[PATCH] D79162: [Analysis] TTI: Add CastContextHint for getCastInstrCost

[Dave Green via Phabricator via llvm-commits]

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


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Comment at: llvm/include/llvm/Analysis/TargetTransformInfo.h:1011
+    Interleave,    ///< The cast is used with an interleaved load/store.
+    Reversed,      ///< The cast is used with a reversed load/store.
+  };
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fhahn wrote:
> Did you consider also excluding non load/store cast uses? For example, it might be worth to include arithmetic instructions into which casts can be folded, e.g. USUBL & co on AArch64.
Yes.. That might be a possibility.

AArch64TTIImpl::getCastInstrCost already has some code to check for isWideningInstruction, which should already catch a lot of these situations I think. That relies on the context instruction being correct, but I believe that will be correct most of the time currently. It's really the type of the load or store that is most often incorrect.

I'm honestly not sure how well this would scale to many different operand kinds. I think in the long run we need something that makes costing multiple nodes together easier, if you imagine something like a trunc(shift(mul(sext(x), sext(y), 16),  all being a single instruction! And with vplan making larger changes during vectorization it would ideally handle "hypothetical" instructions better without relying on "real" context instructions.

But this patch is at least a little step that gets the load/store kinds more correct.


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  https://reviews.llvm.org/D79162/new/

https://reviews.llvm.org/D79162