I reworked abstract.c to prepare work for this issue:

* change 455169e87bb3: Add _PyObject_CallFunctionVa() helper
* change 6e748eb79038: Add _PyObject_VaCallFunctionObjArgs() private function
* change 71876e4abce4: Add _PY_FASTCALL_SMALL_STACK constant


I wrote a function _testcapi to measure the consumption of the C code. I was surprised by the results: calling PyObject_CallFunctionObjArgs(func, arg1, arg2, NULL) consumes 560 bytes! I measured on a Python compiled in release mode.

Attached less_stack.patch rewrites _PyObject_VaCallFunctionObjArgs(), it reduces the stack consumption from 560 bytes to 384 bytes (-176 bytes!).

Changes:

* Remove "va_list countva" variable: the va_list variable itself, va_copy(), etc. consume stack memory. First I tried to move code to a subfunction, it helps. With my patch, it's even simpler.

* Reduce _PY_FASTCALL_SMALL_STACK from 5 to 3. Stack usage is not directly _PY_FASTCALL_SMALL_STACK*sizeof(PyObject*), it's much more, probably because of complex memory alignement rules.

* Use Py_LOCAL_INLINE(). It seems like depending on the size of the object_vacall() function body, the function is inlined or not. If it's not inlined, the stack usage increases from 384 bytes to 544 bytes!? Use Py_LOCAL_INLINE() to force inlining.


Effect of _PY_FASTCALL_SMALL_STACK:

* 1: 368 bytes
* 2: 384 bytes
* 3: 384 bytes -- value chosen in my patch
* 4: 400 bytes
* 5: 416 bytes