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             );
  95
  96         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  97         // constants, instead of just checking for const-folding succeeding.
  98         // That would require an uniform one-def no-mutation analysis
  99         // and RPO (or recursing when needing the value of a local).
 100         let mut optimization_finder = ConstPropagator::new(
 101             body,
 102             dummy_body,
 103             source_scope_local_data,
 104             tcx,
 105             source
 106         );
 107         optimization_finder.visit_body(body);
 108
 109         // put back the data we stole from `mir`
 110         let source_scope_local_data = optimization_finder.release_stolen_data();
 111         std::mem::replace(
 112             &mut body.source_scope_local_data,
 113             source_scope_local_data
 114         );
 115
 116         trace!("ConstProp done for {:?}", source.def_id());
 117     }
 118 }
 119
 120 struct ConstPropMachine;
 121
 122 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 123     type MemoryKinds = !;
 124     type PointerTag = ();
 125     type ExtraFnVal = !;
 126
 127     type FrameExtra = ();
 128     type MemoryExtra = ();
 129     type AllocExtra = ();
 130
 131     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 132
 133     const STATIC_KIND: Option<!> = None;
 134
 135     const CHECK_ALIGN: bool = false;
 136
 137     #[inline(always)]
 138     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 139         false
 140     }
 141
 142     fn find_mir_or_eval_fn(
 143         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 144         _instance: ty::Instance<'tcx>,
 145         _args: &[OpTy<'tcx>],
 146         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 147         _unwind: Option<BasicBlock>,
 148     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 149         Ok(None)
 150     }
 151
 152     fn call_extra_fn(
 153         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 154         fn_val: !,
 155         _args: &[OpTy<'tcx>],
 156         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 157         _unwind: Option<BasicBlock>
 158     ) -> InterpResult<'tcx> {
 159         match fn_val {}
 160     }
 161
 162     fn call_intrinsic(
 163         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 164         _span: Span,
 165         _instance: ty::Instance<'tcx>,
 166         _args: &[OpTy<'tcx>],
 167         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 168         _unwind: Option<BasicBlock>
 169     ) -> InterpResult<'tcx> {
 170         throw_unsup_format!("calling intrinsics isn't supported in ConstProp");
 171     }
 172
 173     fn ptr_to_int(
 174         _mem: &Memory<'mir, 'tcx, Self>,
 175         _ptr: Pointer,
 176     ) -> InterpResult<'tcx, u64> {
 177         throw_unsup_format!("ptr-to-int casts aren't supported in ConstProp");
 178     }
 179
 180     fn binary_ptr_op(
 181         _ecx: &InterpCx<'mir, 'tcx, Self>,
 182         _bin_op: BinOp,
 183         _left: ImmTy<'tcx>,
 184         _right: ImmTy<'tcx>,
 185     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 186         // We can't do this because aliasing of memory can differ between const eval and llvm
 187         throw_unsup_format!("pointer arithmetic or comparisons aren't supported in ConstProp");
 188     }
 189
 190     fn find_foreign_static(
 191         _tcx: TyCtxt<'tcx>,
 192         _def_id: DefId,
 193     ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
 194         throw_unsup!(ReadForeignStatic)
 195     }
 196
 197     #[inline(always)]
 198     fn tag_allocation<'b>(
 199         _memory_extra: &(),
 200         _id: AllocId,
 201         alloc: Cow<'b, Allocation>,
 202         _kind: Option<MemoryKind<!>>,
 203     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 204         // We do not use a tag so we can just cheaply forward the allocation
 205         (alloc, ())
 206     }
 207
 208     #[inline(always)]
 209     fn tag_static_base_pointer(
 210         _memory_extra: &(),
 211         _id: AllocId,
 212     ) -> Self::PointerTag {
 213         ()
 214     }
 215
 216     fn box_alloc(
 217         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 218         _dest: PlaceTy<'tcx>,
 219     ) -> InterpResult<'tcx> {
 220         throw_unsup_format!("can't const prop `box` keyword");
 221     }
 222
 223     fn access_local(
 224         _ecx: &InterpCx<'mir, 'tcx, Self>,
 225         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 226         local: Local,
 227     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 228         let l = &frame.locals[local];
 229
 230         if l.value == LocalValue::Uninitialized {
 231             throw_unsup_format!("tried to access an uninitialized local");
 232         }
 233
 234         l.access()
 235     }
 236
 237     fn before_access_static(
 238         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 239     ) -> InterpResult<'tcx> {
 240         // if the static allocation is mutable or if it has relocations (it may be legal to mutate
 241         // the memory behind that in the future), then we can't const prop it
 242         if allocation.mutability == Mutability::Mutable || allocation.relocations().len() > 0 {
 243             throw_unsup_format!("can't eval mutable statics in ConstProp");
 244         }
 245
 246         Ok(())
 247     }
 248
 249     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 250         Ok(())
 251     }
 252
 253     #[inline(always)]
 254     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 255         Ok(())
 256     }
 257 }
 258
 259 type Const<'tcx> = OpTy<'tcx>;
 260
 261 /// Finds optimization opportunities on the MIR.
 262 struct ConstPropagator<'mir, 'tcx> {
 263     ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
 264     tcx: TyCtxt<'tcx>,
 265     source: MirSource<'tcx>,
 266     can_const_prop: IndexVec<Local, bool>,
 267     param_env: ParamEnv<'tcx>,
 268     source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 269     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 270     ret: Option<OpTy<'tcx, ()>>,
 271 }
 272
 273 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 274     type Ty = Ty<'tcx>;
 275     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 276
 277     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 278         self.tcx.layout_of(self.param_env.and(ty))
 279     }
 280 }
 281
 282 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 283     #[inline]
 284     fn data_layout(&self) -> &TargetDataLayout {
 285         &self.tcx.data_layout
 286     }
 287 }
 288
 289 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 290     #[inline]
 291     fn tcx(&self) -> TyCtxt<'tcx> {
 292         self.tcx
 293     }
 294 }
 295
 296 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 297     fn new(
 298         body: &Body<'tcx>,
 299         dummy_body: &'mir Body<'tcx>,
 300         source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
 301         tcx: TyCtxt<'tcx>,
 302         source: MirSource<'tcx>,
 303     ) -> ConstPropagator<'mir, 'tcx> {
 304         let def_id = source.def_id();
 305         let param_env = tcx.param_env(def_id);
 306         let span = tcx.def_span(def_id);
 307         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
 308         let can_const_prop = CanConstProp::check(body);
 309
 310         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 311
 312         let ret =
 313             ecx
 314                 .layout_of(body.return_ty().subst(tcx, substs))
 315                 .ok()
 316                 // Don't bother allocating memory for ZST types which have no values
 317                 // or for large values.
 318                 .filter(|ret_layout| !ret_layout.is_zst() &&
 319                                      ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT))
 320                 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 321
 322         ecx.push_stack_frame(
 323             Instance::new(def_id, substs),
 324             span,
 325             dummy_body,
 326             ret.map(Into::into),
 327             StackPopCleanup::None {
 328                 cleanup: false,
 329             },
 330         ).expect("failed to push initial stack frame");
 331
 332         ConstPropagator {
 333             ecx,
 334             tcx,
 335             source,
 336             param_env,
 337             can_const_prop,
 338             source_scope_local_data,
 339             //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
 340             local_decls: body.local_decls.clone(),
 341             ret: ret.map(Into::into),
 342         }
 343     }
 344
 345     fn release_stolen_data(self) -> ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>> {
 346         self.source_scope_local_data
 347     }
 348
 349     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
 350         if local == RETURN_PLACE {
 351             // Try to read the return place as an immediate so that if it is representable as a
 352             // scalar, we can handle it as such, but otherwise, just return the value as is.
 353             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 354                 Some(Ok(Ok(imm))) => Some(imm.into()),
 355                 _ => self.ret,
 356             };
 357         }
 358
 359         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 360     }
 361
 362     fn remove_const(&mut self, local: Local) {
 363         self.ecx.frame_mut().locals[local] = LocalState {
 364             value: LocalValue::Uninitialized,
 365             layout: Cell::new(None),
 366         };
 367     }
 368
 369     fn use_ecx<F, T>(
 370         &mut self,
 371         source_info: SourceInfo,
 372         f: F
 373     ) -> Option<T>
 374     where
 375         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 376     {
 377         self.ecx.tcx.span = source_info.span;
 378         let lint_root = match self.source_scope_local_data {
 379             ClearCrossCrate::Set(ref ivs) => {
 380                 //FIXME(#51314): remove this check
 381                 if source_info.scope.index() >= ivs.len() {
 382                     return None;
 383                 }
 384                 ivs[source_info.scope].lint_root
 385             },
 386             ClearCrossCrate::Clear => return None,
 387         };
 388         let r = match f(self) {
 389             Ok(val) => Some(val),
 390             Err(error) => {
 391                 use rustc::mir::interpret::InterpError::*;
 392                 match error.kind {
 393                     Exit(_) => bug!("the CTFE program cannot exit"),
 394                     Unsupported(_)
 395                     | UndefinedBehavior(_)
 396                     | InvalidProgram(_)
 397                     | ResourceExhaustion(_) => {
 398                         // Ignore these errors.
 399                     }
 400                     Panic(_) => {
 401                         let diagnostic = error_to_const_error(&self.ecx, error);
 402                         diagnostic.report_as_lint(
 403                             self.ecx.tcx,
 404                             "this expression will panic at runtime",
 405                             lint_root,
 406                             None,
 407                         );
 408                     }
 409                 }
 410                 None
 411             },
 412         };
 413         self.ecx.tcx.span = DUMMY_SP;
 414         r
 415     }
 416
 417     fn eval_constant(
 418         &mut self,
 419         c: &Constant<'tcx>,
 420     ) -> Option<Const<'tcx>> {
 421         self.ecx.tcx.span = c.span;
 422         match self.ecx.eval_const_to_op(c.literal, None) {
 423             Ok(op) => {
 424                 Some(op)
 425             },
 426             Err(error) => {
 427                 let err = error_to_const_error(&self.ecx, error);
 428                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 429                 None
 430             },
 431         }
 432     }
 433
 434     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 435         trace!("eval_place(place={:?})", place);
 436         self.use_ecx(source_info, |this| {
 437             this.ecx.eval_place_to_op(place, None)
 438         })
 439     }
 440
 441     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 442         match *op {
 443             Operand::Constant(ref c) => self.eval_constant(c),
 444             | Operand::Move(ref place)
 445             | Operand::Copy(ref place) => self.eval_place(place, source_info),
 446         }
 447     }
 448
 449     fn const_prop(
 450         &mut self,
 451         rvalue: &Rvalue<'tcx>,
 452         place_layout: TyLayout<'tcx>,
 453         source_info: SourceInfo,
 454         place: &Place<'tcx>,
 455     ) -> Option<()> {
 456         let span = source_info.span;
 457
 458         // #66397: Don't try to eval into large places as that can cause an OOM
 459         if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
 460             return None;
 461         }
 462
 463         let overflow_check = self.tcx.sess.overflow_checks();
 464
 465         // Perform any special handling for specific Rvalue types.
 466         // Generally, checks here fall into one of two categories:
 467         //   1. Additional checking to provide useful lints to the user
 468         //        - In this case, we will do some validation and then fall through to the
 469         //          end of the function which evals the assignment.
 470         //   2. Working around bugs in other parts of the compiler
 471         //        - In this case, we'll return `None` from this function to stop evaluation.
 472         match rvalue {
 473             // Additional checking: if overflow checks are disabled (which is usually the case in
 474             // release mode), then we need to do additional checking here to give lints to the user
 475             // if an overflow would occur.
 476             Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
 477                 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
 478
 479                 self.use_ecx(source_info, |this| {
 480                     let ty = arg.ty(&this.local_decls, this.tcx);
 481
 482                     if ty.is_integral() {
 483                         let arg = this.ecx.eval_operand(arg, None)?;
 484                         let prim = this.ecx.read_immediate(arg)?;
 485                         // Need to do overflow check here: For actual CTFE, MIR
 486                         // generation emits code that does this before calling the op.
 487                         if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 488                             throw_panic!(OverflowNeg)
 489                         }
 490                     }
 491
 492                     Ok(())
 493                 })?;
 494             }
 495
 496             // Additional checking: check for overflows on integer binary operations and report
 497             // them to the user as lints.
 498             Rvalue::BinaryOp(op, left, right) => {
 499                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 500
 501                 let r = self.use_ecx(source_info, |this| {
 502                     this.ecx.read_immediate(this.ecx.eval_operand(right, None)?)
 503                 })?;
 504                 if *op == BinOp::Shr || *op == BinOp::Shl {
 505                     let left_bits = place_layout.size.bits();
 506                     let right_size = r.layout.size;
 507                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 508                     if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
 509                         let source_scope_local_data = match self.source_scope_local_data {
 510                             ClearCrossCrate::Set(ref data) => data,
 511                             ClearCrossCrate::Clear => return None,
 512                         };
 513                         let dir = if *op == BinOp::Shr {
 514                             "right"
 515                         } else {
 516                             "left"
 517                         };
 518                         let hir_id = source_scope_local_data[source_info.scope].lint_root;
 519                         self.tcx.lint_hir(
 520                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 521                             hir_id,
 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: &Body<'_>) -> IndexVec<Local, bool> {
 683         let mut cpv = CanConstProp {
 684             can_const_prop: IndexVec::from_elem(true, &body.local_decls),
 685             found_assignment: IndexVec::from_elem(false, &body.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.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);
 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 }