Reduce size of Thread

Make insertion fully cold

Cargo.toml

@@ -13,7 +13,7 @@ rust-version = "1.59"
 
 [features]
 # this feature provides performance improvements using nightly features
-nightly = []
+nightly = ["memoffset"]
 
 [badges]
 travis-ci = { repository = "Amanieu/thread_local-rs" }
@@ -23,9 +23,10 @@ once_cell = "1.5.2"
 # this is required to gate `nightly` related code paths
 cfg-if = "1.0.0"
 crossbeam-utils = "0.8.15"
+memoffset = { version = "0.9.0", optional = true }
 
 [dev-dependencies]
-criterion = "0.4.0"
+criterion = "0.4"
 
 [[bench]]
 name = "thread_local"

src/lib.rs

@@ -188,10 +188,11 @@ impl<T: Send> ThreadLocal<T> {
     where
         F: FnOnce() -> T,
     {
-        unsafe {
-            self.get_or_try(|| Ok::<T, ()>(create()))
-                .unchecked_unwrap_ok()
+        if let Some(val) = self.get() {
+            return val;
         }
+
+        self.insert(create)
     }
 
     /// Returns the element for the current thread, or creates it if it doesn't
@@ -201,12 +202,11 @@ impl<T: Send> ThreadLocal<T> {
     where
         F: FnOnce() -> Result<T, E>,
     {
-        let thread = thread_id::get();
-        if let Some(val) = self.get_inner(thread) {
+        if let Some(val) = self.get() {
             return Ok(val);
         }
 
-        Ok(self.insert(create()?))
+        self.insert_maybe(create)
     }
 
     fn get_inner(&self, thread: Thread) -> Option<&T> {
@@ -227,14 +227,22 @@ impl<T: Send> ThreadLocal<T> {
     }
 
     #[cold]
-    fn insert(&self, data: T) -> &T {
+    fn insert_maybe<F: FnOnce() -> Result<T, E>, E>(&self, gen: F) -> Result<&T, E> {
+        let data = gen()?;
+        Ok(self.insert(|| data))
+    }
+
+    #[cold]
+    fn insert<F: FnOnce() -> T>(&self, gen: F) -> &T {
+        // call the generator here, so it is #[cold] as well.
+        let data = gen();
         let thread = thread_id::get();
         let bucket_atomic_ptr = unsafe { self.buckets.get_unchecked(thread.bucket) };
         let bucket_ptr: *const _ = bucket_atomic_ptr.load(Ordering::Acquire);
 
         // If the bucket doesn't already exist, we need to allocate it
         let bucket_ptr = if bucket_ptr.is_null() {
-            let new_bucket = allocate_bucket(thread.bucket_size);
+            let new_bucket = allocate_bucket(thread.bucket_size());
 
             match bucket_atomic_ptr.compare_exchange(
                 ptr::null_mut(),
@@ -247,7 +255,7 @@ impl<T: Send> ThreadLocal<T> {
                 // another thread stored a new bucket before we could,
                 // and we can free our bucket and use that one instead
                 Err(bucket_ptr) => {
-                    unsafe { deallocate_bucket(new_bucket, thread.bucket_size) }
+                    unsafe { deallocate_bucket(new_bucket, thread.bucket_size()) }
                     bucket_ptr
                 }
             }
@@ -496,9 +504,7 @@ impl<T: Send> Iterator for IntoIter<T> {
     fn next(&mut self) -> Option<T> {
         self.raw.next_mut(&mut self.thread_local).map(|entry| {
             *entry.present.get_mut() = false;
-            unsafe {
-                std::mem::replace(&mut *entry.value.get(), MaybeUninit::uninit()).assume_init()
-            }
+            unsafe { mem::replace(&mut *entry.value.get(), MaybeUninit::uninit()).assume_init() }
         })
     }
     fn size_hint(&self) -> (usize, Option<usize>) {

src/thread_id.rs

@@ -49,38 +49,47 @@ static THREAD_ID_MANAGER: Lazy<Mutex<ThreadIdManager>> =
 /// 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 {
+    /// id: The thread ID obtained from the thread ID manager.
+    #[inline]
     fn new(id: usize) -> Self {
         let bucket = usize::from(POINTER_WIDTH) - ((id + 1).leading_zeros() as usize) - 1;
         let bucket_size = 1 << bucket;
         let index = id - (bucket_size - 1);
+        Self { bucket, index }
+    }
 
-        Self {
-            id,
-            bucket,
-            bucket_size,
-            index,
-        }
+    /// The size of the bucket this thread's local storage will be in.
+    #[inline]
+    pub fn bucket_size(&self) -> usize {
+        1 << self.bucket
     }
 }
 
 cfg_if::cfg_if! {
     if #[cfg(feature = "nightly")] {
+        use memoffset::offset_of;
+        use std::ptr::null;
+        use std::cell::UnsafeCell;
+
         // This is split into 2 thread-local variables so that we can check whether the
         // thread is initialized without having to register a thread-local destructor.
         //
         // This makes the fast path smaller.
         #[thread_local]
-        static mut THREAD: Option<Thread> = None;
+        static THREAD: UnsafeCell<ThreadWrapper> = UnsafeCell::new(ThreadWrapper {
+            self_ptr: null(),
+            thread: Thread {
+                index: 0,
+                bucket: 0,
+            },
+        });
         thread_local! { static THREAD_GUARD: ThreadGuard = const { ThreadGuard { id: Cell::new(0) } }; }
 
         // Guard to ensure the thread ID is released on thread exit.
@@ -97,17 +106,41 @@ cfg_if::cfg_if! {
                 // will go through get_slow which will either panic or
                 // initialize a new ThreadGuard.
                 unsafe {
-                    THREAD = None;
+                    (&mut *THREAD.get()).self_ptr = null();
                 }
                 THREAD_ID_MANAGER.lock().free(self.id.get());
             }
         }
 
+        /// Data which is unique to the current thread while it is running.
+        /// A thread ID may be reused after a thread exits.
+        ///
+        /// This wrapper exists to hide multiple accesses to the TLS data
+        /// from the backend as this can lead to inefficient codegen
+        /// (to be precise it can lead to multiple TLS address lookups)
+        #[derive(Clone, Copy)]
+        struct ThreadWrapper {
+            self_ptr: *const Thread,
+            thread: Thread,
+        }
+
+        impl ThreadWrapper {
+            /// The thread ID obtained from the thread ID manager.
+            #[inline]
+            fn new(id: usize) -> Self {
+                Self {
+                    self_ptr: ((THREAD.get().cast_const() as usize) + offset_of!(ThreadWrapper, thread)) as *const Thread,
+                    thread: Thread::new(id),
+                }
+            }
+        }
+
         /// Returns a thread ID for the current thread, allocating one if needed.
         #[inline]
         pub(crate) fn get() -> Thread {
-            if let Some(thread) = unsafe { THREAD } {
-                thread
+            let thread = unsafe { *THREAD.get() };
+            if !thread.self_ptr.is_null() {
+                unsafe { thread.self_ptr.read() }
             } else {
                 get_slow()
             }
@@ -116,12 +149,13 @@ cfg_if::cfg_if! {
         /// Out-of-line slow path for allocating a thread ID.
         #[cold]
          fn get_slow() -> Thread {
-            let new = Thread::new(THREAD_ID_MANAGER.lock().alloc());
+            let id = THREAD_ID_MANAGER.lock().alloc();
+            let new = ThreadWrapper::new(id);
             unsafe {
-                THREAD = Some(new);
+                *THREAD.get() = new;
             }
-            THREAD_GUARD.with(|guard| guard.id.set(new.id));
-            new
+            THREAD_GUARD.with(|guard| guard.id.set(id));
+            new.thread
         }
     } else {
         // This is split into 2 thread-local variables so that we can check whether the
@@ -164,9 +198,10 @@ cfg_if::cfg_if! {
         /// Out-of-line slow path for allocating a thread ID.
         #[cold]
         fn get_slow(thread: &Cell<Option<Thread>>) -> Thread {
-            let new = Thread::new(THREAD_ID_MANAGER.lock().alloc());
+            let id = THREAD_ID_MANAGER.lock().alloc();
+            let new = Thread::new(id);
             thread.set(Some(new));
-            THREAD_GUARD.with(|guard| guard.id.set(new.id));
+            THREAD_GUARD.with(|guard| guard.id.set(id));
             new
         }
     }
@@ -175,32 +210,27 @@ cfg_if::cfg_if! {
 #[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.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, 2);
+    assert_eq!(thread.bucket_size(), 2);
     assert_eq!(thread.index, 0);
 
     let thread = Thread::new(2);
-    assert_eq!(thread.id, 2);
     assert_eq!(thread.bucket, 1);
-    assert_eq!(thread.bucket_size, 2);
+    assert_eq!(thread.bucket_size(), 2);
     assert_eq!(thread.index, 1);
 
     let thread = Thread::new(3);
-    assert_eq!(thread.id, 3);
     assert_eq!(thread.bucket, 2);
-    assert_eq!(thread.bucket_size, 4);
+    assert_eq!(thread.bucket_size(), 4);
     assert_eq!(thread.index, 0);
 
     let thread = Thread::new(19);
-    assert_eq!(thread.id, 19);
     assert_eq!(thread.bucket, 4);
-    assert_eq!(thread.bucket_size, 16);
+    assert_eq!(thread.bucket_size(), 16);
     assert_eq!(thread.index, 4);
 }