src/librustc_trans/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::bitvec::BitVector;
  15 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  16 use rustc::middle::const_val::ConstVal;
  17 use rustc::mir::{self, Location, TerminatorKind, Literal};
  18 use rustc::mir::visit::{Visitor, LvalueContext};
  19 use rustc::mir::traversal;
  20 use common;
  21 use super::MirContext;
  22
  23 pub fn lvalue_locals<'a, 'tcx>(mircx: &MirContext<'a, 'tcx>) -> BitVector {
  24     let mir = mircx.mir;
  25     let mut analyzer = LocalAnalyzer::new(mircx);
  26
  27     analyzer.visit_mir(mir);
  28
  29     for (index, ty) in mir.local_decls.iter().map(|l| l.ty).enumerate() {
  30         let ty = mircx.monomorphize(&ty);
  31         debug!("local {} has type {:?}", index, ty);
  32         if ty.is_scalar() ||
  33             ty.is_box() ||
  34             ty.is_region_ptr() ||
  35             ty.is_simd() ||
  36             common::type_is_zero_size(mircx.ccx, ty)
  37         {
  38             // These sorts of types are immediates that we can store
  39             // in an ValueRef without an alloca.
  40             assert!(common::type_is_immediate(mircx.ccx, ty) ||
  41                     common::type_is_fat_ptr(mircx.ccx, ty));
  42         } else if common::type_is_imm_pair(mircx.ccx, ty) {
  43             // We allow pairs and uses of any of their 2 fields.
  44         } else {
  45             // These sorts of types require an alloca. Note that
  46             // type_is_immediate() may *still* be true, particularly
  47             // for newtypes, but we currently force some types
  48             // (e.g. structs) into an alloca unconditionally, just so
  49             // that we don't have to deal with having two pathways
  50             // (gep vs extractvalue etc).
  51             analyzer.mark_as_lvalue(mir::Local::new(index));
  52         }
  53     }
  54
  55     analyzer.lvalue_locals
  56 }
  57
  58 struct LocalAnalyzer<'mir, 'a: 'mir, 'tcx: 'a> {
  59     cx: &'mir MirContext<'a, 'tcx>,
  60     lvalue_locals: BitVector,
  61     seen_assigned: BitVector
  62 }
  63
  64 impl<'mir, 'a, 'tcx> LocalAnalyzer<'mir, 'a, 'tcx> {
  65     fn new(mircx: &'mir MirContext<'a, 'tcx>) -> LocalAnalyzer<'mir, 'a, 'tcx> {
  66         LocalAnalyzer {
  67             cx: mircx,
  68             lvalue_locals: BitVector::new(mircx.mir.local_decls.len()),
  69             seen_assigned: BitVector::new(mircx.mir.local_decls.len())
  70         }
  71     }
  72
  73     fn mark_as_lvalue(&mut self, local: mir::Local) {
  74         debug!("marking {:?} as lvalue", local);
  75         self.lvalue_locals.insert(local.index());
  76     }
  77
  78     fn mark_assigned(&mut self, local: mir::Local) {
  79         if !self.seen_assigned.insert(local.index()) {
  80             self.mark_as_lvalue(local);
  81         }
  82     }
  83 }
  84
  85 impl<'mir, 'a, 'tcx> Visitor<'tcx> for LocalAnalyzer<'mir, 'a, 'tcx> {
  86     fn visit_assign(&mut self,
  87                     block: mir::BasicBlock,
  88                     lvalue: &mir::Lvalue<'tcx>,
  89                     rvalue: &mir::Rvalue<'tcx>,
  90                     location: Location) {
  91         debug!("visit_assign(block={:?}, lvalue={:?}, rvalue={:?})", block, lvalue, rvalue);
  92
  93         if let mir::Lvalue::Local(index) = *lvalue {
  94             self.mark_assigned(index);
  95             if !self.cx.rvalue_creates_operand(rvalue) {
  96                 self.mark_as_lvalue(index);
  97             }
  98         } else {
  99             self.visit_lvalue(lvalue, LvalueContext::Store, location);
 100         }
 101
 102         self.visit_rvalue(rvalue, location);
 103     }
 104
 105     fn visit_terminator_kind(&mut self,
 106                              block: mir::BasicBlock,
 107                              kind: &mir::TerminatorKind<'tcx>,
 108                              location: Location) {
 109         match *kind {
 110             mir::TerminatorKind::Call {
 111                 func: mir::Operand::Constant(box mir::Constant {
 112                     literal: Literal::Value {
 113                         value: ConstVal::Function(def_id, _), ..
 114                     }, ..
 115                 }),
 116                 ref args, ..
 117             } if Some(def_id) == self.cx.ccx.tcx().lang_items.box_free_fn() => {
 118                 // box_free(x) shares with `drop x` the property that it
 119                 // is not guaranteed to be statically dominated by the
 120                 // definition of x, so x must always be in an alloca.
 121                 if let mir::Operand::Consume(ref lvalue) = args[0] {
 122                     self.visit_lvalue(lvalue, LvalueContext::Drop, location);
 123                 }
 124             }
 125             _ => {}
 126         }
 127
 128         self.super_terminator_kind(block, kind, location);
 129     }
 130
 131     fn visit_lvalue(&mut self,
 132                     lvalue: &mir::Lvalue<'tcx>,
 133                     context: LvalueContext<'tcx>,
 134                     location: Location) {
 135         debug!("visit_lvalue(lvalue={:?}, context={:?})", lvalue, context);
 136
 137         // Allow uses of projections of immediate pair fields.
 138         if let mir::Lvalue::Projection(ref proj) = *lvalue {
 139             if let mir::Lvalue::Local(_) = proj.base {
 140                 let ty = proj.base.ty(self.cx.mir, self.cx.ccx.tcx());
 141
 142                 let ty = self.cx.monomorphize(&ty.to_ty(self.cx.ccx.tcx()));
 143                 if common::type_is_imm_pair(self.cx.ccx, ty) {
 144                     if let mir::ProjectionElem::Field(..) = proj.elem {
 145                         if let LvalueContext::Consume = context {
 146                             return;
 147                         }
 148                     }
 149                 }
 150             }
 151         }
 152
 153         if let mir::Lvalue::Local(index) = *lvalue {
 154             match context {
 155                 LvalueContext::Call => {
 156                     self.mark_assigned(index);
 157                 }
 158
 159                 LvalueContext::StorageLive |
 160                 LvalueContext::StorageDead |
 161                 LvalueContext::Validate |
 162                 LvalueContext::Inspect |
 163                 LvalueContext::Consume => {}
 164
 165                 LvalueContext::Store |
 166                 LvalueContext::Borrow { .. } |
 167                 LvalueContext::Projection(..) => {
 168                     self.mark_as_lvalue(index);
 169                 }
 170
 171                 LvalueContext::Drop => {
 172                     let ty = lvalue.ty(self.cx.mir, self.cx.ccx.tcx());
 173                     let ty = self.cx.monomorphize(&ty.to_ty(self.cx.ccx.tcx()));
 174
 175                     // Only need the lvalue if we're actually dropping it.
 176                     if self.cx.ccx.shared().type_needs_drop(ty) {
 177                         self.mark_as_lvalue(index);
 178                     }
 179                 }
 180             }
 181         }
 182
 183         // A deref projection only reads the pointer, never needs the lvalue.
 184         if let mir::Lvalue::Projection(ref proj) = *lvalue {
 185             if let mir::ProjectionElem::Deref = proj.elem {
 186                 return self.visit_lvalue(&proj.base, LvalueContext::Consume, location);
 187             }
 188         }
 189
 190         self.super_lvalue(lvalue, context, location);
 191     }
 192 }
 193
 194 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 195 pub enum CleanupKind {
 196     NotCleanup,
 197     Funclet,
 198     Internal { funclet: mir::BasicBlock }
 199 }
 200
 201 impl CleanupKind {
 202     pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
 203         match self {
 204             CleanupKind::NotCleanup => None,
 205             CleanupKind::Funclet => Some(for_bb),
 206             CleanupKind::Internal { funclet } => Some(funclet),
 207         }
 208     }
 209 }
 210
 211 pub fn cleanup_kinds<'a, 'tcx>(mir: &mir::Mir<'tcx>) -> IndexVec<mir::BasicBlock, CleanupKind> {
 212     fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 213                               mir: &mir::Mir<'tcx>) {
 214         for (bb, data) in mir.basic_blocks().iter_enumerated() {
 215             match data.terminator().kind {
 216                 TerminatorKind::Goto { .. } |
 217                 TerminatorKind::Resume |
 218                 TerminatorKind::Return |
 219                 TerminatorKind::Unreachable |
 220                 TerminatorKind::SwitchInt { .. } => {
 221                     /* nothing to do */
 222                 }
 223                 TerminatorKind::Call { cleanup: unwind, .. } |
 224                 TerminatorKind::Assert { cleanup: unwind, .. } |
 225                 TerminatorKind::DropAndReplace { unwind, .. } |
 226                 TerminatorKind::Drop { unwind, .. } => {
 227                     if let Some(unwind) = unwind {
 228                         debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
 229                                bb, data, unwind);
 230                         result[unwind] = CleanupKind::Funclet;
 231                     }
 232                 }
 233             }
 234         }
 235     }
 236
 237     fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
 238                        mir: &mir::Mir<'tcx>) {
 239         let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
 240
 241         let mut set_successor = |funclet: mir::BasicBlock, succ| {
 242             match funclet_succs[funclet] {
 243                 ref mut s @ None => {
 244                     debug!("set_successor: updating successor of {:?} to {:?}",
 245                            funclet, succ);
 246                     *s = Some(succ);
 247                 },
 248                 Some(s) => if s != succ {
 249                     span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
 250                               funclet, s, succ);
 251                 }
 252             }
 253         };
 254
 255         for (bb, data) in traversal::reverse_postorder(mir) {
 256             let funclet = match result[bb] {
 257                 CleanupKind::NotCleanup => continue,
 258                 CleanupKind::Funclet => bb,
 259                 CleanupKind::Internal { funclet } => funclet,
 260             };
 261
 262             debug!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
 263                    bb, data, result[bb], funclet);
 264
 265             for &succ in data.terminator().successors().iter() {
 266                 let kind = result[succ];
 267                 debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
 268                        funclet, succ, kind);
 269                 match kind {
 270                     CleanupKind::NotCleanup => {
 271                         result[succ] = CleanupKind::Internal { funclet: funclet };
 272                     }
 273                     CleanupKind::Funclet => {
 274                         if funclet != succ {
 275                             set_successor(funclet, succ);
 276                         }
 277                     }
 278                     CleanupKind::Internal { funclet: succ_funclet } => {
 279                         if funclet != succ_funclet {
 280                             // `succ` has 2 different funclet going into it, so it must
 281                             // be a funclet by itself.
 282
 283                             debug!("promoting {:?} to a funclet and updating {:?}", succ,
 284                                    succ_funclet);
 285                             result[succ] = CleanupKind::Funclet;
 286                             set_successor(succ_funclet, succ);
 287                             set_successor(funclet, succ);
 288                         }
 289                     }
 290                 }
 291             }
 292         }
 293     }
 294
 295     let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
 296
 297     discover_masters(&mut result, mir);
 298     propagate(&mut result, mir);
 299     debug!("cleanup_kinds: result={:?}", result);
 300     result
 301 }