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 use rustc_span::DUMMY_SP;
  18
  19 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  20     fx: &FunctionCx<'a, 'tcx, Bx>,
  21 ) -> BitSet<mir::Local> {
  22     let mir = fx.mir;
  23     let mut analyzer = LocalAnalyzer::new(fx);
  24
  25     analyzer.visit_body(mir);
  26
  27     for (local, decl) in mir.local_decls.iter_enumerated() {
  28         // FIXME(eddyb): We should figure out how to use llvm.dbg.value instead
  29         // of putting everything in allocas just so we can use llvm.dbg.declare.
  30         if fx.cx.sess().opts.debuginfo == DebugInfo::Full {
  31             if fx.mir.local_kind(local) == mir::LocalKind::Arg {
  32                 analyzer.not_ssa(local);
  33                 continue;
  34             }
  35         }
  36
  37         let ty = fx.monomorphize(&decl.ty);
  38         debug!("local {:?} has type `{}`", local, ty);
  39         let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
  40         if fx.cx.is_backend_immediate(layout) {
  41             // These sorts of types are immediates that we can store
  42             // in an Value without an alloca.
  43         } else if fx.cx.is_backend_scalar_pair(layout) {
  44             // We allow pairs and uses of any of their 2 fields.
  45         } else {
  46             // These sorts of types require an alloca. Note that
  47             // is_llvm_immediate() may *still* be true, particularly
  48             // for newtypes, but we currently force some types
  49             // (e.g., structs) into an alloca unconditionally, just so
  50             // that we don't have to deal with having two pathways
  51             // (gep vs extractvalue etc).
  52             analyzer.not_ssa(local);
  53         }
  54     }
  55
  56     analyzer.non_ssa_locals
  57 }
  58
  59 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
  60     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
  61     dominators: Dominators<mir::BasicBlock>,
  62     non_ssa_locals: BitSet<mir::Local>,
  63     // The location of the first visited direct assignment to each
  64     // local, or an invalid location (out of bounds `block` index).
  65     first_assignment: IndexVec<mir::Local, Location>,
  66 }
  67
  68 impl<Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> {
  69     fn new(fx: &'mir FunctionCx<'a, 'tcx, Bx>) -> Self {
  70         let invalid_location = mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
  71         let dominators = fx.mir.dominators();
  72         let mut analyzer = LocalAnalyzer {
  73             fx,
  74             dominators,
  75             non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
  76             first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls),
  77         };
  78
  79         // Arguments get assigned to by means of the function being called
  80         for arg in fx.mir.args_iter() {
  81             analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
  82         }
  83
  84         analyzer
  85     }
  86
  87     fn first_assignment(&self, local: mir::Local) -> Option<Location> {
  88         let location = self.first_assignment[local];
  89         if location.block.index() < self.fx.mir.basic_blocks().len() {
  90             Some(location)
  91         } else {
  92             None
  93         }
  94     }
  95
  96     fn not_ssa(&mut self, local: mir::Local) {
  97         debug!("marking {:?} as non-SSA", local);
  98         self.non_ssa_locals.insert(local);
  99     }
 100
 101     fn assign(&mut self, local: mir::Local, location: Location) {
 102         if self.first_assignment(local).is_some() {
 103             self.not_ssa(local);
 104         } else {
 105             self.first_assignment[local] = location;
 106         }
 107     }
 108
 109     fn process_place(
 110         &mut self,
 111         place_ref: &mir::PlaceRef<'_, 'tcx>,
 112         context: PlaceContext,
 113         location: Location,
 114     ) {
 115         let cx = self.fx.cx;
 116
 117         if let [proj_base @ .., elem] = place_ref.projection {
 118             let mut base_context = if context.is_mutating_use() {
 119                 PlaceContext::MutatingUse(MutatingUseContext::Projection)
 120             } else {
 121                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 122             };
 123
 124             // Allow uses of projections that are ZSTs or from scalar fields.
 125             let is_consume = match context {
 126                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy)
 127                 | PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 128                 _ => false,
 129             };
 130             if is_consume {
 131                 let base_ty =
 132                     mir::Place::ty_from(place_ref.base, proj_base, *self.fx.mir, cx.tcx());
 133                 let base_ty = self.fx.monomorphize(&base_ty);
 134
 135                 // ZSTs don't require any actual memory access.
 136                 let elem_ty = base_ty.projection_ty(cx.tcx(), elem).ty;
 137                 let elem_ty = self.fx.monomorphize(&elem_ty);
 138                 let span = if let mir::PlaceBase::Local(index) = place_ref.base {
 139                     self.fx.mir.local_decls[*index].source_info.span
 140                 } else {
 141                     DUMMY_SP
 142                 };
 143                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 144                     return;
 145                 }
 146
 147                 if let mir::ProjectionElem::Field(..) = elem {
 148                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 149                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 150                         // Recurse with the same context, instead of `Projection`,
 151                         // potentially stopping at non-operand projections,
 152                         // which would trigger `not_ssa` on locals.
 153                         base_context = context;
 154                     }
 155                 }
 156             }
 157
 158             if let mir::ProjectionElem::Deref = elem {
 159                 // Deref projections typically only read the pointer.
 160                 // (the exception being `VarDebugInfo` contexts, handled below)
 161                 base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
 162
 163                 // Indirect debuginfo requires going through memory, that only
 164                 // the debugger accesses, following our emitted DWARF pointer ops.
 165                 //
 166                 // FIXME(eddyb) Investigate the possibility of relaxing this, but
 167                 // note that `llvm.dbg.declare` *must* be used for indirect places,
 168                 // even if we start using `llvm.dbg.value` for all other cases,
 169                 // as we don't necessarily know when the value changes, but only
 170                 // where it lives in memory.
 171                 //
 172                 // It's possible `llvm.dbg.declare` could support starting from
 173                 // a pointer that doesn't point to an `alloca`, but this would
 174                 // only be useful if we know the pointer being `Deref`'d comes
 175                 // from an immutable place, and if `llvm.dbg.declare` calls
 176                 // must be at the very start of the function, then only function
 177                 // arguments could contain such pointers.
 178                 if context == PlaceContext::NonUse(NonUseContext::VarDebugInfo) {
 179                     // We use `NonUseContext::VarDebugInfo` for the base,
 180                     // which might not force the base local to memory,
 181                     // so we have to do it manually.
 182                     if let mir::PlaceBase::Local(local) = place_ref.base {
 183                         self.visit_local(&local, context, location);
 184                     }
 185                 }
 186             }
 187
 188             // `NonUseContext::VarDebugInfo` needs to flow all the
 189             // way down to the base local (see `visit_local`).
 190             if context == PlaceContext::NonUse(NonUseContext::VarDebugInfo) {
 191                 base_context = context;
 192             }
 193
 194             self.process_place(
 195                 &mir::PlaceRef { base: place_ref.base, projection: proj_base },
 196                 base_context,
 197                 location,
 198             );
 199             // HACK(eddyb) this emulates the old `visit_projection_elem`, this
 200             // entire `visit_place`-like `process_place` method should be rewritten,
 201             // now that we have moved to the "slice of projections" representation.
 202             if let mir::ProjectionElem::Index(local) = elem {
 203                 self.visit_local(
 204                     local,
 205                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 206                     location,
 207                 );
 208             }
 209         } else {
 210             // FIXME this is super_place code, is repeated here to avoid cloning place or changing
 211             // visit_place API
 212             let mut context = context;
 213
 214             if !place_ref.projection.is_empty() {
 215                 context = if context.is_mutating_use() {
 216                     PlaceContext::MutatingUse(MutatingUseContext::Projection)
 217                 } else {
 218                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 219                 };
 220             }
 221
 222             self.visit_place_base(place_ref.base, context, location);
 223             self.visit_projection(place_ref.base, place_ref.projection, context, location);
 224         }
 225     }
 226 }
 227
 228 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 229     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 230 {
 231     fn visit_assign(
 232         &mut self,
 233         place: &mir::Place<'tcx>,
 234         rvalue: &mir::Rvalue<'tcx>,
 235         location: Location,
 236     ) {
 237         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 238
 239         if let Some(index) = place.as_local() {
 240             self.assign(index, location);
 241             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 242             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 243                 self.not_ssa(index);
 244             }
 245         } else {
 246             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
 247         }
 248
 249         self.visit_rvalue(rvalue, location);
 250     }
 251
 252     fn visit_terminator_kind(&mut self, kind: &mir::TerminatorKind<'tcx>, location: Location) {
 253         let check = match *kind {
 254             mir::TerminatorKind::Call { func: mir::Operand::Constant(ref c), ref args, .. } => {
 255                 match c.literal.ty.kind {
 256                     ty::FnDef(did, _) => Some((did, args)),
 257                     _ => None,
 258                 }
 259             }
 260             _ => None,
 261         };
 262         if let Some((def_id, args)) = check {
 263             if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
 264                 // box_free(x) shares with `drop x` the property that it
 265                 // is not guaranteed to be statically dominated by the
 266                 // definition of x, so x must always be in an alloca.
 267                 if let mir::Operand::Move(ref place) = args[0] {
 268                     self.visit_place(
 269                         place,
 270                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 271                         location,
 272                     );
 273                 }
 274             }
 275         }
 276
 277         self.super_terminator_kind(kind, location);
 278     }
 279
 280     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
 281         debug!("visit_place(place={:?}, context={:?})", place, context);
 282         self.process_place(&place.as_ref(), context, location);
 283     }
 284
 285     fn visit_local(&mut self, &local: &mir::Local, context: PlaceContext, location: Location) {
 286         match context {
 287             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 288                 self.assign(local, location);
 289             }
 290
 291             PlaceContext::NonUse(NonUseContext::VarDebugInfo) => {
 292                 // We need to keep locals in `alloca`s for debuginfo.
 293                 // FIXME(eddyb): We should figure out how to use `llvm.dbg.value` instead
 294                 // of putting everything in allocas just so we can use `llvm.dbg.declare`.
 295                 if self.fx.cx.sess().opts.debuginfo == DebugInfo::Full {
 296                     self.not_ssa(local);
 297                 }
 298             }
 299
 300             PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 301
 302             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy)
 303             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 304                 // Reads from uninitialized variables (e.g., in dead code, after
 305                 // optimizations) require locals to be in (uninitialized) memory.
 306                 // N.B., there can be uninitialized reads of a local visited after
 307                 // an assignment to that local, if they happen on disjoint paths.
 308                 let ssa_read = match self.first_assignment(local) {
 309                     Some(assignment_location) => {
 310                         assignment_location.dominates(location, &self.dominators)
 311                     }
 312                     None => false,
 313                 };
 314                 if !ssa_read {
 315                     self.not_ssa(local);
 316                 }
 317             }
 318
 319             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect)
 320             | PlaceContext::MutatingUse(MutatingUseContext::Store)
 321             | PlaceContext::MutatingUse(MutatingUseContext::AsmOutput)
 322             | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 323             | PlaceContext::MutatingUse(MutatingUseContext::AddressOf)
 324             | PlaceContext::MutatingUse(MutatingUseContext::Projection)
 325             | PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 326             | PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 327             | PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 328             | PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 329             | PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 330                 self.not_ssa(local);
 331             }
 332
 333             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 334                 let ty = self.fx.mir.local_decls[local].ty;
 335                 let ty = self.fx.monomorphize(&ty);
 336
 337                 // Only need the place if we're actually dropping it.
 338                 if self.fx.cx.type_needs_drop(ty) {
 339                     self.not_ssa(local);
 340                 }
 341             }
 342         }
 343     }
 344 }
 345
 346 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 347 pub enum CleanupKind {
 348     NotCleanup,
 349     Funclet,
 350     Internal { funclet: mir::BasicBlock },
 351 }
 352
 353 impl CleanupKind {
 354     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 355         match self {
 356             CleanupKind::NotCleanup => None,
 357             CleanupKind::Funclet => Some(for_bb),
 358             CleanupKind::Internal { funclet } => Some(funclet),
 359         }
 360     }
 361 }
 362
 363 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 364     fn discover_masters<'tcx>(
 365         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 366         mir: &mir::Body<'tcx>,
 367     ) {
 368         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 369             match data.terminator().kind {
 370                 TerminatorKind::Goto { .. }
 371                 | TerminatorKind::Resume
 372                 | TerminatorKind::Abort
 373                 | TerminatorKind::Return
 374                 | TerminatorKind::GeneratorDrop
 375                 | TerminatorKind::Unreachable
 376                 | TerminatorKind::SwitchInt { .. }
 377                 | TerminatorKind::Yield { .. }
 378                 | TerminatorKind::FalseEdges { .. }
 379                 | TerminatorKind::FalseUnwind { .. } => { /* nothing to do */ }
 380                 TerminatorKind::Call { cleanup: unwind, .. }
 381                 | TerminatorKind::Assert { cleanup: unwind, .. }
 382                 | TerminatorKind::DropAndReplace { unwind, .. }
 383                 | TerminatorKind::Drop { unwind, .. } => {
 384                     if let Some(unwind) = unwind {
 385                         debug!(
 386                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 387                             bb, data, unwind
 388                         );
 389                         result[unwind] = CleanupKind::Funclet;
 390                     }
 391                 }
 392             }
 393         }
 394     }
 395
 396     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
 397         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 398
 399         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
 400             ref mut s @ None => {
 401                 debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ);
 402                 *s = Some(succ);
 403             }
 404             Some(s) => {
 405                 if s != succ {
 406                     span_bug!(
 407                         mir.span,
 408                         "funclet {:?} has 2 parents - {:?} and {:?}",
 409                         funclet,
 410                         s,
 411                         succ
 412                     );
 413                 }
 414             }
 415         };
 416
 417         for (bb, data) in traversal::reverse_postorder(mir) {
 418             let funclet = match result[bb] {
 419                 CleanupKind::NotCleanup => continue,
 420                 CleanupKind::Funclet => bb,
 421                 CleanupKind::Internal { funclet } => funclet,
 422             };
 423
 424             debug!(
 425                 "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 426                 bb, data, result[bb], funclet
 427             );
 428
 429             for &succ in data.terminator().successors() {
 430                 let kind = result[succ];
 431                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind);
 432                 match kind {
 433                     CleanupKind::NotCleanup => {
 434                         result[succ] = CleanupKind::Internal { funclet };
 435                     }
 436                     CleanupKind::Funclet => {
 437                         if funclet != succ {
 438                             set_successor(funclet, succ);
 439                         }
 440                     }
 441                     CleanupKind::Internal { funclet: succ_funclet } => {
 442                         if funclet != succ_funclet {
 443                             // `succ` has 2 different funclet going into it, so it must
 444                             // be a funclet by itself.
 445
 446                             debug!(
 447                                 "promoting {:?} to a funclet and updating {:?}",
 448                                 succ, succ_funclet
 449                             );
 450                             result[succ] = CleanupKind::Funclet;
 451                             set_successor(succ_funclet, succ);
 452                             set_successor(funclet, succ);
 453                         }
 454                     }
 455                 }
 456             }
 457         }
 458     }
 459
 460     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 461
 462     discover_masters(&mut result, mir);
 463     propagate(&mut result, mir);
 464     debug!("cleanup_kinds: result={:?}", result);
 465     result
 466 }