Specify minimum PyGC_Head alignment to fix build failure

As documented in InternalDocs/garbage_collector.md, the garbage collector
stores flags in the least significant two bits of the _gc_prev pointer
in struct PyGC_Head. Consequently, this pointer is only capable of storing
a location that's aligned to a 4-byte boundary.

This alignment requirement is documented but it's not actually encoded.
The code works when python happens to run on a platform that has a large
minimum alignment, but fails otherwise.

Since we know that 2 bits are needed, we also know the minimum alignment
that's needed. Let's make that explicit, so the compiler can then make
sure those 2 bits are available.

This patch fixes a segfault in _bootstrap_python. It also clarifies the
code by explicitly stating the actual requirement, in terms of
_PyGC_PREV_SHIFT.

This bug was previously investigated by Adrian Glaubitz here:
https://lists.debian.org/debian-68k/2024/11/msg00020.html
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600

Although Adrian's patch isn't really correct (because natural alignment
is not needed), he deserves full credit for finding the root cause.

Author: fthain

Include/internal/pycore_gc.h

@@ -133,7 +133,6 @@ 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 */

Include/internal/pycore_interp_structs.h

@@ -155,6 +155,8 @@ struct atexit_state {
 
 /****** Garbage collector **********/
 
+#define _PyGC_PREV_SHIFT 2
+
 /* GC information is stored BEFORE the object structure. */
 typedef struct {
     // Tagged pointer to next object in the list.
@@ -163,8 +165,9 @@ typedef struct {
 
     // Tagged pointer to previous object in the list.
     // Lowest two bits are used for flags documented later.
+    // Those bits are made available by the struct's minimum alignment.
     uintptr_t _gc_prev;
-} PyGC_Head;
+} PyGC_Head Py_ALIGNED(1 << _PyGC_PREV_SHIFT);
 
 #define _PyGC_Head_UNUSED PyGC_Head
 

Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst

@@ -0,0 +1 @@
+Fix crash when building on Linux/m68k.