Using CMake to find OpenMP flags for Fortran compilers

CMake is a wonderful tool for configuring your build system. It has built-in support for C, C++, and Fortran projects. The support for C and C++ is excellent. The support for Fortran is quite good, but it there is some missing functionality in some areas. For me, the most obvious deficiency was OpenMP support for Fortran. This can be easily fixed with a little detective work and elbow grease.

A little background

Pretty much all of the CMake functions are actually implemented as scripts in $SOMEPATH/share/cmake/Modules, where for me $SOMEPATH=/usr/local but your mileage may very (they will likely be in your install path on a Windows machine). All the packages that are known to the FIND_PACKAGE function must be implemented as one of these scripts with the name FindPackage.cmake, where “Package” is the package that is searched for.

If you take a look in $SOMEPATH/share/cmake/Modules/FindOpenMP.cmake, you will notice that it returns the variables OPENMP_FOUND, OpenMP_C_FLAGS, OpenMP_CXX_FLAGS; notice that there is no OpenMP_Fortran_FLAGS. This is OK if the C compiler that CMake found is the same vendor as your Fortran compiler (i.e. gcc & gfortran or icc & ifort) so that you can just use the C flags for the Fortran flags. However, this is not a robust solution; for example, what if the openmp flags between the C and Fortran compilers of a vendor don’t match? Or, what if you are using a gcc C compiler with an intel Fortran compiler (as I was)? This obviously won’t work. The best solution is to tell CMake how to look for Fortran OpenMP flags.

How does CMake determine the Fortran OpenMP flags?

I’m not going to go into details, but basically all known OpenMP compiler flags are implemented. Here is an excerpt from the FindOpenMP.cmake file:

set(OpenMP_FLAG_CANDIDATES
  #GNU
  "-fopenmp"
  #Microsoft Visual Studio
  "/openmp"
  #Intel windows
  "-Qopenmp"
  #PathScale, Intel
  "-openmp"
  #Empty, if compiler automatically accepts openmp
  " "
  #Sun
  "-xopenmp"
  #HP
  "+Oopenmp"
  #IBM XL C/c++
  "-qsmp"
  #Portland Group, MIPSpro
  "-mp"
)

It then uses a simple C file that will fail without OpenMP to check for the correct flag:

#include <omp.h>
int main() {
#ifdef _OPENMP
  return 0;
#else
  breaks_on_purpose
#endif
}

For each flag that is given in the above list, CMake attempts to compile the C snippet using the check_c_source_compiles function that comes with CMake. The first flag that does not cause a compiler error is considered the correct flag.

Adding Fortran support for OpenMP

Note: You can download the file FindOpenMP_Fortran.cmake at my cmake_fortran_template GitHub page.

Instead of adding Fortran to FindOpenMP.cmake, I decided to make a new FindOpenMP_Fortran.cmake file. Essentially, I made a copy of the original FindOpenMP.cmake and placed it in ${CMAKE_SOURCE_DIR}/cmake/Modules/FindOpenMP_Fortran.cmake, where ${CMAKE_SOURCE_DIR} is the top level directory of my Fortran project.

I decided that in addition to simply finding the Fortran flag, I wanted my function to also determine the max number of threads that OpenMP can use on the current system and return that as well. To do that, I used the following Fortran snippet:

program TestOpenMP
 use omp_lib
 write(*,'(I2)',ADVANCE='NO') omp_get_num_procs()
end program TestOpenMP

Interestingly, there is no check_fortran_source_compiles (someone should request this) so all the testing had to be done manually. The business end of this code is given below:

TRY_RUN (OpenMP_RUN_FAILED OpenMP_FLAG_DETECTED ${CMAKE_BINARY_DIR}
    ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/testFortranOpenMP.f90
    COMPILE_DEFINITIONS ${CMAKE_REQUIRED_DEFINITIONS}
    CMAKE_FLAGS -DCOMPILE_DEFINITIONS:STRING=${MACRO_CHECK_FUNCTION_DEFINITIONS}
    COMPILE_OUTPUT_VARIABLE OUTPUT
    RUN_OUTPUT_VARIABLE OMP_NUM_PROCS_INTERNAL)

The correct flag is given in OpenMp_Fortran_FLAGS, and the number of OpenMP threads is given in OMP_NUM_PROCS.

Using this is as simple as:

SET(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/Modules/")
FIND_PACKAGE(OpenMP_Fortran)
SET_TARGET_PROPERTIES(${EXECUTABLE_NAME} PROPERTIES
                      COMPILE_FLAGS "${OpenMP_Fortran_FLAGS}"
                      LINK_FLAGS "${OpenMP_Fortran_FLAGS}")