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