Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!mailrus!nrl-cmf!ames!umd5!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: volatile
Message-ID: <7915@brl-smoke.ARPA>
Date: 18 May 88 14:18:31 GMT
References: <20345@pyramid.pyramid.com> <502@wsccs.UUCP> <51431@sun.uucp> <526@wsccs.UUCP> <390@attila.weitek.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 35

In article <390@attila.weitek.UUCP> mahar@attila.UUCP (Mike Mahar) writes:
>char fifo;
>buf_fill(buffer,cnt)
>    char *buffer;
>    int cnt;
>    {
>    int i;
>    for( i = 0; i < cnt; i++)
>	*buffer++ = fifo;
	^ MISSING * ADDED
>    }
>Question: Is it safe to compile the preceding routine like this:
>	movb	_fifo,d2	#pre-load fifo
>L1:

In this particular case it is safe to pre-load `fifo' before the loop.
(It would be disallowed if `fifo' were declared "volatile".)  However,
if the code were changed to

char fifo[3];
buf_fill(buffer,cnt)
    char *buffer;
    int cnt;
    {
    int i;
    for( i = 0; i < cnt; i++)
	*buffer++ = fifo[1];
    }

Then it would NOT be permitted to pre-load fifo[0],
because the function could be invoked as

	buf_fill(fifo,3);

and the generated code must handle the aliasing correctly.