Building a Copying GC for the Plush Programming Language

Summary

Building a Copying GC for the Plush Programming Language November 29th, 2025 This is the fourth post in a series about Plush, a dynamically-typed programming language I've been building in my spare time. The language draws inspiration from Lox/JavaScript, and I created it as a platform to experiment and have fun with sound and graphics programming. It's very easy to just open a window and draw some pixels with very little boilerplate. One of the key features of this language is that it has actor-based parallelism, which makes it relatively easy, even for beginners, to write multithreaded programs without having to think about locks and memory safety. I took some care to implement the Virtual Machine (VM) in such a way that internally, there is no global VM lock, meaning that independent actors only need to synchronize with each other when they are sending and receiving messages, but otherwise, they can execute fully independently. One of the trickier aspects of the Plush VM design is that this is a garbage collected language, and actors can send messages (meaning heap-allocated objects) to one another. So, from the beginning, there was a design question of how I could make it so that these messages, which can be entire subgraphs of objects with cycles in them, could be sent from one actor to another, with dead objects being garbage-collected, and at the same time keep all of this efficient. In an actor-based system, you don't want two actors holding a mutable reference to the same object. That means if you send an object from one actor to another, you either need to copy it, or you need to freeze the object (make it immutable). I wanted to avoid needing to freeze objects before sending messages because I felt it added complexity to the system and would mean the programmer would need to keep track of what is and isn't frozen. If you want to send a subgraph of objects to another actor, but you want to keep a local copy, then you first need to deepcopy these objects, a...