llvm/llvm-project

[DAG] isKnownToBeAPowerOfTwo - add DemandedElts + OrZero handling to ISD::ZERO_EXTEND cases

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#181648 opened on Feb 16, 2026

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good first issuellvm:SelectionDAG

Description

The zext nodes can safely filter elements using DemandedElts and can correctly identify zero cases when OrZero is true (zext(0) is still 0):

https://github.com/RKSimon/llvm-project/blob/20aff2085c358d3e112b245494161ddb9ffe0410/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L4755-L4757

The actual change to the inner isKnownToBeAPowerOfTwo call is trivial (add DemandedElts + OrZero passthrough args), but this will require test coverage as well - these might be possible by extending existing DAG tests, else maybe add unit tests like we did in https://github.com/llvm/llvm-project/pull/181485

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.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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.
mostly clear
Prerequisites
C++LLVM internalsSelectionDAGunit testing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Examine the isKnownToBeAPowerOfTwo function in llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp around lines 4755-4757. Add DemandedElts and OrZero passthrough arguments. Then ensure test coverage by extending existing DAG tests or following the unit test approach from PR #181485. The comments indicate maintainer interest, so seek guidance if needed.