Description

markshannon

opened on Jul 25, 2022

Issue body actions

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

• gh-95245: Document Py_TPFLAGS_PREHEADER #135861

Reacted by Christian Heimes, Ken Jin, Erlend E. Aasland and Donghee Na

Activity

1. markshannon

   added

   performancePerformance or resource usagePerformance 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

    on Sep 10, 2022

    Member

    More actions

    Is there anything left to do here?
13. iritkatriel

    added

    pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

    on Sep 10, 2022
14. markshannon commented on Sep 12, 2022

    

    markshannon

    on Sep 12, 2022

    MemberAuthor

    More actions

    Nothing more to do.
15. markshannon

    closed this as completedon Sep 12, 2022
16. AA-Turner commented on Jun 24, 2025

    

    AA-Turner

    on Jun 24, 2025

    Member

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    Re-opening per discussion in #135755.

    Reacted by Sergey Miryanov
17. AA-Turner

    reopened this on Jun 24, 2025
18. ZeroIntensity

    removed

    pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

    on Jun 24, 2025
19. encukou commented on Jun 24, 2025

    

    encukou

    on Jun 24, 2025

    Member

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

    Reacted by Peter Bierma
20. picnixz

    added

    interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

    on Jun 24, 2025
21. sergey-miryanov commented on Jun 24, 2025

    

    sergey-miryanov

    on Jun 24, 2025

    Contributor

    More actions

    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

Issue actions