Import 'thread_local' package version 1.0.1platform-tools-30.0.0

* Add OWNERS
* No Android.bp yet
Bug: 152884384
Test: make

Change-Id: I50daca2484475354a0f7696a1ad755a0738697c3

4 files changed, 940 insertions, 0 deletions

diff --git a/src/cached.rs b/src/cached.rs
new file mode 100644
index 0000000..ab43c86
--- /dev/null
+++ b/src/cached.rs
@@ -0,0 +1,198 @@
+use super::{IntoIter, IterMut, ThreadLocal};
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::panic::UnwindSafe;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use thread_id;
+use unreachable::{UncheckedOptionExt, UncheckedResultExt};
+
+/// Wrapper around `ThreadLocal` which adds a fast path for a single thread.
+///
+/// This has the same API as `ThreadLocal`, but will register the first thread
+/// that sets a value as its owner. All accesses by the owner will go through
+/// a special fast path which is much faster than the normal `ThreadLocal` path.
+pub struct CachedThreadLocal<T: Send> {
+    owner: AtomicUsize,
+    local: UnsafeCell<Option<Box<T>>>,
+    global: ThreadLocal<T>,
+}
+
+// CachedThreadLocal is always Sync, even if T isn't
+unsafe impl<T: Send> Sync for CachedThreadLocal<T> {}
+
+impl<T: Send> Default for CachedThreadLocal<T> {
+    fn default() -> CachedThreadLocal<T> {
+        CachedThreadLocal::new()
+    }
+}
+
+impl<T: Send> CachedThreadLocal<T> {
+    /// Creates a new empty `CachedThreadLocal`.
+    pub fn new() -> CachedThreadLocal<T> {
+        CachedThreadLocal {
+            owner: AtomicUsize::new(0),
+            local: UnsafeCell::new(None),
+            global: ThreadLocal::new(),
+        }
+    }
+
+    /// Returns the element for the current thread, if it exists.
+    pub fn get(&self) -> Option<&T> {
+        let id = thread_id::get();
+        let owner = self.owner.load(Ordering::Relaxed);
+        if owner == id {
+            return unsafe { Some((*self.local.get()).as_ref().unchecked_unwrap()) };
+        }
+        if owner == 0 {
+            return None;
+        }
+        self.global.get_fast(id)
+    }
+
+    /// Returns the element for the current thread, or creates it if it doesn't
+    /// exist.
+    #[inline(always)]
+    pub fn get_or<F>(&self, create: F) -> &T
+    where
+        F: FnOnce() -> T,
+    {
+        unsafe {
+            self.get_or_try(|| Ok::<T, ()>(create()))
+                .unchecked_unwrap_ok()
+        }
+    }
+
+    /// Returns the element for the current thread, or creates it if it doesn't
+    /// exist. If `create` fails, that error is returned and no element is
+    /// added.
+    pub fn get_or_try<F, E>(&self, create: F) -> Result<&T, E>
+    where
+        F: FnOnce() -> Result<T, E>,
+    {
+        let id = thread_id::get();
+        let owner = self.owner.load(Ordering::Relaxed);
+        if owner == id {
+            return Ok(unsafe { (*self.local.get()).as_ref().unchecked_unwrap() });
+        }
+        self.get_or_try_slow(id, owner, create)
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn get_or_try_slow<F, E>(&self, id: usize, owner: usize, create: F) -> Result<&T, E>
+    where
+        F: FnOnce() -> Result<T, E>,
+    {
+        if owner == 0 && self.owner.compare_and_swap(0, id, Ordering::Relaxed) == 0 {
+            unsafe {
+                (*self.local.get()) = Some(Box::new(create()?));
+                return Ok((*self.local.get()).as_ref().unchecked_unwrap());
+            }
+        }
+        match self.global.get_fast(id) {
+            Some(x) => Ok(x),
+            None => Ok(self.global.insert(id, Box::new(create()?), true)),
+        }
+    }
+
+    /// Returns a mutable iterator over the local values of all threads.
+    ///
+    /// Since this call borrows the `ThreadLocal` mutably, this operation can
+    /// be done safely---the mutable borrow statically guarantees no other
+    /// threads are currently accessing their associated values.
+    pub fn iter_mut(&mut self) -> CachedIterMut<T> {
+        CachedIterMut {
+            local: unsafe { (*self.local.get()).as_mut().map(|x| &mut **x) },
+            global: self.global.iter_mut(),
+        }
+    }
+
+    /// Removes all thread-specific values from the `ThreadLocal`, effectively
+    /// reseting it to its original state.
+    ///
+    /// Since this call borrows the `ThreadLocal` mutably, this operation can
+    /// be done safely---the mutable borrow statically guarantees no other
+    /// threads are currently accessing their associated values.
+    pub fn clear(&mut self) {
+        *self = CachedThreadLocal::new();
+    }
+}
+
+impl<T: Send> IntoIterator for CachedThreadLocal<T> {
+    type Item = T;
+    type IntoIter = CachedIntoIter<T>;
+
+    fn into_iter(self) -> CachedIntoIter<T> {
+        CachedIntoIter {
+            local: unsafe { (*self.local.get()).take().map(|x| *x) },
+            global: self.global.into_iter(),
+        }
+    }
+}
+
+impl<'a, T: Send + 'a> IntoIterator for &'a mut CachedThreadLocal<T> {
+    type Item = &'a mut T;
+    type IntoIter = CachedIterMut<'a, T>;
+
+    fn into_iter(self) -> CachedIterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+impl<T: Send + Default> CachedThreadLocal<T> {
+    /// Returns the element for the current thread, or creates a default one if
+    /// it doesn't exist.
+    pub fn get_or_default(&self) -> &T {
+        self.get_or(T::default)
+    }
+}
+
+impl<T: Send + fmt::Debug> fmt::Debug for CachedThreadLocal<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ThreadLocal {{ local_data: {:?} }}", self.get())
+    }
+}
+
+impl<T: Send + UnwindSafe> UnwindSafe for CachedThreadLocal<T> {}
+
+/// Mutable iterator over the contents of a `CachedThreadLocal`.
+pub struct CachedIterMut<'a, T: Send + 'a> {
+    local: Option<&'a mut T>,
+    global: IterMut<'a, T>,
+}
+
+impl<'a, T: Send + 'a> Iterator for CachedIterMut<'a, T> {
+    type Item = &'a mut T;
+
+    fn next(&mut self) -> Option<&'a mut T> {
+        self.local.take().or_else(|| self.global.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.global.size_hint().0 + self.local.is_some() as usize;
+        (len, Some(len))
+    }
+}
+
+impl<'a, T: Send + 'a> ExactSizeIterator for CachedIterMut<'a, T> {}
+
+/// An iterator that moves out of a `CachedThreadLocal`.
+pub struct CachedIntoIter<T: Send> {
+    local: Option<T>,
+    global: IntoIter<T>,
+}
+
+impl<T: Send> Iterator for CachedIntoIter<T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        self.local.take().or_else(|| self.global.next())
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.global.size_hint().0 + self.local.is_some() as usize;
+        (len, Some(len))
+    }
+}
+
+impl<T: Send> ExactSizeIterator for CachedIntoIter<T> {}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..9fd6d19
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,607 @@
+// 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.
+
+//! Per-object thread-local storage
+//!
+//! This library provides the `ThreadLocal` type which allows a separate copy of
+//! an object to be used for each thread. This allows for per-object
+//! thread-local storage, unlike the standard library's `thread_local!` macro
+//! which only allows static thread-local storage.
+//!
+//! Per-thread objects are not destroyed when a thread exits. Instead, objects
+//! are only destroyed when the `ThreadLocal` containing them is destroyed.
+//!
+//! You can also iterate over the thread-local values of all thread in a
+//! `ThreadLocal` object using the `iter_mut` and `into_iter` methods. This can
+//! only be done if you have mutable access to the `ThreadLocal` object, which
+//! guarantees that you are the only thread currently accessing it.
+//!
+//! A `CachedThreadLocal` type is also provided which wraps a `ThreadLocal` but
+//! also uses a special fast path for the first thread that writes into it. The
+//! fast path has very low overhead (<1ns per access) while keeping the same
+//! performance as `ThreadLocal` for other threads.
+//!
+//! Note that since thread IDs are recycled when a thread exits, it is possible
+//! for one thread to retrieve the object of another thread. Since this can only
+//! occur after a thread has exited this does not lead to any race conditions.
+//!
+//! # Examples
+//!
+//! Basic usage of `ThreadLocal`:
+//!
+//! ```rust
+//! use thread_local::ThreadLocal;
+//! let tls: ThreadLocal<u32> = ThreadLocal::new();
+//! assert_eq!(tls.get(), None);
+//! assert_eq!(tls.get_or(|| 5), &5);
+//! assert_eq!(tls.get(), Some(&5));
+//! ```
+//!
+//! Combining thread-local values into a single result:
+//!
+//! ```rust
+//! use thread_local::ThreadLocal;
+//! use std::sync::Arc;
+//! use std::cell::Cell;
+//! use std::thread;
+//!
+//! let tls = Arc::new(ThreadLocal::new());
+//!
+//! // Create a bunch of threads to do stuff
+//! for _ in 0..5 {
+//!     let tls2 = tls.clone();
+//!     thread::spawn(move || {
+//!         // Increment a counter to count some event...
+//!         let cell = tls2.get_or(|| Cell::new(0));
+//!         cell.set(cell.get() + 1);
+//!     }).join().unwrap();
+//! }
+//!
+//! // Once all threads are done, collect the counter values and return the
+//! // sum of all thread-local counter values.
+//! let tls = Arc::try_unwrap(tls).unwrap();
+//! let total = tls.into_iter().fold(0, |x, y| x + y.get());
+//! assert_eq!(total, 5);
+//! ```
+
+#![warn(missing_docs)]
+
+#[macro_use]
+extern crate lazy_static;
+
+mod thread_id;
+mod unreachable;
+mod cached;
+
+pub use cached::{CachedIntoIter, CachedIterMut, CachedThreadLocal};
+
+use std::cell::UnsafeCell;
+use std::fmt;
+use std::marker::PhantomData;
+use std::panic::UnwindSafe;
+use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
+use std::sync::Mutex;
+use unreachable::{UncheckedOptionExt, UncheckedResultExt};
+
+/// Thread-local variable wrapper
+///
+/// See the [module-level documentation](index.html) for more.
+pub struct ThreadLocal<T: Send> {
+    // Pointer to the current top-level hash table
+    table: AtomicPtr<Table<T>>,
+
+    // Lock used to guard against concurrent modifications. This is only taken
+    // while writing to the table, not when reading from it. This also guards
+    // the counter for the total number of values in the hash table.
+    lock: Mutex<usize>,
+}
+
+struct Table<T: Send> {
+    // Hash entries for the table
+    entries: Box<[TableEntry<T>]>,
+
+    // Number of bits used for the hash function
+    hash_bits: usize,
+
+    // Previous table, half the size of the current one
+    prev: Option<Box<Table<T>>>,
+}
+
+struct TableEntry<T: Send> {
+    // Current owner of this entry, or 0 if this is an empty entry
+    owner: AtomicUsize,
+
+    // The object associated with this entry. This is only ever accessed by the
+    // owner of the entry.
+    data: UnsafeCell<Option<Box<T>>>,
+}
+
+// ThreadLocal is always Sync, even if T isn't
+unsafe impl<T: Send> Sync for ThreadLocal<T> {}
+
+impl<T: Send> Default for ThreadLocal<T> {
+    fn default() -> ThreadLocal<T> {
+        ThreadLocal::new()
+    }
+}
+
+impl<T: Send> Drop for ThreadLocal<T> {
+    fn drop(&mut self) {
+        unsafe {
+            Box::from_raw(self.table.load(Ordering::Relaxed));
+        }
+    }
+}
+
+// Implementation of Clone for TableEntry, needed to make vec![] work
+impl<T: Send> Clone for TableEntry<T> {
+    fn clone(&self) -> TableEntry<T> {
+        TableEntry {
+            owner: AtomicUsize::new(0),
+            data: UnsafeCell::new(None),
+        }
+    }
+}
+
+// Hash function for the thread id
+#[cfg(target_pointer_width = "32")]
+#[inline]
+fn hash(id: usize, bits: usize) -> usize {
+    id.wrapping_mul(0x9E3779B9) >> (32 - bits)
+}
+#[cfg(target_pointer_width = "64")]
+#[inline]
+fn hash(id: usize, bits: usize) -> usize {
+    id.wrapping_mul(0x9E37_79B9_7F4A_7C15) >> (64 - bits)
+}
+
+impl<T: Send> ThreadLocal<T> {
+    /// Creates a new empty `ThreadLocal`.
+    pub fn new() -> ThreadLocal<T> {
+        let entry = TableEntry {
+            owner: AtomicUsize::new(0),
+            data: UnsafeCell::new(None),
+        };
+        let table = Table {
+            entries: vec![entry; 2].into_boxed_slice(),
+            hash_bits: 1,
+            prev: None,
+        };
+        ThreadLocal {
+            table: AtomicPtr::new(Box::into_raw(Box::new(table))),
+            lock: Mutex::new(0),
+        }
+    }
+
+    /// Returns the element for the current thread, if it exists.
+    pub fn get(&self) -> Option<&T> {
+        let id = thread_id::get();
+        self.get_fast(id)
+    }
+
+    /// Returns the element for the current thread, or creates it if it doesn't
+    /// exist.
+    pub fn get_or<F>(&self, create: F) -> &T
+    where
+        F: FnOnce() -> T,
+    {
+        unsafe {
+            self.get_or_try(|| Ok::<T, ()>(create()))
+                .unchecked_unwrap_ok()
+        }
+    }
+
+    /// Returns the element for the current thread, or creates it if it doesn't
+    /// exist. If `create` fails, that error is returned and no element is
+    /// added.
+    pub fn get_or_try<F, E>(&self, create: F) -> Result<&T, E>
+    where
+        F: FnOnce() -> Result<T, E>,
+    {
+        let id = thread_id::get();
+        match self.get_fast(id) {
+            Some(x) => Ok(x),
+            None => Ok(self.insert(id, Box::new(create()?), true)),
+        }
+    }
+
+    // Simple hash table lookup function
+    fn lookup(id: usize, table: &Table<T>) -> Option<&UnsafeCell<Option<Box<T>>>> {
+        // Because we use a Mutex to prevent concurrent modifications (but not
+        // reads) of the hash table, we can avoid any memory barriers here. No
+        // elements between our hash bucket and our value can have been modified
+        // since we inserted our thread-local value into the table.
+        for entry in table.entries.iter().cycle().skip(hash(id, table.hash_bits)) {
+            let owner = entry.owner.load(Ordering::Relaxed);
+            if owner == id {
+                return Some(&entry.data);
+            }
+            if owner == 0 {
+                return None;
+            }
+        }
+        unreachable!();
+    }
+
+    // Fast path: try to find our thread in the top-level hash table
+    fn get_fast(&self, id: usize) -> Option<&T> {
+        let table = unsafe { &*self.table.load(Ordering::Acquire) };
+        match Self::lookup(id, table) {
+            Some(x) => unsafe { Some((*x.get()).as_ref().unchecked_unwrap()) },
+            None => self.get_slow(id, table),
+        }
+    }
+
+    // Slow path: try to find our thread in the other hash tables, and then
+    // move it to the top-level hash table.
+    #[cold]
+    fn get_slow(&self, id: usize, table_top: &Table<T>) -> Option<&T> {
+        let mut current = &table_top.prev;
+        while let Some(ref table) = *current {
+            if let Some(x) = Self::lookup(id, table) {
+                let data = unsafe { (*x.get()).take().unchecked_unwrap() };
+                return Some(self.insert(id, data, false));
+            }
+            current = &table.prev;
+        }
+        None
+    }
+
+    #[cold]
+    fn insert(&self, id: usize, data: Box<T>, new: bool) -> &T {
+        // Lock the Mutex to ensure only a single thread is modify the hash
+        // table at once.
+        let mut count = self.lock.lock().unwrap();
+        if new {
+            *count += 1;
+        }
+        let table_raw = self.table.load(Ordering::Relaxed);
+        let table = unsafe { &*table_raw };
+
+        // If the current top-level hash table is more than 75% full, add a new
+        // level with 2x the capacity. Elements will be moved up to the new top
+        // level table as they are accessed.
+        let table = if *count > table.entries.len() * 3 / 4 {
+            let entry = TableEntry {
+                owner: AtomicUsize::new(0),
+                data: UnsafeCell::new(None),
+            };
+            let new_table = Box::into_raw(Box::new(Table {
+                entries: vec![entry; table.entries.len() * 2].into_boxed_slice(),
+                hash_bits: table.hash_bits + 1,
+                prev: unsafe { Some(Box::from_raw(table_raw)) },
+            }));
+            self.table.store(new_table, Ordering::Release);
+            unsafe { &*new_table }
+        } else {
+            table
+        };
+
+        // Insert the new element into the top-level hash table
+        for entry in table.entries.iter().cycle().skip(hash(id, table.hash_bits)) {
+            let owner = entry.owner.load(Ordering::Relaxed);
+            if owner == 0 {
+                unsafe {
+                    entry.owner.store(id, Ordering::Relaxed);
+                    *entry.data.get() = Some(data);
+                    return (*entry.data.get()).as_ref().unchecked_unwrap();
+                }
+            }
+            if owner == id {
+                // This can happen if create() inserted a value into this
+                // ThreadLocal between our calls to get_fast() and insert(). We
+                // just return the existing value and drop the newly-allocated
+                // Box.
+                unsafe {
+                    return (*entry.data.get()).as_ref().unchecked_unwrap();
+                }
+            }
+        }
+        unreachable!();
+    }
+
+    fn raw_iter(&mut self) -> RawIter<T> {
+        RawIter {
+            remaining: *self.lock.get_mut().unwrap(),
+            index: 0,
+            table: self.table.load(Ordering::Relaxed),
+        }
+    }
+
+    /// Returns a mutable iterator over the local values of all threads.
+    ///
+    /// Since this call borrows the `ThreadLocal` mutably, this operation can
+    /// be done safely---the mutable borrow statically guarantees no other
+    /// threads are currently accessing their associated values.
+    pub fn iter_mut(&mut self) -> IterMut<T> {
+        IterMut {
+            raw: self.raw_iter(),
+            marker: PhantomData,
+        }
+    }
+
+    /// Removes all thread-specific values from the `ThreadLocal`, effectively
+    /// reseting it to its original state.
+    ///
+    /// Since this call borrows the `ThreadLocal` mutably, this operation can
+    /// be done safely---the mutable borrow statically guarantees no other
+    /// threads are currently accessing their associated values.
+    pub fn clear(&mut self) {
+        *self = ThreadLocal::new();
+    }
+}
+
+impl<T: Send> IntoIterator for ThreadLocal<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    fn into_iter(mut self) -> IntoIter<T> {
+        IntoIter {
+            raw: self.raw_iter(),
+            _thread_local: self,
+        }
+    }
+}
+
+impl<'a, T: Send + 'a> IntoIterator for &'a mut ThreadLocal<T> {
+    type Item = &'a mut T;
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+impl<T: Send + Default> ThreadLocal<T> {
+    /// Returns the element for the current thread, or creates a default one if
+    /// it doesn't exist.
+    pub fn get_or_default(&self) -> &T {
+        self.get_or(Default::default)
+    }
+}
+
+impl<T: Send + fmt::Debug> fmt::Debug for ThreadLocal<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ThreadLocal {{ local_data: {:?} }}", self.get())
+    }
+}
+
+impl<T: Send + UnwindSafe> UnwindSafe for ThreadLocal<T> {}
+
+struct RawIter<T: Send> {
+    remaining: usize,
+    index: usize,
+    table: *const Table<T>,
+}
+
+impl<T: Send> Iterator for RawIter<T> {
+    type Item = *mut Option<Box<T>>;
+
+    fn next(&mut self) -> Option<*mut Option<Box<T>>> {
+        if self.remaining == 0 {
+            return None;
+        }
+
+        loop {
+            let entries = unsafe { &(*self.table).entries[..] };
+            while self.index < entries.len() {
+                let val = entries[self.index].data.get();
+                self.index += 1;
+                if unsafe { (*val).is_some() } {
+                    self.remaining -= 1;
+                    return Some(val);
+                }
+            }
+            self.index = 0;
+            self.table = unsafe { &**(*self.table).prev.as_ref().unchecked_unwrap() };
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+/// Mutable iterator over the contents of a `ThreadLocal`.
+pub struct IterMut<'a, T: Send + 'a> {
+    raw: RawIter<T>,
+    marker: PhantomData<&'a mut ThreadLocal<T>>,
+}
+
+impl<'a, T: Send + 'a> Iterator for IterMut<'a, T> {
+    type Item = &'a mut T;
+
+    fn next(&mut self) -> Option<&'a mut T> {
+        self.raw
+            .next()
+            .map(|x| unsafe { &mut **(*x).as_mut().unchecked_unwrap() })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.raw.size_hint()
+    }
+}
+
+impl<'a, T: Send + 'a> ExactSizeIterator for IterMut<'a, T> {}
+
+/// An iterator that moves out of a `ThreadLocal`.
+pub struct IntoIter<T: Send> {
+    raw: RawIter<T>,
+    _thread_local: ThreadLocal<T>,
+}
+
+impl<T: Send> Iterator for IntoIter<T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        self.raw
+            .next()
+            .map(|x| unsafe { *(*x).take().unchecked_unwrap() })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.raw.size_hint()
+    }
+}
+
+impl<T: Send> ExactSizeIterator for IntoIter<T> {}
+
+#[cfg(test)]
+mod tests {
+    use super::{CachedThreadLocal, ThreadLocal};
+    use std::cell::RefCell;
+    use std::sync::atomic::AtomicUsize;
+    use std::sync::atomic::Ordering::Relaxed;
+    use std::sync::Arc;
+    use std::thread;
+
+    fn make_create() -> Arc<dyn Fn() -> usize + Send + Sync> {
+        let count = AtomicUsize::new(0);
+        Arc::new(move || count.fetch_add(1, Relaxed))
+    }
+
+    #[test]
+    fn same_thread() {
+        let create = make_create();
+        let mut tls = ThreadLocal::new();
+        assert_eq!(None, tls.get());
+        assert_eq!("ThreadLocal { local_data: None }", format!("{:?}", &tls));
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!("ThreadLocal { local_data: Some(0) }", format!("{:?}", &tls));
+        tls.clear();
+        assert_eq!(None, tls.get());
+    }
+
+    #[test]
+    fn same_thread_cached() {
+        let create = make_create();
+        let mut tls = CachedThreadLocal::new();
+        assert_eq!(None, tls.get());
+        assert_eq!("ThreadLocal { local_data: None }", format!("{:?}", &tls));
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!("ThreadLocal { local_data: Some(0) }", format!("{:?}", &tls));
+        tls.clear();
+        assert_eq!(None, tls.get());
+    }
+
+    #[test]
+    fn different_thread() {
+        let create = make_create();
+        let tls = Arc::new(ThreadLocal::new());
+        assert_eq!(None, tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+
+        let tls2 = tls.clone();
+        let create2 = create.clone();
+        thread::spawn(move || {
+            assert_eq!(None, tls2.get());
+            assert_eq!(1, *tls2.get_or(|| create2()));
+            assert_eq!(Some(&1), tls2.get());
+        })
+        .join()
+        .unwrap();
+
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+    }
+
+    #[test]
+    fn different_thread_cached() {
+        let create = make_create();
+        let tls = Arc::new(CachedThreadLocal::new());
+        assert_eq!(None, tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+        assert_eq!(Some(&0), tls.get());
+
+        let tls2 = tls.clone();
+        let create2 = create.clone();
+        thread::spawn(move || {
+            assert_eq!(None, tls2.get());
+            assert_eq!(1, *tls2.get_or(|| create2()));
+            assert_eq!(Some(&1), tls2.get());
+        })
+        .join()
+        .unwrap();
+
+        assert_eq!(Some(&0), tls.get());
+        assert_eq!(0, *tls.get_or(|| create()));
+    }
+
+    #[test]
+    fn iter() {
+        let tls = Arc::new(ThreadLocal::new());
+        tls.get_or(|| Box::new(1));
+
+        let tls2 = tls.clone();
+        thread::spawn(move || {
+            tls2.get_or(|| Box::new(2));
+            let tls3 = tls2.clone();
+            thread::spawn(move || {
+                tls3.get_or(|| Box::new(3));
+            })
+            .join()
+            .unwrap();
+        })
+        .join()
+        .unwrap();
+
+        let mut tls = Arc::try_unwrap(tls).unwrap();
+        let mut v = tls.iter_mut().map(|x| **x).collect::<Vec<i32>>();
+        v.sort();
+        assert_eq!(vec![1, 2, 3], v);
+        let mut v = tls.into_iter().map(|x| *x).collect::<Vec<i32>>();
+        v.sort();
+        assert_eq!(vec![1, 2, 3], v);
+    }
+
+    #[test]
+    fn iter_cached() {
+        let tls = Arc::new(CachedThreadLocal::new());
+        tls.get_or(|| Box::new(1));
+
+        let tls2 = tls.clone();
+        thread::spawn(move || {
+            tls2.get_or(|| Box::new(2));
+            let tls3 = tls2.clone();
+            thread::spawn(move || {
+                tls3.get_or(|| Box::new(3));
+            })
+            .join()
+            .unwrap();
+        })
+        .join()
+        .unwrap();
+
+        let mut tls = Arc::try_unwrap(tls).unwrap();
+        let mut v = tls.iter_mut().map(|x| **x).collect::<Vec<i32>>();
+        v.sort();
+        assert_eq!(vec![1, 2, 3], v);
+        let mut v = tls.into_iter().map(|x| *x).collect::<Vec<i32>>();
+        v.sort();
+        assert_eq!(vec![1, 2, 3], v);
+    }
+
+    #[test]
+    fn is_sync() {
+        fn foo<T: Sync>() {}
+        foo::<ThreadLocal<String>>();
+        foo::<ThreadLocal<RefCell<String>>>();
+        foo::<CachedThreadLocal<String>>();
+        foo::<CachedThreadLocal<RefCell<String>>>();
+    }
+}
diff --git a/src/thread_id.rs b/src/thread_id.rs
new file mode 100644
index 0000000..e757948
--- /dev/null
+++ b/src/thread_id.rs
@@ -0,0 +1,61 @@
+// 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 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 {
+    limit: usize,
+    free_list: BinaryHeap<usize>,
+}
+impl ThreadIdManager {
+    fn new() -> ThreadIdManager {
+        ThreadIdManager {
+            limit: usize::MAX,
+            free_list: BinaryHeap::new(),
+        }
+    }
+    fn alloc(&mut self) -> usize {
+        if let Some(id) = self.free_list.pop() {
+            id
+        } else {
+            let id = self.limit;
+            self.limit = self.limit.checked_sub(1).expect("Ran out of thread IDs");
+            id
+        }
+    }
+    fn free(&mut self, id: usize) {
+        self.free_list.push(id);
+    }
+}
+lazy_static! {
+    static ref THREAD_ID_MANAGER: Mutex<ThreadIdManager> = Mutex::new(ThreadIdManager::new());
+}
+
+// Non-zero integer which is unique to the current thread while it is running.
+// A thread ID may be reused after a thread exits.
+struct ThreadId(usize);
+impl ThreadId {
+    fn new() -> ThreadId {
+        ThreadId(THREAD_ID_MANAGER.lock().unwrap().alloc())
+    }
+}
+impl Drop for ThreadId {
+    fn drop(&mut self) {
+        THREAD_ID_MANAGER.lock().unwrap().free(self.0);
+    }
+}
+thread_local!(static THREAD_ID: ThreadId = ThreadId::new());
+
+/// Returns a non-zero ID for the current thread
+pub fn get() -> usize {
+    THREAD_ID.with(|x| x.0)
+}
diff --git a/src/unreachable.rs b/src/unreachable.rs
new file mode 100644
index 0000000..baff766
--- /dev/null
+++ b/src/unreachable.rs
@@ -0,0 +1,74 @@
+// 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.
+
+//! # unreachable
+//! inlined from https://github.com/reem/rust-unreachable/
+//!
+//! An unreachable code optimization hint in stable rust, and some useful
+//! extension traits for `Option` and `Result`.
+//!
+
+/// Hint to the optimizer that any code path which calls this function is
+/// statically unreachable and can be removed.
+///
+/// Calling this function in reachable code invokes undefined behavior. Be
+/// very, very sure this is what you want; often, a simple `panic!` is more
+/// suitable.
+#[inline]
+pub unsafe fn unreachable() -> ! {
+    /// The empty type for cases which can't occur.
+    enum Void { }
+    let x: &Void = ::std::mem::transmute(1usize);
+    match *x {}
+}
+
+/// An extension trait for `Option<T>` providing unchecked unwrapping methods.
+pub trait UncheckedOptionExt<T> {
+    /// Get the value out of this Option without checking for None.
+    unsafe fn unchecked_unwrap(self) -> T;
+
+    /// Assert that this Option is a None to the optimizer.
+    unsafe fn unchecked_unwrap_none(self);
+}
+
+/// An extension trait for `Result<T, E>` providing unchecked unwrapping methods.
+pub trait UncheckedResultExt<T, E> {
+    /// Get the value out of this Result without checking for Err.
+    unsafe fn unchecked_unwrap_ok(self) -> T;
+
+    /// Get the error out of this Result without checking for Ok.
+    unsafe fn unchecked_unwrap_err(self) -> E;
+}
+
+impl<T> UncheckedOptionExt<T> for Option<T> {
+    unsafe fn unchecked_unwrap(self) -> T {
+        match self {
+            Some(x) => x,
+            None => unreachable()
+        }
+    }
+
+    unsafe fn unchecked_unwrap_none(self) {
+        if self.is_some() { unreachable() }
+    }
+}
+
+impl<T, E> UncheckedResultExt<T, E> for Result<T, E> {
+    unsafe fn unchecked_unwrap_ok(self) -> T {
+        match self {
+            Ok(x) => x,
+            Err(_) => unreachable()
+        }
+    }
+
+    unsafe fn unchecked_unwrap_err(self) -> E {
+        match self {
+            Ok(_) => unreachable(),
+            Err(e) => e
+        }
+    }
+}
\ No newline at end of file