add pml-specific field in OMPI datatype.

PML UCX will use it to cache a handle for UCX datatype.

ompi/datatype/ompi_datatype.h

@@ -73,6 +73,7 @@ struct ompi_datatype_t {
 
     void*              args;                     /**< Data description for the user */
     void*              packed_description;       /**< Packed description of the datatype */
+    uint64_t           pml_data;                 /**< PML-specific information */
     /* --- cacheline 6 boundary (384 bytes) --- */
     char               name[MPI_MAX_OBJECT_NAME];/**< Externally visible name */
     /* --- cacheline 7 boundary (448 bytes) --- */

ompi/datatype/ompi_datatype_create.c

@@ -35,6 +35,7 @@ static void __ompi_datatype_allocate( ompi_datatype_t* datatype )
     datatype->d_keyhash          = NULL;
     datatype->name[0]            = '\0';
     datatype->packed_description = NULL;
+    datatype->pml_data           = 0;
 }
 
 static void __ompi_datatype_release(ompi_datatype_t * datatype)