Refactor push_out_non_seq_scan and remove unnecessary cloning

aesara/scan/opt.py

@@ -203,12 +203,11 @@ def push_out_non_seq_scan(fgraph, node):
     if not isinstance(node.op, Scan):
         return False
 
-    # this flag tells if there was any change during the last iterations
-    clean_inputs, clean_outputs = reconstruct_graph(node.op.inputs, node.op.outputs)
+    node_inputs, node_outputs = node.op.inputs, node.op.outputs
 
-    local_fgraph_topo = io_toposort(clean_inputs, clean_outputs)
-    local_fgraph_outs_set = set(clean_outputs)
-    local_fgraph_outs_map = {v: k for k, v in enumerate(clean_outputs)}
+    local_fgraph_topo = io_toposort(node_inputs, node_outputs)
+    local_fgraph_outs_set = set(node_outputs)
+    local_fgraph_outs_map = {v: k for k, v in enumerate(node_outputs)}
 
     to_remove_set = set()
     to_replace_set = set()
@@ -221,18 +220,21 @@ def push_out_non_seq_scan(fgraph, node):
 
     add_to_replace.n = 0
 
+    # The variables that will replace the variables pushed-out of the
+    # inner-graph
     replace_with_in = []
+    # The variables that have been pushed-out of the graph
     replace_with_out = []
 
     op = node.op
     # Construct the list of non_sequences to simplify a few things
-    inner_non_seqs = op.inner_non_seqs(clean_inputs)
+    inner_non_seqs = op.inner_non_seqs(node_inputs)
     inner_non_seqs_set = set(inner_non_seqs)
     inner_non_seqs_map = {v: k for k, v in enumerate(inner_non_seqs)}
 
     outer_non_seqs = op.outer_non_seqs(node.inputs)
 
-    inner_seqs = op.inner_seqs(clean_inputs)
+    inner_seqs = op.inner_seqs(node_inputs)
     outer_seqs = op.outer_seqs(node.inputs)
 
     assert len(inner_non_seqs) == len(outer_non_seqs)
@@ -242,55 +244,60 @@ def push_out_non_seq_scan(fgraph, node):
         if (  # we haven't already looked at this node
             nd not in to_remove_set
             and all(
-                [
-                    (
-                        (x in inner_non_seqs_set)
-                        or (x.owner in to_remove_set)
-                        or isinstance(x, Constant)
-                    )
-                    for x in nd.inputs
-                ]
+                (
+                    (x in inner_non_seqs_set)
+                    or (x.owner in to_remove_set)
+                    or isinstance(x, Constant)
+                )
+                for x in nd.inputs
             )
-            and
-            # we can do this because the assumption is that a
-            # viewOp or deepCopyOp will be just at the end of the
-            # function and not somewhere in the middle ..
-            not isinstance(nd.op, aesara.compile.ViewOp)
+            # We can (supposedly) do this because the assumption is that a
+            # `ViewOp` or `DeepCopyOp` will be just at the end of the
+            # function and not somewhere in the middle
+            and not isinstance(nd.op, aesara.compile.ViewOp)
             and not isinstance(nd.op, aesara.compile.DeepCopyOp)
         ):
-
-            # We have a candidate node to removable
-            # Step 1. Reconstruct it on outside
+            # We have a candidate node to remove from the inner-graph
+
+            # Step 1. Reconstruct the node using the relevant outer-inputs.
+            #
+            # More specifically, the node's current inputs are either
+            # a) inner-graph input place-holders for non-sequences,
+            # b) the outputs of other nodes being pushed out of the inner-graph,
+            # c) or constants.
             to_remove_set.add(nd)
-            outside_ins = []
-            for x in nd.inputs:
-                if x in inner_non_seqs_set:
-                    _idx = inner_non_seqs_map[x]
-                    outside_ins.append(outer_non_seqs[_idx])
-                elif x in to_replace_set:
-                    outside_ins.append(replace_with_out[to_replace_map[x]])
-                elif isinstance(x, Constant):
-                    outside_ins.append(x.clone())
+            new_inputs = []
+            for old_input in nd.inputs:
+                if old_input in inner_non_seqs_set:
+                    # This is case a), so we want to use the corresponding
+                    # outer-graph input as the input to our new pushed-out node
+                    _idx = inner_non_seqs_map[old_input]
+                    new_input = outer_non_seqs[_idx]
+                elif old_input in to_replace_set:
+                    # This is case b), so we want to use the new pushed-out node
+                    # as the input to this new pushed-out node
+                    new_input = replace_with_out[to_replace_map[old_input]]
                 else:
-                    # TODO: Explain why is this an error, and raise an
-                    # appropriate exception type.
-                    raise RuntimeError()
-            outside_ins = [
-                x.type.filter_variable(y) for x, y in zip(nd.inputs, outside_ins)
-            ]
+                    assert isinstance(old_input, Constant)
+                    new_input = old_input
 
-            nw_outer_node = nd.op.make_node(*outside_ins)
+                new_input = old_input.type.filter_variable(new_input)
+                new_inputs.append(new_input)
+
+            pushed_out_node = nd.op.make_node(*new_inputs)
 
             if config.compute_test_value != "off":
-                compute_test_value(nw_outer_node)
+                compute_test_value(pushed_out_node)
 
-            # Step 2. Create variables for replacements
+            # Step 2. Create variables to replace the old outputs of the node
+            # that we're pushing out of the inner-graph
             for idx, y in enumerate(nd.outputs):
-                y_place_holder = safe_new(y, "_replace")
+                y_place_holder = y.clone()
+                # y_place_holder = safe_new(y, "_replace")
                 add_to_replace(y)
                 replace_with_in.append(y_place_holder)
-                assert isinstance(y, type(nw_outer_node.outputs[idx]))
-                replace_with_out.append(nw_outer_node.outputs[idx])
+                assert isinstance(y, type(pushed_out_node.outputs[idx]))
+                replace_with_out.append(pushed_out_node.outputs[idx])
 
     # We need to check all candidate replacements and choose those that
     # make sense for us
@@ -326,14 +333,14 @@ def push_out_non_seq_scan(fgraph, node):
             clean_to_replace, clean_replace_with_in, clean_replace_with_out
         ):
             if isinstance(repl_out, Constant):
-                repl_in = repl_out.clone()
+                repl_in = repl_out
             else:
                 nw_inner.append(repl_in)
                 nw_outer.append(repl_out)
             givens[to_repl] = repl_in
 
-        op_outs = clone_replace(clean_outputs, replace=givens)
-        op_ins = clean_inputs + nw_inner
+        op_outs = clone_replace(node_outputs, replace=givens)
+        op_ins = node_inputs + nw_inner
 
         # Reconstruct node
         nwScan = Scan(