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

 // Copyright 2014 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/bytealg"
 	"internal/goarch"
 	"unsafe"
 )

 // The constant is known to the compiler.
 // There is no fundamental theory behind this number.
 const tmpStringBufSize = 32

 type tmpBuf [tmpStringBufSize]byte

 // concatstrings implements a Go string concatenation x+y+z+...
 // The operands are passed in the slice a.
 // If buf != nil, the compiler has determined that the result does not
 // escape the calling function, so the string data can be stored in buf
 // if small enough.
 func concatstrings(buf *tmpBuf, a []string) string {
 	idx := 0
 	l := 0
 	count := 0
 	for i, x := range a {
 		n := len(x)
 		if n == 0 {
 			continue
 		}
 		if l+n < l {
 			throw("string concatenation too long")
 		}
 		l += n
 		count++
 		idx = i
 	}
 	if count == 0 {
 		return ""
 	}

 	// If there is just one string and either it is not on the stack
 	// or our result does not escape the calling frame (buf != nil),
 	// then we can return that string directly.
 	if count == 1 && (buf != nil || !stringDataOnStack(a[idx])) {
 		return a[idx]
 	}
 	s, b := rawstringtmp(buf, l)
 	for _, x := range a {
 		copy(b, x)
 		b = b[len(x):]
 	}
 	return s
 }

 func concatstring2(buf *tmpBuf, a0, a1 string) string {
 	return concatstrings(buf, []string{a0, a1})
 }

 func concatstring3(buf *tmpBuf, a0, a1, a2 string) string {
 	return concatstrings(buf, []string{a0, a1, a2})
 }

 func concatstring4(buf *tmpBuf, a0, a1, a2, a3 string) string {
 	return concatstrings(buf, []string{a0, a1, a2, a3})
 }

 func concatstring5(buf *tmpBuf, a0, a1, a2, a3, a4 string) string {
 	return concatstrings(buf, []string{a0, a1, a2, a3, a4})
 }

 // slicebytetostring converts a byte slice to a string.
 // It is inserted by the compiler into generated code.
 // ptr is a pointer to the first element of the slice;
 // n is the length of the slice.
 // Buf is a fixed-size buffer for the result,
 // it is not nil if the result does not escape.
 func slicebytetostring(buf *tmpBuf, ptr *byte, n int) string {
 	if n == 0 {
 		// Turns out to be a relatively common case.
 		// Consider that you want to parse out data between parens in "foo()bar",
 		// you find the indices and convert the subslice to string.
 		return ""
 	}
 	if raceenabled {
 		racereadrangepc(unsafe.Pointer(ptr),
 			uintptr(n),
 			getcallerpc(),
 			abi.FuncPCABIInternal(slicebytetostring))
 	}
 	if msanenabled {
 		msanread(unsafe.Pointer(ptr), uintptr(n))
 	}
 	if asanenabled {
 		asanread(unsafe.Pointer(ptr), uintptr(n))
 	}
 	if n == 1 {
 		p := unsafe.Pointer(&staticuint64s[*ptr])
 		if goarch.BigEndian {
 			p = add(p, 7)
 		}
 		return unsafe.String((*byte)(p), 1)
 	}

 	var p unsafe.Pointer
 	if buf != nil && n <= len(buf) {
 		p = unsafe.Pointer(buf)
 	} else {
 		p = mallocgc(uintptr(n), nil, false)
 	}
 	memmove(p, unsafe.Pointer(ptr), uintptr(n))
 	return unsafe.String((*byte)(p), n)
 }

 // stringDataOnStack reports whether the string's data is
 // stored on the current goroutine's stack.
 func stringDataOnStack(s string) bool {
 	ptr := uintptr(unsafe.Pointer(unsafe.StringData(s)))
 	stk := getg().stack
 	return stk.lo <= ptr && ptr < stk.hi
 }

 func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
 	if buf != nil && l <= len(buf) {
 		b = buf[:l]
 		s = slicebytetostringtmp(&b[0], len(b))
 	} else {
 		s, b = rawstring(l)
 	}
 	return
 }

 // slicebytetostringtmp returns a "string" referring to the actual []byte bytes.
 //
 // Callers need to ensure that the returned string will not be used after
 // the calling goroutine modifies the original slice or synchronizes with
 // another goroutine.
 //
 // The function is only called when instrumenting
 // and otherwise intrinsified by the compiler.
 //
 // Some internal compiler optimizations use this function.
 //   - Used for m[T1{... Tn{..., string(k), ...} ...}] and m[string(k)]
 //     where k is []byte, T1 to Tn is a nesting of struct and array literals.
 //   - Used for "<"+string(b)+">" concatenation where b is []byte.
 //   - Used for string(b)=="foo" comparison where b is []byte.
 func slicebytetostringtmp(ptr *byte, n int) string {
 	if raceenabled && n > 0 {
 		racereadrangepc(unsafe.Pointer(ptr),
 			uintptr(n),
 			getcallerpc(),
 			abi.FuncPCABIInternal(slicebytetostringtmp))
 	}
 	if msanenabled && n > 0 {
 		msanread(unsafe.Pointer(ptr), uintptr(n))
 	}
 	if asanenabled && n > 0 {
 		asanread(unsafe.Pointer(ptr), uintptr(n))
 	}
 	return unsafe.String(ptr, n)
 }

 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
 	var b []byte
 	if buf != nil && len(s) <= len(buf) {
 		*buf = tmpBuf{}
 		b = buf[:len(s)]
 	} else {
 		b = rawbyteslice(len(s))
 	}
 	copy(b, s)
 	return b
 }

 func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	// two passes.
 	// unlike slicerunetostring, no race because strings are immutable.
 	n := 0
 	for range s {
 		n++
 	}

 	var a []rune
 	if buf != nil && n <= len(buf) {
 		*buf = [tmpStringBufSize]rune{}
 		a = buf[:n]
 	} else {
 		a = rawruneslice(n)
 	}

 	n = 0
 	for _, r := range s {
 		a[n] = r
 		n++
 	}
 	return a
 }

 func slicerunetostring(buf *tmpBuf, a []rune) string {
 	if raceenabled && len(a) > 0 {
 		racereadrangepc(unsafe.Pointer(&a[0]),
 			uintptr(len(a))*unsafe.Sizeof(a[0]),
 			getcallerpc(),
 			abi.FuncPCABIInternal(slicerunetostring))
 	}
 	if msanenabled && len(a) > 0 {
 		msanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
 	}
 	if asanenabled && len(a) > 0 {
 		asanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
 	}
 	var dum [4]byte
 	size1 := 0
 	for _, r := range a {
 		size1 += encoderune(dum[:], r)
 	}
 	s, b := rawstringtmp(buf, size1+3)
 	size2 := 0
 	for _, r := range a {
 		// check for race
 		if size2 >= size1 {
 			break
 		}
 		size2 += encoderune(b[size2:], r)
 	}
 	return s[:size2]
 }

 type stringStruct struct {
 	str unsafe.Pointer
 	len int
 }

 // Variant with *byte pointer type for DWARF debugging.
 type stringStructDWARF struct {
 	str *byte
 	len int
 }

 func stringStructOf(sp *string) *stringStruct {
 	return (*stringStruct)(unsafe.Pointer(sp))
 }

 func intstring(buf *[4]byte, v int64) (s string) {
 	var b []byte
 	if buf != nil {
 		b = buf[:]
 		s = slicebytetostringtmp(&b[0], len(b))
 	} else {
 		s, b = rawstring(4)
 	}
 	if int64(rune(v)) != v {
 		v = runeError
 	}
 	n := encoderune(b, rune(v))
 	return s[:n]
 }

 // rawstring allocates storage for a new string. The returned
 // string and byte slice both refer to the same storage.
 // The storage is not zeroed. Callers should use
 // b to set the string contents and then drop b.
 func rawstring(size int) (s string, b []byte) {
 	p := mallocgc(uintptr(size), nil, false)
 	return unsafe.String((*byte)(p), size), unsafe.Slice((*byte)(p), size)
 }

 // rawbyteslice allocates a new byte slice. The byte slice is not zeroed.
 func rawbyteslice(size int) (b []byte) {
 	cap := roundupsize(uintptr(size))
 	p := mallocgc(cap, nil, false)
 	if cap != uintptr(size) {
 		memclrNoHeapPointers(add(p, uintptr(size)), cap-uintptr(size))
 	}

 	*(*slice)(unsafe.Pointer(&b)) = slice{p, size, int(cap)}
 	return
 }

 // rawruneslice allocates a new rune slice. The rune slice is not zeroed.
 func rawruneslice(size int) (b []rune) {
 	if uintptr(size) > maxAlloc/4 {
 		throw("out of memory")
 	}
 	mem := roundupsize(uintptr(size) * 4)
 	p := mallocgc(mem, nil, false)
 	if mem != uintptr(size)*4 {
 		memclrNoHeapPointers(add(p, uintptr(size)*4), mem-uintptr(size)*4)
 	}

 	*(*slice)(unsafe.Pointer(&b)) = slice{p, size, int(mem / 4)}
 	return
 }

 // used by cmd/cgo
 func gobytes(p *byte, n int) (b []byte) {
 	if n == 0 {
 		return make([]byte, 0)
 	}

 	if n < 0 || uintptr(n) > maxAlloc {
 		panic(errorString("gobytes: length out of range"))
 	}

 	bp := mallocgc(uintptr(n), nil, false)
 	memmove(bp, unsafe.Pointer(p), uintptr(n))

 	*(*slice)(unsafe.Pointer(&b)) = slice{bp, n, n}
 	return
 }

 // This is exported via linkname to assembly in syscall (for Plan9).
 //
 //go:linkname gostring
 func gostring(p *byte) string {
 	l := findnull(p)
 	if l == 0 {
 		return ""
 	}
 	s, b := rawstring(l)
 	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
 	return s
 }

 // internal_syscall_gostring is a version of gostring for internal/syscall/unix.
 //
 //go:linkname internal_syscall_gostring internal/syscall/unix.gostring
 func internal_syscall_gostring(p *byte) string {
 	return gostring(p)
 }

 func gostringn(p *byte, l int) string {
 	if l == 0 {
 		return ""
 	}
 	s, b := rawstring(l)
 	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
 	return s
 }

 func hasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
 }

 const (
 	maxUint64 = ^uint64(0)
 	maxInt64  = int64(maxUint64 >> 1)
 )

 // atoi64 parses an int64 from a string s.
 // The bool result reports whether s is a number
 // representable by a value of type int64.
 func atoi64(s string) (int64, bool) {
 	if s == "" {
 		return 0, false
 	}

 	neg := false
 	if s[0] == '-' {
 		neg = true
 		s = s[1:]
 	}

 	un := uint64(0)
 	for i := 0; i < len(s); i++ {
 		c := s[i]
 		if c < '0' || c > '9' {
 			return 0, false
 		}
 		if un > maxUint64/10 {
 			// overflow
 			return 0, false
 		}
 		un *= 10
 		un1 := un + uint64(c) - '0'
 		if un1 < un {
 			// overflow
 			return 0, false
 		}
 		un = un1
 	}

 	if !neg && un > uint64(maxInt64) {
 		return 0, false
 	}
 	if neg && un > uint64(maxInt64)+1 {
 		return 0, false
 	}

 	n := int64(un)
 	if neg {
 		n = -n
 	}

 	return n, true
 }

 // atoi is like atoi64 but for integers
 // that fit into an int.
 func atoi(s string) (int, bool) {
 	if n, ok := atoi64(s); n == int64(int(n)) {
 		return int(n), ok
 	}
 	return 0, false
 }

 // atoi32 is like atoi but for integers
 // that fit into an int32.
 func atoi32(s string) (int32, bool) {
 	if n, ok := atoi64(s); n == int64(int32(n)) {
 		return int32(n), ok
 	}
 	return 0, false
 }

 // parseByteCount parses a string that represents a count of bytes.
 //
 // s must match the following regular expression:
 //
 //	^[0-9]+(([KMGT]i)?B)?$
 //
 // In other words, an integer byte count with an optional unit
 // suffix. Acceptable suffixes include one of
 // - KiB, MiB, GiB, TiB which represent binary IEC/ISO 80000 units, or
 // - B, which just represents bytes.
 //
 // Returns an int64 because that's what its callers want and receive,
 // but the result is always non-negative.
 func parseByteCount(s string) (int64, bool) {
 	// The empty string is not valid.
 	if s == "" {
 		return 0, false
 	}
 	// Handle the easy non-suffix case.
 	last := s[len(s)-1]
 	if last >= '0' && last <= '9' {
 		n, ok := atoi64(s)
 		if !ok || n < 0 {
 			return 0, false
 		}
 		return n, ok
 	}
 	// Failing a trailing digit, this must always end in 'B'.
 	// Also at this point there must be at least one digit before
 	// that B.
 	if last != 'B' || len(s) < 2 {
 		return 0, false
 	}
 	// The one before that must always be a digit or 'i'.
 	if c := s[len(s)-2]; c >= '0' && c <= '9' {
 		// Trivial 'B' suffix.
 		n, ok := atoi64(s[:len(s)-1])
 		if !ok || n < 0 {
 			return 0, false
 		}
 		return n, ok
 	} else if c != 'i' {
 		return 0, false
 	}
 	// Finally, we need at least 4 characters now, for the unit
 	// prefix and at least one digit.
 	if len(s) < 4 {
 		return 0, false
 	}
 	power := 0
 	switch s[len(s)-3] {
 	case 'K':
 		power = 1
 	case 'M':
 		power = 2
 	case 'G':
 		power = 3
 	case 'T':
 		power = 4
 	default:
 		// Invalid suffix.
 		return 0, false
 	}
 	m := uint64(1)
 	for i := 0; i < power; i++ {
 		m *= 1024
 	}
 	n, ok := atoi64(s[:len(s)-3])
 	if !ok || n < 0 {
 		return 0, false
 	}
 	un := uint64(n)
 	if un > maxUint64/m {
 		// Overflow.
 		return 0, false
 	}
 	un *= m
 	if un > uint64(maxInt64) {
 		// Overflow.
 		return 0, false
 	}
 	return int64(un), true
 }

 //go:nosplit
 func findnull(s *byte) int {
 	if s == nil {
 		return 0
 	}

 	// Avoid IndexByteString on Plan 9 because it uses SSE instructions
 	// on x86 machines, and those are classified as floating point instructions,
 	// which are illegal in a note handler.
 	if GOOS == "plan9" {
 		p := (*[maxAlloc/2 - 1]byte)(unsafe.Pointer(s))
 		l := 0
 		for p[l] != 0 {
 			l++
 		}
 		return l
 	}

 	// pageSize is the unit we scan at a time looking for NULL.
 	// It must be the minimum page size for any architecture Go
 	// runs on. It's okay (just a minor performance loss) if the
 	// actual system page size is larger than this value.
 	const pageSize = 4096

 	offset := 0
 	ptr := unsafe.Pointer(s)
 	// IndexByteString uses wide reads, so we need to be careful
 	// with page boundaries. Call IndexByteString on
 	// [ptr, endOfPage) interval.
 	safeLen := int(pageSize - uintptr(ptr)%pageSize)

 	for {
 		t := *(*string)(unsafe.Pointer(&stringStruct{ptr, safeLen}))
 		// Check one page at a time.
 		if i := bytealg.IndexByteString(t, 0); i != -1 {
 			return offset + i
 		}
 		// Move to next page
 		ptr = unsafe.Pointer(uintptr(ptr) + uintptr(safeLen))
 		offset += safeLen
 		safeLen = pageSize
 	}
 }

 func findnullw(s *uint16) int {
 	if s == nil {
 		return 0
 	}
 	p := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(s))
 	l := 0
 	for p[l] != 0 {
 		l++
 	}
 	return l
 }

 //go:nosplit
 func gostringnocopy(str *byte) string {
 	ss := stringStruct{str: unsafe.Pointer(str), len: findnull(str)}
 	s := *(*string)(unsafe.Pointer(&ss))
 	return s
 }

 func gostringw(strw *uint16) string {
 	var buf [8]byte
 	str := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(strw))
 	n1 := 0
 	for i := 0; str[i] != 0; i++ {
 		n1 += encoderune(buf[:], rune(str[i]))
 	}
 	s, b := rawstring(n1 + 4)
 	n2 := 0
 	for i := 0; str[i] != 0; i++ {
 		// check for race
 		if n2 >= n1 {
 			break
 		}
 		n2 += encoderune(b[n2:], rune(str[i]))
 	}
 	b[n2] = 0 // for luck
 	return s[:n2]
 }
	// Copyright 2014 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/bytealg"
	"internal/goarch"
	"unsafe"
	)

	// The constant is known to the compiler.
	// There is no fundamental theory behind this number.
	const tmpStringBufSize = 32

	type tmpBuf [tmpStringBufSize]byte

	// concatstrings implements a Go string concatenation x+y+z+...
	// The operands are passed in the slice a.
	// If buf != nil, the compiler has determined that the result does not
	// escape the calling function, so the string data can be stored in buf
	// if small enough.
	func concatstrings(buf *tmpBuf, a []string) string {
	idx := 0
	l := 0
	count := 0
	for i, x := range a {
	n := len(x)
	if n == 0 {
	continue
	}
	if l+n < l {
	throw("string concatenation too long")
	}
	l += n
	count++
	idx = i
	}
	if count == 0 {
	return ""
	}

	// If there is just one string and either it is not on the stack
	// or our result does not escape the calling frame (buf != nil),
	// then we can return that string directly.
	if count == 1 && (buf != nil \|\| !stringDataOnStack(a[idx])) {
	return a[idx]
	}
	s, b := rawstringtmp(buf, l)
	for _, x := range a {
	copy(b, x)
	b = b[len(x):]
	}
	return s
	}

	func concatstring2(buf *tmpBuf, a0, a1 string) string {
	return concatstrings(buf, []string{a0, a1})
	}

	func concatstring3(buf *tmpBuf, a0, a1, a2 string) string {
	return concatstrings(buf, []string{a0, a1, a2})
	}

	func concatstring4(buf *tmpBuf, a0, a1, a2, a3 string) string {
	return concatstrings(buf, []string{a0, a1, a2, a3})
	}

	func concatstring5(buf *tmpBuf, a0, a1, a2, a3, a4 string) string {
	return concatstrings(buf, []string{a0, a1, a2, a3, a4})
	}

	// slicebytetostring converts a byte slice to a string.
	// It is inserted by the compiler into generated code.
	// ptr is a pointer to the first element of the slice;
	// n is the length of the slice.
	// Buf is a fixed-size buffer for the result,
	// it is not nil if the result does not escape.
	func slicebytetostring(buf tmpBuf, ptr byte, n int) string {
	if n == 0 {
	// Turns out to be a relatively common case.
	// Consider that you want to parse out data between parens in "foo()bar",
	// you find the indices and convert the subslice to string.
	return ""
	}
	if raceenabled {
	racereadrangepc(unsafe.Pointer(ptr),
	uintptr(n),
	getcallerpc(),
	abi.FuncPCABIInternal(slicebytetostring))
	}
	if msanenabled {
	msanread(unsafe.Pointer(ptr), uintptr(n))
	}
	if asanenabled {
	asanread(unsafe.Pointer(ptr), uintptr(n))
	}
	if n == 1 {
	p := unsafe.Pointer(&staticuint64s[*ptr])
	if goarch.BigEndian {
	p = add(p, 7)
	}
	return unsafe.String((*byte)(p), 1)
	}

	var p unsafe.Pointer
	if buf != nil && n <= len(buf) {
	p = unsafe.Pointer(buf)
	} else {
	p = mallocgc(uintptr(n), nil, false)
	}
	memmove(p, unsafe.Pointer(ptr), uintptr(n))
	return unsafe.String((*byte)(p), n)
	}

	// stringDataOnStack reports whether the string's data is
	// stored on the current goroutine's stack.
	func stringDataOnStack(s string) bool {
	ptr := uintptr(unsafe.Pointer(unsafe.StringData(s)))
	stk := getg().stack
	return stk.lo <= ptr && ptr < stk.hi
	}

	func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
	if buf != nil && l <= len(buf) {
	b = buf[:l]
	s = slicebytetostringtmp(&b[0], len(b))
	} else {
	s, b = rawstring(l)
	}
	return
	}

	// slicebytetostringtmp returns a "string" referring to the actual []byte bytes.
	//
	// Callers need to ensure that the returned string will not be used after
	// the calling goroutine modifies the original slice or synchronizes with
	// another goroutine.
	//
	// The function is only called when instrumenting
	// and otherwise intrinsified by the compiler.
	//
	// Some internal compiler optimizations use this function.
	// - Used for m[T1{... Tn{..., string(k), ...} ...}] and m[string(k)]
	// where k is []byte, T1 to Tn is a nesting of struct and array literals.
	// - Used for "<"+string(b)+">" concatenation where b is []byte.
	// - Used for string(b)=="foo" comparison where b is []byte.
	func slicebytetostringtmp(ptr *byte, n int) string {
	if raceenabled && n > 0 {
	racereadrangepc(unsafe.Pointer(ptr),
	uintptr(n),
	getcallerpc(),
	abi.FuncPCABIInternal(slicebytetostringtmp))
	}
	if msanenabled && n > 0 {
	msanread(unsafe.Pointer(ptr), uintptr(n))
	}
	if asanenabled && n > 0 {
	asanread(unsafe.Pointer(ptr), uintptr(n))
	}
	return unsafe.String(ptr, n)
	}

	func stringtoslicebyte(buf *tmpBuf, s string) []byte {
	var b []byte
	if buf != nil && len(s) <= len(buf) {
	*buf = tmpBuf{}
	b = buf[:len(s)]
	} else {
	b = rawbyteslice(len(s))
	}
	copy(b, s)
	return b
	}

	func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
	// two passes.
	// unlike slicerunetostring, no race because strings are immutable.
	n := 0
	for range s {
	n++
	}

	var a []rune
	if buf != nil && n <= len(buf) {
	*buf = [tmpStringBufSize]rune{}
	a = buf[:n]
	} else {
	a = rawruneslice(n)
	}

	n = 0
	for _, r := range s {
	a[n] = r
	n++
	}
	return a
	}

	func slicerunetostring(buf *tmpBuf, a []rune) string {
	if raceenabled && len(a) > 0 {
	racereadrangepc(unsafe.Pointer(&a[0]),
	uintptr(len(a))*unsafe.Sizeof(a[0]),
	getcallerpc(),
	abi.FuncPCABIInternal(slicerunetostring))
	}
	if msanenabled && len(a) > 0 {
	msanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
	}
	if asanenabled && len(a) > 0 {
	asanread(unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(a[0]))
	}
	var dum [4]byte
	size1 := 0
	for _, r := range a {
	size1 += encoderune(dum[:], r)
	}
	s, b := rawstringtmp(buf, size1+3)
	size2 := 0
	for _, r := range a {
	// check for race
	if size2 >= size1 {
	break
	}
	size2 += encoderune(b[size2:], r)
	}
	return s[:size2]
	}

	type stringStruct struct {
	str unsafe.Pointer
	len int
	}

	// Variant with *byte pointer type for DWARF debugging.
	type stringStructDWARF struct {
	str *byte
	len int
	}

	func stringStructOf(sp string) stringStruct {
	return (*stringStruct)(unsafe.Pointer(sp))
	}

	func intstring(buf *[4]byte, v int64) (s string) {
	var b []byte
	if buf != nil {
	b = buf[:]
	s = slicebytetostringtmp(&b[0], len(b))
	} else {
	s, b = rawstring(4)
	}
	if int64(rune(v)) != v {
	v = runeError
	}
	n := encoderune(b, rune(v))
	return s[:n]
	}

	// rawstring allocates storage for a new string. The returned
	// string and byte slice both refer to the same storage.
	// The storage is not zeroed. Callers should use
	// b to set the string contents and then drop b.
	func rawstring(size int) (s string, b []byte) {
	p := mallocgc(uintptr(size), nil, false)
	return unsafe.String((byte)(p), size), unsafe.Slice((byte)(p), size)
	}

	// rawbyteslice allocates a new byte slice. The byte slice is not zeroed.
	func rawbyteslice(size int) (b []byte) {
	cap := roundupsize(uintptr(size))
	p := mallocgc(cap, nil, false)
	if cap != uintptr(size) {
	memclrNoHeapPointers(add(p, uintptr(size)), cap-uintptr(size))
	}

	(slice)(unsafe.Pointer(&b)) = slice{p, size, int(cap)}
	return
	}

	// rawruneslice allocates a new rune slice. The rune slice is not zeroed.
	func rawruneslice(size int) (b []rune) {
	if uintptr(size) > maxAlloc/4 {
	throw("out of memory")
	}
	mem := roundupsize(uintptr(size) * 4)
	p := mallocgc(mem, nil, false)
	if mem != uintptr(size)*4 {
	memclrNoHeapPointers(add(p, uintptr(size)4), mem-uintptr(size)4)
	}

	(slice)(unsafe.Pointer(&b)) = slice{p, size, int(mem / 4)}
	return
	}

	// used by cmd/cgo
	func gobytes(p *byte, n int) (b []byte) {
	if n == 0 {
	return make([]byte, 0)
	}

	if n < 0 \|\| uintptr(n) > maxAlloc {
	panic(errorString("gobytes: length out of range"))
	}

	bp := mallocgc(uintptr(n), nil, false)
	memmove(bp, unsafe.Pointer(p), uintptr(n))

	(slice)(unsafe.Pointer(&b)) = slice{bp, n, n}
	return
	}

	// This is exported via linkname to assembly in syscall (for Plan9).
	//
	//go:linkname gostring
	func gostring(p *byte) string {
	l := findnull(p)
	if l == 0 {
	return ""
	}
	s, b := rawstring(l)
	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
	return s
	}

	// internal_syscall_gostring is a version of gostring for internal/syscall/unix.
	//
	//go:linkname internal_syscall_gostring internal/syscall/unix.gostring
	func internal_syscall_gostring(p *byte) string {
	return gostring(p)
	}

	func gostringn(p *byte, l int) string {
	if l == 0 {
	return ""
	}
	s, b := rawstring(l)
	memmove(unsafe.Pointer(&b[0]), unsafe.Pointer(p), uintptr(l))
	return s
	}

	func hasPrefix(s, prefix string) bool {
	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
	}

	const (
	maxUint64 = ^uint64(0)
	maxInt64 = int64(maxUint64 >> 1)
	)

	// atoi64 parses an int64 from a string s.
	// The bool result reports whether s is a number
	// representable by a value of type int64.
	func atoi64(s string) (int64, bool) {
	if s == "" {
	return 0, false
	}

	neg := false
	if s[0] == '-' {
	neg = true
	s = s[1:]
	}

	un := uint64(0)
	for i := 0; i < len(s); i++ {
	c := s[i]
	if c < '0' \|\| c > '9' {
	return 0, false
	}
	if un > maxUint64/10 {
	// overflow
	return 0, false
	}
	un *= 10
	un1 := un + uint64(c) - '0'
	if un1 < un {
	// overflow
	return 0, false
	}
	un = un1
	}

	if !neg && un > uint64(maxInt64) {
	return 0, false
	}
	if neg && un > uint64(maxInt64)+1 {
	return 0, false
	}

	n := int64(un)
	if neg {
	n = -n
	}

	return n, true
	}

	// atoi is like atoi64 but for integers
	// that fit into an int.
	func atoi(s string) (int, bool) {
	if n, ok := atoi64(s); n == int64(int(n)) {
	return int(n), ok
	}
	return 0, false
	}

	// atoi32 is like atoi but for integers
	// that fit into an int32.
	func atoi32(s string) (int32, bool) {
	if n, ok := atoi64(s); n == int64(int32(n)) {
	return int32(n), ok
	}
	return 0, false
	}

	// parseByteCount parses a string that represents a count of bytes.
	//
	// s must match the following regular expression:
	//
	// ^[0-9]+(([KMGT]i)?B)?$
	//
	// In other words, an integer byte count with an optional unit
	// suffix. Acceptable suffixes include one of
	// - KiB, MiB, GiB, TiB which represent binary IEC/ISO 80000 units, or
	// - B, which just represents bytes.
	//
	// Returns an int64 because that's what its callers want and receive,
	// but the result is always non-negative.
	func parseByteCount(s string) (int64, bool) {
	// The empty string is not valid.
	if s == "" {
	return 0, false
	}
	// Handle the easy non-suffix case.
	last := s[len(s)-1]
	if last >= '0' && last <= '9' {
	n, ok := atoi64(s)
	if !ok \|\| n < 0 {
	return 0, false
	}
	return n, ok
	}
	// Failing a trailing digit, this must always end in 'B'.
	// Also at this point there must be at least one digit before
	// that B.
	if last != 'B' \|\| len(s) < 2 {
	return 0, false
	}
	// The one before that must always be a digit or 'i'.
	if c := s[len(s)-2]; c >= '0' && c <= '9' {
	// Trivial 'B' suffix.
	n, ok := atoi64(s[:len(s)-1])
	if !ok \|\| n < 0 {
	return 0, false
	}
	return n, ok
	} else if c != 'i' {
	return 0, false
	}
	// Finally, we need at least 4 characters now, for the unit
	// prefix and at least one digit.
	if len(s) < 4 {
	return 0, false
	}
	power := 0
	switch s[len(s)-3] {
	case 'K':
	power = 1
	case 'M':
	power = 2
	case 'G':
	power = 3
	case 'T':
	power = 4
	default:
	// Invalid suffix.
	return 0, false
	}
	m := uint64(1)
	for i := 0; i < power; i++ {
	m *= 1024
	}
	n, ok := atoi64(s[:len(s)-3])
	if !ok \|\| n < 0 {
	return 0, false
	}
	un := uint64(n)
	if un > maxUint64/m {
	// Overflow.
	return 0, false
	}
	un *= m
	if un > uint64(maxInt64) {
	// Overflow.
	return 0, false
	}
	return int64(un), true
	}

	//go:nosplit
	func findnull(s *byte) int {
	if s == nil {
	return 0
	}

	// Avoid IndexByteString on Plan 9 because it uses SSE instructions
	// on x86 machines, and those are classified as floating point instructions,
	// which are illegal in a note handler.
	if GOOS == "plan9" {
	p := (*[maxAlloc/2 - 1]byte)(unsafe.Pointer(s))
	l := 0
	for p[l] != 0 {
	l++
	}
	return l
	}

	// pageSize is the unit we scan at a time looking for NULL.
	// It must be the minimum page size for any architecture Go
	// runs on. It's okay (just a minor performance loss) if the
	// actual system page size is larger than this value.
	const pageSize = 4096

	offset := 0
	ptr := unsafe.Pointer(s)
	// IndexByteString uses wide reads, so we need to be careful
	// with page boundaries. Call IndexByteString on
	// [ptr, endOfPage) interval.
	safeLen := int(pageSize - uintptr(ptr)%pageSize)

	for {
	t := (string)(unsafe.Pointer(&stringStruct{ptr, safeLen}))
	// Check one page at a time.
	if i := bytealg.IndexByteString(t, 0); i != -1 {
	return offset + i
	}
	// Move to next page
	ptr = unsafe.Pointer(uintptr(ptr) + uintptr(safeLen))
	offset += safeLen
	safeLen = pageSize
	}
	}

	func findnullw(s *uint16) int {
	if s == nil {
	return 0
	}
	p := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(s))
	l := 0
	for p[l] != 0 {
	l++
	}
	return l
	}

	//go:nosplit
	func gostringnocopy(str *byte) string {
	ss := stringStruct{str: unsafe.Pointer(str), len: findnull(str)}
	s := (string)(unsafe.Pointer(&ss))
	return s
	}

	func gostringw(strw *uint16) string {
	var buf [8]byte
	str := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(strw))
	n1 := 0
	for i := 0; str[i] != 0; i++ {
	n1 += encoderune(buf[:], rune(str[i]))
	}
	s, b := rawstring(n1 + 4)
	n2 := 0
	for i := 0; str[i] != 0; i++ {
	// check for race
	if n2 >= n1 {
	break
	}
	n2 += encoderune(b[n2:], rune(str[i]))
	}
	b[n2] = 0 // for luck
	return s[:n2]
	}