Reverting r4703.

src/spaces.h

 // Copyright 2006-2008 the V8 project authors. 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
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 27 matching lines @@
 // Heap structures:
 //
 // A JS heap consists of a young generation, an old generation, and a large
 // object space. The young generation is divided into two semispaces. A
 // scavenger implements Cheney's copying algorithm. The old generation is
 // separated into a map space and an old object space. The map space contains
 // all (and only) map objects, the rest of old objects go into the old space.
 // The old generation is collected by a mark-sweep-compact collector.
 //
 // The semispaces of the young generation are contiguous.  The old and map
-// spaces consists of a list of pages. A page has a page header and an object
-// area. A page size is deliberately chosen as 8K bytes.
-// The first word of a page is an opaque page header that has the
+// spaces consists of a list of pages. A page has a page header, a remembered
+// set area, and an object area. A page size is deliberately chosen as 8K
+// bytes. The first word of a page is an opaque page header that has the
 // address of the next page and its ownership information. The second word may
-// have the allocation top address of this page. Heap objects are aligned to the
-// pointer size.
+// have the allocation top address of this page. The next 248 bytes are
+// remembered sets. Heap objects are aligned to the pointer size (4 bytes). A
+// remembered set bit corresponds to a pointer in the object area.
 //
 // There is a separate large object space for objects larger than
 // Page::kMaxHeapObjectSize, so that they do not have to move during
-// collection. The large object space is paged. Pages in large object space
-// may be larger than 8K.
+// collection.  The large object space is paged and uses the same remembered
+// set implementation.  Pages in large object space may be larger than 8K.
 //
-// A card marking write barrier is used to keep track of intergenerational
-// references. Old space pages are divided into regions of Page::kRegionSize
-// size. Each region has a corresponding dirty bit in the page header which is
-// set if the region might contain pointers to new space. For details about
-// dirty bits encoding see comments in the Page::GetRegionNumberForAddress()
-// method body.
-//
-// During scavenges and mark-sweep collections we iterate intergenerational
-// pointers without decoding heap object maps so if the page belongs to old
-// pointer space or large object space it is essential to guarantee that
-// the page does not contain any garbage pointers to new space: every pointer
-// aligned word which satisfies the Heap::InNewSpace() predicate must be a
-// pointer to a live heap object in new space. Thus objects in old pointer
-// and large object spaces should have a special layout (e.g. no bare integer
-// fields). This requirement does not apply to map space which is iterated in
-// a special fashion. However we still require pointer fields of dead maps to
-// be cleaned.
-//
-// To enable lazy cleaning of old space pages we use a notion of allocation
-// watermark. Every pointer under watermark is considered to be well formed.
-// Page allocation watermark is not necessarily equal to page allocation top but
-// all alive objects on page should reside under allocation watermark.
-// During scavenge allocation watermark might be bumped and invalid pointers
-// might appear below it. To avoid following them we store a valid watermark
-// into special field in the page header and set a page WATERMARK_INVALIDATED
-// flag. For details see comments in the Page::SetAllocationWatermark() method
-// body.
-//
+// NOTE: The mark-compact collector rebuilds the remembered set after a
+// collection. It reuses first a few words of the remembered set for
+// bookkeeping relocation information.
+
 
 // Some assertion macros used in the debugging mode.
 
 #define ASSERT_PAGE_ALIGNED(address)                                           \
   ASSERT((OffsetFrom(address) & Page::kPageAlignmentMask) == 0)
 
 #define ASSERT_OBJECT_ALIGNED(address)                                         \
   ASSERT((OffsetFrom(address) & kObjectAlignmentMask) == 0)
 
 #define ASSERT_MAP_ALIGNED(address)                                            \
   ASSERT((OffsetFrom(address) & kMapAlignmentMask) == 0)
 
 #define ASSERT_OBJECT_SIZE(size)                                               \
   ASSERT((0 < size) && (size <= Page::kMaxHeapObjectSize))
 
 #define ASSERT_PAGE_OFFSET(offset)                                             \
   ASSERT((Page::kObjectStartOffset <= offset)                                  \
       && (offset <= Page::kPageSize))
 
 #define ASSERT_MAP_PAGE_INDEX(index)                                           \
   ASSERT((0 <= index) && (index <= MapSpace::kMaxMapPageIndex))
 
 
 class PagedSpace;
 class MemoryAllocator;
 class AllocationInfo;
 
 // -----------------------------------------------------------------------------
 // A page normally has 8K bytes. Large object pages may be larger.  A page
-// address is always aligned to the 8K page size.
+// address is always aligned to the 8K page size.  A page is divided into
+// three areas: the first two words are used for bookkeeping, the next 248
+// bytes are used as remembered set, and the rest of the page is the object
+// area.
 //
-// Each page starts with a header of Page::kPageHeaderSize size which contains
-// bookkeeping data.
+// Pointers are aligned to the pointer size (4), only 1 bit is needed
+// for a pointer in the remembered set. Given an address, its remembered set
+// bit position (offset from the start of the page) is calculated by dividing
+// its page offset by 32. Therefore, the object area in a page starts at the
+// 256th byte (8K/32). Bytes 0 to 255 do not need the remembered set, so that
+// the first two words (64 bits) in a page can be used for other purposes.
+//
+// On the 64-bit platform, we add an offset to the start of the remembered set,
+// and pointers are aligned to 8-byte pointer size. This means that we need
+// only 128 bytes for the RSet, and only get two bytes free in the RSet's RSet.
+// For this reason we add an offset to get room for the Page data at the start.
 //
 // The mark-compact collector transforms a map pointer into a page index and a
-// page offset. The exact encoding is described in the comments for
+// page offset. The excact encoding is described in the comments for
 // class MapWord in objects.h.
 //
 // The only way to get a page pointer is by calling factory methods:
 //   Page* p = Page::FromAddress(addr); or
 //   Page* p = Page::FromAllocationTop(top);
 class Page {
  public:
   // Returns the page containing a given address. The address ranges
   // from [page_addr .. page_addr + kPageSize[
   //
@@ skipping 20 matching lines @@
 
   // Checks whether this is a valid page address.
   bool is_valid() { return address() != NULL; }
 
   // Returns the next page of this page.
   inline Page* next_page();
 
   // Return the end of allocation in this page. Undefined for unused pages.
   inline Address AllocationTop();
 
-  // Return the allocation watermark for the page.
-  // For old space pages it is guaranteed that the area under the watermark
-  // does not contain any garbage pointers to new space.
-  inline Address AllocationWatermark();
-
-  // Return the allocation watermark offset from the beginning of the page.
-  inline uint32_t AllocationWatermarkOffset();
-
-  inline void SetAllocationWatermark(Address allocation_watermark);
-
-  inline void SetCachedAllocationWatermark(Address allocation_watermark);
-  inline Address CachedAllocationWatermark();
-
   // Returns the start address of the object area in this page.
   Address ObjectAreaStart() { return address() + kObjectStartOffset; }
 
   // Returns the end address (exclusive) of the object area in this page.
   Address ObjectAreaEnd() { return address() + Page::kPageSize; }
 
+  // Returns the start address of the remembered set area.
+  Address RSetStart() { return address() + kRSetStartOffset; }
+
+  // Returns the end address of the remembered set area (exclusive).
+  Address RSetEnd() { return address() + kRSetEndOffset; }
+
   // Checks whether an address is page aligned.
   static bool IsAlignedToPageSize(Address a) {
     return 0 == (OffsetFrom(a) & kPageAlignmentMask);
   }
 
   // True if this page was in use before current compaction started.
   // Result is valid only for pages owned by paged spaces and
   // only after PagedSpace::PrepareForMarkCompact was called.
   inline bool WasInUseBeforeMC();
 
@@ skipping 11 matching lines @@
     return offset;
   }
 
   // Returns the address for a given offset to the this page.
   Address OffsetToAddress(int offset) {
     ASSERT_PAGE_OFFSET(offset);
     return address() + offset;
   }
 
   // ---------------------------------------------------------------------
-  // Card marking support
+  // Remembered set support
 
-  static const uint32_t kAllRegionsCleanMarks = 0x0;
-  static const uint32_t kAllRegionsDirtyMarks = 0xFFFFFFFF;
+  // Clears remembered set in this page.
+  inline void ClearRSet();
 
-  inline uint32_t GetRegionMarks();
-  inline void SetRegionMarks(uint32_t dirty);
+  // Return the address of the remembered set word corresponding to an
+  // object address/offset pair, and the bit encoded as a single-bit
+  // mask in the output parameter 'bitmask'.
+  INLINE(static Address ComputeRSetBitPosition(Address address, int offset,
+                                               uint32_t* bitmask));
 
-  inline uint32_t GetRegionMaskForAddress(Address addr);
-  inline int GetRegionNumberForAddress(Address addr);
+  // Sets the corresponding remembered set bit for a given address.
+  INLINE(static void SetRSet(Address address, int offset));
 
-  inline void MarkRegionDirty(Address addr);
-  inline bool IsRegionDirty(Address addr);
+  // Clears the corresponding remembered set bit for a given address.
+  static inline void UnsetRSet(Address address, int offset);
 
-  inline void ClearRegionMarks(Address start,
-                               Address end,
-                               bool reaches_limit);
+  // Checks whether the remembered set bit for a given address is set.
+  static inline bool IsRSetSet(Address address, int offset);
+
+#ifdef DEBUG
+  // Use a state to mark whether remembered set space can be used for other
+  // purposes.
+  enum RSetState { IN_USE,  NOT_IN_USE };
+  static bool is_rset_in_use() { return rset_state_ == IN_USE; }
+  static void set_rset_state(RSetState state) { rset_state_ = state; }
+#endif
 
   // Page size in bytes.  This must be a multiple of the OS page size.
   static const int kPageSize = 1 << kPageSizeBits;
 
   // Page size mask.
   static const intptr_t kPageAlignmentMask = (1 << kPageSizeBits) - 1;
 
-  static const int kPageHeaderSize = kPointerSize + kPointerSize + kIntSize +
-    kIntSize + kPointerSize;
+  // The offset of the remembered set in a page, in addition to the empty bytes
+  // formed as the remembered bits of the remembered set itself.
+#ifdef V8_TARGET_ARCH_X64
+  static const int kRSetOffset = 4 * kPointerSize;  // Room for four pointers.
+#else
+  static const int kRSetOffset = 0;
+#endif
+  // The end offset of the remembered set in a page
+  // (heaps are aligned to pointer size).
+  static const int kRSetEndOffset = kRSetOffset + kPageSize / kBitsPerPointer;
 
   // The start offset of the object area in a page.
-  static const int kObjectStartOffset = MAP_POINTER_ALIGN(kPageHeaderSize);
+  // This needs to be at least (bits per uint32_t) * kBitsPerPointer,
+  // to align start of rset to a uint32_t address.
+  static const int kObjectStartOffset = 256;
+
+  // The start offset of the used part of the remembered set in a page.
+  static const int kRSetStartOffset = kRSetOffset +
+      kObjectStartOffset / kBitsPerPointer;
 
   // Object area size in bytes.
   static const int kObjectAreaSize = kPageSize - kObjectStartOffset;
 
   // Maximum object size that fits in a page.
   static const int kMaxHeapObjectSize = kObjectAreaSize;
 
-  static const int kDirtyFlagOffset = 2 * kPointerSize;
-  static const int kRegionSizeLog2 = 8;
-  static const int kRegionSize = 1 << kRegionSizeLog2;
-  static const intptr_t kRegionAlignmentMask = (kRegionSize - 1);
-
-  STATIC_CHECK(kRegionSize == kPageSize / kBitsPerInt);
-
   enum PageFlag {
     IS_NORMAL_PAGE = 1 << 0,
-    WAS_IN_USE_BEFORE_MC = 1 << 1,
-
-    // Page allocation watermark was bumped by preallocation during scavenge.
-    // Correct watermark can be retrieved by CachedAllocationWatermark() method
-    WATERMARK_INVALIDATED = 1 << 2
+    WAS_IN_USE_BEFORE_MC = 1 << 1
   };
 
-  // To avoid an additional WATERMARK_INVALIDATED flag clearing pass during
-  // scavenge we just invalidate the watermark on each old space page after
-  // processing it. And then we flip the meaning of the WATERMARK_INVALIDATED
-  // flag at the beginning of the next scavenge and each page becomes marked as
-  // having a valid watermark.
-  //
-  // The following invariant must hold for pages in old pointer and map spaces:
-  //     If page is in use then page is marked as having invalid watermark at
-  //     the beginning and at the end of any GC.
-  //
-  // This invariant guarantees that after flipping flag meaning at the
-  // beginning of scavenge all pages in use will be marked as having valid
-  // watermark.
-  static inline void FlipMeaningOfInvalidatedWatermarkFlag();
-
-  // Returns true if the page allocation watermark was not altered during
-  // scavenge.
-  inline bool IsWatermarkValid();
-
-  inline void InvalidateWatermark(bool value);
-
   inline bool GetPageFlag(PageFlag flag);
   inline void SetPageFlag(PageFlag flag, bool value);
-  inline void ClearPageFlags();
-
-  static const int kAllocationWatermarkOffsetShift = 3;
-  static const int kAllocationWatermarkOffsetBits  = kPageSizeBits + 1;
-  static const uint32_t kAllocationWatermarkOffsetMask =
-      ((1 << kAllocationWatermarkOffsetBits) - 1) <<
-      kAllocationWatermarkOffsetShift;
-
-  static const uint32_t kFlagsMask =
-    ((1 << kAllocationWatermarkOffsetShift) - 1);
-
-  STATIC_CHECK(kBitsPerInt - kAllocationWatermarkOffsetShift >=
-               kAllocationWatermarkOffsetBits);
-
-  // This field contains the meaning of the WATERMARK_INVALIDATED flag.
-  // Instead of clearing this flag from all pages we just flip
-  // its meaning at the beginning of a scavenge.
-  static intptr_t watermark_invalidated_mark_;
 
   //---------------------------------------------------------------------------
   // Page header description.
   //
   // If a page is not in the large object space, the first word,
   // opaque_header, encodes the next page address (aligned to kPageSize 8K)
   // and the chunk number (0 ~ 8K-1).  Only MemoryAllocator should use
   // opaque_header. The value range of the opaque_header is [0..kPageSize[,
   // or [next_page_start, next_page_end[. It cannot point to a valid address
   // in the current page.  If a page is in the large object space, the first
   // word *may* (if the page start and large object chunk start are the
   // same) contain the address of the next large object chunk.
   intptr_t opaque_header;
 
   // If the page is not in the large object space, the low-order bit of the
   // second word is set. If the page is in the large object space, the
   // second word *may* (if the page start and large object chunk start are
   // the same) contain the large object chunk size.  In either case, the
   // low-order bit for large object pages will be cleared.
-  // For normal pages this word is used to store page flags and
-  // offset of allocation top.
-  intptr_t flags_;
+  // For normal pages this word is used to store various page flags.
+  int flags;
 
-  // This field contains dirty marks for regions covering the page. Only dirty
-  // regions might contain intergenerational references.
-  // Only 32 dirty marks are supported so for large object pages several regions
-  // might be mapped to a single dirty mark.
-  uint32_t dirty_regions_;
+  // The following fields may overlap with remembered set, they can only
+  // be used in the mark-compact collector when remembered set is not
+  // used.
 
   // The index of the page in its owner space.
   int mc_page_index;
 
-  // During mark-compact collections this field contains the forwarding address
-  // of the first live object in this page.
-  // During scavenge collection this field is used to store allocation watermark
-  // if it is altered during scavenge.
+  // The allocation pointer after relocating objects to this page.
+  Address mc_relocation_top;
+
+  // The forwarding address of the first live object in this page.
   Address mc_first_forwarded;
+
+#ifdef DEBUG
+ private:
+  static RSetState rset_state_;  // state of the remembered set
+#endif
 };
 
 
 // ----------------------------------------------------------------------------
 // Space is the abstract superclass for all allocation spaces.
 class Space : public Malloced {
  public:
   Space(AllocationSpace id, Executability executable)
       : id_(id), executable_(executable) {}
 
@@ skipping 602 matching lines @@
 
   // Given an address occupied by a live object, return that object if it is
   // in this space, or Failure::Exception() if it is not. The implementation
   // iterates over objects in the page containing the address, the cost is
   // linear in the number of objects in the page. It may be slow.
   Object* FindObject(Address addr);
 
   // Checks whether page is currently in use by this space.
   bool IsUsed(Page* page);
 
-  void MarkAllPagesClean();
+  // Clears remembered sets of pages in this space.
+  void ClearRSet();
 
   // Prepares for a mark-compact GC.
   virtual void PrepareForMarkCompact(bool will_compact);
 
   // The top of allocation in a page in this space. Undefined if page is unused.
   Address PageAllocationTop(Page* page) {
     return page == TopPageOf(allocation_info_) ? top()
         : PageAllocationLimit(page);
   }
 
   // The limit of allocation for a page in this space.
   virtual Address PageAllocationLimit(Page* page) = 0;
 
-  void FlushTopPageWatermark() {
-    AllocationTopPage()->SetCachedAllocationWatermark(top());
-    AllocationTopPage()->InvalidateWatermark(true);
-  }
-
   // Current capacity without growing (Size() + Available() + Waste()).
   int Capacity() { return accounting_stats_.Capacity(); }
 
   // Total amount of memory committed for this space.  For paged
   // spaces this equals the capacity.
   int CommittedMemory() { return Capacity(); }
 
   // Available bytes without growing.
   int Available() { return accounting_stats_.Available(); }
 
@@ skipping 34 matching lines @@
   }
 
   // ---------------------------------------------------------------------------
   // Mark-compact collection support functions
 
   // Set the relocation point to the beginning of the space.
   void MCResetRelocationInfo();
 
   // Writes relocation info to the top page.
   void MCWriteRelocationInfoToPage() {
-    TopPageOf(mc_forwarding_info_)->
-        SetAllocationWatermark(mc_forwarding_info_.top);
+    TopPageOf(mc_forwarding_info_)->mc_relocation_top = mc_forwarding_info_.top;
   }
 
   // Computes the offset of a given address in this space to the beginning
   // of the space.
   int MCSpaceOffsetForAddress(Address addr);
 
   // Updates the allocation pointer to the relocation top after a mark-compact
   // collection.
   virtual void MCCommitRelocationInfo() = 0;
 
@@ skipping 97 matching lines @@
 
   // Slow path of AllocateRaw.  This function is space-dependent.
   virtual HeapObject* SlowAllocateRaw(int size_in_bytes) = 0;
 
   // Slow path of MCAllocateRaw.
   HeapObject* SlowMCAllocateRaw(int size_in_bytes);
 
 #ifdef DEBUG
   // Returns the number of total pages in this space.
   int CountTotalPages();
+
+  void DoPrintRSet(const char* space_name);
 #endif
  private:
 
   // Returns a pointer to the page of the relocation pointer.
   Page* MCRelocationTopPage() { return TopPageOf(mc_forwarding_info_); }
 
   friend class PageIterator;
 };
 
 
@@ skipping 632 matching lines @@
 
   // Updates the allocation pointer to the relocation top after a mark-compact
   // collection.
   virtual void MCCommitRelocationInfo();
 
   virtual void PutRestOfCurrentPageOnFreeList(Page* current_page);
 
 #ifdef DEBUG
   // Reports statistics for the space
   void ReportStatistics();
+  // Dump the remembered sets in the space to stdout.
+  void PrintRSet();
 #endif
 
  protected:
   // Virtual function in the superclass.  Slow path of AllocateRaw.
   HeapObject* SlowAllocateRaw(int size_in_bytes);
 
   // Virtual function in the superclass.  Allocate linearly at the start of
   // the page after current_page (there is assumed to be one).
   HeapObject* AllocateInNextPage(Page* current_page, int size_in_bytes);
 
@@ skipping 44 matching lines @@
 
   // Updates the allocation pointer to the relocation top after a mark-compact
   // collection.
   virtual void MCCommitRelocationInfo();
 
   virtual void PutRestOfCurrentPageOnFreeList(Page* current_page);
 
 #ifdef DEBUG
   // Reports statistic info of the space
   void ReportStatistics();
+
+  // Dump the remembered sets in the space to stdout.
+  void PrintRSet();
 #endif
 
  protected:
   // Virtual function in the superclass.  Slow path of AllocateRaw.
   HeapObject* SlowAllocateRaw(int size_in_bytes);
 
   // Virtual function in the superclass.  Allocate linearly at the start of
   // the page after current_page (there is assumed to be one).
   HeapObject* AllocateInNextPage(Page* current_page, int size_in_bytes);
 
@@ skipping 48 matching lines @@
     return !MapPointersEncodable() && live_maps <= CompactionThreshold();
   }
 
   Address TopAfterCompaction(int live_maps) {
     ASSERT(NeedsCompaction(live_maps));
 
     int pages_left = live_maps / kMapsPerPage;
     PageIterator it(this, PageIterator::ALL_PAGES);
     while (pages_left-- > 0) {
       ASSERT(it.has_next());
-      it.next()->SetRegionMarks(Page::kAllRegionsCleanMarks);
+      it.next()->ClearRSet();
     }
     ASSERT(it.has_next());
     Page* top_page = it.next();
-    top_page->SetRegionMarks(Page::kAllRegionsCleanMarks);
+    top_page->ClearRSet();
     ASSERT(top_page->is_valid());
 
     int offset = live_maps % kMapsPerPage * Map::kSize;
     Address top = top_page->ObjectAreaStart() + offset;
     ASSERT(top < top_page->ObjectAreaEnd());
     ASSERT(Contains(top));
 
     return top;
   }
 
@@ skipping 70 matching lines @@
 // extra padding bytes (Page::kPageSize + Page::kObjectStartOffset).
 // A large object always starts at Page::kObjectStartOffset to a page.
 // Large objects do not move during garbage collections.
 
 // A LargeObjectChunk holds exactly one large object page with exactly one
 // large object.
 class LargeObjectChunk {
  public:
   // Allocates a new LargeObjectChunk that contains a large object page
   // (Page::kPageSize aligned) that has at least size_in_bytes (for a large
-  // object) bytes after the object area start of that page.
-  // The allocated chunk size is set in the output parameter chunk_size.
+  // object and possibly extra remembered set words) bytes after the object
+  // area start of that page. The allocated chunk size is set in the output
+  // parameter chunk_size.
   static LargeObjectChunk* New(int size_in_bytes,
                                size_t* chunk_size,
                                Executability executable);
 
   // Interpret a raw address as a large object chunk.
   static LargeObjectChunk* FromAddress(Address address) {
     return reinterpret_cast<LargeObjectChunk*>(address);
   }
 
   // Returns the address of this chunk.
   Address address() { return reinterpret_cast<Address>(this); }
 
   // Accessors for the fields of the chunk.
   LargeObjectChunk* next() { return next_; }
   void set_next(LargeObjectChunk* chunk) { next_ = chunk; }
 
   size_t size() { return size_; }
   void set_size(size_t size_in_bytes) { size_ = size_in_bytes; }
 
   // Returns the object in this chunk.
   inline HeapObject* GetObject();
 
-  // Given a requested size returns the physical size of a chunk to be
-  // allocated.
+  // Given a requested size (including any extra remembered set words),
+  // returns the physical size of a chunk to be allocated.
   static int ChunkSizeFor(int size_in_bytes);
 
-  // Given a chunk size, returns the object size it can accommodate.  Used by
-  // LargeObjectSpace::Available.
+  // Given a chunk size, returns the object size it can accommodate (not
+  // including any extra remembered set words).  Used by
+  // LargeObjectSpace::Available.  Note that this can overestimate the size
+  // of object that will fit in a chunk---if the object requires extra
+  // remembered set words (eg, for large fixed arrays), the actual object
+  // size for the chunk will be smaller than reported by this function.
   static int ObjectSizeFor(int chunk_size) {
     if (chunk_size <= (Page::kPageSize + Page::kObjectStartOffset)) return 0;
     return chunk_size - Page::kPageSize - Page::kObjectStartOffset;
   }
 
  private:
   // A pointer to the next large object chunk in the space or NULL.
   LargeObjectChunk* next_;
 
   // The size of this chunk.
@@ skipping 15 matching lines @@
   // Releases internal resources, frees objects in this space.
   void TearDown();
 
   // Allocates a (non-FixedArray, non-Code) large object.
   Object* AllocateRaw(int size_in_bytes);
   // Allocates a large Code object.
   Object* AllocateRawCode(int size_in_bytes);
   // Allocates a large FixedArray.
   Object* AllocateRawFixedArray(int size_in_bytes);
 
-  // Available bytes for objects in this space.
+  // Available bytes for objects in this space, not including any extra
+  // remembered set words.
   int Available() {
     return LargeObjectChunk::ObjectSizeFor(MemoryAllocator::Available());
   }
 
   virtual int Size() {
     return size_;
   }
 
   int PageCount() {
     return page_count_;
   }
 
   // Finds an object for a given address, returns Failure::Exception()
   // if it is not found. The function iterates through all objects in this
   // space, may be slow.
   Object* FindObject(Address a);
 
-  // Iterates objects covered by dirty regions.
-  void IterateDirtyRegions(ObjectSlotCallback func);
+  // Clears remembered sets.
+  void ClearRSet();
+
+  // Iterates objects whose remembered set bits are set.
+  void IterateRSet(ObjectSlotCallback func);
 
   // Frees unmarked objects.
   void FreeUnmarkedObjects();
 
   // Checks whether a heap object is in this space; O(1).
   bool Contains(HeapObject* obj);
 
   // Checks whether the space is empty.
   bool IsEmpty() { return first_chunk_ == NULL; }
 
   // See the comments for ReserveSpace in the Space class.  This has to be
   // called after ReserveSpace has been called on the paged spaces, since they
   // may use some memory, leaving less for large objects.
   virtual bool ReserveSpace(int bytes);
 
 #ifdef ENABLE_HEAP_PROTECTION
   // Protect/unprotect the space by marking it read-only/writable.
   void Protect();
   void Unprotect();
 #endif
 
 #ifdef DEBUG
   virtual void Verify();
   virtual void Print();
   void ReportStatistics();
   void CollectCodeStatistics();
+  // Dump the remembered sets in the space to stdout.
+  void PrintRSet();
 #endif
   // Checks whether an address is in the object area in this space.  It
   // iterates all objects in the space. May be slow.
   bool SlowContains(Address addr) { return !FindObject(addr)->IsFailure(); }
 
  private:
   // The head of the linked list of large object chunks.
   LargeObjectChunk* first_chunk_;
   int size_;  // allocated bytes
   int page_count_;  // number of chunks
 
 
   // Shared implementation of AllocateRaw, AllocateRawCode and
   // AllocateRawFixedArray.
   Object* AllocateRawInternal(int requested_size,
                               int object_size,
                               Executability executable);
 
+  // Returns the number of extra bytes (rounded up to the nearest full word)
+  // required for extra_object_bytes of extra pointers (in bytes).
+  static inline int ExtraRSetBytesFor(int extra_object_bytes);
+
   friend class LargeObjectIterator;
 
  public:
   TRACK_MEMORY("LargeObjectSpace")
 };
 
 
 class LargeObjectIterator: public ObjectIterator {
  public:
   explicit LargeObjectIterator(LargeObjectSpace* space);
   LargeObjectIterator(LargeObjectSpace* space, HeapObjectCallback size_func);
 
   HeapObject* next();
 
   // implementation of ObjectIterator.
   virtual HeapObject* next_object() { return next(); }
 
  private:
   LargeObjectChunk* current_;
   HeapObjectCallback size_func_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_