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

[Bill Lorensen]

Tom,

I am really surprised that C++ does not generate a proper alignment
for double. What flags are you using for your build?

Bill

On Thu, Jun 5, 2008 at 3:49 PM, Tom Vercauteren <tom.vercauteren at m4x.org> wrote:
> Hi,
>
> As shown in the code I sent, the real performance hit does NOT come
> from the destructor itself but from the memory alignment.
>
> The example shows that by using the same vanilla FixedArray class
> (where the destructor is explicitly implemented as empty), the well
> aligned case is faster than the badly aligned case.
>
> What is weird is why the empty destructor leads to bad memory alignment...
>
> What is striking is that if you replace the c array of itk::FixedArray
> by a std::vector<itk::FixedArray>, you don't get any performance hit.
> I guess that STL takes care of aligning things in an optimized manner.
>
> By the way, this also implies that if ITK was using STL vector instead
> of c array in its image class, we wouldn't have stumbled on the issue.
>
> Anyhow, I agree with Karthik and Bill. In this specific case,
> explicitly implementing an empty destructor seems useless.
>
> Tom
>
> On Thu, Jun 5, 2008 at 9:02 PM, Niels Dekker <niels-xtk at xs4all.nl> wrote:
>> Bill Lorensen wrote:
>>>
>>> Here is an FAQ on destructors:
>>> http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=370
>>> They say that an empty destructor is pretty much useless.
>>
>> The destructor of FixedArray<TValueType, VLength> invokes the destructor of
>> TValueType for all of its elements.  So it may not be as empty as it
>> seems...  When the destructor is removed from FixedArray, it will be
>> implicitly defined as an inline function, as the FAQ you're referring to
>> says.  This might give a performance gain, but it might also produce a
>> larger binary.  Things might get worse (or better?) when TValueType itself
>> has also has an inline destructor.
>>
>> In some special cases, the destructor of a class /must/ be implemented in a
>> CXX file, even if it is empty. As is the case discussed at GameDev.Net,
>> "auto_ptr within Pimpl Class",
>> http://www.gamedev.net/community/forums/topic.asp?topic_id=437587
>>
>> Anyway, just my 2 cents.  It might still be okay to remove FixedArray's
>> destructor...
>>
>> Kind regards,
>> --
>> Niels Dekker
>> http://www.xs4all.nl/~nd/dekkerware
>> Scientific programmer at LKEB, Leiden University Medical Center
>>
>