Add no-op rb_gc_impl_zjit_writebarrier_required_p_ptr

See ruby/ruby#18641

Author: tekknolagi

gc/gc_impl.h

@@ -89,6 +89,14 @@ 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.
+ */
+GC_IMPL_FN const uintptr_t *rb_gc_impl_zjit_writebarrier_required_p_ptr(void *objspace_ptr);
 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);

gc/mmtk/mmtk.c

@@ -749,6 +749,14 @@ 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)
+{
+    /* MMTk's write barrier is not a no-op for young, unshareable receivers,
+     * so ZJIT must always call rb_gc_writebarrier(). */
+    return NULL;
+}
+
 void
 rb_gc_impl_init(void)
 {