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Re: GCC performance regression - up to 20% ?
>
> Then I tried to figure out where the slowdown is, using gprof. And
> this is where things get really interesting: gprof tells me that the
> code compiled with 3.1 is faster, but 'time' tells me that the user
> time spent executing that code is higher with 3.1 than with 2.95. I'm
> not sure what to make of this, but I think this might give you some
> clues, so I'll describe it in more detail. I'm not sure what the
> overhead is, but it seems to be right in gprof's blind spot.
gprof "measures" the time that a function takes by probing
which function the program is executing about every 20 ms.
>From that it builds up a statistical histogram.
I wish there would be a more precise profiler that uses the
hardware counters. Does anyone know of one? Hmm, I remember
a Subject: line on the PAPI mailinglist that mentioned gprof,
but I deleted it. I think it asked the same question: whether
or not there existed a 'gprof' that used PAPI.
--
Carlo Wood <carlo@alinoe.com>