[X86] SimplifyDemandedBitsForTargetNode - add X86ISD::BZHI handling · llvm/llvm-project

(5 則留言) (1 個反應) (1 位負責人)C++ (10,782 個分叉)batch import

backend:X86good first issue

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描述

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

貢獻者指南

研究方向: 查看BZHI指令語義（Intel內建指南）。通過將第二個操作數掩碼為8位來實現所需位傳播。使用已知位分析來減少源和結果的DemandedBits：如果掩碼值已知且小於位寬，則getMaxValue以上的位不被需要。在combine bzhi.ll中添加測試。
技術棧: cpp
領域: backend
議題類型: 功能
難度: 3
預計時間: 半天
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