[jira] [Created] (MXNET-507) Two problems in our ordering operators (topk, sort, argsort)

["Xingjian Shi (JIRA)" <ji...@apache.org>]

Xingjian Shi created MXNET-507:
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             Summary: Two problems in our ordering operators (topk, sort, argsort)
                 Key: MXNET-507
                 URL: https://issues.apache.org/jira/browse/MXNET-507
             Project: Apache MXNet
          Issue Type: Bug
            Reporter: Xingjian Shi


There are two problems in the ordering operators, i.e, topk, sort, argsort:

1) Only real_t is supported.

2) The indices are stored as real_t. This will cause error in the backward pass where the gradient are passed to the wrong locations.

For example, the following code cannot be run in the previous version:

```python

import mxnet as mx
import numpy as np
import mxnet.ndarray as nd

ctx = mx.cpu()

a = mx.nd.arange(54686454, ctx=ctx, dtype=np.int32)
a.attach_grad()

k = 10
with mx.autograd.record():
 b = mx.nd.topk(a, k=k, ret_typ='value')
 b.backward(mx.nd.ones((k,), ctx=ctx, dtype=np.int32))
a_grad = a.grad.asnumpy()
for i in range(-1, - k - 1, -1):
 assert a_grad[i] == 1

```

 

I propose to fix this bug by changing the dtype of the indices to int32. However, this will make the code to be backward incompatible.



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