RE: [GD-Windows] VC.NET 'Optimization'

[Simon O'C. <si...@sc...>]

Simply re-ordering the source without forcing optimisations off isn't
guaranteed to do what you expect. Contrived example: imagine your =
profiling
code calls QueryPerformanceFrequency() and then code being profiled =
later on
makes a call to QueryPerformanceFrequency(); it's feasible that an =
optimiser
could detect two calls to that same Windows function and optimise them =
into
one call thus totally skewing your results.

Personally (when not using an already proven profiler) I'd always =
declare
the timer object and even msTime as "volatile" to ensure the compiler
doesn't consider them for optimization.=20

Alternatively you can use #pragma optimize to control exactly which code
does and doesn=92t get optimised.=20

BTW: I'm not entirely sure whether that's a compiler bug or a valid
optimisation - it depends what your Restart() function does.


> -----Original Message-----
> From: gam...@li...=20
> [mailto:gam...@li...] On=20
> Behalf Of yogiwp
> Sent: 11 April 2005 14:47
> To: gam...@li...
> Subject: [GD-Windows] VC.NET 'Optimization'
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> Hi all,
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> I've just waste alot of time debugging, and surprised that=20
> the root of problem come from VC.NET 2003 reordering stuff=20
> too aggresively.
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> This piece of code is supposed to measure time spent in fibonacci():
> ----------------------------------------------------
>     timer.Restart();
>     int fib =3D fibonacci( 42 );
>     float msTime =3D timer.GetElapsed();
>     printf( "time=3D%f, result=3D%d\n", msTime, fib );
> ----------------------------------------------------
> But the optimizer decided to move the fibonacci() call just before
> printf() and after GetElapsed(), causing the timer to measure nothing.
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> Of course the bug does not manifest in debug builds, which=20
> made my head hurt even more :(
>=20
> The fix is simply changing the last line to:
>     printf( "time=3D%f, result=3D%d\n", timer.GetElapsed(), fib );
>=20
> Anyone got bitten by this before? This is a bug, right?
>=20
>=20
> Yogi Wahyu Prasidha
>=20
>=20
>=20
>=20

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