.. _l-onnx-op-loop-d1-11:

Loop - 1 vs 11
==============

Next section compares an older to a newer version of the same operator 
after both definition are converted into markdown text.
Green means an addition to the newer version, red means a deletion.
Anything else is unchanged.

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    --- a/Loop1
    +++ b/Loop11
    @@ -1 +1 @@
      Generic Looping construct. This loop has multiple termination conditions:
  
      1) Trip count. Iteration count specified at runtime. Set by
         specifying the input M. Optional. Set to empty string to omit.
         Note that a static trip count (specified at graph construction time) can be
         specified by passing in a constant node for input M.
      2) Loop termination condition. This is an input to the op that determines
         whether to run the first iteration and also a loop-carried dependency for
         the body graph. The body graph must yield a value for the condition variable,
         whether this input is provided or not.
  
      This table summarizes the operating modes of this operator with equivalent
      C-style code:
  
          Operator inputs defined as (max_trip_count, condition_var).
  
          input ("", ""):
              for (int i=0; ; ++i) {
                cond = ... // Note this value is ignored, but is required in the body
              }
  
          input ("", cond) // Note this is analogous to a while loop
              bool cond = ...;
              for (int i=0; cond; ++i) {
                cond = ...;
              }
  
          input ("", 1) // Note this is analogous to a do-while loop
              bool cond = true
              for (int i=0; cond; ++i) {
                cond = ...;
              }
  
          input (trip_count, "") // Note this is analogous to a for loop
              int trip_count = ...
              for (int i=0; i < trip_count; ++i) {
                cond = ...; // ignored
              }
  
          input (trip_count, cond)
              int trip_count = ...;
              bool cond = ...;
              for (int i=0; i < trip_count &amp;&amp; cond; ++i) {
                cond = ...;
              }
  
      *Sample usage - cond as well as trip count*
  
          graph predict-net {
            %a = Constant[value = <Scalar Tensor [3]>]()
            %b = Constant[value = <Scalar Tensor [6]>]()
            %keepgoing = Constant[value = <Scalar Tensor [1]>]()
            %max_trip_count = Constant[value = <Scalar Tensor [10]>]()
            %keepgoing_out, %b_out, %user_defined_vals = Loop[body = <graph body-net>](%max_trip_count, %keepgoing, %b)
            return
          }
  
          graph body-net (
    -       %i[INT32, scalar]
    -       %keepgoing[BOOL, scalar]
    -       %b[INT32, scalar]
    +       %i[INT32, scalar]           // iteration number
    +       %keepgoing_in[BOOL, scalar] // incoming loop-termination-condition; not used
    +       %b_in[INT32, scalar]        // incoming value of loop-carried-dependency b
          ) {
    -       %my_local = Add(%a, %b)
    +       %my_local = Add(%a, %b_in)
    ?                             +++
    -       %b_out = Sub(%a, %b)
    -       %keepgoing_out = Greater(%my_local, %b_out)
    -       %user_defined_vals = Add(%b, %b)
    +       %b_out = Sub(%a, %b_in) // outgoing value of loop-carried-dependency b
    +       %keepgoing_out = Greater(%my_local, %b_out) // outgoing loop-termination-condition
    +       %user_defined_val = Add(%b_in, %b_in) // scan-output value to be accumulated
    -       return %keepgoing_out, %b_out, %user_defined_vals
    ?                                                       -
    +       return %keepgoing_out, %b_out, %user_defined_val
          }
  
      *Sample equivalent C code*
  
          {
            /* User-defined code (enclosing scope) */
            int a = 3, b = 6;
            bool keepgoing = true; // Analogous to input cond
            /* End user-defined code */
  
            /* Implicitly-defined code */
            const int max_trip_count = 10; // Analogous to input M
            int user_defined_vals[]; // Imagine this is resizable
            /* End implicitly-defined code */
    +       /* initialize loop-carried variables and scan-output variables */
    +       bool keepgoing_out = keepgoing
    +       int b_out = b
    + 
    -       for (int i=0; i < max_trip_count &amp;&amp; keepgoing; ++i) {
    +       for (int i=0; i < max_trip_count &amp;&amp; keepgoing_out; ++i) {
    ?                                                    ++++
    +         /* Implicitly-defined code: bind actual parameter values
    +            to formal parameter variables of loop-body */
    +         bool keepgoing_in = keepgoing_out;
    +         bool b_in = b_out;
    + 
              /* User-defined code (loop body) */
    -         int my_local = a + b; // Reading values in the enclosing scope is fine
    ?                                               - ^^
    +         int my_local = a + b_in; // Reading value "a" from the enclosing scope is fine
    ?                             +++                   ^^^^^^^^
    -         b = a - b; // writes fine if we specify b as a loop-carried dependency
    -         keepgoing = my_local > b; // keepgoing is a loop-carried dependency
    -         user_defined_vals[i] = b + b;
    +         b_out = a - b_in;
    +         keepgoing_out = my_local > b_out;
    +         user_defined_val = b_in + b_in; // b_in and b_out are different variables
              /* End user-defined code */
    + 
    +         /* Implicitly defined-code */
    +         user_defined_vals[i] = user_defined_val // accumulate scan-output values
            }
    -       // my_local = 123; // Can't do this. my_local was defined in the body
    +       // int t = my_local; // Can't do this. my_local is not accessible here.
  
    -       // These below values are live-out from the loop and therefore accessible
    +       // The values below are bound to the output variables of the loop and therefore accessible
    -       b_out; user_defined_vals; keepgoing_out;
    +       // b_out; user_defined_vals; keepgoing_out;
    ?      +++
          }
  
      There are several things of note in this code snippet:
  
    - 1) Values from the enclosing scope (i.e. variable a here) are in scope and can
    + 1) Values from the enclosing scope (i.e. variable "a" here) are in scope and can
    ?                                                   + +
         be referenced in the inputs of the loop.
    - 2) Any variables which you wish to make available in the enclosing scope (i.e.
    -    the variables b and keepgoing) must be declared as either loop-carried
    -    dependencies (both at the op inputs and output and at the body net input and
    -    output) or scan_outputs.
    + 2) Any values computed in the loop body that needs to be used in a subsequent
    +    iteration or after the loop are modelled using a pair of variables in the loop-body,
    +    consisting of an input variable (eg., b_in) and an output variable (eg., b_out).
    +    These are referred to as loop-carried dependences. The loop operation node
    +    supplies the input value of the input variable for the first iteration, and
    +    returns the output value of the output variable produced by the final
    +    iteration.
    + 3) Scan_output variables are used to implicitly concatenate values computed across
    +    all the iterations. In the above example, the value of user_defined_val computed
    +    over all iterations are concatenated and returned as the value of user_defined_vals
    +    after the loop.
    - 3) Values created in the body cannot be accessed in the enclosing scope.
    ? ^                                                                      ^
    + 4) Values created in the body cannot be accessed in the enclosing scope,
    ? ^                                                                      ^
    +    except using the mechanism described above.
  
      Note that the semantics of this op support "diagonal" or "wavefront" execution.
      (See Step 3 here for an example:
      https://devblogs.nvidia.com/optimizing-recurrent-neural-networks-cudnn-5/).
      Frontends should emit multi-layer RNNs as a series of While operators (with
      time being the inner looping dimension), with each successive layer consuming
      the scan_outputs from the previous layer, possibly going through several
      point-wise operators (e.g. dropout, residual connections, linear layer).
  
      ### Attributes
  
      * **body - GRAPH** (required) :
  
        The graph run each iteration. It has 2+N inputs: (iteration_num, condition, loop carried dependencies...). It has 1+N+K outputs: (condition, loop carried dependencies..., scan_outputs...). Each scan_output is created by concatenating the value of the specified output value at the end of each iteration of the loop. It is an error if the dimensions or data type of these scan_outputs change across loop iterations.
  
      ### Inputs
  
    - Between 3 and 2147483647 inputs.
    ?         ^
    + Between 2 and 2147483647 inputs.
    ?         ^
  
      - **M** (optional, heterogeneous) - **I**:
  
        A maximum trip-count for the loop specified at runtime. Optional. Pass empty string to skip.
      - **cond** (optional, heterogeneous) - **B**:
  
        A boolean termination condition. Optional. Pass empty string to skip.
      - **v_initial** (variadic) - **V**:
  
        The initial values of any loop-carried dependencies (values that change across loop iterations)
  
      ### Outputs
  
      Between 1 and 2147483647 outputs.
  
      - **v_final_and_scan_outputs** (variadic) - **V**:
  
        Final N loop carried dependency values then K scan_outputs
  
      ### Type Constraints
  
      * **V** in ( tensor(bool), tensor(complex128), tensor(complex64), tensor(double), tensor(float), tensor(float16), tensor(int16), tensor(int32), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint32), tensor(uint64), tensor(uint8) ):
  
        All Tensor types
      * **I** in ( tensor(int64) ):
  
        tensor of int64, which should be a scalar.
      * **B** in ( tensor(bool) ):
  
        tensor of bool, which should be a scalar.
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