src/shims/foreign_items.rs

   1 use std::{
   2     convert::{TryFrom, TryInto},
   3     iter,
   4 };
   5
   6 use log::trace;
   7
   8 use rustc_apfloat::Float;
   9 use rustc_hir::{
  10     def::DefKind,
  11     def_id::{CrateNum, DefId, LOCAL_CRATE},
  12 };
  13 use rustc_middle::middle::{
  14     codegen_fn_attrs::CodegenFnAttrFlags, dependency_format::Linkage,
  15     exported_symbols::ExportedSymbol,
  16 };
  17 use rustc_middle::mir;
  18 use rustc_middle::ty;
  19 use rustc_session::config::CrateType;
  20 use rustc_span::{symbol::sym, Symbol};
  21 use rustc_target::{
  22     abi::{Align, Size},
  23     spec::abi::Abi,
  24 };
  25
  26 use super::backtrace::EvalContextExt as _;
  27 use crate::*;
  28
  29 /// Returned by `emulate_foreign_item_by_name`.
  30 pub enum EmulateByNameResult {
  31     /// The caller is expected to jump to the return block.
  32     NeedsJumping,
  33     /// Jumping has already been taken care of.
  34     AlreadyJumped,
  35     /// The item is not supported.
  36     NotSupported,
  37 }
  38
  39 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  40 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  41     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  42     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  43         let this = self.eval_context_ref();
  44         // List taken from `libstd/sys_common/alloc.rs`.
  45         let min_align = match this.tcx.sess.target.arch.as_str() {
  46             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  47             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  48             arch => bug!("Unsupported target architecture: {}", arch),
  49         };
  50         // Windows always aligns, even small allocations.
  51         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  52         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  53         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  54             return Align::from_bytes(min_align).unwrap();
  55         }
  56         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  57         fn prev_power_of_two(x: u64) -> u64 {
  58             let next_pow2 = x.next_power_of_two();
  59             if next_pow2 == x {
  60                 // x *is* a power of two, just use that.
  61                 x
  62             } else {
  63                 // x is between two powers, so next = 2*prev.
  64                 next_pow2 / 2
  65             }
  66         }
  67         Align::from_bytes(prev_power_of_two(size)).unwrap()
  68     }
  69
  70     fn malloc(
  71         &mut self,
  72         size: u64,
  73         zero_init: bool,
  74         kind: MiriMemoryKind,
  75     ) -> InterpResult<'tcx, Scalar<Tag>> {
  76         let this = self.eval_context_mut();
  77         if size == 0 {
  78             Ok(Scalar::null_ptr(this))
  79         } else {
  80             let align = this.min_align(size, kind);
  81             let ptr = this.memory.allocate(Size::from_bytes(size), align, kind.into())?;
  82             if zero_init {
  83                 // We just allocated this, the access is definitely in-bounds.
  84                 this.memory.write_bytes(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  85             }
  86             Ok(Scalar::Ptr(ptr))
  87         }
  88     }
  89
  90     fn free(&mut self, ptr: Scalar<Tag>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  91         let this = self.eval_context_mut();
  92         if !this.is_null(ptr)? {
  93             let ptr = this.force_ptr(ptr)?;
  94             this.memory.deallocate(ptr, None, kind.into())?;
  95         }
  96         Ok(())
  97     }
  98
  99     fn realloc(
 100         &mut self,
 101         old_ptr: Scalar<Tag>,
 102         new_size: u64,
 103         kind: MiriMemoryKind,
 104     ) -> InterpResult<'tcx, Scalar<Tag>> {
 105         let this = self.eval_context_mut();
 106         let new_align = this.min_align(new_size, kind);
 107         if this.is_null(old_ptr)? {
 108             if new_size == 0 {
 109                 Ok(Scalar::null_ptr(this))
 110             } else {
 111                 let new_ptr =
 112                     this.memory.allocate(Size::from_bytes(new_size), new_align, kind.into())?;
 113                 Ok(Scalar::Ptr(new_ptr))
 114             }
 115         } else {
 116             let old_ptr = this.force_ptr(old_ptr)?;
 117             if new_size == 0 {
 118                 this.memory.deallocate(old_ptr, None, kind.into())?;
 119                 Ok(Scalar::null_ptr(this))
 120             } else {
 121                 let new_ptr = this.memory.reallocate(
 122                     old_ptr,
 123                     None,
 124                     Size::from_bytes(new_size),
 125                     new_align,
 126                     kind.into(),
 127                 )?;
 128                 Ok(Scalar::Ptr(new_ptr))
 129             }
 130         }
 131     }
 132
 133     /// Lookup the body of a function that has `link_name` as the symbol name.
 134     fn lookup_exported_symbol(
 135         &mut self,
 136         link_name: Symbol,
 137     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
 138         let this = self.eval_context_mut();
 139         let tcx = this.tcx.tcx;
 140
 141         // If the result was cached, just return it.
 142         if let Some(instance) = this.machine.exported_symbols_cache.get(&link_name) {
 143             return instance.map(|instance| this.load_mir(instance.def, None)).transpose();
 144         }
 145
 146         // Find it if it was not cached.
 147         let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
 148         // `dependency_formats` includes all the transitive informations needed to link a crate,
 149         // which is what we need here since we need to dig out `exported_symbols` from all transitive
 150         // dependencies.
 151         let dependency_formats = tcx.dependency_formats(());
 152         let dependency_format = dependency_formats
 153             .iter()
 154             .find(|(crate_type, _)| *crate_type == CrateType::Executable)
 155             .expect("interpreting a non-executable crate");
 156         for cnum in
 157             iter::once(LOCAL_CRATE).chain(dependency_format.1.iter().enumerate().filter_map(
 158                 |(num, &linkage)| (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1)),
 159             ))
 160         {
 161             // We can ignore `_export_level` here: we are a Rust crate, and everything is exported
 162             // from a Rust crate.
 163             for &(symbol, _export_level) in tcx.exported_symbols(cnum) {
 164                 if let ExportedSymbol::NonGeneric(def_id) = symbol {
 165                     let attrs = tcx.codegen_fn_attrs(def_id);
 166                     let symbol_name = if let Some(export_name) = attrs.export_name {
 167                         export_name
 168                     } else if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
 169                         tcx.item_name(def_id)
 170                     } else {
 171                         // Skip over items without an explicitly defined symbol name.
 172                         continue;
 173                     };
 174                     if symbol_name == link_name {
 175                         if let Some((instance, original_cnum)) = instance_and_crate {
 176                             throw_machine_stop!(TerminationInfo::MultipleSymbolDefinitions {
 177                                 link_name,
 178                                 first: tcx.def_span(instance.def_id()).data(),
 179                                 first_crate: tcx.crate_name(original_cnum),
 180                                 second: tcx.def_span(def_id).data(),
 181                                 second_crate: tcx.crate_name(cnum),
 182                             });
 183                         }
 184                         if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 185                             throw_ub_format!(
 186                                 "attempt to call an exported symbol that is not defined as a function"
 187                             );
 188                         }
 189                         instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 190                     }
 191                 }
 192             }
 193         }
 194
 195         let instance = instance_and_crate.map(|ic| ic.0);
 196         // Cache it and load its MIR, if found.
 197         this.machine.exported_symbols_cache.try_insert(link_name, instance).unwrap();
 198         instance.map(|instance| this.load_mir(instance.def, None)).transpose()
 199     }
 200
 201     /// Emulates calling a foreign item, failing if the item is not supported.
 202     /// This function will handle `goto_block` if needed.
 203     /// Returns Ok(None) if the foreign item was completely handled
 204     /// by this function.
 205     /// Returns Ok(Some(body)) if processing the foreign item
 206     /// is delegated to another function.
 207     fn emulate_foreign_item(
 208         &mut self,
 209         def_id: DefId,
 210         abi: Abi,
 211         args: &[OpTy<'tcx, Tag>],
 212         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 213         unwind: StackPopUnwind,
 214     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
 215         let this = self.eval_context_mut();
 216         let attrs = this.tcx.get_attrs(def_id);
 217         let link_name = this
 218             .tcx
 219             .sess
 220             .first_attr_value_str_by_name(&attrs, sym::link_name)
 221             .unwrap_or_else(|| this.tcx.item_name(def_id));
 222         let tcx = this.tcx.tcx;
 223
 224         // First: functions that diverge.
 225         let (dest, ret) = match ret {
 226             None => match &*link_name.as_str() {
 227                 "miri_start_panic" => {
 228                     // `check_shim` happens inside `handle_miri_start_panic`.
 229                     this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 230                     return Ok(None);
 231                 }
 232                 // This matches calls to the foreign item `panic_impl`.
 233                 // The implementation is provided by the function with the `#[panic_handler]` attribute.
 234                 "panic_impl" => {
 235                     // We don't use `check_shim` here because we are just forwarding to the lang
 236                     // item. Argument count checking will be performed when the returned `Body` is
 237                     // called.
 238                     this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 239                     let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 240                     let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 241                     return Ok(Some(&*this.load_mir(panic_impl_instance.def, None)?));
 242                 }
 243                 #[rustfmt::skip]
 244                 | "exit"
 245                 | "ExitProcess"
 246                 => {
 247                     let exp_abi = if link_name.as_str() == "exit" {
 248                         Abi::C { unwind: false }
 249                     } else {
 250                         Abi::System { unwind: false }
 251                     };
 252                     let &[ref code] = this.check_shim(abi, exp_abi, link_name, args)?;
 253                     // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 254                     let code = this.read_scalar(code)?.to_i32()?;
 255                     throw_machine_stop!(TerminationInfo::Exit(code.into()));
 256                 }
 257                 "abort" => {
 258                     let &[] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 259                     throw_machine_stop!(TerminationInfo::Abort(
 260                         "the program aborted execution".to_owned()
 261                     ))
 262                 }
 263                 _ => {
 264                     if let Some(body) = this.lookup_exported_symbol(link_name)? {
 265                         return Ok(Some(body));
 266                     }
 267                     this.handle_unsupported(format!(
 268                         "can't call (diverging) foreign function: {}",
 269                         link_name
 270                     ))?;
 271                     return Ok(None);
 272                 }
 273             },
 274             Some(p) => p,
 275         };
 276
 277         // Second: functions that return.
 278         match this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 279             EmulateByNameResult::NeedsJumping => {
 280                 trace!("{:?}", this.dump_place(**dest));
 281                 this.go_to_block(ret);
 282             }
 283             EmulateByNameResult::AlreadyJumped => (),
 284             EmulateByNameResult::NotSupported => {
 285                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 286                     return Ok(Some(body));
 287                 }
 288
 289                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 290                 return Ok(None);
 291             }
 292         }
 293
 294         Ok(None)
 295     }
 296
 297     /// Emulates calling a foreign item using its name.
 298     fn emulate_foreign_item_by_name(
 299         &mut self,
 300         link_name: Symbol,
 301         abi: Abi,
 302         args: &[OpTy<'tcx, Tag>],
 303         dest: &PlaceTy<'tcx, Tag>,
 304         ret: mir::BasicBlock,
 305     ) -> InterpResult<'tcx, EmulateByNameResult> {
 306         let this = self.eval_context_mut();
 307
 308         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 309         // shim, add it to the corresponding submodule.
 310         match &*link_name.as_str() {
 311             // Miri-specific extern functions
 312             "miri_static_root" => {
 313                 let &[ref ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 314                 let ptr = this.read_scalar(ptr)?.check_init()?;
 315                 let ptr = this.force_ptr(ptr)?;
 316                 if ptr.offset != Size::ZERO {
 317                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 318                 }
 319                 this.machine.static_roots.push(ptr.alloc_id);
 320             }
 321
 322             // Obtains a Miri backtrace. See the README for details.
 323             "miri_get_backtrace" => {
 324                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 325                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 326             }
 327
 328             // Resolves a Miri backtrace frame. See the README for details.
 329             "miri_resolve_frame" => {
 330                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 331                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 332             }
 333
 334
 335             // Standard C allocation
 336             "malloc" => {
 337                 let &[ref size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 338                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 339                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 340                 this.write_scalar(res, dest)?;
 341             }
 342             "calloc" => {
 343                 let &[ref items, ref len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 344                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 345                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 346                 let size =
 347                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 348                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 349                 this.write_scalar(res, dest)?;
 350             }
 351             "free" => {
 352                 let &[ref ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 353                 let ptr = this.read_scalar(ptr)?.check_init()?;
 354                 this.free(ptr, MiriMemoryKind::C)?;
 355             }
 356             "realloc" => {
 357                 let &[ref old_ptr, ref new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 358                 let old_ptr = this.read_scalar(old_ptr)?.check_init()?;
 359                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 360                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 361                 this.write_scalar(res, dest)?;
 362             }
 363
 364             // Rust allocation
 365             // (Usually these would be forwarded to to `#[global_allocator]`; we instead implement a generic
 366             // allocation that also checks that all conditions are met, such as not permitting zero-sized allocations.)
 367             "__rust_alloc" => {
 368                 let &[ref size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 369                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 370                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 371                 Self::check_alloc_request(size, align)?;
 372                 let ptr = this.memory.allocate(
 373                     Size::from_bytes(size),
 374                     Align::from_bytes(align).unwrap(),
 375                     MiriMemoryKind::Rust.into(),
 376                 )?;
 377                 this.write_scalar(ptr, dest)?;
 378             }
 379             "__rust_alloc_zeroed" => {
 380                 let &[ref size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 381                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 382                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 383                 Self::check_alloc_request(size, align)?;
 384                 let ptr = this.memory.allocate(
 385                     Size::from_bytes(size),
 386                     Align::from_bytes(align).unwrap(),
 387                     MiriMemoryKind::Rust.into(),
 388                 )?;
 389                 // We just allocated this, the access is definitely in-bounds.
 390                 this.memory.write_bytes(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 391                 this.write_scalar(ptr, dest)?;
 392             }
 393             "__rust_dealloc" => {
 394                 let &[ref ptr, ref old_size, ref align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 395                 let ptr = this.read_scalar(ptr)?.check_init()?;
 396                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 397                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 398                 // No need to check old_size/align; we anyway check that they match the allocation.
 399                 let ptr = this.force_ptr(ptr)?;
 400                 this.memory.deallocate(
 401                     ptr,
 402                     Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 403                     MiriMemoryKind::Rust.into(),
 404                 )?;
 405             }
 406             "__rust_realloc" => {
 407                 let &[ref ptr, ref old_size, ref align, ref new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 408                 let ptr = this.force_ptr(this.read_scalar(ptr)?.check_init()?)?;
 409                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 410                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 411                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 412                 Self::check_alloc_request(new_size, align)?;
 413                 // No need to check old_size; we anyway check that they match the allocation.
 414                 let align = Align::from_bytes(align).unwrap();
 415                 let new_ptr = this.memory.reallocate(
 416                     ptr,
 417                     Some((Size::from_bytes(old_size), align)),
 418                     Size::from_bytes(new_size),
 419                     align,
 420                     MiriMemoryKind::Rust.into(),
 421                 )?;
 422                 this.write_scalar(new_ptr, dest)?;
 423             }
 424
 425             // C memory handling functions
 426             "memcmp" => {
 427                 let &[ref left, ref right, ref n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 428                 let left = this.read_scalar(left)?.check_init()?;
 429                 let right = this.read_scalar(right)?.check_init()?;
 430                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 431
 432                 let result = {
 433                     let left_bytes = this.memory.read_bytes(left, n)?;
 434                     let right_bytes = this.memory.read_bytes(right, n)?;
 435
 436                     use std::cmp::Ordering::*;
 437                     match left_bytes.cmp(right_bytes) {
 438                         Less => -1i32,
 439                         Equal => 0,
 440                         Greater => 1,
 441                     }
 442                 };
 443
 444                 this.write_scalar(Scalar::from_i32(result), dest)?;
 445             }
 446             "memrchr" => {
 447                 let &[ref ptr, ref val, ref num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 448                 let ptr = this.read_scalar(ptr)?.check_init()?;
 449                 let val = this.read_scalar(val)?.to_i32()? as u8;
 450                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 451                 if let Some(idx) = this
 452                     .memory
 453                     .read_bytes(ptr, Size::from_bytes(num))?
 454                     .iter()
 455                     .rev()
 456                     .position(|&c| c == val)
 457                 {
 458                     let new_ptr = ptr.ptr_offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 459                     this.write_scalar(new_ptr, dest)?;
 460                 } else {
 461                     this.write_null(dest)?;
 462                 }
 463             }
 464             "memchr" => {
 465                 let &[ref ptr, ref val, ref num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 466                 let ptr = this.read_scalar(ptr)?.check_init()?;
 467                 let val = this.read_scalar(val)?.to_i32()? as u8;
 468                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 469                 let idx = this
 470                     .memory
 471                     .read_bytes(ptr, Size::from_bytes(num))?
 472                     .iter()
 473                     .position(|&c| c == val);
 474                 if let Some(idx) = idx {
 475                     let new_ptr = ptr.ptr_offset(Size::from_bytes(idx as u64), this)?;
 476                     this.write_scalar(new_ptr, dest)?;
 477                 } else {
 478                     this.write_null(dest)?;
 479                 }
 480             }
 481             "strlen" => {
 482                 let &[ref ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 483                 let ptr = this.read_scalar(ptr)?.check_init()?;
 484                 let n = this.read_c_str(ptr)?.len();
 485                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 486             }
 487
 488             // math functions
 489             #[rustfmt::skip]
 490             | "cbrtf"
 491             | "coshf"
 492             | "sinhf"
 493             | "tanf"
 494             | "acosf"
 495             | "asinf"
 496             | "atanf"
 497             => {
 498                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 499                 // FIXME: Using host floats.
 500                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 501                 let f = match &*link_name.as_str() {
 502                     "cbrtf" => f.cbrt(),
 503                     "coshf" => f.cosh(),
 504                     "sinhf" => f.sinh(),
 505                     "tanf" => f.tan(),
 506                     "acosf" => f.acos(),
 507                     "asinf" => f.asin(),
 508                     "atanf" => f.atan(),
 509                     _ => bug!(),
 510                 };
 511                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 512             }
 513             #[rustfmt::skip]
 514             | "_hypotf"
 515             | "hypotf"
 516             | "atan2f"
 517             => {
 518                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 519                 // underscore case for windows, here and below
 520                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 521                 // FIXME: Using host floats.
 522                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 523                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 524                 let n = match &*link_name.as_str() {
 525                     "_hypotf" | "hypotf" => f1.hypot(f2),
 526                     "atan2f" => f1.atan2(f2),
 527                     _ => bug!(),
 528                 };
 529                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 530             }
 531             #[rustfmt::skip]
 532             | "cbrt"
 533             | "cosh"
 534             | "sinh"
 535             | "tan"
 536             | "acos"
 537             | "asin"
 538             | "atan"
 539             => {
 540                 let &[ref f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 541                 // FIXME: Using host floats.
 542                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 543                 let f = match &*link_name.as_str() {
 544                     "cbrt" => f.cbrt(),
 545                     "cosh" => f.cosh(),
 546                     "sinh" => f.sinh(),
 547                     "tan" => f.tan(),
 548                     "acos" => f.acos(),
 549                     "asin" => f.asin(),
 550                     "atan" => f.atan(),
 551                     _ => bug!(),
 552                 };
 553                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 554             }
 555             #[rustfmt::skip]
 556             | "_hypot"
 557             | "hypot"
 558             | "atan2"
 559             => {
 560                 let &[ref f1, ref f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 561                 // FIXME: Using host floats.
 562                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 563                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 564                 let n = match &*link_name.as_str() {
 565                     "_hypot" | "hypot" => f1.hypot(f2),
 566                     "atan2" => f1.atan2(f2),
 567                     _ => bug!(),
 568                 };
 569                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 570             }
 571             #[rustfmt::skip]
 572             | "_ldexp"
 573             | "ldexp"
 574             | "scalbn"
 575             => {
 576                 let &[ref x, ref exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 577                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 578                 let x = this.read_scalar(x)?.to_f64()?;
 579                 let exp = this.read_scalar(exp)?.to_i32()?;
 580
 581                 // Saturating cast to i16. Even those are outside the valid exponent range to
 582                 // `scalbn` below will do its over/underflow handling.
 583                 let exp = if exp > i32::from(i16::MAX) {
 584                     i16::MAX
 585                 } else if exp < i32::from(i16::MIN) {
 586                     i16::MIN
 587                 } else {
 588                     exp.try_into().unwrap()
 589                 };
 590
 591                 let res = x.scalbn(exp);
 592                 this.write_scalar(Scalar::from_f64(res), dest)?;
 593             }
 594
 595             // Architecture-specific shims
 596             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 597                 let &[] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 598                 this.yield_active_thread();
 599             }
 600             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 601                 let &[ref arg] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 602                 let arg = this.read_scalar(arg)?.to_i32()?;
 603                 match arg {
 604                     15 => { // SY ("full system scope")
 605                         this.yield_active_thread();
 606                     }
 607                     _ => {
 608                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 609                     }
 610                 }
 611             }
 612
 613             // Platform-specific shims
 614             _ => match this.tcx.sess.target.os.as_str() {
 615                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 616                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 617                 target => throw_unsup_format!("the target `{}` is not supported", target),
 618             }
 619         };
 620
 621         // We only fall through to here if we did *not* hit the `_` arm above,
 622         // i.e., if we actually emulated the function.
 623         Ok(EmulateByNameResult::NeedsJumping)
 624     }
 625
 626     /// Check some basic requirements for this allocation request:
 627     /// non-zero size, power-of-two alignment.
 628     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 629         if size == 0 {
 630             throw_ub_format!("creating allocation with size 0");
 631         }
 632         if !align.is_power_of_two() {
 633             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 634         }
 635         Ok(())
 636     }
 637 }