compiler/rustc_const_eval/src/interpret/intrinsics.rs

   1 //! Intrinsics and other functions that the miri engine executes without
   2 //! looking at their MIR. Intrinsics/functions supported here are shared by CTFE
   3 //! and miri.
   4
   5 use std::convert::TryFrom;
   6
   7 use rustc_hir::def_id::DefId;
   8 use rustc_middle::mir::{
   9     self,
  10     interpret::{ConstValue, GlobalId, InterpResult, Scalar},
  11     BinOp,
  12 };
  13 use rustc_middle::ty;
  14 use rustc_middle::ty::layout::LayoutOf as _;
  15 use rustc_middle::ty::subst::SubstsRef;
  16 use rustc_middle::ty::{Ty, TyCtxt};
  17 use rustc_span::symbol::{sym, Symbol};
  18 use rustc_target::abi::{Abi, Align, Primitive, Size};
  19
  20 use super::{
  21     util::ensure_monomorphic_enough, CheckInAllocMsg, ImmTy, InterpCx, Machine, OpTy, PlaceTy,
  22     Pointer,
  23 };
  24
  25 mod caller_location;
  26 mod type_name;
  27
  28 fn numeric_intrinsic<Prov>(name: Symbol, bits: u128, kind: Primitive) -> Scalar<Prov> {
  29     let size = match kind {
  30         Primitive::Int(integer, _) => integer.size(),
  31         _ => bug!("invalid `{}` argument: {:?}", name, bits),
  32     };
  33     let extra = 128 - u128::from(size.bits());
  34     let bits_out = match name {
  35         sym::ctpop => u128::from(bits.count_ones()),
  36         sym::ctlz => u128::from(bits.leading_zeros()) - extra,
  37         sym::cttz => u128::from((bits << extra).trailing_zeros()) - extra,
  38         sym::bswap => (bits << extra).swap_bytes(),
  39         sym::bitreverse => (bits << extra).reverse_bits(),
  40         _ => bug!("not a numeric intrinsic: {}", name),
  41     };
  42     Scalar::from_uint(bits_out, size)
  43 }
  44
  45 /// The logic for all nullary intrinsics is implemented here. These intrinsics don't get evaluated
  46 /// inside an `InterpCx` and instead have their value computed directly from rustc internal info.
  47 pub(crate) fn eval_nullary_intrinsic<'tcx>(
  48     tcx: TyCtxt<'tcx>,
  49     param_env: ty::ParamEnv<'tcx>,
  50     def_id: DefId,
  51     substs: SubstsRef<'tcx>,
  52 ) -> InterpResult<'tcx, ConstValue<'tcx>> {
  53     let tp_ty = substs.type_at(0);
  54     let name = tcx.item_name(def_id);
  55     Ok(match name {
  56         sym::type_name => {
  57             ensure_monomorphic_enough(tcx, tp_ty)?;
  58             let alloc = type_name::alloc_type_name(tcx, tp_ty);
  59             ConstValue::Slice { data: alloc, start: 0, end: alloc.inner().len() }
  60         }
  61         sym::needs_drop => {
  62             ensure_monomorphic_enough(tcx, tp_ty)?;
  63             ConstValue::from_bool(tp_ty.needs_drop(tcx, param_env))
  64         }
  65         sym::pref_align_of => {
  66             // Correctly handles non-monomorphic calls, so there is no need for ensure_monomorphic_enough.
  67             let layout = tcx.layout_of(param_env.and(tp_ty)).map_err(|e| err_inval!(Layout(e)))?;
  68             ConstValue::from_machine_usize(layout.align.pref.bytes(), &tcx)
  69         }
  70         sym::type_id => {
  71             ensure_monomorphic_enough(tcx, tp_ty)?;
  72             ConstValue::from_u64(tcx.type_id_hash(tp_ty))
  73         }
  74         sym::variant_count => match tp_ty.kind() {
  75             // Correctly handles non-monomorphic calls, so there is no need for ensure_monomorphic_enough.
  76             ty::Adt(ref adt, _) => {
  77                 ConstValue::from_machine_usize(adt.variants().len() as u64, &tcx)
  78             }
  79             ty::Projection(_)
  80             | ty::Opaque(_, _)
  81             | ty::Param(_)
  82             | ty::Bound(_, _)
  83             | ty::Placeholder(_)
  84             | ty::Infer(_) => throw_inval!(TooGeneric),
  85             ty::Bool
  86             | ty::Char
  87             | ty::Int(_)
  88             | ty::Uint(_)
  89             | ty::Float(_)
  90             | ty::Foreign(_)
  91             | ty::Str
  92             | ty::Array(_, _)
  93             | ty::Slice(_)
  94             | ty::RawPtr(_)
  95             | ty::Ref(_, _, _)
  96             | ty::FnDef(_, _)
  97             | ty::FnPtr(_)
  98             | ty::Dynamic(_, _)
  99             | ty::Closure(_, _)
 100             | ty::Generator(_, _, _)
 101             | ty::GeneratorWitness(_)
 102             | ty::Never
 103             | ty::Tuple(_)
 104             | ty::Error(_) => ConstValue::from_machine_usize(0u64, &tcx),
 105         },
 106         other => bug!("`{}` is not a zero arg intrinsic", other),
 107     })
 108 }
 109
 110 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 111     /// Returns `true` if emulation happened.
 112     /// Here we implement the intrinsics that are common to all Miri instances; individual machines can add their own
 113     /// intrinsic handling.
 114     pub fn emulate_intrinsic(
 115         &mut self,
 116         instance: ty::Instance<'tcx>,
 117         args: &[OpTy<'tcx, M::Provenance>],
 118         dest: &PlaceTy<'tcx, M::Provenance>,
 119         ret: Option<mir::BasicBlock>,
 120     ) -> InterpResult<'tcx, bool> {
 121         let substs = instance.substs;
 122         let intrinsic_name = self.tcx.item_name(instance.def_id());
 123
 124         // First handle intrinsics without return place.
 125         let ret = match ret {
 126             None => match intrinsic_name {
 127                 sym::transmute => throw_ub_format!("transmuting to uninhabited type"),
 128                 sym::abort => M::abort(self, "the program aborted execution".to_owned())?,
 129                 // Unsupported diverging intrinsic.
 130                 _ => return Ok(false),
 131             },
 132             Some(p) => p,
 133         };
 134
 135         match intrinsic_name {
 136             sym::caller_location => {
 137                 let span = self.find_closest_untracked_caller_location();
 138                 let location = self.alloc_caller_location_for_span(span);
 139                 self.write_immediate(location.to_ref(self), dest)?;
 140             }
 141
 142             sym::min_align_of_val | sym::size_of_val => {
 143                 // Avoid `deref_operand` -- this is not a deref, the ptr does not have to be
 144                 // dereferenceable!
 145                 let place = self.ref_to_mplace(&self.read_immediate(&args[0])?)?;
 146                 let (size, align) = self
 147                     .size_and_align_of_mplace(&place)?
 148                     .ok_or_else(|| err_unsup_format!("`extern type` does not have known layout"))?;
 149
 150                 let result = match intrinsic_name {
 151                     sym::min_align_of_val => align.bytes(),
 152                     sym::size_of_val => size.bytes(),
 153                     _ => bug!(),
 154                 };
 155
 156                 self.write_scalar(Scalar::from_machine_usize(result, self), dest)?;
 157             }
 158
 159             sym::pref_align_of
 160             | sym::needs_drop
 161             | sym::type_id
 162             | sym::type_name
 163             | sym::variant_count => {
 164                 let gid = GlobalId { instance, promoted: None };
 165                 let ty = match intrinsic_name {
 166                     sym::pref_align_of | sym::variant_count => self.tcx.types.usize,
 167                     sym::needs_drop => self.tcx.types.bool,
 168                     sym::type_id => self.tcx.types.u64,
 169                     sym::type_name => self.tcx.mk_static_str(),
 170                     _ => bug!(),
 171                 };
 172                 let val =
 173                     self.tcx.const_eval_global_id(self.param_env, gid, Some(self.tcx.span))?;
 174                 let val = self.const_val_to_op(val, ty, Some(dest.layout))?;
 175                 self.copy_op(&val, dest, /*allow_transmute*/ false)?;
 176             }
 177
 178             sym::ctpop
 179             | sym::cttz
 180             | sym::cttz_nonzero
 181             | sym::ctlz
 182             | sym::ctlz_nonzero
 183             | sym::bswap
 184             | sym::bitreverse => {
 185                 let ty = substs.type_at(0);
 186                 let layout_of = self.layout_of(ty)?;
 187                 let val = self.read_scalar(&args[0])?.check_init()?;
 188                 let bits = val.to_bits(layout_of.size)?;
 189                 let kind = match layout_of.abi {
 190                     Abi::Scalar(scalar) => scalar.primitive(),
 191                     _ => span_bug!(
 192                         self.cur_span(),
 193                         "{} called on invalid type {:?}",
 194                         intrinsic_name,
 195                         ty
 196                     ),
 197                 };
 198                 let (nonzero, intrinsic_name) = match intrinsic_name {
 199                     sym::cttz_nonzero => (true, sym::cttz),
 200                     sym::ctlz_nonzero => (true, sym::ctlz),
 201                     other => (false, other),
 202                 };
 203                 if nonzero && bits == 0 {
 204                     throw_ub_format!("`{}_nonzero` called on 0", intrinsic_name);
 205                 }
 206                 let out_val = numeric_intrinsic(intrinsic_name, bits, kind);
 207                 self.write_scalar(out_val, dest)?;
 208             }
 209             sym::add_with_overflow | sym::sub_with_overflow | sym::mul_with_overflow => {
 210                 let lhs = self.read_immediate(&args[0])?;
 211                 let rhs = self.read_immediate(&args[1])?;
 212                 let bin_op = match intrinsic_name {
 213                     sym::add_with_overflow => BinOp::Add,
 214                     sym::sub_with_overflow => BinOp::Sub,
 215                     sym::mul_with_overflow => BinOp::Mul,
 216                     _ => bug!(),
 217                 };
 218                 self.binop_with_overflow(
 219                     bin_op, /*force_overflow_checks*/ true, &lhs, &rhs, dest,
 220                 )?;
 221             }
 222             sym::saturating_add | sym::saturating_sub => {
 223                 let l = self.read_immediate(&args[0])?;
 224                 let r = self.read_immediate(&args[1])?;
 225                 let val = self.saturating_arith(
 226                     if intrinsic_name == sym::saturating_add { BinOp::Add } else { BinOp::Sub },
 227                     &l,
 228                     &r,
 229                 )?;
 230                 self.write_scalar(val, dest)?;
 231             }
 232             sym::discriminant_value => {
 233                 let place = self.deref_operand(&args[0])?;
 234                 let discr_val = self.read_discriminant(&place.into())?.0;
 235                 self.write_scalar(discr_val, dest)?;
 236             }
 237             sym::unchecked_shl
 238             | sym::unchecked_shr
 239             | sym::unchecked_add
 240             | sym::unchecked_sub
 241             | sym::unchecked_mul
 242             | sym::unchecked_div
 243             | sym::unchecked_rem => {
 244                 let l = self.read_immediate(&args[0])?;
 245                 let r = self.read_immediate(&args[1])?;
 246                 let bin_op = match intrinsic_name {
 247                     sym::unchecked_shl => BinOp::Shl,
 248                     sym::unchecked_shr => BinOp::Shr,
 249                     sym::unchecked_add => BinOp::Add,
 250                     sym::unchecked_sub => BinOp::Sub,
 251                     sym::unchecked_mul => BinOp::Mul,
 252                     sym::unchecked_div => BinOp::Div,
 253                     sym::unchecked_rem => BinOp::Rem,
 254                     _ => bug!(),
 255                 };
 256                 let (val, overflowed, _ty) = self.overflowing_binary_op(bin_op, &l, &r)?;
 257                 if overflowed {
 258                     let layout = self.layout_of(substs.type_at(0))?;
 259                     let r_val = r.to_scalar()?.to_bits(layout.size)?;
 260                     if let sym::unchecked_shl | sym::unchecked_shr = intrinsic_name {
 261                         throw_ub_format!("overflowing shift by {} in `{}`", r_val, intrinsic_name);
 262                     } else {
 263                         throw_ub_format!("overflow executing `{}`", intrinsic_name);
 264                     }
 265                 }
 266                 self.write_scalar(val, dest)?;
 267             }
 268             sym::rotate_left | sym::rotate_right => {
 269                 // rotate_left: (X << (S % BW)) | (X >> ((BW - S) % BW))
 270                 // rotate_right: (X << ((BW - S) % BW)) | (X >> (S % BW))
 271                 let layout = self.layout_of(substs.type_at(0))?;
 272                 let val = self.read_scalar(&args[0])?.check_init()?;
 273                 let val_bits = val.to_bits(layout.size)?;
 274                 let raw_shift = self.read_scalar(&args[1])?.check_init()?;
 275                 let raw_shift_bits = raw_shift.to_bits(layout.size)?;
 276                 let width_bits = u128::from(layout.size.bits());
 277                 let shift_bits = raw_shift_bits % width_bits;
 278                 let inv_shift_bits = (width_bits - shift_bits) % width_bits;
 279                 let result_bits = if intrinsic_name == sym::rotate_left {
 280                     (val_bits << shift_bits) | (val_bits >> inv_shift_bits)
 281                 } else {
 282                     (val_bits >> shift_bits) | (val_bits << inv_shift_bits)
 283                 };
 284                 let truncated_bits = self.truncate(result_bits, layout);
 285                 let result = Scalar::from_uint(truncated_bits, layout.size);
 286                 self.write_scalar(result, dest)?;
 287             }
 288             sym::copy => {
 289                 self.copy_intrinsic(&args[0], &args[1], &args[2], /*nonoverlapping*/ false)?;
 290             }
 291             sym::write_bytes => {
 292                 self.write_bytes_intrinsic(&args[0], &args[1], &args[2])?;
 293             }
 294             sym::offset => {
 295                 let ptr = self.read_pointer(&args[0])?;
 296                 let offset_count = self.read_scalar(&args[1])?.to_machine_isize(self)?;
 297                 let pointee_ty = substs.type_at(0);
 298
 299                 let offset_ptr = self.ptr_offset_inbounds(ptr, pointee_ty, offset_count)?;
 300                 self.write_pointer(offset_ptr, dest)?;
 301             }
 302             sym::arith_offset => {
 303                 let ptr = self.read_pointer(&args[0])?;
 304                 let offset_count = self.read_scalar(&args[1])?.to_machine_isize(self)?;
 305                 let pointee_ty = substs.type_at(0);
 306
 307                 let pointee_size = i64::try_from(self.layout_of(pointee_ty)?.size.bytes()).unwrap();
 308                 let offset_bytes = offset_count.wrapping_mul(pointee_size);
 309                 let offset_ptr = ptr.wrapping_signed_offset(offset_bytes, self);
 310                 self.write_pointer(offset_ptr, dest)?;
 311             }
 312             sym::ptr_offset_from | sym::ptr_offset_from_unsigned => {
 313                 let a = self.read_pointer(&args[0])?;
 314                 let b = self.read_pointer(&args[1])?;
 315
 316                 let usize_layout = self.layout_of(self.tcx.types.usize)?;
 317                 let isize_layout = self.layout_of(self.tcx.types.isize)?;
 318
 319                 // Get offsets for both that are at least relative to the same base.
 320                 let (a_offset, b_offset) =
 321                     match (self.ptr_try_get_alloc_id(a), self.ptr_try_get_alloc_id(b)) {
 322                         (Err(a), Err(b)) => {
 323                             // Neither poiner points to an allocation.
 324                             // If these are inequal or null, this *will* fail the deref check below.
 325                             (a, b)
 326                         }
 327                         (Err(_), _) | (_, Err(_)) => {
 328                             // We managed to find a valid allocation for one pointer, but not the other.
 329                             // That means they are definitely not pointing to the same allocation.
 330                             throw_ub_format!(
 331                                 "{} called on pointers into different allocations",
 332                                 intrinsic_name
 333                             );
 334                         }
 335                         (Ok((a_alloc_id, a_offset, _)), Ok((b_alloc_id, b_offset, _))) => {
 336                             // Found allocation for both. They must be into the same allocation.
 337                             if a_alloc_id != b_alloc_id {
 338                                 throw_ub_format!(
 339                                     "{} called on pointers into different allocations",
 340                                     intrinsic_name
 341                                 );
 342                             }
 343                             // Use these offsets for distance calculation.
 344                             (a_offset.bytes(), b_offset.bytes())
 345                         }
 346                     };
 347
 348                 // Compute distance.
 349                 let distance = {
 350                     // The subtraction is always done in `isize` to enforce
 351                     // the "no more than `isize::MAX` apart" requirement.
 352                     let a_offset = ImmTy::from_uint(a_offset, isize_layout);
 353                     let b_offset = ImmTy::from_uint(b_offset, isize_layout);
 354                     let (val, overflowed, _ty) =
 355                         self.overflowing_binary_op(BinOp::Sub, &a_offset, &b_offset)?;
 356                     if overflowed {
 357                         throw_ub_format!("pointers were too far apart for {}", intrinsic_name);
 358                     }
 359                     val.to_machine_isize(self)?
 360                 };
 361
 362                 // Check that the range between them is dereferenceable ("in-bounds or one past the
 363                 // end of the same allocation"). This is like the check in ptr_offset_inbounds.
 364                 let min_ptr = if distance >= 0 { b } else { a };
 365                 self.check_ptr_access_align(
 366                     min_ptr,
 367                     Size::from_bytes(distance.unsigned_abs()),
 368                     Align::ONE,
 369                     CheckInAllocMsg::OffsetFromTest,
 370                 )?;
 371
 372                 if intrinsic_name == sym::ptr_offset_from_unsigned && distance < 0 {
 373                     throw_ub_format!(
 374                         "{} called when first pointer has smaller offset than second: {} < {}",
 375                         intrinsic_name,
 376                         a_offset,
 377                         b_offset,
 378                     );
 379                 }
 380
 381                 // Perform division by size to compute return value.
 382                 let ret_layout = if intrinsic_name == sym::ptr_offset_from_unsigned {
 383                     usize_layout
 384                 } else {
 385                     isize_layout
 386                 };
 387                 let pointee_layout = self.layout_of(substs.type_at(0))?;
 388                 // If ret_layout is unsigned, we checked that so is the distance, so we are good.
 389                 let val = ImmTy::from_int(distance, ret_layout);
 390                 let size = ImmTy::from_int(pointee_layout.size.bytes(), ret_layout);
 391                 self.exact_div(&val, &size, dest)?;
 392             }
 393
 394             sym::transmute => {
 395                 self.copy_op(&args[0], dest, /*allow_transmute*/ true)?;
 396             }
 397             sym::assert_inhabited | sym::assert_zero_valid | sym::assert_uninit_valid => {
 398                 let ty = instance.substs.type_at(0);
 399                 let layout = self.layout_of(ty)?;
 400
 401                 // For *all* intrinsics we first check `is_uninhabited` to give a more specific
 402                 // error message.
 403                 if layout.abi.is_uninhabited() {
 404                     // The run-time intrinsic panics just to get a good backtrace; here we abort
 405                     // since there is no problem showing a backtrace even for aborts.
 406                     M::abort(
 407                         self,
 408                         format!(
 409                             "aborted execution: attempted to instantiate uninhabited type `{}`",
 410                             ty
 411                         ),
 412                     )?;
 413                 }
 414
 415                 if intrinsic_name == sym::assert_zero_valid {
 416                     let should_panic = !self.tcx.permits_zero_init(layout);
 417
 418                     if should_panic {
 419                         M::abort(
 420                             self,
 421                             format!(
 422                                 "aborted execution: attempted to zero-initialize type `{}`, which is invalid",
 423                                 ty
 424                             ),
 425                         )?;
 426                     }
 427                 }
 428
 429                 if intrinsic_name == sym::assert_uninit_valid {
 430                     let should_panic = !self.tcx.permits_uninit_init(layout);
 431
 432                     if should_panic {
 433                         M::abort(
 434                             self,
 435                             format!(
 436                                 "aborted execution: attempted to leave type `{}` uninitialized, which is invalid",
 437                                 ty
 438                             ),
 439                         )?;
 440                     }
 441                 }
 442             }
 443             sym::simd_insert => {
 444                 let index = u64::from(self.read_scalar(&args[1])?.to_u32()?);
 445                 let elem = &args[2];
 446                 let (input, input_len) = self.operand_to_simd(&args[0])?;
 447                 let (dest, dest_len) = self.place_to_simd(dest)?;
 448                 assert_eq!(input_len, dest_len, "Return vector length must match input length");
 449                 assert!(
 450                     index < dest_len,
 451                     "Index `{}` must be in bounds of vector with length {}`",
 452                     index,
 453                     dest_len
 454                 );
 455
 456                 for i in 0..dest_len {
 457                     let place = self.mplace_index(&dest, i)?;
 458                     let value = if i == index {
 459                         elem.clone()
 460                     } else {
 461                         self.mplace_index(&input, i)?.into()
 462                     };
 463                     self.copy_op(&value, &place.into(), /*allow_transmute*/ false)?;
 464                 }
 465             }
 466             sym::simd_extract => {
 467                 let index = u64::from(self.read_scalar(&args[1])?.to_u32()?);
 468                 let (input, input_len) = self.operand_to_simd(&args[0])?;
 469                 assert!(
 470                     index < input_len,
 471                     "index `{}` must be in bounds of vector with length `{}`",
 472                     index,
 473                     input_len
 474                 );
 475                 self.copy_op(
 476                     &self.mplace_index(&input, index)?.into(),
 477                     dest,
 478                     /*allow_transmute*/ false,
 479                 )?;
 480             }
 481             sym::likely | sym::unlikely | sym::black_box => {
 482                 // These just return their argument
 483                 self.copy_op(&args[0], dest, /*allow_transmute*/ false)?;
 484             }
 485             sym::assume => {
 486                 let cond = self.read_scalar(&args[0])?.check_init()?.to_bool()?;
 487                 if !cond {
 488                     throw_ub_format!("`assume` intrinsic called with `false`");
 489                 }
 490             }
 491             sym::raw_eq => {
 492                 let result = self.raw_eq_intrinsic(&args[0], &args[1])?;
 493                 self.write_scalar(result, dest)?;
 494             }
 495             _ => return Ok(false),
 496         }
 497
 498         trace!("{:?}", self.dump_place(**dest));
 499         self.go_to_block(ret);
 500         Ok(true)
 501     }
 502
 503     pub fn exact_div(
 504         &mut self,
 505         a: &ImmTy<'tcx, M::Provenance>,
 506         b: &ImmTy<'tcx, M::Provenance>,
 507         dest: &PlaceTy<'tcx, M::Provenance>,
 508     ) -> InterpResult<'tcx> {
 509         // Performs an exact division, resulting in undefined behavior where
 510         // `x % y != 0` or `y == 0` or `x == T::MIN && y == -1`.
 511         // First, check x % y != 0 (or if that computation overflows).
 512         let (res, overflow, _ty) = self.overflowing_binary_op(BinOp::Rem, &a, &b)?;
 513         assert!(!overflow); // All overflow is UB, so this should never return on overflow.
 514         if res.assert_bits(a.layout.size) != 0 {
 515             throw_ub_format!("exact_div: {} cannot be divided by {} without remainder", a, b)
 516         }
 517         // `Rem` says this is all right, so we can let `Div` do its job.
 518         self.binop_ignore_overflow(BinOp::Div, &a, &b, dest)
 519     }
 520
 521     pub fn saturating_arith(
 522         &self,
 523         mir_op: BinOp,
 524         l: &ImmTy<'tcx, M::Provenance>,
 525         r: &ImmTy<'tcx, M::Provenance>,
 526     ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
 527         assert!(matches!(mir_op, BinOp::Add | BinOp::Sub));
 528         let (val, overflowed, _ty) = self.overflowing_binary_op(mir_op, l, r)?;
 529         Ok(if overflowed {
 530             let size = l.layout.size;
 531             let num_bits = size.bits();
 532             if l.layout.abi.is_signed() {
 533                 // For signed ints the saturated value depends on the sign of the first
 534                 // term since the sign of the second term can be inferred from this and
 535                 // the fact that the operation has overflowed (if either is 0 no
 536                 // overflow can occur)
 537                 let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
 538                 let first_term_positive = first_term & (1 << (num_bits - 1)) == 0;
 539                 if first_term_positive {
 540                     // Negative overflow not possible since the positive first term
 541                     // can only increase an (in range) negative term for addition
 542                     // or corresponding negated positive term for subtraction
 543                     Scalar::from_int(size.signed_int_max(), size)
 544                 } else {
 545                     // Positive overflow not possible for similar reason
 546                     // max negative
 547                     Scalar::from_int(size.signed_int_min(), size)
 548                 }
 549             } else {
 550                 // unsigned
 551                 if matches!(mir_op, BinOp::Add) {
 552                     // max unsigned
 553                     Scalar::from_uint(size.unsigned_int_max(), size)
 554                 } else {
 555                     // underflow to 0
 556                     Scalar::from_uint(0u128, size)
 557                 }
 558             }
 559         } else {
 560             val
 561         })
 562     }
 563
 564     /// Offsets a pointer by some multiple of its type, returning an error if the pointer leaves its
 565     /// allocation. For integer pointers, we consider each of them their own tiny allocation of size
 566     /// 0, so offset-by-0 (and only 0) is okay -- except that null cannot be offset by _any_ value.
 567     pub fn ptr_offset_inbounds(
 568         &self,
 569         ptr: Pointer<Option<M::Provenance>>,
 570         pointee_ty: Ty<'tcx>,
 571         offset_count: i64,
 572     ) -> InterpResult<'tcx, Pointer<Option<M::Provenance>>> {
 573         // We cannot overflow i64 as a type's size must be <= isize::MAX.
 574         let pointee_size = i64::try_from(self.layout_of(pointee_ty)?.size.bytes()).unwrap();
 575         // The computed offset, in bytes, must not overflow an isize.
 576         // `checked_mul` enforces a too small bound, but no actual allocation can be big enough for
 577         // the difference to be noticeable.
 578         let offset_bytes =
 579             offset_count.checked_mul(pointee_size).ok_or(err_ub!(PointerArithOverflow))?;
 580         // The offset being in bounds cannot rely on "wrapping around" the address space.
 581         // So, first rule out overflows in the pointer arithmetic.
 582         let offset_ptr = ptr.signed_offset(offset_bytes, self)?;
 583         // ptr and offset_ptr must be in bounds of the same allocated object. This means all of the
 584         // memory between these pointers must be accessible. Note that we do not require the
 585         // pointers to be properly aligned (unlike a read/write operation).
 586         let min_ptr = if offset_bytes >= 0 { ptr } else { offset_ptr };
 587         // This call handles checking for integer/null pointers.
 588         self.check_ptr_access_align(
 589             min_ptr,
 590             Size::from_bytes(offset_bytes.unsigned_abs()),
 591             Align::ONE,
 592             CheckInAllocMsg::PointerArithmeticTest,
 593         )?;
 594         Ok(offset_ptr)
 595     }
 596
 597     /// Copy `count*size_of::<T>()` many bytes from `*src` to `*dst`.
 598     pub(crate) fn copy_intrinsic(
 599         &mut self,
 600         src: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 601         dst: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 602         count: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 603         nonoverlapping: bool,
 604     ) -> InterpResult<'tcx> {
 605         let count = self.read_scalar(&count)?.to_machine_usize(self)?;
 606         let layout = self.layout_of(src.layout.ty.builtin_deref(true).unwrap().ty)?;
 607         let (size, align) = (layout.size, layout.align.abi);
 608         // `checked_mul` enforces a too small bound (the correct one would probably be machine_isize_max),
 609         // but no actual allocation can be big enough for the difference to be noticeable.
 610         let size = size.checked_mul(count, self).ok_or_else(|| {
 611             err_ub_format!(
 612                 "overflow computing total size of `{}`",
 613                 if nonoverlapping { "copy_nonoverlapping" } else { "copy" }
 614             )
 615         })?;
 616
 617         let src = self.read_pointer(&src)?;
 618         let dst = self.read_pointer(&dst)?;
 619
 620         self.mem_copy(src, align, dst, align, size, nonoverlapping)
 621     }
 622
 623     pub(crate) fn write_bytes_intrinsic(
 624         &mut self,
 625         dst: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 626         byte: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 627         count: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 628     ) -> InterpResult<'tcx> {
 629         let layout = self.layout_of(dst.layout.ty.builtin_deref(true).unwrap().ty)?;
 630
 631         let dst = self.read_pointer(&dst)?;
 632         let byte = self.read_scalar(&byte)?.to_u8()?;
 633         let count = self.read_scalar(&count)?.to_machine_usize(self)?;
 634
 635         // `checked_mul` enforces a too small bound (the correct one would probably be machine_isize_max),
 636         // but no actual allocation can be big enough for the difference to be noticeable.
 637         let len = layout
 638             .size
 639             .checked_mul(count, self)
 640             .ok_or_else(|| err_ub_format!("overflow computing total size of `write_bytes`"))?;
 641
 642         let bytes = std::iter::repeat(byte).take(len.bytes_usize());
 643         self.write_bytes_ptr(dst, bytes)
 644     }
 645
 646     pub(crate) fn raw_eq_intrinsic(
 647         &mut self,
 648         lhs: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 649         rhs: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::Provenance>,
 650     ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
 651         let layout = self.layout_of(lhs.layout.ty.builtin_deref(true).unwrap().ty)?;
 652         assert!(!layout.is_unsized());
 653
 654         let lhs = self.read_pointer(lhs)?;
 655         let rhs = self.read_pointer(rhs)?;
 656         let lhs_bytes = self.read_bytes_ptr(lhs, layout.size)?;
 657         let rhs_bytes = self.read_bytes_ptr(rhs, layout.size)?;
 658         Ok(Scalar::from_bool(lhs_bytes == rhs_bytes))
 659     }
 660 }