[GitHub] [incubator-mxnet] DickJC123 commented on issue #15657: Eliminate common expressions

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DickJC123 commented on issue #15657: Eliminate common expressions
URL: https://github.com/apache/incubator-mxnet/pull/15657#issuecomment-540802351
 
 
   I believe the case of having one output driving multiple symbol outputs is handled strangely, not that this is an important use case.  The issue occurs with an operator driving 3 or more symbol outputs.  Here's the progression of model compute graphs:
   ```
   foo ----+----> out0
           |
           +----> out1
           |
           +----> out2
   ```
   copy operators inserted:
   ```
   foo ----+----> out0
           |
           +-- copy --> out1
           |
           +-- copy --> out2
   ```
   More CSE applied:
   ```
   foo ----+----> out0
           |
           +-- copy --+----> out1
                      |
                      +----> out2
   ```
   copy operators inserted:
   ```
   foo ----+----> out0
           |
           +-- copy --+----> out1
                      |
                      +-- copy --> out2
   ```
   So N outputs driven by the same node end up being driven by N - 1 serialized copies, not N-1 parallel copies.
   
   What is the use-case that requires these copies to be inserted?

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