[Bug tree-optimization/77498] [7 regression] Performance drop after r239414 on spec2000/172mgrid

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2016-09-07
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
   Target Milestone|---                         |7.0
     Ever confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note this revision isn't really related to code hoisting.  It merely allows PRE
to perform simple predictive commoning and more PRE in general.  The commoning
can interfere with sinking (see the adjusted testcase).

For the testcase we apply commoning which increases register pressure.

The pcom pass does a better job (well, it was designed for this).

I suppose this PRE improvement raises the general question (again) whether
we want it to introduce loop-carried dependences at all.  In this case
it trades 18 loads for 18 loop-carried dependences - optimally reg colaesced
and thus "free", maybe reg-reg copies or worst spills (as seen here).

I'll need to think about this (again).