[PATCH] D77475: GlobalISel: Combine G_UNMERGE_VALUES with G_TRUNC

[Matt Arsenault via Phabricator via llvm-commits]

arsenm marked an inline comment as done.
arsenm added inline comments.


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Comment at: llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h:401
+
+        //  %1:_(<4 x s8>) = G_TRUNC %0(<4 x s32>)
+        //  %2:_(s8), %3:_(s8), %4:_(s8), %5:_(s8) = G_UNMERGE_VALUES %1
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arsenm wrote:
> aemerson wrote:
> > How do we make the decision that this the resulting output is "better" than the input? Can we check for input legality or something?
> I don't think anywhere in the artifact combiner is deciding what's really "better". I think what makes most sense is checking if the vector truncate is legal, but that doesn't really work due to the special constraints on G_TRUNC legality. Checking if the unmerge skips cases where it's still beneficial to do the combine, so I'm not sure what else I would do here
I've also been wondering if the conventions of the artifact combiner derive from originally missing most legalization actions on merge/unmerge etc. isInstUnsupported only makes sense to me for poorly defined legalization rules that don't try to handle standalone artifacts as legalizable operations


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https://reviews.llvm.org/D77475