[LLVMdev] RFC: R600, a new backend for AMD GPUs

[Tom Stellard]

On Mon, Mar 26, 2012 at 12:50:07PM -0400, Tom Stellard wrote:
> Hi,
> 
> We've been working on an LLVM backend for the previous generation of AMD
> GPUs (HD 2XXX - HD 6XXX) and we would like submit it for inclusion in the
> main LLVM tree.  The latest code can be found in this git repository:
> http://cgit.freedesktop.org/~tstellar/llvm/ in the r600-initial-review
> branch or if you prefer you can download the entire tree with this link:
> http://cgit.freedesktop.org/~tstellar/llvm/snapshot/llvm-r600-initial-review.tar.gz
> The R600 backend is located in lib/Target/AMDIL
> 
> First, a brief description of the backend:
> 
> The r600 backend is being developed as a part of the Open Source compute
> stack in Mesa (http://www.mesa3d.org/), which uses the Gallium API.
> It uses large portions of the AMDIL backend which was open-sourced
> last December and you'll notice the TargetMachine for this backend
> (AMDGPUTargetMachine) is a sub-class of AMDILTargetMachine.  We are also
> currently working on an LLVM backend for Southern Islands GPUs, and we
> would like to get that code into the LLVM tree as well, once it has been
> approved for release.  The Southern Islands backend will be used for
> compute shaders and also graphics shaders in the AMD open source 3D driver.
> 
> One thing that I would like to point out is that all of the
> code from the AMDIL backend is licensed under a BSD license with an
> additional clause that deals with United States export laws (non-AMDIL
> code is licensed with the University of Illinois Open Source License).
> Will the LLVM project accept a backend with code licensed under this BSD
> license?  We would prefer to keep this license, but if it isn't
> acceptable, we can try to relicense it.
> 
> Second, I am looking for two categories of feedback for the r600 backend:
> 
> 1. What changes do I need to make to get the backend included in the LLVM tree.
> 2. What changes can I make to improve the backend overall.
> 
> My top priority is to get the backend into the LLVM tree, so
> when you provide feedback if you could be clear about what
> changes are needed to get the backend into the tree versus what changes
> are just general improvements, I would appreciate it.
> 
> Lastly, I did a very brief run through of the code to check the coding style,
> but I know there are still some violations.  For example, a lot of the
> file headers are missing file descriptions.  I didn't want to spend a
> lot of time on coding style changes prior to the initial review in case
> I was asked to make big changes to the code, so I will address these
> issues once I have received an OK on the organization of the code.
> However, please still point out coding style errors to me, and I'll be
> sure to fix them during the final pass.
> 
> Looking forward to your feedback.
> 
> Thanks,
> Tom Stellard
> 

Has anyone had a chance to review this code yet?  What about the
licensing issue?  Getting this backend included in LLVM is currently
a blocker for the Mesa Open Source compute stack on AMD hardware as
well as the Southern Islands Open Source 3D driver, so I would really
appreciate it if someone could take a look at the code and let me know what
is needed to get it upstream.

Thanks,
Tom Stellard