Describe the reserved header bits with their own bit-field definition.

runtime/vm/raw_object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/token.h"
@@ skipping 165 matching lines @@
 // be dereferenced (e.g. RawSmi).
 class RawObject {
  public:
   // The tags field which is a part of the object header uses the following
   // bit fields for storing tags.
   enum TagBits {
     kFreeBit = 0,
     kMarkBit = 1,
     kCanonicalBit = 2,
     kFromSnapshotBit = 3,
-    kReservedBit10K = 4,
-    kReservedBit100K = 5,
-    kReservedBit1M = 6,
-    kReservedBit10M = 7,
+    kReservedTagBit = 4,  // kReservedBit{10K,100K,1M,10M}
+    kReservedTagSize = 4,
     kSizeTagBit = 8,
     kSizeTagSize = 8,
     kClassIdTagBit = kSizeTagBit + kSizeTagSize,
     kClassIdTagSize = 16
   };
 
   // Encodes the object size in the tag in units of object alignment.
   class SizeTag {
    public:
     static const intptr_t kMaxSizeTag =
@@ skipping 123 matching lines @@
 
  private:
   class FreeBit : public BitField<bool, kFreeBit, 1> {};
 
   class MarkBit : public BitField<bool, kMarkBit, 1> {};
 
   class CanonicalObjectTag : public BitField<bool, kCanonicalBit, 1> {};
 
   class CreatedFromSnapshotTag : public BitField<bool, kFromSnapshotBit, 1> {};
 
+  class ReservedBits : public BitField<intptr_t,
+                                       kReservedTagBit,
+                                       kReservedTagSize> {};  // NOLINT
+
   RawObject* ptr() const {
     ASSERT(IsHeapObject());
     return reinterpret_cast<RawObject*>(
         reinterpret_cast<uword>(this) - kHeapObjectTag);
   }
 
   intptr_t SizeFromClass() const;
 
   intptr_t GetClassId() const {
     uword tags = ptr()->tags_;
@@ skipping 1173 matching lines @@
          kExternalUint64Array == kByteArray + 18 &&
          kExternalFloat32Array == kByteArray + 19 &&
          kExternalFloat64Array == kByteArray + 20 &&
          kClosure == kByteArray + 21);
   return (index >= kByteArray && index <= kClosure);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_