insert write back now tries to avoid inserting if all operations were inplace

exir/passes/insert_write_back_for_buffers_pass.py

@@ -7,6 +7,7 @@
 from typing import Dict, List, Optional, Tuple
 
 import torch
+from executorch.exir.operator.convert import is_inplace_variant
 
 from torch.export.exported_program import (
     ExportedProgram,
@@ -17,6 +18,7 @@
 )
 from torch.export.graph_signature import TensorArgument
 from torch.utils import _pytree as pytree
+from torchgen.model import SchemaKind
 
 
 def _insert_copy(
@@ -70,6 +72,44 @@ def _insert_copy(
     return buffer_output_nodes
 
 
+def _is_inplace_node(node: torch.fx.Node) -> bool:
+    """Check if a node is an inplace node."""
+    return (
+        node.op == "call_function"
+        and isinstance(node.target, torch._ops.OpOverload)
+        and is_inplace_variant(
+            node.target._schema.name, node.target._schema.overload_name
+        )
+    )
+
+
+def _inplace_lineage(
+    output_arg: torch.fx.Node,
+    gm: torch.fx.GraphModule,
+    gs: ExportGraphSignature,
+    kind: SchemaKind,
+) -> bool:
+    """
+    Walk the graph backwards to see if output_arg is ultimately the same as an input.
+    """
+    if kind != OutputKind.BUFFER_MUTATION and kind != OutputKind.USER_INPUT_MUTATION:
+        return False
+
+    while output_arg.op != "placeholder":
+        if _is_inplace_node(output_arg):
+            # From looking at native_functions.yaml, inplace ops always have self as the first arg
+            output_arg = output_arg.args[0]  # pyre-ignore
+        else:
+            return False
+
+    # If the output arg was a buffer then it needs to reach a buffer placeholder
+    if kind == OutputKind.BUFFER_MUTATION:
+        return output_arg.target in gs.inputs_to_buffers
+    # If the output arg was a user input then it needs to reach a user input placeholder
+    assert kind == OutputKind.USER_INPUT_MUTATION
+    return output_arg.target in gs.user_inputs
+
+
 def insert_write_back_for_buffers_pass(
     ep: ExportedProgram,
 ) -> Tuple[torch.fx.GraphModule, ExportGraphSignature]:
@@ -99,9 +139,15 @@ def insert_write_back_for_buffers_pass(
         if lifted_node is not None:
             input_name_to_node[lifted_node] = input_node
 
+    output_node = None
+    for node in gm.graph.nodes:
+        if node.op == "output":
+            output_node = node
+            break
+
     # Grab the mutable buffer nodes in the outputs,
     mutated_outputs: List[Optional[str]] = []
-    for out_spec in ep.graph_signature.output_specs:
+    for i, out_spec in enumerate(ep.graph_signature.output_specs):
         # if the output arg is the input value then all operations on it are in-place
         # so there's no need to add a copy_ node
         if (
@@ -112,7 +158,12 @@ def insert_write_back_for_buffers_pass(
             out_spec.target in input_name_to_node
             and
             # if the arg and target are not the same, we add a copy_ node.
-            out_spec.arg.name != input_name_to_node[out_spec.target].name
+            not _inplace_lineage(
+                output_node.args[0][i],
+                gm,
+                ep.graph_signature,
+                ep.graph_signature.output_specs[i].kind,
+            )
         ):
             mutated_outputs.append(out_spec.target)
         else:

exir/tests/TARGETS

@@ -145,6 +145,7 @@ python_unittest(
         "//caffe2:torch",
         "//executorch/exir:lib",
         "//executorch/exir/passes:lib",
+        "//executorch/extension/pybindings:portable_lib",
     ],
 )