In generated code for x64 don't load object's class directly from class_ field.

runtime/vm/assembler_x64.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 1522 matching lines @@
   if (wide) {
     EmitRegisterREX(operand, REX_W);
   } else {
     EmitRegisterREX(operand, REX_NONE);
   }
   EmitUint8(0xD3);
   EmitOperand(rm, Operand(operand));
 }
 
 
+void Assembler::LoadClassIndexOfObject(Register result, Register object) {
+  ASSERT(RawObject::kClassTagBit == 16);
+  ASSERT(RawObject::kClassTagSize == 16);
+  const intptr_t class_id_offset = Object::tags_offset() +
+      RawObject::kClassTagBit / kBitsPerByte;
+  movzxw(result, FieldAddress(object, class_id_offset));
+}
+
+
+void Assembler::LoadClassByIndex(Register result, Register class_index) {
+  ASSERT(result != class_index);
+  movq(result, FieldAddress(CTX, Context::isolate_offset()));
+  const intptr_t table_offset_in_isolate =
+      Isolate::class_table_offset() + ClassTable::table_offset();
+  movq(result, Address(result, table_offset_in_isolate));
+  movq(result, Address(result, class_index, TIMES_8, 0));
+}
+
+
+void Assembler::LoadClassOfObject(Register result,
+                                  Register object,
+                                  Register scratch) {

[srdjan, 2012/05/30 16:44:00]

You do not need to pass scratch, use TMP register instead.

+  LoadClassIndexOfObject(scratch, object);
+  LoadClassByIndex(result, scratch);
+}
+
+
+void Assembler::CompareClassOfObject(Register object,
+                                     const Class& clazz,
+                                     Register scratch) {

[srdjan, 2012/05/30 16:44:00]

ditto

+  LoadClassIndexOfObject(scratch, object);
+  cmpl(scratch, Immediate(clazz.index()));
+}
+
+
 void Assembler::Comment(const char* format, ...) {
   if (FLAG_code_comments) {
     char buffer[1024];
 
     va_list args;
     va_start(args, format);
     OS::VSNPrint(buffer, sizeof(buffer), format, args);
     va_end(args);
 
     comments_.Add(new CodeComment(buffer_.GetPosition(),
@@ skipping 10 matching lines @@
     comments.SetCommentAt(i, comments_[i]->comment());
   }
 
   return comments;
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64