servo-quickjs

This is an experimental, AI agent-driven experiment of using quickjs instead of spidermonkey in servo. The main goals are to reduce the binary size and memory usage of servo. Assume basically everything in this repo, besides these first sentences are written by AI.

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QuickJS as an alternative JavaScript engine for Servo.

The mozjs crate in this repository implements the API of Servo's mozjs crate, the SpiderMonkey bindings, on QuickJS. Servo switches engines with a Cargo [patch], without changes to its Cargo.toml.

Status: experimental. Servo runs on it with its js-quickjs feature, which is not in upstream Servo. On the WPT directories dom, WebIDL, html/webappapis, html/semantics/scripting-1 and encoding, the results equal SpiderMonkey's, apart from 7 subtests that pass only on QuickJS.

Crates

• servo-quickjs: Fabrice Bellard's QuickJS (upstream commit in servo-quickjs/quickjs/UPSTREAM), vendored with local patches, and pregenerated bindings.
• mozjs: the mozjs 0.26.7 API on QuickJS. Its version must equal the mozjs version Servo pins.
• gen-bindings: regenerates the bindings of servo-quickjs (just gen-bindings).
• fuzz: cargo-fuzz targets.

docs/design.md describes how SpiderMonkey's API maps onto QuickJS, and lists the QuickJS patches.

Building Servo with it

cargo build -p servoshell --features js-quickjs \
    --config 'patch.crates-io.mozjs.git="https://github.com/openharmony-rs/servo-quickjs"'

main implements the mozjs version that servo-ohos's ohos-main pins. The last commit for an older version is tagged mozjs-<version> (e.g. mozjs-0.23.1); add --config 'patch.crates-io.mozjs.tag="mozjs-0.23.1"' to build a Servo that pins it.

Use patch.crates-io.mozjs.path="/path/to/servo-quickjs/mozjs" for a local checkout. mach cannot pass --config; put the [patch.crates-io] entry in Servo's .cargo/config.toml instead.

Differences from SpiderMonkey

• No JIT, WebAssembly, devtools debugger or SharedArrayBuffer.
• Intl is a subset implemented over the OS's ICU on OpenHarmony and glibc Linux, and absent elsewhere (see docs/design.md). On Linux it needs the system ICU, 65 or later (libicu-dev), unless the intl feature is disabled.
• QuickJS frees objects by reference counting and collects cycles. Servo's tracing must report every reference exactly once; QJS_VERIFY_TRACING=1 checks that at run time.

Development

HACKING.md explains how SpiderMonkey functions are implemented. The justfile has the common tasks:

• just test, and just test-verify with the tracing verifier;
• just test-asan: the tests under ASan and LSan (nightly);
• just fuzz <target> and just fuzz-smoke;
• just check-bindings: fails if the bindings are out of date;
• just servo, just run <url> and just wpt <log> <tests> with a Servo checkout in SERVO_DIR.

QJS_GC_STATS prints live objects, roots and retaining paths after each GC (see mozjs/src/engine/runtime.rs).

A build with QJS_POOL_STATS=1 records which API functions put values in the autorelease pool, and whether a root took them before it drained; tools/pool_stats.py summarizes the output (see mozjs/src/engine/pool_stats.rs, just test-pool-stats).

License

• mozjs, gen-bindings, fuzz and tools: MPL-2.0. Much of mozjs is copied from servo/mozjs, under the same license.
• servo-quickjs: MIT, as QuickJS itself (servo-quickjs/quickjs/LICENSE).
• mozjs/src/engine/intl/dtpg.rs (a port of ICU's date pattern generator) and mozjs/src/engine/intl/patterns.rs (CLDR data): Unicode-3.0.