[ParallelRouter] Fixed Bitcoin Bug

The bug in bitcoin_miner was caused by a precision issue where the
congestion cost would get so large that the delay cost would not matter.
This creates zero-cost cycles which the deterministic parallel router
cannot handle.

There was a PR on VTR Master which fixed this by preventing the pres_fac
from going over 1000. Bringing that change in early. This change will
fully be brought in the next time this branch is rebased onto master.

Author: AlexandreSinger

vpr/src/route/parallel_connection_router.cpp

@@ -881,7 +881,7 @@ void ParallelConnectionRouter::add_route_tree_node_to_heap(
                        describe_rr_node(device_ctx.rr_graph, device_ctx.grid, device_ctx.rr_indexed_data, inode, is_flat_).c_str());
 
 
-        if (tot_cost >= rr_node_route_inf_[inode].path_cost)
+        if (prune_node(inode, tot_cost, backward_path_cost, RREdgeId::INVALID(), target_node, rr_node_route_inf_, cost_params))
             return ;
         add_to_mod_list(inode, 0/*main thread*/);
         rr_node_route_inf_[inode].path_cost = tot_cost;

vpr/src/route/route.cpp

@@ -440,7 +440,7 @@ bool route(const Netlist<>& net_list,
             pres_fac = update_draw_pres_fac(router_opts.initial_pres_fac);
         } else {
             pres_fac *= router_opts.pres_fac_mult;
-
+            pres_fac = std::min(pres_fac, 1000.f);
             /* Avoid overflow for high iteration counts, even if acc_cost is big */
             pres_fac = update_draw_pres_fac(std::min(pres_fac, static_cast<float>(HUGE_POSITIVE_FLOAT / 1e5)));
 

vpr/src/route/route_tree.cpp

@@ -7,6 +7,8 @@
 #include "rr_graph_fwd.h"
 #include "vtr_math.h"
 
+// #define DEBUG_CHECK_FOR_LOOP
+
 /* Construct a new RouteTreeNode.
  * Doesn't add the node to parent's child_nodes! (see add_child) */
 RouteTreeNode::RouteTreeNode(RRNodeId _inode, RRSwitchId _parent_switch, RouteTreeNode* parent)
@@ -535,9 +537,25 @@ RouteTree::add_subtree_from_heap(t_heap* hptr, int target_net_pin_index, bool is
     RRNodeId new_inode = rr_graph.edge_src_node(edge);
     RRSwitchId new_iswitch = RRSwitchId(rr_graph.rr_nodes().edge_switch(edge));
 
+#ifdef DEBUG_CHECK_FOR_LOOP
+    std::unordered_set<RRNodeId> seen_nodes;
+#endif
+
     /* build a path, looking up rr nodes and switches from rr_node_route_inf */
     new_branch_inodes.push_back(sink_inode);
     while (!_rr_node_to_rt_node.count(new_inode)) {
+#ifdef DEBUG_CHECK_FOR_LOOP
+        VTR_ASSERT(new_inode.is_valid() && "Invalid node being added to path!");
+        if (seen_nodes.count(new_inode) != 0) {
+            for (const RRNodeId &n : new_branch_inodes) {
+                VTR_LOG("%zu (%.20e), ", (size_t)n, route_ctx.rr_node_route_inf[n].backward_path_cost);
+            }
+            VTR_LOG("\n");
+            VTR_LOG("Duplicate node: %zu\n", new_inode);
+            VTR_ASSERT(false && "Duplicate node in path!");
+        }
+        seen_nodes.insert(new_inode);
+#endif
         new_branch_inodes.push_back(new_inode);
         new_branch_iswitches.push_back(new_iswitch);
         edge = route_ctx.rr_node_route_inf[new_inode].prev_edge;