https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80197
Bug ID: 80197
Summary: pgo dramatically pessimizes scimark2 MonteCarlo
benchmark
Product: gcc
Version: 7.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vincenzo.innocente at cern dot ch
Target Milestone: ---
Created attachment 41053
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41053&action=edit
self contained benchmark of scimark2 MC
while chasing the regression I then found identified and solved in #79389
I discovered that pgo manages to do much worse than the regression above.
The symptom is the same: a huge increase in branch-miss.
This is not a regression: it is the same at least since gcc5.3
Attached a self contained single file, copy of scimark2 MC, and a couple of
scripts to compile and run it
just
tar -xzf fullMC.tgz
cd fullMC
# standard compilation -O2 -O3
./runit
# same with pgo passes
./dopgo
or just do
[innocent@vinavx3 fullMC]$ rm -rf pgo/* ; c++ -O3 fullMC.c -g
-fprofile-generate=pgo ; time ./a.out
1.848u 0.000s 0:01.85 99.4% 0+0k 0+8io 0pf+0w
[innocent@vinavx3 fullMC]$ c++ -O3 fullMC.c -g -fprofile-use=./pgo ; time
./a.out
0.967u 0.001s 0:00.96 100.0% 0+0k 0+0io 0pf+0w
[innocent@vinavx3 fullMC]$ c++ -O3 fullMC.c -g; time ./a.out
0.328u 0.000s 0:00.32 100.0% 0+0k 0+0io 0pf+0w
for reference:
cat dopgo
cat /proc/cpuinfo | grep name | head -n 1
gcc -v
rm -rf pgo/*;gcc -O2 fullMC.c -g -fprofile-generate=pgo; ./a.out
gcc -O2 fullMC.c -g -fprofile-use=pgo; ./a.out
perf stat -e task-clock -e cycles -e instructions -e branches -e branch-misses
./a.out
rm -rf pgo/*;gcc -O3 fullMC.c -g -fprofile-generate=pgo; ./a.out
gcc -O3 fullMC.c -g -fprofile-use=pgo; ./a.out
perf stat -e task-clock -e cycles -e instructions -e branches -e branch-misses
./a.out
on my machine the result is
# standard compilation
[innocent@vinavx3 fullMC]$ ./runit
model name : Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/afs/cern.ch/work/i/innocent/public/w5/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk//configure
--prefix=/afs/cern.ch/user/i/innocent/w5 -enable-languages=c,c++,lto,fortran
--enable-lto -enable-libitm -disable-multilib
Thread model: posix
gcc version 7.0.1 20170326 (experimental) [trunk revision 246482] (GCC)
gcc -O2 fullMC.c -g
real 0m0.489s
user 0m0.485s
sys 0m0.002s
Performance counter stats for './a.out':
486.303424 task-clock (msec) # 0.999 CPUs utilized
1901271534 cycles # 3.910 GHz
6403589598 instructions # 3.37 insn per cycle
700683389 branches # 1440.836 M/sec
13582 branch-misses # 0.00% of all branches
0.486571089 seconds time elapsed
gcc -O3 fullMC.c -g
real 0m0.330s
user 0m0.330s
sys 0m0.000s
Performance counter stats for './a.out':
327.385696 task-clock (msec) # 0.999 CPUs utilized
1279958668 cycles # 3.910 GHz
5009002909 instructions # 3.91 insn per cycle
306481761 branches # 936.149 M/sec
10805 branch-misses # 0.00% of all branches
0.327637485 seconds time elapsed
// pro generation and use (perf after use...)
[innocent@vinavx3 fullMC]$ ./dopgo
model name : Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/afs/cern.ch/work/i/innocent/public/w5/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk//configure
--prefix=/afs/cern.ch/user/i/innocent/w5 -enable-languages=c,c++,lto,fortran
--enable-lto -enable-libitm -disable-multilib
Thread model: posix
gcc version 7.0.1 20170326 (experimental) [trunk revision 246482] (GCC)
Performance counter stats for './a.out':
964.399833 task-clock (msec) # 1.000 CPUs utilized
3770455888 cycles # 3.910 GHz
5007987488 instructions # 1.33 insn per cycle
816525627 branches # 846.667 M/sec
88982233 branch-misses # 10.90% of all branches
0.964699603 seconds time elapsed
Performance counter stats for './a.out':
964.540691 task-clock (msec) # 1.000 CPUs utilized
3771010753 cycles # 3.910 GHz
5007957589 instructions # 1.33 insn per cycle
816522043 branches # 846.540 M/sec
88992086 branch-misses # 10.90% of all branches
0.964758684 seconds time elapsed
