Read/write dependencies out of order

[asujeeth]

The following OptiML code triggers this bug:

val ipiv = DenseVector(5,7,10,8,8,9,10,9,10,10)
val P_indices = (0::10).mutable
println("P_indices before: " )
P_indices.pprint
var i = 0
while (i < ipiv.length) {
  val swap = P_indices(i)
  P_indices(i) = P_indices(ipiv(i)-1)
  P_indices(ipiv(i)-1) = swap
  i += 1
}
println("P_indices after: " )
P_indices.pprint

The P_indices value will not be correctly updated unless there is an effect inside the while loop body like println(P_indices(i)) or changing "val swap" to "var swap". I tried various other things, like disabling fusion and SoA, and trying to hide the structness of P_indices (to see if struct rewrites were the problem), but nothing else worked.

Anyone have any idea what's going on?