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//! Boa's **`boa_gc`** crate implements a garbage collector.
//!
//! # Crate Overview
//! **`boa_gc`** is a mark-sweep garbage collector that implements a [`Trace`] and [`Finalize`] trait
//! for garbage collected values.
#![doc = include_str!("../ABOUT.md")]
#![doc(
html_logo_url = "https://raw.githubusercontent.com/boa-dev/boa/main/assets/logo.svg",
html_favicon_url = "https://raw.githubusercontent.com/boa-dev/boa/main/assets/logo.svg"
)]
#![cfg_attr(not(test), forbid(clippy::unwrap_used))]
#![allow(
clippy::module_name_repetitions,
clippy::redundant_pub_crate,
clippy::let_unit_value
)]
extern crate self as boa_gc;
mod cell;
mod pointers;
mod trace;
pub(crate) mod internals;
use boa_profiler::Profiler;
use internals::{EphemeronBox, ErasedEphemeronBox, ErasedWeakMapBox, WeakMapBox};
use pointers::{NonTraceable, RawWeakMap};
use std::{
cell::{Cell, RefCell},
mem,
ptr::NonNull,
};
pub use crate::trace::{Finalize, Trace, Tracer};
pub use boa_macros::{Finalize, Trace};
pub use cell::{GcRef, GcRefCell, GcRefMut};
pub use internals::GcBox;
pub use pointers::{Ephemeron, Gc, WeakGc, WeakMap};
type GcErasedPointer = NonNull<GcBox<NonTraceable>>;
type EphemeronPointer = NonNull<dyn ErasedEphemeronBox>;
type ErasedWeakMapBoxPointer = NonNull<dyn ErasedWeakMapBox>;
thread_local!(static GC_DROPPING: Cell<bool> = Cell::new(false));
thread_local!(static BOA_GC: RefCell<BoaGc> = RefCell::new( BoaGc {
config: GcConfig::default(),
runtime: GcRuntimeData::default(),
strongs: Vec::default(),
weaks: Vec::default(),
weak_maps: Vec::default(),
}));
#[derive(Debug, Clone, Copy)]
struct GcConfig {
threshold: usize,
used_space_percentage: usize,
}
// Setting the defaults to an arbitrary value currently.
//
// TODO: Add a configure later
impl Default for GcConfig {
fn default() -> Self {
Self {
threshold: 1024,
used_space_percentage: 70,
}
}
}
#[derive(Default, Debug, Clone, Copy)]
struct GcRuntimeData {
collections: usize,
bytes_allocated: usize,
}
#[derive(Debug)]
struct BoaGc {
config: GcConfig,
runtime: GcRuntimeData,
strongs: Vec<GcErasedPointer>,
weaks: Vec<EphemeronPointer>,
weak_maps: Vec<ErasedWeakMapBoxPointer>,
}
impl Drop for BoaGc {
fn drop(&mut self) {
Collector::dump(self);
}
}
// Whether or not the thread is currently in the sweep phase of garbage collection.
// During this phase, attempts to dereference a `Gc<T>` pointer will trigger a panic.
/// `DropGuard` flags whether the Collector is currently running `Collector::sweep()` or `Collector::dump()`
///
/// While the `DropGuard` is active, all `GcBox`s must not be dereferenced or accessed as it could cause Undefined Behavior
#[derive(Debug, Clone)]
struct DropGuard;
impl DropGuard {
fn new() -> Self {
GC_DROPPING.with(|dropping| dropping.set(true));
Self
}
}
impl Drop for DropGuard {
fn drop(&mut self) {
GC_DROPPING.with(|dropping| dropping.set(false));
}
}
/// Returns `true` if it is safe for a type to run [`Finalize::finalize`].
#[must_use]
#[inline]
pub fn finalizer_safe() -> bool {
GC_DROPPING.with(|dropping| !dropping.get())
}
/// The Allocator handles allocation of garbage collected values.
///
/// The allocator can trigger a garbage collection.
#[derive(Debug, Clone, Copy)]
struct Allocator;
impl Allocator {
/// Allocate a new garbage collected value to the Garbage Collector's heap.
fn alloc_gc<T: Trace>(value: GcBox<T>) -> NonNull<GcBox<T>> {
let _timer = Profiler::global().start_event("New GcBox", "BoaAlloc");
let element_size = mem::size_of_val::<GcBox<T>>(&value);
BOA_GC.with(|st| {
let mut gc = st.borrow_mut();
Self::manage_state(&mut gc);
// Safety: value cannot be a null pointer, since `Box` cannot return null pointers.
let ptr = unsafe { NonNull::new_unchecked(Box::into_raw(Box::new(value))) };
let erased: NonNull<GcBox<NonTraceable>> = ptr.cast();
gc.strongs.push(erased);
gc.runtime.bytes_allocated += element_size;
ptr
})
}
fn alloc_ephemeron<K: Trace + ?Sized, V: Trace>(
value: EphemeronBox<K, V>,
) -> NonNull<EphemeronBox<K, V>> {
let _timer = Profiler::global().start_event("New EphemeronBox", "BoaAlloc");
let element_size = mem::size_of_val::<EphemeronBox<K, V>>(&value);
BOA_GC.with(|st| {
let mut gc = st.borrow_mut();
Self::manage_state(&mut gc);
// Safety: value cannot be a null pointer, since `Box` cannot return null pointers.
let ptr = unsafe { NonNull::new_unchecked(Box::into_raw(Box::new(value))) };
let erased: NonNull<dyn ErasedEphemeronBox> = ptr;
gc.weaks.push(erased);
gc.runtime.bytes_allocated += element_size;
ptr
})
}
fn alloc_weak_map<K: Trace + ?Sized, V: Trace + Clone>() -> WeakMap<K, V> {
let _timer = Profiler::global().start_event("New WeakMap", "BoaAlloc");
let weak_map = WeakMap {
inner: Gc::new(GcRefCell::new(RawWeakMap::new())),
};
let weak = WeakGc::new(&weak_map.inner);
BOA_GC.with(|st| {
let mut gc = st.borrow_mut();
let weak_box = WeakMapBox { map: weak };
// Safety: value cannot be a null pointer, since `Box` cannot return null pointers.
let ptr = unsafe { NonNull::new_unchecked(Box::into_raw(Box::new(weak_box))) };
let erased: ErasedWeakMapBoxPointer = ptr;
gc.weak_maps.push(erased);
weak_map
})
}
fn manage_state(gc: &mut BoaGc) {
if gc.runtime.bytes_allocated > gc.config.threshold {
Collector::collect(gc);
if gc.runtime.bytes_allocated
> gc.config.threshold / 100 * gc.config.used_space_percentage
{
gc.config.threshold =
gc.runtime.bytes_allocated / gc.config.used_space_percentage * 100;
}
}
}
}
struct Unreachables {
strong: Vec<GcErasedPointer>,
weak: Vec<NonNull<dyn ErasedEphemeronBox>>,
}
/// This collector currently functions in four main phases
///
/// Mark -> Finalize -> Mark -> Sweep
///
/// 1. Mark nodes as reachable.
/// 2. Finalize the unreachable nodes.
/// 3. Mark again because `Finalize::finalize` can potentially resurrect dead nodes.
/// 4. Sweep and drop all dead nodes.
///
/// A better approach in a more concurrent structure may be to reorder.
///
/// Mark -> Sweep -> Finalize
struct Collector;
impl Collector {
/// Run a collection on the full heap.
fn collect(gc: &mut BoaGc) {
let _timer = Profiler::global().start_event("Gc Full Collection", "gc");
gc.runtime.collections += 1;
Self::trace_non_roots(gc);
let mut tracer = Tracer::new();
let unreachables = Self::mark_heap(&mut tracer, &gc.strongs, &gc.weaks, &gc.weak_maps);
assert!(tracer.is_empty(), "The queue should be empty");
// Only finalize if there are any unreachable nodes.
if !unreachables.strong.is_empty() || !unreachables.weak.is_empty() {
// Finalize all the unreachable nodes.
// SAFETY: All passed pointers are valid, since we won't deallocate until `Self::sweep`.
unsafe { Self::finalize(unreachables) };
// Reuse the tracer's already allocated capacity.
let _final_unreachables =
Self::mark_heap(&mut tracer, &gc.strongs, &gc.weaks, &gc.weak_maps);
}
// SAFETY: The head of our linked list is always valid per the invariants of our GC.
unsafe {
Self::sweep(
&mut gc.strongs,
&mut gc.weaks,
&mut gc.runtime.bytes_allocated,
);
}
// Weak maps have to be cleared after the sweep, since the process dereferences GcBoxes.
gc.weak_maps.retain(|w| {
// SAFETY: The caller must ensure the validity of every node of `heap_start`.
let node_ref = unsafe { w.as_ref() };
if node_ref.is_live() {
node_ref.clear_dead_entries();
true
} else {
// SAFETY:
// The `Allocator` must always ensure its start node is a valid, non-null pointer that
// was allocated by `Box::from_raw(Box::new(..))`.
let _unmarked_node = unsafe { Box::from_raw(w.as_ptr()) };
false
}
});
gc.strongs.shrink_to(gc.strongs.len() >> 2);
gc.weaks.shrink_to(gc.weaks.len() >> 2);
gc.weak_maps.shrink_to(gc.weak_maps.len() >> 2);
}
fn trace_non_roots(gc: &BoaGc) {
// Count all the handles located in GC heap.
// Then, we can find whether there is a reference from other places, and they are the roots.
for node in &gc.strongs {
// SAFETY: node must be valid as this phase cannot drop any node.
let trace_non_roots_fn = unsafe { node.as_ref() }.trace_non_roots_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
trace_non_roots_fn(*node);
}
}
for eph in &gc.weaks {
// SAFETY: node must be valid as this phase cannot drop any node.
let eph_ref = unsafe { eph.as_ref() };
eph_ref.trace_non_roots();
}
}
/// Walk the heap and mark any nodes deemed reachable
fn mark_heap(
tracer: &mut Tracer,
strongs: &[GcErasedPointer],
weaks: &[EphemeronPointer],
weak_maps: &[ErasedWeakMapBoxPointer],
) -> Unreachables {
let _timer = Profiler::global().start_event("Gc Marking", "gc");
// Walk the list, tracing and marking the nodes
let mut strong_dead = Vec::new();
let mut pending_ephemerons = Vec::new();
// === Preliminary mark phase ===
//
// 0. Get the naive list of possibly dead nodes.
for node in strongs {
// SAFETY: node must be valid as this phase cannot drop any node.
let node_ref = unsafe { node.as_ref() };
if node_ref.is_rooted() {
tracer.enqueue(*node);
while let Some(node) = tracer.next() {
// SAFETY: the gc heap object should be alive if there is a root.
let node_ref = unsafe { node.as_ref() };
if !node_ref.header.is_marked() {
node_ref.header.mark();
// SAFETY: if `GcBox::trace_inner()` has been called, then,
// this box must have been deemed as reachable via tracing
// from a root, which by extension means that value has not
// been dropped either.
let trace_fn = node_ref.trace_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe { trace_fn(node, tracer) }
}
}
} else if !node_ref.is_marked() {
strong_dead.push(*node);
}
}
// 0.1. Early return if there are no ephemerons in the GC
if weaks.is_empty() {
strong_dead.retain_mut(|node| {
// SAFETY: node must be valid as this phase cannot drop any node.
unsafe { !node.as_ref().is_marked() }
});
return Unreachables {
strong: strong_dead,
weak: Vec::new(),
};
}
// === Weak mark phase ===
//
//
// 1. Get the naive list of ephemerons that are supposedly dead or their key is dead and
// trace all the ephemerons that have roots and their keys are live. Also remove from
// this list the ephemerons that are marked but their value is dead.
for eph in weaks {
// SAFETY: node must be valid as this phase cannot drop any node.
let eph_ref = unsafe { eph.as_ref() };
let header = eph_ref.header();
if header.is_rooted() {
header.mark();
}
// SAFETY: the garbage collector ensures `eph_ref` always points to valid data.
if unsafe { !eph_ref.trace(tracer) } {
pending_ephemerons.push(*eph);
}
while let Some(node) = tracer.next() {
// SAFETY: node must be valid as this phase cannot drop any node.
let trace_fn = unsafe { node.as_ref() }.trace_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
trace_fn(node, tracer);
}
}
}
// 2. Trace all the weak pointers in the live weak maps to make sure they do not get swept.
for w in weak_maps {
// SAFETY: node must be valid as this phase cannot drop any node.
let node_ref = unsafe { w.as_ref() };
// SAFETY: The garbage collector ensures that all nodes are valid.
unsafe { node_ref.trace(tracer) };
while let Some(node) = tracer.next() {
// SAFETY: node must be valid as this phase cannot drop any node.
let trace_fn = unsafe { node.as_ref() }.trace_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
trace_fn(node, tracer);
}
}
}
// 3. Iterate through all pending ephemerons, removing the ones which have been successfully
// traced. If there are no changes in the pending ephemerons list, it means that there are no
// more reachable ephemerons from the remaining ephemeron values.
let mut previous_len = pending_ephemerons.len();
loop {
pending_ephemerons.retain_mut(|eph| {
// SAFETY: node must be valid as this phase cannot drop any node.
let eph_ref = unsafe { eph.as_ref() };
// SAFETY: the garbage collector ensures `eph_ref` always points to valid data.
let is_key_marked = unsafe { !eph_ref.trace(tracer) };
while let Some(node) = tracer.next() {
// SAFETY: node must be valid as this phase cannot drop any node.
let trace_fn = unsafe { node.as_ref() }.trace_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
trace_fn(node, tracer);
}
}
is_key_marked
});
if previous_len == pending_ephemerons.len() {
break;
}
previous_len = pending_ephemerons.len();
}
// 4. The remaining list should contain the ephemerons that are either unreachable or its key
// is dead. Cleanup the strong pointers since this procedure could have marked some more strong
// pointers.
strong_dead.retain_mut(|node| {
// SAFETY: node must be valid as this phase cannot drop any node.
unsafe { !node.as_ref().is_marked() }
});
Unreachables {
strong: strong_dead,
weak: pending_ephemerons,
}
}
/// # Safety
///
/// Passing a `strong` or a `weak` vec with invalid pointers will result in Undefined Behaviour.
unsafe fn finalize(unreachables: Unreachables) {
let _timer = Profiler::global().start_event("Gc Finalization", "gc");
for node in unreachables.strong {
// SAFETY: The caller must ensure all pointers inside `unreachables.strong` are valid.
let node_ref = unsafe { node.as_ref() };
let run_finalizer_fn = node_ref.run_finalizer_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
run_finalizer_fn(node);
}
}
for node in unreachables.weak {
// SAFETY: The caller must ensure all pointers inside `unreachables.weak` are valid.
let node = unsafe { node.as_ref() };
node.finalize_and_clear();
}
}
/// # Safety
///
/// - Providing an invalid pointer in the `heap_start` or in any of the headers of each
/// node will result in Undefined Behaviour.
/// - Providing a list of pointers that weren't allocated by `Box::into_raw(Box::new(..))`
/// will result in Undefined Behaviour.
unsafe fn sweep(
strong: &mut Vec<GcErasedPointer>,
weak: &mut Vec<EphemeronPointer>,
total_allocated: &mut usize,
) {
let _timer = Profiler::global().start_event("Gc Sweeping", "gc");
let _guard = DropGuard::new();
strong.retain(|node| {
// SAFETY: The caller must ensure the validity of every node of `heap_start`.
let node_ref = unsafe { node.as_ref() };
if node_ref.is_marked() {
node_ref.header.unmark();
node_ref.reset_non_root_count();
true
} else {
// SAFETY: The algorithm ensures only unmarked/unreachable pointers are dropped.
// The caller must ensure all pointers were allocated by `Box::into_raw(Box::new(..))`.
let drop_fn = node_ref.drop_fn();
let size = node_ref.size();
*total_allocated -= size;
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
drop_fn(*node);
}
false
}
});
weak.retain(|eph| {
// SAFETY: The caller must ensure the validity of every node of `heap_start`.
let eph_ref = unsafe { eph.as_ref() };
let header = eph_ref.header();
if header.is_marked() {
header.unmark();
header.reset_non_root_count();
true
} else {
// SAFETY: The algorithm ensures only unmarked/unreachable pointers are dropped.
// The caller must ensure all pointers were allocated by `Box::into_raw(Box::new(..))`.
let unmarked_eph = unsafe { Box::from_raw(eph.as_ptr()) };
let unallocated_bytes = mem::size_of_val(&*unmarked_eph);
*total_allocated -= unallocated_bytes;
false
}
});
}
// Clean up the heap when BoaGc is dropped
fn dump(gc: &mut BoaGc) {
// Weak maps have to be dropped first, since the process dereferences GcBoxes.
// This can be done without initializing a dropguard since no GcBox's are being dropped.
for node in std::mem::take(&mut gc.weak_maps) {
// SAFETY:
// The `Allocator` must always ensure its start node is a valid, non-null pointer that
// was allocated by `Box::from_raw(Box::new(..))`.
let _unmarked_node = unsafe { Box::from_raw(node.as_ptr()) };
}
// Not initializing a dropguard since this should only be invoked when BOA_GC is being dropped.
let _guard = DropGuard::new();
for node in std::mem::take(&mut gc.strongs) {
// SAFETY:
// The `Allocator` must always ensure its start node is a valid, non-null pointer that
// was allocated by `Box::from_raw(Box::new(..))`.
let drop_fn = unsafe { node.as_ref() }.drop_fn();
// SAFETY: The function pointer is appropriate for this node type because we extract it from it's VTable.
unsafe {
drop_fn(node);
}
}
for node in std::mem::take(&mut gc.weaks) {
// SAFETY:
// The `Allocator` must always ensure its start node is a valid, non-null pointer that
// was allocated by `Box::from_raw(Box::new(..))`.
let _unmarked_node = unsafe { Box::from_raw(node.as_ptr()) };
}
}
}
/// Forcefully runs a garbage collection of all unaccessible nodes.
pub fn force_collect() {
BOA_GC.with(|current| {
let mut gc = current.borrow_mut();
if gc.runtime.bytes_allocated > 0 {
Collector::collect(&mut gc);
}
});
}
#[cfg(test)]
mod test;
/// Returns `true` is any weak maps are currently allocated.
#[cfg(test)]
#[must_use]
pub fn has_weak_maps() -> bool {
BOA_GC.with(|current| {
let gc = current.borrow();
!gc.weak_maps.is_empty()
})
}