llvm/llvm-project

GlobalISel should have G_POISON

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#127,486 opened on Feb 17, 2025

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good first issuellvm:globalisel

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Description

Currently GlobalISel has G_IMPLICIT_DEF, which acts like undef. This is used to lower both poison and undef from the IR. To mirror the semantics of the IR, we need a separate representation of poison. We should introduce G_POISON to enable better optimizations, particularly when legalization introduces padding vector elements.

This change would mostly be finding all the places G_IMPLICIT_DEF is handled, and equivalently handling G_POISON. We should then migrate vector widening and other transforms to introduce poison instead of undef.

Contributor guide

Research direction
Search the LLVM source code for all references to G IMPLICIT DEF and add parallel handling for G POISON. Start with the GlobalISel instruction definitions, then update the MachineVerifier, legalizer, combiner, and any other passes that handle G IMPLICIT DEF. Finally, migrate vector widening to use G POISON instead.
Tech stack
cpp
Domain
backend
Issue type
Feature
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.
Half day
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
C++LLVMGlobalISel
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60