Go的slice工作机制
slice是咋工作的?首先我们从一个demo看起:
package main
import (
"fmt"
)
func main() {
a := make([]int, 10)
fmt.Printf("%p\n", &a[0])
for i := 0; i < 100; i++ {
a = append(a, 1)
fmt.Printf("%p\n", &a[0])
}
}
用gdb在 a = append(a, 1)
这一行下个断点,执行:
(gdb) list
1 package main
2
3 import (
4 "fmt"
5 )
6
7 func main() {
8 a := make([]int, 10)
9 fmt.Printf("%p\n", &a[0])
10 for i := 0; i < 100; i++ {
(gdb)
11 a = append(a, 1)
12 fmt.Printf("%p\n", &a[0])
13 }
14 }
(gdb) b 11
Breakpoint 1 at 0x4af7d8: file /home/jiajun/Code/test/main.go, line 11.
(gdb) run
Starting program: /home/jiajun/Code/test/test
[New LWP 12218]
[New LWP 12219]
[New LWP 12220]
[New LWP 12221]
0xc000130000
Thread 1 "test" hit Breakpoint 1, main.main () at /home/jiajun/Code/test/main.go:11
11 a = append(a, 1)
(gdb) s
runtime.growslice (et=0x4bd560, old=..., cap=11, ~r3=...) at /snap/go/5759/src/runtime/slice.go:76
76 func growslice(et *_type, old slice, cap int) slice {
(gdb) quit
可以看到调用了 slice.go 里的 growslice
函数:
func growslice(et *_type, old slice, cap int) slice {
newcap := old.cap
doublecap := newcap + newcap
if cap > doublecap {
newcap = cap
} else {
if old.len < 1024 {
newcap = doublecap
} else {
for 0 < newcap && newcap < cap {
newcap += newcap / 4
}
if newcap <= 0 {
newcap = cap
}
}
}
var overflow bool
var lenmem, newlenmem, capmem uintptr
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)
}
if overflow || capmem > maxAlloc {
panic(errorString("growslice: cap out of range"))
}
var p unsafe.Pointer
if et.ptrdata == 0 {
p = mallocgc(capmem, nil, false)
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 {
bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
}
}
memmove(p, old.array, lenmem)
return slice{p, old.len, newcap}
}
上述代码,是执行append时的代码,但是,从最后几行来看,岂不是每次都新申请了一块内存?我们来执行一下最开始的demo看看:
$ go run main.go
0xc000130000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
0xc000134000
...
可以看到,这里打出来的内存地址,并不是每次都不一样的,而且如果真的这样做,那么append的性能就非常低,所以,growslice 函数只是在容量不足时,才会调用,而平时追加值,可能是直接在汇编里完成的,我们来看看汇编码:
$ go tool compile -N -S main.go | grep main.go:11
0x025e 00606 (main.go:11) MOVQ "".a+376(SP), DX
0x0266 00614 (main.go:11) LEAQ 1(DX), BX
0x026a 00618 (main.go:11) PCDATA $0, $5
0x026a 00618 (main.go:11) MOVQ "".a+368(SP), SI
0x0272 00626 (main.go:11) PCDATA $1, $0
0x0272 00626 (main.go:11) MOVQ "".a+384(SP), DI
0x027a 00634 (main.go:11) CMPQ BX, DI
0x027d 00637 (main.go:11) JLS 644
0x027f 00639 (main.go:11) JMP 1167
0x0284 00644 (main.go:11) PCDATA $0, $-1
0x0284 00644 (main.go:11) PCDATA $1, $-1
0x0284 00644 (main.go:11) JMP 646
0x0286 00646 (main.go:11) PCDATA $0, $5
0x0286 00646 (main.go:11) PCDATA $1, $0
0x0286 00646 (main.go:11) MOVQ $1, (SI)(DX*8)
0x028e 00654 (main.go:11) PCDATA $1, $1
0x028e 00654 (main.go:11) MOVQ SI, "".a+368(SP)
0x0296 00662 (main.go:11) MOVQ BX, "".a+376(SP)
0x029e 00670 (main.go:11) MOVQ DI, "".a+384(SP)
0x048f 01167 (main.go:11) PCDATA $0, $5
0x048f 01167 (main.go:11) PCDATA $1, $0
0x048f 01167 (main.go:11) MOVQ DX, ""..autotmp_27+120(SP)
0x0494 01172 (main.go:11) PCDATA $0, $6
0x0494 01172 (main.go:11) LEAQ type.int(SB), AX
0x049b 01179 (main.go:11) PCDATA $0, $5
0x049b 01179 (main.go:11) MOVQ AX, (SP)
0x049f 01183 (main.go:11) PCDATA $0, $0
0x049f 01183 (main.go:11) MOVQ SI, 8(SP)
0x04a4 01188 (main.go:11) MOVQ DX, 16(SP)
0x04a9 01193 (main.go:11) MOVQ DI, 24(SP)
0x04ae 01198 (main.go:11) MOVQ BX, 32(SP)
0x04b3 01203 (main.go:11) CALL runtime.growslice(SB)
0x04b8 01208 (main.go:11) PCDATA $0, $5
0x04b8 01208 (main.go:11) MOVQ 40(SP), SI
0x04bd 01213 (main.go:11) MOVQ 48(SP), AX
0x04c2 01218 (main.go:11) MOVQ 56(SP), DI
0x04c7 01223 (main.go:11) LEAQ 1(AX), BX
0x04cb 01227 (main.go:11) MOVQ ""..autotmp_27+120(SP), DX
0x04d0 01232 (main.go:11) JMP 646
emmm,可以仔细品味一下这段汇编码,可以看到几个JMP指令,这是跳转指令。还有CMPQ指令,这是判断指令,虽然不能完全看懂生成 出来的汇编码,但是结合上面的试验结果,基本印证了我们的猜测,即append操作是通过汇编完成的,只有当容量不足时,才会调用 growslice函数。
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