int a[100], b[100], c[100], d[100];

void foo ()
{
  int i;
  for(i=1; i< 99; i++)
  {
    a[i] = b[i-1] + c[i];
    b[i] = b[i+1] + d[i];
  }
}

gcc -O3 -ffast-math -ftree-vectorizer-verbose=2 -c foo.c

foo.c:6: note: not vectorized, possible dependence between data-refs b[D.2728_3] and b[i_17]
foo.c:3: note: vectorized 0 loops in function.

However, if we reorder the two statements in the loop, then it can be vectorized. open64 can do this reordering.