LLVM bytecode is very cool, and arguably evidence that a more generic system could be successful, but it is not considered to be instruction-set agnostic. Yes, it allowed Apple to tweak the cpu in new Apple Watches without requiring updated app submissions, which is great. But it would not support a transition from, for example, x86 to ARM. I've seen this topic discussed multiple times, best summary I could find is from a quora post:
Everyone's first thought (including the other two answerers here!) was that this would allow Apple to change processors entirely: if they wanted to make an Intel iPhone, for instance, they could just tell the App Store to start converting apps' bitcode to Intel machine code instead of ARM machine code for the new phones, and every app would instantly be "updated" for the new processor.
But in practice, this doesn't seem likely. Even with LLVM's unusually clean separation between frontend and backend, the frontend actually does know certain things about the processor it's targeting (mainly details about its memory layout, or special processor features it can use to make certain code faster). These things get baked into the bitcode, and in practice ensure you can't really mix-and-match frontends and backends as freely as you would like. So suddenly switching processors entirely probably isn't in the cards.
What's more likely is that it will allow Apple to make smaller improvements to their processors and then roll them out to existing apps. ...
Everyone's first thought (including the other two answerers here!) was that this would allow Apple to change processors entirely: if they wanted to make an Intel iPhone, for instance, they could just tell the App Store to start converting apps' bitcode to Intel machine code instead of ARM machine code for the new phones, and every app would instantly be "updated" for the new processor.
But in practice, this doesn't seem likely. Even with LLVM's unusually clean separation between frontend and backend, the frontend actually does know certain things about the processor it's targeting (mainly details about its memory layout, or special processor features it can use to make certain code faster). These things get baked into the bitcode, and in practice ensure you can't really mix-and-match frontends and backends as freely as you would like. So suddenly switching processors entirely probably isn't in the cards.
What's more likely is that it will allow Apple to make smaller improvements to their processors and then roll them out to existing apps. ...