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From: barmar@mit-eddie.UUCP (Barry Margolin)
Newsgroups: net.lang.c
Subject: Re: Must useless expressions be evaluated?
Message-ID: <817@mit-eddie.UUCP>
Date: Sat, 15-Oct-83 02:33:28 EDT
Article-I.D.: mit-eddi.817
Posted: Sat Oct 15 02:33:28 1983
Date-Received: Sun, 16-Oct-83 10:22:30 EDT
References: <317@ucbtopaz.CC.Berkeley.ARPA>
Organization: MIT, Cambridge, MA
Lines: 31

ether there are any side effects and whether
the value of an expression is used.

PL/I (or at least the Multics version) allows the programmer to give a
function the "reducible" attribute.  A reducible function is one which
does not have any side effects and whose value is solely a function of
the arguments (i.e., not global variables can affect the value).  The
compiler is then free to throw away excess calls, i.e. it may translate
	y = foo (x) + foo (x);
to
	y = 2 * foo (x);
and it may skip over reducible function calls in boolean expressions,
i.e.
	if bool_var | foo (x) then ...
is permitted to not evaluate foo (x) if bool_var is true.
-- 
			Barry Margolin
			ARPA: barmar@MIT-Multics
			UUCP: ..!genrad!mit-eddie!barmar