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;
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::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>,
129 _instance: ty::Instance<'tcx>,
130 _args: &[OpTy<'tcx>],
131 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
132 _unwind: Option<BasicBlock>,
133 ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
138 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
140 _args: &[OpTy<'tcx>],
141 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
142 _unwind: Option<BasicBlock>,
143 ) -> InterpResult<'tcx> {
148 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
150 _instance: ty::Instance<'tcx>,
151 _args: &[OpTy<'tcx>],
152 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
153 _unwind: Option<BasicBlock>,
154 ) -> InterpResult<'tcx> {
155 throw_unsup!(ConstPropUnsupported("calling intrinsics isn't supported in ConstProp"));
159 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
161 _msg: &rustc::mir::interpret::AssertMessage<'tcx>,
162 _unwind: Option<rustc::mir::BasicBlock>,
163 ) -> InterpResult<'tcx> {
164 bug!("panics terminators are not evaluated in ConstProp");
167 fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
168 throw_unsup!(ConstPropUnsupported("ptr-to-int casts aren't supported in ConstProp"));
172 _ecx: &InterpCx<'mir, 'tcx, Self>,
176 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
177 // We can't do this because aliasing of memory can differ between const eval and llvm
178 throw_unsup!(ConstPropUnsupported(
179 "pointer arithmetic or comparisons aren't supported \
184 fn find_foreign_static(
187 ) -> InterpResult<'tcx, Cow<'tcx, Allocation<Self::PointerTag>>> {
188 throw_unsup!(ReadForeignStatic)
192 fn init_allocation_extra<'b>(
195 alloc: Cow<'b, Allocation>,
196 _kind: Option<MemoryKind<!>>,
197 ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
198 // We do not use a tag so we can just cheaply forward the allocation
203 fn tag_static_base_pointer(_memory_extra: &(), _id: AllocId) -> Self::PointerTag {
208 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
209 _dest: PlaceTy<'tcx>,
210 ) -> InterpResult<'tcx> {
211 throw_unsup!(ConstPropUnsupported("can't const prop `box` keyword"));
215 _ecx: &InterpCx<'mir, 'tcx, Self>,
216 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
218 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
219 let l = &frame.locals[local];
221 if l.value == LocalValue::Uninitialized {
222 throw_unsup!(ConstPropUnsupported("tried to access an uninitialized local"));
228 fn before_access_static(
230 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
231 ) -> InterpResult<'tcx> {
232 // if the static allocation is mutable or if it has relocations (it may be legal to mutate
233 // the memory behind that in the future), then we can't const prop it
234 if allocation.mutability == Mutability::Mut || allocation.relocations().len() > 0 {
235 throw_unsup!(ConstPropUnsupported("can't eval mutable statics in ConstProp"));
241 fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
246 fn stack_push(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
251 type Const<'tcx> = OpTy<'tcx>;
253 /// Finds optimization opportunities on the MIR.
254 struct ConstPropagator<'mir, 'tcx> {
255 ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
257 source: MirSource<'tcx>,
258 can_const_prop: IndexVec<Local, ConstPropMode>,
259 param_env: ParamEnv<'tcx>,
260 // FIXME(eddyb) avoid cloning these two fields more than once,
261 // by accessing them through `ecx` instead.
262 source_scopes: IndexVec<SourceScope, SourceScopeData>,
263 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
264 ret: Option<OpTy<'tcx, ()>>,
265 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
266 // the last known `SourceInfo` here and just keep revisiting it.
267 source_info: Option<SourceInfo>,
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),
350 fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
351 if local == RETURN_PLACE {
352 // Try to read the return place as an immediate so that if it is representable as a
353 // scalar, we can handle it as such, but otherwise, just return the value as is.
354 return match self.ret.map(|ret| self.ecx.try_read_immediate(ret)) {
355 Some(Ok(Ok(imm))) => Some(imm.into()),
360 self.ecx.access_local(self.ecx.frame(), local, None).ok()
363 fn remove_const(&mut self, local: Local) {
364 self.ecx.frame_mut().locals[local] =
365 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
368 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
369 match &self.source_scopes[source_info.scope].local_data {
370 ClearCrossCrate::Set(data) => Some(data.lint_root),
371 ClearCrossCrate::Clear => None,
375 fn use_ecx<F, T>(&mut self, source_info: SourceInfo, f: F) -> Option<T>
377 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
379 self.ecx.tcx.span = source_info.span;
380 // FIXME(eddyb) move this to the `Panic(_)` error case, so that
381 // `f(self)` is always called, and that the only difference when the
382 // scope's `local_data` is missing, is that the lint isn't emitted.
383 let lint_root = self.lint_root(source_info)?;
384 let r = match f(self) {
385 Ok(val) => Some(val),
387 use rustc::mir::interpret::{
388 InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
391 MachineStop(_) => bug!("ConstProp does not stop"),
393 // Some error shouldn't come up because creating them causes
394 // an allocation, which we should avoid. When that happens,
395 // dedicated error variants should be introduced instead.
396 // Only test this in debug builds though to avoid disruptions.
397 Unsupported(UnsupportedOpInfo::Unsupported(_))
398 | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
399 | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
400 | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
401 if cfg!(debug_assertions) =>
403 bug!("const-prop encountered allocating error: {:?}", error.kind);
407 | UndefinedBehavior(_)
409 | ResourceExhaustion(_) => {
410 // Ignore these errors.
413 let diagnostic = error_to_const_error(&self.ecx, error);
414 diagnostic.report_as_lint(
416 "this expression will panic at runtime",
425 self.ecx.tcx.span = DUMMY_SP;
432 source_info: SourceInfo,
433 ) -> Option<Const<'tcx>> {
434 self.ecx.tcx.span = c.span;
435 match self.ecx.eval_const_to_op(c.literal, None) {
438 let err = error_to_const_error(&self.ecx, error);
439 match self.lint_root(source_info) {
440 Some(lint_root) if c.literal.needs_subst() => {
441 // Out of backwards compatibility we cannot report hard errors in unused
442 // generic functions using associated constants of the generic parameters.
445 "erroneous constant used",
451 err.report_as_error(self.ecx.tcx, "erroneous constant used");
459 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
460 trace!("eval_place(place={:?})", place);
461 self.use_ecx(source_info, |this| this.ecx.eval_place_to_op(place, None))
464 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
466 Operand::Constant(ref c) => self.eval_constant(c, source_info),
467 Operand::Move(ref place) | Operand::Copy(ref place) => {
468 self.eval_place(place, source_info)
473 fn check_unary_op(&mut self, arg: &Operand<'tcx>, source_info: SourceInfo) -> Option<()> {
474 self.use_ecx(source_info, |this| {
475 let ty = arg.ty(&this.local_decls, this.tcx);
477 if ty.is_integral() {
478 let arg = this.ecx.eval_operand(arg, None)?;
479 let prim = this.ecx.read_immediate(arg)?;
480 // Need to do overflow check here: For actual CTFE, MIR
481 // generation emits code that does this before calling the op.
482 if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
483 throw_panic!(OverflowNeg)
496 left: &Operand<'tcx>,
497 right: &Operand<'tcx>,
498 source_info: SourceInfo,
499 place_layout: TyLayout<'tcx>,
500 overflow_check: bool,
502 let r = self.use_ecx(source_info, |this| {
503 this.ecx.read_immediate(this.ecx.eval_operand(right, None)?)
505 if op == BinOp::Shr || op == BinOp::Shl {
506 let left_bits = place_layout.size.bits();
507 let right_size = r.layout.size;
508 let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
509 if r_bits.map_or(false, |b| b >= left_bits as u128) {
510 let lint_root = self.lint_root(source_info)?;
511 let dir = if op == BinOp::Shr { "right" } else { "left" };
513 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
516 &format!("attempt to shift {} with overflow", dir),
522 // If overflow checking is enabled (like in debug mode by default),
523 // then we'll already catch overflow when we evaluate the `Assert` statement
524 // in MIR. However, if overflow checking is disabled, then there won't be any
525 // `Assert` statement and so we have to do additional checking here.
527 self.use_ecx(source_info, |this| {
528 let l = this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)?;
529 let (_, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
532 let err = err_panic!(Overflow(op)).into();
545 rvalue: &Rvalue<'tcx>,
546 place_layout: TyLayout<'tcx>,
547 source_info: SourceInfo,
550 // #66397: Don't try to eval into large places as that can cause an OOM
551 if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) {
555 let overflow_check = self.tcx.sess.overflow_checks();
557 // Perform any special handling for specific Rvalue types.
558 // Generally, checks here fall into one of two categories:
559 // 1. Additional checking to provide useful lints to the user
560 // - In this case, we will do some validation and then fall through to the
561 // end of the function which evals the assignment.
562 // 2. Working around bugs in other parts of the compiler
563 // - In this case, we'll return `None` from this function to stop evaluation.
565 // Additional checking: if overflow checks are disabled (which is usually the case in
566 // release mode), then we need to do additional checking here to give lints to the user
567 // if an overflow would occur.
568 Rvalue::UnaryOp(UnOp::Neg, arg) if !overflow_check => {
569 trace!("checking UnaryOp(op = Neg, arg = {:?})", arg);
570 self.check_unary_op(arg, source_info)?;
573 // Additional checking: check for overflows on integer binary operations and report
574 // them to the user as lints.
575 Rvalue::BinaryOp(op, left, right) => {
576 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
577 self.check_binary_op(*op, left, right, source_info, place_layout, overflow_check)?;
580 // Work around: avoid ICE in miri. FIXME(wesleywiser)
581 // The Miri engine ICEs when taking a reference to an uninitialized unsized
582 // local. There's nothing it can do here: taking a reference needs an allocation
583 // which needs to know the size. Normally that's okay as during execution
584 // (e.g. for CTFE) it can never happen. But here in const_prop
585 // unknown data is uninitialized, so if e.g. a function argument is unsized
586 // and has a reference taken, we get an ICE.
587 Rvalue::Ref(_, _, place_ref) => {
588 trace!("checking Ref({:?})", place_ref);
590 if let Some(local) = place_ref.as_local() {
591 let alive = if let LocalValue::Live(_) = self.ecx.frame().locals[local].value {
598 trace!("skipping Ref({:?}) to uninitialized local", place);
607 self.use_ecx(source_info, |this| {
608 trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place);
609 this.ecx.eval_rvalue_into_place(rvalue, place)?;
614 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
615 Operand::Constant(Box::new(Constant {
618 literal: self.tcx.mk_const(*ty::Const::from_scalar(self.tcx, scalar, ty)),
622 fn replace_with_const(
624 rval: &mut Rvalue<'tcx>,
626 source_info: SourceInfo,
628 trace!("attepting to replace {:?} with {:?}", rval, value);
629 if let Err(e) = self.ecx.validate_operand(
632 // FIXME: is ref tracking too expensive?
633 Some(&mut interpret::RefTracking::empty()),
635 trace!("validation error, attempt failed: {:?}", e);
639 // FIXME> figure out what tho do when try_read_immediate fails
640 let imm = self.use_ecx(source_info, |this| this.ecx.try_read_immediate(value));
642 if let Some(Ok(imm)) = imm {
644 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
645 *rval = Rvalue::Use(self.operand_from_scalar(
651 Immediate::ScalarPair(
652 ScalarMaybeUndef::Scalar(one),
653 ScalarMaybeUndef::Scalar(two),
655 // Found a value represented as a pair. For now only do cont-prop if type of
656 // Rvalue is also a pair with two scalars. The more general case is more
657 // complicated to implement so we'll do it later.
658 let ty = &value.layout.ty.kind;
659 // Only do it for tuples
660 if let ty::Tuple(substs) = ty {
661 // Only do it if tuple is also a pair with two scalars
662 if substs.len() == 2 {
663 let opt_ty1_ty2 = self.use_ecx(source_info, |this| {
664 let ty1 = substs[0].expect_ty();
665 let ty2 = substs[1].expect_ty();
666 let ty_is_scalar = |ty| {
667 this.ecx.layout_of(ty).ok().map(|ty| ty.details.abi.is_scalar())
670 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
677 if let Some(Some((ty1, ty2))) = opt_ty1_ty2 {
678 *rval = Rvalue::Aggregate(
679 Box::new(AggregateKind::Tuple),
681 self.operand_from_scalar(one, ty1, source_info.span),
682 self.operand_from_scalar(two, ty2, source_info.span),
694 fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
695 let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
697 if mir_opt_level == 0 {
702 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) => {
705 interpret::Operand::Immediate(Immediate::ScalarPair(
706 ScalarMaybeUndef::Scalar(l),
707 ScalarMaybeUndef::Scalar(r),
708 )) => l.is_bits() && r.is_bits(),
709 interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
710 intern_const_alloc_recursive(&mut self.ecx, None, op.assert_mem_place())
711 .expect("failed to intern alloc");
719 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
720 #[derive(Clone, Copy, Debug, PartialEq)]
722 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
724 /// The `Local` can be propagated into but reads cannot be propagated.
726 /// No propagation is allowed at all.
730 struct CanConstProp {
731 can_const_prop: IndexVec<Local, ConstPropMode>,
732 // false at the beginning, once set, there are not allowed to be any more assignments
733 found_assignment: IndexVec<Local, bool>,
737 /// returns true if `local` can be propagated
738 fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
739 let mut cpv = CanConstProp {
740 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
741 found_assignment: IndexVec::from_elem(false, &body.local_decls),
743 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
744 // cannot use args at all
745 // cannot use locals because if x < y { y - x } else { x - y } would
747 // FIXME(oli-obk): lint variables until they are used in a condition
748 // FIXME(oli-obk): lint if return value is constant
749 let local_kind = body.local_kind(local);
751 if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
752 *val = ConstPropMode::OnlyPropagateInto;
753 trace!("local {:?} can't be const propagated because it's not a temporary", local);
756 cpv.visit_body(body);
761 impl<'tcx> Visitor<'tcx> for CanConstProp {
762 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
763 use rustc::mir::visit::PlaceContext::*;
765 // Constants must have at most one write
766 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
767 // only occur in independent execution paths
768 MutatingUse(MutatingUseContext::Store) => {
769 if self.found_assignment[local] {
770 trace!("local {:?} can't be propagated because of multiple assignments", local);
771 self.can_const_prop[local] = ConstPropMode::NoPropagation;
773 self.found_assignment[local] = true
776 // Reading constants is allowed an arbitrary number of times
777 NonMutatingUse(NonMutatingUseContext::Copy)
778 | NonMutatingUse(NonMutatingUseContext::Move)
779 | NonMutatingUse(NonMutatingUseContext::Inspect)
780 | NonMutatingUse(NonMutatingUseContext::Projection)
781 | MutatingUse(MutatingUseContext::Projection)
784 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
785 self.can_const_prop[local] = ConstPropMode::NoPropagation;
791 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
792 fn tcx(&self) -> TyCtxt<'tcx> {
796 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
797 trace!("visit_constant: {:?}", constant);
798 self.super_constant(constant, location);
799 self.eval_constant(constant, self.source_info.unwrap());
802 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
803 trace!("visit_statement: {:?}", statement);
804 let source_info = statement.source_info;
805 self.source_info = Some(source_info);
806 if let StatementKind::Assign(box (ref place, ref mut rval)) = statement.kind {
807 let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty;
808 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
809 if let Some(local) = place.as_local() {
810 let can_const_prop = self.can_const_prop[local];
811 if let Some(()) = self.const_prop(rval, place_layout, source_info, place) {
812 if can_const_prop == ConstPropMode::FullConstProp
813 || can_const_prop == ConstPropMode::OnlyPropagateInto
815 if let Some(value) = self.get_const(local) {
816 if self.should_const_prop(value) {
817 trace!("replacing {:?} with {:?}", rval, value);
818 self.replace_with_const(rval, value, statement.source_info);
820 if can_const_prop == ConstPropMode::FullConstProp {
821 trace!("propagated into {:?}", local);
827 if self.can_const_prop[local] != ConstPropMode::FullConstProp {
828 trace!("can't propagate into {:?}", local);
829 if local != RETURN_PLACE {
830 self.remove_const(local);
836 match statement.kind {
837 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
838 let frame = self.ecx.frame_mut();
839 frame.locals[local].value =
840 if let StatementKind::StorageLive(_) = statement.kind {
841 LocalValue::Uninitialized
850 self.super_statement(statement, location);
853 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
854 let source_info = terminator.source_info;
855 self.source_info = Some(source_info);
856 self.super_terminator(terminator, location);
857 match &mut terminator.kind {
858 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
859 if let Some(value) = self.eval_operand(&cond, source_info) {
860 trace!("assertion on {:?} should be {:?}", value, expected);
861 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
862 let value_const = self.ecx.read_scalar(value).unwrap();
863 if expected != value_const {
864 // poison all places this operand references so that further code
865 // doesn't use the invalid value
867 Operand::Move(ref place) | Operand::Copy(ref place) => {
868 if let PlaceBase::Local(local) = place.base {
869 self.remove_const(local);
872 Operand::Constant(_) => {}
874 let span = terminator.source_info.span;
878 .as_local_hir_id(self.source.def_id())
879 .expect("some part of a failing const eval must be local");
880 let msg = match msg {
881 PanicInfo::Overflow(_)
882 | PanicInfo::OverflowNeg
883 | PanicInfo::DivisionByZero
884 | PanicInfo::RemainderByZero => msg.description().to_owned(),
885 PanicInfo::BoundsCheck { ref len, ref index } => {
887 self.eval_operand(len, source_info).expect("len must be const");
888 let len = match self.ecx.read_scalar(len) {
889 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
890 other => bug!("const len not primitive: {:?}", other),
893 .eval_operand(index, source_info)
894 .expect("index must be const");
895 let index = match self.ecx.read_scalar(index) {
896 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw { data, .. })) => data,
897 other => bug!("const index not primitive: {:?}", other),
900 "index out of bounds: \
901 the len is {} but the index is {}",
905 // Need proper const propagator for these
908 self.tcx.lint_hir(::rustc::lint::builtin::CONST_ERR, hir_id, span, &msg);
910 if self.should_const_prop(value) {
911 if let ScalarMaybeUndef::Scalar(scalar) = value_const {
912 *cond = self.operand_from_scalar(
922 TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
923 if let Some(value) = self.eval_operand(&discr, source_info) {
924 if self.should_const_prop(value) {
925 if let ScalarMaybeUndef::Scalar(scalar) =
926 self.ecx.read_scalar(value).unwrap()
928 *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
933 //none of these have Operands to const-propagate
934 TerminatorKind::Goto { .. }
935 | TerminatorKind::Resume
936 | TerminatorKind::Abort
937 | TerminatorKind::Return
938 | TerminatorKind::Unreachable
939 | TerminatorKind::Drop { .. }
940 | TerminatorKind::DropAndReplace { .. }
941 | TerminatorKind::Yield { .. }
942 | TerminatorKind::GeneratorDrop
943 | TerminatorKind::FalseEdges { .. }
944 | TerminatorKind::FalseUnwind { .. } => {}
945 //FIXME(wesleywiser) Call does have Operands that could be const-propagated
946 TerminatorKind::Call { .. } => {}