Micro VM interface changes precipitated by having C as a client

Created by: eliotmoss

While there will be a bunch of carefully worked out details that need to follow, I thought I would get the ball rolling with a quick summary / highlights of the Modula-3 approach. (See https://www.cs.purdue.edu/homes/hosking/m3/reference/m3.html for a Modula-3 reference.)

First, M3 has ordinary traced references and also untraced ones. REF T is the type of a traced reference to a T, while UNTRACED REF T is the type of an untraced one. Traced vs. untraced is a concept distinct from safe vs. unsafe. Safe use of traced and untraced keep them segregated. Untraced objects come from an explicitly managed storage area. You can use NEW on an untraced reference type to allocate in such an area.

Certain regions of code (interfaces or modules -- probably a bundle is the corresponding thing for the Micro VM) can be marked UNSAFE. Unsafe code can do additional things. These are ones that I remember:

• Cast an expression to an arbitrary type (of the same size in bits) - this clearly allows interconverting between traced and untraced pointers, but also other things; this is called LOOPHOLE(e, T).
• Address arithmetic (Digression: In M3 REFANY is the type of a traced reference to any object, which in safe code requires a dynamically type-checked downcast to turn into a REF T and do more interesting things. The corresponding untraced type is ADDRESS, and it has arithmetic operators.)
• Up/down cast without checking
• Free an untraced reference's referent (the function is called DISPOSE) Safe code must use traced/untraced only is specific ways, described in the language reference.

We may need to add to this sort of design to allow short unsafe code regions to manipulate ordinary (traced) objects safely in the presence of (say) concurrent GC. Also, an untraced version of iref would make sense for the Micro VM.