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, EvalErrorKind};
  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                 match err.kind {
 149                     // don't report these, they make no sense in a const prop context
 150                     EvalErrorKind::MachineError(_) => {},
 151                     _ => {
 152                         let (frames, span) = self.ecx.generate_stacktrace(None);
 153                         let err = ConstEvalErr {
 154                             span,
 155                             error: err,
 156                             stacktrace: frames,
 157                         };
 158                         err.report_as_lint(
 159                             self.ecx.tcx,
 160                             "this expression will panic at runtime",
 161                             lint_root,
 162                         );
 163                     }
 164                 }
 165                 None
 166             },
 167         };
 168         self.ecx.tcx.span = DUMMY_SP;
 169         r
 170     }
 171
 172     fn eval_constant(
 173         &mut self,
 174         c: &Constant<'tcx>,
 175         source_info: SourceInfo,
 176     ) -> Option<Const<'tcx>> {
 177         match c.literal {
 178             Literal::Value { value } => {
 179                 self.ecx.tcx.span = source_info.span;
 180                 match self.ecx.const_to_value(value.val) {
 181                     Ok(val) => Some((val, value.ty, c.span)),
 182                     Err(error) => {
 183                         let (stacktrace, span) = self.ecx.generate_stacktrace(None);
 184                         let err = ConstEvalErr {
 185                             span,
 186                             error,
 187                             stacktrace,
 188                         };
 189                         err.report_as_error(
 190                             self.tcx.at(source_info.span),
 191                             "could not evaluate constant",
 192                         );
 193                         None
 194                     },
 195                 }
 196             },
 197             // evaluate the promoted and replace the constant with the evaluated result
 198             Literal::Promoted { index } => {
 199                 let generics = self.tcx.generics_of(self.source.def_id);
 200                 if generics.requires_monomorphization(self.tcx) {
 201                     // FIXME: can't handle code with generics
 202                     return None;
 203                 }
 204                 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
 205                 let instance = Instance::new(self.source.def_id, substs);
 206                 let cid = GlobalId {
 207                     instance,
 208                     promoted: Some(index),
 209                 };
 210                 // cannot use `const_eval` here, because that would require having the MIR
 211                 // for the current function available, but we're producing said MIR right now
 212                 let (value, _, ty) = self.use_ecx(source_info, |this| {
 213                     eval_promoted(&mut this.ecx, cid, this.mir, this.param_env)
 214                 })?;
 215                 let val = (value, ty, c.span);
 216                 trace!("evaluated {:?} to {:?}", c, val);
 217                 Some(val)
 218             }
 219         }
 220     }
 221
 222     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 223         match *place {
 224             Place::Local(loc) => self.places[loc].clone(),
 225             Place::Projection(ref proj) => match proj.elem {
 226                 ProjectionElem::Field(field, _) => {
 227                     trace!("field proj on {:?}", proj.base);
 228                     let (base, ty, span) = self.eval_place(&proj.base, source_info)?;
 229                     let valty = self.use_ecx(source_info, |this| {
 230                         this.ecx.read_field(base, None, field, ty)
 231                     })?;
 232                     Some((valty.value, valty.ty, span))
 233                 },
 234                 _ => None,
 235             },
 236             _ => None,
 237         }
 238     }
 239
 240     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 241         match *op {
 242             Operand::Constant(ref c) => self.eval_constant(c, source_info),
 243             | Operand::Move(ref place)
 244             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 245         }
 246     }
 247
 248     fn const_prop(
 249         &mut self,
 250         rvalue: &Rvalue<'tcx>,
 251         place_ty: ty::Ty<'tcx>,
 252         source_info: SourceInfo,
 253     ) -> Option<Const<'tcx>> {
 254         let span = source_info.span;
 255         match *rvalue {
 256             // This branch exists for the sanity type check
 257             Rvalue::Use(Operand::Constant(ref c)) => {
 258                 assert_eq!(c.ty, place_ty);
 259                 self.eval_constant(c, source_info)
 260             },
 261             Rvalue::Use(ref op) => {
 262                 self.eval_operand(op, source_info)
 263             },
 264             Rvalue::Repeat(..) |
 265             Rvalue::Ref(..) |
 266             Rvalue::Aggregate(..) |
 267             Rvalue::NullaryOp(NullOp::Box, _) |
 268             Rvalue::Discriminant(..) => None,
 269
 270             Rvalue::Cast(kind, ref operand, _) => {
 271                 let (value, ty, span) = self.eval_operand(operand, source_info)?;
 272                 self.use_ecx(source_info, |this| {
 273                     let dest_ptr = this.ecx.alloc_ptr(place_ty)?;
 274                     let place_align = this.ecx.layout_of(place_ty)?.align;
 275                     let dest = ::interpret::Place::from_ptr(dest_ptr, place_align);
 276                     this.ecx.cast(ValTy { value, ty }, kind, place_ty, dest)?;
 277                     Ok((
 278                         Value::ByRef(dest_ptr.into(), place_align),
 279                         place_ty,
 280                         span,
 281                     ))
 282                 })
 283             }
 284
 285             // FIXME(oli-obk): evaluate static/constant slice lengths
 286             Rvalue::Len(_) => None,
 287             Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
 288                 let param_env = self.tcx.param_env(self.source.def_id);
 289                 type_size_of(self.tcx, param_env, ty).map(|n| (
 290                     Value::Scalar(Scalar::Bits {
 291                         bits: n as u128,
 292                         defined: self.tcx.data_layout.pointer_size.bits() as u8,
 293                     }),
 294                     self.tcx.types.usize,
 295                     span,
 296                 ))
 297             }
 298             Rvalue::UnaryOp(op, ref arg) => {
 299                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 300                     self.tcx.closure_base_def_id(self.source.def_id)
 301                 } else {
 302                     self.source.def_id
 303                 };
 304                 let generics = self.tcx.generics_of(def_id);
 305                 if generics.requires_monomorphization(self.tcx) {
 306                     // FIXME: can't handle code with generics
 307                     return None;
 308                 }
 309
 310                 let val = self.eval_operand(arg, source_info)?;
 311                 let prim = self.use_ecx(source_info, |this| {
 312                     this.ecx.value_to_scalar(ValTy { value: val.0, ty: val.1 })
 313                 })?;
 314                 let val = self.use_ecx(source_info, |this| this.ecx.unary_op(op, prim, val.1))?;
 315                 Some((Value::Scalar(val), place_ty, span))
 316             }
 317             Rvalue::CheckedBinaryOp(op, ref left, ref right) |
 318             Rvalue::BinaryOp(op, ref left, ref right) => {
 319                 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
 320                 let right = self.eval_operand(right, source_info)?;
 321                 let def_id = if self.tcx.is_closure(self.source.def_id) {
 322                     self.tcx.closure_base_def_id(self.source.def_id)
 323                 } else {
 324                     self.source.def_id
 325                 };
 326                 let generics = self.tcx.generics_of(def_id);
 327                 if generics.requires_monomorphization(self.tcx) {
 328                     // FIXME: can't handle code with generics
 329                     return None;
 330                 }
 331
 332                 let r = self.use_ecx(source_info, |this| {
 333                     this.ecx.value_to_scalar(ValTy { value: right.0, ty: right.1 })
 334                 })?;
 335                 if op == BinOp::Shr || op == BinOp::Shl {
 336                     let left_ty = left.ty(self.mir, self.tcx);
 337                     let left_bits = self
 338                         .tcx
 339                         .layout_of(self.param_env.and(left_ty))
 340                         .unwrap()
 341                         .size
 342                         .bits();
 343                     let right_size = self.tcx.layout_of(self.param_env.and(right.1)).unwrap().size;
 344                     if r.to_bits(right_size).ok().map_or(false, |b| b >= left_bits as u128) {
 345                         let source_scope_local_data = match self.mir.source_scope_local_data {
 346                             ClearCrossCrate::Set(ref data) => data,
 347                             ClearCrossCrate::Clear => return None,
 348                         };
 349                         let dir = if op == BinOp::Shr {
 350                             "right"
 351                         } else {
 352                             "left"
 353                         };
 354                         let node_id = source_scope_local_data[source_info.scope].lint_root;
 355                         self.tcx.lint_node(
 356                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 357                             node_id,
 358                             span,
 359                             &format!("attempt to shift {} with overflow", dir));
 360                         return None;
 361                     }
 362                 }
 363                 let left = self.eval_operand(left, source_info)?;
 364                 let l = self.use_ecx(source_info, |this| {
 365                     this.ecx.value_to_scalar(ValTy { value: left.0, ty: left.1 })
 366                 })?;
 367                 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
 368                 let (val, overflow) = self.use_ecx(source_info, |this| {
 369                     this.ecx.binary_op(op, l, left.1, r, right.1)
 370                 })?;
 371                 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
 372                     Value::ScalarPair(
 373                         val,
 374                         Scalar::from_bool(overflow),
 375                     )
 376                 } else {
 377                     if overflow {
 378                         let err = EvalErrorKind::Overflow(op).into();
 379                         let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
 380                         return None;
 381                     }
 382                     Value::Scalar(val)
 383                 };
 384                 Some((val, place_ty, span))
 385             },
 386         }
 387     }
 388 }
 389
 390 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 391                           param_env: ty::ParamEnv<'tcx>,
 392                           ty: ty::Ty<'tcx>) -> Option<u64> {
 393     tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
 394 }
 395
 396 struct CanConstProp {
 397     can_const_prop: IndexVec<Local, bool>,
 398     // false at the beginning, once set, there are not allowed to be any more assignments
 399     found_assignment: IndexVec<Local, bool>,
 400 }
 401
 402 impl CanConstProp {
 403     /// returns true if `local` can be propagated
 404     fn check(mir: &Mir) -> IndexVec<Local, bool> {
 405         let mut cpv = CanConstProp {
 406             can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
 407             found_assignment: IndexVec::from_elem(false, &mir.local_decls),
 408         };
 409         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 410             // cannot use args at all
 411             // cannot use locals because if x < y { y - x } else { x - y } would
 412             //        lint for x != y
 413             // FIXME(oli-obk): lint variables until they are used in a condition
 414             // FIXME(oli-obk): lint if return value is constant
 415             *val = mir.local_kind(local) == LocalKind::Temp;
 416         }
 417         cpv.visit_mir(mir);
 418         cpv.can_const_prop
 419     }
 420 }
 421
 422 impl<'tcx> Visitor<'tcx> for CanConstProp {
 423     fn visit_local(
 424         &mut self,
 425         &local: &Local,
 426         context: PlaceContext<'tcx>,
 427         _: Location,
 428     ) {
 429         use rustc::mir::visit::PlaceContext::*;
 430         match context {
 431             // Constants must have at most one write
 432             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 433             // only occur in independent execution paths
 434             Store => if self.found_assignment[local] {
 435                 self.can_const_prop[local] = false;
 436             } else {
 437                 self.found_assignment[local] = true
 438             },
 439             // Reading constants is allowed an arbitrary number of times
 440             Copy | Move |
 441             StorageDead | StorageLive |
 442             Validate |
 443             Projection(_) |
 444             Inspect => {},
 445             _ => self.can_const_prop[local] = false,
 446         }
 447     }
 448 }
 449
 450 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
 451     fn visit_constant(
 452         &mut self,
 453         constant: &Constant<'tcx>,
 454         location: Location,
 455     ) {
 456         trace!("visit_constant: {:?}", constant);
 457         self.super_constant(constant, location);
 458         let source_info = *self.mir.source_info(location);
 459         self.eval_constant(constant, source_info);
 460     }
 461
 462     fn visit_statement(
 463         &mut self,
 464         block: BasicBlock,
 465         statement: &Statement<'tcx>,
 466         location: Location,
 467     ) {
 468         trace!("visit_statement: {:?}", statement);
 469         if let StatementKind::Assign(ref place, ref rval) = statement.kind {
 470             let place_ty = place
 471                 .ty(&self.mir.local_decls, self.tcx)
 472                 .to_ty(self.tcx);
 473             if let Some(value) = self.const_prop(rval, place_ty, statement.source_info) {
 474                 if let Place::Local(local) = *place {
 475                     trace!("checking whether {:?} can be stored to {:?}", value, local);
 476                     if self.can_const_prop[local] {
 477                         trace!("storing {:?} to {:?}", value, local);
 478                         assert!(self.places[local].is_none());
 479                         self.places[local] = Some(value);
 480                     }
 481                 }
 482             }
 483         }
 484         self.super_statement(block, statement, location);
 485     }
 486
 487     fn visit_terminator_kind(
 488         &mut self,
 489         block: BasicBlock,
 490         kind: &TerminatorKind<'tcx>,
 491         location: Location,
 492     ) {
 493         self.super_terminator_kind(block, kind, location);
 494         let source_info = *self.mir.source_info(location);
 495         if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
 496             if let Some(value) = self.eval_operand(cond, source_info) {
 497                 trace!("assertion on {:?} should be {:?}", value, expected);
 498                 if Value::Scalar(Scalar::from_bool(*expected)) != value.0 {
 499                     // poison all places this operand references so that further code
 500                     // doesn't use the invalid value
 501                     match cond {
 502                         Operand::Move(ref place) | Operand::Copy(ref place) => {
 503                             let mut place = place;
 504                             while let Place::Projection(ref proj) = *place {
 505                                 place = &proj.base;
 506                             }
 507                             if let Place::Local(local) = *place {
 508                                 self.places[local] = None;
 509                             }
 510                         },
 511                         Operand::Constant(_) => {}
 512                     }
 513                     let span = self.mir[block]
 514                         .terminator
 515                         .as_ref()
 516                         .unwrap()
 517                         .source_info
 518                         .span;
 519                     let node_id = self
 520                         .tcx
 521                         .hir
 522                         .as_local_node_id(self.source.def_id)
 523                         .expect("some part of a failing const eval must be local");
 524                     use rustc::mir::interpret::EvalErrorKind::*;
 525                     let msg = match msg {
 526                         Overflow(_) |
 527                         OverflowNeg |
 528                         DivisionByZero |
 529                         RemainderByZero => msg.description().to_owned(),
 530                         BoundsCheck { ref len, ref index } => {
 531                             let len = self
 532                                 .eval_operand(len, source_info)
 533                                 .expect("len must be const");
 534                             let len = match len.0 {
 535                                 Value::Scalar(Scalar::Bits { bits, ..}) => bits,
 536                                 _ => bug!("const len not primitive: {:?}", len),
 537                             };
 538                             let index = self
 539                                 .eval_operand(index, source_info)
 540                                 .expect("index must be const");
 541                             let index = match index.0 {
 542                                 Value::Scalar(Scalar::Bits { bits, .. }) => bits,
 543                                 _ => bug!("const index not primitive: {:?}", index),
 544                             };
 545                             format!(
 546                                 "index out of bounds: \
 547                                 the len is {} but the index is {}",
 548                                 len,
 549                                 index,
 550                             )
 551                         },
 552                         // Need proper const propagator for these
 553                         _ => return,
 554                     };
 555                     self.tcx.lint_node(
 556                         ::rustc::lint::builtin::CONST_ERR,
 557                         node_id,
 558                         span,
 559                         &msg,
 560                     );
 561                 }
 562             }
 563         }
 564     }
 565 }