refactor(joinir): Phase 255 P2 - loop_invariants + var() unification + Phase 256 prep

## Task A: Loop Invariants Architecture (Option A - Boundary Extension) Introduced explicit loop_invariants concept for variables that are: - Used in loop body but never modified (e.g., substring needle in index_of) - Need header PHI (all iterations use same value) - Do NOT need exit PHI (not a LoopState) Implementation: - Added `loop_invariants: Vec<(String, ValueId)>` field to JoinInlineBoundary - Added `with_loop_invariants()` method to JoinInlineBoundaryBuilder - Modified Pattern 6 to use loop_invariants instead of ConditionOnly misuse * s (haystack) and ch (needle) now properly classified * exit_bindings simplified to only LoopState carriers - Extended LoopHeaderPhiBuilder to generate invariant PHIs * Entry PHI: from host * Latch PHI: self-reference (same value every iteration) - Updated instruction_rewriter to handle invariant latch incoming Files Modified: - src/mir/join_ir/lowering/inline_boundary.rs (structure + builder) - src/mir/join_ir/lowering/inline_boundary_builder.rs (builder method) - src/mir/builder/control_flow/joinir/patterns/pattern6_scan_with_init.rs - src/mir/builder/control_flow/joinir/merge/loop_header_phi_builder.rs - src/mir/builder/control_flow/joinir/merge/mod.rs - src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs ## Task B: Code Quality - var() Helper Unification Created common module to eliminate var() duplication: - New module: src/mir/builder/control_flow/joinir/patterns/common/ - Centralized var() helper in ast_helpers.rs - Updated Pattern 6 and Pattern 2 to use common var() - Test code (5 occurrences) deferred to Phase 256+ per 80/20 rule Result: Eliminated 2 duplicate var() functions, 5 test occurrences remain Files Created: - src/mir/builder/control_flow/joinir/patterns/common/ast_helpers.rs - src/mir/builder/control_flow/joinir/patterns/common/mod.rs Files Modified: - src/mir/builder/control_flow/joinir/patterns/mod.rs - src/mir/builder/control_flow/joinir/patterns/pattern6_scan_with_init.rs - src/mir/builder/control_flow/joinir/patterns/policies/balanced_depth_scan_policy.rs ## Task C: Documentation - PostLoopEarlyReturnPlan Usage Examples Enhanced post_loop_early_return_plan.rs with: - Architecture explanation (exit PHI usage prevents DCE) - Pattern 2 example (Less: balanced_depth_scan) - Pattern 6 example (NotEqual: index_of) - Builder defer decision: when 4+ patterns emerge, add builder Files Modified: - src/mir/builder/control_flow/joinir/patterns/policies/post_loop_early_return_plan.rs ## Task D: Phase 256 Preparation Analyzed next failure from smoke tests: - `StringUtils.split/2` with variable-step loop - Not constant step (i += len or similar) - Three implementation options identified Created Phase 256 README with: - Minimal reproduction code - Root cause analysis - Implementation options with trade-offs - Clear next steps Files Created: - docs/development/current/main/phases/phase-256/README.md ## Verification Results ✅ All tests passing: - pattern254_p0_index_of_vm.sh: PASS - No regression in Pattern 1-5 - Smoke test progresses past json_lint_vm to next failure ✅ Architecture achievements: - Semantic clarity: loop_invariants vs exit_bindings properly separated - ConditionOnly misuse eliminated - PHI generation correct for all carrier types - Code quality: var() duplication reduced - Next phase clearly defined ## Summary Phase 255 P2 achieves root-cause fix for multi-param loop support: - Boundary concept expanded (loop_invariants field) - Pattern 6 architecture corrected (no ConditionOnly misuse) - Code quality improved (var() centralized) - Next pattern (variable-step) is now the challenge ✨ Box-first principles maintained: clean separation, explicit roles, minimal coupling 🧠 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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+# Phase 256: StringUtils.split/2 Pattern Support
+
+Status: Planning
+Scope: Loop pattern recognition for split/tokenization operations
+Related:
+- Phase 255 完了（loop_invariants 導入、Pattern 6 完成）
+- Phase 254 完了（Pattern 6 index_of 実装）
+
+## 失敗詳細
+
+**テスト**: json_lint_vm (quick profile)
+**エラー**: `[joinir/freeze] Loop lowering failed: JoinIR does not support this pattern`
+**関数**: `StringUtils.split/2`
+
+### エラーメッセージ全体
+
+```
+[trace:dev] loop_canonicalizer:   Decision: FAIL_FAST
+[trace:dev] loop_canonicalizer:   Missing caps: [ConstStep]
+[trace:dev] loop_canonicalizer:   Reason: Phase 143-P2: Loop does not match read_digits(loop(true)),
+    skip_whitespace, parse_number, continue, parse_string, or parse_array pattern
+[ERROR] ❌ MIR compilation error: [joinir/freeze] Loop lowering failed: JoinIR does not support this pattern,
+    and LoopBuilder has been removed.
+Function: StringUtils.split/2
+Hint: This loop pattern is not supported. All loops must use JoinIR lowering.
+```
+
+## 期待される動作
+
+`StringUtils.split(s, separator)` が正常にコンパイルされ、文字列分割が動作すること。
+
+## 実際の動作
+
+Loop canonicalizer が `ConstStep` を要求しているが、このループはステップが複雑で定数ではない。
+
+## 最小再現コード
+
+```nyash
+split(s, separator) {
+    local result = new ArrayBox()
+
+    // Early return for empty separator
+    if separator.length() == 0 {
+        result.push(s)
+        return result
+    }
+
+    local start = 0
+    local i = 0
+
+    // Main scan loop
+    loop(i <= s.length() - separator.length()) {
+        if s.substring(i, i + separator.length()) == separator {
+            result.push(s.substring(start, i))
+            start = i + separator.length()
+            i = start  // Variable step - moves by separator.length()
+        } else {
+            i = i + 1  // Constant step - moves by 1
+        }
+    }
+
+    // Push remaining segment
+    if start <= s.length() {
+        result.push(s.substring(start, s.length()))
+    }
+
+    return result
+}
+```
+
+## 分析
+
+### ループ構造
+
+1. **条件**: `i <= s.length() - separator.length()`
+2. **ボディ**:
+   - If branch: マッチング検出時
+     - `result.push()` でセグメント追加
+     - `start = i + separator.length()` で次の開始位置更新
+     - `i = start` で大きくジャンプ（可変ステップ）
+   - Else branch: マッチなし
+     - `i = i + 1` で 1 進む（定数ステップ）
+3. **特徴**:
+   - **可変ステップ**: マッチング時は `separator.length()` 分ジャンプ
+   - **複数キャリア**: `i`, `start`, `result` を更新
+   - **MethodCall**: `substring()`, `push()`, `length()` を使用
+
+### Canonicalizer の問題
+
+```
+Missing caps: [ConstStep]
+```
+
+- 既存の Pattern 1-6 は定数ステップを想定
+- このループは条件分岐で異なるステップ幅を使う
+- Pattern 2 (balanced_depth_scan) に近いが、可変ステップがネック
+
+## 実装計画
+
+### Option A: Pattern 7 - Split/Tokenization Pattern
+
+**新しいパターン追加**:
+- 可変ステップサポート
+- 複数キャリア（i, start, accumulator）
+- If-else での異なるステップ幅処理
+
+**検出条件**:
+1. Loop condition: `i <= expr - len`
+2. Body has if statement:
+   - Then: `i = something_big` (可変ジャンプ)
+   - Else: `i = i + 1` (定数ステップ)
+3. Accumulator への追加操作 (`push` など)
+
+### Option B: Pattern 2 拡張
+
+**既存 Pattern 2 を拡張**:
+- ConstStep 要件を緩和
+- If-else で異なるステップ幅を許可
+- balanced_depth_scan_policy を拡張
+
+### Option C: Normalization 経路
+
+**ループ正規化で対応**:
+- 可変ステップを定数ステップに変換
+- Carrier 追加で状態管理
+
+## 次のステップ
+
+1. **StepTree 詳細解析**: split ループの完全な AST 構造確認
+2. **類似パターン調査**: 他の可変ステップループ（indexOf, contains など）
+3. **Option 選択**: Pattern 7 新設 vs Pattern 2 拡張 vs Normalization
+4. **実装戦略策定**: 選択した Option の詳細設計
+
+## 備考
+
+- Phase 255 で loop_invariants が導入されたが、このケースは invariants 以前の問題（可変ステップ）
+- Phase 254-256 の流れで Pattern 6 → Pattern 7 の自然な進化が期待される
+- split/tokenization は一般的なパターンなので、汎用的な解決策が望ましい