Landing: 9f6eb7d29930

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Project / Subsystem

gcc / gcc

Date

2026-09-17

Author

Richard Biener

Commit

9f6eb7d2993046e5720275f590098db0ac0188d4

Source

github

Perf win

No

Breaking

No

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All attributes

project: gcc
subsystem: gcc
patch_id: —
commit_hash: 9f6eb7d2993046e5720275f590098db0ac0188d4
source_type: github
headline: Fix loop unroller epilogue bound when control IV wraps.
tldr: Corrects loop unroller's epilogue iteration count when the loop control variable wraps around.
author: Richard Biener
outcome: committed
performance_win: false
breaking_change: false
series_id: —
series_parts: []
tags: • optimization
• loop unrolling
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/9f6eb7d2993046e5720275f590098db0ac0188d4
date: 2026-09-17T00:00:00.000Z

This commit fixes a bug in GCC’s loop unrolling optimization where the epilogue loop’s iteration bound was incorrectly calculated when the loop’s control variable wraps. Previously, the unroller assumed the main loop consumed all but fewer than FACTOR iterations. However, when the control IV wraps, this assumption breaks, leading to miscompilation. The fix ensures the epilogue bound is only set when the exit test is not a NE_EXPR or when the control IV is known not to wrap, otherwise inheriting the original loop’s bound.