Function back_on_x87 

pub fn back_on_x87(returns: &[Type]) -> bool

Whether what a function gives back is left on the x87 stack, which is one long double in st(0) or a _Complex long double with its real half there and its imaginary half in st(1).

Those two are the whole of what SysV returns on that stack, and a structure holding a long double is not one of them, since the lowering sends that back through memory.