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PEP 590: Clarify the use of PY_VECTORCALL_ARGUMENTS_OFFSET. (GH-980)

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Mark Shannon 2019-04-10 12:22:42 +01:00 committed by Petr Viktorin
parent c8872842e8
commit 349b6d6b8b
1 changed files with 10 additions and 3 deletions
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pep-0590.rst
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@ -88,10 +88,17 @@ The call takes the form ``((vectorcall)(((char *)o)+offset))(o, n, args, kwnames
``offset`` is ``Py_TYPE(o)->tp_vectorcall_offset``.
The caller is responsible for creating the ``kwnames`` tuple and ensuring that there are no duplicates in it.
``n`` is the number of postional arguments plus ``PY_VECTORCALL_ARGUMENTS_OFFSET`` if the argument vector pointer points to argument 1 in the
allocated vector.
allocated vector and the callee is allowed to mutate the contents of the ``args`` vector.
``n = number_postional_args | (offset ? PY_VECTORCALL_ARGUMENTS_OFFSET: 0))``.
Example of how ``PY_VECTORCALL_ARGUMENTS_OFFSET`` is used by a callee to avoid allocation [3]_
PY_VECTORCALL_ARGUMENTS_OFFSET
------------------------------
When a caller sets the ``PY_VECTORCALL_ARGUMENTS_OFFSET`` it is indicating that the ``args`` pointer points to item 1 (counting from 0) of the allocated array
and that the contents of the allocated array can be safely mutated by the callee. The callee still needs to make sure that the reference counts of any objects
in the array remain correct.
Example of how ``PY_VECTORCALL_ARGUMENTS_OFFSET`` is used by a callee is safely used to avoid allocation [3]_
Whenever they can do so cheaply (without allocation) callers are encouraged to offset the arguments.
Doing so will allow callables such as bound methods to make their onward calls cheaply.
@ -221,7 +228,7 @@ References
.. [2] tp_call/PyObject_Call calling convention
https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_call
.. [3] Using PY_VECTORCALL_ARGUMENTS_OFFSET in callee
https://github.com/markshannon/cpython/blob/60e87d3a1978b38da40e7f9b6065e4b6a8f31b7f/Objects/classobject.c#L53
https://github.com/markshannon/cpython/blob/vectorcall-minimal/Objects/classobject.c#L53
.. [4] Argument Clinic
https://docs.python.org/3/howto/clinic.html
.. [5] PEP 576