src/librustc_mir/dataflow/impls/borrows.rs

   1 use crate::borrow_check::borrow_set::{BorrowSet, BorrowData};
   2 use crate::borrow_check::place_ext::PlaceExt;
   3
   4 use rustc::mir::{self, Location, Place, PlaceBase, Body};
   5 use rustc::ty::TyCtxt;
   6 use rustc::ty::RegionVid;
   7
   8 use rustc_data_structures::bit_set::BitSet;
   9 use rustc_data_structures::fx::FxHashMap;
  10 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  11
  12 use crate::dataflow::{BitDenotation, BottomValue, GenKillSet};
  13 use crate::borrow_check::nll::region_infer::RegionInferenceContext;
  14 use crate::borrow_check::nll::ToRegionVid;
  15 use crate::borrow_check::places_conflict;
  16
  17 use std::rc::Rc;
  18
  19 newtype_index! {
  20     pub struct BorrowIndex {
  21         DEBUG_FORMAT = "bw{}"
  22     }
  23 }
  24
  25 /// `Borrows` stores the data used in the analyses that track the flow
  26 /// of borrows.
  27 ///
  28 /// It uniquely identifies every borrow (`Rvalue::Ref`) by a
  29 /// `BorrowIndex`, and maps each such index to a `BorrowData`
  30 /// describing the borrow. These indexes are used for representing the
  31 /// borrows in compact bitvectors.
  32 pub struct Borrows<'a, 'tcx> {
  33     tcx: TyCtxt<'tcx>,
  34     body: &'a Body<'tcx>,
  35
  36     borrow_set: Rc<BorrowSet<'tcx>>,
  37     borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
  38
  39     /// NLL region inference context with which NLL queries should be resolved
  40     _nonlexical_regioncx: Rc<RegionInferenceContext<'tcx>>,
  41 }
  42
  43 struct StackEntry {
  44     bb: mir::BasicBlock,
  45     lo: usize,
  46     hi: usize,
  47     first_part_only: bool
  48 }
  49
  50 fn precompute_borrows_out_of_scope<'tcx>(
  51     body: &Body<'tcx>,
  52     regioncx: &Rc<RegionInferenceContext<'tcx>>,
  53     borrows_out_of_scope_at_location: &mut FxHashMap<Location, Vec<BorrowIndex>>,
  54     borrow_index: BorrowIndex,
  55     borrow_region: RegionVid,
  56     location: Location,
  57 ) {
  58     // We visit one BB at a time. The complication is that we may start in the
  59     // middle of the first BB visited (the one containing `location`), in which
  60     // case we may have to later on process the first part of that BB if there
  61     // is a path back to its start.
  62
  63     // For visited BBs, we record the index of the first statement processed.
  64     // (In fully processed BBs this index is 0.) Note also that we add BBs to
  65     // `visited` once they are added to `stack`, before they are actually
  66     // processed, because this avoids the need to look them up again on
  67     // completion.
  68     let mut visited = FxHashMap::default();
  69     visited.insert(location.block, location.statement_index);
  70
  71     let mut stack = vec![];
  72     stack.push(StackEntry {
  73         bb: location.block,
  74         lo: location.statement_index,
  75         hi: body[location.block].statements.len(),
  76         first_part_only: false,
  77     });
  78
  79     while let Some(StackEntry { bb, lo, hi, first_part_only }) = stack.pop() {
  80         let mut finished_early = first_part_only;
  81         for i in lo ..= hi {
  82             let location = Location { block: bb, statement_index: i };
  83             // If region does not contain a point at the location, then add to list and skip
  84             // successor locations.
  85             if !regioncx.region_contains(borrow_region, location) {
  86                 debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
  87                 borrows_out_of_scope_at_location
  88                     .entry(location)
  89                     .or_default()
  90                     .push(borrow_index);
  91                 finished_early = true;
  92                 break;
  93             }
  94         }
  95
  96         if !finished_early {
  97             // Add successor BBs to the work list, if necessary.
  98             let bb_data = &body[bb];
  99             assert!(hi == bb_data.statements.len());
 100             for &succ_bb in bb_data.terminator.as_ref().unwrap().successors() {
 101                 visited.entry(succ_bb)
 102                     .and_modify(|lo| {
 103                         // `succ_bb` has been seen before. If it wasn't
 104                         // fully processed, add its first part to `stack`
 105                         // for processing.
 106                         if *lo > 0 {
 107                             stack.push(StackEntry {
 108                                 bb: succ_bb,
 109                                 lo: 0,
 110                                 hi: *lo - 1,
 111                                 first_part_only: true,
 112                             });
 113                         }
 114                         // And update this entry with 0, to represent the
 115                         // whole BB being processed.
 116                         *lo = 0;
 117                     })
 118                     .or_insert_with(|| {
 119                         // succ_bb hasn't been seen before. Add it to
 120                         // `stack` for processing.
 121                         stack.push(StackEntry {
 122                             bb: succ_bb,
 123                             lo: 0,
 124                             hi: body[succ_bb].statements.len(),
 125                             first_part_only: false,
 126                         });
 127                         // Insert 0 for this BB, to represent the whole BB
 128                         // being processed.
 129                         0
 130                     });
 131             }
 132         }
 133     }
 134 }
 135
 136 impl<'a, 'tcx> Borrows<'a, 'tcx> {
 137     crate fn new(
 138         tcx: TyCtxt<'tcx>,
 139         body: &'a Body<'tcx>,
 140         nonlexical_regioncx: Rc<RegionInferenceContext<'tcx>>,
 141         borrow_set: &Rc<BorrowSet<'tcx>>,
 142     ) -> Self {
 143         let mut borrows_out_of_scope_at_location = FxHashMap::default();
 144         for (borrow_index, borrow_data) in borrow_set.borrows.iter_enumerated() {
 145             let borrow_region = borrow_data.region.to_region_vid();
 146             let location = borrow_set.borrows[borrow_index].reserve_location;
 147
 148             precompute_borrows_out_of_scope(body, &nonlexical_regioncx,
 149                                             &mut borrows_out_of_scope_at_location,
 150                                             borrow_index, borrow_region, location);
 151         }
 152
 153         Borrows {
 154             tcx: tcx,
 155             body: body,
 156             borrow_set: borrow_set.clone(),
 157             borrows_out_of_scope_at_location,
 158             _nonlexical_regioncx: nonlexical_regioncx,
 159         }
 160     }
 161
 162     crate fn borrows(&self) -> &IndexVec<BorrowIndex, BorrowData<'tcx>> { &self.borrow_set.borrows }
 163
 164     pub fn location(&self, idx: BorrowIndex) -> &Location {
 165         &self.borrow_set.borrows[idx].reserve_location
 166     }
 167
 168     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
 169     /// That means they went out of a nonlexical scope
 170     fn kill_loans_out_of_scope_at_location(&self,
 171                                            trans: &mut GenKillSet<BorrowIndex>,
 172                                            location: Location) {
 173         // NOTE: The state associated with a given `location`
 174         // reflects the dataflow on entry to the statement.
 175         // Iterate over each of the borrows that we've precomputed
 176         // to have went out of scope at this location and kill them.
 177         //
 178         // We are careful always to call this function *before* we
 179         // set up the gen-bits for the statement or
 180         // termanator. That way, if the effect of the statement or
 181         // terminator *does* introduce a new loan of the same
 182         // region, then setting that gen-bit will override any
 183         // potential kill introduced here.
 184         if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
 185             trans.kill_all(indices);
 186         }
 187     }
 188
 189     /// Kill any borrows that conflict with `place`.
 190     fn kill_borrows_on_place(
 191         &self,
 192         trans: &mut GenKillSet<BorrowIndex>,
 193         place: &Place<'tcx>
 194     ) {
 195         debug!("kill_borrows_on_place: place={:?}", place);
 196
 197         if let PlaceBase::Local(local) = place.base {
 198             let other_borrows_of_local = self
 199                 .borrow_set
 200                 .local_map
 201                 .get(&local)
 202                 .into_iter()
 203                 .flat_map(|bs| bs.into_iter());
 204
 205             // If the borrowed place is a local with no projections, all other borrows of this
 206             // local must conflict. This is purely an optimization so we don't have to call
 207             // `places_conflict` for every borrow.
 208             if place.projection.is_none() {
 209                 trans.kill_all(other_borrows_of_local);
 210                 return;
 211             }
 212
 213             // By passing `PlaceConflictBias::NoOverlap`, we conservatively assume that any given
 214             // pair of array indices are unequal, so that when `places_conflict` returns true, we
 215             // will be assured that two places being compared definitely denotes the same sets of
 216             // locations.
 217             let definitely_conflicting_borrows = other_borrows_of_local
 218                 .filter(|&&i| {
 219                     places_conflict::places_conflict(
 220                         self.tcx,
 221                         self.body,
 222                         &self.borrow_set.borrows[i].borrowed_place,
 223                         place,
 224                         places_conflict::PlaceConflictBias::NoOverlap)
 225                 });
 226
 227             trans.kill_all(definitely_conflicting_borrows);
 228         }
 229     }
 230 }
 231
 232 impl<'a, 'tcx> BitDenotation<'tcx> for Borrows<'a, 'tcx> {
 233     type Idx = BorrowIndex;
 234     fn name() -> &'static str { "borrows" }
 235     fn bits_per_block(&self) -> usize {
 236         self.borrow_set.borrows.len() * 2
 237     }
 238
 239     fn start_block_effect(&self, _entry_set: &mut BitSet<Self::Idx>) {
 240         // no borrows of code region_scopes have been taken prior to
 241         // function execution, so this method has no effect.
 242     }
 243
 244     fn before_statement_effect(&self,
 245                                trans: &mut GenKillSet<Self::Idx>,
 246                                location: Location) {
 247         debug!("Borrows::before_statement_effect trans: {:?} location: {:?}",
 248                trans, location);
 249         self.kill_loans_out_of_scope_at_location(trans, location);
 250     }
 251
 252     fn statement_effect(&self,
 253                         trans: &mut GenKillSet<Self::Idx>,
 254                         location: Location) {
 255         debug!("Borrows::statement_effect: trans={:?} location={:?}",
 256                trans, location);
 257
 258         let block = &self.body.basic_blocks().get(location.block).unwrap_or_else(|| {
 259             panic!("could not find block at location {:?}", location);
 260         });
 261         let stmt = block.statements.get(location.statement_index).unwrap_or_else(|| {
 262             panic!("could not find statement at location {:?}");
 263         });
 264
 265         debug!("Borrows::statement_effect: stmt={:?}", stmt);
 266         match stmt.kind {
 267             mir::StatementKind::Assign(ref lhs, ref rhs) => {
 268                 // Make sure there are no remaining borrows for variables
 269                 // that are assigned over.
 270                 self.kill_borrows_on_place(trans, lhs);
 271
 272                 if let mir::Rvalue::Ref(_, _, ref place) = **rhs {
 273                     if place.ignore_borrow(
 274                         self.tcx,
 275                         self.body,
 276                         &self.borrow_set.locals_state_at_exit,
 277                     ) {
 278                         return;
 279                     }
 280                     let index = self.borrow_set.location_map.get(&location).unwrap_or_else(|| {
 281                         panic!("could not find BorrowIndex for location {:?}", location);
 282                     });
 283
 284                     trans.gen(*index);
 285                 }
 286             }
 287
 288             mir::StatementKind::StorageDead(local) => {
 289                 // Make sure there are no remaining borrows for locals that
 290                 // are gone out of scope.
 291                 self.kill_borrows_on_place(trans, &Place::from(local));
 292             }
 293
 294             mir::StatementKind::InlineAsm(ref asm) => {
 295                 for (output, kind) in asm.outputs.iter().zip(&asm.asm.outputs) {
 296                     if !kind.is_indirect && !kind.is_rw {
 297                         self.kill_borrows_on_place(trans, output);
 298                     }
 299                 }
 300             }
 301
 302             mir::StatementKind::FakeRead(..) |
 303             mir::StatementKind::SetDiscriminant { .. } |
 304             mir::StatementKind::StorageLive(..) |
 305             mir::StatementKind::Retag { .. } |
 306             mir::StatementKind::AscribeUserType(..) |
 307             mir::StatementKind::Nop => {}
 308
 309         }
 310     }
 311
 312     fn before_terminator_effect(&self,
 313                                 trans: &mut GenKillSet<Self::Idx>,
 314                                 location: Location) {
 315         debug!("Borrows::before_terminator_effect: trans={:?} location={:?}",
 316                trans, location);
 317         self.kill_loans_out_of_scope_at_location(trans, location);
 318     }
 319
 320     fn terminator_effect(&self,
 321                          _: &mut GenKillSet<Self::Idx>,
 322                          _: Location) {}
 323
 324     fn propagate_call_return(
 325         &self,
 326         _in_out: &mut BitSet<BorrowIndex>,
 327         _call_bb: mir::BasicBlock,
 328         _dest_bb: mir::BasicBlock,
 329         _dest_place: &mir::Place<'tcx>,
 330     ) {
 331     }
 332 }
 333
 334 impl<'a, 'tcx> BottomValue for Borrows<'a, 'tcx> {
 335     /// bottom = nothing is reserved or activated yet;
 336     const BOTTOM_VALUE: bool = false;
 337 }