Go slice的底层实现原理?

news/2025/2/12 8:04:55/

切片是基于数组实现的，它的底层是数组，可以理解为对底层数组的抽象。

源码包中src/runtime/slice.go 定义了slice的数据结构：

type slice struct {array unsafe.Pointerlen   intcap   int
}

slice占用24个字节

array: 指向底层数组的指针，占用8个字节

len: 切片的长度，占用8个字节

cap: 切片的容量，cap 总是大于等于 len 的，占用8个字节

slice有4种初始化方式

// 初始化方式1：直接声明
var slice1 []int// 初始化方式2：使用字面量
slice2 := []int{1, 2, 3, 4}// 初始化方式3：使用make创建slice
slice3 := make([]int, 3, 5)         // 初始化方式4: 从切片或数组“截取”
slcie4 := arr[1:3]

通过一个简单程序，看下slice初始化调用的底层函数

package mainimport "fmt"func main() {slice := make([]int, 0)slice = append(slice, 1)fmt.Println(slice, len(slice), cap(slice))
}

通过 go tool compile -S test.go | grep CALL 得到汇编代码

0x0042 00066 (test.go:6)        CALL    runtime.makeslice(SB)
0x006d 00109 (test.go:7)        CALL    runtime.growslice(SB)
0x00a4 00164 (test.go:8)        CALL    runtime.convTslice(SB)
0x00c0 00192 (test.go:8)        CALL    runtime.convT64(SB)
0x00d8 00216 (test.go:8)        CALL    runtime.convT64(SB)
0x0166 00358 ($GOROOT/src/fmt/print.go:274)     CALL    fmt.Fprintln(SB)
0x0180 00384 (test.go:5)        CALL    runtime.morestack_noctxt(SB)
0x0079 00121 (<autogenerated>:1)        CALL    runtime.efaceeq(SB)
0x00a0 00160 (<autogenerated>:1)        CALL    runtime.morestack_noctxt(SB)

初始化slice调用的是runtime.makeslice，makeslice函数的工作主要就是计算slice所需内存大小，然后调用mallocgc进行内存的分配

所需内存大小 = 切片中元素大小 * 切片的容量

func makeslice(et *_type, len, cap int) unsafe.Pointer {mem, overflow := math.MulUintptr(et.size, uintptr(cap))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()}return mallocgc(mem, et, true)
}