[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 1310 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());
+  if (!isolate->IsArrayIsConcatSpreadableLookupChainIntact()) {

[Toon Verwaest, 2016/04/11 09:30:56]

This only guarantees that Array.prototype and Object.prototype don't have it. It
doesn't guarantee that the receiver itself doesn't have it. Also, it could be
installed on a non-array or array with non-default .__proto__. In all those
cases you need to slow-fetch the property.
Please add tests for all these currently unsupported cases.
Basically you have to
if (IsSimpleArray(obj) && IsArrayIsConcatSpreadableLookupChainIntact()) return
Object::IsArray(obj);

[Camillo Bruni, 2016/04/11 11:55:12]

I think the naming for the protector is confusing. 
Currently I invalidate the protector in lookup.cc whenever you use
@@IsConcatSpreadable, independently from the receiver.


test/mjsunit/es6/array-concat.js added further down already covers the case of
@@IsConcatSpreadable on the receiver.

+    // Slow path if @@isConcatSpreadable has been used.
+    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);
 }
 
 
 Object* Slow_ArrayConcat(Arguments* args, Handle<Object> species,
                          Isolate* isolate) {
   int argument_count = args->length();
 
   bool is_array_species = *species == isolate->context()->array_function();
 
@@ skipping 167 matching lines @@
         isolate, NewRangeError(MessageTemplate::kInvalidArrayLength));
   }
 
   if (is_array_species) {
     return *visitor.ToArray();
   } else {
     return *visitor.storage_jsreceiver();
   }
 }
 
+bool IsSimpleArray(Isolate* isolate, Handle<JSArray> obj) {
+  {
+    DisallowHeapAllocation no_gc;
+    Map* map = obj->map();
+    // If there is only the 'length' property we are fine.
+    if (map->prototype() ==
+            isolate->native_context()->initial_array_prototype() &&
+        map->NumberOfOwnDescriptors() == 1) {
+      return true;
+    }
+  }
+  // TODO(cbruni): slower lookup for array subclasses and support slow
+  // @@IsConcatSpreadable lookup.
+  return false;
+}
 
 MaybeHandle<JSArray> Fast_ArrayConcat(Isolate* isolate, Arguments* args) {
+  if (!isolate->IsArrayIsConcatSpreadableLookupChainIntact()) {
+    return MaybeHandle<JSArray>();
+  }
   // We shouldn't overflow when adding another len.
   const int kHalfOfMaxInt = 1 << (kBitsPerInt - 2);
   STATIC_ASSERT(FixedArray::kMaxLength < kHalfOfMaxInt);
   STATIC_ASSERT(FixedDoubleArray::kMaxLength < kHalfOfMaxInt);
   USE(kHalfOfMaxInt);
 
   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>();
       }
-      if (!HasOnlySimpleReceiverElements(isolate, JSObject::cast(arg))) {
-        return MaybeHandle<JSArray>();
-      }
       Handle<JSArray> array(JSArray::cast(arg), isolate);
-      if (HasConcatSpreadableModifier(isolate, array)) {
+      if (!IsSimpleArray(isolate, array)) {
         return MaybeHandle<JSArray>();
       }
       // The Array length is guaranted to be <= kHalfOfMaxInt thus we won't
       // overflow.
       result_len += Smi::cast(array->length())->value();
       DCHECK(result_len >= 0);
       // Throw an Error if we overflow the FixedArray limits
-      if (FixedDoubleArray::kMaxLength < result_len ||
-          FixedArray::kMaxLength < result_len) {
-        AllowHeapAllocation allow_gc;
+      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 3361 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