[elements, turbofan] Implement simple GrowElements

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 873 matching lines @@
 
   void TransitionElementsKind(Handle<JSObject> object, Handle<Map> map) final {
     Subclass::TransitionElementsKindImpl(object, map);
   }
 
   void GrowCapacityAndConvert(Handle<JSObject> object,
                               uint32_t capacity) final {
     Subclass::GrowCapacityAndConvertImpl(object, capacity);
   }
 
+  bool GrowCapacity(Handle<JSObject> object, uint32_t capacity) final {
+    if (object->map()->is_prototype_map() ||

[Jakob Kummerow, 2016/08/16 15:15:01]

To explain why this code does what it's doing, please add a comment to the tune
of the one you're deleting in runtime-array.cc, roughly:
// This function is intended to be called from optimized code. We don't want
// to trigger lazy deopts there, so refuse to handle cases that would.

+        object->WouldConvertToSlowElements(capacity)) {
+      return false;
+    }
+    Handle<FixedArrayBase> old_elements(object->elements());
+    DCHECK(static_cast<uint32_t>(old_elements->length()) < capacity);
+    Handle<FixedArrayBase> elements =
+        ConvertElementsWithCapacity(object, old_elements, kind(), capacity);
+
+    DCHECK_EQ(object->GetElementsKind(), kind());
+    // Transition through the allocation site as well if present.
+    if (JSObject::UpdateAllocationSite<AllocationSiteUpdateMode::kCheckOnly>(
+            object, kind())) {
+      return false;
+    }
+
+    object->set_elements(*elements);
+    return true;
+  }
+
   void Delete(Handle<JSObject> obj, uint32_t entry) final {
     Subclass::DeleteImpl(obj, entry);
   }
 
   static void CopyElementsImpl(FixedArrayBase* from, uint32_t from_start,
                                FixedArrayBase* to, ElementsKind from_kind,
                                uint32_t to_start, int packed_size,
                                int copy_size) {
     UNREACHABLE();
   }
@@ skipping 2565 matching lines @@
     insertion_index += len;
   }
 
   DCHECK_EQ(insertion_index, result_len);
   return result_array;
 }
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
 }  // namespace internal
 }  // namespace v8