---
id: scan-spidermonkey
priority: medium
status: unscanned
created: 2026-03-23
---

## Target

**Repo:** `https://github.com/mozilla/gecko-dev` (sparse clone: `js/src/jit/`)
**Language:** C++
**Why:** Firefox JIT compiler — linear vector scans in IonMonkey graph coloring or SCC inflate JIT compilation time on complex JS functions.

## Scan command

```bash
# Sparse shallow clone of just the JIT directory
git clone --depth 1 --filter=blob:none --sparse https://github.com/mozilla/gecko-dev /tmp/spidermonkey
cd /tmp/spidermonkey && git sparse-checkout set js/src/jit

# Submit to unsandbox
cat > /tmp/scan-spidermonkey.sh << 'EOF'
#!/usr/bin/env bash
set -euo pipefail
GRAPH_KW="tarjan|strongly.connected|dfs|scc|topolog|topo.sort|cycle|reachab|postorder|dominator|liveness|digraph|shortest.path|path.find"
MEM="std::find|\.count\(|vector.*find|find.*begin.*end|push_back.*visited|visited\.push_back"
find /tmp/spidermonkey/js/src/jit -name "*.cpp" -o -name "*.h" | sort | while read f; do
  graph_lines=$(grep -inE "$GRAPH_KW" "$f" 2>/dev/null | cut -d: -f1 | head -5 || true)
  [ -z "$graph_lines" ] && continue
  while IFS= read -r lineno; do
    s=$(( lineno > 20 ? lineno - 20 : 1 )); e=$(( lineno + 20 ))
    hit=$(sed -n "${s},${e}p" "$f" 2>/dev/null | grep -iE "$MEM" | head -1 | sed 's/^\s*//' || true)
    [ -n "$hit" ] && echo "CANDIDATE\t${f}:${lineno}\t${hit}"
  done <<< "$graph_lines"
done
EOF
export UNSANDBOX_PUBLIC_KEY=unsb-pk-russ-test-isth-best
export UNSANDBOX_SECRET_KEY=unsb-sk-rk46c-3zvpg-zjf6z-hjrge
~/git/un-inception/build/un /tmp/scan-spidermonkey.sh
```

## Key files to check

- `js/src/jit/IonAnalysis.cpp` — IonMonkey IR analysis passes; SCC, dominators, liveness
- `js/src/jit/BacktrackingAllocator.cpp` — Register allocator; live range graph coloring with visited tracking
- `js/src/jit/MIR.cpp` — MIR graph node management; use/def chain traversal

## Expected pattern

`std::find(vec.begin(), vec.end(), block)` where `vec` is a `Vector<MBasicBlock*>` used as a visited set inside `BuildDominatorTree` or `AnalyzeNewScriptDefiniteProperties` — or equivalent loop in `BacktrackingAllocator` over a `LiveRangeVector`.

## Acceptance criteria

- [ ] Scan run and results saved to `tools/scan-results/spidermonkey.txt`
- [ ] All CANDIDATE hits triaged (confirmed defect or false positive)
- [ ] If confirmed: defect ticket created in `tools/tickets/defects/`