cmd/compile: strings.HasSuffix has extra bounds check (len(s)-delta)

[fice-t]

Go version

go version go1.26-devel_704f841 Thu Nov 13 15:16:31 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?

[Note: Code/output below was tested here]

Compile the following function from the strings library:

func HasSuffix(s, suffix string) bool {
	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
}

What did you see happen?

Assembly contains an extra check that jumps to a section containing CALL runtime.panicBounds(SB).

What did you expect to see?

Assembly does not contain that check or section.

The relevant part:

        CMPQ    DI, BX
        JGT     command-line-arguments_HasSuffix_pc86
        MOVQ    BX, DX
        SUBQ    DI, BX
        CMPQ    DX, BX
        JCS     command-line-arguments_HasSuffix_pc94 ; <- panic section

Assuming DI and BX are both non-negative, then the JCS will never jump (as DI <= BX).

In searching for similar issues, I found #27251, which contains this commit explicitly mentioning HasSuffix that caused a regression. Hopefully there's a solution that doesn't regress on that front.

Edit: I misread the commit message: that commit is only for literal suffixes, and indeed the test case for the general case is still marked todo.

[gabyhelp]

Related Issues

• cmd/compile: unnecessary bounds check when indexing relative to the end #51622 (closed)
• cmd/compile: Extra bounds checking when using addition in condition/index #76270
• cmd/compile: teach prove.go that {strings|bytes}.Index* return value in the range [-1 .. len(s)) #25862
• cmd/compile: missing BCE on slice in loop #15203 (closed)
• cmd/compile: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice). #15079 (closed)
• cmd/compile: bounds check not needed #61333
• cmd/compile: unnecessary bounds check with slice offset #66691
• cmd/compile: remove bounds checking for sub-slices #14905 (closed)
• cmd/compile: avoid unnecessary calls to runtime.panicIndex #37740
• cmd/compile: unnecessary bounds check when indexing slice from range #71439 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[gopherbot]

Change https://go.dev/cl/720740 mentions this issue: cmd/compile: teach prove about subtract idioms