Add heuristic for flattening strings before comparing them.

src/heap.h

 // Copyright 2006-2008 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 611 matching lines @@
     return LookupSymbol(CStrVector(str));
   }
   static Object* LookupSymbol(String* str);
   static bool LookupSymbolIfExists(String* str, String** symbol);
   static bool LookupTwoCharsSymbolIfExists(String* str, String** symbol);
 
   // Compute the matching symbol map for a string if possible.
   // NULL is returned if string is in new space or not flattened.
   static Map* SymbolMapForString(String* str);
 
+  // Tries to flatten a string before compare operation.
+  //
+  // Returns a failure in case it was decided that flattening was
+  // necessary and failed.  Note, if flattening is not necessary the
+  // string might stay non-flat even when not a failure is returned.
+  //
+  // Please note this function does not perform a garbage collection.
+  static inline Object* PrepareForCompare(String* str);
+
   // Converts the given boolean condition to JavaScript boolean value.
   static Object* ToBoolean(bool condition) {
     return condition ? true_value() : false_value();
   }
 
   // Code that should be run before and after each GC.  Includes some
   // reporting/verification activities when compiled with DEBUG set.
   static void GarbageCollectionPrologue();
   static void GarbageCollectionEpilogue();
 
@@ skipping 301 matching lines @@
 
   // Returns the size of object residing in non new spaces.
   static int PromotedSpaceSize();
 
   // Returns the amount of external memory registered since last global gc.
   static int PromotedExternalMemorySize();
 
   static int mc_count_;  // how many mark-compact collections happened
   static int gc_count_;  // how many gc happened
 
+  // Total length of the strings we failed to flatten since the last GC.
+  static int unflattended_strings_length_;
+
 #define ROOT_ACCESSOR(type, name, camel_name)                                  \
   static inline void set_##name(type* value) {                                 \
     roots_[k##camel_name##RootIndex] = value;                                  \
   }
   ROOT_LIST(ROOT_ACCESSOR)
 #undef ROOT_ACCESSOR
 
 #ifdef DEBUG
   static bool allocation_allowed_;
 
@@ skipping 778 matching lines @@
 
   // To speed up scavenge collections new space string are kept
   // separate from old space strings.
   static List<Object*> new_space_strings_;
   static List<Object*> old_space_strings_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_