Implement fast path in UTF8ToUTF16 for pure ASCII strings

base/strings/utf_string_conversions.cc

Index: base/strings/utf_string_conversions.cc
diff --git a/base/strings/utf_string_conversions.cc b/base/strings/utf_string_conversions.cc
index f13ed1bb9c61fc6c2211a488bd3568047c5e3fc1..9d520fa5ecf0841a4379a4062de65767bf22e9fd 100644
--- a/base/strings/utf_string_conversions.cc
+++ b/base/strings/utf_string_conversions.cc
@@ -56,13 +56,23 @@ std::string WideToUTF8(const std::wstring& wide) {
 }
 
 bool UTF8ToWide(const char* src, size_t src_len, std::wstring* output) {
-  PrepareForUTF16Or32Output(src, src_len, output);
-  return ConvertUnicode(src, src_len, output);
+  if (IsStringASCII(StringPiece(src, src_len))) {
+    output->assign(src, src + src_len);
+    return true;
+  } else {
+    PrepareForUTF16Or32Output(src, src_len, output);
+    return ConvertUnicode(src, src_len, output);
+  }
 }
 
 std::wstring UTF8ToWide(const StringPiece& utf8) {
+  if (IsStringASCII(utf8)) {
+    return std::wstring(utf8.begin(), utf8.end());
+  }
+
   std::wstring ret;
-  UTF8ToWide(utf8.data(), utf8.length(), &ret);
+  PrepareForUTF16Or32Output(utf8.data(), utf8.length(), &ret);
+  ConvertUnicode(utf8.data(), utf8.length(), &ret);
   return ret;
 }
 
@@ -126,15 +136,25 @@ std::wstring UTF16ToWide(const string16& utf16) {
 #if defined(WCHAR_T_IS_UTF32)
 
 bool UTF8ToUTF16(const char* src, size_t src_len, string16* output) {
-  PrepareForUTF16Or32Output(src, src_len, output);
-  return ConvertUnicode(src, src_len, output);
+  if (IsStringASCII(StringPiece(src, src_len))) {
+    output->assign(src, src + src_len);
+    return true;
+  } else {
+    PrepareForUTF16Or32Output(src, src_len, output);
+    return ConvertUnicode(src, src_len, output);
+  }
 }
 
 string16 UTF8ToUTF16(const StringPiece& utf8) {
+  if (IsStringASCII(utf8)) {
+    return string16(utf8.begin(), utf8.end());
+  }
+
   string16 ret;
+  PrepareForUTF16Or32Output(utf8.data(), utf8.length(), &ret);
   // Ignore the success flag of this call, it will do the best it can for
   // invalid input, which is what we want here.
-  UTF8ToUTF16(utf8.data(), utf8.length(), &ret);
+  ConvertUnicode(utf8.data(), utf8.length(), &ret);
   return ret;
 }