本文是对 Go slice扩容分析之 不是double或1.25那么简单的学习与记录


问题


依据大多数资料,slice的扩容机制是当切片的容量小于1024时,进行双倍扩容;当大于1024时,进行1.25倍扩容,见如下代码:

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var sli = []int{}

sli = append(sli, 666)

fmt.Println(cap(sli))

sli = append(sli, 777)

fmt.Println(cap(sli))

sli = append(sli, 888)

fmt.Println(sli)

fmt.Println(cap(sli))

sli = append(sli, 999)

fmt.Println(cap(sli))

sli = append(sli, 1000)

fmt.Println(cap(sli))

输出为:

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1
2
[666 777 888]
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4
8

又见如下代码:

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var sli2 = []int{}

for i := 0; i < 10; i++ {
sli2 = append(sli2, i)
}

fmt.Println(sli2)

fmt.Println(len(sli2))

fmt.Println(cap(sli2))

输出为:

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[0 1 2 3 4 5 6 7 8 9]
10
16

用更”准确”的话描述,是当cap<1024时,cap的值一定是2的n次方,且cap>=len;每当发生append使len增加,如果导致len>cap,此时cap会先于append操作进行double

即有

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var sli3 = []int{}

for i := 0; i < 512; i++ {
sli3 = append(sli3, i)
}

fmt.Println(len(sli3))

fmt.Println(cap(sli3))

sli3 = append(sli3, 123)

fmt.Println(len(sli3))

fmt.Println(cap(sli3))

结果为:

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512
512
513
1024


对于

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var sli4 = []int{}

for i := 0; i < 1024; i++ {
sli4 = append(sli4, i)
}

fmt.Println(len(sli4))

fmt.Println(cap(sli4))

sli4 = append(sli4, 123)

fmt.Println(len(sli4))

fmt.Println(cap(sli4))

结果为:

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1024
1024
1025
1280


1024*1.25 = 1280

看似无懈可击的结果, 不过, 果真确凿如此吗?




上面的操作是每次append一个元素,考虑另一种情形,一次性append很多元素,会发生什么呢?

当同时append进多个元素时,如下:

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package main

import "fmt"

func main() {
a := []byte{1, 0}
fmt.Println("len of old a is ", len(a))
fmt.Println("cap of old a is ", cap(a))
fmt.Println("")

a = append(a, 1, 1, 1)
fmt.Println("len of a is ", len(a))
fmt.Println("cap of a is ", cap(a))

fmt.Println("------")

b := []int{23, 51}
fmt.Println("len of old b is ", len(b))
fmt.Println("cap of old b is ", cap(b))
fmt.Println("")

b = append(b, 4, 5, 6)
fmt.Println("len of b is ", len(b))
fmt.Println("cap of b is ", cap(b))

fmt.Println("------")

c := []int32{1, 23}
fmt.Println("len of old c is ", len(c))
fmt.Println("cap of old c is ", cap(c))
fmt.Println("")

c = append(c, 2, 5, 6)
fmt.Println("len of c is ", len(c))
fmt.Println("cap of c is ", cap(c))

fmt.Println("------")

type D struct {
age byte
name string
}
d := []D{
{1, "123"},
{2, "234"},
}
fmt.Println("len of old d is ", len(d))
fmt.Println("cap of old d is ", cap(d))
fmt.Println("")

d = append(d, D{4, "456"}, D{5, "567"}, D{6, "678"})
fmt.Println("len of d is ", len(d))
fmt.Println("cap of d is ", cap(d))

}

结果为:

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len of old a is  2
cap of old a is 2

len of a is 5
cap of a is 8
------
len of old b is 2
cap of old b is 2

len of b is 5
cap of b is 6
------
len of old c is 2
cap of old c is 2

len of c is 5
cap of c is 6
------
len of old d is 2
cap of old d is 2

len of d is 5
cap of d is 5

匪夷所思?

其实是因为内存对齐


简化以上代码:

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package main

import "fmt"

func main() {

m := []int64{2, 3}
fmt.Println("len of old m is ", len(m))
fmt.Println("cap of old m is ", cap(m))
fmt.Println("")

m = append(m, 4, 5, 6)
fmt.Println("len of m is ", len(m))
fmt.Println("cap of m is ", cap(m))

fmt.Println()
fmt.Println("------")

n := []int64{2, 3}
fmt.Println("len of old n is ", len(n))
fmt.Println("cap of old n is ", cap(n))
fmt.Println("")

n = append(n, 4)
n = append(n, 5)
n = append(n, 6)
fmt.Println("len of n is ", len(n))
fmt.Println("cap of n is ", cap(n))

fmt.Println("------")

}


输出为:

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len of old m is  2
cap of old m is 2

len of m is 5
cap of m is 6

------
len of old n is 2
cap of old n is 2

len of n is 5
cap of n is 8
------


为什么一次性append多个,最后切片的长度为6; 而多次append单个元素,最后切片的长度为8?

切片扩容的源码,是/src/runtime/slice.go中的growslice方法:

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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.
// 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.kind&kindNoPointers != 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 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}
}




原理


使用gdb调试工具进行调试

强烈建议先阅读此文,学习GDB基本用法


使用l 10可查看第10行附近的源码

使用 b 12 在第12行处设置断点 (可以设置多个断点)

这样会在运行到第12行时停止,可查看变量的值、堆栈情况等;

使用info b 查看断点处情况


可使用 s, 跳入断点,并看执行情况


可使用 r运行代码

可使用 p 变量名,显示变量值



如果调试过程中出现value optimized out,说明编译器进行了内联优化。

可通过go build -gcflags "-N -l" -o 自定义的二进制文件名称 原始Go文件.go命令,禁用编译器优化




gdb调试 出现value optimized out解决方法



可使用n单步运行. 非常好用,可以看到调用链


使用c, 使程序继续往下运行,直到再次遇到断点或程序结束

使用q, 退出gdb


调用roundupsize()函数,进行(向上)内存对齐



roundupsize()位于go/src/runtime/msize.go,更具体介绍与使用可参考

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// 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.

// Malloc small size classes.
//
// See malloc.go for overview.
// See also mksizeclasses.go for how we decide what size classes to use.

package runtime

// Returns size of the memory block that mallocgc will allocate if you ask for the size.
func roundupsize(size uintptr) uintptr {
if size < _MaxSmallSize {
if size <= smallSizeMax-8 {
return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
} else {
return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
}
}
if size+_PageSize < size {
return size
}
return alignUp(size, _PageSize)
}


go/src/runtime/sizeclasses.go

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// Code generated by mksizeclasses.go; DO NOT EDIT.
//go:generate go run mksizeclasses.go

package runtime

// class bytes/obj bytes/span objects tail waste max waste
// 1 8 8192 1024 0 87.50%
// 2 16 8192 512 0 43.75%
// 3 24 8192 341 8 29.24%
// 4 32 8192 256 0 21.88%
// 5 48 8192 170 32 31.52%
// 6 64 8192 128 0 23.44%
// 7 80 8192 102 32 19.07%
// 8 96 8192 85 32 15.95%
// 9 112 8192 73 16 13.56%
// 10 128 8192 64 0 11.72%
// 11 144 8192 56 128 11.82%
// 12 160 8192 51 32 9.73%
// 13 176 8192 46 96 9.59%
// 14 192 8192 42 128 9.25%
// 15 208 8192 39 80 8.12%
// 16 224 8192 36 128 8.15%
// 17 240 8192 34 32 6.62%
// 18 256 8192 32 0 5.86%
// 19 288 8192 28 128 12.16%
// 20 320 8192 25 192 11.80%
// 21 352 8192 23 96 9.88%
// 22 384 8192 21 128 9.51%
// 23 416 8192 19 288 10.71%
// 24 448 8192 18 128 8.37%
// 25 480 8192 17 32 6.82%
// 26 512 8192 16 0 6.05%
// 27 576 8192 14 128 12.33%
// 28 640 8192 12 512 15.48%
// 29 704 8192 11 448 13.93%
// 30 768 8192 10 512 13.94%
// 31 896 8192 9 128 15.52%
// 32 1024 8192 8 0 12.40%
// 33 1152 8192 7 128 12.41%
// 34 1280 8192 6 512 15.55%
// 35 1408 16384 11 896 14.00%
// 36 1536 8192 5 512 14.00%
// 37 1792 16384 9 256 15.57%
// 38 2048 8192 4 0 12.45%
// 39 2304 16384 7 256 12.46%
// 40 2688 8192 3 128 15.59%
// 41 3072 24576 8 0 12.47%
// 42 3200 16384 5 384 6.22%
// 43 3456 24576 7 384 8.83%
// 44 4096 8192 2 0 15.60%
// 45 4864 24576 5 256 16.65%
// 46 5376 16384 3 256 10.92%
// 47 6144 24576 4 0 12.48%
// 48 6528 32768 5 128 6.23%
// 49 6784 40960 6 256 4.36%
// 50 6912 49152 7 768 3.37%
// 51 8192 8192 1 0 15.61%
// 52 9472 57344 6 512 14.28%
// 53 9728 49152 5 512 3.64%
// 54 10240 40960 4 0 4.99%
// 55 10880 32768 3 128 6.24%
// 56 12288 24576 2 0 11.45%
// 57 13568 40960 3 256 9.99%
// 58 14336 57344 4 0 5.35%
// 59 16384 16384 1 0 12.49%
// 60 18432 73728 4 0 11.11%
// 61 19072 57344 3 128 3.57%
// 62 20480 40960 2 0 6.87%
// 63 21760 65536 3 256 6.25%
// 64 24576 24576 1 0 11.45%
// 65 27264 81920 3 128 10.00%
// 66 28672 57344 2 0 4.91%
// 67 32768 32768 1 0 12.50%

const (
_MaxSmallSize = 32768
smallSizeDiv = 8
smallSizeMax = 1024
largeSizeDiv = 128
_NumSizeClasses = 68
_PageShift = 13
)

var class_to_size = [_NumSizeClasses]uint16{0, 8, 16, 24, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256, 288, 320, 352, 384, 416, 448, 480, 512, 576, 640, 704, 768, 896, 1024, 1152, 1280, 1408, 1536, 1792, 2048, 2304, 2688, 3072, 3200, 3456, 4096, 4864, 5376, 6144, 6528, 6784, 6912, 8192, 9472, 9728, 10240, 10880, 12288, 13568, 14336, 16384, 18432, 19072, 20480, 21760, 24576, 27264, 28672, 32768}
var class_to_allocnpages = [_NumSizeClasses]uint8{0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 3, 2, 3, 1, 3, 2, 3, 4, 5, 6, 1, 7, 6, 5, 4, 3, 5, 7, 2, 9, 7, 5, 8, 3, 10, 7, 4}

type divMagic struct {
shift uint8
shift2 uint8
mul uint16
baseMask uint16
}

var class_to_divmagic = [_NumSizeClasses]divMagic{{0, 0, 0, 0}, {3, 0, 1, 65528}, {4, 0, 1, 65520}, {3, 11, 683, 0}, {5, 0, 1, 65504}, {4, 11, 683, 0}, {6, 0, 1, 65472}, {4, 10, 205, 0}, {5, 9, 171, 0}, {4, 11, 293, 0}, {7, 0, 1, 65408}, {4, 13, 911, 0}, {5, 10, 205, 0}, {4, 12, 373, 0}, {6, 9, 171, 0}, {4, 13, 631, 0}, {5, 11, 293, 0}, {4, 13, 547, 0}, {8, 0, 1, 65280}, {5, 9, 57, 0}, {6, 9, 103, 0}, {5, 12, 373, 0}, {7, 7, 43, 0}, {5, 10, 79, 0}, {6, 10, 147, 0}, {5, 11, 137, 0}, {9, 0, 1, 65024}, {6, 9, 57, 0}, {7, 9, 103, 0}, {6, 11, 187, 0}, {8, 7, 43, 0}, {7, 8, 37, 0}, {10, 0, 1, 64512}, {7, 9, 57, 0}, {8, 6, 13, 0}, {7, 11, 187, 0}, {9, 5, 11, 0}, {8, 8, 37, 0}, {11, 0, 1, 63488}, {8, 9, 57, 0}, {7, 10, 49, 0}, {10, 5, 11, 0}, {7, 10, 41, 0}, {7, 9, 19, 0}, {12, 0, 1, 61440}, {8, 9, 27, 0}, {8, 10, 49, 0}, {11, 5, 11, 0}, {7, 13, 161, 0}, {7, 13, 155, 0}, {8, 9, 19, 0}, {13, 0, 1, 57344}, {8, 12, 111, 0}, {9, 9, 27, 0}, {11, 6, 13, 0}, {7, 14, 193, 0}, {12, 3, 3, 0}, {8, 13, 155, 0}, {11, 8, 37, 0}, {14, 0, 1, 49152}, {11, 8, 29, 0}, {7, 13, 55, 0}, {12, 5, 7, 0}, {8, 14, 193, 0}, {13, 3, 3, 0}, {7, 14, 77, 0}, {12, 7, 19, 0}, {15, 0, 1, 32768}}
var size_to_class8 = [smallSizeMax/smallSizeDiv + 1]uint8{0, 1, 2, 3, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32}
var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv + 1]uint8{32, 33, 34, 35, 36, 37, 37, 38, 38, 39, 39, 40, 40, 40, 41, 41, 41, 42, 43, 43, 44, 44, 44, 44, 44, 45, 45, 45, 45, 45, 45, 46, 46, 46, 46, 47, 47, 47, 47, 47, 47, 48, 48, 48, 49, 49, 50, 51, 51, 51, 51, 51, 51, 51, 51, 51, 51, 52, 52, 52, 52, 52, 52, 52, 52, 52, 52, 53, 53, 54, 54, 54, 54, 55, 55, 55, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 57, 57, 57, 57, 57, 57, 57, 57, 57, 58, 58, 58, 58, 58, 58, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 60, 61, 61, 61, 61, 61, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67, 67}


> 当创建一个对象时,需要分配一块内存。假设创建的对象需要52 byte,系统是不会真就给分配52byte大小的内存,首先会根据上面代码中第6行注释部分,来计算需要分配的内存大小。


可参考[go/src/runtime/msize.go] 中的 roundupsize 函数。也就是需要向上取整,即 48<52<64,所以申请52byte,实际会分配64byte大小的内存。


另外还需要了解: 如果每次创建一个对象,Go runtime都向计算机中申请一块内存,而在程序运行时是会频繁的创建对象的,这样效率将会大大降低。 所以程序会预先申请好一些内存块,其大小就是 8、16、32、48 等等,这样在程序申请内存时序就可以把申请好的内存选一块给我们,也就提高了效率



growslice的三个参数: 第一个是类型(上例中为int64),第二个是扩容前切片a(元素为2和3),第三个参数是预估容量5(原有切片容量加上新加元素个数,即2+3=5)

对容量进行计算:

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...
newcap := old.cap //​ 此时newcap = 2
doublecap := newcap + newcap // doublecap 为2+2=4
if cap > doublecap { // cap=5
newcap = cap // 故而newcap 最终为5
} 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
}
}
}
...

又因为int64 类型大小为8字节,故而下面会走到et.size == sys.PtrSize:代码块



对于const PtrSize = 4 << (^uintptr(0) >> 63):


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package main

import "fmt"

func main() {

const PtrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const

fmt.Println("uintptr(0)值为:", uintptr(0))
fmt.Println("^uintptr(0)值为:", ^uintptr(0)) // 对0取反,在64位操作系统得到18446744073709551615; uint64(所有无符号 64 位整数的集合),范围:0 到 18446744073709551615。
fmt.Println("^uintptr(0) >> 63值为:", ^uintptr(0)>>63) // 1

fmt.Println("PtrSize值为:", PtrSize) // 4左移一位,即8
}


执行结果为:

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uintptr(0)值为: 0
^uintptr(0)值为: 18446744073709551615
^uintptr(0) >> 63值为: 1
PtrSize值为: 8



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case et.size == sys.PtrSize:
lenmem = uintptr(old.len) * sys.PtrSize //即 2*8=16
newlenmem = uintptr(cap) * sys.PtrSize // 即 2*8=16
capmem = roundupsize(uintptr(newcap) * sys.PtrSize) // 即5*8=40,经过roundupsize处理后为48
overflow = uintptr(newcap) > maxAlloc/sys.PtrSize //是否超出上限,此处为false,在Darwin Arm64上maxAlloc为281474976710656
newcap = int(capmem / sys.PtrSize) // 关键步骤,48/8=6,故而最终得到的新切片的cap为6
<font size=1 color=”grey>

关于maxAlloc:

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package main

import "fmt"

const GoarchArm64 = 1
const GoarchMips = 0
const GoarchMipsle = 0
const GoarchWasm = 0

const GoosIos = 0

func main() {
// _64bit = 1 on 64-bit systems, 0 on 32-bit systems
_64bit := 1 << (^uintptr(0) >> 63) / 2

fmt.Println("_64bit值为:", _64bit)

heapAddrBits := (_64bit*(1-GoarchWasm)*(1-GoosIos*GoarchArm64))*48 + (1-_64bit+GoarchWasm)*(32-(GoarchMips+GoarchMipsle)) + 33*GoosIos*GoarchArm64

fmt.Println("heapAddrBits值为:", heapAddrBits)

// maxAlloc is the maximum size of an allocation. On 64-bit,
// it's theoretically possible to allocate 1<<heapAddrBits bytes. On
// 32-bit, however, this is one less than 1<<32 because the
// number of bytes in the address space doesn't actually fit
// in a uintptr.
maxAlloc := (1 << heapAddrBits) - (1-_64bit)*1

fmt.Println("maxAlloc值为:", maxAlloc)

}


输出为:

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_64bit值为: 1
heapAddrBits值为: 48
maxAlloc值为: 281474976710656





更多参考:

Go Slice Growth

Golang 切片容量(cap)增长探秘

深度解密Go语言之Slice 搜关键字 roundupsize

讨论群中关于切片的一个问题

Go源码解读-切片slice


Linux内核中有rounddown方法,取数的最高二进制阶数

取数的最高二进制阶数rounddown_pow_of_two