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.
See the documentation.
- Per-object pinning (with
mmccollectors) - Finalization (supporting resuscitation)
- Ephemerons (except on
bdw, which has a polyfill) - Conservative roots (optionally with
mmcor always withbdw) - Precise roots (optionally with
mmcor always withsemi/pcc) - Precise embedder-parameterized heap tracing (except with
bdw) - Conservative heap tracing (optionally with
mmc, always withbdw) - 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
- 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 ofmmc). - benchmarks/: Benchmarks. A work in progress.
- test/: A dusty attic of minimal testing.
As of August 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 future, Whippet's internal collectors need to add support for MacOS and Windows, to fix some usage of the C11 atomics API that clang doesn't like, to test on AArch64, and
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!
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.
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
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distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
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The above copyright notice and this permission notice shall be included
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
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Note that some benchmarks have other licenses; see
benchmarks/README.md for more.