[PATCH] D102828: [GlobalISel] Implement splitting of G_SHUFFLE_VECTOR.

Thu May 20 16:01:24 PDT 2021

aemerson added inline comments.

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Comment at: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:4231
+  // Further legalization attempts will be needed to do split further.
+  NarrowTy = DstTy.changeNumElements(DstTy.getNumElements() / 2);
+  unsigned NewElts = NarrowTy.getNumElements();
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arsenm wrote:
> aemerson wrote:
> > arsenm wrote:
> > > I don't like simply discarding NarrowTy. It would be better to compute the type you do handle, and fail if it doesn't match the requested type
> > But if you do that, fewerElements isn't going to know which narrow type it supports. Legalizing <16 x s64> is going to use a narrow ty of <2 x s64> with the rules as they're written now.
> I don't follow, the parameter tells you which type to use
For AArch64, if you have an oversize shuffle like `<8 x s64>`, fewerElementsVector is called with NarrowTy set to `<2 x s64>` since that's the type that we support. We can't just error out if in that case since this can be legalized, it just needs to the fewerElementsVector action to be done multiple times until we reach <2 x s64>.

Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D102828/new/

https://reviews.llvm.org/D102828