golang slice底层实现

type SliceHeader struct {
    Data uintptr // 指向连续内存数组
    Len  int
    Cap  int
}

// et 是元素的类型
func makeslice(et *_type, len, cap int) unsafe.Pointer {
    mem, overflow := math.MulUintptr(et.size, uintptr(cap))
    // 判断 len 和 cap 参数是否合法
    if overflow || mem > maxAlloc || len < 0 || len > cap {
        // NOTE: Produce a 'len out of range' error instead of a
        // 'cap out of range' error when someone does make([]T, bignumber).
        // 'cap out of range' is true too, but since the cap is only being
        // supplied implicitly, saying len is clearer.
        // See golang.org/issue/4085.
        mem, overflow := math.MulUintptr(et.size, uintptr(len))
        if overflow || mem > maxAlloc || len < 0 {
            panicmakeslicelen()
        }
        panicmakeslicecap()
    }

    // 在堆上分配一段连续的内存
    return mallocgc(mem, et, true)
}

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}
    }

    // 先按旧的slice容量翻倍，如果还不满足实际需要的容量，则按照实际需要的容量扩容
    newcap := old.cap
    doublecap := newcap + newcap
    if cap > doublecap {
        newcap = cap
    } else {
        // 判断旧数组的长度是否小于1024，如果是的话就按旧数组的容量扩容一倍
        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
            }
            // 判断上面newcap的加法是否溢出了
            // 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.
    // 根据元素类型的大小，选择对应的计算逻辑，节省计算量
    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):
        // 2的倍数，可以通过位移计算
        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
    // 分配内存，这里还有一些跟 GC 相关的逻辑
    if et.ptrdata == 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 lenmem > 0 && 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)
        }
    }
    // 将旧数组的数据复制到新新数组中
    memmove(p, old.array, lenmem)

    // 返回新的slice
    return slice{p, old.len, newcap}
}

package main

import "fmt"

func main() {
    a := make([]int, 0, 3)
    b := append(a, 1)
    _ = append(a, 2)
    fmt.Println(b[0])

    c := make([]int, 3, 3)
    fmt.Println(cap(c[1:]))
    fmt.Println(cap(c[:1]))
}