src/shims/intrinsics.rs

   1 use rustc_apfloat::Float;
   2 use rustc::mir;
   3 use rustc::mir::interpret::{InterpResult, PointerArithmetic};
   4 use rustc::ty::layout::{self, LayoutOf, Size, Align};
   5 use rustc::ty;
   6
   7 use crate::{
   8     PlaceTy, OpTy, Immediate, Scalar, Tag,
   9     OperatorEvalContextExt
  10 };
  11
  12 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
  13 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
  14     fn call_intrinsic(
  15         &mut self,
  16         instance: ty::Instance<'tcx>,
  17         args: &[OpTy<'tcx, Tag>],
  18         dest: PlaceTy<'tcx, Tag>,
  19     ) -> InterpResult<'tcx> {
  20         let this = self.eval_context_mut();
  21         if this.emulate_intrinsic(instance, args, dest)? {
  22             return Ok(());
  23         }
  24         let tcx = &{this.tcx.tcx};
  25         let substs = instance.substs;
  26
  27         // All these intrinsics take raw pointers, so if we access memory directly
  28         // (as opposed to through a place), we have to remember to erase any tag
  29         // that might still hang around!
  30
  31         let intrinsic_name = &*tcx.item_name(instance.def_id()).as_str();
  32         match intrinsic_name {
  33             "arith_offset" => {
  34                 let offset = this.read_scalar(args[1])?.to_isize(this)?;
  35                 let ptr = this.read_scalar(args[0])?.not_undef()?;
  36
  37                 let pointee_ty = substs.type_at(0);
  38                 let pointee_size = this.layout_of(pointee_ty)?.size.bytes() as i64;
  39                 let offset = offset.overflowing_mul(pointee_size).0;
  40                 let result_ptr = ptr.ptr_wrapping_signed_offset(offset, this);
  41                 this.write_scalar(result_ptr, dest)?;
  42             }
  43
  44             "assume" => {
  45                 let cond = this.read_scalar(args[0])?.to_bool()?;
  46                 if !cond {
  47                     throw_ub_format!("`assume` intrinsic called with `false`");
  48                 }
  49             }
  50
  51             "volatile_load" => {
  52                 let place = this.deref_operand(args[0])?;
  53                 this.copy_op(place.into(), dest)?;
  54             }
  55
  56             "volatile_store" => {
  57                 let place = this.deref_operand(args[0])?;
  58                 this.copy_op(args[1], place.into())?;
  59             }
  60
  61             "atomic_load" |
  62             "atomic_load_relaxed" |
  63             "atomic_load_acq" => {
  64                 let place = this.deref_operand(args[0])?;
  65                 let val = this.read_scalar(place.into())?; // make sure it fits into a scalar; otherwise it cannot be atomic
  66
  67                 // Check alignment requirements. Atomics must always be aligned to their size,
  68                 // even if the type they wrap would be less aligned (e.g. AtomicU64 on 32bit must
  69                 // be 8-aligned).
  70                 let align = Align::from_bytes(place.layout.size.bytes()).unwrap();
  71                 this.memory.check_ptr_access(place.ptr, place.layout.size, align)?;
  72
  73                 this.write_scalar(val, dest)?;
  74             }
  75
  76             "atomic_store" |
  77             "atomic_store_relaxed" |
  78             "atomic_store_rel" => {
  79                 let place = this.deref_operand(args[0])?;
  80                 let val = this.read_scalar(args[1])?; // make sure it fits into a scalar; otherwise it cannot be atomic
  81
  82                 // Check alignment requirements. Atomics must always be aligned to their size,
  83                 // even if the type they wrap would be less aligned (e.g. AtomicU64 on 32bit must
  84                 // be 8-aligned).
  85                 let align = Align::from_bytes(place.layout.size.bytes()).unwrap();
  86                 this.memory.check_ptr_access(place.ptr, place.layout.size, align)?;
  87
  88                 this.write_scalar(val, place.into())?;
  89             }
  90
  91             "atomic_fence_acq" |
  92             "atomic_fence_rel" |
  93             "atomic_fence_acqrel" |
  94             "atomic_fence" => {
  95                 // we are inherently singlethreaded and singlecored, this is a nop
  96             }
  97
  98             _ if intrinsic_name.starts_with("atomic_xchg") => {
  99                 let place = this.deref_operand(args[0])?;
 100                 let new = this.read_scalar(args[1])?;
 101                 let old = this.read_scalar(place.into())?;
 102
 103                 // Check alignment requirements. Atomics must always be aligned to their size,
 104                 // even if the type they wrap would be less aligned (e.g. AtomicU64 on 32bit must
 105                 // be 8-aligned).
 106                 let align = Align::from_bytes(place.layout.size.bytes()).unwrap();
 107                 this.memory.check_ptr_access(place.ptr, place.layout.size, align)?;
 108
 109                 this.write_scalar(old, dest)?; // old value is returned
 110                 this.write_scalar(new, place.into())?;
 111             }
 112
 113             _ if intrinsic_name.starts_with("atomic_cxchg") => {
 114                 let place = this.deref_operand(args[0])?;
 115                 let expect_old = this.read_immediate(args[1])?; // read as immediate for the sake of `binary_op()`
 116                 let new = this.read_scalar(args[2])?;
 117                 let old = this.read_immediate(place.into())?; // read as immediate for the sake of `binary_op()`
 118
 119                 // Check alignment requirements. Atomics must always be aligned to their size,
 120                 // even if the type they wrap would be less aligned (e.g. AtomicU64 on 32bit must
 121                 // be 8-aligned).
 122                 let align = Align::from_bytes(place.layout.size.bytes()).unwrap();
 123                 this.memory.check_ptr_access(place.ptr, place.layout.size, align)?;
 124
 125                 // binary_op will bail if either of them is not a scalar
 126                 let eq = this.overflowing_binary_op(mir::BinOp::Eq, old, expect_old)?.0;
 127                 let res = Immediate::ScalarPair(old.to_scalar_or_undef(), eq.into());
 128                 this.write_immediate(res, dest)?; // old value is returned
 129                 // update ptr depending on comparison
 130                 if eq.to_bool()? {
 131                     this.write_scalar(new, place.into())?;
 132                 }
 133             }
 134
 135             "atomic_or" |
 136             "atomic_or_acq" |
 137             "atomic_or_rel" |
 138             "atomic_or_acqrel" |
 139             "atomic_or_relaxed" |
 140             "atomic_xor" |
 141             "atomic_xor_acq" |
 142             "atomic_xor_rel" |
 143             "atomic_xor_acqrel" |
 144             "atomic_xor_relaxed" |
 145             "atomic_and" |
 146             "atomic_and_acq" |
 147             "atomic_and_rel" |
 148             "atomic_and_acqrel" |
 149             "atomic_and_relaxed" |
 150             "atomic_nand" |
 151             "atomic_nand_acq" |
 152             "atomic_nand_rel" |
 153             "atomic_nand_acqrel" |
 154             "atomic_nand_relaxed" |
 155             "atomic_xadd" |
 156             "atomic_xadd_acq" |
 157             "atomic_xadd_rel" |
 158             "atomic_xadd_acqrel" |
 159             "atomic_xadd_relaxed" |
 160             "atomic_xsub" |
 161             "atomic_xsub_acq" |
 162             "atomic_xsub_rel" |
 163             "atomic_xsub_acqrel" |
 164             "atomic_xsub_relaxed" => {
 165                 let place = this.deref_operand(args[0])?;
 166                 if !place.layout.ty.is_integral() {
 167                     bug!("Atomic arithmetic operations only work on integer types");
 168                 }
 169                 let rhs = this.read_immediate(args[1])?;
 170                 let old = this.read_immediate(place.into())?;
 171
 172                 // Check alignment requirements. Atomics must always be aligned to their size,
 173                 // even if the type they wrap would be less aligned (e.g. AtomicU64 on 32bit must
 174                 // be 8-aligned).
 175                 let align = Align::from_bytes(place.layout.size.bytes()).unwrap();
 176                 this.memory.check_ptr_access(place.ptr, place.layout.size, align)?;
 177
 178                 this.write_immediate(*old, dest)?; // old value is returned
 179                 let (op, neg) = match intrinsic_name.split('_').nth(1).unwrap() {
 180                     "or" => (mir::BinOp::BitOr, false),
 181                     "xor" => (mir::BinOp::BitXor, false),
 182                     "and" => (mir::BinOp::BitAnd, false),
 183                     "xadd" => (mir::BinOp::Add, false),
 184                     "xsub" => (mir::BinOp::Sub, false),
 185                     "nand" => (mir::BinOp::BitAnd, true),
 186                     _ => bug!(),
 187                 };
 188                 // Atomics wrap around on overflow.
 189                 let val = this.binary_op(op, old, rhs)?;
 190                 let val = if neg {
 191                     this.unary_op(mir::UnOp::Not, val)?
 192                 } else {
 193                     val
 194                 };
 195                 this.write_immediate(*val, place.into())?;
 196             }
 197
 198             "breakpoint" => unimplemented!(), // halt miri
 199
 200             "copy" |
 201             "copy_nonoverlapping" => {
 202                 let elem_ty = substs.type_at(0);
 203                 let elem_layout = this.layout_of(elem_ty)?;
 204                 let elem_size = elem_layout.size.bytes();
 205                 let count = this.read_scalar(args[2])?.to_usize(this)?;
 206                 let elem_align = elem_layout.align.abi;
 207
 208                 let size = Size::from_bytes(count * elem_size);
 209                 let src = this.read_scalar(args[0])?.not_undef()?;
 210                 let src = this.memory.check_ptr_access(src, size, elem_align)?;
 211                 let dest = this.read_scalar(args[1])?.not_undef()?;
 212                 let dest = this.memory.check_ptr_access(dest, size, elem_align)?;
 213
 214                 if let (Some(src), Some(dest)) = (src, dest) {
 215                     this.memory.copy(
 216                         src,
 217                         dest,
 218                         size,
 219                         intrinsic_name.ends_with("_nonoverlapping"),
 220                     )?;
 221                 }
 222             }
 223
 224             "discriminant_value" => {
 225                 let place = this.deref_operand(args[0])?;
 226                 let discr_val = this.read_discriminant(place.into())?.0;
 227                 this.write_scalar(Scalar::from_uint(discr_val, dest.layout.size), dest)?;
 228             }
 229
 230             "sinf32" | "fabsf32" | "cosf32" | "sqrtf32" | "expf32" | "exp2f32" | "logf32" |
 231             "log10f32" | "log2f32" | "floorf32" | "ceilf32" | "truncf32" | "roundf32" => {
 232                 // FIXME: Using host floats.
 233                 let f = f32::from_bits(this.read_scalar(args[0])?.to_u32()?);
 234                 let f = match intrinsic_name {
 235                     "sinf32" => f.sin(),
 236                     "fabsf32" => f.abs(),
 237                     "cosf32" => f.cos(),
 238                     "sqrtf32" => f.sqrt(),
 239                     "expf32" => f.exp(),
 240                     "exp2f32" => f.exp2(),
 241                     "logf32" => f.ln(),
 242                     "log10f32" => f.log10(),
 243                     "log2f32" => f.log2(),
 244                     "floorf32" => f.floor(),
 245                     "ceilf32" => f.ceil(),
 246                     "truncf32" => f.trunc(),
 247                     "roundf32" => f.round(),
 248                     _ => bug!(),
 249                 };
 250                 this.write_scalar(Scalar::from_u32(f.to_bits()), dest)?;
 251             }
 252
 253             "sinf64" | "fabsf64" | "cosf64" | "sqrtf64" | "expf64" | "exp2f64" | "logf64" |
 254             "log10f64" | "log2f64" | "floorf64" | "ceilf64" | "truncf64" | "roundf64" => {
 255                 // FIXME: Using host floats.
 256                 let f = f64::from_bits(this.read_scalar(args[0])?.to_u64()?);
 257                 let f = match intrinsic_name {
 258                     "sinf64" => f.sin(),
 259                     "fabsf64" => f.abs(),
 260                     "cosf64" => f.cos(),
 261                     "sqrtf64" => f.sqrt(),
 262                     "expf64" => f.exp(),
 263                     "exp2f64" => f.exp2(),
 264                     "logf64" => f.ln(),
 265                     "log10f64" => f.log10(),
 266                     "log2f64" => f.log2(),
 267                     "floorf64" => f.floor(),
 268                     "ceilf64" => f.ceil(),
 269                     "truncf64" => f.trunc(),
 270                     "roundf64" => f.round(),
 271                     _ => bug!(),
 272                 };
 273                 this.write_scalar(Scalar::from_u64(f.to_bits()), dest)?;
 274             }
 275
 276             "fadd_fast" | "fsub_fast" | "fmul_fast" | "fdiv_fast" | "frem_fast" => {
 277                 let a = this.read_immediate(args[0])?;
 278                 let b = this.read_immediate(args[1])?;
 279                 let op = match intrinsic_name {
 280                     "fadd_fast" => mir::BinOp::Add,
 281                     "fsub_fast" => mir::BinOp::Sub,
 282                     "fmul_fast" => mir::BinOp::Mul,
 283                     "fdiv_fast" => mir::BinOp::Div,
 284                     "frem_fast" => mir::BinOp::Rem,
 285                     _ => bug!(),
 286                 };
 287                 this.binop_ignore_overflow(op, a, b, dest)?;
 288             }
 289
 290             "minnumf32" | "maxnumf32" => {
 291                 let a = this.read_scalar(args[0])?.to_f32()?;
 292                 let b = this.read_scalar(args[1])?.to_f32()?;
 293                 let res = if intrinsic_name.starts_with("min") {
 294                     a.min(b)
 295                 } else {
 296                     a.max(b)
 297                 };
 298                 this.write_scalar(Scalar::from_f32(res), dest)?;
 299             }
 300
 301             "minnumf64" | "maxnumf64" => {
 302                 let a = this.read_scalar(args[0])?.to_f64()?;
 303                 let b = this.read_scalar(args[1])?.to_f64()?;
 304                 let res = if intrinsic_name.starts_with("min") {
 305                     a.min(b)
 306                 } else {
 307                     a.max(b)
 308                 };
 309                 this.write_scalar(Scalar::from_f64(res), dest)?;
 310             }
 311
 312             "exact_div" => {
 313                 // Performs an exact division, resulting in undefined behavior where
 314                 // `x % y != 0` or `y == 0` or `x == T::min_value() && y == -1`
 315                 let a = this.read_immediate(args[0])?;
 316                 let b = this.read_immediate(args[1])?;
 317                 // check x % y != 0
 318                 if this.overflowing_binary_op(mir::BinOp::Rem, a, b)?.0.to_bits(dest.layout.size)? != 0 {
 319                     // Check if `b` is -1, which is the "min_value / -1" case.
 320                     let minus1 = Scalar::from_int(-1, dest.layout.size);
 321                     return Err(if b.to_scalar().unwrap() == minus1 {
 322                         err_ub_format!("exact_div: result of dividing MIN by -1 cannot be represented")
 323                     } else {
 324                         err_ub_format!("exact_div: {:?} cannot be divided by {:?} without remainder", *a, *b)
 325                     }.into());
 326                 }
 327                 this.binop_ignore_overflow(mir::BinOp::Div, a, b, dest)?;
 328             },
 329
 330             "forget" => {}
 331
 332             "likely" | "unlikely" => {
 333                 // These just return their argument
 334                 let b = this.read_immediate(args[0])?;
 335                 this.write_immediate(*b, dest)?;
 336             }
 337
 338             "init" => {
 339                 // Check fast path: we don't want to force an allocation in case the destination is a simple value,
 340                 // but we also do not want to create a new allocation with 0s and then copy that over.
 341                 // FIXME: We do not properly validate in case of ZSTs and when doing it in memory!
 342                 // However, this only affects direct calls of the intrinsic; calls to the stable
 343                 // functions wrapping them do get their validation.
 344                 // FIXME: should we check that the destination pointer is aligned even for ZSTs?
 345                 if !dest.layout.is_zst() {
 346                     match dest.layout.abi {
 347                         layout::Abi::Scalar(ref s) => {
 348                             let x = Scalar::from_int(0, s.value.size(this));
 349                             this.write_scalar(x, dest)?;
 350                         }
 351                         layout::Abi::ScalarPair(ref s1, ref s2) => {
 352                             let x = Scalar::from_int(0, s1.value.size(this));
 353                             let y = Scalar::from_int(0, s2.value.size(this));
 354                             this.write_immediate(Immediate::ScalarPair(x.into(), y.into()), dest)?;
 355                         }
 356                         _ => {
 357                             // Do it in memory
 358                             let mplace = this.force_allocation(dest)?;
 359                             assert!(mplace.meta.is_none());
 360                             // not a zst, must be valid pointer
 361                             let ptr = mplace.ptr.to_ptr()?;
 362                             this.memory.get_mut(ptr.alloc_id)?.write_repeat(tcx, ptr, 0, dest.layout.size)?;
 363                         }
 364                     }
 365                 }
 366             }
 367
 368             "pref_align_of" => {
 369                 let ty = substs.type_at(0);
 370                 let layout = this.layout_of(ty)?;
 371                 let align = layout.align.pref.bytes();
 372                 let ptr_size = this.pointer_size();
 373                 let align_val = Scalar::from_uint(align as u128, ptr_size);
 374                 this.write_scalar(align_val, dest)?;
 375             }
 376
 377             "move_val_init" => {
 378                 let place = this.deref_operand(args[0])?;
 379                 this.copy_op(args[1], place.into())?;
 380             }
 381
 382             "offset" => {
 383                 let offset = this.read_scalar(args[1])?.to_isize(this)?;
 384                 let ptr = this.read_scalar(args[0])?.not_undef()?;
 385                 let result_ptr = this.pointer_offset_inbounds(ptr, substs.type_at(0), offset)?;
 386                 this.write_scalar(result_ptr, dest)?;
 387             }
 388
 389             "panic_if_uninhabited" => {
 390                 let ty = substs.type_at(0);
 391                 let layout = this.layout_of(ty)?;
 392                 if layout.abi.is_uninhabited() {
 393                     throw_ub_format!("Trying to instantiate uninhabited type {}", ty)
 394                 }
 395             }
 396
 397             "powf32" => {
 398                 // FIXME: Using host floats.
 399                 let f = f32::from_bits(this.read_scalar(args[0])?.to_u32()?);
 400                 let f2 = f32::from_bits(this.read_scalar(args[1])?.to_u32()?);
 401                 this.write_scalar(
 402                     Scalar::from_u32(f.powf(f2).to_bits()),
 403                     dest,
 404                 )?;
 405             }
 406
 407             "powf64" => {
 408                 // FIXME: Using host floats.
 409                 let f = f64::from_bits(this.read_scalar(args[0])?.to_u64()?);
 410                 let f2 = f64::from_bits(this.read_scalar(args[1])?.to_u64()?);
 411                 this.write_scalar(
 412                     Scalar::from_u64(f.powf(f2).to_bits()),
 413                     dest,
 414                 )?;
 415             }
 416
 417             "fmaf32" => {
 418                 let a = this.read_scalar(args[0])?.to_f32()?;
 419                 let b = this.read_scalar(args[1])?.to_f32()?;
 420                 let c = this.read_scalar(args[2])?.to_f32()?;
 421                 let res = a.mul_add(b, c).value;
 422                 this.write_scalar(
 423                     Scalar::from_f32(res),
 424                     dest,
 425                 )?;
 426             }
 427
 428             "fmaf64" => {
 429                 let a = this.read_scalar(args[0])?.to_f64()?;
 430                 let b = this.read_scalar(args[1])?.to_f64()?;
 431                 let c = this.read_scalar(args[2])?.to_f64()?;
 432                 let res = a.mul_add(b, c).value;
 433                 this.write_scalar(
 434                     Scalar::from_f64(res),
 435                     dest,
 436                 )?;
 437             }
 438
 439             "powif32" => {
 440                 // FIXME: Using host floats.
 441                 let f = f32::from_bits(this.read_scalar(args[0])?.to_u32()?);
 442                 let i = this.read_scalar(args[1])?.to_i32()?;
 443                 this.write_scalar(
 444                     Scalar::from_u32(f.powi(i).to_bits()),
 445                     dest,
 446                 )?;
 447             }
 448
 449             "powif64" => {
 450                 // FIXME: Using host floats.
 451                 let f = f64::from_bits(this.read_scalar(args[0])?.to_u64()?);
 452                 let i = this.read_scalar(args[1])?.to_i32()?;
 453                 this.write_scalar(
 454                     Scalar::from_u64(f.powi(i).to_bits()),
 455                     dest,
 456                 )?;
 457             }
 458
 459             "size_of_val" => {
 460                 let mplace = this.deref_operand(args[0])?;
 461                 let (size, _) = this.size_and_align_of_mplace(mplace)?
 462                     .expect("size_of_val called on extern type");
 463                 let ptr_size = this.pointer_size();
 464                 this.write_scalar(
 465                     Scalar::from_uint(size.bytes() as u128, ptr_size),
 466                     dest,
 467                 )?;
 468             }
 469
 470             "min_align_of_val" |
 471             "align_of_val" => {
 472                 let mplace = this.deref_operand(args[0])?;
 473                 let (_, align) = this.size_and_align_of_mplace(mplace)?
 474                     .expect("size_of_val called on extern type");
 475                 let ptr_size = this.pointer_size();
 476                 this.write_scalar(
 477                     Scalar::from_uint(align.bytes(), ptr_size),
 478                     dest,
 479                 )?;
 480             }
 481
 482             "unchecked_div" => {
 483                 let l = this.read_immediate(args[0])?;
 484                 let r = this.read_immediate(args[1])?;
 485                 let rval = r.to_scalar()?.to_bits(args[1].layout.size)?;
 486                 if rval == 0 {
 487                     throw_ub_format!("Division by 0 in unchecked_div");
 488                 }
 489                 this.binop_ignore_overflow(
 490                     mir::BinOp::Div,
 491                     l,
 492                     r,
 493                     dest,
 494                 )?;
 495             }
 496
 497             "unchecked_rem" => {
 498                 let l = this.read_immediate(args[0])?;
 499                 let r = this.read_immediate(args[1])?;
 500                 let rval = r.to_scalar()?.to_bits(args[1].layout.size)?;
 501                 if rval == 0 {
 502                     throw_ub_format!("Division by 0 in unchecked_rem");
 503                 }
 504                 this.binop_ignore_overflow(
 505                     mir::BinOp::Rem,
 506                     l,
 507                     r,
 508                     dest,
 509                 )?;
 510             }
 511
 512             "unchecked_add" | "unchecked_sub" | "unchecked_mul" => {
 513                 let l = this.read_immediate(args[0])?;
 514                 let r = this.read_immediate(args[1])?;
 515                 let op = match intrinsic_name {
 516                     "unchecked_add" => mir::BinOp::Add,
 517                     "unchecked_sub" => mir::BinOp::Sub,
 518                     "unchecked_mul" => mir::BinOp::Mul,
 519                     _ => bug!(),
 520                 };
 521                 let (res, overflowed, _ty) = this.overflowing_binary_op(op, l, r)?;
 522                 if overflowed {
 523                     throw_ub_format!("Overflowing arithmetic in {}", intrinsic_name);
 524                 }
 525                 this.write_scalar(res, dest)?;
 526             }
 527
 528             "uninit" => {
 529                 // Check fast path: we don't want to force an allocation in case the destination is a simple value,
 530                 // but we also do not want to create a new allocation with 0s and then copy that over.
 531                 // FIXME: We do not properly validate in case of ZSTs and when doing it in memory!
 532                 // However, this only affects direct calls of the intrinsic; calls to the stable
 533                 // functions wrapping them do get their validation.
 534                 // FIXME: should we check alignment for ZSTs?
 535                 use crate::ScalarMaybeUndef;
 536                 if !dest.layout.is_zst() {
 537                     match dest.layout.abi {
 538                         layout::Abi::Scalar(..) => {
 539                             let x = ScalarMaybeUndef::Undef;
 540                             this.write_immediate(Immediate::Scalar(x), dest)?;
 541                         }
 542                         layout::Abi::ScalarPair(..) => {
 543                             let x = ScalarMaybeUndef::Undef;
 544                             this.write_immediate(Immediate::ScalarPair(x, x), dest)?;
 545                         }
 546                         _ => {
 547                             // Do it in memory
 548                             let mplace = this.force_allocation(dest)?;
 549                             assert!(mplace.meta.is_none());
 550                             let ptr = mplace.ptr.to_ptr()?;
 551                             this.memory
 552                                 .get_mut(ptr.alloc_id)?
 553                                 .mark_definedness(ptr, dest.layout.size, false);
 554                         }
 555                     }
 556                 }
 557             }
 558
 559             "write_bytes" => {
 560                 let ty = substs.type_at(0);
 561                 let ty_layout = this.layout_of(ty)?;
 562                 let val_byte = this.read_scalar(args[1])?.to_u8()?;
 563                 let ptr = this.read_scalar(args[0])?.not_undef()?;
 564                 let count = this.read_scalar(args[2])?.to_usize(this)?;
 565                 let byte_count = ty_layout.size * count;
 566                 match this.memory.check_ptr_access(ptr, byte_count, ty_layout.align.abi)? {
 567                     Some(ptr) => {
 568                         this.memory
 569                             .get_mut(ptr.alloc_id)?
 570                             .write_repeat(tcx, ptr, val_byte, byte_count)?;
 571                     }
 572                     None => {
 573                         // Size is 0, nothing to do.
 574                     }
 575                 }
 576             }
 577
 578             name => throw_unsup_format!("unimplemented intrinsic: {}", name),
 579         }
 580
 581         Ok(())
 582     }
 583 }