Landing: a9b19ded78ca

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Project / Subsystem

gcc / tree-optimization

Date

2026-10-01

Author

Richard Biener

Commit

a9b19ded78ca2dd8f66b355b05ebbbb149c2d320

Source

github

Perf win

Yes

Breaking

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All attributes

project: gcc
subsystem: tree-optimization
patch_id: —
commit_hash: a9b19ded78ca2dd8f66b355b05ebbbb149c2d320
source_type: github
headline: Vector cost model heuristic misapplied when loop iteration count is constant
tldr: GCC's vectorizer will no longer wrongly assume vectorization is profitable for single-iteration loops when the iteration count is already known.
author: Richard Biener
outcome: committed
performance_win: true
breaking_change: false
series_id: —
series_parts: []
tags: • optimization
• vectorization
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/a9b19ded78ca2dd8f66b355b05ebbbb149c2d320
date: 2026-10-01T00:00:00.000Z

GCC’s vectorizer uses a cost model heuristic that assumes vectorization is profitable even for loops that execute only once, provided the vector cost is cheap. However, this heuristic was being applied even when the loop’s iteration count was constant, a case already handled by a separate profitability assessment. This commit modifies the vector cost analysis to avoid applying the single-iteration heuristic when the iteration count is constant, preventing incorrect vectorization decisions.