rustc_codegen_llvm/
llvm_util.rs

1use std::collections::VecDeque;
2use std::ffi::{CStr, CString};
3use std::fmt::Write;
4use std::path::Path;
5use std::sync::Once;
6use std::{ptr, slice, str};
7
8use libc::c_int;
9use rustc_codegen_ssa::base::wants_wasm_eh;
10use rustc_codegen_ssa::target_features::cfg_target_feature;
11use rustc_codegen_ssa::{TargetConfig, target_features};
12use rustc_data_structures::fx::FxHashSet;
13use rustc_data_structures::small_c_str::SmallCStr;
14use rustc_fs_util::path_to_c_string;
15use rustc_middle::bug;
16use rustc_session::Session;
17use rustc_session::config::{PrintKind, PrintRequest};
18use rustc_target::spec::{Arch, MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
19use smallvec::{SmallVec, smallvec};
20
21use crate::back::write::create_informational_target_machine;
22use crate::{errors, llvm};
23
24static INIT: Once = Once::new();
25
26pub(crate) fn init(sess: &Session) {
27    unsafe {
28        // Before we touch LLVM, make sure that multithreading is enabled.
29        if !llvm::LLVMIsMultithreaded().is_true() {
30            bug!("LLVM compiled without support for threads");
31        }
32        INIT.call_once(|| {
33            configure_llvm(sess);
34        });
35    }
36}
37
38fn require_inited() {
39    if !INIT.is_completed() {
40        bug!("LLVM is not initialized");
41    }
42}
43
44unsafe fn configure_llvm(sess: &Session) {
45    let n_args = sess.opts.cg.llvm_args.len() + sess.target.llvm_args.len();
46    let mut llvm_c_strs = Vec::with_capacity(n_args + 1);
47    let mut llvm_args = Vec::with_capacity(n_args + 1);
48
49    unsafe {
50        llvm::LLVMRustInstallErrorHandlers();
51    }
52    // On Windows, an LLVM assertion will open an Abort/Retry/Ignore dialog
53    // box for the purpose of launching a debugger. However, on CI this will
54    // cause it to hang until it times out, which can take several hours.
55    if std::env::var_os("CI").is_some() {
56        unsafe {
57            llvm::LLVMRustDisableSystemDialogsOnCrash();
58        }
59    }
60
61    fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
62        full_arg.trim().split(|c: char| c == '=' || c.is_whitespace()).next().unwrap_or("")
63    }
64
65    let cg_opts = sess.opts.cg.llvm_args.iter().map(AsRef::as_ref);
66    let tg_opts = sess.target.llvm_args.iter().map(AsRef::as_ref);
67    let sess_args = cg_opts.chain(tg_opts);
68
69    let user_specified_args: FxHashSet<_> =
70        sess_args.clone().map(|s| llvm_arg_to_arg_name(s)).filter(|s| !s.is_empty()).collect();
71
72    {
73        // This adds the given argument to LLVM. Unless `force` is true
74        // user specified arguments are *not* overridden.
75        let mut add = |arg: &str, force: bool| {
76            if force || !user_specified_args.contains(llvm_arg_to_arg_name(arg)) {
77                let s = CString::new(arg).unwrap();
78                llvm_args.push(s.as_ptr());
79                llvm_c_strs.push(s);
80            }
81        };
82        // Set the llvm "program name" to make usage and invalid argument messages more clear.
83        add("rustc -Cllvm-args=\"...\" with", true);
84        if sess.opts.unstable_opts.time_llvm_passes {
85            add("-time-passes", false);
86        }
87        if sess.opts.unstable_opts.print_llvm_passes {
88            add("-debug-pass=Structure", false);
89        }
90        if sess.target.generate_arange_section
91            && !sess.opts.unstable_opts.no_generate_arange_section
92        {
93            add("-generate-arange-section", false);
94        }
95
96        match sess.opts.unstable_opts.merge_functions.unwrap_or(sess.target.merge_functions) {
97            MergeFunctions::Disabled | MergeFunctions::Trampolines => {}
98            MergeFunctions::Aliases => {
99                add("-mergefunc-use-aliases", false);
100            }
101        }
102
103        if wants_wasm_eh(sess) {
104            add("-wasm-enable-eh", false);
105        }
106
107        if sess.target.os == "emscripten"
108            && !sess.opts.unstable_opts.emscripten_wasm_eh
109            && sess.panic_strategy().unwinds()
110        {
111            add("-enable-emscripten-cxx-exceptions", false);
112        }
113
114        // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
115        // during inlining. Unfortunately these may block other optimizations.
116        add("-preserve-alignment-assumptions-during-inlining=false", false);
117
118        // Use non-zero `import-instr-limit` multiplier for cold callsites.
119        add("-import-cold-multiplier=0.1", false);
120
121        if sess.print_llvm_stats() {
122            add("-stats", false);
123        }
124
125        for arg in sess_args {
126            add(&(*arg), true);
127        }
128
129        match (
130            sess.opts.unstable_opts.small_data_threshold,
131            sess.target.small_data_threshold_support(),
132        ) {
133            // Set up the small-data optimization limit for architectures that use
134            // an LLVM argument to control this.
135            (Some(threshold), SmallDataThresholdSupport::LlvmArg(arg)) => {
136                add(&format!("--{arg}={threshold}"), false)
137            }
138            _ => (),
139        };
140    }
141
142    if sess.opts.unstable_opts.llvm_time_trace {
143        unsafe { llvm::LLVMRustTimeTraceProfilerInitialize() };
144    }
145
146    rustc_llvm::initialize_available_targets();
147
148    unsafe { llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int, llvm_args.as_ptr()) };
149}
150
151pub(crate) fn time_trace_profiler_finish(file_name: &Path) {
152    unsafe {
153        let file_name = path_to_c_string(file_name);
154        llvm::LLVMRustTimeTraceProfilerFinish(file_name.as_ptr());
155    }
156}
157
158enum TargetFeatureFoldStrength<'a> {
159    // The feature is only tied when enabling the feature, disabling
160    // this feature shouldn't disable the tied feature.
161    EnableOnly(&'a str),
162    // The feature is tied for both enabling and disabling this feature.
163    Both(&'a str),
164}
165
166impl<'a> TargetFeatureFoldStrength<'a> {
167    fn as_str(&self) -> &'a str {
168        match self {
169            TargetFeatureFoldStrength::EnableOnly(feat) => feat,
170            TargetFeatureFoldStrength::Both(feat) => feat,
171        }
172    }
173}
174
175pub(crate) struct LLVMFeature<'a> {
176    llvm_feature_name: &'a str,
177    dependencies: SmallVec<[TargetFeatureFoldStrength<'a>; 1]>,
178}
179
180impl<'a> LLVMFeature<'a> {
181    fn new(llvm_feature_name: &'a str) -> Self {
182        Self { llvm_feature_name, dependencies: SmallVec::new() }
183    }
184
185    fn with_dependencies(
186        llvm_feature_name: &'a str,
187        dependencies: SmallVec<[TargetFeatureFoldStrength<'a>; 1]>,
188    ) -> Self {
189        Self { llvm_feature_name, dependencies }
190    }
191}
192
193impl<'a> IntoIterator for LLVMFeature<'a> {
194    type Item = &'a str;
195    type IntoIter = impl Iterator<Item = &'a str>;
196
197    fn into_iter(self) -> Self::IntoIter {
198        let dependencies = self.dependencies.into_iter().map(|feat| feat.as_str());
199        std::iter::once(self.llvm_feature_name).chain(dependencies)
200    }
201}
202
203/// Convert a Rust feature name to an LLVM feature name. Returning `None` means the
204/// feature should be skipped, usually because it is not supported by the current
205/// LLVM version.
206///
207/// WARNING: the features after applying `to_llvm_features` must be known
208/// to LLVM or the feature detection code will walk past the end of the feature
209/// array, leading to crashes.
210///
211/// To find a list of LLVM's names, see llvm-project/llvm/lib/Target/{ARCH}/*.td
212/// where `{ARCH}` is the architecture name. Look for instances of `SubtargetFeature`.
213///
214/// Check the current rustc fork of LLVM in the repo at
215/// <https://github.com/rust-lang/llvm-project/>. The commit in use can be found via the
216/// `llvm-project` submodule in <https://github.com/rust-lang/rust/tree/HEAD/src> Though note that
217/// Rust can also be build with an external precompiled version of LLVM which might lead to failures
218/// if the oldest tested / supported LLVM version doesn't yet support the relevant intrinsics.
219pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFeature<'a>> {
220    let (major, _, _) = get_version();
221    match sess.target.arch {
222        Arch::AArch64 | Arch::Arm64EC => {
223            match s {
224                "rcpc2" => Some(LLVMFeature::new("rcpc-immo")),
225                "dpb" => Some(LLVMFeature::new("ccpp")),
226                "dpb2" => Some(LLVMFeature::new("ccdp")),
227                "frintts" => Some(LLVMFeature::new("fptoint")),
228                "fcma" => Some(LLVMFeature::new("complxnum")),
229                "pmuv3" => Some(LLVMFeature::new("perfmon")),
230                "paca" => Some(LLVMFeature::new("pauth")),
231                "pacg" => Some(LLVMFeature::new("pauth")),
232                "flagm2" => Some(LLVMFeature::new("altnzcv")),
233                // Rust ties fp and neon together.
234                "neon" => Some(LLVMFeature::with_dependencies(
235                    "neon",
236                    smallvec![TargetFeatureFoldStrength::Both("fp-armv8")],
237                )),
238                // In LLVM neon implicitly enables fp, but we manually enable
239                // neon when a feature only implicitly enables fp
240                "fhm" => Some(LLVMFeature::new("fp16fml")),
241                "fp16" => Some(LLVMFeature::new("fullfp16")),
242                // Filter out features that are not supported by the current LLVM version
243                "fpmr" => None, // only existed in 18
244                s => Some(LLVMFeature::new(s)),
245            }
246        }
247        Arch::Arm => match s {
248            "fp16" => Some(LLVMFeature::new("fullfp16")),
249            s => Some(LLVMFeature::new(s)),
250        },
251
252        // Filter out features that are not supported by the current LLVM version
253        Arch::LoongArch32 | Arch::LoongArch64 => match s {
254            "32s" if major < 21 => None,
255            s => Some(LLVMFeature::new(s)),
256        },
257        Arch::PowerPC | Arch::PowerPC64 => match s {
258            "power8-crypto" => Some(LLVMFeature::new("crypto")),
259            s => Some(LLVMFeature::new(s)),
260        },
261        Arch::Sparc | Arch::Sparc64 => match s {
262            "leoncasa" => Some(LLVMFeature::new("hasleoncasa")),
263            s => Some(LLVMFeature::new(s)),
264        },
265        Arch::X86 | Arch::X86_64 => {
266            match s {
267                "sse4.2" => Some(LLVMFeature::with_dependencies(
268                    "sse4.2",
269                    smallvec![TargetFeatureFoldStrength::EnableOnly("crc32")],
270                )),
271                "pclmulqdq" => Some(LLVMFeature::new("pclmul")),
272                "rdrand" => Some(LLVMFeature::new("rdrnd")),
273                "bmi1" => Some(LLVMFeature::new("bmi")),
274                "cmpxchg16b" => Some(LLVMFeature::new("cx16")),
275                "lahfsahf" => Some(LLVMFeature::new("sahf")),
276                // Enable the evex512 target feature if an avx512 target feature is enabled.
277                s if s.starts_with("avx512") => Some(LLVMFeature::with_dependencies(
278                    s,
279                    smallvec![TargetFeatureFoldStrength::EnableOnly("evex512")],
280                )),
281                "avx10.1" => Some(LLVMFeature::new("avx10.1-512")),
282                "avx10.2" => Some(LLVMFeature::new("avx10.2-512")),
283                "apxf" => Some(LLVMFeature::with_dependencies(
284                    "egpr",
285                    smallvec![
286                        TargetFeatureFoldStrength::Both("push2pop2"),
287                        TargetFeatureFoldStrength::Both("ppx"),
288                        TargetFeatureFoldStrength::Both("ndd"),
289                        TargetFeatureFoldStrength::Both("ccmp"),
290                        TargetFeatureFoldStrength::Both("cf"),
291                        TargetFeatureFoldStrength::Both("nf"),
292                        TargetFeatureFoldStrength::Both("zu"),
293                    ],
294                )),
295                s => Some(LLVMFeature::new(s)),
296            }
297        }
298        _ => Some(LLVMFeature::new(s)),
299    }
300}
301
302/// Used to generate cfg variables and apply features.
303/// Must express features in the way Rust understands them.
304///
305/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
306pub(crate) fn target_config(sess: &Session) -> TargetConfig {
307    let target_machine = create_informational_target_machine(sess, true);
308
309    let (unstable_target_features, target_features) = cfg_target_feature(
310        sess,
311        |feature| {
312            to_llvm_features(sess, feature)
313                .map(|f| SmallVec::<[&str; 2]>::from_iter(f.into_iter()))
314                .unwrap_or_default()
315        },
316        |feature| {
317            // This closure determines whether the target CPU has the feature according to LLVM. We do
318            // *not* consider the `-Ctarget-feature`s here, as that will be handled later in
319            // `cfg_target_feature`.
320            if let Some(feat) = to_llvm_features(sess, feature) {
321                // All the LLVM features this expands to must be enabled.
322                for llvm_feature in feat {
323                    let cstr = SmallCStr::new(llvm_feature);
324                    // `LLVMRustHasFeature` is moderately expensive. On targets with many
325                    // features (e.g. x86) these calls take a non-trivial fraction of runtime
326                    // when compiling very small programs.
327                    if !unsafe { llvm::LLVMRustHasFeature(target_machine.raw(), cstr.as_ptr()) } {
328                        return false;
329                    }
330                }
331                true
332            } else {
333                false
334            }
335        },
336    );
337
338    let mut cfg = TargetConfig {
339        target_features,
340        unstable_target_features,
341        has_reliable_f16: true,
342        has_reliable_f16_math: true,
343        has_reliable_f128: true,
344        has_reliable_f128_math: true,
345    };
346
347    update_target_reliable_float_cfg(sess, &mut cfg);
348    cfg
349}
350
351/// Determine whether or not experimental float types are reliable based on known bugs.
352fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
353    let target_arch = &sess.target.arch;
354    let target_os = sess.target.options.os.as_ref();
355    let target_env = sess.target.options.env.as_ref();
356    let target_abi = sess.target.options.abi.as_ref();
357    let target_pointer_width = sess.target.pointer_width;
358    let version = get_version();
359    let lt_20_1_1 = version < (20, 1, 1);
360    let lt_21_0_0 = version < (21, 0, 0);
361
362    cfg.has_reliable_f16 = match (target_arch, target_os) {
363        // LLVM crash without neon <https://github.com/llvm/llvm-project/issues/129394> (fixed in llvm20)
364        (Arch::AArch64, _)
365            if !cfg.target_features.iter().any(|f| f.as_str() == "neon") && lt_20_1_1 =>
366        {
367            false
368        }
369        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
370        (Arch::Arm64EC, _) => false,
371        // Selection failure <https://github.com/llvm/llvm-project/issues/50374> (fixed in llvm21)
372        (Arch::S390x, _) if lt_21_0_0 => false,
373        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
374        (Arch::X86_64, "windows") if target_env == "gnu" && target_abi != "llvm" => false,
375        // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
376        (Arch::CSky, _) => false,
377        (Arch::Hexagon, _) if lt_21_0_0 => false, // (fixed in llvm21)
378        (Arch::PowerPC | Arch::PowerPC64, _) => false,
379        (Arch::Sparc | Arch::Sparc64, _) => false,
380        (Arch::Wasm32 | Arch::Wasm64, _) => false,
381        // `f16` support only requires that symbols converting to and from `f32` are available. We
382        // provide these in `compiler-builtins`, so `f16` should be available on all platforms that
383        // do not have other ABI issues or LLVM crashes.
384        _ => true,
385    };
386
387    cfg.has_reliable_f128 = match (target_arch, target_os) {
388        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
389        (Arch::Arm64EC, _) => false,
390        // Selection bug <https://github.com/llvm/llvm-project/issues/96432> (fixed in llvm20)
391        (Arch::Mips64 | Arch::Mips64r6, _) if lt_20_1_1 => false,
392        // Selection bug <https://github.com/llvm/llvm-project/issues/95471>. This issue is closed
393        // but basic math still does not work.
394        (Arch::Nvptx64, _) => false,
395        // Unsupported https://github.com/llvm/llvm-project/issues/121122
396        (Arch::AmdGpu, _) => false,
397        // ABI bugs <https://github.com/rust-lang/rust/issues/125109> et al. (full
398        // list at <https://github.com/rust-lang/rust/issues/116909>)
399        (Arch::PowerPC | Arch::PowerPC64, _) => false,
400        // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
401        (Arch::Sparc, _) => false,
402        // Stack alignment bug <https://github.com/llvm/llvm-project/issues/77401>. NB: tests may
403        // not fail if our compiler-builtins is linked. (fixed in llvm21)
404        (Arch::X86, _) if lt_21_0_0 => false,
405        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
406        (Arch::X86_64, "windows") if target_env == "gnu" && target_abi != "llvm" => false,
407        // There are no known problems on other platforms, so the only requirement is that symbols
408        // are available. `compiler-builtins` provides all symbols required for core `f128`
409        // support, so this should work for everything else.
410        _ => true,
411    };
412
413    // Assume that working `f16` means working `f16` math for most platforms, since
414    // operations just go through `f32`.
415    cfg.has_reliable_f16_math = cfg.has_reliable_f16;
416
417    cfg.has_reliable_f128_math = match (target_arch, target_os) {
418        // LLVM lowers `fp128` math to `long double` symbols even on platforms where
419        // `long double` is not IEEE binary128. See
420        // <https://github.com/llvm/llvm-project/issues/44744>.
421        //
422        // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
423        // (ld is `f64`), anything other than Linux (Windows and MacOS use `f64`), and `x86`
424        // (ld is 80-bit extended precision).
425        //
426        // musl does not implement the symbols required for f128 math at all.
427        _ if target_env == "musl" => false,
428        (Arch::X86_64, _) => false,
429        (_, "linux") if target_pointer_width == 64 => true,
430        _ => false,
431    } && cfg.has_reliable_f128;
432}
433
434pub(crate) fn print_version() {
435    let (major, minor, patch) = get_version();
436    println!("LLVM version: {major}.{minor}.{patch}");
437}
438
439pub(crate) fn get_version() -> (u32, u32, u32) {
440    // Can be called without initializing LLVM
441    unsafe {
442        (llvm::LLVMRustVersionMajor(), llvm::LLVMRustVersionMinor(), llvm::LLVMRustVersionPatch())
443    }
444}
445
446pub(crate) fn print_passes() {
447    // Can be called without initializing LLVM
448    unsafe {
449        llvm::LLVMRustPrintPasses();
450    }
451}
452
453fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
454    let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
455    let mut ret = Vec::with_capacity(len);
456    for i in 0..len {
457        unsafe {
458            let mut feature = ptr::null();
459            let mut desc = ptr::null();
460            llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
461            if feature.is_null() || desc.is_null() {
462                bug!("LLVM returned a `null` target feature string");
463            }
464            let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
465                bug!("LLVM returned a non-utf8 feature string: {}", e);
466            });
467            let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
468                bug!("LLVM returned a non-utf8 feature string: {}", e);
469            });
470            ret.push((feature, desc));
471        }
472    }
473    ret
474}
475
476pub(crate) fn print(req: &PrintRequest, out: &mut String, sess: &Session) {
477    require_inited();
478    let tm = create_informational_target_machine(sess, false);
479    match req.kind {
480        PrintKind::TargetCPUs => print_target_cpus(sess, tm.raw(), out),
481        PrintKind::TargetFeatures => print_target_features(sess, tm.raw(), out),
482        _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
483    }
484}
485
486fn print_target_cpus(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
487    let cpu_names = llvm::build_string(|s| unsafe {
488        llvm::LLVMRustPrintTargetCPUs(&tm, s);
489    })
490    .unwrap();
491
492    struct Cpu<'a> {
493        cpu_name: &'a str,
494        remark: String,
495    }
496    // Compare CPU against current target to label the default.
497    let target_cpu = handle_native(&sess.target.cpu);
498    let make_remark = |cpu_name| {
499        if cpu_name == target_cpu {
500            // FIXME(#132514): This prints the LLVM target string, which can be
501            // different from the Rust target string. Is that intended?
502            let target = &sess.target.llvm_target;
503            format!(
504                " - This is the default target CPU for the current build target (currently {target})."
505            )
506        } else {
507            "".to_owned()
508        }
509    };
510    let mut cpus = cpu_names
511        .lines()
512        .map(|cpu_name| Cpu { cpu_name, remark: make_remark(cpu_name) })
513        .collect::<VecDeque<_>>();
514
515    // Only print the "native" entry when host and target are the same arch,
516    // since otherwise it could be wrong or misleading.
517    if sess.host.arch == sess.target.arch {
518        let host = get_host_cpu_name();
519        cpus.push_front(Cpu {
520            cpu_name: "native",
521            remark: format!(" - Select the CPU of the current host (currently {host})."),
522        });
523    }
524
525    let max_name_width = cpus.iter().map(|cpu| cpu.cpu_name.len()).max().unwrap_or(0);
526    writeln!(out, "Available CPUs for this target:").unwrap();
527    for Cpu { cpu_name, remark } in cpus {
528        // Only pad the CPU name if there's a remark to print after it.
529        let width = if remark.is_empty() { 0 } else { max_name_width };
530        writeln!(out, "    {cpu_name:<width$}{remark}").unwrap();
531    }
532}
533
534fn print_target_features(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
535    let mut llvm_target_features = llvm_target_features(tm);
536    let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
537    let mut rustc_target_features = sess
538        .target
539        .rust_target_features()
540        .iter()
541        .filter_map(|(feature, gate, _implied)| {
542            if !gate.in_cfg() {
543                // Only list (experimentally) supported features.
544                return None;
545            }
546            // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these
547            // strings.
548            let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
549            let desc =
550                match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
551                    Some(index) => {
552                        known_llvm_target_features.insert(llvm_feature);
553                        llvm_target_features[index].1
554                    }
555                    None => "",
556                };
557
558            Some((*feature, desc))
559        })
560        .collect::<Vec<_>>();
561
562    // Since we add this at the end ...
563    rustc_target_features.extend_from_slice(&[(
564        "crt-static",
565        "Enables C Run-time Libraries to be statically linked",
566    )]);
567    // ... we need to sort the list again.
568    rustc_target_features.sort();
569
570    llvm_target_features.retain(|(f, _d)| !known_llvm_target_features.contains(f));
571
572    let max_feature_len = llvm_target_features
573        .iter()
574        .chain(rustc_target_features.iter())
575        .map(|(feature, _desc)| feature.len())
576        .max()
577        .unwrap_or(0);
578
579    writeln!(out, "Features supported by rustc for this target:").unwrap();
580    for (feature, desc) in &rustc_target_features {
581        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
582    }
583    writeln!(out, "\nCode-generation features supported by LLVM for this target:").unwrap();
584    for (feature, desc) in &llvm_target_features {
585        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
586    }
587    if llvm_target_features.is_empty() {
588        writeln!(out, "    Target features listing is not supported by this LLVM version.")
589            .unwrap();
590    }
591    writeln!(out, "\nUse +feature to enable a feature, or -feature to disable it.").unwrap();
592    writeln!(out, "For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n")
593        .unwrap();
594    writeln!(out, "Code-generation features cannot be used in cfg or #[target_feature],").unwrap();
595    writeln!(out, "and may be renamed or removed in a future version of LLVM or rustc.\n").unwrap();
596}
597
598/// Returns the host CPU name, according to LLVM.
599fn get_host_cpu_name() -> &'static str {
600    let mut len = 0;
601    // SAFETY: The underlying C++ global function returns a `StringRef` that
602    // isn't tied to any particular backing buffer, so it must be 'static.
603    let slice: &'static [u8] = unsafe {
604        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
605        assert!(!ptr.is_null());
606        slice::from_raw_parts(ptr, len)
607    };
608    str::from_utf8(slice).expect("host CPU name should be UTF-8")
609}
610
611/// If the given string is `"native"`, returns the host CPU name according to
612/// LLVM. Otherwise, the string is returned as-is.
613fn handle_native(cpu_name: &str) -> &str {
614    match cpu_name {
615        "native" => get_host_cpu_name(),
616        _ => cpu_name,
617    }
618}
619
620pub(crate) fn target_cpu(sess: &Session) -> &str {
621    let cpu_name = sess.opts.cg.target_cpu.as_deref().unwrap_or_else(|| &sess.target.cpu);
622    handle_native(cpu_name)
623}
624
625/// The target features for compiler flags other than `-Ctarget-features`.
626fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
627    target_features::retpoline_features_by_flags(sess, features);
628
629    // -Zfixed-x18
630    if sess.opts.unstable_opts.fixed_x18 {
631        if sess.target.arch != Arch::AArch64 {
632            sess.dcx().emit_fatal(errors::FixedX18InvalidArch { arch: sess.target.arch.desc() });
633        } else {
634            features.push("+reserve-x18".into());
635        }
636    }
637}
638
639/// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
640/// `--target` and similar).
641pub(crate) fn global_llvm_features(sess: &Session, only_base_features: bool) -> Vec<String> {
642    // Features that come earlier are overridden by conflicting features later in the string.
643    // Typically we'll want more explicit settings to override the implicit ones, so:
644    //
645    // * Features from -Ctarget-cpu=*; are overridden by [^1]
646    // * Features implied by --target; are overridden by
647    // * Features from -Ctarget-feature; are overridden by
648    // * function specific features.
649    //
650    // [^1]: target-cpu=native is handled here, other target-cpu values are handled implicitly
651    // through LLVM TargetMachine implementation.
652    //
653    // FIXME(nagisa): it isn't clear what's the best interaction between features implied by
654    // `-Ctarget-cpu` and `--target` are. On one hand, you'd expect CLI arguments to always
655    // override anything that's implicit, so e.g. when there's no `--target` flag, features implied
656    // the host target are overridden by `-Ctarget-cpu=*`. On the other hand, what about when both
657    // `--target` and `-Ctarget-cpu=*` are specified? Both then imply some target features and both
658    // flags are specified by the user on the CLI. It isn't as clear-cut which order of precedence
659    // should be taken in cases like these.
660    let mut features = vec![];
661
662    // -Ctarget-cpu=native
663    match sess.opts.cg.target_cpu {
664        Some(ref s) if s == "native" => {
665            // We have already figured out the actual CPU name with `LLVMRustGetHostCPUName` and set
666            // that for LLVM, so the features implied by that CPU name will be available everywhere.
667            // However, that is not sufficient: e.g. `skylake` alone is not sufficient to tell if
668            // some of the instructions are available or not. So we have to also explicitly ask for
669            // the exact set of features available on the host, and enable all of them.
670            let features_string = unsafe {
671                let ptr = llvm::LLVMGetHostCPUFeatures();
672                let features_string = if !ptr.is_null() {
673                    CStr::from_ptr(ptr)
674                        .to_str()
675                        .unwrap_or_else(|e| {
676                            bug!("LLVM returned a non-utf8 features string: {}", e);
677                        })
678                        .to_owned()
679                } else {
680                    bug!("could not allocate host CPU features, LLVM returned a `null` string");
681                };
682
683                llvm::LLVMDisposeMessage(ptr);
684
685                features_string
686            };
687            features.extend(features_string.split(',').map(String::from));
688        }
689        Some(_) | None => {}
690    };
691
692    // Features implied by an implicit or explicit `--target`.
693    features.extend(sess.target.features.split(',').filter(|v| !v.is_empty()).map(String::from));
694
695    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
696        features.push("+exception-handling".into());
697    }
698
699    // -Ctarget-features
700    if !only_base_features {
701        target_features::flag_to_backend_features(sess, |feature, enable| {
702            let enable_disable = if enable { '+' } else { '-' };
703            // We run through `to_llvm_features` when
704            // passing requests down to LLVM. This means that all in-language
705            // features also work on the command line instead of having two
706            // different names when the LLVM name and the Rust name differ.
707            let Some(llvm_feature) = to_llvm_features(sess, feature) else { return };
708
709            features.extend(
710                std::iter::once(format!("{}{}", enable_disable, llvm_feature.llvm_feature_name))
711                    .chain(llvm_feature.dependencies.into_iter().filter_map(move |feat| {
712                        match (enable, feat) {
713                            (_, TargetFeatureFoldStrength::Both(f))
714                            | (true, TargetFeatureFoldStrength::EnableOnly(f)) => {
715                                Some(format!("{enable_disable}{f}"))
716                            }
717                            _ => None,
718                        }
719                    })),
720            )
721        });
722    }
723
724    // We add this in the "base target" so that these show up in `sess.unstable_target_features`.
725    llvm_features_by_flags(sess, &mut features);
726
727    features
728}
729
730pub(crate) fn tune_cpu(sess: &Session) -> Option<&str> {
731    let name = sess.opts.unstable_opts.tune_cpu.as_ref()?;
732    Some(handle_native(name))
733}
rustc_codegen_llvm/llvm_util.rs

rustc_codegen_llvm/
llvm_util.rs
