Filter out remembered slots that are at the start of an object.

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 3104 matching lines @@
 
   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)));
 
-  // First check if the object is in the current cell.
+  // Check if the slot points to the start of an object. This can happen e.g.
+  // when we left trim a fixed array. Such slots are invalid and we can remove
+  // them.
+  if ((cells[start_index] & index_in_cell) != 0) {
+    return false;
+  }
+
+  // Check if the object is in the current cell.
   MarkBit::CellType slot_mask;
   if ((cells[start_index] == 0) ||
       (base::bits::CountTrailingZeros32(cells[start_index]) >
        base::bits::CountTrailingZeros32(cells[start_index] | index_in_cell))) {
     // If we are already in the first cell, there is no live object.
     if (start_index == cell_base_start_index) return false;
 
     // If not, find a cell in a preceding cell slot that has a mark bit set.
     do {
       start_index--;
     } while (start_index > cell_base_start_index && cells[start_index] == 0);
 
     // The slot must be in a dead object if there are no preceding cells that
     // have mark bits set.
     if (cells[start_index] == 0) {
       return false;
     }
 
     // The object is in a preceding cell. Set the mask to find any object.
     slot_mask = 0xffffffff;
   } else {
+    // The object start is before the the slot index. Hence, in this case the
+    // slot index can not be at the beginning of the cell.
+    CHECK(index_in_cell > 1);
     // We are interested in object mark bits right before the slot.
     slot_mask = index_in_cell - 1;
   }
 
   MarkBit::CellType current_cell = cells[start_index];
-  DCHECK(current_cell != 0);
+  CHECK(current_cell != 0);
 
   // Find the last live object in the cell.
   unsigned int leading_zeros =
       base::bits::CountLeadingZeros32(current_cell & slot_mask);
-  DCHECK(leading_zeros != 32);
+  CHECK(leading_zeros != 32);
   unsigned int offset = Bitmap::kBitIndexMask - leading_zeros;
 
   cell_base += (start_index - cell_base_start_index) * 32 * kPointerSize;
   Address address = cell_base + offset * kPointerSize;
   HeapObject* object = HeapObject::FromAddress(address);
-  DCHECK(object->address() < reinterpret_cast<Address>(slot));
+  CHECK(object->address() < reinterpret_cast<Address>(slot));
   if (object->address() <= slot &&
       (object->address() + object->Size()) > slot) {
     // If the slot is within the last found object in the cell, the slot is
     // in a live object.
     *out_object = object;
     return true;
   }
   return false;
 }
 
@@ skipping 1613 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