[PATCH] D67677: [InstCombine] dropRedundantMaskingOfLeftShiftInput(): pat. a/b with mask (PR42563)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:176-180
+      Type *ExtendedScalarTy = Type::getIntNTy(Ty->getContext(), 2 * BitWidth);
+      Type *ExtendedTy =
+          Ty->isVectorTy()
+              ? VectorType::get(ExtendedScalarTy, Ty->getVectorNumElements())
+              : ExtendedScalarTy;
----------------
spatel wrote:
> Is there a test showing that we need this ext+trunc complexity?
> 
> See if I've botched this Alive somehow, but the simpler constant mask appears to work:
> https://rise4fun.com/Alive/ArQC
Hmm. The reason i've gone forward with ext/trunc is: https://rise4fun.com/Alive/o5l
In your example alive does not complain because those are constants, and somehow the usual poison rules don't apply?
Are we sure this isn't alive limitation, but the correct behavior?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67677/new/

https://reviews.llvm.org/D67677