Remove unused maps during marking garbage collections.

Review URL: http://codereview.chromium.org/8831

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@637 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/flag-definitions.h

@@ -134,6 +134,8 @@ 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

@@ -78,6 +78,8 @@ void MarkCompactCollector::CollectGarbage(GCTracer* tracer) {
 
   MarkLiveObjects();
 
+  if (FLAG_collect_maps) ClearNonLiveTransitions();
+
   SweepLargeObjectSpace();
 
   if (compacting_collection_) {
@@ -135,6 +137,7 @@ void MarkCompactCollector::Prepare() {
   }
 
   if (FLAG_never_compact) compacting_collection_ = false;
+  if (FLAG_collect_maps) CreateBackPointers();
 
 #ifdef DEBUG
   if (compacting_collection_) {
@@ -322,9 +325,10 @@ class MarkingVisitor : public ObjectVisitor {
 
   // Visit an unmarked object.
   void VisitUnmarkedObject(HeapObject* obj) {
-    ASSERT(Heap::Contains(obj));
 #ifdef DEBUG
+    ASSERT(Heap::Contains(obj));
     MarkCompactCollector::UpdateLiveObjectCount(obj);
+    ASSERT(!obj->IsMarked());
 #endif
     Map* map = obj->map();
     obj->SetMark();
@@ -425,14 +429,104 @@ void MarkCompactCollector::MarkUnmarkedObject(HeapObject* object) {
   ASSERT(!object->IsMarked());
   if (object->IsJSGlobalObject()) Counters::global_objects.Increment();
 
-  if (FLAG_cleanup_caches_in_maps_at_gc && object->IsMap()) {
-    Map::cast(object)->ClearCodeCache();
+  tracer_->increment_marked_count();
+  ASSERT(Heap::Contains(object));
+  if (object->IsMap()) {
+    Map* map = Map::cast(object);
+    if (FLAG_cleanup_caches_in_maps_at_gc) {
+      map->ClearCodeCache();
+    }
+    map->SetMark();
+    if (FLAG_collect_maps &&
+        map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
+        map->instance_type() <= JS_FUNCTION_TYPE) {
+      MarkMapContents(map);
+    } else {
+      marking_stack.Push(map);
     }
-
+  } else {
     object->SetMark();
+    marking_stack.Push(object);
+  }
+}
+
+
+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();
-  ASSERT(Heap::Contains(object));
+#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);
+  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() <= JS_FUNCTION_TYPE) {
+        map->CreateBackPointers();
+      } else {
+        ASSERT(map->instance_descriptors() == Heap::empty_descriptor_array());
+      }
+    }
+  }
 }
 
 
@@ -675,6 +769,13 @@ 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();
@@ -697,13 +798,6 @@ 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();
@@ -755,6 +849,63 @@ 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() > JS_FUNCTION_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

@@ -155,6 +155,17 @@ 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);
 
@@ -194,6 +205,13 @@ 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

@@ -557,8 +557,8 @@ Object* Object::GetProperty(String* key, PropertyAttributes* attributes) {
   (*reinterpret_cast<byte*>(FIELD_ADDR(p, offset)) = value)
 
 
-Object* HeapObject::GetHeapObjectField(HeapObject* obj, int index) {
-  return READ_FIELD(obj, HeapObject::kHeaderSize + kPointerSize * index);
+Object** HeapObject::RawField(HeapObject* obj, int byte_offset) {
+  return &READ_FIELD(obj, byte_offset);
 }
 
 

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 the it.
+      // access to it.
       JSObject* checked = JSObject::cast(current);
       if (!Top::MayNamedAccess(checked, name, v8::ACCESS_GET)) {
         return checked->GetPropertyWithFailedAccessCheck(receiver,
@@ -4052,6 +4052,67 @@ 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

@@ -1043,7 +1043,11 @@ class HeapObject: public Object {
   // object is overflowed (ie, partially restore the map pointer).
   inline void ClearOverflow();
 
-  static inline Object* GetHeapObjectField(HeapObject* obj, int index);
+  // 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);
 
   // Casting.
   static inline HeapObject* cast(Object* obj);
@@ -2420,6 +2424,17 @@ 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
@@ -2804,7 +2819,7 @@ 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 objet.
+  // [global context]: the global context corresponding to this global object.
   DECL_ACCESSORS(global_context, Context)
 
   // [global receiver]: the global receiver object of the context

src/property.h

@@ -99,7 +99,10 @@ 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

@@ -320,9 +320,19 @@ 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]);
-  return HeapObject::GetHeapObjectField(templ, field->value());
+  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);
 }