llvm/llvm-project

[DAG] ISD::matchUnaryPredicate / matchUnaryFpPredicate / matchBinaryPredicate - add DemandedElts variant

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#181658 opened on Feb 16, 2026

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good first issuellvm:SelectionDAG

Description

The ISD::match*Predicate methods help us to perform all_of tests on scalar/splat/build_vector constants with a callback test.

It'd be very useful if we could provide variants of these that take a DemandedElts argument (similar to #181485 and other DAG value tracking methods), to allow us to restrict the build_vector tests to specific elements.

For instance, this would allow us to remove the vector constant checks from isKnownToBeAPowerOfTwo entirely and just use the initial matchUnaryPredicate call at the start of the method:

https://github.com/RKSimon/llvm-project/blob/20aff2085c358d3e112b245494161ddb9ffe0410/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L4682-L4696

scalar/splat cases can just ignore the DemandedElts argument, but we should assert the build_vector are the same size as the DemandedElts APInt

Contributor guide

Tech stack
cpp
Domain
backendperformance
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.
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
C++LLVM SelectionDAGAPInt
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Explore the current ISD::matchUnaryPredicate and similar methods in llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp. Understand the existing scalar/splat/build vector handling. Add new overloads that accept a DemandedElts APInt argument, restricting build vector checks to specific elements. Ensure assertions for size consistency. Update callers like isKnownToBeAPowerOfTwo to use the new variants. Reference the linked code lines for context.