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