Added support for using a temporary to implement emulated ABI return from inline asm, could be easier to use, but I think this will do. It's so extremely target dependent in any case that doing a completely generic approach seems hard.

module mini.atomic1;

pragma(intrinsic, "llvm.atomic.swap.i#.p0i#")
    T atomic_swap(T)(T* ptr, T val);

void main()
{
    int i = 42;
    int j = atomic_swap(&i, 43);
    assert(j == 42);
    assert(i == 43);
}