rust/compiler/rustc_codegen_ssa/src/target_features.rs

use rustc_attr_data_structures::InstructionSetAttr;
use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
use rustc_data_structures::unord::{UnordMap, UnordSet};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
use rustc_middle::middle::codegen_fn_attrs::TargetFeature;
use rustc_middle::query::Providers;
use rustc_middle::ty::TyCtxt;
use rustc_session::Session;
use rustc_session::lint::builtin::AARCH64_SOFTFLOAT_NEON;
use rustc_session::parse::feature_err;
use rustc_span::{Span, Symbol, sym};
use rustc_target::target_features::{RUSTC_SPECIFIC_FEATURES, Stability};
use smallvec::SmallVec;

use crate::errors::FeatureNotValid;
use crate::{errors, target_features};

/// Compute the enabled target features from the `#[target_feature]` function attribute.
/// Enabled target features are added to `target_features`.
pub(crate) fn from_target_feature_attr(
    tcx: TyCtxt<'_>,
    did: LocalDefId,
    features: &[(Symbol, Span)],
    rust_target_features: &UnordMap<String, target_features::Stability>,
    target_features: &mut Vec<TargetFeature>,
) {
    let rust_features = tcx.features();
    let abi_feature_constraints = tcx.sess.target.abi_required_features();
    for &(feature, feature_span) in features {
        let feature_str = feature.as_str();
        let Some(stability) = rust_target_features.get(feature_str) else {
            let plus_hint = feature_str
                .strip_prefix('+')
                .is_some_and(|stripped| rust_target_features.contains_key(stripped));
            tcx.dcx().emit_err(FeatureNotValid {
                feature: feature_str,
                span: feature_span,
                plus_hint,
            });
            continue;
        };

        // Only allow target features whose feature gates have been enabled
        // and which are permitted to be toggled.
        if let Err(reason) = stability.toggle_allowed() {
            tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
                span: feature_span,
                feature: feature_str,
                reason,
            });
        } else if let Some(nightly_feature) = stability.requires_nightly()
            && !rust_features.enabled(nightly_feature)
        {
            feature_err(
                &tcx.sess,
                nightly_feature,
                feature_span,
                format!("the target feature `{feature}` is currently unstable"),
            )
            .emit();
        } else {
            // Add this and the implied features.
            for &name in tcx.implied_target_features(feature) {
                // But ensure the ABI does not forbid enabling this.
                // Here we do assume that the backend doesn't add even more implied features
                // we don't know about, at least no features that would have ABI effects!
                // We skip this logic in rustdoc, where we want to allow all target features of
                // all targets, so we can't check their ABI compatibility and anyway we are not
                // generating code so "it's fine".
                if !tcx.sess.opts.actually_rustdoc {
                    if abi_feature_constraints.incompatible.contains(&name.as_str()) {
                        // For "neon" specifically, we emit an FCW instead of a hard error.
                        // See <https://github.com/rust-lang/rust/issues/134375>.
                        if tcx.sess.target.arch == "aarch64" && name.as_str() == "neon" {
                            tcx.emit_node_span_lint(
                                AARCH64_SOFTFLOAT_NEON,
                                tcx.local_def_id_to_hir_id(did),
                                feature_span,
                                errors::Aarch64SoftfloatNeon,
                            );
                        } else {
                            tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
                                span: feature_span,
                                feature: name.as_str(),
                                reason: "this feature is incompatible with the target ABI",
                            });
                        }
                    }
                }
                target_features.push(TargetFeature { name, implied: name != feature })
            }
        }
    }
}

/// Computes the set of target features used in a function for the purposes of
/// inline assembly.
fn asm_target_features(tcx: TyCtxt<'_>, did: DefId) -> &FxIndexSet<Symbol> {
    let mut target_features = tcx.sess.unstable_target_features.clone();
    if tcx.def_kind(did).has_codegen_attrs() {
        let attrs = tcx.codegen_fn_attrs(did);
        target_features.extend(attrs.target_features.iter().map(|feature| feature.name));
        match attrs.instruction_set {
            None => {}
            Some(InstructionSetAttr::ArmA32) => {
                // FIXME(#120456) - is `swap_remove` correct?
                target_features.swap_remove(&sym::thumb_mode);
            }
            Some(InstructionSetAttr::ArmT32) => {
                target_features.insert(sym::thumb_mode);
            }
        }
    }

    tcx.arena.alloc(target_features)
}

/// Checks the function annotated with `#[target_feature]` is not a safe
/// trait method implementation, reporting an error if it is.
pub(crate) fn check_target_feature_trait_unsafe(tcx: TyCtxt<'_>, id: LocalDefId, attr_span: Span) {
    if let DefKind::AssocFn = tcx.def_kind(id) {
        let parent_id = tcx.local_parent(id);
        if let DefKind::Trait | DefKind::Impl { of_trait: true } = tcx.def_kind(parent_id) {
            tcx.dcx().emit_err(errors::TargetFeatureSafeTrait {
                span: attr_span,
                def: tcx.def_span(id),
            });
        }
    }
}

/// Parse the value of `-Ctarget-feature`, also expanding implied features,
/// and call the closure for each (expanded) Rust feature. If the list contains
/// a syntactically invalid item (not starting with `+`/`-`), the error callback is invoked.
fn parse_rust_feature_flag<'a>(
    sess: &'a Session,
    err_callback: impl Fn(&'a str),
    mut callback: impl FnMut(
        /* base_feature */ &'a str,
        /* with_implied */ FxHashSet<&'a str>,
        /* enable */ bool,
    ),
) {
    // A cache for the backwards implication map.
    let mut inverse_implied_features: Option<FxHashMap<&str, FxHashSet<&str>>> = None;

    for feature in sess.opts.cg.target_feature.split(',') {
        if let Some(base_feature) = feature.strip_prefix('+') {
            // Skip features that are not target features, but rustc features.
            if RUSTC_SPECIFIC_FEATURES.contains(&base_feature) {
                return;
            }

            callback(base_feature, sess.target.implied_target_features(base_feature), true)
        } else if let Some(base_feature) = feature.strip_prefix('-') {
            // Skip features that are not target features, but rustc features.
            if RUSTC_SPECIFIC_FEATURES.contains(&base_feature) {
                return;
            }

            // If `f1` implies `f2`, then `!f2` implies `!f1` -- this is standard logical
            // contraposition. So we have to find all the reverse implications of `base_feature` and
            // disable them, too.

            let inverse_implied_features = inverse_implied_features.get_or_insert_with(|| {
                let mut set: FxHashMap<&str, FxHashSet<&str>> = FxHashMap::default();
                for (f, _, is) in sess.target.rust_target_features() {
                    for i in is.iter() {
                        set.entry(i).or_default().insert(f);
                    }
                }
                set
            });

            // Inverse implied target features have their own inverse implied target features, so we
            // traverse the map until there are no more features to add.
            let mut features = FxHashSet::default();
            let mut new_features = vec![base_feature];
            while let Some(new_feature) = new_features.pop() {
                if features.insert(new_feature) {
                    if let Some(implied_features) = inverse_implied_features.get(&new_feature) {
                        new_features.extend(implied_features)
                    }
                }
            }

            callback(base_feature, features, false)
        } else if !feature.is_empty() {
            err_callback(feature)
        }
    }
}

/// Utility function for a codegen backend to compute `cfg(target_feature)`, or more specifically,
/// to populate `sess.unstable_target_features` and `sess.target_features` (these are the first and
/// 2nd component of the return value, respectively).
///
/// `target_base_has_feature` should check whether the given feature (a Rust feature name!) is
/// enabled in the "base" target machine, i.e., without applying `-Ctarget-feature`.
///
/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled elsewhere.
pub fn cfg_target_feature(
    sess: &Session,
    mut target_base_has_feature: impl FnMut(&str) -> bool,
) -> (Vec<Symbol>, Vec<Symbol>) {
    // Compute which of the known target features are enabled in the 'base' target machine. We only
    // consider "supported" features; "forbidden" features are not reflected in `cfg` as of now.
    let mut features: UnordSet<Symbol> = sess
        .target
        .rust_target_features()
        .iter()
        .filter(|(feature, _, _)| target_base_has_feature(feature))
        .map(|(feature, _, _)| Symbol::intern(feature))
        .collect();

    // Add enabled and remove disabled features.
    parse_rust_feature_flag(
        sess,
        /* err_callback */
        |_| {
            // Errors are already emitted in `flag_to_backend_features`; avoid duplicates.
        },
        |_base_feature, new_features, enabled| {
            // Iteration order is irrelevant since this only influences an `UnordSet`.
            #[allow(rustc::potential_query_instability)]
            if enabled {
                features.extend(new_features.into_iter().map(|f| Symbol::intern(f)));
            } else {
                // Remove `new_features` from `features`.
                for new in new_features {
                    features.remove(&Symbol::intern(new));
                }
            }
        },
    );

    // Filter enabled features based on feature gates.
    let f = |allow_unstable| {
        sess.target
            .rust_target_features()
            .iter()
            .filter_map(|(feature, gate, _)| {
                // The `allow_unstable` set is used by rustc internally to determine which target
                // features are truly available, so we want to return even perma-unstable
                // "forbidden" features.
                if allow_unstable
                    || (gate.in_cfg()
                        && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
                {
                    Some(Symbol::intern(feature))
                } else {
                    None
                }
            })
            .filter(|feature| features.contains(&feature))
            .collect()
    };

    (f(true), f(false))
}

/// Given a map from target_features to whether they are enabled or disabled, ensure only valid
/// combinations are allowed.
pub fn check_tied_features(
    sess: &Session,
    features: &FxHashMap<&str, bool>,
) -> Option<&'static [&'static str]> {
    if !features.is_empty() {
        for tied in sess.target.tied_target_features() {
            // Tied features must be set to the same value, or not set at all
            let mut tied_iter = tied.iter();
            let enabled = features.get(tied_iter.next().unwrap());
            if tied_iter.any(|f| enabled != features.get(f)) {
                return Some(tied);
            }
        }
    }
    None
}

/// Translates the `-Ctarget-feature` flag into a backend target feature list.
///
/// `to_backend_features` converts a Rust feature name into a list of backend feature names; this is
/// used for diagnostic purposes only.
///
/// `extend_backend_features` extends the set of backend features (assumed to be in mutable state
/// accessible by that closure) to enable/disable the given Rust feature name.
pub fn flag_to_backend_features<'a, const N: usize>(
    sess: &'a Session,
    diagnostics: bool,
    to_backend_features: impl Fn(&'a str) -> SmallVec<[&'a str; N]>,
    mut extend_backend_features: impl FnMut(&'a str, /* enable */ bool),
) {
    let known_features = sess.target.rust_target_features();

    // Compute implied features
    let mut rust_features = vec![];
    parse_rust_feature_flag(
        sess,
        /* err_callback */
        |feature| {
            if diagnostics {
                sess.dcx().emit_warn(errors::UnknownCTargetFeaturePrefix { feature });
            }
        },
        |base_feature, new_features, enable| {
            rust_features.extend(
                UnordSet::from(new_features).to_sorted_stable_ord().iter().map(|&&s| (enable, s)),
            );
            // Check feature validity.
            if diagnostics {
                let feature_state = known_features.iter().find(|&&(v, _, _)| v == base_feature);
                match feature_state {
                    None => {
                        // This is definitely not a valid Rust feature name. Maybe it is a backend
                        // feature name? If so, give a better error message.
                        let rust_feature =
                            known_features.iter().find_map(|&(rust_feature, _, _)| {
                                let backend_features = to_backend_features(rust_feature);
                                if backend_features.contains(&base_feature)
                                    && !backend_features.contains(&rust_feature)
                                {
                                    Some(rust_feature)
                                } else {
                                    None
                                }
                            });
                        let unknown_feature = if let Some(rust_feature) = rust_feature {
                            errors::UnknownCTargetFeature {
                                feature: base_feature,
                                rust_feature: errors::PossibleFeature::Some { rust_feature },
                            }
                        } else {
                            errors::UnknownCTargetFeature {
                                feature: base_feature,
                                rust_feature: errors::PossibleFeature::None,
                            }
                        };
                        sess.dcx().emit_warn(unknown_feature);
                    }
                    Some((_, stability, _)) => {
                        if let Err(reason) = stability.toggle_allowed() {
                            sess.dcx().emit_warn(errors::ForbiddenCTargetFeature {
                                feature: base_feature,
                                enabled: if enable { "enabled" } else { "disabled" },
                                reason,
                            });
                        } else if stability.requires_nightly().is_some() {
                            // An unstable feature. Warn about using it. It makes little sense
                            // to hard-error here since we just warn about fully unknown
                            // features above.
                            sess.dcx().emit_warn(errors::UnstableCTargetFeature {
                                feature: base_feature,
                            });
                        }
                    }
                }
            }
        },
    );

    if diagnostics {
        // FIXME(nagisa): figure out how to not allocate a full hashmap here.
        if let Some(f) = check_tied_features(
            sess,
            &FxHashMap::from_iter(rust_features.iter().map(|&(enable, feature)| (feature, enable))),
        ) {
            sess.dcx().emit_err(errors::TargetFeatureDisableOrEnable {
                features: f,
                span: None,
                missing_features: None,
            });
        }
    }

    // Add this to the backend features.
    for (enable, feature) in rust_features {
        extend_backend_features(feature, enable);
    }
}

/// Computes the backend target features to be added to account for retpoline flags.
/// Used by both LLVM and GCC since their target features are, conveniently, the same.
pub fn retpoline_features_by_flags(sess: &Session, features: &mut Vec<String>) {
    // -Zretpoline without -Zretpoline-external-thunk enables
    // retpoline-indirect-branches and retpoline-indirect-calls target features
    let unstable_opts = &sess.opts.unstable_opts;
    if unstable_opts.retpoline && !unstable_opts.retpoline_external_thunk {
        features.push("+retpoline-indirect-branches".into());
        features.push("+retpoline-indirect-calls".into());
    }
    // -Zretpoline-external-thunk (maybe, with -Zretpoline too) enables
    // retpoline-external-thunk, retpoline-indirect-branches and
    // retpoline-indirect-calls target features
    if unstable_opts.retpoline_external_thunk {
        features.push("+retpoline-external-thunk".into());
        features.push("+retpoline-indirect-branches".into());
        features.push("+retpoline-indirect-calls".into());
    }
}

pub(crate) fn provide(providers: &mut Providers) {
    *providers = Providers {
        rust_target_features: |tcx, cnum| {
            assert_eq!(cnum, LOCAL_CRATE);
            if tcx.sess.opts.actually_rustdoc {
                // HACK: rustdoc would like to pretend that we have all the target features, so we
                // have to merge all the lists into one. To ensure an unstable target never prevents
                // a stable one from working, we merge the stability info of all instances of the
                // same target feature name, with the "most stable" taking precedence. And then we
                // hope that this doesn't cause issues anywhere else in the compiler...
                let mut result: UnordMap<String, Stability> = Default::default();
                for (name, stability) in rustc_target::target_features::all_rust_features() {
                    use std::collections::hash_map::Entry;
                    match result.entry(name.to_owned()) {
                        Entry::Vacant(vacant_entry) => {
                            vacant_entry.insert(stability);
                        }
                        Entry::Occupied(mut occupied_entry) => {
                            // Merge the two stabilities, "more stable" taking precedence.
                            match (occupied_entry.get(), stability) {
                                (Stability::Stable, _)
                                | (
                                    Stability::Unstable { .. },
                                    Stability::Unstable { .. } | Stability::Forbidden { .. },
                                )
                                | (Stability::Forbidden { .. }, Stability::Forbidden { .. }) => {
                                    // The stability in the entry is at least as good as the new
                                    // one, just keep it.
                                }
                                _ => {
                                    // Overwrite stability.
                                    occupied_entry.insert(stability);
                                }
                            }
                        }
                    }
                }
                result
            } else {
                tcx.sess
                    .target
                    .rust_target_features()
                    .iter()
                    .map(|(a, b, _)| (a.to_string(), *b))
                    .collect()
            }
        },
        implied_target_features: |tcx, feature: Symbol| {
            let feature = feature.as_str();
            UnordSet::from(tcx.sess.target.implied_target_features(feature))
                .into_sorted_stable_ord()
                .into_iter()
                .map(|s| Symbol::intern(s))
                .collect()
        },
        asm_target_features,
        ..*providers
    }
}