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

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/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 3042 matching lines @@
     heap()->IncrementPromotedObjectsSize(object_size);
     return true;
   }
 
   return false;
 }
 
 
 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);
   unsigned int start_index = mark_bit_index >> Bitmap::kBitsPerCellLog2;
   MarkBit::CellType index_in_cell = 1U
                                     << (mark_bit_index & Bitmap::kBitIndexMask);
   MarkBit::CellType* cells = p->markbits()->cells();
   Address cell_base = p->area_start();
   unsigned int cell_base_start_index = Bitmap::IndexToCell(
       Bitmap::CellAlignIndex(p->AddressToMarkbitIndex(cell_base)));
@@ skipping 1670 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }  // namespace internal
 }  // namespace v8