library.py

from ._ops import OpOverload
from typing import Set
import traceback
import torch
import weakref

__all__ = ['Library', 'impl', 'define', 'fallthrough_kernel']

# Set containing the combination of (namespace, operator, DispatchKey) for which a new kernel has been registered
# The keys in the set are of the form `namespace + "/" + op_name + "/" + dispatch_key`.
# This set is maintained to ensure that two libraries don't try to override the exact same functionality to avoid
# libraries calling into kernels not intended to be called.
_impls: Set[str] = set()

# prim is reserved by TorchScript interpreter
_reserved_namespaces = ['prim']

def fallthrough_kernel():
    """
    A dummy function to pass to ``Library.impl`` in order to register a fallthrough.
    """
    raise NotImplementedError("fallthrough_kernel() should never be called.")

class Library:
    """
    A class to create libraries that can be used to register new operators or
    override operators in existing libraries from Python.
    A user can optionally pass in a dispatch keyname if they only want to register
    kernels corresponding to only one specific dispatch key.

    To create a library to override operators in an existing library (with name ns), set the kind to "IMPL".
    To create a new library (with name ns) to register new operators, set the kind to "DEF".
    To create a fragment of a possibly existing library to register operators (and bypass
    the limitation that there is only one library for a given namespace), set the kind to
    "FRAGMENT".

    Args:
        ns: library name
        kind: "DEF", "IMPL" (default: "IMPL"), "FRAGMENT"
        dispatch_key: PyTorch dispatch key (default: "")
    """
    def __init__(self, ns, kind, dispatch_key=""):
        if kind not in ('IMPL', 'DEF', 'FRAGMENT'):
            raise ValueError("Unsupported kind: ", kind)

        if ns in _reserved_namespaces and (kind == "DEF" or kind == 'FRAGMENT'):
            raise ValueError(ns, " is a reserved namespace. Please try creating a library with another name.")

        frame = traceback.extract_stack(limit=3)[0]
        filename, lineno = frame.filename, frame.lineno
        self.m = torch._C._dispatch_library(kind, ns, dispatch_key, filename, lineno)
        self.ns = ns
        self._op_impls: Set[str] = set()
        self.kind = kind
        self.dispatch_key = dispatch_key
        # Use a finalizer to setup the "destructor" instead of __del__.
        # Python __del__ can lead to weird things (globals and locals may already
        # be gone when __del__ actually gets called!). finalizers help the
        # situation because it lets us capture references and keeps them alive
        weakref.finalize(self, _del_library, _impls, self._op_impls)

    def __repr__(self):
        return f"Library(kind={self.kind}, ns={self.ns}, dispatch_key={self.dispatch_key})>"

    def define(self, schema, alias_analysis=""):
        r'''Defines a new operator and its semantics in the ns namespace.

        Args:
            schema: function schema to define a new operator.
            alias_analysis (optional): Indicates if the aliasing properties of the operator arguments can be
                                       inferred from the schema (default behavior) or not ("CONSERVATIVE").
        Returns:
            name of the operator as inferred from the schema.

        Example::
            >>> # xdoctest: +REQUIRES(env:TORCH_DOCTEST_LIBRARY)
            >>> my_lib = Library("foo", "DEF")
            >>> my_lib.define("sum(Tensor self) -> Tensor")
        '''
        # This is added because we also want to disallow PURE_FUNCTION alias analysis which is a valid
        # AliasAnalysis type in C++
        if alias_analysis not in ["", "FROM_SCHEMA", "CONSERVATIVE"]:
            raise RuntimeError(f"Invalid alias_analysis type {alias_analysis}")
        return self.m.define(schema, alias_analysis)

    def impl(self, op_name, fn, dispatch_key=''):
        r'''Registers the function implementation for an operator defined in the library.

        Args:
            op_name: operator name (along with the overload) or OpOverload object.
            fn: function that's the operator implementation for the input dispatch key or :func:`~fallthrough_kernel`
                to register a fallthrough.
            dispatch_key: dispatch key that the input function should be registered for. By default, it uses
                          the dispatch key that the library was created with.

        Example::
            >>> my_lib = Library("aten", "IMPL")
            >>> def div_cpu(self, other):
            >>>     return self * (1 / other)
            >>> my_lib.impl("div.Tensor", div_cpu, "CPU")
        '''
        if not callable(fn):
            raise TypeError(f"Input function is required to be a callable but found type {type(fn)}")
        if dispatch_key == '':
            dispatch_key = self.dispatch_key

        if isinstance(op_name, str):
            name = op_name
        elif isinstance(op_name, OpOverload):
            name = op_name._schema.name
            overload_name = op_name._schema.overload_name
            if overload_name != '':
                name = name + '.' + overload_name
        else:
            raise RuntimeError("impl should be passed either a name or an OpOverload object as the first argument")

        key = self.ns + "/" + name.split("::")[-1] + "/" + dispatch_key
        if key in _impls:
            # TODO: in future, add more info about where the existing function is registered (this info is
            # today already returned by the C++ warning when impl is called but we error out before that)
            raise RuntimeError("This is not allowed since there's already a kernel registered from python overriding {}"
                               "'s behavior for {} dispatch key and {} namespace.".
                               format(name.split("::")[-1], dispatch_key, self.ns))

        if dispatch_key == "Meta":
            dispatcher_op_name = name
            if '::' not in dispatcher_op_name:
                dispatcher_op_name = f'{self.ns}::{dispatcher_op_name}'

            # Internally, we shouldn't be registering meta kernels for any operators that
            # have CompositeImplicitAutograd kernels.
            # Instead, we should be letting those decompositions run, and writing meta kernels
            # only for the base operators.
            if torch._C._dispatch_has_kernel_for_dispatch_key(dispatcher_op_name, "CompositeImplicitAutograd"):
                raise RuntimeError(
                    f"We should not register a meta kernel directly to the operator '{name}',"
                    " because it has a CompositeImplicitAutograd kernel in core."
                    " Instead we should let the operator decompose, and ensure that we have meta kernels"
                    " for the base ops that it decomposes into.")

        self.m.impl(name, dispatch_key if dispatch_key != "" else "CompositeImplicitAutograd", fn)

        _impls.add(key)
        self._op_impls.add(key)


def _del_library(captured_impls, op_impls):
    captured_impls -= op_impls


# decorator to register python functions for library ops
# Note: this decorator API should remain consistent with `Library.impl` API
def impl(lib, name, dispatch_key=""):
    def wrap(f):
        lib.impl(name, f, dispatch_key)
        return f
    return wrap


def define(lib, schema, alias_analysis=""):
    def wrap(f):
        name = lib.define(schema, alias_analysis)
        lib.impl(name, f)
        return f
    return wrap