Rust moving away from GC into reference counting

[evansl]

On 06/03/13 12:41, Paulo Pinto wrote:
> Am 03.06.2013 18:00, schrieb Manu:
>> Haha, wow. Indeed, isn't that well timed with respect to recent
>> discussions! ;)
>> Post-dconf, I gave it some serious thought and I'm kinda convincing
>> myself more and more each day that it's the way to go.
>>
> 
> As I mentioned I prefer GC based solutions, but then again I live in the
> JVM/.NET world, so I don't have the memory/timing pressure you have to
> deal with.
> 
> But when looking at systems programming languages that offer reference
> counting as the main memory management, ATS, Parasail, Objective-C, and
> now Rust, all of them share a common feature:
> 
> - Compiler support to remove extra increment/decrement operations.
> 
> I guess on D's case this support would also be required.
> 
> -- 
> Paulo
> 

IIUC, a system programming language would use threading at least a fair
amount.  In that case, I'm not sure reference counting is appropriate,
at least according to the following:

.> However, I think you are also talking about RC in multi-threaded
.> (not distributed) systems. Here, the cost of making RC thread-safe
.> (both RC and pointer manipulations must be atomic) can be expensive
.> if done naively. It's worth taking a moment to think about
.> this. Consider a scenario with 3 objects, A, B and C, and two
.> threads, T1 and T2. Suppose that T1 writes a reference to B into a
.> slot in A and T2 writes a reference to C into the same slot. (You
.> can say that such a program is broken but memory managers but cope
.> with broken code). Both the reference count operations and the
.> pointer updates must be synchronised so that the reference counts
.> do reflect the final state of the reference held in A. This is
.> prohibitively expensive if used for general memory management
.> (i.e. rather than just for managing a few special objects).

This is from a private email(which Jones said I could forward to
c.l.c++.m, which, I assume means I can quote here also):

.> Date: Fri, 15 Apr 2005 10:50:50 +0100
.> From: Richard Jones <R.E.Jones at kent.ac.uk>

Of course maybe RC has improved since 2005.  I'm no expert :(

-regards,
Larry