incorrect handling of [repr(packed)] and [repr(align(n)] struct attributes

[nobel-sh]

In Rust, the #[repr(packed)] attribute is intended to remove padding between struct fields. Similarly, #[repr(align(n))] is used to adjust the alignment of a struct to n bytes. However, in the case of gccrs, it appears that these attributes are not fully respected.

// uncomment commented code to run with gccrs

// #![feature(intrinsics)]
// #[lang = "sized"]
// pub trait Sized {}

// extern "rust-intrinsic" {
//     fn size_of<T>() -> usize;
// }

extern "C"{
    fn printf(format: *const i8, ...);
}


#[repr(packed)]
struct StructPacked {
    flag: bool,
    number: i32,
    data: [u8; 16],
}

#[repr(align(16))]
struct StructAligned {
    flag: bool,
    number: i32,
    data: [u8; 16],
}

fn main(){
    let matrix = [[1.0; 4]; 4];
    let data = [0u8; 16];

    let struct_packed = StructPacked {
        flag: true,
        number: 42,
        data,
    };

    let struct_aligned = StructAligned {
        flag: true,
        number: 42,
        data,
    };

    println!("StructPacked: {}", size_of::<StructPacked>());
    println!("StructAligned: {}", size_of::<StructAligned>());

    // uncomment below for gccrs

    // unsafe{
    //     printf(
    //         "StructPacked: %zu\n\0" as *const str as *const i8,
    //         size_of::<StructPacked>(),
    //     );
    //     printf(
    //         "StructAligned: %zu\n\0" as *const str as *const i8,
    //         size_of::<StructAligned>(),
    //     );
    // }
}

rustc:

StructPacked: 21
StructAligned: 32

gccrs:

StructPacked: 24
StructAligned: 24

godbolt: https://godbolt.org/z/rornPra3c

[philberty]

I wonder is the bool type throwing off our packed handling iirc I don't know if we have added the aligned support in or not will need to check