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Subject: Re: [OMPI users] Use of __float128 with openmpi
From: Jeff Hammond (jeff.science_at_[hidden])
Date: 2014-02-01 17:07:19

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

There is a proposal to support __float128 in the MPI standard in the
future but it has not been formally considered by the MPI Forum yet



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
>> where they state
>> ----
>> 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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Jeff Hammond