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Path: utzoo!mnetor!seismo!rutgers!ames!hc!hi!josh
From: josh@hi.uucp (Josh Siegel )
Newsgroups: comp.lang.c
Subject: Re: Stupid evaluation ordering.  Is this legal?
Message-ID: <4627@hi.uucp>
Date: Sun, 19-Apr-87 02:34:12 EST
Article-I.D.: hi.4627
Posted: Sun Apr 19 02:34:12 1987
Date-Received: Sun, 19-Apr-87 18:01:22 EST
References: <4395@hi.uucp> <1104@ius2.cs.cmu.edu>
Reply-To: josh@hi.UUCP (Josh Siegel)
Organization: U. of New Mexico, Albuquerque
Lines: 71
Keywords: K&R, Bible, PCs, UNIX

In article <1104@ius2.cs.cmu.edu> edw@ius2.cs.cmu.edu (Eddie Wyatt) writes:
>In article <4395@hi.uucp>, josh@hi.uucp (Josh Siegel ) writes:
>> I just lost a argument and am running to the net for comfort...
>> 
>> According to K&R(Bible) page 185,  a C compiler is allowed to do the
>>  following.  
>> 
>> Given:
>> 
>> 	a(b()+c(),d()+e())
>> 
>> 	It is allowed to run functions b() and d(), then run
>> 	c() and e(), THEN add them (This has shades of
>> 	compilers that optimise on parallel machines).  
>> 	This causes problems with stuff like:
>
>
>   Never assume anything about the order of evaluation of parameters to
>a function.  One thing, you are not guarenteed that b()+c() will be
>evaluated before d()+e().  I know, I had code break because of this
>very same thing.  I'm not sure,but you may not even be guarenteed that 
>b() will be evaluated before c().  Is it consider a legal optimization -
>to reorder the sequence of function calls within an expression?
>
>-- 
>					Eddie Wyatt

FLAME ON!!

	READ THE QUESTION!!!

FLAME OFF!


I said "c() and e()".  Not, "b()+c()".  I am talking about the compiler
doing..

	a(b()+c(),d()+e())

Compiled

b() -> temp1
d() -> temp2
c() -> temp3
e() -> temp4
temp1+temp3->STACK
temp2+temp4->STACK

As apposed to:

e()+d() -> stack
c()+b() -> stack

Oh well... it turns out that we found proof in K&R that says that
 they can't do

a(b(4)->left,b(5)->right) in the following order:

b(4) -> temp1
b(5) -> temp2
follow temp1
follow temp2

Once it starts a expression it will complete it (in what ever
 order it wishes) as restricted by K&R.

			--Josh Siegel
-- 
Josh Siegel		(siegel@hc.dspo.gov)
                        (505) 277-2497  (Home)
		I'm a mathematician, not a programmer!