------- Additional Comments From dmantipov at yandex dot ru 2006-06-22 14:55
-------
(In reply to comment #1)
> 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 ?
Strace shows a lot of gettimeofday() calls during execution of the first test
program above.
Moreover, I've checked an annotated assemler output produced with
'-fverbose-asm' by gcc 4.0.2 and gcc 4.1.1 and found that even with '-O3', the
code is assembled 'as is', i.e.
at least:
- rdtsc and gettimeofday() aren't eliminated
- loops aren't unrolled
> What about main(), is any time spent in that function ?
Flat profile doesn't include data for main(), i.e. it contains f() and g()
only. But the call graph shows something for main() too, for example:
index % time self children called name
<spontaneous>
[1] 100.0 0.00 0.50 main [1]
0.49 0.00 100/100 f [2]
0.01 0.00 100/100 g [3]
-----------------------------------------------
0.49 0.00 100/100 main [1]
[2] 98.0 0.49 0.00 100 f [2]
-----------------------------------------------
0.01 0.00 100/100 main [1]
[3] 2.0 0.01 0.00 100 g [3]
Assembler output says that mcount() is called from main(), so main() should be
accounted too. On the other side, it's possible that main() is just too fast
even for 1 profiling sample (all SIGPROFs was handled when we're in f() or g()
?). I believe this is another question for you - why main() isn't included into
flat profile :-) ?
> Also it appears that the two percentages above add up to more than 100%.
Yes, this is strange.
> 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.
I'll try a few versions of older gcc, starting from gcc 3.2...
--
http://sourceware.org/bugzilla/show_bug.cgi?id=2776
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