[MPI3 Fortran] Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION

[Rolf Rabenseifner]

> A very good question. Instead of referring to SEQUENCE, it should say
> that MPI_DOUBLE_PRECISION would use the numeric storage sequence rules
> (16.4.3.1 in Fortran 2003) for alignment.

Nick, 
please can you explain, what "16.4.3.1 in Fortran 2003" tells
about alignments. 
In SEQUENCE derived types and COMMON blocks,
when I understand this correctly, then DOUBLE PRECISION
must have also one numerical storage unit as alignment,
i.e., with REAL=4 bytes, DOUBLE PRECISION alignment must be
also 4 byte, i.e., exactly what the Intel compiler is doing
in a SEQUENCE derived type or COMMON block.

The implication is that the compiler issues nice "warnings" about
misaligned data.

Do I understand correctly, that the other compilers with 
REAL-size=4 byte and DOUBLE PRECISION size and alignment = 8 byte,
do it wrong, i.e.not according to Fortran 2003 or 2008?

Best regards
Rolf   

----- Original Message -----
> From: "N.M. Maclaren" <nmm1 at cam.ac.uk>
> To: "Fab Tillier" <ftillier at microsoft.com>
> Cc: "Rolf Rabenseifner" <rabenseifner at hlrs.de>, "Jeff Squyres" <jsquyres at cisco.com>, "Shinji Sumimoto"
> <s-sumi at labs.fujitsu.com>, "Hubert Ritzdorf" <hubert.ritzdorf at emea.nec.com>, "Howard Pritchard" <howardp at cray.com>,
> "Brian Smith" <smithbr at us.ibm.com>, "Charles J Archer" <archerc at us.ibm.com>, "Rajeev Thakur" <thakur at mcs.anl.gov>,
> "Bill Long" <longb at cray.com>, "Bill Gropp" <wgropp at uiuc.edu>, "Richard Graham" <rlgraham at ornl.gov>, "Iain Bason"
> <iain.bason at oracle.com>
> Sent: Wednesday, September 28, 2011 10:26:37 AM
> Subject: RE: Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION
> On Sep 27 2011, Fab Tillier wrote:
> >
> > It seems that all the a == 4 results come from using the Intel
> > Fortran
> > compiler suite. Given that Microsoft doesn't ship a Fortran
> > compiler,
> > mandating a == 8 doesn't put us in a position to succeed. Therefore,
> > I
> > cannot support such a proposal.
> 
> I agree that Microsoft should not be forgotten, but it is not a major
> platform for MPI use, and the tail should not wag the dog. We have the
> ghastly example of C99 long long where some people claimed Microsoft
> compatibility required an incompatible change to the standard; largely
> as a result, the official C standard is no longer used as a portable
> programming base by a good half of the C-using community.
> 
> > however, shouldn't a DOUBLE PRECISION BIND(C) derived type map to
> > MPI_DOUBLE, not MPI_DOUBLE_PRECISION?
> >
> > Can't we simply document that MPI_DOUBLE_PRECISION (and any Fortran
> > types
> > that need similar treatment) apply only to SEQUENCE derived types,
> > while
> > BIND(C) derived types should use the C types?
> 
> A very good question. Instead of referring to SEQUENCE, it should say
> that MPI_DOUBLE_PRECISION would use the numeric storage sequence rules
> (16.4.3.1 in Fortran 2003) for alignment.
> 
> 
> Slightly related to this, can't we PLEASE deprecate those horrible
> MPI_INTEGER8 etc. types? They have been a portability disaster for
> half a century, even among Fortran IV compilers (where they started)!
> MPI has provided some perfectly good mechanisms to use Fortran's
> selectable KINDs. The reason that it is related is that there is no
> guarantee that a compiler will give them the same alignment as another
> type even if they use the same storage, because they are non-standard
> and therefore any specification is the compiler's choice.
> 
> 
> Regards,
> Nick Maclaren.

-- 
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: Allmandring 30)