[Insight-developers] Empty FixedArray destructor: Performance hit using gcc (times 2)

[Luis Ibanez]

Hi Gert,

Thanks for the quick report !

It makes sense that -g flag will prevent the method
from being optimized away.

If you have a chance,
could you please test what happens when no -g is
used, and the optimization flag is set to -O3 ?

Those will be the flags that we set when
CMAKE_BUILD_TYPE is set to "Release".


   Thanks for any feedback,


      Luis


-------------------
Gert Wollny wrote:
>>Here is a suggested skeleton for the test:
>>
>>---------------------------------------------
>>#include "itkFixedArray.h"
>>#include "itkTimeProbesCollectorBase.h"
>>
>>int main()
>>{
>>
>>typedef itk::FixedArray< double, 2 >   ArrayType;
>>
>>ArrayType foo[10];
>>ArrayType * p = foo;
>>ArrayType * q = foo;
>>p++;
>>
>>char * cp = (char*)( p );
>>char * cq = (char*)( q );
>>
>>std::cout << "Type size  =" << sizeof( ArrayType ) << std::endl;
>>std::cout << "Pointer step = " << int( cp - cq ) << std::endl;
>>
>>itk::TimeProbesCollectorBase chronometer;
>>
>>chronometer.Start("FixedArray");
>>for(unsigned long t=0; t<1000000L; t++)
>>   {
>>   ArrayType foo;
>>   foo[0] = t;
>>   }
>>chronometer.Stop("FixedArray");
>>
>>chronometer.Report(std::cout);
>>
>>return 0;
>>}
>>
>>---------------------------------------------
>>
> 
> I've did a test with g++  4.2.3 (Ubuntu)
> using compile flags "-g -O2". If the destructor is implemented, it is called (of course), 
> if there is no explicit implementation of the destructor, the call to the destructor is optimized away resulting in a ~30% speedup of above loop. 
> In both cases alignment is 16.
> 
> Best
> 
> Gert
> 
> 
>