Re: PR 25512: pointer overflow defined?

[Richard Guenther <rguenther at suse dot de>]

On Wed, 21 Dec 2005, Robert Dewar wrote:

> Gabriel Dos Reis wrote:
> 
> >with no more qualifications (this definition is slightly different in
> >some nearby languages), it is slippery to found optimizations on
> >"pointer overflows."
> >  
> >
> Well I think unfortunately the standard does allow such "optimizations", but
> that does
> not mean it is a good thing to take advantage of this.

Another example is

char a[2];
void foo(void)
{
  char *p = &a[1];
  if (p + 0xffffffff > p)
    abort ();
}

where, on a 32bit arch, p + 0xffffffff will point to a[0] (does the std
say so?), but we still optimize the test to if (1) abort (); because
we say "pointer overflow" is undefined.  Using p - 1 > p will work as
expected.

So the basic question remains - is pointer overflow defined?

Richard.