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

Tue May 29 08:33:02 PDT 2012


> -----Original Message-----
> From: Stellard, Thomas
> Sent: Monday, May 28, 2012 9:07 AM
> To: Justin Holewinski
> Cc: Villmow, Micah; Tom Stellard; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] RFC: R600, a new backend for AMD GPUs
> 
> 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.
[Villmow, Micah] This is not possible with the R600 code generator, yes, but it 
is possible with some ELF fiddling with the AMDIL code generator which the R600
is based on.
> 
> -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
> > >