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