Guard against undefined fields in global context.

src/incremental-marking.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 659 matching lines @@
 
   if (FLAG_cleanup_code_caches_at_gc) {
     PolymorphicCodeCache* poly_cache = heap_->polymorphic_code_cache();
     Marking::GreyToBlack(Marking::MarkBitFrom(poly_cache));
     MemoryChunk::IncrementLiveBytes(poly_cache->address(),
                                     PolymorphicCodeCache::kSize);
   }
 
   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());
+    // GC can happen when the context is not fully initialized,
+    // so the cache can be undefined.
+    HeapObject* cache = HeapObject::cast(
+        Context::cast(context)->get(Context::NORMALIZED_MAP_CACHE_INDEX));
+    if (!cache->IsUndefined()) {
+      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);
   }
 }
 
 
 void IncrementalMarking::Abort() {
   if (IsStopped()) return;
   if (FLAG_trace_incremental_marking) {
     PrintF("[IncrementalMarking] Aborting.\n");
@@ skipping 216 matching lines @@
   allocation_marking_factor_ = kInitialAllocationMarkingFactor;
   bytes_scanned_ = 0;
 }
 
 
 int64_t IncrementalMarking::SpaceLeftInOldSpace() {
   return heap_->MaxOldGenerationSize() - heap_->PromotedSpaceSize();
 }
 
 } }  // namespace v8::internal