For clarity, give the alignment in terms of _PyGC_PREV_SHIFT rather than a plain number.

Move the definition accordingly.

Author: fthain

Include/internal/pycore_gc.h

@@ -8,6 +8,9 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#define _PyGC_PREV_SHIFT           2
+#define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
+
 /* GC information is stored BEFORE the object structure. */
 typedef struct {
     // Tagged pointer to next object in the list.
@@ -18,7 +21,7 @@ typedef struct {
     // Lowest two bits are used for flags documented later.
     // Those two bits are made available by the struct's minimum alignment.
     uintptr_t _gc_prev;
-} PyGC_Head Py_ALIGNED(4);
+} PyGC_Head Py_ALIGNED(1 << _PyGC_PREV_SHIFT);
 
 #define _PyGC_Head_UNUSED PyGC_Head
 
@@ -142,9 +145,6 @@ static inline void _PyObject_GC_SET_SHARED(PyObject *op) {
 */
 #define _PyGC_NEXT_MASK_OLD_SPACE_1    1
 
-#define _PyGC_PREV_SHIFT           2
-#define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
-
 /* set for debugging information */
 #define _PyGC_DEBUG_STATS             (1<<0) /* print collection statistics */
 #define _PyGC_DEBUG_COLLECTABLE       (1<<1) /* print collectable objects */