It's an implementation-defined register, which means it's up to Apple to define it. We have no idea what it does; we haven't observed any visible effects from flipping those bits. Given that it's per-cluster, we can infer that it has something to do with cluster-specific logic. Perhaps memory interface or power control.

There are hundreds of Apple implementation-defined registers; we're documenting them as we learn more about them [0] [1] [2]

[0] https://github.com/AsahiLinux/m1n1/blob/main/tools/apple_reg...

[1] https://github.com/AsahiLinux/docs/wiki/HW%3AARM-System-Regi...

[2] https://github.com/AsahiLinux/docs/wiki/HW%3AARM-System-Regi...

I googled it for you and err, came up blank, there's just two code references in some ASM code, the rest points to this resource. Weird, I would have thought things like this would have public documentation.

It’s not a standard ARM register; it’s implementation-defined and nobody gave it a meaningful name. It appears to do nothing much.

Then why not unofficially standardize its use in GCC and Clang's register-pooling subsystem?

Because GCC and Clang don't get to decide what hardware does with the register…