[MPIWG Fortran] "ELEMENTAL" keyword on mpi_f08 comparison operator functions

Sat Mar 12 01:45:32 CST 2022

Hello Rolf, 

apologies for the delayed response. Here my comments:

1. If an MPI implementation already implements the specifics with assumed rank dummy arguments (with run-time checks for conformity of rank and shape), 
    making these ELEMENTAL would not be possible. One would need to change the implementation to have scalar arguments. Similar if multiple specifics
    for different ranks are implemented.
2. Adding ELEMENTAL in an implementation will necessitate recompilation of application codes 
3. Lack of portability for application codes seems to me to be less of a  problem than it is now, i.e., adding the ELEMENTAL requirement in the spec would 
    simply assure that implementations are not permitted to limit themselves to the scalar case.

Regards
Reinhold

> -----Ursprüngliche Nachricht-----
> Von: Rolf Rabenseifner <rabenseifner at hlrs.de>
> Gesendet: Dienstag, 8. März 2022 11:59
> An: Bader, Reinhold <Reinhold.Bader at lrz.de>
> Cc: Jeff Hammond <jeff.science at gmail.com>; MPI-WG Fortran working
> group <mpiwg-fortran at lists.mpi-forum.org>
> Betreff: Re: [MPIWG Fortran] "ELEMENTAL" keyword on mpi_f08 comparison
> operator functions
> 
> Hello Reinhold,
> 
> can you give as background information about this question on ELEMENTAL
> below.
> 
> 
> Our MPI handles are defined according to MPi-4.0 page 18 as
> 
>    TYPE, BIND(C) :: MPI_Comm
>      INTEGER :: MPI_VAL
>    END TYPE MPI_Comm
> 
> and same for MPI_Group, ...
> 
> 
> The MPI standard specifies for both the mpi and the mpi_f08 module:
> 
> mpi_f08 in MPI-4.0 Section 19.1.2 page 793 lines 23-24:
> >    * Overload the operators .EQ. and .NE. to allow the comparison of these
> MPI
> >    handles with .EQ. , .NE. , == and /= .
> 
> mpi module in MPI-4.0 Section 19.1.3 page 795 lines 35-37:
>    * Defi
> 
> ne the derived type MPI_Status and all named handle types that are used in
> the
>    mpi_f08 module. For these named handle types, overload the operators
> .EQ. and
>    .NE. to allow handle comparison via the .EQ., .NE., == and /= operators.
> 
> Best regards
> Rolf
> 
> 
> ----- Original Message -----
> > From: "MPI-WG Fortran working group"
> > <mpiwg-fortran at lists.mpi-forum.org>
> > To: "MPI-WG Fortran working group" <mpiwg-fortran at lists.mpi-
> forum.org>
> > Cc: "Jeff Hammond" <jeff.science at gmail.com>
> > Sent: Tuesday, March 8, 2022 11:01:13 AM
> > Subject: Re: [MPIWG Fortran] "ELEMENTAL" keyword on mpi_f08
> comparison
> > operator functions
> 
> > It should be correct to add ELEMENTAL here and we should encourage that.
> >
> > I would argue that we should require ELEMENTAL in the place where it
> says:
> >
> >
> >    * Overload the operators .EQ. and .NE. to allow the comparison of these
> MPI
> >    handles with .EQ. , .NE. , == and /= .
> > [ https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node409.htm |
> > https://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node409.htm ] (I
> > assume the text is unchanged in MPI-4)
> >
> > Jeff
> >
> > On Tue, Mar 8, 2022 at 1:19 AM Jeff Squyres (jsquyres) via
> > mpiwg-fortran < [ mailto:mpiwg-fortran at lists.mpi-forum.org |
> > mpiwg-fortran at lists.mpi-forum.org ]
> > > wrote:
> >
> >
> > Forum Fortran WG --
> >
> > Any comments?
> >
> > I got one off-list reply explaining the use of ELEMENTAL, but I'm
> > curious as to what the Forum's response should be.
> >
> > I ask because the use of ELEMENTAL has an impact on MPI application
> portability.
> > E.g., if the Open MPI family of implementations use ELEMENTAL but the
> > MPICH family of implementations do not, that could be confusing for users.
> >
> > --
> > Jeff Squyres
> > [ mailto:jsquyres at cisco.com | jsquyres at cisco.com ]
> >
> > ________________________________________
> > From: mpiwg-fortran < [
> > mailto:mpiwg-fortran-bounces at lists.mpi-forum.org |
> > mpiwg-fortran-bounces at lists.mpi-forum.org ] > on behalf of Jeff
> > Squyres
> > (jsquyres) via mpiwg-fortran < [
> > mailto:mpiwg-fortran at lists.mpi-forum.org |
> > mpiwg-fortran at lists.mpi-forum.org ] >
> > Sent: Tuesday, March 1, 2022 10:24 AM
> > To: MPI Forum
> > Cc: Jeff Squyres (jsquyres)
> > Subject: [MPIWG Fortran] "ELEMENTAL" keyword on mpi_f08 comparison
> > operator functions
> >
> > An Open MPI user this morning requested that we add the "ELEMENTAL"
> > keyword to the mpi_f08 module eq / neq operator functions: [
> > https://github.com/open-mpi/ompi/issues/10057 |
> > https://github.com/open-mpi/ompi/issues/10057 ]
> >
> > The only statement I can find in MPI-4.0 about the eq / neq operators
> > is section 19.1.2, "Fortran Support Through the mpi_f08 Module", only
> > says the following about the operator functions (p793, 22:23):
> >
> >> Overload the operators .EQ. and .NE. to allow the comparison of these
> >> MPI handles with .EQ., .NE., == and /=.
> >
> > This doesn't seem to allow or disallow the use of ELEMENTAL.
> >
> > What are the pros / cons of adding ELEMENTAL to the eq / neq operator
> functions?
> >
> > --
> > Jeff Squyres
> > [ mailto:jsquyres at cisco.com | jsquyres at cisco.com ]
> > _______________________________________________
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> >
> >
> > --
> > Jeff Hammond
> > [ mailto:jeff.science at gmail.com | jeff.science at gmail.com ] [
> > http://jeffhammond.github.io/ | http://jeffhammond.github.io/ ]
> >
> > _______________________________________________
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> > mpiwg-fortran at lists.mpi-forum.org
> > https://lists.mpi-forum.org/mailman/listinfo/mpiwg-fortran
> 
> --
> Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de .
> High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 .
> University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 .
> Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner .
> Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .