compiler/rustc_codegen_ssa/src/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::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
  11 use rustc_middle::mir::{self, Location, TerminatorKind};
  12 use rustc_middle::ty;
  13 use rustc_middle::ty::layout::HasTyCtxt;
  14 use rustc_target::abi::LayoutOf;
  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     for (bb, data) in mir.basic_blocks().iter_enumerated() {
  23         analyzer.visit_basic_block_data(bb, data);
  24     }
  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 Some((place_base, elem)) = place_ref.last_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 = matches!(
 116                 context,
 117                 PlaceContext::NonMutatingUse(
 118                     NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 119                 )
 120             );
 121             if is_consume {
 122                 let base_ty = place_base.ty(self.fx.mir, cx.tcx());
 123                 let base_ty = self.fx.monomorphize(base_ty);
 124
 125                 // ZSTs don't require any actual memory access.
 126                 let elem_ty = base_ty.projection_ty(cx.tcx(), 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                 base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
 146             }
 147
 148             self.process_place(&place_base, base_context, location);
 149             // HACK(eddyb) this emulates the old `visit_projection_elem`, this
 150             // entire `visit_place`-like `process_place` method should be rewritten,
 151             // now that we have moved to the "slice of projections" representation.
 152             if let mir::ProjectionElem::Index(local) = elem {
 153                 self.visit_local(
 154                     &local,
 155                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 156                     location,
 157                 );
 158             }
 159         } else {
 160             // FIXME this is super_place code, is repeated here to avoid cloning place or changing
 161             // visit_place API
 162             let mut context = context;
 163
 164             if !place_ref.projection.is_empty() {
 165                 context = if context.is_mutating_use() {
 166                     PlaceContext::MutatingUse(MutatingUseContext::Projection)
 167                 } else {
 168                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 169                 };
 170             }
 171
 172             self.visit_local(&place_ref.local, context, location);
 173             self.visit_projection(*place_ref, context, location);
 174         }
 175     }
 176 }
 177
 178 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 179     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 180 {
 181     fn visit_assign(
 182         &mut self,
 183         place: &mir::Place<'tcx>,
 184         rvalue: &mir::Rvalue<'tcx>,
 185         location: Location,
 186     ) {
 187         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 188
 189         if let Some(index) = place.as_local() {
 190             self.assign(index, location);
 191             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 192             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 193                 self.not_ssa(index);
 194             }
 195         } else {
 196             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
 197         }
 198
 199         self.visit_rvalue(rvalue, location);
 200     }
 201
 202     fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
 203         let check = match terminator.kind {
 204             mir::TerminatorKind::Call { func: mir::Operand::Constant(ref c), ref args, .. } => {
 205                 match *c.ty().kind() {
 206                     ty::FnDef(did, _) => Some((did, args)),
 207                     _ => None,
 208                 }
 209             }
 210             _ => None,
 211         };
 212         if let Some((def_id, args)) = check {
 213             if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
 214                 // box_free(x) shares with `drop x` the property that it
 215                 // is not guaranteed to be statically dominated by the
 216                 // definition of x, so x must always be in an alloca.
 217                 if let mir::Operand::Move(ref place) = args[0] {
 218                     self.visit_place(
 219                         place,
 220                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 221                         location,
 222                     );
 223                 }
 224             }
 225         }
 226
 227         self.super_terminator(terminator, location);
 228     }
 229
 230     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
 231         debug!("visit_place(place={:?}, context={:?})", place, context);
 232         self.process_place(&place.as_ref(), context, location);
 233     }
 234
 235     fn visit_local(&mut self, &local: &mir::Local, context: PlaceContext, location: Location) {
 236         match context {
 237             PlaceContext::MutatingUse(MutatingUseContext::Call)
 238             | PlaceContext::MutatingUse(MutatingUseContext::Yield) => {
 239                 self.assign(local, location);
 240             }
 241
 242             PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 243
 244             PlaceContext::NonMutatingUse(
 245                 NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 246             ) => {
 247                 // Reads from uninitialized variables (e.g., in dead code, after
 248                 // optimizations) require locals to be in (uninitialized) memory.
 249                 // N.B., there can be uninitialized reads of a local visited after
 250                 // an assignment to that local, if they happen on disjoint paths.
 251                 let ssa_read = match self.first_assignment(local) {
 252                     Some(assignment_location) => {
 253                         assignment_location.dominates(location, &self.dominators)
 254                     }
 255                     None => {
 256                         debug!("No first assignment found for {:?}", local);
 257                         // We have not seen any assignment to the local yet,
 258                         // but before marking not_ssa, check if it is a ZST,
 259                         // in which case we don't need to initialize the local.
 260                         let ty = self.fx.mir.local_decls[local].ty;
 261                         let ty = self.fx.monomorphize(ty);
 262
 263                         let is_zst = self.fx.cx.layout_of(ty).is_zst();
 264                         debug!("is_zst: {}", is_zst);
 265                         is_zst
 266                     }
 267                 };
 268                 if !ssa_read {
 269                     self.not_ssa(local);
 270                 }
 271             }
 272
 273             PlaceContext::MutatingUse(
 274                 MutatingUseContext::Store
 275                 | MutatingUseContext::AsmOutput
 276                 | MutatingUseContext::Borrow
 277                 | MutatingUseContext::AddressOf
 278                 | MutatingUseContext::Projection,
 279             )
 280             | PlaceContext::NonMutatingUse(
 281                 NonMutatingUseContext::Inspect
 282                 | NonMutatingUseContext::SharedBorrow
 283                 | NonMutatingUseContext::UniqueBorrow
 284                 | NonMutatingUseContext::ShallowBorrow
 285                 | NonMutatingUseContext::AddressOf
 286                 | NonMutatingUseContext::Projection,
 287             ) => {
 288                 self.not_ssa(local);
 289             }
 290
 291             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 292                 let ty = self.fx.mir.local_decls[local].ty;
 293                 let ty = self.fx.monomorphize(ty);
 294
 295                 // Only need the place if we're actually dropping it.
 296                 if self.fx.cx.type_needs_drop(ty) {
 297                     self.not_ssa(local);
 298                 }
 299             }
 300         }
 301     }
 302 }
 303
 304 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 305 pub enum CleanupKind {
 306     NotCleanup,
 307     Funclet,
 308     Internal { funclet: mir::BasicBlock },
 309 }
 310
 311 impl CleanupKind {
 312     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 313         match self {
 314             CleanupKind::NotCleanup => None,
 315             CleanupKind::Funclet => Some(for_bb),
 316             CleanupKind::Internal { funclet } => Some(funclet),
 317         }
 318     }
 319 }
 320
 321 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 322     fn discover_masters<'tcx>(
 323         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 324         mir: &mir::Body<'tcx>,
 325     ) {
 326         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 327             match data.terminator().kind {
 328                 TerminatorKind::Goto { .. }
 329                 | TerminatorKind::Resume
 330                 | TerminatorKind::Abort
 331                 | TerminatorKind::Return
 332                 | TerminatorKind::GeneratorDrop
 333                 | TerminatorKind::Unreachable
 334                 | TerminatorKind::SwitchInt { .. }
 335                 | TerminatorKind::Yield { .. }
 336                 | TerminatorKind::FalseEdge { .. }
 337                 | TerminatorKind::FalseUnwind { .. }
 338                 | TerminatorKind::InlineAsm { .. } => { /* nothing to do */ }
 339                 TerminatorKind::Call { cleanup: unwind, .. }
 340                 | TerminatorKind::Assert { cleanup: unwind, .. }
 341                 | TerminatorKind::DropAndReplace { unwind, .. }
 342                 | TerminatorKind::Drop { unwind, .. } => {
 343                     if let Some(unwind) = unwind {
 344                         debug!(
 345                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 346                             bb, data, unwind
 347                         );
 348                         result[unwind] = CleanupKind::Funclet;
 349                     }
 350                 }
 351             }
 352         }
 353     }
 354
 355     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
 356         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 357
 358         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
 359             ref mut s @ None => {
 360                 debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ);
 361                 *s = Some(succ);
 362             }
 363             Some(s) => {
 364                 if s != succ {
 365                     span_bug!(
 366                         mir.span,
 367                         "funclet {:?} has 2 parents - {:?} and {:?}",
 368                         funclet,
 369                         s,
 370                         succ
 371                     );
 372                 }
 373             }
 374         };
 375
 376         for (bb, data) in traversal::reverse_postorder(mir) {
 377             let funclet = match result[bb] {
 378                 CleanupKind::NotCleanup => continue,
 379                 CleanupKind::Funclet => bb,
 380                 CleanupKind::Internal { funclet } => funclet,
 381             };
 382
 383             debug!(
 384                 "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 385                 bb, data, result[bb], funclet
 386             );
 387
 388             for &succ in data.terminator().successors() {
 389                 let kind = result[succ];
 390                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind);
 391                 match kind {
 392                     CleanupKind::NotCleanup => {
 393                         result[succ] = CleanupKind::Internal { funclet };
 394                     }
 395                     CleanupKind::Funclet => {
 396                         if funclet != succ {
 397                             set_successor(funclet, succ);
 398                         }
 399                     }
 400                     CleanupKind::Internal { funclet: succ_funclet } => {
 401                         if funclet != succ_funclet {
 402                             // `succ` has 2 different funclet going into it, so it must
 403                             // be a funclet by itself.
 404
 405                             debug!(
 406                                 "promoting {:?} to a funclet and updating {:?}",
 407                                 succ, succ_funclet
 408                             );
 409                             result[succ] = CleanupKind::Funclet;
 410                             set_successor(succ_funclet, succ);
 411                             set_successor(funclet, succ);
 412                         }
 413                     }
 414                 }
 415             }
 416         }
 417     }
 418
 419     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 420
 421     discover_masters(&mut result, mir);
 422     propagate(&mut result, mir);
 423     debug!("cleanup_kinds: result={:?}", result);
 424     result
 425 }