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