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

 // Copyright 2009 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/goarch"
 	"runtime/internal/math"
 	"runtime/internal/sys"
 	"unsafe"
 )

 type slice struct {
 	array unsafe.Pointer
 	len   int
 	cap   int
 }

 // A notInHeapSlice is a slice backed by runtime/internal/sys.NotInHeap memory.
 type notInHeapSlice struct {
 	array *notInHeap
 	len   int
 	cap   int
 }

 func panicmakeslicelen() {
 	panic(errorString("makeslice: len out of range"))
 }

 func panicmakeslicecap() {
 	panic(errorString("makeslice: cap out of range"))
 }

 // makeslicecopy allocates a slice of "tolen" elements of type "et",
 // then copies "fromlen" elements of type "et" into that new allocation from "from".
 func makeslicecopy(et *_type, tolen int, fromlen int, from unsafe.Pointer) unsafe.Pointer {
 	var tomem, copymem uintptr
 	if uintptr(tolen) > uintptr(fromlen) {
 		var overflow bool
 		tomem, overflow = math.MulUintptr(et.Size_, uintptr(tolen))
 		if overflow || tomem > maxAlloc || tolen < 0 {
 			panicmakeslicelen()
 		}
 		copymem = et.Size_ * uintptr(fromlen)
 	} else {
 		// fromlen is a known good length providing and equal or greater than tolen,
 		// thereby making tolen a good slice length too as from and to slices have the
 		// same element width.
 		tomem = et.Size_ * uintptr(tolen)
 		copymem = tomem
 	}

 	var to unsafe.Pointer
 	if et.PtrBytes == 0 {
 		to = mallocgc(tomem, nil, false)
 		if copymem < tomem {
 			memclrNoHeapPointers(add(to, copymem), tomem-copymem)
 		}
 	} else {
 		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
 		to = mallocgc(tomem, et, true)
 		if copymem > 0 && writeBarrier.enabled {
 			// Only shade the pointers in old.array since we know the destination slice to
 			// only contains nil pointers because it has been cleared during alloc.
 			//
 			// It's safe to pass a type to this function as an optimization because
 			// from and to only ever refer to memory representing whole values of
 			// type et. See the comment on bulkBarrierPreWrite.
 			bulkBarrierPreWriteSrcOnly(uintptr(to), uintptr(from), copymem, et)
 		}
 	}

 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := abi.FuncPCABIInternal(makeslicecopy)
 		racereadrangepc(from, copymem, callerpc, pc)
 	}
 	if msanenabled {
 		msanread(from, copymem)
 	}
 	if asanenabled {
 		asanread(from, copymem)
 	}

 	memmove(to, from, copymem)

 	return to
 }

 func makeslice(et *_type, len, cap int) unsafe.Pointer {
 	mem, overflow := math.MulUintptr(et.Size_, uintptr(cap))
 	if overflow || mem > maxAlloc || len < 0 || len > cap {
 		// NOTE: Produce a 'len out of range' error instead of a
 		// 'cap out of range' error when someone does make([]T, bignumber).
 		// 'cap out of range' is true too, but since the cap is only being
 		// supplied implicitly, saying len is clearer.
 		// See golang.org/issue/4085.
 		mem, overflow := math.MulUintptr(et.Size_, uintptr(len))
 		if overflow || mem > maxAlloc || len < 0 {
 			panicmakeslicelen()
 		}
 		panicmakeslicecap()
 	}

 	return mallocgc(mem, et, true)
 }

 func makeslice64(et *_type, len64, cap64 int64) unsafe.Pointer {
 	len := int(len64)
 	if int64(len) != len64 {
 		panicmakeslicelen()
 	}

 	cap := int(cap64)
 	if int64(cap) != cap64 {
 		panicmakeslicecap()
 	}

 	return makeslice(et, len, cap)
 }

 // growslice allocates new backing store for a slice.
 //
 // arguments:
 //
 //	oldPtr = pointer to the slice's backing array
 //	newLen = new length (= oldLen + num)
 //	oldCap = original slice's capacity.
 //	   num = number of elements being added
 //	    et = element type
 //
 // return values:
 //
 //	newPtr = pointer to the new backing store
 //	newLen = same value as the argument
 //	newCap = capacity of the new backing store
 //
 // Requires that uint(newLen) > uint(oldCap).
 // Assumes the original slice length is newLen - num
 //
 // A new backing store is allocated with space for at least newLen elements.
 // Existing entries [0, oldLen) are copied over to the new backing store.
 // Added entries [oldLen, newLen) are not initialized by growslice
 // (although for pointer-containing element types, they are zeroed). They
 // must be initialized by the caller.
 // Trailing entries [newLen, newCap) are zeroed.
 //
 // growslice's odd calling convention makes the generated code that calls
 // this function simpler. In particular, it accepts and returns the
 // new length so that the old length is not live (does not need to be
 // spilled/restored) and the new length is returned (also does not need
 // to be spilled/restored).
 func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice {
 	oldLen := newLen - num
 	if raceenabled {
 		callerpc := getcallerpc()
 		racereadrangepc(oldPtr, uintptr(oldLen*int(et.Size_)), callerpc, abi.FuncPCABIInternal(growslice))
 	}
 	if msanenabled {
 		msanread(oldPtr, uintptr(oldLen*int(et.Size_)))
 	}
 	if asanenabled {
 		asanread(oldPtr, uintptr(oldLen*int(et.Size_)))
 	}

 	if newLen < 0 {
 		panic(errorString("growslice: len out of range"))
 	}

 	if et.Size_ == 0 {
 		// append should not create a slice with nil pointer but non-zero len.
 		// We assume that append doesn't need to preserve oldPtr in this case.
 		return slice{unsafe.Pointer(&zerobase), newLen, newLen}
 	}

 	newcap := nextslicecap(newLen, oldCap)

 	var overflow bool
 	var lenmem, newlenmem, capmem uintptr
 	// Specialize for common values of et.Size.
 	// For 1 we don't need any division/multiplication.
 	// For goarch.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
 	// For powers of 2, use a variable shift.
 	noscan := et.PtrBytes == 0
 	switch {
 	case et.Size_ == 1:
 		lenmem = uintptr(oldLen)
 		newlenmem = uintptr(newLen)
 		capmem = roundupsize(uintptr(newcap), noscan)
 		overflow = uintptr(newcap) > maxAlloc
 		newcap = int(capmem)
 	case et.Size_ == goarch.PtrSize:
 		lenmem = uintptr(oldLen) * goarch.PtrSize
 		newlenmem = uintptr(newLen) * goarch.PtrSize
 		capmem = roundupsize(uintptr(newcap)*goarch.PtrSize, noscan)
 		overflow = uintptr(newcap) > maxAlloc/goarch.PtrSize
 		newcap = int(capmem / goarch.PtrSize)
 	case isPowerOfTwo(et.Size_):
 		var shift uintptr
 		if goarch.PtrSize == 8 {
 			// Mask shift for better code generation.
 			shift = uintptr(sys.TrailingZeros64(uint64(et.Size_))) & 63
 		} else {
 			shift = uintptr(sys.TrailingZeros32(uint32(et.Size_))) & 31
 		}
 		lenmem = uintptr(oldLen) << shift
 		newlenmem = uintptr(newLen) << shift
 		capmem = roundupsize(uintptr(newcap)<<shift, noscan)
 		overflow = uintptr(newcap) > (maxAlloc >> shift)
 		newcap = int(capmem >> shift)
 		capmem = uintptr(newcap) << shift
 	default:
 		lenmem = uintptr(oldLen) * et.Size_
 		newlenmem = uintptr(newLen) * et.Size_
 		capmem, overflow = math.MulUintptr(et.Size_, uintptr(newcap))
 		capmem = roundupsize(capmem, noscan)
 		newcap = int(capmem / et.Size_)
 		capmem = uintptr(newcap) * et.Size_
 	}

 	// The check of overflow in addition to capmem > maxAlloc is needed
 	// to prevent an overflow which can be used to trigger a segfault
 	// on 32bit architectures with this example program:
 	//
 	// type T [1<<27 + 1]int64
 	//
 	// var d T
 	// var s []T
 	//
 	// func main() {
 	//   s = append(s, d, d, d, d)
 	//   print(len(s), "\n")
 	// }
 	if overflow || capmem > maxAlloc {
 		panic(errorString("growslice: len out of range"))
 	}

 	var p unsafe.Pointer
 	if et.PtrBytes == 0 {
 		p = mallocgc(capmem, nil, false)
 		// The append() that calls growslice is going to overwrite from oldLen to newLen.
 		// Only clear the part that will not be overwritten.
 		// The reflect_growslice() that calls growslice will manually clear
 		// the region not cleared here.
 		memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
 	} else {
 		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
 		p = mallocgc(capmem, et, true)
 		if lenmem > 0 && writeBarrier.enabled {
 			// Only shade the pointers in oldPtr since we know the destination slice p
 			// only contains nil pointers because it has been cleared during alloc.
 			//
 			// It's safe to pass a type to this function as an optimization because
 			// from and to only ever refer to memory representing whole values of
 			// type et. See the comment on bulkBarrierPreWrite.
 			bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(oldPtr), lenmem-et.Size_+et.PtrBytes, et)
 		}
 	}
 	memmove(p, oldPtr, lenmem)

 	return slice{p, newLen, newcap}
 }

 // nextslicecap computes the next appropriate slice length.
 func nextslicecap(newLen, oldCap int) int {
 	newcap := oldCap
 	doublecap := newcap + newcap
 	if newLen > doublecap {
 		return newLen
 	}

 	const threshold = 256
 	if oldCap < threshold {
 		return doublecap
 	}
 	for {
 		// Transition from growing 2x for small slices
 		// to growing 1.25x for large slices. This formula
 		// gives a smooth-ish transition between the two.
 		newcap += (newcap + 3*threshold) >> 2

 		// We need to check `newcap >= newLen` and whether `newcap` overflowed.
 		// newLen is guaranteed to be larger than zero, hence
 		// when newcap overflows then `uint(newcap) > uint(newLen)`.
 		// This allows to check for both with the same comparison.
 		if uint(newcap) >= uint(newLen) {
 			break
 		}
 	}

 	// Set newcap to the requested cap when
 	// the newcap calculation overflowed.
 	if newcap <= 0 {
 		return newLen
 	}
 	return newcap
 }

 //go:linkname reflect_growslice reflect.growslice
 func reflect_growslice(et *_type, old slice, num int) slice {
 	// Semantically equivalent to slices.Grow, except that the caller
 	// is responsible for ensuring that old.len+num > old.cap.
 	num -= old.cap - old.len // preserve memory of old[old.len:old.cap]
 	new := growslice(old.array, old.cap+num, old.cap, num, et)
 	// growslice does not zero out new[old.cap:new.len] since it assumes that
 	// the memory will be overwritten by an append() that called growslice.
 	// Since the caller of reflect_growslice is not append(),
 	// zero out this region before returning the slice to the reflect package.
 	if et.PtrBytes == 0 {
 		oldcapmem := uintptr(old.cap) * et.Size_
 		newlenmem := uintptr(new.len) * et.Size_
 		memclrNoHeapPointers(add(new.array, oldcapmem), newlenmem-oldcapmem)
 	}
 	new.len = old.len // preserve the old length
 	return new
 }

 func isPowerOfTwo(x uintptr) bool {
 	return x&(x-1) == 0
 }

 // slicecopy is used to copy from a string or slice of pointerless elements into a slice.
 func slicecopy(toPtr unsafe.Pointer, toLen int, fromPtr unsafe.Pointer, fromLen int, width uintptr) int {
 	if fromLen == 0 || toLen == 0 {
 		return 0
 	}

 	n := fromLen
 	if toLen < n {
 		n = toLen
 	}

 	if width == 0 {
 		return n
 	}

 	size := uintptr(n) * width
 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := abi.FuncPCABIInternal(slicecopy)
 		racereadrangepc(fromPtr, size, callerpc, pc)
 		racewriterangepc(toPtr, size, callerpc, pc)
 	}
 	if msanenabled {
 		msanread(fromPtr, size)
 		msanwrite(toPtr, size)
 	}
 	if asanenabled {
 		asanread(fromPtr, size)
 		asanwrite(toPtr, size)
 	}

 	if size == 1 { // common case worth about 2x to do here
 		// TODO: is this still worth it with new memmove impl?
 		*(*byte)(toPtr) = *(*byte)(fromPtr) // known to be a byte pointer
 	} else {
 		memmove(toPtr, fromPtr, size)
 	}
 	return n
 }

 //go:linkname bytealg_MakeNoZero internal/bytealg.MakeNoZero
 func bytealg_MakeNoZero(len int) []byte {
 	if uintptr(len) > maxAlloc {
 		panicmakeslicelen()
 	}
 	return unsafe.Slice((*byte)(mallocgc(uintptr(len), nil, false)), len)
 }
	// Copyright 2009 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/goarch"
	"runtime/internal/math"
	"runtime/internal/sys"
	"unsafe"
	)

	type slice struct {
	array unsafe.Pointer
	len int
	cap int
	}

	// A notInHeapSlice is a slice backed by runtime/internal/sys.NotInHeap memory.
	type notInHeapSlice struct {
	array *notInHeap
	len int
	cap int
	}

	func panicmakeslicelen() {
	panic(errorString("makeslice: len out of range"))
	}

	func panicmakeslicecap() {
	panic(errorString("makeslice: cap out of range"))
	}

	// makeslicecopy allocates a slice of "tolen" elements of type "et",
	// then copies "fromlen" elements of type "et" into that new allocation from "from".
	func makeslicecopy(et *_type, tolen int, fromlen int, from unsafe.Pointer) unsafe.Pointer {
	var tomem, copymem uintptr
	if uintptr(tolen) > uintptr(fromlen) {
	var overflow bool
	tomem, overflow = math.MulUintptr(et.Size_, uintptr(tolen))
	if overflow \|\| tomem > maxAlloc \|\| tolen < 0 {
	panicmakeslicelen()
	}
	copymem = et.Size_ * uintptr(fromlen)
	} else {
	// fromlen is a known good length providing and equal or greater than tolen,
	// thereby making tolen a good slice length too as from and to slices have the
	// same element width.
	tomem = et.Size_ * uintptr(tolen)
	copymem = tomem
	}

	var to unsafe.Pointer
	if et.PtrBytes == 0 {
	to = mallocgc(tomem, nil, false)
	if copymem < tomem {
	memclrNoHeapPointers(add(to, copymem), tomem-copymem)
	}
	} else {
	// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
	to = mallocgc(tomem, et, true)
	if copymem > 0 && writeBarrier.enabled {
	// Only shade the pointers in old.array since we know the destination slice to
	// only contains nil pointers because it has been cleared during alloc.
	//
	// It's safe to pass a type to this function as an optimization because
	// from and to only ever refer to memory representing whole values of
	// type et. See the comment on bulkBarrierPreWrite.
	bulkBarrierPreWriteSrcOnly(uintptr(to), uintptr(from), copymem, et)
	}
	}

	if raceenabled {
	callerpc := getcallerpc()
	pc := abi.FuncPCABIInternal(makeslicecopy)
	racereadrangepc(from, copymem, callerpc, pc)
	}
	if msanenabled {
	msanread(from, copymem)
	}
	if asanenabled {
	asanread(from, copymem)
	}

	memmove(to, from, copymem)

	return to
	}

	func makeslice(et *_type, len, cap int) unsafe.Pointer {
	mem, overflow := math.MulUintptr(et.Size_, uintptr(cap))
	if overflow \|\| mem > maxAlloc \|\| len < 0 \|\| len > cap {
	// NOTE: Produce a 'len out of range' error instead of a
	// 'cap out of range' error when someone does make([]T, bignumber).
	// 'cap out of range' is true too, but since the cap is only being
	// supplied implicitly, saying len is clearer.
	// See golang.org/issue/4085.
	mem, overflow := math.MulUintptr(et.Size_, uintptr(len))
	if overflow \|\| mem > maxAlloc \|\| len < 0 {
	panicmakeslicelen()
	}
	panicmakeslicecap()
	}

	return mallocgc(mem, et, true)
	}

	func makeslice64(et *_type, len64, cap64 int64) unsafe.Pointer {
	len := int(len64)
	if int64(len) != len64 {
	panicmakeslicelen()
	}

	cap := int(cap64)
	if int64(cap) != cap64 {
	panicmakeslicecap()
	}

	return makeslice(et, len, cap)
	}

	// growslice allocates new backing store for a slice.
	//
	// arguments:
	//
	// oldPtr = pointer to the slice's backing array
	// newLen = new length (= oldLen + num)
	// oldCap = original slice's capacity.
	// num = number of elements being added
	// et = element type
	//
	// return values:
	//
	// newPtr = pointer to the new backing store
	// newLen = same value as the argument
	// newCap = capacity of the new backing store
	//
	// Requires that uint(newLen) > uint(oldCap).
	// Assumes the original slice length is newLen - num
	//
	// A new backing store is allocated with space for at least newLen elements.
	// Existing entries [0, oldLen) are copied over to the new backing store.
	// Added entries [oldLen, newLen) are not initialized by growslice
	// (although for pointer-containing element types, they are zeroed). They
	// must be initialized by the caller.
	// Trailing entries [newLen, newCap) are zeroed.
	//
	// growslice's odd calling convention makes the generated code that calls
	// this function simpler. In particular, it accepts and returns the
	// new length so that the old length is not live (does not need to be
	// spilled/restored) and the new length is returned (also does not need
	// to be spilled/restored).
	func growslice(oldPtr unsafe.Pointer, newLen, oldCap, num int, et *_type) slice {
	oldLen := newLen - num
	if raceenabled {
	callerpc := getcallerpc()
	racereadrangepc(oldPtr, uintptr(oldLen*int(et.Size_)), callerpc, abi.FuncPCABIInternal(growslice))
	}
	if msanenabled {
	msanread(oldPtr, uintptr(oldLen*int(et.Size_)))
	}
	if asanenabled {
	asanread(oldPtr, uintptr(oldLen*int(et.Size_)))
	}

	if newLen < 0 {
	panic(errorString("growslice: len out of range"))
	}

	if et.Size_ == 0 {
	// append should not create a slice with nil pointer but non-zero len.
	// We assume that append doesn't need to preserve oldPtr in this case.
	return slice{unsafe.Pointer(&zerobase), newLen, newLen}
	}

	newcap := nextslicecap(newLen, oldCap)

	var overflow bool
	var lenmem, newlenmem, capmem uintptr
	// Specialize for common values of et.Size.
	// For 1 we don't need any division/multiplication.
	// For goarch.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
	// For powers of 2, use a variable shift.
	noscan := et.PtrBytes == 0
	switch {
	case et.Size_ == 1:
	lenmem = uintptr(oldLen)
	newlenmem = uintptr(newLen)
	capmem = roundupsize(uintptr(newcap), noscan)
	overflow = uintptr(newcap) > maxAlloc
	newcap = int(capmem)
	case et.Size_ == goarch.PtrSize:
	lenmem = uintptr(oldLen) * goarch.PtrSize
	newlenmem = uintptr(newLen) * goarch.PtrSize
	capmem = roundupsize(uintptr(newcap)*goarch.PtrSize, noscan)
	overflow = uintptr(newcap) > maxAlloc/goarch.PtrSize
	newcap = int(capmem / goarch.PtrSize)
	case isPowerOfTwo(et.Size_):
	var shift uintptr
	if goarch.PtrSize == 8 {
	// Mask shift for better code generation.
	shift = uintptr(sys.TrailingZeros64(uint64(et.Size_))) & 63
	} else {
	shift = uintptr(sys.TrailingZeros32(uint32(et.Size_))) & 31
	}
	lenmem = uintptr(oldLen) << shift
	newlenmem = uintptr(newLen) << shift
	capmem = roundupsize(uintptr(newcap)<<shift, noscan)
	overflow = uintptr(newcap) > (maxAlloc >> shift)
	newcap = int(capmem >> shift)
	capmem = uintptr(newcap) << shift
	default:
	lenmem = uintptr(oldLen) * et.Size_
	newlenmem = uintptr(newLen) * et.Size_
	capmem, overflow = math.MulUintptr(et.Size_, uintptr(newcap))
	capmem = roundupsize(capmem, noscan)
	newcap = int(capmem / et.Size_)
	capmem = uintptr(newcap) * et.Size_
	}

	// The check of overflow in addition to capmem > maxAlloc is needed
	// to prevent an overflow which can be used to trigger a segfault
	// on 32bit architectures with this example program:
	//
	// type T [1<<27 + 1]int64
	//
	// var d T
	// var s []T
	//
	// func main() {
	// s = append(s, d, d, d, d)
	// print(len(s), "\n")
	// }
	if overflow \|\| capmem > maxAlloc {
	panic(errorString("growslice: len out of range"))
	}

	var p unsafe.Pointer
	if et.PtrBytes == 0 {
	p = mallocgc(capmem, nil, false)
	// The append() that calls growslice is going to overwrite from oldLen to newLen.
	// Only clear the part that will not be overwritten.
	// The reflect_growslice() that calls growslice will manually clear
	// the region not cleared here.
	memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
	} else {
	// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
	p = mallocgc(capmem, et, true)
	if lenmem > 0 && writeBarrier.enabled {
	// Only shade the pointers in oldPtr since we know the destination slice p
	// only contains nil pointers because it has been cleared during alloc.
	//
	// It's safe to pass a type to this function as an optimization because
	// from and to only ever refer to memory representing whole values of
	// type et. See the comment on bulkBarrierPreWrite.
	bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(oldPtr), lenmem-et.Size_+et.PtrBytes, et)
	}
	}
	memmove(p, oldPtr, lenmem)

	return slice{p, newLen, newcap}
	}

	// nextslicecap computes the next appropriate slice length.
	func nextslicecap(newLen, oldCap int) int {
	newcap := oldCap
	doublecap := newcap + newcap
	if newLen > doublecap {
	return newLen
	}

	const threshold = 256
	if oldCap < threshold {
	return doublecap
	}
	for {
	// Transition from growing 2x for small slices
	// to growing 1.25x for large slices. This formula
	// gives a smooth-ish transition between the two.
	newcap += (newcap + 3*threshold) >> 2

	// We need to check `newcap >= newLen` and whether `newcap` overflowed.
	// newLen is guaranteed to be larger than zero, hence
	// when newcap overflows then `uint(newcap) > uint(newLen)`.
	// This allows to check for both with the same comparison.
	if uint(newcap) >= uint(newLen) {
	break
	}
	}

	// Set newcap to the requested cap when
	// the newcap calculation overflowed.
	if newcap <= 0 {
	return newLen
	}
	return newcap
	}

	//go:linkname reflect_growslice reflect.growslice
	func reflect_growslice(et *_type, old slice, num int) slice {
	// Semantically equivalent to slices.Grow, except that the caller
	// is responsible for ensuring that old.len+num > old.cap.
	num -= old.cap - old.len // preserve memory of old[old.len:old.cap]
	new := growslice(old.array, old.cap+num, old.cap, num, et)
	// growslice does not zero out new[old.cap:new.len] since it assumes that
	// the memory will be overwritten by an append() that called growslice.
	// Since the caller of reflect_growslice is not append(),
	// zero out this region before returning the slice to the reflect package.
	if et.PtrBytes == 0 {
	oldcapmem := uintptr(old.cap) * et.Size_
	newlenmem := uintptr(new.len) * et.Size_
	memclrNoHeapPointers(add(new.array, oldcapmem), newlenmem-oldcapmem)
	}
	new.len = old.len // preserve the old length
	return new
	}

	func isPowerOfTwo(x uintptr) bool {
	return x&(x-1) == 0
	}

	// slicecopy is used to copy from a string or slice of pointerless elements into a slice.
	func slicecopy(toPtr unsafe.Pointer, toLen int, fromPtr unsafe.Pointer, fromLen int, width uintptr) int {
	if fromLen == 0 \|\| toLen == 0 {
	return 0
	}

	n := fromLen
	if toLen < n {
	n = toLen
	}

	if width == 0 {
	return n
	}

	size := uintptr(n) * width
	if raceenabled {
	callerpc := getcallerpc()
	pc := abi.FuncPCABIInternal(slicecopy)
	racereadrangepc(fromPtr, size, callerpc, pc)
	racewriterangepc(toPtr, size, callerpc, pc)
	}
	if msanenabled {
	msanread(fromPtr, size)
	msanwrite(toPtr, size)
	}
	if asanenabled {
	asanread(fromPtr, size)
	asanwrite(toPtr, size)
	}

	if size == 1 { // common case worth about 2x to do here
	// TODO: is this still worth it with new memmove impl?
	(byte)(toPtr) = (byte)(fromPtr) // known to be a byte pointer
	} else {
	memmove(toPtr, fromPtr, size)
	}
	return n
	}

	//go:linkname bytealg_MakeNoZero internal/bytealg.MakeNoZero
	func bytealg_MakeNoZero(len int) []byte {
	if uintptr(len) > maxAlloc {
	panicmakeslicelen()
	}
	return unsafe.Slice((*byte)(mallocgc(uintptr(len), nil, false)), len)
	}