Allow partial scanning of large arrays in order to avoid

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 12610)
+++ src/spaces.h	(working copy)
@@ -393,6 +393,15 @@
     WAS_SWEPT_PRECISELY,
     WAS_SWEPT_CONSERVATIVELY,
 
+    // Used for large objects only.  Indicates that the object has been
+    // partially scanned by the incremental mark-sweep GC.  Objects that have
+    // been partially scanned are marked black so that the write barrier
+    // triggers for them, and they are counted as live bytes.  If the mutator
+    // writes to them they may be turned grey and subtracted from the live byte
+    // list.  They move back to the marking deque either by an iteration over
+    // the large object space or in the write barrier.
+    IS_PARTIALLY_SCANNED,
+
     // Last flag, keep at bottom.
     NUM_MEMORY_CHUNK_FLAGS
   };
@@ -413,7 +422,26 @@
       (1 << IN_FROM_SPACE) |
       (1 << IN_TO_SPACE);
 
+  static const int kIsPartiallyScannedMask = 1 << IS_PARTIALLY_SCANNED;
 
+  void SetPartiallyScannedProgress(int progress) {
+    SetFlag(IS_PARTIALLY_SCANNED);
+    partially_scanned_progress_ = progress;
+  }
+
+  bool IsPartiallyScanned() {
+    return IsFlagSet(IS_PARTIALLY_SCANNED);
+  }
+
+  void SetCompletelyScanned() {
+    ClearFlag(IS_PARTIALLY_SCANNED);
+  }
+
+  int PartiallyScannedProgress() {
+    ASSERT(IsPartiallyScanned());
+    return partially_scanned_progress_;
+  }
+
   void SetFlag(int flag) {
     flags_ |= static_cast<uintptr_t>(1) << flag;
   }
@@ -488,9 +516,15 @@
 
   static const size_t kSlotsBufferOffset = kLiveBytesOffset + kIntSize;
 
-  static const size_t kHeaderSize =
-      kSlotsBufferOffset + kPointerSize + kPointerSize;
+  static const size_t kPartiallyScannedProgress =
+        kSlotsBufferOffset + kPointerSize + kPointerSize;
 
+  // Actually the partially_scanned_progress_ member is only an int, but on
+  // 64 bit the size of MemoryChunk gets rounded up to a 64 bit size so we
+  // have to have the header start kPointerSize after the
+  // partially_scanned_progress_ member.
+  static const size_t kHeaderSize = kPartiallyScannedProgress + kPointerSize;
+
   static const int kBodyOffset =
     CODE_POINTER_ALIGN(MAP_POINTER_ALIGN(kHeaderSize + Bitmap::kSize));
 
@@ -625,6 +659,7 @@
   int live_byte_count_;
   SlotsBuffer* slots_buffer_;
   SkipList* skip_list_;
+  int partially_scanned_progress_;
 
   static MemoryChunk* Initialize(Heap* heap,
                                  Address base,