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

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

On Thu, 11 Jun 2015, Marek Polacek wrote:

>>> +  (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).
>
> I don't have a strong preference here but we surely use single_use
> in match.pd elsewhere.

The criterion for single_use up to now has been whether we may end up with 
more operations after the transformation than before. Take:
(x & ~m) | (y & m) -> ((x ^ y) & m) ^ x

If (x & ~m) and (y & m) have other uses, we are going to compute them 
anyway, and the original is essentially a single bit_ior operation. After 
the transformation, we have 2 more operations. That's worse than we 
started with, so we don't do it.

-- 
Marc Glisse