Status: VerificationReport, Historical

# Phase 176-1: Pattern2 Limitation Investigation - Completion Report

**Date**: 2025-12-08
**Status**: ✅ COMPLETE
**Task**: Mark all single-carrier limitations in Pattern2 lowerer

---

## What Was Done

Investigated `src/mir/join_ir/lowering/loop_with_break_minimal.rs` and identified **10 critical points** where the lowerer currently only handles the position carrier (`i`) and ignores `CarrierInfo.carriers`.

### Files Modified

1. **`src/mir/join_ir/lowering/loop_with_break_minimal.rs`**
   - Added 10 TODO comments marking limitation points
   - No code changes (read + memo level only)

2. **`docs/development/current/main/phase176-pattern2-limitations.md`**
   - Created comprehensive limitation report
   - Detailed explanation of each limitation point
   - Impact analysis and next steps

---

## Limitation Points Identified

### Easy Fixes (9 points) - Iteration-based
1. **ValueId Allocation** (Line 172) - Only allocates for position carrier
2. **Main Function Params** (Line 208) - Only takes `i_init`
3. **Loop Step Call Args** (Line 214) - Only passes `i_init`
4. **Loop Step Params** (Line 234) - Only takes `i_param`
5. **Natural Exit Jump** (Line 257) - Only passes `i_param` to k_exit
6. **Break Exit Jump** (Line 272) - Only passes `i_param` to k_exit
7. **Tail Call Args** (Line 304) - Only passes `i_next`
8. **K_Exit Params** (Line 319) - Only takes `i_exit`
9. **ExitMeta Construction** (Line 344) - Only includes position carrier

### Hard Fix (1 point) - Requires AST Body Analysis
10. **Loop Body Updates** (Line 284) - Only computes `i_next = i + 1`
    - **Challenge**: Need to analyze AST body to determine carrier updates
    - **Example**: How do we know `sum = sum + x` updates the `sum` carrier?

---

## Key Findings

### Architecture Issue
The Pattern2 lowerer completely ignores `CarrierInfo.carriers`:

```rust
pub struct CarrierInfo {
    pub loop_var_name: String,    // Used ✅
    pub loop_var_id: ValueId,     // Used ✅
    pub carriers: Vec<CarrierVar>, // IGNORED ❌
    pub trim_helper: Option<TrimLoopHelper>,
}
```

The function signature only takes `loop_var_name` as a separate string parameter, losing access to the full CarrierInfo structure.

### Infrastructure Ready
- ✅ **CarrierInfo**: Already multi-carrier ready (Phase 175)
- ✅ **ExitMeta**: Supports `ExitMeta::multiple(vec![...])` for multi-carrier
- ✅ **LoopHeaderPhiBuilder**: Multi-carrier ready (Phase 175)
- ✅ **ExitPhiBuilder**: Multi-carrier ready (Phase 175)

**Problem**: Pattern2 lowerer doesn't use these capabilities!

---

## Next Phase Roadmap

### Phase 176-2: Iteration-Based Fixes
**Difficulty**: Easy
**Estimate**: 1-2 hours

Fix points 1-6, 8-10 by iterating over `CarrierInfo.carriers`:
- Allocate ValueIds for all carriers
- Extend function params/call args/jump args
- Build multi-carrier ExitMeta

### Phase 176-3: Loop Body Analysis
**Difficulty**: Hard
**Estimate**: 3-4 hours

Fix point 7 by analyzing AST body:
- Track carrier assignments in loop body
- Emit update instructions for each carrier
- Handle complex cases (conditional updates, etc.)

### Integration Test
Pattern 3 (trim) with Pattern 2 shape:
```nyash
loop(pos < len) {
    if ch == ' ' { break }
    pos = pos + 1
}
```

Verify sum/count carriers survive through break exits.

---

## Deliverables

1. ✅ **TODO Comments**: 10 markers added to `loop_with_break_minimal.rs`
2. ✅ **Limitation Report**: `phase176-pattern2-limitations.md`
3. ✅ **Completion Report**: This document

---

## How to Use This Report

For Task 176-2/3 implementers:

1. **Read the limitation report first**: `phase176-pattern2-limitations.md`
2. **Start with easy fixes**: Points 1-6, 8-10 (iteration-based)
3. **Tackle hard fix last**: Point 7 (loop body analysis)
4. **Use TODO markers as guide**: Search for `TODO(Phase 176)` in code
5. **Test with Pattern 3**: Use trim pattern as integration test

---

## Related Files

- **Main file**: `src/mir/join_ir/lowering/loop_with_break_minimal.rs`
- **CarrierInfo**: `src/mir/join_ir/lowering/carrier_info.rs`
- **Limitation report**: `docs/development/current/main/phase176-pattern2-limitations.md`
- **This report**: `docs/development/current/main/phase176-1-completion-report.md`

---

## Conclusion

Task 176-1 successfully identified and documented all 10 single-carrier limitations in the Pattern2 lowerer. The code is now well-marked with TODO comments, and a comprehensive analysis report is available for the next implementation phases.

**Ready for Phase 176-2/3 implementation!** 🚀
