Simplify PyStackRef_FromPyObjectSteal

[colesbury]

Feature or enhancement

Currently, PyStackRef_FromPyObjectSteal checks if the object is immortal in order to set the Py_TAG_DEFERRED bit.

cpython/Include/internal/pycore_stackref.h

Lines 96 to 105 in 29cbcbd

	static inline _PyStackRef
	_PyStackRef_FromPyObjectSteal(PyObject *obj)
	{
	assert(obj != NULL);
	// Make sure we don't take an already tagged value.
	assert(((uintptr_t)obj & Py_TAG_BITS) == 0);
	unsigned int tag = _Py_IsImmortal(obj) ? (Py_TAG_DEFERRED) : Py_TAG_PTR;
	return ((_PyStackRef){.bits = ((uintptr_t)(obj)) | tag});
	}
	# define PyStackRef_FromPyObjectSteal(obj) _PyStackRef_FromPyObjectSteal(_PyObject_CAST(obj))

This check isn't necessary and has a performance cost that's not made up for by the slightly faster PyStackRef_CLOSE() calls or PyStackRef_Is() checks.

We should simplify PyStackRef_FromPyObjectSteal so that it creates _PyStackRef directly from the PyObject * without setting any tag bits.

Linked PRs

• gh-127022: Simplify PyStackRef_FromPyObjectSteal #127024
• gh-127022: Remove _PyEvalFramePushAndInit_UnTagged #127168

[colesbury]

I think this will also allow us to simplify or remove _PyEvalFramePushAndInit_UnTagged.