14.4 栈的拷贝与指针调整

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type adjustinfo struct {
	old   stack
	delta uintptr // ptr distance from old to new stack (newbase - oldbase)

	// 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 adjinfo.old.lo <= p && p < adjinfo.old.hi {
		*pp = p + adjinfo.delta
	}
}

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// 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

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// Note: the argument/return area is adjusted by the callee.
func adjustframe(frame *stkframe, adjinfo *adjustinfo) {
	// Adjust saved frame pointer if there is one.
	if (goarch.ArchFamily == goarch.AMD64 || goarch.ArchFamily == goarch.ARM64) && frame.argp-frame.varp == 2*goarch.PtrSize {
		// ……
		adjustpointer(adjinfo, unsafe.Pointer(frame.varp))
	}
	// ……
	if frame.continpc == 0 {
		// Frame is dead. The program might still see the frame pointer
		// saved in the frame, adjusted above, but we don't need to
		// adjust the rest of the frame.
		return
	}
	f := frame.fn

	locals, args, objs := frame.getStackMap(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 arguments.
	if args.n > 0 {
		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
			}
			// ……按对象的指针位图逐字调用 adjustpointer
		}
	}
}

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// 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 {
		// ……
		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
				throw("invalid pointer found on stack")
			}
			if minp <= p && p < maxp {
				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
				}
			}
		}
	}
}

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func adjustctxt(gp *g, adjinfo *adjustinfo) {
	adjustpointer(adjinfo, unsafe.Pointer(&gp.sched.ctxt))
	if !framepointer_enabled {
		return
	}
	// ……
	adjustpointer(adjinfo, unsafe.Pointer(&gp.sched.bp))
	// ……arm64 上还要处理保存在 SP 之下一个字处的帧指针
}

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.link))
	}
}

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.vu))
		adjustpointer(adjinfo, unsafe.Pointer(&s.elem.vp))
	}
}

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	// activeStackChans indicates that there are unlocked channels
	// pointing into this goroutine's stack. If true, stack
	// copying needs to acquire channel locks to protect these
	// areas of the stack.
	activeStackChans bool

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// 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.get() != lastc {
			// ……
			lockWithRank(&sg.c.get().lock, lockRankHchanLeaf)
		}
		lastc = sg.c.get()
	}

	// 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.
	// ……
	return sgsize
}

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// 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
	// ……
	used := old.hi - gp.sched.sp
	// Add just the difference to gcController.addScannableStack.
	// ……
	gcController.addScannableStack(getg().m.p.ptr(), int64(newsize)-int64(old.hi-old.lo))

	// allocate new stack
	new := stackalloc(uint32(newsize))
	// ……

	// 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() {
			// ……
			throw("racy sudog adjustment due to parking on channel")
		}
		adjustsudogs(gp, &adjinfo)
	} else {
		// ……
		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.
	var u unwinder
	for u.init(gp, 0); u.valid(); u.next() {
		adjustframe(&u.frame, &adjinfo)
	}
	// ……
	stackfree(old)
}

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package main

import (
	"fmt"
	"unsafe"
)

// grow 递归 n 层，每层的栈帧约 256 字节，迫使当前 goroutine 的栈增长。
func grow(n int) int {
	var buf [256]byte
	if n == 0 {
		return 0
	}
	buf[n%256] = byte(n)
	return grow(n-1) + int(buf[0])
}

func main() {
	x := 1
	p := &x                          // 指针：栈映射记录了这个槽位存放指针
	u := uintptr(unsafe.Pointer(&x)) // 整数：栈映射不知道它曾是一个地址

	grow(1000) // 约 250 KiB，栈至少移动一次
	x = 2

	fmt.Println("x 的地址已改变：", u != uintptr(unsafe.Pointer(&x)))
	fmt.Println("经 p 读到的值：", *p)
}