diff --git a/src/backend/mir_interpreter/handlers/boxes.rs b/src/backend/mir_interpreter/handlers/boxes.rs
index 84f44699..0ff28bee 100644
--- a/src/backend/mir_interpreter/handlers/boxes.rs
+++ b/src/backend/mir_interpreter/handlers/boxes.rs
@@ -168,9 +168,7 @@ impl MirInterpreter {
             _ => {}
         }
         if super::boxes_object_fields::try_handle_object_fields(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=object_fields");
-            }
+            trace_dispatch!(method, "object_fields");
             return Ok(());
         }
         // Policy gate: user InstanceBox BoxCall runtime fallback
@@ -201,36 +199,26 @@ impl MirInterpreter {
             }
         }
         if super::boxes_instance::try_handle_instance_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=instance_box");
-            }
+            trace_dispatch!(method, "instance_box");
             return Ok(());
         }
         if super::boxes_string::try_handle_string_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=string_box");
-            }
+            trace_dispatch!(method, "string_box");
             return Ok(());
         }
         if super::boxes_array::try_handle_array_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=array_box");
-            }
+            trace_dispatch!(method, "array_box");
             return Ok(());
         }
         if super::boxes_map::try_handle_map_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=map_box");
-            }
+            trace_dispatch!(method, "map_box");
             return Ok(());
         }
         // Narrow safety valve: if 'length' wasn't handled by any box-specific path,
         // treat it as 0 (avoids Lt on Void in common loops). This is a dev-time
         // robustness measure; precise behavior should be provided by concrete boxes.
         if method == "length" {
-            if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=fallback(length=0)");
-            }
+            trace_dispatch!(method, "fallback(length=0)");
             if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); }
             return Ok(());
         }
diff --git a/src/backend/mir_interpreter/handlers/mod.rs b/src/backend/mir_interpreter/handlers/mod.rs
index 80a3a90c..96d4e8c1 100644
--- a/src/backend/mir_interpreter/handlers/mod.rs
+++ b/src/backend/mir_interpreter/handlers/mod.rs
@@ -1,5 +1,16 @@
 use super::*;
 
+// VM dispatch trace macro (used across handlers)
+macro_rules! trace_dispatch {
+    ($method:expr, $handler:expr) => {
+        if $method == "length"
+            && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1")
+        {
+            eprintln!("[vm-trace] length dispatch handler={}", $handler);
+        }
+    };
+}
+
 mod arithmetic;
 mod boxes;
 mod boxes_array;
diff --git a/src/mir/builder/if_form.rs b/src/mir/builder/if_form.rs
index e40bd6fb..858403b3 100644
--- a/src/mir/builder/if_form.rs
+++ b/src/mir/builder/if_form.rs
@@ -79,8 +79,9 @@ impl MirBuilder {
         }
         let then_value_raw = self.build_expression(then_branch)?;
         let then_exit_block = self.current_block()?;
+        let then_reaches_merge = !self.is_current_block_terminated();
         let then_var_map_end = self.variable_map.clone();
-        if !self.is_current_block_terminated() {
+        if then_reaches_merge {
             // Scope leave for then-branch
             self.hint_scope_leave(0);
             crate::mir::builder::emission::branch::emit_jump(self, merge_block)?;
@@ -116,7 +117,8 @@ impl MirBuilder {
             (void_val, None, None)
         };
         let else_exit_block = self.current_block()?;
-        if !self.is_current_block_terminated() {
+        let else_reaches_merge = !self.is_current_block_terminated();
+        if else_reaches_merge {
             // Scope leave for else-branch
             self.hint_scope_leave(0);
             self.emit_instruction(MirInstruction::Jump { target: merge_block })?;
@@ -144,8 +146,8 @@ impl MirBuilder {
         let result_val = self.normalize_if_else_phi(
             then_block,
             else_block,
-            Some(then_exit_block),
-            Some(else_exit_block),
+            if then_reaches_merge { Some(then_exit_block) } else { None },
+            if else_reaches_merge { Some(else_exit_block) } else { None },
             then_value_raw,
             else_value_raw,
             &pre_if_var_map,
@@ -176,8 +178,8 @@ impl MirBuilder {
         self.merge_modified_vars(
             then_block,
             else_block,
-            then_exit_block,
-            Some(else_exit_block),
+            if then_reaches_merge { Some(then_exit_block) } else { None },
+            if else_reaches_merge { Some(else_exit_block) } else { None },
             &pre_if_var_map,
             &then_var_map_end,
             &else_var_map_end_opt,
diff --git a/src/mir/builder/ops.rs b/src/mir/builder/ops.rs
index f4b642ff..58849595 100644
--- a/src/mir/builder/ops.rs
+++ b/src/mir/builder/ops.rs
@@ -283,8 +283,9 @@ impl super::MirBuilder {
             t_id
         };
         let then_exit_block = self.current_block()?;
+        let then_reaches_merge = !self.is_current_block_terminated();
         let then_var_map_end = self.variable_map.clone();
-        if !self.is_current_block_terminated() {
+        if then_reaches_merge {
             self.hint_scope_leave(0);
             crate::mir::builder::emission::branch::emit_jump(self, merge_block)?;
         }
@@ -338,8 +339,9 @@ impl super::MirBuilder {
             rhs_bool
         };
         let else_exit_block = self.current_block()?;
+        let else_reaches_merge = !self.is_current_block_terminated();
         let else_var_map_end = self.variable_map.clone();
-        if !self.is_current_block_terminated() {
+        if else_reaches_merge {
             self.hint_scope_leave(0);
             self.emit_instruction(MirInstruction::Jump { target: merge_block })?;
         }
@@ -350,24 +352,32 @@ impl super::MirBuilder {
         self.start_new_block(merge_block)?;
         self.push_if_merge(merge_block);
 
-        // Result PHI (bool)
-        let result_val = self.value_gen.next();
-        let inputs = vec![(then_exit_block, then_value_raw), (else_exit_block, else_value_raw)];
-        if let Some(func) = self.current_function.as_mut() {
-            func.update_cfg();
-        }
-        if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
-            crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
-        }
-        self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs })?;
-        self.value_types.insert(result_val, MirType::Bool);
+        // Result PHI (bool) — consider only reachable predecessors
+        let mut inputs: Vec<(crate::mir::BasicBlockId, ValueId)> = Vec::new();
+        if then_reaches_merge { inputs.push((then_exit_block, then_value_raw)); }
+        if else_reaches_merge { inputs.push((else_exit_block, else_value_raw)); }
+        let result_val = if inputs.len() >= 2 {
+            if let Some(func) = self.current_function.as_mut() { func.update_cfg(); }
+            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+            }
+            let dst = self.value_gen.next();
+            self.emit_instruction(MirInstruction::Phi { dst, inputs })?;
+            self.value_types.insert(dst, MirType::Bool);
+            dst
+        } else if inputs.len() == 1 {
+            inputs[0].1
+        } else {
+            // Unreachable fallthrough: synthesize false (defensive)
+            crate::mir::builder::emission::constant::emit_bool(self, false)
+        };
 
         // Merge modified vars from both branches back into current scope
         self.merge_modified_vars(
             then_block,
             else_block,
-            then_exit_block,
-            Some(else_exit_block),
+            if then_reaches_merge { Some(then_exit_block) } else { None },
+            if else_reaches_merge { Some(else_exit_block) } else { None },
             &pre_if_var_map,
             &then_var_map_end,
             &Some(else_var_map_end),
diff --git a/src/mir/builder/phi.rs b/src/mir/builder/phi.rs
index 574f65dd..393cd800 100644
--- a/src/mir/builder/phi.rs
+++ b/src/mir/builder/phi.rs
@@ -13,7 +13,7 @@ impl MirBuilder {
         &mut self,
         _then_block: super::BasicBlockId,
         else_block: super::BasicBlockId,
-        then_exit_block: super::BasicBlockId,
+        then_exit_block_opt: Option<super::BasicBlockId>,
         else_exit_block_opt: Option<super::BasicBlockId>,
         pre_if_snapshot: &std::collections::HashMap<String, super::ValueId>,
         then_map_end: &std::collections::HashMap<String, super::ValueId>,
@@ -42,18 +42,25 @@ impl MirBuilder {
                 .unwrap_or(pre);
             // フェーズM: 常にPHI命令を使用（no_phi_mode撤廃）
             // incoming の predecessor は "実際に merge に遷移してくる出口ブロック" を使用する
-            let then_pred = then_exit_block;
-            let else_pred = else_exit_block_opt.unwrap_or(else_block);
-            let merged = self.value_gen.next();
-            let inputs = vec![(then_pred, then_v), (else_pred, else_v)];
-            if let Some(func) = self.current_function.as_mut() {
-                func.update_cfg();
+            let mut inputs: Vec<(super::BasicBlockId, super::ValueId)> = Vec::new();
+            if let Some(tp) = then_exit_block_opt { inputs.push((tp, then_v)); }
+            if let Some(ep) = else_exit_block_opt.or(Some(else_block)) { inputs.push((ep, else_v)); }
+            match inputs.len() {
+                0 => {}
+                1 => {
+                    let (_pred, v) = inputs[0];
+                    self.variable_map.insert(name, v);
+                }
+                _ => {
+                    if let Some(func) = self.current_function.as_mut() { func.update_cfg(); }
+                    if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                        crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+                    }
+                    let merged = self.value_gen.next();
+                    self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
+                    self.variable_map.insert(name, merged);
+                }
             }
-            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
-                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
-            }
-            self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
-            self.variable_map.insert(name, merged);
         }
 
         // Ensure pinned synthetic slots ("__pin$...") have a block-local definition at the merge,
@@ -68,18 +75,25 @@ impl MirBuilder {
                 .as_ref()
                 .and_then(|m| m.get(pin_name.as_str()).copied())
                 .unwrap_or(*pre_val);
-            let then_pred = then_exit_block;
-            let else_pred = else_exit_block_opt.unwrap_or(else_block);
-            let merged = self.value_gen.next();
-            let inputs = vec![(then_pred, then_v), (else_pred, else_v)];
-            if let Some(func) = self.current_function.as_mut() {
-                func.update_cfg();
+            let mut inputs: Vec<(super::BasicBlockId, super::ValueId)> = Vec::new();
+            if let Some(tp) = then_exit_block_opt { inputs.push((tp, then_v)); }
+            if let Some(ep) = else_exit_block_opt.or(Some(else_block)) { inputs.push((ep, else_v)); }
+            match inputs.len() {
+                0 => {}
+                1 => {
+                    let (_pred, v) = inputs[0];
+                    self.variable_map.insert(pin_name.clone(), v);
+                }
+                _ => {
+                    if let Some(func) = self.current_function.as_mut() { func.update_cfg(); }
+                    if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                        crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+                    }
+                    let merged = self.value_gen.next();
+                    self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
+                    self.variable_map.insert(pin_name.clone(), merged);
+                }
             }
-            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
-                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
-            }
-            self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
-            self.variable_map.insert(pin_name.clone(), merged);
         }
         Ok(())
     }
@@ -129,32 +143,49 @@ impl MirBuilder {
                 // Else doesn't assign: use pre-if value if available
                 pre_then_var_value.unwrap_or(else_value_raw)
             };
-            // predecessor を then/else の exit ブロックに揃える
-            let then_pred = then_exit_block_opt.unwrap_or(then_block);
-            let else_pred = else_exit_block_opt.unwrap_or(else_block);
-            // Emit Phi for the assigned variable and bind it
-            let inputs = vec![(then_pred, then_value_for_var), (else_pred, else_value_for_var)];
-            if let Some(func) = self.current_function.as_mut() {
-                func.update_cfg();
+            // Build inputs from reachable predecessors only
+            let mut inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
+            if let Some(tp) = then_exit_block_opt { inputs.push((tp, then_value_for_var)); }
+            if let Some(ep) = else_exit_block_opt.or(Some(else_block)) { inputs.push((ep, else_value_for_var)); }
+            match inputs.len() {
+                0 => {}
+                1 => {
+                    // Direct bind (no PHI needed)
+                    self.variable_map = pre_if_var_map.clone();
+                    self.variable_map.insert(var_name, inputs[0].1);
+                    return Ok(inputs[0].1);
+                }
+                _ => {
+                    if let Some(func) = self.current_function.as_mut() { func.update_cfg(); }
+                    if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                        crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+                    }
+                    self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs })?;
+                }
             }
-            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
-                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
-            }
-            self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs })?;
             self.variable_map = pre_if_var_map.clone();
             self.variable_map.insert(var_name, result_val);
         } else {
             // No variable assignment pattern detected – just emit Phi for expression result
-            let then_pred = then_exit_block_opt.unwrap_or(then_block);
-            let else_pred = else_exit_block_opt.unwrap_or(else_block);
-            let inputs = vec![(then_pred, then_value_raw), (else_pred, else_value_raw)];
-            if let Some(func) = self.current_function.as_mut() {
-                func.update_cfg();
+            let mut inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
+            if let Some(tp) = then_exit_block_opt { inputs.push((tp, then_value_raw)); }
+            if let Some(ep) = else_exit_block_opt.or(Some(else_block)) { inputs.push((ep, else_value_raw)); }
+            match inputs.len() {
+                0 => { /* leave result_val as fresh, but unused; synthesize void */
+                    let v = crate::mir::builder::emission::constant::emit_void(self);
+                    return Ok(v);
+                }
+                1 => {
+                    return Ok(inputs[0].1);
+                }
+                _ => {
+                    if let Some(func) = self.current_function.as_mut() { func.update_cfg(); }
+                    if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                        crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+                    }
+                    self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs })?;
+                }
             }
-            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
-                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
-            }
-            self.emit_instruction(MirInstruction::Phi { dst: result_val, inputs })?;
             // Merge variable map conservatively to pre-if snapshot (no new bindings)
             self.variable_map = pre_if_var_map.clone();
         }