[go-nuts] Go 1.24 BLooper

[peterGo]

Go 1.24 introduces B.Loop() to replace B.N.

https://pkg.go.dev/testing@go1.24rc2#B.Loop    
https://pkg.go.dev/testing@go1.24rc2#B    

testing: add testing.B.Loop for iteration #61515    
https://github.com/golang/go/issues/61515    
As suggested by @rsc, b.Loop could be a clear signal to the compiler not to 
perform certain optimizations in the loop body that often signal to the 
compiler not to perform certain optimizations in the loop body.

There is a significant regression.

For example, here is an important benchmark that no longer works for a 
function that should be inlined (Strings_TrimNewline since strings.Lines() 
is not OS agnostic). b.Loop() does not optimize. b.N optimizes as is should.


$ go test blooper_test.go -run=! -bench=BenchmarkStrings
devel go1.24-3f4164f508 Mon Jan 20 09:25:11 2025 -0800
goos: linux
goarch: amd64
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
noinline Strings_TrimNewline:
BenchmarkStringsBLoop-12     1000000000          1.044 ns/op
inline Strings_TrimNewline: 
BenchmarkStringsBN-12       1000000000          0.6154 ns/op
$ 


blooper_test.go:

package main

import (
"fmt"
"runtime"
"testing"
)

// TrimNewline returns s without a trailing newline string.
// If s doesn't end with newline, s is returned unchanged.
// A newline string is LF ("\n") or CR+LF ("\r\n").
func Strings_TrimNewline(s string) string {
if len(s) > 0 && s[len(s)-1] == '\n' {
s = s[0 : len(s)-1]
if len(s) > 0 && s[len(s)-1] == '\r' {
s = s[0 : len(s)-1]
}
}
return s
}

var sinkString string

func BenchmarkStringsBLoop(b *testing.B) {
var sink string
for b.Loop() {
line := stringLine
sink = Strings_TrimNewline(line)
}
sinkString = sink
}

func BenchmarkStringsBN(b *testing.B) {
var sink string
for range b.N {
line := stringLine
sink = Strings_TrimNewline(line)
}
sinkString = sink
}

var (
nl         = "\r\n"
byteLine   = append(make([]byte, 256), nl...)
stringLine = string(byteLine)
)

func init() {
fmt.Println(runtime.Version())
}

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