src/librustc_mir/interpret/operand.rs

   1 //! Functions concerning immediate values and operands, and reading from operands.
   2 //! All high-level functions to read from memory work on operands as sources.
   3
   4 use std::convert::{TryFrom, TryInto};
   5
   6 use rustc::ty::layout::{
   7     self, HasDataLayout, IntegerExt, LayoutOf, PrimitiveExt, Size, TyLayout, VariantIdx,
   8 };
   9 use rustc::{mir, ty};
  10
  11 use super::{InterpCx, MPlaceTy, Machine, MemPlace, Place, PlaceTy};
  12 pub use rustc::mir::interpret::ScalarMaybeUndef;
  13 use rustc::mir::interpret::{
  14     sign_extend, truncate, AllocId, ConstValue, GlobalId, InterpResult, Pointer, Scalar,
  15 };
  16 use rustc_ast::ast;
  17 use rustc_macros::HashStable;
  18
  19 /// An `Immediate` represents a single immediate self-contained Rust value.
  20 ///
  21 /// For optimization of a few very common cases, there is also a representation for a pair of
  22 /// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
  23 /// operations and wide pointers. This idea was taken from rustc's codegen.
  24 /// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
  25 /// defined on `Immediate`, and do not have to work with a `Place`.
  26 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
  27 pub enum Immediate<Tag = (), Id = AllocId> {
  28     Scalar(ScalarMaybeUndef<Tag, Id>),
  29     ScalarPair(ScalarMaybeUndef<Tag, Id>, ScalarMaybeUndef<Tag, Id>),
  30 }
  31
  32 impl<Tag> From<ScalarMaybeUndef<Tag>> for Immediate<Tag> {
  33     #[inline(always)]
  34     fn from(val: ScalarMaybeUndef<Tag>) -> Self {
  35         Immediate::Scalar(val)
  36     }
  37 }
  38
  39 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
  40     #[inline(always)]
  41     fn from(val: Scalar<Tag>) -> Self {
  42         Immediate::Scalar(val.into())
  43     }
  44 }
  45
  46 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
  47     #[inline(always)]
  48     fn from(val: Pointer<Tag>) -> Self {
  49         Immediate::Scalar(Scalar::from(val).into())
  50     }
  51 }
  52
  53 impl<'tcx, Tag> Immediate<Tag> {
  54     pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
  55         Immediate::ScalarPair(
  56             val.into(),
  57             Scalar::from_uint(len, cx.data_layout().pointer_size).into(),
  58         )
  59     }
  60
  61     pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
  62         Immediate::ScalarPair(val.into(), vtable.into())
  63     }
  64
  65     #[inline]
  66     pub fn to_scalar_or_undef(self) -> ScalarMaybeUndef<Tag> {
  67         match self {
  68             Immediate::Scalar(val) => val,
  69             Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
  70         }
  71     }
  72
  73     #[inline]
  74     pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
  75         self.to_scalar_or_undef().not_undef()
  76     }
  77
  78     #[inline]
  79     pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Tag>, Scalar<Tag>)> {
  80         match self {
  81             Immediate::Scalar(..) => bug!("Got a thin pointer where a scalar pair was expected"),
  82             Immediate::ScalarPair(a, b) => Ok((a.not_undef()?, b.not_undef()?)),
  83         }
  84     }
  85 }
  86
  87 // ScalarPair needs a type to interpret, so we often have an immediate and a type together
  88 // as input for binary and cast operations.
  89 #[derive(Copy, Clone, Debug)]
  90 pub struct ImmTy<'tcx, Tag = ()> {
  91     pub(crate) imm: Immediate<Tag>,
  92     pub layout: TyLayout<'tcx>,
  93 }
  94
  95 // `Tag: Copy` because some methods on `Scalar` consume them by value
  96 impl<Tag: Copy> std::fmt::Display for ImmTy<'tcx, Tag> {
  97     fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
  98         match &self.imm {
  99             // We cannot use `to_bits_or_ptr` as we do not have a `tcx`.
 100             // So we use `is_bits` and circumvent a bunch of sanity checking -- but
 101             // this is anyway only for printing.
 102             Immediate::Scalar(ScalarMaybeUndef::Scalar(s)) if s.is_ptr() => {
 103                 fmt.write_str("{pointer}")
 104             }
 105             Immediate::Scalar(ScalarMaybeUndef::Scalar(s)) => {
 106                 let s = s.assert_bits(self.layout.size);
 107                 match self.layout.ty.kind {
 108                     ty::Int(_) => {
 109                         return write!(fmt, "{}", super::sign_extend(s, self.layout.size) as i128,);
 110                     }
 111                     ty::Uint(_) => return write!(fmt, "{}", s),
 112                     ty::Bool if s == 0 => return fmt.write_str("false"),
 113                     ty::Bool if s == 1 => return fmt.write_str("true"),
 114                     ty::Char => {
 115                         if let Some(c) = u32::try_from(s).ok().and_then(std::char::from_u32) {
 116                             return write!(fmt, "{}", c);
 117                         }
 118                     }
 119                     ty::Float(ast::FloatTy::F32) => {
 120                         if let Ok(u) = u32::try_from(s) {
 121                             return write!(fmt, "{}", f32::from_bits(u));
 122                         }
 123                     }
 124                     ty::Float(ast::FloatTy::F64) => {
 125                         if let Ok(u) = u64::try_from(s) {
 126                             return write!(fmt, "{}", f64::from_bits(u));
 127                         }
 128                     }
 129                     _ => {}
 130                 }
 131                 write!(fmt, "{:x}", s)
 132             }
 133             Immediate::Scalar(ScalarMaybeUndef::Undef) => fmt.write_str("{undef}"),
 134             Immediate::ScalarPair(..) => fmt.write_str("{wide pointer or tuple}"),
 135         }
 136     }
 137 }
 138
 139 impl<'tcx, Tag> ::std::ops::Deref for ImmTy<'tcx, Tag> {
 140     type Target = Immediate<Tag>;
 141     #[inline(always)]
 142     fn deref(&self) -> &Immediate<Tag> {
 143         &self.imm
 144     }
 145 }
 146
 147 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
 148 /// or still in memory. The latter is an optimization, to delay reading that chunk of
 149 /// memory and to avoid having to store arbitrary-sized data here.
 150 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
 151 pub enum Operand<Tag = (), Id = AllocId> {
 152     Immediate(Immediate<Tag, Id>),
 153     Indirect(MemPlace<Tag, Id>),
 154 }
 155
 156 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 157 pub struct OpTy<'tcx, Tag = ()> {
 158     op: Operand<Tag>, // Keep this private; it helps enforce invariants.
 159     pub layout: TyLayout<'tcx>,
 160 }
 161
 162 impl<'tcx, Tag> ::std::ops::Deref for OpTy<'tcx, Tag> {
 163     type Target = Operand<Tag>;
 164     #[inline(always)]
 165     fn deref(&self) -> &Operand<Tag> {
 166         &self.op
 167     }
 168 }
 169
 170 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
 171     #[inline(always)]
 172     fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
 173         OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
 174     }
 175 }
 176
 177 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
 178     #[inline(always)]
 179     fn from(val: ImmTy<'tcx, Tag>) -> Self {
 180         OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
 181     }
 182 }
 183
 184 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
 185     #[inline]
 186     pub fn from_scalar(val: Scalar<Tag>, layout: TyLayout<'tcx>) -> Self {
 187         ImmTy { imm: val.into(), layout }
 188     }
 189
 190     #[inline]
 191     pub fn try_from_uint(i: impl Into<u128>, layout: TyLayout<'tcx>) -> Option<Self> {
 192         Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
 193     }
 194     #[inline]
 195     pub fn from_uint(i: impl Into<u128>, layout: TyLayout<'tcx>) -> Self {
 196         Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
 197     }
 198
 199     #[inline]
 200     pub fn try_from_int(i: impl Into<i128>, layout: TyLayout<'tcx>) -> Option<Self> {
 201         Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
 202     }
 203
 204     #[inline]
 205     pub fn from_int(i: impl Into<i128>, layout: TyLayout<'tcx>) -> Self {
 206         Self::from_scalar(Scalar::from_int(i, layout.size), layout)
 207     }
 208 }
 209
 210 // Use the existing layout if given (but sanity check in debug mode),
 211 // or compute the layout.
 212 #[inline(always)]
 213 pub(super) fn from_known_layout<'tcx>(
 214     layout: Option<TyLayout<'tcx>>,
 215     compute: impl FnOnce() -> InterpResult<'tcx, TyLayout<'tcx>>,
 216 ) -> InterpResult<'tcx, TyLayout<'tcx>> {
 217     match layout {
 218         None => compute(),
 219         Some(layout) => {
 220             if cfg!(debug_assertions) {
 221                 let layout2 = compute()?;
 222                 assert_eq!(
 223                     layout.details, layout2.details,
 224                     "mismatch in layout of supposedly equal-layout types {:?} and {:?}",
 225                     layout.ty, layout2.ty
 226                 );
 227             }
 228             Ok(layout)
 229         }
 230     }
 231 }
 232
 233 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 234     /// Normalice `place.ptr` to a `Pointer` if this is a place and not a ZST.
 235     /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
 236     #[inline]
 237     pub fn force_op_ptr(
 238         &self,
 239         op: OpTy<'tcx, M::PointerTag>,
 240     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 241         match op.try_as_mplace(self) {
 242             Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
 243             Err(imm) => Ok(imm.into()), // Nothing to cast/force
 244         }
 245     }
 246
 247     /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
 248     /// Returns `None` if the layout does not permit loading this as a value.
 249     fn try_read_immediate_from_mplace(
 250         &self,
 251         mplace: MPlaceTy<'tcx, M::PointerTag>,
 252     ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
 253         if mplace.layout.is_unsized() {
 254             // Don't touch unsized
 255             return Ok(None);
 256         }
 257
 258         let ptr = match self
 259             .check_mplace_access(mplace, None)
 260             .expect("places should be checked on creation")
 261         {
 262             Some(ptr) => ptr,
 263             None => {
 264                 return Ok(Some(ImmTy {
 265                     // zero-sized type
 266                     imm: Scalar::zst().into(),
 267                     layout: mplace.layout,
 268                 }));
 269             }
 270         };
 271
 272         match mplace.layout.abi {
 273             layout::Abi::Scalar(..) => {
 274                 let scalar = self.memory.get_raw(ptr.alloc_id)?.read_scalar(
 275                     self,
 276                     ptr,
 277                     mplace.layout.size,
 278                 )?;
 279                 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
 280             }
 281             layout::Abi::ScalarPair(ref a, ref b) => {
 282                 // We checked `ptr_align` above, so all fields will have the alignment they need.
 283                 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
 284                 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
 285                 let (a, b) = (&a.value, &b.value);
 286                 let (a_size, b_size) = (a.size(self), b.size(self));
 287                 let a_ptr = ptr;
 288                 let b_offset = a_size.align_to(b.align(self).abi);
 289                 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
 290                 let b_ptr = ptr.offset(b_offset, self)?;
 291                 let a_val = self.memory.get_raw(ptr.alloc_id)?.read_scalar(self, a_ptr, a_size)?;
 292                 let b_val = self.memory.get_raw(ptr.alloc_id)?.read_scalar(self, b_ptr, b_size)?;
 293                 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
 294             }
 295             _ => Ok(None),
 296         }
 297     }
 298
 299     /// Try returning an immediate for the operand.
 300     /// If the layout does not permit loading this as an immediate, return where in memory
 301     /// we can find the data.
 302     /// Note that for a given layout, this operation will either always fail or always
 303     /// succeed!  Whether it succeeds depends on whether the layout can be represented
 304     /// in a `Immediate`, not on which data is stored there currently.
 305     pub(crate) fn try_read_immediate(
 306         &self,
 307         src: OpTy<'tcx, M::PointerTag>,
 308     ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
 309         Ok(match src.try_as_mplace(self) {
 310             Ok(mplace) => {
 311                 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
 312                     Ok(val)
 313                 } else {
 314                     Err(mplace)
 315                 }
 316             }
 317             Err(val) => Ok(val),
 318         })
 319     }
 320
 321     /// Read an immediate from a place, asserting that that is possible with the given layout.
 322     #[inline(always)]
 323     pub fn read_immediate(
 324         &self,
 325         op: OpTy<'tcx, M::PointerTag>,
 326     ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
 327         if let Ok(imm) = self.try_read_immediate(op)? {
 328             Ok(imm)
 329         } else {
 330             bug!("primitive read failed for type: {:?}", op.layout.ty);
 331         }
 332     }
 333
 334     /// Read a scalar from a place
 335     pub fn read_scalar(
 336         &self,
 337         op: OpTy<'tcx, M::PointerTag>,
 338     ) -> InterpResult<'tcx, ScalarMaybeUndef<M::PointerTag>> {
 339         Ok(self.read_immediate(op)?.to_scalar_or_undef())
 340     }
 341
 342     // Turn the wide MPlace into a string (must already be dereferenced!)
 343     pub fn read_str(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
 344         let len = mplace.len(self)?;
 345         let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len as u64))?;
 346         let str = ::std::str::from_utf8(bytes)
 347             .map_err(|err| err_unsup!(ValidationFailure(err.to_string())))?;
 348         Ok(str)
 349     }
 350
 351     /// Projection functions
 352     pub fn operand_field(
 353         &self,
 354         op: OpTy<'tcx, M::PointerTag>,
 355         field: u64,
 356     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 357         let base = match op.try_as_mplace(self) {
 358             Ok(mplace) => {
 359                 // The easy case
 360                 let field = self.mplace_field(mplace, field)?;
 361                 return Ok(field.into());
 362             }
 363             Err(value) => value,
 364         };
 365
 366         let field = field.try_into().unwrap();
 367         let field_layout = op.layout.field(self, field)?;
 368         if field_layout.is_zst() {
 369             let immediate = Scalar::zst().into();
 370             return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
 371         }
 372         let offset = op.layout.fields.offset(field);
 373         let immediate = match *base {
 374             // the field covers the entire type
 375             _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
 376             // extract fields from types with `ScalarPair` ABI
 377             Immediate::ScalarPair(a, b) => {
 378                 let val = if offset.bytes() == 0 { a } else { b };
 379                 Immediate::from(val)
 380             }
 381             Immediate::Scalar(val) => {
 382                 bug!("field access on non aggregate {:#?}, {:#?}", val, op.layout)
 383             }
 384         };
 385         Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
 386     }
 387
 388     pub fn operand_downcast(
 389         &self,
 390         op: OpTy<'tcx, M::PointerTag>,
 391         variant: VariantIdx,
 392     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 393         // Downcasts only change the layout
 394         Ok(match op.try_as_mplace(self) {
 395             Ok(mplace) => self.mplace_downcast(mplace, variant)?.into(),
 396             Err(..) => {
 397                 let layout = op.layout.for_variant(self, variant);
 398                 OpTy { layout, ..op }
 399             }
 400         })
 401     }
 402
 403     pub fn operand_projection(
 404         &self,
 405         base: OpTy<'tcx, M::PointerTag>,
 406         proj_elem: &mir::PlaceElem<'tcx>,
 407     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 408         use rustc::mir::ProjectionElem::*;
 409         Ok(match *proj_elem {
 410             Field(field, _) => self.operand_field(base, field.index() as u64)?,
 411             Downcast(_, variant) => self.operand_downcast(base, variant)?,
 412             Deref => self.deref_operand(base)?.into(),
 413             Subslice { .. } | ConstantIndex { .. } | Index(_) => {
 414                 // The rest should only occur as mplace, we do not use Immediates for types
 415                 // allowing such operations.  This matches place_projection forcing an allocation.
 416                 let mplace = base.assert_mem_place(self);
 417                 self.mplace_projection(mplace, proj_elem)?.into()
 418             }
 419         })
 420     }
 421
 422     /// This is used by [priroda](https://github.com/oli-obk/priroda) to get an OpTy from a local
 423     pub fn access_local(
 424         &self,
 425         frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
 426         local: mir::Local,
 427         layout: Option<TyLayout<'tcx>>,
 428     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 429         assert_ne!(local, mir::RETURN_PLACE);
 430         let layout = self.layout_of_local(frame, local, layout)?;
 431         let op = if layout.is_zst() {
 432             // Do not read from ZST, they might not be initialized
 433             Operand::Immediate(Scalar::zst().into())
 434         } else {
 435             M::access_local(&self, frame, local)?
 436         };
 437         Ok(OpTy { op, layout })
 438     }
 439
 440     /// Every place can be read from, so we can turn them into an operand
 441     #[inline(always)]
 442     pub fn place_to_op(
 443         &self,
 444         place: PlaceTy<'tcx, M::PointerTag>,
 445     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 446         let op = match *place {
 447             Place::Ptr(mplace) => Operand::Indirect(mplace),
 448             Place::Local { frame, local } => *self.access_local(&self.stack[frame], local, None)?,
 449         };
 450         Ok(OpTy { op, layout: place.layout })
 451     }
 452
 453     // Evaluate a place with the goal of reading from it.  This lets us sometimes
 454     // avoid allocations.
 455     pub fn eval_place_to_op(
 456         &self,
 457         place: &mir::Place<'tcx>,
 458         layout: Option<TyLayout<'tcx>>,
 459     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 460         let base_op = match place.local {
 461             mir::RETURN_PLACE => throw_unsup!(ReadFromReturnPointer),
 462             local => {
 463                 // Do not use the layout passed in as argument if the base we are looking at
 464                 // here is not the entire place.
 465                 let layout = if place.projection.is_empty() { layout } else { None };
 466
 467                 self.access_local(self.frame(), local, layout)?
 468             }
 469         };
 470
 471         let op = place
 472             .projection
 473             .iter()
 474             .try_fold(base_op, |op, elem| self.operand_projection(op, elem))?;
 475
 476         trace!("eval_place_to_op: got {:?}", *op);
 477         Ok(op)
 478     }
 479
 480     /// Evaluate the operand, returning a place where you can then find the data.
 481     /// If you already know the layout, you can save two table lookups
 482     /// by passing it in here.
 483     pub fn eval_operand(
 484         &self,
 485         mir_op: &mir::Operand<'tcx>,
 486         layout: Option<TyLayout<'tcx>>,
 487     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 488         use rustc::mir::Operand::*;
 489         let op = match *mir_op {
 490             // FIXME: do some more logic on `move` to invalidate the old location
 491             Copy(ref place) | Move(ref place) => self.eval_place_to_op(place, layout)?,
 492
 493             Constant(ref constant) => {
 494                 let val = self.subst_from_frame_and_normalize_erasing_regions(constant.literal);
 495                 self.eval_const_to_op(val, layout)?
 496             }
 497         };
 498         trace!("{:?}: {:?}", mir_op, *op);
 499         Ok(op)
 500     }
 501
 502     /// Evaluate a bunch of operands at once
 503     pub(super) fn eval_operands(
 504         &self,
 505         ops: &[mir::Operand<'tcx>],
 506     ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
 507         ops.iter().map(|op| self.eval_operand(op, None)).collect()
 508     }
 509
 510     // Used when the miri-engine runs into a constant and for extracting information from constants
 511     // in patterns via the `const_eval` module
 512     /// The `val` and `layout` are assumed to already be in our interpreter
 513     /// "universe" (param_env).
 514     crate fn eval_const_to_op(
 515         &self,
 516         val: &ty::Const<'tcx>,
 517         layout: Option<TyLayout<'tcx>>,
 518     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
 519         let tag_scalar = |scalar| match scalar {
 520             Scalar::Ptr(ptr) => Scalar::Ptr(self.tag_static_base_pointer(ptr)),
 521             Scalar::Raw { data, size } => Scalar::Raw { data, size },
 522         };
 523         // Early-return cases.
 524         let val_val = match val.val {
 525             ty::ConstKind::Param(_) => throw_inval!(TooGeneric),
 526             ty::ConstKind::Unevaluated(def_id, substs, promoted) => {
 527                 let instance = self.resolve(def_id, substs)?;
 528                 // We use `const_eval` here and `const_eval_raw` elsewhere in mir interpretation.
 529                 // The reason we use `const_eval_raw` everywhere else is to prevent cycles during
 530                 // validation, because validation automatically reads through any references, thus
 531                 // potentially requiring the current static to be evaluated again. This is not a
 532                 // problem here, because we are building an operand which means an actual read is
 533                 // happening.
 534                 return Ok(self.const_eval(GlobalId { instance, promoted }, val.ty)?);
 535             }
 536             ty::ConstKind::Infer(..)
 537             | ty::ConstKind::Bound(..)
 538             | ty::ConstKind::Placeholder(..) => {
 539                 bug!("eval_const_to_op: Unexpected ConstKind {:?}", val)
 540             }
 541             ty::ConstKind::Value(val_val) => val_val,
 542         };
 543         // Other cases need layout.
 544         let layout = from_known_layout(layout, || self.layout_of(val.ty))?;
 545         let op = match val_val {
 546             ConstValue::ByRef { alloc, offset } => {
 547                 let id = self.tcx.alloc_map.lock().create_memory_alloc(alloc);
 548                 // We rely on mutability being set correctly in that allocation to prevent writes
 549                 // where none should happen.
 550                 let ptr = self.tag_static_base_pointer(Pointer::new(id, offset));
 551                 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
 552             }
 553             ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x).into()),
 554             ConstValue::Slice { data, start, end } => {
 555                 // We rely on mutability being set correctly in `data` to prevent writes
 556                 // where none should happen.
 557                 let ptr = Pointer::new(
 558                     self.tcx.alloc_map.lock().create_memory_alloc(data),
 559                     Size::from_bytes(start as u64), // offset: `start`
 560                 );
 561                 Operand::Immediate(Immediate::new_slice(
 562                     self.tag_static_base_pointer(ptr).into(),
 563                     (end - start) as u64, // len: `end - start`
 564                     self,
 565                 ))
 566             }
 567         };
 568         Ok(OpTy { op, layout })
 569     }
 570
 571     /// Read discriminant, return the runtime value as well as the variant index.
 572     pub fn read_discriminant(
 573         &self,
 574         rval: OpTy<'tcx, M::PointerTag>,
 575     ) -> InterpResult<'tcx, (u128, VariantIdx)> {
 576         trace!("read_discriminant_value {:#?}", rval.layout);
 577
 578         let (discr_layout, discr_kind, discr_index) = match rval.layout.variants {
 579             layout::Variants::Single { index } => {
 580                 let discr_val = rval
 581                     .layout
 582                     .ty
 583                     .discriminant_for_variant(*self.tcx, index)
 584                     .map_or(index.as_u32() as u128, |discr| discr.val);
 585                 return Ok((discr_val, index));
 586             }
 587             layout::Variants::Multiple {
 588                 discr: ref discr_layout,
 589                 ref discr_kind,
 590                 discr_index,
 591                 ..
 592             } => (discr_layout, discr_kind, discr_index),
 593         };
 594
 595         // read raw discriminant value
 596         let discr_op = self.operand_field(rval, discr_index as u64)?;
 597         let discr_val = self.read_immediate(discr_op)?;
 598         let raw_discr = discr_val.to_scalar_or_undef();
 599         trace!("discr value: {:?}", raw_discr);
 600         // post-process
 601         Ok(match *discr_kind {
 602             layout::DiscriminantKind::Tag => {
 603                 let bits_discr = raw_discr
 604                     .not_undef()
 605                     .and_then(|raw_discr| self.force_bits(raw_discr, discr_val.layout.size))
 606                     .map_err(|_| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
 607                 let real_discr = if discr_val.layout.ty.is_signed() {
 608                     // going from layout tag type to typeck discriminant type
 609                     // requires first sign extending with the discriminant layout
 610                     let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;
 611                     // and then zeroing with the typeck discriminant type
 612                     let discr_ty = rval
 613                         .layout
 614                         .ty
 615                         .ty_adt_def()
 616                         .expect("tagged layout corresponds to adt")
 617                         .repr
 618                         .discr_type();
 619                     let size = layout::Integer::from_attr(self, discr_ty).size();
 620                     let truncatee = sexted as u128;
 621                     truncate(truncatee, size)
 622                 } else {
 623                     bits_discr
 624                 };
 625                 // Make sure we catch invalid discriminants
 626                 let index = match rval.layout.ty.kind {
 627                     ty::Adt(adt, _) => {
 628                         adt.discriminants(self.tcx.tcx).find(|(_, var)| var.val == real_discr)
 629                     }
 630                     ty::Generator(def_id, substs, _) => {
 631                         let substs = substs.as_generator();
 632                         substs
 633                             .discriminants(def_id, self.tcx.tcx)
 634                             .find(|(_, var)| var.val == real_discr)
 635                     }
 636                     _ => bug!("tagged layout for non-adt non-generator"),
 637                 }
 638                 .ok_or_else(|| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
 639                 (real_discr, index.0)
 640             }
 641             layout::DiscriminantKind::Niche {
 642                 dataful_variant,
 643                 ref niche_variants,
 644                 niche_start,
 645             } => {
 646                 let variants_start = niche_variants.start().as_u32();
 647                 let variants_end = niche_variants.end().as_u32();
 648                 let raw_discr = raw_discr
 649                     .not_undef()
 650                     .map_err(|_| err_ub!(InvalidDiscriminant(ScalarMaybeUndef::Undef)))?;
 651                 match raw_discr.to_bits_or_ptr(discr_val.layout.size, self) {
 652                     Err(ptr) => {
 653                         // The niche must be just 0 (which an inbounds pointer value never is)
 654                         let ptr_valid = niche_start == 0
 655                             && variants_start == variants_end
 656                             && !self.memory.ptr_may_be_null(ptr);
 657                         if !ptr_valid {
 658                             throw_ub!(InvalidDiscriminant(raw_discr.erase_tag().into()))
 659                         }
 660                         (dataful_variant.as_u32() as u128, dataful_variant)
 661                     }
 662                     Ok(raw_discr) => {
 663                         // We need to use machine arithmetic to get the relative variant idx:
 664                         // variant_index_relative = discr_val - niche_start_val
 665                         let discr_layout =
 666                             self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
 667                         let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
 668                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
 669                         let variant_index_relative_val =
 670                             self.binary_op(mir::BinOp::Sub, discr_val, niche_start_val)?;
 671                         let variant_index_relative = variant_index_relative_val
 672                             .to_scalar()?
 673                             .assert_bits(discr_val.layout.size);
 674                         // Check if this is in the range that indicates an actual discriminant.
 675                         if variant_index_relative <= u128::from(variants_end - variants_start) {
 676                             let variant_index_relative = u32::try_from(variant_index_relative)
 677                                 .expect("we checked that this fits into a u32");
 678                             // Then computing the absolute variant idx should not overflow any more.
 679                             let variant_index = variants_start
 680                                 .checked_add(variant_index_relative)
 681                                 .expect("overflow computing absolute variant idx");
 682                             let variants_len = rval
 683                                 .layout
 684                                 .ty
 685                                 .ty_adt_def()
 686                                 .expect("tagged layout for non adt")
 687                                 .variants
 688                                 .len();
 689                             assert!((variant_index as usize) < variants_len);
 690                             (u128::from(variant_index), VariantIdx::from_u32(variant_index))
 691                         } else {
 692                             (u128::from(dataful_variant.as_u32()), dataful_variant)
 693                         }
 694                     }
 695                 }
 696             }
 697         })
 698     }
 699 }