[PATCH] D78486: [SystemZ] Expand vector zero-extend into a shuffle.

[Jonas Paulsson via Phabricator via llvm-commits]

jonpa updated this revision to Diff 260296.
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> Hi Jonas, I haven't looked into everything in detail, but first one fundmental question: My understanding was that the current GeneralShuffle code would detect the shuffle you generate for a zero-extend as a case of a MERGE. Later, combineMERGE would detect that one input of the MERGE is a zero vector, and replace the merge by an UNPACKL.
> 
> Is there some reason why this doesn't work for you? Why do you have to create the UNPACKL already in GeneralShuffle?

That will only work in cases where only one unpack is needed, and only if the input does not need any permutation of any kind.

Patch changed to use increasing indexes into the zero vector so that the simple case can still be matched with a MERGE_HIGH.

I am not that sure about the actual best tuning yet, but so far I have aimed to reduce the number of VPERMs.  I thought originally that using isByteZero() to check if a single byte was known zero would make for detecting more cases where only one element or byte of was zero. This way, any of the operands could have a zero byte which would be detected, like a high 0 byte in a wider immediate operand, for instance. I see now however that this is no improvement compared to simply finding a zero or undef vector input, so I have changed the patch to to this instead.

lowerExtendVectorInreg() split into lowerSIGN_EXTEND_VECTOR_INREG() and lowerZERO_EXTEND_VECTOR_INREG().


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D78486/new/

https://reviews.llvm.org/D78486

Files:
  llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
  llvm/lib/Target/SystemZ/SystemZISelLowering.h
  llvm/test/CodeGen/SystemZ/vec-move-24.ll

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