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// Copyright 2017 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.
use crate::POINTER_WIDTH;
use once_cell::sync::Lazy;
use std::cmp::Reverse;
use std::collections::BinaryHeap;
use std::sync::Mutex;
use std::usize;
/// Thread ID manager which allocates thread IDs. It attempts to aggressively
/// reuse thread IDs where possible to avoid cases where a ThreadLocal grows
/// indefinitely when it is used by many short-lived threads.
struct ThreadIdManager {
free_from: usize,
free_list: BinaryHeap<Reverse<usize>>,
}
impl ThreadIdManager {
fn new() -> ThreadIdManager {
ThreadIdManager {
free_from: 0,
free_list: BinaryHeap::new(),
}
}
fn alloc(&mut self) -> usize {
if let Some(id) = self.free_list.pop() {
id.0
} else {
let id = self.free_from;
self.free_from = self
.free_from
.checked_add(1)
.expect("Ran out of thread IDs");
id
}
}
fn free(&mut self, id: usize) {
self.free_list.push(Reverse(id));
}
}
static THREAD_ID_MANAGER: Lazy<Mutex<ThreadIdManager>> =
Lazy::new(|| Mutex::new(ThreadIdManager::new()));
/// Data which is unique to the current thread while it is running.
/// A thread ID may be reused after a thread exits.
#[derive(Clone, Copy)]
pub(crate) struct Thread {
/// The thread ID obtained from the thread ID manager.
pub(crate) id: usize,
/// The bucket this thread's local storage will be in.
pub(crate) bucket: usize,
/// The size of the bucket this thread's local storage will be in.
pub(crate) bucket_size: usize,
/// The index into the bucket this thread's local storage is in.
pub(crate) index: usize,
}
impl Thread {
fn new(id: usize) -> Thread {
let bucket = usize::from(POINTER_WIDTH) - id.leading_zeros() as usize;
let bucket_size = 1 << bucket.saturating_sub(1);
let index = if id != 0 { id ^ bucket_size } else { 0 };
Thread {
id,
bucket,
bucket_size,
index,
}
}
}
/// Wrapper around `Thread` that allocates and deallocates the ID.
struct ThreadHolder(Thread);
impl ThreadHolder {
fn new() -> ThreadHolder {
ThreadHolder(Thread::new(THREAD_ID_MANAGER.lock().unwrap().alloc()))
}
}
impl Drop for ThreadHolder {
fn drop(&mut self) {
THREAD_ID_MANAGER.lock().unwrap().free(self.0.id);
}
}
thread_local!(static THREAD_HOLDER: ThreadHolder = ThreadHolder::new());
/// Get the current thread.
pub(crate) fn get() -> Thread {
THREAD_HOLDER.with(|holder| holder.0)
}
#[test]
fn test_thread() {
let thread = Thread::new(0);
assert_eq!(thread.id, 0);
assert_eq!(thread.bucket, 0);
assert_eq!(thread.bucket_size, 1);
assert_eq!(thread.index, 0);
let thread = Thread::new(1);
assert_eq!(thread.id, 1);
assert_eq!(thread.bucket, 1);
assert_eq!(thread.bucket_size, 1);
assert_eq!(thread.index, 0);
let thread = Thread::new(2);
assert_eq!(thread.id, 2);
assert_eq!(thread.bucket, 2);
assert_eq!(thread.bucket_size, 2);
assert_eq!(thread.index, 0);
let thread = Thread::new(3);
assert_eq!(thread.id, 3);
assert_eq!(thread.bucket, 2);
assert_eq!(thread.bucket_size, 2);
assert_eq!(thread.index, 1);
let thread = Thread::new(19);
assert_eq!(thread.id, 19);
assert_eq!(thread.bucket, 5);
assert_eq!(thread.bucket_size, 16);
assert_eq!(thread.index, 3);
}
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