[PATCH] D120353: [OpenMP] Add option to make offloading mandatory

[Joseph Huber via Phabricator via cfe-commits]

jhuber6 added a comment.

In D120353#3340533 <https://reviews.llvm.org/D120353#3340533>, @ABataev wrote:

> In D120353#3338770 <https://reviews.llvm.org/D120353#3338770>, @jhuber6 wrote:
>
>> In D120353#3338718 <https://reviews.llvm.org/D120353#3338718>, @ABataev wrote:
>>
>>> In D120353#3338647 <https://reviews.llvm.org/D120353#3338647>, @jhuber6 wrote:
>>>
>>>> But the main reason I made this patch is for interoperability. Without this if you want to call a CUDA function from the OpenMP device you'd need a variant and a dummy implementation. If you don't write a dummy implementation you'll get a linker error, if you don't use a variant you'll override the CUDA function.
>>>
>>> Ah, ok, I see. How is supposed to be used? In Cuda code or in plain C/C++ code?
>>
>> I haven't finalized the implementation, but the basic support I've tested was calling a `__device__` function compiled from another file with OpenMP, with this patch the source files would look like this for example. I think the inverse would also be possible given some code on the CUDA side. Calling CUDA kernels would take some extra work.
>>
>>   __device__ int cuda() { return 0; }
>>
>>
>>
>>   int cuda(void);
>>   #pragma omp declare target device_type(nohost) to(cuda)
>>   
>>   int main() {
>>     int x = 1;
>>   #pragma omp target map(from : x)
>>     x = cuda();
>>   
>>     return x;
>>   }
>
> What if we have `#pragma omp target if (...)` or `#pragma omp target device(...)`?

If we have `#pragma omp target if (...)` then that requires a host fallback and violates the assertion the user passed in, it will hit the unreachable and fail. If the user passed in `#pragma omp target device(...)` we will assume that a host implementation exists as well.


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