Update tag bits atomically

runtime/vm/scavenger.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/scavenger.h"
 
 #include <algorithm>
 #include <map>
 #include <utility>
 
@@ skipping 95 matching lines @@
     visiting_old_object_ = obj;
   }
 
   void DelayWeakProperty(RawWeakProperty* raw_weak) {
     RawObject* raw_key = raw_weak->ptr()->key_;
     DelaySet::iterator it = delay_set_.find(raw_key);
     if (it != delay_set_.end()) {
       ASSERT(raw_key->IsWatched());
     } else {
       ASSERT(!raw_key->IsWatched());
-      raw_key->SetWatchedBit();
+      raw_key->SetWatchedBitUnsynchronized();
     }
     delay_set_.insert(std::make_pair(raw_key, raw_weak));
   }
 
   void Finalize() {
     DelaySet::iterator it = delay_set_.begin();
     for (; it != delay_set_.end(); ++it) {
       WeakProperty::Clear(it->second);
     }
   }
@@ skipping 44 matching lines @@
     uword raw_addr = RawObject::ToAddr(raw_obj);
     // Read the header word of the object and determine if the object has
     // already been copied.
     uword header = *reinterpret_cast<uword*>(raw_addr);
     uword new_addr = 0;
     if (IsForwarding(header)) {
       // Get the new location of the object.
       new_addr = ForwardedAddr(header);
     } else {
       if (raw_obj->IsWatched()) {
-        raw_obj->ClearWatchedBit();
+        raw_obj->ClearWatchedBitUnsynchronized();
         std::pair<DelaySet::iterator, DelaySet::iterator> ret;
         // Visit all elements with a key equal to this raw_obj.
         ret = delay_set_.equal_range(raw_obj);
         for (DelaySet::iterator it = ret.first; it != ret.second; ++it) {
           // Remember the delayed WeakProperty. These objects have been
           // forwarded, but have not been scavenged because their key was not
           // known to be reachable. Now that the key object is known to be
           // reachable, we need to visit its key and value pointers.
           delayed_weak_stack_.Add(it->second);
         }
@@ skipping 704 matching lines @@
 }
 
 
 void Scavenger::FreeExternal(intptr_t size) {
   ASSERT(size >= 0);
   external_size_ -= size;
   ASSERT(external_size_ >= 0);
 }
 
 }  // namespace dart