"foreach(i, dchar c; s)" vs "decode"

[monarch_dodra]

I spent *all* week benchmarking a string processing function. And 
now, at the end of the week, I can safely say that the compiler's 
"foreach" is slower than a phobos decode based while loop.

Basically, given a
----
foreach(i, dchar c; s)
{codeCodeCode;}
----
  loop, I replaced it with:
----
{
     size_t i;
     size_t j;
     immutable k = s.length;
     dchar c;
     for ( ; i < k ; i = j )
     {
         c = decode(s, j);
         codeCodeCode;
     }
}
----

And my algorithms instantly gained a 10-25% performance 
improvement(!). I benched using varied sources of data, in 
particular, both ASCII only strings, as well as unicode heavy 
text.

Unicode has better gains, but raw ASCII text is *also* has gains 
:/
this holds true for both UTF-8 and UTF-16.

UTF-32 is different, because foreach has the "unfair" advantage 
of not validating the code points...

I got these results on 2.061 alpha release, with phobos in 
release and both -inline and without inline.

So if any of the compiler guys are reading this... I have no idea 
how the unicode foreach is actually implemented, but there 
*should* be substantial gains to be had...