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    //! RDTSC instructions.

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

    /// Reads the current value of the processor’s time-stamp counter.
    ///
    /// The processor monotonically increments the time-stamp counter MSR
    /// every clock cycle and resets it to 0 whenever the processor is
    /// reset.
    ///
    /// The RDTSC instruction is not a serializing instruction. It does
    /// not necessarily wait until all previous instructions have been
    /// executed before reading the counter. Similarly, subsequent
    /// instructions may begin execution before the read operation is
    /// performed.
    ///
    /// On processors that support the Intel 64 architecture, the
    /// high-order 32 bits of each of RAX and RDX are cleared.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_rdtsc)
    #[inline]
    #[cfg_attr(test, assert_instr(rdtsc))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn _rdtsc() -> u64 {
        rdtsc()
    }

    /// Reads the current value of the processor’s time-stamp counter and
    /// the `IA32_TSC_AUX MSR`.
    ///
    /// The processor monotonically increments the time-stamp counter MSR
    /// every clock cycle and resets it to 0 whenever the processor is
    /// reset.
    ///
    /// The RDTSCP instruction waits until all previous instructions have
    /// been executed before reading the counter. However, subsequent
    /// instructions may begin execution before the read operation is
    /// performed.
    ///
    /// On processors that support the Intel 64 architecture, the
    /// high-order 32 bits of each of RAX, RDX, and RCX are cleared.
    ///
    /// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=__rdtscp)
    #[inline]
    #[cfg_attr(test, assert_instr(rdtscp))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn __rdtscp(aux: *mut u32) -> u64 {
        rdtscp(aux as *mut _)
    }

    #[allow(improper_ctypes)]
    extern "C" {
        #[link_name = "llvm.x86.rdtsc"]
        fn rdtsc() -> u64;
        #[link_name = "llvm.x86.rdtscp"]
        fn rdtscp(aux: *mut u8) -> u64;
    }

    #[cfg(test)]
    mod tests {
        use crate::core_arch::x86::*;
        use stdarch_test::simd_test;

        #[simd_test(enable = "sse2")]
        unsafe fn _rdtsc() {
            let r = rdtsc::_rdtsc();
            assert_ne!(r, 0); // The chances of this being 0 are infinitesimal
        }

        #[simd_test(enable = "sse2")]
        unsafe fn _rdtscp() {
            let mut aux = 0;
            let r = rdtsc::__rdtscp(&mut aux);
            assert_ne!(r, 0); // The chances of this being 0 are infinitesimal
        }
    }

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