Save a sqrtf() call per particle in drift task

src/runner.c

@@ -709,16 +709,18 @@ 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 motion since last cell construction */
-      const float dx =
-          sqrtf((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]));
+      /* Compute (square of) motion since last cell construction */
+      const float dx = (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]);
       dx_max = fmaxf(dx_max, dx);
 
       /* Maximal smoothing length */
       h_max = fmaxf(p->h, h_max);
     }
+
+    /* Now, get the maximal particle motion from its square */
+    dx_max = sqrtf(dx_max);
   }
 
   /* Otherwise, aggregate data from children. */