[Router] Removed the R_upstream field in t_rr_node_route_inf

The `R_upstream` field added to the `t_rr_node_route_inf` structure for
convenience in commit b11b2 is removed and kept inside the connection
router class instead.

This allows the size of each `t_rr_node_route_inf` to be as minimal as
possible, which tends to be more cache-friendly when accessing the large
RR node route info data, leading to higher performance.

Author: ueqri

vpr/src/base/vpr_types.h

@@ -1684,13 +1684,6 @@ struct t_rr_node_route_inf {
     float acc_cost;
     float path_cost;
     float backward_path_cost;
-    float R_upstream; // TODO: Investigate the effect of adding the R_upstream field in
-                      //       this struct. It is put in for the fine-grained parallel
-                      //       router's benefits. It is increasing the working set, which
-                      //       can have some performance implications. This could affect
-                      //       the performance of the serial connection router, which will
-                      //       make the Hybrid Connection Router less efficient (but that
-                      //       needs to be investigated).
 
   public: //Accessors
     short occ() const { return occ_; }

vpr/src/route/connection_router.cpp

@@ -373,8 +373,8 @@ void ConnectionRouter<Heap>::timing_driven_expand_cheapest(RRNodeId from_node,
         // element for the corresponding node.
         node_t current;
         current.backward_path_cost = rr_node_route_inf_[from_node].backward_path_cost;
-        current.R_upstream = rr_node_route_inf_[from_node].R_upstream;
         current.prev_edge = rr_node_route_inf_[from_node].prev_edge;
+        current.R_upstream = rr_node_R_upstream[from_node];
 
         VTR_LOGV_DEBUG(router_debug_, "    Better cost to %d\n", from_node);
         VTR_LOGV_DEBUG(router_debug_, "    New total cost: %g\n", new_total_cost);
@@ -887,7 +887,7 @@ void ConnectionRouter<Heap>::add_route_tree_node_to_heap(
         rr_node_route_inf_[inode].path_cost = tot_cost;
         rr_node_route_inf_[inode].prev_edge = RREdgeId::INVALID();
         rr_node_route_inf_[inode].backward_path_cost = backward_path_cost;
-        rr_node_route_inf_[inode].R_upstream = R_upstream;
+        rr_node_R_upstream[inode] = R_upstream;
         heap_.push_back(tot_cost, inode);
 
         // push_back_node(&heap_, rr_node_route_inf_,
@@ -900,7 +900,7 @@ void ConnectionRouter<Heap>::add_route_tree_node_to_heap(
         rr_node_route_inf_[inode].path_cost = expected_total_cost;
         rr_node_route_inf_[inode].prev_edge = RREdgeId::INVALID();
         rr_node_route_inf_[inode].backward_path_cost = backward_path_cost;
-        rr_node_route_inf_[inode].R_upstream = R_upstream;
+        rr_node_R_upstream[inode] = R_upstream;
 
         rcv_path_manager.alloc_path_struct(rcv_path_data[inode]);
         rcv_path_data[inode]->backward_delay = rt_node.Tdel;

vpr/src/route/connection_router.h

@@ -60,6 +60,7 @@ class ConnectionRouter : public ConnectionRouterInterface {
         heap_.init_heap(grid);
         // heap_.set_prune_limit(rr_nodes_.size(), kHeapPruneFactor * rr_nodes_.size());
         only_opin_inter_layer = (grid.get_num_layers() > 1) && inter_layer_connections_limited_to_opin(*rr_graph);
+        rr_node_R_upstream.resize(rr_node_route_inf.size());
         rcv_path_data.resize(rr_node_route_inf.size());
     }
 
@@ -81,6 +82,9 @@ class ConnectionRouter : public ConnectionRouterInterface {
         for (const auto& node : modified_rr_node_inf_) {
             rcv_path_data[node] = nullptr;
         }
+        // Note: R_upstream of each node is intentionally not reset here.
+        // For the reasons and details, please refer to the `Update R_upstream`
+        // in `evaluate_timing_driven_node_costs` in `connection_router.cpp`.
     }
 
     /** Finds a path from the route tree rooted at rt_root to sink_node.
@@ -311,6 +315,11 @@ class ConnectionRouter : public ConnectionRouterInterface {
     // Timing
     std::chrono::microseconds sssp_total_time{0};
 
+    // Used only by the timing-driven router. Stores the upstream resistance to ground from this node, including the resistance
+    // of the node itself (device_ctx.rr_nodes[index].R). Since the `t_heap` is hardly used for heap node structure in the
+    // connection router anymore, the `R_upstream` field of the `t_heap` is kept inside the connection router class instead.
+    vtr::vector<RRNodeId, float> rr_node_R_upstream;
+
     // The path manager for RCV, keeps track of the route tree as a set, also manages the allocation of the heap types
     PathManager rcv_path_manager;
     vtr::vector<RRNodeId, t_heap_path*> rcv_path_data;