What is the cache line size on iPhone and iPad?
And does it vary much between the different devices and CPUs? This is not too easy to find with google.
I need to squeeze some extra performance from my app. :)
Well, the Cortex-A8 has 64-byte lines, Cortex-A9 has 32-byte lines, as for Swift and Cyclone I don't know - looking at comparable cores (A15, A57, Scorpion, Krait) 32 or 64 bytes seems likely. Either way there's at least 2 different lengths across iOS7 machines.
As you're performance-focused though, remember that benchmarking is infinitely more valuable than theorising - try as many reasonable combinations of code on every piece of hardware you can lay your hands on, go with what's fastest in practice regardless of what 'should' be best.