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

[Marc Glisse <marc.glisse@inria.fr>]

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