perf(runtime): skip unchanged spill layout notes

[proggeramlug]

Summary

• route object spill-slot writes through the existing value-aware layout hook
• skip the full GC layout update when an overwrite keeps the same pointer kind
• preserve pointer-mask updates for scalar/pointer transitions and array element-shape bookkeeping for pointer overwrites
• add a regression test that measures full layout-hook calls and verifies the spill pointer mask

Testing

• RUST_TEST_THREADS=1 cargo test --profile perry-dev -p perry-runtime --lib -- --test-threads=1 (4,379 passed; 5 ignored)
• cargo fmt --all -- --check
• ./scripts/check_file_size.sh
• python3 scripts/gc_runtime_root_holders.py
• python3 scripts/check_test_registration.py

Fixes #10872

Summary by CodeRabbit

• Performance
  • Repeated updates to object spill slots now avoid redundant garbage-collection layout bookkeeping when the value’s pointer kind remains unchanged. Changes between scalar and pointer values continue to update the layout tracking, preserving correct garbage-collection behavior.

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: c5fae102-4654-4c99-a13d-e5847283e34c

📥 Commits

Reviewing files that changed from the base of the PR and between 7f4417b and 0b80204.

📒 Files selected for processing (5)

• changelog.d/11098-spill-layout-notes.md
• crates/perry-runtime/src/gc/layout.rs
• crates/perry-runtime/src/object/mod.rs
• crates/perry-runtime/src/object/spill.rs
• scripts/gc_runtime_root_holders.json

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📝 Walkthrough

Walkthrough

Spill-slot writes now skip the full GC layout-note path when an overwrite keeps the same pointer kind. Writes that change the slot between scalar and pointer values still update the slot mask. Tests count layout-note calls and check pointer-slot counts.

Changes

Spill Slot Layout-Note Optimization

Layer / File(s)	Summary
Update spill-store layout-note calls crates/perry-runtime/src/object/spill.rs, crates/perry-runtime/src/gc/layout.rs, crates/perry-runtime/src/object/mod.rs, scripts/gc_runtime_root_holders.json, changelog.d/11098-spill-layout-notes.md	spill_store_slot uses the prior slot bits to skip the full layout note for same-kind overwrites. Test instrumentation and assertions cover scalar and pointer overwrites, kind transitions, and pointer-slot counts. The changelog records the behavior.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Refactor

Merge Risk: ⚪ Minimal · up to 0b802

The spill-store optimization has no identified issue that needs resolution before merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: skipping unchanged spill layout notes for runtime performance.
Description check	✅ Passed	The description explains the optimization, preserves the required transition behavior, identifies the regression test, links issue #10872, and lists verification commands. It omits the template's sepa…
Linked Issues check	✅ Passed	The PR addresses the coding objective in issue [#10872]. spill_store_slot reads the previous slot value and calls layout_note_slot_aware instead of always calling layout_note_slot. Same-kind sca…
Out of Scope Changes check	✅ Passed	The changes stay within issue [#10872]. The test-only call counter, test exports, root-holder entry, layout documentation update, regression tests, and changelog entry support verification or document…
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 3 files. (2 skipped: 2 u…

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[proggeramlug]

Landed on main in merge train 266 (#11109), released as v0.5.1649 at 784ed8e2c4.

Cherry-picked from this PR's head 0b80204f09 and validated as one tree — CI 22/22 green, all 6 gap-suite shards. A train rebase gives the commits new SHAs, so GitHub cannot auto-close the source PR; closing by hand.

Nothing needed from you. Thanks.