Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

3 files changed, 821 insertions, 0 deletions

diff --git a/vendor/rayon-core/src/sleep/README.md b/vendor/rayon-core/src/sleep/README.md
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+# Introduction: the sleep module
+
+The code in this module governs when worker threads should go to
+sleep. The system used in this code was introduced in [Rayon RFC #5].
+There is also a [video walkthrough] available. Both of those may be
+valuable resources to understanding the code, though naturally they
+will also grow stale over time. The comments in this file are
+extracted from the RFC and meant to be kept up to date.
+
+[Rayon RFC #5]: https://github.com/rayon-rs/rfcs/pull/5
+[video walkthrough]: https://youtu.be/HvmQsE5M4cY
+
+# The `Sleep` struct
+
+The `Sleep` struct is embedded into each registry. It performs several functions:
+
+* It tracks when workers are awake or asleep.
+* It decides how long a worker should look for work before it goes to sleep,
+  via a callback that is invoked periodically from the worker's search loop.
+* It is notified when latches are set, jobs are published, or other
+  events occur, and it will go and wake the appropriate threads if
+  they are sleeping.
+
+# Thread states
+
+There are three main thread states:
+
+* An **active** thread is one that is actively executing a job.
+* An **idle** thread is one that is searching for work to do. It will be
+  trying to steal work or pop work from the global injector queue.
+* A **sleeping** thread is one that is blocked on a condition variable,
+  waiting to be awoken.
+
+We sometimes refer to the final two states collectively as **inactive**.
+Threads begin as idle but transition to idle and finally sleeping when
+they're unable to find work to do.
+
+## Sleepy threads
+
+There is one other special state worth mentioning. During the idle state,
+threads can get **sleepy**. A sleepy thread is still idle, in that it is still
+searching for work, but it is *about* to go to sleep after it does one more
+search (or some other number, potentially). When a thread enters the sleepy
+state, it signals (via the **jobs event counter**, described below) that it is
+about to go to sleep. If new work is published, this will lead to the counter
+being adjusted. When the thread actually goes to sleep, it will (hopefully, but
+not guaranteed) see that the counter has changed and elect not to sleep, but
+instead to search again. See the section on the **jobs event counter** for more
+details.
+
+# The counters
+
+One of the key structs in the sleep module is `AtomicCounters`, found in
+`counters.rs`. It packs three counters into one atomically managed value:
+
+* Two **thread counters**, which track the number of threads in a particular state.
+* The **jobs event counter**, which is used to signal when new work is available.
+  It (sort of) tracks the number of jobs posted, but not quite, and it can rollover.
+
+## Thread counters
+
+There are two thread counters, one that tracks **inactive** threads and one that
+tracks **sleeping** threads. From this, one can deduce the number of threads
+that are idle by subtracting sleeping threads from inactive threads. We track
+the counters in this way because it permits simpler atomic operations. One can
+increment the number of sleeping threads (and thus decrease the number of idle
+threads) simply by doing one atomic increment, for example. Similarly, one can
+decrease the number of sleeping threads (and increase the number of idle
+threads) through one atomic decrement.
+
+These counters are adjusted as follows:
+
+* When a thread enters the idle state: increment the inactive thread counter.
+* When a thread enters the sleeping state: increment the sleeping thread counter.
+* When a thread awakens a sleeping thread: decrement the sleeping thread counter.
+  * Subtle point: the thread that *awakens* the sleeping thread decrements the
+    counter, not the thread that is *sleeping*. This is because there is a delay
+    between signaling a thread to wake and the thread actually waking:
+    decrementing the counter when awakening the thread means that other threads
+    that may be posting work will see the up-to-date value that much faster.
+* When a thread finds work, exiting the idle state: decrement the inactive
+  thread counter.
+
+## Jobs event counter
+
+The final counter is the **jobs event counter**. The role of this counter is to
+help sleepy threads detect when new work is posted in a lightweight fashion. In
+its simplest form, we would simply have a counter that gets incremented each
+time a new job is posted. This way, when a thread gets sleepy, it could read the
+counter, and then compare to see if the value has changed before it actually
+goes to sleep. But this [turns out to be too expensive] in practice, so we use a
+somewhat more complex scheme.
+
+[turns out to be too expensive]: https://github.com/rayon-rs/rayon/pull/746#issuecomment-624802747
+
+The idea is that the counter toggles between two states, depending on whether
+its value is even or odd (or, equivalently, on the value of its low bit):
+
+* Even -- If the low bit is zero, then it means that there has been no new work
+  since the last thread got sleepy.
+* Odd -- If the low bit is one, then it means that new work was posted since
+  the last thread got sleepy.
+
+### New work is posted
+
+When new work is posted, we check the value of the counter: if it is even,
+then we increment it by one, so that it becomes odd.
+
+### Worker thread gets sleepy
+
+When a worker thread gets sleepy, it will read the value of the counter. If the
+counter is odd, it will increment the counter so that it is even. Either way, it
+remembers the final value of the counter. The final value will be used later,
+when the thread is going to sleep. If at that time the counter has not changed,
+then we can assume no new jobs have been posted (though note the remote
+possibility of rollover, discussed in detail below).
+
+# Protocol for a worker thread to post work
+
+The full protocol for a thread to post work is as follows
+
+* If the work is posted into the injection queue, then execute a seq-cst fence (see below).
+* Load the counters, incrementing the JEC if it is even so that it is odd.
+* Check if there are idle threads available to handle this new job. If not,
+  and there are sleeping threads, then wake one or more threads.
+
+# Protocol for a worker thread to fall asleep
+
+The full protocol for a thread to fall asleep is as follows:
+
+* After completing all its jobs, the worker goes idle and begins to
+  search for work. As it searches, it counts "rounds". In each round,
+  it searches all other work threads' queues, plus the 'injector queue' for
+  work injected from the outside. If work is found in this search, the thread
+  becomes active again and hence restarts this protocol from the top.
+* After a certain number of rounds, the thread "gets sleepy" and executes `get_sleepy`
+  above, remembering the `final_value` of the JEC. It does one more search for work.
+* If no work is found, the thread atomically:
+  * Checks the JEC to see that it has not changed from `final_value`.
+    * If it has, then the thread goes back to searching for work. We reset to
+      just before we got sleepy, so that we will do one more search
+      before attending to sleep again (rather than searching for many rounds).
+  * Increments the number of sleeping threads by 1.
+* The thread then executes a seq-cst fence operation (see below).
+* The thread then does one final check for injected jobs (see below). If any
+  are available, it returns to the 'pre-sleepy' state as if the JEC had changed.
+* The thread waits to be signaled. Once signaled, it returns to the idle state.
+
+# The jobs event counter and deadlock
+
+As described in the section on the JEC, the main concern around going to sleep
+is avoiding a race condition wherein:
+
+* Thread A looks for work, finds none.
+* Thread B posts work but sees no sleeping threads.
+* Thread A goes to sleep.
+
+The JEC protocol largely prevents this, but due to rollover, this prevention is
+not complete. It is possible -- if unlikely -- that enough activity occurs for
+Thread A to observe the same JEC value that it saw when getting sleepy. If the
+new work being published came from *inside* the thread-pool, then this race
+condition isn't too harmful. It means that we have fewer workers processing the
+work then we should, but we won't deadlock. This seems like an acceptable risk
+given that this is unlikely in practice.
+
+However, if the work was posted as an *external* job, that is a problem. In that
+case, it's possible that all of our workers could go to sleep, and the external
+job would never get processed. To prevent that, the sleeping protocol includes
+one final check to see if the injector queue is empty before fully falling
+asleep. Note that this final check occurs **after** the number of sleeping
+threads has been incremented. We are not concerned therefore with races against
+injections that occur after that increment, only before.
+
+Unfortunately, there is one rather subtle point concerning this final check:
+we wish to avoid the possibility that:
+
+* work is pushed into the injection queue by an outside thread X,
+* the sleepy thread S sees the JEC but it has rolled over and is equal
+* the sleepy thread S reads the injection queue but does not see the work posted by X.
+
+This is possible because the C++ memory model typically offers guarantees of the
+form "if you see the access A, then you must see those other accesses" -- but it
+doesn't guarantee that you will see the access A (i.e., if you think of
+processors with independent caches, you may be operating on very out of date
+cache state). 
+
+## Using seq-cst fences to prevent deadlock
+
+To overcome this problem, we have inserted two sequentially consistent fence
+operations into the protocols above:
+
+* One fence occurs after work is posted into the injection queue, but before the
+  counters are read (including the number of sleeping threads).
+  * Note that no fence is needed for work posted to internal queues, since it is ok
+    to overlook work in that case.
+* One fence occurs after the number of sleeping threads is incremented, but
+  before the injection queue is read.
+
+### Proof sketch
+
+What follows is a "proof sketch" that the protocol is deadlock free. We model
+two relevant bits of memory, the job injector queue J and the atomic counters C.
+
+Consider the actions of the injecting thread:
+
+* PushJob: Job is injected, which can be modeled as an atomic write to J with release semantics.
+* PushFence: A sequentially consistent fence is executed.
+* ReadSleepers: The counters C are read (they may also be incremented, but we just consider the read that comes first).
+
+Meanwhile, the sleepy thread does the following:
+
+* IncSleepers: The number of sleeping threads is incremented, which is atomic exchange to C.
+* SleepFence: A sequentially consistent fence is executed.
+* ReadJob: We look to see if the queue is empty, which is a read of J with acquire semantics.
+
+Either PushFence or SleepFence must come first:
+
+* If PushFence comes first, then PushJob must be visible to ReadJob.
+* If SleepFence comes first, then IncSleepers is visible to ReadSleepers.
\ No newline at end of file
diff --git a/vendor/rayon-core/src/sleep/counters.rs b/vendor/rayon-core/src/sleep/counters.rs
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+++ b/vendor/rayon-core/src/sleep/counters.rs
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+use std::sync::atomic::{AtomicUsize, Ordering};
+
+pub(super) struct AtomicCounters {
+    /// Packs together a number of counters. The counters are ordered as
+    /// follows, from least to most significant bits (here, we assuming
+    /// that [`THREADS_BITS`] is equal to 10):
+    ///
+    /// * Bits 0..10: Stores the number of **sleeping threads**
+    /// * Bits 10..20: Stores the number of **inactive threads**
+    /// * Bits 20..: Stores the **job event counter** (JEC)
+    ///
+    /// This uses 10 bits ([`THREADS_BITS`]) to encode the number of threads. Note
+    /// that the total number of bits (and hence the number of bits used for the
+    /// JEC) will depend on whether we are using a 32- or 64-bit architecture.
+    value: AtomicUsize,
+}
+
+#[derive(Copy, Clone)]
+pub(super) struct Counters {
+    word: usize,
+}
+
+/// A value read from the **Jobs Event Counter**.
+/// See the [`README.md`](README.md) for more
+/// coverage of how the jobs event counter works.
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub(super) struct JobsEventCounter(usize);
+
+impl JobsEventCounter {
+    pub(super) const DUMMY: JobsEventCounter = JobsEventCounter(std::usize::MAX);
+
+    #[inline]
+    pub(super) fn as_usize(self) -> usize {
+        self.0
+    }
+
+    /// The JEC "is sleepy" if the last thread to increment it was in the
+    /// process of becoming sleepy. This is indicated by its value being *even*.
+    /// When new jobs are posted, they check if the JEC is sleepy, and if so
+    /// they incremented it.
+    #[inline]
+    pub(super) fn is_sleepy(self) -> bool {
+        (self.as_usize() & 1) == 0
+    }
+
+    /// The JEC "is active" if the last thread to increment it was posting new
+    /// work. This is indicated by its value being *odd*. When threads get
+    /// sleepy, they will check if the JEC is active, and increment it.
+    #[inline]
+    pub(super) fn is_active(self) -> bool {
+        !self.is_sleepy()
+    }
+}
+
+/// Number of bits used for the thread counters.
+#[cfg(target_pointer_width = "64")]
+const THREADS_BITS: usize = 16;
+
+#[cfg(target_pointer_width = "32")]
+const THREADS_BITS: usize = 8;
+
+/// Bits to shift to select the sleeping threads
+/// (used with `select_bits`).
+#[allow(clippy::erasing_op)]
+const SLEEPING_SHIFT: usize = 0 * THREADS_BITS;
+
+/// Bits to shift to select the inactive threads
+/// (used with `select_bits`).
+#[allow(clippy::identity_op)]
+const INACTIVE_SHIFT: usize = 1 * THREADS_BITS;
+
+/// Bits to shift to select the JEC
+/// (use JOBS_BITS).
+const JEC_SHIFT: usize = 2 * THREADS_BITS;
+
+/// Max value for the thread counters.
+pub(crate) const THREADS_MAX: usize = (1 << THREADS_BITS) - 1;
+
+/// Constant that can be added to add one sleeping thread.
+const ONE_SLEEPING: usize = 1;
+
+/// Constant that can be added to add one inactive thread.
+/// An inactive thread is either idle, sleepy, or sleeping.
+const ONE_INACTIVE: usize = 1 << INACTIVE_SHIFT;
+
+/// Constant that can be added to add one to the JEC.
+const ONE_JEC: usize = 1 << JEC_SHIFT;
+
+impl AtomicCounters {
+    #[inline]
+    pub(super) fn new() -> AtomicCounters {
+        AtomicCounters {
+            value: AtomicUsize::new(0),
+        }
+    }
+
+    /// Load and return the current value of the various counters.
+    /// This value can then be given to other method which will
+    /// attempt to update the counters via compare-and-swap.
+    #[inline]
+    pub(super) fn load(&self, ordering: Ordering) -> Counters {
+        Counters::new(self.value.load(ordering))
+    }
+
+    #[inline]
+    fn try_exchange(&self, old_value: Counters, new_value: Counters, ordering: Ordering) -> bool {
+        self.value
+            .compare_exchange(old_value.word, new_value.word, ordering, Ordering::Relaxed)
+            .is_ok()
+    }
+
+    /// Adds an inactive thread. This cannot fail.
+    ///
+    /// This should be invoked when a thread enters its idle loop looking
+    /// for work. It is decremented when work is found. Note that it is
+    /// not decremented if the thread transitions from idle to sleepy or sleeping;
+    /// so the number of inactive threads is always greater-than-or-equal
+    /// to the number of sleeping threads.
+    #[inline]
+    pub(super) fn add_inactive_thread(&self) {
+        self.value.fetch_add(ONE_INACTIVE, Ordering::SeqCst);
+    }
+
+    /// Increments the jobs event counter if `increment_when`, when applied to
+    /// the current value, is true. Used to toggle the JEC from even (sleepy) to
+    /// odd (active) or vice versa. Returns the final value of the counters, for
+    /// which `increment_when` is guaranteed to return false.
+    pub(super) fn increment_jobs_event_counter_if(
+        &self,
+        increment_when: impl Fn(JobsEventCounter) -> bool,
+    ) -> Counters {
+        loop {
+            let old_value = self.load(Ordering::SeqCst);
+            if increment_when(old_value.jobs_counter()) {
+                let new_value = old_value.increment_jobs_counter();
+                if self.try_exchange(old_value, new_value, Ordering::SeqCst) {
+                    return new_value;
+                }
+            } else {
+                return old_value;
+            }
+        }
+    }
+
+    /// Subtracts an inactive thread. This cannot fail. It is invoked
+    /// when a thread finds work and hence becomes active. It returns the
+    /// number of sleeping threads to wake up (if any).
+    ///
+    /// See `add_inactive_thread`.
+    #[inline]
+    pub(super) fn sub_inactive_thread(&self) -> usize {
+        let old_value = Counters::new(self.value.fetch_sub(ONE_INACTIVE, Ordering::SeqCst));
+        debug_assert!(
+            old_value.inactive_threads() > 0,
+            "sub_inactive_thread: old_value {:?} has no inactive threads",
+            old_value,
+        );
+        debug_assert!(
+            old_value.sleeping_threads() <= old_value.inactive_threads(),
+            "sub_inactive_thread: old_value {:?} had {} sleeping threads and {} inactive threads",
+            old_value,
+            old_value.sleeping_threads(),
+            old_value.inactive_threads(),
+        );
+
+        // Current heuristic: whenever an inactive thread goes away, if
+        // there are any sleeping threads, wake 'em up.
+        let sleeping_threads = old_value.sleeping_threads();
+        std::cmp::min(sleeping_threads, 2)
+    }
+
+    /// Subtracts a sleeping thread. This cannot fail, but it is only
+    /// safe to do if you you know the number of sleeping threads is
+    /// non-zero (i.e., because you have just awoken a sleeping
+    /// thread).
+    #[inline]
+    pub(super) fn sub_sleeping_thread(&self) {
+        let old_value = Counters::new(self.value.fetch_sub(ONE_SLEEPING, Ordering::SeqCst));
+        debug_assert!(
+            old_value.sleeping_threads() > 0,
+            "sub_sleeping_thread: old_value {:?} had no sleeping threads",
+            old_value,
+        );
+        debug_assert!(
+            old_value.sleeping_threads() <= old_value.inactive_threads(),
+            "sub_sleeping_thread: old_value {:?} had {} sleeping threads and {} inactive threads",
+            old_value,
+            old_value.sleeping_threads(),
+            old_value.inactive_threads(),
+        );
+    }
+
+    #[inline]
+    pub(super) fn try_add_sleeping_thread(&self, old_value: Counters) -> bool {
+        debug_assert!(
+            old_value.inactive_threads() > 0,
+            "try_add_sleeping_thread: old_value {:?} has no inactive threads",
+            old_value,
+        );
+        debug_assert!(
+            old_value.sleeping_threads() < THREADS_MAX,
+            "try_add_sleeping_thread: old_value {:?} has too many sleeping threads",
+            old_value,
+        );
+
+        let mut new_value = old_value;
+        new_value.word += ONE_SLEEPING;
+
+        self.try_exchange(old_value, new_value, Ordering::SeqCst)
+    }
+}
+
+#[inline]
+fn select_thread(word: usize, shift: usize) -> usize {
+    (word >> shift) & THREADS_MAX
+}
+
+#[inline]
+fn select_jec(word: usize) -> usize {
+    word >> JEC_SHIFT
+}
+
+impl Counters {
+    #[inline]
+    fn new(word: usize) -> Counters {
+        Counters { word }
+    }
+
+    #[inline]
+    fn increment_jobs_counter(self) -> Counters {
+        // We can freely add to JEC because it occupies the most significant bits.
+        // Thus it doesn't overflow into the other counters, just wraps itself.
+        Counters {
+            word: self.word.wrapping_add(ONE_JEC),
+        }
+    }
+
+    #[inline]
+    pub(super) fn jobs_counter(self) -> JobsEventCounter {
+        JobsEventCounter(select_jec(self.word))
+    }
+
+    /// The number of threads that are not actively
+    /// executing work. They may be idle, sleepy, or asleep.
+    #[inline]
+    pub(super) fn inactive_threads(self) -> usize {
+        select_thread(self.word, INACTIVE_SHIFT)
+    }
+
+    #[inline]
+    pub(super) fn awake_but_idle_threads(self) -> usize {
+        debug_assert!(
+            self.sleeping_threads() <= self.inactive_threads(),
+            "sleeping threads: {} > raw idle threads {}",
+            self.sleeping_threads(),
+            self.inactive_threads()
+        );
+        self.inactive_threads() - self.sleeping_threads()
+    }
+
+    #[inline]
+    pub(super) fn sleeping_threads(self) -> usize {
+        select_thread(self.word, SLEEPING_SHIFT)
+    }
+}
+
+impl std::fmt::Debug for Counters {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let word = format!("{:016x}", self.word);
+        fmt.debug_struct("Counters")
+            .field("word", &word)
+            .field("jobs", &self.jobs_counter().0)
+            .field("inactive", &self.inactive_threads())
+            .field("sleeping", &self.sleeping_threads())
+            .finish()
+    }
+}
diff --git a/vendor/rayon-core/src/sleep/mod.rs b/vendor/rayon-core/src/sleep/mod.rs
new file mode 100644
index 0000000..03d1077
--- /dev/null
+++ b/vendor/rayon-core/src/sleep/mod.rs
@@ -0,0 +1,325 @@
+//! Code that decides when workers should go to sleep. See README.md
+//! for an overview.
+
+use crate::latch::CoreLatch;
+use crossbeam_utils::CachePadded;
+use std::sync::atomic::Ordering;
+use std::sync::{Condvar, Mutex};
+use std::thread;
+use std::usize;
+
+mod counters;
+pub(crate) use self::counters::THREADS_MAX;
+use self::counters::{AtomicCounters, JobsEventCounter};
+
+/// The `Sleep` struct is embedded into each registry. It governs the waking and sleeping
+/// of workers. It has callbacks that are invoked periodically at significant events,
+/// such as when workers are looping and looking for work, when latches are set, or when
+/// jobs are published, and it either blocks threads or wakes them in response to these
+/// events. See the [`README.md`] in this module for more details.
+///
+/// [`README.md`] README.md
+pub(super) struct Sleep {
+    /// One "sleep state" per worker. Used to track if a worker is sleeping and to have
+    /// them block.
+    worker_sleep_states: Vec<CachePadded<WorkerSleepState>>,
+
+    counters: AtomicCounters,
+}
+
+/// An instance of this struct is created when a thread becomes idle.
+/// It is consumed when the thread finds work, and passed by `&mut`
+/// reference for operations that preserve the idle state. (In other
+/// words, producing one of these structs is evidence the thread is
+/// idle.) It tracks state such as how long the thread has been idle.
+pub(super) struct IdleState {
+    /// What is worker index of the idle thread?
+    worker_index: usize,
+
+    /// How many rounds have we been circling without sleeping?
+    rounds: u32,
+
+    /// Once we become sleepy, what was the sleepy counter value?
+    /// Set to `INVALID_SLEEPY_COUNTER` otherwise.
+    jobs_counter: JobsEventCounter,
+}
+
+/// The "sleep state" for an individual worker.
+#[derive(Default)]
+struct WorkerSleepState {
+    /// Set to true when the worker goes to sleep; set to false when
+    /// the worker is notified or when it wakes.
+    is_blocked: Mutex<bool>,
+
+    condvar: Condvar,
+}
+
+const ROUNDS_UNTIL_SLEEPY: u32 = 32;
+const ROUNDS_UNTIL_SLEEPING: u32 = ROUNDS_UNTIL_SLEEPY + 1;
+
+impl Sleep {
+    pub(super) fn new(n_threads: usize) -> Sleep {
+        assert!(n_threads <= THREADS_MAX);
+        Sleep {
+            worker_sleep_states: (0..n_threads).map(|_| Default::default()).collect(),
+            counters: AtomicCounters::new(),
+        }
+    }
+
+    #[inline]
+    pub(super) fn start_looking(&self, worker_index: usize) -> IdleState {
+        self.counters.add_inactive_thread();
+
+        IdleState {
+            worker_index,
+            rounds: 0,
+            jobs_counter: JobsEventCounter::DUMMY,
+        }
+    }
+
+    #[inline]
+    pub(super) fn work_found(&self) {
+        // If we were the last idle thread and other threads are still sleeping,
+        // then we should wake up another thread.
+        let threads_to_wake = self.counters.sub_inactive_thread();
+        self.wake_any_threads(threads_to_wake as u32);
+    }
+
+    #[inline]
+    pub(super) fn no_work_found(
+        &self,
+        idle_state: &mut IdleState,
+        latch: &CoreLatch,
+        has_injected_jobs: impl FnOnce() -> bool,
+    ) {
+        if idle_state.rounds < ROUNDS_UNTIL_SLEEPY {
+            thread::yield_now();
+            idle_state.rounds += 1;
+        } else if idle_state.rounds == ROUNDS_UNTIL_SLEEPY {
+            idle_state.jobs_counter = self.announce_sleepy();
+            idle_state.rounds += 1;
+            thread::yield_now();
+        } else if idle_state.rounds < ROUNDS_UNTIL_SLEEPING {
+            idle_state.rounds += 1;
+            thread::yield_now();
+        } else {
+            debug_assert_eq!(idle_state.rounds, ROUNDS_UNTIL_SLEEPING);
+            self.sleep(idle_state, latch, has_injected_jobs);
+        }
+    }
+
+    #[cold]
+    fn announce_sleepy(&self) -> JobsEventCounter {
+        self.counters
+            .increment_jobs_event_counter_if(JobsEventCounter::is_active)
+            .jobs_counter()
+    }
+
+    #[cold]
+    fn sleep(
+        &self,
+        idle_state: &mut IdleState,
+        latch: &CoreLatch,
+        has_injected_jobs: impl FnOnce() -> bool,
+    ) {
+        let worker_index = idle_state.worker_index;
+
+        if !latch.get_sleepy() {
+            return;
+        }
+
+        let sleep_state = &self.worker_sleep_states[worker_index];
+        let mut is_blocked = sleep_state.is_blocked.lock().unwrap();
+        debug_assert!(!*is_blocked);
+
+        // Our latch was signalled. We should wake back up fully as we
+        // will have some stuff to do.
+        if !latch.fall_asleep() {
+            idle_state.wake_fully();
+            return;
+        }
+
+        loop {
+            let counters = self.counters.load(Ordering::SeqCst);
+
+            // Check if the JEC has changed since we got sleepy.
+            debug_assert!(idle_state.jobs_counter.is_sleepy());
+            if counters.jobs_counter() != idle_state.jobs_counter {
+                // JEC has changed, so a new job was posted, but for some reason
+                // we didn't see it. We should return to just before the SLEEPY
+                // state so we can do another search and (if we fail to find
+                // work) go back to sleep.
+                idle_state.wake_partly();
+                latch.wake_up();
+                return;
+            }
+
+            // Otherwise, let's move from IDLE to SLEEPING.
+            if self.counters.try_add_sleeping_thread(counters) {
+                break;
+            }
+        }
+
+        // Successfully registered as asleep.
+
+        // We have one last check for injected jobs to do. This protects against
+        // deadlock in the very unlikely event that
+        //
+        // - an external job is being injected while we are sleepy
+        // - that job triggers the rollover over the JEC such that we don't see it
+        // - we are the last active worker thread
+        std::sync::atomic::fence(Ordering::SeqCst);
+        if has_injected_jobs() {
+            // If we see an externally injected job, then we have to 'wake
+            // ourselves up'. (Ordinarily, `sub_sleeping_thread` is invoked by
+            // the one that wakes us.)
+            self.counters.sub_sleeping_thread();
+        } else {
+            // If we don't see an injected job (the normal case), then flag
+            // ourselves as asleep and wait till we are notified.
+            //
+            // (Note that `is_blocked` is held under a mutex and the mutex was
+            // acquired *before* we incremented the "sleepy counter". This means
+            // that whomever is coming to wake us will have to wait until we
+            // release the mutex in the call to `wait`, so they will see this
+            // boolean as true.)
+            *is_blocked = true;
+            while *is_blocked {
+                is_blocked = sleep_state.condvar.wait(is_blocked).unwrap();
+            }
+        }
+
+        // Update other state:
+        idle_state.wake_fully();
+        latch.wake_up();
+    }
+
+    /// Notify the given thread that it should wake up (if it is
+    /// sleeping).  When this method is invoked, we typically know the
+    /// thread is asleep, though in rare cases it could have been
+    /// awoken by (e.g.) new work having been posted.
+    pub(super) fn notify_worker_latch_is_set(&self, target_worker_index: usize) {
+        self.wake_specific_thread(target_worker_index);
+    }
+
+    /// Signals that `num_jobs` new jobs were injected into the thread
+    /// pool from outside. This function will ensure that there are
+    /// threads available to process them, waking threads from sleep
+    /// if necessary.
+    ///
+    /// # Parameters
+    ///
+    /// - `num_jobs` -- lower bound on number of jobs available for stealing.
+    ///   We'll try to get at least one thread per job.
+    #[inline]
+    pub(super) fn new_injected_jobs(&self, num_jobs: u32, queue_was_empty: bool) {
+        // This fence is needed to guarantee that threads
+        // as they are about to fall asleep, observe any
+        // new jobs that may have been injected.
+        std::sync::atomic::fence(Ordering::SeqCst);
+
+        self.new_jobs(num_jobs, queue_was_empty)
+    }
+
+    /// Signals that `num_jobs` new jobs were pushed onto a thread's
+    /// local deque. This function will try to ensure that there are
+    /// threads available to process them, waking threads from sleep
+    /// if necessary. However, this is not guaranteed: under certain
+    /// race conditions, the function may fail to wake any new
+    /// threads; in that case the existing thread should eventually
+    /// pop the job.
+    ///
+    /// # Parameters
+    ///
+    /// - `num_jobs` -- lower bound on number of jobs available for stealing.
+    ///   We'll try to get at least one thread per job.
+    #[inline]
+    pub(super) fn new_internal_jobs(&self, num_jobs: u32, queue_was_empty: bool) {
+        self.new_jobs(num_jobs, queue_was_empty)
+    }
+
+    /// Common helper for `new_injected_jobs` and `new_internal_jobs`.
+    #[inline]
+    fn new_jobs(&self, num_jobs: u32, queue_was_empty: bool) {
+        // Read the counters and -- if sleepy workers have announced themselves
+        // -- announce that there is now work available. The final value of `counters`
+        // with which we exit the loop thus corresponds to a state when
+        let counters = self
+            .counters
+            .increment_jobs_event_counter_if(JobsEventCounter::is_sleepy);
+        let num_awake_but_idle = counters.awake_but_idle_threads();
+        let num_sleepers = counters.sleeping_threads();
+
+        if num_sleepers == 0 {
+            // nobody to wake
+            return;
+        }
+
+        // Promote from u16 to u32 so we can interoperate with
+        // num_jobs more easily.
+        let num_awake_but_idle = num_awake_but_idle as u32;
+        let num_sleepers = num_sleepers as u32;
+
+        // If the queue is non-empty, then we always wake up a worker
+        // -- clearly the existing idle jobs aren't enough. Otherwise,
+        // check to see if we have enough idle workers.
+        if !queue_was_empty {
+            let num_to_wake = std::cmp::min(num_jobs, num_sleepers);
+            self.wake_any_threads(num_to_wake);
+        } else if num_awake_but_idle < num_jobs {
+            let num_to_wake = std::cmp::min(num_jobs - num_awake_but_idle, num_sleepers);
+            self.wake_any_threads(num_to_wake);
+        }
+    }
+
+    #[cold]
+    fn wake_any_threads(&self, mut num_to_wake: u32) {
+        if num_to_wake > 0 {
+            for i in 0..self.worker_sleep_states.len() {
+                if self.wake_specific_thread(i) {
+                    num_to_wake -= 1;
+                    if num_to_wake == 0 {
+                        return;
+                    }
+                }
+            }
+        }
+    }
+
+    fn wake_specific_thread(&self, index: usize) -> bool {
+        let sleep_state = &self.worker_sleep_states[index];
+
+        let mut is_blocked = sleep_state.is_blocked.lock().unwrap();
+        if *is_blocked {
+            *is_blocked = false;
+            sleep_state.condvar.notify_one();
+
+            // When the thread went to sleep, it will have incremented
+            // this value. When we wake it, its our job to decrement
+            // it. We could have the thread do it, but that would
+            // introduce a delay between when the thread was
+            // *notified* and when this counter was decremented. That
+            // might mislead people with new work into thinking that
+            // there are sleeping threads that they should try to
+            // wake, when in fact there is nothing left for them to
+            // do.
+            self.counters.sub_sleeping_thread();
+
+            true
+        } else {
+            false
+        }
+    }
+}
+
+impl IdleState {
+    fn wake_fully(&mut self) {
+        self.rounds = 0;
+        self.jobs_counter = JobsEventCounter::DUMMY;
+    }
+
+    fn wake_partly(&mut self) {
+        self.rounds = ROUNDS_UNTIL_SLEEPY;
+        self.jobs_counter = JobsEventCounter::DUMMY;
+    }
+}