Re: [PATCH PR45098] Disallow NULL pointer in pointer arithmetic

[Richard Guenther <richard.guenther@gmail.com>]

2011/6/17 Jeff Law <law@redhat.com>:
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> On 06/17/11 08:12, Zdenek Dvorak wrote:
>
>>>> if we do, we definitely should not -- the only point of such a construction is
>>>> to bypass the pointer arithmetics restrictions,
>>>
>>> Ok, we don't.  Where does the C standard say that NULL + 0 is undefined?
>>
>> I don't think it explicitly states that it is undefined.  But 6.5.6 #7 and #8 only
>> give semantics for pointers to objects,
> Right.  I think we need to review the standard closely, but IIRC
> pointers are allowed to be NULL, point into an object or point to one
> element beyond an object.
>
> If that's correct P+C could only result in a NULL value if P was one
> element beyond the end of an object and C happened to be the exact magic
> value to wrap P to zero.  I have a hard time seeing how code could which
> did that (and relied upon it) could ever be standard conforming.
>
> Closely related, given P+C where the result is a pointer ought to tells
> us that P is non-null since the only way it could be null would be if C
> was the exact magic constant to make the result point to a valid object.

So you propose we compile the following to an abort()?

int __attribute__((noinline)) foo (void *p, int i)
{
  return p + i != NULL;
}
int main()
{
  if (foo(NULL, 0))
    abort ();
  return 0;
}

?  Thus, would it be ok to fold ptr + offset ==/!= NULL to false/true?

I don't think we should move this kind of undefinedness from C to
the GIMPLE semantics.  What do other languages allow that
we have to support (what did K&R C specify?).

Richard.

> Jeff
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