Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!apollo!wanginst!mckeeman
From: mckeeman@wanginst.UUCP
Newsgroups: comp.lang.c
Subject: Re: C and restraints on optimization
Message-ID: <1007@wanginst.EDU>
Date: Mon, 13-Apr-87 15:21:00 EST
Article-I.D.: wanginst.1007
Posted: Mon Apr 13 15:21:00 1987
Date-Received: Sat, 18-Apr-87 08:28:42 EST
References: <15958@sun.uucp> <5716@brl-smoke.ARPA> <14680@cca.CCA.COM> <6264@mimsy.UUCP>
Organization: Wang Institute
Lines: 48
Summary: the semantics of volatile & optimization

Chris Torek writes:
>Here is a question:  Which of those problems are sidestepped by
>volatile variables?...

>Just what does `volatile' *do* in a compiler?  What are the precise
>semantics of volatile variables?  If a volatile variable is just
>one whose value can never be assumed from context, a compiler could
>eliminate `dead' stores, so this definition is clearly wrong:
>
>	volatile v;
>	v=0; v=1;

Bill McKeeman (me) responds:

I worry that volatile, designed to deal with asynchronous processes, is
accidentally picking up some sequential access semantics baggage.  The
relevant paragraph from x3j11 is: 3.5.2.4 const and volatile

	The properties associated with the const and volatile type specifiers
	are meaningful only for expressions that are lvalues.... 

and

	...Therefore any expression referring to such an object [volatile]
	shall be evaluated strictly according to the sequence rules of the
	abstract machine...

That is pretty handy for those who want to keep the optimizer at bay.  I
interpret it to mean that both v=0; and v=1; must be executed in sequence in
Torek's example above.

A cast (volatile) can only be applied to an lvalue.  I interpret this to mean
that as a compiler writer, I am obligated to flag Torek's suggestion:

	#define	h_bar	((volatile double) (6.626e-34 / 2 / PI))

under the present rules.

QUESTION:   Do we want to use volatile variables as a (subtle) way to
	    indicate that our expressions are not to be optimized?

Would this be more intuitive and transparent than ignored parentheses and
unary +?

-- 
W. M. McKeeman            mckeeman@WangInst
Wang Institute            decvax!wanginst!mckeeman
Tyngsboro MA 01879