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

src/elements.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/elements.h"
 
 #include "src/arguments.h"
 #include "src/conversions.h"
 #include "src/factory.h"
 #include "src/isolate-inl.h"
@@ skipping 2949 matching lines @@
 
 
 void ElementsAccessor::TearDown() {
   if (elements_accessors_ == NULL) return;
 #define ACCESSOR_DELETE(Class, Kind, Store) delete elements_accessors_[Kind];
   ELEMENTS_LIST(ACCESSOR_DELETE)
 #undef ACCESSOR_DELETE
   elements_accessors_ = NULL;
 }
 
-
 Handle<JSArray> ElementsAccessor::Concat(Isolate* isolate, Arguments* args,
-                                         uint32_t concat_size) {
-  uint32_t result_len = 0;
+                                         uint32_t concat_size,
+                                         uint32_t result_len) {
+  ElementsKind result_elements_kind = GetInitialFastElementsKind();
   bool has_raw_doubles = false;
-  ElementsKind result_elements_kind = GetInitialFastElementsKind();
   {
     DisallowHeapAllocation no_gc;
     bool is_holey = false;
-    // Iterate through all the arguments performing checks
-    // and calculating total length.
     for (uint32_t i = 0; i < concat_size; i++) {
-      JSArray* array = JSArray::cast((*args)[i]);
-      uint32_t len = 0;
-      array->length()->ToArrayLength(&len);
-
-      // 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(0 <= result_len);
-      DCHECK(result_len <= FixedDoubleArray::kMaxLength);
-
-      ElementsKind arg_kind = array->GetElementsKind();
+      Object* arg = (*args)[i];
+      ElementsKind arg_kind = JSArray::cast(arg)->map()->elements_kind();

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

JSArray::cast(arg)->GetElementsKind()

       has_raw_doubles = has_raw_doubles || IsFastDoubleElementsKind(arg_kind);
       is_holey = is_holey || IsFastHoleyElementsKind(arg_kind);
       result_elements_kind =
           GetMoreGeneralElementsKind(result_elements_kind, arg_kind);
     }
     if (is_holey) {
       result_elements_kind = GetHoleyElementsKind(result_elements_kind);
     }
   }
 
@@ skipping 17 matching lines @@
     // performance degradation.
     JSArray* array = JSArray::cast((*args)[i]);
     uint32_t len = 0;
     array->length()->ToArrayLength(&len);
     if (len == 0) continue;
     ElementsKind from_kind = array->GetElementsKind();
     accessor->CopyElements(array, 0, from_kind, storage, insertion_index, len);
     insertion_index += len;
   }
 
-  DCHECK_EQ(insertion_index, result_len);
+  DCHECK(insertion_index == result_len);

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

Spurious change

   return result_array;
 }
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
 }  // namespace internal
 }  // namespace v8