unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}

unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}

impl<T: ?Sized> Clone for Arc<T> {
    fn clone(&self) -> Arc<T> {
        // Using a relaxed ordering is alright here, as knowledge of the
        // original reference prevents other threads from erroneously deleting
        // the object.
        //
        // As explained in the [Boost documentation][1], Increasing the
        // reference counter can always be done with memory_order_relaxed: New
        // references to an object can only be formed from an existing
        // reference, and passing an existing reference from one thread to
        // another must already provide any required synchronization.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        let old_size = self.inner().strong.fetch_add(1, Relaxed);

        // However we need to guard against massive refcounts in case someone
        // is `mem::forget`ing Arcs. If we don't do this the count can overflow
        // and users will use-after free. We racily saturate to `isize::MAX` on
        // the assumption that there aren't ~2 billion threads incrementing
        // the reference count at once. This branch will never be taken in
        // any realistic program.
        //
        // We abort because such a program is incredibly degenerate, and we
        // don't care to support it.
        if old_size > MAX_REFCOUNT {
            unsafe {
                abort();
            }
        }

        Self::from_inner(self.ptr)
    }
}

/// A soft limit on the amount of references that may be made to an `Arc`.
///
/// Going above this limit will abort your program (although not
/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

let mut old_counter = strong.load(Relaxed);
loop {
    if old_counter >= MAX_REFCOUNT {
        // abort
    }
    let c = strong.compare_and_swap(old_counter, old_counter + 1);
    if c == old_counter {
        break;
    } else {
        old_counter = c;
    }
}

// usizeの最大値の約半分!
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

let old_size = self.inner().strong.fetch_add(1, Relaxed);

if old_size > MAX_REFCOUNT {
    unsafe {
        abort();
    }
}

unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
    fn drop(&mut self) {
        // Because `fetch_sub` is already atomic, we do not need to synchronize
        // with other threads unless we are going to delete the object. This
        // same logic applies to the below `fetch_sub` to the `weak` count.
        if self.inner().strong.fetch_sub(1, Release) != 1 {
            return;
        }

        // This fence is needed to prevent reordering of use of the data and
        // deletion of the data.  Because it is marked `Release`, the decreasing
        // of the reference count synchronizes with this `Acquire` fence. This
        // means that use of the data happens before decreasing the reference
        // count, which happens before this fence, which happens before the
        // deletion of the data.
        //
        // As explained in the [Boost documentation][1],
        //
        // > It is important to enforce any possible access to the object in one
        // > thread (through an existing reference) to *happen before* deleting
        // > the object in a different thread. This is achieved by a "release"
        // > operation after dropping a reference (any access to the object
        // > through this reference must obviously happened before), and an
        // > "acquire" operation before deleting the object.
        //
        // In particular, while the contents of an Arc are usually immutable, it's
        // possible to have interior writes to something like a Mutex<T>. Since a
        // Mutex is not acquired when it is deleted, we can't rely on its
        // synchronization logic to make writes in thread A visible to a destructor
        // running in thread B.
        //
        // Also note that the Acquire fence here could probably be replaced with an
        // Acquire load, which could improve performance in highly-contended
        // situations. See [2].
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        // [2]: (https://github.com/rust-lang/rust/pull/41714)
        atomic::fence(Acquire);

        unsafe {
            self.drop_slow();
        }
    }
}

if self.strong.load(Relaxed) != 1 {
    return None;
}
// ←競合条件
if self.weak.load(Relaxed) != 1 {
    return None;
}
fence(Acquire);
return Some(unsafe { &mut *self.value.get() });

if self.weak.load(Relaxed) != 1 {
    return None;
}
// ←競合条件
if self.strong.load(Relaxed) != 1 {
    return None;
}
fence(Acquire);
return Some(unsafe { &mut *self.value.get() });

if self.strong.compare_and_swap(1, 0, Relaxed) != 1 {
    return None;
}
if self.weak.load(Relaxed) != 1 {
    self.strong.fetch_add(1, Relaxed);
    return None;
}
self.strong.fetch_add(1, Acquire);
return Some(unsafe { &mut *self.value.get() });

struct ArcInner<T: ?Sized> {
    strong: atomic::AtomicUsize,

    // the value usize::MAX acts as a sentinel for temporarily "locking" the
    // ability to upgrade weak pointers or downgrade strong ones; this is used
    // to avoid races in `make_mut` and `get_mut`.
    weak: atomic::AtomicUsize,

    data: T,
}

impl<T: ?Sized> Arc<T> {
    pub fn downgrade(this: &Self) -> Weak<T> {
        // This Relaxed is OK because we're checking the value in the CAS
        // below.
        let mut cur = this.inner().weak.load(Relaxed);

        loop {
            // check if the weak counter is currently "locked"; if so, spin.
            if cur == usize::MAX {
                cur = this.inner().weak.load(Relaxed);
                continue;
            }

            // NOTE: this code currently ignores the possibility of overflow
            // into usize::MAX; in general both Rc and Arc need to be adjusted
            // to deal with overflow.

            // Unlike with Clone(), we need this to be an Acquire read to
            // synchronize with the write coming from `is_unique`, so that the
            // events prior to that write happen before this read.
            match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
                Ok(_) => {
                    // Make sure we do not create a dangling Weak
                    debug_assert!(!is_dangling(this.ptr));
                    return Weak { ptr: this.ptr };
                }
                Err(old) => cur = old,
            }
        }
    }
}

this.inner().weak.fetch_add(1, Acquire);
// Make sure we do not create a dangling Weak
debug_assert!(!is_dangling(this.ptr));
return Weak { ptr: this.ptr };

impl<T: ?Sized> Clone for Weak<T> {
    fn clone(&self) -> Weak<T> {
        let inner = if let Some(inner) = self.inner() {
            inner
        } else {
            return Weak { ptr: self.ptr };
        };
        // See comments in Arc::clone() for why this is relaxed.  This can use a
        // fetch_add (ignoring the lock) because the weak count is only locked
        // where are *no other* weak pointers in existence. (So we can't be
        // running this code in that case).
        let old_size = inner.weak.fetch_add(1, Relaxed);

        // See comments in Arc::clone() for why we do this (for mem::forget).
        if old_size > MAX_REFCOUNT {
            unsafe {
                abort();
            }
        }

        return Weak { ptr: self.ptr };
    }
}

impl<T: ?Sized> Weak<T> {
    pub fn upgrade(&self) -> Option<Arc<T>> {
        // We use a CAS loop to increment the strong count instead of a
        // fetch_add because once the count hits 0 it must never be above 0.
        let inner = self.inner()?;

        // Relaxed load because any write of 0 that we can observe
        // leaves the field in a permanently zero state (so a
        // "stale" read of 0 is fine), and any other value is
        // confirmed via the CAS below.
        let mut n = inner.strong.load(Relaxed);

        loop {
            if n == 0 {
                return None;
            }

            // See comments in `Arc::clone` for why we do this (for `mem::forget`).
            if n > MAX_REFCOUNT {
                unsafe {
                    abort();
                }
            }

            // Relaxed is valid for the same reason it is on Arc's Clone impl
            match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
                Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
                Err(old) => n = old,
            }
        }
    }
}

impl<T: ?Sized> Drop for Weak<T> {
    fn drop(&mut self) {
        // If we find out that we were the last weak pointer, then its time to
        // deallocate the data entirely. See the discussion in Arc::drop() about
        // the memory orderings
        //
        // It's not necessary to check for the locked state here, because the
        // weak count can only be locked if there was precisely one weak ref,
        // meaning that drop could only subsequently run ON that remaining weak
        // ref, which can only happen after the lock is released.
        let inner = if let Some(inner) = self.inner() {
            inner
        } else {
            return
        };

        if inner.weak.fetch_sub(1, Release) == 1 {
            atomic::fence(Acquire);
            unsafe {
                Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
            }
        }
    }
}

impl<T> Arc<T> {
    pub fn try_unwrap(this: Self) -> Result<T, Self> {
        // See `drop` for why all these atomics are like this
        if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
            return Err(this);
        }

        atomic::fence(Acquire);

        unsafe {
            let elem = ptr::read(&this.ptr.as_ref().data);

            // Make a weak pointer to clean up the implicit strong-weak reference
            let _weak = Weak { ptr: this.ptr };
            mem::forget(this);

            Ok(elem)
        }
    }
}

impl<T: ?Sized> Arc<T> {
    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
        if this.is_unique() {
            // This unsafety is ok because we're guaranteed that the pointer
            // returned is the *only* pointer that will ever be returned to T. Our
            // reference count is guaranteed to be 1 at this point, and we required
            // the Arc itself to be `mut`, so we're returning the only possible
            // reference to the inner data.
            unsafe {
                Some(Arc::get_mut_unchecked(this))
            }
        } else {
            None
        }
    }

    fn is_unique(&mut self) -> bool {
        // lock the weak pointer count if we appear to be the sole weak pointer
        // holder.
        //
        // The acquire label here ensures a happens-before relationship with any
        // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
        // of the `weak` count (via `Weak::drop`, which uses release).  If the upgraded
        // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
        if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
            // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
            // counter in `drop` -- the only access that happens when any but the last reference
            // is being dropped.
            let unique = self.inner().strong.load(Acquire) == 1;

            // The release write here synchronizes with a read in `downgrade`,
            // effectively preventing the above read of `strong` from happening
            // after the write.
            self.inner().weak.store(1, Release); // release the lock
            unique
        } else {
            false
        }
    }
}

if weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
    // ...

    weak.store(1, Release);
}

impl<T: Clone> Arc<T> {
    pub fn make_mut(this: &mut Self) -> &mut T {
        // Note that we hold both a strong reference and a weak reference.
        // Thus, releasing our strong reference only will not, by itself, cause
        // the memory to be deallocated.
        //
        // Use Acquire to ensure that we see any writes to `weak` that happen
        // before release writes (i.e., decrements) to `strong`. Since we hold a
        // weak count, there's no chance the ArcInner itself could be
        // deallocated.
        if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
            // Another strong pointer exists; clone
            *this = Arc::new((**this).clone());
        } else if this.inner().weak.load(Relaxed) != 1 {
            // Relaxed suffices in the above because this is fundamentally an
            // optimization: we are always racing with weak pointers being
            // dropped. Worst case, we end up allocated a new Arc unnecessarily.

            // We removed the last strong ref, but there are additional weak
            // refs remaining. We'll move the contents to a new Arc, and
            // invalidate the other weak refs.

            // Note that it is not possible for the read of `weak` to yield
            // usize::MAX (i.e., locked), since the weak count can only be
            // locked by a thread with a strong reference.

            // Materialize our own implicit weak pointer, so that it can clean
            // up the ArcInner as needed.
            let weak = Weak { ptr: this.ptr };

            // mark the data itself as already deallocated
            unsafe {
                // there is no data race in the implicit write caused by `read`
                // here (due to zeroing) because data is no longer accessed by
                // other threads (due to there being no more strong refs at this
                // point).
                let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
                mem::swap(this, &mut swap);
                mem::forget(swap);
            }
        } else {
            // We were the sole reference of either kind; bump back up the
            // strong ref count.
            this.inner().strong.store(1, Release);
        }

        // As with `get_mut()`, the unsafety is ok because our reference was
        // either unique to begin with, or became one upon cloning the contents.
        unsafe {
            &mut this.ptr.as_mut().data
        }
    }
}

impl<T: ?Sized> Arc<T> {
    pub fn weak_count(this: &Self) -> usize {
        let cnt = this.inner().weak.load(SeqCst);
        // If the weak count is currently locked, the value of the
        // count was 0 just before taking the lock.
        if cnt == usize::MAX { 0 } else { cnt - 1 }
    }

    pub fn strong_count(this: &Self) -> usize {
        this.inner().strong.load(SeqCst)
    }
}

impl<T: ?Sized> Weak<T> {
    #[unstable(feature = "weak_counts", issue = "57977")]
    pub fn strong_count(&self) -> usize {
        if let Some(inner) = self.inner() {
            inner.strong.load(SeqCst)
        } else {
            0
        }
    }

    #[unstable(feature = "weak_counts", issue = "57977")]
    pub fn weak_count(&self) -> Option<usize> {
        // Due to the implicit weak pointer added when any strong pointers are
        // around, we cannot implement `weak_count` correctly since it
        // necessarily requires accessing the strong count and weak count in an
        // unsynchronized fashion. So this version is a bit racy.
        self.inner().map(|inner| {
            let strong = inner.strong.load(SeqCst);
            let weak = inner.weak.load(SeqCst);
            if strong == 0 {
                // If the last `Arc` has *just* been dropped, it might not yet
                // have removed the implicit weak count, so the value we get
                // here might be 1 too high.
                weak
            } else {
                // As long as there's still at least 1 `Arc` around, subtract
                // the implicit weak pointer.
                // Note that the last `Arc` might get dropped between the 2
                // loads we do above, removing the implicit weak pointer. This
                // means that the value might be 1 too low here. In order to not
                // return 0 here (which would happen if we're the only weak
                // pointer), we guard against that specifically.
                cmp::max(1, weak - 1)
            }
        })
    }
}