mpl2: apply constraint relaxation until we find a reasonable split when using PAR

src/mpl2/src/clusterEngine.cpp

@@ -1432,33 +1432,44 @@ void ClusteringEngine::breakLargeFlatCluster(Cluster* parent)
   }
 
   const int seed = 0;
-  const float balance_constraint = 1.0;
+  constexpr float default_balance_constraint = 1.0f;
+  float balance_constraint = default_balance_constraint;
   const int num_parts = 2;  // We use two-way partitioning here
   const int num_vertices = static_cast<int>(vertex_weight.size());
   std::vector<float> hyperedge_weights(hyperedges.size(), 1.0f);
 
-  debugPrint(logger_,
-             MPL,
-             "multilevel_autoclustering",
-             1,
-             "Breaking flat cluster {} with TritonPart",
-             parent->getName());
+  // Due to the discrepancy that may exist between the weight of vertices
+  // that represent macros/std cells, the partitioner may fail to meet the
+  // balance constraint. This may cause the output to be completely unbalanced
+  // and lead to infinite partitioning recursion. To handle that, we relax
+  // the constraint until we find a reasonable split.
+  constexpr float balance_constraint_relaxation_factor = 10.0f;
+  std::vector<int> solution;
+  do {
+    debugPrint(
+        logger_,
+        MPL,
+        "multilevel_autoclustering",
+        1,
+        "Attempting flat cluster {} partitioning with balance constraint = {}",
+        parent->getName(),
+        balance_constraint);
 
-  std::vector<int> part
-      = triton_part_->PartitionKWaySimpleMode(num_parts,
-                                              balance_constraint,
-                                              seed,
-                                              hyperedges,
-                                              vertex_weight,
-                                              hyperedge_weights);
+    if (balance_constraint >= 90) {
+      logger_->error(
+          MPL, 45, "Cannot find a balanced partitioning for the clusters.");
+    }
 
-  if (partitionerSolutionIsFullyUnbalanced(part, num_other_cluster_vertices)) {
-    logger_->error(MPL,
-                   37,
-                   "Couldn't break flat cluster {} with PAR. The solution is "
-                   "fully unbalanced.",
-                   parent->getName());
-  }
+    solution = triton_part_->PartitionKWaySimpleMode(num_parts,
+                                                     balance_constraint,
+                                                     seed,
+                                                     hyperedges,
+                                                     vertex_weight,
+                                                     hyperedge_weights);
+
+    balance_constraint += balance_constraint_relaxation_factor;
+  } while (partitionerSolutionIsFullyUnbalanced(solution,
+                                                num_other_cluster_vertices));
 
   parent->clearLeafStdCells();
   parent->clearLeafMacros();
@@ -1470,7 +1481,7 @@ void ClusteringEngine::breakLargeFlatCluster(Cluster* parent)
 
   for (int i = num_other_cluster_vertices; i < num_vertices; i++) {
     odb::dbInst* inst = insts[i - num_other_cluster_vertices];
-    if (part[i] == 0) {
+    if (solution[i] == 0) {
       parent->addLeafInst(inst);
     } else {
       cluster_part_1->addLeafInst(inst);