golang/go

cmd/compile: doesn't optimize float comparison to MINSD

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#72831 opened on Mar 12, 2025

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BugReportNeedsFixPerformancecompiler/runtimehelp wanted

Description

Go version

go version go1.24.1 linux/amd64

Output of go env in your module/workspace:

-

What did you do?

go build -gcflags=-S

func mymin(a, b float64) float64 {
	if a < b {
		return a
	} else {
		return b
	}
}

What did you see happen?

0x0000 00000 (bar.go:4)      UCOMISD X0, X1
0x0004 00004 (bar.go:4)      JLS     7
0x0006 00006 (bar.go:5)      RET
0x0007 00007 (bar.go:7)      MOVUPS  X1, X0
0x000a 00010 (bar.go:7)      RET

What did you expect to see?

I think

MINSD X1, X0
RET

but maybe

MINSD X0, X1
MOVUPS X1, X0
RET

AT&T argument order has me thoroughly confused at times.

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.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go compiler internalsx86 assemblySSA form
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the Go SSA compiler backend in cmd/compile/internal/ssa for x86 architecture to add a rule that replaces a float comparison followed by conditional move with MINSD instruction. The current code generates UCOMISD and JLS; the optimization should detect this pattern and emit MINSD. Review similar optimization rules for MINSS and MAXSS for float32. Also check the issue comments for additional context.