Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!bloom-beacon!think!ames!pasteur!ucbvax!sdcsvax!ucsdhub!esosun!seismo!uunet!mcvax!ukc!its63b!aiva!richard
From: richard@aiva.ed.ac.uk (Richard Tobin)
Newsgroups: comp.lang.c
Subject: Re: noalias; parens honored
Keywords: ANSI C standard
Message-ID: <236@aiva.ed.ac.uk>
Date: 21 Jan 88 15:44:50 GMT
References: <6829@brl-smoke.ARPA> <2845@zeus.TEK.COM> <6860@brl-smoke.ARPA> <12211@orchid.waterloo.edu> <2648@mmintl.UUCP>
Reply-To: richard@uk.ac.ed.aiva (Richard Tobin)
Organization: Bannerman's Bar, Cowgate, Edinburgh
Lines: 20

In article <2648@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
>>    ((a + 1) -1)
>
>In this example, the rule does forbid full optimizing on many machines.
>(One can still compute (a + 1), and then use a, skipping the subtraction.)

What overflow behaviour does optimization have to preserve?  Presumably
the standard does not specify behaviour when overflow would occur, so that
all you have to do is ensure that your re-arrangement doesn't cause
overflow that wouldn't have happened otherwise.  Optimizing the above
expression to 'a' never does this.

Of course, this might not be consistent with what a particular machine
does with overflow, but I doubt that the standard requires consistent
overflow behaviour.

-- 
Richard Tobin,                         JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,             ARPA:  R.Tobin%uk.ac.ed@nss.cs.ucl.ac.uk
Edinburgh University.                  UUCP:  ...!ukc!ed.ac.uk!R.Tobin