Re: [Discuss-gnuradio] SOCIS project update 8

[West, Nathan]

On Fri, Jul 24, 2015 at 11:44 AM, Johannes Demel <address@hidden> wrote:

Hey community,

after last weeks success with channel construction, this week is
calmer. It involves a steep learning curve for SIMD.
So I was able to create my first VOLK kernels [3]. There are two new
kernels for 8bit packing and unpacking. In case someone wants to pack
8 bytes with the LSB active into one byte, there's a new VOLK kernel
to do this for you. At first, I thought, this is as simple as doing a
load+movemask operation. Unfortunately, endianness stopped me from
doing so. Thus it involves shuffling and AND, COMPARE operations too.
Without Shuffling it should have worked with SSE2 but since shuffle is
involved SSSE3 is required.
I'm reading through all the docs and websites which target SIMD and
find new ways to do things all the time. So, I guess it is a long way
to go until I have some decent knowledge about SIMD instructions.
Though, I could achieve a 7x speedup for packing bits compared to the
generic implementation.
Also, I created a kernel for unpacking. I wasn't very successful here.
SSSE3 implementation is slower than the generic one for now. Maybe
someone can give me a hint on what is going wrong here.
I named those two new kernels 'volk_8u_pack8_8u' and
'volk_8u_unpack8_8u'. I hope this explains there operation.
Suggestions on alternative names are welcome here.
I tried to integrate my VOLK kernels into VOLKS test framework, but
that is quite tough. It seems like it doesn't expect any rate changing
kernels.

My aim for next week is to come up with a kernel for polar code
encoding. This will include interleaving a lot of bits which is the
actual issue to overcome.

More info and current project progress can be found in [1], [2] and [3].

Cheers
Johannes

[1] https://github.com/jdemel/gnuradio
[2] https://github.com/jdemel/socis-proposal
[3] https://github.com/jdemel/volk

Hi Johannes,

This is pretty neat-- nice work!

You'll probably need to use a puppet. The VOLK QA creates input and output buffers that are itemsize * num_points for every input and output. I think this is fine for the packer, but as you've discovered will not work for the unpacker. A puppet lets you wrap your actual kernel in a way that works nicely with the VOLK QA. In this case I suspect you want something like the following:

volk_8u_unpack8puppet_8u_generic(uchar* out, uchar* in, num_points{

 volk_8u_unpack8_8u_generic(out, in, num_points/8);

}

You'll get 8x as much buffer space and a bunch of inputs that you'll never operate on, which is OK. This obviously isn't critical for your GSoC project, but we'll want to do this at some point since this looks really useful.

Nathan