Preserve Arm partition tag order

Author: Praneeth Yashovardhan Kadem

backends/arm/test/misc/test_partitioner_tag_order.py

@@ -0,0 +1,80 @@
+# 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 types import SimpleNamespace
+
+from executorch.backends.arm.tosa import partitioner as tosa_partitioner
+from executorch.backends.arm.tosa.compile_spec import TosaCompileSpec
+from executorch.backends.arm.tosa.partitioner import TOSAPartitioner
+
+
+class _FakeCapabilityBasedPartitioner:
+    def __init__(self, *args, **kwargs) -> None:
+        pass
+
+    def propose_partitions(self):
+        return [
+            SimpleNamespace(nodes=[SimpleNamespace(meta={}, target=f"op{idx}")])
+            for idx in range(3)
+        ]
+
+
+def _make_reporter() -> SimpleNamespace:
+    return SimpleNamespace(
+        report_reject=lambda *args, **kwargs: None,
+        get_table_report=lambda: "",
+    )
+
+
+def test_tag_module_preserves_partition_discovery_order(monkeypatch):
+    partitioner = TOSAPartitioner(TosaCompileSpec("TOSA-1.0+FP"))
+
+    monkeypatch.setattr(
+        tosa_partitioner, "get_cond_while_submodules_nested", lambda module: []
+    )
+    monkeypatch.setattr(
+        tosa_partitioner, "tosa_support_factory", lambda *args, **kwargs: object()
+    )
+    monkeypatch.setattr(
+        tosa_partitioner,
+        "CapabilityBasedPartitioner",
+        _FakeCapabilityBasedPartitioner,
+    )
+    monkeypatch.setattr(
+        partitioner,
+        "_partition_has_invalid_uint8",
+        lambda partition, tag: False,
+    )
+    monkeypatch.setattr(
+        partitioner,
+        "_preserve_io_quantization_enabled",
+        lambda: False,
+    )
+
+    tags = partitioner._tag_module(
+        SimpleNamespace(graph=SimpleNamespace(nodes=[])),
+        SimpleNamespace(),
+        _make_reporter(),
+    )
+
+    assert tags == ["tag0", "tag1", "tag2"]
+
+
+def test_partition_preserves_tag_discovery_order(monkeypatch):
+    partitioner = TOSAPartitioner(TosaCompileSpec("TOSA-1.0+FP"))
+
+    monkeypatch.setattr(
+        partitioner,
+        "_tag_module",
+        lambda *args, **kwargs: ["tag2", "tag10"],
+    )
+    monkeypatch.setattr(tosa_partitioner, "tag_constant_data", lambda program: None)
+    monkeypatch.setattr(
+        tosa_partitioner, "WhyNoPartitionReporter", _make_reporter
+    )
+
+    result = partitioner.partition(SimpleNamespace(graph_module=SimpleNamespace()))
+
+    assert list(result.partition_tags) == ["tag2", "tag10"]

backends/arm/tosa/partitioner.py

@@ -289,7 +289,7 @@ def _tag_module(  # noqa
         containing_program: ExportedProgram,
         reporter: WhyNoPartitionReporter,
         tag_iterator: count | None = None,
-    ) -> set[str]:
+    ) -> list[str]:
         """Tag nodes in a module or submodule from the containing program.
 
         Args:
@@ -298,21 +298,25 @@ def _tag_module(  # noqa
             reporter: A reporter to report why nodes were rejected.
 
         Returns:
-            A set of strings with the partition tags.
+            A list of strings with the partition tags in discovery order.
 
         """
-        tags: set[str] = set()
+        # Preserve discovery order so backend lowering sees a deterministic
+        # partition order across Python processes.
+        tags: list[str] = []
+        seen_tags: set[str] = set()
         if tag_iterator is None:
             tag_iterator = count(0)
         for _, submodule, _ in get_cond_while_submodules_nested(module):
             submodule_tags = self._tag_module(
                 submodule, containing_program, reporter, tag_iterator
             )
-            if len(tags & submodule_tags) != 0:
+            if any(tag in seen_tags for tag in submodule_tags):
                 raise RuntimeError(
                     "Got overlapping tags in two different modules, this shouldn't happen."
                 )
-            tags = tags | submodule_tags
+            tags.extend(submodule_tags)
+            seen_tags.update(submodule_tags)
         operator_support = tosa_support_factory(
             self.tosa_spec, containing_program, reporter, self.additional_checks
         )
@@ -335,7 +339,8 @@ def is_node_supported(self, submodules, node: torch.fx.Node) -> bool:
 
         for partition in partition_list:
             tag = f"tag{next(tag_iterator)}"
-            tags.add(tag)
+            tags.append(tag)
+            seen_tags.add(tag)
 
             for node in partition.nodes:
                 node.meta["delegation_tag"] = tag
@@ -364,6 +369,7 @@ def is_node_supported(self, submodules, node: torch.fx.Node) -> bool:
                     reporter,
                 )
                 tags.remove(tag)
+                seen_tags.remove(tag)
                 continue
 
             # Check whether the partition contains only no-op or non-computational ops. Such partitions don't make sense to delegate, and in the worst case may be optimized away during lowering, which can break compilation."
@@ -385,6 +391,7 @@ def is_node_supported(self, submodules, node: torch.fx.Node) -> bool:
                     reporter,
                 )
                 tags.remove(tag)
+                seen_tags.remove(tag)
         return tags
 
     def partition(self, exported_program: ExportedProgram) -> PartitionResult: