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    #[cfg(not(all(
        httparse_simd,
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    )))]
    mod fallback;

    #[cfg(not(all(
        httparse_simd,
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    )))]
    pub use self::fallback::*;

    #[cfg(all(
        httparse_simd,
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    mod sse42;

    #[cfg(all(
        httparse_simd,
        any(
            httparse_simd_target_feature_avx2,
            not(httparse_simd_target_feature_sse42),
        ),
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    mod avx2;

    #[cfg(all(
        httparse_simd,
        not(any(
            httparse_simd_target_feature_sse42,
            httparse_simd_target_feature_avx2,
        )),
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    mod runtime {
        //! Runtime detection of simd features. Used when the build script
        //! doesn't notice any target features at build time.
        //!
        //! While `is_x86_feature_detected!` has it's own caching built-in,
        //! at least in 1.27.0, the functions don't inline, leaving using it
        //! actually *slower* than just using the scalar fallback.

        use core::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};

        static FEATURE: AtomicUsize = ATOMIC_USIZE_INIT;

        const INIT: usize = 0;
        const SSE_42: usize = 1;
        const AVX_2: usize = 2;
        const AVX_2_AND_SSE_42: usize = 3;
        const NONE: usize = ::core::usize::MAX;

        fn detect() -> usize {
            let feat = FEATURE.load(Ordering::Relaxed);
            if feat == INIT {
                if cfg!(target_arch = "x86_64") && is_x86_feature_detected!("avx2") {
                    if is_x86_feature_detected!("sse4.2") {
                        FEATURE.store(AVX_2_AND_SSE_42, Ordering::Relaxed);
                        return AVX_2_AND_SSE_42;
                    } else {
                        FEATURE.store(AVX_2, Ordering::Relaxed);
                        return AVX_2;
                    }
                } else if is_x86_feature_detected!("sse4.2") {
                    FEATURE.store(SSE_42, Ordering::Relaxed);
                    return SSE_42;
                } else {
                    FEATURE.store(NONE, Ordering::Relaxed);
                }
            }
            feat
        }

        pub fn match_uri_vectored(bytes: &mut ::Bytes) {
            unsafe {
                match detect() {
                    SSE_42 => super::sse42::parse_uri_batch_16(bytes),
                    AVX_2 => { super::avx2::parse_uri_batch_32(bytes); },
                    AVX_2_AND_SSE_42 => {
                        if let super::avx2::Scan::Found = super::avx2::parse_uri_batch_32(bytes) {
                            return;
                        }
                        super::sse42::parse_uri_batch_16(bytes)
                    },
                    _ => ()
                }
            }

            // else do nothing
        }

        pub fn match_header_value_vectored(bytes: &mut ::Bytes) {
            unsafe {
                match detect() {
                    SSE_42 => super::sse42::match_header_value_batch_16(bytes),
                    AVX_2 => { super::avx2::match_header_value_batch_32(bytes); },
                    AVX_2_AND_SSE_42 => {
                        if let super::avx2::Scan::Found = super::avx2::match_header_value_batch_32(bytes) {
                            return;
                        }
                        super::sse42::match_header_value_batch_16(bytes)
                    },
                    _ => ()
                }
            }

            // else do nothing
        }
    }

    #[cfg(all(
        httparse_simd,
        not(any(
            httparse_simd_target_feature_sse42,
            httparse_simd_target_feature_avx2,
        )),
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    pub use self::runtime::*;

    #[cfg(all(
        httparse_simd,
        httparse_simd_target_feature_sse42,
        not(httparse_simd_target_feature_avx2),
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    mod sse42_compile_time {
        pub fn match_uri_vectored(bytes: &mut ::Bytes) {
            if is_x86_feature_detected!("sse4.2") {
                unsafe {
                    super::sse42::parse_uri_batch_16(bytes);
                }
            }

            // else do nothing
        }

        pub fn match_header_value_vectored(bytes: &mut ::Bytes) {
            if is_x86_feature_detected!("sse4.2") {
                unsafe {
                    super::sse42::match_header_value_batch_16(bytes);
                }
            }

            // else do nothing
        }
    }

    #[cfg(all(
        httparse_simd,
        httparse_simd_target_feature_sse42,
        not(httparse_simd_target_feature_avx2),
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    pub use self::sse42_compile_time::*;

    #[cfg(all(
        httparse_simd,
        httparse_simd_target_feature_avx2,
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    mod avx2_compile_time {
        pub fn match_uri_vectored(bytes: &mut ::Bytes) {
            // do both, since avx2 only works when bytes.len() >= 32
            if cfg!(target_arch = "x86_64") && is_x86_feature_detected!("avx2") {
                unsafe {
                    super::avx2::parse_uri_batch_32(bytes);
                }

            }
            if is_x86_feature_detected!("sse4.2") {
                unsafe {
                    super::sse42::parse_uri_batch_16(bytes);
                }
            }

            // else do nothing
        }

        pub fn match_header_value_vectored(bytes: &mut ::Bytes) {
            // do both, since avx2 only works when bytes.len() >= 32
            if cfg!(target_arch = "x86_64") && is_x86_feature_detected!("avx2") {
                let scanned = unsafe {
                    super::avx2::match_header_value_batch_32(bytes)
                };

                if let super::avx2::Scan::Found = scanned {
                    return;
                }
            }
            if is_x86_feature_detected!("sse4.2") {
                unsafe {
                    super::sse42::match_header_value_batch_16(bytes);
                }
            }

            // else do nothing
        }
    }

    #[cfg(all(
        httparse_simd,
        httparse_simd_target_feature_avx2,
        any(
            target_arch = "x86",
            target_arch = "x86_64",
        ),
    ))]
    pub use self::avx2_compile_time::*;