[runtime] Avoid @@isConcatSpreadable lookup for fast path Array.prototype.concat

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index e98fb47a74c4224a54cab9018997f58b74de43da..92f66d2c91bea4be62f78c288d5a669a19f46faa 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -266,6 +266,7 @@ inline bool EnsureJSArrayWithWritableFastElements(Isolate* isolate,
                                                   Arguments* args,
                                                   int first_added_arg) {
   if (!receiver->IsJSArray()) return false;
+
   Handle<JSArray> array = Handle<JSArray>::cast(receiver);
   ElementsKind origin_kind = array->GetElementsKind();
   if (IsDictionaryElementsKind(origin_kind)) return false;
@@ -1317,14 +1318,6 @@ bool IterateElements(Isolate* isolate, Handle<JSReceiver> receiver,
   return true;
 }
 
-
-bool HasConcatSpreadableModifier(Isolate* isolate, Handle<JSArray> obj) {
-  Handle<Symbol> key(isolate->factory()->is_concat_spreadable_symbol());
-  Maybe<bool> maybe = JSReceiver::HasProperty(obj, key);
-  return maybe.FromMaybe(false);
-}
-
-
 static Maybe<bool> IsConcatSpreadable(Isolate* isolate, Handle<Object> obj) {
   HandleScope handle_scope(isolate);
   if (!obj->IsJSReceiver()) return Just(false);
@@ -1519,8 +1512,25 @@ Object* Slow_ArrayConcat(Arguments* args, Handle<Object> species,
   }
 }
 
+bool IsSimpleArray(Isolate* isolate, Handle<JSArray> obj) {
+  {
+    DisallowHeapAllocation no_gc;
+    Map* map = obj->map();
+    if (map->prototype() ==
+        isolate->native_context()->initial_array_prototype()) {
+      // If there is only the 'length' property we are fine.
+      if (map->NumberOfOwnDescriptors() == 1) return false;
+    }
+  }
+  // TODO(cbruni): slower lookup for array subclasses and support slow
+  // @@IsConcatSpreadable lookup.
+  return true;
+}
 
 MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
+  if (!isolate->IsArrayIsConcatSpreadableLookupChainIntact()) {
+    return MaybeHandle<JSArray>();
+  }
   int n_arguments = args->length();
   int result_len = 0;
   {
@@ -1538,7 +1548,7 @@ MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
         return MaybeHandle<JSArray>();
       }
       Handle<JSArray> array(JSArray::cast(arg), isolate);
-      if (HasConcatSpreadableModifier(isolate, array)) {
+      if (IsSimpleArray(isolate, array)) {
         return MaybeHandle<JSArray>();
       }
       int len = Smi::cast(array->length())->value();
@@ -1548,7 +1558,7 @@ MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
       STATIC_ASSERT(FixedArray::kMaxLength < kHalfOfMaxInt);
       USE(kHalfOfMaxInt);
       result_len += len;
-      DCHECK(result_len >= 0);
+      DCHECK_LE(0, result_len);
       // Throw an Error if we overflow the FixedArray limits
       if (FixedArray::kMaxLength < result_len) {
         THROW_NEW_ERROR(isolate,
@@ -1557,7 +1567,7 @@ MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
       }
     }
   }
-  return ElementsAccessor::Concat(isolate, args, n_arguments);
+  return ElementsAccessor::Concat(isolate, args, n_arguments, result_len);
 }
 
 }  // namespace