src/librustc_codegen_llvm/attributes.rs

   1 //! Set and unset common attributes on LLVM values.
   2
   3 use std::ffi::CString;
   4
   5 use rustc::hir::{CodegenFnAttrFlags, CodegenFnAttrs};
   6 use rustc::hir::def_id::{DefId, LOCAL_CRATE};
   7 use rustc::session::Session;
   8 use rustc::session::config::{Sanitizer, OptLevel};
   9 use rustc::ty::{self, TyCtxt, PolyFnSig};
  10 use rustc::ty::layout::HasTyCtxt;
  11 use rustc::ty::query::Providers;
  12 use rustc_data_structures::small_c_str::SmallCStr;
  13 use rustc_data_structures::sync::Lrc;
  14 use rustc_data_structures::fx::FxHashMap;
  15 use rustc_target::spec::PanicStrategy;
  16 use rustc_codegen_ssa::traits::*;
  17
  18 use crate::abi::Abi;
  19 use crate::attributes;
  20 use crate::llvm::{self, Attribute};
  21 use crate::llvm::AttributePlace::Function;
  22 use crate::llvm_util;
  23 pub use syntax::attr::{self, InlineAttr, OptimizeAttr};
  24
  25 use crate::context::CodegenCx;
  26 use crate::value::Value;
  27
  28 /// Mark LLVM function to use provided inline heuristic.
  29 #[inline]
  30 pub fn inline(cx: &CodegenCx<'ll, '_>, val: &'ll Value, inline: InlineAttr) {
  31     use self::InlineAttr::*;
  32     match inline {
  33         Hint   => Attribute::InlineHint.apply_llfn(Function, val),
  34         Always => Attribute::AlwaysInline.apply_llfn(Function, val),
  35         Never  => {
  36             if cx.tcx().sess.target.target.arch != "amdgpu" {
  37                 Attribute::NoInline.apply_llfn(Function, val);
  38             }
  39         },
  40         None   => {
  41             Attribute::InlineHint.unapply_llfn(Function, val);
  42             Attribute::AlwaysInline.unapply_llfn(Function, val);
  43             Attribute::NoInline.unapply_llfn(Function, val);
  44         },
  45     };
  46 }
  47
  48 /// Tell LLVM to emit or not emit the information necessary to unwind the stack for the function.
  49 #[inline]
  50 pub fn emit_uwtable(val: &'ll Value, emit: bool) {
  51     Attribute::UWTable.toggle_llfn(Function, val, emit);
  52 }
  53
  54 /// Tell LLVM whether the function can or cannot unwind.
  55 #[inline]
  56 fn unwind(val: &'ll Value, can_unwind: bool) {
  57     Attribute::NoUnwind.toggle_llfn(Function, val, !can_unwind);
  58 }
  59
  60 /// Tell LLVM if this function should be 'naked', i.e., skip the epilogue and prologue.
  61 #[inline]
  62 pub fn naked(val: &'ll Value, is_naked: bool) {
  63     Attribute::Naked.toggle_llfn(Function, val, is_naked);
  64 }
  65
  66 pub fn set_frame_pointer_elimination(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
  67     if cx.sess().must_not_eliminate_frame_pointers() {
  68         llvm::AddFunctionAttrStringValue(
  69             llfn, llvm::AttributePlace::Function,
  70             const_cstr!("no-frame-pointer-elim"), const_cstr!("true"));
  71     }
  72 }
  73
  74 /// Tell LLVM what instrument function to insert.
  75 #[inline]
  76 pub fn set_instrument_function(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
  77     if cx.sess().instrument_mcount() {
  78         // Similar to `clang -pg` behavior. Handled by the
  79         // `post-inline-ee-instrument` LLVM pass.
  80         llvm::AddFunctionAttrStringValue(
  81             llfn, llvm::AttributePlace::Function,
  82             const_cstr!("instrument-function-entry-inlined"), const_cstr!("mcount"));
  83     }
  84 }
  85
  86 pub fn set_probestack(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
  87     // Only use stack probes if the target specification indicates that we
  88     // should be using stack probes
  89     if !cx.sess().target.target.options.stack_probes {
  90         return
  91     }
  92
  93     // Currently stack probes seem somewhat incompatible with the address
  94     // sanitizer. With asan we're already protected from stack overflow anyway
  95     // so we don't really need stack probes regardless.
  96     if let Some(Sanitizer::Address) = cx.sess().opts.debugging_opts.sanitizer {
  97         return
  98     }
  99
 100     // probestack doesn't play nice either with pgo-gen.
 101     if cx.sess().opts.debugging_opts.pgo_gen.is_some() {
 102         return;
 103     }
 104
 105     // probestack doesn't play nice either with gcov profiling.
 106     if cx.sess().opts.debugging_opts.profile {
 107         return;
 108     }
 109
 110     // Flag our internal `__rust_probestack` function as the stack probe symbol.
 111     // This is defined in the `compiler-builtins` crate for each architecture.
 112     llvm::AddFunctionAttrStringValue(
 113         llfn, llvm::AttributePlace::Function,
 114         const_cstr!("probe-stack"), const_cstr!("__rust_probestack"));
 115 }
 116
 117 pub fn llvm_target_features(sess: &Session) -> impl Iterator<Item = &str> {
 118     const RUSTC_SPECIFIC_FEATURES: &[&str] = &[
 119         "crt-static",
 120     ];
 121
 122     let cmdline = sess.opts.cg.target_feature.split(',')
 123         .filter(|f| !RUSTC_SPECIFIC_FEATURES.iter().any(|s| f.contains(s)));
 124     sess.target.target.options.features.split(',')
 125         .chain(cmdline)
 126         .filter(|l| !l.is_empty())
 127 }
 128
 129 pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
 130     let target_cpu = SmallCStr::new(llvm_util::target_cpu(cx.tcx.sess));
 131     llvm::AddFunctionAttrStringValue(
 132             llfn,
 133             llvm::AttributePlace::Function,
 134             const_cstr!("target-cpu"),
 135             target_cpu.as_c_str());
 136 }
 137
 138 /// Sets the `NonLazyBind` LLVM attribute on a given function,
 139 /// assuming the codegen options allow skipping the PLT.
 140 pub fn non_lazy_bind(sess: &Session, llfn: &'ll Value) {
 141     // Don't generate calls through PLT if it's not necessary
 142     if !sess.needs_plt() {
 143         Attribute::NonLazyBind.apply_llfn(Function, llfn);
 144     }
 145 }
 146
 147 pub(crate) fn default_optimisation_attrs(sess: &Session, llfn: &'ll Value) {
 148     match sess.opts.optimize {
 149         OptLevel::Size => {
 150             llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
 151             llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
 152             llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
 153         },
 154         OptLevel::SizeMin => {
 155             llvm::Attribute::MinSize.apply_llfn(Function, llfn);
 156             llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
 157             llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
 158         }
 159         OptLevel::No => {
 160             llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
 161             llvm::Attribute::OptimizeForSize.unapply_llfn(Function, llfn);
 162             llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
 163         }
 164         _ => {}
 165     }
 166 }
 167
 168
 169 /// Composite function which sets LLVM attributes for function depending on its AST (`#[attribute]`)
 170 /// attributes.
 171 pub fn from_fn_attrs(
 172     cx: &CodegenCx<'ll, 'tcx>,
 173     llfn: &'ll Value,
 174     id: Option<DefId>,
 175     sig: PolyFnSig<'tcx>,
 176 ) {
 177     let codegen_fn_attrs = id.map(|id| cx.tcx.codegen_fn_attrs(id))
 178         .unwrap_or_else(|| CodegenFnAttrs::new());
 179
 180     match codegen_fn_attrs.optimize {
 181         OptimizeAttr::None => {
 182             default_optimisation_attrs(cx.tcx.sess, llfn);
 183         }
 184         OptimizeAttr::Speed => {
 185             llvm::Attribute::MinSize.unapply_llfn(Function, llfn);
 186             llvm::Attribute::OptimizeForSize.unapply_llfn(Function, llfn);
 187             llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
 188         }
 189         OptimizeAttr::Size => {
 190             llvm::Attribute::MinSize.apply_llfn(Function, llfn);
 191             llvm::Attribute::OptimizeForSize.apply_llfn(Function, llfn);
 192             llvm::Attribute::OptimizeNone.unapply_llfn(Function, llfn);
 193         }
 194     }
 195
 196     inline(cx, llfn, codegen_fn_attrs.inline);
 197
 198     // The `uwtable` attribute according to LLVM is:
 199     //
 200     //     This attribute indicates that the ABI being targeted requires that an
 201     //     unwind table entry be produced for this function even if we can show
 202     //     that no exceptions passes by it. This is normally the case for the
 203     //     ELF x86-64 abi, but it can be disabled for some compilation units.
 204     //
 205     // Typically when we're compiling with `-C panic=abort` (which implies this
 206     // `no_landing_pads` check) we don't need `uwtable` because we can't
 207     // generate any exceptions! On Windows, however, exceptions include other
 208     // events such as illegal instructions, segfaults, etc. This means that on
 209     // Windows we end up still needing the `uwtable` attribute even if the `-C
 210     // panic=abort` flag is passed.
 211     //
 212     // You can also find more info on why Windows is whitelisted here in:
 213     //      https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
 214     if !cx.sess().no_landing_pads() ||
 215        cx.sess().target.target.options.requires_uwtable {
 216         attributes::emit_uwtable(llfn, true);
 217     }
 218
 219     set_frame_pointer_elimination(cx, llfn);
 220     set_instrument_function(cx, llfn);
 221     set_probestack(cx, llfn);
 222
 223     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {
 224         Attribute::Cold.apply_llfn(Function, llfn);
 225     }
 226     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::FFI_RETURNS_TWICE) {
 227         Attribute::ReturnsTwice.apply_llfn(Function, llfn);
 228     }
 229     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
 230         naked(llfn, true);
 231     }
 232     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR) {
 233         Attribute::NoAlias.apply_llfn(
 234             llvm::AttributePlace::ReturnValue, llfn);
 235     }
 236
 237     unwind(llfn, if cx.tcx.sess.panic_strategy() != PanicStrategy::Unwind {
 238         // In panic=abort mode we assume nothing can unwind anywhere, so
 239         // optimize based on this!
 240         false
 241     } else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::UNWIND) {
 242         // If a specific #[unwind] attribute is present, use that
 243         true
 244     } else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_ALLOCATOR_NOUNWIND) {
 245         // Special attribute for allocator functions, which can't unwind
 246         false
 247     } else if let Some(id) = id {
 248         let sig = cx.tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &sig);
 249         if cx.tcx.is_foreign_item(id) {
 250             // Foreign items like `extern "C" { fn foo(); }` are assumed not to
 251             // unwind
 252             false
 253         } else if sig.abi != Abi::Rust && sig.abi != Abi::RustCall {
 254             // Any items defined in Rust that *don't* have the `extern` ABI are
 255             // defined to not unwind. We insert shims to abort if an unwind
 256             // happens to enforce this.
 257             false
 258         } else {
 259             // Anything else defined in Rust is assumed that it can possibly
 260             // unwind
 261             true
 262         }
 263     } else {
 264         // assume this can possibly unwind, avoiding the application of a
 265         // `nounwind` attribute below.
 266         true
 267     });
 268
 269     // Always annotate functions with the target-cpu they are compiled for.
 270     // Without this, ThinLTO won't inline Rust functions into Clang generated
 271     // functions (because Clang annotates functions this way too).
 272     apply_target_cpu_attr(cx, llfn);
 273
 274     let features = llvm_target_features(cx.tcx.sess)
 275         .map(|s| s.to_string())
 276         .chain(
 277             codegen_fn_attrs.target_features
 278                 .iter()
 279                 .map(|f| {
 280                     let feature = &*f.as_str();
 281                     format!("+{}", llvm_util::to_llvm_feature(cx.tcx.sess, feature))
 282                 })
 283         )
 284         .collect::<Vec<String>>()
 285         .join(",");
 286
 287     if !features.is_empty() {
 288         let val = CString::new(features).unwrap();
 289         llvm::AddFunctionAttrStringValue(
 290             llfn, llvm::AttributePlace::Function,
 291             const_cstr!("target-features"), &val);
 292     }
 293
 294     // Note that currently the `wasm-import-module` doesn't do anything, but
 295     // eventually LLVM 7 should read this and ferry the appropriate import
 296     // module to the output file.
 297     if let Some(id) = id {
 298         if cx.tcx.sess.target.target.arch == "wasm32" {
 299             if let Some(module) = wasm_import_module(cx.tcx, id) {
 300                 llvm::AddFunctionAttrStringValue(
 301                     llfn,
 302                     llvm::AttributePlace::Function,
 303                     const_cstr!("wasm-import-module"),
 304                     &module,
 305                 );
 306             }
 307         }
 308     }
 309 }
 310
 311 pub fn provide(providers: &mut Providers<'_>) {
 312     providers.target_features_whitelist = |tcx, cnum| {
 313         assert_eq!(cnum, LOCAL_CRATE);
 314         if tcx.sess.opts.actually_rustdoc {
 315             // rustdoc needs to be able to document functions that use all the features, so
 316             // whitelist them all
 317             Lrc::new(llvm_util::all_known_features()
 318                 .map(|(a, b)| (a.to_string(), b.map(|s| s.to_string())))
 319                 .collect())
 320         } else {
 321             Lrc::new(llvm_util::target_feature_whitelist(tcx.sess)
 322                 .iter()
 323                 .map(|&(a, b)| (a.to_string(), b.map(|s| s.to_string())))
 324                 .collect())
 325         }
 326     };
 327
 328     provide_extern(providers);
 329 }
 330
 331 pub fn provide_extern(providers: &mut Providers<'_>) {
 332     providers.wasm_import_module_map = |tcx, cnum| {
 333         // Build up a map from DefId to a `NativeLibrary` structure, where
 334         // `NativeLibrary` internally contains information about
 335         // `#[link(wasm_import_module = "...")]` for example.
 336         let native_libs = tcx.native_libraries(cnum);
 337
 338         let def_id_to_native_lib = native_libs.iter().filter_map(|lib|
 339             if let Some(id) = lib.foreign_module {
 340                 Some((id, lib))
 341             } else {
 342                 None
 343             }
 344         ).collect::<FxHashMap<_, _>>();
 345
 346         let mut ret = FxHashMap::default();
 347         for lib in tcx.foreign_modules(cnum).iter() {
 348             let module = def_id_to_native_lib
 349                 .get(&lib.def_id)
 350                 .and_then(|s| s.wasm_import_module);
 351             let module = match module {
 352                 Some(s) => s,
 353                 None => continue,
 354             };
 355             ret.extend(lib.foreign_items.iter().map(|id| {
 356                 assert_eq!(id.krate, cnum);
 357                 (*id, module.to_string())
 358             }));
 359         }
 360
 361         Lrc::new(ret)
 362     };
 363 }
 364
 365 fn wasm_import_module(tcx: TyCtxt<'_, '_, '_>, id: DefId) -> Option<CString> {
 366     tcx.wasm_import_module_map(id.krate)
 367         .get(&id)
 368         .map(|s| CString::new(&s[..]).unwrap())
 369 }