[PATCH] D47980: [InstCombine] Fold (x << y) >> y -> x & (-1 >> y)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp:741-744
         // (X << C1) >>u C2  --> (X >>u (C2 - C1)) & (-1 >> C2)
         Value *NewLShr = Builder.CreateLShr(X, ShiftDiff, "", I.isExact());
         APInt Mask(APInt::getLowBitsSet(BitWidth, BitWidth - ShAmt));
         return BinaryOperator::CreateAnd(NewLShr, ConstantInt::get(Ty, Mask));
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BTW it's interesting to note that all these masks are not fine-grained, isn't it?
Alive says https://rise4fun.com/Alive/Yes (lol)

Though in practice, from what i have seen from the tests, somehow the mask seems to be adjusted later.


Repository:
  rL LLVM

https://reviews.llvm.org/D47980