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