Grouped and arena allocations work really well in Zig. For awhile, we tried to use this pattern almost everywhere in Bun but it gets really tricky when there’s some GC-managed memory and you want to free things incrementally to reduce RSS. Also, using arenas for arrays that grow wastes memory a lot since it keeps every previous version around (mimalloc arenas are slightly better for this)
Grouped allocations works especially well in parsers & ASTs where the lifetime is very bounded. Since the Rust rewrite, we still use arenas for Bun’s parsers and the bundler but not a ton elsewhere.
I have learned so much reading Andrew’s code and as I said in the original post: Bun would never have happened without Zig.
> The post claims they were fuzzing their Zig code, while during our calls the whole Bun team told us that they were not fuzzing anything. This appears to be an outright fabrication.
This PR is an implementation of the design from https://webkit.org/blog/7846/concurrent-javascript-it-can-wo.... I think it would be really cool if JavaScript had true shared object multi-threading without compromises (SharedArrayBuffer, postMessage are not that). If we had both threads and structs, it’s likely the TypeScript compiler would never have needed to be rewritten in Go.
The title should be changed to clarify that it’s a PR to Bun’s JavaScriptCore fork and not the upstream WebKit.
This PR is scarier to merge than Bun’s Rust rewrite PR. There are a good number of benchmarks/stress tests, unit tests, and also TSAN runs and security scanner runs, but this is a more complex change than the Rust rewrite (yes, really). I’m also worried about syncing with upstream - today the “fork” is mostly a bunch of patches, but with this PR, changes to the JIT need to be reviewed for behavior when multiple threads are in use. Our best bet for this to move forward is figuring out a way for some constrained version to be upstreamed into WebKit proper, if that makes sense and if they’re interested.
Dynamic workflows, in my experience, make Claude more effective at complex long-running tasks. They help precisely with getting Claude to do the task correctly.
It feels more like a bespoke build system for the specific task/project than prompting a freeform chat.
`PathString` worked the exact same way in our Zig code, with less visibility from the compiler & type system. And yes, it will be refactored heavily (or deleted overall) in the next week or so.
Not accurate. Bun is a batteries-included JavaScript & CSS transpiler (parser), minifier, bundler, npm-like package manager, Jest-like test runner, as well as runtime APIs like a builtin Postgres, MySQL and Redis client. This is naturally a ton of code.
> it's basically solving the ,,tests not pass'' problem by changing the tests themselves.
False.
0 test files were deleted. 0 pre-existing tests were skipped, todo’d, or had assertions removed. 5 new tests were added in test.skip/test.todo state to track known not-yet-fixed bugs in the port that lacked test coverage before.
The merge changed 28 test files in total.
+1,312 lines
−141 lines
Most of that +1,312 is new tests.
The depth-of-recursion tests for TOML/JSONC parsers went from 25_000 -> 200_000 because Rust’s smaller stack frames (LLVM lifetime annotations let the optimizer reuse stack slots) mean 25k levels no longer reaches the 18 MB stack on Windows.
Still writing the blog post about this. Will share more details.
For where this is coming from, skim the bugfixes in the Bun v1.3.14 and earlier release notes. Rust won’t catch all of these - leaks from holding references too long and anything that re-enters across the JS boundary are still on us. But a large % of that list is use-after-free, double-free, and forgot-to-free-on-error-path, which become compile errors or automatic cleanup.
cargo check reported over 16,000 compiler errors when I wrote that message. It could not print a version number or run JavaScript. I didn’t expect it to work this quickly and I also didn’t expect the performance to be as competitive. There’ll be a blog post with more details.
This whole thread is an overreaction. 302 comments about code that does not work. We haven’t committed to rewriting. There’s a very high chance all this code gets thrown out completely.
I’m curious to see what a working version of this looks, what it feels like, how it performs and if/how hard it’d be to get it to pass Bun’s test suite and be maintainable. I’d like to be able to compare a viable Rust version and a Zig version side by side.
I work on Bun, and this post is confusing to me. Me personally and the Bun team continues to dogfood & make Bun better everyday. Our development pace has only gotten faster. Bun's stability has improved significantly since joining Anthropic.
Here are some things shipping in the next version of Bun:
- 17 MB smaller Windows x64 binaries [0]
- 8 MB smaller Linux binaries [1]
- `--no-orphans` CLI flag to recursively kill any lingering processes spawned [3]
- SSL context caching for client TCP & unix sockets, which significantly reduces memory usage for database clients like Mongoose/MongoDB [4]
- Experimental HTTP/3 & HTTP/2 client in fetch [5]
- Experimental HTTP/3 support in Bun.serve() [6]
- Bun.Image, a builtin image processing library [7]
(Along with several reliability improvements to node:fs, Worker, BroadcastChannel, and MessagePort)
The Anthropic acquisition also means Bun no longer needs to become a revenue-generating business. We are very incentivized to make Bun better because Claude Code depends on it, and so many software engineers depend on Claude Code to help get their work done.
You can find a comparison with `bun build` on Bun's homepage. It hasn't been updated in a little while, but I haven't heard that the relative difference between Bun and Rolldown has changed much in the time since (both have gotten faster).
Bundler Version Time
─────────────────────────────────────────────────────────
Bun v1.3.0 269.1 ms
Rolldown v1.0.0-beta.42 494.9 ms
esbuild v0.25.10 571.9 ms
Farm v1.0.5 1,608.0 ms
Rspack v1.5.8 2,137.0 ms
https://bun.com
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