[issue45947] Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[Mark Shannon]

Change by Mark Shannon :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue45947] Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[Mark Shannon]

Mark Shannon  added the comment:


New changeset 8319114feedd2a5b77378bba24eb9fb2689c5033 by Mark Shannon in 
branch 'main':
bpo-45947: Place dict and values pointer at fixed (negative) offset just before 
GC header. (GH-29879)
https://github.com/python/cpython/commit/8319114feedd2a5b77378bba24eb9fb2689c5033


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[issue45947] Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[Ken Jin]

Change by Ken Jin :


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nosy: +kj

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[issue45947] Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[Mark Shannon]

Change by Mark Shannon :


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keywords: +patch
pull_requests: +28104
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/29879

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[issue45947] Place dict (and values) pointers at a fixed (negative) offset from the base of the object.

[Mark Shannon]

New submission from Mark Shannon :

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 https://github.com/faster-cpython/ideas/issues/80, specifically
https://github.com/faster-cpython/ideas/issues/80#issuecomment-931504847 for 
more details.

--
assignee: Mark.Shannon
components: Interpreter Core
messages: 407457
nosy: Mark.Shannon
priority: normal
severity: normal
status: open
title: Place dict (and values) pointers at a fixed (negative) offset from the 
base of the object.
type: performance
versions: Python 3.11

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