RE: VC.NET 'Optimization'

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. 

Alternatively you can use #pragma optimize to control exactly which code
does and doesn’t get optimised. 

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: [email protected] 
> [mailto:[email protected]] On 
> Behalf Of yogiwp
> Sent: 11 April 2005 14:47
> To: [email protected]
> Subject: [GD-Windows] VC.NET 'Optimization'
> 
> Hi all,
> 
> I've just waste alot of time debugging, and surprised that 
> the root of problem come from VC.NET 2003 reordering stuff 
> too aggresively.
> 
> This piece of code is supposed to measure time spent in fibonacci():
> ----------------------------------------------------
>     timer.Restart();
>     int fib = fibonacci( 42 );
>     float msTime = timer.GetElapsed();
>     printf( "time=%f, result=%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.
> 
> Of course the bug does not manifest in debug builds, which 
> made my head hurt even more :(
> 
> The fix is simply changing the last line to:
>     printf( "time=%f, result=%d\n", timer.GetElapsed(), fib );
> 
> Anyone got bitten by this before? This is a bug, right?
> 
> 
> Yogi Wahyu Prasidha
> 
> 
> 
> 

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