src/librustc_codegen_ssa/mir/analyze.rs

   1 //! An analysis to determine which locals require allocas and
   2 //! which do not.
   3
   4 use super::FunctionCx;
   5 use crate::traits::*;
   6 use rustc::mir::traversal;
   7 use rustc::mir::visit::{
   8     MutatingUseContext, NonMutatingUseContext, NonUseContext, PlaceContext, Visitor,
   9 };
  10 use rustc::mir::{self, Location, TerminatorKind};
  11 use rustc::session::config::DebugInfo;
  12 use rustc::ty;
  13 use rustc::ty::layout::{HasTyCtxt, LayoutOf};
  14 use rustc_data_structures::graph::dominators::Dominators;
  15 use rustc_index::bit_set::BitSet;
  16 use rustc_index::vec::{Idx, IndexVec};
  17
  18 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  19     fx: &FunctionCx<'a, 'tcx, Bx>,
  20 ) -> BitSet<mir::Local> {
  21     let mir = fx.mir;
  22     let mut analyzer = LocalAnalyzer::new(fx);
  23
  24     analyzer.visit_body(mir);
  25
  26     for (local, decl) in mir.local_decls.iter_enumerated() {
  27         // FIXME(eddyb): We should figure out how to use llvm.dbg.value instead
  28         // of putting everything in allocas just so we can use llvm.dbg.declare.
  29         if fx.cx.sess().opts.debuginfo == DebugInfo::Full {
  30             if fx.mir.local_kind(local) == mir::LocalKind::Arg {
  31                 analyzer.not_ssa(local);
  32                 continue;
  33             }
  34         }
  35
  36         let ty = fx.monomorphize(&decl.ty);
  37         debug!("local {:?} has type `{}`", local, ty);
  38         let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
  39         if fx.cx.is_backend_immediate(layout) {
  40             // These sorts of types are immediates that we can store
  41             // in an Value without an alloca.
  42         } else if fx.cx.is_backend_scalar_pair(layout) {
  43             // We allow pairs and uses of any of their 2 fields.
  44         } else {
  45             // These sorts of types require an alloca. Note that
  46             // is_llvm_immediate() may *still* be true, particularly
  47             // for newtypes, but we currently force some types
  48             // (e.g., structs) into an alloca unconditionally, just so
  49             // that we don't have to deal with having two pathways
  50             // (gep vs extractvalue etc).
  51             analyzer.not_ssa(local);
  52         }
  53     }
  54
  55     analyzer.non_ssa_locals
  56 }
  57
  58 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
  59     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
  60     dominators: Dominators<mir::BasicBlock>,
  61     non_ssa_locals: BitSet<mir::Local>,
  62     // The location of the first visited direct assignment to each
  63     // local, or an invalid location (out of bounds `block` index).
  64     first_assignment: IndexVec<mir::Local, Location>,
  65 }
  66
  67 impl<Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> {
  68     fn new(fx: &'mir FunctionCx<'a, 'tcx, Bx>) -> Self {
  69         let invalid_location = mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
  70         let dominators = fx.mir.dominators();
  71         let mut analyzer = LocalAnalyzer {
  72             fx,
  73             dominators,
  74             non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
  75             first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls),
  76         };
  77
  78         // Arguments get assigned to by means of the function being called
  79         for arg in fx.mir.args_iter() {
  80             analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
  81         }
  82
  83         analyzer
  84     }
  85
  86     fn first_assignment(&self, local: mir::Local) -> Option<Location> {
  87         let location = self.first_assignment[local];
  88         if location.block.index() < self.fx.mir.basic_blocks().len() {
  89             Some(location)
  90         } else {
  91             None
  92         }
  93     }
  94
  95     fn not_ssa(&mut self, local: mir::Local) {
  96         debug!("marking {:?} as non-SSA", local);
  97         self.non_ssa_locals.insert(local);
  98     }
  99
 100     fn assign(&mut self, local: mir::Local, location: Location) {
 101         if self.first_assignment(local).is_some() {
 102             self.not_ssa(local);
 103         } else {
 104             self.first_assignment[local] = location;
 105         }
 106     }
 107
 108     fn process_place(
 109         &mut self,
 110         place_ref: &mir::PlaceRef<'_, 'tcx>,
 111         context: PlaceContext,
 112         location: Location,
 113     ) {
 114         let cx = self.fx.cx;
 115
 116         if let [proj_base @ .., elem] = place_ref.projection {
 117             let mut base_context = if context.is_mutating_use() {
 118                 PlaceContext::MutatingUse(MutatingUseContext::Projection)
 119             } else {
 120                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 121             };
 122
 123             // Allow uses of projections that are ZSTs or from scalar fields.
 124             let is_consume = match context {
 125                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy)
 126                 | PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 127                 _ => false,
 128             };
 129             if is_consume {
 130                 let base_ty =
 131                     mir::Place::ty_from(place_ref.local, proj_base, *self.fx.mir, cx.tcx());
 132                 let base_ty = self.fx.monomorphize(&base_ty);
 133
 134                 // ZSTs don't require any actual memory access.
 135                 let elem_ty = base_ty.projection_ty(cx.tcx(), elem).ty;
 136                 let elem_ty = self.fx.monomorphize(&elem_ty);
 137                 let span = self.fx.mir.local_decls[*place_ref.local].source_info.span;
 138                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 139                     return;
 140                 }
 141
 142                 if let mir::ProjectionElem::Field(..) = elem {
 143                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 144                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 145                         // Recurse with the same context, instead of `Projection`,
 146                         // potentially stopping at non-operand projections,
 147                         // which would trigger `not_ssa` on locals.
 148                         base_context = context;
 149                     }
 150                 }
 151             }
 152
 153             if let mir::ProjectionElem::Deref = elem {
 154                 // Deref projections typically only read the pointer.
 155                 // (the exception being `VarDebugInfo` contexts, handled below)
 156                 base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
 157
 158                 // Indirect debuginfo requires going through memory, that only
 159                 // the debugger accesses, following our emitted DWARF pointer ops.
 160                 //
 161                 // FIXME(eddyb) Investigate the possibility of relaxing this, but
 162                 // note that `llvm.dbg.declare` *must* be used for indirect places,
 163                 // even if we start using `llvm.dbg.value` for all other cases,
 164                 // as we don't necessarily know when the value changes, but only
 165                 // where it lives in memory.
 166                 //
 167                 // It's possible `llvm.dbg.declare` could support starting from
 168                 // a pointer that doesn't point to an `alloca`, but this would
 169                 // only be useful if we know the pointer being `Deref`'d comes
 170                 // from an immutable place, and if `llvm.dbg.declare` calls
 171                 // must be at the very start of the function, then only function
 172                 // arguments could contain such pointers.
 173                 if context == PlaceContext::NonUse(NonUseContext::VarDebugInfo) {
 174                     // We use `NonUseContext::VarDebugInfo` for the base,
 175                     // which might not force the base local to memory,
 176                     // so we have to do it manually.
 177                     self.visit_local(place_ref.local, context, location);
 178                 }
 179             }
 180
 181             // `NonUseContext::VarDebugInfo` needs to flow all the
 182             // way down to the base local (see `visit_local`).
 183             if context == PlaceContext::NonUse(NonUseContext::VarDebugInfo) {
 184                 base_context = context;
 185             }
 186
 187             self.process_place(
 188                 &mir::PlaceRef { local: place_ref.local, projection: proj_base },
 189                 base_context,
 190                 location,
 191             );
 192             // HACK(eddyb) this emulates the old `visit_projection_elem`, this
 193             // entire `visit_place`-like `process_place` method should be rewritten,
 194             // now that we have moved to the "slice of projections" representation.
 195             if let mir::ProjectionElem::Index(local) = elem {
 196                 self.visit_local(
 197                     local,
 198                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 199                     location,
 200                 );
 201             }
 202         } else {
 203             // FIXME this is super_place code, is repeated here to avoid cloning place or changing
 204             // visit_place API
 205             let mut context = context;
 206
 207             if !place_ref.projection.is_empty() {
 208                 context = if context.is_mutating_use() {
 209                     PlaceContext::MutatingUse(MutatingUseContext::Projection)
 210                 } else {
 211                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 212                 };
 213             }
 214
 215             self.visit_place_base(place_ref.local, context, location);
 216             self.visit_projection(place_ref.local, place_ref.projection, context, location);
 217         }
 218     }
 219 }
 220
 221 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 222     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 223 {
 224     fn visit_assign(
 225         &mut self,
 226         place: &mir::Place<'tcx>,
 227         rvalue: &mir::Rvalue<'tcx>,
 228         location: Location,
 229     ) {
 230         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 231
 232         if let Some(index) = place.as_local() {
 233             self.assign(index, location);
 234             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 235             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 236                 self.not_ssa(index);
 237             }
 238         } else {
 239             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
 240         }
 241
 242         self.visit_rvalue(rvalue, location);
 243     }
 244
 245     fn visit_terminator_kind(&mut self, kind: &mir::TerminatorKind<'tcx>, location: Location) {
 246         let check = match *kind {
 247             mir::TerminatorKind::Call { func: mir::Operand::Constant(ref c), ref args, .. } => {
 248                 match c.literal.ty.kind {
 249                     ty::FnDef(did, _) => Some((did, args)),
 250                     _ => None,
 251                 }
 252             }
 253             _ => None,
 254         };
 255         if let Some((def_id, args)) = check {
 256             if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
 257                 // box_free(x) shares with `drop x` the property that it
 258                 // is not guaranteed to be statically dominated by the
 259                 // definition of x, so x must always be in an alloca.
 260                 if let mir::Operand::Move(ref place) = args[0] {
 261                     self.visit_place(
 262                         place,
 263                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 264                         location,
 265                     );
 266                 }
 267             }
 268         }
 269
 270         self.super_terminator_kind(kind, location);
 271     }
 272
 273     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
 274         debug!("visit_place(place={:?}, context={:?})", place, context);
 275         self.process_place(&place.as_ref(), context, location);
 276     }
 277
 278     fn visit_local(&mut self, &local: &mir::Local, context: PlaceContext, location: Location) {
 279         match context {
 280             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 281                 self.assign(local, location);
 282             }
 283
 284             PlaceContext::NonUse(NonUseContext::VarDebugInfo) => {
 285                 // We need to keep locals in `alloca`s for debuginfo.
 286                 // FIXME(eddyb): We should figure out how to use `llvm.dbg.value` instead
 287                 // of putting everything in allocas just so we can use `llvm.dbg.declare`.
 288                 if self.fx.cx.sess().opts.debuginfo == DebugInfo::Full {
 289                     self.not_ssa(local);
 290                 }
 291             }
 292
 293             PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 294
 295             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy)
 296             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 297                 // Reads from uninitialized variables (e.g., in dead code, after
 298                 // optimizations) require locals to be in (uninitialized) memory.
 299                 // N.B., there can be uninitialized reads of a local visited after
 300                 // an assignment to that local, if they happen on disjoint paths.
 301                 let ssa_read = match self.first_assignment(local) {
 302                     Some(assignment_location) => {
 303                         assignment_location.dominates(location, &self.dominators)
 304                     }
 305                     None => false,
 306                 };
 307                 if !ssa_read {
 308                     self.not_ssa(local);
 309                 }
 310             }
 311
 312             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect)
 313             | PlaceContext::MutatingUse(MutatingUseContext::Store)
 314             | PlaceContext::MutatingUse(MutatingUseContext::AsmOutput)
 315             | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 316             | PlaceContext::MutatingUse(MutatingUseContext::AddressOf)
 317             | PlaceContext::MutatingUse(MutatingUseContext::Projection)
 318             | PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 319             | PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 320             | PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 321             | PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 322             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 323                 self.not_ssa(local);
 324             }
 325
 326             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 327                 let ty = self.fx.mir.local_decls[local].ty;
 328                 let ty = self.fx.monomorphize(&ty);
 329
 330                 // Only need the place if we're actually dropping it.
 331                 if self.fx.cx.type_needs_drop(ty) {
 332                     self.not_ssa(local);
 333                 }
 334             }
 335         }
 336     }
 337 }
 338
 339 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 340 pub enum CleanupKind {
 341     NotCleanup,
 342     Funclet,
 343     Internal { funclet: mir::BasicBlock },
 344 }
 345
 346 impl CleanupKind {
 347     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 348         match self {
 349             CleanupKind::NotCleanup => None,
 350             CleanupKind::Funclet => Some(for_bb),
 351             CleanupKind::Internal { funclet } => Some(funclet),
 352         }
 353     }
 354 }
 355
 356 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 357     fn discover_masters<'tcx>(
 358         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 359         mir: &mir::Body<'tcx>,
 360     ) {
 361         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 362             match data.terminator().kind {
 363                 TerminatorKind::Goto { .. }
 364                 | TerminatorKind::Resume
 365                 | TerminatorKind::Abort
 366                 | TerminatorKind::Return
 367                 | TerminatorKind::GeneratorDrop
 368                 | TerminatorKind::Unreachable
 369                 | TerminatorKind::SwitchInt { .. }
 370                 | TerminatorKind::Yield { .. }
 371                 | TerminatorKind::FalseEdges { .. }
 372                 | TerminatorKind::FalseUnwind { .. } => { /* nothing to do */ }
 373                 TerminatorKind::Call { cleanup: unwind, .. }
 374                 | TerminatorKind::Assert { cleanup: unwind, .. }
 375                 | TerminatorKind::DropAndReplace { unwind, .. }
 376                 | TerminatorKind::Drop { unwind, .. } => {
 377                     if let Some(unwind) = unwind {
 378                         debug!(
 379                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 380                             bb, data, unwind
 381                         );
 382                         result[unwind] = CleanupKind::Funclet;
 383                     }
 384                 }
 385             }
 386         }
 387     }
 388
 389     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
 390         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 391
 392         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
 393             ref mut s @ None => {
 394                 debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ);
 395                 *s = Some(succ);
 396             }
 397             Some(s) => {
 398                 if s != succ {
 399                     span_bug!(
 400                         mir.span,
 401                         "funclet {:?} has 2 parents - {:?} and {:?}",
 402                         funclet,
 403                         s,
 404                         succ
 405                     );
 406                 }
 407             }
 408         };
 409
 410         for (bb, data) in traversal::reverse_postorder(mir) {
 411             let funclet = match result[bb] {
 412                 CleanupKind::NotCleanup => continue,
 413                 CleanupKind::Funclet => bb,
 414                 CleanupKind::Internal { funclet } => funclet,
 415             };
 416
 417             debug!(
 418                 "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 419                 bb, data, result[bb], funclet
 420             );
 421
 422             for &succ in data.terminator().successors() {
 423                 let kind = result[succ];
 424                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind);
 425                 match kind {
 426                     CleanupKind::NotCleanup => {
 427                         result[succ] = CleanupKind::Internal { funclet };
 428                     }
 429                     CleanupKind::Funclet => {
 430                         if funclet != succ {
 431                             set_successor(funclet, succ);
 432                         }
 433                     }
 434                     CleanupKind::Internal { funclet: succ_funclet } => {
 435                         if funclet != succ_funclet {
 436                             // `succ` has 2 different funclet going into it, so it must
 437                             // be a funclet by itself.
 438
 439                             debug!(
 440                                 "promoting {:?} to a funclet and updating {:?}",
 441                                 succ, succ_funclet
 442                             );
 443                             result[succ] = CleanupKind::Funclet;
 444                             set_successor(succ_funclet, succ);
 445                             set_successor(funclet, succ);
 446                         }
 447                     }
 448                 }
 449             }
 450         }
 451     }
 452
 453     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 454
 455     discover_masters(&mut result, mir);
 456     propagate(&mut result, mir);
 457     debug!("cleanup_kinds: result={:?}", result);
 458     result
 459 }