void sse_sqrt(float *a){
 // We assume N % 4 == 0.
 int nb_iters = N / 4;
 __m128 *ptr = (__m128*)a;
 int i;
 for(i = 0; i < nb_iters; i++, ptr++, a += 4)
 _mm_store_ps(a, _mm_sqrt_ps(*ptr));
}