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From: robison@uiucdcsm.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: hack to fake volatile
Message-ID: <4700010@uiucdcsm>
Date: 30 Mar 88 23:31:00 GMT
Lines: 24
Nf-ID: #N:uiucdcsm:4700010:000:837
Nf-From: uiucdcsm.cs.uiuc.edu!robison    Mar 30 17:31:00 1988


I came in late on the ``volatile'' argument, but the following hack 
would seem to work without adding anything to the language.
The hack may be expressed as:

	``Chain the optimizer's paws by encapsulating the volatile variable''

We declare the variable and two functions for accessing it:

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

	#define WriteX(a) (_X = a)
	#define ReadX     (Munge(&_X), _X)

The function ``Munge'' would be in a library and be implemented as a no-op,
but not known to the optimizer!  Therefore the optimizer must invalidate
any copies of X in registers, as Munge could potentially (on Friday the
13's with a full moons) alter X.  

Arch D. Robison
University of Illinois at Urbana-Champaign
	
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