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From: "grosser at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/58028] [4.9 Regression] Several failures in libgomp.graphite after revision 200946
Date: Wed, 29 Jan 2014 12:52:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58028

--- Comment #9 from Tobias Grosser <grosser at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #8)
> On Wed, 29 Jan 2014, dominiq at lps dot ens.fr wrote:
> 
> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58028
> > 
> > --- Comment #7 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> > > The testsuite failures are.  We have to do sth about them.
> > 
> > Revert r200946?
> 
> Probably - the
> 
> +       * graphite-dependences.c (carries_deps): Do not assume the 
> schedule is
> +         in 2D + 1 form.
> 
> part looks wrong.
> 
> -  idx = 2 * depth + 1;
> -  for (i = 0; i < idx; i++)
> +  for (i = 0; i < depth - 1; i++)
> 
> we now iterate over less dimensions than before.

Only in cases where the schedule is not in 2D + 1 form. In case the schedule
is in 2D + 1 form 

int scheduling_dim = isl_set_n_dim (domain);

will be equal to 2 * depth + 1.

> I'd say we should simply check whether the loop _is_ in 2D + 1 form
> at
> 
> +  isl_set *domain = isl_set_from_cloog_domain (stmt->domain);
> +  int scheduling_dim = isl_set_n_dim (domain);
> +
>    if (flag_loop_parallelize_all
> -      && loop_is_parallel_p (loop, bb_pbb_mapping, level))
> +      && loop_is_parallel_p (loop, bb_pbb_mapping, scheduling_dim))
>      loop->can_be_parallel = true;
>  
> thus
> 
>    if (flag_loop_parallelize_all
>        && scheduling_dim == 2 * level + 1
>        && loop_is_parallel_p (loop, bb_pbb_mapping, level)
>      loop->can_be_parallel = true;

The change you propose seems conservatively correct, as in that loops that
are not in 2D + 1 form are not detected to be parallel. This change may hide
the bug, but I don't see any bug it solves.

> no time to check whether reverting the other hunk plus this will
> resolve the bug the revision fixed and restores the testcases.

I assume it will fix the crash, but it will not detect parallellism in none
2D+1 loops, something the isl scheduler happily creates.