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Eliminate overhead of fetching and testing NULL attributes in STORE_ATTR specializations for new objects #144141
Description
Consider
class C:
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
C(1,2,3)This produces a trace looking something like this:
...
_CHECK_AND_ALLOCATE_OBJECT
_CREATE_INIT_FRAME
_PUSH_FRAME
# Some guards
_LOAD_FAST_BORROW_1
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
# Some more guards
_LOAD_FAST_BORROW_2
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
# Some more guards
_LOAD_FAST_BORROW_3
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
...
Each of those _STORE_ATTR_INSTANCE_VALUE reads the old value out of memory and then conditionally decrefs it.
But in this case we know that the old value was NULL so we can just overwrite it.
So we can replace this:
PyObject **value_ptr = (PyObject**)(((char *)owner_o) + offset);
PyObject *old_value = *value_ptr;
FT_ATOMIC_STORE_PTR_RELEASE(*value_ptr, PyStackRef_AsPyObjectSteal(value));
if (old_value == NULL) {
PyDictValues *values = _PyObject_InlineValues(owner_o);
Py_ssize_t index = value_ptr - values->values;
_PyDictValues_AddToInsertionOrder(values, index);
}
Py_XDECREF(old_value);with this:
PyObject **value_ptr = (PyObject**)(((char *)owner_o) + offset);
FT_ATOMIC_STORE_PTR_RELEASE(*value_ptr, PyStackRef_AsPyObjectSteal(value));
PyDictValues *values = _PyObject_InlineValues(owner_o);
Py_ssize_t index = value_ptr - values->values;
_PyDictValues_AddToInsertionOrder(values, index);On Aarch64, this reduces the number of machine instructions from 48 to 26.
The same reasoning also applies to _STORE_ATTR_SLOT where it reduces the number of machine instructions from 32 to 14.
See also #134584
We can probably remove some of those guards as well, but that's a separate issue.
Activity
markshannon commentedon Jan 22, 2026
As a mechanism for achieving this we can add a new jit optimizer type that tracks the
NULLness of its attributes.It would need to track when it was last modified, to check for escapes, and one bit of information for each slot.
cocolato commentedon Jan 22, 2026
If needed, I can try to implement it.
Fidget-Spinner commentedon Jan 22, 2026
@cocolato your
__slots__PR can already do this. It needs a bit of adjusting to track escapes, but tracking the types of something in slots is a superset of tracking the nullness of the slot.@markshannon let's use #144122 to achieve this. I already discussed with @cocolato on the issue that we should do this. It's a useful thing to have.
__slots__object and its property in the Tier 2 optimizer #144145 as a duplicate of this issue