Clear SMI and non-evacuation candidate entries when filtering the slots buffer.

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/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 4504 matching lines @@
              ->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() && heap->InNewSpace(object)) ||
+        // Slots are invalid when they currently:
+        // - do not point to a heap object (SMI)
+        // - point to a heap object in new space
+        // - are not within a live heap object on a valid pointer slot
+        // - point to a heap object not on an evacuation candidate
+        if (!object->IsHeapObject() || heap->InNewSpace(object) ||
             !heap->mark_compact_collector()->IsSlotInLiveObject(
-                reinterpret_cast<Address>(slot))) {
+                reinterpret_cast<Address>(slot)) ||
+            !Page::FromAddress(reinterpret_cast<Address>(object))
+                 ->IsEvacuationCandidate()) {
+          // TODO(hpayer): Instead of replacing slots with kRemovedEntry we
+          // could shrink the slots buffer in-place.
           slots[slot_idx] = kRemovedEntry;
         }
       } else {
         ++slot_idx;
         DCHECK(slot_idx < slots_count);
       }
     }
     buffer = buffer->next();
   }
 }
@@ skipping 11 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer::ObjectSlot* slots = buffer->slots_;
     intptr_t slots_count = buffer->idx_;
     bool is_typed_slot = false;
 
     for (int slot_idx = 0; slot_idx < slots_count; ++slot_idx) {
       ObjectSlot slot = slots[slot_idx];
       if (!IsTypedSlot(slot)) {
         Address slot_address = reinterpret_cast<Address>(slot);
         if (slot_address >= start_slot && slot_address < end_slot) {
+          // TODO(hpayer): Instead of replacing slots with kRemovedEntry we
+          // could shrink the slots buffer in-place.
           slots[slot_idx] = kRemovedEntry;
           if (is_typed_slot) {
             slots[slot_idx - 1] = kRemovedEntry;
           }
         }
         is_typed_slot = false;
       } else {
         is_typed_slot = true;
         DCHECK(slot_idx < slots_count);
       }
@@ skipping 182 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