[Bug tree-optimization/62171] restrict pointer to struct with restrict pointers parm doesn't prevent aliases

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62171

--- Comment #8 from vries at gcc dot gnu.org ---
(In reply to Richard Biener from comment #7)
> I was thinking about sth like
> 
> struct X { int i; int * __restrict__ q; };
> 
> int foo (X& __restrict__ x, X *p)
> {
>   *x.q = 1;
>   p->i = 0;
>   return *x.q;
> }
> 
> int main()
> {
>   X x;
>   x.q = &x.i;
>   return foo (x, &x);
> }
> 

I think this example's an invalid use of restrict.

By using restrict in the 'X& __restrict__ x' parameter of foo, we promise that
if the object x points to is modified during foo execution (and it is, by both
assignments) we only access the object using pointers based on x during foo
execution.

p is pointing to the same object, and we access the object via p. But p is not
based on x.