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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/60043] -fschedule-insns2 breaks anti-dependency
Date: Tue, 04 Feb 2014 09:26:00 -0000
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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).