[lowrisc-dev] Open GPU for the first CPU

Theo Markettos theo.markettos at cl.cam.ac.uk
Fri Feb 13 01:06:14 GMT 2015

ALadar-V <lowrisc-dev at aggregator.eu> wrote: 
> Hi,
> I was thinking along the (GPU) lines you do a few weeks now. One
> question is whether RISC-V ISA is the right ISA for a GPU (I wish so)
> and whether some community of developers could be established to take
> care of development of a GPU. Something along Rocket cores line -
> personally, I was thinking about gradual rewrite of MESA 3d library into
> hardware, accelerating parts of it until it's a full-fledged GPU-V   

This was posted recently to comp.arch.fpga:

Subject: Open Source GPGPU core
From: Jeff Bush

I've been designing an open source Larrabee-esque GPGPU processor in
SystemVerilog and I thought people might find it interesting. Full source
code, documentation, tests, tools, etc. are available on github:


The processor supports a wide, predicated vector floating point pipeline
with 16 lanes and multiple hardware threads to hide memory and computation
latency. It also supports multiple cache coherent cores. I've created an
LLVM backend for this, so C/C++ code can be compiled for it.  It includes
support for first class vector types using the GCC vector extensions, as  
well as a intrinsics to expose specialized instructions. 

I've written a 3D engine (software/librender) that is optimized to take 
advantage both of the vector unit and multiple cores/hardware threads.
Here's a video of the standard teapot (with ~2300 triangles) rendering on a
single core on FPGA running at 50 Mhz: 


This image is the emulator rendering Dabrovik's Sponza atrium, ~66k
triangles. This took around 200 million instructions to render between 8
virtual cores and 32 hardware threads (at 1024x768):


My main purpose of designing this was to be able to experiment with
processor architecture with real, empirical data. The neat thing about
having all the source to a cycle accurate hardware design is that it is
infinitely instrumentable. I've kept notes about some of my findings here:


Anyway, comments and suggestions are appreciated, and I'm happy to take
contributions if people are interested in hacking on it.

It's GPL/LGPL.  I haven't looked in detail, but it has a cache, a compiler
and a testsuite which is a lot more than many open source hardware projects.


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