src/librustc_mir/transform/promote_consts.rs

   1 //! A pass that promotes borrows of constant rvalues.
   2 //!
   3 //! The rvalues considered constant are trees of temps,
   4 //! each with exactly one initialization, and holding
   5 //! a constant value with no interior mutability.
   6 //! They are placed into a new MIR constant body in
   7 //! `promoted` and the borrow rvalue is replaced with
   8 //! a `Literal::Promoted` using the index into `promoted`
   9 //! of that constant MIR.
  10 //!
  11 //! This pass assumes that every use is dominated by an
  12 //! initialization and can otherwise silence errors, if
  13 //! move analysis runs after promotion on broken MIR.
  14
  15 use rustc::mir::*;
  16 use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor};
  17 use rustc::mir::traversal::ReversePostorder;
  18 use rustc::ty::TyCtxt;
  19 use syntax_pos::Span;
  20
  21 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
  22
  23 use std::{iter, mem, usize};
  24
  25 /// State of a temporary during collection and promotion.
  26 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  27 pub enum TempState {
  28     /// No references to this temp.
  29     Undefined,
  30     /// One direct assignment and any number of direct uses.
  31     /// A borrow of this temp is promotable if the assigned
  32     /// value is qualified as constant.
  33     Defined {
  34         location: Location,
  35         uses: usize
  36     },
  37     /// Any other combination of assignments/uses.
  38     Unpromotable,
  39     /// This temp was part of an rvalue which got extracted
  40     /// during promotion and needs cleanup.
  41     PromotedOut
  42 }
  43
  44 impl TempState {
  45     pub fn is_promotable(&self) -> bool {
  46         debug!("is_promotable: self={:?}", self);
  47         if let TempState::Defined { .. } = *self {
  48             true
  49         } else {
  50             false
  51         }
  52     }
  53 }
  54
  55 /// A "root candidate" for promotion, which will become the
  56 /// returned value in a promoted MIR, unless it's a subset
  57 /// of a larger candidate.
  58 #[derive(Debug)]
  59 pub enum Candidate {
  60     /// Borrow of a constant temporary.
  61     Ref(Location),
  62
  63     /// Currently applied to function calls where the callee has the unstable
  64     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
  65     /// intrinsic. The intrinsic requires the arguments are indeed constant and
  66     /// the attribute currently provides the semantic requirement that arguments
  67     /// must be constant.
  68     Argument { bb: BasicBlock, index: usize },
  69 }
  70
  71 struct TempCollector<'tcx> {
  72     temps: IndexVec<Local, TempState>,
  73     span: Span,
  74     body: &'tcx Body<'tcx>,
  75 }
  76
  77 impl<'tcx> Visitor<'tcx> for TempCollector<'tcx> {
  78     fn visit_local(&mut self,
  79                    &index: &Local,
  80                    context: PlaceContext,
  81                    location: Location) {
  82         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
  83         // We're only interested in temporaries and the return place
  84         match self.body.local_kind(index) {
  85             | LocalKind::Temp
  86             | LocalKind::ReturnPointer
  87             => {},
  88             | LocalKind::Arg
  89             | LocalKind::Var
  90             => return,
  91         }
  92
  93         // Ignore drops, if the temp gets promoted,
  94         // then it's constant and thus drop is noop.
  95         // Non-uses are also irrelevent.
  96         if context.is_drop() || !context.is_use() {
  97             debug!(
  98                 "visit_local: context.is_drop={:?} context.is_use={:?}",
  99                 context.is_drop(), context.is_use(),
 100             );
 101             return;
 102         }
 103
 104         let temp = &mut self.temps[index];
 105         debug!("visit_local: temp={:?}", temp);
 106         if *temp == TempState::Undefined {
 107             match context {
 108                 PlaceContext::MutatingUse(MutatingUseContext::Store) |
 109                 PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 110                     *temp = TempState::Defined {
 111                         location,
 112                         uses: 0
 113                     };
 114                     return;
 115                 }
 116                 _ => { /* mark as unpromotable below */ }
 117             }
 118         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 119             // We always allow borrows, even mutable ones, as we need
 120             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 121             let allowed_use = context.is_borrow() || context.is_nonmutating_use();
 122             debug!("visit_local: allowed_use={:?}", allowed_use);
 123             if allowed_use {
 124                 *uses += 1;
 125                 return;
 126             }
 127             /* mark as unpromotable below */
 128         }
 129         *temp = TempState::Unpromotable;
 130     }
 131
 132     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 133         self.span = source_info.span;
 134     }
 135 }
 136
 137 pub fn collect_temps(body: &Body<'_>,
 138                      rpo: &mut ReversePostorder<'_, '_>) -> IndexVec<Local, TempState> {
 139     let mut collector = TempCollector {
 140         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 141         span: body.span,
 142         body,
 143     };
 144     for (bb, data) in rpo {
 145         collector.visit_basic_block_data(bb, data);
 146     }
 147     collector.temps
 148 }
 149
 150 struct Promoter<'a, 'tcx: 'a> {
 151     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 152     source: &'a mut Body<'tcx>,
 153     promoted: Body<'tcx>,
 154     temps: &'a mut IndexVec<Local, TempState>,
 155
 156     /// If true, all nested temps are also kept in the
 157     /// source MIR, not moved to the promoted MIR.
 158     keep_original: bool
 159 }
 160
 161 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 162     fn new_block(&mut self) -> BasicBlock {
 163         let span = self.promoted.span;
 164         self.promoted.basic_blocks_mut().push(BasicBlockData {
 165             statements: vec![],
 166             terminator: Some(Terminator {
 167                 source_info: SourceInfo {
 168                     span,
 169                     scope: OUTERMOST_SOURCE_SCOPE
 170                 },
 171                 kind: TerminatorKind::Return
 172             }),
 173             is_cleanup: false
 174         })
 175     }
 176
 177     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 178         let last = self.promoted.basic_blocks().last().unwrap();
 179         let data = &mut self.promoted[last];
 180         data.statements.push(Statement {
 181             source_info: SourceInfo {
 182                 span,
 183                 scope: OUTERMOST_SOURCE_SCOPE
 184             },
 185             kind: StatementKind::Assign(Place::Base(PlaceBase::Local(dest)), box rvalue)
 186         });
 187     }
 188
 189     /// Copies the initialization of this temp to the
 190     /// promoted MIR, recursing through temps.
 191     fn promote_temp(&mut self, temp: Local) -> Local {
 192         let old_keep_original = self.keep_original;
 193         let loc = match self.temps[temp] {
 194             TempState::Defined { location, uses } if uses > 0 => {
 195                 if uses > 1 {
 196                     self.keep_original = true;
 197                 }
 198                 location
 199             }
 200             state =>  {
 201                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
 202                           temp, state);
 203             }
 204         };
 205         if !self.keep_original {
 206             self.temps[temp] = TempState::PromotedOut;
 207         }
 208
 209         let no_stmts = self.source[loc.block].statements.len();
 210         let new_temp = self.promoted.local_decls.push(
 211             LocalDecl::new_temp(self.source.local_decls[temp].ty,
 212                                 self.source.local_decls[temp].source_info.span));
 213
 214         debug!("promote({:?} @ {:?}/{:?}, {:?})",
 215                temp, loc, no_stmts, self.keep_original);
 216
 217         // First, take the Rvalue or Call out of the source MIR,
 218         // or duplicate it, depending on keep_original.
 219         if loc.statement_index < no_stmts {
 220             let (rvalue, source_info) = {
 221                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 222                 let rhs = match statement.kind {
 223                     StatementKind::Assign(_, ref mut rhs) => rhs,
 224                     _ => {
 225                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 226                                   statement);
 227                     }
 228                 };
 229
 230                 (if self.keep_original {
 231                     rhs.clone()
 232                 } else {
 233                     let unit = box Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 234                     mem::replace(rhs, unit)
 235                 }, statement.source_info)
 236             };
 237
 238             let mut rvalue = *rvalue;
 239             self.visit_rvalue(&mut rvalue, loc);
 240             self.assign(new_temp, rvalue, source_info.span);
 241         } else {
 242             let terminator = if self.keep_original {
 243                 self.source[loc.block].terminator().clone()
 244             } else {
 245                 let terminator = self.source[loc.block].terminator_mut();
 246                 let target = match terminator.kind {
 247                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 248                     ref kind => {
 249                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 250                     }
 251                 };
 252                 Terminator {
 253                     source_info: terminator.source_info,
 254                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
 255                         target,
 256                     })
 257                 }
 258             };
 259
 260             match terminator.kind {
 261                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 262                     self.visit_operand(&mut func, loc);
 263                     for arg in &mut args {
 264                         self.visit_operand(arg, loc);
 265                     }
 266
 267                     let last = self.promoted.basic_blocks().last().unwrap();
 268                     let new_target = self.new_block();
 269
 270                     *self.promoted[last].terminator_mut() = Terminator {
 271                         kind: TerminatorKind::Call {
 272                             func,
 273                             args,
 274                             cleanup: None,
 275                             destination: Some(
 276                                 (Place::Base(PlaceBase::Local(new_temp)), new_target)
 277                             ),
 278                             from_hir_call,
 279                         },
 280                         ..terminator
 281                     };
 282                 }
 283                 ref kind => {
 284                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 285                 }
 286             };
 287         };
 288
 289         self.keep_original = old_keep_original;
 290         new_temp
 291     }
 292
 293     fn promote_candidate(mut self, candidate: Candidate) {
 294         let mut operand = {
 295             let promoted = &mut self.promoted;
 296             let promoted_id = Promoted::new(self.source.promoted.len());
 297             let mut promoted_place = |ty, span| {
 298                 promoted.span = span;
 299                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 300                 Place::Base(
 301                     PlaceBase::Static(box Static{ kind: StaticKind::Promoted(promoted_id), ty })
 302                 )
 303             };
 304             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 305             match candidate {
 306                 Candidate::Ref(loc) => {
 307                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 308                     match statement.kind {
 309                         StatementKind::Assign(_, box Rvalue::Ref(_, _, ref mut place)) => {
 310                             // Find the underlying local for this (necessarily interior) borrow.
 311                             let mut place = place;
 312                             while let Place::Projection(ref mut proj) = *place {
 313                                 assert_ne!(proj.elem, ProjectionElem::Deref);
 314                                 place = &mut proj.base;
 315                             };
 316
 317                             let ty = place.ty(local_decls, self.tcx).ty;
 318                             let span = statement.source_info.span;
 319
 320                             Operand::Move(mem::replace(place, promoted_place(ty, span)))
 321                         }
 322                         _ => bug!()
 323                     }
 324                 }
 325                 Candidate::Argument { bb, index } => {
 326                     let terminator = blocks[bb].terminator_mut();
 327                     match terminator.kind {
 328                         TerminatorKind::Call { ref mut args, .. } => {
 329                             let ty = args[index].ty(local_decls, self.tcx);
 330                             let span = terminator.source_info.span;
 331                             let operand = Operand::Copy(promoted_place(ty, span));
 332                             mem::replace(&mut args[index], operand)
 333                         }
 334                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 335                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 336                         // we are seeing a `Goto`. That means that the `promote_temps` method
 337                         // already promoted this call away entirely. This case occurs when calling
 338                         // a function requiring a constant argument and as that constant value
 339                         // providing a value whose computation contains another call to a function
 340                         // requiring a constant argument.
 341                         TerminatorKind::Goto { .. } => return,
 342                         _ => bug!()
 343                     }
 344                 }
 345             }
 346         };
 347
 348         assert_eq!(self.new_block(), START_BLOCK);
 349         self.visit_operand(&mut operand, Location {
 350             block: BasicBlock::new(0),
 351             statement_index: usize::MAX
 352         });
 353
 354         let span = self.promoted.span;
 355         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
 356         self.source.promoted.push(self.promoted);
 357     }
 358 }
 359
 360 /// Replaces all temporaries with their promoted counterparts.
 361 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 362     fn visit_local(&mut self,
 363                    local: &mut Local,
 364                    _: PlaceContext,
 365                    _: Location) {
 366         if self.source.local_kind(*local) == LocalKind::Temp {
 367             *local = self.promote_temp(*local);
 368         }
 369     }
 370 }
 371
 372 pub fn promote_candidates<'a, 'tcx>(body: &mut Body<'tcx>,
 373                                     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 374                                     mut temps: IndexVec<Local, TempState>,
 375                                     candidates: Vec<Candidate>) {
 376     // Visit candidates in reverse, in case they're nested.
 377     debug!("promote_candidates({:?})", candidates);
 378
 379     for candidate in candidates.into_iter().rev() {
 380         match candidate {
 381             Candidate::Ref(Location { block, statement_index }) => {
 382                 match body[block].statements[statement_index].kind {
 383                     StatementKind::Assign(Place::Base(PlaceBase::Local(local)), _) => {
 384                         if temps[local] == TempState::PromotedOut {
 385                             // Already promoted.
 386                             continue;
 387                         }
 388                     }
 389                     _ => {}
 390                 }
 391             }
 392             Candidate::Argument { .. } => {}
 393         }
 394
 395
 396         // Declare return place local so that `mir::Body::new` doesn't complain.
 397         let initial_locals = iter::once(
 398             LocalDecl::new_return_place(tcx.types.never, body.span)
 399         ).collect();
 400
 401         let promoter = Promoter {
 402             promoted: Body::new(
 403                 IndexVec::new(),
 404                 // FIXME: maybe try to filter this to avoid blowing up
 405                 // memory usage?
 406                 body.source_scopes.clone(),
 407                 body.source_scope_local_data.clone(),
 408                 IndexVec::new(),
 409                 None,
 410                 initial_locals,
 411                 IndexVec::new(),
 412                 0,
 413                 vec![],
 414                 body.span,
 415                 vec![],
 416             ),
 417             tcx,
 418             source: body,
 419             temps: &mut temps,
 420             keep_original: false
 421         };
 422         promoter.promote_candidate(candidate);
 423     }
 424
 425     // Eliminate assignments to, and drops of promoted temps.
 426     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
 427     for block in body.basic_blocks_mut() {
 428         block.statements.retain(|statement| {
 429             match statement.kind {
 430                 StatementKind::Assign(Place::Base(PlaceBase::Local(index)), _) |
 431                 StatementKind::StorageLive(index) |
 432                 StatementKind::StorageDead(index) => {
 433                     !promoted(index)
 434                 }
 435                 _ => true
 436             }
 437         });
 438         let terminator = block.terminator_mut();
 439         match terminator.kind {
 440             TerminatorKind::Drop { location: Place::Base(PlaceBase::Local(index)), target, .. } => {
 441                 if promoted(index) {
 442                     terminator.kind = TerminatorKind::Goto {
 443                         target,
 444                     };
 445                 }
 446             }
 447             _ => {}
 448         }
 449     }
 450 }