Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!iuvax!pur-ee!uiucdcs!uiucdcsb!robison
From: robison@uiucdcsb.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: hack to fake volatile
Message-ID: <165600036@uiucdcsb>
Date: 3 Apr 88 21:23:00 GMT
References: <4700010@uiucdcsm>
Lines: 25
Nf-ID: #R:uiucdcsm:4700010:uiucdcsb:165600036:000:870
Nf-From: uiucdcsb.cs.uiuc.edu!robison    Apr  3 15:23:00 1988



Scott Daniels (daniels@teklds.UUCP) pointed out a bug in my original hack
to fake "volatile".  There are situations involving writes-only which an 
"optimizer" may incorrectly transform.  

One needs a call to "Munge" in both the read and write routines to
"flush the caches", so to speak.

	int _X;		/* local to WriteX and ReadX */

	#define WriteX(a) (_X=a, Munge(&_X))	/* Munge is a secret no-op */
	#define ReadX     (Munge(&_X), _X)

I'm not sure how to formally verify this hack.  I suppose one could argue
that any asynchronous writes and reads could be simulated by the Munge calls,
and therefore the optimizer must keep its paws off.  But this is far from 
a convincing argument.

Arch D. Robison
University of Illinois at Urbana-Champaign
	
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