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    //! `cpuid` intrinsics

    #![cfg_attr(feature = "cargo-clippy", allow(stutter))]

    use mem;

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

    /// Result of the `cpuid` instruction.
    #[derive(Copy, Clone, Eq, Ord, PartialEq, PartialOrd)]
    #[cfg_attr(feature = "cargo-clippy", allow(stutter))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub struct CpuidResult {
        /// EAX register.
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub eax: u32,
        /// EBX register.
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub ebx: u32,
        /// ECX register.
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub ecx: u32,
        /// EDX register.
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub edx: u32,
    }

    /// Returns the result of the `cpuid` instruction for a given `leaf` (`EAX`)
    /// and
    /// `sub_leaf` (`ECX`).
    ///
    /// The highest-supported leaf value is returned by the first tuple argument of
    /// [`__get_cpuid_max(0)`](fn.__get_cpuid_max.html). For leaves containung
    /// sub-leaves, the second tuple argument returns the highest-supported
    /// sub-leaf
    /// value.
    ///
    /// The [CPUID Wikipedia page][wiki_cpuid] contains how to query which
    /// information using the `EAX` and `ECX` registers, and the interpretation of
    /// the results returned in `EAX`, `EBX`, `ECX`, and `EDX`.
    ///
    /// The references are:
    /// - [Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2:
    ///   Instruction Set Reference, A-Z][intel64_ref].
    /// - [AMD64 Architecture Programmer's Manual, Volume 3: General-Purpose and
    ///   System Instructions][amd64_ref].
    ///
    /// [wiki_cpuid]: https://en.wikipedia.org/wiki/CPUID
    /// [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
    /// [amd64_ref]: http://support.amd.com/TechDocs/24594.pdf
    #[inline]
    #[cfg_attr(test, assert_instr(cpuid))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn __cpuid_count(leaf: u32, sub_leaf: u32) -> CpuidResult {
        let mut r = mem::uninitialized::<CpuidResult>();
        if cfg!(target_arch = "x86") {
            asm!("cpuid"
                 : "={eax}"(r.eax), "={ebx}"(r.ebx), "={ecx}"(r.ecx), "={edx}"(r.edx)
                 : "{eax}"(leaf), "{ecx}"(sub_leaf)
                 : :);
        } else {
            // x86-64 uses %rbx as the base register, so preserve it.
            asm!("cpuid\n"
                 : "={eax}"(r.eax), "={ebx}"(r.ebx), "={ecx}"(r.ecx), "={edx}"(r.edx)
                 : "{eax}"(leaf), "{ecx}"(sub_leaf)
                 : "rbx" :);
        }
        r
    }

    /// See [`__cpuid_count`](fn.__cpuid_count.html).
    #[inline]
    #[cfg_attr(test, assert_instr(cpuid))]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn __cpuid(leaf: u32) -> CpuidResult {
        __cpuid_count(leaf, 0)
    }

    /// Does the host support the `cpuid` instruction?
    #[inline]
    pub fn has_cpuid() -> bool {
        #[cfg(target_arch = "x86_64")]
        {
            true
        }
        #[cfg(target_arch = "x86")]
        {
            // Optimization for i586 and i686 Rust targets which SSE enabled
            // and support cpuid:
            #[cfg(target_feature = "sse")] {
                true
            }

            // If SSE is not enabled, detect whether cpuid is available:
            #[cfg(not(target_feature = "sse"))]
            unsafe {
                // On `x86` the `cpuid` instruction is not always available.
                // This follows the approach indicated in:
                // http://wiki.osdev.org/CPUID#Checking_CPUID_availability
                // https://software.intel.com/en-us/articles/using-cpuid-to-detect-the-presence-of-sse-41-and-sse-42-instruction-sets/
                // which detects whether `cpuid` is available by checking whether the 21st bit of the EFLAGS register is modifiable or not.
                // If it is, then `cpuid` is available.
                let result: u32;
                let _temp: u32;
                asm!(r#"
                     # Read eflags into $0 and copy it into $1:
                     pushfd
                     pop     $0
                     mov     $1, $0
                     # Flip 21st bit of $0.
                     xor     $0, 0x200000
                     # Set eflags to the value of $0
                     #
                     # Bit 21st can only be modified if cpuid is available
                     push    $0
                     popfd          # A
                     # Read eflags into $0:
                     pushfd         # B
                     pop     $0
                     # xor with the original eflags sets the bits that
                     # have been modified:
                     xor     $0, $1
                     "#
                     : "=r"(result), "=r"(_temp)
                     :
                     : "cc", "memory"
                     : "intel");
                // There is a race between popfd (A) and pushfd (B)
                // where other bits beyond 21st may have been modified due to
                // interrupts, a debugger stepping through the asm, etc.
                //
                // Therefore, explicitly check whether the 21st bit
                // was modified or not.
                //
                // If the result is zero, the cpuid bit was not modified.
                // If the result is 0x200000 (non-zero), then the cpuid
                // was correctly modified and the CPU supports the cpuid
                // instruction:
                (result & 0x200000) != 0
            }
        }
    }

    /// Returns the highest-supported `leaf` (`EAX`) and sub-leaf (`ECX`) `cpuid`
    /// values.
    ///
    /// If `cpuid` is supported, and `leaf` is zero, then the first tuple argument
    /// contains the highest `leaf` value that `cpuid` supports. For `leaf`s
    /// containing sub-leafs, the second tuple argument contains the
    /// highest-supported sub-leaf value.
    ///
    /// See also [`__cpuid`](fn.__cpuid.html) and
    /// [`__cpuid_count`](fn.__cpuid_count.html).
    #[inline]
    #[stable(feature = "simd_x86", since = "1.27.0")]
    pub unsafe fn __get_cpuid_max(leaf: u32) -> (u32, u32) {
        let CpuidResult { eax, ebx, .. } = __cpuid(leaf);
        (eax, ebx)
    }

    #[cfg(test)]
    mod tests {
        use coresimd::x86::*;

        #[test]
        fn test_always_has_cpuid() {
            // all currently-tested targets have the instruction
            // FIXME: add targets without `cpuid` to CI
            assert!(cpuid::has_cpuid());
        }

        #[test]
        fn test_has_cpuid_idempotent() {
            assert_eq!(cpuid::has_cpuid(), cpuid::has_cpuid());
        }
    }

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