golang/go

cmd/compile: x86-64: sub-optimal codegen less-than-int sized index from structured slice/array

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#74489 opened on Jul 7, 2025

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BugReportNeedsInvestigationPerformancecompiler/runtimehelp wanted

Description

Go version

go version go1.25-devel_6c3b5a2 Thu Jul 3 18:43:56 2025 -0700 linux/amd64

Output of go env in your module/workspace:

Using service on https://go.godbolt.org/
Selected "x86-64 gc (tip)"

What did you do?

I entered following program

package main

func Foo(a [][4]byte, b []int, i int) int {
	return b[a[i][1]]
}

func main() {
}

https://go.godbolt.org/z/hfvezofxf

What did you see happen?

Codegen for a[i][1] part was following:

        LEAQ    (AX)(R9*4), DX
        LEAQ    1(DX), DX
        MOVBLZX (DX), AX

What did you expect to see?

One-line codegen like MOVBLZX 1(AX)(R9*4), AX was expected

The compiler really worked very hard to optimize codegen regarding various element size of slice a. However, it seems that it missed simple case - when the element size is 2, 4 or 8.

Contributor guide

Tech stack
go
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Gocompiler internalsx86-64 assembly
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the SSA lowering rules for slice/array indexing in the Go compiler's x86-64 backend. The issue occurs when the element size is 2, 4, or 8 bytes. Focus on the LEAQ instruction generation and how the index is computed. Check the relevant files in `src/cmd/compile/internal/ssa` and `src/cmd/compile/internal/amd64`.