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Reduce the size of plain Python objects from 8 to 6 machine words. #95245

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Reduce the size of plain Python objects from 8 to 6 machine words.#95245
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markshannon
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interpreter-core(Objects, Python, Grammar, and Parser dirs)performancePerformance or resource usage

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@markshannon
markshannon
opened on Jul 25, 2022

Currently a plain Python object has the following fields:

  • values
  • (managed) __dict__
  • GC 1
  • GC 2
  • refcount
  • __class__
  • __weakrefs__
  • WASTED

The WASTED field is present because, due to alignment, we must allocate a multiple of 2 words.
We can reduce this to 6 by merging the values and __dict__ pointers into one (only one can be valid at any time),
and then moving the __weakrefs__ field into the empty space.

  • __weakrefs__
  • values or __dict__
  • GC 1
  • GC 2
  • refcount
  • __class__

See faster-cpython/ideas#125 for full discussion

Linked PRs

Activity

  1. markshannon
    added
    performancePerformance or resource usage
    on Jul 25, 2022
  2. markshannon
    changed the title [-]Reduce the size of plain objects from 8 to 6 machines words.[/-] [+]Reduce the size of plain Python objects from 8 to 6 machine words.[/+] on Jul 25, 2022
  3. markshannon
    moved this to Todo in Fancy CPython Boardon Jul 25, 2022
  4. markshannon
    added this to Fancy CPython Boardon Jul 25, 2022
  5. mdboom
    moved this from Todo to In Progress in Fancy CPython Boardon Jul 26, 2022
  6. mdboom
    assigned
    markshannon
    on Jul 26, 2022
  7. markshannon
    added a commit that references this issue on Aug 1, 2022

    GH-95245: Store object values and dict pointers in single tagged poin…

    de388c0
  8. markshannon
    moved this from In Progress to In Review in Fancy CPython Boardon Aug 15, 2022
  9. markshannon
    added a commit that references this issue on Aug 16, 2022

    GH-95245: Move weakreflist into the pre-header. (GH-95996)

    5a8c158
  10. markshannon
    moved this from In Review to Done in Fancy CPython Boardon Aug 22, 2022
  11. markshannon
    added a commit that references this issue on Aug 30, 2022

    GH-95245: Document use of `MANAGED` flags instead of offsets. (GH-96044)

    0f733ff

  12. iritkatriel commented on Sep 10, 2022

    @iritkatriel
    iritkatriel
    on Sep 10, 2022
    Member

    Is there anything left to do here?

  13. iritkatriel
    added
    pendingThe issue will be closed if no feedback is provided
    on Sep 10, 2022

  14. markshannon commented on Sep 12, 2022

    @markshannon
    markshannon
    on Sep 12, 2022
    MemberAuthor

    Nothing more to do.

  15. markshannon
    closed this as completedon Sep 12, 2022

  16. AA-Turner commented on Jun 24, 2025

    @AA-Turner
    AA-Turner
    on Jun 24, 2025
    Member

    Re-opening per discussion in #135755.

  17. AA-Turner
    reopened this on Jun 24, 2025
  18. ZeroIntensity
    removed
    pendingThe issue will be closed if no feedback is provided
    on Jun 24, 2025

  19. encukou commented on Jun 24, 2025

    @encukou
    encukou
    on Jun 24, 2025
    Member

    There's no documentation about Py_TPFLAGS_PREHEADER. What's it for? How should users use it?

  20. picnixz
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Jun 24, 2025

  21. sergey-miryanov commented on Jun 24, 2025

    @sergey-miryanov
    sergey-miryanov
    on Jun 24, 2025
    Contributor

    As I understand it, users are not supposed to use this flag. It is only used for internal purposes.

  22. encukou
    added a commit that references this issue on Aug 25, 2025

    gh-95245: Document Py_TPFLAGS_PREHEADER (GH-135861)

    5baa7a0
  23. encukou
    closed this as completedon Aug 25, 2025
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