[turbofan] Introduce a TransitionElementsKind simplified operator.

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index 3cf8378162844a1fc00525e3ccacdb6f47406ac5..031d5169290bab0c24da0d8b4537fae95b7b11b5 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -783,6 +783,45 @@ class ElementsAccessorBase : public ElementsAccessor {
     return new_elements;
   }
 
+  static void TransitionElementsKindImpl(Handle<JSObject> object,
+                                         Handle<Map> to_map) {
+    Handle<Map> from_map = handle(object->map());
+    ElementsKind from_kind = from_map->elements_kind();
+    ElementsKind to_kind = to_map->elements_kind();
+    if (IsFastHoleyElementsKind(from_kind)) {
+      to_kind = GetHoleyElementsKind(to_kind);
+    }
+    if (from_kind != to_kind) {
+      // This method should never be called for any other case.
+      DCHECK(IsFastElementsKind(from_kind));
+      DCHECK(IsFastElementsKind(to_kind));
+      DCHECK_NE(TERMINAL_FAST_ELEMENTS_KIND, from_kind);
+
+      Handle<FixedArrayBase> from_elements(object->elements());
+      if (object->elements() == object->GetHeap()->empty_fixed_array() ||
+          IsFastDoubleElementsKind(from_kind) ==
+              IsFastDoubleElementsKind(to_kind)) {
+        // No change is needed to the elements() buffer, the transition
+        // only requires a map change.
+        JSObject::MigrateToMap(object, to_map);
+      } else {
+        DCHECK((IsFastSmiElementsKind(from_kind) &&
+                IsFastDoubleElementsKind(to_kind)) ||
+               (IsFastDoubleElementsKind(from_kind) &&
+                IsFastObjectElementsKind(to_kind)));
+        uint32_t capacity = static_cast<uint32_t>(object->elements()->length());
+        Handle<FixedArrayBase> elements = ConvertElementsWithCapacity(
+            object, from_elements, from_kind, capacity);
+        JSObject::SetMapAndElements(object, to_map, elements);
+      }
+      if (FLAG_trace_elements_transitions) {
+        JSObject::PrintElementsTransition(stdout, object, from_kind,
+                                          from_elements, to_kind,
+                                          handle(object->elements()));
+      }
+    }
+  }
+
   static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                          uint32_t capacity) {
     ElementsKind from_kind = object->GetElementsKind();
@@ -822,6 +861,10 @@ class ElementsAccessorBase : public ElementsAccessor {
     }
   }
 
+  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);
@@ -2255,6 +2298,11 @@ class SloppyArgumentsElementsAccessor
     }
   }
 
+  static void TransitionElementsKindImpl(Handle<JSObject> object,
+                                         Handle<Map> map) {
+    UNREACHABLE();
+  }
+
   static void GrowCapacityAndConvertImpl(Handle<JSObject> object,
                                          uint32_t capacity) {
     UNREACHABLE();