llvm/llvm-project

[X86] computeKnownBitsForTargetNode - add X86ISD::GF2P8AFFINEQB handling

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#190,520 opened on Apr 5, 2026

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Description

X86ISD::GF2P8AFFINEQB nodes are being used for many shift/rotate style expansions, it'd be very useful if we can access the known bits to attempt additional folding afterward (e.g #190502).

This should only be attempted by somebody with access to a CPU with GFNI support to allow them to actually test this properly (although constexpr testing can help now that GFNI intrinsics support it).

Reference: https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#ig_expand=172,3053,2650,3028,3046,849,3516,3630,3054,3053,3054,4389,2239,2239,4602,205,196,4461,3954,1601,2140,417,2873,2851,2571,5840,2571,2927,3639&text=VGF2P8AFFINEQB

(see also GFNIAffine() in llvm-project\clang\lib\AST\ExprConstant.cpp)

Contributor guide

Research direction
Study existing computeKnownBitsForTargetNode implementations for other X86 ISD nodes. Understand the semantics of VGF2P8AFFINEQB from the Intel intrinsics guide. Implement known bits computation for the GF2P8AFFINEQB node, considering it as a bitwise affine transformation. Test using constexpr evaluation with GFNI intrinsics on a machine with GFNI support or via LLVM's test infrastructure.
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.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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 internalsx86 GFNI
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25