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

Tue Jun 26 09:20:43 CDT 2012

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
>>>>> 
>>>> 
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>> 
>> 
>> --
>> 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
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-- 
Jeff Squyres
jsquyres at cisco.com
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/