My suspicion is they'll fork the arm architecture at some point (either via a legal agreement with arm or just taking the deploy the lawyers approach and doing it unilaterally). Apple like their walled gardens and closed eco-systems so there's no clear reason for them to switch to RISC-V whilst they're very happy with their arm eco-system.
I worked at a semiconductor company that had an ARM architectural license and we implemented our own ARM CPU. We still had to pass ARM compliance tests.
I heard from some of the people on the CPU team that Apple did not have to pass those tests because they had an exemption from ARM. Apple was one of the original ARM investors when it was spun off from Acorn in 1990. I think that agreement allows them to do things with ARM like adding custom things that other ARM licensees aren't allowed to do.
You are making it sound like a burden, but I do not understand why it would be. Surely you are interested in safety checks giving you more confidence that you are positioning yourself to take advantage of the Arm ecosystem (instead of releasing something that is not supportable by compilers and OS experts)?.. Or am I misunderstanding the nature of the compliance tests?
I think the compliance tests were making sure that your add instruction or memory load instruction followed the spec.
You mention ARM ecosystem and that is precisely the point. Apple controls their ecosystems. How do you write apps for the iPhone? You use Apple's development environment with Apple's compiler. If Apple decided NOT to implement an instruction for some reason they could simply make their compiler never output that instruction.
I worked on chips that ran embedded applications with no ability for ordinary users to change the software. What is the value of meeting an external ARM controlled spec for that?
I also worked on chips that only ran Android and nothing else. If you are also the company porting Android and writing all drivers for your own platform then people may argue whether it is worth only being 99% compatible.
Later I worked on chips where people may run Android, Windows Mobile, or plain Linux on this chip using GCC, Clang, Microsoft's compiler, or whatever. For that you definitely wanted to comply with specs.
In a long time horizon, it's probably in Apple's interest to migrate away from ARM just so that they have one less company to negotiate with or receive restrictions from
Apple helped to design Arm v8 they probably have huge influence on the direction of travel of the ISA. Plus they have huge investment in Arm based software and probably get access to all of Arm’s IP (eg big.LITTLE).
They will migrate when there is a major performance advantage (as in the past).
