Expand the class id to 32 bits and size field to 16 bits on 64-bit platforms.

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/atomic.h"
 #include "vm/globals.h"
@@ skipping 220 matching lines @@
 class RawObject {
  public:
   // The tags field which is a part of the object header uses the following
   // bit fields for storing tags.
   enum TagBits {
     kWatchedBit = 0,
     kMarkBit = 1,
     kCanonicalBit = 2,
     kFromSnapshotBit = 3,
     kRememberedBit = 4,
-    kReservedTagPos = 5,  // kReservedBit{10K,100K,1M,10M}
+#if defined(ARCH_IS_32_BIT)
+    kReservedTagPos = 5,  // kReservedBit{100K,1M,10M}
     kReservedTagSize = 3,
     kSizeTagPos = kReservedTagPos + kReservedTagSize,  // = 8
     kSizeTagSize = 8,
     kClassIdTagPos = kSizeTagPos + kSizeTagSize,  // = 16
     kClassIdTagSize = 16,
+#elif defined(ARCH_IS_64_BIT)
+    kReservedTagPos = 5,  // kReservedBit{100K,1M,10M}
+    kReservedTagSize = 11,
+    kSizeTagPos = kReservedTagPos + kReservedTagSize,  // = 16
+    kSizeTagSize = 16,
+    kClassIdTagPos = kSizeTagPos + kSizeTagSize,  // = 32
+    kClassIdTagSize = 32,
+#else
+#error Unexpected architecture word size
+#endif
   };
 
+  COMPILE_ASSERT(kClassIdTagSize == (sizeof(classid_t) * kBitsPerByte));
+
   // Encodes the object size in the tag in units of object alignment.
   class SizeTag {
    public:
     static const intptr_t kMaxSizeTag =
         ((1 << RawObject::kSizeTagSize) - 1) << kObjectAlignmentLog2;
 
     static uword encode(intptr_t size) {
       return SizeBits::encode(SizeToTagValue(size));
     }
 
@@ skipping 1906 matching lines @@
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kByteBufferCid == kExternalTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kNullCid == kByteBufferCid + 1);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_