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 use interfaces::*;
  26
  27 pub fn non_ssa_locals(fx: &FunctionCx<'a, 'll, 'tcx, &'ll Value>) -> BitSet<mir::Local> {
  28     let mir = fx.mir;
  29     let mut analyzer = LocalAnalyzer::new(fx);
  30
  31     analyzer.visit_mir(mir);
  32
  33     for (index, ty) in mir.local_decls.iter().map(|l| l.ty).enumerate() {
  34         let ty = fx.monomorphize(&ty);
  35         debug!("local {} has type {:?}", index, ty);
  36         let layout = fx.cx.layout_of(ty);
  37         if layout.is_llvm_immediate() {
  38             // These sorts of types are immediates that we can store
  39             // in an Value without an alloca.
  40         } else if layout.is_llvm_scalar_pair() {
  41             // We allow pairs and uses of any of their 2 fields.
  42         } else {
  43             // These sorts of types require an alloca. Note that
  44             // is_llvm_immediate() may *still* be true, particularly
  45             // for newtypes, but we currently force some types
  46             // (e.g. structs) into an alloca unconditionally, just so
  47             // that we don't have to deal with having two pathways
  48             // (gep vs extractvalue etc).
  49             analyzer.not_ssa(mir::Local::new(index));
  50         }
  51     }
  52
  53     analyzer.non_ssa_locals
  54 }
  55
  56 struct LocalAnalyzer<'mir, 'a: 'mir, 'll: 'a, 'tcx: 'll, V: 'll> {
  57     fx: &'mir FunctionCx<'a, 'll, 'tcx, V>,
  58     dominators: Dominators<mir::BasicBlock>,
  59     non_ssa_locals: BitSet<mir::Local>,
  60     // The location of the first visited direct assignment to each
  61     // local, or an invalid location (out of bounds `block` index).
  62     first_assignment: IndexVec<mir::Local, Location>
  63 }
  64
  65 impl LocalAnalyzer<'mir, 'a, 'll, 'tcx, &'ll Value> {
  66     fn new(fx: &'mir FunctionCx<'a, 'll, 'tcx, &'ll Value>) -> Self {
  67         let invalid_location =
  68             mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
  69         let mut analyzer = LocalAnalyzer {
  70             fx,
  71             dominators: fx.mir.dominators(),
  72             non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
  73             first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls)
  74         };
  75
  76         // Arguments get assigned to by means of the function being called
  77         for arg in fx.mir.args_iter() {
  78             analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
  79         }
  80
  81         analyzer
  82     }
  83
  84     fn first_assignment(&self, local: mir::Local) -> Option<Location> {
  85         let location = self.first_assignment[local];
  86         if location.block.index() < self.fx.mir.basic_blocks().len() {
  87             Some(location)
  88         } else {
  89             None
  90         }
  91     }
  92
  93     fn not_ssa(&mut self, local: mir::Local) {
  94         debug!("marking {:?} as non-SSA", local);
  95         self.non_ssa_locals.insert(local);
  96     }
  97
  98     fn assign(&mut self, local: mir::Local, location: Location) {
  99         if self.first_assignment(local).is_some() {
 100             self.not_ssa(local);
 101         } else {
 102             self.first_assignment[local] = location;
 103         }
 104     }
 105 }
 106
 107 impl Visitor<'tcx> for LocalAnalyzer<'mir, 'a, 'll, 'tcx, &'ll Value> {
 108     fn visit_assign(&mut self,
 109                     block: mir::BasicBlock,
 110                     place: &mir::Place<'tcx>,
 111                     rvalue: &mir::Rvalue<'tcx>,
 112                     location: Location) {
 113         debug!("visit_assign(block={:?}, place={:?}, rvalue={:?})", block, place, rvalue);
 114
 115         if let mir::Place::Local(index) = *place {
 116             self.assign(index, location);
 117             if !self.fx.rvalue_creates_operand(rvalue) {
 118                 self.not_ssa(index);
 119             }
 120         } else {
 121             self.visit_place(
 122                 place,
 123                 PlaceContext::MutatingUse(MutatingUseContext::Store),
 124                 location
 125             );
 126         }
 127
 128         self.visit_rvalue(rvalue, location);
 129     }
 130
 131     fn visit_terminator_kind(&mut self,
 132                              block: mir::BasicBlock,
 133                              kind: &mir::TerminatorKind<'tcx>,
 134                              location: Location) {
 135         let check = match *kind {
 136             mir::TerminatorKind::Call {
 137                 func: mir::Operand::Constant(ref c),
 138                 ref args, ..
 139             } => match c.ty.sty {
 140                 ty::FnDef(did, _) => Some((did, args)),
 141                 _ => None,
 142             },
 143             _ => None,
 144         };
 145         if let Some((def_id, args)) = check {
 146             if Some(def_id) == self.fx.cx.tcx.lang_items().box_free_fn() {
 147                 // box_free(x) shares with `drop x` the property that it
 148                 // is not guaranteed to be statically dominated by the
 149                 // definition of x, so x must always be in an alloca.
 150                 if let mir::Operand::Move(ref place) = args[0] {
 151                     self.visit_place(
 152                         place,
 153                         PlaceContext::MutatingUse(MutatingUseContext::Drop),
 154                         location
 155                     );
 156                 }
 157             }
 158         }
 159
 160         self.super_terminator_kind(block, kind, location);
 161     }
 162
 163     fn visit_place(&mut self,
 164                    place: &mir::Place<'tcx>,
 165                    context: PlaceContext<'tcx>,
 166                    location: Location) {
 167         debug!("visit_place(place={:?}, context={:?})", place, context);
 168         let cx = self.fx.cx;
 169
 170         if let mir::Place::Projection(ref proj) = *place {
 171             // Allow uses of projections that are ZSTs or from scalar fields.
 172             let is_consume = match context {
 173                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 174                 PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
 175                 _ => false
 176             };
 177             if is_consume {
 178                 let base_ty = proj.base.ty(self.fx.mir, cx.tcx);
 179                 let base_ty = self.fx.monomorphize(&base_ty);
 180
 181                 // ZSTs don't require any actual memory access.
 182                 let elem_ty = base_ty
 183                     .projection_ty(cx.tcx, &proj.elem)
 184                     .to_ty(cx.tcx);
 185                 let elem_ty = self.fx.monomorphize(&elem_ty);
 186                 if cx.layout_of(elem_ty).is_zst() {
 187                     return;
 188                 }
 189
 190                 if let mir::ProjectionElem::Field(..) = proj.elem {
 191                     let layout = cx.layout_of(base_ty.to_ty(cx.tcx));
 192                     if layout.is_llvm_immediate() || layout.is_llvm_scalar_pair() {
 193                         // Recurse with the same context, instead of `Projection`,
 194                         // potentially stopping at non-operand projections,
 195                         // which would trigger `not_ssa` on locals.
 196                         self.visit_place(&proj.base, context, location);
 197                         return;
 198                     }
 199                 }
 200             }
 201
 202             // A deref projection only reads the pointer, never needs the place.
 203             if let mir::ProjectionElem::Deref = proj.elem {
 204                 return self.visit_place(
 205                     &proj.base,
 206                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
 207                     location
 208                 );
 209             }
 210         }
 211
 212         self.super_place(place, context, location);
 213     }
 214
 215     fn visit_local(&mut self,
 216                    &local: &mir::Local,
 217                    context: PlaceContext<'tcx>,
 218                    location: Location) {
 219         match context {
 220             PlaceContext::MutatingUse(MutatingUseContext::Call) => {
 221                 self.assign(local, location);
 222             }
 223
 224             PlaceContext::NonUse(_) |
 225             PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
 226
 227             PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
 228             PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
 229                 // Reads from uninitialized variables (e.g. in dead code, after
 230                 // optimizations) require locals to be in (uninitialized) memory.
 231                 // NB: there can be uninitialized reads of a local visited after
 232                 // an assignment to that local, if they happen on disjoint paths.
 233                 let ssa_read = match self.first_assignment(local) {
 234                     Some(assignment_location) => {
 235                         assignment_location.dominates(location, &self.dominators)
 236                     }
 237                     None => false
 238                 };
 239                 if !ssa_read {
 240                     self.not_ssa(local);
 241                 }
 242             }
 243
 244             PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
 245             PlaceContext::MutatingUse(MutatingUseContext::Store) |
 246             PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
 247             PlaceContext::MutatingUse(MutatingUseContext::Borrow(..)) |
 248             PlaceContext::MutatingUse(MutatingUseContext::Projection) |
 249             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow(..)) |
 250             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow(..)) |
 251             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow(..)) |
 252             PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
 253                 self.not_ssa(local);
 254             }
 255
 256             PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
 257                 let ty = mir::Place::Local(local).ty(self.fx.mir, self.fx.cx.tcx);
 258                 let ty = self.fx.monomorphize(&ty.to_ty(self.fx.cx.tcx));
 259
 260                 // Only need the place if we're actually dropping it.
 261                 if self.fx.cx.type_needs_drop(ty) {
 262                     self.not_ssa(local);
 263                 }
 264             }
 265         }
 266     }
 267 }
 268
 269 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 270 pub enum CleanupKind {
 271     NotCleanup,
 272     Funclet,
 273     Internal { funclet: mir::BasicBlock }
 274 }
 275
 276 impl CleanupKind {
 277     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 278         match self {
 279             CleanupKind::NotCleanup => None,
 280             CleanupKind::Funclet => Some(for_bb),
 281             CleanupKind::Internal { funclet } => Some(funclet),
 282         }
 283     }
 284 }
 285
 286 pub fn cleanup_kinds<'a, 'tcx>(mir: &mir::Mir<'tcx>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 287     fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 288                               mir: &mir::Mir<'tcx>) {
 289         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 290             match data.terminator().kind {
 291                 TerminatorKind::Goto { .. } |
 292                 TerminatorKind::Resume |
 293                 TerminatorKind::Abort |
 294                 TerminatorKind::Return |
 295                 TerminatorKind::GeneratorDrop |
 296                 TerminatorKind::Unreachable |
 297                 TerminatorKind::SwitchInt { .. } |
 298                 TerminatorKind::Yield { .. } |
 299                 TerminatorKind::FalseEdges { .. } |
 300                 TerminatorKind::FalseUnwind { .. } => {
 301                     /* nothing to do */
 302                 }
 303                 TerminatorKind::Call { cleanup: unwind, .. } |
 304                 TerminatorKind::Assert { cleanup: unwind, .. } |
 305                 TerminatorKind::DropAndReplace { unwind, .. } |
 306                 TerminatorKind::Drop { unwind, .. } => {
 307                     if let Some(unwind) = unwind {
 308                         debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 309                                bb, data, unwind);
 310                         result[unwind] = CleanupKind::Funclet;
 311                     }
 312                 }
 313             }
 314         }
 315     }
 316
 317     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 318                        mir: &mir::Mir<'tcx>) {
 319         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 320
 321         let mut set_successor = |funclet: mir::BasicBlock, succ| {
 322             match funclet_succs[funclet] {
 323                 ref mut s @ None => {
 324                     debug!("set_successor: updating successor of {:?} to {:?}",
 325                            funclet, succ);
 326                     *s = Some(succ);
 327                 },
 328                 Some(s) => if s != succ {
 329                     span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
 330                               funclet, s, succ);
 331                 }
 332             }
 333         };
 334
 335         for (bb, data) in traversal::reverse_postorder(mir) {
 336             let funclet = match result[bb] {
 337                 CleanupKind::NotCleanup => continue,
 338                 CleanupKind::Funclet => bb,
 339                 CleanupKind::Internal { funclet } => funclet,
 340             };
 341
 342             debug!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 343                    bb, data, result[bb], funclet);
 344
 345             for &succ in data.terminator().successors() {
 346                 let kind = result[succ];
 347                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
 348                        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!("promoting {:?} to a funclet and updating {:?}", succ,
 364                                    succ_funclet);
 365                             result[succ] = CleanupKind::Funclet;
 366                             set_successor(succ_funclet, succ);
 367                             set_successor(funclet, succ);
 368                         }
 369                     }
 370                 }
 371             }
 372         }
 373     }
 374
 375     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 376
 377     discover_masters(&mut result, mir);
 378     propagate(&mut result, mir);
 379     debug!("cleanup_kinds: result={:?}", result);
 380     result
 381 }