[Openmp-dev] [Non-DoD Source] Re: [llvm-dev] llvm OpenMP support for Apple Silicon

[Johannes Doerfert via Openmp-dev]

On 10/19/21 18:05, Jon Chesterfield via Openmp-dev wrote:
> Elided the discussion starting:
>> Are there any plans to support OpenMP
>> offload to the integrated GPU in Apple Silicon processors?
> Replying from Johannes' sketch:
>   Given that description one would need two things:
>
>> 1) A Metal API plugin for libomptarget (next to the CUDA and AMDGCN
>> plugin). That is actually pretty easy if
>>      their API is somewhat sane.
>> 2) A backend that can produce device code. I don't think they upstreamed
>> their Metal backend *but* one can
>>      probably use their clang as "backend" given that it can consume
>> LLVM-IR.
>>
> I think this is in the realm of difficult but tractable. One scheme would
> be to add a target to LLVM which approximates the GPU - pointer sizes etc -
> then a heuristically derived pass that mangles the whole-program IR (post
> device lib splice) into IR that the closed source toolchain tolerates.
> Could alternatively use an existing target that looks vaguely similar and
> do more work in the heuristic pass.
>
> An alternative would be to revive a C backend for LLVM and use that as the
> common IR. Essentially need 'something' that exists after the
> openmp-specific rewrites and preferably after the device runtime that can
> be fed to 'something' that creates binaries that the GPU can run.
>
> There's some independent interest in passing IR through to the API plugin
> layer and if Intel's GPU compiler stays out of tree we might end up with
> machinery for feeding an IR to a third party toolchain to 'finalize' it.
>
> Interesting problem, not immediately obvious who would have the motivation
> to make it happen. Could be a side project for an Apple GPU enthusiast.
> Whole thing would be much easier if the backend was added to LLVM trunk,
> presumably by Apple.

The problem is that I might have to do almost all of this for the
Intel case... so why not start here.

~ Johannes


>
> Thanks for the interesting question!
>
> Jon
>
>
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