cmd/compile: unexpected benchmark results #64328
Description
What version of Go are you using (go version)?
$ go version go version go1.21.4 linux/amd64
Does this issue reproduce with the latest release?
Yes
What did you do?
package main
import "testing"
var n = 1024 * 1024 * 16
type Large = int
var r1 []Large
var r2 []Large
func Benchmark_CompactFunc(b *testing.B) {
ss1 := make([]Large, n)
b.ResetTimer()
for i := 0; i < b.N; i++ {
r1 = CompactFunc(ss1, func(a, b Large) bool {return a == b})
}
}
func Benchmark_CompactFunc_2(b *testing.B) {
ss1 := make([]Large, n)
b.ResetTimer()
for i := 0; i < b.N; i++ {
r2 = CompactFunc_2(ss1, func(a, b Large) bool {return a == b})
}
}
func CompactFunc[S ~[]E, E any](s S, eq func(E, E) bool) S {
if len(s) < 2 {
return s
}
i := 1
for k := 1; k < len(s); k++ {
if !eq(s[k], s[k-1]) {
if i != k {
s[i] = s[k]
}
i++
}
}
clear(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}
var global bool
func CompactFunc_2[S ~[]E, E any](s S, eq func(E, E) bool) S {
global = true // This is the only difference
if len(s) < 2 {
return s
}
i := 1
for k := 1; k < len(s); k++ {
if !eq(s[k], s[k-1]) {
if i != k {
s[i] = s[k]
}
i++
}
}
clear(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}What did you expect to see?
Similar performance for the specified test case.
What did you see instead?
Benchmark result shows CompactFunc_2 is (about 15%) slower, which is quite confusing.
goos: linux
goarch: amd64
pkg: example.com
cpu: Intel(R) Core(TM) i5-4210U CPU @ 1.70GHz
Benchmark_CompactFunc-4 19 66914978 ns/op
Benchmark_CompactFunc_2-4 18 75483657 ns/op