[mpich-discuss] Erroneous MPI type tag warning

[Rajeev Thakur]

We will fix it. A ticket has been created.
http://trac.mpich.org/projects/mpich/ticket/1993

Rajeev

On Dec 23, 2013, at 10:55 AM, Markus Geimer <m.geimer at fz-juelich.de> wrote:

> Hi,
> 
>>  I guess the warning has nothing to do with padding.
> 
> Agreed. The warning is due to the type_tag attributes defined in
> MPICH's 'mpi.h' header file.
> 
>> For MPI_Reduce(in, out, 1, MPI_2INT, ...), the compiler tries to
>> match int, i.e., typeof(*in), with MPI_2INT.  Whatever the layout of
>> MPI_2INT is defined (e.g., struct {int i1; int i2} or int[2]), the
>> compiler fails at matching. If you want to use MPI_2INT, then in, out
>> should be defined as of type struct {int v1;  int v2}.
> 
> Well, according to the C99 standard "Each non-bit-field member of a
> structure or union object is aligned in an *implementation-defined*
> manner appropriate to its type." (§6.7.2.1) It is therefore not
> guaranteed that the two ints in the struct are contiguous -- which
> means that, strictly speaking, int[2] is the only portable choice.
> (Although I'm not aware of any compiler that would insert padding
> between the two ints, i.e., in practice the struct should be fine,
> too.)
> 
>>  I agree that MPI_Reduce(in, out, 1, MPI_2INT, ...) is absolutely
>> right.  I think the only reason for the warning is that the compiler is
>> not smart enough.
> 
> Compilers could always be smarter ;-) It would be great if you could
> come up with a datatype that is -- from the compiler's perspective --
> compatible with both struct {int i1; int i2} and int[2]. Otherwise,
> I suggest to remove the type check for MPI_2INT as the warning is
> misleading.
> 
> Thanks,
> Markus
> 
> 
>> On Sun, Dec 22, 2013 at 3:56 AM, Markus Geimer <m.geimer at fz-juelich.de
>> <mailto:m.geimer at fz-juelich.de>> wrote:
>> 
>>    Dear MPICH developers,
>> 
>>    When compiling the attached example with MPICH 3.1rc2 using Clang 3.3,
>>    I get the following compiler warnings:
>> 
>>    -------------------- snip --------------------
>>    mpi2int.c:17:20: warning: argument type 'int *' doesn't match specified
>>    'MPI' type tag [-Wtype-safety]
>>        MPI_Reduce(in, out, 1, MPI_2INT, MPI_MAXLOC, 0, MPI_COMM_WORLD);
>>                       ^~~     ~~~~~~~~
>>    mpi2int.c:17:16: warning: argument type 'int *' doesn't match specified
>>    'MPI' type tag [-Wtype-safety]
>>        MPI_Reduce(in, out, 1, MPI_2INT, MPI_MAXLOC, 0, MPI_COMM_WORLD);
>>                   ^~          ~~~~~~~~
>>    2 warnings generated.
>>    -------------------- snip --------------------
>> 
>>    According to the MPI standard, however, MPI_2INT is a datatype as if
>>    defined by
>> 
>>            MPI_Type_contiguous(2, MPI_INT, MPI_2INT)
>> 
>>    i.e., 'int[2]' should be a perfect match. This is not necessarily true
>>    for the type used for comparison,
>> 
>>            struct mpich_struct_mpi_2int { int i1; int i2; };
>> 
>>    which will only be contiguous if the compiler does not add any padding.
>> 
>>    Is there any chance this gets fixed for the final 3.1 release? Or did
>>    I miss something?
>> 
>>    Thanks,
>>    Markus
>> 
> 
> --
> Dr. Markus Geimer
> Juelich Supercomputing Centre
> Institute for Advanced Simulation
> Forschungszentrum Juelich GmbH
> 52425 Juelich, Germany
> 
> Phone:  +49-2461-61-1773
> Fax:    +49-2461-61-6656
> E-mail: m.geimer at fz-juelich.de
> WWW:    http://www.fz-juelich.de/jsc/
> 
> 
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