Old space cannot be assumed to be iterable between GCs, even if swept precisely.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/cpu-profiler.h"
@@ skipping 1991 matching lines @@
       heap()->IncrementSemiSpaceCopiedObjectSize(size);
     }
     *cells = 0;
   }
   return survivors_size;
 }
 
 
 static void DiscoverGreyObjectsInSpace(Heap* heap, MarkingDeque* marking_deque,
                                        PagedSpace* space) {
-  if (space->swept_precisely()) {
-    HeapObjectIterator it(space);
-    DiscoverGreyObjectsWithIterator(heap, marking_deque, &it);
-  } else {
-    PageIterator it(space);
-    while (it.has_next()) {
-      Page* p = it.next();
-      DiscoverGreyObjectsOnPage(marking_deque, p);
-      if (marking_deque->IsFull()) return;
-    }
+  PageIterator it(space);
+  while (it.has_next()) {
+    Page* p = it.next();
+    DiscoverGreyObjectsOnPage(marking_deque, p);
+    if (marking_deque->IsFull()) return;
   }
 }
 
 
 static void DiscoverGreyObjectsInNewSpace(Heap* heap,
                                           MarkingDeque* marking_deque) {
   NewSpace* space = heap->new_space();
   NewSpacePageIterator it(space->bottom(), space->top());
   while (it.has_next()) {
     NewSpacePage* page = it.next();
@@ skipping 2750 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal