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[torchlib] Precompute the constant for gelu to avoid precision loss #2179

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Apr 10, 2025
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[torchlib] Precompute the constant for gelu to avoid precision loss #2179

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[torchlib] Precompute the constant for gelu to avoid precision loss
justinchuby Apr 10, 2025
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justinchuby Apr 10, 2025
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22 changes: 10 additions & 12 deletions onnxscript/function_libs/torch_lib/ops/nn.py
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Original file line number Diff line number Diff line change
Expand Up @@ -479,33 +479,31 @@ def aten_gelu(self: TReal, approximate: str = "none") -> TReal:
return result


@torch_op("aten::gelu", private=True)
def _aten_gelu_approximate_none(self: TReal) -> TReal:
"""gelu(Tensor self, *, str approximate='none') -> Tensor"""

# GELU(x) = 0.5 * x * [1 + ERF(x/sqrt(2)]
inner = op.Div(self, 1.4142135623730951)
inner = op.Div(self, ir.tensor(1.4142135623730951, dtype=self.dtype))
erf = op.Erf(inner)
inner = op.Add(erf, 1)
inner = op.Mul(0.5, inner)
inner = op.Add(erf, ir.tensor(1, dtype=self.dtype))
inner = op.Mul(ir.tensor(0.5, dtype=self.dtype), inner)
result = op.Mul(self, inner)
return result


@torch_op("aten::gelu", private=True)
def _aten_gelu_approximate_tanh(self: TReal) -> TReal:
"""gelu(Tensor self, *, str approximate='none') -> Tensor"""

# GELU(x) = 0.5 * x * {1 + Tanh[\sqrt(2/pi) * (x + 0.044715 * x^3)]}
cubed = op.Pow(self, 3)
inner = op.Mul(0.044715, cubed)
cubed = op.Pow(self, ir.tensor(3, dtype=self.dtype))
inner = op.Mul(ir.tensor(0.044715, dtype=self.dtype), cubed)
inner = op.Add(self, inner)
# Prefer explicit graph construction over precomputed constants for clarity.
two_over_pi = op.CastLike(op.Div(2.0, _MATH_PI), self)
inner = op.Mul(op.Sqrt(two_over_pi), inner)
# math.sqrt(2.0/math.pi) = 0.7978845608028654
sqrt_two_over_pi = ir.tensor(0.7978845608028654, dtype=self.dtype)
inner = op.Mul(sqrt_two_over_pi, inner)
inner = op.Tanh(inner)
inner = op.Add(inner, 1)
inner = op.Mul(0.5, inner)
inner = op.Add(inner, ir.tensor(1, dtype=self.dtype))
inner = op.Mul(ir.tensor(0.5, dtype=self.dtype), inner)
result = op.Mul(self, inner)
return result

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6 changes: 1 addition & 5 deletions tests/function_libs/torch_lib/ops_test_data.py
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Expand Up @@ -1790,11 +1790,7 @@ def _where_input_wrangler(
core_ops.aten_conv3d,
tolerance={torch.float32: (3.7e-5, 1.8e-4)},
),
TorchLibOpInfo(
"nn.functional.gelu",
nn_ops.aten_gelu,
tolerance={torch.float16: (8e-2, 1e-4)},
),
TorchLibOpInfo("nn.functional.gelu", nn_ops.aten_gelu),
TorchLibOpInfo("nn.functional.glu", nn_ops.aten_glu),
TorchLibOpInfo(
"nn.functional.linear", nn_ops.aten_linear, tolerance={torch.float16: (1e-2, 1e-3)}
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