[mpich-discuss] Erroneous MPI type tag warning

[Junchao Zhang]

Oops, MPI_MAXLOC requires MPI_DOUBLE_INT and the like.
The MPI_Reduce(in, out, 2, INT, MPI_MAXLOC, ...)  approach is incorrect.


--Junchao Zhang


On Sun, Dec 22, 2013 at 10:58 PM, Junchao Zhang <jczhang at mcs.anl.gov> wrote:

> Hi, Markus,
>   I think you should change MPI_Reduce(in, out, 1, MPI_2INT, MPI_MAXLOC,
> 0, MPI_COMM_WORLD)  to  MPI_Reduce(in, out, 2, INT, MPI_MAXLOC, 0,
> MPI_COMM_WORLD).
>   I guess the warning has nothing to do with padding. 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}.
>
>   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. For example, when matching a pointer P with a type T, the compiler
> should try to match typeof(*P) ~ T, typeof(*P)[2] ~T, typeof(*P)[3] ~ T,
> etc.
>
>
> --Junchao Zhang
>
>
> On Sun, Dec 22, 2013 at 3:56 AM, Markus Geimer <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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