C abi compatability

[bjorn3]

• Don't pass return pointer for unit return type (419cbcb)
• Make sure every type has the correct PassMode
  • Return !
  • Option<fn(..)>
  • layout.abi = Abi::ByValPair (3f15b0a)
  • Abi::ScalarPair + non rust abis
• Find out correct way of passing Abi::ByRef values. (ptr or stack offset)

[bjorn3]

Useful link: https://software.intel.com/sites/default/files/article/402129/mpx-linux64-abi.pdf

[bjorn3]

Find out correct way of passing Abi::ByRef values. (ptr or stack offset)

It needs to be stored at a certain stack offset.

[bjorn3]

Found another problem:

use std::os::raw::*;

#[repr(C)]
pub struct Sel {
    ptr: *const c_void,
}

extern {
    pub fn sel_registerName(name: *const c_char) -> Sel;
}

pub unsafe fn call_register_name(res: &mut Sel) {
    *res = sel_registerName(0 as *const _)
}

Uses a return pointer to return Sel instead of returning it in a register.

Expected:

playground::call_register_name:
	pushq	%rbx
	movq	%rdi, %rbx
	xorl	%edi, %edi
	callq	*sel_registerName@GOTPCREL(%rip)
	movq	%rax, (%rbx)
	popq	%rbx
	retq

This causes the objc crate to misbehave.