On 3/3/15 12:35 PM, Zach the Mystic wrote:
On Tuesday, 3 March 2015 at 18:48:36 UTC, Andrei Alexandrescu wrote:
On 3/3/15 9:00 AM, Zach the Mystic wrote:
On Tuesday, 3 March 2015 at 16:31:07 UTC, Andrei Alexandrescu wrote:
I was dazzed, but I'm not anymore. I wrote my concern here:
http://forum.dlang.org/post/[email protected]
There's a misunderstanding here. The object being assigned keeps a
trailing list of past values and defers their deallocation to
destruction. -- Andrei
So you need an extra pointer per instance?
Yah, or define your type to be single-assignment (probably an emerging
idiom). You can massage the extra pointer with other data thus
reducing its cost.
Isn't that a big price to
pay? Is the only problem we're still trying to solve aliasing which is
not recognized as such and therefore doesn't bump the refcounter like it
should? An extra pointer would be overkill for that. Isn't it better to
just recognize the aliasing when it happens?
It's all tradeoffs. This has runtime overhead.
Isn't allocating and collecting a freelist also overhead?
No. I don't have time now for a proof of concept and it seems everybody
wants to hypothesize about code that doesn't exist instead of writing
code and then discussing it.
A static analysis would have the challenges of being permissive
enough, cheap enough, not add notational overhead, etc. etc.
It's certainly permissive: you can do anything, and compiler wraps
uncertain operations with add/release cycles automatically. These are:
passing a global as a mutable reference to an impure function; aliasing
the same variable in two parameters with itself. The unoptimized
lowerings would be:
{
auto tmp = myGlobal; // bump count
impureFun(myGlobal);
} // tmp destroyed, --count
{
auto tmp2 = c; // bump count
fun(c, c);
} // --count
The only addition is an optimization where the compiler elides the
assignments and calls the add/release cycles directly.
Do you have something reviewable, or just your own past posts?
For the time being I want to move forward with DIP25 and deferred freeing.
Andrei
