Actually make this a literal

Author: encukou

Include/internal/pycore_gc.h

@@ -133,7 +133,7 @@ static inline void _PyObject_GC_SET_SHARED(PyObject *op) {
 */
 #define _PyGC_NEXT_MASK_OLD_SPACE_1    1
 
-#define _PyGC_PREV_SHIFT           _PyObject_ALIGNMENT_SHIFT
+#define _PyGC_PREV_SHIFT           2
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
 
 /* set for debugging information */

Include/internal/pycore_interp_structs.h

@@ -159,7 +159,7 @@ struct atexit_state {
 typedef struct {
     // Tagged pointer to next object in the list.
     // 0 means the object is not tracked
-    _Py_ALIGNED_DEF(1 << _PyObject_ALIGNMENT_SHIFT, uintptr_t) _gc_next;
+    _Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, uintptr_t) _gc_next;
 
     // Tagged pointer to previous object in the list.
     // Lowest two bits are used for flags documented later.

Include/object.h

@@ -103,8 +103,9 @@ whose size is determined when the object is allocated.
 
 /* PyObjects are given a minimum alignment so that the least significant bits
  * of an object pointer become available for other purposes.
+ * This must be an integer literal with the value (1 << _PyGC_PREV_SHIFT)
  */
-#define _PyObject_ALIGNMENT_SHIFT       2
+#define _PyObject_MIN_ALIGNMENT 4
 
 /* Nothing is actually declared to be a PyObject, but every pointer to
  * a Python object can be cast to a PyObject*.  This is inheritance built
@@ -141,7 +142,7 @@ struct _object {
 #else
         Py_ssize_t ob_refcnt;
 #endif
-        _Py_ALIGNED_DEF(1 << _PyObject_ALIGNMENT_SHIFT, char) _aligner;
+        _Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, char) _aligner;
     };
 #ifdef _MSC_VER
     __pragma(warning(pop))
@@ -159,7 +160,7 @@ struct _object {
     // ob_tid stores the thread id (or zero). It is also used by the GC and the
     // trashcan mechanism as a linked list pointer and by the GC to store the
     // computed "gc_refs" refcount.
-    _Py_ALIGNED_DEF(1 << _PyObject_ALIGNMENT_SHIFT, uintptr_t) ob_tid;
+    _Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, uintptr_t) ob_tid;
     uint16_t ob_flags;
     PyMutex ob_mutex;           // per-object lock
     uint8_t ob_gc_bits;         // gc-related state