Revert of Oilpan: Decommit backing storage of CallbackStacks

third_party/WebKit/Source/platform/heap/CallbackStack.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CallbackStack_h
 #define CallbackStack_h
 
 #include "platform/heap/ThreadState.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
@@ skipping 23 matching lines @@
 
     private:
         void* m_object;
         VisitorCallback m_callback;
     };
 
     CallbackStack();
     ~CallbackStack();
 
     void clear();
-    void decommit();
 
     Item* allocateEntry();
     Item* pop();
 
     bool isEmpty() const;
 
     void invokeEphemeronCallbacks(Visitor*);
 
 #if ENABLE(ASSERT)
     bool hasCallbackForObject(const void*);
 #endif
 
 private:
-    static const size_t blockSize = (1 << 13);
+    static const size_t blockSize = 8192;
 
     class Block {
         USING_FAST_MALLOC(Block);
     public:
-        explicit Block(Block* next);
-        ~Block();
+        explicit Block(Block* next)
+            : m_limit(&(m_buffer[blockSize]))
+            , m_current(&(m_buffer[0]))
+            , m_next(next)
+        {
+            clearUnused();
+        }
 
-        void decommit();
+        ~Block()
+        {
+            clearUnused();
+        }
+
+        void clear();
 
         Block* next() const { return m_next; }
         void setNext(Block* next) { m_next = next; }
 
         bool isEmptyBlock() const
         {
             return m_current == &(m_buffer[0]);
         }
 
+        size_t size() const
+        {
+            return blockSize - (m_limit - m_current);
+        }
+
         Item* allocateEntry()
         {
             if (LIKELY(m_current < m_limit))
                 return m_current++;
             return nullptr;
         }
 
         Item* pop()
         {
             if (UNLIKELY(isEmptyBlock()))
                 return nullptr;
             return --m_current;
         }
 
         void invokeEphemeronCallbacks(Visitor*);
 #if ENABLE(ASSERT)
         bool hasCallbackForObject(const void*);
 #endif
 
     private:
-        Item* m_buffer;
+        void clearUnused();
+
+        Item m_buffer[blockSize];
         Item* m_limit;
         Item* m_current;
         Block* m_next;
     };
 
     Item* popSlow();
     Item* allocateEntrySlow();
     void invokeOldestCallbacks(Block*, Block*, Visitor*);
     bool hasJustOneBlock() const;
 
@@ skipping 15 matching lines @@
     Item* item = m_first->pop();
     if (LIKELY(!!item))
         return item;
 
     return popSlow();
 }
 
 } // namespace blink
 
 #endif // CallbackStack_h