[J3] [SC22WG5.6583] Fortran type sizes - please help with MPI Fortran

[Brad Richardson]

Hi Jeff,

It is my expectation that there is virtually no existing code using MPI
where "INTEGER is INTEGER(kind=c_int)" is not the case, else it
wouldn't have worked in the first place. Unless you're telling me that
somebody is building a version of an MPI library and the Fortran
interface to it where that isn't the case, but if so I have faith they
could figure out how to continue doing weird things with the right
compiler options.

Brad

On Fri, 2024-03-29 at 17:54 +0000, Jeff Hammond via J3 wrote:
> 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
> > 
>