Changes to set up the object tag bits

vm/raw_object.h

Index: vm/raw_object.h
===================================================================
--- vm/raw_object.h	(revision 2393)
+++ vm/raw_object.h	(working copy)
@@ -101,6 +101,24 @@
   kSmiTagShift = 1,
 };
 
+
+/*
+ * The tags field which is a part of the object header uses the following
+ * bit fields for storing tags.
+ *
+ * bit 0 - SmiTag
+ * bit 1 - Mark bit.
+ * bit 2 - Canonical object.
+ * bit 3 - Created from a full snapshot.
+ */
+class CanonicalObjectTag : public BitField<bool, 2, 1> {
+};
+
+
+class CreatedFromSnapshotTag : public BitField<bool, 3, 1> {
+};
+
+
 #define SNAPSHOT_WRITER_SUPPORT()                                              \
   void WriteTo(                                                                \
       SnapshotWriter* writer, intptr_t object_id, Snapshot::Kind kind);        \
@@ -166,6 +184,22 @@
     ptr()->class_ = reinterpret_cast<RawClass*>(header_bits ^ kMarked);
   }
 
+  // Support for object tags.
+  bool IsCanonical() const {
+    return CanonicalObjectTag::decode(ptr()->tags_);
+  }
+  void SetCanonical() {
+    intptr_t tags = ptr()->tags_;
+    ptr()->tags_ = CanonicalObjectTag::update(true, tags);
+  }
+  bool IsCreatedFromSnapshot() {
+    return CreatedFromSnapshotTag::decode(ptr()->tags_);
+  }
+  void SetCreatedFromSnapshot() {
+    intptr_t tags = ptr()->tags_;
+    ptr()->tags_ = CreatedFromSnapshotTag::update(true, tags);
+  }
+
   void Validate() const;
   intptr_t Size() const;
   intptr_t VisitPointers(ObjectPointerVisitor* visitor);
@@ -180,8 +214,17 @@
     return reinterpret_cast<uword>(raw_obj->ptr());
   }
 
+  static bool IsCreatedFromSnapshot(intptr_t value) {
+    return CreatedFromSnapshotTag::decode(value);
+  }
+
+  static bool IsCanonical(intptr_t value) {
+    return CanonicalObjectTag::decode(value);
+  }
+
  protected:
   RawClass* class_;
+  intptr_t tags_;  // Various object tags (bits).
 
  private:
   enum {
@@ -196,8 +239,6 @@
         reinterpret_cast<uword>(this) - kHeapObjectTag);
   }
 
-  intptr_t tags_;  // Various object tags (bits).
-
   friend class Object;
   friend class Array;
   friend class SnapshotWriter;
@@ -343,7 +384,6 @@
   RawObject** from() {
     return reinterpret_cast<RawObject**>(&ptr()->length_);
   }
-  bool is_canonical_;
   RawSmi* length_;
 
   // Variable length data follows here.