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

[Tom Vercauteren]

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
>