Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18167)

Summary:

Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes:

1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic
   shape_indices on the raw shapes and preserves the last dimension (NHWC
   channel), skip inserting input/output transposes. The view_copy can
   operate directly on NHWC data.

2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose
   permutation matches channels_last_order (NCHW→NHWC) or its inverse
   (NHWC→NCHW) are redundant with the tosa_dim_order annotation that
   already handles format conversion. Replace them with view_copy (identity
   reshape) to avoid generating TOSA TRANSPOSE nodes.

Reviewed By: digantdesai

Differential Revision: D96432610

Author: 3l1

backends/arm/_passes/to_tosa_memory_format_pass.py

@@ -16,6 +16,7 @@
 )
 from executorch.backends.arm.constants import NCHW_ORDER, NNCHW_ORDER, NNNCHW_ORDER
 from executorch.backends.arm.tosa.dialect.shape import is_shape_op_node
+from executorch.backends.arm.tosa.mapping import TosaSpecialDtype
 from executorch.exir import ExportedProgram
 from executorch.exir.dialects._ops import ops as exir_ops
 from executorch.exir.pass_base import ExportPass, PassResult
@@ -271,7 +272,7 @@ def insert_output_transpose(node, graph_module):
         # Guard: mem_format must be a true permutation for the current rank
         assert sorted(mem_format) == list(
             range(rank)
-        ), f"bad perm {mem_format} for rank {rank} in insert_input_transpose"
+        ), f"bad perm {mem_format} for rank {rank} in insert_output_transpose"
 
         with graph_module.graph.inserting_after(node):
             permute_node = create_node(
@@ -296,6 +297,110 @@ def insert_output_transpose(node, graph_module):
             for user in users:
                 user.replace_input_with(node, permute_node)
 
+    @staticmethod
+    def _get_shape_indices(
+        src_shape: list[int], tgt_shape: list[int]
+    ) -> list[list[int]] | None:
+        """Greedy dimension matching for reshape operations.
+
+        For each target dimension, greedily consumes contiguous source
+        dimensions whose product equals the target size.  Size-1 target
+        dimensions that do not correspond to any source dimension produce
+        empty index lists (inserted dims).
+
+        Returns ``None`` when no valid mapping exists.
+
+        """
+        src_idx = 0
+        result: list[list[int]] = []
+
+        for tgt_dim in tgt_shape:
+            if tgt_dim <= 0:
+                return None
+
+            indices: list[int] = []
+            remaining = tgt_dim
+
+            while src_idx < len(src_shape):
+                if src_shape[src_idx] == 0:
+                    return None
+                if remaining % src_shape[src_idx] != 0:
+                    break
+                indices.append(src_idx)
+                remaining //= src_shape[src_idx]
+                src_idx += 1
+                if remaining == 1:
+                    break
+
+            if remaining != 1:
+                return None
+
+            result.append(indices)
+
+        if src_idx != len(src_shape):
+            return None
+
+        return result
+
+    @staticmethod
+    def _is_monotonic(indices: list[list[int]]) -> bool:
+        """Return ``True`` when all non-empty index groups are strictly ordered
+        — i.e. each group's indices follow the previous group's.
+        """
+        last_max = -1
+        for group in indices:
+            if not group:
+                continue
+            if group[0] <= last_max:
+                return False
+            last_max = group[-1]
+        return True
+
+    @staticmethod
+    def _is_nhwc_safe_reshape(
+        input_shape, output_shape, input_sr, output_sr  # noqa: ARG004
+    ) -> bool:
+        """Detect whether a 4-D+ reshape can operate directly on NHWC data.
+
+        By the time ``ToTosaMemoryFormatPass`` runs, 4-D tensor shapes in
+        ``meta["val"]`` are already in NHWC physical order (the channel
+        dimension sits at position ``rank - spatial_rank - 1``, not at
+        position 1 as in NCHW).  We therefore check the shape indices on
+        the **raw** input/output shapes — no extra permutation is needed.
+
+        Returns ``True`` when:
+        1. The reshape has monotonic shape_indices (each output dim maps
+           to a contiguous, in-order group of input dims), AND
+        2. The channel dimension is preserved alone (not merged with
+           spatial dims).
+
+        """
+        rank_in = len(input_shape)
+        rank_out = len(output_shape)
+        if rank_in < 4 or rank_out < 4:
+            return False
+
+        indices = ToTosaMemoryFormatPass._get_shape_indices(
+            list(input_shape), list(output_shape)
+        )
+        if indices is None:
+            return False
+
+        if not ToTosaMemoryFormatPass._is_monotonic(indices):
+            return False
+
+        # In the TOSA pipeline the physical memory order is NHWC.
+        # The channel dimension in NHWC is always the **last** axis
+        # (position ``rank - 1``).  It must appear *alone* in its
+        # output group — if it is merged with spatial dims the reshape
+        # would reorder channel data and the optimisation is invalid.
+        channel_idx = rank_in - 1
+        for group in indices:
+            if channel_idx in group:
+                return len(group) == 1
+        # Channel dim not consumed by any group — conservative reject.
+        return False
+
     @staticmethod
     def _insert_view_transpose(
         input_shape, output_shape, node, input_node, graph_module
@@ -317,6 +422,14 @@ def _insert_view_transpose(
             output_sr,
         )
 
+        # When the NHWC-space reshape has monotonic shape_indices the
+        # view_copy can operate directly on NHWC data — no transposes
+        # are needed.
+        if channel_reshape and ToTosaMemoryFormatPass._is_nhwc_safe_reshape(
+            input_shape, output_shape, input_sr, output_sr
+        ):
+            return
+
         if (
             channel_reshape or nhwc_to_nchw
         ) and ToTosaMemoryFormatPass.memory_format_differs(input_shape, input_sr):
@@ -329,6 +442,59 @@ def _insert_view_transpose(
         ) and ToTosaMemoryFormatPass.memory_format_differs(output_shape, output_sr):
             ToTosaMemoryFormatPass.insert_output_transpose(node, graph_module)
 
+    def _try_replace_redundant_permute(
+        self, node: torch.fx.Node, graph_module: torch.fx.GraphModule
+    ) -> bool:
+        """Replace a permute_copy with view_copy if it duplicates tosa_dim_order.
+
+        When a permute_copy's permutation matches the channels-last order
+        (or its inverse), the permute does the same NCHW<>NHWC conversion
+        that tosa_dim_order already handles — keeping both would
+        double-convert.  Replace with view_copy (identity reshape).
+
+        Returns ``True`` if the node was replaced and erased.
+        """
+        if node.target not in (
+            exir_ops.edge.aten.permute_copy.default,
+            exir_ops.edge.aten.permute.default,
+        ):
+            return False
+
+        perm = list(node.args[1])
+        rank = len(perm)
+        sr = node.meta.get("tosa_spatial_rank", 0)
+
+        if rank < 3 or sr < 1:
+            return False
+
+        cl_order = list(self._channels_last_order(rank, sr))
+        cl_inv = list(self._channels_last_inverse_order(rank, sr))
+        if perm != cl_order and perm != cl_inv:
+            return False
+
+        input_node = node.args[0]
+        output_shape = list(node.meta["val"].shape)
+        with graph_module.graph.inserting_before(node):
+            const_shape_node = graph_module.graph.call_function(
+                exir_ops.backend.tosa.CONST_SHAPE.default,
+                (output_shape,),
+            )
+            const_shape_node.meta["val"] = output_shape
+            const_shape_node.meta["tosa_dim_order"] = node.meta.get(
+                "tosa_dim_order", tuple(range(rank))
+            )
+            const_shape_node.meta[TosaSpecialDtype.meta_key()] = (
+                TosaSpecialDtype.SHAPE
+            )
+            view_node = graph_module.graph.call_function(
+                exir_ops.edge.aten.view_copy.default,
+                (input_node, const_shape_node),
+            )
+            view_node.meta = dict(node.meta)
+        node.replace_all_uses_with(view_node)
+        graph_module.graph.erase_node(node)
+        return True
+
     def insert_tosa_transposes(self, graph_module: torch.fx.GraphModule):
         """Transposes are needed for operators transforming the input to a
         different rank, as 4D and 5D-tensors are assumed to be in (N)NHWC-
@@ -345,12 +511,15 @@ def insert_tosa_transposes(self, graph_module: torch.fx.GraphModule):
         - 1D/2D tensors
 
         """
-        for node in graph_module.graph.nodes:
+        for node in list(graph_module.graph.nodes):
             if node.op != "call_function":
                 continue
 
+            if self._try_replace_redundant_permute(node, graph_module):
+                continue
+
             # Transpose views
-            elif node.target == exir_ops.edge.aten.view_copy.default:
+            if node.target == exir_ops.edge.aten.view_copy.default:
                 input_node = node.args[0]
                 input_shape = input_node.meta["val"].shape
                 output_shape = node.meta["val"].shape

backends/arm/test/passes/test_to_tosa_memory_format.py

@@ -177,11 +177,79 @@ def get_inputs(self) -> input_t:
         return (torch.rand(4, 4, 4, 4),)
 
 
+class NHWCSafeSpatialMerge(torch.nn.Module):
+    """Test-module with a 4D->4D reshape that merges spatial dims H*W while
+    preserving the last-dim channel.
+
+    For models with view_copy shapes [1,2,14,72]->[1,28,1,72] where C=2
+    sits at NCHW position 1 and the last dim (72) is the NHWC channel that gets
+    preserved.  ``_is_nhwc_safe_reshape`` detects that shape_indices on the raw
+    shapes are monotonic with the last dim alone, so no transposes are inserted
+    around the view_copy.
+
+    Setup: conv2d (forces NHWC, C=2) -> view_copy -> add (keeps in NHWC).
+
+    """
+
+    ops_before_pass: Dict[str, int] = {}
+    # Only the 2 I/O transposes for the conv, NO extra transposes from view_copy
+    ops_after_pass: Dict[str, int] = {
+        "executorch_exir_dialects_backend__ops_tosa_TRANSPOSE_default": 2
+    }
+    ops_not_after_pass: List[str] = []
+
+    def __init__(self):
+        super().__init__()
+        self.conv = torch.nn.Conv2d(
+            in_channels=2, out_channels=2, kernel_size=1, bias=False
+        )
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        x = self.conv(x)  # forces NHWC path; output [1, 2, 14, 72]
+        x = x.view(1, 28, 1, 72)  # spatial merge: H*W=2*14->28, last dim 72 preserved
+        return x + x  # keep result 4-D in NHWC
+
+    def get_inputs(self) -> input_t:
+        return (torch.randn(1, 2, 14, 72),)
+
+
+class NHWCUnsafeChannelChange(torch.nn.Module):
+    """Test-module with a 4D->4D reshape that is NOT NHWC-safe because the
+    target shape cannot be produced by a monotonic merge of NHWC input dims.
+
+    The pass MUST still insert transposes around the view_copy.
+
+    """
+
+    ops_before_pass: Dict[str, int] = {}
+    # conv I/O transposes (2) + view_copy transposes (2) = 4
+    ops_after_pass: Dict[str, int] = {
+        "executorch_exir_dialects_backend__ops_tosa_TRANSPOSE_default": 4
+    }
+    ops_not_after_pass: List[str] = []
+
+    def __init__(self):
+        super().__init__()
+        self.conv = torch.nn.Conv2d(
+            in_channels=72, out_channels=72, kernel_size=1, bias=False
+        )
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        x = self.conv(x)  # output [1, 72, 2, 14]
+        x = x.view(1, 14, 2, 72)  # not NHWC-safe (channels shuffled)
+        return x + x
+
+    def get_inputs(self) -> input_t:
+        return (torch.randn(1, 72, 2, 14),)
+
+
 modules: Dict[str, ModuleMetadata] = {
     "no_nhwc": NoNHWC(),
     "parallel_clusters": ParallelClusters(),
     "serial_clusters": SerialClusters(),
     "reshapes": Reshapes(),
+    "nhwc_safe_spatial_merge": NHWCSafeSpatialMerge(),
+    "nhwc_unsafe_channel_change": NHWCUnsafeChannelChange(),
 }
 
 
@@ -209,3 +277,67 @@ 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()
+
+
+# --- Direct unit tests for NHWC-safe reshape helpers ---
+
+
+def test_get_shape_indices_spatial_merge():
+    """[1,2,14,72] -> [1,28,1,72]: merge H*W, insert size-1 dim, preserve C."""
+    indices = ToTosaMemoryFormatPass._get_shape_indices([1, 2, 14, 72], [1, 28, 1, 72])
+    assert indices == [[0], [1, 2], [], [3]]
+
+
+def test_get_shape_indices_identity():
+    """Same shape => each dim maps to itself."""
+    indices = ToTosaMemoryFormatPass._get_shape_indices([2, 3, 4], [2, 3, 4])
+    assert indices == [[0], [1], [2]]
+
+
+def test_get_shape_indices_full_merge():
+    """[2, 3, 4] -> [24]: merge all dims into one."""
+    indices = ToTosaMemoryFormatPass._get_shape_indices([2, 3, 4], [24])
+    assert indices == [[0, 1, 2]]
+
+
+def test_get_shape_indices_incompatible():
+    """Sizes that don't divide => None."""
+    indices = ToTosaMemoryFormatPass._get_shape_indices([2, 3, 5], [6, 4])
+    assert indices is None
+
+
+def test_get_shape_indices_size_one_insert():
+    """[6, 4] -> [6, 1, 4]: inserted size-1 dim in the middle."""
+    indices = ToTosaMemoryFormatPass._get_shape_indices([6, 4], [6, 1, 4])
+    assert indices is not None
+    assert indices == [[0], [], [1]]
+
+
+def test_is_monotonic_true():
+    assert ToTosaMemoryFormatPass._is_monotonic([[0], [1, 2], [], [3]])
+    assert ToTosaMemoryFormatPass._is_monotonic([[0], [], [1], [2, 3]])
+    assert ToTosaMemoryFormatPass._is_monotonic([[], [0, 1, 2]])
+
+
+def test_is_monotonic_false():
+    assert not ToTosaMemoryFormatPass._is_monotonic([[1], [0]])
+    assert not ToTosaMemoryFormatPass._is_monotonic([[0, 2], [1]])
+
+
+def test_is_nhwc_safe_forward():
+    """Shapes already NHWC by the time the pass runs.
+
+    [1,2,14,72] -> [1,28,1,72], sr=2 -> NHWC-safe (spatial merge, C=72
+    preserved).
+
+    """
+    assert ToTosaMemoryFormatPass._is_nhwc_safe_reshape(
+        [1, 2, 14, 72], [1, 28, 1, 72], input_sr=2, output_sr=2
+    )
+
+
+def test_is_nhwc_safe_non_4d():
+    """Reshapes below rank 4 are never NHWC-safe."""
+    assert not ToTosaMemoryFormatPass._is_nhwc_safe_reshape(
+        [6, 4], [24], input_sr=0, output_sr=0
+    )