Intriguing Ideas

Pixilica starts a 3D Open Graphics Alliance initiative; 
We decide to go with a "reconfigurable" pipeline;

# The possibility of a 3D Open Graphics Alliance

At SIGGRAPH 2019 this year there was a very interesting BoF, where the
[idea was put forward]
(https://www.pixilica.com/forum/event/risc-v-graphical-isa-at-siggraph-2019/p-1/dl-5d62b6282dc27100170a4a05)
by Atif, of Pixilica, to use RISC-V as the core
basis of a 3D Embedded flexible GPGPU (hybrid / general purpose GPU). 
Whilst the idea of a GPGPU has been floated before (in particular by
ICubeCorp), the reasons *why* were what particularly caught peoples'
attention at the BoF.

The current 3D GPU designs -  NVIDIA, AMD, Intel, are hugely optimised
for mass volume appeal. Niche markets, by virtue of the profit
opportunities being lower or even negative given the design choices of
the incumbents, are inherently penalised. Not only that but the source
code of the 3D engines is proprietary, meaning that anything outside of
what is dictated by the incumbents is out of the question.

At the BoF, one attendee described how they are implementing *transparent*
shader algorithms. Most shader hardware provides triangle algorithms that
asume a solid surface. Using such hardware for transparent shaders is a
2 pass process which clearly comes with an inherent *100%* performance
penalty. If on the other hand they had some input into a new 3D core,
one that was designed to be flexible...

The level of interest was sufficiently high that Atif is reaching out to
people (including our team) to set up an Open 3D Graphics Alliance. The
basic idea being to have people work together to create an appropriate
efficient "Hybrid CPU/GPU" Instruction Set (ISA) suitable for a diverse
range of architectures and requirements: all the way from small embedded
softcores, to embedded GPUs for use in mobile processors, to HPC servers
to high end Machine Learning and Robotics applications.

One interesting thing that has to be made clear - the lesson from
Nyuzi and Larrabee - is that a good Vector Processor does **not**
automatically make a good 3D GPU. Jeff Bush designed Nyuzi very
specifically to replicate the Larrabee team's work: in particular, their
use of a recursive software-based tiling algorithm.  By deliberately
not including custom 3D Hardware Accelerated Opcodes, Nyuzi has only
25% the performance of a modern GPU consuming the same amount of power.
Put another way: if you want to use a pure Vector Engine to get the same
performance as a commercially-competitive GPU, you need *four times*
the power consumption and four times the silicon area.

Thus we simply cannot use an off-the-shelf Vector extension such as the
upcoming RISC-V Vector Extension, or even SimpleV, and expect to
automatically have a commercially competitive 3D GPU. It takes texture
opcodes, Z-Buffers, pixel conversion, Linear Interpolation, Trascendentals
(sin, cos, exp, log), and much more, all of which has to be designed,
thought through, implemented *and then used behind a suitable API*.

In addition, given that the Alliance is to meet the needs of "unusual"
markets, it is no good creating an ISA that has such a high barrier to
entry and such a power-performance penalty that it inherently excludes 
the very implementors it is targetted at, particularly in Embedded markets.

These are the challenges to be discussed at the upcoming first
[meetup](https://www.meetup.com/Bay-Area-RISC-V-Meetup/events/264231095/)
at Western Digital's Milpitas HQ.

https://youtu.be/HeVz-z4D8os

# Reconfigureable Pipelines

Jacob came up with a fascinating idea: a reconfigureable pipeline. The
basic idea behind pipelines is that combinatorial blocks are separated
by latches.  The reason is because when gates are chained together,
there is a ripple effect which has to have time to stabilise. If the
clock is run too fast, computations no longer have time to become valid.

So the solution is to split the combinatorial blocks into shorter chains,
and have "latches" in between them which capture the intermediary
results. This is termed a "pipeline".  Actually it's more like an
escalator.

The problem comes when you want to vary the clock speed. This is desirable
because if the pipeline is long and the clock rate is slow, the latency
(completion time of an instruction) is also long.

Conversely, if the pipeline is short (large numbers of gates connected
together) then as mentioned above, this can inherently limit the maximum
frequency that the processor could run at.

What if there was a solution which allowed *both* options? What if you
could actually reconfigure tge pipeline to be shorter or longer?

It turns out that by using what is termed "transparent latches" that it
is possible to do precisely that.  The advantages are enormous and were
described in detail on comp.arch

https://groups.google.com/d/msg/comp.arch/fcq-GLQqvas/SY2F9Hd8AQAJ  
Earlier in that thread, someone kindly pointed out that IBM published
papers on the technique.  Basically, the latches normally present in the
pipeline have a combinatorial "bypass" in the form of a Mux. The output
is dynamically selected from either the input *or* the input after it
has been put through a flip-flop. The flip-flop basically stores (and
delays) its input for one clock cycle.

By putting these transparent latches on every other combinatorial stage
in the processing chain, the length of the pipeline may be halved, such
that when the clock rate is also halved the *instruction completion time
remains the same*.

Normally if the processor speed were lowered it would have an adverse
impact on instruction latency.

It's a fantastic idea that will allow us to reconfigure the processor
to reach a 1.5ghz clock rate for high performance bursts.