[Bug tree-optimization/65752] Too strong optimizations int -> pointer casts

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752

--- Comment #58 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #49)
> Related testcase from PR61502:
> 
> #include <stdio.h>
> 
> int main()
> {
>    int x, y = 1;
>    int *volatile v;
>    int *p;
> 
>    v = &y;
>    p = v;
>    if (p == &x + 1) {
>      *p = 2;
>      printf("y = %d\n", y);
>    }
> }
> 
> which shows how propagating conditional equivalences (&x+1 into *p = 2)
> breaks
> alias analysis.

We now optimize the comparison to false via ptrs_compare_unequal given
&x + 1 has provenance &x and p has provenance &y.  Thus we decided to
ignore that sub-sentence of the standard that seems to allow comparing
such pointers.  So we behave as-if we'd put at least one byte of padding
after each object.