Fiber local GC

[rikki cattermole via Digitalmars-d]

On 25/06/2016 11:00 PM, rikki cattermole wrote:
... snip ...

> What we want is to be able to explicitly say what the context is for a
> functions code and declare no globals.
>
> @noglobals
> struct MyContext {
>
>     @disable
>     this(this);
>
>     void func() {
>
>     }
> }
>
> Function calls outside of the struct are ok for: @system, pure and
> @noglobals.
> Making the struct pure wouldn't work since there goes @safe and @system
> calls.
>
> MyContext.func would implicitly have @noglobals applied to it.
>
> Now preferably we would have new hooks added so they could be in the
> context. Specifically if they are there they are used instead of the
> globals for e.g. GC. That way things like allocators could be used
> instead. Given that the context should be heap/stack allocated and never
> copied it would mean that once the fiber dies, all memory it uses is
> deallocated or returned to the pool.
>
> Now I did say about hooks, there are no ways to do that without having a
> bunch of context pointers somewhere saying which function is currently
> set to the hook. If done via a stack it would given history for
> unsetting the current context allowing recursive.
>
> The underlying fiber provider would have to inform the switch on e.g.
> yield. But that isn't hard to do.
>
> Looking back perhaps we can allow globals, but we'd need to instead have
> a hook for when assigning to a global / passing to anything not pure
> that is heap based. That would allow "moving" memory ownership from the
> fiber to the process GC.
>
>
> TLDR: I think we can do this with just a @noglobals attribute, but done
> properly it looks like we need proper hooks in place which are a lot
> harder.

I've thought about this further, the only hook function we don't 
currently have is related to global + TLS assignment for memory.
We can get away with e.g. new overriding and ~ via the GC proxy 
(actually a fairly decent way to go about it).

Since this assignment hook is potentially quite expensive, it definitely 
should be only if used under whatever attribute we use.