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Subject: Re: [OMPI users] Use of __float128 with openmpi
From: George Bosilca (bosilca_at_[hidden])
Date: 2014-02-02 08:15:51


Just go for the most trivial:

MPI_Type_contiguous(sizeof(__float128), MPI_BYTE, &my__float128);

A little bit more info about the optional quad-precision floating-point format is available on Wikipedia (https://en.wikipedia.org/wiki/Double-double_%28arithmetic%29#Double-double_arithmetic).

  George.

On Feb 2, 2014, at 13:41 , Patrick Boehl <Patrick.Boehl_at_[hidden]> wrote:

> Hello Jeff,
>
> thank you a lot for your reply!
>
> On 01.02.2014, at 23:07, Jeff Hammond wrote:
>
>> See Section 5.9.5 of MPI-3 or the section named "User-Defined
>> Reduction Operations" but presumably numbered differently in older
>> copies of the MPI standard.
>>
>> An older but still relevant online reference is
>> http://www.mpi-forum.org/docs/mpi-2.2/mpi22-report/node107.htm
>>
>
> In this example they construct this "datatype"
>
> ---------
> typedef struct {
> double real,imag;
> } Complex
> ---------
>
> and later
>
> ---------
> MPI_Datatype ctype;
> /* explain to MPI how type Complex is defined
> */
> MPI_Type_contiguous(2, MPI_DOUBLE, &ctype);
> ---------
>
> Do I understand correctly that I have to find out how __float128 is constructed internally and
> convert it to a form which is compatible with the standard MPI Datatypes?
> In an analogue way as they do in the example. Up to now, I only found out that __float128 should
> be somehow the sum of two doubles.
>
> Again, I am grateful for any help!
>
> Best regards,
> Patrick
>
>
>
>
>> On Sat, Feb 1, 2014 at 2:28 PM, Tim Prince <n8tm_at_[hidden]> wrote:
>>>
>>> On 02/01/2014 12:42 PM, Patrick Boehl wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I have a question on datatypes in openmpi:
>>>>
>>>> Is there an (easy?) way to use __float128 variables with openmpi?
>>>>
>>>> Specifically, functions like
>>>>
>>>> MPI_Allreduce
>>>>
>>>> seem to give weird results with __float128.
>>>>
>>>> Essentially all I found was
>>>>
>>>> http://beige.ucs.indiana.edu/I590/node100.html
>>>>
>>>> where they state
>>>> ----
>>>> MPI_LONG_DOUBLE
>>>> This is a quadruple precision, 128-bit long floating point number.
>>>> ----
>>>>
>>>> But as far as I have seen, MPI_LONG_DOUBLE is only used for long doubles.
>>>>
>>>> The Open MPI Version is 1.6.3 and gcc is 4.7.3 on a x86_64 machine.
>>>>
>>> It seems unlikely that 10 year old course notes on an unspecified MPI
>>> implementation (hinted to be IBM power3) would deal with specific details of
>>> openmpi on a different architecture.
>>> Where openmpi refers to "portable C types" I would take long double to be
>>> the 80-bit hardware format you would have in a standard build of gcc for
>>> x86_64. You should be able to gain some insight by examining your openmpi
>>> build logs to see if it builds for both __float80 and __float128 (or
>>> neither). gfortran has a 128-bit data type (software floating point
>>> real(16), corresponding to __float128); you should be able to see in the
>>> build logs whether that data type was used.
>>>
>>>
>>> _______________________________________________
>>> users mailing list
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
>>
>>
>> --
>> Jeff Hammond
>> jeff.science_at_[hidden]
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
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