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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());
    }
}