diff --git a/Compiler.java b/Compiler.java
index 6e9e055..35d949d 100755
--- a/Compiler.java
+++ b/Compiler.java
@@ -7,9 +7,8 @@ import frontend.lexer.TokenStream;
 import frontend.parser.Parser;
 import midend.Midend;
 import midend.errorhandle.ErrorHandler;
-
+import midend.optimize.Optimize;
 import error.Errors;
-import midend.symbol.SymbolManager;
 
 public class Compiler {
     public static void main(String[] args) {
@@ -17,6 +16,7 @@ public class Compiler {
         try {
             String content = new String(Files.readAllBytes(Paths.get("testfile.txt")));
             String llvmFile = "llvm_ir.txt";
+            String llvmOpFile = "llvm_op_ir.txt";
             String mipsFile = "mips.txt";
             String errorFile = "error.txt";
             Lexer lexer = new Lexer(content);
@@ -36,10 +36,13 @@ public class Compiler {
             } else {
                 Midend midend = new Midend(parser.getCompUnit());
                 midend.generateLLvmIr();
-                // midend.writeToFile(llvmFile);
-                BackEnd backEnd = new BackEnd(midend.getModule());
-                backEnd.toMips();
-                backEnd.writeToFile(mipsFile);
+                midend.writeToFile(llvmFile);
+                Optimize optimize = new Optimize(midend.getModule());
+                optimize.run();
+                midend.writeToFile(llvmOpFile);
+                // BackEnd backEnd = new BackEnd(midend.getModule());
+                // backEnd.toMips();
+                // backEnd.writeToFile(mipsFile);
             }
         } catch (Exception e) {
             e.printStackTrace();
diff --git a/midend/llvm/instr/PhiInstr.java b/midend/llvm/instr/PhiInstr.java
new file mode 100644
index 0000000..195242a
--- /dev/null
+++ b/midend/llvm/instr/PhiInstr.java
@@ -0,0 +1,57 @@
+package midend.llvm.instr;
+
+import java.util.ArrayList;
+import java.util.StringJoiner;
+
+import midend.llvm.type.IrType;
+import midend.llvm.value.IrBasicBlock;
+import midend.llvm.value.IrValue;
+
+public class PhiInstr extends IrInstr {
+    private ArrayList<IrBasicBlock> predBBs;
+
+    public PhiInstr(IrType type, IrBasicBlock bblock, String name) {
+        super(type, name, IrInstrType.PHI);
+        this.setBBlock(bblock);
+        this.predBBs = new ArrayList<>(bblock.getPreds());
+        for (IrBasicBlock bb : predBBs) {
+            this.addUse(null);
+        }
+    }
+
+    public ArrayList<IrBasicBlock> getPredBBs() {
+        return predBBs;
+    }
+
+    public void setValueForPred(IrBasicBlock pred, IrValue value) {
+        int index = predBBs.indexOf(pred);
+        this.setUse(index, value);
+    }
+
+    public void deletePred(IrBasicBlock pred) {
+        int index = predBBs.indexOf(pred);
+        if (index != -1) {
+            predBBs.remove(index);
+            this.deleteUse(index);
+        }
+    }
+
+    public void replacePred(IrBasicBlock oldPred, IrBasicBlock newPred) {
+        //TODO: whether the function is needed    
+    }
+
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        sb.append(getName());
+        sb.append(" = phi ");
+        sb.append(getType().toString());
+        for (int i = 0; i < this.predBBs.size(); i++) {
+            sb.append("[ ");
+            sb.append(getUse(i).getName());
+            sb.append(", %");
+            sb.append(this.predBBs.get(i).getName());
+            sb.append(" ]");
+        }
+        return sb.toString();
+    }
+}
diff --git a/midend/llvm/use/IrUser.java b/midend/llvm/use/IrUser.java
index c699e5b..15a7b10 100755
--- a/midend/llvm/use/IrUser.java
+++ b/midend/llvm/use/IrUser.java
@@ -18,11 +18,10 @@ public class IrUser extends IrValue {
     }
 
     public void addUse(IrValue value) {
-        if (value == null) {
-            return;
-        }
         uses.add(value);
-        value.addUser(this);
+        if (value != null) {
+            value.addUser(this);
+        }
     }
 
     public IrValue getUse(int index) {
@@ -32,6 +31,44 @@ public class IrUser extends IrValue {
         return uses.get(index);
     }
 
+    public void setUse(int index, IrValue value) {
+        if (index >= uses.size() || index < 0) {
+            return;
+        }
+        IrValue oldValue = uses.get(index);
+        if (oldValue != null) {
+            oldValue.deleteUser(this);
+        }
+        uses.set(index, value);
+        if (value != null) {
+            value.addUser(this);
+        }
+    }
+
+    public void deleteUse(IrValue value) {
+        uses.remove(value);
+        if (value != null) {
+            value.deleteUser(this);
+        }   
+    }
+
+    public void clearUses() {
+        for (IrValue value : uses) {
+            if (value != null) {
+                value.deleteUser(this);
+            }
+        }
+        uses.clear();
+    }
+
+    public void deleteUse(int index) {
+        if (index >= uses.size() || index < 0) {
+            return;
+        }
+        IrValue value = uses.get(index);
+        deleteUse(value);
+    }
+
     public int getNumUses() {
         return uses.size();
     }
diff --git a/midend/llvm/value/IrBasicBlock.java b/midend/llvm/value/IrBasicBlock.java
index 17ca7b0..df57b6c 100755
--- a/midend/llvm/value/IrBasicBlock.java
+++ b/midend/llvm/value/IrBasicBlock.java
@@ -35,6 +35,27 @@ public class IrBasicBlock extends IrValue {
         instr.setBBlock(this);
     }
 
+    public void addInstr(IrInstr instr, int index) {
+        instrs.add(index, instr);
+        instr.setBBlock(this);
+    }
+
+    public void deleteInstr(IrInstr instr) {
+        instrs.remove(instr);
+        instr.setBBlock(null);
+        instr.clearUses();
+        instr.clearUsers();
+    }
+
+    public void clearAllInstrs() {
+        for (IrInstr instr : instrs) {
+            instr.setBBlock(null);
+            instr.clearUses();
+            instr.clearUsers();
+        }
+        instrs.clear();
+    }
+
     public IrFuncValue getFunc() {
         return func;
     }
@@ -87,6 +108,20 @@ public class IrBasicBlock extends IrValue {
         return instrs;
     }
 
+    public IrInstr getFirstInstr() {
+        if (instrs.isEmpty()) {
+            return null;
+        }
+        return instrs.get(0);
+    }
+
+    public IrInstr getLastInstr() {
+        if (instrs.isEmpty()) {
+            return null;
+        }
+        return instrs.get(instrs.size() - 1);
+    }
+
     public void addPred(IrBasicBlock bb) {
         this.preds.add(bb);
     }
@@ -119,6 +154,14 @@ public class IrBasicBlock extends IrValue {
         return this.directDomi;
     }
 
+    public HashSet<IrBasicBlock> getDirectDomies() {
+        return directDomies;
+    }
+
+    public HashSet<IrBasicBlock> getDomiFrontier() {
+        return domiFrontier;
+    }
+
     public void toMips() {
         new MipsLabel(getMipsLabel());
         for (IrInstr instr : instrs) {
diff --git a/midend/llvm/value/IrFuncValue.java b/midend/llvm/value/IrFuncValue.java
index 60a60d6..610bca4 100755
--- a/midend/llvm/value/IrFuncValue.java
+++ b/midend/llvm/value/IrFuncValue.java
@@ -45,6 +45,27 @@ public class IrFuncValue extends IrValue {
         bblocks.add(bblock);
     }
 
+    public void deleteBBlock(IrBasicBlock bblock) {
+        bblocks.remove(bblock);
+        bblock.clearAllInstrs();
+    }
+
+    public void deleteDeadBlock() {
+        ArrayList<IrBasicBlock> liveBlocks = new ArrayList<>();
+        ArrayList<IrBasicBlock> deadBlocks = new ArrayList<>();
+        for (IrBasicBlock bb : bblocks) {
+            if (!bb.getPreds().isEmpty() || bb.isEntry()) {
+                liveBlocks.add(bb);
+            } else {
+                deadBlocks.add(bb);
+            }
+        }
+        bblocks = liveBlocks;
+        for (IrBasicBlock bb : deadBlocks) {
+            bb.clearAllInstrs();
+        }
+    }
+
     public Register getRegister(IrValue value) {
         return valueRegisterMap.get(value);
     }
diff --git a/midend/llvm/value/IrValue.java b/midend/llvm/value/IrValue.java
index f365032..dabc551 100755
--- a/midend/llvm/value/IrValue.java
+++ b/midend/llvm/value/IrValue.java
@@ -32,6 +32,28 @@ public class IrValue {
         users.add(user);
     }
 
+    public void deleteUser(IrUser user) {
+        users.remove(user);
+    }
+
+    public void clearUsers() {
+        ArrayList<IrUser> usersCopy = new ArrayList<>(users);
+        for (IrUser user : usersCopy) {
+            user.deleteUse(this);
+        }
+    }
+
+    public void replaceUserToAnother(IrValue newValue) {
+        ArrayList<IrUser> usersCopy = new ArrayList<>(users);
+        for (IrUser user : usersCopy) {
+            for (int i = 0; i < user.getUses().size(); i++) {
+                if (user.getUse(i) == this) {
+                    user.setUse(i, newValue);
+                }
+            }
+        }
+    }
+
     public String toString() {
         return type.toString() + " " + name;
     }
diff --git a/midend/optimize/CfgMake.java b/midend/optimize/CfgMake.java
index 6ab6f2a..ef43d03 100755
--- a/midend/optimize/CfgMake.java
+++ b/midend/optimize/CfgMake.java
@@ -1,6 +1,7 @@
 package midend.optimize;
 
 import java.util.HashSet;
+import java.util.ArrayList;
 
 import midend.llvm.instr.IrInstr;
 import midend.llvm.instr.IrInstrType;
@@ -16,7 +17,81 @@ public class CfgMake extends Optimizer {
                 bb.clearCfg();
             }
         }
+        deleteUselessInstrs();
+        deleteUnreachedBlocks();
         makeCfg();
+        for (IrFuncValue func : getIrModule().getFuncs()) {
+            func.deleteDeadBlock();
+        }
+        makeDomination();
+        makeDirectDomi();
+        makeDomiFrontier();
+    }
+
+    public void deleteUselessInstrs() {
+        for (IrFuncValue func : getIrModule().getFuncs()) {
+            for (IrBasicBlock bb : func.getBBlocks()) {
+                ArrayList<IrInstr> toDelete = new ArrayList<>();
+                boolean deleteUseless = false;
+                for (IrInstr instr : bb.getInstrs()) {
+                    if (!deleteUseless) {
+                        if (instr.getInstrType() == IrInstrType.RET ||
+                                instr.getInstrType() == IrInstrType.BR ||
+                                instr.getInstrType() == IrInstrType.JUMP) {
+                            deleteUseless = true;
+                        }
+                    } else {
+                        toDelete.add(instr);
+                    }
+                }
+                for (IrInstr instr : toDelete) {
+                    bb.deleteInstr(instr);
+                }
+            }
+        }
+    }
+
+    public void deleteUnreachedBlocks() {
+        for (IrFuncValue func : getIrModule().getFuncs()) {
+            HashSet<IrBasicBlock> canArrive = new HashSet<>();
+            IrBasicBlock entryBlock = func.getBBlock(0);
+            dfsArrive(entryBlock, canArrive);
+            ArrayList<IrBasicBlock> toDelete = new ArrayList<>();
+            for (IrBasicBlock bb : func.getBBlocks()) {
+                if (!canArrive.contains(bb)) {
+                    toDelete.add(bb);
+                }
+            }
+            for (IrBasicBlock bb : toDelete) {
+                func.deleteBBlock(bb);
+            }
+        }
+    }
+
+    public void dfsArrive(IrBasicBlock bb, HashSet<IrBasicBlock> canArrive) {
+        if (!canArrive.contains(bb)) {
+            canArrive.add(bb);
+        }
+        IrInstr lastInstr = bb.getLastInstr();
+        if (lastInstr != null) {
+            if (lastInstr.getInstrType() == IrInstrType.BR) {
+                BranchInstr branchInstr = (BranchInstr) lastInstr;
+                IrBasicBlock trueBlock = branchInstr.getTrueBB();
+                IrBasicBlock falseBlock = branchInstr.getFalseBB();
+                if (!canArrive.contains(trueBlock)) {
+                    dfsArrive(trueBlock, canArrive);
+                }
+                if (!canArrive.contains(falseBlock)) {
+                    dfsArrive(falseBlock, canArrive);
+                }
+            } else if (lastInstr.getInstrType() == IrInstrType.JUMP) {
+                JumpInstr jumpInstr = (JumpInstr) lastInstr;
+                IrBasicBlock jumpBlock = jumpInstr.getTargetBlock();
+                if (!canArrive.contains(jumpBlock)) {
+                    dfsArrive(jumpBlock, canArrive);
+                }
+            }
+        }
     }
 
     public void makeCfg() {
@@ -76,8 +151,8 @@ public class CfgMake extends Optimizer {
         for (IrFuncValue func : getIrModule().getFuncs()) {
             for (IrBasicBlock bb : func.getBBlocks()) {
                 for (IrBasicBlock domi : bb.getDomied()) {
-                    HashSet<IrBasicBlock> bbDomi = bb.getDomied();
-                    HashSet<IrBasicBlock> domiDomi = domi.getDomied();
+                    HashSet<IrBasicBlock> bbDomi = new HashSet<>(bb.getDomied());
+                    HashSet<IrBasicBlock> domiDomi = new HashSet<>(domi.getDomied());
                     for (IrBasicBlock domiDomiBB : domiDomi) {
                         if (bbDomi.contains(domiDomiBB)) {
                             bbDomi.remove(domiDomiBB);
diff --git a/midend/optimize/DeleteDead.java b/midend/optimize/DeleteDead.java
new file mode 100644
index 0000000..9c800cd
--- /dev/null
+++ b/midend/optimize/DeleteDead.java
@@ -0,0 +1,20 @@
+package midend.optimize;
+
+import java.util.HashSet;
+
+import midend.llvm.value.IrFuncValue;
+
+public class DeleteDead extends Optimizer {
+    public void optimize() {
+
+    }
+
+    public void deleteDeadFunc() {
+        IrFuncValue mainFunc = getIrModule().getMainFunc();
+        HashSet<IrFuncValue> liveFuncs = new HashSet<>();
+        liveFuncs.add(mainFunc);
+        findLiveFuncs(mainFunc, liveFuncs);
+    }
+
+    public void findLiveFuncs()
+}
diff --git a/midend/optimize/MemToReg.java b/midend/optimize/MemToReg.java
new file mode 100644
index 0000000..ac81123
--- /dev/null
+++ b/midend/optimize/MemToReg.java
@@ -0,0 +1,36 @@
+package midend.optimize;
+
+import midend.llvm.value.IrBasicBlock;
+import midend.llvm.value.IrFuncValue;
+
+import java.util.ArrayList;
+
+import midend.llvm.instr.AllocateInstr;
+import midend.llvm.instr.IrInstr;
+import midend.llvm.type.IrArrayType;
+
+public class MemToReg extends Optimizer {
+    public void optimize() {
+        for (IrFuncValue func : getIrModule().getFuncs()) {
+            IrBasicBlock entryBlock = func.getBBlock(0);
+            for (IrBasicBlock block : func.getBBlocks()) {
+                ArrayList<IrInstr> instrs = new ArrayList<>(block.getInstrs());
+                for (IrInstr instr : instrs) {
+                    if (normalAlloca(instr)) {
+                        AllocateInstr allocInstr = (AllocateInstr) instr;
+                        PhiInsert phiInsert = new PhiInsert(allocInstr, entryBlock);
+                        phiInsert.run();
+                    }
+                }
+            }
+        }
+    }
+
+    public boolean normalAlloca(IrInstr instr) {
+        if (!(instr instanceof AllocateInstr)) {
+            return false;
+        }
+        AllocateInstr allocInstr = (AllocateInstr) instr;
+        return !(allocInstr.getPointeeType() instanceof IrArrayType);
+    }
+}
diff --git a/midend/optimize/Optimize.java b/midend/optimize/Optimize.java
new file mode 100644
index 0000000..5bfe2e5
--- /dev/null
+++ b/midend/optimize/Optimize.java
@@ -0,0 +1,23 @@
+package midend.optimize;
+
+import java.util.ArrayList;
+
+import midend.llvm.IrModule;
+
+public class Optimize {
+    private ArrayList<Optimizer> optimizers;
+
+    public Optimize(IrModule module) {
+        Optimizer.setIrModule(module);
+        optimizers = new ArrayList<>();
+        optimizers.add(new CfgMake());
+        optimizers.add(new MemToReg());
+        optimizers.add(new CfgMake());
+    }
+
+    public void run() {
+        for (Optimizer optimizer : optimizers) {
+            optimizer.optimize();
+        }
+    }
+}
diff --git a/midend/optimize/Optimizer.java b/midend/optimize/Optimizer.java
index abc317f..9a65ba8 100755
--- a/midend/optimize/Optimizer.java
+++ b/midend/optimize/Optimizer.java
@@ -5,7 +5,7 @@ import midend.llvm.IrModule;
 public class Optimizer {
     private static IrModule irModule;
 
-    public void setIrModule(IrModule irModule) {
+    public static void setIrModule(IrModule irModule) {
         Optimizer.irModule = irModule;
     }
 
diff --git a/midend/optimize/PhiInsert.java b/midend/optimize/PhiInsert.java
new file mode 100644
index 0000000..27bdac6
--- /dev/null
+++ b/midend/optimize/PhiInsert.java
@@ -0,0 +1,133 @@
+package midend.optimize;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.Stack;
+
+import midend.llvm.IrBuilder;
+import midend.llvm.constant.IrConstantInt;
+import midend.llvm.instr.AllocateInstr;
+import midend.llvm.instr.IrInstr;
+import midend.llvm.instr.StoreInstr;
+import midend.llvm.instr.LoadInstr;
+import midend.llvm.instr.PhiInstr;
+import midend.llvm.use.IrUser;
+import midend.llvm.value.IrBasicBlock;
+import midend.llvm.value.IrValue;
+
+public class PhiInsert {
+    private ArrayList<IrInstr> defineInstrs;
+    private ArrayList<IrInstr> useInstrs;
+    private AllocateInstr allocInstr;
+    private IrBasicBlock entryBlock;
+    private ArrayList<IrBasicBlock> defBlocks;
+    private ArrayList<IrBasicBlock> useBlocks;
+    private Stack<IrValue> workList;
+
+    public PhiInsert(AllocateInstr allocInstr, IrBasicBlock entryBlock) {
+        this.defineInstrs = new ArrayList<>();
+        this.useInstrs = new ArrayList<>();
+        this.allocInstr = allocInstr;
+        this.entryBlock = entryBlock;
+        this.defBlocks = new ArrayList<>();
+        this.useBlocks = new ArrayList<>();
+        this.workList = new Stack<>();
+    }
+
+    public IrValue getNewValue() {
+        if (workList.isEmpty()) {
+            return new IrConstantInt(0);
+        }
+        return workList.peek();
+    }
+
+    public void run() {
+        makeDefAndUse();
+        insertPhiNodes();
+        changeLoadAndStoreToReg(entryBlock);
+    }
+
+    public void makeDefAndUse() {
+        ArrayList<IrUser> users = new ArrayList<>(allocInstr.getUsers());
+        for (IrUser user : users) {
+            IrInstr instr = (IrInstr) user;
+            if (instr instanceof StoreInstr) {
+                defineInstrs.add(instr);
+                IrBasicBlock defBlock = instr.getBBlock();
+                if (!defBlocks.contains(defBlock)) {
+                    defBlocks.add(defBlock);
+                }
+            } else if (instr instanceof LoadInstr){
+                useInstrs.add(instr);
+                IrBasicBlock useBlock = instr.getBBlock();
+                if (!useBlocks.contains(useBlock)) {
+                    useBlocks.add(useBlock);
+                }
+            }
+        }
+    }
+
+    public void insertPhiNodes() {
+        HashSet<IrBasicBlock> hasPhiBlocks = new HashSet<>();
+        Stack<IrBasicBlock> defStack = new Stack<>();
+        for (IrBasicBlock defBlock : defBlocks) {
+            defStack.push(defBlock);
+        }
+        while (!defStack.isEmpty()) {
+            IrBasicBlock currBlock = defStack.pop();
+            for (IrBasicBlock frontierBlock : currBlock.getDomiFrontier()) {
+                if (hasPhiBlocks.contains(frontierBlock)) {
+                    continue;
+                }
+                //插入phi节点
+                insertPhiNode(frontierBlock);
+                hasPhiBlocks.add(frontierBlock);
+                if (!defBlocks.contains(frontierBlock)) {
+                    defStack.push(frontierBlock);
+                }
+            }
+        }
+    }
+
+    public void insertPhiNode(IrBasicBlock block) {
+        PhiInstr phiInstr = new PhiInstr(allocInstr.getPointeeType(), block, IrBuilder.getLocalName(block.getFunc()));
+        block.addInstr(phiInstr, 0);
+        defineInstrs.add(phiInstr);
+        useInstrs.add(phiInstr);
+    }
+
+    public void changeLoadAndStoreToReg(IrBasicBlock block) {
+        Stack<IrValue> workListCopy = new Stack<>();
+        workListCopy.addAll(workList);
+        ArrayList<IrInstr> deleteInstrs = new ArrayList<>();
+        for(IrInstr instr : block.getInstrs()) {
+            if (instr instanceof LoadInstr && useInstrs.contains(instr)) {
+                instr.replaceUserToAnother(getNewValue());
+                deleteInstrs.add(instr);
+            } else if (instr instanceof StoreInstr && defineInstrs.contains(instr)) {
+                StoreInstr storeInstr = (StoreInstr) instr;
+                IrValue value = storeInstr.getValue();
+                workList.push(value);
+                deleteInstrs.add(instr);
+            } else if (instr instanceof PhiInstr && defineInstrs.contains(instr)) {
+                workList.push(instr);
+            } else if (instr == allocInstr) {
+                deleteInstrs.add(instr);
+            }
+        }
+        for (IrInstr instr : deleteInstrs) {
+            block.deleteInstr(instr);
+        }
+        for (IrBasicBlock succ : block.getSuccs()) {
+            IrInstr firstInstr = succ.getFirstInstr();
+            if (firstInstr instanceof PhiInstr && useInstrs.contains(firstInstr)) {
+                PhiInstr phiInstr = (PhiInstr) firstInstr;
+                phiInstr.setValueForPred(block, getNewValue());
+            }
+        }
+        for (IrBasicBlock domiBB : block.getDirectDomies()) {
+            changeLoadAndStoreToReg(domiBB);
+        }
+        workList = workListCopy;
+    }
+}