Whippet Garbage Collector

This repository is for development of Whippet, a new garbage collector implementation, initially targetted for use in Guile Scheme.

Whippet is an embed-only C library, designed to be copied into a program's source tree. It exposes an abstract C API for managed memory allocation, and provides a number of implementations of that API.

Documentation

See the documentation.

Features

• Per-object pinning (with mmc collectors)
• Finalization (supporting resuscitation)
• Ephemerons (except on bdw, which has a polyfill)
• Conservative roots (optionally with mmc or always with bdw)
• Precise roots (optionally with mmc or always with semi / pcc)
• Precise embedder-parameterized heap tracing (except with bdw)
• Conservative heap tracing (optionally with mmc, always with bdw)
• Parallel tracing (except semi)
• Parallel mutators (except semi)
• Inline allocation / write barrier / safepoint fast paths (supporting JIT)
• One unified API with no-overhead abstraction: switch collectors when you like
• Three policies for sizing heaps: fixed, proportional to live size, and MemBalancer

Source repository structure

• api/: The user-facing API. Also, the "embedder API"; see the manual for more.
• doc/: Documentation, such as it is.
• src/: The actual GC implementation, containing a number of collector implementations. The embedder chooses which collector to use at compile-time. See the documentation for more on the different collectors (semi, bdw, pcc, and the different flavors of mmc).
• benchmarks/: Benchmarks. A work in progress.
• test/: A dusty attic of minimal testing.

Status and roadmap

As of October 2025, Whippet is feature-complete, in the sense that it is ready to replace Guile's use of the Boehm-Demers-Weiser collector; see the wip-whippet integration branch. We hope to incrementally merge this branch of Guile over to the main Guile branch over the next months.

In the near future, Whippet's internal collectors need to add support for MacOS and Windows, and to test on AArch64.

There are all kinds of new collector designs that it would be fun to experiment with, but we are mostly in a getting-into-production phase at the moment. Feel very free to fork the repo to try out any ideas you might have!

About the name

It sounds better than WIP (work-in-progress) garbage collector, doesn't it? Also apparently a whippet is a kind of dog that is fast for its size. It would be nice if the Whippet collectors turn out to have this property.

License

Copyright (c) 2022-2025 Andy Wingo

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Note that some benchmarks have other licenses; see benchmarks/README.md for more.