// 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 // 指针, array指针指向底层数组
len int // 长度
cap int // 容量
}
// A notInHeapSlice is a slice backed by runtime/internal/sys.NotInHeap memory.
// notInHeapSlice是由runtime/internal/sys.NotInHeap内存支持的切片。
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".
// makeslicecopy分配一片“et”类型的“tolen”元素，然后从“from”将类型为“et”的“fromlen”元素复制到新分配中。
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.
// fromlen是已知的良好长度提供并且等于或大于tolen，从而使得tolen也是良好的切片长度，因为from和to切片具有相同的元件宽度。
tomem = et.size * uintptr(tolen)
copymem = tomem
}
var to unsafe.Pointer
if et.ptrdata == 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.
// 注意：不能使用rawmem（这可以避免内存归零），因为GC可以扫描未初始化的内存。
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.
// 只对old.array中的指针进行阴影处理，因为我们知道目标切片只包含零指针，因为它在分配过程中已被清除。
bulkBarrierPreWriteSrcOnly(uintptr(to), uintptr(from), copymem)
}
}
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.
// 注意：当有人确实犯了（[]T，bignumber）时，会产生“len out-of-range”错误，而不是“cap out-of-lange”错误cap超出范围也是正确的，
// 但由于cap只是隐式提供的，所以说len更清楚。见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)
}
// This is a wrapper over runtime/internal/math.MulUintptr,
// so the compiler can recognize and treat it as an intrinsic.
// 这是对runtime/internal/math.MulUintptr的包装，因此编译器可以识别它并将其视为内在的。
func mulUintptr(a, b uintptr) (uintptr, bool) {
return math.MulUintptr(a, b)
}
// growslice allocates new backing store for a slice.
// growtslice为切片分配新的后备存储。
// 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
// 假设原始切片长度为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.
// 一个新的后备存储器被分配了至少用于newLen元素的空间。现有条目[0，oldLen）被复制到新的后备存储中。
// 添加的条目[oldLen，newLen）不会由growtslice初始化（尽管对于包含指针的元素类型，它们被置零）。
// 它们必须由调用方初始化。后面的条目[newLen，newCap）被置零。
// 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).
// growtslice的奇数调用约定使生成的调用此函数的代码更加简单。特别是，它接受并返回新的长度，从而使旧的长度不活动（不需要溢出/恢复），并返回新长度（也不需要溢出或恢复）。
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 := oldCap
doublecap := newcap + newcap
if newLen > doublecap {
newcap = newLen
} else {
const threshold = 256
if oldCap < threshold {
newcap = doublecap
} else {
// Check 0 < newcap to detect overflow
// and prevent an infinite loop.
// 检查0<newcap以检测溢出并防止无限循环。
for 0 < newcap && newcap < newLen {
// Transition from growing 2x for small slices
// to growing 1.25x for large slices. This formula
// gives a smooth-ish transition between the two.
// 从小切片的2倍增长转变为大切片的1.25倍增长。这个公式在两者之间提供了一个平稳的过渡。
newcap += (newcap + 3*threshold) / 4
}
// Set newcap to the requested cap when
// the newcap calculation overflowed.
// 当newcap计算溢出时，将newcap设置为请求的cap。
if newcap <= 0 {
newcap = newLen
}
}
}
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.
// 专门用于et.size的常用值。对于1，我们不需要任何除法/乘法。对于goarch.PtrSize，编译器会将除法/乘法优化为一个常数的移位。对于2的幂，使用可变移位。
switch {
case et.size == 1:
lenmem = uintptr(oldLen)
newlenmem = uintptr(newLen)
capmem = roundupsize(uintptr(newcap))
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)
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. 掩码移位可更好地生成代码。
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)
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)
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:
// 除了capmem>maxAlloc之外，还需要检查溢出，以防止溢出，该溢出可用于触发32位架构上的segfault，示例程序如下：
// 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.ptrdata == 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.
// 调用growtslice的append（）将从oldLen覆盖到newLen。只清除不会被覆盖的部分。调用growtslice的reflect_growslice（）将手动清除此处未清除的区域。
memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
} else {
// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
// 注意：不能使用rawmem（这可以避免内存归零），因为GC可以扫描未初始化的内存。
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.
// 只对oldPtr中的指针进行阴影处理，因为我们知道目标切片p只包含零指针，因为它在分配期间已被清除。
bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(oldPtr), lenmem-et.size+et.ptrdata)
}
}
memmove(p, oldPtr, lenmem)
return slice{p, newLen, 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.
// 在语义上等同于slices.Grow，只是调用者负责确保old.len+num>old.cab。
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.
// growtslice不会将new〔old.cap:new.len〕清零，因为它假定内存将被调用growtslite的append（）覆盖。
// 由于reflect_growslice的调用方不是append（），因此在将切片返回到反射包之前，请先将该区域清零。
if et.ptrdata == 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.
// slicecopy用于将无指针元素的字符串或切片复制到切片中。
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 普通案例价值约2倍
// 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
}
[Golang1.20 source code reading] runtime/slice.go
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