MarisaKirisame commented on issue #1996: [RFC][WIP] Tensor Expression level 
automatic differentiation
URL: https://github.com/apache/incubator-tvm/issues/1996#issuecomment-595980942
 
 
   @yzhliu let me explain it another way.
   suppose we work only at the scalar level. people had proved that reverse 
mode only take 3x more computation. This does not need any optimization - the 
gradient of f will only be a (small, typicallty less then 10) constant factor 
times slower then the computation of f itself.
   
   In this PR, the gradient of f might be MANY times more expensive.
   This is because it is calculating the jacobian, rather then the product of a 
vector/matrix/tensor with that jacobian, which can be fused, so the computation 
can be expressed in a way simpler form.

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