Landing: a20f4528f3ab

← All landings

Project / Subsystem

gcc / tree-optimization

Date

2026-05-08

Author

Philipp Tomsich

Commit

a20f4528f3ab84afc2568ea1e208f33fe59bec66

Source

github

Perf win

Yes

Breaking

View on GitHub →

All attributes

project: gcc
subsystem: tree-optimization
patch_id: —
commit_hash: a20f4528f3ab84afc2568ea1e208f33fe59bec66
source_type: github
headline: tree-optimization/122569 - recognize CLZ via isolated MSB DeBruijn lookup
tldr: Compiler now recognizes a CLZ idiom using an isolated MSB and DeBruijn lookup for potential performance gains.
author: Philipp Tomsich
outcome: committed
performance_win: true
breaking_change: false
series_id: —
series_parts: []
tags: • optimization
• tree-optimization
• intrinsics
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/a20f4528f3ab84afc2568ea1e208f33fe59bec66
date: 2026-05-08T00:00:00.000Z

GCC can now recognize a count-leading-zeros (CLZ) optimization pattern. This pattern appears after an OR-cascade and uses a value transformation to isolate the most significant bit (MSB) as a power of two, which is then mapped to the bit position using a DeBruijn lookup. The frontend is updated to support this optimization, potentially improving performance for operations involving CLZ where this specific idiom is present.