llvm/llvm-project

[InstCombiner] foldIntrinsicIsFPClass - incorrect submasking of FPClassTest masks

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#189,949 opened on Apr 1, 2026

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floating-pointgood first issuellvm:instcombine

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Description

As mentioned here when creating the DAG equivalent: https://github.com/llvm/llvm-project/pull/189944#discussion_r3021637130

We shouldn't be looking for equality, we should be testing that they are a subset of the known classes:

https://github.com/llvm/llvm-project/blob/03b5539dd58584e6821e11123d5c8546147a6f41/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp#L1180-L1184

Contributor guide

Research direction
Review the code at the referenced location and change the condition from equality (==) to a subset check using bitwise operations such as `(Mask & Known) == Mask`.
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.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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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.
50