compiler/rustc_codegen_llvm/src/llvm_util.rs

   1 use crate::back::write::create_informational_target_machine;
   2 use crate::errors::{
   3     FixedX18InvalidArch, InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
   4     UnknownCTargetFeature, UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
   5 };
   6 use crate::llvm;
   7 use libc::c_int;
   8 use rustc_codegen_ssa::base::wants_wasm_eh;
   9 use rustc_codegen_ssa::traits::PrintBackendInfo;
  10 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  11 use rustc_data_structures::small_c_str::SmallCStr;
  12 use rustc_fs_util::path_to_c_string;
  13 use rustc_middle::bug;
  14 use rustc_session::config::{PrintKind, PrintRequest};
  15 use rustc_session::Session;
  16 use rustc_span::symbol::Symbol;
  17 use rustc_target::spec::{MergeFunctions, PanicStrategy};
  18 use rustc_target::target_features::RUSTC_SPECIFIC_FEATURES;
  19
  20 use std::ffi::{c_char, c_void, CStr, CString};
  21 use std::path::Path;
  22 use std::ptr;
  23 use std::slice;
  24 use std::str;
  25 use std::sync::Once;
  26
  27 static INIT: Once = Once::new();
  28
  29 pub(crate) fn init(sess: &Session) {
  30     unsafe {
  31         // Before we touch LLVM, make sure that multithreading is enabled.
  32         if llvm::LLVMIsMultithreaded() != 1 {
  33             bug!("LLVM compiled without support for threads");
  34         }
  35         INIT.call_once(|| {
  36             configure_llvm(sess);
  37         });
  38     }
  39 }
  40
  41 fn require_inited() {
  42     if !INIT.is_completed() {
  43         bug!("LLVM is not initialized");
  44     }
  45 }
  46
  47 unsafe fn configure_llvm(sess: &Session) {
  48     let n_args = sess.opts.cg.llvm_args.len() + sess.target.llvm_args.len();
  49     let mut llvm_c_strs = Vec::with_capacity(n_args + 1);
  50     let mut llvm_args = Vec::with_capacity(n_args + 1);
  51
  52     llvm::LLVMRustInstallErrorHandlers();
  53     // On Windows, an LLVM assertion will open an Abort/Retry/Ignore dialog
  54     // box for the purpose of launching a debugger. However, on CI this will
  55     // cause it to hang until it times out, which can take several hours.
  56     if std::env::var_os("CI").is_some() {
  57         llvm::LLVMRustDisableSystemDialogsOnCrash();
  58     }
  59
  60     fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
  61         full_arg.trim().split(|c: char| c == '=' || c.is_whitespace()).next().unwrap_or("")
  62     }
  63
  64     let cg_opts = sess.opts.cg.llvm_args.iter().map(AsRef::as_ref);
  65     let tg_opts = sess.target.llvm_args.iter().map(AsRef::as_ref);
  66     let sess_args = cg_opts.chain(tg_opts);
  67
  68     let user_specified_args: FxHashSet<_> =
  69         sess_args.clone().map(|s| llvm_arg_to_arg_name(s)).filter(|s| !s.is_empty()).collect();
  70
  71     {
  72         // This adds the given argument to LLVM. Unless `force` is true
  73         // user specified arguments are *not* overridden.
  74         let mut add = |arg: &str, force: bool| {
  75             if force || !user_specified_args.contains(llvm_arg_to_arg_name(arg)) {
  76                 let s = CString::new(arg).unwrap();
  77                 llvm_args.push(s.as_ptr());
  78                 llvm_c_strs.push(s);
  79             }
  80         };
  81         // Set the llvm "program name" to make usage and invalid argument messages more clear.
  82         add("rustc -Cllvm-args=\"...\" with", true);
  83         if sess.opts.unstable_opts.time_llvm_passes {
  84             add("-time-passes", false);
  85         }
  86         if sess.opts.unstable_opts.print_llvm_passes {
  87             add("-debug-pass=Structure", false);
  88         }
  89         if sess.target.generate_arange_section
  90             && !sess.opts.unstable_opts.no_generate_arange_section
  91         {
  92             add("-generate-arange-section", false);
  93         }
  94
  95         match sess.opts.unstable_opts.merge_functions.unwrap_or(sess.target.merge_functions) {
  96             MergeFunctions::Disabled | MergeFunctions::Trampolines => {}
  97             MergeFunctions::Aliases => {
  98                 add("-mergefunc-use-aliases", false);
  99             }
 100         }
 101
 102         if wants_wasm_eh(sess) {
 103             add("-wasm-enable-eh", false);
 104         }
 105
 106         if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
 107             add("-enable-emscripten-cxx-exceptions", false);
 108         }
 109
 110         // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
 111         // during inlining. Unfortunately these may block other optimizations.
 112         add("-preserve-alignment-assumptions-during-inlining=false", false);
 113
 114         // Use non-zero `import-instr-limit` multiplier for cold callsites.
 115         add("-import-cold-multiplier=0.1", false);
 116
 117         if sess.print_llvm_stats() {
 118             add("-stats", false);
 119         }
 120
 121         for arg in sess_args {
 122             add(&(*arg), true);
 123         }
 124     }
 125
 126     if sess.opts.unstable_opts.llvm_time_trace {
 127         llvm::LLVMRustTimeTraceProfilerInitialize();
 128     }
 129
 130     rustc_llvm::initialize_available_targets();
 131
 132     llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int, llvm_args.as_ptr());
 133 }
 134
 135 pub fn time_trace_profiler_finish(file_name: &Path) {
 136     unsafe {
 137         let file_name = path_to_c_string(file_name);
 138         llvm::LLVMRustTimeTraceProfilerFinish(file_name.as_ptr());
 139     }
 140 }
 141
 142 pub enum TargetFeatureFoldStrength<'a> {
 143     // The feature is only tied when enabling the feature, disabling
 144     // this feature shouldn't disable the tied feature.
 145     EnableOnly(&'a str),
 146     // The feature is tied for both enabling and disabling this feature.
 147     Both(&'a str),
 148 }
 149
 150 impl<'a> TargetFeatureFoldStrength<'a> {
 151     fn as_str(&self) -> &'a str {
 152         match self {
 153             TargetFeatureFoldStrength::EnableOnly(feat) => feat,
 154             TargetFeatureFoldStrength::Both(feat) => feat,
 155         }
 156     }
 157 }
 158
 159 pub struct LLVMFeature<'a> {
 160     pub llvm_feature_name: &'a str,
 161     pub dependency: Option<TargetFeatureFoldStrength<'a>>,
 162 }
 163
 164 impl<'a> LLVMFeature<'a> {
 165     pub fn new(llvm_feature_name: &'a str) -> Self {
 166         Self { llvm_feature_name, dependency: None }
 167     }
 168
 169     pub fn with_dependency(
 170         llvm_feature_name: &'a str,
 171         dependency: TargetFeatureFoldStrength<'a>,
 172     ) -> Self {
 173         Self { llvm_feature_name, dependency: Some(dependency) }
 174     }
 175
 176     pub fn contains(&self, feat: &str) -> bool {
 177         self.iter().any(|dep| dep == feat)
 178     }
 179
 180     pub fn iter(&'a self) -> impl Iterator<Item = &'a str> {
 181         let dependencies = self.dependency.iter().map(|feat| feat.as_str());
 182         std::iter::once(self.llvm_feature_name).chain(dependencies)
 183     }
 184 }
 185
 186 impl<'a> IntoIterator for LLVMFeature<'a> {
 187     type Item = &'a str;
 188     type IntoIter = impl Iterator<Item = &'a str>;
 189
 190     fn into_iter(self) -> Self::IntoIter {
 191         let dependencies = self.dependency.into_iter().map(|feat| feat.as_str());
 192         std::iter::once(self.llvm_feature_name).chain(dependencies)
 193     }
 194 }
 195
 196 // WARNING: the features after applying `to_llvm_features` must be known
 197 // to LLVM or the feature detection code will walk past the end of the feature
 198 // array, leading to crashes.
 199 //
 200 // To find a list of LLVM's names, see llvm-project/llvm/lib/Target/{ARCH}/*.td
 201 // where `{ARCH}` is the architecture name. Look for instances of `SubtargetFeature`.
 202 //
 203 // Check the current rustc fork of LLVM in the repo at https://github.com/rust-lang/llvm-project/.
 204 // The commit in use can be found via the `llvm-project` submodule in https://github.com/rust-lang/rust/tree/master/src
 205 // Though note that Rust can also be build with an external precompiled version of LLVM
 206 // which might lead to failures if the oldest tested / supported LLVM version
 207 // doesn't yet support the relevant intrinsics
 208 pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a> {
 209     let arch = if sess.target.arch == "x86_64" {
 210         "x86"
 211     } else if sess.target.arch == "arm64ec" {
 212         "aarch64"
 213     } else {
 214         &*sess.target.arch
 215     };
 216     match (arch, s) {
 217         ("x86", "sse4.2") => {
 218             LLVMFeature::with_dependency("sse4.2", TargetFeatureFoldStrength::EnableOnly("crc32"))
 219         }
 220         ("x86", "pclmulqdq") => LLVMFeature::new("pclmul"),
 221         ("x86", "rdrand") => LLVMFeature::new("rdrnd"),
 222         ("x86", "bmi1") => LLVMFeature::new("bmi"),
 223         ("x86", "cmpxchg16b") => LLVMFeature::new("cx16"),
 224         ("x86", "lahfsahf") => LLVMFeature::new("sahf"),
 225         ("aarch64", "rcpc2") => LLVMFeature::new("rcpc-immo"),
 226         ("aarch64", "dpb") => LLVMFeature::new("ccpp"),
 227         ("aarch64", "dpb2") => LLVMFeature::new("ccdp"),
 228         ("aarch64", "frintts") => LLVMFeature::new("fptoint"),
 229         ("aarch64", "fcma") => LLVMFeature::new("complxnum"),
 230         ("aarch64", "pmuv3") => LLVMFeature::new("perfmon"),
 231         ("aarch64", "paca") => LLVMFeature::new("pauth"),
 232         ("aarch64", "pacg") => LLVMFeature::new("pauth"),
 233         // Rust ties fp and neon together.
 234         ("aarch64", "neon") => {
 235             LLVMFeature::with_dependency("neon", TargetFeatureFoldStrength::Both("fp-armv8"))
 236         }
 237         // In LLVM neon implicitly enables fp, but we manually enable
 238         // neon when a feature only implicitly enables fp
 239         ("aarch64", "f32mm") => {
 240             LLVMFeature::with_dependency("f32mm", TargetFeatureFoldStrength::EnableOnly("neon"))
 241         }
 242         ("aarch64", "f64mm") => {
 243             LLVMFeature::with_dependency("f64mm", TargetFeatureFoldStrength::EnableOnly("neon"))
 244         }
 245         ("aarch64", "fhm") => {
 246             LLVMFeature::with_dependency("fp16fml", TargetFeatureFoldStrength::EnableOnly("neon"))
 247         }
 248         ("aarch64", "fp16") => {
 249             LLVMFeature::with_dependency("fullfp16", TargetFeatureFoldStrength::EnableOnly("neon"))
 250         }
 251         ("aarch64", "jsconv") => {
 252             LLVMFeature::with_dependency("jsconv", TargetFeatureFoldStrength::EnableOnly("neon"))
 253         }
 254         ("aarch64", "sve") => {
 255             LLVMFeature::with_dependency("sve", TargetFeatureFoldStrength::EnableOnly("neon"))
 256         }
 257         ("aarch64", "sve2") => {
 258             LLVMFeature::with_dependency("sve2", TargetFeatureFoldStrength::EnableOnly("neon"))
 259         }
 260         ("aarch64", "sve2-aes") => {
 261             LLVMFeature::with_dependency("sve2-aes", TargetFeatureFoldStrength::EnableOnly("neon"))
 262         }
 263         ("aarch64", "sve2-sm4") => {
 264             LLVMFeature::with_dependency("sve2-sm4", TargetFeatureFoldStrength::EnableOnly("neon"))
 265         }
 266         ("aarch64", "sve2-sha3") => {
 267             LLVMFeature::with_dependency("sve2-sha3", TargetFeatureFoldStrength::EnableOnly("neon"))
 268         }
 269         ("aarch64", "sve2-bitperm") => LLVMFeature::with_dependency(
 270             "sve2-bitperm",
 271             TargetFeatureFoldStrength::EnableOnly("neon"),
 272         ),
 273         // In LLVM 18, `unaligned-scalar-mem` was merged with `unaligned-vector-mem` into a single feature called
 274         // `fast-unaligned-access`. In LLVM 19, it was split back out.
 275         ("riscv32" | "riscv64", "unaligned-scalar-mem") if get_version().0 == 18 => {
 276             LLVMFeature::new("fast-unaligned-access")
 277         }
 278         // For LLVM 18, enable the evex512 target feature if a avx512 target feature is enabled.
 279         ("x86", s) if get_version().0 >= 18 && s.starts_with("avx512") => {
 280             LLVMFeature::with_dependency(s, TargetFeatureFoldStrength::EnableOnly("evex512"))
 281         }
 282         (_, s) => LLVMFeature::new(s),
 283     }
 284 }
 285
 286 /// Given a map from target_features to whether they are enabled or disabled,
 287 /// ensure only valid combinations are allowed.
 288 pub fn check_tied_features(
 289     sess: &Session,
 290     features: &FxHashMap<&str, bool>,
 291 ) -> Option<&'static [&'static str]> {
 292     if !features.is_empty() {
 293         for tied in sess.target.tied_target_features() {
 294             // Tied features must be set to the same value, or not set at all
 295             let mut tied_iter = tied.iter();
 296             let enabled = features.get(tied_iter.next().unwrap());
 297             if tied_iter.any(|f| enabled != features.get(f)) {
 298                 return Some(tied);
 299             }
 300         }
 301     }
 302     return None;
 303 }
 304
 305 /// Used to generate cfg variables and apply features
 306 /// Must express features in the way Rust understands them
 307 pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
 308     let target_machine = create_informational_target_machine(sess);
 309     sess.target
 310         .supported_target_features()
 311         .iter()
 312         .filter_map(|&(feature, gate)| {
 313             if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
 314                 Some(feature)
 315             } else {
 316                 None
 317             }
 318         })
 319         .filter(|feature| {
 320             // check that all features in a given smallvec are enabled
 321             for llvm_feature in to_llvm_features(sess, feature) {
 322                 let cstr = SmallCStr::new(llvm_feature);
 323                 if !unsafe { llvm::LLVMRustHasFeature(&target_machine, cstr.as_ptr()) } {
 324                     return false;
 325                 }
 326             }
 327             true
 328         })
 329         .map(|feature| Symbol::intern(feature))
 330         .collect()
 331 }
 332
 333 pub fn print_version() {
 334     let (major, minor, patch) = get_version();
 335     println!("LLVM version: {major}.{minor}.{patch}");
 336 }
 337
 338 pub fn get_version() -> (u32, u32, u32) {
 339     // Can be called without initializing LLVM
 340     unsafe {
 341         (llvm::LLVMRustVersionMajor(), llvm::LLVMRustVersionMinor(), llvm::LLVMRustVersionPatch())
 342     }
 343 }
 344
 345 pub fn print_passes() {
 346     // Can be called without initializing LLVM
 347     unsafe {
 348         llvm::LLVMRustPrintPasses();
 349     }
 350 }
 351
 352 fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
 353     let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
 354     let mut ret = Vec::with_capacity(len);
 355     for i in 0..len {
 356         unsafe {
 357             let mut feature = ptr::null();
 358             let mut desc = ptr::null();
 359             llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
 360             if feature.is_null() || desc.is_null() {
 361                 bug!("LLVM returned a `null` target feature string");
 362             }
 363             let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
 364                 bug!("LLVM returned a non-utf8 feature string: {}", e);
 365             });
 366             let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
 367                 bug!("LLVM returned a non-utf8 feature string: {}", e);
 368             });
 369             ret.push((feature, desc));
 370         }
 371     }
 372     ret
 373 }
 374
 375 fn print_target_features(out: &mut dyn PrintBackendInfo, sess: &Session, tm: &llvm::TargetMachine) {
 376     let mut llvm_target_features = llvm_target_features(tm);
 377     let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
 378     let mut rustc_target_features = sess
 379         .target
 380         .supported_target_features()
 381         .iter()
 382         .map(|(feature, _gate)| {
 383             // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these strings.
 384             let llvm_feature = to_llvm_features(sess, *feature).llvm_feature_name;
 385             let desc =
 386                 match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
 387                     Some(index) => {
 388                         known_llvm_target_features.insert(llvm_feature);
 389                         llvm_target_features[index].1
 390                     }
 391                     None => "",
 392                 };
 393
 394             (*feature, desc)
 395         })
 396         .collect::<Vec<_>>();
 397     rustc_target_features.extend_from_slice(&[(
 398         "crt-static",
 399         "Enables C Run-time Libraries to be statically linked",
 400     )]);
 401     llvm_target_features.retain(|(f, _d)| !known_llvm_target_features.contains(f));
 402
 403     let max_feature_len = llvm_target_features
 404         .iter()
 405         .chain(rustc_target_features.iter())
 406         .map(|(feature, _desc)| feature.len())
 407         .max()
 408         .unwrap_or(0);
 409
 410     writeln!(out, "Features supported by rustc for this target:");
 411     for (feature, desc) in &rustc_target_features {
 412         writeln!(out, "    {feature:max_feature_len$} - {desc}.");
 413     }
 414     writeln!(out, "\nCode-generation features supported by LLVM for this target:");
 415     for (feature, desc) in &llvm_target_features {
 416         writeln!(out, "    {feature:max_feature_len$} - {desc}.");
 417     }
 418     if llvm_target_features.is_empty() {
 419         writeln!(out, "    Target features listing is not supported by this LLVM version.");
 420     }
 421     writeln!(out, "\nUse +feature to enable a feature, or -feature to disable it.");
 422     writeln!(out, "For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n");
 423     writeln!(out, "Code-generation features cannot be used in cfg or #[target_feature],");
 424     writeln!(out, "and may be renamed or removed in a future version of LLVM or rustc.\n");
 425 }
 426
 427 pub(crate) fn print(req: &PrintRequest, mut out: &mut dyn PrintBackendInfo, sess: &Session) {
 428     require_inited();
 429     let tm = create_informational_target_machine(sess);
 430     match req.kind {
 431         PrintKind::TargetCPUs => {
 432             // SAFETY generate a C compatible string from a byte slice to pass
 433             // the target CPU name into LLVM, the lifetime of the reference is
 434             // at least as long as the C function
 435             let cpu_cstring = CString::new(handle_native(sess.target.cpu.as_ref()))
 436                 .unwrap_or_else(|e| bug!("failed to convert to cstring: {}", e));
 437             unsafe extern "C" fn callback(out: *mut c_void, string: *const c_char, len: usize) {
 438                 let out = &mut *(out as *mut &mut dyn PrintBackendInfo);
 439                 let bytes = slice::from_raw_parts(string as *const u8, len);
 440                 write!(out, "{}", String::from_utf8_lossy(bytes));
 441             }
 442             unsafe {
 443                 llvm::LLVMRustPrintTargetCPUs(
 444                     &tm,
 445                     cpu_cstring.as_ptr(),
 446                     callback,
 447                     std::ptr::addr_of_mut!(out) as *mut c_void,
 448                 );
 449             }
 450         }
 451         PrintKind::TargetFeatures => print_target_features(out, sess, &tm),
 452         _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
 453     }
 454 }
 455
 456 fn handle_native(name: &str) -> &str {
 457     if name != "native" {
 458         return name;
 459     }
 460
 461     unsafe {
 462         let mut len = 0;
 463         let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
 464         str::from_utf8(slice::from_raw_parts(ptr as *const u8, len)).unwrap()
 465     }
 466 }
 467
 468 pub fn target_cpu(sess: &Session) -> &str {
 469     match sess.opts.cg.target_cpu {
 470         Some(ref name) => handle_native(name),
 471         None => handle_native(sess.target.cpu.as_ref()),
 472     }
 473 }
 474
 475 /// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
 476 /// `--target` and similar).
 477 pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<String> {
 478     // Features that come earlier are overridden by conflicting features later in the string.
 479     // Typically we'll want more explicit settings to override the implicit ones, so:
 480     //
 481     // * Features from -Ctarget-cpu=*; are overridden by [^1]
 482     // * Features implied by --target; are overridden by
 483     // * Features from -Ctarget-feature; are overridden by
 484     // * function specific features.
 485     //
 486     // [^1]: target-cpu=native is handled here, other target-cpu values are handled implicitly
 487     // through LLVM TargetMachine implementation.
 488     //
 489     // FIXME(nagisa): it isn't clear what's the best interaction between features implied by
 490     // `-Ctarget-cpu` and `--target` are. On one hand, you'd expect CLI arguments to always
 491     // override anything that's implicit, so e.g. when there's no `--target` flag, features implied
 492     // the host target are overridden by `-Ctarget-cpu=*`. On the other hand, what about when both
 493     // `--target` and `-Ctarget-cpu=*` are specified? Both then imply some target features and both
 494     // flags are specified by the user on the CLI. It isn't as clear-cut which order of precedence
 495     // should be taken in cases like these.
 496     let mut features = vec![];
 497
 498     // -Ctarget-cpu=native
 499     match sess.opts.cg.target_cpu {
 500         Some(ref s) if s == "native" => {
 501             let features_string = unsafe {
 502                 let ptr = llvm::LLVMGetHostCPUFeatures();
 503                 let features_string = if !ptr.is_null() {
 504                     CStr::from_ptr(ptr)
 505                         .to_str()
 506                         .unwrap_or_else(|e| {
 507                             bug!("LLVM returned a non-utf8 features string: {}", e);
 508                         })
 509                         .to_owned()
 510                 } else {
 511                     bug!("could not allocate host CPU features, LLVM returned a `null` string");
 512                 };
 513
 514                 llvm::LLVMDisposeMessage(ptr);
 515
 516                 features_string
 517             };
 518             features.extend(features_string.split(',').map(String::from));
 519         }
 520         Some(_) | None => {}
 521     };
 522
 523     // Features implied by an implicit or explicit `--target`.
 524     features.extend(
 525         sess.target
 526             .features
 527             .split(',')
 528             .filter(|v| !v.is_empty() && backend_feature_name(sess, v).is_some())
 529             .map(String::from),
 530     );
 531
 532     if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
 533         features.push("+exception-handling".into());
 534     }
 535
 536     // -Ctarget-features
 537     let supported_features = sess.target.supported_target_features();
 538     let mut featsmap = FxHashMap::default();
 539     let feats = sess
 540         .opts
 541         .cg
 542         .target_feature
 543         .split(',')
 544         .filter_map(|s| {
 545             let enable_disable = match s.chars().next() {
 546                 None => return None,
 547                 Some(c @ ('+' | '-')) => c,
 548                 Some(_) => {
 549                     if diagnostics {
 550                         sess.dcx().emit_warn(UnknownCTargetFeaturePrefix { feature: s });
 551                     }
 552                     return None;
 553                 }
 554             };
 555
 556             let feature = backend_feature_name(sess, s)?;
 557             // Warn against use of LLVM specific feature names and unstable features on the CLI.
 558             if diagnostics {
 559                 let feature_state = supported_features.iter().find(|&&(v, _)| v == feature);
 560                 if feature_state.is_none() {
 561                     let rust_feature = supported_features.iter().find_map(|&(rust_feature, _)| {
 562                         let llvm_features = to_llvm_features(sess, rust_feature);
 563                         if llvm_features.contains(feature) && !llvm_features.contains(rust_feature)
 564                         {
 565                             Some(rust_feature)
 566                         } else {
 567                             None
 568                         }
 569                     });
 570                     let unknown_feature = if let Some(rust_feature) = rust_feature {
 571                         UnknownCTargetFeature {
 572                             feature,
 573                             rust_feature: PossibleFeature::Some { rust_feature },
 574                         }
 575                     } else {
 576                         UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
 577                     };
 578                     sess.dcx().emit_warn(unknown_feature);
 579                 } else if feature_state
 580                     .is_some_and(|(_name, feature_gate)| !feature_gate.is_stable())
 581                 {
 582                     // An unstable feature. Warn about using it.
 583                     sess.dcx().emit_warn(UnstableCTargetFeature { feature });
 584                 }
 585             }
 586
 587             if diagnostics {
 588                 // FIXME(nagisa): figure out how to not allocate a full hashset here.
 589                 featsmap.insert(feature, enable_disable == '+');
 590             }
 591
 592             // rustc-specific features do not get passed down to LLVM…
 593             if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
 594                 return None;
 595             }
 596             // ... otherwise though we run through `to_llvm_features` when
 597             // passing requests down to LLVM. This means that all in-language
 598             // features also work on the command line instead of having two
 599             // different names when the LLVM name and the Rust name differ.
 600             let llvm_feature = to_llvm_features(sess, feature);
 601
 602             Some(
 603                 std::iter::once(format!("{}{}", enable_disable, llvm_feature.llvm_feature_name))
 604                     .chain(llvm_feature.dependency.into_iter().filter_map(move |feat| {
 605                         match (enable_disable, feat) {
 606                             ('-' | '+', TargetFeatureFoldStrength::Both(f))
 607                             | ('+', TargetFeatureFoldStrength::EnableOnly(f)) => {
 608                                 Some(format!("{enable_disable}{f}"))
 609                             }
 610                             _ => None,
 611                         }
 612                     })),
 613             )
 614         })
 615         .flatten();
 616     features.extend(feats);
 617
 618     // -Zfixed-x18
 619     if sess.opts.unstable_opts.fixed_x18 {
 620         if sess.target.arch != "aarch64" {
 621             sess.dcx().emit_fatal(FixedX18InvalidArch { arch: &sess.target.arch });
 622         } else {
 623             features.push("+reserve-x18".into());
 624         }
 625     }
 626
 627     if diagnostics && let Some(f) = check_tied_features(sess, &featsmap) {
 628         sess.dcx().emit_err(TargetFeatureDisableOrEnable {
 629             features: f,
 630             span: None,
 631             missing_features: None,
 632         });
 633     }
 634
 635     features
 636 }
 637
 638 /// Returns a feature name for the given `+feature` or `-feature` string.
 639 ///
 640 /// Only allows features that are backend specific (i.e. not [`RUSTC_SPECIFIC_FEATURES`].)
 641 fn backend_feature_name<'a>(sess: &Session, s: &'a str) -> Option<&'a str> {
 642     // features must start with a `+` or `-`.
 643     let feature = s
 644         .strip_prefix(&['+', '-'][..])
 645         .unwrap_or_else(|| sess.dcx().emit_fatal(InvalidTargetFeaturePrefix { feature: s }));
 646     // Rustc-specific feature requests like `+crt-static` or `-crt-static`
 647     // are not passed down to LLVM.
 648     if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
 649         return None;
 650     }
 651     Some(feature)
 652 }
 653
 654 pub fn tune_cpu(sess: &Session) -> Option<&str> {
 655     let name = sess.opts.unstable_opts.tune_cpu.as_ref()?;
 656     Some(handle_native(name))
 657 }