[GitHub] [tvm] wrongtest edited a comment on pull request #9482: Implementation of Common Subexpression Elimination for TIR

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wrongtest edited a comment on pull request #9482:
URL: https://github.com/apache/tvm/pull/9482#issuecomment-972538596


   I have another three questions about the pass.
   
   1.  Since function calls are non-eligible, it seems to prevent many common non-stateful builtin computations from being optimized, such as `tir.floordiv`, `tir.shift_right`, `tir.likely`, `tir.log` and etc.  TVM already have op annotation like `PURE`:
       https://github.com/apache/tvm/blob/main/include/tvm/tir/op_attr_types.h#L60-L67
       what if to add an annotation type like `DETERMINISTIC` to mark the function return same output on same input. It should be 
    safe to eliminate common calls if the call op is both pure and deterministic.
   
   2. Is the let binding semantic in tir explicitly what we want? Is the value expr is specified to be evaluated at let scope entry? otherwise there may be no reuse effect if let-binding allow the value expr get lazily expanded in let body.
   
   3. Is this possible for some complex (but common) expr to be lifted out of control flow scope? eg
       ```python
       if cond0:
           tir.evaluate(complex_e)  # slow path
       else:
          if cond2: 
               tir.evaluate(complex_e)  # slow path
          else:
               ...  # general fast path
       ```
       after CSE maybe comes to
       ```python
       x = tir.Var("int32")
       with tir.let(x, complex_e):  # always evaluate slow path
           if cond0:
               tir.evaluate(x) 
           else:
               if cond2: 
                   tir.evaluate(x)
               else:
                   ... # general fast path
        ```


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