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, TypeVisitable,
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, MemoryKind, OpTy, PlaceTy,
33 Pointer, Scalar, ScalarMaybeUninit, StackPopCleanup, StackPopUnwind,
36 /// The maximum number of bytes that we'll allocate space for a local or the return value.
37 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
38 /// Severely regress performance.
39 const MAX_ALLOC_LIMIT: u64 = 1024;
41 /// Macro for machine-specific `InterpError` without allocation.
42 /// (These will never be shown to the user, but they help diagnose ICEs.)
43 macro_rules! throw_machine_stop_str {
45 // We make a new local type for it. The type itself does not carry any information,
46 // but its vtable (for the `MachineStopType` trait) does.
48 // Printing this type shows the desired string.
49 impl std::fmt::Display for Zst {
50 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
54 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
55 throw_machine_stop!(Zst)
61 impl<'tcx> MirPass<'tcx> for ConstProp {
62 fn is_enabled(&self, _sess: &rustc_session::Session) -> bool {
63 // FIXME(#70073): Unlike the other passes in "optimizations", this one emits errors, so it
64 // runs even when MIR optimizations are disabled. We should separate the lint out from the
65 // transform and move the lint as early in the pipeline as possible.
69 #[instrument(skip(self, tcx), level = "debug")]
70 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
71 // will be evaluated by miri and produce its errors there
72 if body.source.promoted.is_some() {
76 let def_id = body.source.def_id().expect_local();
77 let def_kind = tcx.def_kind(def_id);
78 let is_fn_like = def_kind.is_fn_like();
79 let is_assoc_const = def_kind == DefKind::AssocConst;
81 // Only run const prop on functions, methods, closures and associated constants
82 if !is_fn_like && !is_assoc_const {
83 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
84 trace!("ConstProp skipped for {:?}", def_id);
88 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
89 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
90 // computing their layout.
92 trace!("ConstProp skipped for generator {:?}", def_id);
96 // Check if it's even possible to satisfy the 'where' clauses
98 // This branch will never be taken for any normal function.
99 // However, it's possible to `#!feature(trivial_bounds)]` to write
100 // a function with impossible to satisfy clauses, e.g.:
101 // `fn foo() where String: Copy {}`
103 // We don't usually need to worry about this kind of case,
104 // since we would get a compilation error if the user tried
105 // to call it. However, since we can do const propagation
106 // even without any calls to the function, we need to make
107 // sure that it even makes sense to try to evaluate the body.
108 // If there are unsatisfiable where clauses, then all bets are
109 // off, and we just give up.
111 // We manually filter the predicates, skipping anything that's not
112 // "global". We are in a potentially generic context
113 // (e.g. we are evaluating a function without substituting generic
114 // parameters, so this filtering serves two purposes:
116 // 1. We skip evaluating any predicates that we would
117 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
118 // 2. We avoid trying to normalize predicates involving generic
119 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
120 // the normalization code (leading to cycle errors), since
121 // it's usually never invoked in this way.
123 .predicates_of(def_id.to_def_id())
126 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
127 if traits::impossible_predicates(
129 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
131 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
135 trace!("ConstProp starting for {:?}", def_id);
137 let dummy_body = &Body::new(
139 body.basic_blocks().clone(),
140 body.source_scopes.clone(),
141 body.local_decls.clone(),
146 body.generator_kind(),
147 body.tainted_by_errors,
150 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
151 // constants, instead of just checking for const-folding succeeding.
152 // That would require a uniform one-def no-mutation analysis
153 // and RPO (or recursing when needing the value of a local).
154 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
155 optimization_finder.visit_body(body);
157 trace!("ConstProp done for {:?}", def_id);
161 struct ConstPropMachine<'mir, 'tcx> {
162 /// The virtual call stack.
163 stack: Vec<Frame<'mir, 'tcx>>,
164 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
165 written_only_inside_own_block_locals: FxHashSet<Local>,
166 /// Locals that need to be cleared after every block terminates.
167 only_propagate_inside_block_locals: BitSet<Local>,
168 can_const_prop: IndexVec<Local, ConstPropMode>,
171 impl ConstPropMachine<'_, '_> {
173 only_propagate_inside_block_locals: BitSet<Local>,
174 can_const_prop: IndexVec<Local, ConstPropMode>,
178 written_only_inside_own_block_locals: Default::default(),
179 only_propagate_inside_block_locals,
185 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
186 compile_time_machine!(<'mir, 'tcx>);
187 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
192 _ecx: &InterpCx<'mir, 'tcx, Self>,
193 _instance: ty::InstanceDef<'tcx>,
194 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
195 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
198 fn find_mir_or_eval_fn(
199 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
200 _instance: ty::Instance<'tcx>,
202 _args: &[OpTy<'tcx>],
203 _destination: &PlaceTy<'tcx>,
204 _target: Option<BasicBlock>,
205 _unwind: StackPopUnwind,
206 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
211 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
212 _instance: ty::Instance<'tcx>,
213 _args: &[OpTy<'tcx>],
214 _destination: &PlaceTy<'tcx>,
215 _target: Option<BasicBlock>,
216 _unwind: StackPopUnwind,
217 ) -> InterpResult<'tcx> {
218 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
222 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
223 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
224 _unwind: Option<rustc_middle::mir::BasicBlock>,
225 ) -> InterpResult<'tcx> {
226 bug!("panics terminators are not evaluated in ConstProp")
230 _ecx: &InterpCx<'mir, 'tcx, Self>,
233 _right: &ImmTy<'tcx>,
234 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
235 // We can't do this because aliasing of memory can differ between const eval and llvm
236 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
240 frame: &'a Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
242 ) -> InterpResult<'tcx, &'a interpret::Operand<Self::PointerTag>> {
243 let l = &frame.locals[local];
247 LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit))
249 // For us "uninit" means "we don't know its value, might be initiailized or not".
251 throw_machine_stop_str!("tried to access alocal with unknown value ")
257 fn access_local_mut<'a>(
258 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
261 ) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::PointerTag>> {
262 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
263 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
265 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
267 "mutating local {:?} which is restricted to its block. \
268 Will remove it from const-prop after block is finished.",
271 ecx.machine.written_only_inside_own_block_locals.insert(local);
273 ecx.machine.stack[frame].locals[local].access_mut()
276 fn before_access_global(
280 alloc: ConstAllocation<'tcx, Self::PointerTag, Self::AllocExtra>,
281 _static_def_id: Option<DefId>,
283 ) -> InterpResult<'tcx> {
285 throw_machine_stop_str!("can't write to global");
287 // If the static allocation is mutable, then we can't const prop it as its content
288 // might be different at runtime.
289 if alloc.inner().mutability == Mutability::Mut {
290 throw_machine_stop_str!("can't access mutable globals in ConstProp");
298 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
299 _ptr: Pointer<AllocId>,
300 ) -> InterpResult<'tcx> {
301 throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
306 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
307 frame: Frame<'mir, 'tcx>,
308 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
314 ecx: &'a InterpCx<'mir, 'tcx, Self>,
315 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
321 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
322 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
323 &mut ecx.machine.stack
327 /// Finds optimization opportunities on the MIR.
328 struct ConstPropagator<'mir, 'tcx> {
329 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
331 param_env: ParamEnv<'tcx>,
332 local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
333 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
334 // the last known `SourceInfo` here and just keep revisiting it.
335 source_info: Option<SourceInfo>,
338 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
339 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
342 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
347 impl HasDataLayout for ConstPropagator<'_, '_> {
349 fn data_layout(&self) -> &TargetDataLayout {
350 &self.tcx.data_layout
354 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
356 fn tcx(&self) -> TyCtxt<'tcx> {
361 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
363 fn param_env(&self) -> ty::ParamEnv<'tcx> {
368 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
371 dummy_body: &'mir Body<'tcx>,
373 ) -> ConstPropagator<'mir, 'tcx> {
374 let def_id = body.source.def_id();
375 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
376 let param_env = tcx.param_env_reveal_all_normalized(def_id);
378 let can_const_prop = CanConstProp::check(tcx, param_env, body);
379 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
380 for (l, mode) in can_const_prop.iter_enumerated() {
381 if *mode == ConstPropMode::OnlyInsideOwnBlock {
382 only_propagate_inside_block_locals.insert(l);
385 let mut ecx = InterpCx::new(
387 tcx.def_span(def_id),
389 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
393 .layout_of(EarlyBinder(body.return_ty()).subst(tcx, substs))
395 // Don't bother allocating memory for large values.
396 .filter(|ret_layout| ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT))
397 .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
400 .allocate(ret_layout, MemoryKind::Stack)
401 .expect("couldn't perform small allocation")
404 ecx.push_stack_frame(
405 Instance::new(def_id, substs),
408 StackPopCleanup::Root { cleanup: false },
410 .expect("failed to push initial stack frame");
416 local_decls: &dummy_body.local_decls,
421 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
422 let op = match self.ecx.eval_place_to_op(place, None) {
425 trace!("get_const failed: {}", e);
430 // Try to read the local as an immediate so that if it is representable as a scalar, we can
431 // handle it as such, but otherwise, just return the value as is.
432 Some(match self.ecx.read_immediate_raw(&op, /*force*/ false) {
433 Ok(Ok(imm)) => imm.into(),
438 /// Remove `local` from the pool of `Locals`. Allows writing to them,
439 /// but not reading from them anymore.
440 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
441 ecx.frame_mut().locals[local] = LocalState {
442 value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
443 layout: Cell::new(None),
447 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
449 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
452 Ok(val) => Some(val),
454 trace!("InterpCx operation failed: {:?}", error);
455 // Some errors shouldn't come up because creating them causes
456 // an allocation, which we should avoid. When that happens,
457 // dedicated error variants should be introduced instead.
459 !error.kind().formatted_string(),
460 "const-prop encountered formatting error: {}",
468 /// Returns the value, if any, of evaluating `c`.
469 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
470 // FIXME we need to revisit this for #67176
475 self.ecx.mir_const_to_op(&c.literal, None).ok()
478 /// Returns the value, if any, of evaluating `place`.
479 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
480 trace!("eval_place(place={:?})", place);
481 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
484 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
485 /// or `eval_place`, depending on the variant of `Operand` used.
486 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
488 Operand::Constant(ref c) => self.eval_constant(c),
489 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
493 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
494 if self.use_ecx(|this| {
495 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
496 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
499 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
500 // appropriate to use.
501 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
511 left: &Operand<'tcx>,
512 right: &Operand<'tcx>,
514 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
515 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
516 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
517 if op == BinOp::Shr || op == BinOp::Shl {
519 // We need the type of the LHS. We cannot use `place_layout` as that is the type
520 // of the result, which for checked binops is not the same!
521 let left_ty = left.ty(self.local_decls, self.tcx);
522 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
523 let right_size = r.layout.size;
524 let r_bits = r.to_scalar().ok();
525 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
526 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
531 if let (Some(l), Some(r)) = (&l, &r) {
532 // The remaining operators are handled through `overflowing_binary_op`.
533 if self.use_ecx(|this| {
534 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
543 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
545 Operand::Copy(l) | Operand::Move(l) => {
546 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
547 // FIXME(felix91gr): this code only handles `Scalar` cases.
548 // For now, we're not handling `ScalarPair` cases because
549 // doing so here would require a lot of code duplication.
550 // We should hopefully generalize `Operand` handling into a fn,
551 // and use it to do const-prop here and everywhere else
552 // where it makes sense.
553 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
554 ScalarMaybeUninit::Scalar(scalar),
557 *operand = self.operand_from_scalar(
560 self.source_info.unwrap().span,
565 Operand::Constant(_) => (),
569 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
570 // Perform any special handling for specific Rvalue types.
571 // Generally, checks here fall into one of two categories:
572 // 1. Additional checking to provide useful lints to the user
573 // - In this case, we will do some validation and then fall through to the
574 // end of the function which evals the assignment.
575 // 2. Working around bugs in other parts of the compiler
576 // - In this case, we'll return `None` from this function to stop evaluation.
578 // Additional checking: give lints to the user if an overflow would occur.
579 // We do this here and not in the `Assert` terminator as that terminator is
580 // only sometimes emitted (overflow checks can be disabled), but we want to always
582 Rvalue::UnaryOp(op, arg) => {
583 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
584 self.check_unary_op(*op, arg)?;
586 Rvalue::BinaryOp(op, box (left, right)) => {
587 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
588 self.check_binary_op(*op, left, right)?;
590 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
592 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
597 self.check_binary_op(*op, left, right)?;
600 // Do not try creating references (#67862)
601 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
602 trace!("skipping AddressOf | Ref for {:?}", place);
604 // This may be creating mutable references or immutable references to cells.
605 // If that happens, the pointed to value could be mutated via that reference.
606 // Since we aren't tracking references, the const propagator loses track of what
607 // value the local has right now.
608 // Thus, all locals that have their reference taken
609 // must not take part in propagation.
610 Self::remove_const(&mut self.ecx, place.local);
614 Rvalue::ThreadLocalRef(def_id) => {
615 trace!("skipping ThreadLocalRef({:?})", def_id);
620 // There's no other checking to do at this time.
621 Rvalue::Aggregate(..)
626 | Rvalue::ShallowInitBox(..)
627 | Rvalue::Discriminant(..)
628 | Rvalue::NullaryOp(..) => {}
631 // FIXME we need to revisit this for #67176
632 if rvalue.needs_subst() {
636 if self.tcx.sess.mir_opt_level() >= 4 {
637 self.eval_rvalue_with_identities(rvalue, place)
639 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
643 // Attempt to use algebraic identities to eliminate constant expressions
644 fn eval_rvalue_with_identities(
646 rvalue: &Rvalue<'tcx>,
649 self.use_ecx(|this| match rvalue {
650 Rvalue::BinaryOp(op, box (left, right))
651 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
652 let l = this.ecx.eval_operand(left, None);
653 let r = this.ecx.eval_operand(right, None);
655 let const_arg = match (l, r) {
656 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
657 (Err(e), Err(_)) => return Err(e),
658 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
661 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
662 let dest = this.ecx.eval_place(place)?;
665 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
667 if arg_value == const_arg.layout.size.truncate(u128::MAX)
668 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
670 this.ecx.write_immediate(*const_arg, &dest)
672 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
673 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
674 let val = Immediate::ScalarPair(
675 const_arg.to_scalar()?.into(),
676 Scalar::from_bool(false).into(),
678 this.ecx.write_immediate(val, &dest)
680 this.ecx.write_immediate(*const_arg, &dest)
683 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
686 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
690 /// Creates a new `Operand::Constant` from a `Scalar` value
691 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
692 Operand::Constant(Box::new(Constant {
695 literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
699 fn replace_with_const(
701 rval: &mut Rvalue<'tcx>,
703 source_info: SourceInfo,
705 if let Rvalue::Use(Operand::Constant(c)) = rval {
707 ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
709 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
715 trace!("attempting to replace {:?} with {:?}", rval, value);
716 if let Err(e) = self.ecx.const_validate_operand(
719 // FIXME: is ref tracking too expensive?
720 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
721 &mut interpret::RefTracking::empty(),
722 CtfeValidationMode::Regular,
724 trace!("validation error, attempt failed: {:?}", e);
728 // FIXME> figure out what to do when read_immediate_raw fails
729 let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value, /*force*/ false));
731 if let Some(Ok(imm)) = imm {
733 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
734 *rval = Rvalue::Use(self.operand_from_scalar(
740 Immediate::ScalarPair(
741 ScalarMaybeUninit::Scalar(_),
742 ScalarMaybeUninit::Scalar(_),
744 // Found a value represented as a pair. For now only do const-prop if the type
745 // of `rvalue` is also a tuple with two scalars.
746 // FIXME: enable the general case stated above ^.
747 let ty = value.layout.ty;
748 // Only do it for tuples
749 if let ty::Tuple(types) = ty.kind() {
750 // Only do it if tuple is also a pair with two scalars
751 if let [ty1, ty2] = types[..] {
752 let alloc = self.use_ecx(|this| {
753 let ty_is_scalar = |ty| {
754 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
757 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
760 .intern_with_temp_alloc(value.layout, |ecx, dest| {
761 ecx.write_immediate(*imm, dest)
770 if let Some(Some(alloc)) = alloc {
771 // Assign entire constant in a single statement.
772 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
773 let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
774 let literal = ConstantKind::Val(const_val, ty);
775 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
776 span: source_info.span,
784 // Scalars or scalar pairs that contain undef values are assumed to not have
785 // successfully evaluated and are thus not propagated.
791 /// Returns `true` if and only if this `op` should be const-propagated into.
792 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
793 let mir_opt_level = self.tcx.sess.mir_opt_level();
795 if mir_opt_level == 0 {
799 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
804 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
805 s.try_to_int().is_ok()
807 interpret::Operand::Immediate(Immediate::ScalarPair(
808 ScalarMaybeUninit::Scalar(l),
809 ScalarMaybeUninit::Scalar(r),
810 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
816 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
817 #[derive(Clone, Copy, Debug, PartialEq)]
819 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
821 /// The `Local` can only be propagated into and from its own block.
823 /// The `Local` can be propagated into but reads cannot be propagated.
825 /// The `Local` cannot be part of propagation at all. Any statement
826 /// referencing it either for reading or writing will not get propagated.
830 struct CanConstProp {
831 can_const_prop: IndexVec<Local, ConstPropMode>,
832 // False at the beginning. Once set, no more assignments are allowed to that local.
833 found_assignment: BitSet<Local>,
834 // Cache of locals' information
835 local_kinds: IndexVec<Local, LocalKind>,
839 /// Returns true if `local` can be propagated
842 param_env: ParamEnv<'tcx>,
844 ) -> IndexVec<Local, ConstPropMode> {
845 let mut cpv = CanConstProp {
846 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
847 found_assignment: BitSet::new_empty(body.local_decls.len()),
848 local_kinds: IndexVec::from_fn_n(
849 |local| body.local_kind(local),
850 body.local_decls.len(),
853 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
854 let ty = body.local_decls[local].ty;
855 match tcx.layout_of(param_env.and(ty)) {
856 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
857 // Either the layout fails to compute, then we can't use this local anyway
858 // or the local is too large, then we don't want to.
860 *val = ConstPropMode::NoPropagation;
864 // Cannot use args at all
865 // Cannot use locals because if x < y { y - x } else { x - y } would
867 // FIXME(oli-obk): lint variables until they are used in a condition
868 // FIXME(oli-obk): lint if return value is constant
869 if cpv.local_kinds[local] == LocalKind::Arg {
870 *val = ConstPropMode::OnlyPropagateInto;
872 "local {:?} can't be const propagated because it's a function argument",
875 } else if cpv.local_kinds[local] == LocalKind::Var {
876 *val = ConstPropMode::OnlyInsideOwnBlock;
878 "local {:?} will only be propagated inside its block, because it's a user variable",
883 cpv.visit_body(&body);
888 impl Visitor<'_> for CanConstProp {
889 fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
890 use rustc_middle::mir::visit::PlaceContext::*;
892 // Projections are fine, because `&mut foo.x` will be caught by
893 // `MutatingUseContext::Borrow` elsewhere.
894 MutatingUse(MutatingUseContext::Projection)
895 // These are just stores, where the storing is not propagatable, but there may be later
896 // mutations of the same local via `Store`
897 | MutatingUse(MutatingUseContext::Call)
898 | MutatingUse(MutatingUseContext::AsmOutput)
899 | MutatingUse(MutatingUseContext::Deinit)
900 // Actual store that can possibly even propagate a value
901 | MutatingUse(MutatingUseContext::Store)
902 | MutatingUse(MutatingUseContext::SetDiscriminant) => {
903 if !self.found_assignment.insert(local) {
904 match &mut self.can_const_prop[local] {
905 // If the local can only get propagated in its own block, then we don't have
906 // to worry about multiple assignments, as we'll nuke the const state at the
907 // end of the block anyway, and inside the block we overwrite previous
908 // states as applicable.
909 ConstPropMode::OnlyInsideOwnBlock => {}
910 ConstPropMode::NoPropagation => {}
911 ConstPropMode::OnlyPropagateInto => {}
912 other @ ConstPropMode::FullConstProp => {
914 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
917 *other = ConstPropMode::OnlyInsideOwnBlock;
922 // Reading constants is allowed an arbitrary number of times
923 NonMutatingUse(NonMutatingUseContext::Copy)
924 | NonMutatingUse(NonMutatingUseContext::Move)
925 | NonMutatingUse(NonMutatingUseContext::Inspect)
926 | NonMutatingUse(NonMutatingUseContext::Projection)
929 // These could be propagated with a smarter analysis or just some careful thinking about
930 // whether they'd be fine right now.
931 MutatingUse(MutatingUseContext::Yield)
932 | MutatingUse(MutatingUseContext::Drop)
933 | MutatingUse(MutatingUseContext::Retag)
934 // These can't ever be propagated under any scheme, as we can't reason about indirect
936 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
937 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
938 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
939 | NonMutatingUse(NonMutatingUseContext::AddressOf)
940 | MutatingUse(MutatingUseContext::Borrow)
941 | MutatingUse(MutatingUseContext::AddressOf) => {
942 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
943 self.can_const_prop[local] = ConstPropMode::NoPropagation;
949 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
950 fn tcx(&self) -> TyCtxt<'tcx> {
954 fn visit_body(&mut self, body: &mut Body<'tcx>) {
955 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
956 self.visit_basic_block_data(bb, data);
960 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
961 self.super_operand(operand, location);
963 // Only const prop copies and moves on `mir_opt_level=3` as doing so
964 // currently slightly increases compile time in some cases.
965 if self.tcx.sess.mir_opt_level() >= 3 {
966 self.propagate_operand(operand)
970 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
971 trace!("visit_constant: {:?}", constant);
972 self.super_constant(constant, location);
973 self.eval_constant(constant);
976 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
977 trace!("visit_statement: {:?}", statement);
978 let source_info = statement.source_info;
979 self.source_info = Some(source_info);
980 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
981 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
982 if let Some(()) = self.const_prop(rval, place) {
983 // This will return None if the above `const_prop` invocation only "wrote" a
984 // type whose creation requires no write. E.g. a generator whose initial state
985 // consists solely of uninitialized memory (so it doesn't capture any locals).
986 if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
987 trace!("replacing {:?} with {:?}", rval, value);
988 self.replace_with_const(rval, value, source_info);
989 if can_const_prop == ConstPropMode::FullConstProp
990 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
992 trace!("propagated into {:?}", place);
995 match can_const_prop {
996 ConstPropMode::OnlyInsideOwnBlock => {
998 "found local restricted to its block. \
999 Will remove it from const-prop after block is finished. Local: {:?}",
1003 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1004 trace!("can't propagate into {:?}", place);
1005 if place.local != RETURN_PLACE {
1006 Self::remove_const(&mut self.ecx, place.local);
1009 ConstPropMode::FullConstProp => {}
1012 // Const prop failed, so erase the destination, ensuring that whatever happens
1013 // from here on, does not know about the previous value.
1014 // This is important in case we have
1017 // x = SOME_MUTABLE_STATIC;
1018 // // x must now be uninit
1020 // FIXME: we overzealously erase the entire local, because that's easier to
1023 "propagation into {:?} failed.
1024 Nuking the entire site from orbit, it's the only way to be sure",
1027 Self::remove_const(&mut self.ecx, place.local);
1030 match statement.kind {
1031 StatementKind::SetDiscriminant { ref place, .. } => {
1032 match self.ecx.machine.can_const_prop[place.local] {
1033 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1034 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1035 trace!("propped discriminant into {:?}", place);
1037 Self::remove_const(&mut self.ecx, place.local);
1040 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1041 Self::remove_const(&mut self.ecx, place.local);
1045 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1046 let frame = self.ecx.frame_mut();
1047 frame.locals[local].value =
1048 if let StatementKind::StorageLive(_) = statement.kind {
1049 LocalValue::Live(interpret::Operand::Immediate(
1050 interpret::Immediate::Uninit,
1060 self.super_statement(statement, location);
1063 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1064 let source_info = terminator.source_info;
1065 self.source_info = Some(source_info);
1066 self.super_terminator(terminator, location);
1067 match &mut terminator.kind {
1068 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1069 if let Some(ref value) = self.eval_operand(&cond) {
1070 trace!("assertion on {:?} should be {:?}", value, expected);
1071 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1072 let value_const = self.ecx.read_scalar(&value).unwrap();
1073 if expected != value_const {
1074 // Poison all places this operand references so that further code
1075 // doesn't use the invalid value
1077 Operand::Move(ref place) | Operand::Copy(ref place) => {
1078 Self::remove_const(&mut self.ecx, place.local);
1080 Operand::Constant(_) => {}
1083 if self.should_const_prop(value) {
1084 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1085 *cond = self.operand_from_scalar(
1087 self.tcx.types.bool,
1095 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1096 // FIXME: This is currently redundant with `visit_operand`, but sadly
1097 // always visiting operands currently causes a perf regression in LLVM codegen, so
1098 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1099 self.propagate_operand(discr)
1101 // None of these have Operands to const-propagate.
1102 TerminatorKind::Goto { .. }
1103 | TerminatorKind::Resume
1104 | TerminatorKind::Abort
1105 | TerminatorKind::Return
1106 | TerminatorKind::Unreachable
1107 | TerminatorKind::Drop { .. }
1108 | TerminatorKind::DropAndReplace { .. }
1109 | TerminatorKind::Yield { .. }
1110 | TerminatorKind::GeneratorDrop
1111 | TerminatorKind::FalseEdge { .. }
1112 | TerminatorKind::FalseUnwind { .. }
1113 | TerminatorKind::InlineAsm { .. } => {}
1114 // Every argument in our function calls have already been propagated in `visit_operand`.
1116 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1117 // gated on `mir_opt_level=3`.
1118 TerminatorKind::Call { .. } => {}
1121 // We remove all Locals which are restricted in propagation to their containing blocks and
1122 // which were modified in the current block.
1123 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1124 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1125 for &local in locals.iter() {
1126 Self::remove_const(&mut self.ecx, local);
1129 // Put it back so we reuse the heap of the storage
1130 self.ecx.machine.written_only_inside_own_block_locals = locals;
1131 if cfg!(debug_assertions) {
1132 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1133 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1135 self.get_const(local.into()).is_none()
1137 .layout_of(self.local_decls[local].ty)
1138 .map_or(true, |layout| layout.is_zst())