[Bug c/109967] New: Wrong code at -O2 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=109967

            Bug ID: 109967
           Summary: Wrong code at -O2 on x86_64-linux-gnu
           Product: gcc
           Version: 14.0
            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: ---

For the following code, gcc trunk at -O2 emits the wrong code. This seems to be
a long latent bug since GCC-7 (I tried gcc-7.4 on compiler explorer, which
still emits the wrong result.)

Compiler explorer: https://godbolt.org/z/YGeWedWq7

$ cat a.c
int printf(const char *, ...);
int a, c;
int main() {
  long f[2];
  int e = 0;
  for (; e < 2; e++) {
    {
      char g[20];
      char *b = g;
      int d = 48, e = 0;
      while (d && e < 5)
        b[e++] = d /= 10;
      c = e;
    }
    f[c - 2 + e] = 1;
  }
  a = f[0];
  printf("%d\n", a);
}
$
$ gcc-tk -O0 a.c && ./a.out
1
$ gcc-tk -O2 a.c && ./a.out
4
$
$ gcc-tk -v
Using built-in specs.
COLLECT_GCC=gcc-tk
COLLECT_LTO_WRAPPER=/zdata/shaoli/compilers/ccbuilder-compilers/gcc-8d5f050dabbf6dd3b992c3b46661848dbcf30d9e/libexec/gcc/x86_64-pc-linux-gnu/14.0.0/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-8d5f050dabbf6dd3b992c3b46661848dbcf30d9e
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 14.0.0 20230523 (experimental) (GCC) 
$