Cleanups.

runtime/vm/stub_code.h

 // 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.
 
 #ifndef VM_STUB_CODE_H_
 #define VM_STUB_CODE_H_
 
 #include "vm/allocation.h"
 #include "vm/assembler.h"
 
@@ skipping 15 matching lines @@
   V(AllocateArray)                                                             \
   V(CallNoSuchMethodFunction)                                                  \
   V(MegamorphicLookup)                                                         \
   V(CallStaticFunction)                                                        \
   V(CallClosureFunction)                                                       \
   V(StackOverflow)                                                             \
   V(OptimizeInvokedFunction)                                                   \
   V(FixCallersTarget)                                                          \
   V(Deoptimize)                                                                \
 
-// TODO(regis): Is it OK for the stubs above to refer to Object::null()?
+// Is it permitted for the stubs above to refer to Object::null(), which is
+// allocated in the VM isolate and shared across all isolates.
+// However, in cases where a simple GC-safe placeholder is needed on the stack,
+// using Smi 0 instead of Object::null() is slightly more efficient, since a Smi
+// does not require relocation.
 
 // List of stubs created per isolate, these stubs could potentially contain
 // embedded objects and hence cannot be shared across isolates.
 #define STUB_CODE_LIST(V)                                                      \
   V(InvokeDartCode)                                                            \
   V(AllocateContext)                                                           \
   V(InlineCache)                                                               \
 
 
 // class StubEntry is used to describe stub methods generated in dart to
@@ skipping 114 matching lines @@
 
   static void GenerateAllocationStubForClass(Assembler* assembler,
                                              const Class& cls);
   static void GenerateAllocationStubForClosure(Assembler* assembler,
                                                const Function& func);
 };
 
 }  // namespace dart
 
 #endif  // VM_STUB_CODE_H_