fix: gc_thread_before_fork should return NULL (Address) rather than boolean

rpython/translator/mu/ll2mu.py

@@ -642,11 +642,13 @@ class LL2MuMapper:
         return dst_nor, dst_exc
 
     def _same_as_false(self, llop):
-        llop.__init__('same_as', [self.mapped_const(False)], llop.result)
-        return [llop]
+        return self._same_as_const(llop, self.mapped_const(False))
 
     def _same_as_true(self, llop):
-        llop.__init__('same_as', [self.mapped_const(True)], llop.result)
+        return self._same_as_const(llop, self.mapped_const(True))
+
+    def _same_as_const(self, llop, const):
+        llop.__init__('same_as', [const], llop.result)
         return [llop]
 
     def _rename_to_same_as(self, llop):
@@ -1620,7 +1622,10 @@ class LL2MuMapper:
     map_op_gc_get_rpy_referents = _same_as_false
     map_op_gc_is_rpy_instance = _same_as_false
     map_op_gc_dump_rpy_heap = _same_as_false
-    map_op_gc_thread_before_fork = _same_as_false
+
+    def map_op_gc_thread_before_fork(self, llop):
+        MuT = self.map_type(llmemory.Address)
+        return self._same_as_const(llop, Constant(MuT._null(), MuT))
 
     map_op_gc_stack_bottom = lambda self, llop: []  # no-op