Re: [eigen] New true array class ?

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/1/27 Boris Mansencal <boris.mansencal@xxxxxxxx>:
> Quoting Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>
>> this is because this was not implemented. done now for sin, cos, exp,
>> log, abs, and sqrt. feel free to send a patch to support more (I've no
>> time right now)
>
> Thanks a lot. Perfect.
>
> One more question, the attached test try to compare std::valarray &
> Eigen::Array for a simple computation.
> Eigen::Array is much slower than std::valrray.
>
> It seems that Array copy constructor is called too many times (and
> posix_memalign with it).
>
> Do you have any idea from where it comes from ?
> Is it because of missed inlining ?

Wow, indeed, there's a problem. While the valarray version is
perfectly vectorized and without any temporaries, the Eigen version
has a lot of posix_memalign() and free() going on in the loop.

A small improvement for both valarray and Eigen is to use the v4 that
you constructed instead of declaring a local v4 inside of the for
loop. So:

    for (size_t i=0; i<NB_REPEATS; ++i) {
      v4 = v1 * M_PI * (v2 * 0.321) * (0.5 * v2);
      v3 += v4;
    }

valarray is still easily 4x faster than eigen.

Definitely a big issue! My best guess is that a bug is causing Eigen
to evaluate each product into a temporary.

Benoit