src/librustc_mir/transform/copy_prop.rs

   1 // Copyright 2016 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 //! Trivial copy propagation pass.
  12 //!
  13 //! This uses def-use analysis to remove values that have exactly one def and one use, which must
  14 //! be an assignment.
  15 //!
  16 //! To give an example, we look for patterns that look like:
  17 //!
  18 //!     DEST = SRC
  19 //!     ...
  20 //!     USE(DEST)
  21 //!
  22 //! where `DEST` and `SRC` are both locals of some form. We replace that with:
  23 //!
  24 //!     NOP
  25 //!     ...
  26 //!     USE(SRC)
  27 //!
  28 //! The assignment `DEST = SRC` must be (a) the only mutation of `DEST` and (b) the only
  29 //! (non-mutating) use of `SRC`. These restrictions are conservative and may be relaxed in the
  30 //! future.
  31
  32 use rustc::mir::{Constant, Local, LocalKind, Location, Lvalue, Mir, Operand, Rvalue, StatementKind};
  33 use rustc::mir::transform::{MirPass, MirSource};
  34 use rustc::mir::visit::MutVisitor;
  35 use rustc::ty::TyCtxt;
  36 use util::def_use::DefUseAnalysis;
  37
  38 pub struct CopyPropagation;
  39
  40 impl MirPass for CopyPropagation {
  41     fn run_pass<'a, 'tcx>(&self,
  42                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  43                           source: MirSource,
  44                           mir: &mut Mir<'tcx>) {
  45         match source {
  46             MirSource::Const(_) => {
  47                 // Don't run on constants, because constant qualification might reject the
  48                 // optimized IR.
  49                 return
  50             }
  51             MirSource::Static(..) | MirSource::Promoted(..) => {
  52                 // Don't run on statics and promoted statics, because trans might not be able to
  53                 // evaluate the optimized IR.
  54                 return
  55             }
  56             MirSource::Fn(function_node_id) => {
  57                 if tcx.is_const_fn(tcx.hir.local_def_id(function_node_id)) {
  58                     // Don't run on const functions, as, again, trans might not be able to evaluate
  59                     // the optimized IR.
  60                     return
  61                 }
  62             }
  63             MirSource::GeneratorDrop(_) => (),
  64         }
  65
  66         // We only run when the MIR optimization level is > 1.
  67         // This avoids a slow pass, and messing up debug info.
  68         if tcx.sess.opts.debugging_opts.mir_opt_level <= 1 {
  69             return;
  70         }
  71
  72         loop {
  73             let mut def_use_analysis = DefUseAnalysis::new(mir);
  74             def_use_analysis.analyze(mir);
  75
  76             let mut changed = false;
  77             for dest_local in mir.local_decls.indices() {
  78                 debug!("Considering destination local: {:?}", dest_local);
  79
  80                 let action;
  81                 let location;
  82                 {
  83                     // The destination must have exactly one def.
  84                     let dest_use_info = def_use_analysis.local_info(dest_local);
  85                     let dest_def_count = dest_use_info.def_count_not_including_drop();
  86                     if dest_def_count == 0 {
  87                         debug!("  Can't copy-propagate local: dest {:?} undefined",
  88                                dest_local);
  89                         continue
  90                     }
  91                     if dest_def_count > 1 {
  92                         debug!("  Can't copy-propagate local: dest {:?} defined {} times",
  93                                dest_local,
  94                                dest_use_info.def_count());
  95                         continue
  96                     }
  97                     if dest_use_info.use_count() == 0 {
  98                         debug!("  Can't copy-propagate local: dest {:?} unused",
  99                                dest_local);
 100                         continue
 101                     }
 102                     let dest_lvalue_def = dest_use_info.defs_and_uses.iter().filter(|lvalue_def| {
 103                         lvalue_def.context.is_mutating_use() && !lvalue_def.context.is_drop()
 104                     }).next().unwrap();
 105                     location = dest_lvalue_def.location;
 106
 107                     let basic_block = &mir[location.block];
 108                     let statement_index = location.statement_index;
 109                     let statement = match basic_block.statements.get(statement_index) {
 110                         Some(statement) => statement,
 111                         None => {
 112                             debug!("  Can't copy-propagate local: used in terminator");
 113                             continue
 114                         }
 115                     };
 116
 117                     // That use of the source must be an assignment.
 118                     match statement.kind {
 119                         StatementKind::Assign(Lvalue::Local(local), Rvalue::Use(ref operand)) if
 120                                 local == dest_local => {
 121                             let maybe_action = match *operand {
 122                                 Operand::Consume(ref src_lvalue) => {
 123                                     Action::local_copy(&mir, &def_use_analysis, src_lvalue)
 124                                 }
 125                                 Operand::Constant(ref src_constant) => {
 126                                     Action::constant(src_constant)
 127                                 }
 128                             };
 129                             match maybe_action {
 130                                 Some(this_action) => action = this_action,
 131                                 None => continue,
 132                             }
 133                         }
 134                         _ => {
 135                             debug!("  Can't copy-propagate local: source use is not an \
 136                                     assignment");
 137                             continue
 138                         }
 139                     }
 140                 }
 141
 142                 changed = action.perform(mir, &def_use_analysis, dest_local, location) || changed;
 143                 // FIXME(pcwalton): Update the use-def chains to delete the instructions instead of
 144                 // regenerating the chains.
 145                 break
 146             }
 147             if !changed {
 148                 break
 149             }
 150         }
 151     }
 152 }
 153
 154 enum Action<'tcx> {
 155     PropagateLocalCopy(Local),
 156     PropagateConstant(Constant<'tcx>),
 157 }
 158
 159 impl<'tcx> Action<'tcx> {
 160     fn local_copy(mir: &Mir<'tcx>, def_use_analysis: &DefUseAnalysis, src_lvalue: &Lvalue<'tcx>)
 161                   -> Option<Action<'tcx>> {
 162         // The source must be a local.
 163         let src_local = if let Lvalue::Local(local) = *src_lvalue {
 164             local
 165         } else {
 166             debug!("  Can't copy-propagate local: source is not a local");
 167             return None;
 168         };
 169
 170         // We're trying to copy propagate a local.
 171         // There must be exactly one use of the source used in a statement (not in a terminator).
 172         let src_use_info = def_use_analysis.local_info(src_local);
 173         let src_use_count = src_use_info.use_count();
 174         if src_use_count == 0 {
 175             debug!("  Can't copy-propagate local: no uses");
 176             return None
 177         }
 178         if src_use_count != 1 {
 179             debug!("  Can't copy-propagate local: {} uses", src_use_info.use_count());
 180             return None
 181         }
 182
 183         // Verify that the source doesn't change in between. This is done conservatively for now,
 184         // by ensuring that the source has exactly one mutation. The goal is to prevent things
 185         // like:
 186         //
 187         //     DEST = SRC;
 188         //     SRC = X;
 189         //     USE(DEST);
 190         //
 191         // From being misoptimized into:
 192         //
 193         //     SRC = X;
 194         //     USE(SRC);
 195         let src_def_count = src_use_info.def_count_not_including_drop();
 196         // allow function arguments to be propagated
 197         if src_def_count > 1 ||
 198             (src_def_count == 0 && mir.local_kind(src_local) != LocalKind::Arg) {
 199             debug!("  Can't copy-propagate local: {} defs of src",
 200                    src_use_info.def_count_not_including_drop());
 201             return None
 202         }
 203
 204         Some(Action::PropagateLocalCopy(src_local))
 205     }
 206
 207     fn constant(src_constant: &Constant<'tcx>) -> Option<Action<'tcx>> {
 208         Some(Action::PropagateConstant((*src_constant).clone()))
 209     }
 210
 211     fn perform(self,
 212                mir: &mut Mir<'tcx>,
 213                def_use_analysis: &DefUseAnalysis<'tcx>,
 214                dest_local: Local,
 215                location: Location)
 216                -> bool {
 217         match self {
 218             Action::PropagateLocalCopy(src_local) => {
 219                 // Eliminate the destination and the assignment.
 220                 //
 221                 // First, remove all markers.
 222                 //
 223                 // FIXME(pcwalton): Don't do this. Merge live ranges instead.
 224                 debug!("  Replacing all uses of {:?} with {:?} (local)",
 225                        dest_local,
 226                        src_local);
 227                 for lvalue_use in &def_use_analysis.local_info(dest_local).defs_and_uses {
 228                     if lvalue_use.context.is_storage_marker() {
 229                         mir.make_statement_nop(lvalue_use.location)
 230                     }
 231                 }
 232                 for lvalue_use in &def_use_analysis.local_info(src_local).defs_and_uses {
 233                     if lvalue_use.context.is_storage_marker() {
 234                         mir.make_statement_nop(lvalue_use.location)
 235                     }
 236                 }
 237
 238                 // Replace all uses of the destination local with the source local.
 239                 def_use_analysis.replace_all_defs_and_uses_with(dest_local, mir, src_local);
 240
 241                 // Finally, zap the now-useless assignment instruction.
 242                 debug!("  Deleting assignment");
 243                 mir.make_statement_nop(location);
 244
 245                 true
 246             }
 247             Action::PropagateConstant(src_constant) => {
 248                 // First, remove all markers.
 249                 //
 250                 // FIXME(pcwalton): Don't do this. Merge live ranges instead.
 251                 debug!("  Replacing all uses of {:?} with {:?} (constant)",
 252                        dest_local,
 253                        src_constant);
 254                 let dest_local_info = def_use_analysis.local_info(dest_local);
 255                 for lvalue_use in &dest_local_info.defs_and_uses {
 256                     if lvalue_use.context.is_storage_marker() {
 257                         mir.make_statement_nop(lvalue_use.location)
 258                     }
 259                 }
 260
 261                 // Replace all uses of the destination local with the constant.
 262                 let mut visitor = ConstantPropagationVisitor::new(dest_local,
 263                                                                   src_constant);
 264                 for dest_lvalue_use in &dest_local_info.defs_and_uses {
 265                     visitor.visit_location(mir, dest_lvalue_use.location)
 266                 }
 267
 268                 // Zap the assignment instruction if we eliminated all the uses. We won't have been
 269                 // able to do that if the destination was used in a projection, because projections
 270                 // must have lvalues on their LHS.
 271                 let use_count = dest_local_info.use_count();
 272                 if visitor.uses_replaced == use_count {
 273                     debug!("  {} of {} use(s) replaced; deleting assignment",
 274                            visitor.uses_replaced,
 275                            use_count);
 276                     mir.make_statement_nop(location);
 277                     true
 278                 } else if visitor.uses_replaced == 0 {
 279                     debug!("  No uses replaced; not deleting assignment");
 280                     false
 281                 } else {
 282                     debug!("  {} of {} use(s) replaced; not deleting assignment",
 283                            visitor.uses_replaced,
 284                            use_count);
 285                     true
 286                 }
 287             }
 288         }
 289     }
 290 }
 291
 292 struct ConstantPropagationVisitor<'tcx> {
 293     dest_local: Local,
 294     constant: Constant<'tcx>,
 295     uses_replaced: usize,
 296 }
 297
 298 impl<'tcx> ConstantPropagationVisitor<'tcx> {
 299     fn new(dest_local: Local, constant: Constant<'tcx>)
 300            -> ConstantPropagationVisitor<'tcx> {
 301         ConstantPropagationVisitor {
 302             dest_local,
 303             constant,
 304             uses_replaced: 0,
 305         }
 306     }
 307 }
 308
 309 impl<'tcx> MutVisitor<'tcx> for ConstantPropagationVisitor<'tcx> {
 310     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
 311         self.super_operand(operand, location);
 312
 313         match *operand {
 314             Operand::Consume(Lvalue::Local(local)) if local == self.dest_local => {}
 315             _ => return,
 316         }
 317
 318         *operand = Operand::Constant(box self.constant.clone());
 319         self.uses_replaced += 1
 320     }
 321 }