complex numbers and optimization flags

[Gideon]

The following bit of code uses the complex data type to construct 5 complex numbers of the form

e^{i PI * j/4}

with j = 0, 1, 2, 3,4.

It then stores them in a double array, with the real components in 0-4, and the imaginary components in 5-9. When I compile and run the code with O2 flags, I get what I'd expect:

1 + i 0
0.707107 + i 0.707107
6.12303e-17 + i 1
-0.707107 + i 0.707107
-1 + i 1.22461e-16


But when I compile and run with O3 or Ofast flag, I get:

1 + i 0
0.707107 + i 0.707107
6.12323e-17 + i 1
-0.707107 + i 0.707107
-1 + i -0.00400571

It is this last entry that's troubling. I am using gcc 4.6.3, which I obtained through macports. I use the GSL library, but only to get the constant PI. What I'm trying to understand is:

1. Is this just the price I pay for further optimization, or is it a bug?

2. Can anyone suggest what optimization flag associated with O3 and Ofast that's causing this?

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<complex.h>
#include <gsl/gsl_math.h>

#define N 5

int main(int argc,char *argv[]){

double y[2*N];

double complex b;

int i;

for (i=0;i<N;i++){
b = cexp(_Complex_I * .25 * M_PI * i);
y[i] = creal(b);
y[i+N] = cimag(b);
printf(" y[%i] = %g + i %g \n",i, y[i], y[i+N]);
}

return 0;
}