grt: add option to skip large fanout nets

src/grt/README.md

@@ -24,6 +24,7 @@ global_route
     [-congestion_report_iter_step steps]
     [-grid_origin {x y}]
     [-critical_nets_percentage percent]
+    [-skip_large_fanout_nets fanout]
     [-allow_congestion]
     [-verbose]
     [-start_incremental]
@@ -42,6 +43,7 @@ global_route
 | `-congestion_report_iter_step` | Set the number of iterations to report. The default value is `0`, and the allowed values are integers `[0, MAX_INT]`. |
 | `-grid_origin` | Set the (x, y) origin of the routing grid in DBU. For example, `-grid_origin {1 1}` corresponds to the die (0, 0) + 1 DBU in each x--, y- direction. |
 | `-critical_nets_percentage` | Set the percentage of nets with the worst slack value that are considered timing critical, having preference over other nets during congestion iterations (e.g. `-critical_nets_percentage 30`). The default value is `0`, and the allowed values are integers `[0, MAX_INT]`. |
+| `-skip_large_fanout_nets` | Skips routing for nets with a fanout higher than the specified limit. Nets above this pin count threshold are ignored by the global router and will not have routing guides, meaning they will also be skipped during detailed routing. This option is useful in debugging or estimation flows where high-fanout nets (such as pre-CTS clock nets) can be ignored. The default value is 0, indicating no fanout limit. The default value is `0`, meaning no limit for net fanout. The allowed values are integers `[0, MAX_INT]`. |
 | `-allow_congestion` | Allow global routing results to be generated with remaining congestion. The default is false. |
 | `-verbose` | This flag enables the full reporting of the global routing. |
 | `-start_incremental` | This flag initializes the GRT listener to get the net modified. The default is false. |

src/grt/include/grt/GlobalRouter.h

@@ -154,6 +154,10 @@ class GlobalRouter
   void setResistanceAware(bool resistance_aware);
   void setMacroExtension(int macro_extension);
   void setUseCUGR(bool use_cugr) { use_cugr_ = use_cugr; };
+  void setSkipLargeFanoutNets(int skip_large_fanout)
+  {
+    skip_large_fanout_ = skip_large_fanout;
+  };
 
   // flow functions
   void readGuides(const char* file_name);
@@ -495,6 +499,7 @@ class GlobalRouter
   int total_diodes_count_;
   bool is_congested_{false};
   bool use_cugr_{false};
+  int skip_large_fanout_{0};
 
   // Region adjustment variables
   std::vector<RegionAdjustment> region_adjustments_;

src/grt/src/GlobalRouter.cpp

@@ -3744,6 +3744,9 @@ std::vector<Net*> GlobalRouter::findNets(bool init_clock_nets)
   }
   std::vector<Net*> clk_nets;
   for (odb::dbNet* db_net : db_nets) {
+    if (skip_large_fanout_ > 0 && db_net->getTermCount() > skip_large_fanout_) {
+      continue;
+    }
     Net* net = addNet(db_net);
     // add clock nets not connected to a leaf first
     if (net) {

src/grt/src/GlobalRouter.i

@@ -146,6 +146,12 @@ set_use_cugr(bool use_cugr)
   getGlobalRouter()->setUseCUGR(use_cugr);
 }
 
+void
+set_skip_large_fanout(int skip_large_fanout)
+{
+  getGlobalRouter()->setSkipLargeFanoutNets(skip_large_fanout);
+}
+
 void
 global_route(bool start_incremental, bool end_incremental)
 {

src/grt/src/GlobalRouter.tcl

@@ -146,6 +146,7 @@ sta::define_cmd_args "global_route" {[-guide_file out_file] \
                                   [-congestion_report_iter_step steps] \
                                   [-grid_origin origin] \
                                   [-critical_nets_percentage percent] \
+                                  [-skip_large_fanout_nets fanout] \
                                   [-allow_congestion] \
                                   [-verbose] \
                                   [-start_incremental] \
@@ -157,7 +158,8 @@ sta::define_cmd_args "global_route" {[-guide_file out_file] \
 proc global_route { args } {
   sta::parse_key_args "global_route" args \
     keys {-guide_file -congestion_iterations -congestion_report_file \
-          -grid_origin -critical_nets_percentage -congestion_report_iter_step
+          -grid_origin -critical_nets_percentage -congestion_report_iter_step\
+          -skip_large_fanout_nets
          } \
     flags {-allow_congestion -resistance_aware -verbose -start_incremental -end_incremental \
           -use_cugr}
@@ -214,6 +216,12 @@ proc global_route { args } {
 
   grt::set_use_cugr [info exists flags(-use_cugr)]
 
+  if { [info exists keys(-skip_large_fanout_nets)] } {
+    set fanout $keys(-skip_large_fanout_nets)
+    sta::check_positive_integer "-skip_large_fanout_nets" $fanout
+    grt::set_skip_large_fanout $fanout
+  }
+
   set allow_congestion [info exists flags(-allow_congestion)]
   grt::set_allow_congestion $allow_congestion
 

src/grt/test/BUILD

@@ -78,6 +78,7 @@ TESTS = [
     "set_nets_to_route1",
     "silence",
     "single_row",
+    "skip_large_fanout1",
     "soft_ndr_4w_6s",
     "top_level_term1",
     "top_level_term2",

src/grt/test/CMakeLists.txt

@@ -76,6 +76,7 @@ or_integration_tests(
     set_nets_to_route1
     silence
     single_row
+    skip_large_fanout1
     soft_ndr_4w_6s
     top_level_term1
     top_level_term2