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;
9 use rustc_index::bit_set::BitSet;
10 use rustc_index::vec::IndexVec;
11 use rustc_middle::mir::visit::{
12 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
14 use rustc_middle::mir::{
15 BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
16 Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
19 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
20 use rustc_middle::ty::subst::{InternalSubsts, Subst};
21 use rustc_middle::ty::{
22 self, ConstKind, EarlyBinder, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable,
24 use rustc_span::{def_id::DefId, Span};
25 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
26 use rustc_target::spec::abi::Abi;
27 use rustc_trait_selection::traits;
30 use rustc_const_eval::interpret::{
31 self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
32 ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, MemoryKind, OpTy,
33 Operand as InterpOperand, PlaceTy, Pointer, Scalar, ScalarMaybeUninit, StackPopCleanup,
37 /// The maximum number of bytes that we'll allocate space for a local or the return value.
38 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
39 /// Severely regress performance.
40 const MAX_ALLOC_LIMIT: u64 = 1024;
42 /// Macro for machine-specific `InterpError` without allocation.
43 /// (These will never be shown to the user, but they help diagnose ICEs.)
44 macro_rules! throw_machine_stop_str {
46 // We make a new local type for it. The type itself does not carry any information,
47 // but its vtable (for the `MachineStopType` trait) does.
49 // Printing this type shows the desired string.
50 impl std::fmt::Display for Zst {
51 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
55 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
56 throw_machine_stop!(Zst)
62 impl<'tcx> MirPass<'tcx> for ConstProp {
63 fn is_enabled(&self, _sess: &rustc_session::Session) -> bool {
64 // FIXME(#70073): Unlike the other passes in "optimizations", this one emits errors, so it
65 // runs even when MIR optimizations are disabled. We should separate the lint out from the
66 // transform and move the lint as early in the pipeline as possible.
70 #[instrument(skip(self, tcx), level = "debug")]
71 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
72 // will be evaluated by miri and produce its errors there
73 if body.source.promoted.is_some() {
77 let def_id = body.source.def_id().expect_local();
78 let def_kind = tcx.def_kind(def_id);
79 let is_fn_like = def_kind.is_fn_like();
80 let is_assoc_const = def_kind == DefKind::AssocConst;
82 // Only run const prop on functions, methods, closures and associated constants
83 if !is_fn_like && !is_assoc_const {
84 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
85 trace!("ConstProp skipped for {:?}", def_id);
89 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
90 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
91 // computing their layout.
93 trace!("ConstProp skipped for generator {:?}", def_id);
97 // Check if it's even possible to satisfy the 'where' clauses
99 // This branch will never be taken for any normal function.
100 // However, it's possible to `#!feature(trivial_bounds)]` to write
101 // a function with impossible to satisfy clauses, e.g.:
102 // `fn foo() where String: Copy {}`
104 // We don't usually need to worry about this kind of case,
105 // since we would get a compilation error if the user tried
106 // to call it. However, since we can do const propagation
107 // even without any calls to the function, we need to make
108 // sure that it even makes sense to try to evaluate the body.
109 // If there are unsatisfiable where clauses, then all bets are
110 // off, and we just give up.
112 // We manually filter the predicates, skipping anything that's not
113 // "global". We are in a potentially generic context
114 // (e.g. we are evaluating a function without substituting generic
115 // parameters, so this filtering serves two purposes:
117 // 1. We skip evaluating any predicates that we would
118 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
119 // 2. We avoid trying to normalize predicates involving generic
120 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
121 // the normalization code (leading to cycle errors), since
122 // it's usually never invoked in this way.
124 .predicates_of(def_id.to_def_id())
127 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
128 if traits::impossible_predicates(
130 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
132 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
136 trace!("ConstProp starting for {:?}", def_id);
138 let dummy_body = &Body::new(
140 body.basic_blocks().clone(),
141 body.source_scopes.clone(),
142 body.local_decls.clone(),
147 body.generator_kind(),
148 body.tainted_by_errors,
151 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
152 // constants, instead of just checking for const-folding succeeding.
153 // That would require a uniform one-def no-mutation analysis
154 // and RPO (or recursing when needing the value of a local).
155 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
156 optimization_finder.visit_body(body);
158 trace!("ConstProp done for {:?}", def_id);
162 struct ConstPropMachine<'mir, 'tcx> {
163 /// The virtual call stack.
164 stack: Vec<Frame<'mir, 'tcx>>,
165 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
166 written_only_inside_own_block_locals: FxHashSet<Local>,
167 /// Locals that need to be cleared after every block terminates.
168 only_propagate_inside_block_locals: BitSet<Local>,
169 can_const_prop: IndexVec<Local, ConstPropMode>,
172 impl ConstPropMachine<'_, '_> {
174 only_propagate_inside_block_locals: BitSet<Local>,
175 can_const_prop: IndexVec<Local, ConstPropMode>,
179 written_only_inside_own_block_locals: Default::default(),
180 only_propagate_inside_block_locals,
186 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
187 compile_time_machine!(<'mir, 'tcx>);
188 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
193 _ecx: &InterpCx<'mir, 'tcx, Self>,
194 _instance: ty::InstanceDef<'tcx>,
195 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
196 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
199 fn find_mir_or_eval_fn(
200 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
201 _instance: ty::Instance<'tcx>,
203 _args: &[OpTy<'tcx>],
204 _destination: &PlaceTy<'tcx>,
205 _target: Option<BasicBlock>,
206 _unwind: StackPopUnwind,
207 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
212 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
213 _instance: ty::Instance<'tcx>,
214 _args: &[OpTy<'tcx>],
215 _destination: &PlaceTy<'tcx>,
216 _target: Option<BasicBlock>,
217 _unwind: StackPopUnwind,
218 ) -> InterpResult<'tcx> {
219 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
223 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
224 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
225 _unwind: Option<rustc_middle::mir::BasicBlock>,
226 ) -> InterpResult<'tcx> {
227 bug!("panics terminators are not evaluated in ConstProp")
231 _ecx: &InterpCx<'mir, 'tcx, Self>,
234 _right: &ImmTy<'tcx>,
235 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
236 // We can't do this because aliasing of memory can differ between const eval and llvm
237 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
241 _ecx: &InterpCx<'mir, 'tcx, Self>,
242 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
244 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
245 let l = &frame.locals[local];
247 if l.value == LocalValue::Unallocated {
248 throw_machine_stop_str!("tried to access an unallocated local")
254 fn access_local_mut<'a>(
255 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
258 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
260 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
261 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
263 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
265 "mutating local {:?} which is restricted to its block. \
266 Will remove it from const-prop after block is finished.",
269 ecx.machine.written_only_inside_own_block_locals.insert(local);
271 ecx.machine.stack[frame].locals[local].access_mut()
274 fn before_access_global(
278 alloc: ConstAllocation<'tcx, Self::PointerTag, Self::AllocExtra>,
279 _static_def_id: Option<DefId>,
281 ) -> InterpResult<'tcx> {
283 throw_machine_stop_str!("can't write to global");
285 // If the static allocation is mutable, then we can't const prop it as its content
286 // might be different at runtime.
287 if alloc.inner().mutability == Mutability::Mut {
288 throw_machine_stop_str!("can't access mutable globals in ConstProp");
296 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
297 _ptr: Pointer<AllocId>,
298 ) -> InterpResult<'tcx> {
299 throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
304 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
305 frame: Frame<'mir, 'tcx>,
306 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
312 ecx: &'a InterpCx<'mir, 'tcx, Self>,
313 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
319 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
320 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
321 &mut ecx.machine.stack
325 /// Finds optimization opportunities on the MIR.
326 struct ConstPropagator<'mir, 'tcx> {
327 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
329 param_env: ParamEnv<'tcx>,
330 local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
331 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
332 // the last known `SourceInfo` here and just keep revisiting it.
333 source_info: Option<SourceInfo>,
336 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
337 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
340 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
345 impl HasDataLayout for ConstPropagator<'_, '_> {
347 fn data_layout(&self) -> &TargetDataLayout {
348 &self.tcx.data_layout
352 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
354 fn tcx(&self) -> TyCtxt<'tcx> {
359 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
361 fn param_env(&self) -> ty::ParamEnv<'tcx> {
366 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
369 dummy_body: &'mir Body<'tcx>,
371 ) -> ConstPropagator<'mir, 'tcx> {
372 let def_id = body.source.def_id();
373 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
374 let param_env = tcx.param_env_reveal_all_normalized(def_id);
376 let can_const_prop = CanConstProp::check(tcx, param_env, body);
377 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
378 for (l, mode) in can_const_prop.iter_enumerated() {
379 if *mode == ConstPropMode::OnlyInsideOwnBlock {
380 only_propagate_inside_block_locals.insert(l);
383 let mut ecx = InterpCx::new(
385 tcx.def_span(def_id),
387 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
391 .layout_of(EarlyBinder(body.return_ty()).subst(tcx, substs))
393 // Don't bother allocating memory for large values.
394 .filter(|ret_layout| ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT))
395 .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
398 .allocate(ret_layout, MemoryKind::Stack)
399 .expect("couldn't perform small allocation")
402 ecx.push_stack_frame(
403 Instance::new(def_id, substs),
406 StackPopCleanup::Root { cleanup: false },
408 .expect("failed to push initial stack frame");
414 local_decls: &dummy_body.local_decls,
419 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
420 let op = match self.ecx.eval_place_to_op(place, None) {
423 trace!("get_const failed: {}", e);
428 // Try to read the local as an immediate so that if it is representable as a scalar, we can
429 // handle it as such, but otherwise, just return the value as is.
430 Some(match self.ecx.read_immediate_raw(&op, /*force*/ false) {
431 Ok(Ok(imm)) => imm.into(),
436 /// Remove `local` from the pool of `Locals`. Allows writing to them,
437 /// but not reading from them anymore.
438 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
439 ecx.frame_mut().locals[local] =
440 LocalState { value: LocalValue::Unallocated, layout: Cell::new(None) };
443 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
445 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
448 Ok(val) => Some(val),
450 trace!("InterpCx operation failed: {:?}", error);
451 // Some errors shouldn't come up because creating them causes
452 // an allocation, which we should avoid. When that happens,
453 // dedicated error variants should be introduced instead.
455 !error.kind().formatted_string(),
456 "const-prop encountered formatting error: {}",
464 /// Returns the value, if any, of evaluating `c`.
465 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
466 // FIXME we need to revisit this for #67176
471 self.ecx.mir_const_to_op(&c.literal, None).ok()
474 /// Returns the value, if any, of evaluating `place`.
475 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
476 trace!("eval_place(place={:?})", place);
477 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
480 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
481 /// or `eval_place`, depending on the variant of `Operand` used.
482 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
484 Operand::Constant(ref c) => self.eval_constant(c),
485 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
489 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
490 if self.use_ecx(|this| {
491 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
492 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
495 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
496 // appropriate to use.
497 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
507 left: &Operand<'tcx>,
508 right: &Operand<'tcx>,
510 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
511 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
512 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
513 if op == BinOp::Shr || op == BinOp::Shl {
515 // We need the type of the LHS. We cannot use `place_layout` as that is the type
516 // of the result, which for checked binops is not the same!
517 let left_ty = left.ty(self.local_decls, self.tcx);
518 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
519 let right_size = r.layout.size;
520 let r_bits = r.to_scalar().ok();
521 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
522 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
527 if let (Some(l), Some(r)) = (&l, &r) {
528 // The remaining operators are handled through `overflowing_binary_op`.
529 if self.use_ecx(|this| {
530 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
539 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
541 Operand::Copy(l) | Operand::Move(l) => {
542 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
543 // FIXME(felix91gr): this code only handles `Scalar` cases.
544 // For now, we're not handling `ScalarPair` cases because
545 // doing so here would require a lot of code duplication.
546 // We should hopefully generalize `Operand` handling into a fn,
547 // and use it to do const-prop here and everywhere else
548 // where it makes sense.
549 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
550 ScalarMaybeUninit::Scalar(scalar),
553 *operand = self.operand_from_scalar(
556 self.source_info.unwrap().span,
561 Operand::Constant(_) => (),
565 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
566 // Perform any special handling for specific Rvalue types.
567 // Generally, checks here fall into one of two categories:
568 // 1. Additional checking to provide useful lints to the user
569 // - In this case, we will do some validation and then fall through to the
570 // end of the function which evals the assignment.
571 // 2. Working around bugs in other parts of the compiler
572 // - In this case, we'll return `None` from this function to stop evaluation.
574 // Additional checking: give lints to the user if an overflow would occur.
575 // We do this here and not in the `Assert` terminator as that terminator is
576 // only sometimes emitted (overflow checks can be disabled), but we want to always
578 Rvalue::UnaryOp(op, arg) => {
579 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
580 self.check_unary_op(*op, arg)?;
582 Rvalue::BinaryOp(op, box (left, right)) => {
583 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
584 self.check_binary_op(*op, left, right)?;
586 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
588 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
593 self.check_binary_op(*op, left, right)?;
596 // Do not try creating references (#67862)
597 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
598 trace!("skipping AddressOf | Ref for {:?}", place);
600 // This may be creating mutable references or immutable references to cells.
601 // If that happens, the pointed to value could be mutated via that reference.
602 // Since we aren't tracking references, the const propagator loses track of what
603 // value the local has right now.
604 // Thus, all locals that have their reference taken
605 // must not take part in propagation.
606 Self::remove_const(&mut self.ecx, place.local);
610 Rvalue::ThreadLocalRef(def_id) => {
611 trace!("skipping ThreadLocalRef({:?})", def_id);
616 // There's no other checking to do at this time.
617 Rvalue::Aggregate(..)
622 | Rvalue::ShallowInitBox(..)
623 | Rvalue::Discriminant(..)
624 | Rvalue::NullaryOp(..) => {}
627 // FIXME we need to revisit this for #67176
628 if rvalue.needs_subst() {
632 if self.tcx.sess.mir_opt_level() >= 4 {
633 self.eval_rvalue_with_identities(rvalue, place)
635 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
639 // Attempt to use algebraic identities to eliminate constant expressions
640 fn eval_rvalue_with_identities(
642 rvalue: &Rvalue<'tcx>,
645 self.use_ecx(|this| match rvalue {
646 Rvalue::BinaryOp(op, box (left, right))
647 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
648 let l = this.ecx.eval_operand(left, None);
649 let r = this.ecx.eval_operand(right, None);
651 let const_arg = match (l, r) {
652 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
653 (Err(e), Err(_)) => return Err(e),
654 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
657 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
658 let dest = this.ecx.eval_place(place)?;
661 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
663 if arg_value == const_arg.layout.size.truncate(u128::MAX)
664 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
666 this.ecx.write_immediate(*const_arg, &dest)
668 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
669 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
670 let val = Immediate::ScalarPair(
671 const_arg.to_scalar()?.into(),
672 Scalar::from_bool(false).into(),
674 this.ecx.write_immediate(val, &dest)
676 this.ecx.write_immediate(*const_arg, &dest)
679 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
682 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
686 /// Creates a new `Operand::Constant` from a `Scalar` value
687 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
688 Operand::Constant(Box::new(Constant {
691 literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
695 fn replace_with_const(
697 rval: &mut Rvalue<'tcx>,
699 source_info: SourceInfo,
701 if let Rvalue::Use(Operand::Constant(c)) = rval {
703 ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
705 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
711 trace!("attempting to replace {:?} with {:?}", rval, value);
712 if let Err(e) = self.ecx.const_validate_operand(
715 // FIXME: is ref tracking too expensive?
716 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
717 &mut interpret::RefTracking::empty(),
718 CtfeValidationMode::Regular,
720 trace!("validation error, attempt failed: {:?}", e);
724 // FIXME> figure out what to do when read_immediate_raw fails
725 let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value, /*force*/ false));
727 if let Some(Ok(imm)) = imm {
729 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
730 *rval = Rvalue::Use(self.operand_from_scalar(
736 Immediate::ScalarPair(
737 ScalarMaybeUninit::Scalar(_),
738 ScalarMaybeUninit::Scalar(_),
740 // Found a value represented as a pair. For now only do const-prop if the type
741 // of `rvalue` is also a tuple with two scalars.
742 // FIXME: enable the general case stated above ^.
743 let ty = value.layout.ty;
744 // Only do it for tuples
745 if let ty::Tuple(types) = ty.kind() {
746 // Only do it if tuple is also a pair with two scalars
747 if let [ty1, ty2] = types[..] {
748 let alloc = self.use_ecx(|this| {
749 let ty_is_scalar = |ty| {
750 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
753 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
756 .intern_with_temp_alloc(value.layout, |ecx, dest| {
757 ecx.write_immediate(*imm, dest)
766 if let Some(Some(alloc)) = alloc {
767 // Assign entire constant in a single statement.
768 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
769 let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
770 let literal = ConstantKind::Val(const_val, ty);
771 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
772 span: source_info.span,
780 // Scalars or scalar pairs that contain undef values are assumed to not have
781 // successfully evaluated and are thus not propagated.
787 /// Returns `true` if and only if this `op` should be const-propagated into.
788 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
789 let mir_opt_level = self.tcx.sess.mir_opt_level();
791 if mir_opt_level == 0 {
795 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
800 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
801 s.try_to_int().is_ok()
803 interpret::Operand::Immediate(Immediate::ScalarPair(
804 ScalarMaybeUninit::Scalar(l),
805 ScalarMaybeUninit::Scalar(r),
806 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
812 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
813 #[derive(Clone, Copy, Debug, PartialEq)]
815 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
817 /// The `Local` can only be propagated into and from its own block.
819 /// The `Local` can be propagated into but reads cannot be propagated.
821 /// The `Local` cannot be part of propagation at all. Any statement
822 /// referencing it either for reading or writing will not get propagated.
826 struct CanConstProp {
827 can_const_prop: IndexVec<Local, ConstPropMode>,
828 // False at the beginning. Once set, no more assignments are allowed to that local.
829 found_assignment: BitSet<Local>,
830 // Cache of locals' information
831 local_kinds: IndexVec<Local, LocalKind>,
835 /// Returns true if `local` can be propagated
838 param_env: ParamEnv<'tcx>,
840 ) -> IndexVec<Local, ConstPropMode> {
841 let mut cpv = CanConstProp {
842 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
843 found_assignment: BitSet::new_empty(body.local_decls.len()),
844 local_kinds: IndexVec::from_fn_n(
845 |local| body.local_kind(local),
846 body.local_decls.len(),
849 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
850 let ty = body.local_decls[local].ty;
851 match tcx.layout_of(param_env.and(ty)) {
852 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
853 // Either the layout fails to compute, then we can't use this local anyway
854 // or the local is too large, then we don't want to.
856 *val = ConstPropMode::NoPropagation;
860 // Cannot use args at all
861 // Cannot use locals because if x < y { y - x } else { x - y } would
863 // FIXME(oli-obk): lint variables until they are used in a condition
864 // FIXME(oli-obk): lint if return value is constant
865 if cpv.local_kinds[local] == LocalKind::Arg {
866 *val = ConstPropMode::OnlyPropagateInto;
868 "local {:?} can't be const propagated because it's a function argument",
871 } else if cpv.local_kinds[local] == LocalKind::Var {
872 *val = ConstPropMode::OnlyInsideOwnBlock;
874 "local {:?} will only be propagated inside its block, because it's a user variable",
879 cpv.visit_body(&body);
884 impl Visitor<'_> for CanConstProp {
885 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
886 use rustc_middle::mir::visit::PlaceContext::*;
888 // Projections are fine, because `&mut foo.x` will be caught by
889 // `MutatingUseContext::Borrow` elsewhere.
890 MutatingUse(MutatingUseContext::Projection)
891 // These are just stores, where the storing is not propagatable, but there may be later
892 // mutations of the same local via `Store`
893 | MutatingUse(MutatingUseContext::Call)
894 | MutatingUse(MutatingUseContext::AsmOutput)
895 | MutatingUse(MutatingUseContext::Deinit)
896 // Actual store that can possibly even propagate a value
897 | MutatingUse(MutatingUseContext::Store)
898 | MutatingUse(MutatingUseContext::SetDiscriminant) => {
899 if !self.found_assignment.insert(local) {
900 match &mut self.can_const_prop[local] {
901 // If the local can only get propagated in its own block, then we don't have
902 // to worry about multiple assignments, as we'll nuke the const state at the
903 // end of the block anyway, and inside the block we overwrite previous
904 // states as applicable.
905 ConstPropMode::OnlyInsideOwnBlock => {}
906 ConstPropMode::NoPropagation => {}
907 ConstPropMode::OnlyPropagateInto => {}
908 other @ ConstPropMode::FullConstProp => {
910 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
913 *other = ConstPropMode::OnlyInsideOwnBlock;
918 // Reading constants is allowed an arbitrary number of times
919 NonMutatingUse(NonMutatingUseContext::Copy)
920 | NonMutatingUse(NonMutatingUseContext::Move)
921 | NonMutatingUse(NonMutatingUseContext::Inspect)
922 | NonMutatingUse(NonMutatingUseContext::Projection)
925 // These could be propagated with a smarter analysis or just some careful thinking about
926 // whether they'd be fine right now.
927 MutatingUse(MutatingUseContext::Yield)
928 | MutatingUse(MutatingUseContext::Drop)
929 | MutatingUse(MutatingUseContext::Retag)
930 // These can't ever be propagated under any scheme, as we can't reason about indirect
932 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
933 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
934 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
935 | NonMutatingUse(NonMutatingUseContext::AddressOf)
936 | MutatingUse(MutatingUseContext::Borrow)
937 | MutatingUse(MutatingUseContext::AddressOf) => {
938 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
939 self.can_const_prop[local] = ConstPropMode::NoPropagation;
945 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
946 fn tcx(&self) -> TyCtxt<'tcx> {
950 fn visit_body(&mut self, body: &mut Body<'tcx>) {
951 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
952 self.visit_basic_block_data(bb, data);
956 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
957 self.super_operand(operand, location);
959 // Only const prop copies and moves on `mir_opt_level=3` as doing so
960 // currently slightly increases compile time in some cases.
961 if self.tcx.sess.mir_opt_level() >= 3 {
962 self.propagate_operand(operand)
966 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
967 trace!("visit_constant: {:?}", constant);
968 self.super_constant(constant, location);
969 self.eval_constant(constant);
972 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
973 trace!("visit_statement: {:?}", statement);
974 let source_info = statement.source_info;
975 self.source_info = Some(source_info);
976 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
977 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
978 if let Some(()) = self.const_prop(rval, place) {
979 // This will return None if the above `const_prop` invocation only "wrote" a
980 // type whose creation requires no write. E.g. a generator whose initial state
981 // consists solely of uninitialized memory (so it doesn't capture any locals).
982 if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
983 trace!("replacing {:?} with {:?}", rval, value);
984 self.replace_with_const(rval, value, source_info);
985 if can_const_prop == ConstPropMode::FullConstProp
986 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
988 trace!("propagated into {:?}", place);
991 match can_const_prop {
992 ConstPropMode::OnlyInsideOwnBlock => {
994 "found local restricted to its block. \
995 Will remove it from const-prop after block is finished. Local: {:?}",
999 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1000 trace!("can't propagate into {:?}", place);
1001 if place.local != RETURN_PLACE {
1002 Self::remove_const(&mut self.ecx, place.local);
1005 ConstPropMode::FullConstProp => {}
1008 // Const prop failed, so erase the destination, ensuring that whatever happens
1009 // from here on, does not know about the previous value.
1010 // This is important in case we have
1013 // x = SOME_MUTABLE_STATIC;
1014 // // x must now be uninit
1016 // FIXME: we overzealously erase the entire local, because that's easier to
1019 "propagation into {:?} failed.
1020 Nuking the entire site from orbit, it's the only way to be sure",
1023 Self::remove_const(&mut self.ecx, place.local);
1026 match statement.kind {
1027 StatementKind::SetDiscriminant { ref place, .. } => {
1028 match self.ecx.machine.can_const_prop[place.local] {
1029 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1030 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1031 trace!("propped discriminant into {:?}", place);
1033 Self::remove_const(&mut self.ecx, place.local);
1036 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1037 Self::remove_const(&mut self.ecx, place.local);
1041 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1042 let frame = self.ecx.frame_mut();
1043 frame.locals[local].value =
1044 if let StatementKind::StorageLive(_) = statement.kind {
1045 LocalValue::Unallocated
1054 self.super_statement(statement, location);
1057 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1058 let source_info = terminator.source_info;
1059 self.source_info = Some(source_info);
1060 self.super_terminator(terminator, location);
1061 match &mut terminator.kind {
1062 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1063 if let Some(ref value) = self.eval_operand(&cond) {
1064 trace!("assertion on {:?} should be {:?}", value, expected);
1065 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1066 let value_const = self.ecx.read_scalar(&value).unwrap();
1067 if expected != value_const {
1068 // Poison all places this operand references so that further code
1069 // doesn't use the invalid value
1071 Operand::Move(ref place) | Operand::Copy(ref place) => {
1072 Self::remove_const(&mut self.ecx, place.local);
1074 Operand::Constant(_) => {}
1077 if self.should_const_prop(value) {
1078 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1079 *cond = self.operand_from_scalar(
1081 self.tcx.types.bool,
1089 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1090 // FIXME: This is currently redundant with `visit_operand`, but sadly
1091 // always visiting operands currently causes a perf regression in LLVM codegen, so
1092 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1093 self.propagate_operand(discr)
1095 // None of these have Operands to const-propagate.
1096 TerminatorKind::Goto { .. }
1097 | TerminatorKind::Resume
1098 | TerminatorKind::Abort
1099 | TerminatorKind::Return
1100 | TerminatorKind::Unreachable
1101 | TerminatorKind::Drop { .. }
1102 | TerminatorKind::DropAndReplace { .. }
1103 | TerminatorKind::Yield { .. }
1104 | TerminatorKind::GeneratorDrop
1105 | TerminatorKind::FalseEdge { .. }
1106 | TerminatorKind::FalseUnwind { .. }
1107 | TerminatorKind::InlineAsm { .. } => {}
1108 // Every argument in our function calls have already been propagated in `visit_operand`.
1110 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1111 // gated on `mir_opt_level=3`.
1112 TerminatorKind::Call { .. } => {}
1115 // We remove all Locals which are restricted in propagation to their containing blocks and
1116 // which were modified in the current block.
1117 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1118 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1119 for &local in locals.iter() {
1120 Self::remove_const(&mut self.ecx, local);
1123 // Put it back so we reuse the heap of the storage
1124 self.ecx.machine.written_only_inside_own_block_locals = locals;
1125 if cfg!(debug_assertions) {
1126 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1127 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1129 self.get_const(local.into()).is_none()
1131 .layout_of(self.local_decls[local].ty)
1132 .map_or(true, |layout| layout.is_zst())