src/shims/foreign_items.rs

   1 use std::{collections::hash_map::Entry, iter};
   2
   3 use log::trace;
   4
   5 use rustc_apfloat::Float;
   6 use rustc_ast::expand::allocator::AllocatorKind;
   7 use rustc_hir::{
   8     def::DefKind,
   9     def_id::{CrateNum, DefId, LOCAL_CRATE},
  10 };
  11 use rustc_middle::middle::{
  12     codegen_fn_attrs::CodegenFnAttrFlags, dependency_format::Linkage,
  13     exported_symbols::ExportedSymbol,
  14 };
  15 use rustc_middle::mir;
  16 use rustc_middle::ty;
  17 use rustc_session::config::CrateType;
  18 use rustc_span::Symbol;
  19 use rustc_target::{
  20     abi::{Align, Size},
  21     spec::abi::Abi,
  22 };
  23
  24 use super::backtrace::EvalContextExt as _;
  25 use crate::helpers::convert::Truncate;
  26 use crate::*;
  27
  28 /// Returned by `emulate_foreign_item_by_name`.
  29 pub enum EmulateByNameResult<'mir, 'tcx> {
  30     /// The caller is expected to jump to the return block.
  31     NeedsJumping,
  32     /// Jumping has already been taken care of.
  33     AlreadyJumped,
  34     /// A MIR body has been found for the function
  35     MirBody(&'mir mir::Body<'tcx>, ty::Instance<'tcx>),
  36     /// The item is not supported.
  37     NotSupported,
  38 }
  39
  40 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  41 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  42     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  43     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  44         let this = self.eval_context_ref();
  45         // List taken from `libstd/sys_common/alloc.rs`.
  46         let min_align = match this.tcx.sess.target.arch.as_ref() {
  47             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  48             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  49             arch => bug!("Unsupported target architecture: {}", arch),
  50         };
  51         // Windows always aligns, even small allocations.
  52         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  53         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  54         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  55             return Align::from_bytes(min_align).unwrap();
  56         }
  57         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  58         fn prev_power_of_two(x: u64) -> u64 {
  59             let next_pow2 = x.next_power_of_two();
  60             if next_pow2 == x {
  61                 // x *is* a power of two, just use that.
  62                 x
  63             } else {
  64                 // x is between two powers, so next = 2*prev.
  65                 next_pow2 / 2
  66             }
  67         }
  68         Align::from_bytes(prev_power_of_two(size)).unwrap()
  69     }
  70
  71     fn malloc(
  72         &mut self,
  73         size: u64,
  74         zero_init: bool,
  75         kind: MiriMemoryKind,
  76     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
  77         let this = self.eval_context_mut();
  78         if size == 0 {
  79             Ok(Pointer::null())
  80         } else {
  81             let align = this.min_align(size, kind);
  82             let ptr = this.allocate_ptr(Size::from_bytes(size), align, kind.into())?;
  83             if zero_init {
  84                 // We just allocated this, the access is definitely in-bounds.
  85                 this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  86             }
  87             Ok(ptr.into())
  88         }
  89     }
  90
  91     fn free(&mut self, ptr: Pointer<Option<Tag>>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  92         let this = self.eval_context_mut();
  93         if !this.ptr_is_null(ptr)? {
  94             this.deallocate_ptr(ptr, None, kind.into())?;
  95         }
  96         Ok(())
  97     }
  98
  99     fn realloc(
 100         &mut self,
 101         old_ptr: Pointer<Option<Tag>>,
 102         new_size: u64,
 103         kind: MiriMemoryKind,
 104     ) -> InterpResult<'tcx, Pointer<Option<Tag>>> {
 105         let this = self.eval_context_mut();
 106         let new_align = this.min_align(new_size, kind);
 107         if this.ptr_is_null(old_ptr)? {
 108             if new_size == 0 {
 109                 Ok(Pointer::null())
 110             } else {
 111                 let new_ptr =
 112                     this.allocate_ptr(Size::from_bytes(new_size), new_align, kind.into())?;
 113                 Ok(new_ptr.into())
 114             }
 115         } else {
 116             if new_size == 0 {
 117                 this.deallocate_ptr(old_ptr, None, kind.into())?;
 118                 Ok(Pointer::null())
 119             } else {
 120                 let new_ptr = this.reallocate_ptr(
 121                     old_ptr,
 122                     None,
 123                     Size::from_bytes(new_size),
 124                     new_align,
 125                     kind.into(),
 126                 )?;
 127                 Ok(new_ptr.into())
 128             }
 129         }
 130     }
 131
 132     /// Lookup the body of a function that has `link_name` as the symbol name.
 133     fn lookup_exported_symbol(
 134         &mut self,
 135         link_name: Symbol,
 136     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 137         let this = self.eval_context_mut();
 138         let tcx = this.tcx.tcx;
 139
 140         // If the result was cached, just return it.
 141         // (Cannot use `or_insert` since the code below might have to throw an error.)
 142         let entry = this.machine.exported_symbols_cache.entry(link_name);
 143         let instance = *match entry {
 144             Entry::Occupied(e) => e.into_mut(),
 145             Entry::Vacant(e) => {
 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 iter::once(LOCAL_CRATE).chain(
 157                     dependency_format.1.iter().enumerate().filter_map(|(num, &linkage)| {
 158                         (linkage != Linkage::NotLinked).then_some(CrateNum::new(num + 1))
 159                     }),
 160                 ) {
 161                     // We can ignore `_export_info` here: we are a Rust crate, and everything is exported
 162                     // from a Rust crate.
 163                     for &(symbol, _export_info) 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((original_instance, original_cnum)) = instance_and_crate
 176                                 {
 177                                     // Make sure we are consistent wrt what is 'first' and 'second'.
 178                                     let original_span =
 179                                         tcx.def_span(original_instance.def_id()).data();
 180                                     let span = tcx.def_span(def_id).data();
 181                                     if original_span < span {
 182                                         throw_machine_stop!(
 183                                             TerminationInfo::MultipleSymbolDefinitions {
 184                                                 link_name,
 185                                                 first: original_span,
 186                                                 first_crate: tcx.crate_name(original_cnum),
 187                                                 second: span,
 188                                                 second_crate: tcx.crate_name(cnum),
 189                                             }
 190                                         );
 191                                     } else {
 192                                         throw_machine_stop!(
 193                                             TerminationInfo::MultipleSymbolDefinitions {
 194                                                 link_name,
 195                                                 first: span,
 196                                                 first_crate: tcx.crate_name(cnum),
 197                                                 second: original_span,
 198                                                 second_crate: tcx.crate_name(original_cnum),
 199                                             }
 200                                         );
 201                                     }
 202                                 }
 203                                 if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
 204                                     throw_ub_format!(
 205                                         "attempt to call an exported symbol that is not defined as a function"
 206                                     );
 207                                 }
 208                                 instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
 209                             }
 210                         }
 211                     }
 212                 }
 213
 214                 e.insert(instance_and_crate.map(|ic| ic.0))
 215             }
 216         };
 217         match instance {
 218             None => Ok(None), // no symbol with this name
 219             Some(instance) => Ok(Some((this.load_mir(instance.def, None)?, instance))),
 220         }
 221     }
 222
 223     /// Emulates calling a foreign item, failing if the item is not supported.
 224     /// This function will handle `goto_block` if needed.
 225     /// Returns Ok(None) if the foreign item was completely handled
 226     /// by this function.
 227     /// Returns Ok(Some(body)) if processing the foreign item
 228     /// is delegated to another function.
 229     fn emulate_foreign_item(
 230         &mut self,
 231         def_id: DefId,
 232         abi: Abi,
 233         args: &[OpTy<'tcx, Tag>],
 234         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 235         unwind: StackPopUnwind,
 236     ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
 237         let this = self.eval_context_mut();
 238         let link_name = this.item_link_name(def_id);
 239         let tcx = this.tcx.tcx;
 240
 241         // First: functions that diverge.
 242         let (dest, ret) = match ret {
 243             None =>
 244                 match &*link_name.as_str() {
 245                     "miri_start_panic" => {
 246                         // `check_shim` happens inside `handle_miri_start_panic`.
 247                         this.handle_miri_start_panic(abi, link_name, args, unwind)?;
 248                         return Ok(None);
 249                     }
 250                     // This matches calls to the foreign item `panic_impl`.
 251                     // The implementation is provided by the function with the `#[panic_handler]` attribute.
 252                     "panic_impl" => {
 253                         // We don't use `check_shim` here because we are just forwarding to the lang
 254                         // item. Argument count checking will be performed when the returned `Body` is
 255                         // called.
 256                         this.check_abi_and_shim_symbol_clash(abi, Abi::Rust, link_name)?;
 257                         let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 258                         let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 259                         return Ok(Some((
 260                             &*this.load_mir(panic_impl_instance.def, None)?,
 261                             panic_impl_instance,
 262                         )));
 263                     }
 264                     #[rustfmt::skip]
 265                     | "exit"
 266                     | "ExitProcess"
 267                     => {
 268                         let exp_abi = if link_name.as_str() == "exit" {
 269                             Abi::C { unwind: false }
 270                         } else {
 271                             Abi::System { unwind: false }
 272                         };
 273                         let [code] = this.check_shim(abi, exp_abi, link_name, args)?;
 274                         // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 275                         let code = this.read_scalar(code)?.to_i32()?;
 276                         throw_machine_stop!(TerminationInfo::Exit(code.into()));
 277                     }
 278                     "abort" => {
 279                         let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 280                         throw_machine_stop!(TerminationInfo::Abort(
 281                             "the program aborted execution".to_owned()
 282                         ))
 283                     }
 284                     _ => {
 285                         if let Some(body) = this.lookup_exported_symbol(link_name)? {
 286                             return Ok(Some(body));
 287                         }
 288                         this.handle_unsupported(format!(
 289                             "can't call (diverging) foreign function: {}",
 290                             link_name
 291                         ))?;
 292                         return Ok(None);
 293                     }
 294                 },
 295             Some(p) => p,
 296         };
 297
 298         // Second: functions that return.
 299         match this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 300             EmulateByNameResult::NeedsJumping => {
 301                 trace!("{:?}", this.dump_place(**dest));
 302                 this.go_to_block(ret);
 303             }
 304             EmulateByNameResult::AlreadyJumped => (),
 305             EmulateByNameResult::MirBody(mir, instance) => return Ok(Some((mir, instance))),
 306             EmulateByNameResult::NotSupported => {
 307                 if let Some(body) = this.lookup_exported_symbol(link_name)? {
 308                     return Ok(Some(body));
 309                 }
 310
 311                 this.handle_unsupported(format!("can't call foreign function: {}", link_name))?;
 312                 return Ok(None);
 313             }
 314         }
 315
 316         Ok(None)
 317     }
 318
 319     /// Emulates calling the internal __rust_* allocator functions
 320     fn emulate_allocator(
 321         &mut self,
 322         symbol: Symbol,
 323         default: impl FnOnce(&mut MiriEvalContext<'mir, 'tcx>) -> InterpResult<'tcx>,
 324     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 325         let this = self.eval_context_mut();
 326
 327         let allocator_kind = if let Some(allocator_kind) = this.tcx.allocator_kind(()) {
 328             allocator_kind
 329         } else {
 330             // in real code, this symbol does not exist without an allocator
 331             return Ok(EmulateByNameResult::NotSupported);
 332         };
 333
 334         match allocator_kind {
 335             AllocatorKind::Global => {
 336                 let (body, instance) = this
 337                     .lookup_exported_symbol(symbol)?
 338                     .expect("symbol should be present if there is a global allocator");
 339
 340                 Ok(EmulateByNameResult::MirBody(body, instance))
 341             }
 342             AllocatorKind::Default => {
 343                 default(this)?;
 344                 Ok(EmulateByNameResult::NeedsJumping)
 345             }
 346         }
 347     }
 348
 349     /// Emulates calling a foreign item using its name.
 350     fn emulate_foreign_item_by_name(
 351         &mut self,
 352         link_name: Symbol,
 353         abi: Abi,
 354         args: &[OpTy<'tcx, Tag>],
 355         dest: &PlaceTy<'tcx, Tag>,
 356         ret: mir::BasicBlock,
 357     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
 358         let this = self.eval_context_mut();
 359
 360         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 361         // shim, add it to the corresponding submodule.
 362         match &*link_name.as_str() {
 363             // Miri-specific extern functions
 364             "miri_static_root" => {
 365                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 366                 let ptr = this.read_pointer(ptr)?;
 367                 let (alloc_id, offset, _) = this.ptr_get_alloc_id(ptr)?;
 368                 if offset != Size::ZERO {
 369                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 370                 }
 371                 this.machine.static_roots.push(alloc_id);
 372             }
 373
 374             // Obtains the size of a Miri backtrace. See the README for details.
 375             "miri_backtrace_size" => {
 376                 this.handle_miri_backtrace_size(abi, link_name, args, dest)?;
 377             }
 378
 379             // Obtains a Miri backtrace. See the README for details.
 380             "miri_get_backtrace" => {
 381                 // `check_shim` happens inside `handle_miri_get_backtrace`.
 382                 this.handle_miri_get_backtrace(abi, link_name, args, dest)?;
 383             }
 384
 385             // Resolves a Miri backtrace frame. See the README for details.
 386             "miri_resolve_frame" => {
 387                 // `check_shim` happens inside `handle_miri_resolve_frame`.
 388                 this.handle_miri_resolve_frame(abi, link_name, args, dest)?;
 389             }
 390
 391             // Writes the function and file names of a Miri backtrace frame into a user provided buffer. See the README for details.
 392             "miri_resolve_frame_names" => {
 393                 this.handle_miri_resolve_frame_names(abi, link_name, args)?;
 394             }
 395
 396             // Standard C allocation
 397             "malloc" => {
 398                 let [size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 399                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 400                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C)?;
 401                 this.write_pointer(res, dest)?;
 402             }
 403             "calloc" => {
 404                 let [items, len] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 405                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 406                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 407                 let size =
 408                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 409                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C)?;
 410                 this.write_pointer(res, dest)?;
 411             }
 412             "free" => {
 413                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 414                 let ptr = this.read_pointer(ptr)?;
 415                 this.free(ptr, MiriMemoryKind::C)?;
 416             }
 417             "realloc" => {
 418                 let [old_ptr, new_size] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 419                 let old_ptr = this.read_pointer(old_ptr)?;
 420                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 421                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 422                 this.write_pointer(res, dest)?;
 423             }
 424
 425             // Rust allocation
 426             "__rust_alloc" => {
 427                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 428                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 429                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 430
 431                 return this.emulate_allocator(Symbol::intern("__rg_alloc"), |this| {
 432                     Self::check_alloc_request(size, align)?;
 433
 434                     let ptr = this.allocate_ptr(
 435                         Size::from_bytes(size),
 436                         Align::from_bytes(align).unwrap(),
 437                         MiriMemoryKind::Rust.into(),
 438                     )?;
 439
 440                     this.write_pointer(ptr, dest)
 441                 });
 442             }
 443             "__rust_alloc_zeroed" => {
 444                 let [size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 445                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 446                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 447
 448                 return this.emulate_allocator(Symbol::intern("__rg_alloc_zeroed"), |this| {
 449                     Self::check_alloc_request(size, align)?;
 450
 451                     let ptr = this.allocate_ptr(
 452                         Size::from_bytes(size),
 453                         Align::from_bytes(align).unwrap(),
 454                         MiriMemoryKind::Rust.into(),
 455                     )?;
 456
 457                     // We just allocated this, the access is definitely in-bounds.
 458                     this.write_bytes_ptr(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 459                     this.write_pointer(ptr, dest)
 460                 });
 461             }
 462             "__rust_dealloc" => {
 463                 let [ptr, old_size, align] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 464                 let ptr = this.read_pointer(ptr)?;
 465                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 466                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 467
 468                 return this.emulate_allocator(Symbol::intern("__rg_dealloc"), |this| {
 469                     // No need to check old_size/align; we anyway check that they match the allocation.
 470                     this.deallocate_ptr(
 471                         ptr,
 472                         Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 473                         MiriMemoryKind::Rust.into(),
 474                     )
 475                 });
 476             }
 477             "__rust_realloc" => {
 478                 let [ptr, old_size, align, new_size] = this.check_shim(abi, Abi::Rust, link_name, args)?;
 479                 let ptr = this.read_pointer(ptr)?;
 480                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 481                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 482                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 483                 // No need to check old_size; we anyway check that they match the allocation.
 484
 485                 return this.emulate_allocator(Symbol::intern("__rg_realloc"), |this| {
 486                     Self::check_alloc_request(new_size, align)?;
 487
 488                     let align = Align::from_bytes(align).unwrap();
 489                     let new_ptr = this.reallocate_ptr(
 490                         ptr,
 491                         Some((Size::from_bytes(old_size), align)),
 492                         Size::from_bytes(new_size),
 493                         align,
 494                         MiriMemoryKind::Rust.into(),
 495                     )?;
 496                     this.write_pointer(new_ptr, dest)
 497                 });
 498             }
 499
 500             // C memory handling functions
 501             "memcmp" => {
 502                 let [left, right, n] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 503                 let left = this.read_pointer(left)?;
 504                 let right = this.read_pointer(right)?;
 505                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 506
 507                 let result = {
 508                     let left_bytes = this.read_bytes_ptr(left, n)?;
 509                     let right_bytes = this.read_bytes_ptr(right, n)?;
 510
 511                     use std::cmp::Ordering::*;
 512                     match left_bytes.cmp(right_bytes) {
 513                         Less => -1i32,
 514                         Equal => 0,
 515                         Greater => 1,
 516                     }
 517                 };
 518
 519                 this.write_scalar(Scalar::from_i32(result), dest)?;
 520             }
 521             "memrchr" => {
 522                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 523                 let ptr = this.read_pointer(ptr)?;
 524                 let val = this.read_scalar(val)?.to_i32()? as u8;
 525                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 526                 if let Some(idx) = this
 527                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 528                     .iter()
 529                     .rev()
 530                     .position(|&c| c == val)
 531                 {
 532                     let new_ptr = ptr.offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 533                     this.write_pointer(new_ptr, dest)?;
 534                 } else {
 535                     this.write_null(dest)?;
 536                 }
 537             }
 538             "memchr" => {
 539                 let [ptr, val, num] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 540                 let ptr = this.read_pointer(ptr)?;
 541                 let val = this.read_scalar(val)?.to_i32()? as u8;
 542                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 543                 let idx = this
 544                     .read_bytes_ptr(ptr, Size::from_bytes(num))?
 545                     .iter()
 546                     .position(|&c| c == val);
 547                 if let Some(idx) = idx {
 548                     let new_ptr = ptr.offset(Size::from_bytes(idx as u64), this)?;
 549                     this.write_pointer(new_ptr, dest)?;
 550                 } else {
 551                     this.write_null(dest)?;
 552                 }
 553             }
 554             "strlen" => {
 555                 let [ptr] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 556                 let ptr = this.read_pointer(ptr)?;
 557                 let n = this.read_c_str(ptr)?.len();
 558                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 559             }
 560
 561             // math functions
 562             #[rustfmt::skip]
 563             | "cbrtf"
 564             | "coshf"
 565             | "sinhf"
 566             | "tanf"
 567             | "acosf"
 568             | "asinf"
 569             | "atanf"
 570             => {
 571                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 572                 // FIXME: Using host floats.
 573                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 574                 let f = match &*link_name.as_str() {
 575                     "cbrtf" => f.cbrt(),
 576                     "coshf" => f.cosh(),
 577                     "sinhf" => f.sinh(),
 578                     "tanf" => f.tan(),
 579                     "acosf" => f.acos(),
 580                     "asinf" => f.asin(),
 581                     "atanf" => f.atan(),
 582                     _ => bug!(),
 583                 };
 584                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 585             }
 586             #[rustfmt::skip]
 587             | "_hypotf"
 588             | "hypotf"
 589             | "atan2f"
 590             => {
 591                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 592                 // underscore case for windows, here and below
 593                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 594                 // FIXME: Using host floats.
 595                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 596                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 597                 let n = match &*link_name.as_str() {
 598                     "_hypotf" | "hypotf" => f1.hypot(f2),
 599                     "atan2f" => f1.atan2(f2),
 600                     _ => bug!(),
 601                 };
 602                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 603             }
 604             #[rustfmt::skip]
 605             | "cbrt"
 606             | "cosh"
 607             | "sinh"
 608             | "tan"
 609             | "acos"
 610             | "asin"
 611             | "atan"
 612             => {
 613                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 614                 // FIXME: Using host floats.
 615                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 616                 let f = match &*link_name.as_str() {
 617                     "cbrt" => f.cbrt(),
 618                     "cosh" => f.cosh(),
 619                     "sinh" => f.sinh(),
 620                     "tan" => f.tan(),
 621                     "acos" => f.acos(),
 622                     "asin" => f.asin(),
 623                     "atan" => f.atan(),
 624                     _ => bug!(),
 625                 };
 626                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 627             }
 628             #[rustfmt::skip]
 629             | "_hypot"
 630             | "hypot"
 631             | "atan2"
 632             => {
 633                 let [f1, f2] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 634                 // FIXME: Using host floats.
 635                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 636                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 637                 let n = match &*link_name.as_str() {
 638                     "_hypot" | "hypot" => f1.hypot(f2),
 639                     "atan2" => f1.atan2(f2),
 640                     _ => bug!(),
 641                 };
 642                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 643             }
 644             #[rustfmt::skip]
 645             | "_ldexp"
 646             | "ldexp"
 647             | "scalbn"
 648             => {
 649                 let [x, exp] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 650                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 651                 let x = this.read_scalar(x)?.to_f64()?;
 652                 let exp = this.read_scalar(exp)?.to_i32()?;
 653
 654                 // Saturating cast to i16. Even those are outside the valid exponent range to
 655                 // `scalbn` below will do its over/underflow handling.
 656                 let exp = if exp > i32::from(i16::MAX) {
 657                     i16::MAX
 658                 } else if exp < i32::from(i16::MIN) {
 659                     i16::MIN
 660                 } else {
 661                     exp.try_into().unwrap()
 662                 };
 663
 664                 let res = x.scalbn(exp);
 665                 this.write_scalar(Scalar::from_f64(res), dest)?;
 666             }
 667
 668             // Architecture-specific shims
 669             "llvm.x86.addcarry.64" if this.tcx.sess.target.arch == "x86_64" => {
 670                 // Computes u8+u64+u64, returning tuple (u8,u64) comprising the output carry and truncated sum.
 671                 let [c_in, a, b] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 672                 let c_in = this.read_scalar(c_in)?.to_u8()?;
 673                 let a = this.read_scalar(a)?.to_u64()?;
 674                 let b = this.read_scalar(b)?.to_u64()?;
 675
 676                 let wide_sum = u128::from(c_in) + u128::from(a) + u128::from(b);
 677                 let (c_out, sum) = ((wide_sum >> 64).truncate::<u8>(), wide_sum.truncate::<u64>());
 678
 679                 let c_out_field = this.place_field(dest, 0)?;
 680                 this.write_scalar(Scalar::from_u8(c_out), &c_out_field)?;
 681                 let sum_field = this.place_field(dest, 1)?;
 682                 this.write_scalar(Scalar::from_u64(sum), &sum_field)?;
 683             }
 684             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 685                 let [] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
 686                 this.yield_active_thread();
 687             }
 688             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 689                 let [arg] = this.check_shim(abi, Abi::Unadjusted, link_name, args)?;
 690                 let arg = this.read_scalar(arg)?.to_i32()?;
 691                 match arg {
 692                     15 => { // SY ("full system scope")
 693                         this.yield_active_thread();
 694                     }
 695                     _ => {
 696                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 697                     }
 698                 }
 699             }
 700
 701             // Platform-specific shims
 702             _ => match this.tcx.sess.target.os.as_ref() {
 703                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 704                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 705                 target => throw_unsup_format!("the target `{}` is not supported", target),
 706             }
 707         };
 708
 709         // We only fall through to here if we did *not* hit the `_` arm above,
 710         // i.e., if we actually emulated the function.
 711         Ok(EmulateByNameResult::NeedsJumping)
 712     }
 713
 714     /// Check some basic requirements for this allocation request:
 715     /// non-zero size, power-of-two alignment.
 716     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 717         if size == 0 {
 718             throw_ub_format!("creating allocation with size 0");
 719         }
 720         if !align.is_power_of_two() {
 721             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 722         }
 723         Ok(())
 724     }
 725 }