Landing: c8aaf14bf688
Project / Subsystem
gcc / match.pd
Date
2026-09-29
Author
Dominic P
Commit
c8aaf14bf6882705d2515a5eb9c15de7f4034c5b
Source
github
Perf win
No
Breaking
No
All attributes
- project
- gcc
- subsystem
- match.pd
- patch_id
- —
- commit_hash
- c8aaf14bf6882705d2515a5eb9c15de7f4034c5b
- source_type
- github
- headline
- match.pd: Prevent incorrect folding of minmax(a-c, b) + c
- tldr
- Fixes an optimization that incorrectly folded arithmetic involving unsigned subtraction that wraps around.
- author
- Dominic P
- outcome
- committed
- performance_win
- false
- breaking_change
- false
- series_id
- —
- series_parts
- []
- tags
-
- • optimization
- • compiler
- • correctness
- discussion_id_link
- —
- bugzilla_pr
- —
- date
- 2026-09-29T00:00:00.000Z
An optimization pass in GCC’s pattern matching (match.pd) incorrectly folded expressions of the form minmax(a - c, b) + c. This fold was valid only if a - c did not wrap around (underflow) for unsigned types. The bug allowed incorrect transformations when a was small and c was large, leading to different results for the original and optimized expressions. This change adds a check using range_op_handler to ensure a - c is overflow-free before applying the fold, thus correcting the optimization.