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

//! JoinIR MIR Block Merging Coordinator
//!
//! This module coordinates the merging of JoinIR-generated MIR functions
//! into the host MIR builder. The process is broken into 6 phases:
//!
//! 1. Block ID allocation (block_allocator.rs)
//! 2. Value collection (value_collector.rs)
//! 3. ValueId remapping (uses JoinIrIdRemapper)
//! 4. Instruction rewriting (instruction_rewriter.rs)
//! 5. Exit PHI construction (exit_phi_builder.rs)
//! 6. Boundary reconnection (inline in this file)
//!
//! Phase 4 Refactoring: Breaking down 714-line merge_joinir_mir_blocks() into focused modules

mod block_allocator;
mod value_collector;
mod instruction_rewriter;
mod exit_phi_builder;
pub mod exit_line;
mod loop_header_phi_info;
mod loop_header_phi_builder;
mod tail_call_classifier;
mod merge_result;
mod expr_result_resolver;
#[cfg(debug_assertions)]
mod contract_checks;

// Phase 33-17: Re-export for use by other modules
pub use loop_header_phi_info::LoopHeaderPhiInfo;
pub use loop_header_phi_builder::LoopHeaderPhiBuilder;

use crate::mir::{MirModule, ValueId};
use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;

/// Phase 49-3.2: Merge JoinIR-generated MIR blocks into current_function
///
/// # Phase 189: Multi-Function MIR Merge
///
/// This merges JoinIR-generated blocks by:
/// 1. Remapping all block IDs across ALL functions to avoid conflicts
/// 2. Remapping all value IDs across ALL functions to avoid conflicts
/// 3. Adding all blocks from all functions to current_function
/// 4. Jumping from current_block to the entry block
/// 5. Converting Return → Jump to exit block for all functions
///
/// **Multi-Function Support** (Phase 189):
/// - Pattern 1 (Simple While) generates 3 functions: entry + loop_step + k_exit
/// - All functions are flattened into current_function with global ID remapping
/// - Single exit block receives all Return instructions from all functions
///
/// # Phase 188-Impl-3: JoinInlineBoundary Support
///
/// When `boundary` is provided, injects Copy instructions at the entry block
/// to connect host ValueIds to JoinIR local ValueIds:
///
/// ```text
/// entry_block:
///   // Injected by boundary
///   ValueId(100) = Copy ValueId(4)  // join_input → host_input
///   // Original JoinIR instructions follow...
/// ```
///
/// This enables clean separation: JoinIR uses local IDs (0,1,2...),
/// host uses its own IDs, and Copy instructions bridge the gap.
///
/// # Returns
///
/// Returns `Ok(Some(exit_phi_id))` if the merged JoinIR functions have return values
/// that were collected into an exit block PHI. さらに、`boundary` に
/// host_outputs が指定されている場合は、exit PHI の結果をホスト側の
/// SSA スロットへ再接続する（variable_map 内の ValueId を更新する）。
pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
    builder: &mut crate::mir::builder::MirBuilder,
    mir_module: &MirModule,
    boundary: Option<&JoinInlineBoundary>,
    debug: bool,
) -> Result<Option<ValueId>, String> {
    let verbose = debug || crate::config::env::joinir_dev_enabled();

    if debug {
        eprintln!(
            "[cf_loop/joinir] merge_joinir_mir_blocks called with {} functions",
            mir_module.functions.len()
        );
    }

    if verbose {
        if let Some(boundary) = boundary {
            let exit_summary: Vec<String> = boundary
                .exit_bindings
                .iter()
                .map(|b| {
                    format!(
                        "{}: join {:?} → host {:?} ({:?})",
                        b.carrier_name, b.join_exit_value, b.host_slot, b.role
                    )
                })
                .collect();

            let cond_summary: Vec<String> = boundary
                .condition_bindings
                .iter()
                .map(|b| format!("{}: host {:?} → join {:?}", b.name, b.host_value, b.join_value))
                .collect();

            eprintln!(
                "[cf_loop/joinir] Boundary join_inputs={:?} host_inputs={:?}",
                boundary.join_inputs, boundary.host_inputs
            );
            eprintln!(
                "[cf_loop/joinir] Boundary exit_bindings ({}): {}",
                boundary.exit_bindings.len(),
                exit_summary.join(", ")
            );
            if !cond_summary.is_empty() {
                eprintln!(
                    "[cf_loop/joinir] Boundary condition_bindings ({}): {}",
                    cond_summary.len(),
                    cond_summary.join(", ")
                );
            }
            if let Some(ci) = &boundary.carrier_info {
                let carriers: Vec<String> = ci.carriers.iter().map(|c| c.name.clone()).collect();
                eprintln!(
                    "[cf_loop/joinir] Boundary carrier_info: loop_var='{}', carriers={:?}",
                    ci.loop_var_name,
                    carriers
                );
            }
        } else {
            eprintln!("[cf_loop/joinir] No boundary provided");
        }
    }

    // Phase 1: Allocate block IDs for all functions
    // Phase 177-3: block_allocator now returns exit_block_id to avoid conflicts
    let (mut remapper, exit_block_id) = block_allocator::allocate_blocks(builder, mir_module, debug)?;

    // Phase 2: Collect values from all functions
    let (mut used_values, value_to_func_name, function_params) =
        value_collector::collect_values(mir_module, &remapper, debug)?;

    // Phase 171-fix: Add condition_bindings' join_values to used_values for remapping
    if let Some(boundary) = boundary {
        for binding in &boundary.condition_bindings {
            if debug {
                eprintln!(
                    "[cf_loop/joinir] Phase 171-fix: Adding condition binding '{}' JoinIR {:?} to used_values",
                    binding.name, binding.join_value
                );
            }
            used_values.insert(binding.join_value);
        }

        // Phase 172-3: Add exit_bindings' join_exit_values to used_values for remapping
        for binding in &boundary.exit_bindings {
            if debug {
                eprintln!(
                    "[cf_loop/joinir] Phase 172-3: Adding exit binding '{}' JoinIR {:?} to used_values",
                    binding.carrier_name, binding.join_exit_value
                );
            }
            used_values.insert(binding.join_exit_value);
        }
    }

    // Phase 201-A: Build loop header PHI info BEFORE Phase 3
    //
    // We need to allocate PHI dst ValueIds before remap_values() runs,
    // to prevent conflicts where a Const instruction gets a ValueId that
    // will later be used as a PHI dst, causing carrier value corruption.
    //
    // This is a reordering of Phase 3 and Phase 3.5 logic.
    let mut loop_header_phi_info = if let Some(boundary) = boundary {
        if let Some(loop_var_name) = &boundary.loop_var_name {
            // Get entry function and block for building PHI info
            let (entry_func_name, entry_func) = mir_module
                .functions
                .iter()
                .next()
                .ok_or("JoinIR module has no functions (Phase 201-A)")?;
            let entry_block_remapped = remapper
                .get_block(entry_func_name, entry_func.entry_block)
                .ok_or_else(|| format!("Entry block not found for {} (Phase 201-A)", entry_func_name))?;

            // Get host's current block as the entry edge
            let host_entry_block = builder.current_block.ok_or(
                "Phase 201-A: No current block when building header PHIs"
            )?;

            // Get loop variable's initial value from HOST
            let loop_var_init = boundary.host_inputs.first().copied().ok_or(
                "Phase 201-A: No host_inputs in boundary for loop_var_init"
            )?;

            // Phase 228-4: Extract carriers with their initialization strategy
            let other_carriers: Vec<(String, ValueId, crate::mir::join_ir::lowering::carrier_info::CarrierInit, crate::mir::join_ir::lowering::carrier_info::CarrierRole)> =
                if let Some(ref carrier_info) = boundary.carrier_info {
                    // Use carrier_info if available (Phase 228)
                    carrier_info.carriers
                        .iter()
                        .filter(|c| c.name != *loop_var_name)
                        .map(|c| (c.name.clone(), c.host_id, c.init, c.role))
                        .collect()
                } else {
                    // Fallback: exit_bindings から取得（既存動作）
                    boundary.exit_bindings
                        .iter()
                        .filter(|b| b.carrier_name != *loop_var_name)
                        .map(|b| (b.carrier_name.clone(), b.host_slot, crate::mir::join_ir::lowering::carrier_info::CarrierInit::FromHost, crate::mir::join_ir::lowering::carrier_info::CarrierRole::LoopState))
                        .collect()
                };

            if debug {
                eprintln!(
                    "[cf_loop/joinir] Phase 201-A: Pre-building header PHIs for loop_var='{}' at {:?}",
                    loop_var_name, entry_block_remapped
                );
                eprintln!(
                    "[cf_loop/joinir]   loop_var_init={:?}, carriers={:?}",
                    loop_var_init,
                    other_carriers.iter().map(|(n, _, _, _)| n.as_str()).collect::<Vec<_>>()
                );
            }

            // Build PHI info (this allocates PHI dst ValueIds)
            LoopHeaderPhiBuilder::build(
                builder,
                entry_block_remapped,
                host_entry_block,
                loop_var_name,
                loop_var_init,
                &other_carriers,
                boundary.expr_result.is_some(),
                debug,
            )?
        } else {
            LoopHeaderPhiInfo::empty(remapper.get_block(
                mir_module.functions.iter().next().unwrap().0,
                mir_module.functions.iter().next().unwrap().1.entry_block
            ).unwrap())
        }
    } else {
        LoopHeaderPhiInfo::empty(remapper.get_block(
            mir_module.functions.iter().next().unwrap().0,
            mir_module.functions.iter().next().unwrap().1.entry_block
        ).unwrap())
    };

    // Phase 201-A: Get reserved PHI dst ValueIds and set in MirBuilder
    let reserved_phi_dsts = loop_header_phi_info.reserved_value_ids();
    if debug && !reserved_phi_dsts.is_empty() {
        eprintln!("[cf_loop/joinir] Phase 201-A: Reserved PHI dsts: {:?}", reserved_phi_dsts);
    }

    // Phase 201-A: Set reserved IDs in MirBuilder so next_value_id() skips them
    // This protects against carrier corruption when break conditions emit Const instructions
    builder.reserved_value_ids = reserved_phi_dsts.clone();
    if debug && !builder.reserved_value_ids.is_empty() {
        eprintln!("[cf_loop/joinir] Phase 201-A: Set builder.reserved_value_ids = {:?}", builder.reserved_value_ids);
    }

    // Phase 3: Remap ValueIds (with reserved PHI dsts protection)
    remap_values(builder, &used_values, &mut remapper, &reserved_phi_dsts, debug)?;

    // Phase 177-3 DEBUG: Verify remapper state after Phase 3
    eprintln!("[DEBUG-177] === Remapper state after Phase 3 ===");
    eprintln!("[DEBUG-177] used_values count: {}", used_values.len());
    for value_id in &used_values {
        if let Some(remapped) = remapper.get_value(*value_id) {
            eprintln!("[DEBUG-177]   JoinIR {:?} → Host {:?}", value_id, remapped);
        } else {
            eprintln!("[DEBUG-177]   JoinIR {:?} → NOT FOUND ❌", value_id);
        }
    }

    // Check condition_bindings specifically
    if let Some(boundary) = boundary {
        eprintln!("[DEBUG-177] === Condition bindings check ===");
        for binding in &boundary.condition_bindings {
            let lookup_result = remapper.get_value(binding.join_value);
            eprintln!(
                "[DEBUG-177]   '{}': JoinIR {:?} → {:?}",
                binding.name, binding.join_value, lookup_result
            );
        }
    }
    eprintln!("[DEBUG-177] ==============================");

    // Phase 3.5: Override remapper for function parameters to use PHI dsts
    //
    // Phase 201-A: This phase now uses the loop_header_phi_info built before Phase 3.
    // The PHI dst allocation has been moved earlier to prevent ValueId conflicts.
    if let Some(boundary) = boundary {
        if let Some(loop_var_name) = &boundary.loop_var_name {
            // Phase 201-A: PHI info is already built (before Phase 3) - just use it

            // Phase 33-21: Override remapper for loop_step's parameters
            //
            // JoinIR generates separate parameter ValueIds for each function:
            // - main(): ValueId(0), ValueId(1), ... for (i_init, carrier1_init, ...)
            // - loop_step(): ValueId(3), ValueId(4), ... for (i_param, carrier1_param, ...)
            //
            // The loop body uses loop_step's parameters, so we need to remap THOSE
            // to the header PHI dsts, not main()'s parameters.
            //
            // We get loop_step's parameters from function_params collected earlier.
            // Phase 33-21: Override remapper for ALL functions' parameters
            //
            // JoinIR generates separate parameter ValueIds for each function:
            // - main (join_func_0): ValueId(0), ValueId(1), ... for (i_init, carrier1_init, ...)
            // - loop_step (join_func_1): ValueId(3), ValueId(4), ... for (i_param, carrier1_param, ...)
            //
            // ALL of these need to be mapped to header PHI dsts so that:
            // 1. condition evaluation uses PHI result
            // 2. loop body uses PHI result
            // 3. tail call args are correctly routed

            // Phase 177-3 fix: Protect condition-ONLY bindings from being overridden to PHI dsts
            //
            // Problem: condition_bindings may contain:
            // 1. True condition-only variables (e.g., 'limit' in loop(i < limit)) - NOT carriers
            // 2. Body-only carriers added by Phase 176-5 (e.g., 'result') - ARE carriers
            //
            // We must ONLY protect (1), not (2), because:
            // - Condition-only vars should keep their HOST mapping (e.g., limit = %8)
            // - Body-only carriers MUST be remapped to PHI dsts (e.g., result = %24)
            //
            // Solution: Protect condition_bindings that are NOT in exit_bindings (i.e., not carriers)
            let carrier_names: std::collections::HashSet<&str> = boundary
                .exit_bindings
                .iter()
                .map(|eb| eb.carrier_name.as_str())
                .collect();

            let condition_binding_ids: std::collections::HashSet<ValueId> = boundary
                .condition_bindings
                .iter()
                .filter(|cb| !carrier_names.contains(cb.name.as_str()))
                .map(|cb| cb.join_value)
                .collect();

            if !condition_binding_ids.is_empty() {
                eprintln!(
                    "[cf_loop/joinir] Phase 177-3: Protected ValueIds (condition-only, not carriers): {:?}",
                    condition_binding_ids
                );
                for cb in &boundary.condition_bindings {
                    let is_carrier = carrier_names.contains(cb.name.as_str());
                    eprintln!(
                        "[cf_loop/joinir] Phase 177-3:   '{}': JoinIR {:?} (carrier={})",
                        cb.name, cb.join_value, is_carrier
                    );
                }
            }

            let main_func_name = "join_func_0";
            let loop_step_func_name = "join_func_1";

            if function_params.get(main_func_name).is_none() {
                eprintln!(
                    "[cf_loop/joinir] WARNING: function_params.get('{}') returned None. Available keys: {:?}",
                    main_func_name,
                    function_params.keys().collect::<Vec<_>>()
                );
            }
            if let Some(main_params) = function_params.get(main_func_name) {
                eprintln!(
                    "[DEBUG-177] Phase 33-21: main ({}) params: {:?}",
                    main_func_name, main_params
                );
                eprintln!(
                    "[DEBUG-177] Phase 33-21: carrier_phis count: {}, names: {:?}",
                    loop_header_phi_info.carrier_phis.len(),
                    loop_header_phi_info.carrier_phis.iter().map(|(n, _)| n.as_str()).collect::<Vec<_>>()
                );
                // Map main's parameters to header PHI dsts
                // main params: [i_init, carrier1_init, ...]
                // carrier_phis: [("i", entry), ("sum", entry), ...]
                for (idx, (carrier_name, entry)) in loop_header_phi_info.carrier_phis.iter().enumerate() {
                    if let Some(&main_param) = main_params.get(idx) {
                        // Phase 177-3: Don't override condition_bindings
                        if condition_binding_ids.contains(&main_param) {
                            eprintln!(
                                "[cf_loop/joinir] Phase 177-3: Skipping override for condition_binding {:?} ('{}')",
                                main_param, carrier_name
                            );
                            continue;
                        }
                        eprintln!(
                            "[DEBUG-177] Phase 33-21: REMAP main param[{}] {:?} → {:?} ('{}')",
                            idx, main_param, entry.phi_dst, carrier_name
                        );
                        remapper.set_value(main_param, entry.phi_dst);
                    }
                }
            }

            // Phase 177-3-B: Handle body-only carriers
            // These are carriers in carrier_phis that are NOT in main function params.
            // They appear in condition_bindings (added by Phase 176-5) but need PHI remapping.
            for (carrier_name, entry) in &loop_header_phi_info.carrier_phis {
                // Check if this carrier has a condition_binding
                if let Some(binding) = boundary.condition_bindings.iter().find(|cb| cb.name == *carrier_name) {
                    // Skip if it's a true condition-only variable (already protected above)
                    if condition_binding_ids.contains(&binding.join_value) {
                        continue;
                    }
                    // This is a body-only carrier - remap it to PHI dst
                    eprintln!(
                        "[cf_loop/joinir] Phase 177-3-B: Body-only carrier '{}': JoinIR {:?} → PHI {:?}",
                        carrier_name, binding.join_value, entry.phi_dst
                    );
                    remapper.set_value(binding.join_value, entry.phi_dst);
                }
            }

            // Map loop_step's parameters
            // DEBUG-177: Always log function_params keys to diagnose multi-carrier issue
            eprintln!(
                "[DEBUG-177] Phase 33-21: function_params keys: {:?}",
                function_params.keys().collect::<Vec<_>>()
            );
            if function_params.get(loop_step_func_name).is_none() {
                eprintln!(
                    "[cf_loop/joinir] WARNING: function_params.get('{}') returned None. Available keys: {:?}",
                    loop_step_func_name,
                    function_params.keys().collect::<Vec<_>>()
                );
            }
            if let Some(loop_step_params) = function_params.get(loop_step_func_name) {
                // DEBUG-177: Always log loop_step params
                eprintln!(
                    "[DEBUG-177] Phase 33-21: loop_step ({}) params: {:?}",
                    loop_step_func_name, loop_step_params
                );
                // Phase 177-FIX: Process loop_step params but skip if already mapped
                //
                // We use a name-based approach: for each carrier_phi, check if
                // its join_value was already set in Phase 177-3-B (body-only carriers).
                // Only process loop_step params for carriers NOT already handled.
                for loop_step_param in loop_step_params {
                    // Phase 177-3: Don't override condition_bindings
                    if condition_binding_ids.contains(loop_step_param) {
                        eprintln!(
                            "[DEBUG-177] Phase 177-FIX: Skipping condition_binding {:?}",
                            loop_step_param
                        );
                        continue;
                    }
                    // Find which carrier this param belongs to by matching join_value
                    // Check if this param was already handled by Phase 177-3-B
                    let already_mapped = boundary.condition_bindings.iter().any(|cb| {
                        cb.join_value == *loop_step_param &&
                        loop_header_phi_info.carrier_phis.iter().any(|(name, _)| name == &cb.name)
                    });
                    if already_mapped {
                        eprintln!(
                            "[DEBUG-177] Phase 177-FIX: Skipping {:?} (already mapped by Phase 177-3-B)",
                            loop_step_param
                        );
                        continue;
                    }
                    // Phase 177-STRUCT-2: Use carrier_order for index-based matching
                    //
                    // Problem: BTreeMap iterates in alphabetical order, but JoinIR
                    // generates params in exit_bindings order.
                    //
                    // Solution: Use carrier_order (Vec<String>) which preserves insertion order.
                    if let Some(param_idx) = loop_step_params.iter().position(|p| p == loop_step_param) {
                        // Map params[i] to carrier_order[i]
                        if let (Some(carrier_name), Some(entry)) = (
                            loop_header_phi_info.get_carrier_at_index(param_idx),
                            loop_header_phi_info.get_entry_at_index(param_idx),
                        ) {
                            eprintln!(
                                "[DEBUG-177] Phase 177-STRUCT-2: REMAP loop_step param[{}] {:?} → {:?} (carrier '{}')",
                                param_idx, loop_step_param, entry.phi_dst, carrier_name
                            );
                            remapper.set_value(*loop_step_param, entry.phi_dst);
                        }
                    }
                }
            }

            if function_params.get(main_func_name).is_none() && function_params.get(loop_step_func_name).is_none() {
                // Fallback: Use old behavior (ValueId(0), ValueId(1), ...)
                // This handles patterns that don't have loop_step function
                if let Some(phi_dst) = loop_header_phi_info.get_carrier_phi(loop_var_name) {
                    // Phase 177-3: Don't override condition_bindings
                    if !condition_binding_ids.contains(&ValueId(0)) {
                        remapper.set_value(ValueId(0), phi_dst);
                        if debug {
                            eprintln!(
                                "[cf_loop/joinir] Phase 33-16 fallback: Override remap ValueId(0) → {:?} (PHI dst)",
                                phi_dst
                            );
                        }
                    } else {
                        eprintln!(
                            "[cf_loop/joinir] Phase 177-3 fallback: Skipping override for condition_binding ValueId(0)"
                        );
                    }
                }
                // Phase 177-STRUCT-2: Use carrier_order for deterministic iteration
                for (idx, carrier_name) in loop_header_phi_info.carrier_order.iter().enumerate() {
                    if carrier_name == loop_var_name {
                        continue;
                    }
                    let entry = match loop_header_phi_info.carrier_phis.get(carrier_name) {
                        Some(e) => e,
                        None => continue,
                    };
                    let join_value_id = ValueId(idx as u32);
                    // Phase 177-3: Don't override condition_bindings
                    if !condition_binding_ids.contains(&join_value_id) {
                        remapper.set_value(join_value_id, entry.phi_dst);
                        if debug {
                            eprintln!(
                                "[cf_loop/joinir] Phase 33-20 fallback: Override remap {:?} → {:?} (carrier '{}' PHI dst)",
                                join_value_id, entry.phi_dst, carrier_name
                            );
                        }
                    } else {
                        eprintln!(
                            "[cf_loop/joinir] Phase 177-3 fallback: Skipping override for condition_binding {:?} ('{}')",
                            join_value_id, carrier_name
                        );
                    }
                }
            }

            // Phase 177-3 DEBUG: Check remapper after Phase 33-21 overrides
            eprintln!("[DEBUG-177] === Remapper state after Phase 33-21 ===");
            for binding in &boundary.condition_bindings {
                let lookup_result = remapper.get_value(binding.join_value);
                eprintln!(
                    "[DEBUG-177]   '{}': JoinIR {:?} → {:?} (after 33-21)",
                    binding.name, binding.join_value, lookup_result
                );
            }

            // Phase 201-A: loop_header_phi_info already built (no assignment needed)
        }
    }

    // Phase 4: Merge blocks and rewrite instructions
    // Phase 33-16: Pass mutable loop_header_phi_info for latch_incoming tracking
    // Phase 177-3: Pass exit_block_id from allocator to avoid conflicts
    let merge_result = instruction_rewriter::merge_and_rewrite(
        builder,
        mir_module,
        &mut remapper,
        &value_to_func_name,
        &function_params,
        boundary,
        &mut loop_header_phi_info,
        exit_block_id,
        debug,
    )?;

    // Phase 4.5: Finalize loop header PHIs (insert into header block)
    //
    // By now, instruction_rewriter has set latch_incoming for all carriers.
    // We can finalize the PHIs and insert them into the header block.
    if !loop_header_phi_info.carrier_phis.is_empty() {
        if debug {
            eprintln!(
                "[cf_loop/joinir] Phase 4.5: Finalizing {} header PHIs",
                loop_header_phi_info.carrier_phis.len()
            );
        }
        LoopHeaderPhiBuilder::finalize(
            builder,
            &loop_header_phi_info,
            debug,
        )?;
    }

    // Phase 5: Build exit PHI (expr result only, not carrier PHIs)
    // Phase 33-20: Carrier PHIs are now taken from header PHI info, not exit block
    // Phase 246-EX: REVERT Phase 33-20 - Use EXIT PHI dsts, not header PHI dsts!
    let (exit_phi_result_id, exit_carrier_phis) = exit_phi_builder::build_exit_phi(
        builder,
        merge_result.exit_block_id,
        &merge_result.exit_phi_inputs,
        &merge_result.carrier_inputs,
        debug,
    )?;

    // Phase 246-EX: CRITICAL FIX - Use exit PHI dsts for variable_map reconnection
    //
    // **Why EXIT PHI, not HEADER PHI?**
    //
    // Header PHI represents the value at the BEGINNING of each iteration.
    // Exit PHI represents the FINAL value when leaving the loop (from any exit path).
    //
    // For Pattern 2 loops with multiple exit paths (natural exit + break):
    // - Header PHI: `%15 = phi [%3, bb7], [%42, bb14]` (loop variable at iteration start)
    // - Exit PHI:   `%5 = phi [%15, bb11], [%15, bb13]` (final value from exit paths)
    //
    // When we exit the loop, we want the FINAL value (%5), not the iteration-start value (%15).
    // Phase 33-20 incorrectly used header PHI, causing loops to return initial values (e.g., 0 instead of 42).
    //
    // Example (_atoi):
    // - Initial: result=0 (header PHI)
    // - After iteration 1: result=4 (updated in loop body)
    // - After iteration 2: result=42 (updated in loop body)
    // - Exit: Should return 42 (exit PHI), not 0 (header PHI initial value)
    //
    // The exit PHI correctly merges values from both exit paths, giving us the final result.
    let carrier_phis = &exit_carrier_phis;

    if debug && !carrier_phis.is_empty() {
        eprintln!(
            "[cf_loop/joinir] Phase 246-EX: Using EXIT PHI dsts for variable_map (not header): {:?}",
            carrier_phis.iter().map(|(n, v)| (n.as_str(), v)).collect::<Vec<_>>()
        );
    }

    // Phase 6: Reconnect boundary (if specified)
    // Phase 197-B: Pass remapper to enable per-carrier exit value lookup
    // Phase 33-10-Refactor-P3: Delegate to ExitLineOrchestrator
    // Phase 246-EX: Now uses EXIT PHI dsts (reverted Phase 33-20)
    if let Some(boundary) = boundary {
        exit_line::ExitLineOrchestrator::execute(builder, boundary, carrier_phis, debug)?;
    }

    let exit_block_id = merge_result.exit_block_id;

    // Phase 201-A: Get entry block from loop_header_phi_info
    // The header_block in loop_header_phi_info is the remapped entry block
    let entry_block = loop_header_phi_info.header_block;

    if debug {
        eprintln!("[cf_loop/joinir]   Entry block (from loop_header_phi_info): {:?}", entry_block);
        eprintln!(
            "[cf_loop/joinir]   Current block before emit_jump: {:?}",
            builder.current_block
        );
        eprintln!("[cf_loop/joinir]   Jumping to entry block: {:?}", entry_block);
    }

    crate::mir::builder::emission::branch::emit_jump(builder, entry_block)?;

    if debug {
        eprintln!(
            "[cf_loop/joinir]   After emit_jump, current_block: {:?}",
            builder.current_block
        );
    }

    // Switch to exit block for subsequent code
    builder.start_new_block(exit_block_id)?;

    if debug {
        eprintln!(
            "[cf_loop/joinir] Phase 189: Merge complete: {} functions merged, continuing from {:?}",
            mir_module.functions.len(),
            exit_block_id
        );
    }

    // Phase 200-3: Verify JoinIR contracts (debug only)
    #[cfg(debug_assertions)]
    {
        if let Some(boundary) = boundary {
            if let Some(ref func) = builder.current_function {
                verify_joinir_contracts(
                    func,
                    entry_block,
                    exit_block_id,
                    &loop_header_phi_info,
                    boundary,
                );
            }
            if debug {
                eprintln!("[cf_loop/joinir] Phase 200-3: Contract verification passed");
            }
        }
    }

    // Phase 201-A: Clear reserved ValueIds after merge completes
    // Future loops will set their own reserved IDs
    if !builder.reserved_value_ids.is_empty() {
        if debug {
            eprintln!("[cf_loop/joinir] Phase 201-A: Clearing reserved_value_ids (was {:?})", builder.reserved_value_ids);
        }
        builder.reserved_value_ids.clear();
    }

    // Phase 246-EX-FIX: Handle loop variable expr_result separately from carrier expr_result
    //
    // The loop variable (e.g., 'i') is returned via exit_phi_result_id, not carrier_phis.
    // Other carriers use carrier_phis. We need to check which case we're in.
    let expr_result_value = if let Some(b) = boundary {
        if let Some(expr_result_id) = b.expr_result {
            // Check if expr_result is the loop variable
            if let Some(loop_var_name) = &b.loop_var_name {
                // Find the exit binding for the loop variable
                let loop_var_binding = b.exit_bindings.iter()
                    .find(|binding| binding.carrier_name == *loop_var_name);

                if let Some(binding) = loop_var_binding {
                    if binding.join_exit_value == expr_result_id {
                        // expr_result is the loop variable! Use exit_phi_result_id
                        if debug {
                            eprintln!(
                                "[cf_loop/joinir] Phase 246-EX-FIX: expr_result {:?} is loop variable '{}', using exit_phi_result_id {:?}",
                                expr_result_id, loop_var_name, exit_phi_result_id
                            );
                        }
                        exit_phi_result_id
                    } else {
                        // expr_result is not the loop variable, resolve as carrier
                        expr_result_resolver::ExprResultResolver::resolve(
                            Some(expr_result_id),
                            b.exit_bindings.as_slice(),
                            &carrier_phis,
                            &remapper,
                            debug,
                        )?
                    }
                } else {
                    // No loop variable binding, resolve normally
                    expr_result_resolver::ExprResultResolver::resolve(
                        Some(expr_result_id),
                        b.exit_bindings.as_slice(),
                        &carrier_phis,
                        &remapper,
                        debug,
                    )?
                }
            } else {
                // No loop variable name, resolve normally
                expr_result_resolver::ExprResultResolver::resolve(
                    Some(expr_result_id),
                    b.exit_bindings.as_slice(),
                    &carrier_phis,
                    &remapper,
                    debug,
                )?
            }
        } else {
            None
        }
    } else {
        None
    };

    // Return expr_result if present, otherwise fall back to exit_phi_result_id
    if let Some(resolved) = expr_result_value {
        if debug {
            eprintln!("[cf_loop/joinir] Phase 246-EX-FIX: Returning expr_result_value {:?}", resolved);
        }
        Ok(Some(resolved))
    } else {
        // Fallback: return exit_phi_result_id (for legacy patterns or carrier-only loops)
        if debug && exit_phi_result_id.is_some() {
            eprintln!(
                "[cf_loop/joinir] Phase 221-R: Returning exit_phi_result_id (fallback): {:?}",
                exit_phi_result_id
            );
        }
        Ok(exit_phi_result_id)
    }
}

/// Phase 3: Allocate new ValueIds for all collected values
///
/// Phase 201-A: Accept reserved ValueIds that must not be reused.
/// These are PHI dst ValueIds that will be created by LoopHeaderPhiBuilder.
/// We must skip these IDs to prevent carrier value corruption.
fn remap_values(
    builder: &mut crate::mir::builder::MirBuilder,
    used_values: &std::collections::BTreeSet<ValueId>,
    remapper: &mut crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper,
    reserved_ids: &std::collections::HashSet<ValueId>,
    debug: bool,
) -> Result<(), String> {
    if debug {
        eprintln!("[cf_loop/joinir] Phase 3: Remapping {} ValueIds (reserved: {})",
                  used_values.len(), reserved_ids.len());
    }

    for old_value in used_values {
        // Phase 201-A: Allocate new ValueId, skipping reserved PHI dsts
        let new_value = loop {
            let candidate = builder.next_value_id();
            if !reserved_ids.contains(&candidate) {
                break candidate;
            }
            // Skip reserved ID - will try next one
            if debug {
                eprintln!("[cf_loop/joinir] Phase 201-A: Skipping reserved PHI dst {:?}", candidate);
            }
        };

        remapper.set_value(*old_value, new_value);
        if debug {
            eprintln!(
                "[cf_loop/joinir]   Value remap: {:?} → {:?}",
                old_value, new_value
            );
        }
    }

    Ok(())
}

/// Verify that PHI dst values are not overwritten by later instructions (Phase 204-2)
///
/// # Checks
///
/// 1. PHI instructions define dst values in header block
/// 2. No subsequent instruction in the same block overwrites these dsts
///
/// # Rationale
///
/// PHI dst overwrite violates SSA invariant (single assignment) and causes undefined behavior.
/// While Phase 201 JoinValueSpace prevents ValueId collisions, manual coding errors can still
/// occur during pattern lowering.
///
/// # Panics
///
/// Panics in debug mode if PHI dst is overwritten by a later instruction.
#[cfg(debug_assertions)]
fn verify_no_phi_dst_overwrite(
    func: &crate::mir::MirFunction,
    header_block: crate::mir::BasicBlockId,
    loop_info: &LoopHeaderPhiInfo,
) {
    if loop_info.carrier_phis.is_empty() {
        return; // No PHIs to verify
    }

    let header_block_data = func.blocks.get(&header_block).unwrap_or_else(|| {
        panic!(
            "[JoinIRVerifier] Header block {} not found ({} blocks in func)",
            header_block,
            func.blocks.len()
        )
    });

    // 1. Collect all PHI dsts
    let phi_dsts: std::collections::HashSet<crate::mir::ValueId> = loop_info
        .carrier_phis
        .values()
        .map(|entry| entry.phi_dst)
        .collect();

    // 2. Check instructions after PHI definitions
    let mut after_phis = false;
    for instr in &header_block_data.instructions {
        match instr {
            crate::mir::MirInstruction::Phi { dst, .. } => {
                // PHI instructions come first in basic block
                if after_phis {
                    panic!(
                        "[JoinIRVerifier] PHI instruction {:?} appears after non-PHI instructions in block {}",
                        dst, header_block.0
                    );
                }
            }
            _ => {
                after_phis = true;
                // Check if this instruction writes to a PHI dst
                if let Some(dst) = get_instruction_dst(instr) {
                    if phi_dsts.contains(&dst) {
                        panic!(
                            "[JoinIRVerifier/Phase204] PHI dst {:?} is overwritten by instruction in header block {}: {:?}",
                            dst, header_block.0, instr
                        );
                    }
                }
            }
        }
    }
}

/// Helper: Extract dst ValueId from MirInstruction (Phase 204-2)
#[cfg(debug_assertions)]
fn get_instruction_dst(instr: &crate::mir::MirInstruction) -> Option<crate::mir::ValueId> {
    use crate::mir::MirInstruction;
    match instr {
        // Instructions with always-present dst
        MirInstruction::Const { dst, .. }
        | MirInstruction::Load { dst, .. }
        | MirInstruction::UnaryOp { dst, .. }
        | MirInstruction::BinOp { dst, .. }
        | MirInstruction::Compare { dst, .. }
        | MirInstruction::TypeOp { dst, .. }
        | MirInstruction::NewBox { dst, .. }
        | MirInstruction::NewClosure { dst, .. }
        | MirInstruction::Copy { dst, .. }
        | MirInstruction::Cast { dst, .. }
        | MirInstruction::TypeCheck { dst, .. }
        | MirInstruction::Phi { dst, .. }
        | MirInstruction::ArrayGet { dst, .. }
        | MirInstruction::RefNew { dst, .. }
        | MirInstruction::RefGet { dst, .. }
        | MirInstruction::WeakNew { dst, .. }
        | MirInstruction::WeakLoad { dst, .. }
        | MirInstruction::WeakRef { dst, .. }
        | MirInstruction::FutureNew { dst, .. }
        | MirInstruction::Await { dst, .. } => Some(*dst),
        // Instructions with Option<ValueId> dst
        MirInstruction::BoxCall { dst, .. }
        | MirInstruction::ExternCall { dst, .. }
        | MirInstruction::Call { dst, .. }
        | MirInstruction::PluginInvoke { dst, .. } => *dst,
        // Instructions without dst (side-effects only)
        _ => None,
    }
}

/// Verify PHI inputs are defined (Phase 204-3 - Conservative sanity checks)
///
/// # Checks
///
/// 1. PHI inputs have reasonable ValueId values (< threshold)
/// 2. No obviously undefined values (e.g., suspiciously large IDs)
///
/// # Note
///
/// Full data-flow analysis (DFA) verification is deferred to Phase 205+.
/// This function only performs conservative sanity checks.
///
/// # Panics
///
/// Panics in debug mode if suspicious PHI inputs are detected.
#[cfg(debug_assertions)]
fn verify_phi_inputs_defined(
    func: &crate::mir::MirFunction,
    header_block: crate::mir::BasicBlockId,
) {
    let header_block_data = func.blocks.get(&header_block).unwrap_or_else(|| {
        panic!(
            "[JoinIRVerifier] Header block {} not found ({} blocks in func)",
            header_block,
            func.blocks.len()
        )
    });

    for instr in &header_block_data.instructions {
        if let crate::mir::MirInstruction::Phi { dst, inputs, type_hint: _ } = instr {
            for (value_id, pred_block) in inputs {
                // Conservative sanity check: ValueId should not be suspiciously large
                // Phase 201 JoinValueSpace uses regions:
                // - PHI Reserved: 0-99
                // - Param: 100-999
                // - Local: 1000+
                // - Reasonable max: 100000 (arbitrary but catches obvious bugs)
                if value_id.0 >= 100000 {
                    panic!(
                        "[JoinIRVerifier/Phase204-3] PHI {:?} has suspiciously large input {:?} from predecessor block {:?}",
                        dst, value_id, pred_block
                    );
                }
            }
        }
    }
}

/// Verify all loop contracts for a merged JoinIR function
///
/// This is the main entry point for verification. It runs all checks
/// and panics if any contract violation is found.
///
/// # Panics
///
/// Panics in debug mode if any contract violation is detected.
#[cfg(debug_assertions)]
fn verify_joinir_contracts(
    func: &crate::mir::MirFunction,
    header_block: crate::mir::BasicBlockId,
    exit_block: crate::mir::BasicBlockId,
    loop_info: &LoopHeaderPhiInfo,
    boundary: &JoinInlineBoundary,
) {
    contract_checks::verify_loop_header_phis(func, header_block, loop_info, boundary);
    verify_no_phi_dst_overwrite(func, header_block, loop_info); // Phase 204-2
    verify_phi_inputs_defined(func, header_block); // Phase 204-3
    contract_checks::verify_exit_line(func, exit_block, boundary);
    contract_checks::verify_valueid_regions(loop_info, boundary); // Phase 205-4
}