A thought about garbage collection

[Jonathan M Davis]

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.

- Jonathan M Davis