short circuit evaluation

[chris at mimsy.UUCP]

In article <14479 at amdcad.UUCP> tim at amdcad.UUCP (Tim Olson) writes:
>What about if either the left side expression or the right side
>expression [of an otherwise short-circuitable expression] contained
>a side-effect (or a procedure call, which also may have a side-
>effect)?  These cannot be short-circuited when bit-wise operators
>are used.

	c = *p++ & *q++;
	/* are both p and q always incremented? */

I can find no promise in K&R that bitwise expressions are not short
circuited even in the presence of side effects.  The ANSI draft
may have more to say.  In any case, I would advise not counting on
full evaluation.  It is even conceivable that a compiler might
generate

	if ((c = *p) != 0)
		c &= *q;
	p++;
	q++;

which, if p and q point to device registers, is not the same!

In general, existing C compilers will not attempt to optimise
away the AND above if *p++ (or *q++) is zero, simply because the
`C philosophy' is that if you *meant*

	c = *p++ ? p[-1] & *q++ : 0;

you would have written that.  But is it mandated?  I cannot say.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris at mimsy.umd.edu