People frequently ask me for status updates on generational GC, and I usually say I’ll tell them when something notable happens. Well, something notable just happened: exact rooting landed.

What is exact rooting? In order to support generational and/or compacting GC, you need to be able to move GC-allocated things such as objects around. This means you can’t have raw C++ pointers to any objects that might move; instead, you need some kind of indirect pointer that can be updated when necessary.

Unfortunately, both the JS engine and Gecko have a lot of pointers to GC-allocated things. The process of checking and converting them has been the main part of a task called “exact rooting”, and that’s what just finished. This has required an enormous amount of what is essentially very tedious work. Jim Blandy summarized it nicely, as follows.

I’ve never heard of a major project escaping from conservative GC once it had entered that state of sin; nor have I heard of anyone implementing a moving collector after starting with a non-moving collector. So, doing *both* is impressive. I hope it pays off big!

Major kudos to Terrence Cole, Steve Fink, Jon Coppeard, Brian Hackett, and the small army of other helpers who did this. Now that they’ve finished eating this gigantic serving of vegetables, they can move onto dessert, i.e. making the GC generational and compacting.

https://blog.mozilla.org/nnethercote/2014/01/20/a-big-step-towards-generational-and-compacting-gc/