WIP: Queue Gc's rather than recursively tracing them #1
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| @@ -52,6 +52,7 @@ const ROOTS_MAX: usize = ROOTS_MASK; // max allowed value of roots | |||
| pub(crate) struct GcBoxHeader { | |||
| roots: Cell<usize>, // high bit is used as mark flag | |||
| next: Cell<Option<NonNull<GcBox<dyn Trace>>>>, | |||
| dyn_trace_ptr: Cell<Option<NonNull<GcBox<dyn Trace>>>>, | |||
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If NonNull<GcBox<dyn Trace>>'s are what it makes sense to put in the "queue" then I wasn't clear on how to get ahold of one at the time of calling Gc's Trace::trace() implementation (I seemed to be running into the fact that since there T can be ?Sized the compiler is saying it doesn't know how to make a trait object/fat pointer for something it doesn't know the size of or something along those lines?) so I resorted to storing one "up front" at the time of creation of Gc/GcBox/GcBoxHeader
Maybe another option to circumvent that would be to push closures (eg Box<dyn Fn()>) onto a queue instead of NonNull<GcBox<dyn Trace>>'s?
| } | ||
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| #[repr(C)] // to justify the layout computation in Gc::from_raw | ||
| pub(crate) struct GcBox<T: Trace + ?Sized + 'static> { | ||
| header: GcBoxHeader, | ||
| pub header: GcBoxHeader, |
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Just doing dirty stuff to wire it up
| while let Some(node) = mark_head { | ||
| if (*node.as_ptr()).header.roots() > 0 { | ||
| (*node.as_ptr()).trace_inner(); | ||
| MARK_QUEUE.with(|queue| { |
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My understanding is that this head is a linked list containing all currently known Gc's effectively, and what we're interested in doing here is only marking + calling into the nested Trace::trace() for ones that are rooted, and now we're trying to do that while avoiding "drilling down" into any nested Gc's that we encounter during tracing (but we still need to accomplish marking them and calling into their nested Trace::trace()'s)
So the basic idea is to modify Gc's Trace::trace() implementation to "push itself onto a queue" that then gets processed as a second pass
| } | ||
| while let Some(node) = { | ||
| let value = queue.borrow_mut().pop(); | ||
| value |
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This is just boilerplate to convince the compiler to drop the mutable borrow before calling into the while body (I don't know if there's a nicer/more idiomatic way to accomplish that)
This is addressing Manishearth#161
I doubt it would make sense to merge any version of this branch anywhere but am just opening it as a PR for the sake of being able to leave a couple comments