Change Loop op with maximum iterations input M equals to empty string (#1971)

* make Loop op with maximum iterations M equal to empty string to match onnx spec

Signed-off-by: Deyu Huang <[email protected]>

Author: hwangdeyu

tf2onnx/onnx_opset/controlflow.py

@@ -381,29 +381,32 @@ def version_7(cls, ctx, node, **kwargs):
         # may be removed from output_names below
         output_names = node.output.copy()
 
-        # Make maximum_iterations int64 and replace -1(tf) with maxsize(onnx). If the const node has no other
+        # Make maximum_iterations int64. If the const node has no other
         # consumers, modify it in place. Otherwise, make a new const node and leave the original unchanged.
         # if maximum_iterations is not const,should add an cast node(cast to int64)
         maximum_iterations_name = node.input[1]
         if node.inputs[1].is_const():
             maximum_iterations = node.inputs[1].get_tensor_value()
-            if maximum_iterations == -1:
-                maximum_iterations = np.iinfo(np.int64).max
-            consumers = ctx.find_output_consumers(maximum_iterations_name)
-            external_consumers = [c for c in consumers if c != node and c.type != 'TensorListReserve']
-            if len(external_consumers) == 0:
-                ctx.remove_node(node.inputs[1].name)
+            # maximum_iterations with -1(tf) means it doesn't set the maximum count.
+            # For onnx Loop op optional input `M`(int64), represents a maximum trip-count. Set empty string to skip.
+            if maximum_iterations != -1:
+                consumers = ctx.find_output_consumers(maximum_iterations_name)
+                external_consumers = [c for c in consumers if c != node and c.type != 'TensorListReserve']
+                if len(external_consumers) == 0:
+                    ctx.remove_node(node.inputs[1].name)
+                else:
+                    maximum_iterations_name = utils.make_name(node.inputs[1].name)
+                ctx.make_const(maximum_iterations_name, np.array(maximum_iterations, dtype=np.int64))
+                ctx.replace_input(node, node.input[1], maximum_iterations_name, 1)
+                maximum_iterations_m = maximum_iterations_name
             else:
-                maximum_iterations_name = utils.make_name(node.inputs[1].name)
-            ctx.make_const(maximum_iterations_name, np.array(maximum_iterations, dtype=np.int64))
-            ctx.replace_input(node, node.input[1], maximum_iterations_name, 1)
-            maximum_iterations_int64 = maximum_iterations_name
+                maximum_iterations_m = ""
         else:
             cast_inputs = [maximum_iterations_name]
             attr = {"to": onnx_pb.TensorProto.INT64}
             cast_name = node.name + "_cast"
             cast_node = ctx.make_node("Cast", cast_inputs, attr, name=cast_name)
-            maximum_iterations_int64 = cast_node.output[0]
+            maximum_iterations_m = cast_node.output[0]
 
         cond_name = node.get_attr_str("cond")
         cond_graph = find_function(cond_name)
@@ -427,7 +430,7 @@ def version_7(cls, ctx, node, **kwargs):
                 cond_input_to_state_var[cond_graph.input_names[idx]] = maximum_iterations_name
                 continue
             if idx < 2:
-                # skip  [0,1] loop_counter, max_iterations
+                # skip [0,1] loop_counter, max_iterations
                 continue
             n = node.inputs[idx]
             if n.type in ["TensorListReserve", "TensorListResize"]:
@@ -511,7 +514,7 @@ def version_7(cls, ctx, node, **kwargs):
         output_names = output_names[2:]
 
         branches = {"body": body}
-        loop_node = ctx.make_node("Loop", [maximum_iterations_int64, cond_outputs[0]] + loop_vars,
+        loop_node = ctx.make_node("Loop", [maximum_iterations_m, cond_outputs[0]] + loop_vars,
                                   output_count=len(output_shapes), name=node.name + "_loop",
                                   shapes=output_shapes, dtypes=output_dtypes, skip_conversion=True,
                                   branches=branches)