src/librustc_mir/build/matches/test.rs

   1 // Copyright 2015 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 // Testing candidates
  12 //
  13 // After candidates have been simplified, the only match pairs that
  14 // remain are those that require some sort of test. The functions here
  15 // identify what tests are needed, perform the tests, and then filter
  16 // the candidates based on the result.
  17
  18 use build::Builder;
  19 use build::matches::{Candidate, MatchPair, Test, TestKind};
  20 use hair::*;
  21 use rustc_data_structures::fnv::FnvHashMap;
  22 use rustc::middle::const_eval::ConstVal;
  23 use rustc::middle::ty::{self, Ty};
  24 use rustc::mir::repr::*;
  25 use syntax::codemap::Span;
  26
  27 impl<'a,'tcx> Builder<'a,'tcx> {
  28     /// Identifies what test is needed to decide if `match_pair` is applicable.
  29     ///
  30     /// It is a bug to call this with a simplifyable pattern.
  31     pub fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<'tcx> {
  32         match *match_pair.pattern.kind {
  33             PatternKind::Variant { ref adt_def, variant_index: _, subpatterns: _ } => {
  34                 Test {
  35                     span: match_pair.pattern.span,
  36                     kind: TestKind::Switch { adt_def: adt_def.clone() },
  37                 }
  38             }
  39
  40             PatternKind::Constant { .. }
  41             if is_switch_ty(match_pair.pattern.ty) => {
  42                 // for integers, we use a SwitchInt match, which allows
  43                 // us to handle more cases
  44                 Test {
  45                     span: match_pair.pattern.span,
  46                     kind: TestKind::SwitchInt {
  47                         switch_ty: match_pair.pattern.ty,
  48
  49                         // these maps are empty to start; cases are
  50                         // added below in add_cases_to_switch
  51                         options: vec![],
  52                         indices: FnvHashMap(),
  53                     }
  54                 }
  55             }
  56
  57             PatternKind::Constant { ref value } => {
  58                 Test {
  59                     span: match_pair.pattern.span,
  60                     kind: TestKind::Eq {
  61                         value: value.clone(),
  62                         ty: match_pair.pattern.ty.clone()
  63                     }
  64                 }
  65             }
  66
  67             PatternKind::Range { ref lo, ref hi } => {
  68                 Test {
  69                     span: match_pair.pattern.span,
  70                     kind: TestKind::Range {
  71                         lo: lo.clone(),
  72                         hi: hi.clone(),
  73                         ty: match_pair.pattern.ty.clone(),
  74                     },
  75                 }
  76             }
  77
  78             PatternKind::Slice { ref prefix, ref slice, ref suffix }
  79                     if !match_pair.slice_len_checked => {
  80                 let len = prefix.len() + suffix.len();
  81                 let op = if slice.is_some() {
  82                     BinOp::Ge
  83                 } else {
  84                     BinOp::Eq
  85                 };
  86                 Test {
  87                     span: match_pair.pattern.span,
  88                     kind: TestKind::Len { len: len as u64, op: op },
  89                 }
  90             }
  91
  92             PatternKind::Array { .. } |
  93             PatternKind::Slice { .. } |
  94             PatternKind::Wild |
  95             PatternKind::Binding { .. } |
  96             PatternKind::Leaf { .. } |
  97             PatternKind::Deref { .. } => {
  98                 self.error_simplifyable(match_pair)
  99             }
 100         }
 101     }
 102
 103     pub fn add_cases_to_switch<'pat>(&mut self,
 104                                      test_lvalue: &Lvalue<'tcx>,
 105                                      candidate: &Candidate<'pat, 'tcx>,
 106                                      switch_ty: Ty<'tcx>,
 107                                      options: &mut Vec<ConstVal>,
 108                                      indices: &mut FnvHashMap<ConstVal, usize>)
 109                                      -> bool
 110     {
 111         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
 112             Some(match_pair) => match_pair,
 113             _ => { return false; }
 114         };
 115
 116         match *match_pair.pattern.kind {
 117             PatternKind::Constant { ref value } => {
 118                 // if the lvalues match, the type should match
 119                 assert_eq!(match_pair.pattern.ty, switch_ty);
 120
 121                 indices.entry(value.clone())
 122                        .or_insert_with(|| {
 123                            options.push(value.clone());
 124                            options.len() - 1
 125                        });
 126                 true
 127             }
 128
 129             PatternKind::Range { .. } |
 130             PatternKind::Variant { .. } |
 131             PatternKind::Slice { .. } |
 132             PatternKind::Array { .. } |
 133             PatternKind::Wild |
 134             PatternKind::Binding { .. } |
 135             PatternKind::Leaf { .. } |
 136             PatternKind::Deref { .. } => {
 137                 // don't know how to add these patterns to a switch
 138                 false
 139             }
 140         }
 141     }
 142
 143     /// Generates the code to perform a test.
 144     pub fn perform_test(&mut self,
 145                         block: BasicBlock,
 146                         lvalue: &Lvalue<'tcx>,
 147                         test: &Test<'tcx>)
 148                         -> Vec<BasicBlock> {
 149         let scope_id = self.innermost_scope_id();
 150         match test.kind {
 151             TestKind::Switch { adt_def } => {
 152                 let num_enum_variants = self.hir.num_variants(adt_def);
 153                 let target_blocks: Vec<_> =
 154                     (0..num_enum_variants).map(|_| self.cfg.start_new_block())
 155                                           .collect();
 156                 self.cfg.terminate(block, TerminatorKind::Switch {
 157                     discr: lvalue.clone(),
 158                     adt_def: adt_def,
 159                     targets: target_blocks.clone()
 160                 });
 161                 target_blocks
 162             }
 163
 164             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 165                 let otherwise = self.cfg.start_new_block();
 166                 let targets: Vec<_> =
 167                     options.iter()
 168                            .map(|_| self.cfg.start_new_block())
 169                            .chain(Some(otherwise))
 170                            .collect();
 171                 self.cfg.terminate(block, TerminatorKind::SwitchInt {
 172                     discr: lvalue.clone(),
 173                     switch_ty: switch_ty,
 174                     values: options.clone(),
 175                     targets: targets.clone(),
 176                 });
 177                 targets
 178             }
 179
 180             TestKind::Eq { ref value, mut ty } => {
 181                 let mut val = Operand::Consume(lvalue.clone());
 182
 183                 // If we're using b"..." as a pattern, we need to insert an
 184                 // unsizing coercion, as the byte string has the type &[u8; N].
 185                 let expect = if let ConstVal::ByteStr(ref bytes) = *value {
 186                     let tcx = self.hir.tcx();
 187
 188                     // Unsize the lvalue to &[u8], too, if necessary.
 189                     if let ty::TyRef(region, mt) = ty.sty {
 190                         if let ty::TyArray(_, _) = mt.ty.sty {
 191                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(tcx.types.u8));
 192                             let val_slice = self.temp(ty);
 193                             self.cfg.push_assign(block, scope_id, test.span, &val_slice,
 194                                                  Rvalue::Cast(CastKind::Unsize, val, ty));
 195                             val = Operand::Consume(val_slice);
 196                         }
 197                     }
 198
 199                     assert!(ty.is_slice());
 200
 201                     let array_ty = tcx.mk_array(tcx.types.u8, bytes.len());
 202                     let array_ref = tcx.mk_imm_ref(tcx.mk_region(ty::ReStatic), array_ty);
 203                     let array = self.literal_operand(test.span, array_ref, Literal::Value {
 204                         value: value.clone()
 205                     });
 206
 207                     let slice = self.temp(ty);
 208                     self.cfg.push_assign(block, scope_id, test.span, &slice,
 209                                          Rvalue::Cast(CastKind::Unsize, array, ty));
 210                     Operand::Consume(slice)
 211                 } else {
 212                     self.literal_operand(test.span, ty, Literal::Value {
 213                         value: value.clone()
 214                     })
 215                 };
 216
 217                 // Use PartialEq::eq for &str and &[u8] slices, instead of BinOp::Eq.
 218                 let fail = self.cfg.start_new_block();
 219                 if let ty::TyRef(_, mt) = ty.sty {
 220                     assert!(ty.is_slice());
 221                     let eq_def_id = self.hir.tcx().lang_items.eq_trait().unwrap();
 222                     let ty = mt.ty;
 223                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, vec![ty]);
 224
 225                     let bool_ty = self.hir.bool_ty();
 226                     let eq_result = self.temp(bool_ty);
 227                     let eq_block = self.cfg.start_new_block();
 228                     let cleanup = self.diverge_cleanup();
 229                     self.cfg.terminate(block, Terminator::Call {
 230                         func: Operand::Constant(Constant {
 231                             span: test.span,
 232                             ty: mty,
 233                             literal: method
 234                         }),
 235                         args: vec![val, expect],
 236                         destination: Some((eq_result.clone(), eq_block)),
 237                         cleanup: cleanup,
 238                     });
 239
 240                     // check the result
 241                     let block = self.cfg.start_new_block();
 242                     self.cfg.terminate(eq_block, Terminator::If {
 243                         cond: Operand::Consume(eq_result),
 244                         targets: (block, fail),
 245                     });
 246
 247                     vec![block, fail]
 248                 } else {
 249                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 250                     vec![block, fail]
 251                 }
 252             }
 253
 254             TestKind::Range { ref lo, ref hi, ty } => {
 255                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 256                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 257                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 258                 let val = Operand::Consume(lvalue.clone());
 259
 260                 let fail = self.cfg.start_new_block();
 261                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 262                 let block = self.compare(block, fail, test.span, BinOp::Le, val, hi);
 263
 264                 vec![block, fail]
 265             }
 266
 267             TestKind::Len { len, op } => {
 268                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 269                 let (actual, result) = (self.temp(usize_ty), self.temp(bool_ty));
 270
 271                 // actual = len(lvalue)
 272                 self.cfg.push_assign(block, scope_id, test.span, &actual, Rvalue::Len(lvalue.clone()));
 273
 274                 // expected = <N>
 275                 let expected = self.push_usize(block, scope_id, test.span, len);
 276
 277                 // result = actual == expected OR result = actual < expected
 278                 self.cfg.push_assign(block,
 279                                      scope_id,
 280                                      test.span,
 281                                      &result,
 282                                      Rvalue::BinaryOp(op,
 283                                                       Operand::Consume(actual),
 284                                                       Operand::Consume(expected)));
 285
 286                 // branch based on result
 287                 let target_blocks: Vec<_> = vec![self.cfg.start_new_block(),
 288                                                  self.cfg.start_new_block()];
 289                 self.cfg.terminate(block, TerminatorKind::If {
 290                     cond: Operand::Consume(result),
 291                     targets: (target_blocks[0], target_blocks[1])
 292                 });
 293
 294                 target_blocks
 295             }
 296         }
 297     }
 298
 299     fn compare(&mut self,
 300                block: BasicBlock,
 301                fail_block: BasicBlock,
 302                span: Span,
 303                op: BinOp,
 304                left: Operand<'tcx>,
 305                right: Operand<'tcx>) -> BasicBlock {
 306         let bool_ty = self.hir.bool_ty();
 307         let result = self.temp(bool_ty);
 308
 309         // result = op(left, right)
 310         let scope_id = self.innermost_scope_id();
 311         self.cfg.push_assign(block, scope_id, span, &result,
 312                              Rvalue::BinaryOp(op, left, right));
 313
 314         // branch based on result
 315         let target_block = self.cfg.start_new_block();
 316         self.cfg.terminate(block, TerminatorKind::If {
 317             cond: Operand::Consume(result),
 318             targets: (target_block, fail_block)
 319         });
 320
 321         target_block
 322     }
 323
 324     /// Given that we are performing `test` against `test_lvalue`,
 325     /// this job sorts out what the status of `candidate` will be
 326     /// after the test. The `resulting_candidates` vector stores, for
 327     /// each possible outcome of `test`, a vector of the candidates
 328     /// that will result. This fn should add a (possibly modified)
 329     /// clone of candidate into `resulting_candidates` wherever
 330     /// appropriate.
 331     ///
 332     /// So, for example, if this candidate is `x @ Some(P0)` and the
 333     /// test is a variant test, then we would add `(x as Option).0 @
 334     /// P0` to the `resulting_candidates` entry corresponding to the
 335     /// variant `Some`.
 336     ///
 337     /// However, in some cases, the test may just not be relevant to
 338     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 339     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 340     /// the test for what value `x` has has no particular relevance
 341     /// to this candidate. In such cases, this function just returns false
 342     /// without doing anything. This is used by the overall `match_candidates`
 343     /// algorithm to structure the match as a whole. See `match_candidates` for
 344     /// more details.
 345     ///
 346     /// FIXME(#29623). In some cases, we have some tricky choices to
 347     /// make.  for example, if we are testing that `x == 22`, but the
 348     /// candidate is `x @ 13..55`, what should we do? In the event
 349     /// that the test is true, we know that the candidate applies, but
 350     /// in the event of false, we don't know that it *doesn't*
 351     /// apply. For now, we return false, indicate that the test does
 352     /// not apply to this candidate, but it might be we can get
 353     /// tighter match code if we do something a bit different.
 354     pub fn sort_candidate<'pat>(&mut self,
 355                                 test_lvalue: &Lvalue<'tcx>,
 356                                 test: &Test<'tcx>,
 357                                 candidate: &Candidate<'pat, 'tcx>,
 358                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 359                                 -> bool {
 360         // Find the match_pair for this lvalue (if any). At present,
 361         // afaik, there can be at most one. (In the future, if we
 362         // adopted a more general `@` operator, there might be more
 363         // than one, but it'd be very unusual to have two sides that
 364         // both require tests; you'd expect one side to be simplified
 365         // away.)
 366         let tested_match_pair = candidate.match_pairs.iter()
 367                                                      .enumerate()
 368                                                      .filter(|&(_, mp)| mp.lvalue == *test_lvalue)
 369                                                      .next();
 370         let (match_pair_index, match_pair) = match tested_match_pair {
 371             Some(pair) => pair,
 372             None => {
 373                 // We are not testing this lvalue. Therefore, this
 374                 // candidate applies to ALL outcomes.
 375                 return false;
 376             }
 377         };
 378
 379         match test.kind {
 380             // If we are performing a variant switch, then this
 381             // informs variant patterns, but nothing else.
 382             TestKind::Switch { adt_def: tested_adt_def } => {
 383                 match *match_pair.pattern.kind {
 384                     PatternKind::Variant { adt_def, variant_index, ref subpatterns } => {
 385                         assert_eq!(adt_def, tested_adt_def);
 386                         let new_candidate =
 387                             self.candidate_after_variant_switch(match_pair_index,
 388                                                                 adt_def,
 389                                                                 variant_index,
 390                                                                 subpatterns,
 391                                                                 candidate);
 392                         resulting_candidates[variant_index].push(new_candidate);
 393                         true
 394                     }
 395                     _ => {
 396                         false
 397                     }
 398                 }
 399             }
 400
 401             // If we are performing a switch over integers, then this informs integer
 402             // equality, but nothing else.
 403             //
 404             // FIXME(#29623) we could use TestKind::Range to rule
 405             // things out here, in some cases.
 406             TestKind::SwitchInt { switch_ty: _, options: _, ref indices } => {
 407                 match *match_pair.pattern.kind {
 408                     PatternKind::Constant { ref value }
 409                     if is_switch_ty(match_pair.pattern.ty) => {
 410                         let index = indices[value];
 411                         let new_candidate = self.candidate_without_match_pair(match_pair_index,
 412                                                                               candidate);
 413                         resulting_candidates[index].push(new_candidate);
 414                         true
 415                     }
 416                     _ => {
 417                         false
 418                     }
 419                 }
 420             }
 421
 422             // If we are performing a length check, then this
 423             // informs slice patterns, but nothing else.
 424             TestKind::Len { .. } => {
 425                 let pattern_test = self.test(&match_pair);
 426                 match *match_pair.pattern.kind {
 427                     PatternKind::Slice { .. } if pattern_test.kind == test.kind => {
 428                         let mut new_candidate = candidate.clone();
 429
 430                         // Set up the MatchKind to simplify this like an array.
 431                         new_candidate.match_pairs[match_pair_index]
 432                                      .slice_len_checked = true;
 433                         resulting_candidates[0].push(new_candidate);
 434                         true
 435                     }
 436                     _ => false
 437                 }
 438             }
 439
 440             TestKind::Eq { .. } |
 441             TestKind::Range { .. } => {
 442                 // These are all binary tests.
 443                 //
 444                 // FIXME(#29623) we can be more clever here
 445                 let pattern_test = self.test(&match_pair);
 446                 if pattern_test.kind == test.kind {
 447                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 448                                                                           candidate);
 449                     resulting_candidates[0].push(new_candidate);
 450                     true
 451                 } else {
 452                     false
 453                 }
 454             }
 455         }
 456     }
 457
 458     fn candidate_without_match_pair<'pat>(&mut self,
 459                                           match_pair_index: usize,
 460                                           candidate: &Candidate<'pat, 'tcx>)
 461                                           -> Candidate<'pat, 'tcx> {
 462         let other_match_pairs =
 463             candidate.match_pairs.iter()
 464                                  .enumerate()
 465                                  .filter(|&(index, _)| index != match_pair_index)
 466                                  .map(|(_, mp)| mp.clone())
 467                                  .collect();
 468         Candidate {
 469             match_pairs: other_match_pairs,
 470             bindings: candidate.bindings.clone(),
 471             guard: candidate.guard.clone(),
 472             arm_index: candidate.arm_index,
 473         }
 474     }
 475
 476     fn candidate_after_variant_switch<'pat>(&mut self,
 477                                             match_pair_index: usize,
 478                                             adt_def: ty::AdtDef<'tcx>,
 479                                             variant_index: usize,
 480                                             subpatterns: &'pat [FieldPattern<'tcx>],
 481                                             candidate: &Candidate<'pat, 'tcx>)
 482                                             -> Candidate<'pat, 'tcx> {
 483         let match_pair = &candidate.match_pairs[match_pair_index];
 484
 485         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 486         // we want to create a set of derived match-patterns like
 487         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 488         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 489         let downcast_lvalue = match_pair.lvalue.clone().elem(elem); // `(x as Variant)`
 490         let consequent_match_pairs =
 491             subpatterns.iter()
 492                        .map(|subpattern| {
 493                            // e.g., `(x as Variant).0`
 494                            let lvalue = downcast_lvalue.clone().field(subpattern.field,
 495                                                                       subpattern.pattern.ty);
 496                            // e.g., `(x as Variant).0 @ P1`
 497                            MatchPair::new(lvalue, &subpattern.pattern)
 498                        });
 499
 500         // In addition, we need all the other match pairs from the old candidate.
 501         let other_match_pairs =
 502             candidate.match_pairs.iter()
 503                                  .enumerate()
 504                                  .filter(|&(index, _)| index != match_pair_index)
 505                                  .map(|(_, mp)| mp.clone());
 506
 507         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 508
 509         Candidate {
 510             match_pairs: all_match_pairs,
 511             bindings: candidate.bindings.clone(),
 512             guard: candidate.guard.clone(),
 513             arm_index: candidate.arm_index,
 514         }
 515     }
 516
 517     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 518         self.hir.span_bug(match_pair.pattern.span,
 519                           &format!("simplifyable pattern found: {:?}", match_pair.pattern))
 520     }
 521 }
 522
 523 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 524     ty.is_integral() || ty.is_char() || ty.is_bool()
 525 }