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

src/builtins.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins.h"
 
 #include "src/api.h"
 #include "src/api-arguments.h"
 #include "src/api-natives.h"
 #include "src/base/once.h"
@@ skipping 248 matching lines @@
   return true;
 }
 
 // Returns |false| if not applicable.
 MUST_USE_RESULT
 inline bool EnsureJSArrayWithWritableFastElements(Isolate* isolate,
                                                   Handle<Object> receiver,
                                                   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;
   if (array->map()->is_observed()) return false;
   if (!array->map()->is_extensible()) return false;
   if (args == nullptr) return true;
 
   // If there may be elements accessors in the prototype chain, the fast path
   // cannot be used if there arguments to add to the array.
   if (!IsJSArrayFastElementMovingAllowed(isolate, *array)) return false;
@@ skipping 926 matching lines @@
     case FAST_STRING_WRAPPER_ELEMENTS:
     case SLOW_STRING_WRAPPER_ELEMENTS:
       // |array| is guaranteed to be an array or typed array.
       UNREACHABLE();
       break;
   }
   visitor->increase_index_offset(length);
   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);
   Handle<Symbol> key(isolate->factory()->is_concat_spreadable_symbol());
   Handle<Object> value;
   MaybeHandle<Object> maybeValue =
       i::Runtime::GetObjectProperty(isolate, obj, key);
   if (!maybeValue.ToHandle(&value)) return Nothing<bool>();
   if (!value->IsUndefined()) return Just(value->BooleanValue());
   return Object::IsArray(obj);
@@ skipping 174 matching lines @@
         isolate, NewRangeError(MessageTemplate::kInvalidArrayLength));
   }
 
   if (is_array_species) {
     return *visitor.ToArray();
   } else {
     return *visitor.storage_jsreceiver();
   }
 }
 
+bool FastArrayHasIsConcatSpreadable(Isolate* isolate, Handle<JSArray> obj) {
+  {
+    DisallowHeapAllocation no_gc;
+    Map* map = obj->map();
+    if (!isolate->IsArrayIsConcatSpreadableSet() &&
+        map->prototype() == *isolate->initial_array_prototype()) {

[Toon Verwaest, 2016/03/31 08:09:46]

isolate->[NATIVE_CONTEXT_FIELD]() does
Handle<...>(isolate->native_context()->[NATIVE_CONTEXT_FIELD](), isolate)
internally, so that allocates a handle. Directly use
isolate->native_context()->...() instead to avoid the unnecessary handle.

+      // If there is only the 'length' property we are fine.
+      if (map->NumberOfOwnDescriptors() == 1) return false;
+    }
+  }
+  // TODO(cbruni): slower lookup for array subclasses
+  return true;
+}
 
 MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
   int n_arguments = args->length();
   int result_len = 0;
   {
     DisallowHeapAllocation no_gc;
     // Iterate through all the arguments performing checks
     // and calculating total length.
     for (int i = 0; i < n_arguments; i++) {
       Object* arg = (*args)[i];
       if (!arg->IsJSArray()) return MaybeHandle<JSArray>();
       if (!HasOnlySimpleReceiverElements(isolate, JSObject::cast(arg))) {
         return MaybeHandle<JSArray>();
       }
       // TODO(cbruni): support fast concatenation of DICTIONARY_ELEMENTS.
       if (!JSObject::cast(arg)->HasFastElements()) {
         return MaybeHandle<JSArray>();
       }
       Handle<JSArray> array(JSArray::cast(arg), isolate);
-      if (HasConcatSpreadableModifier(isolate, array)) {
+      if (FastArrayHasIsConcatSpreadable(isolate, array)) {
         return MaybeHandle<JSArray>();
       }
       int len = Smi::cast(array->length())->value();
 
       // We shouldn't overflow when adding another len.
       const int kHalfOfMaxInt = 1 << (kBitsPerInt - 2);
       STATIC_ASSERT(FixedArray::kMaxLength < kHalfOfMaxInt);
       USE(kHalfOfMaxInt);
       result_len += len;
-      DCHECK(result_len >= 0);
+      DCHECK(0 <= result_len);

[Toon Verwaest, 2016/03/31 08:09:46]

DCHECK_LE(...)

       // Throw an Error if we overflow the FixedArray limits
       if (FixedArray::kMaxLength < result_len) {
         THROW_NEW_ERROR(isolate,
                         NewRangeError(MessageTemplate::kInvalidArrayLength),
                         JSArray);
       }
     }
   }
-  return ElementsAccessor::Concat(isolate, args, n_arguments);
+  return ElementsAccessor::Concat(isolate, args, n_arguments, result_len);
 }
 
 }  // namespace
 
 
 // ES6 22.1.3.1 Array.prototype.concat
 BUILTIN(ArrayConcat) {
   HandleScope scope(isolate);
 
   Handle<Object> receiver = args.receiver();
@@ skipping 3166 matching lines @@
 BUILTIN_LIST_T(DEFINE_BUILTIN_ACCESSOR_T)
 BUILTIN_LIST_H(DEFINE_BUILTIN_ACCESSOR_H)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 #undef DEFINE_BUILTIN_ACCESSOR_T
 #undef DEFINE_BUILTIN_ACCESSOR_H
 
 }  // namespace internal
 }  // namespace v8