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GeezerVison is showing github.com — gh-131798: Add tier-2 list specialization for STORE_SLICE by eendebakpt · Pull Request #149446 · python/cpython

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@eendebakpt

@eendebakpt eendebakpt commented May 6, 2026 •
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The JIT currently dispatches STORE_SLICE (container[start:stop] = value) through PyObject_SetItem even when the trace has proven the container is an exact list. Adding a tier-2 specialization _STORE_SLICE_LIST skips the slice-object allocation and the generic lookup.

The optimizer rewrites _STORE_SLICE → _STORE_SLICE_LIST only when the container's type is statically known, so non-list callers and tier-1 are unaffected. A second commit for bytearray was added but reverted.

Benchmark results on pyperformance fannkuch (tier-2 / JIT enabled):

main (252 ± 3 ms)  ->  PR (223 ± 2 ms):  1.13x faster
  • Issue: Better uop coverage in the JIT optimizer #131798

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eendebakpt and others added 4 commits May 5, 2026 21:17
@eendebakpt @claude
bytecodes: add tier-2 _STORE_SLICE_LIST optimizer specialization
5748806 
Adds a tier-2 only `_STORE_SLICE_LIST` op and a guard `_GUARD_THIRD_LIST`,
plus an optimizer rule that rewrites generic `_STORE_SLICE` to the list
fast path when the LHS container is (or can be guarded as) an exact list.

The list path calls a new `_PyList_StoreSlice` helper that adjusts the
slice indices and dispatches into `list_ass_slice_lock_held` directly,
skipping the slice-object allocation and the generic
`PyObject_SetItem` lookup.

The tier-1 `_STORE_SLICE` opcode is left unchanged so non-list callers
incur no extra branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
@eendebakpt
remove speculative guards
3ee86eb
@eendebakpt
add bytearray slice assignments
a2f6b84
@eendebakpt
fix memory leak
dacab18
@eendebakpt
eendebakpt requested review from Fidget-Spinner, markshannon, savannahostrowski and tomasr8 as code owners May 6, 2026 07:32
@bedevere-app bedevere-app Bot mentioned this pull request May 6, 2026
Better uop coverage in the JIT optimizer #131798
Open
@bedevere-app bedevere-app Bot added the awaiting review label May 6, 2026
@eendebakpt
remove bytearray opcode
dede9fa
@read-the-docs-community

read-the-docs-community Bot commented May 6, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #35284756 | 📁 Comparing 0bc13a1 against main (970b043)

  🔍 Preview build  

1 file changed
± whatsnew/changelog.html
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@eendebakpt eendebakpt changed the title gh-131798: Add tier2 coverage for STORE_SLICE gh-131798: Add tier-2 list specialization for STORE_SLICE May 6, 2026
eendebakpt and others added 3 commits May 6, 2026 09:44
@eendebakpt
add news entry
e9f81cb
@eendebakpt @claude
Merge branch 'main' into fannkuch_store_slice_tier2
0a93906 
Resolved generated pycore_uop_ids.h conflict by regenerating from
bytecodes.c / optimizer_bytecodes.c.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
@eendebakpt
add PyAPI_FUNC
2988266
markshannon
markshannon reviewed May 22, 2026
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One question, otherwise looks good.

It would be good to optimize constant slices, but that would need a change to the bytecode compiler. For another PR.

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Comment thread Python/optimizer_bytecodes.c Resolved

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eendebakpt
eendebakpt commented May 22, 2026
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Comment thread Python/optimizer_bytecodes.c Resolved

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eendebakpt and others added 2 commits May 22, 2026 15:08
@eendebakpt
Apply suggestion from @eendebakpt
ba66173
@eendebakpt @claude
Regenerate optimizer_cases.c.h
0bc13a1 
Picks up the dead-local removal from the latest commit
('Apply suggestion from @eendebakpt') in optimizer_bytecodes.c.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
@eendebakpt

eendebakpt commented Jun 2, 2026 •
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One question, otherwise looks good.

It would be good to optimize constant slices, but that would need a change to the bytecode compiler. For another PR.

Ah yes. So the mechanism is: for x[1:2] = ... there is a constant slice. The compiler optimizes this to LOAD_CONST and STORE_SUBSCR (which is not specialized), which means we will never get to the specialized STORE_SLICE introduced in this PR.

We could:

  1. Skip the optimization now that STORE_SLICE has more specializations (compiler change).
  2. Specialize STORE_SUBSCR for lists (and more types?)

I am leaning towards the second option (initially only in tier2 to avoid adding a tier1 opcode).

Update: a branch with STORE_SUBSCR specialized is main...eendebakpt:cpython:gh-131798-store-subscr-list-slice, but there is hardly any performance gain.
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@eendebakpt
eendebakpt requested a review from markshannon June 2, 2026 12:30
@github-actions

github-actions Bot commented Sep 5, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Sep 5, 2026
@ambv

ambv commented Oct 20, 2026 •
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This PR is superseded by #157011, a newer open pull request for the same issue (gh-131798), and it no longer merges cleanly. Closing to cull the number of open pull requests. Do not reopen this pull request without getting a go-ahead from a triager or core developer.

Sorry, incorrectly flagged as a duplicate.
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@ambv ambv closed this Oct 20, 2026
@ambv ambv reopened this Oct 21, 2026
@bedevere-app bedevere-app Bot added the type-feature A feature request or enhancement label Oct 21, 2026
@savannahostrowski
savannahostrowski removed their request for review October 21, 2026 03:11
@eendebakpt

eendebakpt commented Oct 23, 2026

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This is is a jit optimization, so closing for now.
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@eendebakpt eendebakpt closed this Oct 23, 2026
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@tomasr8 tomasr8 Awaiting requested review from tomasr8 tomasr8 is a code owner
@Fidget-Spinner Fidget-Spinner Awaiting requested review from Fidget-Spinner Fidget-Spinner is a code owner
@markshannon markshannon Awaiting requested review from markshannon markshannon is a code owner

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