rustc_codegen_llvm/
llvm_util.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
use std::ffi::{CStr, CString, c_char, c_void};
use std::fmt::Write;
use std::path::Path;
use std::sync::Once;
use std::{ptr, slice, str};

use libc::c_int;
use rustc_codegen_ssa::base::wants_wasm_eh;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::small_c_str::SmallCStr;
use rustc_data_structures::unord::UnordSet;
use rustc_fs_util::path_to_c_string;
use rustc_middle::bug;
use rustc_session::Session;
use rustc_session::config::{PrintKind, PrintRequest};
use rustc_span::symbol::Symbol;
use rustc_target::spec::{MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};

use crate::back::write::create_informational_target_machine;
use crate::errors::{
    FixedX18InvalidArch, InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
    UnknownCTargetFeature, UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
};
use crate::llvm;

static INIT: Once = Once::new();

pub(crate) fn init(sess: &Session) {
    unsafe {
        // Before we touch LLVM, make sure that multithreading is enabled.
        if llvm::LLVMIsMultithreaded() != 1 {
            bug!("LLVM compiled without support for threads");
        }
        INIT.call_once(|| {
            configure_llvm(sess);
        });
    }
}

fn require_inited() {
    if !INIT.is_completed() {
        bug!("LLVM is not initialized");
    }
}

unsafe fn configure_llvm(sess: &Session) {
    let n_args = sess.opts.cg.llvm_args.len() + sess.target.llvm_args.len();
    let mut llvm_c_strs = Vec::with_capacity(n_args + 1);
    let mut llvm_args = Vec::with_capacity(n_args + 1);

    unsafe {
        llvm::LLVMRustInstallErrorHandlers();
    }
    // On Windows, an LLVM assertion will open an Abort/Retry/Ignore dialog
    // box for the purpose of launching a debugger. However, on CI this will
    // cause it to hang until it times out, which can take several hours.
    if std::env::var_os("CI").is_some() {
        unsafe {
            llvm::LLVMRustDisableSystemDialogsOnCrash();
        }
    }

    fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
        full_arg.trim().split(|c: char| c == '=' || c.is_whitespace()).next().unwrap_or("")
    }

    let cg_opts = sess.opts.cg.llvm_args.iter().map(AsRef::as_ref);
    let tg_opts = sess.target.llvm_args.iter().map(AsRef::as_ref);
    let sess_args = cg_opts.chain(tg_opts);

    let user_specified_args: FxHashSet<_> =
        sess_args.clone().map(|s| llvm_arg_to_arg_name(s)).filter(|s| !s.is_empty()).collect();

    {
        // This adds the given argument to LLVM. Unless `force` is true
        // user specified arguments are *not* overridden.
        let mut add = |arg: &str, force: bool| {
            if force || !user_specified_args.contains(llvm_arg_to_arg_name(arg)) {
                let s = CString::new(arg).unwrap();
                llvm_args.push(s.as_ptr());
                llvm_c_strs.push(s);
            }
        };
        // Set the llvm "program name" to make usage and invalid argument messages more clear.
        add("rustc -Cllvm-args=\"...\" with", true);
        if sess.opts.unstable_opts.time_llvm_passes {
            add("-time-passes", false);
        }
        if sess.opts.unstable_opts.print_llvm_passes {
            add("-debug-pass=Structure", false);
        }
        if sess.target.generate_arange_section
            && !sess.opts.unstable_opts.no_generate_arange_section
        {
            add("-generate-arange-section", false);
        }

        match sess.opts.unstable_opts.merge_functions.unwrap_or(sess.target.merge_functions) {
            MergeFunctions::Disabled | MergeFunctions::Trampolines => {}
            MergeFunctions::Aliases => {
                add("-mergefunc-use-aliases", false);
            }
        }

        if wants_wasm_eh(sess) {
            add("-wasm-enable-eh", false);
        }

        if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
            add("-enable-emscripten-cxx-exceptions", false);
        }

        // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
        // during inlining. Unfortunately these may block other optimizations.
        add("-preserve-alignment-assumptions-during-inlining=false", false);

        // Use non-zero `import-instr-limit` multiplier for cold callsites.
        add("-import-cold-multiplier=0.1", false);

        if sess.print_llvm_stats() {
            add("-stats", false);
        }

        for arg in sess_args {
            add(&(*arg), true);
        }

        match (
            sess.opts.unstable_opts.small_data_threshold,
            sess.target.small_data_threshold_support(),
        ) {
            // Set up the small-data optimization limit for architectures that use
            // an LLVM argument to control this.
            (Some(threshold), SmallDataThresholdSupport::LlvmArg(arg)) => {
                add(&format!("--{arg}={threshold}"), false)
            }
            _ => (),
        };
    }

    if sess.opts.unstable_opts.llvm_time_trace {
        unsafe { llvm::LLVMRustTimeTraceProfilerInitialize() };
    }

    rustc_llvm::initialize_available_targets();

    unsafe { llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int, llvm_args.as_ptr()) };
}

pub(crate) fn time_trace_profiler_finish(file_name: &Path) {
    unsafe {
        let file_name = path_to_c_string(file_name);
        llvm::LLVMRustTimeTraceProfilerFinish(file_name.as_ptr());
    }
}

enum TargetFeatureFoldStrength<'a> {
    // The feature is only tied when enabling the feature, disabling
    // this feature shouldn't disable the tied feature.
    EnableOnly(&'a str),
    // The feature is tied for both enabling and disabling this feature.
    Both(&'a str),
}

impl<'a> TargetFeatureFoldStrength<'a> {
    fn as_str(&self) -> &'a str {
        match self {
            TargetFeatureFoldStrength::EnableOnly(feat) => feat,
            TargetFeatureFoldStrength::Both(feat) => feat,
        }
    }
}

pub(crate) struct LLVMFeature<'a> {
    llvm_feature_name: &'a str,
    dependency: Option<TargetFeatureFoldStrength<'a>>,
}

impl<'a> LLVMFeature<'a> {
    fn new(llvm_feature_name: &'a str) -> Self {
        Self { llvm_feature_name, dependency: None }
    }

    fn with_dependency(
        llvm_feature_name: &'a str,
        dependency: TargetFeatureFoldStrength<'a>,
    ) -> Self {
        Self { llvm_feature_name, dependency: Some(dependency) }
    }

    fn contains(&self, feat: &str) -> bool {
        self.iter().any(|dep| dep == feat)
    }

    fn iter(&'a self) -> impl Iterator<Item = &'a str> {
        let dependencies = self.dependency.iter().map(|feat| feat.as_str());
        std::iter::once(self.llvm_feature_name).chain(dependencies)
    }
}

impl<'a> IntoIterator for LLVMFeature<'a> {
    type Item = &'a str;
    type IntoIter = impl Iterator<Item = &'a str>;

    fn into_iter(self) -> Self::IntoIter {
        let dependencies = self.dependency.into_iter().map(|feat| feat.as_str());
        std::iter::once(self.llvm_feature_name).chain(dependencies)
    }
}

// WARNING: the features after applying `to_llvm_features` must be known
// to LLVM or the feature detection code will walk past the end of the feature
// array, leading to crashes.
//
// To find a list of LLVM's names, see llvm-project/llvm/lib/Target/{ARCH}/*.td
// where `{ARCH}` is the architecture name. Look for instances of `SubtargetFeature`.
//
// Check the current rustc fork of LLVM in the repo at https://github.com/rust-lang/llvm-project/.
// The commit in use can be found via the `llvm-project` submodule in
// https://github.com/rust-lang/rust/tree/master/src Though note that Rust can also be build with
// an external precompiled version of LLVM which might lead to failures if the oldest tested /
// supported LLVM version doesn't yet support the relevant intrinsics.
pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFeature<'a>> {
    let arch = if sess.target.arch == "x86_64" {
        "x86"
    } else if sess.target.arch == "arm64ec" {
        "aarch64"
    } else {
        &*sess.target.arch
    };
    match (arch, s) {
        ("x86", "sse4.2") => Some(LLVMFeature::with_dependency(
            "sse4.2",
            TargetFeatureFoldStrength::EnableOnly("crc32"),
        )),
        ("x86", "pclmulqdq") => Some(LLVMFeature::new("pclmul")),
        ("x86", "rdrand") => Some(LLVMFeature::new("rdrnd")),
        ("x86", "bmi1") => Some(LLVMFeature::new("bmi")),
        ("x86", "cmpxchg16b") => Some(LLVMFeature::new("cx16")),
        ("x86", "lahfsahf") => Some(LLVMFeature::new("sahf")),
        ("aarch64", "rcpc2") => Some(LLVMFeature::new("rcpc-immo")),
        ("aarch64", "dpb") => Some(LLVMFeature::new("ccpp")),
        ("aarch64", "dpb2") => Some(LLVMFeature::new("ccdp")),
        ("aarch64", "frintts") => Some(LLVMFeature::new("fptoint")),
        ("aarch64", "fcma") => Some(LLVMFeature::new("complxnum")),
        ("aarch64", "pmuv3") => Some(LLVMFeature::new("perfmon")),
        ("aarch64", "paca") => Some(LLVMFeature::new("pauth")),
        ("aarch64", "pacg") => Some(LLVMFeature::new("pauth")),
        ("aarch64", "sve-b16b16") => Some(LLVMFeature::new("b16b16")),
        ("aarch64", "flagm2") => Some(LLVMFeature::new("altnzcv")),
        // Rust ties fp and neon together.
        ("aarch64", "neon") => {
            Some(LLVMFeature::with_dependency("neon", TargetFeatureFoldStrength::Both("fp-armv8")))
        }
        // In LLVM neon implicitly enables fp, but we manually enable
        // neon when a feature only implicitly enables fp
        ("aarch64", "fhm") => Some(LLVMFeature::new("fp16fml")),
        ("aarch64", "fp16") => Some(LLVMFeature::new("fullfp16")),
        // Filter out features that are not supported by the current LLVM version
        ("aarch64", "fpmr") if get_version().0 != 18 => None,
        // In LLVM 18, `unaligned-scalar-mem` was merged with `unaligned-vector-mem` into a single
        // feature called `fast-unaligned-access`. In LLVM 19, it was split back out.
        ("riscv32" | "riscv64", "unaligned-scalar-mem") if get_version().0 == 18 => {
            Some(LLVMFeature::new("fast-unaligned-access"))
        }
        // Filter out features that are not supported by the current LLVM version
        ("riscv32" | "riscv64", "zaamo") if get_version().0 < 19 => None,
        ("riscv32" | "riscv64", "zabha") if get_version().0 < 19 => None,
        ("riscv32" | "riscv64", "zalrsc") if get_version().0 < 19 => None,
        // Enable the evex512 target feature if an avx512 target feature is enabled.
        ("x86", s) if s.starts_with("avx512") => {
            Some(LLVMFeature::with_dependency(s, TargetFeatureFoldStrength::EnableOnly("evex512")))
        }
        (_, s) => Some(LLVMFeature::new(s)),
    }
}

/// Given a map from target_features to whether they are enabled or disabled,
/// ensure only valid combinations are allowed.
pub(crate) 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
}

/// Used to generate cfg variables and apply features
/// Must express features in the way Rust understands them
pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
    let mut features = vec![];

    // Add base features for the target
    let target_machine = create_informational_target_machine(sess, true);
    features.extend(
        sess.target
            .supported_target_features()
            .iter()
            .filter(|(feature, _, _)| {
                // skip checking special features, as LLVM may not understands them
                if RUSTC_SPECIAL_FEATURES.contains(feature) {
                    return true;
                }
                // check that all features in a given smallvec are enabled
                if let Some(feat) = to_llvm_features(sess, feature) {
                    for llvm_feature in feat {
                        let cstr = SmallCStr::new(llvm_feature);
                        if !unsafe { llvm::LLVMRustHasFeature(&target_machine, cstr.as_ptr()) } {
                            return false;
                        }
                    }
                    true
                } else {
                    false
                }
            })
            .map(|(feature, _, _)| Symbol::intern(feature)),
    );

    // Add enabled features
    for (enabled, feature) in
        sess.opts.cg.target_feature.split(',').filter_map(|s| match s.chars().next() {
            Some('+') => Some((true, Symbol::intern(&s[1..]))),
            Some('-') => Some((false, Symbol::intern(&s[1..]))),
            _ => None,
        })
    {
        if enabled {
            features.extend(sess.target.implied_target_features(std::iter::once(feature)));
        } else {
            features.retain(|f| {
                !sess.target.implied_target_features(std::iter::once(*f)).contains(&feature)
            });
        }
    }

    // Filter enabled features based on feature gates
    sess.target
        .supported_target_features()
        .iter()
        .filter_map(|&(feature, gate, _)| {
            if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
                Some(feature)
            } else {
                None
            }
        })
        .filter(|feature| features.contains(&Symbol::intern(feature)))
        .map(|feature| Symbol::intern(feature))
        .collect()
}

pub(crate) fn print_version() {
    let (major, minor, patch) = get_version();
    println!("LLVM version: {major}.{minor}.{patch}");
}

pub(crate) fn get_version() -> (u32, u32, u32) {
    // Can be called without initializing LLVM
    unsafe {
        (llvm::LLVMRustVersionMajor(), llvm::LLVMRustVersionMinor(), llvm::LLVMRustVersionPatch())
    }
}

pub(crate) fn print_passes() {
    // Can be called without initializing LLVM
    unsafe {
        llvm::LLVMRustPrintPasses();
    }
}

fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
    let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
    let mut ret = Vec::with_capacity(len);
    for i in 0..len {
        unsafe {
            let mut feature = ptr::null();
            let mut desc = ptr::null();
            llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
            if feature.is_null() || desc.is_null() {
                bug!("LLVM returned a `null` target feature string");
            }
            let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
                bug!("LLVM returned a non-utf8 feature string: {}", e);
            });
            let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
                bug!("LLVM returned a non-utf8 feature string: {}", e);
            });
            ret.push((feature, desc));
        }
    }
    ret
}

fn print_target_features(out: &mut String, sess: &Session, tm: &llvm::TargetMachine) {
    let mut llvm_target_features = llvm_target_features(tm);
    let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
    let mut rustc_target_features = sess
        .target
        .supported_target_features()
        .iter()
        .filter_map(|(feature, _gate, _implied)| {
            // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these
            // strings.
            let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
            let desc =
                match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
                    Some(index) => {
                        known_llvm_target_features.insert(llvm_feature);
                        llvm_target_features[index].1
                    }
                    None => "",
                };

            Some((*feature, desc))
        })
        .collect::<Vec<_>>();

    // Since we add this at the end ...
    rustc_target_features.extend_from_slice(&[(
        "crt-static",
        "Enables C Run-time Libraries to be statically linked",
    )]);
    // ... we need to sort the list again.
    rustc_target_features.sort();

    llvm_target_features.retain(|(f, _d)| !known_llvm_target_features.contains(f));

    let max_feature_len = llvm_target_features
        .iter()
        .chain(rustc_target_features.iter())
        .map(|(feature, _desc)| feature.len())
        .max()
        .unwrap_or(0);

    writeln!(out, "Features supported by rustc for this target:").unwrap();
    for (feature, desc) in &rustc_target_features {
        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
    }
    writeln!(out, "\nCode-generation features supported by LLVM for this target:").unwrap();
    for (feature, desc) in &llvm_target_features {
        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
    }
    if llvm_target_features.is_empty() {
        writeln!(out, "    Target features listing is not supported by this LLVM version.")
            .unwrap();
    }
    writeln!(out, "\nUse +feature to enable a feature, or -feature to disable it.").unwrap();
    writeln!(out, "For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n")
        .unwrap();
    writeln!(out, "Code-generation features cannot be used in cfg or #[target_feature],").unwrap();
    writeln!(out, "and may be renamed or removed in a future version of LLVM or rustc.\n").unwrap();
}

pub(crate) fn print(req: &PrintRequest, mut out: &mut String, sess: &Session) {
    require_inited();
    let tm = create_informational_target_machine(sess, false);
    match req.kind {
        PrintKind::TargetCPUs => {
            // SAFETY generate a C compatible string from a byte slice to pass
            // the target CPU name into LLVM, the lifetime of the reference is
            // at least as long as the C function
            let cpu_cstring = CString::new(handle_native(sess.target.cpu.as_ref()))
                .unwrap_or_else(|e| bug!("failed to convert to cstring: {}", e));
            unsafe extern "C" fn callback(out: *mut c_void, string: *const c_char, len: usize) {
                let out = unsafe { &mut *(out as *mut &mut String) };
                let bytes = unsafe { slice::from_raw_parts(string as *const u8, len) };
                write!(out, "{}", String::from_utf8_lossy(bytes)).unwrap();
            }
            unsafe {
                llvm::LLVMRustPrintTargetCPUs(
                    &tm,
                    cpu_cstring.as_ptr(),
                    callback,
                    (&raw mut out) as *mut c_void,
                );
            }
        }
        PrintKind::TargetFeatures => print_target_features(out, sess, &tm),
        _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
    }
}

fn handle_native(name: &str) -> &str {
    if name != "native" {
        return name;
    }

    unsafe {
        let mut len = 0;
        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
        str::from_utf8(slice::from_raw_parts(ptr as *const u8, len)).unwrap()
    }
}

pub(crate) fn target_cpu(sess: &Session) -> &str {
    match sess.opts.cg.target_cpu {
        Some(ref name) => handle_native(name),
        None => handle_native(sess.target.cpu.as_ref()),
    }
}

/// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
/// `--target` and similar).
pub(crate) fn global_llvm_features(
    sess: &Session,
    diagnostics: bool,
    only_base_features: bool,
) -> Vec<String> {
    // Features that come earlier are overridden by conflicting features later in the string.
    // Typically we'll want more explicit settings to override the implicit ones, so:
    //
    // * Features from -Ctarget-cpu=*; are overridden by [^1]
    // * Features implied by --target; are overridden by
    // * Features from -Ctarget-feature; are overridden by
    // * function specific features.
    //
    // [^1]: target-cpu=native is handled here, other target-cpu values are handled implicitly
    // through LLVM TargetMachine implementation.
    //
    // FIXME(nagisa): it isn't clear what's the best interaction between features implied by
    // `-Ctarget-cpu` and `--target` are. On one hand, you'd expect CLI arguments to always
    // override anything that's implicit, so e.g. when there's no `--target` flag, features implied
    // the host target are overridden by `-Ctarget-cpu=*`. On the other hand, what about when both
    // `--target` and `-Ctarget-cpu=*` are specified? Both then imply some target features and both
    // flags are specified by the user on the CLI. It isn't as clear-cut which order of precedence
    // should be taken in cases like these.
    let mut features = vec![];

    // -Ctarget-cpu=native
    match sess.opts.cg.target_cpu {
        Some(ref s) if s == "native" => {
            // We have already figured out the actual CPU name with `LLVMRustGetHostCPUName` and set
            // that for LLVM, so the features implied by that CPU name will be available everywhere.
            // However, that is not sufficient: e.g. `skylake` alone is not sufficient to tell if
            // some of the instructions are available or not. So we have to also explicitly ask for
            // the exact set of features available on the host, and enable all of them.
            let features_string = unsafe {
                let ptr = llvm::LLVMGetHostCPUFeatures();
                let features_string = if !ptr.is_null() {
                    CStr::from_ptr(ptr)
                        .to_str()
                        .unwrap_or_else(|e| {
                            bug!("LLVM returned a non-utf8 features string: {}", e);
                        })
                        .to_owned()
                } else {
                    bug!("could not allocate host CPU features, LLVM returned a `null` string");
                };

                llvm::LLVMDisposeMessage(ptr);

                features_string
            };
            features.extend(features_string.split(',').map(String::from));
        }
        Some(_) | None => {}
    };

    // Features implied by an implicit or explicit `--target`.
    features.extend(
        sess.target
            .features
            .split(',')
            .filter(|v| !v.is_empty() && backend_feature_name(sess, v).is_some())
            .map(String::from),
    );

    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
        features.push("+exception-handling".into());
    }

    // -Ctarget-features
    if !only_base_features {
        let supported_features = sess.target.supported_target_features();
        let mut featsmap = FxHashMap::default();

        // insert implied features
        let mut all_rust_features = vec![];
        for feature in sess.opts.cg.target_feature.split(',') {
            match feature.strip_prefix('+') {
                Some(feature) => all_rust_features.extend(
                    UnordSet::from(
                        sess.target
                            .implied_target_features(std::iter::once(Symbol::intern(feature))),
                    )
                    .to_sorted_stable_ord()
                    .iter()
                    .map(|s| format!("+{}", s.as_str())),
                ),
                _ => all_rust_features.push(feature.to_string()),
            }
        }

        let feats = all_rust_features
            .iter()
            .filter_map(|s| {
                let enable_disable = match s.chars().next() {
                    None => return None,
                    Some(c @ ('+' | '-')) => c,
                    Some(_) => {
                        if diagnostics {
                            sess.dcx().emit_warn(UnknownCTargetFeaturePrefix { feature: s });
                        }
                        return None;
                    }
                };

                let feature = backend_feature_name(sess, s)?;
                // Warn against use of LLVM specific feature names and unstable features on the CLI.
                if diagnostics {
                    let feature_state = supported_features.iter().find(|&&(v, _, _)| v == feature);
                    if feature_state.is_none() {
                        let rust_feature =
                            supported_features.iter().find_map(|&(rust_feature, _, _)| {
                                let llvm_features = to_llvm_features(sess, rust_feature)?;
                                if llvm_features.contains(feature)
                                    && !llvm_features.contains(rust_feature)
                                {
                                    Some(rust_feature)
                                } else {
                                    None
                                }
                            });
                        let unknown_feature = if let Some(rust_feature) = rust_feature {
                            UnknownCTargetFeature {
                                feature,
                                rust_feature: PossibleFeature::Some { rust_feature },
                            }
                        } else {
                            UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
                        };
                        sess.dcx().emit_warn(unknown_feature);
                    } else if feature_state
                        .is_some_and(|(_name, feature_gate, _implied)| !feature_gate.is_stable())
                    {
                        // An unstable feature. Warn about using it.
                        sess.dcx().emit_warn(UnstableCTargetFeature { feature });
                    }
                }

                if diagnostics {
                    // FIXME(nagisa): figure out how to not allocate a full hashset here.
                    featsmap.insert(feature, enable_disable == '+');
                }

                // rustc-specific features do not get passed down to LLVM…
                if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
                    return None;
                }

                // ... otherwise though we run through `to_llvm_features` when
                // passing requests down to LLVM. This means that all in-language
                // features also work on the command line instead of having two
                // different names when the LLVM name and the Rust name differ.
                let llvm_feature = to_llvm_features(sess, feature)?;

                Some(
                    std::iter::once(format!(
                        "{}{}",
                        enable_disable, llvm_feature.llvm_feature_name
                    ))
                    .chain(llvm_feature.dependency.into_iter().filter_map(
                        move |feat| match (enable_disable, feat) {
                            ('-' | '+', TargetFeatureFoldStrength::Both(f))
                            | ('+', TargetFeatureFoldStrength::EnableOnly(f)) => {
                                Some(format!("{enable_disable}{f}"))
                            }
                            _ => None,
                        },
                    )),
                )
            })
            .flatten();
        features.extend(feats);

        if diagnostics && let Some(f) = check_tied_features(sess, &featsmap) {
            sess.dcx().emit_err(TargetFeatureDisableOrEnable {
                features: f,
                span: None,
                missing_features: None,
            });
        }
    }

    // -Zfixed-x18
    if sess.opts.unstable_opts.fixed_x18 {
        if sess.target.arch != "aarch64" {
            sess.dcx().emit_fatal(FixedX18InvalidArch { arch: &sess.target.arch });
        } else {
            features.push("+reserve-x18".into());
        }
    }

    features
}

/// Returns a feature name for the given `+feature` or `-feature` string.
///
/// Only allows features that are backend specific (i.e. not [`RUSTC_SPECIFIC_FEATURES`].)
fn backend_feature_name<'a>(sess: &Session, s: &'a str) -> Option<&'a str> {
    // features must start with a `+` or `-`.
    let feature = s
        .strip_prefix(&['+', '-'][..])
        .unwrap_or_else(|| sess.dcx().emit_fatal(InvalidTargetFeaturePrefix { feature: s }));
    if s.is_empty() {
        return None;
    }
    // Rustc-specific feature requests like `+crt-static` or `-crt-static`
    // are not passed down to LLVM.
    if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
        return None;
    }
    Some(feature)
}

pub(crate) fn tune_cpu(sess: &Session) -> Option<&str> {
    let name = sess.opts.unstable_opts.tune_cpu.as_ref()?;
    Some(handle_native(name))
}