Safe reference counting cannot be implemented as a library

[deadalnix via Digitalmars-d]

On Wednesday, 28 October 2015 at 01:13:16 UTC, Walter Bright 
wrote:
>
> It's not just safety. If the compiler knows that reference 
> counting is going on, it can potentially elide a lot of the 
> overhead. If it is faced with an arbitrary library solution, it 
> only has a worm's eye view of it, and cannot do higher level 
> optimizations.

I don't think the compiler can do that much more, but before I 
address that point, let me mention that intrinsic can be added 
for inc and dec, which would be much more lightweight for the 
language at large.

Now as to why I think this wouldn't give that much. First, if 
exception can be thrown, then all bets are pretty much off, as 
inc and dec do not go by pairs anymore. So we are down to the no 
exception situation. In that case, pairs are fairly visible to 
the compiler and can be optimized away or combined, that is 
already the kind of things that optimizer are good at.

But if, so, how do you explain the C++ situation, where nothing 
is elided (or ObjC's) ?

Well, there is a major difference with these languages: sharing 
by default. It means that inc and dec must be (atomic and 
ordered) or synchronized, which means that, as far as the 
compiler is concerned, all bets are off and the optimizer can't 
do its job.

This doesn't really apply to D, so I don't expect this to be a 
problem. And even if there is: intrinsic can save the day to hint 
the optimizer, no need for heavyweight language addition.

Now, let's get back to the exception case, as it is IMO the most 
interesting one. What if one is willing to accept leakage on 
exception throwing. That would get the optimizer back into the 
game and remove a lot of "dark matter" as Andrei calls it, which 
have a real cost on term of icache pressure and exception 
unwinding cost (one now doesn't have to resume each frame to 
maintain refcount).

If I had to go about this, I'd rather see the introduction a 
scope(exit/success/failure) like mechanism for destructors rather 
than something ref counting specific.