Some string optimizations:

src/heap.cc

 // Copyright 2009 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 1943 matching lines @@
 
   // Optimization for 2-byte strings often used as keys in a decompression
   // dictionary.  Check whether we already have the string in the symbol
   // table to prevent creation of many unneccesary strings.
   if (length == 2) {
     unsigned c1 = first->Get(0);
     unsigned c2 = second->Get(0);
     return MakeOrFindTwoCharacterString(c1, c2);
   }
 
-  bool is_ascii = first->IsAsciiRepresentation()
-      && second->IsAsciiRepresentation();
+  bool first_is_ascii = first->IsAsciiRepresentation();
+  bool second_is_ascii = second->IsAsciiRepresentation();
+  bool is_ascii = first_is_ascii && second_is_ascii;
 
   // Make sure that an out of memory exception is thrown if the length
   // of the new cons string is too large.
   if (length > String::kMaxLength || length < 0) {
     Top::context()->mark_out_of_memory();
     return Failure::OutOfMemoryException();
   }
 
   // If the resulting string is small make a flat string.
   if (length < String::kMinNonFlatLength) {
@@ skipping 14 matching lines @@
       for (int i = 0; i < first_length; i++) *dest++ = src[i];
       // Copy second part.
       if (second->IsExternalString()) {
         src = ExternalAsciiString::cast(second)->resource()->data();
       } else {
         src = SeqAsciiString::cast(second)->GetChars();
       }
       for (int i = 0; i < second_length; i++) *dest++ = src[i];
       return result;
     } else {
+      // For short external two-byte strings we check whether they can
+      // be represented using ascii.
+      if (!first_is_ascii) {
+        first_is_ascii = first->IsExternalTwoByteStringWithAsciiChars();
+      }
+      if (first_is_ascii && !second_is_ascii) {
+        second_is_ascii = second->IsExternalTwoByteStringWithAsciiChars();
+      }
+      if (first_is_ascii && second_is_ascii) {
+        Object* result = AllocateRawAsciiString(length);
+        if (result->IsFailure()) return result;
+        // Copy the characters into the new object.
+        char* dest = SeqAsciiString::cast(result)->GetChars();
+        String::WriteToFlat(first, dest, 0, first_length);
+        String::WriteToFlat(second, dest + first_length, 0, second_length);
+        Counters::string_add_runtime_ext_to_ascii.Increment();
+        return result;
+      }
+
       Object* result = AllocateRawTwoByteString(length);
       if (result->IsFailure()) return result;
       // Copy the characters into the new object.
       uc16* dest = SeqTwoByteString::cast(result)->GetChars();
       String::WriteToFlat(first, dest, 0, first_length);
       String::WriteToFlat(second, dest + first_length, 0, second_length);
       return result;
     }
   }
 
@@ skipping 2356 matching lines @@
 void ExternalStringTable::TearDown() {
   new_space_strings_.Free();
   old_space_strings_.Free();
 }
 
 
 List<Object*> ExternalStringTable::new_space_strings_;
 List<Object*> ExternalStringTable::old_space_strings_;
 
 } }  // namespace v8::internal