Non-atomic access to atomic through a union

I would like to manipulate the same memory location both atomically and non-atomically.

Let's suppose that I'm working with a simple type, like an int, and in particular that std::atomic<T>::is_lock_free() returns true, and that sizeof(T) == sizeof(std::atomic<T>).

I thought that a reinterpret_cast should work:

std::atomic<int> x;
int& xx = reinterpret_cast<int&>(x);

But N4013 explains that this may confuse type-based alias analysis in the compiler and therefore is not reliable.

My question is: what about a union? If I create the following:

union AtomicInt
{
    int nonatomic;
    std::atomic<int> atomic;
};

AtomicInt x;
x.nonatomic = 5;
x.atomic.compare_exchange_weak(...);

Will this work as intended? Can I manipulate the same memory atomically and non-atomically?

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To preempt suggestions about using load(std::memory_order_relaxed) in lieu of non-atomic operations, I've tried the suggestion in this answer to a related question, but it slowed my code down by 50%.