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gh-127545: Specify minimum PyGC_Head and PyObject alignment to fix build failure #135016

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gh-127545: Specify minimum PyGC_Head and PyObject alignment to fix build failure #135016

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2 changes: 1 addition & 1 deletion Include/internal/pycore_gc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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 2
#define _PyGC_PREV_SHIFT _PyObject_ALIGNMENT_SHIFT
#define _PyGC_PREV_MASK (((uintptr_t) -1) << _PyGC_PREV_SHIFT)

/* set for debugging information */
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3 changes: 2 additions & 1 deletion Include/internal/pycore_interp_structs.h
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Expand Up @@ -163,8 +163,9 @@ 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 << _PyObject_ALIGNMENT_SHIFT);
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Note that Py_ALIGNED is only for GCC. I think we want the _Py_ALIGN_AS macro, but that seems to only be on the free-threaded build for some reason. See capi-workgroup/decisions#61.


#define _PyGC_Head_UNUSED PyGC_Head

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9 changes: 7 additions & 2 deletions Include/object.h
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Expand Up @@ -101,6 +101,11 @@ whose size is determined when the object is allocated.
#define PyObject_VAR_HEAD PyVarObject ob_base;
#define Py_INVALID_SIZE (Py_ssize_t)-1

/* PyObjects are given a minimum alignment so that the least significant bits
* of an object pointer become available for other purposes.
*/
#define _PyObject_ALIGNMENT_SHIFT 2

/* 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
* by hand. Similarly every pointer to a variable-size Python object can,
Expand Down Expand Up @@ -142,7 +147,7 @@ struct _object {
#endif

PyTypeObject *ob_type;
};
} Py_ALIGNED(1 << _PyObject_ALIGNMENT_SHIFT);
#else
// Objects that are not owned by any thread use a thread id (tid) of zero.
// This includes both immortal objects and objects whose reference count
Expand All @@ -160,7 +165,7 @@ struct _object {
uint32_t ob_ref_local; // local reference count
Py_ssize_t ob_ref_shared; // shared (atomic) reference count
PyTypeObject *ob_type;
};
} Py_ALIGNED(1 << _PyObject_ALIGNMENT_SHIFT);
#endif

/* Cast argument to PyObject* type. */
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst
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@@ -0,0 +1 @@
Fix crash when building on Linux/m68k.
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