feat(joinir): Phase 175 - P5 multi-carrier architecture validation

Task 175-1: Analyzed _parse_string carrier candidates
- Identified 3 carriers: pos (position), result (buffer), is_ch_match (promoted)
- Categorized as: required carriers (pos, result), promoted carrier (is_ch_match)

Task 175-2: Validated existing boxes support multi-carrier
- CarrierInfo: Vec<CarrierData> supports arbitrary carriers ✅
- LoopUpdateAnalyzer: Loops over all carriers ✅
- ExitMeta: Vec<(String, ValueId)> supports all exit bindings ✅
- ExitLineReconnector: Reconnects all carriers to variable_map ✅
- No code changes needed - architecture already supports it!

Task 175-3: PoC test revealed Pattern2 limitation
- Test: test_jsonparser_parse_string_min2.hako (pos + result carriers)
- CarrierInfo detected 3 carriers correctly (pos, result, is_ch_match)
- variable_map contains all carriers at pattern2_start
- BUT: Pattern2's Trim optimization only emits pos carrier in MIR
- MIR shows result stays as empty string (no loop update emitted)
- Root cause: Trim pattern focuses on position-only optimization

Task 175-4: Documentation updates
- Created: phase175-multicarrier-design.md (comprehensive analysis)
- Updated: CURRENT_TASK.md (Phase 175 completion)
- Updated: routing.rs (added JsonParserStringTest2 whitelist)

Key Finding:
- Architecture is sound ✅ - all boxes support multi-carrier
- Pattern2 implementation gap ❌ - Trim optimization ignores non-position carriers
- Phase 176 scope: Extend Pattern2 to emit all carrier updates

Next: Phase 176 for escape sequence handling and full multi-carrier emission

CURRENT_TASK.md

@@ -69,6 +69,14 @@
         → Task 174-5: ドキュメント更新（phase174-jsonparser-p5b-design.md + CURRENT_TASK）。
         → **成果**: Trim P5 パイプラインが `_parse_string` 最小化版でも機能することを実証。
               文字比較対象が `"\""`（終端クォート）でも Trim と同じ昇格パターンで動作確認。
+  - [x] Phase 175: P5 複数キャリア対応（_parse_string 向け） ✅ (2025-12-08)
+        → Task 175-1: _parse_string のキャリア候補を洗い出す（pos, result, is_ch_match の 3 キャリア特定）。
+        → Task 175-2: CarrierInfo / ExitMeta に複数キャリアを載せる設計を固める（既存箱がすべて複数対応済みと確認）。
+        → Task 175-3: 小さい PoC で 2 キャリアだけ通してみる（MIR 検証で Pattern2 の制約を特定）。
+        → Task 175-4: CURRENT_TASK / joinir-architecture-overview 更新。
+        → **成果**: P5 アーキテクチャが複数キャリアに対応済みであることを確認。
+              CarrierInfo は 3 キャリア検出済み（pos, result, is_ch_match）。
+              Pattern2 実装が Trim 最適化により `pos` のみ emit する制約を発見（Phase 176 改善対象）。
 
 ---
 

docs/development/current/main/phase175-multicarrier-design.md

@@ -0,0 +1,259 @@
+# Phase 175: P5 Multiple Carrier Support Design
+
+**Date**: 2025-12-08
+**Purpose**: Extend P5 pipeline to support multiple carriers for complex loops like `_parse_string`
+
+---
+
+## 1. Target: _parse_string Carrier Analysis
+
+### 1.1 Loop Structure (lines 150-178)
+
+```hako
+_parse_string(s, pos) {
+    if s.substring(pos, pos+1) != '"' { return null }
+
+    local p = pos + 1      // Carrier 1: position index
+    local str = ""         // Carrier 2: result buffer
+
+    loop(p < s.length()) {
+        local ch = s.substring(p, p+1)  // LoopBodyLocal (promotion candidate)
+
+        if ch == '"' {
+            // End of string
+            local result = new MapBox()
+            result.set("value", me._unescape_string(str))  // Uses str carrier
+            result.set("pos", p + 1)                        // Uses p carrier
+            result.set("type", "string")
+            return result
+        }
+
+        if ch == "\\" {
+            // Escape sequence (Phase 176 scope)
+            local has_next = 0
+            if p + 1 < s.length() { has_next = 1 }
+            if has_next == 0 { return null }
+
+            str = str + ch
+            p = p + 1
+            str = str + s.substring(p, p+1)
+            p = p + 1
+            continue  // ⚠️ Phase 176 scope
+        }
+
+        str = str + ch  // Update carrier 2
+        p = p + 1       // Update carrier 1
+    }
+
+    return null
+}
+```
+
+### 1.2 Carrier Candidates Table
+
+| Variable | Type | Update Pattern | Exit Usage | Carrier Status |
+|---------|------|----------------|------------|----------------|
+| `p` | IntegerBox | `p = p + 1` | Position in `result.set("pos", p + 1)` | ✅ **Required Carrier** |
+| `str` | StringBox | `str = str + ch` | String buffer in `result.set("value", me._unescape_string(str))` | ✅ **Required Carrier** |
+| `ch` | StringBox | `local ch = s.substring(p, p+1)` | Loop body comparison | ❌ **LoopBodyLocal** (promotion target) |
+| `has_next` | IntegerBox | `local has_next = 0` | Escape processing guard | ❌ **Loop body only** (Phase 176) |
+
+### 1.3 Carrier Classification
+
+**Required Carriers (Exit-dependent)**:
+1. **`p`**: Position index - final value used in `result.set("pos", p + 1)`
+2. **`str`**: Result buffer - final value used in `result.set("value", me._unescape_string(str))`
+
+**Promoted Carriers (P5 mechanism)**:
+3. **`is_ch_match`**: Bool carrier promoted from `ch` (Trim pattern detection)
+   - Pattern: `ch = s.substring(p, p+1)` → `ch == "\""` equality chain
+   - Promotion: `LoopBodyCarrierPromoter` converts to bool carrier
+
+**Loop-Internal Only (No carrier needed)**:
+- `ch`: LoopBodyLocal, promotion target → becomes `is_ch_match` carrier
+- `has_next`: Escape sequence guard (Phase 176 scope)
+
+---
+
+## 2. Phase 175 Minimal PoC Scope
+
+**Goal**: Prove multi-carrier support with 2 carriers (`p` + `str`), excluding escape handling
+
+### 2.1 Minimal PoC Structure
+
+```hako
+_parse_string_min2() {
+    me.s = "hello world\""
+    me.pos = 0
+    me.len = me.s.length()
+    me.result = ""
+
+    // 2-carrier version: p + result updated together
+    loop(me.pos < me.len) {
+        local ch = me.s.substring(me.pos, me.pos+1)
+        if ch == "\"" {
+            break
+        } else {
+            me.result = me.result + ch  // Carrier 2 update
+            me.pos = me.pos + 1         // Carrier 1 update
+        }
+    }
+
+    // Exit: both pos and result are used
+    print("Parsed string: ")
+    print(me.result)
+    print(", final pos: ")
+    print(me.pos)
+}
+```
+
+**Carrier Count**: 2 (`pos`, `result`) + 1 promoted (`is_ch_match`) = **3 total**
+
+**Excluded from Phase 175**:
+- ❌ Escape sequence handling (`\\"`, `continue` path)
+- ❌ Complex nested conditionals
+- ✅ Focus: Simple char accumulation + position increment
+
+---
+
+## 3. P5 Multi-Carrier Architecture
+
+### 3.1 Existing Boxes Already Support Multi-Carrier ✅
+
+#### 3.1.1 LoopUpdateAnalyzer (Multi-carrier ready ✅)
+
+**File**: `src/mir/join_ir/lowering/loop_update_analyzer.rs`
+**API**: `identify_updated_carriers(body, all_carriers) -> CarrierInfo`
+**Multi-carrier support**: ✅ Loops over `all_carriers.carriers`
+
+**Code**:
+```rust
+pub fn identify_updated_carriers(
+    body: &[ASTNode],
+    all_carriers: &CarrierInfo,
+) -> Result<CarrierInfo, String> {
+    let mut updated = CarrierInfo::new();
+    for carrier in &all_carriers.carriers {  // ✅ Multi-carrier loop
+        if is_updated_in_body(body, &carrier.name) {
+            updated.add_carrier(carrier.clone());
+        }
+    }
+    Ok(updated)
+}
+```
+
+#### 3.1.2 LoopBodyCarrierPromoter (Adds carriers ✅)
+
+**File**: `src/mir/loop_pattern_detection/loop_body_carrier_promoter.rs`
+**API**: `try_promote(request) -> PromotionResult`
+**Multi-carrier support**: ✅ Generates **additional carriers** from promotion
+
+**Behavior**:
+```rust
+let promoted = LoopBodyCarrierPromoter::try_promote(&request)?;
+carrier_info.merge_from(&promoted.to_carrier_info());  // Add promoted carrier
+// Result: carrier_info.carriers = [pos, result, is_ch_match]
+```
+
+#### 3.1.3 CarrierInfo (Multi-carrier container ✅)
+
+**File**: `src/mir/join_ir/lowering/carrier_info.rs`
+**API**: `carriers: Vec<CarrierData>`, `merge_from(&other)`
+**Multi-carrier support**: ✅ `Vec` holds arbitrary number of carriers
+
+**Phase 175-3 Usage**:
+```rust
+let mut carrier_info = CarrierInfo::new();
+carrier_info.add_carrier(CarrierData {  // Carrier 1
+    name: "pos".to_string(),
+    update_expr: UpdateExpr::Simple { ... },
+});
+carrier_info.add_carrier(CarrierData {  // Carrier 2
+    name: "result".to_string(),
+    update_expr: UpdateExpr::Simple { ... },
+});
+// carrier_info.carriers.len() == 2 ✅
+```
+
+#### 3.1.4 ExitMeta / ExitBinding (Multi-carrier ready ✅)
+
+**File**: `src/mir/builder/control_flow/joinir/merge/exit_phi_builder.rs`
+**API**: `carrier_exits: Vec<(String, ValueId)>`
+**Multi-carrier support**: ✅ `Vec` holds all carrier exits
+
+**ExitMetaCollector Behavior**:
+```rust
+for carrier in &carrier_info.carriers {  // ✅ Multi-carrier loop
+    exit_bindings.push((carrier.name.clone(), exit_value_id));
+}
+// exit_bindings = [("pos", ValueId(10)), ("result", ValueId(20)), ("is_ch_match", ValueId(30))]
+```
+
+#### 3.1.5 ExitLineReconnector (Multi-carrier ready ✅)
+
+**File**: `src/mir/builder/control_flow/joinir/merge/mod.rs`
+**API**: `reconnect_exit_bindings(exit_bindings, loop_header_phi_info, variable_map)`
+**Multi-carrier support**: ✅ Loops over all `exit_bindings`
+
+**Behavior**:
+```rust
+for (carrier_name, _) in exit_bindings {  // ✅ Multi-carrier loop
+    if let Some(phi_dst) = loop_header_phi_info.get_carrier_phi_dst(carrier_name) {
+        variable_map.insert(carrier_name.clone(), phi_dst);
+    }
+}
+// variable_map: {"pos" -> ValueId(100), "result" -> ValueId(200), "is_ch_match" -> ValueId(300)}
+```
+
+---
+
+## 4. Conclusion: Architecture Supports Multi-Carrier ✅ (Pattern2 limitation found)
+
+### 4.1 Phase 175-3 Test Results
+
+**Test**: `local_tests/test_jsonparser_parse_string_min2.hako`
+
+**MIR Analysis**:
+```mir
+bb3:
+    %9 = copy %8  // result = "" (initialization)
+
+bb5:  // Loop header
+    %25 = phi [%4, bb3], [%21, bb10]  // Only pos carrier!
+    // ❌ Missing: result carrier PHI
+
+bb10:  // Loop update
+    %21 = %25 Add %20  // pos = pos + 1
+    // ❌ Missing: result = result + ch update
+
+bb12:  // Exit block
+    %29 = copy %9  // Still uses original %9 (empty string)!
+```
+
+**Root Cause**: Pattern2's Trim optimization only emits `pos` carrier, ignoring `result` updates in loop body.
+
+**Architecture Validation** ✅:
+- ✅ `CarrierInfo` detected 3 carriers (`pos`, `result`, `is_ch_match`)
+- ✅ `variable_map` contains all carriers at pattern2_start
+- ✅ Existing boxes (ExitMeta, ExitLineReconnector) support multi-carrier
+- ❌ Pattern2 lowerer only emits loop update for `pos`, not `result`
+
+**Conclusion**:
+- **Architecture is sound** - all boxes support multi-carrier
+- **Pattern2 implementation gap** - Trim optimization doesn't emit body updates for non-position carriers
+- **Phase 176 scope** - Extend Pattern2 to emit all carrier updates, not just position
+
+### 4.2 Next Steps
+
+- **Phase 175-3**: Run PoC test (`test_jsonparser_parse_string_min2.hako`)
+- **Phase 176**: Add escape sequence handling (`continue` path, Phase 176 scope)
+- **Phase 177**: Full `_parse_string` with all edge cases
+
+---
+
+## 5. References
+
+- **Phase 170**: LoopUpdateSummary design
+- **Phase 171**: LoopBodyCarrierPromoter implementation
+- **Phase 174**: P5 minimal PoC (quote detection only)
+- **Pattern Space**: [docs/development/current/main/loop_pattern_space.md](loop_pattern_space.md)

src/mir/builder/control_flow/joinir/routing.rs

@@ -94,6 +94,9 @@ impl MirBuilder {
                 // Phase 174: JsonParser complex loop P5B extension test
                 "JsonParserStringTest.parse_string_min/0" => true,
                 "JsonParserStringTest.main/0" => true,
+                // Phase 175: P5 multi-carrier support (2 carriers: pos + result)
+                "JsonParserStringTest2.parse_string_min2/0" => true,
+                "JsonParserStringTest2.main/0" => true,
                 _ => false,
             };