llvm/llvm-project

missed optimization of and + csel + sub

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#201,328 opened on Jun 3, 2026

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good first issuellvm:instcombinemissed-optimization

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Description

the case I find: https://alive2.llvm.org/ce/z/eVF5DP

general version: https://alive2.llvm.org/ce/z/jv3rGf

Contributor guide

Research direction
Examine the alive2 links to understand the missed optimization pattern. Look at the code generation path for `and`, `csel`, and `sub` instructions in the LLVM backend. Identify where the optimization should occur and propose a fix.
Tech stack
cpp
Domain
backend
Issue type
Bug
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.
3-5 days
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
GitC++
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20