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::{
19    Abi, Arch, Env, MergeFunctions, Os, PanicStrategy, SmallDataThresholdSupport,
20};
21use smallvec::{SmallVec, smallvec};
22
23use crate::back::write::create_informational_target_machine;
24use crate::{errors, llvm};
25
26static INIT: Once = Once::new();
27
28pub(crate) fn init(sess: &Session) {
29    unsafe {
30        // Before we touch LLVM, make sure that multithreading is enabled.
31        if !llvm::LLVMIsMultithreaded().is_true() {
32            bug!("LLVM compiled without support for threads");
33        }
34        INIT.call_once(|| {
35            configure_llvm(sess);
36        });
37    }
38}
39
40fn require_inited() {
41    if !INIT.is_completed() {
42        bug!("LLVM is not initialized");
43    }
44}
45
46unsafe fn configure_llvm(sess: &Session) {
47    let n_args = sess.opts.cg.llvm_args.len() + sess.target.llvm_args.len();
48    let mut llvm_c_strs = Vec::with_capacity(n_args + 1);
49    let mut llvm_args = Vec::with_capacity(n_args + 1);
50
51    unsafe {
52        llvm::LLVMRustInstallErrorHandlers();
53    }
54    // On Windows, an LLVM assertion will open an Abort/Retry/Ignore dialog
55    // box for the purpose of launching a debugger. However, on CI this will
56    // cause it to hang until it times out, which can take several hours.
57    if std::env::var_os("CI").is_some() {
58        unsafe {
59            llvm::LLVMRustDisableSystemDialogsOnCrash();
60        }
61    }
62
63    fn llvm_arg_to_arg_name(full_arg: &str) -> &str {
64        full_arg.trim().split(|c: char| c == '=' || c.is_whitespace()).next().unwrap_or("")
65    }
66
67    let cg_opts = sess.opts.cg.llvm_args.iter().map(AsRef::as_ref);
68    let tg_opts = sess.target.llvm_args.iter().map(AsRef::as_ref);
69    let sess_args = cg_opts.chain(tg_opts);
70
71    let user_specified_args: FxHashSet<_> =
72        sess_args.clone().map(|s| llvm_arg_to_arg_name(s)).filter(|s| !s.is_empty()).collect();
73
74    {
75        // This adds the given argument to LLVM. Unless `force` is true
76        // user specified arguments are *not* overridden.
77        let mut add = |arg: &str, force: bool| {
78            if force || !user_specified_args.contains(llvm_arg_to_arg_name(arg)) {
79                let s = CString::new(arg).unwrap();
80                llvm_args.push(s.as_ptr());
81                llvm_c_strs.push(s);
82            }
83        };
84        // Set the llvm "program name" to make usage and invalid argument messages more clear.
85        add("rustc -Cllvm-args=\"...\" with", true);
86        if sess.opts.unstable_opts.time_llvm_passes {
87            add("-time-passes", false);
88        }
89        if sess.opts.unstable_opts.print_llvm_passes {
90            add("-debug-pass=Structure", false);
91        }
92        if sess.target.generate_arange_section
93            && !sess.opts.unstable_opts.no_generate_arange_section
94        {
95            add("-generate-arange-section", false);
96        }
97
98        match sess.opts.unstable_opts.merge_functions.unwrap_or(sess.target.merge_functions) {
99            MergeFunctions::Disabled | MergeFunctions::Trampolines => {}
100            MergeFunctions::Aliases => {
101                add("-mergefunc-use-aliases", false);
102            }
103        }
104
105        if wants_wasm_eh(sess) {
106            add("-wasm-enable-eh", false);
107        }
108
109        if sess.target.os == Os::Emscripten
110            && !sess.opts.unstable_opts.emscripten_wasm_eh
111            && sess.panic_strategy().unwinds()
112        {
113            add("-enable-emscripten-cxx-exceptions", false);
114        }
115
116        // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes
117        // during inlining. Unfortunately these may block other optimizations.
118        add("-preserve-alignment-assumptions-during-inlining=false", false);
119
120        // Use non-zero `import-instr-limit` multiplier for cold callsites.
121        add("-import-cold-multiplier=0.1", false);
122
123        if sess.print_llvm_stats() {
124            add("-stats", false);
125        }
126
127        for arg in sess_args {
128            add(&(*arg), true);
129        }
130
131        match (
132            sess.opts.unstable_opts.small_data_threshold,
133            sess.target.small_data_threshold_support(),
134        ) {
135            // Set up the small-data optimization limit for architectures that use
136            // an LLVM argument to control this.
137            (Some(threshold), SmallDataThresholdSupport::LlvmArg(arg)) => {
138                add(&format!("--{arg}={threshold}"), false)
139            }
140            _ => (),
141        };
142    }
143
144    if sess.opts.unstable_opts.llvm_time_trace {
145        unsafe { llvm::LLVMRustTimeTraceProfilerInitialize() };
146    }
147
148    rustc_llvm::initialize_available_targets();
149
150    unsafe { llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int, llvm_args.as_ptr()) };
151}
152
153pub(crate) fn time_trace_profiler_finish(file_name: &Path) {
154    unsafe {
155        let file_name = path_to_c_string(file_name);
156        llvm::LLVMRustTimeTraceProfilerFinish(file_name.as_ptr());
157    }
158}
159
160enum TargetFeatureFoldStrength<'a> {
161    // The feature is only tied when enabling the feature, disabling
162    // this feature shouldn't disable the tied feature.
163    EnableOnly(&'a str),
164    // The feature is tied for both enabling and disabling this feature.
165    Both(&'a str),
166}
167
168impl<'a> TargetFeatureFoldStrength<'a> {
169    fn as_str(&self) -> &'a str {
170        match self {
171            TargetFeatureFoldStrength::EnableOnly(feat) => feat,
172            TargetFeatureFoldStrength::Both(feat) => feat,
173        }
174    }
175}
176
177pub(crate) struct LLVMFeature<'a> {
178    llvm_feature_name: &'a str,
179    dependencies: SmallVec<[TargetFeatureFoldStrength<'a>; 1]>,
180}
181
182impl<'a> LLVMFeature<'a> {
183    fn new(llvm_feature_name: &'a str) -> Self {
184        Self { llvm_feature_name, dependencies: SmallVec::new() }
185    }
186
187    fn with_dependencies(
188        llvm_feature_name: &'a str,
189        dependencies: SmallVec<[TargetFeatureFoldStrength<'a>; 1]>,
190    ) -> Self {
191        Self { llvm_feature_name, dependencies }
192    }
193}
194
195impl<'a> IntoIterator for LLVMFeature<'a> {
196    type Item = &'a str;
197    type IntoIter = impl Iterator<Item = &'a str>;
198
199    fn into_iter(self) -> Self::IntoIter {
200        let dependencies = self.dependencies.into_iter().map(|feat| feat.as_str());
201        std::iter::once(self.llvm_feature_name).chain(dependencies)
202    }
203}
204
205/// Convert a Rust feature name to an LLVM feature name. Returning `None` means the
206/// feature should be skipped, usually because it is not supported by the current
207/// LLVM version.
208///
209/// WARNING: the features after applying `to_llvm_features` must be known
210/// to LLVM or the feature detection code will walk past the end of the feature
211/// array, leading to crashes.
212///
213/// To find a list of LLVM's names, see llvm-project/llvm/lib/Target/{ARCH}/*.td
214/// where `{ARCH}` is the architecture name. Look for instances of `SubtargetFeature`.
215///
216/// Check the current rustc fork of LLVM in the repo at
217/// <https://github.com/rust-lang/llvm-project/>. The commit in use can be found via the
218/// `llvm-project` submodule in <https://github.com/rust-lang/rust/tree/HEAD/src> Though note that
219/// Rust can also be build with an external precompiled version of LLVM which might lead to failures
220/// if the oldest tested / supported LLVM version doesn't yet support the relevant intrinsics.
221pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFeature<'a>> {
222    let (major, _, _) = get_version();
223    match sess.target.arch {
224        Arch::AArch64 | Arch::Arm64EC => {
225            match s {
226                "rcpc2" => Some(LLVMFeature::new("rcpc-immo")),
227                "dpb" => Some(LLVMFeature::new("ccpp")),
228                "dpb2" => Some(LLVMFeature::new("ccdp")),
229                "frintts" => Some(LLVMFeature::new("fptoint")),
230                "fcma" => Some(LLVMFeature::new("complxnum")),
231                "pmuv3" => Some(LLVMFeature::new("perfmon")),
232                "paca" => Some(LLVMFeature::new("pauth")),
233                "pacg" => Some(LLVMFeature::new("pauth")),
234                "flagm2" => Some(LLVMFeature::new("altnzcv")),
235                // Rust ties fp and neon together.
236                "neon" => Some(LLVMFeature::with_dependencies(
237                    "neon",
238                    smallvec![TargetFeatureFoldStrength::Both("fp-armv8")],
239                )),
240                // In LLVM neon implicitly enables fp, but we manually enable
241                // neon when a feature only implicitly enables fp
242                "fhm" => Some(LLVMFeature::new("fp16fml")),
243                "fp16" => Some(LLVMFeature::new("fullfp16")),
244                // Filter out features that are not supported by the current LLVM version
245                "fpmr" => None, // only existed in 18
246                // Withdrawn by ARM; removed from LLVM in 22
247                "tme" if major >= 22 => None,
248                s => Some(LLVMFeature::new(s)),
249            }
250        }
251        Arch::Arm => match s {
252            "fp16" => Some(LLVMFeature::new("fullfp16")),
253            s => Some(LLVMFeature::new(s)),
254        },
255
256        // Filter out features that are not supported by the current LLVM version
257        Arch::LoongArch32 | Arch::LoongArch64 => match s {
258            "32s" if major < 21 => None,
259            s => Some(LLVMFeature::new(s)),
260        },
261        Arch::PowerPC | Arch::PowerPC64 => match s {
262            "power8-crypto" => Some(LLVMFeature::new("crypto")),
263            s => Some(LLVMFeature::new(s)),
264        },
265        Arch::Sparc | Arch::Sparc64 => match s {
266            "leoncasa" => Some(LLVMFeature::new("hasleoncasa")),
267            s => Some(LLVMFeature::new(s)),
268        },
269        Arch::X86 | Arch::X86_64 => {
270            match s {
271                "sse4.2" => Some(LLVMFeature::with_dependencies(
272                    "sse4.2",
273                    smallvec![TargetFeatureFoldStrength::EnableOnly("crc32")],
274                )),
275                "pclmulqdq" => Some(LLVMFeature::new("pclmul")),
276                "rdrand" => Some(LLVMFeature::new("rdrnd")),
277                "bmi1" => Some(LLVMFeature::new("bmi")),
278                "cmpxchg16b" => Some(LLVMFeature::new("cx16")),
279                "lahfsahf" => Some(LLVMFeature::new("sahf")),
280                // Enable the evex512 target feature if an avx512 target feature is enabled.
281                s if s.starts_with("avx512") => Some(LLVMFeature::with_dependencies(
282                    s,
283                    smallvec![TargetFeatureFoldStrength::EnableOnly("evex512")],
284                )),
285                "avx10.1" => Some(LLVMFeature::new("avx10.1-512")),
286                "avx10.2" => Some(LLVMFeature::new("avx10.2-512")),
287                "apxf" => Some(LLVMFeature::with_dependencies(
288                    "egpr",
289                    smallvec![
290                        TargetFeatureFoldStrength::Both("push2pop2"),
291                        TargetFeatureFoldStrength::Both("ppx"),
292                        TargetFeatureFoldStrength::Both("ndd"),
293                        TargetFeatureFoldStrength::Both("ccmp"),
294                        TargetFeatureFoldStrength::Both("cf"),
295                        TargetFeatureFoldStrength::Both("nf"),
296                        TargetFeatureFoldStrength::Both("zu"),
297                    ],
298                )),
299                s => Some(LLVMFeature::new(s)),
300            }
301        }
302        _ => Some(LLVMFeature::new(s)),
303    }
304}
305
306/// Used to generate cfg variables and apply features.
307/// Must express features in the way Rust understands them.
308///
309/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
310pub(crate) fn target_config(sess: &Session) -> TargetConfig {
311    let target_machine = create_informational_target_machine(sess, true);
312
313    let (unstable_target_features, target_features) = cfg_target_feature(
314        sess,
315        |feature| {
316            to_llvm_features(sess, feature)
317                .map(|f| SmallVec::<[&str; 2]>::from_iter(f.into_iter()))
318                .unwrap_or_default()
319        },
320        |feature| {
321            // This closure determines whether the target CPU has the feature according to LLVM. We do
322            // *not* consider the `-Ctarget-feature`s here, as that will be handled later in
323            // `cfg_target_feature`.
324            if let Some(feat) = to_llvm_features(sess, feature) {
325                // All the LLVM features this expands to must be enabled.
326                for llvm_feature in feat {
327                    let cstr = SmallCStr::new(llvm_feature);
328                    // `LLVMRustHasFeature` is moderately expensive. On targets with many
329                    // features (e.g. x86) these calls take a non-trivial fraction of runtime
330                    // when compiling very small programs.
331                    if !unsafe { llvm::LLVMRustHasFeature(target_machine.raw(), cstr.as_ptr()) } {
332                        return false;
333                    }
334                }
335                true
336            } else {
337                false
338            }
339        },
340    );
341
342    let mut cfg = TargetConfig {
343        target_features,
344        unstable_target_features,
345        has_reliable_f16: true,
346        has_reliable_f16_math: true,
347        has_reliable_f128: true,
348        has_reliable_f128_math: true,
349    };
350
351    update_target_reliable_float_cfg(sess, &mut cfg);
352    cfg
353}
354
355/// Determine whether or not experimental float types are reliable based on known bugs.
356fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
357    let target_arch = &sess.target.arch;
358    let target_os = &sess.target.options.os;
359    let target_env = &sess.target.options.env;
360    let target_abi = &sess.target.options.abi;
361    let target_pointer_width = sess.target.pointer_width;
362    let version = get_version();
363    let lt_20_1_1 = version < (20, 1, 1);
364    let lt_21_0_0 = version < (21, 0, 0);
365
366    cfg.has_reliable_f16 = match (target_arch, target_os) {
367        // LLVM crash without neon <https://github.com/llvm/llvm-project/issues/129394> (fixed in llvm20)
368        (Arch::AArch64, _)
369            if !cfg.target_features.iter().any(|f| f.as_str() == "neon") && lt_20_1_1 =>
370        {
371            false
372        }
373        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
374        (Arch::Arm64EC, _) => false,
375        // Selection failure <https://github.com/llvm/llvm-project/issues/50374> (fixed in llvm21)
376        (Arch::S390x, _) if lt_21_0_0 => false,
377        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
378        (Arch::X86_64, Os::Windows) if *target_env == Env::Gnu && *target_abi != Abi::Llvm => false,
379        // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
380        (Arch::CSky, _) => false,
381        (Arch::Hexagon, _) if lt_21_0_0 => false, // (fixed in llvm21)
382        (Arch::PowerPC | Arch::PowerPC64, _) => false,
383        (Arch::Sparc | Arch::Sparc64, _) => false,
384        (Arch::Wasm32 | Arch::Wasm64, _) => false,
385        // `f16` support only requires that symbols converting to and from `f32` are available. We
386        // provide these in `compiler-builtins`, so `f16` should be available on all platforms that
387        // do not have other ABI issues or LLVM crashes.
388        _ => true,
389    };
390
391    cfg.has_reliable_f128 = match (target_arch, target_os) {
392        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
393        (Arch::Arm64EC, _) => false,
394        // Selection bug <https://github.com/llvm/llvm-project/issues/96432> (fixed in llvm20)
395        (Arch::Mips64 | Arch::Mips64r6, _) if lt_20_1_1 => false,
396        // Selection bug <https://github.com/llvm/llvm-project/issues/95471>. This issue is closed
397        // but basic math still does not work.
398        (Arch::Nvptx64, _) => false,
399        // Unsupported https://github.com/llvm/llvm-project/issues/121122
400        (Arch::AmdGpu, _) => false,
401        // ABI bugs <https://github.com/rust-lang/rust/issues/125109> et al. (full
402        // list at <https://github.com/rust-lang/rust/issues/116909>)
403        (Arch::PowerPC | Arch::PowerPC64, _) => false,
404        // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
405        (Arch::Sparc, _) => false,
406        // Stack alignment bug <https://github.com/llvm/llvm-project/issues/77401>. NB: tests may
407        // not fail if our compiler-builtins is linked. (fixed in llvm21)
408        (Arch::X86, _) if lt_21_0_0 => false,
409        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
410        (Arch::X86_64, Os::Windows) if *target_env == Env::Gnu && *target_abi != Abi::Llvm => false,
411        // There are no known problems on other platforms, so the only requirement is that symbols
412        // are available. `compiler-builtins` provides all symbols required for core `f128`
413        // support, so this should work for everything else.
414        _ => true,
415    };
416
417    // Assume that working `f16` means working `f16` math for most platforms, since
418    // operations just go through `f32`.
419    cfg.has_reliable_f16_math = cfg.has_reliable_f16;
420
421    cfg.has_reliable_f128_math = match (target_arch, target_os) {
422        // LLVM lowers `fp128` math to `long double` symbols even on platforms where
423        // `long double` is not IEEE binary128. See
424        // <https://github.com/llvm/llvm-project/issues/44744>.
425        //
426        // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
427        // (ld is `f64`), anything other than Linux (Windows and MacOS use `f64`), and `x86`
428        // (ld is 80-bit extended precision).
429        //
430        // musl does not implement the symbols required for f128 math at all.
431        _ if *target_env == Env::Musl => false,
432        (Arch::X86_64, _) => false,
433        (_, Os::Linux) if target_pointer_width == 64 => true,
434        _ => false,
435    } && cfg.has_reliable_f128;
436}
437
438pub(crate) fn print_version() {
439    let (major, minor, patch) = get_version();
440    println!("LLVM version: {major}.{minor}.{patch}");
441}
442
443pub(crate) fn get_version() -> (u32, u32, u32) {
444    // Can be called without initializing LLVM
445    unsafe {
446        (llvm::LLVMRustVersionMajor(), llvm::LLVMRustVersionMinor(), llvm::LLVMRustVersionPatch())
447    }
448}
449
450pub(crate) fn print_passes() {
451    // Can be called without initializing LLVM
452    unsafe {
453        llvm::LLVMRustPrintPasses();
454    }
455}
456
457fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
458    let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
459    let mut ret = Vec::with_capacity(len);
460    for i in 0..len {
461        unsafe {
462            let mut feature = ptr::null();
463            let mut desc = ptr::null();
464            llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
465            if feature.is_null() || desc.is_null() {
466                bug!("LLVM returned a `null` target feature string");
467            }
468            let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
469                bug!("LLVM returned a non-utf8 feature string: {}", e);
470            });
471            let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
472                bug!("LLVM returned a non-utf8 feature string: {}", e);
473            });
474            ret.push((feature, desc));
475        }
476    }
477    ret
478}
479
480pub(crate) fn print(req: &PrintRequest, out: &mut String, sess: &Session) {
481    require_inited();
482    let tm = create_informational_target_machine(sess, false);
483    match req.kind {
484        PrintKind::TargetCPUs => print_target_cpus(sess, tm.raw(), out),
485        PrintKind::TargetFeatures => print_target_features(sess, tm.raw(), out),
486        _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
487    }
488}
489
490fn print_target_cpus(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
491    let cpu_names = llvm::build_string(|s| unsafe {
492        llvm::LLVMRustPrintTargetCPUs(&tm, s);
493    })
494    .unwrap();
495
496    struct Cpu<'a> {
497        cpu_name: &'a str,
498        remark: String,
499    }
500    // Compare CPU against current target to label the default.
501    let target_cpu = handle_native(&sess.target.cpu);
502    let make_remark = |cpu_name| {
503        if cpu_name == target_cpu {
504            // FIXME(#132514): This prints the LLVM target string, which can be
505            // different from the Rust target string. Is that intended?
506            let target = &sess.target.llvm_target;
507            format!(
508                " - This is the default target CPU for the current build target (currently {target})."
509            )
510        } else {
511            "".to_owned()
512        }
513    };
514    let mut cpus = cpu_names
515        .lines()
516        .map(|cpu_name| Cpu { cpu_name, remark: make_remark(cpu_name) })
517        .collect::<VecDeque<_>>();
518
519    // Only print the "native" entry when host and target are the same arch,
520    // since otherwise it could be wrong or misleading.
521    if sess.host.arch == sess.target.arch {
522        let host = get_host_cpu_name();
523        cpus.push_front(Cpu {
524            cpu_name: "native",
525            remark: format!(" - Select the CPU of the current host (currently {host})."),
526        });
527    }
528
529    let max_name_width = cpus.iter().map(|cpu| cpu.cpu_name.len()).max().unwrap_or(0);
530    writeln!(out, "Available CPUs for this target:").unwrap();
531    for Cpu { cpu_name, remark } in cpus {
532        // Only pad the CPU name if there's a remark to print after it.
533        let width = if remark.is_empty() { 0 } else { max_name_width };
534        writeln!(out, "    {cpu_name:<width$}{remark}").unwrap();
535    }
536}
537
538fn print_target_features(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
539    let mut llvm_target_features = llvm_target_features(tm);
540    let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
541    let mut rustc_target_features = sess
542        .target
543        .rust_target_features()
544        .iter()
545        .filter_map(|(feature, gate, _implied)| {
546            if !gate.in_cfg() {
547                // Only list (experimentally) supported features.
548                return None;
549            }
550            // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these
551            // strings.
552            let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
553            let desc =
554                match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
555                    Some(index) => {
556                        known_llvm_target_features.insert(llvm_feature);
557                        llvm_target_features[index].1
558                    }
559                    None => "",
560                };
561
562            Some((*feature, desc))
563        })
564        .collect::<Vec<_>>();
565
566    // Since we add this at the end ...
567    rustc_target_features.extend_from_slice(&[(
568        "crt-static",
569        "Enables C Run-time Libraries to be statically linked",
570    )]);
571    // ... we need to sort the list again.
572    rustc_target_features.sort();
573
574    llvm_target_features.retain(|(f, _d)| !known_llvm_target_features.contains(f));
575
576    let max_feature_len = llvm_target_features
577        .iter()
578        .chain(rustc_target_features.iter())
579        .map(|(feature, _desc)| feature.len())
580        .max()
581        .unwrap_or(0);
582
583    writeln!(out, "Features supported by rustc for this target:").unwrap();
584    for (feature, desc) in &rustc_target_features {
585        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
586    }
587    writeln!(out, "\nCode-generation features supported by LLVM for this target:").unwrap();
588    for (feature, desc) in &llvm_target_features {
589        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
590    }
591    if llvm_target_features.is_empty() {
592        writeln!(out, "    Target features listing is not supported by this LLVM version.")
593            .unwrap();
594    }
595    writeln!(out, "\nUse +feature to enable a feature, or -feature to disable it.").unwrap();
596    writeln!(out, "For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n")
597        .unwrap();
598    writeln!(out, "Code-generation features cannot be used in cfg or #[target_feature],").unwrap();
599    writeln!(out, "and may be renamed or removed in a future version of LLVM or rustc.\n").unwrap();
600}
601
602/// Returns the host CPU name, according to LLVM.
603fn get_host_cpu_name() -> &'static str {
604    let mut len = 0;
605    // SAFETY: The underlying C++ global function returns a `StringRef` that
606    // isn't tied to any particular backing buffer, so it must be 'static.
607    let slice: &'static [u8] = unsafe {
608        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
609        assert!(!ptr.is_null());
610        slice::from_raw_parts(ptr, len)
611    };
612    str::from_utf8(slice).expect("host CPU name should be UTF-8")
613}
614
615/// If the given string is `"native"`, returns the host CPU name according to
616/// LLVM. Otherwise, the string is returned as-is.
617fn handle_native(cpu_name: &str) -> &str {
618    match cpu_name {
619        "native" => get_host_cpu_name(),
620        _ => cpu_name,
621    }
622}
623
624pub(crate) fn target_cpu(sess: &Session) -> &str {
625    let cpu_name = sess.opts.cg.target_cpu.as_deref().unwrap_or_else(|| &sess.target.cpu);
626    handle_native(cpu_name)
627}
628
629/// The target features for compiler flags other than `-Ctarget-features`.
630fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
631    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
632        features.push("+exception-handling".into());
633    }
634
635    target_features::retpoline_features_by_flags(sess, features);
636
637    // -Zfixed-x18
638    if sess.opts.unstable_opts.fixed_x18 {
639        if sess.target.arch != Arch::AArch64 {
640            sess.dcx().emit_fatal(errors::FixedX18InvalidArch { arch: sess.target.arch.desc() });
641        } else {
642            features.push("+reserve-x18".into());
643        }
644    }
645}
646
647/// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
648/// `--target` and similar).
649pub(crate) fn global_llvm_features(sess: &Session, only_base_features: bool) -> Vec<String> {
650    // Features that come earlier are overridden by conflicting features later in the string.
651    // Typically we'll want more explicit settings to override the implicit ones, so:
652    //
653    // * Features from -Ctarget-cpu=*; are overridden by [^1]
654    // * Features implied by --target; are overridden by
655    // * Features from -Ctarget-feature; are overridden by
656    // * function specific features.
657    //
658    // [^1]: target-cpu=native is handled here, other target-cpu values are handled implicitly
659    // through LLVM TargetMachine implementation.
660    //
661    // FIXME(nagisa): it isn't clear what's the best interaction between features implied by
662    // `-Ctarget-cpu` and `--target` are. On one hand, you'd expect CLI arguments to always
663    // override anything that's implicit, so e.g. when there's no `--target` flag, features implied
664    // the host target are overridden by `-Ctarget-cpu=*`. On the other hand, what about when both
665    // `--target` and `-Ctarget-cpu=*` are specified? Both then imply some target features and both
666    // flags are specified by the user on the CLI. It isn't as clear-cut which order of precedence
667    // should be taken in cases like these.
668    let mut features = vec![];
669
670    // -Ctarget-cpu=native
671    match sess.opts.cg.target_cpu {
672        Some(ref s) if s == "native" => {
673            // We have already figured out the actual CPU name with `LLVMRustGetHostCPUName` and set
674            // that for LLVM, so the features implied by that CPU name will be available everywhere.
675            // However, that is not sufficient: e.g. `skylake` alone is not sufficient to tell if
676            // some of the instructions are available or not. So we have to also explicitly ask for
677            // the exact set of features available on the host, and enable all of them.
678            let features_string = unsafe {
679                let ptr = llvm::LLVMGetHostCPUFeatures();
680                let features_string = if !ptr.is_null() {
681                    CStr::from_ptr(ptr)
682                        .to_str()
683                        .unwrap_or_else(|e| {
684                            bug!("LLVM returned a non-utf8 features string: {}", e);
685                        })
686                        .to_owned()
687                } else {
688                    bug!("could not allocate host CPU features, LLVM returned a `null` string");
689                };
690
691                llvm::LLVMDisposeMessage(ptr);
692
693                features_string
694            };
695            features.extend(features_string.split(',').map(String::from));
696        }
697        Some(_) | None => {}
698    };
699
700    let mut extend_backend_features = |feature: &str, enable: bool| {
701        let enable_disable = if enable { '+' } else { '-' };
702        // We run through `to_llvm_features` when
703        // passing requests down to LLVM. This means that all in-language
704        // features also work on the command line instead of having two
705        // different names when the LLVM name and the Rust name differ.
706        let Some(llvm_feature) = to_llvm_features(sess, feature) else { return };
707
708        features.extend(
709            std::iter::once(format!("{}{}", enable_disable, llvm_feature.llvm_feature_name)).chain(
710                llvm_feature.dependencies.into_iter().filter_map(move |feat| {
711                    match (enable, feat) {
712                        (_, TargetFeatureFoldStrength::Both(f))
713                        | (true, TargetFeatureFoldStrength::EnableOnly(f)) => {
714                            Some(format!("{enable_disable}{f}"))
715                        }
716                        _ => None,
717                    }
718                }),
719            ),
720        );
721    };
722
723    // Features implied by an implicit or explicit `--target`.
724    target_features::target_spec_to_backend_features(sess, &mut extend_backend_features);
725
726    // -Ctarget-features
727    if !only_base_features {
728        target_features::flag_to_backend_features(sess, extend_backend_features);
729    }
730
731    // We add this in the "base target" so that these show up in `sess.unstable_target_features`.
732    llvm_features_by_flags(sess, &mut features);
733
734    features
735}
736
737pub(crate) fn tune_cpu(sess: &Session) -> Option<&str> {
738    let name = sess.opts.unstable_opts.tune_cpu.as_ref()?;
739    Some(handle_native(name))
740}
rustc_codegen_llvm/llvm_util.rs

rustc_codegen_llvm/
llvm_util.rs
