[PATCH] D61307: [InstCombine] Add new combine to sub folding

[Chris Dawson via Phabricator via llvm-commits]

cdawson added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp:1535-1536
+  // (Y | X) - Y --> (Y | X) ^ Y
+  if (match(Op0, m_OneUse(m_c_Or(m_Value(X), m_Specific(Op1)))))
+    return BinaryOperator::CreateXor(Builder.CreateOr(X, Op1), Op1);
+
----------------
lebedev.ri wrote:
> I do not understand. You don't change the inner pattern at all.
> Just do:
> ```
>   if (match(Op0, m_c_Or(m_Specific(Op1), m_Value())))
>     return BinaryOperator::CreateXor(Op0, Op1);
> ```
I totally agree with the change to the return statement and the m_c_Or check. 

One point I'd like clarification on is the removal of the m_OneUse. I was including that since the transformation is to facilitate a further (X | Y) ^ Y --> X & ~Y which will only happen if there are no extra uses of (X | Y). If you think we should always transform (X | Y) - Y --> (X | Y) ^ Y I'm happy to remove the m_OneUse.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61307/new/

https://reviews.llvm.org/D61307