Revert changes 601 and 602. TBR

Review URL: http://codereview.chromium.org/8190

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@605 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/flag-definitions.h

@@ -134,8 +134,6 @@ DEFINE_bool(gc_global, false, "always perform global GCs")
 DEFINE_int(gc_interval, -1, "garbage collect after <n> allocations")
 DEFINE_bool(trace_gc, false,
             "print one trace line following each garbage collection")
-DEFINE_bool(collect_maps, true,
-            "garbage collect maps from which no objects can be reached")
 
 // ic.cc
 DEFINE_bool(use_ic, true, "use inline caching")

src/mark-compact.cc

@@ -78,8 +78,6 @@ void MarkCompactCollector::CollectGarbage(GCTracer* tracer) {
 
   MarkLiveObjects();
 
-  if (FLAG_collect_maps) ClearNonLiveTransitions();
-
   SweepLargeObjectSpace();
 
   if (compacting_collection_) {
@@ -137,7 +135,6 @@ void MarkCompactCollector::Prepare() {
   }
 
   if (FLAG_never_compact) compacting_collection_ = false;
-  if (FLAG_collect_maps) CreateBackPointers();
 
 #ifdef DEBUG
   if (compacting_collection_) {
@@ -325,13 +322,9 @@ class MarkingVisitor : public ObjectVisitor {
 
   // Visit an unmarked object.
   void VisitUnmarkedObject(HeapObject* obj) {
-#ifdef DEBUG
     ASSERT(Heap::Contains(obj));
+#ifdef DEBUG
     MarkCompactCollector::UpdateLiveObjectCount(obj);
-    ASSERT(!obj->IsMarked());
-    // ASSERT(!obj->IsMap());  // Some maps are processed here.
-    // Their map transitions will be followed.  If we do a test
-    // here to treat maps separately, will there be a performance impact?
 #endif
     Map* map = obj->map();
     obj->SetMark();
@@ -432,99 +425,14 @@ void MarkCompactCollector::MarkUnmarkedObject(HeapObject* object) {
   ASSERT(!object->IsMarked());
   if (object->IsJSGlobalObject()) Counters::global_objects.Increment();
 
-  tracer_->increment_marked_count();
-  ASSERT(Heap::Contains(object));
-  if (object->IsMap()) {
-    if (FLAG_cleanup_caches_in_maps_at_gc) {
-      Map::cast(object)->ClearCodeCache();
-    }
-    object->SetMark();
-    if (FLAG_collect_maps) {
-      MarkMapContents(reinterpret_cast<Map*>(object));
-    } else {
-      marking_stack.Push(object);
-    }
-  } else {
-    object->SetMark();
-    marking_stack.Push(object);
+  if (FLAG_cleanup_caches_in_maps_at_gc && object->IsMap()) {
+    Map::cast(object)->ClearCodeCache();
   }
-}
-
-
-void MarkCompactCollector::MarkMapContents(Map* map) {
-  MarkDescriptorArray(reinterpret_cast<DescriptorArray*>(
-      HeapObject::RawField(map, Map::kInstanceDescriptorsOffset)));
 
-  // Mark the Object* fields of the Map.
-  // Since the descriptor array has been marked already, it is fine
-  // that one of these fields contains a pointer to it.
-  MarkingVisitor visitor;  // Has no state or contents.
-  visitor.VisitPointers(&HeapObject::RawField(map, Map::kPrototypeOffset),
-                        &HeapObject::RawField(map, Map::kSize));
-}
-
-
-void MarkCompactCollector::MarkDescriptorArray(
-    DescriptorArray *descriptors) {
-  if (descriptors->IsMarked()) return;
-  // Empty descriptor array is marked as a root before any maps are marked.
-  ASSERT(descriptors != Heap::empty_descriptor_array());
-
-  tracer_->increment_marked_count();
-#ifdef DEBUG
-  UpdateLiveObjectCount(descriptors);
-#endif
-  descriptors->SetMark();
-
-  FixedArray* contents = reinterpret_cast<FixedArray*>(
-      descriptors->get(DescriptorArray::kContentArrayIndex));
-  ASSERT(contents->IsHeapObject());
-  ASSERT(!contents->IsMarked());
-  ASSERT(contents->IsFixedArray());
-  ASSERT(contents->length() >= 2);
+  object->SetMark();
   tracer_->increment_marked_count();
-#ifdef DEBUG
-  UpdateLiveObjectCount(contents);
-#endif
-  contents->SetMark();
-  // Contents contains (value, details) pairs.  If the details say
-  // that the type of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION,
-  // or NULL_DESCRIPTOR, we don't mark the value as live.  Only for
-  // type MAP_TRANSITION is the value a Object* (a Map*).
-  for (int i = 0; i < contents->length(); i += 2) {
-    // If the pair (value, details) at index i, i+1 is not
-    // a transition or null descriptor, mark the value.
-    PropertyDetails details(Smi::cast(contents->get(i + 1)));
-    if (details.type() < FIRST_PHANTOM_PROPERTY_TYPE) {
-      HeapObject* object = reinterpret_cast<HeapObject*>(contents->get(i));
-      if (object->IsHeapObject() && !object->IsMarked()) {
-        tracer_->increment_marked_count();
-#ifdef DEBUG
-        UpdateLiveObjectCount(object);
-#endif
-        object->SetMark();
-        marking_stack.Push(object);
-      }
-    }
-  }
-  // The DescriptorArray descriptors contains a pointer to its contents array,
-  // but the contents array is already marked.
-  marking_stack.Push(descriptors);
-}
-
-
-void MarkCompactCollector::CreateBackPointers() {
-  HeapObjectIterator iterator(Heap::map_space());
-  while (iterator.has_next()) {
-    Object* next_object = iterator.next();
-    if (next_object->IsMap()) {  // Could also be ByteArray on free list.
-      Map* map = Map::cast(next_object);
-      if (map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
-          map->instance_type() <= LAST_JS_OBJECT_TYPE) {
-        map->CreateBackPointers();
-      }
-    }
-  }
+  ASSERT(Heap::Contains(object));
+  marking_stack.Push(object);
 }
 
 
@@ -767,13 +675,6 @@ void MarkCompactCollector::MarkLiveObjects() {
 }
 
 
-static int CountMarkedCallback(HeapObject* obj) {
-  MapWord map_word = obj->map_word();
-  map_word.ClearMark();
-  return obj->SizeFromMap(map_word.ToMap());
-}
-
-
 #ifdef DEBUG
 void MarkCompactCollector::UpdateLiveObjectCount(HeapObject* obj) {
   live_bytes_ += obj->Size();
@@ -796,6 +697,13 @@ void MarkCompactCollector::UpdateLiveObjectCount(HeapObject* obj) {
 }
 
 
+static int CountMarkedCallback(HeapObject* obj) {
+  MapWord map_word = obj->map_word();
+  map_word.ClearMark();
+  return obj->SizeFromMap(map_word.ToMap());
+}
+
+
 void MarkCompactCollector::VerifyHeapAfterMarkingPhase() {
   Heap::new_space()->Verify();
   Heap::old_pointer_space()->Verify();
@@ -847,63 +755,6 @@ void MarkCompactCollector::SweepLargeObjectSpace() {
   Heap::lo_space()->FreeUnmarkedObjects();
 }
 
-// Safe to use during marking phase only.
-bool MarkCompactCollector::SafeIsMap(HeapObject* object) {
-  MapWord metamap = object->map_word();
-  metamap.ClearMark();
-  return metamap.ToMap()->instance_type() == MAP_TYPE;
-}
-
-void MarkCompactCollector::ClearNonLiveTransitions() {
-  HeapObjectIterator map_iterator(Heap::map_space(), &CountMarkedCallback);
-  // Iterate over the map space, setting map transitions that go from
-  // a marked map to an unmarked map to null transitions.  At the same time,
-  // set all the prototype fields of maps back to their original value,
-  // dropping the back pointers temporarily stored in the prototype field.
-  // Setting the prototype field requires following the linked list of
-  // back pointers, reversing them all at once.  This allows us to find
-  // those maps with map transitions that need to be nulled, and only
-  // scan the descriptor arrays of those maps, not all maps.
-  // All of these actions are carried out only on maps of JSObects
-  // and related subtypes.
-  while (map_iterator.has_next()) {
-    Map* map = reinterpret_cast<Map*>(map_iterator.next());
-    if (!map->IsMarked() && map->IsByteArray()) continue;
-
-    ASSERT(SafeIsMap(map));
-    // Only JSObject and subtypes have map transitions and back pointers.
-    if (map->instance_type() < FIRST_JS_OBJECT_TYPE) continue;
-    if (map->instance_type() > LAST_JS_OBJECT_TYPE) continue;
-    // Follow the chain of back pointers to find the prototype.
-    Map* current = map;
-    while (SafeIsMap(current)) {
-      current = reinterpret_cast<Map*>(current->prototype());
-      ASSERT(current->IsHeapObject());
-    }
-    Object* real_prototype = current;
-
-    // Follow back pointers, setting them to prototype,
-    // clearing map transitions when necessary.
-    current = map;
-    bool on_dead_path = !current->IsMarked();
-    Object *next;
-    while (SafeIsMap(current)) {
-      next = current->prototype();
-      // There should never be a dead map above a live map.
-      ASSERT(on_dead_path || current->IsMarked());
-
-      // A live map above a dead map indicates a dead transition.
-      // This test will always be false on the first iteration.
-      if (on_dead_path && current->IsMarked()) {
-        on_dead_path = false;
-        current->ClearNonLiveTransitions(real_prototype);
-      }
-      HeapObject::RawField(current, Map::kPrototypeOffset) =
-          real_prototype;
-      current = reinterpret_cast<Map*>(next);
-    }
-  }
-}
 
 // -------------------------------------------------------------------------
 // Phase 2: Encode forwarding addresses.

src/mark-compact.h

@@ -155,17 +155,6 @@ class MarkCompactCollector : public AllStatic {
      if (!obj->IsMarked()) MarkUnmarkedObject(obj);
   }
 
-  // Creates back pointers for all map transitions, stores them in
-  // the prototype field.  The original prototype pointers are restored
-  // in ClearNonLiveTransitions().  All JSObject maps
-  // connected by map transitions have the same prototype object, which
-  // is why we can use this field temporarily for back pointers.
-  static void CreateBackPointers();
-
-  // Mark a Map and its DescriptorArray together, skipping transitions.
-  static void MarkMapContents(Map* map);
-  static void MarkDescriptorArray(DescriptorArray* descriptors);
-
   // Mark the heap roots and all objects reachable from them.
   static void ProcessRoots(RootMarkingVisitor* visitor);
 
@@ -205,13 +194,6 @@ class MarkCompactCollector : public AllStatic {
   // compacting or not, because the large object space is never compacted.
   static void SweepLargeObjectSpace();
 
-  // Test whether a (possibly marked) object is a Map.
-  static inline bool SafeIsMap(HeapObject* object);
-
-    // Map transitions from a live map to a dead map must be killed.
-  // We replace them with a null descriptor, with the same key.
-  static void ClearNonLiveTransitions();
-
   // --------------------------------------------------------------------------
   // Phase 2: functions related to computing and encoding forwarding pointers
   //   before: live objects' map pointers are marked as '00'

src/objects-inl.h

@@ -552,8 +552,8 @@ Object* Object::GetProperty(String* key, PropertyAttributes* attributes) {
   (*reinterpret_cast<byte*>(FIELD_ADDR(p, offset)) = value)
 
 
-Object*& HeapObject::RawField(HeapObject* obj, int byte_offset) {
-  return READ_FIELD(obj, byte_offset);
+Object* HeapObject::GetHeapObjectField(HeapObject* obj, int index) {
+  return READ_FIELD(obj, HeapObject::kHeaderSize + kPointerSize * index);
 }
 
 

src/objects.cc

@@ -333,7 +333,7 @@ Object* Object::GetProperty(Object* receiver,
       // Check if we're allowed to read from the current object. Note
       // that even though we may not actually end up loading the named
       // property from the current object, we still check that we have
-      // access to it.
+      // access to the it.
       JSObject* checked = JSObject::cast(current);
       if (!Top::MayNamedAccess(checked, name, v8::ACCESS_GET)) {
         return checked->GetPropertyWithFailedAccessCheck(receiver,
@@ -4052,67 +4052,6 @@ void String::PrintOn(FILE* file) {
 }
 
 
-void Map::CreateBackPointers() {
-  DescriptorArray* descriptors = instance_descriptors();
-  for (DescriptorReader r(descriptors); !r.eos(); r.advance()) {
-    if (r.type() == MAP_TRANSITION) {
-      // Get target.
-      Map* target = Map::cast(r.GetValue());
-#ifdef DEBUG
-      // Verify target.
-      Object* source_prototype = prototype();
-      Object* target_prototype = target->prototype();
-      ASSERT(source_prototype->IsJSObject() ||
-             source_prototype->IsMap() ||
-             source_prototype->IsNull());
-      ASSERT(target_prototype->IsJSObject() ||
-             target_prototype->IsNull());
-      ASSERT(source_prototype->IsMap() ||
-          source_prototype == target_prototype);
-#endif
-      // Point target back to source.  set_prototype() will not let us set
-      // the prototype to a map, as we do here.
-      RawField(target, kPrototypeOffset) = this;
-    }
-  }
-}
-
-
-void Map::ClearNonLiveTransitions(Object* real_prototype) {
-  // Live DescriptorArray objects will be marked, so we must use
-  // low-level accessors to get and modify their data.
-  DescriptorArray* d = reinterpret_cast<DescriptorArray*>(
-      RawField(this, Map::kInstanceDescriptorsOffset));
-  if (d == Heap::empty_descriptor_array()) return;
-  Smi* NullDescriptorDetails =
-    PropertyDetails(NONE, NULL_DESCRIPTOR).AsSmi();
-  FixedArray* contents = reinterpret_cast<FixedArray*>(
-      d->get(DescriptorArray::kContentArrayIndex));
-  ASSERT(contents->length() >= 2);
-  for (int i = 0; i < contents->length(); i += 2) {
-    // If the pair (value, details) is a map transition,
-    // check if the target is live.  If not, null the descriptor.
-    // Also drop the back pointer for that map transition, so that this
-    // map is not reached again by following a back pointer from a
-    // non-live object.
-    PropertyDetails details(Smi::cast(contents->get(i + 1)));
-    if (details.type() == MAP_TRANSITION) {
-      Map* target = reinterpret_cast<Map*>(contents->get(i));
-      ASSERT(target->IsHeapObject());
-      if (!target->IsMarked()) {
-        ASSERT(target->IsMap());
-        contents->set(i + 1, NullDescriptorDetails, SKIP_WRITE_BARRIER);
-        contents->set(i, Heap::null_value(), SKIP_WRITE_BARRIER);
-        ASSERT(target->prototype() == this ||
-               target->prototype() == real_prototype);
-        // Getter prototype() is read-only, set_prototype() has side effects.
-        RawField(target, Map::kPrototypeOffset) = real_prototype;
-      }
-    }
-  }
-}
-
-
 void Map::MapIterateBody(ObjectVisitor* v) {
   // Assumes all Object* members are contiguously allocated!
   IteratePointers(v, kPrototypeOffset, kCodeCacheOffset + kPointerSize);

src/objects.h

@@ -1042,11 +1042,7 @@ class HeapObject: public Object {
   // object is overflowed (ie, partially restore the map pointer).
   inline void ClearOverflow();
 
-  // Returns the field at offset in obj, as a read/write Object* reference.
-  // Does no checking, and is safe to use during GC, while maps are invalid.
-  // Does not update remembered sets, so should only be assigned to
-  // during marking GC.
-  static inline Object* &RawField(HeapObject* obj, int offset);
+  static inline Object* GetHeapObjectField(HeapObject* obj, int index);
 
   // Casting.
   static inline HeapObject* cast(Object* obj);
@@ -2426,17 +2422,6 @@ class Map: public HeapObject {
   // Removes a code object from the code cache at the given index.
   void RemoveFromCodeCache(int index);
 
-  // For every transition in this map, makes the transition's
-  // target's prototype pointer point back to this map.
-  // This is undone in MarkCompactCollector::ClearNonLiveTransitions().
-  void CreateBackPointers();
-
-  // Set all map transitions from this map to dead maps to null.
-  // Also, restore the original prototype on the targets of these
-  // transitions, so that we do not process this map again while
-  // following back pointers.
-  void ClearNonLiveTransitions(Object* real_prototype);
-
   // Dispatched behavior.
   void MapIterateBody(ObjectVisitor* v);
 #ifdef DEBUG
@@ -2821,7 +2806,7 @@ class GlobalObject: public JSObject {
   // [builtins]: the object holding the runtime routines written in JS.
   DECL_ACCESSORS(builtins, JSBuiltinsObject)
 
-  // [global context]: the global context corresponding to this global object.
+  // [global context]: the global context corresponding to this global objet.
   DECL_ACCESSORS(global_context, Context)
 
   // [global receiver]: the global receiver object of the context

src/property.h

@@ -99,10 +99,7 @@ class Descriptor BASE_EMBEDDED {
   friend class DescriptorArray;
 };
 
-// A pointer from a map to the new map that is created by adding
-// a named property.  These are key to the speed and functioning of V8.
-// The two maps should always have the same prototype, since
-// MapSpace::CreateBackPointers depends on this.
+
 class MapTransitionDescriptor: public Descriptor {
  public:
   MapTransitionDescriptor(String* key, Map* map, PropertyAttributes attributes)

src/runtime.cc

@@ -320,19 +320,9 @@ static Object* Runtime_IsTemplate(Arguments args) {
 static Object* Runtime_GetTemplateField(Arguments args) {
   ASSERT(args.length() == 2);
   CONVERT_CHECKED(HeapObject, templ, args[0]);
+  RUNTIME_ASSERT(templ->IsStruct());
   CONVERT_CHECKED(Smi, field, args[1]);
-  int index = field->value();
-  int offset = index * kPointerSize + HeapObject::kHeaderSize;
-  InstanceType type = templ->map()->instance_type();
-  RUNTIME_ASSERT(type ==  FUNCTION_TEMPLATE_INFO_TYPE ||
-                 type ==  OBJECT_TEMPLATE_INFO_TYPE);
-  RUNTIME_ASSERT(offset > 0);
-  if (type ==  FUNCTION_TEMPLATE_INFO_TYPE) {
-    RUNTIME_ASSERT(offset < FunctionTemplateInfo::kSize);
-  } else {
-    RUNTIME_ASSERT(offset < ObjectTemplateInfo::kSize);
-  }
-  return HeapObject::RawField(templ, offset);
+  return HeapObject::GetHeapObjectField(templ, field->value());
 }