[Bug tree-optimization/128269] New: wrong code at -O1 and above: snprintf "%lc" with a wint_t in [0, 200] is assumed not to return a negative value

[kyuwoncho18 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128269

            Bug ID: 128269
           Summary: wrong code at -O1 and above: snprintf "%lc" with a
                    wint_t in [0, 200] is assumed not to return a negative
                    value
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kyuwoncho18 at gmail dot com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

#include <stdio.h>
#include <wchar.h>

volatile int v = 128;

int main(void)
{
  char b[32];
  int x = v;
  if (x < 0 || x > 200)
    return 0;
  int r = snprintf(b, sizeof b, "%lc", (wint_t)x);
  printf("%d %d\n", r, r < 0);
  return 0;
}

godbolt: https://godbolt.org/z/7fjdf5TKE

With glibc the startup "C" locale cannot encode the wide character 128 (wcrtomb
fails with EILSEQ), so snprintf returns -1 and the program must print "-1 1",
as it does at -O0 and with -O1 -fno-printf-return-value. At -O1, -O2, -O3 and
-Os it prints "-1 0".