Prototype of mark-and-compact support for Harmony weak maps.

R=vegorov@chromium.org
BUG=v8:1565
TEST=cctest/test-weakmaps

Review URL: http://codereview.chromium.org/7553012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8817 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -1292,6 +1292,10 @@ class ScavengingVisitor : public StaticVisitorBase {
                     &ObjectEvacuationStrategy<POINTER_OBJECT>::
                         template VisitSpecialized<SharedFunctionInfo::kSize>);
 
+    table_.Register(kVisitJSWeakMap,
+                    &ObjectEvacuationStrategy<POINTER_OBJECT>::
+                    Visit);
+
     table_.Register(kVisitJSRegExp,
                     &ObjectEvacuationStrategy<POINTER_OBJECT>::
                     Visit);

src/heap.h

@@ -1646,6 +1646,7 @@ class Heap {
   friend class Page;
   friend class Isolate;
   friend class MarkCompactCollector;
+  friend class StaticMarkingVisitor;
   friend class MapCompact;
 
   DISALLOW_COPY_AND_ASSIGN(Heap);

src/mark-compact.cc

@@ -64,13 +64,15 @@ MarkCompactCollector::MarkCompactCollector() :  // NOLINT
       live_bytes_(0),
 #endif
       heap_(NULL),
-      code_flusher_(NULL) { }
+      code_flusher_(NULL),
+      encountered_weak_maps_(NULL) { }
 
 
 void MarkCompactCollector::CollectGarbage() {
   // Make sure that Prepare() has been called. The individual steps below will
   // update the state as they proceed.
   ASSERT(state_ == PREPARE_GC);
+  ASSERT(encountered_weak_maps_ == Smi::FromInt(0));
 
   // Prepare has selected whether to compact the old generation or not.
   // Tell the tracer.
@@ -80,6 +82,8 @@ void MarkCompactCollector::CollectGarbage() {
 
   if (FLAG_collect_maps) ClearNonLiveTransitions();
 
+  ClearWeakMaps();
+
   SweepLargeObjectSpace();
 
   if (IsCompacting()) {
@@ -407,6 +411,8 @@ class StaticMarkingVisitor : public StaticVisitorBase {
     table_.Register(kVisitSeqAsciiString, &DataObjectVisitor::Visit);
     table_.Register(kVisitSeqTwoByteString, &DataObjectVisitor::Visit);
 
+    table_.Register(kVisitJSWeakMap, &VisitJSWeakMap);
+
     table_.Register(kVisitOddball,
                     &FixedBodyVisitor<StaticMarkingVisitor,
                                       Oddball::BodyDescriptor,
@@ -556,6 +562,34 @@ class StaticMarkingVisitor : public StaticVisitorBase {
                               StructBodyDescriptor,
                               void> StructObjectVisitor;
 
+  static void VisitJSWeakMap(Map* map, HeapObject* object) {
+    MarkCompactCollector* collector = map->heap()->mark_compact_collector();
+    JSWeakMap* weak_map = reinterpret_cast<JSWeakMap*>(object);
+
+    // Enqueue weak map in linked list of encountered weak maps.
+    ASSERT(weak_map->next() == Smi::FromInt(0));
+    weak_map->set_next(collector->encountered_weak_maps());
+    collector->set_encountered_weak_maps(weak_map);
+
+    // Skip visiting the backing hash table containing the mappings.
+    int object_size = JSWeakMap::BodyDescriptor::SizeOf(map, object);
+    BodyVisitorBase<StaticMarkingVisitor>::IteratePointers(
+        map->heap(),
+        object,
+        JSWeakMap::BodyDescriptor::kStartOffset,
+        JSWeakMap::kTableOffset);
+    BodyVisitorBase<StaticMarkingVisitor>::IteratePointers(
+        map->heap(),
+        object,
+        JSWeakMap::kTableOffset + kPointerSize,
+        object_size);
+
+    // Mark the backing hash table without pushing it on the marking stack.
+    ASSERT(!weak_map->unchecked_table()->IsMarked());
+    ASSERT(weak_map->unchecked_table()->map()->IsMarked());
+    collector->SetMark(weak_map->unchecked_table());
+  }
+
   static void VisitCode(Map* map, HeapObject* object) {
     reinterpret_cast<Code*>(object)->CodeIterateBody<StaticMarkingVisitor>(
         map->heap());
@@ -1369,20 +1403,26 @@ void MarkCompactCollector::MarkImplicitRefGroups() {
 // marking stack have been marked, or are overflowed in the heap.
 void MarkCompactCollector::EmptyMarkingStack() {
   while (!marking_stack_.is_empty()) {
-    HeapObject* object = marking_stack_.Pop();
-    ASSERT(object->IsHeapObject());
-    ASSERT(heap()->Contains(object));
-    ASSERT(object->IsMarked());
-    ASSERT(!object->IsOverflowed());
-
-    // Because the object is marked, we have to recover the original map
-    // pointer and use it to mark the object's body.
-    MapWord map_word = object->map_word();
-    map_word.ClearMark();
-    Map* map = map_word.ToMap();
-    MarkObject(map);
+    while (!marking_stack_.is_empty()) {
+      HeapObject* object = marking_stack_.Pop();
+      ASSERT(object->IsHeapObject());
+      ASSERT(heap()->Contains(object));
+      ASSERT(object->IsMarked());
+      ASSERT(!object->IsOverflowed());
 
-    StaticMarkingVisitor::IterateBody(map, object);
+      // Because the object is marked, we have to recover the original map
+      // pointer and use it to mark the object's body.
+      MapWord map_word = object->map_word();
+      map_word.ClearMark();
+      Map* map = map_word.ToMap();
+      MarkObject(map);
+
+      StaticMarkingVisitor::IterateBody(map, object);
+    }
+
+    // Process encountered weak maps, mark objects only reachable by those
+    // weak maps and repeat until fix-point is reached.
+    ProcessWeakMaps();
   }
 }
 
@@ -1735,6 +1775,45 @@ void MarkCompactCollector::ClearNonLiveTransitions() {
   }
 }
 
+
+void MarkCompactCollector::ProcessWeakMaps() {
+  Object* weak_map_obj = encountered_weak_maps();
+  while (weak_map_obj != Smi::FromInt(0)) {
+    ASSERT(HeapObject::cast(weak_map_obj)->IsMarked());
+    JSWeakMap* weak_map = reinterpret_cast<JSWeakMap*>(weak_map_obj);
+    ObjectHashTable* table = weak_map->unchecked_table();
+    for (int i = 0; i < table->Capacity(); i++) {
+      if (HeapObject::cast(table->KeyAt(i))->IsMarked()) {
+        Object* value = table->get(table->EntryToValueIndex(i));
+        StaticMarkingVisitor::MarkObjectByPointer(heap(), &value);
+        table->set_unchecked(heap(),
+                             table->EntryToValueIndex(i),
+                             value,
+                             UPDATE_WRITE_BARRIER);
+      }
+    }
+    weak_map_obj = weak_map->next();
+  }
+}
+
+
+void MarkCompactCollector::ClearWeakMaps() {
+  Object* weak_map_obj = encountered_weak_maps();
+  while (weak_map_obj != Smi::FromInt(0)) {
+    ASSERT(HeapObject::cast(weak_map_obj)->IsMarked());
+    JSWeakMap* weak_map = reinterpret_cast<JSWeakMap*>(weak_map_obj);
+    ObjectHashTable* table = weak_map->unchecked_table();
+    for (int i = 0; i < table->Capacity(); i++) {
+      if (!HeapObject::cast(table->KeyAt(i))->IsMarked()) {
+        table->RemoveEntry(i, heap());
+      }
+    }
+    weak_map_obj = weak_map->next();
+    weak_map->set_next(Smi::FromInt(0));
+  }
+  set_encountered_weak_maps(Smi::FromInt(0));
+}
+
 // -------------------------------------------------------------------------
 // Phase 2: Encode forwarding addresses.
 // When compacting, forwarding addresses for objects in old space and map

src/mark-compact.h

@@ -193,6 +193,11 @@ class MarkCompactCollector {
   inline bool is_code_flushing_enabled() const { return code_flusher_ != NULL; }
   void EnableCodeFlushing(bool enable);
 
+  inline Object* encountered_weak_maps() { return encountered_weak_maps_; }
+  inline void set_encountered_weak_maps(Object* weak_map) {
+    encountered_weak_maps_ = weak_map;
+  }
+
  private:
   MarkCompactCollector();
   ~MarkCompactCollector();
@@ -329,6 +334,16 @@ class MarkCompactCollector {
   // We replace them with a null descriptor, with the same key.
   void ClearNonLiveTransitions();
 
+  // Mark all values associated with reachable keys in weak maps encountered
+  // so far.  This might push new object or even new weak maps onto the
+  // marking stack.
+  void ProcessWeakMaps();
+
+  // After all reachable objects have been marked those weak map entries
+  // with an unreachable key are removed from all encountered weak maps.
+  // The linked list of all encountered weak maps is destroyed.
+  void ClearWeakMaps();
+
   // -----------------------------------------------------------------------
   // Phase 2: Sweeping to clear mark bits and free non-live objects for
   // a non-compacting collection, or else computing and encoding
@@ -499,6 +514,7 @@ class MarkCompactCollector {
   Heap* heap_;
   MarkingStack marking_stack_;
   CodeFlusher* code_flusher_;
+  Object* encountered_weak_maps_;
 
   friend class Heap;
   friend class OverflowedObjectsScanner;

src/objects-inl.h

@@ -3861,6 +3861,12 @@ ACCESSORS(JSProxy, padding, Object, kPaddingOffset)
 
 
 ACCESSORS(JSWeakMap, table, ObjectHashTable, kTableOffset)
+ACCESSORS_GCSAFE(JSWeakMap, next, Object, kNextOffset)
+
+
+ObjectHashTable* JSWeakMap::unchecked_table() {
+  return reinterpret_cast<ObjectHashTable*>(READ_FIELD(this, kTableOffset));
+}
 
 
 Address Foreign::address() {
@@ -4471,6 +4477,11 @@ MaybeObject* ObjectHashTableShape::AsObject(JSObject* key) {
 }
 
 
+void ObjectHashTable::RemoveEntry(int entry) {
+  RemoveEntry(entry, GetHeap());
+}
+
+
 void Map::ClearCodeCache(Heap* heap) {
   // No write barrier is needed since empty_fixed_array is not in new space.
   // Please note this function is used during marking:

src/objects-visiting.cc

@@ -88,6 +88,9 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
     case JS_GLOBAL_PROPERTY_CELL_TYPE:
       return kVisitPropertyCell;
 
+    case JS_WEAK_MAP_TYPE:
+      return kVisitJSWeakMap;
+
     case JS_REGEXP_TYPE:
       return kVisitJSRegExp;
 
@@ -115,7 +118,6 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
     case JS_GLOBAL_OBJECT_TYPE:
     case JS_BUILTINS_OBJECT_TYPE:
     case JS_MESSAGE_OBJECT_TYPE:
-    case JS_WEAK_MAP_TYPE:
       return GetVisitorIdForSize(kVisitJSObject,
                                  kVisitJSObjectGeneric,
                                  instance_size);

src/objects-visiting.h

@@ -121,6 +121,7 @@ class StaticVisitorBase : public AllStatic {
     kVisitPropertyCell,
     kVisitSharedFunctionInfo,
     kVisitJSFunction,
+    kVisitJSWeakMap,
     kVisitJSRegExp,
 
     kVisitorIdCount,
@@ -317,7 +318,9 @@ class StaticNewSpaceVisitor : public StaticVisitorBase {
                                       SharedFunctionInfo::BodyDescriptor,
                                       int>::Visit);
 
-    table_.Register(kVisitJSRegExp, &VisitJSRegExp);
+    table_.Register(kVisitJSWeakMap, &VisitJSObject);
+
+    table_.Register(kVisitJSRegExp, &VisitJSObject);
 
     table_.Register(kVisitSeqAsciiString, &VisitSeqAsciiString);
 
@@ -356,15 +359,15 @@ class StaticNewSpaceVisitor : public StaticVisitorBase {
     return FixedDoubleArray::SizeFor(length);
   }
 
+  static inline int VisitJSObject(Map* map, HeapObject* object) {
+    return JSObjectVisitor::Visit(map, object);
+  }
+
   static inline int VisitSeqAsciiString(Map* map, HeapObject* object) {
     return SeqAsciiString::cast(object)->
         SeqAsciiStringSize(map->instance_type());
   }
 
-  static inline int VisitJSRegExp(Map* map, HeapObject* object) {
-    return JSObjectVisitor::Visit(map, object);
-  }
-
   static inline int VisitSeqTwoByteString(Map* map, HeapObject* object) {
     return SeqTwoByteString::cast(object)->
         SeqTwoByteStringSize(map->instance_type());

src/objects.cc

@@ -11640,10 +11640,9 @@ void ObjectHashTable::AddEntry(int entry, JSObject* key, Object* value) {
 }
 
 
-void ObjectHashTable::RemoveEntry(int entry) {
-  Object* null_value = GetHeap()->null_value();
-  set(EntryToIndex(entry), null_value);
-  set(EntryToIndex(entry) + 1, null_value);
+void ObjectHashTable::RemoveEntry(int entry, Heap* heap) {
+  set_null(heap, EntryToIndex(entry));
+  set_null(heap, EntryToIndex(entry) + 1);
   ElementRemoved();
 }
 

src/objects.h

@@ -2984,8 +2984,16 @@ class ObjectHashTable: public HashTable<ObjectHashTableShape, JSObject*> {
   MUST_USE_RESULT MaybeObject* Put(JSObject* key, Object* value);
 
  private:
+  friend class MarkCompactCollector;
+
   void AddEntry(int entry, JSObject* key, Object* value);
-  void RemoveEntry(int entry);
+  void RemoveEntry(int entry, Heap* heap);
+  inline void RemoveEntry(int entry);
+
+  // Returns the index to the value of an entry.
+  static inline int EntryToValueIndex(int entry) {
+    return EntryToIndex(entry) + 1;
+  }
 };
 
 
@@ -6642,6 +6650,12 @@ class JSWeakMap: public JSObject {
   // [table]: the backing hash table mapping keys to values.
   DECL_ACCESSORS(table, ObjectHashTable)
 
+  // [next]: linked list of encountered weak maps during GC.
+  DECL_ACCESSORS(next, Object)
+
+  // Unchecked accessors to be used during GC.
+  inline ObjectHashTable* unchecked_table();
+
   // Casting.
   static inline JSWeakMap* cast(Object* obj);
 
@@ -6656,7 +6670,8 @@ class JSWeakMap: public JSObject {
 #endif
 
   static const int kTableOffset = JSObject::kHeaderSize;
-  static const int kSize = kTableOffset + kPointerSize;
+  static const int kNextOffset = kTableOffset + kPointerSize;
+  static const int kSize = kNextOffset + kPointerSize;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSWeakMap);

src/runtime.cc

@@ -642,6 +642,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_WeakMapInitialize) {
   ASSERT(weakmap->map()->inobject_properties() == 0);
   Handle<ObjectHashTable> table = isolate->factory()->NewObjectHashTable(0);
   weakmap->set_table(*table);
+  weakmap->set_next(Smi::FromInt(0));
   return *weakmap;
 }
 

test/cctest/SConscript

@@ -96,7 +96,8 @@ SOURCES = {
     'test-threads.cc',
     'test-unbound-queue.cc',
     'test-utils.cc',
-    'test-version.cc'
+    'test-version.cc',
+    'test-weakmaps.cc'
   ],
   'arch:arm':  [
     'test-assembler-arm.cc',

test/cctest/cctest.status

@@ -38,6 +38,9 @@ test-api/ApplyInterruption: PASS || TIMEOUT
 test-serialize/TestThatAlwaysFails: FAIL
 test-serialize/DependentTestThatAlwaysFails: FAIL
 
+# We do not yet shrink weak maps after they have been emptied by the GC
+test-weakmaps/Shrinking: FAIL
+
 ##############################################################################
 [ $arch == arm ]
 

test/cctest/test-weakmaps.cc

+// Copyright 2011 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "v8.h"
+
+#include "global-handles.h"
+#include "snapshot.h"
+#include "cctest.h"
+
+using namespace v8::internal;
+
+
+static Handle<JSWeakMap> AllocateJSWeakMap() {
+  Handle<Map> map = FACTORY->NewMap(JS_WEAK_MAP_TYPE, JSWeakMap::kSize);
+  Handle<JSObject> weakmap_obj = FACTORY->NewJSObjectFromMap(map);
+  Handle<JSWeakMap> weakmap(JSWeakMap::cast(*weakmap_obj));
+  // Do not use handles for the hash table, it would make entries strong.
+  Object* table_obj = ObjectHashTable::Allocate(1)->ToObjectChecked();
+  ObjectHashTable* table = ObjectHashTable::cast(table_obj);
+  weakmap->set_table(table);
+  weakmap->set_next(Smi::FromInt(0));
+  return weakmap;
+}
+
+static void PutIntoWeakMap(Handle<JSWeakMap> weakmap,
+                           Handle<JSObject> key,
+                           int value) {
+  Handle<ObjectHashTable> table = PutIntoObjectHashTable(
+      Handle<ObjectHashTable>(weakmap->table()),
+      Handle<JSObject>(JSObject::cast(*key)),
+      Handle<Smi>(Smi::FromInt(value)));
+  weakmap->set_table(*table);
+}
+
+static int NumberOfWeakCalls = 0;
+static void WeakPointerCallback(v8::Persistent<v8::Value> handle, void* id) {
+  ASSERT(id == reinterpret_cast<void*>(1234));
+  NumberOfWeakCalls++;
+  handle.Dispose();
+}
+
+
+TEST(Weakness) {
+  LocalContext context;
+  v8::HandleScope scope;
+  Handle<JSWeakMap> weakmap = AllocateJSWeakMap();
+  GlobalHandles* global_handles = Isolate::Current()->global_handles();
+
+  // Keep global reference to the key.
+  Handle<Object> key;
+  {
+    v8::HandleScope scope;
+    Handle<Map> map = FACTORY->NewMap(JS_OBJECT_TYPE, JSObject::kHeaderSize);
+    Handle<JSObject> object = FACTORY->NewJSObjectFromMap(map);
+    key = global_handles->Create(*object);
+  }
+  CHECK(!global_handles->IsWeak(key.location()));
+
+  // Put entry into weak map.
+  {
+    v8::HandleScope scope;
+    PutIntoWeakMap(weakmap, Handle<JSObject>(JSObject::cast(*key)), 23);
+  }
+  CHECK_EQ(1, weakmap->table()->NumberOfElements());
+
+  // Force a full GC.
+  HEAP->CollectAllGarbage(false);
+  CHECK_EQ(0, NumberOfWeakCalls);
+  CHECK_EQ(1, weakmap->table()->NumberOfElements());
+  CHECK_EQ(0, weakmap->table()->NumberOfDeletedElements());
+
+  // Make the global reference to the key weak.
+  {
+    v8::HandleScope scope;
+    global_handles->MakeWeak(key.location(),
+                             reinterpret_cast<void*>(1234),
+                             &WeakPointerCallback);
+  }
+  CHECK(global_handles->IsWeak(key.location()));
+
+  // Force a full GC.
+  // Perform two consecutive GCs because the first one will only clear
+  // weak references whereas the second one will also clear weak maps.
+  HEAP->CollectAllGarbage(false);
+  CHECK_EQ(1, NumberOfWeakCalls);
+  CHECK_EQ(1, weakmap->table()->NumberOfElements());
+  CHECK_EQ(0, weakmap->table()->NumberOfDeletedElements());
+  HEAP->CollectAllGarbage(false);
+  CHECK_EQ(1, NumberOfWeakCalls);
+  CHECK_EQ(0, weakmap->table()->NumberOfElements());
+  CHECK_EQ(1, weakmap->table()->NumberOfDeletedElements());
+}
+
+
+TEST(Shrinking) {
+  LocalContext context;
+  v8::HandleScope scope;
+  Handle<JSWeakMap> weakmap = AllocateJSWeakMap();
+
+  // Check initial capacity.
+  CHECK_EQ(32, weakmap->table()->Capacity());
+
+  // Fill up weak map to trigger capacity change.
+  {
+    v8::HandleScope scope;
+    Handle<Map> map = FACTORY->NewMap(JS_OBJECT_TYPE, JSObject::kHeaderSize);
+    for (int i = 0; i < 32; i++) {
+      Handle<JSObject> object = FACTORY->NewJSObjectFromMap(map);
+      PutIntoWeakMap(weakmap, object, i);
+    }
+  }
+
+  // Check increased capacity.
+  CHECK_EQ(128, weakmap->table()->Capacity());
+
+  // Force a full GC.
+  CHECK_EQ(32, weakmap->table()->NumberOfElements());
+  CHECK_EQ(0, weakmap->table()->NumberOfDeletedElements());
+  HEAP->CollectAllGarbage(false);
+  CHECK_EQ(0, weakmap->table()->NumberOfElements());
+  CHECK_EQ(32, weakmap->table()->NumberOfDeletedElements());
+
+  // Check shrunk capacity.
+  CHECK_EQ(32, weakmap->table()->Capacity());
+}

test/mjsunit/harmony/weakmaps.js

@@ -112,3 +112,9 @@ var m = new WeakMap;
 assertTrue(m instanceof WeakMap);
 assertTrue(WeakMap.prototype.set instanceof Function)
 assertTrue(WeakMap.prototype.get instanceof Function)
+
+
+// Stress Test
+// There is a proposed stress-test available at the es-discuss mailing list
+// which cannot be reasonably automated.  Check it out by hand if you like:
+// https://mail.mozilla.org/pipermail/es-discuss/2011-May/014096.html