docs(phi): Phase 25.1 - LoopForm v2 コメント整備 + ケース表完成

- ✅ [LoopForm] タグで統一コメント追加
  - src/mir/loop_builder.rs
    - header-cond: Case A/B分岐説明
    - exit-break path / continue-backedge path
    - exit PHI for Case A/B
  - src/mir/phi_core/loop_snapshot_merge.rs
    - Case A/B分岐: header ∈ exit_preds判定ロジック
  - src/mir/phi_core/exit_phi_builder.rs
    - LoopForm Process ステップバイステップ説明

- ✅ UsingCollectorBox Region+next_i化
  - lang/src/compiler/parser/using/using_collector_box.hako
    - 全ループをLoopForm v2形式に統一
    - next_i, next_j, next_k, next_t パターン導入
    - SSA安全化（未定義変数撲滅）

- ✅ LoopForm v2 ケース表完成
  - docs/development/architecture/loops/loopform_ssot.md
    - Case A/B/C/D の完全な表
    - テスト対応マッピング
    - 実装ファイル対応表

🎯 成果: LoopForm v2の「形」をソース・テスト・ドキュメントで完全固定

docs/development/architecture/loops/loopform_ssot.md

@@ -27,7 +27,58 @@ SSOT（Single Source of Truth）
 - Bridge 側に `LoopPhiOps` 実装を追加し、`prepare/seal/exit` を直接呼ぶ。
 - ループ形状の生成をユーティリティ化（builder/bridge 双方から共通呼び出し）。
 
+---
+
+## LoopForm v2 ケース表
+
+| Case | loop 条件形 | exit preds の構成 | 想定される PHI 入力の形 | 対応テスト | 対応 .hako |
+|------|-----------|-----------------|---------------------|----------|-----------|
+| **A** | `loop(i < n)` | header / body | header fallthrough + break | `loop_conditional_reassign_exit_phi_header_and_break` | - |
+| **B** | `loop(1 == 1)` | body のみ | break のみ | `loop_constant_true_exit_phi_dominates` | `apps/tests/minimal_ssa_skip_ws.hako` |
+| **C** | `loop(1 == 1)` + body-local | body のみ（一部の経路のみ定義） | break のみ（BodyLocalInternal は除外） | `loop_body_local_exit_phi_body_only` | - |
+| **D** | `loop(i < n)` + continue | header / body / continue_merge | header + break + continue_merge | `loop_continue_merge_header_exit` | - |
+
+### ケース説明
+
+#### Case A: header+break（標準パターン）
+- **条件**: `loop(i < n)` のような動的条件
+- **特徴**: header→exit と body→exit の両方が CFG 上存在
+- **exit PHI**: header fallthrough + break 経路の両方を含む
+- **検証**: `loop_conditional_reassign_exit_phi_header_and_break`
+
+#### Case B: constant-true+break-only（header 除外パターン）
+- **条件**: `loop(1 == 1)` のような定数 true
+- **特徴**: exit pred は break 経路のみ（`header→exit` は無い）
+- **exit PHI**: break 経路のみ（header は CFG predecessor でないため除外）
+- **検証**: `loop_constant_true_exit_phi_dominates` + `minimal_ssa_skip_ws.hako`
+
+#### Case C: body-local変数（BodyLocalInternal 除外パターン）
+- **条件**: body 内で宣言された変数が一部の exit 経路でのみ定義される
+- **特徴**: 変数が全 exit predecessors で定義されていない
+- **exit PHI**: BodyLocalInternal 変数は除外（PHI 生成しない）
+- **検証**: `loop_body_local_exit_phi_body_only`
+
+#### Case D: continue+break（continue_merge パターン）
+- **条件**: continue 文を含むループ
+- **特徴**: `continue_merge → header → exit` の経路あり
+- **exit PHI**: header + break + continue_merge の 3 系統
+- **検証**: `loop_continue_merge_header_exit`
+
+### 実装ファイル
+
+| ファイル | 役割 |
+|---------|-----|
+| `src/mir/loop_builder.rs` | ループ構造生成・[LoopForm] コメント付き |
+| `src/mir/phi_core/loop_snapshot_merge.rs` | Case A/B 分岐ロジック |
+| `src/mir/phi_core/exit_phi_builder.rs` | Exit PHI 生成・Phantom block 除外 |
+| `src/tests/mir_loopform_conditional_reassign.rs` | 4 ケース全てのテスト（[LoopForm-Test] タグ付き） |
+
+---
+
 関連
 - `src/mir/loop_builder.rs`
 - `src/runner/json_v0_bridge/lowering/loop_.rs`
 - `src/mir/phi_core/common.rs`
+- `src/mir/phi_core/loop_snapshot_merge.rs`
+- `src/mir/phi_core/exit_phi_builder.rs`
+- `src/tests/mir_loopform_conditional_reassign.rs`

lang/src/compiler/parser/using/using_collector_box.hako

@@ -11,28 +11,47 @@ using lang.compiler.parser.scan.parser_common_utils_box as ParserCommonUtilsBox
 static box UsingCollectorBox {
 
   // Public API: collect line-based using declarations to JSON array string
+  // Refactored to LoopForm v2 / Region+next_i pattern (Phase 25.1)
   collect(src) {
     if src == null { return "[]" }
     local n = src.length()
     local i = 0
     local first = 1
     local out = "["
+
+    // Main line-scanning loop: Region+next_i形
     loop(i < n) {
-      // line slice [i, j)
+      local next_i = i
+
+      // Find line boundaries [i, j) - Region+next_j形
       local j = i
-      loop(j < n && src.substring(j, j+1) != "\n") { j = j + 1 }
+      loop(j < n && src.substring(j, j+1) != "\n") {
+        local next_j = j + 1
+        j = next_j
+      }
       local line = src.substring(i, j)
-      // trim left spaces/tabs
+
+      // Trim left spaces/tabs - Region+next_k形
       local k = 0
-      loop(k < line.length() && (line.substring(k,k+1) == " " || line.substring(k,k+1) == "\t")) { k = k + 1 }
+      loop(k < line.length() && (line.substring(k,k+1) == " " || line.substring(k,k+1) == "\t")) {
+        local next_k = k + 1
+        k = next_k
+      }
+
+      // Process if line starts with "using "
       if ParserCommonUtilsBox.starts_with(line, k, "using ") == 1 {
         local rest = ParserCommonUtilsBox.trim(line.substring(k + 6, line.length()))
-        // split on ' as '
+
+        // Split on ' as ' - initialize all variables before conditional use
         local as_pos = ParserCommonUtilsBox.index_of(rest, 0, " as ")
         local target = rest
         local alias = null
-        if as_pos >= 0 { target = ParserCommonUtilsBox.trim(rest.substring(0, as_pos)) alias = ParserCommonUtilsBox.trim(rest.substring(as_pos + 4, rest.length())) }
-        // path or namespace
+        if as_pos >= 0 {
+          target = ParserCommonUtilsBox.trim(rest.substring(0, as_pos))
+          alias = ParserCommonUtilsBox.trim(rest.substring(as_pos + 4, rest.length()))
+        }
+
+        // Determine if target is a path or namespace
         local is_path = 0
         if target.length() > 0 {
           if ParserCommonUtilsBox.starts_with(target, 0, ParserCommonUtilsBox.dq()) == 1 { is_path = 1 }
@@ -41,38 +60,71 @@ static box UsingCollectorBox {
           if target.length() >= 5 && ParserCommonUtilsBox.starts_with(target, target.length()-5, ".hako") == 1 { is_path = 1 }
           if target.length() >= 6 && ParserCommonUtilsBox.starts_with(target, target.length()-6, ".hako") == 1 { is_path = 1 }
         }
+
+        // Initialize name and path before conditional blocks
         local name = ""
         local path = null
+
         if is_path == 1 {
-          // strip quotes
+          // Strip quotes if present - use temp var to avoid SSA issues
+          local clean_target = target
           if ParserCommonUtilsBox.starts_with(target, 0, ParserCommonUtilsBox.dq()) == 1 {
-            target = target.substring(1, target.length())
-            if target.length() > 0 && target.substring(target.length()-1, target.length()) == ParserCommonUtilsBox.dq() { target = target.substring(0, target.length()-1) }
+            local temp1 = target.substring(1, target.length())
+            clean_target = temp1
+            if temp1.length() > 0 && temp1.substring(temp1.length()-1, temp1.length()) == ParserCommonUtilsBox.dq() {
+              clean_target = temp1.substring(0, temp1.length()-1)
+            }
           }
-          path = target
-          if alias != null { name = alias } else {
-            // basename
-            local p = target
+          path = clean_target
+
+          // Determine name from alias or basename
+          if alias != null {
+            name = alias
+          } else {
+            // Extract basename: Region+next_t形 - use temp vars to avoid SSA issues
+            local p = clean_target
             local idx = -1
             local t = 0
-            loop(t < p.length()) { if p.substring(t,t+1) == "/" { idx = t } t = t + 1 }
-            if idx >= 0 { p = p.substring(idx+1, p.length()) }
-            // strip extension
-            if p.length() > 5 && ParserCommonUtilsBox.starts_with(p, p.length()-5, ".hako") == 1 { p = p.substring(0, p.length()-5) }
-            else { if p.length() > 6 && ParserCommonUtilsBox.starts_with(p, p.length()-6, ".hako") == 1 { p = p.substring(0, p.length()-6) } }
-            name = p
+            loop(t < p.length()) {
+              local next_t = t + 1
+              if p.substring(t,t+1) == "/" { idx = t }
+              t = next_t
+            }
+
+            // Extract substring only if idx found
+            local p2 = p
+            if idx >= 0 { p2 = p.substring(idx+1, p.length()) }
+
+            // Strip extension - use temp vars
+            local p3 = p2
+            if p2.length() > 5 && ParserCommonUtilsBox.starts_with(p2, p2.length()-5, ".hako") == 1 {
+              p3 = p2.substring(0, p2.length()-5)
+            } else if p2.length() > 6 && ParserCommonUtilsBox.starts_with(p2, p2.length()-6, ".hako") == 1 {
+              p3 = p2.substring(0, p2.length()-6)
+            }
+            name = p3
           }
         } else {
           name = target
         }
-        // append entry
-        if first == 0 { out = out + "," } else { first = 0 }
+
+        // Append entry to output
+        if first == 0 {
+          out = out + ","
+        } else {
+          first = 0
+        }
         out = out + "{" + ParserCommonUtilsBox.dq() + "name" + ParserCommonUtilsBox.dq() + ":" + ParserCommonUtilsBox.dq() + ParserCommonUtilsBox.esc_json(name) + ParserCommonUtilsBox.dq()
-        if path != null { out = out + "," + ParserCommonUtilsBox.dq() + "path" + ParserCommonUtilsBox.dq() + ":" + ParserCommonUtilsBox.dq() + ParserCommonUtilsBox.esc_json(path) + ParserCommonUtilsBox.dq() }
+        if path != null {
+          out = out + "," + ParserCommonUtilsBox.dq() + "path" + ParserCommonUtilsBox.dq() + ":" + ParserCommonUtilsBox.dq() + ParserCommonUtilsBox.esc_json(path) + ParserCommonUtilsBox.dq()
+        }
         out = out + "}"
       }
-      i = j + 1
+
+      next_i = j + 1
+      i = next_i
     }
+
     out = out + "]"
     return out
   }

src/mir/loop_builder.rs

@@ -1,23 +1,35 @@
 /*!
  * MIR Loop Builder - SSA形式でのループ構築専用モジュール
  *
- * Sealed/Unsealed blockとPhi nodeを使った正しいループ実装
- * Based on Gemini's recommendation for proper SSA loop handling
+ * Sealed/Unsealed blockとPhi nodeを使った正しいループ実装。
+ *
+ * LoopForm v2 の「形」をここで固定している:
+ * - preheader: ループに入る直前のブロック（初期値の copy 発生源）
+ * - header   : ループ条件を評価するブロック（`loop(cond)` の `cond` 部分）
+ * - body     : ループ本体（ユーザーコードが書いたブロック）
+ * - latch    : body の末尾から header へ戻る backedge 用ブロック
+ * - exit     : ループ脱出先（`break` / `cond == false` が合流するブロック）
+ *
+ * 典型パターン（ControlForm::LoopShape）:
+ * - Case A: header-cond + header→exit + body→exit（`loop(i < n) { if (...) break }`）
+ * - Case B: constant-true + body→exit のみ（`loop(1 == 1) { if (...) break }`）
+ *   - この場合、header→exit のエッジは存在しないので、exit PHI に header 値を入れてはいけない。
+ * - Case C: continue_merge を経由して header に戻る経路あり（`continue` を含むループ）。
+ *
+ * それぞれのケースは ControlForm / LoopSnapshotMergeBox / ExitPhiBuilder に伝搬され、
+ * exit PHI の入力選択や BodyLocalInternal 変数の扱いに反映される。
  */
 
 use super::{BasicBlockId, ConstValue, MirInstruction, ValueId};
-use crate::mir::control_form::{ControlForm, IfShape, LoopShape, is_control_form_trace_on};
-use crate::mir::phi_core::loopform_builder::{LoopFormBuilder, LoopFormOps};
-use crate::mir::phi_core::loop_snapshot_merge::LoopSnapshotMergeBox;
-use crate::mir::phi_core::phi_input_collector::PhiInputCollector;
 use crate::ast::ASTNode;
+use crate::mir::control_form::{is_control_form_trace_on, ControlForm, IfShape, LoopShape};
+use crate::mir::phi_core::loop_snapshot_merge::LoopSnapshotMergeBox;
+use crate::mir::phi_core::loopform_builder::{LoopFormBuilder, LoopFormOps};
+use crate::mir::phi_core::phi_input_collector::PhiInputCollector;
 use std::collections::HashMap;
 
 // Phase 15 段階的根治戦略：制御フローユーティリティ
-use super::utils::{
-    is_current_block_terminated,
-    capture_actual_predecessor_and_jump,
-};
+use super::utils::{capture_actual_predecessor_and_jump, is_current_block_terminated};
 
 /// ループ脱出の種類（箱化・共通化のための型）
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -50,11 +62,9 @@ pub struct LoopBuilder<'a> {
 
     /// break文からの変数スナップショット（exit PHI生成用）
     exit_snapshots: Vec<(BasicBlockId, HashMap<String, ValueId>)>,
-
     // フェーズM: no_phi_modeフィールド削除（常にPHI使用）
 }
 
-
 impl<'a> LoopBuilder<'a> {
     // Implement phi_core LoopPhiOps on LoopBuilder for in-place delegation
 
@@ -83,27 +93,31 @@ impl<'a> LoopBuilder<'a> {
         Ok(void_id)
     }
 
-    /// 【箱化】ループ脱出の共通処理（break/continue統一化）
+    /// [LoopForm] 【箱化】ループ脱出の共通処理（break/continue統一化）
     ///
     /// Phase 25.1o: break と continue の共通パターンを抽出し、
     /// LoopExitKind で振る舞いを切り替える統一メソッド。
     ///
     /// # 処理フロー
     /// 1. 現在の変数マップをスナップショット
-    /// 2. 適切なスナップショットリストに追加（Break → exit_snapshots, Continue → continue_snapshots）
-    /// 3. ターゲットブロックへジャンプ（Break → exit, Continue → header）
+    /// 2. [LoopForm] スナップショット保存（Break → exit_snapshots, Continue → continue_snapshots）
+    /// 3. [LoopForm] ターゲットブロックへジャンプ（Break → exit, Continue → header/continue_merge）
     /// 4. unreachable ブロックに切り替え
     fn do_loop_exit(&mut self, kind: LoopExitKind) -> Result<ValueId, String> {
         // 1. スナップショット取得（共通処理）
         let snapshot = self.get_current_variable_map();
         let cur_block = self.current_block()?;
 
-        // 2. スナップショット保存（kind別処理）
+        // 2. [LoopForm] exit-break path: スナップショット保存（exit PHI入力用）
+        // [LoopForm] continue-backedge path: スナップショット保存（continue_merge → header）
         match kind {
             LoopExitKind::Break => {
                 if std::env::var("NYASH_LOOPFORM_DEBUG").ok().as_deref() == Some("1") {
-                    eprintln!("[DEBUG/do_break] Saved snapshot from block {:?}, vars: {:?}",
-                        cur_block, snapshot.keys().collect::<Vec<_>>());
+                    eprintln!(
+                        "[DEBUG/do_break] Saved snapshot from block {:?}, vars: {:?}",
+                        cur_block,
+                        snapshot.keys().collect::<Vec<_>>()
+                    );
                 }
                 self.exit_snapshots.push((cur_block, snapshot));
             }
@@ -116,7 +130,8 @@ impl<'a> LoopBuilder<'a> {
         // 3. ターゲットブロックへジャンプ（kind別処理）
         match kind {
             LoopExitKind::Break => {
-                if let Some(exit_bb) = crate::mir::builder::loops::current_exit(self.parent_builder) {
+                if let Some(exit_bb) = crate::mir::builder::loops::current_exit(self.parent_builder)
+                {
                     self.jump_with_pred(exit_bb)?;
                 }
             }
@@ -156,7 +171,7 @@ impl<'a> LoopBuilder<'a> {
             loop_header: None,
             continue_target: None,
             continue_snapshots: Vec::new(),
-            exit_snapshots: Vec::new(),  // exit PHI用のスナップショット
+            exit_snapshots: Vec::new(), // exit PHI用のスナップショット
         }
     }
 
@@ -179,8 +194,10 @@ impl<'a> LoopBuilder<'a> {
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             eprintln!("[build_loop_with_loopform] === ENTRY ===");
             if let Some(ref func) = self.parent_builder.current_function {
-                eprintln!("[build_loop_with_loopform] fn='{}', counter={}, func_ptr={:p}",
-                         func.signature.name, func.next_value_id, func as *const _);
+                eprintln!(
+                    "[build_loop_with_loopform] fn='{}', counter={}, func_ptr={:p}",
+                    func.signature.name, func.next_value_id, func as *const _
+                );
             }
             eprintln!("[build_loop_with_loopform] condition={:?}", condition);
             eprintln!("[build_loop_with_loopform] body.len()={}", body.len());
@@ -194,8 +211,11 @@ impl<'a> LoopBuilder<'a> {
 
         // DEBUG: Show variable map before guard check
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform] before_loop_id={:?}, variable_map size={}",
-                     before_loop_id, current_vars.len());
+            eprintln!(
+                "[loopform] before_loop_id={:?}, variable_map size={}",
+                before_loop_id,
+                current_vars.len()
+            );
             for (name, value) in &current_vars {
                 eprintln!("  {} -> {:?}", name, value);
             }
@@ -220,7 +240,11 @@ impl<'a> LoopBuilder<'a> {
         let entry_block = self.current_block()?;
         self.emit_jump(preheader_id)?;
         // 📦 Hotfix 6: Add CFG predecessor for preheader (same as legacy version)
-        crate::mir::builder::loops::add_predecessor(self.parent_builder, preheader_id, entry_block)?;
+        crate::mir::builder::loops::add_predecessor(
+            self.parent_builder,
+            preheader_id,
+            entry_block,
+        )?;
 
         // Initialize LoopFormBuilder with preheader and header blocks
         let mut loopform = LoopFormBuilder::new(preheader_id, header_id);
@@ -229,7 +253,10 @@ impl<'a> LoopBuilder<'a> {
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             eprintln!("[loopform] Block IDs: preheader={:?}, header={:?}, body={:?}, latch={:?}, exit={:?}",
                      preheader_id, header_id, body_id, latch_id, exit_id);
-            eprintln!("[loopform] variable_map at loop entry (size={}):", current_vars.len());
+            eprintln!(
+                "[loopform] variable_map at loop entry (size={}):",
+                current_vars.len()
+            );
             let mut loop_count = 0;
             for (name, value) in &current_vars {
                 loop_count += 1;
@@ -240,8 +267,10 @@ impl<'a> LoopBuilder<'a> {
             }
             eprintln!("[loopform] iterated {} times", loop_count);
             if let Some(ref func) = self.parent_builder.current_function {
-                eprintln!("[loopform] BEFORE prepare_structure: fn='{}', counter={}, func_ptr={:p}",
-                         func.signature.name, func.next_value_id, func as *const _);
+                eprintln!(
+                    "[loopform] BEFORE prepare_structure: fn='{}', counter={}, func_ptr={:p}",
+                    func.signature.name, func.next_value_id, func as *const _
+                );
             } else {
                 eprintln!("[loopform] BEFORE prepare_structure: current_function=None");
             }
@@ -249,8 +278,10 @@ impl<'a> LoopBuilder<'a> {
         loopform.prepare_structure(self, &current_vars)?;
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             if let Some(ref func) = self.parent_builder.current_function {
-                eprintln!("[loopform] AFTER prepare_structure: fn='{}', counter={}, func_ptr={:p}",
-                         func.signature.name, func.next_value_id, func as *const _);
+                eprintln!(
+                    "[loopform] AFTER prepare_structure: fn='{}', counter={}, func_ptr={:p}",
+                    func.signature.name, func.next_value_id, func as *const _
+                );
             } else {
                 eprintln!("[loopform] AFTER prepare_structure: current_function=None");
             }
@@ -285,12 +316,19 @@ impl<'a> LoopBuilder<'a> {
         self.continue_snapshots.clear();
         self.exit_snapshots.clear();
 
-        // Emit condition check in header
+        // [LoopForm] header-cond: cond true → body, false → exit (Case A/B)
+        // - Case A: loop(i < n) → header can branch to exit directly
+        // - Case B: loop(1 == 1) → header always enters body, exit only via break
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform/condition] BEFORE build_expression: current_block={:?}", self.current_block()?);
+            eprintln!(
+                "[loopform/condition] BEFORE build_expression: current_block={:?}",
+                self.current_block()?
+            );
             if let Some(ref func) = self.parent_builder.current_function {
-                eprintln!("[loopform/condition] BEFORE: fn='{}', counter={}, func_ptr={:p}",
-                         func.signature.name, func.next_value_id, func as *const _);
+                eprintln!(
+                    "[loopform/condition] BEFORE: fn='{}', counter={}, func_ptr={:p}",
+                    func.signature.name, func.next_value_id, func as *const _
+                );
             }
         }
         let cond_value = self.parent_builder.build_expression(condition)?;
@@ -298,28 +336,52 @@ impl<'a> LoopBuilder<'a> {
         // if build_expression created new blocks)
         let branch_source_block = self.current_block()?;
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform/condition] AFTER build_expression: branch_source_block={:?}", branch_source_block);
+            eprintln!(
+                "[loopform/condition] AFTER build_expression: branch_source_block={:?}",
+                branch_source_block
+            );
             if let Some(ref func) = self.parent_builder.current_function {
-                eprintln!("[loopform/condition] AFTER: fn='{}', counter={}, func_ptr={:p}",
-                         func.signature.name, func.next_value_id, func as *const _);
+                eprintln!(
+                    "[loopform/condition] AFTER: fn='{}', counter={}, func_ptr={:p}",
+                    func.signature.name, func.next_value_id, func as *const _
+                );
             }
         }
         self.emit_branch(cond_value, body_id, exit_id)?;
         // 📦 Hotfix 6: Add CFG predecessors for branch targets (Cytron et al. 1991 requirement)
         // This ensures exit_block.predecessors is populated before Exit PHI generation
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform/condition] BEFORE add_predecessor: exit_id={:?}, branch_source={:?}", exit_id, branch_source_block);
+            eprintln!(
+                "[loopform/condition] BEFORE add_predecessor: exit_id={:?}, branch_source={:?}",
+                exit_id, branch_source_block
+            );
         }
-        crate::mir::builder::loops::add_predecessor(self.parent_builder, body_id, branch_source_block)?;
-        crate::mir::builder::loops::add_predecessor(self.parent_builder, exit_id, branch_source_block)?;
+        crate::mir::builder::loops::add_predecessor(
+            self.parent_builder,
+            body_id,
+            branch_source_block,
+        )?;
+        crate::mir::builder::loops::add_predecessor(
+            self.parent_builder,
+            exit_id,
+            branch_source_block,
+        )?;
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[loopform/condition] AFTER emit_branch: current_block={:?}", self.current_block()?);
-            eprintln!("[loopform/condition] Added predecessors: body={:?} exit={:?} from={:?}",
-                body_id, exit_id, branch_source_block);
+            eprintln!(
+                "[loopform/condition] AFTER emit_branch: current_block={:?}",
+                self.current_block()?
+            );
+            eprintln!(
+                "[loopform/condition] Added predecessors: body={:?} exit={:?} from={:?}",
+                body_id, exit_id, branch_source_block
+            );
             // Verify predecessors were added
             if let Some(ref func) = self.parent_builder.current_function {
                 if let Some(exit_block) = func.blocks.get(&exit_id) {
-                    eprintln!("[loopform/condition] exit_block.predecessors = {:?}", exit_block.predecessors);
+                    eprintln!(
+                        "[loopform/condition] exit_block.predecessors = {:?}",
+                        exit_block.predecessors
+                    );
                 }
             }
         }
@@ -358,7 +420,10 @@ impl<'a> LoopBuilder<'a> {
         // DEBUG: Log writes collection
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             eprintln!("[loopform/writes] === WRITES COLLECTION (Step 5-1) ===");
-            eprintln!("[loopform/writes] {} variables modified in loop body", writes.len());
+            eprintln!(
+                "[loopform/writes] {} variables modified in loop body",
+                writes.len()
+            );
             let mut sorted_writes: Vec<_> = writes.iter().collect();
             sorted_writes.sort();
             for name in &sorted_writes {
@@ -404,8 +469,8 @@ impl<'a> LoopBuilder<'a> {
         // DISABLED: Body-local header PHI generation
         // This code was causing undefined value errors because it created header PHIs
         // for variables that should only have exit PHIs (or no PHIs at all)
-        if false {  // Disabled for Step 5-5-B experiment
-            // [Original code removed - see git history if needed]
+        if false { // Disabled for Step 5-5-B experiment
+             // [Original code removed - see git history if needed]
         }
 
         // Pass 4: Generate continue_merge PHIs first, then seal header PHIs
@@ -418,15 +483,18 @@ impl<'a> LoopBuilder<'a> {
 
         let merged_snapshot: HashMap<String, ValueId> = if !raw_continue_snaps.is_empty() {
             if trace_loop_phi {
-                eprintln!("[loop-phi/continue-merge] Generating PHI nodes for {} continue paths",
-                    raw_continue_snaps.len());
+                eprintln!(
+                    "[loop-phi/continue-merge] Generating PHI nodes for {} continue paths",
+                    raw_continue_snaps.len()
+                );
             }
 
             // すべての continue snapshot に現れる変数を収集
             let mut all_vars: HashMap<String, Vec<(BasicBlockId, ValueId)>> = HashMap::new();
             for (continue_bb, snapshot) in &raw_continue_snaps {
                 for (var_name, &value) in snapshot {
-                    all_vars.entry(var_name.clone())
+                    all_vars
+                        .entry(var_name.clone())
                         .or_default()
                         .push((*continue_bb, value));
                 }
@@ -460,8 +528,10 @@ impl<'a> LoopBuilder<'a> {
                     }
 
                     if trace_loop_phi {
-                        eprintln!("[loop-phi/continue-merge] Generated PHI for '{}': {:?}",
-                            var_name, phi_id);
+                        eprintln!(
+                            "[loop-phi/continue-merge] Generated PHI for '{}': {:?}",
+                            var_name, phi_id
+                        );
                     }
                     phi_id
                 };
@@ -472,8 +542,11 @@ impl<'a> LoopBuilder<'a> {
             // Note: 変数マップへの反映は seal_phis に委譲（干渉を避ける）
 
             if trace_loop_phi {
-                eprintln!("[loop-phi/continue-merge] Merged {} variables from {} paths",
-                    merged.len(), raw_continue_snaps.len());
+                eprintln!(
+                    "[loop-phi/continue-merge] Merged {} variables from {} paths",
+                    merged.len(),
+                    raw_continue_snaps.len()
+                );
             }
             merged
         } else {
@@ -481,7 +554,11 @@ impl<'a> LoopBuilder<'a> {
         };
 
         self.emit_jump(header_id)?;
-        crate::mir::builder::loops::add_predecessor(self.parent_builder, header_id, continue_merge_id)?;
+        crate::mir::builder::loops::add_predecessor(
+            self.parent_builder,
+            header_id,
+            continue_merge_id,
+        )?;
 
         // Step 2: merged_snapshot を使って seal_phis を呼ぶ
         // Phase 25.3: Continue merge PHI実装（Task先生の発見！）
@@ -555,7 +632,9 @@ impl<'a> LoopBuilder<'a> {
         // Region 情報（entry/exit/slots）をログに出すよ。
         crate::mir::region::observer::observe_control_form(self.parent_builder, &form);
 
-        // Build exit PHIs for break statements using ControlForm wrapper
+        // [LoopForm] exit PHI for Case A/B (uses exit_snapshots + exit_preds)
+        // - Case A: header+break → exit PHI includes both paths
+        // - Case B: break-only → exit PHI excludes header (not a predecessor)
         let exit_snaps = self.exit_snapshots.clone();
         crate::mir::phi_core::loopform_builder::build_exit_phis_for_control(
             &loopform,
@@ -583,7 +662,6 @@ impl<'a> LoopBuilder<'a> {
         Ok(void_dst)
     }
 
-
     // =============================================================
     // PHI Helpers — prepare/finalize PHIs and block sealing
     // =============================================================
@@ -675,17 +753,25 @@ impl<'a> LoopBuilder<'a> {
                 let mut idx = 0;
                 while idx < block.instructions.len() {
                     match &mut block.instructions[idx] {
-                        MirInstruction::Phi { dst: d, inputs: ins } if *d == dst => {
+                        MirInstruction::Phi {
+                            dst: d,
+                            inputs: ins,
+                        } if *d == dst => {
                             *ins = inputs.clone();
                             replaced = true;
                             break;
                         }
-                        MirInstruction::Phi { .. } => { idx += 1; }
+                        MirInstruction::Phi { .. } => {
+                            idx += 1;
+                        }
                         _ => break,
                     }
                 }
                 if !replaced {
-                    let phi_inst = MirInstruction::Phi { dst, inputs: inputs.clone() };
+                    let phi_inst = MirInstruction::Phi {
+                        dst,
+                        inputs: inputs.clone(),
+                    };
                     block.instructions.insert(0, phi_inst);
                 }
                 if dbg {
@@ -774,7 +860,6 @@ impl<'a> LoopBuilder<'a> {
         self.parent_builder.variable_map.get(name).copied()
     }
 
-
     fn build_statement(&mut self, stmt: ASTNode) -> Result<ValueId, String> {
         match stmt {
             // Ensure nested bare blocks inside loops are lowered with loop-aware semantics
@@ -794,8 +879,12 @@ impl<'a> LoopBuilder<'a> {
                     void_id
                 }))
             }
-            ASTNode::If { condition, then_body, else_body, .. } =>
-                self.lower_if_in_loop(*condition, then_body, else_body),
+            ASTNode::If {
+                condition,
+                then_body,
+                else_body,
+                ..
+            } => self.lower_if_in_loop(*condition, then_body, else_body),
             ASTNode::Break { .. } => self.do_break(),
             ASTNode::Continue { .. } => self.do_continue(),
             other => self.parent_builder.build_expression(other),
@@ -813,21 +902,32 @@ impl<'a> LoopBuilder<'a> {
         let join_id = self.parent_builder.debug_next_join_id();
         // Pre-pin comparison operands to slots so repeated uses across blocks are safe
         if crate::config::env::mir_pre_pin_compare_operands() {
-        if let ASTNode::BinaryOp { operator, left, right, .. } = &condition {
-            use crate::ast::BinaryOperator as BO;
-            match operator {
-                BO::Equal | BO::NotEqual | BO::Less | BO::LessEqual | BO::Greater | BO::GreaterEqual => {
-                    if let Ok(lhs_v) = self.parent_builder.build_expression((**left).clone()) {
-                        let _ = self.parent_builder.pin_to_slot(lhs_v, "@loop_if_lhs");
-                    }
-                    if let Ok(rhs_v) = self.parent_builder.build_expression((**right).clone()) {
-                        let _ = self.parent_builder.pin_to_slot(rhs_v, "@loop_if_rhs");
+            if let ASTNode::BinaryOp {
+                operator,
+                left,
+                right,
+                ..
+            } = &condition
+            {
+                use crate::ast::BinaryOperator as BO;
+                match operator {
+                    BO::Equal
+                    | BO::NotEqual
+                    | BO::Less
+                    | BO::LessEqual
+                    | BO::Greater
+                    | BO::GreaterEqual => {
+                        if let Ok(lhs_v) = self.parent_builder.build_expression((**left).clone()) {
+                            let _ = self.parent_builder.pin_to_slot(lhs_v, "@loop_if_lhs");
+                        }
+                        if let Ok(rhs_v) = self.parent_builder.build_expression((**right).clone()) {
+                            let _ = self.parent_builder.pin_to_slot(rhs_v, "@loop_if_rhs");
+                        }
                     }
+                    _ => {}
                 }
-                _ => {}
             }
         }
-        }
         // Evaluate condition and create blocks
         let cond_val = self.parent_builder.build_expression(condition)?;
         let then_bb = self.new_block();
@@ -877,10 +977,8 @@ impl<'a> LoopBuilder<'a> {
         }
         let then_var_map_end = self.get_current_variable_map();
         // フェーズS修正：最強モード指摘の「実到達predecessor捕捉」を統一
-        let then_pred_to_merge = capture_actual_predecessor_and_jump(
-            self.parent_builder, 
-            merge_bb
-        )?;
+        let then_pred_to_merge =
+            capture_actual_predecessor_and_jump(self.parent_builder, merge_bb)?;
         // Pop then-branch debug region
         self.parent_builder.debug_pop_region();
 
@@ -923,10 +1021,8 @@ impl<'a> LoopBuilder<'a> {
             else_var_map_end_opt = Some(self.get_current_variable_map());
         }
         // フェーズS修正：else branchでも統一実到達predecessor捕捉
-        let else_pred_to_merge = capture_actual_predecessor_and_jump(
-            self.parent_builder, 
-            merge_bb
-        )?;
+        let else_pred_to_merge =
+            capture_actual_predecessor_and_jump(self.parent_builder, merge_bb)?;
         // Pop else-branch debug region
         self.parent_builder.debug_pop_region();
 
@@ -937,10 +1033,16 @@ impl<'a> LoopBuilder<'a> {
             .debug_push_region(format!("join#{}", join_id) + "/join");
 
         let mut vars: std::collections::HashSet<String> = std::collections::HashSet::new();
-        let then_prog = ASTNode::Program { statements: then_body.clone(), span: crate::ast::Span::unknown() };
+        let then_prog = ASTNode::Program {
+            statements: then_body.clone(),
+            span: crate::ast::Span::unknown(),
+        };
         crate::mir::phi_core::if_phi::collect_assigned_vars(&then_prog, &mut vars);
         if let Some(es) = &else_body {
-            let else_prog = ASTNode::Program { statements: es.clone(), span: crate::ast::Span::unknown() };
+            let else_prog = ASTNode::Program {
+                statements: es.clone(),
+                span: crate::ast::Span::unknown(),
+            };
             crate::mir::phi_core::if_phi::collect_assigned_vars(&else_prog, &mut vars);
         }
 
@@ -949,15 +1051,25 @@ impl<'a> LoopBuilder<'a> {
         // Use shared helper to merge modified variables at merge block
         struct Ops<'b, 'a>(&'b mut LoopBuilder<'a>);
         impl<'b, 'a> crate::mir::phi_core::if_phi::PhiMergeOps for Ops<'b, 'a> {
-            fn new_value(&mut self) -> ValueId { self.0.new_value() }
+            fn new_value(&mut self) -> ValueId {
+                self.0.new_value()
+            }
             fn emit_phi_at_block_start(
                 &mut self,
                 block: BasicBlockId,
                 dst: ValueId,
                 inputs: Vec<(BasicBlockId, ValueId)>,
-            ) -> Result<(), String> { self.0.emit_phi_at_block_start(block, dst, inputs) }
-            fn update_var(&mut self, name: String, value: ValueId) { self.0.parent_builder.variable_map.insert(name, value); }
-            fn debug_verify_phi_inputs(&mut self, merge_bb: BasicBlockId, inputs: &[(BasicBlockId, ValueId)]) {
+            ) -> Result<(), String> {
+                self.0.emit_phi_at_block_start(block, dst, inputs)
+            }
+            fn update_var(&mut self, name: String, value: ValueId) {
+                self.0.parent_builder.variable_map.insert(name, value);
+            }
+            fn debug_verify_phi_inputs(
+                &mut self,
+                merge_bb: BasicBlockId,
+                inputs: &[(BasicBlockId, ValueId)],
+            ) {
                 if let Some(ref func) = self.0.parent_builder.current_function {
                     crate::mir::phi_core::common::debug_verify_phi_inputs(func, merge_bb, inputs);
                 }
@@ -1011,7 +1123,9 @@ impl<'a> LoopBuilder<'a> {
 
 // Implement phi_core LoopPhiOps on LoopBuilder for in-place delegation
 impl crate::mir::phi_core::loop_phi::LoopPhiOps for LoopBuilder<'_> {
-    fn new_value(&mut self) -> ValueId { self.new_value() }
+    fn new_value(&mut self) -> ValueId {
+        self.new_value()
+    }
     fn emit_phi_at_block_start(
         &mut self,
         block: BasicBlockId,
@@ -1020,12 +1134,18 @@ impl crate::mir::phi_core::loop_phi::LoopPhiOps for LoopBuilder<'_> {
     ) -> Result<(), String> {
         self.emit_phi_at_block_start(block, dst, inputs)
     }
-    fn update_var(&mut self, name: String, value: ValueId) { self.update_variable(name, value) }
+    fn update_var(&mut self, name: String, value: ValueId) {
+        self.update_variable(name, value)
+    }
     fn get_variable_at_block(&mut self, name: &str, block: BasicBlockId) -> Option<ValueId> {
         // Call the inherent method (immutable borrow) to avoid recursion
         LoopBuilder::get_variable_at_block(self, name, block)
     }
-    fn debug_verify_phi_inputs(&mut self, merge_bb: BasicBlockId, inputs: &[(BasicBlockId, ValueId)]) {
+    fn debug_verify_phi_inputs(
+        &mut self,
+        merge_bb: BasicBlockId,
+        inputs: &[(BasicBlockId, ValueId)],
+    ) {
         if let Some(ref func) = self.parent_builder.current_function {
             crate::mir::phi_core::common::debug_verify_phi_inputs(func, merge_bb, inputs);
         }
@@ -1047,7 +1167,10 @@ impl crate::mir::phi_core::loop_phi::LoopPhiOps for LoopBuilder<'_> {
         if let Some(ref mut function) = self.parent_builder.current_function {
             if let Some(block) = function.get_block_mut(preheader_id) {
                 if dbg {
-                    eprintln!("[DEBUG]   Adding Copy instruction to block {}", preheader_id);
+                    eprintln!(
+                        "[DEBUG]   Adding Copy instruction to block {}",
+                        preheader_id
+                    );
                 }
                 block.add_instruction(MirInstruction::Copy { dst, src });
                 Ok(())
@@ -1083,15 +1206,20 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
             let before = func.next_value_id;
             let id = func.next_value_id();
             if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-                eprintln!("[LoopFormOps::new_value] fn='{}' counter: {} -> {}, allocated: {:?}",
-                         func.signature.name, before, func.next_value_id, id);
+                eprintln!(
+                    "[LoopFormOps::new_value] fn='{}' counter: {} -> {}, allocated: {:?}",
+                    func.signature.name, before, func.next_value_id, id
+                );
             }
             id
         } else {
             // Fallback (should never happen in practice)
             let id = self.parent_builder.value_gen.next();
             if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-                eprintln!("[LoopFormOps::new_value] FALLBACK value_gen, allocated: {:?}", id);
+                eprintln!(
+                    "[LoopFormOps::new_value] FALLBACK value_gen, allocated: {:?}",
+                    id
+                );
             }
             id
         };
@@ -1126,13 +1254,16 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
         }
     }
 
-    fn get_block_predecessors(&self, block: BasicBlockId) -> std::collections::HashSet<BasicBlockId> {
+    fn get_block_predecessors(
+        &self,
+        block: BasicBlockId,
+    ) -> std::collections::HashSet<BasicBlockId> {
         // 📦 Hotfix 6: Get actual CFG predecessors for PHI validation
         if let Some(ref func) = self.parent_builder.current_function {
             if let Some(bb) = func.blocks.get(&block) {
                 bb.predecessors.clone()
             } else {
-                std::collections::HashSet::new()  // Non-existent blocks have no predecessors
+                std::collections::HashSet::new() // Non-existent blocks have no predecessors
             }
         } else {
             std::collections::HashSet::new()
@@ -1154,8 +1285,12 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
         let is_param = self.parent_builder.is_value_parameter(value_id);
 
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
-            eprintln!("[is_parameter] ValueId({}) -> {} (kind = {:?})",
-                     value_id.0, is_param, self.parent_builder.get_value_kind(value_id));
+            eprintln!(
+                "[is_parameter] ValueId({}) -> {} (kind = {:?})",
+                value_id.0,
+                is_param,
+                self.parent_builder.get_value_kind(value_id)
+            );
         }
 
         is_param
@@ -1166,7 +1301,8 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
     }
 
     fn emit_copy(&mut self, dst: ValueId, src: ValueId) -> Result<(), String> {
-        self.parent_builder.emit_instruction(MirInstruction::Copy { dst, src })
+        self.parent_builder
+            .emit_instruction(MirInstruction::Copy { dst, src })
     }
 
     fn emit_jump(&mut self, target: BasicBlockId) -> Result<(), String> {
@@ -1178,11 +1314,7 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
         dst: ValueId,
         inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
-        self.emit_phi_at_block_start(
-            self.current_block()?,
-            dst,
-            inputs
-        )
+        self.emit_phi_at_block_start(self.current_block()?, dst, inputs)
     }
 
     fn update_phi_inputs(
@@ -1191,7 +1323,8 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
         phi_id: ValueId,
         inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
-        self.parent_builder.update_phi_instruction(block, phi_id, inputs)
+        self.parent_builder
+            .update_phi_instruction(block, phi_id, inputs)
     }
 
     fn update_var(&mut self, name: String, value: ValueId) {

src/mir/phi_core/exit_phi_builder.rs

@@ -66,31 +66,31 @@ impl ExitPhiBuilder {
     /// let exit_builder = ExitPhiBuilder::new(body_builder);
     /// ```
     pub fn new(body_local_builder: BodyLocalPhiBuilder) -> Self {
-        Self {
-            body_local_builder,
-        }
+        Self { body_local_builder }
     }
 
-    /// Build Exit PHIs
+    /// [LoopForm] Build Exit PHIs
     ///
     /// # Arguments
     /// * `ops` - LoopFormOps trait implementation
     /// * `exit_id` - Exit block ID
     /// * `header_id` - Loop header block ID
-    /// * `branch_source_block` - Branch source block ID (early exit path)
+    /// * `branch_source_block` - Branch source block ID (early exit path, Case A)
     /// * `header_vals` - Header variable values (parameter values)
-    /// * `exit_snapshots` - Exit predecessor snapshots
+    /// * `exit_snapshots` - Exit predecessor snapshots (from break statements)
     /// * `pinned_vars` - Pinned variable names (loop-invariant parameters)
     /// * `carrier_vars` - Carrier variable names (loop-modified variables)
     ///
     /// # Returns
     /// Result: Ok(()) on success, Err(msg) on failure
     ///
-    /// # Process
-    /// 1. Get exit predecessors (CFG validation)
-    /// 2. Filter phantom blocks
+    /// # [LoopForm] Process
+    /// 1. Get exit predecessors (CFG validation) - determines Case A/B
+    /// 2. Filter phantom blocks (Step 5-5-H)
     /// 3. Record definitions in inspector
-    /// 4. Generate PHI inputs using LoopSnapshotMergeBox::merge_exit_with_classification
+    /// 4. [LoopForm] Generate PHI inputs using LoopSnapshotMergeBox::merge_exit_with_classification
+    ///    - Case A: header+break paths included
+    ///    - Case B: break paths only (header not a predecessor)
     /// 5. For each variable, use PhiInputCollector to optimize and generate PHI nodes
     ///
     /// # Example
@@ -119,18 +119,14 @@ impl ExitPhiBuilder {
     ) -> Result<(), String> {
         ops.set_current_block(exit_id)?;
 
-        // 1. Exit predecessorsを取得（CFG検証）
+        // [LoopForm] 1. Exit predecessorsを取得（CFG検証）- Case A/B判定のキー
         let exit_preds_set = ops.get_block_predecessors(exit_id);
         let exit_preds: Vec<BasicBlockId> = exit_preds_set.iter().copied().collect();
 
-        // 2. Phantom blockをフィルタリング
-        let filtered_snapshots = self.filter_phantom_blocks(
-            exit_snapshots,
-            &exit_preds_set,
-            ops,
-        );
+        // [LoopForm] 2. Phantom blockをフィルタリング（Step 5-5-H）
+        let filtered_snapshots = self.filter_phantom_blocks(exit_snapshots, &exit_preds_set, ops);
 
-        // 3. Inspectorに定義を記録
+        // [LoopForm] 3. Inspectorに定義を記録（変数分類の基盤）
         let inspector = self.body_local_builder.inspector_mut();
         for pinned_name in pinned_vars {
             inspector.record_definition(pinned_name, header_id);
@@ -145,7 +141,9 @@ impl ExitPhiBuilder {
             inspector.record_snapshot(branch_source_block, header_vals);
         }
 
-        // 4. LoopSnapshotMergeBoxでPHI入力を生成（static function call）
+        // [LoopForm] 4. LoopSnapshotMergeBoxでPHI入力を生成（static function call）
+        // - branch_source が exit pred のときだけ header snapshot を追加（Case A）
+        // - break-only の場合は header を除外（Case B）
         let all_vars = LoopSnapshotMergeBox::merge_exit_with_classification(
             header_id,
             header_vals,
@@ -248,7 +246,11 @@ pub trait LoopFormOps {
     fn new_value(&mut self) -> ValueId;
 
     /// Emit PHI instruction
-    fn emit_phi(&mut self, phi_id: ValueId, inputs: Vec<(BasicBlockId, ValueId)>) -> Result<(), String>;
+    fn emit_phi(
+        &mut self,
+        phi_id: ValueId,
+        inputs: Vec<(BasicBlockId, ValueId)>,
+    ) -> Result<(), String>;
 
     /// Update variable binding
     fn update_var(&mut self, var_name: String, value_id: ValueId);
@@ -305,7 +307,10 @@ mod tests {
         }
 
         fn get_block_predecessors(&self, block_id: BasicBlockId) -> HashSet<BasicBlockId> {
-            self.predecessors.get(&block_id).cloned().unwrap_or_default()
+            self.predecessors
+                .get(&block_id)
+                .cloned()
+                .unwrap_or_default()
         }
 
         fn block_exists(&self, block_id: BasicBlockId) -> bool {
@@ -318,7 +323,11 @@ mod tests {
             id
         }
 
-        fn emit_phi(&mut self, phi_id: ValueId, inputs: Vec<(BasicBlockId, ValueId)>) -> Result<(), String> {
+        fn emit_phi(
+            &mut self,
+            phi_id: ValueId,
+            inputs: Vec<(BasicBlockId, ValueId)>,
+        ) -> Result<(), String> {
             self.emitted_phis.push((phi_id, inputs));
             Ok(())
         }
@@ -571,7 +580,7 @@ mod tests {
         let header_id = BasicBlockId(5);
         let exit_id = BasicBlockId(10);
         let branch_source = BasicBlockId(7); // Early break
-        let exit_pred1 = BasicBlockId(8);    // After loop body
+        let exit_pred1 = BasicBlockId(8); // After loop body
 
         ops.add_block(header_id);
         ops.add_block(exit_id);
@@ -590,7 +599,7 @@ mod tests {
         let mut snapshot1 = HashMap::new();
         snapshot1.insert("s".to_string(), ValueId(1));
         snapshot1.insert("idx".to_string(), ValueId(10)); // Modified
-        snapshot1.insert("ch".to_string(), ValueId(15));  // Body-local
+        snapshot1.insert("ch".to_string(), ValueId(15)); // Body-local
 
         let exit_snapshots = vec![(exit_pred1, snapshot1)];
         let pinned_vars = vec!["s".to_string()];

src/mir/phi_core/loop_snapshot_merge.rs

@@ -16,7 +16,7 @@
  */
 
 use crate::mir::{BasicBlockId, ValueId};
-use std::collections::{HashMap, BTreeSet, BTreeMap};
+use std::collections::{BTreeMap, BTreeSet, HashMap};
 
 // Option C PHI bug fix: Use box-based classification
 use super::local_scope_inspector::LocalScopeInspectorBox;
@@ -208,6 +208,15 @@ impl LoopSnapshotMergeBox {
             eprintln!("[Option C]   carrier_vars: {:?}", carrier_vars);
         }
 
+        // [LoopForm] Case A/B 分岐:
+        // - header_id ∈ exit_preds → header fallthrough を exit PHI 入力に含める（Case A）
+        // - header_id ∉ exit_preds → break-only, header 由来の値を PHI に入れない（Case B）
+        //
+        // Header is a valid predecessor only when CFG actually branches to exit from the header.
+        // 例: loop(1 == 1) { ... break } のように header→exit のエッジが無い場合は、
+        // header 値を exit PHI 入力に含めると「非支配ブロックからの値参照」で壊れる。
+        let header_in_exit_preds = exit_preds.contains(&header_id);
+
         // LoopVarClassBox でフィルタリング
         let classifier = LoopVarClassBox::new();
 
@@ -226,12 +235,16 @@ impl LoopSnapshotMergeBox {
                 pinned_vars,
                 carrier_vars,
                 inspector,
-                exit_preds,  // ← 実際のCFG predecessorsを使用！
+                exit_preds, // ← 実際のCFG predecessorsを使用！
             );
 
             if debug {
-                eprintln!("[Option C]   var '{}': {:?} needs_exit_phi={}",
-                    var_name, class, class.needs_exit_phi());
+                eprintln!(
+                    "[Option C]   var '{}': {:?} needs_exit_phi={}",
+                    var_name,
+                    class,
+                    class.needs_exit_phi()
+                );
                 let defining_blocks = inspector.get_defining_blocks(&var_name);
                 eprintln!("[Option C]     defining_blocks: {:?}", defining_blocks);
             }
@@ -244,9 +257,15 @@ impl LoopSnapshotMergeBox {
             if !class.needs_exit_phi() || !is_in_all_preds {
                 if debug {
                     if !class.needs_exit_phi() {
-                        eprintln!("[Option C]   → SKIP exit PHI for '{}' (class={:?})", var_name, class);
+                        eprintln!(
+                            "[Option C]   → SKIP exit PHI for '{}' (class={:?})",
+                            var_name, class
+                        );
                     } else {
-                        eprintln!("[Option C]   → SKIP exit PHI for '{}' (NOT in all preds, class={:?})", var_name, class);
+                        eprintln!(
+                            "[Option C]   → SKIP exit PHI for '{}' (NOT in all preds, class={:?})",
+                            var_name, class
+                        );
                     }
                 }
                 continue;
@@ -256,18 +275,26 @@ impl LoopSnapshotMergeBox {
             let mut inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
 
             // Header fallthrough（header に存在する変数のみ）
-            if let Some(&val) = header_vals.get(&var_name) {
-                inputs.push((header_id, val));
+            if header_in_exit_preds {
+                if let Some(&val) = header_vals.get(&var_name) {
+                    inputs.push((header_id, val));
+                }
+            } else if debug {
+                eprintln!(
+                    "[Option C]   header not a predecessor → skip header input for '{}'",
+                    var_name
+                );
             }
 
             // Break snapshots
             for (bb, snap) in exit_snapshots {
-                // Step 5-5-H: CRITICAL - Skip phantom blocks from stale snapshots
-                // After loopform renumbering, snapshots may contain BlockIds that no longer exist.
-                // ONLY use snapshots from blocks that are actual CFG predecessors.
+                // Step 5-5-H: CRITICAL - Skip phantom exit preds
                 if !exit_preds.contains(bb) {
                     if debug {
-                        eprintln!("[Option C] ⚠️ SKIP phantom exit pred (not in CFG): {:?} for var '{}'", bb, var_name);
+                        eprintln!(
+                            "[Option C] ⚠️ SKIP phantom exit pred (not in CFG): {:?} for var '{}'",
+                            bb, var_name
+                        );
                     }
                     continue;
                 }