[elements, turbofan] Implement simple GrowElements

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index 29fa4fb08c8bdaef6bda5d26794b4deab44a962c..c945f8e68545f3fe4b6e75adc173b7260f98e84c 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -891,6 +891,27 @@ class ElementsAccessorBase : public ElementsAccessor {
     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);
   }