compiler/rustc_mir/src/dataflow/impls/borrows.rs

   1 use rustc_middle::mir::{self, Body, Location, Place};
   2 use rustc_middle::ty::RegionVid;
   3 use rustc_middle::ty::TyCtxt;
   4
   5 use rustc_data_structures::fx::FxHashMap;
   6 use rustc_index::bit_set::BitSet;
   7
   8 use crate::borrow_check::{
   9     places_conflict, BorrowSet, PlaceConflictBias, PlaceExt, RegionInferenceContext, ToRegionVid,
  10 };
  11 use crate::dataflow::{self, fmt::DebugWithContext, GenKill};
  12
  13 use std::fmt;
  14
  15 rustc_index::newtype_index! {
  16     pub struct BorrowIndex {
  17         DEBUG_FORMAT = "bw{}"
  18     }
  19 }
  20
  21 /// `Borrows` stores the data used in the analyses that track the flow
  22 /// of borrows.
  23 ///
  24 /// It uniquely identifies every borrow (`Rvalue::Ref`) by a
  25 /// `BorrowIndex`, and maps each such index to a `BorrowData`
  26 /// describing the borrow. These indexes are used for representing the
  27 /// borrows in compact bitvectors.
  28 pub struct Borrows<'a, 'tcx> {
  29     tcx: TyCtxt<'tcx>,
  30     body: &'a Body<'tcx>,
  31
  32     borrow_set: &'a BorrowSet<'tcx>,
  33     borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
  34 }
  35
  36 struct StackEntry {
  37     bb: mir::BasicBlock,
  38     lo: usize,
  39     hi: usize,
  40 }
  41
  42 struct OutOfScopePrecomputer<'a, 'tcx> {
  43     visited: BitSet<mir::BasicBlock>,
  44     visit_stack: Vec<StackEntry>,
  45     body: &'a Body<'tcx>,
  46     regioncx: &'a RegionInferenceContext<'tcx>,
  47     borrows_out_of_scope_at_location: FxHashMap<Location, Vec<BorrowIndex>>,
  48 }
  49
  50 impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
  51     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
  52         OutOfScopePrecomputer {
  53             visited: BitSet::new_empty(body.basic_blocks().len()),
  54             visit_stack: vec![],
  55             body,
  56             regioncx,
  57             borrows_out_of_scope_at_location: FxHashMap::default(),
  58         }
  59     }
  60 }
  61
  62 impl<'tcx> OutOfScopePrecomputer<'_, 'tcx> {
  63     fn precompute_borrows_out_of_scope(
  64         &mut self,
  65         borrow_index: BorrowIndex,
  66         borrow_region: RegionVid,
  67         location: Location,
  68     ) {
  69         // We visit one BB at a time. The complication is that we may start in the
  70         // middle of the first BB visited (the one containing `location`), in which
  71         // case we may have to later on process the first part of that BB if there
  72         // is a path back to its start.
  73
  74         // For visited BBs, we record the index of the first statement processed.
  75         // (In fully processed BBs this index is 0.) Note also that we add BBs to
  76         // `visited` once they are added to `stack`, before they are actually
  77         // processed, because this avoids the need to look them up again on
  78         // completion.
  79         self.visited.insert(location.block);
  80
  81         let mut first_lo = location.statement_index;
  82         let first_hi = self.body[location.block].statements.len();
  83
  84         self.visit_stack.push(StackEntry { bb: location.block, lo: first_lo, hi: first_hi });
  85
  86         while let Some(StackEntry { bb, lo, hi }) = self.visit_stack.pop() {
  87             // If we process the first part of the first basic block (i.e. we encounter that block
  88             // for the second time), we no longer have to visit its successors again.
  89             let mut finished_early = bb == location.block && hi != first_hi;
  90             for i in lo..=hi {
  91                 let location = Location { block: bb, statement_index: i };
  92                 // If region does not contain a point at the location, then add to list and skip
  93                 // successor locations.
  94                 if !self.regioncx.region_contains(borrow_region, location) {
  95                     debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
  96                     self.borrows_out_of_scope_at_location
  97                         .entry(location)
  98                         .or_default()
  99                         .push(borrow_index);
 100                     finished_early = true;
 101                     break;
 102                 }
 103             }
 104
 105             if !finished_early {
 106                 // Add successor BBs to the work list, if necessary.
 107                 let bb_data = &self.body[bb];
 108                 debug_assert!(hi == bb_data.statements.len());
 109                 for &succ_bb in bb_data.terminator().successors() {
 110                     if self.visited.insert(succ_bb) == false {
 111                         if succ_bb == location.block && first_lo > 0 {
 112                             // `succ_bb` has been seen before. If it wasn't
 113                             // fully processed, add its first part to `stack`
 114                             // for processing.
 115                             self.visit_stack.push(StackEntry {
 116                                 bb: succ_bb,
 117                                 lo: 0,
 118                                 hi: first_lo - 1,
 119                             });
 120
 121                             // And update this entry with 0, to represent the
 122                             // whole BB being processed.
 123                             first_lo = 0;
 124                         }
 125                     } else {
 126                         // succ_bb hasn't been seen before. Add it to
 127                         // `stack` for processing.
 128                         self.visit_stack.push(StackEntry {
 129                             bb: succ_bb,
 130                             lo: 0,
 131                             hi: self.body[succ_bb].statements.len(),
 132                         });
 133                     }
 134                 }
 135             }
 136         }
 137
 138         self.visited.clear();
 139     }
 140 }
 141
 142 impl<'a, 'tcx> Borrows<'a, 'tcx> {
 143     crate fn new(
 144         tcx: TyCtxt<'tcx>,
 145         body: &'a Body<'tcx>,
 146         nonlexical_regioncx: &'a RegionInferenceContext<'tcx>,
 147         borrow_set: &'a BorrowSet<'tcx>,
 148     ) -> Self {
 149         let mut prec = OutOfScopePrecomputer::new(body, nonlexical_regioncx);
 150         for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
 151             let borrow_region = borrow_data.region.to_region_vid();
 152             let location = borrow_data.reserve_location;
 153
 154             prec.precompute_borrows_out_of_scope(borrow_index, borrow_region, location);
 155         }
 156
 157         Borrows {
 158             tcx,
 159             body,
 160             borrow_set,
 161             borrows_out_of_scope_at_location: prec.borrows_out_of_scope_at_location,
 162         }
 163     }
 164
 165     pub fn location(&self, idx: BorrowIndex) -> &Location {
 166         &self.borrow_set[idx].reserve_location
 167     }
 168
 169     /// Add all borrows to the kill set, if those borrows are out of scope at `location`.
 170     /// That means they went out of a nonlexical scope
 171     fn kill_loans_out_of_scope_at_location(
 172         &self,
 173         trans: &mut impl GenKill<BorrowIndex>,
 174         location: Location,
 175     ) {
 176         // NOTE: The state associated with a given `location`
 177         // reflects the dataflow on entry to the statement.
 178         // Iterate over each of the borrows that we've precomputed
 179         // to have went out of scope at this location and kill them.
 180         //
 181         // We are careful always to call this function *before* we
 182         // set up the gen-bits for the statement or
 183         // terminator. That way, if the effect of the statement or
 184         // terminator *does* introduce a new loan of the same
 185         // region, then setting that gen-bit will override any
 186         // potential kill introduced here.
 187         if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
 188             trans.kill_all(indices.iter().copied());
 189         }
 190     }
 191
 192     /// Kill any borrows that conflict with `place`.
 193     fn kill_borrows_on_place(&self, trans: &mut impl GenKill<BorrowIndex>, place: Place<'tcx>) {
 194         debug!("kill_borrows_on_place: place={:?}", place);
 195
 196         let other_borrows_of_local = self
 197             .borrow_set
 198             .local_map
 199             .get(&place.local)
 200             .into_iter()
 201             .flat_map(|bs| bs.iter())
 202             .copied();
 203
 204         // If the borrowed place is a local with no projections, all other borrows of this
 205         // local must conflict. This is purely an optimization so we don't have to call
 206         // `places_conflict` for every borrow.
 207         if place.projection.is_empty() {
 208             if !self.body.local_decls[place.local].is_ref_to_static() {
 209                 trans.kill_all(other_borrows_of_local);
 210             }
 211             return;
 212         }
 213
 214         // By passing `PlaceConflictBias::NoOverlap`, we conservatively assume that any given
 215         // pair of array indices are unequal, so that when `places_conflict` returns true, we
 216         // will be assured that two places being compared definitely denotes the same sets of
 217         // locations.
 218         let definitely_conflicting_borrows = other_borrows_of_local.filter(|&i| {
 219             places_conflict(
 220                 self.tcx,
 221                 self.body,
 222                 self.borrow_set[i].borrowed_place,
 223                 place,
 224                 PlaceConflictBias::NoOverlap,
 225             )
 226         });
 227
 228         trans.kill_all(definitely_conflicting_borrows);
 229     }
 230 }
 231
 232 impl<'tcx> dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
 233     type Domain = BitSet<BorrowIndex>;
 234
 235     const NAME: &'static str = "borrows";
 236
 237     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
 238         // bottom = nothing is reserved or activated yet;
 239         BitSet::new_empty(self.borrow_set.len() * 2)
 240     }
 241
 242     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
 243         // no borrows of code region_scopes have been taken prior to
 244         // function execution, so this method has no effect.
 245     }
 246 }
 247
 248 impl<'tcx> dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
 249     type Idx = BorrowIndex;
 250
 251     fn before_statement_effect(
 252         &self,
 253         trans: &mut impl GenKill<Self::Idx>,
 254         _statement: &mir::Statement<'tcx>,
 255         location: Location,
 256     ) {
 257         self.kill_loans_out_of_scope_at_location(trans, location);
 258     }
 259
 260     fn statement_effect(
 261         &self,
 262         trans: &mut impl GenKill<Self::Idx>,
 263         stmt: &mir::Statement<'tcx>,
 264         location: Location,
 265     ) {
 266         match stmt.kind {
 267             mir::StatementKind::Assign(box (lhs, ref rhs)) => {
 268                 if let mir::Rvalue::Ref(_, _, place) = *rhs {
 269                     if place.ignore_borrow(
 270                         self.tcx,
 271                         self.body,
 272                         &self.borrow_set.locals_state_at_exit,
 273                     ) {
 274                         return;
 275                     }
 276                     let index = self.borrow_set.get_index_of(&location).unwrap_or_else(|| {
 277                         panic!("could not find BorrowIndex for location {:?}", location);
 278                     });
 279
 280                     trans.gen(index);
 281                 }
 282
 283                 // Make sure there are no remaining borrows for variables
 284                 // that are assigned over.
 285                 self.kill_borrows_on_place(trans, lhs);
 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::LlvmInlineAsm(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::Coverage(..)
 308             | mir::StatementKind::CopyNonOverlapping(..)
 309             | mir::StatementKind::Nop => {}
 310         }
 311     }
 312
 313     fn before_terminator_effect(
 314         &self,
 315         trans: &mut impl GenKill<Self::Idx>,
 316         _terminator: &mir::Terminator<'tcx>,
 317         location: Location,
 318     ) {
 319         self.kill_loans_out_of_scope_at_location(trans, location);
 320     }
 321
 322     fn terminator_effect(
 323         &self,
 324         trans: &mut impl GenKill<Self::Idx>,
 325         teminator: &mir::Terminator<'tcx>,
 326         _location: Location,
 327     ) {
 328         if let mir::TerminatorKind::InlineAsm { operands, .. } = &teminator.kind {
 329             for op in operands {
 330                 if let mir::InlineAsmOperand::Out { place: Some(place), .. }
 331                 | mir::InlineAsmOperand::InOut { out_place: Some(place), .. } = *op
 332                 {
 333                     self.kill_borrows_on_place(trans, place);
 334                 }
 335             }
 336         }
 337     }
 338
 339     fn call_return_effect(
 340         &self,
 341         _trans: &mut impl GenKill<Self::Idx>,
 342         _block: mir::BasicBlock,
 343         _func: &mir::Operand<'tcx>,
 344         _args: &[mir::Operand<'tcx>],
 345         _dest_place: mir::Place<'tcx>,
 346     ) {
 347     }
 348 }
 349
 350 impl DebugWithContext<Borrows<'_, '_>> for BorrowIndex {
 351     fn fmt_with(&self, ctxt: &Borrows<'_, '_>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 352         write!(f, "{:?}", ctxt.location(*self))
 353     }
 354 }