1 //! Propagates constants for early reporting of statically known
8 use rustc_ast::Mutability;
9 use rustc_data_structures::fx::FxHashSet;
10 use rustc_hir::def::DefKind;
11 use rustc_index::bit_set::BitSet;
12 use rustc_index::vec::IndexVec;
13 use rustc_middle::mir::visit::{
14 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
16 use rustc_middle::mir::{
17 BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
18 Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
21 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
22 use rustc_middle::ty::InternalSubsts;
23 use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitable};
24 use rustc_span::{def_id::DefId, Span};
25 use rustc_target::abi::{self, HasDataLayout, Size, TargetDataLayout};
26 use rustc_target::spec::abi::Abi as CallAbi;
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, 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 sess.mir_opt_level() >= 1
66 #[instrument(skip(self, tcx), level = "debug")]
67 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
68 // will be evaluated by miri and produce its errors there
69 if body.source.promoted.is_some() {
73 let def_id = body.source.def_id().expect_local();
74 let def_kind = tcx.def_kind(def_id);
75 let is_fn_like = def_kind.is_fn_like();
76 let is_assoc_const = def_kind == DefKind::AssocConst;
78 // Only run const prop on functions, methods, closures and associated constants
79 if !is_fn_like && !is_assoc_const {
80 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
81 trace!("ConstProp skipped for {:?}", def_id);
85 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
86 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
87 // computing their layout.
89 trace!("ConstProp skipped for generator {:?}", def_id);
93 // Check if it's even possible to satisfy the 'where' clauses
95 // This branch will never be taken for any normal function.
96 // However, it's possible to `#!feature(trivial_bounds)]` to write
97 // a function with impossible to satisfy clauses, e.g.:
98 // `fn foo() where String: Copy {}`
100 // We don't usually need to worry about this kind of case,
101 // since we would get a compilation error if the user tried
102 // to call it. However, since we can do const propagation
103 // even without any calls to the function, we need to make
104 // sure that it even makes sense to try to evaluate the body.
105 // If there are unsatisfiable where clauses, then all bets are
106 // off, and we just give up.
108 // We manually filter the predicates, skipping anything that's not
109 // "global". We are in a potentially generic context
110 // (e.g. we are evaluating a function without substituting generic
111 // parameters, so this filtering serves two purposes:
113 // 1. We skip evaluating any predicates that we would
114 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
115 // 2. We avoid trying to normalize predicates involving generic
116 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
117 // the normalization code (leading to cycle errors), since
118 // it's usually never invoked in this way.
120 .predicates_of(def_id.to_def_id())
123 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
124 if traits::impossible_predicates(
126 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
128 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
132 trace!("ConstProp starting for {:?}", def_id);
134 let dummy_body = &Body::new(
136 (*body.basic_blocks).clone(),
137 body.source_scopes.clone(),
138 body.local_decls.clone(),
143 body.generator_kind(),
144 body.tainted_by_errors,
147 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
148 // constants, instead of just checking for const-folding succeeding.
149 // That would require a uniform one-def no-mutation analysis
150 // and RPO (or recursing when needing the value of a local).
151 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
152 optimization_finder.visit_body(body);
154 trace!("ConstProp done for {:?}", def_id);
158 pub struct ConstPropMachine<'mir, 'tcx> {
159 /// The virtual call stack.
160 stack: Vec<Frame<'mir, 'tcx>>,
161 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
162 pub written_only_inside_own_block_locals: FxHashSet<Local>,
163 /// Locals that need to be cleared after every block terminates.
164 pub only_propagate_inside_block_locals: BitSet<Local>,
165 pub can_const_prop: IndexVec<Local, ConstPropMode>,
168 impl ConstPropMachine<'_, '_> {
170 only_propagate_inside_block_locals: BitSet<Local>,
171 can_const_prop: IndexVec<Local, ConstPropMode>,
175 written_only_inside_own_block_locals: Default::default(),
176 only_propagate_inside_block_locals,
182 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
183 compile_time_machine!(<'mir, 'tcx>);
184 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
189 fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
190 // We do not check for alignment to avoid having to carry an `Align`
191 // in `ConstValue::ByRef`.
196 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
197 false // for now, we don't enforce validity
201 _ecx: &InterpCx<'mir, 'tcx, Self>,
202 _instance: ty::InstanceDef<'tcx>,
203 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
204 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
207 fn find_mir_or_eval_fn(
208 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
209 _instance: ty::Instance<'tcx>,
211 _args: &[OpTy<'tcx>],
212 _destination: &PlaceTy<'tcx>,
213 _target: Option<BasicBlock>,
214 _unwind: StackPopUnwind,
215 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
220 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
221 _instance: ty::Instance<'tcx>,
222 _args: &[OpTy<'tcx>],
223 _destination: &PlaceTy<'tcx>,
224 _target: Option<BasicBlock>,
225 _unwind: StackPopUnwind,
226 ) -> InterpResult<'tcx> {
227 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
231 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
232 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
233 _unwind: Option<rustc_middle::mir::BasicBlock>,
234 ) -> InterpResult<'tcx> {
235 bug!("panics terminators are not evaluated in ConstProp")
239 _ecx: &InterpCx<'mir, 'tcx, Self>,
242 _right: &ImmTy<'tcx>,
243 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
244 // We can't do this because aliasing of memory can differ between const eval and llvm
245 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
248 fn access_local_mut<'a>(
249 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
252 ) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::Provenance>> {
253 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
254 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
256 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
258 "mutating local {:?} which is restricted to its block. \
259 Will remove it from const-prop after block is finished.",
262 ecx.machine.written_only_inside_own_block_locals.insert(local);
264 ecx.machine.stack[frame].locals[local].access_mut()
267 fn before_access_global(
271 alloc: ConstAllocation<'tcx>,
272 _static_def_id: Option<DefId>,
274 ) -> InterpResult<'tcx> {
276 throw_machine_stop_str!("can't write to global");
278 // If the static allocation is mutable, then we can't const prop it as its content
279 // might be different at runtime.
280 if alloc.inner().mutability == Mutability::Mut {
281 throw_machine_stop_str!("can't access mutable globals in ConstProp");
289 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
290 _ptr: Pointer<AllocId>,
291 ) -> InterpResult<'tcx> {
292 throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
297 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
298 frame: Frame<'mir, 'tcx>,
299 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
305 ecx: &'a InterpCx<'mir, 'tcx, Self>,
306 ) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] {
312 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
313 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>> {
314 &mut ecx.machine.stack
318 /// Finds optimization opportunities on the MIR.
319 struct ConstPropagator<'mir, 'tcx> {
320 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
322 param_env: ParamEnv<'tcx>,
323 local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
324 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
325 // the last known `SourceInfo` here and just keep revisiting it.
326 source_info: Option<SourceInfo>,
329 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
330 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
333 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
338 impl HasDataLayout for ConstPropagator<'_, '_> {
340 fn data_layout(&self) -> &TargetDataLayout {
341 &self.tcx.data_layout
345 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
347 fn tcx(&self) -> TyCtxt<'tcx> {
352 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
354 fn param_env(&self) -> ty::ParamEnv<'tcx> {
359 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
362 dummy_body: &'mir Body<'tcx>,
364 ) -> ConstPropagator<'mir, 'tcx> {
365 let def_id = body.source.def_id();
366 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
367 let param_env = tcx.param_env_reveal_all_normalized(def_id);
369 let can_const_prop = CanConstProp::check(tcx, param_env, body);
370 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
371 for (l, mode) in can_const_prop.iter_enumerated() {
372 if *mode == ConstPropMode::OnlyInsideOwnBlock {
373 only_propagate_inside_block_locals.insert(l);
376 let mut ecx = InterpCx::new(
378 tcx.def_span(def_id),
380 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
384 .layout_of(body.bound_return_ty().subst(tcx, substs))
386 // Don't bother allocating memory for large values.
387 // I don't know how return types can seem to be unsized but this happens in the
388 // `type/type-unsatisfiable.rs` test.
389 .filter(|ret_layout| {
390 ret_layout.is_sized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
392 .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
395 .allocate(ret_layout, MemoryKind::Stack)
396 .expect("couldn't perform small allocation")
399 ecx.push_stack_frame(
400 Instance::new(def_id, substs),
403 StackPopCleanup::Root { cleanup: false },
405 .expect("failed to push initial stack frame");
411 local_decls: &dummy_body.local_decls,
416 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
417 let op = match self.ecx.eval_place_to_op(place, None) {
419 if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
420 // Make sure nobody accidentally uses this value.
426 trace!("get_const failed: {}", e);
431 // Try to read the local as an immediate so that if it is representable as a scalar, we can
432 // handle it as such, but otherwise, just return the value as is.
433 Some(match self.ecx.read_immediate_raw(&op) {
434 Ok(Right(imm)) => imm.into(),
439 /// Remove `local` from the pool of `Locals`. Allows writing to them,
440 /// but not reading from them anymore.
441 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
442 ecx.frame_mut().locals[local] = LocalState {
443 value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
444 layout: Cell::new(None),
448 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
450 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
453 Ok(val) => Some(val),
455 trace!("InterpCx operation failed: {:?}", error);
456 // Some errors shouldn't come up because creating them causes
457 // an allocation, which we should avoid. When that happens,
458 // dedicated error variants should be introduced instead.
460 !error.kind().formatted_string(),
461 "const-prop encountered formatting error: {}",
469 /// Returns the value, if any, of evaluating `c`.
470 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
471 // FIXME we need to revisit this for #67176
476 // No span, we don't want errors to be shown.
477 self.ecx.eval_mir_constant(&c.literal, None, None).ok()
480 /// Returns the value, if any, of evaluating `place`.
481 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
482 trace!("eval_place(place={:?})", place);
483 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
486 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
487 /// or `eval_place`, depending on the variant of `Operand` used.
488 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
490 Operand::Constant(ref c) => self.eval_constant(c),
491 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
495 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
496 if self.use_ecx(|this| {
497 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
498 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
501 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
502 // appropriate to use.
503 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
513 left: &Operand<'tcx>,
514 right: &Operand<'tcx>,
516 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
517 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
518 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
519 if op == BinOp::Shr || op == BinOp::Shl {
521 // We need the type of the LHS. We cannot use `place_layout` as that is the type
522 // of the result, which for checked binops is not the same!
523 let left_ty = left.ty(self.local_decls, self.tcx);
524 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
525 let right_size = r.layout.size;
526 let r_bits = r.to_scalar().to_bits(right_size).ok();
527 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
532 if let (Some(l), Some(r)) = (&l, &r) {
533 // The remaining operators are handled through `overflowing_binary_op`.
534 if self.use_ecx(|this| {
535 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
544 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
546 Operand::Copy(l) | Operand::Move(l) => {
547 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
548 // FIXME(felix91gr): this code only handles `Scalar` cases.
549 // For now, we're not handling `ScalarPair` cases because
550 // doing so here would require a lot of code duplication.
551 // We should hopefully generalize `Operand` handling into a fn,
552 // and use it to do const-prop here and everywhere else
553 // where it makes sense.
554 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
558 *operand = self.operand_from_scalar(
561 self.source_info.unwrap().span,
566 Operand::Constant(_) => (),
570 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
571 // Perform any special handling for specific Rvalue types.
572 // Generally, checks here fall into one of two categories:
573 // 1. Additional checking to provide useful lints to the user
574 // - In this case, we will do some validation and then fall through to the
575 // end of the function which evals the assignment.
576 // 2. Working around bugs in other parts of the compiler
577 // - In this case, we'll return `None` from this function to stop evaluation.
579 // Additional checking: give lints to the user if an overflow would occur.
580 // We do this here and not in the `Assert` terminator as that terminator is
581 // only sometimes emitted (overflow checks can be disabled), but we want to always
583 Rvalue::UnaryOp(op, arg) => {
584 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
585 self.check_unary_op(*op, arg)?;
587 Rvalue::BinaryOp(op, box (left, right)) => {
588 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
589 self.check_binary_op(*op, left, right)?;
591 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
593 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
598 self.check_binary_op(*op, left, right)?;
601 // Do not try creating references (#67862)
602 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
603 trace!("skipping AddressOf | Ref for {:?}", place);
605 // This may be creating mutable references or immutable references to cells.
606 // If that happens, the pointed to value could be mutated via that reference.
607 // Since we aren't tracking references, the const propagator loses track of what
608 // value the local has right now.
609 // Thus, all locals that have their reference taken
610 // must not take part in propagation.
611 Self::remove_const(&mut self.ecx, place.local);
615 Rvalue::ThreadLocalRef(def_id) => {
616 trace!("skipping ThreadLocalRef({:?})", def_id);
621 // There's no other checking to do at this time.
622 Rvalue::Aggregate(..)
624 | Rvalue::CopyForDeref(..)
628 | Rvalue::ShallowInitBox(..)
629 | Rvalue::Discriminant(..)
630 | Rvalue::NullaryOp(..) => {}
633 // FIXME we need to revisit this for #67176
634 if rvalue.needs_subst() {
638 .ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
639 .is_sized(*self.ecx.tcx, self.param_env)
641 // the interpreter doesn't support unsized locals (only unsized arguments),
642 // but rustc does (in a kinda broken way), so we have to skip them here
646 if self.tcx.sess.mir_opt_level() >= 4 {
647 self.eval_rvalue_with_identities(rvalue, place)
649 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
653 // Attempt to use algebraic identities to eliminate constant expressions
654 fn eval_rvalue_with_identities(
656 rvalue: &Rvalue<'tcx>,
659 self.use_ecx(|this| match rvalue {
660 Rvalue::BinaryOp(op, box (left, right))
661 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
662 let l = this.ecx.eval_operand(left, None).and_then(|x| this.ecx.read_immediate(&x));
664 this.ecx.eval_operand(right, None).and_then(|x| this.ecx.read_immediate(&x));
666 let const_arg = match (l, r) {
667 (Ok(x), Err(_)) | (Err(_), Ok(x)) => x, // exactly one side is known
668 (Err(e), Err(_)) => return Err(e), // neither side is known
669 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place), // both sides are known
672 if !matches!(const_arg.layout.abi, abi::Abi::Scalar(..)) {
673 // We cannot handle Scalar Pair stuff.
674 // No point in calling `eval_rvalue_into_place`, since only one side is known
675 throw_machine_stop_str!("cannot optimize this")
678 let arg_value = const_arg.to_scalar().to_bits(const_arg.layout.size)?;
679 let dest = this.ecx.eval_place(place)?;
682 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
684 if arg_value == const_arg.layout.size.truncate(u128::MAX)
685 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
687 this.ecx.write_immediate(*const_arg, &dest)
689 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
690 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
691 let val = Immediate::ScalarPair(
692 const_arg.to_scalar().into(),
693 Scalar::from_bool(false).into(),
695 this.ecx.write_immediate(val, &dest)
697 this.ecx.write_immediate(*const_arg, &dest)
700 _ => throw_machine_stop_str!("cannot optimize this"),
703 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
707 /// Creates a new `Operand::Constant` from a `Scalar` value
708 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
709 Operand::Constant(Box::new(Constant {
712 literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
716 fn replace_with_const(
718 rval: &mut Rvalue<'tcx>,
720 source_info: SourceInfo,
722 if let Rvalue::Use(Operand::Constant(c)) = rval {
724 ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
726 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
732 trace!("attempting to replace {:?} with {:?}", rval, value);
733 if let Err(e) = self.ecx.const_validate_operand(
736 // FIXME: is ref tracking too expensive?
737 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
738 &mut interpret::RefTracking::empty(),
739 CtfeValidationMode::Regular,
741 trace!("validation error, attempt failed: {:?}", e);
745 // FIXME> figure out what to do when read_immediate_raw fails
746 let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value));
748 if let Some(Right(imm)) = imm {
750 interpret::Immediate::Scalar(scalar) => {
751 *rval = Rvalue::Use(self.operand_from_scalar(
757 Immediate::ScalarPair(..) => {
758 // Found a value represented as a pair. For now only do const-prop if the type
759 // of `rvalue` is also a tuple with two scalars.
760 // FIXME: enable the general case stated above ^.
761 let ty = value.layout.ty;
762 // Only do it for tuples
763 if let ty::Tuple(types) = ty.kind() {
764 // Only do it if tuple is also a pair with two scalars
765 if let [ty1, ty2] = types[..] {
766 let alloc = self.use_ecx(|this| {
767 let ty_is_scalar = |ty| {
768 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
771 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
774 .intern_with_temp_alloc(value.layout, |ecx, dest| {
775 ecx.write_immediate(*imm, dest)
784 if let Some(Some(alloc)) = alloc {
785 // Assign entire constant in a single statement.
786 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
787 let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
788 let literal = ConstantKind::Val(const_val, ty);
789 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
790 span: source_info.span,
798 // Scalars or scalar pairs that contain undef values are assumed to not have
799 // successfully evaluated and are thus not propagated.
805 /// Returns `true` if and only if this `op` should be const-propagated into.
806 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
807 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
812 interpret::Operand::Immediate(Immediate::Scalar(s)) => s.try_to_int().is_ok(),
813 interpret::Operand::Immediate(Immediate::ScalarPair(l, r)) => {
814 l.try_to_int().is_ok() && r.try_to_int().is_ok()
821 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
822 #[derive(Clone, Copy, Debug, PartialEq)]
823 pub enum ConstPropMode {
824 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
826 /// The `Local` can only be propagated into and from its own block.
828 /// The `Local` can be propagated into but reads cannot be propagated.
830 /// The `Local` cannot be part of propagation at all. Any statement
831 /// referencing it either for reading or writing will not get propagated.
835 pub struct CanConstProp {
836 can_const_prop: IndexVec<Local, ConstPropMode>,
837 // False at the beginning. Once set, no more assignments are allowed to that local.
838 found_assignment: BitSet<Local>,
839 // Cache of locals' information
840 local_kinds: IndexVec<Local, LocalKind>,
844 /// Returns true if `local` can be propagated
847 param_env: ParamEnv<'tcx>,
849 ) -> IndexVec<Local, ConstPropMode> {
850 let mut cpv = CanConstProp {
851 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
852 found_assignment: BitSet::new_empty(body.local_decls.len()),
853 local_kinds: IndexVec::from_fn_n(
854 |local| body.local_kind(local),
855 body.local_decls.len(),
858 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
859 let ty = body.local_decls[local].ty;
860 match tcx.layout_of(param_env.and(ty)) {
861 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
862 // Either the layout fails to compute, then we can't use this local anyway
863 // or the local is too large, then we don't want to.
865 *val = ConstPropMode::NoPropagation;
869 // Cannot use args at all
870 // Cannot use locals because if x < y { y - x } else { x - y } would
872 // FIXME(oli-obk): lint variables until they are used in a condition
873 // FIXME(oli-obk): lint if return value is constant
874 if cpv.local_kinds[local] == LocalKind::Arg {
875 *val = ConstPropMode::OnlyPropagateInto;
877 "local {:?} can't be const propagated because it's a function argument",
880 } else if cpv.local_kinds[local] == LocalKind::Var {
881 *val = ConstPropMode::OnlyInsideOwnBlock;
883 "local {:?} will only be propagated inside its block, because it's a user variable",
888 cpv.visit_body(&body);
893 impl Visitor<'_> for CanConstProp {
894 fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
895 use rustc_middle::mir::visit::PlaceContext::*;
897 // Projections are fine, because `&mut foo.x` will be caught by
898 // `MutatingUseContext::Borrow` elsewhere.
899 MutatingUse(MutatingUseContext::Projection)
900 // These are just stores, where the storing is not propagatable, but there may be later
901 // mutations of the same local via `Store`
902 | MutatingUse(MutatingUseContext::Call)
903 | MutatingUse(MutatingUseContext::AsmOutput)
904 | MutatingUse(MutatingUseContext::Deinit)
905 // Actual store that can possibly even propagate a value
906 | MutatingUse(MutatingUseContext::Store)
907 | MutatingUse(MutatingUseContext::SetDiscriminant) => {
908 if !self.found_assignment.insert(local) {
909 match &mut self.can_const_prop[local] {
910 // If the local can only get propagated in its own block, then we don't have
911 // to worry about multiple assignments, as we'll nuke the const state at the
912 // end of the block anyway, and inside the block we overwrite previous
913 // states as applicable.
914 ConstPropMode::OnlyInsideOwnBlock => {}
915 ConstPropMode::NoPropagation => {}
916 ConstPropMode::OnlyPropagateInto => {}
917 other @ ConstPropMode::FullConstProp => {
919 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
922 *other = ConstPropMode::OnlyInsideOwnBlock;
927 // Reading constants is allowed an arbitrary number of times
928 NonMutatingUse(NonMutatingUseContext::Copy)
929 | NonMutatingUse(NonMutatingUseContext::Move)
930 | NonMutatingUse(NonMutatingUseContext::Inspect)
931 | NonMutatingUse(NonMutatingUseContext::Projection)
934 // These could be propagated with a smarter analysis or just some careful thinking about
935 // whether they'd be fine right now.
936 MutatingUse(MutatingUseContext::Yield)
937 | MutatingUse(MutatingUseContext::Drop)
938 | MutatingUse(MutatingUseContext::Retag)
939 // These can't ever be propagated under any scheme, as we can't reason about indirect
941 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
942 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
943 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
944 | NonMutatingUse(NonMutatingUseContext::AddressOf)
945 | MutatingUse(MutatingUseContext::Borrow)
946 | MutatingUse(MutatingUseContext::AddressOf) => {
947 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
948 self.can_const_prop[local] = ConstPropMode::NoPropagation;
954 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
955 fn tcx(&self) -> TyCtxt<'tcx> {
959 fn visit_body(&mut self, body: &mut Body<'tcx>) {
960 for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
961 self.visit_basic_block_data(bb, data);
965 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
966 self.super_operand(operand, location);
968 // Only const prop copies and moves on `mir_opt_level=3` as doing so
969 // currently slightly increases compile time in some cases.
970 if self.tcx.sess.mir_opt_level() >= 3 {
971 self.propagate_operand(operand)
975 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
976 trace!("visit_constant: {:?}", constant);
977 self.super_constant(constant, location);
978 self.eval_constant(constant);
981 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
982 trace!("visit_statement: {:?}", statement);
983 let source_info = statement.source_info;
984 self.source_info = Some(source_info);
985 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
986 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
987 if let Some(()) = self.const_prop(rval, place) {
988 // This will return None if the above `const_prop` invocation only "wrote" a
989 // type whose creation requires no write. E.g. a generator whose initial state
990 // consists solely of uninitialized memory (so it doesn't capture any locals).
991 if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
992 trace!("replacing {:?} with {:?}", rval, value);
993 self.replace_with_const(rval, value, source_info);
994 if can_const_prop == ConstPropMode::FullConstProp
995 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
997 trace!("propagated into {:?}", place);
1000 match can_const_prop {
1001 ConstPropMode::OnlyInsideOwnBlock => {
1003 "found local restricted to its block. \
1004 Will remove it from const-prop after block is finished. Local: {:?}",
1008 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1009 trace!("can't propagate into {:?}", place);
1010 if place.local != RETURN_PLACE {
1011 Self::remove_const(&mut self.ecx, place.local);
1014 ConstPropMode::FullConstProp => {}
1017 // Const prop failed, so erase the destination, ensuring that whatever happens
1018 // from here on, does not know about the previous value.
1019 // This is important in case we have
1022 // x = SOME_MUTABLE_STATIC;
1023 // // x must now be uninit
1025 // FIXME: we overzealously erase the entire local, because that's easier to
1028 "propagation into {:?} failed.
1029 Nuking the entire site from orbit, it's the only way to be sure",
1032 Self::remove_const(&mut self.ecx, place.local);
1035 match statement.kind {
1036 StatementKind::SetDiscriminant { ref place, .. } => {
1037 match self.ecx.machine.can_const_prop[place.local] {
1038 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1039 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1040 trace!("propped discriminant into {:?}", place);
1042 Self::remove_const(&mut self.ecx, place.local);
1045 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1046 Self::remove_const(&mut self.ecx, place.local);
1050 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1051 let frame = self.ecx.frame_mut();
1052 frame.locals[local].value =
1053 if let StatementKind::StorageLive(_) = statement.kind {
1054 LocalValue::Live(interpret::Operand::Immediate(
1055 interpret::Immediate::Uninit,
1065 self.super_statement(statement, location);
1068 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1069 let source_info = terminator.source_info;
1070 self.source_info = Some(source_info);
1071 self.super_terminator(terminator, location);
1072 // Do NOT early return in this function, it does some crucial fixup of the state at the end!
1073 match &mut terminator.kind {
1074 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1075 if let Some(ref value) = self.eval_operand(&cond) {
1076 trace!("assertion on {:?} should be {:?}", value, expected);
1077 let expected = Scalar::from_bool(*expected);
1078 // FIXME should be used use_ecx rather than a local match... but we have
1079 // quite a few of these read_scalar/read_immediate that need fixing.
1080 if let Ok(value_const) = self.ecx.read_scalar(&value) {
1081 if expected != value_const {
1082 // Poison all places this operand references so that further code
1083 // doesn't use the invalid value
1085 Operand::Move(ref place) | Operand::Copy(ref place) => {
1086 Self::remove_const(&mut self.ecx, place.local);
1088 Operand::Constant(_) => {}
1091 if self.should_const_prop(value) {
1092 *cond = self.operand_from_scalar(
1094 self.tcx.types.bool,
1102 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1103 // FIXME: This is currently redundant with `visit_operand`, but sadly
1104 // always visiting operands currently causes a perf regression in LLVM codegen, so
1105 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1106 self.propagate_operand(discr)
1108 // None of these have Operands to const-propagate.
1109 TerminatorKind::Goto { .. }
1110 | TerminatorKind::Resume
1111 | TerminatorKind::Abort
1112 | TerminatorKind::Return
1113 | TerminatorKind::Unreachable
1114 | TerminatorKind::Drop { .. }
1115 | TerminatorKind::DropAndReplace { .. }
1116 | TerminatorKind::Yield { .. }
1117 | TerminatorKind::GeneratorDrop
1118 | TerminatorKind::FalseEdge { .. }
1119 | TerminatorKind::FalseUnwind { .. }
1120 | TerminatorKind::InlineAsm { .. } => {}
1121 // Every argument in our function calls have already been propagated in `visit_operand`.
1123 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1124 // gated on `mir_opt_level=3`.
1125 TerminatorKind::Call { .. } => {}
1128 // We remove all Locals which are restricted in propagation to their containing blocks and
1129 // which were modified in the current block.
1130 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1131 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1132 for &local in locals.iter() {
1133 Self::remove_const(&mut self.ecx, local);
1136 // Put it back so we reuse the heap of the storage
1137 self.ecx.machine.written_only_inside_own_block_locals = locals;
1138 if cfg!(debug_assertions) {
1139 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1140 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1142 self.get_const(local.into()).is_none()
1144 .layout_of(self.local_decls[local].ty)
1145 .map_or(true, |layout| layout.is_zst())