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

[Jose Fonseca]

----- Original Message -----
> 
> Hi,
> 
> On Thursday, April 25, 2013 10:29:27 Jose Fonseca wrote:
> > - There are a bunch of options that need to be set via globals, (see
> > lp_set_target_options), so app/drivers could tamper with each other
> > options.
> >
> > - llvm::cl::ParseCommandLineOptions will complain if called multiple times
> > -- I think we no longer need to call it these days though
> >
> > In short, LLVM was not designed for multiple users in the same process.
> Yep.
> 
> Also llvm is still emerging too fast to assume a specific version to be
> available. At least with r600 we do currently need a somewhat recent version
> and kind of have this assumption.
> But due to the api not kept strictly backwards compatible and all the
> pitfalls
> that happen while emerging fast its very likely that a potential application
> that also tries to make use of the driver modules just brings its own
> probably
> incompatible llvm version in some way. So shielding this in any way makes
> sense ...
> 
> 
> For the Mesa wrappers:
> I have attached a shell script again as a rapid proof that is able to build a
> linker script that builds up a wrapper shared library that contains a private
> llvm copy. 
>
> That's again non optimal - it contains just all static libs that I
> have in my current test environment... It's just to sketch how this could
> work.
> 
> The MesaLLVM-with-prefix.link script can by used with the command
> 
> g++ -shared -o libMesaLLVM.so MesaLLVM-with-prefix.link
> 
> to produce a libMesaLLVM.so that contains all C symbols starting with LLVM
> from libLLVMCore.a. All of them get prefixed with Mesa and are the only
> exported symbols then.
> That's close to Joses suggestion but with less work to be done in sources.
> 
> The MesaLLVM-with-version.link script can by used with the command
> 
> g++ -shared -o libMesaLLVM.so MesaLLVM-with-version.link
> 
> to produce a libMesaLLVM.so shared library that uses symbol versioning to
> distinguish between the llvm versions. I got this idea yesterday and this
> might simplify the problem a lot.
> By this variant we do not even need to prefix all the callers by Mesa. What
> this does is to explicitly assign a symbol version to all these calls. At
> static link symbol resolve time with this libMesaLLVM.so, this symbol version
> (the 'A MesaLLVM_1.0' entry) is then pulled out of this shared object and all
> users, in libllvmradeon.so for example, are linked against
> LLVMCreateContext at MesaLLVM_1,0 instead of just LLVMCreateContext. So we
> should
> get a private copy of llvm in libMesaLLVM.so with just the same call names
> than usual source code wise.
> That's to be tested and verified, but if this works like I think it should,
> this is the easiest way to get our own LLVM version on linux at least.

Wow. I confess I wasn't aware that linker scripts were this powerful. And if it works, using a custom version instead of a custom symbol prefix seems a genial idea IMO.

Jose