Adding upstream version 1.16.10.upstream/1.16.10upstream

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

1 files changed, 1413 insertions, 0 deletions

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
new file mode 100644
index 0000000..5b2ccdd
--- /dev/null
+++ b/src/runtime/panic.go
@@ -0,0 +1,1413 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import (
+	"runtime/internal/atomic"
+	"runtime/internal/sys"
+	"unsafe"
+)
+
+// We have two different ways of doing defers. The older way involves creating a
+// defer record at the time that a defer statement is executing and adding it to a
+// defer chain. This chain is inspected by the deferreturn call at all function
+// exits in order to run the appropriate defer calls. A cheaper way (which we call
+// open-coded defers) is used for functions in which no defer statements occur in
+// loops. In that case, we simply store the defer function/arg information into
+// specific stack slots at the point of each defer statement, as well as setting a
+// bit in a bitmask. At each function exit, we add inline code to directly make
+// the appropriate defer calls based on the bitmask and fn/arg information stored
+// on the stack. During panic/Goexit processing, the appropriate defer calls are
+// made using extra funcdata info that indicates the exact stack slots that
+// contain the bitmask and defer fn/args.
+
+// Check to make sure we can really generate a panic. If the panic
+// was generated from the runtime, or from inside malloc, then convert
+// to a throw of msg.
+// pc should be the program counter of the compiler-generated code that
+// triggered this panic.
+func panicCheck1(pc uintptr, msg string) {
+	if sys.GoarchWasm == 0 && hasPrefix(funcname(findfunc(pc)), "runtime.") {
+		// Note: wasm can't tail call, so we can't get the original caller's pc.
+		throw(msg)
+	}
+	// TODO: is this redundant? How could we be in malloc
+	// but not in the runtime? runtime/internal/*, maybe?
+	gp := getg()
+	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
+		throw(msg)
+	}
+}
+
+// Same as above, but calling from the runtime is allowed.
+//
+// Using this function is necessary for any panic that may be
+// generated by runtime.sigpanic, since those are always called by the
+// runtime.
+func panicCheck2(err string) {
+	// panic allocates, so to avoid recursive malloc, turn panics
+	// during malloc into throws.
+	gp := getg()
+	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
+		throw(err)
+	}
+}
+
+// Many of the following panic entry-points turn into throws when they
+// happen in various runtime contexts. These should never happen in
+// the runtime, and if they do, they indicate a serious issue and
+// should not be caught by user code.
+//
+// The panic{Index,Slice,divide,shift} functions are called by
+// code generated by the compiler for out of bounds index expressions,
+// out of bounds slice expressions, division by zero, and shift by negative.
+// The panicdivide (again), panicoverflow, panicfloat, and panicmem
+// functions are called by the signal handler when a signal occurs
+// indicating the respective problem.
+//
+// Since panic{Index,Slice,shift} are never called directly, and
+// since the runtime package should never have an out of bounds slice
+// or array reference or negative shift, if we see those functions called from the
+// runtime package we turn the panic into a throw. That will dump the
+// entire runtime stack for easier debugging.
+//
+// The entry points called by the signal handler will be called from
+// runtime.sigpanic, so we can't disallow calls from the runtime to
+// these (they always look like they're called from the runtime).
+// Hence, for these, we just check for clearly bad runtime conditions.
+//
+// The panic{Index,Slice} functions are implemented in assembly and tail call
+// to the goPanic{Index,Slice} functions below. This is done so we can use
+// a space-minimal register calling convention.
+
+// failures in the comparisons for s[x], 0 <= x < y (y == len(s))
+func goPanicIndex(x int, y int) {
+	panicCheck1(getcallerpc(), "index out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
+}
+func goPanicIndexU(x uint, y int) {
+	panicCheck1(getcallerpc(), "index out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsIndex})
+}
+
+// failures in the comparisons for s[:x], 0 <= x <= y (y == len(s) or cap(s))
+func goPanicSliceAlen(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAlen})
+}
+func goPanicSliceAlenU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAlen})
+}
+func goPanicSliceAcap(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAcap})
+}
+func goPanicSliceAcapU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAcap})
+}
+
+// failures in the comparisons for s[x:y], 0 <= x <= y
+func goPanicSliceB(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceB})
+}
+func goPanicSliceBU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceB})
+}
+
+// failures in the comparisons for s[::x], 0 <= x <= y (y == len(s) or cap(s))
+func goPanicSlice3Alen(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Alen})
+}
+func goPanicSlice3AlenU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Alen})
+}
+func goPanicSlice3Acap(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Acap})
+}
+func goPanicSlice3AcapU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Acap})
+}
+
+// failures in the comparisons for s[:x:y], 0 <= x <= y
+func goPanicSlice3B(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3B})
+}
+func goPanicSlice3BU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3B})
+}
+
+// failures in the comparisons for s[x:y:], 0 <= x <= y
+func goPanicSlice3C(x int, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3C})
+}
+func goPanicSlice3CU(x uint, y int) {
+	panicCheck1(getcallerpc(), "slice bounds out of range")
+	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
+}
+
+// Implemented in assembly, as they take arguments in registers.
+// Declared here to mark them as ABIInternal.
+func panicIndex(x int, y int)
+func panicIndexU(x uint, y int)
+func panicSliceAlen(x int, y int)
+func panicSliceAlenU(x uint, y int)
+func panicSliceAcap(x int, y int)
+func panicSliceAcapU(x uint, y int)
+func panicSliceB(x int, y int)
+func panicSliceBU(x uint, y int)
+func panicSlice3Alen(x int, y int)
+func panicSlice3AlenU(x uint, y int)
+func panicSlice3Acap(x int, y int)
+func panicSlice3AcapU(x uint, y int)
+func panicSlice3B(x int, y int)
+func panicSlice3BU(x uint, y int)
+func panicSlice3C(x int, y int)
+func panicSlice3CU(x uint, y int)
+
+var shiftError = error(errorString("negative shift amount"))
+
+func panicshift() {
+	panicCheck1(getcallerpc(), "negative shift amount")
+	panic(shiftError)
+}
+
+var divideError = error(errorString("integer divide by zero"))
+
+func panicdivide() {
+	panicCheck2("integer divide by zero")
+	panic(divideError)
+}
+
+var overflowError = error(errorString("integer overflow"))
+
+func panicoverflow() {
+	panicCheck2("integer overflow")
+	panic(overflowError)
+}
+
+var floatError = error(errorString("floating point error"))
+
+func panicfloat() {
+	panicCheck2("floating point error")
+	panic(floatError)
+}
+
+var memoryError = error(errorString("invalid memory address or nil pointer dereference"))
+
+func panicmem() {
+	panicCheck2("invalid memory address or nil pointer dereference")
+	panic(memoryError)
+}
+
+func panicmemAddr(addr uintptr) {
+	panicCheck2("invalid memory address or nil pointer dereference")
+	panic(errorAddressString{msg: "invalid memory address or nil pointer dereference", addr: addr})
+}
+
+// Create a new deferred function fn with siz bytes of arguments.
+// The compiler turns a defer statement into a call to this.
+//go:nosplit
+func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
+	gp := getg()
+	if gp.m.curg != gp {
+		// go code on the system stack can't defer
+		throw("defer on system stack")
+	}
+
+	// the arguments of fn are in a perilous state. The stack map
+	// for deferproc does not describe them. So we can't let garbage
+	// collection or stack copying trigger until we've copied them out
+	// to somewhere safe. The memmove below does that.
+	// Until the copy completes, we can only call nosplit routines.
+	sp := getcallersp()
+	argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
+	callerpc := getcallerpc()
+
+	d := newdefer(siz)
+	if d._panic != nil {
+		throw("deferproc: d.panic != nil after newdefer")
+	}
+	d.link = gp._defer
+	gp._defer = d
+	d.fn = fn
+	d.pc = callerpc
+	d.sp = sp
+	switch siz {
+	case 0:
+		// Do nothing.
+	case sys.PtrSize:
+		*(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
+	default:
+		memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))
+	}
+
+	// deferproc returns 0 normally.
+	// a deferred func that stops a panic
+	// makes the deferproc return 1.
+	// the code the compiler generates always
+	// checks the return value and jumps to the
+	// end of the function if deferproc returns != 0.
+	return0()
+	// No code can go here - the C return register has
+	// been set and must not be clobbered.
+}
+
+// deferprocStack queues a new deferred function with a defer record on the stack.
+// The defer record must have its siz and fn fields initialized.
+// All other fields can contain junk.
+// The defer record must be immediately followed in memory by
+// the arguments of the defer.
+// Nosplit because the arguments on the stack won't be scanned
+// until the defer record is spliced into the gp._defer list.
+//go:nosplit
+func deferprocStack(d *_defer) {
+	gp := getg()
+	if gp.m.curg != gp {
+		// go code on the system stack can't defer
+		throw("defer on system stack")
+	}
+	// siz and fn are already set.
+	// The other fields are junk on entry to deferprocStack and
+	// are initialized here.
+	d.started = false
+	d.heap = false
+	d.openDefer = false
+	d.sp = getcallersp()
+	d.pc = getcallerpc()
+	d.framepc = 0
+	d.varp = 0
+	// The lines below implement:
+	//   d.panic = nil
+	//   d.fd = nil
+	//   d.link = gp._defer
+	//   gp._defer = d
+	// But without write barriers. The first three are writes to
+	// the stack so they don't need a write barrier, and furthermore
+	// are to uninitialized memory, so they must not use a write barrier.
+	// The fourth write does not require a write barrier because we
+	// explicitly mark all the defer structures, so we don't need to
+	// keep track of pointers to them with a write barrier.
+	*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
+	*(*uintptr)(unsafe.Pointer(&d.fd)) = 0
+	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
+	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
+
+	return0()
+	// No code can go here - the C return register has
+	// been set and must not be clobbered.
+}
+
+// Small malloc size classes >= 16 are the multiples of 16: 16, 32, 48, 64, 80, 96, 112, 128, 144, ...
+// Each P holds a pool for defers with small arg sizes.
+// Assign defer allocations to pools by rounding to 16, to match malloc size classes.
+
+const (
+	deferHeaderSize = unsafe.Sizeof(_defer{})
+	minDeferAlloc   = (deferHeaderSize + 15) &^ 15
+	minDeferArgs    = minDeferAlloc - deferHeaderSize
+)
+
+// defer size class for arg size sz
+//go:nosplit
+func deferclass(siz uintptr) uintptr {
+	if siz <= minDeferArgs {
+		return 0
+	}
+	return (siz - minDeferArgs + 15) / 16
+}
+
+// total size of memory block for defer with arg size sz
+func totaldefersize(siz uintptr) uintptr {
+	if siz <= minDeferArgs {
+		return minDeferAlloc
+	}
+	return deferHeaderSize + siz
+}
+
+// Ensure that defer arg sizes that map to the same defer size class
+// also map to the same malloc size class.
+func testdefersizes() {
+	var m [len(p{}.deferpool)]int32
+
+	for i := range m {
+		m[i] = -1
+	}
+	for i := uintptr(0); ; i++ {
+		defersc := deferclass(i)
+		if defersc >= uintptr(len(m)) {
+			break
+		}
+		siz := roundupsize(totaldefersize(i))
+		if m[defersc] < 0 {
+			m[defersc] = int32(siz)
+			continue
+		}
+		if m[defersc] != int32(siz) {
+			print("bad defer size class: i=", i, " siz=", siz, " defersc=", defersc, "\n")
+			throw("bad defer size class")
+		}
+	}
+}
+
+// The arguments associated with a deferred call are stored
+// immediately after the _defer header in memory.
+//go:nosplit
+func deferArgs(d *_defer) unsafe.Pointer {
+	if d.siz == 0 {
+		// Avoid pointer past the defer allocation.
+		return nil
+	}
+	return add(unsafe.Pointer(d), unsafe.Sizeof(*d))
+}
+
+var deferType *_type // type of _defer struct
+
+func init() {
+	var x interface{}
+	x = (*_defer)(nil)
+	deferType = (*(**ptrtype)(unsafe.Pointer(&x))).elem
+}
+
+// Allocate a Defer, usually using per-P pool.
+// Each defer must be released with freedefer.  The defer is not
+// added to any defer chain yet.
+//
+// This must not grow the stack because there may be a frame without
+// stack map information when this is called.
+//
+//go:nosplit
+func newdefer(siz int32) *_defer {
+	var d *_defer
+	sc := deferclass(uintptr(siz))
+	gp := getg()
+	if sc < uintptr(len(p{}.deferpool)) {
+		pp := gp.m.p.ptr()
+		if len(pp.deferpool[sc]) == 0 && sched.deferpool[sc] != nil {
+			// Take the slow path on the system stack so
+			// we don't grow newdefer's stack.
+			systemstack(func() {
+				lock(&sched.deferlock)
+				for len(pp.deferpool[sc]) < cap(pp.deferpool[sc])/2 && sched.deferpool[sc] != nil {
+					d := sched.deferpool[sc]
+					sched.deferpool[sc] = d.link
+					d.link = nil
+					pp.deferpool[sc] = append(pp.deferpool[sc], d)
+				}
+				unlock(&sched.deferlock)
+			})
+		}
+		if n := len(pp.deferpool[sc]); n > 0 {
+			d = pp.deferpool[sc][n-1]
+			pp.deferpool[sc][n-1] = nil
+			pp.deferpool[sc] = pp.deferpool[sc][:n-1]
+		}
+	}
+	if d == nil {
+		// Allocate new defer+args.
+		systemstack(func() {
+			total := roundupsize(totaldefersize(uintptr(siz)))
+			d = (*_defer)(mallocgc(total, deferType, true))
+		})
+	}
+	d.siz = siz
+	d.heap = true
+	return d
+}
+
+// Free the given defer.
+// The defer cannot be used after this call.
+//
+// This must not grow the stack because there may be a frame without a
+// stack map when this is called.
+//
+//go:nosplit
+func freedefer(d *_defer) {
+	if d._panic != nil {
+		freedeferpanic()
+	}
+	if d.fn != nil {
+		freedeferfn()
+	}
+	if !d.heap {
+		return
+	}
+	sc := deferclass(uintptr(d.siz))
+	if sc >= uintptr(len(p{}.deferpool)) {
+		return
+	}
+	pp := getg().m.p.ptr()
+	if len(pp.deferpool[sc]) == cap(pp.deferpool[sc]) {
+		// Transfer half of local cache to the central cache.
+		//
+		// Take this slow path on the system stack so
+		// we don't grow freedefer's stack.
+		systemstack(func() {
+			var first, last *_defer
+			for len(pp.deferpool[sc]) > cap(pp.deferpool[sc])/2 {
+				n := len(pp.deferpool[sc])
+				d := pp.deferpool[sc][n-1]
+				pp.deferpool[sc][n-1] = nil
+				pp.deferpool[sc] = pp.deferpool[sc][:n-1]
+				if first == nil {
+					first = d
+				} else {
+					last.link = d
+				}
+				last = d
+			}
+			lock(&sched.deferlock)
+			last.link = sched.deferpool[sc]
+			sched.deferpool[sc] = first
+			unlock(&sched.deferlock)
+		})
+	}
+
+	// These lines used to be simply `*d = _defer{}` but that
+	// started causing a nosplit stack overflow via typedmemmove.
+	d.siz = 0
+	d.started = false
+	d.openDefer = false
+	d.sp = 0
+	d.pc = 0
+	d.framepc = 0
+	d.varp = 0
+	d.fd = nil
+	// d._panic and d.fn must be nil already.
+	// If not, we would have called freedeferpanic or freedeferfn above,
+	// both of which throw.
+	d.link = nil
+
+	pp.deferpool[sc] = append(pp.deferpool[sc], d)
+}
+
+// Separate function so that it can split stack.
+// Windows otherwise runs out of stack space.
+func freedeferpanic() {
+	// _panic must be cleared before d is unlinked from gp.
+	throw("freedefer with d._panic != nil")
+}
+
+func freedeferfn() {
+	// fn must be cleared before d is unlinked from gp.
+	throw("freedefer with d.fn != nil")
+}
+
+// Run a deferred function if there is one.
+// The compiler inserts a call to this at the end of any
+// function which calls defer.
+// If there is a deferred function, this will call runtime·jmpdefer,
+// which will jump to the deferred function such that it appears
+// to have been called by the caller of deferreturn at the point
+// just before deferreturn was called. The effect is that deferreturn
+// is called again and again until there are no more deferred functions.
+//
+// Declared as nosplit, because the function should not be preempted once we start
+// modifying the caller's frame in order to reuse the frame to call the deferred
+// function.
+//
+// The single argument isn't actually used - it just has its address
+// taken so it can be matched against pending defers.
+//go:nosplit
+func deferreturn(arg0 uintptr) {
+	gp := getg()
+	d := gp._defer
+	if d == nil {
+		return
+	}
+	sp := getcallersp()
+	if d.sp != sp {
+		return
+	}
+	if d.openDefer {
+		done := runOpenDeferFrame(gp, d)
+		if !done {
+			throw("unfinished open-coded defers in deferreturn")
+		}
+		gp._defer = d.link
+		freedefer(d)
+		return
+	}
+
+	// Moving arguments around.
+	//
+	// Everything called after this point must be recursively
+	// nosplit because the garbage collector won't know the form
+	// of the arguments until the jmpdefer can flip the PC over to
+	// fn.
+	switch d.siz {
+	case 0:
+		// Do nothing.
+	case sys.PtrSize:
+		*(*uintptr)(unsafe.Pointer(&arg0)) = *(*uintptr)(deferArgs(d))
+	default:
+		memmove(unsafe.Pointer(&arg0), deferArgs(d), uintptr(d.siz))
+	}
+	fn := d.fn
+	d.fn = nil
+	gp._defer = d.link
+	freedefer(d)
+	// If the defer function pointer is nil, force the seg fault to happen
+	// here rather than in jmpdefer. gentraceback() throws an error if it is
+	// called with a callback on an LR architecture and jmpdefer is on the
+	// stack, because the stack trace can be incorrect in that case - see
+	// issue #8153).
+	_ = fn.fn
+	jmpdefer(fn, uintptr(unsafe.Pointer(&arg0)))
+}
+
+// Goexit terminates the goroutine that calls it. No other goroutine is affected.
+// Goexit runs all deferred calls before terminating the goroutine. Because Goexit
+// is not a panic, any recover calls in those deferred functions will return nil.
+//
+// Calling Goexit from the main goroutine terminates that goroutine
+// without func main returning. Since func main has not returned,
+// the program continues execution of other goroutines.
+// If all other goroutines exit, the program crashes.
+func Goexit() {
+	// Run all deferred functions for the current goroutine.
+	// This code is similar to gopanic, see that implementation
+	// for detailed comments.
+	gp := getg()
+
+	// Create a panic object for Goexit, so we can recognize when it might be
+	// bypassed by a recover().
+	var p _panic
+	p.goexit = true
+	p.link = gp._panic
+	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
+
+	addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
+	for {
+		d := gp._defer
+		if d == nil {
+			break
+		}
+		if d.started {
+			if d._panic != nil {
+				d._panic.aborted = true
+				d._panic = nil
+			}
+			if !d.openDefer {
+				d.fn = nil
+				gp._defer = d.link
+				freedefer(d)
+				continue
+			}
+		}
+		d.started = true
+		d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
+		if d.openDefer {
+			done := runOpenDeferFrame(gp, d)
+			if !done {
+				// We should always run all defers in the frame,
+				// since there is no panic associated with this
+				// defer that can be recovered.
+				throw("unfinished open-coded defers in Goexit")
+			}
+			if p.aborted {
+				// Since our current defer caused a panic and may
+				// have been already freed, just restart scanning
+				// for open-coded defers from this frame again.
+				addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
+			} else {
+				addOneOpenDeferFrame(gp, 0, nil)
+			}
+		} else {
+
+			// Save the pc/sp in reflectcallSave(), so we can "recover" back to this
+			// loop if necessary.
+			reflectcallSave(&p, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz))
+		}
+		if p.aborted {
+			// We had a recursive panic in the defer d we started, and
+			// then did a recover in a defer that was further down the
+			// defer chain than d. In the case of an outstanding Goexit,
+			// we force the recover to return back to this loop. d will
+			// have already been freed if completed, so just continue
+			// immediately to the next defer on the chain.
+			p.aborted = false
+			continue
+		}
+		if gp._defer != d {
+			throw("bad defer entry in Goexit")
+		}
+		d._panic = nil
+		d.fn = nil
+		gp._defer = d.link
+		freedefer(d)
+		// Note: we ignore recovers here because Goexit isn't a panic
+	}
+	goexit1()
+}
+
+// Call all Error and String methods before freezing the world.
+// Used when crashing with panicking.
+func preprintpanics(p *_panic) {
+	defer func() {
+		if recover() != nil {
+			throw("panic while printing panic value")
+		}
+	}()
+	for p != nil {
+		switch v := p.arg.(type) {
+		case error:
+			p.arg = v.Error()
+		case stringer:
+			p.arg = v.String()
+		}
+		p = p.link
+	}
+}
+
+// Print all currently active panics. Used when crashing.
+// Should only be called after preprintpanics.
+func printpanics(p *_panic) {
+	if p.link != nil {
+		printpanics(p.link)
+		if !p.link.goexit {
+			print("\t")
+		}
+	}
+	if p.goexit {
+		return
+	}
+	print("panic: ")
+	printany(p.arg)
+	if p.recovered {
+		print(" [recovered]")
+	}
+	print("\n")
+}
+
+// addOneOpenDeferFrame scans the stack for the first frame (if any) with
+// open-coded defers and if it finds one, adds a single record to the defer chain
+// for that frame. If sp is non-nil, it starts the stack scan from the frame
+// specified by sp. If sp is nil, it uses the sp from the current defer record
+// (which has just been finished). Hence, it continues the stack scan from the
+// frame of the defer that just finished. It skips any frame that already has an
+// open-coded _defer record, which would have been been created from a previous
+// (unrecovered) panic.
+//
+// Note: All entries of the defer chain (including this new open-coded entry) have
+// their pointers (including sp) adjusted properly if the stack moves while
+// running deferred functions. Also, it is safe to pass in the sp arg (which is
+// the direct result of calling getcallersp()), because all pointer variables
+// (including arguments) are adjusted as needed during stack copies.
+func addOneOpenDeferFrame(gp *g, pc uintptr, sp unsafe.Pointer) {
+	var prevDefer *_defer
+	if sp == nil {
+		prevDefer = gp._defer
+		pc = prevDefer.framepc
+		sp = unsafe.Pointer(prevDefer.sp)
+	}
+	systemstack(func() {
+		gentraceback(pc, uintptr(sp), 0, gp, 0, nil, 0x7fffffff,
+			func(frame *stkframe, unused unsafe.Pointer) bool {
+				if prevDefer != nil && prevDefer.sp == frame.sp {
+					// Skip the frame for the previous defer that
+					// we just finished (and was used to set
+					// where we restarted the stack scan)
+					return true
+				}
+				f := frame.fn
+				fd := funcdata(f, _FUNCDATA_OpenCodedDeferInfo)
+				if fd == nil {
+					return true
+				}
+				// Insert the open defer record in the
+				// chain, in order sorted by sp.
+				d := gp._defer
+				var prev *_defer
+				for d != nil {
+					dsp := d.sp
+					if frame.sp < dsp {
+						break
+					}
+					if frame.sp == dsp {
+						if !d.openDefer {
+							throw("duplicated defer entry")
+						}
+						return true
+					}
+					prev = d
+					d = d.link
+				}
+				if frame.fn.deferreturn == 0 {
+					throw("missing deferreturn")
+				}
+
+				maxargsize, _ := readvarintUnsafe(fd)
+				d1 := newdefer(int32(maxargsize))
+				d1.openDefer = true
+				d1._panic = nil
+				// These are the pc/sp to set after we've
+				// run a defer in this frame that did a
+				// recover. We return to a special
+				// deferreturn that runs any remaining
+				// defers and then returns from the
+				// function.
+				d1.pc = frame.fn.entry + uintptr(frame.fn.deferreturn)
+				d1.varp = frame.varp
+				d1.fd = fd
+				// Save the SP/PC associated with current frame,
+				// so we can continue stack trace later if needed.
+				d1.framepc = frame.pc
+				d1.sp = frame.sp
+				d1.link = d
+				if prev == nil {
+					gp._defer = d1
+				} else {
+					prev.link = d1
+				}
+				// Stop stack scanning after adding one open defer record
+				return false
+			},
+			nil, 0)
+	})
+}
+
+// readvarintUnsafe reads the uint32 in varint format starting at fd, and returns the
+// uint32 and a pointer to the byte following the varint.
+//
+// There is a similar function runtime.readvarint, which takes a slice of bytes,
+// rather than an unsafe pointer. These functions are duplicated, because one of
+// the two use cases for the functions would get slower if the functions were
+// combined.
+func readvarintUnsafe(fd unsafe.Pointer) (uint32, unsafe.Pointer) {
+	var r uint32
+	var shift int
+	for {
+		b := *(*uint8)((unsafe.Pointer(fd)))
+		fd = add(fd, unsafe.Sizeof(b))
+		if b < 128 {
+			return r + uint32(b)<<shift, fd
+		}
+		r += ((uint32(b) &^ 128) << shift)
+		shift += 7
+		if shift > 28 {
+			panic("Bad varint")
+		}
+	}
+}
+
+// runOpenDeferFrame runs the active open-coded defers in the frame specified by
+// d. It normally processes all active defers in the frame, but stops immediately
+// if a defer does a successful recover. It returns true if there are no
+// remaining defers to run in the frame.
+func runOpenDeferFrame(gp *g, d *_defer) bool {
+	done := true
+	fd := d.fd
+
+	// Skip the maxargsize
+	_, fd = readvarintUnsafe(fd)
+	deferBitsOffset, fd := readvarintUnsafe(fd)
+	nDefers, fd := readvarintUnsafe(fd)
+	deferBits := *(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset)))
+
+	for i := int(nDefers) - 1; i >= 0; i-- {
+		// read the funcdata info for this defer
+		var argWidth, closureOffset, nArgs uint32
+		argWidth, fd = readvarintUnsafe(fd)
+		closureOffset, fd = readvarintUnsafe(fd)
+		nArgs, fd = readvarintUnsafe(fd)
+		if deferBits&(1<<i) == 0 {
+			for j := uint32(0); j < nArgs; j++ {
+				_, fd = readvarintUnsafe(fd)
+				_, fd = readvarintUnsafe(fd)
+				_, fd = readvarintUnsafe(fd)
+			}
+			continue
+		}
+		closure := *(**funcval)(unsafe.Pointer(d.varp - uintptr(closureOffset)))
+		d.fn = closure
+		deferArgs := deferArgs(d)
+		// If there is an interface receiver or method receiver, it is
+		// described/included as the first arg.
+		for j := uint32(0); j < nArgs; j++ {
+			var argOffset, argLen, argCallOffset uint32
+			argOffset, fd = readvarintUnsafe(fd)
+			argLen, fd = readvarintUnsafe(fd)
+			argCallOffset, fd = readvarintUnsafe(fd)
+			memmove(unsafe.Pointer(uintptr(deferArgs)+uintptr(argCallOffset)),
+				unsafe.Pointer(d.varp-uintptr(argOffset)),
+				uintptr(argLen))
+		}
+		deferBits = deferBits &^ (1 << i)
+		*(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset))) = deferBits
+		p := d._panic
+		reflectcallSave(p, unsafe.Pointer(closure), deferArgs, argWidth)
+		if p != nil && p.aborted {
+			break
+		}
+		d.fn = nil
+		// These args are just a copy, so can be cleared immediately
+		memclrNoHeapPointers(deferArgs, uintptr(argWidth))
+		if d._panic != nil && d._panic.recovered {
+			done = deferBits == 0
+			break
+		}
+	}
+
+	return done
+}
+
+// reflectcallSave calls reflectcall after saving the caller's pc and sp in the
+// panic record. This allows the runtime to return to the Goexit defer processing
+// loop, in the unusual case where the Goexit may be bypassed by a successful
+// recover.
+func reflectcallSave(p *_panic, fn, arg unsafe.Pointer, argsize uint32) {
+	if p != nil {
+		p.argp = unsafe.Pointer(getargp(0))
+		p.pc = getcallerpc()
+		p.sp = unsafe.Pointer(getcallersp())
+	}
+	reflectcall(nil, fn, arg, argsize, argsize)
+	if p != nil {
+		p.pc = 0
+		p.sp = unsafe.Pointer(nil)
+	}
+}
+
+// The implementation of the predeclared function panic.
+func gopanic(e interface{}) {
+	gp := getg()
+	if gp.m.curg != gp {
+		print("panic: ")
+		printany(e)
+		print("\n")
+		throw("panic on system stack")
+	}
+
+	if gp.m.mallocing != 0 {
+		print("panic: ")
+		printany(e)
+		print("\n")
+		throw("panic during malloc")
+	}
+	if gp.m.preemptoff != "" {
+		print("panic: ")
+		printany(e)
+		print("\n")
+		print("preempt off reason: ")
+		print(gp.m.preemptoff)
+		print("\n")
+		throw("panic during preemptoff")
+	}
+	if gp.m.locks != 0 {
+		print("panic: ")
+		printany(e)
+		print("\n")
+		throw("panic holding locks")
+	}
+
+	var p _panic
+	p.arg = e
+	p.link = gp._panic
+	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
+
+	atomic.Xadd(&runningPanicDefers, 1)
+
+	// By calculating getcallerpc/getcallersp here, we avoid scanning the
+	// gopanic frame (stack scanning is slow...)
+	addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
+
+	for {
+		d := gp._defer
+		if d == nil {
+			break
+		}
+
+		// If defer was started by earlier panic or Goexit (and, since we're back here, that triggered a new panic),
+		// take defer off list. An earlier panic will not continue running, but we will make sure below that an
+		// earlier Goexit does continue running.
+		if d.started {
+			if d._panic != nil {
+				d._panic.aborted = true
+			}
+			d._panic = nil
+			if !d.openDefer {
+				// For open-coded defers, we need to process the
+				// defer again, in case there are any other defers
+				// to call in the frame (not including the defer
+				// call that caused the panic).
+				d.fn = nil
+				gp._defer = d.link
+				freedefer(d)
+				continue
+			}
+		}
+
+		// Mark defer as started, but keep on list, so that traceback
+		// can find and update the defer's argument frame if stack growth
+		// or a garbage collection happens before reflectcall starts executing d.fn.
+		d.started = true
+
+		// Record the panic that is running the defer.
+		// If there is a new panic during the deferred call, that panic
+		// will find d in the list and will mark d._panic (this panic) aborted.
+		d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
+
+		done := true
+		if d.openDefer {
+			done = runOpenDeferFrame(gp, d)
+			if done && !d._panic.recovered {
+				addOneOpenDeferFrame(gp, 0, nil)
+			}
+		} else {
+			p.argp = unsafe.Pointer(getargp(0))
+			reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
+		}
+		p.argp = nil
+
+		// reflectcall did not panic. Remove d.
+		if gp._defer != d {
+			throw("bad defer entry in panic")
+		}
+		d._panic = nil
+
+		// trigger shrinkage to test stack copy. See stack_test.go:TestStackPanic
+		//GC()
+
+		pc := d.pc
+		sp := unsafe.Pointer(d.sp) // must be pointer so it gets adjusted during stack copy
+		if done {
+			d.fn = nil
+			gp._defer = d.link
+			freedefer(d)
+		}
+		if p.recovered {
+			gp._panic = p.link
+			if gp._panic != nil && gp._panic.goexit && gp._panic.aborted {
+				// A normal recover would bypass/abort the Goexit.  Instead,
+				// we return to the processing loop of the Goexit.
+				gp.sigcode0 = uintptr(gp._panic.sp)
+				gp.sigcode1 = uintptr(gp._panic.pc)
+				mcall(recovery)
+				throw("bypassed recovery failed") // mcall should not return
+			}
+			atomic.Xadd(&runningPanicDefers, -1)
+
+			// Remove any remaining non-started, open-coded
+			// defer entries after a recover, since the
+			// corresponding defers will be executed normally
+			// (inline). Any such entry will become stale once
+			// we run the corresponding defers inline and exit
+			// the associated stack frame.
+			d := gp._defer
+			var prev *_defer
+			if !done {
+				// Skip our current frame, if not done. It is
+				// needed to complete any remaining defers in
+				// deferreturn()
+				prev = d
+				d = d.link
+			}
+			for d != nil {
+				if d.started {
+					// This defer is started but we
+					// are in the middle of a
+					// defer-panic-recover inside of
+					// it, so don't remove it or any
+					// further defer entries
+					break
+				}
+				if d.openDefer {
+					if prev == nil {
+						gp._defer = d.link
+					} else {
+						prev.link = d.link
+					}
+					newd := d.link
+					freedefer(d)
+					d = newd
+				} else {
+					prev = d
+					d = d.link
+				}
+			}
+
+			gp._panic = p.link
+			// Aborted panics are marked but remain on the g.panic list.
+			// Remove them from the list.
+			for gp._panic != nil && gp._panic.aborted {
+				gp._panic = gp._panic.link
+			}
+			if gp._panic == nil { // must be done with signal
+				gp.sig = 0
+			}
+			// Pass information about recovering frame to recovery.
+			gp.sigcode0 = uintptr(sp)
+			gp.sigcode1 = pc
+			mcall(recovery)
+			throw("recovery failed") // mcall should not return
+		}
+	}
+
+	// ran out of deferred calls - old-school panic now
+	// Because it is unsafe to call arbitrary user code after freezing
+	// the world, we call preprintpanics to invoke all necessary Error
+	// and String methods to prepare the panic strings before startpanic.
+	preprintpanics(gp._panic)
+
+	fatalpanic(gp._panic) // should not return
+	*(*int)(nil) = 0      // not reached
+}
+
+// getargp returns the location where the caller
+// writes outgoing function call arguments.
+//go:nosplit
+//go:noinline
+func getargp(x int) uintptr {
+	// x is an argument mainly so that we can return its address.
+	return uintptr(noescape(unsafe.Pointer(&x)))
+}
+
+// The implementation of the predeclared function recover.
+// Cannot split the stack because it needs to reliably
+// find the stack segment of its caller.
+//
+// TODO(rsc): Once we commit to CopyStackAlways,
+// this doesn't need to be nosplit.
+//go:nosplit
+func gorecover(argp uintptr) interface{} {
+	// Must be in a function running as part of a deferred call during the panic.
+	// Must be called from the topmost function of the call
+	// (the function used in the defer statement).
+	// p.argp is the argument pointer of that topmost deferred function call.
+	// Compare against argp reported by caller.
+	// If they match, the caller is the one who can recover.
+	gp := getg()
+	p := gp._panic
+	if p != nil && !p.goexit && !p.recovered && argp == uintptr(p.argp) {
+		p.recovered = true
+		return p.arg
+	}
+	return nil
+}
+
+//go:linkname sync_throw sync.throw
+func sync_throw(s string) {
+	throw(s)
+}
+
+//go:nosplit
+func throw(s string) {
+	// Everything throw does should be recursively nosplit so it
+	// can be called even when it's unsafe to grow the stack.
+	systemstack(func() {
+		print("fatal error: ", s, "\n")
+	})
+	gp := getg()
+	if gp.m.throwing == 0 {
+		gp.m.throwing = 1
+	}
+	fatalthrow()
+	*(*int)(nil) = 0 // not reached
+}
+
+// runningPanicDefers is non-zero while running deferred functions for panic.
+// runningPanicDefers is incremented and decremented atomically.
+// This is used to try hard to get a panic stack trace out when exiting.
+var runningPanicDefers uint32
+
+// panicking is non-zero when crashing the program for an unrecovered panic.
+// panicking is incremented and decremented atomically.
+var panicking uint32
+
+// paniclk is held while printing the panic information and stack trace,
+// so that two concurrent panics don't overlap their output.
+var paniclk mutex
+
+// Unwind the stack after a deferred function calls recover
+// after a panic. Then arrange to continue running as though
+// the caller of the deferred function returned normally.
+func recovery(gp *g) {
+	// Info about defer passed in G struct.
+	sp := gp.sigcode0
+	pc := gp.sigcode1
+
+	// d's arguments need to be in the stack.
+	if sp != 0 && (sp < gp.stack.lo || gp.stack.hi < sp) {
+		print("recover: ", hex(sp), " not in [", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
+		throw("bad recovery")
+	}
+
+	// Make the deferproc for this d return again,
+	// this time returning 1. The calling function will
+	// jump to the standard return epilogue.
+	gp.sched.sp = sp
+	gp.sched.pc = pc
+	gp.sched.lr = 0
+	gp.sched.ret = 1
+	gogo(&gp.sched)
+}
+
+// fatalthrow implements an unrecoverable runtime throw. It freezes the
+// system, prints stack traces starting from its caller, and terminates the
+// process.
+//
+//go:nosplit
+func fatalthrow() {
+	pc := getcallerpc()
+	sp := getcallersp()
+	gp := getg()
+	// Switch to the system stack to avoid any stack growth, which
+	// may make things worse if the runtime is in a bad state.
+	systemstack(func() {
+		startpanic_m()
+
+		if dopanic_m(gp, pc, sp) {
+			// crash uses a decent amount of nosplit stack and we're already
+			// low on stack in throw, so crash on the system stack (unlike
+			// fatalpanic).
+			crash()
+		}
+
+		exit(2)
+	})
+
+	*(*int)(nil) = 0 // not reached
+}
+
+// fatalpanic implements an unrecoverable panic. It is like fatalthrow, except
+// that if msgs != nil, fatalpanic also prints panic messages and decrements
+// runningPanicDefers once main is blocked from exiting.
+//
+//go:nosplit
+func fatalpanic(msgs *_panic) {
+	pc := getcallerpc()
+	sp := getcallersp()
+	gp := getg()
+	var docrash bool
+	// Switch to the system stack to avoid any stack growth, which
+	// may make things worse if the runtime is in a bad state.
+	systemstack(func() {
+		if startpanic_m() && msgs != nil {
+			// There were panic messages and startpanic_m
+			// says it's okay to try to print them.
+
+			// startpanic_m set panicking, which will
+			// block main from exiting, so now OK to
+			// decrement runningPanicDefers.
+			atomic.Xadd(&runningPanicDefers, -1)
+
+			printpanics(msgs)
+		}
+
+		docrash = dopanic_m(gp, pc, sp)
+	})
+
+	if docrash {
+		// By crashing outside the above systemstack call, debuggers
+		// will not be confused when generating a backtrace.
+		// Function crash is marked nosplit to avoid stack growth.
+		crash()
+	}
+
+	systemstack(func() {
+		exit(2)
+	})
+
+	*(*int)(nil) = 0 // not reached
+}
+
+// startpanic_m prepares for an unrecoverable panic.
+//
+// It returns true if panic messages should be printed, or false if
+// the runtime is in bad shape and should just print stacks.
+//
+// It must not have write barriers even though the write barrier
+// explicitly ignores writes once dying > 0. Write barriers still
+// assume that g.m.p != nil, and this function may not have P
+// in some contexts (e.g. a panic in a signal handler for a signal
+// sent to an M with no P).
+//
+//go:nowritebarrierrec
+func startpanic_m() bool {
+	_g_ := getg()
+	if mheap_.cachealloc.size == 0 { // very early
+		print("runtime: panic before malloc heap initialized\n")
+	}
+	// Disallow malloc during an unrecoverable panic. A panic
+	// could happen in a signal handler, or in a throw, or inside
+	// malloc itself. We want to catch if an allocation ever does
+	// happen (even if we're not in one of these situations).
+	_g_.m.mallocing++
+
+	// If we're dying because of a bad lock count, set it to a
+	// good lock count so we don't recursively panic below.
+	if _g_.m.locks < 0 {
+		_g_.m.locks = 1
+	}
+
+	switch _g_.m.dying {
+	case 0:
+		// Setting dying >0 has the side-effect of disabling this G's writebuf.
+		_g_.m.dying = 1
+		atomic.Xadd(&panicking, 1)
+		lock(&paniclk)
+		if debug.schedtrace > 0 || debug.scheddetail > 0 {
+			schedtrace(true)
+		}
+		freezetheworld()
+		return true
+	case 1:
+		// Something failed while panicking.
+		// Just print a stack trace and exit.
+		_g_.m.dying = 2
+		print("panic during panic\n")
+		return false
+	case 2:
+		// This is a genuine bug in the runtime, we couldn't even
+		// print the stack trace successfully.
+		_g_.m.dying = 3
+		print("stack trace unavailable\n")
+		exit(4)
+		fallthrough
+	default:
+		// Can't even print! Just exit.
+		exit(5)
+		return false // Need to return something.
+	}
+}
+
+var didothers bool
+var deadlock mutex
+
+func dopanic_m(gp *g, pc, sp uintptr) bool {
+	if gp.sig != 0 {
+		signame := signame(gp.sig)
+		if signame != "" {
+			print("[signal ", signame)
+		} else {
+			print("[signal ", hex(gp.sig))
+		}
+		print(" code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
+	}
+
+	level, all, docrash := gotraceback()
+	_g_ := getg()
+	if level > 0 {
+		if gp != gp.m.curg {
+			all = true
+		}
+		if gp != gp.m.g0 {
+			print("\n")
+			goroutineheader(gp)
+			traceback(pc, sp, 0, gp)
+		} else if level >= 2 || _g_.m.throwing > 0 {
+			print("\nruntime stack:\n")
+			traceback(pc, sp, 0, gp)
+		}
+		if !didothers && all {
+			didothers = true
+			tracebackothers(gp)
+		}
+	}
+	unlock(&paniclk)
+
+	if atomic.Xadd(&panicking, -1) != 0 {
+		// Some other m is panicking too.
+		// Let it print what it needs to print.
+		// Wait forever without chewing up cpu.
+		// It will exit when it's done.
+		lock(&deadlock)
+		lock(&deadlock)
+	}
+
+	printDebugLog()
+
+	return docrash
+}
+
+// canpanic returns false if a signal should throw instead of
+// panicking.
+//
+//go:nosplit
+func canpanic(gp *g) bool {
+	// Note that g is m->gsignal, different from gp.
+	// Note also that g->m can change at preemption, so m can go stale
+	// if this function ever makes a function call.
+	_g_ := getg()
+	_m_ := _g_.m
+
+	// Is it okay for gp to panic instead of crashing the program?
+	// Yes, as long as it is running Go code, not runtime code,
+	// and not stuck in a system call.
+	if gp == nil || gp != _m_.curg {
+		return false
+	}
+	if _m_.locks != 0 || _m_.mallocing != 0 || _m_.throwing != 0 || _m_.preemptoff != "" || _m_.dying != 0 {
+		return false
+	}
+	status := readgstatus(gp)
+	if status&^_Gscan != _Grunning || gp.syscallsp != 0 {
+		return false
+	}
+	if GOOS == "windows" && _m_.libcallsp != 0 {
+		return false
+	}
+	return true
+}
+
+// shouldPushSigpanic reports whether pc should be used as sigpanic's
+// return PC (pushing a frame for the call). Otherwise, it should be
+// left alone so that LR is used as sigpanic's return PC, effectively
+// replacing the top-most frame with sigpanic. This is used by
+// preparePanic.
+func shouldPushSigpanic(gp *g, pc, lr uintptr) bool {
+	if pc == 0 {
+		// Probably a call to a nil func. The old LR is more
+		// useful in the stack trace. Not pushing the frame
+		// will make the trace look like a call to sigpanic
+		// instead. (Otherwise the trace will end at sigpanic
+		// and we won't get to see who faulted.)
+		return false
+	}
+	// If we don't recognize the PC as code, but we do recognize
+	// the link register as code, then this assumes the panic was
+	// caused by a call to non-code. In this case, we want to
+	// ignore this call to make unwinding show the context.
+	//
+	// If we running C code, we're not going to recognize pc as a
+	// Go function, so just assume it's good. Otherwise, traceback
+	// may try to read a stale LR that looks like a Go code
+	// pointer and wander into the woods.
+	if gp.m.incgo || findfunc(pc).valid() {
+		// This wasn't a bad call, so use PC as sigpanic's
+		// return PC.
+		return true
+	}
+	if findfunc(lr).valid() {
+		// This was a bad call, but the LR is good, so use the
+		// LR as sigpanic's return PC.
+		return false
+	}
+	// Neither the PC or LR is good. Hopefully pushing a frame
+	// will work.
+	return true
+}
+
+// isAbortPC reports whether pc is the program counter at which
+// runtime.abort raises a signal.
+//
+// It is nosplit because it's part of the isgoexception
+// implementation.
+//
+//go:nosplit
+func isAbortPC(pc uintptr) bool {
+	return pc == funcPC(abort) || ((GOARCH == "arm" || GOARCH == "arm64") && pc == funcPC(abort)+sys.PCQuantum)
+}