[GuB-42]

I fixed your example a bit and here is what I get

  /tmp>cat t.c
  #include <stdio.h>
  int main() {
      float acc = 0;
      float den = 1.40129846432e-45;
      for (size_t i = 0; i < (1ul<<33); i++) acc += den;
      printf("%g\n", acc);
      return 0;
  }
  /tmp>gcc -O3 t.c && time ./a.out
  2.35099e-38
  ./a.out  5.94s user 0.00s system 99% cpu 5.944 total
  /tmp>gcc -O3 -ffast-math t.c && time ./a.out
  0
  ./a.out  1.50s user 0.00s system 99% cpu 1.502 total

So subnormal numbers are supported in -O3 unless you specify -ffast-math. And it definitely makes a difference (gcc 9.3, Debian 11, Ryzen 7 3700X).

EDIT That one is interesting too

  /tmp>clang -O3 t.c && time ./a.out
  2.35099e-38
  ./a.out  6.04s user 0.00s system 99% cpu 6.044 total
  /tmp>clang -O3 -ffast-math t.c && time ./a.out
  0
  ./a.out  0.10s user 0.00s system 99% cpu 0.101 total

clang (9.0.1) performs about the same without -ffast-math; but with it, it managed to optimize the loop away.