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From: jon@maui.cs.ucla.edu (Jonathan Gingerich)
Newsgroups: comp.lang.c
Subject: Re: Can analysis detect undefined expressions?
Message-ID: <1991Jun21.165226.15192@cs.ucla.edu>
Date: 21 Jun 91 16:52:26 GMT
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In article <14489.2861906B@stjhmc.fidonet.org> Dave.Harris@f14.n15.z1.fidonet.org (Dave Harris) writes:
>In a message of <Jun 18 20:31>, Dale Worley (1:114/15) writes: 
>> An extended example so that I can think clearly here:
>>      (j = ((i=1) == (i=2))) == (j = ((i=3) == (i=4)))
>> Assumedly, i can end up as 1,2,3, or 4.  j should be 0.  The grouping is 
>such 
>> that i=4, i=2, i=3, i=1 won't happen without breaking any laws.  right?
> >Sorry to belabor this yet again, but there is no requirement in Ansi C
> >that i have one of the values 1, 2, 3, or 4.  The effect of this
> >statement is "undefined", which means that the implementation can do
> >*anything*, including giving i the value 100, core dumping, or
> >starting World War III.  Ditto for j.  The mere fact that none of
> >these actions are mentioned in the statement is irrelevant.
>Not arguing that the result is undefined as you say.  But....
>I for one would quickly scrap any compiler that went to the additional work of 
>embedding code to yield a value of anything other than 1,2,3 or 4 for i.  It 
>would mean the compiler would have to detect the undefined statement first 
>before it could even do this.  ...
Not if it were setting i on 4 CPUs run in parallel!                    Jon.