diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5f3017e..301640f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.15)
 
 project(SysYFCompiler)
 
@@ -23,6 +23,15 @@ else()
     message(WARNING "Unknown build type: ${CMAKE_BUILD_TYPE}. Using default flags.")
 endif()
 
+include(FetchContent)
+FetchContent_Declare(
+  googletest
+  URL https://github.com/google/googletest/archive/03597a01ee50ed33e9dfd640b249b4be3799d395.zip
+)
+# For Windows: Prevent overriding the parent project's compiler/linker settings
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
+
 # for ninja color, see https://stackoverflow.com/questions/73349743/ninja-build-system-gcc-clang-doesnt-output-diagnostic-colors
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fdiagnostics-color=always")
 
@@ -43,6 +52,8 @@ add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/src/SysYFIR)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/src/SysYFIRBuilder)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/Student/task2)
 
+add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/unittest)
+
 add_executable(
   SysYFCompiler
   ${CMAKE_CURRENT_SOURCE_DIR}/src/main.cpp
diff --git a/Student/task2/CMakeLists.txt b/Student/task2/CMakeLists.txt
index 4e7c863..bbc5620 100644
--- a/Student/task2/CMakeLists.txt
+++ b/Student/task2/CMakeLists.txt
@@ -1,6 +1,6 @@
 project(task2)
 
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.15)
 
 set(CMAKE_CXX_STANDARD 14)
 
diff --git a/include/SysYFIR/User.h b/include/SysYFIR/User.h
index c0cc4cb..d5a3816 100644
--- a/include/SysYFIR/User.h
+++ b/include/SysYFIR/User.h
@@ -32,7 +32,6 @@ class User : public Value
 
 private:
     std::vector<std::weak_ptr<Value>> operands_;   // operands of this value
-    unsigned num_ops_;
 };
 
 }
diff --git a/src/SysYFIR/User.cpp b/src/SysYFIR/User.cpp
index 08fb528..e9d69bd 100644
--- a/src/SysYFIR/User.cpp
+++ b/src/SysYFIR/User.cpp
@@ -1,6 +1,7 @@
 #include "User.h"
 #include "internal_types.h"
 #include <memory>
+#include <vector>
 #ifdef DEBUG
 #include <cassert>
 #endif
@@ -10,11 +11,11 @@ namespace SysYF
 namespace IR
 {
 User::User(Ptr<Type> ty, const std::string &name , unsigned num_ops)
-    : Value(ty, name), num_ops_(num_ops)
+    : Value(ty, name)
 {
     // if (num_ops_ > 0)
     //   operands_.reset(new std::list<Ptr<Value> >());
-    operands_.resize(num_ops_);
+    operands_.resize(num_ops);
 }
 
 Ptr<Value> User::get_operand(unsigned i) const
@@ -35,13 +36,12 @@ void User::set_operand(unsigned i, Ptr<Value> v)
 void User::add_operand(Ptr<Value> v)
 {
     operands_.push_back(v);  
-    v->add_use(shared_from_this(), num_ops_);
-    num_ops_++;
+    v->add_use(shared_from_this(), operands_.size()-1);
 }
 
 unsigned User::get_num_operand() const
 {
-    return num_ops_;
+    return operands_.size();
 }
 
 void User::remove_use_of_ops()
@@ -51,13 +51,19 @@ void User::remove_use_of_ops()
     }
 }
 
-void User::remove_operands(int index1,int index2){
-    for(int i=index1;i<=index2;i++){
-        operands_[i].lock()->remove_use(shared_from_this());
+void User::remove_operands(int index1, int index2) {
+    auto backup = std::vector<std::weak_ptr<Value>>{};
+    for (auto iter = operands_.cbegin() + index2 + 1; iter != operands_.cend(); iter++) {
+        backup.push_back(*iter);
+    }
+    for (auto iter = operands_.cbegin() + index1; iter != operands_.cend(); iter++) {
+        iter->lock()->remove_use(shared_from_this());
+    }
+    operands_.erase(operands_.begin()+index1, operands_.end());
+    for (auto op: backup) {
+        add_operand(op.lock());
     }
-    operands_.erase(operands_.begin()+index1,operands_.begin()+index2+1);
     // std::cout<<operands_.size()<<std::endl;
-    num_ops_=operands_.size();
 }
 
 }
diff --git a/unittest/CMakeLists.txt b/unittest/CMakeLists.txt
new file mode 100644
index 0000000..aa18366
--- /dev/null
+++ b/unittest/CMakeLists.txt
@@ -0,0 +1,15 @@
+enable_testing()
+
+add_executable(
+    ir_test
+    ir_test.cpp
+)
+
+target_link_libraries(
+    ir_test
+    GTest::gtest_main
+    IRLib
+)
+
+include(GoogleTest)
+gtest_discover_tests(ir_test)
diff --git a/unittest/ir_test.cpp b/unittest/ir_test.cpp
new file mode 100644
index 0000000..936f3d5
--- /dev/null
+++ b/unittest/ir_test.cpp
@@ -0,0 +1,27 @@
+#include <gtest/gtest.h>
+#include <iostream>
+
+#include "Constant.h"
+#include "Function.h"
+#include "Instruction.h"
+#include "Module.h"
+#include "Type.h"
+
+using namespace SysYF::IR;
+
+TEST(IRTest, UserRemoveOperands) {
+    // https://github.com/ustc-compiler/IRGen/issues/2
+    std::cout << "user remove operands";
+    auto m = Module::create("test");
+    auto fn = Function::create(FunctionType::create(Type::create(SysYF::IR::Type::VoidTyID, m), {}, m), "foo", m);
+    auto bb = BasicBlock::create(m, "entry", fn);
+    auto const_1 = ConstantInt::create(1, m);
+    auto const_2 = ConstantInt::create(2, m);
+    auto add = BinaryInst::create_add(const_1, const_2, bb, m);
+    add->remove_operands(0, 0); // remove the first operand
+    for (unsigned int i = 0; i < add->get_num_operand(); i++) {
+        auto op = add->get_operand(i);
+        ASSERT_EQ(op->get_use_list().front().arg_no_, i);
+    }
+}
+