llvm/llvm-project

[InstCombine] SimplifyDemandedBits - add basic bitcast handling

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#173780 opened on Dec 28, 2025

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good first issuellvm:instcombine

Description

Noticed on #173723 - we could avoid the isIntOrIntVectorTy / dyn_cast code if the IC.SimplifyDemandedBits call was able to peek through bitcasts, at least of the same scalar bit size (e.g. f32 -> i32 / v1f64 -> i64 etc.).

Contributor guide

Tech stack
cpp
Domain
performance
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
LLVM IRC++InstCombine pass
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the SimplifyDemandedBits function in llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp. Understand how it currently handles bitcasts and identify where to add peeking logic for same size bitcasts (e.g., f32 >i32). Refer to issue #173723 for additional context and discussion.