Re: [Boost-users] [Boost.Array] surprising performance issue

From: Bryan Green (bgreen_at_[hidden])
Date: 2007-11-28 18:19:19

KSpam writes:
> > I've discovered a situation where using boost::array results in a
> > 2-fold performance degradation over other array types, including C arrays,
> > tr1::array, and a simple handwritten array class.
>
> In your build environment, ensure that you ARE NOT defining _DEBUG, and that 
> you ARE defining NDEBUG.  These flags can make a significant difference in 
> run-time performance.

Setting NDEBUG took care of it.  Thanks.  :)

> > While the tr1::array performs well, I was suprised to find that it pads the
> > array, so sizeof(tr1::array<3,float>) == sizeof(tr1::array<4,float>).  Is
> > that a requirement of tr1::array?
> 
> It is very strange that you see a performance difference between tr1::array 
> and boost::array.  I think that boost::array was used as a submission for 
> tr1::array (they are likely one and the same).

The tr1::array does not do range checking or an assert in operator[].

As for the alignment, this explains it:

from gcc tr1/array:

      value_type _M_instance[_Nm ? _Nm : 1] __attribute__((__aligned__));

This is in the implementation of tr1::array, for gcc's standard C++ library,
which is used by Intel C++.

I sure would like to know the rational for putting in the alignment
attribute, as it changes the code in a very non-trivial way.

-bgreen