try to recreate https://github.com/JuliaLang/julia/pull/9925 in C

fmatest.c

#include <stdio.h>
#include <math.h>

int main() {
  volatile double a = 0.1;
  volatile double x = a + 0.2;
  printf("%.20f\n", x);
  volatile double y = fma(a, 1.0, 0.0);
  printf("%.20f\n", y);
  volatile double z = y + 0.2;
  printf("%.20f\n", z);
  return 0;
}

Compiled with gcc fmatest.c -o fmatest -lm, on GCC 4.8.2, gives:

0.30000000000000004441
0.10000000000000000555
0.30000000000000004441