[PATCH] D24419: [InstCombine] use commutative matchers for patterns with commutative operators

[Eli Friedman via llvm-commits]

efriedma added inline comments.


================
Comment at: lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:2350
+  if (match(Op1, m_c_Xor(m_Not(m_Value(A)), m_Value(B))) &&
+      match(Op0, m_c_And(m_Specific(A), m_Specific(B))))
     return BinaryOperator::CreateXor(Builder->CreateNot(A), B);
----------------
spatel wrote:
> efriedma wrote:
> > This isn't actually equivalent to four separate checks in the case where both operands of the xor are "not" operations.  I guess that doesn't really matter much in practice for this particular case, but we need a better approach in general.
> Is it possible to have both operands 'notted'? We should always be able to fold:
>   %negx = xor i32 %x, -1
>   %negy = xor i32 %y, -1
>   %xor = xor i32 %negx, %negy
> 
> to:
>   xor i32 %x, %y
> 
> before we reach here?
Yes, in this particular case, it probably doesn't matter.  I don't want to have to reason that out for every possible use of commuted patterns, though.


https://reviews.llvm.org/D24419