Thanks for the additional comments. I have adjusted the comments.
I think it'd be nice to have that on the other arch as well, if you don't mind having a look.
I can have a look at the `i386` case, as it should be fairly straight-forward. I haven't good expertise about ARM, though. So I don't think I can spent the time to actually write the unwind codes for that. I might help out to argue about the CFI instructions, though.
Also, I think the change itself would be already very beneficial without ARM support.
I also discovered that we already had some .cfi_escape statements on the PE side, for the same kind of purpose, though they are just written by hand.
They also describe vector registers locations, maybe there's some value in trying to do it here too?
So `XMM6-XMM15` would be callee-save. But unless I am missing something, we *should* be fine by the default.