Revert r4715.

src/spaces.cc

Index: src/spaces.cc
===================================================================
--- src/spaces.cc	(revision 4722)
+++ src/spaces.cc	(working copy)
@@ -41,7 +41,6 @@
          && (info).top <= (space).high()              \
          && (info).limit == (space).high())
 
-intptr_t Page::watermark_invalidated_mark_ = Page::WATERMARK_INVALIDATED;
 
 // ----------------------------------------------------------------------------
 // HeapObjectIterator
@@ -140,6 +139,13 @@
 
 
 // -----------------------------------------------------------------------------
+// Page
+
+#ifdef DEBUG
+Page::RSetState Page::rset_state_ = Page::IN_USE;
+#endif
+
+// -----------------------------------------------------------------------------
 // CodeRange
 
 List<CodeRange::FreeBlock> CodeRange::free_list_(0);
@@ -518,10 +524,7 @@
   for (int i = 0; i < pages_in_chunk; i++) {
     Page* p = Page::FromAddress(page_addr);
     p->opaque_header = OffsetFrom(page_addr + Page::kPageSize) | chunk_id;
-    p->InvalidateWatermark(true);
     p->SetIsLargeObjectPage(false);
-    p->SetAllocationWatermark(p->ObjectAreaStart());
-    p->SetCachedAllocationWatermark(p->ObjectAreaStart());
     page_addr += Page::kPageSize;
   }
 
@@ -678,7 +681,6 @@
     p->opaque_header = OffsetFrom(page_addr + Page::kPageSize) | chunk_id;
     page_addr += Page::kPageSize;
 
-    p->InvalidateWatermark(true);
     if (p->WasInUseBeforeMC()) {
       *last_page_in_use = p;
     }
@@ -742,10 +744,10 @@
   accounting_stats_.ExpandSpace(num_pages * Page::kObjectAreaSize);
   ASSERT(Capacity() <= max_capacity_);
 
-  // Sequentially clear region marks in the newly allocated
+  // Sequentially initialize remembered sets in the newly allocated
   // pages and cache the current last page in the space.
   for (Page* p = first_page_; p->is_valid(); p = p->next_page()) {
-    p->SetRegionMarks(Page::kAllRegionsCleanMarks);
+    p->ClearRSet();
     last_page_ = p;
   }
 
@@ -792,10 +794,10 @@
 #endif
 
 
-void PagedSpace::MarkAllPagesClean() {
+void PagedSpace::ClearRSet() {
   PageIterator it(this, PageIterator::ALL_PAGES);
   while (it.has_next()) {
-    it.next()->SetRegionMarks(Page::kAllRegionsCleanMarks);
+    it.next()->ClearRSet();
   }
 }
 
@@ -898,8 +900,7 @@
   // of forwarding addresses is as an offset in terms of live bytes, so we
   // need quick access to the allocation top of each page to decode
   // forwarding addresses.
-  current_page->SetAllocationWatermark(mc_forwarding_info_.top);
-  current_page->next_page()->InvalidateWatermark(true);
+  current_page->mc_relocation_top = mc_forwarding_info_.top;
   SetAllocationInfo(&mc_forwarding_info_, current_page->next_page());
   return AllocateLinearly(&mc_forwarding_info_, size_in_bytes);
 }
@@ -927,10 +928,10 @@
 
   MemoryAllocator::SetNextPage(last_page, p);
 
-  // Sequentially clear region marks of new pages and and cache the
+  // Sequentially clear remembered set of new pages and and cache the
   // new last page in the space.
   while (p->is_valid()) {
-    p->SetRegionMarks(Page::kAllRegionsCleanMarks);
+    p->ClearRSet();
     last_page_ = p;
     p = p->next_page();
   }
@@ -1029,11 +1030,16 @@
     if (above_allocation_top) {
       // We don't care what's above the allocation top.
     } else {
+      // Unless this is the last page in the space containing allocated
+      // objects, the allocation top should be at a constant offset from the
+      // object area end.
       Address top = current_page->AllocationTop();
       if (current_page == top_page) {
         ASSERT(top == allocation_info_.top);
         // The next page will be above the allocation top.
         above_allocation_top = true;
+      } else {
+        ASSERT(top == PageAllocationLimit(current_page));
       }
 
       // It should be packed with objects from the bottom to the top.
@@ -1054,8 +1060,8 @@
         object->Verify();
 
         // All the interior pointers should be contained in the heap and
-        // have page regions covering intergenerational references should be
-        // marked dirty.
+        // have their remembered set bits set if required as determined
+        // by the visitor.
         int size = object->Size();
         object->IterateBody(map->instance_type(), size, visitor);
 
@@ -1114,7 +1120,7 @@
 
   start_ = start;
   address_mask_ = ~(size - 1);
-  object_mask_ = address_mask_ | kHeapObjectTagMask;
+  object_mask_ = address_mask_ | kHeapObjectTag;
   object_expected_ = reinterpret_cast<uintptr_t>(start) | kHeapObjectTag;
 
   allocation_info_.top = to_space_.low();
@@ -1318,7 +1324,7 @@
 
   start_ = start;
   address_mask_ = ~(maximum_capacity - 1);
-  object_mask_ = address_mask_ | kHeapObjectTagMask;
+  object_mask_ = address_mask_ | kHeapObjectTag;
   object_expected_ = reinterpret_cast<uintptr_t>(start) | kHeapObjectTag;
   age_mark_ = start_;
 
@@ -1628,7 +1634,7 @@
   // If the block is too small (eg, one or two words), to hold both a size
   // field and a next pointer, we give it a filler map that gives it the
   // correct size.
-  if (size_in_bytes > ByteArray::kHeaderSize) {
+  if (size_in_bytes > ByteArray::kAlignedSize) {
     set_map(Heap::raw_unchecked_byte_array_map());
     // Can't use ByteArray::cast because it fails during deserialization.
     ByteArray* this_as_byte_array = reinterpret_cast<ByteArray*>(this);
@@ -1901,14 +1907,15 @@
     Page* p = it.next();
     // Space below the relocation pointer is allocated.
     computed_size +=
-        static_cast<int>(p->AllocationWatermark() - p->ObjectAreaStart());
+        static_cast<int>(p->mc_relocation_top - p->ObjectAreaStart());
     if (it.has_next()) {
-      // Free the space at the top of the page.
+      // Free the space at the top of the page.  We cannot use
+      // p->mc_relocation_top after the call to Free (because Free will clear
+      // remembered set bits).
       int extra_size =
-          static_cast<int>(p->ObjectAreaEnd() - p->AllocationWatermark());
+          static_cast<int>(p->ObjectAreaEnd() - p->mc_relocation_top);
       if (extra_size > 0) {
-        int wasted_bytes = free_list_.Free(p->AllocationWatermark(),
-                                           extra_size);
+        int wasted_bytes = free_list_.Free(p->mc_relocation_top, extra_size);
         // The bytes we have just "freed" to add to the free list were
         // already accounted as available.
         accounting_stats_.WasteBytes(wasted_bytes);
@@ -1956,10 +1963,7 @@
 
   // Clean them up.
   do {
-    first->InvalidateWatermark(true);
-    first->SetAllocationWatermark(first->ObjectAreaStart());
-    first->SetCachedAllocationWatermark(first->ObjectAreaStart());
-    first->SetRegionMarks(Page::kAllRegionsCleanMarks);
+    first->ClearRSet();
     first = first->next_page();
   } while (first != NULL);
 
@@ -1999,7 +2003,6 @@
         // Current allocation top points to a page which is now in the middle
         // of page list. We should move allocation top forward to the new last
         // used page so various object iterators will continue to work properly.
-        last_in_use->SetAllocationWatermark(last_in_use->AllocationTop());
 
         int size_in_bytes = static_cast<int>(PageAllocationLimit(last_in_use) -
                                              last_in_use->AllocationTop());
@@ -2032,7 +2035,6 @@
           int size_in_bytes = static_cast<int>(PageAllocationLimit(p) -
                                                p->ObjectAreaStart());
 
-          p->SetAllocationWatermark(p->ObjectAreaStart());
           Heap::CreateFillerObjectAt(p->ObjectAreaStart(), size_in_bytes);
         }
       }
@@ -2064,7 +2066,6 @@
     if (!reserved_page->is_valid()) return false;
   }
   ASSERT(TopPageOf(allocation_info_)->next_page()->is_valid());
-  TopPageOf(allocation_info_)->next_page()->InvalidateWatermark(true);
   SetAllocationInfo(&allocation_info_,
                     TopPageOf(allocation_info_)->next_page());
   return true;
@@ -2099,15 +2100,7 @@
     accounting_stats_.WasteBytes(wasted_bytes);
     if (!result->IsFailure()) {
       accounting_stats_.AllocateBytes(size_in_bytes);
-
-      HeapObject* obj = HeapObject::cast(result);
-      Page* p = Page::FromAddress(obj->address());
-
-      if (obj->address() >= p->AllocationWatermark()) {
-        p->SetAllocationWatermark(obj->address() + size_in_bytes);
-      }
-
-      return obj;
+      return HeapObject::cast(result);
     }
   }
 
@@ -2130,7 +2123,6 @@
 
 
 void OldSpace::PutRestOfCurrentPageOnFreeList(Page* current_page) {
-  current_page->SetAllocationWatermark(allocation_info_.top);
   int free_size =
       static_cast<int>(current_page->ObjectAreaEnd() - allocation_info_.top);
   if (free_size > 0) {
@@ -2141,7 +2133,6 @@
 
 
 void FixedSpace::PutRestOfCurrentPageOnFreeList(Page* current_page) {
-  current_page->SetAllocationWatermark(allocation_info_.top);
   int free_size =
       static_cast<int>(current_page->ObjectAreaEnd() - allocation_info_.top);
   // In the fixed space free list all the free list items have the right size.
@@ -2161,7 +2152,6 @@
 HeapObject* OldSpace::AllocateInNextPage(Page* current_page,
                                          int size_in_bytes) {
   ASSERT(current_page->next_page()->is_valid());
-  current_page->next_page()->InvalidateWatermark(true);
   PutRestOfCurrentPageOnFreeList(current_page);
   SetAllocationInfo(&allocation_info_, current_page->next_page());
   return AllocateLinearly(&allocation_info_, size_in_bytes);
@@ -2306,12 +2296,160 @@
   PrintF("  capacity: %d, waste: %d, available: %d, %%%d\n",
          Capacity(), Waste(), Available(), pct);
 
+  // Report remembered set statistics.
+  int rset_marked_pointers = 0;
+  int rset_marked_arrays = 0;
+  int rset_marked_array_elements = 0;
+  int cross_gen_pointers = 0;
+  int cross_gen_array_elements = 0;
+
+  PageIterator page_it(this, PageIterator::PAGES_IN_USE);
+  while (page_it.has_next()) {
+    Page* p = page_it.next();
+
+    for (Address rset_addr = p->RSetStart();
+         rset_addr < p->RSetEnd();
+         rset_addr += kIntSize) {
+      int rset = Memory::int_at(rset_addr);
+      if (rset != 0) {
+        // Bits were set
+        int intoff =
+            static_cast<int>(rset_addr - p->address() - Page::kRSetOffset);
+        int bitoff = 0;
+        for (; bitoff < kBitsPerInt; ++bitoff) {
+          if ((rset & (1 << bitoff)) != 0) {
+            int bitpos = intoff*kBitsPerByte + bitoff;
+            Address slot = p->OffsetToAddress(bitpos << kObjectAlignmentBits);
+            Object** obj = reinterpret_cast<Object**>(slot);
+            if (*obj == Heap::raw_unchecked_fixed_array_map()) {
+              rset_marked_arrays++;
+              FixedArray* fa = FixedArray::cast(HeapObject::FromAddress(slot));
+
+              rset_marked_array_elements += fa->length();
+              // Manually inline FixedArray::IterateBody
+              Address elm_start = slot + FixedArray::kHeaderSize;
+              Address elm_stop = elm_start + fa->length() * kPointerSize;
+              for (Address elm_addr = elm_start;
+                   elm_addr < elm_stop; elm_addr += kPointerSize) {
+                // Filter non-heap-object pointers
+                Object** elm_p = reinterpret_cast<Object**>(elm_addr);
+                if (Heap::InNewSpace(*elm_p))
+                  cross_gen_array_elements++;
+              }
+            } else {
+              rset_marked_pointers++;
+              if (Heap::InNewSpace(*obj))
+                cross_gen_pointers++;
+            }
+          }
+        }
+      }
+    }
+  }
+
+  pct = rset_marked_pointers == 0 ?
+        0 : cross_gen_pointers * 100 / rset_marked_pointers;
+  PrintF("  rset-marked pointers %d, to-new-space %d (%%%d)\n",
+            rset_marked_pointers, cross_gen_pointers, pct);
+  PrintF("  rset_marked arrays %d, ", rset_marked_arrays);
+  PrintF("  elements %d, ", rset_marked_array_elements);
+  pct = rset_marked_array_elements == 0 ? 0
+           : cross_gen_array_elements * 100 / rset_marked_array_elements;
+  PrintF("  pointers to new space %d (%%%d)\n", cross_gen_array_elements, pct);
+  PrintF("  total rset-marked bits %d\n",
+            (rset_marked_pointers + rset_marked_arrays));
+  pct = (rset_marked_pointers + rset_marked_array_elements) == 0 ? 0
+        : (cross_gen_pointers + cross_gen_array_elements) * 100 /
+          (rset_marked_pointers + rset_marked_array_elements);
+  PrintF("  total rset pointers %d, true cross generation ones %d (%%%d)\n",
+         (rset_marked_pointers + rset_marked_array_elements),
+         (cross_gen_pointers + cross_gen_array_elements),
+         pct);
+
   ClearHistograms();
   HeapObjectIterator obj_it(this);
   for (HeapObject* obj = obj_it.next(); obj != NULL; obj = obj_it.next())
     CollectHistogramInfo(obj);
   ReportHistogram(true);
 }
+
+
+// Dump the range of remembered set words between [start, end) corresponding
+// to the pointers starting at object_p.  The allocation_top is an object
+// pointer which should not be read past.  This is important for large object
+// pages, where some bits in the remembered set range do not correspond to
+// allocated addresses.
+static void PrintRSetRange(Address start, Address end, Object** object_p,
+                           Address allocation_top) {
+  Address rset_address = start;
+
+  // If the range starts on on odd numbered word (eg, for large object extra
+  // remembered set ranges), print some spaces.
+  if ((reinterpret_cast<uintptr_t>(start) / kIntSize) % 2 == 1) {
+    PrintF("                                    ");
+  }
+
+  // Loop over all the words in the range.
+  while (rset_address < end) {
+    uint32_t rset_word = Memory::uint32_at(rset_address);
+    int bit_position = 0;
+
+    // Loop over all the bits in the word.
+    while (bit_position < kBitsPerInt) {
+      if (object_p == reinterpret_cast<Object**>(allocation_top)) {
+        // Print a bar at the allocation pointer.
+        PrintF("|");
+      } else if (object_p > reinterpret_cast<Object**>(allocation_top)) {
+        // Do not dereference object_p past the allocation pointer.
+        PrintF("#");
+      } else if ((rset_word & (1 << bit_position)) == 0) {
+        // Print a dot for zero bits.
+        PrintF(".");
+      } else if (Heap::InNewSpace(*object_p)) {
+        // Print an X for one bits for pointers to new space.
+        PrintF("X");
+      } else {
+        // Print a circle for one bits for pointers to old space.
+        PrintF("o");
+      }
+
+      // Print a space after every 8th bit except the last.
+      if (bit_position % 8 == 7 && bit_position != (kBitsPerInt - 1)) {
+        PrintF(" ");
+      }
+
+      // Advance to next bit.
+      bit_position++;
+      object_p++;
+    }
+
+    // Print a newline after every odd numbered word, otherwise a space.
+    if ((reinterpret_cast<uintptr_t>(rset_address) / kIntSize) % 2 == 1) {
+      PrintF("\n");
+    } else {
+      PrintF(" ");
+    }
+
+    // Advance to next remembered set word.
+    rset_address += kIntSize;
+  }
+}
+
+
+void PagedSpace::DoPrintRSet(const char* space_name) {
+  PageIterator it(this, PageIterator::PAGES_IN_USE);
+  while (it.has_next()) {
+    Page* p = it.next();
+    PrintF("%s page 0x%x:\n", space_name, p);
+    PrintRSetRange(p->RSetStart(), p->RSetEnd(),
+                   reinterpret_cast<Object**>(p->ObjectAreaStart()),
+                   p->AllocationTop());
+    PrintF("\n");
+  }
+}
+
+
+void OldSpace::PrintRSet() { DoPrintRSet("old"); }
 #endif
 
 // -----------------------------------------------------------------------------
@@ -2361,7 +2499,6 @@
     if (it.has_next()) {
       accounting_stats_.WasteBytes(
           static_cast<int>(page->ObjectAreaEnd() - page_top));
-      page->SetAllocationWatermark(page_top);
     }
   }
 
@@ -2391,14 +2528,7 @@
     Object* result = free_list_.Allocate();
     if (!result->IsFailure()) {
       accounting_stats_.AllocateBytes(size_in_bytes);
-      HeapObject* obj = HeapObject::cast(result);
-      Page* p = Page::FromAddress(obj->address());
-
-      if (obj->address() >= p->AllocationWatermark()) {
-        p->SetAllocationWatermark(obj->address() + size_in_bytes);
-      }
-
-      return obj;
+      return HeapObject::cast(result);
     }
   }
 
@@ -2428,8 +2558,6 @@
   ASSERT(current_page->next_page()->is_valid());
   ASSERT(allocation_info_.top == PageAllocationLimit(current_page));
   ASSERT_EQ(object_size_in_bytes_, size_in_bytes);
-  current_page->next_page()->InvalidateWatermark(true);
-  current_page->SetAllocationWatermark(allocation_info_.top);
   accounting_stats_.WasteBytes(page_extra_);
   SetAllocationInfo(&allocation_info_, current_page->next_page());
   return AllocateLinearly(&allocation_info_, size_in_bytes);
@@ -2442,12 +2570,51 @@
   PrintF("  capacity: %d, waste: %d, available: %d, %%%d\n",
          Capacity(), Waste(), Available(), pct);
 
+  // Report remembered set statistics.
+  int rset_marked_pointers = 0;
+  int cross_gen_pointers = 0;
+
+  PageIterator page_it(this, PageIterator::PAGES_IN_USE);
+  while (page_it.has_next()) {
+    Page* p = page_it.next();
+
+    for (Address rset_addr = p->RSetStart();
+         rset_addr < p->RSetEnd();
+         rset_addr += kIntSize) {
+      int rset = Memory::int_at(rset_addr);
+      if (rset != 0) {
+        // Bits were set
+        int intoff =
+            static_cast<int>(rset_addr - p->address() - Page::kRSetOffset);
+        int bitoff = 0;
+        for (; bitoff < kBitsPerInt; ++bitoff) {
+          if ((rset & (1 << bitoff)) != 0) {
+            int bitpos = intoff*kBitsPerByte + bitoff;
+            Address slot = p->OffsetToAddress(bitpos << kObjectAlignmentBits);
+            Object** obj = reinterpret_cast<Object**>(slot);
+            rset_marked_pointers++;
+            if (Heap::InNewSpace(*obj))
+              cross_gen_pointers++;
+          }
+        }
+      }
+    }
+  }
+
+  pct = rset_marked_pointers == 0 ?
+          0 : cross_gen_pointers * 100 / rset_marked_pointers;
+  PrintF("  rset-marked pointers %d, to-new-space %d (%%%d)\n",
+            rset_marked_pointers, cross_gen_pointers, pct);
+
   ClearHistograms();
   HeapObjectIterator obj_it(this);
   for (HeapObject* obj = obj_it.next(); obj != NULL; obj = obj_it.next())
     CollectHistogramInfo(obj);
   ReportHistogram(false);
 }
+
+
+void FixedSpace::PrintRSet() { DoPrintRSet(name_); }
 #endif
 
 
@@ -2626,7 +2793,8 @@
   chunk->set_size(chunk_size);
   first_chunk_ = chunk;
 
-  // Initialize page header.
+  // Set the object address and size in the page header and clear its
+  // remembered set.
   Page* page = Page::FromAddress(RoundUp(chunk->address(), Page::kPageSize));
   Address object_address = page->ObjectAreaStart();
   // Clear the low order bit of the second word in the page to flag it as a
@@ -2634,7 +2802,13 @@
   // low order bit should already be clear.
   ASSERT((chunk_size & 0x1) == 0);
   page->SetIsLargeObjectPage(true);
-  page->SetRegionMarks(Page::kAllRegionsCleanMarks);
+  page->ClearRSet();
+  int extra_bytes = requested_size - object_size;
+  if (extra_bytes > 0) {
+    // The extra memory for the remembered set should be cleared.
+    memset(object_address + object_size, 0, extra_bytes);
+  }
+
   return HeapObject::FromAddress(object_address);
 }
 
@@ -2649,7 +2823,8 @@
 
 Object* LargeObjectSpace::AllocateRawFixedArray(int size_in_bytes) {
   ASSERT(0 < size_in_bytes);
-  return AllocateRawInternal(size_in_bytes,
+  int extra_rset_bytes = ExtraRSetBytesFor(size_in_bytes);
+  return AllocateRawInternal(size_in_bytes + extra_rset_bytes,
                              size_in_bytes,
                              NOT_EXECUTABLE);
 }
@@ -2676,62 +2851,60 @@
   return Failure::Exception();
 }
 
-void LargeObjectSpace::IterateDirtyRegions(ObjectSlotCallback copy_object) {
+
+void LargeObjectSpace::ClearRSet() {
+  ASSERT(Page::is_rset_in_use());
+
   LargeObjectIterator it(this);
   for (HeapObject* object = it.next(); object != NULL; object = it.next()) {
     // We only have code, sequential strings, or fixed arrays in large
-    // object space, and only fixed arrays can possibly contain pointers to
-    // the young generation.
+    // object space, and only fixed arrays need remembered set support.
     if (object->IsFixedArray()) {
+      // Clear the normal remembered set region of the page;
       Page* page = Page::FromAddress(object->address());
-      uint32_t marks = page->GetRegionMarks();
-      uint32_t newmarks = Page::kAllRegionsCleanMarks;
+      page->ClearRSet();
 
-      if (marks != Page::kAllRegionsCleanMarks) {
-        // For a large page a single dirty mark corresponds to several
-        // regions (modulo 32). So we treat a large page as a sequence of
-        // normal pages of size Page::kPageSize having same dirty marks
-        // and subsequently iterate dirty regions on each of these pages.
-        Address start = object->address();
-        Address end = page->ObjectAreaEnd();
-        Address object_end = start + object->Size();
+      // Clear the extra remembered set.
+      int size = object->Size();
+      int extra_rset_bytes = ExtraRSetBytesFor(size);
+      memset(object->address() + size, 0, extra_rset_bytes);
+    }
+  }
+}
 
-        // Iterate regions of the first normal page covering object.
-        uint32_t first_region_number = page->GetRegionNumberForAddress(start);
-        newmarks |=
-            Heap::IterateDirtyRegions(marks >> first_region_number,
-                                      start,
-                                      end,
-                                      &Heap::IteratePointersInDirtyRegion,
-                                      copy_object) << first_region_number;
 
-        start = end;
-        end = start + Page::kPageSize;
-        while (end <= object_end) {
-          // Iterate next 32 regions.
-          newmarks |=
-              Heap::IterateDirtyRegions(marks,
-                                        start,
-                                        end,
-                                        &Heap::IteratePointersInDirtyRegion,
-                                        copy_object);
-          start = end;
-          end = start + Page::kPageSize;
-        }
+void LargeObjectSpace::IterateRSet(ObjectSlotCallback copy_object_func) {
+  ASSERT(Page::is_rset_in_use());
 
-        if (start != object_end) {
-          // Iterate the last piece of an object which is less than
-          // Page::kPageSize.
-          newmarks |=
-              Heap::IterateDirtyRegions(marks,
-                                        start,
-                                        object_end,
-                                        &Heap::IteratePointersInDirtyRegion,
-                                        copy_object);
-        }
+  static void* lo_rset_histogram = StatsTable::CreateHistogram(
+      "V8.RSetLO",
+      0,
+      // Keeping this histogram's buckets the same as the paged space histogram.
+      Page::kObjectAreaSize / kPointerSize,
+      30);
 
-        page->SetRegionMarks(newmarks);
+  LargeObjectIterator it(this);
+  for (HeapObject* object = it.next(); object != NULL; object = it.next()) {
+    // We only have code, sequential strings, or fixed arrays in large
+    // object space, and only fixed arrays can possibly contain pointers to
+    // the young generation.
+    if (object->IsFixedArray()) {
+      // Iterate the normal page remembered set range.
+      Page* page = Page::FromAddress(object->address());
+      Address object_end = object->address() + object->Size();
+      int count = Heap::IterateRSetRange(page->ObjectAreaStart(),
+                                         Min(page->ObjectAreaEnd(), object_end),
+                                         page->RSetStart(),
+                                         copy_object_func);
+
+      // Iterate the extra array elements.
+      if (object_end > page->ObjectAreaEnd()) {
+        count += Heap::IterateRSetRange(page->ObjectAreaEnd(), object_end,
+                                        object_end, copy_object_func);
       }
+      if (lo_rset_histogram != NULL) {
+        StatsTable::AddHistogramSample(lo_rset_histogram, count);
+      }
     }
   }
 }
@@ -2822,7 +2995,7 @@
     } else if (object->IsFixedArray()) {
       // We loop over fixed arrays ourselves, rather then using the visitor,
       // because the visitor doesn't support the start/offset iteration
-      // needed for IsRegionDirty.
+      // needed for IsRSetSet.
       FixedArray* array = FixedArray::cast(object);
       for (int j = 0; j < array->length(); j++) {
         Object* element = array->get(j);
@@ -2831,11 +3004,8 @@
           ASSERT(Heap::Contains(element_object));
           ASSERT(element_object->map()->IsMap());
           if (Heap::InNewSpace(element_object)) {
-            Address array_addr = object->address();
-            Address element_addr = array_addr + FixedArray::kHeaderSize +
-                j * kPointerSize;
-
-            ASSERT(Page::FromAddress(array_addr)->IsRegionDirty(element_addr));
+            ASSERT(Page::IsRSetSet(object->address(),
+                                   FixedArray::kHeaderSize + j * kPointerSize));
           }
         }
       }
@@ -2876,6 +3046,33 @@
     }
   }
 }
+
+
+void LargeObjectSpace::PrintRSet() {
+  LargeObjectIterator it(this);
+  for (HeapObject* object = it.next(); object != NULL; object = it.next()) {
+    if (object->IsFixedArray()) {
+      Page* page = Page::FromAddress(object->address());
+
+      Address allocation_top = object->address() + object->Size();
+      PrintF("large page 0x%x:\n", page);
+      PrintRSetRange(page->RSetStart(), page->RSetEnd(),
+                     reinterpret_cast<Object**>(object->address()),
+                     allocation_top);
+      int extra_array_bytes = object->Size() - Page::kObjectAreaSize;
+      int extra_rset_bits = RoundUp(extra_array_bytes / kPointerSize,
+                                    kBitsPerInt);
+      PrintF("------------------------------------------------------------"
+             "-----------\n");
+      PrintRSetRange(allocation_top,
+                     allocation_top + extra_rset_bits / kBitsPerByte,
+                     reinterpret_cast<Object**>(object->address()
+                                                + Page::kObjectAreaSize),
+                     allocation_top);
+      PrintF("\n");
+    }
+  }
+}
 #endif  // DEBUG
 
 } }  // namespace v8::internal