Path: utzoo!attcan!uunet!nuchat!moray!mybest!ut-emx!cs.utexas.edu!meyering
From: meyering@cs.utexas.edu (Jim Meyering)
Newsgroups: comp.lang.c
Subject: Re: Does your compiler get this program right?
Summary: standard doesn't define "right"
Keywords: float, increment
Message-ID: <4082@cs.utexas.edu>
Date: 23 Nov 88 23:29:47 GMT
References: <2298@cbnews.ATT.COM>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 29

In article <2298@cbnews.ATT.COM> lvc@cbnews.ATT.COM (Lawrence V. Cipriani) writes:
	>A friend of mine found a bug in his C compiler.  He found

It's not a bug.

	[...deleted commentary, code]
	>*f++ += *g++;		/* miscompiled line */

The standard does not specify the order of evaluation for such
statements.  It's easier to see the ambiguity if you try to rewrite
it without the += notation.  Which do you choose?

 1) *f++ = *f++ + *g++;
 2) *f++ = *f + *g++;
 3) *f = *f++ + *g++;

It can't be (1) since the side effect, f++, may be realized only once,
but it's up to the compiler writer to choose between (2) and (3).

You might be interested to know that while the Sun3/os3.2
(or an HP, don't remember which) C compiler produced code
that gave the "correct" results for your code, when I replaced
that statement by the two:

	*f += *g++;      or      *f = *f + *g++;
	f++;                     f++;

I found that the size of the object code was actually reduced.
Chalk one up for readability *and* efficiency.