Stream: platform development

Topic: Memorylayout of a Tag with payload

Oskar Hahn (Jan 29 2024 at 10:43):

Since there is no glue for go yet, I have to translate the roc-types to go for my platform manually. I was not able to do it for a tag with payload. For example

MyType : [Foo Str, Bar U64]

I would guess, that it first contains a Number, which Tag is active and then enough bytes for the biggest type (for example 24 byte for Str on a 64bit system or 8 byte for U64).

I also guess, that the type is on the stack and therefore does not need a refcount-prefix.

Are my assumptions correct?

For this to work in go, I have to write a .h file. How would you define such a type in C when the payload can be different types?

Luke Boswell (Jan 29 2024 at 10:50):

Sometimes I generate the glue for rust, and then compare notes... maybe that will help?

Oskar Hahn (Jan 29 2024 at 13:45):

That helped. So the rust code looks like this:

#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Hash, )]
#[repr(u8)]
pub enum discriminant_MyType {
    Bar = 0,
    Foo = 1,
}

#[repr(C, align(8))]
pub union union_MyType {
    Bar: u64,
    Foo: core::mem::ManuallyDrop<roc_std::RocStr>,
}

#[repr(C)]
pub struct MyType {
    payload: union_MyType,
    discriminant: discriminant_MyType,
}

When I understand this correctly, then my guess was correct. But the u8 value, that tells the type, is after the value.

Brendan Hansknecht (Jan 29 2024 at 15:49):

Yeah, tag is at the end cause it is more likely to fit in a gap instead of requiring alignment extra bytes.

Oskar Hahn (Jan 29 2024 at 15:49):

It works :tada:

In the .h file, it looks like this:

union MyTypeUnion {
    struct RocStr foo;
    long long unsigned int bar;
};

struct MyType {
    union MyTypeUnion payload;
    unsigned char discriminant;
};

This is not so helpful. Go converts a C-union to a byte-array, with enough space for the biggest type. So in this case [24]byte.

The the following code was necessary, to get the actual payload:

switch mytype.discriminant {
case 0:
    payload := *(*C.struct_RocStr)(unsafe.Pointer(&mytype.payload))
    foo := rocStrRead(payload)
    ...

case 1:
    payload := *(*C.longlong)(unsafe.Pointer(&mytype.payload))
    bar := uint64(payload)
       ...

default:
    panic("invalid payload")
}

Oskar Hahn (Jan 29 2024 at 15:51):

Brendan Hansknecht said:

Yeah, tag is at the end cause it is more likely to fit in a gap instead of requiring alignment extra bytes.

This makes sense.

Last updated: Jul 26 2025 at 12:14 UTC