------- Additional Comments From nickc at redhat dot com 2006-06-22 14:03
-------
Subject: Re: New: Strange profiling results
Hi Dmitry,
> void f (int n)
> {
> rdtscll (t);
> void g (int n)
> {
> gettimeofday (&tv, 0);
> int main (int argc, char *argv[])
> {
> int n;
>
> for (n = 0; n < 200; n++) {
> if (n % 2)
> f (n);
> else
> g (n);
> My typical results are:
>
> $ gcc -O2 -g -pg -o test test.c
> $ gprof test | head -n 10
> Flat profile:
>
> Each sample counts as 0.01 seconds.
> % cumulative self self total
> time seconds seconds calls ms/call ms/call name
> 95.85 0.40 0.40 100 4.03 4.03 f
> 4.79 0.42 0.02 100 0.20 0.20 g
Have you checked the assembler output of the compiler ? Perhaps it is
being clever. Maybe it knows that all but the calls to gettimeofday are
redundant ?
What about main(), is any time spent in that function ?
Also it appears that the two percentages above add up to more than 100%.
The most likely though is that bad profiling data is being generated.
ie it is not gprof's fault but either gcc's (for not inserting the calls
to the profiling hooks correctly) or else the run time C library's (for
not implementing the profiling hooks correctly). Check you version of
gcc. If it is an old one, then try updating it. If it is a new one,
then try using an older one and see if the problem goes away.
Cheers
Nick
--
http://sourceware.org/bugzilla/show_bug.cgi?id=2776
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