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

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index d7e8aac433060be3d82abea01de8859063d74a56..eb47498980675467607d18959c5fa8fa3349c38f 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -1540,6 +1540,41 @@ void Assembler::EmitGenericShift(bool wide,
 }
 
 
+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];