Stream: cranelift

Topic: What is the order of multiple return values?

Adel Prokurov (Mar 11 2023 at 19:25):

In what order do I need to return multiple values in order to return this enum in registers? (I know that it is exactly 16 bytes in size when aligned)

#[repr(C, u8)]
enum Trampoline {
   Ret(Value) = 0,
   Throw(Value),
    TailCall,
}

bjorn3 (Mar 11 2023 at 20:25):

@Adel Prokurov #[repr(C)] types don't get passed in registers unless the C ABI says so. In your case the C ABI on at least x86_64 says to pass it on the stack so it will be passed on the stack. You could split it into two u64 you pass manually if you really need. Or if the exact abi doesn't matter you can remove the #[repr] in which case it will be passed in two registers provided that you are using the rust calling convention. (no guarantees, but it just so happens to be the case right now)

bjorn3 (Mar 11 2023 at 20:26):

By the way do you want this for perf or abi reasons?

Adel Prokurov (Mar 12 2023 at 04:51):

bjorn3 said:

By the way do you want this for perf or abi reasons?

I want it for Scheme -> Rust calls that are not in tail position. To do Scheme -> Rust call I first invoke a thunk that checks arity of a procedure and the type of procedure and then actually invokes the code, it is not done in a tail call because my thunk signature is like this:

fn(vm: &mut Vm, proc: Value, argc: usize, tag: &mut u8) -> Value { ... }

proc actually stores the fn(...) -> Trampoline pointer. On the return I update tag ref to trampoline tag and because of this tail call is not done. So it is more of a performance thing

bjorn3 (Mar 12 2023 at 14:21):

For returning into rust code with multiple return values I did recommend creating a trampoline which writes the values to a location specified as pointer argument. This is also what Wasmtime does for handling multiple return values.

Adel Prokurov (Mar 12 2023 at 14:29):

Yea that is what I am doing right now but then thunk written in Rust does not perform tail call, probably I'll stick to it then and just will wait for return_call support in Cranelift to rewrite thunks in Cranelift IR :D

Last updated: Dec 23 2024 at 12:05 UTC