1 files changed, 78 insertions, 0 deletions
diff --git a/servo/components/style/scoped_tls.rs b/servo/components/style/scoped_tls.rs
new file mode 100644
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+++ b/servo/components/style/scoped_tls.rs
@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Stack-scoped thread-local storage for rayon thread pools.
+
+#![allow(unsafe_code)]
+#![deny(missing_docs)]
+
+use rayon;
+use std::cell::{Ref, RefCell, RefMut};
+use std::ops::DerefMut;
+
+/// A scoped TLS set, that is alive during the `'scope` lifetime.
+///
+/// We use this on Servo to construct thread-local contexts, but clear them once
+/// we're done with restyling.
+///
+/// Note that the cleanup is done on the thread that owns the scoped TLS, thus
+/// the Send bound.
+pub struct ScopedTLS<'scope, T: Send> {
+    pool: &'scope rayon::ThreadPool,
+    slots: Box<[RefCell<Option<T>>]>,
+}
+
+/// The scoped TLS is `Sync` because no more than one worker thread can access a
+/// given slot.
+unsafe impl<'scope, T: Send> Sync for ScopedTLS<'scope, T> {}
+
+impl<'scope, T: Send> ScopedTLS<'scope, T> {
+    /// Create a new scoped TLS that will last as long as this rayon threadpool
+    /// reference.
+    pub fn new(p: &'scope rayon::ThreadPool) -> Self {
+        let count = p.current_num_threads();
+        let mut v = Vec::with_capacity(count);
+        for _ in 0..count {
+            v.push(RefCell::new(None));
+        }
+
+        ScopedTLS {
+            pool: p,
+            slots: v.into_boxed_slice(),
+        }
+    }
+
+    /// Return an immutable reference to the `Option<T>` that this thread owns.
+    pub fn borrow(&self) -> Ref<Option<T>> {
+        let idx = self.pool.current_thread_index().unwrap();
+        self.slots[idx].borrow()
+    }
+
+    /// Return a mutable reference to the `Option<T>` that this thread owns.
+    pub fn borrow_mut(&self) -> RefMut<Option<T>> {
+        let idx = self.pool.current_thread_index().unwrap();
+        self.slots[idx].borrow_mut()
+    }
+
+    /// Ensure that the current data this thread owns is initialized, or
+    /// initialize it using `f`.  We want ensure() to be fast and inline, and we
+    /// want to inline the memmove that initializes the Option<T>.  But we don't
+    /// want to inline space for the entire large T struct in our stack frame.
+    /// That's why we hand `f` a mutable borrow to write to instead of just
+    /// having it return a T.
+    #[inline(always)]
+    pub fn ensure<F: FnOnce(&mut Option<T>)>(&self, f: F) -> RefMut<T> {
+        let mut opt = self.borrow_mut();
+        if opt.is_none() {
+            f(opt.deref_mut());
+        }
+
+        RefMut::map(opt, |x| x.as_mut().unwrap())
+    }
+
+    /// Returns the slots, consuming the scope.
+    pub fn into_slots(self) -> Box<[RefCell<Option<T>>]> {
+        self.slots
+    }
+}