lock-free 8bit mask

gistfile1.cpp

// we only allow no more than 8 worker in pool
std::atomic_uint_fast8_t idle_mask = {0};

// this function is called by each thread when it is about to sleep
void register_idle(const size_t thread_id)
{
	idle_mask.fetch_or(1u << thread_id, std::memory_order_release);
}

// this function can be called from anywhere at anytime
void wakeup_idle()
{
	uint_fast8_t snap(idle_mask.load(std::memory_order_relaxed));
	// Turn off the rightmost 1-bit.
	while(!idle_mask.compare_exchange_weak(snap, snap & (snap-1), 
						std::memory_order_acquire, std::memory_order_relaxed))
	{}
	// snap now should be the state before we turn off 1-bit.
	if(snap == 0) return;
	// Isolate the rightmost 1-bit.
	uint_fast8_t idle_bit = snap & (-snap);
	// find the bit
	for(size_t i = 0; i < 8; ++i)
	{
		if((idle_bit&(1u<<i)) != 0)
		{
			signal_thread(i);
			break;
		}
	}
}