mountinfo_openbsd: Rewrite int8SliceToString with unsafe

[Red54]

#114 (comment)

[kolyshkin]

I have submitted https://go-review.googlesource.com/c/sys/+/407195 and hope it will be merged soon. Let's wait a couple of days.

[kolyshkin]

The alternative is using something like BytePtrToString((*byte)(unsafe.Pointer(&is)))

[Red54]

package main

import (
        "testing"
        "unsafe"

        "golang.org/x/sys/unix"
)

var is = [90]int8{'a', 'b', 'c'}
var es = "abc"

func int8SliceToString(is []int8) string {
        for i := range is {
                if is[i] == 0 {
                        is = is[:i]
                        break
                }
        }
        return *(*string)(unsafe.Pointer(&is))
}

func TestingInt8SliceToString(t *testing.T) {
        s := int8SliceToString(is[:])
        if s != es {
                t.Fail()
        }
}

func BenchmarkInt8SliceToString(b *testing.B) {
        for i := 0; i < b.N; i++ {
                int8SliceToString(is[:])
        }
}

func TestingBytePtrToString(t *testing.T) {
        s := unix.BytePtrToString((*byte)(unsafe.Pointer(&is)))
        if s != es {
                t.Fail()
        }
}

func BenchmarkBytePtrToString(b *testing.B) {
        for i := 0; i < b.N; i++ {
                unix.BytePtrToString((*byte)(unsafe.Pointer(&is)))
        }
}

$ go test -bench=. -benchmem  
goos: openbsd
goarch: amd64
pkg: bench
cpu: Intel Core Processor (Skylake, IBRS)
BenchmarkInt8SliceToString-2    458439867                2.585 ns/op           0 B/op          0 allocs/op
BenchmarkBytePtrToString-2      194815534                6.158 ns/op           0 B/op          0 allocs/op
PASS
ok      bench   3.317s
$

BytePtrToString((*byte)(unsafe.Pointer(&is))) is more than twice as slow.

[Red54]

I have submitted https://go-review.googlesource.com/c/go/+/407436 and if it is merged there is no need to split bsd into freebsdlike and openbsd.

[kolyshkin]

Guess we should go with what I outlined in #114 (comment) instead

[kolyshkin]

Closing in favor of #118