Xref: utzoo comp.lang.c:12276 comp.arch:6214
Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!pacbell!varian!zehntel!bales!nat
From: nat@bales.UUCP (Nathaniel Stitt)
Newsgroups: comp.lang.c,comp.arch
Subject: Re: Explanation, please!
Message-ID: <189@bales.UUCP>
Date: 30 Aug 88 21:23:24 GMT
References: <638@paris.ics.uci.edu> <dpmuY#2EBC4R=eric@snark.UUCP>
Reply-To: nat@bales.UUCP (Nathaniel Stitt)
Organization: Bales Scientific Inc., Walnut Creek, CA
Lines: 92

In article <dpmuY#2EBC4R=eric@snark.UUCP> eric@snark.UUCP (Eric S. Raymond) writes:
>(Code below reproduced so that comp.arch people seeing this followup only
>won't get terminally frustrated. This is *really neat*, gang...)
>
>In article <638@paris.ics.uci.edu> Douglas C. Schmidt writes:
>> 
>> void send(int *to,int *from, int count) {
>>    int n = (count + 7) / 8;
>>    
>>    switch(count % 8) {
>>       case 0:  do { *to++ = *from++;
>>       case 7:       *to++ = *from++;
>>       case 6:       *to++ = *from++;
>>       case 5:       *to++ = *from++;
>>       case 4:       *to++ = *from++;
>>       case 3:       *to++ = *from++;
>>       case 2:       *to++ = *from++;
>>       case 1:       *to++ = *from++;
>>                } while (--n > 0);
>>    }
>> 
>> }
>> 
>> Finally, Stroustrup asks the rhetorical question ``why would anyone
>> want to write something like this.''  Any guesses?!
>
>Yeah. That's the most hackish way of trying to write a portable optimized
>copy routine I've ever seen. I gather the whole point of the shenanigans
>is to get all the *from++ -> *to++ instructions in the generated code to be
>adjacent.
>

Here is my own personal version of the "Portable Optimized Copy" routine.
It certainly seems more clear than the above example, and I would expect
it to be at least as fast on virtually any machine.  (Note the use of division
and mod in the above routine.)

Code does NOT have to be ugly to be fast.


/* Copy 'count' ints from *from to *to. */
void nats_send(to, from, count)
register int *to;
register int *from;
register int count;
{
	/* Copy the bulk of the data */
	while(count >= 8)
	{
		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;

		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;

		count -= 8;
	}

	/* Copy the rest. */

	if(count >= 4)
	{
		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;
		*to++ = *from++;

		count -= 4;
	}

	if(count >= 2)
	{
		*to++ = *from++;
		*to++ = *from++;

		count -= 2;
	}

	if(count)
		*to++ = *from++;
}

Hope this helps.

-- 
Nathaniel Stitt           | This life is a test.  It is only a test.  Had
Guidelines Software, Inc. | this been an actual life, you would have received
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