On Jan 18, 2008 7:18 PM, Peter Dimov <pdimov@pdimov.com> wrote:
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; }
Don't you need a mutex in the slow path to prevent two threads that both find s_ps null to try initialize s_x withoug mutual exclusion? Or are you assuming C++0x thread safe initialization of statics? In the latter case, isn't safe to assume that the compiler will already use a form of the above pattern anyway? BTW, slightly off-topic, will C++0x standardize load dependent memory barriers? According to the latest memory model paper, the then-current consensus was not to. gpd