whippet: towards a new local maximum -- wingolog
Friends, you might have noted, but over the last year or so I really caught the GC bug. Today's post sums up that year, in the form of a talk I gave yesterday at FOSDEM. It's long! If you prefer video, you can have a look instead to the at the FOSDEM event page.
So the context for the whole effort is that Guile has this part of its implementation which is in C. It also exposes a lot of that implementation to users as an API.
Let's start by considering the simple cons call above. In a garbage-collected environment, the GC is responsible for reclaiming unused memory. How does the GC know that the result of a scm_cons call is in use?
Generally speaking there are two main strategies for automatic memory management. One is reference counting: you associate a count with an object, incremented once for each referrer; in this case, the stack would hold a reference to x. When removing the reference, you decrement the count, and if it goes to 0 the object is unused and can be freed.
We GC people used to laugh at reference-counting as a memory management solution because it over-approximates the live object set in the presence of cycles, but it would seem that refcounting is coming back. Anyway, this isn't what Guile does, not right now anyway.
