llvm/llvm-project

[DAG] isGuaranteedNotToBeUndefOrPoison - add ISD::BITCAST handling with DemandedElts

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#161,512 opened on Oct 1, 2025

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good first issuellvm:SelectionDAG

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Description

Currently ISD::BITCAST nodes fallback to the default implementation (which tests all elements), but we should be rescaling the input demanded elts to match the vector element count of the input operand, typically with APIntOps::ScaleBitMask but there will need to be special case handling of src/dst scalar types as well.

Test coverage will be the main issue

Contributor guide

Research direction
Study the implementation of isGuaranteedNotToBeUndefOrPoisonDAG in SelectionDAG/DAGCombiner.cpp and understand how DemandedElts is used for other ISD nodes. Look at APIntOps::ScaleBitMask for resizing. For scalar types, handle special cases where source or destination is scalar.
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.
5
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.
Mostly clear
Prerequisites
C++LLVMCompiler Optimization
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20