Vector API benefits for small matrices/vectors?

[Mark Raynsford]

Hello!

On 2021-12-17T17:08:07 +0000
Paul Sandoz <paul.sandoz at oracle.com> wrote:

> Hi Mark,
> 
> I think your use-case is very relevant to see if the Vector API can be used effectively.
> 
> In the Matrix4x4D example you mention I think a different layout or access will be required, namely double[] or ByteBuffer holding the 16 elements rather than 16 independent fields, so the values can be loaded into vectors. 

Yeah, I thought that might be the case. I've avoided a backing array up
until now precisely to avoid that extra pointer. In terms of API,
there's also the issue that I'd need to expose the backing array in
order to operate on it using the vector API, but arrays are obviously
mutable and so users shouldn't have access to it. I think the first
step will be to try writing some versions of all of these functions
that just mindlessly move the 16 fields to/from a local array in each
static method. This will at least tell me if the API *could* be faster
even in the presence of all of that extra moving around of data.

> You might wanna start assuming say AVX2 and a vector size of 256-bits, then after see if it can be generalized. Writing shape (as in vector bit-size) independent code can be tricker, so that experience would be very useful as a further experiment.
> 
> For, say Matrix4x4D multiplication, it will of require that C2 inline the multiply. We don’t yet have vector calling convention support.

For reference, here's what the heavily warmed up C2 code looks like
right now, without any use of the vector API:

  https://gist.github.com/io7m/27de1e5068f953cba228ac3a4323d7ea

There appear to be 16 scalar multiplications in there, which is at
least better than the 64 multiplications that appear in the original
program text. :)

> Using 17 and the second incubator is a good place to start. It will allow you to slot in an 18-ea or a Panama build if necessary (the API has not changed), since we are making continual improvements to C2. Use-cases are really helpful in that regard.

Right!

-- 
Mark Raynsford | https://www.io7m.com