[Bug tree-optimization/38998] New: Missed full redundancies during PRE

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

extern double cos(double);
void test2(double x, double y)
{
 if (cos(y<10 ? x : -y) != cos(y<10 ? x : y))
     link_error ();
}

PRE figures out new full redundancies because it phi-translates.  This leads
to missed optimizations wrt the SCCVN value-numbers because we do not iterate
with the new redundancy.  The immediate effect is that PRE inserts a
duplicate PHI:

<bb 4>:
  # x_2 = PHI <x_5(D)(3), y_3(D)(7)>
  # D.1601_16 = PHI <D.1601_8(3), D.1601_9(7)>
  # prephitmp.13_15 = PHI <D.1601_8(3), D.1601_9(7)>

which only DOM removes later.  The second-order missed optimization is
that elimination does not optimize the following predicate

  D.1606_12 = prephitmp.13_15;
  if (D.1601_16 != D.1606_12)

See also

http://gcc.gnu.org/ml/gcc-patches/2008-08/msg01545.html

which added a broken patch that caused PR38926.


-- 
           Summary: Missed full redundancies during PRE
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38998