perf(interp): fold loop branch legs into native back-edges by siyul-park · Pull Request #160 · siyul-park/minivm

siyul-park · 2026-07-18T02:00:26Z

Record native loop fallback exits as branches so loop anchors recompile
through the side-exit machinery, fold partial legs whose cut lands on the
plan's own header into the committing-flush native back-edge, and share
identical scheduled continuations so folded legs that cycle through a loop
nest converge instead of exhausting the continuation limit. Handle the
trapNone return a folded RETURN or completed leg produces in the loop
wrapper: tear the frame down like call() for a function loop and mark the
module frame exhausted like start() for top-level code.

Sieve(256) drops from 4.19us to 1.61us (default) and 4.17us to 1.57us
(eager jit) on M4 Pro interleaved A/B; scan-loop native entries fall from
~55 to ~1 per run. All other benchmarks stay at parity.

Closes #155

Summary by CodeRabbit

Performance
- Reduced native loop re-entry overhead and unnecessary exits, improving loop execution efficiency.
- Refreshed Sieve(256) benchmark figures based on recent re-measurements.
Bug Fixes
- Fixed loop completion and native execution teardown/return handling, including fallback exits.
- Corrected trace-plan branching/cut behavior to avoid incorrect folding and spurious blocks.
Tests
- Added ARM64 coverage for loop-leg folding and updated trace-plan scenarios for loop-header cuts and related edge cases.
Documentation
- Updated JIT internals docs to reflect revised loop, branch folding, and continuation scheduling semantics.

coderabbitai · 2026-07-18T02:01:08Z

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 513a1cc3-f865-44cf-8af6-094f3abdc928

📥 Commits

Reviewing files that changed from the base of the PR and between 7bf65ad and 46a2ed2.

📒 Files selected for processing (6)

interp/interp.go
interp/interp_test.go
interp/jit.go
interp/jit_arm64.go
interp/jit_arm64_test.go
interp/jit_plan.go

📝 Walkthrough

Walkthrough

Changes

Native loop-leg folding

Layer / File(s)	Summary
Loop plan back-edge folding `interp/jit_plan.go`, `interp/jit_plan_test.go`, `docs/jit-internals.md`	Loop-header cuts and rejoining legs now wire to native back-edges, while unrelated cuts retain fallback exits and explicit branches no longer create spurious blocks.
Continuation snapshot reuse `interp/jit.go`, `interp/jit_arm64.go`, `docs/jit-internals.md`	ARM64 lowering records scheduled work and reuses continuation labels only for matching block, tail, value snapshots, and frame shapes.
Loop completion and exit handling `interp/interp.go`, `AGENTS.md`, `docs/jit-internals.md`	Native loop completion handles top-level exhaustion and function-return teardown separately, and fallback exits are recorded before threaded resumption.
Behavioral validation and measurements `interp/jit_arm64_test.go`, `interp/jit_plan_test.go`, `docs/benchmarks.md`	Tests compare folded loop, return, and completion legs with threaded execution and validate plan wiring; Sieve benchmark results are updated with July 18, 2026 measurements.

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

Sequence Diagram(s)

sequenceDiagram
  participant NativeLoop
  participant tracePlan
  participant arm64Lowerer
  participant Interpreter
  NativeLoop->>tracePlan: process loop leg and header cut
  tracePlan->>arm64Lowerer: emit native back-edge or fallback edge
  arm64Lowerer->>NativeLoop: reuse matching continuation or schedule work
  NativeLoop->>Interpreter: trapNone or trapFallback
  Interpreter->>Interpreter: exhaust module or tear down frame

Possibly related PRs

siyul-park/minivm#154: Related native-loop trap handling and header-resumption changes.
siyul-park/minivm#159: Related native loop back-edge emission and safepoint bookkeeping.
siyul-park/minivm#137: Related trace cut and loop-leg representation.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description omits the required template headings and sections, so it doesn't match the repository format.	Add the Changes Made, Related Issues, and Additional Information sections, and summarize the implementation and issue reference.
Docstring Coverage	⚠️ Warning	Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title concisely describes the main change: folding loop branch legs into native back-edges.
Linked Issues check	✅ Passed	The code updates match `#155` by folding loop-kind legs, wiring header cuts correctly, and reusing continuations to avoid limit exhaustion.
Out of Scope Changes check	✅ Passed	The docs, tests, and interpreter/JIT changes all support the loop-leg folding objective, with no clear unrelated changes.

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Commit unit tests in branch perf/jit-loop-leg-fold

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Actionable comments posted: 2

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Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@interp/jit_arm64_test.go`:
- Around line 2671-2680: The native-entry assertion in the test currently
aggregates entries from both the scan and fill loops, so it does not verify
scan-loop behavior. Update the metrics iteration around profile.Metrics() to
filter entries using the scan loop’s identifying func/ip/kind labels, then apply
the existing count assertions only to that scoped metric.

In `@interp/jit_plan.go`:
- Around line 626-634: Restrict the shortcut in the trace split logic around the
hot-edge check to the plan’s own depth-0 loop cut by applying the same
entryLoop, depth, and plan-header conditions used near Lines 564-568. Ensure
matching branch/cut pairs at depth > 0 follow fallback instead of folding onto
the native loop back-edge, and add a regression case covering that inlined-frame
scenario.

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Push a commit to this branch (recommended)
Create a new PR with the fixes

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 13c9e2c3-1ff9-4b5f-aa48-19bc79f98f2e

📥 Commits

Reviewing files that changed from the base of the PR and between b4719c5 and 08a6c74.

📒 Files selected for processing (9)

AGENTS.md
docs/benchmarks.md
docs/jit-internals.md
interp/interp.go
interp/jit.go
interp/jit_arm64.go
interp/jit_arm64_test.go
interp/jit_plan.go
interp/jit_plan_test.go

codecov · 2026-07-18T02:05:34Z

Codecov Report

❌ Patch coverage is 36.00000% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.74%. Comparing base (b4719c5) to head (29521b2).

Files with missing lines	Patch %	Lines
interp/interp.go	0.00%	20 Missing ⚠️
interp/jit.go	0.00%	9 Missing ⚠️
interp/jit_plan.go	85.71%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   29.70%   29.74%   +0.03%     
==========================================
  Files          86       86              
  Lines       60357    60386      +29     
==========================================
+ Hits        17930    17962      +32     
  Misses      41156    41156              
+ Partials     1271     1268       -3

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perf(interp): fold loop branch legs into native back-edges

08a6c74

coderabbitai Bot reviewed Jul 18, 2026

View reviewed changes

Comment thread interp/jit_arm64_test.go Outdated

Comment thread interp/jit_plan.go

siyul-park added 6 commits July 18, 2026 22:12

fix(interp): restrict loop leg folding

7bf65ad

refactor(interp): align loop folding patterns

8f5f812

refactor(interp): simplify loop continuation state

2adc4e0

refactor(interp): unify deferred work scheduling

53bfb58

refactor(interp): simplify deferred work ownership

29521b2

refactor(interp): align JIT symbol roles

46a2ed2

siyul-park merged commit 8c413ae into main Jul 18, 2026
5 checks passed

siyul-park deleted the perf/jit-loop-leg-fold branch July 18, 2026 14:07

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perf(interp): fold loop branch legs into native back-edges#160

perf(interp): fold loop branch legs into native back-edges#160
siyul-park merged 7 commits into
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perf/jit-loop-leg-fold

siyul-park commented Jul 18, 2026 •

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