Adding upstream version 1.20.14.upstream/1.20.14upstream

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

1 files changed, 1345 insertions, 0 deletions

diff --git a/src/runtime/stack.go b/src/runtime/stack.go
new file mode 100644
index 0000000..d5e587a
--- /dev/null
+++ b/src/runtime/stack.go
@@ -0,0 +1,1345 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import (
+	"internal/abi"
+	"internal/cpu"
+	"internal/goarch"
+	"internal/goos"
+	"runtime/internal/atomic"
+	"runtime/internal/sys"
+	"unsafe"
+)
+
+/*
+Stack layout parameters.
+Included both by runtime (compiled via 6c) and linkers (compiled via gcc).
+
+The per-goroutine g->stackguard is set to point StackGuard bytes
+above the bottom of the stack.  Each function compares its stack
+pointer against g->stackguard to check for overflow.  To cut one
+instruction from the check sequence for functions with tiny frames,
+the stack is allowed to protrude StackSmall bytes below the stack
+guard.  Functions with large frames don't bother with the check and
+always call morestack.  The sequences are (for amd64, others are
+similar):
+
+	guard = g->stackguard
+	frame = function's stack frame size
+	argsize = size of function arguments (call + return)
+
+	stack frame size <= StackSmall:
+		CMPQ guard, SP
+		JHI 3(PC)
+		MOVQ m->morearg, $(argsize << 32)
+		CALL morestack(SB)
+
+	stack frame size > StackSmall but < StackBig
+		LEAQ (frame-StackSmall)(SP), R0
+		CMPQ guard, R0
+		JHI 3(PC)
+		MOVQ m->morearg, $(argsize << 32)
+		CALL morestack(SB)
+
+	stack frame size >= StackBig:
+		MOVQ m->morearg, $((argsize << 32) | frame)
+		CALL morestack(SB)
+
+The bottom StackGuard - StackSmall bytes are important: there has
+to be enough room to execute functions that refuse to check for
+stack overflow, either because they need to be adjacent to the
+actual caller's frame (deferproc) or because they handle the imminent
+stack overflow (morestack).
+
+For example, deferproc might call malloc, which does one of the
+above checks (without allocating a full frame), which might trigger
+a call to morestack.  This sequence needs to fit in the bottom
+section of the stack.  On amd64, morestack's frame is 40 bytes, and
+deferproc's frame is 56 bytes.  That fits well within the
+StackGuard - StackSmall bytes at the bottom.
+The linkers explore all possible call traces involving non-splitting
+functions to make sure that this limit cannot be violated.
+*/
+
+const (
+	// StackSystem is a number of additional bytes to add
+	// to each stack below the usual guard area for OS-specific
+	// purposes like signal handling. Used on Windows, Plan 9,
+	// and iOS because they do not use a separate stack.
+	_StackSystem = goos.IsWindows*512*goarch.PtrSize + goos.IsPlan9*512 + goos.IsIos*goarch.IsArm64*1024
+
+	// The minimum size of stack used by Go code
+	_StackMin = 2048
+
+	// The minimum stack size to allocate.
+	// The hackery here rounds FixedStack0 up to a power of 2.
+	_FixedStack0 = _StackMin + _StackSystem
+	_FixedStack1 = _FixedStack0 - 1
+	_FixedStack2 = _FixedStack1 | (_FixedStack1 >> 1)
+	_FixedStack3 = _FixedStack2 | (_FixedStack2 >> 2)
+	_FixedStack4 = _FixedStack3 | (_FixedStack3 >> 4)
+	_FixedStack5 = _FixedStack4 | (_FixedStack4 >> 8)
+	_FixedStack6 = _FixedStack5 | (_FixedStack5 >> 16)
+	_FixedStack  = _FixedStack6 + 1
+
+	// Functions that need frames bigger than this use an extra
+	// instruction to do the stack split check, to avoid overflow
+	// in case SP - framesize wraps below zero.
+	// This value can be no bigger than the size of the unmapped
+	// space at zero.
+	_StackBig = 4096
+
+	// The stack guard is a pointer this many bytes above the
+	// bottom of the stack.
+	//
+	// The guard leaves enough room for one _StackSmall frame plus
+	// a _StackLimit chain of NOSPLIT calls plus _StackSystem
+	// bytes for the OS.
+	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackLimit.
+	_StackGuard = 928*sys.StackGuardMultiplier + _StackSystem
+
+	// After a stack split check the SP is allowed to be this
+	// many bytes below the stack guard. This saves an instruction
+	// in the checking sequence for tiny frames.
+	_StackSmall = 128
+
+	// The maximum number of bytes that a chain of NOSPLIT
+	// functions can use.
+	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackLimit.
+	_StackLimit = _StackGuard - _StackSystem - _StackSmall
+)
+
+const (
+	// stackDebug == 0: no logging
+	//            == 1: logging of per-stack operations
+	//            == 2: logging of per-frame operations
+	//            == 3: logging of per-word updates
+	//            == 4: logging of per-word reads
+	stackDebug       = 0
+	stackFromSystem  = 0 // allocate stacks from system memory instead of the heap
+	stackFaultOnFree = 0 // old stacks are mapped noaccess to detect use after free
+	stackPoisonCopy  = 0 // fill stack that should not be accessed with garbage, to detect bad dereferences during copy
+	stackNoCache     = 0 // disable per-P small stack caches
+
+	// check the BP links during traceback.
+	debugCheckBP = false
+)
+
+const (
+	uintptrMask = 1<<(8*goarch.PtrSize) - 1
+
+	// The values below can be stored to g.stackguard0 to force
+	// the next stack check to fail.
+	// These are all larger than any real SP.
+
+	// Goroutine preemption request.
+	// 0xfffffade in hex.
+	stackPreempt = uintptrMask & -1314
+
+	// Thread is forking. Causes a split stack check failure.
+	// 0xfffffb2e in hex.
+	stackFork = uintptrMask & -1234
+
+	// Force a stack movement. Used for debugging.
+	// 0xfffffeed in hex.
+	stackForceMove = uintptrMask & -275
+
+	// stackPoisonMin is the lowest allowed stack poison value.
+	stackPoisonMin = uintptrMask & -4096
+)
+
+// Global pool of spans that have free stacks.
+// Stacks are assigned an order according to size.
+//
+//	order = log_2(size/FixedStack)
+//
+// There is a free list for each order.
+var stackpool [_NumStackOrders]struct {
+	item stackpoolItem
+	_    [(cpu.CacheLinePadSize - unsafe.Sizeof(stackpoolItem{})%cpu.CacheLinePadSize) % cpu.CacheLinePadSize]byte
+}
+
+type stackpoolItem struct {
+	_    sys.NotInHeap
+	mu   mutex
+	span mSpanList
+}
+
+// Global pool of large stack spans.
+var stackLarge struct {
+	lock mutex
+	free [heapAddrBits - pageShift]mSpanList // free lists by log_2(s.npages)
+}
+
+func stackinit() {
+	if _StackCacheSize&_PageMask != 0 {
+		throw("cache size must be a multiple of page size")
+	}
+	for i := range stackpool {
+		stackpool[i].item.span.init()
+		lockInit(&stackpool[i].item.mu, lockRankStackpool)
+	}
+	for i := range stackLarge.free {
+		stackLarge.free[i].init()
+		lockInit(&stackLarge.lock, lockRankStackLarge)
+	}
+}
+
+// stacklog2 returns ⌊log_2(n)⌋.
+func stacklog2(n uintptr) int {
+	log2 := 0
+	for n > 1 {
+		n >>= 1
+		log2++
+	}
+	return log2
+}
+
+// Allocates a stack from the free pool. Must be called with
+// stackpool[order].item.mu held.
+func stackpoolalloc(order uint8) gclinkptr {
+	list := &stackpool[order].item.span
+	s := list.first
+	lockWithRankMayAcquire(&mheap_.lock, lockRankMheap)
+	if s == nil {
+		// no free stacks. Allocate another span worth.
+		s = mheap_.allocManual(_StackCacheSize>>_PageShift, spanAllocStack)
+		if s == nil {
+			throw("out of memory")
+		}
+		if s.allocCount != 0 {
+			throw("bad allocCount")
+		}
+		if s.manualFreeList.ptr() != nil {
+			throw("bad manualFreeList")
+		}
+		osStackAlloc(s)
+		s.elemsize = _FixedStack << order
+		for i := uintptr(0); i < _StackCacheSize; i += s.elemsize {
+			x := gclinkptr(s.base() + i)
+			x.ptr().next = s.manualFreeList
+			s.manualFreeList = x
+		}
+		list.insert(s)
+	}
+	x := s.manualFreeList
+	if x.ptr() == nil {
+		throw("span has no free stacks")
+	}
+	s.manualFreeList = x.ptr().next
+	s.allocCount++
+	if s.manualFreeList.ptr() == nil {
+		// all stacks in s are allocated.
+		list.remove(s)
+	}
+	return x
+}
+
+// Adds stack x to the free pool. Must be called with stackpool[order].item.mu held.
+func stackpoolfree(x gclinkptr, order uint8) {
+	s := spanOfUnchecked(uintptr(x))
+	if s.state.get() != mSpanManual {
+		throw("freeing stack not in a stack span")
+	}
+	if s.manualFreeList.ptr() == nil {
+		// s will now have a free stack
+		stackpool[order].item.span.insert(s)
+	}
+	x.ptr().next = s.manualFreeList
+	s.manualFreeList = x
+	s.allocCount--
+	if gcphase == _GCoff && s.allocCount == 0 {
+		// Span is completely free. Return it to the heap
+		// immediately if we're sweeping.
+		//
+		// If GC is active, we delay the free until the end of
+		// GC to avoid the following type of situation:
+		//
+		// 1) GC starts, scans a SudoG but does not yet mark the SudoG.elem pointer
+		// 2) The stack that pointer points to is copied
+		// 3) The old stack is freed
+		// 4) The containing span is marked free
+		// 5) GC attempts to mark the SudoG.elem pointer. The
+		//    marking fails because the pointer looks like a
+		//    pointer into a free span.
+		//
+		// By not freeing, we prevent step #4 until GC is done.
+		stackpool[order].item.span.remove(s)
+		s.manualFreeList = 0
+		osStackFree(s)
+		mheap_.freeManual(s, spanAllocStack)
+	}
+}
+
+// stackcacherefill/stackcacherelease implement a global pool of stack segments.
+// The pool is required to prevent unlimited growth of per-thread caches.
+//
+//go:systemstack
+func stackcacherefill(c *mcache, order uint8) {
+	if stackDebug >= 1 {
+		print("stackcacherefill order=", order, "\n")
+	}
+
+	// Grab some stacks from the global cache.
+	// Grab half of the allowed capacity (to prevent thrashing).
+	var list gclinkptr
+	var size uintptr
+	lock(&stackpool[order].item.mu)
+	for size < _StackCacheSize/2 {
+		x := stackpoolalloc(order)
+		x.ptr().next = list
+		list = x
+		size += _FixedStack << order
+	}
+	unlock(&stackpool[order].item.mu)
+	c.stackcache[order].list = list
+	c.stackcache[order].size = size
+}
+
+//go:systemstack
+func stackcacherelease(c *mcache, order uint8) {
+	if stackDebug >= 1 {
+		print("stackcacherelease order=", order, "\n")
+	}
+	x := c.stackcache[order].list
+	size := c.stackcache[order].size
+	lock(&stackpool[order].item.mu)
+	for size > _StackCacheSize/2 {
+		y := x.ptr().next
+		stackpoolfree(x, order)
+		x = y
+		size -= _FixedStack << order
+	}
+	unlock(&stackpool[order].item.mu)
+	c.stackcache[order].list = x
+	c.stackcache[order].size = size
+}
+
+//go:systemstack
+func stackcache_clear(c *mcache) {
+	if stackDebug >= 1 {
+		print("stackcache clear\n")
+	}
+	for order := uint8(0); order < _NumStackOrders; order++ {
+		lock(&stackpool[order].item.mu)
+		x := c.stackcache[order].list
+		for x.ptr() != nil {
+			y := x.ptr().next
+			stackpoolfree(x, order)
+			x = y
+		}
+		c.stackcache[order].list = 0
+		c.stackcache[order].size = 0
+		unlock(&stackpool[order].item.mu)
+	}
+}
+
+// stackalloc allocates an n byte stack.
+//
+// stackalloc must run on the system stack because it uses per-P
+// resources and must not split the stack.
+//
+//go:systemstack
+func stackalloc(n uint32) stack {
+	// Stackalloc must be called on scheduler stack, so that we
+	// never try to grow the stack during the code that stackalloc runs.
+	// Doing so would cause a deadlock (issue 1547).
+	thisg := getg()
+	if thisg != thisg.m.g0 {
+		throw("stackalloc not on scheduler stack")
+	}
+	if n&(n-1) != 0 {
+		throw("stack size not a power of 2")
+	}
+	if stackDebug >= 1 {
+		print("stackalloc ", n, "\n")
+	}
+
+	if debug.efence != 0 || stackFromSystem != 0 {
+		n = uint32(alignUp(uintptr(n), physPageSize))
+		v := sysAlloc(uintptr(n), &memstats.stacks_sys)
+		if v == nil {
+			throw("out of memory (stackalloc)")
+		}
+		return stack{uintptr(v), uintptr(v) + uintptr(n)}
+	}
+
+	// Small stacks are allocated with a fixed-size free-list allocator.
+	// If we need a stack of a bigger size, we fall back on allocating
+	// a dedicated span.
+	var v unsafe.Pointer
+	if n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
+		order := uint8(0)
+		n2 := n
+		for n2 > _FixedStack {
+			order++
+			n2 >>= 1
+		}
+		var x gclinkptr
+		if stackNoCache != 0 || thisg.m.p == 0 || thisg.m.preemptoff != "" {
+			// thisg.m.p == 0 can happen in the guts of exitsyscall
+			// or procresize. Just get a stack from the global pool.
+			// Also don't touch stackcache during gc
+			// as it's flushed concurrently.
+			lock(&stackpool[order].item.mu)
+			x = stackpoolalloc(order)
+			unlock(&stackpool[order].item.mu)
+		} else {
+			c := thisg.m.p.ptr().mcache
+			x = c.stackcache[order].list
+			if x.ptr() == nil {
+				stackcacherefill(c, order)
+				x = c.stackcache[order].list
+			}
+			c.stackcache[order].list = x.ptr().next
+			c.stackcache[order].size -= uintptr(n)
+		}
+		v = unsafe.Pointer(x)
+	} else {
+		var s *mspan
+		npage := uintptr(n) >> _PageShift
+		log2npage := stacklog2(npage)
+
+		// Try to get a stack from the large stack cache.
+		lock(&stackLarge.lock)
+		if !stackLarge.free[log2npage].isEmpty() {
+			s = stackLarge.free[log2npage].first
+			stackLarge.free[log2npage].remove(s)
+		}
+		unlock(&stackLarge.lock)
+
+		lockWithRankMayAcquire(&mheap_.lock, lockRankMheap)
+
+		if s == nil {
+			// Allocate a new stack from the heap.
+			s = mheap_.allocManual(npage, spanAllocStack)
+			if s == nil {
+				throw("out of memory")
+			}
+			osStackAlloc(s)
+			s.elemsize = uintptr(n)
+		}
+		v = unsafe.Pointer(s.base())
+	}
+
+	if raceenabled {
+		racemalloc(v, uintptr(n))
+	}
+	if msanenabled {
+		msanmalloc(v, uintptr(n))
+	}
+	if asanenabled {
+		asanunpoison(v, uintptr(n))
+	}
+	if stackDebug >= 1 {
+		print("  allocated ", v, "\n")
+	}
+	return stack{uintptr(v), uintptr(v) + uintptr(n)}
+}
+
+// stackfree frees an n byte stack allocation at stk.
+//
+// stackfree must run on the system stack because it uses per-P
+// resources and must not split the stack.
+//
+//go:systemstack
+func stackfree(stk stack) {
+	gp := getg()
+	v := unsafe.Pointer(stk.lo)
+	n := stk.hi - stk.lo
+	if n&(n-1) != 0 {
+		throw("stack not a power of 2")
+	}
+	if stk.lo+n < stk.hi {
+		throw("bad stack size")
+	}
+	if stackDebug >= 1 {
+		println("stackfree", v, n)
+		memclrNoHeapPointers(v, n) // for testing, clobber stack data
+	}
+	if debug.efence != 0 || stackFromSystem != 0 {
+		if debug.efence != 0 || stackFaultOnFree != 0 {
+			sysFault(v, n)
+		} else {
+			sysFree(v, n, &memstats.stacks_sys)
+		}
+		return
+	}
+	if msanenabled {
+		msanfree(v, n)
+	}
+	if asanenabled {
+		asanpoison(v, n)
+	}
+	if n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
+		order := uint8(0)
+		n2 := n
+		for n2 > _FixedStack {
+			order++
+			n2 >>= 1
+		}
+		x := gclinkptr(v)
+		if stackNoCache != 0 || gp.m.p == 0 || gp.m.preemptoff != "" {
+			lock(&stackpool[order].item.mu)
+			stackpoolfree(x, order)
+			unlock(&stackpool[order].item.mu)
+		} else {
+			c := gp.m.p.ptr().mcache
+			if c.stackcache[order].size >= _StackCacheSize {
+				stackcacherelease(c, order)
+			}
+			x.ptr().next = c.stackcache[order].list
+			c.stackcache[order].list = x
+			c.stackcache[order].size += n
+		}
+	} else {
+		s := spanOfUnchecked(uintptr(v))
+		if s.state.get() != mSpanManual {
+			println(hex(s.base()), v)
+			throw("bad span state")
+		}
+		if gcphase == _GCoff {
+			// Free the stack immediately if we're
+			// sweeping.
+			osStackFree(s)
+			mheap_.freeManual(s, spanAllocStack)
+		} else {
+			// If the GC is running, we can't return a
+			// stack span to the heap because it could be
+			// reused as a heap span, and this state
+			// change would race with GC. Add it to the
+			// large stack cache instead.
+			log2npage := stacklog2(s.npages)
+			lock(&stackLarge.lock)
+			stackLarge.free[log2npage].insert(s)
+			unlock(&stackLarge.lock)
+		}
+	}
+}
+
+var maxstacksize uintptr = 1 << 20 // enough until runtime.main sets it for real
+
+var maxstackceiling = maxstacksize
+
+var ptrnames = []string{
+	0: "scalar",
+	1: "ptr",
+}
+
+// Stack frame layout
+//
+// (x86)
+// +------------------+
+// | args from caller |
+// +------------------+ <- frame->argp
+// |  return address  |
+// +------------------+
+// |  caller's BP (*) | (*) if framepointer_enabled && varp < sp
+// +------------------+ <- frame->varp
+// |     locals       |
+// +------------------+
+// |  args to callee  |
+// +------------------+ <- frame->sp
+//
+// (arm)
+// +------------------+
+// | args from caller |
+// +------------------+ <- frame->argp
+// | caller's retaddr |
+// +------------------+ <- frame->varp
+// |     locals       |
+// +------------------+
+// |  args to callee  |
+// +------------------+
+// |  return address  |
+// +------------------+ <- frame->sp
+
+type adjustinfo struct {
+	old   stack
+	delta uintptr // ptr distance from old to new stack (newbase - oldbase)
+	cache pcvalueCache
+
+	// sghi is the highest sudog.elem on the stack.
+	sghi uintptr
+}
+
+// adjustpointer checks whether *vpp is in the old stack described by adjinfo.
+// If so, it rewrites *vpp to point into the new stack.
+func adjustpointer(adjinfo *adjustinfo, vpp unsafe.Pointer) {
+	pp := (*uintptr)(vpp)
+	p := *pp
+	if stackDebug >= 4 {
+		print("        ", pp, ":", hex(p), "\n")
+	}
+	if adjinfo.old.lo <= p && p < adjinfo.old.hi {
+		*pp = p + adjinfo.delta
+		if stackDebug >= 3 {
+			print("        adjust ptr ", pp, ":", hex(p), " -> ", hex(*pp), "\n")
+		}
+	}
+}
+
+// Information from the compiler about the layout of stack frames.
+// Note: this type must agree with reflect.bitVector.
+type bitvector struct {
+	n        int32 // # of bits
+	bytedata *uint8
+}
+
+// ptrbit returns the i'th bit in bv.
+// ptrbit is less efficient than iterating directly over bitvector bits,
+// and should only be used in non-performance-critical code.
+// See adjustpointers for an example of a high-efficiency walk of a bitvector.
+func (bv *bitvector) ptrbit(i uintptr) uint8 {
+	b := *(addb(bv.bytedata, i/8))
+	return (b >> (i % 8)) & 1
+}
+
+// bv describes the memory starting at address scanp.
+// Adjust any pointers contained therein.
+func adjustpointers(scanp unsafe.Pointer, bv *bitvector, adjinfo *adjustinfo, f funcInfo) {
+	minp := adjinfo.old.lo
+	maxp := adjinfo.old.hi
+	delta := adjinfo.delta
+	num := uintptr(bv.n)
+	// If this frame might contain channel receive slots, use CAS
+	// to adjust pointers. If the slot hasn't been received into
+	// yet, it may contain stack pointers and a concurrent send
+	// could race with adjusting those pointers. (The sent value
+	// itself can never contain stack pointers.)
+	useCAS := uintptr(scanp) < adjinfo.sghi
+	for i := uintptr(0); i < num; i += 8 {
+		if stackDebug >= 4 {
+			for j := uintptr(0); j < 8; j++ {
+				print("        ", add(scanp, (i+j)*goarch.PtrSize), ":", ptrnames[bv.ptrbit(i+j)], ":", hex(*(*uintptr)(add(scanp, (i+j)*goarch.PtrSize))), " # ", i, " ", *addb(bv.bytedata, i/8), "\n")
+			}
+		}
+		b := *(addb(bv.bytedata, i/8))
+		for b != 0 {
+			j := uintptr(sys.TrailingZeros8(b))
+			b &= b - 1
+			pp := (*uintptr)(add(scanp, (i+j)*goarch.PtrSize))
+		retry:
+			p := *pp
+			if f.valid() && 0 < p && p < minLegalPointer && debug.invalidptr != 0 {
+				// Looks like a junk value in a pointer slot.
+				// Live analysis wrong?
+				getg().m.traceback = 2
+				print("runtime: bad pointer in frame ", funcname(f), " at ", pp, ": ", hex(p), "\n")
+				throw("invalid pointer found on stack")
+			}
+			if minp <= p && p < maxp {
+				if stackDebug >= 3 {
+					print("adjust ptr ", hex(p), " ", funcname(f), "\n")
+				}
+				if useCAS {
+					ppu := (*unsafe.Pointer)(unsafe.Pointer(pp))
+					if !atomic.Casp1(ppu, unsafe.Pointer(p), unsafe.Pointer(p+delta)) {
+						goto retry
+					}
+				} else {
+					*pp = p + delta
+				}
+			}
+		}
+	}
+}
+
+// Note: the argument/return area is adjusted by the callee.
+func adjustframe(frame *stkframe, arg unsafe.Pointer) bool {
+	adjinfo := (*adjustinfo)(arg)
+	if frame.continpc == 0 {
+		// Frame is dead.
+		return true
+	}
+	f := frame.fn
+	if stackDebug >= 2 {
+		print("    adjusting ", funcname(f), " frame=[", hex(frame.sp), ",", hex(frame.fp), "] pc=", hex(frame.pc), " continpc=", hex(frame.continpc), "\n")
+	}
+	if f.funcID == funcID_systemstack_switch {
+		// A special routine at the bottom of stack of a goroutine that does a systemstack call.
+		// We will allow it to be copied even though we don't
+		// have full GC info for it (because it is written in asm).
+		return true
+	}
+
+	locals, args, objs := frame.getStackMap(&adjinfo.cache, true)
+
+	// Adjust local variables if stack frame has been allocated.
+	if locals.n > 0 {
+		size := uintptr(locals.n) * goarch.PtrSize
+		adjustpointers(unsafe.Pointer(frame.varp-size), &locals, adjinfo, f)
+	}
+
+	// Adjust saved base pointer if there is one.
+	// TODO what about arm64 frame pointer adjustment?
+	if goarch.ArchFamily == goarch.AMD64 && frame.argp-frame.varp == 2*goarch.PtrSize {
+		if stackDebug >= 3 {
+			print("      saved bp\n")
+		}
+		if debugCheckBP {
+			// Frame pointers should always point to the next higher frame on
+			// the Go stack (or be nil, for the top frame on the stack).
+			bp := *(*uintptr)(unsafe.Pointer(frame.varp))
+			if bp != 0 && (bp < adjinfo.old.lo || bp >= adjinfo.old.hi) {
+				println("runtime: found invalid frame pointer")
+				print("bp=", hex(bp), " min=", hex(adjinfo.old.lo), " max=", hex(adjinfo.old.hi), "\n")
+				throw("bad frame pointer")
+			}
+		}
+		adjustpointer(adjinfo, unsafe.Pointer(frame.varp))
+	}
+
+	// Adjust arguments.
+	if args.n > 0 {
+		if stackDebug >= 3 {
+			print("      args\n")
+		}
+		adjustpointers(unsafe.Pointer(frame.argp), &args, adjinfo, funcInfo{})
+	}
+
+	// Adjust pointers in all stack objects (whether they are live or not).
+	// See comments in mgcmark.go:scanframeworker.
+	if frame.varp != 0 {
+		for i := range objs {
+			obj := &objs[i]
+			off := obj.off
+			base := frame.varp // locals base pointer
+			if off >= 0 {
+				base = frame.argp // arguments and return values base pointer
+			}
+			p := base + uintptr(off)
+			if p < frame.sp {
+				// Object hasn't been allocated in the frame yet.
+				// (Happens when the stack bounds check fails and
+				// we call into morestack.)
+				continue
+			}
+			ptrdata := obj.ptrdata()
+			gcdata := obj.gcdata()
+			var s *mspan
+			if obj.useGCProg() {
+				// See comments in mgcmark.go:scanstack
+				s = materializeGCProg(ptrdata, gcdata)
+				gcdata = (*byte)(unsafe.Pointer(s.startAddr))
+			}
+			for i := uintptr(0); i < ptrdata; i += goarch.PtrSize {
+				if *addb(gcdata, i/(8*goarch.PtrSize))>>(i/goarch.PtrSize&7)&1 != 0 {
+					adjustpointer(adjinfo, unsafe.Pointer(p+i))
+				}
+			}
+			if s != nil {
+				dematerializeGCProg(s)
+			}
+		}
+	}
+
+	return true
+}
+
+func adjustctxt(gp *g, adjinfo *adjustinfo) {
+	adjustpointer(adjinfo, unsafe.Pointer(&gp.sched.ctxt))
+	if !framepointer_enabled {
+		return
+	}
+	if debugCheckBP {
+		bp := gp.sched.bp
+		if bp != 0 && (bp < adjinfo.old.lo || bp >= adjinfo.old.hi) {
+			println("runtime: found invalid top frame pointer")
+			print("bp=", hex(bp), " min=", hex(adjinfo.old.lo), " max=", hex(adjinfo.old.hi), "\n")
+			throw("bad top frame pointer")
+		}
+	}
+	adjustpointer(adjinfo, unsafe.Pointer(&gp.sched.bp))
+}
+
+func adjustdefers(gp *g, adjinfo *adjustinfo) {
+	// Adjust pointers in the Defer structs.
+	// We need to do this first because we need to adjust the
+	// defer.link fields so we always work on the new stack.
+	adjustpointer(adjinfo, unsafe.Pointer(&gp._defer))
+	for d := gp._defer; d != nil; d = d.link {
+		adjustpointer(adjinfo, unsafe.Pointer(&d.fn))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.sp))
+		adjustpointer(adjinfo, unsafe.Pointer(&d._panic))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.link))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.varp))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.fd))
+	}
+}
+
+func adjustpanics(gp *g, adjinfo *adjustinfo) {
+	// Panics are on stack and already adjusted.
+	// Update pointer to head of list in G.
+	adjustpointer(adjinfo, unsafe.Pointer(&gp._panic))
+}
+
+func adjustsudogs(gp *g, adjinfo *adjustinfo) {
+	// the data elements pointed to by a SudoG structure
+	// might be in the stack.
+	for s := gp.waiting; s != nil; s = s.waitlink {
+		adjustpointer(adjinfo, unsafe.Pointer(&s.elem))
+	}
+}
+
+func fillstack(stk stack, b byte) {
+	for p := stk.lo; p < stk.hi; p++ {
+		*(*byte)(unsafe.Pointer(p)) = b
+	}
+}
+
+func findsghi(gp *g, stk stack) uintptr {
+	var sghi uintptr
+	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
+		p := uintptr(sg.elem) + uintptr(sg.c.elemsize)
+		if stk.lo <= p && p < stk.hi && p > sghi {
+			sghi = p
+		}
+	}
+	return sghi
+}
+
+// syncadjustsudogs adjusts gp's sudogs and copies the part of gp's
+// stack they refer to while synchronizing with concurrent channel
+// operations. It returns the number of bytes of stack copied.
+func syncadjustsudogs(gp *g, used uintptr, adjinfo *adjustinfo) uintptr {
+	if gp.waiting == nil {
+		return 0
+	}
+
+	// Lock channels to prevent concurrent send/receive.
+	var lastc *hchan
+	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
+		if sg.c != lastc {
+			// There is a ranking cycle here between gscan bit and
+			// hchan locks. Normally, we only allow acquiring hchan
+			// locks and then getting a gscan bit. In this case, we
+			// already have the gscan bit. We allow acquiring hchan
+			// locks here as a special case, since a deadlock can't
+			// happen because the G involved must already be
+			// suspended. So, we get a special hchan lock rank here
+			// that is lower than gscan, but doesn't allow acquiring
+			// any other locks other than hchan.
+			lockWithRank(&sg.c.lock, lockRankHchanLeaf)
+		}
+		lastc = sg.c
+	}
+
+	// Adjust sudogs.
+	adjustsudogs(gp, adjinfo)
+
+	// Copy the part of the stack the sudogs point in to
+	// while holding the lock to prevent races on
+	// send/receive slots.
+	var sgsize uintptr
+	if adjinfo.sghi != 0 {
+		oldBot := adjinfo.old.hi - used
+		newBot := oldBot + adjinfo.delta
+		sgsize = adjinfo.sghi - oldBot
+		memmove(unsafe.Pointer(newBot), unsafe.Pointer(oldBot), sgsize)
+	}
+
+	// Unlock channels.
+	lastc = nil
+	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
+		if sg.c != lastc {
+			unlock(&sg.c.lock)
+		}
+		lastc = sg.c
+	}
+
+	return sgsize
+}
+
+// Copies gp's stack to a new stack of a different size.
+// Caller must have changed gp status to Gcopystack.
+func copystack(gp *g, newsize uintptr) {
+	if gp.syscallsp != 0 {
+		throw("stack growth not allowed in system call")
+	}
+	old := gp.stack
+	if old.lo == 0 {
+		throw("nil stackbase")
+	}
+	used := old.hi - gp.sched.sp
+	// Add just the difference to gcController.addScannableStack.
+	// g0 stacks never move, so this will never account for them.
+	// It's also fine if we have no P, addScannableStack can deal with
+	// that case.
+	gcController.addScannableStack(getg().m.p.ptr(), int64(newsize)-int64(old.hi-old.lo))
+
+	// allocate new stack
+	new := stackalloc(uint32(newsize))
+	if stackPoisonCopy != 0 {
+		fillstack(new, 0xfd)
+	}
+	if stackDebug >= 1 {
+		print("copystack gp=", gp, " [", hex(old.lo), " ", hex(old.hi-used), " ", hex(old.hi), "]", " -> [", hex(new.lo), " ", hex(new.hi-used), " ", hex(new.hi), "]/", newsize, "\n")
+	}
+
+	// Compute adjustment.
+	var adjinfo adjustinfo
+	adjinfo.old = old
+	adjinfo.delta = new.hi - old.hi
+
+	// Adjust sudogs, synchronizing with channel ops if necessary.
+	ncopy := used
+	if !gp.activeStackChans {
+		if newsize < old.hi-old.lo && gp.parkingOnChan.Load() {
+			// It's not safe for someone to shrink this stack while we're actively
+			// parking on a channel, but it is safe to grow since we do that
+			// ourselves and explicitly don't want to synchronize with channels
+			// since we could self-deadlock.
+			throw("racy sudog adjustment due to parking on channel")
+		}
+		adjustsudogs(gp, &adjinfo)
+	} else {
+		// sudogs may be pointing in to the stack and gp has
+		// released channel locks, so other goroutines could
+		// be writing to gp's stack. Find the highest such
+		// pointer so we can handle everything there and below
+		// carefully. (This shouldn't be far from the bottom
+		// of the stack, so there's little cost in handling
+		// everything below it carefully.)
+		adjinfo.sghi = findsghi(gp, old)
+
+		// Synchronize with channel ops and copy the part of
+		// the stack they may interact with.
+		ncopy -= syncadjustsudogs(gp, used, &adjinfo)
+	}
+
+	// Copy the stack (or the rest of it) to the new location
+	memmove(unsafe.Pointer(new.hi-ncopy), unsafe.Pointer(old.hi-ncopy), ncopy)
+
+	// Adjust remaining structures that have pointers into stacks.
+	// We have to do most of these before we traceback the new
+	// stack because gentraceback uses them.
+	adjustctxt(gp, &adjinfo)
+	adjustdefers(gp, &adjinfo)
+	adjustpanics(gp, &adjinfo)
+	if adjinfo.sghi != 0 {
+		adjinfo.sghi += adjinfo.delta
+	}
+
+	// Swap out old stack for new one
+	gp.stack = new
+	gp.stackguard0 = new.lo + _StackGuard // NOTE: might clobber a preempt request
+	gp.sched.sp = new.hi - used
+	gp.stktopsp += adjinfo.delta
+
+	// Adjust pointers in the new stack.
+	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, adjustframe, noescape(unsafe.Pointer(&adjinfo)), 0)
+
+	// free old stack
+	if stackPoisonCopy != 0 {
+		fillstack(old, 0xfc)
+	}
+	stackfree(old)
+}
+
+// round x up to a power of 2.
+func round2(x int32) int32 {
+	s := uint(0)
+	for 1<<s < x {
+		s++
+	}
+	return 1 << s
+}
+
+// Called from runtime·morestack when more stack is needed.
+// Allocate larger stack and relocate to new stack.
+// Stack growth is multiplicative, for constant amortized cost.
+//
+// g->atomicstatus will be Grunning or Gscanrunning upon entry.
+// If the scheduler is trying to stop this g, then it will set preemptStop.
+//
+// This must be nowritebarrierrec because it can be called as part of
+// stack growth from other nowritebarrierrec functions, but the
+// compiler doesn't check this.
+//
+//go:nowritebarrierrec
+func newstack() {
+	thisg := getg()
+	// TODO: double check all gp. shouldn't be getg().
+	if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
+		throw("stack growth after fork")
+	}
+	if thisg.m.morebuf.g.ptr() != thisg.m.curg {
+		print("runtime: newstack called from g=", hex(thisg.m.morebuf.g), "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
+		morebuf := thisg.m.morebuf
+		traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
+		throw("runtime: wrong goroutine in newstack")
+	}
+
+	gp := thisg.m.curg
+
+	if thisg.m.curg.throwsplit {
+		// Update syscallsp, syscallpc in case traceback uses them.
+		morebuf := thisg.m.morebuf
+		gp.syscallsp = morebuf.sp
+		gp.syscallpc = morebuf.pc
+		pcname, pcoff := "(unknown)", uintptr(0)
+		f := findfunc(gp.sched.pc)
+		if f.valid() {
+			pcname = funcname(f)
+			pcoff = gp.sched.pc - f.entry()
+		}
+		print("runtime: newstack at ", pcname, "+", hex(pcoff),
+			" sp=", hex(gp.sched.sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
+			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
+			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")
+
+		thisg.m.traceback = 2 // Include runtime frames
+		traceback(morebuf.pc, morebuf.sp, morebuf.lr, gp)
+		throw("runtime: stack split at bad time")
+	}
+
+	morebuf := thisg.m.morebuf
+	thisg.m.morebuf.pc = 0
+	thisg.m.morebuf.lr = 0
+	thisg.m.morebuf.sp = 0
+	thisg.m.morebuf.g = 0
+
+	// NOTE: stackguard0 may change underfoot, if another thread
+	// is about to try to preempt gp. Read it just once and use that same
+	// value now and below.
+	stackguard0 := atomic.Loaduintptr(&gp.stackguard0)
+
+	// Be conservative about where we preempt.
+	// We are interested in preempting user Go code, not runtime code.
+	// If we're holding locks, mallocing, or preemption is disabled, don't
+	// preempt.
+	// This check is very early in newstack so that even the status change
+	// from Grunning to Gwaiting and back doesn't happen in this case.
+	// That status change by itself can be viewed as a small preemption,
+	// because the GC might change Gwaiting to Gscanwaiting, and then
+	// this goroutine has to wait for the GC to finish before continuing.
+	// If the GC is in some way dependent on this goroutine (for example,
+	// it needs a lock held by the goroutine), that small preemption turns
+	// into a real deadlock.
+	preempt := stackguard0 == stackPreempt
+	if preempt {
+		if !canPreemptM(thisg.m) {
+			// Let the goroutine keep running for now.
+			// gp->preempt is set, so it will be preempted next time.
+			gp.stackguard0 = gp.stack.lo + _StackGuard
+			gogo(&gp.sched) // never return
+		}
+	}
+
+	if gp.stack.lo == 0 {
+		throw("missing stack in newstack")
+	}
+	sp := gp.sched.sp
+	if goarch.ArchFamily == goarch.AMD64 || goarch.ArchFamily == goarch.I386 || goarch.ArchFamily == goarch.WASM {
+		// The call to morestack cost a word.
+		sp -= goarch.PtrSize
+	}
+	if stackDebug >= 1 || sp < gp.stack.lo {
+		print("runtime: newstack sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
+			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
+			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")
+	}
+	if sp < gp.stack.lo {
+		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->status=", hex(readgstatus(gp)), "\n ")
+		print("runtime: split stack overflow: ", hex(sp), " < ", hex(gp.stack.lo), "\n")
+		throw("runtime: split stack overflow")
+	}
+
+	if preempt {
+		if gp == thisg.m.g0 {
+			throw("runtime: preempt g0")
+		}
+		if thisg.m.p == 0 && thisg.m.locks == 0 {
+			throw("runtime: g is running but p is not")
+		}
+
+		if gp.preemptShrink {
+			// We're at a synchronous safe point now, so
+			// do the pending stack shrink.
+			gp.preemptShrink = false
+			shrinkstack(gp)
+		}
+
+		if gp.preemptStop {
+			preemptPark(gp) // never returns
+		}
+
+		// Act like goroutine called runtime.Gosched.
+		gopreempt_m(gp) // never return
+	}
+
+	// Allocate a bigger segment and move the stack.
+	oldsize := gp.stack.hi - gp.stack.lo
+	newsize := oldsize * 2
+
+	// Make sure we grow at least as much as needed to fit the new frame.
+	// (This is just an optimization - the caller of morestack will
+	// recheck the bounds on return.)
+	if f := findfunc(gp.sched.pc); f.valid() {
+		max := uintptr(funcMaxSPDelta(f))
+		needed := max + _StackGuard
+		used := gp.stack.hi - gp.sched.sp
+		for newsize-used < needed {
+			newsize *= 2
+		}
+	}
+
+	if stackguard0 == stackForceMove {
+		// Forced stack movement used for debugging.
+		// Don't double the stack (or we may quickly run out
+		// if this is done repeatedly).
+		newsize = oldsize
+	}
+
+	if newsize > maxstacksize || newsize > maxstackceiling {
+		if maxstacksize < maxstackceiling {
+			print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")
+		} else {
+			print("runtime: goroutine stack exceeds ", maxstackceiling, "-byte limit\n")
+		}
+		print("runtime: sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
+		throw("stack overflow")
+	}
+
+	// The goroutine must be executing in order to call newstack,
+	// so it must be Grunning (or Gscanrunning).
+	casgstatus(gp, _Grunning, _Gcopystack)
+
+	// The concurrent GC will not scan the stack while we are doing the copy since
+	// the gp is in a Gcopystack status.
+	copystack(gp, newsize)
+	if stackDebug >= 1 {
+		print("stack grow done\n")
+	}
+	casgstatus(gp, _Gcopystack, _Grunning)
+	gogo(&gp.sched)
+}
+
+//go:nosplit
+func nilfunc() {
+	*(*uint8)(nil) = 0
+}
+
+// adjust Gobuf as if it executed a call to fn
+// and then stopped before the first instruction in fn.
+func gostartcallfn(gobuf *gobuf, fv *funcval) {
+	var fn unsafe.Pointer
+	if fv != nil {
+		fn = unsafe.Pointer(fv.fn)
+	} else {
+		fn = unsafe.Pointer(abi.FuncPCABIInternal(nilfunc))
+	}
+	gostartcall(gobuf, fn, unsafe.Pointer(fv))
+}
+
+// isShrinkStackSafe returns whether it's safe to attempt to shrink
+// gp's stack. Shrinking the stack is only safe when we have precise
+// pointer maps for all frames on the stack.
+func isShrinkStackSafe(gp *g) bool {
+	// We can't copy the stack if we're in a syscall.
+	// The syscall might have pointers into the stack and
+	// often we don't have precise pointer maps for the innermost
+	// frames.
+	//
+	// We also can't copy the stack if we're at an asynchronous
+	// safe-point because we don't have precise pointer maps for
+	// all frames.
+	//
+	// We also can't *shrink* the stack in the window between the
+	// goroutine calling gopark to park on a channel and
+	// gp.activeStackChans being set.
+	return gp.syscallsp == 0 && !gp.asyncSafePoint && !gp.parkingOnChan.Load()
+}
+
+// Maybe shrink the stack being used by gp.
+//
+// gp must be stopped and we must own its stack. It may be in
+// _Grunning, but only if this is our own user G.
+func shrinkstack(gp *g) {
+	if gp.stack.lo == 0 {
+		throw("missing stack in shrinkstack")
+	}
+	if s := readgstatus(gp); s&_Gscan == 0 {
+		// We don't own the stack via _Gscan. We could still
+		// own it if this is our own user G and we're on the
+		// system stack.
+		if !(gp == getg().m.curg && getg() != getg().m.curg && s == _Grunning) {
+			// We don't own the stack.
+			throw("bad status in shrinkstack")
+		}
+	}
+	if !isShrinkStackSafe(gp) {
+		throw("shrinkstack at bad time")
+	}
+	// Check for self-shrinks while in a libcall. These may have
+	// pointers into the stack disguised as uintptrs, but these
+	// code paths should all be nosplit.
+	if gp == getg().m.curg && gp.m.libcallsp != 0 {
+		throw("shrinking stack in libcall")
+	}
+
+	if debug.gcshrinkstackoff > 0 {
+		return
+	}
+	f := findfunc(gp.startpc)
+	if f.valid() && f.funcID == funcID_gcBgMarkWorker {
+		// We're not allowed to shrink the gcBgMarkWorker
+		// stack (see gcBgMarkWorker for explanation).
+		return
+	}
+
+	oldsize := gp.stack.hi - gp.stack.lo
+	newsize := oldsize / 2
+	// Don't shrink the allocation below the minimum-sized stack
+	// allocation.
+	if newsize < _FixedStack {
+		return
+	}
+	// Compute how much of the stack is currently in use and only
+	// shrink the stack if gp is using less than a quarter of its
+	// current stack. The currently used stack includes everything
+	// down to the SP plus the stack guard space that ensures
+	// there's room for nosplit functions.
+	avail := gp.stack.hi - gp.stack.lo
+	if used := gp.stack.hi - gp.sched.sp + _StackLimit; used >= avail/4 {
+		return
+	}
+
+	if stackDebug > 0 {
+		print("shrinking stack ", oldsize, "->", newsize, "\n")
+	}
+
+	copystack(gp, newsize)
+}
+
+// freeStackSpans frees unused stack spans at the end of GC.
+func freeStackSpans() {
+	// Scan stack pools for empty stack spans.
+	for order := range stackpool {
+		lock(&stackpool[order].item.mu)
+		list := &stackpool[order].item.span
+		for s := list.first; s != nil; {
+			next := s.next
+			if s.allocCount == 0 {
+				list.remove(s)
+				s.manualFreeList = 0
+				osStackFree(s)
+				mheap_.freeManual(s, spanAllocStack)
+			}
+			s = next
+		}
+		unlock(&stackpool[order].item.mu)
+	}
+
+	// Free large stack spans.
+	lock(&stackLarge.lock)
+	for i := range stackLarge.free {
+		for s := stackLarge.free[i].first; s != nil; {
+			next := s.next
+			stackLarge.free[i].remove(s)
+			osStackFree(s)
+			mheap_.freeManual(s, spanAllocStack)
+			s = next
+		}
+	}
+	unlock(&stackLarge.lock)
+}
+
+// A stackObjectRecord is generated by the compiler for each stack object in a stack frame.
+// This record must match the generator code in cmd/compile/internal/liveness/plive.go:emitStackObjects.
+type stackObjectRecord struct {
+	// offset in frame
+	// if negative, offset from varp
+	// if non-negative, offset from argp
+	off       int32
+	size      int32
+	_ptrdata  int32  // ptrdata, or -ptrdata is GC prog is used
+	gcdataoff uint32 // offset to gcdata from moduledata.rodata
+}
+
+func (r *stackObjectRecord) useGCProg() bool {
+	return r._ptrdata < 0
+}
+
+func (r *stackObjectRecord) ptrdata() uintptr {
+	x := r._ptrdata
+	if x < 0 {
+		return uintptr(-x)
+	}
+	return uintptr(x)
+}
+
+// gcdata returns pointer map or GC prog of the type.
+func (r *stackObjectRecord) gcdata() *byte {
+	ptr := uintptr(unsafe.Pointer(r))
+	var mod *moduledata
+	for datap := &firstmoduledata; datap != nil; datap = datap.next {
+		if datap.gofunc <= ptr && ptr < datap.end {
+			mod = datap
+			break
+		}
+	}
+	// If you get a panic here due to a nil mod,
+	// you may have made a copy of a stackObjectRecord.
+	// You must use the original pointer.
+	res := mod.rodata + uintptr(r.gcdataoff)
+	return (*byte)(unsafe.Pointer(res))
+}
+
+// This is exported as ABI0 via linkname so obj can call it.
+//
+//go:nosplit
+//go:linkname morestackc
+func morestackc() {
+	throw("attempt to execute system stack code on user stack")
+}
+
+// startingStackSize is the amount of stack that new goroutines start with.
+// It is a power of 2, and between _FixedStack and maxstacksize, inclusive.
+// startingStackSize is updated every GC by tracking the average size of
+// stacks scanned during the GC.
+var startingStackSize uint32 = _FixedStack
+
+func gcComputeStartingStackSize() {
+	if debug.adaptivestackstart == 0 {
+		return
+	}
+	// For details, see the design doc at
+	// https://docs.google.com/document/d/1YDlGIdVTPnmUiTAavlZxBI1d9pwGQgZT7IKFKlIXohQ/edit?usp=sharing
+	// The basic algorithm is to track the average size of stacks
+	// and start goroutines with stack equal to that average size.
+	// Starting at the average size uses at most 2x the space that
+	// an ideal algorithm would have used.
+	// This is just a heuristic to avoid excessive stack growth work
+	// early in a goroutine's lifetime. See issue 18138. Stacks that
+	// are allocated too small can still grow, and stacks allocated
+	// too large can still shrink.
+	var scannedStackSize uint64
+	var scannedStacks uint64
+	for _, p := range allp {
+		scannedStackSize += p.scannedStackSize
+		scannedStacks += p.scannedStacks
+		// Reset for next time
+		p.scannedStackSize = 0
+		p.scannedStacks = 0
+	}
+	if scannedStacks == 0 {
+		startingStackSize = _FixedStack
+		return
+	}
+	avg := scannedStackSize/scannedStacks + _StackGuard
+	// Note: we add _StackGuard to ensure that a goroutine that
+	// uses the average space will not trigger a growth.
+	if avg > uint64(maxstacksize) {
+		avg = uint64(maxstacksize)
+	}
+	if avg < _FixedStack {
+		avg = _FixedStack
+	}
+	// Note: maxstacksize fits in 30 bits, so avg also does.
+	startingStackSize = uint32(round2(int32(avg)))
+}