[cfe-commits] [PATCH] Compile-time MPI_Datatype checking, type arguments in attributes

[Dmitri Gribenko]

Hi Hal,

Thank you for looking into this!

On Sun, Feb 12, 2012 at 6:22 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 1. How much of this is realy MPI specific? Maybe I'm wrong, but the
> general idea is to add type checking for APIs that take void* along with
> some kind of tag indicating the actual data type. MPI may be a primary
> use case, but that general situation seems not uncommon. I wonder if we
> could take 'mpi' out of the syntax and replace it with some kind of
> domain tag. For example, instead of mpi_typed_arg(1,3) we might name it
> pwt_arg("mpi",1,3) [pwt = pointer with tag] and use pwt_type("mpi",int).

There are two MPI-specific points:
1. void* pointers don't trigger a warning (to support wrapping MPI
calls and passing NULL, MPI_IN_PLACE).
2. void datatype is tied to the special case of MPI_DATATYPE_NULL and
we complain if a non-NULL pointer was passed.

If we are going to generalize this (which is a good idea in itself),
we should define the semantics of pwt_arg(kind,ptr-idx,type-idx) where
`kind' is not known to the compiler.  If the compiler is required to
make checks, we have to specify them (and that should be a subset of
the checks that are done by every known tag).

If the compiler should ignore such attribute, then what does this buy
us compared with MPI-specific attribute name?

> 2. Supporting MPICH-derived implementations will be important. Maybe we
> could add a pragma to allow the definition of integer-type tags. [I may
> be slightly biased here ;) -- a bunch of the MPICH developers work down
> the hall from me -- but it would be great to have something like this
> installed for our users here]. Especially if we make this slightly more
> generic, supporting integer/enum tags will be necessary.

Here's what MPICH currently does:
#define MPI_INT ((MPI_Datatype) 0xMAGIC)

I don't think pragmas can create a good solution.

Here's an example of what can be done.  In addition to defining the
macros, MPICH could add some private constants to mpi.h:
static const int mpich_datatype_int __attribute__(( mpi_datatype(int)
)) = 0xMAGIC;

The compiler could build a map (integer tag -> type) from that.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/