Re: RFA: PATCH to make fold_indirect_ref_1 fold more things

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

On Thu, Oct 28, 2010 at 2:45 PM, Jason Merrill <jason@redhat.com> wrote:
> On 10/27/2010 02:29 PM, Richard Guenther wrote:
>>
>> On Tue, Oct 26, 2010 at 6:35 PM, Jason Merrill<jason@redhat.com>  wrote:
>>>
>>> For constexpr I need to be able to fold some tree forms that
>>> fold_indirect_ref_1 didn't handle; this patch extends it to handle
>>> folding
>>> POINTER_PLUS_EXPR to an ARRAY_REF, and also folding to COMPONENT_REF.
>>
>> I think by making this kind of transforms you are prone to bugs like
>> PR44468.
>
> Ah, interesting.  But it seems to me that this case is different because we
> are starting from an ADDR_EXPR around an expression of the aggregate type in
> the folded expression, not just any pointer to the aggregate type.  What do
> you think?

Hm, I think what might save you is that you only look into immediate
fields (not fields
in sub-structs as we originally did).

Now I am concerned about sth like

struct S {
    int i;
    int j;
};
struct R {
    struct S a;
    int k;
};
struct S s;
int main()
{
  struct R *p = (struct R *)&s;
  s.i = 1;
  s.j = 2;
  (*(struct S *)&*p).i = 0;
  if (s.i != 0)
    abort ();
  return 0;
}

where if we end up folding the obfuscated access to p->a.i = 0 we will generate
wrong code (one might argue that *p is invalid in C, but I'm viewing this from
a middle-end POV, not a C one).  Now if &* is already folded it will look as
p and so your code wouldn't trigger, but I guess this is just a matter of
obfuscating (like using &(*p + offset)) - the point is that the actual access
would change from one via struct S to one via struct R (and the alias set
of struct is a subset of that of R, but not the other way around).

So - what kind of testcases are you trying to handle?  (the middle-end will
optimize this stuff keeping TBAA info correct during forwprop)

Richard.

> Jason
>