Also called WarpBuilder, Warp improves responsiveness and memory usage and speeds up page loads by making changes to JiT (just-in-time) compilers. Optimizing JiT is changed to rely solely on the CacheIR simple linear bytecode format, specifically, CacheIR data collected by the baseline tiers. The new architecture leveraged in the browser also is described as being more maintainable and unlocking additional SpiderMonkey improvements.
Basing Warp on CacheIR enabled removal of code throughout the engine that was needed to track global type inference data used by IonBuilder, resulting in speedups. Although IonBuilder and WarpBuilder both produce Ion MIR, an intermediate representation used by the optimizing JiT back end, IonBuilder had a lot of complex code not needed in WarpBuilder. Warp also can do more work off-thread and needs fewer recompilations. Plans call for continued optimizations on Warp, which at present is slower than Ion on some synthetic benchmarks.