Arm backend: Add U55 operator check for cast

backends/arm/operator_support/ethos_u55_support.py

@@ -384,3 +384,63 @@ def is_node_supported(
             return False
 
         return True
+
+
+class EthosU55CastCheck(OperatorSupportBase):
+    """Reject unsupported casts on U55.
+
+    U55 does not support casting from INT32 or any casts involving BOOL. Note that
+    casting from one dtype to the same dtype is a no-op and is supported.
+
+
+    Attributes:
+        reporter (WhyNoPartitionReporter): Reporter for rejection reasons.
+
+    """
+
+    targets = [
+        exir_ops.edge.dim_order_ops._to_dim_order_copy.default,
+    ]
+
+    def __init__(self, reporter: WhyNoPartitionReporter):
+        """Initialize the check with a reporter.
+
+        Args:
+            reporter (WhyNoPartitionReporter): Reporter for rejection reasons.
+
+        """
+        super().__init__()
+        self.reporter = reporter
+
+    def is_node_supported(
+        self, submodules: typing.Mapping[str, torch.nn.Module], node: fx.Node
+    ) -> bool:
+        """Return True if the node satisfies the cast constraints of U55.
+
+        Args:
+            submodules (typing.Mapping[str, torch.nn.Module]): Exported modules.
+            node (fx.Node): FX node to check.
+
+        Returns:
+            bool: True if supported; otherwise, False.
+
+        """
+        if node.target not in self.targets:
+            return True
+        input_dtype = get_first_fake_tensor(node.all_input_nodes[0]).dtype
+        output_dtype = get_first_fake_tensor(node).dtype
+        if input_dtype == output_dtype:
+            # This is ok as this will not result in a cast
+            return True
+        if input_dtype in (torch.bool, torch.int32):
+            self.reporter.report_reject(
+                node, f"Casting from {input_dtype} is not supported on U55."
+            )
+            return False
+        if output_dtype in (torch.bool,):
+            self.reporter.report_reject(
+                node, f"Casting to {output_dtype} is not supported on U55."
+            )
+            return False
+
+        return True

backends/arm/operator_support/tosa_supported_operators.py

@@ -21,6 +21,7 @@
 from executorch.backends.arm._passes.insert_table_ops import TableOps
 from executorch.backends.arm.constants import DQ_OPS, Q_OPS
 from executorch.backends.arm.operator_support.ethos_u55_support import (
+    EthosU55CastCheck,
     EthosU55DtypeSupport,
     EthosU55NotSupported,
     EthosU55TransposeCheck,
@@ -141,6 +142,7 @@ def tosa_support_factory(
         negative_checks.append(EthosU55DtypeSupport(reporter))
         negative_checks.append(EthosU55TransposeCheck(reporter))
         negative_checks.append(EthosU55ViewCheck(reporter))
+        negative_checks.append(EthosU55CastCheck(reporter))
 
     return chain(
         reporter.wrap_check(

backends/arm/test/ops/test_to_copy.py

@@ -244,3 +244,32 @@ def test_to_tosa_INT_not_delegated_REDUNDANT_CAST(test_data: Tuple):
         non_delegated_ops={},  # These are removed outside of the Arm backend so the graph is empty
     )
     pipeline.run()
+
+
+_TO_COPY_DATA_INT_U55_REJECT = {
+    "rand_bool_int8": lambda: (
+        torch.randint(0, 2, (1, 2, 3, 4), dtype=torch.bool),
+        torch.int8,
+    ),
+    "rand_int16_bool": lambda: (
+        torch.randint(-1000, 1000, (1, 2, 3, 4), dtype=torch.int16),
+        torch.bool,
+    ),
+    "rand_int32_int8": lambda: (
+        torch.randint(-1000, 1000, (1, 2, 3, 4), dtype=torch.int32),
+        torch.int8,
+    ),
+}
+
+
+@common.parametrize("test_data", _TO_COPY_DATA_INT_U55_REJECT)
+def test_to_u55_INT(test_data: Tuple):
+    test_tensor, new_dtype = test_data()
+    pipeline = OpNotSupportedPipeline[input_t1](
+        Cast(new_dtype),
+        (test_tensor,),
+        u55_subset=True,
+        quantize=True,
+        non_delegated_ops={},  # These are removed outside of the Arm backend so the graph is empty
+    )
+    pipeline.run()