Landing: 804ff22702ec

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Project / Subsystem

gcc / phiopt

Date

2026-08-22

Author

Aldy Hernandez

Commit

804ff22702eca4e389e1a2f3659010190504a757

Source

github

Perf win

Yes

Breaking

No

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All attributes

project: gcc
subsystem: phiopt
patch_id: —
commit_hash: 804ff22702eca4e389e1a2f3659010190504a757
source_type: github
headline: Replicate conditionals over a PHI of comparisons
tldr: Restores an optimization that replicates conditional logic over PHI nodes.
author: Aldy Hernandez
outcome: committed
performance_win: true
breaking_change: false
series_id: —
series_parts: []
tags: • optimization
• phiopt
• control flow
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/804ff22702eca4e389e1a2f3659010190504a757
date: 2026-08-22T00:00:00.000Z

This commit reintroduces an optimization in the phiopt pass that replicates conditional logic over a PHI node formed by comparisons. This optimization was lost during a rewrite of GCC’s jump threading machinery but was partially recovered by the DOM (Dominator) pass. The phiopt pass now handles these cases directly, improving performance by avoiding unnecessary conditional merges. The optimization is deliberately not applied to loop boundaries to preserve loop structure, though the DOM pass’s version remains available for such cases. This change recovers over 94% of the joins previously handled by DOM’s special case.