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.
4 use std::convert::TryFrom;
7 use rustc_errors::ErrorReported;
8 use rustc_hir::def::Namespace;
9 use rustc_macros::HashStable;
10 use rustc_middle::ty::layout::{PrimitiveExt, TyAndLayout};
11 use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Printer};
12 use rustc_middle::ty::{ConstInt, Ty};
13 use rustc_middle::{mir, ty};
14 use rustc_target::abi::{Abi, HasDataLayout, LayoutOf, Size, TagEncoding};
15 use rustc_target::abi::{VariantIdx, Variants};
18 alloc_range, from_known_layout, mir_assign_valid_types, AllocId, ConstValue, GlobalId,
19 InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer, Provenance,
20 Scalar, ScalarMaybeUninit,
23 /// An `Immediate` represents a single immediate self-contained Rust value.
25 /// For optimization of a few very common cases, there is also a representation for a pair of
26 /// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
27 /// operations and wide pointers. This idea was taken from rustc's codegen.
28 /// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
29 /// defined on `Immediate`, and do not have to work with a `Place`.
30 #[derive(Copy, Clone, PartialEq, Eq, HashStable, Hash)]
31 pub enum Immediate<Tag = AllocId> {
32 Scalar(ScalarMaybeUninit<Tag>),
33 ScalarPair(ScalarMaybeUninit<Tag>, ScalarMaybeUninit<Tag>),
36 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
37 //FIXME rustc_data_structures::static_assert_size!(Immediate, 56);
39 impl<Tag: Provenance> std::fmt::Debug for Immediate<Tag> {
40 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
43 Scalar(s) => f.debug_tuple("Scalar").field(s).finish(),
44 ScalarPair(s1, s2) => f.debug_tuple("ScalarPair").field(s1).field(s2).finish(),
49 impl<Tag> From<ScalarMaybeUninit<Tag>> for Immediate<Tag> {
51 fn from(val: ScalarMaybeUninit<Tag>) -> Self {
52 Immediate::Scalar(val)
56 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
58 fn from(val: Scalar<Tag>) -> Self {
59 Immediate::Scalar(val.into())
63 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
65 fn from(val: Pointer<Tag>) -> Self {
66 Immediate::Scalar(Scalar::from(val).into())
70 impl<'tcx, Tag> Immediate<Tag> {
71 pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
72 Immediate::ScalarPair(val.into(), Scalar::from_machine_usize(len, cx).into())
75 pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
76 Immediate::ScalarPair(val.into(), vtable.into())
80 pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Tag> {
82 Immediate::Scalar(val) => val,
83 Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
88 pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
89 self.to_scalar_or_uninit().check_init()
93 // ScalarPair needs a type to interpret, so we often have an immediate and a type together
94 // as input for binary and cast operations.
95 #[derive(Copy, Clone)]
96 pub struct ImmTy<'tcx, Tag = AllocId> {
98 pub layout: TyAndLayout<'tcx>,
101 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
102 //FIXME rustc_data_structures::static_assert_size!(ImmTy<'_>, 72);
104 impl<'tcx, Tag: Provenance> std::fmt::Debug for ImmTy<'tcx, Tag> {
105 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
106 let ImmTy { imm, layout } = self;
107 f.debug_struct("ImmTy").field("imm", imm).field("layout", layout).finish()
111 impl<Tag: Provenance> std::fmt::Display for ImmTy<'tcx, Tag> {
112 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
113 /// Helper function for printing a scalar to a FmtPrinter
114 fn p<'a, 'tcx, F: std::fmt::Write, Tag: Provenance>(
115 cx: FmtPrinter<'a, 'tcx, F>,
116 s: ScalarMaybeUninit<Tag>,
118 ) -> Result<FmtPrinter<'a, 'tcx, F>, std::fmt::Error> {
120 ScalarMaybeUninit::Scalar(s) => {
121 cx.pretty_print_const_scalar(s.erase_for_fmt(), ty, true)
123 ScalarMaybeUninit::Uninit => cx.typed_value(
125 this.write_str("uninit ")?;
128 |this| this.print_type(ty),
133 ty::tls::with(|tcx| {
135 Immediate::Scalar(s) => {
136 if let Some(ty) = tcx.lift(self.layout.ty) {
137 let cx = FmtPrinter::new(tcx, f, Namespace::ValueNS);
141 write!(f, "{}: {}", s.erase_for_fmt(), self.layout.ty)
143 Immediate::ScalarPair(a, b) => {
144 // FIXME(oli-obk): at least print tuples and slices nicely
145 write!(f, "({}, {}): {}", a.erase_for_fmt(), b.erase_for_fmt(), self.layout.ty,)
152 impl<'tcx, Tag> std::ops::Deref for ImmTy<'tcx, Tag> {
153 type Target = Immediate<Tag>;
155 fn deref(&self) -> &Immediate<Tag> {
160 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
161 /// or still in memory. The latter is an optimization, to delay reading that chunk of
162 /// memory and to avoid having to store arbitrary-sized data here.
163 #[derive(Copy, Clone, PartialEq, Eq, HashStable, Hash)]
164 pub enum Operand<Tag = AllocId> {
165 Immediate(Immediate<Tag>),
166 Indirect(MemPlace<Tag>),
169 impl<Tag: Provenance> std::fmt::Debug for Operand<Tag> {
170 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
173 Immediate(i) => f.debug_tuple("Immediate").field(i).finish(),
174 Indirect(p) => f.debug_tuple("Indirect").field(p).finish(),
179 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
180 pub struct OpTy<'tcx, Tag = AllocId> {
181 op: Operand<Tag>, // Keep this private; it helps enforce invariants.
182 pub layout: TyAndLayout<'tcx>,
185 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
186 rustc_data_structures::static_assert_size!(OpTy<'_, ()>, 80);
188 impl<'tcx, Tag: Provenance> std::fmt::Debug for OpTy<'tcx, Tag> {
189 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
190 let OpTy { op, layout } = self;
191 f.debug_struct("OpTy").field("op", op).field("layout", layout).finish()
195 impl<'tcx, Tag> std::ops::Deref for OpTy<'tcx, Tag> {
196 type Target = Operand<Tag>;
198 fn deref(&self) -> &Operand<Tag> {
203 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
205 fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
206 OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
210 impl<'tcx, Tag: Copy> From<&'_ MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
212 fn from(mplace: &MPlaceTy<'tcx, Tag>) -> Self {
213 OpTy { op: Operand::Indirect(**mplace), layout: mplace.layout }
217 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
219 fn from(val: ImmTy<'tcx, Tag>) -> Self {
220 OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
224 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
226 pub fn from_scalar(val: Scalar<Tag>, layout: TyAndLayout<'tcx>) -> Self {
227 ImmTy { imm: val.into(), layout }
231 pub fn from_immediate(imm: Immediate<Tag>, layout: TyAndLayout<'tcx>) -> Self {
232 ImmTy { imm, layout }
236 pub fn try_from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
237 Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
240 pub fn from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Self {
241 Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
245 pub fn try_from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
246 Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
250 pub fn from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Self {
251 Self::from_scalar(Scalar::from_int(i, layout.size), layout)
255 pub fn to_const_int(self) -> ConstInt {
256 assert!(self.layout.ty.is_integral());
257 let int = self.to_scalar().expect("to_const_int doesn't work on scalar pairs").assert_int();
258 ConstInt::new(int, self.layout.ty.is_signed(), self.layout.ty.is_ptr_sized_integral())
262 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
263 /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
264 /// Returns `None` if the layout does not permit loading this as a value.
265 fn try_read_immediate_from_mplace(
267 mplace: &MPlaceTy<'tcx, M::PointerTag>,
268 ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
269 if mplace.layout.is_unsized() {
270 // Don't touch unsized
274 let alloc = match self.get_alloc(mplace)? {
277 return Ok(Some(ImmTy {
279 imm: Scalar::ZST.into(),
280 layout: mplace.layout,
285 match mplace.layout.abi {
287 let scalar = alloc.read_scalar(alloc_range(Size::ZERO, mplace.layout.size))?;
288 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
290 Abi::ScalarPair(ref a, ref b) => {
291 // We checked `ptr_align` above, so all fields will have the alignment they need.
292 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
293 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
294 let (a, b) = (&a.value, &b.value);
295 let (a_size, b_size) = (a.size(self), b.size(self));
296 let b_offset = a_size.align_to(b.align(self).abi);
297 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
298 let a_val = alloc.read_scalar(alloc_range(Size::ZERO, a_size))?;
299 let b_val = alloc.read_scalar(alloc_range(b_offset, b_size))?;
300 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
306 /// Try returning an immediate for the operand.
307 /// If the layout does not permit loading this as an immediate, return where in memory
308 /// we can find the data.
309 /// Note that for a given layout, this operation will either always fail or always
310 /// succeed! Whether it succeeds depends on whether the layout can be represented
311 /// in a `Immediate`, not on which data is stored there currently.
312 pub(crate) fn try_read_immediate(
314 src: &OpTy<'tcx, M::PointerTag>,
315 ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
316 Ok(match src.try_as_mplace() {
318 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
328 /// Read an immediate from a place, asserting that that is possible with the given layout.
330 pub fn read_immediate(
332 op: &OpTy<'tcx, M::PointerTag>,
333 ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
334 if let Ok(imm) = self.try_read_immediate(op)? {
337 span_bug!(self.cur_span(), "primitive read failed for type: {:?}", op.layout.ty);
341 /// Read a scalar from a place
344 op: &OpTy<'tcx, M::PointerTag>,
345 ) -> InterpResult<'tcx, ScalarMaybeUninit<M::PointerTag>> {
346 Ok(self.read_immediate(op)?.to_scalar_or_uninit())
349 /// Read a pointer from a place.
352 op: &OpTy<'tcx, M::PointerTag>,
353 ) -> InterpResult<'tcx, Pointer<Option<M::PointerTag>>> {
354 Ok(self.scalar_to_ptr(self.read_scalar(op)?.check_init()?))
357 // Turn the wide MPlace into a string (must already be dereferenced!)
358 pub fn read_str(&self, mplace: &MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
359 let len = mplace.len(self)?;
360 let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len))?;
361 let str = std::str::from_utf8(bytes).map_err(|err| err_ub!(InvalidStr(err)))?;
365 /// Projection functions
366 pub fn operand_field(
368 op: &OpTy<'tcx, M::PointerTag>,
370 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
371 let base = match op.try_as_mplace() {
373 // We can reuse the mplace field computation logic for indirect operands.
374 let field = self.mplace_field(mplace, field)?;
375 return Ok(field.into());
380 let field_layout = op.layout.field(self, field)?;
381 if field_layout.is_zst() {
382 let immediate = Scalar::ZST.into();
383 return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
385 let offset = op.layout.fields.offset(field);
386 let immediate = match *base {
387 // the field covers the entire type
388 _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
389 // extract fields from types with `ScalarPair` ABI
390 Immediate::ScalarPair(a, b) => {
391 let val = if offset.bytes() == 0 { a } else { b };
394 Immediate::Scalar(val) => span_bug!(
396 "field access on non aggregate {:#?}, {:#?}",
401 Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
404 pub fn operand_index(
406 op: &OpTy<'tcx, M::PointerTag>,
408 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
409 if let Ok(index) = usize::try_from(index) {
410 // We can just treat this as a field.
411 self.operand_field(op, index)
413 // Indexing into a big array. This must be an mplace.
414 let mplace = op.assert_mem_place();
415 Ok(self.mplace_index(&mplace, index)?.into())
419 pub fn operand_downcast(
421 op: &OpTy<'tcx, M::PointerTag>,
423 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
424 // Downcasts only change the layout
425 Ok(match op.try_as_mplace() {
426 Ok(ref mplace) => self.mplace_downcast(mplace, variant)?.into(),
428 let layout = op.layout.for_variant(self, variant);
429 OpTy { layout, ..*op }
434 pub fn operand_projection(
436 base: &OpTy<'tcx, M::PointerTag>,
437 proj_elem: mir::PlaceElem<'tcx>,
438 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
439 use rustc_middle::mir::ProjectionElem::*;
441 Field(field, _) => self.operand_field(base, field.index())?,
442 Downcast(_, variant) => self.operand_downcast(base, variant)?,
443 Deref => self.deref_operand(base)?.into(),
444 Subslice { .. } | ConstantIndex { .. } | Index(_) => {
445 // The rest should only occur as mplace, we do not use Immediates for types
446 // allowing such operations. This matches place_projection forcing an allocation.
447 let mplace = base.assert_mem_place();
448 self.mplace_projection(&mplace, proj_elem)?.into()
453 /// Read from a local. Will not actually access the local if reading from a ZST.
454 /// Will not access memory, instead an indirect `Operand` is returned.
456 /// This is public because it is used by [priroda](https://github.com/oli-obk/priroda) to get an
457 /// OpTy from a local
460 frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
462 layout: Option<TyAndLayout<'tcx>>,
463 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
464 let layout = self.layout_of_local(frame, local, layout)?;
465 let op = if layout.is_zst() {
466 // Do not read from ZST, they might not be initialized
467 Operand::Immediate(Scalar::ZST.into())
469 M::access_local(&self, frame, local)?
471 Ok(OpTy { op, layout })
474 /// Every place can be read from, so we can turn them into an operand.
475 /// This will definitely return `Indirect` if the place is a `Ptr`, i.e., this
476 /// will never actually read from memory.
480 place: &PlaceTy<'tcx, M::PointerTag>,
481 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
482 let op = match **place {
483 Place::Ptr(mplace) => Operand::Indirect(mplace),
484 Place::Local { frame, local } => {
485 *self.access_local(&self.stack()[frame], local, None)?
488 Ok(OpTy { op, layout: place.layout })
491 // Evaluate a place with the goal of reading from it. This lets us sometimes
492 // avoid allocations.
493 pub fn eval_place_to_op(
495 place: mir::Place<'tcx>,
496 layout: Option<TyAndLayout<'tcx>>,
497 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
498 // Do not use the layout passed in as argument if the base we are looking at
499 // here is not the entire place.
500 let layout = if place.projection.is_empty() { layout } else { None };
502 let base_op = self.access_local(self.frame(), place.local, layout)?;
507 .try_fold(base_op, |op, elem| self.operand_projection(&op, elem))?;
509 trace!("eval_place_to_op: got {:?}", *op);
510 // Sanity-check the type we ended up with.
511 debug_assert!(mir_assign_valid_types(
514 self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
515 place.ty(&self.frame().body.local_decls, *self.tcx).ty
522 /// Evaluate the operand, returning a place where you can then find the data.
523 /// If you already know the layout, you can save two table lookups
524 /// by passing it in here.
528 mir_op: &mir::Operand<'tcx>,
529 layout: Option<TyAndLayout<'tcx>>,
530 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
531 use rustc_middle::mir::Operand::*;
532 let op = match *mir_op {
533 // FIXME: do some more logic on `move` to invalidate the old location
534 Copy(place) | Move(place) => self.eval_place_to_op(place, layout)?,
536 Constant(ref constant) => {
538 self.subst_from_current_frame_and_normalize_erasing_regions(constant.literal);
539 // This can still fail:
540 // * During ConstProp, with `TooGeneric` or since the `requried_consts` were not all
542 // * During CTFE, since promoteds in `const`/`static` initializer bodies can fail.
544 self.mir_const_to_op(&val, layout)?
547 trace!("{:?}: {:?}", mir_op, *op);
551 /// Evaluate a bunch of operands at once
552 pub(super) fn eval_operands(
554 ops: &[mir::Operand<'tcx>],
555 ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
556 ops.iter().map(|op| self.eval_operand(op, None)).collect()
559 // Used when the miri-engine runs into a constant and for extracting information from constants
560 // in patterns via the `const_eval` module
561 /// The `val` and `layout` are assumed to already be in our interpreter
562 /// "universe" (param_env).
563 crate fn const_to_op(
565 val: &ty::Const<'tcx>,
566 layout: Option<TyAndLayout<'tcx>>,
567 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
569 ty::ConstKind::Param(_) | ty::ConstKind::Bound(..) => throw_inval!(TooGeneric),
570 ty::ConstKind::Error(_) => throw_inval!(AlreadyReported(ErrorReported)),
571 ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }) => {
572 let instance = self.resolve(def, substs)?;
573 Ok(self.eval_to_allocation(GlobalId { instance, promoted })?.into())
575 ty::ConstKind::Infer(..) | ty::ConstKind::Placeholder(..) => {
576 span_bug!(self.cur_span(), "const_to_op: Unexpected ConstKind {:?}", val)
578 ty::ConstKind::Value(val_val) => self.const_val_to_op(val_val, val.ty, layout),
582 crate fn mir_const_to_op(
584 val: &mir::ConstantKind<'tcx>,
585 layout: Option<TyAndLayout<'tcx>>,
586 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
588 mir::ConstantKind::Ty(ct) => self.const_to_op(ct, layout),
589 mir::ConstantKind::Val(val, ty) => self.const_val_to_op(*val, ty, layout),
593 crate fn const_val_to_op(
595 val_val: ConstValue<'tcx>,
597 layout: Option<TyAndLayout<'tcx>>,
598 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
599 // Other cases need layout.
600 let tag_scalar = |scalar| -> InterpResult<'tcx, _> {
602 Scalar::Ptr(ptr) => Scalar::Ptr(self.global_base_pointer(ptr)?),
603 Scalar::Int(int) => Scalar::Int(int),
606 let layout = from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(ty))?;
607 let op = match val_val {
608 ConstValue::ByRef { alloc, offset } => {
609 let id = self.tcx.create_memory_alloc(alloc);
610 // We rely on mutability being set correctly in that allocation to prevent writes
611 // where none should happen.
612 let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
613 Operand::Indirect(MemPlace::from_ptr(ptr.into(), layout.align.abi))
615 ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x.into())?.into()),
616 ConstValue::Slice { data, start, end } => {
617 // We rely on mutability being set correctly in `data` to prevent writes
618 // where none should happen.
619 let ptr = Pointer::new(
620 self.tcx.create_memory_alloc(data),
621 Size::from_bytes(start), // offset: `start`
623 Operand::Immediate(Immediate::new_slice(
624 self.global_base_pointer(ptr)?.into(),
625 u64::try_from(end.checked_sub(start).unwrap()).unwrap(), // len: `end - start`
630 Ok(OpTy { op, layout })
633 /// Read discriminant, return the runtime value as well as the variant index.
634 pub fn read_discriminant(
636 op: &OpTy<'tcx, M::PointerTag>,
637 ) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
638 trace!("read_discriminant_value {:#?}", op.layout);
639 // Get type and layout of the discriminant.
640 let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
641 trace!("discriminant type: {:?}", discr_layout.ty);
643 // We use "discriminant" to refer to the value associated with a particular enum variant.
644 // This is not to be confused with its "variant index", which is just determining its position in the
645 // declared list of variants -- they can differ with explicitly assigned discriminants.
646 // We use "tag" to refer to how the discriminant is encoded in memory, which can be either
647 // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
648 let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
649 Variants::Single { index } => {
650 let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
652 // This type actually has discriminants.
653 assert_eq!(discr.ty, discr_layout.ty);
654 Scalar::from_uint(discr.val, discr_layout.size)
657 // On a type without actual discriminants, variant is 0.
658 assert_eq!(index.as_u32(), 0);
659 Scalar::from_uint(index.as_u32(), discr_layout.size)
662 return Ok((discr, index));
664 Variants::Multiple { ref tag, ref tag_encoding, tag_field, .. } => {
665 (tag, tag_encoding, tag_field)
669 // There are *three* layouts that come into play here:
670 // - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
671 // the `Scalar` we return.
672 // - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
673 // and used to interpret the value we read from the tag field.
674 // For the return value, a cast to `discr_layout` is performed.
675 // - The field storing the tag has a layout, which is very similar to `tag_layout` but
676 // may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
678 // Get layout for tag.
679 let tag_layout = self.layout_of(tag_scalar_layout.value.to_int_ty(*self.tcx))?;
681 // Read tag and sanity-check `tag_layout`.
682 let tag_val = self.read_immediate(&self.operand_field(op, tag_field)?)?;
683 assert_eq!(tag_layout.size, tag_val.layout.size);
684 assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
685 let tag_val = tag_val.to_scalar()?;
686 trace!("tag value: {:?}", tag_val);
688 // Figure out which discriminant and variant this corresponds to.
689 Ok(match *tag_encoding {
690 TagEncoding::Direct => {
691 let tag_bits = tag_val
692 .to_bits(tag_layout.size)
693 .map_err(|_| err_ub!(InvalidTag(tag_val.erase_for_fmt())))?;
694 // Cast bits from tag layout to discriminant layout.
695 let discr_val = self.cast_from_scalar(tag_bits, tag_layout, discr_layout.ty);
696 let discr_bits = discr_val.assert_bits(discr_layout.size);
697 // Convert discriminant to variant index, and catch invalid discriminants.
698 let index = match *op.layout.ty.kind() {
700 adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
702 ty::Generator(def_id, substs, _) => {
703 let substs = substs.as_generator();
705 .discriminants(def_id, *self.tcx)
706 .find(|(_, var)| var.val == discr_bits)
708 _ => span_bug!(self.cur_span(), "tagged layout for non-adt non-generator"),
710 .ok_or_else(|| err_ub!(InvalidTag(tag_val.erase_for_fmt())))?;
711 // Return the cast value, and the index.
714 TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => {
715 // Compute the variant this niche value/"tag" corresponds to. With niche layout,
716 // discriminant (encoded in niche/tag) and variant index are the same.
717 let variants_start = niche_variants.start().as_u32();
718 let variants_end = niche_variants.end().as_u32();
719 let variant = match tag_val.to_bits_or_ptr(tag_layout.size, self) {
721 // The niche must be just 0 (which an inbounds pointer value never is)
722 let ptr_valid = niche_start == 0
723 && variants_start == variants_end
724 && !self.memory.ptr_may_be_null(ptr.into());
726 throw_ub!(InvalidTag(tag_val.erase_for_fmt()))
731 // We need to use machine arithmetic to get the relative variant idx:
732 // variant_index_relative = tag_val - niche_start_val
733 let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
734 let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
735 let variant_index_relative_val =
736 self.binary_op(mir::BinOp::Sub, &tag_val, &niche_start_val)?;
737 let variant_index_relative = variant_index_relative_val
739 .assert_bits(tag_val.layout.size);
740 // Check if this is in the range that indicates an actual discriminant.
741 if variant_index_relative <= u128::from(variants_end - variants_start) {
742 let variant_index_relative = u32::try_from(variant_index_relative)
743 .expect("we checked that this fits into a u32");
744 // Then computing the absolute variant idx should not overflow any more.
745 let variant_index = variants_start
746 .checked_add(variant_index_relative)
747 .expect("overflow computing absolute variant idx");
748 let variants_len = op
752 .expect("tagged layout for non adt")
755 assert!(usize::try_from(variant_index).unwrap() < variants_len);
756 VariantIdx::from_u32(variant_index)
762 // Compute the size of the scalar we need to return.
763 // No need to cast, because the variant index directly serves as discriminant and is
764 // encoded in the tag.
765 (Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)