[mypyc] feat: make Final values immortal on py3.12+

mypyc/irbuild/builder.py

@@ -429,6 +429,9 @@ def set_immortal_if_free_threaded(self, v: Value, line: int) -> None:
         """Make an object immortal on free-threaded builds (to avoid contention)."""
         self.builder.set_immortal_if_free_threaded(v, line)
 
+    def set_immortal_if_py312plus(self, v: Value, line: int) -> None:
+        self.builder.set_immortal_if_py312plus(v, line)
+
     # Helpers for IR building
 
     def add_to_non_ext_dict(
@@ -572,6 +575,7 @@ def init_final_static(
             coerced = self.coerce(rvalue_reg, type_override or self.node_type(lvalue), lvalue.line)
             self.final_names.append((name, coerced.type))
             self.add(InitStatic(coerced, name, self.module_name))
+            self.set_immortal_if_py312plus(coerced, lvalue.line)
 
     def load_final_static(
         self, fullname: str, typ: RType, line: int, error_name: str | None = None

mypyc/irbuild/classdef.py

@@ -262,8 +262,7 @@ def finalize(self, ir: ClassIR) -> None:
         non_ext_class = load_non_ext_class(self.builder, ir, self.non_ext, self.cdef.line)
         non_ext_class = load_decorated_class(self.builder, self.cdef, non_ext_class)
 
-        # Try to avoid contention when using free threading.
-        self.builder.set_immortal_if_free_threaded(non_ext_class, self.cdef.line)
+        self.builder.set_immortal_if_py312plus(non_ext_class, self.cdef.line)
 
         # Save the decorated class
         self.builder.add(

mypyc/irbuild/ll_builder.py

@@ -2492,6 +2492,11 @@ def set_immortal_if_free_threaded(self, v: Value, line: int) -> None:
         if IS_FREE_THREADED and sys.version_info >= (3, 14):
             self.primitive_op(set_immortal_op, [v], line)
 
+    def set_immortal_if_py312plus(self, v: Value, line: int) -> None:
+        """Emit IR to mark an object as immortal in Python 3.12+ (calls CPy_SetImmortal)."""
+        if sys.version_info >= (3, 12):
+            self.primitive_op(set_immortal_op, [v], line)
+
     # Internal helpers
 
     def decompose_union_helper(

mypyc/lib-rt/misc_ops.c

@@ -1129,12 +1129,21 @@ void CPy_SetTypeAliasTypeComputeFunction(PyObject *alias, PyObject *compute_valu
 
 #endif
 
-#if CPY_3_14_FEATURES
-
-#include "internal/pycore_object.h"
+#if CPY_3_12_FEATURES
+    #if CPY_3_14_FEATURES
+        #include "internal/pycore_object.h"
+    #else
+        // The immortal refcount value is -1 (see PEP 683 and CPython source)
+        // This is safe for statically allocated objects and matches CPython's logic.
+        #define IMMORTAL_REFCNT ((Py_ssize_t)-1)
+    #endif
 
 void CPy_SetImmortal(PyObject *obj) {
+#if CPY_3_14_FEATURES
     _Py_SetImmortal(obj);
+#elif CPY_3_12_FEATURES
+    Py_SET_REFCNT(obj, IMMORTAL_REFCNT);
+#endif
 }
 
 #endif

mypyc/primitives/misc_ops.py

@@ -324,9 +324,9 @@
 # threading, since this eliminates contention from concurrent reference count
 # updates.
 #
-# Needs at least Python 3.14.
+# Available for Python 3.12+ (uses public API in 3.14+, internal logic in 3.12/3.13).
 set_immortal_op = custom_primitive_op(
-    name="set_immmortal",
+    name="set_immortal",
     c_function_name="CPy_SetImmortal",
     arg_types=[object_rprimitive],
     return_type=void_rtype,