golang 设计与实现 slice

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Golang 动态数组 slice。

定义

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type slice struct {
truearray unsafe.Pointer // 数据
truelen   int            // 长度（元素个数）
truecap   int            // 容量
}

创建 slice

采用 mallocgc 分配内存。

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

truereturn mallocgc(mem, et, true)
}

容量扩容策略

总结：

  1. 新容量 cap 大于旧容量 old.cap 的 2 倍时，直接采用 cap；
  2. 旧容量 old.cap 小于 1024 时，扩容为旧容量 old.cap 的 2 倍；
  3. 旧容量 old.cap 大于等于 1024 时，按照旧容量 old.cap 的 1.25 倍指数幂增长；
  4. 当步骤 3 溢出时，直接采用新容量 cap 的值； 
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// 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.
// 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.
//
// slice 在追加数据时使用 growslice 处理容量增长
// 入参：slice 元素类型，旧 slice，期望容量
// 返回：新 slice
func growslice(et *_type, old slice, cap int) slice {
trueif raceenabled {
truetruecallerpc := getcallerpc()
truetrueracereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
true}
trueif msanenabled {
truetruemsanread(old.array, uintptr(old.len*int(et.size)))
true}

trueif cap < old.cap {
truetruepanic(errorString("growslice: cap out of range"))
true}

trueif et.size == 0 {
truetrue// append should not create a slice with nil pointer but non-zero len.
truetrue// We assume that append doesn't need to preserve old.array in this case.
truetruereturn slice{unsafe.Pointer(&zerobase), old.len, cap}
true}

truenewcap := old.cap
truedoublecap := newcap + newcap
true// 期望容量 cap 超过旧容量 2 倍时，直接使用 cap 作为新 slice 的容量
trueif cap > doublecap {
truetruenewcap = cap
true} else {
truetrue// 就容量小于 1024 时，空间按 2 倍增长
truetrueif old.len < 1024 {
truetruetruenewcap = doublecap
truetrue} else {
truetruetrue// Check 0 < newcap to detect overflow
truetruetrue// and prevent an infinite loop.
truetruetrue// 新容量以 (1 + 1 / 4) 倍率增加，直到大于期望容量
truetruetruefor 0 < newcap && newcap < cap {
truetruetruetruenewcap += newcap / 4
truetruetrue}
truetruetrue// Set newcap to the requested cap when
truetruetrue// the newcap calculation overflowed.
truetruetrue// 防止溢出
truetruetrueif newcap <= 0 {
truetruetruetruenewcap = cap
truetruetrue}
truetrue}
true}

truevar overflow bool
truevar lenmem, newlenmem, capmem uintptr
true// Specialize for common values of et.size.
true// For 1 we don't need any division/multiplication.
true// For sys.PtrSize, compiler will optimize division/multiplication into a shift by a constant.
true// For powers of 2, use a variable shift.
true// 计算新容量
trueswitch {
truecase et.size == 1:
truetruelenmem = uintptr(old.len)
truetruenewlenmem = uintptr(cap)
truetruecapmem = roundupsize(uintptr(newcap))
truetrueoverflow = uintptr(newcap) > maxAlloc
truetruenewcap = int(capmem)
truecase et.size == sys.PtrSize:
truetruelenmem = uintptr(old.len) * sys.PtrSize
truetruenewlenmem = uintptr(cap) * sys.PtrSize
truetruecapmem = roundupsize(uintptr(newcap) * sys.PtrSize)
truetrueoverflow = uintptr(newcap) > maxAlloc/sys.PtrSize
truetruenewcap = int(capmem / sys.PtrSize)
truecase isPowerOfTwo(et.size):
truetruevar shift uintptr
truetrueif sys.PtrSize == 8 {
truetruetrue// Mask shift for better code generation.
truetruetrueshift = uintptr(sys.Ctz64(uint64(et.size))) & 63
truetrue} else {
truetruetrueshift = uintptr(sys.Ctz32(uint32(et.size))) & 31
truetrue}
truetruelenmem = uintptr(old.len) << shift
truetruenewlenmem = uintptr(cap) << shift
truetruecapmem = roundupsize(uintptr(newcap) << shift)
truetrueoverflow = uintptr(newcap) > (maxAlloc >> shift)
truetruenewcap = int(capmem >> shift)
truedefault:
truetruelenmem = uintptr(old.len) * et.size
truetruenewlenmem = uintptr(cap) * et.size
truetruecapmem, overflow = math.MulUintptr(et.size, uintptr(newcap))
truetruecapmem = roundupsize(capmem)
truetruenewcap = int(capmem / et.size)
true}

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

truevar p unsafe.Pointer
trueif et.ptrdata == 0 {
truetruep = mallocgc(capmem, nil, false)
truetrue// The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length).
truetrue// Only clear the part that will not be overwritten.
truetruememclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem)
true} else {
truetrue// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
truetruep = mallocgc(capmem, et, true)
truetrueif lenmem > 0 && writeBarrier.enabled {
truetruetrue// Only shade the pointers in old.array since we know the destination slice p
truetruetrue// only contains nil pointers because it has been cleared during alloc.
truetruetruebulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
truetrue}
true}
true// 复制旧数据
truememmove(p, old.array, lenmem)

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

来源：https://leunggeorge.github.io/

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