src/librustc_mir/transform/const_prop.rs

   1 // Copyright 2017 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 //! Propagates constants for early reporting of statically known
  12 //! assertion failures
  13
  14
  15 use rustc::hir::def::Def;
  16 use rustc::mir::{Constant, Literal, Location, Place, Mir, Operand, Rvalue, Local};
  17 use rustc::mir::{NullOp, StatementKind, Statement, BasicBlock, LocalKind};
  18 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
  19 use rustc::mir::visit::{Visitor, PlaceContext};
  20 use rustc::mir::interpret::ConstEvalErr;
  21 use rustc::ty::{TyCtxt, self, Instance};
  22 use rustc::mir::interpret::{Value, Scalar, GlobalId, EvalResult};
  23 use interpret::EvalContext;
  24 use interpret::CompileTimeEvaluator;
  25 use interpret::{eval_promoted, mk_borrowck_eval_cx, ValTy};
  26 use transform::{MirPass, MirSource};
  27 use syntax::codemap::{Span, DUMMY_SP};
  28 use rustc::ty::subst::Substs;
  29 use rustc_data_structures::indexed_vec::IndexVec;
  30 use rustc::ty::ParamEnv;
  31 use rustc::ty::layout::{
  32     LayoutOf, TyLayout, LayoutError,
  33     HasTyCtxt, TargetDataLayout, HasDataLayout,
  34 };
  35
  36 pub struct ConstProp;
  37
  38 impl MirPass for ConstProp {
  39     fn run_pass<'a, 'tcx>(&self,
  40                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  41                           source: MirSource,
  42                           mir: &mut Mir<'tcx>) {
  43         // will be evaluated by miri and produce its errors there
  44         if source.promoted.is_some() {
  45             return;
  46         }
  47         match tcx.describe_def(source.def_id) {
  48             // skip statics/consts because they'll be evaluated by miri anyway
  49             Some(Def::Const(..)) |
  50             Some(Def::Static(..)) => return,
  51             // we still run on associated constants, because they might not get evaluated
  52             // within the current crate
  53             _ => {},
  54         }
  55         trace!("ConstProp starting for {:?}", source.def_id);
  56
  57         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  58         // constants, instead of just checking for const-folding succeeding.
  59         // That would require an uniform one-def no-mutation analysis
  60         // and RPO (or recursing when needing the value of a local).
  61         let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
  62         optimization_finder.visit_mir(mir);
  63
  64         trace!("ConstProp done for {:?}", source.def_id);
  65     }
  66 }
  67
  68 type Const<'tcx> = (Value, ty::Ty<'tcx>, Span);
  69
  70 /// Finds optimization opportunities on the MIR.
  71 struct ConstPropagator<'b, 'a, 'tcx:'a+'b> {
  72     ecx: EvalContext<'a, 'b, 'tcx, CompileTimeEvaluator>,
  73     mir: &'b Mir<'tcx>,
  74     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  75     source: MirSource,
  76     places: IndexVec<Local, Option<Const<'tcx>>>,
  77     can_const_prop: IndexVec<Local, bool>,
  78     param_env: ParamEnv<'tcx>,
  79 }
  80
  81 impl<'a, 'b, 'tcx> LayoutOf for &'a ConstPropagator<'a, 'b, 'tcx> {
  82     type Ty = ty::Ty<'tcx>;
  83     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
  84
  85     fn layout_of(self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
  86         self.tcx.layout_of(self.param_env.and(ty))
  87     }
  88 }
  89
  90 impl<'a, 'b, 'tcx> HasDataLayout for &'a ConstPropagator<'a, 'b, 'tcx> {
  91     #[inline]
  92     fn data_layout(&self) -> &TargetDataLayout {
  93         &self.tcx.data_layout
  94     }
  95 }
  96
  97 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for &'a ConstPropagator<'a, 'b, 'tcx> {
  98     #[inline]
  99     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
 100         self.tcx
 101     }
 102 }
 103
 104 impl<'b, 'a, 'tcx:'b> ConstPropagator<'b, 'a, 'tcx> {
 105     fn new(
 106         mir: &'b Mir<'tcx>,
 107         tcx: TyCtxt<'a, 'tcx, 'tcx>,
 108         source: MirSource,
 109     ) -> ConstPropagator<'b, 'a, 'tcx> {
 110         let param_env = tcx.param_env(source.def_id);
 111         let substs = Substs::identity_for_item(tcx, source.def_id);
 112         let instance = Instance::new(source.def_id, substs);
 113         let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
 114         ConstPropagator {
 115             ecx,
 116             mir,
 117             tcx,
 118             source,
 119             param_env,
 120             can_const_prop: CanConstProp::check(mir),
 121             places: IndexVec::from_elem(None, &mir.local_decls),
 122         }
 123     }
 124
 125     fn use_ecx<F, T>(
 126         &mut self,
 127         source_info: SourceInfo,
 128         f: F
 129     ) -> Option<T>
 130     where
 131         F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
 132     {
 133         self.ecx.tcx.span = source_info.span;
 134         let lint_root = match self.mir.source_scope_local_data {
 135             ClearCrossCrate::Set(ref ivs) => {
 136                 use rustc_data_structures::indexed_vec::Idx;
 137                 //FIXME(#51314): remove this check
 138                 if source_info.scope.index() >= ivs.len() {
 139                     return None;
 140                 }
 141                 ivs[source_info.scope].lint_root
 142             },
 143             ClearCrossCrate::Clear => return None,
 144         };
 145         let r = match f(self) {
 146             Ok(val) => Some(val),
 147             Err(err) => {
 148                 let (frames, span) = self.ecx.generate_stacktrace(None);
 149                 let err = ConstEvalErr {
 150                     span,
 151                     error: err,
 152                     stacktrace: frames,
 153                 };
 154                 err.report_as_lint(
 155                     self.ecx.tcx,
 156                     "this expression will panic at runtime",
 157                     lint_root,
 158                 );
 159                 None
 160             },
 161         };
 162         self.ecx.tcx.span = DUMMY_SP;
 163         r
 164     }
 165
 166     fn eval_constant(
 167         &mut self,
 168         c: &Constant<'tcx>,
 169         source_info: SourceInfo,
 170     ) -> Option<Const<'tcx>> {
 171         match c.literal {
 172             Literal::Value { value } => {
 173                 let v = self.use_ecx(source_info, |this| {
 174                     this.ecx.const_to_value(value.val)
 175                 })?;
 176                 Some((v, value.ty, c.span))
 177             },
 178             // evaluate the promoted and replace the constant with the evaluated result
 179             Literal::Promoted { index } => {
 180                 let generics = self.tcx.generics_of(self.source.def_id);
 181                 if generics.requires_monomorphization(self.tcx) {
 182                     // FIXME: can't handle code with generics
 183                     return None;
 184                 }
 185                 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
 186                 let instance = Instance::new(self.source.def_id, substs);
 187                 let cid = GlobalId {
 188                     instance,
 189                     promoted: Some(index),
 190                 };
 191                 // cannot use `const_eval` here, because that would require having the MIR
 192                 // for the current function available, but we're producing said MIR right now
 193                 let (value, _, ty) = self.use_ecx(source_info, |this| {
 194                     eval_promoted(&mut this.ecx, cid, this.mir, this.param_env)
 195                 })?;
 196                 let val = (value, ty, c.span);
 197                 trace!("evaluated {:?} to {:?}", c, val);
 198                 Some(val)
 199             }
 200         }
 201     }
 202
 203     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 204         match *place {
 205             Place::Local(loc) => self.places[loc].clone(),
 206             Place::Projection(ref proj) => match proj.elem {
 207                 ProjectionElem::Field(field, _) => {
 208                     trace!("field proj on {:?}", proj.base);
 209                     let (base, ty, span) = self.eval_place(&proj.base, source_info)?;
 210                     let valty = self.use_ecx(source_info, |this| {
 211                         this.ecx.read_field(base, None, field, ty)
 212                     })?;
 213                     Some((valty.value, valty.ty, span))
 214                 },
 215                 _ => None,
 216             },
 217             _ => None,
 218         }
 219     }
 220
 221     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 222         match *op {
 223             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 224             | Operand::Move(ref place)
 225             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 226         }
 227     }
 228
 229     fn const_prop(
 230         &mut self,
 231         rvalue: &Rvalue<'tcx>,
 232         place_ty: ty::Ty<'tcx>,
 233         source_info: SourceInfo,
 234     ) -> Option<Const<'tcx>> {
 235         let span = source_info.span;
 236         match *rvalue {
 237             // This branch exists for the sanity type check
 238             Rvalue::Use(Operand::Constant(ref c)) => {
 239                 assert_eq!(c.ty, place_ty);
 240                 self.eval_constant(c, source_info)
 241             },
 242             Rvalue::Use(ref op) => {
 243                 self.eval_operand(op, source_info)
 244             },
 245             Rvalue::Repeat(..) |
 246             Rvalue::Ref(..) |
 247             Rvalue::Cast(..) |
 248             Rvalue::Aggregate(..) |
 249             Rvalue::NullaryOp(NullOp::Box, _) |
 250             Rvalue::Discriminant(..) => None,
 251             // FIXME(oli-obk): evaluate static/constant slice lengths
 252             Rvalue::Len(_) => None,
 253             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 254                 let param_env = self.tcx.param_env(self.source.def_id);
 255                 type_size_of(self.tcx, param_env, ty).map(|n| (
 256                     Value::Scalar(Scalar::Bits {
 257                         bits: n as u128,
 258                         defined: self.tcx.data_layout.pointer_size.bits() as u8,
 259                     }),
 260                     self.tcx.types.usize,
 261                     span,
 262                 ))
 263             }
 264             Rvalue::UnaryOp(op, ref arg) => {
 265                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 266                     self.tcx.closure_base_def_id(self.source.def_id)
 267                 } else {
 268                     self.source.def_id
 269                 };
 270                 let generics = self.tcx.generics_of(def_id);
 271                 if generics.requires_monomorphization(self.tcx) {
 272                     // FIXME: can't handle code with generics
 273                     return None;
 274                 }
 275
 276                 let val = self.eval_operand(arg, source_info)?;
 277                 let prim = self.use_ecx(source_info, |this| {
 278                     this.ecx.value_to_scalar(ValTy { value: val.0, ty: val.1 })
 279                 })?;
 280                 let val = self.use_ecx(source_info, |this| this.ecx.unary_op(op, prim, val.1))?;
 281                 Some((Value::Scalar(val), place_ty, span))
 282             }
 283             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 284             Rvalue::BinaryOp(op, ref left, ref right) => {
 285                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 286                 let right = self.eval_operand(right, source_info)?;
 287                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 288                     self.tcx.closure_base_def_id(self.source.def_id)
 289                 } else {
 290                     self.source.def_id
 291                 };
 292                 let generics = self.tcx.generics_of(def_id);
 293                 if generics.requires_monomorphization(self.tcx) {
 294                     // FIXME: can't handle code with generics
 295                     return None;
 296                 }
 297
 298                 let r = self.use_ecx(source_info, |this| {
 299                     this.ecx.value_to_scalar(ValTy { value: right.0, ty: right.1 })
 300                 })?;
 301                 if op == BinOp::Shr || op == BinOp::Shl {
 302                     let left_ty = left.ty(self.mir, self.tcx);
 303                     let left_bits = self
 304                         .tcx
 305                         .layout_of(self.param_env.and(left_ty))
 306                         .unwrap()
 307                         .size
 308                         .bits();
 309                     let right_size = self.tcx.layout_of(self.param_env.and(right.1)).unwrap().size;
 310                     if r.to_bits(right_size).ok().map_or(false, |b| b >= left_bits as u128) {
 311                         let source_scope_local_data = match self.mir.source_scope_local_data {
 312                             ClearCrossCrate::Set(ref data) => data,
 313                             ClearCrossCrate::Clear => return None,
 314                         };
 315                         let dir = if op == BinOp::Shr {
 316                             "right"
 317                         } else {
 318                             "left"
 319                         };
 320                         let node_id = source_scope_local_data[source_info.scope].lint_root;
 321                         self.tcx.lint_node(
 322                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 323                             node_id,
 324                             span,
 325                             &format!("attempt to shift {} with overflow", dir));
 326                         return None;
 327                     }
 328                 }
 329                 let left = self.eval_operand(left, source_info)?;
 330                 let l = self.use_ecx(source_info, |this| {
 331                     this.ecx.value_to_scalar(ValTy { value: left.0, ty: left.1 })
 332                 })?;
 333                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 334                 let (val, overflow) = self.use_ecx(source_info, |this| {
 335                     this.ecx.binary_op(op, l, left.1, r, right.1)
 336                 })?;
 337                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 338                     Value::ScalarPair(
 339                         val,
 340                         Scalar::from_bool(overflow),
 341                     )
 342                 } else {
 343                     if overflow {
 344                         use rustc::mir::interpret::EvalErrorKind;
 345                         let err = EvalErrorKind::Overflow(op).into();
 346                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 347                         return None;
 348                     }
 349                     Value::Scalar(val)
 350                 };
 351                 Some((val, place_ty, span))
 352             },
 353         }
 354     }
 355 }
 356
 357 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 358                           param_env: ty::ParamEnv<'tcx>,
 359                           ty: ty::Ty<'tcx>) -> Option<u64> {
 360     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 361 }
 362
 363 struct CanConstProp {
 364     can_const_prop: IndexVec<Local, bool>,
 365     // false at the beginning, once set, there are not allowed to be any more assignments
 366     found_assignment: IndexVec<Local, bool>,
 367 }
 368
 369 impl CanConstProp {
 370     /// returns true if `local` can be propagated
 371     fn check(mir: &Mir) -> IndexVec<Local, bool> {
 372         let mut cpv = CanConstProp {
 373             can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
 374             found_assignment: IndexVec::from_elem(false, &mir.local_decls),
 375         };
 376         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 377             // cannot use args at all
 378             // cannot use locals because if x < y { y - x } else { x - y } would
 379             //        lint for x != y
 380             // FIXME(oli-obk): lint variables until they are used in a condition
 381             // FIXME(oli-obk): lint if return value is constant
 382             *val = mir.local_kind(local) == LocalKind::Temp;
 383         }
 384         cpv.visit_mir(mir);
 385         cpv.can_const_prop
 386     }
 387 }
 388
 389 impl<'tcx> Visitor<'tcx> for CanConstProp {
 390     fn visit_local(
 391         &mut self,
 392         &local: &Local,
 393         context: PlaceContext<'tcx>,
 394         _: Location,
 395     ) {
 396         use rustc::mir::visit::PlaceContext::*;
 397         match context {
 398             // Constants must have at most one write
 399             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 400             // only occur in independent execution paths
 401             Store => if self.found_assignment[local] {
 402                 self.can_const_prop[local] = false;
 403             } else {
 404                 self.found_assignment[local] = true
 405             },
 406             // Reading constants is allowed an arbitrary number of times
 407             Copy | Move |
 408             StorageDead | StorageLive |
 409             Validate |
 410             Projection(_) |
 411             Inspect => {},
 412             _ => self.can_const_prop[local] = false,
 413         }
 414     }
 415 }
 416
 417 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
 418     fn visit_constant(
 419         &mut self,
 420         constant: &Constant<'tcx>,
 421         location: Location,
 422     ) {
 423         trace!("visit_constant: {:?}", constant);
 424         self.super_constant(constant, location);
 425         let source_info = *self.mir.source_info(location);
 426         self.eval_constant(constant, source_info);
 427     }
 428
 429     fn visit_statement(
 430         &mut self,
 431         block: BasicBlock,
 432         statement: &Statement<'tcx>,
 433         location: Location,
 434     ) {
 435         trace!("visit_statement: {:?}", statement);
 436         if let StatementKind::Assign(ref place, ref rval) = statement.kind {
 437             let place_ty = place
 438                 .ty(&self.mir.local_decls, self.tcx)
 439                 .to_ty(self.tcx);
 440             if let Some(value) = self.const_prop(rval, place_ty, statement.source_info) {
 441                 if let Place::Local(local) = *place {
 442                     trace!("checking whether {:?} can be stored to {:?}", value, local);
 443                     if self.can_const_prop[local] {
 444                         trace!("storing {:?} to {:?}", value, local);
 445                         assert!(self.places[local].is_none());
 446                         self.places[local] = Some(value);
 447                     }
 448                 }
 449             }
 450         }
 451         self.super_statement(block, statement, location);
 452     }
 453
 454     fn visit_terminator_kind(
 455         &mut self,
 456         block: BasicBlock,
 457         kind: &TerminatorKind<'tcx>,
 458         location: Location,
 459     ) {
 460         self.super_terminator_kind(block, kind, location);
 461         let source_info = *self.mir.source_info(location);
 462         if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
 463             if let Some(value) = self.eval_operand(cond, source_info) {
 464                 trace!("assertion on {:?} should be {:?}", value, expected);
 465                 if Value::Scalar(Scalar::from_bool(*expected)) != value.0 {
 466                     // poison all places this operand references so that further code
 467                     // doesn't use the invalid value
 468                     match cond {
 469                         Operand::Move(ref place) | Operand::Copy(ref place) => {
 470                             let mut place = place;
 471                             while let Place::Projection(ref proj) = *place {
 472                                 place = &proj.base;
 473                             }
 474                             if let Place::Local(local) = *place {
 475                                 self.places[local] = None;
 476                             }
 477                         },
 478                         Operand::Constant(_) => {}
 479                     }
 480                     let span = self.mir[block]
 481                         .terminator
 482                         .as_ref()
 483                         .unwrap()
 484                         .source_info
 485                         .span;
 486                     let node_id = self
 487                         .tcx
 488                         .hir
 489                         .as_local_node_id(self.source.def_id)
 490                         .expect("some part of a failing const eval must be local");
 491                     use rustc::mir::interpret::EvalErrorKind::*;
 492                     let msg = match msg {
 493                         Overflow(_) |
 494                         OverflowNeg |
 495                         DivisionByZero |
 496                         RemainderByZero => msg.description().to_owned(),
 497                         BoundsCheck { ref len, ref index } => {
 498                             let len = self
 499                                 .eval_operand(len, source_info)
 500                                 .expect("len must be const");
 501                             let len = match len.0 {
 502                                 Value::Scalar(Scalar::Bits { bits, ..}) => bits,
 503                                 _ => bug!("const len not primitive: {:?}", len),
 504                             };
 505                             let index = self
 506                                 .eval_operand(index, source_info)
 507                                 .expect("index must be const");
 508                             let index = match index.0 {
 509                                 Value::Scalar(Scalar::Bits { bits, .. }) => bits,
 510                                 _ => bug!("const index not primitive: {:?}", index),
 511                             };
 512                             format!(
 513                                 "index out of bounds: \
 514                                 the len is {} but the index is {}",
 515                                 len,
 516                                 index,
 517                             )
 518                         },
 519                         // Need proper const propagator for these
 520                         _ => return,
 521                     };
 522                     self.tcx.lint_node(
 523                         ::rustc::lint::builtin::CONST_ERR,
 524                         node_id,
 525                         span,
 526                         &msg,
 527                     );
 528                 }
 529             }
 530         }
 531     }
 532 }