perf(codegen): elide GC write barrier for non-pointer class-field stores (#5334 lever D)

[proggeramlug]

What

Tier-2 lever of the IR-efficiency roadmap (#5334, lever D): skip the GC write barrier on boxed class-field stores whose value is a non-pointer by construction.

The boxed class-field-SET fast path emitted js_write_barrier_slot unconditionally:

class_field_set.fast:
  store double %val, ptr %slot
  call void @js_gc_note_slot_layout(...)
  call void @js_write_barrier_slot(...)   ; <- always

But the generational write barrier only matters when the stored value is a heap pointer — it records the parent→child reference so the minor GC scans the parent. A value that is a non-pointer by construction (number / bool / undefined / null / comparison / arithmetic) creates no such reference, so the barrier is a semantic no-op:

class_field_set.fast:                      ; value is numeric/boolean
  store double %val, ptr %slot
  call void @js_gc_note_slot_layout(...)   ; layout note kept
                                           ; barrier elided

Safety

• Reuses expr_produces_non_pointer_bits_by_construction — the same predicate the array-store paths already trust for barrier elision, so the GC soundness standard is unchanged.
• The layout note stays (it tracks the slot's pointer-ness for minor-scan skipping; a non-pointer write into a slot that previously held a pointer is a real transition the GC must observe).
• The barrier is kept for any value that may be a heap pointer.

Verification

• A numeric-heavy class drops 6 of 9 boxed field-store barriers (the numeric/boolean stores), keeps all 3 genuine pointer (string) stores.
• Runs to the correct result (5999997) under a 2M-iteration GC-exercising loop — barrier elision doesn't corrupt the heap.
• New unit test class_field_set_elides_write_barrier_for_nonpointer_value asserts both directions (numeric → no barrier, string literal → barrier kept).
• Full perry-codegen suite green, including large_object_barriers and the runtime barrier-metadata tests.

Refs #5334. (Levers A #5351 and C #5350 already merged.)

Summary by CodeRabbit

• Performance Improvements

  • Optimized boxed class field writes to elide unnecessary generational write barriers when the assigned value is provably non-pointer, while preserving barrier emission for pointer values.

• Tests

  • Added coverage ensuring non-pointer class field assignments use the optimized path without emitting a write barrier, and pointer assignments still emit the required barrier.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 11e4facc-55ba-49ca-bc7a-a934211dcf5d

📥 Commits

Reviewing files that changed from the base of the PR and between 4b47b09 and 4ab7e3a.

📒 Files selected for processing (1)

• crates/perry-codegen/src/expr/property_set.rs

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📝 Walkthrough

Walkthrough

The PropertySet fast path now imports expr_produces_non_pointer_bits_by_construction and uses it to compute field_set_barrier_needed. When the assigned value is provably non-pointer (numbers, booleans, null, undefined), the generational write barrier is skipped while the slot's pointer-ness note is still recorded for GC correctness. A new test verifies the elision for numeric literals and retention for string literals.

Changes

Write-barrier elision for non-pointer class field sets

Layer / File(s)	Summary
Barrier elision logic in PropertySet fast path crates/perry-codegen/src/expr/property_set.rs	Adds import of expr_produces_non_pointer_bits_by_construction, computes field_set_barrier_needed = !expr_produces_non_pointer_bits_by_construction(ctx, value) in the class-field-set fast path, and passes it to emit_jsvalue_slot_store_on_block instead of the previous hardcoded true.
Test: barrier elision vs. retention crates/perry-codegen/tests/typed_feedback.rs	Adds class_field_set_elides_write_barrier_for_nonpointer_value, asserting class_field_set.fast omits js_write_barrier_slot for a numeric literal and includes it for a string literal.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• PerryTS/perry#5098: Both PRs modify the slot-store/write-barrier lowering to conditionally emit GC/barrier behavior based on whether stored/previous values are provably non-pointer (retrieved PR adds js_gc_note_slot_layout_aware + scalar-aware slot-store helper; main PR uses the non-pointer-by-construction predicate to pass field_set_barrier_needed into the slot store emission).

Poem

A bunny hops past the write barrier gate,
"No pointer here!" it thumps — no need to wait.
Numbers and booleans skip the GC queue,
Strings still pay the toll, as all heap values do.
The test confirms the elision is neat and true. 🐇✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: eliding GC write barriers for non-pointer class-field stores, with a reference to the broader IR-efficiency roadmap (#5334 lever D).
Description check	✅ Passed	The description provides comprehensive coverage including summary, concrete changes, related issue, verification details, and test plan with checkboxes completed, closely following the repository template structure.
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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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/elide-nonpointer-field-barrier

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