Don't flatten every time we call CharCodeAt

src/isolate.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 976 matching lines @@
         // in JS or not.
         ASSERT((current_state == JS) == (state == JS));
       }
     }
     thread_local_top_.current_vm_state_ = state;
   }
 #endif
 
   void ResetEagerOptimizingData();
 
+  bool ShouldFlattenString() {
+    if (--flatten_counter_ > 0) {
+      return false;
+    }
+    flatten_counter_ = kStringFlattenInterval;
+    return true;
+  }
+
  private:
   Isolate();
 
   // The per-process lock should be acquired before the ThreadDataTable is
   // modified.
   class ThreadDataTable {
    public:
     ThreadDataTable();
     ~ThreadDataTable();
 
@@ skipping 143 matching lines @@
   StringInputBuffer objects_string_compare_buffer_a_;
   StringInputBuffer objects_string_compare_buffer_b_;
   StaticResource<StringInputBuffer> objects_string_input_buffer_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize>
       regexp_macro_assembler_canonicalize_;
   RegExpStack* regexp_stack_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   ZoneObjectList frame_element_constant_list_;
   ZoneObjectList result_constant_list_;
 
+  static const int kStringFlattenInterval = 257;
+  int flatten_counter_;
+
 #if defined(V8_TARGET_ARCH_ARM) && !defined(__arm__) || \
     defined(V8_TARGET_ARCH_MIPS) && !defined(__mips__)
   bool simulator_initialized_;
   HashMap* simulator_i_cache_;
   Redirection* simulator_redirection_;
 #endif
 
 #ifdef DEBUG
   // A static array of histogram info for each type.
   HistogramInfo heap_histograms_[LAST_TYPE + 1];
@@ skipping 213 matching lines @@
 
 } }  // namespace v8::internal
 
 // TODO(isolates): Get rid of these -inl.h includes and place them only where
 //                 they're needed.
 #include "allocation-inl.h"
 #include "zone-inl.h"
 #include "frames-inl.h"
 
 #endif  // V8_ISOLATE_H_