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

[Tom Stellard]

On Mon, May 28, 2012 at 08:54:41AM -0700, Justin Holewinski wrote:
> On May 28, 2012 6:44 AM, "Tom Stellard" <thomas.stellard at amd.com> wrote:
> >
> > 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.
> 
> Okay, so there is no way to use the backend to produce loadable compute
> kernels for the proprietary drivers, or on Mac/windows?
>

Right, this is not currently possible.

-Tom

> >
> > 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
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > > >
> > >
> > >
> > >
> > > --
> > >
> > > Thanks,
> > >
> > > Justin Holewinski
> >