[3.15] gh-149459: Fix segfault when _LOAD_SPECIAL guard deoptimizes (GH-149478) (#149552)

gh-149459: Fix segfault when `_LOAD_SPECIAL` guard deoptimizes (GH-149478)
(cherry picked from commit c341e34)

Co-authored-by: Hai Zhu <haiizhu@outlook.com>

Author: miss-islington

Lib/test/test_capi/test_opt.py

@@ -6138,6 +6138,20 @@ def __init__(self, x):
                 C(0) if i else str(0)
         """))
 
+    def test_load_special_type_guard_deopt(self):
+        script_helper.assert_python_ok("-s", "-c", textwrap.dedent(f"""
+            def f1():
+                class Context:
+                    def __enter__(self): ...
+                    def __exit__(self, e, v, t): ...
+
+                with Context():
+                    pass
+
+            for _ in range({TIER2_THRESHOLD + 5}):
+                f1()
+        """), PYTHON_JIT="1")
+
 def global_identity(x):
     return x
 

Misc/NEWS.d/next/Core_and_Builtins/2026-05-07-03-18-59.gh-issue-149459.5fhAqP.rst

@@ -0,0 +1 @@
+Fix a crash in the JIT optimizer when a specialized ``LOAD_SPECIAL`` guard deoptimized after inserting the synthetic ``NULL`` stack entry.

Python/optimizer_bytecodes.c

@@ -2043,7 +2043,16 @@ dummy_func(void) {
             PyObject *name = _Py_SpecialMethods[oparg].name;
             PyObject *descr = _PyType_Lookup(type, name);
             if (descr != NULL && (Py_TYPE(descr)->tp_flags & Py_TPFLAGS_METHOD_DESCRIPTOR)) {
-                ADD_OP(_GUARD_TYPE_VERSION, 0, type->tp_version_tag);
+                /* LOAD_SPECIAL expands to _RECORD_TOS_TYPE + _INSERT_NULL +
+                 * _LOAD_SPECIAL. Insert _GUARD_TYPE_VERSION before the
+                 * already-emitted _INSERT_NULL so deopt sees the original
+                 * stack shape.*/
+                _PyUOpInstruction *insert_null = uop_buffer_last(&ctx->out_buffer);
+                assert(insert_null->opcode == _INSERT_NULL);
+                assert(insert_null->target == this_instr->target);
+                REPLACE_OP(insert_null, _GUARD_TYPE_VERSION, 0, type->tp_version_tag);
+                ADD_OP(_INSERT_NULL, 0, 0);
+
                 bool immortal = _Py_IsImmortal(descr) || (type->tp_flags & Py_TPFLAGS_IMMUTABLETYPE);
                 ADD_OP(immortal ? _LOAD_CONST_INLINE_BORROW : _LOAD_CONST_INLINE,
                        0, (uintptr_t)descr);