group: make MPIR_Lpid int64_t

The unsigned type uint64_t is dangerous as we perform pmap stride math.
Signed integer should always work and we can use assertions to check its
range.

Author: hzhou

src/include/mpir_group.h

@@ -47,7 +47,7 @@
  * in principle, but 64-bit is simpler since we can trivially combine
  * (world_idx, world_rank).
  */
-typedef uint64_t MPIR_Lpid;
+typedef int64_t MPIR_Lpid;
 
 struct MPIR_Pmap {
     int size;                   /* same as group->size, duplicate here so Pmap is logically complete */

src/mpi/group/grouputil.c

@@ -129,7 +129,10 @@ int MPIR_Group_create_stride(int size, int rank, MPIR_Session * session_ptr,
          * for others it is implied */
         *new_group_ptr = MPIR_Group_empty;
     } else {
-        MPIR_Assert(stride > 0 && blocksize > 0);
+        /* NOTE: stride may be negative */
+        MPIR_Assert(offset >= 0);
+        MPIR_Assert(stride != 0);
+        MPIR_Assert(blocksize > 0);
 
         MPIR_Group *newgrp;
         mpi_errno = MPIR_Group_create(size, &newgrp);