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    // Data structures for storing garbage to be freed later (once the
    // epochs have sufficiently advanced).
    //
    // In general, we try to manage the garbage thread locally whenever
    // possible. Each thread keep track of three bags of garbage. But if a
    // thread is exiting, these bags must be moved into the global garbage
    // bags.

    use std::ptr;
    use std::mem;
    use std::sync::atomic::AtomicPtr;
    use std::sync::atomic::Ordering::{Relaxed, Release, Acquire};

    use mem::ZerosValid;

    /// One item of garbage.
    ///
    /// Stores enough information to do a deallocation.
    struct Item {
        ptr: *mut u8,
        free: unsafe fn(*mut u8),
    }

    /// A single, thread-local bag of garbage.
    pub struct Bag(Vec<Item>);

    impl Bag {
        fn new() -> Bag {
            Bag(vec![])
        }

        fn insert<T>(&mut self, elem: *mut T) {
            let size = mem::size_of::<T>();
            if size > 0 {
                self.0.push(Item {
                    ptr: elem as *mut u8,
                    free: free::<T>,
                })
            }
            unsafe fn free<T>(t: *mut u8) {
                drop(Vec::from_raw_parts(t as *mut T, 0, 1));
            }
        }

        fn len(&self) -> usize {
            self.0.len()
        }

        /// Deallocate all garbage in the bag
        pub unsafe fn collect(&mut self) {
            let mut data = mem::replace(&mut self.0, Vec::new());
            for item in data.iter() {
                (item.free)(item.ptr);
            }
            data.truncate(0);
            self.0 = data;
        }
    }

    // needed because the bags store raw pointers.
    unsafe impl Send for Bag {}
    unsafe impl Sync for Bag {}

    /// A thread-local set of garbage bags.
    pub struct Local {
        /// Garbage added at least one epoch behind the current local epoch
        pub old: Bag,
        /// Garbage added in the current local epoch or earlier
        pub cur: Bag,
        /// Garbage added in the current *global* epoch
        pub new: Bag,
    }

    impl Local {
        pub fn new() -> Local {
            Local {
                old: Bag::new(),
                cur: Bag::new(),
                new: Bag::new(),
            }
        }

        pub fn insert<T>(&mut self, elem: *mut T) {
            self.new.insert(elem)
        }

        /// Collect one epoch of garbage, rotating the local garbage bags.
        pub unsafe fn collect(&mut self) {
            let ret = self.old.collect();
            mem::swap(&mut self.old, &mut self.cur);
            mem::swap(&mut self.cur, &mut self.new);
            ret
        }

        pub fn size(&self) -> usize {
            self.old.len() + self.cur.len() + self.new.len()
        }
    }

    /// A concurrent garbage bag, currently based on Treiber's stack.
    ///
    /// The elements are themselves owned `Bag`s.
    pub struct ConcBag {
        head: AtomicPtr<Node>,
    }

    unsafe impl ZerosValid for ConcBag {}

    struct Node {
        data: Bag,
        next: AtomicPtr<Node>,
    }

    impl ConcBag {
        pub fn insert(&self, t: Bag){
            let n = Box::into_raw(Box::new(
                Node { data: t, next: AtomicPtr::new(ptr::null_mut()) }));
            loop {
                let head = self.head.load(Acquire);
                unsafe { (*n).next.store(head, Relaxed) };
                if self.head.compare_and_swap(head, n, Release) == head { break }
            }
        }

        pub unsafe fn collect(&self) {
            // check to avoid xchg instruction
            // when no garbage exists
            let mut head = self.head.load(Relaxed);
            if head != ptr::null_mut() {
                head = self.head.swap(ptr::null_mut(), Acquire);

                while head != ptr::null_mut() {
                    let mut n = Box::from_raw(head);
                    n.data.collect();
                    head = n.next.load(Relaxed);
                }
            }
        }
    }

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