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

[Justin Holewinski]

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?

>
> 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
>
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