[mpich-discuss] Erroneous MPI type tag warning

[Junchao Zhang]

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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