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Args in empty no-op functions cause allocations #30953

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fmstephe opened this issue Mar 20, 2019 · 3 comments
Closed

Args in empty no-op functions cause allocations #30953

fmstephe opened this issue Mar 20, 2019 · 3 comments
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@fmstephe
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fmstephe commented Mar 20, 2019 •
edited

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

go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Yes, also tried it with 1.12.1

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

go env Output
$ GOARCH="amd64"
GOBIN="/Users/fmstephe/golang/go/bin"
GOCACHE="/Users/fmstephe/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/fmstephe/golang/proj"
GOPROXY=""
GORACE=""
GOROOT="/Users/fmstephe/golang/go"
GOTMPDIR=""
GOTOOLDIR="/Users/fmstephe/golang/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/5b/mmf8wp713214hcyzh0vdcjph0000gn/T/go-build469919168=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

This benchmark allocates on every loop.

https://gist.github.com/fmstephe/a23ddff062f3b96df5e634ccf21e0106

func BenchmarkDebugNoop(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		Debugf("This does nothing %d", i)
	}
}

func BenchmarkDebugNoop_NoArgs(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		Debugf("This is also does nothing")
	}
}

func Debugf(s string, a ...interface{}) {} // disable debug logs unless `debug` tag is provided

What did you expect to see?

No allocations.

What did you see instead?

Allocations. I would like the compiler to observe that the allocating args are unused and not allocate.
@fmstephe
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Curiously this seems to be tied to the use of a vararg in Debugf. With just a straight a interface{} it doesn't allocate at all.

package main

import (
	"testing"
)
/*
goos: darwin
goarch: amd64
pkg: github.com/fmstephe/test_empty_method
BenchmarkDebugNoop-4            2000000000               0.26 ns/op            0 B/op          0 allocs/op
BenchmarkDebugNoop_NoArgs-4     2000000000               0.26 ns/op            0 B/op          0 allocs/op
PASS
*/

func BenchmarkDebugNoop(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		Debugf("This does nothing %d", i)
	}
}

func BenchmarkDebugNoop_NoArgs(b *testing.B) {
	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		Debugf("This is also does nothing", nil)
	}
}

func Debugf(s string, a interface{}) {} // disable debug logs unless `debug` tag is provided

@cherrymui
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Looks like a dup of #30898 ?

@fmstephe
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Closing as dupe of #30898

@golang golang locked and limited conversation to collaborators Mar 19, 2020
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@fmstephe @gopherbot @cherrymui