[runtime] Speed up allocating instances in the runtime by having a quick-check for inobject slack t…

src/objects.h

 // 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.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 5564 matching lines @@
   class IsUnstable : public BitField<bool, 24, 1> {};
   class IsMigrationTarget : public BitField<bool, 25, 1> {};
   class IsStrong : public BitField<bool, 26, 1> {};
   class NewTargetIsBase : public BitField<bool, 27, 1> {};
   // Bit 28 is free.
 
   // Keep this bit field at the very end for better code in
   // Builtins::kJSConstructStubGeneric stub.
   // This counter is used for in-object slack tracking.
   // The in-object slack tracking is considered enabled when the counter is
-  // non zero.
+  // non zero. The counter only has a valid count for initial maps. For
+  // transitioned maps only kNoSlackTracking has a meaning, namely that inobject
+  // slack tracking already finished for the transition tree. Any other value
+  // indicates that either inobject slack tracking is still in progress, or that
+  // the map isn't part of the transition tree anymore.
   class ConstructionCounter : public BitField<int, 29, 3> {};
   static const int kSlackTrackingCounterStart = 7;
   static const int kSlackTrackingCounterEnd = 1;
   static const int kNoSlackTracking = 0;
   STATIC_ASSERT(kSlackTrackingCounterStart <= ConstructionCounter::kMax);
 
 
   // Inobject slack tracking is the way to reclaim unused inobject space.
   //
   // The instance size is initially determined by adding some slack to
@@ skipping 5206 matching lines @@
     }
     return value;
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #endif  // V8_OBJECTS_H_