On Monday, 2 March 2015 at 20:04:49 UTC, deadalnix wrote:
I let the night go over that one. Here is what I think is the
best road forward :
- triggering postblit and/or ref count bump/decrease is
prohibited on borrowed.
- Acquiring and releasing ownership does.
Now that we have this, let's get back to the exemple :
class C {
C c;
// Make ti refconted somehow, doesn't matter. Andrei's
proposal for instance.
}
void boom() {
C c = new C();
c.c = new C();
foo(c, c.c);
}
void foo(ref C c1, ref C c2) {
// Here is where things get different. c1 is borrowed, so
you can't
// do c1.c = null before acquiring c1.c beforehand.
Right, I agree with this.
That means the
// compiler needs to get a local copy of c1.c, bump the
refcount
// to get ownership before executing c1.c = null and
decrease
// the refcount.
Yeah, but should it do this inside foo() or in bump() right
before it calls foo. I think in bump, and only for a parameter
which might be aliased by another parameter (an extremely rare
case). For any other case, the refcount has already been
preserved:
void boom() {
C c = new C(); // refcount(c) == 1
c.c = new C(); // refcount(c.c) == 1
auto d = c.c; // refcount(c.c) == 2 now
foo(c, d); // safe
}
The only problem is the rare case when the exact same identifier
is getting sent to two different parameters.
I'm sure there will be opportunities to elide a lot of refcount
calls, but in this case, I don't see much to left to elide.
