Semantics of preserve_all calling convention

The ir reference for preserve_all states:

This calling convention also behaves identical to the Ccalling convention on how arguments and return values are passed

I tried the following:

uint64_t return_zero() attribute((preserve_all))
return 0;

and got

0000000000000020 <return_zero>:
20: 50 push %rax
21: 31 c0 xor %eax,%eax
23: 58 pop %rax
24: c3 retq

as you can see, the rax register is restored to its value upon entry to the function and does not contain the return value. What are the semantics here? Should this work? Should we add a verifier error for functions with this calling convention that return values?