A high resolution device that's moving fast can potentially generate

an int overflow, making dx*dx+dy*dy negative. Now pow(negative,
non-integer) yields NaN, so you loose.  Use fp math to avoid that.

dix/getevents.c

@@ -250,7 +250,7 @@ acceleratePointer(DeviceIntPtr pDev, int first_valuator, int num_valuators,
             }
         }
         else {
-            mult = pow((float)(dx * dx + dy * dy),
+		mult = pow((float)((float)dx * (float)dx + (float)dy * (float)dy),
                        ((float)(pDev->ptrfeed->ctrl.num) /
                         (float)(pDev->ptrfeed->ctrl.den) - 1.0) /
                        2.0) / 2.0;