mod.rs.html -- source
Description: Source to the Rust file `libstd/../stdsimd/stdsimd/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    //! `stdsimd`

    /// SIMD and vendor intrinsics module.
    ///
    /// This module is intended to be the gateway to architecture-specific
    /// intrinsic functions, typically related to SIMD (but not always!). Each
    /// architecture that Rust compiles to may contain a submodule here, which
    /// means that this is not a portable module! If you're writing a portable
    /// library take care when using these APIs!
    ///
    /// Under this module you'll find an architecture-named module, such as
    /// `x86_64`. Each `#[cfg(target_arch)]` that Rust can compile to may have a
    /// module entry here, only present on that particular target. For example the
    /// `i686-pc-windows-msvc` target will have an `x86` module here, whereas
    /// `x86_64-pc-windows-msvc` has `x86_64`.
    ///
    /// [rfc]: https://github.com/rust-lang/rfcs/pull/2325
    /// [tracked]: https://github.com/rust-lang/rust/issues/48556
    ///
    /// # Overview
    ///
    /// This module exposes vendor-specific intrinsics that typically correspond to
    /// a single machine instruction. These intrinsics are not portable: their
    /// availability is architecture-dependent, and not all machines of that
    /// architecture might provide the intrinsic.
    ///
    /// The `arch` module is intended to be a low-level implementation detail for
    /// higher-level APIs. Using it correctly can be quite tricky as you need to
    /// ensure at least a few guarantees are upheld:
    ///
    /// * The correct architecture's module is used. For example the `arm` module
    ///   isn't available on the `x86_64-unknown-linux-gnu` target. This is
    ///   typically done by ensuring that `#[cfg]` is used appropriately when using
    ///   this module.
    /// * The CPU the program is currently running on supports the function being
    ///   called. For example it is unsafe to call an AVX2 function on a CPU that
    ///   doesn't actually support AVX2.
    ///
    /// As a result of the latter of these guarantees all intrinsics in this module
    /// are `unsafe` and extra care needs to be taken when calling them!
    ///
    /// # CPU Feature Detection
    ///
    /// In order to call these APIs in a safe fashion there's a number of
    /// mechanisms available to ensure that the correct CPU feature is available
    /// to call an intrinsic. Let's consider, for example, the `_mm256_add_epi64`
    /// intrinsics on the `x86` and `x86_64` architectures. This function requires
    /// the AVX2 feature as [documented by Intel][intel-dox] so to correctly call
    /// this function we need to (a) guarantee we only call it on `x86`/`x86_64`
    /// and (b) ensure that the CPU feature is available
    ///
    /// [intel-dox]: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm256_add_epi64&expand=100
    ///
    /// ## Static CPU Feature Detection
    ///
    /// The first option available to us is to conditionally compile code via the
    /// `#[cfg]` attribute. CPU features correspond to the `target_feature` cfg
    /// available, and can be used like so:
    ///
    /// ```ignore
    /// #[cfg(all(any(target_arch = "x86", target_arch = "x86_64"),
    ///       target_feature = "avx2"))]
    /// fn foo() {
    ///     #[cfg(target_arch = "x86")]
    ///     use std::arch::x86::_mm256_add_epi64;
    ///     #[cfg(target_arch = "x86_64")]
    ///     use std::arch::x86_64::_mm256_add_epi64;
    ///
    ///     unsafe {
    ///         _mm256_add_epi64(...);
    ///     }
    /// }
    /// ```
    ///
    /// Here we're using `#[cfg(target_feature = "avx2")]` to conditionally compile
    /// this function into our module. This means that if the `avx2` feature is
    /// *enabled statically* then we'll use the `_mm256_add_epi64` function at
    /// runtime. The `unsafe` block here can be justified through the usage of
    /// `#[cfg]` to only compile the code in situations where the safety guarantees
    /// are upheld.
    ///
    /// Statically enabling a feature is typically done with the `-C
    /// target-feature` or `-C target-cpu` flags to the compiler. For example if
    /// your local CPU supports AVX2 then you can compile the above function with:
    ///
    /// ```sh
    /// $ RUSTFLAGS='-C target-cpu=native' cargo build
    /// ```
    ///
    /// Or otherwise you can specifically enable just the AVX2 feature:
    ///
    /// ```sh
    /// $ RUSTFLAGS='-C target-feature=+avx2' cargo build
    /// ```
    ///
    /// Note that when you compile a binary with a particular feature enabled it's
    /// important to ensure that you only run the binary on systems which satisfy
    /// the required feature set.
    ///
    /// ## Dynamic CPU Feature Detection
    ///
    /// Sometimes statically dispatching isn't quite what you want. Instead you
    /// might want to build a portable binary that runs across a variety of CPUs,
    /// but at runtime it selects the most optimized implementation available. This
    /// allows you to build a "least common denominator" binary which has certain
    /// sections more optimized for different CPUs.
    ///
    /// Taking our previous example from before, we're going to compile our binary
    /// *without* AVX2 support, but we'd like to enable it for just one function.
    /// We can do that in a manner like:
    ///
    /// ```ignore
    /// fn foo() {
    ///     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    ///     {
    ///         if is_x86_feature_detected!("avx2") {
    ///             return unsafe { foo_avx2() };
    ///         }
    ///     }
    ///
    ///     // fallback implementation without using AVX2
    /// }
    ///
    /// #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    /// #[target_feature(enable = "avx2")]
    /// unsafe fn foo_avx2() {
    ///     #[cfg(target_arch = "x86")]
    ///     use std::arch::x86::_mm256_add_epi64;
    ///     #[cfg(target_arch = "x86_64")]
    ///     use std::arch::x86_64::_mm256_add_epi64;
    ///
    ///     _mm256_add_epi64(...);
    /// }
    /// ```
    ///
    /// There's a couple of components in play here, so let's go through them in
    /// detail!
    ///
    /// * First up we notice the `is_x86_feature_detected!` macro. Provided by
    ///   the standard library, this macro will perform necessary runtime detection
    ///   to determine whether the CPU the program is running on supports the
    ///   specified feature. In this case the macro will expand to a boolean
    /// expression evaluating to whether the local CPU has the AVX2 feature or
    /// not.
    ///
    ///   Note that this macro, like the `arch` module, is platform-specific. For
    ///   example calling `is_x86_feature_detected!("avx2")` on ARM will be a
    ///   compile time error. To ensure we don't hit this error a statement level
    ///   `#[cfg]` is used to only compile usage of the macro on `x86`/`x86_64`.
    ///
    /// * Next up we see our AVX2-enabled function, `foo_avx2`. This function is
    ///   decorated with the `#[target_feature]` attribute which enables a CPU
    ///   feature for just this one function. Using a compiler flag like `-C
    ///   target-feature=+avx2` will enable AVX2 for the entire program, but using
    ///   an attribute will only enable it for the one function. Usage of the
    ///   `#[target_feature]` attribute currently requires the function to also be
    ///   `unsafe`, as we see here. This is because the function can only be
    ///   correctly called on systems which have the AVX2 (like the intrinsics
    ///   themselves).
    ///
    /// And with all that we should have a working program! This program will run
    /// across all machines and it'll use the optimized AVX2 implementation on
    /// machines where support is detected.
    ///
    /// # Ergonomics
    ///
    /// It's important to note that using the `arch` module is not the easiest
    /// thing in the world, so if you're curious to try it out you may want to
    /// brace yourself for some wordiness!
    ///
    /// The primary purpose of this module is to enable stable crates on crates.io
    /// to build up much more ergonomic abstractions which end up using SIMD under
    /// the hood. Over time these abstractions may also move into the standard
    /// library itself, but for now this module is tasked with providing the bare
    /// minimum necessary to use vendor intrinsics on stable Rust.
    ///
    /// # Other architectures
    ///
    /// This documentation is only for one particular architecture, you can find
    /// others at:
    ///
    /// * [`x86`]
    /// * [`x86_64`]
    /// * [`arm`]
    /// * [`aarch64`]
    /// * [`mips`]
    /// * [`mips64`]
    /// * [`powerpc`]
    /// * [`powerpc64`]
    ///
    /// [`x86`]: x86/index.html
    /// [`x86_64`]: x86_64/index.html
    /// [`arm`]: arm/index.html
    /// [`aarch64`]: aarch64/index.html
    /// [`mips`]: mips/index.html
    /// [`mips64`]: mips64/index.html
    /// [`powerpc`]: powerpc/index.html
    /// [`powerpc64`]: powerpc64/index.html
    ///
    /// # Examples
    ///
    /// First let's take a look at not actually using any intrinsics but instead
    /// using LLVM's auto-vectorization to produce optimized vectorized code for
    /// AVX2 and also for the default platform.
    ///
    /// ```rust
    /// # #![cfg_attr(not(dox), feature(cfg_target_feature, target_feature, stdsimd))]
    ///
    /// # #[cfg(not(dox))]
    /// # #[macro_use]
    /// # extern crate stdsimd;
    ///
    /// fn main() {
    ///     let mut dst = [0];
    ///     add_quickly(&[1], &[2], &mut dst);
    ///     assert_eq!(dst[0], 3);
    /// }
    ///
    /// fn add_quickly(a: &[u8], b: &[u8], c: &mut [u8]) {
    ///     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    ///     {
    ///         // Note that this `unsafe` block is safe because we're testing
    ///         // that the `avx2` feature is indeed available on our CPU.
    ///         if is_x86_feature_detected!("avx2") {
    ///             return unsafe { add_quickly_avx2(a, b, c) }
    ///         }
    ///     }
    ///
    ///     add_quickly_fallback(a, b, c)
    /// }
    ///
    /// #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    /// #[target_feature(enable = "avx2")]
    /// unsafe fn add_quickly_avx2(a: &[u8], b: &[u8], c: &mut [u8]) {
    ///     add_quickly_fallback(a, b, c) // the function below is inlined here
    /// }
    ///
    /// fn add_quickly_fallback(a: &[u8], b: &[u8], c: &mut [u8]) {
    ///     for ((a, b), c) in a.iter().zip(b).zip(c) {
    ///         *c = *a + *b;
    ///     }
    /// }
    /// ```
    ///
    /// Next up let's take a look at an example of manually using intrinsics. Here
    /// we'll be using SSE4.1 features to implement hex encoding.
    ///
    /// ```
    /// # #![cfg_attr(not(dox), feature(cfg_target_feature, target_feature, stdsimd))]
    /// # #![cfg_attr(not(dox), no_std)]
    /// # #[cfg(not(dox))]
    /// # extern crate std as real_std;
    /// # #[cfg(not(dox))]
    /// # #[macro_use]
    /// # extern crate stdsimd as std;
    ///
    /// fn main() {
    ///     let mut dst = [0; 32];
    ///     hex_encode(b"\x01\x02\x03", &mut dst);
    ///     assert_eq!(&dst[..6], b"010203");
    ///
    ///     let mut src = [0; 16];
    ///     for i in 0..16 {
    ///         src[i] = (i + 1) as u8;
    ///     }
    ///     hex_encode(&src, &mut dst);
    ///     assert_eq!(&dst, b"0102030405060708090a0b0c0d0e0f10");
    /// }
    ///
    /// pub fn hex_encode(src: &[u8], dst: &mut [u8]) {
    ///     let len = src.len().checked_mul(2).unwrap();
    ///     assert!(dst.len() >= len);
    ///
    ///     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    ///     {
    ///         if is_x86_feature_detected!("sse4.1") {
    ///             return unsafe { hex_encode_sse41(src, dst) };
    ///         }
    ///     }
    ///
    ///     hex_encode_fallback(src, dst)
    /// }
    ///
    /// // translated from https://github.com/Matherunner/bin2hex-sse/blob/master/base16_sse4.cpp
    /// #[target_feature(enable = "sse4.1")]
    /// #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    /// unsafe fn hex_encode_sse41(mut src: &[u8], dst: &mut [u8]) {
    ///     #[cfg(target_arch = "x86")]
    ///     use std::arch::x86::*;
    ///     #[cfg(target_arch = "x86_64")]
    ///     use std::arch::x86_64::*;
    ///
    ///     let ascii_zero = _mm_set1_epi8(b'0' as i8);
    ///     let nines = _mm_set1_epi8(9);
    ///     let ascii_a = _mm_set1_epi8((b'a' - 9 - 1) as i8);
    ///     let and4bits = _mm_set1_epi8(0xf);
    ///
    ///     let mut i = 0_isize;
    ///     while src.len() >= 16 {
    ///         let invec = _mm_loadu_si128(src.as_ptr() as *const _);
    ///
    ///         let masked1 = _mm_and_si128(invec, and4bits);
    ///         let masked2 = _mm_and_si128(_mm_srli_epi64(invec, 4), and4bits);
    ///
    ///         // return 0xff corresponding to the elements > 9, or 0x00 otherwise
    ///         let cmpmask1 = _mm_cmpgt_epi8(masked1, nines);
    ///         let cmpmask2 = _mm_cmpgt_epi8(masked2, nines);
    ///
    ///         // add '0' or the offset depending on the masks
    ///         let masked1 = _mm_add_epi8(
    ///             masked1,
    ///             _mm_blendv_epi8(ascii_zero, ascii_a, cmpmask1),
    ///         );
    ///         let masked2 = _mm_add_epi8(
    ///             masked2,
    ///             _mm_blendv_epi8(ascii_zero, ascii_a, cmpmask2),
    ///         );
    ///
    ///         // interleave masked1 and masked2 bytes
    ///         let res1 = _mm_unpacklo_epi8(masked2, masked1);
    ///         let res2 = _mm_unpackhi_epi8(masked2, masked1);
    ///
    ///         _mm_storeu_si128(dst.as_mut_ptr().offset(i * 2) as *mut _, res1);
    /// _mm_storeu_si128(dst.as_mut_ptr().offset(i * 2 + 16) as *mut _,
    /// res2);         src = &src[16..];
    ///         i += 16;
    ///     }
    ///
    ///     let i = i as usize;
    ///     hex_encode_fallback(src, &mut dst[i * 2..]);
    /// }
    ///
    /// fn hex_encode_fallback(src: &[u8], dst: &mut [u8]) {
    ///     fn hex(byte: u8) -> u8 {
    ///         static TABLE: &[u8] = b"0123456789abcdef";
    ///         TABLE[byte as usize]
    ///     }
    ///
    ///     for (byte, slots) in src.iter().zip(dst.chunks_mut(2)) {
    ///         slots[0] = hex((*byte >> 4) & 0xf);
    ///         slots[1] = hex(*byte & 0xf);
    ///     }
    /// }
    /// ```
    #[stable(feature = "simd_arch", since = "1.27.0")]
    pub mod arch {
        #[cfg(all(not(dox), target_arch = "x86"))]
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub use coresimd::arch::x86;

        #[cfg(all(not(dox), target_arch = "x86_64"))]
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub use coresimd::arch::x86_64;

        #[cfg(all(not(dox), target_arch = "arm"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::arm;

        #[cfg(all(not(dox), target_arch = "aarch64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::aarch64;

        #[cfg(target_arch = "wasm32")]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::wasm32;

        #[cfg(all(not(dox), target_arch = "mips"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::mips;

        #[cfg(all(not(dox), target_arch = "mips64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::mips64;

        #[cfg(all(not(dox), target_arch = "powerpc"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::powerpc;

        #[cfg(all(not(dox), target_arch = "powerpc64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub use coresimd::arch::powerpc64;

        #[doc(hidden)] // unstable implementation detail
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod detect;

        /// Platform-specific intrinsics for the `x86` platform.
        ///
        /// The documentation with the full listing of `x86` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/x86/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "x86"))]
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub mod x86 {}

        /// Platform-specific intrinsics for the `x86_64` platform.
        ///
        /// The documentation with the full listing of `x86_64` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/x86_64/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "x86_64"))]
        #[stable(feature = "simd_x86", since = "1.27.0")]
        pub mod x86_64 {}

        /// Platform-specific intrinsics for the `arm` platform.
        ///
        /// The documentation with the full listing of `arm` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/arm/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "arm"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod arm {}

        /// Platform-specific intrinsics for the `aarch64` platform.
        ///
        /// The documentation with the full listing of `aarch64` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/aarch64/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "aarch64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod aarch64 {}

        /// Platform-specific intrinsics for the `mips` platform.
        ///
        /// The documentation with the full listing of `mips` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/mips/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "mips"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod mips {}

        /// Platform-specific intrinsics for the `mips64` platform.
        ///
        /// The documentation with the full listing of `mips64` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/mips64/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "mips64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod mips64 {}

        /// Platform-specific intrinsics for the `powerpc` platform.
        ///
        /// The documentation with the full listing of `powerpc` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/powerpc/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "powerpc"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod powerpc {}

        /// Platform-specific intrinsics for the `powerpc64` platform.
        ///
        /// The documentation with the full listing of `powerpc64` intrinsics is
        /// available in [libcore], but the module is re-exported here in std
        /// as well.
        ///
        /// [libcore]: ../../../core/arch/powerpc64/index.html
        #[cfg(dox)]
        #[doc(cfg(target_arch = "powerpc64"))]
        #[unstable(feature = "stdsimd", issue = "27731")]
        pub mod powerpc64 {}
    }

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