# Rawrefcount and the GC

## GC Interface

“PyObject” is a raw structure with at least two fields, ob_refcnt and ob_pypy_link. The ob_refcnt is the reference counter as used on CPython. If the PyObject structure is linked to a live PyPy object, its current address is stored in ob_pypy_link and ob_refcnt is bumped by either the constant REFCNT_FROM_PYPY, or the constant REFCNT_FROM_PYPY_LIGHT (== REFCNT_FROM_PYPY + SOME_HUGE_VALUE) (to mean “light finalizer”).

Most PyPy objects exist outside cpyext, and conversely in cpyext it is possible that a lot of PyObjects exist without being seen by the rest of PyPy. At the interface, however, we can “link” a PyPy object and a PyObject. There are two kinds of link:

rawrefcount.create_link_pypy(p, ob)

> Makes a link between an exising object gcref ‘p’ and a newly allocated PyObject structure ‘ob’. ob->ob_refcnt must be initialized to either REFCNT_FROM_PYPY, or REFCNT_FROM_PYPY_LIGHT. (The second case is an optimization: when the GC finds the PyPy object and PyObject no longer referenced, it can just free() the PyObject.)

rawrefcount.create_link_pyobj(p, ob)

> Makes a link from an existing PyObject structure ‘ob’ to a newly allocated W_CPyExtPlaceHolderObject ‘p’. You must also add REFCNT_FROM_PYPY to ob->ob_refcnt. For cases where the PyObject contains all the data, and the PyPy object is just a proxy. The W_CPyExtPlaceHolderObject should have only a field that contains the address of the PyObject, but that’s outside the scope of the GC.

rawrefcount.from_obj(p)

> If there is a link from object ‘p’ made with create_link_pypy(), returns the corresponding ‘ob’. Otherwise, returns NULL.

rawrefcount.to_obj(Class, ob)

> Returns ob->ob_pypy_link, cast to an instance of ‘Class’.

## Collection logic

Objects existing purely on the C side have ob->ob_pypy_link == 0; these are purely reference counted. On the other hand, if ob->ob_pypy_link != 0, then ob->ob_refcnt is at least REFCNT_FROM_PYPY and the object is part of a “link”.

The idea is that links whose ‘p’ is not reachable from other PyPy objects and whose ‘ob->ob_refcnt’ is REFCNT_FROM_PYPY or REFCNT_FROM_PYPY_LIGHT are the ones who die. But it is more messy because PyObjects still (usually) need to have a tp_dealloc called, and this cannot occur immediately (and can do random things like accessing other references this object points to, or resurrecting the object).

Let P = list of links created with rawrefcount.create_link_pypy() and O = list of links created with rawrefcount.create_link_pyobj(). The PyPy objects in the list O are all W_CPyExtPlaceHolderObject: all the data is in the PyObjects, and all outsite references (if any) are in C, as “PyObject *” fields.

So, during the collection we do this about P links:

> for (p, ob) in P:
>   if ob->ob_refcnt != REFCNT_FROM_PYPY

>     > and ob->ob_refcnt != REFCNT_FROM_PYPY_LIGHT:

>     mark ‘p’ as surviving, as well as all its dependencies

At the end of the collection, the P and O links are both handled like this:

> for (p, ob) in P + O:
>   if p is not surviving: # even if ‘ob’ might be surviving

>     unlink p and ob if ob->ob_refcnt == REFCNT_FROM_PYPY_LIGHT:

>     > free(ob)

>     elif ob->ob_refcnt > REFCNT_FROM_PYPY_LIGHT:
>       ob->ob_refcnt -= REFCNT_FROM_PYPY_LIGHT
>     else:

>       ob->ob_refcnt -= REFCNT_FROM_PYPY if ob->ob_refcnt == 0:

>       > invoke _Py_Dealloc(ob) later, outside the GC

## GC Implementation

We need two copies of both the P list and O list, for young or old objects. All four lists can be regular AddressLists of ‘ob’ objects.

We also need an AddressDict mapping ‘p’ to ‘ob’ for all links in the P list, and update it when PyPy objects move.

## Further notes

XXX XXX the rest is the ideal world, but as a first step, we’ll look XXX for the minimal tweaks needed to adapt the existing cpyext XXX

For objects that are opaque in CPython, like <dict>, we always create a PyPy object, and then when needed we make an empty PyObject and attach it with create_link_pypy()/REFCNT_FROM_PYPY_LIGHT.

For <int> and <float> objects, the corresponding PyObjects contain a “long” or “double” field too. We link them with create_link_pypy() and we can use REFCNT_FROM_PYPY_LIGHT too: ‘tp_dealloc’ doesn’t need to be called, and instead just calling free() is fine.

For <type> objects, we need both a PyPy and a PyObject side. These are made with create_link_pypy()/REFCNT_FROM_PYPY.

For custom PyXxxObjects allocated from the C extension module, we need create_link_pyobj().

For <str> or <unicode> objects coming from PyPy, we use create_link_pypy()/REFCNT_FROM_PYPY_LIGHT with a PyObject preallocated with the size of the string. We copy the string lazily into that area if PyString_AS_STRING() is called.

For <str>, <unicode>, <tuple> or <list> objects in the C extension module, we first allocate it as only a PyObject, which supports mutation of the data from C, like CPython. When it is exported to PyPy we could make a W_CPyExtPlaceHolderObject with create_link_pyobj().

For <tuple> objects coming from PyPy, if they are not specialized, then the PyPy side holds a regular reference to the items. Then we can allocate a PyTupleObject and store in it borrowed PyObject pointers to the items. Such a case is created with create_link_pypy()/REFCNT_FROM_PYPY_LIGHT. If it is specialized, then it doesn’t work because the items are created just-in-time on the PyPy side. In this case, the PyTupleObject needs to hold real references to the PyObject items, and we use create_link_pypy()/ REFCNT_FROM_PYPY. In all cases, we have a C array of PyObjects that we can directly return from PySequence_Fast_ITEMS, PyTuple_ITEMS, PyTuple_GetItem, and so on.

For <list> objects coming from PyPy, we can use a cpyext list strategy. The list turns into a PyListObject, as if it had been allocated from C in the first place. The special strategy can hold (only) a direct reference to the PyListObject, and we can use either create_link_pyobj() or create_link_pypy() (to be decided). PySequence_Fast_ITEMS then works for lists too, and PyList_GetItem can return a borrowed reference, and so on.