1 //! Propagates constants for early reporting of statically known
6 use rustc_ast::Mutability;
7 use rustc_data_structures::fx::FxHashSet;
8 use rustc_hir::def::DefKind;
10 use rustc_index::bit_set::BitSet;
11 use rustc_index::vec::IndexVec;
12 use rustc_middle::mir::visit::{
13 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
15 use rustc_middle::mir::{
16 AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
17 Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
18 StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
20 use rustc_middle::ty::layout::{HasTyCtxt, LayoutError, TyAndLayout};
21 use rustc_middle::ty::subst::{InternalSubsts, Subst};
22 use rustc_middle::ty::{
23 self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
25 use rustc_session::lint;
26 use rustc_span::{def_id::DefId, Span};
27 use rustc_target::abi::{HasDataLayout, LayoutOf, Size, TargetDataLayout};
28 use rustc_target::spec::abi::Abi;
29 use rustc_trait_selection::traits;
31 use crate::const_eval::ConstEvalErr;
32 use crate::interpret::{
33 self, compile_time_machine, AllocId, Allocation, ConstValue, CtfeValidationMode, Frame, ImmTy,
34 Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, Memory, MemoryKind, OpTy,
35 Operand as InterpOperand, PlaceTy, Pointer, Scalar, ScalarMaybeUninit, StackPopCleanup,
37 use crate::transform::MirPass;
39 /// The maximum number of bytes that we'll allocate space for a local or the return value.
40 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
41 /// Severely regress performance.
42 const MAX_ALLOC_LIMIT: u64 = 1024;
44 /// Macro for machine-specific `InterpError` without allocation.
45 /// (These will never be shown to the user, but they help diagnose ICEs.)
46 macro_rules! throw_machine_stop_str {
48 // We make a new local type for it. The type itself does not carry any information,
49 // but its vtable (for the `MachineStopType` trait) does.
51 // Printing this type shows the desired string.
52 impl std::fmt::Display for Zst {
53 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
57 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
58 throw_machine_stop!(Zst)
64 impl<'tcx> MirPass<'tcx> for ConstProp {
65 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
66 // will be evaluated by miri and produce its errors there
67 if body.source.promoted.is_some() {
71 use rustc_middle::hir::map::blocks::FnLikeNode;
72 let def_id = body.source.def_id().expect_local();
73 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
75 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
76 let is_assoc_const = tcx.def_kind(def_id.to_def_id()) == DefKind::AssocConst;
78 // Only run const prop on functions, methods, closures and associated constants
79 if !is_fn_like && !is_assoc_const {
80 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
81 trace!("ConstProp skipped for {:?}", def_id);
85 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
86 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
87 // computing their layout.
89 trace!("ConstProp skipped for generator {:?}", def_id);
93 // Check if it's even possible to satisfy the 'where' clauses
95 // This branch will never be taken for any normal function.
96 // However, it's possible to `#!feature(trivial_bounds)]` to write
97 // a function with impossible to satisfy clauses, e.g.:
98 // `fn foo() where String: Copy {}`
100 // We don't usually need to worry about this kind of case,
101 // since we would get a compilation error if the user tried
102 // to call it. However, since we can do const propagation
103 // even without any calls to the function, we need to make
104 // sure that it even makes sense to try to evaluate the body.
105 // If there are unsatisfiable where clauses, then all bets are
106 // off, and we just give up.
108 // We manually filter the predicates, skipping anything that's not
109 // "global". We are in a potentially generic context
110 // (e.g. we are evaluating a function without substituting generic
111 // parameters, so this filtering serves two purposes:
113 // 1. We skip evaluating any predicates that we would
114 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
115 // 2. We avoid trying to normalize predicates involving generic
116 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
117 // the normalization code (leading to cycle errors), since
118 // it's usually never invoked in this way.
120 .predicates_of(def_id.to_def_id())
123 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
124 if traits::impossible_predicates(
126 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
128 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
132 trace!("ConstProp starting for {:?}", def_id);
134 let dummy_body = &Body::new(
136 body.basic_blocks().clone(),
137 body.source_scopes.clone(),
138 body.local_decls.clone(),
143 body.generator_kind(),
146 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
147 // constants, instead of just checking for const-folding succeeding.
148 // That would require an uniform one-def no-mutation analysis
149 // and RPO (or recursing when needing the value of a local).
150 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
151 optimization_finder.visit_body(body);
153 trace!("ConstProp done for {:?}", def_id);
157 struct ConstPropMachine<'mir, 'tcx> {
158 /// The virtual call stack.
159 stack: Vec<Frame<'mir, 'tcx, (), ()>>,
160 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
161 written_only_inside_own_block_locals: FxHashSet<Local>,
162 /// Locals that need to be cleared after every block terminates.
163 only_propagate_inside_block_locals: BitSet<Local>,
164 can_const_prop: IndexVec<Local, ConstPropMode>,
167 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
169 only_propagate_inside_block_locals: BitSet<Local>,
170 can_const_prop: IndexVec<Local, ConstPropMode>,
174 written_only_inside_own_block_locals: Default::default(),
175 only_propagate_inside_block_locals,
181 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
182 compile_time_machine!(<'mir, 'tcx>);
186 type MemoryExtra = ();
189 _ecx: &InterpCx<'mir, 'tcx, Self>,
190 _instance: ty::InstanceDef<'tcx>,
191 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
192 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
195 fn find_mir_or_eval_fn(
196 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
197 _instance: ty::Instance<'tcx>,
199 _args: &[OpTy<'tcx>],
200 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
201 _unwind: Option<BasicBlock>,
202 ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
207 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
208 _instance: ty::Instance<'tcx>,
209 _args: &[OpTy<'tcx>],
210 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
211 _unwind: Option<BasicBlock>,
212 ) -> InterpResult<'tcx> {
213 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
217 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
218 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
219 _unwind: Option<rustc_middle::mir::BasicBlock>,
220 ) -> InterpResult<'tcx> {
221 bug!("panics terminators are not evaluated in ConstProp")
224 fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
225 throw_unsup!(ReadPointerAsBytes)
229 _ecx: &InterpCx<'mir, 'tcx, Self>,
232 _right: &ImmTy<'tcx>,
233 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
234 // We can't do this because aliasing of memory can differ between const eval and llvm
235 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
239 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
240 _dest: &PlaceTy<'tcx>,
241 ) -> InterpResult<'tcx> {
242 throw_machine_stop_str!("can't const prop heap allocations")
246 _ecx: &InterpCx<'mir, 'tcx, Self>,
247 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
249 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
250 let l = &frame.locals[local];
252 if l.value == LocalValue::Uninitialized {
253 throw_machine_stop_str!("tried to access an uninitialized local")
259 fn access_local_mut<'a>(
260 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
263 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
265 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
266 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
268 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
270 "mutating local {:?} which is restricted to its block. \
271 Will remove it from const-prop after block is finished.",
274 ecx.machine.written_only_inside_own_block_locals.insert(local);
276 ecx.machine.stack[frame].locals[local].access_mut()
279 fn before_access_global(
282 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
283 _static_def_id: Option<DefId>,
285 ) -> InterpResult<'tcx> {
287 throw_machine_stop_str!("can't write to global");
289 // If the static allocation is mutable, then we can't const prop it as its content
290 // might be different at runtime.
291 if allocation.mutability == Mutability::Mut {
292 throw_machine_stop_str!("can't access mutable globals in ConstProp");
300 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
301 frame: Frame<'mir, 'tcx>,
302 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
308 ecx: &'a InterpCx<'mir, 'tcx, Self>,
309 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
315 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
316 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
317 &mut ecx.machine.stack
321 /// Finds optimization opportunities on the MIR.
322 struct ConstPropagator<'mir, 'tcx> {
323 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
325 param_env: ParamEnv<'tcx>,
326 // FIXME(eddyb) avoid cloning these two fields more than once,
327 // by accessing them through `ecx` instead.
328 source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
329 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
330 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
331 // the last known `SourceInfo` here and just keep revisiting it.
332 source_info: Option<SourceInfo>,
335 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
337 type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
339 fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
340 self.tcx.layout_of(self.param_env.and(ty))
344 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
346 fn data_layout(&self) -> &TargetDataLayout {
347 &self.tcx.data_layout
351 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
353 fn tcx(&self) -> TyCtxt<'tcx> {
358 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
361 dummy_body: &'mir Body<'tcx>,
363 ) -> ConstPropagator<'mir, 'tcx> {
364 let def_id = body.source.def_id();
365 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
366 let param_env = tcx.param_env_reveal_all_normalized(def_id);
368 let span = tcx.def_span(def_id);
369 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
370 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
371 // `layout_of` query invocations.
372 let can_const_prop = CanConstProp::check(tcx, param_env, body);
373 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
374 for (l, mode) in can_const_prop.iter_enumerated() {
375 if *mode == ConstPropMode::OnlyInsideOwnBlock {
376 only_propagate_inside_block_locals.insert(l);
379 let mut ecx = InterpCx::new(
383 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
388 .layout_of(body.return_ty().subst(tcx, substs))
390 // Don't bother allocating memory for ZST types which have no values
391 // or for large values.
392 .filter(|ret_layout| {
393 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
395 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack).into());
397 ecx.push_stack_frame(
398 Instance::new(def_id, substs),
401 StackPopCleanup::None { cleanup: false },
403 .expect("failed to push initial stack frame");
409 // FIXME(eddyb) avoid cloning these two fields more than once,
410 // by accessing them through `ecx` instead.
411 source_scopes: body.source_scopes.clone(),
412 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
413 local_decls: body.local_decls.clone(),
418 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
419 let op = match self.ecx.eval_place_to_op(place, None) {
422 trace!("get_const failed: {}", e);
427 // Try to read the local as an immediate so that if it is representable as a scalar, we can
428 // handle it as such, but otherwise, just return the value as is.
429 Some(match self.ecx.try_read_immediate(&op) {
430 Ok(Ok(imm)) => imm.into(),
435 /// Remove `local` from the pool of `Locals`. Allows writing to them,
436 /// but not reading from them anymore.
437 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
438 ecx.frame_mut().locals[local] =
439 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
442 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
443 source_info.scope.lint_root(&self.source_scopes)
446 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
448 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
451 Ok(val) => Some(val),
453 trace!("InterpCx operation failed: {:?}", error);
454 // Some errors shouldn't come up because creating them causes
455 // an allocation, which we should avoid. When that happens,
456 // dedicated error variants should be introduced instead.
458 !error.kind().formatted_string(),
459 "const-prop encountered formatting error: {}",
467 /// Returns the value, if any, of evaluating `c`.
468 fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
469 // FIXME we need to revisit this for #67176
474 match self.ecx.mir_const_to_op(&c.literal, None) {
477 let tcx = self.ecx.tcx.at(c.span);
478 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
479 if let Some(lint_root) = self.lint_root(source_info) {
480 let lint_only = match c.literal {
481 ConstantKind::Ty(ct) => match ct.val {
482 // Promoteds must lint and not error as the user didn't ask for them
483 ConstKind::Unevaluated(ty::Unevaluated {
488 // Out of backwards compatibility we cannot report hard errors in unused
489 // generic functions using associated constants of the generic parameters.
490 _ => c.literal.needs_subst(),
492 ConstantKind::Val(_, ty) => ty.needs_subst(),
495 // Out of backwards compatibility we cannot report hard errors in unused
496 // generic functions using associated constants of the generic parameters.
497 err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
499 err.report_as_error(tcx, "erroneous constant used");
502 err.report_as_error(tcx, "erroneous constant used");
509 /// Returns the value, if any, of evaluating `place`.
510 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
511 trace!("eval_place(place={:?})", place);
512 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
515 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
516 /// or `eval_place`, depending on the variant of `Operand` used.
517 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
519 Operand::Constant(ref c) => self.eval_constant(c, source_info),
520 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
524 fn report_assert_as_lint(
526 lint: &'static lint::Lint,
527 source_info: SourceInfo,
528 message: &'static str,
529 panic: AssertKind<impl std::fmt::Debug>,
531 let lint_root = self.lint_root(source_info)?;
532 self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
533 let mut err = lint.build(message);
534 err.span_label(source_info.span, format!("{:?}", panic));
544 source_info: SourceInfo,
546 if let (val, true) = self.use_ecx(|this| {
547 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
548 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
551 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
552 // appropriate to use.
553 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
554 self.report_assert_as_lint(
555 lint::builtin::ARITHMETIC_OVERFLOW,
557 "this arithmetic operation will overflow",
558 AssertKind::OverflowNeg(val.to_const_int()),
568 left: &Operand<'tcx>,
569 right: &Operand<'tcx>,
570 source_info: SourceInfo,
572 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
573 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
574 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
575 if op == BinOp::Shr || op == BinOp::Shl {
577 // We need the type of the LHS. We cannot use `place_layout` as that is the type
578 // of the result, which for checked binops is not the same!
579 let left_ty = left.ty(&self.local_decls, self.tcx);
580 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
581 let right_size = r.layout.size;
582 let r_bits = r.to_scalar().ok();
583 // This is basically `force_bits`.
584 let r_bits = r_bits.and_then(|r| r.to_bits_or_ptr(right_size, &self.tcx).ok());
585 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
586 debug!("check_binary_op: reporting assert for {:?}", source_info);
587 self.report_assert_as_lint(
588 lint::builtin::ARITHMETIC_OVERFLOW,
590 "this arithmetic operation will overflow",
591 AssertKind::Overflow(
594 Some(l) => l.to_const_int(),
595 // Invent a dummy value, the diagnostic ignores it anyway
596 None => ConstInt::new(
597 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
599 left_ty.is_ptr_sized_integral(),
608 if let (Some(l), Some(r)) = (&l, &r) {
609 // The remaining operators are handled through `overflowing_binary_op`.
610 if self.use_ecx(|this| {
611 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
614 self.report_assert_as_lint(
615 lint::builtin::ARITHMETIC_OVERFLOW,
617 "this arithmetic operation will overflow",
618 AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
625 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
627 Operand::Copy(l) | Operand::Move(l) => {
628 if let Some(value) = self.get_const(l) {
629 if self.should_const_prop(&value) {
630 // FIXME(felix91gr): this code only handles `Scalar` cases.
631 // For now, we're not handling `ScalarPair` cases because
632 // doing so here would require a lot of code duplication.
633 // We should hopefully generalize `Operand` handling into a fn,
634 // and use it to do const-prop here and everywhere else
635 // where it makes sense.
636 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
637 ScalarMaybeUninit::Scalar(scalar),
640 *operand = self.operand_from_scalar(
643 self.source_info.unwrap().span,
649 Operand::Constant(_) => (),
655 rvalue: &Rvalue<'tcx>,
656 source_info: SourceInfo,
659 // Perform any special handling for specific Rvalue types.
660 // Generally, checks here fall into one of two categories:
661 // 1. Additional checking to provide useful lints to the user
662 // - In this case, we will do some validation and then fall through to the
663 // end of the function which evals the assignment.
664 // 2. Working around bugs in other parts of the compiler
665 // - In this case, we'll return `None` from this function to stop evaluation.
667 // Additional checking: give lints to the user if an overflow would occur.
668 // We do this here and not in the `Assert` terminator as that terminator is
669 // only sometimes emitted (overflow checks can be disabled), but we want to always
671 Rvalue::UnaryOp(op, arg) => {
672 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
673 self.check_unary_op(*op, arg, source_info)?;
675 Rvalue::BinaryOp(op, box (left, right)) => {
676 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
677 self.check_binary_op(*op, left, right, source_info)?;
679 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
681 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
686 self.check_binary_op(*op, left, right, source_info)?;
689 // Do not try creating references (#67862)
690 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
691 trace!("skipping AddressOf | Ref for {:?}", place);
693 // This may be creating mutable references or immutable references to cells.
694 // If that happens, the pointed to value could be mutated via that reference.
695 // Since we aren't tracking references, the const propagator loses track of what
696 // value the local has right now.
697 // Thus, all locals that have their reference taken
698 // must not take part in propagation.
699 Self::remove_const(&mut self.ecx, place.local);
703 Rvalue::ThreadLocalRef(def_id) => {
704 trace!("skipping ThreadLocalRef({:?})", def_id);
709 // There's no other checking to do at this time.
710 Rvalue::Aggregate(..)
715 | Rvalue::Discriminant(..)
716 | Rvalue::NullaryOp(..) => {}
719 // FIXME we need to revisit this for #67176
720 if rvalue.needs_subst() {
724 if self.tcx.sess.mir_opt_level() >= 4 {
725 self.eval_rvalue_with_identities(rvalue, place)
727 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
731 // Attempt to use albegraic identities to eliminate constant expressions
732 fn eval_rvalue_with_identities(
734 rvalue: &Rvalue<'tcx>,
737 self.use_ecx(|this| {
739 Rvalue::BinaryOp(op, box (left, right))
740 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
741 let l = this.ecx.eval_operand(left, None);
742 let r = this.ecx.eval_operand(right, None);
744 let const_arg = match (l, r) {
745 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
746 (Err(e), Err(_)) => return Err(e),
748 this.ecx.eval_rvalue_into_place(rvalue, place)?;
754 this.ecx.force_bits(const_arg.to_scalar()?, const_arg.layout.size)?;
755 let dest = this.ecx.eval_place(place)?;
760 this.ecx.write_immediate(*const_arg, &dest)?;
764 if arg_value == const_arg.layout.size.truncate(u128::MAX)
765 || (const_arg.layout.ty.is_bool() && arg_value == 1)
767 this.ecx.write_immediate(*const_arg, &dest)?;
771 if const_arg.layout.ty.is_integral() && arg_value == 0 {
772 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
773 let val = Immediate::ScalarPair(
774 const_arg.to_scalar()?.into(),
775 Scalar::from_bool(false).into(),
777 this.ecx.write_immediate(val, &dest)?;
779 this.ecx.write_immediate(*const_arg, &dest)?;
784 this.ecx.eval_rvalue_into_place(rvalue, place)?;
789 this.ecx.eval_rvalue_into_place(rvalue, place)?;
797 /// Creates a new `Operand::Constant` from a `Scalar` value
798 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
799 Operand::Constant(Box::new(Constant {
802 literal: ty::Const::from_scalar(self.tcx, scalar, ty).into(),
806 fn replace_with_const(
808 rval: &mut Rvalue<'tcx>,
810 source_info: SourceInfo,
812 if let Rvalue::Use(Operand::Constant(c)) = rval {
814 ConstantKind::Ty(c) if matches!(c.val, ConstKind::Unevaluated(..)) => {}
816 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
822 trace!("attempting to replace {:?} with {:?}", rval, value);
823 if let Err(e) = self.ecx.const_validate_operand(
826 // FIXME: is ref tracking too expensive?
827 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
828 &mut interpret::RefTracking::empty(),
829 CtfeValidationMode::Regular,
831 trace!("validation error, attempt failed: {:?}", e);
835 // FIXME> figure out what to do when try_read_immediate fails
836 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
838 if let Some(Ok(imm)) = imm {
840 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
841 *rval = Rvalue::Use(self.operand_from_scalar(
847 Immediate::ScalarPair(
848 ScalarMaybeUninit::Scalar(_),
849 ScalarMaybeUninit::Scalar(_),
851 // Found a value represented as a pair. For now only do const-prop if the type
852 // of `rvalue` is also a tuple with two scalars.
853 // FIXME: enable the general case stated above ^.
854 let ty = &value.layout.ty;
855 // Only do it for tuples
856 if let ty::Tuple(substs) = ty.kind() {
857 // Only do it if tuple is also a pair with two scalars
858 if substs.len() == 2 {
859 let alloc = self.use_ecx(|this| {
860 let ty1 = substs[0].expect_ty();
861 let ty2 = substs[1].expect_ty();
862 let ty_is_scalar = |ty| {
863 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
866 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
869 .intern_with_temp_alloc(value.layout, |ecx, dest| {
870 ecx.write_immediate_to_mplace(*imm, dest)
879 if let Some(Some(alloc)) = alloc {
880 // Assign entire constant in a single statement.
881 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
882 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
883 span: source_info.span,
888 .mk_const(ty::Const {
890 val: ty::ConstKind::Value(ConstValue::ByRef {
901 // Scalars or scalar pairs that contain undef values are assumed to not have
902 // successfully evaluated and are thus not propagated.
908 /// Returns `true` if and only if this `op` should be const-propagated into.
909 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
910 let mir_opt_level = self.tcx.sess.mir_opt_level();
912 if mir_opt_level == 0 {
916 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
921 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
924 interpret::Operand::Immediate(Immediate::ScalarPair(
925 ScalarMaybeUninit::Scalar(l),
926 ScalarMaybeUninit::Scalar(r),
927 )) => l.is_bits() && r.is_bits(),
933 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
934 #[derive(Clone, Copy, Debug, PartialEq)]
936 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
938 /// The `Local` can only be propagated into and from its own block.
940 /// The `Local` can be propagated into but reads cannot be propagated.
942 /// The `Local` cannot be part of propagation at all. Any statement
943 /// referencing it either for reading or writing will not get propagated.
947 struct CanConstProp {
948 can_const_prop: IndexVec<Local, ConstPropMode>,
949 // False at the beginning. Once set, no more assignments are allowed to that local.
950 found_assignment: BitSet<Local>,
951 // Cache of locals' information
952 local_kinds: IndexVec<Local, LocalKind>,
956 /// Returns true if `local` can be propagated
959 param_env: ParamEnv<'tcx>,
961 ) -> IndexVec<Local, ConstPropMode> {
962 let mut cpv = CanConstProp {
963 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
964 found_assignment: BitSet::new_empty(body.local_decls.len()),
965 local_kinds: IndexVec::from_fn_n(
966 |local| body.local_kind(local),
967 body.local_decls.len(),
970 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
971 let ty = body.local_decls[local].ty;
972 match tcx.layout_of(param_env.and(ty)) {
973 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
974 // Either the layout fails to compute, then we can't use this local anyway
975 // or the local is too large, then we don't want to.
977 *val = ConstPropMode::NoPropagation;
981 // Cannot use args at all
982 // Cannot use locals because if x < y { y - x } else { x - y } would
984 // FIXME(oli-obk): lint variables until they are used in a condition
985 // FIXME(oli-obk): lint if return value is constant
986 if cpv.local_kinds[local] == LocalKind::Arg {
987 *val = ConstPropMode::OnlyPropagateInto;
989 "local {:?} can't be const propagated because it's a function argument",
992 } else if cpv.local_kinds[local] == LocalKind::Var {
993 *val = ConstPropMode::OnlyInsideOwnBlock;
995 "local {:?} will only be propagated inside its block, because it's a user variable",
1000 cpv.visit_body(&body);
1005 impl<'tcx> Visitor<'tcx> for CanConstProp {
1006 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
1007 use rustc_middle::mir::visit::PlaceContext::*;
1009 // Projections are fine, because `&mut foo.x` will be caught by
1010 // `MutatingUseContext::Borrow` elsewhere.
1011 MutatingUse(MutatingUseContext::Projection)
1012 // These are just stores, where the storing is not propagatable, but there may be later
1013 // mutations of the same local via `Store`
1014 | MutatingUse(MutatingUseContext::Call)
1015 // Actual store that can possibly even propagate a value
1016 | MutatingUse(MutatingUseContext::Store) => {
1017 if !self.found_assignment.insert(local) {
1018 match &mut self.can_const_prop[local] {
1019 // If the local can only get propagated in its own block, then we don't have
1020 // to worry about multiple assignments, as we'll nuke the const state at the
1021 // end of the block anyway, and inside the block we overwrite previous
1022 // states as applicable.
1023 ConstPropMode::OnlyInsideOwnBlock => {}
1024 ConstPropMode::NoPropagation => {}
1025 ConstPropMode::OnlyPropagateInto => {}
1026 other @ ConstPropMode::FullConstProp => {
1028 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
1031 *other = ConstPropMode::OnlyInsideOwnBlock;
1036 // Reading constants is allowed an arbitrary number of times
1037 NonMutatingUse(NonMutatingUseContext::Copy)
1038 | NonMutatingUse(NonMutatingUseContext::Move)
1039 | NonMutatingUse(NonMutatingUseContext::Inspect)
1040 | NonMutatingUse(NonMutatingUseContext::Projection)
1043 // These could be propagated with a smarter analysis or just some careful thinking about
1044 // whether they'd be fine right now.
1045 MutatingUse(MutatingUseContext::AsmOutput)
1046 | MutatingUse(MutatingUseContext::Yield)
1047 | MutatingUse(MutatingUseContext::Drop)
1048 | MutatingUse(MutatingUseContext::Retag)
1049 // These can't ever be propagated under any scheme, as we can't reason about indirect
1051 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1052 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1053 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1054 | NonMutatingUse(NonMutatingUseContext::AddressOf)
1055 | MutatingUse(MutatingUseContext::Borrow)
1056 | MutatingUse(MutatingUseContext::AddressOf) => {
1057 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1058 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1064 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
1065 fn tcx(&self) -> TyCtxt<'tcx> {
1069 fn visit_body(&mut self, body: &mut Body<'tcx>) {
1070 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1071 self.visit_basic_block_data(bb, data);
1075 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1076 self.super_operand(operand, location);
1078 // Only const prop copies and moves on `mir_opt_level=3` as doing so
1079 // currently slightly increases compile time in some cases.
1080 if self.tcx.sess.mir_opt_level() >= 3 {
1081 self.propagate_operand(operand)
1085 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1086 trace!("visit_constant: {:?}", constant);
1087 self.super_constant(constant, location);
1088 self.eval_constant(constant, self.source_info.unwrap());
1091 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1092 trace!("visit_statement: {:?}", statement);
1093 let source_info = statement.source_info;
1094 self.source_info = Some(source_info);
1095 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1096 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1097 if let Some(()) = self.const_prop(rval, source_info, place) {
1098 // This will return None if the above `const_prop` invocation only "wrote" a
1099 // type whose creation requires no write. E.g. a generator whose initial state
1100 // consists solely of uninitialized memory (so it doesn't capture any locals).
1101 if let Some(ref value) = self.get_const(place) {
1102 if self.should_const_prop(value) {
1103 trace!("replacing {:?} with {:?}", rval, value);
1104 self.replace_with_const(rval, value, source_info);
1105 if can_const_prop == ConstPropMode::FullConstProp
1106 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1108 trace!("propagated into {:?}", place);
1112 match can_const_prop {
1113 ConstPropMode::OnlyInsideOwnBlock => {
1115 "found local restricted to its block. \
1116 Will remove it from const-prop after block is finished. Local: {:?}",
1120 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1121 trace!("can't propagate into {:?}", place);
1122 if place.local != RETURN_PLACE {
1123 Self::remove_const(&mut self.ecx, place.local);
1126 ConstPropMode::FullConstProp => {}
1129 // Const prop failed, so erase the destination, ensuring that whatever happens
1130 // from here on, does not know about the previous value.
1131 // This is important in case we have
1134 // x = SOME_MUTABLE_STATIC;
1135 // // x must now be uninit
1137 // FIXME: we overzealously erase the entire local, because that's easier to
1140 "propagation into {:?} failed.
1141 Nuking the entire site from orbit, it's the only way to be sure",
1144 Self::remove_const(&mut self.ecx, place.local);
1147 match statement.kind {
1148 StatementKind::SetDiscriminant { ref place, .. } => {
1149 match self.ecx.machine.can_const_prop[place.local] {
1150 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1151 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1152 trace!("propped discriminant into {:?}", place);
1154 Self::remove_const(&mut self.ecx, place.local);
1157 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1158 Self::remove_const(&mut self.ecx, place.local);
1162 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1163 let frame = self.ecx.frame_mut();
1164 frame.locals[local].value =
1165 if let StatementKind::StorageLive(_) = statement.kind {
1166 LocalValue::Uninitialized
1175 self.super_statement(statement, location);
1178 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1179 let source_info = terminator.source_info;
1180 self.source_info = Some(source_info);
1181 self.super_terminator(terminator, location);
1182 match &mut terminator.kind {
1183 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1184 if let Some(ref value) = self.eval_operand(&cond, source_info) {
1185 trace!("assertion on {:?} should be {:?}", value, expected);
1186 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1187 let value_const = self.ecx.read_scalar(&value).unwrap();
1188 if expected != value_const {
1193 impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1194 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1196 Self::Val(val) => val.fmt(fmt),
1197 Self::Underscore => fmt.write_str("_"),
1201 let mut eval_to_int = |op| {
1202 // This can be `None` if the lhs wasn't const propagated and we just
1203 // triggered the assert on the value of the rhs.
1204 match self.eval_operand(op, source_info) {
1205 Some(op) => DbgVal::Val(
1206 self.ecx.read_immediate(&op).unwrap().to_const_int(),
1208 None => DbgVal::Underscore,
1211 let msg = match msg {
1212 AssertKind::DivisionByZero(op) => {
1213 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1215 AssertKind::RemainderByZero(op) => {
1216 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1218 AssertKind::BoundsCheck { ref len, ref index } => {
1219 let len = eval_to_int(len);
1220 let index = eval_to_int(index);
1221 Some(AssertKind::BoundsCheck { len, index })
1223 // Overflow is are already covered by checks on the binary operators.
1224 AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1225 // Need proper const propagator for these.
1228 // Poison all places this operand references so that further code
1229 // doesn't use the invalid value
1231 Operand::Move(ref place) | Operand::Copy(ref place) => {
1232 Self::remove_const(&mut self.ecx, place.local);
1234 Operand::Constant(_) => {}
1236 if let Some(msg) = msg {
1237 self.report_assert_as_lint(
1238 lint::builtin::UNCONDITIONAL_PANIC,
1240 "this operation will panic at runtime",
1245 if self.should_const_prop(value) {
1246 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1247 *cond = self.operand_from_scalar(
1249 self.tcx.types.bool,
1257 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1258 // FIXME: This is currently redundant with `visit_operand`, but sadly
1259 // always visiting operands currently causes a perf regression in LLVM codegen, so
1260 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1261 self.propagate_operand(discr)
1263 // None of these have Operands to const-propagate.
1264 TerminatorKind::Goto { .. }
1265 | TerminatorKind::Resume
1266 | TerminatorKind::Abort
1267 | TerminatorKind::Return
1268 | TerminatorKind::Unreachable
1269 | TerminatorKind::Drop { .. }
1270 | TerminatorKind::DropAndReplace { .. }
1271 | TerminatorKind::Yield { .. }
1272 | TerminatorKind::GeneratorDrop
1273 | TerminatorKind::FalseEdge { .. }
1274 | TerminatorKind::FalseUnwind { .. }
1275 | TerminatorKind::InlineAsm { .. } => {}
1276 // Every argument in our function calls have already been propagated in `visit_operand`.
1278 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1279 // gated on `mir_opt_level=3`.
1280 TerminatorKind::Call { .. } => {}
1283 // We remove all Locals which are restricted in propagation to their containing blocks and
1284 // which were modified in the current block.
1285 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1286 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1287 for &local in locals.iter() {
1288 Self::remove_const(&mut self.ecx, local);
1291 // Put it back so we reuse the heap of the storage
1292 self.ecx.machine.written_only_inside_own_block_locals = locals;
1293 if cfg!(debug_assertions) {
1294 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1295 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1297 self.get_const(local.into()).is_none()
1299 .layout_of(self.local_decls[local].ty)
1300 .map_or(true, |layout| layout.is_zst())