library/std_detect/src/detect/os/riscv.rs

//! Run-time feature detection utility for RISC-V.
//!
//! On RISC-V, full feature detection needs a help of one or more
//! feature detection mechanisms (usually provided by the operating system).
//!
//! RISC-V architecture defines many extensions and some have dependency to others.
//! More importantly, some of them cannot be enabled without resolving such
//! dependencies due to limited set of features that such mechanisms provide.
//!
//! This module provides an OS-independent utility to process such relations
//! between RISC-V extensions.

use crate::detect::{Feature, cache};

/// Imply features by the given set of enabled features.
///
/// Note that it does not perform any consistency checks including existence of
/// conflicting extensions and/or complicated requirements.  Eliminating such
/// inconsistencies is the responsibility of the feature detection logic and
/// its provider(s).
pub(crate) fn imply_features(mut value: cache::Initializer) -> cache::Initializer {
    loop {
        // Check convergence of the feature flags later.
        let prev = value;

        // Expect that the optimizer turns repeated operations into
        // a fewer number of bit-manipulation operations.
        macro_rules! imply {
            // Regular implication:
            // A1 => (B1[, B2...]), A2 => (B1[, B2...]) and so on.
            ($($from: ident)|+ => $($to: ident)&+) => {
                if [$(Feature::$from as u32),+].iter().any(|&x| value.test(x)) {
                    $(
                        value.set(Feature::$to as u32);
                    )+
                }
            };
            // Implication with multiple requirements:
            // A1 && A2 ... => (B1[, B2...]).
            ($($from: ident)&+ => $($to: ident)&+) => {
                if [$(Feature::$from as u32),+].iter().all(|&x| value.test(x)) {
                    $(
                        value.set(Feature::$to as u32);
                    )+
                }
            };
        }
        macro_rules! group {
            ($group: ident == $($member: ident)&+) => {
                // Forward implication as defined in the specifications.
                imply!($group => $($member)&+);
                // Reverse implication to "group extension" from its members.
                // This is not a part of specifications but convenient for
                // feature detection and implemented in e.g. LLVM.
                imply!($($member)&+ => $group);
            };
        }

        /*
            If a dependency/implication is not explicitly stated in the
            specification, it is denoted as a comment as follows:
            "defined as subset":
                The latter extension is described as a subset of the former
                (but the evidence is weak).
            "functional":
                The former extension is functionally a superset of the latter
                (no direct references though).
        */

        imply!(zvbb => zvkb);

        // Certain set of vector cryptography extensions form a group.
        group!(zvkn == zvkned & zvknhb & zvkb & zvkt);
        group!(zvknc == zvkn & zvbc);
        group!(zvkng == zvkn & zvkg);
        group!(zvks == zvksed & zvksh & zvkb & zvkt);
        group!(zvksc == zvks & zvbc);
        group!(zvksg == zvks & zvkg);

        imply!(zvknhb => zvknha); // functional

        // For vector cryptography, Zvknhb and Zvbc require integer arithmetic
        // with EEW=64 (Zve64x) while others not depending on them
        // require EEW=32 (Zve32x).
        imply!(zvknhb | zvbc => zve64x);
        imply!(zvbb | zvkb | zvkg | zvkned | zvknha | zvksed | zvksh => zve32x);

        imply!(zbc => zbkc); // defined as subset
        group!(zkn == zbkb & zbkc & zbkx & zkne & zknd & zknh);
        group!(zks == zbkb & zbkc & zbkx & zksed & zksh);
        group!(zk == zkn & zkr & zkt);

        imply!(zabha | zacas => zaamo);
        group!(a == zalrsc & zaamo);

        group!(b == zba & zbb & zbs);

        imply!(zcf => zca & f);
        imply!(zcd => zca & d);
        imply!(zcmop | zcb => zca);

        imply!(zhinx => zhinxmin);
        imply!(zdinx | zhinxmin => zfinx);

        imply!(zvfh => zvfhmin); // functional
        imply!(zvfh => zve32f & zfhmin);
        imply!(zvfhmin => zve32f);
        imply!(zvfbfwma => zvfbfmin & zfbfmin);
        imply!(zvfbfmin => zve32f);

        imply!(v => zve64d);
        imply!(zve64d => zve64f & d);
        imply!(zve64f => zve64x & zve32f);
        imply!(zve64x => zve32x);
        imply!(zve32f => zve32x & f);

        imply!(zfh => zfhmin);
        imply!(q => d);
        imply!(d | zfhmin | zfa => f);
        imply!(zfbfmin => f); // and some of (not all) "Zfh" instructions.

        // Relatively complex implication rules around the "C" extension.
        // (from "C" and some others)
        imply!(c => zca);
        imply!(c & d => zcd);
        #[cfg(target_arch = "riscv32")]
        imply!(c & f => zcf);
        // (to "C"; defined as superset)
        cfg_select! {
            target_arch = "riscv32" => {
                if value.test(Feature::d as u32) {
                    imply!(zcf & zcd => c);
                } else if value.test(Feature::f as u32) {
                    imply!(zcf => c);
                } else {
                    imply!(zca => c);
                }
            }
            _ => {
                if value.test(Feature::d as u32) {
                    imply!(zcd => c);
                } else {
                    imply!(zca => c);
                }
            }
        }

        imply!(zicntr | zihpm | f | zfinx | zve32x => zicsr);

        // Loop until the feature flags converge.
        if prev == value {
            return value;
        }
    }
}

#[cfg(test)]
#[path = "riscv/tests.rs"]
mod tests;