PEP 699: Remove private dict version field added in PEP 509 (#2811)

Remove private dict version field added in PEP 509

.github/CODEOWNERS

@@ -579,6 +579,7 @@ pep-0695.rst  @gvanrossum
 pep-0696.rst  @jellezijlstra
 pep-0697.rst  @encukou
 pep-0698.rst  @jellezijlstra
+pep-0699.rst  @Fidget-Spinner
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pep-0699.rst

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+PEP: 699
+Title: Remove private dict version field added in PEP 509
+Author: Ken Jin <kenjin@python.org>
+Discussions-To:
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 03-Oct-2022
+Python-Version: 3.12
+Post-History:
+Replaces: 509
+Resolution:
+
+
+
+Abstract
+========
+
+:pep:`509` introduced a private ``ma_version_tag`` field for dictionaries to
+allow optimizations in CPython and extension libraries. This PEP proposes to
+rescind :pep:`509` and declare the field an implementation detail, as it has
+already been superseded by alternatives. This will further allow the field to
+be reused for future optimization.
+
+
+Motivation
+==========
+
+:pep:`509` introduced the ``ma_version_tag`` field to dictionaries. This 64-bit
+field stored a version counter that updates every time a dictionary was
+modified. The original proposal was to use this version counter as a
+guard for optimizations.
+
+Since CPython 3.11, this field has become unused by internal optimization
+efforts. :pep:`659`-specialized instructions use other methods of verifying
+that certain optimisations are safe.
+
+To enable further optimizations in CPython, this PEP proposes that the
+``ma_version_tag`` field no longer conform to the :pep:`509` specification.
+This will allow the CPython developers to store other optimization information,
+such as dictionary write watchers.
+
+
+Rationale
+=========
+This PEP does not specify what the field may be used for in the future. This is
+intentional, as implementation details are subject to change, and the field
+should be used only for internal consumption by CPython.
+
+
+Specification
+=============
+
+This specification rescinds that in :pep:`509`. The ``ma_version_tag`` field of
+the Python :class:`dict` class is declared to be an internal implementation
+detail and may be removed altogether, or may have a different representation.
+C extensions should not rely on this field.
+
+
+Backwards Compatibility
+=======================
+
+Certain extensions use ``ma_version_tag`` for fast dictionary or globals
+lookups. For example,
+`Cython uses the field for fast dynamic module variable lookups <https://github.com/cython/cython/blob/169876872f3cb6198971a1db07e5b8a9d12b3dac/Cython/Utility/ObjectHandling.c#L1556>`_.
+
+Since this field is private and makes no guarantees on backwards compatibility,
+this PEP proposes to repeal :pep:`509` immediately within the next CPython
+version. A function for fast cached dictionary lookups may be provided as an
+alternative feature to replace directly accessing ``ma_version_tag``.
+
+The biggest user the author could find for this field was Cython.
+Discussions with a Cython maintainer indicated that
+`removing support for it from Cython is trivial <https://github.com/faster-cpython/ideas/issues/461#issuecomment-1250358596>`_.
+
+
+Security Implications
+=====================
+
+:pep:`509` was concerned with integer overflow. However, this PEP does not
+introduce any additional security concerns.
+
+
+Rejected Ideas
+==============
+
+A possible alternative is to preserve the field for backwards compatibility.
+This PEP rejects that alternative as future optimizations will consume more
+memory, and the field was always considered private and undocumented aside
+from the PEP, with no backward compatibility guarantees. Dictionaries in Python
+are commonly used, and any increase in their memory consumption will adversely
+affect Python’s performance.
+
+
+Special Thanks
+==============
+
+Thanks to C.A.M. Gerlach for edits and wording changes to this document.
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.
+
+