"Preserve none" calling convention

Clang/LLVM provides some non-standard calling conventions on x86-64, among them preserve_most and preserve_all. They inform the callee that it must preserve additional registers than would be required in C calling convention on x86-64.

I’m looking for a way to get opposite effect: to allow the callee not preserving most of the general purpose registers (e.g. r12). I have not found any calling convention which would give me this. The solution does not have to be “global” because I only need to apply it to a single function.