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

runtime/platform/globals.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 PLATFORM_GLOBALS_H_
 #define PLATFORM_GLOBALS_H_
 
 // __STDC_FORMAT_MACROS has to be defined before including <inttypes.h> to
 // enable platform independent printf format specifiers.
 #ifndef __STDC_FORMAT_MACROS
@@ skipping 323 matching lines @@
 const int64_t kMinInt64 = DART_INT64_C(0x8000000000000000);
 const int64_t kMaxInt64 = DART_INT64_C(0x7FFFFFFFFFFFFFFF);
 const uint64_t kMaxUint64 = DART_2PART_UINT64_C(0xFFFFFFFF, FFFFFFFF);
 const int64_t kSignBitDouble = DART_INT64_C(0x8000000000000000);
 
 // Types for native machine words. Guaranteed to be able to hold pointers and
 // integers.
 typedef intptr_t word;
 typedef uintptr_t uword;
 
-// Size of a class id. We may make this 32-bit on 64-bit architectures.
+// Size of a class id.
+#if defined(ARCH_IS_32_BIT)
 typedef uint16_t classid_t;
+#elif defined(ARCH_IS_64_BIT)
+typedef uint32_t classid_t;
+#else
+#error Unexpected architecture word size
+#endif
 
 // Byte sizes.
 const int kWordSize = sizeof(word);
 const int kDoubleSize = sizeof(double);  // NOLINT
 const int kFloatSize = sizeof(float);  // NOLINT
 const int kQuadSize = 4 * kFloatSize;
 const int kSimd128Size = sizeof(simd128_value_t);  // NOLINT
 const int kInt32Size = sizeof(int32_t);  // NOLINT
 #ifdef ARCH_IS_32_BIT
 const int kWordSizeLog2 = 2;
@@ skipping 226 matching lines @@
 // should be counted from two, not one."
 #define PRINTF_ATTRIBUTE(string_index, first_to_check) \
   __attribute__((__format__(__printf__, string_index, first_to_check)))
 #else
 #define PRINTF_ATTRIBUTE(string_index, first_to_check)
 #endif
 
 }  // namespace dart
 
 #endif  // PLATFORM_GLOBALS_H_