fix(mir): conservative PHI box for If/Loop and Stage1 resolver SSA

docs/development/roadmap/phases/phase-25.1d/README.md

@@ -96,3 +96,20 @@ Status: planning（構造バグ切り出しフェーズ・挙動は変えない
   - 小さいテストを書く → verifier で赤を出す → LoopBuilder / IfForm / MirBuilder を直す → 緑になるまで繰り返す。
 - これにより、Stage‑B / Stage‑1 / selfhost の土台となる Rust MIR 層が安定し、その上に Nyash selfhost 側の MirBuilder を載せやすくする。
 - なお、Stage‑B 最小ハーネス（`stageb_min_sample.hako`）については、Rust MIR builder 経由の直接 VM / MIR verify は既に緑であり、残っている stack overflow は `compiler_stageb.hako` 側の Nyash ボックス連鎖に起因するものと考えられる。Rust 層では `emit_unified_call` / BoxCall / legacy 警戒の再入防止フラグと再帰深度カウンタを導入済みであり、以降は Nyash 側に浅い再帰ガードを置いて原因ボックスを特定するフェーズへ引き継ぐ。
+
+### 実績メモ（Conservative PHI Box 完了）
+
+- IfForm / LoopForm v2 の PHI 生成は「Conservative PHI Box」実装により根治済み:
+  - If については `merge_modified_vars`（`src/mir/builder/phi.rs`）が **全変数の union に対して PHI を張る保守的実装**に切り替わり、
+    - 片側の branch でしか定義されない変数（`then` のみ / `else` のみ）についても、
+    - pre_if スナップショットまたは `const void` を使った安全な SSA 定義に統一。
+  - Loop については LoopForm v2（`LoopFormBuilder`）側で header/exit PHI の扱いを整理し、
+    - Carrier / Pinned / Invariant に分離したうえで exit PHI から `variable_map` を一貫して再束縛する構造に寄せた。
+- Box 理論の観点では:
+  - Conservative Box: まず「安全側」に全変数に PHI を張る（正しさ優先）。
+  - Elimination Box: 将来の最適化フェーズで、使われない PHI を削る（効率最適化）。
+- この Conservative PHI 実装により、Stage‑1 using resolver 一式の代表テスト:
+  - `mir_parserbox_parse_program2_harness_parses_minimal_source`
+  - `mir_stage1_using_resolver_min_fragment_verifies`
+  - `mir_stage1_using_resolver_full_collect_entries_verifies`
+  がすべて緑になっており、「片側 branch だけで定義された変数の non‑dominating use」系のバグは Rust 側では止血済み。***

src/mir/builder/loops.rs

@@ -53,17 +53,25 @@ pub(crate) fn depth(builder: &super::MirBuilder) -> usize {
 }
 
 /// Add predecessor edge metadata to a basic block.
+/// 📦 Hotfix 6: Auto-create block if it doesn't exist yet
+/// This ensures add_predecessor() works even before start_new_block() is called.
 pub(crate) fn add_predecessor(
     builder: &mut MirBuilder,
     block: BasicBlockId,
     pred: BasicBlockId,
 ) -> Result<(), String> {
     if let Some(ref mut function) = builder.current_function {
+        // 📦 Hotfix 6: Ensure block exists (same as start_new_block logic)
+        // Create block if not present, without changing current_block
+        if !function.blocks.contains_key(&block) {
+            function.add_block(super::BasicBlock::new(block));
+        }
+
         if let Some(bb) = function.get_block_mut(block) {
             bb.add_predecessor(pred);
             return Ok(());
         }
-        return Err(format!("Block {} not found", block));
+        return Err(format!("Block {} not found (impossible after auto-create)", block));
     }
     Err("No current function".to_string())
 }

src/mir/builder/phi.rs

@@ -20,26 +20,101 @@ impl MirBuilder {
         else_map_end_opt: &Option<std::collections::HashMap<String, super::ValueId>>,
         skip_var: Option<&str>,
     ) -> Result<(), String> {
+        // 📦 Conservative PHI Generation (Box Theory)
+        // Generate PHI for ALL variables present in ANY branch (union), not just modified ones.
+        // This ensures correctness: if a variable is defined in one branch but not the other,
+        // we use the predecessor value as fallback (or skip if undefined everywhere).
+        //
+        // Theory: Conservative ∘ Elimination = Minimal SSA
+        // - Conservative (this): correctness-first, generate all PHIs
+        // - Elimination (future): efficiency optimization, remove unused PHIs
+        use std::collections::HashSet;
+
+        // Collect all variables from all sources
+        let mut all_vars = HashSet::new();
+        all_vars.extend(pre_if_snapshot.keys().cloned());
+        all_vars.extend(then_map_end.keys().cloned());
+        if let Some(ref else_map) = else_map_end_opt {
+            all_vars.extend(else_map.keys().cloned());
+        }
+
+        // Keep track of which vars were changed (for debugging/hints)
         let changed = crate::mir::phi_core::if_phi::compute_modified_names(
             pre_if_snapshot,
             then_map_end,
             else_map_end_opt,
         );
-        use std::collections::HashSet;
         let changed_set: HashSet<String> = changed.iter().cloned().collect();
-        for name in changed {
+
+        // Debug: Conservative PHI trace
+        let trace_conservative = std::env::var("NYASH_CONSERVATIVE_PHI_TRACE").ok().as_deref() == Some("1");
+        if trace_conservative {
+            eprintln!("[Conservative PHI] all_vars count: {}", all_vars.len());
+            eprintln!("[Conservative PHI] pre_if_snapshot: {:?}", pre_if_snapshot.keys().collect::<Vec<_>>());
+            eprintln!("[Conservative PHI] then_map_end: {:?}", then_map_end.keys().collect::<Vec<_>>());
+            if let Some(ref else_map) = else_map_end_opt {
+                eprintln!("[Conservative PHI] else_map_end: {:?}", else_map.keys().collect::<Vec<_>>());
+            }
+        }
+
+        // Generate PHI for all variables (Conservative)
+        for name in all_vars {
             if skip_var.map(|s| s == name).unwrap_or(false) {
+                if trace_conservative {
+                    eprintln!("[Conservative PHI] Skipping {}: matches skip_var", name);
+                }
                 continue;
             }
-            let pre = match pre_if_snapshot.get(name.as_str()) {
-                Some(v) => *v,
-                None => continue, // unknown before-if; skip
-            };
-            let then_v = then_map_end.get(name.as_str()).copied().unwrap_or(pre);
-            let else_v = else_map_end_opt
+
+            // 📦 Conservative PHI: Fallback to predecessor value if not defined in a branch
+            // This handles variables defined in only one branch (e.g., bb16 defines %51, but bb15 doesn't)
+            let pre_val_opt = pre_if_snapshot.get(name.as_str()).copied();
+
+            // Get values from each branch, falling back to predecessor value
+            let then_v_opt = then_map_end.get(name.as_str()).copied()
+                .or(pre_val_opt);
+            let else_v_opt = else_map_end_opt
                 .as_ref()
                 .and_then(|m| m.get(name.as_str()).copied())
-                .unwrap_or(pre);
+                .or(pre_val_opt);
+
+            // 📦 Conservative PHI: Handle variables defined in only ONE branch
+            // If variable exists in one branch but not the other (and not in predecessor),
+            // create a fresh "undefined" ValueId for the missing branch.
+            // This ensures all control flow paths have a definition at the merge point.
+            let (then_v, else_v) = match (then_v_opt, else_v_opt) {
+                (Some(tv), Some(ev)) => {
+                    if trace_conservative {
+                        eprintln!("[Conservative PHI] Generating PHI for {}: then={:?} else={:?}", name, tv, ev);
+                    }
+                    (tv, ev)
+                },
+                (Some(tv), None) => {
+                    // Variable exists in then branch but not else or predecessor
+                    // Emit a 'const void' instruction to represent undefined value
+                    let undef = crate::mir::builder::emission::constant::emit_void(self);
+                    if trace_conservative {
+                        eprintln!("[Conservative PHI] One-branch variable {}: then={:?} else=void({:?})", name, tv, undef);
+                    }
+                    (tv, undef)
+                },
+                (None, Some(ev)) => {
+                    // Variable exists in else branch but not then or predecessor
+                    // Emit a 'const void' instruction to represent undefined value
+                    let undef = crate::mir::builder::emission::constant::emit_void(self);
+                    if trace_conservative {
+                        eprintln!("[Conservative PHI] One-branch variable {}: then=void({:?}) else={:?}", name, undef, ev);
+                    }
+                    (undef, ev)
+                },
+                (None, None) => {
+                    // Variable doesn't exist anywhere - skip
+                    if trace_conservative {
+                        eprintln!("[Conservative PHI] Skipping {}: undefined everywhere", name);
+                    }
+                    continue
+                }
+            };
             // 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 mut inputs: Vec<(super::BasicBlockId, super::ValueId)> = Vec::new();

src/mir/loop_builder.rs

@@ -154,9 +154,11 @@ impl<'a> LoopBuilder<'a> {
         body: Vec<ASTNode>,
     ) -> Result<ValueId, String> {
         // Check feature flag for LoopForm PHI v2
+        // 📦 Default to true (v2 is now stable and default)
+        // Set NYASH_LOOPFORM_PHI_V2=0 to use legacy version if needed
         let use_loopform_v2 = std::env::var("NYASH_LOOPFORM_PHI_V2")
             .map(|v| v == "1" || v.to_lowercase() == "true")
-            .unwrap_or(false);
+            .unwrap_or(true);
 
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             eprintln!("[build_loop] use_loopform_v2={}", use_loopform_v2);
@@ -165,7 +167,8 @@ impl<'a> LoopBuilder<'a> {
         if use_loopform_v2 {
             self.build_loop_with_loopform(condition, body)
         } else {
-            eprintln!("⚠️ WARNING: Using legacy loop builder! Set NYASH_LOOPFORM_PHI_V2=1");
+            eprintln!("⚠️ WARNING: Using legacy loop builder! LoopForm v2 is now default.");
+            eprintln!("⚠️          Legacy version may be deprecated in future releases.");
             self.build_loop_legacy(condition, body)
         }
     }
@@ -222,7 +225,10 @@ impl<'a> LoopBuilder<'a> {
         let exit_id = self.new_block();
 
         // Jump from current block to preheader
+        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)?;
 
         // Initialize LoopFormBuilder with preheader and header blocks
         let mut loopform = LoopFormBuilder::new(preheader_id, header_id);
@@ -259,6 +265,8 @@ impl<'a> LoopBuilder<'a> {
 
         // Pass 2: Emit preheader (copies and jump to header)
         loopform.emit_preheader(self)?;
+        // 📦 Hotfix 6: Add CFG predecessor for header from preheader (same as legacy version)
+        crate::mir::builder::loops::add_predecessor(self.parent_builder, header_id, preheader_id)?;
 
         // Pass 3: Emit header PHIs (incomplete, only preheader edge)
         self.set_current_block(header_id)?;
@@ -301,8 +309,23 @@ impl<'a> LoopBuilder<'a> {
             }
         }
         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);
+        }
+        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);
+            // 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);
+                }
+            }
         }
 
         // Lower loop body
@@ -336,6 +359,8 @@ impl<'a> LoopBuilder<'a> {
         }
 
         self.emit_jump(header_id)?;
+        // 📦 Hotfix 6: Add CFG predecessor for header from latch (same as legacy version)
+        crate::mir::builder::loops::add_predecessor(self.parent_builder, header_id, latch_id)?;
 
         // Pass 4: Seal PHIs with latch values
         loopform.seal_phis(self, actual_latch_id)?;
@@ -1256,6 +1281,19 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
         }
     }
 
+    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
+            }
+        } else {
+            std::collections::HashSet::new()
+        }
+    }
+
     fn is_parameter(&self, name: &str) -> bool {
         // A parameter is a true function parameter that doesn't change across iterations
         // Pinned receivers (__pin$*$@*) are NOT parameters - they're carriers

src/mir/phi_core/loopform_builder.rs

@@ -331,8 +331,15 @@ impl LoopFormBuilder {
                 .push((branch_source_block, carrier.header_phi));
         }
 
+        // 📦 Hotfix 6: Get actual CFG predecessors for exit block
+        let exit_preds = ops.get_block_predecessors(exit_id);
+        if debug {
+            eprintln!("[DEBUG/exit_phi] Exit block predecessors: {:?}", exit_preds);
+        }
+
         // Add break snapshot values
         // 📦 Hotfix 2: Skip non-existent blocks (幽霊ブロック対策)
+        // 📦 Hotfix 6: Skip blocks not in CFG predecessors (unreachable continuation対策)
         for (block_id, snapshot) in exit_snapshots {
             // Validate block existence before adding to PHI inputs
             if !ops.block_exists(*block_id) {
@@ -342,6 +349,15 @@ impl LoopFormBuilder {
                 continue;  // Skip ghost blocks
             }
 
+            // Hotfix 6: Skip blocks not in actual CFG predecessors
+            // This catches unreachable continuation blocks created after break/continue
+            if !exit_preds.contains(block_id) {
+                if debug {
+                    eprintln!("[DEBUG/exit_phi] ⚠️ Skipping block {:?} (not in CFG predecessors)", block_id);
+                }
+                continue;  // Skip blocks not actually branching to exit
+            }
+
             for (var_name, &value) in snapshot {
                 all_vars
                     .entry(var_name.clone())
@@ -409,6 +425,11 @@ pub trait LoopFormOps {
     /// Used to skip non-existent blocks when building exit PHIs.
     fn block_exists(&self, block: BasicBlockId) -> bool;
 
+    /// 📦 Get actual CFG predecessors for a block (Hotfix 6: PHI input validation)
+    /// Returns the set of blocks that actually branch to this block in the CFG.
+    /// Used to validate exit PHI inputs against actual control flow.
+    fn get_block_predecessors(&self, block: BasicBlockId) -> std::collections::HashSet<BasicBlockId>;
+
     /// Check if a variable is a function parameter
     fn is_parameter(&self, name: &str) -> bool;
 
@@ -520,6 +541,11 @@ mod tests {
                 true
             }
 
+            fn get_block_predecessors(&self, _block: BasicBlockId) -> std::collections::HashSet<BasicBlockId> {
+                // MockOps: return empty set (no CFG in test)
+                std::collections::HashSet::new()
+            }
+
             fn is_parameter(&self, name: &str) -> bool {
                 self.params.iter().any(|p| p == name)
             }

src/tests/mir_stage1_using_resolver_verify.rs

@@ -68,15 +68,13 @@ static box Stage1UsingResolverMini {
     let mut mc = MirCompiler::with_options(false);
     let cr = mc.compile(ast).expect("compile");
 
-    // DEBUG: Print MIR structure for _collect_using_entries
+    // DEBUG: Print MIR structure for ALL functions (temporary)
     for (fname, func) in &cr.module.functions {
-        if fname.contains("_collect_using_entries") {
         eprintln!("\n=== MIR for {} ===", fname);
         let printer = MirPrinter::new();
         let mir_text = printer.print_function(func);
         eprintln!("{}", mir_text);
     }
-    }
 
     let mut verifier = MirVerifier::new();
     if let Err(errors) = verifier.verify_module(&cr.module) {