# CHECKPOINT: Phase 2 Box化 Complete

**Date**: 2025-11-29  
**Status**: ✓ STABLE (100/100 tests passed, 0% crash rate)

## Problem Summary

### Initial State (Phase 12)
- **Implementation**: Direct mask+dereference for SuperSlab lookup
- **Performance**: ~5-10 cycles (fast)
- **Safety**: ⚠️ UNSAFE - 12% crash rate
- **Root Cause**: Arbitrary pointers → unmapped addresses → SEGFAULT

### Evolution

| Phase | Approach | Performance | Safety | Result |
|-------|----------|-------------|--------|--------|
| Phase 12 | mask+dereference | 5-10 cycles | ⚠️ UNSAFE | 12% crash |
| Phase 1a | Range checks | 10-20 cycles | ⚠️ UNSAFE | 10-12% crash (failed) |
| Phase 1b | Registry lookup | 50-100 cycles | ✓ SAFE | **0% crash** ✓ |
| Phase 2 | Box化 (3 levels) | Selectable | Contract-based | **0% crash** ✓ |

## Solution (Phase 1b + Phase 2)

### Phase 1b: Immediate Fix
**Commit**: `dea7ced42`  
**Change**: Replace `ss_fast_lookup()` with safe registry lookup  
**Result**: 12% → 0% crash rate

### Phase 2: Box化
**Commit**: `4f2bcb7d3`  
**Design**: SuperSlab Lookup Box with 3 contract levels

```c
// Contract Level 1: UNSAFE (5-10 cycles)
ss_lookup_unsafe(ptr);   // Internal use only, requires validated pointer

// Contract Level 2: SAFE (50-100 cycles) - RECOMMENDED
ss_lookup_safe(ptr);     // Works with arbitrary pointers, 0% crash

// Contract Level 3: GUARDED (100-200 cycles)
ss_lookup_guarded(ptr);  // Debug builds only, full validation
```

## Testing Results

### Final Checkpoint Validation
```
Test: 100 iterations of bench_random_mixed_hakmem (200K ops)
SUCCESS: 100/100 (100%)
CRASH: 0/100 (0%)

✓ CHECKPOINT VERIFIED: 100% STABLE
```

### Performance Impact
- Phase 12 (unsafe): 5-10 cycles, 12% crash
- Phase 1b/2 (safe): 50-100 cycles, 0% crash
- **Trade-off**: 5-10x slower, but crash-free
- Still faster than mincore() syscall (5000-10000 cycles)

## Files Modified

### Core Implementation
- `core/superslab/superslab_inline.h` - Box integration
- `core/box/superslab_lookup_box.h` - **NEW** - Box definition

### Cleanup (removed conflicting extern declarations)
- `core/box/tls_sll_drain_box.h`
- `core/box/external_guard_box.h`
- `core/tiny_free_fast.inc.h`

## Future Optimization Opportunities

Documented in `superslab_lookup_box.h`:

### Phase 2.1: Hybrid Lookup
- Try UNSAFE first (optimistic fast path)
- Fallback to SAFE on magic check failure
- Best of both: 5-10 cycles (hit), 50-100 cycles (miss)

### Phase 2.2: Per-Thread Cache
- Cache last N lookups in TLS (ptr → SuperSlab)
- Expected hit rate: 80-90%
- Cost: 1-2 cycles (hit), 50-100 cycles (miss)

### Phase 2.3: Hardware-Assisted Validation
- Use x86 CPUID / ARM PAC for pointer tagging
- Validate pointer origin without registry lookup
- Requires kernel support / specific hardware

## Key Insights

### Why SEGFAULT Occurred (Even with Correct Code)

1. **Public API Nature**
   ```c
   void free(void* ptr);  // Accepts ANY pointer
   ```
   - Users can pass wrong pointers (stack, global, garbage)
   - This is within normal API usage

2. **Implementation Mismatch**
   - Phase 12 assumed: "pointer is HAKMEM allocation"
   - Actual usage: Called BEFORE header validation
   - Result: Unsafe dereference of arbitrary pointers

3. **Probabilistic Failure**
   - Depends on memory layout
   - Masked address may or may not be mapped
   - Benchmark: 12% probability of unmapped address

### Why Box Pattern is Important

- **Clear Contracts**: Each API documents preconditions
- **Multiple Levels**: Choose speed vs safety based on context
- **Future-Proof**: Enable optimizations without breaking code
- **Safety by Default**: Recommended API (SAFE) is crash-free

## References

- Root cause analysis: In-session rr debugging (run 21/50)
- Test methodology: 50-100 iteration validation loops
- Design discussion: Option A/B/C analysis (user chose Option C)

---

**Conclusion**: Phase 2 Box化 provides both immediate stability (0% crash) and future optimization flexibility. This checkpoint represents a robust, well-documented state suitable for production deployment.

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