undetected gprof counter overflow

[Martien de Jong]

Hi,

I am using gprof 2.15.92.0.2

If the entry count of a function has overflowed, I get *very* strange
results without being notified that something is wrong. For instance,
the total cumulative time from flat profile is 2161.29 s, and the graph
profile states:

granularity: each sample hit covers 4 byte(s) for 0.00% of 9803.48 seconds

My theory is this:
Suppose I have a small leaf function f, that is called by tens of
functions, each with a non-overflowing call count. f itself has an entry
count that overflows. I suspect that gprof computes the total time of
f by something like

time(f) =  sum over callers:c {callcount(c) / entrycount(f)} * flattime(f)

Somehow gprof fails to notice that the sum of the callcounts exceeds the
entrycount of f, and comes (in my specific case) with a time(f) that is
more than flattime(f) by a factor of 120 (yes, I have loads
of callers to f). Since this is then accounted for in the importance of
all callers of f, I get profile which is complete nonsense.

In my opinion, time(f) <= flattime(f), always. Moreover, by detecting
violation of this, it should frequently be possible to reconstruct the
(incorrect by overflow) entry count of f, as the total sum of profiled
calls to f. For a fully profiled program this is accurate provided that
the call counts have not overflowed. Even if they have, the results will
make more sense than without correction.

BTW I usually only profile program runs that take long (because they get
noticed). That makes counter overflow be the default case, rather than
the exception.  I appreciate that it is hard to have an efficient mcount
implementation that guards against overflow, but the offline tool should
try very hard to report detectable inconsistencies.

Kind regards,
Martien

--
Martien de Jong, Senior Software Engineer, Associated Compiler Experts bv,
De Ruyterkade 113, 1011 AB  Amsterdam, The Netherlands.
Tel: +31 20 6646416, Fax: +31 20 6750389,
mailto:address@hidden, http://www.ace.nl.
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