[PATCH] D76500: GlobalISel: Lower funnel shifts

Mon Mar 30 11:57:06 PDT 2020

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

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Comment at: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:4196-4197
+  auto CCTy = Ty.changeElementSize(1);
+  // If (Z % BW == 0), then the opposite direction shift is shift-by-bitwidth,
+  // and that is undefined. We must compare and select to avoid UB.
+
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foad wrote:
> It would be slicker to implement `(X << (BW - (Z % BW)))` as `(X << 1 << ((BW - 1) - (Z % BW)))`.
I copied this from the DAG version. Maybe that should be updated first? (or maybe it happens to turn into this anyway from other combines)

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  https://reviews.llvm.org/D76500/new/

https://reviews.llvm.org/D76500