Add support for large object IsSlotInBlackObject to filter out all dead slots correctly.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 603c294749b151b6dfe311e8171b7f24d326e38f..d6a2f15887ab2ac2844f12bffdb982728e4f60bd 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -3060,11 +3060,18 @@ bool MarkCompactCollector::TryPromoteObject(HeapObject* object,
 
 bool MarkCompactCollector::IsSlotInBlackObject(Page* p, Address slot,
                                                HeapObject** out_object) {
-  // This function does not support large objects right now.
   Space* owner = p->owner();

[Hannes Payer (out of office), 2015/07/30 12:38:12]

owner() could actually return lo

   if (owner == heap_->lo_space() || owner == NULL) {

[ulan, 2015/07/30 12:45:23]

Did you mean if (owner == heap_->lo_space()) { ?

[Hannes Payer (out of office), 2015/07/30 13:40:29]

As discussed offline, NULL is returned when we somewhere after 1M in a large
object. I will clean the owner() function up in a different cl.

-    *out_object = NULL;
-    return true;
+    Object* large_object = heap_->lo_space()->FindObject(slot);
+    // This object has to exist, otherwise we would not have recorded a slot
+    // for it.
+    CHECK(large_object->IsHeapObject());
+    HeapObject* large_heap_object = HeapObject::cast(large_object);
+    if (IsMarked(large_heap_object)) {
+      *out_object = large_heap_object;
+      return true;
+    }
+    return false;
   }
 
   uint32_t mark_bit_index = p->AddressToMarkbitIndex(slot);