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Subject: Re: [OMPI devel] Compile-time MPI_Datatype checking
From: Dmitri Gribenko (gribozavr_at_[hidden])
Date: 2012-10-31 03:45:29

On Wed, Oct 31, 2012 at 4:25 AM, Jeff Squyres <jsquyres_at_[hidden]> wrote:
> On Oct 28, 2012, at 10:28 AM, Dmitri Gribenko wrote:
>> Thank you for the feedback! Hopefully the attached patch fixes both of these.
>> 1. There are two helper structs with complex numbers. I predicated
>> the struct declarations and use to appear only in C99.
>> 2. These macros were indeed missing.
> I did a few tests and this now looks good; no more warnings.
> I brought up this functionality on the weekly OMPI dev telecon today and got an important piece of feedback: apparently there are a large class of apps that wrap their messages as transparent blobs, and then use either non-blob-like or derived MPI datatypes. (I said something similar to this earlier in the thread, but I didn't know that there was a large class of apps that actually did it)
> A very simple example is:
> char *foo = malloc(...);
> // ...fill foo...
> MPI_Send(foo, x, MPI_INT, ...);
> Another not-uncommon example is:
> char *foo = malloc(...);
> // Receive some INTEGERs from a Fortran sender
> MPI_Recv(foo, x, MPI_INTEGER, ...);
> With this patch, they'd get warnings about these uses, even though they are completely valid according to MPI.
> A suggestion was that this functionality could be disabled by default, and enabled with a magic macro. Perhaps something like:
> or something like that.
> Thoughts?

This is a vaild concern and a good idea for a FAQ entry.

Q: How to silence warnings about buffer type/type tag mismatch?

A: There are multiple ways for an MPI application to silence these
warnings. To silence a particular warning, change the type of the
buffer to 'void *' or explicitly cast it to 'void *':

     void *foo = malloc(...);
     // ...fill foo...
     MPI_Send(foo, x, MPI_INT, ...);


     char *foo = malloc(...);
     // ...fill foo...
     MPI_Send((void *) foo, x, MPI_INT, ...);

To turn off all warnings of this kind in clang, use the
-Wno-type-safety command line option.

It is also possible to completely remove annotations from mpi.h by
defining a macro OMPI_NO_ATTR_TYPE_TAGS in the source code or on the
compiler's command line.


(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr_at_[hidden]>*/