On Mon, Aug 3, 2015 at 5:37 PM, Kyrill Tkachov <kyrylo.tkac...@arm.com> wrote:
>> Looking at the x86 movcc expansion code (ix86_expand_int_movcc) I >> don't think this is a good idea. In the expander, there is already >> quite some target-dependent code that goes great length to utilize sbb >> insn as much as possible, before cmove is used. >> >> IMO, as far as x86 is concerned, the best solution would be to revert >> the change. ix86_expand_int_movcc already does some tricks from your >> patch in a target-efficient way. Generic change that was introduced by >> your patch now interferes with this expansion. > > > Well, technically the transformation was already there, it was just never > reached for an x86 compilation because noce_try_cmove was tried in front of > it > and used a target-specific expansion. > In any case, how's this proposal? > The transformation noce_try_store_flag_constants > /* if (test) x = a; else x = b; > => x = (-(test != 0) & (b - a)) + a; */ > > Is a catch-all-immediates transformation in noce_try_store_flag_constants. > What if we moved it to noce_try_cmove and performed it only if the > target-specific > conditional move expansion there failed? > > That way we can try the x86_64-specific sequence first and still give the > opportunity > to noce_try_store_flag_constants to perform the transformations that can > benefit targets > that don't have highly specific conditional move expanders. Yes, let's try this approach. As was found out, some targets (e.g. x86) hide lots of different target-dependent expansion strategies into movcc expander. Perhaps this fact should be documented in the comment in the generic code? Uros.
