[GitHub] [incubator-mxnet] kshitij12345 commented on a change in pull request #14992: [MXNET-978] Support higher order gradient for `log`.

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kshitij12345 commented on a change in pull request #14992: [MXNET-978] Support higher order gradient for `log`.
URL: https://github.com/apache/incubator-mxnet/pull/14992#discussion_r285357016
 
 

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 File path: src/operator/tensor/elemwise_unary_op_basic.cc
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 @@ -1016,7 +1016,28 @@ The storage type of ``log2`` output is always dense
 .set_attr<nnvm::FGradient>("FGradient", ElemwiseGradUseIn{"_backward_log2"});
 
 MXNET_OPERATOR_REGISTER_BINARY_WITH_SPARSE_CPU_DR(_backward_log,
-                                                  unary_bwd<mshadow_op::log_grad>);
+                                                  unary_bwd<mshadow_op::log_grad>)
 
 Review comment:
   I don't know much about this library but,
   
   I believe it would be better to have gradients defined for  existing backward, instead of a differentiable gradient (relying on  autograd machinery) at least on `ops`  where backward is not trivial. It will allow to use existing optimised fused kernels and make sure there is no regression in the backward.
   
   Note: `log` is relatively trivial (single reciprocal). But maybe we may see a performance regression for `sigmoid`, if we do it by relying on autograd machinery instead of the existing `_backward_sigmoid`.

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