Reducing the cost of autodecoding

[Stefan Koch via Digitalmars-d]

On Thursday, 13 October 2016 at 21:49:22 UTC, safety0ff wrote:
>> Bad benchmark! Bad! -- Andrei
>
> Also, I suspect a benchmark with a larger loop body might not 
> benefit as significantly from branch hints as this one.

I disagree in longer loops code compactness is as important as in 
small ones.

This is about the smallest inline version of decode I could come 
up with :

__gshared static immutable ubyte[] charWidthTab = [
             2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
             2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
             3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
             4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 1, 1
];

dchar myFront(ref char[] str) pure nothrow
{
     dchar c = cast(dchar) str[0];
     if ((c & 128))
     {
         if (c & 64)
         	final switch(charWidthTab[c - 192])
         {
             case 2 :
                 c |= ((str[1] & 0x80) >> 5);
             break;
             case 3 :
                c |= ((str[1] & 0x80) >> 4);
                c |= ((str[2] & 0x80) >> 10);
             break;
             case 4 :
                c |= ((str[1] & 0x80) >> 3);
                c |= ((str[2] & 0x80) >> 9);
                c |= ((str[3] & 0x80) >> 15);
             break;
             case 5,6,1 :
               goto Linvalid;
         }
         else
         Linvalid :
         	c = dchar.init;

     }
	return c;
}