golang/go

cmd/compile: optimize string concatenation to avoid runtime calls

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#77325 opened on Jan 27, 2026

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FeatureRequestNeedsInvestigationPerformancecompiler/runtimehelp wanted

Description

Hi team, I've identified a potential optimization opportunity to eliminate concat calls for constant strings and replace them directly with a LEAQ straddr instruction, similar to what the Java JIT does.

This is a cheap optimization that can enable further optimizations down the line, such as compile-time evaluation of len(const_string).

A local run of ./make.bash shows 200+ potential sites for this optimization.

Here is an example:

func ff(a, b int) string {
	y := "abc"
	t := y + "def"
	if t == "abcdef" {
		return t + "ghi"
	}
	return t
}

before:

after applying opts:

For implementation, we could either

  • add a dedicated stringopt pass before expand_call (which centralizes the code for future optimizations like strcmp) or
  • integrate it into the sccp pass after expand_call (which uses its framework but disperses the code) or
  • add some generic rules

Do you have a preference on the approach? Would the community be interested in this?

Contributor guide

Tech stack
go
Domain
backendperformance
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 internalsSSA optimization pass knowledge
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Study the Go compiler's string concatenation handling in cmd/compile/internal/ssa/expand calls.go and related files. Review the two linked prototype commits (stringopt pass and sccp pass) to understand the implementation approaches. Discuss with maintainers in the issue comments to decide on the preferred approach (dedicated pass, integration into sccp, or generic rules). Then implement the optimization, add tests, and ensure no regression in performance benchmarks.