An obscure feature in Apple’s Xcode development software enabled Apple Watch apps to make an instant transition from 32-bit to 64-bit last month, an unheralded win for Apple Watch developers inside and outside the company. The “Enable Bitcode” feature was introduced to developers three years ago, but the Accidental Tech Podcast suggests that it was quietly responsible for the smooth launch of software for the Apple Watch Series 4 last month.
Support for Bitcode was originally added to Xcode 7 in November 2015, subsequently becoming optional for iOS apps but mandatory for watchOS and tvOS apps. Bitcode is an “intermediate representation” halfway between human-written app code and machine code. Rather than the developer sending a completely compiled app to the App Store, enabling Bitcode provides Apple with a partially compiled app that it can then finish compiling for whatever processors it wants to support.
The feature was forward-thinking enough that reports of its existence in 2015 called its most obvious use, “recompil[ing] bitcode-encoded App Store apps without any work from developers … unlikely to happen.” But that’s exactly what did happen in September 2018 with the release of the Apple Watch Series 4, which transitioned from the 32-bit Apple S3 processor to the 64-bit Apple S4. There was no waiting period for new 64-bit apps after the release of the new Watch last month, and developers didn’t even have to recompile their 32-bit apps; the apps just worked, and noticeably faster than before, on the new devices.
By contrast, Apple, developers, and users have had to endure comparatively messy switches of both iOS devices and Macs from 32-bit to 64-bit. After the iPhone 5S unexpectedly arrived with a 64-bit processor, users experienced software glitches with some earlier 32-bit apps, Apple quickly mandated that app developers start work on 64-bit versions, requiring support within five months of the device’s release and cutting 32-bit support last year. On the Mac side, the company is still in the process of phasing out 32-bit app support this year.
Thus far, there have been no major complaints about 64-bit apps on the Apple Watch, though the silence may be somewhat attributable to the comparative lack of apps for Apple’s wearable platform. Some developers abandoned their Apple Watch apps due to lack of resources, interest, or sufficient performance — but the fast and well-reviewed new Series 4 could address these problems with an infusion of new customers.