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::def_id::DefId;
  10 use rustc_middle::mir;
  11 use rustc_middle::ty;
  12 use rustc_span::symbol::sym;
  13 use rustc_target::{
  14     abi::{Align, Size},
  15     spec::{abi::Abi, PanicStrategy},
  16 };
  17
  18 use super::backtrace::EvalContextExt as _;
  19 use crate::*;
  20 use helpers::{check_abi, check_arg_count};
  21
  22 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  23 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  24     /// Returns the minimum alignment for the target architecture for allocations of the given size.
  25     fn min_align(&self, size: u64, kind: MiriMemoryKind) -> Align {
  26         let this = self.eval_context_ref();
  27         // List taken from `libstd/sys_common/alloc.rs`.
  28         let min_align = match this.tcx.sess.target.arch.as_str() {
  29             "x86" | "arm" | "mips" | "powerpc" | "powerpc64" | "asmjs" | "wasm32" => 8,
  30             "x86_64" | "aarch64" | "mips64" | "s390x" | "sparc64" => 16,
  31             arch => bug!("Unsupported target architecture: {}", arch),
  32         };
  33         // Windows always aligns, even small allocations.
  34         // Source: <https://support.microsoft.com/en-us/help/286470/how-to-use-pageheap-exe-in-windows-xp-windows-2000-and-windows-server>
  35         // But jemalloc does not, so for the C heap we only align if the allocation is sufficiently big.
  36         if kind == MiriMemoryKind::WinHeap || size >= min_align {
  37             return Align::from_bytes(min_align).unwrap();
  38         }
  39         // We have `size < min_align`. Round `size` *down* to the next power of two and use that.
  40         fn prev_power_of_two(x: u64) -> u64 {
  41             let next_pow2 = x.next_power_of_two();
  42             if next_pow2 == x {
  43                 // x *is* a power of two, just use that.
  44                 x
  45             } else {
  46                 // x is between two powers, so next = 2*prev.
  47                 next_pow2 / 2
  48             }
  49         }
  50         Align::from_bytes(prev_power_of_two(size)).unwrap()
  51     }
  52
  53     fn malloc(&mut self, size: u64, zero_init: bool, kind: MiriMemoryKind) -> Scalar<Tag> {
  54         let this = self.eval_context_mut();
  55         if size == 0 {
  56             Scalar::null_ptr(this)
  57         } else {
  58             let align = this.min_align(size, kind);
  59             let ptr = this.memory.allocate(Size::from_bytes(size), align, kind.into());
  60             if zero_init {
  61                 // We just allocated this, the access is definitely in-bounds.
  62                 this.memory.write_bytes(ptr.into(), iter::repeat(0u8).take(size as usize)).unwrap();
  63             }
  64             Scalar::Ptr(ptr)
  65         }
  66     }
  67
  68     fn free(&mut self, ptr: Scalar<Tag>, kind: MiriMemoryKind) -> InterpResult<'tcx> {
  69         let this = self.eval_context_mut();
  70         if !this.is_null(ptr)? {
  71             let ptr = this.force_ptr(ptr)?;
  72             this.memory.deallocate(ptr, None, kind.into())?;
  73         }
  74         Ok(())
  75     }
  76
  77     fn realloc(
  78         &mut self,
  79         old_ptr: Scalar<Tag>,
  80         new_size: u64,
  81         kind: MiriMemoryKind,
  82     ) -> InterpResult<'tcx, Scalar<Tag>> {
  83         let this = self.eval_context_mut();
  84         let new_align = this.min_align(new_size, kind);
  85         if this.is_null(old_ptr)? {
  86             if new_size == 0 {
  87                 Ok(Scalar::null_ptr(this))
  88             } else {
  89                 let new_ptr =
  90                     this.memory.allocate(Size::from_bytes(new_size), new_align, kind.into());
  91                 Ok(Scalar::Ptr(new_ptr))
  92             }
  93         } else {
  94             let old_ptr = this.force_ptr(old_ptr)?;
  95             if new_size == 0 {
  96                 this.memory.deallocate(old_ptr, None, kind.into())?;
  97                 Ok(Scalar::null_ptr(this))
  98             } else {
  99                 let new_ptr = this.memory.reallocate(
 100                     old_ptr,
 101                     None,
 102                     Size::from_bytes(new_size),
 103                     new_align,
 104                     kind.into(),
 105                 )?;
 106                 Ok(Scalar::Ptr(new_ptr))
 107             }
 108         }
 109     }
 110
 111     /// Emulates calling a foreign item, failing if the item is not supported.
 112     /// This function will handle `goto_block` if needed.
 113     /// Returns Ok(None) if the foreign item was completely handled
 114     /// by this function.
 115     /// Returns Ok(Some(body)) if processing the foreign item
 116     /// is delegated to another function.
 117     fn emulate_foreign_item(
 118         &mut self,
 119         def_id: DefId,
 120         abi: Abi,
 121         args: &[OpTy<'tcx, Tag>],
 122         ret: Option<(&PlaceTy<'tcx, Tag>, mir::BasicBlock)>,
 123         unwind: StackPopUnwind,
 124     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
 125         let this = self.eval_context_mut();
 126         let attrs = this.tcx.get_attrs(def_id);
 127         let link_name = match this.tcx.sess.first_attr_value_str_by_name(&attrs, sym::link_name) {
 128             Some(name) => name.as_str(),
 129             None => this.tcx.item_name(def_id).as_str(),
 130         };
 131         // Strip linker suffixes (seen on 32-bit macOS).
 132         let link_name = link_name.trim_end_matches("$UNIX2003");
 133         let tcx = this.tcx.tcx;
 134
 135         // First: functions that diverge.
 136         let (dest, ret) = match ret {
 137             None => match link_name {
 138                 "miri_start_panic" => {
 139                     check_abi(this, abi, Abi::Rust)?;
 140                     this.handle_miri_start_panic(args, unwind)?;
 141                     return Ok(None);
 142                 }
 143                 // This matches calls to the foreign item `panic_impl`.
 144                 // The implementation is provided by the function with the `#[panic_handler]` attribute.
 145                 "panic_impl" => {
 146                     check_abi(this, abi, Abi::Rust)?;
 147                     let panic_impl_id = tcx.lang_items().panic_impl().unwrap();
 148                     let panic_impl_instance = ty::Instance::mono(tcx, panic_impl_id);
 149                     return Ok(Some(&*this.load_mir(panic_impl_instance.def, None)?));
 150                 }
 151                 #[rustfmt::skip]
 152                 | "exit"
 153                 | "ExitProcess"
 154                 => {
 155                     check_abi(this, abi, if link_name == "exit" { Abi::C { unwind: false } } else { Abi::System { unwind: false } })?;
 156                     let &[ref code] = check_arg_count(args)?;
 157                     // it's really u32 for ExitProcess, but we have to put it into the `Exit` variant anyway
 158                     let code = this.read_scalar(code)?.to_i32()?;
 159                     throw_machine_stop!(TerminationInfo::Exit(code.into()));
 160                 }
 161                 "abort" => {
 162                     check_abi(this, abi, Abi::C { unwind: false })?;
 163                     throw_machine_stop!(TerminationInfo::Abort(
 164                         "the program aborted execution".to_owned()
 165                     ))
 166                 }
 167                 _ => throw_unsup_format!("can't call (diverging) foreign function: {}", link_name),
 168             },
 169             Some(p) => p,
 170         };
 171
 172         // Second: some functions that we forward to MIR implementations.
 173         match link_name {
 174             // This matches calls to the foreign item `__rust_start_panic`, that is,
 175             // calls to `extern "Rust" { fn __rust_start_panic(...) }`
 176             // (and `__rust_panic_cleanup`, respectively).
 177             // We forward this to the underlying *implementation* in the panic runtime crate.
 178             // Normally, this will be either `libpanic_unwind` or `libpanic_abort`, but it could
 179             // also be a custom user-provided implementation via `#![feature(panic_runtime)]`
 180             #[rustfmt::skip]
 181             "__rust_start_panic" |
 182             "__rust_panic_cleanup" => {
 183                 check_abi(this, abi, Abi::C { unwind: false })?;
 184                 // This replicates some of the logic in `inject_panic_runtime`.
 185                 // FIXME: is there a way to reuse that logic?
 186                 let panic_runtime = match this.tcx.sess.panic_strategy() {
 187                     PanicStrategy::Unwind => sym::panic_unwind,
 188                     PanicStrategy::Abort => sym::panic_abort,
 189                 };
 190                 let start_panic_instance =
 191                     this.resolve_path(&[&*panic_runtime.as_str(), link_name]);
 192                 return Ok(Some(&*this.load_mir(start_panic_instance.def, None)?));
 193             }
 194             _ => {}
 195         }
 196
 197         // Third: functions that return.
 198         if this.emulate_foreign_item_by_name(link_name, abi, args, dest, ret)? {
 199             trace!("{:?}", this.dump_place(**dest));
 200             this.go_to_block(ret);
 201         }
 202
 203         Ok(None)
 204     }
 205
 206     /// Emulates calling a foreign item using its name, failing if the item is not supported.
 207     /// Returns `true` if the caller is expected to jump to the return block, and `false` if
 208     /// jumping has already been taken care of.
 209     fn emulate_foreign_item_by_name(
 210         &mut self,
 211         link_name: &str,
 212         abi: Abi,
 213         args: &[OpTy<'tcx, Tag>],
 214         dest: &PlaceTy<'tcx, Tag>,
 215         ret: mir::BasicBlock,
 216     ) -> InterpResult<'tcx, bool> {
 217         let this = self.eval_context_mut();
 218
 219         // Here we dispatch all the shims for foreign functions. If you have a platform specific
 220         // shim, add it to the corresponding submodule.
 221         match link_name {
 222             // Miri-specific extern functions
 223             "miri_static_root" => {
 224                 check_abi(this, abi, Abi::Rust)?;
 225                 let &[ref ptr] = check_arg_count(args)?;
 226                 let ptr = this.read_scalar(ptr)?.check_init()?;
 227                 let ptr = this.force_ptr(ptr)?;
 228                 if ptr.offset != Size::ZERO {
 229                     throw_unsup_format!("pointer passed to miri_static_root must point to beginning of an allocated block");
 230                 }
 231                 this.machine.static_roots.push(ptr.alloc_id);
 232             }
 233
 234             // Obtains a Miri backtrace. See the README for details.
 235             "miri_get_backtrace" => {
 236                 check_abi(this, abi, Abi::Rust)?;
 237                 this.handle_miri_get_backtrace(args, dest)?;
 238             }
 239
 240             // Resolves a Miri backtrace frame. See the README for details.
 241             "miri_resolve_frame" => {
 242                 check_abi(this, abi, Abi::Rust)?;
 243                 this.handle_miri_resolve_frame(args, dest)?;
 244             }
 245
 246
 247             // Standard C allocation
 248             "malloc" => {
 249                 check_abi(this, abi, Abi::C { unwind: false })?;
 250                 let &[ref size] = check_arg_count(args)?;
 251                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 252                 let res = this.malloc(size, /*zero_init:*/ false, MiriMemoryKind::C);
 253                 this.write_scalar(res, dest)?;
 254             }
 255             "calloc" => {
 256                 check_abi(this, abi, Abi::C { unwind: false })?;
 257                 let &[ref items, ref len] = check_arg_count(args)?;
 258                 let items = this.read_scalar(items)?.to_machine_usize(this)?;
 259                 let len = this.read_scalar(len)?.to_machine_usize(this)?;
 260                 let size =
 261                     items.checked_mul(len).ok_or_else(|| err_ub_format!("overflow during calloc size computation"))?;
 262                 let res = this.malloc(size, /*zero_init:*/ true, MiriMemoryKind::C);
 263                 this.write_scalar(res, dest)?;
 264             }
 265             "free" => {
 266                 check_abi(this, abi, Abi::C { unwind: false })?;
 267                 let &[ref ptr] = check_arg_count(args)?;
 268                 let ptr = this.read_scalar(ptr)?.check_init()?;
 269                 this.free(ptr, MiriMemoryKind::C)?;
 270             }
 271             "realloc" => {
 272                 check_abi(this, abi, Abi::C { unwind: false })?;
 273                 let &[ref old_ptr, ref new_size] = check_arg_count(args)?;
 274                 let old_ptr = this.read_scalar(old_ptr)?.check_init()?;
 275                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 276                 let res = this.realloc(old_ptr, new_size, MiriMemoryKind::C)?;
 277                 this.write_scalar(res, dest)?;
 278             }
 279
 280             // Rust allocation
 281             // (Usually these would be forwarded to to `#[global_allocator]`; we instead implement a generic
 282             // allocation that also checks that all conditions are met, such as not permitting zero-sized allocations.)
 283             "__rust_alloc" => {
 284                 check_abi(this, abi, Abi::Rust)?;
 285                 let &[ref size, ref align] = check_arg_count(args)?;
 286                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 287                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 288                 Self::check_alloc_request(size, align)?;
 289                 let ptr = this.memory.allocate(
 290                     Size::from_bytes(size),
 291                     Align::from_bytes(align).unwrap(),
 292                     MiriMemoryKind::Rust.into(),
 293                 );
 294                 this.write_scalar(ptr, dest)?;
 295             }
 296             "__rust_alloc_zeroed" => {
 297                 check_abi(this, abi, Abi::Rust)?;
 298                 let &[ref size, ref align] = check_arg_count(args)?;
 299                 let size = this.read_scalar(size)?.to_machine_usize(this)?;
 300                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 301                 Self::check_alloc_request(size, align)?;
 302                 let ptr = this.memory.allocate(
 303                     Size::from_bytes(size),
 304                     Align::from_bytes(align).unwrap(),
 305                     MiriMemoryKind::Rust.into(),
 306                 );
 307                 // We just allocated this, the access is definitely in-bounds.
 308                 this.memory.write_bytes(ptr.into(), iter::repeat(0u8).take(usize::try_from(size).unwrap())).unwrap();
 309                 this.write_scalar(ptr, dest)?;
 310             }
 311             "__rust_dealloc" => {
 312                 check_abi(this, abi, Abi::Rust)?;
 313                 let &[ref ptr, ref old_size, ref align] = check_arg_count(args)?;
 314                 let ptr = this.read_scalar(ptr)?.check_init()?;
 315                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 316                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 317                 // No need to check old_size/align; we anyway check that they match the allocation.
 318                 let ptr = this.force_ptr(ptr)?;
 319                 this.memory.deallocate(
 320                     ptr,
 321                     Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
 322                     MiriMemoryKind::Rust.into(),
 323                 )?;
 324             }
 325             "__rust_realloc" => {
 326                 check_abi(this, abi, Abi::Rust)?;
 327                 let &[ref ptr, ref old_size, ref align, ref new_size] = check_arg_count(args)?;
 328                 let ptr = this.force_ptr(this.read_scalar(ptr)?.check_init()?)?;
 329                 let old_size = this.read_scalar(old_size)?.to_machine_usize(this)?;
 330                 let align = this.read_scalar(align)?.to_machine_usize(this)?;
 331                 let new_size = this.read_scalar(new_size)?.to_machine_usize(this)?;
 332                 Self::check_alloc_request(new_size, align)?;
 333                 // No need to check old_size; we anyway check that they match the allocation.
 334                 let align = Align::from_bytes(align).unwrap();
 335                 let new_ptr = this.memory.reallocate(
 336                     ptr,
 337                     Some((Size::from_bytes(old_size), align)),
 338                     Size::from_bytes(new_size),
 339                     align,
 340                     MiriMemoryKind::Rust.into(),
 341                 )?;
 342                 this.write_scalar(new_ptr, dest)?;
 343             }
 344
 345             // C memory handling functions
 346             "memcmp" => {
 347                 check_abi(this, abi, Abi::C { unwind: false })?;
 348                 let &[ref left, ref right, ref n] = check_arg_count(args)?;
 349                 let left = this.read_scalar(left)?.check_init()?;
 350                 let right = this.read_scalar(right)?.check_init()?;
 351                 let n = Size::from_bytes(this.read_scalar(n)?.to_machine_usize(this)?);
 352
 353                 let result = {
 354                     let left_bytes = this.memory.read_bytes(left, n)?;
 355                     let right_bytes = this.memory.read_bytes(right, n)?;
 356
 357                     use std::cmp::Ordering::*;
 358                     match left_bytes.cmp(right_bytes) {
 359                         Less => -1i32,
 360                         Equal => 0,
 361                         Greater => 1,
 362                     }
 363                 };
 364
 365                 this.write_scalar(Scalar::from_i32(result), dest)?;
 366             }
 367             "memrchr" => {
 368                 check_abi(this, abi, Abi::C { unwind: false })?;
 369                 let &[ref ptr, ref val, ref num] = check_arg_count(args)?;
 370                 let ptr = this.read_scalar(ptr)?.check_init()?;
 371                 let val = this.read_scalar(val)?.to_i32()? as u8;
 372                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 373                 if let Some(idx) = this
 374                     .memory
 375                     .read_bytes(ptr, Size::from_bytes(num))?
 376                     .iter()
 377                     .rev()
 378                     .position(|&c| c == val)
 379                 {
 380                     let new_ptr = ptr.ptr_offset(Size::from_bytes(num - idx as u64 - 1), this)?;
 381                     this.write_scalar(new_ptr, dest)?;
 382                 } else {
 383                     this.write_null(dest)?;
 384                 }
 385             }
 386             "memchr" => {
 387                 check_abi(this, abi, Abi::C { unwind: false })?;
 388                 let &[ref ptr, ref val, ref num] = check_arg_count(args)?;
 389                 let ptr = this.read_scalar(ptr)?.check_init()?;
 390                 let val = this.read_scalar(val)?.to_i32()? as u8;
 391                 let num = this.read_scalar(num)?.to_machine_usize(this)?;
 392                 let idx = this
 393                     .memory
 394                     .read_bytes(ptr, Size::from_bytes(num))?
 395                     .iter()
 396                     .position(|&c| c == val);
 397                 if let Some(idx) = idx {
 398                     let new_ptr = ptr.ptr_offset(Size::from_bytes(idx as u64), this)?;
 399                     this.write_scalar(new_ptr, dest)?;
 400                 } else {
 401                     this.write_null(dest)?;
 402                 }
 403             }
 404             "strlen" => {
 405                 check_abi(this, abi, Abi::C { unwind: false })?;
 406                 let &[ref ptr] = check_arg_count(args)?;
 407                 let ptr = this.read_scalar(ptr)?.check_init()?;
 408                 let n = this.read_c_str(ptr)?.len();
 409                 this.write_scalar(Scalar::from_machine_usize(u64::try_from(n).unwrap(), this), dest)?;
 410             }
 411
 412             // math functions
 413             #[rustfmt::skip]
 414             | "cbrtf"
 415             | "coshf"
 416             | "sinhf"
 417             | "tanf"
 418             | "acosf"
 419             | "asinf"
 420             | "atanf"
 421             => {
 422                 check_abi(this, abi, Abi::C { unwind: false })?;
 423                 let &[ref f] = check_arg_count(args)?;
 424                 // FIXME: Using host floats.
 425                 let f = f32::from_bits(this.read_scalar(f)?.to_u32()?);
 426                 let f = match link_name {
 427                     "cbrtf" => f.cbrt(),
 428                     "coshf" => f.cosh(),
 429                     "sinhf" => f.sinh(),
 430                     "tanf" => f.tan(),
 431                     "acosf" => f.acos(),
 432                     "asinf" => f.asin(),
 433                     "atanf" => f.atan(),
 434                     _ => bug!(),
 435                 };
 436                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 437             }
 438             #[rustfmt::skip]
 439             | "_hypotf"
 440             | "hypotf"
 441             | "atan2f"
 442             => {
 443                 check_abi(this, abi, Abi::C { unwind: false })?;
 444                 let &[ref f1, ref f2] = check_arg_count(args)?;
 445                 // underscore case for windows, here and below
 446                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
 447                 // FIXME: Using host floats.
 448                 let f1 = f32::from_bits(this.read_scalar(f1)?.to_u32()?);
 449                 let f2 = f32::from_bits(this.read_scalar(f2)?.to_u32()?);
 450                 let n = match link_name {
 451                     "_hypotf" | "hypotf" => f1.hypot(f2),
 452                     "atan2f" => f1.atan2(f2),
 453                     _ => bug!(),
 454                 };
 455                 this.write_scalar(Scalar::from_u32(n.to_bits()), dest)?;
 456             }
 457             #[rustfmt::skip]
 458             | "cbrt"
 459             | "cosh"
 460             | "sinh"
 461             | "tan"
 462             | "acos"
 463             | "asin"
 464             | "atan"
 465             => {
 466                 check_abi(this, abi, Abi::C { unwind: false })?;
 467                 let &[ref f] = check_arg_count(args)?;
 468                 // FIXME: Using host floats.
 469                 let f = f64::from_bits(this.read_scalar(f)?.to_u64()?);
 470                 let f = match link_name {
 471                     "cbrt" => f.cbrt(),
 472                     "cosh" => f.cosh(),
 473                     "sinh" => f.sinh(),
 474                     "tan" => f.tan(),
 475                     "acos" => f.acos(),
 476                     "asin" => f.asin(),
 477                     "atan" => f.atan(),
 478                     _ => bug!(),
 479                 };
 480                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 481             }
 482             #[rustfmt::skip]
 483             | "_hypot"
 484             | "hypot"
 485             | "atan2"
 486             => {
 487                 check_abi(this, abi, Abi::C { unwind: false })?;
 488                 let &[ref f1, ref f2] = check_arg_count(args)?;
 489                 // FIXME: Using host floats.
 490                 let f1 = f64::from_bits(this.read_scalar(f1)?.to_u64()?);
 491                 let f2 = f64::from_bits(this.read_scalar(f2)?.to_u64()?);
 492                 let n = match link_name {
 493                     "_hypot" | "hypot" => f1.hypot(f2),
 494                     "atan2" => f1.atan2(f2),
 495                     _ => bug!(),
 496                 };
 497                 this.write_scalar(Scalar::from_u64(n.to_bits()), dest)?;
 498             }
 499             #[rustfmt::skip]
 500             | "_ldexp"
 501             | "ldexp"
 502             | "scalbn"
 503             => {
 504                 check_abi(this, abi, Abi::C { unwind: false })?;
 505                 let &[ref x, ref exp] = check_arg_count(args)?;
 506                 // For radix-2 (binary) systems, `ldexp` and `scalbn` are the same.
 507                 let x = this.read_scalar(x)?.to_f64()?;
 508                 let exp = this.read_scalar(exp)?.to_i32()?;
 509
 510                 // Saturating cast to i16. Even those are outside the valid exponent range to
 511                 // `scalbn` below will do its over/underflow handling.
 512                 let exp = if exp > i32::from(i16::MAX) {
 513                     i16::MAX
 514                 } else if exp < i32::from(i16::MIN) {
 515                     i16::MIN
 516                 } else {
 517                     exp.try_into().unwrap()
 518                 };
 519
 520                 let res = x.scalbn(exp);
 521                 this.write_scalar(Scalar::from_f64(res), dest)?;
 522             }
 523
 524             // Architecture-specific shims
 525             "llvm.x86.sse2.pause" if this.tcx.sess.target.arch == "x86" || this.tcx.sess.target.arch == "x86_64" => {
 526                 check_abi(this, abi, Abi::C { unwind: false })?;
 527                 let &[] = check_arg_count(args)?;
 528                 this.yield_active_thread();
 529             }
 530             "llvm.aarch64.isb" if this.tcx.sess.target.arch == "aarch64" => {
 531                 check_abi(this, abi, Abi::C { unwind: false })?;
 532                 let &[ref arg] = check_arg_count(args)?;
 533                 let arg = this.read_scalar(arg)?.to_i32()?;
 534                 match arg {
 535                     15 => { // SY ("full system scope")
 536                         this.yield_active_thread();
 537                     }
 538                     _ => {
 539                         throw_unsup_format!("unsupported llvm.aarch64.isb argument {}", arg);
 540                     }
 541                 }
 542             }
 543
 544             // Platform-specific shims
 545             _ => match this.tcx.sess.target.os.as_str() {
 546                 "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 547                 "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, abi, args, dest, ret),
 548                 target => throw_unsup_format!("the target `{}` is not supported", target),
 549             }
 550         };
 551
 552         Ok(true)
 553     }
 554
 555     /// Check some basic requirements for this allocation request:
 556     /// non-zero size, power-of-two alignment.
 557     fn check_alloc_request(size: u64, align: u64) -> InterpResult<'tcx> {
 558         if size == 0 {
 559             throw_ub_format!("creating allocation with size 0");
 560         }
 561         if !align.is_power_of_two() {
 562             throw_ub_format!("creating allocation with non-power-of-two alignment {}", align);
 563         }
 564         Ok(())
 565     }
 566 }