While fixing various bindgen bugs related to long double, int128, (rust-lang/rust-bindgen#1370, etc).

I realized that the following Rust program, in my x86_64 Linux machine:

#[repr(C)]
struct Foo {
    f: u128,
}

fn main() {
    println!("Align: {}", ::std::mem::align_of::<Foo>());
}

Prints 8.

While the following C program:

#include <stdio.h>

struct foo {
  unsigned __int128 t;
};

int main() {
  printf("Align: %ld\n", _Alignof(struct foo));
}

Prints 16 on the same system. This is pretty unexpected, and means that i128 / u128 are not really usable for FFI / alignment purposes.