[Bug tree-optimization/50904] [4.7 regression] pessimization when -fno-protect-parens is enabled by -Ofast

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904

--- Comment #18 from rguenther at suse dot de <rguenther at suse dot de> 2011-12-01 08:51:07 UTC ---
On Sat, 12 Nov 2011, ebotcazou at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50904
> 
> Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Status|ASSIGNED                    |NEW
>                  CC|                            |rguenth at gcc dot gnu.org
>           Component|rtl-optimization            |tree-optimization
>          AssignedTo|ebotcazou at gcc dot        |unassigned at gcc dot
>                    |gnu.org                     |gnu.org
> 
> --- Comment #15 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-11-12 16:45:13 UTC ---
> > I don't see why RTL invariant motion should move the one variant but not
> > the other.  Of course this also shows that we should, after loop unrolling
> > on the tree level, also perform loop invariant motion again ...
> 
> AFAICS we already do that (lim3 is run after cunroll).  The problem is that
> lim3 considers that the loads cannot be hoisted because they are "dependent". 
> And it looks like a ccp pass is missing after cunroll as there is a lot of
> cruft...

lim3 was added as a "hack", now yes, cunroll needs ccp after it (but it's
there in the form of DOM and VRP).  It's a pass ordering issue that we
cannot ever solve.

> Recategorizing, at least temporarily, for further investigation.  If nothing
> can be done at the Tree level, we could consider applying the RTL patch.

Please - it seems like a missed optimization there, too.

Richard.