llvm/llvm-project

[InstCombiner] foldIntrinsicIsFPClass - incorrect submasking of FPClassTest masks

Open

#189949 opened on Apr 1, 2026

View on GitHub
 (15 comments) (0 reactions) (1 assignee)C++ (26,378 stars) (10,782 forks)batch import
floating-pointgood first issuellvm:instcombine

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

Tech stack
cpp
Domain
backendperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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 IRInstCombine passFP class test masks
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the InstCombineCalls.cpp file at lines 1180-1184 where the equality check is used for FP class test masks. The correct behavior should be testing that the masks are a subset of known classes, not equality. Review the linked PR discussion for context. Implement the fix and add corresponding test cases in the appropriate test file.