[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448 Bill Schmidt changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |WORKSFORME --- Comment #7 from Bill Schmidt --- Not confirmed at this time. Let's close it until we have something more definitive to look at.
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448 --- Comment #6 from kelvin at gcc dot gnu.org --- I should also clarify regarding all of the above comments that the numbers I have been reporting are the spec ratios. I had misunderstood that these ratios were encoded such that smaller values represented better performance. So some of my "interpretation remarks" are incorrect. Still, my measurements do not show the 2% difference that Michael observed, so there remains a question of whether there is enough of a performance change to merit further exploration.
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448 --- Comment #5 from kelvin at gcc dot gnu.org --- I apologize for an error in the previous comment. The two columns should have been labeled -mcpu=power8 (left) and -mcpu=power9 (right) instead of gcc8 and gcc9.
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448
--- Comment #4 from kelvin at gcc dot gnu.org ---
There are aspects of Michael's recent comment that I may not fully understand.
I checked the source for milc, and it is C, so I added -fgnu89-inline to the
list of OPTIMIZE options. Then I reran my tests with gcc8 (svn version 262483)
on a DD2.2 power9 machine.
OPTIMIZE= -O3 -fpeel-loops -funroll-loops -ftree-vectorize
-fvect-cost-\
model -fno-strict-aliasing -msave-toc-indirect
-mno-pointers-to-nested-function\
s -fno-aggressive-loop-optimizations -ffast-math -mveclibabi=mass
-mrecip=rsqrt\
-fgnu89-inline -mcpu=power9 (vs. -mcpu=power8)
LDOPT = -m64 -Wl,-q -Wl,-rpath=%{BASE_DIR}/lib64
I'm still not seeing the performance degradation Michael saw. Here are my most
recent results:
gcc8gcc9
28.79 28.14
29.01 28.84
28.51 28.5
28.55 28.39
29.02 29.07
29.128.51
delta % delta
average 28.83 28.575 0.255 0.88%
Does anyone see anything I may be doing wrong?
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448
--- Comment #3 from Michael Meissner ---
The options I use for spec are:
-O3 -fpeel-loops -funroll-loops -ftree-vectorize -fvect-cost-model
-msave-toc-indirect -mno-pointers-to-nested-functions
-fno-aggressive-loop-optimizations -ffast-math -mveclibabi=mass -mrecip=rsqrt
-mcpu=power
For C files I use: -fgnu89-inline
For C++ files I use: -std=gnu++98
For Fortran files I use: -fstack-arrays
I use -fno-strict-aliasing on milc (and perlbench) due to it playing pointer
games that earlier compilers would generate the wrong code for. If memory
serves, the -fno-strict-aliasing may not show the bug on power{7,8,9} systems.
I know in the perlbench case, the code in spec violates the ISO C standard. I
don't recall what the milc code is.
I use -fno-aggressive-loop-optimizations because some of the benchmarks as
written go beyond the end of arrays, and GCC over-optimizes these.
I use version 8.1.3 of the MASS library. However, milc is not one of the
benchmarks that heavily use the math library, so you can omit using MASS and
-mveclibabi=mass.
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
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
average28.5525 28.231250.32125 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...
[Bug target/86448] GCC 9 compiler generates slower code for spec 2006 milc on a power9 using -mcpu=power9 than using -mcpu=power8
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86448 kelvin at gcc dot gnu.org changed: What|Removed |Added CC||kelvin at gcc dot gnu.org --- Comment #1 from kelvin at gcc dot gnu.org --- Using trunk on a dedicated DD2.2 power9, I get the following performance comparisons: -mcpu=power8-mcpu=power9 28.92 28.97 28.37 28.99 28.13 28.26 29.06 28.12 28.8 28.23 28.9 28.69 28.37 28.48 28.3 28.08 delta Percent average 28.6062528.4775 0.12875 0.45%
