draft proposal for ref counting in D

[Walter Bright]

On 6/30/2013 7:36 PM, Steven Schveighoffer wrote:
 > On Jun 30, 2013, at 10:26 PM, Walter Bright wrote:
 >
 >> On 6/30/2013 5:25 PM, Steven Schveighoffer wrote:
 >>> On Jun 30, 2013, at 8:18 PM, Walter Bright wrote:
 >>>
 >>>> I very much want to avoid requiring atomic counts - it's a major 
performance penalty. Note that if the GC is reaping a cycle, nobody else is 
referencing the object, so this should not be an issue.
 >>> I think you didn't understand what Michel was saying.
 >>>
 >>> Take for example:
 >>>
 >>> A->B->C->A
 >>>
 >>> this is a cycle.  Imagine that nobody else is pointing at A, B or C.  Fine. 
  The GC starts to collect this cycle.
 >>>
 >>> But let's say that D is not being collected *AND* B has a reference to D.
 >>>
 >>> B could be getting destroyed in one thread, and decrementing D's reference 
count, while someone else in another thread is incrementing/decrementing D's 
reference count.
 >>>
 >>> I agree that RC optimally is thread-local.  But if you involve the GC, then 
ref incs and decs have to be atomic.
 >> This is actually a problem right now with the GC, as destructors may be run 
in another thread than they belong in. The situation you describe is not worse 
or better than that, it's the same thing. The solution is to run the destructors 
in the same thread the objects belong in.
 > I think that's a tall order presently.  For instance, on linux, the threads 
are all stopped using a signal.  It's a very bad idea to run destructors in a 
signal handler.
 >
 > What it seems like you are saying is that a prerequisite for ref counting is 
to have thread-local GC working.  If that is the case, we need to start a 
thread-local GC "thread" before this goes any further.

Not really. This doesn't make anything worse. Also, the proposed solution to 
this issue is to post the "destruct" list to the appropriate thread, and that 
thread runs it next time it calls the GC.

 >
 >>> I don't think this is that bad.  iOS on ARM which has terrible atomic 
primitives uses atomic reference counts.
 >> It's bad. ARM is not the only processor out there.
 >
 > Pragmatically, I think if D targets x86 variants and ARM, it is well-situated 
in the mainstream of existing devices.  Yes, it would be nice if it could target 
other obscure platforms, but if we are talking ref counting works poorly on 
those, I don't think we are any worse off than today.  Note that we can keep the 
options open, and implement atomic RC now without many headaches.
 >

We don't need to require atomic RC for these.