Do not reuse tags_ field to store next_ pointer of FreeListElement.

runtime/vm/class_table.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/class_table.h"
 #include "vm/flags.h"
+#include "vm/freelist.h"
 #include "vm/object.h"
 #include "vm/raw_object.h"
 #include "vm/visitor.h"
 
 namespace dart {
 
 DEFINE_FLAG(bool, print_class_table, false, "Print initial class table.");
 
 ClassTable::ClassTable()
     : top_(kNumPredefinedKinds), capacity_(0), table_(NULL) {
   if (Dart::vm_isolate() == NULL) {
     capacity_ = initial_capacity_;
     table_ = reinterpret_cast<RawClass**>(
         calloc(capacity_, sizeof(RawClass*)));  // NOLINT
+    table_[kFreeListElement] = FreeListElement::element_class();
   } else {
     // Duplicate the class table from the VM isolate.
     ClassTable* vm_class_table = Dart::vm_isolate()->class_table();
     capacity_ = vm_class_table->capacity_;
     table_ = reinterpret_cast<RawClass**>(
         calloc(capacity_, sizeof(RawClass*)));  // NOLINT
     for (intptr_t i = kObject; i < kInstance; i++) {
       table_[i] = vm_class_table->At(i);
     }
+    table_[kFreeListElement] = vm_class_table->At(kFreeListElement);
     table_[kNullClassId] = vm_class_table->At(kNullClassId);
     table_[kDynamicClassId] = vm_class_table->At(kDynamicClassId);
     table_[kVoidClassId] = vm_class_table->At(kVoidClassId);
   }
 }
 
 
 ClassTable::~ClassTable() {
   free(table_);
 }
@@ skipping 22 matching lines @@
     ASSERT(top_ < capacity_);
     cls.set_id(top_);
     table_[top_] = cls.raw();
     top_++;  // Increment next index.
   }
 }
 
 
 void ClassTable::VisitObjectPointers(ObjectPointerVisitor* visitor) {
   ASSERT(visitor != NULL);
-  visitor->VisitPointers(reinterpret_cast<RawObject**>(&table_[0]), top_);
+  // Class stored at the index kFreeListElement is a fake object
+  // residing outside of the heap.  Do not visit it.

[Ivan Posva, 2012/06/08 07:19:20]

Does the FreeListElement class have to reside outside the heap? Since it is not
pointed to directly by the FreeListElements any longer we do not need to update
it in the free blocks if it was ever moved.

+  visitor->VisitPointers(
+    reinterpret_cast<RawObject**>(&table_[0]),
+    reinterpret_cast<RawObject**>(&table_[kFreeListElement - 1]));
+  visitor->VisitPointers(
+    reinterpret_cast<RawObject**>(&table_[kFreeListElement + 1]),
+    reinterpret_cast<RawObject**>(&table_[top_ - 1]));
 }
 
 
 void ClassTable::Print() {
   Class& cls = Class::Handle();
   String& name = String::Handle();
 
   for (intptr_t i = 1; i < top_; i++) {
     cls = At(i);
     if (cls.raw() != reinterpret_cast<RawClass*>(0)) {
       name = cls.Name();
       OS::Print("%d: %s\n", i, name.ToCString());
     }
   }
 }
 
 }  // namespace dart