Re: [eigen] eigen short support: discussion thread

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

2009/8/29 Rohit Garg <rpg.314@xxxxxxxxx>:
> 1) With regard to the __m128i being the common sse type for short
> vectors and int vectors, how about we let just the Packet8s be a
> typedef for a struct/class containing a __m128i. This way, they'll be
> different types.
>
> I want to ask if people here think this would work and if it will be
> cheaper to implement than other ways?

Actually, you raise a very good point. The solution that I proposed
looks like this except that I only proposed to make such a struct wrap
the packet type, not a packet object. I proposed an empty struct. Now
it's tru that when we want to implement e.g. ei_padd, it looks like

packet_type ei_padd(const packet_type&, const packet_type&)

and so if we want " z = ei_padd(x,y) " to work, we need packet_type to
actually contain an object, not just a typedef. My solution would have
forced one to pass a "Scalar" template parameter to all the ei_p...
methods taking packets -- since these packet _objects_ still didn't
remember about the scalar type.

So, +1 to your idea. Gael?

Your struct could be called ei_packet. It remains to be seen if
ei_packet_traits can be merged into it, or not.

> 2) What extra needs to be done for the basic short support to be
> merged into eigen mainline? There's a unit test (a simple one) in my
> branch.
>
> http://bitbucket.org/rpg/eigen2-rpg-branch/src/tip/test/short.cpp
>
> Is it enough for the code to be commited? If not, what else is
> missing? (vectorization will obviously take a bit more time)

First, are you sure that this should go into Core? Why not make this a
separate module? It could go directly into Eigen/ skipping
unsupported/ as it's obvious that there are many users interested in
this.

I'd rather have this as a separate module (not a very strong opinion
though) and I don't remember a decision on this being made, perhaps
this is something you already discussed with Gael?

Second, your unit test is not enough. It should at least cover basic
matrix arithmetic. So a good starting point for a unit test is to copy
and adapt large parts of basicstuff.cpp and/or linearstructure.cpp.
When you do vectorization, add also packetmath.cpp. Keep all that
templated, so it can easily be extended to cover more new small
integer types. Just wipe what is not wanted / not applicable for small
integer types.

Do you want write access to eigen2 so you can eventually push
yourself? (nontrivial additions into Core, especially, still need to
be discussed on this list)

Benoit