[PATCH] D89100: [GlobalISel] Look through truncs and extends in narrowScalarShift

[Konstantin Schwarz via Phabricator via llvm-commits]

kschwarz created this revision.
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If a G_SHL is fed by a G_CONSTANT, the lower and upper bits of the source can be
shifted individually by the constant shift amount.

However in case the shift amount came from a G_TRUNC(G_CONSTANT), the generic shift legalization
code was used, producing potentially illegal intermediate shifts.

Later code in GIselKnownBits would assert that the constant shift amount is bigger than the bit-width
of the source type.

This change teaches narrowScalarShift to look through G_TRUNCs and G_*EXTs.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D89100

Files:
  llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
  llvm/test/CodeGen/AMDGPU/GlobalISel/saddsat.ll
  llvm/test/CodeGen/AMDGPU/GlobalISel/ssubsat.ll
  llvm/unittests/CodeGen/GlobalISel/LegalizerHelperTest.cpp

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