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::rustc::ty::TypeFoldable;
  37 use crate::transform::{MirPass, MirSource};
  38
  39 /// The maximum number of bytes that we'll allocate space for a return value.
  40 const MAX_ALLOC_LIMIT: u64 = 1024;
  41
  42 pub struct ConstProp;
  43
  44 impl<'tcx> MirPass<'tcx> for ConstProp {
  45     fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
  46         // will be evaluated by miri and produce its errors there
  47         if source.promoted.is_some() {
  48             return;
  49         }
  50
  51         use rustc::hir::map::blocks::FnLikeNode;
  52         let hir_id = tcx
  53             .hir()
  54             .as_local_hir_id(source.def_id())
  55             .expect("Non-local call to local provider is_const_fn");
  56
  57         let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
  58         let is_assoc_const = match tcx.def_kind(source.def_id()) {
  59             Some(DefKind::AssocConst) => true,
  60             _ => false,
  61         };
  62
  63         // Only run const prop on functions, methods, closures and associated constants
  64         if !is_fn_like && !is_assoc_const {
  65             // skip anon_const/statics/consts because they'll be evaluated by miri anyway
  66             trace!("ConstProp skipped for {:?}", source.def_id());
  67             return;
  68         }
  69
  70         let is_generator = tcx.type_of(source.def_id()).is_generator();
  71         // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
  72         // computing their layout.
  73         if is_generator {
  74             trace!("ConstProp skipped for generator {:?}", source.def_id());
  75             return;
  76         }
  77
  78         trace!("ConstProp starting for {:?}", source.def_id());
  79
  80         let dummy_body = &Body::new(
  81             body.basic_blocks().clone(),
  82             body.source_scopes.clone(),
  83             body.local_decls.clone(),
  84             Default::default(),
  85             body.arg_count,
  86             Default::default(),
  87             tcx.def_span(source.def_id()),
  88             Default::default(),
  89             body.generator_kind,
  90         );
  91
  92         // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
  93         // constants, instead of just checking for const-folding succeeding.
  94         // That would require an uniform one-def no-mutation analysis
  95         // and RPO (or recursing when needing the value of a local).
  96         let mut optimization_finder =
  97             ConstPropagator::new(read_only!(body), dummy_body, tcx, source);
  98         optimization_finder.visit_body(body);
  99
 100         trace!("ConstProp done for {:?}", source.def_id());
 101     }
 102 }
 103
 104 struct ConstPropMachine;
 105
 106 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
 107     type MemoryKinds = !;
 108     type PointerTag = ();
 109     type ExtraFnVal = !;
 110
 111     type FrameExtra = ();
 112     type MemoryExtra = ();
 113     type AllocExtra = ();
 114
 115     type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
 116
 117     const STATIC_KIND: Option<!> = None;
 118
 119     const CHECK_ALIGN: bool = false;
 120
 121     #[inline(always)]
 122     fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
 123         false
 124     }
 125
 126     fn find_mir_or_eval_fn(
 127         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 128         _instance: ty::Instance<'tcx>,
 129         _args: &[OpTy<'tcx>],
 130         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 131         _unwind: Option<BasicBlock>,
 132     ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
 133         Ok(None)
 134     }
 135
 136     fn call_extra_fn(
 137         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 138         fn_val: !,
 139         _args: &[OpTy<'tcx>],
 140         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 141         _unwind: Option<BasicBlock>,
 142     ) -> InterpResult<'tcx> {
 143         match fn_val {}
 144     }
 145
 146     fn call_intrinsic(
 147         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 148         _span: Span,
 149         _instance: ty::Instance<'tcx>,
 150         _args: &[OpTy<'tcx>],
 151         _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
 152         _unwind: Option<BasicBlock>,
 153     ) -> InterpResult<'tcx> {
 154         throw_unsup!(ConstPropUnsupported("calling intrinsics isn't supported in ConstProp"));
 155     }
 156
 157     fn assert_panic(
 158         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 159         _span: Span,
 160         _msg: &rustc::mir::interpret::AssertMessage<'tcx>,
 161         _unwind: Option<rustc::mir::BasicBlock>,
 162     ) -> InterpResult<'tcx> {
 163         bug!("panics terminators are not evaluated in ConstProp");
 164     }
 165
 166     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
 167         throw_unsup!(ConstPropUnsupported("ptr-to-int casts aren't supported in ConstProp"));
 168     }
 169
 170     fn binary_ptr_op(
 171         _ecx: &InterpCx<'mir, 'tcx, Self>,
 172         _bin_op: BinOp,
 173         _left: ImmTy<'tcx>,
 174         _right: ImmTy<'tcx>,
 175     ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
 176         // We can't do this because aliasing of memory can differ between const eval and llvm
 177         throw_unsup!(ConstPropUnsupported(
 178             "pointer arithmetic or comparisons aren't supported \
 179             in ConstProp"
 180         ));
 181     }
 182
 183     fn find_foreign_static(
 184         _tcx: TyCtxt<'tcx>,
 185         _def_id: DefId,
 186     ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
 187         throw_unsup!(ReadForeignStatic)
 188     }
 189
 190     #[inline(always)]
 191     fn init_allocation_extra<'b>(
 192         _memory_extra: &(),
 193         _id: AllocId,
 194         alloc: Cow<'b, Allocation>,
 195         _kind: Option<MemoryKind<!>>,
 196     ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
 197         // We do not use a tag so we can just cheaply forward the allocation
 198         (alloc, ())
 199     }
 200
 201     #[inline(always)]
 202     fn tag_static_base_pointer(_memory_extra: &(), _id: AllocId) -> Self::PointerTag {
 203         ()
 204     }
 205
 206     fn box_alloc(
 207         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
 208         _dest: PlaceTy<'tcx>,
 209     ) -> InterpResult<'tcx> {
 210         throw_unsup!(ConstPropUnsupported("can't const prop `box` keyword"));
 211     }
 212
 213     fn access_local(
 214         _ecx: &InterpCx<'mir, 'tcx, Self>,
 215         frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
 216         local: Local,
 217     ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
 218         let l = &frame.locals[local];
 219
 220         if l.value == LocalValue::Uninitialized {
 221             throw_unsup!(ConstPropUnsupported("tried to access an uninitialized local"));
 222         }
 223
 224         l.access()
 225     }
 226
 227     fn before_access_static(
 228         _memory_extra: &(),
 229         allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
 230     ) -> InterpResult<'tcx> {
 231         // if the static allocation is mutable or if it has relocations (it may be legal to mutate
 232         // the memory behind that in the future), then we can't const prop it
 233         if allocation.mutability == Mutability::Mut || allocation.relocations().len() > 0 {
 234             throw_unsup!(ConstPropUnsupported("can't eval mutable statics in ConstProp"));
 235         }
 236
 237         Ok(())
 238     }
 239
 240     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 241         Ok(())
 242     }
 243
 244     #[inline(always)]
 245     fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
 246         Ok(())
 247     }
 248 }
 249
 250 type Const<'tcx> = OpTy<'tcx>;
 251
 252 /// Finds optimization opportunities on the MIR.
 253 struct ConstPropagator<'mir, 'tcx> {
 254     ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
 255     tcx: TyCtxt<'tcx>,
 256     source: MirSource<'tcx>,
 257     can_const_prop: IndexVec<Local, ConstPropMode>,
 258     param_env: ParamEnv<'tcx>,
 259     // FIXME(eddyb) avoid cloning these two fields more than once,
 260     // by accessing them through `ecx` instead.
 261     source_scopes: IndexVec<SourceScope, SourceScopeData>,
 262     local_decls: IndexVec<Local, LocalDecl<'tcx>>,
 263     ret: Option<OpTy<'tcx, ()>>,
 264 }
 265
 266 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
 267     type Ty = Ty<'tcx>;
 268     type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
 269
 270     fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
 271         self.tcx.layout_of(self.param_env.and(ty))
 272     }
 273 }
 274
 275 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
 276     #[inline]
 277     fn data_layout(&self) -> &TargetDataLayout {
 278         &self.tcx.data_layout
 279     }
 280 }
 281
 282 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
 283     #[inline]
 284     fn tcx(&self) -> TyCtxt<'tcx> {
 285         self.tcx
 286     }
 287 }
 288
 289 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 290     fn new(
 291         body: ReadOnlyBodyAndCache<'_, 'tcx>,
 292         dummy_body: &'mir Body<'tcx>,
 293         tcx: TyCtxt<'tcx>,
 294         source: MirSource<'tcx>,
 295     ) -> ConstPropagator<'mir, 'tcx> {
 296         let def_id = source.def_id();
 297         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
 298         let mut param_env = tcx.param_env(def_id);
 299
 300         // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
 301         // we want to see the same instances that codegen will see. This allows us to `resolve()`
 302         // specializations.
 303         if !substs.needs_subst() {
 304             param_env = param_env.with_reveal_all();
 305         }
 306
 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 ret = ecx
 312             .layout_of(body.return_ty().subst(tcx, substs))
 313             .ok()
 314             // Don't bother allocating memory for ZST types which have no values
 315             // or for large values.
 316             .filter(|ret_layout| {
 317                 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
 318             })
 319             .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
 320
 321         ecx.push_stack_frame(
 322             Instance::new(def_id, substs),
 323             span,
 324             dummy_body,
 325             ret.map(Into::into),
 326             StackPopCleanup::None { cleanup: false },
 327         )
 328         .expect("failed to push initial stack frame");
 329
 330         ConstPropagator {
 331             ecx,
 332             tcx,
 333             source,
 334             param_env,
 335             can_const_prop,
 336             // FIXME(eddyb) avoid cloning these two fields more than once,
 337             // by accessing them through `ecx` instead.
 338             source_scopes: body.source_scopes.clone(),
 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 get_const(&self, local: Local) -> Option<Const<'tcx>> {
 346         if local == RETURN_PLACE {
 347             // Try to read the return place as an immediate so that if it is representable as a
 348             // scalar, we can handle it as such, but otherwise, just return the value as is.
 349             return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
 350                 Some(Ok(Ok(imm))) => Some(imm.into()),
 351                 _ => self.ret,
 352             };
 353         }
 354
 355         self.ecx.access_local(self.ecx.frame(), local, None).ok()
 356     }
 357
 358     fn remove_const(&mut self, local: Local) {
 359         self.ecx.frame_mut().locals[local] =
 360             LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
 361     }
 362
 363     fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
 364     where
 365         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
 366     {
 367         self.ecx.tcx.span = source_info.span;
 368         // FIXME(eddyb) move this to the `Panic(_)` error case, so that
 369         // `f(self)` is always called, and that the only difference when the
 370         // scope's `local_data` is missing, is that the lint isn't emitted.
 371         let lint_root = match &self.source_scopes[source_info.scope].local_data {
 372             ClearCrossCrate::Set(data) => data.lint_root,
 373             ClearCrossCrate::Clear => return None,
 374         };
 375         let r = match f(self) {
 376             Ok(val) => Some(val),
 377             Err(error) => {
 378                 use rustc::mir::interpret::{
 379                     InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
 380                 };
 381                 match error.kind {
 382                     MachineStop(_) => bug!("ConstProp does not stop"),
 383
 384                     // Some error shouldn't come up because creating them causes
 385                     // an allocation, which we should avoid. When that happens,
 386                     // dedicated error variants should be introduced instead.
 387                     // Only test this in debug builds though to avoid disruptions.
 388                     Unsupported(UnsupportedOpInfo::Unsupported(_))
 389                     | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
 390                     | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
 391                     | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
 392                         if cfg!(debug_assertions) =>
 393                     {
 394                         bug!("const-prop encountered allocating error: {:?}", error.kind);
 395                     }
 396
 397                     Unsupported(_)
 398                     | UndefinedBehavior(_)
 399                     | InvalidProgram(_)
 400                     | ResourceExhaustion(_) => {
 401                         // Ignore these errors.
 402                     }
 403                     Panic(_) => {
 404                         let diagnostic = error_to_const_error(&self.ecx, error);
 405                         diagnostic.report_as_lint(
 406                             self.ecx.tcx,
 407                             "this expression will panic at runtime",
 408                             lint_root,
 409                             None,
 410                         );
 411                     }
 412                 }
 413                 None
 414             }
 415         };
 416         self.ecx.tcx.span = DUMMY_SP;
 417         r
 418     }
 419
 420     fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<Const<'tcx>> {
 421         self.ecx.tcx.span = c.span;
 422         match self.ecx.eval_const_to_op(c.literal, None) {
 423             Ok(op) => Some(op),
 424             Err(error) => {
 425                 let err = error_to_const_error(&self.ecx, error);
 426                 err.report_as_error(self.ecx.tcx, "erroneous constant used");
 427                 None
 428             }
 429         }
 430     }
 431
 432     fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 433         trace!("eval_place(place={:?})", place);
 434         self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
 435     }
 436
 437     fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
 438         match *op {
 439             Operand::Constant(ref c) => self.eval_constant(c),
 440             Operand::Move(ref place) | Operand::Copy(ref place) => {
 441                 self.eval_place(place, source_info)
 442             }
 443         }
 444     }
 445
 446     fn const_prop(
 447         &mut self,
 448         rvalue: &Rvalue<'tcx>,
 449         place_layout: TyLayout<'tcx>,
 450         source_info: SourceInfo,
 451         place: &Place<'tcx>,
 452     ) -> Option<()> {
 453         let span = source_info.span;
 454
 455         // #66397: Don't try to eval into large places as that can cause an OOM
 456         if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
 457             return None;
 458         }
 459
 460         let overflow_check = self.tcx.sess.overflow_checks();
 461
 462         // Perform any special handling for specific Rvalue types.
 463         // Generally, checks here fall into one of two categories:
 464         //   1. Additional checking to provide useful lints to the user
 465         //        - In this case, we will do some validation and then fall through to the
 466         //          end of the function which evals the assignment.
 467         //   2. Working around bugs in other parts of the compiler
 468         //        - In this case, we'll return `None` from this function to stop evaluation.
 469         match rvalue {
 470             // Additional checking: if overflow checks are disabled (which is usually the case in
 471             // release mode), then we need to do additional checking here to give lints to the user
 472             // if an overflow would occur.
 473             Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
 474                 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
 475
 476                 self.use_ecx(source_info, |this| {
 477                     let ty = arg.ty(&this.local_decls, this.tcx);
 478
 479                     if ty.is_integral() {
 480                         let arg = this.ecx.eval_operand(arg, None)?;
 481                         let prim = this.ecx.read_immediate(arg)?;
 482                         // Need to do overflow check here: For actual CTFE, MIR
 483                         // generation emits code that does this before calling the op.
 484                         if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
 485                             throw_panic!(OverflowNeg)
 486                         }
 487                     }
 488
 489                     Ok(())
 490                 })?;
 491             }
 492
 493             // Additional checking: check for overflows on integer binary operations and report
 494             // them to the user as lints.
 495             Rvalue::BinaryOp(op, left, right) => {
 496                 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
 497
 498                 let r = self.use_ecx(source_info, |this| {
 499                     this.ecx.read_immediate(this.ecx.eval_operand(right, None)?)
 500                 })?;
 501                 if *op == BinOp::Shr || *op == BinOp::Shl {
 502                     let left_bits = place_layout.size.bits();
 503                     let right_size = r.layout.size;
 504                     let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
 505                     if r_bits.map_or(false, |b| b >= left_bits as u128) {
 506                         let lint_root = match &self.source_scopes[source_info.scope].local_data {
 507                             ClearCrossCrate::Set(data) => data.lint_root,
 508                             ClearCrossCrate::Clear => return None,
 509                         };
 510                         let dir = if *op == BinOp::Shr { "right" } else { "left" };
 511                         self.tcx.lint_hir(
 512                             ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
 513                             lint_root,
 514                             span,
 515                             &format!("attempt to shift {} with overflow", dir),
 516                         );
 517                         return None;
 518                     }
 519                 }
 520
 521                 // If overflow checking is enabled (like in debug mode by default),
 522                 // then we'll already catch overflow when we evaluate the `Assert` statement
 523                 // in MIR. However, if overflow checking is disabled, then there won't be any
 524                 // `Assert` statement and so we have to do additional checking here.
 525                 if !overflow_check {
 526                     self.use_ecx(source_info, |this| {
 527                         let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
 528                         let (_, overflow, _ty) = this.ecx.overflowing_binary_op(*op, l, r)?;
 529
 530                         if overflow {
 531                             let err = err_panic!(Overflow(*op)).into();
 532                             return Err(err);
 533                         }
 534
 535                         Ok(())
 536                     })?;
 537                 }
 538             }
 539
 540             // Work around: avoid ICE in miri. FIXME(wesleywiser)
 541             // The Miri engine ICEs when taking a reference to an uninitialized unsized
 542             // local. There's nothing it can do here: taking a reference needs an allocation
 543             // which needs to know the size. Normally that's okay as during execution
 544             // (e.g. for CTFE) it can never happen. But here in const_prop
 545             // unknown data is uninitialized, so if e.g. a function argument is unsized
 546             // and has a reference taken, we get an ICE.
 547             Rvalue::Ref(_, _, place_ref) => {
 548                 trace!("checking Ref({:?})", place_ref);
 549
 550                 if let Some(local) = place_ref.as_local() {
 551                     let alive = if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
 552                         true
 553                     } else {
 554                         false
 555                     };
 556
 557                     if !alive {
 558                         trace!("skipping Ref({:?}) to uninitialized local", place);
 559                         return None;
 560                     }
 561                 }
 562             }
 563
 564             _ => {}
 565         }
 566
 567         self.use_ecx(source_info, |this| {
 568             trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
 569             this.ecx.eval_rvalue_into_place(rvalue, place)?;
 570             Ok(())
 571         })
 572     }
 573
 574     fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
 575         Operand::Constant(Box::new(Constant {
 576             span,
 577             user_ty: None,
 578             literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
 579         }))
 580     }
 581
 582     fn replace_with_const(
 583         &mut self,
 584         rval: &mut Rvalue<'tcx>,
 585         value: Const<'tcx>,
 586         source_info: SourceInfo,
 587     ) {
 588         trace!("attepting to replace {:?} with {:?}", rval, value);
 589         if let Err(e) = self.ecx.validate_operand(
 590             value,
 591             vec![],
 592             // FIXME: is ref tracking too expensive?
 593             Some(&mut interpret::RefTracking::empty()),
 594         ) {
 595             trace!("validation error, attempt failed: {:?}", e);
 596             return;
 597         }
 598
 599         // FIXME> figure out what tho do when try_read_immediate fails
 600         let imm = self.use_ecx(source_info, |this| this.ecx.try_read_immediate(value));
 601
 602         if let Some(Ok(imm)) = imm {
 603             match *imm {
 604                 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
 605                     *rval = Rvalue::Use(self.operand_from_scalar(
 606                         scalar,
 607                         value.layout.ty,
 608                         source_info.span,
 609                     ));
 610                 }
 611                 Immediate::ScalarPair(
 612                     ScalarMaybeUndef::Scalar(one),
 613                     ScalarMaybeUndef::Scalar(two),
 614                 ) => {
 615                     // Found a value represented as a pair. For now only do cont-prop if type of
 616                     // Rvalue is also a pair with two scalars. The more general case is more
 617                     // complicated to implement so we'll do it later.
 618                     let ty = &value.layout.ty.kind;
 619                     // Only do it for tuples
 620                     if let ty::Tuple(substs) = ty {
 621                         // Only do it if tuple is also a pair with two scalars
 622                         if substs.len() == 2 {
 623                             let opt_ty1_ty2 = self.use_ecx(source_info, |this| {
 624                                 let ty1 = substs[0].expect_ty();
 625                                 let ty2 = substs[1].expect_ty();
 626                                 let ty_is_scalar = |ty| {
 627                                     this.ecx.layout_of(ty).ok().map(|ty| ty.details.abi.is_scalar())
 628                                         == Some(true)
 629                                 };
 630                                 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
 631                                     Ok(Some((ty1, ty2)))
 632                                 } else {
 633                                     Ok(None)
 634                                 }
 635                             });
 636
 637                             if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
 638                                 *rval = Rvalue::Aggregate(
 639                                     Box::new(AggregateKind::Tuple),
 640                                     vec![
 641                                         self.operand_from_scalar(one, ty1, source_info.span),
 642                                         self.operand_from_scalar(two, ty2, source_info.span),
 643                                     ],
 644                                 );
 645                             }
 646                         }
 647                     }
 648                 }
 649                 _ => {}
 650             }
 651         }
 652     }
 653
 654     fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
 655         let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
 656
 657         if mir_opt_level == 0 {
 658             return false;
 659         }
 660
 661         match *op {
 662             interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
 663                 s.is_bits()
 664             }
 665             interpret::Operand::Immediate(Immediate::ScalarPair(
 666                 ScalarMaybeUndef::Scalar(l),
 667                 ScalarMaybeUndef::Scalar(r),
 668             )) => l.is_bits() && r.is_bits(),
 669             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
 670                 intern_const_alloc_recursive(&mut self.ecx, None, op.assert_mem_place())
 671                     .expect("failed to intern alloc");
 672                 true
 673             }
 674             _ => false,
 675         }
 676     }
 677 }
 678
 679 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
 680 #[derive(Clone, Copy, Debug, PartialEq)]
 681 enum ConstPropMode {
 682     /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
 683     FullConstProp,
 684     /// The `Local` can be propagated into but reads cannot be propagated.
 685     OnlyPropagateInto,
 686     /// No propagation is allowed at all.
 687     NoPropagation,
 688 }
 689
 690 struct CanConstProp {
 691     can_const_prop: IndexVec<Local, ConstPropMode>,
 692     // false at the beginning, once set, there are not allowed to be any more assignments
 693     found_assignment: IndexVec<Local, bool>,
 694 }
 695
 696 impl CanConstProp {
 697     /// returns true if `local` can be propagated
 698     fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
 699         let mut cpv = CanConstProp {
 700             can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
 701             found_assignment: IndexVec::from_elem(false, &body.local_decls),
 702         };
 703         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
 704             // cannot use args at all
 705             // cannot use locals because if x < y { y - x } else { x - y } would
 706             //        lint for x != y
 707             // FIXME(oli-obk): lint variables until they are used in a condition
 708             // FIXME(oli-obk): lint if return value is constant
 709             let local_kind = body.local_kind(local);
 710
 711             if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
 712                 *val = ConstPropMode::OnlyPropagateInto;
 713                 trace!("local {:?} can't be const propagated because it's not a temporary", local);
 714             }
 715         }
 716         cpv.visit_body(body);
 717         cpv.can_const_prop
 718     }
 719 }
 720
 721 impl<'tcx> Visitor<'tcx> for CanConstProp {
 722     fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
 723         use rustc::mir::visit::PlaceContext::*;
 724         match context {
 725             // Constants must have at most one write
 726             // FIXME(oli-obk): we could be more powerful here, if the multiple writes
 727             // only occur in independent execution paths
 728             MutatingUse(MutatingUseContext::Store) => {
 729                 if self.found_assignment[local] {
 730                     trace!("local {:?} can't be propagated because of multiple assignments", local);
 731                     self.can_const_prop[local] = ConstPropMode::NoPropagation;
 732                 } else {
 733                     self.found_assignment[local] = true
 734                 }
 735             }
 736             // Reading constants is allowed an arbitrary number of times
 737             NonMutatingUse(NonMutatingUseContext::Copy)
 738             | NonMutatingUse(NonMutatingUseContext::Move)
 739             | NonMutatingUse(NonMutatingUseContext::Inspect)
 740             | NonMutatingUse(NonMutatingUseContext::Projection)
 741             | MutatingUse(MutatingUseContext::Projection)
 742             | NonUse(_) => {}
 743             _ => {
 744                 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
 745                 self.can_const_prop[local] = ConstPropMode::NoPropagation;
 746             }
 747         }
 748     }
 749 }
 750
 751 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
 752     fn tcx(&self) -> TyCtxt<'tcx> {
 753         self.tcx
 754     }
 755
 756     fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
 757         trace!("visit_constant: {:?}", constant);
 758         self.super_constant(constant, location);
 759         self.eval_constant(constant);
 760     }
 761
 762     fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
 763         trace!("visit_statement: {:?}", statement);
 764         if let StatementKind::Assign(box (ref place, ref mut rval)) = statement.kind {
 765             let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
 766             if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
 767                 if let Some(local) = place.as_local() {
 768                     let source = statement.source_info;
 769                     let can_const_prop = self.can_const_prop[local];
 770                     if let Some(()) = self.const_prop(rval, place_layout, source, place) {
 771                         if can_const_prop == ConstPropMode::FullConstProp
 772                             || can_const_prop == ConstPropMode::OnlyPropagateInto
 773                         {
 774                             if let Some(value) = self.get_const(local) {
 775                                 if self.should_const_prop(value) {
 776                                     trace!("replacing {:?} with {:?}", rval, value);
 777                                     self.replace_with_const(rval, value, statement.source_info);
 778
 779                                     if can_const_prop == ConstPropMode::FullConstProp {
 780                                         trace!("propagated into {:?}", local);
 781                                     }
 782                                 }
 783                             }
 784                         }
 785                     }
 786                     if self.can_const_prop[local] != ConstPropMode::FullConstProp {
 787                         trace!("can't propagate into {:?}", local);
 788                         if local != RETURN_PLACE {
 789                             self.remove_const(local);
 790                         }
 791                     }
 792                 }
 793             }
 794         } else {
 795             match statement.kind {
 796                 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
 797                     let frame = self.ecx.frame_mut();
 798                     frame.locals[local].value =
 799                         if let StatementKind::StorageLive(_) = statement.kind {
 800                             LocalValue::Uninitialized
 801                         } else {
 802                             LocalValue::Dead
 803                         };
 804                 }
 805                 _ => {}
 806             }
 807         }
 808
 809         self.super_statement(statement, location);
 810     }
 811
 812     fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
 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 => msg.description().to_owned(),
 843                             PanicInfo::BoundsCheck { ref len, ref index } => {
 844                                 let len =
 845                                     self.eval_operand(len, source_info).expect("len must be const");
 846                                 let len = match self.ecx.read_scalar(len) {
 847                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
 848                                     other => bug!("const len not primitive: {:?}", other),
 849                                 };
 850                                 let index = self
 851                                     .eval_operand(index, source_info)
 852                                     .expect("index must be const");
 853                                 let index = match self.ecx.read_scalar(index) {
 854                                     Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
 855                                     other => bug!("const index not primitive: {:?}", other),
 856                                 };
 857                                 format!(
 858                                     "index out of bounds: \
 859                                     the len is {} but the index is {}",
 860                                     len, index,
 861                                 )
 862                             }
 863                             // Need proper const propagator for these
 864                             _ => return,
 865                         };
 866                         self.tcx.lint_hir(::rustc::lint::builtin::CONST_ERR, hir_id, span, &msg);
 867                     } else {
 868                         if self.should_const_prop(value) {
 869                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
 870                                 *cond = self.operand_from_scalar(
 871                                     scalar,
 872                                     self.tcx.types.bool,
 873                                     source_info.span,
 874                                 );
 875                             }
 876                         }
 877                     }
 878                 }
 879             }
 880             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
 881                 if let Some(value) = self.eval_operand(&discr, source_info) {
 882                     if self.should_const_prop(value) {
 883                         if let ScalarMaybeUndef::Scalar(scalar) =
 884                             self.ecx.read_scalar(value).unwrap()
 885                         {
 886                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
 887                         }
 888                     }
 889                 }
 890             }
 891             //none of these have Operands to const-propagate
 892             TerminatorKind::Goto { .. }
 893             | TerminatorKind::Resume
 894             | TerminatorKind::Abort
 895             | TerminatorKind::Return
 896             | TerminatorKind::Unreachable
 897             | TerminatorKind::Drop { .. }
 898             | TerminatorKind::DropAndReplace { .. }
 899             | TerminatorKind::Yield { .. }
 900             | TerminatorKind::GeneratorDrop
 901             | TerminatorKind::FalseEdges { .. }
 902             | TerminatorKind::FalseUnwind { .. } => {}
 903             //FIXME(wesleywiser) Call does have Operands that could be const-propagated
 904             TerminatorKind::Call { .. } => {}
 905         }
 906     }
 907 }