Luca's meaningless thoughts  

Accurate Garbage Collection in an Uncooperative Environment

by Leandro Lucarella on 2009- 03- 21 17:23 (updated on 2009- 03- 22 00:05)
tagged accurate, d, dgc, en, henderson, paper, tracing, uncooperative environment - with 0 comment(s)

I just read Accurate Garbage Collection in an Uncooperative Environment paper.

Unfortunately this paper try to solve mostly problems D don't see as problems, like portability (targeting languages that emit C code instead of native machine code, like the Mercury language mentioned in the paper). Based on the problem of tracing the C stack in a portable way, it suggests to inject some code to functions to construct a linked list of stack information (which contains local variables information) to be able to trace the stack in an accurate way.

I think none of the ideas presented by this paper are suitable for D, because the GC already can trace the stack in D (in an unportable way, but it can), and it can get the type info from better places too.

In terms of (time) performance, benchmarks shows that is a little worse than Boehm (et al) GC, but they argue that Boehm has years of fine grained optimizations and it's tightly coupled with the underlying architecture while this new approach is almost unoptimized yet and it's completely portable.

The only thing it mentions that could apply to D (and any conservative GC in general) is the issues that compiler optimizations can introduce. But I'm not aware of any of this issues, so I can't say anything about it.

In case you wonder, I've added this paper to my papers playground page =)

Update

I think I missed the point with this paper. Current D GC can't possibly do accurate tracing of the stack, because there is no way to get a type info from there (I was thinking only in the heap, where some degree of accuracy is achieved by setting the noscan bit for a bin that don't have pointers, as mentioned in my previous post).

So this paper could help getting accurate GC into D, but it doesn't seems a great deal when you can add type information about local variables when emitting machine code instead of adding the shadow stack linked list. The only advantage I see is that I think it should be possible to implement the linked list in the front-end.