Insert write-back copy_ nodes at the earliest safe point

exir/passes/BUCK

@@ -107,6 +107,7 @@ fbcode_target(_kind = runtime.python_library,
     ],
     deps = [
         "//caffe2:torch",
+        "//executorch/exir/passes:replace_view_copy_with_view_pass",
     ],
 )
 

exir/passes/insert_write_back_for_buffers_pass.py

@@ -4,10 +4,12 @@
 # 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 Dict, List, Optional, Tuple
+import operator
+from typing import Dict, List, Optional, Set, Tuple
 
 import torch
 from executorch.exir.operator.convert import is_inplace_variant
+from executorch.exir.passes.replace_view_copy_with_view_pass import _is_view_copy
 
 from torch.export.exported_program import (
     ExportedProgram,
@@ -21,22 +23,244 @@
 from torchgen.model import SchemaKind
 
 
+def _fx_nodes_in(value: object) -> List[torch.fx.Node]:
+    """The FX nodes contained in value, looking through lists and tuples."""
+    if isinstance(value, torch.fx.Node):
+        return [value]
+    if isinstance(value, (list, tuple)):
+        return [n for v in value for n in _fx_nodes_in(v)]
+    return []
+
+
+def _schema_is_trusted(schema: torch.FunctionSchema) -> bool:
+    """
+    Only aten:: schemas are trusted for alias/mutation introspection. Custom
+    op schemas (mlx::, torchao::, etc.) may not accurately annotate mutation
+    or aliasing (the same policy cse_pass.py applies), so they are treated as
+    unknown.
+    """
+    return schema.name.startswith("aten::")
+
+
+def _alias_sets(alias_info: Optional[torch._C._AliasInfo]) -> Set[str]:
+    """The alias-set annotations of a schema argument or return."""
+    if alias_info is None:
+        return set()
+    return set(alias_info.before_set) | set(alias_info.after_set)
+
+
+def _schema_paired_args(
+    node: torch.fx.Node, schema: torch.FunctionSchema
+) -> List[Tuple[object, torch.Argument]]:
+    """Each FX arg/kwarg of node paired with its schema argument."""
+    schema_kwargs = {a.name: a for a in schema.arguments}
+    return [
+        (arg, schema.arguments[i])
+        for i, arg in enumerate(node.args)
+        if i < len(schema.arguments)
+    ] + [
+        (arg, schema_kwargs[name])
+        for name, arg in node.kwargs.items()
+        if name in schema_kwargs
+    ]
+
+
+def _schemaless_aliasing_inputs(
+    node: torch.fx.Node,
+) -> Optional[List[torch.fx.Node]]:
+    """
+    Aliasing inputs for the nodes that cannot be answered from a schema, or
+    None when the node has a schema to consult.
+    """
+    if node.op == "output":
+        # The output node produces no value of its own, so it cannot alias.
+        return []
+    if node.op != "call_function":
+        return list(node.all_input_nodes)
+    if node.target is operator.getitem:
+        return list(node.all_input_nodes)
+    if _is_view_copy(node):
+        # view_copy produces a fresh tensor here, but ReplaceViewCopyWithViewPass
+        # later rewrites non-output view_copy nodes into true aliases of their
+        # base, the first argument.
+        return _fx_nodes_in(node.args[0] if node.args else None)
+    schema = getattr(node.target, "_schema", None)
+    if schema is None or not _schema_is_trusted(schema):
+        return list(node.all_input_nodes)
+    return None
+
+
+def _aliasing_inputs(node: torch.fx.Node) -> List[torch.fx.Node]:
+    """
+    The subset of node's FX inputs that the value produced by node may alias.
+    When we cannot tell (no schema, getitem, submodule calls, etc.) we
+    conservatively answer all inputs; for schema-annotated ops we answer only
+    the inputs whose alias set is shared with a return, so that e.g. the
+    shape-supplying argument of expand_as does not count as an alias.
+    """
+    special = _schemaless_aliasing_inputs(node)
+    if special is not None:
+        return special
+    schema = node.target._schema  # pyre-ignore[16]
+    ret_sets: Set[str] = set()
+    for ret in schema.returns:
+        ret_sets |= _alias_sets(ret.alias_info)
+    if not ret_sets:
+        return []
+    if "*" in ret_sets:
+        # A wildcard return may alias any input.
+        return list(node.all_input_nodes)
+    aliasing: List[torch.fx.Node] = []
+    for arg, schema_arg in _schema_paired_args(node, schema):
+        arg_sets = _alias_sets(schema_arg.alias_info)
+        if arg_sets & ret_sets or "*" in arg_sets:
+            aliasing.extend(_fx_nodes_in(arg))
+    return aliasing
+
+
+def _contains_node(value: object, input_node: torch.fx.Node) -> bool:
+    """
+    Whether input_node appears in value, looking through lists and tuples so
+    that container arguments (e.g. foreach-style ops) are handled.
+    """
+    if value is input_node:
+        return True
+    if isinstance(value, (list, tuple)):
+        return any(_contains_node(v, input_node) for v in value)
+    return False
+
+
+def _mutates_input(node: torch.fx.Node, input_node: torch.fx.Node) -> bool:
+    """
+    Whether this node may mutate the value passed to it as input_node. When we
+    cannot tell we conservatively answer True.
+    """
+    if node.op == "output":
+        return False
+    if node.op != "call_function":
+        return True
+    schema = getattr(node.target, "_schema", None)
+    if schema is None or not _schema_is_trusted(schema):
+        return True
+    for i, arg in enumerate(node.args):
+        if _contains_node(arg, input_node) and i < len(schema.arguments):
+            alias_info = schema.arguments[i].alias_info
+            if alias_info is not None and alias_info.is_write:
+                return True
+    schema_kwargs = {a.name: a for a in schema.arguments}
+    for name, arg in node.kwargs.items():
+        if _contains_node(arg, input_node) and name in schema_kwargs:
+            alias_info = schema_kwargs[name].alias_info
+            if alias_info is not None and alias_info.is_write:
+                return True
+    return False
+
+
+class _AliasIndex:
+    """
+    Alias closures over the graph. The undirected adjacency (each node joined
+    to the inputs its value may alias) is built once, and each closure is a
+    breadth-first walk cached per seed. The closure is symmetric on purpose:
+    the base of a view must count as an alias of the view's value, since a
+    mutation of the base is a mutation of the view once
+    ReplaceViewCopyWithViewPass has run.
+    """
+
+    def __init__(self, nodes: List[torch.fx.Node]) -> None:
+        self._adjacency: Dict[torch.fx.Node, List[torch.fx.Node]] = {}
+        for node in nodes:
+            for arg in _aliasing_inputs(node):
+                self._adjacency.setdefault(node, []).append(arg)
+                self._adjacency.setdefault(arg, []).append(node)
+        self._cache: Dict[torch.fx.Node, Set[torch.fx.Node]] = {}
+
+    def aliases(self, seed: torch.fx.Node) -> Set[torch.fx.Node]:
+        cached = self._cache.get(seed)
+        if cached is not None:
+            return cached
+        seen = {seed}
+        frontier = [seed]
+        while frontier:
+            node = frontier.pop()
+            for other in self._adjacency.get(node, ()):
+                if other not in seen:
+                    seen.add(other)
+                    frontier.append(other)
+        for node in seen:
+            self._cache[node] = seen
+        return seen
+
+
+def _insertion_point(
+    mutated_node: torch.fx.Node,
+    return_node: torch.fx.Node,
+    node_order: Dict[torch.fx.Node, int],
+    last_placeholder: Optional[torch.fx.Node],
+    alias_index: _AliasIndex,
+) -> torch.fx.Node:
+    """
+    The earliest node after which it is safe to insert
+    copy_(mutated_node, return_node), preserving the semantics of inserting it
+    at the end of the graph. The copy_ must come after:
+
+     * return_node itself, and any node that may mutate it (or an alias of
+       it), so that we write back the final value;
+     * every reader of mutated_node or an alias of it, since they must observe
+       the old value of the buffer (this also orders us after anything that
+       may mutate the buffer);
+     * all placeholders.
+    """
+    latest = return_node
+    if (
+        last_placeholder is not None
+        and node_order[last_placeholder] > node_order[latest]
+    ):
+        latest = last_placeholder
+
+    for alias in alias_index.aliases(mutated_node):
+        for user in alias.users:
+            # Users not in node_order are copy_ nodes inserted by us for other
+            # buffers; ordering with respect to them is handled by the
+            # independence check in _insert_copy.
+            if (
+                user.op != "output"
+                and user in node_order
+                and node_order[user] > node_order[latest]
+            ):
+                latest = user
+
+    for alias in alias_index.aliases(return_node):
+        for user in alias.users:
+            if (
+                user in node_order
+                and _mutates_input(user, alias)
+                and node_order[user] > node_order[latest]
+            ):
+                latest = user
+
+    return latest
+
+
 def _insert_copy(
     gm: torch.fx.GraphModule,
     mutated_outputs: List[Optional[str]],
     input_name_to_node: Dict[str, torch.fx.Node],
 ):
     """
     Find the all the buffers and inputs that were mutated and insert copy_
-    operators to reflect mutations.
+    operators to reflect mutations. Each copy_ is inserted at the earliest
+    point at which it is safe, rather than at the end of the graph, so that
+    the memory planner does not have to arbitrarily extend the lifetime of the
+    value written back.
     """
     output_node = gm.graph.output_node()
     assert output_node is not None
     outputs = pytree.tree_flatten(output_node.args)[0]
     assert len(outputs) == len(mutated_outputs)
 
+    # Pair up the returns with the nodes they mutate.
+    copies: List[Tuple[torch.fx.Node, torch.fx.Node]] = []
     user_output_nodes = []
-    buffer_output_nodes = []
     for return_node, mutated_node_name in zip(outputs, mutated_outputs):
         # User output, leave alone
         if mutated_node_name is None:
@@ -50,14 +274,60 @@ def _insert_copy(
             raise RuntimeError(
                 f"Could not find {mutated_node_name} in either buffer or input nodes"
             )
+        copies.append((mutated_node, return_node))
 
-        # insert copy
-        with gm.graph.inserting_before(output_node):
-            buffer_output = gm.graph.call_function(
-                torch.ops.aten.copy_.default, (mutated_node, return_node)
-            )
-            # add output of copy to graph outputs
-            buffer_output_nodes.append(buffer_output)
+    # insert the copies
+    buffer_output_nodes: List[torch.fx.Node] = []
+    # The alias analysis is only needed to place copies, so graphs with no
+    # write-backs (the common case for models without mutable state) skip its
+    # cost entirely.
+    if copies:
+        node_order: Dict[torch.fx.Node, int] = {
+            node: i for i, node in enumerate(gm.graph.nodes)
+        }
+        placeholders = [node for node in gm.graph.nodes if node.op == "placeholder"]
+        last_placeholder = placeholders[-1] if placeholders else None
+        alias_index = _AliasIndex(list(gm.graph.nodes))
+
+        # The copies themselves mutate the buffers. If the value written back
+        # by one copy may alias the buffer mutated by another, then the order
+        # of the copies (and their position relative to everything else)
+        # matters in ways the insertion points below do not track, so fall
+        # back to inserting all of them at the end of the graph, in their
+        # original order, as before.
+        independent = True
+        if len(copies) > 1:
+            mutated_aliases: Set[torch.fx.Node] = set()
+            return_aliases: Set[torch.fx.Node] = set()
+            for mutated_node, return_node in copies:
+                mutated_alias = alias_index.aliases(mutated_node)
+                return_alias = alias_index.aliases(return_node)
+                if (
+                    mutated_alias & return_aliases
+                    or return_alias & mutated_aliases
+                    # Two destinations that may share storage must also keep
+                    # their original write order.
+                    or mutated_alias & mutated_aliases
+                ):
+                    independent = False
+                    break
+                mutated_aliases |= mutated_alias
+                return_aliases |= return_alias
+
+        for mutated_node, return_node in copies:
+            if independent:
+                insert_after = _insertion_point(
+                    mutated_node, return_node, node_order, last_placeholder, alias_index
+                )
+                insertion = gm.graph.inserting_after(insert_after)
+            else:
+                insertion = gm.graph.inserting_before(output_node)
+            with insertion:
+                buffer_output = gm.graph.call_function(
+                    torch.ops.aten.copy_.default, (mutated_node, return_node)
+                )
+                # add output of copy to graph outputs
+                buffer_output_nodes.append(buffer_output)
 
     with gm.graph.inserting_before(output_node):
         buffer_output_nodes.extend(user_output_nodes)