feat: MIR builder TypeOp lowering for is/as methods and isType/asType functions

- Add early TypeOp lowering in build_expression for method-style is()/as()
- Add early TypeOp lowering in build_expression for function-style isType()/asType()
- Add special handling in build_print_statement for print(isType/asType(...))
- Fix MIR optimizer borrow checker issues and remove obsolete BoxFieldLoad
- Extract string literal helper supports both direct literals and StringBox wrappers
- Note: isType call generation still has issues (undefined SSA value in print)

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

docs/development/current/CURRENT_TASK.md

@@ -36,26 +36,26 @@
 
 ## 🚧 次にやること（再開方針）
 
-1) 命令セットダイエットのPoC実装（短期）
+1) MIR26 前進（短期）
-   - 現状: VMに `TypeOp/WeakRef/Barrier` 実行経路（等価）とPrinter対応。Builderに補助APIを追加済（未置換）。
+   - プリンタ拡張: `TypeOp/WeakRef/Barrier` を `--mir-verbose` に明示表示
-   - 次: Builder内の該当箇所を補助APIに置換（flag onで新命令を吐く／offで従来どおり）
+   - スナップショット整備: 代表ケースで flag ON/OFF のMIR差分固定化
+   - vm-stats差分: `weak_field_poc.nyash` 等で JSON 取得・比較（キー: TypeOp/WeakRef/Barrier）
    - 旗: `mir_typeop_poc`（TypeCheck/Cast→TypeOp）、`mir_refbarrier_unify_poc`（Weak*/Barrier→統合）
-   - 成果物: スナップショット（flag on/off）＋ vm-statsのキー確認（TypeOp/WeakRef/Barrier）
 
-2) VM×プラグインのE2E拡張（短期）
+2) Builder適用拡大（短期〜中期）
-   - HTTP: 遅延応答・大ボディの計測、到達不能時のERR安定化の再検証（代表は追加済）
+   - 言語 `is/as` 導線（最小でも擬似ノード）→ `emit_type_check/emit_cast` へ配線
-   - Socket: 反復タイムアウトの追加ケース（代表は追加済）
+   - 弱参照: 既存の `RefGet/RefSet` パスは弱フィールドで `WeakLoad/WeakNew`＋Barrier（flag ONで統合命令）
-   - 成果物: 必要に応じてE2E追補と `VM_README.md` のTips更新
 
-3) ResultBox単一路線への統合（中期）
+3) VM/Verifierの補強（中期）
-- 新`NyashResultBox`へ統合、旧`ResultBox`は薄いラッパーとして段階移行
+   - `TypeOp(Cast)` の数値キャスト（Int/Float）安全化、誤用時TypeError整備
-- 成果物: 実装整理・移行メモ・影響調査
+   - Verifierに26命令整合（Barrier位置、WeakRef整合、支配関係）チェックを追加
 
-4) Array系の本実装（必要時・中期）
+4) VM×プラグインE2Eの維持（短期）
-   - VMの `ArrayGet/ArraySet` 実装済み。BoxCall fast-pathの整合性と回帰テストを充実
+   - HTTP/Socketの回帰確認（Void防御・遅延サーバ軽量化は済）
+   - 必要に応じて `VM_README.md` にTips追記
 
 5) BoxCall高速化（性能段階）
-- vm-statsでホットなBoxCallの高速化（命令セット統合より効果大の可能性）
+   - `--vm-stats` ホットパス特定後、Fast-path/キャッシュ適用
 
 ## ▶ 実行コマンド例
 
@@ -73,6 +73,21 @@ cargo test -q --features plugins e2e_interpreter_plugin_filebox_close_void
 cargo test -q --features plugins e2e_vm_plugin_filebox_close_void
 ```
 
+MIR26 PoC（弱参照・Barrier統合）:
+```bash
+# 弱フィールドPoC（flag OFF: WeakNew/WeakLoad/BarrierRead/Write）
+NYASH_VM_STATS=1 NYASH_VM_STATS_JSON=1 ./target/release/nyash --backend vm --vm-stats --vm-stats-json local_tests/weak_field_poc.nyash > vm_stats_weak_default.json
+
+# flag ON: WeakRef/Barrier 統合
+cargo build --release --features mir_refbarrier_unify_poc -q
+NYASH_VM_STATS=1 NYASH_VM_STATS_JSON=1 ./target/release/nyash --backend vm --vm-stats --vm-stats-json local_tests/weak_field_poc.nyash > vm_stats_weak_unified.json
+```
+
+MIRダンプ（プリンタ拡張後の確認）:
+```bash
+./target/release/nyash --dump-mir --mir-verbose local_tests/weak_field_poc.nyash | sed -n '1,200p'
+```
+
 MIRダンプ/検証:
 ```bash
 nyash --dump-mir --mir-verbose examples/plugin_box_sample.nyash
@@ -84,7 +99,7 @@ nyash --verify examples/plugin_box_sample.nyash
 - メタ降格: Debug / Nop / Safepoint（ビルドモードで制御）
 
 ---
-最終更新: 2025年8月23日（VM強化・E2E拡張・me参照安定化・TypeOp/WeakRef/Barrier PoC完了／次段はBuilder置換とスナップショット）
+最終更新: 2025年8月23日（VM強化・E2E拡張・me参照安定化・TypeOp/WeakRef/Barrier PoC完了／次段はプリンタ拡張・スナップショット・is/as導線）
 
 ## 🔁 再起動後の再開手順（ショート）
 ```bash

src/interpreter/functions.rs

@@ -12,6 +12,26 @@ impl NyashInterpreter {
     /// 関数呼び出しを実行 - 🌍 革命的実装：GlobalBoxのメソッド呼び出し
     pub(super) fn execute_function_call(&mut self, name: &str, arguments: &[ASTNode]) 
         -> Result<Box<dyn NyashBox>, RuntimeError> {
+        // Fallback: built-in type ops as global functions: isType(value, "Type"), asType(value, "Type")
+        if (name == "isType" || name == "asType") && arguments.len() == 2 {
+            // Evaluate args
+            let val = self.execute_expression(&arguments[0])?;
+            let ty_box = self.execute_expression(&arguments[1])?;
+            // Get type name string
+            let type_name = if let Some(s) = ty_box.as_any().downcast_ref::<crate::box_trait::StringBox>() {
+                s.value.clone()
+            } else {
+                return Err(RuntimeError::InvalidOperation { message: "Type name must be a string".to_string() });
+            };
+
+            if name == "isType" {
+                let matched = Self::matches_type_name(&val, &type_name);
+                return Ok(Box::new(crate::box_trait::BoolBox::new(matched)));
+            } else {
+                // asType: minimal safe cast (int<->float), otherwise identity
+                return Self::cast_to_type(val, &type_name);
+            }
+        }
         // コンストラクタ内での親コンストラクタ呼び出しチェック
         if let Some(context) = self.current_constructor_context.clone() {
             if let Some(parent_class) = context.parent_class {
@@ -94,4 +114,43 @@ impl NyashInterpreter {
             eprintln!("Warning: Failed to register global function: {}", err);
         });
     }
+
+    /// Helper: match a NyashBox value against a simple type name
+    fn matches_type_name(val: &Box<dyn NyashBox>, type_name: &str) -> bool {
+        let tn = val.type_name();
+        match type_name {
+            "Integer" | "Int" | "I64" => tn == "IntegerBox",
+            "Float" | "F64" => tn == "FloatBox",
+            "Bool" | "Boolean" => tn == "BoolBox",
+            "String" => tn == "StringBox",
+            "Void" | "Unit" => tn == "VoidBox",
+            other => tn == other || tn == format!("{}Box", other),
+        }
+    }
+
+    /// Helper: cast box to a target type name (minimal support)
+    fn cast_to_type(val: Box<dyn NyashBox>, type_name: &str) -> Result<Box<dyn NyashBox>, RuntimeError> {
+        match type_name {
+            "Integer" | "Int" | "I64" => {
+                // Float -> Integer (truncate), Integer -> Integer, else error
+                if let Some(i) = val.as_any().downcast_ref::<crate::box_trait::IntegerBox>() {
+                    Ok(Box::new(crate::box_trait::IntegerBox::new(i.value)))
+                } else if let Some(f) = val.as_any().downcast_ref::<crate::boxes::FloatBox>() {
+                    Ok(Box::new(crate::box_trait::IntegerBox::new(f.value as i64)))
+                } else {
+                    Ok(val) // identity fallback for now
+                }
+            }
+            "Float" | "F64" => {
+                if let Some(f) = val.as_any().downcast_ref::<crate::boxes::FloatBox>() {
+                    Ok(Box::new(crate::boxes::FloatBox::new(f.value)))
+                } else if let Some(i) = val.as_any().downcast_ref::<crate::box_trait::IntegerBox>() {
+                    Ok(Box::new(crate::boxes::FloatBox::new(i.value as f64)))
+                } else {
+                    Ok(val)
+                }
+            }
+            _ => Ok(val),
+        }
+    }
 }

src/interpreter/method_dispatch.rs.legacy

@@ -47,6 +47,22 @@ impl NyashInterpreter {
         // オブジェクトを評価（通常のメソッド呼び出し）
         let obj_value = self.execute_expression(object)?;
 
+        // Fallback: built-in type ops as instance methods: value.is("Type"), value.as("Type")
+        if (method == "is" || method == "as") && arguments.len() == 1 {
+            let ty_box = self.execute_expression(&arguments[0])?;
+            let type_name = if let Some(s) = ty_box.as_any().downcast_ref::<crate::box_trait::StringBox>() {
+                s.value.clone()
+            } else {
+                return Err(RuntimeError::InvalidOperation { message: "Type name must be a string".to_string() });
+            };
+            if method == "is" {
+                let matched = super::functions::NyashInterpreter::matches_type_name(&obj_value, &type_name);
+                return Ok(Box::new(crate::box_trait::BoolBox::new(matched)));
+            } else {
+                return super::functions::NyashInterpreter::cast_to_type(obj_value, &type_name);
+            }
+        }
+        
         eprintln!("🔍 DEBUG: execute_method_call - object evaluated to type_name='{}', box_id={}", 
                  obj_value.type_name(), obj_value.box_id());
         

src/mir/builder.rs

@@ -43,6 +43,12 @@ pub struct MirBuilder {
 
     /// Names of user-defined boxes declared in the current module
     pub(super) user_defined_boxes: HashSet<String>,
+
+    /// Weak field registry: BoxName -> {weak field names}
+    pub(super) weak_fields_by_box: HashMap<String, HashSet<String>>,
+
+    /// Remember class of object fields after assignments: (base_id, field) -> class_name
+    pub(super) field_origin_class: HashMap<(ValueId, String), String>,
 }
 
 impl MirBuilder {
@@ -58,6 +64,8 @@ impl MirBuilder {
             pending_phis: Vec::new(),
             value_origin_newbox: HashMap::new(),
             user_defined_boxes: HashSet::new(),
+            weak_fields_by_box: HashMap::new(),
+            field_origin_class: HashMap::new(),
         }
     }
 
@@ -203,6 +211,8 @@ impl MirBuilder {
             let me_id = self.value_gen.next();
             f.params.push(me_id);
             self.variable_map.insert("me".to_string(), me_id);
+            // Record origin: 'me' belongs to this box type (enables weak field wiring)
+            self.value_origin_newbox.insert(me_id, box_name.clone());
             // user parameters continue as %1..N
             for p in &params {
                 let pid = self.value_gen.next();
@@ -318,6 +328,17 @@ impl MirBuilder {
             },
             
             ASTNode::MethodCall { object, method, arguments, .. } => {
+                // Early TypeOp lowering for method-style is()/as()
+                if (method == "is" || method == "as") && arguments.len() == 1 {
+                    if let Some(type_name) = Self::extract_string_literal(&arguments[0]) {
+                        let obj_val = self.build_expression(*object.clone())?;
+                        let ty = Self::parse_type_name_to_mir(&type_name);
+                        let dst = self.value_gen.next();
+                        let op = if method == "is" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                        self.emit_instruction(MirInstruction::TypeOp { dst, op, value: obj_val, ty })?;
+                        return Ok(dst);
+                    }
+                }
                 self.build_method_call(*object.clone(), method.clone(), arguments.clone())
             },
             
@@ -338,6 +359,17 @@ impl MirBuilder {
             },
             
             ASTNode::FunctionCall { name, arguments, .. } => {
+                // Early TypeOp lowering for function-style isType()/asType()
+                if (name == "isType" || name == "asType") && arguments.len() == 2 {
+                    if let Some(type_name) = Self::extract_string_literal(&arguments[1]) {
+                        let val = self.build_expression(arguments[0].clone())?;
+                        let ty = Self::parse_type_name_to_mir(&type_name);
+                        let dst = self.value_gen.next();
+                        let op = if name == "isType" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                        self.emit_instruction(MirInstruction::TypeOp { dst, op, value: val, ty })?;
+                        return Ok(dst);
+                    }
+                }
                 self.build_function_call(name.clone(), arguments.clone())
             },
             
@@ -389,14 +421,14 @@ impl MirBuilder {
                 self.build_local_statement(variables.clone(), initial_values.clone())
             },
             
-            ASTNode::BoxDeclaration { name, methods, is_static, fields, constructors, .. } => {
+            ASTNode::BoxDeclaration { name, methods, is_static, fields, constructors, weak_fields, .. } => {
                 if is_static && name == "Main" {
                     self.build_static_main_box(methods.clone())
                 } else {
                     // Support user-defined boxes - handle as statement, return void
                     // Track as user-defined (eligible for method lowering)
                     self.user_defined_boxes.insert(name.clone());
-                    self.build_box_declaration(name.clone(), methods.clone(), fields.clone())?;
+                    self.build_box_declaration(name.clone(), methods.clone(), fields.clone(), weak_fields.clone())?;
 
                     // Phase 2: Lower constructors (birth/N) into MIR functions
                     // Function name pattern: "{BoxName}.{constructor_key}" (e.g., "Person.birth/1")
@@ -535,6 +567,17 @@ impl MirBuilder {
     
     /// Build function call
     fn build_function_call(&mut self, name: String, args: Vec<ASTNode>) -> Result<ValueId, String> {
+        // Minimal TypeOp wiring via function-style: isType(value, "Type"), asType(value, "Type")
+        if (name == "isType" || name == "asType") && args.len() == 2 {
+            if let Some(type_name) = Self::extract_string_literal(&args[1]) {
+                let val = self.build_expression(args[0].clone())?;
+                let ty = Self::parse_type_name_to_mir(&type_name);
+                let dst = self.value_gen.next();
+                let op = if name == "isType" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                self.emit_instruction(MirInstruction::TypeOp { dst, op, value: val, ty })?;
+                return Ok(dst);
+            }
+        }
         // Build argument values
         let mut arg_values = Vec::new();
         for arg in args {
@@ -563,6 +606,24 @@ impl MirBuilder {
     
     /// Build print statement - converts to console output
     fn build_print_statement(&mut self, expression: ASTNode) -> Result<ValueId, String> {
+        // Early lowering for print(isType(...)) / print(asType(...)) to avoid undefined SSA when optimizations run
+        if let ASTNode::FunctionCall { name, arguments, .. } = &expression {
+            if (name == "isType" || name == "asType") && arguments.len() == 2 {
+                if let Some(type_name) = Self::extract_string_literal(&arguments[1]) {
+                    let val = self.build_expression(arguments[0].clone())?;
+                    let ty = Self::parse_type_name_to_mir(&type_name);
+                    let dst = self.value_gen.next();
+                    let op = if name == "isType" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                    self.emit_instruction(MirInstruction::TypeOp { dst, op, value: val, ty })?;
+                    self.emit_instruction(MirInstruction::Print {
+                        value: dst,
+                        effects: EffectMask::PURE.add(Effect::Io),
+                    })?;
+                    return Ok(dst);
+                }
+            }
+        }
+
         let value = self.build_expression(expression)?;
         
         // For now, use a special Print instruction (minimal scope)
@@ -885,18 +946,51 @@ impl MirBuilder {
     
     /// Build field access: object.field
     fn build_field_access(&mut self, object: ASTNode, field: String) -> Result<ValueId, String> {
+        // Clone the object before building expression if we need to check it later
+        let object_clone = object.clone();
+        
         // First, build the object expression to get its ValueId
         let object_value = self.build_expression(object)?;
 
         // Get the field from the object using RefGet
-        let result_id = self.value_gen.next();
+        let field_val = self.value_gen.next();
         self.emit_instruction(MirInstruction::RefGet {
-            dst: result_id,
+            dst: field_val,
             reference: object_value,
-            field,
+            field: field.clone(),
         })?;
 
-        Ok(result_id)
+        // If we recorded origin class for this field on this base object, propagate it to this value id
+        if let Some(class_name) = self.field_origin_class.get(&(object_value, field.clone())).cloned() {
+            self.value_origin_newbox.insert(field_val, class_name);
+        }
+
+        // If we can infer the box type and the field is weak, emit WeakLoad (+ optional barrier)
+        let mut inferred_class: Option<String> = self.value_origin_newbox.get(&object_value).cloned();
+        // Fallback: if the object is a nested field access like (X.Y).Z, consult recorded field origins for X.Y
+        if inferred_class.is_none() {
+            if let ASTNode::FieldAccess { object: inner_obj, field: ref parent_field, .. } = object_clone {
+                // Build inner base to get a stable id and consult mapping
+                if let Ok(base_id) = self.build_expression(*inner_obj.clone()) {
+                    if let Some(cls) = self.field_origin_class.get(&(base_id, parent_field.clone())) {
+                        inferred_class = Some(cls.clone());
+                    }
+                }
+            }
+        }
+        if let Some(class_name) = inferred_class {
+            if let Some(weak_set) = self.weak_fields_by_box.get(&class_name) {
+                if weak_set.contains(&field) {
+                    // Barrier (read) PoC
+                    let _ = self.emit_barrier_read(field_val);
+                    // WeakLoad
+                    let loaded = self.emit_weak_load(field_val)?;
+                    return Ok(loaded);
+                }
+            }
+        }
+
+        Ok(field_val)
     }
     
     /// Build new expression: new ClassName(arguments)
@@ -941,15 +1035,38 @@ impl MirBuilder {
     fn build_field_assignment(&mut self, object: ASTNode, field: String, value: ASTNode) -> Result<ValueId, String> {
         // Build the object and value expressions
         let object_value = self.build_expression(object)?;
-        let value_result = self.build_expression(value)?;
+        let mut value_result = self.build_expression(value)?;
+
+        // If we can infer the box type and the field is weak, create WeakRef before store
+        if let Some(class_name) = self.value_origin_newbox.get(&object_value).cloned() {
+            if let Some(weak_set) = self.weak_fields_by_box.get(&class_name) {
+                if weak_set.contains(&field) {
+                    value_result = self.emit_weak_new(value_result)?;
+                }
+            }
+        }
 
         // Set the field using RefSet
         self.emit_instruction(MirInstruction::RefSet {
             reference: object_value,
-            field,
+            field: field.clone(),
             value: value_result,
         })?;
 
+        // Emit a write barrier for weak fields (PoC)
+        if let Some(class_name) = self.value_origin_newbox.get(&object_value).cloned() {
+            if let Some(weak_set) = self.weak_fields_by_box.get(&class_name) {
+                if weak_set.contains(&field) {
+                    let _ = self.emit_barrier_write(value_result);
+                }
+            }
+        }
+
+        // Record origin class for this field (if the value originates from NewBox of a known class)
+        if let Some(class_name) = self.value_origin_newbox.get(&value_result).cloned() {
+            self.field_origin_class.insert((object_value, field.clone()), class_name);
+        }
+
         // Return the assigned value
         Ok(value_result)
     }
@@ -1059,6 +1176,19 @@ impl MirBuilder {
     
     /// Build method call: object.method(arguments)
     fn build_method_call(&mut self, object: ASTNode, method: String, arguments: Vec<ASTNode>) -> Result<ValueId, String> {
+        // Minimal TypeOp wiring via method-style syntax: value.is("Type") / value.as("Type")
+        if (method == "is" || method == "as") && arguments.len() == 1 {
+            if let ASTNode::Literal { value: crate::ast::LiteralValue::String(type_name), .. } = &arguments[0] {
+                // Build the object expression
+                let object_value = self.build_expression(object.clone())?;
+                // Map string to MIR type
+                let mir_ty = Self::parse_type_name_to_mir(type_name);
+                let dst = self.value_gen.next();
+                let op = if method == "is" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                self.emit_instruction(MirInstruction::TypeOp { dst, op, value: object_value, ty: mir_ty })?;
+                return Ok(dst);
+            }
+        }
         // ExternCall判定はobjectの変数解決より先に行う（未定義変数で落とさない）
         if let ASTNode::Variable { name: object_name, .. } = object.clone() {
             // Build argument expressions first (externはobject自体を使わない)
@@ -1098,6 +1228,17 @@ impl MirBuilder {
         // Build the object expression
         let object_value = self.build_expression(object.clone())?;
 
+        // Secondary interception for is/as in case early path did not trigger
+        if (method == "is" || method == "as") && arguments.len() == 1 {
+            if let ASTNode::Literal { value: crate::ast::LiteralValue::String(type_name), .. } = &arguments[0] {
+                let mir_ty = Self::parse_type_name_to_mir(type_name);
+                let dst = self.value_gen.next();
+                let op = if method == "is" { super::TypeOpKind::Check } else { super::TypeOpKind::Cast };
+                self.emit_instruction(MirInstruction::TypeOp { dst, op, value: object_value, ty: mir_ty })?;
+                return Ok(dst);
+            }
+        }
+
         // Build argument expressions
         let mut arg_values = Vec::new();
         for arg in &arguments {
@@ -1162,6 +1303,35 @@ impl MirBuilder {
         Ok(result_id)
     }
 
+    /// Map a user-facing type name to MIR type
+    fn parse_type_name_to_mir(name: &str) -> super::MirType {
+        match name {
+            "Integer" | "Int" | "I64" => super::MirType::Integer,
+            "Float" | "F64" => super::MirType::Float,
+            "Bool" | "Boolean" => super::MirType::Bool,
+            "String" => super::MirType::String,
+            "Void" | "Unit" => super::MirType::Void,
+            other => super::MirType::Box(other.to_string()),
+        }
+    }
+    
+    /// Extract string literal from AST node if possible
+    /// Supports: Literal("Type") and new StringBox("Type")
+    fn extract_string_literal(node: &ASTNode) -> Option<String> {
+        match node {
+            ASTNode::Literal { value: crate::ast::LiteralValue::String(s), .. } => Some(s.clone()),
+            ASTNode::New { class, arguments, .. } if class == "StringBox" => {
+                if arguments.len() == 1 {
+                    if let ASTNode::Literal { value: crate::ast::LiteralValue::String(s), .. } = &arguments[0] {
+                        return Some(s.clone());
+                    }
+                }
+                None
+            }
+            _ => None,
+        }
+    }
+    
     /// Build from expression: from Parent.method(arguments)
     fn build_from_expression(&mut self, parent: String, method: String, arguments: Vec<ASTNode>) -> Result<ValueId, String> {
         // Build argument expressions
@@ -1193,7 +1363,7 @@ impl MirBuilder {
     }
     
     /// Build box declaration: box Name { fields... methods... }
-    fn build_box_declaration(&mut self, name: String, methods: std::collections::HashMap<String, ASTNode>, fields: Vec<String>) -> Result<(), String> {
+    fn build_box_declaration(&mut self, name: String, methods: std::collections::HashMap<String, ASTNode>, fields: Vec<String>, weak_fields: Vec<String>) -> Result<(), String> {
         // For Phase 8.4, we'll emit metadata instructions to register the box type
         // In a full implementation, this would register type information for later use
         
@@ -1213,6 +1383,12 @@ impl MirBuilder {
             })?;
         }
 
+        // Record weak fields for this box
+        if !weak_fields.is_empty() {
+            let set: HashSet<String> = weak_fields.into_iter().collect();
+            self.weak_fields_by_box.insert(name.clone(), set);
+        }
+        
         // Process methods - now methods is a HashMap
         for (method_name, method_ast) in methods {
             if let ASTNode::FunctionDeclaration { .. } = method_ast {

src/mir/mod.rs

@@ -16,6 +16,7 @@ pub mod ownership_verifier_simple; // Simple ownership forest verification for c
 pub mod printer;
 pub mod value_id;
 pub mod effect;
+pub mod optimizer;
 
 // Re-export main types for easy access
 pub use instruction::{MirInstruction, BinaryOp, CompareOp, UnaryOp, ConstValue, MirType, TypeOpKind, WeakRefOp, BarrierOp};
@@ -28,6 +29,7 @@ pub use ownership_verifier_simple::{OwnershipVerifier, OwnershipError, Ownership
 pub use printer::MirPrinter;
 pub use value_id::{ValueId, LocalId, ValueIdGenerator};
 pub use effect::{EffectMask, Effect};
+pub use optimizer::MirOptimizer;
 
 /// MIR compilation result
 #[derive(Debug, Clone)]
@@ -54,7 +56,11 @@ impl MirCompiler {
     /// Compile AST to MIR module with verification
     pub fn compile(&mut self, ast: crate::ast::ASTNode) -> Result<MirCompileResult, String> {
         // Convert AST to MIR using builder
-        let module = self.builder.build_module(ast)?;
+        let mut module = self.builder.build_module(ast)?;
+        
+        // Optimize (safety net lowering for is/as → TypeOp 等)
+        let mut optimizer = MirOptimizer::new();
+        let _ = optimizer.optimize_module(&mut module);
         
         // Verify the generated MIR
         let verification_result = self.verifier.verify_module(&module);

src/mir/optimizer.rs

@@ -8,7 +8,7 @@
  * - Dead code elimination
  */
 
-use super::{MirModule, MirFunction, MirInstruction, ValueId, EffectMask, Effect};
+use super::{MirModule, MirFunction, MirInstruction, ValueId, MirType, TypeOpKind};
 use std::collections::{HashMap, HashSet};
 
 /// MIR optimization passes
@@ -51,6 +51,9 @@ impl MirOptimizer {
         // Pass 4: Intrinsic function optimization
         stats.merge(self.optimize_intrinsic_calls(module));
 
+        // Pass 4.5: Lower well-known intrinsics (is/as → TypeOp) as a safety net
+        stats.merge(self.lower_type_ops(module));
+        
         // Pass 5: BoxField dependency optimization
         stats.merge(self.optimize_boxfield_operations(module));
         
@@ -196,7 +199,8 @@ impl MirOptimizer {
             MirInstruction::Const { value, .. } => format!("const_{:?}", value),
             MirInstruction::BinOp { op, lhs, rhs, .. } => format!("binop_{:?}_{}_{}", op, lhs.as_u32(), rhs.as_u32()),
             MirInstruction::Compare { op, lhs, rhs, .. } => format!("cmp_{:?}_{}_{}", op, lhs.as_u32(), rhs.as_u32()),
-            MirInstruction::BoxFieldLoad { box_val, field, .. } => format!("boxload_{}_{}", box_val.as_u32(), field),
+            // BoxFieldLoad removed from instruction set
+            // MirInstruction::BoxFieldLoad { box_val, field, .. } => format!("boxload_{}_{}", box_val.as_u32(), field),
             MirInstruction::Call { func, args, .. } => {
                 let args_str = args.iter().map(|v| v.as_u32().to_string()).collect::<Vec<_>>().join(",");
                 format!("call_{}_{}", func.as_u32(), args_str)
@@ -254,6 +258,102 @@ impl MirOptimizer {
         0
     }
 
+    /// Safety-net lowering: convert known is/as patterns into TypeOp when possible
+    fn lower_type_ops(&mut self, module: &mut MirModule) -> OptimizationStats {
+        let mut stats = OptimizationStats::new();
+        for (_name, function) in &mut module.functions {
+            stats.intrinsic_optimizations += self.lower_type_ops_in_function(function);
+        }
+        stats
+    }
+
+    fn lower_type_ops_in_function(&mut self, function: &mut MirFunction) -> usize {
+        // Build a simple definition map: ValueId -> (block_id, index)
+        let mut def_map: std::collections::HashMap<ValueId, (super::basic_block::BasicBlockId, usize)> = std::collections::HashMap::new();
+        for (bb_id, block) in &function.blocks {
+            for (i, inst) in block.instructions.iter().enumerate() {
+                if let Some(dst) = inst.dst_value() {
+                    def_map.insert(dst, (*bb_id, i));
+                }
+            }
+            if let Some(term) = &block.terminator {
+                if let Some(dst) = term.dst_value() {
+                    def_map.insert(dst, (*bb_id, usize::MAX));
+                }
+            }
+        }
+
+        let mut lowered = 0;
+        
+        // Collect replacements first to avoid borrow checker issues
+        let mut replacements: Vec<(super::basic_block::BasicBlockId, usize, MirInstruction)> = Vec::new();
+        
+        // First pass: identify replacements
+        for (bb_id, block) in &function.blocks {
+            for i in 0..block.instructions.len() {
+                let replace = match &block.instructions[i] {
+                    MirInstruction::BoxCall { dst, box_val, method, args, .. } => {
+                        let is_is = method == "is" || method == "isType";
+                        let is_as = method == "as" || method == "asType";
+                        if (is_is || is_as) && args.len() == 1 {
+                            // Try to resolve type name from arg
+                            let arg_id = args[0];
+                            if let Some((def_bb, def_idx)) = def_map.get(&arg_id).copied() {
+                                // Look for pattern: NewBox StringBox(Const String)
+                                if let Some(def_block) = function.blocks.get(&def_bb) {
+                                    if def_idx < def_block.instructions.len() {
+                                        if let MirInstruction::NewBox { box_type, args: sb_args, .. } = &def_block.instructions[def_idx] {
+                                            if box_type == "StringBox" && sb_args.len() == 1 {
+                                                let str_id = sb_args[0];
+                                                if let Some((sbb, sidx)) = def_map.get(&str_id).copied() {
+                                                    if let Some(sblock) = function.blocks.get(&sbb) {
+                                                        if sidx < sblock.instructions.len() {
+                                                            if let MirInstruction::Const { value, .. } = &sblock.instructions[sidx] {
+                                                                if let super::instruction::ConstValue::String(s) = value {
+                                                                    // Build TypeOp to replace
+                                                                    let ty = map_type_name(s);
+                                                                    let op = if is_is { TypeOpKind::Check } else { TypeOpKind::Cast };
+                                                                    let new_inst = MirInstruction::TypeOp {
+                                                                        dst: dst.unwrap_or(*box_val),
+                                                                        op,
+                                                                        value: *box_val,
+                                                                        ty,
+                                                                    };
+                                                                    Some(new_inst)
+                                                                } else { None }
+                                                            } else { None }
+                                                        } else { None }
+                                                    } else { None }
+                                                } else { None }
+                                            } else { None }
+                                        } else { None }
+                                    } else { None }
+                                } else { None }
+                            } else { None }
+                        } else { None }
+                    }
+                    _ => None,
+                };
+
+                if let Some(new_inst) = replace {
+                    replacements.push((*bb_id, i, new_inst));
+                }
+            }
+        }
+        
+        // Second pass: apply replacements
+        for (bb_id, idx, new_inst) in replacements {
+            if let Some(block) = function.blocks.get_mut(&bb_id) {
+                if idx < block.instructions.len() {
+                    block.instructions[idx] = new_inst;
+                    lowered += 1;
+                }
+            }
+        }
+        
+        lowered
+    }
+    
     /// Optimize BoxField operations
     fn optimize_boxfield_operations(&mut self, module: &mut MirModule) -> OptimizationStats {
         let mut stats = OptimizationStats::new();
@@ -280,6 +380,18 @@ impl MirOptimizer {
     }
 }
 
+/// Map string type name to MIR type (optimizer-level helper)
+fn map_type_name(name: &str) -> MirType {
+    match name {
+        "Integer" | "Int" | "I64" => MirType::Integer,
+        "Float" | "F64" => MirType::Float,
+        "Bool" | "Boolean" => MirType::Bool,
+        "String" => MirType::String,
+        "Void" | "Unit" => MirType::Void,
+        other => MirType::Box(other.to_string()),
+    }
+}
+
 impl Default for MirOptimizer {
     fn default() -> Self {
         Self::new()

tools/vm_stats_diff.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [ $# -ne 2 ]; then
+  echo "Usage: $0 <stats_before.json> <stats_after.json>" >&2
+  exit 1
+fi
+
+A="$1"
+B="$2"
+
+if [ ! -f "$A" ] || [ ! -f "$B" ]; then
+  echo "Input files not found" >&2
+  exit 1
+fi
+
+# Extract counts objects and join keys
+KEYS=$(jq -r '.counts | keys[]' "$A" "$B" | sort -u)
+
+printf "%-14s %8s %8s %8s\n" "OP" "A" "B" "+/-"
+for k in $KEYS; do
+  va=$(jq -r --arg k "$k" '.counts[$k] // 0' "$A")
+  vb=$(jq -r --arg k "$k" '.counts[$k] // 0' "$B")
+  d=$(( vb - va ))
+  printf "%-14s %8d %8d %8d\n" "$k" "$va" "$vb" "$d"
+done | sort -k1,1
+

vm_stats_weak_default.json

@@ -0,0 +1,75 @@
+🔌 v2 plugin system initialized from nyash.toml
+✅ v2 plugin system fully configured
+🚀 Nyash VM Backend - Executing file: local_tests/weak_field_poc.nyash 🚀
+0
+{
+  "counts": {
+    "BarrierRead": 1,
+    "BarrierWrite": 1,
+    "BoxCall": 2,
+    "Const": 9,
+    "NewBox": 2,
+    "Print": 1,
+    "RefGet": 4,
+    "RefSet": 3,
+    "Return": 3,
+    "Safepoint": 1,
+    "WeakLoad": 1,
+    "WeakNew": 1
+  },
+  "elapsed_ms": 0.194731,
+  "timestamp_ms": 1755956955634,
+  "top20": [
+    {
+      "count": 9,
+      "op": "Const"
+    },
+    {
+      "count": 4,
+      "op": "RefGet"
+    },
+    {
+      "count": 3,
+      "op": "RefSet"
+    },
+    {
+      "count": 3,
+      "op": "Return"
+    },
+    {
+      "count": 2,
+      "op": "BoxCall"
+    },
+    {
+      "count": 2,
+      "op": "NewBox"
+    },
+    {
+      "count": 1,
+      "op": "BarrierRead"
+    },
+    {
+      "count": 1,
+      "op": "BarrierWrite"
+    },
+    {
+      "count": 1,
+      "op": "Print"
+    },
+    {
+      "count": 1,
+      "op": "Safepoint"
+    },
+    {
+      "count": 1,
+      "op": "WeakLoad"
+    },
+    {
+      "count": 1,
+      "op": "WeakNew"
+    }
+  ],
+  "total": 29
+}
+✅ VM execution completed successfully!
+Result: IntegerBox { value: 0, base: BoxBase { id: 4, parent_type_id: None } }

vm_stats_weak_unified.json

@@ -0,0 +1,65 @@
+🔌 v2 plugin system initialized from nyash.toml
+✅ v2 plugin system fully configured
+🚀 Nyash VM Backend - Executing file: local_tests/weak_field_poc.nyash 🚀
+0
+{
+  "counts": {
+    "Barrier": 2,
+    "BoxCall": 2,
+    "Const": 9,
+    "NewBox": 2,
+    "Print": 1,
+    "RefGet": 4,
+    "RefSet": 3,
+    "Return": 3,
+    "Safepoint": 1,
+    "WeakRef": 2
+  },
+  "elapsed_ms": 0.16804799999999998,
+  "timestamp_ms": 1755957108401,
+  "top20": [
+    {
+      "count": 9,
+      "op": "Const"
+    },
+    {
+      "count": 4,
+      "op": "RefGet"
+    },
+    {
+      "count": 3,
+      "op": "RefSet"
+    },
+    {
+      "count": 3,
+      "op": "Return"
+    },
+    {
+      "count": 2,
+      "op": "Barrier"
+    },
+    {
+      "count": 2,
+      "op": "BoxCall"
+    },
+    {
+      "count": 2,
+      "op": "NewBox"
+    },
+    {
+      "count": 2,
+      "op": "WeakRef"
+    },
+    {
+      "count": 1,
+      "op": "Print"
+    },
+    {
+      "count": 1,
+      "op": "Safepoint"
+    }
+  ],
+  "total": 29
+}
+✅ VM execution completed successfully!
+Result: IntegerBox { value: 0, base: BoxBase { id: 4, parent_type_id: None } }