[Bug c/107493] New: Wrong code at -Os on x86_64-linux-gnu

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107493

            Bug ID: 107493
           Summary: Wrong code at -Os on x86_64-linux-gnu
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shaohua.li at inf dot ethz.ch
  Target Milestone: ---

% gcc-tk -v
Using built-in specs.
COLLECT_GCC=gcc-tk
COLLECT_LTO_WRAPPER=/zdata/shaoli/compilers/ccbuilder-compilers/gcc-db55f1dda2692c3f778ae783bc7121891c79aec4/libexec/gcc/x86_64-pc-linux-gnu/13.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure --disable-multilib --disable-bootstrap
--enable-languages=c,c++
--prefix=/zdata/shaoli/compilers/ccbuilder-compilers/gcc-db55f1dda2692c3f778ae783bc7121891c79aec4
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.0.0 20221031 (experimental) (GCC) 
% 
% gcc-tk -O0 a.c && ./a.out
65525
% gcc-tk -Os a.c && ./a.out
65515
%
% cat a.c
int printf(const char *format, ...);
int a;
int b(int c) { 
    return c; 
}
int main() {
  a = -21;
  for (; a <= 0; a = (unsigned short)(b(a + 2) + 8))
    ;
  printf("%d\n", a);
}
%

Compiler explorer: https://godbolt.org/z/hhqrv7hWh