Whippet Garbage Collector

This repository is for development of Whippet, a new garbage collector implementation, eventually 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 fast paths (supporting JIT)
• One unified API with no-overhead abstraction: switch collectors when you like

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 September 2024, Whippet is almost feature-complete. The main missing feature is dynamic heap growth and shrinkage (#5), which should land soon.

After that, the next phase on the roadmap is support for tracing, and some performance noodling.

Once that is done, the big task is integrating Whippet into the Guile Scheme language run-time, replacing BDW-GC. Fingers crossed!

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) 2022-2024 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.