Add macros to manipulate error bits in tag space.

These macros are used to manipulate the error bit that has been added
to the tag space to propagate error notification on top of other
communication.
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

src/include/mpiimpl.h

@@ -3851,6 +3851,29 @@ int MPID_VCR_Get_lpid(MPID_VCR vcr, int * lpid_ptr);
 #define MPIR_ERROR_TAG                29
 #define MPIR_FIRST_NBC_TAG            30
 
+/* These macros must be used carefully. These macros will not work with
+ * negative tags. By definition, users are not to use negative tags and the
+ * only negative tag in MPICH is MPI_ANY_TAG which is checked seperately, but
+ * if there is a time where negative tags become more common, this setup won't
+ * work anymore. */
+
+/* This bitmask can be used to manually mask the tag space wherever it might
+ * be necessary to do so (for instance in the receive queue */
+#define MPIR_TAG_ERROR_BIT (1 << 30)
+
+/* This macro checks the value of the error bit in the MPI tag and returns 1
+ * if the tag is set and 0 if it is not. */
+#define MPIR_TAG_CHECK_ERROR_BIT(tag) ((MPIR_TAG_ERROR_BIT & tag) == MPIR_TAG_ERROR_BIT ? 1 : 0)
+
+/* This macro sets the value of the error bit in the MPI tag to 1 */
+#define MPIR_TAG_SET_ERROR_BIT(tag) (tag |= MPIR_TAG_ERROR_BIT)
+
+/* This macro clears the value of the error bit in the MPI tag */
+#define MPIR_TAG_CLEAR_ERROR_BIT(tag) (tag &= ~MPIR_TAG_ERROR_BIT)
+
+/* This macro masks the value of the error bit in the MPI tag */
+#define MPIR_TAG_MASK_ERROR_BIT(tag) (tag & ~MPIR_TAG_ERROR_BIT)
+
 /* These functions are used in the implementation of collective and
    other internal operations. They are wrappers around MPID send/recv
    functions. They do sends/receives by setting the context offset to

src/mpid/ch3/include/mpidpre.h

@@ -119,7 +119,9 @@ typedef union {
 /* The first bit of the tag space is reserved for error propagation. We are
  * still well over the required tag size and this allows us to retain the
  * matching semantics while piggybacking the notification. If the message is
- * propagating an error, the data being transmitted is probably garbage.
+ * propagating an error, the data being transmitted is probably garbage. The
+ * macros to set/access this bit are in src/include/mpiimpl.h. If the
+ * location of this bit changes, those macros also need to be adjusted.
  */
 #define MPIDI_TAG_UB (0x3fffffff)