Overview

Introduced with Xcode 7, ENABLE_BITCODE is a build setting that fundamentally changes how apps are compiled and optimized for Apple's platforms, including iOS, macOS, watchOS, and tvOS. When enabled, bitcode allows apps to be compiled into an intermediate representation before being uploaded to the App Store, providing Apple the flexibility to re-optimize apps without needing new submissions from developers. This capability can be crucial for future-proofing and maintaining compatibility with future hardware and software updates.

What is Bitcode?

Bitcode is an intermediate representation of an app's source code. In essence, rather than compiling an app directly to machine code, Xcode compiles it into bitcode. This transformation is analogous to how Java compiles to bytecode for execution by the Java Virtual Machine (JVM).

When an app is submitted to the App Store, Apple retains the bitcode version. This allows Apple to recompile the app for different architectures or optimization settings without requiring the developer to recompile and resubmit the app.

Technical Explanation

Why Use Bitcode?

1. Future Hardware Support: Apple frequently releases new hardware with updated processors and architectures. Bitcode allows apps to be automatically recompiled to support these new environments.
2. Optimization: Apple's team can apply improved compiler optimizations over time, automatically updating apps with these enhancements without requiring developers to update their apps.
3. Bug Fixes: Any compiler bugs discovered and fixed after an app's submission can be bypassed by recompiling the app from its bitcode.

Enabling & Disabling Bitcode

ENABLE_BITCODE has two possible settings:

• YES: Bitcode is enabled. The app compiles to bitcode first before being converted to the final binary format.
• NO: Bitcode is disabled. The app directly compiles to the target architecture's machine code.

Example Configuration

In Xcode 7 and later, developers can configure the ENABLE_BITCODE setting through their project's Build Settings:

11. Open your Xcode project
22. Navigate to "Build Settings"
33. Locate the "Build Options" section
44. Find "Enable Bitcode"
55. Set it to "YES" or "NO"

For command-line enthusiasts using xcodebuild, you can pass the setting like this:

xcodebuild -workspace MyApp.xcworkspace -scheme MyScheme ENABLE_BITCODE=YES

Impact on Libraries

An essential consideration is the impact of bitcode on libraries. All static and dynamic libraries used in a project must also include bitcode. If any library lacks bitcode, the build process will result in an error. Developers may need to verify with library providers for bitcode support if they run into issues.

Challenges and Considerations

Increased Build Time

Enabling bitcode often results in longer build times. This is because an additional compilation step to bitcode is required before generating a final binary.

Larger Binary Size

Bitcode-enabled binaries might appear larger during upload. This increase is not reflective of the final app size available in the App Store; however, developers should be aware of potential impacts on upload times.

Limited Impact on WatchKit Apps

Initially, enabling bitcode was mandatory for watchOS apps because they frequently require recompiling to adjust for the device's often-updated hardware.

Debugging

Because bitcode represents an abstract level above machine code, debugging at the machine level might differ slightly from traditional debugging techniques. However, Xcode provides sufficient tools to mitigate most of these challenges.

Table: Summary of Key Points

Conclusion

ENABLE_BITCODE is a pivotal feature introduced in Xcode 7, devised for long-term app optimization and compatibility across Apple's evolving hardware landscape. While it presents some challenges, such as increased build time and larger upload sizes, its benefits make it a worthwhile consideration for enhancing app longevity and performance.