Prevent modification of cached normalized maps.

Finally sovles the problem that r5342 attempted to solve.
When adding a stub to a map's code cache we need to make
sure that this map is not used by object that do not need
this stub.

Existing solution had 2 flaws:
1. It checked that the map is cached by asking the current context.
If the object escaped into another context then NormalizedMapCache::Contains
returns false negative.

2. If a map gets evicted from the cache we should not try to modify it
even though Contains returns false.

This patch implements much less fragile solution of the same problem:
A map now has a flag (is_shared) that is set once the map is added
to a cache, stays set even after the cache eviction, and is cleared
if the object goes back to fast mode.

Added a regression test.

Review URL: http://codereview.chromium.org/3472006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5518 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 6d49d75..078b23f 100644 (file)
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -649,8 +649,9 @@ void Map::MapVerify() {
 }
 
 
-void Map::NormalizedMapVerify() {
+void Map::SharedMapVerify() {
   MapVerify();
+  ASSERT(is_shared());
   ASSERT_EQ(Heap::empty_descriptor_array(), instance_descriptors());
   ASSERT_EQ(Heap::empty_fixed_array(), code_cache());
   ASSERT_EQ(0, pre_allocated_property_fields());
@@ -1381,7 +1382,7 @@ void NormalizedMapCache::NormalizedMapCacheVerify() {
     for (int i = 0; i < length(); i++) {
       Object* e = get(i);
       if (e->IsMap()) {
-        Map::cast(e)->NormalizedMapVerify();
+        Map::cast(e)->SharedMapVerify();
       } else {
         ASSERT(e->IsUndefined());
       }

diff --git a/src/objects-inl.h b/src/objects-inl.h
index 93f752f..2bbdeaa 100644 (file)
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2292,6 +2292,19 @@ bool Map::attached_to_shared_function_info() {
 }
 
 
+void Map::set_is_shared(bool value) {
+  if (value) {
+    set_bit_field2(bit_field2() | (1 << kIsShared));
+  } else {
+    set_bit_field2(bit_field2() & ~(1 << kIsShared));
+  }
+}
+
+bool Map::is_shared() {
+  return ((1 << kIsShared) & bit_field2()) != 0;
+}
+
+
 JSFunction* Map::unchecked_constructor() {
   return reinterpret_cast<JSFunction*>(READ_FIELD(this, kConstructorOffset));
 }

diff --git a/src/objects.cc b/src/objects.cc
index 3ab6ef5..737bf57 100644 (file)
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2099,61 +2099,34 @@ PropertyAttributes JSObject::GetLocalPropertyAttribute(String* name) {
 }
 
 
-bool NormalizedMapCache::IsCacheable(JSObject* object) {
-  // Caching for global objects is not worth it (there are too few of them).
-  return !object->IsGlobalObject();
-}
-
-
 Object* NormalizedMapCache::Get(JSObject* obj, PropertyNormalizationMode mode) {
-  Object* result;
-
   Map* fast = obj->map();
-  if (!IsCacheable(obj)) {
-    result = fast->CopyNormalized(mode);
-    if (result->IsFailure()) return result;
-  } else {
-    int index = Hash(fast) % kEntries;
-    result = get(index);
-
-    if (result->IsMap() && CheckHit(Map::cast(result), fast, mode)) {
+  int index = Hash(fast) % kEntries;
+  Object* result = get(index);
+  if (result->IsMap() && CheckHit(Map::cast(result), fast, mode)) {
 #ifdef DEBUG
-      if (FLAG_enable_slow_asserts) {
-        // Make sure that the new slow map has exactly the same hash as the
-        // original fast map. This way we can use hash to check if a slow map
-        // is already in the hash (see Contains method).
-        ASSERT(Hash(fast) == Hash(Map::cast(result)));
-        // The cached map should match newly created normalized map bit-by-bit.
-        Object* fresh = fast->CopyNormalized(mode);
-        if (!fresh->IsFailure()) {
-          ASSERT(memcmp(Map::cast(fresh)->address(),
-                        Map::cast(result)->address(),
-                        Map::kSize) == 0);
-        }
+    if (FLAG_enable_slow_asserts) {
+      // The cached map should match newly created normalized map bit-by-bit.
+      Object* fresh = fast->CopyNormalized(mode, SHARED_NORMALIZED_MAP);
+      if (!fresh->IsFailure()) {
+        ASSERT(memcmp(Map::cast(fresh)->address(),
+                      Map::cast(result)->address(),
+                      Map::kSize) == 0);
       }
-#endif
-      return result;
     }
-
-    result = fast->CopyNormalized(mode);
-    if (result->IsFailure()) return result;
-    set(index, result);
+#endif
+    return result;
   }
+
+  result = fast->CopyNormalized(mode, SHARED_NORMALIZED_MAP);
+  if (result->IsFailure()) return result;
+  set(index, result);
   Counters::normalized_maps.Increment();
 
   return result;
 }
 
 
-bool NormalizedMapCache::Contains(Map* map) {
-  // If the map is present in the cache it can only be at one place:
-  // at the index calculated from the hash. We assume that a slow map has the
-  // same hash as a fast map it has been generated from.
-  int index = Hash(map) % kEntries;
-  return get(index) == map;
-}
-
-
 void NormalizedMapCache::Clear() {
   int entries = length();
   for (int i = 0; i != entries; i++) {
@@ -2184,7 +2157,7 @@ bool NormalizedMapCache::CheckHit(Map* slow,
                                   Map* fast,
                                   PropertyNormalizationMode mode) {
 #ifdef DEBUG
-  slow->NormalizedMapVerify();
+  slow->SharedMapVerify();
 #endif
   return
     slow->constructor() == fast->constructor() &&
@@ -2194,17 +2167,17 @@ bool NormalizedMapCache::CheckHit(Map* slow,
                                     fast->inobject_properties()) &&
     slow->instance_type() == fast->instance_type() &&
     slow->bit_field() == fast->bit_field() &&
-    slow->bit_field2() == fast->bit_field2();
+    (slow->bit_field2() & ~(1<<Map::kIsShared)) == fast->bit_field2();
 }
 
 
 Object* JSObject::UpdateMapCodeCache(String* name, Code* code) {
-  if (!HasFastProperties() &&
-      NormalizedMapCache::IsCacheable(this) &&
-      Top::context()->global_context()->normalized_map_cache()->
-          Contains(map())) {
-    // Replace the map with the identical copy that can be safely modified.
-    Object* obj = map()->CopyNormalized(KEEP_INOBJECT_PROPERTIES);
+  if (map()->is_shared()) {
+    // Fast case maps are never marked as shared.
+    ASSERT(!HasFastProperties());
+    // Replace the map with an identical copy that can be safely modified.
+    Object* obj = map()->CopyNormalized(KEEP_INOBJECT_PROPERTIES,
+                                        UNIQUE_NORMALIZED_MAP);
     if (obj->IsFailure()) return obj;
     Counters::normalized_maps.Increment();
 
@@ -3189,12 +3162,14 @@ Object* Map::CopyDropDescriptors() {
   }
   Map::cast(result)->set_bit_field(bit_field());
   Map::cast(result)->set_bit_field2(bit_field2());
+  Map::cast(result)->set_is_shared(false);
   Map::cast(result)->ClearCodeCache();
   return result;
 }
 
 
-Object* Map::CopyNormalized(PropertyNormalizationMode mode) {
+Object* Map::CopyNormalized(PropertyNormalizationMode mode,
+                            NormalizedMapSharingMode sharing) {
   int new_instance_size = instance_size();
   if (mode == CLEAR_INOBJECT_PROPERTIES) {
     new_instance_size -= inobject_properties() * kPointerSize;
@@ -3213,8 +3188,12 @@ Object* Map::CopyNormalized(PropertyNormalizationMode mode) {
   Map::cast(result)->set_bit_field(bit_field());
   Map::cast(result)->set_bit_field2(bit_field2());
 
+  Map::cast(result)->set_is_shared(sharing == SHARED_NORMALIZED_MAP);
+
 #ifdef DEBUG
-  Map::cast(result)->NormalizedMapVerify();
+  if (Map::cast(result)->is_shared()) {
+    Map::cast(result)->SharedMapVerify();
+  }
 #endif
 
   return result;

diff --git a/src/objects.h b/src/objects.h
index 6c4af25..307f9cd 100644 (file)
--- a/src/objects.h
+++ b/src/objects.h
@@ -200,6 +200,14 @@ enum PropertyNormalizationMode {
 };
 
 
+// NormalizedMapSharingMode is used to specify whether a map may be shared
+// by different objects with normalized properties.
+enum NormalizedMapSharingMode {
+  UNIQUE_NORMALIZED_MAP,
+  SHARED_NORMALIZED_MAP
+};
+
+
 // Instance size sentinel for objects of variable size.
 static const int kVariableSizeSentinel = 0;
 
@@ -2509,12 +2517,8 @@ class NormalizedMapCache: public FixedArray {
  public:
   static const int kEntries = 64;
 
-  static bool IsCacheable(JSObject* object);
-
   Object* Get(JSObject* object, PropertyNormalizationMode mode);
 
-  bool Contains(Map* map);
-
   void Clear();
 
   // Casting
@@ -3176,6 +3180,13 @@ class Map: public HeapObject {
 
   inline bool attached_to_shared_function_info();
 
+  // Tells whether the map is shared between objects that may have different
+  // behavior. If true, the map should never be modified, instead a clone
+  // should be created and modified.
+  inline void set_is_shared(bool value);
+
+  inline bool is_shared();
+
   // Tells whether the instance needs security checks when accessing its
   // properties.
   inline void set_is_access_check_needed(bool access_check_needed);
@@ -3197,7 +3208,8 @@ class Map: public HeapObject {
 
   MUST_USE_RESULT Object* CopyDropDescriptors();
 
-  MUST_USE_RESULT Object* CopyNormalized(PropertyNormalizationMode mode);
+  MUST_USE_RESULT Object* CopyNormalized(PropertyNormalizationMode mode,
+                                         NormalizedMapSharingMode sharing);
 
   // Returns a copy of the map, with all transitions dropped from the
   // instance descriptors.
@@ -3261,7 +3273,7 @@ class Map: public HeapObject {
 #ifdef DEBUG
   void MapPrint();
   void MapVerify();
-  void NormalizedMapVerify();
+  void SharedMapVerify();
 #endif
 
   inline int visitor_id();
@@ -3325,6 +3337,7 @@ class Map: public HeapObject {
   static const int kHasFastElements = 2;
   static const int kStringWrapperSafeForDefaultValueOf = 3;
   static const int kAttachedToSharedFunctionInfo = 4;
+  static const int kIsShared = 5;
 
   // Layout of the default cache. It holds alternating name and code objects.
   static const int kCodeCacheEntrySize = 2;

diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 1963ff0..1b6359b 100644 (file)
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -2937,6 +2937,49 @@ THREADED_TEST(NamedInterceptorDictionaryIC) {
 }
 
 
+THREADED_TEST(NamedInterceptorDictionaryICMultipleContext) {
+  v8::HandleScope scope;
+
+  v8::Persistent<Context> context1 = Context::New();
+
+  context1->Enter();
+  Local<ObjectTemplate> templ = ObjectTemplate::New();
+  templ->SetNamedPropertyHandler(XPropertyGetter);
+  // Create an object with a named interceptor.
+  v8::Local<v8::Object> object = templ->NewInstance();
+  context1->Global()->Set(v8_str("interceptor_obj"), object);
+
+  // Force the object into the slow case.
+  CompileRun("interceptor_obj.y = 0;"
+             "delete interceptor_obj.y;");
+  context1->Exit();
+
+  {
+    // Introduce the object into a different context.
+    // Repeat named loads to exercise ICs.
+    LocalContext context2;
+    context2->Global()->Set(v8_str("interceptor_obj"), object);
+    Local<Value> result =
+      CompileRun("function get_x(o) { return o.x; }"
+                 "interceptor_obj.x = 42;"
+                 "for (var i=0; i != 10; i++) {"
+                 "  get_x(interceptor_obj);"
+                 "}"
+                 "get_x(interceptor_obj)");
+    // Check that the interceptor was actually invoked.
+    CHECK_EQ(result, v8_str("x"));
+  }
+
+  // Return to the original context and force some object to the slow case
+  // to cause the NormalizedMapCache to verify.
+  context1->Enter();
+  CompileRun("var obj = { x : 0 }; delete obj.x;");
+  context1->Exit();
+
+  context1.Dispose();
+}
+
+
 static v8::Handle<Value> SetXOnPrototypeGetter(Local<String> property,
                                                const AccessorInfo& info) {
   // Set x on the prototype object and do not handle the get request.