String copy idiom.

[Chris Torek]

>    while (*s++ = *t++);

> ... this code produces a loop with 50% more assembler instructions
> than the slightly clearer sequence:

>    while ((*s = *t) != '\0')
>    {   s++;
>	 t++;
>    }

Not necessarily.  It depends on whether s and t are "register" variables.

(The casual reader should type 'n' at this point . . . .)

Proof:

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

generates (optimized):

	.globl	_f
_f:	.word	0xc00
	movl	4(ap),r11
	movl	8(ap),r10
L16:	movb	(r10)+,(r11)+
	jneq	L16
	ret

while

g(s,t)
char *s, *t;
{
	while (*s = *t) s++, t++;
}

generates (also optimized):

	.globl	_g
_g:	.word	0
	jbr	L16
L2000001:
	incl	4(ap)
	incl	8(ap)
L16:	movb	*8(ap),*4(ap)
	jneq	L2000001
	ret

Changing s and t above to "register char *" gives

	.globl	_g
_g:	.word	0xc00
	movl	4(ap),r11
	movl	8(ap),r10
	jbr	L16
L2000001:
	incl	r11
	incl	r10
L16:	movb	(r10),(r11)
	jneq	L2000001
	ret

which is faster most of the time (for strings of length 0 and 1 it's
probably slower).

It is true, however, that using postincrement on non-register pointer
variables is generally less efficient than doing the same thing "by hand",
since the compiler has to put the original value in a scratch register
so that the increment doesn't clobber the condition codes.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251)
UUCP:	{seismo,allegra,brl-bmd}!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at maryland