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

src/heap-inl.h

 // 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 67 matching lines @@
 void PromotionQueue::ActivateGuardIfOnTheSamePage() {
   guard_ = guard_ ||
       heap_->new_space()->active_space()->current_page()->address() ==
       GetHeadPage()->address();
 }
 
 
 MaybeObject* Heap::AllocateStringFromUtf8(Vector<const char> str,
                                           PretenureFlag pretenure) {
   // Check for ASCII first since this is the common case.
-  if (String::IsAscii(str.start(), str.length())) {
+  const char* start = str.start();
+  int length = str.length();
+  int non_ascii_start = String::NonAsciiStart(start, length);
+  if (non_ascii_start >= length) {
     // If the string is ASCII, we do not need to convert the characters
     // since UTF8 is backwards compatible with ASCII.
     return AllocateStringFromAscii(str, pretenure);
   }
   // Non-ASCII and we need to decode.
-  return AllocateStringFromUtf8Slow(str, pretenure);
+  return AllocateStringFromUtf8Slow(str, non_ascii_start, pretenure);
 }
 
 
 MaybeObject* Heap::AllocateSymbol(Vector<const char> str,
                                   int chars,
                                   uint32_t hash_field) {
   unibrow::Utf8InputBuffer<> buffer(str.start(),
                                     static_cast<unsigned>(str.length()));
   return AllocateInternalSymbol(&buffer, chars, hash_field);
 }
@@ skipping 696 matching lines @@
 AssertNoAllocation::~AssertNoAllocation() { }
 DisableAssertNoAllocation::DisableAssertNoAllocation() { }
 DisableAssertNoAllocation::~DisableAssertNoAllocation() { }
 
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_INL_H_