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

[Dmitri Gribenko]

Hello,

=== Objective ===
Implement a set of annotations for MPI libraries and a diagnostic for
clang that checks that specified buffer type matches the passed
MPI_Datatype.

=== Background ===
Many MPI functions take a void* `buffer' and an MPI_Datatype (like
MPI_INT, MPI_LONG etc) that describes how to send/receive that buffer.
 For example,

int MPI_Send(void *buf, int count, MPI_Datatype datatype, int dest,
    int tag, MPI_Comm comm);

Users call it like this:
int *int_buf = ...;
MPI_Send(int_buf, 1, MPI_INT, ...);

Thus, user is on its own to ensure that buffer type matches the MPI_Datatype.

=== Annotation design ===
I decided to annotate MPI functions that have buffer arguments and
declarations of built-in `MPI_Datatype's.  Functions are easy: we use
mpi_typed_arg(buffer-arg-index, type-arg-index) attribute:

int MPI_Send(void *buf, int count, MPI_Datatype datatype, ...etc...)
    __attribute__(( mpi_typed_arg(1,3) ));

`MPI_Datatype's are trickier.  From the grammar of GNU attributes in
lib/Parse/ParseDecl.cpp we see that there is no easy way to pass a
type argument to an attribute.  For example, this is not currently
parseable by clang:

#define MPI_INT ((MPI_Datatype) &ompi_mpi_int)
extern struct ompi_predefined_datatype_t ompi_mpi_int
        __attribute__(( mpi_datatype(int) ));

Matching C datatype for MPI_DATATYPE_NULL is void.

#define MPI_DATATYPE_NULL ((MPI_Datatype) &ompi_mpi_int)
extern struct ompi_predefined_datatype_t ompi_mpi_void
        __attribute__(( mpi_datatype(void) ));

While checking a function call we recognize mpi_datatype attribute if
datatype argument is either:
* an OpenMPI-style expression `((MPI_Datatype) &ident)';
* a user-specified type `ident';
where ident has mpi_datatype attribute.

We emit a diagnostic in 2 cases.
Case 1.  (Any type except MPI_DATATYPE_NULL)  All of the following is true:
* buffer is not of `void *' type;
* buffer's element type doesn't match specified MPI_Datatype.

Case 2.  (MPI_DATATYPE_NULL)  All of the following is true:
* buffer is not of `void *' type;
* buffer is not a null pointer constant;
* mpi_datatype(void) was specified.

=== Clang modifications ===
We need to allow arbitrary types as attribute arguments.  If I
understand everything correctly, GNU attribute grammar becomes
ambiguous in that case, so attribute argument parsing will be driven
by attribute name.  Fortunately, that is compatible with C++11
attributes where attribute argument list is just a sequence of tokens
with balanced delimiters.

I added AttributeList::TypeArg member, the required AttributeList
constructor and factory method.  In Attr.td a new field,
HasTypeArgument, was added and a tablegen pass was implemented.  The
parser uses the generated table to decide if it should parse a list of
expressions or a type name.

The check itself is called from Sema::CheckFunctionCall, which seems
to be designed for similar purposes.

=== Compatibility ===
The annotation-based approach is compatible with OpenMPI and LAM MPI.
I already have a prototype patch for OpenMPI.

MPICH is not compatible without refactoring their mpi.h header (they
have MPI_XXX macros defined to magic numbers).

=== Miscellaneous ===
There is an `iboutletcollection' attribute that takes a type argument.
 It would be good if this implementation of type arguments could be
used for that, too.

Please see attached patch.  Parsing diagnostics are not very good (see
testcase), advice is welcome.

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>*/
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