Handle potential unavailability of MPI_Neighbor_alltoallw more gracefully.

* This way, the constructor cannot ever be invoked if the MPI version is < 3.

configure.ac

@@ -253,6 +253,9 @@ FCFLAGS="$FCFLAGS ${FPP_DEFOPT}XT_MPI_AINT_KIND=$xt_mpi_aint_kind"
 AC_CHECK_SIZEOF([MPI_Datatype],,[@%:@include <mpi.h>])
 AS_IF([test $ac_cv_sizeof_MPI_Datatype -eq 0],
   [AC_MSG_FAILURE([Could not detect size of MPI_Datatype!])])
+AC_COMPUTE_INT([MPI_VERSION], [MPI_VERSION], [@%:@include <mpi.h>],
+  [AC_MSG_ERROR([cannot determine MPI version!])])
+AM_CONDITIONAL([USE_MPI3],[test $MPI_VERSION -ge 3])
 CFLAGS=$save_CFLAGS
 AS_CASE(["$MPI_FINT"],
   [int],[XT_MPI_FINT_MAX=INT_MAX XT_MPI_FINT_FC_KIND=c_int],

src/Makefile.am

@@ -149,11 +149,15 @@ libyaxt_c_la_SOURCES =						\
 	xt_exchanger_irecv_isend.h				\
 	xt_exchanger_irecv_isend_packed.c			\
 	xt_exchanger_irecv_isend_packed.h			\
-	xt_exchanger_neigh_alltoall.c				\
 	xt_exchanger_neigh_alltoall.h				\
 	xt_exchanger_mix_isend_irecv.c				\
 	xt_exchanger_mix_isend_irecv.h
 
+if USE_MPI3
+libyaxt_c_la_SOURCES +=						\
+	xt_exchanger_neigh_alltoall.c
+endif
+
 libyaxt_la_SOURCES =						\
 	xt_idxlist_collection_f.f90				\
 	xt_xmap_intersection_f.f90				\

src/xt_config.c

@@ -92,7 +92,12 @@ static const struct {
   { "mix_irecv_isend",
     xt_exchanger_mix_isend_irecv_new, xt_exchanger_mix_isend_irecv },
   { "neigh_alltoall",
-    xt_exchanger_neigh_alltoall_new, xt_exchanger_neigh_alltoall },
+#if MPI_VERSION >= 3
+    xt_exchanger_neigh_alltoall_new,
+#else
+    (Xt_exchanger_new)0,
+#endif
+    xt_exchanger_neigh_alltoall },
 };
 
 enum {
@@ -108,15 +113,25 @@ xt_exchanger_id_by_name(const char *name)
   return -1;
 }
 
-int xt_config_get_exchange_method(Xt_config config)
+static inline size_t
+exchanger_by_function(Xt_exchanger_new exchanger_new)
 {
-  Xt_exchanger_new exchanger_new = config->exchanger_new;
   for (size_t i = 0; i < num_exchanger; ++i)
     if (exchanger_table[i].f == exchanger_new)
-      return (int)i;
+      return i;
+  return SIZE_MAX;
+}
+
+
+int xt_config_get_exchange_method(Xt_config config)
+{
+  Xt_exchanger_new exchanger_new = config->exchanger_new;
+  size_t eentry = exchanger_by_function(exchanger_new);
+  if (eentry != SIZE_MAX)
+    return exchanger_table[eentry].code;
   static const char fmt[]
-    = "error: unexpected exchanger function (%p)!\n";
-  char buf[sizeof (fmt) + 9];
+    = "error: unexpected exchanger function (%p)!";
+  char buf[sizeof (fmt) + 3*sizeof(void *)];
   sprintf(buf, fmt, (void *)exchanger_new);
   Xt_abort(Xt_default_comm, buf, "xt_config.c", __LINE__);
 }
@@ -124,16 +139,32 @@ int xt_config_get_exchange_method(Xt_config config)
 
 void xt_config_set_exchange_method(Xt_config config, int method)
 {
+  static const char fmt[]
+    = "error: user-requested exchanger code (%d) does not exist!";
+  char buf[sizeof (fmt) + 3*sizeof(int)];
+  const char *msg = buf;
   for (size_t i = 0; i < num_exchanger; ++i)
     if (exchanger_table[i].code == method) {
-      config->exchanger_new = exchanger_table[i].f;
+      Xt_exchanger_new exchanger_new;
+      if (exchanger_table[i].f) {
+        exchanger_new = exchanger_table[i].f;
+      } else {
+        exchanger_new = xt_default_config.exchanger_new;
+        size_t default_entry = exchanger_by_function(exchanger_new);
+        if (default_entry == SIZE_MAX) {
+          msg = "error: invalid default exchanger constructor!";
+          goto abort;
+        }
+        fprintf(stderr, "warning: %s exchanger unavailable, using "
+                "%s instead\n",
+                exchanger_table[i].name, exchanger_table[default_entry].name);
+      }
+      config->exchanger_new = exchanger_new;
       return;
     }
-  static const char fmt[]
-    = "error: user-requested exchanger code (%d) does not exist!\n";
-  char buf[sizeof (fmt) + 9];
   sprintf(buf, fmt, method);
-  Xt_abort(Xt_default_comm, buf, "xt_config.c", __LINE__);
+abort:
+  Xt_abort(Xt_default_comm, msg, "xt_config.c", __LINE__);
 }
 
 void
@@ -144,6 +175,9 @@ xt_config_defaults_init(void)
     int exchanger_id = xt_exchanger_id_by_name(config_env);
     if (exchanger_id != -1)
       xt_config_set_exchange_method(&xt_default_config, exchanger_id);
+    else
+      fprintf(stderr, "warning: Unexpected value "
+              "for XT_CONFIG_DEFAULT_EXCHANGE_METHOD=%s\n", config_env);
   }
 }
 

src/xt_exchanger_neigh_alltoall.c

@@ -47,17 +47,12 @@
 #include <config.h>
 #endif
 
-#include <mpi.h>
-
-#include "xt_exchanger_neigh_alltoall.h"
-
-#include "core/core.h"
-
-#if MPI_VERSION >= 3
-
 #include <assert.h>
 #include <string.h>
 
+#include <mpi.h>
+
+#include "core/core.h"
 #include "core/ppm_xfuncs.h"
 #include "xt/xt_mpi.h"
 #include "xt_mpi_internal.h"
@@ -67,6 +62,7 @@
 #include "xt/xt_request.h"
 #include "xt/xt_request_msgs.h"
 #include "xt_exchanger.h"
+#include "xt_exchanger_neigh_alltoall.h"
 
 #define MAX(a,b) ((a) >= (b) ? (a) : (b))
 
@@ -309,26 +305,6 @@ xt_exchanger_neigh_alltoall_get_msg_ranks(Xt_exchanger exchanger,
   return (int)nmsg;
 }
 
-// #if MPI_VERSION >= 3
-#else
-
-Xt_exchanger
-xt_exchanger_neigh_alltoall_new(int nsend, int nrecv,
-                                const struct Xt_redist_msg *send_msgs,
-                                const struct Xt_redist_msg *recv_msgs,
-                                MPI_Comm comm, int tag_offset) {
-
-  (void)nsend; (void)nrecv; (void)send_msgs; (void)recv_msgs; (void)tag_offset;
-  Xt_abort(comm, "ERROR(xt_exchanger_neigh_alltoall_new): "
-           "exchanger_neigh_alltoall requires MPI version 3.0 or higher",
-           __FILE__, __LINE__);
-
-  return NULL;
-}
-
-// #if MPI_VERSION >= 3
-#endif
-
 /*
  * Local Variables:
  * c-basic-offset: 2

tests/test_exchanger_parallel.c

@@ -130,7 +130,9 @@ static Xt_exchanger_new *parse_options(int *argc, char ***argv)
     [xt_exchanger_irecv_isend] = xt_exchanger_irecv_isend_new,
     [xt_exchanger_irecv_isend_packed] = xt_exchanger_irecv_isend_packed_new,
     [xt_exchanger_mix_isend_irecv] = xt_exchanger_mix_isend_irecv_new,
+#if MPI_VERSION >= 3
     [xt_exchanger_neigh_alltoall] = xt_exchanger_neigh_alltoall_new,
+#endif
   };
   enum {
     num_exchanger = sizeof (exchanger_table) / sizeof (exchanger_table[0]),
@@ -479,10 +481,11 @@ test_rr(MPI_Comm comm, Xt_exchanger_new exchanger_new)
 static void
 test_intercomm_all2all(MPI_Comm comm, Xt_exchanger_new exchanger_new)
 {
-
+#if MPI_VERSION >= 3
   // inter-communicator's are not defined for virtual topologies, which are
   // used by xt_exchanger_neigh_alltoall_new
   if (exchanger_new == xt_exchanger_neigh_alltoall_new) return;
+#endif
 
   int my_rank, comm_size;
   xt_mpi_call(MPI_Comm_rank(comm, &my_rank), comm);