Reorder slice before permute (#19129)

Differential Revision: D102426699

Pull Request resolved: #19129

Author: DrJessop

backends/cadence/aot/reorder_ops.py

@@ -11,13 +11,14 @@
 
 from collections import defaultdict
 from math import prod
-from typing import DefaultDict, List, Tuple
+from typing import cast, DefaultDict, List, Tuple
 
 import torch
 import torch.fx
 from executorch.backends.cadence.aot.compiler_utils import get_placeholders, get_shape
 from executorch.backends.cadence.aot.pass_utils import (
     CadencePassAttribute,
+    get_arg,
     get_overload_packet,
     register_cadence_pass,
     RemoveOrReplacePassInterface,
@@ -641,6 +642,83 @@ class PostponePermuteOpBelowSqueezeOrUnsqueezeLikeView(
     pass
 
 
+@register_cadence_pass(CadencePassAttribute(opt_level=1))
+class MoveSliceBeforePermutePass(RemoveOrReplacePassInterface):
+    """Move slice_copy ops before permute_copy to reduce permute data volume.
+
+    Rewrites permute(input, perm) -> slice(dim=D) into
+    slice(input, dim=perm[D]) -> permute(sliced, perm), so the permute
+    operates on a smaller tensor.
+
+    Scans slice nodes and matches upstream permutes. This also handles
+    chained cases (permute -> slice -> slice) in one pass: each slice
+    independently checks its input for a permute.
+
+    Cost model: dim-0 slices are nop-eligible (zero-copy pointer offset
+    after MakeSliceAndCatDimOutermostPass).  Moving such a slice loses the
+    nop, so we only move it when the permute savings outweigh the nop loss,
+    i.e. when the slice removes more than half the data (full > 2 * sliced).
+    Non-dim-0 slices have no nop opportunity, so any permute savings is
+    pure win.
+    """
+
+    STRIDED_SLICE_COST_FACTOR: int = 2
+
+    @property
+    def targets(self) -> list[EdgeOpOverload]:
+        return [exir_ops.edge.aten.slice_copy.Tensor]
+
+    def maybe_remove_or_replace(self, node: torch.fx.Node) -> bool:
+        permute_node = get_arg(node, "input", torch.fx.Node)
+        if permute_node.target != exir_ops.edge.aten.permute_copy.default:
+            return False
+
+        if len(permute_node.users) != 1:
+            return False
+
+        perm = cast(list[int], permute_node.args[1])
+        permute_input = permute_node.args[0]
+        assert isinstance(permute_input, torch.fx.Node)
+
+        slice_dim = get_arg(node, "dim", int)
+
+        full_size = prod(permute_node.meta["val"].shape)
+        sliced_size = prod(node.meta["val"].shape)
+        if slice_dim == 0 and full_size <= self.STRIDED_SLICE_COST_FACTOR * sliced_size:
+            return False
+
+        new_dim = perm[slice_dim]
+        graph = node.graph
+
+        with graph.inserting_before(permute_node):
+            new_slice_args = (
+                permute_input,
+                new_dim,
+                get_arg(node, "start"),
+                get_arg(node, "end"),
+                get_arg(node, "step", int),
+            )
+            new_slice = graph.create_node(
+                "call_function",
+                exir_ops.edge.aten.slice_copy.Tensor,
+                args=new_slice_args,
+            )
+            new_slice.meta["val"] = exir_ops.edge.aten.slice_copy.Tensor(
+                permute_input.meta["val"], *new_slice_args[1:]
+            )
+            new_permute = graph.create_node(
+                "call_function",
+                exir_ops.edge.aten.permute_copy.default,
+                args=(new_slice, perm),
+            )
+            new_permute.meta["val"] = exir_ops.edge.aten.permute_copy.default(
+                new_slice.meta["val"], perm
+            )
+
+        node.replace_all_uses_with(new_permute)
+        return True
+
+
 # The following class consolidates functions to reoder ops (i.e., either hoist
 # or sink some ops in the graph).
 class CadenceReorderOpsInGraph:

backends/cadence/aot/tests/test_reorder_ops_passes.py

@@ -23,6 +23,7 @@
 from executorch.backends.cadence.aot.reorder_ops import (
     AdvanceQuantizeOpAboveDefChainPass,
     AdvanceQuantizeOpAboveDefInBranchPass,
+    MoveSliceBeforePermutePass,
     PostponeDequantizeOpBelowUseChainPass,
     PostponePermuteOpBelowSqueezeOrUnsqueezeLikeView,
     SinkOpsCloserToUsePass,
@@ -633,3 +634,130 @@ def test_permute_view_chains_neg(self) -> None:
         self.assertTrue(nodes[1] == exir_ops.edge.aten.permute_copy)
         self.assertTrue(nodes[2] == exir_ops.edge.aten.view_copy)
         self.assertTrue(nodes[3] == exir_ops.edge.aten.permute_copy)
+
+
+class TestMoveSliceBeforePermutePass(unittest.TestCase):
+    def test_basic_move(self) -> None:
+        """permute → slice becomes slice → permute."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(2, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        sliced = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 1, 0, 2, 1),
+        )
+        builder.output([sliced])
+        original = builder.get_graph_module()
+
+        result = transform_and_check_numerics(
+            original,
+            (torch.randn(2, 3, 4, 5),),
+            MoveSliceBeforePermutePass(),
+        )
+        self.assertTrue(result.modified)
+
+        nodes = get_compute_nodes_in_gm(result.graph_module)
+        self.assertEqual(len(nodes), 2)
+        self.assertEqual(nodes[0], exir_ops.edge.aten.slice_copy)
+        self.assertEqual(nodes[1], exir_ops.edge.aten.permute_copy)
+
+    def test_multi_user_permute_no_change(self) -> None:
+        """Permute with multiple users → no change (only single-user supported)."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(2, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        slice1 = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 1, 0, 2, 1),
+        )
+        slice2 = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 2, 1, 3, 1),
+        )
+        builder.output([slice1, slice2])
+        original = builder.get_graph_module()
+
+        result = cast(PassResult, MoveSliceBeforePermutePass()(original))
+        self.assertFalse(result.modified)
+
+    def test_no_slice_users_no_change(self) -> None:
+        """Permute with no slice users → no change."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(2, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        neg = builder.call_operator(op=exir_ops.edge.aten.neg.default, args=(permuted,))
+        builder.output([neg])
+        original = builder.get_graph_module()
+
+        result = cast(PassResult, MoveSliceBeforePermutePass()(original))
+        self.assertFalse(result.modified)
+
+    def test_dim0_slice_large_reduction_moved(self) -> None:
+        """Dim-0 slice removing >50% of data → profitable, moved."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(10, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        sliced = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 0, 0, 2, 1),
+        )
+        builder.output([sliced])
+        original = builder.get_graph_module()
+
+        result = transform_and_check_numerics(
+            original,
+            (torch.randn(10, 3, 4, 5),),
+            MoveSliceBeforePermutePass(),
+        )
+        self.assertTrue(result.modified)
+
+        nodes = get_compute_nodes_in_gm(result.graph_module)
+        self.assertEqual(len(nodes), 2)
+        self.assertEqual(nodes[0], exir_ops.edge.aten.slice_copy)
+        self.assertEqual(nodes[1], exir_ops.edge.aten.permute_copy)
+
+    def test_dim0_slice_small_reduction_not_moved(self) -> None:
+        """Dim-0 slice removing <50% of data → not profitable, kept."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(10, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        sliced = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 0, 0, 8, 1),
+        )
+        builder.output([sliced])
+        original = builder.get_graph_module()
+
+        result = cast(PassResult, MoveSliceBeforePermutePass()(original))
+        self.assertFalse(result.modified)
+
+    def test_non_dim0_slice_always_moved(self) -> None:
+        """Non-dim-0 slice → always profitable, moved regardless of reduction."""
+        builder = GraphBuilder()
+        x = builder.placeholder("x", torch.randn(10, 3, 4, 5))
+        permuted = builder.call_operator(
+            op=exir_ops.edge.aten.permute_copy.default, args=(x, [0, 2, 3, 1])
+        )
+        sliced = builder.call_operator(
+            op=exir_ops.edge.aten.slice_copy.Tensor,
+            args=(permuted, 2, 0, 3, 1),
+        )
+        builder.output([sliced])
+        original = builder.get_graph_module()
+
+        result = transform_and_check_numerics(
+            original,
+            (torch.randn(10, 3, 4, 5),),
+            MoveSliceBeforePermutePass(),
+        )
+        self.assertTrue(result.modified)