We should avoid index x into itself since doing so risks collision.

torch-index-collision.txt

th> i = torch.LongTensor(10)
                                                                      [0.0001s]
th> i:random(10)
  8
  2
  2
  2
  7
  2
  4
 10
  5
 10
[torch.LongTensor of size 10]

th> x:uniform()
 0.8798
 0.2683
 0.2149
 0.5722
 0.6652
 0.5961
 0.6528
 0.0276
 0.4766
 0.6235
[torch.DoubleTensor of size 10]

                                                                      [0.0001s]
th> x2=x:index(1, i)
                                                                      [0.0000s]
th> x:index(x, 1, i)
  8
  2
  2
  2
  7
  2
  4
 10
  5
 10
[torch.LongTensor of size 10]

                                                                      [0.0001s]
th> x:eq(x2)
 1
 1
 1
 1
 1
 1
 0
 1
 0
 1
[torch.ByteTensor of size 10]