Keep track of which maps are associated with prototype objects so we can tune the fast-case vs. has…

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 
 #include <stdlib.h>
 
 #include "v8.h"
 
 #include "execution.h"
 #include "factory.h"
 #include "macro-assembler.h"
 #include "global-handles.h"
@@ skipping 1561 matching lines @@
       "  var prototype = {};"
       "  object.__proto__ = prototype;"
       "  if (i >= 3) live.push(object, prototype);"
       "}");
 
   Handle<JSObject> baseObject =
       v8::Utils::OpenHandle(
           *v8::Handle<v8::Object>::Cast(
               v8::Context::GetCurrent()->Global()->Get(v8_str("base"))));
 
-  // Verify that only dead prototype transitions are cleared.
-  CHECK_EQ(10, baseObject->map()->NumberOfProtoTransitions());
+  // Verify that only dead prototype transitions are cleared.  There is an
+  // extra, 11th, prototype transition on the Object map, which is the
+  // transition to a map with the used_for_prototype flag set (the key is
+  // the_hole).
+  CHECK_EQ(11, baseObject->map()->NumberOfProtoTransitions());
   HEAP->CollectAllGarbage(Heap::kNoGCFlags);
-  CHECK_EQ(10 - 3, baseObject->map()->NumberOfProtoTransitions());
+  const int transitions = 11 - 3;
+  CHECK_EQ(transitions, baseObject->map()->NumberOfProtoTransitions());
 
   // Verify that prototype transitions array was compacted.
   FixedArray* trans = baseObject->map()->prototype_transitions();
-  for (int i = 0; i < 10 - 3; i++) {
+  for (int i = 0; i < transitions; i++) {
     int j = Map::kProtoTransitionHeaderSize +
         i * Map::kProtoTransitionElementsPerEntry;
     CHECK(trans->get(j + Map::kProtoTransitionMapOffset)->IsMap());
-    CHECK(trans->get(j + Map::kProtoTransitionPrototypeOffset)->IsJSObject());
+    Object* proto = trans->get(j + Map::kProtoTransitionPrototypeOffset);
+    CHECK(proto->IsTheHole() || proto->IsJSObject());
   }
 
   // Make sure next prototype is placed on an old-space evacuation candidate.
   Handle<JSObject> prototype;
   PagedSpace* space = HEAP->old_pointer_space();
   do {
     prototype = FACTORY->NewJSArray(32 * KB, FAST_HOLEY_ELEMENTS, TENURED);
   } while (space->FirstPage() == space->LastPage() ||
       !space->LastPage()->Contains(prototype->address()));
 
@@ skipping 285 matching lines @@
 
   Handle<JSObject> root =
       v8::Utils::OpenHandle(
           *v8::Handle<v8::Object>::Cast(
               v8::Context::GetCurrent()->Global()->Get(v8_str("root"))));
 
   // The root object should be in a sane state.
   CHECK(root->IsJSObject());
   CHECK(root->map()->IsMap());
 }