Introducing the JetStream 3 Benchmark Suite

JetStream 3.0 is a new cross-browser benchmark suite developed collaboratively by Apple, Google, and Mozilla. It overhauls how WebAssembly and JavaScript performance is measured by replacing isolated startup/runtime phases with full lifecycle scoring (first iteration, worst case, average case), and shifting from microbenchmarks to larger real-world workloads. The WebKit team used JetStream 3 as a driver for major JavaScriptCore improvements: WasmGC allocation inlining (eliminating two-allocation patterns for ~40% gains), GC type reference counting overhaul, Cohen's type display algorithm for fast type checks, a new greedy register allocator with B+ tree interval tracking (~2x faster for large functions), polymorphic indirect call inlining, BigInt arithmetic improvements (Comba multiplication, DIV2BY1 division), and a full rewrite of the microtask queue and Promise combinators from JS builtins to C++. These changes collectively delivered roughly a 10% performance improvement from Safari 26.0 to 26.4.