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