Adding upstream version 1.16.10.upstream/1.16.10upstream

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

12 files changed, 8397 insertions, 0 deletions

diff --git a/src/cmd/go/internal/work/action.go b/src/cmd/go/internal/work/action.go
new file mode 100644
index 0000000..9d141ae
--- /dev/null
+++ b/src/cmd/go/internal/work/action.go
@@ -0,0 +1,859 @@
+// 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.
+
+// Action graph creation (planning).
+
+package work
+
+import (
+	"bufio"
+	"bytes"
+	"container/heap"
+	"context"
+	"debug/elf"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/trace"
+	"cmd/internal/buildid"
+)
+
+// A Builder holds global state about a build.
+// It does not hold per-package state, because we
+// build packages in parallel, and the builder is shared.
+type Builder struct {
+	WorkDir     string               // the temporary work directory (ends in filepath.Separator)
+	actionCache map[cacheKey]*Action // a cache of already-constructed actions
+	mkdirCache  map[string]bool      // a cache of created directories
+	flagCache   map[[2]string]bool   // a cache of supported compiler flags
+	Print       func(args ...interface{}) (int, error)
+
+	IsCmdList           bool // running as part of go list; set p.Stale and additional fields below
+	NeedError           bool // list needs p.Error
+	NeedExport          bool // list needs p.Export
+	NeedCompiledGoFiles bool // list needs p.CompiledGoFiles
+
+	objdirSeq int // counter for NewObjdir
+	pkgSeq    int
+
+	output    sync.Mutex
+	scriptDir string // current directory in printed script
+
+	exec      sync.Mutex
+	readySema chan bool
+	ready     actionQueue
+
+	id           sync.Mutex
+	toolIDCache  map[string]string // tool name -> tool ID
+	buildIDCache map[string]string // file name -> build ID
+}
+
+// NOTE: Much of Action would not need to be exported if not for test.
+// Maybe test functionality should move into this package too?
+
+// An Action represents a single action in the action graph.
+type Action struct {
+	Mode       string                                         // description of action operation
+	Package    *load.Package                                  // the package this action works on
+	Deps       []*Action                                      // actions that must happen before this one
+	Func       func(*Builder, context.Context, *Action) error // the action itself (nil = no-op)
+	IgnoreFail bool                                           // whether to run f even if dependencies fail
+	TestOutput *bytes.Buffer                                  // test output buffer
+	Args       []string                                       // additional args for runProgram
+
+	triggers []*Action // inverse of deps
+
+	buggyInstall bool // is this a buggy install (see -linkshared)?
+
+	TryCache func(*Builder, *Action) bool // callback for cache bypass
+
+	// Generated files, directories.
+	Objdir   string         // directory for intermediate objects
+	Target   string         // goal of the action: the created package or executable
+	built    string         // the actual created package or executable
+	actionID cache.ActionID // cache ID of action input
+	buildID  string         // build ID of action output
+
+	VetxOnly  bool       // Mode=="vet": only being called to supply info about dependencies
+	needVet   bool       // Mode=="build": need to fill in vet config
+	needBuild bool       // Mode=="build": need to do actual build (can be false if needVet is true)
+	vetCfg    *vetConfig // vet config
+	output    []byte     // output redirect buffer (nil means use b.Print)
+
+	// Execution state.
+	pending      int               // number of deps yet to complete
+	priority     int               // relative execution priority
+	Failed       bool              // whether the action failed
+	json         *actionJSON       // action graph information
+	nonGoOverlay map[string]string // map from non-.go source files to copied files in objdir. Nil if no overlay is used.
+	traceSpan    *trace.Span
+}
+
+// BuildActionID returns the action ID section of a's build ID.
+func (a *Action) BuildActionID() string { return actionID(a.buildID) }
+
+// BuildContentID returns the content ID section of a's build ID.
+func (a *Action) BuildContentID() string { return contentID(a.buildID) }
+
+// BuildID returns a's build ID.
+func (a *Action) BuildID() string { return a.buildID }
+
+// BuiltTarget returns the actual file that was built. This differs
+// from Target when the result was cached.
+func (a *Action) BuiltTarget() string { return a.built }
+
+// An actionQueue is a priority queue of actions.
+type actionQueue []*Action
+
+// Implement heap.Interface
+func (q *actionQueue) Len() int           { return len(*q) }
+func (q *actionQueue) Swap(i, j int)      { (*q)[i], (*q)[j] = (*q)[j], (*q)[i] }
+func (q *actionQueue) Less(i, j int) bool { return (*q)[i].priority < (*q)[j].priority }
+func (q *actionQueue) Push(x interface{}) { *q = append(*q, x.(*Action)) }
+func (q *actionQueue) Pop() interface{} {
+	n := len(*q) - 1
+	x := (*q)[n]
+	*q = (*q)[:n]
+	return x
+}
+
+func (q *actionQueue) push(a *Action) {
+	if a.json != nil {
+		a.json.TimeReady = time.Now()
+	}
+	heap.Push(q, a)
+}
+
+func (q *actionQueue) pop() *Action {
+	return heap.Pop(q).(*Action)
+}
+
+type actionJSON struct {
+	ID         int
+	Mode       string
+	Package    string
+	Deps       []int     `json:",omitempty"`
+	IgnoreFail bool      `json:",omitempty"`
+	Args       []string  `json:",omitempty"`
+	Link       bool      `json:",omitempty"`
+	Objdir     string    `json:",omitempty"`
+	Target     string    `json:",omitempty"`
+	Priority   int       `json:",omitempty"`
+	Failed     bool      `json:",omitempty"`
+	Built      string    `json:",omitempty"`
+	VetxOnly   bool      `json:",omitempty"`
+	NeedVet    bool      `json:",omitempty"`
+	NeedBuild  bool      `json:",omitempty"`
+	ActionID   string    `json:",omitempty"`
+	BuildID    string    `json:",omitempty"`
+	TimeReady  time.Time `json:",omitempty"`
+	TimeStart  time.Time `json:",omitempty"`
+	TimeDone   time.Time `json:",omitempty"`
+
+	Cmd     []string      // `json:",omitempty"`
+	CmdReal time.Duration `json:",omitempty"`
+	CmdUser time.Duration `json:",omitempty"`
+	CmdSys  time.Duration `json:",omitempty"`
+}
+
+// cacheKey is the key for the action cache.
+type cacheKey struct {
+	mode string
+	p    *load.Package
+}
+
+func actionGraphJSON(a *Action) string {
+	var workq []*Action
+	var inWorkq = make(map[*Action]int)
+
+	add := func(a *Action) {
+		if _, ok := inWorkq[a]; ok {
+			return
+		}
+		inWorkq[a] = len(workq)
+		workq = append(workq, a)
+	}
+	add(a)
+
+	for i := 0; i < len(workq); i++ {
+		for _, dep := range workq[i].Deps {
+			add(dep)
+		}
+	}
+
+	var list []*actionJSON
+	for id, a := range workq {
+		if a.json == nil {
+			a.json = &actionJSON{
+				Mode:       a.Mode,
+				ID:         id,
+				IgnoreFail: a.IgnoreFail,
+				Args:       a.Args,
+				Objdir:     a.Objdir,
+				Target:     a.Target,
+				Failed:     a.Failed,
+				Priority:   a.priority,
+				Built:      a.built,
+				VetxOnly:   a.VetxOnly,
+				NeedBuild:  a.needBuild,
+				NeedVet:    a.needVet,
+			}
+			if a.Package != nil {
+				// TODO(rsc): Make this a unique key for a.Package somehow.
+				a.json.Package = a.Package.ImportPath
+			}
+			for _, a1 := range a.Deps {
+				a.json.Deps = append(a.json.Deps, inWorkq[a1])
+			}
+		}
+		list = append(list, a.json)
+	}
+
+	js, err := json.MarshalIndent(list, "", "\t")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "go: writing debug action graph: %v\n", err)
+		return ""
+	}
+	return string(js)
+}
+
+// BuildMode specifies the build mode:
+// are we just building things or also installing the results?
+type BuildMode int
+
+const (
+	ModeBuild BuildMode = iota
+	ModeInstall
+	ModeBuggyInstall
+
+	ModeVetOnly = 1 << 8
+)
+
+func (b *Builder) Init() {
+	b.Print = func(a ...interface{}) (int, error) {
+		return fmt.Fprint(os.Stderr, a...)
+	}
+	b.actionCache = make(map[cacheKey]*Action)
+	b.mkdirCache = make(map[string]bool)
+	b.toolIDCache = make(map[string]string)
+	b.buildIDCache = make(map[string]string)
+
+	if cfg.BuildN {
+		b.WorkDir = "$WORK"
+	} else {
+		tmp, err := os.MkdirTemp(cfg.Getenv("GOTMPDIR"), "go-build")
+		if err != nil {
+			base.Fatalf("go: creating work dir: %v", err)
+		}
+		if !filepath.IsAbs(tmp) {
+			abs, err := filepath.Abs(tmp)
+			if err != nil {
+				os.RemoveAll(tmp)
+				base.Fatalf("go: creating work dir: %v", err)
+			}
+			tmp = abs
+		}
+		b.WorkDir = tmp
+		if cfg.BuildX || cfg.BuildWork {
+			fmt.Fprintf(os.Stderr, "WORK=%s\n", b.WorkDir)
+		}
+		if !cfg.BuildWork {
+			workdir := b.WorkDir
+			base.AtExit(func() {
+				start := time.Now()
+				for {
+					err := os.RemoveAll(workdir)
+					if err == nil {
+						return
+					}
+
+					// On some configurations of Windows, directories containing executable
+					// files may be locked for a while after the executable exits (perhaps
+					// due to antivirus scans?). It's probably worth a little extra latency
+					// on exit to avoid filling up the user's temporary directory with leaked
+					// files. (See golang.org/issue/30789.)
+					if runtime.GOOS != "windows" || time.Since(start) >= 500*time.Millisecond {
+						fmt.Fprintf(os.Stderr, "go: failed to remove work dir: %s\n", err)
+						return
+					}
+					time.Sleep(5 * time.Millisecond)
+				}
+			})
+		}
+	}
+
+	if err := CheckGOOSARCHPair(cfg.Goos, cfg.Goarch); err != nil {
+		fmt.Fprintf(os.Stderr, "cmd/go: %v\n", err)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+
+	for _, tag := range cfg.BuildContext.BuildTags {
+		if strings.Contains(tag, ",") {
+			fmt.Fprintf(os.Stderr, "cmd/go: -tags space-separated list contains comma\n")
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+	}
+}
+
+func CheckGOOSARCHPair(goos, goarch string) error {
+	if _, ok := cfg.OSArchSupportsCgo[goos+"/"+goarch]; !ok && cfg.BuildContext.Compiler == "gc" {
+		return fmt.Errorf("unsupported GOOS/GOARCH pair %s/%s", goos, goarch)
+	}
+	return nil
+}
+
+// NewObjdir returns the name of a fresh object directory under b.WorkDir.
+// It is up to the caller to call b.Mkdir on the result at an appropriate time.
+// The result ends in a slash, so that file names in that directory
+// can be constructed with direct string addition.
+//
+// NewObjdir must be called only from a single goroutine at a time,
+// so it is safe to call during action graph construction, but it must not
+// be called during action graph execution.
+func (b *Builder) NewObjdir() string {
+	b.objdirSeq++
+	return filepath.Join(b.WorkDir, fmt.Sprintf("b%03d", b.objdirSeq)) + string(filepath.Separator)
+}
+
+// readpkglist returns the list of packages that were built into the shared library
+// at shlibpath. For the native toolchain this list is stored, newline separated, in
+// an ELF note with name "Go\x00\x00" and type 1. For GCCGO it is extracted from the
+// .go_export section.
+func readpkglist(shlibpath string) (pkgs []*load.Package) {
+	var stk load.ImportStack
+	if cfg.BuildToolchainName == "gccgo" {
+		f, _ := elf.Open(shlibpath)
+		sect := f.Section(".go_export")
+		data, _ := sect.Data()
+		scanner := bufio.NewScanner(bytes.NewBuffer(data))
+		for scanner.Scan() {
+			t := scanner.Text()
+			if strings.HasPrefix(t, "pkgpath ") {
+				t = strings.TrimPrefix(t, "pkgpath ")
+				t = strings.TrimSuffix(t, ";")
+				pkgs = append(pkgs, load.LoadImportWithFlags(t, base.Cwd, nil, &stk, nil, 0))
+			}
+		}
+	} else {
+		pkglistbytes, err := buildid.ReadELFNote(shlibpath, "Go\x00\x00", 1)
+		if err != nil {
+			base.Fatalf("readELFNote failed: %v", err)
+		}
+		scanner := bufio.NewScanner(bytes.NewBuffer(pkglistbytes))
+		for scanner.Scan() {
+			t := scanner.Text()
+			pkgs = append(pkgs, load.LoadImportWithFlags(t, base.Cwd, nil, &stk, nil, 0))
+		}
+	}
+	return
+}
+
+// cacheAction looks up {mode, p} in the cache and returns the resulting action.
+// If the cache has no such action, f() is recorded and returned.
+// TODO(rsc): Change the second key from *load.Package to interface{},
+// to make the caching in linkShared less awkward?
+func (b *Builder) cacheAction(mode string, p *load.Package, f func() *Action) *Action {
+	a := b.actionCache[cacheKey{mode, p}]
+	if a == nil {
+		a = f()
+		b.actionCache[cacheKey{mode, p}] = a
+	}
+	return a
+}
+
+// AutoAction returns the "right" action for go build or go install of p.
+func (b *Builder) AutoAction(mode, depMode BuildMode, p *load.Package) *Action {
+	if p.Name == "main" {
+		return b.LinkAction(mode, depMode, p)
+	}
+	return b.CompileAction(mode, depMode, p)
+}
+
+// CompileAction returns the action for compiling and possibly installing
+// (according to mode) the given package. The resulting action is only
+// for building packages (archives), never for linking executables.
+// depMode is the action (build or install) to use when building dependencies.
+// To turn package main into an executable, call b.Link instead.
+func (b *Builder) CompileAction(mode, depMode BuildMode, p *load.Package) *Action {
+	vetOnly := mode&ModeVetOnly != 0
+	mode &^= ModeVetOnly
+
+	if mode != ModeBuild && (p.Internal.Local || p.Module != nil) && p.Target == "" {
+		// Imported via local path or using modules. No permanent target.
+		mode = ModeBuild
+	}
+	if mode != ModeBuild && p.Name == "main" {
+		// We never install the .a file for a main package.
+		mode = ModeBuild
+	}
+
+	// Construct package build action.
+	a := b.cacheAction("build", p, func() *Action {
+		a := &Action{
+			Mode:    "build",
+			Package: p,
+			Func:    (*Builder).build,
+			Objdir:  b.NewObjdir(),
+		}
+
+		if p.Error == nil || !p.Error.IsImportCycle {
+			for _, p1 := range p.Internal.Imports {
+				a.Deps = append(a.Deps, b.CompileAction(depMode, depMode, p1))
+			}
+		}
+
+		if p.Standard {
+			switch p.ImportPath {
+			case "builtin", "unsafe":
+				// Fake packages - nothing to build.
+				a.Mode = "built-in package"
+				a.Func = nil
+				return a
+			}
+
+			// gccgo standard library is "fake" too.
+			if cfg.BuildToolchainName == "gccgo" {
+				// the target name is needed for cgo.
+				a.Mode = "gccgo stdlib"
+				a.Target = p.Target
+				a.Func = nil
+				return a
+			}
+		}
+
+		return a
+	})
+
+	// Find the build action; the cache entry may have been replaced
+	// by the install action during (*Builder).installAction.
+	buildAction := a
+	switch buildAction.Mode {
+	case "build", "built-in package", "gccgo stdlib":
+		// ok
+	case "build-install":
+		buildAction = a.Deps[0]
+	default:
+		panic("lost build action: " + buildAction.Mode)
+	}
+	buildAction.needBuild = buildAction.needBuild || !vetOnly
+
+	// Construct install action.
+	if mode == ModeInstall || mode == ModeBuggyInstall {
+		a = b.installAction(a, mode)
+	}
+
+	return a
+}
+
+// VetAction returns the action for running go vet on package p.
+// It depends on the action for compiling p.
+// If the caller may be causing p to be installed, it is up to the caller
+// to make sure that the install depends on (runs after) vet.
+func (b *Builder) VetAction(mode, depMode BuildMode, p *load.Package) *Action {
+	a := b.vetAction(mode, depMode, p)
+	a.VetxOnly = false
+	return a
+}
+
+func (b *Builder) vetAction(mode, depMode BuildMode, p *load.Package) *Action {
+	// Construct vet action.
+	a := b.cacheAction("vet", p, func() *Action {
+		a1 := b.CompileAction(mode|ModeVetOnly, depMode, p)
+
+		// vet expects to be able to import "fmt".
+		var stk load.ImportStack
+		stk.Push("vet")
+		p1 := load.LoadImportWithFlags("fmt", p.Dir, p, &stk, nil, 0)
+		stk.Pop()
+		aFmt := b.CompileAction(ModeBuild, depMode, p1)
+
+		var deps []*Action
+		if a1.buggyInstall {
+			// (*Builder).vet expects deps[0] to be the package
+			// and deps[1] to be "fmt". If we see buggyInstall
+			// here then a1 is an install of a shared library,
+			// and the real package is a1.Deps[0].
+			deps = []*Action{a1.Deps[0], aFmt, a1}
+		} else {
+			deps = []*Action{a1, aFmt}
+		}
+		for _, p1 := range p.Internal.Imports {
+			deps = append(deps, b.vetAction(mode, depMode, p1))
+		}
+
+		a := &Action{
+			Mode:       "vet",
+			Package:    p,
+			Deps:       deps,
+			Objdir:     a1.Objdir,
+			VetxOnly:   true,
+			IgnoreFail: true, // it's OK if vet of dependencies "fails" (reports problems)
+		}
+		if a1.Func == nil {
+			// Built-in packages like unsafe.
+			return a
+		}
+		deps[0].needVet = true
+		a.Func = (*Builder).vet
+		return a
+	})
+	return a
+}
+
+// LinkAction returns the action for linking p into an executable
+// and possibly installing the result (according to mode).
+// depMode is the action (build or install) to use when compiling dependencies.
+func (b *Builder) LinkAction(mode, depMode BuildMode, p *load.Package) *Action {
+	// Construct link action.
+	a := b.cacheAction("link", p, func() *Action {
+		a := &Action{
+			Mode:    "link",
+			Package: p,
+		}
+
+		a1 := b.CompileAction(ModeBuild, depMode, p)
+		a.Func = (*Builder).link
+		a.Deps = []*Action{a1}
+		a.Objdir = a1.Objdir
+
+		// An executable file. (This is the name of a temporary file.)
+		// Because we run the temporary file in 'go run' and 'go test',
+		// the name will show up in ps listings. If the caller has specified
+		// a name, use that instead of a.out. The binary is generated
+		// in an otherwise empty subdirectory named exe to avoid
+		// naming conflicts. The only possible conflict is if we were
+		// to create a top-level package named exe.
+		name := "a.out"
+		if p.Internal.ExeName != "" {
+			name = p.Internal.ExeName
+		} else if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" && p.Target != "" {
+			// On OS X, the linker output name gets recorded in the
+			// shared library's LC_ID_DYLIB load command.
+			// The code invoking the linker knows to pass only the final
+			// path element. Arrange that the path element matches what
+			// we'll install it as; otherwise the library is only loadable as "a.out".
+			// On Windows, DLL file name is recorded in PE file
+			// export section, so do like on OS X.
+			_, name = filepath.Split(p.Target)
+		}
+		a.Target = a.Objdir + filepath.Join("exe", name) + cfg.ExeSuffix
+		a.built = a.Target
+		b.addTransitiveLinkDeps(a, a1, "")
+
+		// Sequence the build of the main package (a1) strictly after the build
+		// of all other dependencies that go into the link. It is likely to be after
+		// them anyway, but just make sure. This is required by the build ID-based
+		// shortcut in (*Builder).useCache(a1), which will call b.linkActionID(a).
+		// In order for that linkActionID call to compute the right action ID, all the
+		// dependencies of a (except a1) must have completed building and have
+		// recorded their build IDs.
+		a1.Deps = append(a1.Deps, &Action{Mode: "nop", Deps: a.Deps[1:]})
+		return a
+	})
+
+	if mode == ModeInstall || mode == ModeBuggyInstall {
+		a = b.installAction(a, mode)
+	}
+
+	return a
+}
+
+// installAction returns the action for installing the result of a1.
+func (b *Builder) installAction(a1 *Action, mode BuildMode) *Action {
+	// Because we overwrite the build action with the install action below,
+	// a1 may already be an install action fetched from the "build" cache key,
+	// and the caller just doesn't realize.
+	if strings.HasSuffix(a1.Mode, "-install") {
+		if a1.buggyInstall && mode == ModeInstall {
+			//  Congratulations! The buggy install is now a proper install.
+			a1.buggyInstall = false
+		}
+		return a1
+	}
+
+	// If there's no actual action to build a1,
+	// there's nothing to install either.
+	// This happens if a1 corresponds to reusing an already-built object.
+	if a1.Func == nil {
+		return a1
+	}
+
+	p := a1.Package
+	return b.cacheAction(a1.Mode+"-install", p, func() *Action {
+		// The install deletes the temporary build result,
+		// so we need all other actions, both past and future,
+		// that attempt to depend on the build to depend instead
+		// on the install.
+
+		// Make a private copy of a1 (the build action),
+		// no longer accessible to any other rules.
+		buildAction := new(Action)
+		*buildAction = *a1
+
+		// Overwrite a1 with the install action.
+		// This takes care of updating past actions that
+		// point at a1 for the build action; now they will
+		// point at a1 and get the install action.
+		// We also leave a1 in the action cache as the result
+		// for "build", so that actions not yet created that
+		// try to depend on the build will instead depend
+		// on the install.
+		*a1 = Action{
+			Mode:    buildAction.Mode + "-install",
+			Func:    BuildInstallFunc,
+			Package: p,
+			Objdir:  buildAction.Objdir,
+			Deps:    []*Action{buildAction},
+			Target:  p.Target,
+			built:   p.Target,
+
+			buggyInstall: mode == ModeBuggyInstall,
+		}
+
+		b.addInstallHeaderAction(a1)
+		return a1
+	})
+}
+
+// addTransitiveLinkDeps adds to the link action a all packages
+// that are transitive dependencies of a1.Deps.
+// That is, if a is a link of package main, a1 is the compile of package main
+// and a1.Deps is the actions for building packages directly imported by
+// package main (what the compiler needs). The linker needs all packages
+// transitively imported by the whole program; addTransitiveLinkDeps
+// makes sure those are present in a.Deps.
+// If shlib is non-empty, then a corresponds to the build and installation of shlib,
+// so any rebuild of shlib should not be added as a dependency.
+func (b *Builder) addTransitiveLinkDeps(a, a1 *Action, shlib string) {
+	// Expand Deps to include all built packages, for the linker.
+	// Use breadth-first search to find rebuilt-for-test packages
+	// before the standard ones.
+	// TODO(rsc): Eliminate the standard ones from the action graph,
+	// which will require doing a little bit more rebuilding.
+	workq := []*Action{a1}
+	haveDep := map[string]bool{}
+	if a1.Package != nil {
+		haveDep[a1.Package.ImportPath] = true
+	}
+	for i := 0; i < len(workq); i++ {
+		a1 := workq[i]
+		for _, a2 := range a1.Deps {
+			// TODO(rsc): Find a better discriminator than the Mode strings, once the dust settles.
+			if a2.Package == nil || (a2.Mode != "build-install" && a2.Mode != "build") || haveDep[a2.Package.ImportPath] {
+				continue
+			}
+			haveDep[a2.Package.ImportPath] = true
+			a.Deps = append(a.Deps, a2)
+			if a2.Mode == "build-install" {
+				a2 = a2.Deps[0] // walk children of "build" action
+			}
+			workq = append(workq, a2)
+		}
+	}
+
+	// If this is go build -linkshared, then the link depends on the shared libraries
+	// in addition to the packages themselves. (The compile steps do not.)
+	if cfg.BuildLinkshared {
+		haveShlib := map[string]bool{shlib: true}
+		for _, a1 := range a.Deps {
+			p1 := a1.Package
+			if p1 == nil || p1.Shlib == "" || haveShlib[filepath.Base(p1.Shlib)] {
+				continue
+			}
+			haveShlib[filepath.Base(p1.Shlib)] = true
+			// TODO(rsc): The use of ModeInstall here is suspect, but if we only do ModeBuild,
+			// we'll end up building an overall library or executable that depends at runtime
+			// on other libraries that are out-of-date, which is clearly not good either.
+			// We call it ModeBuggyInstall to make clear that this is not right.
+			a.Deps = append(a.Deps, b.linkSharedAction(ModeBuggyInstall, ModeBuggyInstall, p1.Shlib, nil))
+		}
+	}
+}
+
+// addInstallHeaderAction adds an install header action to a, if needed.
+// The action a should be an install action as generated by either
+// b.CompileAction or b.LinkAction with mode=ModeInstall,
+// and so a.Deps[0] is the corresponding build action.
+func (b *Builder) addInstallHeaderAction(a *Action) {
+	// Install header for cgo in c-archive and c-shared modes.
+	p := a.Package
+	if p.UsesCgo() && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
+		hdrTarget := a.Target[:len(a.Target)-len(filepath.Ext(a.Target))] + ".h"
+		if cfg.BuildContext.Compiler == "gccgo" && cfg.BuildO == "" {
+			// For the header file, remove the "lib"
+			// added by go/build, so we generate pkg.h
+			// rather than libpkg.h.
+			dir, file := filepath.Split(hdrTarget)
+			file = strings.TrimPrefix(file, "lib")
+			hdrTarget = filepath.Join(dir, file)
+		}
+		ah := &Action{
+			Mode:    "install header",
+			Package: a.Package,
+			Deps:    []*Action{a.Deps[0]},
+			Func:    (*Builder).installHeader,
+			Objdir:  a.Deps[0].Objdir,
+			Target:  hdrTarget,
+		}
+		a.Deps = append(a.Deps, ah)
+	}
+}
+
+// buildmodeShared takes the "go build" action a1 into the building of a shared library of a1.Deps.
+// That is, the input a1 represents "go build pkgs" and the result represents "go build -buildmode=shared pkgs".
+func (b *Builder) buildmodeShared(mode, depMode BuildMode, args []string, pkgs []*load.Package, a1 *Action) *Action {
+	name, err := libname(args, pkgs)
+	if err != nil {
+		base.Fatalf("%v", err)
+	}
+	return b.linkSharedAction(mode, depMode, name, a1)
+}
+
+// linkSharedAction takes a grouping action a1 corresponding to a list of built packages
+// and returns an action that links them together into a shared library with the name shlib.
+// If a1 is nil, shlib should be an absolute path to an existing shared library,
+// and then linkSharedAction reads that library to find out the package list.
+func (b *Builder) linkSharedAction(mode, depMode BuildMode, shlib string, a1 *Action) *Action {
+	fullShlib := shlib
+	shlib = filepath.Base(shlib)
+	a := b.cacheAction("build-shlib "+shlib, nil, func() *Action {
+		if a1 == nil {
+			// TODO(rsc): Need to find some other place to store config,
+			// not in pkg directory. See golang.org/issue/22196.
+			pkgs := readpkglist(fullShlib)
+			a1 = &Action{
+				Mode: "shlib packages",
+			}
+			for _, p := range pkgs {
+				a1.Deps = append(a1.Deps, b.CompileAction(mode, depMode, p))
+			}
+		}
+
+		// Fake package to hold ldflags.
+		// As usual shared libraries are a kludgy, abstraction-violating special case:
+		// we let them use the flags specified for the command-line arguments.
+		p := &load.Package{}
+		p.Internal.CmdlinePkg = true
+		p.Internal.Ldflags = load.BuildLdflags.For(p)
+		p.Internal.Gccgoflags = load.BuildGccgoflags.For(p)
+
+		// Add implicit dependencies to pkgs list.
+		// Currently buildmode=shared forces external linking mode, and
+		// external linking mode forces an import of runtime/cgo (and
+		// math on arm). So if it was not passed on the command line and
+		// it is not present in another shared library, add it here.
+		// TODO(rsc): Maybe this should only happen if "runtime" is in the original package set.
+		// TODO(rsc): This should probably be changed to use load.LinkerDeps(p).
+		// TODO(rsc): We don't add standard library imports for gccgo
+		// because they are all always linked in anyhow.
+		// Maybe load.LinkerDeps should be used and updated.
+		a := &Action{
+			Mode:    "go build -buildmode=shared",
+			Package: p,
+			Objdir:  b.NewObjdir(),
+			Func:    (*Builder).linkShared,
+			Deps:    []*Action{a1},
+		}
+		a.Target = filepath.Join(a.Objdir, shlib)
+		if cfg.BuildToolchainName != "gccgo" {
+			add := func(a1 *Action, pkg string, force bool) {
+				for _, a2 := range a1.Deps {
+					if a2.Package != nil && a2.Package.ImportPath == pkg {
+						return
+					}
+				}
+				var stk load.ImportStack
+				p := load.LoadImportWithFlags(pkg, base.Cwd, nil, &stk, nil, 0)
+				if p.Error != nil {
+					base.Fatalf("load %s: %v", pkg, p.Error)
+				}
+				// Assume that if pkg (runtime/cgo or math)
+				// is already accounted for in a different shared library,
+				// then that shared library also contains runtime,
+				// so that anything we do will depend on that library,
+				// so we don't need to include pkg in our shared library.
+				if force || p.Shlib == "" || filepath.Base(p.Shlib) == pkg {
+					a1.Deps = append(a1.Deps, b.CompileAction(depMode, depMode, p))
+				}
+			}
+			add(a1, "runtime/cgo", false)
+			if cfg.Goarch == "arm" {
+				add(a1, "math", false)
+			}
+
+			// The linker step still needs all the usual linker deps.
+			// (For example, the linker always opens runtime.a.)
+			for _, dep := range load.LinkerDeps(nil) {
+				add(a, dep, true)
+			}
+		}
+		b.addTransitiveLinkDeps(a, a1, shlib)
+		return a
+	})
+
+	// Install result.
+	if (mode == ModeInstall || mode == ModeBuggyInstall) && a.Func != nil {
+		buildAction := a
+
+		a = b.cacheAction("install-shlib "+shlib, nil, func() *Action {
+			// Determine the eventual install target.
+			// The install target is root/pkg/shlib, where root is the source root
+			// in which all the packages lie.
+			// TODO(rsc): Perhaps this cross-root check should apply to the full
+			// transitive package dependency list, not just the ones named
+			// on the command line?
+			pkgDir := a1.Deps[0].Package.Internal.Build.PkgTargetRoot
+			for _, a2 := range a1.Deps {
+				if dir := a2.Package.Internal.Build.PkgTargetRoot; dir != pkgDir {
+					base.Fatalf("installing shared library: cannot use packages %s and %s from different roots %s and %s",
+						a1.Deps[0].Package.ImportPath,
+						a2.Package.ImportPath,
+						pkgDir,
+						dir)
+				}
+			}
+			// TODO(rsc): Find out and explain here why gccgo is different.
+			if cfg.BuildToolchainName == "gccgo" {
+				pkgDir = filepath.Join(pkgDir, "shlibs")
+			}
+			target := filepath.Join(pkgDir, shlib)
+
+			a := &Action{
+				Mode:   "go install -buildmode=shared",
+				Objdir: buildAction.Objdir,
+				Func:   BuildInstallFunc,
+				Deps:   []*Action{buildAction},
+				Target: target,
+			}
+			for _, a2 := range buildAction.Deps[0].Deps {
+				p := a2.Package
+				if p.Target == "" {
+					continue
+				}
+				a.Deps = append(a.Deps, &Action{
+					Mode:    "shlibname",
+					Package: p,
+					Func:    (*Builder).installShlibname,
+					Target:  strings.TrimSuffix(p.Target, ".a") + ".shlibname",
+					Deps:    []*Action{a.Deps[0]},
+				})
+			}
+			return a
+		})
+	}
+
+	return a
+}
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
new file mode 100644
index 0000000..f024b07
--- /dev/null
+++ b/src/cmd/go/internal/work/build.go
@@ -0,0 +1,929 @@
+// 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 work
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	exec "internal/execabs"
+	"internal/goroot"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/search"
+	"cmd/go/internal/trace"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+var CmdBuild = &base.Command{
+	UsageLine: "go build [-o output] [build flags] [packages]",
+	Short:     "compile packages and dependencies",
+	Long: `
+Build compiles the packages named by the import paths,
+along with their dependencies, but it does not install the results.
+
+If the arguments to build are a list of .go files from a single directory,
+build treats them as a list of source files specifying a single package.
+
+When compiling packages, build ignores files that end in '_test.go'.
+
+When compiling a single main package, build writes
+the resulting executable to an output file named after
+the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe')
+or the source code directory ('go build unix/sam' writes 'sam' or 'sam.exe').
+The '.exe' suffix is added when writing a Windows executable.
+
+When compiling multiple packages or a single non-main package,
+build compiles the packages but discards the resulting object,
+serving only as a check that the packages can be built.
+
+The -o flag forces build to write the resulting executable or object
+to the named output file or directory, instead of the default behavior described
+in the last two paragraphs. If the named output is an existing directory or
+ends with a slash or backslash, then any resulting executables
+will be written to that directory.
+
+The -i flag installs the packages that are dependencies of the target.
+The -i flag is deprecated. Compiled packages are cached automatically.
+
+The build flags are shared by the build, clean, get, install, list, run,
+and test commands:
+
+	-a
+		force rebuilding of packages that are already up-to-date.
+	-n
+		print the commands but do not run them.
+	-p n
+		the number of programs, such as build commands or
+		test binaries, that can be run in parallel.
+		The default is the number of CPUs available.
+	-race
+		enable data race detection.
+		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, windows/amd64,
+		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
+	-msan
+		enable interoperation with memory sanitizer.
+		Supported only on linux/amd64, linux/arm64
+		and only with Clang/LLVM as the host C compiler.
+		On linux/arm64, pie build mode will be used.
+	-v
+		print the names of packages as they are compiled.
+	-work
+		print the name of the temporary work directory and
+		do not delete it when exiting.
+	-x
+		print the commands.
+
+	-asmflags '[pattern=]arg list'
+		arguments to pass on each go tool asm invocation.
+	-buildmode mode
+		build mode to use. See 'go help buildmode' for more.
+	-compiler name
+		name of compiler to use, as in runtime.Compiler (gccgo or gc).
+	-gccgoflags '[pattern=]arg list'
+		arguments to pass on each gccgo compiler/linker invocation.
+	-gcflags '[pattern=]arg list'
+		arguments to pass on each go tool compile invocation.
+	-installsuffix suffix
+		a suffix to use in the name of the package installation directory,
+		in order to keep output separate from default builds.
+		If using the -race flag, the install suffix is automatically set to race
+		or, if set explicitly, has _race appended to it. Likewise for the -msan
+		flag. Using a -buildmode option that requires non-default compile flags
+		has a similar effect.
+	-ldflags '[pattern=]arg list'
+		arguments to pass on each go tool link invocation.
+	-linkshared
+		build code that will be linked against shared libraries previously
+		created with -buildmode=shared.
+	-mod mode
+		module download mode to use: readonly, vendor, or mod.
+		By default, if a vendor directory is present and the go version in go.mod
+		is 1.14 or higher, the go command acts as if -mod=vendor were set.
+		Otherwise, the go command acts as if -mod=readonly were set.
+		See https://golang.org/ref/mod#build-commands for details.
+	-modcacherw
+		leave newly-created directories in the module cache read-write
+		instead of making them read-only.
+	-modfile file
+		in module aware mode, read (and possibly write) an alternate go.mod
+		file instead of the one in the module root directory. A file named
+		"go.mod" must still be present in order to determine the module root
+		directory, but it is not accessed. When -modfile is specified, an
+		alternate go.sum file is also used: its path is derived from the
+		-modfile flag by trimming the ".mod" extension and appending ".sum".
+	-overlay file
+		read a JSON config file that provides an overlay for build operations.
+		The file is a JSON struct with a single field, named 'Replace', that
+		maps each disk file path (a string) to its backing file path, so that
+		a build will run as if the disk file path exists with the contents
+		given by the backing file paths, or as if the disk file path does not
+		exist if its backing file path is empty. Support for the -overlay flag
+		has some limitations:importantly, cgo files included from outside the
+		include path must be  in the same directory as the Go package they are
+		included from, and overlays will not appear when binaries and tests are
+		run through go run and go test respectively.
+	-pkgdir dir
+		install and load all packages from dir instead of the usual locations.
+		For example, when building with a non-standard configuration,
+		use -pkgdir to keep generated packages in a separate location.
+	-tags tag,list
+		a comma-separated list of build tags to consider satisfied during the
+		build. For more information about build tags, see the description of
+		build constraints in the documentation for the go/build package.
+		(Earlier versions of Go used a space-separated list, and that form
+		is deprecated but still recognized.)
+	-trimpath
+		remove all file system paths from the resulting executable.
+		Instead of absolute file system paths, the recorded file names
+		will begin with either "go" (for the standard library),
+		or a module path@version (when using modules),
+		or a plain import path (when using GOPATH).
+	-toolexec 'cmd args'
+		a program to use to invoke toolchain programs like vet and asm.
+		For example, instead of running asm, the go command will run
+		'cmd args /path/to/asm <arguments for asm>'.
+
+The -asmflags, -gccgoflags, -gcflags, and -ldflags flags accept a
+space-separated list of arguments to pass to an underlying tool
+during the build. To embed spaces in an element in the list, surround
+it with either single or double quotes. The argument list may be
+preceded by a package pattern and an equal sign, which restricts
+the use of that argument list to the building of packages matching
+that pattern (see 'go help packages' for a description of package
+patterns). Without a pattern, the argument list applies only to the
+packages named on the command line. The flags may be repeated
+with different patterns in order to specify different arguments for
+different sets of packages. If a package matches patterns given in
+multiple flags, the latest match on the command line wins.
+For example, 'go build -gcflags=-S fmt' prints the disassembly
+only for package fmt, while 'go build -gcflags=all=-S fmt'
+prints the disassembly for fmt and all its dependencies.
+
+For more about specifying packages, see 'go help packages'.
+For more about where packages and binaries are installed,
+run 'go help gopath'.
+For more about calling between Go and C/C++, run 'go help c'.
+
+Note: Build adheres to certain conventions such as those described
+by 'go help gopath'. Not all projects can follow these conventions,
+however. Installations that have their own conventions or that use
+a separate software build system may choose to use lower-level
+invocations such as 'go tool compile' and 'go tool link' to avoid
+some of the overheads and design decisions of the build tool.
+
+See also: go install, go get, go clean.
+	`,
+}
+
+const concurrentGCBackendCompilationEnabledByDefault = true
+
+func init() {
+	// break init cycle
+	CmdBuild.Run = runBuild
+	CmdInstall.Run = runInstall
+
+	CmdBuild.Flag.BoolVar(&cfg.BuildI, "i", false, "")
+	CmdBuild.Flag.StringVar(&cfg.BuildO, "o", "", "output file or directory")
+
+	CmdInstall.Flag.BoolVar(&cfg.BuildI, "i", false, "")
+
+	AddBuildFlags(CmdBuild, DefaultBuildFlags)
+	AddBuildFlags(CmdInstall, DefaultBuildFlags)
+}
+
+// Note that flags consulted by other parts of the code
+// (for example, buildV) are in cmd/go/internal/cfg.
+
+var (
+	forcedAsmflags   []string // internally-forced flags for cmd/asm
+	forcedGcflags    []string // internally-forced flags for cmd/compile
+	forcedLdflags    []string // internally-forced flags for cmd/link
+	forcedGccgoflags []string // internally-forced flags for gccgo
+)
+
+var BuildToolchain toolchain = noToolchain{}
+var ldBuildmode string
+
+// buildCompiler implements flag.Var.
+// It implements Set by updating both
+// BuildToolchain and buildContext.Compiler.
+type buildCompiler struct{}
+
+func (c buildCompiler) Set(value string) error {
+	switch value {
+	case "gc":
+		BuildToolchain = gcToolchain{}
+	case "gccgo":
+		BuildToolchain = gccgoToolchain{}
+	default:
+		return fmt.Errorf("unknown compiler %q", value)
+	}
+	cfg.BuildToolchainName = value
+	cfg.BuildToolchainCompiler = BuildToolchain.compiler
+	cfg.BuildToolchainLinker = BuildToolchain.linker
+	cfg.BuildContext.Compiler = value
+	return nil
+}
+
+func (c buildCompiler) String() string {
+	return cfg.BuildContext.Compiler
+}
+
+func init() {
+	switch build.Default.Compiler {
+	case "gc", "gccgo":
+		buildCompiler{}.Set(build.Default.Compiler)
+	}
+}
+
+type BuildFlagMask int
+
+const (
+	DefaultBuildFlags BuildFlagMask = 0
+	OmitModFlag       BuildFlagMask = 1 << iota
+	OmitModCommonFlags
+	OmitVFlag
+)
+
+// AddBuildFlags adds the flags common to the build, clean, get,
+// install, list, run, and test commands.
+func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
+	base.AddBuildFlagsNX(&cmd.Flag)
+	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
+	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
+	if mask&OmitVFlag == 0 {
+		cmd.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	}
+
+	cmd.Flag.Var(&load.BuildAsmflags, "asmflags", "")
+	cmd.Flag.Var(buildCompiler{}, "compiler", "")
+	cmd.Flag.StringVar(&cfg.BuildBuildmode, "buildmode", "default", "")
+	cmd.Flag.Var(&load.BuildGcflags, "gcflags", "")
+	cmd.Flag.Var(&load.BuildGccgoflags, "gccgoflags", "")
+	if mask&OmitModFlag == 0 {
+		base.AddModFlag(&cmd.Flag)
+	}
+	if mask&OmitModCommonFlags == 0 {
+		base.AddModCommonFlags(&cmd.Flag)
+	} else {
+		// Add the overlay flag even when we don't add the rest of the mod common flags.
+		// This only affects 'go get' in GOPATH mode, but add the flag anyway for
+		// consistency.
+		cmd.Flag.StringVar(&fsys.OverlayFile, "overlay", "", "")
+	}
+	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
+	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
+	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")
+	cmd.Flag.StringVar(&cfg.BuildPkgdir, "pkgdir", "", "")
+	cmd.Flag.BoolVar(&cfg.BuildRace, "race", false, "")
+	cmd.Flag.BoolVar(&cfg.BuildMSan, "msan", false, "")
+	cmd.Flag.Var((*tagsFlag)(&cfg.BuildContext.BuildTags), "tags", "")
+	cmd.Flag.Var((*base.StringsFlag)(&cfg.BuildToolexec), "toolexec", "")
+	cmd.Flag.BoolVar(&cfg.BuildTrimpath, "trimpath", false, "")
+	cmd.Flag.BoolVar(&cfg.BuildWork, "work", false, "")
+
+	// Undocumented, unstable debugging flags.
+	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
+	cmd.Flag.StringVar(&cfg.DebugTrace, "debug-trace", "", "")
+}
+
+// tagsFlag is the implementation of the -tags flag.
+type tagsFlag []string
+
+func (v *tagsFlag) Set(s string) error {
+	// For compatibility with Go 1.12 and earlier, allow "-tags='a b c'" or even just "-tags='a'".
+	if strings.Contains(s, " ") || strings.Contains(s, "'") {
+		return (*base.StringsFlag)(v).Set(s)
+	}
+
+	// Split on commas, ignore empty strings.
+	*v = []string{}
+	for _, s := range strings.Split(s, ",") {
+		if s != "" {
+			*v = append(*v, s)
+		}
+	}
+	return nil
+}
+
+func (v *tagsFlag) String() string {
+	return "<TagsFlag>"
+}
+
+// fileExtSplit expects a filename and returns the name
+// and ext (without the dot). If the file has no
+// extension, ext will be empty.
+func fileExtSplit(file string) (name, ext string) {
+	dotExt := filepath.Ext(file)
+	name = file[:len(file)-len(dotExt)]
+	if dotExt != "" {
+		ext = dotExt[1:]
+	}
+	return
+}
+
+func pkgsMain(pkgs []*load.Package) (res []*load.Package) {
+	for _, p := range pkgs {
+		if p.Name == "main" {
+			res = append(res, p)
+		}
+	}
+	return res
+}
+
+func pkgsNotMain(pkgs []*load.Package) (res []*load.Package) {
+	for _, p := range pkgs {
+		if p.Name != "main" {
+			res = append(res, p)
+		}
+	}
+	return res
+}
+
+func oneMainPkg(pkgs []*load.Package) []*load.Package {
+	if len(pkgs) != 1 || pkgs[0].Name != "main" {
+		base.Fatalf("-buildmode=%s requires exactly one main package", cfg.BuildBuildmode)
+	}
+	return pkgs
+}
+
+var pkgsFilter = func(pkgs []*load.Package) []*load.Package { return pkgs }
+
+var runtimeVersion = runtime.Version()
+
+func runBuild(ctx context.Context, cmd *base.Command, args []string) {
+	BuildInit()
+	var b Builder
+	b.Init()
+
+	pkgs := load.PackagesAndErrors(ctx, args)
+	load.CheckPackageErrors(pkgs)
+
+	explicitO := len(cfg.BuildO) > 0
+
+	if len(pkgs) == 1 && pkgs[0].Name == "main" && cfg.BuildO == "" {
+		cfg.BuildO = pkgs[0].DefaultExecName()
+		cfg.BuildO += cfg.ExeSuffix
+	}
+
+	// sanity check some often mis-used options
+	switch cfg.BuildContext.Compiler {
+	case "gccgo":
+		if load.BuildGcflags.Present() {
+			fmt.Println("go build: when using gccgo toolchain, please pass compiler flags using -gccgoflags, not -gcflags")
+		}
+		if load.BuildLdflags.Present() {
+			fmt.Println("go build: when using gccgo toolchain, please pass linker flags using -gccgoflags, not -ldflags")
+		}
+	case "gc":
+		if load.BuildGccgoflags.Present() {
+			fmt.Println("go build: when using gc toolchain, please pass compile flags using -gcflags, and linker flags using -ldflags")
+		}
+	}
+
+	depMode := ModeBuild
+	if cfg.BuildI {
+		depMode = ModeInstall
+		fmt.Fprint(os.Stderr, "go build: -i flag is deprecated\n")
+	}
+
+	pkgs = omitTestOnly(pkgsFilter(pkgs))
+
+	// Special case -o /dev/null by not writing at all.
+	if cfg.BuildO == os.DevNull {
+		cfg.BuildO = ""
+	}
+
+	if cfg.BuildO != "" {
+		// If the -o name exists and is a directory or
+		// ends with a slash or backslash, then
+		// write all main packages to that directory.
+		// Otherwise require only a single package be built.
+		if fi, err := os.Stat(cfg.BuildO); (err == nil && fi.IsDir()) ||
+			strings.HasSuffix(cfg.BuildO, "/") ||
+			strings.HasSuffix(cfg.BuildO, string(os.PathSeparator)) {
+			if !explicitO {
+				base.Fatalf("go build: build output %q already exists and is a directory", cfg.BuildO)
+			}
+			a := &Action{Mode: "go build"}
+			for _, p := range pkgs {
+				if p.Name != "main" {
+					continue
+				}
+
+				p.Target = filepath.Join(cfg.BuildO, p.DefaultExecName())
+				p.Target += cfg.ExeSuffix
+				p.Stale = true
+				p.StaleReason = "build -o flag in use"
+				a.Deps = append(a.Deps, b.AutoAction(ModeInstall, depMode, p))
+			}
+			if len(a.Deps) == 0 {
+				base.Fatalf("go build: no main packages to build")
+			}
+			b.Do(ctx, a)
+			return
+		}
+		if len(pkgs) > 1 {
+			base.Fatalf("go build: cannot write multiple packages to non-directory %s", cfg.BuildO)
+		} else if len(pkgs) == 0 {
+			base.Fatalf("no packages to build")
+		}
+		p := pkgs[0]
+		p.Target = cfg.BuildO
+		p.Stale = true // must build - not up to date
+		p.StaleReason = "build -o flag in use"
+		a := b.AutoAction(ModeInstall, depMode, p)
+		b.Do(ctx, a)
+		return
+	}
+
+	a := &Action{Mode: "go build"}
+	for _, p := range pkgs {
+		a.Deps = append(a.Deps, b.AutoAction(ModeBuild, depMode, p))
+	}
+	if cfg.BuildBuildmode == "shared" {
+		a = b.buildmodeShared(ModeBuild, depMode, args, pkgs, a)
+	}
+	b.Do(ctx, a)
+}
+
+var CmdInstall = &base.Command{
+	UsageLine: "go install [build flags] [packages]",
+	Short:     "compile and install packages and dependencies",
+	Long: `
+Install compiles and installs the packages named by the import paths.
+
+Executables are installed in the directory named by the GOBIN environment
+variable, which defaults to $GOPATH/bin or $HOME/go/bin if the GOPATH
+environment variable is not set. Executables in $GOROOT
+are installed in $GOROOT/bin or $GOTOOLDIR instead of $GOBIN.
+
+If the arguments have version suffixes (like @latest or @v1.0.0), "go install"
+builds packages in module-aware mode, ignoring the go.mod file in the current
+directory or any parent directory, if there is one. This is useful for
+installing executables without affecting the dependencies of the main module.
+To eliminate ambiguity about which module versions are used in the build, the
+arguments must satisfy the following constraints:
+
+- Arguments must be package paths or package patterns (with "..." wildcards).
+They must not be standard packages (like fmt), meta-patterns (std, cmd,
+all), or relative or absolute file paths.
+
+- All arguments must have the same version suffix. Different queries are not
+allowed, even if they refer to the same version.
+
+- All arguments must refer to packages in the same module at the same version.
+
+- No module is considered the "main" module. If the module containing
+packages named on the command line has a go.mod file, it must not contain
+directives (replace and exclude) that would cause it to be interpreted
+differently than if it were the main module. The module must not require
+a higher version of itself.
+
+- Package path arguments must refer to main packages. Pattern arguments
+will only match main packages.
+
+If the arguments don't have version suffixes, "go install" may run in
+module-aware mode or GOPATH mode, depending on the GO111MODULE environment
+variable and the presence of a go.mod file. See 'go help modules' for details.
+If module-aware mode is enabled, "go install" runs in the context of the main
+module.
+
+When module-aware mode is disabled, other packages are installed in the
+directory $GOPATH/pkg/$GOOS_$GOARCH. When module-aware mode is enabled,
+other packages are built and cached but not installed.
+
+The -i flag installs the dependencies of the named packages as well.
+The -i flag is deprecated. Compiled packages are cached automatically.
+
+For more about the build flags, see 'go help build'.
+For more about specifying packages, see 'go help packages'.
+
+See also: go build, go get, go clean.
+	`,
+}
+
+// libname returns the filename to use for the shared library when using
+// -buildmode=shared. The rules we use are:
+// Use arguments for special 'meta' packages:
+//	std --> libstd.so
+//	std cmd --> libstd,cmd.so
+// A single non-meta argument with trailing "/..." is special cased:
+//	foo/... --> libfoo.so
+//	(A relative path like "./..."  expands the "." first)
+// Use import paths for other cases, changing '/' to '-':
+//	somelib --> libsubdir-somelib.so
+//	./ or ../ --> libsubdir-somelib.so
+//	gopkg.in/tomb.v2 -> libgopkg.in-tomb.v2.so
+//	a/... b/... ---> liba/c,b/d.so - all matching import paths
+// Name parts are joined with ','.
+func libname(args []string, pkgs []*load.Package) (string, error) {
+	var libname string
+	appendName := func(arg string) {
+		if libname == "" {
+			libname = arg
+		} else {
+			libname += "," + arg
+		}
+	}
+	var haveNonMeta bool
+	for _, arg := range args {
+		if search.IsMetaPackage(arg) {
+			appendName(arg)
+		} else {
+			haveNonMeta = true
+		}
+	}
+	if len(libname) == 0 { // non-meta packages only. use import paths
+		if len(args) == 1 && strings.HasSuffix(args[0], "/...") {
+			// Special case of "foo/..." as mentioned above.
+			arg := strings.TrimSuffix(args[0], "/...")
+			if build.IsLocalImport(arg) {
+				cwd, _ := os.Getwd()
+				bp, _ := cfg.BuildContext.ImportDir(filepath.Join(cwd, arg), build.FindOnly)
+				if bp.ImportPath != "" && bp.ImportPath != "." {
+					arg = bp.ImportPath
+				}
+			}
+			appendName(strings.ReplaceAll(arg, "/", "-"))
+		} else {
+			for _, pkg := range pkgs {
+				appendName(strings.ReplaceAll(pkg.ImportPath, "/", "-"))
+			}
+		}
+	} else if haveNonMeta { // have both meta package and a non-meta one
+		return "", errors.New("mixing of meta and non-meta packages is not allowed")
+	}
+	// TODO(mwhudson): Needs to change for platforms that use different naming
+	// conventions...
+	return "lib" + libname + ".so", nil
+}
+
+func runInstall(ctx context.Context, cmd *base.Command, args []string) {
+	// TODO(golang.org/issue/41696): print a deprecation message for the -i flag
+	// whenever it's set (or just remove it). For now, we don't print a message
+	// if all named packages are in GOROOT. cmd/dist (run by make.bash) uses
+	// 'go install -i' when bootstrapping, and we don't want to show deprecation
+	// messages in that case.
+	for _, arg := range args {
+		if strings.Contains(arg, "@") && !build.IsLocalImport(arg) && !filepath.IsAbs(arg) {
+			if cfg.BuildI {
+				fmt.Fprint(os.Stderr, "go install: -i flag is deprecated\n")
+			}
+			installOutsideModule(ctx, args)
+			return
+		}
+	}
+
+	BuildInit()
+	pkgs := load.PackagesAndErrors(ctx, args)
+	if cfg.ModulesEnabled && !modload.HasModRoot() {
+		haveErrors := false
+		allMissingErrors := true
+		for _, pkg := range pkgs {
+			if pkg.Error == nil {
+				continue
+			}
+			haveErrors = true
+			if missingErr := (*modload.ImportMissingError)(nil); !errors.As(pkg.Error, &missingErr) {
+				allMissingErrors = false
+				break
+			}
+		}
+		if haveErrors && allMissingErrors {
+			latestArgs := make([]string, len(args))
+			for i := range args {
+				latestArgs[i] = args[i] + "@latest"
+			}
+			hint := strings.Join(latestArgs, " ")
+			base.Fatalf("go install: version is required when current directory is not in a module\n\tTry 'go install %s' to install the latest version", hint)
+		}
+	}
+	load.CheckPackageErrors(pkgs)
+	if cfg.BuildI {
+		allGoroot := true
+		for _, pkg := range pkgs {
+			if !pkg.Goroot {
+				allGoroot = false
+				break
+			}
+		}
+		if !allGoroot {
+			fmt.Fprint(os.Stderr, "go install: -i flag is deprecated\n")
+		}
+	}
+
+	InstallPackages(ctx, args, pkgs)
+}
+
+// omitTestOnly returns pkgs with test-only packages removed.
+func omitTestOnly(pkgs []*load.Package) []*load.Package {
+	var list []*load.Package
+	for _, p := range pkgs {
+		if len(p.GoFiles)+len(p.CgoFiles) == 0 && !p.Internal.CmdlinePkgLiteral {
+			// Package has no source files,
+			// perhaps due to build tags or perhaps due to only having *_test.go files.
+			// Also, it is only being processed as the result of a wildcard match
+			// like ./..., not because it was listed as a literal path on the command line.
+			// Ignore it.
+			continue
+		}
+		list = append(list, p)
+	}
+	return list
+}
+
+func InstallPackages(ctx context.Context, patterns []string, pkgs []*load.Package) {
+	ctx, span := trace.StartSpan(ctx, "InstallPackages "+strings.Join(patterns, " "))
+	defer span.Done()
+
+	if cfg.GOBIN != "" && !filepath.IsAbs(cfg.GOBIN) {
+		base.Fatalf("cannot install, GOBIN must be an absolute path")
+	}
+
+	pkgs = omitTestOnly(pkgsFilter(pkgs))
+	for _, p := range pkgs {
+		if p.Target == "" {
+			switch {
+			case p.Standard && p.ImportPath == "unsafe":
+				// unsafe is a built-in package, has no target
+			case p.Name != "main" && p.Internal.Local && p.ConflictDir == "":
+				// Non-executables outside GOPATH need not have a target:
+				// we can use the cache to hold the built package archive for use in future builds.
+				// The ones inside GOPATH should have a target (in GOPATH/pkg)
+				// or else something is wrong and worth reporting (like a ConflictDir).
+			case p.Name != "main" && p.Module != nil:
+				// Non-executables have no target (except the cache) when building with modules.
+			case p.Internal.GobinSubdir:
+				base.Errorf("go %s: cannot install cross-compiled binaries when GOBIN is set", cfg.CmdName)
+			case p.Internal.CmdlineFiles:
+				base.Errorf("go %s: no install location for .go files listed on command line (GOBIN not set)", cfg.CmdName)
+			case p.ConflictDir != "":
+				base.Errorf("go %s: no install location for %s: hidden by %s", cfg.CmdName, p.Dir, p.ConflictDir)
+			default:
+				base.Errorf("go %s: no install location for directory %s outside GOPATH\n"+
+					"\tFor more details see: 'go help gopath'", cfg.CmdName, p.Dir)
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	var b Builder
+	b.Init()
+	depMode := ModeBuild
+	if cfg.BuildI {
+		depMode = ModeInstall
+	}
+	a := &Action{Mode: "go install"}
+	var tools []*Action
+	for _, p := range pkgs {
+		// If p is a tool, delay the installation until the end of the build.
+		// This avoids installing assemblers/compilers that are being executed
+		// by other steps in the build.
+		a1 := b.AutoAction(ModeInstall, depMode, p)
+		if load.InstallTargetDir(p) == load.ToTool {
+			a.Deps = append(a.Deps, a1.Deps...)
+			a1.Deps = append(a1.Deps, a)
+			tools = append(tools, a1)
+			continue
+		}
+		a.Deps = append(a.Deps, a1)
+	}
+	if len(tools) > 0 {
+		a = &Action{
+			Mode: "go install (tools)",
+			Deps: tools,
+		}
+	}
+
+	if cfg.BuildBuildmode == "shared" {
+		// Note: If buildmode=shared then only non-main packages
+		// are present in the pkgs list, so all the special case code about
+		// tools above did not apply, and a is just a simple Action
+		// with a list of Deps, one per package named in pkgs,
+		// the same as in runBuild.
+		a = b.buildmodeShared(ModeInstall, ModeInstall, patterns, pkgs, a)
+	}
+
+	b.Do(ctx, a)
+	base.ExitIfErrors()
+
+	// Success. If this command is 'go install' with no arguments
+	// and the current directory (the implicit argument) is a command,
+	// remove any leftover command binary from a previous 'go build'.
+	// The binary is installed; it's not needed here anymore.
+	// And worse it might be a stale copy, which you don't want to find
+	// instead of the installed one if $PATH contains dot.
+	// One way to view this behavior is that it is as if 'go install' first
+	// runs 'go build' and the moves the generated file to the install dir.
+	// See issue 9645.
+	if len(patterns) == 0 && len(pkgs) == 1 && pkgs[0].Name == "main" {
+		// Compute file 'go build' would have created.
+		// If it exists and is an executable file, remove it.
+		targ := pkgs[0].DefaultExecName()
+		targ += cfg.ExeSuffix
+		if filepath.Join(pkgs[0].Dir, targ) != pkgs[0].Target { // maybe $GOBIN is the current directory
+			fi, err := os.Stat(targ)
+			if err == nil {
+				m := fi.Mode()
+				if m.IsRegular() {
+					if m&0111 != 0 || cfg.Goos == "windows" { // windows never sets executable bit
+						os.Remove(targ)
+					}
+				}
+			}
+		}
+	}
+}
+
+// installOutsideModule implements 'go install pkg@version'. It builds and
+// installs one or more main packages in module mode while ignoring any go.mod
+// in the current directory or parent directories.
+//
+// See golang.org/issue/40276 for details and rationale.
+func installOutsideModule(ctx context.Context, args []string) {
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NoRoot
+	modload.AllowMissingModuleImports()
+	modload.Init()
+
+	// Check that the arguments satisfy syntactic constraints.
+	var version string
+	for _, arg := range args {
+		if i := strings.Index(arg, "@"); i >= 0 {
+			version = arg[i+1:]
+			if version == "" {
+				base.Fatalf("go install %s: version must not be empty", arg)
+			}
+			break
+		}
+	}
+	patterns := make([]string, len(args))
+	for i, arg := range args {
+		if !strings.HasSuffix(arg, "@"+version) {
+			base.Errorf("go install %s: all arguments must have the same version (@%s)", arg, version)
+			continue
+		}
+		p := arg[:len(arg)-len(version)-1]
+		switch {
+		case build.IsLocalImport(p):
+			base.Errorf("go install %s: argument must be a package path, not a relative path", arg)
+		case filepath.IsAbs(p):
+			base.Errorf("go install %s: argument must be a package path, not an absolute path", arg)
+		case search.IsMetaPackage(p):
+			base.Errorf("go install %s: argument must be a package path, not a meta-package", arg)
+		case path.Clean(p) != p:
+			base.Errorf("go install %s: argument must be a clean package path", arg)
+		case !strings.Contains(p, "...") && search.IsStandardImportPath(p) && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, p):
+			base.Errorf("go install %s: argument must not be a package in the standard library", arg)
+		default:
+			patterns[i] = p
+		}
+	}
+	base.ExitIfErrors()
+	BuildInit()
+
+	// Query the module providing the first argument, load its go.mod file, and
+	// check that it doesn't contain directives that would cause it to be
+	// interpreted differently if it were the main module.
+	//
+	// If multiple modules match the first argument, accept the longest match
+	// (first result). It's possible this module won't provide packages named by
+	// later arguments, and other modules would. Let's not try to be too
+	// magical though.
+	allowed := modload.CheckAllowed
+	if modload.IsRevisionQuery(version) {
+		// Don't check for retractions if a specific revision is requested.
+		allowed = nil
+	}
+	noneSelected := func(path string) (version string) { return "none" }
+	qrs, err := modload.QueryPackages(ctx, patterns[0], version, noneSelected, allowed)
+	if err != nil {
+		base.Fatalf("go install %s: %v", args[0], err)
+	}
+	installMod := qrs[0].Mod
+	data, err := modfetch.GoMod(installMod.Path, installMod.Version)
+	if err != nil {
+		base.Fatalf("go install %s: %v", args[0], err)
+	}
+	f, err := modfile.Parse("go.mod", data, nil)
+	if err != nil {
+		base.Fatalf("go install %s: %s: %v", args[0], installMod, err)
+	}
+	directiveFmt := "go install %s: %s\n" +
+		"\tThe go.mod file for the module providing named packages contains one or\n" +
+		"\tmore %s directives. It must not contain directives that would cause\n" +
+		"\tit to be interpreted differently than if it were the main module."
+	if len(f.Replace) > 0 {
+		base.Fatalf(directiveFmt, args[0], installMod, "replace")
+	}
+	if len(f.Exclude) > 0 {
+		base.Fatalf(directiveFmt, args[0], installMod, "exclude")
+	}
+
+	// Since we are in NoRoot mode, the build list initially contains only
+	// the dummy command-line-arguments module. Add a requirement on the
+	// module that provides the packages named on the command line.
+	if err := modload.EditBuildList(ctx, nil, []module.Version{installMod}); err != nil {
+		base.Fatalf("go install %s: %v", args[0], err)
+	}
+
+	// Load packages for all arguments. Ignore non-main packages.
+	// Print a warning if an argument contains "..." and matches no main packages.
+	// PackagesAndErrors already prints warnings for patterns that don't match any
+	// packages, so be careful not to double print.
+	matchers := make([]func(string) bool, len(patterns))
+	for i, p := range patterns {
+		if strings.Contains(p, "...") {
+			matchers[i] = search.MatchPattern(p)
+		}
+	}
+
+	// TODO(golang.org/issue/40276): don't report errors loading non-main packages
+	// matched by a pattern.
+	pkgs := load.PackagesAndErrors(ctx, patterns)
+	load.CheckPackageErrors(pkgs)
+	mainPkgs := make([]*load.Package, 0, len(pkgs))
+	mainCount := make([]int, len(patterns))
+	nonMainCount := make([]int, len(patterns))
+	for _, pkg := range pkgs {
+		if pkg.Name == "main" {
+			mainPkgs = append(mainPkgs, pkg)
+			for i := range patterns {
+				if matchers[i] != nil && matchers[i](pkg.ImportPath) {
+					mainCount[i]++
+				}
+			}
+		} else {
+			for i := range patterns {
+				if matchers[i] == nil && patterns[i] == pkg.ImportPath {
+					base.Errorf("go install: package %s is not a main package", pkg.ImportPath)
+				} else if matchers[i] != nil && matchers[i](pkg.ImportPath) {
+					nonMainCount[i]++
+				}
+			}
+		}
+	}
+	base.ExitIfErrors()
+	for i, p := range patterns {
+		if matchers[i] != nil && mainCount[i] == 0 && nonMainCount[i] > 0 {
+			fmt.Fprintf(os.Stderr, "go: warning: %q matched no main packages\n", p)
+		}
+	}
+
+	// Check that named packages are all provided by the same module.
+	for _, pkg := range mainPkgs {
+		if pkg.Module == nil {
+			// Packages in std, cmd, and their vendored dependencies
+			// don't have this field set.
+			base.Errorf("go install: package %s not provided by module %s", pkg.ImportPath, installMod)
+		} else if pkg.Module.Path != installMod.Path || pkg.Module.Version != installMod.Version {
+			base.Errorf("go install: package %s provided by module %s@%s\n\tAll packages must be provided by the same module (%s).", pkg.ImportPath, pkg.Module.Path, pkg.Module.Version, installMod)
+		}
+	}
+	base.ExitIfErrors()
+
+	// Build and install the packages.
+	InstallPackages(ctx, patterns, mainPkgs)
+}
+
+// ExecCmd is the command to use to run user binaries.
+// Normally it is empty, meaning run the binaries directly.
+// If cross-compiling and running on a remote system or
+// simulator, it is typically go_GOOS_GOARCH_exec, with
+// the target GOOS and GOARCH substituted.
+// The -exec flag overrides these defaults.
+var ExecCmd []string
+
+// FindExecCmd derives the value of ExecCmd to use.
+// It returns that value and leaves ExecCmd set for direct use.
+func FindExecCmd() []string {
+	if ExecCmd != nil {
+		return ExecCmd
+	}
+	ExecCmd = []string{} // avoid work the second time
+	if cfg.Goos == runtime.GOOS && cfg.Goarch == runtime.GOARCH {
+		return ExecCmd
+	}
+	path, err := exec.LookPath(fmt.Sprintf("go_%s_%s_exec", cfg.Goos, cfg.Goarch))
+	if err == nil {
+		ExecCmd = []string{path}
+	}
+	return ExecCmd
+}
diff --git a/src/cmd/go/internal/work/build_test.go b/src/cmd/go/internal/work/build_test.go
new file mode 100644
index 0000000..eaf2639
--- /dev/null
+++ b/src/cmd/go/internal/work/build_test.go
@@ -0,0 +1,277 @@
+// 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 work
+
+import (
+	"bytes"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+)
+
+func TestRemoveDevNull(t *testing.T) {
+	fi, err := os.Lstat(os.DevNull)
+	if err != nil {
+		t.Skip(err)
+	}
+	if fi.Mode().IsRegular() {
+		t.Errorf("Lstat(%s).Mode().IsRegular() = true; expected false", os.DevNull)
+	}
+	mayberemovefile(os.DevNull)
+	_, err = os.Lstat(os.DevNull)
+	if err != nil {
+		t.Errorf("mayberemovefile(%s) did remove it; oops", os.DevNull)
+	}
+}
+
+func TestSplitPkgConfigOutput(t *testing.T) {
+	for _, test := range []struct {
+		in   []byte
+		want []string
+	}{
+		{[]byte(`-r:foo -L/usr/white\ space/lib -lfoo\ bar -lbar\ baz`), []string{"-r:foo", "-L/usr/white space/lib", "-lfoo bar", "-lbar baz"}},
+		{[]byte(`-lextra\ fun\ arg\\`), []string{`-lextra fun arg\`}},
+		{[]byte("\textra     whitespace\r\n"), []string{"extra", "whitespace"}},
+		{[]byte("     \r\n      "), nil},
+		{[]byte(`"-r:foo" "-L/usr/white space/lib" "-lfoo bar" "-lbar baz"`), []string{"-r:foo", "-L/usr/white space/lib", "-lfoo bar", "-lbar baz"}},
+		{[]byte(`"-lextra fun arg\\"`), []string{`-lextra fun arg\`}},
+		{[]byte(`"     \r\n\      "`), []string{`     \r\n\      `}},
+		{[]byte(`""`), nil},
+		{[]byte(``), nil},
+		{[]byte(`"\\"`), []string{`\`}},
+		{[]byte(`"\x"`), []string{`\x`}},
+		{[]byte(`"\\x"`), []string{`\x`}},
+		{[]byte(`'\\'`), []string{`\`}},
+		{[]byte(`'\x'`), []string{`\x`}},
+		{[]byte(`"\\x"`), []string{`\x`}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED='"/test/share/doc"'`), []string{"-fPIC", "-I/test/include/foo", `-DQUOTED="/test/share/doc"`}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED="/test/share/doc"`), []string{"-fPIC", "-I/test/include/foo", "-DQUOTED=/test/share/doc"}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED=\"/test/share/doc\"`), []string{"-fPIC", "-I/test/include/foo", `-DQUOTED="/test/share/doc"`}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED='/test/share/doc'`), []string{"-fPIC", "-I/test/include/foo", "-DQUOTED=/test/share/doc"}},
+		{[]byte(`-DQUOTED='/te\st/share/d\oc'`), []string{`-DQUOTED=/te\st/share/d\oc`}},
+		{[]byte(`-Dhello=10 -Dworld=+32 -DDEFINED_FROM_PKG_CONFIG=hello\ world`), []string{"-Dhello=10", "-Dworld=+32", "-DDEFINED_FROM_PKG_CONFIG=hello world"}},
+		{[]byte(`"broken\"" \\\a "a"`), []string{"broken\"", "\\a", "a"}},
+	} {
+		got, err := splitPkgConfigOutput(test.in)
+		if err != nil {
+			t.Errorf("splitPkgConfigOutput on %v failed with error %v", test.in, err)
+			continue
+		}
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("splitPkgConfigOutput(%v) = %v; want %v", test.in, got, test.want)
+		}
+	}
+
+	for _, test := range []struct {
+		in   []byte
+		want []string
+	}{
+		// broken quotation
+		{[]byte(`"     \r\n      `), nil},
+		{[]byte(`"-r:foo" "-L/usr/white space/lib "-lfoo bar" "-lbar baz"`), nil},
+		{[]byte(`"-lextra fun arg\\`), nil},
+		// broken char escaping
+		{[]byte(`broken flag\`), nil},
+		{[]byte(`extra broken flag \`), nil},
+		{[]byte(`\`), nil},
+		{[]byte(`"broken\"" "extra" \`), nil},
+	} {
+		got, err := splitPkgConfigOutput(test.in)
+		if err == nil {
+			t.Errorf("splitPkgConfigOutput(%v) = %v; haven't failed with error as expected.", test.in, got)
+		}
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("splitPkgConfigOutput(%v) = %v; want %v", test.in, got, test.want)
+		}
+	}
+
+}
+
+func TestSharedLibName(t *testing.T) {
+	// TODO(avdva) - make these values platform-specific
+	prefix := "lib"
+	suffix := ".so"
+	testData := []struct {
+		args      []string
+		pkgs      []*load.Package
+		expected  string
+		expectErr bool
+		rootedAt  string
+	}{
+		{
+			args:     []string{"std"},
+			pkgs:     []*load.Package{},
+			expected: "std",
+		},
+		{
+			args:     []string{"std", "cmd"},
+			pkgs:     []*load.Package{},
+			expected: "std,cmd",
+		},
+		{
+			args:     []string{},
+			pkgs:     []*load.Package{pkgImportPath("gopkg.in/somelib")},
+			expected: "gopkg.in-somelib",
+		},
+		{
+			args:     []string{"./..."},
+			pkgs:     []*load.Package{pkgImportPath("somelib")},
+			expected: "somelib",
+			rootedAt: "somelib",
+		},
+		{
+			args:     []string{"../somelib", "../somelib"},
+			pkgs:     []*load.Package{pkgImportPath("somelib")},
+			expected: "somelib",
+		},
+		{
+			args:     []string{"../lib1", "../lib2"},
+			pkgs:     []*load.Package{pkgImportPath("gopkg.in/lib1"), pkgImportPath("gopkg.in/lib2")},
+			expected: "gopkg.in-lib1,gopkg.in-lib2",
+		},
+		{
+			args: []string{"./..."},
+			pkgs: []*load.Package{
+				pkgImportPath("gopkg.in/dir/lib1"),
+				pkgImportPath("gopkg.in/lib2"),
+				pkgImportPath("gopkg.in/lib3"),
+			},
+			expected: "gopkg.in",
+			rootedAt: "gopkg.in",
+		},
+		{
+			args:      []string{"std", "../lib2"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+		{
+			args:      []string{"all", "./"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+		{
+			args:      []string{"cmd", "fmt"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+	}
+	for _, data := range testData {
+		func() {
+			if data.rootedAt != "" {
+				tmpGopath, err := os.MkdirTemp("", "gopath")
+				if err != nil {
+					t.Fatal(err)
+				}
+				oldGopath := cfg.BuildContext.GOPATH
+				defer func() {
+					cfg.BuildContext.GOPATH = oldGopath
+					os.Chdir(base.Cwd)
+					err := os.RemoveAll(tmpGopath)
+					if err != nil {
+						t.Error(err)
+					}
+				}()
+				root := filepath.Join(tmpGopath, "src", data.rootedAt)
+				err = os.MkdirAll(root, 0755)
+				if err != nil {
+					t.Fatal(err)
+				}
+				cfg.BuildContext.GOPATH = tmpGopath
+				os.Chdir(root)
+			}
+			computed, err := libname(data.args, data.pkgs)
+			if err != nil {
+				if !data.expectErr {
+					t.Errorf("libname returned an error %q, expected a name", err.Error())
+				}
+			} else if data.expectErr {
+				t.Errorf("libname returned %q, expected an error", computed)
+			} else {
+				expected := prefix + data.expected + suffix
+				if expected != computed {
+					t.Errorf("libname returned %q, expected %q", computed, expected)
+				}
+			}
+		}()
+	}
+}
+
+func pkgImportPath(pkgpath string) *load.Package {
+	return &load.Package{
+		PackagePublic: load.PackagePublic{
+			ImportPath: pkgpath,
+		},
+	}
+}
+
+// When installing packages, the installed package directory should
+// respect the SetGID bit and group name of the destination
+// directory.
+// See https://golang.org/issue/18878.
+func TestRespectSetgidDir(t *testing.T) {
+	switch runtime.GOOS {
+	case "ios":
+		t.Skip("can't set SetGID bit with chmod on iOS")
+	case "windows", "plan9":
+		t.Skip("chown/chmod setgid are not supported on Windows or Plan 9")
+	}
+
+	var b Builder
+
+	// Check that `cp` is called instead of `mv` by looking at the output
+	// of `(*Builder).ShowCmd` afterwards as a sanity check.
+	cfg.BuildX = true
+	var cmdBuf bytes.Buffer
+	b.Print = func(a ...interface{}) (int, error) {
+		return cmdBuf.WriteString(fmt.Sprint(a...))
+	}
+
+	setgiddir, err := os.MkdirTemp("", "SetGroupID")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(setgiddir)
+
+	// BSD mkdir(2) inherits the parent directory group, and other platforms
+	// can inherit the parent directory group via setgid. The test setup (chmod
+	// setgid) will fail if the process does not have the group permission to
+	// the new temporary directory.
+	err = os.Chown(setgiddir, os.Getuid(), os.Getgid())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Change setgiddir's permissions to include the SetGID bit.
+	if err := os.Chmod(setgiddir, 0755|fs.ModeSetgid); err != nil {
+		t.Fatal(err)
+	}
+
+	pkgfile, err := os.CreateTemp("", "pkgfile")
+	if err != nil {
+		t.Fatalf("os.CreateTemp(\"\", \"pkgfile\"): %v", err)
+	}
+	defer os.Remove(pkgfile.Name())
+	defer pkgfile.Close()
+
+	dirGIDFile := filepath.Join(setgiddir, "setgid")
+	if err := b.moveOrCopyFile(dirGIDFile, pkgfile.Name(), 0666, true); err != nil {
+		t.Fatalf("moveOrCopyFile: %v", err)
+	}
+
+	got := strings.TrimSpace(cmdBuf.String())
+	want := b.fmtcmd("", "cp %s %s", pkgfile.Name(), dirGIDFile)
+	if got != want {
+		t.Fatalf("moveOrCopyFile(%q, %q): want %q, got %q", dirGIDFile, pkgfile.Name(), want, got)
+	}
+}
diff --git a/src/cmd/go/internal/work/buildid.go b/src/cmd/go/internal/work/buildid.go
new file mode 100644
index 0000000..c555d4a
--- /dev/null
+++ b/src/cmd/go/internal/work/buildid.go
@@ -0,0 +1,698 @@
+// 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 work
+
+import (
+	"bytes"
+	"fmt"
+	exec "internal/execabs"
+	"os"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"cmd/internal/buildid"
+)
+
+// Build IDs
+//
+// Go packages and binaries are stamped with build IDs that record both
+// the action ID, which is a hash of the inputs to the action that produced
+// the packages or binary, and the content ID, which is a hash of the action
+// output, namely the archive or binary itself. The hash is the same one
+// used by the build artifact cache (see cmd/go/internal/cache), but
+// truncated when stored in packages and binaries, as the full length is not
+// needed and is a bit unwieldy. The precise form is
+//
+//	actionID/[.../]contentID
+//
+// where the actionID and contentID are prepared by buildid.HashToString below.
+// and are found by looking for the first or last slash.
+// Usually the buildID is simply actionID/contentID, but see below for an
+// exception.
+//
+// The build ID serves two primary purposes.
+//
+// 1. The action ID half allows installed packages and binaries to serve as
+// one-element cache entries. If we intend to build math.a with a given
+// set of inputs summarized in the action ID, and the installed math.a already
+// has that action ID, we can reuse the installed math.a instead of rebuilding it.
+//
+// 2. The content ID half allows the easy preparation of action IDs for steps
+// that consume a particular package or binary. The content hash of every
+// input file for a given action must be included in the action ID hash.
+// Storing the content ID in the build ID lets us read it from the file with
+// minimal I/O, instead of reading and hashing the entire file.
+// This is especially effective since packages and binaries are typically
+// the largest inputs to an action.
+//
+// Separating action ID from content ID is important for reproducible builds.
+// The compiler is compiled with itself. If an output were represented by its
+// own action ID (instead of content ID) when computing the action ID of
+// the next step in the build process, then the compiler could never have its
+// own input action ID as its output action ID (short of a miraculous hash collision).
+// Instead we use the content IDs to compute the next action ID, and because
+// the content IDs converge, so too do the action IDs and therefore the
+// build IDs and the overall compiler binary. See cmd/dist's cmdbootstrap
+// for the actual convergence sequence.
+//
+// The “one-element cache” purpose is a bit more complex for installed
+// binaries. For a binary, like cmd/gofmt, there are two steps: compile
+// cmd/gofmt/*.go into main.a, and then link main.a into the gofmt binary.
+// We do not install gofmt's main.a, only the gofmt binary. Being able to
+// decide that the gofmt binary is up-to-date means computing the action ID
+// for the final link of the gofmt binary and comparing it against the
+// already-installed gofmt binary. But computing the action ID for the link
+// means knowing the content ID of main.a, which we did not keep.
+// To sidestep this problem, each binary actually stores an expanded build ID:
+//
+//	actionID(binary)/actionID(main.a)/contentID(main.a)/contentID(binary)
+//
+// (Note that this can be viewed equivalently as:
+//
+//	actionID(binary)/buildID(main.a)/contentID(binary)
+//
+// Storing the buildID(main.a) in the middle lets the computations that care
+// about the prefix or suffix halves ignore the middle and preserves the
+// original build ID as a contiguous string.)
+//
+// During the build, when it's time to build main.a, the gofmt binary has the
+// information needed to decide whether the eventual link would produce
+// the same binary: if the action ID for main.a's inputs matches and then
+// the action ID for the link step matches when assuming the given main.a
+// content ID, then the binary as a whole is up-to-date and need not be rebuilt.
+//
+// This is all a bit complex and may be simplified once we can rely on the
+// main cache, but at least at the start we will be using the content-based
+// staleness determination without a cache beyond the usual installed
+// package and binary locations.
+
+const buildIDSeparator = "/"
+
+// actionID returns the action ID half of a build ID.
+func actionID(buildID string) string {
+	i := strings.Index(buildID, buildIDSeparator)
+	if i < 0 {
+		return buildID
+	}
+	return buildID[:i]
+}
+
+// contentID returns the content ID half of a build ID.
+func contentID(buildID string) string {
+	return buildID[strings.LastIndex(buildID, buildIDSeparator)+1:]
+}
+
+// toolID returns the unique ID to use for the current copy of the
+// named tool (asm, compile, cover, link).
+//
+// It is important that if the tool changes (for example a compiler bug is fixed
+// and the compiler reinstalled), toolID returns a different string, so that old
+// package archives look stale and are rebuilt (with the fixed compiler).
+// This suggests using a content hash of the tool binary, as stored in the build ID.
+//
+// Unfortunately, we can't just open the tool binary, because the tool might be
+// invoked via a wrapper program specified by -toolexec and we don't know
+// what the wrapper program does. In particular, we want "-toolexec toolstash"
+// to continue working: it does no good if "-toolexec toolstash" is executing a
+// stashed copy of the compiler but the go command is acting as if it will run
+// the standard copy of the compiler. The solution is to ask the tool binary to tell
+// us its own build ID using the "-V=full" flag now supported by all tools.
+// Then we know we're getting the build ID of the compiler that will actually run
+// during the build. (How does the compiler binary know its own content hash?
+// We store it there using updateBuildID after the standard link step.)
+//
+// A final twist is that we'd prefer to have reproducible builds for release toolchains.
+// It should be possible to cross-compile for Windows from either Linux or Mac
+// or Windows itself and produce the same binaries, bit for bit. If the tool ID,
+// which influences the action ID half of the build ID, is based on the content ID,
+// then the Linux compiler binary and Mac compiler binary will have different tool IDs
+// and therefore produce executables with different action IDs.
+// To avoid this problem, for releases we use the release version string instead
+// of the compiler binary's content hash. This assumes that all compilers built
+// on all different systems are semantically equivalent, which is of course only true
+// modulo bugs. (Producing the exact same executables also requires that the different
+// build setups agree on details like $GOROOT and file name paths, but at least the
+// tool IDs do not make it impossible.)
+func (b *Builder) toolID(name string) string {
+	b.id.Lock()
+	id := b.toolIDCache[name]
+	b.id.Unlock()
+
+	if id != "" {
+		return id
+	}
+
+	path := base.Tool(name)
+	desc := "go tool " + name
+
+	// Special case: undocumented -vettool overrides usual vet,
+	// for testing vet or supplying an alternative analysis tool.
+	if name == "vet" && VetTool != "" {
+		path = VetTool
+		desc = VetTool
+	}
+
+	cmdline := str.StringList(cfg.BuildToolexec, path, "-V=full")
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		base.Fatalf("%s: %v\n%s%s", desc, err, stdout.Bytes(), stderr.Bytes())
+	}
+
+	line := stdout.String()
+	f := strings.Fields(line)
+	if len(f) < 3 || f[0] != name && path != VetTool || f[1] != "version" || f[2] == "devel" && !strings.HasPrefix(f[len(f)-1], "buildID=") {
+		base.Fatalf("%s -V=full: unexpected output:\n\t%s", desc, line)
+	}
+	if f[2] == "devel" {
+		// On the development branch, use the content ID part of the build ID.
+		id = contentID(f[len(f)-1])
+	} else {
+		// For a release, the output is like: "compile version go1.9.1 X:framepointer".
+		// Use the whole line.
+		id = strings.TrimSpace(line)
+	}
+
+	b.id.Lock()
+	b.toolIDCache[name] = id
+	b.id.Unlock()
+
+	return id
+}
+
+// gccToolID returns the unique ID to use for a tool that is invoked
+// by the GCC driver. This is used particularly for gccgo, but this can also
+// be used for gcc, g++, gfortran, etc.; those tools all use the GCC
+// driver under different names. The approach used here should also
+// work for sufficiently new versions of clang. Unlike toolID, the
+// name argument is the program to run. The language argument is the
+// type of input file as passed to the GCC driver's -x option.
+//
+// For these tools we have no -V=full option to dump the build ID,
+// but we can run the tool with -v -### to reliably get the compiler proper
+// and hash that. That will work in the presence of -toolexec.
+//
+// In order to get reproducible builds for released compilers, we
+// detect a released compiler by the absence of "experimental" in the
+// --version output, and in that case we just use the version string.
+func (b *Builder) gccgoToolID(name, language string) (string, error) {
+	key := name + "." + language
+	b.id.Lock()
+	id := b.toolIDCache[key]
+	b.id.Unlock()
+
+	if id != "" {
+		return id, nil
+	}
+
+	// Invoke the driver with -### to see the subcommands and the
+	// version strings. Use -x to set the language. Pretend to
+	// compile an empty file on standard input.
+	cmdline := str.StringList(cfg.BuildToolexec, name, "-###", "-x", language, "-c", "-")
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+	// Force untranslated output so that we see the string "version".
+	cmd.Env = append(cmd.Env, "LC_ALL=C")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", fmt.Errorf("%s: %v; output: %q", name, err, out)
+	}
+
+	version := ""
+	lines := strings.Split(string(out), "\n")
+	for _, line := range lines {
+		if fields := strings.Fields(line); len(fields) > 1 && fields[1] == "version" {
+			version = line
+			break
+		}
+	}
+	if version == "" {
+		return "", fmt.Errorf("%s: can not find version number in %q", name, out)
+	}
+
+	if !strings.Contains(version, "experimental") {
+		// This is a release. Use this line as the tool ID.
+		id = version
+	} else {
+		// This is a development version. The first line with
+		// a leading space is the compiler proper.
+		compiler := ""
+		for _, line := range lines {
+			if len(line) > 1 && line[0] == ' ' {
+				compiler = line
+				break
+			}
+		}
+		if compiler == "" {
+			return "", fmt.Errorf("%s: can not find compilation command in %q", name, out)
+		}
+
+		fields := strings.Fields(compiler)
+		if len(fields) == 0 {
+			return "", fmt.Errorf("%s: compilation command confusion %q", name, out)
+		}
+		exe := fields[0]
+		if !strings.ContainsAny(exe, `/\`) {
+			if lp, err := exec.LookPath(exe); err == nil {
+				exe = lp
+			}
+		}
+		id, err = buildid.ReadFile(exe)
+		if err != nil {
+			return "", err
+		}
+
+		// If we can't find a build ID, use a hash.
+		if id == "" {
+			id = b.fileHash(exe)
+		}
+	}
+
+	b.id.Lock()
+	b.toolIDCache[key] = id
+	b.id.Unlock()
+
+	return id, nil
+}
+
+// Check if assembler used by gccgo is GNU as.
+func assemblerIsGas() bool {
+	cmd := exec.Command(BuildToolchain.compiler(), "-print-prog-name=as")
+	assembler, err := cmd.Output()
+	if err == nil {
+		cmd := exec.Command(strings.TrimSpace(string(assembler)), "--version")
+		out, err := cmd.Output()
+		return err == nil && strings.Contains(string(out), "GNU")
+	} else {
+		return false
+	}
+}
+
+// gccgoBuildIDFile creates an assembler file that records the
+// action's build ID in an SHF_EXCLUDE section for ELF files or
+// in a CSECT in XCOFF files.
+func (b *Builder) gccgoBuildIDFile(a *Action) (string, error) {
+	sfile := a.Objdir + "_buildid.s"
+
+	var buf bytes.Buffer
+	if cfg.Goos == "aix" {
+		fmt.Fprintf(&buf, "\t.csect .go.buildid[XO]\n")
+	} else if (cfg.Goos != "solaris" && cfg.Goos != "illumos") || assemblerIsGas() {
+		fmt.Fprintf(&buf, "\t"+`.section .go.buildid,"e"`+"\n")
+	} else if cfg.Goarch == "sparc" || cfg.Goarch == "sparc64" {
+		fmt.Fprintf(&buf, "\t"+`.section ".go.buildid",#exclude`+"\n")
+	} else { // cfg.Goarch == "386" || cfg.Goarch == "amd64"
+		fmt.Fprintf(&buf, "\t"+`.section .go.buildid,#exclude`+"\n")
+	}
+	fmt.Fprintf(&buf, "\t.byte ")
+	for i := 0; i < len(a.buildID); i++ {
+		if i > 0 {
+			if i%8 == 0 {
+				fmt.Fprintf(&buf, "\n\t.byte ")
+			} else {
+				fmt.Fprintf(&buf, ",")
+			}
+		}
+		fmt.Fprintf(&buf, "%#02x", a.buildID[i])
+	}
+	fmt.Fprintf(&buf, "\n")
+	if cfg.Goos != "solaris" && cfg.Goos != "illumos" && cfg.Goos != "aix" {
+		secType := "@progbits"
+		if cfg.Goarch == "arm" {
+			secType = "%progbits"
+		}
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-stack,"",%s`+"\n", secType)
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-split-stack,"",%s`+"\n", secType)
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		for _, line := range bytes.Split(buf.Bytes(), []byte("\n")) {
+			b.Showcmd("", "echo '%s' >> %s", line, sfile)
+		}
+		if cfg.BuildN {
+			return sfile, nil
+		}
+	}
+
+	if err := os.WriteFile(sfile, buf.Bytes(), 0666); err != nil {
+		return "", err
+	}
+
+	return sfile, nil
+}
+
+// buildID returns the build ID found in the given file.
+// If no build ID is found, buildID returns the content hash of the file.
+func (b *Builder) buildID(file string) string {
+	b.id.Lock()
+	id := b.buildIDCache[file]
+	b.id.Unlock()
+
+	if id != "" {
+		return id
+	}
+
+	id, err := buildid.ReadFile(file)
+	if err != nil {
+		id = b.fileHash(file)
+	}
+
+	b.id.Lock()
+	b.buildIDCache[file] = id
+	b.id.Unlock()
+
+	return id
+}
+
+// fileHash returns the content hash of the named file.
+func (b *Builder) fileHash(file string) string {
+	file, _ = fsys.OverlayPath(file)
+	sum, err := cache.FileHash(file)
+	if err != nil {
+		return ""
+	}
+	return buildid.HashToString(sum)
+}
+
+// useCache tries to satisfy the action a, which has action ID actionHash,
+// by using a cached result from an earlier build. At the moment, the only
+// cached result is the installed package or binary at target.
+// If useCache decides that the cache can be used, it sets a.buildID
+// and a.built for use by parent actions and then returns true.
+// Otherwise it sets a.buildID to a temporary build ID for use in the build
+// and returns false. When useCache returns false the expectation is that
+// the caller will build the target and then call updateBuildID to finish the
+// build ID computation.
+// When useCache returns false, it may have initiated buffering of output
+// during a's work. The caller should defer b.flushOutput(a), to make sure
+// that flushOutput is eventually called regardless of whether the action
+// succeeds. The flushOutput call must happen after updateBuildID.
+func (b *Builder) useCache(a *Action, actionHash cache.ActionID, target string) bool {
+	// The second half of the build ID here is a placeholder for the content hash.
+	// It's important that the overall buildID be unlikely verging on impossible
+	// to appear in the output by chance, but that should be taken care of by
+	// the actionID half; if it also appeared in the input that would be like an
+	// engineered 120-bit partial SHA256 collision.
+	a.actionID = actionHash
+	actionID := buildid.HashToString(actionHash)
+	if a.json != nil {
+		a.json.ActionID = actionID
+	}
+	contentID := actionID // temporary placeholder, likely unique
+	a.buildID = actionID + buildIDSeparator + contentID
+
+	// Executable binaries also record the main build ID in the middle.
+	// See "Build IDs" comment above.
+	if a.Mode == "link" {
+		mainpkg := a.Deps[0]
+		a.buildID = actionID + buildIDSeparator + mainpkg.buildID + buildIDSeparator + contentID
+	}
+
+	// Check to see if target exists and matches the expected action ID.
+	// If so, it's up to date and we can reuse it instead of rebuilding it.
+	var buildID string
+	if target != "" && !cfg.BuildA {
+		buildID, _ = buildid.ReadFile(target)
+		if strings.HasPrefix(buildID, actionID+buildIDSeparator) {
+			a.buildID = buildID
+			if a.json != nil {
+				a.json.BuildID = a.buildID
+			}
+			a.built = target
+			// Poison a.Target to catch uses later in the build.
+			a.Target = "DO NOT USE - " + a.Mode
+			return true
+		}
+	}
+
+	// Special case for building a main package: if the only thing we
+	// want the package for is to link a binary, and the binary is
+	// already up-to-date, then to avoid a rebuild, report the package
+	// as up-to-date as well. See "Build IDs" comment above.
+	// TODO(rsc): Rewrite this code to use a TryCache func on the link action.
+	if target != "" && !cfg.BuildA && !b.NeedExport && a.Mode == "build" && len(a.triggers) == 1 && a.triggers[0].Mode == "link" {
+		buildID, err := buildid.ReadFile(target)
+		if err == nil {
+			id := strings.Split(buildID, buildIDSeparator)
+			if len(id) == 4 && id[1] == actionID {
+				// Temporarily assume a.buildID is the package build ID
+				// stored in the installed binary, and see if that makes
+				// the upcoming link action ID a match. If so, report that
+				// we built the package, safe in the knowledge that the
+				// link step will not ask us for the actual package file.
+				// Note that (*Builder).LinkAction arranged that all of
+				// a.triggers[0]'s dependencies other than a are also
+				// dependencies of a, so that we can be sure that,
+				// other than a.buildID, b.linkActionID is only accessing
+				// build IDs of completed actions.
+				oldBuildID := a.buildID
+				a.buildID = id[1] + buildIDSeparator + id[2]
+				linkID := buildid.HashToString(b.linkActionID(a.triggers[0]))
+				if id[0] == linkID {
+					// Best effort attempt to display output from the compile and link steps.
+					// If it doesn't work, it doesn't work: reusing the cached binary is more
+					// important than reprinting diagnostic information.
+					if c := cache.Default(); c != nil {
+						showStdout(b, c, a.actionID, "stdout")      // compile output
+						showStdout(b, c, a.actionID, "link-stdout") // link output
+					}
+
+					// Poison a.Target to catch uses later in the build.
+					a.Target = "DO NOT USE - main build pseudo-cache Target"
+					a.built = "DO NOT USE - main build pseudo-cache built"
+					if a.json != nil {
+						a.json.BuildID = a.buildID
+					}
+					return true
+				}
+				// Otherwise restore old build ID for main build.
+				a.buildID = oldBuildID
+			}
+		}
+	}
+
+	// Special case for linking a test binary: if the only thing we
+	// want the binary for is to run the test, and the test result is cached,
+	// then to avoid the link step, report the link as up-to-date.
+	// We avoid the nested build ID problem in the previous special case
+	// by recording the test results in the cache under the action ID half.
+	if !cfg.BuildA && len(a.triggers) == 1 && a.triggers[0].TryCache != nil && a.triggers[0].TryCache(b, a.triggers[0]) {
+		// Best effort attempt to display output from the compile and link steps.
+		// If it doesn't work, it doesn't work: reusing the test result is more
+		// important than reprinting diagnostic information.
+		if c := cache.Default(); c != nil {
+			showStdout(b, c, a.Deps[0].actionID, "stdout")      // compile output
+			showStdout(b, c, a.Deps[0].actionID, "link-stdout") // link output
+		}
+
+		// Poison a.Target to catch uses later in the build.
+		a.Target = "DO NOT USE -  pseudo-cache Target"
+		a.built = "DO NOT USE - pseudo-cache built"
+		return true
+	}
+
+	if b.IsCmdList {
+		// Invoked during go list to compute and record staleness.
+		if p := a.Package; p != nil && !p.Stale {
+			p.Stale = true
+			if cfg.BuildA {
+				p.StaleReason = "build -a flag in use"
+			} else {
+				p.StaleReason = "build ID mismatch"
+				for _, p1 := range p.Internal.Imports {
+					if p1.Stale && p1.StaleReason != "" {
+						if strings.HasPrefix(p1.StaleReason, "stale dependency: ") {
+							p.StaleReason = p1.StaleReason
+							break
+						}
+						if strings.HasPrefix(p.StaleReason, "build ID mismatch") {
+							p.StaleReason = "stale dependency: " + p1.ImportPath
+						}
+					}
+				}
+			}
+		}
+
+		// Fall through to update a.buildID from the build artifact cache,
+		// which will affect the computation of buildIDs for targets
+		// higher up in the dependency graph.
+	}
+
+	// Check the build artifact cache.
+	// We treat hits in this cache as being "stale" for the purposes of go list
+	// (in effect, "stale" means whether p.Target is up-to-date),
+	// but we're still happy to use results from the build artifact cache.
+	if c := cache.Default(); c != nil {
+		if !cfg.BuildA {
+			if file, _, err := c.GetFile(actionHash); err == nil {
+				if buildID, err := buildid.ReadFile(file); err == nil {
+					if err := showStdout(b, c, a.actionID, "stdout"); err == nil {
+						a.built = file
+						a.Target = "DO NOT USE - using cache"
+						a.buildID = buildID
+						if a.json != nil {
+							a.json.BuildID = a.buildID
+						}
+						if p := a.Package; p != nil {
+							// Clearer than explaining that something else is stale.
+							p.StaleReason = "not installed but available in build cache"
+						}
+						return true
+					}
+				}
+			}
+		}
+
+		// Begin saving output for later writing to cache.
+		a.output = []byte{}
+	}
+
+	return false
+}
+
+func showStdout(b *Builder, c *cache.Cache, actionID cache.ActionID, key string) error {
+	stdout, stdoutEntry, err := c.GetBytes(cache.Subkey(actionID, key))
+	if err != nil {
+		return err
+	}
+
+	if len(stdout) > 0 {
+		if cfg.BuildX || cfg.BuildN {
+			b.Showcmd("", "%s  # internal", joinUnambiguously(str.StringList("cat", c.OutputFile(stdoutEntry.OutputID))))
+		}
+		if !cfg.BuildN {
+			b.Print(string(stdout))
+		}
+	}
+	return nil
+}
+
+// flushOutput flushes the output being queued in a.
+func (b *Builder) flushOutput(a *Action) {
+	b.Print(string(a.output))
+	a.output = nil
+}
+
+// updateBuildID updates the build ID in the target written by action a.
+// It requires that useCache was called for action a and returned false,
+// and that the build was then carried out and given the temporary
+// a.buildID to record as the build ID in the resulting package or binary.
+// updateBuildID computes the final content ID and updates the build IDs
+// in the binary.
+//
+// Keep in sync with src/cmd/buildid/buildid.go
+func (b *Builder) updateBuildID(a *Action, target string, rewrite bool) error {
+	if cfg.BuildX || cfg.BuildN {
+		if rewrite {
+			b.Showcmd("", "%s # internal", joinUnambiguously(str.StringList(base.Tool("buildid"), "-w", target)))
+		}
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	// Cache output from compile/link, even if we don't do the rest.
+	if c := cache.Default(); c != nil {
+		switch a.Mode {
+		case "build":
+			c.PutBytes(cache.Subkey(a.actionID, "stdout"), a.output)
+		case "link":
+			// Even though we don't cache the binary, cache the linker text output.
+			// We might notice that an installed binary is up-to-date but still
+			// want to pretend to have run the linker.
+			// Store it under the main package's action ID
+			// to make it easier to find when that's all we have.
+			for _, a1 := range a.Deps {
+				if p1 := a1.Package; p1 != nil && p1.Name == "main" {
+					c.PutBytes(cache.Subkey(a1.actionID, "link-stdout"), a.output)
+					break
+				}
+			}
+		}
+	}
+
+	// Find occurrences of old ID and compute new content-based ID.
+	r, err := os.Open(target)
+	if err != nil {
+		return err
+	}
+	matches, hash, err := buildid.FindAndHash(r, a.buildID, 0)
+	r.Close()
+	if err != nil {
+		return err
+	}
+	newID := a.buildID[:strings.LastIndex(a.buildID, buildIDSeparator)] + buildIDSeparator + buildid.HashToString(hash)
+	if len(newID) != len(a.buildID) {
+		return fmt.Errorf("internal error: build ID length mismatch %q vs %q", a.buildID, newID)
+	}
+
+	// Replace with new content-based ID.
+	a.buildID = newID
+	if a.json != nil {
+		a.json.BuildID = a.buildID
+	}
+	if len(matches) == 0 {
+		// Assume the user specified -buildid= to override what we were going to choose.
+		return nil
+	}
+
+	if rewrite {
+		w, err := os.OpenFile(target, os.O_RDWR, 0)
+		if err != nil {
+			return err
+		}
+		err = buildid.Rewrite(w, matches, newID)
+		if err != nil {
+			w.Close()
+			return err
+		}
+		if err := w.Close(); err != nil {
+			return err
+		}
+	}
+
+	// Cache package builds, but not binaries (link steps).
+	// The expectation is that binaries are not reused
+	// nearly as often as individual packages, and they're
+	// much larger, so the cache-footprint-to-utility ratio
+	// of binaries is much lower for binaries.
+	// Not caching the link step also makes sure that repeated "go run" at least
+	// always rerun the linker, so that they don't get too fast.
+	// (We don't want people thinking go is a scripting language.)
+	// Note also that if we start caching binaries, then we will
+	// copy the binaries out of the cache to run them, and then
+	// that will mean the go process is itself writing a binary
+	// and then executing it, so we will need to defend against
+	// ETXTBSY problems as discussed in exec.go and golang.org/issue/22220.
+	if c := cache.Default(); c != nil && a.Mode == "build" {
+		r, err := os.Open(target)
+		if err == nil {
+			if a.output == nil {
+				panic("internal error: a.output not set")
+			}
+			outputID, _, err := c.Put(a.actionID, r)
+			r.Close()
+			if err == nil && cfg.BuildX {
+				b.Showcmd("", "%s # internal", joinUnambiguously(str.StringList("cp", target, c.OutputFile(outputID))))
+			}
+			if b.NeedExport {
+				if err != nil {
+					return err
+				}
+				a.Package.Export = c.OutputFile(outputID)
+				a.Package.BuildID = a.buildID
+			}
+		}
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
new file mode 100644
index 0000000..422e83c
--- /dev/null
+++ b/src/cmd/go/internal/work/exec.go
@@ -0,0 +1,3316 @@
+// 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.
+
+// Action graph execution.
+
+package work
+
+import (
+	"bytes"
+	"cmd/go/internal/fsys"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	exec "internal/execabs"
+	"internal/lazyregexp"
+	"io"
+	"io/fs"
+	"log"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+)
+
+// actionList returns the list of actions in the dag rooted at root
+// as visited in a depth-first post-order traversal.
+func actionList(root *Action) []*Action {
+	seen := map[*Action]bool{}
+	all := []*Action{}
+	var walk func(*Action)
+	walk = func(a *Action) {
+		if seen[a] {
+			return
+		}
+		seen[a] = true
+		for _, a1 := range a.Deps {
+			walk(a1)
+		}
+		all = append(all, a)
+	}
+	walk(root)
+	return all
+}
+
+// do runs the action graph rooted at root.
+func (b *Builder) Do(ctx context.Context, root *Action) {
+	ctx, span := trace.StartSpan(ctx, "exec.Builder.Do ("+root.Mode+" "+root.Target+")")
+	defer span.Done()
+
+	if !b.IsCmdList {
+		// If we're doing real work, take time at the end to trim the cache.
+		c := cache.Default()
+		defer c.Trim()
+	}
+
+	// Build list of all actions, assigning depth-first post-order priority.
+	// The original implementation here was a true queue
+	// (using a channel) but it had the effect of getting
+	// distracted by low-level leaf actions to the detriment
+	// of completing higher-level actions. The order of
+	// work does not matter much to overall execution time,
+	// but when running "go test std" it is nice to see each test
+	// results as soon as possible. The priorities assigned
+	// ensure that, all else being equal, the execution prefers
+	// to do what it would have done first in a simple depth-first
+	// dependency order traversal.
+	all := actionList(root)
+	for i, a := range all {
+		a.priority = i
+	}
+
+	// Write action graph, without timing information, in case we fail and exit early.
+	writeActionGraph := func() {
+		if file := cfg.DebugActiongraph; file != "" {
+			if strings.HasSuffix(file, ".go") {
+				// Do not overwrite Go source code in:
+				//	go build -debug-actiongraph x.go
+				base.Fatalf("go: refusing to write action graph to %v\n", file)
+			}
+			js := actionGraphJSON(root)
+			if err := os.WriteFile(file, []byte(js), 0666); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing action graph: %v\n", err)
+				base.SetExitStatus(1)
+			}
+		}
+	}
+	writeActionGraph()
+
+	b.readySema = make(chan bool, len(all))
+
+	// Initialize per-action execution state.
+	for _, a := range all {
+		for _, a1 := range a.Deps {
+			a1.triggers = append(a1.triggers, a)
+		}
+		a.pending = len(a.Deps)
+		if a.pending == 0 {
+			b.ready.push(a)
+			b.readySema <- true
+		}
+	}
+
+	// Handle runs a single action and takes care of triggering
+	// any actions that are runnable as a result.
+	handle := func(ctx context.Context, a *Action) {
+		if a.json != nil {
+			a.json.TimeStart = time.Now()
+		}
+		var err error
+		if a.Func != nil && (!a.Failed || a.IgnoreFail) {
+			// TODO(matloob): Better action descriptions
+			desc := "Executing action "
+			if a.Package != nil {
+				desc += "(" + a.Mode + " " + a.Package.Desc() + ")"
+			}
+			ctx, span := trace.StartSpan(ctx, desc)
+			a.traceSpan = span
+			for _, d := range a.Deps {
+				trace.Flow(ctx, d.traceSpan, a.traceSpan)
+			}
+			err = a.Func(b, ctx, a)
+			span.Done()
+		}
+		if a.json != nil {
+			a.json.TimeDone = time.Now()
+		}
+
+		// The actions run in parallel but all the updates to the
+		// shared work state are serialized through b.exec.
+		b.exec.Lock()
+		defer b.exec.Unlock()
+
+		if err != nil {
+			if err == errPrintedOutput {
+				base.SetExitStatus(2)
+			} else {
+				base.Errorf("%s", err)
+			}
+			a.Failed = true
+		}
+
+		for _, a0 := range a.triggers {
+			if a.Failed {
+				a0.Failed = true
+			}
+			if a0.pending--; a0.pending == 0 {
+				b.ready.push(a0)
+				b.readySema <- true
+			}
+		}
+
+		if a == root {
+			close(b.readySema)
+		}
+	}
+
+	var wg sync.WaitGroup
+
+	// Kick off goroutines according to parallelism.
+	// If we are using the -n flag (just printing commands)
+	// drop the parallelism to 1, both to make the output
+	// deterministic and because there is no real work anyway.
+	par := cfg.BuildP
+	if cfg.BuildN {
+		par = 1
+	}
+	for i := 0; i < par; i++ {
+		wg.Add(1)
+		go func() {
+			ctx := trace.StartGoroutine(ctx)
+			defer wg.Done()
+			for {
+				select {
+				case _, ok := <-b.readySema:
+					if !ok {
+						return
+					}
+					// Receiving a value from b.readySema entitles
+					// us to take from the ready queue.
+					b.exec.Lock()
+					a := b.ready.pop()
+					b.exec.Unlock()
+					handle(ctx, a)
+				case <-base.Interrupted:
+					base.SetExitStatus(1)
+					return
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	// Write action graph again, this time with timing information.
+	writeActionGraph()
+}
+
+// buildActionID computes the action ID for a build action.
+func (b *Builder) buildActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("build " + p.ImportPath)
+
+	// Configuration independent of compiler toolchain.
+	// Note: buildmode has already been accounted for in buildGcflags
+	// and should not be inserted explicitly. Most buildmodes use the
+	// same compiler settings and can reuse each other's results.
+	// If not, the reason is already recorded in buildGcflags.
+	fmt.Fprintf(h, "compile\n")
+	// Only include the package directory if it may affect the output.
+	// We trim workspace paths for all packages when -trimpath is set.
+	// The compiler hides the exact value of $GOROOT
+	// when building things in GOROOT.
+	// Assume b.WorkDir is being trimmed properly.
+	// When -trimpath is used with a package built from the module cache,
+	// use the module path and version instead of the directory.
+	if !p.Goroot && !cfg.BuildTrimpath && !strings.HasPrefix(p.Dir, b.WorkDir) {
+		fmt.Fprintf(h, "dir %s\n", p.Dir)
+	} else if cfg.BuildTrimpath && p.Module != nil {
+		fmt.Fprintf(h, "module %s@%s\n", p.Module.Path, p.Module.Version)
+	}
+	if p.Module != nil {
+		fmt.Fprintf(h, "go %s\n", p.Module.GoVersion)
+	}
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+	if p.Internal.ForceLibrary {
+		fmt.Fprintf(h, "forcelibrary\n")
+	}
+	if len(p.CgoFiles)+len(p.SwigFiles) > 0 {
+		fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
+		cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)
+		fmt.Fprintf(h, "CC=%q %q %q %q\n", b.ccExe(), cppflags, cflags, ldflags)
+		if len(p.CXXFiles)+len(p.SwigFiles) > 0 {
+			fmt.Fprintf(h, "CXX=%q %q\n", b.cxxExe(), cxxflags)
+		}
+		if len(p.FFiles) > 0 {
+			fmt.Fprintf(h, "FC=%q %q\n", b.fcExe(), fflags)
+		}
+		// TODO(rsc): Should we include the SWIG version or Fortran/GCC/G++/Objective-C compiler versions?
+	}
+	if p.Internal.CoverMode != "" {
+		fmt.Fprintf(h, "cover %q %q\n", p.Internal.CoverMode, b.toolID("cover"))
+	}
+	fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
+
+	// Configuration specific to compiler toolchain.
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("buildActionID: unknown build toolchain %q", cfg.BuildToolchainName)
+	case "gc":
+		fmt.Fprintf(h, "compile %s %q %q\n", b.toolID("compile"), forcedGcflags, p.Internal.Gcflags)
+		if len(p.SFiles) > 0 {
+			fmt.Fprintf(h, "asm %q %q %q\n", b.toolID("asm"), forcedAsmflags, p.Internal.Asmflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		// TODO(rsc): Convince compiler team not to add more magic environment variables,
+		// or perhaps restrict the environment variables passed to subprocesses.
+		// Because these are clumsy, undocumented special-case hacks
+		// for debugging the compiler, they are not settable using 'go env -w',
+		// and so here we use os.Getenv, not cfg.Getenv.
+		magic := []string{
+			"GOCLOBBERDEADHASH",
+			"GOSSAFUNC",
+			"GO_SSA_PHI_LOC_CUTOFF",
+			"GOSSAHASH",
+		}
+		for _, env := range magic {
+			if x := os.Getenv(env); x != "" {
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+		if os.Getenv("GOSSAHASH") != "" {
+			for i := 0; ; i++ {
+				env := fmt.Sprintf("GOSSAHASH%d", i)
+				x := os.Getenv(env)
+				if x == "" {
+					break
+				}
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+		if os.Getenv("GSHS_LOGFILE") != "" {
+			// Clumsy hack. Compiler writes to this log file,
+			// so do not allow use of cache at all.
+			// We will still write to the cache but it will be
+			// essentially unfindable.
+			fmt.Fprintf(h, "nocache %d\n", time.Now().UnixNano())
+		}
+
+	case "gccgo":
+		id, err := b.gccgoToolID(BuildToolchain.compiler(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "compile %s %q %q\n", id, forcedGccgoflags, p.Internal.Gccgoflags)
+		fmt.Fprintf(h, "pkgpath %s\n", gccgoPkgpath(p))
+		fmt.Fprintf(h, "ar %q\n", BuildToolchain.(gccgoToolchain).ar())
+		if len(p.SFiles) > 0 {
+			id, _ = b.gccgoToolID(BuildToolchain.compiler(), "assembler-with-cpp")
+			// Ignore error; different assembler versions
+			// are unlikely to make any difference anyhow.
+			fmt.Fprintf(h, "asm %q\n", id)
+		}
+	}
+
+	// Input files.
+	inputFiles := str.StringList(
+		p.GoFiles,
+		p.CgoFiles,
+		p.CFiles,
+		p.CXXFiles,
+		p.FFiles,
+		p.MFiles,
+		p.HFiles,
+		p.SFiles,
+		p.SysoFiles,
+		p.SwigFiles,
+		p.SwigCXXFiles,
+		p.EmbedFiles,
+	)
+	for _, file := range inputFiles {
+		fmt.Fprintf(h, "file %s %s\n", file, b.fileHash(filepath.Join(p.Dir, file)))
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			fmt.Fprintf(h, "import %s %s\n", p1.ImportPath, contentID(a1.buildID))
+		}
+	}
+
+	return h.Sum()
+}
+
+// needCgoHdr reports whether the actions triggered by this one
+// expect to be able to access the cgo-generated header file.
+func (b *Builder) needCgoHdr(a *Action) bool {
+	// If this build triggers a header install, run cgo to get the header.
+	if !b.IsCmdList && (a.Package.UsesCgo() || a.Package.UsesSwig()) && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
+		for _, t1 := range a.triggers {
+			if t1.Mode == "install header" {
+				return true
+			}
+		}
+		for _, t1 := range a.triggers {
+			for _, t2 := range t1.triggers {
+				if t2.Mode == "install header" {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+// allowedVersion reports whether the version v is an allowed version of go
+// (one that we can compile).
+// v is known to be of the form "1.23".
+func allowedVersion(v string) bool {
+	// Special case: no requirement.
+	if v == "" {
+		return true
+	}
+	// Special case "1.0" means "go1", which is OK.
+	if v == "1.0" {
+		return true
+	}
+	// Otherwise look through release tags of form "go1.23" for one that matches.
+	for _, tag := range cfg.BuildContext.ReleaseTags {
+		if strings.HasPrefix(tag, "go") && tag[2:] == v {
+			return true
+		}
+	}
+	return false
+}
+
+const (
+	needBuild uint32 = 1 << iota
+	needCgoHdr
+	needVet
+	needCompiledGoFiles
+	needStale
+)
+
+// build is the action for building a single package.
+// Note that any new influence on this logic must be reported in b.buildActionID above as well.
+func (b *Builder) build(ctx context.Context, a *Action) (err error) {
+	p := a.Package
+
+	bit := func(x uint32, b bool) uint32 {
+		if b {
+			return x
+		}
+		return 0
+	}
+
+	cachedBuild := false
+	need := bit(needBuild, !b.IsCmdList && a.needBuild || b.NeedExport) |
+		bit(needCgoHdr, b.needCgoHdr(a)) |
+		bit(needVet, a.needVet) |
+		bit(needCompiledGoFiles, b.NeedCompiledGoFiles)
+
+	if !p.BinaryOnly {
+		if b.useCache(a, b.buildActionID(a), p.Target) {
+			// We found the main output in the cache.
+			// If we don't need any other outputs, we can stop.
+			// Otherwise, we need to write files to a.Objdir (needVet, needCgoHdr).
+			// Remember that we might have them in cache
+			// and check again after we create a.Objdir.
+			cachedBuild = true
+			a.output = []byte{} // start saving output in case we miss any cache results
+			need &^= needBuild
+			if b.NeedExport {
+				p.Export = a.built
+				p.BuildID = a.buildID
+			}
+			if need&needCompiledGoFiles != 0 {
+				if err := b.loadCachedSrcFiles(a); err == nil {
+					need &^= needCompiledGoFiles
+				}
+			}
+		}
+
+		// Source files might be cached, even if the full action is not
+		// (e.g., go list -compiled -find).
+		if !cachedBuild && need&needCompiledGoFiles != 0 {
+			if err := b.loadCachedSrcFiles(a); err == nil {
+				need &^= needCompiledGoFiles
+			}
+		}
+
+		if need == 0 {
+			return nil
+		}
+		defer b.flushOutput(a)
+	}
+
+	defer func() {
+		if err != nil && err != errPrintedOutput {
+			err = fmt.Errorf("go build %s: %v", a.Package.ImportPath, err)
+		}
+		if err != nil && b.IsCmdList && b.NeedError && p.Error == nil {
+			p.Error = &load.PackageError{Err: err}
+		}
+	}()
+	if cfg.BuildN {
+		// In -n mode, print a banner between packages.
+		// The banner is five lines so that when changes to
+		// different sections of the bootstrap script have to
+		// be merged, the banners give patch something
+		// to use to find its context.
+		b.Print("\n#\n# " + a.Package.ImportPath + "\n#\n\n")
+	}
+
+	if cfg.BuildV {
+		b.Print(a.Package.ImportPath + "\n")
+	}
+
+	if a.Package.BinaryOnly {
+		p.Stale = true
+		p.StaleReason = "binary-only packages are no longer supported"
+		if b.IsCmdList {
+			return nil
+		}
+		return errors.New("binary-only packages are no longer supported")
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+	objdir := a.Objdir
+
+	// Load cached cgo header, but only if we're skipping the main build (cachedBuild==true).
+	if cachedBuild && need&needCgoHdr != 0 {
+		if err := b.loadCachedCgoHdr(a); err == nil {
+			need &^= needCgoHdr
+		}
+	}
+
+	// Load cached vet config, but only if that's all we have left
+	// (need == needVet, not testing just the one bit).
+	// If we are going to do a full build anyway,
+	// we're going to regenerate the files below anyway.
+	if need == needVet {
+		if err := b.loadCachedVet(a); err == nil {
+			need &^= needVet
+		}
+	}
+	if need == 0 {
+		return nil
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	gofiles := str.StringList(a.Package.GoFiles)
+	cgofiles := str.StringList(a.Package.CgoFiles)
+	cfiles := str.StringList(a.Package.CFiles)
+	sfiles := str.StringList(a.Package.SFiles)
+	cxxfiles := str.StringList(a.Package.CXXFiles)
+	var objects, cgoObjects, pcCFLAGS, pcLDFLAGS []string
+
+	if a.Package.UsesCgo() || a.Package.UsesSwig() {
+		if pcCFLAGS, pcLDFLAGS, err = b.getPkgConfigFlags(a.Package); err != nil {
+			return
+		}
+	}
+
+	// Compute overlays for .c/.cc/.h/etc. and if there are any overlays
+	// put correct contents of all those files in the objdir, to ensure
+	// the correct headers are included. nonGoOverlay is the overlay that
+	// points from nongo files to the copied files in objdir.
+	nonGoFileLists := [][]string{a.Package.CFiles, a.Package.SFiles, a.Package.CXXFiles, a.Package.HFiles, a.Package.FFiles}
+OverlayLoop:
+	for _, fs := range nonGoFileLists {
+		for _, f := range fs {
+			if _, ok := fsys.OverlayPath(mkAbs(p.Dir, f)); ok {
+				a.nonGoOverlay = make(map[string]string)
+				break OverlayLoop
+			}
+		}
+	}
+	if a.nonGoOverlay != nil {
+		for _, fs := range nonGoFileLists {
+			for i := range fs {
+				from := mkAbs(p.Dir, fs[i])
+				opath, _ := fsys.OverlayPath(from)
+				dst := objdir + filepath.Base(fs[i])
+				if err := b.copyFile(dst, opath, 0666, false); err != nil {
+					return err
+				}
+				a.nonGoOverlay[from] = dst
+			}
+		}
+	}
+
+	// Run SWIG on each .swig and .swigcxx file.
+	// Each run will generate two files, a .go file and a .c or .cxx file.
+	// The .go file will use import "C" and is to be processed by cgo.
+	if a.Package.UsesSwig() {
+		outGo, outC, outCXX, err := b.swig(a, a.Package, objdir, pcCFLAGS)
+		if err != nil {
+			return err
+		}
+		cgofiles = append(cgofiles, outGo...)
+		cfiles = append(cfiles, outC...)
+		cxxfiles = append(cxxfiles, outCXX...)
+	}
+
+	// If we're doing coverage, preprocess the .go files and put them in the work directory
+	if a.Package.Internal.CoverMode != "" {
+		for i, file := range str.StringList(gofiles, cgofiles) {
+			var sourceFile string
+			var coverFile string
+			var key string
+			if strings.HasSuffix(file, ".cgo1.go") {
+				// cgo files have absolute paths
+				base := filepath.Base(file)
+				sourceFile = file
+				coverFile = objdir + base
+				key = strings.TrimSuffix(base, ".cgo1.go") + ".go"
+			} else {
+				sourceFile = filepath.Join(a.Package.Dir, file)
+				coverFile = objdir + file
+				key = file
+			}
+			coverFile = strings.TrimSuffix(coverFile, ".go") + ".cover.go"
+			cover := a.Package.Internal.CoverVars[key]
+			if cover == nil || base.IsTestFile(file) {
+				// Not covering this file.
+				continue
+			}
+			if err := b.cover(a, coverFile, sourceFile, cover.Var); err != nil {
+				return err
+			}
+			if i < len(gofiles) {
+				gofiles[i] = coverFile
+			} else {
+				cgofiles[i-len(gofiles)] = coverFile
+			}
+		}
+	}
+
+	// Run cgo.
+	if a.Package.UsesCgo() || a.Package.UsesSwig() {
+		// In a package using cgo, cgo compiles the C, C++ and assembly files with gcc.
+		// There is one exception: runtime/cgo's job is to bridge the
+		// cgo and non-cgo worlds, so it necessarily has files in both.
+		// In that case gcc only gets the gcc_* files.
+		var gccfiles []string
+		gccfiles = append(gccfiles, cfiles...)
+		cfiles = nil
+		if a.Package.Standard && a.Package.ImportPath == "runtime/cgo" {
+			filter := func(files, nongcc, gcc []string) ([]string, []string) {
+				for _, f := range files {
+					if strings.HasPrefix(f, "gcc_") {
+						gcc = append(gcc, f)
+					} else {
+						nongcc = append(nongcc, f)
+					}
+				}
+				return nongcc, gcc
+			}
+			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
+		} else {
+			for _, sfile := range sfiles {
+				data, err := os.ReadFile(filepath.Join(a.Package.Dir, sfile))
+				if err == nil {
+					if bytes.HasPrefix(data, []byte("TEXT")) || bytes.Contains(data, []byte("\nTEXT")) ||
+						bytes.HasPrefix(data, []byte("DATA")) || bytes.Contains(data, []byte("\nDATA")) ||
+						bytes.HasPrefix(data, []byte("GLOBL")) || bytes.Contains(data, []byte("\nGLOBL")) {
+						return fmt.Errorf("package using cgo has Go assembly file %s", sfile)
+					}
+				}
+			}
+			gccfiles = append(gccfiles, sfiles...)
+			sfiles = nil
+		}
+
+		outGo, outObj, err := b.cgo(a, base.Tool("cgo"), objdir, pcCFLAGS, pcLDFLAGS, mkAbsFiles(a.Package.Dir, cgofiles), gccfiles, cxxfiles, a.Package.MFiles, a.Package.FFiles)
+		if err != nil {
+			return err
+		}
+		if cfg.BuildToolchainName == "gccgo" {
+			cgoObjects = append(cgoObjects, a.Objdir+"_cgo_flags")
+		}
+		cgoObjects = append(cgoObjects, outObj...)
+		gofiles = append(gofiles, outGo...)
+
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared":
+			b.cacheCgoHdr(a)
+		}
+	}
+
+	var srcfiles []string // .go and non-.go
+	srcfiles = append(srcfiles, gofiles...)
+	srcfiles = append(srcfiles, sfiles...)
+	srcfiles = append(srcfiles, cfiles...)
+	srcfiles = append(srcfiles, cxxfiles...)
+	b.cacheSrcFiles(a, srcfiles)
+
+	// Running cgo generated the cgo header.
+	need &^= needCgoHdr
+
+	// Sanity check only, since Package.load already checked as well.
+	if len(gofiles) == 0 {
+		return &load.NoGoError{Package: a.Package}
+	}
+
+	// Prepare Go vet config if needed.
+	if need&needVet != 0 {
+		buildVetConfig(a, srcfiles)
+		need &^= needVet
+	}
+	if need&needCompiledGoFiles != 0 {
+		if err := b.loadCachedSrcFiles(a); err != nil {
+			return fmt.Errorf("loading compiled Go files from cache: %w", err)
+		}
+		need &^= needCompiledGoFiles
+	}
+	if need == 0 {
+		// Nothing left to do.
+		return nil
+	}
+
+	// Collect symbol ABI requirements from assembly.
+	symabis, err := BuildToolchain.symabis(b, a, sfiles)
+	if err != nil {
+		return err
+	}
+
+	// Prepare Go import config.
+	// We start it off with a comment so it can't be empty, so icfg.Bytes() below is never nil.
+	// It should never be empty anyway, but there have been bugs in the past that resulted
+	// in empty configs, which then unfortunately turn into "no config passed to compiler",
+	// and the compiler falls back to looking in pkg itself, which mostly works,
+	// except when it doesn't.
+	var icfg bytes.Buffer
+	fmt.Fprintf(&icfg, "# import config\n")
+	for i, raw := range a.Package.Internal.RawImports {
+		final := a.Package.Imports[i]
+		if final != raw {
+			fmt.Fprintf(&icfg, "importmap %s=%s\n", raw, final)
+		}
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" || a1.built == "" {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+	}
+
+	// Prepare Go embed config if needed.
+	// Unlike the import config, it's okay for the embed config to be empty.
+	var embedcfg []byte
+	if len(p.Internal.Embed) > 0 {
+		var embed struct {
+			Patterns map[string][]string
+			Files    map[string]string
+		}
+		embed.Patterns = p.Internal.Embed
+		embed.Files = make(map[string]string)
+		for _, file := range p.EmbedFiles {
+			embed.Files[file] = filepath.Join(p.Dir, file)
+		}
+		js, err := json.MarshalIndent(&embed, "", "\t")
+		if err != nil {
+			return fmt.Errorf("marshal embedcfg: %v", err)
+		}
+		embedcfg = js
+	}
+
+	if p.Internal.BuildInfo != "" && cfg.ModulesEnabled {
+		if err := b.writeFile(objdir+"_gomod_.go", modload.ModInfoProg(p.Internal.BuildInfo, cfg.BuildToolchainName == "gccgo")); err != nil {
+			return err
+		}
+		gofiles = append(gofiles, objdir+"_gomod_.go")
+	}
+
+	// Compile Go.
+	objpkg := objdir + "_pkg_.a"
+	ofile, out, err := BuildToolchain.gc(b, a, objpkg, icfg.Bytes(), embedcfg, symabis, len(sfiles) > 0, gofiles)
+	if len(out) > 0 {
+		output := b.processOutput(out)
+		if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
+			output += "note: module requires Go " + p.Module.GoVersion + "\n"
+		}
+		b.showOutput(a, a.Package.Dir, a.Package.Desc(), output)
+		if err != nil {
+			return errPrintedOutput
+		}
+	}
+	if err != nil {
+		if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
+			b.showOutput(a, a.Package.Dir, a.Package.Desc(), "note: module requires Go "+p.Module.GoVersion+"\n")
+		}
+		return err
+	}
+	if ofile != objpkg {
+		objects = append(objects, ofile)
+	}
+
+	// Copy .h files named for goos or goarch or goos_goarch
+	// to names using GOOS and GOARCH.
+	// For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
+	_goos_goarch := "_" + cfg.Goos + "_" + cfg.Goarch
+	_goos := "_" + cfg.Goos
+	_goarch := "_" + cfg.Goarch
+	for _, file := range a.Package.HFiles {
+		name, ext := fileExtSplit(file)
+		switch {
+		case strings.HasSuffix(name, _goos_goarch):
+			targ := file[:len(name)-len(_goos_goarch)] + "_GOOS_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goarch):
+			targ := file[:len(name)-len(_goarch)] + "_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goos):
+			targ := file[:len(name)-len(_goos)] + "_GOOS." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		}
+	}
+
+	for _, file := range cfiles {
+		out := file[:len(file)-len(".c")] + ".o"
+		if err := BuildToolchain.cc(b, a, objdir+out, file); err != nil {
+			return err
+		}
+		objects = append(objects, out)
+	}
+
+	// Assemble .s files.
+	if len(sfiles) > 0 {
+		ofiles, err := BuildToolchain.asm(b, a, sfiles)
+		if err != nil {
+			return err
+		}
+		objects = append(objects, ofiles...)
+	}
+
+	// For gccgo on ELF systems, we write the build ID as an assembler file.
+	// This lets us set the SHF_EXCLUDE flag.
+	// This is read by readGccgoArchive in cmd/internal/buildid/buildid.go.
+	if a.buildID != "" && cfg.BuildToolchainName == "gccgo" {
+		switch cfg.Goos {
+		case "aix", "android", "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
+			asmfile, err := b.gccgoBuildIDFile(a)
+			if err != nil {
+				return err
+			}
+			ofiles, err := BuildToolchain.asm(b, a, []string{asmfile})
+			if err != nil {
+				return err
+			}
+			objects = append(objects, ofiles...)
+		}
+	}
+
+	// NOTE(rsc): On Windows, it is critically important that the
+	// gcc-compiled objects (cgoObjects) be listed after the ordinary
+	// objects in the archive. I do not know why this is.
+	// https://golang.org/issue/2601
+	objects = append(objects, cgoObjects...)
+
+	// Add system object files.
+	for _, syso := range a.Package.SysoFiles {
+		objects = append(objects, filepath.Join(a.Package.Dir, syso))
+	}
+
+	// Pack into archive in objdir directory.
+	// If the Go compiler wrote an archive, we only need to add the
+	// object files for non-Go sources to the archive.
+	// If the Go compiler wrote an archive and the package is entirely
+	// Go sources, there is no pack to execute at all.
+	if len(objects) > 0 {
+		if err := BuildToolchain.pack(b, a, objpkg, objects); err != nil {
+			return err
+		}
+	}
+
+	if err := b.updateBuildID(a, objpkg, true); err != nil {
+		return err
+	}
+
+	a.built = objpkg
+	return nil
+}
+
+func (b *Builder) cacheObjdirFile(a *Action, c *cache.Cache, name string) error {
+	f, err := os.Open(a.Objdir + name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	_, _, err = c.Put(cache.Subkey(a.actionID, name), f)
+	return err
+}
+
+func (b *Builder) findCachedObjdirFile(a *Action, c *cache.Cache, name string) (string, error) {
+	file, _, err := c.GetFile(cache.Subkey(a.actionID, name))
+	if err != nil {
+		return "", fmt.Errorf("loading cached file %s: %w", name, err)
+	}
+	return file, nil
+}
+
+func (b *Builder) loadCachedObjdirFile(a *Action, c *cache.Cache, name string) error {
+	cached, err := b.findCachedObjdirFile(a, c, name)
+	if err != nil {
+		return err
+	}
+	return b.copyFile(a.Objdir+name, cached, 0666, true)
+}
+
+func (b *Builder) cacheCgoHdr(a *Action) {
+	c := cache.Default()
+	b.cacheObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) loadCachedCgoHdr(a *Action) error {
+	c := cache.Default()
+	return b.loadCachedObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) cacheSrcFiles(a *Action, srcfiles []string) {
+	c := cache.Default()
+	var buf bytes.Buffer
+	for _, file := range srcfiles {
+		if !strings.HasPrefix(file, a.Objdir) {
+			// not generated
+			buf.WriteString("./")
+			buf.WriteString(file)
+			buf.WriteString("\n")
+			continue
+		}
+		name := file[len(a.Objdir):]
+		buf.WriteString(name)
+		buf.WriteString("\n")
+		if err := b.cacheObjdirFile(a, c, name); err != nil {
+			return
+		}
+	}
+	c.PutBytes(cache.Subkey(a.actionID, "srcfiles"), buf.Bytes())
+}
+
+func (b *Builder) loadCachedVet(a *Action) error {
+	c := cache.Default()
+	list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var srcfiles []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			srcfiles = append(srcfiles, name[2:])
+			continue
+		}
+		if err := b.loadCachedObjdirFile(a, c, name); err != nil {
+			return err
+		}
+		srcfiles = append(srcfiles, a.Objdir+name)
+	}
+	buildVetConfig(a, srcfiles)
+	return nil
+}
+
+func (b *Builder) loadCachedSrcFiles(a *Action) error {
+	c := cache.Default()
+	list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var files []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			files = append(files, name[len("./"):])
+			continue
+		}
+		file, err := b.findCachedObjdirFile(a, c, name)
+		if err != nil {
+			return fmt.Errorf("finding %s: %w", name, err)
+		}
+		files = append(files, file)
+	}
+	a.Package.CompiledGoFiles = files
+	return nil
+}
+
+// vetConfig is the configuration passed to vet describing a single package.
+type vetConfig struct {
+	ID           string   // package ID (example: "fmt [fmt.test]")
+	Compiler     string   // compiler name (gc, gccgo)
+	Dir          string   // directory containing package
+	ImportPath   string   // canonical import path ("package path")
+	GoFiles      []string // absolute paths to package source files
+	NonGoFiles   []string // absolute paths to package non-Go files
+	IgnoredFiles []string // absolute paths to ignored source files
+
+	ImportMap   map[string]string // map import path in source code to package path
+	PackageFile map[string]string // map package path to .a file with export data
+	Standard    map[string]bool   // map package path to whether it's in the standard library
+	PackageVetx map[string]string // map package path to vetx data from earlier vet run
+	VetxOnly    bool              // only compute vetx data; don't report detected problems
+	VetxOutput  string            // write vetx data to this output file
+
+	SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
+}
+
+func buildVetConfig(a *Action, srcfiles []string) {
+	// Classify files based on .go extension.
+	// srcfiles does not include raw cgo files.
+	var gofiles, nongofiles []string
+	for _, name := range srcfiles {
+		if strings.HasSuffix(name, ".go") {
+			gofiles = append(gofiles, name)
+		} else {
+			nongofiles = append(nongofiles, name)
+		}
+	}
+
+	ignored := str.StringList(a.Package.IgnoredGoFiles, a.Package.IgnoredOtherFiles)
+
+	// Pass list of absolute paths to vet,
+	// so that vet's error messages will use absolute paths,
+	// so that we can reformat them relative to the directory
+	// in which the go command is invoked.
+	vcfg := &vetConfig{
+		ID:           a.Package.ImportPath,
+		Compiler:     cfg.BuildToolchainName,
+		Dir:          a.Package.Dir,
+		GoFiles:      mkAbsFiles(a.Package.Dir, gofiles),
+		NonGoFiles:   mkAbsFiles(a.Package.Dir, nongofiles),
+		IgnoredFiles: mkAbsFiles(a.Package.Dir, ignored),
+		ImportPath:   a.Package.ImportPath,
+		ImportMap:    make(map[string]string),
+		PackageFile:  make(map[string]string),
+		Standard:     make(map[string]bool),
+	}
+	a.vetCfg = vcfg
+	for i, raw := range a.Package.Internal.RawImports {
+		final := a.Package.Imports[i]
+		vcfg.ImportMap[raw] = final
+	}
+
+	// Compute the list of mapped imports in the vet config
+	// so that we can add any missing mappings below.
+	vcfgMapped := make(map[string]bool)
+	for _, p := range vcfg.ImportMap {
+		vcfgMapped[p] = true
+	}
+
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" {
+			continue
+		}
+		// Add import mapping if needed
+		// (for imports like "runtime/cgo" that appear only in generated code).
+		if !vcfgMapped[p1.ImportPath] {
+			vcfg.ImportMap[p1.ImportPath] = p1.ImportPath
+		}
+		if a1.built != "" {
+			vcfg.PackageFile[p1.ImportPath] = a1.built
+		}
+		if p1.Standard {
+			vcfg.Standard[p1.ImportPath] = true
+		}
+	}
+}
+
+// VetTool is the path to an alternate vet tool binary.
+// The caller is expected to set it (if needed) before executing any vet actions.
+var VetTool string
+
+// VetFlags are the default flags to pass to vet.
+// The caller is expected to set them before executing any vet actions.
+var VetFlags []string
+
+// VetExplicit records whether the vet flags were set explicitly on the command line.
+var VetExplicit bool
+
+func (b *Builder) vet(ctx context.Context, a *Action) error {
+	// a.Deps[0] is the build of the package being vetted.
+	// a.Deps[1] is the build of the "fmt" package.
+
+	a.Failed = false // vet of dependency may have failed but we can still succeed
+
+	if a.Deps[0].Failed {
+		// The build of the package has failed. Skip vet check.
+		// Vet could return export data for non-typecheck errors,
+		// but we ignore it because the package cannot be compiled.
+		return nil
+	}
+
+	vcfg := a.Deps[0].vetCfg
+	if vcfg == nil {
+		// Vet config should only be missing if the build failed.
+		return fmt.Errorf("vet config not found")
+	}
+
+	vcfg.VetxOnly = a.VetxOnly
+	vcfg.VetxOutput = a.Objdir + "vet.out"
+	vcfg.PackageVetx = make(map[string]string)
+
+	h := cache.NewHash("vet " + a.Package.ImportPath)
+	fmt.Fprintf(h, "vet %q\n", b.toolID("vet"))
+
+	vetFlags := VetFlags
+
+	// In GOROOT, we enable all the vet tests during 'go test',
+	// not just the high-confidence subset. This gets us extra
+	// checking for the standard library (at some compliance cost)
+	// and helps us gain experience about how well the checks
+	// work, to help decide which should be turned on by default.
+	// The command-line still wins.
+	//
+	// Note that this flag change applies even when running vet as
+	// a dependency of vetting a package outside std.
+	// (Otherwise we'd have to introduce a whole separate
+	// space of "vet fmt as a dependency of a std top-level vet"
+	// versus "vet fmt as a dependency of a non-std top-level vet".)
+	// This is OK as long as the packages that are farther down the
+	// dependency tree turn on *more* analysis, as here.
+	// (The unsafeptr check does not write any facts for use by
+	// later vet runs, nor does unreachable.)
+	if a.Package.Goroot && !VetExplicit && VetTool == "" {
+		// Turn off -unsafeptr checks.
+		// There's too much unsafe.Pointer code
+		// that vet doesn't like in low-level packages
+		// like runtime, sync, and reflect.
+		// Note that $GOROOT/src/buildall.bash
+		// does the same for the misc-compile trybots
+		// and should be updated if these flags are
+		// changed here.
+		vetFlags = []string{"-unsafeptr=false"}
+
+		// Also turn off -unreachable checks during go test.
+		// During testing it is very common to make changes
+		// like hard-coded forced returns or panics that make
+		// code unreachable. It's unreasonable to insist on files
+		// not having any unreachable code during "go test".
+		// (buildall.bash still runs with -unreachable enabled
+		// for the overall whole-tree scan.)
+		if cfg.CmdName == "test" {
+			vetFlags = append(vetFlags, "-unreachable=false")
+		}
+	}
+
+	// Note: We could decide that vet should compute export data for
+	// all analyses, in which case we don't need to include the flags here.
+	// But that would mean that if an analysis causes problems like
+	// unexpected crashes there would be no way to turn it off.
+	// It seems better to let the flags disable export analysis too.
+	fmt.Fprintf(h, "vetflags %q\n", vetFlags)
+
+	fmt.Fprintf(h, "pkg %q\n", a.Deps[0].actionID)
+	for _, a1 := range a.Deps {
+		if a1.Mode == "vet" && a1.built != "" {
+			fmt.Fprintf(h, "vetout %q %s\n", a1.Package.ImportPath, b.fileHash(a1.built))
+			vcfg.PackageVetx[a1.Package.ImportPath] = a1.built
+		}
+	}
+	key := cache.ActionID(h.Sum())
+
+	if vcfg.VetxOnly && !cfg.BuildA {
+		c := cache.Default()
+		if file, _, err := c.GetFile(key); err == nil {
+			a.built = file
+			return nil
+		}
+	}
+
+	js, err := json.MarshalIndent(vcfg, "", "\t")
+	if err != nil {
+		return fmt.Errorf("internal error marshaling vet config: %v", err)
+	}
+	js = append(js, '\n')
+	if err := b.writeFile(a.Objdir+"vet.cfg", js); err != nil {
+		return err
+	}
+
+	// TODO(rsc): Why do we pass $GCCGO to go vet?
+	env := b.cCompilerEnv()
+	if cfg.BuildToolchainName == "gccgo" {
+		env = append(env, "GCCGO="+BuildToolchain.compiler())
+	}
+
+	p := a.Package
+	tool := VetTool
+	if tool == "" {
+		tool = base.Tool("vet")
+	}
+	runErr := b.run(a, p.Dir, p.ImportPath, env, cfg.BuildToolexec, tool, vetFlags, a.Objdir+"vet.cfg")
+
+	// If vet wrote export data, save it for input to future vets.
+	if f, err := os.Open(vcfg.VetxOutput); err == nil {
+		a.built = vcfg.VetxOutput
+		cache.Default().Put(key, f)
+		f.Close()
+	}
+
+	return runErr
+}
+
+// linkActionID computes the action ID for a link action.
+func (b *Builder) linkActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("link " + p.ImportPath)
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "link\n")
+	fmt.Fprintf(h, "buildmode %s goos %s goarch %s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+
+	// Toolchain-dependent configuration, shared with b.linkSharedActionID.
+	b.printLinkerConfig(h, p)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			if a1.built != "" || a1.buildID != "" {
+				buildID := a1.buildID
+				if buildID == "" {
+					buildID = b.buildID(a1.built)
+				}
+				fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(buildID))
+			}
+			// Because we put package main's full action ID into the binary's build ID,
+			// we must also put the full action ID into the binary's action ID hash.
+			if p1.Name == "main" {
+				fmt.Fprintf(h, "packagemain %s\n", a1.buildID)
+			}
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+
+	return h.Sum()
+}
+
+// printLinkerConfig prints the linker config into the hash h,
+// as part of the computation of a linker-related action ID.
+func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("linkActionID: unknown toolchain %q", cfg.BuildToolchainName)
+
+	case "gc":
+		fmt.Fprintf(h, "link %s %q %s\n", b.toolID("link"), forcedLdflags, ldBuildmode)
+		if p != nil {
+			fmt.Fprintf(h, "linkflags %q\n", p.Internal.Ldflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		// The linker writes source file paths that say GOROOT_FINAL, but
+		// only if -trimpath is not specified (see ld() in gc.go).
+		gorootFinal := cfg.GOROOT_FINAL
+		if cfg.BuildTrimpath {
+			gorootFinal = trimPathGoRootFinal
+		}
+		fmt.Fprintf(h, "GOROOT=%s\n", gorootFinal)
+
+		// GO_EXTLINK_ENABLED controls whether the external linker is used.
+		fmt.Fprintf(h, "GO_EXTLINK_ENABLED=%s\n", cfg.Getenv("GO_EXTLINK_ENABLED"))
+
+		// TODO(rsc): Do cgo settings and flags need to be included?
+		// Or external linker settings and flags?
+
+	case "gccgo":
+		id, err := b.gccgoToolID(BuildToolchain.linker(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "link %s %s\n", id, ldBuildmode)
+		// TODO(iant): Should probably include cgo flags here.
+	}
+}
+
+// link is the action for linking a single command.
+// Note that any new influence on this logic must be reported in b.linkActionID above as well.
+func (b *Builder) link(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkActionID(a), a.Package.Target) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if err := BuildToolchain.ld(b, a, a.Target, importcfg, a.Deps[0].built); err != nil {
+		return err
+	}
+
+	// Update the binary with the final build ID.
+	// But if OmitDebug is set, don't rewrite the binary, because we set OmitDebug
+	// on binaries that we are going to run and then delete.
+	// There's no point in doing work on such a binary.
+	// Worse, opening the binary for write here makes it
+	// essentially impossible to safely fork+exec due to a fundamental
+	// incompatibility between ETXTBSY and threads on modern Unix systems.
+	// See golang.org/issue/22220.
+	// We still call updateBuildID to update a.buildID, which is important
+	// for test result caching, but passing rewrite=false (final arg)
+	// means we don't actually rewrite the binary, nor store the
+	// result into the cache. That's probably a net win:
+	// less cache space wasted on large binaries we are not likely to
+	// need again. (On the other hand it does make repeated go test slower.)
+	// It also makes repeated go run slower, which is a win in itself:
+	// we don't want people to treat go run like a scripting environment.
+	if err := b.updateBuildID(a, a.Target, !a.Package.Internal.OmitDebug); err != nil {
+		return err
+	}
+
+	a.built = a.Target
+	return nil
+}
+
+func (b *Builder) writeLinkImportcfg(a *Action, file string) error {
+	// Prepare Go import cfg.
+	var icfg bytes.Buffer
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+		if p1.Shlib != "" {
+			fmt.Fprintf(&icfg, "packageshlib %s=%s\n", p1.ImportPath, p1.Shlib)
+		}
+	}
+	return b.writeFile(file, icfg.Bytes())
+}
+
+// PkgconfigCmd returns a pkg-config binary name
+// defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) PkgconfigCmd() string {
+	return envList("PKG_CONFIG", cfg.DefaultPkgConfig)[0]
+}
+
+// splitPkgConfigOutput parses the pkg-config output into a slice of
+// flags. This implements the algorithm from pkgconf/libpkgconf/argvsplit.c.
+func splitPkgConfigOutput(out []byte) ([]string, error) {
+	if len(out) == 0 {
+		return nil, nil
+	}
+	var flags []string
+	flag := make([]byte, 0, len(out))
+	escaped := false
+	quote := byte(0)
+
+	for _, c := range out {
+		if escaped {
+			if quote != 0 {
+				switch c {
+				case '$', '`', '"', '\\':
+				default:
+					flag = append(flag, '\\')
+				}
+				flag = append(flag, c)
+			} else {
+				flag = append(flag, c)
+			}
+			escaped = false
+		} else if quote != 0 {
+			if c == quote {
+				quote = 0
+			} else {
+				switch c {
+				case '\\':
+					escaped = true
+				default:
+					flag = append(flag, c)
+				}
+			}
+		} else if strings.IndexByte(" \t\n\v\f\r", c) < 0 {
+			switch c {
+			case '\\':
+				escaped = true
+			case '\'', '"':
+				quote = c
+			default:
+				flag = append(flag, c)
+			}
+		} else if len(flag) != 0 {
+			flags = append(flags, string(flag))
+			flag = flag[:0]
+		}
+	}
+	if escaped {
+		return nil, errors.New("broken character escaping in pkgconf output ")
+	}
+	if quote != 0 {
+		return nil, errors.New("unterminated quoted string in pkgconf output ")
+	} else if len(flag) != 0 {
+		flags = append(flags, string(flag))
+	}
+
+	return flags, nil
+}
+
+// Calls pkg-config if needed and returns the cflags/ldflags needed to build the package.
+func (b *Builder) getPkgConfigFlags(p *load.Package) (cflags, ldflags []string, err error) {
+	if pcargs := p.CgoPkgConfig; len(pcargs) > 0 {
+		// pkg-config permits arguments to appear anywhere in
+		// the command line. Move them all to the front, before --.
+		var pcflags []string
+		var pkgs []string
+		for _, pcarg := range pcargs {
+			if pcarg == "--" {
+				// We're going to add our own "--" argument.
+			} else if strings.HasPrefix(pcarg, "--") {
+				pcflags = append(pcflags, pcarg)
+			} else {
+				pkgs = append(pkgs, pcarg)
+			}
+		}
+		for _, pkg := range pkgs {
+			if !load.SafeArg(pkg) {
+				return nil, nil, fmt.Errorf("invalid pkg-config package name: %s", pkg)
+			}
+		}
+		var out []byte
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--cflags", pcflags, "--", pkgs)
+		if err != nil {
+			b.showOutput(nil, p.Dir, b.PkgconfigCmd()+" --cflags "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
+			b.Print(err.Error() + "\n")
+			return nil, nil, errPrintedOutput
+		}
+		if len(out) > 0 {
+			cflags, err = splitPkgConfigOutput(out)
+			if err != nil {
+				return nil, nil, err
+			}
+			if err := checkCompilerFlags("CFLAGS", "pkg-config --cflags", cflags); err != nil {
+				return nil, nil, err
+			}
+		}
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--libs", pcflags, "--", pkgs)
+		if err != nil {
+			b.showOutput(nil, p.Dir, b.PkgconfigCmd()+" --libs "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
+			b.Print(err.Error() + "\n")
+			return nil, nil, errPrintedOutput
+		}
+		if len(out) > 0 {
+			ldflags = strings.Fields(string(out))
+			if err := checkLinkerFlags("LDFLAGS", "pkg-config --libs", ldflags); err != nil {
+				return nil, nil, err
+			}
+		}
+	}
+
+	return
+}
+
+func (b *Builder) installShlibname(ctx context.Context, a *Action) error {
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// TODO: BuildN
+	a1 := a.Deps[0]
+	err := os.WriteFile(a.Target, []byte(filepath.Base(a1.Target)+"\n"), 0666)
+	if err != nil {
+		return err
+	}
+	if cfg.BuildX {
+		b.Showcmd("", "echo '%s' > %s # internal", filepath.Base(a1.Target), a.Target)
+	}
+	return nil
+}
+
+func (b *Builder) linkSharedActionID(a *Action) cache.ActionID {
+	h := cache.NewHash("linkShared")
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "linkShared\n")
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+
+	// Toolchain-dependent configuration, shared with b.linkActionID.
+	b.printLinkerConfig(h, nil)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if a1.built == "" {
+			continue
+		}
+		if p1 != nil {
+			fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+	// Files named on command line are special.
+	for _, a1 := range a.Deps[0].Deps {
+		p1 := a1.Package
+		fmt.Fprintf(h, "top %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+	}
+
+	return h.Sum()
+}
+
+func (b *Builder) linkShared(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkSharedActionID(a), a.Target) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	// TODO(rsc): There is a missing updateBuildID here,
+	// but we have to decide where to store the build ID in these files.
+	a.built = a.Target
+	return BuildToolchain.ldShared(b, a, a.Deps[0].Deps, a.Target, importcfg, a.Deps)
+}
+
+// BuildInstallFunc is the action for installing a single package or executable.
+func BuildInstallFunc(b *Builder, ctx context.Context, a *Action) (err error) {
+	defer func() {
+		if err != nil && err != errPrintedOutput {
+			// a.Package == nil is possible for the go install -buildmode=shared
+			// action that installs libmangledname.so, which corresponds to
+			// a list of packages, not just one.
+			sep, path := "", ""
+			if a.Package != nil {
+				sep, path = " ", a.Package.ImportPath
+			}
+			err = fmt.Errorf("go %s%s%s: %v", cfg.CmdName, sep, path, err)
+		}
+	}()
+
+	a1 := a.Deps[0]
+	a.buildID = a1.buildID
+	if a.json != nil {
+		a.json.BuildID = a.buildID
+	}
+
+	// If we are using the eventual install target as an up-to-date
+	// cached copy of the thing we built, then there's no need to
+	// copy it into itself (and that would probably fail anyway).
+	// In this case a1.built == a.Target because a1.built == p.Target,
+	// so the built target is not in the a1.Objdir tree that b.cleanup(a1) removes.
+	if a1.built == a.Target {
+		a.built = a.Target
+		if !a.buggyInstall {
+			b.cleanup(a1)
+		}
+		// Whether we're smart enough to avoid a complete rebuild
+		// depends on exactly what the staleness and rebuild algorithms
+		// are, as well as potentially the state of the Go build cache.
+		// We don't really want users to be able to infer (or worse start depending on)
+		// those details from whether the modification time changes during
+		// "go install", so do a best-effort update of the file times to make it
+		// look like we rewrote a.Target even if we did not. Updating the mtime
+		// may also help other mtime-based systems that depend on our
+		// previous mtime updates that happened more often.
+		// This is still not perfect - we ignore the error result, and if the file was
+		// unwritable for some reason then pretending to have written it is also
+		// confusing - but it's probably better than not doing the mtime update.
+		//
+		// But don't do that for the special case where building an executable
+		// with -linkshared implicitly installs all its dependent libraries.
+		// We want to hide that awful detail as much as possible, so don't
+		// advertise it by touching the mtimes (usually the libraries are up
+		// to date).
+		if !a.buggyInstall && !b.IsCmdList {
+			if cfg.BuildN {
+				b.Showcmd("", "touch %s", a.Target)
+			} else if err := allowInstall(a); err == nil {
+				now := time.Now()
+				os.Chtimes(a.Target, now, now)
+			}
+		}
+		return nil
+	}
+
+	// If we're building for go list -export,
+	// never install anything; just keep the cache reference.
+	if b.IsCmdList {
+		a.built = a1.built
+		return nil
+	}
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	perm := fs.FileMode(0666)
+	if a1.Mode == "link" {
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared", "plugin":
+		default:
+			perm = 0777
+		}
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if !a.buggyInstall {
+		defer b.cleanup(a1)
+	}
+
+	return b.moveOrCopyFile(a.Target, a1.built, perm, false)
+}
+
+// allowInstall returns a non-nil error if this invocation of the go command is
+// allowed to install a.Target.
+//
+// (The build of cmd/go running under its own test is forbidden from installing
+// to its original GOROOT.)
+var allowInstall = func(*Action) error { return nil }
+
+// cleanup removes a's object dir to keep the amount of
+// on-disk garbage down in a large build. On an operating system
+// with aggressive buffering, cleaning incrementally like
+// this keeps the intermediate objects from hitting the disk.
+func (b *Builder) cleanup(a *Action) {
+	if !cfg.BuildWork {
+		if cfg.BuildX {
+			// Don't say we are removing the directory if
+			// we never created it.
+			if _, err := os.Stat(a.Objdir); err == nil || cfg.BuildN {
+				b.Showcmd("", "rm -r %s", a.Objdir)
+			}
+		}
+		os.RemoveAll(a.Objdir)
+	}
+}
+
+// moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
+func (b *Builder) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN {
+		b.Showcmd("", "mv %s %s", src, dst)
+		return nil
+	}
+
+	// If we can update the mode and rename to the dst, do it.
+	// Otherwise fall back to standard copy.
+
+	// If the source is in the build cache, we need to copy it.
+	if strings.HasPrefix(src, cache.DefaultDir()) {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// On Windows, always copy the file, so that we respect the NTFS
+	// permissions of the parent folder. https://golang.org/issue/22343.
+	// What matters here is not cfg.Goos (the system we are building
+	// for) but runtime.GOOS (the system we are building on).
+	if runtime.GOOS == "windows" {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// If the destination directory has the group sticky bit set,
+	// we have to copy the file to retain the correct permissions.
+	// https://golang.org/issue/18878
+	if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
+		if fi.IsDir() && (fi.Mode()&fs.ModeSetgid) != 0 {
+			return b.copyFile(dst, src, perm, force)
+		}
+	}
+
+	// The perm argument is meant to be adjusted according to umask,
+	// but we don't know what the umask is.
+	// Create a dummy file to find out.
+	// This avoids build tags and works even on systems like Plan 9
+	// where the file mask computation incorporates other information.
+	mode := perm
+	f, err := os.OpenFile(filepath.Clean(dst)+"-go-tmp-umask", os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
+	if err == nil {
+		fi, err := f.Stat()
+		if err == nil {
+			mode = fi.Mode() & 0777
+		}
+		name := f.Name()
+		f.Close()
+		os.Remove(name)
+	}
+
+	if err := os.Chmod(src, mode); err == nil {
+		if err := os.Rename(src, dst); err == nil {
+			if cfg.BuildX {
+				b.Showcmd("", "mv %s %s", src, dst)
+			}
+			return nil
+		}
+	}
+
+	return b.copyFile(dst, src, perm, force)
+}
+
+// copyFile is like 'cp src dst'.
+func (b *Builder) copyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cp %s %s", src, dst)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	sf, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer sf.Close()
+
+	// Be careful about removing/overwriting dst.
+	// Do not remove/overwrite if dst exists and is a directory
+	// or a non-empty non-object file.
+	if fi, err := os.Stat(dst); err == nil {
+		if fi.IsDir() {
+			return fmt.Errorf("build output %q already exists and is a directory", dst)
+		}
+		if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
+			return fmt.Errorf("build output %q already exists and is not an object file", dst)
+		}
+	}
+
+	// On Windows, remove lingering ~ file from last attempt.
+	if base.ToolIsWindows {
+		if _, err := os.Stat(dst + "~"); err == nil {
+			os.Remove(dst + "~")
+		}
+	}
+
+	mayberemovefile(dst)
+	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil && base.ToolIsWindows {
+		// Windows does not allow deletion of a binary file
+		// while it is executing. Try to move it out of the way.
+		// If the move fails, which is likely, we'll try again the
+		// next time we do an install of this binary.
+		if err := os.Rename(dst, dst+"~"); err == nil {
+			os.Remove(dst + "~")
+		}
+		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	}
+	if err != nil {
+		return fmt.Errorf("copying %s: %w", src, err) // err should already refer to dst
+	}
+
+	_, err = io.Copy(df, sf)
+	df.Close()
+	if err != nil {
+		mayberemovefile(dst)
+		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
+	}
+	return nil
+}
+
+// writeFile writes the text to file.
+func (b *Builder) writeFile(file string, text []byte) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	return os.WriteFile(file, text, 0666)
+}
+
+// Install the cgo export header file, if there is one.
+func (b *Builder) installHeader(ctx context.Context, a *Action) error {
+	src := a.Objdir + "_cgo_install.h"
+	if _, err := os.Stat(src); os.IsNotExist(err) {
+		// If the file does not exist, there are no exported
+		// functions, and we do not install anything.
+		// TODO(rsc): Once we know that caching is rebuilding
+		// at the right times (not missing rebuilds), here we should
+		// probably delete the installed header, if any.
+		if cfg.BuildX {
+			b.Showcmd("", "# %s not created", src)
+		}
+		return nil
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	return b.moveOrCopyFile(a.Target, src, 0666, true)
+}
+
+// cover runs, in effect,
+//	go tool cover -mode=b.coverMode -var="varName" -o dst.go src.go
+func (b *Builder) cover(a *Action, dst, src string, varName string) error {
+	return b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
+		cfg.BuildToolexec,
+		base.Tool("cover"),
+		"-mode", a.Package.Internal.CoverMode,
+		"-var", varName,
+		"-o", dst,
+		src)
+}
+
+var objectMagic = [][]byte{
+	{'!', '<', 'a', 'r', 'c', 'h', '>', '\n'}, // Package archive
+	{'<', 'b', 'i', 'g', 'a', 'f', '>', '\n'}, // Package AIX big archive
+	{'\x7F', 'E', 'L', 'F'},                   // ELF
+	{0xFE, 0xED, 0xFA, 0xCE},                  // Mach-O big-endian 32-bit
+	{0xFE, 0xED, 0xFA, 0xCF},                  // Mach-O big-endian 64-bit
+	{0xCE, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 32-bit
+	{0xCF, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 64-bit
+	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
+	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
+	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
+	{0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
+	{0x00, 0x61, 0x73, 0x6D},                  // WASM
+	{0x01, 0xDF},                              // XCOFF 32bit
+	{0x01, 0xF7},                              // XCOFF 64bit
+}
+
+func isObject(s string) bool {
+	f, err := os.Open(s)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	buf := make([]byte, 64)
+	io.ReadFull(f, buf)
+	for _, magic := range objectMagic {
+		if bytes.HasPrefix(buf, magic) {
+			return true
+		}
+	}
+	return false
+}
+
+// mayberemovefile removes a file only if it is a regular file
+// When running as a user with sufficient privileges, we may delete
+// even device files, for example, which is not intended.
+func mayberemovefile(s string) {
+	if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
+		return
+	}
+	os.Remove(s)
+}
+
+// fmtcmd formats a command in the manner of fmt.Sprintf but also:
+//
+//	If dir is non-empty and the script is not in dir right now,
+//	fmtcmd inserts "cd dir\n" before the command.
+//
+//	fmtcmd replaces the value of b.WorkDir with $WORK.
+//	fmtcmd replaces the value of goroot with $GOROOT.
+//	fmtcmd replaces the value of b.gobin with $GOBIN.
+//
+//	fmtcmd replaces the name of the current directory with dot (.)
+//	but only when it is at the beginning of a space-separated token.
+//
+func (b *Builder) fmtcmd(dir string, format string, args ...interface{}) string {
+	cmd := fmt.Sprintf(format, args...)
+	if dir != "" && dir != "/" {
+		dot := " ."
+		if dir[len(dir)-1] == filepath.Separator {
+			dot += string(filepath.Separator)
+		}
+		cmd = strings.ReplaceAll(" "+cmd, " "+dir, dot)[1:]
+		if b.scriptDir != dir {
+			b.scriptDir = dir
+			cmd = "cd " + dir + "\n" + cmd
+		}
+	}
+	if b.WorkDir != "" {
+		cmd = strings.ReplaceAll(cmd, b.WorkDir, "$WORK")
+		escaped := strconv.Quote(b.WorkDir)
+		escaped = escaped[1 : len(escaped)-1] // strip quote characters
+		if escaped != b.WorkDir {
+			cmd = strings.ReplaceAll(cmd, escaped, "$WORK")
+		}
+	}
+	return cmd
+}
+
+// showcmd prints the given command to standard output
+// for the implementation of -n or -x.
+func (b *Builder) Showcmd(dir string, format string, args ...interface{}) {
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(b.fmtcmd(dir, format, args...) + "\n")
+}
+
+// showOutput prints "# desc" followed by the given output.
+// The output is expected to contain references to 'dir', usually
+// the source directory for the package that has failed to build.
+// showOutput rewrites mentions of dir with a relative path to dir
+// when the relative path is shorter. This is usually more pleasant.
+// For example, if fmt doesn't compile and we are in src/html,
+// the output is
+//
+//	$ go build
+//	# fmt
+//	../fmt/print.go:1090: undefined: asdf
+//	$
+//
+// instead of
+//
+//	$ go build
+//	# fmt
+//	/usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
+//	$
+//
+// showOutput also replaces references to the work directory with $WORK.
+//
+// If a is not nil and a.output is not nil, showOutput appends to that slice instead of
+// printing to b.Print.
+//
+func (b *Builder) showOutput(a *Action, dir, desc, out string) {
+	prefix := "# " + desc
+	suffix := "\n" + out
+	if reldir := base.ShortPath(dir); reldir != dir {
+		suffix = strings.ReplaceAll(suffix, " "+dir, " "+reldir)
+		suffix = strings.ReplaceAll(suffix, "\n"+dir, "\n"+reldir)
+	}
+	suffix = strings.ReplaceAll(suffix, " "+b.WorkDir, " $WORK")
+
+	if a != nil && a.output != nil {
+		a.output = append(a.output, prefix...)
+		a.output = append(a.output, suffix...)
+		return
+	}
+
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(prefix, suffix)
+}
+
+// errPrintedOutput is a special error indicating that a command failed
+// but that it generated output as well, and that output has already
+// been printed, so there's no point showing 'exit status 1' or whatever
+// the wait status was. The main executor, builder.do, knows not to
+// print this error.
+var errPrintedOutput = errors.New("already printed output - no need to show error")
+
+var cgoLine = lazyregexp.New(`\[[^\[\]]+\.(cgo1|cover)\.go:[0-9]+(:[0-9]+)?\]`)
+var cgoTypeSigRe = lazyregexp.New(`\b_C2?(type|func|var|macro)_\B`)
+
+// run runs the command given by cmdline in the directory dir.
+// If the command fails, run prints information about the failure
+// and returns a non-nil error.
+func (b *Builder) run(a *Action, dir string, desc string, env []string, cmdargs ...interface{}) error {
+	out, err := b.runOut(a, dir, env, cmdargs...)
+	if len(out) > 0 {
+		if desc == "" {
+			desc = b.fmtcmd(dir, "%s", strings.Join(str.StringList(cmdargs...), " "))
+		}
+		b.showOutput(a, dir, desc, b.processOutput(out))
+		if err != nil {
+			err = errPrintedOutput
+		}
+	}
+	return err
+}
+
+// processOutput prepares the output of runOut to be output to the console.
+func (b *Builder) processOutput(out []byte) string {
+	if out[len(out)-1] != '\n' {
+		out = append(out, '\n')
+	}
+	messages := string(out)
+	// Fix up output referring to cgo-generated code to be more readable.
+	// Replace x.go:19[/tmp/.../x.cgo1.go:18] with x.go:19.
+	// Replace *[100]_Ctype_foo with *[100]C.foo.
+	// If we're using -x, assume we're debugging and want the full dump, so disable the rewrite.
+	if !cfg.BuildX && cgoLine.MatchString(messages) {
+		messages = cgoLine.ReplaceAllString(messages, "")
+		messages = cgoTypeSigRe.ReplaceAllString(messages, "C.")
+	}
+	return messages
+}
+
+// runOut runs the command given by cmdline in the directory dir.
+// It returns the command output and any errors that occurred.
+// It accumulates execution time in a.
+func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...interface{}) ([]byte, error) {
+	cmdline := str.StringList(cmdargs...)
+
+	for _, arg := range cmdline {
+		// GNU binutils commands, including gcc and gccgo, interpret an argument
+		// @foo anywhere in the command line (even following --) as meaning
+		// "read and insert arguments from the file named foo."
+		// Don't say anything that might be misinterpreted that way.
+		if strings.HasPrefix(arg, "@") {
+			return nil, fmt.Errorf("invalid command-line argument %s in command: %s", arg, joinUnambiguously(cmdline))
+		}
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		var envcmdline string
+		for _, e := range env {
+			if j := strings.IndexByte(e, '='); j != -1 {
+				if strings.ContainsRune(e[j+1:], '\'') {
+					envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
+				} else {
+					envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
+				}
+				envcmdline += " "
+			}
+		}
+		envcmdline += joinUnambiguously(cmdline)
+		b.Showcmd(dir, "%s", envcmdline)
+		if cfg.BuildN {
+			return nil, nil
+		}
+	}
+
+	var buf bytes.Buffer
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	if cmd.Path != "" {
+		cmd.Args[0] = cmd.Path
+	}
+	cmd.Stdout = &buf
+	cmd.Stderr = &buf
+	cleanup := passLongArgsInResponseFiles(cmd)
+	defer cleanup()
+	cmd.Dir = dir
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+
+	// Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
+	// It doesn't really matter if -toolexec isn't being used.
+	if a != nil && a.Package != nil {
+		cmd.Env = append(cmd.Env, "TOOLEXEC_IMPORTPATH="+a.Package.ImportPath)
+	}
+
+	cmd.Env = append(cmd.Env, env...)
+	start := time.Now()
+	err := cmd.Run()
+	if a != nil && a.json != nil {
+		aj := a.json
+		aj.Cmd = append(aj.Cmd, joinUnambiguously(cmdline))
+		aj.CmdReal += time.Since(start)
+		if ps := cmd.ProcessState; ps != nil {
+			aj.CmdUser += ps.UserTime()
+			aj.CmdSys += ps.SystemTime()
+		}
+	}
+
+	// err can be something like 'exit status 1'.
+	// Add information about what program was running.
+	// Note that if buf.Bytes() is non-empty, the caller usually
+	// shows buf.Bytes() and does not print err at all, so the
+	// prefix here does not make most output any more verbose.
+	if err != nil {
+		err = errors.New(cmdline[0] + ": " + err.Error())
+	}
+	return buf.Bytes(), err
+}
+
+// joinUnambiguously prints the slice, quoting where necessary to make the
+// output unambiguous.
+// TODO: See issue 5279. The printing of commands needs a complete redo.
+func joinUnambiguously(a []string) string {
+	var buf bytes.Buffer
+	for i, s := range a {
+		if i > 0 {
+			buf.WriteByte(' ')
+		}
+		q := strconv.Quote(s)
+		// A gccgo command line can contain -( and -).
+		// Make sure we quote them since they are special to the shell.
+		// The trimpath argument can also contain > (part of =>) and ;. Quote those too.
+		if s == "" || strings.ContainsAny(s, " ()>;") || len(q) > len(s)+2 {
+			buf.WriteString(q)
+		} else {
+			buf.WriteString(s)
+		}
+	}
+	return buf.String()
+}
+
+// cCompilerEnv returns environment variables to set when running the
+// C compiler. This is needed to disable escape codes in clang error
+// messages that confuse tools like cgo.
+func (b *Builder) cCompilerEnv() []string {
+	return []string{"TERM=dumb"}
+}
+
+// mkdir makes the named directory.
+func (b *Builder) Mkdir(dir string) error {
+	// Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
+	if dir == "" {
+		return nil
+	}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	// We can be a little aggressive about being
+	// sure directories exist. Skip repeated calls.
+	if b.mkdirCache[dir] {
+		return nil
+	}
+	b.mkdirCache[dir] = true
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "mkdir -p %s", dir)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return err
+	}
+	return nil
+}
+
+// symlink creates a symlink newname -> oldname.
+func (b *Builder) Symlink(oldname, newname string) error {
+	// It's not an error to try to recreate an existing symlink.
+	if link, err := os.Readlink(newname); err == nil && link == oldname {
+		return nil
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "ln -s %s %s", oldname, newname)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+	return os.Symlink(oldname, newname)
+}
+
+// mkAbs returns an absolute path corresponding to
+// evaluating f in the directory dir.
+// We always pass absolute paths of source files so that
+// the error messages will include the full path to a file
+// in need of attention.
+func mkAbs(dir, f string) string {
+	// Leave absolute paths alone.
+	// Also, during -n mode we use the pseudo-directory $WORK
+	// instead of creating an actual work directory that won't be used.
+	// Leave paths beginning with $WORK alone too.
+	if filepath.IsAbs(f) || strings.HasPrefix(f, "$WORK") {
+		return f
+	}
+	return filepath.Join(dir, f)
+}
+
+type toolchain interface {
+	// gc runs the compiler in a specific directory on a set of files
+	// and returns the name of the generated output file.
+	gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error)
+	// cc runs the toolchain's C compiler in a directory on a C file
+	// to produce an output file.
+	cc(b *Builder, a *Action, ofile, cfile string) error
+	// asm runs the assembler in a specific directory on specific files
+	// and returns a list of named output files.
+	asm(b *Builder, a *Action, sfiles []string) ([]string, error)
+	// symabis scans the symbol ABIs from sfiles and returns the
+	// path to the output symbol ABIs file, or "" if none.
+	symabis(b *Builder, a *Action, sfiles []string) (string, error)
+	// pack runs the archive packer in a specific directory to create
+	// an archive from a set of object files.
+	// typically it is run in the object directory.
+	pack(b *Builder, a *Action, afile string, ofiles []string) error
+	// ld runs the linker to create an executable starting at mainpkg.
+	ld(b *Builder, root *Action, out, importcfg, mainpkg string) error
+	// ldShared runs the linker to create a shared library containing the pkgs built by toplevelactions
+	ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error
+
+	compiler() string
+	linker() string
+}
+
+type noToolchain struct{}
+
+func noCompiler() error {
+	log.Fatalf("unknown compiler %q", cfg.BuildContext.Compiler)
+	return nil
+}
+
+func (noToolchain) compiler() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) linker() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error) {
+	return "", nil, noCompiler()
+}
+
+func (noToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	return nil, noCompiler()
+}
+
+func (noToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	return "", noCompiler()
+}
+
+func (noToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	return noCompiler()
+}
+
+func (noToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	return noCompiler()
+}
+
+// gcc runs the gcc C compiler to create an object from a single C file.
+func (b *Builder) gcc(a *Action, p *load.Package, workdir, out string, flags []string, cfile string) error {
+	return b.ccompile(a, p, out, flags, cfile, b.GccCmd(p.Dir, workdir))
+}
+
+// gxx runs the g++ C++ compiler to create an object from a single C++ file.
+func (b *Builder) gxx(a *Action, p *load.Package, workdir, out string, flags []string, cxxfile string) error {
+	return b.ccompile(a, p, out, flags, cxxfile, b.GxxCmd(p.Dir, workdir))
+}
+
+// gfortran runs the gfortran Fortran compiler to create an object from a single Fortran file.
+func (b *Builder) gfortran(a *Action, p *load.Package, workdir, out string, flags []string, ffile string) error {
+	return b.ccompile(a, p, out, flags, ffile, b.gfortranCmd(p.Dir, workdir))
+}
+
+// ccompile runs the given C or C++ compiler and creates an object from a single source file.
+func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []string, file string, compiler []string) error {
+	file = mkAbs(p.Dir, file)
+	desc := p.ImportPath
+	outfile = mkAbs(p.Dir, outfile)
+
+	// Elide source directory paths if -trimpath or GOROOT_FINAL is set.
+	// This is needed for source files (e.g., a .c file in a package directory).
+	// TODO(golang.org/issue/36072): cgo also generates files with #line
+	// directives pointing to the source directory. It should not generate those
+	// when -trimpath is enabled.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if cfg.BuildTrimpath {
+			// Keep in sync with Action.trimpath.
+			// The trimmed paths are a little different, but we need to trim in the
+			// same situations.
+			var from, toPath string
+			if m := p.Module; m != nil {
+				from = m.Dir
+				toPath = m.Path + "@" + m.Version
+			} else {
+				from = p.Dir
+				toPath = p.ImportPath
+			}
+			// -fdebug-prefix-map requires an absolute "to" path (or it joins the path
+			// with the working directory). Pick something that makes sense for the
+			// target platform.
+			var to string
+			if cfg.BuildContext.GOOS == "windows" {
+				to = filepath.Join(`\\_\_`, toPath)
+			} else {
+				to = filepath.Join("/_", toPath)
+			}
+			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+from+"="+to)
+		} else if p.Goroot && cfg.GOROOT_FINAL != cfg.GOROOT {
+			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+cfg.GOROOT+"="+cfg.GOROOT_FINAL)
+		}
+	}
+
+	overlayPath := file
+	if p, ok := a.nonGoOverlay[overlayPath]; ok {
+		overlayPath = p
+	}
+	output, err := b.runOut(a, filepath.Dir(overlayPath), b.cCompilerEnv(), compiler, flags, "-o", outfile, "-c", filepath.Base(overlayPath))
+	if len(output) > 0 {
+		// On FreeBSD 11, when we pass -g to clang 3.8 it
+		// invokes its internal assembler with -dwarf-version=2.
+		// When it sees .section .note.GNU-stack, it warns
+		// "DWARF2 only supports one section per compilation unit".
+		// This warning makes no sense, since the section is empty,
+		// but it confuses people.
+		// We work around the problem by detecting the warning
+		// and dropping -g and trying again.
+		if bytes.Contains(output, []byte("DWARF2 only supports one section per compilation unit")) {
+			newFlags := make([]string, 0, len(flags))
+			for _, f := range flags {
+				if !strings.HasPrefix(f, "-g") {
+					newFlags = append(newFlags, f)
+				}
+			}
+			if len(newFlags) < len(flags) {
+				return b.ccompile(a, p, outfile, newFlags, file, compiler)
+			}
+		}
+
+		b.showOutput(a, p.Dir, desc, b.processOutput(output))
+		if err != nil {
+			err = errPrintedOutput
+		} else if os.Getenv("GO_BUILDER_NAME") != "" {
+			return errors.New("C compiler warning promoted to error on Go builders")
+		}
+	}
+	return err
+}
+
+// gccld runs the gcc linker to create an executable from a set of object files.
+func (b *Builder) gccld(a *Action, p *load.Package, objdir, outfile string, flags []string, objs []string) error {
+	var cmd []string
+	if len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0 {
+		cmd = b.GxxCmd(p.Dir, objdir)
+	} else {
+		cmd = b.GccCmd(p.Dir, objdir)
+	}
+
+	cmdargs := []interface{}{cmd, "-o", outfile, objs, flags}
+	dir := p.Dir
+	out, err := b.runOut(a, base.Cwd, b.cCompilerEnv(), cmdargs...)
+
+	if len(out) > 0 {
+		// Filter out useless linker warnings caused by bugs outside Go.
+		// See also cmd/link/internal/ld's hostlink method.
+		var save [][]byte
+		var skipLines int
+		for _, line := range bytes.SplitAfter(out, []byte("\n")) {
+			// golang.org/issue/26073 - Apple Xcode bug
+			if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
+				continue
+			}
+
+			if skipLines > 0 {
+				skipLines--
+				continue
+			}
+
+			// Remove duplicate main symbol with runtime/cgo on AIX.
+			// With runtime/cgo, two main are available:
+			// One is generated by cgo tool with {return 0;}.
+			// The other one is the main calling runtime.rt0_go
+			// in runtime/cgo.
+			// The second can't be used by cgo programs because
+			// runtime.rt0_go is unknown to them.
+			// Therefore, we let ld remove this main version
+			// and used the cgo generated one.
+			if p.ImportPath == "runtime/cgo" && bytes.Contains(line, []byte("ld: 0711-224 WARNING: Duplicate symbol: .main")) {
+				skipLines = 1
+				continue
+			}
+
+			save = append(save, line)
+		}
+		out = bytes.Join(save, nil)
+		if len(out) > 0 {
+			b.showOutput(nil, dir, p.ImportPath, b.processOutput(out))
+			if err != nil {
+				err = errPrintedOutput
+			}
+		}
+	}
+	return err
+}
+
+// Grab these before main helpfully overwrites them.
+var (
+	origCC  = cfg.Getenv("CC")
+	origCXX = cfg.Getenv("CXX")
+)
+
+// gccCmd returns a gcc command line prefix
+// defaultCC is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GccCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.ccExe(), incdir, workdir)
+}
+
+// gxxCmd returns a g++ command line prefix
+// defaultCXX is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GxxCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.cxxExe(), incdir, workdir)
+}
+
+// gfortranCmd returns a gfortran command line prefix.
+func (b *Builder) gfortranCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.fcExe(), incdir, workdir)
+}
+
+// ccExe returns the CC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) ccExe() []string {
+	return b.compilerExe(origCC, cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+}
+
+// cxxExe returns the CXX compiler setting without all the extra flags we add implicitly.
+func (b *Builder) cxxExe() []string {
+	return b.compilerExe(origCXX, cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+}
+
+// fcExe returns the FC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) fcExe() []string {
+	return b.compilerExe(cfg.Getenv("FC"), "gfortran")
+}
+
+// compilerExe returns the compiler to use given an
+// environment variable setting (the value not the name)
+// and a default. The resulting slice is usually just the name
+// of the compiler but can have additional arguments if they
+// were present in the environment value.
+// For example if CC="gcc -DGOPHER" then the result is ["gcc", "-DGOPHER"].
+func (b *Builder) compilerExe(envValue string, def string) []string {
+	compiler := strings.Fields(envValue)
+	if len(compiler) == 0 {
+		compiler = strings.Fields(def)
+	}
+	return compiler
+}
+
+// compilerCmd returns a command line prefix for the given environment
+// variable and using the default command when the variable is empty.
+func (b *Builder) compilerCmd(compiler []string, incdir, workdir string) []string {
+	// NOTE: env.go's mkEnv knows that the first three
+	// strings returned are "gcc", "-I", incdir (and cuts them off).
+	a := []string{compiler[0], "-I", incdir}
+	a = append(a, compiler[1:]...)
+
+	// Definitely want -fPIC but on Windows gcc complains
+	// "-fPIC ignored for target (all code is position independent)"
+	if cfg.Goos != "windows" {
+		a = append(a, "-fPIC")
+	}
+	a = append(a, b.gccArchArgs()...)
+	// gcc-4.5 and beyond require explicit "-pthread" flag
+	// for multithreading with pthread library.
+	if cfg.BuildContext.CgoEnabled {
+		switch cfg.Goos {
+		case "windows":
+			a = append(a, "-mthreads")
+		default:
+			a = append(a, "-pthread")
+		}
+	}
+
+	if cfg.Goos == "aix" {
+		// mcmodel=large must always be enabled to allow large TOC.
+		a = append(a, "-mcmodel=large")
+	}
+
+	// disable ASCII art in clang errors, if possible
+	if b.gccSupportsFlag(compiler, "-fno-caret-diagnostics") {
+		a = append(a, "-fno-caret-diagnostics")
+	}
+	// clang is too smart about command-line arguments
+	if b.gccSupportsFlag(compiler, "-Qunused-arguments") {
+		a = append(a, "-Qunused-arguments")
+	}
+
+	// disable word wrapping in error messages
+	a = append(a, "-fmessage-length=0")
+
+	// Tell gcc not to include the work directory in object files.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if workdir == "" {
+			workdir = b.WorkDir
+		}
+		workdir = strings.TrimSuffix(workdir, string(filepath.Separator))
+		a = append(a, "-fdebug-prefix-map="+workdir+"=/tmp/go-build")
+	}
+
+	// Tell gcc not to include flags in object files, which defeats the
+	// point of -fdebug-prefix-map above.
+	if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
+		a = append(a, "-gno-record-gcc-switches")
+	}
+
+	// On OS X, some of the compilers behave as if -fno-common
+	// is always set, and the Mach-O linker in 6l/8l assumes this.
+	// See https://golang.org/issue/3253.
+	if cfg.Goos == "darwin" || cfg.Goos == "ios" {
+		a = append(a, "-fno-common")
+	}
+
+	return a
+}
+
+// gccNoPie returns the flag to use to request non-PIE. On systems
+// with PIE (position independent executables) enabled by default,
+// -no-pie must be passed when doing a partial link with -Wl,-r.
+// But -no-pie is not supported by all compilers, and clang spells it -nopie.
+func (b *Builder) gccNoPie(linker []string) string {
+	if b.gccSupportsFlag(linker, "-no-pie") {
+		return "-no-pie"
+	}
+	if b.gccSupportsFlag(linker, "-nopie") {
+		return "-nopie"
+	}
+	return ""
+}
+
+// gccSupportsFlag checks to see if the compiler supports a flag.
+func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
+	key := [2]string{compiler[0], flag}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	if b, ok := b.flagCache[key]; ok {
+		return b
+	}
+	if b.flagCache == nil {
+		b.flagCache = make(map[[2]string]bool)
+	}
+
+	tmp := os.DevNull
+	if runtime.GOOS == "windows" {
+		f, err := os.CreateTemp(b.WorkDir, "")
+		if err != nil {
+			return false
+		}
+		f.Close()
+		tmp = f.Name()
+		defer os.Remove(tmp)
+	}
+
+	// We used to write an empty C file, but that gets complicated with
+	// go build -n. We tried using a file that does not exist, but that
+	// fails on systems with GCC version 4.2.1; that is the last GPLv2
+	// version of GCC, so some systems have frozen on it.
+	// Now we pass an empty file on stdin, which should work at least for
+	// GCC and clang.
+	cmdArgs := str.StringList(compiler, flag, "-c", "-x", "c", "-", "-o", tmp)
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
+		if cfg.BuildN {
+			return false
+		}
+	}
+	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
+	cmd.Dir = b.WorkDir
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+	cmd.Env = append(cmd.Env, "LC_ALL=C")
+	out, _ := cmd.CombinedOutput()
+	// GCC says "unrecognized command line option".
+	// clang says "unknown argument".
+	// Older versions of GCC say "unrecognised debug output level".
+	// For -fsplit-stack GCC says "'-fsplit-stack' is not supported".
+	supported := !bytes.Contains(out, []byte("unrecognized")) &&
+		!bytes.Contains(out, []byte("unknown")) &&
+		!bytes.Contains(out, []byte("unrecognised")) &&
+		!bytes.Contains(out, []byte("is not supported"))
+	b.flagCache[key] = supported
+	return supported
+}
+
+// gccArchArgs returns arguments to pass to gcc based on the architecture.
+func (b *Builder) gccArchArgs() []string {
+	switch cfg.Goarch {
+	case "386":
+		return []string{"-m32"}
+	case "amd64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
+		return []string{"-m64"}
+	case "arm64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
+	case "arm":
+		return []string{"-marm"} // not thumb
+	case "s390x":
+		return []string{"-m64", "-march=z196"}
+	case "mips64", "mips64le":
+		return []string{"-mabi=64"}
+	case "mips", "mipsle":
+		return []string{"-mabi=32", "-march=mips32"}
+	case "ppc64":
+		if cfg.Goos == "aix" {
+			return []string{"-maix64"}
+		}
+	}
+	return nil
+}
+
+// envList returns the value of the given environment variable broken
+// into fields, using the default value when the variable is empty.
+func envList(key, def string) []string {
+	v := cfg.Getenv(key)
+	if v == "" {
+		v = def
+	}
+	return strings.Fields(v)
+}
+
+// CFlags returns the flags to use when invoking the C, C++ or Fortran compilers, or cgo.
+func (b *Builder) CFlags(p *load.Package) (cppflags, cflags, cxxflags, fflags, ldflags []string, err error) {
+	defaults := "-g -O2"
+
+	if cppflags, err = buildFlags("CPPFLAGS", "", p.CgoCPPFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cflags, err = buildFlags("CFLAGS", defaults, p.CgoCFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cxxflags, err = buildFlags("CXXFLAGS", defaults, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if fflags, err = buildFlags("FFLAGS", defaults, p.CgoFFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if ldflags, err = buildFlags("LDFLAGS", defaults, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
+		return
+	}
+
+	return
+}
+
+func buildFlags(name, defaults string, fromPackage []string, check func(string, string, []string) error) ([]string, error) {
+	if err := check(name, "#cgo "+name, fromPackage); err != nil {
+		return nil, err
+	}
+	return str.StringList(envList("CGO_"+name, defaults), fromPackage), nil
+}
+
+var cgoRe = lazyregexp.New(`[/\\:]`)
+
+func (b *Builder) cgo(a *Action, cgoExe, objdir string, pcCFLAGS, pcLDFLAGS, cgofiles, gccfiles, gxxfiles, mfiles, ffiles []string) (outGo, outObj []string, err error) {
+	p := a.Package
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS, cgoLDFLAGS, err := b.CFlags(p)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cgoCPPFLAGS = append(cgoCPPFLAGS, pcCFLAGS...)
+	cgoLDFLAGS = append(cgoLDFLAGS, pcLDFLAGS...)
+	// If we are compiling Objective-C code, then we need to link against libobjc
+	if len(mfiles) > 0 {
+		cgoLDFLAGS = append(cgoLDFLAGS, "-lobjc")
+	}
+
+	// Likewise for Fortran, except there are many Fortran compilers.
+	// Support gfortran out of the box and let others pass the correct link options
+	// via CGO_LDFLAGS
+	if len(ffiles) > 0 {
+		fc := cfg.Getenv("FC")
+		if fc == "" {
+			fc = "gfortran"
+		}
+		if strings.Contains(fc, "gfortran") {
+			cgoLDFLAGS = append(cgoLDFLAGS, "-lgfortran")
+		}
+	}
+
+	if cfg.BuildMSan {
+		cgoCFLAGS = append([]string{"-fsanitize=memory"}, cgoCFLAGS...)
+		cgoLDFLAGS = append([]string{"-fsanitize=memory"}, cgoLDFLAGS...)
+	}
+
+	// Allows including _cgo_export.h, as well as the user's .h files,
+	// from .[ch] files in the package.
+	cgoCPPFLAGS = append(cgoCPPFLAGS, "-I", objdir)
+
+	// cgo
+	// TODO: CGO_FLAGS?
+	gofiles := []string{objdir + "_cgo_gotypes.go"}
+	cfiles := []string{"_cgo_export.c"}
+	for _, fn := range cgofiles {
+		f := strings.TrimSuffix(filepath.Base(fn), ".go")
+		gofiles = append(gofiles, objdir+f+".cgo1.go")
+		cfiles = append(cfiles, f+".cgo2.c")
+	}
+
+	// TODO: make cgo not depend on $GOARCH?
+
+	cgoflags := []string{}
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = append(cgoflags, "-import_runtime_cgo=false")
+	}
+	if p.Standard && (p.ImportPath == "runtime/race" || p.ImportPath == "runtime/msan" || p.ImportPath == "runtime/cgo") {
+		cgoflags = append(cgoflags, "-import_syscall=false")
+	}
+
+	// Update $CGO_LDFLAGS with p.CgoLDFLAGS.
+	// These flags are recorded in the generated _cgo_gotypes.go file
+	// using //go:cgo_ldflag directives, the compiler records them in the
+	// object file for the package, and then the Go linker passes them
+	// along to the host linker. At this point in the code, cgoLDFLAGS
+	// consists of the original $CGO_LDFLAGS (unchecked) and all the
+	// flags put together from source code (checked).
+	cgoenv := b.cCompilerEnv()
+	if len(cgoLDFLAGS) > 0 {
+		flags := make([]string, len(cgoLDFLAGS))
+		for i, f := range cgoLDFLAGS {
+			flags[i] = strconv.Quote(f)
+		}
+		cgoenv = append(cgoenv, "CGO_LDFLAGS="+strings.Join(flags, " "))
+	}
+
+	if cfg.BuildToolchainName == "gccgo" {
+		if b.gccSupportsFlag([]string{BuildToolchain.compiler()}, "-fsplit-stack") {
+			cgoCFLAGS = append(cgoCFLAGS, "-fsplit-stack")
+		}
+		cgoflags = append(cgoflags, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			cgoflags = append(cgoflags, "-gccgopkgpath="+pkgpath)
+		}
+	}
+
+	switch cfg.BuildBuildmode {
+	case "c-archive", "c-shared":
+		// Tell cgo that if there are any exported functions
+		// it should generate a header file that C code can
+		// #include.
+		cgoflags = append(cgoflags, "-exportheader="+objdir+"_cgo_install.h")
+	}
+
+	execdir := p.Dir
+
+	// Rewrite overlaid paths in cgo files.
+	// cgo adds //line and #line pragmas in generated files with these paths.
+	var trimpath []string
+	for i := range cgofiles {
+		path := mkAbs(p.Dir, cgofiles[i])
+		if opath, ok := fsys.OverlayPath(path); ok {
+			cgofiles[i] = opath
+			trimpath = append(trimpath, opath+"=>"+path)
+		}
+	}
+	if len(trimpath) > 0 {
+		cgoflags = append(cgoflags, "-trimpath", strings.Join(trimpath, ";"))
+	}
+
+	if err := b.run(a, execdir, p.ImportPath, cgoenv, cfg.BuildToolexec, cgoExe, "-objdir", objdir, "-importpath", p.ImportPath, cgoflags, "--", cgoCPPFLAGS, cgoCFLAGS, cgofiles); err != nil {
+		return nil, nil, err
+	}
+	outGo = append(outGo, gofiles...)
+
+	// Use sequential object file names to keep them distinct
+	// and short enough to fit in the .a header file name slots.
+	// We no longer collect them all into _all.o, and we'd like
+	// tools to see both the .o suffix and unique names, so
+	// we need to make them short enough not to be truncated
+	// in the final archive.
+	oseq := 0
+	nextOfile := func() string {
+		oseq++
+		return objdir + fmt.Sprintf("_x%03d.o", oseq)
+	}
+
+	// gcc
+	cflags := str.StringList(cgoCPPFLAGS, cgoCFLAGS)
+	for _, cfile := range cfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, objdir+cfile); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range gccfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	cxxflags := str.StringList(cgoCPPFLAGS, cgoCXXFLAGS)
+	for _, file := range gxxfiles {
+		ofile := nextOfile()
+		if err := b.gxx(a, p, a.Objdir, ofile, cxxflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range mfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	fflags := str.StringList(cgoCPPFLAGS, cgoFFLAGS)
+	for _, file := range ffiles {
+		ofile := nextOfile()
+		if err := b.gfortran(a, p, a.Objdir, ofile, fflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	switch cfg.BuildToolchainName {
+	case "gc":
+		importGo := objdir + "_cgo_import.go"
+		if err := b.dynimport(a, p, objdir, importGo, cgoExe, cflags, cgoLDFLAGS, outObj); err != nil {
+			return nil, nil, err
+		}
+		outGo = append(outGo, importGo)
+
+	case "gccgo":
+		defunC := objdir + "_cgo_defun.c"
+		defunObj := objdir + "_cgo_defun.o"
+		if err := BuildToolchain.cc(b, a, defunObj, defunC); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, defunObj)
+
+	default:
+		noCompiler()
+	}
+
+	// Double check the //go:cgo_ldflag comments in the generated files.
+	// The compiler only permits such comments in files whose base name
+	// starts with "_cgo_". Make sure that the comments in those files
+	// are safe. This is a backstop against people somehow smuggling
+	// such a comment into a file generated by cgo.
+	if cfg.BuildToolchainName == "gc" && !cfg.BuildN {
+		var flags []string
+		for _, f := range outGo {
+			if !strings.HasPrefix(filepath.Base(f), "_cgo_") {
+				continue
+			}
+
+			src, err := os.ReadFile(f)
+			if err != nil {
+				return nil, nil, err
+			}
+
+			const cgoLdflag = "//go:cgo_ldflag"
+			idx := bytes.Index(src, []byte(cgoLdflag))
+			for idx >= 0 {
+				// We are looking at //go:cgo_ldflag.
+				// Find start of line.
+				start := bytes.LastIndex(src[:idx], []byte("\n"))
+				if start == -1 {
+					start = 0
+				}
+
+				// Find end of line.
+				end := bytes.Index(src[idx:], []byte("\n"))
+				if end == -1 {
+					end = len(src)
+				} else {
+					end += idx
+				}
+
+				// Check for first line comment in line.
+				// We don't worry about /* */ comments,
+				// which normally won't appear in files
+				// generated by cgo.
+				commentStart := bytes.Index(src[start:], []byte("//"))
+				commentStart += start
+				// If that line comment is //go:cgo_ldflag,
+				// it's a match.
+				if bytes.HasPrefix(src[commentStart:], []byte(cgoLdflag)) {
+					// Pull out the flag, and unquote it.
+					// This is what the compiler does.
+					flag := string(src[idx+len(cgoLdflag) : end])
+					flag = strings.TrimSpace(flag)
+					flag = strings.Trim(flag, `"`)
+					flags = append(flags, flag)
+				}
+				src = src[end:]
+				idx = bytes.Index(src, []byte(cgoLdflag))
+			}
+		}
+
+		// We expect to find the contents of cgoLDFLAGS in flags.
+		if len(cgoLDFLAGS) > 0 {
+		outer:
+			for i := range flags {
+				for j, f := range cgoLDFLAGS {
+					if f != flags[i+j] {
+						continue outer
+					}
+				}
+				flags = append(flags[:i], flags[i+len(cgoLDFLAGS):]...)
+				break
+			}
+		}
+
+		if err := checkLinkerFlags("LDFLAGS", "go:cgo_ldflag", flags); err != nil {
+			return nil, nil, err
+		}
+	}
+
+	return outGo, outObj, nil
+}
+
+// dynimport creates a Go source file named importGo containing
+// //go:cgo_import_dynamic directives for each symbol or library
+// dynamically imported by the object files outObj.
+func (b *Builder) dynimport(a *Action, p *load.Package, objdir, importGo, cgoExe string, cflags, cgoLDFLAGS, outObj []string) error {
+	cfile := objdir + "_cgo_main.c"
+	ofile := objdir + "_cgo_main.o"
+	if err := b.gcc(a, p, objdir, ofile, cflags, cfile); err != nil {
+		return err
+	}
+
+	linkobj := str.StringList(ofile, outObj, mkAbsFiles(p.Dir, p.SysoFiles))
+	dynobj := objdir + "_cgo_.o"
+
+	// we need to use -pie for Linux/ARM to get accurate imported sym
+	ldflags := cgoLDFLAGS
+	if (cfg.Goarch == "arm" && cfg.Goos == "linux") || cfg.Goos == "android" {
+		// -static -pie doesn't make sense, and causes link errors.
+		// Issue 26197.
+		n := make([]string, 0, len(ldflags))
+		for _, flag := range ldflags {
+			if flag != "-static" {
+				n = append(n, flag)
+			}
+		}
+		ldflags = append(n, "-pie")
+	}
+	if err := b.gccld(a, p, objdir, dynobj, ldflags, linkobj); err != nil {
+		return err
+	}
+
+	// cgo -dynimport
+	var cgoflags []string
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = []string{"-dynlinker"} // record path to dynamic linker
+	}
+	return b.run(a, base.Cwd, p.ImportPath, b.cCompilerEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
+}
+
+// Run SWIG on all SWIG input files.
+// TODO: Don't build a shared library, once SWIG emits the necessary
+// pragmas for external linking.
+func (b *Builder) swig(a *Action, p *load.Package, objdir string, pcCFLAGS []string) (outGo, outC, outCXX []string, err error) {
+	if err := b.swigVersionCheck(); err != nil {
+		return nil, nil, nil, err
+	}
+
+	intgosize, err := b.swigIntSize(objdir)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	for _, f := range p.SwigFiles {
+		goFile, cFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, false, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cFile != "" {
+			outC = append(outC, cFile)
+		}
+	}
+	for _, f := range p.SwigCXXFiles {
+		goFile, cxxFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, true, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cxxFile != "" {
+			outCXX = append(outCXX, cxxFile)
+		}
+	}
+	return outGo, outC, outCXX, nil
+}
+
+// Make sure SWIG is new enough.
+var (
+	swigCheckOnce sync.Once
+	swigCheck     error
+)
+
+func (b *Builder) swigDoVersionCheck() error {
+	out, err := b.runOut(nil, "", nil, "swig", "-version")
+	if err != nil {
+		return err
+	}
+	re := regexp.MustCompile(`[vV]ersion +([\d]+)([.][\d]+)?([.][\d]+)?`)
+	matches := re.FindSubmatch(out)
+	if matches == nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+
+	major, err := strconv.Atoi(string(matches[1]))
+	if err != nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+	const errmsg = "must have SWIG version >= 3.0.6"
+	if major < 3 {
+		return errors.New(errmsg)
+	}
+	if major > 3 {
+		// 4.0 or later
+		return nil
+	}
+
+	// We have SWIG version 3.x.
+	if len(matches[2]) > 0 {
+		minor, err := strconv.Atoi(string(matches[2][1:]))
+		if err != nil {
+			return nil
+		}
+		if minor > 0 {
+			// 3.1 or later
+			return nil
+		}
+	}
+
+	// We have SWIG version 3.0.x.
+	if len(matches[3]) > 0 {
+		patch, err := strconv.Atoi(string(matches[3][1:]))
+		if err != nil {
+			return nil
+		}
+		if patch < 6 {
+			// Before 3.0.6.
+			return errors.New(errmsg)
+		}
+	}
+
+	return nil
+}
+
+func (b *Builder) swigVersionCheck() error {
+	swigCheckOnce.Do(func() {
+		swigCheck = b.swigDoVersionCheck()
+	})
+	return swigCheck
+}
+
+// Find the value to pass for the -intgosize option to swig.
+var (
+	swigIntSizeOnce  sync.Once
+	swigIntSize      string
+	swigIntSizeError error
+)
+
+// This code fails to build if sizeof(int) <= 32
+const swigIntSizeCode = `
+package main
+const i int = 1 << 32
+`
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9. Run only once.
+func (b *Builder) swigDoIntSize(objdir string) (intsize string, err error) {
+	if cfg.BuildN {
+		return "$INTBITS", nil
+	}
+	src := filepath.Join(b.WorkDir, "swig_intsize.go")
+	if err = os.WriteFile(src, []byte(swigIntSizeCode), 0666); err != nil {
+		return
+	}
+	srcs := []string{src}
+
+	p := load.GoFilesPackage(context.TODO(), srcs)
+
+	if _, _, e := BuildToolchain.gc(b, &Action{Mode: "swigDoIntSize", Package: p, Objdir: objdir}, "", nil, nil, "", false, srcs); e != nil {
+		return "32", nil
+	}
+	return "64", nil
+}
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9.
+func (b *Builder) swigIntSize(objdir string) (intsize string, err error) {
+	swigIntSizeOnce.Do(func() {
+		swigIntSize, swigIntSizeError = b.swigDoIntSize(objdir)
+	})
+	return swigIntSize, swigIntSizeError
+}
+
+// Run SWIG on one SWIG input file.
+func (b *Builder) swigOne(a *Action, p *load.Package, file, objdir string, pcCFLAGS []string, cxx bool, intgosize string) (outGo, outC string, err error) {
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, _, _, err := b.CFlags(p)
+	if err != nil {
+		return "", "", err
+	}
+
+	var cflags []string
+	if cxx {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCXXFLAGS)
+	} else {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCFLAGS)
+	}
+
+	n := 5 // length of ".swig"
+	if cxx {
+		n = 8 // length of ".swigcxx"
+	}
+	base := file[:len(file)-n]
+	goFile := base + ".go"
+	gccBase := base + "_wrap."
+	gccExt := "c"
+	if cxx {
+		gccExt = "cxx"
+	}
+
+	gccgo := cfg.BuildToolchainName == "gccgo"
+
+	// swig
+	args := []string{
+		"-go",
+		"-cgo",
+		"-intgosize", intgosize,
+		"-module", base,
+		"-o", objdir + gccBase + gccExt,
+		"-outdir", objdir,
+	}
+
+	for _, f := range cflags {
+		if len(f) > 3 && f[:2] == "-I" {
+			args = append(args, f)
+		}
+	}
+
+	if gccgo {
+		args = append(args, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			args = append(args, "-go-pkgpath", pkgpath)
+		}
+	}
+	if cxx {
+		args = append(args, "-c++")
+	}
+
+	out, err := b.runOut(a, p.Dir, nil, "swig", args, file)
+	if err != nil {
+		if len(out) > 0 {
+			if bytes.Contains(out, []byte("-intgosize")) || bytes.Contains(out, []byte("-cgo")) {
+				return "", "", errors.New("must have SWIG version >= 3.0.6")
+			}
+			b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig error
+			return "", "", errPrintedOutput
+		}
+		return "", "", err
+	}
+	if len(out) > 0 {
+		b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig warning
+	}
+
+	// If the input was x.swig, the output is x.go in the objdir.
+	// But there might be an x.go in the original dir too, and if it
+	// uses cgo as well, cgo will be processing both and will
+	// translate both into x.cgo1.go in the objdir, overwriting one.
+	// Rename x.go to _x_swig.go to avoid this problem.
+	// We ignore files in the original dir that begin with underscore
+	// so _x_swig.go cannot conflict with an original file we were
+	// going to compile.
+	goFile = objdir + goFile
+	newGoFile := objdir + "_" + base + "_swig.go"
+	if err := os.Rename(goFile, newGoFile); err != nil {
+		return "", "", err
+	}
+	return newGoFile, objdir + gccBase + gccExt, nil
+}
+
+// disableBuildID adjusts a linker command line to avoid creating a
+// build ID when creating an object file rather than an executable or
+// shared library. Some systems, such as Ubuntu, always add
+// --build-id to every link, but we don't want a build ID when we are
+// producing an object file. On some of those system a plain -r (not
+// -Wl,-r) will turn off --build-id, but clang 3.0 doesn't support a
+// plain -r. I don't know how to turn off --build-id when using clang
+// other than passing a trailing --build-id=none. So that is what we
+// do, but only on systems likely to support it, which is to say,
+// systems that normally use gold or the GNU linker.
+func (b *Builder) disableBuildID(ldflags []string) []string {
+	switch cfg.Goos {
+	case "android", "dragonfly", "linux", "netbsd":
+		ldflags = append(ldflags, "-Wl,--build-id=none")
+	}
+	return ldflags
+}
+
+// mkAbsFiles converts files into a list of absolute files,
+// assuming they were originally relative to dir,
+// and returns that new list.
+func mkAbsFiles(dir string, files []string) []string {
+	abs := make([]string, len(files))
+	for i, f := range files {
+		if !filepath.IsAbs(f) {
+			f = filepath.Join(dir, f)
+		}
+		abs[i] = f
+	}
+	return abs
+}
+
+// passLongArgsInResponseFiles modifies cmd such that, for
+// certain programs, long arguments are passed in "response files", a
+// file on disk with the arguments, with one arg per line. An actual
+// argument starting with '@' means that the rest of the argument is
+// a filename of arguments to expand.
+//
+// See issues 18468 (Windows) and 37768 (Darwin).
+func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
+	cleanup = func() {} // no cleanup by default
+
+	var argLen int
+	for _, arg := range cmd.Args {
+		argLen += len(arg)
+	}
+
+	// If we're not approaching 32KB of args, just pass args normally.
+	// (use 30KB instead to be conservative; not sure how accounting is done)
+	if !useResponseFile(cmd.Path, argLen) {
+		return
+	}
+
+	tf, err := os.CreateTemp("", "args")
+	if err != nil {
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cleanup = func() { os.Remove(tf.Name()) }
+	var buf bytes.Buffer
+	for _, arg := range cmd.Args[1:] {
+		fmt.Fprintf(&buf, "%s\n", encodeArg(arg))
+	}
+	if _, err := tf.Write(buf.Bytes()); err != nil {
+		tf.Close()
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	if err := tf.Close(); err != nil {
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cmd.Args = []string{cmd.Args[0], "@" + tf.Name()}
+	return cleanup
+}
+
+// Windows has a limit of 32 KB arguments. To be conservative and not worry
+// about whether that includes spaces or not, just use 30 KB. Darwin's limit is
+// less clear. The OS claims 256KB, but we've seen failures with arglen as
+// small as 50KB.
+const ArgLengthForResponseFile = (30 << 10)
+
+func useResponseFile(path string, argLen int) bool {
+	// Unless the program uses objabi.Flagparse, which understands
+	// response files, don't use response files.
+	// TODO: do we need more commands? asm? cgo? For now, no.
+	prog := strings.TrimSuffix(filepath.Base(path), ".exe")
+	switch prog {
+	case "compile", "link":
+	default:
+		return false
+	}
+
+	if argLen > ArgLengthForResponseFile {
+		return true
+	}
+
+	// On the Go build system, use response files about 10% of the
+	// time, just to exercise this codepath.
+	isBuilder := os.Getenv("GO_BUILDER_NAME") != ""
+	if isBuilder && rand.Intn(10) == 0 {
+		return true
+	}
+
+	return false
+}
+
+// encodeArg encodes an argument for response file writing.
+func encodeArg(arg string) string {
+	// If there aren't any characters we need to reencode, fastpath out.
+	if !strings.ContainsAny(arg, "\\\n") {
+		return arg
+	}
+	var b strings.Builder
+	for _, r := range arg {
+		switch r {
+		case '\\':
+			b.WriteByte('\\')
+			b.WriteByte('\\')
+		case '\n':
+			b.WriteByte('\\')
+			b.WriteByte('n')
+		default:
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}
diff --git a/src/cmd/go/internal/work/exec_test.go b/src/cmd/go/internal/work/exec_test.go
new file mode 100644
index 0000000..4eb762c
--- /dev/null
+++ b/src/cmd/go/internal/work/exec_test.go
@@ -0,0 +1,86 @@
+// 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 work
+
+import (
+	"bytes"
+	"cmd/internal/objabi"
+	"fmt"
+	"math/rand"
+	"testing"
+	"time"
+	"unicode/utf8"
+)
+
+func TestEncodeArgs(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		arg, want string
+	}{
+		{"", ""},
+		{"hello", "hello"},
+		{"hello\n", "hello\\n"},
+		{"hello\\", "hello\\\\"},
+		{"hello\nthere", "hello\\nthere"},
+		{"\\\n", "\\\\\\n"},
+	}
+	for _, test := range tests {
+		if got := encodeArg(test.arg); got != test.want {
+			t.Errorf("encodeArg(%q) = %q, want %q", test.arg, got, test.want)
+		}
+	}
+}
+
+func TestEncodeDecode(t *testing.T) {
+	t.Parallel()
+	tests := []string{
+		"",
+		"hello",
+		"hello\\there",
+		"hello\nthere",
+		"hello 中国",
+		"hello \n中\\国",
+	}
+	for _, arg := range tests {
+		if got := objabi.DecodeArg(encodeArg(arg)); got != arg {
+			t.Errorf("objabi.DecodeArg(encodeArg(%q)) = %q", arg, got)
+		}
+	}
+}
+
+func TestEncodeDecodeFuzz(t *testing.T) {
+	if testing.Short() {
+		t.Skip("fuzz test is slow")
+	}
+	t.Parallel()
+
+	nRunes := ArgLengthForResponseFile + 100
+	rBuffer := make([]rune, nRunes)
+	buf := bytes.NewBuffer([]byte(string(rBuffer)))
+
+	seed := time.Now().UnixNano()
+	t.Logf("rand seed: %v", seed)
+	rng := rand.New(rand.NewSource(seed))
+
+	for i := 0; i < 50; i++ {
+		// Generate a random string of runes.
+		buf.Reset()
+		for buf.Len() < ArgLengthForResponseFile+1 {
+			var r rune
+			for {
+				r = rune(rng.Intn(utf8.MaxRune + 1))
+				if utf8.ValidRune(r) {
+					break
+				}
+			}
+			fmt.Fprintf(buf, "%c", r)
+		}
+		arg := buf.String()
+
+		if got := objabi.DecodeArg(encodeArg(arg)); got != arg {
+			t.Errorf("[%d] objabi.DecodeArg(encodeArg(%q)) = %q [seed: %v]", i, arg, got, seed)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
new file mode 100644
index 0000000..cc4e2b2
--- /dev/null
+++ b/src/cmd/go/internal/work/gc.go
@@ -0,0 +1,693 @@
+// 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 work
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/str"
+	"cmd/internal/objabi"
+	"cmd/internal/sys"
+	"crypto/sha1"
+)
+
+// The 'path' used for GOROOT_FINAL when -trimpath is specified
+const trimPathGoRootFinal = "go"
+
+// The Go toolchain.
+
+type gcToolchain struct{}
+
+func (gcToolchain) compiler() string {
+	return base.Tool("compile")
+}
+
+func (gcToolchain) linker() string {
+	return base.Tool("link")
+}
+
+func pkgPath(a *Action) string {
+	p := a.Package
+	ppath := p.ImportPath
+	if cfg.BuildBuildmode == "plugin" {
+		ppath = pluginPath(a)
+	} else if p.Name == "main" && !p.Internal.ForceLibrary {
+		ppath = "main"
+	}
+	return ppath
+}
+
+func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
+	p := a.Package
+	objdir := a.Objdir
+	if archive != "" {
+		ofile = archive
+	} else {
+		out := "_go_.o"
+		ofile = objdir + out
+	}
+
+	pkgpath := pkgPath(a)
+	gcargs := []string{"-p", pkgpath}
+	if p.Module != nil && p.Module.GoVersion != "" && allowedVersion(p.Module.GoVersion) {
+		gcargs = append(gcargs, "-lang=go"+p.Module.GoVersion)
+	}
+	if p.Standard {
+		gcargs = append(gcargs, "-std")
+	}
+	compilingRuntime := p.Standard && (p.ImportPath == "runtime" || strings.HasPrefix(p.ImportPath, "runtime/internal"))
+	// The runtime package imports a couple of general internal packages.
+	if p.Standard && (p.ImportPath == "internal/cpu" || p.ImportPath == "internal/bytealg") {
+		compilingRuntime = true
+	}
+	if compilingRuntime {
+		// runtime compiles with a special gc flag to check for
+		// memory allocations that are invalid in the runtime package,
+		// and to implement some special compiler pragmas.
+		gcargs = append(gcargs, "-+")
+	}
+
+	// If we're giving the compiler the entire package (no C etc files), tell it that,
+	// so that it can give good error messages about forward declarations.
+	// Exceptions: a few standard packages have forward declarations for
+	// pieces supplied behind-the-scenes by package runtime.
+	extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.CXXFiles) + len(p.MFiles) + len(p.FFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
+	if p.Standard {
+		switch p.ImportPath {
+		case "bytes", "internal/poll", "net", "os":
+			fallthrough
+		case "runtime/metrics", "runtime/pprof", "runtime/trace":
+			fallthrough
+		case "sync", "syscall", "time":
+			extFiles++
+		}
+	}
+	if extFiles == 0 {
+		gcargs = append(gcargs, "-complete")
+	}
+	if cfg.BuildContext.InstallSuffix != "" {
+		gcargs = append(gcargs, "-installsuffix", cfg.BuildContext.InstallSuffix)
+	}
+	if a.buildID != "" {
+		gcargs = append(gcargs, "-buildid", a.buildID)
+	}
+	if p.Internal.OmitDebug || cfg.Goos == "plan9" || cfg.Goarch == "wasm" {
+		gcargs = append(gcargs, "-dwarf=false")
+	}
+	if strings.HasPrefix(runtimeVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
+		gcargs = append(gcargs, "-goversion", runtimeVersion)
+	}
+	if symabis != "" {
+		gcargs = append(gcargs, "-symabis", symabis)
+	}
+
+	gcflags := str.StringList(forcedGcflags, p.Internal.Gcflags)
+	if compilingRuntime {
+		// Remove -N, if present.
+		// It is not possible to build the runtime with no optimizations,
+		// because the compiler cannot eliminate enough write barriers.
+		for i := 0; i < len(gcflags); i++ {
+			if gcflags[i] == "-N" {
+				copy(gcflags[i:], gcflags[i+1:])
+				gcflags = gcflags[:len(gcflags)-1]
+				i--
+			}
+		}
+	}
+
+	args := []interface{}{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), gcflags, gcargs, "-D", p.Internal.LocalPrefix}
+	if importcfg != nil {
+		if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-importcfg", objdir+"importcfg")
+	}
+	if embedcfg != nil {
+		if err := b.writeFile(objdir+"embedcfg", embedcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-embedcfg", objdir+"embedcfg")
+	}
+	if ofile == archive {
+		args = append(args, "-pack")
+	}
+	if asmhdr {
+		args = append(args, "-asmhdr", objdir+"go_asm.h")
+	}
+
+	// Add -c=N to use concurrent backend compilation, if possible.
+	if c := gcBackendConcurrency(gcflags); c > 1 {
+		args = append(args, fmt.Sprintf("-c=%d", c))
+	}
+
+	for _, f := range gofiles {
+		f := mkAbs(p.Dir, f)
+
+		// Handle overlays. Convert path names using OverlayPath
+		// so these paths can be handed directly to tools.
+		// Deleted files won't show up in when scanning directories earlier,
+		// so OverlayPath will never return "" (meaning a deleted file) here.
+		// TODO(#39958): Handle cases where the package directory
+		// doesn't exist on disk (this can happen when all the package's
+		// files are in an overlay): the code expects the package directory
+		// to exist and runs some tools in that directory.
+		// TODO(#39958): Process the overlays when the
+		// gofiles, cgofiles, cfiles, sfiles, and cxxfiles variables are
+		// created in (*Builder).build. Doing that requires rewriting the
+		// code that uses those values to expect absolute paths.
+		f, _ = fsys.OverlayPath(f)
+
+		args = append(args, f)
+	}
+
+	output, err = b.runOut(a, base.Cwd, nil, args...)
+	return ofile, output, err
+}
+
+// gcBackendConcurrency returns the backend compiler concurrency level for a package compilation.
+func gcBackendConcurrency(gcflags []string) int {
+	// First, check whether we can use -c at all for this compilation.
+	canDashC := concurrentGCBackendCompilationEnabledByDefault
+
+	switch e := os.Getenv("GO19CONCURRENTCOMPILATION"); e {
+	case "0":
+		canDashC = false
+	case "1":
+		canDashC = true
+	case "":
+		// Not set. Use default.
+	default:
+		log.Fatalf("GO19CONCURRENTCOMPILATION must be 0, 1, or unset, got %q", e)
+	}
+
+CheckFlags:
+	for _, flag := range gcflags {
+		// Concurrent compilation is presumed incompatible with any gcflags,
+		// except for known commonly used flags.
+		// If the user knows better, they can manually add their own -c to the gcflags.
+		switch flag {
+		case "-N", "-l", "-S", "-B", "-C", "-I":
+			// OK
+		default:
+			canDashC = false
+			break CheckFlags
+		}
+	}
+
+	// TODO: Test and delete these conditions.
+	if objabi.Fieldtrack_enabled != 0 || objabi.Preemptibleloops_enabled != 0 {
+		canDashC = false
+	}
+
+	if !canDashC {
+		return 1
+	}
+
+	// Decide how many concurrent backend compilations to allow.
+	//
+	// If we allow too many, in theory we might end up with p concurrent processes,
+	// each with c concurrent backend compiles, all fighting over the same resources.
+	// However, in practice, that seems not to happen too much.
+	// Most build graphs are surprisingly serial, so p==1 for much of the build.
+	// Furthermore, concurrent backend compilation is only enabled for a part
+	// of the overall compiler execution, so c==1 for much of the build.
+	// So don't worry too much about that interaction for now.
+	//
+	// However, in practice, setting c above 4 tends not to help very much.
+	// See the analysis in CL 41192.
+	//
+	// TODO(josharian): attempt to detect whether this particular compilation
+	// is likely to be a bottleneck, e.g. when:
+	//   - it has no successor packages to compile (usually package main)
+	//   - all paths through the build graph pass through it
+	//   - critical path scheduling says it is high priority
+	// and in such a case, set c to runtime.NumCPU.
+	// We do this now when p==1.
+	if cfg.BuildP == 1 {
+		// No process parallelism. Max out c.
+		return runtime.NumCPU()
+	}
+	// Some process parallelism. Set c to min(4, numcpu).
+	c := 4
+	if ncpu := runtime.NumCPU(); ncpu < c {
+		c = ncpu
+	}
+	return c
+}
+
+// trimpath returns the -trimpath argument to use
+// when compiling the action.
+func (a *Action) trimpath() string {
+	// Keep in sync with Builder.ccompile
+	// The trimmed paths are a little different, but we need to trim in the
+	// same situations.
+
+	// Strip the object directory entirely.
+	objdir := a.Objdir
+	if len(objdir) > 1 && objdir[len(objdir)-1] == filepath.Separator {
+		objdir = objdir[:len(objdir)-1]
+	}
+	rewrite := ""
+
+	rewriteDir := a.Package.Dir
+	if cfg.BuildTrimpath {
+		if m := a.Package.Module; m != nil && m.Version != "" {
+			rewriteDir = m.Path + "@" + m.Version + strings.TrimPrefix(a.Package.ImportPath, m.Path)
+		} else {
+			rewriteDir = a.Package.ImportPath
+		}
+		rewrite += a.Package.Dir + "=>" + rewriteDir + ";"
+	}
+
+	// Add rewrites for overlays. The 'from' and 'to' paths in overlays don't need to have
+	// same basename, so go from the overlay contents file path (passed to the compiler)
+	// to the path the disk path would be rewritten to.
+
+	cgoFiles := make(map[string]bool)
+	for _, f := range a.Package.CgoFiles {
+		cgoFiles[f] = true
+	}
+
+	// TODO(matloob): Higher up in the stack, when the logic for deciding when to make copies
+	// of c/c++/m/f/hfiles is consolidated, use the same logic that Build uses to determine
+	// whether to create the copies in objdir to decide whether to rewrite objdir to the
+	// package directory here.
+	var overlayNonGoRewrites string // rewrites for non-go files
+	hasCgoOverlay := false
+	if fsys.OverlayFile != "" {
+		for _, filename := range a.Package.AllFiles() {
+			path := filename
+			if !filepath.IsAbs(path) {
+				path = filepath.Join(a.Package.Dir, path)
+			}
+			base := filepath.Base(path)
+			isGo := strings.HasSuffix(filename, ".go") || strings.HasSuffix(filename, ".s")
+			isCgo := cgoFiles[filename] || !isGo
+			overlayPath, isOverlay := fsys.OverlayPath(path)
+			if isCgo && isOverlay {
+				hasCgoOverlay = true
+			}
+			if !isCgo && isOverlay {
+				rewrite += overlayPath + "=>" + filepath.Join(rewriteDir, base) + ";"
+			} else if isCgo {
+				// Generate rewrites for non-Go files copied to files in objdir.
+				if filepath.Dir(path) == a.Package.Dir {
+					// This is a file copied to objdir.
+					overlayNonGoRewrites += filepath.Join(objdir, base) + "=>" + filepath.Join(rewriteDir, base) + ";"
+				}
+			} else {
+				// Non-overlay Go files are covered by the a.Package.Dir rewrite rule above.
+			}
+		}
+	}
+	if hasCgoOverlay {
+		rewrite += overlayNonGoRewrites
+	}
+	rewrite += objdir + "=>"
+
+	return rewrite
+}
+
+func asmArgs(a *Action, p *load.Package) []interface{} {
+	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
+	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
+	pkgpath := pkgPath(a)
+	args := []interface{}{cfg.BuildToolexec, base.Tool("asm"), "-p", pkgpath, "-trimpath", a.trimpath(), "-I", a.Objdir, "-I", inc, "-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch, forcedAsmflags, p.Internal.Asmflags}
+	if p.ImportPath == "runtime" && cfg.Goarch == "386" {
+		for _, arg := range forcedAsmflags {
+			if arg == "-dynlink" {
+				args = append(args, "-D=GOBUILDMODE_shared=1")
+			}
+		}
+	}
+	if objabi.IsRuntimePackagePath(pkgpath) {
+		args = append(args, "-compiling-runtime")
+		if objabi.Regabi_enabled != 0 {
+			// In order to make it easier to port runtime assembly
+			// to the register ABI, we introduce a macro
+			// indicating the experiment is enabled.
+			//
+			// Note: a similar change also appears in
+			// cmd/dist/build.go.
+			//
+			// TODO(austin): Remove this once we commit to the
+			// register ABI (#40724).
+			args = append(args, "-D=GOEXPERIMENT_REGABI=1")
+		}
+	}
+
+	if cfg.Goarch == "mips" || cfg.Goarch == "mipsle" {
+		// Define GOMIPS_value from cfg.GOMIPS.
+		args = append(args, "-D", "GOMIPS_"+cfg.GOMIPS)
+	}
+
+	if cfg.Goarch == "mips64" || cfg.Goarch == "mips64le" {
+		// Define GOMIPS64_value from cfg.GOMIPS64.
+		args = append(args, "-D", "GOMIPS64_"+cfg.GOMIPS64)
+	}
+
+	return args
+}
+
+func (gcToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	p := a.Package
+	args := asmArgs(a, p)
+
+	var ofiles []string
+	for _, sfile := range sfiles {
+		overlayPath, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
+		ofile := a.Objdir + sfile[:len(sfile)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
+		args1 := append(args, "-o", ofile, overlayPath)
+		if err := b.run(a, p.Dir, p.ImportPath, nil, args1...); err != nil {
+			return nil, err
+		}
+	}
+	return ofiles, nil
+}
+
+func (gcToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	mkSymabis := func(p *load.Package, sfiles []string, path string) error {
+		args := asmArgs(a, p)
+		args = append(args, "-gensymabis", "-o", path)
+		for _, sfile := range sfiles {
+			if p.ImportPath == "runtime/cgo" && strings.HasPrefix(sfile, "gcc_") {
+				continue
+			}
+			op, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
+			args = append(args, op)
+		}
+
+		// Supply an empty go_asm.h as if the compiler had been run.
+		// -gensymabis parsing is lax enough that we don't need the
+		// actual definitions that would appear in go_asm.h.
+		if err := b.writeFile(a.Objdir+"go_asm.h", nil); err != nil {
+			return err
+		}
+
+		return b.run(a, p.Dir, p.ImportPath, nil, args...)
+	}
+
+	var symabis string // Only set if we actually create the file
+	p := a.Package
+	if len(sfiles) != 0 {
+		symabis = a.Objdir + "symabis"
+		if err := mkSymabis(p, sfiles, symabis); err != nil {
+			return "", err
+		}
+	}
+
+	return symabis, nil
+}
+
+// toolVerify checks that the command line args writes the same output file
+// if run using newTool instead.
+// Unused now but kept around for future use.
+func toolVerify(a *Action, b *Builder, p *load.Package, newTool string, ofile string, args []interface{}) error {
+	newArgs := make([]interface{}, len(args))
+	copy(newArgs, args)
+	newArgs[1] = base.Tool(newTool)
+	newArgs[3] = ofile + ".new" // x.6 becomes x.6.new
+	if err := b.run(a, p.Dir, p.ImportPath, nil, newArgs...); err != nil {
+		return err
+	}
+	data1, err := os.ReadFile(ofile)
+	if err != nil {
+		return err
+	}
+	data2, err := os.ReadFile(ofile + ".new")
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(data1, data2) {
+		return fmt.Errorf("%s and %s produced different output files:\n%s\n%s", filepath.Base(args[1].(string)), newTool, strings.Join(str.StringList(args...), " "), strings.Join(str.StringList(newArgs...), " "))
+	}
+	os.Remove(ofile + ".new")
+	return nil
+}
+
+func (gcToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	var absOfiles []string
+	for _, f := range ofiles {
+		absOfiles = append(absOfiles, mkAbs(a.Objdir, f))
+	}
+	absAfile := mkAbs(a.Objdir, afile)
+
+	// The archive file should have been created by the compiler.
+	// Since it used to not work that way, verify.
+	if !cfg.BuildN {
+		if _, err := os.Stat(absAfile); err != nil {
+			base.Fatalf("os.Stat of archive file failed: %v", err)
+		}
+	}
+
+	p := a.Package
+	if cfg.BuildN || cfg.BuildX {
+		cmdline := str.StringList(base.Tool("pack"), "r", absAfile, absOfiles)
+		b.Showcmd(p.Dir, "%s # internal", joinUnambiguously(cmdline))
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	if err := packInternal(absAfile, absOfiles); err != nil {
+		b.showOutput(a, p.Dir, p.Desc(), err.Error()+"\n")
+		return errPrintedOutput
+	}
+	return nil
+}
+
+func packInternal(afile string, ofiles []string) error {
+	dst, err := os.OpenFile(afile, os.O_WRONLY|os.O_APPEND, 0)
+	if err != nil {
+		return err
+	}
+	defer dst.Close() // only for error returns or panics
+	w := bufio.NewWriter(dst)
+
+	for _, ofile := range ofiles {
+		src, err := os.Open(ofile)
+		if err != nil {
+			return err
+		}
+		fi, err := src.Stat()
+		if err != nil {
+			src.Close()
+			return err
+		}
+		// Note: Not using %-16.16s format because we care
+		// about bytes, not runes.
+		name := fi.Name()
+		if len(name) > 16 {
+			name = name[:16]
+		} else {
+			name += strings.Repeat(" ", 16-len(name))
+		}
+		size := fi.Size()
+		fmt.Fprintf(w, "%s%-12d%-6d%-6d%-8o%-10d`\n",
+			name, 0, 0, 0, 0644, size)
+		n, err := io.Copy(w, src)
+		src.Close()
+		if err == nil && n < size {
+			err = io.ErrUnexpectedEOF
+		} else if err == nil && n > size {
+			err = fmt.Errorf("file larger than size reported by stat")
+		}
+		if err != nil {
+			return fmt.Errorf("copying %s to %s: %v", ofile, afile, err)
+		}
+		if size&1 != 0 {
+			w.WriteByte(0)
+		}
+	}
+
+	if err := w.Flush(); err != nil {
+		return err
+	}
+	return dst.Close()
+}
+
+// setextld sets the appropriate linker flags for the specified compiler.
+func setextld(ldflags []string, compiler []string) []string {
+	for _, f := range ldflags {
+		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
+			// don't override -extld if supplied
+			return ldflags
+		}
+	}
+	ldflags = append(ldflags, "-extld="+compiler[0])
+	if len(compiler) > 1 {
+		extldflags := false
+		add := strings.Join(compiler[1:], " ")
+		for i, f := range ldflags {
+			if f == "-extldflags" && i+1 < len(ldflags) {
+				ldflags[i+1] = add + " " + ldflags[i+1]
+				extldflags = true
+				break
+			} else if strings.HasPrefix(f, "-extldflags=") {
+				ldflags[i] = "-extldflags=" + add + " " + ldflags[i][len("-extldflags="):]
+				extldflags = true
+				break
+			}
+		}
+		if !extldflags {
+			ldflags = append(ldflags, "-extldflags="+add)
+		}
+	}
+	return ldflags
+}
+
+// pluginPath computes the package path for a plugin main package.
+//
+// This is typically the import path of the main package p, unless the
+// plugin is being built directly from source files. In that case we
+// combine the package build ID with the contents of the main package
+// source files. This allows us to identify two different plugins
+// built from two source files with the same name.
+func pluginPath(a *Action) string {
+	p := a.Package
+	if p.ImportPath != "command-line-arguments" {
+		return p.ImportPath
+	}
+	h := sha1.New()
+	buildID := a.buildID
+	if a.Mode == "link" {
+		// For linking, use the main package's build ID instead of
+		// the binary's build ID, so it is the same hash used in
+		// compiling and linking.
+		// When compiling, we use actionID/actionID (instead of
+		// actionID/contentID) as a temporary build ID to compute
+		// the hash. Do the same here. (See buildid.go:useCache)
+		// The build ID matters because it affects the overall hash
+		// in the plugin's pseudo-import path returned below.
+		// We need to use the same import path when compiling and linking.
+		id := strings.Split(buildID, buildIDSeparator)
+		buildID = id[1] + buildIDSeparator + id[1]
+	}
+	fmt.Fprintf(h, "build ID: %s\n", buildID)
+	for _, file := range str.StringList(p.GoFiles, p.CgoFiles, p.SFiles) {
+		data, err := os.ReadFile(filepath.Join(p.Dir, file))
+		if err != nil {
+			base.Fatalf("go: %s", err)
+		}
+		h.Write(data)
+	}
+	return fmt.Sprintf("plugin/unnamed-%x", h.Sum(nil))
+}
+
+func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	cxx := len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
+	for _, a := range root.Deps {
+		if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
+			cxx = true
+		}
+	}
+	var ldflags []string
+	if cfg.BuildContext.InstallSuffix != "" {
+		ldflags = append(ldflags, "-installsuffix", cfg.BuildContext.InstallSuffix)
+	}
+	if root.Package.Internal.OmitDebug {
+		ldflags = append(ldflags, "-s", "-w")
+	}
+	if cfg.BuildBuildmode == "plugin" {
+		ldflags = append(ldflags, "-pluginpath", pluginPath(root))
+	}
+
+	// Store BuildID inside toolchain binaries as a unique identifier of the
+	// tool being run, for use by content-based staleness determination.
+	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
+		// External linking will include our build id in the external
+		// linker's build id, which will cause our build id to not
+		// match the next time the tool is built.
+		// Rely on the external build id instead.
+		if !sys.MustLinkExternal(cfg.Goos, cfg.Goarch) {
+			ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
+		}
+	}
+
+	// If the user has not specified the -extld option, then specify the
+	// appropriate linker. In case of C++ code, use the compiler named
+	// by the CXX environment variable or defaultCXX if CXX is not set.
+	// Else, use the CC environment variable and defaultCC as fallback.
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+	} else {
+		compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	}
+	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
+	if root.buildID != "" {
+		ldflags = append(ldflags, "-buildid="+root.buildID)
+	}
+	ldflags = append(ldflags, forcedLdflags...)
+	ldflags = append(ldflags, root.Package.Internal.Ldflags...)
+	ldflags = setextld(ldflags, compiler)
+
+	// On OS X when using external linking to build a shared library,
+	// the argument passed here to -o ends up recorded in the final
+	// shared library in the LC_ID_DYLIB load command.
+	// To avoid putting the temporary output directory name there
+	// (and making the resulting shared library useless),
+	// run the link in the output directory so that -o can name
+	// just the final path element.
+	// On Windows, DLL file name is recorded in PE file
+	// export section, so do like on OS X.
+	dir := "."
+	if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" {
+		dir, out = filepath.Split(out)
+	}
+
+	env := []string{}
+	if cfg.BuildTrimpath {
+		env = append(env, "GOROOT_FINAL="+trimPathGoRootFinal)
+	}
+	return b.run(root, dir, root.Package.ImportPath, env, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags, mainpkg)
+}
+
+func (gcToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	ldflags := []string{"-installsuffix", cfg.BuildContext.InstallSuffix}
+	ldflags = append(ldflags, "-buildmode=shared")
+	ldflags = append(ldflags, forcedLdflags...)
+	ldflags = append(ldflags, root.Package.Internal.Ldflags...)
+	cxx := false
+	for _, a := range allactions {
+		if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
+			cxx = true
+		}
+	}
+	// If the user has not specified the -extld option, then specify the
+	// appropriate linker. In case of C++ code, use the compiler named
+	// by the CXX environment variable or defaultCXX if CXX is not set.
+	// Else, use the CC environment variable and defaultCC as fallback.
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+	} else {
+		compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	}
+	ldflags = setextld(ldflags, compiler)
+	for _, d := range toplevelactions {
+		if !strings.HasSuffix(d.Target, ".a") { // omit unsafe etc and actions for other shared libraries
+			continue
+		}
+		ldflags = append(ldflags, d.Package.ImportPath+"="+d.Target)
+	}
+	return b.run(root, ".", out, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags)
+}
+
+func (gcToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	return fmt.Errorf("%s: C source files not supported without cgo", mkAbs(a.Package.Dir, cfile))
+}
diff --git a/src/cmd/go/internal/work/gccgo.go b/src/cmd/go/internal/work/gccgo.go
new file mode 100644
index 0000000..b58c8aa
--- /dev/null
+++ b/src/cmd/go/internal/work/gccgo.go
@@ -0,0 +1,618 @@
+// 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 work
+
+import (
+	"fmt"
+	exec "internal/execabs"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/str"
+	"cmd/internal/pkgpath"
+)
+
+// The Gccgo toolchain.
+
+type gccgoToolchain struct{}
+
+var GccgoName, GccgoBin string
+var gccgoErr error
+
+func init() {
+	GccgoName = cfg.Getenv("GCCGO")
+	if GccgoName == "" {
+		GccgoName = "gccgo"
+	}
+	GccgoBin, gccgoErr = exec.LookPath(GccgoName)
+}
+
+func (gccgoToolchain) compiler() string {
+	checkGccgoBin()
+	return GccgoBin
+}
+
+func (gccgoToolchain) linker() string {
+	checkGccgoBin()
+	return GccgoBin
+}
+
+func (gccgoToolchain) ar() string {
+	ar := cfg.Getenv("AR")
+	if ar == "" {
+		ar = "ar"
+	}
+	return ar
+}
+
+func checkGccgoBin() {
+	if gccgoErr == nil {
+		return
+	}
+	fmt.Fprintf(os.Stderr, "cmd/go: gccgo: %s\n", gccgoErr)
+	base.SetExitStatus(2)
+	base.Exit()
+}
+
+func (tools gccgoToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
+	p := a.Package
+	objdir := a.Objdir
+	out := "_go_.o"
+	ofile = objdir + out
+	gcargs := []string{"-g"}
+	gcargs = append(gcargs, b.gccArchArgs()...)
+	gcargs = append(gcargs, "-fdebug-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	gcargs = append(gcargs, "-gno-record-gcc-switches")
+	if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+		gcargs = append(gcargs, "-fgo-pkgpath="+pkgpath)
+	}
+	if p.Internal.LocalPrefix != "" {
+		gcargs = append(gcargs, "-fgo-relative-import-path="+p.Internal.LocalPrefix)
+	}
+
+	args := str.StringList(tools.compiler(), "-c", gcargs, "-o", ofile, forcedGccgoflags)
+	if importcfg != nil {
+		if b.gccSupportsFlag(args[:1], "-fgo-importcfg=/dev/null") {
+			if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {
+				return "", nil, err
+			}
+			args = append(args, "-fgo-importcfg="+objdir+"importcfg")
+		} else {
+			root := objdir + "_importcfgroot_"
+			if err := buildImportcfgSymlinks(b, root, importcfg); err != nil {
+				return "", nil, err
+			}
+			args = append(args, "-I", root)
+		}
+	}
+	if embedcfg != nil && b.gccSupportsFlag(args[:1], "-fgo-embedcfg=/dev/null") {
+		if err := b.writeFile(objdir+"embedcfg", embedcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-fgo-embedcfg="+objdir+"embedcfg")
+	}
+
+	if b.gccSupportsFlag(args[:1], "-ffile-prefix-map=a=b") {
+		if cfg.BuildTrimpath {
+			args = append(args, "-ffile-prefix-map="+base.Cwd+"=.")
+			args = append(args, "-ffile-prefix-map="+b.WorkDir+"=/tmp/go-build")
+		}
+		if fsys.OverlayFile != "" {
+			for _, name := range gofiles {
+				absPath := mkAbs(p.Dir, name)
+				overlayPath, ok := fsys.OverlayPath(absPath)
+				if !ok {
+					continue
+				}
+				toPath := absPath
+				// gccgo only applies the last matching rule, so also handle the case where
+				// BuildTrimpath is true and the path is relative to base.Cwd.
+				if cfg.BuildTrimpath && str.HasFilePathPrefix(toPath, base.Cwd) {
+					toPath = "." + toPath[len(base.Cwd):]
+				}
+				args = append(args, "-ffile-prefix-map="+overlayPath+"="+toPath)
+			}
+		}
+	}
+
+	args = append(args, a.Package.Internal.Gccgoflags...)
+	for _, f := range gofiles {
+		f := mkAbs(p.Dir, f)
+		// Overlay files if necessary.
+		// See comment on gctoolchain.gc about overlay TODOs
+		f, _ = fsys.OverlayPath(f)
+		args = append(args, f)
+	}
+
+	output, err = b.runOut(a, p.Dir, nil, args)
+	return ofile, output, err
+}
+
+// buildImportcfgSymlinks builds in root a tree of symlinks
+// implementing the directives from importcfg.
+// This serves as a temporary transition mechanism until
+// we can depend on gccgo reading an importcfg directly.
+// (The Go 1.9 and later gc compilers already do.)
+func buildImportcfgSymlinks(b *Builder, root string, importcfg []byte) error {
+	for lineNum, line := range strings.Split(string(importcfg), "\n") {
+		lineNum++ // 1-based
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		var verb, args string
+		if i := strings.Index(line, " "); i < 0 {
+			verb = line
+		} else {
+			verb, args = line[:i], strings.TrimSpace(line[i+1:])
+		}
+		var before, after string
+		if i := strings.Index(args, "="); i >= 0 {
+			before, after = args[:i], args[i+1:]
+		}
+		switch verb {
+		default:
+			base.Fatalf("importcfg:%d: unknown directive %q", lineNum, verb)
+		case "packagefile":
+			if before == "" || after == "" {
+				return fmt.Errorf(`importcfg:%d: invalid packagefile: syntax is "packagefile path=filename": %s`, lineNum, line)
+			}
+			archive := gccgoArchive(root, before)
+			if err := b.Mkdir(filepath.Dir(archive)); err != nil {
+				return err
+			}
+			if err := b.Symlink(after, archive); err != nil {
+				return err
+			}
+		case "importmap":
+			if before == "" || after == "" {
+				return fmt.Errorf(`importcfg:%d: invalid importmap: syntax is "importmap old=new": %s`, lineNum, line)
+			}
+			beforeA := gccgoArchive(root, before)
+			afterA := gccgoArchive(root, after)
+			if err := b.Mkdir(filepath.Dir(beforeA)); err != nil {
+				return err
+			}
+			if err := b.Mkdir(filepath.Dir(afterA)); err != nil {
+				return err
+			}
+			if err := b.Symlink(afterA, beforeA); err != nil {
+				return err
+			}
+		case "packageshlib":
+			return fmt.Errorf("gccgo -importcfg does not support shared libraries")
+		}
+	}
+	return nil
+}
+
+func (tools gccgoToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	p := a.Package
+	var ofiles []string
+	for _, sfile := range sfiles {
+		base := filepath.Base(sfile)
+		ofile := a.Objdir + base[:len(base)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
+		sfile, _ = fsys.OverlayPath(mkAbs(p.Dir, sfile))
+		defs := []string{"-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch}
+		if pkgpath := tools.gccgoCleanPkgpath(b, p); pkgpath != "" {
+			defs = append(defs, `-D`, `GOPKGPATH=`+pkgpath)
+		}
+		defs = tools.maybePIC(defs)
+		defs = append(defs, b.gccArchArgs()...)
+		err := b.run(a, p.Dir, p.ImportPath, nil, tools.compiler(), "-xassembler-with-cpp", "-I", a.Objdir, "-c", "-o", ofile, defs, sfile)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return ofiles, nil
+}
+
+func (gccgoToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	return "", nil
+}
+
+func gccgoArchive(basedir, imp string) string {
+	end := filepath.FromSlash(imp + ".a")
+	afile := filepath.Join(basedir, end)
+	// add "lib" to the final element
+	return filepath.Join(filepath.Dir(afile), "lib"+filepath.Base(afile))
+}
+
+func (tools gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	p := a.Package
+	objdir := a.Objdir
+	var absOfiles []string
+	for _, f := range ofiles {
+		absOfiles = append(absOfiles, mkAbs(objdir, f))
+	}
+	var arArgs []string
+	if cfg.Goos == "aix" && cfg.Goarch == "ppc64" {
+		// AIX puts both 32-bit and 64-bit objects in the same archive.
+		// Tell the AIX "ar" command to only care about 64-bit objects.
+		arArgs = []string{"-X64"}
+	}
+	absAfile := mkAbs(objdir, afile)
+	// Try with D modifier first, then without if that fails.
+	output, err := b.runOut(a, p.Dir, nil, tools.ar(), arArgs, "rcD", absAfile, absOfiles)
+	if err != nil {
+		return b.run(a, p.Dir, p.ImportPath, nil, tools.ar(), arArgs, "rc", absAfile, absOfiles)
+	}
+
+	if len(output) > 0 {
+		// Show the output if there is any even without errors.
+		b.showOutput(a, p.Dir, p.ImportPath, b.processOutput(output))
+	}
+
+	return nil
+}
+
+func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string, allactions []*Action, buildmode, desc string) error {
+	// gccgo needs explicit linking with all package dependencies,
+	// and all LDFLAGS from cgo dependencies.
+	afiles := []string{}
+	shlibs := []string{}
+	ldflags := b.gccArchArgs()
+	cgoldflags := []string{}
+	usesCgo := false
+	cxx := false
+	objc := false
+	fortran := false
+	if root.Package != nil {
+		cxx = len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
+		objc = len(root.Package.MFiles) > 0
+		fortran = len(root.Package.FFiles) > 0
+	}
+
+	readCgoFlags := func(flagsFile string) error {
+		flags, err := os.ReadFile(flagsFile)
+		if err != nil {
+			return err
+		}
+		const ldflagsPrefix = "_CGO_LDFLAGS="
+		for _, line := range strings.Split(string(flags), "\n") {
+			if strings.HasPrefix(line, ldflagsPrefix) {
+				newFlags := strings.Fields(line[len(ldflagsPrefix):])
+				for _, flag := range newFlags {
+					// Every _cgo_flags file has -g and -O2 in _CGO_LDFLAGS
+					// but they don't mean anything to the linker so filter
+					// them out.
+					if flag != "-g" && !strings.HasPrefix(flag, "-O") {
+						cgoldflags = append(cgoldflags, flag)
+					}
+				}
+			}
+		}
+		return nil
+	}
+
+	var arArgs []string
+	if cfg.Goos == "aix" && cfg.Goarch == "ppc64" {
+		// AIX puts both 32-bit and 64-bit objects in the same archive.
+		// Tell the AIX "ar" command to only care about 64-bit objects.
+		arArgs = []string{"-X64"}
+	}
+
+	newID := 0
+	readAndRemoveCgoFlags := func(archive string) (string, error) {
+		newID++
+		newArchive := root.Objdir + fmt.Sprintf("_pkg%d_.a", newID)
+		if err := b.copyFile(newArchive, archive, 0666, false); err != nil {
+			return "", err
+		}
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "ar d %s _cgo_flags", newArchive)
+			if cfg.BuildN {
+				// TODO(rsc): We could do better about showing the right _cgo_flags even in -n mode.
+				// Either the archive is already built and we can read them out,
+				// or we're printing commands to build the archive and can
+				// forward the _cgo_flags directly to this step.
+				return "", nil
+			}
+		}
+		err := b.run(root, root.Objdir, desc, nil, tools.ar(), arArgs, "x", newArchive, "_cgo_flags")
+		if err != nil {
+			return "", err
+		}
+		err = b.run(root, ".", desc, nil, tools.ar(), arArgs, "d", newArchive, "_cgo_flags")
+		if err != nil {
+			return "", err
+		}
+		err = readCgoFlags(filepath.Join(root.Objdir, "_cgo_flags"))
+		if err != nil {
+			return "", err
+		}
+		return newArchive, nil
+	}
+
+	// If using -linkshared, find the shared library deps.
+	haveShlib := make(map[string]bool)
+	targetBase := filepath.Base(root.Target)
+	if cfg.BuildLinkshared {
+		for _, a := range root.Deps {
+			p := a.Package
+			if p == nil || p.Shlib == "" {
+				continue
+			}
+
+			// The .a we are linking into this .so
+			// will have its Shlib set to this .so.
+			// Don't start thinking we want to link
+			// this .so into itself.
+			base := filepath.Base(p.Shlib)
+			if base != targetBase {
+				haveShlib[base] = true
+			}
+		}
+	}
+
+	// Arrange the deps into afiles and shlibs.
+	addedShlib := make(map[string]bool)
+	for _, a := range root.Deps {
+		p := a.Package
+		if p != nil && p.Shlib != "" && haveShlib[filepath.Base(p.Shlib)] {
+			// This is a package linked into a shared
+			// library that we will put into shlibs.
+			continue
+		}
+
+		if haveShlib[filepath.Base(a.Target)] {
+			// This is a shared library we want to link against.
+			if !addedShlib[a.Target] {
+				shlibs = append(shlibs, a.Target)
+				addedShlib[a.Target] = true
+			}
+			continue
+		}
+
+		if p != nil {
+			target := a.built
+			if p.UsesCgo() || p.UsesSwig() {
+				var err error
+				target, err = readAndRemoveCgoFlags(target)
+				if err != nil {
+					continue
+				}
+			}
+
+			afiles = append(afiles, target)
+		}
+	}
+
+	for _, a := range allactions {
+		// Gather CgoLDFLAGS, but not from standard packages.
+		// The go tool can dig up runtime/cgo from GOROOT and
+		// think that it should use its CgoLDFLAGS, but gccgo
+		// doesn't use runtime/cgo.
+		if a.Package == nil {
+			continue
+		}
+		if !a.Package.Standard {
+			cgoldflags = append(cgoldflags, a.Package.CgoLDFLAGS...)
+		}
+		if len(a.Package.CgoFiles) > 0 {
+			usesCgo = true
+		}
+		if a.Package.UsesSwig() {
+			usesCgo = true
+		}
+		if len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0 {
+			cxx = true
+		}
+		if len(a.Package.MFiles) > 0 {
+			objc = true
+		}
+		if len(a.Package.FFiles) > 0 {
+			fortran = true
+		}
+	}
+
+	wholeArchive := []string{"-Wl,--whole-archive"}
+	noWholeArchive := []string{"-Wl,--no-whole-archive"}
+	if cfg.Goos == "aix" {
+		wholeArchive = nil
+		noWholeArchive = nil
+	}
+	ldflags = append(ldflags, wholeArchive...)
+	ldflags = append(ldflags, afiles...)
+	ldflags = append(ldflags, noWholeArchive...)
+
+	ldflags = append(ldflags, cgoldflags...)
+	ldflags = append(ldflags, envList("CGO_LDFLAGS", "")...)
+	if root.Package != nil {
+		ldflags = append(ldflags, root.Package.CgoLDFLAGS...)
+	}
+	if cfg.Goos != "aix" {
+		ldflags = str.StringList("-Wl,-(", ldflags, "-Wl,-)")
+	}
+
+	if root.buildID != "" {
+		// On systems that normally use gold or the GNU linker,
+		// use the --build-id option to write a GNU build ID note.
+		switch cfg.Goos {
+		case "android", "dragonfly", "linux", "netbsd":
+			ldflags = append(ldflags, fmt.Sprintf("-Wl,--build-id=0x%x", root.buildID))
+		}
+	}
+
+	var rLibPath string
+	if cfg.Goos == "aix" {
+		rLibPath = "-Wl,-blibpath="
+	} else {
+		rLibPath = "-Wl,-rpath="
+	}
+	for _, shlib := range shlibs {
+		ldflags = append(
+			ldflags,
+			"-L"+filepath.Dir(shlib),
+			rLibPath+filepath.Dir(shlib),
+			"-l"+strings.TrimSuffix(
+				strings.TrimPrefix(filepath.Base(shlib), "lib"),
+				".so"))
+	}
+
+	var realOut string
+	goLibBegin := str.StringList(wholeArchive, "-lgolibbegin", noWholeArchive)
+	switch buildmode {
+	case "exe":
+		if usesCgo && cfg.Goos == "linux" {
+			ldflags = append(ldflags, "-Wl,-E")
+		}
+
+	case "c-archive":
+		// Link the Go files into a single .o, and also link
+		// in -lgolibbegin.
+		//
+		// We need to use --whole-archive with -lgolibbegin
+		// because it doesn't define any symbols that will
+		// cause the contents to be pulled in; it's just
+		// initialization code.
+		//
+		// The user remains responsible for linking against
+		// -lgo -lpthread -lm in the final link. We can't use
+		// -r to pick them up because we can't combine
+		// split-stack and non-split-stack code in a single -r
+		// link, and libgo picks up non-split-stack code from
+		// libffi.
+		ldflags = append(ldflags, "-Wl,-r", "-nostdlib")
+		ldflags = append(ldflags, goLibBegin...)
+
+		if nopie := b.gccNoPie([]string{tools.linker()}); nopie != "" {
+			ldflags = append(ldflags, nopie)
+		}
+
+		// We are creating an object file, so we don't want a build ID.
+		if root.buildID == "" {
+			ldflags = b.disableBuildID(ldflags)
+		}
+
+		realOut = out
+		out = out + ".o"
+
+	case "c-shared":
+		ldflags = append(ldflags, "-shared", "-nostdlib")
+		ldflags = append(ldflags, goLibBegin...)
+		ldflags = append(ldflags, "-lgo", "-lgcc_s", "-lgcc", "-lc", "-lgcc")
+
+	case "shared":
+		if cfg.Goos != "aix" {
+			ldflags = append(ldflags, "-zdefs")
+		}
+		ldflags = append(ldflags, "-shared", "-nostdlib", "-lgo", "-lgcc_s", "-lgcc", "-lc")
+
+	default:
+		base.Fatalf("-buildmode=%s not supported for gccgo", buildmode)
+	}
+
+	switch buildmode {
+	case "exe", "c-shared":
+		if cxx {
+			ldflags = append(ldflags, "-lstdc++")
+		}
+		if objc {
+			ldflags = append(ldflags, "-lobjc")
+		}
+		if fortran {
+			fc := cfg.Getenv("FC")
+			if fc == "" {
+				fc = "gfortran"
+			}
+			// support gfortran out of the box and let others pass the correct link options
+			// via CGO_LDFLAGS
+			if strings.Contains(fc, "gfortran") {
+				ldflags = append(ldflags, "-lgfortran")
+			}
+		}
+	}
+
+	if err := b.run(root, ".", desc, nil, tools.linker(), "-o", out, ldflags, forcedGccgoflags, root.Package.Internal.Gccgoflags); err != nil {
+		return err
+	}
+
+	switch buildmode {
+	case "c-archive":
+		if err := b.run(root, ".", desc, nil, tools.ar(), arArgs, "rc", realOut, out); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (tools gccgoToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	return tools.link(b, root, out, importcfg, root.Deps, ldBuildmode, root.Package.ImportPath)
+}
+
+func (tools gccgoToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	return tools.link(b, root, out, importcfg, allactions, "shared", out)
+}
+
+func (tools gccgoToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	p := a.Package
+	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
+	cfile = mkAbs(p.Dir, cfile)
+	defs := []string{"-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch}
+	defs = append(defs, b.gccArchArgs()...)
+	if pkgpath := tools.gccgoCleanPkgpath(b, p); pkgpath != "" {
+		defs = append(defs, `-D`, `GOPKGPATH="`+pkgpath+`"`)
+	}
+	compiler := envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	if b.gccSupportsFlag(compiler, "-fsplit-stack") {
+		defs = append(defs, "-fsplit-stack")
+	}
+	defs = tools.maybePIC(defs)
+	if b.gccSupportsFlag(compiler, "-ffile-prefix-map=a=b") {
+		defs = append(defs, "-ffile-prefix-map="+base.Cwd+"=.")
+		defs = append(defs, "-ffile-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	} else if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		defs = append(defs, "-fdebug-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	}
+	if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
+		defs = append(defs, "-gno-record-gcc-switches")
+	}
+	return b.run(a, p.Dir, p.ImportPath, nil, compiler, "-Wall", "-g",
+		"-I", a.Objdir, "-I", inc, "-o", ofile, defs, "-c", cfile)
+}
+
+// maybePIC adds -fPIC to the list of arguments if needed.
+func (tools gccgoToolchain) maybePIC(args []string) []string {
+	switch cfg.BuildBuildmode {
+	case "c-shared", "shared", "plugin":
+		args = append(args, "-fPIC")
+	}
+	return args
+}
+
+func gccgoPkgpath(p *load.Package) string {
+	if p.Internal.Build.IsCommand() && !p.Internal.ForceLibrary {
+		return ""
+	}
+	return p.ImportPath
+}
+
+var gccgoToSymbolFuncOnce sync.Once
+var gccgoToSymbolFunc func(string) string
+
+func (tools gccgoToolchain) gccgoCleanPkgpath(b *Builder, p *load.Package) string {
+	gccgoToSymbolFuncOnce.Do(func() {
+		fn, err := pkgpath.ToSymbolFunc(tools.compiler(), b.WorkDir)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "cmd/go: %v\n", err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		gccgoToSymbolFunc = fn
+	})
+
+	return gccgoToSymbolFunc(gccgoPkgpath(p))
+}
diff --git a/src/cmd/go/internal/work/init.go b/src/cmd/go/internal/work/init.go
new file mode 100644
index 0000000..ba7c7c2
--- /dev/null
+++ b/src/cmd/go/internal/work/init.go
@@ -0,0 +1,286 @@
+// 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.
+
+// Build initialization (after flag parsing).
+
+package work
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/modload"
+	"cmd/internal/objabi"
+	"cmd/internal/sys"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+func BuildInit() {
+	modload.Init()
+	instrumentInit()
+	buildModeInit()
+	if err := fsys.Init(base.Cwd); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	// Make sure -pkgdir is absolute, because we run commands
+	// in different directories.
+	if cfg.BuildPkgdir != "" && !filepath.IsAbs(cfg.BuildPkgdir) {
+		p, err := filepath.Abs(cfg.BuildPkgdir)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "go %s: evaluating -pkgdir: %v\n", flag.Args()[0], err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		cfg.BuildPkgdir = p
+	}
+
+	// Make sure CC and CXX are absolute paths
+	for _, key := range []string{"CC", "CXX"} {
+		if path := cfg.Getenv(key); !filepath.IsAbs(path) && path != "" && path != filepath.Base(path) {
+			base.Fatalf("go %s: %s environment variable is relative; must be absolute path: %s\n", flag.Args()[0], key, path)
+		}
+	}
+
+	// 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. [2:] strips the
+	// leading "X:" from objabi.Expstring().
+	exp := objabi.Expstring()[2:]
+	if exp != "none" {
+		experiments := strings.Split(exp, ",")
+		for _, expt := range experiments {
+			cfg.BuildContext.BuildTags = append(cfg.BuildContext.BuildTags, "goexperiment."+expt)
+		}
+	}
+}
+
+func instrumentInit() {
+	if !cfg.BuildRace && !cfg.BuildMSan {
+		return
+	}
+	if cfg.BuildRace && cfg.BuildMSan {
+		fmt.Fprintf(os.Stderr, "go %s: may not use -race and -msan simultaneously\n", flag.Args()[0])
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildMSan && !sys.MSanSupported(cfg.Goos, cfg.Goarch) {
+		fmt.Fprintf(os.Stderr, "-msan is not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildRace {
+		if !sys.RaceDetectorSupported(cfg.Goos, cfg.Goarch) {
+			fmt.Fprintf(os.Stderr, "go %s: -race is only supported on linux/amd64, linux/ppc64le, linux/arm64, freebsd/amd64, netbsd/amd64, darwin/amd64, darwin/arm64, and windows/amd64\n", flag.Args()[0])
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+	}
+	mode := "race"
+	if cfg.BuildMSan {
+		mode = "msan"
+		// MSAN does not support non-PIE binaries on ARM64.
+		// See issue #33712 for details.
+		if cfg.Goos == "linux" && cfg.Goarch == "arm64" && cfg.BuildBuildmode == "default" {
+			cfg.BuildBuildmode = "pie"
+		}
+	}
+	modeFlag := "-" + mode
+
+	if !cfg.BuildContext.CgoEnabled {
+		if runtime.GOOS != cfg.Goos || runtime.GOARCH != cfg.Goarch {
+			fmt.Fprintf(os.Stderr, "go %s: %s requires cgo\n", flag.Args()[0], modeFlag)
+		} else {
+			fmt.Fprintf(os.Stderr, "go %s: %s requires cgo; enable cgo by setting CGO_ENABLED=1\n", flag.Args()[0], modeFlag)
+		}
+
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	forcedGcflags = append(forcedGcflags, modeFlag)
+	forcedLdflags = append(forcedLdflags, modeFlag)
+
+	if cfg.BuildContext.InstallSuffix != "" {
+		cfg.BuildContext.InstallSuffix += "_"
+	}
+	cfg.BuildContext.InstallSuffix += mode
+	cfg.BuildContext.BuildTags = append(cfg.BuildContext.BuildTags, mode)
+}
+
+func buildModeInit() {
+	gccgo := cfg.BuildToolchainName == "gccgo"
+	var codegenArg string
+
+	// Configure the build mode first, then verify that it is supported.
+	// That way, if the flag is completely bogus we will prefer to error out with
+	// "-buildmode=%s not supported" instead of naming the specific platform.
+
+	switch cfg.BuildBuildmode {
+	case "archive":
+		pkgsFilter = pkgsNotMain
+	case "c-archive":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			switch cfg.Goos {
+			case "darwin", "ios":
+				switch cfg.Goarch {
+				case "arm64":
+					codegenArg = "-shared"
+				}
+
+			case "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
+				// Use -shared so that the result is
+				// suitable for inclusion in a PIE or
+				// shared library.
+				codegenArg = "-shared"
+			}
+		}
+		cfg.ExeSuffix = ".a"
+		ldBuildmode = "c-archive"
+	case "c-shared":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			switch cfg.Goos {
+			case "linux", "android", "freebsd":
+				codegenArg = "-shared"
+			case "windows":
+				// Do not add usual .exe suffix to the .dll file.
+				cfg.ExeSuffix = ""
+			}
+		}
+		ldBuildmode = "c-shared"
+	case "default":
+		switch cfg.Goos {
+		case "android":
+			codegenArg = "-shared"
+			ldBuildmode = "pie"
+		case "windows":
+			ldBuildmode = "pie"
+		case "ios":
+			codegenArg = "-shared"
+			ldBuildmode = "pie"
+		case "darwin":
+			switch cfg.Goarch {
+			case "arm64":
+				codegenArg = "-shared"
+			}
+			fallthrough
+		default:
+			ldBuildmode = "exe"
+		}
+		if gccgo {
+			codegenArg = ""
+		}
+	case "exe":
+		pkgsFilter = pkgsMain
+		ldBuildmode = "exe"
+		// Set the pkgsFilter to oneMainPkg if the user passed a specific binary output
+		// and is using buildmode=exe for a better error message.
+		// See issue #20017.
+		if cfg.BuildO != "" {
+			pkgsFilter = oneMainPkg
+		}
+	case "pie":
+		if cfg.BuildRace {
+			base.Fatalf("-buildmode=pie not supported when -race is enabled")
+		}
+		if gccgo {
+			codegenArg = "-fPIE"
+		} else {
+			switch cfg.Goos {
+			case "aix", "windows":
+			default:
+				codegenArg = "-shared"
+			}
+		}
+		ldBuildmode = "pie"
+	case "shared":
+		pkgsFilter = pkgsNotMain
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			codegenArg = "-dynlink"
+		}
+		if cfg.BuildO != "" {
+			base.Fatalf("-buildmode=shared and -o not supported together")
+		}
+		ldBuildmode = "shared"
+	case "plugin":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			codegenArg = "-dynlink"
+		}
+		cfg.ExeSuffix = ".so"
+		ldBuildmode = "plugin"
+	default:
+		base.Fatalf("buildmode=%s not supported", cfg.BuildBuildmode)
+	}
+
+	if !sys.BuildModeSupported(cfg.BuildToolchainName, cfg.BuildBuildmode, cfg.Goos, cfg.Goarch) {
+		base.Fatalf("-buildmode=%s not supported on %s/%s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
+	}
+
+	if cfg.BuildLinkshared {
+		if !sys.BuildModeSupported(cfg.BuildToolchainName, "shared", cfg.Goos, cfg.Goarch) {
+			base.Fatalf("-linkshared not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		}
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			forcedAsmflags = append(forcedAsmflags, "-D=GOBUILDMODE_shared=1",
+				"-linkshared")
+			codegenArg = "-dynlink"
+			forcedGcflags = append(forcedGcflags, "-linkshared")
+			// TODO(mwhudson): remove -w when that gets fixed in linker.
+			forcedLdflags = append(forcedLdflags, "-linkshared", "-w")
+		}
+	}
+	if codegenArg != "" {
+		if gccgo {
+			forcedGccgoflags = append([]string{codegenArg}, forcedGccgoflags...)
+		} else {
+			forcedAsmflags = append([]string{codegenArg}, forcedAsmflags...)
+			forcedGcflags = append([]string{codegenArg}, forcedGcflags...)
+		}
+		// Don't alter InstallSuffix when modifying default codegen args.
+		if cfg.BuildBuildmode != "default" || cfg.BuildLinkshared {
+			if cfg.BuildContext.InstallSuffix != "" {
+				cfg.BuildContext.InstallSuffix += "_"
+			}
+			cfg.BuildContext.InstallSuffix += codegenArg[1:]
+		}
+	}
+
+	switch cfg.BuildMod {
+	case "":
+		// Behavior will be determined automatically, as if no flag were passed.
+	case "readonly", "vendor", "mod":
+		if !cfg.ModulesEnabled && !base.InGOFLAGS("-mod") {
+			base.Fatalf("build flag -mod=%s only valid when using modules", cfg.BuildMod)
+		}
+	default:
+		base.Fatalf("-mod=%s not supported (can be '', 'mod', 'readonly', or 'vendor')", cfg.BuildMod)
+	}
+	if !cfg.ModulesEnabled {
+		if cfg.ModCacheRW && !base.InGOFLAGS("-modcacherw") {
+			base.Fatalf("build flag -modcacherw only valid when using modules")
+		}
+		if cfg.ModFile != "" && !base.InGOFLAGS("-mod") {
+			base.Fatalf("build flag -modfile only valid when using modules")
+		}
+	}
+}
diff --git a/src/cmd/go/internal/work/security.go b/src/cmd/go/internal/work/security.go
new file mode 100644
index 0000000..36bbab3
--- /dev/null
+++ b/src/cmd/go/internal/work/security.go
@@ -0,0 +1,310 @@
+// 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.
+
+// Checking of compiler and linker flags.
+// We must avoid flags like -fplugin=, which can allow
+// arbitrary code execution during the build.
+// Do not make changes here without carefully
+// considering the implications.
+// (That's why the code is isolated in a file named security.go.)
+//
+// Note that -Wl,foo means split foo on commas and pass to
+// the linker, so that -Wl,-foo,bar means pass -foo bar to
+// the linker. Similarly -Wa,foo for the assembler and so on.
+// If any of these are permitted, the wildcard portion must
+// disallow commas.
+//
+// Note also that GNU binutils accept any argument @foo
+// as meaning "read more flags from the file foo", so we must
+// guard against any command-line argument beginning with @,
+// even things like "-I @foo".
+// We use load.SafeArg (which is even more conservative)
+// to reject these.
+//
+// Even worse, gcc -I@foo (one arg) turns into cc1 -I @foo (two args),
+// so although gcc doesn't expand the @foo, cc1 will.
+// So out of paranoia, we reject @ at the beginning of every
+// flag argument that might be split into its own argument.
+
+package work
+
+import (
+	"fmt"
+	"internal/lazyregexp"
+	"regexp"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+)
+
+var re = lazyregexp.New
+
+var validCompilerFlags = []*lazyregexp.Regexp{
+	re(`-D([A-Za-z_][A-Za-z0-9_]*)(=[^@\-]*)?`),
+	re(`-U([A-Za-z_][A-Za-z0-9_]*)`),
+	re(`-F([^@\-].*)`),
+	re(`-I([^@\-].*)`),
+	re(`-O`),
+	re(`-O([^@\-].*)`),
+	re(`-W`),
+	re(`-W([^@,]+)`), // -Wall but not -Wa,-foo.
+	re(`-Wa,-mbig-obj`),
+	re(`-Wp,-D([A-Za-z_][A-Za-z0-9_]*)(=[^@,\-]*)?`),
+	re(`-Wp,-U([A-Za-z_][A-Za-z0-9_]*)`),
+	re(`-ansi`),
+	re(`-f(no-)?asynchronous-unwind-tables`),
+	re(`-f(no-)?blocks`),
+	re(`-f(no-)builtin-[a-zA-Z0-9_]*`),
+	re(`-f(no-)?common`),
+	re(`-f(no-)?constant-cfstrings`),
+	re(`-fdiagnostics-show-note-include-stack`),
+	re(`-f(no-)?eliminate-unused-debug-types`),
+	re(`-f(no-)?exceptions`),
+	re(`-f(no-)?fast-math`),
+	re(`-f(no-)?inline-functions`),
+	re(`-finput-charset=([^@\-].*)`),
+	re(`-f(no-)?fat-lto-objects`),
+	re(`-f(no-)?keep-inline-dllexport`),
+	re(`-f(no-)?lto`),
+	re(`-fmacro-backtrace-limit=(.+)`),
+	re(`-fmessage-length=(.+)`),
+	re(`-f(no-)?modules`),
+	re(`-f(no-)?objc-arc`),
+	re(`-f(no-)?objc-nonfragile-abi`),
+	re(`-f(no-)?objc-legacy-dispatch`),
+	re(`-f(no-)?omit-frame-pointer`),
+	re(`-f(no-)?openmp(-simd)?`),
+	re(`-f(no-)?permissive`),
+	re(`-f(no-)?(pic|PIC|pie|PIE)`),
+	re(`-f(no-)?plt`),
+	re(`-f(no-)?rtti`),
+	re(`-f(no-)?split-stack`),
+	re(`-f(no-)?stack-(.+)`),
+	re(`-f(no-)?strict-aliasing`),
+	re(`-f(un)signed-char`),
+	re(`-f(no-)?use-linker-plugin`), // safe if -B is not used; we don't permit -B
+	re(`-f(no-)?visibility-inlines-hidden`),
+	re(`-fsanitize=(.+)`),
+	re(`-ftemplate-depth-(.+)`),
+	re(`-fvisibility=(.+)`),
+	re(`-g([^@\-].*)?`),
+	re(`-m32`),
+	re(`-m64`),
+	re(`-m(abi|arch|cpu|fpu|tune)=([^@\-].*)`),
+	re(`-m(no-)?v?aes`),
+	re(`-marm`),
+	re(`-m(no-)?avx[0-9a-z]*`),
+	re(`-mfloat-abi=([^@\-].*)`),
+	re(`-mfpmath=[0-9a-z,+]*`),
+	re(`-m(no-)?avx[0-9a-z.]*`),
+	re(`-m(no-)?ms-bitfields`),
+	re(`-m(no-)?stack-(.+)`),
+	re(`-mmacosx-(.+)`),
+	re(`-mios-simulator-version-min=(.+)`),
+	re(`-miphoneos-version-min=(.+)`),
+	re(`-mtvos-simulator-version-min=(.+)`),
+	re(`-mtvos-version-min=(.+)`),
+	re(`-mwatchos-simulator-version-min=(.+)`),
+	re(`-mwatchos-version-min=(.+)`),
+	re(`-mnop-fun-dllimport`),
+	re(`-m(no-)?sse[0-9.]*`),
+	re(`-m(no-)?ssse3`),
+	re(`-mthumb(-interwork)?`),
+	re(`-mthreads`),
+	re(`-mwindows`),
+	re(`--param=ssp-buffer-size=[0-9]*`),
+	re(`-pedantic(-errors)?`),
+	re(`-pipe`),
+	re(`-pthread`),
+	re(`-?-std=([^@\-].*)`),
+	re(`-?-stdlib=([^@\-].*)`),
+	re(`--sysroot=([^@\-].*)`),
+	re(`-w`),
+	re(`-x([^@\-].*)`),
+	re(`-v`),
+}
+
+var validCompilerFlagsWithNextArg = []string{
+	"-arch",
+	"-D",
+	"-U",
+	"-I",
+	"-framework",
+	"-include",
+	"-isysroot",
+	"-isystem",
+	"--sysroot",
+	"-target",
+	"-x",
+}
+
+var validLinkerFlags = []*lazyregexp.Regexp{
+	re(`-F([^@\-].*)`),
+	re(`-l([^@\-].*)`),
+	re(`-L([^@\-].*)`),
+	re(`-O`),
+	re(`-O([^@\-].*)`),
+	re(`-f(no-)?(pic|PIC|pie|PIE)`),
+	re(`-f(no-)?openmp(-simd)?`),
+	re(`-fsanitize=([^@\-].*)`),
+	re(`-flat_namespace`),
+	re(`-g([^@\-].*)?`),
+	re(`-headerpad_max_install_names`),
+	re(`-m(abi|arch|cpu|fpu|tune)=([^@\-].*)`),
+	re(`-mfloat-abi=([^@\-].*)`),
+	re(`-mmacosx-(.+)`),
+	re(`-mios-simulator-version-min=(.+)`),
+	re(`-miphoneos-version-min=(.+)`),
+	re(`-mthreads`),
+	re(`-mwindows`),
+	re(`-(pic|PIC|pie|PIE)`),
+	re(`-pthread`),
+	re(`-rdynamic`),
+	re(`-shared`),
+	re(`-?-static([-a-z0-9+]*)`),
+	re(`-?-stdlib=([^@\-].*)`),
+	re(`-v`),
+
+	// Note that any wildcards in -Wl need to exclude comma,
+	// since -Wl splits its argument at commas and passes
+	// them all to the linker uninterpreted. Allowing comma
+	// in a wildcard would allow tunnelling arbitrary additional
+	// linker arguments through one of these.
+	re(`-Wl,--(no-)?allow-multiple-definition`),
+	re(`-Wl,--(no-)?allow-shlib-undefined`),
+	re(`-Wl,--(no-)?as-needed`),
+	re(`-Wl,-Bdynamic`),
+	re(`-Wl,-berok`),
+	re(`-Wl,-Bstatic`),
+	re(`-Wl,-Bsymbolic-functions`),
+	re(`-Wl,-O([^@,\-][^,]*)?`),
+	re(`-Wl,-d[ny]`),
+	re(`-Wl,--disable-new-dtags`),
+	re(`-Wl,-e[=,][a-zA-Z0-9]*`),
+	re(`-Wl,--enable-new-dtags`),
+	re(`-Wl,--end-group`),
+	re(`-Wl,--(no-)?export-dynamic`),
+	re(`-Wl,-E`),
+	re(`-Wl,-framework,[^,@\-][^,]+`),
+	re(`-Wl,--hash-style=(sysv|gnu|both)`),
+	re(`-Wl,-headerpad_max_install_names`),
+	re(`-Wl,--no-undefined`),
+	re(`-Wl,-R([^@\-][^,@]*$)`),
+	re(`-Wl,--just-symbols[=,]([^,@\-][^,@]+)`),
+	re(`-Wl,-rpath(-link)?[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-s`),
+	re(`-Wl,-search_paths_first`),
+	re(`-Wl,-sectcreate,([^,@\-][^,]+),([^,@\-][^,]+),([^,@\-][^,]+)`),
+	re(`-Wl,--start-group`),
+	re(`-Wl,-?-static`),
+	re(`-Wl,-?-subsystem,(native|windows|console|posix|xbox)`),
+	re(`-Wl,-syslibroot[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-undefined[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-?-unresolved-symbols=[^,]+`),
+	re(`-Wl,--(no-)?warn-([^,]+)`),
+	re(`-Wl,-?-wrap[=,][^,@\-][^,]*`),
+	re(`-Wl,-z,(no)?execstack`),
+	re(`-Wl,-z,relro`),
+
+	re(`[a-zA-Z0-9_/].*\.(a|o|obj|dll|dylib|so)`), // direct linker inputs: x.o or libfoo.so (but not -foo.o or @foo.o)
+	re(`\./.*\.(a|o|obj|dll|dylib|so)`),
+}
+
+var validLinkerFlagsWithNextArg = []string{
+	"-arch",
+	"-F",
+	"-l",
+	"-L",
+	"-framework",
+	"-isysroot",
+	"--sysroot",
+	"-target",
+	"-Wl,-framework",
+	"-Wl,-rpath",
+	"-Wl,-R",
+	"-Wl,--just-symbols",
+	"-Wl,-undefined",
+}
+
+func checkCompilerFlags(name, source string, list []string) error {
+	return checkFlags(name, source, list, validCompilerFlags, validCompilerFlagsWithNextArg)
+}
+
+func checkLinkerFlags(name, source string, list []string) error {
+	return checkFlags(name, source, list, validLinkerFlags, validLinkerFlagsWithNextArg)
+}
+
+func checkFlags(name, source string, list []string, valid []*lazyregexp.Regexp, validNext []string) error {
+	// Let users override rules with $CGO_CFLAGS_ALLOW, $CGO_CFLAGS_DISALLOW, etc.
+	var (
+		allow    *regexp.Regexp
+		disallow *regexp.Regexp
+	)
+	if env := cfg.Getenv("CGO_" + name + "_ALLOW"); env != "" {
+		r, err := regexp.Compile(env)
+		if err != nil {
+			return fmt.Errorf("parsing $CGO_%s_ALLOW: %v", name, err)
+		}
+		allow = r
+	}
+	if env := cfg.Getenv("CGO_" + name + "_DISALLOW"); env != "" {
+		r, err := regexp.Compile(env)
+		if err != nil {
+			return fmt.Errorf("parsing $CGO_%s_DISALLOW: %v", name, err)
+		}
+		disallow = r
+	}
+
+Args:
+	for i := 0; i < len(list); i++ {
+		arg := list[i]
+		if disallow != nil && disallow.FindString(arg) == arg {
+			goto Bad
+		}
+		if allow != nil && allow.FindString(arg) == arg {
+			continue Args
+		}
+		for _, re := range valid {
+			if re.FindString(arg) == arg { // must be complete match
+				continue Args
+			}
+		}
+		for _, x := range validNext {
+			if arg == x {
+				if i+1 < len(list) && load.SafeArg(list[i+1]) {
+					i++
+					continue Args
+				}
+
+				// Permit -Wl,-framework -Wl,name.
+				if i+1 < len(list) &&
+					strings.HasPrefix(arg, "-Wl,") &&
+					strings.HasPrefix(list[i+1], "-Wl,") &&
+					load.SafeArg(list[i+1][4:]) &&
+					!strings.Contains(list[i+1][4:], ",") {
+					i++
+					continue Args
+				}
+
+				// Permit -I= /path, -I $SYSROOT.
+				if i+1 < len(list) && arg == "-I" {
+					if (strings.HasPrefix(list[i+1], "=") || strings.HasPrefix(list[i+1], "$SYSROOT")) &&
+						load.SafeArg(list[i+1][1:]) {
+						i++
+						continue Args
+					}
+				}
+
+				if i+1 < len(list) {
+					return fmt.Errorf("invalid flag in %s: %s %s (see https://golang.org/s/invalidflag)", source, arg, list[i+1])
+				}
+				return fmt.Errorf("invalid flag in %s: %s without argument (see https://golang.org/s/invalidflag)", source, arg)
+			}
+		}
+	Bad:
+		return fmt.Errorf("invalid flag in %s: %s", source, arg)
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/work/security_test.go b/src/cmd/go/internal/work/security_test.go
new file mode 100644
index 0000000..4f2e0eb
--- /dev/null
+++ b/src/cmd/go/internal/work/security_test.go
@@ -0,0 +1,277 @@
+// 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 work
+
+import (
+	"os"
+	"testing"
+)
+
+var goodCompilerFlags = [][]string{
+	{"-DFOO"},
+	{"-Dfoo=bar"},
+	{"-Ufoo"},
+	{"-Ufoo1"},
+	{"-F/Qt"},
+	{"-I/"},
+	{"-I/etc/passwd"},
+	{"-I."},
+	{"-O"},
+	{"-O2"},
+	{"-Osmall"},
+	{"-W"},
+	{"-Wall"},
+	{"-Wp,-Dfoo=bar"},
+	{"-Wp,-Ufoo"},
+	{"-Wp,-Dfoo1"},
+	{"-Wp,-Ufoo1"},
+	{"-fobjc-arc"},
+	{"-fno-objc-arc"},
+	{"-fomit-frame-pointer"},
+	{"-fno-omit-frame-pointer"},
+	{"-fpic"},
+	{"-fno-pic"},
+	{"-fPIC"},
+	{"-fno-PIC"},
+	{"-fpie"},
+	{"-fno-pie"},
+	{"-fPIE"},
+	{"-fno-PIE"},
+	{"-fsplit-stack"},
+	{"-fno-split-stack"},
+	{"-fstack-xxx"},
+	{"-fno-stack-xxx"},
+	{"-fsanitize=hands"},
+	{"-g"},
+	{"-ggdb"},
+	{"-march=souza"},
+	{"-mcpu=123"},
+	{"-mfpu=123"},
+	{"-mtune=happybirthday"},
+	{"-mstack-overflow"},
+	{"-mno-stack-overflow"},
+	{"-mmacosx-version"},
+	{"-mnop-fun-dllimport"},
+	{"-pthread"},
+	{"-std=c99"},
+	{"-xc"},
+	{"-D", "FOO"},
+	{"-D", "foo=bar"},
+	{"-I", "."},
+	{"-I", "/etc/passwd"},
+	{"-I", "世界"},
+	{"-I", "=/usr/include/libxml2"},
+	{"-I", "dir"},
+	{"-I", "$SYSROOT/dir"},
+	{"-isystem", "/usr/include/mozjs-68"},
+	{"-include", "/usr/include/mozjs-68/RequiredDefines.h"},
+	{"-framework", "Chocolate"},
+	{"-x", "c"},
+	{"-v"},
+}
+
+var badCompilerFlags = [][]string{
+	{"-D@X"},
+	{"-D-X"},
+	{"-Ufoo=bar"},
+	{"-F@dir"},
+	{"-F-dir"},
+	{"-I@dir"},
+	{"-I-dir"},
+	{"-O@1"},
+	{"-Wa,-foo"},
+	{"-W@foo"},
+	{"-Wp,-DX,-D@X"},
+	{"-Wp,-UX,-U@X"},
+	{"-g@gdb"},
+	{"-g-gdb"},
+	{"-march=@dawn"},
+	{"-march=-dawn"},
+	{"-std=@c99"},
+	{"-std=-c99"},
+	{"-x@c"},
+	{"-x-c"},
+	{"-D", "@foo"},
+	{"-D", "-foo"},
+	{"-I", "@foo"},
+	{"-I", "-foo"},
+	{"-I", "=@obj"},
+	{"-include", "@foo"},
+	{"-framework", "-Caffeine"},
+	{"-framework", "@Home"},
+	{"-x", "--c"},
+	{"-x", "@obj"},
+}
+
+func TestCheckCompilerFlags(t *testing.T) {
+	for _, f := range goodCompilerFlags {
+		if err := checkCompilerFlags("test", "test", f); err != nil {
+			t.Errorf("unexpected error for %q: %v", f, err)
+		}
+	}
+	for _, f := range badCompilerFlags {
+		if err := checkCompilerFlags("test", "test", f); err == nil {
+			t.Errorf("missing error for %q", f)
+		}
+	}
+}
+
+var goodLinkerFlags = [][]string{
+	{"-Fbar"},
+	{"-lbar"},
+	{"-Lbar"},
+	{"-fpic"},
+	{"-fno-pic"},
+	{"-fPIC"},
+	{"-fno-PIC"},
+	{"-fpie"},
+	{"-fno-pie"},
+	{"-fPIE"},
+	{"-fno-PIE"},
+	{"-fsanitize=hands"},
+	{"-g"},
+	{"-ggdb"},
+	{"-march=souza"},
+	{"-mcpu=123"},
+	{"-mfpu=123"},
+	{"-mtune=happybirthday"},
+	{"-pic"},
+	{"-pthread"},
+	{"-Wl,--hash-style=both"},
+	{"-Wl,-rpath,foo"},
+	{"-Wl,-rpath,$ORIGIN/foo"},
+	{"-Wl,-R", "/foo"},
+	{"-Wl,-R", "foo"},
+	{"-Wl,-R,foo"},
+	{"-Wl,--just-symbols=foo"},
+	{"-Wl,--just-symbols,foo"},
+	{"-Wl,--warn-error"},
+	{"-Wl,--no-warn-error"},
+	{"foo.so"},
+	{"_世界.dll"},
+	{"./x.o"},
+	{"libcgosotest.dylib"},
+	{"-F", "framework"},
+	{"-l", "."},
+	{"-l", "/etc/passwd"},
+	{"-l", "世界"},
+	{"-L", "framework"},
+	{"-framework", "Chocolate"},
+	{"-v"},
+	{"-Wl,-sectcreate,__TEXT,__info_plist,${SRCDIR}/Info.plist"},
+	{"-Wl,-framework", "-Wl,Chocolate"},
+	{"-Wl,-framework,Chocolate"},
+	{"-Wl,-unresolved-symbols=ignore-all"},
+}
+
+var badLinkerFlags = [][]string{
+	{"-DFOO"},
+	{"-Dfoo=bar"},
+	{"-W"},
+	{"-Wall"},
+	{"-fobjc-arc"},
+	{"-fno-objc-arc"},
+	{"-fomit-frame-pointer"},
+	{"-fno-omit-frame-pointer"},
+	{"-fsplit-stack"},
+	{"-fno-split-stack"},
+	{"-fstack-xxx"},
+	{"-fno-stack-xxx"},
+	{"-mstack-overflow"},
+	{"-mno-stack-overflow"},
+	{"-mnop-fun-dllimport"},
+	{"-std=c99"},
+	{"-xc"},
+	{"-D", "FOO"},
+	{"-D", "foo=bar"},
+	{"-I", "FOO"},
+	{"-L", "@foo"},
+	{"-L", "-foo"},
+	{"-x", "c"},
+	{"-D@X"},
+	{"-D-X"},
+	{"-I@dir"},
+	{"-I-dir"},
+	{"-O@1"},
+	{"-Wa,-foo"},
+	{"-W@foo"},
+	{"-g@gdb"},
+	{"-g-gdb"},
+	{"-march=@dawn"},
+	{"-march=-dawn"},
+	{"-std=@c99"},
+	{"-std=-c99"},
+	{"-x@c"},
+	{"-x-c"},
+	{"-D", "@foo"},
+	{"-D", "-foo"},
+	{"-I", "@foo"},
+	{"-I", "-foo"},
+	{"-l", "@foo"},
+	{"-l", "-foo"},
+	{"-framework", "-Caffeine"},
+	{"-framework", "@Home"},
+	{"-Wl,-framework,-Caffeine"},
+	{"-Wl,-framework", "-Wl,@Home"},
+	{"-Wl,-framework", "@Home"},
+	{"-Wl,-framework,Chocolate,@Home"},
+	{"-Wl,--hash-style=foo"},
+	{"-x", "--c"},
+	{"-x", "@obj"},
+	{"-Wl,-rpath,@foo"},
+	{"-Wl,-R,foo,bar"},
+	{"-Wl,-R,@foo"},
+	{"-Wl,--just-symbols,@foo"},
+	{"../x.o"},
+}
+
+func TestCheckLinkerFlags(t *testing.T) {
+	for _, f := range goodLinkerFlags {
+		if err := checkLinkerFlags("test", "test", f); err != nil {
+			t.Errorf("unexpected error for %q: %v", f, err)
+		}
+	}
+	for _, f := range badLinkerFlags {
+		if err := checkLinkerFlags("test", "test", f); err == nil {
+			t.Errorf("missing error for %q", f)
+		}
+	}
+}
+
+func TestCheckFlagAllowDisallow(t *testing.T) {
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err == nil {
+		t.Fatalf("missing error for -disallow")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-disallo")
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err == nil {
+		t.Fatalf("missing error for -disallow with CGO_TEST_ALLOW=-disallo")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-disallow")
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err != nil {
+		t.Fatalf("unexpected error for -disallow with CGO_TEST_ALLOW=-disallow: %v", err)
+	}
+	os.Unsetenv("CGO_TEST_ALLOW")
+
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err != nil {
+		t.Fatalf("unexpected error for -Wall: %v", err)
+	}
+	os.Setenv("CGO_TEST_DISALLOW", "-Wall")
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err == nil {
+		t.Fatalf("missing error for -Wall with CGO_TEST_DISALLOW=-Wall")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-Wall") // disallow wins
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err == nil {
+		t.Fatalf("missing error for -Wall with CGO_TEST_DISALLOW=-Wall and CGO_TEST_ALLOW=-Wall")
+	}
+
+	os.Setenv("CGO_TEST_ALLOW", "-fplugin.*")
+	os.Setenv("CGO_TEST_DISALLOW", "-fplugin=lint.so")
+	if err := checkCompilerFlags("TEST", "test", []string{"-fplugin=faster.so"}); err != nil {
+		t.Fatalf("unexpected error for -fplugin=faster.so: %v", err)
+	}
+	if err := checkCompilerFlags("TEST", "test", []string{"-fplugin=lint.so"}); err == nil {
+		t.Fatalf("missing error for -fplugin=lint.so: %v", err)
+	}
+}
diff --git a/src/cmd/go/internal/work/testgo.go b/src/cmd/go/internal/work/testgo.go
new file mode 100644
index 0000000..931f49a
--- /dev/null
+++ b/src/cmd/go/internal/work/testgo.go
@@ -0,0 +1,48 @@
+// 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.
+
+// This file contains extra hooks for testing the go command.
+
+// +build testgo
+
+package work
+
+import (
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/search"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+func init() {
+	if v := os.Getenv("TESTGO_VERSION"); v != "" {
+		runtimeVersion = v
+	}
+
+	if testGOROOT := os.Getenv("TESTGO_GOROOT"); testGOROOT != "" {
+		// Disallow installs to the GOROOT from which testgo was built.
+		// Installs to other GOROOTs — such as one set explicitly within a test — are ok.
+		allowInstall = func(a *Action) error {
+			if cfg.BuildN {
+				return nil
+			}
+
+			rel := search.InDir(a.Target, testGOROOT)
+			if rel == "" {
+				return nil
+			}
+
+			callerPos := ""
+			if _, file, line, ok := runtime.Caller(1); ok {
+				if shortFile := search.InDir(file, filepath.Join(testGOROOT, "src")); shortFile != "" {
+					file = shortFile
+				}
+				callerPos = fmt.Sprintf("%s:%d: ", file, line)
+			}
+			return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s)", callerPos, filepath.Join("GOROOT", rel))
+		}
+	}
+}