Do not clear the sort flag when receiving parts after the ghost. We can re-use...

Do not clear the sort flag when receiving parts after the ghost. We can re-use the sort from the density.

src/runner.c

@@ -1447,9 +1447,10 @@ void runner_do_end_force(struct runner *r, struct cell *c, int timer) {
  *
  * @param r The runner thread.
  * @param c The cell.
+ * @param clear_sorts Should we clear the sort flag and hence trigger a sort ?
  * @param timer Are we timing this ?
  */
-void runner_do_recv_part(struct runner *r, struct cell *c, int timer) {
+void runner_do_recv_part(struct runner *r, struct cell *c, int clear_sorts, int timer) {
 
 #ifdef WITH_MPI
 
@@ -1466,7 +1467,8 @@ void runner_do_recv_part(struct runner *r, struct cell *c, int timer) {
   float h_max = 0.f;
 
   /* Clear this cell's sorted mask. */
-  c->sorted = 0;
+  if(clear_sorts)
+    c->sorted = 0;
 
   /* If this cell is a leaf, collect the particle data. */
   if (!c->split) {
@@ -1493,7 +1495,7 @@ void runner_do_recv_part(struct runner *r, struct cell *c, int timer) {
   else {
     for (int k = 0; k < 8; k++) {
       if (c->progeny[k] != NULL) {
-        runner_do_recv_part(r, c->progeny[k], 0);
+        runner_do_recv_part(r, c->progeny[k], clear_sorts, 0);
         ti_end_min = min(ti_end_min, c->progeny[k]->ti_end_min);
         ti_end_max = max(ti_end_max, c->progeny[k]->ti_end_max);
         h_max = max(h_max, c->progeny[k]->h_max);
@@ -1872,9 +1874,10 @@ void *runner_main(void *data) {
           if (t->subtype == task_subtype_tend) {
             cell_unpack_ti_ends(ci, t->buff);
             free(t->buff);
-          } else if (t->subtype == task_subtype_xv ||
-                     t->subtype == task_subtype_rho) {
-            runner_do_recv_part(r, ci, 1);
+          } else if (t->subtype == task_subtype_xv) {
+            runner_do_recv_part(r, ci, 1, 1);
+	  } else if  (t->subtype == task_subtype_rho) {
+            runner_do_recv_part(r, ci, 0, 1);
           } else if (t->subtype == task_subtype_gpart) {
             runner_do_recv_gpart(r, ci, 1);
           } else if (t->subtype == task_subtype_spart) {