Compile as cmake language

[JBludau]

Some documentation for the Kokkos as CMake language that Kokkos already provides and that solve the problem described in kokkos/kokkos#7144

Will stay a draft for now as we might need to update the example in core and add stuff to the tutorial

[masterleinad]

Looks like we need something like

diff --git a/cmake/KokkosConfigCommon.cmake.in b/cmake/KokkosConfigCommon.cmake.in
index d3ac39ffa..fe021199e 100644
--- a/cmake/KokkosConfigCommon.cmake.in
+++ b/cmake/KokkosConfigCommon.cmake.in
@@ -33,6 +33,9 @@ ENDFOREACH()
 
 IF(Kokkos_ENABLE_CUDA)
   SET(Kokkos_CUDA_ARCHITECTURES @KOKKOS_CUDA_ARCHITECTURES@)
+  IF(Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE)
+    SET(CMAKE_CUDA_ARCHITECTURES @KOKKOS_CUDA_ARCHITECTURES@)
+  ENDIF()
 ENDIF()
 
 IF(Kokkos_ENABLE_HIP)
@@ -300,8 +303,8 @@ FUNCTION(kokkos_compilation)
             # set the properties if defined
             IF(COMP_${_TYPE})
                 # MESSAGE(STATUS "Using ${COMP_COMPILER} :: ${_TYPE} :: ${COMP_${_TYPE}}")
-                SET_PROPERTY(${_TYPE} ${COMP_${_TYPE}} PROPERTY RULE_LAUNCH_COMPILE "${Kokkos_COMPILE_LAUNCHER} ${COMP_COMPILER} ${CMAKE_CXX_COMPILER}")
-                SET_PROPERTY(${_TYPE} ${COMP_${_TYPE}} PROPERTY RULE_LAUNCH_LINK "${Kokkos_COMPILE_LAUNCHER} ${COMP_COMPILER} ${CMAKE_CXX_COMPILER}")
+                SET_PROPERTY(${_TYPE} ${COMP_${_TYPE}} PROPERTY LANGUAGE ${Kokkos_COMPILE_LANGUAGE})
+                SET_PROPERTY(${_TYPE} ${COMP_${_TYPE}} PROPERTY CUDA_ARCHITECTURES ${Kokkos_CUDA_ARCHITECTURES})
             ENDIF()
         ENDFOREACH()
     ENDIF()

for this to be usable with Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE.

[masterleinad]

The challenge is basically that we should set CUDA_ARCHITECTURES for every target that is to be compiled with Kokkos (and we can't set the property for a source file). On the other hand, we can only set the compile language for a source file, not for a target and we can't really iterate over the source files of a target in general. That means we have to use kokkos_compilation on all the source files and we must find Kokkos before declaring any targets (or also use kokkos_compilation on all targets).

[ajpowelsnl]

@cedricchevalier19 - this entry is relevant to 7144, CMAKE_AS_LANGUAGE and kokkos_compilation()

[dalg24]

@cedricchevalier19 - this entry is possibly relevant to 7144, CMAKE_AS_LANGUAGE and kokkos_compilation()

Jakob clearly states in the description that this work is a proposed resolution for that very issue so yes it is (hopefully) relevant

[masterleinad]

kokkoc_compilation in its current form only sets the launch compiler. Hence, it's not useful with Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE=ON. I think we should just document the current behavior for now.

[cedricchevalier19]

I know it is now from you, but I am wondering why "separate compilation"?

[masterleinad]

It's technically "separable compilation" and was introduced in kokkos/kokkos#3136 with meaning such as "the compilation settings can be different for every target; they are separable".

[JBludau]

I have no hard feelings about this. My line of reasoning was: Currently we don't offer the possibility to separate the compilation but rather we kind of force the separation when Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE=ON, as the source files and targets that use kokkos will require to have properties set by cmake manually or might not compile

[cedricchevalier19]

I understand, but most people will read this as "it is the way to do libraries." Separable is slightly better.

[JBludau]

kokkoc_compilation in its current form only sets the launch compiler. Hence, it's not useful with Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE=ON. I think we should just document the current behavior for now.

I agree with your assessment, I ended at the same places in the cmake files.
Nevertheless, I would vote for trying to support this feature, even if the user needs to annotate both the source and target. HIP seems to recommend the use of the CMake language feature (see first Attention box)

[masterleinad]

Nevertheless, I would vote for trying to support this feature, even if the user needs to annotate both the source and target.

Sure. I was just suggesting a more incremental approach. First, document the current behavior of kokkos_compilation, then try to accommodate it when using Kokkos_ENABLE_COMPILE_AS_CMAKE_LANGUAGE (or even define a different function since the behavior needed is very different), and finally document it.

[masterleinad]

HIP seems to recommend the use of the CMake language feature (see first Attention box)

Kokkos just works better when not enabling HIP (or CUDA) as a feature, though.