Use LFortran to compile eCLM

[Leon-Degel-Koehn]

LFortran Support for eCLM

This issue tracks the investigation of using LFortran to compile eCLM.
This was investigated as part of the Research Software Engineering Winter of Code.
It provides guidance for future developers who may want to attempt migrating eCLM to LFortran and documents the current limitations and workarounds discovered during the investigation.

Current Status

• eCLM cannot currently be compiled using LFortran.

• Critical limitations include:

  • No official OpenMPI / MPI support in LFortran.
  • No support for Fortran namelists.

• Minor issues also exist, including:

  • Some unsupported statements such as call abort (upstream fix merged but not in conda release yet as of writing this report: #9595).
  • Some unsupported statements such as call flush_ would still have to be implemented upstream.
  • Compiler crashes due to unexpected memory errors even after fixes are applied.
  • As the compiler becomes capable of progressing futher into the project during compilation, more issues may be discovered.

Despite these limitations, it has become clear that CMake infrastructure changes for LFortran would be minimal once the compiler matures.
LFortran should eventually be close to a drop-in replacement for gfortran.

MPI Workaround

To partially address MPI-related compilation issues:

• An extended MPI wrapper is available for LFortran: fortran_mpi.
• This provides implementations of MPI functions that LFortran currently lacks.
• Some non-trivial MPI interfaces remain to be implemented. (See logical BCast for example)
• If LFortran gains official MPI support, this wrapper may no longer be required.

Namelist Support

• LFortran currently does not support Fortran namelists.

• Full compilation is blocked until namelist support is implemented.

• Once available, further issues may be revealed during testing and compilation.

Compilation of External Projects

• eCLM depends on several external projects, which require careful handling:

  • Some projects (e.g., GPTL) break if the --cpp flag is enabled.
  • Other projects require --cpp for compilation.
  • Solution: Configure --cpp on a per-external-project basis in CMake.

• Minor fixes required for unsupported statements or functions in these external projects. (Diff is provided)

CMake Integration

• Minimal changes are required to integrate LFortran into eCLM’s CMake workflow. (Documented in diff)
• LFortran is already recognized by CMake as a compiler.
• Once LFortran is functional, only a new conditional branch is needed alongside the existing gfortran and Intel compiler options.
• No additional compiler flags are needed other than --cpp where required.

Summary of Findings

Issue	Status / Workaround
MPI support	Partial workaround via fortran_mpi
Namelist support	Not supported; blocks compilation
External projects & --cpp	Needs per-project CMake configuration
Compiler crashes	Still present after fixes; blocks compilation

Recommendations for Future Work

1. Monitor LFortran releases for:
   • Namelist support
   • Stable MPI integration
2. Test compilation of external projects individually to handle --cpp inconsistencies.
3. Once LFortran reaches a usable state, update CMake with a branch for LFortran and test full eCLM compilation.

To execute compilation to the stage where we got it to work so far, apply the provided diff and
run the script.

Usage

Use the following script to compile eCLM with the required mpi wrapper to the point where it is possible at the moment.

#!/bin/bash
# Run from root directory of eCLM.
# The build is expected to fail.
# If you see it fail at the `call abort` line, you may build a
# newer version of lfortran from source which should fix that.

BUILD_DIR="bld"
INSTALL_DIR="install"
CC="gcc"
MPICC="gcc"
FC="lfortran"
MPIFC="lfortran"

# Set this to the path where mpi.h is located
# if you have installed open-mpi via conda:
MPI_C_INCLUDE_PATH="$CONDA_PREFIX/include"
# Otherwise, an installation might be at
# MPI_C_INCLUDE_PATH="/usr/lib/x86_64-linux-gnu/openmpi/include"


# Set this to the path to where you have cloned the fortran_mpi repo
# For example:
MPI_FORTRAN="../fortran_mpi"
 
# For lfortran mpi mock
$CC -I$MPI_C_INCLUDE_PATH -c $MPI_FORTRAN/src/mpi_constants.c
$FC --cpp -c $MPI_FORTRAN/src/mpi_c_bindings.f90
$FC --cpp -c $MPI_FORTRAN/src/mpi.f90
cp mpi* src/externals/gptl
cp mpi* src/externals/pio1

# Cleanup
rm -rf $BUILD_DIR
rm -rf $INSTALL_DIR

cmake -S src -B $BUILD_DIR                                \
	-DCMAKE_INSTALL_PREFIX=$INSTALL_DIR                 \
	-DCMAKE_C_COMPILER=$CC                              \
	-DCMAKE_Fortran_COMPILER=$FC \
	-DCOMPILER=LFortran

cmake --build $BUILD_DIR
cmake --install $BUILD_DIR

References

• Extended MPI wrapper
• LFortran PR fixing call abort

[kvrigor]

@Leon-Degel-Koehn Highly appreciate your investigative efforts! LFortran lacking namelist support surprised me the most. I guess we could only wait until LFortran devs support this feature.

Do you mind creating a small PR which adds LFortran support on SetBuildOptions.cmake? Just add a comment that its not fully supported and a link to this GitHub issue. Also it would be nice to document the steps to install LFortran + build steps in that PR.