odb: makeNewNetName must avoid instance name collisions

dbBlock::makeNewNetName uniquified a new net/port name against ModNets,
ModBTerms, and flat nets in the scope, but not against instance names.
A net/port and an instance cannot share a name in one Verilog scope.

OpenROAD's hierarchical flow promotes some anonymous nets ("_NNNNN_") to
module boundary ports, and yosys/ABC name anonymous cells "_NNNNN_" too,
so a promoted boundary port could be handed a name already owned by a
leaf (dbInst) or hierarchical (dbModInst) instance in the same module.
write_verilog then emits a module with a port and an instance of the
same name -- illegal Verilog that Verilator rejects with "Instance has
the same name as port", breaking gate-level (e.g. SAIF) simulation of
the placed netlist.

Extend the collision check in makeNewNetName to also reject names that
match a dbModInst or dbInst in the scope, so the generated name is
uniquified (suffixed) when it would collide with an instance.

Add a TestModule regression covering both the leaf-instance and
module-instance collision cases, plus the no-collision passthrough.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

src/odb/src/db/dbBlock.cpp

@@ -3883,7 +3883,15 @@ std::string dbBlock::makeNewNetName(const dbModule* parent,
   auto exists = [this, scope, corresponding_flat_net](const char* name) {
     if (scope != nullptr) {
       const char* base = getBaseName(name);
-      if (scope->getModNet(base) || scope->findModBTerm(base)) {
+      // A net/port name must also be unique against instance names in the
+      // scope: a net/port and an instance cannot share a name in one Verilog
+      // scope.  OpenROAD can promote an anonymous net ("_NNNNN_") to a
+      // module boundary port, and yosys/ABC name anonymous cells "_NNNNN_"
+      // too, so without this check the promoted port collides with a leaf or
+      // hierarchical instance of the same name -- illegal Verilog that
+      // Verilator rejects with "Instance has the same name as port".
+      if (scope->getModNet(base) || scope->findModBTerm(base)
+          || scope->findModInst(base) || scope->findDbInst(base)) {
         return true;
       }
     }

src/odb/test/cpp/TestModule.cpp

@@ -291,6 +291,39 @@ TEST_F(ModuleFixture, test_find_modinst)
   auto minst2 = odb::dbModInst::create(master1, master2, "minst2");
   EXPECT_EQ(block_->findModInst("minst1/minst2"), minst2);
 }
+
+// makeNewNetName must not hand out a name that already belongs to an
+// instance in the scope: a net/port and an instance cannot share a name in
+// one Verilog scope.  OpenROAD promotes anonymous nets ("_NNNNN_") to
+// module boundary ports, and yosys/ABC name anonymous cells "_NNNNN_" too,
+// so without an instance-name check the promoted port collides with a leaf
+// or hierarchical instance -- illegal Verilog ("Instance has the same name
+// as port").
+TEST_F(ModuleFixture, makeNewNetName_avoids_instance_collision)
+{
+  auto top = block_->getTopModule();
+  ASSERT_NE(top, nullptr);
+
+  // Leaf (dbInst) collision: an instance named "_42_" exists in scope.
+  dbInst::create(block_, lib_->findMaster("and2"), "_42_");
+  std::string leaf = block_->makeNewNetName(
+      top, "_42_", dbNameUniquifyType::IF_NEEDED_WITH_UNDERSCORE);
+  EXPECT_STRNE(block_->getBaseName(leaf.c_str()), "_42_")
+      << "net name must not collide with leaf instance '_42_'";
+
+  // Hierarchical (dbModInst) collision: a module instance named "_99_".
+  auto master = dbModule::create(block_, "anon_master");
+  dbModInst::create(top, master, "_99_");
+  std::string hier = block_->makeNewNetName(
+      top, "_99_", dbNameUniquifyType::IF_NEEDED_WITH_UNDERSCORE);
+  EXPECT_STRNE(block_->getBaseName(hier.c_str()), "_99_")
+      << "net name must not collide with module instance '_99_'";
+
+  // No collision: a fresh name is returned unchanged.
+  std::string fresh = block_->makeNewNetName(
+      top, "no_collision_here", dbNameUniquifyType::IF_NEEDED_WITH_UNDERSCORE);
+  EXPECT_STREQ(block_->getBaseName(fresh.c_str()), "no_collision_here");
+}
 class DetailedFixture : public SimpleDbFixture
 {
  protected: