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cmd/compile: optimize append([]byte(string), ...) to avoid a duplication #47454

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go101 opened this issue Jul 29, 2021 · 2 comments
Open

cmd/compile: optimize append([]byte(string), ...) to avoid a duplication #47454

go101 opened this issue Jul 29, 2021 · 2 comments
Labels
compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
Milestone
Unplanned

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@go101
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go101 commented Jul 29, 2021

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

$ go version
go version go1.17rc1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package main

import "testing"

var str = "1234567890123456789012345678901234567890"

func f() {
	_ = append([]byte(str), str...)
}

func main() {
	stat := func( f func() ) int {
		allocs := testing.AllocsPerRun(1, f)
		return int(allocs)
	}
	println(stat(f)) // 2
}

What did you expect to see?

One allocation.

What did you see instead?

Two instead.

The allocation for []byte(str) could be saved.

Partially related to #43429.
@randall77 randall77 added this to the Unplanned milestone Jul 29, 2021
@cagedmantis cagedmantis added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 30, 2021
quasilyte added a commit to quasilyte/go-perfguard that referenced this issue Mar 20, 2022
@quasilyte
perfguard/_rules: add sliceClone pattern
152ec9f 
For hot paths (o2 level) suggest this rewrite:

	append([]byte(str1), str2...)
	=>
	append(append(make([]byte, 0, len(str1)+len(str2)), str1...), str2...)

This will make the operation work with only 1 allocation instead of 2.

See golang/go#47454
@quasilyte quasilyte mentioned this issue Mar 20, 2022
Merged
quasilyte added a commit to quasilyte/go-perfguard that referenced this issue Mar 20, 2022
@quasilyte
perfguard/_rules: add sliceClone pattern (#185)
a894e04 
For hot paths (o2 level) suggest this rewrite:

	append([]byte(str1), str2...)
	=>
	append(append(make([]byte, 0, len(str1)+len(str2)), str1...), str2...)

This will make the operation work with only 1 allocation instead of 2.

See golang/go#47454
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jul 13, 2022
@go101 go101 mentioned this issue Jun 8, 2023
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@earthboundkid
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Interestingly, append([]byte(nil), str...) and append([]byte(str), 'a') both only do one alloc, so it's only the combination which the compiler fails to optimize.

@go101
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go101 commented Jun 8, 2023

If len(str) > 32, then there will be two allocs,

Also see #56353 (comment).

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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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@cagedmantis @earthboundkid @randall77 @gopherbot @go101