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use parking_lot::Mutex;
use std::cell::Cell;
use std::cell::OnceCell;
use std::ops::Deref;
use std::ptr;
use std::sync::Arc;
#[cfg(parallel_compiler)]
use {crate::outline, crate::sync::CacheAligned};
/// A pointer to the `RegistryData` which uniquely identifies a registry.
/// This identifier can be reused if the registry gets freed.
#[derive(Clone, Copy, PartialEq)]
struct RegistryId(*const RegistryData);
impl RegistryId {
#[inline(always)]
/// Verifies that the current thread is associated with the registry and returns its unique
/// index within the registry. This panics if the current thread is not associated with this
/// registry.
///
/// Note that there's a race possible where the identifer in `THREAD_DATA` could be reused
/// so this can succeed from a different registry.
#[cfg(parallel_compiler)]
fn verify(self) -> usize {
let (id, index) = THREAD_DATA.with(|data| (data.registry_id.get(), data.index.get()));
if id == self { index } else { outline(|| panic!("Unable to verify registry association")) }
}
}
struct RegistryData {
thread_limit: usize,
threads: Mutex<usize>,
}
/// Represents a list of threads which can access worker locals.
#[derive(Clone)]
pub struct Registry(Arc<RegistryData>);
thread_local! {
/// The registry associated with the thread.
/// This allows the `WorkerLocal` type to clone the registry in its constructor.
static REGISTRY: OnceCell<Registry> = OnceCell::new();
}
struct ThreadData {
registry_id: Cell<RegistryId>,
index: Cell<usize>,
}
thread_local! {
/// A thread local which contains the identifer of `REGISTRY` but allows for faster access.
/// It also holds the index of the current thread.
static THREAD_DATA: ThreadData = const { ThreadData {
registry_id: Cell::new(RegistryId(ptr::null())),
index: Cell::new(0),
}};
}
impl Registry {
/// Creates a registry which can hold up to `thread_limit` threads.
pub fn new(thread_limit: usize) -> Self {
Registry(Arc::new(RegistryData { thread_limit, threads: Mutex::new(0) }))
}
/// Gets the registry associated with the current thread. Panics if there's no such registry.
pub fn current() -> Self {
REGISTRY.with(|registry| registry.get().cloned().expect("No assocated registry"))
}
/// Registers the current thread with the registry so worker locals can be used on it.
/// Panics if the thread limit is hit or if the thread already has an associated registry.
pub fn register(&self) {
let mut threads = self.0.threads.lock();
if *threads < self.0.thread_limit {
REGISTRY.with(|registry| {
if registry.get().is_some() {
drop(threads);
panic!("Thread already has a registry");
}
registry.set(self.clone()).ok();
THREAD_DATA.with(|data| {
data.registry_id.set(self.id());
data.index.set(*threads);
});
*threads += 1;
});
} else {
drop(threads);
panic!("Thread limit reached");
}
}
/// Gets the identifer of this registry.
fn id(&self) -> RegistryId {
RegistryId(&*self.0)
}
}
/// Holds worker local values for each possible thread in a registry. You can only access the
/// worker local value through the `Deref` impl on the registry associated with the thread it was
/// created on. It will panic otherwise.
pub struct WorkerLocal<T> {
#[cfg(not(parallel_compiler))]
local: T,
#[cfg(parallel_compiler)]
locals: Box<[CacheAligned<T>]>,
#[cfg(parallel_compiler)]
registry: Registry,
}
// This is safe because the `deref` call will return a reference to a `T` unique to each thread
// or it will panic for threads without an associated local. So there isn't a need for `T` to do
// it's own synchronization. The `verify` method on `RegistryId` has an issue where the id
// can be reused, but `WorkerLocal` has a reference to `Registry` which will prevent any reuse.
#[cfg(parallel_compiler)]
unsafe impl<T: Send> Sync for WorkerLocal<T> {}
impl<T> WorkerLocal<T> {
/// Creates a new worker local where the `initial` closure computes the
/// value this worker local should take for each thread in the registry.
#[inline]
pub fn new<F: FnMut(usize) -> T>(mut initial: F) -> WorkerLocal<T> {
#[cfg(parallel_compiler)]
{
let registry = Registry::current();
WorkerLocal {
locals: (0..registry.0.thread_limit).map(|i| CacheAligned(initial(i))).collect(),
registry,
}
}
#[cfg(not(parallel_compiler))]
{
WorkerLocal { local: initial(0) }
}
}
/// Returns the worker-local values for each thread
#[inline]
pub fn into_inner(self) -> impl Iterator<Item = T> {
#[cfg(parallel_compiler)]
{
self.locals.into_vec().into_iter().map(|local| local.0)
}
#[cfg(not(parallel_compiler))]
{
std::iter::once(self.local)
}
}
}
impl<T> Deref for WorkerLocal<T> {
type Target = T;
#[inline(always)]
#[cfg(not(parallel_compiler))]
fn deref(&self) -> &T {
&self.local
}
#[inline(always)]
#[cfg(parallel_compiler)]
fn deref(&self) -> &T {
// This is safe because `verify` will only return values less than
// `self.registry.thread_limit` which is the size of the `self.locals` array.
unsafe { &self.locals.get_unchecked(self.registry.id().verify()).0 }
}
}
impl<T: Default> Default for WorkerLocal<T> {
fn default() -> Self {
WorkerLocal::new(|_| T::default())
}
}