x_old is now a x_diff since the last tree build. It is also implemented for the gparts.

src/cell.c

@@ -114,7 +114,7 @@ int cell_unpack(struct pcell *pc, struct cell *c, struct space *s) {
       if (k & 1) temp->loc[2] += temp->h[2];
       temp->depth = c->depth + 1;
       temp->split = 0;
-      temp->dx_max = 0.0;
+      temp->dx_max = 0.f;
       temp->nodeID = c->nodeID;
       temp->parent = c;
       c->progeny[k] = temp;

src/gravity/Default/gravity_part.h

@@ -25,6 +25,9 @@ struct gpart {
   /* Particle position. */
   double x[3];
 
+  /* Offset between current position and position at last tree rebuild. */
+  float x_diff[3];
+
   /* Particle velocity. */
   float v_full[3];
 

src/hydro/Default/hydro_part.h

@@ -20,8 +20,8 @@
 /* Extra particle data not needed during the SPH loops over neighbours. */
 struct xpart {
 
-  /* Old position, at last tree rebuild. */
-  double x_old[3];
+  /* Offset between current position and position at last tree rebuild. */
+  float x_diff[3];
 
   /* Velocity at the last full step. */
   float v_full[3];

src/hydro/Gadget2/hydro_part.h

@@ -20,8 +20,8 @@
 /* Extra particle data not needed during the SPH loops over neighbours. */
 struct xpart {
 
-  /* Old position, at last tree rebuild. */
-  double x_old[3];
+  /* Offset between current position and position at last tree rebuild. */
+  float x_diff[3];
 
   /* Velocity at the last full step. */
   float v_full[3];

src/hydro/Minimal/hydro_part.h

@@ -26,7 +26,8 @@
  */
 struct xpart {
 
-  double x_old[3]; /*!< Old position, at last tree rebuild. */
+  float x_diff[3]; /*!< Offset between current position and position at last
+                      tree rebuild. */
 
   float v_full[3]; /*!< Velocity at the last full step. */
 

src/runner.c

@@ -697,6 +697,17 @@ void runner_dodrift(struct runner *r, struct cell *c, int timer) {
       gp->x[0] += gp->v_full[0] * dt;
       gp->x[1] += gp->v_full[1] * dt;
       gp->x[2] += gp->v_full[2] * dt;
+
+      /* Compute offset since last cell construction */
+      gp->x_diff[0] -= gp->v_full[0] * dt;
+      gp->x_diff[1] -= gp->v_full[1] * dt;
+      gp->x_diff[2] -= gp->v_full[2] * dt;
+
+      /* Compute (square of) motion since last cell construction */
+      const float dx2 = gp->x_diff[0] * gp->x_diff[0] +
+                        gp->x_diff[1] * gp->x_diff[1] +
+                        gp->x_diff[2] * gp->x_diff[2];
+      dx2_max = fmaxf(dx2_max, dx2);
     }
 
     /* Loop over all the particles in the cell (more work for these !) */
@@ -737,10 +748,15 @@ void runner_dodrift(struct runner *r, struct cell *c, int timer) {
       /* Predict the values of the extra fields */
       hydro_predict_extra(p, xp, ti_old, ti_current, timeBase);
 
+      /* Compute offset since last cell construction */
+      xp->x_diff[0] -= xp->v_full[0] * dt;
+      xp->x_diff[1] -= xp->v_full[1] * dt;
+      xp->x_diff[2] -= xp->v_full[2] * dt;
+
       /* Compute (square of) motion since last cell construction */
-      const float dx2 = (p->x[0] - xp->x_old[0]) * (p->x[0] - xp->x_old[0]) +
-                        (p->x[1] - xp->x_old[1]) * (p->x[1] - xp->x_old[1]) +
-                        (p->x[2] - xp->x_old[2]) * (p->x[2] - xp->x_old[2]);
+      const float dx2 = xp->x_diff[0] * xp->x_diff[0] +
+                        xp->x_diff[1] * xp->x_diff[1] +
+                        xp->x_diff[2] * xp->x_diff[2];
       dx2_max = fmaxf(dx2_max, dx2);
 
       /* Maximal smoothing length */

src/space.c

@@ -200,9 +200,9 @@ void space_regrid(struct space *s, double cell_max, int verbose) {
         "Must have at least 3 cells in each spatial dimension when periodicity "
         "is switched on.");
 
-  /* In MPI-Land, changing the top-level cell size requires that the
-   * global partition is recomputed and the particles redistributed.
-   * Be prepared to do that. */
+/* In MPI-Land, changing the top-level cell size requires that the
+ * global partition is recomputed and the particles redistributed.
+ * Be prepared to do that. */
 #ifdef WITH_MPI
   double oldh[3];
   double oldcdim[3];
@@ -296,10 +296,11 @@ void space_regrid(struct space *s, double cell_max, int verbose) {
        * cells around the nodes. We repartition using the old space node
        * positions as a grid to resample. */
       if (s->e->nodeID == 0)
-        message("basic cell dimensions have increased - recalculating the "
-                "global partition.");
+        message(
+            "basic cell dimensions have increased - recalculating the "
+            "global partition.");
 
-      if (!partition_space_to_space(oldh, oldcdim, oldnodeIDs, s) ) {
+      if (!partition_space_to_space(oldh, oldcdim, oldnodeIDs, s)) {
 
         /* Failed, try another technique that requires no settings. */
         message("Failed to get a new partition, trying less optimal method");
@@ -1209,7 +1210,7 @@ void space_do_split(struct space *s, struct cell *c) {
       temp->depth = c->depth + 1;
       temp->split = 0;
       temp->h_max = 0.0;
-      temp->dx_max = 0.0;
+      temp->dx_max = 0.f;
       temp->nodeID = c->nodeID;
       temp->parent = c;
       c->progeny[k] = temp;
@@ -1254,16 +1255,19 @@ void space_do_split(struct space *s, struct cell *c) {
       struct xpart *xp = &xparts[k];
       const float h = p->h;
       const int ti_end = p->ti_end;
-      xp->x_old[0] = p->x[0];
-      xp->x_old[1] = p->x[1];
-      xp->x_old[2] = p->x[2];
+      xp->x_diff[0] = 0.f;
+      xp->x_diff[1] = 0.f;
+      xp->x_diff[2] = 0.f;
       if (h > h_max) h_max = h;
       if (ti_end < ti_end_min) ti_end_min = ti_end;
       if (ti_end > ti_end_max) ti_end_max = ti_end;
     }
     for (int k = 0; k < gcount; k++) {
-      struct gpart *p = &gparts[k];
-      const int ti_end = p->ti_end;
+      struct gpart *gp = &gparts[k];
+      const int ti_end = gp->ti_end;
+      gp->x_diff[0] = 0.f;
+      gp->x_diff[1] = 0.f;
+      gp->x_diff[2] = 0.f;
       if (ti_end < ti_end_min) ti_end_min = ti_end;
       if (ti_end > ti_end_max) ti_end_max = ti_end;
     }
@@ -1400,9 +1404,9 @@ void space_init(struct space *s, const struct swift_params *params,
   space_maxsize = parser_get_param_int(params, "Scheduler:cell_max_size");
   space_subsize = parser_get_param_int(params, "Scheduler:cell_sub_size");
   space_splitsize = parser_get_param_int(params, "Scheduler:cell_split_size");
-  if(verbose)
+  if (verbose)
     message("max_size set to %d, sub_size set to %d, split_size set to %d",
-	    space_maxsize, space_subsize, space_splitsize);
+            space_maxsize, space_subsize, space_splitsize);
 
   /* Check that we have enough cells */
   if (s->cell_min * 3 > dim[0] || s->cell_min * 3 > dim[1] ||