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Subject: Re: [OMPI users] Use of __float128 with openmpi
From: Patrick Boehl (Patrick.Boehl_at_[hidden])
Date: 2014-02-02 07:41:13


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.
>>
>>
>> _______________________________________________
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>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
>
> --
> Jeff Hammond
> jeff.science_at_[hidden]
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