compiler/rustc_mir/src/transform/early_otherwise_branch.rs

   1 use crate::{transform::MirPass, util::patch::MirPatch};
   2 use rustc_middle::mir::*;
   3 use rustc_middle::ty::{Ty, TyCtxt};
   4 use std::fmt::Debug;
   5
   6 use super::simplify::simplify_cfg;
   7
   8 /// This pass optimizes something like
   9 /// ```text
  10 /// let x: Option<()>;
  11 /// let y: Option<()>;
  12 /// match (x,y) {
  13 ///     (Some(_), Some(_)) => {0},
  14 ///     _ => {1}
  15 /// }
  16 /// ```
  17 /// into something like
  18 /// ```text
  19 /// let x: Option<()>;
  20 /// let y: Option<()>;
  21 /// let discriminant_x = // get discriminant of x
  22 /// let discriminant_y = // get discriminant of y
  23 /// if discriminant_x != discriminant_y || discriminant_x == None {1} else {0}
  24 /// ```
  25 pub struct EarlyOtherwiseBranch;
  26
  27 impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
  28     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  29         if tcx.sess.opts.debugging_opts.mir_opt_level < 2 {
  30             return;
  31         }
  32         trace!("running EarlyOtherwiseBranch on {:?}", body.source);
  33         // we are only interested in this bb if the terminator is a switchInt
  34         let bbs_with_switch =
  35             body.basic_blocks().iter_enumerated().filter(|(_, bb)| is_switch(bb.terminator()));
  36
  37         let opts_to_apply: Vec<OptimizationToApply<'tcx>> = bbs_with_switch
  38             .flat_map(|(bb_idx, bb)| {
  39                 let switch = bb.terminator();
  40                 let helper = Helper { body, tcx };
  41                 let infos = helper.go(bb, switch)?;
  42                 Some(OptimizationToApply { infos, basic_block_first_switch: bb_idx })
  43             })
  44             .collect();
  45
  46         let should_cleanup = !opts_to_apply.is_empty();
  47
  48         for opt_to_apply in opts_to_apply {
  49             if !tcx.consider_optimizing(|| format!("EarlyOtherwiseBranch {:?}", &opt_to_apply)) {
  50                 break;
  51             }
  52
  53             trace!("SUCCESS: found optimization possibility to apply: {:?}", &opt_to_apply);
  54
  55             let statements_before =
  56                 body.basic_blocks()[opt_to_apply.basic_block_first_switch].statements.len();
  57             let end_of_block_location = Location {
  58                 block: opt_to_apply.basic_block_first_switch,
  59                 statement_index: statements_before,
  60             };
  61
  62             let mut patch = MirPatch::new(body);
  63
  64             // create temp to store second discriminant in
  65             let discr_type = opt_to_apply.infos[0].second_switch_info.discr_ty;
  66             let discr_span = opt_to_apply.infos[0].second_switch_info.discr_source_info.span;
  67             let second_discriminant_temp = patch.new_temp(discr_type, discr_span);
  68
  69             patch.add_statement(
  70                 end_of_block_location,
  71                 StatementKind::StorageLive(second_discriminant_temp),
  72             );
  73
  74             // create assignment of discriminant
  75             let place_of_adt_to_get_discriminant_of =
  76                 opt_to_apply.infos[0].second_switch_info.place_of_adt_discr_read;
  77             patch.add_assign(
  78                 end_of_block_location,
  79                 Place::from(second_discriminant_temp),
  80                 Rvalue::Discriminant(place_of_adt_to_get_discriminant_of),
  81             );
  82
  83             // create temp to store NotEqual comparison between the two discriminants
  84             let not_equal = BinOp::Ne;
  85             let not_equal_res_type = not_equal.ty(tcx, discr_type, discr_type);
  86             let not_equal_temp = patch.new_temp(not_equal_res_type, discr_span);
  87             patch.add_statement(end_of_block_location, StatementKind::StorageLive(not_equal_temp));
  88
  89             // create NotEqual comparison between the two discriminants
  90             let first_descriminant_place =
  91                 opt_to_apply.infos[0].first_switch_info.discr_used_in_switch;
  92             let not_equal_rvalue = Rvalue::BinaryOp(
  93                 not_equal,
  94                 Operand::Copy(Place::from(second_discriminant_temp)),
  95                 Operand::Copy(first_descriminant_place),
  96             );
  97             patch.add_statement(
  98                 end_of_block_location,
  99                 StatementKind::Assign(box (Place::from(not_equal_temp), not_equal_rvalue)),
 100             );
 101
 102             let new_targets = opt_to_apply
 103                 .infos
 104                 .iter()
 105                 .flat_map(|x| x.second_switch_info.targets_with_values.iter())
 106                 .cloned();
 107
 108             let targets = SwitchTargets::new(
 109                 new_targets,
 110                 opt_to_apply.infos[0].first_switch_info.otherwise_bb,
 111             );
 112
 113             // new block that jumps to the correct discriminant case. This block is switched to if the discriminants are equal
 114             let new_switch_data = BasicBlockData::new(Some(Terminator {
 115                 source_info: opt_to_apply.infos[0].second_switch_info.discr_source_info,
 116                 kind: TerminatorKind::SwitchInt {
 117                     // the first and second discriminants are equal, so just pick one
 118                     discr: Operand::Copy(first_descriminant_place),
 119                     switch_ty: discr_type,
 120                     targets,
 121                 },
 122             }));
 123
 124             let new_switch_bb = patch.new_block(new_switch_data);
 125
 126             // switch on the NotEqual. If true, then jump to the `otherwise` case.
 127             // If false, then jump to a basic block that then jumps to the correct disciminant case
 128             let true_case = opt_to_apply.infos[0].first_switch_info.otherwise_bb;
 129             let false_case = new_switch_bb;
 130             patch.patch_terminator(
 131                 opt_to_apply.basic_block_first_switch,
 132                 TerminatorKind::if_(
 133                     tcx,
 134                     Operand::Move(Place::from(not_equal_temp)),
 135                     true_case,
 136                     false_case,
 137                 ),
 138             );
 139
 140             // generate StorageDead for the second_discriminant_temp not in use anymore
 141             patch.add_statement(
 142                 end_of_block_location,
 143                 StatementKind::StorageDead(second_discriminant_temp),
 144             );
 145
 146             // Generate a StorageDead for not_equal_temp in each of the targets, since we moved it into the switch
 147             for bb in [false_case, true_case].iter() {
 148                 patch.add_statement(
 149                     Location { block: *bb, statement_index: 0 },
 150                     StatementKind::StorageDead(not_equal_temp),
 151                 );
 152             }
 153
 154             patch.apply(body);
 155         }
 156
 157         // Since this optimization adds new basic blocks and invalidates others,
 158         // clean up the cfg to make it nicer for other passes
 159         if should_cleanup {
 160             simplify_cfg(body);
 161         }
 162     }
 163 }
 164
 165 fn is_switch<'tcx>(terminator: &Terminator<'tcx>) -> bool {
 166     match terminator.kind {
 167         TerminatorKind::SwitchInt { .. } => true,
 168         _ => false,
 169     }
 170 }
 171
 172 struct Helper<'a, 'tcx> {
 173     body: &'a Body<'tcx>,
 174     tcx: TyCtxt<'tcx>,
 175 }
 176
 177 #[derive(Debug, Clone)]
 178 struct SwitchDiscriminantInfo<'tcx> {
 179     /// Type of the discriminant being switched on
 180     discr_ty: Ty<'tcx>,
 181     /// The basic block that the otherwise branch points to
 182     otherwise_bb: BasicBlock,
 183     /// Target along with the value being branched from. Otherwise is not included
 184     targets_with_values: Vec<(u128, BasicBlock)>,
 185     discr_source_info: SourceInfo,
 186     /// The place of the discriminant used in the switch
 187     discr_used_in_switch: Place<'tcx>,
 188     /// The place of the adt that has its discriminant read
 189     place_of_adt_discr_read: Place<'tcx>,
 190     /// The type of the adt that has its discriminant read
 191     type_adt_matched_on: Ty<'tcx>,
 192 }
 193
 194 #[derive(Debug)]
 195 struct OptimizationToApply<'tcx> {
 196     infos: Vec<OptimizationInfo<'tcx>>,
 197     /// Basic block of the original first switch
 198     basic_block_first_switch: BasicBlock,
 199 }
 200
 201 #[derive(Debug)]
 202 struct OptimizationInfo<'tcx> {
 203     /// Info about the first switch and discriminant
 204     first_switch_info: SwitchDiscriminantInfo<'tcx>,
 205     /// Info about the second switch and discriminant
 206     second_switch_info: SwitchDiscriminantInfo<'tcx>,
 207 }
 208
 209 impl<'a, 'tcx> Helper<'a, 'tcx> {
 210     pub fn go(
 211         &self,
 212         bb: &BasicBlockData<'tcx>,
 213         switch: &Terminator<'tcx>,
 214     ) -> Option<Vec<OptimizationInfo<'tcx>>> {
 215         // try to find the statement that defines the discriminant that is used for the switch
 216         let discr = self.find_switch_discriminant_info(bb, switch)?;
 217
 218         // go through each target, finding a discriminant read, and a switch
 219         let results = discr
 220             .targets_with_values
 221             .iter()
 222             .map(|(value, target)| self.find_discriminant_switch_pairing(&discr, *target, *value));
 223
 224         // if the optimization did not apply for one of the targets, then abort
 225         if results.clone().any(|x| x.is_none()) || results.len() == 0 {
 226             trace!("NO: not all of the targets matched the pattern for optimization");
 227             return None;
 228         }
 229
 230         Some(results.flatten().collect())
 231     }
 232
 233     fn find_discriminant_switch_pairing(
 234         &self,
 235         discr_info: &SwitchDiscriminantInfo<'tcx>,
 236         target: BasicBlock,
 237         value: u128,
 238     ) -> Option<OptimizationInfo<'tcx>> {
 239         let bb = &self.body.basic_blocks()[target];
 240         // find switch
 241         let terminator = bb.terminator();
 242         if is_switch(terminator) {
 243             let this_bb_discr_info = self.find_switch_discriminant_info(bb, terminator)?;
 244
 245             // the types of the two adts matched on have to be equalfor this optimization to apply
 246             if discr_info.type_adt_matched_on != this_bb_discr_info.type_adt_matched_on {
 247                 trace!(
 248                     "NO: types do not match. LHS: {:?}, RHS: {:?}",
 249                     discr_info.type_adt_matched_on,
 250                     this_bb_discr_info.type_adt_matched_on
 251                 );
 252                 return None;
 253             }
 254
 255             // the otherwise branch of the two switches have to point to the same bb
 256             if discr_info.otherwise_bb != this_bb_discr_info.otherwise_bb {
 257                 trace!("NO: otherwise target is not the same");
 258                 return None;
 259             }
 260
 261             // check that the value being matched on is the same. The
 262             if this_bb_discr_info.targets_with_values.iter().find(|x| x.0 == value).is_none() {
 263                 trace!("NO: values being matched on are not the same");
 264                 return None;
 265             }
 266
 267             // only allow optimization if the left and right of the tuple being matched are the same variants.
 268             // so the following should not optimize
 269             //  ```rust
 270             // let x: Option<()>;
 271             // let y: Option<()>;
 272             // match (x,y) {
 273             //     (Some(_), None) => {},
 274             //     _ => {}
 275             // }
 276             //  ```
 277             // We check this by seeing that the value of the first discriminant is the only other discriminant value being used as a target in the second switch
 278             if !(this_bb_discr_info.targets_with_values.len() == 1
 279                 && this_bb_discr_info.targets_with_values[0].0 == value)
 280             {
 281                 trace!(
 282                     "NO: The second switch did not have only 1 target (besides otherwise) that had the same value as the value from the first switch that got us here"
 283                 );
 284                 return None;
 285             }
 286
 287             // when the second place is a projection of the first one, it's not safe to calculate their discriminant values sequentially.
 288             // for example, this should not be optimized:
 289             //
 290             // ```rust
 291             // enum E<'a> { Empty, Some(&'a E<'a>), }
 292             // let Some(Some(_)) = e;
 293             // ```
 294             //
 295             // ```mir
 296             // bb0: {
 297             //   _2 = discriminant(*_1)
 298             //   switchInt(_2) -> [...]
 299             // }
 300             // bb1: {
 301             //   _3 = discriminant(*(((*_1) as Some).0: &E))
 302             //   switchInt(_3) -> [...]
 303             // }
 304             // ```
 305             let discr_place = discr_info.place_of_adt_discr_read;
 306             let this_discr_place = this_bb_discr_info.place_of_adt_discr_read;
 307             if discr_place.local == this_discr_place.local
 308                 && this_discr_place.projection.starts_with(discr_place.projection)
 309             {
 310                 trace!("NO: one target is the projection of another");
 311                 return None;
 312             }
 313
 314             // if we reach this point, the optimization applies, and we should be able to optimize this case
 315             // store the info that is needed to apply the optimization
 316
 317             Some(OptimizationInfo {
 318                 first_switch_info: discr_info.clone(),
 319                 second_switch_info: this_bb_discr_info,
 320             })
 321         } else {
 322             None
 323         }
 324     }
 325
 326     fn find_switch_discriminant_info(
 327         &self,
 328         bb: &BasicBlockData<'tcx>,
 329         switch: &Terminator<'tcx>,
 330     ) -> Option<SwitchDiscriminantInfo<'tcx>> {
 331         match &switch.kind {
 332             TerminatorKind::SwitchInt { discr, targets, .. } => {
 333                 let discr_local = discr.place()?.as_local()?;
 334                 // the declaration of the discriminant read. Place of this read is being used in the switch
 335                 let discr_decl = &self.body.local_decls()[discr_local];
 336                 let discr_ty = discr_decl.ty;
 337                 // the otherwise target lies as the last element
 338                 let otherwise_bb = targets.otherwise();
 339                 let targets_with_values = targets.iter().collect();
 340
 341                 // find the place of the adt where the discriminant is being read from
 342                 // assume this is the last statement of the block
 343                 let place_of_adt_discr_read = match bb.statements.last()?.kind {
 344                     StatementKind::Assign(box (_, Rvalue::Discriminant(adt_place))) => {
 345                         Some(adt_place)
 346                     }
 347                     _ => None,
 348                 }?;
 349
 350                 let type_adt_matched_on = place_of_adt_discr_read.ty(self.body, self.tcx).ty;
 351
 352                 Some(SwitchDiscriminantInfo {
 353                     discr_used_in_switch: discr.place()?,
 354                     discr_ty,
 355                     otherwise_bb,
 356                     targets_with_values,
 357                     discr_source_info: discr_decl.source_info,
 358                     place_of_adt_discr_read,
 359                     type_adt_matched_on,
 360                 })
 361             }
 362             _ => unreachable!("must only be passed terminator that is a switch"),
 363         }
 364     }
 365 }