A thought about garbage collection

[deadalnix]

On Thursday, 20 December 2012 at 02:29:09 UTC, Jonathan M Davis 
wrote:
> On Thursday, December 20, 2012 03:19:17 bearophile wrote:
>> Jonathan M Davis:
>> > The problem is casting. It's not uncommon to create something
>> > as mutable and
>> > then cast it to immutable when done. It's stuff like that
>> > screws with being
>> > able to per-thread GCs. Which thread owns what isn't always
>> > clear.
>> 
>> I nearly never cast mutables to immutable. The purpose of casts
>> is to subvert the type system. If you accept that some type
>> system assumption doesn't hold just because casts exist, then 
>> you
>> are doomed to ignore every kind of type system assumption, like
>> immutability, safety, purity, nothrowness, and so on. And this 
>> is
>> so wrong. So I can't accept your justification that the 
>> existence
>> casts disallow per-thread GCs. Do not use cast that way, and 
>> give
>> us a "better" GC.
>
> It's perfectly legal to cast from mutable to immutable. You 
> just have to make
> sure that you don't have any other references to that data. In 
> many cases,
> it's the only way to get an immutable version of an object 
> (e.g. if you wanted
> an immutable AA). There's even a helper function for this in 
> the standard
> library: std.exception.assumeUnique. There's no way that this 
> is going away,
> and the GC is going to have to take that into account with 
> however it works.
> It may still be possible to have a per-thread GC, but it 
> definitely complicates
> things.
>
> Casting to and from shared has similar issues and happens for 
> similar reasons.
> And such casts are essentially forced by std.concurrency. So, 
> you're overly
> optimistic if you think that the GC can ignore such casts. It 
> must take them
> into account in whatever it does.
>

I think the right way to do that is to actually clone the object. 
You can argue that is a performance cost, but considering the 
implication on the GC, I'm pretty sure it is a win.

Another way to do it safely is to use pure function, but it 
require that notion of isolated island, and as much as I love the 
idea, I won't be in favor of introducing it right now into D.