rr_nodes: add fan-in list generator

vpr/src/route/rr_graph_util.cpp

@@ -177,3 +177,20 @@ void reorder_rr_graph_nodes(const t_router_opts& router_opts) {
                                std::get<2>(edge));
     });
 }
+
+vtr::vector<RRNodeId, std::vector<RREdgeId>> get_fan_in_list() {
+    auto& rr_nodes = g_vpr_ctx.device().rr_nodes;
+
+    vtr::vector<RRNodeId, std::vector<RREdgeId>> node_fan_in_list;
+
+    node_fan_in_list.resize(rr_nodes.size(), std::vector<RREdgeId>(0));
+    node_fan_in_list.shrink_to_fit();
+
+    //Walk the graph and increment fanin on all dwnstream nodes
+    rr_nodes.for_each_edge(
+        [&](RREdgeId edge, __attribute__((unused)) RRNodeId src, RRNodeId sink) {
+            node_fan_in_list[sink].push_back(edge);
+        });
+
+    return node_fan_in_list;
+}

vpr/src/route/rr_graph_util.h

@@ -1,10 +1,16 @@
 #ifndef RR_GRAPH_UTIL_H
 #define RR_GRAPH_UTIL_H
 
+#include "vpr_types.h"
+
 int seg_index_of_cblock(t_rr_type from_rr_type, int to_node);
 
 int seg_index_of_sblock(int from_node, int to_node);
 
 void reorder_rr_graph_nodes(const t_router_opts& router_opts);
 
+// This function generates and returns a vector indexed by RRNodeId
+// containing a list of fan-in edges for each node.
+vtr::vector<RRNodeId, std::vector<RREdgeId>> get_fan_in_list();
+
 #endif