This testcase:

#define N 32

int a[N], b[N];

int
foo ()
{
  for (int i = 0; i < N ; i++)
  {
    int m = (a[i] & i) ? 5 : 4;
    b[i] = a[i] * m;
  }
}

does not vectorize at -O3 on x86_64 or other platforms. Following dom1, jump threading partially peels the loop to give:


  <bb 2>:
  goto <bb 8>;

  <bb 3>:
  # i_11 = PHI <i_9(6)>
  _5 = a[i_11];
  _6 = i_11 & _5;
  if (_6 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

  <bb 4>:

  <bb 5>:
  # m_14 = PHI <5(4), 4(3)>

  <bb 6>:
  # m_2 = PHI <m_14(5), 4(8)>
  # _15 = PHI <_5(5), _10(8)>
  # i_16 = PHI <i_11(5), i_1(8)>
  _7 = m_2 * _15;
  b[i_16] = _7;
  i_9 = i_16 + 1;
  if (i_9 != 32)
    goto <bb 3>;
  else
    goto <bb 7>;

  <bb 7>:
  return;

  <bb 8>:
  # i_1 = PHI <0(2)>
  _10 = a[i_1];
  _3 = i_1 & _10;
  goto <bb 6>;

which form cannot be if-converted (tree-if-conv.c):

  /* If one of the loop header's edge is an exit edge then do not
     apply if-conversion.  */
  FOR_EACH_EDGE (e, ei, loop->header->succs)
    if (loop_exit_edge_p (loop, e))
      return false;

and even if it were, the PHI nodes at loop entry cannot be handled by the vectorizer.