[GitHub] [incubator-mxnet] ptrendx commented on a change in pull request #17849: Fix SoftReLU fused operator numerical stability

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ptrendx commented on a change in pull request #17849: Fix SoftReLU fused operator numerical stability
URL: https://github.com/apache/incubator-mxnet/pull/17849#discussion_r393270412
 
 

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 File path: src/operator/fusion/fused_op-inl.h
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 @@ -550,7 +550,10 @@ __device__ inline DType sigmoid(const DType val) {
 
 template <typename DType>
 __device__ inline DType softrelu(const DType val) {
-  return logf(1 + expf(val));
+  // Avoid overflow of exp for large inputs.
+  // The threshold 20 is chosen such that softrelu(a) = a
+  // for a > 20 using floating precision.
+  return val > 20 ? val : logf(1 + expf(val));
 
 Review comment:
   From the test I did - difference between `logf(1 + expf(20)) - 20` is 0 already, so I think there should not be any difference (and if anything, going to max float should give less precision).

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