synchronized (this[.classinfo]) in druntime and phobos

[Artur Skawina]

On 05/30/12 17:50, Steven Schveighoffer wrote:
> So I propose it does this instead (entirely via lowering):
> 
> synchronized(x)
> {
> }
> 
> translates to:
> 
> auto l = x.__getMutex();
> l.lock();
> try
> {
> }
> finally
> {
>    l.unlock();
> }

I like this, it's slightly more complex, but can reduce the amount of boilerplate
(by not requiring lock/unlock forwarders etc) and is more powerful than my proposal.

> So we have a few benefits here:
> 
> 1. if typeof(x) does not define __getMutex, the above is a compiler error.
> 2. We can control the visibility of the mutex, because we can attribute private or protected to the __getMutex function.
> 3. We can control what kinds of objects of typeof(x) use mutexes.  e.g. define __getMutex() shared only.
> 4. We have control over the type of the lock, it can be anything and does not need to fit into some runtime function's interface.

5. The lock doesn't have to be a real lock, but eg a lock manager. Hmm, lowering to
l.lock(x) and l.unlock(x) might be better; inlining will make it zero-cost for the
simple case, but this will make certain schemes much simplier (no need to encode
'x' inside the returned 'l').

> Second, I propose a somewhat gradual transition.  I think on first release, we *still* keep the allocated word in the class instance, and keep the druntime function which lazily allocates the mutex.  Therefore, changing an object that currently *properly* can use synchronized(this) can be made to work:
> 
> class Synchronizable
> {
>    xxx __getMutex() shared { return _d_getObjectMonitor(this);}
> }
> 
> We can deprecate that functionality, and the allocated space for the mutex later if desired.

Yes, UFCS could help too.

artur