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From: shannon@sun.uucp (Bill Shannon)
Newsgroups: net.lang.c
Subject: Re: String copy idiom.
Message-ID: <2040@sun.uucp>
Date: Mon, 11-Mar-85 04:16:59 EST
Article-I.D.: sun.2040
Posted: Mon Mar 11 04:16:59 1985
Date-Received: Wed, 13-Mar-85 00:49:59 EST
References: <7044@watdaisy.UUCP> <3448@alice.UUCP> <464@petsd.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 46

> In article <3448@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
> >If s and t are char pointers in registers,
> >
> >	while (*s++ = *t++) ;
> >
> >generates the best code I could possibly imagine.
> >
> >	while ((*s = *t) != '\0') {s++; t++;}
> >
> >is considerably worse.  Try it with register variables on your compiler.
> 
> Ok, I did.  The second sequence generates less code than the first sequence
> on our machine (Perkin-Elmer).  This is due to the fact that our machine
> doesn't support auto-increment in hardware.  The C compiler has to "fake" it.
> 
> 					regards,
> 					joe

On the Sun the first generates the obvious two instruction loop
while the second generates a five instruction loop:

	register char *s, *t;
	while (*s++ = *t++) ;

L14:
	movb	a4@+,a5@+
	jne	L14

	while ((*s = *t) != '\0') {s++; t++;}
L17:
	movb	a4@,a5@
	jeq	LE12
	addql	#1,a5
	addql	#1,a4
	jra	L17
LE12:

Note that the two loops differ in the values of s and t at loop
termination.  I consider the first loop to be more obvious, it
is after all the standard C idiom for copying a string.  If you
"optimize" your code for one machine by writing the second loop,
you may be pessimizing it for other machines.  Such is the nature
of C and C programmers.

					Bill Shannon
					Sun Microsystems, Inc.