[Mesa-dev] [PATCH] radeon/llvm: Use LLVM C API for compiling LLVM IR to ISA.

[Tom Stellard]

On Wed, Apr 24, 2013 at 09:54:02PM -0700, Jose Fonseca wrote:
> ----- Original Message -----
> > On Wed, Apr 24, 2013 at 09:40:44PM +0200, Mathias Fröhlich wrote:
> > > 
> > > Hi Tom,
> > > 
> > > On Tuesday, April 23, 2013 20:47:24 Tom Stellard wrote:
> > > > First of all, thanks for investigating this.  The information you've
> > > > provided has helped me a lot.
> > > Good to hear that it helps.
> > > 
> > > > I took a shot at implementing it this way with private static copies of
> > > > llvm.  I've pushed the initial work to this branch:
> > > > git://people.freedesktop.org/~tstellar/mesa llvm-build
> > > > 
> > > > I was able to hide the LLVM symbols by using the --exclude-libs linker
> > > > flag and so far I've tested with glxgears, an opencl example and also
> > > > eglgears and they all work.  I still need to do some more testing, but
> > > > any thoughts on what I've done so far in this branch?
> > > 
> > > I will look at this as soon as I find time. Probably not before the
> > > beginning
> > > of next week.
> > > 
> > 
> > I've thought about this some more, and I think that the best solution
> > might be to move all LLVM API calls into gallivm and build it as a
> > shared object with it's own private copy of LLVM statically linked.  This way
> > we
> > would still have a private copy of LLVM to work with, but without having
> > to statically link every single gallium target with LLVM.  Any thoughts
> > about this?
> 
> I don't see how this would work -- llvmpipe/draw has LLVMBuildXxxx calls too.  So to prevent symbol collision with apps that use them, we'd need to expose all LLVM calls we need under nome unique prefix.
>

I didn't realize this, and actually the radeon drivers use a lot of
the LLVMBuildXxxx calls too.
 
> Also note that gallivm has a lot of function calls. This would also pollute the app's namespace.
> 
> In short, for this to work we should do it not for whole gallivm, but with a new module, just for LLVM C calls + the additional helpers, and use an unique prefix (e.g., MesaLLVMXxxx) for everything. 
> 
> And we should have a new header, that #defines LLVMFoo MesaLLVMFoo, so that Mesa code doesn't need to be changed all over the place.
> 

I think this is a good approach, however before we make big changes to
Mesa I want to make sure we know what problems we are trying to solve
with these changes.  As far as I understand it, the current problems
are:

1. If an application is using the LLVM API outside of Mesa, then it is
impossible for Mesa to safely use the llvm_*_multithreaded() functions.

2. If an application is using a different version of LLVM than Mesa,
then it might incorrectly use the LLVM symbols that are included in
the Mesa libraries.

Are there others?

-Tom