diff --git a/gc.c b/gc.c
index 7fcfdbcccbec4b..3d61b062a34b5f 100644
--- a/gc.c
+++ b/gc.c
@@ -158,10 +158,10 @@ RubyUpcalls ruby_upcalls;
 #endif
 
 #ifdef USE_THIRD_PARTY_HEAP
-static char *mmtk_env_plan = NULL;
-static char *mmtk_pre_arg_plan = NULL;
-static char *mmtk_post_arg_plan = NULL;
-static char *mmtk_chosen_plan = NULL;
+static const char *mmtk_env_plan = NULL;
+static const char *mmtk_pre_arg_plan = NULL;
+static const char *mmtk_post_arg_plan = NULL;
+static const char *mmtk_chosen_plan = NULL;
 #endif
 
 static inline struct rbimpl_size_mul_overflow_tag
@@ -1124,6 +1124,7 @@ heap_allocatable_slots(rb_objspace_t *objspace)
     return count;
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static inline size_t
 total_allocated_pages(rb_objspace_t *objspace)
 {
@@ -1145,6 +1146,7 @@ total_freed_pages(rb_objspace_t *objspace)
     }
     return count;
 }
+#endif
 
 #define gc_mode(objspace)                gc_mode_verify((enum gc_mode)(objspace)->flags.mode)
 #define gc_mode_set(objspace, mode)      ((objspace)->flags.mode = (unsigned int)gc_mode_verify(mode))
@@ -1212,7 +1214,6 @@ NORETURN(static void *gc_vraise(void *ptr));
 NORETURN(static void gc_raise(VALUE exc, const char *fmt, ...));
 NORETURN(static void negative_size_allocation_error(const char *));
 
-static void init_mark_stack(mark_stack_t *stack);
 
 static int ready_to_gc(rb_objspace_t *objspace);
 
@@ -1252,7 +1253,10 @@ static void gc_sweep_continue(rb_objspace_t *objspace, rb_size_pool_t *size_pool
 static inline void gc_mark(rb_objspace_t *objspace, VALUE ptr);
 static inline void gc_pin(rb_objspace_t *objspace, VALUE ptr);
 static inline void gc_mark_and_pin(rb_objspace_t *objspace, VALUE ptr);
+#ifndef USE_THIRD_PARTY_HEAP
+static void init_mark_stack(mark_stack_t *stack);
 static void gc_mark_ptr(rb_objspace_t *objspace, VALUE ptr);
+#endif
 NO_SANITIZE("memory", static void gc_mark_maybe(rb_objspace_t *objspace, VALUE ptr));
 static void gc_mark_children(rb_objspace_t *objspace, VALUE ptr);
 
@@ -1528,7 +1532,9 @@ check_rvalue_consistency_force(const VALUE obj, int terminate)
             bp();
             fprintf(stderr, "check_rvalue_consistency: %p is not a Ruby object.\n", (void *)obj);
             err++;
+#ifndef USE_THIRD_PARTY_HEAP
           skip:
+#endif
             ;
         }
         else {
@@ -1812,6 +1818,7 @@ RVALUE_AGE_RESET_RAW(VALUE obj)
     RBASIC(obj)->flags = RVALUE_FLAGS_AGE_SET(RBASIC(obj)->flags, 0);
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static inline void
 RVALUE_AGE_RESET(VALUE obj)
 {
@@ -1821,6 +1828,7 @@ RVALUE_AGE_RESET(VALUE obj)
     RVALUE_AGE_RESET_RAW(obj);
     check_rvalue_consistency(obj);
 }
+#endif
 
 static inline int
 RVALUE_BLACK_P(VALUE obj)
@@ -3201,6 +3209,7 @@ rb_objspace_data_type_name(VALUE obj)
     }
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static int
 ptr_in_page_body_p(const void *ptr, const void *memb)
 {
@@ -3237,6 +3246,7 @@ heap_page_for_ptr(rb_objspace_t *objspace, uintptr_t ptr)
         return NULL;
     }
 }
+#endif
 
 PUREFUNC(static inline int is_pointer_to_heap(rb_objspace_t *objspace, void *ptr);)
 static inline int
@@ -4581,6 +4591,7 @@ struct force_finalize_list {
     struct force_finalize_list *next;
 };
 
+#ifndef USE_THIRD_PARTY_HEAP
 static int
 force_chain_object(st_data_t key, st_data_t val, st_data_t arg)
 {
@@ -4592,6 +4603,7 @@ force_chain_object(st_data_t key, st_data_t val, st_data_t arg)
     *prev = curr;
     return ST_CONTINUE;
 }
+#endif
 
 bool rb_obj_is_main_ractor(VALUE gv);
 
@@ -4691,7 +4703,6 @@ rb_objspace_call_finalizer(rb_objspace_t *objspace)
     while ((resurrected = mmtk_poll_finalizable(true)) != NULL) {
 	VALUE obj = (VALUE)resurrected;
 	if (USE_RUBY_DEBUG_LOG) {
-	    VALUE klass = CLASS_OF(obj);
 	    RUBY_DEBUG_LOG("Resurrected for obj_free: %p: %s %s",
 		    resurrected,
 		    rb_type_str(RB_BUILTIN_TYPE(obj)),
@@ -5312,6 +5323,7 @@ count_objects(int argc, VALUE *argv, VALUE os)
 
 /* Sweeping */
 
+#ifndef USE_THIRD_PARTY_HEAP
 static size_t
 objspace_available_slots(rb_objspace_t *objspace)
 {
@@ -5327,6 +5339,7 @@ objspace_available_slots(rb_objspace_t *objspace)
     return total_slots;
 #endif
 }
+#endif
 
 static size_t
 objspace_live_slots(rb_objspace_t *objspace)
@@ -5334,6 +5347,7 @@ objspace_live_slots(rb_objspace_t *objspace)
     return (objspace->total_allocated_objects - objspace->profile.total_freed_objects) - heap_pages_final_slots;
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static size_t
 objspace_free_slots(rb_objspace_t *objspace)
 {
@@ -5343,6 +5357,7 @@ objspace_free_slots(rb_objspace_t *objspace)
     return objspace_available_slots(objspace) - objspace_live_slots(objspace) - heap_pages_final_slots;
 #endif
 }
+#endif //USE_THIRD_PARTY_HEAP
 
 static void
 gc_setup_mark_bits(struct heap_page *page)
@@ -6533,6 +6548,7 @@ pop_mark_stack(mark_stack_t *stack, VALUE *data)
     return TRUE;
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static void
 init_mark_stack(mark_stack_t *stack)
 {
@@ -6546,6 +6562,7 @@ init_mark_stack(mark_stack_t *stack)
     }
     stack->unused_cache_size = stack->cache_size;
 }
+#endif
 
 /* Marking */
 
@@ -6953,6 +6970,7 @@ mark_current_machine_context(rb_objspace_t *objspace, rb_execution_context_t *ec
 
 #else // !defined(__wasm__)
 
+#ifndef USE_THIRD_PARTY_HEAP
 static void
 mark_current_machine_context(rb_objspace_t *objspace, rb_execution_context_t *ec)
 {
@@ -6977,6 +6995,7 @@ mark_current_machine_context(rb_objspace_t *objspace, rb_execution_context_t *ec
 
     each_stack_location(objspace, ec, stack_start, stack_end, gc_mark_maybe);
 }
+#endif // USE_THIRD_PARTY_HEAP
 #endif
 
 static void
@@ -7162,6 +7181,8 @@ gc_aging(rb_objspace_t *objspace, VALUE obj)
     objspace->marked_slots++;
 }
 
+
+#ifndef USE_THIRD_PARTY_HEAP
 NOINLINE(static void gc_mark_ptr(rb_objspace_t *objspace, VALUE obj));
 static void reachable_objects_from_callback(VALUE obj);
 
@@ -7194,6 +7215,7 @@ gc_mark_ptr(rb_objspace_t *objspace, VALUE obj)
         reachable_objects_from_callback(obj);
     }
 }
+#endif
 
 static inline void
 gc_pin(rb_objspace_t *objspace, VALUE obj)
@@ -7206,11 +7228,13 @@ gc_pin(rb_objspace_t *objspace, VALUE obj)
     }
 }
 
+#ifdef USE_THIRD_PARTY_HEAP
 static inline void
 rb_mmtk_mark_movable(VALUE obj);
 
 static inline void
 rb_mmtk_mark_pin(VALUE obj);
+#endif
 
 static inline void
 gc_mark_and_pin(rb_objspace_t *objspace, VALUE obj)
@@ -7628,8 +7652,10 @@ show_mark_ticks(void)
 
 #endif /* PRINT_ROOT_TICKS */
 
+#ifdef USE_THIRD_PARTY_HEAP
 static void
 rb_mmtk_assert_mmtk_worker();
+#endif
 
 static void
 gc_mark_roots(rb_objspace_t *objspace, const char **categoryp)
@@ -9932,6 +9958,9 @@ static VALUE
 gc_start_internal(rb_execution_context_t *ec, VALUE self, VALUE full_mark, VALUE immediate_mark, VALUE immediate_sweep, VALUE compact)
 {
     rb_objspace_t *objspace = &rb_objspace;
+#ifdef USE_THIRD_PARTY_HEAP
+    mmtk_handle_user_collection_request(GET_THREAD());
+#else
     unsigned int reason = (GPR_FLAG_FULL_MARK |
                            GPR_FLAG_IMMEDIATE_MARK |
                            GPR_FLAG_IMMEDIATE_SWEEP |
@@ -9949,9 +9978,6 @@ gc_start_internal(rb_execution_context_t *ec, VALUE self, VALUE full_mark, VALUE
         if (!RTEST(immediate_sweep)) reason &= ~GPR_FLAG_IMMEDIATE_SWEEP;
     }
 
-#ifdef USE_THIRD_PARTY_HEAP
-    mmtk_handle_user_collection_request(GET_THREAD());
-#else
     garbage_collect(objspace, reason);
 #endif // USE_THIRD_PARTY_HEAP
 
@@ -11890,12 +11916,14 @@ ruby_gc_set_params(void)
 #endif
 }
 
+#ifndef USE_THIRD_PARTY_HEAP
 static void
 reachable_objects_from_callback(VALUE obj)
 {
     rb_ractor_t *cr = GET_RACTOR();
     cr->mfd->mark_func(obj, cr->mfd->data);
 }
+#endif
 
 void
 rb_objspace_reachable_objects_from(VALUE obj, void (func)(VALUE, void *), void *data)
@@ -14997,6 +15025,7 @@ rb_mmtk_scan_vm_specific_roots(void)
     gc_mark_roots(vm->objspace, &phase);
 }
 
+RBIMPL_ATTR_NORETURN()
 static void
 rb_mmtk_scan_thread_roots(void)
 {
@@ -15013,7 +15042,7 @@ rb_mmtk_scan_thread_root(MMTk_VMMutatorThread mutator, MMTk_VMWorkerThread worke
 
     RUBY_DEBUG_LOG("[Worker: %p] We will scan thread root for thread: %p, ec: %p", worker, thread, ec);
 
-    rb_execution_context_mark(thread->ec);
+    rb_execution_context_mark(ec);
 
     RUBY_DEBUG_LOG("[Worker: %p] Finished scanning thread for thread: %p, ec: %p", worker, thread, ec);
 }
@@ -15179,7 +15208,7 @@ void rb_mmtk_pre_process_opts(int argc, char **argv) {
 #define opt_match_arg(s, l, name) \
     opt_match(s, l, name) && (*(s) ? 1 : (rb_raise(rb_eRuntimeError, "--mmtk-" name " needs an argument"), 0))
 
-void rb_mmtk_post_process_opts(char *s) {
+void rb_mmtk_post_process_opts(const char *s) {
     const size_t l = strlen(s);
     if (l == 0) {
         return;
diff --git a/gc.h b/gc.h
index ad3296d457a49b..171b07fdb34dd2 100644
--- a/gc.h
+++ b/gc.h
@@ -122,7 +122,7 @@ struct rb_thread_struct;
 #define MMTK_DEFAULT_PLAN "MarkSweep"
 void rb_gc_init_collection();
 void rb_mmtk_pre_process_opts(int argc, char **argv);
-void rb_mmtk_post_process_opts(char *arg);
+void rb_mmtk_post_process_opts(const char *arg);
 void rb_mmtk_post_process_opts_finish(void);
 #endif // USE_THIRD_PARTY_HEAP
 
diff --git a/mmtk.h b/mmtk.h
index 654a818f9b2e11..daf7a35030b798 100644
--- a/mmtk.h
+++ b/mmtk.h
@@ -63,7 +63,7 @@ extern void* mmtk_alloc_slow(MMTk_Mutator mutator, size_t size,
     size_t align, ssize_t offset, int semantics);
 
 extern void mmtk_post_alloc(MMTk_Mutator mutator, void* refer,
-    int bytes, int semantics);
+    size_t bytes, int semantics);
 
 /**
  * Tracing
diff --git a/string.c b/string.c
index 02acddd9322b9d..8b0f0561fdecae 100644
--- a/string.c
+++ b/string.c
@@ -462,7 +462,7 @@ fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t data, int exist
 	RBASIC(str)->flags |= RSTRING_FSTR;
 
 #ifdef USE_THIRD_PARTY_HEAP
-        mmtk_register_finalizable(str);
+        mmtk_register_finalizable((void *)str);
 #endif // USE_THIRD_PARTY_HEAP
 
 	*key = *value = arg->fstr = str;