1 //! Propagates constants for early reporting of statically known
8 use rustc_ast::Mutability;
9 use rustc_const_eval::const_eval::CheckAlignment;
10 use rustc_data_structures::fx::FxHashSet;
11 use rustc_hir::def::DefKind;
12 use rustc_index::bit_set::BitSet;
13 use rustc_index::vec::IndexVec;
14 use rustc_middle::mir::visit::{
15 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
17 use rustc_middle::mir::{
18 BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
19 Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
22 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
23 use rustc_middle::ty::InternalSubsts;
24 use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitable};
25 use rustc_span::{def_id::DefId, Span};
26 use rustc_target::abi::{self, HasDataLayout, Size, TargetDataLayout};
27 use rustc_target::spec::abi::Abi as CallAbi;
28 use rustc_trait_selection::traits;
31 use rustc_const_eval::interpret::{
32 self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
33 ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, PlaceTy,
34 Pointer, Scalar, StackPopCleanup, StackPopUnwind,
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 sess.mir_opt_level() >= 1
67 #[instrument(skip(self, tcx), level = "debug")]
68 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
69 // will be evaluated by miri and produce its errors there
70 if body.source.promoted.is_some() {
74 let def_id = body.source.def_id().expect_local();
75 let def_kind = tcx.def_kind(def_id);
76 let is_fn_like = def_kind.is_fn_like();
77 let is_assoc_const = def_kind == DefKind::AssocConst;
79 // Only run const prop on functions, methods, closures and associated constants
80 if !is_fn_like && !is_assoc_const {
81 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
82 trace!("ConstProp skipped for {:?}", def_id);
86 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
87 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
88 // computing their layout.
90 trace!("ConstProp skipped for generator {:?}", def_id);
94 // Check if it's even possible to satisfy the 'where' clauses
96 // This branch will never be taken for any normal function.
97 // However, it's possible to `#!feature(trivial_bounds)]` to write
98 // a function with impossible to satisfy clauses, e.g.:
99 // `fn foo() where String: Copy {}`
101 // We don't usually need to worry about this kind of case,
102 // since we would get a compilation error if the user tried
103 // to call it. However, since we can do const propagation
104 // even without any calls to the function, we need to make
105 // sure that it even makes sense to try to evaluate the body.
106 // If there are unsatisfiable where clauses, then all bets are
107 // off, and we just give up.
109 // We manually filter the predicates, skipping anything that's not
110 // "global". We are in a potentially generic context
111 // (e.g. we are evaluating a function without substituting generic
112 // parameters, so this filtering serves two purposes:
114 // 1. We skip evaluating any predicates that we would
115 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
116 // 2. We avoid trying to normalize predicates involving generic
117 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
118 // the normalization code (leading to cycle errors), since
119 // it's usually never invoked in this way.
121 .predicates_of(def_id.to_def_id())
124 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
125 if traits::impossible_predicates(
127 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
129 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
133 trace!("ConstProp starting for {:?}", def_id);
135 let dummy_body = &Body::new(
137 (*body.basic_blocks).clone(),
138 body.source_scopes.clone(),
139 body.local_decls.clone(),
144 body.generator_kind(),
145 body.tainted_by_errors,
148 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
149 // constants, instead of just checking for const-folding succeeding.
150 // That would require a uniform one-def no-mutation analysis
151 // and RPO (or recursing when needing the value of a local).
152 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
153 optimization_finder.visit_body(body);
155 trace!("ConstProp done for {:?}", def_id);
159 pub struct ConstPropMachine<'mir, 'tcx> {
160 /// The virtual call stack.
161 stack: Vec<Frame<'mir, 'tcx>>,
162 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
163 pub written_only_inside_own_block_locals: FxHashSet<Local>,
164 /// Locals that need to be cleared after every block terminates.
165 pub only_propagate_inside_block_locals: BitSet<Local>,
166 pub can_const_prop: IndexVec<Local, ConstPropMode>,
169 impl ConstPropMachine<'_, '_> {
171 only_propagate_inside_block_locals: BitSet<Local>,
172 can_const_prop: IndexVec<Local, ConstPropMode>,
176 written_only_inside_own_block_locals: Default::default(),
177 only_propagate_inside_block_locals,
183 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
184 compile_time_machine!(<'mir, 'tcx>);
185 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
190 fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment {
191 // We do not check for alignment to avoid having to carry an `Align`
192 // in `ConstValue::ByRef`.
197 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
198 false // for now, we don't enforce validity
202 _ecx: &InterpCx<'mir, 'tcx, Self>,
203 _instance: ty::InstanceDef<'tcx>,
204 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
205 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
208 fn find_mir_or_eval_fn(
209 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
210 _instance: ty::Instance<'tcx>,
212 _args: &[OpTy<'tcx>],
213 _destination: &PlaceTy<'tcx>,
214 _target: Option<BasicBlock>,
215 _unwind: StackPopUnwind,
216 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
221 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
222 _instance: ty::Instance<'tcx>,
223 _args: &[OpTy<'tcx>],
224 _destination: &PlaceTy<'tcx>,
225 _target: Option<BasicBlock>,
226 _unwind: StackPopUnwind,
227 ) -> InterpResult<'tcx> {
228 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
232 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
233 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
234 _unwind: Option<rustc_middle::mir::BasicBlock>,
235 ) -> InterpResult<'tcx> {
236 bug!("panics terminators are not evaluated in ConstProp")
240 _ecx: &InterpCx<'mir, 'tcx, Self>,
243 _right: &ImmTy<'tcx>,
244 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
245 // We can't do this because aliasing of memory can differ between const eval and llvm
246 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
249 fn access_local_mut<'a>(
250 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
253 ) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::Provenance>> {
254 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
255 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
257 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
259 "mutating local {:?} which is restricted to its block. \
260 Will remove it from const-prop after block is finished.",
263 ecx.machine.written_only_inside_own_block_locals.insert(local);
265 ecx.machine.stack[frame].locals[local].access_mut()
268 fn before_access_global(
272 alloc: ConstAllocation<'tcx>,
273 _static_def_id: Option<DefId>,
275 ) -> InterpResult<'tcx> {
277 throw_machine_stop_str!("can't write to global");
279 // If the static allocation is mutable, then we can't const prop it as its content
280 // might be different at runtime.
281 if alloc.inner().mutability == Mutability::Mut {
282 throw_machine_stop_str!("can't access mutable globals in ConstProp");
290 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
291 _ptr: Pointer<AllocId>,
292 ) -> InterpResult<'tcx> {
293 throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
298 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
299 frame: Frame<'mir, 'tcx>,
300 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
306 ecx: &'a InterpCx<'mir, 'tcx, Self>,
307 ) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] {
313 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
314 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>> {
315 &mut ecx.machine.stack
319 /// Finds optimization opportunities on the MIR.
320 struct ConstPropagator<'mir, 'tcx> {
321 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
323 param_env: ParamEnv<'tcx>,
324 local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
325 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
326 // the last known `SourceInfo` here and just keep revisiting it.
327 source_info: Option<SourceInfo>,
330 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
331 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
334 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
339 impl HasDataLayout for ConstPropagator<'_, '_> {
341 fn data_layout(&self) -> &TargetDataLayout {
342 &self.tcx.data_layout
346 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
348 fn tcx(&self) -> TyCtxt<'tcx> {
353 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
355 fn param_env(&self) -> ty::ParamEnv<'tcx> {
360 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
363 dummy_body: &'mir Body<'tcx>,
365 ) -> ConstPropagator<'mir, 'tcx> {
366 let def_id = body.source.def_id();
367 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
368 let param_env = tcx.param_env_reveal_all_normalized(def_id);
370 let can_const_prop = CanConstProp::check(tcx, param_env, body);
371 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
372 for (l, mode) in can_const_prop.iter_enumerated() {
373 if *mode == ConstPropMode::OnlyInsideOwnBlock {
374 only_propagate_inside_block_locals.insert(l);
377 let mut ecx = InterpCx::new(
379 tcx.def_span(def_id),
381 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
385 .layout_of(body.bound_return_ty().subst(tcx, substs))
387 // Don't bother allocating memory for large values.
388 // I don't know how return types can seem to be unsized but this happens in the
389 // `type/type-unsatisfiable.rs` test.
390 .filter(|ret_layout| {
391 ret_layout.is_sized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
393 .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
396 .allocate(ret_layout, MemoryKind::Stack)
397 .expect("couldn't perform small allocation")
400 ecx.push_stack_frame(
401 Instance::new(def_id, substs),
404 StackPopCleanup::Root { cleanup: false },
406 .expect("failed to push initial stack frame");
412 local_decls: &dummy_body.local_decls,
417 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
418 let op = match self.ecx.eval_place_to_op(place, None) {
420 if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
421 // Make sure nobody accidentally uses this value.
427 trace!("get_const failed: {}", e);
432 // Try to read the local as an immediate so that if it is representable as a scalar, we can
433 // handle it as such, but otherwise, just return the value as is.
434 Some(match self.ecx.read_immediate_raw(&op) {
435 Ok(Right(imm)) => imm.into(),
440 /// Remove `local` from the pool of `Locals`. Allows writing to them,
441 /// but not reading from them anymore.
442 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
443 ecx.frame_mut().locals[local] = LocalState {
444 value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
445 layout: Cell::new(None),
449 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
451 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
454 Ok(val) => Some(val),
456 trace!("InterpCx operation failed: {:?}", error);
457 // Some errors shouldn't come up because creating them causes
458 // an allocation, which we should avoid. When that happens,
459 // dedicated error variants should be introduced instead.
461 !error.kind().formatted_string(),
462 "const-prop encountered formatting error: {}",
470 /// Returns the value, if any, of evaluating `c`.
471 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
472 // FIXME we need to revisit this for #67176
477 // No span, we don't want errors to be shown.
478 self.ecx.eval_mir_constant(&c.literal, None, None).ok()
481 /// Returns the value, if any, of evaluating `place`.
482 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
483 trace!("eval_place(place={:?})", place);
484 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
487 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
488 /// or `eval_place`, depending on the variant of `Operand` used.
489 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
491 Operand::Constant(ref c) => self.eval_constant(c),
492 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
496 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
497 if self.use_ecx(|this| {
498 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
499 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
502 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
503 // appropriate to use.
504 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
514 left: &Operand<'tcx>,
515 right: &Operand<'tcx>,
517 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
518 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
519 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
520 if op == BinOp::Shr || op == BinOp::Shl {
522 // We need the type of the LHS. We cannot use `place_layout` as that is the type
523 // of the result, which for checked binops is not the same!
524 let left_ty = left.ty(self.local_decls, self.tcx);
525 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
526 let right_size = r.layout.size;
527 let r_bits = r.to_scalar().to_bits(right_size).ok();
528 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
533 if let (Some(l), Some(r)) = (&l, &r) {
534 // The remaining operators are handled through `overflowing_binary_op`.
535 if self.use_ecx(|this| {
536 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
545 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
547 Operand::Copy(l) | Operand::Move(l) => {
548 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
549 // FIXME(felix91gr): this code only handles `Scalar` cases.
550 // For now, we're not handling `ScalarPair` cases because
551 // doing so here would require a lot of code duplication.
552 // We should hopefully generalize `Operand` handling into a fn,
553 // and use it to do const-prop here and everywhere else
554 // where it makes sense.
555 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
559 *operand = self.operand_from_scalar(
562 self.source_info.unwrap().span,
567 Operand::Constant(_) => (),
571 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
572 // Perform any special handling for specific Rvalue types.
573 // Generally, checks here fall into one of two categories:
574 // 1. Additional checking to provide useful lints to the user
575 // - In this case, we will do some validation and then fall through to the
576 // end of the function which evals the assignment.
577 // 2. Working around bugs in other parts of the compiler
578 // - In this case, we'll return `None` from this function to stop evaluation.
580 // Additional checking: give lints to the user if an overflow would occur.
581 // We do this here and not in the `Assert` terminator as that terminator is
582 // only sometimes emitted (overflow checks can be disabled), but we want to always
584 Rvalue::UnaryOp(op, arg) => {
585 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
586 self.check_unary_op(*op, arg)?;
588 Rvalue::BinaryOp(op, box (left, right)) => {
589 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
590 self.check_binary_op(*op, left, right)?;
592 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
594 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
599 self.check_binary_op(*op, left, right)?;
602 // Do not try creating references (#67862)
603 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
604 trace!("skipping AddressOf | Ref for {:?}", place);
606 // This may be creating mutable references or immutable references to cells.
607 // If that happens, the pointed to value could be mutated via that reference.
608 // Since we aren't tracking references, the const propagator loses track of what
609 // value the local has right now.
610 // Thus, all locals that have their reference taken
611 // must not take part in propagation.
612 Self::remove_const(&mut self.ecx, place.local);
616 Rvalue::ThreadLocalRef(def_id) => {
617 trace!("skipping ThreadLocalRef({:?})", def_id);
622 // There's no other checking to do at this time.
623 Rvalue::Aggregate(..)
625 | Rvalue::CopyForDeref(..)
629 | Rvalue::ShallowInitBox(..)
630 | Rvalue::Discriminant(..)
631 | Rvalue::NullaryOp(..) => {}
634 // FIXME we need to revisit this for #67176
635 if rvalue.needs_subst() {
639 .ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
640 .is_sized(*self.ecx.tcx, self.param_env)
642 // the interpreter doesn't support unsized locals (only unsized arguments),
643 // but rustc does (in a kinda broken way), so we have to skip them here
647 if self.tcx.sess.mir_opt_level() >= 4 {
648 self.eval_rvalue_with_identities(rvalue, place)
650 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
654 // Attempt to use algebraic identities to eliminate constant expressions
655 fn eval_rvalue_with_identities(
657 rvalue: &Rvalue<'tcx>,
660 self.use_ecx(|this| match rvalue {
661 Rvalue::BinaryOp(op, box (left, right))
662 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
663 let l = this.ecx.eval_operand(left, None).and_then(|x| this.ecx.read_immediate(&x));
665 this.ecx.eval_operand(right, None).and_then(|x| this.ecx.read_immediate(&x));
667 let const_arg = match (l, r) {
668 (Ok(x), Err(_)) | (Err(_), Ok(x)) => x, // exactly one side is known
669 (Err(e), Err(_)) => return Err(e), // neither side is known
670 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place), // both sides are known
673 if !matches!(const_arg.layout.abi, abi::Abi::Scalar(..)) {
674 // We cannot handle Scalar Pair stuff.
675 // No point in calling `eval_rvalue_into_place`, since only one side is known
676 throw_machine_stop_str!("cannot optimize this")
679 let arg_value = const_arg.to_scalar().to_bits(const_arg.layout.size)?;
680 let dest = this.ecx.eval_place(place)?;
683 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
685 if arg_value == const_arg.layout.size.truncate(u128::MAX)
686 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
688 this.ecx.write_immediate(*const_arg, &dest)
690 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
691 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
692 let val = Immediate::ScalarPair(
693 const_arg.to_scalar().into(),
694 Scalar::from_bool(false).into(),
696 this.ecx.write_immediate(val, &dest)
698 this.ecx.write_immediate(*const_arg, &dest)
701 _ => throw_machine_stop_str!("cannot optimize this"),
704 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
708 /// Creates a new `Operand::Constant` from a `Scalar` value
709 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
710 Operand::Constant(Box::new(Constant {
713 literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
717 fn replace_with_const(
719 rval: &mut Rvalue<'tcx>,
721 source_info: SourceInfo,
723 if let Rvalue::Use(Operand::Constant(c)) = rval {
725 ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
727 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
733 trace!("attempting to replace {:?} with {:?}", rval, value);
734 if let Err(e) = self.ecx.const_validate_operand(
737 // FIXME: is ref tracking too expensive?
738 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
739 &mut interpret::RefTracking::empty(),
740 CtfeValidationMode::Regular,
742 trace!("validation error, attempt failed: {:?}", e);
746 // FIXME> figure out what to do when read_immediate_raw fails
747 let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value));
749 if let Some(Right(imm)) = imm {
751 interpret::Immediate::Scalar(scalar) => {
752 *rval = Rvalue::Use(self.operand_from_scalar(
758 Immediate::ScalarPair(..) => {
759 // Found a value represented as a pair. For now only do const-prop if the type
760 // of `rvalue` is also a tuple with two scalars.
761 // FIXME: enable the general case stated above ^.
762 let ty = value.layout.ty;
763 // Only do it for tuples
764 if let ty::Tuple(types) = ty.kind() {
765 // Only do it if tuple is also a pair with two scalars
766 if let [ty1, ty2] = types[..] {
767 let alloc = self.use_ecx(|this| {
768 let ty_is_scalar = |ty| {
769 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
772 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
775 .intern_with_temp_alloc(value.layout, |ecx, dest| {
776 ecx.write_immediate(*imm, dest)
785 if let Some(Some(alloc)) = alloc {
786 // Assign entire constant in a single statement.
787 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
788 let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
789 let literal = ConstantKind::Val(const_val, ty);
790 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
791 span: source_info.span,
799 // Scalars or scalar pairs that contain undef values are assumed to not have
800 // successfully evaluated and are thus not propagated.
806 /// Returns `true` if and only if this `op` should be const-propagated into.
807 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
808 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
813 interpret::Operand::Immediate(Immediate::Scalar(s)) => s.try_to_int().is_ok(),
814 interpret::Operand::Immediate(Immediate::ScalarPair(l, r)) => {
815 l.try_to_int().is_ok() && r.try_to_int().is_ok()
822 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
823 #[derive(Clone, Copy, Debug, PartialEq)]
824 pub enum ConstPropMode {
825 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
827 /// The `Local` can only be propagated into and from its own block.
829 /// The `Local` can be propagated into but reads cannot be propagated.
831 /// The `Local` cannot be part of propagation at all. Any statement
832 /// referencing it either for reading or writing will not get propagated.
836 pub struct CanConstProp {
837 can_const_prop: IndexVec<Local, ConstPropMode>,
838 // False at the beginning. Once set, no more assignments are allowed to that local.
839 found_assignment: BitSet<Local>,
840 // Cache of locals' information
841 local_kinds: IndexVec<Local, LocalKind>,
845 /// Returns true if `local` can be propagated
848 param_env: ParamEnv<'tcx>,
850 ) -> IndexVec<Local, ConstPropMode> {
851 let mut cpv = CanConstProp {
852 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
853 found_assignment: BitSet::new_empty(body.local_decls.len()),
854 local_kinds: IndexVec::from_fn_n(
855 |local| body.local_kind(local),
856 body.local_decls.len(),
859 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
860 let ty = body.local_decls[local].ty;
861 match tcx.layout_of(param_env.and(ty)) {
862 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
863 // Either the layout fails to compute, then we can't use this local anyway
864 // or the local is too large, then we don't want to.
866 *val = ConstPropMode::NoPropagation;
870 // Cannot use args at all
871 // Cannot use locals because if x < y { y - x } else { x - y } would
873 // FIXME(oli-obk): lint variables until they are used in a condition
874 // FIXME(oli-obk): lint if return value is constant
875 if cpv.local_kinds[local] == LocalKind::Arg {
876 *val = ConstPropMode::OnlyPropagateInto;
878 "local {:?} can't be const propagated because it's a function argument",
881 } else if cpv.local_kinds[local] == LocalKind::Var {
882 *val = ConstPropMode::OnlyInsideOwnBlock;
884 "local {:?} will only be propagated inside its block, because it's a user variable",
889 cpv.visit_body(&body);
894 impl Visitor<'_> for CanConstProp {
895 fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
896 use rustc_middle::mir::visit::PlaceContext::*;
898 // Projections are fine, because `&mut foo.x` will be caught by
899 // `MutatingUseContext::Borrow` elsewhere.
900 MutatingUse(MutatingUseContext::Projection)
901 // These are just stores, where the storing is not propagatable, but there may be later
902 // mutations of the same local via `Store`
903 | MutatingUse(MutatingUseContext::Call)
904 | MutatingUse(MutatingUseContext::AsmOutput)
905 | MutatingUse(MutatingUseContext::Deinit)
906 // Actual store that can possibly even propagate a value
907 | MutatingUse(MutatingUseContext::Store)
908 | MutatingUse(MutatingUseContext::SetDiscriminant) => {
909 if !self.found_assignment.insert(local) {
910 match &mut self.can_const_prop[local] {
911 // If the local can only get propagated in its own block, then we don't have
912 // to worry about multiple assignments, as we'll nuke the const state at the
913 // end of the block anyway, and inside the block we overwrite previous
914 // states as applicable.
915 ConstPropMode::OnlyInsideOwnBlock => {}
916 ConstPropMode::NoPropagation => {}
917 ConstPropMode::OnlyPropagateInto => {}
918 other @ ConstPropMode::FullConstProp => {
920 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
923 *other = ConstPropMode::OnlyInsideOwnBlock;
928 // Reading constants is allowed an arbitrary number of times
929 NonMutatingUse(NonMutatingUseContext::Copy)
930 | NonMutatingUse(NonMutatingUseContext::Move)
931 | NonMutatingUse(NonMutatingUseContext::Inspect)
932 | NonMutatingUse(NonMutatingUseContext::Projection)
935 // These could be propagated with a smarter analysis or just some careful thinking about
936 // whether they'd be fine right now.
937 MutatingUse(MutatingUseContext::Yield)
938 | MutatingUse(MutatingUseContext::Drop)
939 | MutatingUse(MutatingUseContext::Retag)
940 // These can't ever be propagated under any scheme, as we can't reason about indirect
942 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
943 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
944 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
945 | NonMutatingUse(NonMutatingUseContext::AddressOf)
946 | MutatingUse(MutatingUseContext::Borrow)
947 | MutatingUse(MutatingUseContext::AddressOf) => {
948 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
949 self.can_const_prop[local] = ConstPropMode::NoPropagation;
955 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
956 fn tcx(&self) -> TyCtxt<'tcx> {
960 fn visit_body(&mut self, body: &mut Body<'tcx>) {
961 for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
962 self.visit_basic_block_data(bb, data);
966 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
967 self.super_operand(operand, location);
969 // Only const prop copies and moves on `mir_opt_level=3` as doing so
970 // currently slightly increases compile time in some cases.
971 if self.tcx.sess.mir_opt_level() >= 3 {
972 self.propagate_operand(operand)
976 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
977 trace!("visit_constant: {:?}", constant);
978 self.super_constant(constant, location);
979 self.eval_constant(constant);
982 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
983 trace!("visit_statement: {:?}", statement);
984 let source_info = statement.source_info;
985 self.source_info = Some(source_info);
986 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
987 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
988 if let Some(()) = self.const_prop(rval, place) {
989 // This will return None if the above `const_prop` invocation only "wrote" a
990 // type whose creation requires no write. E.g. a generator whose initial state
991 // consists solely of uninitialized memory (so it doesn't capture any locals).
992 if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
993 trace!("replacing {:?} with {:?}", rval, value);
994 self.replace_with_const(rval, value, source_info);
995 if can_const_prop == ConstPropMode::FullConstProp
996 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
998 trace!("propagated into {:?}", place);
1001 match can_const_prop {
1002 ConstPropMode::OnlyInsideOwnBlock => {
1004 "found local restricted to its block. \
1005 Will remove it from const-prop after block is finished. Local: {:?}",
1009 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1010 trace!("can't propagate into {:?}", place);
1011 if place.local != RETURN_PLACE {
1012 Self::remove_const(&mut self.ecx, place.local);
1015 ConstPropMode::FullConstProp => {}
1018 // Const prop failed, so erase the destination, ensuring that whatever happens
1019 // from here on, does not know about the previous value.
1020 // This is important in case we have
1023 // x = SOME_MUTABLE_STATIC;
1024 // // x must now be uninit
1026 // FIXME: we overzealously erase the entire local, because that's easier to
1029 "propagation into {:?} failed.
1030 Nuking the entire site from orbit, it's the only way to be sure",
1033 Self::remove_const(&mut self.ecx, place.local);
1036 match statement.kind {
1037 StatementKind::SetDiscriminant { ref place, .. } => {
1038 match self.ecx.machine.can_const_prop[place.local] {
1039 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1040 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1041 trace!("propped discriminant into {:?}", place);
1043 Self::remove_const(&mut self.ecx, place.local);
1046 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1047 Self::remove_const(&mut self.ecx, place.local);
1051 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1052 let frame = self.ecx.frame_mut();
1053 frame.locals[local].value =
1054 if let StatementKind::StorageLive(_) = statement.kind {
1055 LocalValue::Live(interpret::Operand::Immediate(
1056 interpret::Immediate::Uninit,
1066 self.super_statement(statement, location);
1069 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1070 let source_info = terminator.source_info;
1071 self.source_info = Some(source_info);
1072 self.super_terminator(terminator, location);
1073 // Do NOT early return in this function, it does some crucial fixup of the state at the end!
1074 match &mut terminator.kind {
1075 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1076 if let Some(ref value) = self.eval_operand(&cond) {
1077 trace!("assertion on {:?} should be {:?}", value, expected);
1078 let expected = Scalar::from_bool(*expected);
1079 // FIXME should be used use_ecx rather than a local match... but we have
1080 // quite a few of these read_scalar/read_immediate that need fixing.
1081 if let Ok(value_const) = self.ecx.read_scalar(&value) {
1082 if expected != value_const {
1083 // Poison all places this operand references so that further code
1084 // doesn't use the invalid value
1086 Operand::Move(ref place) | Operand::Copy(ref place) => {
1087 Self::remove_const(&mut self.ecx, place.local);
1089 Operand::Constant(_) => {}
1092 if self.should_const_prop(value) {
1093 *cond = self.operand_from_scalar(
1095 self.tcx.types.bool,
1103 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1104 // FIXME: This is currently redundant with `visit_operand`, but sadly
1105 // always visiting operands currently causes a perf regression in LLVM codegen, so
1106 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1107 self.propagate_operand(discr)
1109 // None of these have Operands to const-propagate.
1110 TerminatorKind::Goto { .. }
1111 | TerminatorKind::Resume
1112 | TerminatorKind::Abort
1113 | TerminatorKind::Return
1114 | TerminatorKind::Unreachable
1115 | TerminatorKind::Drop { .. }
1116 | TerminatorKind::DropAndReplace { .. }
1117 | TerminatorKind::Yield { .. }
1118 | TerminatorKind::GeneratorDrop
1119 | TerminatorKind::FalseEdge { .. }
1120 | TerminatorKind::FalseUnwind { .. }
1121 | TerminatorKind::InlineAsm { .. } => {}
1122 // Every argument in our function calls have already been propagated in `visit_operand`.
1124 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1125 // gated on `mir_opt_level=3`.
1126 TerminatorKind::Call { .. } => {}
1129 // We remove all Locals which are restricted in propagation to their containing blocks and
1130 // which were modified in the current block.
1131 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1132 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1133 for &local in locals.iter() {
1134 Self::remove_const(&mut self.ecx, local);
1137 // Put it back so we reuse the heap of the storage
1138 self.ecx.machine.written_only_inside_own_block_locals = locals;
1139 if cfg!(debug_assertions) {
1140 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1141 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1143 self.get_const(local.into()).is_none()
1145 .layout_of(self.local_decls[local].ty)
1146 .map_or(true, |layout| layout.is_zst())