refactor(joinir): Phase 33-17-B LoopHeaderPhiInfo extraction

## Changes
- Extract LoopHeaderPhiInfo Box (116 lines) from loop_header_phi_builder
- loop_header_phi_builder.rs reduced: 318 → 217 lines (-32%)
- Proper separation of data structures from builder logic

## Module Structure
- loop_header_phi_info.rs: LoopHeaderPhiInfo, CarrierPhiEntry structs + tests
- loop_header_phi_builder.rs: LoopHeaderPhiBuilder implementation

## Box Theory Compliance
- Data structures separated from builder logic
- Each file has clear single responsibility
- Clean re-exports through mod.rs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -9,7 +9,7 @@
 use crate::mir::{BasicBlock, BasicBlockId, MirInstruction, MirModule, ValueId};
 use crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper;
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
-use super::loop_header_phi_builder::LoopHeaderPhiInfo;
+use super::loop_header_phi_info::LoopHeaderPhiInfo;
 use super::tail_call_classifier::{TailCallKind, classify_tail_call};
 use super::merge_result::MergeResult;
 use std::collections::HashMap;

src/mir/builder/control_flow/joinir/merge/loop_header_phi_builder.rs

@@ -1,6 +1,7 @@
 //! Loop Header PHI Builder
 //!
 //! Phase 33-16: Generates PHI nodes at loop header blocks to track carrier values.
+//! Phase 33-17-B: Refactored to separate data structures from builder logic.
 //!
 //! ## Problem
 //!
@@ -26,74 +27,8 @@
 //! Called from merge pipeline between Phase 3 (remap_values) and Phase 4
 //! (instruction_rewriter).
 
-use crate::mir::{BasicBlock, BasicBlockId, MirInstruction, ValueId};
-use std::collections::BTreeMap;
-
-/// Information about loop header PHIs
-///
-/// Generated by LoopHeaderPhiBuilder and used by:
-/// - exit_phi_builder: to reference the current loop value
-/// - ExitLineReconnector: to update variable_map with final values
-#[derive(Debug, Clone)]
-pub struct LoopHeaderPhiInfo {
-    /// The block where header PHIs are placed
-    pub header_block: BasicBlockId,
-
-    /// Carrier variable PHI mappings: carrier_name → PHI dst
-    ///
-    /// The PHI dst is the ValueId that represents the "current value"
-    /// of this carrier during loop iteration.
-    pub carrier_phis: BTreeMap<String, CarrierPhiEntry>,
-
-    /// Expression result PHI dst (if loop is used as expression)
-    ///
-    /// For Pattern 2 (joinir_min_loop), this is the same as the loop
-    /// variable's PHI dst. For carrier-only patterns (trim), this is None.
-    pub expr_result_phi: Option<ValueId>,
-}
-
-/// Entry for a single carrier's header PHI
-#[derive(Debug, Clone)]
-pub struct CarrierPhiEntry {
-    /// PHI destination ValueId (the "current value" during iteration)
-    pub phi_dst: ValueId,
-
-    /// Entry edge: (from_block, init_value)
-    pub entry_incoming: (BasicBlockId, ValueId),
-
-    /// Latch edge: (from_block, updated_value) - set after instruction rewrite
-    pub latch_incoming: Option<(BasicBlockId, ValueId)>,
-}
-
-impl LoopHeaderPhiInfo {
-    /// Create empty LoopHeaderPhiInfo
-    pub fn empty(header_block: BasicBlockId) -> Self {
-        Self {
-            header_block,
-            carrier_phis: BTreeMap::new(),
-            expr_result_phi: None,
-        }
-    }
-
-    /// Get the PHI dst for a carrier variable
-    pub fn get_carrier_phi(&self, name: &str) -> Option<ValueId> {
-        self.carrier_phis.get(name).map(|e| e.phi_dst)
-    }
-
-    /// Set latch incoming for a carrier
-    ///
-    /// Called from instruction_rewriter after processing tail call Copy instructions.
-    pub fn set_latch_incoming(&mut self, name: &str, from_block: BasicBlockId, value: ValueId) {
-        if let Some(entry) = self.carrier_phis.get_mut(name) {
-            entry.latch_incoming = Some((from_block, value));
-        }
-    }
-
-    /// Check if all carriers have latch incoming set
-    pub fn all_latch_set(&self) -> bool {
-        self.carrier_phis.values().all(|e| e.latch_incoming.is_some())
-    }
-}
+use crate::mir::{MirInstruction, ValueId, BasicBlockId};
+use super::loop_header_phi_info::{LoopHeaderPhiInfo, CarrierPhiEntry};
 
 /// Builder for loop header PHIs
 ///
@@ -280,39 +215,3 @@ impl LoopHeaderPhiBuilder {
         Ok(())
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn test_loop_header_phi_info_creation() {
-        let header = BasicBlockId(10);
-        let info = LoopHeaderPhiInfo::empty(header);
-
-        assert_eq!(info.header_block, header);
-        assert!(info.carrier_phis.is_empty());
-        assert!(info.expr_result_phi.is_none());
-    }
-
-    #[test]
-    fn test_carrier_phi_entry() {
-        let mut info = LoopHeaderPhiInfo::empty(BasicBlockId(10));
-
-        info.carrier_phis.insert(
-            "i".to_string(),
-            CarrierPhiEntry {
-                phi_dst: ValueId(100),
-                entry_incoming: (BasicBlockId(1), ValueId(5)),
-                latch_incoming: None,
-            },
-        );
-
-        assert_eq!(info.get_carrier_phi("i"), Some(ValueId(100)));
-        assert_eq!(info.get_carrier_phi("x"), None);
-        assert!(!info.all_latch_set());
-
-        info.set_latch_incoming("i", BasicBlockId(20), ValueId(50));
-        assert!(info.all_latch_set());
-    }
-}

src/mir/builder/control_flow/joinir/merge/loop_header_phi_info.rs

@@ -0,0 +1,116 @@
+//! Loop Header PHI Information Structures
+//!
+//! Phase 33-17-B: Data structures for loop header PHI tracking.
+//!
+//! ## Overview
+//!
+//! These structures hold metadata about PHI nodes generated at loop headers
+//! to track carrier variables. Used by:
+//! - LoopHeaderPhiBuilder: to build PHIs
+//! - exit_phi_builder: to reference current loop values
+//! - ExitLineReconnector: to update variable_map with final values
+
+use crate::mir::{BasicBlockId, ValueId};
+use std::collections::BTreeMap;
+
+/// Information about loop header PHIs
+///
+/// Generated by LoopHeaderPhiBuilder and used by:
+/// - exit_phi_builder: to reference the current loop value
+/// - ExitLineReconnector: to update variable_map with final values
+#[derive(Debug, Clone)]
+pub struct LoopHeaderPhiInfo {
+    /// The block where header PHIs are placed
+    pub header_block: BasicBlockId,
+
+    /// Carrier variable PHI mappings: carrier_name → PHI dst
+    ///
+    /// The PHI dst is the ValueId that represents the "current value"
+    /// of this carrier during loop iteration.
+    pub carrier_phis: BTreeMap<String, CarrierPhiEntry>,
+
+    /// Expression result PHI dst (if loop is used as expression)
+    ///
+    /// For Pattern 2 (joinir_min_loop), this is the same as the loop
+    /// variable's PHI dst. For carrier-only patterns (trim), this is None.
+    pub expr_result_phi: Option<ValueId>,
+}
+
+/// Entry for a single carrier's header PHI
+#[derive(Debug, Clone)]
+pub struct CarrierPhiEntry {
+    /// PHI destination ValueId (the "current value" during iteration)
+    pub phi_dst: ValueId,
+
+    /// Entry edge: (from_block, init_value)
+    pub entry_incoming: (BasicBlockId, ValueId),
+
+    /// Latch edge: (from_block, updated_value) - set after instruction rewrite
+    pub latch_incoming: Option<(BasicBlockId, ValueId)>,
+}
+
+impl LoopHeaderPhiInfo {
+    /// Create empty LoopHeaderPhiInfo
+    pub fn empty(header_block: BasicBlockId) -> Self {
+        Self {
+            header_block,
+            carrier_phis: BTreeMap::new(),
+            expr_result_phi: None,
+        }
+    }
+
+    /// Get the PHI dst for a carrier variable
+    pub fn get_carrier_phi(&self, name: &str) -> Option<ValueId> {
+        self.carrier_phis.get(name).map(|e| e.phi_dst)
+    }
+
+    /// Set latch incoming for a carrier
+    ///
+    /// Called from instruction_rewriter after processing tail call Copy instructions.
+    pub fn set_latch_incoming(&mut self, name: &str, from_block: BasicBlockId, value: ValueId) {
+        if let Some(entry) = self.carrier_phis.get_mut(name) {
+            entry.latch_incoming = Some((from_block, value));
+        }
+    }
+
+    /// Check if all carriers have latch incoming set
+    pub fn all_latch_set(&self) -> bool {
+        self.carrier_phis.values().all(|e| e.latch_incoming.is_some())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_loop_header_phi_info_creation() {
+        let header = BasicBlockId(10);
+        let info = LoopHeaderPhiInfo::empty(header);
+
+        assert_eq!(info.header_block, header);
+        assert!(info.carrier_phis.is_empty());
+        assert!(info.expr_result_phi.is_none());
+    }
+
+    #[test]
+    fn test_carrier_phi_entry() {
+        let mut info = LoopHeaderPhiInfo::empty(BasicBlockId(10));
+
+        info.carrier_phis.insert(
+            "i".to_string(),
+            CarrierPhiEntry {
+                phi_dst: ValueId(100),
+                entry_incoming: (BasicBlockId(1), ValueId(5)),
+                latch_incoming: None,
+            },
+        );
+
+        assert_eq!(info.get_carrier_phi("i"), Some(ValueId(100)));
+        assert_eq!(info.get_carrier_phi("x"), None);
+        assert!(!info.all_latch_set());
+
+        info.set_latch_incoming("i", BasicBlockId(20), ValueId(50));
+        assert!(info.all_latch_set());
+    }
+}

src/mir/builder/control_flow/joinir/merge/mod.rs

@@ -17,13 +17,16 @@ mod value_collector;
 mod instruction_rewriter;
 mod exit_phi_builder;
 pub mod exit_line;
-pub mod loop_header_phi_builder;
+mod loop_header_phi_info;
+mod loop_header_phi_builder;
 mod tail_call_classifier;
 mod merge_result;
 
 // Phase 33-17: Re-export for use by other modules
 pub use merge_result::MergeResult;
 pub use tail_call_classifier::{TailCallKind, classify_tail_call};
+pub use loop_header_phi_info::{LoopHeaderPhiInfo, CarrierPhiEntry};
+pub use loop_header_phi_builder::LoopHeaderPhiBuilder;
 
 use crate::mir::{BasicBlockId, MirModule, ValueId};
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
@@ -154,7 +157,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 );
             }
 
-            let phi_info = loop_header_phi_builder::LoopHeaderPhiBuilder::build(
+            let phi_info = LoopHeaderPhiBuilder::build(
                 builder,
                 entry_block_remapped,  // header_block (JoinIR's entry block = loop header)
                 host_entry_block,      // entry_block (host's block that jumps to loop header)
@@ -182,10 +185,10 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
 
             phi_info
         } else {
-            loop_header_phi_builder::LoopHeaderPhiInfo::empty(entry_block_remapped)
+            LoopHeaderPhiInfo::empty(entry_block_remapped)
         }
     } else {
-        loop_header_phi_builder::LoopHeaderPhiInfo::empty(entry_block_remapped)
+        LoopHeaderPhiInfo::empty(entry_block_remapped)
     };
 
     // Phase 4: Merge blocks and rewrite instructions
@@ -212,7 +215,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 loop_header_phi_info.carrier_phis.len()
             );
         }
-        loop_header_phi_builder::LoopHeaderPhiBuilder::finalize(
+        LoopHeaderPhiBuilder::finalize(
             builder,
             &loop_header_phi_info,
             debug,