Why slice kept escaping from stack?

Question

I am trying to solve leetcode problem permutations. But when i test with -benchmem, i found it allocs too much which reach 1957 allocs/op when permute([]int{1,2,3,4,5,6})

I found it escape to heap when generating sub-nums target. Even i try to allocate [6]int, and use unsafe package to build the slice, it still moved to heap.

My question is, why the slice escape to heap, and how could i allocate the slice on stack?

Here's my code:

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 function without unsafe but escapes to heap:

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
}

And my test case:

package main

import "testing"

func Benchmark(b *testing.B){
    for i:=0;i<b.N;i++{
        permute([]int{1,2,3,4,5,6})
    }
}

When i run go build -gcflags="-m", it reports ./main.go:32:8: moved to heap: ab

Answer 1

Trying to subvert the compiler using unsafe.Pointer is only making it harder for the escape analysis to do its job, preventing the slice from being stack allocated. Simply allocate a single slice and reuse it for each loop iteration:

func build(res [][]int, targets []int) {
    buff := make([]int, 0, 6)
    step := len(res) / len(targets)
    for i := range targets {
        buff = buff[:0]
        for j := i * step; j < (i+1)*step; j++ {
            res[j] = append(res[j], targets[i])
        }
        if len(targets) != 1 {
            buff = append(buff, targets[:i]...)
            buff = append(buff, targets[i+1:]...)
            build(res[i*step:(i+1)*step], buff)
        }
    }
    return
}

This can be correctly optimized by the compiler

./main.go:26:17: make([]int, 0, 6) does not escape

And will result in only the desired allocations:

Benchmark-8  44607  26838 ns/op  52992 B/op  721 allocs/op