'branches' perf event mapping differs on ARC and ARM

[Eugeniy Paltsev]

Hi,

While playing with perf tool on ARMv7 and ARCv2 processors and profiling the
same application I got interesting results. Even if we got pretty
similar total
execution time and instructions number the number of branches on ARC is about
three times more then on ARM.

I dug into architecture
specific perf sources and found that we map different
HW counters into generic 'branches' event on ARC and ARM.
- We use "ijmp" event on ARC which
counts all jump and branch instructions (regardless
of real execution flow - even if no real jump happens)
- We use "pc_write_retired" event on ARM
which counts only taken branches (Instruction
architecturally executed, condition check pass - software change of the PC)

So I was wondering do you know
which approach is correct?



I guess counting all jump and branch instructions is correct because we use
'branches' event value to calculate relative value of 'branch-misses' using

following formula:
----------------------------8----------------------------
branch-misses-ration = 'branch-misses' / 'branches' * 100.0
----------------
------------8----------------------------
And using only taken branches here is incorrect IMHO. So I guess we should
map 'br_immed_retired' instead of
"pc_write_retired" into generic 'branches'
event on ARM.
-- 
 Eugeniy Paltsev