Garbage collection in Rust got a little better

52 points by polywolf

olliej

I really wish there was a function to force the GC to go named “fire”

mcherm

It took me a disturbingly long time to figure out what you meant here -- I had to recall the name of this particular GC library.
- olliej
  
  you're welcome :D
mplant

I’ve been working in this space for a long time for the garbage collector of my embedded scheme implementation scheme-rs, which has gone through many iterations. The solution I had for Trace was to instead make the visitor function itself a visitor: &mut dyn FnMut(OpaqueGcPtr). This is acceptable to me since I need to type-erase these functions anyway
elihunter173

For instance, I want to see if I can get niche-optimization for zero-overhead Option<Gc> and, of course, I always want to crank out extra performance.

From briefly reading the source code, it seems like this is blocked because null is used to indicate a dead pointer (see Gc::is_dead). Would it be possible to switch to a different sentinel value? E.g. -1.

My gut tells me this is fine on all(?) hardware. But I don't know if that runs afoul with some undefined behavior or Rust safety rules.
- claytonwramsey
  
  This isn’t the only blocking issue; otherwise I would use an unaligned pointer or a the maxed address and call it a day. The other problem is that UnsafeCell is exempt from niche optimization, so using a nonzero type internal still won’t allow Option<Gc> to be niche-optimized.