MPI ABI discussion

MPI-ABI.md

Current ABI draft -

• All fortran interop C API are included in MPI C ABI, e.g. MPI_INTEGER, MPI_Comm_f2c, MPI_F_STATUS_IGNORE
• Jeff intended an MPI ABI implementation that does not depend on Fortran
• Thus the ABI standard need fix all relevant Fortran ABIs
• The key requirement: Fortran INTEGER is equivalent to C int, but also need fix all Fortran datatypes with C equivalents.
• MPI C ABI need implement MPI_Comm_f2c etc. with a dictionary mechanism
• MPI C ABI does not do any Fortran compiler checks, only via assumptions

My rejection:

• MPI should not specifiy INTEGER is C int if Fortran doesn't say so
• MPI Fortran binding should implement proper MPI Fortran inter-op, via compiler feature checks and value conversions if necessary.
• A C-only MPI Fortran inter-op is useless. For example, to inter-op with Fortran INTEGER, just use MPI_Int.

My proposal:

• Do not change any MPI API
• MPI C ABI does not contain any Fortran parts
  • mpi_abi.h only contains non-fortran APIs
  • mpi.h includes mpi_abi.h + mpi_fort.h
  • libmpi_abi.so does not contain any Fortran related symbols
  • libmpifort.so contains or Fortran related symbols include Fortran interface and inter-op C APIs
• Fortran related API is provided in header via an opt-in option and in linker via libmpifort.so

Examples

case 1: t.c is a C-only MPI code

$ mpicc -mpi-abi -o t t.c
# t will link with `libmpi_abi.so`

case 2: t.c is a C code that uses MPI_INTEGER (or MPI_Comm_f2c)

$ mpicc -mpi-abi -o t t.c
ERROR: name undefined

$ mpicc -mpi-abi -fort -o t t.c
# SUCCESS. t will link with `libmpifort.so libmpi_abi.so`

case 3: t.f is a FORTRAN code

$ mpifort -mpi-abi -o t t.f
# SUCCESS. t will link with `libmpifort.so libmpi_abi.so`