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Re: match.pd: Optimize (x & y) ^ (x | y)


On Thu, 11 Jun 2015, Marek Polacek wrote:

I have verified this transformation on a toy testcase (tried x and y
in the range [-1000,1000]) and it does a correct thing for all integers.

Note that for pure bitop (only involving &|^), testing the range [0,1] is sufficient.

+/* (x & y) ^ (x | y) -> x ^ y */
+(simplify
+ (bit_xor:c (bit_and@2 @0 @1) (bit_ior@3 @0 @1))

Make either bit_and or bit_ior commutative? Or do we canonicalize in a way that makes it unnecessary?

+  (if (single_use (@2) && single_use (@3))
+   (bit_xor @0 @1)))

I don't think we should use single_use here. The result is never more complicated than the original. Sure, it might increase register pressure a bit in some cases, but we have not used that as a criterion for other simplifications in match.pd yet (LLVM does though).

--
Marc Glisse


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