Improve Py_CHECKWRITE codegen by moving cold checks out of line

Include/refcount.h

@@ -142,11 +142,24 @@ static inline Py_ALWAYS_INLINE int _Py_IsImmutable(PyObject *op)
 }
 #define _Py_IsImmutable(op) _Py_IsImmutable(_PyObject_CAST(op))
 
-// Artifact[Implementation]: The definition of the `Py_CHECKWRITE` macro
+#ifndef Py_LIMITED_API
+// Cold path for Py_CHECKWRITE: called when the object is immutable.
+// Returns 1 if the object is still writable (ImmModule or finalizing).
+PyAPI_FUNC(int) _Py_CheckWriteImmutable(PyObject *op);
+
 // Check whether an object is writeable.
-// This check will always succeed during runtime finalization.
-#define Py_CHECKWRITE(op) ((op) && (!_Py_IsImmutable(op) || _PyImmModule_Check(op) || Py_IsFinalizing()))
+// The fast path just checks the immutable flag; the ImmModule and
+// finalizing checks are pushed into a cold out-of-line function.
+static inline int Py_CHECKWRITE(PyObject *op)
+{
+    if (_Py_IsImmutable(op)) {
+        return _Py_CheckWriteImmutable(op);
+    }
+    return 1;
+}
+#define Py_CHECKWRITE(op) Py_CHECKWRITE(_PyObject_CAST(op))
 #define Py_REQUIREWRITE(op, msg) {if (Py_CHECKWRITE(op)) { _PyObject_ASSERT_FAILED_MSG(op, msg); }}
+#endif  // !Py_LIMITED_API
 
 static inline Py_ALWAYS_INLINE void _Py_CLEAR_IMMUTABLE(PyObject *op)
 {

Python/immutability.c

@@ -675,7 +675,7 @@
     return true;
 }
 
 static PyObject* get_next(PyObject* obj, struct FreezeState *freeze_state)
 {
     (void)freeze_state;
     PyObject* next = scc_next(obj);
@@ -1846,6 +1846,15 @@
     return 1;
 }
 
+// Cold path for Py_CHECKWRITE: the object is known to be immutable.
+// Returns 1 (writable) if the object's type is _PyImmModule_Type or
+// the runtime is finalizing.
+int
+_Py_CheckWriteImmutable(PyObject *op)
+{
+    return _PyImmModule_Check(op) || Py_IsFinalizing();
+}
+
 // Perform a decref on an immutable object
 // returns true if the object should be deallocated.
 int _Py_DecRef_Immutable(PyObject *op)
@@ -2130,7 +2139,7 @@
         return 0;
     }
 #else
 #error "Immutability currently only works on 64bit platforms"
 #endif
 
     // Mark pre-freeze hook as completed. This has to be set before calling

Python/pystate.c

@@ -1386,7 +1386,7 @@ PyModuleObject* _PyInterpreterState_GetModuleState(PyObject *mod) {
             if (modules_mod == mod) {
                 // The modules in `sys.modules` is this frozen mod but there is
                 // no mutable state in `sys.mut_modules`, probably because it was
-                // unimported? Either way: 
+                // unimported? Either way:
                 // Remove `mod` from `sys.modules` and trigger a reimport.
                 if (PyDict_DelItem(modules, self->md_name) < 0) {
                     Py_DECREF(modules_mod);