[Bug tree-optimization/57051] [4.8/4.9 Regression] Optimization regression in 4.8.0 from 4.7.2

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57051

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |4.7.3
            Version|unknown                     |4.8.0
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2013-04-24
                 CC|                            |glisse at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever Confirmed|0                           |1
            Summary|Optimization regression in  |[4.8/4.9 Regression]
                   |4.8.0 from 4.7.2            |Optimization regression in
                   |                            |4.8.0 from 4.7.2
   Target Milestone|---                         |4.8.1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> 2013-04-24 08:30:06 UTC ---
It seems scev_cprop doesn't do its job here.

(get_scalar_evolution
  (scalar = total.1_8)
  (scalar_evolution = {pretmp_14, +, _6 + 10}_1))

that's ok - SCCP doesn't provide any info in dumps, but I suppose it
fails to compute the number of iterations here(!?).

And we don't constant-fold the vectorized code:

  pretmp_14 = total;
  vect_total.18_44 = { 40, 60, 80, 100 } v>> 64;
  vect_total.18_45 = vect_total.18_44 + { 40, 60, 80, 100 };
  vect_total.18_46 = vect_total.18_45 v>> 32;
  vect_total.18_47 = vect_total.18_46 + vect_total.18_45;
  stmp_total.17_48 = BIT_FIELD_REF <vect_total.18_47, 32, 0>;
  vect_total.19_49 = stmp_total.17_48 + pretmp_14;
  total.1_9 = vect_total.19_49 + 80;
  total.1_20 = vect_total.19_49 + 170;
  total = total.1_20;

because we do not constant-fold vector shifts.  Two ways to attack the
problem (SCCP failure needs to be analyzed).  In 4.7 SCCP works.