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    //! Parses ELF auxiliary vectors.
    #![cfg_attr(not(target_arch = "aarch64"), allow(dead_code))]

    use mem;
    use prelude::v1::*;
    use fs::File;
    use io::Read;

    /// Key to access the CPU Hardware capabilities bitfield.
    pub(crate) const AT_HWCAP: usize = 16;
    /// Key to access the CPU Hardware capabilities 2 bitfield.
    #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
    pub(crate) const AT_HWCAP2: usize = 26;

    /// Cache HWCAP bitfields of the ELF Auxiliary Vector.
    ///
    /// If an entry cannot be read all the bits in the bitfield are set to zero.
    /// This should be interpreted as all the features being disabled.
    #[derive(Debug, Copy, Clone)]
    pub(crate) struct AuxVec {
        pub hwcap: usize,
        #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
        pub hwcap2: usize,
    }

    /// ELF Auxiliary Vector
    ///
    /// The auxiliary vector is a memory region in a running ELF program's stack
    /// composed of (key: usize, value: usize) pairs.
    ///
    /// The keys used in the aux vector are platform dependent. For Linux, they are
    /// defined in [linux/auxvec.h][auxvec_h]. The hardware capabilities of a given
    /// CPU can be queried with the  `AT_HWCAP` and `AT_HWCAP2` keys.
    ///
    /// There is no perfect way of reading the auxiliary vector.
    ///
    /// - If the `getauxval` is dynamically linked to this binary, it will be used.
    /// - Otherwise, try to read `/proc/self/auxv`.
    /// - If that fails, this function returns an error.
    ///
    /// Note that run-time feature detection is not invoked for features that can
    /// be detected at compile-time. Also note that if this function returns an
    /// error, cpuinfo still can (and will) be used to try to perform run-time
    /// feature detecton on some platforms.
    ///
    /// For more information about when `getauxval` is available check the great
    /// [`auxv` crate documentation][auxv_docs].
    ///
    /// [auxvec_h]: https://github.com/torvalds/linux/blob/master/include/uapi/linux/auxvec.h
    /// [auxv_docs]: https://docs.rs/auxv/0.3.3/auxv/
    pub(crate) fn auxv() -> Result<AuxVec, ()> {
        // Try to call a dynamically-linked getauxval function.
        if let Ok(hwcap) = getauxval(AT_HWCAP) {
            // Targets with only AT_HWCAP:
            #[cfg(any(target_arch = "aarch64", target_arch = "mips",
                      target_arch = "mips64"))]
            {
                if hwcap != 0 {
                    return Ok(AuxVec { hwcap });
                }
            }

            // Targets with AT_HWCAP and AT_HWCAP2:
            #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
            {
                if let Ok(hwcap2) = getauxval(AT_HWCAP2) {
                    if hwcap != 0 && hwcap2 != 0 {
                        return Ok(AuxVec { hwcap, hwcap2 });
                    }
                }
            }
            drop(hwcap);
        }
        // If calling getauxval fails, try to read the auxiliary vector from
        // its file:
        auxv_from_file("/proc/self/auxv")
    }

    /// Tries to read the `key` from the auxiliary vector by calling the
    /// dynamically-linked `getauxval` function. If the function is not linked,
    /// this function return `Err`.
    fn getauxval(key: usize) -> Result<usize, ()> {
        use libc;
        pub type F = unsafe extern "C" fn(usize) -> usize;
        unsafe {
            let ptr = libc::dlsym(
                libc::RTLD_DEFAULT,
                "getauxval\0".as_ptr() as *const _,
            );
            if ptr.is_null() {
                return Err(());
            }

            let ffi_getauxval: F = mem::transmute(ptr);
            Ok(ffi_getauxval(key))
        }
    }

    /// Tries to read the auxiliary vector from the `file`. If this fails, this
    /// function returns `Err`.
    fn auxv_from_file(file: &str) -> Result<AuxVec, ()> {
        let mut file = File::open(file).map_err(|_| ())?;

        // See https://github.com/torvalds/linux/blob/v3.19/include/uapi/linux/auxvec.h
        //
        // The auxiliary vector contains at most 32 (key,value) fields: from
        // `AT_EXECFN = 31` to `AT_NULL = 0`. That is, a buffer of
        // 2*32 `usize` elements is enough to read the whole vector.
        let mut buf = [0_usize; 64];
        {
            let raw: &mut [u8; 64 * mem::size_of::<usize>()] =
                unsafe { mem::transmute(&mut buf) };
            file.read(raw).map_err(|_| ())?;
        }
        auxv_from_buf(&buf)
    }

    /// Tries to interpret the `buffer` as an auxiliary vector. If that fails, this
    /// function returns `Err`.
    fn auxv_from_buf(buf: &[usize; 64]) -> Result<AuxVec, ()> {
        // Targets with only AT_HWCAP:
        #[cfg(any(target_arch = "aarch64", target_arch = "mips",
                  target_arch = "mips64"))]
        {
            for el in buf.chunks(2) {
                match el[0] {
                    AT_HWCAP => return Ok(AuxVec { hwcap: el[1] }),
                    _ => (),
                }
            }
        }
        // Targets with AT_HWCAP and AT_HWCAP2:
        #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
        {
            let mut hwcap = None;
            let mut hwcap2 = None;
            for el in buf.chunks(2) {
                match el[0] {
                    AT_HWCAP => hwcap = Some(el[1]),
                    AT_HWCAP2 => hwcap2 = Some(el[1]),
                    _ => (),
                }
            }

            if let (Some(hwcap), Some(hwcap2)) = (hwcap, hwcap2) {
                return Ok(AuxVec { hwcap, hwcap2 });
            }
        }
        drop(buf);
        Err(())
    }

    #[cfg(test)]
    mod tests {
        extern crate auxv as auxv_crate;
        use super::*;

        // Reads the Auxiliary Vector key from /proc/self/auxv
        // using the auxv crate.
        fn auxv_crate_getprocfs(key: usize) -> Option<usize> {
            use self::auxv_crate::AuxvType;
            use self::auxv_crate::procfs::search_procfs_auxv;
            let k = key as AuxvType;
            match search_procfs_auxv(&[k]) {
                Ok(v) => Some(v[&k] as usize),
                Err(_) => None,
            }
        }

        // Reads the Auxiliary Vector key from getauxval()
        // using the auxv crate.
        #[cfg(not(any(target_arch = "mips", target_arch = "mips64")))]
        fn auxv_crate_getauxval(key: usize) -> Option<usize> {
            use self::auxv_crate::AuxvType;
            use self::auxv_crate::getauxval::Getauxval;
            let q = auxv_crate::getauxval::NativeGetauxval {};
            match q.getauxval(key as AuxvType) {
                Ok(v) => Some(v as usize),
                Err(_) => None,
            }
        }

        // FIXME: on mips/mips64 getauxval returns 0, and /proc/self/auxv
        // does not always contain the AT_HWCAP key under qemu.
        #[cfg(not(any(target_arch = "mips", target_arch = "mips64", target_arch = "powerpc")))]
        #[test]
        fn auxv_crate() {
            let v = auxv();
            if let Some(hwcap) = auxv_crate_getauxval(AT_HWCAP) {
                let rt_hwcap = v.expect("failed to find hwcap key").hwcap;
                assert_eq!(rt_hwcap, hwcap);
            }

            // Targets with AT_HWCAP and AT_HWCAP2:
            #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
            {
                if let Some(hwcap2) = auxv_crate_getauxval(AT_HWCAP2) {
                    let rt_hwcap2 = v.expect("failed to find hwcap2 key").hwcap2;
                    assert_eq!(rt_hwcap2, hwcap2);
                }
            }
        }

        #[test]
        fn auxv_dump() {
            if let Ok(auxvec) = auxv() {
                println!("{:?}", auxvec);
            } else {
                println!("both getauxval() and reading /proc/self/auxv failed!");
            }
        }

        cfg_if! {
            if #[cfg(target_arch = "arm")] {
                #[test]
                fn linux_rpi3() {
                    let v = auxv_from_file(
                        "../../stdsimd/arch/detect/test_data/linux-rpi3.auxv",
                    ).unwrap();
                    assert_eq!(v.hwcap, 4174038);
                    assert_eq!(v.hwcap2, 16);
                }

                #[test]
                #[should_panic]
                fn linux_macos_vb() {
                    let _ = auxv_from_file(
                        "../../stdsimd/arch/detect/test_data/macos-virtualbox-linux-x86-4850HQ.auxv"
                    ).unwrap();
                    // this file is incomplete (contains hwcap but not hwcap2), we
                    // want to fall back to /proc/cpuinfo in this case, so
                    // reading should fail. assert_eq!(v.hwcap, 126614527);
                    // assert_eq!(v.hwcap2, 0);
                }
            } else if #[cfg(target_arch = "aarch64")] {
                #[test]
                fn linux_x64() {
                    let v = auxv_from_file(
                        "../../stdsimd/arch/detect/test_data/linux-x64-i7-6850k.auxv",
                    ).unwrap();
                    assert_eq!(v.hwcap, 3219913727);
                }
            }
        }

        #[test]
        fn auxv_dump_procfs() {
            if let Ok(auxvec) = auxv_from_file("/proc/self/auxv") {
                println!("{:?}", auxvec);
            } else {
                println!("reading /proc/self/auxv failed!");
            }
        }

        #[test]
        fn auxv_crate_procfs() {
            let v = auxv();
            if let Some(hwcap) = auxv_crate_getprocfs(AT_HWCAP) {
                assert_eq!(v.unwrap().hwcap, hwcap);
            }

            // Targets with AT_HWCAP and AT_HWCAP2:
            #[cfg(any(target_arch = "arm", target_arch = "powerpc64"))]
            {
                if let Some(hwcap2) = auxv_crate_getprocfs(AT_HWCAP2) {
                    assert_eq!(v.unwrap().hwcap2, hwcap2);
                }
            }
        }
    }

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