import os
from typing import List, Dict, Optional, Tuple, Set, Any, Union, Sequence, TypeVar
from typing_extensions import Literal
import yaml
from collections import OrderedDict, defaultdict, namedtuple
import argparse
import pathlib
import json
from dataclasses import dataclass
import functools

from torchgen.model import (
    STRUCTURED_DISPATCH_KEYS,
    Argument,
    DispatchKey,
    FunctionSchema,
    Location,
    NativeFunction,
    NativeFunctionsGroup,
    OperatorName,
    BackendIndex,
    BackendMetadata,
    OptionalType,
    SchemaKind,
    SelfArgument,
    TensorOptionsArguments,
    Type,
    Variant,
    is_cuda_dispatch_key,
    is_generic_dispatch_key,
    is_ufunc_dispatch_key,
    NativeFunctionsViewGroup,
    ViewSchemaKind,
    BaseOperatorName,
    DEFAULT_KERNEL_NAMESPACE,
)
from torchgen.native_function_generation import (
    pre_group_native_functions,
    add_generated_native_functions,
    gen_composite_functional_kernel,
    gen_composite_out_kernel,
)
from torchgen.api.types import (
    Binding,
    CppSignatureGroup,
    DispatcherSignature,
    NamedCType,
    NativeSignature,
    SpecialArgName,
)
from torchgen.api import cpp
import torchgen.api.dispatcher as dispatcher
import torchgen.api.native as native
import torchgen.api.meta as meta
import torchgen.api.structured as structured
from torchgen.api.translate import translate
from torchgen.selective_build.selector import SelectiveBuilder
from torchgen.utils import (
    Target,
    concatMap,
    context,
    mapMaybe,
    YamlDumper,
    YamlLoader,
    FileManager,
    assert_never,
    make_file_manager,
    NamespaceHelper,
)
from torchgen.context import (
    method_with_native_function,
    native_function_manager,
    with_native_function_and_indices,
    with_native_function,
)
import torchgen.dest as dest
from torchgen.gen_functionalization_type import (
    gen_functionalization_definition,
    gen_functionalization_registration,
    gen_functionalization_view_inverse_declaration,
    gen_composite_view_copy_kernel,
    gen_symint_view_copy_kernel,
)

T = TypeVar("T")

# Welcome to the ATen code generator v2!  The ATen code generator is
# responsible for parsing native_functions.yaml and then generating
# various generated files (e.g., TypeDefault.cpp) based on the operators
# defined in this file.  This means that the code generator knows how to
# parse function schema, and then translate this into various C++ types
# and boilerplate code.
#
# Some things to know about this file when you modify it:
#
# - This file has STRICT mypy typechecking.  Typecheck it with
#   `mypy --config mypy-strict.ini` in the root source directory
#
# - Most of the heavy lifting lives in external modules:
#   - 'model' has the data model for native_functions.yaml.  The classes
#     in those file represent what you see when you look at
#     a native_functions.yaml
#   - 'api' has conversions for how to translate JIT schema into
#     the various C++ APIs that the codegen interacts with.  There
#     are in fact THREE different C++ APIs: the public C++ API,
#     the dispatcher API, and the legacy disaptcher API.  See each
#     of these respective files for more information

# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
#                         HELPER FUNCTIONS
#
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #


# A custom loader for YAML to let us also keep track of line numbers
# of each entry in the YAML file
class LineLoader(YamlLoader):
    def construct_mapping(self, node, deep=False):  # type: ignore[no-untyped-def]
        mapping = super().construct_mapping(node, deep=deep)  # type: ignore[no-untyped-call]
        # Add 1 so line numbering starts at 1
        mapping["__line__"] = node.start_mark.line + 1
        return mapping


_GLOBAL_PARSE_NATIVE_YAML_CACHE = {}
_GLOBAL_PARSE_TAGS_YAML_CACHE = {}

# Parse native_functions.yaml into a sequence of NativeFunctions and Backend Indices.
ParsedYaml = namedtuple("ParsedYaml", ["native_functions", "backend_indices"])


def parse_native_yaml_struct(
    es: object,
    valid_tags: Set[str],
    ignore_keys: Optional[Set[DispatchKey]] = None,
    path: str = "<stdin>",
    skip_native_fns_gen: bool = False,
) -> ParsedYaml:
    assert isinstance(es, list)
    rs: List[NativeFunction] = []
    bs: Dict[DispatchKey, Dict[OperatorName, BackendMetadata]] = defaultdict(dict)
    for e in es:
        assert isinstance(e.get("__line__"), int), e
        loc = Location(path, e["__line__"])
        funcs = e.get("func")
        with context(lambda: f"in {loc}:\n  {funcs}"):
            func, m = NativeFunction.from_yaml(e, loc, valid_tags, ignore_keys)
            rs.append(func)
            BackendIndex.grow_index(bs, m)
    error_check_native_functions(rs)
    # Default dict is to prevent the codegen from barfing when we have a dispatch key that has no kernels yet.
    indices: Dict[DispatchKey, BackendIndex] = defaultdict(
        lambda: BackendIndex(
            dispatch_key=DispatchKey.Undefined,
            use_out_as_primary=True,
            external=False,
            device_guard=False,
            index={},
        )
    )
    if not skip_native_fns_gen:
        add_generated_native_functions(rs, bs)
    for k, v in bs.items():
        # All structured in-tree operators are implemented in terms of their out operator.
        indices[k] = BackendIndex(
            dispatch_key=k,
            use_out_as_primary=True,
            external=False,
            # Only cuda-like devices in tree require device guards
            device_guard=is_cuda_dispatch_key(k),
            index=v,
        )
    return ParsedYaml(rs, indices)


def parse_tags_yaml_struct(es: object, path: str = "<stdin>") -> Set[str]:
    assert isinstance(es, list)
    rs: Set[str] = set()
    for e in es:
        assert isinstance(e.get("__line__"), int), e
        loc = Location(path, e["__line__"])
        tags = e.get("tag")
        with context(lambda: f"in {loc}:\n  {tags}"):
            e_i = e.copy()
            name = e_i.pop("tag")
            desc = e_i.pop("desc", "")
            # ensure that each tag has a non-empty description
            assert desc != ""
            rs.add(name)
    return rs


@functools.lru_cache(maxsize=None)
def parse_tags_yaml(path: str) -> Set[str]:
    global _GLOBAL_PARSE_TAGS_YAML_CACHE
    if path not in _GLOBAL_PARSE_TAGS_YAML_CACHE:
        with open(path, "r") as f:
            es = yaml.load(f, Loader=LineLoader)
            _GLOBAL_PARSE_TAGS_YAML_CACHE[path] = parse_tags_yaml_struct(es, path=path)

    return _GLOBAL_PARSE_TAGS_YAML_CACHE[path]


def parse_native_yaml(
    path: str,
    tags_yaml_path: str,
    ignore_keys: Optional[Set[DispatchKey]] = None,
    *,
    skip_native_fns_gen: bool = False,
) -> ParsedYaml:
    global _GLOBAL_PARSE_NATIVE_YAML_CACHE
    if path not in _GLOBAL_PARSE_NATIVE_YAML_CACHE:
        valid_tags = parse_tags_yaml(tags_yaml_path)
        with open(path, "r") as f:
            es = yaml.load(f, Loader=LineLoader)
        _GLOBAL_PARSE_NATIVE_YAML_CACHE[path] = parse_native_yaml_struct(
            es,
            valid_tags,
            ignore_keys,
            path=path,
            skip_native_fns_gen=skip_native_fns_gen,
        )

    return _GLOBAL_PARSE_NATIVE_YAML_CACHE[path]


# Some assertions are already performed during parsing, but those are only within a single NativeFunction.
# Assertions here are meant to be performed across NativeFunctions.
def error_check_native_functions(funcs: Sequence[NativeFunction]) -> None:
    func_map: Dict[OperatorName, NativeFunction] = {}
    base_func_map: Dict[BaseOperatorName, List[NativeFunction]] = defaultdict(list)
    for f in funcs:
        func_map[f.func.name] = f
        base_func_map[f.func.name.name].append(f)
    for f in funcs:
        if f.structured_delegate is not None:
            delegate_func = func_map[f.structured_delegate]
            assert delegate_func.structured, (
                f"{f.func.name} is marked as a structured_delegate pointing to "
                f"{f.structured_delegate}, but {f.structured_delegate} is not marked as structured. "
                f"Consider adding 'structured=True' to the delegated operator"
            )
        if "inplace_view" in f.tags:
            base_name = f.func.name.name
            overload_name = f.func.name.overload_name
            assert base_name.inplace, (
                f"{f.func.name} is marked with tag: inplace_view, but it doesn't follow the naming "
                "convention for inplace ops - the codegen expects the base name to have a trailing underscore. "
            )
            out_of_place_base_name = BaseOperatorName(
                base_name.base, False, base_name.dunder_method
            )
            assert len(base_func_map[out_of_place_base_name]) > 0, (
                f"{f.func.name} is marked with tag: inplace_view. The codegen expects there to be a corresponding "
                f"out-of-place view op with the name '{base_name}' and matching schema, but it didn't find one. "
            )


def cpp_string(s: str) -> str:
    """Convert a python string into a c++ string literal"""
    s = s.replace("\\", "\\\\")
    s = s.replace('"', '\\"')
    s = s.replace("\a", "\\a")
    s = s.replace("\b", "\\b")
    s = s.replace("\f", "\\f")
    s = s.replace("\n", "\\n")
    s = s.replace("\v", "\\v")
    s = s.replace("\t", "\\t")
    return f'"{s}"'


# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
#                        C++ CODE GENERATION
#
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #

# Most functions in this section are curried: they consist of a function
# that takes some parameters (e.g., what is to be generated) which itself
# returns a function that actually maps NativeFunction to the code
# to be generated.  This pattern makes it convenient to use map, concatMap
# and similar functional combinators.


def static_dispatch_keys(backends: List[BackendIndex]) -> List[DispatchKey]:
    if len(backends) == 0:
        return []
    else:
        return [backend.dispatch_key for backend in backends] + [
            DispatchKey.CompositeImplicitAutograd,
            DispatchKey.CompositeExplicitAutograd,
            DispatchKey.CompositeExplicitAutogradNonFunctional,
        ]


def get_static_dispatch_backend(
    f: NativeFunction, backend_index: BackendIndex
) -> Optional[DispatchKey]:
    if f.structured_delegate is not None or backend_index.has_kernel(f):
        # TODO: for ops with structured_delegate it should check the dispatch table of
        # the out variant instead. For now, these structured ops all have CPU/CUDA kernels
        # so we always dispatch to the `backend`, but this could be wrong when we
        # migrate math/default_backend ops to use structured delegate.
        return backend_index.dispatch_key
    elif f.has_composite_explicit_autograd_kernel:
        return DispatchKey.CompositeExplicitAutograd
    elif f.has_composite_explicit_autograd_non_functional_kernel:
        return DispatchKey.CompositeExplicitAutogradNonFunctional
    elif f.has_composite_implicit_autograd_kernel:
        return DispatchKey.CompositeImplicitAutograd
    return None


def static_dispatch_ops_header(
    f: NativeFunction, backend_index: List[BackendIndex]
) -> Optional[str]:
    if backend_index is None or f.manual_kernel_registration:
        return None

    output = []
    for index in backend_index:
        dispatch_key = get_static_dispatch_backend(f, index)
        if dispatch_key is not None:
            output.append(
                f"#include <ATen/ops/{f.root_name}_{dispatch_key.lower()}_dispatch.h>"
            )
    return "\n".join(output)


def static_dispatch_extra_headers(backends: List[BackendIndex]) -> List[str]:
    return [
        f"#include <ATen/{dispatch_key}Functions.h>"
        for dispatch_key in static_dispatch_keys(backends)
    ]


# Translates arguments of a native function from DispatcherSignature form to CppSignature form with support for
# supporting usecases even when there is a memory_format argument along with tensor_option arguments.
# This usecase is not covered by tools.codegen.api.translate() yet as its application is limited to static dispatch
def translate_args_dispatcher_to_cpp(
    f: NativeFunction,
) -> str:

    # Adds SpecialArgName.possibly_redundant_memory_format NamedCType for memory_format bindings
    def add_spl_memory_format_binding(input_bindings: List[Binding]) -> List[Binding]:
        output_bindings: List[Binding] = []
        for binding in input_bindings:
            if binding.name == "memory_format":
                spl_mem_format_binding = Binding(
                    nctype=NamedCType(
                        SpecialArgName.possibly_redundant_memory_format,
                        binding.nctype.type,
                    ),
                    name=binding.name,
                    default=binding.default,
                    argument=binding.argument,
                )
                output_bindings.append(spl_mem_format_binding)
            else:
                output_bindings.append(binding)
        return output_bindings

    disp_sig = DispatcherSignature.from_schema(f.func)
    cpp_sig = CppSignatureGroup.from_native_function(
        f, method=False, fallback_binding=False
    ).signature
    disp_bindings = disp_sig.arguments()
    # When last argument of CPP signature has SpecialArgName.possibly_redundant_memory_format NCType,
    # get memory_format bindings of dispatcher signature to have the same NCType as well
    for arg in cpp_sig.arguments():
        if arg.nctype.name == SpecialArgName.possibly_redundant_memory_format:
            disp_bindings = add_spl_memory_format_binding(disp_sig.arguments())
            break
    exprs = translate(disp_bindings, cpp_sig.arguments())
    return ", ".join(a.expr for a in exprs)


def generate_static_dispatch_backend_call(
    f: NativeFunction,
    backend_index: BackendIndex,
    ns: str = "at",
) -> str:
    name = DispatcherSignature.from_schema(f.func).name()
    exprs = translate_args_dispatcher_to_cpp(f)
    return f"return {ns}::{backend_index.dispatch_key.lower()}::{name}({exprs});"


def generate_static_dispatch_fallback_call(
    f: NativeFunction,
    backend_indices: List[BackendIndex],
    ns: str = "at",
) -> str:
    name = DispatcherSignature.from_schema(f.func).name()
    exprs = translate_args_dispatcher_to_cpp(f)
    if f.has_composite_explicit_autograd_kernel:
        return f"return {ns}::{DispatchKey.CompositeExplicitAutograd.lower()}::{name}({exprs});"
    elif f.has_composite_explicit_autograd_non_functional_kernel:
        return f"return {ns}::{DispatchKey.CompositeExplicitAutogradNonFunctional.lower()}::{name}({exprs});"
    elif f.has_composite_implicit_autograd_kernel:
        return f"return {ns}::{DispatchKey.CompositeImplicitAutograd.lower()}::{name}({exprs});"
    else:
        return f"""TORCH_CHECK(false, "Static dispatch does not support {name} for\
{', '.join([str(index.dispatch_key)for index in backend_indices])} ");"""


def static_dispatch(
    f: NativeFunction,
    backend_indices: List[BackendIndex],
    namespace: str = "at",
) -> str:
    if len(backend_indices) == 0 or f.manual_kernel_registration:
        return ""

    keys = [
        b
        for b in backend_indices
        if b.has_kernel(f)
        or (
            f.structured_delegate is not None
            and b.dispatch_key in STRUCTURED_DISPATCH_KEYS
        )
    ]
    if len(keys) == 1:
        return generate_static_dispatch_backend_call(f, keys[0], namespace)
    elif len(keys) == 0:
        return generate_static_dispatch_fallback_call(f, backend_indices, namespace)

    sig = DispatcherSignature.from_schema(f.func)
    native_tensor_args = [
        a.name
        for a in sig.arguments()
        if isinstance(a.argument, SelfArgument)
        or isinstance(a.argument, Argument)
        and a.argument.type.is_tensor_like()
    ]
    tensor_args = ", ".join(native_tensor_args)
    tensor_opts = f.func.arguments.tensor_options

    stmts = []
    subexprs: List[str] = []
    if tensor_opts is not None:
        subexprs.append(
            "DispatchKeySet(c10::computeDispatchKey(dtype, layout, device))"
        )
    if tensor_args != "":
        subexprs.append(f"c10::detail::multi_dispatch_key_set({tensor_args})")
    stmts.append(f"""DispatchKeySet _dk_set = {' | '.join(subexprs)};""")
    stmts.append("DispatchKey _dk = c10::highestPriorityBackendTypeId(_dk_set);")

    dispatch_code = []
    for index in keys:
        dispatch_code.append(f"""case DispatchKey::{index.dispatch_key}:""")
        dispatch_code.append(
            f"""\t{generate_static_dispatch_backend_call(f, index, namespace)};"""
        )

    fallback = generate_static_dispatch_fallback_call(f, backend_indices, namespace)
    connector = "\n\t\t"

    return f"""
    {connector.join(stmts)}
    switch (_dk) {{
        {connector.join(dispatch_code)}
        default:
            {fallback}
    }}
    """


# Generates RegisterSchema.cpp.  Depending on the selector, either
# all schemas are registered, or only some are (in the case of
# selective build)
@dataclass(frozen=True)
class RegisterSchema:
    selector: SelectiveBuilder

    @method_with_native_function
    def __call__(self, f: NativeFunction) -> Optional[str]:
        if not self.selector.is_native_function_selected(f):
            return None
        tags = "{" + ", ".join([f"at::Tag::{tag}" for tag in f.tags]) + "}"
        return f"m.def({cpp_string(str(f.func))}, {tags});\n"


# Generates Operators.h and Operators.cpp.
# These provide macros that, given an operator and overload name, allow users
# to access an "un-overloaded" function version of the operator. This
# is useful for extension writers who want to (1) want to decltype the operator
# and (2) don't want to worry about method-only operators.
@dataclass(frozen=True)
class ComputeOperators:
    target: Union[Literal[Target.DECLARATION], Literal[Target.DEFINITION]]
    static_dispatch_backend_indices: List[BackendIndex]

    @method_with_native_function
    def __call__(self, f: NativeFunction) -> str:
        sig = DispatcherSignature.from_schema(f.func)
        name = f.func.name.unambiguous_name()
        call_method_name = "call"
        redispatch_method_name = "redispatch"

        if self.target is Target.DECLARATION:
            # Note [The ATen Operators API]
            # The ATen Operators API lives in the at::_ops namespace, and contains compile-time
            # metadata about each operator + entry points into the Dispatcher.
            # The C++ function, method, and redispatch API's are all implemented as wrappers
            # into various bits of the structs defined here.
            #
            # Important characteristics about the Operators API:
            # (1) It follows the Dispatcher API.
            #     This is kind of necessary to avoid overhead.
            #     For example: if it followed the C++ API, then all of the faithful C++ factory functions
            #     would need to wrap their arguments into TensorOptions only to unwrap them again.
            # (2) Overload names are disambiguated.
            #     This is helpful for pytorch extenders who would like to decltype() an aten operator,
            #     that has overloads, e.g. decltype(at::_ops::mul_Tensor::call)
            # (3) No argument defaulting is allowed.
            #     This is more of an implementation detail to avoid #include cycles,
            #     since TensorBody.h (which defines the Tensor class) needs to include this file.
            # (4) manual_cpp_bindings and faithful names are not included in the API.
            #     This applies to stuff like __dispatch__is_complex(), and add_outf().
            #     These aren't "real aten ops", they're just additional functions provided by the C++ API.
            #     They're implemented as wrappers in Functions.h that call into the actual operators
            #     defined here, i.e. at::_ops::is_complex::call() and at::_ops::add_out::call().
            #     This means that ATEN_OP(is_complex) will not fastpath, and will go through the dispatcher.
            return f"""
struct TORCH_API {name} {{
  using schema = {sig.type()};
  using ptr_schema = schema*;
  // See Note [static constexpr char* members for windows NVCC]
  STATIC_CONSTEXPR_STR_INL_EXCEPT_WIN_CUDA(name, "aten::{f.func.name.name}")
  STATIC_CONSTEXPR_STR_INL_EXCEPT_WIN_CUDA(overload_name, "{f.func.name.overload_name}")
  STATIC_CONSTEXPR_STR_INL_EXCEPT_WIN_CUDA(schema_str, {cpp_string(str(f.func))})
  static {sig.defn(name=call_method_name, is_redispatching_fn=False)};
  static {sig.defn(name=redispatch_method_name, is_redispatching_fn=True)};
}};"""

        elif self.target is Target.DEFINITION:
            defns = f"""
STATIC_CONST_STR_OUT_OF_LINE_FOR_WIN_CUDA({name}, name, "aten::{f.func.name.name}")
STATIC_CONST_STR_OUT_OF_LINE_FOR_WIN_CUDA({name}, overload_name, "{f.func.name.overload_name}")
STATIC_CONST_STR_OUT_OF_LINE_FOR_WIN_CUDA({name}, schema_str, {cpp_string(str(f.func))})

// aten::{f.func}
static C10_NOINLINE c10::TypedOperatorHandle<{name}::schema> create_{name}_typed_handle() {{
  return c10::Dispatcher::singleton()
      .findSchemaOrThrow({name}::name, {name}::overload_name)
      .typed<{name}::schema>();
}}
"""
            for is_redispatching_fn in [False, True]:
                if is_redispatching_fn:
                    dispatcher_exprs_str = ", ".join(
                        ["dispatchKeySet"] + [a.name for a in sig.arguments()]
                    )
                    dispatcher_call = "redispatch"
                    method_name = f"{name}::{redispatch_method_name}"
                else:
                    method_name = f"{name}::{call_method_name}"
                    dispatcher_exprs_str = ", ".join([a.name for a in sig.arguments()])
                    dispatcher_call = "call"

                fn_body = f"""
    static auto op = create_{name}_typed_handle();
    return op.{dispatcher_call}({dispatcher_exprs_str});"""

                if (
                    not is_redispatching_fn
                    and len(self.static_dispatch_backend_indices) > 0
                ):
                    # call() should go through static dispatch
                    fn_body = static_dispatch(
                        f, backend_indices=self.static_dispatch_backend_indices
                    )
                defns += f"""
// aten::{f.func}
{sig.defn(name=method_name, is_redispatching_fn=is_redispatching_fn)} {{
    {fn_body}
}}
"""
            return defns
        else:
            assert_never(self.target)


# Generates Functions.h, which provides the functional public C++ API,
# and the scaffolding to call into the dispatcher from these functions.
@dataclass(frozen=True)
class ComputeFunction:
    @method_with_native_function
    def __call__(self, f: NativeFunction) -> Optional[str]:
        if Variant.function not in f.variants:
            return None

        sig_group = CppSignatureGroup.from_native_function(
            f, method=False, fallback_binding=f.manual_cpp_binding
        )

        def generate_defn(faithful: bool) -> str:
            if faithful:
                sig = sig_group.faithful_signature
                assert sig is not None
            else:
                sig = sig_group.signature

            # See Note [The ATen Operators API]
            target_sig = DispatcherSignature.from_schema(f.func)
            exprs = translate(sig.arguments(), target_sig.arguments())
            exprs_str = ", ".join([e.expr for e in exprs])

            return f"""
// aten::{f.func}
inline {sig.decl()} {{
    return at::_ops::{f.func.name.unambiguous_name()}::call({exprs_str});
}}
"""

        result = generate_defn(False)
        if sig_group.faithful_signature is not None:
            result += generate_defn(True)

        return result


# Generates TensorBody.h. This file provides the object-oriented (method-based)
# public C++ API, and the scaffolding to call into the dispatcher from these functions.
@dataclass(frozen=True)
class ComputeTensorMethod:
    target: Union[Literal[Target.DECLARATION], Literal[Target.DEFINITION]]
    static_dispatch_backend_indices: List[BackendIndex]

    @method_with_native_function
    def __call__(self, f: NativeFunction) -> Optional[str]:
        if Variant.method not in f.variants:
            return None

        assert not f.func.is_out_fn()
        assert f.func.arguments.self_arg is not None

        sig_group = CppSignatureGroup.from_native_function(
            f, method=True, fallback_binding=f.manual_cpp_binding
        )

        if self.target is Target.DECLARATION:
            result = f"{sig_group.signature.decl()} const;\n"
            if sig_group.faithful_signature is not None:
                result += f"{sig_group.faithful_signature.decl()} const;\n"
            return result

        if self.target is not Target.DEFINITION:
            assert_never(self.target)

        def generate_defn(faithful: bool) -> str:
            if faithful:
                sig = sig_group.faithful_signature
                assert sig is not None
            else:
                sig = sig_group.signature

            target_sig = DispatcherSignature.from_schema(f.func)
            exprs = translate(sig.arguments(), target_sig.arguments(), method=True)
            exprs_str = ", ".join([e.expr for e in exprs])

            return f"""
// aten::{f.func}
inline {sig.defn(prefix="Tensor::")} const {{
    return at::_ops::{f.func.name.unambiguous_name()}::call({exprs_str});
}}
"""

        result = generate_defn(faithful=False)
        if sig_group.faithful_signature is not None:
            result += generate_defn(faithful=True)

        return result


# Generates RedispatchFunctions.h.
# This is similar to the C++ API defined in Functions.h, but provides access
# to the dispatcher's redispatch API.
@dataclass(frozen=True)
class ComputeRedispatchFunction:
    @method_with_native_function
    def __call__(self, f: NativeFunction) -> Optional[str]:
        # We unconditionally generate function variants of the redispatch API.
        # This is mainly because we can namespace functions separately, but not methods,
        sig_group = CppSignatureGroup.from_native_function(
            f, method=False, fallback_binding=f.manual_cpp_binding
        )

        def generate_defn(faithful: bool) -> str:
            if faithful:
                sig = sig_group.faithful_signature
                assert sig is not None
            else:
                sig = sig_group.signature

            target_sig = DispatcherSignature.from_schema(f.func)
            exprs = translate(sig.arguments(), target_sig.arguments())
            exprs_str = ", ".join(["dispatchKeySet"] + [a.expr for a in exprs])

            return f"""
// aten::{f.func}
inline {sig.decl(is_redispatching_fn=True)} {{
    return at::_ops::{f.func.name.unambiguous_name()}::redispatch({exprs_str});
}}
"""

        result = generate_defn(False)
        if sig_group.faithful_signature is not None:
            result += generate_defn(True)

        return result


# Generates ATenOpList.cpp, a runtime accessible list of all aten
# operators.
# TODO: This was historically used to help some JIT interop code
# figure out whether or not to treat aten namespace'd operators
# one way or another, we should reevaluate if this is actually needed.
@with_native_function
def compute_aten_op(f: NativeFunction) -> str:
    return f'{{"aten::{f.func.name.name}", "{f.func.name.overload_name}"}},'


# Generates MetaFunctions.h
def compute_meta_function_declaration(g: NativeFunctionsGroup) -> Optional[str]:
    if not g.structured:
        return None
    with native_function_manager(g.out):
        name = meta.name(g)
        args = structured.meta_arguments(g)
        args_str = ", ".join(a.decl() for a in args)
        parent_class = g.out.structured_inherits
        if parent_class is None:
            parent_class = "at::impl::MetaBase"
        meta_return = "void"
        precomputed = g.out.precomputed if g.structured else None

        if precomputed:
            # Generate the template declaration with one bool parameter for each
            # precomputed element. Each parameter is true if the corresponding (in
            # terms of position) precomputed element has been set.
            precomputed_values = [*precomputed.replace.values(), precomputed.add]
            precomputed_elements = [
                elem for replace_list in precomputed_values for elem in replace_list
            ]
            precomputed_template_parameters = [
                elem.name.upper() for elem in precomputed_elements
            ]
            precomputed_template_params_str = ", ".join(
                f"bool {param} = false" for param in precomputed_template_parameters
            )
            precompute_template_decl = f"template <{precomputed_template_params_str}>"

            # Generate a string containing declarations of all precomputed elements.
            precomputed_elements_with_cpp_types = [
                structured.argument_type(elem, binds=elem.name)
                for elem in precomputed_elements
            ]

            precomputed_elements_decl = ";\n".join(
                f"{elem.cpp_type(strip_ref=True)} {elem.name}"
                for elem in precomputed_elements_with_cpp_types
            )

            # Generate "setter" methods for each precomputed element. Each method will return
            # a new instance of precompute_out with the template parameter that corresponds to
            # the member set by the method to true (to indicate that it has been set).
            setter_methods = []
            for i, elem in enumerate(precomputed_elements):
                # Generate the signature. The return type will be the same
                # as the type of `this` but with the template parameter
                # corresponding to the element set by this method set to true.
                # The assert generated below will ensure that this template
                # parameter is false on the type of `this`.
                return_ty_templates = ", ".join(
                    precomputed_template_parameters[:i]
                    + ["true"]
                    + precomputed_template_parameters[i + 1 :]
                )
                return_ty = f"precompute_out<{return_ty_templates}>"
                elem_cpp_ty = precomputed_elements_with_cpp_types[i].cpp_type(
                    strip_ref=True
                )
                signature = f"{return_ty} set_{elem.name}({elem_cpp_ty} value)"

                # Generate an assert which checks that the
                # template parameter corresponding to the precomputed
                # element that is set by this method is false on the
                # class corresponding to the object that `this` points to.
                # This ensures that each element can be set only once.
                assert_msg = f'"{precomputed_elements[i].name} already set"'
                assert_stmt = f"static_assert({precomputed_template_parameters[i]} == false, {assert_msg});"

                # Generate the new object construction block. All state
                # except the element that this method sets is copied from the
                # object that `this` points to. The value for the element that
                # the method sets is taken from a method parameter.
                construction_stmts = []
                construction_stmts.append(f"{return_ty} ret;")

                for j, elem in enumerate(precomputed_elements):
                    if i == j:
                        construction_stmts.append(f"ret.{elem.name} = value;")
                    else:
                        construction_stmts.append(
                            f"ret.{elem.name} = this->{elem.name};"
                        )

                construction_stmts.append("return ret;")
                construction_block = "\n".join(construction_stmts)

                setter_methods.append(
                    f"""
                    {signature} {{
                        {assert_stmt}
                        {construction_block}
                    }}
                """
                )
            setter_methods_decl = "\n".join(setter_methods)

            # Meta should return an instance of the struct containing the precomputed elements.
            meta_return_template_params = ", ".join(
                ["true"] * len(precomputed_template_parameters)
            )
            # This typedef (actually a using statement) is needed so that TORCH_META_FUNC can reuse the return
            # type (which has a variable number of template parameters).
            meta_return_typedef = f"using meta_return_ty = precompute_out <{meta_return_template_params}>;"
            meta_return = "meta_return_ty"
            precomputed_decl = f"""
                {precompute_template_decl}
                struct TORCH_API precompute_out {{
                    {setter_methods_decl}
                    {precomputed_elements_decl};
            }};"""
        else:
            meta_return_typedef = ""
            precomputed_decl = ""

        return f"""\
struct TORCH_API structured_{name} : public {parent_class} {{
    {precomputed_decl}
    {meta_return_typedef}
    {meta_return} meta({args_str});
}};
"""


def needs_backend_select(f: NativeFunction, selector: SelectiveBuilder) -> bool:
    name = str(f.func.name.name)
    if name.endswith("_like") or name.startswith("new_"):
        return False
    if f.func.arguments.tensor_options is None:
        return False
    return selector.is_native_function_selected(f)


# Generates RegisterBackendSelect.cpp, a series of kernels which provide
# specialized computation of dispatch key for operator signatures which cannot
# be easily done automatically using templating.
@dataclass(frozen=True)
class ComputeBackendSelect:
    target: Union[Literal[Target.DEFINITION], Literal[Target.REGISTRATION]]

    # Selector object to determine which operators to generate
    # registration code for.
    selector: SelectiveBuilder

    @method_with_native_function
    def __call__(self, f: NativeFunction) -> Optional[str]:
        if not needs_backend_select(f, self.selector):
            return None

        name = native.name(f.func)
        native_sig = NativeSignature(f.func)

        native_tensor_args = [
            a
            for a in native_sig.arguments()
            if isinstance(a.argument, Argument) and a.argument.type.is_tensor_like()
        ]

        dispatcher_sig = DispatcherSignature.from_schema(f.func)

        sig: Union[NativeSignature, DispatcherSignature]
        sig = dispatcher_sig
        dispatcher_exprs = dispatcher_sig.exprs()
        dispatch_key = "c10::computeDispatchKey(dtype, layout, device)"

        if self.target is Target.DEFINITION:
            # I don't think there's actually a good reason to generate
            # these two cases differently
            # The first case could probably be improved though- it calls computeDispatchKeySet(),
            # which looks at TLS dispatch keys- there should not be any by the time we reach backend select.
            if native_tensor_args:
                tensor_args = ", ".join(a.name for a in native_tensor_args)
                compute_dk = f"""\
DispatchKeySet _dk_set = c10::DispatchKeySet({dispatch_key}) | c10::detail::multi_dispatch_key_set({tensor_args});
DispatchKeySet _dk_mask = c10::DispatchKeySet(DispatchKeySet::FULL_AFTER, DispatchKey::BackendSelect);
DispatchKeySet _dk = c10::impl::computeDispatchKeySet(_dk_set, _dk_mask);"""
            else:
                compute_dk = (
                    f"DispatchKeySet _dk = c10::DispatchKeySet({dispatch_key});"
                )
            return f"""\
// aten::{f.func}
C10_ALWAYS_INLINE
{sig.defn(name)} {{
  {compute_dk}
  return at::_ops::{f.func.name.unambiguous_name()}::redispatch(
      _dk, {', '.join(a.expr for a in dispatcher_exprs)});
}}
"""
        elif self.target is Target.REGISTRATION:
            return f"""m.impl("aten::{f.func.name}", TORCH_FN({name}));"""
        else:
            assert_never(self.target)


# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
#                       YAML CODE GENERATION
#
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #


def format_yaml(data: object) -> str:
    # Ignore alias in Dumper
    YamlDumper.ignore_aliases = lambda self, data: True  # type: ignore[assignment]

    # Support serializing OrderedDict
    def dict_representer(dumper: Any, data: Any) -> Any:
        return dumper.represent_dict(data.items())

    YamlDumper.add_representer(OrderedDict, dict_representer)  # type: ignore[no-untyped-call]
    # Some yaml parsers (e.g. Haskell's) don't understand line breaks.
    # width=1e9 turns off optional line breaks and improves
    # the portability of the outputted yaml.
    return yaml.dump(data, default_flow_style=False, Dumper=YamlDumper, width=1e9)  # type: ignore[no-any-return, call-overload]


# For some reason, some defaults we write to YAML are written as native
# YAML objects, rather than doing them uniformly as strings.  This
# function detects those cases and converts them into native Python
# objects.
def pythonify_default(s: str) -> object:
    if s == "true":
        return True
    elif s == "false":
        return False

    try:
        return int(s)
    except ValueError:
        try:
            return float(s)
        except ValueError:
            return s


# What is a dynamic type?  Over time, the semantic meaning of
# dynamic type has degraded to meaninglessness (in the old days,
# it captured dtype-ness of types, but that has gone away with
# the removal of TH).  These days, it's mostly the same thing as
# the C++ API argument type, except that Tensor and Tensor?
# arguments simply present as Tensor.
#
# TODO: Get rid of dynamic_type, after getting tools/autograd
# to use the new codegen framework
def dynamic_type(t: Type) -> str:
    if isinstance(t, OptionalType):
        return dynamic_type(t.elem)
    # Note we don't use t.is_tensor_like() here because it would
    # also include Tensor[]
    if str(t) == "Tensor":
        return "at::Tensor"
    return cpp.argumenttype_type(t, mutable=False, binds="__placeholder__").cpp_type()


def compute_method_of_yaml(variants: Set[Variant]) -> List[str]:
    # This is written out explicitly to ensure that Tensor and
    # namespace are put into the list in the right order
    method_of = ["Type"]
    if Variant.method in variants:
        method_of.append("Tensor")
    if Variant.function in variants:
        method_of.append("namespace")
    return method_of


def compute_returns_yaml(
    f: NativeFunction,
) -> Tuple[List[Dict[str, str]], Dict[str, str]]:
    # Note [name and field_name]
    # ~~~~~~~~~~~~~~~~~~~~~~~~~~
    # To understand name_to_field_name, we must first talk about this
    # schema:
    #
    #   lstsq.X(Tensor self, Tensor A, *, Tensor(a!) X, Tensor(b!) qr) -> (Tensor(a!) solution, Tensor(b!) QR)
    #
    # There is something very odd about this schema: it is an out
    # variant of the function (that is to say, it will convert into
    # at::lstsq_out() in the C++ API), but the names of the output
    # return arguments don't match the keyword argument names of
    # the inputs.  It TURNS OUT that in this situation, the historical
    # Declarations.yaml we want to output is this (abbreviated to
    # only show relevant fields):
    #
    #   arguments:
    #     ...
    #   - field_name: solution
    #     name: X
    #   - field_name: QR
    #     name: qr
    #     ...
    #
    #   returns:
    #   - field_name: solution
    #     name: X
    #   - field_name: QR
    #     name: qr
    #
    # The name of the return fields is stored in 'field_name', and the
    # name of the arguments is stored in 'name'.  So when we process
    # arguments, we need a way to get at the corresponding return.  At
    # the moment, this is most conveniently done by constructing a
    # mapping from name (the argument concept) to field_name (the
    # return concept) while processing return arguments, since we don't
    # directly maintain this correspondence in the modeling of function
    # schema itself.
    #
    # See also https://github.com/pytorch/pytorch/issues/43114
    name_to_field_name: Dict[str, str] = {}

    # Compute the returns field of the YAML entry
    names = cpp.return_names(f)
    returns = []
    for i, (r, name) in enumerate(zip(f.func.returns, names)):
        ret = {
            "dynamic_type": dynamic_type(r.type),
            "name": name,
            "type": cpp.return_type(r).cpp_type(),
        }

        if r.name:
            # See Note [name and field_name]
            ret["field_name"] = r.name
            if f.func.is_out_fn():
                name_to_field_name[f.func.arguments.out[i].name] = r.name

        returns.append(ret)

    return returns, name_to_field_name


# arguments in yaml roughly corresponds to the public C++ API
def compute_cpp_argument_yaml(
    cpp_a: Binding,
    *,
    schema_order: bool,
    kwarg_only_set: Set[str],
    out_arg_set: Set[str],
    name_to_field_name: Dict[str, str],
) -> object:
    if isinstance(cpp_a.argument, TensorOptionsArguments):
        arg: Dict[str, object] = {
            "annotation": None,
            "dynamic_type": "at::TensorOptions",
            "is_nullable": False,
            "name": cpp_a.name,
            "type": cpp_a.type,
            "kwarg_only": True,
        }
        if cpp_a.default is not None:
            arg["default"] = cpp_a.default
        return arg
    elif isinstance(cpp_a.argument, SelfArgument):
        raise AssertionError()
    elif isinstance(cpp_a.argument, Argument):
        return compute_argument_yaml(
            cpp_a.argument,
            schema_order=schema_order,
            kwarg_only_set=kwarg_only_set,
            out_arg_set=out_arg_set,
            name_to_field_name=name_to_field_name,
        )


def compute_argument_yaml(
    a: Argument,
    *,
    schema_order: bool,
    kwarg_only_set: Set[str],
    out_arg_set: Set[str],
    name_to_field_name: Dict[str, str],
) -> object:
    arg: Dict[str, object] = {
        "annotation": str(a.annotation) if a.annotation else None,
        "dynamic_type": dynamic_type(a.type),
        "is_nullable": a.type.is_nullable(),
        "name": a.name,
        "type": cpp.argument_type(a, binds="__placeholder__").cpp_type(),
    }
    if a.default is not None:
        arg["default"] = pythonify_default(cpp.default_expr(a.default, a.type))
    if a.name in kwarg_only_set:
        arg["kwarg_only"] = True
    if a.name in out_arg_set:
        arg["output"] = True
        arg["allocate"] = True
        # See Note [name and field_name]
        if a.name in name_to_field_name:
            arg["field_name"] = name_to_field_name[a.name]
    # Historically, booleans don't get their size recorded, because it
    # is already built into the cpp type (e.g., std::array<bool, 4>)
    l = a.type.is_list_like()
    if l is not None and l.size is not None and str(l.elem) != "bool":
        arg["size"] = l.size
    return arg


@with_native_function
def compute_declaration_yaml(f: NativeFunction) -> object:
    returns, name_to_field_name = compute_returns_yaml(f)

    # These sets are used to conveniently test if an argument is a
    # kwarg-only or out argument
    kwarg_only_set = set(a.name for a in f.func.arguments.flat_kwarg_only)
    out_arg_set = set(a.name for a in f.func.arguments.out)

    sig_group = CppSignatureGroup.from_native_function(
        f, method=False, fallback_binding=False
    )
    cpp_args = sig_group.signature.arguments()
    arguments = [
        compute_cpp_argument_yaml(
            cpp_a,
            schema_order=False,
            kwarg_only_set=kwarg_only_set,
            out_arg_set=out_arg_set,
            name_to_field_name=name_to_field_name,
        )
        for cpp_a in cpp_args
    ]

    schema_order_jit_arguments = list(f.func.schema_order_arguments())

    schema_order_arguments = [
        compute_argument_yaml(
            a,
            schema_order=True,
            kwarg_only_set=kwarg_only_set,
            out_arg_set=out_arg_set,
            name_to_field_name=name_to_field_name,
        )
        for a in schema_order_jit_arguments
    ]

    cpp_schema_order_types = [
        # NB: method here doesn't matter
        r.type
        for a in schema_order_jit_arguments
        for r in cpp.argument(
            a,
            method=False,
            cpp_no_default_args=set(),
            faithful=False,
            has_tensor_options=False,
        )
    ]

    cpp_returns = cpp.returns_type(f.func.returns).cpp_type()
    schema_order_cpp_signature = f"{cpp_returns} ({', '.join(cpp_schema_order_types)})"

    is_factory_method = (
        any(isinstance(a.argument, TensorOptionsArguments) for a in cpp_args)
        and Variant.method not in f.variants
    )

    return OrderedDict(
        [
            ("name", cpp.name(f.func)),
            ("operator_name", str(f.func.name.name)),
            ("overload_name", str(f.func.name.overload_name)),
            ("manual_kernel_registration", f.manual_kernel_registration),
            (
                "category_override",
                f.category_override if f.category_override is not None else "",
            ),
            ("schema_string", f"aten::{f.func}"),
            ("arguments", arguments),
            ("schema_order_cpp_signature", schema_order_cpp_signature),
            ("schema_order_arguments", schema_order_arguments),
            ("method_of", compute_method_of_yaml(f.variants)),
            ("mode", "native"),
            ("python_module", "" if f.python_module is None else f.python_module),
            ("returns", returns),
            ("inplace", f.func.name.name.inplace),
            ("is_factory_method", is_factory_method),
            ("abstract", f.is_abstract),
            ("device_guard", f.device_guard),
            ("with_gil", False),
            ("deprecated", False),
            ("has_math_kernel", f.has_composite_implicit_autograd_kernel),
        ]
    )


# See Note [Auto generated composite kernels]
def has_autogenerated_composite_kernel(f: NativeFunction) -> bool:
    return (f.structured or f.structured_delegate is not None) and (
        f.func.kind() == SchemaKind.functional or f.func.kind() == SchemaKind.inplace
    )


@with_native_function_and_indices
def compute_registration_declarations(
    f: NativeFunction, backend_indices: Dict[DispatchKey, BackendIndex]
) -> str:
    name = dispatcher.name(f.func)
    returns_type = dispatcher.returns_type(
        f.func.returns
    ).cpp_type_registration_declarations()
    args = dispatcher.arguments(f.func)
    args_str = ", ".join(a.no_default().decl_registration_declarations() for a in args)
    comment_data: Dict[str, str] = {
        "schema": f"aten::{f.func}",
        # TODO: What exactly is the semantics of the 'dispatch' field?
        "dispatch": str(
            {k for k, v in backend_indices.items() if v.has_kernel(f)}
            != {DispatchKey.CompositeImplicitAutograd}
        ),
        "default": str(f.has_composite_kernel or has_autogenerated_composite_kernel(f)),
    }
    return f"""{returns_type} {name}({args_str}); // {json.dumps(comment_data)}
"""


# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #
#
#                           RUN IT ALL
#
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ #


def get_custom_build_selector(
    provided_op_registration_allowlist: Optional[List[str]],
    op_selection_yaml_path: Optional[str],
) -> SelectiveBuilder:
    assert not (
        provided_op_registration_allowlist is not None
        and op_selection_yaml_path is not None
    ), (
        "Both provided_op_registration_allowlist and "
        + "op_selection_yaml_path can NOT be provided at the "
        + "same time."
    )

    op_registration_allowlist: Optional[Set[str]] = None
    if provided_op_registration_allowlist is not None:
        op_registration_allowlist = set(provided_op_registration_allowlist)

    if op_registration_allowlist is not None:
        selector = SelectiveBuilder.from_legacy_op_registration_allow_list(
            op_registration_allowlist,
            True,
            False,
        )
    elif op_selection_yaml_path is not None:
        selector = SelectiveBuilder.from_yaml_path(op_selection_yaml_path)
    else:
        selector = SelectiveBuilder.get_nop_selector()

    return selector


def get_grouped_by_view_native_functions(
    native_functions: Sequence[NativeFunction],
) -> Sequence[Union[NativeFunction, NativeFunctionsViewGroup]]:
    def maybe_create_view_group(
        d: Dict[Union[ViewSchemaKind, SchemaKind], NativeFunction]
    ) -> List[Union[NativeFunction, NativeFunctionsViewGroup]]:
        funcs: List[Union[NativeFunction, NativeFunctionsViewGroup]] = []
        if ViewSchemaKind.aliasing in d:
            view = d.pop(ViewSchemaKind.aliasing)
            view_inplace = d.pop(ViewSchemaKind.aliasing_inplace, None)
            view_copy = d.pop(SchemaKind.functional, None)

            funcs.append(
                NativeFunctionsViewGroup(
                    view=view,
                    view_copy=view_copy,
                    view_inplace=view_inplace,
                )
            )
        # Take the remaining functions that weren't part of the view group
        # and emit them separately
        for func in d.values():
            funcs.append(func)
        return funcs

    grouped_by_views: Dict[
        FunctionSchema, Dict[Union[SchemaKind, ViewSchemaKind], NativeFunction]
    ] = defaultdict(dict)
    for f in native_functions:
        schema = f.func.view_signature()
        view_kind: ViewSchemaKind = f.view_schema_kind
        # We need to group up ops relevant to the same "view", consisting of:
        # view op (ViewSchemaKind.aliasing)
        # view_inplace op (ViewSchemaKind.aliasing_inplace)
        # view_copy op (SchemaKind.functional)
        if view_kind == ViewSchemaKind.non_aliasing:
            kind = f.func.kind()
            assert kind not in grouped_by_views[schema]
            grouped_by_views[schema][kind] = f
        else:
            assert view_kind not in grouped_by_views[schema]
            grouped_by_views[schema][view_kind] = f

    return list(concatMap(maybe_create_view_group, grouped_by_views.values()))


def get_grouped_native_functions(
    native_functions: Sequence[NativeFunction],
) -> Sequence[Union[NativeFunction, NativeFunctionsGroup]]:
    def flatten_pre_group(
        d: Dict[SchemaKind, NativeFunction]
    ) -> Sequence[Union[NativeFunction, NativeFunctionsGroup]]:
        r = NativeFunctionsGroup.from_dict(d)
        if r is None:
            # Invariant: any NativeFunctions that are code-generated
            # should have been grouped into NativeFunctionsGroup objects
            assert not any("generated" in f.tags for f in d.values())
            return list(d.values())
        else:
            return [r]

    # TODO: how come ValuesView isn't a Sequence lol
    pre_grouped_native_functions = pre_group_native_functions(native_functions)
    return list(
        concatMap(flatten_pre_group, list(pre_grouped_native_functions.values()))
    )


# Return native function declarations grouped by their namespaces.
def get_native_function_declarations(
    *,
    grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
    backend_indices: Dict[DispatchKey, BackendIndex],
) -> List[str]:
    declarations: List[str] = []
    ns_grouped_kernels: Dict[str, List[str]] = defaultdict(list)
    newline = "\n"
    for f in grouped_native_functions:
        native_function_namespaces = set()
        for backend_idx in backend_indices.values():
            backend_metadata = backend_idx.get_kernel(f)
            namespace = (
                backend_metadata.cpp_namespace
                if backend_metadata
                else DEFAULT_KERNEL_NAMESPACE
            )
            native_function_namespaces.add(namespace)
            assert (
                len(native_function_namespaces) == 1
            ), "Codegen only supports one namespace per operator."
            ns_grouped_kernels[namespace].extend(
                dest.compute_native_function_declaration(f, backend_idx)
            )

    for namespace, kernels in ns_grouped_kernels.items():
        ns_helper = NamespaceHelper(
            namespace_str=namespace,
            entity_name="",
            max_level=3,
        )
        # Convert to a set first to remove duplicate kernel names. Backends are
        # allowed to repeat kernel names; only generate the declaration once!
        ordered_kernels = list(OrderedDict.fromkeys(kernels))
        declarations.extend(
            f"""
{ns_helper.prologue}
{newline.join(ordered_kernels)}
{ns_helper.epilogue}
        """.split(
                newline
            )
        )
    return declarations


def gen_aggregated_headers(
    *,
    native_functions: Sequence[NativeFunction],
    grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
    structured_native_functions: Sequence[NativeFunctionsGroup],
    static_dispatch_idx: List[BackendIndex],
    selector: SelectiveBuilder,
    backend_indices: Dict[DispatchKey, BackendIndex],
    cpu_fm: FileManager,
    cuda_fm: FileManager,
    functions_keys: Set[DispatchKey],
    dispatch_keys: Sequence[DispatchKey],
    rocm: bool,
) -> None:
    # Buck doesn't support dynamic output files, so we aggregate all operator
    # headers into a single file
    cpu_fm.write(
        "NativeMetaFunctions.h",
        lambda: {
            "NativeMetaFunctions_includes": [],
            "NativeMetaFunctions_declarations": list(
                mapMaybe(compute_meta_function_declaration, structured_native_functions)
            ),
        },
    )
    method_native_functions = [
        fn for fn in native_functions if Variant.method in fn.variants
    ]
    non_method_native_functions = [
        fn for fn in native_functions if fn not in method_native_functions
    ]
    cpu_fm.write(
        "MethodOperators.h",
        lambda: {
            "MethodOperators_includes": [],
            "MethodOperators_declarations": list(
                mapMaybe(
                    ComputeOperators(
                        Target.DECLARATION,
                        static_dispatch_backend_indices=static_dispatch_idx,
                    ),
                    method_native_functions,
                )
            ),
        },
    )
    cpu_fm.write(
        "Operators.h",
        lambda: {
            "Operators_includes": ["#include <ATen/MethodOperators.h>"],
            "Operators_declarations": list(
                mapMaybe(
                    ComputeOperators(
                        Target.DECLARATION,
                        static_dispatch_backend_indices=static_dispatch_idx,
                    ),
                    non_method_native_functions,
                )
            ),
        },
    )
    cpu_fm.write(
        "Functions.h",
        lambda: {
            "static_dispatch_extra_headers": static_dispatch_extra_headers(
                static_dispatch_idx
            ),
            "Functions_includes": ["#include <ATen/Operators.h>"],
            "Functions_declarations": list(
                mapMaybe(
                    ComputeFunction(),
                    native_functions,
                )
            ),
        },
    )
    declarations = get_native_function_declarations(
        grouped_native_functions=grouped_native_functions,
        backend_indices=backend_indices,
    )
    cpu_fm.write(
        "NativeFunctions.h",
        lambda: {
            "NativeFunctions_includes": ["#include <ATen/NativeMetaFunctions.h>"],
            "NativeFunctions_declarations": declarations,
        },
    )

    for dispatch_key in dispatch_keys:
        fm = cuda_fm if is_cuda_dispatch_key(dispatch_key) else cpu_fm
        if dispatch_key in functions_keys:
            inl_headers = f"#include <ATen/{dispatch_key}Functions_inl.h>"

            fm.write_with_template(
                f"{dispatch_key}Functions.h",
                "DispatchKeyFunctions.h",
                lambda: {
                    "dispatch_key": str(dispatch_key),
                    "inline_headers": inl_headers,
                },
            )
            fm.write_with_template(
                f"{dispatch_key}Functions_inl.h",
                "DispatchKeyFunctions_inl.h",
                lambda: {
                    "DispatchKeyFunctions_inl_includes": [],
                    "dispatch_namespace": dispatch_key.lower(),
                    "dispatch_namespaced_declarations": list(
                        concatMap(
                            dest.RegisterDispatchKey(
                                backend_indices[dispatch_key],
                                Target.NAMESPACED_DECLARATION,
                                selector,
                                rocm=rocm,
                                class_method_name=None,
                                skip_dispatcher_op_registration=False,
                            ),
                            grouped_native_functions,
                        )
                    ),
                },
            )

        del fm


def gen_per_operator_headers(
    *,
    native_functions: Sequence[NativeFunction],
    grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
    static_dispatch_idx: List[BackendIndex],
    selector: SelectiveBuilder,
    backend_indices: Dict[DispatchKey, BackendIndex],
    cpu_fm: FileManager,
    cuda_fm: FileManager,
    ops_fm: FileManager,
    functions_keys: Set[DispatchKey],
    dispatch_keys: Sequence[DispatchKey],
    rocm: bool,
) -> None:
    # For CMake builds, split operator declarations into separate headers in
    # the ATen/ops folder to split up header dependencies
    functions_by_root_name: Dict[str, List[NativeFunction]] = defaultdict(lambda: [])
    for fn in native_functions:
        functions_by_root_name[fn.root_name].append(fn)

    grouped_functions_by_root_name: Dict[
        str, List[Union[NativeFunction, NativeFunctionsGroup]]
    ] = defaultdict(lambda: [])
    for group in grouped_native_functions:
        name = group.root_name
        grouped_functions_by_root_name[name].append(group)

    for name, functions in functions_by_root_name.items():
        ops_fm.write_with_template(
            f"{name}_ops.h",
            "Operator.h",
            lambda: {
                "declarations": list(
                    mapMaybe(
                        ComputeOperators(
                            Target.DECLARATION,
                            static_dispatch_backend_indices=static_dispatch_idx,
                        ),
                        functions,
                    )
                ),
            },
        )

        ops_fm.write_with_template(
            f"{name}.h",
            "Function.h",
            lambda: {
                "static_dispatch_ops_headers": list(
                    mapMaybe(
                        lambda fn: static_dispatch_ops_header(
                            fn, backend_index=static_dispatch_idx
                        ),
                        functions,
                    )
                ),
                "operator_includes": f"#include <ATen/ops/{name}_ops.h>",
                "function_definitions": list(
                    mapMaybe(
                        ComputeFunction(),
                        functions,
                    )
                ),
            },
        )

        grouped_functions = grouped_functions_by_root_name.get(name, [])
        structured_functions = [
            fn
            for fn in grouped_functions
            if isinstance(fn, NativeFunctionsGroup) and fn.structured
        ]
        is_structured = len(structured_functions) > 0

        if is_structured:
            ops_fm.write_with_template(
                f"{name}_meta.h",
                "NativeMetaFunction.h",
                lambda: {
                    "meta_function_declarations": list(
                        mapMaybe(
                            compute_meta_function_declaration, structured_functions
                        )
                    ),
                },
            )
        declarations = get_native_function_declarations(
            grouped_native_functions=grouped_functions, backend_indices=backend_indices
        )
        ops_fm.write_with_template(
            f"{name}_native.h",
            "NativeFunction.h",
            lambda: {
                "extra_includes": (
                    f"#include <ATen/ops/{name}_meta.h>" if is_structured else []
                ),
                "native_function_declarations": declarations,
            },
        )

    for category, suffix in [
        ("Functions", ""),
        ("Operators", "_ops"),
        ("NativeMetaFunctions", "_meta"),
        ("NativeFunctions", "_native"),
    ]:
        cpu_fm.write(
            f"{category}.h",
            lambda: {
                f"{category}_includes": [
                    f"#include <ATen/ops/{name}{suffix}.h>"
                    for name in sorted(functions_by_root_name.keys())
                ],
                f"{category}_declarations": [],
            },
        )

    for dispatch_key in dispatch_keys:
        if dispatch_key not in functions_keys:
            continue

        dispatch_namespace = dispatch_key.lower()
        dispatch_names = []

        for name, functions in functions_by_root_name.items():
            grouped_functions = grouped_functions_by_root_name.get(name, [])
            declarations = list(
                concatMap(
                    dest.RegisterDispatchKey(
                        backend_indices[dispatch_key],
                        Target.NAMESPACED_DECLARATION,
                        selector,
                        rocm=rocm,
                        class_method_name=None,
                        skip_dispatcher_op_registration=False,
                    ),
                    grouped_functions,
                )
            )

            if len(declarations) == 0:
                continue

            dispatch_names.append(name)
            ops_fm.write_with_template(
                f"{name}_{dispatch_namespace}_dispatch.h",
                "DispatchKeyFunction.h",
                lambda: {
                    "dispatch_namespace": dispatch_namespace,
                    "dispatch_namespaced_declarations": declarations,
                },
            )

        fm = cuda_fm if is_cuda_dispatch_key(dispatch_key) else cpu_fm
        inl_headers = f"#include <ATen/{dispatch_key}Functions_inl.h>"

        fm.write_with_template(
            f"{dispatch_key}Functions.h",
            "DispatchKeyFunctions.h",
            lambda: {
                "dispatch_key": str(dispatch_key),
                "inline_headers": inl_headers,
            },
        )
        fm.write_with_template(
            f"{dispatch_key}Functions_inl.h",
            "DispatchKeyFunctions_inl.h",
            lambda: {
                "dispatch_namespace": dispatch_namespace,
                "DispatchKeyFunctions_inl_includes": [
                    f"#include <ATen/ops/{name}_{dispatch_namespace}_dispatch.h>"
                    for name in sorted(dispatch_names)
                ],
                "dispatch_namespaced_declarations": [],
            },
        )
        del fm

    cpu_fm.write(
        "MethodOperators.h",
        lambda: {
            "MethodOperators_includes": sorted(
                f"#include <ATen/ops/{name}_ops.h>"
                for name, functions in functions_by_root_name.items()
                if any(Variant.method in fn.variants for fn in functions)
            ),
            "MethodOperators_declarations": [],
        },
    )


def gen_headers(
    *,
    native_functions: Sequence[NativeFunction],
    valid_tags: Set[str],
    grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
    structured_native_functions: Sequence[NativeFunctionsGroup],
    static_dispatch_idx: List[BackendIndex],
    selector: SelectiveBuilder,
    backend_indices: Dict[DispatchKey, BackendIndex],
    core_fm: FileManager,
    cpu_fm: FileManager,
    cuda_fm: FileManager,
    ops_fm: FileManager,
    dispatch_keys: Sequence[DispatchKey],
    functions_keys: Set[DispatchKey],
    rocm: bool,
    per_operator_headers: bool,
) -> None:
    if per_operator_headers:
        gen_per_operator_headers(
            native_functions=native_functions,
            grouped_native_functions=grouped_native_functions,
            static_dispatch_idx=static_dispatch_idx,
            selector=selector,
            backend_indices=backend_indices,
            cpu_fm=cpu_fm,
            cuda_fm=cuda_fm,
            ops_fm=ops_fm,
            dispatch_keys=dispatch_keys,
            functions_keys=functions_keys,
            rocm=rocm,
        )
    else:
        gen_aggregated_headers(
            native_functions=native_functions,
            grouped_native_functions=grouped_native_functions,
            structured_native_functions=structured_native_functions,
            static_dispatch_idx=static_dispatch_idx,
            selector=selector,
            backend_indices=backend_indices,
            cpu_fm=cpu_fm,
            cuda_fm=cuda_fm,
            dispatch_keys=dispatch_keys,
            functions_keys=functions_keys,
            rocm=rocm,
        )

    core_fm.write(
        "TensorBody.h",
        lambda: {
            "tensor_method_declarations": list(
                mapMaybe(
                    ComputeTensorMethod(
                        target=Target.DECLARATION,
                        static_dispatch_backend_indices=static_dispatch_idx,
                    ),
                    native_functions,
                )
            ),
            "tensor_method_definitions": list(
                mapMaybe(
                    ComputeTensorMethod(
                        target=Target.DEFINITION,
                        static_dispatch_backend_indices=static_dispatch_idx,
                    ),
                    native_functions,
                )
            ),
        },
    )

    cpu_fm.write(
        "RedispatchFunctions.h",
        lambda: {
            "function_redispatch_definitions": list(
                mapMaybe(ComputeRedispatchFunction(), native_functions)
            ),
        },
    )

    cpu_fm.write(
        "RegistrationDeclarations.h",
        lambda: {
            "registration_declarations": [
                compute_registration_declarations(f, backend_indices)
                for f in native_functions
            ],
        },
    )

    def gen_aten_interned_strings() -> Dict[str, str]:
        attrs = set()  # All function argument names
        names = set()  # All ATen function names
        for func in native_functions:
            names.add(str(func.func.name.name))
            # Some operators don't have a functional variant but we still create a
            # symbol without the underscore
            names.add(func.func.name.name.base)

            for arg in func.func.schema_order_arguments():
                attrs.add(arg.name)

        # These are keywords in C++, so aren't valid symbol names
        # https://en.cppreference.com/w/cpp/language/operator_alternative
        names -= set(
            [
                "and",
                "and_eq",
                "bitand",
                "bitor",
                "compl",
                "not",
                "not_eq",
                "or",
                "or_eq",
                "xor",
                "xor_eq",
            ]
        )

        return {
            "aten_symbols": " \\\n".join(
                [f"_(aten, {name})" for name in sorted(names)]
            ),
            "attr_symbols": " \\\n".join(
                [f"_(attr, {name})" for name in sorted(attrs)]
            ),
        }

    core_fm.write("aten_interned_strings.h", gen_aten_interned_strings)

    def gen_tags_enum() -> Dict[str, str]:
        return {"enum_of_valid_tags": (",\n".join([f"{tag}" for tag in valid_tags]))}

    core_fm.write("enum_tag.h", gen_tags_enum)


def gen_source_files(
    *,
    native_functions: Sequence[NativeFunction],
    grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
    structured_native_functions: Sequence[NativeFunctionsGroup],
    view_groups: Sequence[NativeFunctionsViewGroup],
    selector: SelectiveBuilder,
    static_dispatch_idx: List[BackendIndex],
    backend_indices: Dict[DispatchKey, BackendIndex],
    core_fm: FileManager,
    cpu_fm: FileManager,
    cpu_vec_fm: FileManager,
    cuda_fm: FileManager,
    dispatch_keys: Sequence[DispatchKey],
    functions_keys: Set[DispatchKey],
    rocm: bool,
    force_schema_registration: bool,
    per_operator_headers: bool,
    skip_dispatcher_op_registration: bool,
) -> None:
    extra_cuda_headers = """\
#include <c10/cuda/CUDAGuard.h>
#include <ATen/cuda/ATenCUDAGeneral.h>
#include <ATen/cuda/CUDADevice.h>
#include <ATen/cuda/CUDAContext.h>"""
    if rocm:
        extra_cuda_headers = """\
#include <ATen/hip/impl/HIPGuardImplMasqueradingAsCUDA.h>
#include <ATen/hip/ATenHIPGeneral.h>
#include <ATen/hip/HIPDevice.h>
#include <ATen/hip/HIPContext.h>"""

    for dispatch_key in dispatch_keys:
        fm = cuda_fm if is_cuda_dispatch_key(dispatch_key) else cpu_fm

        if per_operator_headers:

            def operator_headers() -> List[str]:
                headers = []
                for g in grouped_native_functions:
                    is_registered = False
                    if backend_index.has_kernel(g):
                        is_registered = True
                    # The above has_kernel test on a group will only test for
                    # the existence of out dispatch, because that's how
                    # structured kernels work. But sometimes functions can be
                    # grouped but not be structured, and then you need to check
                    # each individual piece, as they may have manual dispatch
                    # entries.
                    elif isinstance(g, NativeFunctionsGroup) and any(
                        backend_index.has_kernel(fn) for fn in g.functions()
                    ):
                        is_registered = True
                    # TODO: this condition is a bit questionable
                    # (It has to do with the fact that structured kernels get generated kernels
                    # to the Meta + CompositeExplicitAutogradNonFunctional keys).
                    elif g.structured and dispatch_key in (
                        DispatchKey.Meta,
                        DispatchKey.CompositeExplicitAutogradNonFunctional,
                    ):
                        is_registered = True
                    if not is_registered:
                        continue

                    headers.append(f"#include <ATen/ops/{g.root_name}_native.h>")
                    if (
                        dispatch_key
                        == DispatchKey.CompositeExplicitAutogradNonFunctional
                    ):
                        headers.append(f"#include <ATen/ops/{g.root_name}.h>")
                    if dispatch_key in functions_keys:
                        headers.append(
                            f"#include <ATen/ops/{g.root_name}_{dispatch_namespace}_dispatch.h>"
                        )

                return sorted(set(headers))

        else:

            def operator_headers() -> List[str]:
                headers = ["#include <ATen/NativeFunctions.h>"]
                if dispatch_key == DispatchKey.CompositeExplicitAutogradNonFunctional:
                    headers.append("#include <ATen/Functions.h>")
                if dispatch_key in functions_keys:
                    headers.append(f"#include <ATen/{dispatch_key!s}Functions.h>")
                return headers

        backend_index = backend_indices[dispatch_key]
        ns_grouped_native_functions = defaultdict(list)
        for grouped_native_function in grouped_native_functions:
            namespace = (
                grouped_native_function.namespace
                if isinstance(grouped_native_function, NativeFunction)
                else grouped_native_function.functional.namespace
            )
            ns_grouped_native_functions[namespace].append(grouped_native_function)

        static_init_dispatch_registrations = ""
        for namespace, functions in ns_grouped_native_functions.items():
            dispatch_registrations_body = (
                ""
                if skip_dispatcher_op_registration
                else "\n".join(
                    list(
                        concatMap(
                            dest.RegisterDispatchKey(
                                backend_index,
                                Target.REGISTRATION,
                                selector,
                                rocm=rocm,
                                class_method_name=None,
                                skip_dispatcher_op_registration=skip_dispatcher_op_registration,
                            ),
                            functions,
                        )
                    )
                )
            )

            static_init_dispatch_registrations += f"""
TORCH_LIBRARY_IMPL({namespace}, {dispatch_key}, m) {{
    {dispatch_registrations_body}
}};"""
        dispatch_namespace = str(dispatch_key).lower()
        fm.write_with_template(
            f"Register{dispatch_key}.cpp",
            "RegisterDispatchKey.cpp",
            lambda: {
                "extra_cuda_headers": extra_cuda_headers
                if is_cuda_dispatch_key(dispatch_key)
                else "",
                "external_backend_headers": "",
                "dispatch_headers": dest.gen_registration_headers(
                    backend_index, per_operator_headers, rocm
                ),
                "ops_headers": operator_headers(),
                "DispatchKey": dispatch_key,
                "dispatch_namespace": dispatch_key.lower(),
                "dispatch_helpers": dest.gen_registration_helpers(backend_index),
                "dispatch_namespaced_definitions": list(
                    concatMap(
                        dest.RegisterDispatchKey(
                            backend_index,
                            Target.NAMESPACED_DEFINITION,
                            selector,
                            rocm=rocm,
                            class_method_name=None,
                            skip_dispatcher_op_registration=skip_dispatcher_op_registration,
                        ),
                        grouped_native_functions,
                    )
                ),
                "dispatch_anonymous_definitions": list(
                    concatMap(
                        dest.RegisterDispatchKey(
                            backend_index,
                            Target.ANONYMOUS_DEFINITION,
                            selector,
                            rocm=rocm,
                            class_method_name=None,
                            skip_dispatcher_op_registration=skip_dispatcher_op_registration,
                        ),
                        grouped_native_functions,
                    )
                ),
                "static_init_dispatch_registrations": static_init_dispatch_registrations,
                "deferred_dispatch_registrations": "",
            },
        )

        for g in structured_native_functions:
            if not g.out.ufunc_inner_loop or not is_ufunc_dispatch_key(dispatch_key):
                continue
            name = g.functional.func.name.name
            if dispatch_key is DispatchKey.CPU:
                assert fm is cpu_fm
                fm.write_with_template(
                    f"UfuncCPU_{name}.cpp",
                    "UfuncCPU.cpp",
                    lambda: {
                        "meta_declaration": compute_meta_function_declaration(g),
                        "native_declaration": dest.compute_native_function_declaration(
                            g, backend_indices[dispatch_key]
                        ),
                        "native_definitions": dest.compute_ufunc_cpu(g),
                    },
                )
                cpu_vec_fm.write_with_template(
                    f"UfuncCPUKernel_{name}.cpp",
                    "UfuncCPUKernel.cpp",
                    lambda: {
                        "name": name,
                        "native_definitions": dest.compute_ufunc_cpu_kernel(g),
                    },
                )
            elif dispatch_key is DispatchKey.CUDA:
                cuda_headers = "#include <ATen/native/cuda/Loops.cuh>"
                if rocm:
                    cuda_headers = "#include <ATen/native/hip/Loops.cuh>"
                fm.write_with_template(
                    f"UfuncCUDA_{name}.cu",
                    "UfuncCUDA.cu",
                    lambda: {
                        "name": name,
                        "cuda_headers": cuda_headers,
                        "meta_declaration": compute_meta_function_declaration(g),
                        "native_declaration": dest.compute_native_function_declaration(
                            g, backend_indices[dispatch_key]
                        ),
                        "native_definitions": dest.compute_ufunc_cuda(g),
                    },
                )
            else:
                raise AssertionError(f"unrecognized {dispatch_key} for ufunc")

        del fm

    # BackendSelect is generated specially
    def gen_backend_select() -> Dict[str, List[str]]:
        relevant_fns = [
            fn for fn in native_functions if needs_backend_select(fn, selector)
        ]
        return {
            "ops_headers": [
                f"#include <ATen/ops/{fn.root_name}_ops.h>" for fn in relevant_fns
            ],
            "backend_select_method_definitions": list(
                mapMaybe(
                    ComputeBackendSelect(Target.DEFINITION, selector), relevant_fns
                )
            ),
            "backend_select_function_registrations": list(
                mapMaybe(
                    ComputeBackendSelect(Target.REGISTRATION, selector), relevant_fns
                )
            ),
        }

    cpu_fm.write("RegisterBackendSelect.cpp", gen_backend_select)

    schema_selector = selector
    if force_schema_registration:
        schema_selector = SelectiveBuilder.get_nop_selector()

    ns_native_functions: Dict[str, List[NativeFunction]] = defaultdict(list)
    for native_function in native_functions:
        ns_native_functions[native_function.namespace].append(native_function)
    schema_registrations = ""
    aten_schema_registrations = []
    custom_namespace = None
    for namespace, funcs in ns_native_functions.items():

        schema_registrations_body = list(
            mapMaybe(RegisterSchema(schema_selector), funcs)
        )
        # NB: we have to separate aten namespace registration from other namespaces,
        # because in the template we hardcoded an operator for ATen already.
        if namespace == "aten":
            aten_schema_registrations = schema_registrations_body
        else:
            assert custom_namespace is None or namespace == custom_namespace, (
                "Only one custom namespace (other than 'aten') is currently supported, "
                f" but getting {namespace} and {custom_namespace}"
            )
            custom_namespace = namespace
            tab = "\t"
            schema_registrations += f"""
TORCH_LIBRARY({custom_namespace}, m) {{
  {tab.join(schema_registrations_body)}
}};"""
    cpu_fm.write(
        "RegisterSchema.cpp",
        lambda: {
            "aten_schema_registrations": []
            if skip_dispatcher_op_registration
            else aten_schema_registrations,
            "schema_registrations": []
            if skip_dispatcher_op_registration
            else schema_registrations,
        },
    )

    def key_func(
        fn: Union[NativeFunction, NativeFunctionsGroup, NativeFunctionsViewGroup]
    ) -> str:
        return fn.root_name

    cpu_fm.write_sharded(
        "Operators.cpp",
        native_functions,
        key_fn=key_func,
        env_callable=lambda fn: {
            "operator_headers": [f"#include <ATen/ops/{fn.root_name}.h>"],
            "definitions": [
                ComputeOperators(
                    Target.DEFINITION,
                    static_dispatch_backend_indices=static_dispatch_idx,
                )(fn)
            ],
        },
        base_env={
            "static_dispatch_extra_headers": static_dispatch_extra_headers(
                static_dispatch_idx
            ),
        },
        num_shards=5,
        sharded_keys={
            "operator_headers",
            "definitions",
            "static_dispatch_extra_headers",
        },
    )

    cpu_fm.write("Functions.cpp", lambda: {})

    core_fm.write("TensorMethods.cpp", lambda: {})

    core_fm.write(
        "ATenOpList.cpp",
        lambda: {
            "aten_ops": list(mapMaybe(compute_aten_op, native_functions)),
        },
    )

    def functionalization_env_callable(
        g: Union[NativeFunction, NativeFunctionsGroup, NativeFunctionsViewGroup]
    ) -> Dict[str, List[str]]:
        def gen_op_headers(
            g: Union[NativeFunction, NativeFunctionsGroup, NativeFunctionsViewGroup]
        ) -> List[str]:
            if isinstance(g, NativeFunctionsViewGroup):
                # view ops always get a functionalization kernel
                headers = [
                    f"#include <ATen/ops/{g.view.root_name}_native.h>",
                    f"#include <ATen/ops/{g.view.root_name}_ops.h>",
                ]
                if g.view_copy is not None:
                    headers += [
                        f"#include <ATen/ops/{g.view_copy.root_name}_native.h>",
                        f"#include <ATen/ops/{g.view_copy.root_name}_ops.h>",
                    ]
                return headers
            elif isinstance(g, NativeFunctionsGroup):
                headers = [
                    f"#include <ATen/ops/{g.functional.root_name}_native.h>",
                    f"#include <ATen/ops/{g.functional.root_name}_ops.h>",
                    f"#include <ATen/ops/{g.out.root_name}_native.h>",
                    f"#include <ATen/ops/{g.out.root_name}_ops.h>",
                ]
                if g.inplace is not None:
                    headers += [
                        f"#include <ATen/ops/{g.inplace.root_name}_native.h>",
                        f"#include <ATen/ops/{g.inplace.root_name}_ops.h>",
                    ]
                if g.mutable is not None:
                    headers += [
                        f"#include <ATen/ops/{g.mutable.root_name}_native.h>",
                        f"#include <ATen/ops/{g.mutable.root_name}_ops.h>",
                    ]
                return headers
            else:
                return [
                    f"#include <ATen/ops/{g.root_name}_native.h>",
                    f"#include <ATen/ops/{g.root_name}_ops.h>",
                ]

        return {
            "ops_headers": gen_op_headers(g),
            "func_definitions": gen_functionalization_definition(
                selector,
                g,
            ),
            "func_registrations": gen_functionalization_registration(
                selector,
                g,
                backend_indices[DispatchKey.CompositeImplicitAutograd],
            ),
        }

    all_groups: List[
        Union[NativeFunction, NativeFunctionsGroup, NativeFunctionsViewGroup]
    ] = list(structured_native_functions) + list(
        view_groups  # type: ignore[assignment, arg-type, operator]
    )
    # Note: all operators that functionalization needs to handle (mutable and aliasing ops) should be grouped properly.
    # The only reason we really need to deal with direct NativeFunctions here (instead of the groups) is because:
    # (1) We can provide better error checking (error out if someone introduces a mutable op that doesn't obey the grouping logic)
    # (2) functionalization needs to manually register CompositeImplicitAutograd kernels, which might not be grouped.
    #     Although this could go away long-term if we add a dedicated dispatch key for decompositions.
    structured_map: Dict[OperatorName, NativeFunction] = {
        f.func.name: f
        for f in concatMap(lambda g: list(g.functions()), structured_native_functions)
    }
    view_map: Dict[OperatorName, NativeFunction] = {
        f.func.name: f for f in concatMap(lambda g: list(g.functions()), view_groups)
    }
    for f in native_functions:
        if f.func.name not in structured_map and f.func.name not in view_map:
            all_groups.append(f)

    cpu_fm.write_sharded(
        "RegisterFunctionalization.cpp",
        all_groups,
        key_fn=key_func,
        env_callable=functionalization_env_callable,
        num_shards=4,
        sharded_keys={
            "ops_headers",
            "func_definitions",
            "func_registrations",
            "func_add_back_views_definitions",
            "func_add_back_views_registrations",
        },
    )

    cpu_fm.write(
        "FunctionalInverses.h",
        lambda: {
            "view_inverse_declarations": list(
                mapMaybe(
                    lambda g: gen_functionalization_view_inverse_declaration(
                        selector, g
                    ),
                    view_groups,
                )
            )
        },
    )
    view_copy_with_symint_pairs: List[Tuple[NativeFunction, NativeFunction]] = []
    for g1 in view_groups:
        for g2 in view_groups:
            if g1.view_copy is None or g2.view_copy is None:
                continue
            # TODO: make this more first class in the data model
            same_base_op = str(g1.view_copy.func.name.name) == str(
                g2.view_copy.func.name.name
            )
            op1_not_symint = "SymInt" not in str(g1.view_copy.func.name.overload_name)
            op2_symint = "SymInt" in str(g2.view_copy.func.name.overload_name)
            if same_base_op and op1_not_symint and op2_symint:
                view_copy_with_symint_pairs.append(
                    (
                        g1.view_copy,
                        g2.view_copy,
                    )
                )

    # Note [view_copy NativeFunctions]
    # Every view operator in native_functions.yaml that is not CompositeImplicitAutograd
    # needs to have a corresponding non-aliasing {view}_copy variant.
    # Backends that use functionalization and don't know how to handle aliasing ops
    # are expected to implement kernels for these {view}_copy kernels instead.
    # The code for {view}_copy operators in core is pretty boilerplate-heavy however,
    # so we codegen the following:
    # (1) A CompositeExplicitAutogradNonFunctional kernel for every {view}_copy operator.
    #     These are never explicitly invoked by the functionalization pass,
    #     but they could theoretically be called from user code (I added these kernels for completeness,
    #     since the ops are part of the public API).
    # (2) A derivative formula for every {view}_copy operator
    #     {view}_copy operators can re-use the same derivative formulas as their {view} op counterparts,
    #     so rather than stamping all of the entries out in derivatives.yaml,
    #     we codegen them in.
    #     This is similar to how autograd codegen doesn't require inplace ops to have a derivatives.yaml entry.
    cpu_fm.write(
        "CompositeViewCopyKernels.cpp",
        lambda: {
            "ops_headers": [
                "\n".join(
                    f"#include <ATen/ops/{f.root_name}_ops.h>"
                    for f in (
                        [g.view] if g.view_copy is None else [g.view, g.view_copy]
                    )
                )
                for g in view_groups
            ]
            + [
                "\n".join(
                    f"#include <ATen/ops/{f.root_name}_ops.h>"
                    for f in [g.inplace, g.mutable]
                    if f is not None and "generated" not in f.tags
                )
                for g in structured_native_functions
            ],
            "CompositeViewCopyKernel_Definitions": list(
                mapMaybe(gen_composite_view_copy_kernel, view_groups)
            ),
            "SymIntViewCopyKernel_Definitions": list(
                mapMaybe(
                    lambda pair: gen_symint_view_copy_kernel(pair[0], pair[1]),
                    view_copy_with_symint_pairs,
                )
            ),
            "GeneratedCompositeFunctional_Definitions": list(
                mapMaybe(
                    gen_composite_functional_kernel,
                    structured_native_functions,
                )
            ),
            "GeneratedCompositeOut_Definitions": list(
                mapMaybe(
                    gen_composite_out_kernel,
                    structured_native_functions,
                )
            ),
        },
    )


def gen_declarations_yaml(
    cpu_fm: FileManager, native_functions: Sequence[NativeFunction]
) -> None:
    cpu_fm.write(
        "Declarations.yaml",
        lambda: format_yaml([compute_declaration_yaml(f) for f in native_functions]),
    )


def get_torchgen_root() -> pathlib.Path:
    """
    If you're depending on torchgen out-of-tree, you can use the root to figure
    out the path to native_functions.yaml
    """
    return pathlib.Path(__file__).parent.resolve()


def main() -> None:
    parser = argparse.ArgumentParser(description="Generate ATen source files")
    parser.add_argument(
        "-s",
        "--source-path",
        help="path to source directory for ATen",
        default="aten/src/ATen",
    )
    parser.add_argument(
        "-o",
        "--output-dependencies",
        help="output a list of dependencies into the given file and exit",
    )
    parser.add_argument(
        "--dry-run",
        action="store_true",
        help="run without writing any files (still updates outputs)",
    )
    parser.add_argument(
        "--per-operator-headers",
        action="store_true",
        help="generate separate headers per operator in ATen/ops",
    )
    parser.add_argument(
        "-d", "--install_dir", help="output directory", default="build/aten/src/ATen"
    )
    parser.add_argument(
        "--rocm",
        action="store_true",
        help="reinterpret CUDA as ROCm/HIP and adjust filepaths accordingly",
    )
    parser.add_argument(
        "--mps",
        action="store_true",
        help="Generate MPS registration code when set",
    )
    # TODO: --op_registration_whitelist will be removed when all call-sites
    # for gen.py are moved over to using the operator YAML file for mobile
    # custom build.
    parser.add_argument(
        "--op_registration_whitelist",
        nargs="*",
        help="filter op registrations by the whitelist (if set); "
        "each item is `namespace`::`operator name` without overload name; "
        "e.g.: aten::empty aten::conv2d ...",
    )
    parser.add_argument(
        "--op_selection_yaml_path",
        help="Provide a path to the operator selection (for custom build) YAML "
        "that contains the information about the set of selected operators "
        "and their categories (training, ...). Each operator is either a "
        "full operator name with overload or just a bare operator name. "
        "The operator names also contain the namespace prefix (e.g. aten::)",
    )
    parser.add_argument(
        "--backend_whitelist",
        nargs="*",
        help="filter dispatch backend by the whitelist (if set), "
        "e.g.: CPU CUDA QuantizedCPU ...",
    )
    parser.add_argument(
        "--static_dispatch_backend",
        nargs="*",
        help="generate static dispatch code for the specific backend (if set)",
    )
    parser.add_argument(
        "--skip_dispatcher_op_registration",
        action="store_true",
        help="Avoid registering operators into the dispatcher.",
    )
    parser.add_argument(
        "--force_schema_registration",
        action="store_true",
        help="force it to generate schema-only registrations for all ops, including"
        "those that are not listed on --op_registration_whitelist",
    )
    parser.add_argument(
        "--generate",
        type=str,
        nargs="*",
        choices=["headers", "sources", "declarations_yaml"],
        default=["headers", "sources", "declarations_yaml"],
        help="Generate only a subset of files",
    )

    options = parser.parse_args()

    selector = get_custom_build_selector(
        options.op_registration_whitelist,
        options.op_selection_yaml_path,
    )

    native_yaml_path = os.path.join(options.source_path, "native/native_functions.yaml")
    tags_yaml_path = os.path.join(options.source_path, "native/tags.yaml")

    from torchgen.model import dispatch_keys

    # TODO: stop generating CUDA kernels for non-CUDA builds
    ignore_keys = set()
    if not options.mps:
        ignore_keys.add(DispatchKey.MPS)

        if DispatchKey.MPS in dispatch_keys:
            del dispatch_keys[dispatch_keys.index(DispatchKey.MPS)]

    parsed_yaml = parse_native_yaml(native_yaml_path, tags_yaml_path, ignore_keys)
    valid_tags = _GLOBAL_PARSE_TAGS_YAML_CACHE[tags_yaml_path]
    native_functions, backend_indices = (
        parsed_yaml.native_functions,
        parsed_yaml.backend_indices,
    )

    grouped_native_functions = get_grouped_native_functions(native_functions)

    structured_native_functions = [
        g for g in grouped_native_functions if isinstance(g, NativeFunctionsGroup)
    ]
    native_functions_with_view_groups = get_grouped_by_view_native_functions(
        native_functions
    )
    view_groups = [
        g
        for g in native_functions_with_view_groups
        if isinstance(g, NativeFunctionsViewGroup)
    ]

    template_dir = os.path.join(options.source_path, "templates")

    # NB: It is mandatory to NOT use os.path.join here, as the install directory
    # will eventually be ingested by cmake, which does not respect Windows style
    # path slashes.  If you switch this to use os.path.join, you'll get an error
    # like:
    #
    #   Syntax error in cmake code when parsing string
    #
    #     C:/Jenkins/workspace/pytorch-builds/pytorch-win-ws2016-cuda9-cudnn7-py3-build/build/aten/src/ATen\core/TensorMethods.h
    #
    #   Invalid character escape '\c'.
    core_install_dir = f"{options.install_dir}/core"
    pathlib.Path(core_install_dir).mkdir(parents=True, exist_ok=True)
    ops_install_dir = f"{options.install_dir}/ops"
    pathlib.Path(ops_install_dir).mkdir(parents=True, exist_ok=True)

    core_fm = make_file_manager(options=options, install_dir=core_install_dir)
    cpu_fm = make_file_manager(options=options)
    cpu_vec_fm = make_file_manager(options=options)
    cuda_fm = make_file_manager(options=options)
    ops_fm = make_file_manager(options=options, install_dir=ops_install_dir)

    extra_cuda_headers = """\
#include <c10/cuda/CUDAGuard.h>
#include <ATen/cuda/ATenCUDAGeneral.h>
#include <ATen/cuda/CUDADevice.h>
#include <ATen/cuda/CUDAContext.h>"""
    if options.rocm:
        extra_cuda_headers = """\
#include <ATen/hip/impl/HIPGuardImplMasqueradingAsCUDA.h>
#include <ATen/hip/ATenHIPGeneral.h>
#include <ATen/hip/HIPDevice.h>
#include <ATen/hip/HIPContext.h>"""

    # Only a limited set of dispatch keys get CPUFunctions.h headers generated
    # for them; this is the set
    functions_keys = {
        DispatchKey.CPU,
        DispatchKey.CUDA,
        DispatchKey.CompositeImplicitAutograd,
        DispatchKey.CompositeExplicitAutograd,
        DispatchKey.CompositeExplicitAutogradNonFunctional,
        DispatchKey.Meta,
    }
    if options.mps:
        functions_keys.add(DispatchKey.MPS)

    if options.backend_whitelist:
        dispatch_keys = [
            k
            for k in dispatch_keys
            if is_generic_dispatch_key(k) or str(k) in options.backend_whitelist
        ]

    static_dispatch_idx: List[BackendIndex] = []
    if options.static_dispatch_backend:
        static_dispatch_idx = [
            backend_indices[DispatchKey.parse(key)]
            for key in options.static_dispatch_backend
        ]
        for key in options.static_dispatch_backend:
            dp_key = DispatchKey.parse(key)
            if dp_key not in functions_keys:
                functions_keys.add(dp_key)

    if "sources" in options.generate:
        gen_source_files(
            native_functions=native_functions,
            grouped_native_functions=grouped_native_functions,
            structured_native_functions=structured_native_functions,
            view_groups=view_groups,
            selector=selector,
            static_dispatch_idx=static_dispatch_idx,
            backend_indices=backend_indices,
            core_fm=core_fm,
            cpu_fm=cpu_fm,
            cpu_vec_fm=cpu_vec_fm,
            cuda_fm=cuda_fm,
            dispatch_keys=dispatch_keys,
            functions_keys=functions_keys,
            rocm=options.rocm,
            force_schema_registration=options.force_schema_registration,
            per_operator_headers=options.per_operator_headers,
            skip_dispatcher_op_registration=options.skip_dispatcher_op_registration,
        )

    if "headers" in options.generate:
        gen_headers(
            native_functions=native_functions,
            valid_tags=valid_tags,
            grouped_native_functions=grouped_native_functions,
            structured_native_functions=structured_native_functions,
            static_dispatch_idx=static_dispatch_idx,
            selector=selector,
            backend_indices=backend_indices,
            core_fm=core_fm,
            cpu_fm=cpu_fm,
            cuda_fm=cuda_fm,
            ops_fm=ops_fm,
            dispatch_keys=dispatch_keys,
            functions_keys=functions_keys,
            rocm=options.rocm,
            per_operator_headers=options.per_operator_headers,
        )

    if "declarations_yaml" in options.generate:
        gen_declarations_yaml(native_functions=native_functions, cpu_fm=cpu_fm)

    if options.output_dependencies:
        depfile_path = pathlib.Path(options.output_dependencies).resolve()
        depfile_name = depfile_path.name
        depfile_stem = depfile_path.stem

        for fm, prefix in [
            (cpu_fm, ""),
            (cpu_vec_fm, "cpu_vec_"),
            (core_fm, "core_"),
            (cuda_fm, "cuda_"),
            (ops_fm, "ops_"),
        ]:
            varname = prefix + depfile_stem
            path = depfile_path.parent / (prefix + depfile_name)
            fm.write_outputs(varname, str(path))


if __name__ == "__main__":
    main()