Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ll-xn!mit-eddie!killer!jfh
From: jfh@killer.UUCP (The Beach Bum)
Newsgroups: comp.lang.c
Subject: Re: Associativity -- what is it?
Message-ID: <3466@killer.UUCP>
Date: 24 Feb 88 05:20:14 GMT
References: <226@mccc.UUCP> <224@sdrc.UUCP> <234@mccc.UUCP>
Reply-To: jfh@killer.UUCP (The Beach Bum)
Organization: Big "D" Home for Wayward Hackers
Lines: 42
Summary: Postfix ++ explained some more.
         Not for real programmers.

In article <234@mccc.UUCP> pjh@mccc.UUCP (Peter J. Holsberg) writes:

[ one explaination down, twenty five more to go ;-) ]

>Well, that still leaves me confused.  If i has the value 7, it is 7 that
>is added to 3, so it seems to be that the ++ *is* deferred until later. 
>Also, ++ has higher precedence than +, so why is the incrementation
>delayed until after the current value of i is used?
> 
>I think we're getting close, though.  :-)  Thanks for the help.
>-- 
>Peter Holsberg                  UUCP: {rutgers!}princeton!mccc!pjh

This comes from a discussion which I believe was given in the document
explaining the implementation of the portable C compiler.

Consider ++ in it's two forms as a short-hand notation (don't believe
it's true, just consider it.  Flames to /dev/null) for:

pre-fix:	++ X	->	(X = X + 1)
post-fix:	X ++	->	((X = X + 1), X - 1)

So, the increment gets done, but, the value you get has been `adjusted'
if you will.

Try this:
		x = 5;
		printf ("3 * x ++ = %d\n", 3 * x ++);
		printf ("now x = %d\n", x);

You should get 15 and 4.  I suspect you expected 18 or 16 or something
like that.

[ I believe the context was that the compiler generated the trees using
  the long-hand, and then hoped to optimize to using increment instructions
  later. ]

- John.
-- 
John F. Haugh II                  SNAIL:  HECI Exploration Co. Inc.
UUCP: ...!ihnp4!killer!jfh                11910 Greenville Ave, Suite 600
"You can't threaten us, we're             Dallas, TX. 75243
  the Oil Company!"                       (214) 231-0993 Ext 260