# Integration of PyPy with host Virtual Machines

This document is based on the discussion I had with Samuele during the Duesseldorf sprint. It's not much more than random thoughts -- to be reviewed!

Terminology disclaimer: both PyPy and .NET have the concept of "wrapped" or "boxed" objects. To avoid confusion I will use "wrapping" on the PyPy side and "boxing" on the .NET side.

## General idea

The goal is to find a way to efficiently integrate the PyPy interpreter with the hosting environment such as .NET. What we would like to do includes but it's not limited to:

• calling .NET methods and instantiate .NET classes from Python
• subclass a .NET class from Python
• handle native .NET objects as transparently as possible
• automatically apply obvious Python <--> .NET conversions when crossing the borders (e.g. integers, string, etc.)

One possible solution is the "proxy" approach, in which we manually (un)wrap/(un)box all the objects when they cross the border.

### Example

public static int foo(int x) { return x}

>>>> from somewhere import foo
>>>> print foo(42)


In this case we need to take the intval field of W_IntObject, box it to .NET System.Int32, call foo using reflection, then unbox the return value and reconstruct a new (or reuse an existing one) W_IntObject.

### The other approach

The general idea to solve handle this problem is to split the "stateful" and "behavioral" parts of wrapped objects, and use already boxed values for storing the state.

This way when we cross the Python --> .NET border we can just throw away the behavioral part; when crossing .NET --> Python we have to find the correct behavioral part for that kind of boxed object and reconstruct the pair.

## Split state and behaviour in the flowgraphs

The idea is to write a graph transformation that takes an usual ootyped flowgraph and split the classes and objects we want into a stateful part and a behavioral part.

We need to introduce the new ootypesystem type Pair: it acts like a Record but it hasn't its own identity: the id of the Pair is the id of its first member.

XXX about Pair: I'm not sure this is totally right. It means that an object can change identity simply by changing the value of a field??? Maybe we could add the constraint that the "id" field can't be modified after initialization (but it's not easy to enforce).

XXX-2 about Pair: how to implement it in the backends? One possibility is to use "struct-like" types if available (as in .NET). But in this case it's hard to implement methods/functions that modify the state of the object (such as __init__, usually). The other possibility is to use a reference type (i.e., a class), but in this case there will be a gap between the RPython identity (in which two Pairs with the same state are indistinguishable) and the .NET identity (in which the two objects will have a different identity, of course).

### Step 1: RPython source code

class W_IntObject:
def __init__(self, intval):
self.intval = intval

def foo(self, x):
return self.intval + x

def bar():
x = W_IntObject(41)
return x.foo(1)


### Step 2: RTyping

Sometimes the following examples are not 100% accurate for the sake of simplicity (e.g: we directly list the type of methods instead of the ootype._meth instances that contains it).

Low level types

W_IntObject = Instance(
"W_IntObject",                   # name
ootype.OBJECT,                   # base class
{"intval": (Signed, 0)},         # attributes
{"foo": Meth([Signed], Signed)}  # methods
)


Prebuilt constants (referred by name in the flowgraphs)

W_IntObject_meta_pbc = (...)
W_IntObject.__init__ = (static method pbc - see below for the graph)


Flowgraphs

bar() {
1.    x = new(W_IntObject)
2.    oosetfield(x, "meta", W_IntObject_meta_pbc)
3.    direct_call(W_IntObject.__init__, x, 41)
4.    result = oosend("foo", x, 1)
5.    return result
}

W_IntObject.__init__(W_IntObject self, Signed intval) {
1.    oosetfield(self, "intval", intval)
}

W_IntObject.foo(W_IntObject self, Signed x) {
1.    value = oogetfield(self, "value")
3.    return result
}


### Step 3: Transformation

This step is done before the backend plays any role, but it's still driven by its need, because at this time we want a mapping that tell us what classes to split and how (i.e., which boxed value we want to use).

Let's suppose we want to map W_IntObject.intvalue to the .NET boxed System.Int32. This is possible just because W_IntObject contains only one field. Note that the "meta" field inherited from ootype.OBJECT is special-cased because we know that it will never change, so we can store it in the behaviour.

Low level types

W_IntObject_bhvr = Instance(
"W_IntObject_bhvr",
ootype.OBJECT,
{},                                               # no more fields!
{"foo": Meth([W_IntObject_pair, Signed], Signed)} # the Pair is also explicitly passed
)

W_IntObject_pair = Pair(
("value", (System.Int32, 0)),  # (name, (TYPE, default))
("behaviour", (W_IntObject_bhvr, W_IntObject_bhvr_pbc))
)


Prebuilt constants

W_IntObject_meta_pbc = (...)
W_IntObject.__init__ = (static method pbc - see below for the graph)
W_IntObject_bhvr_pbc = new(W_IntObject_bhvr); W_IntObject_bhvr_pbc.meta = W_IntObject_meta_pbc
W_IntObject_value_default = new System.Int32(0)


Flowgraphs

bar() {
1.    x = new(W_IntObject_pair) # the behaviour has been already set because
# it's the default value of the field

2.    # skipped (meta is already set in the W_IntObject_bhvr_pbc)

3.    direct_call(W_IntObject.__init__, x, 41)

4.    bhvr = oogetfield(x, "behaviour")
result = oosend("foo", bhvr, x, 1) # note that "x" is explicitly passed to foo

5.    return result
}

W_IntObject.__init__(W_IntObjectPair self, Signed value) {
1.    boxed = clibox(value)             # boxed is of type System.Int32
oosetfield(self, "value", boxed)
}

W_IntObject.foo(W_IntObject_bhvr bhvr, W_IntObject_pair self, Signed x) {
1.    boxed = oogetfield(self, "value")
value = unbox(boxed, Signed)

3.    return result
}


### Inheritance

Apply the transformation to a whole class (sub)hierarchy is a bit more complex. Basically we want to mimic the same hierarchy also on the Pairs, but we have to fight the VM limitations. In .NET for example, we can't have "covariant fields":

class Base {
public Base field;
}

class Derived: Base {
public Derived field;
}


A solution is to use only kind of Pair, whose value and behaviour type are of the most precise type that can hold all the values needed by the subclasses:

class W_Object: pass
class W_IntObject(W_Object): ...
class W_StringObject(W_Object): ...

...

W_Object_pair = Pair(System.Object, W_Object_bhvr)


Where System.Object is of course the most precise type that can hold both System.Int32 and System.String.

This means that the low level type of all the W_Object subclasses will be W_Object_pair, but it also means that we will need to insert the appropriate downcasts every time we want to access its fields. I'm not sure how much this can impact performances.

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