Example for passing pre-allocated buf to hash.Sum(b []byte) in Go

console.log

$ go test -bench=. -benchmem
goos: linux
goarch: amd64
BenchmarkHashNewSum-2            5807700               205 ns/op             112 B/op          2 allocs/op
BenchmarkHashNewSum2-2           8323488               142 ns/op               0 B/op          0 allocs/op
BenchmarkHashSum-2               8817842               134 ns/op               0 B/op          0 allocs/op
BenchmarkHashSum2-2              7696573               155 ns/op              16 B/op          1 allocs/op
PASS
ok      _/home/hnakamur/hash-benchmark  5.402s
$ go version
go version go1.13.5 linux/amd64

hash_test.go

package sum

import (
    "crypto/md5"
    "testing"
)

var input = []byte("あめんぼあかいなあいうえお")

func BenchmarkHashNewSum(b *testing.B) {
    for i := 0; i < b.N; i++ {
        hash := md5.New()
        hash.Write(input)
        buf := hash.Sum(nil)
        dummy(buf)
    }
}

func BenchmarkHashNewSum2(b *testing.B) {
    hash := md5.New()
    buf := make([]byte, md5.Size)
    for i := 0; i < b.N; i++ {
        hash.Write(input)
        buf = hash.Sum(buf)
        dummy(buf)
        buf = buf[:0]
        hash.Reset()
    }
}

func BenchmarkHashSum(b *testing.B) {
    for i := 0; i < b.N; i++ {
        _ = md5.Sum(input)
    }
}

func BenchmarkHashSum2(b *testing.B) {
    var buf []byte
    for i := 0; i < b.N; i++ {
        s := md5.Sum(input)
        buf = s[:]
    }
    dummy(buf)
}

func dummy(buf []byte) {}