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From: kwh@bentley.UUCP (KW Heuer)
Newsgroups: net.lang.c
Subject: Re: More extensions to C
Message-ID: <856@bentley.UUCP>
Date: Mon, 26-May-86 15:34:12 EDT
Article-I.D.: bentley.856
Posted: Mon May 26 15:34:12 1986
Date-Received: Wed, 28-May-86 01:49:53 EDT
References: <1483@mmintl.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner
Lines: 32

In article <1483@mmintl.UUCP> mmintl!franka (Frank Adams) writes:
>In article <826@bentley.UUCP> kwh@bentley.UUCP writes:
>>[Swap] could be considered a special case of my ",," operator: to exchange
>>x and y, write "x = (y ,, (y = x))".
>
>I would prefer a syntax like "{x, y} = {y, x};"  It's more readable.

Yes, for a swap.  The real power of ",," is for removing explicit temporary
once-only variables in general; for example, it allows the function
	int pop() {
		int temp1;
		struct stack *temp2;
		temp1 = stk->top;
		temp2 = stk->next;
		free(stk);
		stk = temp2;
		return (temp1);
	}
to be abbreviated to the macro
	#define	pop()		(stk->top,,(stk=(stk->next,,free(stk))))
(A sort of generalization of the postfix operators.  "++x" means "x=x+1", but
"x++" means "x,,(x=x+1)".)

>(BTW, a really good optimizing compiler can optimize out the temporary.  I'm
>not fully up on what optimizers are doing these days, but I doubt that there
>are many compilers which do this.  Optimizing it out is easier if it is
>declared locally ("{int temp = x; x = y; y = temp;}") instead of being
>declared at the top of the routine.)

Unfortunately, I think the "clue" gets lost before the optimizing phase.

Karl W. Z. Heuer (ihnp4!bentley!kwh), The Walking Lint