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    use std::marker;
    use std::cell::UnsafeCell;
    use std::fmt;
    use std::mem;
    use std::ptr;
    use std::ops::{Deref, DerefMut};

    // For now, treat this as an arch-independent constant.
    const CACHE_LINE: usize = 32;

    #[cfg_attr(feature = "nightly",
               repr(simd))]
    #[derive(Debug)]
    struct Padding(u64, u64, u64, u64);

    /// Pad `T` to the length of a cacheline.
    ///
    /// Sometimes concurrent programming requires a piece of data to be padded out
    /// to the size of a cacheline to avoid "false sharing": cachelines being
    /// invalidated due to unrelated concurrent activity. Use the `CachePadded` type
    /// when you want to *avoid* cache locality.
    ///
    /// At the moment, cache lines are assumed to be 32 * sizeof(usize) on all
    /// architectures.
    ///
    /// **Warning**: the wrapped data is never dropped; move out using `ptr::read`
    /// if you need to run dtors.
    pub struct CachePadded<T> {
        data: UnsafeCell<[usize; CACHE_LINE]>,
        _marker: ([Padding; 0], marker::PhantomData<T>),
    }

    impl<T> fmt::Debug for CachePadded<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "CachePadded {{ ... }}")
        }
    }

    unsafe impl<T: Send> Send for CachePadded<T> {}
    unsafe impl<T: Sync> Sync for CachePadded<T> {}

    #[cfg(not(feature = "nightly"))]
    macro_rules! declare_zeros_valid {
        () => {
            /// Types for which mem::zeroed() is safe.
            ///
            /// If a type `T: ZerosValid`, then a sequence of zeros the size of `T` must be
            /// a valid member of the type `T`.
            pub unsafe trait ZerosValid {}
        }
    }

    #[cfg(feature = "nightly")]
    macro_rules! declare_zeros_valid {
        () => {
            /// Types for which mem::zeroed() is safe.
            ///
            /// If a type `T: ZerosValid`, then a sequence of zeros the size of `T` must be
            /// a valid member of the type `T`.
            pub unsafe auto trait ZerosValid {}
        }
    }

    declare_zeros_valid!();

    macro_rules! zeros_valid { ($( $T:ty )*) => ($(
        unsafe impl ZerosValid for $T {}
    )*)}

    zeros_valid!(u8 u16 u32 u64 usize);
    zeros_valid!(i8 i16 i32 i64 isize);

    unsafe impl ZerosValid for ::std::sync::atomic::AtomicUsize {}
    unsafe impl<T> ZerosValid for ::std::sync::atomic::AtomicPtr<T> {}

    impl<T: ZerosValid> CachePadded<T> {
        /// A const fn equivalent to mem::zeroed().
        #[cfg(not(feature = "nightly"))]
        pub fn zeroed() -> CachePadded<T> {
            CachePadded {
                data: UnsafeCell::new(([0; CACHE_LINE])),
                _marker: ([], marker::PhantomData),
            }
        }

        /// A const fn equivalent to mem::zeroed().
        #[cfg(feature = "nightly")]
        pub const fn zeroed() -> CachePadded<T> {
            CachePadded {
                data: UnsafeCell::new(([0; CACHE_LINE])),
                _marker: ([], marker::PhantomData),
            }
        }
    }

    #[inline]
    /// Assert that the size and alignment of `T` are consistent with `CachePadded<T>`.
    fn assert_valid<T>() {
        assert!(mem::size_of::<T>() <= mem::size_of::<CachePadded<T>>());
        assert!(mem::align_of::<T>() <= mem::align_of::<CachePadded<T>>());
    }

    impl<T> CachePadded<T> {
        /// Wrap `t` with cacheline padding.
        ///
        /// **Warning**: the wrapped data is never dropped; move out using
        /// `ptr:read` if you need to run dtors.
        pub fn new(t: T) -> CachePadded<T> {
            assert_valid::<T>();
            let ret = CachePadded {
                data: UnsafeCell::new(([0; CACHE_LINE])),
                _marker: ([], marker::PhantomData),
            };
            unsafe {
                let p: *mut T = mem::transmute(&ret.data);
                ptr::write(p, t);
            }
            ret
        }
    }

    impl<T> Deref for CachePadded<T> {
        type Target = T;
        fn deref(&self) -> &T {
            assert_valid::<T>();
            unsafe { mem::transmute(&self.data) }
        }
    }

    impl<T> DerefMut for CachePadded<T> {
        fn deref_mut(&mut self) -> &mut T {
            assert_valid::<T>();
            unsafe { mem::transmute(&mut self.data) }
        }
    }

    // FIXME: support Drop by pulling out a version usable for statics
    /*
    impl<T> Drop for CachePadded<T> {
        fn drop(&mut self) {
            assert_valid::<T>();
            let p: *mut T = mem::transmute(&self.data);
            mem::drop(ptr::read(p));
        }
    }
    */

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn cache_padded_store_u64() {
            let x: CachePadded<u64> = CachePadded::new(17);
            assert_eq!(*x, 17);
        }

        #[test]
        fn cache_padded_store_pair() {
            let x: CachePadded<(u64, u64)> = CachePadded::new((17, 37));
            assert_eq!(x.0, 17);
            assert_eq!(x.1, 37);
        }
    }

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