[cpp-threads] modes, interlude

[Peter Dimov]

Doug Lea wrote:

> If we are going to do this, is there any reason not go all the way and
> have four flavors of explicit barrier functions in the atomic classes

We will.

template<class M> void barrier( M msync );

> and not bother adding modes to load, store etc?

A library can't fold the explicit barrier() calls into the preceding or 
subsequent atomic operation, so providing only barrier() may be inefficient 
for the first ten years or so. :-)

On platforms where a bidirectional barrier is the natural primitive, the 
modes can easily be implemented in terms of barriers:

template<class M, class T> T atomic_load( M msync, T * addr )
{
    barrier( (msync & msync_rel) | msync_hlb );

    T r = _load_none( addr );

    barrier( (msync & msync_acq) | msync_slb );

    return r;
}

("constraint algebra" at work.)