Guard against undefined fields in global context.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index dd54c630733f9c74905dcd7928641e9f9e0fb2f7..5ef3a14372f39cf59827b313eb1d5e536f7287ab 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -677,11 +677,15 @@ void IncrementalMarking::Hurry() {
 
   Object* context = heap_->global_contexts_list();
   while (!context->IsUndefined()) {
-    NormalizedMapCache* cache = Context::cast(context)->normalized_map_cache();
-    MarkBit mark_bit = Marking::MarkBitFrom(cache);
-    if (Marking::IsGrey(mark_bit)) {
-      Marking::GreyToBlack(mark_bit);
-      MemoryChunk::IncrementLiveBytes(cache->address(), cache->Size());
+    int index = Context::NORMALIZED_MAP_CACHE_INDEX;

[Vyacheslav Egorov (Chromium), 2011/12/13 12:26:20]

I don't think we need separate index variable.

[ulan, 2011/12/13 13:02:58]

Done.

+    Object* cache_or_undefined = Context::cast(context)->get(index);
+    if (!cache_or_undefined->IsUndefined()) {

[Vyacheslav Egorov (Chromium), 2011/12/13 12:26:20]

Please comment why can it be undefined.

[ulan, 2011/12/13 13:02:58]

Done.

+      NormalizedMapCache* cache = NormalizedMapCache::cast(cache_or_undefined);

[Vyacheslav Egorov (Chromium), 2011/12/13 12:26:20]

you don't need a separate cache variable. just make cache_or_undefined cache.
(make it HeapObject as it is guaranteed to be one).

[ulan, 2011/12/13 13:02:58]

Done.

+      MarkBit mark_bit = Marking::MarkBitFrom(cache);
+      if (Marking::IsGrey(mark_bit)) {
+        Marking::GreyToBlack(mark_bit);
+        MemoryChunk::IncrementLiveBytes(cache->address(), cache->Size());
+      }
     }
     context = Context::cast(context)->get(Context::NEXT_CONTEXT_LINK);
   }