Calls across stack chunks perform badly

[markshannon]

The Python stack is composed of a series of chunks of memory. These chunks are large, so we cross the boundaries infrequently and assume that it will never be on the fast path.

Unfortunately, it is possible to cross the boundary repeatedly if looping deep in the stack.

We should adjust the stack when crossing the boundary, to avoid doing so repeatedly.

Possible options include:

1. Move the caller to the new chunk, so repeated calls do not cross the boundary.
2. Instead of using a chunked stack, double the size of the stack and copy the old stack.

Both of these assume that we can move the frame. This is only possible if there are no pointers into the frame.
If a frames calls a native function taking an array of arguments, then there will be pointers into the frame.
So if we do any moving, we need to check for these pointers.

While we cannot move the frame, we can copy it, as long as the original remains. This is a bit wasteful, but shouldn't happen too often or waste too much space.

[colesbury]

If a frames calls a native function taking an array of arguments, then there will be pointers into the frame.

Is this still true? Previously, PyObject_Vectorcall used the localsplus array from the frame, but I think it now makes a copy to a local variable via STACKREFS_TO_PYOBJECTS.

cpython/Python/bytecodes.c

Lines 3748 to 3757 in 4172644

	STACKREFS_TO_PYOBJECTS(arguments, total_args, args_o);
	if (CONVERSION_FAILED(args_o)) {
	DECREF_INPUTS();
	ERROR_IF(true);
	}
	PyObject *res_o = PyObject_Vectorcall(
	callable_o, args_o,
	total_args | PY_VECTORCALL_ARGUMENTS_OFFSET,
	NULL);
	STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);

[markshannon]

Is this still true?

It isn't currently true, but it might become true again with PyNI or similar, and I don't want to prevent that use case.
Plus, we often want to pass pointers to the stack to instrumentation and debug functions.