Never mark free list elements.

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 739 matching lines @@
   ASSERT(branch_offset_error_->IsLanguageError());
 }
 
 
 // An object visitor which will mark all visited objects. This is used to
 // premark all objects in the vm_isolate_ heap.
 class PremarkingVisitor : public ObjectVisitor {
  public:
   explicit PremarkingVisitor(Isolate* isolate) : ObjectVisitor(isolate) {}
 
   void VisitObject(RawObject* obj) {

[Ivan Posva, 2015/04/15 21:57:01]

Another question could be: Why are we visiting free list elements in the first
place?

-    // RawInstruction objects are premarked on allocation.
-    if (!obj->IsMarked()) {
+    ASSERT(!obj->IsMarked());
+    // Free list elements should never be marked.
+    if (!obj->IsFreeListElement()) {
       obj->SetMarkBitUnsynchronized();
     }
   }
 };
 
 
 #define SET_CLASS_NAME(class_name, name)                                       \
   cls = class_name##_class();                                                  \
   cls.set_name(Symbols::name());                                               \
 
@@ skipping 19880 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart