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Avoid temporary big-integer storage for small getrandbits calls - #9233

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youknowone merged 1 commit into
RustPython:mainfrom
1ndahous3:random_small_bits
Oct 20, 2026
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Avoid temporary big-integer storage for small getrandbits calls#9233
youknowone merged 1 commit into
RustPython:mainfrom
1ndahous3:random_small_bits

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@1ndahous3

@1ndahous3 1ndahous3 commented Oct 19, 2026 •
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Summary

Generate getrandbits(1..32) directly from one MT19937 word, avoiding the temporary Vec and BigUint. Fixed-seed output and generator state are unchanged.

Performance

Release builds, median of three paired runs; each run uses five timing samples.

Windows 11 x64

Operation Main, ns/op Patched, ns/op Speedup
getrandbits(8) 221 183 1.21×
getrandbits(32) 250 208 1.20×
randrange(100) 1,140 1,035 1.10×
choice(64 items) 1,209 1,119 1.08×
shuffle(64 items) 50,734 46,302 1.10×

Debian 12 amd64

Operation Main, ns/op Patched, ns/op Speedup
getrandbits(8) 229 170 1.35×
getrandbits(32) 239 191 1.25×
randrange(100) 1,042 945 1.10×
choice(64 items) 1,132 980 1.16×
shuffle(64 items) 50,453 43,845 1.15×

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Written with Codex (GPT-6), reviewed by a human before submission.

Summary by CodeRabbit

  • Bug Fixes
    • Corrected random bit generation for requests of 1–32 bits. These requests now return the requested number of high-order bits from a generated random value.

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chatgpt-codex-connector Bot commented Oct 19, 2026 •
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Codex Review Summary

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Review Status Commit Review trigger
📝 Code Review ✅ Completed 2026-10-19T18:23:14.489332Z 6bcf8af PR opened
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coderabbitai Bot commented Oct 19, 2026 •
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Review in Change Stack →

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info
⚙️ Run configuration
  • Configuration used: Repository: RustPython/RustPython/.coderabbit.yml
  • Review profile: CHILL
  • Plan: Advanced
  • Run ID: 5f15360c-3535-414d-b514-d203c7adb84d
📥 Commits

Reviewing files that changed from the base of the PR and between c08375b and 6bcf8af.

📒 Files selected for processing: 1
  • crates/stdlib/src/random.rs

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 4 remain after this review.

📝 Walkthrough

getrandbits now handles requests for 1–32 bits by returning the high-order bits of one generated u32. Requests larger than 32 bits continue through the existing multiword path.

Changes: Random Bit Generation

Layer / File(s) Summary
Handle requests of 1–32 bits
crates/stdlib/src/random.rs		 For k from 1 through 32, getrandbits shifts one generated u32 right by 32 - k.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Refactor

Suggested reviewers: youknowone

Merge Risk: ⚪ Minimal · up to 6bcf8

The small-request path is intended to preserve seeded values and generator state while avoiding temporary big-integer storage. No actionable merge risk is evident from the supplied context.

Pre-merge checks: Passed 5
Check name Status Explanation
Description Check Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check Passed The title clearly and concisely describes the main change: removing temporary big-integer storage for small getrandbits calls.
Docstring Coverage Passed Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
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.
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youknowone
youknowone approved these changes Oct 20, 2026
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@youknowone
youknowone merged commit c025843 into RustPython:main Oct 20, 2026
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