gh-144145: Track nullness of properties in the Tier 2 JIT optimizer#144122

cocolato

This PR implements symbolic tracking for __slots__ object attributes in the Tier 2 JIT optimizer.

Currently, when the JIT optimizer encounters _LOAD_ATTR_SLOT, it creates a new unknown symbol (<!NULL>) even if the same slot was previously written in the same trace. This prevents the optimizer from:

Knowing the type of values loaded from slots
Eliminating redundant type guards
Performing further type-based optimizations

Before(set PYTHON_OPT_DEBUG=5):

51 abs: _LOAD_ATTR_SLOT (4, target=48, operand0=0x10, ...)
    stack=[..., <Point at 0x...440>]
52 abs: _POP_TOP
    stack=[..., <!NULL at 0x7fb981d874c0>, <Point at 0x...440>]
                ↑↑↑ NOT NULL TYPE

Now:

51 abs: _LOAD_ATTR_SLOT (4, target=48, operand0=0x10, ...)
    stack=[..., <Point slots[2] v131145 at 0x...440>]
52 abs: _POP_TOP
    stack=[..., <compact_int at 0x7f02ba1d6490>, <Point slots[2]>]
                ↑↑↑ we know the exact type

Issue: Support track __slots__ object and its property in the Tier 2 optimizer #144145

markshannon

This PR seems unrelated to the issue. It tracks the internal state of objects, not the uniqueness of references.
Please create a new issue.

Tracking the internal state of objects is tricky, as we need to keep track of where code might escape and invalidate any knowledge prior to the escape. So far, we have only tracked properties of stateless objects, so it isn't an issue.
There is value in tracking the state of objects, but it needs to be done carefully.

In future, I would also advise waiting to see if there is general support on the issue before spending time on the implementation.

cocolato

Thanks for reply!

This PR seems unrelated to the issue. It tracks the internal state of objects, not the uniqueness of references.
Please create a new issue.

Sorry, I'm linking to this issue because I discussed the implementation logic with @Fidget-Spinner in this issue, and I think this should be a pre-task for unique reference tracking (if I'm wrong, please point it out, thanks!). I'll create a new issue to link to this PR.

Tracking the internal state of objects is tricky, as we need to keep track of where code might escape and invalidate any knowledge prior to the escape. So far, we have only tracked properties of stateless objects, so it isn't an issue.
There is value in tracking the state of objects, but it needs to be done carefully.

Sorry again for overlooking object escape. I think may be Ken Jin's original requirement was just to add tracking for the Slots object(if I'm wrong, please point it out again, thanks!). Perhaps I should remove the optimization in the _LOAD_ATTR_SLOT section?

In future, I would also advise waiting to see if there is general support on the issue before spending time on the implementation.

Thanks for the reminder. I'll be more careful in the future.

Fidget-Spinner

Sorry again for overlooking object escape. I think may be Ken Jin's original requirement was just to add tracking for the Slots object(if I'm wrong, please point it out again, thanks!). Perhaps I should remove the optimization in the _LOAD_ATTR_SLOT section?

Yeah we need to track for escapes. I mentioned it here how to do it #143414 (comment). It got lost in the noise though, so no worries.

@markshannon

In future, I would also advise waiting to see if there is general support on the issue before spending time on the implementation.

I advised @cocolato to embark on this issue. I think there is value in doing it. They waited for me to give my go-ahead before doing it. So I think they didn't do anything wrong.

cocolato

Thanks! I will refactor this to also support #144141.

cocolato

@Fidget-Spinner Hi, one thing i want to know. Should this PR flag which property escape during escape handling UOPs (e.g. _CALL_*)? This would allow us to reset only those objects instead of resetting all __slot__ objects tracked in s_arena. However, this will made this PR more complex.

Fidget-Spinner

Sorry I'm busy for these 2 days. I'll try to review on Saturday. If Mark reviews it earlier, we can go with his ideas.

Fidget-Spinner

@Fidget-Spinner Hi, one thing i want to know. Should this PR flag which property escape during escape handling UOPs (e.g. _CALL_*)? This would allow us to reset only those objects instead of resetting all __slot__ objects tracked in s_arena. However, this will made this PR more complex.

No. The problem is any uop that escapes must be handled. For example, anything with POP_TOP if not eliminated is escaping, because it might call __del__ which can trigger arbitrary code.

cocolato

Updated, added _STORE_ATTR_INSTANCE_VALUE_NULL and now only use is_init_shim flag to avoid _CREATE_INIT_FRAME escaping.

Fidget-Spinner

Cool, this LGTM now I think. This deserves a news entry. Potentially call it:

Track nullness of attributes of objects in the JIT optimizer. Patch by Hai Zhu.

I know it's not just nullness, but in all practicality due to the escapes, it's just nullness for now.

…e-gh-144145.mxJyUj.rst

Fidget-Spinner

Thanks

Fidget-Spinner

Thanks! Do you want to handle STORE_ATTR_WITH_HINT Too?

cocolato

Thanks! Do you want to handle STORE_ATTR_WITH_HINT Too?

Sure, I will work on it :)

cocolato

@Fidget-Spinner I noticed a slight decrease in JIT performance yesterday: https://doesjitgobrrr.com, which seems related to this PR. Should we look into it?

Fidget-Spinner

@cocolato I suggest waiting for one more day. If it's real, please investigate. Thank you!

markshannon

I'm going to have to ask for this PR to be reverted, because:

it has a number of bugs: gh-144145: Cleanups for object property tracking in JIT optimizer #144366, and maybe others,
it does not improve performance, and
it increases the complexity of optimizer considerably.

We also need to decide whether we want to track whether execution has escaped.
Tracking escapes may allow us some extra optimizations, but it adds a lot of complexity.
Until the front-end and optimizer is more robust, I really don't want to be making the optimizer more complex.

markshannon

@cocolato if you are looking for ways to boost performance, I've created a meta-issue with some ideas: #144388

…IT optimizer (pythonGH-144122)" This reverts commit 1dc12b2.

cocolato

Sorry about that, revert it by #144391. I will do some more safe changes in the future.

markshannon

Don't worry. Occasionally things need to be reverted.

I think something like this is worth doing, but we should discuss it a bit more on the issue first.

…44391) Revert "gh-144145: Track nullness of properties in the Tier 2 JIT optimizer (GH-144122)" This reverts commit 1dc12b2.

…pythonGH-144391) Revert "pythongh-144145: Track nullness of properties in the Tier 2 JIT optimizer (pythonGH-144122)" This reverts commit 1dc12b2.

…izer (pythonGH-144122)

…pythonGH-144391) Revert "pythongh-144145: Track nullness of properties in the Tier 2 JIT optimizer (pythonGH-144122)" This reverts commit 1dc12b2.

cocolato added 2 commits January 20, 2026 22:09

Add slots object property tracking for the Tier 2 JIT optimizer

dfd99d7

use s_arena to save slots mapping

7bb2c20

cocolato requested review from Fidget-Spinner, markshannon, savannahostrowski and tomasr8 as code owners January 21, 2026 17:47

bedevere-app Bot mentioned this pull request Jan 21, 2026

JIT: Implement unique reference tracking in Tier 2 for reference count optimizations #143414

Closed

bedevere-app Bot added the awaiting review label Jan 21, 2026

cocolato commented Jan 21, 2026

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fix switch syntax error

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cocolato changed the title ~~gh-143414: Add __slots__ object property tracking for the Tier 2 JIT optimizer~~ Jan 22, 2026

bedevere-app Bot mentioned this pull request Jan 22, 2026

Support track __slots__ object and its property in the Tier 2 optimizer #144145

Closed

cocolato marked this pull request as draft January 22, 2026 10:56

bedevere-app Bot removed the awaiting review label Jan 22, 2026

Fidget-Spinner mentioned this pull request Jan 22, 2026

Eliminate overhead of fetching and testing NULL attributes in STORE_ATTR specializations for new objects #144141

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Merge branch 'main' into dev_143414

2e9c5a6

fix arena struct

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Fidget-Spinner reviewed Jan 24, 2026

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cocolato and others added 3 commits January 26, 2026 09:46

support track escape

6a828d7

check null ptr

90e4800

fix escape check

bf29c13

cocolato commented Jan 26, 2026

View reviewed changes

Fidget-Spinner changed the title ~~gh-144145: Add object property tracking for the Tier 2 JIT optimizer~~ Jan 30, 2026

blurb-it Bot and others added 3 commits January 30, 2026 09:42

📜🤖 Added by blurb_it.

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Delete Misc/NEWS.d/next/Core_and_Builtins/2026-01-30-09-42-30.gh-issu… …

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…e-gh-144145.mxJyUj.rst

📜🤖 Added by blurb_it.

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Fidget-Spinner reviewed Jan 30, 2026

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cocolato added 2 commits January 30, 2026 11:41

add comment

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format comment

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Fidget-Spinner approved these changes Jan 30, 2026

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bedevere-app Bot added awaiting merge and removed awaiting review labels Jan 30, 2026

bedevere-app Bot removed the awaiting merge label Jan 30, 2026

Fidget-Spinner mentioned this pull request Jan 31, 2026

gh-144145: Cleanups for object property tracking in JIT optimizer #144366

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cocolato added a commit to cocolato/cpython that referenced this pull request Feb 2, 2026

Revert "pythongh-144145: Track nullness of properties in the Tier 2 J… …

abb3e2b

…IT optimizer (pythonGH-144122)" This reverts commit 1dc12b2.

cocolato mentioned this pull request Feb 2, 2026

gh-144145: Revert PR#144122 for performance and potential bugs. #144391

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markshannon pushed a commit that referenced this pull request Feb 2, 2026

gh-144145: Revert PR#144122 for performance and potential bugs. (GH-1… …

ebbb2ca

…44391) Revert "gh-144145: Track nullness of properties in the Tier 2 JIT optimizer (GH-144122)" This reverts commit 1dc12b2.

thunder-coding pushed a commit to thunder-coding/cpython that referenced this pull request Feb 15, 2026

pythongh-144145: Track nullness of properties in the Tier 2 JIT optim… …

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…izer (pythonGH-144122)

cocolato deleted the dev_143414 branch March 17, 2026 08:12

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