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. The main write barrier for
 // individual pointer writes is gcWriteBarrier and is implemented in
 // assembly. This file contains write barrier entry points for bulk
 // operations. See also mwbbuf.go.

 package runtime

 import (
 	"internal/abi"
 	"internal/goarch"
 	"internal/goexperiment"
 	"unsafe"
 )

 // Go uses 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).
 //
 //
 // 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.
 //
 //
 // Signal handler pointer writes:
 //
 // In general, the signal handler cannot safely invoke the write
 // barrier because it may run without a P or even during the write
 // barrier.
 //
 // There is exactly one exception: profbuf.go omits a barrier during
 // signal handler profile logging. That's safe only because of the
 // deletion barrier. See profbuf.go for a detailed argument. If we
 // remove the deletion barrier, we'll have to work out a new way to
 // handle the profile logging.

 // typedmemmove copies a value of type typ to dst from src.
 // Must be nosplit, see #16026.
 //
 // TODO: Perfect for go:nosplitrec since we can't have a safe point
 // anywhere in the bulk barrier or memmove.
 //
 //go:nosplit
 func typedmemmove(typ *abi.Type, dst, src unsafe.Pointer) {
 	if dst == src {
 		return
 	}
 	if writeBarrier.enabled && typ.Pointers() {
 		// This always copies a full value of type typ so it's safe
 		// to pass typ along as an optimization. See the comment on
 		// bulkBarrierPreWrite.
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes, typ)
 	}
 	// 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 goexperiment.CgoCheck2 {
 		cgoCheckMemmove2(typ, dst, src, 0, typ.Size_)
 	}
 }

 // wbZero performs the write barrier operations necessary before
 // zeroing a region of memory at address dst of type typ.
 // Does not actually do the zeroing.
 //
 //go:nowritebarrierrec
 //go:nosplit
 func wbZero(typ *_type, dst unsafe.Pointer) {
 	// This always copies a full value of type typ so it's safe
 	// to pass typ along as an optimization. See the comment on
 	// bulkBarrierPreWrite.
 	bulkBarrierPreWrite(uintptr(dst), 0, typ.PtrBytes, typ)
 }

 // wbMove performs the write barrier operations necessary before
 // copying a region of memory from src to dst of type typ.
 // Does not actually do the copying.
 //
 //go:nowritebarrierrec
 //go:nosplit
 func wbMove(typ *_type, dst, src unsafe.Pointer) {
 	// This always copies a full value of type typ so it's safe to
 	// pass a type here.
 	//
 	// See the comment on bulkBarrierPreWrite.
 	bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes, typ)
 }

 //go:linkname reflect_typedmemmove reflect.typedmemmove
 func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
 	if raceenabled {
 		raceWriteObjectPC(typ, dst, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
 		raceReadObjectPC(typ, src, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
 	}
 	if msanenabled {
 		msanwrite(dst, typ.Size_)
 		msanread(src, typ.Size_)
 	}
 	if asanenabled {
 		asanwrite(dst, typ.Size_)
 		asanread(src, typ.Size_)
 	}
 	typedmemmove(typ, dst, src)
 }

 //go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove
 func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
 	reflect_typedmemmove(typ, dst, src)
 }

 // 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, regs *abi.RegArgs) {
 	if writeBarrier.enabled && typ != nil && typ.Pointers() && size >= goarch.PtrSize {
 		// Pass nil for the type. dst does not point to value of type typ,
 		// but rather points into one, so applying the optimization is not
 		// safe. See the comment on this function.
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size, nil)
 	}
 	memmove(dst, src, size)

 	// Move pointers returned in registers to a place where the GC can see them.
 	for i := range regs.Ints {
 		if regs.ReturnIsPtr.Get(i) {
 			regs.Ptrs[i] = unsafe.Pointer(regs.Ints[i])
 		}
 	}
 }

 //go:nosplit
 func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int {
 	n := dstLen
 	if n > srcLen {
 		n = srcLen
 	}
 	if n == 0 {
 		return 0
 	}

 	// The compiler emits calls to typedslicecopy before
 	// instrumentation runs, so unlike the other copying and
 	// assignment operations, it's not instrumented in the calling
 	// code and needs its own instrumentation.
 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := abi.FuncPCABIInternal(slicecopy)
 		racewriterangepc(dstPtr, uintptr(n)*typ.Size_, callerpc, pc)
 		racereadrangepc(srcPtr, uintptr(n)*typ.Size_, callerpc, pc)
 	}
 	if msanenabled {
 		msanwrite(dstPtr, uintptr(n)*typ.Size_)
 		msanread(srcPtr, uintptr(n)*typ.Size_)
 	}
 	if asanenabled {
 		asanwrite(dstPtr, uintptr(n)*typ.Size_)
 		asanread(srcPtr, uintptr(n)*typ.Size_)
 	}

 	if goexperiment.CgoCheck2 {
 		cgoCheckSliceCopy(typ, dstPtr, srcPtr, n)
 	}

 	if dstPtr == srcPtr {
 		return n
 	}

 	// Note: No point in checking typ.PtrBytes here:
 	// compiler only emits calls to typedslicecopy for types with pointers,
 	// and growslice and reflect_typedslicecopy check for pointers
 	// before calling typedslicecopy.
 	size := uintptr(n) * typ.Size_
 	if writeBarrier.enabled {
 		// This always copies one or more full values of type typ so
 		// it's safe to pass typ along as an optimization. See the comment on
 		// bulkBarrierPreWrite.
 		pwsize := size - typ.Size_ + typ.PtrBytes
 		bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize, typ)
 	}
 	// See typedmemmove for a discussion of the race between the
 	// barrier and memmove.
 	memmove(dstPtr, srcPtr, size)
 	return n
 }

 //go:linkname reflect_typedslicecopy reflect.typedslicecopy
 func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
 	if !elemType.Pointers() {
 		return slicecopy(dst.array, dst.len, src.array, src.len, elemType.Size_)
 	}
 	return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len)
 }

 // 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.
 //
 // TODO: A "go:nosplitrec" annotation would be perfect for this.
 //
 //go:nosplit
 func typedmemclr(typ *_type, ptr unsafe.Pointer) {
 	if writeBarrier.enabled && typ.Pointers() {
 		// This always clears a whole value of type typ, so it's
 		// safe to pass a type here and apply the optimization.
 		// See the comment on bulkBarrierPreWrite.
 		bulkBarrierPreWrite(uintptr(ptr), 0, typ.PtrBytes, typ)
 	}
 	memclrNoHeapPointers(ptr, typ.Size_)
 }

 //go:linkname reflect_typedmemclr reflect.typedmemclr
 func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
 	typedmemclr(typ, ptr)
 }

 //go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
 func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
 	if writeBarrier.enabled && typ.Pointers() {
 		// Pass nil for the type. ptr does not point to value of type typ,
 		// but rather points into one so it's not safe to apply the optimization.
 		// See the comment on this function in the reflect package and the
 		// comment on bulkBarrierPreWrite.
 		bulkBarrierPreWrite(uintptr(ptr), 0, size, nil)
 	}
 	memclrNoHeapPointers(ptr, size)
 }

 //go:linkname reflect_typedarrayclear reflect.typedarrayclear
 func reflect_typedarrayclear(typ *_type, ptr unsafe.Pointer, len int) {
 	size := typ.Size_ * uintptr(len)
 	if writeBarrier.enabled && typ.Pointers() {
 		// This always clears whole elements of an array, so it's
 		// safe to pass a type here. See the comment on bulkBarrierPreWrite.
 		bulkBarrierPreWrite(uintptr(ptr), 0, size, typ)
 	}
 	memclrNoHeapPointers(ptr, 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.PtrBytes. However, ptr
 // does not have to point to the start of the allocation.
 //
 //go:nosplit
 func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
 	// Pass nil for the type since we don't have one here anyway.
 	bulkBarrierPreWrite(uintptr(ptr), 0, n, nil)
 	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. The main write barrier for
	// individual pointer writes is gcWriteBarrier and is implemented in
	// assembly. This file contains write barrier entry points for bulk
	// operations. See also mwbbuf.go.

	package runtime

	import (
	"internal/abi"
	"internal/goarch"
	"internal/goexperiment"
	"unsafe"
	)

	// Go uses 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).
	//
	//
	// 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.
	//
	//
	// Signal handler pointer writes:
	//
	// In general, the signal handler cannot safely invoke the write
	// barrier because it may run without a P or even during the write
	// barrier.
	//
	// There is exactly one exception: profbuf.go omits a barrier during
	// signal handler profile logging. That's safe only because of the
	// deletion barrier. See profbuf.go for a detailed argument. If we
	// remove the deletion barrier, we'll have to work out a new way to
	// handle the profile logging.

	// typedmemmove copies a value of type typ to dst from src.
	// Must be nosplit, see #16026.
	//
	// TODO: Perfect for go:nosplitrec since we can't have a safe point
	// anywhere in the bulk barrier or memmove.
	//
	//go:nosplit
	func typedmemmove(typ *abi.Type, dst, src unsafe.Pointer) {
	if dst == src {
	return
	}
	if writeBarrier.enabled && typ.Pointers() {
	// This always copies a full value of type typ so it's safe
	// to pass typ along as an optimization. See the comment on
	// bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes, typ)
	}
	// 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 goexperiment.CgoCheck2 {
	cgoCheckMemmove2(typ, dst, src, 0, typ.Size_)
	}
	}

	// wbZero performs the write barrier operations necessary before
	// zeroing a region of memory at address dst of type typ.
	// Does not actually do the zeroing.
	//
	//go:nowritebarrierrec
	//go:nosplit
	func wbZero(typ *_type, dst unsafe.Pointer) {
	// This always copies a full value of type typ so it's safe
	// to pass typ along as an optimization. See the comment on
	// bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(dst), 0, typ.PtrBytes, typ)
	}

	// wbMove performs the write barrier operations necessary before
	// copying a region of memory from src to dst of type typ.
	// Does not actually do the copying.
	//
	//go:nowritebarrierrec
	//go:nosplit
	func wbMove(typ *_type, dst, src unsafe.Pointer) {
	// This always copies a full value of type typ so it's safe to
	// pass a type here.
	//
	// See the comment on bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes, typ)
	}

	//go:linkname reflect_typedmemmove reflect.typedmemmove
	func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	if raceenabled {
	raceWriteObjectPC(typ, dst, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
	raceReadObjectPC(typ, src, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
	}
	if msanenabled {
	msanwrite(dst, typ.Size_)
	msanread(src, typ.Size_)
	}
	if asanenabled {
	asanwrite(dst, typ.Size_)
	asanread(src, typ.Size_)
	}
	typedmemmove(typ, dst, src)
	}

	//go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove
	func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
	reflect_typedmemmove(typ, dst, src)
	}

	// 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, regs abi.RegArgs) {
	if writeBarrier.enabled && typ != nil && typ.Pointers() && size >= goarch.PtrSize {
	// Pass nil for the type. dst does not point to value of type typ,
	// but rather points into one, so applying the optimization is not
	// safe. See the comment on this function.
	bulkBarrierPreWrite(uintptr(dst), uintptr(src), size, nil)
	}
	memmove(dst, src, size)

	// Move pointers returned in registers to a place where the GC can see them.
	for i := range regs.Ints {
	if regs.ReturnIsPtr.Get(i) {
	regs.Ptrs[i] = unsafe.Pointer(regs.Ints[i])
	}
	}
	}

	//go:nosplit
	func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int {
	n := dstLen
	if n > srcLen {
	n = srcLen
	}
	if n == 0 {
	return 0
	}

	// The compiler emits calls to typedslicecopy before
	// instrumentation runs, so unlike the other copying and
	// assignment operations, it's not instrumented in the calling
	// code and needs its own instrumentation.
	if raceenabled {
	callerpc := getcallerpc()
	pc := abi.FuncPCABIInternal(slicecopy)
	racewriterangepc(dstPtr, uintptr(n)*typ.Size_, callerpc, pc)
	racereadrangepc(srcPtr, uintptr(n)*typ.Size_, callerpc, pc)
	}
	if msanenabled {
	msanwrite(dstPtr, uintptr(n)*typ.Size_)
	msanread(srcPtr, uintptr(n)*typ.Size_)
	}
	if asanenabled {
	asanwrite(dstPtr, uintptr(n)*typ.Size_)
	asanread(srcPtr, uintptr(n)*typ.Size_)
	}

	if goexperiment.CgoCheck2 {
	cgoCheckSliceCopy(typ, dstPtr, srcPtr, n)
	}

	if dstPtr == srcPtr {
	return n
	}

	// Note: No point in checking typ.PtrBytes here:
	// compiler only emits calls to typedslicecopy for types with pointers,
	// and growslice and reflect_typedslicecopy check for pointers
	// before calling typedslicecopy.
	size := uintptr(n) * typ.Size_
	if writeBarrier.enabled {
	// This always copies one or more full values of type typ so
	// it's safe to pass typ along as an optimization. See the comment on
	// bulkBarrierPreWrite.
	pwsize := size - typ.Size_ + typ.PtrBytes
	bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize, typ)
	}
	// See typedmemmove for a discussion of the race between the
	// barrier and memmove.
	memmove(dstPtr, srcPtr, size)
	return n
	}

	//go:linkname reflect_typedslicecopy reflect.typedslicecopy
	func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
	if !elemType.Pointers() {
	return slicecopy(dst.array, dst.len, src.array, src.len, elemType.Size_)
	}
	return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len)
	}

	// 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.
	//
	// TODO: A "go:nosplitrec" annotation would be perfect for this.
	//
	//go:nosplit
	func typedmemclr(typ *_type, ptr unsafe.Pointer) {
	if writeBarrier.enabled && typ.Pointers() {
	// This always clears a whole value of type typ, so it's
	// safe to pass a type here and apply the optimization.
	// See the comment on bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(ptr), 0, typ.PtrBytes, typ)
	}
	memclrNoHeapPointers(ptr, typ.Size_)
	}

	//go:linkname reflect_typedmemclr reflect.typedmemclr
	func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
	typedmemclr(typ, ptr)
	}

	//go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
	func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
	if writeBarrier.enabled && typ.Pointers() {
	// Pass nil for the type. ptr does not point to value of type typ,
	// but rather points into one so it's not safe to apply the optimization.
	// See the comment on this function in the reflect package and the
	// comment on bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(ptr), 0, size, nil)
	}
	memclrNoHeapPointers(ptr, size)
	}

	//go:linkname reflect_typedarrayclear reflect.typedarrayclear
	func reflect_typedarrayclear(typ *_type, ptr unsafe.Pointer, len int) {
	size := typ.Size_ * uintptr(len)
	if writeBarrier.enabled && typ.Pointers() {
	// This always clears whole elements of an array, so it's
	// safe to pass a type here. See the comment on bulkBarrierPreWrite.
	bulkBarrierPreWrite(uintptr(ptr), 0, size, typ)
	}
	memclrNoHeapPointers(ptr, 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.PtrBytes. However, ptr
	// does not have to point to the start of the allocation.
	//
	//go:nosplit
	func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
	// Pass nil for the type since we don't have one here anyway.
	bulkBarrierPreWrite(uintptr(ptr), 0, n, nil)
	memclrNoHeapPointers(ptr, n)
	}