llvm/llvm-project

[X86] computeKnownBitsForTargetNode - add X86ISD::GF2P8AFFINEQB handling

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#190520 opened on Apr 5, 2026

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backend:X86good first issue

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

Tech stack
cpp
Domain
tooling
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.
over 1 week
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
LLVM codegen infrastructureX86 backend knowledgeknown bits analysisGFNI intrinsics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
The issue requests adding handling for X86ISD::GF2P8AFFINEQB in the computeKnownBitsForTargetNode function. Start by studying the existing implementations for other X86ISD nodes in the X86 backend's target specific lowering. The reference implementation for GFNI intrinsics in clang's ExprConstant.cpp (GFNIAffine) can provide insight. The Intel intrinsics guide details the behavior of the VGF2P8AFFINEQB instruction. Ensure testing is done either on a CPU with GFNI support or using constexpr evaluation as mentioned.