> I wrote a simple Fibonacci and counted the #of instructions (inst_retired)
> using both pin and performance counter.
> As you can see, it seems like perf_counter undercount the #of instructions
> and the result is non-deterministic (sometimes 94730 but sometimes 94729)
> Any reason for this?
Did you dynamically link your code? The dynamic linker under Linux does a
lot of stuff which might not be deterministic.
I compiled your code on an AMD Phenom machine.
Dynamically linked, compiled with gcc 4.4 and -O2 it varied from
86558 to 86561.
But if I statically linked, that is compile with "-static" I got
a very consistent "7749" instructions each time.
If you happen to have valgrind installed, version 3.5 or later, you can
also count instructions using something like:
valgrind --tool=exp-bbv --instr-count-only=yes -- ./fib
although on your fib code the results are too high by a factor of two...
weird. I need to find out why that happens.
One thing to watch out for when using pin... newer versions of Pin (my
notes say pinkit more recent than 29972) change the way that rep-prefixed
string instructions are counted. Before that Pin matches what hardware
does (each rep-prefixed instruction counts as "1") wheras current Pin
counts each rep as a separate instruction. It is quite possible the
overcount you see with Pin is due to this; the valgrind tool I mention
above will tell you how many rep-prefixed instructions were executed in
the code.
One note on your test code... you should probably use the result of the
"fib" calculation, as a printf or such. Otherwise the C compiler could
optimize out the whole routine, as the result is unused.
> main()
> {
> fib(10)
> }
>
>
>
>
Vince
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