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Tsukasa OI db188b33b3 RISC-V: OS-independent implication logic 
This commit adds the OS-independent extension implication logic for RISC-V.
It implements:

1.  Regular implication (A → B)
    a.  "the extension A implies the extension B"
    b.  "the extension A requires the extension B"
    c.  "the extension A depends on the extension B"
2.  Extension group or shorthand (A == B1 & B2...)
    a.  "the extension A is shorthand for other extensions: B1, B2..."
    b.  "the extension A comprises instructions provided by B1, B2..."
    This is implemented as (A → B1 & B2... + B1 & B2... → A)
    where the former is a regular implication as required by specifications
    and the latter is a "reverse" implication to improve usability.

and prepares for:

3.  Implication with multiple requirements (A1 & A2... → B)
    a.  "A1 + A2 implies B"
    b.  (implicitly used to implement reverse implication of case 2)

Although it uses macros and iterators, good optimizers turn the series of
implications into fast bit-manipulation operations.

In the case 2 (extension group or shorthand; where a superset extension
is just a collection of other subextensions and provides no features by
a superset itself), specifications do specify that an extension group
implies its members but not vice versa.  However, implying an extension
group from its members would improve usability on the feature detection
(especially when the feature provider does not provide existence of such
extension group but provides existence of its members).

Similar "reverse implication" on RISC-V is implemented on LLVM.

Case 3 is implicitly used to implement reverse implication of case 2 but
there's another use case: implication with multiple requirements like
"Zcf" and "Zcd" extensions (not yet implemented in this crate for now).

To handle extension groups perfectly, we need to loop implication several
times (until they converge; normally 2 times and up to 4 times when we add
most of `riscv_hwprobe`-based features).
To make implementation of that loop possible, `cache::Initializer` is
modified to implement `PartialEq` and `Eq`.
2025-04-16 00:56:48 +00:00
..
src
RISC-V: OS-independent implication logic
2025-04-16 00:56:48 +00:00
tests
RISC-V: Add RISC-V + Linux / Android test
2025-04-16 00:56:48 +00:00
Cargo.toml
Update all crates to Rust 2024
2025-02-09 12:31:33 -08:00
LICENSE-APACHE
Readme from std_detect
2019-01-22 18:49:24 +01:00
LICENSE-MIT
Readme from std_detect
2019-01-22 18:49:24 +01:00
README.md
std_detect: Always avoid dlsym on *-linux-{musl,ohos}* targets
2025-03-26 13:55:33 +00:00

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.

  • std_detect_file_io (enabled by default, requires std): Enable to perform run-time feature detection using file APIs (e.g. /proc/cpuinfo, 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.

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. If you need raw support for querying cpuid, consider using the cupid crate.

  • Linux/Android:

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

  • 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

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.