1 //! Propagates constants for early reporting of statically known
7 use rustc::mir::interpret::{InterpResult, PanicInfo, Scalar};
8 use rustc::mir::visit::{
9 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
12 read_only, AggregateKind, BasicBlock, BinOp, Body, BodyAndCache, ClearCrossCrate, Constant,
13 Local, LocalDecl, LocalKind, Location, Operand, Place, PlaceBase, ReadOnlyBodyAndCache, Rvalue,
14 SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
17 use rustc::ty::layout::{
18 HasDataLayout, HasTyCtxt, LayoutError, LayoutOf, Size, TargetDataLayout, TyLayout,
20 use rustc::ty::subst::{InternalSubsts, Subst};
21 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
22 use rustc_data_structures::fx::FxHashMap;
23 use rustc_hir::def::DefKind;
24 use rustc_hir::def_id::DefId;
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::transform::{MirPass, MirSource};
38 /// The maximum number of bytes that we'll allocate space for a return value.
39 const MAX_ALLOC_LIMIT: u64 = 1024;
43 impl<'tcx> MirPass<'tcx> for ConstProp {
44 fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
45 // will be evaluated by miri and produce its errors there
46 if source.promoted.is_some() {
50 use rustc::hir::map::blocks::FnLikeNode;
53 .as_local_hir_id(source.def_id())
54 .expect("Non-local call to local provider is_const_fn");
56 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
57 let is_assoc_const = match tcx.def_kind(source.def_id()) {
58 Some(DefKind::AssocConst) => true,
62 // Only run const prop on functions, methods, closures and associated constants
63 if !is_fn_like && !is_assoc_const {
64 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
65 trace!("ConstProp skipped for {:?}", source.def_id());
69 let is_generator = tcx.type_of(source.def_id()).is_generator();
70 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
71 // computing their layout.
73 trace!("ConstProp skipped for generator {:?}", source.def_id());
77 trace!("ConstProp starting for {:?}", source.def_id());
79 let dummy_body = &Body::new(
80 body.basic_blocks().clone(),
81 body.source_scopes.clone(),
82 body.local_decls.clone(),
86 tcx.def_span(source.def_id()),
91 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
92 // constants, instead of just checking for const-folding succeeding.
93 // That would require an uniform one-def no-mutation analysis
94 // and RPO (or recursing when needing the value of a local).
95 let mut optimization_finder =
96 ConstPropagator::new(read_only!(body), dummy_body, tcx, source);
97 optimization_finder.visit_body(body);
99 trace!("ConstProp done for {:?}", source.def_id());
103 struct ConstPropMachine;
105 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
106 type MemoryKinds = !;
107 type PointerTag = ();
110 type FrameExtra = ();
111 type MemoryExtra = ();
112 type AllocExtra = ();
114 type MemoryMap = FxHashMap<AllocId, (MemoryKind<!>, Allocation)>;
116 const STATIC_KIND: Option<!> = None;
118 const CHECK_ALIGN: bool = false;
121 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
125 fn find_mir_or_eval_fn(
126 _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>,
267 lint_root: Option<HirId>,
270 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
272 type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
274 fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
275 self.tcx.layout_of(self.param_env.and(ty))
279 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
281 fn data_layout(&self) -> &TargetDataLayout {
282 &self.tcx.data_layout
286 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
288 fn tcx(&self) -> TyCtxt<'tcx> {
293 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
295 body: ReadOnlyBodyAndCache<'_, 'tcx>,
296 dummy_body: &'mir Body<'tcx>,
298 source: MirSource<'tcx>,
299 ) -> ConstPropagator<'mir, 'tcx> {
300 let def_id = source.def_id();
301 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
302 let mut param_env = tcx.param_env(def_id);
304 // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
305 // we want to see the same instances that codegen will see. This allows us to `resolve()`
307 if !substs.needs_subst() {
308 param_env = param_env.with_reveal_all();
311 let span = tcx.def_span(def_id);
312 let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
313 let can_const_prop = CanConstProp::check(body);
316 .layout_of(body.return_ty().subst(tcx, substs))
318 // Don't bother allocating memory for ZST types which have no values
319 // or for large values.
320 .filter(|ret_layout| {
321 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
323 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
325 ecx.push_stack_frame(
326 Instance::new(def_id, substs),
330 StackPopCleanup::None { cleanup: false },
332 .expect("failed to push initial stack frame");
340 // FIXME(eddyb) avoid cloning these two fields more than once,
341 // by accessing them through `ecx` instead.
342 source_scopes: body.source_scopes.clone(),
343 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
344 local_decls: body.local_decls.clone(),
345 ret: ret.map(Into::into),
351 fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
352 if local == RETURN_PLACE {
353 // Try to read the return place as an immediate so that if it is representable as a
354 // scalar, we can handle it as such, but otherwise, just return the value as is.
355 return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
356 Some(Ok(Ok(imm))) => Some(imm.into()),
361 self.ecx.access_local(self.ecx.frame(), local, None).ok()
364 fn remove_const(&mut self, local: Local) {
365 self.ecx.frame_mut().locals[local] =
366 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
369 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
370 match &self.source_scopes[source_info.scope].local_data {
371 ClearCrossCrate::Set(data) => Some(data.lint_root),
372 ClearCrossCrate::Clear => None,
376 fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
378 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
380 self.ecx.tcx.span = source_info.span;
381 let r = match f(self) {
382 Ok(val) => Some(val),
384 use rustc::mir::interpret::{
385 InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
388 MachineStop(_) => bug!("ConstProp does not stop"),
390 // Some error shouldn't come up because creating them causes
391 // an allocation, which we should avoid. When that happens,
392 // dedicated error variants should be introduced instead.
393 // Only test this in debug builds though to avoid disruptions.
394 Unsupported(UnsupportedOpInfo::Unsupported(_))
395 | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
396 | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
397 | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
398 if cfg!(debug_assertions) =>
400 bug!("const-prop encountered allocating error: {:?}", error.kind);
404 | UndefinedBehavior(_)
406 | ResourceExhaustion(_) => {
407 // Ignore these errors.
410 let diagnostic = error_to_const_error(&self.ecx, error);
411 diagnostic.report_as_lint(
413 "this expression will panic at runtime",
422 self.ecx.tcx.span = DUMMY_SP;
426 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<Const<'tcx>> {
427 self.ecx.tcx.span = c.span;
429 // FIXME we need to revisit this for #67176
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 {
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),
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 // FIXME we need to revisit this for #67176
555 if rvalue.needs_subst() {
559 let overflow_check = self.tcx.sess.overflow_checks();
561 // Perform any special handling for specific Rvalue types.
562 // Generally, checks here fall into one of two categories:
563 // 1. Additional checking to provide useful lints to the user
564 // - In this case, we will do some validation and then fall through to the
565 // end of the function which evals the assignment.
566 // 2. Working around bugs in other parts of the compiler
567 // - In this case, we'll return `None` from this function to stop evaluation.
569 // Additional checking: if overflow checks are disabled (which is usually the case in
570 // release mode), then we need to do additional checking here to give lints to the user
571 // if an overflow would occur.
572 Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
573 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
574 self.check_unary_op(arg, source_info)?;
577 // Additional checking: check for overflows on integer binary operations and report
578 // them to the user as lints.
579 Rvalue::BinaryOp(op, left, right) => {
580 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
581 self.check_binary_op(*op, left, right, source_info, place_layout, overflow_check)?;
584 // Work around: avoid ICE in miri. FIXME(wesleywiser)
585 // The Miri engine ICEs when taking a reference to an uninitialized unsized
586 // local. There's nothing it can do here: taking a reference needs an allocation
587 // which needs to know the size. Normally that's okay as during execution
588 // (e.g. for CTFE) it can never happen. But here in const_prop
589 // unknown data is uninitialized, so if e.g. a function argument is unsized
590 // and has a reference taken, we get an ICE.
591 Rvalue::Ref(_, _, place_ref) => {
592 trace!("checking Ref({:?})", place_ref);
594 if let Some(local) = place_ref.as_local() {
595 let alive = if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
602 trace!("skipping Ref({:?}) to uninitialized local", place);
611 self.use_ecx(source_info, |this| {
612 trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
613 this.ecx.eval_rvalue_into_place(rvalue, place)?;
618 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
619 Operand::Constant(Box::new(Constant {
622 literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
626 fn replace_with_const(
628 rval: &mut Rvalue<'tcx>,
630 source_info: SourceInfo,
632 trace!("attepting to replace {:?} with {:?}", rval, value);
633 if let Err(e) = self.ecx.validate_operand(
636 // FIXME: is ref tracking too expensive?
637 Some(&mut interpret::RefTracking::empty()),
639 trace!("validation error, attempt failed: {:?}", e);
643 // FIXME> figure out what tho do when try_read_immediate fails
644 let imm = self.use_ecx(source_info, |this| this.ecx.try_read_immediate(value));
646 if let Some(Ok(imm)) = imm {
648 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
649 *rval = Rvalue::Use(self.operand_from_scalar(
655 Immediate::ScalarPair(
656 ScalarMaybeUndef::Scalar(one),
657 ScalarMaybeUndef::Scalar(two),
659 // Found a value represented as a pair. For now only do cont-prop if type of
660 // Rvalue is also a pair with two scalars. The more general case is more
661 // complicated to implement so we'll do it later.
662 let ty = &value.layout.ty.kind;
663 // Only do it for tuples
664 if let ty::Tuple(substs) = ty {
665 // Only do it if tuple is also a pair with two scalars
666 if substs.len() == 2 {
667 let opt_ty1_ty2 = self.use_ecx(source_info, |this| {
668 let ty1 = substs[0].expect_ty();
669 let ty2 = substs[1].expect_ty();
670 let ty_is_scalar = |ty| {
671 this.ecx.layout_of(ty).ok().map(|ty| ty.details.abi.is_scalar())
674 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
681 if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
682 *rval = Rvalue::Aggregate(
683 Box::new(AggregateKind::Tuple),
685 self.operand_from_scalar(one, ty1, source_info.span),
686 self.operand_from_scalar(two, ty2, source_info.span),
698 fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
699 let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
701 if mir_opt_level == 0 {
706 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
709 interpret::Operand::Immediate(Immediate::ScalarPair(
710 ScalarMaybeUndef::Scalar(l),
711 ScalarMaybeUndef::Scalar(r),
712 )) => l.is_bits() && r.is_bits(),
713 interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
714 let mplace = op.assert_mem_place(&self.ecx);
715 intern_const_alloc_recursive(&mut self.ecx, None, mplace, false)
716 .expect("failed to intern alloc");
724 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
725 #[derive(Clone, Copy, Debug, PartialEq)]
727 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
729 /// The `Local` can be propagated into but reads cannot be propagated.
731 /// No propagation is allowed at all.
735 struct CanConstProp {
736 can_const_prop: IndexVec<Local, ConstPropMode>,
737 // false at the beginning, once set, there are not allowed to be any more assignments
738 found_assignment: IndexVec<Local, bool>,
742 /// returns true if `local` can be propagated
743 fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
744 let mut cpv = CanConstProp {
745 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
746 found_assignment: IndexVec::from_elem(false, &body.local_decls),
748 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
749 // cannot use args at all
750 // cannot use locals because if x < y { y - x } else { x - y } would
752 // FIXME(oli-obk): lint variables until they are used in a condition
753 // FIXME(oli-obk): lint if return value is constant
754 let local_kind = body.local_kind(local);
756 if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
757 *val = ConstPropMode::OnlyPropagateInto;
758 trace!("local {:?} can't be const propagated because it's not a temporary", local);
761 cpv.visit_body(body);
766 impl<'tcx> Visitor<'tcx> for CanConstProp {
767 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
768 use rustc::mir::visit::PlaceContext::*;
770 // Constants must have at most one write
771 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
772 // only occur in independent execution paths
773 MutatingUse(MutatingUseContext::Store) => {
774 if self.found_assignment[local] {
775 trace!("local {:?} can't be propagated because of multiple assignments", local);
776 self.can_const_prop[local] = ConstPropMode::NoPropagation;
778 self.found_assignment[local] = true
781 // Reading constants is allowed an arbitrary number of times
782 NonMutatingUse(NonMutatingUseContext::Copy)
783 | NonMutatingUse(NonMutatingUseContext::Move)
784 | NonMutatingUse(NonMutatingUseContext::Inspect)
785 | NonMutatingUse(NonMutatingUseContext::Projection)
786 | MutatingUse(MutatingUseContext::Projection)
789 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
790 self.can_const_prop[local] = ConstPropMode::NoPropagation;
796 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
797 fn tcx(&self) -> TyCtxt<'tcx> {
801 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
802 trace!("visit_constant: {:?}", constant);
803 self.super_constant(constant, location);
804 self.eval_constant(constant);
807 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
808 trace!("visit_statement: {:?}", statement);
809 let source_info = statement.source_info;
810 self.source_info = Some(source_info);
811 self.lint_root = self.lint_root(source_info);
812 if let StatementKind::Assign(box (ref place, ref mut rval)) = statement.kind {
813 let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
814 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
815 if let Some(local) = place.as_local() {
816 let can_const_prop = self.can_const_prop[local];
817 if let Some(()) = self.const_prop(rval, place_layout, source_info, place) {
818 if can_const_prop == ConstPropMode::FullConstProp
819 || can_const_prop == ConstPropMode::OnlyPropagateInto
821 if let Some(value) = self.get_const(local) {
822 if self.should_const_prop(value) {
823 trace!("replacing {:?} with {:?}", rval, value);
824 self.replace_with_const(rval, value, statement.source_info);
826 if can_const_prop == ConstPropMode::FullConstProp {
827 trace!("propagated into {:?}", local);
833 if self.can_const_prop[local] != ConstPropMode::FullConstProp {
834 trace!("can't propagate into {:?}", local);
835 if local != RETURN_PLACE {
836 self.remove_const(local);
842 match statement.kind {
843 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
844 let frame = self.ecx.frame_mut();
845 frame.locals[local].value =
846 if let StatementKind::StorageLive(_) = statement.kind {
847 LocalValue::Uninitialized
856 self.super_statement(statement, location);
859 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
860 let source_info = terminator.source_info;
861 self.source_info = Some(source_info);
862 self.super_terminator(terminator, location);
863 self.lint_root = self.lint_root(source_info);
864 match &mut terminator.kind {
865 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
866 if let Some(value) = self.eval_operand(&cond, source_info) {
867 trace!("assertion on {:?} should be {:?}", value, expected);
868 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
869 let value_const = self.ecx.read_scalar(value).unwrap();
870 if expected != value_const {
871 // poison all places this operand references so that further code
872 // doesn't use the invalid value
874 Operand::Move(ref place) | Operand::Copy(ref place) => {
876 PlaceBase::Local(local) => self.remove_const(local),
879 Operand::Constant(_) => {}
881 let span = terminator.source_info.span;
885 .as_local_hir_id(self.source.def_id())
886 .expect("some part of a failing const eval must be local");
887 let msg = match msg {
888 PanicInfo::Overflow(_)
889 | PanicInfo::OverflowNeg
890 | PanicInfo::DivisionByZero
891 | PanicInfo::RemainderByZero => msg.description().to_owned(),
892 PanicInfo::BoundsCheck { ref len, ref index } => {
894 self.eval_operand(len, source_info).expect("len must be const");
895 let len = match self.ecx.read_scalar(len) {
896 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
897 other => bug!("const len not primitive: {:?}", other),
900 .eval_operand(index, source_info)
901 .expect("index must be const");
902 let index = match self.ecx.read_scalar(index) {
903 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
904 other => bug!("const index not primitive: {:?}", other),
907 "index out of bounds: \
908 the len is {} but the index is {}",
912 // Need proper const propagator for these
915 self.tcx.lint_hir(::rustc::lint::builtin::CONST_ERR, hir_id, span, &msg);
917 if self.should_const_prop(value) {
918 if let ScalarMaybeUndef::Scalar(scalar) = value_const {
919 *cond = self.operand_from_scalar(
929 TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
930 if let Some(value) = self.eval_operand(&discr, source_info) {
931 if self.should_const_prop(value) {
932 if let ScalarMaybeUndef::Scalar(scalar) =
933 self.ecx.read_scalar(value).unwrap()
935 *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
940 //none of these have Operands to const-propagate
941 TerminatorKind::Goto { .. }
942 | TerminatorKind::Resume
943 | TerminatorKind::Abort
944 | TerminatorKind::Return
945 | TerminatorKind::Unreachable
946 | TerminatorKind::Drop { .. }
947 | TerminatorKind::DropAndReplace { .. }
948 | TerminatorKind::Yield { .. }
949 | TerminatorKind::GeneratorDrop
950 | TerminatorKind::FalseEdges { .. }
951 | TerminatorKind::FalseUnwind { .. } => {}
952 //FIXME(wesleywiser) Call does have Operands that could be const-propagated
953 TerminatorKind::Call { .. } => {}