/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

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

I've been fighting the compiler to generate a minimal working example of the subnormals, but didn't have any success.

Some things take need to be taken in account (from the top of my head):

- Rounding. You don't want to get stuck in the same number. - The FPU have some accumulator register that are larger than the floating point register. - Using more register than the architecture has it not trivial because the register renaming and code reordering. The CPU might optimize in a way that the data never leaves those register.

Trying to make a mwe, I found this code:

    #include <stdio.h>
    
    int
    main ()
    {
        double x = 5e-324;
        double acc = x;
    
        for (size_t i; i < (1ul<<46); i++) {
                acc += x;
    
        }
    
        printf ("%e\n", acc);
    
        return 0;
    }

    gcc double.c -o double -O0

    gcc double.c -o double -O1

I also manage to create a code that sometimes run in 1s, but in others would take 30s. Didn't matter if I recompiled.

Floating point is hard.