diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5f03dd59..5de9cabf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,15 +5,11 @@ project(pocketpy)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED ON)
 
-# use IPO
-option(PK_BUILD_WITH_IPO "" TRUE)
-include(CheckIPOSupported)
-check_ipo_supported(RESULT result)
-
-if(result AND PK_BUILD_WITH_IPO)
-    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+# use UNITY_BUILD if CMake version >= 3.16
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.16")
+    option(PK_BUILD_WITH_UNITY "" TRUE)
 else()
-    message(WARNING ">> IPO disabled. You will not get the best performance.")
+    option(PK_BUILD_WITH_UNITY "" FALSE)
 endif()
 
 if(WIN32)
@@ -94,13 +90,20 @@ else()
     target_link_libraries(main ${PROJECT_NAME})
 endif()
 
-if(UNIX)
+if(UNIX AND NOT APPLE)
     target_link_libraries(${PROJECT_NAME} m)
     if(PK_ENABLE_OS)
         target_link_libraries(${PROJECT_NAME} dl)
     endif()
 endif()
 
+if(PK_BUILD_WITH_UNITY)
+    set_source_files_properties(${POCKETPY_SRC} PROPERTIES UNITY_GROUP "pocketpy")
+    set_target_properties(${PROJECT_NAME} PROPERTIES UNITY_BUILD_MODE GROUP)
+    set_target_properties(${PROJECT_NAME} PROPERTIES UNITY_BUILD ON)
+endif()
+
+############################################
 if(PK_BUILD_MODULE_LZ4)
     target_link_libraries(${PROJECT_NAME} lz4)
 endif()
diff --git a/build_ios.sh b/build_ios.sh
index 95715a73..e5e23391 100644
--- a/build_ios.sh
+++ b/build_ios.sh
@@ -8,10 +8,7 @@ cd build
 
 FLAGS="-DCMAKE_TOOLCHAIN_FILE=3rd/ios.toolchain.cmake \
     -DDEPLOYMENT_TARGET=13.0 \
-    -DPK_BUILD_STATIC_LIB=ON \
-    -DPK_BUILD_WITH_IPO=OFF \
-    -DPK_BUILD_MODULE_LZ4=ON \
-    -DPK_BUILD_MODULE_LIBHV=ON"
+    -DPK_BUILD_STATIC_LIB=ON"
 
 cmake -B os64 -G Xcode $FLAGS -DPLATFORM=OS64 ..
 cmake --build os64 --config Release
diff --git a/include/pocketpy/interpreter/heap.h b/include/pocketpy/interpreter/heap.h
index 0a682058..fe1fc058 100644
--- a/include/pocketpy/interpreter/heap.h
+++ b/include/pocketpy/interpreter/heap.h
@@ -1,3 +1,5 @@
+#pragma once
+
 #include "pocketpy/objects/object.h"
 #include "pocketpy/interpreter/objectpool.h"