Use a slot that is located on a heap page when removing invalid entries from the SlotsBuffer.

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 4540 matching lines @@
     buffer = allocator->AllocateBuffer(buffer);
     *buffer_address = buffer;
   }
   DCHECK(buffer->HasSpaceForTypedSlot());
   buffer->Add(reinterpret_cast<ObjectSlot>(type));
   buffer->Add(reinterpret_cast<ObjectSlot>(addr));
   return true;
 }
 
 
-static Object* g_smi_slot = NULL;
-
-
 void SlotsBuffer::RemoveInvalidSlots(Heap* heap, SlotsBuffer* buffer) {
-  DCHECK_EQ(Smi::FromInt(0), g_smi_slot);
-
-  // Remove entries by replacing them with a dummy slot containing a smi.
-  const ObjectSlot kRemovedEntry = &g_smi_slot;
+  // Remove entries by replacing them with an old-space slot containing a smi
+  // that is located in an unmovable page.
+  const ObjectSlot kRemovedEntry =
+      HeapObject::RawField(heap->empty_fixed_array(),
+                           FixedArrayBase::kLengthOffset);
+  DCHECK(Page::FromAddress(
+      reinterpret_cast<Address>(kRemovedEntry))->NeverEvacuate());
 
   while (buffer != NULL) {
     SlotsBuffer::ObjectSlot* slots = buffer->slots_;
     intptr_t slots_count = buffer->idx_;
 
     for (int slot_idx = 0; slot_idx < slots_count; ++slot_idx) {
       ObjectSlot slot = slots[slot_idx];
       if (!IsTypedSlot(slot)) {
         Object* object = *slot;
         if (object->IsHeapObject()) {
           if (heap->InNewSpace(object) ||
               !heap->mark_compact_collector()->IsSlotInLiveObject(
                   reinterpret_cast<Address>(slot))) {
             slots[slot_idx] = kRemovedEntry;
           }
         }
       } else {
         ++slot_idx;
         DCHECK(slot_idx < slots_count);
       }
     }
     buffer = buffer->next();
   }
 }
 
 
 void SlotsBuffer::VerifySlots(Heap* heap, SlotsBuffer* buffer) {
-  DCHECK_EQ(Smi::FromInt(0), g_smi_slot);
-
   while (buffer != NULL) {
     SlotsBuffer::ObjectSlot* slots = buffer->slots_;
     intptr_t slots_count = buffer->idx_;
 
     for (int slot_idx = 0; slot_idx < slots_count; ++slot_idx) {
       ObjectSlot slot = slots[slot_idx];
       if (!IsTypedSlot(slot)) {
         Object* object = *slot;
         if (object->IsHeapObject()) {
           CHECK(!heap->InNewSpace(object));
@@ skipping 144 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