Golang slice 源码阅读

翻Golang代码：

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

slice 的array是一个指针，指向一块连续内存。

再看 growslice 函数，这是append调用的函数

func growslice(et *_type, old slice, cap int) slice {
    if raceenabled {
        callerpc := getcallerpc(unsafe.Pointer(&et))
        racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice))
    }
    if msanenabled {
        msanread(old.array, uintptr(old.len*int(et.size)))
    }

    if et.size == 0 {
        if cap < old.cap {
            panic(errorString("growslice: cap out of range"))
        }
        // 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 {
            for newcap < cap {
                newcap += newcap / 4
            }
        }
    }

    var lenmem, newlenmem, capmem uintptr
    const ptrSize = unsafe.Sizeof((*byte)(nil))
    switch et.size {
    case 1:
        lenmem = uintptr(old.len)
        newlenmem = uintptr(cap)
        capmem = roundupsize(uintptr(newcap))
        newcap = int(capmem)
    case ptrSize:
        lenmem = uintptr(old.len) * ptrSize
        newlenmem = uintptr(cap) * ptrSize
        capmem = roundupsize(uintptr(newcap) * ptrSize)
        newcap = int(capmem / ptrSize)
    default:
        lenmem = uintptr(old.len) * et.size
        newlenmem = uintptr(cap) * et.size
        capmem = roundupsize(uintptr(newcap) * et.size)
        newcap = int(capmem / et.size)
    }

    if cap < old.cap || uintptr(newcap) > maxSliceCap(et.size) {
        panic(errorString("growslice: cap out of range"))
    }

    var p unsafe.Pointer
    if et.kind&kindNoPointers != 0 {
        p = mallocgc(capmem, nil, false)
        memmove(p, old.array, lenmem)
        // 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 {
            memmove(p, old.array, lenmem)
        } else {
            for i := uintptr(0); i < lenmem; i += et.size {
                typedmemmove(et, add(p, i), add(old.array, i))
            }
        }
    }

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

可以看出slice每次扩容的实现是在小于1024的时候每次乘以2，之后在小于cap的时候每次 加1/4，一直到超过为止。

所以来看看同事发的一段代码：

package main

import "fmt"

func main() {
    s := []int{5}
    s = append(s, 7)
    s = append(s, 9)
    x := append(s, 11)
    y := append(s, 12)
    fmt.Println(s, x, y)
}

最开始同事问我你猜这段代码会输出什么的时候我答错了，本以为Golang的语义不会 实现成这样的。不过翻了实现才发现，well, ahh...

root@arch test: go run test.go 
[5 7 9] [5 7 9 12] [5 7 9 12]

按照上面的内存翻倍策略， s := []int{5} 的时候，array容量是1， s = append(s, 7) 时为2， s = append(s, 9) 时为3，执行 x := append(s, 11) 时为4，但是 执行 y := append(s, 12) 时容量仍然为4，因为尚未执行 x := append(s, 11) 时， x.array 指向了连续内存（数组），len为3，cap为4，s最后一个元素是9， append 之后会把9后面的元素填成11，然后返回这样一个 slice 对象给x, 同样， y := append(s, 12) 时一样，执行之前，len为3，cap为4，s的最后一个元素是9， 所以执行之后，就把原来的11给覆盖了。

还是挺坑的，内存直接被盖掉了。