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cmd/compile: Iterating a slice backwards is a tiny bit faster than forwards #35140

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mlh758 opened this issue Oct 24, 2019 · 2 comments
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cmd/compile: Iterating a slice backwards is a tiny bit faster than forwards #35140

mlh758 opened this issue Oct 24, 2019 · 2 comments

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@mlh758
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@mlh758 mlh758 commented Oct 24, 2019

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

go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Need to try.

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

go env Output
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/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/hl/8pj040l94nlfzdfz8yzxjmr82d2t40/T/go-build481657979=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import "testing"

func BenchmarkCap(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 0, 500)
		for s := 0; s < 500; s++ {
			x = append(x, "taco")
		}
	}
}

func BenchmarkLen(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 500)
		for s := 0; s < 500; s++ {
			x[s] = "taco"
		}
	}
}

func BenchmarkLenReverse(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 500)
		for s := 499; s >= 0; s-- {
			x[s] = "taco"
		}
	}
}

What did you expect to see?

I figured the second and third benchmarks would be about the same

What did you see instead?

Going through the array in reverse is consistently a tiny bit faster. Is this just a quirk of my CPU architecture? I tried switching it around a little to use 1 instead of 0 for the comparison and that narrowed the gap slightly but in reverse was still consistently faster.

@dmitshur dmitshur added this to the Backlog milestone Oct 24, 2019
@dmitshur

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@dmitshur dmitshur commented Oct 24, 2019

Thanks for the report.

a tiny bit faster

Can you provide the results?

I suggest using benchstat to get a better sense of whether the difference is statistically significant.

@dmitshur dmitshur changed the title Iterating a slice backwards is a tiny bit faster than forwards cmd/compile: Iterating a slice backwards is a tiny bit faster than forwards Oct 24, 2019
@mlh758

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@mlh758 mlh758 commented Oct 24, 2019

When I run benchstat (20 runs) it seems to consistently reverse:

Len-8         146ns ± 1%
LenReverse-8  159ns ± 1%

It's probably just a fluke with the numbers being so small.

Edit: Bumping it up to 5000 items gives me

Len-8         16.3µs ± 3%
LenReverse-8  16.3µs ± 1%

I'm definitely looking at nothing here.

@mlh758 mlh758 closed this Oct 24, 2019
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