DConf 2013 Day 1 Talk 6: Concurrent Garbage Collection for D by Leandro Lucarella

[Diggory]

On Monday, 27 May 2013 at 17:56:10 UTC, Brian Rogoff wrote:
> On Friday, 24 May 2013 at 19:44:19 UTC, Jonathan M Davis wrote:
>> On Friday, May 24, 2013 20:30:54 Juan Manuel Cabo wrote:
>>> I'd like to know if there is interest in a precise garbage
>>> collector.
>>
>> There is interest in it, and Rainer Schütze did a talk on it 
>> at DConf. At the
>> current pace (assuming that Andrei actually posts one on 
>> Monday even though
>> it's a federal holiday in the US), it'll be posted on June 3rd 
>> (and if he
>> skips Monday, then it'll probably be June 5th). And actually, 
>> the precise GC
>> changes stand a much better chance of making it into druntime 
>> in the short
>> term than any concurrency changes do.
>>
>> - Jonathan M Davis
>
> That's very promising. The lack of precise garbage collection 
> and the unclear story with regards to programming sans-GC 
> (maybe it's clear to someone, but not to me) is far more of a 
> "deal breaker" for me than the lack of non-nullable pointers. I 
> hope that you're right and that this gets sorted out soon.
>
> -- Brian

It's actually possible to improve the precision of the GC without 
any additional type info. As long as you can give some unique ID 
to each type when you allocate it then the GC can learn the 
layout of that type on the fly.

For example a simple algorithm would be:
- When a new ID is first see create new type-info that is all 
pointers.
- While scanning an instance of that type, if a pointer points to 
a value in the higher half, or a sufficiently low value which is 
not equal to zero, then remove this pointer from the type-info.

You would have to disable this for unions, but for the rest it 
should work fine. Plus with more intelligent algorithms you can 
handle more cases. You could even save the type-info to a file 
and reuse it later to improve performance.