[tracing] Adding class-wise memory statistics to blink gc memory dumps

Source/platform/heap/ThreadState.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 471 matching lines @@
     // Visit local thread stack and trace all pointers conservatively.
     void visitStack(Visitor*);
 
     // Visit the asan fake stack frame corresponding to a slot on the
     // real machine stack if there is one.
     void visitAsanFakeStackForPointer(Visitor*, Address);
 
     // Visit all persistents allocated on this thread.
     void visitPersistents(Visitor*);
 
+    struct GCSnapshotInfo {
+        // Map from base-classes to a snapshot class-ids (used as index below).
+        using ClassTagMap = HashMap<const GCInfo*, size_t>;

[haraken, 2015/06/26 05:35:28]

How about just using gcInfoIndex for the tag? gcInfoIndex is already a tag for
GCInfo.

[ssid, 2015/06/26 10:10:28]

Hm, I think it is better to store map of GcInfo* here since everytime
takeSnapshot() method is called it needs to get GcInfo from GcInfoIndex. 
ie, instead of key->m_className i would have to do Heap::gcInfo(key)->className.

This adds table look ups everytime dump is taken.

+        ClassTagMap classTags;
+
+        // Map from class-id (index) to count/size.
+        Vector<int> liveCount;
+        Vector<int> deadCount;
+        Vector<size_t> liveSize;
+        Vector<size_t> deadSize;
+
+        size_t getClassTag(const GCInfo*);
+    };
+
 #if ENABLE(GC_PROFILING)
     const GCInfo* findGCInfo(Address);
     static const GCInfo* findGCInfoFromAllThreads(Address);
 
     struct SnapshotInfo {
         ThreadState* state;
 
         size_t freeSize;
         size_t pageCount;
 
@@ skipping 278 matching lines @@
 };
 
 template<> class ThreadStateFor<AnyThread> {
 public:
     static ThreadState* state() { return ThreadState::current(); }
 };
 
 } // namespace blink
 
 #endif // ThreadState_h