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_r287109398
##########
File path: src/operator/tensor/elemwise_unary_op_basic.cc
##########
@@ -1069,13 +1069,72 @@ 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>)
+.set_attr<nnvm::FGradient>("FGradient",
+ [](const nnvm::NodePtr& n, const std::vector<nnvm::NodeEntry>& ograds) {
+ // For g(x) -> g = log
+ // g''(x) = -1 * (g'(x) * g'(x))
+ auto gx = nnvm::NodeEntry{n, 0, 0};
+ auto ggx_mid = MakeNode("elemwise_mul", n->attrs.name +
"_backward_mid_grad_grad",
+ {gx, gx}, nullptr, &n);
+ auto ggx = MakeNode("negative", n->attrs.name + "_backward_grad_grad",
+ {nnvm::NodeEntry{ggx_mid, 0, 0}}, nullptr, &n);
+
+ std::vector<nnvm::NodeEntry> ret;
+
+ ret.emplace_back(MakeNode("elemwise_mul", n->attrs.name +
"_backward_grad_grad",
+ {ograds[0], gx}, nullptr, &n));
+ ret.emplace_back(MakeNode("elemwise_mul", n->attrs.name +
"_backward_grad_grad_inp",
Review comment:
`log` is unary, but the `_backward_log` isn't.
https://github.com/apache/incubator-mxnet/blob/016556553ced0c9c9400efec2f08a3c39cbb00ba/src/operator/tensor/elemwise_unary_op_basic.cc#L1071
It takes as input the output gradient and input to log.
https://github.com/apache/incubator-mxnet/blob/016556553ced0c9c9400efec2f08a3c39cbb00ba/src/operator/tensor/elemwise_unary_op_basic.cc#L1045
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