src/runtime/mbarrier.go - go - Git at Google

 // Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Garbage collector: write barriers.
 //
 // For the concurrent garbage collector, the Go compiler implements
 // updates to pointer-valued fields that may be in heap objects by
 // emitting calls to write barriers. This file contains the actual write barrier
 // implementation, gcmarkwb_m, and the various wrappers called by the
 // compiler to implement pointer assignment, slice assignment,
 // typed memmove, and so on.

 package runtime

 import (
 	"runtime/internal/sys"
 	"unsafe"
 )

 // gcmarkwb_m is the mark-phase write barrier, the only barrier we have.
 // The rest of this file exists only to make calls to this function.
 //
 // This is a hybrid barrier that combines a Yuasa-style deletion
 // barrier—which shades the object whose reference is being
 // overwritten—with Dijkstra insertion barrier—which shades the object
 // whose reference is being written. The insertion part of the barrier
 // is necessary while the calling goroutine's stack is grey. In
 // pseudocode, the barrier is:
 //
 //     writePointer(slot, ptr):
 //         shade(*slot)
 //         if current stack is grey:
 //             shade(ptr)
 //         *slot = ptr
 //
 // slot is the destination in Go code.
 // ptr is the value that goes into the slot in Go code.
 //
 // Shade indicates that it has seen a white pointer by adding the referent
 // to wbuf as well as marking it.
 //
 // The two shades and the condition work together to prevent a mutator
 // from hiding an object from the garbage collector:
 //
 // 1. shade(*slot) prevents a mutator from hiding an object by moving
 // the sole pointer to it from the heap to its stack. If it attempts
 // to unlink an object from the heap, this will shade it.
 //
 // 2. shade(ptr) prevents a mutator from hiding an object by moving
 // the sole pointer to it from its stack into a black object in the
 // heap. If it attempts to install the pointer into a black object,
 // this will shade it.
 //
 // 3. Once a goroutine's stack is black, the shade(ptr) becomes
 // unnecessary. shade(ptr) prevents hiding an object by moving it from
 // the stack to the heap, but this requires first having a pointer
 // hidden on the stack. Immediately after a stack is scanned, it only
 // points to shaded objects, so it's not hiding anything, and the
 // shade(*slot) prevents it from hiding any other pointers on its
 // stack.
 //
 // For a detailed description of this barrier and proof of
 // correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
 //
 //
 //
 // Dealing with memory ordering:
 //
 // Both the Yuasa and Dijkstra barriers can be made conditional on the
 // color of the object containing the slot. We chose not to make these
 // conditional because the cost of ensuring that the object holding
 // the slot doesn't concurrently change color without the mutator
 // noticing seems prohibitive.
 //
 // Consider the following example where the mutator writes into
 // a slot and then loads the slot's mark bit while the GC thread
 // writes to the slot's mark bit and then as part of scanning reads
 // the slot.
 //
 // Initially both [slot] and [slotmark] are 0 (nil)
 // Mutator thread          GC thread
 // st [slot], ptr          st [slotmark], 1
 //
 // ld r1, [slotmark]       ld r2, [slot]
 //
 // Without an expensive memory barrier between the st and the ld, the final
 // result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
 // example of what can happen when loads are allowed to be reordered with older
 // stores (avoiding such reorderings lies at the heart of the classic
 // Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
 // barriers, which will slow down both the mutator and the GC, we always grey
 // the ptr object regardless of the slot's color.
 //
 // Another place where we intentionally omit memory barriers is when
 // accessing mheap_.arena_used to check if a pointer points into the
 // heap. On relaxed memory machines, it's possible for a mutator to
 // extend the size of the heap by updating arena_used, allocate an
 // object from this new region, and publish a pointer to that object,
 // but for tracing running on another processor to observe the pointer
 // but use the old value of arena_used. In this case, tracing will not
 // mark the object, even though it's reachable. However, the mutator
 // is guaranteed to execute a write barrier when it publishes the
 // pointer, so it will take care of marking the object. A general
 // consequence of this is that the garbage collector may cache the
 // value of mheap_.arena_used. (See issue #9984.)
 //
 //
 // Stack writes:
 //
 // The compiler omits write barriers for writes to the current frame,
 // but if a stack pointer has been passed down the call stack, the
 // compiler will generate a write barrier for writes through that
 // pointer (because it doesn't know it's not a heap pointer).
 //
 // One might be tempted to ignore the write barrier if slot points
 // into to the stack. Don't do it! Mark termination only re-scans
 // frames that have potentially been active since the concurrent scan,
 // so it depends on write barriers to track changes to pointers in
 // stack frames that have not been active.
 //
 //
 // Global writes:
 //
 // The Go garbage collector requires write barriers when heap pointers
 // are stored in globals. Many garbage collectors ignore writes to
 // globals and instead pick up global -> heap pointers during
 // termination. This increases pause time, so we instead rely on write
 // barriers for writes to globals so that we don't have to rescan
 // global during mark termination.
 //
 //
 // Publication ordering:
 //
 // The write barrier is *pre-publication*, meaning that the write
 // barrier happens prior to the *slot = ptr write that may make ptr
 // reachable by some goroutine that currently cannot reach it.
 //
 //
 //go:nowritebarrierrec
 //go:systemstack
 func gcmarkwb_m(slot *uintptr, ptr uintptr) {
 	if writeBarrier.needed {
 		// Note: This turns bad pointer writes into bad
 		// pointer reads, which could be confusing. We avoid
 		// reading from obviously bad pointers, which should
 		// take care of the vast majority of these. We could
 		// patch this up in the signal handler, or use XCHG to
 		// combine the read and the write. Checking inheap is
 		// insufficient since we need to track changes to
 		// roots outside the heap.
 		//
 		// Note: profbuf.go omits a barrier during signal handler
 		// profile logging; that's safe only because this deletion barrier exists.
 		// If we remove the deletion barrier, we'll have to work out
 		// a new way to handle the profile logging.
 		if slot1 := uintptr(unsafe.Pointer(slot)); slot1 >= minPhysPageSize {
 			if optr := *slot; optr != 0 {
 				shade(optr)
 			}
 		}
 		// TODO: Make this conditional on the caller's stack color.
 		if ptr != 0 && inheap(ptr) {
 			shade(ptr)
 		}
 	}
 }

 // writebarrierptr_prewrite1 invokes a write barrier for *dst = src
 // prior to the write happening.
 //
 // Write barrier calls must not happen during critical GC and scheduler
 // related operations. In particular there are times when the GC assumes
 // that the world is stopped but scheduler related code is still being
 // executed, dealing with syscalls, dealing with putting gs on runnable
 // queues and so forth. This code cannot execute write barriers because
 // the GC might drop them on the floor. Stopping the world involves removing
 // the p associated with an m. We use the fact that m.p == nil to indicate
 // that we are in one these critical section and throw if the write is of
 // a pointer to a heap object.
 //go:nosplit
 func writebarrierptr_prewrite1(dst *uintptr, src uintptr) {
 	mp := acquirem()
 	if mp.inwb || mp.dying > 0 {
 		releasem(mp)
 		return
 	}
 	systemstack(func() {
 		if mp.p == 0 && memstats.enablegc && !mp.inwb && inheap(src) {
 			throw("writebarrierptr_prewrite1 called with mp.p == nil")
 		}
 		mp.inwb = true
 		gcmarkwb_m(dst, src)
 	})
 	mp.inwb = false
 	releasem(mp)
 }

 // NOTE: Really dst *unsafe.Pointer, src unsafe.Pointer,
 // but if we do that, Go inserts a write barrier on *dst = src.
 //go:nosplit
 func writebarrierptr(dst *uintptr, src uintptr) {
 	if writeBarrier.cgo {
 		cgoCheckWriteBarrier(dst, src)
 	}
 	if !writeBarrier.needed {
 		*dst = src
 		return
 	}
 	if src != 0 && src < minPhysPageSize {
 		systemstack(func() {
 			print("runtime: writebarrierptr *", dst, " = ", hex(src), "\n")
 			throw("bad pointer in write barrier")
 		})
 	}
 	writebarrierptr_prewrite1(dst, src)
 	*dst = src
 }

 // writebarrierptr_prewrite is like writebarrierptr, but the store
 // will be performed by the caller after this call. The caller must
 // not allow preemption between this call and the write.
 //
 //go:nosplit
 func writebarrierptr_prewrite(dst *uintptr, src uintptr) {
 	if writeBarrier.cgo {
 		cgoCheckWriteBarrier(dst, src)
 	}
 	if !writeBarrier.needed {
 		return
 	}
 	if src != 0 && src < minPhysPageSize {
 		systemstack(func() { throw("bad pointer in write barrier") })
 	}
 	writebarrierptr_prewrite1(dst, src)
 }

 // typedmemmove copies a value of type t to dst from src.
 // Must be nosplit, see #16026.
 //go:nosplit
 func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
 	if typ.kind&kindNoPointers == 0 {
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.size)
 	}
 	// There's a race here: if some other goroutine can write to
 	// src, it may change some pointer in src after we've
 	// performed the write barrier but before we perform the
 	// memory copy. This safe because the write performed by that
 	// other goroutine must also be accompanied by a write
 	// barrier, so at worst we've unnecessarily greyed the old
 	// pointer that was in src.
 	memmove(dst, src, typ.size)
 	if writeBarrier.cgo {
 		cgoCheckMemmove(typ, dst, src, 0, typ.size)
 	}
 }

 //go:linkname reflect_typedmemmove reflect.typedmemmove
 func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
 	if raceenabled {
 		raceWriteObjectPC(typ, dst, getcallerpc(unsafe.Pointer(&typ)), funcPC(reflect_typedmemmove))
 		raceReadObjectPC(typ, src, getcallerpc(unsafe.Pointer(&typ)), funcPC(reflect_typedmemmove))
 	}
 	if msanenabled {
 		msanwrite(dst, typ.size)
 		msanread(src, typ.size)
 	}
 	typedmemmove(typ, dst, src)
 }

 // typedmemmovepartial is like typedmemmove but assumes that
 // dst and src point off bytes into the value and only copies size bytes.
 //go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
 func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
 	if writeBarrier.needed && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
 		// Pointer-align start address for bulk barrier.
 		adst, asrc, asize := dst, src, size
 		if frag := -off & (sys.PtrSize - 1); frag != 0 {
 			adst = add(dst, frag)
 			asrc = add(src, frag)
 			asize -= frag
 		}
 		bulkBarrierPreWrite(uintptr(adst), uintptr(asrc), asize&^(sys.PtrSize-1))
 	}

 	memmove(dst, src, size)
 	if writeBarrier.cgo {
 		cgoCheckMemmove(typ, dst, src, off, size)
 	}
 }

 // reflectcallmove is invoked by reflectcall to copy the return values
 // out of the stack and into the heap, invoking the necessary write
 // barriers. dst, src, and size describe the return value area to
 // copy. typ describes the entire frame (not just the return values).
 // typ may be nil, which indicates write barriers are not needed.
 //
 // It must be nosplit and must only call nosplit functions because the
 // stack map of reflectcall is wrong.
 //
 //go:nosplit
 func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr) {
 	if writeBarrier.needed && typ != nil && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
 	}
 	memmove(dst, src, size)
 }

 //go:nosplit
 func typedslicecopy(typ *_type, dst, src slice) int {
 	// TODO(rsc): If typedslicecopy becomes faster than calling
 	// typedmemmove repeatedly, consider using during func growslice.
 	n := dst.len
 	if n > src.len {
 		n = src.len
 	}
 	if n == 0 {
 		return 0
 	}
 	dstp := dst.array
 	srcp := src.array

 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&typ))
 		pc := funcPC(slicecopy)
 		racewriterangepc(dstp, uintptr(n)*typ.size, callerpc, pc)
 		racereadrangepc(srcp, uintptr(n)*typ.size, callerpc, pc)
 	}
 	if msanenabled {
 		msanwrite(dstp, uintptr(n)*typ.size)
 		msanread(srcp, uintptr(n)*typ.size)
 	}

 	if writeBarrier.cgo {
 		cgoCheckSliceCopy(typ, dst, src, n)
 	}

 	// Note: No point in checking typ.kind&kindNoPointers here:
 	// compiler only emits calls to typedslicecopy for types with pointers,
 	// and growslice and reflect_typedslicecopy check for pointers
 	// before calling typedslicecopy.
 	if !writeBarrier.needed {
 		memmove(dstp, srcp, uintptr(n)*typ.size)
 		return n
 	}

 	systemstack(func() {
 		if uintptr(srcp) < uintptr(dstp) && uintptr(srcp)+uintptr(n)*typ.size > uintptr(dstp) {
 			// Overlap with src before dst.
 			// Copy backward, being careful not to move dstp/srcp
 			// out of the array they point into.
 			dstp = add(dstp, uintptr(n-1)*typ.size)
 			srcp = add(srcp, uintptr(n-1)*typ.size)
 			i := 0
 			for {
 				typedmemmove(typ, dstp, srcp)
 				if i++; i >= n {
 					break
 				}
 				dstp = add(dstp, -typ.size)
 				srcp = add(srcp, -typ.size)
 			}
 		} else {
 			// Copy forward, being careful not to move dstp/srcp
 			// out of the array they point into.
 			i := 0
 			for {
 				typedmemmove(typ, dstp, srcp)
 				if i++; i >= n {
 					break
 				}
 				dstp = add(dstp, typ.size)
 				srcp = add(srcp, typ.size)
 			}
 		}
 	})
 	return n
 }

 //go:linkname reflect_typedslicecopy reflect.typedslicecopy
 func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
 	if elemType.kind&kindNoPointers != 0 {
 		n := dst.len
 		if n > src.len {
 			n = src.len
 		}
 		if n == 0 {
 			return 0
 		}

 		size := uintptr(n) * elemType.size
 		if raceenabled {
 			callerpc := getcallerpc(unsafe.Pointer(&elemType))
 			pc := funcPC(reflect_typedslicecopy)
 			racewriterangepc(dst.array, size, callerpc, pc)
 			racereadrangepc(src.array, size, callerpc, pc)
 		}
 		if msanenabled {
 			msanwrite(dst.array, size)
 			msanread(src.array, size)
 		}

 		memmove(dst.array, src.array, size)
 		return n
 	}
 	return typedslicecopy(elemType, dst, src)
 }

 // typedmemclr clears the typed memory at ptr with type typ. The
 // memory at ptr must already be initialized (and hence in type-safe
 // state). If the memory is being initialized for the first time, see
 // memclrNoHeapPointers.
 //
 // If the caller knows that typ has pointers, it can alternatively
 // call memclrHasPointers.
 //
 //go:nosplit
 func typedmemclr(typ *_type, ptr unsafe.Pointer) {
 	if typ.kind&kindNoPointers == 0 {
 		bulkBarrierPreWrite(uintptr(ptr), 0, typ.size)
 	}
 	memclrNoHeapPointers(ptr, typ.size)
 }

 // memclrHasPointers clears n bytes of typed memory starting at ptr.
 // The caller must ensure that the type of the object at ptr has
 // pointers, usually by checking typ.kind&kindNoPointers. However, ptr
 // does not have to point to the start of the allocation.
 //
 //go:nosplit
 func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
 	bulkBarrierPreWrite(uintptr(ptr), 0, n)
 	memclrNoHeapPointers(ptr, n)
 }
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Garbage collector: write barriers.
	//
	// For the concurrent garbage collector, the Go compiler implements
	// updates to pointer-valued fields that may be in heap objects by
	// emitting calls to write barriers. This file contains the actual write barrier
	// implementation, gcmarkwb_m, and the various wrappers called by the
	// compiler to implement pointer assignment, slice assignment,
	// typed memmove, and so on.

	package runtime

	import (
	"runtime/internal/sys"
	"unsafe"
	)

	// gcmarkwb_m is the mark-phase write barrier, the only barrier we have.
	// The rest of this file exists only to make calls to this function.
	//
	// This is a hybrid barrier that combines a Yuasa-style deletion
	// barrier—which shades the object whose reference is being
	// overwritten—with Dijkstra insertion barrier—which shades the object
	// whose reference is being written. The insertion part of the barrier
	// is necessary while the calling goroutine's stack is grey. In
	// pseudocode, the barrier is:
	//
	// writePointer(slot, ptr):
	// shade(*slot)
	// if current stack is grey:
	// shade(ptr)
	// *slot = ptr
	//
	// slot is the destination in Go code.
	// ptr is the value that goes into the slot in Go code.
	//
	// Shade indicates that it has seen a white pointer by adding the referent
	// to wbuf as well as marking it.
	//
	// The two shades and the condition work together to prevent a mutator
	// from hiding an object from the garbage collector:
	//
	// 1. shade(*slot) prevents a mutator from hiding an object by moving
	// the sole pointer to it from the heap to its stack. If it attempts
	// to unlink an object from the heap, this will shade it.
	//
	// 2. shade(ptr) prevents a mutator from hiding an object by moving
	// the sole pointer to it from its stack into a black object in the
	// heap. If it attempts to install the pointer into a black object,
	// this will shade it.
	//
	// 3. Once a goroutine's stack is black, the shade(ptr) becomes
	// unnecessary. shade(ptr) prevents hiding an object by moving it from
	// the stack to the heap, but this requires first having a pointer
	// hidden on the stack. Immediately after a stack is scanned, it only
	// points to shaded objects, so it's not hiding anything, and the
	// shade(*slot) prevents it from hiding any other pointers on its
	// stack.
	//
	// For a detailed description of this barrier and proof of
	// correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
	//
	//
	//
	// Dealing with memory ordering:
	//
	// Both the Yuasa and Dijkstra barriers can be made conditional on the
	// color of the object containing the slot. We chose not to make these
	// conditional because the cost of ensuring that the object holding
	// the slot doesn't concurrently change color without the mutator
	// noticing seems prohibitive.
	//
	// Consider the following example where the mutator writes into
	// a slot and then loads the slot's mark bit while the GC thread
	// writes to the slot's mark bit and then as part of scanning reads
	// the slot.
	//
	// Initially both [slot] and [slotmark] are 0 (nil)
	// Mutator thread GC thread
	// st [slot], ptr st [slotmark], 1
	//
	// ld r1, [slotmark] ld r2, [slot]
	//
	// Without an expensive memory barrier between the st and the ld, the final
	// result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
	// example of what can happen when loads are allowed to be reordered with older
	// stores (avoiding such reorderings lies at the heart of the classic
	// Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
	// barriers, which will slow down both the mutator and the GC, we always grey
	// the ptr object regardless of the slot's color.
	//
	// Another place where we intentionally omit memory barriers is when
	// accessing mheap_.arena_used to check if a pointer points into the
	// heap. On relaxed memory machines, it's possible for a mutator to
	// extend the size of the heap by updating arena_used, allocate an
	// object from this new region, and publish a pointer to that object,
	// but for tracing running on another processor to observe the pointer
	// but use the old value of arena_used. In this case, tracing will not
	// mark the object, even though it's reachable. However, the mutator
	// is guaranteed to execute a write barrier when it publishes the
	// pointer, so it will take care of marking the object. A general
	// consequence of this is that the garbage collector may cache the
	// value of mheap_.arena_used. (See issue #9984.)
	//
	//
	// Stack writes:
	//
	// The compiler omits write barriers for writes to the current frame,
	// but if a stack pointer has been passed down the call stack, the
	// compiler will generate a write barrier for writes through that
	// pointer (because it doesn't know it's not a heap pointer).
	//
	// One might be tempted to ignore the write barrier if slot points
	// into to the stack. Don't do it! Mark termination only re-scans
	// frames that have potentially been active since the concurrent scan,
	// so it depends on write barriers to track changes to pointers in
	// stack frames that have not been active.
	//
	//
	// Global writes:
	//
	// The Go garbage collector requires write barriers when heap pointers
	// are stored in globals. Many garbage collectors ignore writes to
	// globals and instead pick up global -> heap pointers during
	// termination. This increases pause time, so we instead rely on write
	// barriers for writes to globals so that we don't have to rescan
	// global during mark termination.
	//
	//
	// Publication ordering:
	//
	// The write barrier is pre-publication, meaning that the write
	// barrier happens prior to the *slot = ptr write that may make ptr
	// reachable by some goroutine that currently cannot reach it.
	//
	//
	//go:nowritebarrierrec
	//go:systemstack
	func gcmarkwb_m(slot *uintptr, ptr uintptr) {
	if writeBarrier.needed {
	// Note: This turns bad pointer writes into bad
	// pointer reads, which could be confusing. We avoid
	// reading from obviously bad pointers, which should
	// take care of the vast majority of these. We could
	// patch this up in the signal handler, or use XCHG to
	// combine the read and the write. Checking inheap is
	// insufficient since we need to track changes to
	// roots outside the heap.
	//
	// Note: profbuf.go omits a barrier during signal handler
	// profile logging; that's safe only because this deletion barrier exists.
	// If we remove the deletion barrier, we'll have to work out
	// a new way to handle the profile logging.
	if slot1 := uintptr(unsafe.Pointer(slot)); slot1 >= minPhysPageSize {
	if optr := *slot; optr != 0 {
	shade(optr)
	}
	}
	// TODO: Make this conditional on the caller's stack color.
	if ptr != 0 && inheap(ptr) {
	shade(ptr)
	}
	}
	}

	// writebarrierptr_prewrite1 invokes a write barrier for *dst = src
	// prior to the write happening.
	//
	// Write barrier calls must not happen during critical GC and scheduler
	// related operations. In particular there are times when the GC assumes
	// that the world is stopped but scheduler related code is still being
	// executed, dealing with syscalls, dealing with putting gs on runnable
	// queues and so forth. This code cannot execute write barriers because
	// the GC might drop them on the floor. Stopping the world involves removing
	// the p associated with an m. We use the fact that m.p == nil to indicate
	// that we are in one these critical section and throw if the write is of
	// a pointer to a heap object.
	//go:nosplit
	func writebarrierptr_prewrite1(dst *uintptr, src uintptr) {
	mp := acquirem()
	if mp.inwb \|\| mp.dying > 0 {
	releasem(mp)
	return
	}
	systemstack(func() {
	if mp.p == 0 && memstats.enablegc && !mp.inwb && inheap(src) {
	throw("writebarrierptr_prewrite1 called with mp.p == nil")
	}
	mp.inwb = true
	gcmarkwb_m(dst, src)
	})
	mp.inwb = false
	releasem(mp)
	}

	// NOTE: Really dst *unsafe.Pointer, src unsafe.Pointer,
	// but if we do that, Go inserts a write barrier on *dst = src.
	//go:nosplit
	func writebarrierptr(dst *uintptr, src uintptr) {
	if writeBarrier.cgo {
	cgoCheckWriteBarrier(dst, src)
	}
	if !writeBarrier.needed {
	*dst = src
	return
	}
	if src != 0 && src < minPhysPageSize {
	systemstack(func() {
	print("runtime: writebarrierptr *", dst, " = ", hex(src), "\n")
	throw("bad pointer in write barrier")
	})
	}
	writebarrierptr_prewrite1(dst, src)
	*dst = src
	}

	// writebarrierptr_prewrite is like writebarrierptr, but the store
	// will be performed by the caller after this call. The caller must
	// not allow preemption between this call and the write.
	//
	//go:nosplit
	func writebarrierptr_prewrite(dst *uintptr, src uintptr) {
	if writeBarrier.cgo {
	cgoCheckWriteBarrier(dst, src)
	}
	if !writeBarrier.needed {
	return
	}
	if src != 0 && src < minPhysPageSize {
	systemstack(func() { throw("bad pointer in write barrier") })
	}
	writebarrierptr_prewrite1(dst, src)
	}

	// typedmemmove copies a value of type t to dst from src.
	// Must be nosplit, see #16026.
	//go:nosplit
	func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	if typ.kind&kindNoPointers == 0 {
	bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.size)
	}
	// There's a race here: if some other goroutine can write to
	// src, it may change some pointer in src after we've
	// performed the write barrier but before we perform the
	// memory copy. This safe because the write performed by that
	// other goroutine must also be accompanied by a write
	// barrier, so at worst we've unnecessarily greyed the old
	// pointer that was in src.
	memmove(dst, src, typ.size)
	if writeBarrier.cgo {
	cgoCheckMemmove(typ, dst, src, 0, typ.size)
	}
	}

	//go:linkname reflect_typedmemmove reflect.typedmemmove
	func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	if raceenabled {
	raceWriteObjectPC(typ, dst, getcallerpc(unsafe.Pointer(&typ)), funcPC(reflect_typedmemmove))
	raceReadObjectPC(typ, src, getcallerpc(unsafe.Pointer(&typ)), funcPC(reflect_typedmemmove))
	}
	if msanenabled {
	msanwrite(dst, typ.size)
	msanread(src, typ.size)
	}
	typedmemmove(typ, dst, src)
	}

	// typedmemmovepartial is like typedmemmove but assumes that
	// dst and src point off bytes into the value and only copies size bytes.
	//go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
	func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
	if writeBarrier.needed && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
	// Pointer-align start address for bulk barrier.
	adst, asrc, asize := dst, src, size
	if frag := -off & (sys.PtrSize - 1); frag != 0 {
	adst = add(dst, frag)
	asrc = add(src, frag)
	asize -= frag
	}
	bulkBarrierPreWrite(uintptr(adst), uintptr(asrc), asize&^(sys.PtrSize-1))
	}

	memmove(dst, src, size)
	if writeBarrier.cgo {
	cgoCheckMemmove(typ, dst, src, off, size)
	}
	}

	// reflectcallmove is invoked by reflectcall to copy the return values
	// out of the stack and into the heap, invoking the necessary write
	// barriers. dst, src, and size describe the return value area to
	// copy. typ describes the entire frame (not just the return values).
	// typ may be nil, which indicates write barriers are not needed.
	//
	// It must be nosplit and must only call nosplit functions because the
	// stack map of reflectcall is wrong.
	//
	//go:nosplit
	func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr) {
	if writeBarrier.needed && typ != nil && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
	bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
	}
	memmove(dst, src, size)
	}

	//go:nosplit
	func typedslicecopy(typ *_type, dst, src slice) int {
	// TODO(rsc): If typedslicecopy becomes faster than calling
	// typedmemmove repeatedly, consider using during func growslice.
	n := dst.len
	if n > src.len {
	n = src.len
	}
	if n == 0 {
	return 0
	}
	dstp := dst.array
	srcp := src.array

	if raceenabled {
	callerpc := getcallerpc(unsafe.Pointer(&typ))
	pc := funcPC(slicecopy)
	racewriterangepc(dstp, uintptr(n)*typ.size, callerpc, pc)
	racereadrangepc(srcp, uintptr(n)*typ.size, callerpc, pc)
	}
	if msanenabled {
	msanwrite(dstp, uintptr(n)*typ.size)
	msanread(srcp, uintptr(n)*typ.size)
	}

	if writeBarrier.cgo {
	cgoCheckSliceCopy(typ, dst, src, n)
	}

	// Note: No point in checking typ.kind&kindNoPointers here:
	// compiler only emits calls to typedslicecopy for types with pointers,
	// and growslice and reflect_typedslicecopy check for pointers
	// before calling typedslicecopy.
	if !writeBarrier.needed {
	memmove(dstp, srcp, uintptr(n)*typ.size)
	return n
	}

	systemstack(func() {
	if uintptr(srcp) < uintptr(dstp) && uintptr(srcp)+uintptr(n)*typ.size > uintptr(dstp) {
	// Overlap with src before dst.
	// Copy backward, being careful not to move dstp/srcp
	// out of the array they point into.
	dstp = add(dstp, uintptr(n-1)*typ.size)
	srcp = add(srcp, uintptr(n-1)*typ.size)
	i := 0
	for {
	typedmemmove(typ, dstp, srcp)
	if i++; i >= n {
	break
	}
	dstp = add(dstp, -typ.size)
	srcp = add(srcp, -typ.size)
	}
	} else {
	// Copy forward, being careful not to move dstp/srcp
	// out of the array they point into.
	i := 0
	for {
	typedmemmove(typ, dstp, srcp)
	if i++; i >= n {
	break
	}
	dstp = add(dstp, typ.size)
	srcp = add(srcp, typ.size)
	}
	}
	})
	return n
	}

	//go:linkname reflect_typedslicecopy reflect.typedslicecopy
	func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
	if elemType.kind&kindNoPointers != 0 {
	n := dst.len
	if n > src.len {
	n = src.len
	}
	if n == 0 {
	return 0
	}

	size := uintptr(n) * elemType.size
	if raceenabled {
	callerpc := getcallerpc(unsafe.Pointer(&elemType))
	pc := funcPC(reflect_typedslicecopy)
	racewriterangepc(dst.array, size, callerpc, pc)
	racereadrangepc(src.array, size, callerpc, pc)
	}
	if msanenabled {
	msanwrite(dst.array, size)
	msanread(src.array, size)
	}

	memmove(dst.array, src.array, size)
	return n
	}
	return typedslicecopy(elemType, dst, src)
	}

	// typedmemclr clears the typed memory at ptr with type typ. The
	// memory at ptr must already be initialized (and hence in type-safe
	// state). If the memory is being initialized for the first time, see
	// memclrNoHeapPointers.
	//
	// If the caller knows that typ has pointers, it can alternatively
	// call memclrHasPointers.
	//
	//go:nosplit
	func typedmemclr(typ *_type, ptr unsafe.Pointer) {
	if typ.kind&kindNoPointers == 0 {
	bulkBarrierPreWrite(uintptr(ptr), 0, typ.size)
	}
	memclrNoHeapPointers(ptr, typ.size)
	}

	// memclrHasPointers clears n bytes of typed memory starting at ptr.
	// The caller must ensure that the type of the object at ptr has
	// pointers, usually by checking typ.kind&kindNoPointers. However, ptr
	// does not have to point to the start of the allocation.
	//
	//go:nosplit
	func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
	bulkBarrierPreWrite(uintptr(ptr), 0, n)
	memclrNoHeapPointers(ptr, n)
	}