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::generic::{self as dataflow, Analysis};
53 use crate::dataflow::{MaybeBorrowedLocals, MaybeRequiresStorage, MaybeStorageLive};
54 use crate::transform::no_landing_pads::no_landing_pads;
55 use crate::transform::simplify;
56 use crate::transform::{MirPass, MirSource};
57 use crate::util::dump_mir;
58 use crate::util::liveness;
59 use rustc::mir::visit::{MutVisitor, PlaceContext, Visitor};
61 use rustc::ty::layout::VariantIdx;
62 use rustc::ty::subst::SubstsRef;
63 use rustc::ty::GeneratorSubsts;
64 use rustc::ty::{self, AdtDef, Ty, TyCtxt};
65 use rustc_data_structures::fx::FxHashMap;
67 use rustc_hir::def_id::DefId;
68 use rustc_index::bit_set::{BitMatrix, BitSet};
69 use rustc_index::vec::{Idx, IndexVec};
73 pub struct StateTransform;
75 struct RenameLocalVisitor<'tcx> {
81 impl<'tcx> MutVisitor<'tcx> for RenameLocalVisitor<'tcx> {
82 fn tcx(&self) -> TyCtxt<'tcx> {
86 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
87 if *local == self.from {
92 fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
94 PlaceElem::Index(local) if *local == self.from => Some(PlaceElem::Index(self.to)),
100 struct DerefArgVisitor<'tcx> {
104 impl<'tcx> MutVisitor<'tcx> for DerefArgVisitor<'tcx> {
105 fn tcx(&self) -> TyCtxt<'tcx> {
109 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
110 assert_ne!(*local, self_arg());
113 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
114 if place.local == self_arg() {
119 projection: self.tcx().intern_place_elems(&[ProjectionElem::Deref]),
124 self.visit_place_base(&mut place.local, context, location);
126 for elem in place.projection.iter() {
127 if let PlaceElem::Index(local) = elem {
128 assert_ne!(*local, self_arg());
135 struct PinArgVisitor<'tcx> {
136 ref_gen_ty: Ty<'tcx>,
140 impl<'tcx> MutVisitor<'tcx> for PinArgVisitor<'tcx> {
141 fn tcx(&self) -> TyCtxt<'tcx> {
145 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
146 assert_ne!(*local, self_arg());
149 fn visit_place(&mut self, place: &mut Place<'tcx>, context: PlaceContext, location: Location) {
150 if place.local == self_arg() {
155 projection: self.tcx().intern_place_elems(&[ProjectionElem::Field(
163 self.visit_place_base(&mut place.local, context, location);
165 for elem in place.projection.iter() {
166 if let PlaceElem::Index(local) = elem {
167 assert_ne!(*local, self_arg());
174 fn replace_base<'tcx>(place: &mut Place<'tcx>, new_base: Place<'tcx>, tcx: TyCtxt<'tcx>) {
175 place.local = new_base.local;
177 let mut new_projection = new_base.projection.to_vec();
178 new_projection.append(&mut place.projection.to_vec());
180 place.projection = tcx.intern_place_elems(&new_projection);
183 fn self_arg() -> Local {
187 /// Generator has not been resumed yet.
188 const UNRESUMED: usize = GeneratorSubsts::UNRESUMED;
189 /// Generator has returned / is completed.
190 const RETURNED: usize = GeneratorSubsts::RETURNED;
191 /// Generator has panicked and is poisoned.
192 const POISONED: usize = GeneratorSubsts::POISONED;
194 /// A `yield` point in the generator.
195 struct SuspensionPoint<'tcx> {
196 /// State discriminant used when suspending or resuming at this point.
198 /// The block to jump to after resumption.
200 /// Where to move the resume argument after resumption.
201 resume_arg: Place<'tcx>,
202 /// Which block to jump to if the generator is dropped in this state.
203 drop: Option<BasicBlock>,
204 /// Set of locals that have live storage while at this suspension point.
205 storage_liveness: liveness::LiveVarSet,
208 struct TransformVisitor<'tcx> {
210 state_adt_ref: &'tcx AdtDef,
211 state_substs: SubstsRef<'tcx>,
213 // The type of the discriminant in the generator struct
216 // Mapping from Local to (type of local, generator struct index)
217 // FIXME(eddyb) This should use `IndexVec<Local, Option<_>>`.
218 remap: FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
220 // A map from a suspension point in a block to the locals which have live storage at that point
221 // FIXME(eddyb) This should use `IndexVec<BasicBlock, Option<_>>`.
222 storage_liveness: FxHashMap<BasicBlock, liveness::LiveVarSet>,
224 // A list of suspension points, generated during the transform
225 suspension_points: Vec<SuspensionPoint<'tcx>>,
227 // The original RETURN_PLACE local
228 new_ret_local: Local,
231 impl TransformVisitor<'tcx> {
232 // Make a GeneratorState rvalue
233 fn make_state(&self, idx: VariantIdx, val: Operand<'tcx>) -> Rvalue<'tcx> {
234 let adt = AggregateKind::Adt(self.state_adt_ref, idx, self.state_substs, None, None);
235 Rvalue::Aggregate(box adt, vec![val])
238 // Create a Place referencing a generator struct field
239 fn make_field(&self, variant_index: VariantIdx, idx: usize, ty: Ty<'tcx>) -> Place<'tcx> {
240 let self_place = Place::from(self_arg());
241 let base = self.tcx.mk_place_downcast_unnamed(self_place, variant_index);
242 let mut projection = base.projection.to_vec();
243 projection.push(ProjectionElem::Field(Field::new(idx), ty));
245 Place { local: base.local, projection: self.tcx.intern_place_elems(&projection) }
248 // Create a statement which changes the discriminant
249 fn set_discr(&self, state_disc: VariantIdx, source_info: SourceInfo) -> Statement<'tcx> {
250 let self_place = Place::from(self_arg());
253 kind: StatementKind::SetDiscriminant {
254 place: box self_place,
255 variant_index: state_disc,
260 // Create a statement which reads the discriminant into a temporary
261 fn get_discr(&self, body: &mut Body<'tcx>) -> (Statement<'tcx>, Place<'tcx>) {
262 let temp_decl = LocalDecl::new_internal(self.tcx.types.isize, body.span);
263 let local_decls_len = body.local_decls.push(temp_decl);
264 let temp = Place::from(local_decls_len);
266 let self_place = Place::from(self_arg());
267 let assign = Statement {
268 source_info: source_info(body),
269 kind: StatementKind::Assign(box (temp, Rvalue::Discriminant(self_place))),
275 impl MutVisitor<'tcx> for TransformVisitor<'tcx> {
276 fn tcx(&self) -> TyCtxt<'tcx> {
280 fn visit_local(&mut self, local: &mut Local, _: PlaceContext, _: Location) {
281 assert_eq!(self.remap.get(local), None);
286 place: &mut Place<'tcx>,
287 _context: PlaceContext,
290 // Replace an Local in the remap with a generator struct access
291 if let Some(&(ty, variant_index, idx)) = self.remap.get(&place.local) {
292 replace_base(place, self.make_field(variant_index, idx, ty), self.tcx);
296 fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) {
297 // Remove StorageLive and StorageDead statements for remapped locals
298 data.retain_statements(|s| match s.kind {
299 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
300 !self.remap.contains_key(&l)
305 let ret_val = match data.terminator().kind {
306 TerminatorKind::Return => Some((
309 Operand::Move(Place::from(self.new_ret_local)),
312 TerminatorKind::Yield { ref value, resume, resume_arg, drop } => {
313 Some((VariantIdx::new(0), Some((resume, resume_arg)), value.clone(), drop))
318 if let Some((state_idx, resume, v, drop)) = ret_val {
319 let source_info = data.terminator().source_info;
320 // We must assign the value first in case it gets declared dead below
321 data.statements.push(Statement {
323 kind: StatementKind::Assign(box (
324 Place::return_place(),
325 self.make_state(state_idx, v),
328 let state = if let Some((resume, resume_arg)) = resume {
330 let state = 3 + self.suspension_points.len();
332 // The resume arg target location might itself be remapped if its base local is
333 // live across a yield.
335 if let Some(&(ty, variant, idx)) = self.remap.get(&resume_arg.local) {
336 self.make_field(variant, idx, ty)
341 self.suspension_points.push(SuspensionPoint {
346 storage_liveness: self.storage_liveness.get(&block).unwrap().clone(),
349 VariantIdx::new(state)
352 VariantIdx::new(RETURNED) // state for returned
354 data.statements.push(self.set_discr(state, source_info));
355 data.terminator_mut().kind = TerminatorKind::Return;
358 self.super_basic_block_data(block, data);
362 fn make_generator_state_argument_indirect<'tcx>(
365 body: &mut BodyAndCache<'tcx>,
367 let gen_ty = body.local_decls.raw[1].ty;
369 let region = ty::ReFree(ty::FreeRegion { scope: def_id, bound_region: ty::BoundRegion::BrEnv });
371 let region = tcx.mk_region(region);
373 let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut });
375 // Replace the by value generator argument
376 body.local_decls.raw[1].ty = ref_gen_ty;
378 // Add a deref to accesses of the generator state
379 DerefArgVisitor { tcx }.visit_body(body);
382 fn make_generator_state_argument_pinned<'tcx>(tcx: TyCtxt<'tcx>, body: &mut BodyAndCache<'tcx>) {
383 let ref_gen_ty = body.local_decls.raw[1].ty;
385 let pin_did = tcx.lang_items().pin_type().unwrap();
386 let pin_adt_ref = tcx.adt_def(pin_did);
387 let substs = tcx.intern_substs(&[ref_gen_ty.into()]);
388 let pin_ref_gen_ty = tcx.mk_adt(pin_adt_ref, substs);
390 // Replace the by ref generator argument
391 body.local_decls.raw[1].ty = pin_ref_gen_ty;
393 // Add the Pin field access to accesses of the generator state
394 PinArgVisitor { ref_gen_ty, tcx }.visit_body(body);
397 /// Allocates a new local and replaces all references of `local` with it. Returns the new local.
399 /// `local` will be changed to a new local decl with type `ty`.
401 /// Note that the new local will be uninitialized. It is the caller's responsibility to assign some
402 /// valid value to it before its first use.
403 fn replace_local<'tcx>(
406 body: &mut BodyAndCache<'tcx>,
409 let source_info = source_info(body);
410 let new_decl = LocalDecl {
411 mutability: Mutability::Mut,
413 user_ty: UserTypeProjections::none(),
417 local_info: LocalInfo::Other,
419 let new_local = Local::new(body.local_decls.len());
420 body.local_decls.push(new_decl);
421 body.local_decls.swap(local, new_local);
423 RenameLocalVisitor { from: local, to: new_local, tcx }.visit_body(body);
428 struct StorageIgnored(liveness::LiveVarSet);
430 impl<'tcx> Visitor<'tcx> for StorageIgnored {
431 fn visit_statement(&mut self, statement: &Statement<'tcx>, _location: Location) {
432 match statement.kind {
433 StatementKind::StorageLive(l) | StatementKind::StorageDead(l) => {
441 struct LivenessInfo {
442 /// Which locals are live across any suspension point.
444 /// GeneratorSavedLocal is indexed in terms of the elements in this set;
445 /// i.e. GeneratorSavedLocal::new(1) corresponds to the second local
446 /// included in this set.
447 live_locals: liveness::LiveVarSet,
449 /// The set of saved locals live at each suspension point.
450 live_locals_at_suspension_points: Vec<BitSet<GeneratorSavedLocal>>,
452 /// For every saved local, the set of other saved locals that are
453 /// storage-live at the same time as this local. We cannot overlap locals in
454 /// the layout which have conflicting storage.
455 storage_conflicts: BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal>,
457 /// For every suspending block, the locals which are storage-live across
458 /// that suspension point.
459 storage_liveness: FxHashMap<BasicBlock, liveness::LiveVarSet>,
462 fn locals_live_across_suspend_points(
464 body: ReadOnlyBodyAndCache<'_, 'tcx>,
465 source: MirSource<'tcx>,
468 let def_id = source.def_id();
469 let body_ref: &Body<'_> = &body;
471 // Calculate when MIR locals have live storage. This gives us an upper bound of their
473 let mut storage_live = MaybeStorageLive
474 .into_engine(tcx, body_ref, def_id)
475 .iterate_to_fixpoint()
476 .into_results_cursor(body_ref);
478 // Find the MIR locals which do not use StorageLive/StorageDead statements.
479 // The storage of these locals are always live.
480 let mut ignored = StorageIgnored(BitSet::new_filled(body.local_decls.len()));
481 ignored.visit_body(body);
483 // Calculate the MIR locals which have been previously
484 // borrowed (even if they are still active).
485 let borrowed_locals_results =
486 MaybeBorrowedLocals::all_borrows().into_engine(tcx, body_ref, def_id).iterate_to_fixpoint();
488 let mut borrowed_locals_cursor =
489 dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results);
491 // Calculate the MIR locals that we actually need to keep storage around
493 let requires_storage_results = MaybeRequiresStorage::new(body, &borrowed_locals_results)
494 .into_engine(tcx, body_ref, def_id)
495 .iterate_to_fixpoint();
496 let mut requires_storage_cursor =
497 dataflow::ResultsCursor::new(body_ref, &requires_storage_results);
499 // Calculate the liveness of MIR locals ignoring borrows.
500 let mut live_locals = liveness::LiveVarSet::new_empty(body.local_decls.len());
501 let mut liveness = liveness::liveness_of_locals(body);
502 liveness::dump_mir(tcx, "generator_liveness", source, body_ref, &liveness);
504 let mut storage_liveness_map = FxHashMap::default();
505 let mut live_locals_at_suspension_points = Vec::new();
507 for (block, data) in body.basic_blocks().iter_enumerated() {
508 if let TerminatorKind::Yield { .. } = data.terminator().kind {
509 let loc = Location { block, statement_index: data.statements.len() };
512 // The `liveness` variable contains the liveness of MIR locals ignoring borrows.
513 // This is correct for movable generators since borrows cannot live across
514 // suspension points. However for immovable generators we need to account for
515 // borrows, so we conseratively assume that all borrowed locals are live until
516 // we find a StorageDead statement referencing the locals.
517 // To do this we just union our `liveness` result with `borrowed_locals`, which
518 // contains all the locals which has been borrowed before this suspension point.
519 // If a borrow is converted to a raw reference, we must also assume that it lives
520 // forever. Note that the final liveness is still bounded by the storage liveness
521 // of the local, which happens using the `intersect` operation below.
522 borrowed_locals_cursor.seek_before(loc);
523 liveness.outs[block].union(borrowed_locals_cursor.get());
526 storage_live.seek_before(loc);
527 let storage_liveness = storage_live.get();
529 // Store the storage liveness for later use so we can restore the state
530 // after a suspension point
531 storage_liveness_map.insert(block, storage_liveness.clone());
533 requires_storage_cursor.seek_before(loc);
534 let storage_required = requires_storage_cursor.get().clone();
536 // Locals live are live at this point only if they are used across
537 // suspension points (the `liveness` variable)
538 // and their storage is required (the `storage_required` variable)
539 let mut live_locals_here = storage_required;
540 live_locals_here.intersect(&liveness.outs[block]);
542 // The generator argument is ignored
543 live_locals_here.remove(self_arg());
545 debug!("loc = {:?}, live_locals_here = {:?}", loc, live_locals_here);
547 // Add the locals live at this suspension point to the set of locals which live across
548 // any suspension points
549 live_locals.union(&live_locals_here);
551 live_locals_at_suspension_points.push(live_locals_here);
554 debug!("live_locals = {:?}", live_locals);
556 // Renumber our liveness_map bitsets to include only the locals we are
558 let live_locals_at_suspension_points = live_locals_at_suspension_points
560 .map(|live_here| renumber_bitset(&live_here, &live_locals))
563 let storage_conflicts =
564 compute_storage_conflicts(body_ref, &live_locals, &ignored, requires_storage_results);
568 live_locals_at_suspension_points,
570 storage_liveness: storage_liveness_map,
574 /// Renumbers the items present in `stored_locals` and applies the renumbering
577 /// For example, if `stored_locals = [1, 3, 5]`, this would be renumbered to
578 /// `[0, 1, 2]`. Thus, if `input = [3, 5]` we would return `[1, 2]`.
580 input: &BitSet<Local>,
581 stored_locals: &liveness::LiveVarSet,
582 ) -> BitSet<GeneratorSavedLocal> {
583 assert!(stored_locals.superset(&input), "{:?} not a superset of {:?}", stored_locals, input);
584 let mut out = BitSet::new_empty(stored_locals.count());
585 for (idx, local) in stored_locals.iter().enumerate() {
586 let saved_local = GeneratorSavedLocal::from(idx);
587 if input.contains(local) {
588 out.insert(saved_local);
591 debug!("renumber_bitset({:?}, {:?}) => {:?}", input, stored_locals, out);
595 /// For every saved local, looks for which locals are StorageLive at the same
596 /// time. Generates a bitset for every local of all the other locals that may be
597 /// StorageLive simultaneously with that local. This is used in the layout
598 /// computation; see `GeneratorLayout` for more.
599 fn compute_storage_conflicts(
600 body: &'mir Body<'tcx>,
601 stored_locals: &liveness::LiveVarSet,
602 ignored: &StorageIgnored,
603 requires_storage: dataflow::Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
604 ) -> BitMatrix<GeneratorSavedLocal, GeneratorSavedLocal> {
605 assert_eq!(body.local_decls.len(), ignored.0.domain_size());
606 assert_eq!(body.local_decls.len(), stored_locals.domain_size());
607 debug!("compute_storage_conflicts({:?})", body.span);
608 debug!("ignored = {:?}", ignored.0);
610 // Storage ignored locals are not eligible for overlap, since their storage
612 let mut ineligible_locals = ignored.0.clone();
613 ineligible_locals.intersect(&stored_locals);
615 // Compute the storage conflicts for all eligible locals.
616 let mut visitor = StorageConflictVisitor {
618 stored_locals: &stored_locals,
619 local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
622 // Visit only reachable basic blocks. The exact order is not important.
623 let reachable_blocks = traversal::preorder(body).map(|(bb, _)| bb);
624 requires_storage.visit_with(body, reachable_blocks, &mut visitor);
626 let local_conflicts = visitor.local_conflicts;
628 // Compress the matrix using only stored locals (Local -> GeneratorSavedLocal).
630 // NOTE: Today we store a full conflict bitset for every local. Technically
631 // this is twice as many bits as we need, since the relation is symmetric.
632 // However, in practice these bitsets are not usually large. The layout code
633 // also needs to keep track of how many conflicts each local has, so it's
634 // simpler to keep it this way for now.
635 let mut storage_conflicts = BitMatrix::new(stored_locals.count(), stored_locals.count());
636 for (idx_a, local_a) in stored_locals.iter().enumerate() {
637 let saved_local_a = GeneratorSavedLocal::new(idx_a);
638 if ineligible_locals.contains(local_a) {
639 // Conflicts with everything.
640 storage_conflicts.insert_all_into_row(saved_local_a);
642 // Keep overlap information only for stored locals.
643 for (idx_b, local_b) in stored_locals.iter().enumerate() {
644 let saved_local_b = GeneratorSavedLocal::new(idx_b);
645 if local_conflicts.contains(local_a, local_b) {
646 storage_conflicts.insert(saved_local_a, saved_local_b);
654 struct StorageConflictVisitor<'mir, 'tcx, 's> {
655 body: &'mir Body<'tcx>,
656 stored_locals: &'s liveness::LiveVarSet,
657 // FIXME(tmandry): Consider using sparse bitsets here once we have good
658 // benchmarks for generators.
659 local_conflicts: BitMatrix<Local, Local>,
662 impl dataflow::ResultsVisitor<'mir, 'tcx> for StorageConflictVisitor<'mir, 'tcx, '_> {
663 type FlowState = BitSet<Local>;
667 state: &Self::FlowState,
668 _statement: &'mir Statement<'tcx>,
671 self.apply_state(state, loc);
676 state: &Self::FlowState,
677 _terminator: &'mir Terminator<'tcx>,
680 self.apply_state(state, loc);
684 impl<'body, 'tcx, 's> StorageConflictVisitor<'body, 'tcx, 's> {
685 fn apply_state(&mut self, flow_state: &BitSet<Local>, loc: Location) {
686 // Ignore unreachable blocks.
687 match self.body.basic_blocks()[loc.block].terminator().kind {
688 TerminatorKind::Unreachable => return,
692 let mut eligible_storage_live = flow_state.clone();
693 eligible_storage_live.intersect(&self.stored_locals);
695 for local in eligible_storage_live.iter() {
696 self.local_conflicts.union_row_with(&eligible_storage_live, local);
699 if eligible_storage_live.count() > 1 {
700 trace!("at {:?}, eligible_storage_live={:?}", loc, eligible_storage_live);
705 fn compute_layout<'tcx>(
707 source: MirSource<'tcx>,
708 upvars: &Vec<Ty<'tcx>>,
711 body: &mut BodyAndCache<'tcx>,
713 FxHashMap<Local, (Ty<'tcx>, VariantIdx, usize)>,
714 GeneratorLayout<'tcx>,
715 FxHashMap<BasicBlock, liveness::LiveVarSet>,
717 // Use a liveness analysis to compute locals which are live across a suspension point
720 live_locals_at_suspension_points,
723 } = locals_live_across_suspend_points(tcx, read_only!(body), source, movable);
725 // Erase regions from the types passed in from typeck so we can compare them with
727 let allowed_upvars = tcx.erase_regions(upvars);
728 let allowed = match interior.kind {
729 ty::GeneratorWitness(s) => tcx.erase_late_bound_regions(&s),
733 for (local, decl) in body.local_decls.iter_enumerated() {
734 // Ignore locals which are internal or not live
735 if !live_locals.contains(local) || decl.internal {
739 // Sanity check that typeck knows about the type of locals which are
740 // live across a suspension point
741 if !allowed.contains(&decl.ty) && !allowed_upvars.contains(&decl.ty) {
744 "Broken MIR: generator contains type {} in MIR, \
745 but typeck only knows about {}",
752 // Gather live local types and their indices.
753 let mut locals = IndexVec::<GeneratorSavedLocal, _>::new();
754 let mut tys = IndexVec::<GeneratorSavedLocal, _>::new();
755 for (idx, local) in live_locals.iter().enumerate() {
757 tys.push(body.local_decls[local].ty);
758 debug!("generator saved local {:?} => {:?}", GeneratorSavedLocal::from(idx), local);
761 // Leave empty variants for the UNRESUMED, RETURNED, and POISONED states.
762 const RESERVED_VARIANTS: usize = 3;
764 // Build the generator variant field list.
765 // Create a map from local indices to generator struct indices.
766 let mut variant_fields: IndexVec<VariantIdx, IndexVec<Field, GeneratorSavedLocal>> =
767 iter::repeat(IndexVec::new()).take(RESERVED_VARIANTS).collect();
768 let mut remap = FxHashMap::default();
769 for (suspension_point_idx, live_locals) in live_locals_at_suspension_points.iter().enumerate() {
770 let variant_index = VariantIdx::from(RESERVED_VARIANTS + suspension_point_idx);
771 let mut fields = IndexVec::new();
772 for (idx, saved_local) in live_locals.iter().enumerate() {
773 fields.push(saved_local);
774 // Note that if a field is included in multiple variants, we will
775 // just use the first one here. That's fine; fields do not move
776 // around inside generators, so it doesn't matter which variant
777 // index we access them by.
778 remap.entry(locals[saved_local]).or_insert((tys[saved_local], variant_index, idx));
780 variant_fields.push(fields);
782 debug!("generator variant_fields = {:?}", variant_fields);
783 debug!("generator storage_conflicts = {:#?}", storage_conflicts);
785 let layout = GeneratorLayout { field_tys: tys, variant_fields, storage_conflicts };
787 (remap, layout, storage_liveness)
790 /// Replaces the entry point of `body` with a block that switches on the generator discriminant and
791 /// dispatches to blocks according to `cases`.
793 /// After this function, the former entry point of the function will be bb1.
794 fn insert_switch<'tcx>(
795 body: &mut BodyAndCache<'tcx>,
796 cases: Vec<(usize, BasicBlock)>,
797 transform: &TransformVisitor<'tcx>,
798 default: TerminatorKind<'tcx>,
800 let default_block = insert_term_block(body, default);
801 let (assign, discr) = transform.get_discr(body);
802 let switch = TerminatorKind::SwitchInt {
803 discr: Operand::Move(discr),
804 switch_ty: transform.discr_ty,
805 values: Cow::from(cases.iter().map(|&(i, _)| i as u128).collect::<Vec<_>>()),
806 targets: cases.iter().map(|&(_, d)| d).chain(iter::once(default_block)).collect(),
809 let source_info = source_info(body);
810 body.basic_blocks_mut().raw.insert(
813 statements: vec![assign],
814 terminator: Some(Terminator { source_info, kind: switch }),
819 let blocks = body.basic_blocks_mut().iter_mut();
821 for target in blocks.flat_map(|b| b.terminator_mut().successors_mut()) {
822 *target = BasicBlock::new(target.index() + 1);
826 fn elaborate_generator_drops<'tcx>(
829 body: &mut BodyAndCache<'tcx>,
831 use crate::shim::DropShimElaborator;
832 use crate::util::elaborate_drops::{elaborate_drop, Unwind};
833 use crate::util::patch::MirPatch;
835 // Note that `elaborate_drops` only drops the upvars of a generator, and
836 // this is ok because `open_drop` can only be reached within that own
837 // generator's resume function.
839 let param_env = tcx.param_env(def_id);
840 let gen = self_arg();
842 let mut elaborator = DropShimElaborator { body, patch: MirPatch::new(body), tcx, param_env };
844 for (block, block_data) in body.basic_blocks().iter_enumerated() {
845 let (target, unwind, source_info) = match block_data.terminator() {
846 Terminator { source_info, kind: TerminatorKind::Drop { location, target, unwind } } => {
847 if let Some(local) = location.as_local() {
849 (target, unwind, source_info)
859 let unwind = if block_data.is_cleanup {
862 Unwind::To(unwind.unwrap_or_else(|| elaborator.patch.resume_block()))
874 elaborator.patch.apply(body);
877 fn create_generator_drop_shim<'tcx>(
879 transform: &TransformVisitor<'tcx>,
881 source: MirSource<'tcx>,
883 body: &mut BodyAndCache<'tcx>,
884 drop_clean: BasicBlock,
885 ) -> BodyAndCache<'tcx> {
886 let mut body = body.clone();
887 body.arg_count = 1; // make sure the resume argument is not included here
889 let source_info = source_info(&body);
891 let mut cases = create_cases(&mut body, transform, Operation::Drop);
893 cases.insert(0, (UNRESUMED, drop_clean));
895 // The returned state and the poisoned state fall through to the default
896 // case which is just to return
898 insert_switch(&mut body, cases, &transform, TerminatorKind::Return);
900 for block in body.basic_blocks_mut() {
901 let kind = &mut block.terminator_mut().kind;
902 if let TerminatorKind::GeneratorDrop = *kind {
903 *kind = TerminatorKind::Return;
907 // Replace the return variable
908 body.local_decls[RETURN_PLACE] = LocalDecl {
909 mutability: Mutability::Mut,
911 user_ty: UserTypeProjections::none(),
915 local_info: LocalInfo::Other,
918 make_generator_state_argument_indirect(tcx, def_id, &mut body);
920 // Change the generator argument from &mut to *mut
921 body.local_decls[self_arg()] = LocalDecl {
922 mutability: Mutability::Mut,
923 ty: tcx.mk_ptr(ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut }),
924 user_ty: UserTypeProjections::none(),
928 local_info: LocalInfo::Other,
930 if tcx.sess.opts.debugging_opts.mir_emit_retag {
931 // Alias tracking must know we changed the type
932 body.basic_blocks_mut()[START_BLOCK].statements.insert(
936 kind: StatementKind::Retag(RetagKind::Raw, box Place::from(self_arg())),
941 no_landing_pads(tcx, &mut body);
943 // Make sure we remove dead blocks to remove
944 // unrelated code from the resume part of the function
945 simplify::remove_dead_blocks(&mut body);
947 dump_mir(tcx, None, "generator_drop", &0, source, &mut body, |_, _| Ok(()));
952 fn insert_term_block<'tcx>(
953 body: &mut BodyAndCache<'tcx>,
954 kind: TerminatorKind<'tcx>,
956 let term_block = BasicBlock::new(body.basic_blocks().len());
957 let source_info = source_info(body);
958 body.basic_blocks_mut().push(BasicBlockData {
959 statements: Vec::new(),
960 terminator: Some(Terminator { source_info, kind }),
966 fn insert_panic_block<'tcx>(
968 body: &mut BodyAndCache<'tcx>,
969 message: AssertMessage<'tcx>,
971 let assert_block = BasicBlock::new(body.basic_blocks().len());
972 let term = TerminatorKind::Assert {
973 cond: Operand::Constant(box Constant {
976 literal: ty::Const::from_bool(tcx, false),
980 target: assert_block,
984 let source_info = source_info(body);
985 body.basic_blocks_mut().push(BasicBlockData {
986 statements: Vec::new(),
987 terminator: Some(Terminator { source_info, kind: term }),
994 fn create_generator_resume_function<'tcx>(
996 transform: TransformVisitor<'tcx>,
998 source: MirSource<'tcx>,
999 body: &mut BodyAndCache<'tcx>,
1001 // Poison the generator when it unwinds
1002 for block in body.basic_blocks_mut() {
1003 let source_info = block.terminator().source_info;
1004 if let &TerminatorKind::Resume = &block.terminator().kind {
1005 block.statements.push(transform.set_discr(VariantIdx::new(POISONED), source_info));
1009 let mut cases = create_cases(body, &transform, Operation::Resume);
1011 use rustc::mir::AssertKind::{ResumedAfterPanic, ResumedAfterReturn};
1013 // Jump to the entry point on the unresumed
1014 cases.insert(0, (UNRESUMED, BasicBlock::new(0)));
1016 // Panic when resumed on the returned or poisoned state
1017 let generator_kind = body.generator_kind.unwrap();
1018 cases.insert(1, (RETURNED, insert_panic_block(tcx, body, ResumedAfterReturn(generator_kind))));
1019 cases.insert(2, (POISONED, insert_panic_block(tcx, body, ResumedAfterPanic(generator_kind))));
1021 insert_switch(body, cases, &transform, TerminatorKind::Unreachable);
1023 make_generator_state_argument_indirect(tcx, def_id, body);
1024 make_generator_state_argument_pinned(tcx, body);
1026 no_landing_pads(tcx, body);
1028 // Make sure we remove dead blocks to remove
1029 // unrelated code from the drop part of the function
1030 simplify::remove_dead_blocks(body);
1032 dump_mir(tcx, None, "generator_resume", &0, source, body, |_, _| Ok(()));
1035 fn source_info(body: &Body<'_>) -> SourceInfo {
1036 SourceInfo { span: body.span, scope: OUTERMOST_SOURCE_SCOPE }
1039 fn insert_clean_drop(body: &mut BodyAndCache<'_>) -> BasicBlock {
1040 let return_block = insert_term_block(body, TerminatorKind::Return);
1042 // Create a block to destroy an unresumed generators. This can only destroy upvars.
1043 let drop_clean = BasicBlock::new(body.basic_blocks().len());
1044 let term = TerminatorKind::Drop {
1045 location: Place::from(self_arg()),
1046 target: return_block,
1049 let source_info = source_info(body);
1050 body.basic_blocks_mut().push(BasicBlockData {
1051 statements: Vec::new(),
1052 terminator: Some(Terminator { source_info, kind: term }),
1059 /// An operation that can be performed on a generator.
1060 #[derive(PartialEq, Copy, Clone)]
1067 fn target_block(self, point: &SuspensionPoint<'_>) -> Option<BasicBlock> {
1069 Operation::Resume => Some(point.resume),
1070 Operation::Drop => point.drop,
1075 fn create_cases<'tcx>(
1076 body: &mut BodyAndCache<'tcx>,
1077 transform: &TransformVisitor<'tcx>,
1078 operation: Operation,
1079 ) -> Vec<(usize, BasicBlock)> {
1080 let source_info = source_info(body);
1085 .filter_map(|point| {
1086 // Find the target for this suspension point, if applicable
1087 operation.target_block(point).map(|target| {
1088 let block = BasicBlock::new(body.basic_blocks().len());
1089 let mut statements = Vec::new();
1091 // Create StorageLive instructions for locals with live storage
1092 for i in 0..(body.local_decls.len()) {
1094 // The resume argument is live on function entry. Don't insert a
1095 // `StorageLive`, or the following `Assign` will read from uninitialized
1100 let l = Local::new(i);
1101 if point.storage_liveness.contains(l) && !transform.remap.contains_key(&l) {
1103 .push(Statement { source_info, kind: StatementKind::StorageLive(l) });
1107 if operation == Operation::Resume {
1108 // Move the resume argument to the destination place of the `Yield` terminator
1109 let resume_arg = Local::new(2); // 0 = return, 1 = self
1110 statements.push(Statement {
1112 kind: StatementKind::Assign(box (
1114 Rvalue::Use(Operand::Move(resume_arg.into())),
1119 // Then jump to the real target
1120 body.basic_blocks_mut().push(BasicBlockData {
1122 terminator: Some(Terminator {
1124 kind: TerminatorKind::Goto { target },
1129 (point.state, block)
1135 impl<'tcx> MirPass<'tcx> for StateTransform {
1136 fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
1137 let yield_ty = if let Some(yield_ty) = body.yield_ty {
1140 // This only applies to generators
1144 assert!(body.generator_drop.is_none());
1146 let def_id = source.def_id();
1148 // The first argument is the generator type passed by value
1149 let gen_ty = body.local_decls.raw[1].ty;
1151 // Get the interior types and substs which typeck computed
1152 let (upvars, interior, discr_ty, movable) = match gen_ty.kind {
1153 ty::Generator(_, substs, movability) => {
1154 let substs = substs.as_generator();
1156 substs.upvar_tys(def_id, tcx).collect(),
1157 substs.witness(def_id, tcx),
1158 substs.discr_ty(tcx),
1159 movability == hir::Movability::Movable,
1165 // Compute GeneratorState<yield_ty, return_ty>
1166 let state_did = tcx.lang_items().gen_state().unwrap();
1167 let state_adt_ref = tcx.adt_def(state_did);
1168 let state_substs = tcx.intern_substs(&[yield_ty.into(), body.return_ty().into()]);
1169 let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
1171 // We rename RETURN_PLACE which has type mir.return_ty to new_ret_local
1172 // RETURN_PLACE then is a fresh unused local with type ret_ty.
1173 let new_ret_local = replace_local(RETURN_PLACE, ret_ty, body, tcx);
1175 // We also replace the resume argument and insert an `Assign`.
1176 // This is needed because the resume argument `_2` might be live across a `yield`, in which
1177 // case there is no `Assign` to it that the transform can turn into a store to the generator
1178 // state. After the yield the slot in the generator state would then be uninitialized.
1179 let resume_local = Local::new(2);
1180 let new_resume_local =
1181 replace_local(resume_local, body.local_decls[resume_local].ty, body, tcx);
1183 // When first entering the generator, move the resume argument into its new local.
1184 let source_info = source_info(body);
1185 let stmts = &mut body.basic_blocks_mut()[BasicBlock::new(0)].statements;
1190 kind: StatementKind::Assign(box (
1191 new_resume_local.into(),
1192 Rvalue::Use(Operand::Move(resume_local.into())),
1197 // Extract locals which are live across suspension point into `layout`
1198 // `remap` gives a mapping from local indices onto generator struct indices
1199 // `storage_liveness` tells us which locals have live storage at suspension points
1200 let (remap, layout, storage_liveness) =
1201 compute_layout(tcx, source, &upvars, interior, movable, body);
1203 // Run the transformation which converts Places from Local to generator struct
1204 // accesses for locals in `remap`.
1205 // It also rewrites `return x` and `yield y` as writing a new generator state and returning
1206 // GeneratorState::Complete(x) and GeneratorState::Yielded(y) respectively.
1207 let mut transform = TransformVisitor {
1213 suspension_points: Vec::new(),
1217 transform.visit_body(body);
1219 // Update our MIR struct to reflect the changes we've made
1220 body.yield_ty = None;
1221 body.arg_count = 2; // self, resume arg
1222 body.spread_arg = None;
1223 body.generator_layout = Some(layout);
1225 // Insert `drop(generator_struct)` which is used to drop upvars for generators in
1226 // the unresumed state.
1227 // This is expanded to a drop ladder in `elaborate_generator_drops`.
1228 let drop_clean = insert_clean_drop(body);
1230 dump_mir(tcx, None, "generator_pre-elab", &0, source, body, |_, _| Ok(()));
1232 // Expand `drop(generator_struct)` to a drop ladder which destroys upvars.
1233 // If any upvars are moved out of, drop elaboration will handle upvar destruction.
1234 // However we need to also elaborate the code generated by `insert_clean_drop`.
1235 elaborate_generator_drops(tcx, def_id, body);
1237 dump_mir(tcx, None, "generator_post-transform", &0, source, body, |_, _| Ok(()));
1239 // Create a copy of our MIR and use it to create the drop shim for the generator
1241 create_generator_drop_shim(tcx, &transform, def_id, source, gen_ty, body, drop_clean);
1243 body.generator_drop = Some(box drop_shim);
1245 // Create the Generator::resume function
1246 create_generator_resume_function(tcx, transform, def_id, source, body);