src/librustc_codegen_ssa/mir/analyze.rs

   1 //! An analysis to determine which locals require allocas and
   2 //! which do not.
   3
   4 use rustc_data_structures::bit_set::BitSet;
   5 use rustc_data_structures::graph::dominators::Dominators;
   6 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
   7 use rustc::mir::{self, Location, TerminatorKind};
   8 use rustc::mir::visit::{Visitor, PlaceContext, MutatingUseContext, NonMutatingUseContext};
   9 use rustc::mir::traversal;
  10 use rustc::ty;
  11 use rustc::ty::layout::{LayoutOf, HasTyCtxt};
  12 use syntax_pos::DUMMY_SP;
  13 use super::FunctionCx;
  14 use crate::traits::*;
  15
  16 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  17     fx: &FunctionCx<'a, 'tcx, Bx>,
  18 ) -> BitSet<mir::Local> {
  19     let mir = fx.mir;
  20     let mut analyzer = LocalAnalyzer::new(fx);
  21
  22     analyzer.visit_body(mir);
  23
  24     for (index, (ty, span)) in mir.local_decls.iter()
  25         .map(|l| (l.ty, l.source_info.span))
  26         .enumerate()
  27     {
  28         let ty = fx.monomorphize(&ty);
  29         debug!("local {} has type {:?}", index, ty);
  30         let layout = fx.cx.spanned_layout_of(ty, span);
  31         if fx.cx.is_backend_immediate(layout) {
  32             // These sorts of types are immediates that we can store
  33             // in an Value without an alloca.
  34         } else if fx.cx.is_backend_scalar_pair(layout) {
  35             // We allow pairs and uses of any of their 2 fields.
  36         } else {
  37             // These sorts of types require an alloca. Note that
  38             // is_llvm_immediate() may *still* be true, particularly
  39             // for newtypes, but we currently force some types
  40             // (e.g., structs) into an alloca unconditionally, just so
  41             // that we don't have to deal with having two pathways
  42             // (gep vs extractvalue etc).
  43             analyzer.not_ssa(mir::Local::new(index));
  44         }
  45     }
  46
  47     analyzer.non_ssa_locals
  48 }
  49
  50 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
  51     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
  52     dominators: Dominators<mir::BasicBlock>,
  53     non_ssa_locals: BitSet<mir::Local>,
  54     // The location of the first visited direct assignment to each
  55     // local, or an invalid location (out of bounds `block` index).
  56     first_assignment: IndexVec<mir::Local, Location>,
  57 }
  58
  59 impl<Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> {
  60     fn new(fx: &'mir FunctionCx<'a, 'tcx, Bx>) -> Self {
  61         let invalid_location =
  62             mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
  63         let mut analyzer = LocalAnalyzer {
  64             fx,
  65             dominators: fx.mir.dominators(),
  66             non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
  67             first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls)
  68         };
  69
  70         // Arguments get assigned to by means of the function being called
  71         for arg in fx.mir.args_iter() {
  72             analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
  73         }
  74
  75         analyzer
  76     }
  77
  78     fn first_assignment(&self, local: mir::Local) -> Option<Location> {
  79         let location = self.first_assignment[local];
  80         if location.block.index() < self.fx.mir.basic_blocks().len() {
  81             Some(location)
  82         } else {
  83             None
  84         }
  85     }
  86
  87     fn not_ssa(&mut self, local: mir::Local) {
  88         debug!("marking {:?} as non-SSA", local);
  89         self.non_ssa_locals.insert(local);
  90     }
  91
  92     fn assign(&mut self, local: mir::Local, location: Location) {
  93         if self.first_assignment(local).is_some() {
  94             self.not_ssa(local);
  95         } else {
  96             self.first_assignment[local] = location;
  97         }
  98     }
  99
 100     fn process_place(
 101         &mut self,
 102         place_ref: &mir::PlaceRef<'_, 'tcx>,
 103         context: PlaceContext,
 104         location: Location,
 105     ) {
 106         let cx = self.fx.cx;
 107
 108         if let [.., elem] = place_ref.projection {
 109             // FIXME(spastorino) include this in the pattern when stabilized
 110             let proj_base = &place_ref.projection[..place_ref.projection.len() - 1];
 111
 112             // Allow uses of projections that are ZSTs or from scalar fields.
 113             let is_consume = match context {
 114                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 115                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 116                 _ => false
 117             };
 118             if is_consume {
 119                 let base_ty =
 120                     mir::Place::ty_from(place_ref.base, proj_base, self.fx.mir, cx.tcx());
 121                 let base_ty = self.fx.monomorphize(&base_ty);
 122
 123                 // ZSTs don't require any actual memory access.
 124                 let elem_ty = base_ty
 125                     .projection_ty(cx.tcx(), elem)
 126                     .ty;
 127                 let elem_ty = self.fx.monomorphize(&elem_ty);
 128                 let span = if let mir::PlaceBase::Local(index) = place_ref.base {
 129                     self.fx.mir.local_decls[*index].source_info.span
 130                 } else {
 131                     DUMMY_SP
 132                 };
 133                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 134                     return;
 135                 }
 136
 137                 if let mir::ProjectionElem::Field(..) = elem {
 138                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 139                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 140                         // Recurse with the same context, instead of `Projection`,
 141                         // potentially stopping at non-operand projections,
 142                         // which would trigger `not_ssa` on locals.
 143                         self.process_place(
 144                             &mir::PlaceRef {
 145                                 base: place_ref.base,
 146                                 projection: proj_base,
 147                             },
 148                             context,
 149                             location,
 150                         );
 151                         return;
 152                     }
 153                 }
 154             }
 155
 156             // A deref projection only reads the pointer, never needs the place.
 157             if let mir::ProjectionElem::Deref = elem {
 158                 self.process_place(
 159                     &mir::PlaceRef {
 160                         base: place_ref.base,
 161                         projection: proj_base,
 162                     },
 163                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 164                     location
 165                 );
 166                 return;
 167             }
 168         }
 169
 170         // FIXME this is super_place code, is repeated here to avoid cloning place or changing
 171         // visit_place API
 172         let mut context = context;
 173
 174         if !place_ref.projection.is_empty() {
 175             context = if context.is_mutating_use() {
 176                 PlaceContext::MutatingUse(MutatingUseContext::Projection)
 177             } else {
 178                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 179             };
 180         }
 181
 182         self.visit_place_base(place_ref.base, context, location);
 183         self.visit_projection(place_ref.base, place_ref.projection, context, location);
 184     }
 185
 186 }
 187
 188 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 189     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 190 {
 191     fn visit_assign(&mut self,
 192                     place: &mir::Place<'tcx>,
 193                     rvalue: &mir::Rvalue<'tcx>,
 194                     location: Location) {
 195         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 196
 197         if let mir::Place {
 198             base: mir::PlaceBase::Local(index),
 199             projection: box [],
 200         } = *place {
 201             self.assign(index, location);
 202             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 203             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 204                 self.not_ssa(index);
 205             }
 206         } else {
 207             self.visit_place(
 208                 place,
 209                 PlaceContext::MutatingUse(MutatingUseContext::Store),
 210                 location
 211             );
 212         }
 213
 214         self.visit_rvalue(rvalue, location);
 215     }
 216
 217     fn visit_terminator_kind(&mut self,
 218                              kind: &mir::TerminatorKind<'tcx>,
 219                              location: Location) {
 220         let check = match *kind {
 221             mir::TerminatorKind::Call {
 222                 func: mir::Operand::Constant(ref c),
 223                 ref args, ..
 224             } => match c.literal.ty.sty {
 225                 ty::FnDef(did, _) => Some((did, args)),
 226                 _ => None,
 227             },
 228             _ => None,
 229         };
 230         if let Some((def_id, args)) = check {
 231             if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
 232                 // box_free(x) shares with `drop x` the property that it
 233                 // is not guaranteed to be statically dominated by the
 234                 // definition of x, so x must always be in an alloca.
 235                 if let mir::Operand::Move(ref place) = args[0] {
 236                     self.visit_place(
 237                         place,
 238                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 239                         location
 240                     );
 241                 }
 242             }
 243         }
 244
 245         self.super_terminator_kind(kind, location);
 246     }
 247
 248     fn visit_place(&mut self,
 249                    place: &mir::Place<'tcx>,
 250                    context: PlaceContext,
 251                    location: Location) {
 252         debug!("visit_place(place={:?}, context={:?})", place, context);
 253         self.process_place(&place.as_ref(), context, location);
 254     }
 255
 256     fn visit_local(&mut self,
 257                    &local: &mir::Local,
 258                    context: PlaceContext,
 259                    location: Location) {
 260         match context {
 261             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 262                 self.assign(local, location);
 263             }
 264
 265             PlaceContext::NonUse(_) |
 266             PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 267
 268             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 269             PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 270                 // Reads from uninitialized variables (e.g., in dead code, after
 271                 // optimizations) require locals to be in (uninitialized) memory.
 272                 // N.B., there can be uninitialized reads of a local visited after
 273                 // an assignment to that local, if they happen on disjoint paths.
 274                 let ssa_read = match self.first_assignment(local) {
 275                     Some(assignment_location) => {
 276                         assignment_location.dominates(location, &self.dominators)
 277                     }
 278                     None => false
 279                 };
 280                 if !ssa_read {
 281                     self.not_ssa(local);
 282                 }
 283             }
 284
 285             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
 286             PlaceContext::MutatingUse(MutatingUseContext::Store) |
 287             PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
 288             PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
 289             PlaceContext::MutatingUse(MutatingUseContext::Projection) |
 290             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
 291             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
 292             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
 293             PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 294                 self.not_ssa(local);
 295             }
 296
 297             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 298                 let ty = self.fx.mir.local_decls[local].ty;
 299                 let ty = self.fx.monomorphize(&ty);
 300
 301                 // Only need the place if we're actually dropping it.
 302                 if self.fx.cx.type_needs_drop(ty) {
 303                     self.not_ssa(local);
 304                 }
 305             }
 306         }
 307     }
 308 }
 309
 310 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 311 pub enum CleanupKind {
 312     NotCleanup,
 313     Funclet,
 314     Internal { funclet: mir::BasicBlock }
 315 }
 316
 317 impl CleanupKind {
 318     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 319         match self {
 320             CleanupKind::NotCleanup => None,
 321             CleanupKind::Funclet => Some(for_bb),
 322             CleanupKind::Internal { funclet } => Some(funclet),
 323         }
 324     }
 325 }
 326
 327 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 328     fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 329                               mir: &mir::Body<'tcx>) {
 330         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 331             match data.terminator().kind {
 332                 TerminatorKind::Goto { .. } |
 333                 TerminatorKind::Resume |
 334                 TerminatorKind::Abort |
 335                 TerminatorKind::Return |
 336                 TerminatorKind::GeneratorDrop |
 337                 TerminatorKind::Unreachable |
 338                 TerminatorKind::SwitchInt { .. } |
 339                 TerminatorKind::Yield { .. } |
 340                 TerminatorKind::FalseEdges { .. } |
 341                 TerminatorKind::FalseUnwind { .. } => {
 342                     /* nothing to do */
 343                 }
 344                 TerminatorKind::Call { cleanup: unwind, .. } |
 345                 TerminatorKind::Assert { cleanup: unwind, .. } |
 346                 TerminatorKind::DropAndReplace { unwind, .. } |
 347                 TerminatorKind::Drop { unwind, .. } => {
 348                     if let Some(unwind) = unwind {
 349                         debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 350                                bb, data, unwind);
 351                         result[unwind] = CleanupKind::Funclet;
 352                     }
 353                 }
 354             }
 355         }
 356     }
 357
 358     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 359                        mir: &mir::Body<'tcx>) {
 360         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 361
 362         let mut set_successor = |funclet: mir::BasicBlock, succ| {
 363             match funclet_succs[funclet] {
 364                 ref mut s @ None => {
 365                     debug!("set_successor: updating successor of {:?} to {:?}",
 366                            funclet, succ);
 367                     *s = Some(succ);
 368                 },
 369                 Some(s) => if s != succ {
 370                     span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
 371                               funclet, s, succ);
 372                 }
 373             }
 374         };
 375
 376         for (bb, data) in traversal::reverse_postorder(mir) {
 377             let funclet = match result[bb] {
 378                 CleanupKind::NotCleanup => continue,
 379                 CleanupKind::Funclet => bb,
 380                 CleanupKind::Internal { funclet } => funclet,
 381             };
 382
 383             debug!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 384                    bb, data, result[bb], funclet);
 385
 386             for &succ in data.terminator().successors() {
 387                 let kind = result[succ];
 388                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
 389                        funclet, succ, kind);
 390                 match kind {
 391                     CleanupKind::NotCleanup => {
 392                         result[succ] = CleanupKind::Internal { funclet };
 393                     }
 394                     CleanupKind::Funclet => {
 395                         if funclet != succ {
 396                             set_successor(funclet, succ);
 397                         }
 398                     }
 399                     CleanupKind::Internal { funclet: succ_funclet } => {
 400                         if funclet != succ_funclet {
 401                             // `succ` has 2 different funclet going into it, so it must
 402                             // be a funclet by itself.
 403
 404                             debug!("promoting {:?} to a funclet and updating {:?}", succ,
 405                                    succ_funclet);
 406                             result[succ] = CleanupKind::Funclet;
 407                             set_successor(succ_funclet, succ);
 408                             set_successor(funclet, succ);
 409                         }
 410                     }
 411                 }
 412             }
 413         }
 414     }
 415
 416     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 417
 418     discover_masters(&mut result, mir);
 419     propagate(&mut result, mir);
 420     debug!("cleanup_kinds: result={:?}", result);
 421     result
 422 }