Use CMake FetchContent to build Kokkos Core

[dalg24]

Enables doing

cmake -B build-cuda -DKokkos_ENABLE_CUDA=ON
cmake --build build-cuda
./build-cuda/<exe>
cmake -B build-openmp -DKokkos_ENABLE_OPENMP=ON
...

One caveat is that, as currently proposed would download Kokkos instead of using an external install. If using CMake 3.24 one can do

cmake -B build -DKokkos_ROOT=/path/to/kokkos/install
...

which would bypass the fetch content.

Let me know what you think. I would do the same for KokkosKernels once we can agree on something

[masterleinad]

For me,

diff --git a/Exercises/common.cmake b/Exercises/common.cmake
index ed081b7..933fcf0 100644
--- a/Exercises/common.cmake
+++ b/Exercises/common.cmake
@@ -8,21 +8,12 @@ endif()

 set(Kokkos_COMMON_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../dep/Kokkos)

-include(FetchContent)
-if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.24)
-  FetchContent_Declare(
-    Kokkos
-    GIT_REPOSITORY https://github.com/kokkos/kokkos.git
-    GIT_TAG        4.0.01
-    SOURCE_DIR ${Kokkos_COMMON_SOURCE_DIR}
-    FIND_PACKAGE_ARGS
-  )
-  FetchContent_MakeAvailable(Kokkos)
-  
-  find_package(Kokkos REQUIRED)
-else()
-  find_package(Kokkos)
-  if(NOT Kokkos_FOUND)
+find_package(Kokkos QUIET)
+if(NOT Kokkos_FOUND)
+  if(EXISTS ${Kokkos_COMMON_SOURCE_DIR})
+    add_subdirectory(${Kokkos_COMMON_SOURCE_DIR} Kokkos)
+  else()
+    include(FetchContent)
     FetchContent_Declare(
       Kokkos
       GIT_REPOSITORY https://github.com/kokkos/kokkos.git

works well and only downloads Kokkos once.

[dalg24]

One downside with the approach you suggested is that if Kokkos is found, because of the QUIET option, it will not print what backends are enabled.

[masterleinad]

One downside with the approach you suggested is that if Kokkos is found, because of the QUIET option, it will not print what backends are enabled.

Then do

find_package (Kokkos CONFIG)