bits
+	R_TOCL = 0x31 // Relative to TOC - low order bits
+)