[Bug target/84743] New: default widths for parallel reassociation now hurt rather than help

[acsawdey at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84743

            Bug ID: 84743
           Summary: default widths for parallel reassociation now hurt
                    rather than help
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: acsawdey at gcc dot gnu.org
          Reporter: acsawdey at gcc dot gnu.org
  Target Milestone: ---
            Target: powerpc64le power8

The width settings in rs6000_reassociation_width() were chosen to help
performance for SPEC CPU in gcc 7.

Testing on power8 shows that in gcc 8 (258101) there are now major degradations
in CPU2017 int with the default reassociation widths as compared to using
--param tree-reassoc-width=1 to disable reassociation.

Benchmark               
500.perlbench_r         -5.98%
502.gcc_r               -1.16%
505.mcf_r               -12.44%
520.omnetpp_r           -39.00%
523.xalancbmk_r         -9.78%
525.x264_r              -1.76%
531.deepsjeng_r         -4.23%
548.exchange2_r         -0.66%
557.xz_r                -2.04%