native_function_generation.py

from __future__ import annotations

import string
from collections import defaultdict
from typing import TYPE_CHECKING

import torchgen.api.dispatcher as dispatcher
from torchgen.api.translate import translate
from torchgen.api.types import Binding, DispatcherSignature, Expr
from torchgen.context import with_native_function
from torchgen.model import (
    Annotation,
    Argument,
    BackendIndex,
    BackendMetadata,
    BaseOperatorName,
    BaseTy,
    BaseType,
    DEFAULT_KERNEL_NAMESPACE,
    DeviceCheckType,
    DispatchKey,
    FunctionSchema,
    NativeFunction,
    NativeFunctionsGroup,
    OperatorName,
    Return,
    SchemaKind,
    Variant,
)
from torchgen.utils import concatMap


if TYPE_CHECKING:
    from collections.abc import Sequence


# See Note: [Out ops with functional variants that don't get grouped properly]
OUT_OPS_THAT_DONT_GET_GROUPED_PROPERLY = [
    # This has a functional variant, but it's currently marked private.
    # This function should be marked private as well (*_backward ops aren't exposed to python anyway).
    "adaptive_avg_pool3d_backward.grad_input",
    # There's a functional variant, _slow_conv2d_backward.output_mask, that isn't grouped properly.
    # Maybe we can kill this operator in favor of convolution_backward?
    "_slow_conv2d_backward.grad_input",
]


# See Note: [Mutable ops that cannot get an out variant]
MUTABLE_OPS_THAT_CANNOT_GET_AN_OUT_VARIANT = [
    # should be out=?
    "_cummax_helper",
    # should be out=?
    "_cummin_helper",
]

# All of these operators don't have any tensor like returns
FUNCTIONAL_OPS_THAT_CANNOT_GET_AN_OUT_VARIANT = [
    "_assert_async",  # no return
    "_assert_async.msg",  # no return
    "_assert_tensor_metadata",  # no return
    "_cslt_sparse_mm_search",  # returns an int
    "_assert_scalar",  # no return
    "_dimI",  # returns an int
    "_dimV",  # returns an int
    "_has_same_storage_numel",  # returns a boolean
    "_linalg_check_errors",  # no return
    "_local_scalar_dense",  # returns a Scalar
    "_nested_tensor_from_mask_left_aligned",  # returns a boolean
    "_nnz",  # returns an int
    "_use_cudnn_ctc_loss",  # returns a boolean
    "_use_cudnn_ctc_loss.Tensor",  # returns a boolean
    "_validate_compressed_sparse_indices",  # no return
    "allclose",  # returns a boolean
    "dense_dim",  # returns an int
    "equal",  # returns a boolean
    "is_coalesced",  # returns an boolean
    "is_pinned",  # returns a boolean
    "is_same_size",  # returns a boolean
    "is_set_to",  # returns a boolean
    "q_per_channel_axis",  # returns an int
    "q_scale",  # returns a float
    "q_zero_point",  # returns an int
    "qscheme",  # returns a QScheme
    "record_stream",  # no return
    "sparse_dim",  # returns an int
    "sym_constrain_range",  # no return
    "sym_constrain_range_for_size",  # no return
    "_nested_tensor_storage_offsets",  # returns a vector of ints
    "_chunk_grad_outputs_efficient_attention",  # returns a bool
    "_fused_sdp_choice",  # returns an int
    "_print",  # no return
    "_sink_tokens",  # no return
    "_nested_get_ragged_idx",  # returns an int
]

INPLACE_OPS_THAT_DONT_GET_GROUPED_PROPERLY = [
    # polygamma and polygamma.out both exist, but have a
    # pre-self arg (while polygamma_ does not)
    # We should either fix this schema so it can be grouped properly,
    # or allow the codegen to generate new functional/out= NativeFunctions for this op
    # (which would require changing its overload name to prevent overload ambiguity).
    "polygamma_"
]


# Groups "similar" NativeFunctions together
# example add.Tensor, add_.Tensor, add.out
# "similar" NativeFunctions are all expected to have an identical `signature()`,
# But have differing SchemaKinds.
def pre_group_native_functions(
    native_functions: Sequence[NativeFunction],
) -> dict[FunctionSchema, dict[SchemaKind, NativeFunction]]:
    pre_grouped_native_functions: dict[
        FunctionSchema, dict[SchemaKind, NativeFunction]
    ] = defaultdict(dict)
    for f in native_functions:
        d = pre_grouped_native_functions[f.func.signature()]
        assert f.func.kind() not in d
        d[f.func.kind()] = f
    return pre_grouped_native_functions


# Returns the out variant overload name given a base function overload name
def get_expected_out_variant_overload_name(overload_name: str | None) -> str:
    return "out" if not overload_name else f"{overload_name}_out"


# Helper function: given an inplace FunctionSchema, generate its corresponding out= variant
# Example before:
#   _add_relu_.Scalar(Tensor(a!) self, Scalar other, Scalar alpha=1) -> Tensor(a!)
# Example after:
#   _add_relu.Scalar_out(Tensor self, Scalar other, Scalar alpha=1, *, Tensor(a!) out)
def self_to_out_signature(func: FunctionSchema) -> FunctionSchema:
    # Generating an out= schema from an inplace schema.
    assert func.kind() == SchemaKind.inplace
    assert func.arguments.self_arg is not None
    # The new out= schema has:
    # - a new out argument with the same type as "func" (but with a mutable annotation)
    # - The returns (if any) now alias the out= argument instead of "func"
    # - an "out" overload name
    return FunctionSchema(
        name=func.name.remove_inplace().with_overload(
            get_expected_out_variant_overload_name(func.name.overload_name)
        ),
        arguments=func.arguments.remove_self_annotation().with_out_args(
            [
                Argument(
                    name="out",
                    type=func.arguments.self_arg.argument.type,
                    default=None,
                    annotation=func.arguments.self_arg.argument.annotation,
                )
            ]
        ),
        returns=func.returns,
    )


# Helper function: given a functional FunctionSchema, generate its corresponding out= variant
# Example before:
#   _to_copy(Tensor self, *, ScalarType? dtype=None, Layout? layout=None, Device? device=None,
#       bool? pin_memory=None, bool non_blocking=False, MemoryFormat? memory_format=None) -> Tensor
# Example after:
#   _to_copy._out(Tensor self, *, bool non_blocking=False, MemoryFormat? memory_format=None,
#       Tensor(a!) out) -> Tensor(a!)
def functional_to_out_signature(func: FunctionSchema) -> FunctionSchema:
    # Generating an out= schema from a functional schema.
    assert func.kind() == SchemaKind.functional

    new_returns, new_out_args = generate_out_args_from_schema(func)
    # The new out= schema has:
    # - one or more new out argument(s) with the same type as returns (but with a mutable annotation)
    # - The returns now alias the out= arguments
    # - an "_out" overload name
    return FunctionSchema(
        name=func.name.with_overload(
            get_expected_out_variant_overload_name(func.name.overload_name)
        ),
        arguments=func.arguments.signature().with_out_args(
            new_out_args,
        ),
        returns=tuple(new_returns),
    )


# Helper function: given a function schema, generate corresponding out arguments, also the updated return annotations.
def generate_out_args_from_schema(
    func: FunctionSchema,
) -> tuple[list[Return], list[Argument]]:
    # More of a sanity check - our existing restrictions on schemas should enforce that
    # mutable schema kinds never return their mutable arguments.
    assert not any(
        r.annotation is not None and r.annotation.is_write for r in func.returns
    )

    tensorlike_rets = [r for r in func.returns if r.type.is_tensor_like()]
    assert len(tensorlike_rets) > 0

    used_annotations = concatMap(
        lambda a: [] if a.annotation is None else a.annotation.alias_set,
        func.arguments.flat_all,
    )
    valid_annotations = [x for x in string.ascii_lowercase if x not in used_annotations]

    all_rets_are_tensors = all(r.type == BaseType(BaseTy.Tensor) for r in func.returns)

    new_out_args: list[Argument] = []
    # The end result of new_returns is that:
    # - If every return is a plain tensor, then the new returns == the old returns, but with the out= alias annotations added.
    # - Otherwise, none of the out arguments show up in the returns (and we're only left with non-tensor-like returns, if any).
    new_returns: list[Return] = []
    for i, r in enumerate(func.returns):
        if r.type.is_tensor_like():
            new_out = Argument(
                name="out" if len(func.returns) == 1 else f"out{i}",
                type=r.type,
                default=None,
                annotation=Annotation.parse(f"{valid_annotations[i]}!"),
            )
            new_out_args.append(new_out)
            if all_rets_are_tensors:
                # The convention for out= schemas is that they only return their out arguments
                # if the return is a plain Tensor (or if it's a tuple of plain Tensors)
                new_ret = Return(
                    name=None, type=new_out.type, annotation=new_out.annotation
                )
                new_returns.append(new_ret)
        else:
            new_returns.append(r)
    return new_returns, new_out_args


# Helper function: given a mutable FunctionSchema, generate its corresponding out= variant
# Example before:
#   _fused_moving_avg_obs_fq_helper(Tensor self, Tensor observer_on, Tensor fake_quant_on, Tensor(a!) running_min, Tensor(b!) running_max, Tensor(c!) scale, Tensor(d!) zero_point, float averaging_const, int quant_min, int quant_max, int ch_axis, bool per_row_fake_quant=False, bool symmetric_quant=False) -> (Tensor output, Tensor mask)  # noqa: B950
# Example after:
#   _fused_moving_avg_obs_fq_helper._out(Tensor self, Tensor observer_on, Tensor fake_quant_on, Tensor(a!) running_min, Tensor(b!) running_max, Tensor(c!) scale, Tensor(d!) zero_point, float averaging_const, int quant_min, int quant_max, int ch_axis, bool per_row_fake_quant=False, bool symmetric_quant=False, *, Tensor(e!) out0, Tensor(f!) out1) -> (Tensor(e!), Tensor(f!))  # noqa: B950
def mutable_to_out_signature(func: FunctionSchema) -> FunctionSchema:
    # Generating an out= schema from a mutable schema.
    assert func.kind() == SchemaKind.mutable
    # The new out= schema has:
    # - Any non-aliased tensor-like returns are converted to mutable, aliased out= arguments
    #   (if the argument is a tensor then we also return it for method chaining,
    #   otherwise we return nothing)
    # - an "out" overload name
    #
    # Note that:
    # (1) This also means that we can *only* generate an out= variant from a mutable schema
    #     if the mutable schema has at least one tensor-like non-aliasing return.
    # (2) The generated out= variant still has mutable positional arguments,
    #     but if necessary we could probably add another out= variant that also
    #     functionalizes the mutable arguments (a functional_out variant)

    new_returns, new_out_args = generate_out_args_from_schema(func)

    return FunctionSchema(
        name=func.name.remove_inplace().with_overload(
            get_expected_out_variant_overload_name(func.name.overload_name)
        ),
        arguments=func.arguments.with_out_args(new_out_args),
        returns=tuple(new_returns),
    )


# This function, given function of one SchemaKind, as well as a target SchemaKind,
# generates a new NativeFunction with the same properties, but using the target SchemaKind.
# We only actually generate functions for either functional or out= SchemaKinds.
# This function returns a tuple, with:
# - The generated NativeFunction
# - a dictionary of `BackendIndex` objects, describing which dispatch keys
#   we will generate kernels for, for the new NativeFunction.
#   Details are in the function, but we only generate composite kernels (in some cases) today.
def generate_function(
    f: NativeFunction, k: SchemaKind
) -> tuple[NativeFunction, dict[DispatchKey, dict[OperatorName, BackendMetadata]]]:
    from torchgen.api import cpp

    if k == SchemaKind.functional:
        assert f.func.kind() != SchemaKind.functional
        # The new "functional" NativeFunction has:
        # - any mutable arguments have been converted into (immutable) returns.
        #   (if a mutable argument was not also a return, it gets converted to one)
        # - "_functional" appended to the base name, ONLY IF this op has a mutable variant.
        #   See Note [Overload Ambiguity With Functional Variants]
        # The default grouping logic in signature() actually already does this,
        # so we can piggy-back off it (but we still want return names)
        func = f.func.signature(keep_return_names=True).with_name(
            OperatorName(
                name=BaseOperatorName(
                    base=f.func.name.name.base,
                    inplace=False,
                    dunder_method=f.func.name.name.dunder_method,
                    # See Note [Overload Ambiguity With Functional Variants]
                    functional_overload=f.func.kind() == SchemaKind.mutable,
                ),
                overload_name=f.func.name.overload_name,
            )
        )
    elif k == SchemaKind.out:
        # We generate out= ops mostly just so that we can pair up NativeFunctions into groups easily,
        # but at least today, there is no good reason to actually use them.
        # we'll generate a dispatcher entry for them, but won't actually register any kernels for them.
        if f.func.kind() == SchemaKind.inplace:
            func = self_to_out_signature(f.func)
        elif f.func.kind() == SchemaKind.mutable:
            func = mutable_to_out_signature(f.func)
        elif f.func.kind() == SchemaKind.functional:
            func = functional_to_out_signature(f.func)
        else:
            raise AssertionError(
                "We only bother generating out= functions from either inplace or mutable or functional variants"
            )
    else:
        raise AssertionError(
            "We currently only generate either functional or out= NativeFunctions"
        )

    # Generated kernel naming convention for out: <op_name>_<overload_name>. The reason for this is to
    # disambiguate operator with the same name but different overload name, e.g., `randn.names_out` and
    # `randn.generator_with_names_out`.
    kernel_name = (
        func.name.unambiguous_name()
        if func.kind() == SchemaKind.out
        else cpp.name(func)
    )
    if f.func.has_symint():
        kernel_name += "_symint"
    backend_metadata = {
        DispatchKey.CompositeExplicitAutograd: {
            func.name: BackendMetadata(
                kernel=kernel_name,
                structured=False,
                cpp_namespace=DEFAULT_KERNEL_NAMESPACE,
            )
        }
    }
    tags = {"generated"} | set(
        f.tags & {"nondeterministic_seeded", "view_copy", "pt2_compliant_tag"}
    )

    return (
        NativeFunction(
            func=func,
            use_const_ref_for_mutable_tensors=f.use_const_ref_for_mutable_tensors,
            # These generated fn's aren't meant to be user friendly- don't generate methods.
            variants={Variant.function},
            structured=False,
            structured_delegate=None,
            structured_inherits=None,
            precomputed=None,
            autogen=[],
            ufunc_inner_loop={},
            manual_kernel_registration=False,
            manual_cpp_binding=False,
            python_module=None,
            category_override=None,
            device_guard=False,
            device_check=DeviceCheckType.NoCheck,
            loc=f.loc,
            cpp_no_default_args=set(),
            is_abstract=f.is_abstract,
            has_composite_implicit_autograd_kernel=False,
            has_composite_implicit_autograd_nested_tensor_kernel=False,
            has_composite_explicit_autograd_kernel=True,
            has_composite_explicit_autograd_non_functional_kernel=False,
            # Every generated NativeFunction gets a "generated" tag, so it's easy to tell
            # which NativeFunction objects did not come directly from native_functions.yaml.
            tags=tags,
            namespace=f.namespace,
        ),
        backend_metadata,
    )


# This function is responsible for adding generated NativeFunctions which don't appear
# explicitly in the codegen.
# You can inspect the full list of NativeFunctions yourself with the torchgen package, by running
# torchgen.parse_native_yaml("aten/src/ATen/native/native_functions.yaml", "aten/src/ATen/native/tags.yaml")
# (Maybe we should make a friendly API for this)
#
# Note: this function *mutates* its two inputs,
# adding the new NativeFunctions / BackendMetadata to them
def add_generated_native_functions(
    rs: list[NativeFunction],
    indices: dict[DispatchKey, dict[OperatorName, BackendMetadata]],
) -> None:
    # The main code for generating new NativeFunctions
    # First we group of NativeFunctions by schema kind,
    # then we detect which ones are missing and generate them.
    pre_grouped_native_functions = pre_group_native_functions(rs)
    for d in pre_grouped_native_functions.values():
        has_functional = SchemaKind.functional in d
        has_inplace = SchemaKind.inplace in d
        has_mutable = SchemaKind.mutable in d
        has_out = SchemaKind.out in d
        is_core = any("core" in variant.tags for variant in d.values())

        # We automatically generate a few native functions that don't exist in the yaml, for a few reasons:
        # (1) If an operator has an inplace/out= variant but no functional variant, we can generate
        #     a simple functional variant that the functionalization pass can consume.
        # (2) If an operator has an inplace or functional but no out= variant, we generate an out=
        #     variant, mostly so we can easily pair up functions into NativeFunctionsGroup,
        #     while maintaining the constraint that the out= variant is "required".
        if has_mutable or has_inplace or has_out or has_functional:
            # Don't bother generating functions trio's for native functions that bypass the dispatcher.
            are_manual = all(f.manual_cpp_binding for f in d.values())
            # Don't bother generating functional + out= variants for view operators
            # set_ is technically an inplace_view, but for now it is treated
            # as a normal inplace op in the codegen
            has_view_ops = any(
                f.is_view_op and str(f.func.name.name) != "set_" for f in d.values()
            )
            # Don't generate the other variants for non-core CompositeImplicitAutograd operators.
            # We could probably do this, but the main benefit of generating the function triplets
            # is for transforms that need them, and transforms don't need to act directly
            # on CompositeImplicitAutograd operators (since we let them decompose).
            are_composite_implicit = all(
                f.has_composite_implicit_autograd_kernel for f in d.values()
            )
            if are_manual or has_view_ops or are_composite_implicit and not is_core:
                continue
            if has_out and len(d.values()) == 1:
                # Note: [Out ops with functional variants that don't get grouped properly]
                # In theory we could validly have an out= operator in native_functions.yaml
                # that has no other variants.
                # But today, all of the operators where that's the case actually do have
                # functional variants, that we are just unable to pair up properly.
                # I think banning this all together is probably safer
                # (you can always add a functional variant yourself if you want to add a new out= operator).
                #
                # We should probably fix the existing cases; this check is to prevent us from adding more over time.
                if (
                    str(d[SchemaKind.out].func.name)
                    not in OUT_OPS_THAT_DONT_GET_GROUPED_PROPERLY
                ):
                    raise AssertionError(
                        f"Found an out= operator that we could not find any other variants of: {str(d[SchemaKind.out].func)}"
                    )
                continue

            # Some inplace ops that have problematic schemas (that we should fix), which prevent us
            # from generating out= and functional variants
            if (
                has_inplace
                and str(d[SchemaKind.inplace].func.name)
                in INPLACE_OPS_THAT_DONT_GET_GROUPED_PROPERLY
            ):
                continue

            base_fn = (
                d[SchemaKind.mutable]
                if has_mutable
                else d[SchemaKind.inplace]
                if has_inplace
                else d[SchemaKind.out]
                if has_out
                else d[SchemaKind.functional]
            )

            # Note: [Mutable ops that cannot get an out variant]
            # We can only generate an out= variant if either:
            # - the original function has tensor-like returns (since we can convert them to out kwargs)
            # - or it's inplace (since we can convert `self` to an out kwarg)
            # There are only two functions that don't fit this criteria today though,
            # and they both look like they should be fixed to be out= variants,
            # so if feels safer to ban this schema all-together
            base_fn_valid = base_fn.func.kind() == SchemaKind.inplace or any(
                r.type.is_tensor_like() for r in base_fn.func.returns
            )
            # Note: [Loosen the assertion that all functional should have out variant]
            # By design all functional operators should have our variants. The needs_out check
            # is loosening this requirement, changing it to only generate out variant if there's
            # an `autogen` block in the native function, in the long run it should be removed.
            # FIXME: Remove this after figuring out CI job failures related to min, max, mean
            needs_out = any("out" in str(op_name) for op_name in base_fn.autogen)
            gets_out_variant = not has_out and base_fn_valid and needs_out
            if not has_out and not base_fn_valid:
                if (
                    str(base_fn.func.name)
                    not in MUTABLE_OPS_THAT_CANNOT_GET_AN_OUT_VARIANT
                    and str(base_fn.func.name)
                    not in FUNCTIONAL_OPS_THAT_CANNOT_GET_AN_OUT_VARIANT
                ):
                    raise AssertionError(
                        f"""Found an operator that we could not generate an out= variant for: {str(base_fn.func)}.
This type of operators don't have tensor-like return, making it difficult to generate a proper out= variant. If
out= variant is not needed, please add the function name into FUNCTIONAL_OPS_THAT_CANNOT_GET_AN_OUT_VARIANT list."""
                    )

            # Generate an out= variant
            if gets_out_variant:
                fn, metadata = generate_function(base_fn, SchemaKind.out)
                d[SchemaKind.out] = fn
                BackendIndex.grow_index(indices, metadata)
                rs.append(fn)

            # Generate a functional variant, but only do it if the operator got an out= variant
            # (Functional variants are only useful if we can group up the variants,
            # which we can only do if they have an out= variant)
            if not has_functional and (has_out or gets_out_variant):
                fn, metadata = generate_function(base_fn, SchemaKind.functional)
                d[SchemaKind.functional] = fn
                BackendIndex.grow_index(indices, metadata)
                rs.append(fn)


def return_str(rets: tuple[Return, ...], names: list[str]) -> str:
    assert len(rets) == len(names)
    if len(rets) == 0:
        return ""
    elif len(rets) == 1:
        return f"return {names[0]};"
    else:
        return f"return {dispatcher.returns_type(rets).cpp_type()}({', '.join(names)});"


# Given a function, and the name of a variable corresponding to the output of that function,
# gather up all of the individual returns that are not aliased
def gather_nonaliased_inner_rets(func: FunctionSchema, out_var: str) -> list[str]:
    aliased_rets = func.aliased_return_names()
    non_aliased_names = []
    is_out_var_a_tuple = len(func.returns) > 1
    for i, r in enumerate(aliased_rets):
        if r is None:
            non_aliased_names.append(
                f"std::get<{i}>({out_var})" if is_out_var_a_tuple else out_var
            )
    return non_aliased_names


# Generates functional kernels in terms of their inplace.mutable counterparts.
# We only do this for "generated" NativeFunctions
@with_native_function
def gen_composite_functional_kernel(g: NativeFunctionsGroup) -> str | None:
    # We should only be generating these for code-generated NativeFunctions
    if "generated" not in g.functional.tags:
        return None
    # And we always write the kernel for a generated op in terms of a non-generated op.
    if g.inplace is not None and "generated" not in g.inplace.tags:
        target_f = g.inplace
    elif g.mutable is not None and "generated" not in g.mutable.tags:
        target_f = g.mutable
    else:
        # We should be guaranteed to have a valid inplace/mutable variant to call into.
        # See Note: [Mutable Ops Not Using Functionalization]
        raise AssertionError(str(g.functional.func))

    sig = DispatcherSignature(g.functional.func)
    target_sig = DispatcherSignature(target_f.func)

    context: list[Binding | Expr] = []
    clone_mutable_inputs = []
    cloned_return_names = []
    # We can't just directly pass all of the arguments from the functional op into the mutating op.
    # We need to check for which inputs to the mutating operator are mutable,
    # and clone those inputs first.
    for a_curr, a_tgt in zip(
        dispatcher.jit_arguments(g.functional.func),
        dispatcher.jit_arguments(target_f.func),
    ):
        if a_tgt.annotation is not None and a_tgt.annotation.is_write:
            clone_mutable_inputs.append(
                f"auto {a_curr.name}_clone = clone_arg({a_curr.name});"
            )
            context.append(
                Expr(
                    expr=f"{a_curr.name}_clone",
                    type=dispatcher.argument_type(a_curr, binds=a_curr.name),
                )
            )
            # Invariant: mutable arguments on the inner mutable op are always returns on the functional op.
            cloned_return_names.append(f"{a_curr.name}_clone")
        else:
            context.append(dispatcher.argument(a_curr))
    exprs = ", ".join([e.expr for e in translate(context, target_sig.arguments())])

    out_name = "output"
    maybe_assign = f"auto {out_name} = " if len(target_f.func.returns) > 0 else ""
    inner_return_names = gather_nonaliased_inner_rets(target_f.func, out_name)
    ret_str = return_str(
        g.functional.func.returns, inner_return_names + cloned_return_names
    )

    clone_mutable_inputs_str = "\n".join(clone_mutable_inputs)
    return f"""
{sig.defn(name=sig.name() + ("_symint" if g.out.func.has_symint() else ""))} {{
  {clone_mutable_inputs_str}
  {maybe_assign}at::_ops::{target_f.func.name.unambiguous_name()}::call({exprs});
  {ret_str}
}}
"""


# Generates out= kernels in terms of their functional counterparts.
# We only do this for "generated" NativeFunctions
@with_native_function
def gen_composite_out_kernel(g: NativeFunctionsGroup) -> str | None:
    # We should only be generating these for code-generated NativeFunctions
    if "generated" not in g.out.tags:
        return None
    # And we always write the kernel for the out= op in terms of the functional.
    # Note that the functional op might have also been generated, but we don't have to
    # worry about cycles, because the generated functional kernels are always implemented
    # in terms of non-generated kernels (see gen_composite_functional_kernel).

    sig = DispatcherSignature(g.out.func)
    target_sig = DispatcherSignature(g.functional.func)

    exprs = ", ".join(
        [e.expr for e in translate(sig.arguments(), target_sig.arguments())]
    )

    copy_outs = []
    out_name = "tmp_output"
    for i, out_arg in enumerate(g.out.func.arguments.out):
        functional_return_name = (
            out_name
            if len(g.functional.func.returns) == 1
            else f"std::get<{i}>({out_name})"
        )
        copy_outs.append(
            f"""\
  resize_out_helper({out_arg.name}, {functional_return_name});
  copy_arg({out_arg.name}, {functional_return_name});"""
        )

    rets = []
    # For each return arg in the calling (out=) operator,
    # If it corresponds to an aliased input, return the input.
    # Otherwise, return the corresponding output from calling the functional operator.
    for i, ret_name in enumerate(g.out.func.aliased_return_names()):
        if ret_name is not None:
            rets.append(ret_name)
        else:
            functional_return_name = (
                out_name
                if len(g.functional.func.returns) == 1
                else f"std::get<{i}>({out_name})"
            )
            rets.append(functional_return_name)

    copy_outs_str = "\n".join(copy_outs)

    # Kernel name needs to follow the naming convention defined in `generate_function()`
    return f"""
{sig.defn(name=g.out.func.name.unambiguous_name() + ("_symint" if g.out.func.has_symint() else ""))} {{
  auto {out_name} = at::_ops::{g.functional.func.name.unambiguous_name()}::call({exprs});
  {copy_outs_str}
  {return_str(g.out.func.returns, rets)}
}}
"""