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

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.

Comment From: randall77

I think this is just a case of missing strides. We have only implemented strides of 1 and the load size so far. It is a lot of extra opcodes and rewrite rules :( In this case, it is a load size of 1 but a stride of 4.

There are two LEAQs instead of one because of https://github.com/golang/go/issues/21735. Presumably we could fold the constant offset into the load itself to save one of those LEAQs.

Comment From: gabyhelp

Related Issues

• cmd/compile: improve code generation for temporary slice copy and inlining #18529
• cmd/compile: optimize slice access with constant upper/lower bound #14266 (closed)
• cmd/compile: remove bounds checking for sub-slices #14905 (closed)
• cmd/compile: unexpected difference in compiled code for returned array #31198
• cmd/internal/gc: accessing an array with power of two 2 size should use left shifts not multiplications #10638 (closed)
• cmd/compile: assembly generated is bigger than previous versions #30229 (closed)
• cmd/compile: unsafe conversion from slice to struct pointer generates worse code on amd64 than on 386 #65330
• cmd/compile: unnecessary array copying in for range loop #33838 (closed)

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Comment From: Jorropo

Aside of what @randall77 already said, should this snippet of code not use INCQ DX since the source and destination match ?

Comment From: bupjae

Then, correct way would be adding SSA opcodes such as MOVBloadidx4 and rewrite rules.

Looks like this issue is marked as TODO long time ago https://github.com/golang/go/blob/86b5ba731044dcbb0400f03293989796ed5807fe/src/cmd/compile/internal/ssa/_gen/AMD64Ops.go#L841-L865

Related work - adding SSA opcode: #40426 . Although it seems that how to write rewrite rule has been changed significantly since then.

Comment From: gopherbot

Change https://go.dev/cl/687976 mentions this issue: cmd/internal/pkgsite: increase x/pkgsite dependency to CL 687918

Comment From: randall77

Note that now that we have the addressingmodes pass, the combinatorial explosion, particularly in the rules files, won't be as bad.