[PATCH] D47981: [InstCombine] Fold (x >> y) << y -> x & (-1 << y)

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In https://reviews.llvm.org/D47981#1127526, @lebedev.ri wrote:

> Even in light of https://reviews.llvm.org/D46760#1127287, i believe we still can do *this* transform, ...


Agreed. I don't think a variable shift amount is a problem, but if it is, then maybe we should enhance SCEV rather than cripple instcombine? (And I think the same could be said for the constant shift amount).



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Comment at: lib/Transforms/InstCombine/InstCombineShifts.cpp:620-624
     // (X >> C) << C --> X & (-1 << C)
     if (match(Op0, m_Shr(m_Value(X), m_Specific(Op1)))) {
       APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmt));
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
     }
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lebedev.ri wrote:
> ... because we already do it here for when the `y` is constant, regardless of `exact`ness of `shr`.
> Also, maybe this block should be dropped, now that the more general fold will be done, one not limited to constants?
Right - I saw that block. I think it's a just a matter of taste because we don't need the one-use check if the shift amount is constant (so you'd have to add an 'if' for that possibility in the later block).


Repository:
  rL LLVM

https://reviews.llvm.org/D47981