compiler/rustc_mir_transform/src/early_otherwise_branch.rs

   1 use rustc_middle::mir::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 is_enabled(&self, sess: &rustc_session::Session) -> bool {
  29         //  FIXME(#78496)
  30         sess.opts.debugging_opts.unsound_mir_opts && sess.mir_opt_level() >= 3
  31     }
  32
  33     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  34         trace!("running EarlyOtherwiseBranch on {:?}", body.source);
  35
  36         // we are only interested in this bb if the terminator is a switchInt
  37         let bbs_with_switch =
  38             body.basic_blocks().iter_enumerated().filter(|(_, bb)| is_switch(bb.terminator()));
  39
  40         let opts_to_apply: Vec<OptimizationToApply<'tcx>> = bbs_with_switch
  41             .flat_map(|(bb_idx, bb)| {
  42                 let switch = bb.terminator();
  43                 let helper = Helper { body, tcx };
  44                 let infos = helper.go(bb, switch)?;
  45                 Some(OptimizationToApply { infos, basic_block_first_switch: bb_idx })
  46             })
  47             .collect();
  48
  49         let should_cleanup = !opts_to_apply.is_empty();
  50
  51         for opt_to_apply in opts_to_apply {
  52             if !tcx.consider_optimizing(|| format!("EarlyOtherwiseBranch {:?}", &opt_to_apply)) {
  53                 break;
  54             }
  55
  56             trace!("SUCCESS: found optimization possibility to apply: {:?}", &opt_to_apply);
  57
  58             let statements_before =
  59                 body.basic_blocks()[opt_to_apply.basic_block_first_switch].statements.len();
  60             let end_of_block_location = Location {
  61                 block: opt_to_apply.basic_block_first_switch,
  62                 statement_index: statements_before,
  63             };
  64
  65             let mut patch = MirPatch::new(body);
  66
  67             // create temp to store second discriminant in
  68             let discr_type = opt_to_apply.infos[0].second_switch_info.discr_ty;
  69             let discr_span = opt_to_apply.infos[0].second_switch_info.discr_source_info.span;
  70             let second_discriminant_temp = patch.new_temp(discr_type, discr_span);
  71
  72             patch.add_statement(
  73                 end_of_block_location,
  74                 StatementKind::StorageLive(second_discriminant_temp),
  75             );
  76
  77             // create assignment of discriminant
  78             let place_of_adt_to_get_discriminant_of =
  79                 opt_to_apply.infos[0].second_switch_info.place_of_adt_discr_read;
  80             patch.add_assign(
  81                 end_of_block_location,
  82                 Place::from(second_discriminant_temp),
  83                 Rvalue::Discriminant(place_of_adt_to_get_discriminant_of),
  84             );
  85
  86             // create temp to store NotEqual comparison between the two discriminants
  87             let not_equal = BinOp::Ne;
  88             let not_equal_res_type = not_equal.ty(tcx, discr_type, discr_type);
  89             let not_equal_temp = patch.new_temp(not_equal_res_type, discr_span);
  90             patch.add_statement(end_of_block_location, StatementKind::StorageLive(not_equal_temp));
  91
  92             // create NotEqual comparison between the two discriminants
  93             let first_descriminant_place =
  94                 opt_to_apply.infos[0].first_switch_info.discr_used_in_switch;
  95             let not_equal_rvalue = Rvalue::BinaryOp(
  96                 not_equal,
  97                 Box::new((
  98                     Operand::Copy(Place::from(second_discriminant_temp)),
  99                     Operand::Copy(first_descriminant_place),
 100                 )),
 101             );
 102             patch.add_statement(
 103                 end_of_block_location,
 104                 StatementKind::Assign(Box::new((Place::from(not_equal_temp), not_equal_rvalue))),
 105             );
 106
 107             let new_targets = opt_to_apply
 108                 .infos
 109                 .iter()
 110                 .flat_map(|x| x.second_switch_info.targets_with_values.iter())
 111                 .cloned();
 112
 113             let targets = SwitchTargets::new(
 114                 new_targets,
 115                 opt_to_apply.infos[0].first_switch_info.otherwise_bb,
 116             );
 117
 118             // new block that jumps to the correct discriminant case. This block is switched to if the discriminants are equal
 119             let new_switch_data = BasicBlockData::new(Some(Terminator {
 120                 source_info: opt_to_apply.infos[0].second_switch_info.discr_source_info,
 121                 kind: TerminatorKind::SwitchInt {
 122                     // the first and second discriminants are equal, so just pick one
 123                     discr: Operand::Copy(first_descriminant_place),
 124                     switch_ty: discr_type,
 125                     targets,
 126                 },
 127             }));
 128
 129             let new_switch_bb = patch.new_block(new_switch_data);
 130
 131             // switch on the NotEqual. If true, then jump to the `otherwise` case.
 132             // If false, then jump to a basic block that then jumps to the correct disciminant case
 133             let true_case = opt_to_apply.infos[0].first_switch_info.otherwise_bb;
 134             let false_case = new_switch_bb;
 135             patch.patch_terminator(
 136                 opt_to_apply.basic_block_first_switch,
 137                 TerminatorKind::if_(
 138                     tcx,
 139                     Operand::Move(Place::from(not_equal_temp)),
 140                     true_case,
 141                     false_case,
 142                 ),
 143             );
 144
 145             // generate StorageDead for the second_discriminant_temp not in use anymore
 146             patch.add_statement(
 147                 end_of_block_location,
 148                 StatementKind::StorageDead(second_discriminant_temp),
 149             );
 150
 151             // Generate a StorageDead for not_equal_temp in each of the targets, since we moved it into the switch
 152             for bb in [false_case, true_case].iter() {
 153                 patch.add_statement(
 154                     Location { block: *bb, statement_index: 0 },
 155                     StatementKind::StorageDead(not_equal_temp),
 156                 );
 157             }
 158
 159             patch.apply(body);
 160         }
 161
 162         // Since this optimization adds new basic blocks and invalidates others,
 163         // clean up the cfg to make it nicer for other passes
 164         if should_cleanup {
 165             simplify_cfg(tcx, body);
 166         }
 167     }
 168 }
 169
 170 fn is_switch(terminator: &Terminator<'_>) -> bool {
 171     matches!(terminator.kind, TerminatorKind::SwitchInt { .. })
 172 }
 173
 174 struct Helper<'a, 'tcx> {
 175     body: &'a Body<'tcx>,
 176     tcx: TyCtxt<'tcx>,
 177 }
 178
 179 #[derive(Debug, Clone)]
 180 struct SwitchDiscriminantInfo<'tcx> {
 181     /// Type of the discriminant being switched on
 182     discr_ty: Ty<'tcx>,
 183     /// The basic block that the otherwise branch points to
 184     otherwise_bb: BasicBlock,
 185     /// Target along with the value being branched from. Otherwise is not included
 186     targets_with_values: Vec<(u128, BasicBlock)>,
 187     discr_source_info: SourceInfo,
 188     /// The place of the discriminant used in the switch
 189     discr_used_in_switch: Place<'tcx>,
 190     /// The place of the adt that has its discriminant read
 191     place_of_adt_discr_read: Place<'tcx>,
 192     /// The type of the adt that has its discriminant read
 193     type_adt_matched_on: Ty<'tcx>,
 194 }
 195
 196 #[derive(Debug)]
 197 struct OptimizationToApply<'tcx> {
 198     infos: Vec<OptimizationInfo<'tcx>>,
 199     /// Basic block of the original first switch
 200     basic_block_first_switch: BasicBlock,
 201 }
 202
 203 #[derive(Debug)]
 204 struct OptimizationInfo<'tcx> {
 205     /// Info about the first switch and discriminant
 206     first_switch_info: SwitchDiscriminantInfo<'tcx>,
 207     /// Info about the second switch and discriminant
 208     second_switch_info: SwitchDiscriminantInfo<'tcx>,
 209 }
 210
 211 impl<'tcx> Helper<'_, 'tcx> {
 212     pub fn go(
 213         &self,
 214         bb: &BasicBlockData<'tcx>,
 215         switch: &Terminator<'tcx>,
 216     ) -> Option<Vec<OptimizationInfo<'tcx>>> {
 217         // try to find the statement that defines the discriminant that is used for the switch
 218         let discr = self.find_switch_discriminant_info(bb, switch)?;
 219
 220         // go through each target, finding a discriminant read, and a switch
 221         let results = discr
 222             .targets_with_values
 223             .iter()
 224             .map(|(value, target)| self.find_discriminant_switch_pairing(&discr, *target, *value));
 225
 226         // if the optimization did not apply for one of the targets, then abort
 227         if results.clone().any(|x| x.is_none()) || results.len() == 0 {
 228             trace!("NO: not all of the targets matched the pattern for optimization");
 229             return None;
 230         }
 231
 232         Some(results.flatten().collect())
 233     }
 234
 235     fn find_discriminant_switch_pairing(
 236         &self,
 237         discr_info: &SwitchDiscriminantInfo<'tcx>,
 238         target: BasicBlock,
 239         value: u128,
 240     ) -> Option<OptimizationInfo<'tcx>> {
 241         let bb = &self.body.basic_blocks()[target];
 242         // find switch
 243         let terminator = bb.terminator();
 244         if is_switch(terminator) {
 245             let this_bb_discr_info = self.find_switch_discriminant_info(bb, terminator)?;
 246
 247             // the types of the two adts matched on have to be equalfor this optimization to apply
 248             if discr_info.type_adt_matched_on != this_bb_discr_info.type_adt_matched_on {
 249                 trace!(
 250                     "NO: types do not match. LHS: {:?}, RHS: {:?}",
 251                     discr_info.type_adt_matched_on,
 252                     this_bb_discr_info.type_adt_matched_on
 253                 );
 254                 return None;
 255             }
 256
 257             // the otherwise branch of the two switches have to point to the same bb
 258             if discr_info.otherwise_bb != this_bb_discr_info.otherwise_bb {
 259                 trace!("NO: otherwise target is not the same");
 260                 return None;
 261             }
 262
 263             // check that the value being matched on is the same. The
 264             if !this_bb_discr_info.targets_with_values.iter().any(|x| x.0 == value) {
 265                 trace!("NO: values being matched on are not the same");
 266                 return None;
 267             }
 268
 269             // only allow optimization if the left and right of the tuple being matched are the same variants.
 270             // so the following should not optimize
 271             //  ```rust
 272             // let x: Option<()>;
 273             // let y: Option<()>;
 274             // match (x,y) {
 275             //     (Some(_), None) => {},
 276             //     _ => {}
 277             // }
 278             //  ```
 279             // 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
 280             if !(this_bb_discr_info.targets_with_values.len() == 1
 281                 && this_bb_discr_info.targets_with_values[0].0 == value)
 282             {
 283                 trace!(
 284                     "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"
 285                 );
 286                 return None;
 287             }
 288
 289             // when the second place is a projection of the first one, it's not safe to calculate their discriminant values sequentially.
 290             // for example, this should not be optimized:
 291             //
 292             // ```rust
 293             // enum E<'a> { Empty, Some(&'a E<'a>), }
 294             // let Some(Some(_)) = e;
 295             // ```
 296             //
 297             // ```mir
 298             // bb0: {
 299             //   _2 = discriminant(*_1)
 300             //   switchInt(_2) -> [...]
 301             // }
 302             // bb1: {
 303             //   _3 = discriminant(*(((*_1) as Some).0: &E))
 304             //   switchInt(_3) -> [...]
 305             // }
 306             // ```
 307             let discr_place = discr_info.place_of_adt_discr_read;
 308             let this_discr_place = this_bb_discr_info.place_of_adt_discr_read;
 309             if discr_place.local == this_discr_place.local
 310                 && this_discr_place.projection.starts_with(discr_place.projection)
 311             {
 312                 trace!("NO: one target is the projection of another");
 313                 return None;
 314             }
 315
 316             // if we reach this point, the optimization applies, and we should be able to optimize this case
 317             // store the info that is needed to apply the optimization
 318
 319             Some(OptimizationInfo {
 320                 first_switch_info: discr_info.clone(),
 321                 second_switch_info: this_bb_discr_info,
 322             })
 323         } else {
 324             None
 325         }
 326     }
 327
 328     fn find_switch_discriminant_info(
 329         &self,
 330         bb: &BasicBlockData<'tcx>,
 331         switch: &Terminator<'tcx>,
 332     ) -> Option<SwitchDiscriminantInfo<'tcx>> {
 333         match &switch.kind {
 334             TerminatorKind::SwitchInt { discr, targets, .. } => {
 335                 let discr_local = discr.place()?.as_local()?;
 336                 // the declaration of the discriminant read. Place of this read is being used in the switch
 337                 let discr_decl = &self.body.local_decls()[discr_local];
 338                 let discr_ty = discr_decl.ty;
 339                 // the otherwise target lies as the last element
 340                 let otherwise_bb = targets.otherwise();
 341                 let targets_with_values = targets.iter().collect();
 342
 343                 // find the place of the adt where the discriminant is being read from
 344                 // assume this is the last statement of the block
 345                 let place_of_adt_discr_read = match bb.statements.last()?.kind {
 346                     StatementKind::Assign(box (_, Rvalue::Discriminant(adt_place))) => {
 347                         Some(adt_place)
 348                     }
 349                     _ => None,
 350                 }?;
 351
 352                 let type_adt_matched_on = place_of_adt_discr_read.ty(self.body, self.tcx).ty;
 353
 354                 Some(SwitchDiscriminantInfo {
 355                     discr_used_in_switch: discr.place()?,
 356                     discr_ty,
 357                     otherwise_bb,
 358                     targets_with_values,
 359                     discr_source_info: discr_decl.source_info,
 360                     place_of_adt_discr_read,
 361                     type_adt_matched_on,
 362                 })
 363             }
 364             _ => unreachable!("must only be passed terminator that is a switch"),
 365         }
 366     }
 367 }