[Bug rtl-optimization/121852] New: wrong code at -O2 with "-fno-gcse" on x86_64-linux-gnu

[zhendong.su at inf dot ethz.ch via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121852

            Bug ID: 121852
           Summary: wrong code at -O2 with "-fno-gcse" on x86_64-linux-gnu
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
  Target Milestone: ---

Compiler Explorer: https://godbolt.org/z/6MrKz8cfr

Note:
- fails: 12.1 and later
- works: 11.4 and earlier
- -fno-tree-slsr and -fno-ivopts do not make it disappear

[675] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/home/suz/suz-local/software/local/gcc-trunk/bin/../libexec/gcc/x86_64-pc-linux-gnu/16.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--enable-checking=yes --prefix=/local/suz-local/software/local/gcc-trunk
--enable-sanitizers --enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 16.0.0 20250908 (experimental) (GCC)
[676] %
[676] % gcctk -O2 small.c; ./a.out
[677] % gcctk -O2 -fno-gcse small.c
[678] % ./a.out
Floating point exception
[679] % cat small.c
int a[] = {0}, d, e, h, i, j, k, l, n[1], *o = n;
volatile int m;
int p(char q) { return a[e ^ (q & 5)]; }
int s(int q[]) {
  int f, g = 0;
  unsigned b = 5;
  for (; g < d; ++g) {
    int c = p(q[g] >> 6);
    b = f = (c & 4095) ^ a[c & 5];
  }
  return b;
}
int u(volatile int q) {
  k = 5 % q;
  int r[] = {h, i, k, j};
  return s(r);
}
int main() {
  int t;
  do {
    if (u(5))
      m = 4;
    l--;
    t = l - 1 % m + 1;
  } while (!u(5));
  o[0] = 2 % t;
  return 0;
}