python-peps/pep-0670.rst

PEP: 670
Title: Convert macros to functions in the Python C API
Author: Erlend Egeberg Aasland <erlend.aasland@protonmail.com>,
        Victor Stinner <vstinner@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 19-Oct-2021
Python-Version: 3.11


Abstract
========

Convert macros to static inline functions or regular functions.

Remove the return value of macros having a return value, whereas they
should not, to detect bugs in C extensions when the C API is misused.

Some function arguments are still casted to ``PyObject*`` to prevent
emitting new compiler warnings.


Rationale
=========

The use of macros may have unintended adverse effects that are hard to
avoid, even for the experienced C developers. Some issues have been
known for years, while others have been discovered recently in Python.
Working around macro pitfalls makes the macro coder harder to read and
to maintain.

Converting macros to functions has multiple advantages:

* By design, functions don't have macro pitfalls.
* Arguments type and return type are well defined.
* Debuggers and profilers can retrieve the name of inlined functions.
* Debuggers can put breakpoints on inlined functions.
* Variables have a well defined scope.
* Code is usually easier to read and to maintain than similar macro
  code.  Functions don't need the following workarounds for macro
  pitfalls:

  * Add parentheses around arguments.
  * Use line continuation characters if the function is written on
    multiple lines.
  * Add commas to execute multiple expressions.
  * Use ``do { ... } while (0)`` to write multiple statements.


Macro Pitfalls
==============

The `GCC documentation
<https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html>`_ lists several
common macro pitfalls:

- Misnesting;
- Operator precedence problems;
- Swallowing the semicolon;
- Duplication of side effects;
- Self-referential macros;
- Argument prescan;
- Newlines in arguments.


Performance and inlining
========================

Static inline functions is a feature added to the C99 standard. In 2021,
C compilers can inline them and have efficient heuristics to decide if a
function should be inlined or not.

When a C compiler decides to not inline, there is likely a good reason.
For example, inlining would reuse a register which require to
save/restore the register value on the stack and so increase the stack
memory usage or be less efficient.


Debug mode
----------

When Python is built in debug mode, most compiler optimizations are
disabled.  For example, Visual Studio disables inlining. Benchmarks must
not be run on a Python debug build, only on release build: using LTO and
PGO is recommended for reliable benchmarks. PGO helps the compiler to
decide if function should be inlined or not.


Force inlining
--------------

If a developer is convinced to know better machine code than C compiler,
which is very unlikely, it is still possible to mark the function with
the ``Py_ALWAYS_INLINE`` macro. This macro uses
``__attribute__((always_inline))`` with GCC and Clang, and
``__forceinline`` with MSC.

So far, previous attempts to use ``Py_ALWAYS_INLINE`` didn't show any
benefit and were abandoned. See for example: `bpo-45094
<https://bugs.python.org/issue45094>`_: "Consider using
``__forceinline`` and ``__attribute__((always_inline))`` on static
inline functions (``Py_INCREF``, ``Py_TYPE``) for debug builds".

When the ``Py_INCREF()`` macro was converted to a static inline
functions in 2018 (`commit
<https://github.com/python/cpython/commit/2aaf0c12041bcaadd7f2cc5a54450eefd7a6ff12>`__),
it was decided not to force inlining. The machine code was analyzed with
multiple C compilers and compiler options: ``Py_INCREF()`` was always
inlined without having to force inlining. The only case where it was not
inlined was debug builds, but this is acceptable for a debug build. See
discussion in the `bpo-35059 <https://bugs.python.org/issue35059>`_:
"Convert Py_INCREF() and PyObject_INIT() to inlined functions".


Prevent inlining
----------------

On the other side, the ``Py_NO_INLINE`` macro can be used to prevent
inlining.  It is useful to reduce the stack memory usage, it is
especially useful on LTO+PGO builds which heavily inlines code: see
`bpo-33720 <https://bugs.python.org/issue33720>`_. This macro uses
``__attribute__ ((noinline))`` with GCC and Clang, and
``__declspec(noinline)`` with MSC.


Convert macros and static inline functions to regular functions
---------------------------------------------------------------

There are projects embedding Python or using Python which cannot use
macros and static inline functions. For example, projects using
programming languages other than C and C++. There are also projects
written in C which make the deliberate choice of only getting
``libpython`` symbols (functions and variables).

Converting macros and static inline functions to regular functions make
these functions accessible to these projects.


Specification
=============

Convert macros to static inline functions
-----------------------------------------

Most macros should be converted to static inline functions to prevent
`macro pitfalls`_.

The following macros should not be converted:

* Empty macros. Example: ``#define Py_HAVE_CONDVAR``.
* Macros only defining a number, even if a constant with a well defined
  type can better. Example: ``#define METH_VARARGS 0x0001``.
* Compatibility layer for different C compilers, C language extensions,
  or recent C features.
  Example: ``#define Py_ALWAYS_INLINE __attribute__((always_inline))``.


Convert static inline functions to regular functions
----------------------------------------------------

Converting static inline functions to regular functions give access to
these functions for projects which cannot use macros and static inline
functions.

The performance impact of such conversion should be measured with
benchmarks.  If there is a significant slowdown, there should be a good
reason to do the conversion. One reason can be hiding some
implementation details.

Using static inline functions in the internal C API is fine: the
internal C API exposes implemenation details by design and should not be
used outside Python.

Cast to PyObject*
-----------------

When a macro is converted to a function and the macro casts its
arguments to ``PyObject*``, the new function comes with a new macro
which cast arguments to ``PyObject*`` to prevent emitting new compiler
warnings. So the converted functions still accept pointers to structures
inheriting from ``PyObject`` (ex: ``PyTupleObject``).

For example, the ``Py_TYPE(obj)`` macro casts its ``obj`` argument to
``PyObject*``::

    #define _PyObject_CAST_CONST(op) ((const PyObject*)(op))

    static inline PyTypeObject* _Py_TYPE(const PyObject *ob) {
        return ob->ob_type;
    }
    #define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))

The undocumented private ``_Py_TYPE()`` function must not be called
directly. Only the documented public ``Py_TYPE()`` macro must be used.

Later, the cast can be removed on a case by case basis, but it is out of
this PEP scope.

Remove the return value
-----------------------

When a macro is implemented as an expression, it has an implicit return
value. In some cases, the macro must not have a return value and can be
misused in third party C extensions. See `bpo-30459
<https://bugs.python.org/issue30459>`_ for the example of
``PyList_SET_ITEM()`` and ``PyCell_SET()`` macros. It is not easy to
notice this issue while reviewing a macro code.

These macros are converted to functions using the ``void`` return type
to remove their return value. Removing the return value detects bugs in
C extensions when the C API is misused.


Backwards Compatibility
=======================

Removing the return value of macros is an incompatible change made on
purpose: see the `Remove the return value`_ section.


Rejected Ideas
==============

Keep macros, but fix some macro issues
--------------------------------------

Converting macros to functions is not needed to `remove the return
value`_: casting a macro return value to ``void`` also fix the issue.
For example, the ``PyList_SET_ITEM()`` macro was already fixed like
that.

Macros are always "inlined" with any C compiler.

The duplication of side effects can be worked around in the caller of
the macro.

People using macros should be considered "consenting adults". People who
feel unsafe with macros should simply not use them.

Example of hard to read macros
==============================

_Py_NewReference()
------------------

Example showing the usage of an ``#ifdef`` inside a macro.

Python 3.7 macro (simplified code)::

    #ifdef COUNT_ALLOCS
    #  define _Py_INC_TPALLOCS(OP) inc_count(Py_TYPE(OP))
    #  define _Py_COUNT_ALLOCS_COMMA  ,
    #else
    #  define _Py_INC_TPALLOCS(OP)
    #  define _Py_COUNT_ALLOCS_COMMA
    #endif /* COUNT_ALLOCS */

    #define _Py_NewReference(op) (                   \
        _Py_INC_TPALLOCS(op) _Py_COUNT_ALLOCS_COMMA  \
        Py_REFCNT(op) = 1)

Python 3.8 function (simplified code)::

    static inline void _Py_NewReference(PyObject *op)
    {
        _Py_INC_TPALLOCS(op);
        Py_REFCNT(op) = 1;
    }

PyObject_INIT()
---------------

Example showing the usage of commas in a macro.

Python 3.7 macro::

    #define PyObject_INIT(op, typeobj) \
        ( Py_TYPE(op) = (typeobj), _Py_NewReference((PyObject *)(op)), (op) )

Python 3.8 function (simplified code)::

    static inline PyObject*
    _PyObject_INIT(PyObject *op, PyTypeObject *typeobj)
    {
        Py_TYPE(op) = typeobj;
        _Py_NewReference(op);
        return op;
    }

    #define PyObject_INIT(op, typeobj) \
        _PyObject_INIT(_PyObject_CAST(op), (typeobj))

The function doesn't need the line continuation character. It has an
explicit ``"return op;"`` rather than a surprising ``", (op)"`` at the
end of the macro.  It uses one short statement per line, rather than a
single long line. Inside the function, the *op* argument has a well
defined type: ``PyObject*``.


Discussions
===========

* `bpo-45490 <https://bugs.python.org/issue45490>`_:
  [meta][C API] Avoid C macro pitfalls and usage of static inline
  functions (October 2021).
* `What to do with unsafe macros
  <https://discuss.python.org/t/what-to-do-with-unsafe-macros/7771>`_
  (March 2021).
* `bpo-43502 <https://bugs.python.org/issue43502>`_:
  [C-API] Convert obvious unsafe macros to static inline functions
  (March 2021).


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.