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From: "pthaugen at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/54497] New: Revision 190015 causes 22% degradation on 172.mgrid on PowerPC
Date: Wed, 05 Sep 2012 21:46:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54497

             Bug #: 54497
           Summary: Revision 190015 causes 22% degradation on 172.mgrid on
                    PowerPC
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pthaugen@gcc.gnu.org
                CC: bergner@gcc.gnu.org, dehao@gcc.gnu.org
              Host: powerpc64-linux
            Target: powerpc64-linux
             Build: powerpc64-linux


Created attachment 28135
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28135
Reduced testcase

Starting with revision 190015 I'm seeing a major degradation on cpu2000
benchmark 172.mgrid. The core loop of the benchmark is in procedure RESID().
The main differences I see in the tree dumps, compared with the prior revision,
which result in the degradation are the following.

1) IPA-CP creates a clone of RESID to be used at certain call sites.

2) Predictive Commoning doesn't optimize the hot loop in the cloned version of
RESID.

The attatched testcase was reduced from mgrid.f and can be compiled with
gfortran -S -m64 -O3 -mcpu=power7 -mno-vsx -fno-inline.

In the original version, resid_, predictive commoning has unrolled the loop by
a factor of 2 so that memory refs can be resued (hot loop has 20 loads and 2
stfd). In the cloned version, resid_.constprop.0, no predictive commoning is
done and we end up with a single copy of the the loop with 32 loads and a
single store.