[eigen] AVX integer and half-packet support (was Altivec port status)

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 15.07.2014 10:10, Christoph Hertzberg wrote:
> My change actually exposes that integer vectorization with AVX is
> somewhat undefined. We provide Packet8i but don't implement
> vectorization, since it's only available with AVX2:
> http://manao.inria.fr/CDash/testDetails.php?test=2551040&build=13031

The problem is that the alignment_bit in Matrix::Flags is set if
  (((MaxCols*MaxRows*int(sizeof(Scalar))) % EIGEN_ALIGN_BYTES) == 0))
This should actually be dependent on packet_traits<Scalar>::size not 
EIGEN_ALIGN_BYTES. And actually, if HasHalfPacket==1, we should also 
enable vectorization if the number of elements is a multiple of half 
that size or we unnecessarily lose a lot of performance for small 
objects. I'm aware that this will make things complicated ... :(

Christoph


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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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28359 Bremen

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