------- Comment #10 from rakdver at atrey dot karlin dot mff dot cuni dot cz
2006-04-03 17:22 -------
Subject: Re: [4.1 Regression] wrong final value of induction variable
calculated
> > (In reply to comment #6)
> > > I believe c-common.c:pointer_int_sum is wrong in relying on pointer
> > > overflow
> > > during conversion of the integer offset to an unsigned pointer. I'm
> > > sending
> > > a patch that fixes this for comments.
> >
> > The patch seems a bit too conservative to me; perhaps just always comparing
> > the
> > offsets as signed could work?
>
> I'm not a language lawyer here - and as this is the second (or third)
> patch to this folding to correct problems I'd rather be safe than sorry
> this time. I'm sure jsm can construct a testcase where comparing offsets
> as signed leads to wrong code. Maybe
>
> char *memory = 0;
>
> int foo(void)
> {
> return memory + 0x80000000 < memory;
> }
>
> int main()
> {
> if (foo())
> abort ();
> }
>
> i.e. have a mapping >2Gb on a 32bit machine. A corner case, but valid I
> guess.
no -- the result in this example is undefined. The comparisons are only
defined for pointers in the same object. I guess nothing really
prevents having an object whose size is more than half of the address
space, though.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26763
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