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# Virtualizables

Note: this document does not have a proper introduction as to how to understand the basics. We should write some. If you happen to be here and you’re missing context, feel free to pester us on IRC.

## Problem description

The JIT is very good at making sure some objects are never allocated if they don’t escape from the trace. Such objects are called `virtuals`. However, if we’re dealing with frames, virtuals are often not good enough. Frames can escape and they can also be allocated already at the moment we enter the JIT. In such cases we need some extra object that can still be optimized away, despite existing on the heap.

## Solution

We introduce virtualizables. They’re objects that exist on the heap, but their fields are not always in sync with whatever happens in the assembler. One example is that virtualizable fields can store virtual objects without forcing them. This is very useful for frames. Declaring an object to be virtualizable works like this:

    class Frame(object):
       _virtualizable_ = ['locals[*]', 'stackdepth']

And we use them in `JitDriver` like this:

    jitdriver = JitDriver(greens=[], reds=['frame'], virtualizables=['frame'])

This declaration means that `stackdepth` is a virtualizable field, while `locals` is a virtualizable array (a list stored on a virtualizable). There are various rules about using virtualizables, especially using virtualizable arrays that can be very confusing. Those will usually end up with a compile-time error (as opposed to strange behavior). The rules are:

- Each array access must be with a known positive index that cannot raise an `IndexError`. Using `no = jit.hint(no, promote=True)` might be useful to get a constant-number access. This is only safe if the index is actually constant or changing rarely within the context of the user’s code.

- If you initialize a new virtualizable in the JIT, it has to be done like this (for example if we’re in `Frame.__init__`):

      self = hint(self, access_directly=True, fresh_virtualizable=True)

  that way you can populate the fields directly.

- If you use virtualizable outside of the JIT – it’s very expensive and sometimes aborts tracing. Consider it carefully as to how do it only for debugging purposes and not every time (e.g. `sys._getframe` call).

- If you have something equivalent of a Python generator, where the virtualizable survives for longer, you want to force it before returning. It’s better to do it that way than by an external call some time later. It’s done using `jit.hint(frame, force_virtualizable=True)`

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