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|
//! Global `Arc`-based object interning infrastructure.
//!
//! Eventually this should probably be replaced with salsa-based interning.
use std::{
collections::HashMap,
fmt::{self, Debug},
hash::{BuildHasherDefault, Hash, Hasher},
ops::Deref,
sync::Arc,
};
use dashmap::{lock::RwLockWriteGuard, DashMap, SharedValue};
use once_cell::sync::OnceCell;
use rustc_hash::FxHasher;
use crate::generics::GenericParams;
type InternMap<T> = DashMap<Arc<T>, (), BuildHasherDefault<FxHasher>>;
type Guard<T> =
RwLockWriteGuard<'static, HashMap<Arc<T>, SharedValue<()>, BuildHasherDefault<FxHasher>>>;
pub struct Interned<T: Internable + ?Sized> {
arc: Arc<T>,
}
impl<T: Internable> Interned<T> {
pub fn new(obj: T) -> Self {
match Interned::lookup(&obj) {
Ok(this) => this,
Err(shard) => {
let arc = Arc::new(obj);
Self::alloc(arc, shard)
}
}
}
}
impl<T: Internable + ?Sized> Interned<T> {
fn lookup(obj: &T) -> Result<Self, Guard<T>> {
let storage = T::storage().get();
let shard_idx = storage.determine_map(obj);
let shard = &storage.shards()[shard_idx];
let shard = shard.write();
// Atomically,
// - check if `obj` is already in the map
// - if so, clone its `Arc` and return it
// - if not, box it up, insert it, and return a clone
// This needs to be atomic (locking the shard) to avoid races with other thread, which could
// insert the same object between us looking it up and inserting it.
// FIXME: avoid double lookup/hashing by using raw entry API (once stable, or when
// hashbrown can be plugged into dashmap)
match shard.get_key_value(obj) {
Some((arc, _)) => Ok(Self { arc: arc.clone() }),
None => Err(shard),
}
}
fn alloc(arc: Arc<T>, mut shard: Guard<T>) -> Self {
let arc2 = arc.clone();
shard.insert(arc2, SharedValue::new(()));
Self { arc }
}
}
impl Interned<str> {
pub fn new_str(s: &str) -> Self {
match Interned::lookup(s) {
Ok(this) => this,
Err(shard) => {
let arc = Arc::<str>::from(s);
Self::alloc(arc, shard)
}
}
}
}
impl<T: Internable + ?Sized> Drop for Interned<T> {
#[inline]
fn drop(&mut self) {
// When the last `Ref` is dropped, remove the object from the global map.
if Arc::strong_count(&self.arc) == 2 {
// Only `self` and the global map point to the object.
self.drop_slow();
}
}
}
impl<T: Internable + ?Sized> Interned<T> {
#[cold]
fn drop_slow(&mut self) {
let storage = T::storage().get();
let shard_idx = storage.determine_map(&self.arc);
let shard = &storage.shards()[shard_idx];
let mut shard = shard.write();
// FIXME: avoid double lookup
let (arc, _) = shard.get_key_value(&self.arc).expect("interned value removed prematurely");
if Arc::strong_count(arc) != 2 {
// Another thread has interned another copy
return;
}
shard.remove(&self.arc);
// Shrink the backing storage if the shard is less than 50% occupied.
if shard.len() * 2 < shard.capacity() {
shard.shrink_to_fit();
}
}
}
/// Compares interned `Ref`s using pointer equality.
impl<T: Internable> PartialEq for Interned<T> {
// NOTE: No `?Sized` because `ptr_eq` doesn't work right with trait objects.
#[inline]
fn eq(&self, other: &Self) -> bool {
Arc::ptr_eq(&self.arc, &other.arc)
}
}
impl<T: Internable> Eq for Interned<T> {}
impl PartialEq for Interned<str> {
fn eq(&self, other: &Self) -> bool {
Arc::ptr_eq(&self.arc, &other.arc)
}
}
impl Eq for Interned<str> {}
impl<T: Internable + ?Sized> Hash for Interned<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
// NOTE: Cast disposes vtable pointer / slice/str length.
state.write_usize(Arc::as_ptr(&self.arc) as *const () as usize)
}
}
impl<T: Internable + ?Sized> AsRef<T> for Interned<T> {
#[inline]
fn as_ref(&self) -> &T {
&self.arc
}
}
impl<T: Internable + ?Sized> Deref for Interned<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
&self.arc
}
}
impl<T: Internable + ?Sized> Clone for Interned<T> {
fn clone(&self) -> Self {
Self { arc: self.arc.clone() }
}
}
impl<T: Debug + Internable + ?Sized> Debug for Interned<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(*self.arc).fmt(f)
}
}
pub struct InternStorage<T: ?Sized> {
map: OnceCell<InternMap<T>>,
}
impl<T: ?Sized> InternStorage<T> {
pub const fn new() -> Self {
Self { map: OnceCell::new() }
}
}
impl<T: Internable + ?Sized> InternStorage<T> {
fn get(&self) -> &InternMap<T> {
self.map.get_or_init(DashMap::default)
}
}
pub trait Internable: Hash + Eq + 'static {
fn storage() -> &'static InternStorage<Self>;
}
/// Implements `Internable` for a given list of types, making them usable with `Interned`.
#[macro_export]
#[doc(hidden)]
macro_rules! _impl_internable {
( $($t:path),+ $(,)? ) => { $(
impl Internable for $t {
fn storage() -> &'static InternStorage<Self> {
static STORAGE: InternStorage<$t> = InternStorage::new();
&STORAGE
}
}
)+ };
}
pub use crate::_impl_internable as impl_internable;
impl_internable!(
crate::type_ref::TypeRef,
crate::type_ref::TraitRef,
crate::path::ModPath,
GenericParams,
str,
);
|
