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::hir::def_id::DefId;
  16 use rustc::mir::*;
  17 use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor};
  18 use rustc::mir::traversal::ReversePostorder;
  19 use rustc::ty::subst::InternalSubsts;
  20 use rustc::ty::TyCtxt;
  21 use syntax_pos::Span;
  22
  23 use rustc_index::vec::{IndexVec, Idx};
  24
  25 use std::{iter, mem, usize};
  26
  27 /// State of a temporary during collection and promotion.
  28 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  29 pub enum TempState {
  30     /// No references to this temp.
  31     Undefined,
  32     /// One direct assignment and any number of direct uses.
  33     /// A borrow of this temp is promotable if the assigned
  34     /// value is qualified as constant.
  35     Defined {
  36         location: Location,
  37         uses: usize
  38     },
  39     /// Any other combination of assignments/uses.
  40     Unpromotable,
  41     /// This temp was part of an rvalue which got extracted
  42     /// during promotion and needs cleanup.
  43     PromotedOut
  44 }
  45
  46 impl TempState {
  47     pub fn is_promotable(&self) -> bool {
  48         debug!("is_promotable: self={:?}", self);
  49         if let TempState::Defined { .. } = *self {
  50             true
  51         } else {
  52             false
  53         }
  54     }
  55 }
  56
  57 /// A "root candidate" for promotion, which will become the
  58 /// returned value in a promoted MIR, unless it's a subset
  59 /// of a larger candidate.
  60 #[derive(Debug)]
  61 pub enum Candidate {
  62     /// Borrow of a constant temporary.
  63     Ref(Location),
  64
  65     /// Promotion of the `x` in `[x; 32]`.
  66     Repeat(Location),
  67
  68     /// Currently applied to function calls where the callee has the unstable
  69     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
  70     /// intrinsic. The intrinsic requires the arguments are indeed constant and
  71     /// the attribute currently provides the semantic requirement that arguments
  72     /// must be constant.
  73     Argument { bb: BasicBlock, index: usize },
  74 }
  75
  76 struct TempCollector<'tcx> {
  77     temps: IndexVec<Local, TempState>,
  78     span: Span,
  79     body: &'tcx Body<'tcx>,
  80 }
  81
  82 impl<'tcx> Visitor<'tcx> for TempCollector<'tcx> {
  83     fn visit_local(&mut self,
  84                    &index: &Local,
  85                    context: PlaceContext,
  86                    location: Location) {
  87         debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
  88         // We're only interested in temporaries and the return place
  89         match self.body.local_kind(index) {
  90             | LocalKind::Temp
  91             | LocalKind::ReturnPointer
  92             => {},
  93             | LocalKind::Arg
  94             | LocalKind::Var
  95             => return,
  96         }
  97
  98         // Ignore drops, if the temp gets promoted,
  99         // then it's constant and thus drop is noop.
 100         // Non-uses are also irrelevent.
 101         if context.is_drop() || !context.is_use() {
 102             debug!(
 103                 "visit_local: context.is_drop={:?} context.is_use={:?}",
 104                 context.is_drop(), context.is_use(),
 105             );
 106             return;
 107         }
 108
 109         let temp = &mut self.temps[index];
 110         debug!("visit_local: temp={:?}", temp);
 111         if *temp == TempState::Undefined {
 112             match context {
 113                 PlaceContext::MutatingUse(MutatingUseContext::Store) |
 114                 PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 115                     *temp = TempState::Defined {
 116                         location,
 117                         uses: 0
 118                     };
 119                     return;
 120                 }
 121                 _ => { /* mark as unpromotable below */ }
 122             }
 123         } else if let TempState::Defined { ref mut uses, .. } = *temp {
 124             // We always allow borrows, even mutable ones, as we need
 125             // to promote mutable borrows of some ZSTs e.g., `&mut []`.
 126             let allowed_use = context.is_borrow() || context.is_nonmutating_use();
 127             debug!("visit_local: allowed_use={:?}", allowed_use);
 128             if allowed_use {
 129                 *uses += 1;
 130                 return;
 131             }
 132             /* mark as unpromotable below */
 133         }
 134         *temp = TempState::Unpromotable;
 135     }
 136
 137     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 138         self.span = source_info.span;
 139     }
 140 }
 141
 142 pub fn collect_temps(body: &Body<'_>,
 143                      rpo: &mut ReversePostorder<'_, '_>) -> IndexVec<Local, TempState> {
 144     let mut collector = TempCollector {
 145         temps: IndexVec::from_elem(TempState::Undefined, &body.local_decls),
 146         span: body.span,
 147         body,
 148     };
 149     for (bb, data) in rpo {
 150         collector.visit_basic_block_data(bb, data);
 151     }
 152     collector.temps
 153 }
 154
 155 struct Promoter<'a, 'tcx> {
 156     tcx: TyCtxt<'tcx>,
 157     source: &'a mut Body<'tcx>,
 158     promoted: Body<'tcx>,
 159     temps: &'a mut IndexVec<Local, TempState>,
 160
 161     /// If true, all nested temps are also kept in the
 162     /// source MIR, not moved to the promoted MIR.
 163     keep_original: bool,
 164 }
 165
 166 impl<'a, 'tcx> Promoter<'a, 'tcx> {
 167     fn new_block(&mut self) -> BasicBlock {
 168         let span = self.promoted.span;
 169         self.promoted.basic_blocks_mut().push(BasicBlockData {
 170             statements: vec![],
 171             terminator: Some(Terminator {
 172                 source_info: SourceInfo {
 173                     span,
 174                     scope: OUTERMOST_SOURCE_SCOPE
 175                 },
 176                 kind: TerminatorKind::Return
 177             }),
 178             is_cleanup: false
 179         })
 180     }
 181
 182     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
 183         let last = self.promoted.basic_blocks().last().unwrap();
 184         let data = &mut self.promoted[last];
 185         data.statements.push(Statement {
 186             source_info: SourceInfo {
 187                 span,
 188                 scope: OUTERMOST_SOURCE_SCOPE
 189             },
 190             kind: StatementKind::Assign(box(Place::from(dest), rvalue))
 191         });
 192     }
 193
 194     /// Copies the initialization of this temp to the
 195     /// promoted MIR, recursing through temps.
 196     fn promote_temp(&mut self, temp: Local) -> Local {
 197         let old_keep_original = self.keep_original;
 198         let loc = match self.temps[temp] {
 199             TempState::Defined { location, uses } if uses > 0 => {
 200                 if uses > 1 {
 201                     self.keep_original = true;
 202                 }
 203                 location
 204             }
 205             state =>  {
 206                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
 207                           temp, state);
 208             }
 209         };
 210         if !self.keep_original {
 211             self.temps[temp] = TempState::PromotedOut;
 212         }
 213
 214         let no_stmts = self.source[loc.block].statements.len();
 215         let new_temp = self.promoted.local_decls.push(
 216             LocalDecl::new_temp(self.source.local_decls[temp].ty,
 217                                 self.source.local_decls[temp].source_info.span));
 218
 219         debug!("promote({:?} @ {:?}/{:?}, {:?})",
 220                temp, loc, no_stmts, self.keep_original);
 221
 222         // First, take the Rvalue or Call out of the source MIR,
 223         // or duplicate it, depending on keep_original.
 224         if loc.statement_index < no_stmts {
 225             let (mut rvalue, source_info) = {
 226                 let statement = &mut self.source[loc.block].statements[loc.statement_index];
 227                 let rhs = match statement.kind {
 228                     StatementKind::Assign(box(_, ref mut rhs)) => rhs,
 229                     _ => {
 230                         span_bug!(statement.source_info.span, "{:?} is not an assignment",
 231                                   statement);
 232                     }
 233                 };
 234
 235                 (if self.keep_original {
 236                     rhs.clone()
 237                 } else {
 238                     let unit = Rvalue::Aggregate(box AggregateKind::Tuple, vec![]);
 239                     mem::replace(rhs, unit)
 240                 }, statement.source_info)
 241             };
 242
 243             self.visit_rvalue(&mut rvalue, loc);
 244             self.assign(new_temp, rvalue, source_info.span);
 245         } else {
 246             let terminator = if self.keep_original {
 247                 self.source[loc.block].terminator().clone()
 248             } else {
 249                 let terminator = self.source[loc.block].terminator_mut();
 250                 let target = match terminator.kind {
 251                     TerminatorKind::Call { destination: Some((_, target)), .. } => target,
 252                     ref kind => {
 253                         span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 254                     }
 255                 };
 256                 Terminator {
 257                     source_info: terminator.source_info,
 258                     kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
 259                         target,
 260                     })
 261                 }
 262             };
 263
 264             match terminator.kind {
 265                 TerminatorKind::Call { mut func, mut args, from_hir_call, .. } => {
 266                     self.visit_operand(&mut func, loc);
 267                     for arg in &mut args {
 268                         self.visit_operand(arg, loc);
 269                     }
 270
 271                     let last = self.promoted.basic_blocks().last().unwrap();
 272                     let new_target = self.new_block();
 273
 274                     *self.promoted[last].terminator_mut() = Terminator {
 275                         kind: TerminatorKind::Call {
 276                             func,
 277                             args,
 278                             cleanup: None,
 279                             destination: Some(
 280                                 (Place::from(new_temp), new_target)
 281                             ),
 282                             from_hir_call,
 283                         },
 284                         ..terminator
 285                     };
 286                 }
 287                 ref kind => {
 288                     span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
 289                 }
 290             };
 291         };
 292
 293         self.keep_original = old_keep_original;
 294         new_temp
 295     }
 296
 297     fn promote_candidate(
 298         mut self,
 299         def_id: DefId,
 300         candidate: Candidate,
 301         next_promoted_id: usize,
 302     ) -> Option<Body<'tcx>> {
 303         let mut operand = {
 304             let promoted = &mut self.promoted;
 305             let promoted_id = Promoted::new(next_promoted_id);
 306             let tcx = self.tcx;
 307             let mut promoted_place = |ty, span| {
 308                 promoted.span = span;
 309                 promoted.local_decls[RETURN_PLACE] = LocalDecl::new_return_place(ty, span);
 310                 Place {
 311                     base: PlaceBase::Static(box Static {
 312                         kind:
 313                             StaticKind::Promoted(
 314                                 promoted_id,
 315                                 InternalSubsts::identity_for_item(tcx, def_id),
 316                             ),
 317                         ty,
 318                         def_id,
 319                     }),
 320                     projection: box [],
 321                 }
 322             };
 323             let (blocks, local_decls) = self.source.basic_blocks_and_local_decls_mut();
 324             match candidate {
 325                 Candidate::Ref(loc) => {
 326                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 327                     match statement.kind {
 328                         StatementKind::Assign(box(_, Rvalue::Ref(_, _, ref mut place))) => {
 329                             // Use the underlying local for this (necessarily interior) borrow.
 330                             let ty = place.base.ty(local_decls).ty;
 331                             let span = statement.source_info.span;
 332
 333                             Operand::Move(Place {
 334                                 base: mem::replace(
 335                                     &mut place.base,
 336                                     promoted_place(ty, span).base,
 337                                 ),
 338                                 projection: box [],
 339                             })
 340                         }
 341                         _ => bug!()
 342                     }
 343                 }
 344                 Candidate::Repeat(loc) => {
 345                     let ref mut statement = blocks[loc.block].statements[loc.statement_index];
 346                     match statement.kind {
 347                         StatementKind::Assign(box(_, Rvalue::Repeat(ref mut operand, _))) => {
 348                             let ty = operand.ty(local_decls, self.tcx);
 349                             let span = statement.source_info.span;
 350                             mem::replace(
 351                                 operand,
 352                                 Operand::Copy(promoted_place(ty, span))
 353                             )
 354                         }
 355                         _ => bug!()
 356                     }
 357                 },
 358                 Candidate::Argument { bb, index } => {
 359                     let terminator = blocks[bb].terminator_mut();
 360                     match terminator.kind {
 361                         TerminatorKind::Call { ref mut args, .. } => {
 362                             let ty = args[index].ty(local_decls, self.tcx);
 363                             let span = terminator.source_info.span;
 364                             let operand = Operand::Copy(promoted_place(ty, span));
 365                             mem::replace(&mut args[index], operand)
 366                         }
 367                         // We expected a `TerminatorKind::Call` for which we'd like to promote an
 368                         // argument. `qualify_consts` saw a `TerminatorKind::Call` here, but
 369                         // we are seeing a `Goto`. That means that the `promote_temps` method
 370                         // already promoted this call away entirely. This case occurs when calling
 371                         // a function requiring a constant argument and as that constant value
 372                         // providing a value whose computation contains another call to a function
 373                         // requiring a constant argument.
 374                         TerminatorKind::Goto { .. } => return None,
 375                         _ => bug!()
 376                     }
 377                 }
 378             }
 379         };
 380
 381         assert_eq!(self.new_block(), START_BLOCK);
 382         self.visit_operand(&mut operand, Location {
 383             block: BasicBlock::new(0),
 384             statement_index: usize::MAX
 385         });
 386
 387         let span = self.promoted.span;
 388         self.assign(RETURN_PLACE, Rvalue::Use(operand), span);
 389         Some(self.promoted)
 390     }
 391 }
 392
 393 /// Replaces all temporaries with their promoted counterparts.
 394 impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 395     fn visit_local(&mut self,
 396                    local: &mut Local,
 397                    _: PlaceContext,
 398                    _: Location) {
 399         if self.source.local_kind(*local) == LocalKind::Temp {
 400             *local = self.promote_temp(*local);
 401         }
 402     }
 403 }
 404
 405 pub fn promote_candidates<'tcx>(
 406     def_id: DefId,
 407     body: &mut Body<'tcx>,
 408     tcx: TyCtxt<'tcx>,
 409     mut temps: IndexVec<Local, TempState>,
 410     candidates: Vec<Candidate>,
 411 ) -> IndexVec<Promoted, Body<'tcx>> {
 412     // Visit candidates in reverse, in case they're nested.
 413     debug!("promote_candidates({:?})", candidates);
 414
 415     let mut promotions = IndexVec::new();
 416
 417     for candidate in candidates.into_iter().rev() {
 418         match candidate {
 419             Candidate::Repeat(Location { block, statement_index }) |
 420             Candidate::Ref(Location { block, statement_index }) => {
 421                 match body[block].statements[statement_index].kind {
 422                     StatementKind::Assign(box(Place {
 423                         base: PlaceBase::Local(local),
 424                         projection: box [],
 425                     }, _)) => {
 426                         if temps[local] == TempState::PromotedOut {
 427                             // Already promoted.
 428                             continue;
 429                         }
 430                     }
 431                     _ => {}
 432                 }
 433             }
 434             Candidate::Argument { .. } => {}
 435         }
 436
 437
 438         // Declare return place local so that `mir::Body::new` doesn't complain.
 439         let initial_locals = iter::once(
 440             LocalDecl::new_return_place(tcx.types.never, body.span)
 441         ).collect();
 442
 443         let promoter = Promoter {
 444             promoted: Body::new(
 445                 IndexVec::new(),
 446                 // FIXME: maybe try to filter this to avoid blowing up
 447                 // memory usage?
 448                 body.source_scopes.clone(),
 449                 body.source_scope_local_data.clone(),
 450                 None,
 451                 initial_locals,
 452                 IndexVec::new(),
 453                 0,
 454                 vec![],
 455                 body.span,
 456                 vec![],
 457             ),
 458             tcx,
 459             source: body,
 460             temps: &mut temps,
 461             keep_original: false
 462         };
 463
 464         //FIXME(oli-obk): having a `maybe_push()` method on `IndexVec` might be nice
 465         if let Some(promoted) = promoter.promote_candidate(def_id, candidate, promotions.len()) {
 466             promotions.push(promoted);
 467         }
 468     }
 469
 470     // Eliminate assignments to, and drops of promoted temps.
 471     let promoted = |index: Local| temps[index] == TempState::PromotedOut;
 472     for block in body.basic_blocks_mut() {
 473         block.statements.retain(|statement| {
 474             match statement.kind {
 475                 StatementKind::Assign(box(Place {
 476                     base: PlaceBase::Local(index),
 477                     projection: box [],
 478                 }, _)) |
 479                 StatementKind::StorageLive(index) |
 480                 StatementKind::StorageDead(index) => {
 481                     !promoted(index)
 482                 }
 483                 _ => true
 484             }
 485         });
 486         let terminator = block.terminator_mut();
 487         match terminator.kind {
 488             TerminatorKind::Drop { location: Place {
 489                 base: PlaceBase::Local(index),
 490                 projection: box [],
 491             }, target, .. } => {
 492                 if promoted(index) {
 493                     terminator.kind = TerminatorKind::Goto {
 494                         target,
 495                     };
 496                 }
 497             }
 498             _ => {}
 499         }
 500     }
 501
 502     promotions
 503 }