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 [proj_base @ .., elem] = place_ref.projection {
 109             // Allow uses of projections that are ZSTs or from scalar fields.
 110             let is_consume = match context {
 111                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 112                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 113                 _ => false
 114             };
 115             if is_consume {
 116                 let base_ty =
 117                     mir::Place::ty_from(place_ref.base, proj_base, self.fx.mir, cx.tcx());
 118                 let base_ty = self.fx.monomorphize(&base_ty);
 119
 120                 // ZSTs don't require any actual memory access.
 121                 let elem_ty = base_ty
 122                     .projection_ty(cx.tcx(), elem)
 123                     .ty;
 124                 let elem_ty = self.fx.monomorphize(&elem_ty);
 125                 let span = if let mir::PlaceBase::Local(index) = place_ref.base {
 126                     self.fx.mir.local_decls[*index].source_info.span
 127                 } else {
 128                     DUMMY_SP
 129                 };
 130                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 131                     return;
 132                 }
 133
 134                 if let mir::ProjectionElem::Field(..) = elem {
 135                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 136                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 137                         // Recurse with the same context, instead of `Projection`,
 138                         // potentially stopping at non-operand projections,
 139                         // which would trigger `not_ssa` on locals.
 140                         self.process_place(
 141                             &mir::PlaceRef {
 142                                 base: place_ref.base,
 143                                 projection: proj_base,
 144                             },
 145                             context,
 146                             location,
 147                         );
 148                         return;
 149                     }
 150                 }
 151             }
 152
 153             // A deref projection only reads the pointer, never needs the place.
 154             if let mir::ProjectionElem::Deref = elem {
 155                 self.process_place(
 156                     &mir::PlaceRef {
 157                         base: place_ref.base,
 158                         projection: proj_base,
 159                     },
 160                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 161                     location
 162                 );
 163                 return;
 164             }
 165         }
 166
 167         // FIXME this is super_place code, is repeated here to avoid cloning place or changing
 168         // visit_place API
 169         let mut context = context;
 170
 171         if !place_ref.projection.is_empty() {
 172             context = if context.is_mutating_use() {
 173                 PlaceContext::MutatingUse(MutatingUseContext::Projection)
 174             } else {
 175                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 176             };
 177         }
 178
 179         self.visit_place_base(place_ref.base, context, location);
 180         self.visit_projection(place_ref.base, place_ref.projection, context, location);
 181     }
 182
 183 }
 184
 185 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 186     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 187 {
 188     fn visit_assign(&mut self,
 189                     place: &mir::Place<'tcx>,
 190                     rvalue: &mir::Rvalue<'tcx>,
 191                     location: Location) {
 192         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 193
 194         if let mir::Place {
 195             base: mir::PlaceBase::Local(index),
 196             projection: box [],
 197         } = *place {
 198             self.assign(index, location);
 199             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 200             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 201                 self.not_ssa(index);
 202             }
 203         } else {
 204             self.visit_place(
 205                 place,
 206                 PlaceContext::MutatingUse(MutatingUseContext::Store),
 207                 location
 208             );
 209         }
 210
 211         self.visit_rvalue(rvalue, location);
 212     }
 213
 214     fn visit_terminator_kind(&mut self,
 215                              kind: &mir::TerminatorKind<'tcx>,
 216                              location: Location) {
 217         let check = match *kind {
 218             mir::TerminatorKind::Call {
 219                 func: mir::Operand::Constant(ref c),
 220                 ref args, ..
 221             } => match c.literal.ty.sty {
 222                 ty::FnDef(did, _) => Some((did, args)),
 223                 _ => None,
 224             },
 225             _ => None,
 226         };
 227         if let Some((def_id, args)) = check {
 228             if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
 229                 // box_free(x) shares with `drop x` the property that it
 230                 // is not guaranteed to be statically dominated by the
 231                 // definition of x, so x must always be in an alloca.
 232                 if let mir::Operand::Move(ref place) = args[0] {
 233                     self.visit_place(
 234                         place,
 235                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 236                         location
 237                     );
 238                 }
 239             }
 240         }
 241
 242         self.super_terminator_kind(kind, location);
 243     }
 244
 245     fn visit_place(&mut self,
 246                    place: &mir::Place<'tcx>,
 247                    context: PlaceContext,
 248                    location: Location) {
 249         debug!("visit_place(place={:?}, context={:?})", place, context);
 250         self.process_place(&place.as_ref(), context, location);
 251     }
 252
 253     fn visit_local(&mut self,
 254                    &local: &mir::Local,
 255                    context: PlaceContext,
 256                    location: Location) {
 257         match context {
 258             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 259                 self.assign(local, location);
 260             }
 261
 262             PlaceContext::NonUse(_) |
 263             PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 264
 265             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 266             PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 267                 // Reads from uninitialized variables (e.g., in dead code, after
 268                 // optimizations) require locals to be in (uninitialized) memory.
 269                 // N.B., there can be uninitialized reads of a local visited after
 270                 // an assignment to that local, if they happen on disjoint paths.
 271                 let ssa_read = match self.first_assignment(local) {
 272                     Some(assignment_location) => {
 273                         assignment_location.dominates(location, &self.dominators)
 274                     }
 275                     None => false
 276                 };
 277                 if !ssa_read {
 278                     self.not_ssa(local);
 279                 }
 280             }
 281
 282             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
 283             PlaceContext::MutatingUse(MutatingUseContext::Store) |
 284             PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
 285             PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
 286             PlaceContext::MutatingUse(MutatingUseContext::Projection) |
 287             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
 288             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
 289             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
 290             PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 291                 self.not_ssa(local);
 292             }
 293
 294             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 295                 let ty = self.fx.mir.local_decls[local].ty;
 296                 let ty = self.fx.monomorphize(&ty);
 297
 298                 // Only need the place if we're actually dropping it.
 299                 if self.fx.cx.type_needs_drop(ty) {
 300                     self.not_ssa(local);
 301                 }
 302             }
 303         }
 304     }
 305 }
 306
 307 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 308 pub enum CleanupKind {
 309     NotCleanup,
 310     Funclet,
 311     Internal { funclet: mir::BasicBlock }
 312 }
 313
 314 impl CleanupKind {
 315     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 316         match self {
 317             CleanupKind::NotCleanup => None,
 318             CleanupKind::Funclet => Some(for_bb),
 319             CleanupKind::Internal { funclet } => Some(funclet),
 320         }
 321     }
 322 }
 323
 324 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 325     fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 326                               mir: &mir::Body<'tcx>) {
 327         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 328             match data.terminator().kind {
 329                 TerminatorKind::Goto { .. } |
 330                 TerminatorKind::Resume |
 331                 TerminatorKind::Abort |
 332                 TerminatorKind::Return |
 333                 TerminatorKind::GeneratorDrop |
 334                 TerminatorKind::Unreachable |
 335                 TerminatorKind::SwitchInt { .. } |
 336                 TerminatorKind::Yield { .. } |
 337                 TerminatorKind::FalseEdges { .. } |
 338                 TerminatorKind::FalseUnwind { .. } => {
 339                     /* nothing to do */
 340                 }
 341                 TerminatorKind::Call { cleanup: unwind, .. } |
 342                 TerminatorKind::Assert { cleanup: unwind, .. } |
 343                 TerminatorKind::DropAndReplace { unwind, .. } |
 344                 TerminatorKind::Drop { unwind, .. } => {
 345                     if let Some(unwind) = unwind {
 346                         debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 347                                bb, data, unwind);
 348                         result[unwind] = CleanupKind::Funclet;
 349                     }
 350                 }
 351             }
 352         }
 353     }
 354
 355     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 356                        mir: &mir::Body<'tcx>) {
 357         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 358
 359         let mut set_successor = |funclet: mir::BasicBlock, succ| {
 360             match funclet_succs[funclet] {
 361                 ref mut s @ None => {
 362                     debug!("set_successor: updating successor of {:?} to {:?}",
 363                            funclet, succ);
 364                     *s = Some(succ);
 365                 },
 366                 Some(s) => if s != succ {
 367                     span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
 368                               funclet, s, succ);
 369                 }
 370             }
 371         };
 372
 373         for (bb, data) in traversal::reverse_postorder(mir) {
 374             let funclet = match result[bb] {
 375                 CleanupKind::NotCleanup => continue,
 376                 CleanupKind::Funclet => bb,
 377                 CleanupKind::Internal { funclet } => funclet,
 378             };
 379
 380             debug!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 381                    bb, data, result[bb], funclet);
 382
 383             for &succ in data.terminator().successors() {
 384                 let kind = result[succ];
 385                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
 386                        funclet, succ, kind);
 387                 match kind {
 388                     CleanupKind::NotCleanup => {
 389                         result[succ] = CleanupKind::Internal { funclet };
 390                     }
 391                     CleanupKind::Funclet => {
 392                         if funclet != succ {
 393                             set_successor(funclet, succ);
 394                         }
 395                     }
 396                     CleanupKind::Internal { funclet: succ_funclet } => {
 397                         if funclet != succ_funclet {
 398                             // `succ` has 2 different funclet going into it, so it must
 399                             // be a funclet by itself.
 400
 401                             debug!("promoting {:?} to a funclet and updating {:?}", succ,
 402                                    succ_funclet);
 403                             result[succ] = CleanupKind::Funclet;
 404                             set_successor(succ_funclet, succ);
 405                             set_successor(funclet, succ);
 406                         }
 407                     }
 408                 }
 409             }
 410         }
 411     }
 412
 413     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 414
 415     discover_masters(&mut result, mir);
 416     propagate(&mut result, mir);
 417     debug!("cleanup_kinds: result={:?}", result);
 418     result
 419 }