crates/std_detect/README.md

`std::detect` - Rust's standard library run-time CPU feature detection
=======

The private `std::detect` module implements run-time feature detection in Rust's
standard library. This allows detecting whether the CPU the binary runs on
supports certain features, like SIMD instructions.

# Usage

`std::detect` APIs are available as part of `libstd`. Prefer using it via the
standard library than through this crate. Unstable features of `std::detect` are
available on nightly Rust behind various feature-gates.

If you need run-time feature detection in `#[no_std]` environments, Rust `core`
library cannot help you. By design, Rust `core` is platform independent, but
performing run-time feature detection requires a certain level of cooperation
from the platform.

You can then manually include `std_detect` as a dependency to get similar
run-time feature detection support than the one offered by Rust's standard
library. We intend to make `std_detect` more flexible and configurable in this
regard to better serve the needs of `#[no_std]` targets.

# Features

* `std_detect_dlsym_getauxval` (enabled by default, requires `libc`): Enable to
use `libc::dlsym` to query whether [`getauxval`] is linked into the binary. When
this is not the case, this feature allows other fallback methods to perform
run-time feature detection. When this feature is disabled, `std_detect` assumes
that [`getauxval`] is linked to the binary. If that is not the case the behavior
is undefined.

  Note: This feature is ignored on `*-linux-{gnu,musl,ohos}*` and `*-android*` targets
  because we can safely assume `getauxval` is linked to the binary.
  * `*-linux-gnu*` targets ([since Rust 1.64](https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html))
    have glibc requirements higher than [glibc 2.16 that added `getauxval`](https://sourceware.org/legacy-ml/libc-announce/2012/msg00000.html).
  * `*-linux-musl*` targets ([at least since Rust 1.15](https://github.com/rust-lang/rust/blob/1.15.0/src/ci/docker/x86_64-musl/build-musl.sh#L15))
    use musl newer than [musl 1.1.0 that added `getauxval`](https://git.musl-libc.org/cgit/musl/tree/WHATSNEW?h=v1.1.0#n1197)
  * `*-linux-ohos*` targets use a [fork of musl 1.2](https://gitee.com/openharmony/docs/blob/master/en/application-dev/reference/native-lib/musl.md)
  * `*-android*` targets ([since Rust 1.68](https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html))
    have the minimum supported API level higher than [Android 4.3 (API level 18) that added `getauxval`](https://github.com/aosp-mirror/platform_bionic/blob/d3ebc2f7c49a9893b114124d4a6b315f3a328764/libc/include/sys/auxv.h#L49).

* `std_detect_file_io` (enabled by default, requires `std`): Enable to perform run-time feature
detection using file APIs (e.g. `/proc/self/auxv`, etc.) if other more performant
methods fail. This feature requires `libstd` as a dependency, preventing the
crate from working on applications in which `std` is not available.

[`getauxval`]: https://man7.org/linux/man-pages/man3/getauxval.3.html

# Platform support

* All `x86`/`x86_64` targets are supported on all platforms by querying the
  `cpuid` instruction directly for the features supported by the hardware and
  the operating system. `std_detect` assumes that the binary is an user-space
  application.

* Linux/Android:
  * `arm{32, 64}`, `mips{32,64}{,el}`, `powerpc{32,64}{,le}`, `loongarch64`, `s390x`:
    `std_detect` supports these on Linux by querying ELF auxiliary vectors (using `getauxval`
    when available), and if that fails, by querying `/proc/self/auxv`.
  * `arm64`: partial support for doing run-time feature detection by directly
    querying `mrs` is implemented for Linux >= 4.11, but not enabled by default.
  * `riscv{32,64}`:
    `std_detect` supports these on Linux by querying `riscv_hwprobe`, and
    by querying ELF auxiliary vectors (using `getauxval` when available).

* FreeBSD:
  * `arm32`, `powerpc64`: `std_detect` supports these on FreeBSD by querying ELF
    auxiliary vectors using `sysctl`.
  * `arm64`: run-time feature detection is implemented by directly querying `mrs`.

* OpenBSD:
  * `arm64`: run-time feature detection is implemented by querying `sysctl`.

* Windows:
  * `arm64`: run-time feature detection is implemented by querying `IsProcessorFeaturePresent`.

# License

This project is licensed under either of

 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
   http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
   http://opensource.org/licenses/MIT)

at your option.

# Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in `std_detect` by you, as defined in the Apache-2.0 license,
shall be dual licensed as above, without any additional terms or conditions.