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From: edw@IUS1.CS.CMU.EDU (Eddie Wyatt)
Newsgroups: comp.unix.questions
Subject: Re: Wierd Compilers
Message-ID: <318@PT.CS.CMU.EDU>
Date: Fri, 6-Nov-87 17:33:48 EST
Article-I.D.: PT.318
Posted: Fri Nov  6 17:33:48 1987
Date-Received: Sun, 8-Nov-87 21:50:52 EST
References: <367@white.gcm> <18964@amdcad.AMD.COM> <44177@beno.seismo.CSS.GOV> <1182@tut.cis.ohio-state.edu>
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Organization: Carnegie-Mellon University, CS/RI
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> 
> Forgive a possibly stupid question, but in what way is there any
> difference between a++ and ++a in the generated code, even with the
> optimiser disabled.
> 
> Note:  I am not arguing with you, merely asking for more information.
> 
> 


  In case 1 (a++) the value store in "a" may have to be saved before executing
the increment if the value of the expression is used.  In case case 2
(++a) the value stored in  "a" after the operation is preformed.
Hence no extra save must be preformed.

Example:

case 1	i = a++;

	save a in b
	increment a
	i gets  b


case 2	 i = ++a;

	increment a
	i gets a;

Note there is an optimization that can be made for the first case that is  :

	i get a
	increment a

But this optimization assumes that the order of the evaluation ++ and
assignment don't matter (however in some cases it does).

	a = 1;
	a = a++;

The unoptimized case would yields an "a" value of  1.  The optimized
case would yield an "a" value of 2.  This really shows that there
is an ambiguity in the language definition.  They should have  used
denotational semantics.
-- 
That one looks Jewish -- And that one's a coon -- Who let all this riff raff
into the room -- There's one smoking a joint -- and Anoter with spots -- If
I had my way -- I'd have all of you shot ... Pink Floyd, "In the Flesh"
Eddie Wyatt 				e-mail: edw@ius1.cs.cmu.edu