diff --git a/src/opt/mem2reg.rs b/src/opt/mem2reg.rs
index eda5bb8..70fa5de 100644
--- a/src/opt/mem2reg.rs
+++ b/src/opt/mem2reg.rs
@@ -1,4 +1,5 @@
 use core::ops::{Deref, DerefMut};
+use std::cmp::Ordering;
 use std::collections::{BTreeMap, HashMap, HashSet};
 
 use crate::ir::*;
@@ -12,6 +13,572 @@ pub struct Mem2regInner {}
 
 impl Optimize<FunctionDefinition> for Mem2regInner {
     fn optimize(&mut self, code: &mut FunctionDefinition) -> bool {
-        todo!()
+        let mut inpromotable = HashSet::new();
+        let mut stores = HashMap::new();
+
+        for (bid, block) in &code.blocks {
+            for inst in &block.instructions {
+                match inst.deref() {
+                    Instruction::BinOp { lhs, rhs, .. } => {
+                        mark_inpromotable(&mut inpromotable, lhs);
+                        mark_inpromotable(&mut inpromotable, rhs);
+                    }
+                    Instruction::UnaryOp { operand, .. } => {
+                        mark_inpromotable(&mut inpromotable, operand);
+                    }
+                    Instruction::Store { ptr, value } => {
+                        mark_inpromotable(&mut inpromotable, value);
+                        if let Some((RegisterId::Local { aid }, _)) = ptr.get_register() {
+                            stores.entry(*aid).or_insert_with(Vec::new).push(*bid);
+                        }
+                    }
+                    Instruction::Load { .. } => (),
+                    Instruction::Call { callee, args, .. } => {
+                        mark_inpromotable(&mut inpromotable, callee);
+                        for arg in args {
+                            mark_inpromotable(&mut inpromotable, arg);
+                        }
+                    }
+                    Instruction::TypeCast { value, .. } => {
+                        mark_inpromotable(&mut inpromotable, value);
+                    }
+                    Instruction::GetElementPtr { ptr, .. } => {
+                        mark_inpromotable(&mut inpromotable, ptr);
+                    }
+                    Instruction::Nop => (),
+                    _ => todo!()
+                }
+            }
+        }
+
+        if inpromotable.len() == code.allocations.len() {
+            return false;
+        }
+
+        let cfg = make_cfg(code);
+        let reverse_cfg = reverse_cfg(&cfg);
+        let domtree = Domtree::new(code.bid_init, &cfg, &reverse_cfg);
+
+        let joins = stores
+            .iter()
+            .filter(|(aid, _)| !inpromotable.contains(*aid))
+            .map(|(aid, bids)| {
+                (*aid, {
+                    let mut stack = bids.clone();
+                    let mut visited = HashSet::new();
+                    while let Some(bid) = stack.pop() {
+                        if let Some(bid_frontiers) = domtree.frontiers(&bid) {
+                            for bid_frontier in bid_frontiers {
+                                if visited.insert(*bid_frontier) {
+                                    stack.push(*bid_frontier);
+                                }
+                            }
+                        }
+                    }
+                    visited
+                })
+            })
+            .collect::<HashMap<_, _>>();
+
+        let mut inv_joins  = HashMap::new();
+        for (aid, bids) in &joins {
+            for bid in bids {
+                inv_joins.entry(bid).or_insert_with(Vec::new).push(*aid);
+            }
+        }
+        for (_, aids) in inv_joins.iter_mut() {
+            aids.sort();
+        }
+
+        let mut phinode_indexes = HashMap::new();
+        let mut phinode_allocs = HashMap::new();
+        for (bid, aids) in &inv_joins {
+            let block = code.blocks.get_mut(bid).unwrap();
+            let index = block.phinodes.len();
+            for (i, aid) in aids.iter().enumerate() {
+                block.phinodes.push(code.allocations[*aid].clone());
+                let _ = phinode_indexes.insert((*aid, **bid), index + i);
+                phinode_allocs
+                    .entry(**bid)
+                    .or_insert_with(Vec::new)
+                    .push((*aid, code.allocations[*aid].deref().clone()));
+            }
+        }
+        
+        
+
+        // for (aid, bids) in &joins {
+        //     let alloc = code.allocations.get(*aid).unwrap();
+        //     for bid in bids {
+        //         // aid를 담는 Phinode를 넣을 곳
+        //         let block = code.blocks.get_mut(bid).unwrap();
+        //         let index = block.phinodes.len();
+        //         block.phinodes.push(alloc.clone());
+        //         let _ = phinode_indexes.insert((*aid, *bid), index);
+        //         phinode_allocs
+        //             .entry(*bid)
+        //             .or_insert_with(Vec::new)
+        //             .push((*aid, alloc.deref().clone()));
+        //     }
+        // }
+
+        let mut inv_domtree = HashMap::new();
+        for (bid, idom) in &domtree.idoms {
+            inv_domtree.entry(*idom).or_insert_with(Vec::new).push(*bid);
+        }
+
+        println!("{:?}", domtree.idoms);
+        println!("{:?}", inv_domtree);
+
+        let mut stack = HashMap::new();
+        let mut replaces = HashMap::new();
+
+        fn find_initial(
+            aid: usize,
+            dtype: Dtype,
+            code: &FunctionDefinition,
+            inpromotable: &HashSet<usize>,
+            bid: &BlockId,
+            stack: &HashMap<BlockId, HashMap<usize, Vec<Operand>>>,
+            phinode_indexes: &HashMap<(usize, BlockId), usize>,
+            domtree: &Domtree,
+        ) -> Operand {
+            if let Some(block_stack) = stack.get(bid) {
+                if let Some(block_stack) = block_stack.get(&aid) {
+                    block_stack.last().unwrap().clone()
+                } else {
+                    if let Some(phinode_index) = phinode_indexes.get(&(aid, *bid)) {
+                        Operand::register(RegisterId::arg(*bid, *phinode_index), dtype)
+                    } else if let Some(bid_idom) = domtree.idoms.get(bid) {
+                        find_initial(
+                            aid,
+                            dtype,
+                            code,
+                            inpromotable,
+                            bid_idom,
+                            stack,
+                            phinode_indexes,
+                            domtree,
+                        )
+                    } else {
+                        Operand::constant(Constant::undef(dtype))
+                    }
+                }
+            } else if let Some(phinode_index) = phinode_indexes.get(&(aid, *bid)) {
+                Operand::register(RegisterId::arg(*bid, *phinode_index), dtype)
+            } else if let Some(bid_idom) = domtree.idoms.get(bid) {
+                find_initial(
+                    aid,
+                    dtype,
+                    code,
+                    inpromotable,
+                    bid_idom,
+                    stack,
+                    phinode_indexes,
+                    domtree,
+                )
+            } else {
+                Operand::constant(Constant::undef(dtype))
+            }
+        }
+
+        fn traverse_preorder(
+            code: &mut FunctionDefinition,
+            phinode_indexes: &HashMap<(usize, BlockId), usize>,
+            phinode_allocs: &HashMap<BlockId, Vec<(usize, Dtype)>>,
+            inv_domtree: &HashMap<BlockId, Vec<BlockId>>,
+            bid: &BlockId,
+            stack: &mut HashMap<BlockId, HashMap<usize, Vec<Operand>>>,
+            replaces: &mut HashMap<RegisterId, Operand>,
+            inpromotable: &HashSet<usize>,
+            domtree: &Domtree,
+        ) {
+            println!("bid: {}", bid);
+
+            let stack_org = stack.clone();
+            // let block_stack = stack.entry(*bid).or_insert_with(HashMap::new);
+
+            for (aid, dtype) in code.allocations.iter().enumerate() {
+                if !inpromotable.contains(&aid) {
+                    let initial = find_initial(
+                        aid,
+                        dtype.deref().clone(),
+                        code,
+                        inpromotable,
+                        bid,
+                        stack,
+                        phinode_indexes,
+                        domtree,
+                    );
+
+                    stack
+                        .entry(*bid)
+                        .or_insert_with(HashMap::new)
+                        .entry(aid)
+                        .or_insert_with(Vec::new)
+                        .push(initial);
+                    // if let Some(phinode_index) = phinode_indexes.get(&(aid, *bid)) {
+                    //     entry.push(Operand::register(
+                    //         RegisterId::arg(*bid, *phinode_index),
+                    //         dtype.deref().clone(),
+                    //     ));
+                    // } else {
+                    //     entry.push(Operand::constant(Constant::undef(dtype.deref().clone())));
+                    // }
+                }
+            }
+            // find_initial(code, inpromotable, bid, stack, phinode_indexes, domtree);
+            let block_stack = stack.entry(*bid).or_insert_with(HashMap::new);
+
+            for (i, inst) in code.blocks[bid].instructions.iter().enumerate() {
+                match inst.deref() {
+                    Instruction::Store { ptr, value } => {
+                        if let Some((RegisterId::Local { aid }, _)) = ptr.get_register() {
+                            if !inpromotable.contains(aid) {
+                                block_stack.get_mut(aid).unwrap().push(value.clone());
+                            }
+                        }
+                    }
+                    Instruction::Load { ptr } => {
+                        if let Some((RegisterId::Local { aid }, _)) = ptr.get_register() {
+                            if !inpromotable.contains(aid) {
+                                let _unused = replaces.insert(
+                                    RegisterId::temp(*bid, i),
+                                    block_stack[aid].last().unwrap().clone(),
+                                );
+                            }
+                        }
+                    }
+                    _ => (),
+                }
+            }
+
+            let block = code.blocks.get_mut(bid).unwrap();
+            match &mut block.exit {
+                BlockExit::Jump { arg } => {
+                    fill_jump_args(arg, phinode_allocs, &block_stack);
+                }
+                BlockExit::ConditionalJump {
+                    arg_then, arg_else, ..
+                } => {
+                    fill_jump_args(arg_then, phinode_allocs, &block_stack);
+                    fill_jump_args(arg_else, phinode_allocs, &block_stack);
+                }
+                BlockExit::Switch { default, cases, .. } => {
+                    fill_jump_args(default, phinode_allocs, &block_stack);
+                    for (_, arg) in cases {
+                        fill_jump_args(arg, phinode_allocs, &block_stack);
+                    }
+                }
+                _ => (),
+            }
+
+            if let Some(succs) = inv_domtree.get(bid) {
+                for succ in succs {
+                    traverse_preorder(
+                        code,
+                        phinode_indexes,
+                        phinode_allocs,
+                        inv_domtree,
+                        succ,
+                        stack,
+                        replaces,
+                        inpromotable,
+                        domtree,
+                    );
+                }
+            }
+
+            *stack = stack_org;
+        }
+
+        let bid_init = code.bid_init;
+
+        traverse_preorder(
+            code,
+            &phinode_indexes,
+            &phinode_allocs,
+            &inv_domtree,
+            &bid_init,
+            &mut stack,
+            &mut replaces,
+            &inpromotable,
+            &domtree,
+        );
+
+        for (bid, block) in &mut code.blocks {
+            for inst in block.instructions.iter_mut() {
+                replace_instruction_operands(inst, &replaces);
+            }
+            replace_exit_operands(&mut block.exit, &replaces);
+        }
+
+        for block in code.blocks.values_mut() {
+            for inst in block.instructions.iter_mut() {
+                match inst.deref().deref() {
+                    Instruction::Store { ptr, .. } => {
+                        if let Some((RegisterId::Local { aid }, _)) = ptr.get_register() {
+                            if !inpromotable.contains(aid) {
+                                *inst.deref_mut() = Instruction::Nop;
+                            }
+                        }
+                    }
+                    Instruction::Load { ptr } => {
+                        if let Some((RegisterId::Local { aid }, _)) = ptr.get_register() {
+                            if !inpromotable.contains(aid) {
+                                *inst.deref_mut() = Instruction::Nop;
+                            }
+                        }
+                    }
+                    _ => (),
+                }
+            }
+        }
+
+        println!("replaces: {:?}\n", replaces);
+
+        true // TODO
+    }
+}
+
+fn fill_jump_args(
+    arg: &mut JumpArg,
+    phinode_allocs: &HashMap<BlockId, Vec<(usize, Dtype)>>,
+    block_stack: &HashMap<usize, Vec<Operand>>,
+) {
+    if let Some(target_phinode_args) = phinode_allocs.get(&arg.bid) {
+        for (target_phinode_arg, dtype) in target_phinode_args {
+            arg.args
+                .push(block_stack[target_phinode_arg].last().unwrap().clone());
+        }
+    }
+}
+
+fn mark_inpromotable(inpromotable: &mut HashSet<usize>, operand: &Operand) {
+    if let Some((RegisterId::Local { aid }, _)) = operand.get_register() {
+        let _ = inpromotable.insert(*aid);
+    }
+}
+
+fn make_cfg(fdef: &FunctionDefinition) -> HashMap<BlockId, Vec<JumpArg>> {
+    fdef.blocks
+        .iter()
+        .map(|(bid, block)| {
+            let mut args = Vec::new();
+            match &block.exit {
+                BlockExit::Jump { arg } => args.push(arg.clone()),
+                BlockExit::ConditionalJump {
+                    arg_then, arg_else, ..
+                } => {
+                    args.push(arg_then.clone());
+                    args.push(arg_else.clone());
+                }
+                BlockExit::Switch { default, cases, .. } => {
+                    args.push(default.clone());
+                    for (_, arg) in cases {
+                        args.push(arg.clone());
+                    }
+                }
+                _ => (),
+            }
+            (*bid, args)
+        })
+        .collect()
+}
+
+fn reverse_cfg(cfg: &HashMap<BlockId, Vec<JumpArg>>) -> HashMap<BlockId, Vec<(BlockId, JumpArg)>> {
+    let mut result = HashMap::new();
+    for (bid, jumps) in cfg {
+        for jump in jumps {
+            result
+                .entry(jump.bid)
+                .or_insert_with(Vec::new)
+                .push((*bid, jump.clone()));
+        }
+    }
+    result
+}
+
+fn traverse_rpo(bid_init: BlockId, cfg: &HashMap<BlockId, Vec<JumpArg>>) -> Vec<BlockId> {
+    fn traverse_po(
+        bid: BlockId,
+        cfg: &HashMap<BlockId, Vec<JumpArg>>,
+        visited: &mut HashSet<BlockId>,
+        post_order: &mut Vec<BlockId>,
+    ) {
+        for jump in &cfg[&bid] {
+            if visited.insert(jump.bid) {
+                traverse_po(jump.bid, cfg, visited, post_order);
+            }
+        }
+        post_order.push(bid);
+    }
+
+    let mut visited = HashSet::new();
+    let _ = visited.insert(bid_init);
+    let mut order = Vec::new();
+
+    traverse_po(bid_init, cfg, &mut visited, &mut order);
+    order.reverse();
+    order
+}
+
+struct Domtree {
+    idoms: HashMap<BlockId, BlockId>,
+    frontiers: HashMap<BlockId, Vec<BlockId>>,
+    rpo: Vec<BlockId>,
+}
+
+impl Domtree {
+    fn new(
+        bid_init: BlockId,
+        cfg: &HashMap<BlockId, Vec<JumpArg>>,
+        reverse_cfg: &HashMap<BlockId, Vec<(BlockId, JumpArg)>>,
+    ) -> Self {
+        let rpo = traverse_rpo(bid_init, cfg);
+        let irpo = rpo.iter().enumerate().map(|(i, bid)| (*bid, i)).collect();
+        let mut idoms: HashMap<BlockId, BlockId> = HashMap::new();
+
+        loop {
+            let mut changed = false;
+
+            for bid in &rpo {
+                if *bid == bid_init {
+                    continue;
+                }
+
+                let mut idom = None;
+                for (bid_pred, _) in &reverse_cfg[bid] {
+                    if *bid_pred == bid_init || idoms.contains_key(bid_pred) {
+                        idom = Some(intersect_idom(idom, *bid_pred, &irpo, &idoms));
+                    }
+                }
+
+                if let Some(idom) = idom {
+                    let _ = idoms
+                        .entry(*bid)
+                        .and_modify(|v| {
+                            if *v != idom {
+                                changed = true;
+                                *v = idom;
+                            }
+                        })
+                        .or_insert_with(|| {
+                            changed = true;
+                            idom
+                        });
+                }
+            }
+
+            if !changed {
+                break;
+            }
+        }
+
+        let mut frontiers = HashMap::new();
+        for (bid, preds) in reverse_cfg.iter().filter(|(_, preds)| preds.len() > 1) {
+            let idom = if let Some(idom) = idoms.get(bid) {
+                idom
+            } else {
+                continue;
+            };
+            for (bid_pred, _) in preds {
+                let mut runner = *bid_pred;
+                while !Self::dominates(&idoms, runner, *bid) {
+                    frontiers.entry(runner).or_insert_with(Vec::new).push(*bid);
+                    runner = idoms[&runner];
+                }
+            }
+        }
+
+        Self {
+            idoms,
+            frontiers,
+            rpo,
+        }
+    }
+
+    fn dominates(idoms: &HashMap<BlockId, BlockId>, bid1: BlockId, mut bid2: BlockId) -> bool {
+        loop {
+            bid2 = if let Some(idom2) = idoms.get(&bid2) {
+                *idom2
+            } else {
+                return false;
+            };
+            if bid1 == bid2 {
+                return true;
+            }
+        }
+    }
+
+    fn idom(&self, bid: &BlockId) -> Option<&BlockId> {
+        self.idoms.get(bid)
+    }
+
+    fn frontiers(&self, bid: &BlockId) -> Option<&Vec<BlockId>> {
+        self.frontiers.get(bid)
+    }
+}
+
+fn intersect_idom(
+    lhs: Option<BlockId>,
+    mut rhs: BlockId,
+    irpo: &HashMap<BlockId, usize>,
+    idoms: &HashMap<BlockId, BlockId>,
+) -> BlockId {
+    let mut lhs = if let Some(lhs) = lhs { lhs } else { return rhs };
+    loop {
+        if lhs == rhs {
+            return lhs;
+        }
+        match irpo[&lhs].cmp(&irpo[&rhs]) {
+            Ordering::Less => rhs = idoms[&rhs],
+            Ordering::Greater => lhs = idoms[&lhs],
+            Ordering::Equal => unreachable!(),
+        }
+    }
+}
+
+fn replace_instruction_operands(inst: &mut Instruction, replaces: &HashMap<RegisterId, Operand>) {
+    match inst {
+        Instruction::BinOp { lhs, rhs, .. } => {
+            replace_operand(lhs, replaces);
+            replace_operand(rhs, replaces);
+        }
+        Instruction::UnaryOp { operand, .. } => replace_operand(operand, replaces),
+        Instruction::Store { ptr, value } => {
+            replace_operand(ptr, replaces);
+            replace_operand(value, replaces);
+        }
+        Instruction::Load { ptr } => replace_operand(ptr, replaces),
+        Instruction::Call { callee, args, .. } => {
+            replace_operand(callee, replaces);
+            for arg in args.iter_mut() {
+                replace_operand(arg, replaces);
+            }
+        }
+        Instruction::TypeCast { value, .. } => replace_operand(value, replaces),
+        Instruction::GetElementPtr { ptr, offset, .. } => {
+            replace_operand(ptr, replaces);
+            replace_operand(offset, replaces);
+        }
+        _ => unreachable!(),
+    }
+}
+
+fn replace_exit_operands(exit: &mut BlockExit, replaces: &HashMap<RegisterId, Operand>) {
+    match exit {
+        BlockExit::ConditionalJump { condition, .. } => replace_operand(condition, replaces),
+        BlockExit::Switch { value, .. } => replace_operand(value, replaces),
+        BlockExit::Return { value } => replace_operand(value, replaces),
+        _ => (),
+    }
+}
+
+fn replace_operand(operand: &mut Operand, replaces: &HashMap<RegisterId, Operand>) {
+    if let Operand::Register { rid, .. } = operand {
+        if let Some(new_operand) = replaces.get(rid) {
+            *operand = new_operand.clone();
+        }
     }
 }