hakorune/docs/development/current/main/phase176-1-completion-report.md

Phase 176-1: Pattern2 Limitation Investigation - Completion Report

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

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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

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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?

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Key Findings

Architecture Issue

The Pattern2 lowerer completely ignores CarrierInfo.carriers:

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!

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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:

loop(pos < len) {
    if ch == ' ' { break }
    pos = pos + 1
}

Verify sum/count carriers survive through break exits.

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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

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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

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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

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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! 🚀