[Bug tree-optimization/50969] New: 17% degradation in 168.wupwise for interleave via permutation

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50969

             Bug #: 50969
           Summary: 17% degradation in 168.wupwise for interleave via
                    permutation
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pthaugen@gcc.gnu.org
                CC: bergner@gcc.gnu.org, rth@gcc.gnu.org
              Host: powerpc64-linux
            Target: powerpc64-linux
             Build: powerpc64-linux


Created attachment 25694
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25694
benchmark file

Revision 180450 (along with 180567 to fix the ICE) causes a large degradation
in cpu2000 benchmark wupwise. Additional loops are now being vectorized but
result in worse performance, not sure it that means a cost issue or what. Based
on prior observations the degradation is most likely due to the permute
instructions being used which are restricted to a single VSU pipe, so two of
them can't be executed in parallel.

Attatched file zaxpy.f is just one of the files containing a function that
degraded (zscal.f is another). The second loop is where the time is spent in
the function. Following degradations (compared to revision 180449) were
observed with oprofile.

-m64 -O3 -mcpu=power7
zaxpy : -24%
zscal : -79%

-m64 -O3 -mcpu=power7 -funroll-loops
zaxpy : -65%
zscal : -61%