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gh-112529: Remove PyGC_Head from object pre-header in free-threaded build (#114564)

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* gh-112529: Remove PyGC_Head from object pre-header in free-threaded build

This avoids allocating space for PyGC_Head in the free-threaded build.
The GC implementation for free-threaded CPython does not use the
PyGC_Head structure.

 * The trashcan mechanism uses the `ob_tid` field instead of `_gc_prev`
   in the free-threaded build.
 * The GDB libpython.py file now determines the offset of the managed
   dict field based on whether the running process is a free-threaded
   build. Those are identified by the `ob_ref_local` field in PyObject.
 * Fixes `_PySys_GetSizeOf()` which incorrectly incorrectly included the
   size of `PyGC_Head` in the size of static `PyTypeObject`.
This commit is contained in:
Sam Gross 2024-02-01 15:29:19 -05:00 • committed by GitHub
parent 500ede0117
commit 587d480203
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GPG key ID: B5690EEEBB952194
9 changed files with 86 additions and 26 deletions
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27
Include/internal/pycore_object.h
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@ -315,16 +315,15 @@ static inline void _PyObject_GC_TRACK(
_PyObject_ASSERT_FROM(op, !_PyObject_GC_IS_TRACKED(op),
"object already tracked by the garbage collector",
filename, lineno, __func__);
#ifdef Py_GIL_DISABLED
op->ob_gc_bits |= _PyGC_BITS_TRACKED;
#else
PyGC_Head *gc = _Py_AS_GC(op);
_PyObject_ASSERT_FROM(op,
(gc->_gc_prev & _PyGC_PREV_MASK_COLLECTING) == 0,
"object is in generation which is garbage collected",
filename, lineno, __func__);
#ifdef Py_GIL_DISABLED
op->ob_gc_bits |= _PyGC_BITS_TRACKED;
#else
PyInterpreterState *interp = _PyInterpreterState_GET();
PyGC_Head *generation0 = interp->gc.generation0;
PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
@ -594,8 +593,12 @@ _PyObject_IS_GC(PyObject *obj)
static inline size_t
_PyType_PreHeaderSize(PyTypeObject *tp)
{
return _PyType_IS_GC(tp) * sizeof(PyGC_Head) +
_PyType_HasFeature(tp, Py_TPFLAGS_PREHEADER) * 2 * sizeof(PyObject *);
return (
#ifndef Py_GIL_DISABLED
_PyType_IS_GC(tp) * sizeof(PyGC_Head) +
#endif
_PyType_HasFeature(tp, Py_TPFLAGS_PREHEADER) * 2 * sizeof(PyObject *)
);
}
void _PyObject_GC_Link(PyObject *op);
@ -625,6 +628,14 @@ extern int _PyObject_StoreInstanceAttribute(PyObject *obj, PyDictValues *values,
PyObject * _PyObject_GetInstanceAttribute(PyObject *obj, PyDictValues *values,
PyObject *name);
#ifdef Py_GIL_DISABLED
# define MANAGED_DICT_OFFSET (((Py_ssize_t)sizeof(PyObject *))*-1)
# define MANAGED_WEAKREF_OFFSET (((Py_ssize_t)sizeof(PyObject *))*-2)
#else
# define MANAGED_DICT_OFFSET (((Py_ssize_t)sizeof(PyObject *))*-3)
# define MANAGED_WEAKREF_OFFSET (((Py_ssize_t)sizeof(PyObject *))*-4)
#endif
typedef union {
PyObject *dict;
/* Use a char* to generate a warning if directly assigning a PyDictValues */
@ -635,7 +646,7 @@ static inline PyDictOrValues *
_PyObject_DictOrValuesPointer(PyObject *obj)
{
assert(Py_TYPE(obj)->tp_flags & Py_TPFLAGS_MANAGED_DICT);
return ((PyDictOrValues *)obj)-3;
return (PyDictOrValues *)((char *)obj + MANAGED_DICT_OFFSET);
}
static inline int
@ -664,8 +675,6 @@ _PyDictOrValues_SetValues(PyDictOrValues *ptr, PyDictValues *values)
ptr->values = ((char *)values) - 1;
}
#define MANAGED_WEAKREF_OFFSET (((Py_ssize_t)sizeof(PyObject *))*-4)
extern PyObject ** _PyObject_ComputedDictPointer(PyObject *);
extern void _PyObject_FreeInstanceAttributes(PyObject *obj);
extern int _PyObject_IsInstanceDictEmpty(PyObject *);