> This means that apps can automatically “take advantage of new processor capabilities we might be adding in the future, without you re-submitting to the store.”
It doesn't. Bitcode is arch specific and unportable. It allows some instruction-set level optimizations, but not changing e.g. endianness (exactly what x64 and arm differ in) or type sizes.
ARM cores, although technically bi-endian, are in practice little-endian.
I play writing LLVM backends, and its entirely viable to even do things like rewrite float80 and other assumptions that a front-end has made for a particular target.
But what about things like non-aligned memory access (allowed on x86, not allowed on ARM)? Porting code across architectures can't be as easy as just swapping LLVM backends, when not even recompiling the code is sufficient.
There will be ISAs sufficiently different that you cannot make bitcode generated when targetting one not be massaged to fit another, but they are not mainstream. The mainstream are all increasingly similar 64-bit targets.
Here's a nice article describing it:
> This means that apps can automatically “take advantage of new processor capabilities we might be adding in the future, without you re-submitting to the store.”
http://thenextweb.com/apple/2015/06/17/apples-biggest-develo...