cmd/compile: x86-64: sub-optimal codegen less-than-int sized index from structured slice/array · golang/go#74489

(6 留言) (0 反應) (0 負責人)Go (133,883 star) (19,008 fork)batch import

BugReportNeedsInvestigationPerformancecompiler/runtimehelp wanted

描述

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.

貢獻者指南

技術棧: go
領域: performance
議題類型: performance
難度: 5
預計時間: over 1 week
活動狀態: fresh
清晰度: clear
前置要求: Gocompiler internalsx86-64 assembly
新手友善度: 10
研究方向: 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`.

描述