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, Operand, Rvalue, Local, UnOp,
  11     StatementKind, Statement, LocalKind, TerminatorKind, Terminator,  ClearCrossCrate, SourceInfo,
  12     BinOp, SourceScope, SourceScopeLocalData, LocalDecl, BasicBlock, RETURN_PLACE,
  13 };
  14 use rustc::mir::visit::{
  15     Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
  16 };
  17 use rustc::mir::interpret::{Scalar, InterpResult, PanicInfo};
  18 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
  19 use syntax::ast::Mutability;
  20 use syntax_pos::{Span, DUMMY_SP};
  21 use rustc::ty::subst::InternalSubsts;
  22 use rustc_data_structures::fx::FxHashMap;
  23 use rustc_index::vec::IndexVec;
  24 use rustc::ty::layout::{
  25     LayoutOf, TyLayout, LayoutError, HasTyCtxt, TargetDataLayout, HasDataLayout, Size,
  26 };
  27
  28 use crate::rustc::ty::subst::Subst;
  29 use crate::interpret::{
  30     self, InterpCx, ScalarMaybeUndef, Immediate, OpTy,
  31     StackPopCleanup, LocalValue, LocalState, AllocId, Frame,
  32     Allocation, MemoryKind, ImmTy, Pointer, Memory, PlaceTy,
  33     Operand as InterpOperand, intern_const_alloc_recursive,
  34 };
  35 use crate::const_eval::error_to_const_error;
  36 use crate::transform::{MirPass, MirSource};
  37
  38 /// The maximum number of bytes that we'll allocate space for a return value.
  39 const MAX_ALLOC_LIMIT: u64 = 1024;
  40
  41 pub struct ConstProp;
  42
  43 impl<'tcx> MirPass<'tcx> for ConstProp {
  44     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
  45         // will be evaluated by miri and produce its errors there
  46         if source.promoted.is_some() {
  47             return;
  48         }
  49
  50         use rustc::hir::map::blocks::FnLikeNode;
  51         let hir_id = tcx.hir().as_local_hir_id(source.def_id())
  52                               .expect("Non-local call to local provider is_const_fn");
  53
  54         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  55         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  56             Some(DefKind::AssocConst) => true,
  57             _ => false,
  58         };
  59
  60         // Only run const prop on functions, methods, closures and associated constants
  61         if !is_fn_like && !is_assoc_const  {
  62             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  63             trace!("ConstProp skipped for {:?}", source.def_id());
  64             return
  65         }
  66
  67         let is_generator = tcx.type_of(source.def_id()).is_generator();
  68         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  69         // computing their layout.
  70         if is_generator {
  71             trace!("ConstProp skipped for generator {:?}", source.def_id());
  72             return
  73         }
  74
  75         trace!("ConstProp starting for {:?}", source.def_id());
  76
  77         // Steal some data we need from `body`.
  78         let source_scope_local_data = std::mem::replace(
  79             &mut body.source_scope_local_data,
  80             ClearCrossCrate::Clear
  81         );
  82
  83         let dummy_body =
  84             &Body::new(
  85                 body.basic_blocks().clone(),
  86                 Default::default(),
  87                 ClearCrossCrate::Clear,
  88                 body.local_decls.clone(),
  89                 Default::default(),
  90                 body.arg_count,
  91                 Default::default(),
  92                 tcx.def_span(source.def_id()),
  93                 Default::default(),
  94                 body.generator_kind,
  95             );
  96
  97         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  98         // constants, instead of just checking for const-folding succeeding.
  99         // That would require an uniform one-def no-mutation analysis
 100         // and RPO (or recursing when needing the value of a local).
 101         let mut optimization_finder = ConstPropagator::new(
 102             body,
 103             dummy_body,
 104             source_scope_local_data,
 105             tcx,
 106             source
 107         );
 108         optimization_finder.visit_body(body);
 109
 110         // put back the data we stole from `mir`
 111         let source_scope_local_data = optimization_finder.release_stolen_data();
 112         std::mem::replace(
 113             &mut body.source_scope_local_data,
 114             source_scope_local_data
 115         );
 116
 117         trace!("ConstProp done for {:?}", source.def_id());
 118     }
 119 }
 120
 121 struct ConstPropMachine;
 122
 123 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 124     type MemoryKinds = !;
 125     type PointerTag = ();
 126     type ExtraFnVal = !;
 127
 128     type FrameExtra = ();
 129     type MemoryExtra = ();
 130     type AllocExtra = ();
 131
 132     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 133
 134     const STATIC_KIND: Option<!> = None;
 135
 136     const CHECK_ALIGN: bool = false;
 137
 138     #[inline(always)]
 139     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 140         false
 141     }
 142
 143     fn find_fn(
 144         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 145         _instance: ty::Instance<'tcx>,
 146         _args: &[OpTy<'tcx>],
 147         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 148         _unwind: Option<BasicBlock>,
 149     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 150         Ok(None)
 151     }
 152
 153     fn call_extra_fn(
 154         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 155         fn_val: !,
 156         _args: &[OpTy<'tcx>],
 157         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 158         _unwind: Option<BasicBlock>
 159     ) -> InterpResult<'tcx> {
 160         match fn_val {}
 161     }
 162
 163     fn call_intrinsic(
 164         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 165         _span: Span,
 166         _instance: ty::Instance<'tcx>,
 167         _args: &[OpTy<'tcx>],
 168         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 169         _unwind: Option<BasicBlock>
 170     ) -> InterpResult<'tcx> {
 171         throw_unsup_format!("calling intrinsics isn't supported in ConstProp");
 172     }
 173
 174     fn ptr_to_int(
 175         _mem: &Memory<'mir, 'tcx, Self>,
 176         _ptr: Pointer,
 177     ) -> InterpResult<'tcx, u64> {
 178         throw_unsup_format!("ptr-to-int casts aren't supported in ConstProp");
 179     }
 180
 181     fn binary_ptr_op(
 182         _ecx: &InterpCx<'mir, 'tcx, Self>,
 183         _bin_op: BinOp,
 184         _left: ImmTy<'tcx>,
 185         _right: ImmTy<'tcx>,
 186     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 187         // We can't do this because aliasing of memory can differ between const eval and llvm
 188         throw_unsup_format!("pointer arithmetic or comparisons aren't supported in ConstProp");
 189     }
 190
 191     fn find_foreign_static(
 192         _tcx: TyCtxt<'tcx>,
 193         _def_id: DefId,
 194     ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
 195         throw_unsup!(ReadForeignStatic)
 196     }
 197
 198     #[inline(always)]
 199     fn tag_allocation<'b>(
 200         _memory_extra: &(),
 201         _id: AllocId,
 202         alloc: Cow<'b, Allocation>,
 203         _kind: Option<MemoryKind<!>>,
 204     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 205         // We do not use a tag so we can just cheaply forward the allocation
 206         (alloc, ())
 207     }
 208
 209     #[inline(always)]
 210     fn tag_static_base_pointer(
 211         _memory_extra: &(),
 212         _id: AllocId,
 213     ) -> Self::PointerTag {
 214         ()
 215     }
 216
 217     fn box_alloc(
 218         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 219         _dest: PlaceTy<'tcx>,
 220     ) -> InterpResult<'tcx> {
 221         throw_unsup_format!("can't const prop `box` keyword");
 222     }
 223
 224     fn access_local(
 225         _ecx: &InterpCx<'mir, 'tcx, Self>,
 226         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 227         local: Local,
 228     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 229         let l = &frame.locals[local];
 230
 231         if l.value == LocalValue::Uninitialized {
 232             throw_unsup_format!("tried to access an uninitialized local");
 233         }
 234
 235         l.access()
 236     }
 237
 238     fn before_access_static(
 239         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 240     ) -> InterpResult<'tcx> {
 241         // if the static allocation is mutable or if it has relocations (it may be legal to mutate
 242         // the memory behind that in the future), then we can't const prop it
 243         if allocation.mutability == Mutability::Mutable || allocation.relocations().len() > 0 {
 244             throw_unsup_format!("can't eval mutable statics in ConstProp");
 245         }
 246
 247         Ok(())
 248     }
 249
 250     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 251         Ok(())
 252     }
 253
 254     #[inline(always)]
 255     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 256         Ok(())
 257     }
 258 }
 259
 260 type Const<'tcx> = OpTy<'tcx>;
 261
 262 /// Finds optimization opportunities on the MIR.
 263 struct ConstPropagator<'mir, 'tcx> {
 264     ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
 265     tcx: TyCtxt<'tcx>,
 266     source: MirSource<'tcx>,
 267     can_const_prop: IndexVec<Local, bool>,
 268     param_env: ParamEnv<'tcx>,
 269     source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 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: &Body<'tcx>,
 300         dummy_body: &'mir Body<'tcx>,
 301         source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 302         tcx: TyCtxt<'tcx>,
 303         source: MirSource<'tcx>,
 304     ) -> ConstPropagator<'mir, 'tcx> {
 305         let def_id = source.def_id();
 306         let param_env = tcx.param_env(def_id);
 307         let span = tcx.def_span(def_id);
 308         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
 309         let can_const_prop = CanConstProp::check(body);
 310
 311         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 312
 313         let ret =
 314             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| !ret_layout.is_zst() &&
 320                                      ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT))
 321                 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 322
 323         ecx.push_stack_frame(
 324             Instance::new(def_id, substs),
 325             span,
 326             dummy_body,
 327             ret.map(Into::into),
 328             StackPopCleanup::None {
 329                 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             source_scope_local_data,
 340             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 341             local_decls: body.local_decls.clone(),
 342             ret: ret.map(Into::into),
 343         }
 344     }
 345
 346     fn release_stolen_data(self) -> ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>> {
 347         self.source_scope_local_data
 348     }
 349
 350     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 351         if local == RETURN_PLACE {
 352             // Try to read the return place as an immediate so that if it is representable as a
 353             // scalar, we can handle it as such, but otherwise, just return the value as is.
 354             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 355                 Some(Ok(Ok(imm))) => Some(imm.into()),
 356                 _ => self.ret,
 357             };
 358         }
 359
 360         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 361     }
 362
 363     fn remove_const(&mut self, local: Local) {
 364         self.ecx.frame_mut().locals[local] = LocalState {
 365             value: LocalValue::Uninitialized,
 366             layout: Cell::new(None),
 367         };
 368     }
 369
 370     fn use_ecx<F, T>(
 371         &mut self,
 372         source_info: SourceInfo,
 373         f: F
 374     ) -> Option<T>
 375     where
 376         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 377     {
 378         self.ecx.tcx.span = source_info.span;
 379         let lint_root = match self.source_scope_local_data {
 380             ClearCrossCrate::Set(ref ivs) => {
 381                 //FIXME(#51314): remove this check
 382                 if source_info.scope.index() >= ivs.len() {
 383                     return None;
 384                 }
 385                 ivs[source_info.scope].lint_root
 386             },
 387             ClearCrossCrate::Clear => return None,
 388         };
 389         let r = match f(self) {
 390             Ok(val) => Some(val),
 391             Err(error) => {
 392                 use rustc::mir::interpret::InterpError::*;
 393                 match error.kind {
 394                     Exit(_) => bug!("the CTFE program cannot exit"),
 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 source_scope_local_data = match self.source_scope_local_data {
 511                             ClearCrossCrate::Set(ref data) => data,
 512                             ClearCrossCrate::Clear => return None,
 513                         };
 514                         let dir = if *op == BinOp::Shr {
 515                             "right"
 516                         } else {
 517                             "left"
 518                         };
 519                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 520                         self.tcx.lint_hir(
 521                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 522                             hir_id,
 523                             span,
 524                             &format!("attempt to shift {} with overflow", dir));
 525                         return None;
 526                     }
 527                 }
 528
 529                 // If overflow checking is enabled (like in debug mode by default),
 530                 // then we'll already catch overflow when we evaluate the `Assert` statement
 531                 // in MIR. However, if overflow checking is disabled, then there won't be any
 532                 // `Assert` statement and so we have to do additional checking here.
 533                 if !overflow_check {
 534                     self.use_ecx(source_info, |this| {
 535                         let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
 536                         let (_, overflow, _ty) = this.ecx.overflowing_binary_op(*op, l, r)?;
 537
 538                         if overflow {
 539                             let err = err_panic!(Overflow(*op)).into();
 540                             return Err(err);
 541                         }
 542
 543                         Ok(())
 544                     })?;
 545                 }
 546             }
 547
 548             // Work around: avoid ICE in miri. FIXME(wesleywiser)
 549             // The Miri engine ICEs when taking a reference to an uninitialized unsized
 550             // local. There's nothing it can do here: taking a reference needs an allocation
 551             // which needs to know the size. Normally that's okay as during execution
 552             // (e.g. for CTFE) it can never happen. But here in const_prop
 553             // unknown data is uninitialized, so if e.g. a function argument is unsized
 554             // and has a reference taken, we get an ICE.
 555             Rvalue::Ref(_, _, place_ref) => {
 556                 trace!("checking Ref({:?})", place_ref);
 557
 558                 if let Some(local) = place_ref.as_local() {
 559                     let alive =
 560                         if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
 561                             true
 562                         } else {
 563                             false
 564                         };
 565
 566                     if !alive {
 567                         trace!("skipping Ref({:?}) to uninitialized local", place);
 568                         return None;
 569                     }
 570                 }
 571             }
 572
 573             _ => { }
 574         }
 575
 576         self.use_ecx(source_info, |this| {
 577             trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
 578             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 579             Ok(())
 580         })
 581     }
 582
 583     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 584         Operand::Constant(Box::new(
 585             Constant {
 586                 span,
 587                 user_ty: None,
 588                 literal: self.tcx.mk_const(*ty::Const::from_scalar(
 589                     self.tcx,
 590                     scalar,
 591                     ty,
 592                 ))
 593             }
 594         ))
 595     }
 596
 597     fn replace_with_const(
 598         &mut self,
 599         rval: &mut Rvalue<'tcx>,
 600         value: Const<'tcx>,
 601         source_info: SourceInfo,
 602     ) {
 603         trace!("attepting to replace {:?} with {:?}", rval, value);
 604         if let Err(e) = self.ecx.validate_operand(
 605             value,
 606             vec![],
 607             // FIXME: is ref tracking too expensive?
 608             Some(&mut interpret::RefTracking::empty()),
 609         ) {
 610             trace!("validation error, attempt failed: {:?}", e);
 611             return;
 612         }
 613
 614         // FIXME> figure out what tho do when try_read_immediate fails
 615         let imm = self.use_ecx(source_info, |this| {
 616             this.ecx.try_read_immediate(value)
 617         });
 618
 619         if let Some(Ok(imm)) = imm {
 620             match *imm {
 621                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 622                     *rval = Rvalue::Use(
 623                         self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
 624                 },
 625                 Immediate::ScalarPair(
 626                     ScalarMaybeUndef::Scalar(one),
 627                     ScalarMaybeUndef::Scalar(two)
 628                 ) => {
 629                     let ty = &value.layout.ty.kind;
 630                     if let ty::Tuple(substs) = ty {
 631                         *rval = Rvalue::Aggregate(
 632                             Box::new(AggregateKind::Tuple),
 633                             vec![
 634                                 self.operand_from_scalar(
 635                                     one, substs[0].expect_ty(), source_info.span
 636                                 ),
 637                                 self.operand_from_scalar(
 638                                     two, substs[1].expect_ty(), source_info.span
 639                                 ),
 640                             ],
 641                         );
 642                     }
 643                 },
 644                 _ => { }
 645             }
 646         }
 647     }
 648
 649     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 650         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 651
 652         if mir_opt_level == 0 {
 653             return false;
 654         }
 655
 656         match *op {
 657             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) =>
 658                 s.is_bits(),
 659             interpret::Operand::Immediate(Immediate::ScalarPair(ScalarMaybeUndef::Scalar(l),
 660                                                                 ScalarMaybeUndef::Scalar(r))) =>
 661                 l.is_bits() && r.is_bits(),
 662             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
 663                 intern_const_alloc_recursive(
 664                     &mut self.ecx,
 665                     None,
 666                     op.assert_mem_place()
 667                 ).expect("failed to intern alloc");
 668                 true
 669             },
 670             _ => false
 671         }
 672     }
 673 }
 674
 675 struct CanConstProp {
 676     can_const_prop: IndexVec<Local, bool>,
 677     // false at the beginning, once set, there are not allowed to be any more assignments
 678     found_assignment: IndexVec<Local, bool>,
 679 }
 680
 681 impl CanConstProp {
 682     /// returns true if `local` can be propagated
 683     fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
 684         let mut cpv = CanConstProp {
 685             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 686             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 687         };
 688         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 689             // cannot use args at all
 690             // cannot use locals because if x < y { y - x } else { x - y } would
 691             //        lint for x != y
 692             // FIXME(oli-obk): lint variables until they are used in a condition
 693             // FIXME(oli-obk): lint if return value is constant
 694             let local_kind = body.local_kind(local);
 695             *val = local_kind == LocalKind::Temp || local_kind == LocalKind::ReturnPointer;
 696
 697             if !*val {
 698                 trace!("local {:?} can't be propagated because it's not a temporary", local);
 699             }
 700         }
 701         cpv.visit_body(body);
 702         cpv.can_const_prop
 703     }
 704 }
 705
 706 impl<'tcx> Visitor<'tcx> for CanConstProp {
 707     fn visit_local(
 708         &mut self,
 709         &local: &Local,
 710         context: PlaceContext,
 711         _: Location,
 712     ) {
 713         use rustc::mir::visit::PlaceContext::*;
 714         match context {
 715             // Constants must have at most one write
 716             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 717             // only occur in independent execution paths
 718             MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
 719                 trace!("local {:?} can't be propagated because of multiple assignments", local);
 720                 self.can_const_prop[local] = false;
 721             } else {
 722                 self.found_assignment[local] = true
 723             },
 724             // Reading constants is allowed an arbitrary number of times
 725             NonMutatingUse(NonMutatingUseContext::Copy) |
 726             NonMutatingUse(NonMutatingUseContext::Move) |
 727             NonMutatingUse(NonMutatingUseContext::Inspect) |
 728             NonMutatingUse(NonMutatingUseContext::Projection) |
 729             MutatingUse(MutatingUseContext::Projection) |
 730             NonUse(_) => {},
 731             _ => {
 732                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 733                 self.can_const_prop[local] = false;
 734             },
 735         }
 736     }
 737 }
 738
 739 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 740     fn tcx(&self) -> TyCtxt<'tcx> {
 741         self.tcx
 742     }
 743
 744     fn visit_constant(
 745         &mut self,
 746         constant: &mut Constant<'tcx>,
 747         location: Location,
 748     ) {
 749         trace!("visit_constant: {:?}", constant);
 750         self.super_constant(constant, location);
 751         self.eval_constant(constant);
 752     }
 753
 754     fn visit_statement(
 755         &mut self,
 756         statement: &mut Statement<'tcx>,
 757         location: Location,
 758     ) {
 759         trace!("visit_statement: {:?}", statement);
 760         if let StatementKind::Assign(box(ref place, ref mut rval)) = statement.kind {
 761             let place_ty: Ty<'tcx> = place
 762                 .ty(&self.local_decls, self.tcx)
 763                 .ty;
 764             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 765                 if let Some(local) = place.as_local() {
 766                     let source = statement.source_info;
 767                     if let Some(()) = self.const_prop(rval, place_layout, source, place) {
 768                         if self.can_const_prop[local] {
 769                             trace!("propagated into {:?}", local);
 770
 771                             if let Some(value) = self.get_const(local) {
 772                                 if self.should_const_prop(value) {
 773                                     trace!("replacing {:?} with {:?}", rval, value);
 774                                     self.replace_with_const(
 775                                         rval,
 776                                         value,
 777                                         statement.source_info,
 778                                     );
 779                                 }
 780                             }
 781                         } else {
 782                             trace!("can't propagate into {:?}", local);
 783                             if local != RETURN_PLACE {
 784                                 self.remove_const(local);
 785                             }
 786                         }
 787                     }
 788                 }
 789             }
 790         } else {
 791             match statement.kind {
 792                 StatementKind::StorageLive(local) |
 793                 StatementKind::StorageDead(local) if self.can_const_prop[local] => {
 794                     let frame = self.ecx.frame_mut();
 795                     frame.locals[local].value =
 796                         if let StatementKind::StorageLive(_) = statement.kind {
 797                             LocalValue::Uninitialized
 798                         } else {
 799                             LocalValue::Dead
 800                         };
 801                 }
 802                 _ => {}
 803             }
 804         }
 805
 806         self.super_statement(statement, location);
 807     }
 808
 809     fn visit_terminator(
 810         &mut self,
 811         terminator: &mut Terminator<'tcx>,
 812         location: Location,
 813     ) {
 814         self.super_terminator(terminator, location);
 815         let source_info = terminator.source_info;
 816         match &mut terminator.kind {
 817             TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
 818                 if let Some(value) = self.eval_operand(&cond, source_info) {
 819                     trace!("assertion on {:?} should be {:?}", value, expected);
 820                     let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
 821                     let value_const = self.ecx.read_scalar(value).unwrap();
 822                     if expected != value_const {
 823                         // poison all places this operand references so that further code
 824                         // doesn't use the invalid value
 825                         match cond {
 826                             Operand::Move(ref place) | Operand::Copy(ref place) => {
 827                                 if let PlaceBase::Local(local) = place.base {
 828                                     self.remove_const(local);
 829                                 }
 830                             },
 831                             Operand::Constant(_) => {}
 832                         }
 833                         let span = terminator.source_info.span;
 834                         let hir_id = self
 835                             .tcx
 836                             .hir()
 837                             .as_local_hir_id(self.source.def_id())
 838                             .expect("some part of a failing const eval must be local");
 839                         let msg = match msg {
 840                             PanicInfo::Overflow(_) |
 841                             PanicInfo::OverflowNeg |
 842                             PanicInfo::DivisionByZero |
 843                             PanicInfo::RemainderByZero =>
 844                                 msg.description().to_owned(),
 845                             PanicInfo::BoundsCheck { ref len, ref index } => {
 846                                 let len = self
 847                                     .eval_operand(len, source_info)
 848                                     .expect("len must be const");
 849                                 let len = match self.ecx.read_scalar(len) {
 850                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 851                                         data, ..
 852                                     })) => data,
 853                                     other => bug!("const len not primitive: {:?}", other),
 854                                 };
 855                                 let index = self
 856                                     .eval_operand(index, source_info)
 857                                     .expect("index must be const");
 858                                 let index = match self.ecx.read_scalar(index) {
 859                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
 860                                         data, ..
 861                                     })) => data,
 862                                     other => bug!("const index not primitive: {:?}", other),
 863                                 };
 864                                 format!(
 865                                     "index out of bounds: \
 866                                     the len is {} but the index is {}",
 867                                     len,
 868                                     index,
 869                                 )
 870                             },
 871                             // Need proper const propagator for these
 872                             _ => return,
 873                         };
 874                         self.tcx.lint_hir(
 875                             ::rustc::lint::builtin::CONST_ERR,
 876                             hir_id,
 877                             span,
 878                             &msg,
 879                         );
 880                     } else {
 881                         if self.should_const_prop(value) {
 882                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 883                                 *cond = self.operand_from_scalar(
 884                                     scalar,
 885                                     self.tcx.types.bool,
 886                                     source_info.span,
 887                                 );
 888                             }
 889                         }
 890                     }
 891                 }
 892             },
 893             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 894                 if let Some(value) = self.eval_operand(&discr, source_info) {
 895                     if self.should_const_prop(value) {
 896                         if let ScalarMaybeUndef::Scalar(scalar) =
 897                                 self.ecx.read_scalar(value).unwrap() {
 898                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 899                         }
 900                     }
 901                 }
 902             },
 903             //none of these have Operands to const-propagate
 904             TerminatorKind::Goto { .. } |
 905             TerminatorKind::Resume |
 906             TerminatorKind::Abort |
 907             TerminatorKind::Return |
 908             TerminatorKind::Unreachable |
 909             TerminatorKind::Drop { .. } |
 910             TerminatorKind::DropAndReplace { .. } |
 911             TerminatorKind::Yield { .. } |
 912             TerminatorKind::GeneratorDrop |
 913             TerminatorKind::FalseEdges { .. } |
 914             TerminatorKind::FalseUnwind { .. } => { }
 915             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 916             TerminatorKind::Call { .. } => { }
 917         }
 918     }
 919 }