Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!columbia!topaz!ll-xn!mit-amt!mit-eddie!genrad!decvax!tektronix!teklds!copper!stevesu
From: stevesu@copper.UUCP (Steve Summit)
Newsgroups: net.lang.c
Subject: Re: C Compiler bug
Message-ID: <500@copper.UUCP>
Date: Thu, 24-Jul-86 02:56:39 EDT
Article-I.D.: copper.500
Posted: Thu Jul 24 02:56:39 1986
Date-Received: Thu, 24-Jul-86 21:25:23 EDT
References: <501@bunny.UUCP>
Distribution: net
Organization: Tektronix, Inc., Beaverton, OR
Lines: 46
Summary: Yes, it's a bug

In article <501@bunny.UUCP>, mlr0@bunny.UUCP (Martin Resnick) writes:
> Please try this code on your flavor of C compiler...
(followed by code demonstrating essentially
>    i = i/2.5;
vs.
>    i /= 2.5;

The answer is absolutely unequivocal on this point.  A compiler
that does not treat i=i/float and i/=float identically is broken,
although many compilers are so broken.  This question was
discussed to death on the net a year or so ago.  The discussion
came to a screeching halt, quite unlike most net discussions,
when the following article appeared from one dmr@research:

> From: dmr@research.UUCP
> Newsgroups: net.lang.c
> Subject: bug in type conversion
> Date: Wed, 4-Jan-84 21:32:17 PST
>
> Mike O'Brien points out that in the C compilers he has available, the
> expression
> 	i *= d;
> where  i  is int and  d  is double is evaluated in fixed point, and
> wonders why.  The answer: it is a compiler bug.  I fixed it in
> (a post V7 version) of the 11 compiler, and it is fixed in the current
> System V compiler (by "current" I mean the one I tried;
> I don't know what is being shipped at this instant.)
>
> The manual is reasonably clear and unambiguous on the point, but
> it's not surprising that people for search for definition problems when
> the compilers are unanimously wrong.
>
> 	Dennis Ritchie

What the manual says is (K&R page 191):

	"The behavior of an expression of the form E1 op= E2 may
	be inferred by taking it as equivalent to E1 = E1 op (E2);
	however, E1 is evaluated only once."

The single evaluation of E1 is the only difference between the
two forms; any type casting, including truncation, should be done
exactly as if E1 = E1 op (E2) had been performed.

                                         Steve Summit
                                         tektronix!copper!stevesu