src/librustc_mir/transform/const_prop.rs

   1 //! Propagates constants for early reporting of statically known
   2 //! assertion failures
   3
   4 use std::borrow::Cow;
   5 use std::cell::Cell;
   6
   7 use rustc::hir::def::DefKind;
   8 use rustc::hir::def_id::DefId;
   9 use rustc::mir::{
  10     AggregateKind, Constant, Location, Place, PlaceBase, Body, BodyAndCache, Operand, Local, UnOp,
  11     Rvalue, StatementKind, Statement, LocalKind, TerminatorKind, Terminator, ClearCrossCrate,
  12     SourceInfo, BinOp, SourceScope, SourceScopeData, LocalDecl, BasicBlock, ReadOnlyBodyAndCache,
  13     read_only, RETURN_PLACE
  14 };
  15 use rustc::mir::visit::{
  16     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  17 };
  18 use rustc::mir::interpret::{Scalar, InterpResult, PanicInfo};
  19 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
  20 use syntax::ast::Mutability;
  21 use syntax_pos::{Span, DUMMY_SP};
  22 use rustc::ty::subst::InternalSubsts;
  23 use rustc_data_structures::fx::FxHashMap;
  24 use rustc_index::vec::IndexVec;
  25 use rustc::ty::layout::{
  26     LayoutOf, TyLayout, LayoutError, HasTyCtxt, TargetDataLayout, HasDataLayout, Size,
  27 };
  28
  29 use crate::rustc::ty::subst::Subst;
  30 use crate::interpret::{
  31     self, InterpCx, ScalarMaybeUndef, Immediate, OpTy,
  32     StackPopCleanup, LocalValue, LocalState, AllocId, Frame,
  33     Allocation, MemoryKind, ImmTy, Pointer, Memory, PlaceTy,
  34     Operand as InterpOperand, intern_const_alloc_recursive,
  35 };
  36 use crate::const_eval::error_to_const_error;
  37 use crate::transform::{MirPass, MirSource};
  38
  39 /// The maximum number of bytes that we'll allocate space for a return value.
  40 const MAX_ALLOC_LIMIT: u64 = 1024;
  41
  42 pub struct ConstProp;
  43
  44 impl<'tcx> MirPass<'tcx> for ConstProp {
  45     fn run_pass(
  46         &self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>
  47     ) {
  48         // will be evaluated by miri and produce its errors there
  49         if source.promoted.is_some() {
  50             return;
  51         }
  52
  53         use rustc::hir::map::blocks::FnLikeNode;
  54         let hir_id = tcx.hir().as_local_hir_id(source.def_id())
  55                               .expect("Non-local call to local provider is_const_fn");
  56
  57         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  58         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  59             Some(DefKind::AssocConst) => true,
  60             _ => false,
  61         };
  62
  63         // Only run const prop on functions, methods, closures and associated constants
  64         if !is_fn_like && !is_assoc_const  {
  65             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  66             trace!("ConstProp skipped for {:?}", source.def_id());
  67             return
  68         }
  69
  70         let is_generator = tcx.type_of(source.def_id()).is_generator();
  71         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  72         // computing their layout.
  73         if is_generator {
  74             trace!("ConstProp skipped for generator {:?}", source.def_id());
  75             return
  76         }
  77
  78         trace!("ConstProp starting for {:?}", source.def_id());
  79
  80         let dummy_body =
  81             &Body::new(
  82                 body.basic_blocks().clone(),
  83                 body.source_scopes.clone(),
  84                 body.local_decls.clone(),
  85                 Default::default(),
  86                 body.arg_count,
  87                 Default::default(),
  88                 tcx.def_span(source.def_id()),
  89                 Default::default(),
  90                 body.generator_kind,
  91             );
  92
  93         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  94         // constants, instead of just checking for const-folding succeeding.
  95         // That would require an uniform one-def no-mutation analysis
  96         // and RPO (or recursing when needing the value of a local).
  97         let mut optimization_finder = ConstPropagator::new(
  98             read_only!(body),
  99             dummy_body,
 100             tcx,
 101             source
 102         );
 103         optimization_finder.visit_body(body);
 104
 105         trace!("ConstProp done for {:?}", source.def_id());
 106     }
 107 }
 108
 109 struct ConstPropMachine;
 110
 111 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 112     type MemoryKinds = !;
 113     type PointerTag = ();
 114     type ExtraFnVal = !;
 115
 116     type FrameExtra = ();
 117     type MemoryExtra = ();
 118     type AllocExtra = ();
 119
 120     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 121
 122     const STATIC_KIND: Option<!> = None;
 123
 124     const CHECK_ALIGN: bool = false;
 125
 126     #[inline(always)]
 127     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 128         false
 129     }
 130
 131     fn find_mir_or_eval_fn(
 132         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 133         _instance: ty::Instance<'tcx>,
 134         _args: &[OpTy<'tcx>],
 135         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 136         _unwind: Option<BasicBlock>,
 137     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 138         Ok(None)
 139     }
 140
 141     fn call_extra_fn(
 142         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 143         fn_val: !,
 144         _args: &[OpTy<'tcx>],
 145         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 146         _unwind: Option<BasicBlock>
 147     ) -> InterpResult<'tcx> {
 148         match fn_val {}
 149     }
 150
 151     fn call_intrinsic(
 152         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 153         _span: Span,
 154         _instance: ty::Instance<'tcx>,
 155         _args: &[OpTy<'tcx>],
 156         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 157         _unwind: Option<BasicBlock>
 158     ) -> InterpResult<'tcx> {
 159         throw_unsup!(ConstPropUnsupported("calling intrinsics isn't supported in ConstProp"));
 160     }
 161
 162     fn assert_panic(
 163         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 164         _span: Span,
 165         _msg: &rustc::mir::interpret::AssertMessage<'tcx>,
 166         _unwind: Option<rustc::mir::BasicBlock>,
 167     ) -> InterpResult<'tcx> {
 168         bug!("panics terminators are not evaluated in ConstProp");
 169     }
 170
 171     fn ptr_to_int(
 172         _mem: &Memory<'mir, 'tcx, Self>,
 173         _ptr: Pointer,
 174     ) -> InterpResult<'tcx, u64> {
 175         throw_unsup!(ConstPropUnsupported("ptr-to-int casts aren't supported in ConstProp"));
 176     }
 177
 178     fn binary_ptr_op(
 179         _ecx: &InterpCx<'mir, 'tcx, Self>,
 180         _bin_op: BinOp,
 181         _left: ImmTy<'tcx>,
 182         _right: ImmTy<'tcx>,
 183     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 184         // We can't do this because aliasing of memory can differ between const eval and llvm
 185         throw_unsup!(ConstPropUnsupported("pointer arithmetic or comparisons aren't supported \
 186             in ConstProp"));
 187     }
 188
 189     fn find_foreign_static(
 190         _tcx: TyCtxt<'tcx>,
 191         _def_id: DefId,
 192     ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
 193         throw_unsup!(ReadForeignStatic)
 194     }
 195
 196     #[inline(always)]
 197     fn init_allocation_extra<'b>(
 198         _memory_extra: &(),
 199         _id: AllocId,
 200         alloc: Cow<'b, Allocation>,
 201         _kind: Option<MemoryKind<!>>,
 202     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 203         // We do not use a tag so we can just cheaply forward the allocation
 204         (alloc, ())
 205     }
 206
 207     #[inline(always)]
 208     fn tag_static_base_pointer(
 209         _memory_extra: &(),
 210         _id: AllocId,
 211     ) -> Self::PointerTag {
 212         ()
 213     }
 214
 215     fn box_alloc(
 216         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 217         _dest: PlaceTy<'tcx>,
 218     ) -> InterpResult<'tcx> {
 219         throw_unsup!(ConstPropUnsupported("can't const prop `box` keyword"));
 220     }
 221
 222     fn access_local(
 223         _ecx: &InterpCx<'mir, 'tcx, Self>,
 224         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 225         local: Local,
 226     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 227         let l = &frame.locals[local];
 228
 229         if l.value == LocalValue::Uninitialized {
 230             throw_unsup!(ConstPropUnsupported("tried to access an uninitialized local"));
 231         }
 232
 233         l.access()
 234     }
 235
 236     fn before_access_static(
 237         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 238     ) -> InterpResult<'tcx> {
 239         // if the static allocation is mutable or if it has relocations (it may be legal to mutate
 240         // the memory behind that in the future), then we can't const prop it
 241         if allocation.mutability == Mutability::Mutable || allocation.relocations().len() > 0 {
 242             throw_unsup!(ConstPropUnsupported("can't eval mutable statics in ConstProp"));
 243         }
 244
 245         Ok(())
 246     }
 247
 248     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 249         Ok(())
 250     }
 251
 252     #[inline(always)]
 253     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 254         Ok(())
 255     }
 256 }
 257
 258 type Const<'tcx> = OpTy<'tcx>;
 259
 260 /// Finds optimization opportunities on the MIR.
 261 struct ConstPropagator<'mir, 'tcx> {
 262     ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
 263     tcx: TyCtxt<'tcx>,
 264     source: MirSource<'tcx>,
 265     can_const_prop: IndexVec<Local, bool>,
 266     param_env: ParamEnv<'tcx>,
 267     // FIXME(eddyb) avoid cloning these two fields more than once,
 268     // by accessing them through `ecx` instead.
 269     source_scopes: IndexVec<SourceScope, SourceScopeData>,
 270     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 271     ret: Option<OpTy<'tcx, ()>>,
 272 }
 273
 274 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 275     type Ty = Ty<'tcx>;
 276     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 277
 278     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 279         self.tcx.layout_of(self.param_env.and(ty))
 280     }
 281 }
 282
 283 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 284     #[inline]
 285     fn data_layout(&self) -> &TargetDataLayout {
 286         &self.tcx.data_layout
 287     }
 288 }
 289
 290 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 291     #[inline]
 292     fn tcx(&self) -> TyCtxt<'tcx> {
 293         self.tcx
 294     }
 295 }
 296
 297 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 298     fn new(
 299         body: ReadOnlyBodyAndCache<'_, 'tcx>,
 300         dummy_body: &'mir Body<'tcx>,
 301         tcx: TyCtxt<'tcx>,
 302         source: MirSource<'tcx>,
 303     ) -> ConstPropagator<'mir, 'tcx> {
 304         let def_id = source.def_id();
 305         let param_env = tcx.param_env(def_id);
 306         let span = tcx.def_span(def_id);
 307         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
 308         let can_const_prop = CanConstProp::check(body);
 309
 310         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 311
 312         let ret =
 313             ecx
 314                 .layout_of(body.return_ty().subst(tcx, substs))
 315                 .ok()
 316                 // Don't bother allocating memory for ZST types which have no values
 317                 // or for large values.
 318                 .filter(|ret_layout| !ret_layout.is_zst() &&
 319                                      ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT))
 320                 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 321
 322         ecx.push_stack_frame(
 323             Instance::new(def_id, substs),
 324             span,
 325             dummy_body,
 326             ret.map(Into::into),
 327             StackPopCleanup::None {
 328                 cleanup: false,
 329             },
 330         ).expect("failed to push initial stack frame");
 331
 332         ConstPropagator {
 333             ecx,
 334             tcx,
 335             source,
 336             param_env,
 337             can_const_prop,
 338             // FIXME(eddyb) avoid cloning these two fields more than once,
 339             // by accessing them through `ecx` instead.
 340             source_scopes: body.source_scopes.clone(),
 341             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 342             local_decls: body.local_decls.clone(),
 343             ret: ret.map(Into::into),
 344         }
 345     }
 346
 347     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 348         if local == RETURN_PLACE {
 349             // Try to read the return place as an immediate so that if it is representable as a
 350             // scalar, we can handle it as such, but otherwise, just return the value as is.
 351             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 352                 Some(Ok(Ok(imm))) => Some(imm.into()),
 353                 _ => self.ret,
 354             };
 355         }
 356
 357         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 358     }
 359
 360     fn remove_const(&mut self, local: Local) {
 361         self.ecx.frame_mut().locals[local] = LocalState {
 362             value: LocalValue::Uninitialized,
 363             layout: Cell::new(None),
 364         };
 365     }
 366
 367     fn use_ecx<F, T>(
 368         &mut self,
 369         source_info: SourceInfo,
 370         f: F
 371     ) -> Option<T>
 372     where
 373         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 374     {
 375         self.ecx.tcx.span = source_info.span;
 376         // FIXME(eddyb) move this to the `Panic(_)` error case, so that
 377         // `f(self)` is always called, and that the only difference when the
 378         // scope's `local_data` is missing, is that the lint isn't emitted.
 379         let lint_root = match &self.source_scopes[source_info.scope].local_data {
 380             ClearCrossCrate::Set(data) => data.lint_root,
 381             ClearCrossCrate::Clear => return None,
 382         };
 383         let r = match f(self) {
 384             Ok(val) => Some(val),
 385             Err(error) => {
 386                 use rustc::mir::interpret::{
 387                     UnsupportedOpInfo,
 388                     UndefinedBehaviorInfo,
 389                     InterpError::*
 390                 };
 391                 match error.kind {
 392                     MachineStop(_) => bug!("ConstProp does not stop"),
 393
 394                     // Some error shouldn't come up because creating them causes
 395                     // an allocation, which we should avoid. When that happens,
 396                     // dedicated error variants should be introduced instead.
 397                     // Only test this in debug builds though to avoid disruptions.
 398                     Unsupported(UnsupportedOpInfo::Unsupported(_))
 399                     | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
 400                     | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
 401                     | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
 402                       if cfg!(debug_assertions) => {
 403                         bug!("const-prop encountered allocating error: {:?}", error.kind);
 404                     }
 405
 406                     Unsupported(_)
 407                     | UndefinedBehavior(_)
 408                     | InvalidProgram(_)
 409                     | ResourceExhaustion(_) => {
 410                         // Ignore these errors.
 411                     }
 412                     Panic(_) => {
 413                         let diagnostic = error_to_const_error(&self.ecx, error);
 414                         diagnostic.report_as_lint(
 415                             self.ecx.tcx,
 416                             "this expression will panic at runtime",
 417                             lint_root,
 418                             None,
 419                         );
 420                     }
 421                 }
 422                 None
 423             },
 424         };
 425         self.ecx.tcx.span = DUMMY_SP;
 426         r
 427     }
 428
 429     fn eval_constant(
 430         &mut self,
 431         c: &Constant<'tcx>,
 432     ) -> Option<Const<'tcx>> {
 433         self.ecx.tcx.span = c.span;
 434         match self.ecx.eval_const_to_op(c.literal, None) {
 435             Ok(op) => {
 436                 Some(op)
 437             },
 438             Err(error) => {
 439                 let err = error_to_const_error(&self.ecx, error);
 440                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 441                 None
 442             },
 443         }
 444     }
 445
 446     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 447         trace!("eval_place(place={:?})", place);
 448         self.use_ecx(source_info, |this| {
 449             this.ecx.eval_place_to_op(place, None)
 450         })
 451     }
 452
 453     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 454         match *op {
 455             Operand::Constant(ref c) => self.eval_constant(c),
 456             | Operand::Move(ref place)
 457             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 458         }
 459     }
 460
 461     fn const_prop(
 462         &mut self,
 463         rvalue: &Rvalue<'tcx>,
 464         place_layout: TyLayout<'tcx>,
 465         source_info: SourceInfo,
 466         place: &Place<'tcx>,
 467     ) -> Option<()> {
 468         let span = source_info.span;
 469
 470         // #66397: Don't try to eval into large places as that can cause an OOM
 471         if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
 472             return None;
 473         }
 474
 475         let overflow_check = self.tcx.sess.overflow_checks();
 476
 477         // Perform any special handling for specific Rvalue types.
 478         // Generally, checks here fall into one of two categories:
 479         //   1. Additional checking to provide useful lints to the user
 480         //        - In this case, we will do some validation and then fall through to the
 481         //          end of the function which evals the assignment.
 482         //   2. Working around bugs in other parts of the compiler
 483         //        - In this case, we'll return `None` from this function to stop evaluation.
 484         match rvalue {
 485             // Additional checking: if overflow checks are disabled (which is usually the case in
 486             // release mode), then we need to do additional checking here to give lints to the user
 487             // if an overflow would occur.
 488             Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
 489                 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
 490
 491                 self.use_ecx(source_info, |this| {
 492                     let ty = arg.ty(&this.local_decls, this.tcx);
 493
 494                     if ty.is_integral() {
 495                         let arg = this.ecx.eval_operand(arg, None)?;
 496                         let prim = this.ecx.read_immediate(arg)?;
 497                         // Need to do overflow check here: For actual CTFE, MIR
 498                         // generation emits code that does this before calling the op.
 499                         if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 500                             throw_panic!(OverflowNeg)
 501                         }
 502                     }
 503
 504                     Ok(())
 505                 })?;
 506             }
 507
 508             // Additional checking: check for overflows on integer binary operations and report
 509             // them to the user as lints.
 510             Rvalue::BinaryOp(op, left, right) => {
 511                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 512
 513                 let r = self.use_ecx(source_info, |this| {
 514                     this.ecx.read_immediate(this.ecx.eval_operand(right, None)?)
 515                 })?;
 516                 if *op == BinOp::Shr || *op == BinOp::Shl {
 517                     let left_bits = place_layout.size.bits();
 518                     let right_size = r.layout.size;
 519                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 520                     if r_bits.map_or(false, |b| b >= left_bits as u128) {
 521                         let lint_root = match &self.source_scopes[source_info.scope].local_data {
 522                             ClearCrossCrate::Set(data) => data.lint_root,
 523                             ClearCrossCrate::Clear => return None,
 524                         };
 525                         let dir = if *op == BinOp::Shr {
 526                             "right"
 527                         } else {
 528                             "left"
 529                         };
 530                         self.tcx.lint_hir(
 531                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 532                             lint_root,
 533                             span,
 534                             &format!("attempt to shift {} with overflow", dir));
 535                         return None;
 536                     }
 537                 }
 538
 539                 // If overflow checking is enabled (like in debug mode by default),
 540                 // then we'll already catch overflow when we evaluate the `Assert` statement
 541                 // in MIR. However, if overflow checking is disabled, then there won't be any
 542                 // `Assert` statement and so we have to do additional checking here.
 543                 if !overflow_check {
 544                     self.use_ecx(source_info, |this| {
 545                         let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
 546                         let (_, overflow, _ty) = this.ecx.overflowing_binary_op(*op, l, r)?;
 547
 548                         if overflow {
 549                             let err = err_panic!(Overflow(*op)).into();
 550                             return Err(err);
 551                         }
 552
 553                         Ok(())
 554                     })?;
 555                 }
 556             }
 557
 558             // Work around: avoid ICE in miri. FIXME(wesleywiser)
 559             // The Miri engine ICEs when taking a reference to an uninitialized unsized
 560             // local. There's nothing it can do here: taking a reference needs an allocation
 561             // which needs to know the size. Normally that's okay as during execution
 562             // (e.g. for CTFE) it can never happen. But here in const_prop
 563             // unknown data is uninitialized, so if e.g. a function argument is unsized
 564             // and has a reference taken, we get an ICE.
 565             Rvalue::Ref(_, _, place_ref) => {
 566                 trace!("checking Ref({:?})", place_ref);
 567
 568                 if let Some(local) = place_ref.as_local() {
 569                     let alive =
 570                         if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
 571                             true
 572                         } else {
 573                             false
 574                         };
 575
 576                     if !alive {
 577                         trace!("skipping Ref({:?}) to uninitialized local", place);
 578                         return None;
 579                     }
 580                 }
 581             }
 582
 583             _ => { }
 584         }
 585
 586         self.use_ecx(source_info, |this| {
 587             trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
 588             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 589             Ok(())
 590         })
 591     }
 592
 593     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 594         Operand::Constant(Box::new(
 595             Constant {
 596                 span,
 597                 user_ty: None,
 598                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 599                     self.tcx,
 600                     scalar,
 601                     ty,
 602                 ))
 603             }
 604         ))
 605     }
 606
 607     fn replace_with_const(
 608         &mut self,
 609         rval: &mut Rvalue<'tcx>,
 610         value: Const<'tcx>,
 611         source_info: SourceInfo,
 612     ) {
 613         trace!("attepting to replace {:?} with {:?}", rval, value);
 614         if let Err(e) = self.ecx.validate_operand(
 615             value,
 616             vec![],
 617             // FIXME: is ref tracking too expensive?
 618             Some(&mut interpret::RefTracking::empty()),
 619         ) {
 620             trace!("validation error, attempt failed: {:?}", e);
 621             return;
 622         }
 623
 624         // FIXME> figure out what tho do when try_read_immediate fails
 625         let imm = self.use_ecx(source_info, |this| {
 626             this.ecx.try_read_immediate(value)
 627         });
 628
 629         if let Some(Ok(imm)) = imm {
 630             match *imm {
 631                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 632                     *rval = Rvalue::Use(
 633                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 634                 },
 635                 Immediate::ScalarPair(
 636                     ScalarMaybeUndef::Scalar(one),
 637                     ScalarMaybeUndef::Scalar(two)
 638                 ) => {
 639                     // Found a value represented as a pair. For now only do cont-prop if type of
 640                     // Rvalue is also a pair with two scalars. The more general case is more
 641                     // complicated to implement so we'll do it later.
 642                     let ty = &value.layout.ty.kind;
 643                     // Only do it for tuples
 644                     if let ty::Tuple(substs) = ty {
 645                         // Only do it if tuple is also a pair with two scalars
 646                         if substs.len() == 2 {
 647                             let opt_ty1_ty2 = self.use_ecx(source_info, |this| {
 648                                 let ty1 = substs[0].expect_ty();
 649                                 let ty2 = substs[1].expect_ty();
 650                                 let ty_is_scalar = |ty| {
 651                                     this.ecx
 652                                         .layout_of(ty)
 653                                         .ok()
 654                                         .map(|ty| ty.details.abi.is_scalar())
 655                                         == Some(true)
 656                                 };
 657                                 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
 658                                     Ok(Some((ty1, ty2)))
 659                                 } else {
 660                                     Ok(None)
 661                                 }
 662                             });
 663
 664                             if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
 665                                 *rval = Rvalue::Aggregate(
 666                                     Box::new(AggregateKind::Tuple),
 667                                     vec![
 668                                         self.operand_from_scalar(
 669                                             one, ty1, source_info.span
 670                                         ),
 671                                         self.operand_from_scalar(
 672                                             two, ty2, source_info.span
 673                                         ),
 674                                     ],
 675                                 );
 676                             }
 677                         }
 678                     }
 679                 },
 680                 _ => { }
 681             }
 682         }
 683     }
 684
 685     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 686         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 687
 688         if mir_opt_level == 0 {
 689             return false;
 690         }
 691
 692         match *op {
 693             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) =>
 694                 s.is_bits(),
 695             interpret::Operand::Immediate(Immediate::ScalarPair(ScalarMaybeUndef::Scalar(l),
 696                                                                 ScalarMaybeUndef::Scalar(r))) =>
 697                 l.is_bits() && r.is_bits(),
 698             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
 699                 intern_const_alloc_recursive(
 700                     &mut self.ecx,
 701                     None,
 702                     op.assert_mem_place()
 703                 ).expect("failed to intern alloc");
 704                 true
 705             },
 706             _ => false
 707         }
 708     }
 709 }
 710
 711 struct CanConstProp {
 712     can_const_prop: IndexVec<Local, bool>,
 713     // false at the beginning, once set, there are not allowed to be any more assignments
 714     found_assignment: IndexVec<Local, bool>,
 715 }
 716
 717 impl CanConstProp {
 718     /// returns true if `local` can be propagated
 719     fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, bool> {
 720         let mut cpv = CanConstProp {
 721             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 722             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 723         };
 724         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 725             // cannot use args at all
 726             // cannot use locals because if x < y { y - x } else { x - y } would
 727             //        lint for x != y
 728             // FIXME(oli-obk): lint variables until they are used in a condition
 729             // FIXME(oli-obk): lint if return value is constant
 730             let local_kind = body.local_kind(local);
 731             *val = local_kind == LocalKind::Temp || local_kind == LocalKind::ReturnPointer;
 732
 733             if !*val {
 734                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 735             }
 736         }
 737         cpv.visit_body(body);
 738         cpv.can_const_prop
 739     }
 740 }
 741
 742 impl<'tcx> Visitor<'tcx> for CanConstProp {
 743     fn visit_local(
 744         &mut self,
 745         &local: &Local,
 746         context: PlaceContext,
 747         _: Location,
 748     ) {
 749         use rustc::mir::visit::PlaceContext::*;
 750         match context {
 751             // Constants must have at most one write
 752             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 753             // only occur in independent execution paths
 754             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 755                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 756                 self.can_const_prop[local] = false;
 757             } else {
 758                 self.found_assignment[local] = true
 759             },
 760             // Reading constants is allowed an arbitrary number of times
 761             NonMutatingUse(NonMutatingUseContext::Copy) |
 762             NonMutatingUse(NonMutatingUseContext::Move) |
 763             NonMutatingUse(NonMutatingUseContext::Inspect) |
 764             NonMutatingUse(NonMutatingUseContext::Projection) |
 765             MutatingUse(MutatingUseContext::Projection) |
 766             NonUse(_) => {},
 767             _ => {
 768                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 769                 self.can_const_prop[local] = false;
 770             },
 771         }
 772     }
 773 }
 774
 775 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 776     fn tcx(&self) -> TyCtxt<'tcx> {
 777         self.tcx
 778     }
 779
 780     fn visit_constant(
 781         &mut self,
 782         constant: &mut Constant<'tcx>,
 783         location: Location,
 784     ) {
 785         trace!("visit_constant: {:?}", constant);
 786         self.super_constant(constant, location);
 787         self.eval_constant(constant);
 788     }
 789
 790     fn visit_statement(
 791         &mut self,
 792         statement: &mut Statement<'tcx>,
 793         location: Location,
 794     ) {
 795         trace!("visit_statement: {:?}", statement);
 796         if let StatementKind::Assign(box(ref place, ref mut rval)) = statement.kind {
 797             let place_ty: Ty<'tcx> = place
 798                 .ty(&self.local_decls, self.tcx)
 799                 .ty;
 800             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 801                 if let Some(local) = place.as_local() {
 802                     let source = statement.source_info;
 803                     if let Some(()) = self.const_prop(rval, place_layout, source, place) {
 804                         if self.can_const_prop[local] {
 805                             trace!("propagated into {:?}", local);
 806
 807                             if let Some(value) = self.get_const(local) {
 808                                 if self.should_const_prop(value) {
 809                                     trace!("replacing {:?} with {:?}", rval, value);
 810                                     self.replace_with_const(
 811                                         rval,
 812                                         value,
 813                                         statement.source_info,
 814                                     );
 815                                 }
 816                             }
 817                         } else {
 818                             trace!("can't propagate into {:?}", local);
 819                             if local != RETURN_PLACE {
 820                                 self.remove_const(local);
 821                             }
 822                         }
 823                     }
 824                 }
 825             }
 826         } else {
 827             match statement.kind {
 828                 StatementKind::StorageLive(local) |
 829                 StatementKind::StorageDead(local) if self.can_const_prop[local] => {
 830                     let frame = self.ecx.frame_mut();
 831                     frame.locals[local].value =
 832                         if let StatementKind::StorageLive(_) = statement.kind {
 833                             LocalValue::Uninitialized
 834                         } else {
 835                             LocalValue::Dead
 836                         };
 837                 }
 838                 _ => {}
 839             }
 840         }
 841
 842         self.super_statement(statement, location);
 843     }
 844
 845     fn visit_terminator(
 846         &mut self,
 847         terminator: &mut Terminator<'tcx>,
 848         location: Location,
 849     ) {
 850         self.super_terminator(terminator, location);
 851         let source_info = terminator.source_info;
 852         match &mut terminator.kind {
 853             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 854                 if let Some(value) = self.eval_operand(&cond, source_info) {
 855                     trace!("assertion on {:?} should be {:?}", value, expected);
 856                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 857                     let value_const = self.ecx.read_scalar(value).unwrap();
 858                     if expected != value_const {
 859                         // poison all places this operand references so that further code
 860                         // doesn't use the invalid value
 861                         match cond {
 862                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 863                                 if let PlaceBase::Local(local) = place.base {
 864                                     self.remove_const(local);
 865                                 }
 866                             },
 867                             Operand::Constant(_) => {}
 868                         }
 869                         let span = terminator.source_info.span;
 870                         let hir_id = self
 871                             .tcx
 872                             .hir()
 873                             .as_local_hir_id(self.source.def_id())
 874                             .expect("some part of a failing const eval must be local");
 875                         let msg = match msg {
 876                             PanicInfo::Overflow(_) |
 877                             PanicInfo::OverflowNeg |
 878                             PanicInfo::DivisionByZero |
 879                             PanicInfo::RemainderByZero =>
 880                                 msg.description().to_owned(),
 881                             PanicInfo::BoundsCheck { ref len, ref index } => {
 882                                 let len = self
 883                                     .eval_operand(len, source_info)
 884                                     .expect("len must be const");
 885                                 let len = match self.ecx.read_scalar(len) {
 886                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 887                                         data, ..
 888                                     })) => data,
 889                                     other => bug!("const len not primitive: {:?}", other),
 890                                 };
 891                                 let index = self
 892                                     .eval_operand(index, source_info)
 893                                     .expect("index must be const");
 894                                 let index = match self.ecx.read_scalar(index) {
 895                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 896                                         data, ..
 897                                     })) => data,
 898                                     other => bug!("const index not primitive: {:?}", other),
 899                                 };
 900                                 format!(
 901                                     "index out of bounds: \
 902                                     the len is {} but the index is {}",
 903                                     len,
 904                                     index,
 905                                 )
 906                             },
 907                             // Need proper const propagator for these
 908                             _ => return,
 909                         };
 910                         self.tcx.lint_hir(
 911                             ::rustc::lint::builtin::CONST_ERR,
 912                             hir_id,
 913                             span,
 914                             &msg,
 915                         );
 916                     } else {
 917                         if self.should_const_prop(value) {
 918                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 919                                 *cond = self.operand_from_scalar(
 920                                     scalar,
 921                                     self.tcx.types.bool,
 922                                     source_info.span,
 923                                 );
 924                             }
 925                         }
 926                     }
 927                 }
 928             },
 929             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 930                 if let Some(value) = self.eval_operand(&discr, source_info) {
 931                     if self.should_const_prop(value) {
 932                         if let ScalarMaybeUndef::Scalar(scalar) =
 933                                 self.ecx.read_scalar(value).unwrap() {
 934                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 935                         }
 936                     }
 937                 }
 938             },
 939             //none of these have Operands to const-propagate
 940             TerminatorKind::Goto { .. } |
 941             TerminatorKind::Resume |
 942             TerminatorKind::Abort |
 943             TerminatorKind::Return |
 944             TerminatorKind::Unreachable |
 945             TerminatorKind::Drop { .. } |
 946             TerminatorKind::DropAndReplace { .. } |
 947             TerminatorKind::Yield { .. } |
 948             TerminatorKind::GeneratorDrop |
 949             TerminatorKind::FalseEdges { .. } |
 950             TerminatorKind::FalseUnwind { .. } => { }
 951             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 952             TerminatorKind::Call { .. } => { }
 953         }
 954     }
 955 }