Keep track of the first non-ascii word/char to avoid redoing the work.

src/heap.cc

 // Copyright 2012 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 4410 matching lines @@
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
 
   // Copy the characters into the new object.
   CopyChars(SeqAsciiString::cast(result)->GetChars(), string.start(), length);
   return result;
 }
 
 
 MaybeObject* Heap::AllocateStringFromUtf8Slow(Vector<const char> string,
+                                              int non_ascii_start,
                                               PretenureFlag pretenure) {
-  // Count the number of characters in the UTF-8 string and check if
-  // it is an ASCII string.
+  // Continue counting the number of characters in the UTF-8 string, starting
+  // from the first non-ascii character or word.
+  int chars = non_ascii_start;
   Access<UnicodeCache::Utf8Decoder>
       decoder(isolate_->unicode_cache()->utf8_decoder());
-  decoder->Reset(string.start(), string.length());
-  int chars = 0;
+  decoder->Reset(string.start() + non_ascii_start, string.length() - chars);
   while (decoder->has_more()) {
     uint32_t r = decoder->GetNext();
     if (r <= unibrow::Utf16::kMaxNonSurrogateCharCode) {
       chars++;
     } else {
       chars += 2;
     }
   }
 
   Object* result;
@@ skipping 2889 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   FIXED_ARRAY_SUB_INSTANCE_TYPE_LIST(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   memcpy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   memcpy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal