Sibling has an amazing long response, but tl;dr, the live registers that are clobbered are exactly the same as those that would need to be saved in the async case.
I see the confusion now. I wrote caller-saved when I meant callee-saved. In general you don’t need to preserve callee-saved registers if you don’t use them during normal control flow but in the “sync” case you always have to save callee-saved registers on task switch. In the Async case, you simple return to the executor.
