[Mpi3-rma] Target displacement sign issue

[Pavan Balaji]

FYI, casting back MPI_Aint to a pointer was never safe (even in 
MPI-2.2), since MPI_Aint can be larger than void *.

  -- Pavan

On 09/05/2012 09:13 AM, Jim Dinan wrote:
> Hi Hubert,
>
> If I understand this correctly, then one would no longer be able to cast
> an MPI_Aint back to a pointer.  We would have to add another routine,
> MPI_Get_pointer_dynamic() to do this conversion.
>
>    ~Jim.
>
> On 9/4/12 6:25 PM, Hubert Ritzdorf wrote:
>>
>> Hi,
>>
>> Possibly another solution:
>>
>> (*) Let MPI_AINT_MAX the maximal value of MPI_Aint variable
>> (*) Let the window_base of a window created by MPI_WIN_CREATE_DYNAMIC
>>       be MPI_AINT_MAX + 1
>> (*) Define a new function MPI_Get_address_dynamic returning the
>>       address of a location relative to MPI_AINT_MAX + 1
>> (*) Remove the non-negative qualification in the description
>>       of target_disp in MPI_{Put/Get/Accumulate/...).
>>
>> Hubert
>>
>>> -----Original Message-----
>>> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
>>> bounces at lists.mpi-forum.org] On Behalf Of Hubert Ritzdorf
>>> Sent: Tuesday, September 04, 2012 7:23 PM
>>> To: MPI 3.0 Remote Memory Access working group
>>> Subject: Re: [Mpi3-rma] Target displacement sign issue
>>>
>>> Hi,
>>>
>>> MPI_Get_address returns an MPI_Aint which is signed.(integer).
>>>
>>> C:
>>> MPI_Aint's may be used in Reduce Operations with Min/Max
>>> or the application can internally use min/max computations.
>>> Can anybody guarantee that applications don't perform
>>> min/max operations on these displacements (which results in different
>>> results for unsigned and signed variables) or use derived values of these
>>> displacements in other signed/unsigned significant cases.
>>>
>>> Fortran:
>>> MPI_BOTTOM is a variable in a common block/module. Thus,
>>> MPI_Get_address might really return a signed integer (depending on the
>>> compiler).
>>> Fortran doesn't have unsigned integers and cannot handle them.
>>> Thus, the non-negative integer specification for target_disp in
>>> MPI_{Put/Get/Accumulate/...)
>>> might be incorrect for Fortran and MPI_Win_Create_dynamic().
>>>
>>> Hubert
>>> ________________________________________
>>> From: mpi3-rma-bounces at lists.mpi-forum.org [mpi3-rma-bounces at lists.mpi-
>>> forum.org] on behalf of Jim Dinan [dinan at mcs.anl.gov]
>>> Sent: Tuesday, September 04, 2012 5:10 PM
>>> To: mpi3-rma at lists.mpi-forum.org
>>> Subject: Re: [Mpi3-rma] Target displacement sign issue
>>>
>>> Hi All,
>>>
>>> This is an important issue that we need to resolve before releasing the
>>> 3.0 spec.  Please take a look and send feedback.
>>>
>>> Dave has voiced a concern that the implicit type conversion in my
>>> proposed text below may not be portable.  If anyone has more experience
>>> with this aspect (signed->unsigned conversion) of C/Fortran/etc type
>>> systems, please chime in.
>>>
>>>     ~Jim.
>>>
>>> On 8/28/12 2:29 PM, Jim Dinan wrote:
>>>> Hi All,
>>>>
>>>> Here is the suggested change to address this issue.  In the paragraph on
>>>> page 425, line 1, add the bracketed sentence:
>>>>
>>>> In the case of a window created with MPI_WIN_CREATE_DYNAMIC, the
>>>> target_disp for all RMA functions is the address at the target; i.e.,
>>>> the effective window_base is MPI_BOTTOM and the disp_unit is one. [Thus,
>>>> for dynamic windows, the target_disp argument to all RMA functions in
>>>> interpreted as unsigned.] Users should use MPI_GET_ADDRESS at the target
>>>> process to determine the address of a target memory location and
>>>> communicate this address to the origin process.
>>>>
>>>>      ~Jim.
>>>>
>>>> On 8/28/12 9:46 AM, Jim Dinan wrote:
>>>>> Hi All,
>>>>>
>>>>> Currently, target displacements in put/get/acc/etc are defined to be
>>>>> non-negative integers of type, MPI_Aint.  MPI_Aint is defined to be a
>>>>> signed integer.  However, displacements associated with dynamic windows
>>>>> are addresses (unsigned) and can range into the space of negative
>>>>> integers.
>>>>>
>>>>> It seems like the least impact change would be to add a special case
>>>>> that the displacement argument will be interpreted as unsigned for
>>>>> dynamic windows.  Changing the interface to take an 'unsigned MPI_Aint'
>>>>> would be the most clear (and preserve the existing and intended new
>>>>> semantics), but would break backward compatibility.
>>>>>
>>>>> Interested to hear your thoughts.
>>>>>
>>>>>     ~Jim.
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-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji