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    //! Caches run-time feature detection so that it only needs to be computed
    //! once.

    #![allow(dead_code)] // not used on all platforms

    use core::sync::atomic::Ordering;

    #[cfg(target_pointer_width = "64")]
    use core::sync::atomic::AtomicU64;

    #[cfg(target_pointer_width = "32")]
    use core::sync::atomic::AtomicU32;

    /// Sets the `bit` of `x`.
    #[inline]
    const fn set_bit(x: u64, bit: u32) -> u64 {
        x | 1 << bit
    }

    /// Tests the `bit` of `x`.
    #[inline]
    const fn test_bit(x: u64, bit: u32) -> bool {
        x & (1 << bit) != 0
    }

    /// Maximum number of features that can be cached.
    const CACHE_CAPACITY: u32 = 63;

    /// This type is used to initialize the cache
    #[derive(Copy, Clone)]
    pub(crate) struct Initializer(u64);

    impl Default for Initializer {
        fn default() -> Self {
            Initializer(0)
        }
    }

    impl Initializer {
        /// Tests the `bit` of the cache.
        #[allow(dead_code)]
        #[inline]
        pub(crate) fn test(self, bit: u32) -> bool {
            // FIXME: this way of making sure that the cache is large enough is
            // brittle.
            debug_assert!(
                bit < CACHE_CAPACITY,
                "too many features, time to increase the cache size!"
            );
            test_bit(self.0, bit)
        }

        /// Sets the `bit` of the cache.
        #[inline]
        pub(crate) fn set(&mut self, bit: u32) {
            // FIXME: this way of making sure that the cache is large enough is
            // brittle.
            debug_assert!(
                bit < CACHE_CAPACITY,
                "too many features, time to increase the cache size!"
            );
            let v = self.0;
            self.0 = set_bit(v, bit);
        }
    }

    /// This global variable is a cache of the features supported by the CPU.
    static CACHE: Cache = Cache::uninitialized();

    /// Feature cache with capacity for `CACHE_CAPACITY` features.
    ///
    /// Note: the last feature bit is used to represent an
    /// uninitialized cache.
    #[cfg(target_pointer_width = "64")]
    struct Cache(AtomicU64);

    #[cfg(target_pointer_width = "64")]
    impl Cache {
        /// Creates an uninitialized cache.
        const fn uninitialized() -> Self {
            Cache(AtomicU64::new(u64::max_value()))
        }
        /// Is the cache uninitialized?
        #[inline]
        pub(crate) fn is_uninitialized(&self) -> bool {
            self.0.load(Ordering::Relaxed) == u64::max_value()
        }

        /// Is the `bit` in the cache set?
        #[inline]
        pub(crate) fn test(&self, bit: u32) -> bool {
            test_bit(CACHE.0.load(Ordering::Relaxed), bit)
        }

        /// Initializes the cache.
        #[inline]
        pub(crate) fn initialize(&self, value: Initializer) {
            self.0.store(value.0, Ordering::Relaxed);
        }
    }

    /// Feature cache with capacity for `CACHE_CAPACITY` features.
    ///
    /// Note: the last feature bit is used to represent an
    /// uninitialized cache.
    #[cfg(target_pointer_width = "32")]
    struct Cache(AtomicU32, AtomicU32);

    #[cfg(target_pointer_width = "32")]
    impl Cache {
        /// Creates an uninitialized cache.
        const fn uninitialized() -> Self {
            Cache(
                AtomicU32::new(u32::max_value()),
                AtomicU32::new(u32::max_value()),
            )
        }
        /// Is the cache uninitialized?
        #[inline]
        pub(crate) fn is_uninitialized(&self) -> bool {
            self.1.load(Ordering::Relaxed) == u32::max_value()
        }

        /// Is the `bit` in the cache set?
        #[inline]
        pub(crate) fn test(&self, bit: u32) -> bool {
            if bit < 32 {
                test_bit(CACHE.0.load(Ordering::Relaxed) as u64, bit)
            } else {
                test_bit(CACHE.1.load(Ordering::Relaxed) as u64, bit - 32)
            }
        }

        /// Initializes the cache.
        #[inline]
        pub(crate) fn initialize(&self, value: Initializer) {
            let lo: u32 = value.0 as u32;
            let hi: u32 = (value.0 >> 32) as u32;
            self.0.store(lo, Ordering::Relaxed);
            self.1.store(hi, Ordering::Relaxed);
        }
    }

    /// Test the `bit` of the storage. If the storage has not been initialized,
    /// initializes it with the result of `f()`.
    ///
    /// On its first invocation, it detects the CPU features and caches them in the
    /// `CACHE` global variable as an `AtomicU64`.
    ///
    /// It uses the `Feature` variant to index into this variable as a bitset. If
    /// the bit is set, the feature is enabled, and otherwise it is disabled.
    #[inline]
    pub(crate) fn test<F>(bit: u32, f: F) -> bool
    where
        F: FnOnce() -> Initializer,
    {
        if CACHE.is_uninitialized() {
            CACHE.initialize(f());
        }
        CACHE.test(bit)
    }

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