Low-Lock Singletons In D

[deadalnix]

On Tuesday, 7 May 2013 at 16:14:50 UTC, Steven Schveighoffer 
wrote:
> Not really.  Whether it is entered or not is dictated by the 
> vtable.  Even classic double-check locking doesn't need an 
> acquire outside the lock.  Even if your CPU's view of the 
> variable is outdated, the check after the memory barrier inside 
> the lock only occurs once.  After that, steady state is 
> achieved.  All subsequent reads need no memory barriers, 
> because the singleton object will never change after that.
>
> The only thing we need to guard against is non-atomic writes, 
> and out of order writes of the static variable (fixed with a 
> memory barrier).  Instruction ordering OUTSIDE the lock is 
> irrelevant, because if we don't get the "steady state" value 
> (not null), then we go into the lock to perform the careful 
> initialization with barriers.
>
> I think aligned native word writes are atomic, so we don't have 
> to worry about that.
>

That is incorrect as the thread not going into the lock can see a 
partially initialized object.