[oilpan]: optimize the way we allocate persistent handles in wrappers.

Source/platform/heap/Handle.h

Index: Source/platform/heap/Handle.h
diff --git a/Source/platform/heap/Handle.h b/Source/platform/heap/Handle.h
index 62c7734053274e02f73723c59bfffb6f39cc9841..3cf1dca4bea5fa21d4826c6a03dd2e5b53668f63 100644
--- a/Source/platform/heap/Handle.h
+++ b/Source/platform/heap/Handle.h
@@ -150,6 +150,189 @@ private:
     friend class ThreadState;
 };
 
+
+const int wrapperPersistentsPerRegion = 256;
+const size_t wrapperPersistentOffsetMask = ~static_cast<size_t>(3);
+const size_t wrapperPersistentLiveBitMask = 1;
+
+class WrapperPersistentNode {
+    WTF_MAKE_NONCOPYABLE(WrapperPersistentNode);
+public:
+    bool isAlive() { return m_regionOffset & wrapperPersistentLiveBitMask; }
+
+    WrapperPersistentRegion* region()
+    {
+        return reinterpret_cast<WrapperPersistentRegion*>(
+            reinterpret_cast<Address>(this) - (m_regionOffset & wrapperPersistentOffsetMask));
+    }
+
+    virtual ~WrapperPersistentNode()
+    {
+        m_regionOffset &= ~wrapperPersistentLiveBitMask;
+    }
+
+    virtual void trace(Visitor* visitor) { }
+
+protected:
+    WrapperPersistentNode() : m_raw(0), m_regionOffset(0) { }
+
+    explicit WrapperPersistentNode(void* raw)
+    {
+        // When the constructor is called the slot should have been taken (takeSlot)
+        // as part of allocating the memory (via operator new). Hence the m_raw
+        // pointer should be 0.
+        ASSERT(!m_raw);
+        m_raw = raw;
+        // The m_regionOffset should always be set as an offset to the containing
+        // region. However it should not have the live bit set when the constructor
+        // is called.
+        ASSERT(m_regionOffset && !isAlive());
+        m_regionOffset |= wrapperPersistentLiveBitMask;
+    }
+
+private:
+    void initSlot(size_t regionOffset, WrapperPersistentNode* nextFree)
+    {
+        ASSERT(!m_raw);
+        ASSERT(!m_regionOffset);
+        ASSERT(!(regionOffset & ~wrapperPersistentOffsetMask));
+        m_raw = nextFree;
+        m_regionOffset = regionOffset;
+    }
+
+    WrapperPersistentNode* takeSlot()
+    {
+        // The slot should not be alive at the point where it is allocated.
+        ASSERT(!isAlive());
+        WrapperPersistentNode* nextFree = reinterpret_cast<WrapperPersistentNode*>(m_raw);
+        m_raw = 0;
+        return nextFree;
+    }
+
+    WrapperPersistentNode* freeSlot(WrapperPersistentNode* nextFree)
+    {
+        // When the slot is freed the destructor should already have cleared the live bit.
+        ASSERT(!isAlive());
+        m_raw = nextFree;
+        return this;
+    }
+
+protected:
+    // m_raw is used both to point to the object when the WrapperPersistentNode is used/alive
+    // and to point to the next free wrapperPersistentNode in the region when the node is
+    // unused/dead.
+    void* m_raw;
+
+    // The m_regionOffset field is an offset from this node to the base of the containing
+    // WrapperPersistentRegion.
+    size_t m_regionOffset;
+
+    friend class WrapperPersistentRegion;
+};
+
+template<typename T>
+class WrapperPersistent FINAL : public WrapperPersistentNode {
+public:
+    WrapperPersistent() : WrapperPersistentNode(0) { }
+    WrapperPersistent(std::nullptr_t) : WrapperPersistentNode(0) { }
+    WrapperPersistent(T* raw) : WrapperPersistentNode(raw) { }
+    WrapperPersistent(T& raw) : WrapperPersistentNode(&raw) { }
+
+    void* operator new(size_t);
+    void operator delete(void*);
+
+    virtual void trace(Visitor* visitor) { visitor->mark(static_cast<T*>(m_raw)); }
+};
+
+class WrapperPersistentRegion {
+    WTF_MAKE_NONCOPYABLE(WrapperPersistentRegion);
+public:
+    WrapperPersistentRegion()
+    {
+        WrapperPersistentNode* nextFree = 0;
+        for (int i = wrapperPersistentsPerRegion - 1; i >= 0; --i) {
+            size_t regionOffset = reinterpret_cast<Address>(&m_entries[i]) - reinterpret_cast<Address>(this);
+            // Setup the free slot with an offset to the containing region's base and a pointer to the next
+            // free slot in the region.
+            m_entries[i].initSlot(regionOffset, nextFree);
+            nextFree = &m_entries[i];
+        }
+        m_prev = 0;
+        m_next = 0;
+        m_freeHead = nextFree;
+        m_count = 0;
+    }
+
+    void* allocate()
+    {
+        if (!m_freeHead)
+            return 0;
+        // We have a free persistent slot in this region.
+        WrapperPersistentNode* freeSlot = m_freeHead;
+        // Take the slot and advance m_freeHead to the next free slot.
+        m_freeHead = freeSlot->takeSlot();
+        m_count++;
+        return reinterpret_cast<void*>(freeSlot);
+    }
+
+    void free(WrapperPersistentNode* object)
+    {
+        ASSERT(object);
+        ASSERT(!object->isAlive());
+        m_freeHead = object->freeSlot(m_freeHead);
+        m_count--;
+        if (!m_count)
+            ThreadState::current()->removeWrapperPersistentRegion(this);
+    }
+
+    bool removeIfNotLast(WrapperPersistentRegion** headPtr);
+    static void insertHead(WrapperPersistentRegion** headPtr, WrapperPersistentRegion* newHead);
+    static WrapperPersistentRegion* removeHead(WrapperPersistentRegion** headPtr);
+    static void* outOfLineAllocate(ThreadState*, WrapperPersistentRegion*);
+    static void trace(WrapperPersistentRegion* head, Visitor* visitor)
+    {
+        for (WrapperPersistentRegion* current = head; current; current = current->m_next)
+            current->traceRegion(visitor);
+    }
+
+private:
+    void traceRegion(Visitor* visitor)
+    {
+        size_t live = 0;
+        for (int i = 0; i < wrapperPersistentsPerRegion && live < m_count; ++i) {
+            if (m_entries[i].isAlive()) {
+                m_entries[i].trace(visitor);
+                live++;
+            }
+        }
+    }
+
+    WrapperPersistentRegion* m_prev;
+    WrapperPersistentRegion* m_next;
+    WrapperPersistentNode* m_freeHead;
+    size_t m_count;
+    WrapperPersistentNode m_entries[wrapperPersistentsPerRegion];
+};
+
+template<typename T>
+void* WrapperPersistent<T>::operator new(size_t size)
+{
+    ThreadState* state = ThreadState::current();
+    WrapperPersistentRegion* region = state->wrapperRoots();
+    ASSERT(region);
+    void* persistent = region->allocate();
+    if (!persistent)
+        return WrapperPersistentRegion::outOfLineAllocate(state, region);
+    return persistent;
+}
+
+template<typename T>
+void WrapperPersistent<T>::operator delete(void* object)
+{
+    WrapperPersistentNode* persistent = static_cast<WrapperPersistentNode*>(object);
+    persistent->region()->free(persistent);
+}
+
 // RootsAccessor for Persistent that provides access to thread-local list
 // of persistent handles. Can only be used to create handles that
 // are constructed and destructed on the same thread.