doc: MPI Cray

Author: scivision

README.md

@@ -44,6 +44,27 @@ re: setting PATH and LD_LIBRARY_PATH if CMake has trouble finding OpenMPI for a
 
 * https://hpc-forge.cineca.it/files/CoursesDev/public/2017/MasterCS/CalcoloParallelo/MPI_Master2017.pdf
 
+### Cray
+
+FindMPI.cmake on Cray defines a rather sparse imported target MPI::MPI_C and MPI::MPI_Fortran like
+
+```
+-- Target: MPI::MPI_Fortran properties
+-- MPI::MPI_Fortran IMPORTED = TRUE
+-- MPI::MPI_Fortran NAME = MPI::MPI_Fortran
+-- MPI::MPI_Fortran SYSTEM = ON
+-- MPI::MPI_Fortran TYPE = INTERFACE_LIBRARY
+
+-- Target: MPI::MPI_C properties
+-- MPI::MPI_C IMPORTED = TRUE
+-- MPI::MPI_C NAME = MPI::MPI_C
+-- MPI::MPI_C SYSTEM = ON
+-- MPI::MPI_C TYPE = INTERFACE_LIBRARY
+```
+
+while on most non-Cray platforms the typical properties are also defined: INTERFACE_INCLUDE_DIRECTORIES, INTERFACE_LINK_LIBRARIES, INTERFACE_LINK_OPTIONS.
+Also on Cray. these typically defined variables are empty: MPI_Fortran_LIBRARIES, MPI_Fortran_MODULE_DIR, MPI_Fortran_INCLUDE_DIRS, MPI_Fortran_COMPILE_OPTIONS, MPI_Fortran_LINK_FLAGS.
+
 ### MPI-3 Fortran
 
 If "mpi_f08.mod" is not found, typically the MPI include directories are also missing.