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

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

 // An notInHeapSlice is a slice backed by go: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"))
 }

 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 handles slice growth during append.
 // It is passed the slice element type, the old slice, and the desired new minimum capacity,
 // and it returns a new slice with at least that capacity, with the old data
 // copied into it.
 // The new slice's length is set to the old slice's length,
 // NOT to the new requested capacity.
 // This is for codegen convenience. The old slice's length is used immediately
 // to calculate where to write new values during an append.
 // TODO: When the old backend is gone, reconsider this decision.
 // The SSA backend might prefer the new length or to return only ptr/cap and save stack space.
 func growslice(et *_type, old slice, cap int) slice {
 	if raceenabled {
 		callerpc := getcallerpc()
 		racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
 	}
 	if msanenabled {
 		msanread(old.array, uintptr(old.len*int(et.size)))
 	}

 	if cap < old.cap {
 		panic(errorString("growslice: cap 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 old.array in this case.
 		return slice{unsafe.Pointer(&zerobase), old.len, cap}
 	}

 	newcap := old.cap
 	doublecap := newcap + newcap
 	if cap > doublecap {
 		newcap = cap
 	} else {
 		if old.len < 1024 {
 			newcap = doublecap
 		} else {
 			// Check 0 < newcap to detect overflow
 			// and prevent an infinite loop.
 			for 0 < newcap && newcap < cap {
 				newcap += newcap / 4
 			}
 			// Set newcap to the requested cap when
 			// the newcap calculation overflowed.
 			if newcap <= 0 {
 				newcap = cap
 			}
 		}
 	}

 	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 sys.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
 	// For powers of 2, use a variable shift.
 	switch {
 	case et.size == 1:
 		lenmem = uintptr(old.len)
 		newlenmem = uintptr(cap)
 		capmem = roundupsize(uintptr(newcap))
 		overflow = uintptr(newcap) > maxAlloc
 		newcap = int(capmem)
 	case et.size == sys.PtrSize:
 		lenmem = uintptr(old.len) * sys.PtrSize
 		newlenmem = uintptr(cap) * sys.PtrSize
 		capmem = roundupsize(uintptr(newcap) * sys.PtrSize)
 		overflow = uintptr(newcap) > maxAlloc/sys.PtrSize
 		newcap = int(capmem / sys.PtrSize)
 	case isPowerOfTwo(et.size):
 		var shift uintptr
 		if sys.PtrSize == 8 {
 			// Mask shift for better code generation.
 			shift = uintptr(sys.Ctz64(uint64(et.size))) & 63
 		} else {
 			shift = uintptr(sys.Ctz32(uint32(et.size))) & 31
 		}
 		lenmem = uintptr(old.len) << shift
 		newlenmem = uintptr(cap) << shift
 		capmem = roundupsize(uintptr(newcap) << shift)
 		overflow = uintptr(newcap) > (maxAlloc >> shift)
 		newcap = int(capmem >> shift)
 	default:
 		lenmem = uintptr(old.len) * et.size
 		newlenmem = uintptr(cap) * et.size
 		capmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
 		capmem = roundupsize(capmem)
 		newcap = int(capmem / 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: cap out of range"))
 	}

 	var p unsafe.Pointer
 	if et.ptrdata == 0 {
 		p = mallocgc(capmem, nil, false)
 		// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
 		// Only clear the part that will not be overwritten.
 		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 old.array since we know the destination slice p
 			// only contains nil pointers because it has been cleared during alloc.
 			bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
 		}
 	}
 	memmove(p, old.array, lenmem)

 	return slice{p, old.len, newcap}
 }

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

 func slicecopy(to, fm slice, width uintptr) int {
 	if fm.len == 0 || to.len == 0 {
 		return 0
 	}

 	n := fm.len
 	if to.len < n {
 		n = to.len
 	}

 	if width == 0 {
 		return n
 	}

 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := funcPC(slicecopy)
 		racewriterangepc(to.array, uintptr(n*int(width)), callerpc, pc)
 		racereadrangepc(fm.array, uintptr(n*int(width)), callerpc, pc)
 	}
 	if msanenabled {
 		msanwrite(to.array, uintptr(n*int(width)))
 		msanread(fm.array, uintptr(n*int(width)))
 	}

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

 func slicestringcopy(to []byte, fm string) int {
 	if len(fm) == 0 || len(to) == 0 {
 		return 0
 	}

 	n := len(fm)
 	if len(to) < n {
 		n = len(to)
 	}

 	if raceenabled {
 		callerpc := getcallerpc()
 		pc := funcPC(slicestringcopy)
 		racewriterangepc(unsafe.Pointer(&to[0]), uintptr(n), callerpc, pc)
 	}
 	if msanenabled {
 		msanwrite(unsafe.Pointer(&to[0]), uintptr(n))
 	}

 	memmove(unsafe.Pointer(&to[0]), stringStructOf(&fm).str, uintptr(n))
 	return n
 }
	// 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 (
	"runtime/internal/math"
	"runtime/internal/sys"
	"unsafe"
	)

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

	// An notInHeapSlice is a slice backed by go: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"))
	}

	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 handles slice growth during append.
	// It is passed the slice element type, the old slice, and the desired new minimum capacity,
	// and it returns a new slice with at least that capacity, with the old data
	// copied into it.
	// The new slice's length is set to the old slice's length,
	// NOT to the new requested capacity.
	// This is for codegen convenience. The old slice's length is used immediately
	// to calculate where to write new values during an append.
	// TODO: When the old backend is gone, reconsider this decision.
	// The SSA backend might prefer the new length or to return only ptr/cap and save stack space.
	func growslice(et *_type, old slice, cap int) slice {
	if raceenabled {
	callerpc := getcallerpc()
	racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
	}
	if msanenabled {
	msanread(old.array, uintptr(old.len*int(et.size)))
	}

	if cap < old.cap {
	panic(errorString("growslice: cap 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 old.array in this case.
	return slice{unsafe.Pointer(&zerobase), old.len, cap}
	}

	newcap := old.cap
	doublecap := newcap + newcap
	if cap > doublecap {
	newcap = cap
	} else {
	if old.len < 1024 {
	newcap = doublecap
	} else {
	// Check 0 < newcap to detect overflow
	// and prevent an infinite loop.
	for 0 < newcap && newcap < cap {
	newcap += newcap / 4
	}
	// Set newcap to the requested cap when
	// the newcap calculation overflowed.
	if newcap <= 0 {
	newcap = cap
	}
	}
	}

	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 sys.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
	// For powers of 2, use a variable shift.
	switch {
	case et.size == 1:
	lenmem = uintptr(old.len)
	newlenmem = uintptr(cap)
	capmem = roundupsize(uintptr(newcap))
	overflow = uintptr(newcap) > maxAlloc
	newcap = int(capmem)
	case et.size == sys.PtrSize:
	lenmem = uintptr(old.len) * sys.PtrSize
	newlenmem = uintptr(cap) * sys.PtrSize
	capmem = roundupsize(uintptr(newcap) * sys.PtrSize)
	overflow = uintptr(newcap) > maxAlloc/sys.PtrSize
	newcap = int(capmem / sys.PtrSize)
	case isPowerOfTwo(et.size):
	var shift uintptr
	if sys.PtrSize == 8 {
	// Mask shift for better code generation.
	shift = uintptr(sys.Ctz64(uint64(et.size))) & 63
	} else {
	shift = uintptr(sys.Ctz32(uint32(et.size))) & 31
	}
	lenmem = uintptr(old.len) << shift
	newlenmem = uintptr(cap) << shift
	capmem = roundupsize(uintptr(newcap) << shift)
	overflow = uintptr(newcap) > (maxAlloc >> shift)
	newcap = int(capmem >> shift)
	default:
	lenmem = uintptr(old.len) * et.size
	newlenmem = uintptr(cap) * et.size
	capmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
	capmem = roundupsize(capmem)
	newcap = int(capmem / 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: cap out of range"))
	}

	var p unsafe.Pointer
	if et.ptrdata == 0 {
	p = mallocgc(capmem, nil, false)
	// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
	// Only clear the part that will not be overwritten.
	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 old.array since we know the destination slice p
	// only contains nil pointers because it has been cleared during alloc.
	bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
	}
	}
	memmove(p, old.array, lenmem)

	return slice{p, old.len, newcap}
	}

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

	func slicecopy(to, fm slice, width uintptr) int {
	if fm.len == 0 \|\| to.len == 0 {
	return 0
	}

	n := fm.len
	if to.len < n {
	n = to.len
	}

	if width == 0 {
	return n
	}

	if raceenabled {
	callerpc := getcallerpc()
	pc := funcPC(slicecopy)
	racewriterangepc(to.array, uintptr(n*int(width)), callerpc, pc)
	racereadrangepc(fm.array, uintptr(n*int(width)), callerpc, pc)
	}
	if msanenabled {
	msanwrite(to.array, uintptr(n*int(width)))
	msanread(fm.array, uintptr(n*int(width)))
	}

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

	func slicestringcopy(to []byte, fm string) int {
	if len(fm) == 0 \|\| len(to) == 0 {
	return 0
	}

	n := len(fm)
	if len(to) < n {
	n = len(to)
	}

	if raceenabled {
	callerpc := getcallerpc()
	pc := funcPC(slicestringcopy)
	racewriterangepc(unsafe.Pointer(&to[0]), uintptr(n), callerpc, pc)
	}
	if msanenabled {
	msanwrite(unsafe.Pointer(&to[0]), uintptr(n))
	}

	memmove(unsafe.Pointer(&to[0]), stringStructOf(&fm).str, uintptr(n))
	return n
	}