src/librustc_codegen_llvm/mir/analyze.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! An analysis to determine which locals require allocas and
  12 //! which do not.
  13
  14 use rustc_data_structures::bit_set::BitSet;
  15 use rustc_data_structures::graph::dominators::Dominators;
  16 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  17 use rustc::mir::{self, Location, TerminatorKind};
  18 use rustc::mir::visit::{Visitor, PlaceContext, MutatingUseContext, NonMutatingUseContext};
  19 use rustc::mir::traversal;
  20 use rustc::ty;
  21 use rustc::ty::layout::LayoutOf;
  22 use type_of::LayoutLlvmExt;
  23 use super::FunctionCx;
  24 use value::Value;
  25
  26 pub fn non_ssa_locals(fx: &FunctionCx<'a, 'll, 'tcx, &'ll Value>) -> BitSet<mir::Local> {
  27     let mir = fx.mir;
  28     let mut analyzer = LocalAnalyzer::new(fx);
  29
  30     analyzer.visit_mir(mir);
  31
  32     for (index, ty) in mir.local_decls.iter().map(|l| l.ty).enumerate() {
  33         let ty = fx.monomorphize(&ty);
  34         debug!("local {} has type {:?}", index, ty);
  35         let layout = fx.cx.layout_of(ty);
  36         if layout.is_llvm_immediate() {
  37             // These sorts of types are immediates that we can store
  38             // in an Value without an alloca.
  39         } else if layout.is_llvm_scalar_pair() {
  40             // We allow pairs and uses of any of their 2 fields.
  41         } else {
  42             // These sorts of types require an alloca. Note that
  43             // is_llvm_immediate() may *still* be true, particularly
  44             // for newtypes, but we currently force some types
  45             // (e.g. structs) into an alloca unconditionally, just so
  46             // that we don't have to deal with having two pathways
  47             // (gep vs extractvalue etc).
  48             analyzer.not_ssa(mir::Local::new(index));
  49         }
  50     }
  51
  52     analyzer.non_ssa_locals
  53 }
  54
  55 struct LocalAnalyzer<'mir, 'a: 'mir, 'll: 'a, 'tcx: 'll, V: 'll> {
  56     fx: &'mir FunctionCx<'a, 'll, 'tcx, V>,
  57     dominators: Dominators<mir::BasicBlock>,
  58     non_ssa_locals: BitSet<mir::Local>,
  59     // The location of the first visited direct assignment to each
  60     // local, or an invalid location (out of bounds `block` index).
  61     first_assignment: IndexVec<mir::Local, Location>
  62 }
  63
  64 impl LocalAnalyzer<'mir, 'a, 'll, 'tcx, &'ll Value> {
  65     fn new(fx: &'mir FunctionCx<'a, 'll, 'tcx, &'ll Value>) -> Self {
  66         let invalid_location =
  67             mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
  68         let mut analyzer = LocalAnalyzer {
  69             fx,
  70             dominators: fx.mir.dominators(),
  71             non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
  72             first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls)
  73         };
  74
  75         // Arguments get assigned to by means of the function being called
  76         for arg in fx.mir.args_iter() {
  77             analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
  78         }
  79
  80         analyzer
  81     }
  82
  83     fn first_assignment(&self, local: mir::Local) -> Option<Location> {
  84         let location = self.first_assignment[local];
  85         if location.block.index() < self.fx.mir.basic_blocks().len() {
  86             Some(location)
  87         } else {
  88             None
  89         }
  90     }
  91
  92     fn not_ssa(&mut self, local: mir::Local) {
  93         debug!("marking {:?} as non-SSA", local);
  94         self.non_ssa_locals.insert(local);
  95     }
  96
  97     fn assign(&mut self, local: mir::Local, location: Location) {
  98         if self.first_assignment(local).is_some() {
  99             self.not_ssa(local);
 100         } else {
 101             self.first_assignment[local] = location;
 102         }
 103     }
 104 }
 105
 106 impl Visitor<'tcx> for LocalAnalyzer<'mir, 'a, 'll, 'tcx, &'ll Value> {
 107     fn visit_assign(&mut self,
 108                     block: mir::BasicBlock,
 109                     place: &mir::Place<'tcx>,
 110                     rvalue: &mir::Rvalue<'tcx>,
 111                     location: Location) {
 112         debug!("visit_assign(block={:?}, place={:?}, rvalue={:?})", block, place, rvalue);
 113
 114         if let mir::Place::Local(index) = *place {
 115             self.assign(index, location);
 116             if !self.fx.rvalue_creates_operand(rvalue) {
 117                 self.not_ssa(index);
 118             }
 119         } else {
 120             self.visit_place(
 121                 place,
 122                 PlaceContext::MutatingUse(MutatingUseContext::Store),
 123                 location
 124             );
 125         }
 126
 127         self.visit_rvalue(rvalue, location);
 128     }
 129
 130     fn visit_terminator_kind(&mut self,
 131                              block: mir::BasicBlock,
 132                              kind: &mir::TerminatorKind<'tcx>,
 133                              location: Location) {
 134         let check = match *kind {
 135             mir::TerminatorKind::Call {
 136                 func: mir::Operand::Constant(ref c),
 137                 ref args, ..
 138             } => match c.ty.sty {
 139                 ty::FnDef(did, _) => Some((did, args)),
 140                 _ => None,
 141             },
 142             _ => None,
 143         };
 144         if let Some((def_id, args)) = check {
 145             if Some(def_id) == self.fx.cx.tcx.lang_items().box_free_fn() {
 146                 // box_free(x) shares with `drop x` the property that it
 147                 // is not guaranteed to be statically dominated by the
 148                 // definition of x, so x must always be in an alloca.
 149                 if let mir::Operand::Move(ref place) = args[0] {
 150                     self.visit_place(
 151                         place,
 152                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 153                         location
 154                     );
 155                 }
 156             }
 157         }
 158
 159         self.super_terminator_kind(block, kind, location);
 160     }
 161
 162     fn visit_place(&mut self,
 163                    place: &mir::Place<'tcx>,
 164                    context: PlaceContext<'tcx>,
 165                    location: Location) {
 166         debug!("visit_place(place={:?}, context={:?})", place, context);
 167         let cx = self.fx.cx;
 168
 169         if let mir::Place::Projection(ref proj) = *place {
 170             // Allow uses of projections that are ZSTs or from scalar fields.
 171             let is_consume = match context {
 172                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 173                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 174                 _ => false
 175             };
 176             if is_consume {
 177                 let base_ty = proj.base.ty(self.fx.mir, cx.tcx);
 178                 let base_ty = self.fx.monomorphize(&base_ty);
 179
 180                 // ZSTs don't require any actual memory access.
 181                 let elem_ty = base_ty
 182                     .projection_ty(cx.tcx, &proj.elem)
 183                     .to_ty(cx.tcx);
 184                 let elem_ty = self.fx.monomorphize(&elem_ty);
 185                 if cx.layout_of(elem_ty).is_zst() {
 186                     return;
 187                 }
 188
 189                 if let mir::ProjectionElem::Field(..) = proj.elem {
 190                     let layout = cx.layout_of(base_ty.to_ty(cx.tcx));
 191                     if layout.is_llvm_immediate() || layout.is_llvm_scalar_pair() {
 192                         // Recurse with the same context, instead of `Projection`,
 193                         // potentially stopping at non-operand projections,
 194                         // which would trigger `not_ssa` on locals.
 195                         self.visit_place(&proj.base, context, location);
 196                         return;
 197                     }
 198                 }
 199             }
 200
 201             // A deref projection only reads the pointer, never needs the place.
 202             if let mir::ProjectionElem::Deref = proj.elem {
 203                 return self.visit_place(
 204                     &proj.base,
 205                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 206                     location
 207                 );
 208             }
 209         }
 210
 211         self.super_place(place, context, location);
 212     }
 213
 214     fn visit_local(&mut self,
 215                    &local: &mir::Local,
 216                    context: PlaceContext<'tcx>,
 217                    location: Location) {
 218         match context {
 219             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 220                 self.assign(local, location);
 221             }
 222
 223             PlaceContext::NonUse(_) |
 224             PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 225
 226             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 227             PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 228                 // Reads from uninitialized variables (e.g. in dead code, after
 229                 // optimizations) require locals to be in (uninitialized) memory.
 230                 // NB: there can be uninitialized reads of a local visited after
 231                 // an assignment to that local, if they happen on disjoint paths.
 232                 let ssa_read = match self.first_assignment(local) {
 233                     Some(assignment_location) => {
 234                         assignment_location.dominates(location, &self.dominators)
 235                     }
 236                     None => false
 237                 };
 238                 if !ssa_read {
 239                     self.not_ssa(local);
 240                 }
 241             }
 242
 243             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
 244             PlaceContext::MutatingUse(MutatingUseContext::Store) |
 245             PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
 246             PlaceContext::MutatingUse(MutatingUseContext::Borrow(..)) |
 247             PlaceContext::MutatingUse(MutatingUseContext::Projection) |
 248             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow(..)) |
 249             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow(..)) |
 250             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow(..)) |
 251             PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 252                 self.not_ssa(local);
 253             }
 254
 255             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 256                 let ty = mir::Place::Local(local).ty(self.fx.mir, self.fx.cx.tcx);
 257                 let ty = self.fx.monomorphize(&ty.to_ty(self.fx.cx.tcx));
 258
 259                 // Only need the place if we're actually dropping it.
 260                 if self.fx.cx.type_needs_drop(ty) {
 261                     self.not_ssa(local);
 262                 }
 263             }
 264         }
 265     }
 266 }
 267
 268 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 269 pub enum CleanupKind {
 270     NotCleanup,
 271     Funclet,
 272     Internal { funclet: mir::BasicBlock }
 273 }
 274
 275 impl CleanupKind {
 276     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 277         match self {
 278             CleanupKind::NotCleanup => None,
 279             CleanupKind::Funclet => Some(for_bb),
 280             CleanupKind::Internal { funclet } => Some(funclet),
 281         }
 282     }
 283 }
 284
 285 pub fn cleanup_kinds<'a, 'tcx>(mir: &mir::Mir<'tcx>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 286     fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 287                               mir: &mir::Mir<'tcx>) {
 288         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 289             match data.terminator().kind {
 290                 TerminatorKind::Goto { .. } |
 291                 TerminatorKind::Resume |
 292                 TerminatorKind::Abort |
 293                 TerminatorKind::Return |
 294                 TerminatorKind::GeneratorDrop |
 295                 TerminatorKind::Unreachable |
 296                 TerminatorKind::SwitchInt { .. } |
 297                 TerminatorKind::Yield { .. } |
 298                 TerminatorKind::FalseEdges { .. } |
 299                 TerminatorKind::FalseUnwind { .. } => {
 300                     /* nothing to do */
 301                 }
 302                 TerminatorKind::Call { cleanup: unwind, .. } |
 303                 TerminatorKind::Assert { cleanup: unwind, .. } |
 304                 TerminatorKind::DropAndReplace { unwind, .. } |
 305                 TerminatorKind::Drop { unwind, .. } => {
 306                     if let Some(unwind) = unwind {
 307                         debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 308                                bb, data, unwind);
 309                         result[unwind] = CleanupKind::Funclet;
 310                     }
 311                 }
 312             }
 313         }
 314     }
 315
 316     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 317                        mir: &mir::Mir<'tcx>) {
 318         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 319
 320         let mut set_successor = |funclet: mir::BasicBlock, succ| {
 321             match funclet_succs[funclet] {
 322                 ref mut s @ None => {
 323                     debug!("set_successor: updating successor of {:?} to {:?}",
 324                            funclet, succ);
 325                     *s = Some(succ);
 326                 },
 327                 Some(s) => if s != succ {
 328                     span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
 329                               funclet, s, succ);
 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!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 342                    bb, data, result[bb], funclet);
 343
 344             for &succ in data.terminator().successors() {
 345                 let kind = result[succ];
 346                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
 347                        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!("promoting {:?} to a funclet and updating {:?}", succ,
 363                                    succ_funclet);
 364                             result[succ] = CleanupKind::Funclet;
 365                             set_successor(succ_funclet, succ);
 366                             set_successor(funclet, succ);
 367                         }
 368                     }
 369                 }
 370             }
 371         }
 372     }
 373
 374     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 375
 376     discover_masters(&mut result, mir);
 377     propagate(&mut result, mir);
 378     debug!("cleanup_kinds: result={:?}", result);
 379     result
 380 }