Changing the three kahan_* variables to volatile makes this work (slowly) with -Ofast.

  #include <stdio.h>

  int main(int argc, char **argv) {
    int i;
    double sample, sum;
    double kahan_y, kahan_t, kahan_c;

    // initial values
    sum=0.0;
    sample=1.0; // start with "large" value

    for (i=0; i <= 1000000000; i++) { // add 1 large value plus 1 billion small values
      // Kahan summation algorithm
      kahan_y=sample - kahan_c;
      kahan_t=sum + kahan_y;
      kahan_c=(kahan_t - sum) - kahan_y;
      sum=kahan_t;

      // pre-load next small value
      sample=1.0E-20;
    }
    printf("sum: %.15f\n", sum);
  }