Use the correct CoM when testing cells in the MAC at rebuild time

src/cell.c

@@ -3327,7 +3327,7 @@ 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_sizes=*/0)) {
+    if (cell_can_use_pair_mm(ci, cj, e, sp, /*use_rebuild_data=*/0)) {
       /* Ok, no need to drift anything */
       return;
     }
@@ -6381,12 +6381,12 @@ void cell_reorder_extra_gparts(struct cell *c, struct part *parts,
  * @param cj The second #cell.
  * @param e The #engine.
  * @param s The #space.
- * @param use_rebuild_sizes Are we considering the sizes at the last tree-build
- * (1) or current sizes (0)?
+ * @param use_rebuild_data Are we considering the data at the last tree-build
+ * (1) or current data (0)?
  */
 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_sizes) {
+                         const int use_rebuild_data) {
 
   const struct gravity_props *props = e->gravity_properties;
   const int periodic = s->periodic;
@@ -6396,10 +6396,18 @@ int cell_can_use_pair_mm(const struct cell *ci, const struct cell *cj,
   const struct gravity_tensors *const multi_i = ci->grav.multipole;
   const struct gravity_tensors *const multi_j = cj->grav.multipole;
 
+  double dx, dy, dz;
+
   /* Get the distance between the CoMs */
-  double dx = multi_i->CoM[0] - multi_j->CoM[0];
-  double dy = multi_i->CoM[1] - multi_j->CoM[1];
-  double dz = multi_i->CoM[2] - multi_j->CoM[2];
+  if (use_rebuild_data) {
+    dx = multi_i->CoM_rebuild[0] - multi_j->CoM_rebuild[0];
+    dy = multi_i->CoM_rebuild[1] - multi_j->CoM_rebuild[1];
+    dz = multi_i->CoM_rebuild[2] - multi_j->CoM_rebuild[2];
+  } else {
+    dx = multi_i->CoM[0] - multi_j->CoM[0];
+    dy = multi_i->CoM[1] - multi_j->CoM[1];
+    dz = multi_i->CoM[2] - multi_j->CoM[2];
+  }
 
   /* Apply BC */
   if (periodic) {
@@ -6410,5 +6418,5 @@ int cell_can_use_pair_mm(const struct cell *ci, const struct cell *cj,
   const double r2 = dx * dx + dy * dy + dz * dz;
 
   return gravity_M2L_accept_symmetric(props, multi_i, multi_j, r2,
-                                      use_rebuild_sizes);
+                                      use_rebuild_data);
 }

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_sizes);
+                         const int use_rebuild_data);
 
 /**
  * @brief Compute the square of the minimal distance between any two points in

src/engine_maketasks.c

@@ -1381,7 +1381,7 @@ 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_sizes=*/1)) {
+          if (!cell_can_use_pair_mm(ci, cj, e, s, /*use_rebuild_data=*/1)) {
 
             /* Ok, we need to add a direct pair calculation */
             scheduler_addtask(sched, task_type_pair, task_subtype_grav, 0, 0,

src/scheduler.c

@@ -873,7 +873,7 @@ 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_sizes=*/1)) {
+                                           sp, /*use_rebuild_data=*/1)) {
 
                     /* Flag this pair as being treated by the M-M task.
                      * We use the 64 bits in the task->flags field to store