Block off the highest bit of tag space.

The highest bit of tag space will now be reserved for failure
notification. We are far above the required 32767, so this shouldn't be
a problem.
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

src/mpid/ch3/include/mpidpre.h

@@ -116,7 +116,12 @@ typedef union {
  * upper level code may then modify this value after MPID_Init and before the
  * end of MPIR_Init_thread.  Don't use this value directly, always check the
  * runtime global value. */
-#define MPIDI_TAG_UB (0x7fffffff)
+/* 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.
+ */
+#define MPIDI_TAG_UB (0x3fffffff)
 
 /* Provides MPIDI_CH3_Pkt_t.  Must come after MPIDI_Message_match definition. */
 #include "mpidpkt.h"