1 //! This is the implementation of the pass which transforms generators into state machines.
3 //! MIR generation for generators creates a function which has a self argument which
4 //! passes by value. This argument is effectively a generator type which only contains upvars and
5 //! is only used for this argument inside the MIR for the generator.
6 //! It is passed by value to enable upvars to be moved out of it. Drop elaboration runs on that
7 //! MIR before this pass and creates drop flags for MIR locals.
8 //! It will also drop the generator argument (which only consists of upvars) if any of the upvars
9 //! are moved out of. This pass elaborates the drops of upvars / generator argument in the case
10 //! that none of the upvars were moved out of. This is because we cannot have any drops of this
11 //! generator in the MIR, since it is used to create the drop glue for the generator. We'd get
12 //! infinite recursion otherwise.
14 //! This pass creates the implementation for the Generator::resume function and the drop shim
15 //! for the generator based on the MIR input. It converts the generator argument from Self to
16 //! &mut Self adding derefs in the MIR as needed. It computes the final layout of the generator
17 //! struct which looks like this:
18 //! First upvars are stored
19 //! It is followed by the generator state field.
20 //! Then finally the MIR locals which are live across a suspension point are stored.
22 //! struct Generator {
28 //! This pass computes the meaning of the state field and the MIR locals which are live
29 //! across a suspension point. There are however three hardcoded generator states:
30 //! 0 - Generator have not been resumed yet
31 //! 1 - Generator has returned / is completed
32 //! 2 - Generator has been poisoned
34 //! It also rewrites `return x` and `yield y` as setting a new generator state and returning
35 //! GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
36 //! MIR locals which are live across a suspension point are moved to the generator struct
37 //! with references to them being updated with references to the generator struct.
39 //! The pass creates two functions which have a switch on the generator state giving
40 //! the action to take.
42 //! One of them is the implementation of Generator::resume.
43 //! For generators with state 0 (unresumed) it starts the execution of the generator.
44 //! For generators with state 1 (returned) and state 2 (poisoned) it panics.
45 //! Otherwise it continues the execution from the last suspension point.
47 //! The other function is the drop glue for the generator.
48 //! For generators with state 0 (unresumed) it drops the upvars of the generator.
49 //! For generators with state 1 (returned) and state 2 (poisoned) it does nothing.
50 //! Otherwise it drops all the values in scope at the last suspension point.
52 use crate::dataflow
::impls
::{
53 MaybeBorrowedLocals
, MaybeLiveLocals
, MaybeRequiresStorage
, MaybeStorageLive
,
55 use crate::dataflow
::{self, Analysis}
;
56 use crate::transform
::no_landing_pads
::no_landing_pads
;
57 use crate::transform
::simplify
;
58 use crate::transform
::MirPass
;
59 use crate::util
::dump_mir
;
60 use crate::util
::expand_aggregate
;
61 use crate::util
::storage
;
62 use rustc_data_structures
::fx
::FxHashMap
;
64 use rustc_hir
::lang_items
::LangItem
;
65 use rustc_index
::bit_set
::{BitMatrix, BitSet}
;
66 use rustc_index
::vec
::{Idx, IndexVec}
;
67 use rustc_middle
::mir
::visit
::{MutVisitor, PlaceContext, Visitor}
;
68 use rustc_middle
::mir
::*;
69 use rustc_middle
::ty
::subst
::{Subst, SubstsRef}
;
70 use rustc_middle
::ty
::GeneratorSubsts
;
71 use rustc_middle
::ty
::{self, AdtDef, Ty, TyCtxt}
;
72 use rustc_target
::abi
::VariantIdx
;
73 use rustc_target
::spec
::PanicStrategy
;
76 pub struct StateTransform
;
78 struct RenameLocalVisitor
<'tcx
> {
84 impl<'tcx
> MutVisitor
<'tcx
> for RenameLocalVisitor
<'tcx
> {
85 fn tcx(&self) -> TyCtxt
<'tcx
> {
89 fn visit_local(&mut self, local
: &mut Local
, _
: PlaceContext
, _
: Location
) {
90 if *local
== self.from
{
95 fn visit_terminator(&mut self, terminator
: &mut Terminator
<'tcx
>, location
: Location
) {
96 match terminator
.kind
{
97 TerminatorKind
::Return
=> {
98 // Do not replace the implicit `_0` access here, as that's not possible. The
99 // transform already handles `return` correctly.
101 _
=> self.super_terminator(terminator
, location
),
106 struct DerefArgVisitor
<'tcx
> {
110 impl<'tcx
> MutVisitor
<'tcx
> for DerefArgVisitor
<'tcx
> {
111 fn tcx(&self) -> TyCtxt
<'tcx
> {
115 fn visit_local(&mut self, local
: &mut Local
, _
: PlaceContext
, _
: Location
) {
116 assert_ne
!(*local
, SELF_ARG
);
119 fn visit_place(&mut self, place
: &mut Place
<'tcx
>, context
: PlaceContext
, location
: Location
) {
120 if place
.local
== SELF_ARG
{
125 projection
: self.tcx().intern_place_elems(&[ProjectionElem
::Deref
]),
130 self.visit_local(&mut place
.local
, context
, location
);
132 for elem
in place
.projection
.iter() {
133 if let PlaceElem
::Index(local
) = elem
{
134 assert_ne
!(local
, SELF_ARG
);
141 struct PinArgVisitor
<'tcx
> {
142 ref_gen_ty
: Ty
<'tcx
>,
146 impl<'tcx
> MutVisitor
<'tcx
> for PinArgVisitor
<'tcx
> {
147 fn tcx(&self) -> TyCtxt
<'tcx
> {
151 fn visit_local(&mut self, local
: &mut Local
, _
: PlaceContext
, _
: Location
) {
152 assert_ne
!(*local
, SELF_ARG
);
155 fn visit_place(&mut self, place
: &mut Place
<'tcx
>, context
: PlaceContext
, location
: Location
) {
156 if place
.local
== SELF_ARG
{
161 projection
: self.tcx().intern_place_elems(&[ProjectionElem
::Field(
169 self.visit_local(&mut place
.local
, context
, location
);
171 for elem
in place
.projection
.iter() {
172 if let PlaceElem
::Index(local
) = elem
{
173 assert_ne
!(local
, SELF_ARG
);
180 fn replace_base
<'tcx
>(place
: &mut Place
<'tcx
>, new_base
: Place
<'tcx
>, tcx
: TyCtxt
<'tcx
>) {
181 place
.local
= new_base
.local
;
183 let mut new_projection
= new_base
.projection
.to_vec();
184 new_projection
.append(&mut place
.projection
.to_vec());
186 place
.projection
= tcx
.intern_place_elems(&new_projection
);
189 const SELF_ARG
: Local
= Local
::from_u32(1);
191 /// Generator has not been resumed yet.
192 const UNRESUMED
: usize = GeneratorSubsts
::UNRESUMED
;
193 /// Generator has returned / is completed.
194 const RETURNED
: usize = GeneratorSubsts
::RETURNED
;
195 /// Generator has panicked and is poisoned.
196 const POISONED
: usize = GeneratorSubsts
::POISONED
;
198 /// A `yield` point in the generator.
199 struct SuspensionPoint
<'tcx
> {
200 /// State discriminant used when suspending or resuming at this point.
202 /// The block to jump to after resumption.
204 /// Where to move the resume argument after resumption.
205 resume_arg
: Place
<'tcx
>,
206 /// Which block to jump to if the generator is dropped in this state.
207 drop
: Option
<BasicBlock
>,
208 /// Set of locals that have live storage while at this suspension point.
209 storage_liveness
: BitSet
<Local
>,
212 struct TransformVisitor
<'tcx
> {
214 state_adt_ref
: &'tcx AdtDef
,
215 state_substs
: SubstsRef
<'tcx
>,
217 // The type of the discriminant in the generator struct
220 // Mapping from Local to (type of local, generator struct index)
221 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
222 remap
: FxHashMap
<Local
, (Ty
<'tcx
>, VariantIdx
, usize)>,
224 // A map from a suspension point in a block to the locals which have live storage at that point
225 storage_liveness
: IndexVec
<BasicBlock
, Option
<BitSet
<Local
>>>,
227 // A list of suspension points, generated during the transform
228 suspension_points
: Vec
<SuspensionPoint
<'tcx
>>,
230 // The set of locals that have no `StorageLive`/`StorageDead` annotations.
231 always_live_locals
: storage
::AlwaysLiveLocals
,
233 // The original RETURN_PLACE local
234 new_ret_local
: Local
,
237 impl TransformVisitor
<'tcx
> {
238 // Make a GeneratorState variant assignment. `core::ops::GeneratorState` only has single
239 // element tuple variants, so we can just write to the downcasted first field and then set the
240 // discriminant to the appropriate variant.
245 source_info
: SourceInfo
,
246 ) -> impl Iterator
<Item
= Statement
<'tcx
>> {
247 let kind
= AggregateKind
::Adt(self.state_adt_ref
, idx
, self.state_substs
, None
, None
);
248 assert_eq
!(self.state_adt_ref
.variants
[idx
].fields
.len(), 1);
251 .type_of(self.state_adt_ref
.variants
[idx
].fields
[0].did
)
252 .subst(self.tcx
, self.state_substs
);
254 Place
::return_place(),
255 std
::iter
::once((val
, ty
)),
262 // Create a Place referencing a generator struct field
263 fn make_field(&self, variant_index
: VariantIdx
, idx
: usize, ty
: Ty
<'tcx
>) -> Place
<'tcx
> {
264 let self_place
= Place
::from(SELF_ARG
);
265 let base
= self.tcx
.mk_place_downcast_unnamed(self_place
, variant_index
);
266 let mut projection
= base
.projection
.to_vec();
267 projection
.push(ProjectionElem
::Field(Field
::new(idx
), ty
));
269 Place { local: base.local, projection: self.tcx.intern_place_elems(&projection) }
272 // Create a statement which changes the discriminant
273 fn set_discr(&self, state_disc
: VariantIdx
, source_info
: SourceInfo
) -> Statement
<'tcx
> {
274 let self_place
= Place
::from(SELF_ARG
);
277 kind
: StatementKind
::SetDiscriminant
{
278 place
: box self_place
,
279 variant_index
: state_disc
,
284 // Create a statement which reads the discriminant into a temporary
285 fn get_discr(&self, body
: &mut Body
<'tcx
>) -> (Statement
<'tcx
>, Place
<'tcx
>) {
286 let temp_decl
= LocalDecl
::new(self.discr_ty
, body
.span
).internal();
287 let local_decls_len
= body
.local_decls
.push(temp_decl
);
288 let temp
= Place
::from(local_decls_len
);
290 let self_place
= Place
::from(SELF_ARG
);
291 let assign
= Statement
{
292 source_info
: SourceInfo
::outermost(body
.span
),
293 kind
: StatementKind
::Assign(box (temp
, Rvalue
::Discriminant(self_place
))),
299 impl MutVisitor
<'tcx
> for TransformVisitor
<'tcx
> {
300 fn tcx(&self) -> TyCtxt
<'tcx
> {
304 fn visit_local(&mut self, local
: &mut Local
, _
: PlaceContext
, _
: Location
) {
305 assert_eq
!(self.remap
.get(local
), None
);
310 place
: &mut Place
<'tcx
>,
311 _context
: PlaceContext
,
314 // Replace an Local in the remap with a generator struct access
315 if let Some(&(ty
, variant_index
, idx
)) = self.remap
.get(&place
.local
) {
316 replace_base(place
, self.make_field(variant_index
, idx
, ty
), self.tcx
);
320 fn visit_basic_block_data(&mut self, block
: BasicBlock
, data
: &mut BasicBlockData
<'tcx
>) {
321 // Remove StorageLive and StorageDead statements for remapped locals
322 data
.retain_statements(|s
| match s
.kind
{
323 StatementKind
::StorageLive(l
) | StatementKind
::StorageDead(l
) => {
324 !self.remap
.contains_key(&l
)
329 let ret_val
= match data
.terminator().kind
{
330 TerminatorKind
::Return
=> Some((
333 Operand
::Move(Place
::from(self.new_ret_local
)),
336 TerminatorKind
::Yield { ref value, resume, resume_arg, drop }
=> {
337 Some((VariantIdx
::new(0), Some((resume
, resume_arg
)), value
.clone(), drop
))
342 if let Some((state_idx
, resume
, v
, drop
)) = ret_val
{
343 let source_info
= data
.terminator().source_info
;
344 // We must assign the value first in case it gets declared dead below
345 data
.statements
.extend(self.make_state(state_idx
, v
, source_info
));
346 let state
= if let Some((resume
, resume_arg
)) = resume
{
348 let state
= 3 + self.suspension_points
.len();
350 // The resume arg target location might itself be remapped if its base local is
351 // live across a yield.
353 if let Some(&(ty
, variant
, idx
)) = self.remap
.get(&resume_arg
.local
) {
354 self.make_field(variant
, idx
, ty
)
359 self.suspension_points
.push(SuspensionPoint
{
364 storage_liveness
: self.storage_liveness
[block
].clone().unwrap(),
367 VariantIdx
::new(state
)
370 VariantIdx
::new(RETURNED
) // state for returned
372 data
.statements
.push(self.set_discr(state
, source_info
));
373 data
.terminator_mut().kind
= TerminatorKind
::Return
;
376 self.super_basic_block_data(block
, data
);
380 fn make_generator_state_argument_indirect
<'tcx
>(tcx
: TyCtxt
<'tcx
>, body
: &mut Body
<'tcx
>) {
381 let gen_ty
= body
.local_decls
.raw
[1].ty
;
384 tcx
.mk_ref(tcx
.lifetimes
.re_erased
, ty
::TypeAndMut { ty: gen_ty, mutbl: Mutability::Mut }
);
386 // Replace the by value generator argument
387 body
.local_decls
.raw
[1].ty
= ref_gen_ty
;
389 // Add a deref to accesses of the generator state
390 DerefArgVisitor { tcx }
.visit_body(body
);
393 fn make_generator_state_argument_pinned
<'tcx
>(tcx
: TyCtxt
<'tcx
>, body
: &mut Body
<'tcx
>) {
394 let ref_gen_ty
= body
.local_decls
.raw
[1].ty
;
396 let pin_did
= tcx
.require_lang_item(LangItem
::Pin
, Some(body
.span
));
397 let pin_adt_ref
= tcx
.adt_def(pin_did
);
398 let substs
= tcx
.intern_substs(&[ref_gen_ty
.into()]);
399 let pin_ref_gen_ty
= tcx
.mk_adt(pin_adt_ref
, substs
);
401 // Replace the by ref generator argument
402 body
.local_decls
.raw
[1].ty
= pin_ref_gen_ty
;
404 // Add the Pin field access to accesses of the generator state
405 PinArgVisitor { ref_gen_ty, tcx }
.visit_body(body
);
408 /// Allocates a new local and replaces all references of `local` with it. Returns the new local.
410 /// `local` will be changed to a new local decl with type `ty`.
412 /// Note that the new local will be uninitialized. It is the caller's responsibility to assign some
413 /// valid value to it before its first use.
414 fn replace_local
<'tcx
>(
417 body
: &mut Body
<'tcx
>,
420 let new_decl
= LocalDecl
::new(ty
, body
.span
);
421 let new_local
= body
.local_decls
.push(new_decl
);
422 body
.local_decls
.swap(local
, new_local
);
424 RenameLocalVisitor { from: local, to: new_local, tcx }
.visit_body(body
);
429 struct LivenessInfo
{
430 /// Which locals are live across any suspension point.
431 saved_locals
: GeneratorSavedLocals
,
433 /// The set of saved locals live at each suspension point.
434 live_locals_at_suspension_points
: Vec
<BitSet
<GeneratorSavedLocal
>>,
436 /// Parallel vec to the above with SourceInfo for each yield terminator.
437 source_info_at_suspension_points
: Vec
<SourceInfo
>,
439 /// For every saved local, the set of other saved locals that are
440 /// storage-live at the same time as this local. We cannot overlap locals in
441 /// the layout which have conflicting storage.
442 storage_conflicts
: BitMatrix
<GeneratorSavedLocal
, GeneratorSavedLocal
>,
444 /// For every suspending block, the locals which are storage-live across
445 /// that suspension point.
446 storage_liveness
: IndexVec
<BasicBlock
, Option
<BitSet
<Local
>>>,
449 fn locals_live_across_suspend_points(
452 always_live_locals
: &storage
::AlwaysLiveLocals
,
455 let body_ref
: &Body
<'_
> = &body
;
457 // Calculate when MIR locals have live storage. This gives us an upper bound of their
459 let mut storage_live
= MaybeStorageLive
::new(always_live_locals
.clone())
460 .into_engine(tcx
, body_ref
)
461 .iterate_to_fixpoint()
462 .into_results_cursor(body_ref
);
464 // Calculate the MIR locals which have been previously
465 // borrowed (even if they are still active).
466 let borrowed_locals_results
= MaybeBorrowedLocals
::all_borrows()
467 .into_engine(tcx
, body_ref
)
468 .pass_name("generator")
469 .iterate_to_fixpoint();
471 let mut borrowed_locals_cursor
=
472 dataflow
::ResultsCursor
::new(body_ref
, &borrowed_locals_results
);
474 // Calculate the MIR locals that we actually need to keep storage around
476 let requires_storage_results
= MaybeRequiresStorage
::new(body
, &borrowed_locals_results
)
477 .into_engine(tcx
, body_ref
)
478 .iterate_to_fixpoint();
479 let mut requires_storage_cursor
=
480 dataflow
::ResultsCursor
::new(body_ref
, &requires_storage_results
);
482 // Calculate the liveness of MIR locals ignoring borrows.
483 let mut liveness
= MaybeLiveLocals
484 .into_engine(tcx
, body_ref
)
485 .pass_name("generator")
486 .iterate_to_fixpoint()
487 .into_results_cursor(body_ref
);
489 let mut storage_liveness_map
= IndexVec
::from_elem(None
, body
.basic_blocks());
490 let mut live_locals_at_suspension_points
= Vec
::new();
491 let mut source_info_at_suspension_points
= Vec
::new();
492 let mut live_locals_at_any_suspension_point
= BitSet
::new_empty(body
.local_decls
.len());
494 for (block
, data
) in body
.basic_blocks().iter_enumerated() {
495 if let TerminatorKind
::Yield { .. }
= data
.terminator().kind
{
496 let loc
= Location { block, statement_index: data.statements.len() }
;
498 liveness
.seek_to_block_end(block
);
499 let mut live_locals
= liveness
.get().clone();
502 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
503 // This is correct for movable generators since borrows cannot live across
504 // suspension points. However for immovable generators we need to account for
505 // borrows, so we conseratively assume that all borrowed locals are live until
506 // we find a StorageDead statement referencing the locals.
507 // To do this we just union our `liveness` result with `borrowed_locals`, which
508 // contains all the locals which has been borrowed before this suspension point.
509 // If a borrow is converted to a raw reference, we must also assume that it lives
510 // forever. Note that the final liveness is still bounded by the storage liveness
511 // of the local, which happens using the `intersect` operation below.
512 borrowed_locals_cursor
.seek_before_primary_effect(loc
);
513 live_locals
.union(borrowed_locals_cursor
.get());
516 // Store the storage liveness for later use so we can restore the state
517 // after a suspension point
518 storage_live
.seek_before_primary_effect(loc
);
519 storage_liveness_map
[block
] = Some(storage_live
.get().clone());
521 // Locals live are live at this point only if they are used across
522 // suspension points (the `liveness` variable)
523 // and their storage is required (the `storage_required` variable)
524 requires_storage_cursor
.seek_before_primary_effect(loc
);
525 live_locals
.intersect(requires_storage_cursor
.get());
527 // The generator argument is ignored.
528 live_locals
.remove(SELF_ARG
);
530 debug
!("loc = {:?}, live_locals = {:?}", loc
, live_locals
);
532 // Add the locals live at this suspension point to the set of locals which live across
533 // any suspension points
534 live_locals_at_any_suspension_point
.union(&live_locals
);
536 live_locals_at_suspension_points
.push(live_locals
);
537 source_info_at_suspension_points
.push(data
.terminator().source_info
);
541 debug
!("live_locals_anywhere = {:?}", live_locals_at_any_suspension_point
);
542 let saved_locals
= GeneratorSavedLocals(live_locals_at_any_suspension_point
);
544 // Renumber our liveness_map bitsets to include only the locals we are
546 let live_locals_at_suspension_points
= live_locals_at_suspension_points
548 .map(|live_here
| saved_locals
.renumber_bitset(&live_here
))
551 let storage_conflicts
= compute_storage_conflicts(
554 always_live_locals
.clone(),
555 requires_storage_results
,
560 live_locals_at_suspension_points
,
561 source_info_at_suspension_points
,
563 storage_liveness
: storage_liveness_map
,
567 /// The set of `Local`s that must be saved across yield points.
569 /// `GeneratorSavedLocal` is indexed in terms of the elements in this set;
570 /// i.e. `GeneratorSavedLocal::new(1)` corresponds to the second local
571 /// included in this set.
572 struct GeneratorSavedLocals(BitSet
<Local
>);
574 impl GeneratorSavedLocals
{
575 /// Returns an iterator over each `GeneratorSavedLocal` along with the `Local` it corresponds
577 fn iter_enumerated(&self) -> impl '_
+ Iterator
<Item
= (GeneratorSavedLocal
, Local
)> {
578 self.iter().enumerate().map(|(i
, l
)| (GeneratorSavedLocal
::from(i
), l
))
581 /// Transforms a `BitSet<Local>` that contains only locals saved across yield points to the
582 /// equivalent `BitSet<GeneratorSavedLocal>`.
583 fn renumber_bitset(&self, input
: &BitSet
<Local
>) -> BitSet
<GeneratorSavedLocal
> {
584 assert
!(self.superset(&input
), "{:?} not a superset of {:?}", self.0, input
);
585 let mut out
= BitSet
::new_empty(self.count());
586 for (saved_local
, local
) in self.iter_enumerated() {
587 if input
.contains(local
) {
588 out
.insert(saved_local
);
594 fn get(&self, local
: Local
) -> Option
<GeneratorSavedLocal
> {
595 if !self.contains(local
) {
599 let idx
= self.iter().take_while(|&l
| l
< local
).count();
600 Some(GeneratorSavedLocal
::new(idx
))
604 impl ops
::Deref
for GeneratorSavedLocals
{
605 type Target
= BitSet
<Local
>;
607 fn deref(&self) -> &Self::Target
{
612 /// For every saved local, looks for which locals are StorageLive at the same
613 /// time. Generates a bitset for every local of all the other locals that may be
614 /// StorageLive simultaneously with that local. This is used in the layout
615 /// computation; see `GeneratorLayout` for more.
616 fn compute_storage_conflicts(
617 body
: &'mir Body
<'tcx
>,
618 saved_locals
: &GeneratorSavedLocals
,
619 always_live_locals
: storage
::AlwaysLiveLocals
,
620 requires_storage
: dataflow
::Results
<'tcx
, MaybeRequiresStorage
<'mir
, 'tcx
>>,
621 ) -> BitMatrix
<GeneratorSavedLocal
, GeneratorSavedLocal
> {
622 assert_eq
!(body
.local_decls
.len(), saved_locals
.domain_size());
624 debug
!("compute_storage_conflicts({:?})", body
.span
);
625 debug
!("always_live = {:?}", always_live_locals
);
627 // Locals that are always live or ones that need to be stored across
628 // suspension points are not eligible for overlap.
629 let mut ineligible_locals
= always_live_locals
.into_inner();
630 ineligible_locals
.intersect(saved_locals
);
632 // Compute the storage conflicts for all eligible locals.
633 let mut visitor
= StorageConflictVisitor
{
635 saved_locals
: &saved_locals
,
636 local_conflicts
: BitMatrix
::from_row_n(&ineligible_locals
, body
.local_decls
.len()),
639 requires_storage
.visit_reachable_with(body
, &mut visitor
);
641 let local_conflicts
= visitor
.local_conflicts
;
643 // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal).
645 // NOTE: Today we store a full conflict bitset for every local. Technically
646 // this is twice as many bits as we need, since the relation is symmetric.
647 // However, in practice these bitsets are not usually large. The layout code
648 // also needs to keep track of how many conflicts each local has, so it's
649 // simpler to keep it this way for now.
650 let mut storage_conflicts
= BitMatrix
::new(saved_locals
.count(), saved_locals
.count());
651 for (saved_local_a
, local_a
) in saved_locals
.iter_enumerated() {
652 if ineligible_locals
.contains(local_a
) {
653 // Conflicts with everything.
654 storage_conflicts
.insert_all_into_row(saved_local_a
);
656 // Keep overlap information only for stored locals.
657 for (saved_local_b
, local_b
) in saved_locals
.iter_enumerated() {
658 if local_conflicts
.contains(local_a
, local_b
) {
659 storage_conflicts
.insert(saved_local_a
, saved_local_b
);
667 struct StorageConflictVisitor
<'mir
, 'tcx
, 's
> {
668 body
: &'mir Body
<'tcx
>,
669 saved_locals
: &'s GeneratorSavedLocals
,
670 // FIXME(tmandry): Consider using sparse bitsets here once we have good
671 // benchmarks for generators.
672 local_conflicts
: BitMatrix
<Local
, Local
>,
675 impl dataflow
::ResultsVisitor
<'mir
, 'tcx
> for StorageConflictVisitor
<'mir
, 'tcx
, '_
> {
676 type FlowState
= BitSet
<Local
>;
678 fn visit_statement_before_primary_effect(
680 state
: &Self::FlowState
,
681 _statement
: &'mir Statement
<'tcx
>,
684 self.apply_state(state
, loc
);
687 fn visit_terminator_before_primary_effect(
689 state
: &Self::FlowState
,
690 _terminator
: &'mir Terminator
<'tcx
>,
693 self.apply_state(state
, loc
);
697 impl<'body
, 'tcx
, 's
> StorageConflictVisitor
<'body
, 'tcx
, 's
> {
698 fn apply_state(&mut self, flow_state
: &BitSet
<Local
>, loc
: Location
) {
699 // Ignore unreachable blocks.
700 if self.body
.basic_blocks()[loc
.block
].terminator().kind
== TerminatorKind
::Unreachable
{
704 let mut eligible_storage_live
= flow_state
.clone();
705 eligible_storage_live
.intersect(&self.saved_locals
);
707 for local
in eligible_storage_live
.iter() {
708 self.local_conflicts
.union_row_with(&eligible_storage_live
, local
);
711 if eligible_storage_live
.count() > 1 {
712 trace
!("at {:?}, eligible_storage_live={:?}", loc
, eligible_storage_live
);
717 /// Validates the typeck view of the generator against the actual set of types saved between
719 fn sanitize_witness
<'tcx
>(
723 upvars
: &Vec
<Ty
<'tcx
>>,
724 saved_locals
: &GeneratorSavedLocals
,
726 let did
= body
.source
.def_id();
727 let allowed_upvars
= tcx
.erase_regions(upvars
);
728 let allowed
= match witness
.kind() {
729 ty
::GeneratorWitness(s
) => tcx
.erase_late_bound_regions(&s
),
731 tcx
.sess
.delay_span_bug(
733 &format
!("unexpected generator witness type {:?}", witness
.kind()),
739 let param_env
= tcx
.param_env(did
);
741 for (local
, decl
) in body
.local_decls
.iter_enumerated() {
742 // Ignore locals which are internal or not saved between yields.
743 if !saved_locals
.contains(local
) || decl
.internal
{
746 let decl_ty
= tcx
.normalize_erasing_regions(param_env
, decl
.ty
);
748 // Sanity check that typeck knows about the type of locals which are
749 // live across a suspension point
750 if !allowed
.contains(&decl_ty
) && !allowed_upvars
.contains(&decl_ty
) {
753 "Broken MIR: generator contains type {} in MIR, \
754 but typeck only knows about {}",
762 fn compute_layout
<'tcx
>(
763 liveness
: LivenessInfo
,
764 body
: &mut Body
<'tcx
>,
766 FxHashMap
<Local
, (Ty
<'tcx
>, VariantIdx
, usize)>,
767 GeneratorLayout
<'tcx
>,
768 IndexVec
<BasicBlock
, Option
<BitSet
<Local
>>>,
772 live_locals_at_suspension_points
,
773 source_info_at_suspension_points
,
778 // Gather live local types and their indices.
779 let mut locals
= IndexVec
::<GeneratorSavedLocal
, _
>::new();
780 let mut tys
= IndexVec
::<GeneratorSavedLocal
, _
>::new();
781 for (saved_local
, local
) in saved_locals
.iter_enumerated() {
783 tys
.push(body
.local_decls
[local
].ty
);
784 debug
!("generator saved local {:?} => {:?}", saved_local
, local
);
787 // Leave empty variants for the UNRESUMED, RETURNED, and POISONED states.
788 // In debuginfo, these will correspond to the beginning (UNRESUMED) or end
789 // (RETURNED, POISONED) of the function.
790 const RESERVED_VARIANTS
: usize = 3;
791 let body_span
= body
.source_scopes
[OUTERMOST_SOURCE_SCOPE
].span
;
792 let mut variant_source_info
: IndexVec
<VariantIdx
, SourceInfo
> = [
793 SourceInfo
::outermost(body_span
.shrink_to_lo()),
794 SourceInfo
::outermost(body_span
.shrink_to_hi()),
795 SourceInfo
::outermost(body_span
.shrink_to_hi()),
801 // Build the generator variant field list.
802 // Create a map from local indices to generator struct indices.
803 let mut variant_fields
: IndexVec
<VariantIdx
, IndexVec
<Field
, GeneratorSavedLocal
>> =
804 iter
::repeat(IndexVec
::new()).take(RESERVED_VARIANTS
).collect();
805 let mut remap
= FxHashMap
::default();
806 for (suspension_point_idx
, live_locals
) in live_locals_at_suspension_points
.iter().enumerate() {
807 let variant_index
= VariantIdx
::from(RESERVED_VARIANTS
+ suspension_point_idx
);
808 let mut fields
= IndexVec
::new();
809 for (idx
, saved_local
) in live_locals
.iter().enumerate() {
810 fields
.push(saved_local
);
811 // Note that if a field is included in multiple variants, we will
812 // just use the first one here. That's fine; fields do not move
813 // around inside generators, so it doesn't matter which variant
814 // index we access them by.
815 remap
.entry(locals
[saved_local
]).or_insert((tys
[saved_local
], variant_index
, idx
));
817 variant_fields
.push(fields
);
818 variant_source_info
.push(source_info_at_suspension_points
[suspension_point_idx
]);
820 debug
!("generator variant_fields = {:?}", variant_fields
);
821 debug
!("generator storage_conflicts = {:#?}", storage_conflicts
);
824 GeneratorLayout { field_tys: tys, variant_fields, variant_source_info, storage_conflicts }
;
826 (remap
, layout
, storage_liveness
)
829 /// Replaces the entry point of `body` with a block that switches on the generator discriminant and
830 /// dispatches to blocks according to `cases`.
832 /// After this function, the former entry point of the function will be bb1.
833 fn insert_switch
<'tcx
>(
834 body
: &mut Body
<'tcx
>,
835 cases
: Vec
<(usize, BasicBlock
)>,
836 transform
: &TransformVisitor
<'tcx
>,
837 default: TerminatorKind
<'tcx
>,
839 let default_block
= insert_term_block(body
, default);
840 let (assign
, discr
) = transform
.get_discr(body
);
842 SwitchTargets
::new(cases
.iter().map(|(i
, bb
)| ((*i
) as u128
, *bb
)), default_block
);
843 let switch
= TerminatorKind
::SwitchInt
{
844 discr
: Operand
::Move(discr
),
845 switch_ty
: transform
.discr_ty
,
846 targets
: switch_targets
,
849 let source_info
= SourceInfo
::outermost(body
.span
);
850 body
.basic_blocks_mut().raw
.insert(
853 statements
: vec
![assign
],
854 terminator
: Some(Terminator { source_info, kind: switch }
),
859 let blocks
= body
.basic_blocks_mut().iter_mut();
861 for target
in blocks
.flat_map(|b
| b
.terminator_mut().successors_mut()) {
862 *target
= BasicBlock
::new(target
.index() + 1);
866 fn elaborate_generator_drops
<'tcx
>(tcx
: TyCtxt
<'tcx
>, body
: &mut Body
<'tcx
>) {
867 use crate::shim
::DropShimElaborator
;
868 use crate::util
::elaborate_drops
::{elaborate_drop, Unwind}
;
869 use crate::util
::patch
::MirPatch
;
871 // Note that `elaborate_drops` only drops the upvars of a generator, and
872 // this is ok because `open_drop` can only be reached within that own
873 // generator's resume function.
875 let def_id
= body
.source
.def_id();
876 let param_env
= tcx
.param_env(def_id
);
878 let mut elaborator
= DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env }
;
880 for (block
, block_data
) in body
.basic_blocks().iter_enumerated() {
881 let (target
, unwind
, source_info
) = match block_data
.terminator() {
882 Terminator { source_info, kind: TerminatorKind::Drop { place, target, unwind }
} => {
883 if let Some(local
) = place
.as_local() {
884 if local
== SELF_ARG
{
885 (target
, unwind
, source_info
)
895 let unwind
= if block_data
.is_cleanup
{
898 Unwind
::To(unwind
.unwrap_or_else(|| elaborator
.patch
.resume_block()))
903 Place
::from(SELF_ARG
),
910 elaborator
.patch
.apply(body
);
913 fn create_generator_drop_shim
<'tcx
>(
915 transform
: &TransformVisitor
<'tcx
>,
917 body
: &mut Body
<'tcx
>,
918 drop_clean
: BasicBlock
,
920 let mut body
= body
.clone();
921 body
.arg_count
= 1; // make sure the resume argument is not included here
923 let source_info
= SourceInfo
::outermost(body
.span
);
925 let mut cases
= create_cases(&mut body
, transform
, Operation
::Drop
);
927 cases
.insert(0, (UNRESUMED
, drop_clean
));
929 // The returned state and the poisoned state fall through to the default
930 // case which is just to return
932 insert_switch(&mut body
, cases
, &transform
, TerminatorKind
::Return
);
934 for block
in body
.basic_blocks_mut() {
935 let kind
= &mut block
.terminator_mut().kind
;
936 if let TerminatorKind
::GeneratorDrop
= *kind
{
937 *kind
= TerminatorKind
::Return
;
941 // Replace the return variable
942 body
.local_decls
[RETURN_PLACE
] = LocalDecl
::with_source_info(tcx
.mk_unit(), source_info
);
944 make_generator_state_argument_indirect(tcx
, &mut body
);
946 // Change the generator argument from &mut to *mut
947 body
.local_decls
[SELF_ARG
] = LocalDecl
::with_source_info(
948 tcx
.mk_ptr(ty
::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }
),
951 if tcx
.sess
.opts
.debugging_opts
.mir_emit_retag
{
952 // Alias tracking must know we changed the type
953 body
.basic_blocks_mut()[START_BLOCK
].statements
.insert(
957 kind
: StatementKind
::Retag(RetagKind
::Raw
, box Place
::from(SELF_ARG
)),
962 no_landing_pads(tcx
, &mut body
);
964 // Make sure we remove dead blocks to remove
965 // unrelated code from the resume part of the function
966 simplify
::remove_dead_blocks(&mut body
);
968 dump_mir(tcx
, None
, "generator_drop", &0, &body
, |_
, _
| Ok(()));
973 fn insert_term_block
<'tcx
>(body
: &mut Body
<'tcx
>, kind
: TerminatorKind
<'tcx
>) -> BasicBlock
{
974 let source_info
= SourceInfo
::outermost(body
.span
);
975 body
.basic_blocks_mut().push(BasicBlockData
{
976 statements
: Vec
::new(),
977 terminator
: Some(Terminator { source_info, kind }
),
982 fn insert_panic_block
<'tcx
>(
984 body
: &mut Body
<'tcx
>,
985 message
: AssertMessage
<'tcx
>,
987 let assert_block
= BasicBlock
::new(body
.basic_blocks().len());
988 let term
= TerminatorKind
::Assert
{
989 cond
: Operand
::Constant(box Constant
{
992 literal
: ty
::Const
::from_bool(tcx
, false),
996 target
: assert_block
,
1000 let source_info
= SourceInfo
::outermost(body
.span
);
1001 body
.basic_blocks_mut().push(BasicBlockData
{
1002 statements
: Vec
::new(),
1003 terminator
: Some(Terminator { source_info, kind: term }
),
1010 fn can_return
<'tcx
>(tcx
: TyCtxt
<'tcx
>, body
: &Body
<'tcx
>) -> bool
{
1011 // Returning from a function with an uninhabited return type is undefined behavior.
1012 if body
.return_ty().conservative_is_privately_uninhabited(tcx
) {
1016 // If there's a return terminator the function may return.
1017 for block
in body
.basic_blocks() {
1018 if let TerminatorKind
::Return
= block
.terminator().kind
{
1023 // Otherwise the function can't return.
1027 fn can_unwind
<'tcx
>(tcx
: TyCtxt
<'tcx
>, body
: &Body
<'tcx
>) -> bool
{
1028 // Nothing can unwind when landing pads are off.
1029 if tcx
.sess
.panic_strategy() == PanicStrategy
::Abort
{
1033 // Unwinds can only start at certain terminators.
1034 for block
in body
.basic_blocks() {
1035 match block
.terminator().kind
{
1036 // These never unwind.
1037 TerminatorKind
::Goto { .. }
1038 | TerminatorKind
::SwitchInt { .. }
1039 | TerminatorKind
::Abort
1040 | TerminatorKind
::Return
1041 | TerminatorKind
::Unreachable
1042 | TerminatorKind
::GeneratorDrop
1043 | TerminatorKind
::FalseEdge { .. }
1044 | TerminatorKind
::FalseUnwind { .. }
1045 | TerminatorKind
::InlineAsm { .. }
=> {}
1047 // Resume will *continue* unwinding, but if there's no other unwinding terminator it
1048 // will never be reached.
1049 TerminatorKind
::Resume
=> {}
1051 TerminatorKind
::Yield { .. }
=> {
1052 unreachable
!("`can_unwind` called before generator transform")
1055 // These may unwind.
1056 TerminatorKind
::Drop { .. }
1057 | TerminatorKind
::DropAndReplace { .. }
1058 | TerminatorKind
::Call { .. }
1059 | TerminatorKind
::Assert { .. }
=> return true,
1063 // If we didn't find an unwinding terminator, the function cannot unwind.
1067 fn create_generator_resume_function
<'tcx
>(
1069 transform
: TransformVisitor
<'tcx
>,
1070 body
: &mut Body
<'tcx
>,
1073 let can_unwind
= can_unwind(tcx
, body
);
1075 // Poison the generator when it unwinds
1077 let source_info
= SourceInfo
::outermost(body
.span
);
1078 let poison_block
= body
.basic_blocks_mut().push(BasicBlockData
{
1079 statements
: vec
![transform
.set_discr(VariantIdx
::new(POISONED
), source_info
)],
1080 terminator
: Some(Terminator { source_info, kind: TerminatorKind::Resume }
),
1084 for (idx
, block
) in body
.basic_blocks_mut().iter_enumerated_mut() {
1085 let source_info
= block
.terminator().source_info
;
1087 if let TerminatorKind
::Resume
= block
.terminator().kind
{
1088 // An existing `Resume` terminator is redirected to jump to our dedicated
1089 // "poisoning block" above.
1090 if idx
!= poison_block
{
1091 *block
.terminator_mut() = Terminator
{
1093 kind
: TerminatorKind
::Goto { target: poison_block }
,
1096 } else if !block
.is_cleanup
{
1097 // Any terminators that *can* unwind but don't have an unwind target set are also
1098 // pointed at our poisoning block (unless they're part of the cleanup path).
1099 if let Some(unwind @ None
) = block
.terminator_mut().unwind_mut() {
1100 *unwind
= Some(poison_block
);
1106 let mut cases
= create_cases(body
, &transform
, Operation
::Resume
);
1108 use rustc_middle
::mir
::AssertKind
::{ResumedAfterPanic, ResumedAfterReturn}
;
1110 // Jump to the entry point on the unresumed
1111 cases
.insert(0, (UNRESUMED
, BasicBlock
::new(0)));
1113 // Panic when resumed on the returned or poisoned state
1114 let generator_kind
= body
.generator_kind
.unwrap();
1119 (POISONED
, insert_panic_block(tcx
, body
, ResumedAfterPanic(generator_kind
))),
1126 (RETURNED
, insert_panic_block(tcx
, body
, ResumedAfterReturn(generator_kind
))),
1130 insert_switch(body
, cases
, &transform
, TerminatorKind
::Unreachable
);
1132 make_generator_state_argument_indirect(tcx
, body
);
1133 make_generator_state_argument_pinned(tcx
, body
);
1135 no_landing_pads(tcx
, body
);
1137 // Make sure we remove dead blocks to remove
1138 // unrelated code from the drop part of the function
1139 simplify
::remove_dead_blocks(body
);
1141 dump_mir(tcx
, None
, "generator_resume", &0, body
, |_
, _
| Ok(()));
1144 fn insert_clean_drop(body
: &mut Body
<'_
>) -> BasicBlock
{
1145 let return_block
= insert_term_block(body
, TerminatorKind
::Return
);
1148 TerminatorKind
::Drop { place: Place::from(SELF_ARG), target: return_block, unwind: None }
;
1149 let source_info
= SourceInfo
::outermost(body
.span
);
1151 // Create a block to destroy an unresumed generators. This can only destroy upvars.
1152 body
.basic_blocks_mut().push(BasicBlockData
{
1153 statements
: Vec
::new(),
1154 terminator
: Some(Terminator { source_info, kind: term }
),
1159 /// An operation that can be performed on a generator.
1160 #[derive(PartialEq, Copy, Clone)]
1167 fn target_block(self, point
: &SuspensionPoint
<'_
>) -> Option
<BasicBlock
> {
1169 Operation
::Resume
=> Some(point
.resume
),
1170 Operation
::Drop
=> point
.drop
,
1175 fn create_cases
<'tcx
>(
1176 body
: &mut Body
<'tcx
>,
1177 transform
: &TransformVisitor
<'tcx
>,
1178 operation
: Operation
,
1179 ) -> Vec
<(usize, BasicBlock
)> {
1180 let source_info
= SourceInfo
::outermost(body
.span
);
1185 .filter_map(|point
| {
1186 // Find the target for this suspension point, if applicable
1187 operation
.target_block(point
).map(|target
| {
1188 let mut statements
= Vec
::new();
1190 // Create StorageLive instructions for locals with live storage
1191 for i
in 0..(body
.local_decls
.len()) {
1193 // The resume argument is live on function entry. Don't insert a
1194 // `StorageLive`, or the following `Assign` will read from uninitialized
1199 let l
= Local
::new(i
);
1200 let needs_storage_live
= point
.storage_liveness
.contains(l
)
1201 && !transform
.remap
.contains_key(&l
)
1202 && !transform
.always_live_locals
.contains(l
);
1203 if needs_storage_live
{
1205 .push(Statement { source_info, kind: StatementKind::StorageLive(l) }
);
1209 if operation
== Operation
::Resume
{
1210 // Move the resume argument to the destination place of the `Yield` terminator
1211 let resume_arg
= Local
::new(2); // 0 = return, 1 = self
1212 statements
.push(Statement
{
1214 kind
: StatementKind
::Assign(box (
1216 Rvalue
::Use(Operand
::Move(resume_arg
.into())),
1221 // Then jump to the real target
1222 let block
= body
.basic_blocks_mut().push(BasicBlockData
{
1224 terminator
: Some(Terminator
{
1226 kind
: TerminatorKind
::Goto { target }
,
1231 (point
.state
, block
)
1237 impl<'tcx
> MirPass
<'tcx
> for StateTransform
{
1238 fn run_pass(&self, tcx
: TyCtxt
<'tcx
>, body
: &mut Body
<'tcx
>) {
1239 let yield_ty
= if let Some(yield_ty
) = body
.yield_ty
{
1242 // This only applies to generators
1246 assert
!(body
.generator_drop
.is_none());
1248 // The first argument is the generator type passed by value
1249 let gen_ty
= body
.local_decls
.raw
[1].ty
;
1251 // Get the interior types and substs which typeck computed
1252 let (upvars
, interior
, discr_ty
, movable
) = match *gen_ty
.kind() {
1253 ty
::Generator(_
, substs
, movability
) => {
1254 let substs
= substs
.as_generator();
1256 substs
.upvar_tys().collect(),
1258 substs
.discr_ty(tcx
),
1259 movability
== hir
::Movability
::Movable
,
1264 .delay_span_bug(body
.span
, &format
!("unexpected generator type {}", gen_ty
));
1269 // Compute GeneratorState<yield_ty, return_ty>
1270 let state_did
= tcx
.require_lang_item(LangItem
::GeneratorState
, None
);
1271 let state_adt_ref
= tcx
.adt_def(state_did
);
1272 let state_substs
= tcx
.intern_substs(&[yield_ty
.into(), body
.return_ty().into()]);
1273 let ret_ty
= tcx
.mk_adt(state_adt_ref
, state_substs
);
1275 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
1276 // RETURN_PLACE then is a fresh unused local with type ret_ty.
1277 let new_ret_local
= replace_local(RETURN_PLACE
, ret_ty
, body
, tcx
);
1279 // We also replace the resume argument and insert an `Assign`.
1280 // This is needed because the resume argument `_2` might be live across a `yield`, in which
1281 // case there is no `Assign` to it that the transform can turn into a store to the generator
1282 // state. After the yield the slot in the generator state would then be uninitialized.
1283 let resume_local
= Local
::new(2);
1284 let new_resume_local
=
1285 replace_local(resume_local
, body
.local_decls
[resume_local
].ty
, body
, tcx
);
1287 // When first entering the generator, move the resume argument into its new local.
1288 let source_info
= SourceInfo
::outermost(body
.span
);
1289 let stmts
= &mut body
.basic_blocks_mut()[BasicBlock
::new(0)].statements
;
1294 kind
: StatementKind
::Assign(box (
1295 new_resume_local
.into(),
1296 Rvalue
::Use(Operand
::Move(resume_local
.into())),
1301 let always_live_locals
= storage
::AlwaysLiveLocals
::new(&body
);
1304 locals_live_across_suspend_points(tcx
, body
, &always_live_locals
, movable
);
1306 sanitize_witness(tcx
, body
, interior
, &upvars
, &liveness_info
.saved_locals
);
1308 if tcx
.sess
.opts
.debugging_opts
.validate_mir
{
1309 let mut vis
= EnsureGeneratorFieldAssignmentsNeverAlias
{
1310 assigned_local
: None
,
1311 saved_locals
: &liveness_info
.saved_locals
,
1312 storage_conflicts
: &liveness_info
.storage_conflicts
,
1315 vis
.visit_body(body
);
1318 // Extract locals which are live across suspension point into `layout`
1319 // `remap` gives a mapping from local indices onto generator struct indices
1320 // `storage_liveness` tells us which locals have live storage at suspension points
1321 let (remap
, layout
, storage_liveness
) = compute_layout(liveness_info
, body
);
1323 let can_return
= can_return(tcx
, body
);
1325 // Run the transformation which converts Places from Local to generator struct
1326 // accesses for locals in `remap`.
1327 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
1328 // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
1329 let mut transform
= TransformVisitor
{
1336 suspension_points
: Vec
::new(),
1340 transform
.visit_body(body
);
1342 // Update our MIR struct to reflect the changes we've made
1343 body
.yield_ty
= None
;
1344 body
.arg_count
= 2; // self, resume arg
1345 body
.spread_arg
= None
;
1346 body
.generator_layout
= Some(layout
);
1348 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
1349 // the unresumed state.
1350 // This is expanded to a drop ladder in `elaborate_generator_drops`.
1351 let drop_clean
= insert_clean_drop(body
);
1353 dump_mir(tcx
, None
, "generator_pre-elab", &0, body
, |_
, _
| Ok(()));
1355 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
1356 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
1357 // However we need to also elaborate the code generated by `insert_clean_drop`.
1358 elaborate_generator_drops(tcx
, body
);
1360 dump_mir(tcx
, None
, "generator_post-transform", &0, body
, |_
, _
| Ok(()));
1362 // Create a copy of our MIR and use it to create the drop shim for the generator
1363 let drop_shim
= create_generator_drop_shim(tcx
, &transform
, gen_ty
, body
, drop_clean
);
1365 body
.generator_drop
= Some(box drop_shim
);
1367 // Create the Generator::resume function
1368 create_generator_resume_function(tcx
, transform
, body
, can_return
);
1372 /// Looks for any assignments between locals (e.g., `_4 = _5`) that will both be converted to fields
1373 /// in the generator state machine but whose storage is not marked as conflicting
1375 /// Validation needs to happen immediately *before* `TransformVisitor` is invoked, not after.
1377 /// This condition would arise when the assignment is the last use of `_5` but the initial
1378 /// definition of `_4` if we weren't extra careful to mark all locals used inside a statement as
1379 /// conflicting. Non-conflicting generator saved locals may be stored at the same location within
1380 /// the generator state machine, which would result in ill-formed MIR: the left-hand and right-hand
1381 /// sides of an assignment may not alias. This caused a miscompilation in [#73137].
1383 /// [#73137]: https://github.com/rust-lang/rust/issues/73137
1384 struct EnsureGeneratorFieldAssignmentsNeverAlias
<'a
> {
1385 saved_locals
: &'a GeneratorSavedLocals
,
1386 storage_conflicts
: &'a BitMatrix
<GeneratorSavedLocal
, GeneratorSavedLocal
>,
1387 assigned_local
: Option
<GeneratorSavedLocal
>,
1390 impl EnsureGeneratorFieldAssignmentsNeverAlias
<'_
> {
1391 fn saved_local_for_direct_place(&self, place
: Place
<'_
>) -> Option
<GeneratorSavedLocal
> {
1392 if place
.is_indirect() {
1396 self.saved_locals
.get(place
.local
)
1399 fn check_assigned_place(&mut self, place
: Place
<'tcx
>, f
: impl FnOnce(&mut Self)) {
1400 if let Some(assigned_local
) = self.saved_local_for_direct_place(place
) {
1401 assert
!(self.assigned_local
.is_none(), "`check_assigned_place` must not recurse");
1403 self.assigned_local
= Some(assigned_local
);
1405 self.assigned_local
= None
;
1410 impl Visitor
<'tcx
> for EnsureGeneratorFieldAssignmentsNeverAlias
<'_
> {
1411 fn visit_place(&mut self, place
: &Place
<'tcx
>, context
: PlaceContext
, location
: Location
) {
1412 let lhs
= match self.assigned_local
{
1415 // This visitor only invokes `visit_place` for the right-hand side of an assignment
1416 // and only after setting `self.assigned_local`. However, the default impl of
1417 // `Visitor::super_body` may call `visit_place` with a `NonUseContext` for places
1418 // with debuginfo. Ignore them here.
1419 assert
!(!context
.is_use());
1424 let rhs
= match self.saved_local_for_direct_place(*place
) {
1429 if !self.storage_conflicts
.contains(lhs
, rhs
) {
1431 "Assignment between generator saved locals whose storage is not \
1432 marked as conflicting: {:?}: {:?} = {:?}",
1440 fn visit_statement(&mut self, statement
: &Statement
<'tcx
>, location
: Location
) {
1441 match &statement
.kind
{
1442 StatementKind
::Assign(box (lhs
, rhs
)) => {
1443 self.check_assigned_place(*lhs
, |this
| this
.visit_rvalue(rhs
, location
));
1446 // FIXME: Does `llvm_asm!` have any aliasing requirements?
1447 StatementKind
::LlvmInlineAsm(_
) => {}
1449 StatementKind
::FakeRead(..)
1450 | StatementKind
::SetDiscriminant { .. }
1451 | StatementKind
::StorageLive(_
)
1452 | StatementKind
::StorageDead(_
)
1453 | StatementKind
::Retag(..)
1454 | StatementKind
::AscribeUserType(..)
1455 | StatementKind
::Coverage(..)
1456 | StatementKind
::Nop
=> {}
1460 fn visit_terminator(&mut self, terminator
: &Terminator
<'tcx
>, location
: Location
) {
1461 // Checking for aliasing in terminators is probably overkill, but until we have actual
1462 // semantics, we should be conservative here.
1463 match &terminator
.kind
{
1464 TerminatorKind
::Call
{
1467 destination
: Some((dest
, _
)),
1472 self.check_assigned_place(*dest
, |this
| {
1473 this
.visit_operand(func
, location
);
1475 this
.visit_operand(arg
, location
);
1480 TerminatorKind
::Yield { value, resume: _, resume_arg, drop: _ }
=> {
1481 self.check_assigned_place(*resume_arg
, |this
| this
.visit_operand(value
, location
));
1484 // FIXME: Does `asm!` have any aliasing requirements?
1485 TerminatorKind
::InlineAsm { .. }
=> {}
1487 TerminatorKind
::Call { .. }
1488 | TerminatorKind
::Goto { .. }
1489 | TerminatorKind
::SwitchInt { .. }
1490 | TerminatorKind
::Resume
1491 | TerminatorKind
::Abort
1492 | TerminatorKind
::Return
1493 | TerminatorKind
::Unreachable
1494 | TerminatorKind
::Drop { .. }
1495 | TerminatorKind
::DropAndReplace { .. }
1496 | TerminatorKind
::Assert { .. }
1497 | TerminatorKind
::GeneratorDrop
1498 | TerminatorKind
::FalseEdge { .. }
1499 | TerminatorKind
::FalseUnwind { .. }
=> {}