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    //! Bit Manipulation Instruction (BMI) Set 2.0.
    //!
    //! The reference is [Intel 64 and IA-32 Architectures Software Developer's
    //! Manual Volume 2: Instruction Set Reference, A-Z][intel64_ref].
    //!
    //! [Wikipedia][wikipedia_bmi] provides a quick overview of the instructions
    //! available.
    //!
    //! [intel64_ref]: http://www.intel.de/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
    //! [wikipedia_bmi]:
    //! https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets#ABM_.28Advanced_Bit_Manipulation.29

    #[cfg(test)]
    use stdarch_test::assert_instr;

    /// Unsigned multiply without affecting flags.
    ///
    /// Unsigned multiplication of `a` with `b` returning a pair `(lo, hi)` with
    /// the low half and the high half of the result.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mulx_u64)
    #[inline]
    #[cfg_attr(test, assert_instr(mul))]
    #[target_feature(enable = "bmi2")]
    #[cfg(not(target_arch = "x86"))] // calls an intrinsic
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn _mulx_u64(a: u64, b: u64, hi: &mut u64) -> u64 {
        let result: u128 = (a as u128) * (b as u128);
        *hi = (result >> 64) as u64;
        result as u64
    }

    /// Zeroes higher bits of `a` >= `index`.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_bzhi_u64)
    #[inline]
    #[target_feature(enable = "bmi2")]
    #[cfg_attr(test, assert_instr(bzhi))]
    #[cfg(not(target_arch = "x86"))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn _bzhi_u64(a: u64, index: u32) -> u64 {
        x86_bmi2_bzhi_64(a, index as u64)
    }

    /// Scatter contiguous low order bits of `a` to the result at the positions
    /// specified by the `mask`.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_pdep_u64)
    #[inline]
    #[target_feature(enable = "bmi2")]
    #[cfg_attr(test, assert_instr(pdep))]
    #[cfg(not(target_arch = "x86"))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn _pdep_u64(a: u64, mask: u64) -> u64 {
        x86_bmi2_pdep_64(a, mask)
    }

    /// Gathers the bits of `x` specified by the `mask` into the contiguous low
    /// order bit positions of the result.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_pext_u64)
    #[inline]
    #[target_feature(enable = "bmi2")]
    #[cfg_attr(test, assert_instr(pext))]
    #[cfg(not(target_arch = "x86"))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn _pext_u64(a: u64, mask: u64) -> u64 {
        x86_bmi2_pext_64(a, mask)
    }

    extern "C" {
        #[link_name = "llvm.x86.bmi.bzhi.64"]
        fn x86_bmi2_bzhi_64(x: u64, y: u64) -> u64;
        #[link_name = "llvm.x86.bmi.pdep.64"]
        fn x86_bmi2_pdep_64(x: u64, y: u64) -> u64;
        #[link_name = "llvm.x86.bmi.pext.64"]
        fn x86_bmi2_pext_64(x: u64, y: u64) -> u64;
    }

    #[cfg(test)]
    mod tests {
        use stdarch_test::simd_test;

        use crate::core_arch::x86_64::*;

        #[simd_test(enable = "bmi2")]
        unsafe fn test_pext_u64() {
            let n = 0b1011_1110_1001_0011u64;

            let m0 = 0b0110_0011_1000_0101u64;
            let s0 = 0b0000_0000_0011_0101u64;

            let m1 = 0b1110_1011_1110_1111u64;
            let s1 = 0b0001_0111_0100_0011u64;

            assert_eq!(_pext_u64(n, m0), s0);
            assert_eq!(_pext_u64(n, m1), s1);
        }

        #[simd_test(enable = "bmi2")]
        unsafe fn test_pdep_u64() {
            let n = 0b1011_1110_1001_0011u64;

            let m0 = 0b0110_0011_1000_0101u64;
            let s0 = 0b0000_0010_0000_0101u64;

            let m1 = 0b1110_1011_1110_1111u64;
            let s1 = 0b1110_1001_0010_0011u64;

            assert_eq!(_pdep_u64(n, m0), s0);
            assert_eq!(_pdep_u64(n, m1), s1);
        }

        #[simd_test(enable = "bmi2")]
        unsafe fn test_bzhi_u64() {
            let n = 0b1111_0010u64;
            let s = 0b0001_0010u64;
            assert_eq!(_bzhi_u64(n, 5), s);
        }

        #[simd_test(enable = "bmi2")]
        #[rustfmt::skip]
        unsafe fn test_mulx_u64() {
            let a: u64 = 9_223_372_036_854_775_800;
            let b: u64 = 100;
            let mut hi = 0;
            let lo = _mulx_u64(a, b, &mut hi);
            /*
    result = 922337203685477580000 =
    0b00110001_1111111111111111_1111111111111111_1111111111111111_1111110011100000
      ^~hi~~~~ ^~lo~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            */
            assert_eq!(
                lo,
                0b11111111_11111111_11111111_11111111_11111111_11111111_11111100_11100000u64
            );
            assert_eq!(hi, 0b00110001u64);
        }
    }