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Drop Py_IMMORTAL_CONST_REFCOUNTS and make immortal objects internal.
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‎Doc/c-api/refcounting.rst‎

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@@ -119,47 +119,3 @@ The following functions or macros are only for use within the interpreter core:
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:c:func:`_Py_Dealloc`, :c:func:`_Py_ForgetReference`, :c:func:`_Py_NewReference`,
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as well as the global variable :c:data:`_Py_RefTotal`.
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.. _immortal-objects:
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Immortal Objects
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================
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"Immortal" objects are those that are expected to never be deallocated
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by the runtime (due to their reference count reaching 0). In the public
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C-API examples of such objects includes the singletons and many builtin
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types. For such objects the reference count is essentially irrelevant.
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Immortal objects are especially useful if otherwise immutable.
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Note that for now the API for immortal objects is not available
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for general use, by default. Users of the public C-API (but not
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the limited API) may opt in by defining ``_Py_IMMORTAL_OBJECTS``.
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This API should not be considered stable yet.
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.. c:function:: int _PyObject_IsImmortal(PyObject *o)
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Return non-zero if the object is immortal.
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.. versionadded:: 3.10
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.. c:function:: void _PyObject_SetImmortal(PyObject *o)
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Mark an object as immortal.
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.. versionadded:: 3.10
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Also see :c:macro:`_PyObject_HEAD_IMMORTAL_INIT` and
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:c:macro:`_PyVarObject_HEAD_IMMORTAL_INIT`.
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.. _immutable-refcounts:
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Immutable Refcounts
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-------------------
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If ``Py_IMMORTAL_CONST_REFCOUNTS`` is defined then the following
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happens:
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* the immortal objects API is enabled
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* the runtime never changes reference counts for immortal objects
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This mode can help with copy-on-write semantics when forking.
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‎Doc/c-api/structures.rst‎

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@@ -98,7 +98,6 @@ the definition of all other Python objects.
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.. c:function:: void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)
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Set the object *o* reference counter to *refcnt*.
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:ref:`immortal-objects` are not affected.
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.. versionadded:: 3.9
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@@ -136,39 +135,6 @@ the definition of all other Python objects.
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1, type, size,
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.. c:macro:: _PyObject_HEAD_IMMORTAL_INIT(type)
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This is a macro which expands to initialization values for a new
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:c:type:`PyObject` type. It makes the object
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:ref:`immortal <immortal-objects>`. This macro expands to::
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_PyObject_EXTRA_INIT
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_PyObject_IMMORTAL_INIT_REFCNT, type,
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For now you must opt in to use this by defining
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``_Py_IMMORTAL_OBJECTS``.
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.. versionadded:: 3.10
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.. c:macro:: PyVarObject_HEAD_IMMORTAL_INIT(type, size)
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This is a macro which expands to initialization values for a new
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:c:type:`PyVarObject` type, including the :attr:`ob_size` field.
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It makes the object :ref:`immortal <immortal-objects>`. This
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macro expands to::
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_PyObject_EXTRA_INIT
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_PyObject_IMMORTAL_INIT_REFCNT, type, size,
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This is especially useful for static types.
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For now you must opt in to use this by defining
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``_Py_IMMORTAL_OBJECTS``.
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.. versionadded:: 3.10
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Implementing functions and methods
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----------------------------------
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‎Include/cpython/object.h‎

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@@ -376,60 +376,6 @@ PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
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} while (0)
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/* Immortal Objects
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*
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* An "immortal" object is one for which Py_DECREF() will never try
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* to deallocate it. */
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#if defined(Py_IMMORTAL_CONST_REFCOUNTS) || defined(Py_BUILD_CORE)
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#define _Py_IMMORTAL_OBJECTS 1
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#endif
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#ifdef _Py_IMMORTAL_OBJECTS
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/* The implementation-independent API is only the following functions: */
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PyAPI_FUNC(int) _PyObject_IsImmortal(PyObject *);
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PyAPI_FUNC(void) _PyObject_SetImmortal(PyObject *);
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/* In the actual implementation we set the refcount to some positive
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* value that we would never expect to be reachable through use of
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* Py_INCREF() in a program.
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*
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* The only parts that should be used directly are the two
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* _Py*Object_HEAD_IMMORTAL_INIT() macros.
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*/
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/* _PyObject_IMMORTAL_BIT is the bit in the refcount value (Py_ssize_t)
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* that we use to mark an object as immortal. It shouldn't ever be
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* part of the public API.
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*
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* The GC bit-shifts refcounts left by two, and after that shift we still
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* need this to be >> 0, so leave three high zero bits (the sign bit and
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* room for a shift of two.) */
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#define _PyObject_IMMORTAL_BIT (1LL << (8 * sizeof(Py_ssize_t) - 4))
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/* _PyObject_IMMORTAL_INIT_REFCNT is the initial value we use for
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* immortal objects. It shouldn't ever be part of the public API. */
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#ifdef Py_IMMORTAL_CONST_REFCOUNTS
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#define _PyObject_IMMORTAL_INIT_REFCNT \
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_PyObject_IMMORTAL_BIT
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#else
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// We leave plenty of room to preserve _PyObject_IMMORTAL_BIT.
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#define _PyObject_IMMORTAL_INIT_REFCNT \
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(_PyObject_IMMORTAL_BIT + (_PyObject_IMMORTAL_BIT / 2))
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#endif
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/* These macros are drop-in replacements for the corresponding
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* Py*Object_HEAD_INIT() macros. They will probably become
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* part of the public API. */
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#define _PyObject_HEAD_IMMORTAL_INIT(type) \
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{ _PyObject_EXTRA_INIT _PyObject_IMMORTAL_INIT_REFCNT, type },
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#define _PyVarObject_HEAD_IMMORTAL_INIT(type, size) \
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{ PyObject_HEAD_IMMORTAL_INIT(type) size },
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#endif /* defined(_Py_IMMORTAL_OBJECTS) */
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PyAPI_DATA(PyTypeObject) _PyNone_Type;
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PyAPI_DATA(PyTypeObject) _PyNotImplemented_Type;
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‎Include/internal/pycore_object.h‎

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@@ -30,6 +30,55 @@ _PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
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extern void _PyType_InitCache(PyInterpreterState *interp);
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/* Immortal Objects
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*
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* An "immortal" object is one for which Py_DECREF() will never try
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* to deallocate it.
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*
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* At the moment this API is strictly internal. However, if it proves
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* helpful for extension authors we may move it to the public API. */
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#define _Py_IMMORTAL_OBJECTS 1
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/* The implementation-independent API is only the following functions: */
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PyAPI_FUNC(int) _PyObject_IsImmortal(PyObject *);
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PyAPI_FUNC(void) _PyObject_SetImmortal(PyObject *);
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/* In the actual implementation we set the refcount to some positive
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* value that we would never expect to be reachable through use of
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* Py_INCREF() in a program.
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*
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* The only parts that should be used directly are the two
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* _Py*Object_HEAD_IMMORTAL_INIT() macros.
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*/
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/* _PyObject_IMMORTAL_BIT is the bit in the refcount value (Py_ssize_t)
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* that we use to mark an object as immortal. It shouldn't ever be
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* part of the public API.
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*
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* The GC bit-shifts refcounts left by two, and after that shift we still
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* need this to be >> 0, so leave three high zero bits (the sign bit and
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* room for a shift of two.) */
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#define _PyObject_IMMORTAL_BIT (1LL << (8 * sizeof(Py_ssize_t) - 4))
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/* _PyObject_IMMORTAL_INIT_REFCNT is the initial value we use for
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* immortal objects. It shouldn't ever be part of the public API.
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*
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* We leave plenty of room to preserve _PyObject_IMMORTAL_BIT. */
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#define _PyObject_IMMORTAL_INIT_REFCNT \
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(_PyObject_IMMORTAL_BIT + (_PyObject_IMMORTAL_BIT / 2))
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/* These macros are drop-in replacements for the corresponding
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* Py*Object_HEAD_INIT() macros. They will probably become
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* part of the public API. */
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#define _PyObject_HEAD_IMMORTAL_INIT(type) \
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{ _PyObject_EXTRA_INIT _PyObject_IMMORTAL_INIT_REFCNT, type },
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#define _PyVarObject_HEAD_IMMORTAL_INIT(type, size) \
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{ PyObject_HEAD_IMMORTAL_INIT(type) size },
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/* end Immortal Objects */
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/* Inline functions trading binary compatibility for speed:
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_PyObject_Init() is the fast version of PyObject_Init(), and
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_PyObject_InitVar() is the fast version of PyObject_InitVar().

‎Include/object.h‎

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{
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#ifdef Py_REF_DEBUG
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_Py_RefTotal++;
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#endif
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#ifdef Py_IMMORTAL_CONST_REFCOUNTS
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if (_py_is_immortal(op)) {
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return;
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}
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#endif
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op->ob_refcnt++;
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}
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{
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#ifdef Py_REF_DEBUG
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_Py_RefTotal--;
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#endif
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#ifdef Py_IMMORTAL_CONST_REFCOUNTS
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if (_py_is_immortal(op)) {
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return;
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}
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#endif
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if (--op->ob_refcnt != 0) {
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#ifdef Py_REF_DEBUG
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#endif
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#if defined(Py_LIMITED_API) && \
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(defined(_Py_IMMORTAL_OBJECTS) || defined(Py_IMMORTAL_CONST_REFCOUNTS))
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#error "the immortal objects API is not available in the limited API"
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#endif
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#ifdef Py_IMMORTAL_CONST_REFCOUNTS
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// We need this function since _PyObject_IMMORTAL_BIT is defined
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// in cpython/object.h and not available for use above. Otherwise
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// we wouldn't need this function.
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static inline int _py_is_immortal(PyObject *op)
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{
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return (op->ob_refcnt & _PyObject_IMMORTAL_BIT) != 0;
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}
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#endif
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static inline int
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PyType_HasFeature(PyTypeObject *type, unsigned long feature)
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{

‎Misc/NEWS.d/next/Core and Builtins/2021-03-11-13-03-15.bpo-40255._1LfiG.rst‎

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There is now internal support for "immortal" objects. Those are objects
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that will never be deleted, like the singletons and static types. This has
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benefits for the C-API and subinterpreters, as well as allowing for better
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copy-on-write behavior for forking (if Py_IMMORTAL_CONST_REFCOUNTS is used).
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The feature is currently intended for internal use, though you can try it
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out by building with _Py_IMMORTAL_OBJECTS defined.
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There is now internal support for "immortal" objects (with a "private"
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C-API). Those are objects that will never be deleted, like the
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singletons and static types. This will benefit the C-API and
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subinterpreters. For now the API is currently intended only
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for internal use.

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