llvm/llvm-project

[X86] SimplifyDemandedBitsForTargetNode - add X86ISD::BZHI handling

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#177369 opened on Jan 22, 2026

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backend:X86good first issue

Description

  • Only the bottom 8 bits of the mask (second operand) are required
  • Based off the known bits of the mask, we can tweak the src DemandedBits - as well as the incoming DemandedBits mask, if the 8 bit mask is known to be less than bitwidth (getMaxValue.ult(BitWidth)) then bits above getMaxValue are not demanded.

Supporting tests can be added to combine-bzhi.ll

See also #177347 and #177364 for refs

https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=bzhi&ig_expand=634,633,5,634

Contributor guide

Tech stack
cpp
Domain
backendperformance
Issue type
performance
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.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
LLVM SelectionDAGX86 ISAC++compiler optimization
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Study the existing BZHI handling in combine bzhi.ll and the referenced issues #177347 and #177364. Implement the demanded bits optimization for X86ISD::BZHI in the SimplifyDemandedBitsForTargetNode function, located in the X86 target code. Ensure that the mask's bottom 8 bits are used and adjust DemandedBits accordingly. Add test cases to combine bzhi.ll to verify the optimization.