[J3] [SC22WG5.6576] Fortran type sizes - please help with MPI Fortran

[Jeff Hammond]

MPI has three types of Fortran support.  The (deprecated) mpif.h and MPI.mod support an old-fashioned API, where we only use the primitive types of Fortran 77.  At least with MPI.mod, the compiler can do interface checking, although the type-safety is quite weak given most arguments are INTEGER.  The newer MPI_F08.mod, which Bill Long was instrumental in creating and which WG5 made possible with the Fortran 2018 CFI extensions, is a properly typed, modern Fortran API.

The idea that "it will work" is equivalent to MPI asserting that INTEGER is INTEGER(kind=c_int) or the MPI ABI doesn’t work, which certain people will not accept unless WG5 provides some sort of evidence that I am not a lunatic for wanting to assert this.

Jeff

> On 29. Mar 2024, at 19.33, Thomas König <tk at tkoenig.net> wrote:
>
> External email: Use caution opening links or attachments
>
>
> Am 29.03.24 um 16:24 schrieb Jeff Hammond via J3:
>> The issue is, we have 30 years of MPI Fortran code that does things
>> that are correct according to MPI and Fortran, but which are broken if
>> we do certain things that are otherwise very nice from a software
>> design perspective.  Breaking user code is bad for business, as
>> everybody here knows :-)
>>
>> If we could redefine MPI Fortran to use ISO_C_BINDING types, we’d have
>> zero problems, but ISO_C_BINDING came a decade after MPI was created
>> and Fortran 2003 compilers weren’t widely available until well after 2003.
>
> How does MPI define its interfaces?  Does it use Fortran modules or
> the traditional F77-style ABI (with no checking) that f2c etc use?
>
> For the former, C interoperable types could be used; I think all
> major compilers support it by now.
>
> For Linux and at least some BSD variants on AMD64, the latter is
> defined in the x86-64-ABI psABI, which can be found at
> https://gitlab.com/x86-psABIs/x86-64-ABI (section 9.2),
> and major other architectures and operating systems effectively do
> the same.
>
> Maybe defining the MPI interface with respect to an existing ABI
> would be less onerous than just saying "it will work".
>
> Best regards
>
>    Thomas
>