Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: `noalias' vs `register'
Message-ID: <6895@brl-smoke.ARPA>
Date: 21 Dec 87 08:55:41 GMT
References: <8712170057.AA17035@decwrl.dec.com> <6858@brl-smoke.ARPA> <2270@dasys1.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 11
Keywords: noalias ANSI

In article <2270@dasys1.UUCP> jsb@dasys1.UUCP (Jim Baumbach) writes:
>Before the invention of volatile, all variables were potentially volatile
>and optimizers which treated them otherwise were WRONG.

No, there was simply no guarantee one way or the other.
Some flavors of PCC had a special hack that detected a probable
reference to a Unibus device register and disabled optimizations
for the expression containing it, much as "volatile" now does.
The original (Ritchie) PDP-11 C compiler didn't do enough
optimization for this to ever be an issue.  (Of course, it didn't
need to; the PDP-11 instruction set and C were a close match.)