Mozilla Firefox boosts JavaScript performance


Firefox users can expect improved JavaScript performance in the Firefox 83 browser, with the SpiderMonkey JavaScript Engine Warp update enabled by default.

Also known as WarpBuilder, Warp improves responsiveness and memory usage and speeds up page loading by making changes to JiT (just in time) compilers. JiT optimization is changed to rely only on the CacheIR simple linear bytecode format, specifically CacheIR data collected by base levels. The new architecture operated in the browser is also described as being more manageable and unlocking additional improvements to SpiderMonkey.

Firefox 83 was released in beta on October 20 and is scheduled for release on November 17. Warp was faster than Ion, SpiderMonkey’s previous optimization JiT, with a 20% improvement in Google Docs load time. Other JavaScript-intensive websites like Netflix and Reddit also showed improvement.

Basing Warp on CacheIR allowed for the removal of engine-wide code that was needed to track the global type inference data used by IonBuilder, resulting in speedups. Although both IonBuilder and WarpBuilder produce Ion MIR, an intermediate representation used by the JiT optimization backend, IonBuilder had a lot of unnecessary complex code in WarpBuilder. Warp can also do more non-threading work and require fewer recompilations. Plans call for continued optimizations on Warp, which is currently slower than Ion on some synthetic benchmarks.

Warp replaced the front-end – the MIR build phase – of the IonMonkey JiT. Plans call for the removal of old code and architecture, which is likely to occur in Firefox 85. Further improvements in performance and memory usage are expected as a result. Mozilla will also continue to progressively optimize the back-end of IonMonkey JiT, as Mozilla believes there is still room for improvement for JavaScript-intensive workloads. A tool for Web and Mozilla developers is also under development to explore CacheIR data for a JavaScript function.

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