perf(repsel): bound loop accumulators for i32

[proggeramlug]

Closes #7123

Summary

• derive saturating trip-count upper bounds from proven constant-bounded for-loop induction variables, including inclusive/non-unit and nested loops
• admit accumulators only when the literal entry magnitude plus every bounded write contribution fits signed i32
• bound literal/const, remainder, non-negative bit-mask, and loop-bounded-local steps; conservatively reject unknown loops, writes, and expression forms
• add the census liveness promotion and registered runtime-observable overflow witness

Validation

• cargo test -p perry-codegen --no-default-features --lib collectors::loop_bounded_i32::tests (45 passed; used a temporary, restored test-only manifest setting to avoid the packaged Windows LLVM all-target linker gap)
• cargo test -p perry-codegen --no-default-features --lib collectors::hir_facts::tests::bounded_modulo_accumulator_seeds_integer_provenance -- --exact (passed)
• cargo check -p perry-codegen --tests
• cargo clippy -p perry-codegen --tests (passed; existing warnings only)
• cargo fmt -p perry-codegen -- --check
• python -m py_compile scripts/compiler_output_harness/repsel_census.py
• sabotage: suppressing the new result fails the bounded-promotion assertion; removing the final i32 range gate fails the 3-billion overflow refusal

No version bump.

Summary by CodeRabbit

• New Features

  • Improved optimization of loop accumulators when their values can be safely proven to remain within 32-bit integer limits.
  • Supports bounded loop steps, nested loops, modulo operations, and bit-masked increments.
  • Preserves correct behavior for accumulators that may overflow.

• Bug Fixes

  • Improved handling of bounded accumulator values while conservatively rejecting cases with unknown or unsafe limits.

• Documentation

  • Added guidance covering supported bounded-loop and accumulator scenarios.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: b6f656e4-e7c2-4f34-93df-aef7b464cb53

📥 Commits

Reviewing files that changed from the base of the PR and between a40ae16 and 1345a36.

📒 Files selected for processing (11)

• benchmarks/repsel_census/baseline.json
• benchmarks/repsel_census/fixtures/fixture_loop_bounded_i32.ts
• changelog.d/7973-repsel-loop-accumulator.md
• crates/perry-codegen/src/collectors/hir_facts.rs
• crates/perry-codegen/src/collectors/loop_bounded_i32.rs
• crates/perry-codegen/src/collectors/loop_bounded_i32/tests.rs
• crates/perry-codegen/src/expr/slot_rep.rs
• crates/perry-codegen/src/stmt/let_stmt.rs
• scripts/compiler_output_harness/repsel_census.py
• test-files/test_gap_repsel_loop_bounded_i32.ts
• test-parity/gc_repsel_corpus.txt

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📝 Walkthrough

Walkthrough

The PR adds sound bounded-accumulator analysis for loop-local i32 values. It supports trip-count and step-magnitude proofs, integrates accepted locals into canonical i32 selection, and adds compiler, runtime, fixture, and census coverage.

Changes

Bounded accumulator promotion

Layer / File(s)	Summary
Accumulator range analysis crates/perry-codegen/src/collectors/loop_bounded_i32.rs, crates/perry-codegen/src/collectors/loop_bounded_i32/tests.rs	The collector derives saturating loop execution bounds and step magnitudes. It admits accumulators whose worst-case values fit i32 and rejects unknown, unsupported, or overflowing writes.
Representation eligibility integration crates/perry-codegen/src/collectors/hir_facts.rs, crates/perry-codegen/src/expr/slot_rep.rs, crates/perry-codegen/src/stmt/let_stmt.rs	Accepted accumulator locals now contribute integer facts and canonical i32 eligibility. Documentation and denial expectations describe the expanded proof rules.
Fixture and census validation benchmarks/repsel_census/fixtures/fixture_loop_bounded_i32.ts, test-files/test_gap_repsel_loop_bounded_i32.ts, test-parity/gc_repsel_corpus.txt, scripts/compiler_output_harness/repsel_census.py, benchmarks/repsel_census/baseline.json, changelog.d/7973-repsel-loop-accumulator.md	Coverage distinguishes bounded and overflowing accumulators. Runtime probes verify output behavior, and the expected canonical-i32 promotion count increases from three to four.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant LoopAnalyzer as loop_bounded_i32 collector
  participant HIRFacts as collect_type_facts
  participant SlotRep as slot_rep
  participant Fixture as runtime fixture
  LoopAnalyzer->>HIRFacts: report bounded accumulator locals
  HIRFacts->>SlotRep: merge integer provenance
  SlotRep->>Fixture: select canonical i32 representation
  Fixture->>Fixture: compare bounded and overflowing results

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Possibly related PRs

• PerryTS/perry#6903: This PR extends its canonical-i32 representation selection with loop-bounded accumulator analysis.
• PerryTS/perry#7122: This PR extends its loop-bounded i32 analysis from induction variables to accumulators.
• PerryTS/perry#7132: Both PRs modify shared loop-bounded representation-selection logic and related fixtures.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the primary change: bounding loop accumulators for canonical i32 representation.
Description check	✅ Passed	The description includes the summary, issue reference, implementation details, validation commands, sabotage evidence, and version note, although it omits some template headings.
Linked Issues check	✅ Passed	The changes implement the linked issue's bounded-accumulator proof, conservative rejection, liveness fixture, runtime overflow witness, parity registration, and sabotage evidence.
Out of Scope Changes check	✅ Passed	The modified code, tests, fixtures, census data, parity documentation, and changelog entry all support the linked accumulator-bounding objective.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ 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 perf/7123-loop-accumulator

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