Free one bit in Map by removing unused retaining counter.

src/heap/heap.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/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/ast/scopeinfo.h"
 #include "src/base/bits.h"
@@ skipping 2022 matching lines @@
         ->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
   }
   reinterpret_cast<Map*>(result)->clear_unused();
   reinterpret_cast<Map*>(result)
       ->set_inobject_properties_or_constructor_function_index(0);
   reinterpret_cast<Map*>(result)->set_unused_property_fields(0);
   reinterpret_cast<Map*>(result)->set_bit_field(0);
   reinterpret_cast<Map*>(result)->set_bit_field2(0);
   int bit_field3 = Map::EnumLengthBits::encode(kInvalidEnumCacheSentinel) |
                    Map::OwnsDescriptors::encode(true) |
-                   Map::Counter::encode(Map::kRetainingCounterStart);
+                   Map::ConstructionCounter::encode(Map::kNoSlackTracking);
   reinterpret_cast<Map*>(result)->set_bit_field3(bit_field3);
   reinterpret_cast<Map*>(result)->set_weak_cell_cache(Smi::FromInt(0));
   return result;
 }
 
 
 AllocationResult Heap::AllocateMap(InstanceType instance_type,
                                    int instance_size,
                                    ElementsKind elements_kind) {
   HeapObject* result = nullptr;
@@ skipping 18 matching lines @@
   if (FLAG_unbox_double_fields) {
     map->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
   }
   // Must be called only after |instance_type|, |instance_size| and
   // |layout_descriptor| are set.
   map->set_visitor_id(Heap::GetStaticVisitorIdForMap(map));
   map->set_bit_field(0);
   map->set_bit_field2(1 << Map::kIsExtensible);
   int bit_field3 = Map::EnumLengthBits::encode(kInvalidEnumCacheSentinel) |
                    Map::OwnsDescriptors::encode(true) |
-                   Map::Counter::encode(Map::kRetainingCounterStart);
+                   Map::ConstructionCounter::encode(Map::kNoSlackTracking);
   map->set_bit_field3(bit_field3);
   map->set_elements_kind(elements_kind);
   map->set_new_target_is_base(true);
 
   return map;
 }
 
 
 AllocationResult Heap::AllocateFillerObject(int size, bool double_align,
                                             AllocationSpace space) {
@@ skipping 4024 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8