[Bug c/106920] New: -Warray-bound false positive regression with -O2 or -Os

[npfhrotynz-ptnqh.myvf at noclue dot notk.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106920

            Bug ID: 106920
           Summary: -Warray-bound false positive regression with -O2 or
                    -Os
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: npfhrotynz-ptnqh.myvf at noclue dot notk.org
  Target Milestone: ---

Hello,

I think I've run into a false positive on this file:
https://source.codeaurora.org/external/imx/imx-atf/tree/plat/imx/imx8m/hab.c?h=lf_v2.6

I could trim it down to this
----
#include <stdint.h>

typedef void hab_rvt_entry_t(void);

int main() {
        hab_rvt_entry_t *a;
        a = ((hab_rvt_entry_t *)(*(unsigned long *)(0x908)));
        a();
        return 0;
}
----
$ gcc -O2 -Warray-bounds -c t.c
t.c: In function ‘main’:
t.c:7:34: warning: array subscript 0 is outside array bounds of ‘long unsigned
int[0]’ [-Warray-bounds]
    7 |         a = ((hab_rvt_entry_t *)(*(unsigned long *)(0x908)));
      |                                 ~^~~~~~~~~~~~~~~~~~~~~~~~~~
----

According to godbolt this passed on 11.3 and starts emitting the warning on
12.1 (it doesn't have 12.0) and still emits it on trunk.

Note the warning requires -O2, -O3 or -Os to be emitted.


The problem seems to be that it considers an arbitrary address casted to u64*
to be a u64[0] ?

If so that might be a problem for quite a few embedded products as that is
quite common when dealing with hardware registers.
(and who doesn't love products that compile with -Werror for release builds...)

Thanks!