More radical ideas about gc and reference counting

[Andrei Alexandrescu via Digitalmars-d]

On 5/12/14, 10:25 AM, bearophile wrote:
> Walter Bright:
>
>> Unions of pointers are so rare in actual code that treating them
>> conservatively is not a big problem.
>
> std.variant.Algebraic is based on on std.variant.VariantN, and on
> std.variant.VariantN is based on an union, and often you use algebraic
> data types to represent trees and similar data structures that contain
> many references/pointers. Adding Adding an onGC() method to
> std.variant.VariantN you allow the GC to manage Algebraic well enough.

I, too, felt the need of onGC() - actually preGC() - in my allocators 
implementation.

Specifically, a thread-local freelist would save a pointer to the root 
in thread-local storage (i.e. a traditional D global variable). That 
would thread through a number of free nodes available for allocation.

When a GC cycle occurs, it's okay if the list stays referenced; the GC 
will consider it "used" and won't do anything in particular about it. 
However, the GC cycle is a good opportunity to clean these freelists and 
offer the memory for other size classes, seeing as the freelists may 
grow unreasonably large and then just hold memory for no good reason.

A hook that nulls all freelist heads just as the collection process 
starts would be helpful.


Andrei