On Fri, Mar 11, 2022 at 8:21 PM Haoxin Tu via Gcc <gcc@gcc.gnu.org> wrote:
>
> Dear developers,
>
> May I seek your confirmation to check whether the following program
> triggers a true wrong-code issue or not? I don't want to make noise to the
> bug repository so I'd like to seek your confirmation here first.
>
> The following case makes GCC outputs differ under -O1 below vs -O1 above
> optimization levels. Here is the program (s.c):
> ```
> #include <stdint.h>
> union {
> int32_t a;
> uint16_t b;
> } c;
> static uint16_t *d = &c.b;
> int32_t *e = &c.a;
> int32_t **f = &e;
> int32_t ***g = &f;
> int32_t *h = &c.a;
> int64_t i() {
> int8_t j = 5;
> *h = j;
> ***g = *d;
> return 0;
> }
> int main() {
> c.a = 1;
> c.b = 1;
> i();
> printf("%d\n", c.a);
> }
> ```
>
> ```
> $gcc-trunk -O0 -w s.c ; ./a.out
> 5
> $gcc-trunk -O1 -w s.c ; ./a.out
> 5
> $gcc-trunk -O2 -w s.c ; ./a.out
> 1
> $gcc-trunk -O3 -w s.c ; ./a.out
> 1
> $gcc-trunk -Os -w s.c ; ./a.out
> 1
> ```
>
> What surprised me was that almost all GCC versions produce different
> results (please check more details here: https://godbolt.org/z/vTzhhvnGE).
> Besides, clang also has the same issue with -O0 vs the above versions. So,
> does this program have any UB? If not, I would like to file new bug reports
> then. Thank you so much!
Yes it looks like a strict aliasing issue though GCC does not
implement some aliasing rules sometimes with unions, see PR 82224.
Thanks,
Adnrew Pinski
>
>
> Best regards,
> Haoxin
