go: bytes: IndexAny is slower than a naive implementation in some cases

What version of Go are you using (go version)?

$ go version
go version go1.20.3 windows/amd64

What operating system and processor architecture are you using (go env)?

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Kroum\AppData\Local\go-build
set GOENV=C:\Users\Kroum\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=D:\MyPrograms\golibs\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=D:\MyPrograms\golibs
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.20.3
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=D:\MyDocuments\Programming\go\test\bench\go.mod
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=D:\Temp\System\Kroum\go-build2837998563=/tmp/go-build -gno-record-gcc-switches

What did you do?

I consider the following naive implementation of bytes.IndexAny

func IndexAny(s []byte, chars string) int {
	min := -1
	for _, c := range chars {
		if i := bytes.IndexRune(s, c); uint(i) < uint(min) {
			min = i
		}
	}
	return min
}

I benchmark it for 10 delimiters on a data where the first delimiter appears at position 1000, like this

package indexany

import (
	"bytes"
	"strings"
	"testing"
)

func IndexAny(s []byte, chars string) int {
	min := -1
	for _, c := range chars {
		if i := bytes.IndexRune(s, c); uint(i) < uint(min) {
			min = i
		}
	}
	return min
}

const (
	t             = "🚆"
	firstchar     = 1000
	numdelimiters = 10
)

var (
	data       []byte
	separators string
)

// create the data and the separators fot the benchmark
func init() {
	// data = "xxx...🚆"
	data = bytes.Repeat([]byte("x"), firstchar)
	data = append(data, t...)
	// separators = "yyy...🚆"
	separators = strings.Repeat("y", numdelimiters)
	separators = separators + t
}

func BenchmarkNaiveIndexAny(b *testing.B) {
	for i := 0; i < b.N; i++ {
		IndexAny(data, separators)
	}
}

func BenchmarkBytesIndexAny(b *testing.B) {
	for i := 0; i < b.N; i++ {
		bytes.IndexAny(data, separators)
	}
}

What did you expect to see?

That bytes.IndexAny is faster then this “naive” IndexAny.

What did you see instead?

$ go test -benchmem -bench=. .
goos: windows
goarch: amd64
pkg: kpym.github.com/gobench/indexany
cpu: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
BenchmarkNaiveIndexAny-8         3033126               522.9 ns/op             0 B/op          0 allocs/op
BenchmarkBytesIndexAny-8          223227              7233 ns/op               0 B/op          0 allocs/op
PASS
ok      kpym.github.com/gobench/indexany        4.325s

So this “naive” implementation is more than 10 times faster than the standard bytes.IndexAny.

Am I doing something wrong ? Is this relevant ?

Remark : If we take a lot of delimiters (1000) the standard function is 30% better (on my computer).