fix(slice): the reaching-definitions walk is iterative, so nesting depth grows the heap, never the thread stack

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Summary

SliceRdWalker (the --slice reaching-definitions walker) and the occurrence scan's own sliceWalk
no longer recurse per AST nesting level. Every descent into a child node now pushes a continuation onto
an explicit heap-backed work stack instead of calling itself, so nesting depth grows a std::vector,
never the calling thread's stack. This replaces closed PR #266, which instead ran deep definitions on a
64 MB pthread stack — relocating the recursion rather than removing it.

Two defects surfaced during the rewrite and are fixed before this shipped:

1. O(depth²) closure copies. stmt()'s one dispatch point reached on every nesting level passed its
   pending continuation by value into stmtC/stmtPy. A continuation at depth k is itself a chain of
   k nested closures, and std::function's copy constructor deep-copies whatever it closed over — so
   copying it once per level is O(depth) there alone, O(depth²) total (measured: 1,000 nested for loops
   went from 0.02 s to 12 s). Fixed by passing it by reference; only the one branch that actually matches
   ever moves out of it.
2. Unbounded memory retention across fixpoint redos. A loop's own per-round locals were allocated in
   the same arena as everything else, which never frees. When an outer level's reaching-definitions
   fixpoint needs a second round (a binding with 2 reaching defs always does), it redoes its entire body —
   including every loop nested inside it — and none of the superseded round's states were ever reclaimed.
   Measured: 2,040 nested for loops retained 2.6 GB against the recursive form's 13 MB. Fixed by giving a
   loop's own locals shared_ptr ownership instead of the arena, so a superseded round frees the moment
   its closures finish running — the same lifetime the recursive form's stack frames gave for free.

Verification

• Parity: byte-identical to the recursive form (origin/main @ b5ef1700) on 198 real definitions / 790
  --slice invocations (bare, :VAR, and :VAR --slice-flow=both) — ripwire's own src/, two other
  local C++/Python repos (two other local C++/Python codebases), plus the
  #252 re-review's parity fixture set (CPython, MLflow, a GPU kernel repo). 0 diffs.
• Timing: interleaved MAIN vs HEAD over the same 198-definition real-world set — within ~1% total wall
  time. The pathological nested-for-loop synthetic (which both binaries already scale super-linearly on
  — an existing property of the reaching-definitions fixpoint, not new here) runs ~1.5-1.7x slower at
  2,040 levels; real code never approaches that depth.
• Memory: after the shared_ptr fix, peak RSS on 2,040 nested for loops matches the recursive form
  (13.5 MB vs 13.6 MB, was 2.6 GB before the fix). On real functions it was already within ~13% (crawl
  memory dominates on a large repo either way).
• The 2,048-level guard is unchanged. Measured whether it could be raised now that it is a heap
  structure rather than a stack depth: 8,192 nested for loops took 48 s (super-linear, confirmed — this
  is inherent to the fixpoint, present on main too, just untestable there past the guard). Raising the
  ceiling is a time risk, not a safety win, so it stays at 2,048.
• Gates: test/slicecheck.sh (15a)-(15d) — PASS, including under ulimit -s 1024 (RED on the
  pre-change binary at (15b): SIGSEGV, exit 139). mcpslicecheck.sh, sliceflowcheck.sh,
  sliceflowsenscheck.sh, slicediffcheck.sh — ALL PASS. gateexitcheck.sh, manifestcheck.sh,
  gatecountcheck.sh, docs/limits_build.py --check — PASS (no new gate, no cap change). One ASan run
  (through the shared sanitizer-build lock): slicecheck.sh ALL PASS including (15a)/(15b) with the
  caller's stack held to 1 MB — the exact condition that aborted before this change — plus a targeted
  LeakSanitizer check (lsan_suppressions.txt) on both a normal --slice call and the 2,040-loop case:
  clean, no leaks. --quality-delta against the merge-base: gating="0", zero findings of any kind.

Note for the coordinator

origin/main advanced (to bcd3b016, an integration-train merge) partway through this lane's work; per
CI_HANDOFF.txt's standing instruction not to merge origin/main into an in-flight branch, this stays
based on the branch's original merge-base (b5ef1700) and was not rebased.

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Summary by CodeRabbit

• Bug Fixes

  • Improved slicing reliability for deeply nested code, reducing the risk of stack overflow during reaching-definition analysis and occurrence scanning.
  • Improved performance and memory usage when processing deeply nested control-flow structures.

• Compatibility

  • Slice results and control-flow behavior remain unchanged.
  • The existing 2,048-level nesting guard remains in place.

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📝 Walkthrough

Walkthrough

Changes

The slice occurrence and reaching-definitions walks now use heap-backed continuation stacks instead of recursive traversal. Loop state uses shared ownership, and stack capacity is reserved from descendant counts. Traversal output and control-flow semantics remain unchanged.

Slice traversal

Layer / File(s)	Summary
Occurrence walk conversion src/slice.h	sliceWalk and preprocessor traversal now use explicit SliceWalkItem work items. Child order, pruning, and occurrence classification remain preserved.
Reaching-definitions work stack src/slice.h	SliceRdWalker now runs SliceRdStep continuations from a vector, stores branch states in a deque, and handles sequences, structures, branches, and loop fixpoints without recursive descent.
Construct handlers and integration src/slice.h, CHANGELOG.md	C and Python control-flow handlers use continuations. sliceComputeReach drains the work stack, and callers provide node-count hints for stack reservation.

Priority: ⬇️ Low

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

Change: Bug fix

Suggested reviewers: andriytyurnikov

Merge Risk: 🟡 Moderate · up to 796c2

Nested loops containing branches can still consume excessive memory, undermining the PR’s memory-safety objective; this should be fixed before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 43.10% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 1 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: converting the reaching-definitions walk to iterative heap-backed traversal to prevent thread-stack growth from deep nesting.
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.

Full details: Docstring Coverage

Explanation

Docstring coverage is 43.10% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 1 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
In `@src/slice.h`:
- Around line 1428-1431: Make branch states allocated by newState within
loopRound round-scoped rather than retaining them in the walker-wide arena.
Ensure each round releases or shares ownership of its branch states only after
all stmt callbacks and closures for that round have drained, while preserving
loop-local state validity across callbacks and nested loops.

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[joyful-ii-V-I]

Disposition — 1 commit pushed: f63dda0c

• Coordinator's stale-comment item (kMaxSliceDepth in src/slice.h, arm-15 header in test/slicecheck.sh, plus the (15b)/(15d) messages that repeated the same framing): reworded — the guard is a time/memory guard on SliceRdWalker's fixpoint now, not a stack one, both walks are iterative, and the 8,192-loop / 48s measurement is stated as the reason for the bound.
• CodeRabbit's one actionable finding ("make branch states allocated by newState within loopRound round-scoped rather than retaining them in the walker-wide arena"): not applicable to the reviewed commit — loop()/loopRound() already own their states via shared_ptr (stateBox), never arena/newState (confirmed by grep: zero newState calls in either function). Verified with a new stress case closest to what the finding describes — 800 nested for loops each wrapping its own if/else — which shows no memory growth (12.4 MB). No code change made for it.

Gates re-run on f63dda0c: slicecheck.sh (ALL PASS, incl. under ulimit -s 1024), mcpslicecheck.sh, sliceflowcheck.sh, sliceflowsenscheck.sh, slicediffcheck.sh, gateexitcheck.sh, manifestcheck.sh, gatecountcheck.sh, docs/limits_build.py --check — all PASS.

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