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Reduce the size of plain Python objects from 8 to 6 machine words. #95245
Description
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
Activity
[-]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.[/+]GH-95245: Store object values and dict pointers in single tagged poin…
GH-95245: Move weakreflist into the pre-header. (GH-95996)
GH-95245: Document use of `MANAGED` flags instead of offsets. (GH-96044)
iritkatriel commentedon Sep 10, 2022
Is there anything left to do here?
markshannon commentedon Sep 12, 2022
Nothing more to do.
AA-Turner commentedon Jun 24, 2025
Re-opening per discussion in #135755.
encukou commentedon Jun 24, 2025
There's no documentation about
Py_TPFLAGS_PREHEADER. What's it for? How should users use it?sergey-miryanov commentedon Jun 24, 2025
As I understand it, users are not supposed to use this flag. It is only used for internal purposes.
gh-95245: Document Py_TPFLAGS_PREHEADER (GH-135861)