[vpr][placement] comment on median move generator

Author: amin1377

vpr/src/place/median_move_generator.cpp

@@ -117,7 +117,10 @@ e_create_move MedianMoveGenerator::propose_move(t_pl_blocks_to_be_moved& blocks_
             } else {
                 layer_new = net_bb_coords.layer_min;
             }
-
+            
+            // If the mvoing block is on the border of the bounding box, we cannot get
+            // the bounding box incrementatlly. In that case, bounding box should be calculated 
+            // from scratch.
             if (!get_bb_incrementally(net_id,
                                       coords,
                                       xold,
@@ -324,6 +327,12 @@ static bool get_bb_incrementally(ClusterNetId net_id,
     t_bb union_bb_edge;
     t_bb union_bb;
     const bool& cube_bb = g_vpr_ctx.placement().cube_bb;
+    /* Calculating per-layer bounding box is more time consuming compared to cube bounding box. To speed up
+    * this move, the bounding box used for this move is of the type cube bounding box even if the per-layer
+    * bounding box is used by placement SA engine. 
+    * If per-layer bounding box is used, we take a union of boundinx boxes on each layer to make a cube bounding box.
+    * For example, the xmax of this cube boundix box is determined by the maximim x coordinate across all blocks on all layers.
+    */
     if (!cube_bb) {
         std::tie(union_bb_edge, union_bb) = union_2d_bb_incr(place_move_ctx.layer_bb_num_on_edges[net_id],
                                                              place_move_ctx.layer_bb_coords[net_id]);