OpenMPTarget: Use OpenMP atomics for atomic_add.

[rgayatri23]

The PR uses atomic operation from OpenMP spec instead of DESUL atomics for llvm/18 as they cause compiler segfaults.

[dalg24]

Please confirm that the issue is only affecting atomic_add and none of the other atomic functions we provide.
Also please elaborate how you got Clang to segfault. What were you building? Was it Kokkos test suite? Are you able to reproduce with a simple Kokkos::parallel_for? etc.

[masterleinad]

Wouldn't it make more sense to fix this in desul directly?

[rgayatri23]

Please confirm that the issue is only affecting atomic_add and none of the other atomic functions we provide. Also please elaborate how you got Clang to segfault. What were you building? Was it Kokkos test suite? Are you able to reproduce with a simple Kokkos::parallel_for? etc.

Did not try anything other than atomic_add yet.
I am trying to come up with a small example where this fails but of-course it passes for simple cases :-/
This might also be due to the use of new dynamic shared memory extensions that are used with llvm/18. The failing kernel was using that feature to atomically update main memory by reading value from scratch memory.

[rgayatri23]

Wouldn't it make more sense to fix this in desul directly?

Yeah that can also work. I am fine with either.

[rgayatri23]

I guess the first issue is do we want to do this if the problem only arises in very specific cases and with just one version of the compiler?
As mentioned before, the upstream llvm seems to work fine which will be the basis for llvm/19

[masterleinad]

I guess the first issue is do we want to do this if the problem only arises in very specific cases and with just one version of the compiler?

We should try fixing for the minimum compiler version supported for OpenMPTarget or raise the minimum compiler version.

[rgayatri23]

Yeah we are setting the minimum version to 17 as of now https://github.com/kokkos/kokkos/blob/develop/cmake/kokkos_enable_devices.cmake#L95-L98

17 passes most of the enabled tests on both V100 and A100. With 18, I saw a few failing tests on A100.