# UNDF: UNDF-2026-000000399
# UNDF: (pending)
--- a/ompi/group/group.c
+++ b/ompi/group/group.c
@@ -98,6 +98,17 @@ int ompi_group_translate_ranks ( ompi_group_t *group1,
 #endif

+    /* Build reverse-hash of group2: (jobid<<32|vpid) -> rank+1 */
+    opal_hash_table_t *g2_ht = OBJ_NEW(opal_hash_table_t);
+    if (NULL == g2_ht) return MPI_ERR_NO_MEM;
+    opal_hash_table_init(g2_ht, group2->grp_proc_count * 2 + 1);
+    for (int _i = 0; _i < group2->grp_proc_count; _i++) {
+        opal_process_name_t _n = ompi_group_get_proc_name(group2, _i);
+        uint64_t _k = (((uint64_t)_n.jobid) << 32) | (uint64_t)_n.vpid;
+        opal_hash_table_set_value_uint64(g2_ht, _k, (void *)(uintptr_t)(_i + 1));
+    }
+
     /* loop over all ranks */
     for (int proc = 0; proc < n_ranks; ++proc) {
         ompi_process_name_t proc1_name, proc2_name;
@@ -110,17 +121,13 @@ int ompi_group_translate_ranks ( ompi_group_t *group1,
         proc1_name = ompi_group_get_proc_name(group1, rank);
         /* initialize to no "match" */
         ranks2[proc] = MPI_UNDEFINED;
-        for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
-            proc2_name = ompi_group_get_proc_name(group2, proc2);
-            if(0 == opal_compare_proc(proc1_name, proc2_name)) {
-                ranks2[proc] = proc2;
-                break;
-            }
-        }  /* end proc2 loop */
+        uint64_t _k = (((uint64_t)proc1_name.jobid) << 32) | (uint64_t)proc1_name.vpid;
+        void *_v = NULL;
+        opal_hash_table_get_value_uint64(g2_ht, _k, &_v);
+        ranks2[proc] = _v ? (int)((uintptr_t)_v - 1) : MPI_UNDEFINED;
     } /* end proc loop */

+    OBJ_RELEASE(g2_ht);
     return MPI_SUCCESS;
 }

@@ -453,6 +460,17 @@ int ompi_group_intersection(ompi_group_t* group1,ompi_group_t* group2,
     k = 0;
+
+    /* Build reverse-hash of group2 for O(1) membership test */
+    opal_hash_table_t *g2_ht = OBJ_NEW(opal_hash_table_t);
+    if (NULL == g2_ht) { free(ranks_included); return MPI_ERR_NO_MEM; }
+    opal_hash_table_init(g2_ht, group2_pointer->grp_proc_count * 2 + 1);
+    for (int _i = 0; _i < group2_pointer->grp_proc_count; _i++) {
+        opal_process_name_t _n = ompi_group_get_proc_name(group2_pointer, _i);
+        uint64_t _k = (((uint64_t)_n.jobid) << 32) | (uint64_t)_n.vpid;
+        opal_hash_table_set_value_uint64(g2_ht, _k, (void *)(uintptr_t)1);
+    }
+
     /* determine the list of included processes for the incl-method */
     for (proc1 = 0; proc1 < group1_pointer->grp_proc_count; proc1++) {
         proc1_name = ompi_group_get_proc_name(group1_pointer , proc1);
-        /* check to see if this proc is in group2 */
-        for (proc2 = 0; proc2 < group2_pointer->grp_proc_count; proc2++) {
-            proc2_name = ompi_group_get_proc_name(group2_pointer ,proc2);
-            if(0 == opal_compare_proc(proc1_name, proc2_name)) {
-                ranks_included[k] = proc1;
-                k++;
-                break;
-            }
-        }  /* end proc2 loop */
+        uint64_t _k = (((uint64_t)proc1_name.jobid) << 32) | (uint64_t)proc1_name.vpid;
+        void *_v = NULL;
+        opal_hash_table_get_value_uint64(g2_ht, _k, &_v);
+        if (_v) { ranks_included[k++] = proc1; }
     }  /* end proc1 loop */

+    OBJ_RELEASE(g2_ht);
     result = ompi_group_incl(group1, k, ranks_included, new_group);

@@ -629,14 +643,20 @@ bool ompi_group_overlap (const ompi_group_t *group1, const ompi_group_t *group2)
 {
-    for (int i = 0 ; i < group1->grp_proc_count ; ++i) {
-        opal_process_name_t proc1 = ompi_group_get_proc_name (group1, i);
-        for (int j = 0 ; j < group2->grp_proc_count ; ++j) {
-            opal_process_name_t proc2 = ompi_group_get_proc_name (group2, j);
-            if (0 == opal_compare_proc (proc1, proc2)) {
-                return true;
-            }
-        }
-    }
-    return false;
+    /* Build hash of smaller group for O(N+M) instead of O(N*M) */
+    const ompi_group_t *small = (group1->grp_proc_count <= group2->grp_proc_count) ? group1 : group2;
+    const ompi_group_t *large = (small == group1) ? group2 : group1;
+    opal_hash_table_t *ht = OBJ_NEW(opal_hash_table_t);
+    if (NULL == ht) return false;  /* conservative: assume no overlap on OOM */
+    opal_hash_table_init(ht, small->grp_proc_count * 2 + 1);
+    for (int i = 0; i < small->grp_proc_count; i++) {
+        opal_process_name_t n = ompi_group_get_proc_name(small, i);
+        uint64_t k = (((uint64_t)n.jobid) << 32) | (uint64_t)n.vpid;
+        opal_hash_table_set_value_uint64(ht, k, (void *)(uintptr_t)1);
+    }
+    bool found = false;
+    for (int i = 0; i < large->grp_proc_count && !found; i++) {
+        opal_process_name_t n = ompi_group_get_proc_name(large, i);
+        uint64_t k = (((uint64_t)n.jobid) << 32) | (uint64_t)n.vpid;
+        void *v = NULL;
+        opal_hash_table_get_value_uint64(ht, k, &v);
+        found = (v != NULL);
+    }
+    OBJ_RELEASE(ht);
+    return found;
 }