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