我正在尝试解决 leetcode 问题排列。但是当我用 -benchmem 进行测试时,我发现它分配太多,达到 1957 年分配/操作时permute([]int{1,2,3,4,5,6})
我发现它在生成 sub-nums 目标时逃到了堆上。即使我尝试分配 [6]int,并使用 unsafe 包来构建切片,它仍然是moved to heap
。
我的问题是,为什么切片逃逸到堆,我怎么能在堆栈上分配切片?
这是我的代码:
package main
import (
"fmt"
"reflect"
"unsafe"
)
func permute(nums []int) [][]int {
resLen := 1
for i := 1; i<= len(nums);i ++{
resLen *= i
}
// pre allocate
res := make([][]int, resLen)
for i := range res{
res[i] = make([]int, 0, len(nums))
}
build(res, nums)
return res
}
func build(res [][]int,targets []int){
step := len(res) / len(targets)
for i := range targets{
for j := i*step; j < (i+1) * step; j ++{
res[j] = append(res[j], targets[i])
}
if len(targets) != 1{
var ab = [6]int{}
var buff []int
var bp *reflect.SliceHeader
bp = (*reflect.SliceHeader)(unsafe.Pointer(&buff))
bp.Data = uintptr(unsafe.Pointer(&ab))
bp.Cap = 6
buff = append(buff, targets[:i]...)
buff = append(buff, targets[i+1:]...)
build(res[i*step:(i+1)*step], buff)
}
}
return
}
func main() {
nums := []int{1,2,3}
res := permute(nums)
fmt.Println(res)
}
build
没有不安全但逃逸到堆的函数:
func build(res [][]int, targets []int) {
step := len(res) / len(targets)
for i := range targets {
for j := i * step; j < (i+1)*step; j++ {
res[j] = append(res[j], targets[i])
}
if len(targets) != 1 {
buff := make([]int, 0, 6) // make([]int, 0, 6) escapes to heap
buff = append(buff, targets[:i]...)
buff = append(buff, targets[i+1:]...)
build(res[i*step:(i+1)*step], buff)
}
}
return
}
还有我的测试用例:
package main
import "testing"
func Benchmark(b *testing.B){
for i:=0;i<b.N;i++{
permute([]int{1,2,3,4,5,6})
}
}
当我运行时go build -gcflags="-m"
,它会报告./main.go:32:8: moved to heap: ab