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

[Tom Stellard]

On Fri, May 25, 2012 at 02:37:26PM -0700, Justin Holewinski wrote:
> Hi Tom,
> 
> I have a higher-level question regarding this back-end.  If I have an LLVM
> IR module and run it through this back-end, it seems like the only output
> option is a binary format.  Is this a device binary, or another
> intermediate format?
> 
> If the input LLVM IR module was a compute kernel, how would I go about
> executing it on an AMD GPU?  Can I use the APP SDK to load the binary,
> perhaps through the CAL interfaces?  How about the OpenCL binary interface?
>

Hi Justin,

The binary produced by this backend is meant to be consumed by AMD's Open
Source 3D/Compute drivers which are part of the Mesa3D[1] project.  The
backend is integrated into the driver, so you don't need to compile
shaders offline.  Currently we are using the backend for graphics and
compute shaders in our r600g driver (HD2xxx-HD6xxx GPUs) and for graphics
in our radeonsi (HD7xxx GPUs).  In the future we will use it for compute
shaders on radensi too.

In order to use the backend for graphics on r600g, you need to build
Mesa with the --enable-r600-llvm-compiler option.  For compute the
installation instructions are here:
http://dri.freedesktop.org/wiki/GalliumCompute

We're working hard to get everything upstream into LLVM to so we can
have compute shaders working out of the box, so our users don't need to
manually apply patches.

Let me know if you have any other questions.

-Tom

[1] http://www.mesa3d.org/

> On Tue, Apr 24, 2012 at 2:52 PM, Tom Stellard <tstellar at gmail.com> wrote:
> 
> > 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.
> > >
> >
> > Hi,
> >
> > Based on some feedback I received on IRC today, it sounds like this
> > is just too much code to review at once.  I'm going to try to strip
> > the R600 backend down to the bare minimum that is needed for our Mesa
> > drivers and then repost the code to the list.  Hopefully, this will make it
> > easier for people to review.
> >
> > -Tom
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> >
> 
> 
> 
> -- 
> 
> Thanks,
> 
> Justin Holewinski