Add approximate gelu replacement to opt level 2 (#10129)

Summary:
Pull Request resolved: #10129

As titled. Gelu is prohibitively expensive to run on DSPs, due to the std::erf call in the function. The PT approximate version using an approximation based on `tanh`, which is  faster on the ASR encoder 27M model for example.

Seems like BUCK files (even with just on_call commands, the linter is complaining).

Differential Revision: D72935935

Author: mcremon-meta

Diff for: backends/cadence/aot/replace_ops.py

@@ -2110,6 +2110,102 @@ def call_operator(
         return super().call_operator(op, args, kwargs, meta)
 
 
+@register_cadence_pass(CadencePassAttribute(opt_level=2))
+class ReplaceGeluWithApproximateGeluPass(ExportPass):
+    """
+    Replace the gelu op with an approximate gelu op. The approximate gelu op
+    is more efficient on DSP backends.
+    """
+
+    def call_operator(
+        self,
+        op,
+        args: Tuple[Argument, ...],
+        kwargs: Dict[str, Argument],
+        meta: NodeMetadata,
+    ) -> ProxyValue:
+        if op not in {
+            exir_ops.edge.aten.gelu.default,
+        }:
+            return super().call_operator(op, args, kwargs, meta)
+
+        # compute the approximate gelu (0.7978845608028654 is sqrt(2 / pi))
+        # as 0.5 * x * (1 + torch.tanh(0.7978845608028654 * ( x + 0.044715 * x^3)))
+
+        # Get 0.5 * x
+        half = super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (args[0], 0.5),
+            {},
+            meta,
+        )
+
+        scaled = super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (args[0], 0.044715),
+            {},
+            meta,
+        )
+
+        # Get x^2 (note that we use mul.Tensor twice instead of pow.Tensor because
+        # it is much more efficient on DSP backends)
+        scaled_square = super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (scaled, args[0]),
+            {},
+            meta,
+        )
+
+        # Get x^3
+        scaled_cubed = super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (scaled_square, args[0]),
+            {},
+            meta,
+        )
+
+        # Get x + 0.044715 * x^3
+        inner_sum = super().call_operator(
+            exir_ops.edge.aten.add.Tensor,
+            (scaled_cubed, args[0]),
+            {},
+            meta,
+        )
+
+        # Get 0.7978845608028654 * ( x + 0.044715 * x^3)
+        scaled_sum = super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (inner_sum, 0.7978845608028654),
+            {},
+            meta,
+        )
+
+        # Get torch.tanh(0.7978845608028654 * ( x + 0.044715 * x^3))
+        tanh = super().call_operator(
+            exir_ops.edge.aten.tanh.default,
+            (scaled_sum,),
+            {},
+            meta,
+        )
+
+        # Get 1 + torch.tanh(0.79788456 * ( x + 0.044715 * x^3))
+        # TODO(): Check why this is not working properly with integer values (e.g. 1 instead of 1.)
+        outer_sum = super().call_operator(
+            exir_ops.edge.aten.add.Tensor,
+            (tanh, 1.0),
+            {},
+            meta,
+        )
+
+        # Retunr the final result
+        return super().call_operator(
+            exir_ops.edge.aten.mul.Tensor,
+            (half, outer_sum),
+            {},
+            meta,
+        )
+
+
 # This class encapsulates all the functions that replace/switch one op in the
 # graph with another.
 class CadenceReplaceOpsInGraph:
@@ -2149,4 +2245,5 @@ class CadenceReplaceOpsInGraph:
         ReplaceAtenAvgPoolWithJarvisAvgPoolPass,
         ReplaceAtenLinalgVectorNormWithCadenceLinalgVectorNormPass,
         ReplaceWhereWithFullArgsWithWhereScalar,
+        # ReplaceGeluWithApproximateGeluPass,
     ]

Diff for: backends/cadence/aot/tests/test_replace_ops_passes.py

@@ -29,6 +29,7 @@
     ReplaceConvWithIm2RowAndLinear,
     ReplaceEmptyTensorsWithFullPass,
     ReplaceFunctionallyEquivalentOpTargets,
+    ReplaceGeluWithApproximateGeluPass,
     ReplaceIm2RowWithViewPass,
     ReplaceLinearWithFullyConnectedOpPass,
     ReplaceMMWithAddMMPass,
@@ -1301,6 +1302,41 @@ def forward(self, cond: torch.Tensor):
             1,
         )
 
+    def test_replace_aten_gelu_with_approximate_gelu(self):
+        class Gelu(torch.nn.Module):
+            def forward(self, input):
+                return torch.nn.functional.gelu(input)
+
+        inputs = torch.randn(2, 1, 64)
+
+        graph_module = export_to_edge(Gelu(), (inputs,)).exported_program().graph_module
+
+        p = ReplaceGeluWithApproximateGeluPass()
+        graph_after_passes = cast(PassResult, p(graph_module)).graph_module
+
+        # Assert that aten.gelu op was decomposed
+        self.assertEqual(
+            count_node(
+                graph_after_passes,
+                exir_ops.edge.aten.gelu.default,
+            ),
+            0,
+        )
+
+        # The decomposition should have one tanh, 2 add and 6 mul
+        self.assertEqual(
+            count_node(graph_after_passes, exir_ops.edge.aten.tanh.default),
+            1,
+        )
+        self.assertEqual(
+            count_node(graph_after_passes, exir_ops.edge.aten.add.Tensor),
+            2,
+        )
+        self.assertEqual(
+            count_node(graph_after_passes, exir_ops.edge.aten.mul.Tensor),
+            6,
+        )
+
 
 class TestReplaceIm2rowWithViewPass(unittest.TestCase):
     def test_no_replacement_for_conv(self):