In September, machine learning engineers at Mozilla filed a bug report indicating that Firefox was consuming excessive memory and CPU resources while running Microsoft’s ONNX Runtime (a machine learning library) compiled to WebAssembly.

SpiderMonkey is the JavaScript engine used in Mozilla Firefox and other Mozilla products. Readers can learn about JavaScript language features, engine internals, and performance optimizations. With documentation, blog posts, and developer tools, SpiderMonkey provides resources for JavaScript developers and browser engine enthusiasts.

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Mozilla's engineers improved the performance of the Ion compiler backend in Firefox, addressing issues with excessive memory and CPU usage when running Microsoft's ONNX Runtime. Key optimizations included switching from sorted linked lists to optionally-sorted vectors for VirtualRegister live ranges, implementing the Semi-NCA algorithm for dominator tree building, using Sparse BitSets for better memory management, and optimizing move resolution functions. These changes resulted in significant performance gains, reducing compilation time from minutes to mere seconds. Future plans involve further refining the WebAssembly compilation pipeline for even better performance.

75x faster: optimizing the Ion compiler backend