Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!umd5!brl-adm!adm!christiansen@chewi.che.wisc.EDU
From: christiansen@chewi.che.wisc.EDU (REED CHRISTIANSEN)
Newsgroups: comp.lang.c
Subject: A different view of volatile
Message-ID: <13120@brl-adm.ARPA>
Date: 26 Apr 88 18:23:02 GMT
Sender: news@brl-adm.ARPA
Lines: 48

>
>From: Hank Dietz <hankd@pur-ee.uucp>
>Subject: A different view of volatile
>Message-ID: <7996@pur-ee.UUCP>
>Date: 26 Apr 88 00:29:37 GMT
>
> [Text Deleted.  Professor Dietz proposes the use of REGISTER to denote
>  data objects that cannot possibly be VOLATILE, since a REGISTER item
>  does not have an address.]
>
>although I have nothing against volatile, per se, isn't it better to go with
>the more aggressive compiler optimization only when a person has specifically
>hinted that it's ok, as opposed to the "volatile" scheme of aggressively
>optimizing unless you're told no?  Besides, by now we all know how to spell
>"register."

My observation is that the number of data items in a program that are
VOLATILE (that is, whose value(s) can change asynchronously due to
action by another process or by hardware action) is very small.  Even in
implementing real-time process control systems, the D/A and A/D handlers
are a small portion of the entire package.  It seems much easier to
declare these few variables as special cases than to declare all
non-special variables to be REGISTER.

I have no problem with aggressive optimization--I would prefer that the
compiler do as much optimization as it can, as long as I can point out
the VERY FEW data items that are non-deterministic.  In fact, our
never-ending problem is to squeeze more complicated mathematical models
into our real-time packages in order to do state-estimation, etc.  The
faster these computational loops run, the more realistic our models can
become.

If you hate VOLATILE and need to do real-time work, you have these
options:

(1) Optimize and use REGISTER, which is a burdensome but workable
alternative, requiring one to modify 90+% of declarations in one's code
from conventional C technique [TEDIOUS BUT SUITABLE CHOICE],

(2) Don't optimize, which will limit your ability to do computations in
real-time loops [VERY BAD CHOICE],

(3) Optimize computational modules; don't optimize the control loops;
make sure there is no module with both, document what you're doing, and
hope that the next fool to work on your project understands what is going
on [POTENTIALLY DANGEROUS CHOICE].

I prefer VOLATILE.
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