diff --git a/gc/gc_impl.h b/gc/gc_impl.h
index 80da58d..1bcb744 100644
--- a/gc/gc_impl.h
+++ b/gc/gc_impl.h
@@ -89,14 +89,15 @@ GC_IMPL_FN VALUE rb_gc_impl_new_obj(void *objspace_ptr, void *cache_ptr, VALUE k
  * generated instruction sequence; see zjit/src/gc_fastpath.rs.
  */
 GC_IMPL_FN bool rb_gc_impl_zjit_new_obj_fastpath(void *objspace_ptr, size_t alloc_size, VALUE flags, VALUE klass, struct rb_gc_zjit_fastpath *fastpath);
-/* Returns a pointer to a word that is nonzero while incremental marking is in
- * progress, or NULL if the implementation does not support ZJIT's inline write
- * barrier fast path. ZJIT-generated code skips the call to rb_gc_writebarrier()
- * when the receiver has neither RUBY_FL_PROMOTED nor RUBY_FL_SHAREABLE set and
- * this word is zero, so only implementations whose write barrier is a no-op
- * under exactly those conditions may return non-NULL.
+/* Returns the offset of a word inside the objspace that is nonzero while that
+ * objspace is incremental-marking, or -1 if the implementation does not support
+ * ZJIT's inline write barrier fast path. ZJIT-generated code skips the call to
+ * rb_gc_writebarrier() when the receiver has neither RUBY_FL_PROMOTED nor
+ * RUBY_FL_SHAREABLE set and the word of the current Ractor's objspace is zero,
+ * so only implementations whose write barrier is a no-op under exactly those
+ * conditions may return a nonnegative offset.
  */
-GC_IMPL_FN const uintptr_t *rb_gc_impl_zjit_writebarrier_required_p_ptr(void *objspace_ptr);
+GC_IMPL_FN int32_t rb_gc_impl_zjit_writebarrier_required_offset(void);
 GC_IMPL_FN size_t rb_gc_impl_obj_slot_size(VALUE obj);
 GC_IMPL_FN size_t rb_gc_impl_size_slot_size(void *objspace_ptr, size_t size);
 GC_IMPL_FN bool rb_gc_impl_size_allocatable_p(size_t size);
@@ -152,7 +153,7 @@ GC_IMPL_FN bool rb_gc_impl_during_postmortem_p(void *objspace_ptr);
  * objspace impl. */
 GC_IMPL_FN bool rb_gc_impl_obj_foreign_p(void *objspace_ptr, VALUE obj);
 GC_IMPL_FN bool rb_gc_impl_shref_marked_p(void *objspace_ptr, VALUE obj);
-GC_IMPL_FN size_t rb_gc_impl_heap_page_count(void *objspace_ptr);
+GC_IMPL_FN size_t rb_gc_impl_zombie_heap_page_count(void *objspace_ptr);
 GC_IMPL_FN void rb_gc_impl_objspace_absorb(void *dst_ptr, void *src_ptr);
 GC_IMPL_FN void rb_gc_impl_each_object(void *objspace_ptr, void (*func)(VALUE obj, void *data), void *data);
 // Finalizers
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index cf80d98..c379354 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -742,12 +742,12 @@ rb_gc_impl_zjit_new_obj_fastpath(void *objspace_ptr, size_t alloc_size, VALUE fl
 #endif
 }
 
-const uintptr_t *
-rb_gc_impl_zjit_writebarrier_required_p_ptr(void *objspace_ptr)
+int32_t
+rb_gc_impl_zjit_writebarrier_required_offset(void)
 {
     /* MMTk's write barrier is not a no-op for young, unshareable receivers,
      * so ZJIT must always call rb_gc_writebarrier(). */
-    return NULL;
+    return -1;
 }
 
 void
@@ -1859,7 +1859,7 @@ rb_gc_impl_shref_marked_p(void *objspace_ptr, VALUE obj)
 }
 
 size_t
-rb_gc_impl_heap_page_count(void *objspace_ptr)
+rb_gc_impl_zombie_heap_page_count(void *objspace_ptr)
 {
     /* With a single objspace zombie_objspaces is always empty. */
     return 0;