Kokkos launch compiler updates

[jrmadsen]

• Creates new find_package(Kokkos COMPONENTS launch_compiler) mode for using the launch compiler feature regardless of whether CUDA is enabled
• kokkos_launch_compiler was extended to support forwarding to the original compiler Kokkos was compiled with
• There is only one known caveat: kokkos is installed with nvcc_wrapper and downstream application uses clang
  • This results in the default host compiler to use NVCC_WRAPPER_DEFAULT_COMPILER from environment or nothing and without specify NVCC_WRAPPER_DEFAULT_COMPILER during compilation clang will be used and it will fail
  • Moral of the story: don't use nvcc_wrapper anymore when installing kokkos

[jrmadsen]

Retest this please

[jrmadsen]

Retest this please

[crtrott]

I guess this is ok? But I just don't know. What happened before if you installed Kokkos with nvcc_wrapper and then downstream use clang? But we can just say we don't support that.

[jrmadsen]

What happened before if you installed Kokkos with nvcc_wrapper and then downstream use clang?

Before this, i don't know. You probably just got compiler-flag errors since Kokkos would export compiler flags for NVCC. After this PR, the problem with installing Kokkos via nvcc_wrapper is that we don't actually know the underlying host compiler at CMake time (since they could configure and then set NVCC_WRAPPER_DEFAULT_COMPILER in the env later) so it can't save that compiler for downstream projects. Thus if the default g++ is version 4.9 and the user set NVCC_WRAPPER_DEFAULT_COMPILER=g++-8 when building Kokkos after cmake was run, kokkos_launch_compiler in the install tree would not "know" to set NVCC_WRAPPER_DEFAULT_COMPILER to g++-8.

[jrmadsen]

I actually think this makes a rather strong argument for us deprecating the direct use of nvcc_wrapper. E.g. make a really annoying warning in the Kokkos CMake that says stop setting the C++ compiler to nvcc_wrapper.

[jrmadsen]

And it would be a win in my and @DavidPoliakoff war against compiler-wrappers.

[jrmadsen]

But we can just say we don't support that.

Also, now it is supported. Since you could do find_package(Kokkos COMPONENTS launch_compiler) and even if the downstream compiler were clang, you would end up compiling your kokkos code with nvcc_wrapper.