Should X15/Y15 be left untouched?

[Jille]

Per https://go.googlesource.com/go/+/refs/heads/master/src/cmd/compile/abi-internal.md#amd64-architecture, X15/Y15 should always be zero. So should the Avo register allocator avoid using it?

[dominikh]

Assembly code uses ABI0, not ABIInternal. In ABI0, X15's value is undefined. That means the runtime has to restore X15 when leaving ABI0, anyway. So no, no Avo change should be needed.