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