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, ConstValue, GlobalId, InterpCx,
19 InterpResult, 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 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
36 rustc_data_structures::static_assert_size!(Immediate, 56);
38 impl<Tag> From<ScalarMaybeUninit<Tag>> for Immediate<Tag> {
40 fn from(val: ScalarMaybeUninit<Tag>) -> Self {
41 Immediate::Scalar(val)
45 impl<Tag> From<Scalar<Tag>> for Immediate<Tag> {
47 fn from(val: Scalar<Tag>) -> Self {
48 Immediate::Scalar(val.into())
52 impl<Tag> From<Pointer<Tag>> for Immediate<Tag> {
54 fn from(val: Pointer<Tag>) -> Self {
55 Immediate::Scalar(Scalar::from(val).into())
59 impl<'tcx, Tag> Immediate<Tag> {
60 pub fn new_slice(val: Scalar<Tag>, len: u64, cx: &impl HasDataLayout) -> Self {
61 Immediate::ScalarPair(val.into(), Scalar::from_machine_usize(len, cx).into())
64 pub fn new_dyn_trait(val: Scalar<Tag>, vtable: Pointer<Tag>) -> Self {
65 Immediate::ScalarPair(val.into(), vtable.into())
69 pub fn to_scalar_or_uninit(self) -> ScalarMaybeUninit<Tag> {
71 Immediate::Scalar(val) => val,
72 Immediate::ScalarPair(..) => bug!("Got a wide pointer where a scalar was expected"),
77 pub fn to_scalar(self) -> InterpResult<'tcx, Scalar<Tag>> {
78 self.to_scalar_or_uninit().check_init()
82 // ScalarPair needs a type to interpret, so we often have an immediate and a type together
83 // as input for binary and cast operations.
84 #[derive(Copy, Clone, Debug)]
85 pub struct ImmTy<'tcx, Tag = ()> {
87 pub layout: TyAndLayout<'tcx>,
90 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
91 rustc_data_structures::static_assert_size!(ImmTy<'_>, 72);
93 impl<Tag: Copy> std::fmt::Display for ImmTy<'tcx, Tag> {
94 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
95 /// Helper function for printing a scalar to a FmtPrinter
96 fn p<'a, 'tcx, F: std::fmt::Write, Tag>(
97 cx: FmtPrinter<'a, 'tcx, F>,
98 s: ScalarMaybeUninit<Tag>,
100 ) -> Result<FmtPrinter<'a, 'tcx, F>, std::fmt::Error> {
102 ScalarMaybeUninit::Scalar(s) => {
103 cx.pretty_print_const_scalar(s.erase_tag(), ty, true)
105 ScalarMaybeUninit::Uninit => cx.typed_value(
107 this.write_str("uninit ")?;
110 |this| this.print_type(ty),
115 ty::tls::with(|tcx| {
117 Immediate::Scalar(s) => {
118 if let Some(ty) = tcx.lift(self.layout.ty) {
119 let cx = FmtPrinter::new(tcx, f, Namespace::ValueNS);
123 write!(f, "{}: {}", s.erase_tag(), self.layout.ty)
125 Immediate::ScalarPair(a, b) => {
126 // FIXME(oli-obk): at least print tuples and slices nicely
127 write!(f, "({}, {}): {}", a.erase_tag(), b.erase_tag(), self.layout.ty,)
134 impl<'tcx, Tag> std::ops::Deref for ImmTy<'tcx, Tag> {
135 type Target = Immediate<Tag>;
137 fn deref(&self) -> &Immediate<Tag> {
142 /// An `Operand` is the result of computing a `mir::Operand`. It can be immediate,
143 /// or still in memory. The latter is an optimization, to delay reading that chunk of
144 /// memory and to avoid having to store arbitrary-sized data here.
145 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, Hash)]
146 pub enum Operand<Tag = ()> {
147 Immediate(Immediate<Tag>),
148 Indirect(MemPlace<Tag>),
151 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
152 pub struct OpTy<'tcx, Tag = ()> {
153 op: Operand<Tag>, // Keep this private; it helps enforce invariants.
154 pub layout: TyAndLayout<'tcx>,
157 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
158 rustc_data_structures::static_assert_size!(OpTy<'_, ()>, 80);
160 impl<'tcx, Tag> std::ops::Deref for OpTy<'tcx, Tag> {
161 type Target = Operand<Tag>;
163 fn deref(&self) -> &Operand<Tag> {
168 impl<'tcx, Tag: Copy> From<MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
170 fn from(mplace: MPlaceTy<'tcx, Tag>) -> Self {
171 OpTy { op: Operand::Indirect(*mplace), layout: mplace.layout }
175 impl<'tcx, Tag: Copy> From<&'_ MPlaceTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
177 fn from(mplace: &MPlaceTy<'tcx, Tag>) -> Self {
178 OpTy { op: Operand::Indirect(**mplace), layout: mplace.layout }
182 impl<'tcx, Tag> From<ImmTy<'tcx, Tag>> for OpTy<'tcx, Tag> {
184 fn from(val: ImmTy<'tcx, Tag>) -> Self {
185 OpTy { op: Operand::Immediate(val.imm), layout: val.layout }
189 impl<'tcx, Tag: Copy> ImmTy<'tcx, Tag> {
191 pub fn from_scalar(val: Scalar<Tag>, layout: TyAndLayout<'tcx>) -> Self {
192 ImmTy { imm: val.into(), layout }
196 pub fn from_immediate(imm: Immediate<Tag>, layout: TyAndLayout<'tcx>) -> Self {
197 ImmTy { imm, layout }
201 pub fn try_from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
202 Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
205 pub fn from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Self {
206 Self::from_scalar(Scalar::from_uint(i, layout.size), layout)
210 pub fn try_from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
211 Some(Self::from_scalar(Scalar::try_from_int(i, layout.size)?, layout))
215 pub fn from_int(i: impl Into<i128>, layout: TyAndLayout<'tcx>) -> Self {
216 Self::from_scalar(Scalar::from_int(i, layout.size), layout)
220 pub fn to_const_int(self) -> ConstInt {
221 assert!(self.layout.ty.is_integral());
222 let int = self.to_scalar().expect("to_const_int doesn't work on scalar pairs").assert_int();
223 ConstInt::new(int, self.layout.ty.is_signed(), self.layout.ty.is_ptr_sized_integral())
227 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
228 /// Normalize `place.ptr` to a `Pointer` if this is a place and not a ZST.
229 /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
233 op: &OpTy<'tcx, M::PointerTag>,
234 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
235 match op.try_as_mplace(self) {
236 Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
237 Err(imm) => Ok(imm.into()), // Nothing to cast/force
241 /// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
242 /// Returns `None` if the layout does not permit loading this as a value.
243 fn try_read_immediate_from_mplace(
245 mplace: &MPlaceTy<'tcx, M::PointerTag>,
246 ) -> InterpResult<'tcx, Option<ImmTy<'tcx, M::PointerTag>>> {
247 if mplace.layout.is_unsized() {
248 // Don't touch unsized
252 let alloc = match self.get_alloc(mplace)? {
255 return Ok(Some(ImmTy {
257 imm: Scalar::ZST.into(),
258 layout: mplace.layout,
263 match mplace.layout.abi {
265 let scalar = alloc.read_scalar(alloc_range(Size::ZERO, mplace.layout.size))?;
266 Ok(Some(ImmTy { imm: scalar.into(), layout: mplace.layout }))
268 Abi::ScalarPair(ref a, ref b) => {
269 // We checked `ptr_align` above, so all fields will have the alignment they need.
270 // We would anyway check against `ptr_align.restrict_for_offset(b_offset)`,
271 // which `ptr.offset(b_offset)` cannot possibly fail to satisfy.
272 let (a, b) = (&a.value, &b.value);
273 let (a_size, b_size) = (a.size(self), b.size(self));
274 let b_offset = a_size.align_to(b.align(self).abi);
275 assert!(b_offset.bytes() > 0); // we later use the offset to tell apart the fields
276 let a_val = alloc.read_scalar(alloc_range(Size::ZERO, a_size))?;
277 let b_val = alloc.read_scalar(alloc_range(b_offset, b_size))?;
278 Ok(Some(ImmTy { imm: Immediate::ScalarPair(a_val, b_val), layout: mplace.layout }))
284 /// Try returning an immediate for the operand.
285 /// If the layout does not permit loading this as an immediate, return where in memory
286 /// we can find the data.
287 /// Note that for a given layout, this operation will either always fail or always
288 /// succeed! Whether it succeeds depends on whether the layout can be represented
289 /// in a `Immediate`, not on which data is stored there currently.
290 pub(crate) fn try_read_immediate(
292 src: &OpTy<'tcx, M::PointerTag>,
293 ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
294 Ok(match src.try_as_mplace(self) {
296 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
306 /// Read an immediate from a place, asserting that that is possible with the given layout.
308 pub fn read_immediate(
310 op: &OpTy<'tcx, M::PointerTag>,
311 ) -> InterpResult<'tcx, ImmTy<'tcx, M::PointerTag>> {
312 if let Ok(imm) = self.try_read_immediate(op)? {
315 span_bug!(self.cur_span(), "primitive read failed for type: {:?}", op.layout.ty);
319 /// Read a scalar from a place
322 op: &OpTy<'tcx, M::PointerTag>,
323 ) -> InterpResult<'tcx, ScalarMaybeUninit<M::PointerTag>> {
324 Ok(self.read_immediate(op)?.to_scalar_or_uninit())
327 // Turn the wide MPlace into a string (must already be dereferenced!)
328 pub fn read_str(&self, mplace: &MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
329 let len = mplace.len(self)?;
330 let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len))?;
331 let str = std::str::from_utf8(bytes).map_err(|err| err_ub!(InvalidStr(err)))?;
335 /// Projection functions
336 pub fn operand_field(
338 op: &OpTy<'tcx, M::PointerTag>,
340 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
341 let base = match op.try_as_mplace(self) {
343 // We can reuse the mplace field computation logic for indirect operands.
344 let field = self.mplace_field(mplace, field)?;
345 return Ok(field.into());
350 let field_layout = op.layout.field(self, field)?;
351 if field_layout.is_zst() {
352 let immediate = Scalar::ZST.into();
353 return Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout });
355 let offset = op.layout.fields.offset(field);
356 let immediate = match *base {
357 // the field covers the entire type
358 _ if offset.bytes() == 0 && field_layout.size == op.layout.size => *base,
359 // extract fields from types with `ScalarPair` ABI
360 Immediate::ScalarPair(a, b) => {
361 let val = if offset.bytes() == 0 { a } else { b };
364 Immediate::Scalar(val) => span_bug!(
366 "field access on non aggregate {:#?}, {:#?}",
371 Ok(OpTy { op: Operand::Immediate(immediate), layout: field_layout })
374 pub fn operand_index(
376 op: &OpTy<'tcx, M::PointerTag>,
378 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
379 if let Ok(index) = usize::try_from(index) {
380 // We can just treat this as a field.
381 self.operand_field(op, index)
383 // Indexing into a big array. This must be an mplace.
384 let mplace = op.assert_mem_place(self);
385 Ok(self.mplace_index(&mplace, index)?.into())
389 pub fn operand_downcast(
391 op: &OpTy<'tcx, M::PointerTag>,
393 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
394 // Downcasts only change the layout
395 Ok(match op.try_as_mplace(self) {
396 Ok(ref mplace) => self.mplace_downcast(mplace, variant)?.into(),
398 let layout = op.layout.for_variant(self, variant);
399 OpTy { layout, ..*op }
404 pub fn operand_projection(
406 base: &OpTy<'tcx, M::PointerTag>,
407 proj_elem: mir::PlaceElem<'tcx>,
408 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
409 use rustc_middle::mir::ProjectionElem::*;
411 Field(field, _) => self.operand_field(base, field.index())?,
412 Downcast(_, variant) => self.operand_downcast(base, variant)?,
413 Deref => self.deref_operand(base)?.into(),
414 Subslice { .. } | ConstantIndex { .. } | Index(_) => {
415 // The rest should only occur as mplace, we do not use Immediates for types
416 // allowing such operations. This matches place_projection forcing an allocation.
417 let mplace = base.assert_mem_place(self);
418 self.mplace_projection(&mplace, proj_elem)?.into()
423 /// Read from a local. Will not actually access the local if reading from a ZST.
424 /// Will not access memory, instead an indirect `Operand` is returned.
426 /// This is public because it is used by [priroda](https://github.com/oli-obk/priroda) to get an
427 /// OpTy from a local
430 frame: &super::Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>,
432 layout: Option<TyAndLayout<'tcx>>,
433 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
434 let layout = self.layout_of_local(frame, local, layout)?;
435 let op = if layout.is_zst() {
436 // Do not read from ZST, they might not be initialized
437 Operand::Immediate(Scalar::ZST.into())
439 M::access_local(&self, frame, local)?
441 Ok(OpTy { op, layout })
444 /// Every place can be read from, so we can turn them into an operand.
445 /// This will definitely return `Indirect` if the place is a `Ptr`, i.e., this
446 /// will never actually read from memory.
450 place: &PlaceTy<'tcx, M::PointerTag>,
451 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
452 let op = match **place {
453 Place::Ptr(mplace) => Operand::Indirect(mplace),
454 Place::Local { frame, local } => {
455 *self.access_local(&self.stack()[frame], local, None)?
458 Ok(OpTy { op, layout: place.layout })
461 // Evaluate a place with the goal of reading from it. This lets us sometimes
462 // avoid allocations.
463 pub fn eval_place_to_op(
465 place: mir::Place<'tcx>,
466 layout: Option<TyAndLayout<'tcx>>,
467 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
468 // Do not use the layout passed in as argument if the base we are looking at
469 // here is not the entire place.
470 let layout = if place.projection.is_empty() { layout } else { None };
472 let base_op = self.access_local(self.frame(), place.local, layout)?;
477 .try_fold(base_op, |op, elem| self.operand_projection(&op, elem))?;
479 trace!("eval_place_to_op: got {:?}", *op);
480 // Sanity-check the type we ended up with.
481 debug_assert!(mir_assign_valid_types(
484 self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
485 place.ty(&self.frame().body.local_decls, *self.tcx).ty
492 /// Evaluate the operand, returning a place where you can then find the data.
493 /// If you already know the layout, you can save two table lookups
494 /// by passing it in here.
498 mir_op: &mir::Operand<'tcx>,
499 layout: Option<TyAndLayout<'tcx>>,
500 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
501 use rustc_middle::mir::Operand::*;
502 let op = match *mir_op {
503 // FIXME: do some more logic on `move` to invalidate the old location
504 Copy(place) | Move(place) => self.eval_place_to_op(place, layout)?,
506 Constant(ref constant) => {
508 self.subst_from_current_frame_and_normalize_erasing_regions(constant.literal);
509 // This can still fail:
510 // * During ConstProp, with `TooGeneric` or since the `requried_consts` were not all
512 // * During CTFE, since promoteds in `const`/`static` initializer bodies can fail.
514 self.mir_const_to_op(&val, layout)?
517 trace!("{:?}: {:?}", mir_op, *op);
521 /// Evaluate a bunch of operands at once
522 pub(super) fn eval_operands(
524 ops: &[mir::Operand<'tcx>],
525 ) -> InterpResult<'tcx, Vec<OpTy<'tcx, M::PointerTag>>> {
526 ops.iter().map(|op| self.eval_operand(op, None)).collect()
529 // Used when the miri-engine runs into a constant and for extracting information from constants
530 // in patterns via the `const_eval` module
531 /// The `val` and `layout` are assumed to already be in our interpreter
532 /// "universe" (param_env).
533 crate fn const_to_op(
535 val: &ty::Const<'tcx>,
536 layout: Option<TyAndLayout<'tcx>>,
537 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
539 ty::ConstKind::Param(_) | ty::ConstKind::Bound(..) => throw_inval!(TooGeneric),
540 ty::ConstKind::Error(_) => throw_inval!(AlreadyReported(ErrorReported)),
541 ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }) => {
542 let instance = self.resolve(def, substs)?;
543 Ok(self.eval_to_allocation(GlobalId { instance, promoted })?.into())
545 ty::ConstKind::Infer(..) | ty::ConstKind::Placeholder(..) => {
546 span_bug!(self.cur_span(), "const_to_op: Unexpected ConstKind {:?}", val)
548 ty::ConstKind::Value(val_val) => self.const_val_to_op(val_val, val.ty, layout),
552 crate fn mir_const_to_op(
554 val: &mir::ConstantKind<'tcx>,
555 layout: Option<TyAndLayout<'tcx>>,
556 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
558 mir::ConstantKind::Ty(ct) => self.const_to_op(ct, layout),
559 mir::ConstantKind::Val(val, ty) => self.const_val_to_op(*val, ty, layout),
563 crate fn const_val_to_op(
565 val_val: ConstValue<'tcx>,
567 layout: Option<TyAndLayout<'tcx>>,
568 ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
569 // Other cases need layout.
570 let tag_scalar = |scalar| -> InterpResult<'tcx, _> {
572 Scalar::Ptr(ptr) => Scalar::Ptr(self.global_base_pointer(ptr)?),
573 Scalar::Int(int) => Scalar::Int(int),
576 let layout = from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(ty))?;
577 let op = match val_val {
578 ConstValue::ByRef { alloc, offset } => {
579 let id = self.tcx.create_memory_alloc(alloc);
580 // We rely on mutability being set correctly in that allocation to prevent writes
581 // where none should happen.
582 let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
583 Operand::Indirect(MemPlace::from_ptr(ptr, layout.align.abi))
585 ConstValue::Scalar(x) => Operand::Immediate(tag_scalar(x)?.into()),
586 ConstValue::Slice { data, start, end } => {
587 // We rely on mutability being set correctly in `data` to prevent writes
588 // where none should happen.
589 let ptr = Pointer::new(
590 self.tcx.create_memory_alloc(data),
591 Size::from_bytes(start), // offset: `start`
593 Operand::Immediate(Immediate::new_slice(
594 self.global_base_pointer(ptr)?.into(),
595 u64::try_from(end.checked_sub(start).unwrap()).unwrap(), // len: `end - start`
600 Ok(OpTy { op, layout })
603 /// Read discriminant, return the runtime value as well as the variant index.
604 pub fn read_discriminant(
606 op: &OpTy<'tcx, M::PointerTag>,
607 ) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
608 trace!("read_discriminant_value {:#?}", op.layout);
609 // Get type and layout of the discriminant.
610 let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
611 trace!("discriminant type: {:?}", discr_layout.ty);
613 // We use "discriminant" to refer to the value associated with a particular enum variant.
614 // This is not to be confused with its "variant index", which is just determining its position in the
615 // declared list of variants -- they can differ with explicitly assigned discriminants.
616 // We use "tag" to refer to how the discriminant is encoded in memory, which can be either
617 // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
618 let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
619 Variants::Single { index } => {
620 let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
622 // This type actually has discriminants.
623 assert_eq!(discr.ty, discr_layout.ty);
624 Scalar::from_uint(discr.val, discr_layout.size)
627 // On a type without actual discriminants, variant is 0.
628 assert_eq!(index.as_u32(), 0);
629 Scalar::from_uint(index.as_u32(), discr_layout.size)
632 return Ok((discr, index));
634 Variants::Multiple { ref tag, ref tag_encoding, tag_field, .. } => {
635 (tag, tag_encoding, tag_field)
639 // There are *three* layouts that come into play here:
640 // - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
641 // the `Scalar` we return.
642 // - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
643 // and used to interpret the value we read from the tag field.
644 // For the return value, a cast to `discr_layout` is performed.
645 // - The field storing the tag has a layout, which is very similar to `tag_layout` but
646 // may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
648 // Get layout for tag.
649 let tag_layout = self.layout_of(tag_scalar_layout.value.to_int_ty(*self.tcx))?;
651 // Read tag and sanity-check `tag_layout`.
652 let tag_val = self.read_immediate(&self.operand_field(op, tag_field)?)?;
653 assert_eq!(tag_layout.size, tag_val.layout.size);
654 assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
655 let tag_val = tag_val.to_scalar()?;
656 trace!("tag value: {:?}", tag_val);
658 // Figure out which discriminant and variant this corresponds to.
659 Ok(match *tag_encoding {
660 TagEncoding::Direct => {
662 .force_bits(tag_val, tag_layout.size)
663 .map_err(|_| err_ub!(InvalidTag(tag_val.erase_tag())))?;
664 // Cast bits from tag layout to discriminant layout.
665 let discr_val = self.cast_from_scalar(tag_bits, tag_layout, discr_layout.ty);
666 let discr_bits = discr_val.assert_bits(discr_layout.size);
667 // Convert discriminant to variant index, and catch invalid discriminants.
668 let index = match *op.layout.ty.kind() {
670 adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
672 ty::Generator(def_id, substs, _) => {
673 let substs = substs.as_generator();
675 .discriminants(def_id, *self.tcx)
676 .find(|(_, var)| var.val == discr_bits)
678 _ => span_bug!(self.cur_span(), "tagged layout for non-adt non-generator"),
680 .ok_or_else(|| err_ub!(InvalidTag(tag_val.erase_tag())))?;
681 // Return the cast value, and the index.
684 TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => {
685 // Compute the variant this niche value/"tag" corresponds to. With niche layout,
686 // discriminant (encoded in niche/tag) and variant index are the same.
687 let variants_start = niche_variants.start().as_u32();
688 let variants_end = niche_variants.end().as_u32();
689 let variant = match tag_val.to_bits_or_ptr(tag_layout.size, self) {
691 // The niche must be just 0 (which an inbounds pointer value never is)
692 let ptr_valid = niche_start == 0
693 && variants_start == variants_end
694 && !self.memory.ptr_may_be_null(ptr);
696 throw_ub!(InvalidTag(tag_val.erase_tag()))
701 // We need to use machine arithmetic to get the relative variant idx:
702 // variant_index_relative = tag_val - niche_start_val
703 let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
704 let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
705 let variant_index_relative_val =
706 self.binary_op(mir::BinOp::Sub, &tag_val, &niche_start_val)?;
707 let variant_index_relative = variant_index_relative_val
709 .assert_bits(tag_val.layout.size);
710 // Check if this is in the range that indicates an actual discriminant.
711 if variant_index_relative <= u128::from(variants_end - variants_start) {
712 let variant_index_relative = u32::try_from(variant_index_relative)
713 .expect("we checked that this fits into a u32");
714 // Then computing the absolute variant idx should not overflow any more.
715 let variant_index = variants_start
716 .checked_add(variant_index_relative)
717 .expect("overflow computing absolute variant idx");
718 let variants_len = op
722 .expect("tagged layout for non adt")
725 assert!(usize::try_from(variant_index).unwrap() < variants_len);
726 VariantIdx::from_u32(variant_index)
732 // Compute the size of the scalar we need to return.
733 // No need to cast, because the variant index directly serves as discriminant and is
734 // encoded in the tag.
735 (Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)