Adding api to get last gc object statistics for chrome://tracing.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 73e5e5c4ee0da19cdbbaf2e320c270b9743f72de..eb138403b870d033fea2f5bfecf7609c18ba0847 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -4813,6 +4813,22 @@ class V8_EXPORT HeapSpaceStatistics {
 };
 
 
+class V8_EXPORT HeapObjectStatistics {
+ public:
+  HeapObjectStatistics();
+  const char* object_type() { return object_type_; }
+  size_t object_count() { return object_count_; }
+  size_t object_size() { return object_size_; }

[ulan, 2015/05/06 12:46:02]

You probably also want a notion of sub-type or secondary type.  For example a
single code object can have multiple HeapObjectStatistics:
- one with type "CODE_TYPE",
- one with type "CODE_TYPE_<code-kind>",
- one with type "CODE_TYPE_<code-age".

So HeapObjectStatistics can share objects. Sum of object_size() will not be the
correct total as it double counts some objects.

[ssid, 2015/05/07 13:53:03]

As discussed offline, the sub-types are just given by the way the object is
named, so this object remains the same.

+
+ private:
+  const char* object_type_;
+  size_t object_count_;
+  size_t object_size_;
+
+  friend class Isolate;
+};
+
+
 class RetainedObjectInfo;
 
 
@@ -5201,6 +5217,23 @@ class V8_EXPORT Isolate {
                               size_t index);
 
   /**
+   * Returns the number of types of objects in the heap.
+   */
+  size_t NumberOfHeapObjectsTypes();

[ulan, 2015/05/06 12:46:02]

Strictly speaking this is not the number of heap object types. This is the
number of heap object statistics that we track (which is more than the actual
heap object types). Can we make it more explicit in the name that this is about
statistics and not generic heap object types?

[ssid, 2015/05/07 13:53:03]

Done.

+
+  /**
+   * Get statistics about objects in the heap.
+   *
+   * \param object_statistics The HeapObjectStatistics object to fill in
+   *   statistics of objects of given type, which were live in the previous GC.
+   * \param type_index The index of the type of object to fill details about,
+   *   which ranges from 0 to NumberOfHeapObjectsTypes() - 1.
+   * \returns true on success.
+   */
+  bool GetLastGcObjectStatistics(HeapObjectStatistics* object_statistics,

[ulan, 2015/05/06 12:46:02]

GetHeapObjectStatisticsAtLastGC(...) to be consistent with other
GetHeap*Statistics functions.

[ssid, 2015/05/07 13:53:03]

Done.

+                                 size_t type_index);
+
+  /**
    * Get a call stack sample from the isolate.
    * \param state Execution state.
    * \param frames Caller allocated buffer to store stack frames.