1 //! Propagates constants for early reporting of statically known
7 use rustc::hir::def::DefKind;
8 use rustc::hir::def_id::DefId;
10 use rustc::mir::interpret::{InterpResult, PanicInfo, Scalar};
11 use rustc::mir::visit::{
12 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
15 read_only, AggregateKind, BasicBlock, BinOp, Body, BodyAndCache, ClearCrossCrate, Constant,
16 Local, LocalDecl, LocalKind, Location, Operand, Place, PlaceBase, ReadOnlyBodyAndCache, Rvalue,
17 SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
20 use rustc::ty::layout::{
21 HasDataLayout, HasTyCtxt, LayoutError, LayoutOf, Size, TargetDataLayout, TyLayout,
23 use rustc::ty::subst::InternalSubsts;
24 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
25 use rustc_data_structures::fx::FxHashMap;
26 use rustc_index::vec::IndexVec;
27 use rustc_span::{Span, DUMMY_SP};
28 use syntax::ast::Mutability;
30 use crate::const_eval::error_to_const_error;
31 use crate::interpret::{
32 self, intern_const_alloc_recursive, AllocId, Allocation, Frame, ImmTy, Immediate, InterpCx,
33 LocalState, LocalValue, Memory, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy, Pointer,
34 ScalarMaybeUndef, StackPopCleanup,
36 use crate::rustc::ty::subst::Subst;
37 use crate::transform::{MirPass, MirSource};
39 /// The maximum number of bytes that we'll allocate space for a return value.
40 const MAX_ALLOC_LIMIT: u64 = 1024;
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() {
51 use rustc::hir::map::blocks::FnLikeNode;
54 .as_local_hir_id(source.def_id())
55 .expect("Non-local call to local provider is_const_fn");
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,
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());
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.
74 trace!("ConstProp skipped for generator {:?}", source.def_id());
78 trace!("ConstProp starting for {:?}", source.def_id());
80 let dummy_body = &Body::new(
81 body.basic_blocks().clone(),
82 body.source_scopes.clone(),
83 body.local_decls.clone(),
87 tcx.def_span(source.def_id()),
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);
100 trace!("ConstProp done for {:?}", source.def_id());
104 struct ConstPropMachine;
106 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
107 type MemoryKinds = !;
108 type PointerTag = ();
111 type FrameExtra = ();
112 type MemoryExtra = ();
113 type AllocExtra = ();
115 type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
117 const STATIC_KIND: Option<!> = None;
119 const CHECK_ALIGN: bool = false;
122 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
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>>> {
137 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
139 _args: &[OpTy<'tcx>],
140 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
141 _unwind: Option<BasicBlock>,
142 ) -> InterpResult<'tcx> {
147 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
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"));
158 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
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");
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"));
171 _ecx: &InterpCx<'mir, 'tcx, Self>,
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 \
183 fn find_foreign_static(
186 ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
187 throw_unsup!(ReadForeignStatic)
191 fn init_allocation_extra<'b>(
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
202 fn tag_static_base_pointer(_memory_extra: &(), _id: AllocId) -> Self::PointerTag {
207 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
208 _dest: PlaceTy<'tcx>,
209 ) -> InterpResult<'tcx> {
210 throw_unsup!(ConstPropUnsupported("can't const prop `box` keyword"));
214 _ecx: &InterpCx<'mir, 'tcx, Self>,
215 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
217 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
218 let l = &frame.locals[local];
220 if l.value == LocalValue::Uninitialized {
221 throw_unsup!(ConstPropUnsupported("tried to access an uninitialized local"));
227 fn before_access_static(
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"));
240 fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
245 fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
250 type Const<'tcx> = OpTy<'tcx>;
252 /// Finds optimization opportunities on the MIR.
253 struct ConstPropagator<'mir, 'tcx> {
254 ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
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 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
265 // the last known `SourceInfo` here and just keep revisiting it.
266 source_info: Option<SourceInfo>,
269 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
271 type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
273 fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
274 self.tcx.layout_of(self.param_env.and(ty))
278 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
280 fn data_layout(&self) -> &TargetDataLayout {
281 &self.tcx.data_layout
285 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
287 fn tcx(&self) -> TyCtxt<'tcx> {
292 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
294 body: ReadOnlyBodyAndCache<'_, 'tcx>,
295 dummy_body: &'mir Body<'tcx>,
297 source: MirSource<'tcx>,
298 ) -> ConstPropagator<'mir, 'tcx> {
299 let def_id = source.def_id();
300 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
301 let mut param_env = tcx.param_env(def_id);
303 // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
304 // we want to see the same instances that codegen will see. This allows us to `resolve()`
306 if !substs.needs_subst() {
307 param_env = param_env.with_reveal_all();
310 let span = tcx.def_span(def_id);
311 let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
312 let can_const_prop = CanConstProp::check(body);
315 .layout_of(body.return_ty().subst(tcx, substs))
317 // Don't bother allocating memory for ZST types which have no values
318 // or for large values.
319 .filter(|ret_layout| {
320 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
322 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
324 ecx.push_stack_frame(
325 Instance::new(def_id, substs),
329 StackPopCleanup::None { cleanup: false },
331 .expect("failed to push initial stack frame");
339 // FIXME(eddyb) avoid cloning these two fields more than once,
340 // by accessing them through `ecx` instead.
341 source_scopes: body.source_scopes.clone(),
342 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
343 local_decls: body.local_decls.clone(),
344 ret: ret.map(Into::into),
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()),
359 self.ecx.access_local(self.ecx.frame(), local, None).ok()
362 fn remove_const(&mut self, local: Local) {
363 self.ecx.frame_mut().locals[local] =
364 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
367 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
368 match &self.source_scopes[source_info.scope].local_data {
369 ClearCrossCrate::Set(data) => Some(data.lint_root),
370 ClearCrossCrate::Clear => None,
374 fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
376 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
378 self.ecx.tcx.span = source_info.span;
379 // FIXME(eddyb) move this to the `Panic(_)` error case, so that
380 // `f(self)` is always called, and that the only difference when the
381 // scope's `local_data` is missing, is that the lint isn't emitted.
382 let lint_root = self.lint_root(source_info)?;
383 let r = match f(self) {
384 Ok(val) => Some(val),
386 use rustc::mir::interpret::{
387 InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
390 MachineStop(_) => bug!("ConstProp does not stop"),
392 // Some error shouldn't come up because creating them causes
393 // an allocation, which we should avoid. When that happens,
394 // dedicated error variants should be introduced instead.
395 // Only test this in debug builds though to avoid disruptions.
396 Unsupported(UnsupportedOpInfo::Unsupported(_))
397 | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
398 | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
399 | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
400 if cfg!(debug_assertions) =>
402 bug!("const-prop encountered allocating error: {:?}", error.kind);
406 | UndefinedBehavior(_)
408 | ResourceExhaustion(_) => {
409 // Ignore these errors.
412 let diagnostic = error_to_const_error(&self.ecx, error);
413 diagnostic.report_as_lint(
415 "this expression will panic at runtime",
424 self.ecx.tcx.span = DUMMY_SP;
431 source_info: SourceInfo,
432 ) -> Option<Const<'tcx>> {
433 self.ecx.tcx.span = c.span;
434 match self.ecx.eval_const_to_op(c.literal, None) {
437 let err = error_to_const_error(&self.ecx, error);
438 match self.lint_root(source_info) {
439 Some(lint_root) if c.literal.needs_subst() => {
440 // Out of backwards compatibility we cannot report hard errors in unused
441 // generic functions using associated constants of the generic parameters.
444 "erroneous constant used",
450 err.report_as_error(self.ecx.tcx, "erroneous constant used");
458 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
459 trace!("eval_place(place={:?})", place);
460 self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
463 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
465 Operand::Constant(ref c) => self.eval_constant(c, source_info),
466 Operand::Move(ref place) | Operand::Copy(ref place) => {
467 self.eval_place(place, source_info)
472 fn check_unary_op(&mut self, arg: &Operand<'tcx>, source_info: SourceInfo) -> Option<()> {
473 self.use_ecx(source_info, |this| {
474 let ty = arg.ty(&this.local_decls, this.tcx);
476 if ty.is_integral() {
477 let arg = this.ecx.eval_operand(arg, None)?;
478 let prim = this.ecx.read_immediate(arg)?;
479 // Need to do overflow check here: For actual CTFE, MIR
480 // generation emits code that does this before calling the op.
481 if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
482 throw_panic!(OverflowNeg)
495 left: &Operand<'tcx>,
496 right: &Operand<'tcx>,
497 source_info: SourceInfo,
498 place_layout: TyLayout<'tcx>,
499 overflow_check: bool,
501 let r = self.use_ecx(source_info, |this| {
502 this.ecx.read_immediate(this.ecx.eval_operand(right, None)?)
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.map_or(false, |b| b >= left_bits as u128) {
509 let lint_root = self.lint_root(source_info)?;
510 let dir = if op == BinOp::Shr { "right" } else { "left" };
512 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
515 &format!("attempt to shift {} with overflow", dir),
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.
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)?;
531 let err = err_panic!(Overflow(op)).into();
544 rvalue: &Rvalue<'tcx>,
545 place_layout: TyLayout<'tcx>,
546 source_info: SourceInfo,
549 // #66397: Don't try to eval into large places as that can cause an OOM
550 if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
554 let overflow_check = self.tcx.sess.overflow_checks();
556 // Perform any special handling for specific Rvalue types.
557 // Generally, checks here fall into one of two categories:
558 // 1. Additional checking to provide useful lints to the user
559 // - In this case, we will do some validation and then fall through to the
560 // end of the function which evals the assignment.
561 // 2. Working around bugs in other parts of the compiler
562 // - In this case, we'll return `None` from this function to stop evaluation.
564 // Additional checking: if overflow checks are disabled (which is usually the case in
565 // release mode), then we need to do additional checking here to give lints to the user
566 // if an overflow would occur.
567 Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
568 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
569 self.check_unary_op(arg, source_info)?;
572 // Additional checking: check for overflows on integer binary operations and report
573 // them to the user as lints.
574 Rvalue::BinaryOp(op, left, right) => {
575 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
576 self.check_binary_op(*op, left, right, source_info, place_layout, overflow_check)?;
579 // Work around: avoid ICE in miri. FIXME(wesleywiser)
580 // The Miri engine ICEs when taking a reference to an uninitialized unsized
581 // local. There's nothing it can do here: taking a reference needs an allocation
582 // which needs to know the size. Normally that's okay as during execution
583 // (e.g. for CTFE) it can never happen. But here in const_prop
584 // unknown data is uninitialized, so if e.g. a function argument is unsized
585 // and has a reference taken, we get an ICE.
586 Rvalue::Ref(_, _, place_ref) => {
587 trace!("checking Ref({:?})", place_ref);
589 if let Some(local) = place_ref.as_local() {
590 let alive = if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
597 trace!("skipping Ref({:?}) to uninitialized local", place);
606 self.use_ecx(source_info, |this| {
607 trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
608 this.ecx.eval_rvalue_into_place(rvalue, place)?;
613 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
614 Operand::Constant(Box::new(Constant {
617 literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
621 fn replace_with_const(
623 rval: &mut Rvalue<'tcx>,
625 source_info: SourceInfo,
627 trace!("attepting to replace {:?} with {:?}", rval, value);
628 if let Err(e) = self.ecx.validate_operand(
631 // FIXME: is ref tracking too expensive?
632 Some(&mut interpret::RefTracking::empty()),
634 trace!("validation error, attempt failed: {:?}", e);
638 // FIXME> figure out what tho do when try_read_immediate fails
639 let imm = self.use_ecx(source_info, |this| this.ecx.try_read_immediate(value));
641 if let Some(Ok(imm)) = imm {
643 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
644 *rval = Rvalue::Use(self.operand_from_scalar(
650 Immediate::ScalarPair(
651 ScalarMaybeUndef::Scalar(one),
652 ScalarMaybeUndef::Scalar(two),
654 // Found a value represented as a pair. For now only do cont-prop if type of
655 // Rvalue is also a pair with two scalars. The more general case is more
656 // complicated to implement so we'll do it later.
657 let ty = &value.layout.ty.kind;
658 // Only do it for tuples
659 if let ty::Tuple(substs) = ty {
660 // Only do it if tuple is also a pair with two scalars
661 if substs.len() == 2 {
662 let opt_ty1_ty2 = self.use_ecx(source_info, |this| {
663 let ty1 = substs[0].expect_ty();
664 let ty2 = substs[1].expect_ty();
665 let ty_is_scalar = |ty| {
666 this.ecx.layout_of(ty).ok().map(|ty| ty.details.abi.is_scalar())
669 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
676 if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
677 *rval = Rvalue::Aggregate(
678 Box::new(AggregateKind::Tuple),
680 self.operand_from_scalar(one, ty1, source_info.span),
681 self.operand_from_scalar(two, ty2, source_info.span),
693 fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
694 let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
696 if mir_opt_level == 0 {
701 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
704 interpret::Operand::Immediate(Immediate::ScalarPair(
705 ScalarMaybeUndef::Scalar(l),
706 ScalarMaybeUndef::Scalar(r),
707 )) => l.is_bits() && r.is_bits(),
708 interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
709 intern_const_alloc_recursive(&mut self.ecx, None, op.assert_mem_place())
710 .expect("failed to intern alloc");
718 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
719 #[derive(Clone, Copy, Debug, PartialEq)]
721 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
723 /// The `Local` can be propagated into but reads cannot be propagated.
725 /// No propagation is allowed at all.
729 struct CanConstProp {
730 can_const_prop: IndexVec<Local, ConstPropMode>,
731 // false at the beginning, once set, there are not allowed to be any more assignments
732 found_assignment: IndexVec<Local, bool>,
736 /// returns true if `local` can be propagated
737 fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
738 let mut cpv = CanConstProp {
739 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
740 found_assignment: IndexVec::from_elem(false, &body.local_decls),
742 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
743 // cannot use args at all
744 // cannot use locals because if x < y { y - x } else { x - y } would
746 // FIXME(oli-obk): lint variables until they are used in a condition
747 // FIXME(oli-obk): lint if return value is constant
748 let local_kind = body.local_kind(local);
750 if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
751 *val = ConstPropMode::OnlyPropagateInto;
752 trace!("local {:?} can't be const propagated because it's not a temporary", local);
755 cpv.visit_body(body);
760 impl<'tcx> Visitor<'tcx> for CanConstProp {
761 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
762 use rustc::mir::visit::PlaceContext::*;
764 // Constants must have at most one write
765 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
766 // only occur in independent execution paths
767 MutatingUse(MutatingUseContext::Store) => {
768 if self.found_assignment[local] {
769 trace!("local {:?} can't be propagated because of multiple assignments", local);
770 self.can_const_prop[local] = ConstPropMode::NoPropagation;
772 self.found_assignment[local] = true
775 // Reading constants is allowed an arbitrary number of times
776 NonMutatingUse(NonMutatingUseContext::Copy)
777 | NonMutatingUse(NonMutatingUseContext::Move)
778 | NonMutatingUse(NonMutatingUseContext::Inspect)
779 | NonMutatingUse(NonMutatingUseContext::Projection)
780 | MutatingUse(MutatingUseContext::Projection)
783 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
784 self.can_const_prop[local] = ConstPropMode::NoPropagation;
790 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
791 fn tcx(&self) -> TyCtxt<'tcx> {
795 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
796 trace!("visit_constant: {:?}", constant);
797 self.super_constant(constant, location);
798 self.eval_constant(constant, self.source_info.unwrap());
801 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
802 trace!("visit_statement: {:?}", statement);
803 let source_info = statement.source_info;
804 self.source_info = Some(source_info);
805 if let StatementKind::Assign(box (ref place, ref mut rval)) = statement.kind {
806 let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
807 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
808 if let Some(local) = place.as_local() {
809 let can_const_prop = self.can_const_prop[local];
810 if let Some(()) = self.const_prop(rval, place_layout, source_info, place) {
811 if can_const_prop == ConstPropMode::FullConstProp
812 || can_const_prop == ConstPropMode::OnlyPropagateInto
814 if let Some(value) = self.get_const(local) {
815 if self.should_const_prop(value) {
816 trace!("replacing {:?} with {:?}", rval, value);
817 self.replace_with_const(rval, value, statement.source_info);
819 if can_const_prop == ConstPropMode::FullConstProp {
820 trace!("propagated into {:?}", local);
826 if self.can_const_prop[local] != ConstPropMode::FullConstProp {
827 trace!("can't propagate into {:?}", local);
828 if local != RETURN_PLACE {
829 self.remove_const(local);
835 match statement.kind {
836 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
837 let frame = self.ecx.frame_mut();
838 frame.locals[local].value =
839 if let StatementKind::StorageLive(_) = statement.kind {
840 LocalValue::Uninitialized
849 self.super_statement(statement, location);
852 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
853 let source_info = terminator.source_info;
854 self.source_info = Some(source_info);
855 self.super_terminator(terminator, location);
856 match &mut terminator.kind {
857 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
858 if let Some(value) = self.eval_operand(&cond, source_info) {
859 trace!("assertion on {:?} should be {:?}", value, expected);
860 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
861 let value_const = self.ecx.read_scalar(value).unwrap();
862 if expected != value_const {
863 // poison all places this operand references so that further code
864 // doesn't use the invalid value
866 Operand::Move(ref place) | Operand::Copy(ref place) => {
867 if let PlaceBase::Local(local) = place.base {
868 self.remove_const(local);
871 Operand::Constant(_) => {}
873 let span = terminator.source_info.span;
877 .as_local_hir_id(self.source.def_id())
878 .expect("some part of a failing const eval must be local");
879 let msg = match msg {
880 PanicInfo::Overflow(_)
881 | PanicInfo::OverflowNeg
882 | PanicInfo::DivisionByZero
883 | PanicInfo::RemainderByZero => msg.description().to_owned(),
884 PanicInfo::BoundsCheck { ref len, ref index } => {
886 self.eval_operand(len, source_info).expect("len must be const");
887 let len = match self.ecx.read_scalar(len) {
888 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
889 other => bug!("const len not primitive: {:?}", other),
892 .eval_operand(index, source_info)
893 .expect("index must be const");
894 let index = match self.ecx.read_scalar(index) {
895 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
896 other => bug!("const index not primitive: {:?}", other),
899 "index out of bounds: \
900 the len is {} but the index is {}",
904 // Need proper const propagator for these
907 self.tcx.lint_hir(::rustc::lint::builtin::CONST_ERR, hir_id, span, &msg);
909 if self.should_const_prop(value) {
910 if let ScalarMaybeUndef::Scalar(scalar) = value_const {
911 *cond = self.operand_from_scalar(
921 TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
922 if let Some(value) = self.eval_operand(&discr, source_info) {
923 if self.should_const_prop(value) {
924 if let ScalarMaybeUndef::Scalar(scalar) =
925 self.ecx.read_scalar(value).unwrap()
927 *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
932 //none of these have Operands to const-propagate
933 TerminatorKind::Goto { .. }
934 | TerminatorKind::Resume
935 | TerminatorKind::Abort
936 | TerminatorKind::Return
937 | TerminatorKind::Unreachable
938 | TerminatorKind::Drop { .. }
939 | TerminatorKind::DropAndReplace { .. }
940 | TerminatorKind::Yield { .. }
941 | TerminatorKind::GeneratorDrop
942 | TerminatorKind::FalseEdges { .. }
943 | TerminatorKind::FalseUnwind { .. } => {}
944 //FIXME(wesleywiser) Call does have Operands that could be const-propagated
945 TerminatorKind::Call { .. } => {}