Add heuristic for flattening strings before comparing them.

src/heap-inl.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 255 matching lines @@
   if (first_word.IsForwardingAddress()) {
     *p = first_word.ToForwardingAddress();
     return;
   }
 
   // Call the slow part of scavenge object.
   return ScavengeObjectSlow(p, object);
 }
 
 
+Object* Heap::PrepareForCompare(String* str) {
+  // Always flatten small strings and force flattening of long strings
+  // after we have accumulated a certain amount we failed to flatten.
+  static const int kMaxAlwaysFlattenLength = 32;
+  static const int kFlattenLongThreshold = 16*KB;
+
+  const int length = str->length();
+  Object* obj = str->TryFlatten();
+  if (length <= kMaxAlwaysFlattenLength ||
+      unflattended_strings_length_ >= kFlattenLongThreshold) {
+    return obj;
+  }
+  if (obj->IsFailure()) {
+    unflattended_strings_length_ += length;
+  }
+  return str;
+}
+
+
 int Heap::AdjustAmountOfExternalAllocatedMemory(int change_in_bytes) {
   ASSERT(HasBeenSetup());
   int amount = amount_of_external_allocated_memory_ + change_in_bytes;
   if (change_in_bytes >= 0) {
     // Avoid overflow.
     if (amount > amount_of_external_allocated_memory_) {
       amount_of_external_allocated_memory_ = amount;
     }
     int amount_since_last_global_gc =
         amount_of_external_allocated_memory_ -
@@ skipping 128 matching lines @@
 
 
 void ExternalStringTable::ShrinkNewStrings(int position) {
   new_space_strings_.Rewind(position);
   Verify();
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_INL_H_