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::layout::HasTyCtxt;
  13 use rustc_target::abi::LayoutOf;
  14
  15 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  16     fx: &FunctionCx<'a, 'tcx, Bx>,
  17 ) -> BitSet<mir::Local> {
  18     let mir = fx.mir;
  19     let mut analyzer = LocalAnalyzer::new(fx);
  20
  21     for (bb, data) in mir.basic_blocks().iter_enumerated() {
  22         analyzer.visit_basic_block_data(bb, data);
  23     }
  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 Some((place_base, elem)) = place_ref.last_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 = matches!(
 115                 context,
 116                 PlaceContext::NonMutatingUse(
 117                     NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 118                 )
 119             );
 120             if is_consume {
 121                 let base_ty = place_base.ty(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(), self.fx.monomorphize(elem)).ty;
 126                 let span = self.fx.mir.local_decls[place_ref.local].source_info.span;
 127                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 128                     return;
 129                 }
 130
 131                 if let mir::ProjectionElem::Field(..) = elem {
 132                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 133                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 134                         // Recurse with the same context, instead of `Projection`,
 135                         // potentially stopping at non-operand projections,
 136                         // which would trigger `not_ssa` on locals.
 137                         base_context = context;
 138                     }
 139                 }
 140             }
 141
 142             if let mir::ProjectionElem::Deref = elem {
 143                 // Deref projections typically only read the pointer.
 144                 base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
 145             }
 146
 147             self.process_place(&place_base, base_context, location);
 148             // HACK(eddyb) this emulates the old `visit_projection_elem`, this
 149             // entire `visit_place`-like `process_place` method should be rewritten,
 150             // now that we have moved to the "slice of projections" representation.
 151             if let mir::ProjectionElem::Index(local) = elem {
 152                 self.visit_local(
 153                     &local,
 154                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 155                     location,
 156                 );
 157             }
 158         } else {
 159             self.visit_local(&place_ref.local, context, location);
 160         }
 161     }
 162 }
 163
 164 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 165     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 166 {
 167     fn visit_assign(
 168         &mut self,
 169         place: &mir::Place<'tcx>,
 170         rvalue: &mir::Rvalue<'tcx>,
 171         location: Location,
 172     ) {
 173         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 174
 175         if let Some(index) = place.as_local() {
 176             self.assign(index, location);
 177             let decl_span = self.fx.mir.local_decls[index].source_info.span;
 178             if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 179                 self.not_ssa(index);
 180             }
 181         } else {
 182             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
 183         }
 184
 185         self.visit_rvalue(rvalue, location);
 186     }
 187
 188     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
 189         debug!("visit_place(place={:?}, context={:?})", place, context);
 190         self.process_place(&place.as_ref(), context, location);
 191     }
 192
 193     fn visit_local(&mut self, &local: &mir::Local, context: PlaceContext, location: Location) {
 194         match context {
 195             PlaceContext::MutatingUse(MutatingUseContext::Call)
 196             | PlaceContext::MutatingUse(MutatingUseContext::Yield) => {
 197                 self.assign(local, location);
 198             }
 199
 200             PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 201
 202             PlaceContext::NonMutatingUse(
 203                 NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 204             ) => {
 205                 // Reads from uninitialized variables (e.g., in dead code, after
 206                 // optimizations) require locals to be in (uninitialized) memory.
 207                 // N.B., there can be uninitialized reads of a local visited after
 208                 // an assignment to that local, if they happen on disjoint paths.
 209                 let ssa_read = match self.first_assignment(local) {
 210                     Some(assignment_location) => {
 211                         assignment_location.dominates(location, &self.dominators)
 212                     }
 213                     None => {
 214                         debug!("No first assignment found for {:?}", local);
 215                         // We have not seen any assignment to the local yet,
 216                         // but before marking not_ssa, check if it is a ZST,
 217                         // in which case we don't need to initialize the local.
 218                         let ty = self.fx.mir.local_decls[local].ty;
 219                         let ty = self.fx.monomorphize(ty);
 220
 221                         let is_zst = self.fx.cx.layout_of(ty).is_zst();
 222                         debug!("is_zst: {}", is_zst);
 223                         is_zst
 224                     }
 225                 };
 226                 if !ssa_read {
 227                     self.not_ssa(local);
 228                 }
 229             }
 230
 231             PlaceContext::MutatingUse(
 232                 MutatingUseContext::Store
 233                 | MutatingUseContext::AsmOutput
 234                 | MutatingUseContext::Borrow
 235                 | MutatingUseContext::AddressOf
 236                 | MutatingUseContext::Projection,
 237             )
 238             | PlaceContext::NonMutatingUse(
 239                 NonMutatingUseContext::Inspect
 240                 | NonMutatingUseContext::SharedBorrow
 241                 | NonMutatingUseContext::UniqueBorrow
 242                 | NonMutatingUseContext::ShallowBorrow
 243                 | NonMutatingUseContext::AddressOf
 244                 | NonMutatingUseContext::Projection,
 245             ) => {
 246                 self.not_ssa(local);
 247             }
 248
 249             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 250                 let ty = self.fx.mir.local_decls[local].ty;
 251                 let ty = self.fx.monomorphize(ty);
 252
 253                 // Only need the place if we're actually dropping it.
 254                 if self.fx.cx.type_needs_drop(ty) {
 255                     self.not_ssa(local);
 256                 }
 257             }
 258         }
 259     }
 260 }
 261
 262 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 263 pub enum CleanupKind {
 264     NotCleanup,
 265     Funclet,
 266     Internal { funclet: mir::BasicBlock },
 267 }
 268
 269 impl CleanupKind {
 270     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 271         match self {
 272             CleanupKind::NotCleanup => None,
 273             CleanupKind::Funclet => Some(for_bb),
 274             CleanupKind::Internal { funclet } => Some(funclet),
 275         }
 276     }
 277 }
 278
 279 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 280     fn discover_masters<'tcx>(
 281         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 282         mir: &mir::Body<'tcx>,
 283     ) {
 284         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 285             match data.terminator().kind {
 286                 TerminatorKind::Goto { .. }
 287                 | TerminatorKind::Resume
 288                 | TerminatorKind::Abort
 289                 | TerminatorKind::Return
 290                 | TerminatorKind::GeneratorDrop
 291                 | TerminatorKind::Unreachable
 292                 | TerminatorKind::SwitchInt { .. }
 293                 | TerminatorKind::Yield { .. }
 294                 | TerminatorKind::FalseEdge { .. }
 295                 | TerminatorKind::FalseUnwind { .. }
 296                 | TerminatorKind::InlineAsm { .. } => { /* nothing to do */ }
 297                 TerminatorKind::Call { cleanup: unwind, .. }
 298                 | TerminatorKind::Assert { cleanup: unwind, .. }
 299                 | TerminatorKind::DropAndReplace { unwind, .. }
 300                 | TerminatorKind::Drop { unwind, .. } => {
 301                     if let Some(unwind) = unwind {
 302                         debug!(
 303                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 304                             bb, data, unwind
 305                         );
 306                         result[unwind] = CleanupKind::Funclet;
 307                     }
 308                 }
 309             }
 310         }
 311     }
 312
 313     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
 314         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 315
 316         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
 317             ref mut s @ None => {
 318                 debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ);
 319                 *s = Some(succ);
 320             }
 321             Some(s) => {
 322                 if s != succ {
 323                     span_bug!(
 324                         mir.span,
 325                         "funclet {:?} has 2 parents - {:?} and {:?}",
 326                         funclet,
 327                         s,
 328                         succ
 329                     );
 330                 }
 331             }
 332         };
 333
 334         for (bb, data) in traversal::reverse_postorder(mir) {
 335             let funclet = match result[bb] {
 336                 CleanupKind::NotCleanup => continue,
 337                 CleanupKind::Funclet => bb,
 338                 CleanupKind::Internal { funclet } => funclet,
 339             };
 340
 341             debug!(
 342                 "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 343                 bb, data, result[bb], funclet
 344             );
 345
 346             for &succ in data.terminator().successors() {
 347                 let kind = result[succ];
 348                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind);
 349                 match kind {
 350                     CleanupKind::NotCleanup => {
 351                         result[succ] = CleanupKind::Internal { funclet };
 352                     }
 353                     CleanupKind::Funclet => {
 354                         if funclet != succ {
 355                             set_successor(funclet, succ);
 356                         }
 357                     }
 358                     CleanupKind::Internal { funclet: succ_funclet } => {
 359                         if funclet != succ_funclet {
 360                             // `succ` has 2 different funclet going into it, so it must
 361                             // be a funclet by itself.
 362
 363                             debug!(
 364                                 "promoting {:?} to a funclet and updating {:?}",
 365                                 succ, succ_funclet
 366                             );
 367                             result[succ] = CleanupKind::Funclet;
 368                             set_successor(succ_funclet, succ);
 369                             set_successor(funclet, succ);
 370                         }
 371                     }
 372                 }
 373             }
 374         }
 375     }
 376
 377     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 378
 379     discover_masters(&mut result, mir);
 380     propagate(&mut result, mir);
 381     debug!("cleanup_kinds: result={:?}", result);
 382     result
 383 }