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

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