Impact of Xcode build options "Enable bitcode" Yes/No

• What does the ENABLE_BITCODE actually do, will it be a non-optional requirement in the future?

I'm not sure at what level you are looking for an answer at, so let's take a little trip. Some of this you may already know.

When you build your project, Xcode invokes clang for Objective-C targets and swift/swiftc for Swift targets. Both of these compilers compile the app to an intermediate representation (IR), one of these IRs is bitcode. From this IR, a program called LLVM takes over and creates the binaries needed for x86 32 and 64 bit modes (for the simulator) and arm6/arm7/arm7s/arm64 (for the device). Normally, all of these different binaries are lumped together in a single file called a fat binary.

The ENABLE_BITCODE option cuts out this final step. It creates a version of the app with an IR bitcode binary. This has a number of nice features, but one giant drawback: it can't run anywhere. In order to get an app with a bitcode binary to run, the bitcode needs to be recompiled (maybe assembled or transcoded… I'm not sure of the correct verb) into an x86 or ARM binary.

When a bitcode app is submitted to the App Store, Apple will do this final step and create the finished binaries.

Right now, bitcode apps are optional, but history has shown Apple turns optional things into requirements (like 64 bit support). This usually takes a few years, so third party developers (like Parse) have time to update.

• can I use the above method without any negative impact and without compromising a future appstore submission?

Yes, you can turn off ENABLE_BITCODE and everything will work just like before. Until Apple makes bitcode apps a requirement for the App Store, you will be fine.

• Are there any performance impacts if I enable / disable it?

There will never be negative performance impacts for enabling it, but internal distribution of an app for testing may get more complicated.

As for positive impacts… well that's complicated.

For distribution in the App Store, Apple will create separate versions of your app for each machine architecture (arm6/arm7/arm7s/arm64) instead of one app with a fat binary. This means the app installed on iOS devices will be smaller.

In addition, when bitcode is recompiled (maybe assembled or transcoded… again, I'm not sure of the correct verb), it is optimized. LLVM is always working on creating new a better optimizations. In theory, the App Store could recreate the separate version of the app in the App Store with each new release of LLVM, so your app could be re-optimized with the latest LLVM technology.

Make sure to select "All" to find the enable bitcode build settings:

Bitcode is a new feature of iOS 9

Bitcode is an intermediate representation of a compiled program. Apps you upload to iTunes Connect that contain bitcode will be compiled and linked on the App Store. Including bitcode will allow Apple to re-optimize your app binary in the future without the need to submit a new version of your app to the store.

Note: For iOS apps, bitcode is the default, but optional. If you provide bitcode, all apps and frameworks in the app bundle need to include bitcode. For watchOS apps, bitcode is required

So you should disabled bitcode until all the frameworks of your app have bitcode enabled.