Incremental mode now works for x64. The only difference

src/heap.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4608 matching lines @@
   Address visitable_start = page->ObjectAreaStart();
   Address end_of_page = page->ObjectAreaEnd();
 
   Address visitable_end = visitable_start;
 
   // TODO(gc) ISOLATES
   Object* free_space_map = HEAP->free_space_map();
   Object* two_pointer_filler_map = HEAP->two_pointer_filler_map();
 
   while (visitable_end < end_of_page) {
+#ifdef DEBUG
+    int offset = visitable_start - page->ObjectAreaStart();
+    ASSERT(offset == space->RoundSizeDownToObjectAlignment(offset));
+#endif
     Object* o = *reinterpret_cast<Object**>(visitable_end);
     // Skip fillers but not things that look like fillers in the special
     // garbage section which can contain anything.
     if (o == free_space_map ||
         o == two_pointer_filler_map ||
         visitable_end == space->top()) {
       if (visitable_start != visitable_end) {
         // After calling this the special garbage section may have moved.
         visit_pointer_region(HEAP,
                              visitable_start,
@@ skipping 1372 matching lines @@
 }
 
 
 void ExternalStringTable::TearDown() {
   new_space_strings_.Free();
   old_space_strings_.Free();
 }
 
 
 } }  // namespace v8::internal