[Bug tree-optimization/127966] New: [15/16/17 Regression] wrong code at -O1 and above with a loop over string literal bytes >= 0x80

[kyuwoncho18 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127966

            Bug ID: 127966
           Summary: [15/16/17 Regression] wrong code at -O1 and above with
                    a loop over string literal bytes >= 0x80
           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

int main(void)
{
  int i = 0;
  while (((unsigned char *)"\x80\x80")[i] == 128)
    i++;
  return i;
}

godbolt: https://godbolt.org/z/oM5Wsxbd8

With -O0, GCC 14 or clang -O3, it returns 2, but with GCC -O1 it returns 1 and
with -O2/-O3/-Os it returns 0. GCC 15.1 and later, including current trunk, are
affected; -funsigned-char does not change the result.

Disclaimer: This test case was generated by a fuzzer, and the report was
written with LLM assistance.