Make instance_size immediately useful for all fixed size objects.

src/objects-inl.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2093 matching lines @@
   return READ_BYTE_FIELD(this, kInObjectPropertiesOffset);
 }
 
 
 int Map::pre_allocated_property_fields() {
   return READ_BYTE_FIELD(this, kPreAllocatedPropertyFieldsOffset);
 }
 
 
 int HeapObject::SizeFromMap(Map* map) {
-  InstanceType instance_type = map->instance_type();
+  int instance_size = map->instance_size();
+  if (instance_size != kVariableSizeSentinel) return instance_size;
+  // We can ignore the "symbol" bit becase it is only set for symbols
+  // and implies a string type.
+  int instance_type = static_cast<int>(map->instance_type()) & ~kIsSymbolMask;
   // Only inline the most frequent cases.
-  if (instance_type == JS_OBJECT_TYPE ||
-      (instance_type & (kIsNotStringMask | kStringRepresentationMask)) ==
-      (kStringTag | kConsStringTag) ||
-      instance_type == JS_ARRAY_TYPE) return map->instance_size();
   if (instance_type == FIXED_ARRAY_TYPE) {
     return FixedArray::BodyDescriptor::SizeOf(map, this);
   }
+  if (instance_type == ASCII_STRING_TYPE) {
+    return SeqAsciiString::SizeFor(
+        reinterpret_cast<SeqAsciiString*>(this)->length());
+  }
   if (instance_type == BYTE_ARRAY_TYPE) {
     return reinterpret_cast<ByteArray*>(this)->ByteArraySize();
   }
-  // Otherwise do the general size computation.
-  return SlowSizeFromMap(map);
+  if (instance_type == STRING_TYPE) {
+    return SeqTwoByteString::SizeFor(
+        reinterpret_cast<SeqTwoByteString*>(this)->length());
+  }
+  ASSERT(instance_type == CODE_TYPE);
+  return reinterpret_cast<Code*>(this)->CodeSize();
 }
 
 
 void Map::set_instance_size(int value) {
   ASSERT_EQ(0, value & (kPointerSize - 1));
   value >>= kPointerSizeLog2;
   ASSERT(0 <= value && value < 256);
   WRITE_BYTE_FIELD(this, kInstanceSizeOffset, static_cast<byte>(value));
 }
 
@@ skipping 1311 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_