Re: [eigen] Status of AVX support

On Sat, Dec 10, 2011 at 18:58, Rohit Garg <rpg.314@xxxxxxxxx> wrote:

On Wed, Dec 7, 2011 at 11:31 AM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
> 2011/12/7 Rhys Ulerich <rhys.ulerich@xxxxxxxxx>:
>>> W.r.t porting to AVX: Be aware that there might be some pitfalls with
>>> AVX-performance:
>>> http://www.agner.org/optimize/blog/read.php?i=142
>>
>> Interesting tidbit from that link "If the programmer inadvertently
>> mixes AVX and non-AVX vector instructions in the same code then there
>> is a penalty of 70 clock cycles for each transition between the two
>> forms."
>
> Between this, and the fact that we can't 32-byte-align Vector4d
> without breaking the ABI, I'm starting to wonder if maybe we should
> treat AVX as a dynamic-size-only thing and completely give up on AVX
> for fixed-size objects? For dynamic-size objects, the situation is
> much simpler, we can increase the alignment without breaking the ABI
> and we can assume that objects are large so that AVX is always better
> than SSE.

That is a good idea. The fixed size objects would be very small
anyway, so not using AVX wouldn't hurt much.

Would it be any easier for implementing AVX for just the dynamic objects?

>
> In any case, I think we should start by doing AVX for dynamic-size
> objects only, it will be time to think about fixed-size later.
>
> Benoit
>
>>
>> Thank you for the pointer to the blog,
>> Rhys
>>
>>
>
>

--
Rohit Garg

http://rpg-314.blogspot.com/

Graduate Student
Applied and Engineering Physics
Cornell University