Apple’s new A7 processors support 64bit under iOS 7 – but only if your app is specifically compiled for 64bit. Ideally you want to compile once and use 64bit if the architecture supports it, and use 32bit for devices that have A4 – A6x processors and hence do not support 64bit.

Since Xcode 5.0.2 you can do this, and here’s how:

• head over to your app’s target and choose Build Settings
• under Architectures, choose “Standard architectures, including 64bit”
• Xcode will offer to “Change the deployment target to iOS 5.1.1”
• agree to it and you’re done

Now your app will choose the 64bit frameworks where available, and otherwise use 32bit.

• https://developer.apple.com/LIBRARY/IOS/documentation/General/Conceptual/CocoaTouch64BitGuide/Introduction/Introduction.html