[Insight-developers] assert and explicit instantiations

[Gaëtan Lehmann]

Le 15 juin 09 à 18:14, Luis Ibanez a écrit :

>
> Hi Gaetan,
>
>
> Yes, I see your point.
> Adding more conditions in the operator++() will degrade performance.
>
> Just to clarify here,
> The penalty in performance will come from the extra "if" that
> guards that call to the Exception, not from the actual Exception.
>
> Exceptions themselves will only be thrown when there is an error,
> and in that case, performance is no longer an issue. That is,  
> Exceptions
> will not make part of the normal flow of the code.

I thought like you until I tried to implement the  
ProjectionImageFilter with exceptions to avoid iterating along the  
full line when the result can be know with a few pixels. The results  
where worth with the try catch code.

The attached progsam does the same number of iterations with and  
without try/catch. Note that no exception is thrown.
Here are the results I get, with gcc 4.0.1, Release mode:

exception: 1.3971
if: 0.568379

The result is very stable - I also tried to run swap the order of the  
measures, without relevant changes.

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>
> The effect of the "if" may be not be that significant,
> given that the current code in the operator++() calls Next()
> which is a virtual function.
>
>
>  Self & operator++()
>    {
>    this->Next();
>    return *this;
>    }

Good point. Virtual functions may have a more important impact than  
try/catch blocks.

Gaëtan

>
>
> That said, I fully agree with you in that before taking such
> route we should profile the impact on performance.
>
>
> Going with pure asserts, as you suggest, seems to be a more
> reasonable approach.
>
>
>   Luis
>
>
>
> --------------------
> Gaëtan Lehmann wrote:
>> Le 14 juin 09 à 18:43, Luis Ibanez a écrit :
>>>
>>>
>>> Hi Gaetan,
>>>
>> Hi Luis,
>>> You should probably use the new Macro:
>>>
>>>          itkAssertOrThrowMacro
>>>
>>>
>>> That is defined in
>>>
>>>     Insight/Code/Common/itkMacro.h
>>>
>>>
>>> It will use Asserts when compiled in Debug mode,
>>> and it will throw Exceptions when compiled in
>>> Release mode.
>> Well, we should carefully use that macro, because exceptions can  
>> have  serious impact on performance.
>> In that case, the assert() is in operator++() and Set() methods of  
>> an  iterator.
>>  http://public.kitware.com/cgi-bin/viewcvs.cgi/Code/Common/itkTreeIteratorBase.h?root=Insight&r1=1.12&r2=1.13&sortby=date 
>>    http://public.kitware.com/cgi-bin/viewcvs.cgi/Code/Common/itkTreeIteratorBase.txx?root=Insight&r1=1.14&r2=1.15&sortby=date 
>>  This is the kind of case where I wouldn't replace an assert() by   
>> itkAssertOrThrowMacro(), or at least without carefully measuring  
>> the  execution times.
>> Regards,
>> Gaëtan
>>>
>>>
>>>
>>> Luis
>>>
>>>
>>> -----------------------
>>> Gaëtan Lehmann wrote:
>>>
>>>> Hi Julien,
>>>> I have commented an assert() in
>>>> Code/Common/itkTreeIteratorBase.h
>>>> Code/Common/itkTreeIteratorBase.h
>>>> to fix build with explicit instantiation.
>>>> The compiler was giving that error:
>>>> /export/home/glehmann/src/Insight/Code/Common/  
>>>> itkTreeIteratorBase.h: 182: erreur: ?assert? was not declared in   
>>>> this scope
>>>> Is it the expected behavior?
>>>> Thans,
>>>> Gaëtan
>>>> ------------------------------------------------------------------------
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-- 
Gaëtan Lehmann
Biologie du Développement et de la Reproduction
INRA de Jouy-en-Josas (France)
tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
http://voxel.jouy.inra.fr  http://www.itk.org
http://www.mandriva.org  http://www.bepo.fr

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