[GitHub] szha commented on issue #14062: Bug in Hybridize w/ Concat

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szha commented on issue #14062: Bug in Hybridize w/ Concat
URL: https://github.com/apache/incubator-mxnet/issues/14062#issuecomment-461292694
 
 
   @stephenrawls after taking a close look at the code, I think the problem mainly comes from the call on [slice](https://mxnet.incubator.apache.org/versions/master/api/python/ndarray/ndarray.html#mxnet.ndarray.slice). Its doc says:
   
   >For an input array of shape=(d_0, d_1, ..., d_n-1), slice operation with begin=(b_0, b_1...b_m-1), end=(e_0, e_1, ..., e_m-1), and step=(s_0, s_1, ..., s_m-1), where m <= n, results in an array with the shape (|e_0-b_0|/|s_0|, ..., |e_m-1-b_m-1|/|s_m-1|, d_m, ..., d_n-1).
   
   Since in the code snippet, it has `begin=(0,0,0), end=(0,1,0)` without step, this means the output shape would be `(0,1,0)`. This is not supported and slice should throw an error on that.
   
   However, since the code did't check for it, NDArray actually returned result as in MXNet the dimension value 0 is used as a placeholder value for unknown size, which in symbolic mode (i.e. Gluon hybridized mode) would trigger further shape inference, which properly triggers the shape inference error.
   
   I haven't yet traced what happened in slice after getting 0 in size in the slice kernel, though that might need fix too.

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