[PATCH] D104355: [GlobalISel] Add a new artifact combiner for unmerge which looks through general artifact expressions.

[Petar Avramovic via Phabricator via llvm-commits]

Petar.Avramovic added inline comments.


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Comment at: llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h:645-646
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+      // Only support finding values from either the container source or the
+      // inserted reg, can't find the value if it spans both.
+      // E.g. the following is disallowed:
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I don't think we want to combine G_INSERT but instead not generate them at all. Is this insert from ir or from legalizer? 
>can't find the value if it spans both.
Did you consider this case? I had also encountered infinite loops when trying to deal with this combine when one of the vector sizes is odd. 
I wanted to avoid making G_INSERT and use LCM merge/unmerge pad with undef instead.  AMDGPU can deal with merge/unmerge when sizes are even (in the end it would turn everything into vectors of size 2 in worst case).
With odd vector size at some point it has to do "pad with undef".  U guess that we could make combine that recognizes value that is spanned over real element and undef but it looks unnecessary complicated to combine. 

I thought about adding new artifacts equivalent to G_ANYEXT/G_TRUNC but for vectors (these would be simplified versions of sdag's INSERT_SUBVECTOR and EXTRACT_SUBVECTOR) with intent to be trivial to combine. They would pad vector with undef elements/trunc elements. These do not exist in llvm-ir, and when used in more/fewer_elements_vector would always end up combining with each other if we consistently ask for same number of elements in all more/fewer_elements_vector legalizer rules.
Would this work for you (I would expect that there would be no need to combine insert)?





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