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::{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/master/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 raw_arch = &*sess.target.arch;
221    let arch = match raw_arch {
222        "x86_64" => "x86",
223        "arm64ec" => "aarch64",
224        "sparc64" => "sparc",
225        "powerpc64" => "powerpc",
226        _ => raw_arch,
227    };
228    let (major, _, _) = get_version();
229    match (arch, s) {
230        ("aarch64", "rcpc2") => Some(LLVMFeature::new("rcpc-immo")),
231        ("aarch64", "dpb") => Some(LLVMFeature::new("ccpp")),
232        ("aarch64", "dpb2") => Some(LLVMFeature::new("ccdp")),
233        ("aarch64", "frintts") => Some(LLVMFeature::new("fptoint")),
234        ("aarch64", "fcma") => Some(LLVMFeature::new("complxnum")),
235        ("aarch64", "pmuv3") => Some(LLVMFeature::new("perfmon")),
236        ("aarch64", "paca") => Some(LLVMFeature::new("pauth")),
237        ("aarch64", "pacg") => Some(LLVMFeature::new("pauth")),
238        ("aarch64", "flagm2") => Some(LLVMFeature::new("altnzcv")),
239        // Rust ties fp and neon together.
240        ("aarch64", "neon") => Some(LLVMFeature::with_dependencies(
241            "neon",
242            smallvec![TargetFeatureFoldStrength::Both("fp-armv8")],
243        )),
244        // In LLVM neon implicitly enables fp, but we manually enable
245        // neon when a feature only implicitly enables fp
246        ("aarch64", "fhm") => Some(LLVMFeature::new("fp16fml")),
247        ("aarch64", "fp16") => Some(LLVMFeature::new("fullfp16")),
248        // Filter out features that are not supported by the current LLVM version
249        ("aarch64", "fpmr") => None, // only existed in 18
250        ("arm", "fp16") => Some(LLVMFeature::new("fullfp16")),
251        // Filter out features that are not supported by the current LLVM version
252        ("loongarch32" | "loongarch64", "32s") if major < 21 => None,
253        ("powerpc", "power8-crypto") => Some(LLVMFeature::new("crypto")),
254        ("sparc", "leoncasa") => Some(LLVMFeature::new("hasleoncasa")),
255        ("x86", "sse4.2") => Some(LLVMFeature::with_dependencies(
256            "sse4.2",
257            smallvec![TargetFeatureFoldStrength::EnableOnly("crc32")],
258        )),
259        ("x86", "pclmulqdq") => Some(LLVMFeature::new("pclmul")),
260        ("x86", "rdrand") => Some(LLVMFeature::new("rdrnd")),
261        ("x86", "bmi1") => Some(LLVMFeature::new("bmi")),
262        ("x86", "cmpxchg16b") => Some(LLVMFeature::new("cx16")),
263        ("x86", "lahfsahf") => Some(LLVMFeature::new("sahf")),
264        // Enable the evex512 target feature if an avx512 target feature is enabled.
265        ("x86", s) if s.starts_with("avx512") => Some(LLVMFeature::with_dependencies(
266            s,
267            smallvec![TargetFeatureFoldStrength::EnableOnly("evex512")],
268        )),
269        ("x86", "avx10.1") => Some(LLVMFeature::new("avx10.1-512")),
270        ("x86", "avx10.2") => Some(LLVMFeature::new("avx10.2-512")),
271        ("x86", "apxf") => Some(LLVMFeature::with_dependencies(
272            "egpr",
273            smallvec![
274                TargetFeatureFoldStrength::Both("push2pop2"),
275                TargetFeatureFoldStrength::Both("ppx"),
276                TargetFeatureFoldStrength::Both("ndd"),
277                TargetFeatureFoldStrength::Both("ccmp"),
278                TargetFeatureFoldStrength::Both("cf"),
279                TargetFeatureFoldStrength::Both("nf"),
280                TargetFeatureFoldStrength::Both("zu"),
281            ],
282        )),
283        (_, s) => Some(LLVMFeature::new(s)),
284    }
285}
286
287/// Used to generate cfg variables and apply features.
288/// Must express features in the way Rust understands them.
289///
290/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
291pub(crate) fn target_config(sess: &Session) -> TargetConfig {
292    let target_machine = create_informational_target_machine(sess, true);
293
294    let (unstable_target_features, target_features) = cfg_target_feature(sess, |feature| {
295        // This closure determines whether the target CPU has the feature according to LLVM. We do
296        // *not* consider the `-Ctarget-feature`s here, as that will be handled later in
297        // `cfg_target_feature`.
298        if let Some(feat) = to_llvm_features(sess, feature) {
299            // All the LLVM features this expands to must be enabled.
300            for llvm_feature in feat {
301                let cstr = SmallCStr::new(llvm_feature);
302                // `LLVMRustHasFeature` is moderately expensive. On targets with many
303                // features (e.g. x86) these calls take a non-trivial fraction of runtime
304                // when compiling very small programs.
305                if !unsafe { llvm::LLVMRustHasFeature(target_machine.raw(), cstr.as_ptr()) } {
306                    return false;
307                }
308            }
309            true
310        } else {
311            false
312        }
313    });
314
315    let mut cfg = TargetConfig {
316        target_features,
317        unstable_target_features,
318        has_reliable_f16: true,
319        has_reliable_f16_math: true,
320        has_reliable_f128: true,
321        has_reliable_f128_math: true,
322    };
323
324    update_target_reliable_float_cfg(sess, &mut cfg);
325    cfg
326}
327
328/// Determine whether or not experimental float types are reliable based on known bugs.
329fn update_target_reliable_float_cfg(sess: &Session, cfg: &mut TargetConfig) {
330    let target_arch = sess.target.arch.as_ref();
331    let target_os = sess.target.options.os.as_ref();
332    let target_env = sess.target.options.env.as_ref();
333    let target_abi = sess.target.options.abi.as_ref();
334    let target_pointer_width = sess.target.pointer_width;
335    let version = get_version();
336    let lt_20_1_1 = version < (20, 1, 1);
337    let lt_21_0_0 = version < (21, 0, 0);
338
339    cfg.has_reliable_f16 = match (target_arch, target_os) {
340        // LLVM crash without neon <https://github.com/llvm/llvm-project/issues/129394> (fixed in llvm20)
341        ("aarch64", _)
342            if !cfg.target_features.iter().any(|f| f.as_str() == "neon") && lt_20_1_1 =>
343        {
344            false
345        }
346        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
347        ("arm64ec", _) => false,
348        // Selection failure <https://github.com/llvm/llvm-project/issues/50374> (fixed in llvm21)
349        ("s390x", _) if lt_21_0_0 => false,
350        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
351        ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
352        // Infinite recursion <https://github.com/llvm/llvm-project/issues/97981>
353        ("csky", _) => false,
354        ("hexagon", _) if lt_21_0_0 => false, // (fixed in llvm21)
355        ("powerpc" | "powerpc64", _) => false,
356        ("sparc" | "sparc64", _) => false,
357        ("wasm32" | "wasm64", _) => false,
358        // `f16` support only requires that symbols converting to and from `f32` are available. We
359        // provide these in `compiler-builtins`, so `f16` should be available on all platforms that
360        // do not have other ABI issues or LLVM crashes.
361        _ => true,
362    };
363
364    cfg.has_reliable_f128 = match (target_arch, target_os) {
365        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
366        ("arm64ec", _) => false,
367        // Selection bug <https://github.com/llvm/llvm-project/issues/96432> (fixed in llvm20)
368        ("mips64" | "mips64r6", _) if lt_20_1_1 => false,
369        // Selection bug <https://github.com/llvm/llvm-project/issues/95471>. This issue is closed
370        // but basic math still does not work.
371        ("nvptx64", _) => false,
372        // Unsupported https://github.com/llvm/llvm-project/issues/121122
373        ("amdgpu", _) => false,
374        // ABI bugs <https://github.com/rust-lang/rust/issues/125109> et al. (full
375        // list at <https://github.com/rust-lang/rust/issues/116909>)
376        ("powerpc" | "powerpc64", _) => false,
377        // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
378        ("sparc", _) => false,
379        // Stack alignment bug <https://github.com/llvm/llvm-project/issues/77401>. NB: tests may
380        // not fail if our compiler-builtins is linked. (fixed in llvm21)
381        ("x86", _) if lt_21_0_0 => false,
382        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
383        ("x86_64", "windows") if target_env == "gnu" && target_abi != "llvm" => false,
384        // There are no known problems on other platforms, so the only requirement is that symbols
385        // are available. `compiler-builtins` provides all symbols required for core `f128`
386        // support, so this should work for everything else.
387        _ => true,
388    };
389
390    // Assume that working `f16` means working `f16` math for most platforms, since
391    // operations just go through `f32`.
392    cfg.has_reliable_f16_math = cfg.has_reliable_f16;
393
394    cfg.has_reliable_f128_math = match (target_arch, target_os) {
395        // LLVM lowers `fp128` math to `long double` symbols even on platforms where
396        // `long double` is not IEEE binary128. See
397        // <https://github.com/llvm/llvm-project/issues/44744>.
398        //
399        // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
400        // (ld is `f64`), anything other than Linux (Windows and MacOS use `f64`), and `x86`
401        // (ld is 80-bit extended precision).
402        //
403        // musl does not implement the symbols required for f128 math at all.
404        _ if target_env == "musl" => false,
405        ("x86_64", _) => false,
406        (_, "linux") if target_pointer_width == 64 => true,
407        _ => false,
408    } && cfg.has_reliable_f128;
409}
410
411pub(crate) fn print_version() {
412    let (major, minor, patch) = get_version();
413    println!("LLVM version: {major}.{minor}.{patch}");
414}
415
416pub(crate) fn get_version() -> (u32, u32, u32) {
417    // Can be called without initializing LLVM
418    unsafe {
419        (llvm::LLVMRustVersionMajor(), llvm::LLVMRustVersionMinor(), llvm::LLVMRustVersionPatch())
420    }
421}
422
423pub(crate) fn print_passes() {
424    // Can be called without initializing LLVM
425    unsafe {
426        llvm::LLVMRustPrintPasses();
427    }
428}
429
430fn llvm_target_features(tm: &llvm::TargetMachine) -> Vec<(&str, &str)> {
431    let len = unsafe { llvm::LLVMRustGetTargetFeaturesCount(tm) };
432    let mut ret = Vec::with_capacity(len);
433    for i in 0..len {
434        unsafe {
435            let mut feature = ptr::null();
436            let mut desc = ptr::null();
437            llvm::LLVMRustGetTargetFeature(tm, i, &mut feature, &mut desc);
438            if feature.is_null() || desc.is_null() {
439                bug!("LLVM returned a `null` target feature string");
440            }
441            let feature = CStr::from_ptr(feature).to_str().unwrap_or_else(|e| {
442                bug!("LLVM returned a non-utf8 feature string: {}", e);
443            });
444            let desc = CStr::from_ptr(desc).to_str().unwrap_or_else(|e| {
445                bug!("LLVM returned a non-utf8 feature string: {}", e);
446            });
447            ret.push((feature, desc));
448        }
449    }
450    ret
451}
452
453pub(crate) fn print(req: &PrintRequest, out: &mut String, sess: &Session) {
454    require_inited();
455    let tm = create_informational_target_machine(sess, false);
456    match req.kind {
457        PrintKind::TargetCPUs => print_target_cpus(sess, tm.raw(), out),
458        PrintKind::TargetFeatures => print_target_features(sess, tm.raw(), out),
459        _ => bug!("rustc_codegen_llvm can't handle print request: {:?}", req),
460    }
461}
462
463fn print_target_cpus(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
464    let cpu_names = llvm::build_string(|s| unsafe {
465        llvm::LLVMRustPrintTargetCPUs(&tm, s);
466    })
467    .unwrap();
468
469    struct Cpu<'a> {
470        cpu_name: &'a str,
471        remark: String,
472    }
473    // Compare CPU against current target to label the default.
474    let target_cpu = handle_native(&sess.target.cpu);
475    let make_remark = |cpu_name| {
476        if cpu_name == target_cpu {
477            // FIXME(#132514): This prints the LLVM target string, which can be
478            // different from the Rust target string. Is that intended?
479            let target = &sess.target.llvm_target;
480            format!(
481                " - This is the default target CPU for the current build target (currently {target})."
482            )
483        } else {
484            "".to_owned()
485        }
486    };
487    let mut cpus = cpu_names
488        .lines()
489        .map(|cpu_name| Cpu { cpu_name, remark: make_remark(cpu_name) })
490        .collect::<VecDeque<_>>();
491
492    // Only print the "native" entry when host and target are the same arch,
493    // since otherwise it could be wrong or misleading.
494    if sess.host.arch == sess.target.arch {
495        let host = get_host_cpu_name();
496        cpus.push_front(Cpu {
497            cpu_name: "native",
498            remark: format!(" - Select the CPU of the current host (currently {host})."),
499        });
500    }
501
502    let max_name_width = cpus.iter().map(|cpu| cpu.cpu_name.len()).max().unwrap_or(0);
503    writeln!(out, "Available CPUs for this target:").unwrap();
504    for Cpu { cpu_name, remark } in cpus {
505        // Only pad the CPU name if there's a remark to print after it.
506        let width = if remark.is_empty() { 0 } else { max_name_width };
507        writeln!(out, "    {cpu_name:<width$}{remark}").unwrap();
508    }
509}
510
511fn print_target_features(sess: &Session, tm: &llvm::TargetMachine, out: &mut String) {
512    let mut llvm_target_features = llvm_target_features(tm);
513    let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
514    let mut rustc_target_features = sess
515        .target
516        .rust_target_features()
517        .iter()
518        .filter_map(|(feature, gate, _implied)| {
519            if !gate.in_cfg() {
520                // Only list (experimentally) supported features.
521                return None;
522            }
523            // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these
524            // strings.
525            let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
526            let desc =
527                match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
528                    Some(index) => {
529                        known_llvm_target_features.insert(llvm_feature);
530                        llvm_target_features[index].1
531                    }
532                    None => "",
533                };
534
535            Some((*feature, desc))
536        })
537        .collect::<Vec<_>>();
538
539    // Since we add this at the end ...
540    rustc_target_features.extend_from_slice(&[(
541        "crt-static",
542        "Enables C Run-time Libraries to be statically linked",
543    )]);
544    // ... we need to sort the list again.
545    rustc_target_features.sort();
546
547    llvm_target_features.retain(|(f, _d)| !known_llvm_target_features.contains(f));
548
549    let max_feature_len = llvm_target_features
550        .iter()
551        .chain(rustc_target_features.iter())
552        .map(|(feature, _desc)| feature.len())
553        .max()
554        .unwrap_or(0);
555
556    writeln!(out, "Features supported by rustc for this target:").unwrap();
557    for (feature, desc) in &rustc_target_features {
558        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
559    }
560    writeln!(out, "\nCode-generation features supported by LLVM for this target:").unwrap();
561    for (feature, desc) in &llvm_target_features {
562        writeln!(out, "    {feature:max_feature_len$} - {desc}.").unwrap();
563    }
564    if llvm_target_features.is_empty() {
565        writeln!(out, "    Target features listing is not supported by this LLVM version.")
566            .unwrap();
567    }
568    writeln!(out, "\nUse +feature to enable a feature, or -feature to disable it.").unwrap();
569    writeln!(out, "For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2\n")
570        .unwrap();
571    writeln!(out, "Code-generation features cannot be used in cfg or #[target_feature],").unwrap();
572    writeln!(out, "and may be renamed or removed in a future version of LLVM or rustc.\n").unwrap();
573}
574
575/// Returns the host CPU name, according to LLVM.
576fn get_host_cpu_name() -> &'static str {
577    let mut len = 0;
578    // SAFETY: The underlying C++ global function returns a `StringRef` that
579    // isn't tied to any particular backing buffer, so it must be 'static.
580    let slice: &'static [u8] = unsafe {
581        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
582        assert!(!ptr.is_null());
583        slice::from_raw_parts(ptr, len)
584    };
585    str::from_utf8(slice).expect("host CPU name should be UTF-8")
586}
587
588/// If the given string is `"native"`, returns the host CPU name according to
589/// LLVM. Otherwise, the string is returned as-is.
590fn handle_native(cpu_name: &str) -> &str {
591    match cpu_name {
592        "native" => get_host_cpu_name(),
593        _ => cpu_name,
594    }
595}
596
597pub(crate) fn target_cpu(sess: &Session) -> &str {
598    let cpu_name = sess.opts.cg.target_cpu.as_deref().unwrap_or_else(|| &sess.target.cpu);
599    handle_native(cpu_name)
600}
601
602/// The target features for compiler flags other than `-Ctarget-features`.
603fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
604    target_features::retpoline_features_by_flags(sess, features);
605
606    // -Zfixed-x18
607    if sess.opts.unstable_opts.fixed_x18 {
608        if sess.target.arch != "aarch64" {
609            sess.dcx().emit_fatal(errors::FixedX18InvalidArch { arch: &sess.target.arch });
610        } else {
611            features.push("+reserve-x18".into());
612        }
613    }
614}
615
616/// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
617/// `--target` and similar).
618pub(crate) fn global_llvm_features(
619    sess: &Session,
620    diagnostics: bool,
621    only_base_features: bool,
622) -> Vec<String> {
623    // Features that come earlier are overridden by conflicting features later in the string.
624    // Typically we'll want more explicit settings to override the implicit ones, so:
625    //
626    // * Features from -Ctarget-cpu=*; are overridden by [^1]
627    // * Features implied by --target; are overridden by
628    // * Features from -Ctarget-feature; are overridden by
629    // * function specific features.
630    //
631    // [^1]: target-cpu=native is handled here, other target-cpu values are handled implicitly
632    // through LLVM TargetMachine implementation.
633    //
634    // FIXME(nagisa): it isn't clear what's the best interaction between features implied by
635    // `-Ctarget-cpu` and `--target` are. On one hand, you'd expect CLI arguments to always
636    // override anything that's implicit, so e.g. when there's no `--target` flag, features implied
637    // the host target are overridden by `-Ctarget-cpu=*`. On the other hand, what about when both
638    // `--target` and `-Ctarget-cpu=*` are specified? Both then imply some target features and both
639    // flags are specified by the user on the CLI. It isn't as clear-cut which order of precedence
640    // should be taken in cases like these.
641    let mut features = vec![];
642
643    // -Ctarget-cpu=native
644    match sess.opts.cg.target_cpu {
645        Some(ref s) if s == "native" => {
646            // We have already figured out the actual CPU name with `LLVMRustGetHostCPUName` and set
647            // that for LLVM, so the features implied by that CPU name will be available everywhere.
648            // However, that is not sufficient: e.g. `skylake` alone is not sufficient to tell if
649            // some of the instructions are available or not. So we have to also explicitly ask for
650            // the exact set of features available on the host, and enable all of them.
651            let features_string = unsafe {
652                let ptr = llvm::LLVMGetHostCPUFeatures();
653                let features_string = if !ptr.is_null() {
654                    CStr::from_ptr(ptr)
655                        .to_str()
656                        .unwrap_or_else(|e| {
657                            bug!("LLVM returned a non-utf8 features string: {}", e);
658                        })
659                        .to_owned()
660                } else {
661                    bug!("could not allocate host CPU features, LLVM returned a `null` string");
662                };
663
664                llvm::LLVMDisposeMessage(ptr);
665
666                features_string
667            };
668            features.extend(features_string.split(',').map(String::from));
669        }
670        Some(_) | None => {}
671    };
672
673    // Features implied by an implicit or explicit `--target`.
674    features.extend(sess.target.features.split(',').filter(|v| !v.is_empty()).map(String::from));
675
676    if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
677        features.push("+exception-handling".into());
678    }
679
680    // -Ctarget-features
681    if !only_base_features {
682        target_features::flag_to_backend_features(
683            sess,
684            diagnostics,
685            |feature| {
686                to_llvm_features(sess, feature)
687                    .map(|f| SmallVec::<[&str; 2]>::from_iter(f.into_iter()))
688                    .unwrap_or_default()
689            },
690            |feature, enable| {
691                let enable_disable = if enable { '+' } else { '-' };
692                // We run through `to_llvm_features` when
693                // passing requests down to LLVM. This means that all in-language
694                // features also work on the command line instead of having two
695                // different names when the LLVM name and the Rust name differ.
696                let Some(llvm_feature) = to_llvm_features(sess, feature) else { return };
697
698                features.extend(
699                    std::iter::once(format!(
700                        "{}{}",
701                        enable_disable, llvm_feature.llvm_feature_name
702                    ))
703                    .chain(llvm_feature.dependencies.into_iter().filter_map(
704                        move |feat| match (enable, feat) {
705                            (_, TargetFeatureFoldStrength::Both(f))
706                            | (true, TargetFeatureFoldStrength::EnableOnly(f)) => {
707                                Some(format!("{enable_disable}{f}"))
708                            }
709                            _ => None,
710                        },
711                    )),
712                )
713            },
714        );
715    }
716
717    // We add this in the "base target" so that these show up in `sess.unstable_target_features`.
718    llvm_features_by_flags(sess, &mut features);
719
720    features
721}
722
723pub(crate) fn tune_cpu(sess: &Session) -> Option<&str> {
724    let name = sess.opts.unstable_opts.tune_cpu.as_ref()?;
725    Some(handle_native(name))
726}
rustc_codegen_llvm/llvm_util.rs

rustc_codegen_llvm/
llvm_util.rs
