Implement fast path in UTF8ToUTF16 for pure ASCII strings

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index e20bbf0a40c59f9440a23812ca7e8723dd2c9dec..21e0d72acb8e7fe6ee88687b4251025e7d0e67f0 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -245,9 +245,14 @@ BASE_EXPORT bool ContainsOnlyChars(const StringPiece16& input,
 // to have the maximum 'discriminating' power from other encodings. If
 // there's a use case for just checking the structural validity, we have to
 // add a new function for that.
+//
+// IsStringASCII assumes the input is likely all ASCII, and does not leave early
+// if it is not the case.
 BASE_EXPORT bool IsStringUTF8(const std::string& str);
 BASE_EXPORT bool IsStringASCII(const StringPiece& str);
 BASE_EXPORT bool IsStringASCII(const string16& str);
+BASE_EXPORT bool IsStringASCII(const char* src, size_t src_len);

[brettw, 2014/09/16 17:14:28]

The string piece version should cover this case. Actually, I think we should
have a a StringPiece and a StringPiece16 version of this function and nothing
else. StringPiece is supposed to handle exactly what you're doing.

[mnaganov (inactive), 2014/09/17 15:15:01]

You are right, I didn't think about this, thanks!

Overloaded version taking StringPiece16 is useful for testing various input
alignments, so I added it. However, removing the string16 version will require
changing all the places where IsStringASCII is called on a WebString.

WebString provides conversions to string / string16, but not to a StringPiece,
which makes sense, because StringPiece doesn't own the data. I guess, this is
the reason why UTF16To... functions are also operating on string16, and not on
StringPiece16.

I added a comment justifying the existence of the string16 version.

+BASE_EXPORT bool IsStringASCII(const char16* src, size_t src_len);
 
 // Converts the elements of the given string.  This version uses a pointer to
 // clearly differentiate it from the non-pointer variant.