Re: [boost] [review] Fast track review ofBoost.Utility/Singletonbegins today

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-01-18 13:18:35

Martin Bonner:

> This has been raised before.  I normally write single-threaded code.
> Occasionally I am forced to write a worker thread to perform a
> particular function.  That means that I need BOOST_HAS_THREADS defined.
> On the other hand, I /know/ that most of my singletons will only be used
> by the main thread, so why do I have to pay for synchronization on those
> singletons?

This might be a relevant objection to the specific implementation under 
discussion, but I just want to point out that you don't need to pay for 
thread-safe initialization of function-level statics, especially in a 
"Singleton" context. Illustration:

X* X::instance()
{
    static X* s_px = 0;

    if( X* px = atomic_load_relaxed( &s_px ) )
    {
        return px; // fast path
    }

    static X s_x; // "slow" thread-safe init

    atomic_store_release( &s_px, &s_x );

    return s_px;
}

A sufficiently good compiler will fold this code into "static X s_x" so you 
won't need to do anything.

The reason this works (except on Alpha) is that the accesses through the 
returned pointer are data-dependent and carry implicit acquire semantics.