Install the internalized string as flattened version into cons-strings for fast re-internalization.

src/objects.cc

 // Copyright 2015 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/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 17939 matching lines @@
   Handle<StringTable> table = isolate->factory()->string_table();
   // We need a key instance for the virtual hash function.
   InternalizedStringKey dummy_key(isolate->factory()->empty_string());
   table = StringTable::EnsureCapacity(table, expected, &dummy_key);
   isolate->heap()->SetRootStringTable(*table);
 }
 
 
 Handle<String> StringTable::LookupString(Isolate* isolate,
                                          Handle<String> string) {
+  if (string->IsConsString() && string->IsFlat()) {
+    string = String::Flatten(string);
+    if (string->IsInternalizedString()) return string;
+  }
+
   InternalizedStringKey key(string);
-  return LookupKey(isolate, &key);
+  Handle<String> result = LookupKey(isolate, &key);
+
+  if (string->IsConsString()) {
+    Handle<ConsString> cons = Handle<ConsString>::cast(string);
+    cons->set_first(*result);
+    cons->set_second(isolate->heap()->empty_string());
+  }
+  return result;
 }
 
 
 Handle<String> StringTable::LookupKey(Isolate* isolate, HashTableKey* key) {
   Handle<StringTable> table = isolate->factory()->string_table();
   int entry = table->FindEntry(key);
 
   // String already in table.
   if (entry != kNotFound) {
     return handle(String::cast(table->KeyAt(entry)), isolate);
@@ skipping 1856 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8