[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8

[kelvin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448

--- Comment #2 from kelvin at gcc dot gnu.org ---

Using the GCC8 branch, svn version id 262483 (the same version tested by
Michael), I'm getting the following results:

         -mcpu=power8    -mcpu=power9           
                28.57   28.79           
                28.41   28.61           
                28.54   28.21           
                28.53   28.55           
                29.02   28.59           
                28.54   27.34           
                28.25   26.63           
                28.56   29.13           
                                        delta   Percent
 average        28.5525 28.23125        0.32125 1.13%

As with my trunk measurements, I'm not seeing a 2% difference.  Rnd I am seeing
that targeting power9 produces slightly better performance than targeting
power8.

It may be that we're running with different optimization flags.  I used

OPTIMIZE        = -Ofast -mcpu=power9   (or -mcpu=power8)                  
LDOPT           = -m64 -Wl,-q  -Wl,-rpath=%{BASE_DIR}/lib64

I'm inclined to close this issue unless Michael can point me to a different set
of options to explore...