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cmd/compile: unnecessary runtime.growslice with provable capacity #66692

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dsnet opened this issue Apr 5, 2024 · 4 comments
Closed

cmd/compile: unnecessary runtime.growslice with provable capacity #66692

dsnet opened this issue Apr 5, 2024 · 4 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsFix The path to resolution is known, but the work has not been done. Performance
Milestone
Go1.24

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@dsnet
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dsnet commented Apr 5, 2024

Go version

go1.22

Output of go env in your module/workspace:

GOARCH=amd64
GOOS=linux

What did you do?

Compile the following:

package main

import "encoding/binary"

var src uint64
var dst []byte

func main() {
	if cap(dst)-len(dst) < 8 {
		return
	}
	dst = binary.BigEndian.AppendUint64(dst, src)
}

What did you see happen?

I see the following in the assembly:

	0x004a 00074 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	MOVL	$8, DI
	0x004f 00079 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	LEAQ	type:uint8(SB), SI
	0x0056 00086 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	PCDATA	$1, $0
	0x0056 00086 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	CALL	runtime.growslice(SB)
	0x005b 00091 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	MOVQ	encoding/binary.v+64(SP), DX
	0x0060 00096 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	BSWAPQ	DX
	0x0063 00099 (/usr/local/go1.22.0/src/encoding/binary/binary.go:201)	MOVQ	DX, -8(BX)(AX*1)

What did you expect to see?

No call to runtime.growslice.

This particular example seems silly, but the capacity check already happens elsewhere in code beforehand so there is no possible way that growslice would be necessary once we get to AppendUint64.
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Apr 5, 2024
@dsnet dsnet added Performance compiler/runtime Issues related to the Go compiler and/or runtime. and removed compiler/runtime Issues related to the Go compiler and/or runtime. labels Apr 5, 2024
@dsnet
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dsnet commented Apr 5, 2024 •
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A fix for this should hopefully take #66691 into account such that the following avoids both growslice calls for subsequent append calls:

if cap(dst)-len(dst) < 1+8 {
	dst = slices.Grow(dst, 1+8)
}
dst = append(dst, flag)
dst = binary.BigEndian.AppendUint64(dst, src)

(Obviously, slices.Grow would need to call growslice, but after the conditional check we should be safe for subsequent append operations)

@Jorropo
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Jorropo commented Apr 5, 2024 •
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With lots of massaging this works:

package main

import "encoding/binary"

var src uint64
var dst []byte

func main() {
	if uint(len(dst)+8) > uint(cap(dst)) {
		return
	}
	dst = binary.BigEndian.AppendUint64(dst, src)
}

This works because uint(len(dst)+8) > uint(cap(dst)) is the exact check generated by the frontend for this append.

I see at least two things here:

  • SliceLen and SliceCap doesn't seems to be recognized as uint63 or uint31 as else Less64 and Less64U should already prove each other https://godbolt.org/z/h9dYfY19M.
  • More general equivalence or canonicalization issues.

@dmitshur dmitshur added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Apr 5, 2024
@dmitshur dmitshur added this to the Backlog milestone Apr 5, 2024
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dmitshur commented Apr 5, 2024

CC @golang/compiler.

@mknyszek mknyszek modified the milestones: Backlog, Unplanned Apr 10, 2024
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Change https://go.dev/cl/599096 mentions this issue: cmd/compile: rewrite the constant parts of the prove pass

@dmitshur dmitshur modified the milestones: Unplanned, Go1.24 Aug 7, 2024
@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Aug 7, 2024
@gabyhelp gabyhelp mentioned this issue Apr 3, 2025
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsFix The path to resolution is known, but the work has not been done. Performance
Projects
Go Compiler / Runtime
Status: Done
Milestone
Go1.24
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@mknyszek @dmitshur @dsnet @gopherbot @Jorropo