https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90128
Bug ID: 90128
Summary: 507.cactuBSSN_r is 9-11% slower at -Ofast and native
march/tuning on Zen CPUs
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jamborm at gcc dot gnu.org
CC: rguenth at gcc dot gnu.org
Blocks: 26163
Target Milestone: ---
Host: x86_64-linux
Target: x86_64-linux
In my own measurements, 507.cactuBSSN_r is about 9.4% slower on an AMD
Zen CPU when compiled with GCC 9 with -Ofast and native march/mtune
than when it si compiled with GCC 8. LNT currently even shows 11.4%
regression: https://lnt.opensuse.org/db_default/v4/SPEC/spec_report/branch
I have done some bisecting and the slowdown happened in three steps.
First, the benchmark slowed by about 2% at some point before r262510
which I have not tracked down yet. Second, it then dived 3% with
r263874 but this seems to be some code-placement issue again because
the assembly of the functions which gained perf samples has not
changed in that revision and perf reported stalled-cycles-frontend
went from 4.58% to 5.02%.
However, the third regression was caused by the immediately following
revision r263875, the difference is 4.5% (7.5% is compared to GCC 8
run-time) while perf reported stalled-cycles-frontend were only 4.05%.
r263872 (good) perf stat and report:
Performance counter stats for 'numactl -C 0 -l specinvoke':
238848.989836 task-clock:u (msec) # 0.999 CPUs utilized
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
92923 page-faults:u # 0.389 K/sec
758195547230 cycles:u # 3.174 GHz
(83.33%)
34727040659 stalled-cycles-frontend:u # 4.58% frontend cycles
idle (83.33%)
15457735869 stalled-cycles-backend:u # 2.04% backend cycles
idle (83.33%)
1225370192228 instructions:u # 1.62 insn per cycle
# 0.03 stalled cycles per
insn (83.33%)
23031544594 branches:u # 96.427 M/sec
(83.34%)
18985096 branch-misses:u # 0.08% of all branches
(83.33%)
239.158442295 seconds time elapsed
# Event count (approx.): 758374775503
#
# Overhead Samples Command Shared Object Symbol
# ........ ......... ............ .................
.........................................
#
40.51% 387505 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_RHS_Body
22.34% 214782 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_Advect_Body
8.42% 80594 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_constraints_Body
7.40% 70897 cactusBSSN_r libm-2.26.so __ieee754_exp_avx
5.77% 55393 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBaseDtLapseShift_Body
4.99% 47952 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBase_Body
2.98% 28573 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_InitRHS_Body
2.44% 23623 cactusBSSN_r cactusBSSN_r_peak MoL_LinearCombination
r263874 (worse) perf stat and report:
Performance counter stats for 'numactl -C 0 -l specinvoke':
244036.523777 task-clock:u (msec) # 0.999 CPUs utilized
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
93013 page-faults:u # 0.381 K/sec
774757677736 cycles:u # 3.175 GHz
(83.33%)
38930288027 stalled-cycles-frontend:u # 5.02% frontend cycles
idle (83.33%)
15508961324 stalled-cycles-backend:u # 2.00% backend cycles
idle (83.34%)
1226167776333 instructions:u # 1.58 insn per cycle
# 0.03 stalled cycles per
insn (83.33%)
23218262947 branches:u # 95.143 M/sec
(83.33%)
18890390 branch-misses:u # 0.08% of all branches
(83.33%)
244.344340731 seconds time elapsed
# Samples: 979K of event 'cycles'
# Event count (approx.): 775138268715
#
# Overhead Samples Command Shared Object Symbol
# ........ ......... ............ .................
.........................................
#
41.43% 404835 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_RHS_Body
22.04% 216520 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_Advect_Body
8.22% 80341 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_constraints_Body
7.26% 71052 cactusBSSN_r libm-2.26.so __ieee754_exp_avx
5.86% 57419 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBaseDtLapseShift_Body
4.89% 48084 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBase_Body
2.92% 28579 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_InitRHS_Body
2.38% 23520 cactusBSSN_r cactusBSSN_r_peak MoL_LinearCombination
r263875 (bad) perf stat and report (note that branch misses grew by 6%):
Performance counter stats for 'numactl -C 0 -l specinvoke':
254984.828108 task-clock:u (msec) # 0.999 CPUs utilized
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
92949 page-faults:u # 0.365 K/sec
809505457529 cycles:u # 3.175 GHz
(83.33%)
32784020923 stalled-cycles-frontend:u # 4.05% frontend cycles
idle (83.33%)
15658463714 stalled-cycles-backend:u # 1.93% backend cycles
idle (83.33%)
1225361873924 instructions:u # 1.51 insn per cycle
# 0.03 stalled cycles per
insn (83.33%)
23461309363 branches:u # 92.011 M/sec
(83.34%)
20152382 branch-misses:u # 0.09% of all branches
(83.33%)
255.313012246 seconds time elapsed
# Event count (approx.): 812138555051
#
# Overhead Samples Command Shared Object Symbol
# ........ ......... ............ .................
.........................................
#
37.54% 384512 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_RHS_Body
27.51% 282987 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_Advect_Body
7.80% 79887 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_constraints_Body
6.86% 70384 cactusBSSN_r libm-2.26.so __ieee754_exp_avx
5.73% 58878 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBaseDtLapseShift_Body
4.66% 47990 cactusBSSN_r cactusBSSN_r_peak
ML_BSSN_convertToADMBase_Body
2.79% 28638 cactusBSSN_r cactusBSSN_r_peak ML_BSSN_InitRHS_Body
2.28% 23615 cactusBSSN_r cactusBSSN_r_peak MoL_LinearCombination
I did the bisecting on a machine with glibc 2.26 but the issue was
detected on one with glibc 2.29.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26163
[Bug 26163] [meta-bug] missed optimization in SPEC (2k17, 2k and 2k6 and 95)
