golang中slice扩容的具体实现

// append() 源码简化版
func append(slice []T, elems ...T) []T {
    // 1. 检查当前切片是否有足够的容量
    if len(slice)+len(elems) <= cap(slice) {
        // 如果容量足够，直接追加元素
        slice = slice[:len(slice)+len(elems)]
        copy(slice[len(slice)-len(elems):], elems)
        return slice
    }
    // 2. 如果容量不足，进行扩容
    return growslice(reflect.TypeOf(slice).Elem(), slice, len(slice)+len(elems))
}

// growslice 扩容的实现
func growslice(et *_type, old slice, cap int) slice {
    if cap < old.cap {
        panic("growslice: cap out of range")
    }

    // 如果元素大小为 0，直接返回新的切片
    if et.size == 0 {
        return slice{unsafe.Pointer(&zerobase), old.len, cap}
    }

    // 扩容策略：根据当前容量和预期的新容量来决定新容量
    newcap := old.cap
    doublecap := newcap + newcap
    if cap > doublecap {
        newcap = cap
    } else {
        const threshold = 256
        if old.cap < threshold {
            newcap = doublecap
        } else {
            for 0 < newcap && newcap < cap {
                newcap += (newcap + 3*threshold) / 4
            }
            if newcap <= 0 {
                newcap = cap
            }
        }
    }

    // 根据扩容后的容量计算所需的内存空间
    var lenmem, newlenmem, capmem uintptr
    switch {
    case et.size == 1:
        lenmem = uintptr(old.len)
        newlenmem = uintptr(cap)
        capmem = roundupsize(uintptr(newcap))
        newcap = int(capmem)
    case et.size == goarch.PtrSize:
        lenmem = uintptr(old.len) * goarch.PtrSize
        newlenmem = uintptr(cap) * goarch.PtrSize
        capmem = roundupsize(uintptr(newcap) * goarch.PtrSize)
        newcap = int(capmem / goarch.PtrSize)
    case isPowerOfTwo(et.size):
        var shift uintptr
        if goarch.PtrSize == 8 {
            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)
        newcap = int(capmem >> shift)
    default:
        lenmem = uintptr(old.len) * et.size
        newlenmem = uintptr(cap) * et.size
        capmem, _ = math.MulUintptr(et.size, uintptr(newcap))
        capmem = roundupsize(capmem)
        newcap = int(capmem / et.size)
    }

    // 分配新的内存，并拷贝旧的数据
    var p unsafe.Pointer
    if et.ptrdata == 0 {
        p = mallocgc(capmem, nil, false)
    } else {
        p = mallocgc(capmem, et, true)
    }

    memmove(p, old.array, lenmem)
    return slice{p, old.len, newcap}
}

package main

import "fmt"

func main() {
    var slice []int
    for i := 0; i < 10; i++ {
        slice = append(slice, i)
        fmt.Printf("Length: %d, Capacity: %d, Slice: %v\n", len(slice), cap(slice), slice)
    }
}