[AP][Timing] Updated Comments for Timing

Author: AlexandreSinger

vpr/src/analytical_place/analytical_placement_flow.cpp

@@ -49,7 +49,7 @@ static void print_ap_netlist_stats(const APNetlist& netlist) {
     float average_fanout = 0.f;
     unsigned net_count = 0;
     for (APNetId net_id : netlist.nets()) {
-        if (netlist.net_is_global(net_id) || netlist.net_is_ignored(net_id))
+        if (netlist.net_is_ignored(net_id))
             continue;
         size_t net_fanout = netlist.net_pins(net_id).size();
         if (net_fanout > highest_fanout)

vpr/src/analytical_place/analytical_solver.cpp

@@ -152,14 +152,21 @@ void AnalyticalSolver::update_net_weights(const PreClusterTimingManager& pre_clu
         // Note: To save time, we do not compute the weights of nets that we
         //       do not care about for AP. This leaves their weights at 1.0 just
         //       in case they are accidentally used.
-        if (netlist_.net_is_global(net_id) || netlist_.net_is_ignored(net_id))
+        if (netlist_.net_is_ignored(net_id))
             continue;
 
         AtomNetId atom_net_id = netlist_.net_atom_net(net_id);
         VTR_ASSERT_SAFE(atom_net_id.is_valid());
 
         float crit = pre_cluster_timing_manager.calc_net_setup_criticality(atom_net_id, atom_netlist_);
 
+        // When optimizing for WL, the net weights are just set to 1 (meaning
+        // that we want to minimize the WL of nets).
+        // When optimizing for timing, the net weights are set to the cirticality.
+        // The intuition is that we care more about shrinking the wirelength of
+        // more critical connections than less critical ones.
+        // Use the AP timing trade-off term to linearly interpolate between these
+        // weighting terms.
         net_weights_[net_id] = ap_timing_tradeoff_ * crit + (1.0f - ap_timing_tradeoff_);
     }
 }
@@ -239,7 +246,7 @@ void QPHybridSolver::init_linear_system() {
     size_t num_star_nodes = 0;
     unsigned num_nets = 0;
     for (APNetId net_id : netlist_.nets()) {
-        if (netlist_.net_is_global(net_id) || netlist_.net_is_ignored(net_id))
+        if (netlist_.net_is_ignored(net_id))
             continue;
         num_nets++;
         if (netlist_.net_pins(net_id).size() > star_num_pins_threshold)
@@ -270,7 +277,7 @@ void QPHybridSolver::init_linear_system() {
     // clique connnection models.
     size_t star_node_offset = 0;
     for (APNetId net_id : netlist_.nets()) {
-        if (netlist_.net_is_global(net_id) || netlist_.net_is_ignored(net_id))
+        if (netlist_.net_is_ignored(net_id))
             continue;
         size_t num_pins = netlist_.net_pins(net_id).size();
         VTR_ASSERT_DEBUG(num_pins > 1);
@@ -789,7 +796,7 @@ void B2BSolver::init_linear_system(PartialPlacement& p_placement) {
     triplet_list_y.reserve(num_nets);
 
     for (APNetId net_id : netlist_.nets()) {
-        if (netlist_.net_is_global(net_id) || netlist_.net_is_ignored(net_id))
+        if (netlist_.net_is_ignored(net_id))
             continue;
         size_t num_pins = netlist_.net_pins(net_id).size();
         VTR_ASSERT_SAFE_MSG(num_pins > 1, "net must have at least 2 pins");

vpr/src/analytical_place/gen_ap_netlist_from_atoms.cpp

@@ -124,9 +124,11 @@ APNetlist gen_ap_netlist_from_atoms(const AtomNetlist& atom_netlist,
             ap_netlist.set_net_is_ignored(ap_net_id, true);
             continue;
         }
-        // Is the net global, if so mark as global for AP
+        // Is the net global, if so mark as global for AP (also ignored)
         if (atom_netlist.net_is_global(atom_net_id)) {
             ap_netlist.set_net_is_global(ap_net_id, true);
+            // Global nets are also ignored by the AP flow.
+            ap_netlist.set_net_is_ignored(ap_net_id, true);
             continue;
         }
         // Get the unique blocks connectioned to this net

vpr/src/analytical_place/global_placer.cpp

@@ -241,10 +241,10 @@ static void print_SimPL_status(size_t iteration,
  *  @param ap_netlist
  *      The AP netlist the p_placement uses.
  */
-static void update_timing_info_with_placement(PreClusterTimingManager& pre_cluster_timing_manager,
-                                              const PlaceDelayModel& place_delay_model,
-                                              const PartialPlacement& p_placement,
-                                              const APNetlist& ap_netlist) {
+static void update_timing_info_with_gp_placement(PreClusterTimingManager& pre_cluster_timing_manager,
+                                                 const PlaceDelayModel& place_delay_model,
+                                                 const PartialPlacement& p_placement,
+                                                 const APNetlist& ap_netlist) {
     // If the timing manager is invalid (i.e. timing analysis is off), do not
     // update.
     if (!pre_cluster_timing_manager.is_valid())
@@ -341,10 +341,10 @@ PartialPlacement SimPLGlobalPlacer::place() {
 
         // Perform a timing update
         float timing_update_start_time = runtime_timer.elapsed_sec();
-        update_timing_info_with_placement(pre_cluster_timing_manager_,
-                                          *place_delay_model_.get(),
-                                          p_placement,
-                                          ap_netlist_);
+        update_timing_info_with_gp_placement(pre_cluster_timing_manager_,
+                                             *place_delay_model_.get(),
+                                             p_placement,
+                                             ap_netlist_);
         solver_->update_net_weights(pre_cluster_timing_manager_);
         float timing_update_end_time = runtime_timer.elapsed_sec();
 
@@ -390,10 +390,10 @@ PartialPlacement SimPLGlobalPlacer::place() {
     // Update the setup slacks. This is performed down here (as well as being
     // inside the GP loop) since the best_p_placement may not be the p_placement
     // from the last iteration of GP.
-    update_timing_info_with_placement(pre_cluster_timing_manager_,
-                                      *place_delay_model_.get(),
-                                      best_p_placement,
-                                      ap_netlist_);
+    update_timing_info_with_gp_placement(pre_cluster_timing_manager_,
+                                         *place_delay_model_.get(),
+                                         best_p_placement,
+                                         ap_netlist_);
 
     // Print statistics on the solver used.
     solver_->print_statistics();

vpr/src/analytical_place/partial_placement.cpp

@@ -14,7 +14,7 @@
 double PartialPlacement::get_hpwl(const APNetlist& netlist) const {
     double hpwl = 0.0;
     for (APNetId net_id : netlist.nets()) {
-        if (netlist.net_is_global(net_id) || netlist.net_is_ignored(net_id))
+        if (netlist.net_is_ignored(net_id))
             continue;
         double min_x = std::numeric_limits<double>::max();
         double max_x = std::numeric_limits<double>::lowest();