Avoid decoding overhead when allocating ascii strings....

src/heap.cc

 // Copyright 2010 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 3289 matching lines @@
 
   // Copy the characters into the new object.
   SeqAsciiString* string_result = SeqAsciiString::cast(result);
   for (int i = 0; i < string.length(); i++) {
     string_result->SeqAsciiStringSet(i, string[i]);
   }
   return result;
 }
 
 
-MaybeObject* Heap::AllocateStringFromUtf8(Vector<const char> string,
-                                          PretenureFlag pretenure) {
+MaybeObject* Heap::AllocateStringFromUtf8Slow(Vector<const char> string,
+                                              PretenureFlag pretenure) {
   // V8 only supports characters in the Basic Multilingual Plane.
   const uc32 kMaxSupportedChar = 0xFFFF;
   // Count the number of characters in the UTF-8 string and check if
   // it is an ASCII string.
   Access<ScannerConstants::Utf8Decoder>
       decoder(ScannerConstants::utf8_decoder());
   decoder->Reset(string.start(), string.length());
   int chars = 0;
-  bool is_ascii = true;
   while (decoder->has_more()) {
-    uc32 r = decoder->GetNext();
-    if (r > String::kMaxAsciiCharCode) is_ascii = false;
+    decoder->GetNext();
     chars++;
   }
 
-  // If the string is ascii, we do not need to convert the characters
-  // since UTF8 is backwards compatible with ascii.
-  if (is_ascii) return AllocateStringFromAscii(string, pretenure);
-
   Object* result;
   { MaybeObject* maybe_result = AllocateRawTwoByteString(chars, pretenure);
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
 
   // Convert and copy the characters into the new object.
   String* string_result = String::cast(result);
   decoder->Reset(string.start(), string.length());
   for (int i = 0; i < chars; i++) {
     uc32 r = decoder->GetNext();
@@ skipping 2178 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