Optimize flattening of cons strings with equal left and right-side.

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 11521 matching lines @@
       }
       case kOneByteStringTag | kConsStringTag:
       case kTwoByteStringTag | kConsStringTag: {
         ConsString* cons_string = ConsString::cast(source);
         String* first = cons_string->first();
         int boundary = first->length();
         if (to - boundary >= boundary - from) {
           // Right hand side is longer.  Recurse over left.
           if (from < boundary) {
             WriteToFlat(first, sink, from, boundary);
+            if (from == 0 && cons_string->second() == first) {
+              CopyChars(sink + boundary, sink, boundary);
+              return;
+            }
             sink += boundary - from;
             from = 0;
           } else {
             from -= boundary;
           }
           to -= boundary;
           source = cons_string->second();
         } else {
           // Left hand side is longer.  Recurse over right.
           if (to > boundary) {
@@ skipping 8232 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