JavaScriptCore (JSC) is the JavaScript engine that powers WebKit (and thus Safari). I was recently browsing through its source and noticed a few interesting things:

ARM64_32 Support

Apple Watch Series 4 uses the S4 64-bit ARM CPU, but running in a mode where pointers are still 32 bits (to save on the memory overhead of a 64 -bit architecture). The watch (and its new CPU) were announced in September 2018, but support for the new ARM64_32 architecture was added in December 2017. That the architecture transition was planned in advance is no surprise (it's been in the works since the original Apple Watch was announced in 2015). However, it does show that JSC/WebKit is a good place to watch for future Apple ISA changes.

ARMv8.3 Support

The iPhone XS and other new devices that use the A12 CPU have significantly improved JavaScript performance when compared to their predecessors. It has been speculated that this is due to the A12 supporting the ARM v8.3 instruction set, which has a new floating point instruction that operates with JavaScript rounding semantics. However, it looks like support for that instruction was only added a couple of weeks ago, after the new phone launch. Furthermore, the benchmarking by the Apple engineer after the change landed showed that it was responsible for a 0.5%-2% speed increase, which while nice, does not explain most of the gain.

Further digging into the JSC source led to my noticing that JIT for the ARMv8.3 ISA (ARM64E in Apple's parlance) is not part of the open source components of JSC/WebKit (the commit that added it references a file in WebKitSupport, which is internal to Apple). So perhaps there are further changes for this new CPU, but we don't know what they are. It's an interesting counterpoint to the previous item, where Apple appears to want extra secrecy in this area. As a side note, initial support for this architecture was also added several months before the announcement (and references to ARM64E showed up more than 18 months earlier), thus another advance notice of upcoming CPU changes.

Fuschia Support

Googler Adam Barth (hi Adam!) added support for running JSC on Fuschia (Google's not-Android, not-Chrome OS operating system). Given that Google has its own JavaScript engine (V8), it's interesting to wonder why they would also want another engine running. A 9to5 Google article has the same observation, and some more speculation as to the motivation.