Retain maps that have live prototypes.

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 2135 matching lines @@
         // The map has aged. Do not retain this map.
         continue;
       }
       Object* constructor = map->GetConstructor();
       if (!constructor->IsHeapObject() ||
           !Marking::MarkBitFrom(HeapObject::cast(constructor)).Get()) {
         // The constructor is dead, no new objects with this map can
         // be created. Do not retain this map.
         continue;
       }
-      new_age = age - 1;
+      Object* prototype = map->prototype();
+      if (prototype->IsHeapObject() &&
+          !Marking::MarkBitFrom(HeapObject::cast(prototype)).Get()) {
+        // The prototype is not marked, age the map.
+        new_age = age - 1;
+      } else {
+        // The prototype and the constructor are marked, this map keeps only
+        // transition tree alive, not JSObjects. Do not age the map.
+        new_age = age;
+      }
       MarkObject(map, map_mark);
     } else {
       new_age = FLAG_retain_maps_for_n_gc;
     }
     if (i != new_length) {
       retained_maps->Set(new_length, cell);
       Object** slot = retained_maps->Slot(new_length);
       RecordSlot(slot, slot, cell);
       retained_maps->Set(new_length + 1, Smi::FromInt(new_age));
     } else if (new_age != age) {
@@ skipping 2450 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