src/librustc_mir/transform/uniform_array_move_out.rs

   1 // This pass converts move out from array by Subslice and
   2 // ConstIndex{.., from_end: true} to ConstIndex move out(s) from begin
   3 // of array. It allows detect error by mir borrowck and elaborate
   4 // drops for array without additional work.
   5 //
   6 // Example:
   7 //
   8 // let a = [ box 1,box 2, box 3];
   9 // if b {
  10 //  let [_a.., _] = a;
  11 // } else {
  12 //  let [.., _b] = a;
  13 // }
  14 //
  15 //  mir statement _10 = move _2[:-1]; replaced by:
  16 //  StorageLive(_12);
  17 //  _12 = move _2[0 of 3];
  18 //  StorageLive(_13);
  19 //  _13 = move _2[1 of 3];
  20 //  _10 = [move _12, move _13]
  21 //  StorageDead(_12);
  22 //  StorageDead(_13);
  23 //
  24 //  and mir statement _11 = move _2[-1 of 1]; replaced by:
  25 //  _11 = move _2[2 of 3];
  26 //
  27 // FIXME: integrate this transformation to the mir build
  28
  29 use rustc::ty;
  30 use rustc::ty::TyCtxt;
  31 use rustc::mir::*;
  32 use rustc::mir::visit::{Visitor, PlaceContext, NonUseContext};
  33 use rustc_data_structures::indexed_vec::{IndexVec};
  34 use crate::transform::{MirPass, MirSource};
  35 use crate::util::patch::MirPatch;
  36
  37 pub struct UniformArrayMoveOut;
  38
  39 impl MirPass for UniformArrayMoveOut {
  40     fn run_pass<'a, 'tcx>(&self,
  41                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  42                           _src: MirSource<'tcx>,
  43                           mir: &mut Body<'tcx>) {
  44         let mut patch = MirPatch::new(mir);
  45         {
  46             let mut visitor = UniformArrayMoveOutVisitor{mir, patch: &mut patch, tcx};
  47             visitor.visit_body(mir);
  48         }
  49         patch.apply(mir);
  50     }
  51 }
  52
  53 struct UniformArrayMoveOutVisitor<'a, 'tcx: 'a> {
  54     mir: &'a Body<'tcx>,
  55     patch: &'a mut MirPatch<'tcx>,
  56     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  57 }
  58
  59 impl<'a, 'tcx> Visitor<'tcx> for UniformArrayMoveOutVisitor<'a, 'tcx> {
  60     fn visit_assign(&mut self,
  61                     dst_place: &Place<'tcx>,
  62                     rvalue: &Rvalue<'tcx>,
  63                     location: Location) {
  64         if let Rvalue::Use(Operand::Move(ref src_place)) = rvalue {
  65             if let Place::Projection(ref proj) = *src_place {
  66                 if let ProjectionElem::ConstantIndex{offset: _,
  67                                                      min_length: _,
  68                                                      from_end: false} = proj.elem {
  69                     // no need to transformation
  70                 } else {
  71                     let place_ty = proj.base.ty(self.mir, self.tcx).ty;
  72                     if let ty::Array(item_ty, const_size) = place_ty.sty {
  73                         if let Some(size) = const_size.assert_usize(self.tcx) {
  74                             assert!(size <= u32::max_value() as u64,
  75                                     "uniform array move out doesn't supported
  76                                      for array bigger then u32");
  77                             self.uniform(location, dst_place, proj, item_ty, size as u32);
  78                         }
  79                     }
  80
  81                 }
  82             }
  83         }
  84         self.super_assign(dst_place, rvalue, location)
  85     }
  86 }
  87
  88 impl<'a, 'tcx> UniformArrayMoveOutVisitor<'a, 'tcx> {
  89     fn uniform(&mut self,
  90                location: Location,
  91                dst_place: &Place<'tcx>,
  92                proj: &Projection<'tcx>,
  93                item_ty: &'tcx ty::TyS<'tcx>,
  94                size: u32) {
  95         match proj.elem {
  96             // uniforms statements like_10 = move _2[:-1];
  97             ProjectionElem::Subslice{from, to} => {
  98                 self.patch.make_nop(location);
  99                 let temps : Vec<_> = (from..(size-to)).map(|i| {
 100                     let temp = self.patch.new_temp(item_ty, self.mir.source_info(location).span);
 101                     self.patch.add_statement(location, StatementKind::StorageLive(temp));
 102                     self.patch.add_assign(location,
 103                                           Place::Base(PlaceBase::Local(temp)),
 104                                           Rvalue::Use(
 105                                               Operand::Move(
 106                                                   Place::Projection(box Projection{
 107                                                       base: proj.base.clone(),
 108                                                       elem: ProjectionElem::ConstantIndex{
 109                                                           offset: i,
 110                                                           min_length: size,
 111                                                           from_end: false}
 112                                                   }))));
 113                     temp
 114                 }).collect();
 115                 self.patch.add_assign(
 116                     location,
 117                     dst_place.clone(),
 118                     Rvalue::Aggregate(
 119                         box AggregateKind::Array(item_ty),
 120                         temps.iter().map(
 121                             |x| Operand::Move(Place::Base(PlaceBase::Local(*x)))
 122                         ).collect()
 123                     )
 124                 );
 125                 for temp in temps {
 126                     self.patch.add_statement(location, StatementKind::StorageDead(temp));
 127                 }
 128             }
 129             // uniforms statements like _11 = move _2[-1 of 1];
 130             ProjectionElem::ConstantIndex{offset, min_length: _, from_end: true} => {
 131                 self.patch.make_nop(location);
 132                 self.patch.add_assign(location,
 133                                       dst_place.clone(),
 134                                       Rvalue::Use(
 135                                           Operand::Move(
 136                                               Place::Projection(box Projection{
 137                                                   base: proj.base.clone(),
 138                                                   elem: ProjectionElem::ConstantIndex{
 139                                                       offset: size - offset,
 140                                                       min_length: size,
 141                                                       from_end: false }}))));
 142             }
 143             _ => {}
 144         }
 145     }
 146 }
 147
 148 // Restore Subslice move out after analysis
 149 // Example:
 150 //
 151 //  next statements:
 152 //   StorageLive(_12);
 153 //   _12 = move _2[0 of 3];
 154 //   StorageLive(_13);
 155 //   _13 = move _2[1 of 3];
 156 //   _10 = [move _12, move _13]
 157 //   StorageDead(_12);
 158 //   StorageDead(_13);
 159 //
 160 // replaced by _10 = move _2[:-1];
 161
 162 pub struct RestoreSubsliceArrayMoveOut;
 163
 164 impl MirPass for RestoreSubsliceArrayMoveOut {
 165     fn run_pass<'a, 'tcx>(&self,
 166                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
 167                           _src: MirSource<'tcx>,
 168                           mir: &mut Body<'tcx>) {
 169         let mut patch = MirPatch::new(mir);
 170         {
 171             let mut visitor = RestoreDataCollector {
 172                 locals_use: IndexVec::from_elem(LocalUse::new(), &mir.local_decls),
 173                 candidates: vec![],
 174             };
 175             visitor.visit_body(mir);
 176
 177             for candidate in &visitor.candidates {
 178                 let statement = &mir[candidate.block].statements[candidate.statement_index];
 179                 if let StatementKind::Assign(ref dst_place, ref rval) = statement.kind {
 180                     if let Rvalue::Aggregate(box AggregateKind::Array(_), ref items) = **rval {
 181                         let items : Vec<_> = items.iter().map(|item| {
 182                             if let Operand::Move(Place::Base(PlaceBase::Local(local))) = item {
 183                                 let local_use = &visitor.locals_use[*local];
 184                                 let opt_index_and_place = Self::try_get_item_source(local_use, mir);
 185                                 // each local should be used twice:
 186                                 //  in assign and in aggregate statements
 187                                 if local_use.use_count == 2 && opt_index_and_place.is_some() {
 188                                     let (index, src_place) = opt_index_and_place.unwrap();
 189                                     return Some((local_use, index, src_place));
 190                                 }
 191                             }
 192                             None
 193                         }).collect();
 194
 195                         let opt_src_place = items.first().and_then(|x| *x).map(|x| x.2);
 196                         let opt_size = opt_src_place.and_then(|src_place| {
 197                             let src_ty = src_place.ty(mir, tcx).ty;
 198                             if let ty::Array(_, ref size_o) = src_ty.sty {
 199                                 size_o.assert_usize(tcx)
 200                             } else {
 201                                 None
 202                             }
 203                         });
 204                         Self::check_and_patch(*candidate, &items, opt_size, &mut patch, dst_place);
 205                     }
 206                 }
 207             }
 208         }
 209         patch.apply(mir);
 210     }
 211 }
 212
 213 impl RestoreSubsliceArrayMoveOut {
 214     // Checks that source has size, all locals are inited from same source place and
 215     // indices is an integer interval. If all checks pass do the replacent.
 216     // items are Vec<Option<LocalUse, index in source array, source place for init local>>
 217     fn check_and_patch<'tcx>(candidate: Location,
 218                              items: &[Option<(&LocalUse, u32, &Place<'tcx>)>],
 219                              opt_size: Option<u64>,
 220                              patch: &mut MirPatch<'tcx>,
 221                              dst_place: &Place<'tcx>) {
 222         let opt_src_place = items.first().and_then(|x| *x).map(|x| x.2);
 223
 224         if opt_size.is_some() && items.iter().all(
 225             |l| l.is_some() && l.unwrap().2 == opt_src_place.unwrap()) {
 226
 227             let indices: Vec<_> = items.iter().map(|x| x.unwrap().1).collect();
 228             for i in 1..indices.len() {
 229                 if indices[i - 1] + 1 != indices[i] {
 230                     return;
 231                 }
 232             }
 233
 234             let min = *indices.first().unwrap();
 235             let max = *indices.last().unwrap();
 236
 237             for item in items {
 238                 let locals_use = item.unwrap().0;
 239                 patch.make_nop(locals_use.alive.unwrap());
 240                 patch.make_nop(locals_use.dead.unwrap());
 241                 patch.make_nop(locals_use.first_use.unwrap());
 242             }
 243             patch.make_nop(candidate);
 244             let size = opt_size.unwrap() as u32;
 245             patch.add_assign(candidate,
 246                              dst_place.clone(),
 247                              Rvalue::Use(
 248                                  Operand::Move(
 249                                      Place::Projection(box Projection{
 250                                          base: opt_src_place.unwrap().clone(),
 251                                          elem: ProjectionElem::Subslice{
 252                                              from: min, to: size - max - 1}}))));
 253         }
 254     }
 255
 256     fn try_get_item_source<'a, 'tcx>(local_use: &LocalUse,
 257                                      mir: &'a Body<'tcx>) -> Option<(u32, &'a Place<'tcx>)> {
 258         if let Some(location) = local_use.first_use {
 259             let block = &mir[location.block];
 260             if block.statements.len() > location.statement_index {
 261                 let statement = &block.statements[location.statement_index];
 262                 if let StatementKind::Assign(
 263                     Place::Base(PlaceBase::Local(_)),
 264                     box Rvalue::Use(Operand::Move(Place::Projection(box Projection{
 265                         ref base, elem: ProjectionElem::ConstantIndex{
 266                             offset, min_length: _, from_end: false}})))) = statement.kind {
 267                     return Some((offset, base))
 268                 }
 269             }
 270         }
 271         None
 272     }
 273 }
 274
 275 #[derive(Copy, Clone, Debug)]
 276 struct LocalUse {
 277     alive: Option<Location>,
 278     dead: Option<Location>,
 279     use_count: u32,
 280     first_use: Option<Location>,
 281 }
 282
 283 impl LocalUse {
 284     pub fn new() -> Self {
 285         LocalUse{alive: None, dead: None, use_count: 0, first_use: None}
 286     }
 287 }
 288
 289 struct RestoreDataCollector {
 290     locals_use: IndexVec<Local, LocalUse>,
 291     candidates: Vec<Location>,
 292 }
 293
 294 impl<'tcx> Visitor<'tcx> for RestoreDataCollector {
 295     fn visit_assign(&mut self,
 296                     place: &Place<'tcx>,
 297                     rvalue: &Rvalue<'tcx>,
 298                     location: Location) {
 299         if let Rvalue::Aggregate(box AggregateKind::Array(_), _) = *rvalue {
 300             self.candidates.push(location);
 301         }
 302         self.super_assign(place, rvalue, location)
 303     }
 304
 305     fn visit_local(&mut self,
 306                    local: &Local,
 307                    context: PlaceContext,
 308                    location: Location) {
 309         let local_use = &mut self.locals_use[*local];
 310         match context {
 311             PlaceContext::NonUse(NonUseContext::StorageLive) => local_use.alive = Some(location),
 312             PlaceContext::NonUse(NonUseContext::StorageDead) => local_use.dead = Some(location),
 313             _ => {
 314                 local_use.use_count += 1;
 315                 if local_use.first_use.is_none() {
 316                     local_use.first_use = Some(location);
 317                 }
 318             }
 319         }
 320     }
 321 }