Use CMake target-style MPI dependencies

The former global flags are a legacy from CMake 2.6 when we did not use
CMake targets. One should generally use a wrapper compiler and the
CMake target, and as such have no need of appending to global flags.

Likewise when using a CMake target, the MPI target should be added
on an as-needs basis.

There's no need to add MPI C flags

Author: mabraham

CMakeLists.txt

@@ -952,31 +952,21 @@ endif()
 
 # # # # # # # # # # NO MORE TESTS AFTER THIS LINE! # # # # # # # # # # #
 # these are set after everything else
-if (GMX_LIB_MPI)
-    # TODO: Restrict the scope of MPI dependence.
-    # Targets that actually need MPI headers and build tool flags should
-    # manage their own `target_link_libraries` locally. Such a change is beyond
-    # the scope of the bug fix for #4678.
-    set(_gmx_mpi_cxx_link_flags "${MPI_CXX_LINK_FLAGS}")
-    set(_gmx_mpi_c_compile_flags "${MPI_C_COMPILE_OPTIONS};${MPI_C_COMPILE_DEFINITIONS}")
-    set(_gmx_mpi_cxx_compile_flags "${MPI_CXX_COMPILE_OPTIONS};${MPI_CXX_COMPILE_DEFINITIONS}")
-endif ()
 if (NOT GMX_SKIP_DEFAULT_CFLAGS)
-    #TODO(#3672): Use target_link_libraries(... MPI::MPI_CXX) instead of ${MPI_CXX_LINK_FLAGS}
-    set(CMAKE_EXE_LINKER_FLAGS "${FFT_LINKER_FLAGS} ${_gmx_mpi_cxx_link_flags} ${CMAKE_EXE_LINKER_FLAGS}")
-    set(CMAKE_SHARED_LINKER_FLAGS "${FFT_LINKER_FLAGS} ${_gmx_mpi_cxx_link_flags} ${CMAKE_SHARED_LINKER_FLAGS}")
+    set(CMAKE_EXE_LINKER_FLAGS "${FFT_LINKER_FLAGS} ${CMAKE_EXE_LINKER_FLAGS}")
+    set(CMAKE_SHARED_LINKER_FLAGS "${FFT_LINKER_FLAGS} ${CMAKE_SHARED_LINKER_FLAGS}")
 else()
     message("Recommended flags which are not added because GMX_SKIP_DEFAULT_CFLAGS=yes:")
-    message("CMAKE_C_FLAGS: ${SIMD_C_FLAGS};${_gmx_mpi_c_compile_flags};${EXTRA_C_FLAGS};${GMXC_CFLAGS}")
+    message("CMAKE_C_FLAGS: ${SIMD_C_FLAGS};${EXTRA_C_FLAGS};${GMXC_CFLAGS}")
     foreach(build_type ${build_types_with_explicit_flags})
         message("CMAKE_C_FLAGS_${build_type}: ${GMXC_CFLAGS_${build_type}}")
     endforeach()
-    message("CMAKE_CXX_FLAGS: ${SIMD_CXX_FLAGS};${_gmx_mpi_cxx_compile_flags};${EXTRA_CXX_FLAGS};${GMXC_CXXFLAGS}")
+    message("CMAKE_CXX_FLAGS: ${SIMD_CXX_FLAGS};${EXTRA_CXX_FLAGS};${GMXC_CXXFLAGS}")
     foreach(build_type ${build_types_with_explicit_flags})
         message("CMAKE_CXX_FLAGS_${build_type}: ${GMXC_CXXFLAGS_${build_type}}")
     endforeach()
-    message("CMAKE_EXE_LINKER_FLAGS: ${FFT_LINKER_FLAGS} ${_gmx_mpi_cxx_link_flags}")
-    message("CMAKE_SHARED_LINKER_FLAGS: ${FFT_LINKER_FLAGS} ${_gmx_mpi_cxx_link_flags}")
+    message("CMAKE_EXE_LINKER_FLAGS: ${FFT_LINKER_FLAGS}")
+    message("CMAKE_SHARED_LINKER_FLAGS: ${FFT_LINKER_FLAGS}")
 endif()
 # Allow `admin` directory to be easily conveyed to nested CMake commands.
 set(GMX_ADMIN_DIR ${CMAKE_SOURCE_DIR}/admin)

cmake/gmxCFlags.cmake

@@ -105,20 +105,6 @@ function(gmx_target_compile_options TARGET)
             $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CONFIG:${build_type}>>:${GMXC_CXXFLAGS_${build_type}}>
             )
     endforeach()
-    # TODO: Restrict the scope of MPI dependence.
-    # Targets that actually need MPI headers and build tool flags should
-    # manage their own `target_link_libraries` locally. Such a change is beyond
-    # the scope of the bug fix for #4678.
-    if (GMX_LIB_MPI AND TARGET ${TARGET})
-        target_link_libraries(
-            ${TARGET} PRIVATE
-            $<$<LINK_LANGUAGE:CXX>:MPI::MPI_CXX>
-            # We don't know whether we have sought the MPI::C component at all, or at least
-            # by the time we process these lines.
-            $<$<AND:$<TARGET_EXISTS:MPI::MPI_C>,$<LINK_LANGUAGE:C>>:MPI::MPI_C>
-            $<$<LINK_LANGUAGE:CUDA>:MPI::MPI_CXX>
-        )
-    endif ()
     # Add the release-configuration compiler options to build
     # configurations that derive from it.
     foreach(build_type RELWITHDEBINFO RELWITHASSERT MINSIZEREL PROFILE)

src/gromacs/CMakeLists.txt

@@ -234,7 +234,11 @@ if(GMX_GPU_FFT_ONEMATH)
 endif()
 
 if (TARGET MPI::MPI_CXX)
-    target_link_libraries(libgromacs PRIVATE MPI::MPI_CXX)
+    target_link_libraries(libgromacs PRIVATE
+        $<$<LINK_LANGUAGE:CXX>:MPI::MPI_CXX>
+        $<$<LINK_LANGUAGE:CUDA>:MPI::MPI_CXX>
+        $<$<LINK_LANGUAGE:HIP>:MPI::MPI_CXX>
+        )
 endif ()
 
 target_link_libraries(libgromacs PRIVATE $<BUILD_INTERFACE:common>)