http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043
--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 4 Feb 2014, abel at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60043 > > Andrey Belevantsev <abel at gcc dot gnu.org> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |abel at gcc dot gnu.org > > --- Comment #1 from Andrey Belevantsev <abel at gcc dot gnu.org> --- > I don't follow the reasoning of this example and the original ML thread. The > load of *b follows the store to *a, thus the scheduler is checking for the > presence of the _true_ dependence between them: > > gcc/sched-deps.c: > 2660 if (true_dependence (XEXP (pending_mem, 0), VOIDmode, t) > 2661 && ! sched_insns_conditions_mutex_p (insn, > 2662 XEXP (pending, > 0))) > 2663 note_mem_dep (t, XEXP (pending_mem, 0), XEXP (pending, > 0), > 2664 sched_deps_info->generate_spec_deps > 2665 ? BEGIN_DATA | DEP_TRUE : DEP_TRUE); > > which does not exist because the mems have different alias sets. But you have > agreed that TBAA can be used for true dependences in the ML thread, no? What > is then required from the scheduler? Yes, TBAA can be used for true dependences - but a true dependence is read-after-write. Here we have an anti-dependence, write-after-read. When the scheduler wants to exchange two mems then it needs to use the predicate that is correct _before_ the transform, not after. Thus in the above code it seems that it does not check whether the write from pending[_mem] is before or after the read in 't' - the used predicate needs to change dependent on the order of the insns (or conservatively assume an anti-dependence and thus disable TBAA).
