[Mpi-forum] [EXTERNAL] Re: C++ types inaccessible after #281

Tue Jun 26 09:43:25 CDT 2012

In MPI-2, we supported float, double, and long double as the underlying
type for std::complex with MPI::COMPLEX, MPI::DOUBLE_COMPLEX, and
MPI::LONG_DOUBLE_COMPLEX, respectively.  That seems like a good middle
ground...

Brian

On 6/26/12 8:33 AM, "Jeff Hammond" <jhammond at alcf.anl.gov> wrote:

>So one issue to consider here is that std::complex is a template, not
>a single type.  One can presumably stick lots of types inside the
>brackets.
>
>So the MPI Forum needs to figure out which of the existing C99 types
>that will work with this.  Do we support std::complex<long double>?
>How about std::complex<uint64_t>?
>
>My reference is http://www.cplusplus.com/reference/std/complex/complex/,
>btw.
>
>Jeff
>
>On Tue, Jun 26, 2012 at 9:20 AM, Jeff Squyres <jsquyres at cisco.com> wrote:
>> Not for many years.  :-)  (/me puts finger on nose)
>>
>> I originally authored the C++ bindings in the mid-90's (ouch!), when I
>>was *much* more active in the C++ community than I am today.
>>
>>
>>
>> On Jun 26, 2012, at 10:16 AM, Jeff Hammond wrote:
>>
>>> Um, aren't you sort of the C++ expert attending the Forum right now?
>>>
>>> :-)
>>>
>>> Jeff
>>>
>>> On Tue, Jun 26, 2012 at 9:13 AM, Jeff Squyres <jsquyres at cisco.com>
>>>wrote:
>>>> Yes, I think that Jed has a good point here.
>>>>
>>>> There are a few C++-specific datatypes for which it would be worth
>>>>introducing new C datatypes (e.g., MPI_CXX_COMPLEX).  It's too bad
>>>>that no one realized this until now, because this won't make the MPI-3
>>>>deadline.  But it could be part of MPI-3.1 or errata, or some such.
>>>>
>>>> Is there a C++ person out there who could draft a proposal about this?
>>>>
>>>>
>>>> On Jun 26, 2012, at 1:02 AM, Jed Brown wrote:
>>>>
>>>>> I'm all for getting rid of the C++ bindings and see no compelling
>>>>>reason for anyone to use them.
>>>>>
>>>>> Predefined types are inconsequential to MPI-1 functionality, but
>>>>>they are special for one-sided, so availability of types is highly
>>>>>relevant to applications. This is a non-cosmetic change imposed by
>>>>>#281 that doesn't seem to have been discussed.
>>>>>
>>>>> On Jun 25, 2012 8:51 PM, "Fab Tillier" <ftillier at microsoft.com>
>>>>>wrote:
>>>>> Ah, thanks, sorry for being dense (I'm not familiar with the C++
>>>>>bindings as we don't ship them for MSMPI).
>>>>>
>>>>> Ticket 281 removes the entire C++ bindings from the standard,
>>>>>MPI::COMPLEX included.  You'd need to convince your MPI library
>>>>>vendor to keep supplying you with an MPI-2.2 compliant version of the
>>>>>C++ bindings (which is still allowed by ticket #281).
>>>>>
>>>>> Hopefully someone with more experience with the C++ bindings can
>>>>>chime in, but I'd expect the answer to your original question is yes,
>>>>>ticket #281 intentionally removes all C++ support from MPI-3.
>>>>>
>>>>> -Fab
>>>>>
>>>>> Jed Brown wrote on Mon, 25 Jun 2012 at 21:31:55
>>>>>
>>>>>> MPI::COMPLEX is not the same as MPI_C_COMPLEX (and the latter is not
>>>>>> available on systems without C99).
>>>>>
>>>>>> On Jun 25, 2012 8:21 PM, "Fab Tillier" <ftillier at microsoft.com>
>>>>>>wrote:
>>>>>> Hi Jed,
>>>>>>
>>>>>> Maybe I'm not following you, but what predefined MPI datatype
>>>>>>supports
>>>>>> std::complex today?  My understanding is that there is no support
>>>>>>for
>>>>>> std::complex in MPI today, and that ticket 281 does not change this
>>>>>>at
>>>>>> all.
>>>>>>
>>>>>> Thanks,
>>>>>> -Fab
>>>>>>
>>>>>> Jed Brown wrote on Mon, 25 Jun 2012 at 20:42:53
>>>>>>
>>>>>>> I see that std::complex will not be accessible in a predefined
>>>>>>>type if
>>>>>>> #281 passes in its current form. Do you intend for there to be a
>>>>>>> supported/recommended way for a C++ caller to get a complex
>>>>>>>predefined
>>>>>>> (so that one-sided can be used) type? Note that C99 is not a
>>>>>>>subset of
>>>>>>> any C++ and C99 complex cannot be used portably from C++ (even with
>>>>>>> compiler suites that happen to support both).
>>>>>>>
>>>>>>> On Jun 25, 2012 7:07 PM, "Fab Tillier" <ftillier at microsoft.com
>>>>>>> <mailto:ftillier at microsoft.com> > wrote: Hi Jed,
>>>>>>>
>>>>>>> Jed Brown wrote on Mon, 25 Jun 2012 at 16:43:54
>>>>>>>
>>>>>>>> Is it intended that #281 would make std::complex inaccessible?
>>>>>>>>Should
>>>>>>>> there be a MPI_CXX_COMPLEX, MPI_CXX_DOUBLE_COMPLEX,
>>>>>>>> MPI_CXX_LONG_DOUBLE_COMPLEX, and (possibly) MPI_CXX_BOOL?
>>>>>>>
>>>>>>> I don't think ticket #281 does anything with respect to
>>>>>>>std::complex
>>>>>>> accessibility.  The MPI standard defines the MPI_C_COMPLEX and
>>>>>>>family
>>>>>>> as mapping to the C99 _Complex type (see Table 3.2, Predefined MPI
>>>>>>> datatypes corresponding to C datatypes).  The standard is clear
>>>>>>>that
>>>>>>> there are no corresponding C++ bindings.
>>>>>>>
>>>>>>>> Note that C99 complex is not a substitute because Microsoft does
>>>>>>>>not
>>>>>>>> implement it and it has different semantics regarding numerical
>>>>>>>> stability. (Well, C99 semantics are specified and useful,
>>>>>>>>std::complex
>>>>>>>> stability is not specified by any C++ standard and in practice,
>>>>>>>>is not
>>>>>>>> implemented in a stable way.) This is not such a big deal for use
>>>>>>>>with
>>>>>>>> collectives because user-defined MPI_Ops can be used, but only
>>>>>>>> predefined types can be used with one-sided operations, so
>>>>>>>>presence of
>>>>>>>> complex datatypes (or lack thereof) is relevant to applications.
>>>>>>>
>>>>>>> The fact that Microsoft does not support C99 is lame.  You can
>>>>>>>probably
>>>>>>> get away using std::complex as a stand-in for C99 _Complex (but
>>>>>>>then
>>>>>>> you're on your own), or move to a compiler that does (I believe the
>>>>>>> Intel compiler supports C99 on Windows).
>>>>>>>
>>>>>>> -Fab
>>>>>>>
>>>>>>
>>>>> _______________________________________________
>>>>> mpi-forum mailing list
>>>>> mpi-forum at lists.mpi-forum.org
>>>>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>>>>
>>>>
>>>> --
>>>> Jeff Squyres
>>>> jsquyres at cisco.com
>>>> For corporate legal information go to:
>>>>http://www.cisco.com/web/about/doing_business/legal/cri/
>>>>
>>>>
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>>>
>>>
>>>
>>> --
>>> Jeff Hammond
>>> Argonne Leadership Computing Facility
>>> University of Chicago Computation Institute
>>> jhammond at alcf.anl.gov / (630) 252-5381
>>> http://www.linkedin.com/in/jeffhammond
>>> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
>>>
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>>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>>
>>
>> --
>> Jeff Squyres
>> jsquyres at cisco.com
>> For corporate legal information go to:
>>http://www.cisco.com/web/about/doing_business/legal/cri/
>>
>>
>> _______________________________________________
>> mpi-forum mailing list
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>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>
>
>
>-- 
>Jeff Hammond
>Argonne Leadership Computing Facility
>University of Chicago Computation Institute
>jhammond at alcf.anl.gov / (630) 252-5381
>http://www.linkedin.com/in/jeffhammond
>https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
>
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>

-- 
  Brian W. Barrett
  Dept. 1423: Scalable System Software
  Sandia National Laboratories