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

runtime/vm/assembler_x64.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_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 501 matching lines @@
     lock();
     cmpxchgl(address, reg);
   }
 
   void EnterFrame(intptr_t frame_space);
   void LeaveFrame();
 
   void CallRuntime(const RuntimeEntry& entry);
 
   /*
+   * Loading and comparing classes of objects.
+   */
+  void LoadClassIndexOfObject(Register result, Register object);

[Ivan Posva, 2012/05/30 17:13:42]

How about?
LoadClassId(Register result, Register object)

[Vyacheslav Egorov (Google), 2012/05/30 17:29:57]

I am using name "class index" for consistency. It is called ClassIndex
everywhere else (e.g. there is a getter in RawObject that is named
GetClassIndex).

Should I rename class index to class id everywhere?

[Ivan Posva, 2012/05/30 18:00:30]

I can change the use of index to id in other areas of the VM.

+
+  void LoadClassByIndex(Register result, Register class_index);

[Ivan Posva, 2012/05/30 17:13:42]

LoadClassById

+
+  void LoadClassOfObject(Register result, Register object, Register scratch);

[Ivan Posva, 2012/05/30 17:13:42]

LoadClass? Might be a bit too generic, better suggestions appreciated.

+
+  void CompareClassOfObject(Register object,

[Ivan Posva, 2012/05/30 17:13:42]

How about renaming this to:
CompareClassId(Register object, const Class& clazz, Register scratch)

When I read this at use sites I always feel that we are really comparing the
class objects which is not the case.

[Vyacheslav Egorov (Google), 2012/05/30 17:29:57]

But it does not accept index. It is implementation detail of this function that
it compares indexes/ids not objects. It should not matter for external user.

I can rename it, but I feel wrong exposing implementation detail through it's
name instead of describing what it does --- compares classes.

[Ivan Posva, 2012/05/30 18:00:30]

Understand the consistency argument about passing a class. Instead it would make
sense to pass an id.

+                            const Class& clazz,
+                            Register scratch);
+
+  /*
    * Misc. functionality.
    */
   void SmiTag(Register reg) {
     addq(reg, reg);
   }
 
   void SmiUntag(Register reg) {
     sarq(reg, Immediate(kSmiTagSize));
   }
 
@@ skipping 104 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_