diff --git a/src/config/env.rs b/src/config/env.rs
index f0f9b9a3..3b1447cc 100644
--- a/src/config/env.rs
+++ b/src/config/env.rs
@@ -265,6 +265,18 @@ pub fn joinir_debug_level() -> u8 {
     0
 }
 
+/// Phase 61-2: If-in-loop JoinIR dry-run有効化
+///
+/// `HAKO_JOINIR_IF_IN_LOOP_DRYRUN=1` でdry-runモードを有効化
+///
+/// dry-runモード:
+/// - JoinIR経路でPHI仕様を計算
+/// - PhiBuilderBox経路と比較
+/// - 実際のPHI生成はPhiBuilderBoxを使用（安全）
+pub fn joinir_if_in_loop_dryrun_enabled() -> bool {
+    env_bool("HAKO_JOINIR_IF_IN_LOOP_DRYRUN")
+}
+
 // VM legacy by-name call fallback was removed (Phase 2 complete).
 // Phase 40-4.1: use_joinir_for_array_filter() removed (Route B now default).
 
diff --git a/src/mir/join_ir/lowering/if_phi_spec.rs b/src/mir/join_ir/lowering/if_phi_spec.rs
new file mode 100644
index 00000000..e24534ee
--- /dev/null
+++ b/src/mir/join_ir/lowering/if_phi_spec.rs
@@ -0,0 +1,188 @@
+//! Phase 61-2: JoinIR経路でのPHI仕様計算
+//!
+//! JoinInst（Select/IfMerge）から、どの変数がPHIを必要とするかを計算する。
+
+use crate::mir::join_ir::JoinInst;
+use crate::mir::join_ir::lowering::if_phi_context::IfPhiContext;
+use crate::mir::ValueId;
+use std::collections::{BTreeMap, BTreeSet};
+
+/// PHI仕様（どの変数がPHIを持つべきか）
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PhiSpec {
+    /// Header PHI候補（ループキャリア変数）
+    pub header_phis: BTreeSet<String>,
+
+    /// Exit PHI候補（ループ脱出時の値）
+    pub exit_phis: BTreeSet<String>,
+}
+
+impl PhiSpec {
+    pub fn new() -> Self {
+        Self {
+            header_phis: BTreeSet::new(),
+            exit_phis: BTreeSet::new(),
+        }
+    }
+
+    /// Header PHI数を取得
+    pub fn header_count(&self) -> usize {
+        self.header_phis.len()
+    }
+
+    /// Exit PHI数を取得
+    pub fn exit_count(&self) -> usize {
+        self.exit_phis.len()
+    }
+
+    /// 2つのPhiSpecが一致するか検証
+    pub fn matches(&self, other: &PhiSpec) -> bool {
+        self.header_phis == other.header_phis && self.exit_phis == other.exit_phis
+    }
+}
+
+/// JoinInstからPHI仕様を計算
+///
+/// # Arguments
+///
+/// * `ctx` - If-in-loopコンテキスト（carrier_names情報）
+/// * `join_inst` - JoinIR命令（Select/IfMerge）
+///
+/// # Returns
+///
+/// PHI仕様（header/exit PHI候補）
+pub fn compute_phi_spec_from_joinir(ctx: &IfPhiContext, join_inst: &JoinInst) -> PhiSpec {
+    let mut spec = PhiSpec::new();
+
+    match join_inst {
+        JoinInst::Select { dst, .. } => {
+            // Select命令: 単一変数のPHI
+            // carrier_namesに含まれる変数をheader PHIとして扱う
+            // TODO Phase 61-3: dstからvariable_nameを逆引き（MIR Builderのvariable_map参照）
+            spec.header_phis = ctx.carrier_names.clone();
+        }
+        JoinInst::IfMerge { merges, .. } => {
+            // IfMerge命令: 複数変数のPHI
+            // TODO Phase 61-3: merge_pair.dstからvariable_nameを逆引き
+            // 暫定: carrier_namesに含まれる変数をheader PHIとして扱う
+            spec.header_phis = ctx.carrier_names.clone();
+
+            if ctx.in_loop_body {
+                eprintln!(
+                    "[Phase 61-2] IfMerge with {} merge pairs in loop body",
+                    merges.len()
+                );
+            }
+        }
+        _ => {
+            eprintln!("[Phase 61-2] ⚠️ Unexpected JoinInst variant for PHI spec");
+        }
+    }
+
+    spec
+}
+
+/// PhiBuilderBoxから実際に生成されたPHI仕様を取得
+///
+/// # Arguments
+///
+/// * `pre_if_var_map` - If前の変数マップ
+/// * `post_snapshots` - If後のスナップショット
+/// * `carrier_names` - ループキャリア変数名
+///
+/// # Note
+///
+/// Phase 61-2では、PhiBuilderBox経路で実際に生成されたPHIを観察する。
+/// Phase 61-3で、このロジックがJoinIR経路に置き換わる。
+pub fn extract_phi_spec_from_builder(
+    pre_if_var_map: &BTreeMap<String, ValueId>,
+    _post_snapshots: &[BTreeMap<String, ValueId>],
+    carrier_names: &BTreeSet<String>,
+) -> PhiSpec {
+    let mut spec = PhiSpec::new();
+
+    // carrier_namesに含まれる変数をPHI候補として扱う
+    for var_name in carrier_names {
+        if pre_if_var_map.contains_key(var_name) {
+            spec.header_phis.insert(var_name.clone());
+        }
+    }
+
+    // Exit PHIは別途計算が必要（Phase 61-3で詳細実装）
+    // 現状はheader PHIのみを比較対象とする
+
+    spec
+}
+
+/// 2つのPhiSpecを比較し、結果をログ出力
+///
+/// # Returns
+///
+/// true if specs match, false otherwise
+pub fn compare_and_log_phi_specs(joinir_spec: &PhiSpec, builder_spec: &PhiSpec) -> bool {
+    let matches = joinir_spec.matches(builder_spec);
+
+    if matches {
+        eprintln!("[Phase 61-2] ✅ PHI spec matches!");
+        eprintln!(
+            "[Phase 61-2]   Header PHIs: {}",
+            joinir_spec.header_count()
+        );
+        eprintln!(
+            "[Phase 61-2]   Exit PHIs: {}",
+            joinir_spec.exit_count()
+        );
+    } else {
+        eprintln!("[Phase 61-2] ❌ PHI spec mismatch detected!");
+        eprintln!("[Phase 61-2] JoinIR spec:");
+        eprintln!(
+            "[Phase 61-2]   Header: {:?}",
+            joinir_spec.header_phis
+        );
+        eprintln!("[Phase 61-2]   Exit: {:?}", joinir_spec.exit_phis);
+        eprintln!("[Phase 61-2] PhiBuilderBox spec:");
+        eprintln!(
+            "[Phase 61-2]   Header: {:?}",
+            builder_spec.header_phis
+        );
+        eprintln!("[Phase 61-2]   Exit: {:?}", builder_spec.exit_phis);
+    }
+
+    matches
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_phi_spec_creation() {
+        let spec = PhiSpec::new();
+        assert_eq!(spec.header_count(), 0);
+        assert_eq!(spec.exit_count(), 0);
+    }
+
+    #[test]
+    fn test_phi_spec_matches() {
+        let mut spec1 = PhiSpec::new();
+        spec1.header_phis.insert("x".to_string());
+        spec1.header_phis.insert("y".to_string());
+
+        let mut spec2 = PhiSpec::new();
+        spec2.header_phis.insert("x".to_string());
+        spec2.header_phis.insert("y".to_string());
+
+        assert!(spec1.matches(&spec2));
+    }
+
+    #[test]
+    fn test_phi_spec_mismatch() {
+        let mut spec1 = PhiSpec::new();
+        spec1.header_phis.insert("x".to_string());
+
+        let mut spec2 = PhiSpec::new();
+        spec2.header_phis.insert("y".to_string());
+
+        assert!(!spec1.matches(&spec2));
+    }
+}
diff --git a/src/mir/join_ir/lowering/mod.rs b/src/mir/join_ir/lowering/mod.rs
index fb6c24a4..aa81ce5b 100644
--- a/src/mir/join_ir/lowering/mod.rs
+++ b/src/mir/join_ir/lowering/mod.rs
@@ -23,6 +23,7 @@ pub mod generic_case_a;
 pub mod if_dry_runner; // Phase 33-10.0
 pub mod if_merge; // Phase 33-7
 pub mod if_phi_context; // Phase 61-1
+pub mod if_phi_spec; // Phase 61-2
 pub mod if_select; // Phase 33
 pub mod loop_form_intake;
 pub mod loop_scope_shape;
diff --git a/src/mir/loop_builder.rs b/src/mir/loop_builder.rs
index 3f68123e..76e2149e 100644
--- a/src/mir/loop_builder.rs
+++ b/src/mir/loop_builder.rs
@@ -1159,7 +1159,12 @@ impl<'a> LoopBuilder<'a> {
             .cloned()
             .collect();
 
-        // Phase 61-1: JoinIR経路を試行（`Ops`作成前に実行）
+        // Phase 61-2: JoinIR dry-run検証モード
+        // dry-run用: JoinInstとPhiSpecを保存（A/B比較用）
+        let mut joinir_phi_spec_opt: Option<
+            crate::mir::join_ir::lowering::if_phi_spec::PhiSpec,
+        > = None;
+
         let joinir_success = if crate::config::env::joinir_if_select_enabled() {
             // IfPhiContext作成
             let if_phi_context =
@@ -1176,15 +1181,42 @@ impl<'a> LoopBuilder<'a> {
                     Some(&if_phi_context),
                 ) {
                     Some(join_inst) => {
-                        eprintln!("[Phase 61-1] ✅ If-in-loop lowered via JoinIR: {:?}", join_inst);
+                        eprintln!("[Phase 61-2] ✅ If-in-loop lowered via JoinIR: {:?}", join_inst);
 
-                        // TODO: join_inst を dry-run して PHI 生成
-                        // Phase 61-1では一旦スキップし、Phase 61-2で実装
-                        eprintln!("[Phase 61-1] ⚠️ JoinIR dry-run not yet implemented, using fallback");
-                        false // 一旦フォールバック
+                        // Phase 61-2: dry-runモードでPHI仕様を検証
+                        if crate::config::env::joinir_if_in_loop_dryrun_enabled() {
+                            eprintln!("[Phase 61-2] 🔍 dry-run mode enabled");
+                            eprintln!("[Phase 61-2]   Carrier variables: {:?}", carrier_names);
+                            eprintln!(
+                                "[Phase 61-2]   JoinInst type: {}",
+                                match &join_inst {
+                                    crate::mir::join_ir::JoinInst::Select { .. } => "Select",
+                                    crate::mir::join_ir::JoinInst::IfMerge { .. } => "IfMerge",
+                                    _ => "Other"
+                                }
+                            );
+
+                            // Phase 61-2.3: JoinInstからPhiSpecを計算
+                            let joinir_spec = crate::mir::join_ir::lowering::if_phi_spec::compute_phi_spec_from_joinir(
+                                &if_phi_context,
+                                &join_inst,
+                            );
+                            eprintln!(
+                                "[Phase 61-2] JoinIR PhiSpec: header={}, exit={}",
+                                joinir_spec.header_count(),
+                                joinir_spec.exit_count()
+                            );
+
+                            // A/B比較用に保存
+                            joinir_phi_spec_opt = Some(joinir_spec);
+                        }
+
+                        false // Phase 61-2では検証のみ、本番切り替えはPhase 61-3
                     }
                     None => {
-                        eprintln!("[Phase 61-1] ⏭️ JoinIR pattern not matched, using fallback");
+                        if crate::config::env::joinir_if_in_loop_dryrun_enabled() {
+                            eprintln!("[Phase 61-2] ⏭️ JoinIR pattern not matched, using fallback");
+                        }
                         false
                     }
                 }
@@ -1210,7 +1242,7 @@ impl<'a> LoopBuilder<'a> {
             // Phase 26-F-3: ループ内if-mergeコンテキスト設定（ChatGPT設計）
             phi_builder.set_if_context(
                 true, // in_loop_body = true
-                carrier_names,
+                carrier_names.clone(),
             );
 
             // Phase 35-5: if_body_local_merge.rs削除、ロジックはPhiBuilderBox内に統合
@@ -1220,6 +1252,31 @@ impl<'a> LoopBuilder<'a> {
                 vec![then_var_map_end.clone()]
             };
             phi_builder.generate_phis(&mut ops, &form, &pre_if_var_map, &post_snapshots)?;
+
+            // Phase 61-2: A/B比較（JoinIR vs PhiBuilderBox）
+            if crate::config::env::joinir_if_in_loop_dryrun_enabled() {
+                if let Some(ref joinir_spec) = joinir_phi_spec_opt {
+                    // PhiBuilderBox経路でのPhiSpecを抽出
+                    let builder_spec =
+                        crate::mir::join_ir::lowering::if_phi_spec::extract_phi_spec_from_builder(
+                            &pre_if_var_map,
+                            &post_snapshots,
+                            &carrier_names,
+                        );
+
+                    eprintln!(
+                        "[Phase 61-2] PhiBuilderBox PhiSpec: header={}, exit={}",
+                        builder_spec.header_count(),
+                        builder_spec.exit_count()
+                    );
+
+                    // A/B比較実行
+                    let _matches = crate::mir::join_ir::lowering::if_phi_spec::compare_and_log_phi_specs(
+                        joinir_spec,
+                        &builder_spec,
+                    );
+                }
+            }
         }
 
         // Phase 26-E-4: PHI生成後に variable_map をリセット（ChatGPT/Task先生指示）
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 2869d0bb..2ee80d43 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -17,6 +17,7 @@ pub mod mir;
 pub mod namingbox_static_method_id; // Phase 21.7++ Phase 1: StaticMethodId structure tests
 pub mod nyash_abi_basic;
 pub mod parser;
+pub mod phase61_if_in_loop_dryrun; // Phase 61-2: If-in-loop JoinIR dry-run tests
 pub mod plugin_hygiene;
 pub mod policy_mutdeny;
 pub mod refcell_assignment_test;
diff --git a/src/tests/phase61_if_in_loop_dryrun.rs b/src/tests/phase61_if_in_loop_dryrun.rs
new file mode 100644
index 00000000..57257d11
--- /dev/null
+++ b/src/tests/phase61_if_in_loop_dryrun.rs
@@ -0,0 +1,48 @@
+//! Phase 61-2: If-in-loop JoinIR dry-run + PHI生成 A/B比較テスト
+//!
+//! 目的: JoinIR経路でPHI仕様を計算し、PhiBuilderBox経路との一致を検証
+//!
+//! 注意: Phase 61-2はdry-run検証のみ。実行結果は変わらない。
+//!       JoinIRパターンマッチは Phase 33の厳格な条件に依存する。
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn phase61_2_dry_run_flag_available() {
+        // Phase 61-2: dry-runフラグが正しく読み取れることを確認
+        std::env::set_var("HAKO_JOINIR_IF_IN_LOOP_DRYRUN", "1");
+        assert_eq!(crate::config::env::joinir_if_in_loop_dryrun_enabled(), true);
+
+        std::env::remove_var("HAKO_JOINIR_IF_IN_LOOP_DRYRUN");
+        assert_eq!(
+            crate::config::env::joinir_if_in_loop_dryrun_enabled(),
+            false
+        );
+
+        eprintln!("[Test] phase61_2_dry_run_flag_available passed");
+    }
+
+    #[test]
+    fn phase61_2_phi_spec_creation() {
+        use crate::mir::join_ir::lowering::if_phi_spec::PhiSpec;
+
+        let mut spec1 = PhiSpec::new();
+        assert_eq!(spec1.header_count(), 0);
+        assert_eq!(spec1.exit_count(), 0);
+
+        spec1.header_phis.insert("x".to_string());
+        spec1.header_phis.insert("y".to_string());
+        assert_eq!(spec1.header_count(), 2);
+
+        let mut spec2 = PhiSpec::new();
+        spec2.header_phis.insert("x".to_string());
+        spec2.header_phis.insert("y".to_string());
+
+        assert!(spec1.matches(&spec2));
+
+        eprintln!("[Test] phase61_2_phi_spec_creation passed");
+    }
+}
+
+// Note: E2E tests for actual if-in-loop JoinIR lowering will be added in Phase 61-3
+// when the production switch is made. Phase 61-2 focuses on dry-run infrastructure.