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::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 from_known_layout, mir_assign_valid_types, ConstValue, GlobalId, InterpCx, InterpResult,
19 MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer, Scalar, ScalarMaybeUninit,
22 /// An `Immediate` represents a single immediate self-contained Rust value.
24 /// For optimization of a few very common cases, there is also a representation for a pair of
25 /// primitive values (`ScalarPair`). It allows Miri to avoid making allocations for checked binary
26 /// operations and wide pointers. This idea was taken from rustc's codegen.
27 /// In particular, thanks to `ScalarPair`, arithmetic operations and casts can be entirely
28 /// defined on `Immediate`, and do not have to work with a `Place`.
29 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
30 pub enum Immediate<Tag = ()> {
31 Scalar(ScalarMaybeUninit<Tag>),
32 ScalarPair(ScalarMaybeUninit<Tag>, ScalarMaybeUninit<Tag>),
35 impl<Tag> From<ScalarMaybeUninit<Tag>> for Immediate<Tag> {
37 fn from(val: ScalarMaybeUninit<Tag>) -> Self {
38 Immediate::Scalar(val)
42 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
44 fn from(val: Scalar<Tag>) -> Self {
45 Immediate::Scalar(val.into())
49 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
51 fn from(val: Pointer<Tag>) -> Self {
52 Immediate::Scalar(Scalar::from(val).into())
56 impl<'tcx, Tag> Immediate<Tag> {
57 pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
58 Immediate::ScalarPair(val.into(), Scalar::from_machine_usize(len, cx).into())
61 pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
62 Immediate::ScalarPair(val.into(), vtable.into())
66 pub fn to_scalar_or_undef(self) -> ScalarMaybeUninit<Tag> {
68 Immediate::Scalar(val) => val,
69 Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
74 pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
75 self.to_scalar_or_undef().not_undef()
79 pub fn to_scalar_pair(self) -> InterpResult<'tcx, (Scalar<Tag>, Scalar<Tag>)> {
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()?)),
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 = ()> {
92 pub layout: TyAndLayout<'tcx>,
95 impl<Tag: Copy> std::fmt::Display for ImmTy<'tcx, Tag> {
96 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
97 /// Helper function for printing a scalar to a FmtPrinter
98 fn p<'a, 'tcx, F: std::fmt::Write, Tag>(
99 cx: FmtPrinter<'a, 'tcx, F>,
100 s: ScalarMaybeUninit<Tag>,
102 ) -> Result<FmtPrinter<'a, 'tcx, F>, std::fmt::Error> {
104 ScalarMaybeUninit::Scalar(s) => {
105 cx.pretty_print_const_scalar(s.erase_tag(), ty, true)
107 ScalarMaybeUninit::Uninit => cx.typed_value(
109 this.write_str("{undef ")?;
112 |this| this.print_type(ty),
117 ty::tls::with(|tcx| {
119 Immediate::Scalar(s) => {
120 if let Some(ty) = tcx.lift(&self.layout.ty) {
121 let cx = FmtPrinter::new(tcx, f, Namespace::ValueNS);
125 write!(f, "{}: {}", s.erase_tag(), self.layout.ty)
127 Immediate::ScalarPair(a, b) => {
128 // FIXME(oli-obk): at least print tuples and slices nicely
129 write!(f, "({}, {}): {}", a.erase_tag(), b.erase_tag(), self.layout.ty,)
136 impl<'tcx, Tag> ::std::ops::Deref for ImmTy<'tcx, Tag> {
137 type Target = Immediate<Tag>;
139 fn deref(&self) -> &Immediate<Tag> {
144 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
145 /// or still in memory. The latter is an optimization, to delay reading that chunk of
146 /// memory and to avoid having to store arbitrary-sized data here.
147 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
148 pub enum Operand<Tag = ()> {
149 Immediate(Immediate<Tag>),
150 Indirect(MemPlace<Tag>),
153 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
154 pub struct OpTy<'tcx, Tag = ()> {
155 op: Operand<Tag>, // Keep this private; it helps enforce invariants.
156 pub layout: TyAndLayout<'tcx>,
159 impl<'tcx, Tag> ::std::ops::Deref for OpTy<'tcx, Tag> {
160 type Target = Operand<Tag>;
162 fn deref(&self) -> &Operand<Tag> {
167 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
169 fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
170 OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
174 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
176 fn from(val: ImmTy<'tcx, Tag>) -> Self {
177 OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
181 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
183 pub fn from_scalar(val: Scalar<Tag>, layout: TyAndLayout<'tcx>) -> Self {
184 ImmTy { imm: val.into(), layout }
188 pub fn from_immediate(imm: Immediate<Tag>, layout: TyAndLayout<'tcx>) -> Self {
189 ImmTy { imm, layout }
193 pub fn try_from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
194 Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
197 pub fn from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Self {
198 Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
202 pub fn try_from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
203 Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
207 pub fn from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Self {
208 Self::from_scalar(Scalar::from_int(i, layout.size), layout)
212 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
213 /// Normalice `place.ptr` to a `Pointer` if this is a place and not a ZST.
214 /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
218 op: OpTy<'tcx, M::PointerTag>,
219 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
220 match op.try_as_mplace(self) {
221 Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
222 Err(imm) => Ok(imm.into()), // Nothing to cast/force
226 /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
227 /// Returns `None` if the layout does not permit loading this as a value.
228 fn try_read_immediate_from_mplace(
230 mplace: MPlaceTy<'tcx, M::PointerTag>,
231 ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
232 if mplace.layout.is_unsized() {
233 // Don't touch unsized
238 .check_mplace_access(mplace, None)
239 .expect("places should be checked on creation")
243 if let Scalar::Ptr(ptr) = mplace.ptr {
244 // We may be reading from a static.
245 // In order to ensure that `static FOO: Type = FOO;` causes a cycle error
246 // instead of magically pulling *any* ZST value from the ether, we need to
247 // actually access the referenced allocation.
248 self.memory.get_raw(ptr.alloc_id)?;
250 return Ok(Some(ImmTy {
252 imm: Scalar::zst().into(),
253 layout: mplace.layout,
258 let alloc = self.memory.get_raw(ptr.alloc_id)?;
260 match mplace.layout.abi {
262 let scalar = alloc.read_scalar(self, ptr, mplace.layout.size)?;
263 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
265 Abi::ScalarPair(ref a, ref b) => {
266 // We checked `ptr_align` above, so all fields will have the alignment they need.
267 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
268 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
269 let (a, b) = (&a.value, &b.value);
270 let (a_size, b_size) = (a.size(self), b.size(self));
272 let b_offset = a_size.align_to(b.align(self).abi);
273 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
274 let b_ptr = ptr.offset(b_offset, self)?;
275 let a_val = alloc.read_scalar(self, a_ptr, a_size)?;
276 let b_val = alloc.read_scalar(self, b_ptr, b_size)?;
277 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
283 /// Try returning an immediate for the operand.
284 /// If the layout does not permit loading this as an immediate, return where in memory
285 /// we can find the data.
286 /// Note that for a given layout, this operation will either always fail or always
287 /// succeed! Whether it succeeds depends on whether the layout can be represented
288 /// in a `Immediate`, not on which data is stored there currently.
289 pub(crate) fn try_read_immediate(
291 src: OpTy<'tcx, M::PointerTag>,
292 ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
293 Ok(match src.try_as_mplace(self) {
295 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
305 /// Read an immediate from a place, asserting that that is possible with the given layout.
307 pub fn read_immediate(
309 op: OpTy<'tcx, M::PointerTag>,
310 ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
311 if let Ok(imm) = self.try_read_immediate(op)? {
314 bug!("primitive read failed for type: {:?}", op.layout.ty);
318 /// Read a scalar from a place
321 op: OpTy<'tcx, M::PointerTag>,
322 ) -> InterpResult<'tcx, ScalarMaybeUninit<M::PointerTag>> {
323 Ok(self.read_immediate(op)?.to_scalar_or_undef())
326 // Turn the wide MPlace into a string (must already be dereferenced!)
327 pub fn read_str(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
328 let len = mplace.len(self)?;
329 let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len))?;
330 let str = ::std::str::from_utf8(bytes).map_err(|err| err_ub!(InvalidStr(err)))?;
334 /// Projection functions
335 pub fn operand_field(
337 op: OpTy<'tcx, M::PointerTag>,
339 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
340 let base = match op.try_as_mplace(self) {
342 // We can reuse the mplace field computation logic for indirect operands.
343 let field = self.mplace_field(mplace, field)?;
344 return Ok(field.into());
349 let field_layout = op.layout.field(self, field)?;
350 if field_layout.is_zst() {
351 let immediate = Scalar::zst().into();
352 return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
354 let offset = op.layout.fields.offset(field);
355 let immediate = match *base {
356 // the field covers the entire type
357 _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
358 // extract fields from types with `ScalarPair` ABI
359 Immediate::ScalarPair(a, b) => {
360 let val = if offset.bytes() == 0 { a } else { b };
363 Immediate::Scalar(val) => {
364 bug!("field access on non aggregate {:#?}, {:#?}", val, op.layout)
367 Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
370 pub fn operand_index(
372 op: OpTy<'tcx, M::PointerTag>,
374 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
375 if let Ok(index) = usize::try_from(index) {
376 // We can just treat this as a field.
377 self.operand_field(op, index)
379 // Indexing into a big array. This must be an mplace.
380 let mplace = op.assert_mem_place(self);
381 Ok(self.mplace_index(mplace, index)?.into())
385 pub fn operand_downcast(
387 op: OpTy<'tcx, M::PointerTag>,
389 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
390 // Downcasts only change the layout
391 Ok(match op.try_as_mplace(self) {
392 Ok(mplace) => self.mplace_downcast(mplace, variant)?.into(),
394 let layout = op.layout.for_variant(self, variant);
395 OpTy { layout, ..op }
400 pub fn operand_projection(
402 base: OpTy<'tcx, M::PointerTag>,
403 proj_elem: mir::PlaceElem<'tcx>,
404 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
405 use rustc_middle::mir::ProjectionElem::*;
407 Field(field, _) => self.operand_field(base, field.index())?,
408 Downcast(_, variant) => self.operand_downcast(base, variant)?,
409 Deref => self.deref_operand(base)?.into(),
410 Subslice { .. } | ConstantIndex { .. } | Index(_) => {
411 // The rest should only occur as mplace, we do not use Immediates for types
412 // allowing such operations. This matches place_projection forcing an allocation.
413 let mplace = base.assert_mem_place(self);
414 self.mplace_projection(mplace, proj_elem)?.into()
419 /// This is used by [priroda](https://github.com/oli-obk/priroda) to get an OpTy from a local
422 frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
424 layout: Option<TyAndLayout<'tcx>>,
425 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
426 let layout = self.layout_of_local(frame, local, layout)?;
427 let op = if layout.is_zst() {
428 // Do not read from ZST, they might not be initialized
429 Operand::Immediate(Scalar::zst().into())
431 M::access_local(&self, frame, local)?
433 Ok(OpTy { op, layout })
436 /// Every place can be read from, so we can turn them into an operand.
437 /// This will definitely return `Indirect` if the place is a `Ptr`, i.e., this
438 /// will never actually read from memory.
442 place: PlaceTy<'tcx, M::PointerTag>,
443 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
444 let op = match *place {
445 Place::Ptr(mplace) => Operand::Indirect(mplace),
446 Place::Local { frame, local } => {
447 *self.access_local(&self.stack()[frame], local, None)?
450 Ok(OpTy { op, layout: place.layout })
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(
457 place: mir::Place<'tcx>,
458 layout: Option<TyAndLayout<'tcx>>,
459 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
460 // Do not use the layout passed in as argument if the base we are looking at
461 // here is not the entire place.
462 let layout = if place.projection.is_empty() { layout } else { None };
464 let base_op = self.access_local(self.frame(), place.local, layout)?;
469 .try_fold(base_op, |op, elem| self.operand_projection(op, elem))?;
471 trace!("eval_place_to_op: got {:?}", *op);
472 // Sanity-check the type we ended up with.
473 debug_assert!(mir_assign_valid_types(
475 self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
476 place.ty(&self.frame().body.local_decls, *self.tcx).ty
483 /// Evaluate the operand, returning a place where you can then find the data.
484 /// If you already know the layout, you can save two table lookups
485 /// by passing it in here.
488 mir_op: &mir::Operand<'tcx>,
489 layout: Option<TyAndLayout<'tcx>>,
490 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
491 use rustc_middle::mir::Operand::*;
492 let op = match *mir_op {
493 // FIXME: do some more logic on `move` to invalidate the old location
494 Copy(place) | Move(place) => self.eval_place_to_op(place, layout)?,
496 Constant(ref constant) => {
498 self.subst_from_current_frame_and_normalize_erasing_regions(constant.literal);
499 self.eval_const_to_op(val, layout)?
502 trace!("{:?}: {:?}", mir_op, *op);
506 /// Evaluate a bunch of operands at once
507 pub(super) fn eval_operands(
509 ops: &[mir::Operand<'tcx>],
510 ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
511 ops.iter().map(|op| self.eval_operand(op, None)).collect()
514 // Used when the miri-engine runs into a constant and for extracting information from constants
515 // in patterns via the `const_eval` module
516 /// The `val` and `layout` are assumed to already be in our interpreter
517 /// "universe" (param_env).
518 crate fn eval_const_to_op(
520 val: &ty::Const<'tcx>,
521 layout: Option<TyAndLayout<'tcx>>,
522 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
523 let tag_scalar = |scalar| match scalar {
524 Scalar::Ptr(ptr) => Scalar::Ptr(self.tag_global_base_pointer(ptr)),
525 Scalar::Raw { data, size } => Scalar::Raw { data, size },
527 // Early-return cases.
528 let val_val = match val.val {
529 ty::ConstKind::Param(_) => throw_inval!(TooGeneric),
530 ty::ConstKind::Error(_) => throw_inval!(TypeckError(ErrorReported)),
531 ty::ConstKind::Unevaluated(def_id, substs, promoted) => {
532 let instance = self.resolve(def_id, substs)?;
533 // We use `const_eval` here and `const_eval_raw` elsewhere in mir interpretation.
534 // The reason we use `const_eval_raw` everywhere else is to prevent cycles during
535 // validation, because validation automatically reads through any references, thus
536 // potentially requiring the current static to be evaluated again. This is not a
537 // problem here, because we are building an operand which means an actual read is
540 // The machine callback `adjust_global_const` below is guaranteed to
541 // be called for all constants because `const_eval` calls
542 // `eval_const_to_op` recursively.
543 return Ok(self.const_eval(GlobalId { instance, promoted }, val.ty)?);
545 ty::ConstKind::Infer(..)
546 | ty::ConstKind::Bound(..)
547 | ty::ConstKind::Placeholder(..) => {
548 bug!("eval_const_to_op: Unexpected ConstKind {:?}", val)
550 ty::ConstKind::Value(val_val) => val_val,
552 // This call allows the machine to create fresh allocation ids for
553 // thread-local statics (see the `adjust_global_const` function
555 let val_val = M::adjust_global_const(self, val_val)?;
556 // Other cases need layout.
557 let layout = from_known_layout(self.tcx, layout, || self.layout_of(val.ty))?;
558 let op = match val_val {
559 ConstValue::ByRef { alloc, offset } => {
560 let id = self.tcx.create_memory_alloc(alloc);
561 // We rely on mutability being set correctly in that allocation to prevent writes
562 // where none should happen.
563 let ptr = self.tag_global_base_pointer(Pointer::new(id, offset));
564 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
566 ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x).into()),
567 ConstValue::Slice { data, start, end } => {
568 // We rely on mutability being set correctly in `data` to prevent writes
569 // where none should happen.
570 let ptr = Pointer::new(
571 self.tcx.create_memory_alloc(data),
572 Size::from_bytes(start), // offset: `start`
574 Operand::Immediate(Immediate::new_slice(
575 self.tag_global_base_pointer(ptr).into(),
576 u64::try_from(end.checked_sub(start).unwrap()).unwrap(), // len: `end - start`
581 Ok(OpTy { op, layout })
584 /// Read discriminant, return the runtime value as well as the variant index.
585 pub fn read_discriminant(
587 op: OpTy<'tcx, M::PointerTag>,
588 ) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
589 trace!("read_discriminant_value {:#?}", op.layout);
590 // Get type and layout of the discriminant.
591 let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
592 trace!("discriminant type: {:?}", discr_layout.ty);
594 // We use "discriminant" to refer to the value associated with a particular enum variant.
595 // This is not to be confused with its "variant index", which is just determining its position in the
596 // declared list of variants -- they can differ with explicitly assigned discriminants.
597 // We use "tag" to refer to how the discriminant is encoded in memory, which can be either
598 // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
599 let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
600 Variants::Single { index } => {
601 let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
603 // This type actually has discriminants.
604 assert_eq!(discr.ty, discr_layout.ty);
605 Scalar::from_uint(discr.val, discr_layout.size)
608 // On a type without actual discriminants, variant is 0.
609 assert_eq!(index.as_u32(), 0);
610 Scalar::from_uint(index.as_u32(), discr_layout.size)
613 return Ok((discr, index));
615 Variants::Multiple { ref tag, ref tag_encoding, tag_field, .. } => {
616 (tag, tag_encoding, tag_field)
620 // There are *three* layouts that come into play here:
621 // - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
622 // the `Scalar` we return.
623 // - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
624 // and used to interpret the value we read from the tag field.
625 // For the return value, a cast to `discr_layout` is performed.
626 // - The field storing the tag has a layout, which is very similar to `tag_layout` but
627 // may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
629 // Get layout for tag.
630 let tag_layout = self.layout_of(tag_scalar_layout.value.to_int_ty(*self.tcx))?;
632 // Read tag and sanity-check `tag_layout`.
633 let tag_val = self.read_immediate(self.operand_field(op, tag_field)?)?;
634 assert_eq!(tag_layout.size, tag_val.layout.size);
635 assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
636 let tag_val = tag_val.to_scalar()?;
637 trace!("tag value: {:?}", tag_val);
639 // Figure out which discriminant and variant this corresponds to.
640 Ok(match *tag_encoding {
641 TagEncoding::Direct => {
643 .force_bits(tag_val, tag_layout.size)
644 .map_err(|_| err_ub!(InvalidTag(tag_val.erase_tag())))?;
645 // Cast bits from tag layout to discriminant layout.
646 let discr_val = self.cast_from_scalar(tag_bits, tag_layout, discr_layout.ty);
647 let discr_bits = discr_val.assert_bits(discr_layout.size);
648 // Convert discriminant to variant index, and catch invalid discriminants.
649 let index = match op.layout.ty.kind {
651 adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
653 ty::Generator(def_id, substs, _) => {
654 let substs = substs.as_generator();
656 .discriminants(def_id, *self.tcx)
657 .find(|(_, var)| var.val == discr_bits)
659 _ => bug!("tagged layout for non-adt non-generator"),
661 .ok_or_else(|| err_ub!(InvalidTag(tag_val.erase_tag())))?;
662 // Return the cast value, and the index.
665 TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => {
666 // Compute the variant this niche value/"tag" corresponds to. With niche layout,
667 // discriminant (encoded in niche/tag) and variant index are the same.
668 let variants_start = niche_variants.start().as_u32();
669 let variants_end = niche_variants.end().as_u32();
670 let variant = match tag_val.to_bits_or_ptr(tag_layout.size, self) {
672 // The niche must be just 0 (which an inbounds pointer value never is)
673 let ptr_valid = niche_start == 0
674 && variants_start == variants_end
675 && !self.memory.ptr_may_be_null(ptr);
677 throw_ub!(InvalidTag(tag_val.erase_tag()))
682 // We need to use machine arithmetic to get the relative variant idx:
683 // variant_index_relative = tag_val - niche_start_val
684 let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
685 let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
686 let variant_index_relative_val =
687 self.binary_op(mir::BinOp::Sub, tag_val, niche_start_val)?;
688 let variant_index_relative = variant_index_relative_val
690 .assert_bits(tag_val.layout.size);
691 // Check if this is in the range that indicates an actual discriminant.
692 if variant_index_relative <= u128::from(variants_end - variants_start) {
693 let variant_index_relative = u32::try_from(variant_index_relative)
694 .expect("we checked that this fits into a u32");
695 // Then computing the absolute variant idx should not overflow any more.
696 let variant_index = variants_start
697 .checked_add(variant_index_relative)
698 .expect("overflow computing absolute variant idx");
699 let variants_len = op
703 .expect("tagged layout for non adt")
706 assert!(usize::try_from(variant_index).unwrap() < variants_len);
707 VariantIdx::from_u32(variant_index)
713 // Compute the size of the scalar we need to return.
714 // No need to cast, because the variant index directly serves as discriminant and is
715 // encoded in the tag.
716 (Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)