coll/libnbc: add Rabenseifner's algorithm for MPI_Ireduce

An implementation of R. Rabenseifner's algorithm for MPI_Ireduce.
This algorithm is a combination of a reduce-scatter implemented with recursive vector halving
and recursive distance doubling, followed either by a gather.

Limitations:
-- count >= 2^{\floor{\log_2 p}}
-- commutative operations only
-- intra-communicators only

Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>

Author: mkurnosov

ompi/mca/coll/libnbc/coll_libnbc.h

@@ -71,6 +71,7 @@ BEGIN_C_DECLS
 
 extern bool libnbc_ibcast_skip_dt_decision;
 extern int libnbc_iexscan_algorithm;
+extern int libnbc_ireduce_algorithm;
 extern int libnbc_iscan_algorithm;
 
 struct ompi_coll_libnbc_component_t {

ompi/mca/coll/libnbc/coll_libnbc_component.c

@@ -54,6 +54,15 @@ static mca_base_var_enum_value_t iexscan_algorithms[] = {
     {0, NULL}
 };
 
+int libnbc_ireduce_algorithm = 0;            /* ireduce user forced algorithm */
+static mca_base_var_enum_value_t ireduce_algorithms[] = {
+    {0, "ignore"},
+    {1, "chain"},
+    {2, "binomial"},
+    {3, "rabenseifner"},
+    {0, NULL}
+};
+
 int libnbc_iscan_algorithm = 0;             /* iscan user forced algorithm */
 static mca_base_var_enum_value_t iscan_algorithms[] = {
     {0, "ignore"},
@@ -185,6 +194,16 @@ libnbc_register(void)
                                     &libnbc_iexscan_algorithm);
     OBJ_RELEASE(new_enum);
 
+    libnbc_ireduce_algorithm = 0;
+    (void) mca_base_var_enum_create("coll_libnbc_ireduce_algorithms", ireduce_algorithms, &new_enum);
+    mca_base_component_var_register(&mca_coll_libnbc_component.super.collm_version,
+                                    "ireduce_algorithm",
+                                    "Which ireduce algorithm is used: 0 ignore, 1 chain, 2 binomial, 3 rabenseifner",
+                                    MCA_BASE_VAR_TYPE_INT, new_enum, 0, MCA_BASE_VAR_FLAG_SETTABLE,
+                                    OPAL_INFO_LVL_5, MCA_BASE_VAR_SCOPE_ALL,
+                                    &libnbc_ireduce_algorithm);
+    OBJ_RELEASE(new_enum);
+
     libnbc_iscan_algorithm = 0;
     (void) mca_base_var_enum_create("coll_libnbc_iscan_algorithms", iscan_algorithms, &new_enum);
     mca_base_component_var_register(&mca_coll_libnbc_component.super.collm_version,