String.prototype.toLower should be optimized for 8 bit strings

String.protoype.toLower() uses UString.characters() and should be optimized for the underlying character size.

Created an attachment (id=116647) [details]
Patch

This speeds up sun spider string-tagcloud by 7%.

(From update of attachment 116647 [details])
Can you put the perf effect in the ChangeLog?

Committed r101187: <http://trac.webkit.org/changeset/101187>

This removes the optimization for the case where the string was already all lowercase. It seems we could easily retain that by checking if the result of s.impl()->lower() is == s.impl() and if so, returning JSValue::encode(sVal).

(In reply to comment #4)
> This removes the optimization for the case where the string was already all lowercase. It seems we could easily retain that by checking if the result of s.impl()->lower() is == s.impl() and if so, returning JSValue::encode(sVal).

I had played with this a little and didn't get the return right so it failed some sputnik tests.  I just coded up a correct "already lower case" return and will be posting it in another bug.

It appears that the fast return is worth maybe a little in sun spider performance (.2ms overall).