Plan 175: check-performance optimizations (−38% latency)

[jeduden]

Implements performance optimizations from plan 175's trace analysis, achieving 38% latency reduction on the check gate (1677 → 1046 us/file, p95 1006 → 627 ms).

Summary

This branch lands six behaviour-preserving performance wins identified through profiling the single-core check path:

1. Link reference caching — MDS053/MDS054 no longer re-parse the whole file; they read link reference definitions from the parse NewFile already ran via new lint.File.LinkReferences() method (~10% CPU savings)

2. Parser pooling — The goldmark parser is taken from a sync.Pool instead of rebuilt per file (~5% allocations)

3. AST walk memoization — CollectCodeBlockLines / CollectPIBlockLines are cached per File (eliminates ~20 redundant AST walks per file)

4. Source context skipping — Per-diagnostic source-context strings are skipped when the caller discards them (the benchmark, machine output)

5. Filesystem cache — crossfilereferenceintegrity memoizes per-link os.Stat / filepath.EvalSymlinks in package-level sync.Maps (Syscall6 was ~5.7% flat)

6. Word counting — mdtext.CountWords counts in an allocation-free rune scan instead of len(strings.Fields(...)) (~0.48 GB saved)

Key Changes

• internal/lint/file.go: Added LinkReferences() method backed by sync.Once to expose goldmark's parsed link reference definitions without re-parsing. Captures parser.Context during NewFile and reuses it. Added codeBlockLines / piBlockLines caches with sync.Once guards.

• internal/lint/file.go: Introduced parserPool (sync.Pool) and parseWithPooledParser() to reuse goldmark parser instances across calls, reducing per-file allocation overhead.

• internal/lint/codeblocks.go: Wrapped CollectCodeBlockLines / CollectPIBlockLines with sync.Once memoization; moved implementation to private collectCodeBlockLines / collectPIBlockLines functions.

• internal/rules/noundefinedreferencelabels/rule.go and internal/rules/nounusedlinkdefinitions/rule.go: Replaced manual re-parsing with calls to f.LinkReferences(), eliminating redundant parses.

• internal/rules/crossfilereferenceintegrity/fscache.go (new): Added cachedStatExists() and cachedEvalSymlinks() backed by package-level sync.Map caches to memoize filesystem operations.

• internal/mdtext/mdtext.go: Rewrote CountWords() to count in a single rune scan instead of allocating strings.Fields() slice.

• internal/engine/check.go: Added skipSourceContext parameter to checkRules() to suppress populateSourceContext() when callers don't render source lines (benchmark, machine output).

• internal/engine/runner.go: Added SkipSourceContext field to Runner to allow callers to opt out of source context population.

• internal/engine/bench_test.go: Updated benchmark to set SkipSourceContext=true to measure rule execution without allocation overhead from unused source windows.

Testing

All caches are guarded by sync.Once, sync.Pool, or sync.Map to remain race-free under concurrent access (verified with -race). New tests cover:

• TestNewFile_ConcurrentParseRaceFree — 32 goroutines parsing concurrently
• TestNewFile_SharedFileConcurrentReaders — 16 goroutines reading one *File
• TestLinkReferences_FromNewFileParse —

https://claude.ai/code/session_018yuh89L418oRxz5V2QyyXg

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.63%. Comparing base (4e63d6a) to head (691fb87).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

Components	Coverage Δ
Go	96.60% <100.00%> (+<0.01%)	⬆️
TypeScript	99.35% <ø> (ø)

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