Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <10293@mimsy.UUCP>
Date: 22 Jan 88 03:45:05 GMT
References: <3819@sigi.Colorado.EDU> <8599@ism780c.UUCP> <1866@bsu-cs.UUCP> <4037@june.cs.washington.edu>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 30

[One-line summary: some people are suffering from the illusion that
`;' allows optimisations that `,' does not.]

In article <4037@june.cs.washington.edu> pardo@june.cs.washington.edu
(David Keppel) provides the following example:
>    int	*ctrl_p = DISK_CTRL_REGISTER;
>    int	*out_p = DISK_OUT_REGISTER;
>
>    *ctrl_p = SETUP, *out_p = GO;
>
>which _relies_ on the evaluation order being guaranteed in a
>stronger way than is provided by sequence points.

Yes.  However, the same code written this way:

	*ctlr_p = SETUP;
	*out_p = GO;

*also* depends on order in a stronger way than is provided by
sequence points.  The *only* way to guarantee proper operation
under dpANS rules is to write

	volatile int *ctlr_p = DISK_CTRL_REGISTER;
	volatile int *out_p = DISK_OUT_REGISTER;

and in such a case the compiler is required to generate
equivalently ordered code for either version (`,' or `;').
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris