In C++, is there any effective difference between a acquire/release atomic access and a relaxed access combined with a fence?

Question

Specifically, is there any effective difference between:

i = a.load(memory_order_acquire);

or

a.store(5, memory_order_release);

and

atomic_thread_fence(memory_order_acquire);
i = a.load(memory_order_relaxed);

or

a.store(5, memory_order_relaxed);
atomic_thread_fence(memory_order_release);

respectively?

Do non-relaxed atomic accesses provide signal fences as well as thread fences?

Answer 1

You need

atomic_thread_fence(memory_order_release);
a.store(5, memory_order_relaxed);

and

i = a.load(memory_order_relaxed);
atomic_thread_fence(memory_order_acquire);

To replace

a.store(5, memory_order_release);

and

i = a.load(memory_order_acquire);

Non-relaxed atomic accesses do provide signal fences as well as thread fences.