[PATCH] D105879: Refine logic of MaskedElementsAreZero

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


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Comment at: llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:2450
-  APInt DemandedBits = APInt::getAllOnesValue(BitWidth);
-  return MaskedValueIsZero(Op, DemandedBits, DemandedElts, Depth);
 }
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xiangzhangllvm wrote:
> > it just analyses the common bits of all the vector elements specified in the DemandedElts mask
> Thanks for your explain, in fact, I read the computeKnownBits  code yesterday, it makes me a lot of puzzle. I thought here the DemandedBits should have the same BitWidth with Op (e.g.  128 for V16xi8)
> 
> 
> Let me take a example:
> So, if the Op is type V16xi8, and the DemandedElts is 0x2222 (16 bits, demanded index is 1, 5, 9, 13), you mean computeKnownBits will return the common zeros/ones of Op's elements  with index 1, 5, 9, 13 ?  ( zeros/ones = element1 & element5 & element9 & element13).
Yes the KnownBits result is the common bits in those demanded elements - if any are set differently or unknown in any element then the KnownBits result doesn't 'know' that bit.

If you have a usecase for a 'computeAllKnownBits' style wrapper that returns the known bits for each element (either as an array of KnownBits or a concatenated KnownBits struct) then it'd be definitely worth a patch along with suitable test coverage.


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  https://reviews.llvm.org/D105879/new/

https://reviews.llvm.org/D105879