On 14/08/14 09:45, Kyrill Tkachov wrote: > > On 13/08/14 18:32, Segher Boessenkool wrote: >> On Wed, Aug 13, 2014 at 03:57:31PM +0100, Richard Earnshaw wrote: >>> The problem with the frankenmonster patterns is that they tend to >>> proliferate into the machine description, and before you know where you >>> are the back-end is full of them. >>> >>> Furthermore, they are very sensitive to the greedy first-match nature of >>> combine: a better, later, combination is missed because a less good, >>> earlier, optimization matched. If the first insn in the sequence is >>> merged into an earlier instruction then you can end up with a junk >>> sequence that completely fails to simplify. That ends up with >>> super-frankenmonster patterns to deal with all the subcases and the >>> problems grow exponentially from there. >> Right. Of course, combine should be fixed, yadda yadda. >> >>> I really do think that the best solution would be to try and catch this >>> during expand if possible and generate the right pattern from the start; >>> then you don't risk combine failing to come to the rescue after several >>> intermediate transformations have taken place. >> I think ssa-phiopt should simply not do this obfuscation at all. Without >> it, RTL ifcvt picks it up just fine on targets with conditional assignment >> instructions. I agree on targets without expand should do a better job >> (also for more generic conditional assignment). > > That particular transformation was added to tree-ssa-phiopt.c for PR > 45685, the problem it was trying to solve was a missed vectorisation > opportunity and transforming it made it into straightline code that was > more amenable to vectorisation, that's why I'm somewhat reluctant to > completely disable it. > > Hmm... I noticed in the midend we guard some optimisations on > HAVE_conditional_move. Maybe we can guard this one on something like > !HAVE_conditional_negation ? >
Can't we just guard it on HAVE_conditional_move? With such an instruction expand would then generate t1 = -a r = <cond> ? b : t1 and combine will do the rest. R. > Kyrill > >> >> Instruction selection belongs in RTL land. >> >> >> Segher >> > >