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Browse Source
This PR changes the following: - Modifies `EphemeronBox` to be more akin to `GcBox`, with its own header, roots and markers. This also makes it more similar to [Racket's](https://docs.racket-lang.org/reference/ephemerons.html) implementation. - Removes `EPHEMERON_QUEUE`. - Ephemerons are now tracked on a special `weak_start` linked list, instead of `strong_start` which is where all other GC boxes live. - Documents all unsafe blocks. - Documents our current garbage collection algorithm. I hope this'll clarify a bit what exactly are we doing on every garbage collection. - Renames/removes some functions.pull/2559/head
José Julián Espina
2 years ago
28 changed files with 838 additions and 577 deletions
@ -1,115 +1,210 @@
|
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use crate::{finalizer_safe, trace::Trace, Finalize, Gc, GcBox}; |
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use std::{cell::Cell, ptr::NonNull}; |
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/// The inner allocation of an [`Ephemeron`][crate::Ephemeron] pointer.
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pub(crate) struct EphemeronBox<K: Trace + ?Sized + 'static, V: Trace + ?Sized + 'static> { |
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key: Cell<Option<NonNull<GcBox<K>>>>, |
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value: V, |
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use crate::{trace::Trace, Gc, GcBox}; |
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use std::{ |
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cell::Cell, |
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ptr::{self, NonNull}, |
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}; |
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// Age and Weak Flags
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const MARK_MASK: usize = 1 << (usize::BITS - 1); |
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const ROOTS_MASK: usize = !MARK_MASK; |
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const ROOTS_MAX: usize = ROOTS_MASK; |
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/// The `EphemeronBoxHeader` contains the `EphemeronBoxHeader`'s current state for the `Collector`'s
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/// Mark/Sweep as well as a pointer to the next ephemeron in the heap.
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///
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/// These flags include:
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/// - Root Count
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/// - Mark Flag Bit
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///
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/// The next node is set by the `Allocator` during initialization and by the
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/// `Collector` during the sweep phase.
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pub(crate) struct EphemeronBoxHeader { |
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roots: Cell<usize>, |
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pub(crate) next: Cell<Option<NonNull<dyn ErasedEphemeronBox>>>, |
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} |
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impl<K: Trace + ?Sized, V: Trace> EphemeronBox<K, V> { |
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pub(crate) fn new(key: &Gc<K>, value: V) -> Self { |
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impl EphemeronBoxHeader { |
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/// Creates a new `EphemeronBoxHeader` with a root of 1 and next set to None.
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pub(crate) fn new() -> Self { |
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Self { |
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key: Cell::new(Some(key.inner_ptr())), |
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value, |
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roots: Cell::new(1), |
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next: Cell::new(None), |
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} |
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} |
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} |
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impl<K: Trace + ?Sized, V: Trace + ?Sized> EphemeronBox<K, V> { |
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/// Checks if the key pointer is marked by Trace
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pub(crate) fn is_marked(&self) -> bool { |
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self.inner_key().map_or(false, GcBox::is_marked) |
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/// Returns the `EphemeronBoxHeader`'s current root count
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pub(crate) fn roots(&self) -> usize { |
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self.roots.get() & ROOTS_MASK |
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} |
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/// Returns some pointer to the `key`'s `GcBox` or None
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/// # Panics
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/// This method will panic if called while the garbage collector is dropping.
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pub(crate) fn inner_key_ptr(&self) -> Option<*mut GcBox<K>> { |
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assert!(finalizer_safe()); |
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self.key.get().map(NonNull::as_ptr) |
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/// Increments `EphemeronBoxHeader`'s root count.
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pub(crate) fn inc_roots(&self) { |
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let roots = self.roots.get(); |
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if (roots & ROOTS_MASK) < ROOTS_MAX { |
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self.roots.set(roots + 1); |
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} else { |
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// TODO: implement a better way to handle root overload.
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panic!("roots counter overflow"); |
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} |
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} |
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/// Returns some reference to `key`'s `GcBox` or None
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pub(crate) fn inner_key(&self) -> Option<&GcBox<K>> { |
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// SAFETY: This is safe as `EphemeronBox::inner_key_ptr()` will
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// fetch either a live `GcBox` or None. The value of `key` is set
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// to None in the case where `EphemeronBox` and `key`'s `GcBox`
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// entered into `Collector::sweep()` as unmarked.
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unsafe { self.inner_key_ptr().map(|inner_key| &*inner_key) } |
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/// Decreases `EphemeronBoxHeader`'s current root count.
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pub(crate) fn dec_roots(&self) { |
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// Underflow check as a stop gap for current issue when dropping.
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if self.roots.get() > 0 { |
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self.roots.set(self.roots.get() - 1); |
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} |
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} |
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/// Returns a reference to the value of `key`'s `GcBox`
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pub(crate) fn key(&self) -> Option<&K> { |
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self.inner_key().map(GcBox::value) |
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/// Returns a bool for whether `EphemeronBoxHeader`'s mark bit is 1.
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pub(crate) fn is_marked(&self) -> bool { |
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self.roots.get() & MARK_MASK != 0 |
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} |
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/// Returns a reference to `value`
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pub(crate) const fn value(&self) -> &V { |
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&self.value |
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/// Sets `EphemeronBoxHeader`'s mark bit to 1.
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pub(crate) fn mark(&self) { |
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self.roots.set(self.roots.get() | MARK_MASK); |
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} |
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/// Calls [`Trace::weak_trace()`][crate::Trace] on key
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fn weak_trace_key(&self) { |
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if let Some(key) = self.inner_key() { |
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key.weak_trace_inner(); |
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} |
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/// Sets `EphemeronBoxHeader`'s mark bit to 0.
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pub(crate) fn unmark(&self) { |
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self.roots.set(self.roots.get() & !MARK_MASK); |
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} |
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} |
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/// Calls [`Trace::weak_trace()`][crate::Trace] on value
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fn weak_trace_value(&self) { |
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// SAFETY: Value is a sized element that must implement trace. The
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// operation is safe as EphemeronBox owns value and `Trace::weak_trace`
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// must be implemented on it
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unsafe { |
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self.value().weak_trace(); |
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} |
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impl core::fmt::Debug for EphemeronBoxHeader { |
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fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { |
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f.debug_struct("EphemeronBoxHeader") |
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.field("roots", &self.roots()) |
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.field("marked", &self.is_marked()) |
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.finish() |
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} |
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} |
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// `EphemeronBox`'s Finalize is special in that if it is determined to be unreachable
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// and therefore so has the `GcBox` that `key`stores the pointer to, then we set `key`
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// to None to guarantee that we do not access freed memory.
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impl<K: Trace + ?Sized, V: Trace + ?Sized> Finalize for EphemeronBox<K, V> { |
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fn finalize(&self) { |
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self.key.set(None); |
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/// The inner allocation of an [`Ephemeron`][crate::Ephemeron] pointer.
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pub(crate) struct EphemeronBox<K: Trace + ?Sized + 'static, V: Trace + 'static> { |
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pub(crate) header: EphemeronBoxHeader, |
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data: Cell<Option<NonNull<Data<K, V>>>>, |
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} |
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impl<K: Trace + ?Sized + 'static, V: Trace + 'static> Drop for EphemeronBox<K, V> { |
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fn drop(&mut self) { |
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if let Some(data) = self.data.take() { |
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// SAFETY: `data` comes from an `into_raw` call, so this pointer is safe to pass to
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// `from_raw`.
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drop(unsafe { Box::from_raw(data.as_ptr()) }); |
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} |
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} |
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} |
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// SAFETY: EphemeronBox implements primarly two methods of trace `Trace::is_marked_ephemeron`
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// to determine whether the key field is stored and `Trace::weak_trace` which continues the `Trace::weak_trace()`
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// into `key` and `value`.
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unsafe impl<K: Trace + ?Sized, V: Trace + ?Sized> Trace for EphemeronBox<K, V> { |
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unsafe fn trace(&self) { |
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/* An ephemeron is never traced with Phase One Trace */ |
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struct Data<K: Trace + ?Sized + 'static, V: Trace + 'static> { |
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key: NonNull<GcBox<K>>, |
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value: V, |
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} |
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impl<K: Trace + ?Sized, V: Trace> EphemeronBox<K, V> { |
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pub(crate) fn new(key: &Gc<K>, value: V) -> Self { |
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let data = Box::into_raw(Box::new(Data { |
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key: key.inner_ptr(), |
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value, |
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})); |
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// SAFETY: `Box::into_raw` must always return a non-null pointer.
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let data = unsafe { NonNull::new_unchecked(data) }; |
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Self { |
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header: EphemeronBoxHeader::new(), |
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data: Cell::new(Some(data)), |
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} |
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} |
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/// Checks if the `key`'s `GcBox` has been marked by `Trace::trace()` or `Trace::weak_trace`.
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fn is_marked_ephemeron(&self) -> bool { |
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self.is_marked() |
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/// Returns `true` if the two references refer to the same `GcBox`.
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pub(crate) fn ptr_eq(this: &Self, other: &Self) -> bool { |
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// Use .header to ignore fat pointer vtables, to work around
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// https://github.com/rust-lang/rust/issues/46139
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ptr::eq(&this.header, &other.header) |
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} |
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/// Checks if this `EphemeronBox` has already been determined reachable. If so, continue to trace
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/// value in `key` and `value`.
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unsafe fn weak_trace(&self) { |
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if self.is_marked() { |
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self.weak_trace_key(); |
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self.weak_trace_value(); |
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} |
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/// Returns a reference to the ephemeron's value or None.
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pub(crate) fn value(&self) -> Option<&V> { |
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// SAFETY: the garbage collector ensures `ptr` is valid as long as `data` is `Some`.
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unsafe { self.data.get().map(|ptr| &ptr.as_ref().value) } |
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} |
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/// Marks this `EphemeronBox` as live.
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///
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/// This doesn't mark the inner value of the ephemeron. [`ErasedEphemeronBox::trace`]
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/// does this, and it's called by the garbage collector on demand.
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pub(crate) unsafe fn mark(&self) { |
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self.header.mark(); |
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} |
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/// Increases the root count on this `EphemeronBox`.
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///
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/// Roots prevent the `EphemeronBox` from being destroyed by the garbage collector.
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pub(crate) fn root(&self) { |
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self.header.inc_roots(); |
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} |
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/// Decreases the root count on this `EphemeronBox`.
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///
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/// Roots prevent the `EphemeronBox` from being destroyed by the garbage collector.
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pub(crate) fn unroot(&self) { |
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self.header.dec_roots(); |
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} |
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} |
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pub(crate) trait ErasedEphemeronBox { |
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/// Gets the header of the `EphemeronBox`.
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fn header(&self) -> &EphemeronBoxHeader; |
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/// Traces through the `EphemeronBox`'s held value, but only if it's marked and its key is also
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/// marked. Returns `true` if the ephemeron successfuly traced through its value. This also
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/// considers ephemerons that are marked but don't have their value anymore as
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/// "successfully traced".
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unsafe fn trace(&self) -> bool; |
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/// Runs the finalization logic of the `EphemeronBox`'s held value, if the key is still live,
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/// and clears its contents.
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fn finalize_and_clear(&self); |
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} |
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impl<K: Trace + ?Sized, V: Trace> ErasedEphemeronBox for EphemeronBox<K, V> { |
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fn header(&self) -> &EphemeronBoxHeader { |
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&self.header |
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} |
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// EphemeronBox does not implement root.
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unsafe fn root(&self) {} |
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unsafe fn trace(&self) -> bool { |
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if !self.header.is_marked() { |
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return false; |
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} |
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let Some(data) = self.data.get() else { |
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return true; |
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}; |
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// EphemeronBox does not implement unroot
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unsafe fn unroot(&self) {} |
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// SAFETY: `data` comes from a `Box`, so it is safe to dereference.
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let data = unsafe { data.as_ref() }; |
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// SAFETY: `key` comes from a `Gc`, and the garbage collector only invalidates
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// `key` when it is unreachable, making `key` always valid.
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let key = unsafe { data.key.as_ref() }; |
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// An `EphemeronBox`'s key is set to None once it has been finalized.
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//
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// NOTE: while it is possible for the `key`'s pointer value to be
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// resurrected, we should still consider the finalize the ephemeron
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// box and set the `key` to None.
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fn run_finalizer(&self) { |
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Finalize::finalize(self); |
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let is_key_marked = key.is_marked(); |
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if is_key_marked { |
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// SAFETY: this is safe to call, since we want to trace all reachable objects
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// from a marked ephemeron that holds a live `key`.
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unsafe { data.value.trace() } |
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} |
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is_key_marked |
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} |
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fn finalize_and_clear(&self) { |
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if let Some(data) = self.data.take() { |
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// SAFETY: `data` comes from an `into_raw` call, so this pointer is safe to pass to
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// `from_raw`.
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let contents = unsafe { Box::from_raw(data.as_ptr()) }; |
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contents.value.finalize(); |
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} |
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} |
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} |
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@ -1,5 +1,5 @@
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mod ephemeron_box; |
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mod gc_box; |
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pub(crate) use self::ephemeron_box::EphemeronBox; |
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pub(crate) use self::ephemeron_box::{EphemeronBox, ErasedEphemeronBox}; |
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pub use self::gc_box::GcBox; |
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@ -0,0 +1,87 @@
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use std::ptr::{self, addr_of_mut, NonNull}; |
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/// A [`NonNull`] pointer with a `rooted` tag.
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///
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/// This pointer can be created only from pointers that are 2-byte aligned. In other words,
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/// the pointer must point to an address that is a multiple of 2.
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pub(crate) struct Rootable<T: ?Sized> { |
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ptr: NonNull<T>, |
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} |
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impl<T: ?Sized> Copy for Rootable<T> {} |
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impl<T: ?Sized> Clone for Rootable<T> { |
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fn clone(&self) -> Self { |
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Self { ptr: self.ptr } |
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} |
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} |
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impl<T: ?Sized> std::fmt::Debug for Rootable<T> { |
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { |
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f.debug_struct("Rootable") |
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.field("ptr", &self.as_ptr()) |
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.field("is_rooted", &self.is_rooted()) |
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.finish() |
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} |
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} |
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impl<T: ?Sized> Rootable<T> { |
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/// Creates a new `Rootable` without checking if the [`NonNull`] is properly aligned.
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///
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/// # Safety
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///
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/// `ptr` must be 2-byte aligned.
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pub(crate) const unsafe fn new_unchecked(ptr: NonNull<T>) -> Self { |
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Self { ptr } |
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} |
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/// Returns `true` if the pointer is rooted.
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pub(crate) fn is_rooted(self) -> bool { |
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self.ptr.as_ptr().cast::<u8>() as usize & 1 != 0 |
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} |
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/// Returns a pointer with the same address as `self` but rooted.
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pub(crate) fn rooted(self) -> Self { |
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let ptr = self.ptr.as_ptr(); |
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let data = ptr.cast::<u8>(); |
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let addr = data as isize; |
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let ptr = set_data_ptr(ptr, data.wrapping_offset((addr | 1) - addr)); |
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// SAFETY: ptr must be a non null value.
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unsafe { Self::new_unchecked(NonNull::new_unchecked(ptr)) } |
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} |
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/// Returns a pointer with the same address as `self` but unrooted.
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pub(crate) fn unrooted(self) -> Self { |
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let ptr = self.ptr.as_ptr(); |
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let data = ptr.cast::<u8>(); |
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let addr = data as isize; |
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let ptr = set_data_ptr(ptr, data.wrapping_offset((addr & !1) - addr)); |
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// SAFETY: ptr must be a non null value
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unsafe { Self::new_unchecked(NonNull::new_unchecked(ptr)) } |
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} |
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/// Acquires the underlying `NonNull` pointer.
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pub(crate) fn as_ptr(self) -> NonNull<T> { |
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self.unrooted().ptr |
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} |
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/// Returns a shared reference to the pointee.
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///
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/// # Safety
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///
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/// See [`NonNull::as_ref`].
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pub(crate) unsafe fn as_ref(&self) -> &T { |
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// SAFETY: it is the caller's job to ensure the safety of this operation.
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unsafe { self.as_ptr().as_ref() } |
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} |
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} |
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// Technically, this function is safe, since we're just modifying the address of a pointer without
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// dereferencing it.
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fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T { |
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// SAFETY: this should be safe as ptr must be a valid nonnull
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unsafe { |
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ptr::write(addr_of_mut!(ptr).cast::<*mut u8>(), data.cast::<u8>()); |
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} |
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ptr |
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} |
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