Arm backend: Preserve duplicate output slots with TOSA identity fanout (#18866)

When FuseEqualPlaceholdersPass fuses equal constant placeholders, the
graph output can contain the same node in multiple output slots. In this
case ToTosaMemoryFormatPass was rewriting the output node with
replace_input_with() while inserting output transposes.

That rewrote all matching occurrences at once, so duplicated logical
output slots were collapsed onto the same transpose node instead of
remaining distinct.

Fix this by handling duplicate outputs in the output rewrite path. For
shared output nodes, create a single boundary TOSA TRANSPOSE and
preserve distinct output slots by inserting TOSA IDENTITY fanout nodes
for later duplicates.

This keeps insert_input_transpose() focused on normal input rewrites,
avoids duplicating equivalent transposes for shared outputs, and
preserves the output slot structure expected by later lowering and
serialization stages.

Add regression coverage for FuseEqualPlaceholdersPass +
ToTosaMemoryFormatPass with duplicate outputs, and add TOSA IDENTITY
dialect and visitor coverage.

---------

Signed-off-by: Baris Demir <baris.demir@arm.com>
Signed-off-by: Adrian Lundell <adrian.lundell@arm.com>
Co-authored-by: Baris Demir <baris.demir@arm.com>

Author: AdrianLundell

backends/arm/_passes/__init__.py

@@ -100,6 +100,7 @@
 from .decompose_var_pass import DecomposeVarPass  # noqa
 from .decompose_where_scalar_other_pass import DecomposeWhereScalarOtherPass  # noqa
 from .decorate_fp32_to_int32_casting_pass import DecorateFp32toInt32CastingPass  # noqa
+from .ensure_unique_output_nodes_pass import EnsureUniqueOutputNodesPass  # noqa
 from .fold_qdq_with_annotated_qparams_pass import (  # noqa
     FoldAndAnnotateQParamsPass,
     QuantizeClampArgumentsPass,

backends/arm/_passes/arm_pass_manager.py

@@ -98,6 +98,7 @@
     DecomposeVarPass,
     DecomposeWhereScalarOtherPass,
     DecorateFp32toInt32CastingPass,
+    EnsureUniqueOutputNodesPass,
     FoldAndAnnotateQParamsPass,
     FuseBatchNorm2dPass,
     FuseConsecutiveConcatShapesPass,
@@ -544,6 +545,7 @@ def _tosa_pipeline(
                 FuseEqualPlaceholdersPass(exported_program),
                 FuseConsecutiveConcatShapesPass(),
                 ToTosaMemoryFormatPass(exported_program),
+                EnsureUniqueOutputNodesPass(),
                 RemoveNoopPass(),
                 InsertRescalePass(),
                 InsertDataLayoutCastsPass(),

backends/arm/_passes/ensure_unique_output_nodes_pass.py

@@ -0,0 +1,82 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+from collections import Counter
+from typing import Any, Set, Type
+
+import torch
+from executorch.backends.arm._passes import ArmPass
+from executorch.backends.arm._passes.arm_pass_utils import create_node
+from executorch.exir.dialects._ops import ops as exir_ops
+from executorch.exir.pass_base import ExportPass, PassResult
+
+
+class EnsureUniqueOutputNodesPass(ArmPass):
+    """Ensure each graph output leaf references a unique producer node.
+
+    If the same node appears multiple times in the output structure, insert a
+    ``tosa.IDENTITY`` node for each occurrence and replace the repeated output
+    entries with those identity nodes.
+
+    """
+
+    _passes_required_after: Set[Type[ExportPass]] = set()
+
+    @staticmethod
+    def _collect_output_nodes(
+        output_value: Any, counts: Counter[torch.fx.Node]
+    ) -> None:
+        if isinstance(output_value, torch.fx.Node):
+            counts[output_value] += 1
+            return
+        if isinstance(output_value, (list, tuple)):
+            for value in output_value:
+                EnsureUniqueOutputNodesPass._collect_output_nodes(value, counts)
+
+    def call(self, graph_module: torch.fx.GraphModule) -> PassResult:
+        graph = graph_module.graph
+        output_node = graph.output_node()
+        output_value = output_node.args[0]
+
+        counts: Counter[torch.fx.Node] = Counter()
+        self._collect_output_nodes(output_value, counts)
+        repeated_nodes = {node for node, count in counts.items() if count > 1}
+        if not repeated_nodes:
+            return PassResult(graph_module, False)
+
+        modified = False
+
+        def _replace_repeated_outputs(value: Any) -> Any:
+            nonlocal modified
+            if isinstance(value, torch.fx.Node):
+                if value not in repeated_nodes:
+                    return value
+                with graph.inserting_before(output_node):
+                    identity_node = create_node(
+                        graph,
+                        exir_ops.backend.tosa.IDENTITY.default,
+                        args=(value,),
+                        from_node=value,
+                    )
+                modified = True
+                return identity_node
+
+            if isinstance(value, tuple):
+                return tuple(_replace_repeated_outputs(v) for v in value)
+
+            if isinstance(value, list):
+                return [_replace_repeated_outputs(v) for v in value]
+
+            return value
+
+        new_output_value = _replace_repeated_outputs(output_value)
+        if modified:
+            output_node.args = (new_output_value,)
+            graph.eliminate_dead_code()
+            graph.lint()
+            graph_module.recompile()
+            graph_module = super().call(graph_module).graph_module
+
+        return PassResult(graph_module, modified)

backends/arm/_passes/to_tosa_memory_format_pass.py

@@ -264,7 +264,6 @@ def insert_output_transpose(node, graph_module):
         """Convert a producer's output to channels-last by appending a backend
         `TRANSPOSE` node and rewiring its users.
         """
-
         rank = len(get_first_fake_tensor(node).size())
         spatial_rank = node.meta["tosa_spatial_rank"]
         mem_format = ToTosaMemoryFormatPass._channels_last_order(rank, spatial_rank)
@@ -383,17 +382,18 @@ def insert_tosa_transposes(self, graph_module: torch.fx.GraphModule):
         if output_dim_orders is None:
             raise RuntimeError(f"{output_dim_orders=} is not supported.")
 
+        transposed_output_inputs: set[torch.fx.Node] = set()
         for output_node_input, output_dim_order in zip(
             outputs, output_dim_orders, strict=True
         ):
-            if output_dim_order in (
-                NCHW_ORDER,
-                NNCHW_ORDER,
-                NNNCHW_ORDER,
+            if (
+                output_dim_order in (NCHW_ORDER, NNCHW_ORDER, NNNCHW_ORDER)
+                and output_node_input not in transposed_output_inputs
             ):
                 self.insert_input_transpose(
                     output_node, output_node_input, graph_module
                 )
+                transposed_output_inputs.add(output_node_input)
 
     def remove_dim_order_kwargs(
         self, graph_module: torch.fx.GraphModule, node: torch.fx.Node

backends/arm/operators/__init__.py

@@ -52,6 +52,7 @@
     op_tosa_custom,
     op_tosa_depthwise_conv2d,
     op_tosa_gather,
+    op_tosa_identity,
     op_tosa_matmul,
     op_tosa_max_pool2d,
     op_tosa_pad,

backends/arm/operators/op_tosa_identity.py

@@ -0,0 +1,62 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+#
+# 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 Any, List
+
+import torch
+import tosa_serializer as ts
+
+from executorch.backends.arm.operators.node_visitor import (
+    NodeVisitor,
+    register_node_visitor,
+)
+from executorch.backends.arm.operators.operator_validation_utils import (
+    validate_num_inputs,
+    validate_same_dtype,
+    validate_valid_dtype,
+)
+from executorch.backends.arm.tosa.mapping import TosaArg
+
+
+@register_node_visitor
+class IdentityVisitor(NodeVisitor):
+    """Lower the TOSA IDENTITY op."""
+
+    target = "tosa.IDENTITY.default"
+
+    def define_node(
+        self,
+        node: torch.fx.Node,
+        tosa_graph: Any,
+        inputs: List[TosaArg],
+        output: TosaArg,
+    ) -> None:
+        validate_num_inputs(self.target, inputs, 1)
+        validate_same_dtype(self.target, [inputs[0], output], ts)
+        validate_valid_dtype(
+            self.target,
+            [inputs[0], output],
+            [
+                ts.DType.BOOL,
+                ts.DType.INT8,
+                ts.DType.INT16,
+                ts.DType.INT32,
+                ts.DType.FP16,
+                ts.DType.FP32,
+                ts.DType.BF16,
+            ],
+            self.tosa_spec,
+        )
+
+        attr = ts.TosaSerializerAttribute()
+        attr.IdentityAttribute()
+        self._serialize_operator(
+            node,
+            tosa_graph,
+            ts.Op.IDENTITY,
+            [inputs[0].name],
+            [output.name],
+            attr,
+        )

backends/arm/test/misc/test_tosa_dialect_identity.py

@@ -0,0 +1,25 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import executorch.backends.arm.tosa.dialect  # noqa: F401
+import torch
+from executorch.backends.arm.tosa.specification import (
+    TosaLoweringContext,
+    TosaSpecification,
+)
+from executorch.exir.dialects._ops import ops as exir_ops
+from torch._subclasses.fake_tensor import FakeTensorMode
+
+
+def test_identity_tosa_FP() -> None:
+    sample_input = torch.randn((1, 2, 3, 4), dtype=torch.float32)
+
+    with TosaLoweringContext(
+        TosaSpecification.create_from_string("TOSA-1.0+FP")
+    ), FakeTensorMode() as mode:
+        output = exir_ops.backend.tosa.IDENTITY.default(mode.from_tensor(sample_input))
+
+        assert output.dtype == sample_input.dtype
+        assert tuple(output.shape) == tuple(sample_input.shape)

backends/arm/test/passes/test_ensure_unique_output_nodes_pass.py

@@ -0,0 +1,66 @@
+# Copyright 2026 Arm Limited and/or its affiliates.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import torch
+from executorch.backends.arm._passes import EnsureUniqueOutputNodesPass
+from executorch.backends.arm.test.tester.test_pipeline import PassPipeline
+from executorch.backends.test.harness.stages import StageType
+from executorch.exir.dialects._ops import ops as exir_ops
+
+
+class DuplicateOutputModule(torch.nn.Module):
+    def forward(self, x: torch.Tensor):
+        y = x + 1.0
+        return y, y
+
+
+class UniqueOutputModule(torch.nn.Module):
+    def forward(self, x: torch.Tensor):
+        y = x + 1.0
+        z = x + 2.0
+        return y, z
+
+
+def test_ensure_unique_output_nodes_no_target_inserts_identity_per_repeated_output() -> (
+    None
+):
+    pipeline = PassPipeline[tuple[torch.Tensor]](
+        DuplicateOutputModule(),
+        (torch.rand(2, 2),),
+        quantize=False,
+        pass_list=[EnsureUniqueOutputNodesPass],
+        ops_after_pass={
+            "executorch_exir_dialects_backend__ops_tosa_IDENTITY_default": 2,
+        },
+    )
+    pipeline.pop_stage("run_method_and_compare_outputs")
+    pipeline.run()
+
+    graph_module = (
+        pipeline.tester.get_artifact(StageType.RUN_PASSES)
+        .exported_program()
+        .graph_module
+    )
+    output_node = graph_module.graph.output_node()
+    outputs = list(output_node.args[0])
+
+    assert outputs[0] is not outputs[1]
+    assert outputs[0].target == exir_ops.backend.tosa.IDENTITY.default
+    assert outputs[1].target == exir_ops.backend.tosa.IDENTITY.default
+    assert outputs[0].args[0] is outputs[1].args[0]
+
+
+def test_ensure_unique_output_nodes_no_target_keeps_unique_outputs_unchanged() -> None:
+    pipeline = PassPipeline[tuple[torch.Tensor]](
+        UniqueOutputModule(),
+        (torch.rand(2, 2),),
+        quantize=False,
+        pass_list=[EnsureUniqueOutputNodesPass],
+        ops_not_after_pass=[
+            "executorch_exir_dialects_backend__ops_tosa_IDENTITY_default",
+        ],
+    )
+    pipeline.pop_stage("run_method_and_compare_outputs")
+    pipeline.run()

backends/arm/test/passes/test_to_tosa_memory_format.py

@@ -8,15 +8,19 @@
 import torch
 from executorch.backends.arm._passes import (
     AnnotateOutputDimOrderPass,
+    EnsureUniqueOutputNodesPass,
+    FuseEqualPlaceholdersPass,
     ToTosaMemoryFormatPass,
 )
 
 from executorch.backends.arm.test import common
 from executorch.backends.arm.test.tester.test_pipeline import (
     PassPipeline,
+    TosaPipelineFP,
     TosaPipelineINT,
 )
 from executorch.backends.transforms.remove_getitem_op import RemoveGetItemPass
+from executorch.exir.dialects._ops import ops as exir_ops
 
 input_t = Tuple[torch.Tensor]  # Input x
 
@@ -177,6 +181,26 @@ def get_inputs(self) -> input_t:
         return (torch.rand(4, 4, 4, 4),)
 
 
+class DuplicateConstantOutputs(torch.nn.Module):
+    def __init__(self) -> None:
+        super().__init__()
+        self.register_buffer("grid0", torch.zeros(1, 32, 32, 2))
+        self.register_buffer("grid1", torch.zeros(1, 32, 32, 2))
+
+    def forward(self, x: torch.Tensor):
+        return self.grid0, self.grid1, x
+
+
+class DuplicateConstantOutputsWithAdd(torch.nn.Module):
+    def __init__(self) -> None:
+        super().__init__()
+        self.register_buffer("grid0", torch.zeros(1, 32, 32, 2))
+        self.register_buffer("grid1", torch.zeros(1, 32, 32, 2))
+
+    def forward(self, x: torch.Tensor):
+        return self.grid0, self.grid1, x + x
+
+
 modules: Dict[str, ModuleMetadata] = {
     "no_nhwc": NoNHWC(),
     "parallel_clusters": ParallelClusters(),
@@ -209,3 +233,38 @@ def test_to_tosa_memory_format_tosa_INT_functional(module: ModuleMetadata) -> No
     module_nn = cast(torch.nn.Module, module)
     pipeline = TosaPipelineINT[input_t](module_nn, module.get_inputs(), [])
     pipeline.run()
+
+
+def test_to_tosa_memory_format_no_target_preserves_duplicate_output_slots() -> None:
+    pipeline = PassPipeline[input_t](
+        DuplicateConstantOutputs(),
+        (torch.rand(1, 2, 32, 32),),
+        quantize=False,
+        pass_list=[RemoveGetItemPass, AnnotateOutputDimOrderPass],
+        passes_with_exported_program=[
+            FuseEqualPlaceholdersPass,
+            ToTosaMemoryFormatPass,
+            EnsureUniqueOutputNodesPass,
+        ],
+    )
+    pipeline.pop_stage("run_method_and_compare_outputs")
+    pipeline.run()
+
+    graph_module = pipeline.tester.get_artifact().exported_program().graph_module
+    output_node = graph_module.graph.output_node()
+    outputs = list(output_node.args[0])
+
+    assert outputs[0] is not outputs[1]
+    assert outputs[0].target == exir_ops.backend.tosa.IDENTITY.default
+    assert outputs[1].target == exir_ops.backend.tosa.IDENTITY.default
+    assert outputs[0].args[0] is outputs[1].args[0]
+
+
+def test_to_tosa_memory_format_tosa_FP_duplicate_output_identity() -> None:
+    pipeline = TosaPipelineFP[input_t](
+        DuplicateConstantOutputsWithAdd(),
+        (torch.rand(1, 2, 32, 32),),
+        [],
+        [],
+    )
+    pipeline.run()

backends/arm/tosa/dialect/__init__.py

@@ -10,6 +10,7 @@
     custom,
     depthwise_conv2d,
     gather,
+    identity,
     matmul,
     max_pool2d,
     pad,