Reducing the cost of autodecoding

[Uplink_Coder via Digitalmars-d]

It can also be written like this producing smaller code.
But it the cost of slower decoding.

dchar myFront(ref char[] str) pure
{
     dchar c = cast(dchar) str.ptr[0];
     if (c & 128)
     {
         if (c & 64)
         {
             int idx = 0;
             int l = charWidthTab.ptr[c - 192];
             if (str.length < l)
                 goto Linvalid;
             c = 0;
           l--;
             while(l) {
               l--;
                c |= str.ptr[idx++];
                c <<= 6;
             }
             c |= str.ptr[idx];

        }
         else
     Linvalid : throw new Exception("yadayada");

     }
     return c;
}