[Bug sanitizer/108060] New: UBsan missed an out-of-bound bug at -O0

["shaohua.li at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108060

            Bug ID: 108060
           Summary: UBsan missed an out-of-bound bug at -O0
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shaohua.li at inf dot ethz.ch
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

For the following code, UBsan at -O0 missed reporting the out-of-bound access,
while -O1 and above caught it. 

Clang could detect it at all optimization levels.

Compiler explorer: https://godbolt.org/z/Tb9Mern7M

% cat a.c
int a[8];
short b;
char c;
int main() {
  b = -32768;
  a[b] |= c;
}
%
% gcc-tk -O0 -fsanitize=undefined -fno-sanitize-recover=all a.c && ./a.out
Segmentation fault
% gcc-tk -O1 -fsanitize=undefined -fno-sanitize-recover=all a.c && ./a.out
a.c:6:4: runtime error: index -32768 out of bounds for type 'int [8]'
%

Interestingly, if you don't use `-fno-sanitize-recover=all`, none of opt levels
could detect it:
% gcc-tk -O1 -fsanitize=undefined a.c && ./a.out
Segmentation fault
%