libgc

libgc is a garbage collection library for Rust. It can be used as a standalone library, but it is highly recommended that programs are compiled with the companion rustc fork, which offers language support for much better performance.

libgc is in active development - there will be bugs!

Example

libgc provides a smart pointer, Gc<T>, which can be used to make garbage collected values. An example, with the necessary global allocator setup, looks as follows:

use libgc::{Gc, GcAllocator};

#[global_allocator]
static ALLOCATOR: GcAllocator = GcAllocator;


fn foo() -> Gc<Vec<usize>> {
    let foo = Gc::new(vec![1,2,3]);  
    let a = foo; // GC pointers are copyable
    let b = foo;

    foo 
}

fn main() {
    let gc = foo();
}

Overview

If you want to write code with shared ownership in Rust, Rc makes this possible. Unfortunately, managing cyclic data structures with reference counting is hard: weak pointers are needed to break strong cycles and thus prevent memory leaks. In programs where these sorts of structures are common, garbage collection is a natural fit.

libgc is not a replacement to the single ownership model - it is intended to complement it by providing a garbage collected alternative for values which might be too difficult to manage with Rc. Values must opt-in to using garbage collection with the Gc::new(x) constructor. This tells libgc to heap allocate x, and GC it for you when you're done with it. Gc can be thought of as a special Box type, where x's lifetime is unknown at compile-time. Periodically, the garbage collector will interrupt the program (known as "stopping the world") to see which Gc values are still in use, and drop those which aren't.

Garbage collection involves scanning parts of the stack and heap to look for live references to Gc values. This means that libgc must be aware of all heap allocated values, even those which aren't Gc. To do this, libgc has its own allocator, GcAllocator, which must be set as the global allocator when using libgc.

use libgc::GcAllocator;

#[global_allocator]
static ALLOCATOR: GcAllocator = GcAllocator;

Finalization

A Gc can be used to manage values which have a drop method. Like all tracing garbage collectors, libgc can not provide any guarantees about exactly when a 'dead' value is dropped. Instead, once libgc has determined that a value is unreachable, its drop method is added to a drop queue, which is ran on a parallel finalization thread at some point in the future. The order of finalization is intentionally undefined to allow libgc to run drop methods on values which contain cycles of Gc.

⚠️ You must not dereference a field of type Gc<T> inside Drop::drop. Doing so is unsound and can lead to dangling pointers. TODO: Add a lint for this and explain why in further details.

Implementation

libgc is implemented using the Boehm-Demers-Weiser collector. It is a conservative, stop-the-world, parallel, mark-sweep collector.

TODO: Expand

Known Issues

• Single-threaded support only.
• No Drop Lint to prevent unsound dereferencing of Gc typed fields.

Using libgc with rustgc

There are two repositories which make up the gc infrastructure:

• libgc the main library which provides the Gc<T> smart pointer and its API.
• rustgc a fork of rustc with GC-aware optimisations. This can be used to compile user programs which use libgc, giving them better GC performance. Use of rustgc is not mandated, but it enables further optimisations for programs which use libgc.

This seperation between libgc and rustgc exists so that a stripped-down form of garbage collection can be used without compiler support.

TODO: Explain rustgc and it's optimizations.