style/
scoped_tls.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5//! Stack-scoped thread-local storage for rayon thread pools.
6
7#![allow(unsafe_code)]
8#![deny(missing_docs)]
9
10use crate::global_style_data::STYLO_MAX_THREADS;
11use std::cell::{Ref, RefCell, RefMut};
12use std::ops::DerefMut;
13
14/// A scoped TLS set, that is alive during the `'scope` lifetime.
15///
16/// We use this on Servo to construct thread-local contexts, but clear them once
17/// we're done with restyling.
18///
19/// Note that the cleanup is done on the thread that owns the scoped TLS, thus
20/// the Send bound.
21pub struct ScopedTLS<'scope, T: Send> {
22    pool: Option<&'scope rayon::ThreadPool>,
23    slots: [RefCell<Option<T>>; STYLO_MAX_THREADS],
24}
25
26/// The scoped TLS is `Sync` because no more than one worker thread can access a
27/// given slot.
28unsafe impl<'scope, T: Send> Sync for ScopedTLS<'scope, T> {}
29
30impl<'scope, T: Send> ScopedTLS<'scope, T> {
31    /// Create a new scoped TLS that will last as long as this rayon threadpool
32    /// reference.
33    pub fn new(pool: Option<&'scope rayon::ThreadPool>) -> Self {
34        debug_assert!(pool.map_or(true, |p| p.current_num_threads() <= STYLO_MAX_THREADS));
35        ScopedTLS {
36            pool,
37            slots: Default::default(),
38        }
39    }
40
41    /// Returns the index corresponding to the calling thread in the thread pool.
42    #[inline]
43    pub fn current_thread_index(&self) -> usize {
44        self.pool.map_or(0, |p| p.current_thread_index().unwrap())
45    }
46
47    /// Return an immutable reference to the `Option<T>` that this thread owns.
48    pub fn borrow(&self) -> Ref<'_, Option<T>> {
49        let idx = self.current_thread_index();
50        self.slots[idx].borrow()
51    }
52
53    /// Return a mutable reference to the `Option<T>` that this thread owns.
54    pub fn borrow_mut(&self) -> RefMut<'_, Option<T>> {
55        let idx = self.current_thread_index();
56        self.slots[idx].borrow_mut()
57    }
58
59    /// Ensure that the current data this thread owns is initialized, or
60    /// initialize it using `f`.  We want ensure() to be fast and inline, and we
61    /// want to inline the memmove that initializes the Option<T>.  But we don't
62    /// want to inline space for the entire large T struct in our stack frame.
63    /// That's why we hand `f` a mutable borrow to write to instead of just
64    /// having it return a T.
65    #[inline(always)]
66    pub fn ensure<F: FnOnce(&mut Option<T>)>(&self, f: F) -> RefMut<'_, T> {
67        let mut opt = self.borrow_mut();
68        if opt.is_none() {
69            f(opt.deref_mut());
70        }
71
72        RefMut::map(opt, |x| x.as_mut().unwrap())
73    }
74
75    /// Returns the slots. Safe because if we have a mut reference the tls can't be referenced by
76    /// any other thread.
77    pub fn slots(&mut self) -> &mut [RefCell<Option<T>>] {
78        &mut self.slots
79    }
80}
style/scoped_tls.rs

style/
scoped_tls.rs
