llvm/llvm-project

[X86] SimplifyDemandedBitsForTargetNode - add X86ISD::BZHI handling

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#177,369 opened on Jan 22, 2026

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backend:X86good first issue

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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

Research direction
Examine the BZHI instruction semantics (Intel Intrinsics Guide). Implement demanded bits propagation by masking the second operand to 8 bits. Use known bits analysis to reduce DemandedBits for source and result: if mask value is known and less than bit width, bits above getMaxValue are not demanded. Add tests in combine bzhi.ll.
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.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++LLVMGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20