NXP backend: Add pre-processing pass to replace nodes with known outputs with their data. (#13937)

### Summary
Sometimes nodes will always produce the same data at runtime, regardless
of the model inputs. Add a pre-processing aten dialect pass
which identifies these cases and replaces the nodes with their static
data. This allows for more advanced graph optimizations down the line.

### Test plan
Unit tests provided.

Author: MartinPavella

backends/nxp/aten_passes/neutron_aten_pass_manager.py

@@ -13,6 +13,9 @@
 from executorch.backends.nxp.aten_passes.fuse_batch_norm_with_linear_pass import (
     FuseBatchNormWithLinearPass,
 )
+from executorch.backends.nxp.aten_passes.remove_nodes_with_known_outputs import (
+    RemoveNodesWithKnownOutputs,
+)
 from executorch.backends.nxp.aten_passes.split_group_convolution import (
     SplitGroupConvolution,
 )
@@ -34,6 +37,7 @@ def __init__(self, passes: list[PassType] = None):
             FuseBatchNormWithLinearPass(),
             SplitGroupConvolution(),
             SplitGRUBasedOnNumLayers(),
+            RemoveNodesWithKnownOutputs(),
         ]
 
         super().__init__(passes)

backends/nxp/aten_passes/remove_nodes_with_known_outputs.py

@@ -0,0 +1,196 @@
+# Copyright 2025 NXP
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+from typing import Collection
+
+import torch
+
+from executorch.backends.nxp.backend.edge_helper import (
+    try_get_tensor_constant_from_node,
+)
+from torch._subclasses import FakeTensor, FakeTensorMode
+from torch.ao.quantization.fx.utils import get_new_attr_name_with_prefix
+from torch.export.unflatten import _assign_attr, _AttrKind
+from torch.fx import GraphModule, Node
+from torch.fx.passes.infra.pass_base import PassBase, PassResult
+from torch.nn import Parameter
+
+
+class RemoveNodesWithKnownOutputs(PassBase):
+    """In some situations, a node will always produce the same output data at runtime. If these cases are identified,
+     the nodes can simply be removed and replaced by a static parameter node, which holds the data the original
+     node would produce.
+    This pass identifies some of these cases and performs the replacement.
+    """
+
+    # Nodes which don't have the `.meta['val']` attribute. The datatype and shape of their inferred output data will
+    #  therefore not be checked against the expected values in the `.meta['val']`.
+    nodes_without_val_meta = [
+        torch.ops.aten.empty.memory_format,
+    ]
+
+    module: GraphModule
+
+    def replace_nodes_in_list_with_their_data(self, list_of_args: list) -> list | None:
+        """Replace the nodes in `list_of_args` by their static data. If not all data is available, return `None`.
+
+        :param list_of_args: List of arguments of an aten operator. Can include nodes, generic arguments, lists...
+        :return:`list_of_args` but with tensors replaced by their static data, or `None` if not all data is available.
+        """
+        args_with_data = []
+        for arg in list_of_args:
+            match arg:
+                case Node():
+                    # `arg` is either another operator, a model input, or a static parameter.
+                    data = try_get_tensor_constant_from_node(self.module, arg)
+                    if data is None:
+                        # No static data is available.
+                        return None
+
+                    args_with_data.append(data)
+                case list():
+                    nested = self.replace_nodes_in_list_with_their_data(arg)
+                    if nested is None:
+                        return None
+                    args_with_data.append(nested)
+
+                case _:
+                    # Generic argument. Not an input from a previous node.
+                    args_with_data.append(arg)
+
+        return args_with_data
+
+    @staticmethod
+    def node_is_followed_only_by_getitem_nodes(node: Node) -> bool:
+        def _is_getitem(node_: Node) -> bool:
+            return node_.op == "call_function" and node_.target.__name__ == "getitem"
+
+        users = list(node.users.keys())
+        return all(_is_getitem(user) for user in users)
+
+    def replace_node_with_static_data(self, node: Node, static_data: Parameter):
+        """Remove the given `node` from the graph and replace it with a parameter node containing the `static_data`."""
+        # Generate a unique name for the new static parameter.
+        new_name = get_new_attr_name_with_prefix(node.name)(self.module)
+
+        # Create the node for the parameter.
+        param = torch.nn.Parameter(static_data, False)
+        _assign_attr(param, self.module, str(new_name), _AttrKind.PARAMETER)
+        with self.module.graph.inserting_before(node):
+            static_parameter_node = self.module.graph.get_attr(new_name)
+
+        with FakeTensorMode() as mode:
+            # Assign the parameter node its shape and data type.
+            static_parameter_node.meta["val"] = FakeTensor.from_tensor(
+                torch.empty(static_data.shape, dtype=static_data.dtype), mode
+            )
+
+        # Replace the old node with the new static parameter.
+        node.replace_all_uses_with(static_parameter_node)
+        self.module.graph.erase_node(node)
+
+    def replace_following_getitem_nodes_with_static_data(
+        self, root_node: Node, static_data: Collection[Parameter]
+    ) -> bool:
+        """Remove the `root_node` and all `GetItem` nodes that consume its output from the graph, and replace their
+             uses with parameter nodes containing the provided `static_data`.
+            If something other than just `GetItem` nodes follow after the `root_node`, nothing is done and `False` is
+             returned.
+
+        :param root_node: The main compute node which is followed only by `GetItem` nodes.
+        :param static_data: A tuple of static tensors with the data that will be used to replace the `GetItem` nodes
+                             after the `root_node`.
+        :return: `True` if the replacement was successfully executed. `False` otherwise.
+        """
+
+        if not self.node_is_followed_only_by_getitem_nodes(root_node):
+            return False  # Unexpected case.
+
+        users = list(root_node.users.keys())
+        if len(users) != len(static_data):
+            return False  # Unexpected missmatch.
+
+        # Replace the individual `GetItem` nodes.
+        for get_item_node in users:
+            idx = get_item_node.args[1]
+            self.replace_node_with_static_data(get_item_node, static_data[idx])
+
+        # Finally remove the root node from the graph.
+        self.module.graph.erase_node(root_node)
+
+        return True
+
+    def data_matches_node_meta(self, node: Node, data: Parameter) -> bool:
+        """Verify that the provided `data` tensor has the same shape and datatype as the `node`."""
+        if node.target not in self.nodes_without_val_meta:
+            if node.meta["val"].shape != data.shape:
+                return False  # The inferred data has a different shape than expected.
+
+            if node.meta["val"].dtype != data.dtype:
+                return (
+                    False  # The inferred data has a different data type than expected.
+                )
+
+        return True
+
+    def data_matches_meta_of_following_getitem_nodes(
+        self, root_node: Node, data: Collection[Parameter]
+    ) -> bool:
+        """Verify that the provided `data` tensor has the same shape and datatype as the `GetItem` nodes which consume
+        the output of the `root_node`.
+        """
+        if not self.node_is_followed_only_by_getitem_nodes(root_node):
+            return False  # Unexpected case
+
+        users = list(root_node.users.keys())
+        return all(
+            self.data_matches_node_meta(get_item, data[get_item.args[1]])
+            for get_item in users
+        )
+
+    def call(self, module: GraphModule) -> bool:
+        self.module = module
+        made_changes = False
+
+        for node in module.graph.nodes:
+            if node.op != "call_function":
+                continue  # Not a compute operator.
+
+            # Try to access the static data for the inputs of the node.
+            args_with_data = self.replace_nodes_in_list_with_their_data(node.args)
+
+            if args_with_data is None:
+                # Output data inference is not possible.
+                continue
+
+            # All input data is static. Run the operator to compute the input it would produce at runtime.
+            # noinspection PyBroadException
+            try:
+                output = node.target(*args_with_data, **node.kwargs)
+
+                if isinstance(output, tuple) or isinstance(output, list):
+                    if not self.data_matches_meta_of_following_getitem_nodes(
+                        node, output
+                    ):
+                        continue  # The inferred data does not have the expected type/shape.
+                else:
+                    if not self.data_matches_node_meta(node, output):
+                        continue  # The inferred data does not have the expected type/shape.
+
+            except Exception:
+                continue  # Failed to infer the data. Continue with the other nodes.
+            # The output data appears to have been correctly inferred. Create a static parameter node for it.
+
+            if isinstance(output, tuple) or isinstance(output, list):
+                # The node produces multiple outputs (e.g. `split`). If the node is followed only by `GetItem` nodes
+                #  which extract the individual outputs, replace them by the static data.
+                if self.replace_following_getitem_nodes_with_static_data(node, output):
+                    made_changes = True
+
+            else:
+                self.replace_node_with_static_data(node, output)
+                made_changes = True  # Indicate that changes were made.
+
+        return PassResult(module, made_changes)

backends/nxp/backend/edge_helper.py

@@ -4,7 +4,7 @@
 # LICENSE file in the root directory of this source tree.
 
 import torch
-from torch.fx import Node
+from torch.fx import GraphModule, Node
 from torch.nn import Parameter
 
 
@@ -71,3 +71,19 @@ def _is_dequantize(node_: Node) -> bool:
     return _is_dequantize(node) and node_is_static_tensor(
         node.args[0], parameters_mapping
     )
+
+
+def try_get_tensor_constant_from_node(
+    graph_module: GraphModule, node: Node
+) -> Parameter | None:
+    """Get the static data from a given node. If it doesn't have any data, return `None`."""
+    if node is None or node.op != "get_attr":
+        return None
+
+    target_atoms = node.target.split(".")
+    attr_itr = graph_module
+    for atom in target_atoms:
+        if not hasattr(attr_itr, atom):
+            return None
+        attr_itr = getattr(attr_itr, atom)
+    return attr_itr

backends/nxp/tests/test_removing_nodes_with_known_outputs.py

@@ -0,0 +1,109 @@
+# Copyright 2025 NXP
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import unittest
+
+import numpy as np
+import torch
+
+from executorch.backends.nxp.aten_passes.neutron_aten_pass_manager import (
+    NeutronAtenPassManager,
+)
+from executorch.backends.nxp.aten_passes.remove_nodes_with_known_outputs import (
+    RemoveNodesWithKnownOutputs,
+)
+from executorch.backends.nxp.aten_passes.split_gru_based_on_num_layers import (
+    SplitGRUBasedOnNumLayers,
+)
+from executorch.backends.nxp.tests.executors import graph_contains_any_of_ops
+from parameterized import parameterized
+from torch import nn
+
+
+class GRUModel(nn.Module):
+    def __init__(self, num_layers=1):
+        super().__init__()
+        self.gru = torch.nn.GRU(8, 8, num_layers=num_layers)
+
+    def forward(self, input_):
+        # `input_` has shape [sequence_length, batch_size, input_size] ([8, 1, 8])
+        return self.gru(
+            input_, None
+        )  # The initial hidden is `None`, which will result in a `Zeros` node being added.
+
+
+class TestRemovingNodesWithKnownOutputs(unittest.TestCase):
+    __test__ = False  # Prevent interfering with PyTest tests.
+
+    @classmethod
+    def setUpClass(cls):
+        torch.manual_seed(23)
+        np.random.seed(42)
+
+    def test_removing_nodes__zeros(self):
+        model = GRUModel()
+
+        input_shape = (8, 1, 8)
+        example_input = (torch.ones(input_shape),)
+
+        exir_program_aten = torch.export.export(model, example_input).module()
+
+        # Make sure the `aten.zeros` is in the model.
+        assert graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.zeros.default]
+        )
+        outputs_before = [o.detach().numpy() for o in exir_program_aten(*example_input)]
+
+        # Apply the optimization.
+        NeutronAtenPassManager([RemoveNodesWithKnownOutputs()])(exir_program_aten)
+
+        # Make sure the `aten.zeros` is no longer in the model.
+        assert not graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.zeros.default]
+        )
+        outputs_after = [o.detach().numpy() for o in exir_program_aten(*example_input)]
+
+        # Make sure the model still produces the exact same output.
+        assert np.allclose(outputs_before[0], outputs_after[0])
+        assert np.allclose(outputs_before[1], outputs_after[1])
+
+    @parameterized.expand([2, 8])
+    def test_removing_nodes__split(self, num_layers):
+        # `num_layers > 1` will result in a `split` operator being added. It's input will be a `zeros` operator, which
+        #  provides the static 0s input data.
+        model = GRUModel(num_layers).eval()
+
+        input_shape = (8, 1, 8)
+        example_input = (torch.ones(input_shape),)
+
+        exir_program_aten = torch.export.export(model, example_input).module()
+
+        # Apply the pass to split the `aten.gru.input` into multiple instances, and add a `split` node.
+        NeutronAtenPassManager([SplitGRUBasedOnNumLayers()])(exir_program_aten)
+
+        # Make sure the `aten.zeros` and `torch.split` are in the model.
+        assert graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.zeros.default]
+        )
+        assert graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.split.default]
+        )
+        outputs_before = [o.detach().numpy() for o in exir_program_aten(*example_input)]
+
+        # Apply the optimization.
+        NeutronAtenPassManager([RemoveNodesWithKnownOutputs()])(exir_program_aten)
+
+        # Make sure the `aten.zeros` and `torch.split` are no longer in the model.
+        assert not graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.zeros.default]
+        )
+        assert not graph_contains_any_of_ops(
+            exir_program_aten.graph, [torch.ops.aten.split.default]
+        )
+        outputs_after = [o.detach().numpy() for o in exir_program_aten(*example_input)]
+
+        # Make sure the model still produces the exact same output.
+        assert np.allclose(outputs_before[0], outputs_after[0])
+        assert np.allclose(outputs_before[1], outputs_after[1])