Filter out invalid slots in store buffer eagerly during object transition.

src/objects.cc

 // Copyright 2015 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/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 2980 matching lines @@
       value = isolate->factory()->uninitialized_value();
     }
     int target_index = new_descriptors->GetFieldIndex(i) - inobject;
     if (target_index < 0) target_index += total_size;
     array->set(target_index, *value);
   }
 
   // From here on we cannot fail and we shouldn't GC anymore.
   DisallowHeapAllocation no_allocation;
 
+  Heap* heap = isolate->heap();
+
   // Copy (real) inobject properties. If necessary, stop at number_of_fields to
   // avoid overwriting |one_pointer_filler_map|.
   int limit = Min(inobject, number_of_fields);
   for (int i = 0; i < limit; i++) {
     FieldIndex index = FieldIndex::ForPropertyIndex(*new_map, i);
     Object* value = array->get(external + i);
     // Can't use JSObject::FastPropertyAtPut() because proper map was not set
     // yet.
     if (new_map->IsUnboxedDoubleField(index)) {
       DCHECK(value->IsMutableHeapNumber());
       object->RawFastDoublePropertyAtPut(index,
                                          HeapNumber::cast(value)->value());
+      if (i < old_number_of_fields && !old_map->IsUnboxedDoubleField(index)) {
+        // Transition from tagged to untagged slot.
+        heap->ClearRecordedSlot(*object,
+                                HeapObject::RawField(*object, index.offset()));
+      }
     } else {
       object->RawFastPropertyAtPut(index, value);
     }
   }
 
-  Heap* heap = isolate->heap();
 
   // If there are properties in the new backing store, trim it to the correct
   // size and install the backing store into the object.
   if (external > 0) {
     heap->RightTrimFixedArray<Heap::CONCURRENT_TO_SWEEPER>(*array, inobject);
     object->set_properties(*array);
   }
 
   // Create filler object past the new instance size.
   int new_instance_size = new_map->instance_size();
@@ skipping 16894 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8