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::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, LayoutOf};
  13
  14 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  15     fx: &FunctionCx<'a, 'tcx, Bx>,
  16 ) -> BitSet<mir::Local> {
  17     let mir = fx.mir;
  18     let dominators = mir.basic_blocks.dominators();
  19     let locals = mir
  20         .local_decls
  21         .iter()
  22         .map(|decl| {
  23             let ty = fx.monomorphize(decl.ty);
  24             let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
  25             if layout.is_zst() {
  26                 LocalKind::ZST
  27             } else if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) {
  28                 LocalKind::Unused
  29             } else {
  30                 LocalKind::Memory
  31             }
  32         })
  33         .collect();
  34
  35     let mut analyzer = LocalAnalyzer { fx, dominators, locals };
  36
  37     // Arguments get assigned to by means of the function being called
  38     for arg in mir.args_iter() {
  39         analyzer.assign(arg, DefLocation::Argument);
  40     }
  41
  42     // If there exists a local definition that dominates all uses of that local,
  43     // the definition should be visited first. Traverse blocks in an order that
  44     // is a topological sort of dominance partial order.
  45     for (bb, data) in traversal::reverse_postorder(&mir) {
  46         analyzer.visit_basic_block_data(bb, data);
  47     }
  48
  49     let mut non_ssa_locals = BitSet::new_empty(analyzer.locals.len());
  50     for (local, kind) in analyzer.locals.iter_enumerated() {
  51         if matches!(kind, LocalKind::Memory) {
  52             non_ssa_locals.insert(local);
  53         }
  54     }
  55
  56     non_ssa_locals
  57 }
  58
  59 #[derive(Copy, Clone, PartialEq, Eq)]
  60 enum LocalKind {
  61     ZST,
  62     /// A local that requires an alloca.
  63     Memory,
  64     /// A scalar or a scalar pair local that is neither defined nor used.
  65     Unused,
  66     /// A scalar or a scalar pair local with a single definition that dominates all uses.
  67     SSA(DefLocation),
  68 }
  69
  70 #[derive(Copy, Clone, PartialEq, Eq)]
  71 enum DefLocation {
  72     Argument,
  73     Body(Location),
  74 }
  75
  76 impl DefLocation {
  77     fn dominates(self, location: Location, dominators: &Dominators<mir::BasicBlock>) -> bool {
  78         match self {
  79             DefLocation::Argument => true,
  80             DefLocation::Body(def) => def.successor_within_block().dominates(location, dominators),
  81         }
  82     }
  83 }
  84
  85 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
  86     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
  87     dominators: Dominators<mir::BasicBlock>,
  88     locals: IndexVec<mir::Local, LocalKind>,
  89 }
  90
  91 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> {
  92     fn assign(&mut self, local: mir::Local, location: DefLocation) {
  93         let kind = &mut self.locals[local];
  94         match *kind {
  95             LocalKind::ZST => {}
  96             LocalKind::Memory => {}
  97             LocalKind::Unused => *kind = LocalKind::SSA(location),
  98             LocalKind::SSA(_) => *kind = LocalKind::Memory,
  99         }
 100     }
 101
 102     fn process_place(
 103         &mut self,
 104         place_ref: &mir::PlaceRef<'tcx>,
 105         context: PlaceContext,
 106         location: Location,
 107     ) {
 108         let cx = self.fx.cx;
 109
 110         if let Some((place_base, elem)) = place_ref.last_projection() {
 111             let mut base_context = if context.is_mutating_use() {
 112                 PlaceContext::MutatingUse(MutatingUseContext::Projection)
 113             } else {
 114                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
 115             };
 116
 117             // Allow uses of projections that are ZSTs or from scalar fields.
 118             let is_consume = matches!(
 119                 context,
 120                 PlaceContext::NonMutatingUse(
 121                     NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 122                 )
 123             );
 124             if is_consume {
 125                 let base_ty = place_base.ty(self.fx.mir, cx.tcx());
 126                 let base_ty = self.fx.monomorphize(base_ty);
 127
 128                 // ZSTs don't require any actual memory access.
 129                 let elem_ty = base_ty.projection_ty(cx.tcx(), self.fx.monomorphize(elem)).ty;
 130                 let span = self.fx.mir.local_decls[place_ref.local].source_info.span;
 131                 if cx.spanned_layout_of(elem_ty, span).is_zst() {
 132                     return;
 133                 }
 134
 135                 if let mir::ProjectionElem::Field(..) = elem {
 136                     let layout = cx.spanned_layout_of(base_ty.ty, span);
 137                     if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
 138                         // Recurse with the same context, instead of `Projection`,
 139                         // potentially stopping at non-operand projections,
 140                         // which would trigger `not_ssa` on locals.
 141                         base_context = context;
 142                     }
 143                 }
 144             }
 145
 146             if let mir::ProjectionElem::Deref = elem {
 147                 // Deref projections typically only read the pointer.
 148                 base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
 149             }
 150
 151             self.process_place(&place_base, base_context, location);
 152             // HACK(eddyb) this emulates the old `visit_projection_elem`, this
 153             // entire `visit_place`-like `process_place` method should be rewritten,
 154             // now that we have moved to the "slice of projections" representation.
 155             if let mir::ProjectionElem::Index(local) = elem {
 156                 self.visit_local(
 157                     local,
 158                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 159                     location,
 160                 );
 161             }
 162         } else {
 163             self.visit_local(place_ref.local, context, location);
 164         }
 165     }
 166 }
 167
 168 impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
 169     for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
 170 {
 171     fn visit_assign(
 172         &mut self,
 173         place: &mir::Place<'tcx>,
 174         rvalue: &mir::Rvalue<'tcx>,
 175         location: Location,
 176     ) {
 177         debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
 178
 179         if let Some(local) = place.as_local() {
 180             self.assign(local, DefLocation::Body(location));
 181             if self.locals[local] != LocalKind::Memory {
 182                 let decl_span = self.fx.mir.local_decls[local].source_info.span;
 183                 if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
 184                     self.locals[local] = LocalKind::Memory;
 185                 }
 186             }
 187         } else {
 188             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
 189         }
 190
 191         self.visit_rvalue(rvalue, location);
 192     }
 193
 194     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
 195         debug!("visit_place(place={:?}, context={:?})", place, context);
 196         self.process_place(&place.as_ref(), context, location);
 197     }
 198
 199     fn visit_local(&mut self, local: mir::Local, context: PlaceContext, location: Location) {
 200         match context {
 201             PlaceContext::MutatingUse(MutatingUseContext::Call)
 202             | PlaceContext::MutatingUse(MutatingUseContext::Yield) => {
 203                 self.assign(local, DefLocation::Body(location));
 204             }
 205
 206             PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 207
 208             PlaceContext::NonMutatingUse(
 209                 NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
 210             ) => match &mut self.locals[local] {
 211                 LocalKind::ZST => {}
 212                 LocalKind::Memory => {}
 213                 LocalKind::SSA(def) if def.dominates(location, &self.dominators) => {}
 214                 // Reads from uninitialized variables (e.g., in dead code, after
 215                 // optimizations) require locals to be in (uninitialized) memory.
 216                 // N.B., there can be uninitialized reads of a local visited after
 217                 // an assignment to that local, if they happen on disjoint paths.
 218                 kind @ (LocalKind::Unused | LocalKind::SSA(_)) => {
 219                     *kind = LocalKind::Memory;
 220                 }
 221             },
 222
 223             PlaceContext::MutatingUse(
 224                 MutatingUseContext::Store
 225                 | MutatingUseContext::Deinit
 226                 | MutatingUseContext::SetDiscriminant
 227                 | MutatingUseContext::AsmOutput
 228                 | MutatingUseContext::Borrow
 229                 | MutatingUseContext::AddressOf
 230                 | MutatingUseContext::Projection,
 231             )
 232             | PlaceContext::NonMutatingUse(
 233                 NonMutatingUseContext::Inspect
 234                 | NonMutatingUseContext::SharedBorrow
 235                 | NonMutatingUseContext::UniqueBorrow
 236                 | NonMutatingUseContext::ShallowBorrow
 237                 | NonMutatingUseContext::AddressOf
 238                 | NonMutatingUseContext::Projection,
 239             ) => {
 240                 self.locals[local] = LocalKind::Memory;
 241             }
 242
 243             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 244                 let kind = &mut self.locals[local];
 245                 if *kind != LocalKind::Memory {
 246                     let ty = self.fx.mir.local_decls[local].ty;
 247                     let ty = self.fx.monomorphize(ty);
 248                     if self.fx.cx.type_needs_drop(ty) {
 249                         // Only need the place if we're actually dropping it.
 250                         *kind = LocalKind::Memory;
 251                     }
 252                 }
 253             }
 254         }
 255     }
 256 }
 257
 258 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 259 pub enum CleanupKind {
 260     NotCleanup,
 261     Funclet,
 262     Internal { funclet: mir::BasicBlock },
 263 }
 264
 265 impl CleanupKind {
 266     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 267         match self {
 268             CleanupKind::NotCleanup => None,
 269             CleanupKind::Funclet => Some(for_bb),
 270             CleanupKind::Internal { funclet } => Some(funclet),
 271         }
 272     }
 273 }
 274
 275 /// MSVC requires unwinding code to be split to a tree of *funclets*, where each funclet can only
 276 /// branch to itself or to its parent. Luckily, the code we generates matches this pattern.
 277 /// Recover that structure in an analyze pass.
 278 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 279     fn discover_masters<'tcx>(
 280         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 281         mir: &mir::Body<'tcx>,
 282     ) {
 283         for (bb, data) in mir.basic_blocks.iter_enumerated() {
 284             match data.terminator().kind {
 285                 TerminatorKind::Goto { .. }
 286                 | TerminatorKind::Resume
 287                 | TerminatorKind::Abort
 288                 | TerminatorKind::Return
 289                 | TerminatorKind::GeneratorDrop
 290                 | TerminatorKind::Unreachable
 291                 | TerminatorKind::SwitchInt { .. }
 292                 | TerminatorKind::Yield { .. }
 293                 | TerminatorKind::FalseEdge { .. }
 294                 | TerminatorKind::FalseUnwind { .. } => { /* nothing to do */ }
 295                 TerminatorKind::Call { cleanup: unwind, .. }
 296                 | TerminatorKind::InlineAsm { cleanup: unwind, .. }
 297                 | TerminatorKind::Assert { cleanup: unwind, .. }
 298                 | TerminatorKind::DropAndReplace { unwind, .. }
 299                 | TerminatorKind::Drop { unwind, .. } => {
 300                     if let Some(unwind) = unwind {
 301                         debug!(
 302                             "cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 303                             bb, data, unwind
 304                         );
 305                         result[unwind] = CleanupKind::Funclet;
 306                     }
 307                 }
 308             }
 309         }
 310     }
 311
 312     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>, mir: &mir::Body<'tcx>) {
 313         let mut funclet_succs = IndexVec::from_elem(None, &mir.basic_blocks);
 314
 315         let mut set_successor = |funclet: mir::BasicBlock, succ| match funclet_succs[funclet] {
 316             ref mut s @ None => {
 317                 debug!("set_successor: updating successor of {:?} to {:?}", funclet, succ);
 318                 *s = Some(succ);
 319             }
 320             Some(s) => {
 321                 if s != succ {
 322                     span_bug!(
 323                         mir.span,
 324                         "funclet {:?} has 2 parents - {:?} and {:?}",
 325                         funclet,
 326                         s,
 327                         succ
 328                     );
 329                 }
 330             }
 331         };
 332
 333         for (bb, data) in traversal::reverse_postorder(mir) {
 334             let funclet = match result[bb] {
 335                 CleanupKind::NotCleanup => continue,
 336                 CleanupKind::Funclet => bb,
 337                 CleanupKind::Internal { funclet } => funclet,
 338             };
 339
 340             debug!(
 341                 "cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 342                 bb, data, result[bb], funclet
 343             );
 344
 345             for succ in data.terminator().successors() {
 346                 let kind = result[succ];
 347                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}", funclet, succ, kind);
 348                 match kind {
 349                     CleanupKind::NotCleanup => {
 350                         result[succ] = CleanupKind::Internal { funclet };
 351                     }
 352                     CleanupKind::Funclet => {
 353                         if funclet != succ {
 354                             set_successor(funclet, succ);
 355                         }
 356                     }
 357                     CleanupKind::Internal { funclet: succ_funclet } => {
 358                         if funclet != succ_funclet {
 359                             // `succ` has 2 different funclet going into it, so it must
 360                             // be a funclet by itself.
 361
 362                             debug!(
 363                                 "promoting {:?} to a funclet and updating {:?}",
 364                                 succ, succ_funclet
 365                             );
 366                             result[succ] = CleanupKind::Funclet;
 367                             set_successor(succ_funclet, succ);
 368                             set_successor(funclet, succ);
 369                         }
 370                     }
 371                 }
 372             }
 373         }
 374     }
 375
 376     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, &mir.basic_blocks);
 377
 378     discover_masters(&mut result, mir);
 379     propagate(&mut result, mir);
 380     debug!("cleanup_kinds: result={:?}", result);
 381     result
 382 }