coll: refactor barrier_intra_k_dissemination

Because the compiler can't figure out the arithmetic, it is warning:
    ‘MPIC_Waitall’ accessing 8 bytes in a region of size 0
[-Wstringop-overflow=]

Refactor to suppress warning and for better readability.

Author: hzhou

src/mpi/coll/barrier/barrier_intra_k_dissemination.c

@@ -51,7 +51,7 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
     int p_of_k;                 /* minimum power of k that is greater than or equal to number of ranks */
     int shift, to, from;
     int nphases = 0;
-    MPIR_Request *sreqs[MAX_RADIX], *rreqs[MAX_RADIX * 2];
+    MPIR_Request *static_sreqs[MAX_RADIX], *static_rreqs[MAX_RADIX * 2];
     MPIR_Request **send_reqs = NULL, **recv_reqs = NULL;
 
     MPIR_COLL_RANK_SIZE(comm, coll_group, rank, nranks);
@@ -76,8 +76,8 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
         send_reqs = (MPIR_Request **) MPL_malloc((k - 1) * sizeof(MPIR_Request *), MPL_MEM_BUFFER);
         MPIR_ERR_CHKANDJUMP(!send_reqs, mpi_errno, MPI_ERR_OTHER, "**nomem");
     } else {
-        send_reqs = sreqs;
-        recv_reqs = rreqs;
+        send_reqs = static_sreqs;
+        recv_reqs = static_rreqs;
     }
 
     p_of_k = 1;
@@ -86,6 +86,8 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
         nphases++;
     }
 
+    MPIR_Request **rreqs = recv_reqs;
+    MPIR_Request **prev_rreqs = recv_reqs + (k - 1);
     shift = 1;
     for (i = 0; i < nphases; i++) {
         for (j = 1; j < k; j++) {
@@ -97,14 +99,12 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
             MPIR_Assert(to >= 0 && to < nranks);
 
             /* recv from (k-1) nbrs */
-            mpi_errno =
-                MPIC_Irecv(NULL, 0, MPI_BYTE, from, MPIR_BARRIER_TAG, comm, coll_group,
-                           &recv_reqs[(j - 1) + ((k - 1) * (i & 1))]);
+            mpi_errno = MPIC_Irecv(NULL, 0, MPI_BYTE, from, MPIR_BARRIER_TAG, comm, coll_group,
+                                   &rreqs[j - 1]);
             MPIR_ERR_CHECK(mpi_errno);
             /* wait on recvs from prev phase */
             if (i > 0 && j == 1) {
-                mpi_errno =
-                    MPIC_Waitall(k - 1, &recv_reqs[((k - 1) * ((i - 1) & 1))], MPI_STATUSES_IGNORE);
+                mpi_errno = MPIC_Waitall(k - 1, prev_rreqs, MPI_STATUSES_IGNORE);
                 MPIR_ERR_CHECK(mpi_errno);
             }
 
@@ -115,10 +115,13 @@ int MPIR_Barrier_intra_k_dissemination(MPIR_Comm * comm, int coll_group, int k,
         mpi_errno = MPIC_Waitall(k - 1, send_reqs, MPI_STATUSES_IGNORE);
         MPIR_ERR_CHECK(mpi_errno);
         shift *= k;
+
+        MPIR_Request **tmp = rreqs;
+        rreqs = prev_rreqs;
+        prev_rreqs = tmp;
     }
 
-    mpi_errno =
-        MPIC_Waitall(k - 1, recv_reqs + ((k - 1) * ((nphases - 1) & 1)), MPI_STATUSES_IGNORE);
+    mpi_errno = MPIC_Waitall(k - 1, prev_rreqs, MPI_STATUSES_IGNORE);
     MPIR_ERR_CHECK(mpi_errno);
 
   fn_exit: