diff --git a/include/pocketpy/interpreter/vm.h b/include/pocketpy/interpreter/vm.h
index 4d136774..61c30880 100644
--- a/include/pocketpy/interpreter/vm.h
+++ b/include/pocketpy/interpreter/vm.h
@@ -35,8 +35,8 @@ typedef struct VM {
     BinTree modules;
     TypeList types;
 
-    py_TValue builtins;  // builtins module
-    py_TValue main;      // __main__ module
+    py_GlobalRef builtins;  // builtins module
+    py_GlobalRef main;      // __main__ module
 
     py_Callbacks callbacks;
 
@@ -152,7 +152,7 @@ py_Type pk_generator__register();
 py_Type pk_namedict__register();
 py_Type pk_code__register();
 
-py_TValue pk_builtins__register();
+py_GlobalRef pk_builtins__register();
 
 /* mappingproxy */
 void pk_mappingproxy__namedict(py_Ref out, py_Ref object);
\ No newline at end of file
diff --git a/include/pocketpy/pocketpy.h b/include/pocketpy/pocketpy.h
index fcb3c5da..798985dd 100644
--- a/include/pocketpy/pocketpy.h
+++ b/include/pocketpy/pocketpy.h
@@ -118,6 +118,8 @@ PK_API void py_setvmctx(void* ctx);
 PK_API void py_sys_setargv(int argc, char** argv);
 /// Set the trace function for the current VM.
 PK_API void py_sys_settrace(py_TraceFunc func, bool reset);
+/// Invoke the garbage collector.
+PK_API int py_gc_collect();
 /// Setup the callbacks for the current VM.
 PK_API py_Callbacks* py_callbacks();
 
diff --git a/src/interpreter/ceval.c b/src/interpreter/ceval.c
index c2aec98c..cf355cd0 100644
--- a/src/interpreter/ceval.c
+++ b/src/interpreter/ceval.c
@@ -255,7 +255,7 @@ FrameResult VM__run_top_frame(VM* self) {
                 }
                 if(res == -1) goto __ERROR;
                 // builtins
-                py_Ref tmp = py_getdict(&self->builtins, name);
+                py_Ref tmp = py_getdict(self->builtins, name);
                 if(tmp != NULL) {
                     PUSH(tmp);
                     DISPATCH();
@@ -277,7 +277,7 @@ FrameResult VM__run_top_frame(VM* self) {
                 }
                 if(res == -1) goto __ERROR;
 
-                tmp = py_getdict(&self->builtins, name);
+                tmp = py_getdict(self->builtins, name);
                 if(tmp != NULL) {
                     PUSH(tmp);
                     DISPATCH();
@@ -293,7 +293,7 @@ FrameResult VM__run_top_frame(VM* self) {
                     DISPATCH();
                 }
                 if(res == -1) goto __ERROR;
-                py_Ref tmp = py_getdict(&self->builtins, name);
+                py_Ref tmp = py_getdict(self->builtins, name);
                 if(tmp != NULL) {
                     PUSH(tmp);
                     DISPATCH();
@@ -325,7 +325,7 @@ FrameResult VM__run_top_frame(VM* self) {
                     DISPATCH();
                 }
                 if(res == -1) goto __ERROR;
-                tmp = py_getdict(&self->builtins, name);
+                tmp = py_getdict(self->builtins, name);
                 if(tmp) {
                     PUSH(tmp);
                     DISPATCH();
@@ -505,7 +505,7 @@ FrameResult VM__run_top_frame(VM* self) {
             /*****************************************/
             case OP_BUILD_IMAG: {
                 // [x]
-                py_Ref f = py_getdict(&self->builtins, py_name("complex"));
+                py_Ref f = py_getdict(self->builtins, py_name("complex"));
                 assert(f != NULL);
                 py_TValue tmp = *TOP();
                 *TOP() = *f;           // [complex]
@@ -558,7 +558,7 @@ FrameResult VM__run_top_frame(VM* self) {
             }
             case OP_BUILD_SET: {
                 py_TValue* begin = SP() - byte.arg;
-                py_Ref typeobject_set = py_getdict(&self->builtins, py_name("set"));
+                py_Ref typeobject_set = py_getdict(self->builtins, py_name("set"));
                 assert(typeobject_set != NULL);
                 py_push(typeobject_set);
                 py_pushnil();
@@ -1079,8 +1079,6 @@ FrameResult VM__run_top_frame(VM* self) {
             }
             case OP_STORE_CLASS_ATTR: {
                 assert(self->curr_class);
-                py_Type type = py_totype(self->curr_class);
-                py_TypeInfo* ti = TypeList__get(&self->types, type);
                 py_Name name = co_names[byte.arg];
                 // TOP() can be a function, classmethod or custom decorator
                 py_Ref actual_func = TOP();
diff --git a/src/interpreter/frame.c b/src/interpreter/frame.c
index 47853917..2d5ee253 100644
--- a/src/interpreter/frame.c
+++ b/src/interpreter/frame.c
@@ -194,7 +194,7 @@ const char* py_Frame_sourceloc(py_Frame* self, int* lineno) {
 
 void py_Frame_newglobals(py_Frame* frame, py_OutRef out) {
     if(!frame) {
-        pk_mappingproxy__namedict(out, &pk_current_vm->main);
+        pk_mappingproxy__namedict(out, pk_current_vm->main);
         return;
     }
     if(frame->globals->type == tp_module) {
diff --git a/src/interpreter/vm.c b/src/interpreter/vm.c
index dafac22b..952e873b 100644
--- a/src/interpreter/vm.c
+++ b/src/interpreter/vm.c
@@ -83,8 +83,8 @@ void VM__ctor(VM* self) {
     BinTree__ctor(&self->modules, c11_strdup(""), py_NIL(), &modules_config);
     TypeList__ctor(&self->types);
 
-    self->builtins = *py_NIL();
-    self->main = *py_NIL();
+    self->builtins = NULL;
+    self->main = NULL;
 
     self->callbacks.importfile = pk_default_importfile;
     self->callbacks.print = pk_default_print;
@@ -175,8 +175,8 @@ void VM__ctor(VM* self) {
     // inject some builtin exceptions
 #define INJECT_BUILTIN_EXC(name, TBase)                                                            \
     do {                                                                                           \
-        py_Type type = pk_newtype(#name, TBase, &self->builtins, NULL, false, true);               \
-        py_setdict(&self->builtins, py_name(#name), py_tpobject(type));                            \
+        py_Type type = pk_newtype(#name, TBase, self->builtins, NULL, false, true);                \
+        py_setdict(self->builtins, py_name(#name), py_tpobject(type));                             \
         validate(tp_##name, type);                                                                 \
     } while(0)
 
@@ -184,7 +184,7 @@ void VM__ctor(VM* self) {
     INJECT_BUILTIN_EXC(KeyboardInterrupt, tp_BaseException);
 
     validate(tp_StopIteration, pk_StopIteration__register());
-    py_setdict(&self->builtins, py_name("StopIteration"), py_tpobject(tp_StopIteration));
+    py_setdict(self->builtins, py_name("StopIteration"), py_tpobject(tp_StopIteration));
 
     INJECT_BUILTIN_EXC(SyntaxError, tp_Exception);
     INJECT_BUILTIN_EXC(RecursionError, tp_Exception);
@@ -230,10 +230,10 @@ void VM__ctor(VM* self) {
 
     for(int i = 0; i < c11__count_array(public_types); i++) {
         py_TypeInfo* ti = pk__type_info(public_types[i]);
-        py_setdict(&self->builtins, ti->name, &ti->self);
+        py_setdict(self->builtins, ti->name, &ti->self);
     }
 
-    py_newnotimplemented(py_emplacedict(&self->builtins, py_name("NotImplemented")));
+    py_newnotimplemented(py_emplacedict(self->builtins, py_name("NotImplemented")));
 
     pk__add_module_vmath();
     pk__add_module_array2d();
@@ -266,7 +266,7 @@ void VM__ctor(VM* self) {
     // add python builtins
     do {
         bool ok;
-        ok = py_exec(kPythonLibs_builtins, "<builtins>", EXEC_MODE, &self->builtins);
+        ok = py_exec(kPythonLibs_builtins, "<builtins>", EXEC_MODE, self->builtins);
         if(!ok) goto __ABORT;
         break;
     __ABORT:
@@ -274,7 +274,7 @@ void VM__ctor(VM* self) {
         c11__abort("failed to load python builtins!");
     } while(0);
 
-    self->main = *py_newmodule("__main__");
+    self->main = py_newmodule("__main__");
 }
 
 void VM__dtor(VM* self) {
@@ -766,7 +766,7 @@ void ManagedHeap__mark(ManagedHeap* self) {
 
 void pk_print_stack(VM* self, py_Frame* frame, Bytecode byte) {
     return;
-    if(frame == NULL || py_isnil(&self->main)) return;
+    if(frame == NULL || py_isnil(self->main)) return;
 
     py_TValue* sp = self->stack.sp;
     c11_sbuf buf;
diff --git a/src/modules/gc.c b/src/modules/gc.c
index 0116609b..5c5b358f 100644
--- a/src/modules/gc.c
+++ b/src/modules/gc.c
@@ -1,7 +1,7 @@
 #include "pocketpy/pocketpy.h"
 #include "pocketpy/interpreter/vm.h"
 
-static bool gc_collect(int argc, py_Ref argv){
+static bool gc_collect(int argc, py_Ref argv) {
     PY_CHECK_ARGC(0);
     ManagedHeap* heap = &pk_current_vm->heap;
     int res = ManagedHeap__collect(heap);
@@ -9,7 +9,7 @@ static bool gc_collect(int argc, py_Ref argv){
     return true;
 }
 
-static bool gc_enable(int argc, py_Ref argv){
+static bool gc_enable(int argc, py_Ref argv) {
     PY_CHECK_ARGC(0);
     ManagedHeap* heap = &pk_current_vm->heap;
     heap->gc_enabled = true;
@@ -17,7 +17,7 @@ static bool gc_enable(int argc, py_Ref argv){
     return true;
 }
 
-static bool gc_disable(int argc, py_Ref argv){
+static bool gc_disable(int argc, py_Ref argv) {
     PY_CHECK_ARGC(0);
     ManagedHeap* heap = &pk_current_vm->heap;
     heap->gc_enabled = false;
@@ -25,7 +25,7 @@ static bool gc_disable(int argc, py_Ref argv){
     return true;
 }
 
-static bool gc_isenabled(int argc, py_Ref argv){
+static bool gc_isenabled(int argc, py_Ref argv) {
     PY_CHECK_ARGC(0);
     ManagedHeap* heap = &pk_current_vm->heap;
     py_newbool(py_retval(), heap->gc_enabled);
@@ -39,4 +39,9 @@ void pk__add_module_gc() {
     py_bindfunc(mod, "enable", gc_enable);
     py_bindfunc(mod, "disable", gc_disable);
     py_bindfunc(mod, "isenabled", gc_isenabled);
+}
+
+int py_gc_collect() {
+    ManagedHeap* heap = &pk_current_vm->heap;
+    return ManagedHeap__collect(heap);
 }
\ No newline at end of file
diff --git a/src/modules/os.c b/src/modules/os.c
index 38cc3a78..45804407 100644
--- a/src/modules/os.c
+++ b/src/modules/os.c
@@ -229,7 +229,7 @@ void pk__add_module_io() {
     py_newint(py_emplacedict(mod, py_name("SEEK_CUR")), SEEK_CUR);
     py_newint(py_emplacedict(mod, py_name("SEEK_END")), SEEK_END);
 
-    py_setdict(&pk_current_vm->builtins, py_name("open"), py_tpobject(FileIO));
+    py_setdict(pk_current_vm->builtins, py_name("open"), py_tpobject(FileIO));
 }
 
 #else
diff --git a/src/public/cast.c b/src/public/cast.c
index ea5169ca..dd1b44bb 100644
--- a/src/public/cast.c
+++ b/src/public/cast.c
@@ -85,7 +85,7 @@ py_Type py_gettype(const char* module, py_Name name) {
         mod = py_getmodule(module);
         if(!mod) return 0;
     } else {
-        mod = &pk_current_vm->builtins;
+        mod = pk_current_vm->builtins;
     }
     py_Ref object = py_getdict(mod, name);
     if(object && py_istype(object, tp_type)) return py_totype(object);
diff --git a/src/public/exec.c b/src/public/exec.c
index 0bf1d4e6..d1cf96da 100644
--- a/src/public/exec.c
+++ b/src/public/exec.c
@@ -49,7 +49,7 @@ bool py_compile(const char* source,
 
 bool pk_exec(CodeObject* co, py_Ref module) {
     VM* vm = pk_current_vm;
-    if(!module) module = &vm->main;
+    if(!module) module = vm->main;
     assert(module->type == tp_module);
 
     py_StackRef sp = vm->stack.sp;
@@ -63,7 +63,7 @@ bool pk_exec(CodeObject* co, py_Ref module) {
 
 bool pk_execdyn(CodeObject* co, py_Ref module, py_Ref globals, py_Ref locals) {
     VM* vm = pk_current_vm;
-    if(!module) module = &vm->main;
+    if(!module) module = vm->main;
     assert(module->type == tp_module);
 
     py_StackRef sp = vm->stack.sp;
diff --git a/src/public/modules.c b/src/public/modules.c
index e060dd7e..a1d0d98d 100644
--- a/src/public/modules.c
+++ b/src/public/modules.c
@@ -16,11 +16,11 @@ py_Ref py_getmodule(const char* path) {
     return BinTree__try_get(&vm->modules, (void*)path);
 }
 
-py_Ref py_getbuiltin(py_Name name) { return py_getdict(&pk_current_vm->builtins, name); }
+py_Ref py_getbuiltin(py_Name name) { return py_getdict(pk_current_vm->builtins, name); }
 
-py_Ref py_getglobal(py_Name name) { return py_getdict(&pk_current_vm->main, name); }
+py_Ref py_getglobal(py_Name name) { return py_getdict(pk_current_vm->main, name); }
 
-void py_setglobal(py_Name name, py_Ref val) { py_setdict(&pk_current_vm->main, name, val); }
+void py_setglobal(py_Name name, py_Ref val) { py_setdict(pk_current_vm->main, name, val); }
 
 py_Ref py_newmodule(const char* path) {
     ManagedHeap* heap = &pk_current_vm->heap;
@@ -608,7 +608,7 @@ static bool builtins_eval(int argc, py_Ref argv) {
 
 static bool
     pk_smartexec(const char* source, py_Ref module, enum py_CompileMode mode, va_list args) {
-    if(module == NULL) module = &pk_current_vm->main;
+    if(module == NULL) module = pk_current_vm->main;
     pk_mappingproxy__namedict(py_pushtmp(), module);  // globals
     py_newdict(py_pushtmp());                         // locals
     bool ok = py_compile(source, "<string>", mode, true);
@@ -716,8 +716,8 @@ static bool NotImplementedType__repr__(int argc, py_Ref argv) {
     return true;
 }
 
-py_TValue pk_builtins__register() {
-    py_Ref builtins = py_newmodule("builtins");
+py_GlobalRef pk_builtins__register() {
+    py_GlobalRef builtins = py_newmodule("builtins");
     py_bindfunc(builtins, "exit", builtins_exit);
     py_bindfunc(builtins, "input", builtins_input);
     py_bindfunc(builtins, "repr", builtins_repr);
@@ -760,7 +760,7 @@ py_TValue pk_builtins__register() {
     py_setdict(py_tpobject(tp_ellipsis), __hash__, py_None());
     py_bindmagic(tp_NotImplementedType, __repr__, NotImplementedType__repr__);
     py_setdict(py_tpobject(tp_NotImplementedType), __hash__, py_None());
-    return *builtins;
+    return builtins;
 }
 
 void function__gc_mark(void* ud, c11_vector* p_stack) {