Escape small cells in cell_can_use_pair_mm() when doing the tree walk

src/cell.c

@@ -3328,7 +3328,8 @@ void cell_activate_subcell_grav_tasks(struct cell *ci, struct cell *cj,
     if (lock_unlock(&cj->grav.mlock) != 0) error("Impossible to unlock m-pole");
 
     /* Can we use multipoles ? */
-    if (cell_can_use_pair_mm(ci, cj, e, sp, /*use_rebuild_data=*/0)) {
+    if (cell_can_use_pair_mm(ci, cj, e, sp, /*use_rebuild_data=*/0,
+                             /*is_top_level=*/0)) {
       /* Ok, no need to drift anything */
       return;
     }
@@ -6390,12 +6391,17 @@ void cell_reorder_extra_gparts(struct cell *c, struct part *parts,
  */
 int cell_can_use_pair_mm(const struct cell *restrict ci,
                          const struct cell *restrict cj, const struct engine *e,
-                         const struct space *s, const int use_rebuild_data) {
+                         const struct space *s, const int use_rebuild_data,
+                         const int is_top_level) {
 
   const struct gravity_props *props = e->gravity_properties;
   const int periodic = s->periodic;
   const double dim[3] = {s->dim[0], s->dim[1], s->dim[2]};
 
+  /* Check for trivial cases */
+  if (!is_top_level && ci->grav.count <= 1) return 0;
+  if (!is_top_level && cj->grav.count <= 1) return 0;
+
   /* Recover the multipole information */
   const struct gravity_tensors *restrict multi_i = ci->grav.multipole;
   const struct gravity_tensors *restrict multi_j = cj->grav.multipole;

src/cell.h

@@ -960,7 +960,7 @@ void cell_reorder_extra_gparts(struct cell *c, struct part *parts,
 void cell_reorder_extra_sparts(struct cell *c, const ptrdiff_t sparts_offset);
 int cell_can_use_pair_mm(const struct cell *ci, const struct cell *cj,
                          const struct engine *e, const struct space *s,
-                         const int use_rebuild_data);
+                         const int use_rebuild_data, const int is_top_level);
 
 /**
  * @brief Compute the square of the minimal distance between any two points in

src/engine_maketasks.c

@@ -1381,7 +1381,8 @@ void engine_make_self_gravity_tasks_mapper(void *map_data, int num_elements,
           if (periodic && min_radius2 > max_distance2) continue;
 
           /* Are the cells too close for a MM interaction ? */
-          if (!cell_can_use_pair_mm(ci, cj, e, s, /*use_rebuild_data=*/1)) {
+          if (!cell_can_use_pair_mm(ci, cj, e, s, /*use_rebuild_data=*/1,
+                                    /*is_top_level=*/1)) {
 
             /* Ok, we need to add a direct pair calculation */
             scheduler_addtask(sched, task_type_pair, task_subtype_grav, 0, 0,

src/runner_doiact_grav.c

@@ -2401,7 +2401,8 @@ void runner_do_grav_long_range(struct runner *r, struct cell *ci,
       }
     }
 
-    if (cell_can_use_pair_mm(top, cj, e, e->s, /*use_rebuild_data=*/1)) {
+    if (cell_can_use_pair_mm(top, cj, e, e->s, /*use_rebuild_data=*/1,
+                             /*is_top_level=*/1)) {
 
       /* Call the PM interaction fucntion on the active sub-cells of ci */
       runner_dopair_grav_mm_nonsym(r, ci, cj);

src/scheduler.c

@@ -873,7 +873,8 @@ static void scheduler_splittask_gravity(struct task *t, struct scheduler *s) {
                 if (cj->progeny[j] != NULL) {
                   /* Can we use a M-M interaction here? */
                   if (cell_can_use_pair_mm(ci->progeny[i], cj->progeny[j], e,
-                                           sp, /*use_rebuild_data=*/1)) {
+                                           sp, /*use_rebuild_data=*/1,
+                                           /*is_top_level=*/0)) {
 
                     /* Flag this pair as being treated by the M-M task.
                      * We use the 64 bits in the task->flags field to store