Re: gprof, ITIMER_PROF

[James E Wilson <wilson at specifixinc dot com>]

Kris Demuynck wrote:
> In the best case, gprof reports the actual timings x10. The reason seems 
> to be that the ITIMER_PROF interval timer is set to 999usec instead of 
> 9999usec.

I don't see how this can be a gcc problem.  Gcc has nothing to do with 
the timer interval used by gprof.

This is perhaps a kernel problem, or perhaps a glibc problem, or perhaps 
a binutils (gprof) problem.  Or this could even perhaps be an 
inconsistency on your system, if you have replaced one of these things, 
but not recompiled the others to match.

>The main problem is not the factor 10 (my math is still that >good), 
but the overflow of the 16 bit counters gprof uses to >derive it 
statistics.

Again, this isn't a gcc problem, as gcc has nothing to do with the 
histogram size.

This stems from a limitation of the profil() system call, which requires 
an unsigned short * parameter.  Thus fixing this means a kernel change 
(and perhaps a new system call to avoid breaking standards conformance), 
a glibc change, and a binutils change.  No gcc changes should be needed. 
 This will probably be difficult to coordinate and accomplish.

Note that the gcc gcov profiler does not have this problem.  It is using 
64-bit (long long) counters.  gcov is implemented entirely within gcc, 
and as a result we are able to control how it works.  This gives you 
line number counts instead of timings.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com