[MPIWG Fortran] FP16 16-bit-floating-point

[Bill Long]

Hi Rolf,

The Fortran 2015 draft is close to finished.  Its IEEE-isms are based on ISO/IEC/IEEE 60559:2011, and includes a lot of changes based on that standard.   If this FP16 feature is part of that standard, then we should look into making a change now.  Otherwise, it would likely wait for the next revision.

As Reinhold pointed out, the parameterized KIND scheme Fortran uses already permits identifying a KIND value for 16-bit floats, assuming the processor (i.e. compiler) knows about such a type. 

Changes to the standard would be minimal - add REAL16 to the iso_fortran_env module, with the value of -1 if that kind of real is not supported, and the value of 2 for most compilers if it is.  Changes to the compiler would be significant.  So implementation would likely wait for customer demand. 

Cheers,
Bill



> On Jun 16, 2017, at 2:44 PM, Rolf Rabenseifner <rabenseifner at hlrs.de> wrote:
> 
> (Because I'm not member of j3, only Reinhold's or Rich's answers will be visible for j3)
> 
> Based on Reinhold's answer, I make my questions more precise:
> 
> - Are there already some activities in J3 on this FP16 topic?
> 
> - When will the Fortran standard include REAL16 to support the new IEEE FP16 standard? 
> 
> Best regards
> Rolf
> 
> 
> ----- Original Message -----
>> From: "Reinhold Bader" <Reinhold.Bader at lrz.de>
>> To: "Rolf Rabenseifner" <rabenseifner at hlrs.de>, "fortran standards email list for J3" <j3 at j3-fortran.org>
>> Cc: "MPI-WG Fortran working group" <mpiwg-fortran at lists.mpi-forum.org>, "Atsushi Hori" <ahori at riken.jp>, "Martin Schulz"
>> <schulzm at llnl.gov>, "MPI WG Point To Point Communications working group" <mpiwg-p2p at lists.mpi-forum.org>
>> Sent: Friday, June 16, 2017 2:31:03 PM
>> Subject: AW: FP16 16-bit-floating-point
> 
>> Hello Rolf,
>> 
>> to the best of my knowledge, the answers are as follows:
>> 
>> Q1: Not in the sense of having an explicit name like the REAL32, REAL64, REAL128
>> names for specific integer kinds that were added to Fortran 2008.
>>       However, it is left free to implementors to create a new kind number in the
>>       implementation that could be returned by SELECTED_REAL_KIND() and,
>>        if IEEE conforming, by IEEE_SELECTED_REAL_KIND() and consequently all relevant
>>        IEEE support routines.
>> 
>> Q2: REAL*2 is (like REAL*8) a non-conforming notation (REAL*8 is supported as an
>> extension by many implementations), so the answer is no.
>>       A proper declaration might look like
>> 
>>       integer, parameter :: rk = selected_real_kind(2,6)    ! at least 2 decimal
>>       digits and 6 maximal decimal exponent - hope I have this right ...
>>       real(KIND=rk) :: x
>> 
>>      with the following caveats:
>>     * the KIND that fits the requirement most closely will be used, so it might be
>>     necessary to use the STORAGE_SIZE intrinsic to check what one gets.
>>     * KIND numbers are non-portable, so might vary for one and the same kind between
>>     implementations (at least one compiler I know deviates from all others I know).
>> 
>> I checked three of the compilers I know: they currently do not support FP16.
>> 
>> Regards
>> Reinhold
>> 
>> 
>>> -----Ursprüngliche Nachricht-----
>>> Von: Rolf Rabenseifner [mailto:rabenseifner at hlrs.de]
>>> Gesendet: Freitag, 16. Juni 2017 13:53
>>> An: fortran standards email list for J3 <j3 at j3-fortran.org>; Bader, Reinhold
>>> <Reinhold.Bader at lrz.de>
>>> Cc: MPI-WG Fortran working group <mpiwg-fortran at lists.mpi-forum.org>;
>>> Atsushi Hori <ahori at riken.jp>; Martin Schulz <schulzm at llnl.gov>; MPI WG
>>> Point To Point Communications working group <mpiwg-p2p at lists.mpi-
>>> forum.org>
>>> Betreff: FP16 16-bit-floating-point
>>> 
>>> Hi all together,
>>> 
>>> I'm sitting in the MPI Forum Meeting and we discuss the new IEEE FP16
>>> datatype.
>>> 
>>> Questions to the Fortran atandardization body:
>>> 
>>> - Does the Fortran Standardization Body has already a proposal for
>>>  the new name in Fortran for this IEEE FP16?
>>> 
>>> - Will be REAL*2 identical to IEEE FP16?
>>> 
>>> Best regards
>>> Rolf
>>> 
>>> --
>>> 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) .
> 
> -- 
> 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) .
> _______________________________________________
> mpiwg-fortran mailing list
> mpiwg-fortran at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-fortran

Bill Long                                                                       longb at cray.com
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Bioinformatics Software Development                      fax:  651-605-9143
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