PEP: 580 Title: The C call protocol Author: Jeroen Demeyer Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 14-Jun-2018 Python-Version: 3.8 Post-History: 20-Jun-2018, 22-Jun-2018, 16-Jul-2018 Abstract ======== A new "C call" protocol is proposed. It is meant for classes representing functions or methods which need to implement fast calling. The goal is to generalize all existing optimizations for built-in functions to arbitrary extension types. In the reference implementation, this new protocol is used for the existing classes ``builtin_function_or_method`` and ``method_descriptor``. However, in the future, more classes may implement it. **NOTE**: This PEP deals only with the Python/C API, it does not affect the Python language or standard library. Motivation ========== Currently, the Python bytecode interpreter has various optimizations for calling instances of ``builtin_function_or_method``, ``method_descriptor``, ``method`` and ``function``. However, none of these classes is subclassable. Therefore, these optimizations are not available to user-defined extension types. If this PEP is implemented, then the checks for ``builtin_function_or_method`` and ``method_descriptor`` could be replaced by simply checking for and using the C call protocol. This simplifies existing code. We also design the C call protocol such that it can easily be extended with new features in the future. For more background and motivation, see PEP 579. Basic idea ========== Currently, CPython has multiple optimizations for fast calling for a few specific function classes. Calling instances of these classes using the ``tp_call`` slot is slower than using the optimizations. The basic idea of this PEP is to allow user-defined extension types (not Python classes) to use these optimizations also, both as caller and as callee. The existing class ``builtin_function_or_method`` and a few others use a ``PyMethodDef`` structure for describing the underlying C function and its signature. The first concrete change is that this is replaced by a new structure ``PyCCallDef``. This stores some of the same information as a ``PyMethodDef``, but with one important addition: the "parent" of the function (the class or module where it is defined). Note that ``PyMethodDef`` arrays are still used to construct functions/methods but no longer for calling them. Second, we want that every class can use such a ``PyCCallDef`` for optimizing calls, so the ``PyTypeObject`` structure gains a ``tp_ccalloffset`` field giving an offset to a ``PyCCallDef *`` in the object structure and a flag ``Py_TPFLAGS_HAVE_CCALL`` indicating that ``tp_ccalloffset`` is valid. Third, since we want to deal efficiently with unbound and bound methods too (as opposed to only plain functions), we need to handle ``__self__`` in the protocol: after the ``PyCCallDef *`` in the object structure, there is a ``PyObject *self`` field. These two fields together are referred to as a ``PyCCallRoot`` structure. The new protocol for efficiently calling objects using these new structures is called the "C call protocol". New data structures =================== The ``PyTypeObject`` structure gains a new field ``Py_ssize_t tp_ccalloffset`` and a new flag ``Py_TPFLAGS_HAVE_CCALL``. If this flag is set, then ``tp_ccalloffset`` is assumed to be a valid offset inside the object structure (similar to ``tp_dictoffset`` and ``tp_weaklistoffset``). It must be a strictly positive integer. At that offset, a ``PyCCallRoot`` structure appears:: typedef struct { PyCCallDef *cr_ccall; PyObject *cr_self; /* __self__ argument for methods */ } PyCCallRoot; The ``PyCCallDef`` structure contains everything needed to describe how the function can be called:: typedef struct { uint32_t cc_flags; PyCFunc cc_func; /* C function to call */ PyObject *cc_parent; /* class or module */ } PyCCallDef; The reason for putting ``__self__`` outside of ``PyCCallDef`` is that ``PyCCallDef`` is not meant to be changed after creating the function. A single ``PyCCallDef`` can be shared by an unbound method and multiple bound methods. This wouldn't work if we would put ``__self__`` inside that structure. **NOTE**: unlike ``tp_dictoffset`` we do not allow negative numbers for ``tp_ccalloffset`` to mean counting from the end. There does not seem to be a use case for it and it would only complicate the implementation. Parent ------ The ``cc_parent`` field (accessed for example by a ``__parent__`` or ``__objclass__`` descriptor from Python code) can be any Python object, or NULL. Custom classes are free to set ``cc_parent`` to whatever they want. It is only used by the C call protocol if the ``CCALL_OBJCLASS`` flag is set. For methods of extension types, ``cc_parent`` points to the class that defines the method (which may be a superclass of ``type(self)``). This is currently is non-trivial to retreive from a method's code. (In the future, this can be used to access the module state via the defining class. See the rationale of PEP 573 for details.) When the flag ``CCALL_OBJCLASS`` is set (as it will be for methods of extension types), ``cc_parent`` is used for type checks like the following:: >>> list.append({}, "x") Traceback (most recent call last): File "", line 1, in TypeError: descriptor 'append' requires a 'list' object but received a 'dict' For functions of modules, ``cc_parent`` is set to the module. Currently, this is exactly the same as ``__self__``. However, using ``__self__`` for the module is a quirk of the current implementation: in the future, we want to allow functions which use ``__self__`` in the normal way, for implementing methods. Such functions can still use ``cc_parent`` instead to refer to the module. The parent would also typically be used to implement ``__qualname__``. The new C API function ``PyCCall_GenericGetQualname()`` does exactly that. Using tp_print -------------- We propose to replace the existing unused field ``tp_print`` by ``tp_ccalloffset``. Since ``Py_TPFLAGS_HAVE_CCALL`` would *not* be added to ``Py_TPFLAGS_DEFAULT``, this ensures full backwards compatibility for existing extension modules setting ``tp_print``. It also means that we can require that ``tp_ccalloffset`` is a valid offset when ``Py_TPFLAGS_HAVE_CCALL`` is specified: we do not need to check ``tp_ccalloffset != 0``. In future Python versions, we may decide that ``tp_print`` becomes ``tp_ccalloffset`` unconditionally, drop the ``Py_TPFLAGS_HAVE_CCALL`` flag and instead check for ``tp_ccalloffset != 0``. **NOTE**: the exact layout of ``PyTypeObject`` is not part of the stable ABI ([#pep384]_). Therefore, changing the ``tp_print`` field from a ``printfunc`` (a function pointer) to a ``Py_ssize_t`` should not be a problem, even if this changes the memory layout of the ``PyTypeObject`` structure. Moreover, on all systems for which binaries are commonly built (Windows, Linux, macOS), the size of ``printfunc`` and ``Py_ssize_t`` are the same, so the issue of binary compatibility will not come up anyway. The C call protocol =================== We say that a class implements the C call protocol if it has the ``Py_TPFLAGS_HAVE_CCALL`` flag set (as explained above, it must then set ``tp_ccalloffset > 0``). Such a class must implement ``__call__`` as described in this section (in practice, this just means setting ``tp_call`` to ``PyCCall_Call``). The ``cc_func`` field is a C function pointer, which plays the same role as the existing ``ml_meth`` field of ``PyMethodDef``. Its precise signature depends on flags. The subset of flags influencing the signature of ``cc_func`` is given by the bitmask ``CCALL_SIGNATURE``. Below are the possible values for ``cc_flags & CCALL_SIGNATURE`` together with the arguments that the C function takes. The return value is always ``PyObject *``. The following are analogous to the existing ``PyMethodDef`` signature flags: - ``CCALL_VARARGS``: ``cc_func(PyObject *self, PyObject *args)`` - ``CCALL_VARARGS | CCALL_KEYWORDS``: ``cc_func(PyObject *self, PyObject *args, PyObject *kwds)`` (``kwds`` is either ``NULL`` or a dict; this dict must not be modified by the callee) - ``CCALL_FASTCALL``: ``cc_func(PyObject *self, PyObject *const *args, Py_ssize_t nargs)`` - ``CCALL_FASTCALL | CCALL_KEYWORDS``: ``cc_func(PyObject *self, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)`` (``kwnames`` is either ``NULL`` or a non-empty tuple of keyword names) - ``CCALL_NOARGS``: ``cc_func(PyObject *self, PyObject *unused)`` (second argument is always ``NULL``) - ``CCALL_O``: ``cc_func(PyObject *self, PyObject *arg)`` The flag ``CCALL_FUNCARG`` may be combined with any of these. If so, the C function takes an additional argument as first argument before ``self``. This argument is used to pass the function object (see NOTE 1 below). For example, we have the following signature: - ``CCALL_FUNCARG | CCALL_VARARGS``: ``cc_func(PyObject *func, PyObject *self, PyObject *args)`` One exception is ``CCALL_FUNCARG | CCALL_NOARGS``: the ``unused`` argument is dropped, so the signature becomes - ``CCALL_FUNCARG | CCALL_NOARGS``: ``cc_func(PyObject *func, PyObject *self)`` **NOTE 1**: with "function object", we mean the ``self`` in ``__call__``. In the case of bound methods, it is currently unspecified whether this refers to the bound method or the original function (which is wrapped by the bound method). In the reference implementation, the bound method is passed. In the future, this may change to the wrapped function. Despite this ambiguity, the implementation of bound methods guarantees that ``PyCCall_CCALLDEF(func)`` points to the ``PyCCallDef`` of the original function. **NOTE 2**: unlike the existing ``METH_...`` flags, the ``CCALL_...`` constants do not necessarily represent single bits. So checking ``if (cc_flags & CCALL_VARARGS)`` is not a valid way for checking the signature. There are also no guarantees of binary compatibility for these flags between Python versions. This allows the implementation to choose the most efficient numerical values of the flags. In the reference implementation, the legal values for ``cc_flags & CCALL_SIGNATURE`` form exactly the interval [0, …, 11]. This means that the compiler can easily optimize a ``switch`` statement for those cases using a computed goto. Checking __objclass__ --------------------- If the ``CCALL_OBJCLASS`` flag is set and if ``cr_self`` is NULL (this is the case for unbound methods of extension types), then a type check is done: the function must be called with at least one positional argument and the first (typically called ``self``) must be an instance of ``cc_parent`` (which must be a class). If not, a ``TypeError`` is raised. Self slicing ------------ If ``cr_self`` is not NULL or if the flag ``CCALL_SELFARG`` is not set in ``cc_flags``, then the argument passed as ``self`` is simply ``cr_self``. If ``cr_self`` is NULL and the flag ``CCALL_SELFARG`` is set, then the first positional argument is removed from ``args`` and instead passed as ``self`` argument to the C function. Effectively, the first positional argument is treated as ``__self__``. If there are no positional arguments, ``TypeError`` is raised. This process is called "self slicing" and a function is said to have self slicing if ``cr_self`` is NULL and ``CCALL_SELFARG`` is set. Note that a ``CCALL_NOARGS`` function with self slicing effectively has one argument, namely ``self``. Analogously, a ``CCALL_O`` function with self slicing has two arguments. Descriptor behavior ------------------- Classes supporting the C call protocol must implement the descriptor protocol in a specific way. This is required for an efficient implementation of bound methods: if other code can make assumptions on what ``__get__`` does, it enables optimizations which would not be possible otherwise. In particular, we want to allow sharing the ``PyCCallDef`` structure between bound and unbound methods. We also need a correct implementation of ``_PyObject_GetMethod`` which is used by the ``LOAD_METHOD``/``CALL_METHOD`` optimization. First of all, if ``func`` supports the C call protocol, then ``func.__set__`` and ``func.__delete__`` must not be implemented. Second, ``func.__get__`` must behave as follows: - If ``cr_self`` is not NULL, then ``__get__`` must be a no-op in the sense that ``func.__get__(obj, cls)(*args, **kwds)`` behaves exactly the same as ``func(*args, **kwds)``. It is also allowed for ``__get__`` to be not implemented at all. - If ``cr_self`` is NULL, then ``func.__get__(obj, cls)(*args, **kwds)`` (with ``obj`` not None) must be equivalent to ``func(obj, *args, **kwds)``. In particular, ``__get__`` must be implemented in this case. This is unrelated to `self slicing`_: ``obj`` may be passed as ``self`` argument to the C function or it may be the first positional argument. - If ``cr_self`` is NULL, then ``func.__get__(None, cls)(*args, **kwds)`` must be equivalent to ``func(*args, **kwds)``. There are no restrictions on the object ``func.__get__(obj, cls)``. The latter is not required to implement the C call protocol for example. We only specify what ``func.__get__(obj, cls).__call__`` does. For classes that do not care about ``__self__`` and ``__get__`` at all, the easiest solution is to assign ``cr_self = Py_None`` (or any other non-NULL value). The __name__ attribute ---------------------- The C call protocol requires that the function has a ``__name__`` attribute which is of type ``str`` (not a subclass). Furthermore, this must be idempotent in the sense that getting the ``__name__`` attribute twice in a row must return exactly the same Python object. This implies that it cannot be a temporary object, it must be stored somewhere. This is required because ``PyEval_GetFuncName`` uses a borrowed reference to the ``__name__`` attribute. Generic API functions --------------------- This section lists the new public API functions or macros dealing with the C call protocol. - ``int PyCCall_Check(PyObject *op)``: return true if ``op`` implements the C call protocol. All the functions and macros below apply to any instance supporting the C call protocol. In other words, ``PyCCall_Check(func)`` must be true. - ``PyObject *PyCCall_Call(PyObject *func, PyObject *args, PyObject *kwds)``: call ``func`` with positional arguments ``args`` and keyword arguments ``kwds`` (``kwds`` may be NULL). This function is meant to be put in the ``tp_call`` slot. - ``PyObject *PyCCall_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs, PyObject *kwds)``: call ``func`` with ``nargs`` positional arguments given by ``args[0]``, …, ``args[nargs-1]``. The parameter ``kwds`` can be NULL (no keyword arguments), a dict with ``name:value`` items or a tuple with keyword names. In the latter case, the keyword values are stored in the ``args`` array, starting at ``args[nargs]``. Macros to access the ``PyCCallRoot`` and ``PyCCallDef`` structures: - ``PyCCallRoot *PyCCall_CCALLROOT(PyObject *func)``: pointer to the ``PyCCallRoot`` structure inside ``func``. - ``PyCCallDef *PyCCall_CCALLDEF(PyObject *func)``: shorthand for ``PyCCall_CCALLROOT(func)->cr_ccall``. - ``PyCCallDef *PyCCall_FLAGS(PyObject *func)``: shorthand for ``PyCCall_CCALLROOT(func)->cr_ccall->cc_flags``. - ``PyObject *PyCCall_SELF(PyOject *func)``: shorthand for ``PyCCall_CCALLROOT(func)->cr_self``. Generic getters, meant to be put into the ``tp_getset`` array: - ``PyObject *PyCCall_GenericGetParent(PyObject *func, void *closure)``: return ``cc_parent``. Raise ``AttributeError`` if ``cc_parent`` is NULL. - ``PyObject *PyCCall_GenericGetQualname(PyObject *func, void *closure)``: return a string suitable for using as ``__qualname__``. This uses the ``__qualname__`` of ``cc_parent`` if possible. It also uses the ``__name__`` attribute. Profiling --------- The profiling events ``c_call``, ``c_return`` and ``c_exception`` are only generated when calling actual instances of ``builtin_function_or_method`` or ``method_descriptor``. This is done for simplicity and also for backwards compatibility (such that the profile function does not receive objects that it does not recognize). In a future PEP, we may extend C-level profiling to arbitrary classes implementing the C call protocol. Changes to built-in functions and methods ========================================= The reference implementation of this PEP changes the existing classes ``builtin_function_or_method`` and ``method_descriptor`` to use the C call protocol. In fact, those two classes are almost merged: the implementation becomes very similar, but they remain separate classes (mostly for backwards compatibility). The ``PyCCallDef`` structure is simply stored as part of the object structure. Both classes use ``PyCFunctionObject`` as object structure. This is the new layout for both classes:: typedef struct { PyObject_HEAD PyCCallDef *m_ccall; PyObject *m_self; /* Passed as 'self' arg to the C function */ PyCCallDef _ccalldef; /* Storage for m_ccall */ PyObject *m_name; /* __name__; str object (not NULL) */ PyObject *m_module; /* __module__; can be anything */ const char *m_doc; /* __text_signature__ and __doc__ */ PyObject *m_weakreflist; /* List of weak references */ } PyCFunctionObject; For functions of a module and for unbound methods of extension types, ``m_ccall`` points to the ``_ccalldef`` field. For bound methods, ``m_ccall`` points to the ``PyCCallDef`` of the unbound method. **NOTE**: the new layout of ``method_descriptor`` changes it such that it no longer starts with ``PyDescr_COMMON``. This is purely an implementation detail and it should cause few (if any) compatibility problems. C API functions --------------- The following function is added (also to the stable ABI [#pep384]_): - ``PyObject * PyCFunction_ClsNew(PyTypeObject *cls, PyMethodDef *ml, PyObject *self, PyObject *module, PyObject *parent)``: create a new object with object structure ``PyCFunctionObject`` and class ``cls``. The entries of the ``PyMethodDef`` structure are used to construct the new object, but the pointer to the ``PyMethodDef`` structure is not stored. The flags for the C call protocol are automatically determined in terms of ``ml->ml_flags``, ``self`` and ``parent``. The existing functions ``PyCFunction_New``, ``PyCFunction_NewEx`` and ``PyDescr_NewMethod`` are implemented in terms of ``PyCFunction_ClsNew``. The undocumented functions ``PyCFunction_GetFlags`` and ``PyCFunction_GET_FLAGS`` are deprecated. They are still artificially supported by storing the original ``METH_...`` flags in a bitfield inside ``cc_flags``. Despite the fact that ``PyCFunction_GetFlags`` is technically part of the stable ABI [#pep384]_, it is highly unlikely to be used that way: first of all, it is not even documented. Second, the flag ``METH_FASTCALL`` is not part of the stable ABI but it is very common (because of Argument Clinic). So, if one cannot support ``METH_FASTCALL``, it is hard to imagine a use case for ``PyCFunction_GetFlags``. The fact that ``PyCFunction_GET_FLAGS`` and ``PyCFunction_GetFlags`` are not used at all by CPython outside of ``Objects/call.c`` further shows that these functions are not particularly useful. Inheritance =========== Extension types inherit the type flag ``Py_TPFLAGS_HAVE_CCALL`` and the value ``tp_ccalloffset`` from the base class, provided that they implement ``tp_call`` and ``tp_descr_get`` the same way as the base class. Heap types never inherit the C call protocol because that would not be safe (heap types can be changed dynamically). Performance =========== This PEP should not impact the performance of existing code (in the positive or negative sense). It is meant to allow efficient new code to be written, not to make existing code faster. Here are a few pointers to the ``python-dev`` mailing list where performance improvements are discussed: - https://mail.python.org/pipermail/python-dev/2018-July/154571.html - https://mail.python.org/pipermail/python-dev/2018-July/154740.html - https://mail.python.org/pipermail/python-dev/2018-July/154775.html Stable ABI ========== The function ``PyCFunction_ClsNew`` is added to the stable ABI [#pep384]_. None of the functions, structures or constants dealing with the C call protocol are added to the stable ABI. There are two reasons for this: first of all, the most useful feature of the C call protocol is probably the ``METH_FASTCALL`` calling convention. Given that this is not even part of the public API (see also PEP 579, issue 6), it would be strange to add anything else from the C call protocol to the stable ABI. Second, we want the C call protocol to be extensible in the future. By not adding anything to the stable ABI, we are free to do that without restrictions. Backwards compatibility ======================= There is no difference at all for the Python interface, nor for the documented C API (in the sense that all functions remain supported with the same functionality). The only potential breakage is with C code which accesses the internals of ``PyCFunctionObject`` and ``PyMethodDescrObject``. We expect very few problems because of this. Rationale ========= Why is this better than PEP 575? -------------------------------- One of the major complaints of PEP 575 was that is was coupling functionality (the calling and introspection protocol) with the class hierarchy: a class could only benefit from the new features if it was a subclass of ``base_function``. It may be difficult for existing classes to do that because they may have other constraints on the layout of the C object structure, coming from an existing base class or implementation details. For example, ``functools.lru_cache`` cannot implement PEP 575 as-is. It also complicated the implementation precisely because changes were needed both in the implementation details and in the class hierarchy. The current PEP does not have these problems. Why store the function pointer in the instance? ----------------------------------------------- The actual information needed for calling an object is stored in the instance (in the ``PyCCallDef`` structure) instead of the class. This is different from the ``tp_call`` slot or earlier attempts at implementing a ``tp_fastcall`` slot [#bpo29259]_. The main use case is built-in functions and methods. For those, the C function to be called does depend on the instance. Note that the current protocol makes it easy to support the case where the same C function is called for all instances: just use a single static ``PyCCallDef`` structure for every instance. Why CCALL_OBJCLASS? ------------------- The flag ``CCALL_OBJCLASS`` is meant to support various cases where the class of a ``self`` argument must be checked, such as:: >>> list.append({}, None) Traceback (most recent call last): File "", line 1, in TypeError: append() requires a 'list' object but received a 'dict' >>> list.__len__({}) Traceback (most recent call last): File "", line 1, in TypeError: descriptor '__len__' requires a 'list' object but received a 'dict' >>> float.__dict__["fromhex"](list, "0xff") Traceback (most recent call last): File "", line 1, in TypeError: descriptor 'fromhex' for type 'float' doesn't apply to type 'list' In the reference implementation, only the first of these uses the new code. The other examples show that these kind of checks appear in multiple places, so it makes sense to add generic support for them. Why CCALL_SELFARG? ------------------ The flag ``CCALL_SELFARG`` and the concept of self slicing are needed to support methods: the C function should not care whether it is called as unbound method or as bound method. In both cases, there should be a ``self`` argument and this is simply the first positional argument of an unbound method call. For example, ``list.append`` is a ``METH_O`` method. Both the calls ``list.append([], 42)`` and ``[].append(42)`` should translate to the C call ``list_append([], 42)``. Thanks to the proposed C call protocol, we can support this in such a way that both the unbound and the bound method share a ``PyCCallDef`` structure (with the ``CCALL_SELFARG`` flag set). So, ``CCALL_SELFARG`` has two advantages: there is no extra layer of indirection for calling methods and constructing bound methods does not require setting up a ``PyCCallDef`` structure. Another minor advantage is that we could make the error messages for a wrong call signature more uniform between Python methods and built-in methods. In the following example, Python is undecided whether a method takes 2 or 3 arguments:: >>> class List(list): ... def myappend(self, item): ... self.append(item) >>> List().myappend(1, 2) Traceback (most recent call last): File "", line 1, in TypeError: myappend() takes 2 positional arguments but 3 were given >>> List().append(1, 2) Traceback (most recent call last): File "", line 1, in TypeError: append() takes exactly one argument (2 given) It is currently impossible for ``PyCFunction_Call`` to know the actual number of user-visible arguments since it cannot distinguish at runtime between a function (without ``self`` argument) and a bound method (with ``self`` argument). The ``CCALL_SELFARG`` flag makes this difference explicit. Replacing tp_print ------------------ We repurpose ``tp_print`` as ``tp_ccalloffset`` because this makes it easier for external projects to backport the C call protocol to earlier Python versions. In particular, the Cython project has shown interest in doing that (see https://mail.python.org/pipermail/python-dev/2018-June/153927.html). Alternative suggestions ======================= PEP 576 is an alternative approach to solving the same problem as this PEP. See https://mail.python.org/pipermail/python-dev/2018-July/154238.html for comments on the difference between PEP 576 and PEP 580. Reference implementation ======================== The reference implementation can be found at https://github.com/jdemeyer/cpython/tree/pep580 References ========== .. [#pep384] Löwis, PEP 384 – Defining a Stable ABI, https://www.python.org/dev/peps/pep-0384/ .. [#bpo29259] Add tp_fastcall to PyTypeObject: support FASTCALL calling convention for all callable objects, https://bugs.python.org/issue29259 Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: