Pizlo also explained, "B3 is designed to enable the same kinds of optimizations as we previously got from LLVM while generating code in less time. It also enables optimizations specific to the FTL JIT, like precise modeling of the effects of self-modifying code." And concluded with, "B3 generates code that is as good as LLVM on the benchmarks we tried, and makes WebKit faster overall by reducing the amount of time spent compiling. We’re happy to have enabled the new B3 compiler in the FTL JIT. Having our own low-level compiler backend gives us an immediate boost on some benchmarks. We hope that in the future it will allow us to better tune our compiler infrastructure for the web. B3 is not yet complete. We still need to finish porting B3 to ARM64. B3 passes all tests, but we haven’t finished optimizing performance on ARM. Once all platforms that used the FTL switch to B3, we plan to remove LLVM support from the FTL JIT."
If you are curious about WebKit's B3 JIT compiler, learn more via WebKit.org.