Which makes developing a large backend a lot of pointless busywork with patchsets touching parts of the codebase that really ought not need touching at all :(

(I'm someone who was stuck on 3.6 for ages because of the pain of keeping up with the trunk)

Why is this beneficial? It means you need to change several repositories to add one target triple. This may be a win in terms of in-tree stability, but it reduces accessibility to everyone else.

It is beneficial in that it discourages proprietary backends to LLVM which are not contributed back to LLVM. It is the same benefit GCC enjoys by using GPL.

Does Apple have some proprietary backends? I guess they have the resources to maintain such.

Yes, they definitely do.

Like what, nowadays? I know their ARM64 backend was, but what is now? Do they still maintain forks of them? (With more target data for their SoCs?)

Swift is still out-of-tree.

GPUs.

There is a reason proprietary backends would always be necessary.

Hardware companies are very touchy about the patents, and not publishing an ISA-related work may often be the only way to protect from the trolls.

Such companies would often encourage (privately or publicly) clean room 3rd party backends, and I'm pretty sure that the one we're discussing here is exactly of this kind. But they would be scared to death to allow any of their employees to even send a tiny patch to a compiler backend, if it could expose some microarchitecture knowledge that was not published.

Sounds like the LLVM project should be using the GPL.

This constant churn kind of turns the MIT license into the GPL but only for small companies and individuals who can't keep up with the churn. I guess that is why Apple likes LLVM.