Paul Brook wrote:
It is actually the FGX processor which would be of interest to us. I
have wondered if it would be possible to have a graphics processor based
on multiple SIMD processors from standard MPUs. I was thinking of the
AltiVec; however, the SPARC is available free.
This has been disucussed here several times before. It isn't a viable option
for two reasons:
a) Fixed function hardware is relatively easy to pipeline, and is smaller so
we can fit more units in the same area. Variable function hardware (aka
shaders) is more complicated and you generally need much higher clock and
shorter pipelines, which aren't feasible to implement on current FPGAs.
I have to admit that I am in the dark as to exactly what we are planning
to implement, is there someplace where it is explained?
The UltraSparc can be implemented on a Xiliix FPGA -- have you seen:
http://www.opensparc.net/
I guess that I could just let you and Nicolas Boulay fight it out since
he says that fixed use hardware is a bad idea. But, I would have to
take the orthodox engineering position that it is an optimization
problem that I don't pretend to know the answer to. Fixed function
hardware can only perform a fixed function which suggests that you might
need more of of it -- even to the point of having 4-word vector
processors limits what can be done with the hardware. OTOH, to the
extent that hardware is general purpose, you can do more things with it
which suggests that you would need less of it. To me, having several
4-word processors is a reasonable compromise. I don't see how the clock
rate and pipeline length (which are usually antithetical) are relevant
here except that faster clocks and shorter pipes are always nice.
As far as area is concerned, it appears to me that most of the hardware
area is going to be consumed by the hardware multiplier arrays. So,
other minor differences would be minor. This also means that you should
avoid making fixed use of a multiplier array if that meant that it would
stand idle part of the time.
b) 3D rendering is a very specialised task. Using a general purpose core adds
lots of overhead for things that we just don't need. Even a relatively slow
special-purpose FPGA can get much higher thoughput and better
performance/watt than the fastest general purpose CPU.
Perhaps you missed this. The VIS and AltiVec units are NOT general
purpose cores. They are 4-word vector processors designed for 3D graphics.
http://www.sun.com/processors/vis/
--
JRT
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