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
