add comments in is_turn_legal() overrides

Author: soheilshahrouz

vpr/src/noc/negative_first_routing.cpp

@@ -119,6 +119,7 @@ bool NegativeFirstRouting::is_turn_legal(const std::array<std::reference_wrapper
     const int x3 = noc_routers[2].get().get_router_grid_position_x();
     const int y3 = noc_routers[2].get().get_router_grid_position_y();
 
+    // check if the given routers can be traversed one after another
     VTR_ASSERT(x2 == x1 || y2 == y1);
     VTR_ASSERT(x3 == x2 || y3 == y2);
 
@@ -127,10 +128,16 @@ bool NegativeFirstRouting::is_turn_legal(const std::array<std::reference_wrapper
         return false;
     }
 
+    /* In negative-first routing algorithm, a traffic flow
+     * can't take a downward turn if it is travelling toward right direction.
+     */
     if (x2 > x1 && y3 < y2) {
         return false;
     }
 
+    /* In negative-first routing algorithm, a traffic flow
+     * can't take a left turn if it is travelling upwards.
+     */
     if (y2 > y1 && x3 < x2) {
         return false;
     }

vpr/src/noc/north_last_routing.cpp

@@ -108,6 +108,7 @@ bool NorthLastRouting::is_turn_legal(const std::array<std::reference_wrapper<con
     const int x3 = noc_routers[2].get().get_router_grid_position_x();
     const int y3 = noc_routers[2].get().get_router_grid_position_y();
 
+    // check if the given routers can be traversed one after another
     VTR_ASSERT(x2 == x1 || y2 == y1);
     VTR_ASSERT(x3 == x2 || y3 == y2);
 
@@ -116,6 +117,11 @@ bool NorthLastRouting::is_turn_legal(const std::array<std::reference_wrapper<con
         return false;
     }
 
+    /* In the north-last algorithm, once the north direction is taken, no other
+     * direction can be followed. Therefore, if the first link moves upward, the
+     * second one cannot move horizontally. The case where the second link goes
+     * back to the first router was checked in the previous if statement.
+     */
     if (y2 > y1 && x2 != x3) {
         return false;
     }

vpr/src/noc/odd_even_routing.cpp

@@ -171,13 +171,13 @@ bool OddEvenRouting::is_turn_legal(const std::array<std::reference_wrapper<const
     const int x3 = noc_routers[2].get().get_router_grid_position_x();
     const int y3 = noc_routers[2].get().get_router_grid_position_y();
 
+    // check if the given routers can be traversed one after another
     VTR_ASSERT(x2 == x1 || y2 == y1);
     VTR_ASSERT(x3 == x2 || y3 == y2);
 
     // get the position of the second NoC routers
     const auto router2_pos = noc_routers[1].get().get_router_physical_location();
 
-
     // Get the logical block type for router
     const auto router_block_type = cluster_ctx.clb_nlist.block_type(noc_ctx.noc_traffic_flows_storage.get_router_clusters_in_netlist()[0]);
 
@@ -192,6 +192,7 @@ bool OddEvenRouting::is_turn_legal(const std::array<std::reference_wrapper<const
         return false;
     }
 
+    // check if the turn is compatible with odd-even routing algorithm turn restrictions
     if (is_odd(compressed_2_loc.x)) {
         if (y2 != y1 && x3 < x2) {
             return false;

vpr/src/noc/west_first_routing.cpp

@@ -105,6 +105,7 @@ bool WestFirstRouting::is_turn_legal(const std::array<std::reference_wrapper<con
     const int x3 = noc_routers[2].get().get_router_grid_position_x();
     const int y3 = noc_routers[2].get().get_router_grid_position_y();
 
+    // check if the given routers can be traversed one after another
     VTR_ASSERT(x2 == x1 || y2 == y1);
     VTR_ASSERT(x3 == x2 || y3 == y2);
 
@@ -113,6 +114,11 @@ bool WestFirstRouting::is_turn_legal(const std::array<std::reference_wrapper<con
         return false;
     }
 
+    /* In the west-first routing algorithm, once the traffic flow
+     * moved in a vertical direction, it is no longer allowed to move
+     * towards west. Therefore, if the first link was travelling in a
+     * vertical direction, the second link can't move towards left.
+     */
     if (y2 != y1 && x3 < x2) {
         return false;
     }

vpr/src/noc/xy_routing.cpp

@@ -92,6 +92,7 @@ bool XYRouting::is_turn_legal(const std::array<std::reference_wrapper<const NocR
     const int x3 = noc_routers[2].get().get_router_grid_position_x();
     const int y3 = noc_routers[2].get().get_router_grid_position_y();
 
+    // check if the given routers can be traversed one after another
     VTR_ASSERT(x2 == x1 || y2 == y1);
     VTR_ASSERT(x3 == x2 || y3 == y2);