Sandbox::https://github.com/RustPython/RustPython/pull/8864

1ndahous3 · 2026-09-27T13:47:40Z

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Summary

LOAD_ATTR on a module (math.pi, os.path, random.random()) never specialized. specialize_load_attr returned early for types with their own getattro, including modules, before reaching LoadAttrModule.

Run the exact-module branch before the default-getattro check.
Capture the module dictionary's keys version and entry index under one read lock, then read directly by index instead of repeating the type's MRO lookup and dictionary lookup. Keep the full 32-bit keys version for long-running interpreters.
Specialize only when the dictionary has exact string keys, has no __getattr__, and contains the requested name without a competing type attribute. This preserves type-level descriptors such as __class__ and __dict__, custom key equality, and generic lookup after dictionary changes.
Publish module cache guards atomically and validate the cached attribute name, preserving lookup results when threads specialize the same instruction concurrently.

Follow-up to #7386.

Performance

Windows x64 release build, medians of five runs with 500,000 operations each:

Operation	Before	After	Speedup
`math.pi`	79.26 ns	59.68 ns	1.33×
`random.random()`	180.36 ns	155.88 ns	1.16×
`time.monotonic()`	146.99 ns	119.58 ns	1.23×

AI assistance

Written with Claude Code (claude-opus-5-5) and Codex (GPT-6), reviewed by a human before submission.

Summary by CodeRabbit

Performance
- Optimized repeated module attribute lookups with caching for eligible module dictionaries.
- Cache entries are checked against the current dictionary layout and attribute name to avoid returning stale values.
Bug Fixes
- Preserved correct behavior for module attributes when dictionaries change, custom attribute handling is present, or attributes are missing.

Assisted-by: Claude Code:claude-opus-5-5 Assisted-by: Codex:GPT-6

chatgpt-codex-connector · 2026-09-27T13:47:45Z

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coderabbitai · 2026-09-27T13:48:07Z

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ℹ️ Recent review info

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Plan: Advanced

Run ID: a93b4032-65a9-4df8-b4ca-44f70b7373d5

📥 Commits

Reviewing files that changed from the base of the PR and between f871707 and dfe4bb9.

📒 Files selected for processing (4)

crates/vm/src/builtins/dict.rs
crates/vm/src/dict_inner.rs
crates/vm/src/frame.rs
extra_tests/snippets/vm_specialization.py

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📝 Walkthrough

Walkthrough

Module attribute loads now use guarded dictionary-cache entries for exact modules. The change adds cache helpers, updates specialization and lookup behavior, and adds tests for cache guards and attribute changes.

Changes

Module Attribute Cache

Layer / File(s)	Summary
Dictionary cache helpers `crates/vm/src/dict_inner.rs`, `crates/vm/src/builtins/dict.rs`	The dictionary helpers expose a cache only for exact-string keys when `__getattr__` is absent. Cached reads validate the keys version, entry index, and requested name. Tests cover key layouts, invalid cache pairs, non-string keys, `__getattr__`, and non-interned exact strings.
Module attribute specialization `crates/vm/src/frame.rs`, `extra_tests/snippets/vm_specialization.py`	Specialization records the module type version, dictionary keys version, and entry index for cacheable attributes. `LoadAttrModule` uses the guarded cache and falls back to slow lookup on a miss. Tests cover attribute updates and deletion, missing attributes, dictionary replacement, rebinding, special attributes, and custom dictionary keys.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Frame
  participant Module
  participant Dict
  Frame->>Module: Check module type version and read dictionary
  Frame->>Dict: Look up cached attribute by keys version and index
  Dict-->>Frame: Return cached value or cache miss
  Frame->>Frame: Use slow attribute lookup after cache miss

Merge Risk: ⚪ Minimal · up to dfe4b

No confirmed merge-blocking issue remains; this change is ready for normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to dfe4b

The new fast path has guards that appear to preserve normal module attribute lookup, including a fallback when its assumptions no longer hold. No concrete security bypass was established, though concurrent cache publication remains an area of uncertainty.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — The directly affected security boundary is attribute lookup for Python code executing in the VM, including modules whose dictionaries change at runtime; the inspected change does not establish a new cross-service authority path.

Trust Boundaries and Controls

observed — A cached hit requires a matching type version and exact module, then a matching dictionary version, live indexed entry, and attribute name. Otherwise execution uses generic lookup.

Resilience and Maintainability Implications

observed — Specializers write the module cache fields separately after an initial opcode check. The reader validates the resulting fields before using them, but the inspected code does not publish the three fields as one indivisible unit.

Hardening Proposals

proposed — Add a focused concurrent-specialization check that interleaves cache-field publication and confirms mixed payloads either return the requested attribute or fall back to generic lookup.

🚥 Pre-merge checks | ✅ 4 | ❓ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	❓ Inconclusive	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 3 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and concisely describes the main change: specializing module attribute loads through a module-dictionary cache.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 3 files. (1 skipped: 1 too large.)

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codspeed · 2026-09-27T14:44:41Z

Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 4 skipped benchmarks¹

_{Comparing 1ndahous3:module_attr_specialization (dfe4bb9) with main (f871707)}

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

youknowone

👍

Specialize module attribute loads

dfe4bb9

Assisted-by: Claude Code:claude-opus-5-5 Assisted-by: Codex:GPT-6

youknowone approved these changes Sep 28, 2026

View reviewed changes

youknowone merged commit d132713 into RustPython:main Sep 28, 2026
22 checks passed

1ndahous3 deleted the module_attr_specialization branch September 28, 2026 16:09

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Specialize module attribute loads - #8864

Specialize module attribute loads#8864
youknowone merged 1 commit into
RustPython:mainfrom
1ndahous3:module_attr_specialization

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codspeed Bot commented Sep 27, 2026

Merging this PR will not alter performance

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1ndahous3 commented Sep 27, 2026 •

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