[cpp-threads] Yet another visibility question

[Peter Dimov]

Consider:

// initial values 0

// thread 1

x = 1; // ordinary store
store_release( y, 1 );

// thread 2

r1 = load_raw( y ); // assume this reads 1
store_release( y, r1+1 ); // ... and this stores 2

// thread 3

r2 = load_acquire( y ); // assume this reads 2

if( r2 == 2 )
{
    assert( x == 1 ); // ordinary load
}

Will the assert pass? Is there a data race on x? What if thread 2's load had 
acquire semantics?

Thread 2 can be rephrased as performing

fetchadd_release( y, 1 );

and

fetchadd_ordered( y, 1 );

respectively. Is this an equivalent transformation (from visibility 
standpoint, atomicity does not concern us for the moment)?