Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[markshannon]

BPO	45947
Nosy	@markshannon, @Fidget-Spinner
PRs	bpo-45947: Place dict and values pointer at fixed (negative) offset just before GC header. #29879

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/markshannon'
closed_at = <Date 2022-01-20.11:51:02.049>
created_at = <Date 2021-12-01.13:30:48.955>
labels = ['interpreter-core', '3.11', 'performance']
title = 'Place dict (and values) pointers at a fixed (negative) offset from the base of the object.'
updated_at = <Date 2022-01-20.11:51:02.049>
user = 'https://github.com/markshannon'

bugs.python.org fields:

activity = <Date 2022-01-20.11:51:02.049>
actor = 'Mark.Shannon'
assignee = 'Mark.Shannon'
closed = True
closed_date = <Date 2022-01-20.11:51:02.049>
closer = 'Mark.Shannon'
components = ['Interpreter Core']
creation = <Date 2021-12-01.13:30:48.955>
creator = 'Mark.Shannon'
dependencies = []
files = []
hgrepos = []
issue_num = 45947
keywords = ['patch']
message_count = 2.0
messages = ['407457', '407946']
nosy_count = 2.0
nosy_names = ['Mark.Shannon', 'kj']
pr_nums = ['29879']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue45947'
versions = ['Python 3.11']

[markshannon]

To get to the dict (or values array) of an object, we currently need 3 dependent loads.
By placing the dict at a fixed offset, this can be reduced to one.

What was obj[obj->type->dictoffset] becomes obj[FIXED_OFFSET]

See faster-cpython/ideas#80, specifically
faster-cpython/ideas#80 (comment) for more details.

[markshannon]

New changeset 8319114 by Mark Shannon in branch 'main':
bpo-45947: Place dict and values pointer at fixed (negative) offset just before GC header. (GH-29879)
8319114