Re: Timing code blocks

[Alex Mentis <asmentis@gmail.com>]

On Mar 5, 3:49 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Fri, 5 Mar 2010 00:16:52 -0800 (PST), deadlyhead wrote:
> > On Mar 4, 11:55 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> > wrote:
>
> >> This is an ancient bug, which managed to survive a number of GNAT compiler
> >> versions.
>
> >> As a workaround, add delay 0.0 at the beginning of your program:
>
> >>> ----------------------------------------------------------------------
>
> >>> --  A test of the cost of conditionals
>
> >>> with Ada.Text_IO;  use Ada.Text_IO;
> >>> with Ada.Real_Time; use Ada.Real_Time;
> >>> procedure Conditional_Test is
>
> > Exactly what was needed.  Thank you!
>
> > Do you happen to know if this bug has been fixed in the current
> > development branch? (At home I compile GNAT based on GCC 4.4, but
> > haven't done any real-time tests with it yet.)
>
> AFAIK it is still present in GNAT Pro 6.3.1, which is the latest version of
> GNAT.
>
> > This seems like an
> > unfortunate bug to have lying around in an otherwise respected
> > compiler.
>
> Yes, but in real-life applications it does not show itself, because tasking
> stuff somehow wakes the RTL up. That is probably the reason, I suggest, why
> none of paying customers had yet reported it to AdaCore.
>
> > BTW, my test results with -O3 and -Os, there is no difference in
> > performance between the two loops, and -Os produces code that is about
> > 33% faster than -O3.  With -O0, the second loop is faster by an
> > average of 10%.  I would have thought the extra conditional would have
> > been costlier.
>
> Niklas has posted an excellent comment regarding performance measures. It
> is quite difficult to do time measurements right in presence of -O2/3.
>
> As a small addition, here is a technique I am using to subtract looping
> overhead:
>
> T := Clock;
> for I in 1..N loop
>    ... -- measured stuff
> end loop;
> D1 := Clock - T;
>
> T := Clock;
> for I in 1..N loop
>    ... -- measured stuff
>    ... -- measured stuff (do it twice)
> end loop;
> D2 := Clock - T;
>
> (D2 - D1) / N is the average time spent on measured stuff without the
> overhead caused by looping. Important, as Niklas has pointed out, to fool
> the compiler so, that it will not optimize out the things you are
> measuring...
>
> --
> Regards,
> Dmitry A. Kazakovhttp://www.dmitry-kazakov.de

Confirmed: the bug is still present in GNAT GPL 2009.  The same code,
timed using Ada.Calendar seems to work just fine, though.
Interesting, and good to know about.