[cfe-dev] [RFC] Unified offloading option for CUDA/HIP/OpenMP

[Doerfert, Johannes via cfe-dev]

Hi Sam, thanks for driving this, I really like the idea!

Here are some thoughts:

Make the "kind" optional if it can be deduced, as Alexey noted. So
   -offload=amd-gfx900
should work fine if we have an -x c and -fopenmp set. Error out if
it is ambiguous.

Allow multiple -offload occurrences.

Keep the support of the old ways for now as well.

Allow to pass the kind + triple + arch as first part of the new -offload
flag and any options as a second part, so:
  -offload=hip-amd-gfx906
works but also does
  -offload="amd-gfx906 -fvectorize" -x hip
as well as
  -offload="amd -march=gfx906 -fno-vectorize" -fopenmp
This will make it way easier to use.

I hope some of these make some sense :)

~ Johannes


On 12/12/20 8:11 AM, Liu, Yaxun (Sam) wrote:
> [AMD Public Use]
>
> Currently CUDA/HIP and OpenMP has different offloading options, e.g.
>
> clang++  -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx900 test.cpp
>
> clang++ -offload-arch=gfx906 test.hip
>
> Our users request to have a concise way to specify offloading options for OpenMP. Ideally, one option to convey offloading kind, offloading triple, and offloading device arch.
>
> On the other hand, there are some limitations of the current offloading option for CUDA/HIP:
>
> 1. It does not specify offloading kind whereas relies on file type to infer offloading kind. If input file is not CUDA/HIP source code (e.g. bundled LLVM bit code), there needs a way to specify offloading kind.
>
> 2. It does not specify offloading target triple whereas relies on device arch to infer target triple. As HIP is ported to different targets, there needs a way to specify offloading target triple.
>
> In summary, a unified offloading option is preferred, which conveys offloading kind, offloading target triple and offloading device arch.
>
> I would like to propose to either have a new option or extend the existing -offload-arch option for that, in the format kind-triple-arch, e.g.
>
> -offload=omp-amd-gfx900
>
> -offload=hip-amd-gfx906
>
> Whereas kind and triple can be abbreviations for conciseness, e.g. omp expands to openmp, amd expands to amdgcn-amd-amdhsa. Arch can be omitted, in which case clang will use the default arch for the triple.
>
> Your feedbacks are welcome.
>
> Thanks.
>
> Sam
>