[Bug ipa/84737] New: 20% degradation in CPU2000 172.mgrid starting with r256888

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84737

            Bug ID: 84737
           Summary: 20% degradation in CPU2000 172.mgrid starting with
                    r256888
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pthaugen at gcc dot gnu.org
                CC: dje at gcc dot gnu.org, marxin at gcc dot gnu.org,
                    segher at gcc dot gnu.org, wschmidt at gcc dot gnu.org
  Target Milestone: ---
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu
             Build: powerpc64-unknown-linux-gnu

I'm seeing a 20% degradation on 172.mgrid with r256888. Benchmark was built
with "-O3 -mcpu=power7 -ffast-math". Profiling shows the difference comes from
function resid() and its clone.

r256887
-------
Counted PM_RUN_CYC events (Run_cycles.) with a unit mask of 0x00 (No unit mask)
count 100000
samples  %        image name               symbol name
658215   48.2563  mgrid_base.pat_test_64   .resid_
367381   26.9341  mgrid_base.pat_test_64   .psinv_
153587   11.2601  mgrid_base.pat_test_64   .interp_
109785    8.0488  mgrid_base.pat_test_64   .rprj3_
52642     3.8594  mgrid_base.pat_test_64   .comm3_
7912      0.5801  mgrid_base.pat_test_64   .MAIN__
3796      0.2783  libc-2.17.so             .__memset_power8



r256888
-------
Counted PM_RUN_CYC events (Run_cycles.) with a unit mask of 0x00 (No unit mask)
count 100000
samples  %        image name               symbol name
1109100  59.2023  mgrid_base.gcc_hunt_64   .resid_.constprop.1
368930   19.6930  mgrid_base.gcc_hunt_64   .psinv_
160102    8.5460  mgrid_base.gcc_hunt_64   .interp_
114954    6.1361  mgrid_base.gcc_hunt_64   .MAIN__
55253     2.9493  mgrid_base.gcc_hunt_64   .comm3_
46903     2.5036  mgrid_base.gcc_hunt_64   .resid_
5103      0.2724  libc-2.17.so             .__memset_power8