cyb70289 commented on issue #6986:
URL: https://github.com/apache/arrow/pull/6986#issuecomment-616978252
This change introduces severe branch misses in certain conditions. See perf
logs below. I changed benchmark code to run only the problematic test case.
Without this patch
```bash
807.415826 task-clock (msec) # 0.979 CPUs utilized
83 context-switches # 0.103 K/sec
0 cpu-migrations # 0.000 K/sec
427 page-faults # 0.529 K/sec
2,285,801,407 cycles # 2.831 GHz
(83.17%)
2,313,785 stalled-cycles-frontend # 0.10% frontend cycles
idle (83.16%)
915,631,177 stalled-cycles-backend # 40.06% backend cycles
idle (82.93%)
9,997,208,858 instructions # 4.37 insn per cycle
# 0.09 stalled cycles
per insn (83.66%)
1,679,799,451 branches # 2080.464 M/sec
(83.66%)
106,599 branch-misses # 0.01% of all branches
(83.41%)
```
With this patch
```bash
902.557236 task-clock (msec) # 0.980 CPUs utilized
94 context-switches # 0.104 K/sec
0 cpu-migrations # 0.000 K/sec
427 page-faults # 0.473 K/sec
2,567,879,767 cycles # 2.845 GHz
(83.17%)
88,266,680 stalled-cycles-frontend # 3.44% frontend cycles
idle (83.17%)
20,826,862 stalled-cycles-backend # 0.81% backend cycles
idle (83.03%)
2,518,949,193 instructions # 0.98 insn per cycle
# 0.04 stalled cycles
per insn (83.62%)
847,459,928 branches # 938.954 M/sec
(83.61%)
75,187,208 branch-misses # 8.87% of all branches
(83.39%)
```
Absolute counts are not comparable as gtest runs different loops for each
test.
The point is branch-misses jumps from 0.01% to 8.87%, which causes high
frontend stall(cpu wait for fetching code to execute), and ipc(instructions per
cycle) drops from 4.37 to 0.98.
I didn't figure out which branch is miss predicted and why. My haswell cpu
is too old to support branch tracing. My guess is [this
line](https://github.com/apache/arrow/blob/5093b809d63ac8db99aec9caa7ad7e723f277c46/cpp/src/arrow/util/bit_util.cc#L285),
no concrete justification.
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