https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752
--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Chung-Kil Hur from comment #15)
> Hi Richard,
>
> Thanks for the explanation.
> But, what I wonder was how to justify such an optimization, rather than how
> it works.
>
> I have a better example. This might be a real bug of GCC.
>
> #include <stdio.h>
>
> int main() {
> int x = 0;
> uintptr_t pi = (uintptr_t) &x;
> uintptr_t i, j;
>
> for (i = 0; i < pi; i++) { }
> j = i;
> /* Note that the following "if" statement is never executed because j ==
> pi. */
Wrong, j == i != pi.
> if (j != pi) {
> j = pi;
> }
>
> *(int*)((pi+i)-j) = 15;
>
> printf("%d\n", x);
> }
>
> This program prints out "0" instead of "15".
> Here, "pi" contains the address of the variable x; and "i" and "j" contain
> the same integer.
> So, it seems that "(pi+i)-j" should have a proper provenance of "x" and thus
> the variable "x" should be updated to 15.
> However, GCC seems to think that "(pi+i)-j" has no provenance.
>
> So, as a programmer, I wonder how I should calculate the provenance of an
> integer in order to see whether casting it to a pointer is valid or not.
>
> Thanks.
