Use a bitmap to record PageMemoryRegion usage.

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

 // Copyright 2015 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 PageMemory_h
 #define PageMemory_h
 
 #include "platform/heap/HeapPage.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
@@ skipping 50 matching lines @@
 
     void pageDeleted(Address page)
     {
         markPageUnused(page);
         if (!--m_numPages)
             delete this;
     }
 
     void markPageUsed(Address page)
     {
-        ASSERT(!m_inUse[index(page)]);
-        m_inUse[index(page)] = true;
+        ASSERT(!(m_inUseBitmap & index(page)));
+        m_inUseBitmap |= index(page);
     }
 
     void markPageUnused(Address page)
     {
-        m_inUse[index(page)] = false;
+        m_inUseBitmap &= ~index(page);
     }
 
     static PageMemoryRegion* allocateLargePage(size_t size)
     {
         return allocate(size, 1);
     }
 
     static PageMemoryRegion* allocateNormalPages()
     {
         return allocate(blinkPageSize * blinkPagesPerRegion, blinkPagesPerRegion);
     }
 
     BasePage* pageFromAddress(Address address)
     {
         ASSERT(contains(address));
-        if (!m_inUse[index(address)])
+        if (!(m_inUseBitmap & index(address)))
             return nullptr;
         if (m_isLargePage)
             return pageFromObject(base());
         return pageFromObject(address);
     }
 
 private:
     PageMemoryRegion(Address base, size_t, unsigned numPages);
 
-    unsigned index(Address address)
+    // Returns word with the bit set which corresponds to the |address|'
+    // page within a region.
+    unsigned index(Address address) const
     {
         ASSERT(contains(address));
         if (m_isLargePage)
-            return 0;
+            return 0x1;
         size_t offset = blinkPageAddress(address) - base();
         ASSERT(offset % blinkPageSize == 0);
-        return offset / blinkPageSize;
+        return 0x1 << (offset / blinkPageSize);
     }
 
     static PageMemoryRegion* allocate(size_t, unsigned numPages);
 
     bool m_isLargePage;
-    bool m_inUse[blinkPagesPerRegion];
+    unsigned m_inUseBitmap;
     unsigned m_numPages;
 };
 
 // A RegionTree is a simple binary search tree of PageMemoryRegions sorted
 // by base addresses.
 class RegionTree {
     USING_FAST_MALLOC(RegionTree);
 public:
     explicit RegionTree(PageMemoryRegion* region)
         : m_region(region)
@@ skipping 77 matching lines @@
 private:
     PageMemory(PageMemoryRegion* reserved, const MemoryRegion& writable);
 
     PageMemoryRegion* m_reserved;
     MemoryRegion m_writable;
 };
 
 } // namespace blink
 
 #endif