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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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21
Python/gc_free_threading.c
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@ -25,7 +25,10 @@ typedef struct _gc_runtime_state GCState;
// Automatically choose the generation that needs collecting.
#define GENERATION_AUTO (-1)
// A linked-list of objects using the `ob_tid` field as the next pointer.
// A linked list of objects using the `ob_tid` field as the next pointer.
// The linked list pointers are distinct from any real thread ids, because the
// thread ids returned by _Py_ThreadId() are also pointers to distinct objects.
// No thread will confuse its own id with a linked list pointer.
struct worklist {
uintptr_t head;
};
@ -221,7 +224,7 @@ gc_visit_heaps_lock_held(PyInterpreterState *interp, mi_block_visit_fun *visitor
struct visitor_args *arg)
{
// Offset of PyObject header from start of memory block.
Py_ssize_t offset_base = sizeof(PyGC_Head);
Py_ssize_t offset_base = 0;
if (_PyMem_DebugEnabled()) {
// The debug allocator adds two words at the beginning of each block.
offset_base += 2 * sizeof(size_t);
@ -331,8 +334,14 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area,
Py_ssize_t refcount = Py_REFCNT(op);
_PyObject_ASSERT(op, refcount >= 0);
// Add the actual refcount to ob_tid.
// We repurpose ob_tid to compute "gc_refs", the number of external
// references to the object (i.e., from outside the GC heaps). This means
// that ob_tid is no longer a valid thread id until it is restored by
// scan_heap_visitor(). Until then, we cannot use the standard reference
// counting functions or allow other threads to run Python code.
gc_maybe_init_refs(op);
// Add the actual refcount to ob_tid.
gc_add_refs(op, refcount);
// Subtract internal references from ob_tid. Objects with ob_tid > 0
@ -1508,8 +1517,10 @@ gc_alloc(PyTypeObject *tp, size_t basicsize, size_t presize)
if (mem == NULL) {
return _PyErr_NoMemory(tstate);
}
((PyObject **)mem)[0] = NULL;
((PyObject **)mem)[1] = NULL;
if (presize) {
((PyObject **)mem)[0] = NULL;
((PyObject **)mem)[1] = NULL;
}
PyObject *op = (PyObject *)(mem + presize);
_PyObject_GC_Link(op);
return op;