[PATCH] D124183: [InstCombine] Add one use limitation for (X * C2) << C1 --> X * (C2 << C1)

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:971
     // (C2 << X) << C1 --> (C2 << C1) << X
     if (match(Op0, m_OneUse(m_Shl(m_Constant(C2), m_Value(X)))))
       return BinaryOperator::CreateShl(ConstantExpr::getShl(C2, C1), X);
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bcl5980 wrote:
> @lebedev.ri @spatel 
> Based on the rules, can we remove this one-use also ?
> (C2 << X) << C1 dependent on C2 << X
> (C2 << C1) << X is independent with C2 << X
> 
> One similar case udiv+lshr, do we need add one-use if we add the transform?
> https://alive2.llvm.org/ce/z/wk6n3q
add general proof for udiv+lshr:
https://alive2.llvm.org/ce/z/CfMqxN


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  https://reviews.llvm.org/D124183/new/

https://reviews.llvm.org/D124183