llvm/llvm-project

[x86] Eliminate `movzx` when upper part of register is irrelevant or overwritten

Open

#168,828 opened on Nov 20, 2025

View on GitHub
 (16 comments) (0 reactions) (1 assignee)C++ (10,782 forks)batch import
backend:X86good first issuellvm:SelectionDAGmissed-optimization

Repository metrics

Stars
 (26,378 stars)
PR merge metrics
 (Avg merge 1d 2h) (1,000 merged PRs in 30d)

Description

export fn foo(x: u8) u32 {
    return @ctz(x >> 1);
}

foo:
        shr     dil
        movzx   eax, dil
        or      eax, 256
        tzcnt   eax, eax
        ret

Could be:

foo:
        shr     dil
        or      edi, 256
        tzcnt   eax, edi
        ret

The compiler could also look for things like or eax, -256 and eliminate movzx eax, al since the relevant bits are overwritten anyway.

Contributor guide

Research direction
Investigate the x86 backend of LLVM to find where movzx is generated and determine if the upper bits are irrelevant or overwritten. Look into the DAG or ISel lowering for patterns like movzx followed by or, and see if the movzx can be eliminated when the destination is overwritten.
Tech stack
cpp
Domain
backend
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitLLVM
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60