[mpiwg-abi] MPI_Fint in the ABI

[Erik Schnetter]

Jeff

As per the Fortran standard, the sizes of integer and real need to be
the same. In practice this means that integers have 4 bytes. If
someone chooses to change this – either changing the size of real as
well or not – then they are breaking the Fortran standard, and
probably know what they are doing, and are aware of the various
incompatibilities and problems this will introduce. I would thus
suggest keeping things simple for the common case. I would not expect
there to be some code that interoperates with Fortran, and which is
unaware of the Fortran integer size and still expects to get things
right. People will either assume that integer = int, or will know the
Fortran ABI details.

In the MPI ABI we assume that the caller knows sizeof(int) and
sizeof(void*), and I would also assume that they know sizeof(integer).
I would keep `MPI_Fint` a fixed type.

To cover all bases I also suggest that we provide a global constant
struct with as much ABI information as we can think of, such as

const struct MPIABI_info_t {
  int32_t MPIABI_version_major, MPIABI_version_minor;
  int32_t sizeof_bool;
  int32_t sizeof_char;
  int32_t sizeof_short;
  int32_t sizeof_int;
  int32_t sizeof_long;
  int32_t sizeof_long_long;
  int32_t sizeof_float;
  int32_t sizeof_double;
  int32_t sizeof_long_double;
  int32_t sizeof_void_p;
  int32_t sizeof_logical;
  int32_t sizeof_integer;
  int32_t sizeof_real;
  int32_t sizeof_char;
} MPIABI_info;

This information would not be necessary to use the ABI, but it will be
convenient to check assumptions. Unsupported types report a size of
-1.

This struct can be extended in the future.

-erik

On Wed, Aug 16, 2023 at 4:09 AM Jeff Hammond <jeff.science at gmail.com> wrote:
>
> As we all know, Fortran users are permitted to change the size of INTEGER with compiler flags.  This creates a problem for the MPI C ABI.
>
> I have considered three options and would like your opinions on them:
>
> 1. MPI_Fint is defined to be a C int in the standard ABI.  This means MPI implementations that elect to provide an ILP64 MPI Fortran ABI on an I32LP64 system, for example, must not call this an implementation of the standard ABI.
>
> This would make a lot of things easier, but it might cause problems in the rare cases where users are using ILP64 in Fortran and using C code that actually uses MPI_Fint.
>
> 2. MPI_Fint ceases to be a compile-time fixed type in the standard ABI.  The C code that uses MPI_Fint must make an MPI library call to figure out the size and do all of the necessary things to call the correct size.
>
> 3. MPI_Fint is not a fixed size but the standard ABI specifies that Fortran handle values must always fit into the range of a C int and we rely on the pass-by-value behavior of all the f2c/c2f APIs to behave properly when they are truncated.
>
> This is my preference but it breaks Example 19.11 in a serious way, because that user code makes use of a MPI_Fint pointer.  However, this code isn't standard-compliant and relies on universal but non-standard argument passing conventions in Fortran.  Writing a standard-compliant interface from Fortran to C - by doing the cast to INTEGER(kind=c_int) in Fortran - would resolve this.
>
> Example 19.11 is awful (https://github.com/mpi-forum/mpi-issues/issues/739) but it is nonetheless important to instruct users that if Option 3 is implemented, they need to do the casting to INTEGER(kind=c_int) and know that this is safe with the standard ABI.
>
>
> None of these options is perfect.  I'm not sure how much code is even impacted by MPI_Fint so it's hard to know which option breaks the most stuff, although Option 2 seems the least desirable in any case.
>
> Thanks,
>
> Jeff
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
> --
> mpiwg-abi mailing list
> mpiwg-abi at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-abi



-- 
Erik Schnetter <schnetter at gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/