Copy-on-write class table for concurrent reading.

runtime/vm/class_table.h

 // 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.
 
 #ifndef VM_CLASS_TABLE_H_
 #define VM_CLASS_TABLE_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 
 namespace dart {
 
 class Class;
 class ClassStats;
 class JSONArray;
 class JSONObject;
 class JSONStream;
+template<typename T> class MallocGrowableArray;
 class ObjectPointerVisitor;
 class RawClass;
 
 template<typename T>
 class AllocStats {
  public:
   T new_count;
   T new_size;
   T old_count;
   T old_size;
@@ skipping 86 matching lines @@
  private:
   // Recent old at start of last new GC (used to compute promoted_*).
   intptr_t old_pre_new_gc_count_;
   intptr_t old_pre_new_gc_size_;
 };
 
 
 class ClassTable {
  public:
   ClassTable();
-  // Create a copy of the original class table only, without copying any of the
-  // stats data.
+  // Creates a shallow copy of the original class table for some read-only
+  // access, without support for stats data.
   explicit ClassTable(ClassTable* original);
   ~ClassTable();
 
+  // Thread-safe.
   RawClass* At(intptr_t index) const {
     ASSERT(IsValidIndex(index));
     return table_[index];
   }
 
   intptr_t IsValidIndex(intptr_t index) const {
     return (index > 0) && (index < top_);
   }
 
   bool HasValidClassAt(intptr_t index) const {
@@ skipping 43 matching lines @@
   // Used by generated code.
   uword ClassStatsTableAddress() {
     return reinterpret_cast<uword>(&class_heap_stats_table_);
   }
 
   ClassHeapStats* StatsWithUpdatedSize(intptr_t cid);
 
   void AllocationProfilePrintJSON(JSONStream* stream);
   void ResetAllocationAccumulators();
 
+  // Deallocates table copies. Do not call during concurrent access to table.
+  void FreeOldTables();
+
  private:
   friend class MarkingVisitor;
   friend class ScavengerVisitor;
   friend class ClassHeapStatsTestHelper;
   static const int initial_capacity_ = 512;
   static const int capacity_increment_ = 256;
 
   static bool ShouldUpdateSizeForClassId(intptr_t cid);
 
   intptr_t top_;
   intptr_t capacity_;
 
+  // Copy-on-write is used for table_, with old copies stored in old_tables_.
   RawClass** table_;
+  MallocGrowableArray<RawClass**>* old_tables_;
+
   ClassHeapStats* class_heap_stats_table_;
 
   ClassHeapStats* predefined_class_heap_stats_table_;
 
   // May not have updated size for variable size classes.
   ClassHeapStats* PreliminaryStatsAt(intptr_t cid);
   void UpdateLiveOld(intptr_t cid, intptr_t size);
   void UpdateLiveNew(intptr_t cid, intptr_t size);
 
   DISALLOW_COPY_AND_ASSIGN(ClassTable);
 };
 
 }  // namespace dart
 
 #endif  // VM_CLASS_TABLE_H_