Update comments on pruning functions.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

vpr/src/route/route_tree_timing.cpp

diff --git a/vpr/src/route/route_tree_timing.cpp b/vpr/src/route/route_tree_timing.cpp
index d51aa5d..0e1ac4f 100644
--- a/vpr/src/route/route_tree_timing.cpp
+++ b/vpr/src/route/route_tree_timing.cpp
@@ -1070,9 +1070,41 @@
         //   3. The node set is not active
         //
         //  Then prune this node.
+        //
         if (non_config_node_set_usage != nullptr && node_set != -1 && device_ctx.rr_switch_inf[node->parent_switch].configurable() && (*non_config_node_set_usage)[node_set] == 0) {
             // This node should be pruned, re-prune edges once more.
-            return prune_route_tree_recurr(node, connections_inf, /*force_prune=*/false, non_config_node_set_usage);
+            //
+            // If the following is true:
+            //
+            //  - The node set is unused
+            //    (e.g. (*non_config_node_set_usage)[node_set] == 0)
+            //  - This particular node still had children
+            //    (which is true by virtue of being in this else statement)
+            //
+            // Then that indicates that the node set became unused during the
+            // pruning. One or more of the children of this node will be
+            // pruned if prune_route_tree_recurr is called again, and
+            // eventually the whole node will be prunable.
+            //
+            //  Consider the following graph:
+            //
+            //  1 -> 2
+            //  2 -> 3 [non-configurable]
+            //  2 -> 4 [non-configurable]
+            //  3 -> 5
+            //  4 -> 6
+            //
+            //  Assume that nodes 5 and 6 do not connect to a sink, so they
+            //  will be pruned (as normal). When prune_route_tree_recurr
+            //  visits 2 for the first time, node 3 or 4 will remain. This is
+            //  because when prune_route_tree_recurr visits 3 or 4, it will
+            //  not have visited 4 or 3 (respectively). As a result, either
+            //  node 3 or 4 will not be pruned on the first pass, because the
+            //  node set usage count will be > 0. However after
+            //  prune_route_tree_recurr visits 2, 3 and 4, the node set usage
+            //  will be 0, so everything can be pruned.
+            return prune_route_tree_recurr(node, connections_inf,
+                                           /*force_prune=*/false, non_config_node_set_usage);
         }
 
         //An unpruned intermediate node

vpr/src/route/route_tree_timing.h

diff --git a/vpr/src/route/route_tree_timing.h b/vpr/src/route/route_tree_timing.h
index ae67a15..01e4d88 100644
--- a/vpr/src/route/route_tree_timing.h
+++ b/vpr/src/route/route_tree_timing.h
@@ -55,7 +55,17 @@
 t_rt_node* traceback_to_route_tree(t_trace* head, std::vector<int>* non_config_node_set_usage);
 t_trace* traceback_from_route_tree(ClusterNetId inet, const t_rt_node* root, int num_remaining_sinks);
 
+// Prune route tree
+//
+//  Note that non-configurable node will not be pruned unless the node is
+//  being totally ripped up, or the node is congested.
 t_rt_node* prune_route_tree(t_rt_node* rt_root, CBRR& connections_inf);
+
+// Prune route tree
+//
+//  Note that non-configurable nodes will be pruned if
+//  non_config_node_set_usage is provided.  prune_route_tree will update
+//  non_config_node_set_usage after pruning.
 t_rt_node* prune_route_tree(t_rt_node* rt_root, CBRR& connections_inf, std::vector<int>* non_config_node_set_usage);
 
 void pathfinder_update_cost_from_route_tree(const t_rt_node* rt_root, int add_or_sub, float pres_fac);