golang/go

cmd/compile: extra bounds check when accessing string using index from range over its runes

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#76354 opened on Nov 19, 2025

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BugReportNeedsInvestigationPerformancecompiler/runtimehelp wanted

Description

Go version

go version go1.26-devel_e1a12c7 Mon Nov 17 13:45:54 2025 -0800 linux/amd64

Output of go env in your module/workspace:

Workspace is `go.godbolt.org` on `x86-64 gc (tip)` as above.

What did you do?

Compile this simple example from the standard library:

func hasUpperCase(s string) bool {
	for i := range s {
		if 'A' <= s[i] && s[i] <= 'Z' {
			return true
		}
	}
	return false
}

(As an aside: is there a reason why the above doesn't iterate over the bytes of s instead?)

What did you see happen?

Extra bounds check on s[i], even though per-spec i should still always be 0 <= i < len(s) even on invalid UTF-8.

What did you expect to see?

No extra bounds checks.

This also appears elsewhere in the standard library, such as strings.ToValidUTF8, fmt.truncateString, and time.tzsetNum.

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
Go programmingcompiler design fundamentals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the Go compiler's bounds check elimination pass in cmd/compile/internal/ssa. Look at how range loops over strings are handled and why the guard on s[i] is not eliminated. Consider modifying the pass to recognize that i from range s is always a valid index. Test with examples from the standard library: src/net/parse.go, src/strings/strings.go, src/fmt/format.go, src/time/zoneinfo.go. Refer to the existing discussions in the issue comments for additional insights.