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::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable};
22 use rustc_span::{def_id::DefId, Span};
23 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
24 use rustc_target::spec::abi::Abi;
25 use rustc_trait_selection::traits;
28 use rustc_const_eval::interpret::{
29 self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
30 ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, MemoryKind, OpTy,
31 Operand as InterpOperand, PlaceTy, Scalar, ScalarMaybeUninit, StackPopCleanup, StackPopUnwind,
34 /// The maximum number of bytes that we'll allocate space for a local or the return value.
35 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
36 /// Severely regress performance.
37 const MAX_ALLOC_LIMIT: u64 = 1024;
39 /// Macro for machine-specific `InterpError` without allocation.
40 /// (These will never be shown to the user, but they help diagnose ICEs.)
41 macro_rules! throw_machine_stop_str {
43 // We make a new local type for it. The type itself does not carry any information,
44 // but its vtable (for the `MachineStopType` trait) does.
46 // Printing this type shows the desired string.
47 impl std::fmt::Display for Zst {
48 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
52 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
53 throw_machine_stop!(Zst)
59 impl<'tcx> MirPass<'tcx> for ConstProp {
60 fn is_enabled(&self, _sess: &rustc_session::Session) -> bool {
61 // FIXME(#70073): Unlike the other passes in "optimizations", this one emits errors, so it
62 // runs even when MIR optimizations are disabled. We should separate the lint out from the
63 // transform and move the lint as early in the pipeline as possible.
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 is_fn_like = tcx.hir().get_by_def_id(def_id).fn_kind().is_some();
75 let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
77 // Only run const prop on functions, methods, closures and associated constants
78 if !is_fn_like && !is_assoc_const {
79 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
80 trace!("ConstProp skipped for {:?}", def_id);
84 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
85 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
86 // computing their layout.
88 trace!("ConstProp skipped for generator {:?}", def_id);
92 // Check if it's even possible to satisfy the 'where' clauses
94 // This branch will never be taken for any normal function.
95 // However, it's possible to `#!feature(trivial_bounds)]` to write
96 // a function with impossible to satisfy clauses, e.g.:
97 // `fn foo() where String: Copy {}`
99 // We don't usually need to worry about this kind of case,
100 // since we would get a compilation error if the user tried
101 // to call it. However, since we can do const propagation
102 // even without any calls to the function, we need to make
103 // sure that it even makes sense to try to evaluate the body.
104 // If there are unsatisfiable where clauses, then all bets are
105 // off, and we just give up.
107 // We manually filter the predicates, skipping anything that's not
108 // "global". We are in a potentially generic context
109 // (e.g. we are evaluating a function without substituting generic
110 // parameters, so this filtering serves two purposes:
112 // 1. We skip evaluating any predicates that we would
113 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
114 // 2. We avoid trying to normalize predicates involving generic
115 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
116 // the normalization code (leading to cycle errors), since
117 // it's usually never invoked in this way.
119 .predicates_of(def_id.to_def_id())
122 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
123 if traits::impossible_predicates(
125 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
127 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
131 trace!("ConstProp starting for {:?}", def_id);
133 let dummy_body = &Body::new(
135 body.basic_blocks().clone(),
136 body.source_scopes.clone(),
137 body.local_decls.clone(),
142 body.generator_kind(),
143 body.tainted_by_errors,
146 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
147 // constants, instead of just checking for const-folding succeeding.
148 // That would require a uniform one-def no-mutation analysis
149 // and RPO (or recursing when needing the value of a local).
150 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
151 optimization_finder.visit_body(body);
153 trace!("ConstProp done for {:?}", def_id);
157 struct ConstPropMachine<'mir, 'tcx> {
158 /// The virtual call stack.
159 stack: Vec<Frame<'mir, 'tcx>>,
160 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
161 written_only_inside_own_block_locals: FxHashSet<Local>,
162 /// Locals that need to be cleared after every block terminates.
163 only_propagate_inside_block_locals: BitSet<Local>,
164 can_const_prop: IndexVec<Local, ConstPropMode>,
167 impl ConstPropMachine<'_, '_> {
169 only_propagate_inside_block_locals: BitSet<Local>,
170 can_const_prop: IndexVec<Local, ConstPropMode>,
174 written_only_inside_own_block_locals: Default::default(),
175 only_propagate_inside_block_locals,
181 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
182 compile_time_machine!(<'mir, 'tcx>);
183 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
187 type MemoryExtra = ();
190 _ecx: &InterpCx<'mir, 'tcx, Self>,
191 _instance: ty::InstanceDef<'tcx>,
192 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
193 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
196 fn find_mir_or_eval_fn(
197 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
198 _instance: ty::Instance<'tcx>,
200 _args: &[OpTy<'tcx>],
201 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
202 _unwind: StackPopUnwind,
203 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
208 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
209 _instance: ty::Instance<'tcx>,
210 _args: &[OpTy<'tcx>],
211 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
212 _unwind: StackPopUnwind,
213 ) -> InterpResult<'tcx> {
214 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
218 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
219 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
220 _unwind: Option<rustc_middle::mir::BasicBlock>,
221 ) -> InterpResult<'tcx> {
222 bug!("panics terminators are not evaluated in ConstProp")
226 _ecx: &InterpCx<'mir, 'tcx, Self>,
229 _right: &ImmTy<'tcx>,
230 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
231 // We can't do this because aliasing of memory can differ between const eval and llvm
232 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
236 _ecx: &InterpCx<'mir, 'tcx, Self>,
237 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
239 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
240 let l = &frame.locals[local];
242 if l.value == LocalValue::Unallocated {
243 throw_machine_stop_str!("tried to access an unallocated local")
249 fn access_local_mut<'a>(
250 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
253 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
255 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
256 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
258 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
260 "mutating local {:?} which is restricted to its block. \
261 Will remove it from const-prop after block is finished.",
264 ecx.machine.written_only_inside_own_block_locals.insert(local);
266 ecx.machine.stack[frame].locals[local].access_mut()
269 fn before_access_global(
272 alloc: ConstAllocation<'tcx, Self::PointerTag, Self::AllocExtra>,
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 frame: Frame<'mir, 'tcx>,
292 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
298 ecx: &'a InterpCx<'mir, 'tcx, Self>,
299 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
305 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
306 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
307 &mut ecx.machine.stack
311 /// Finds optimization opportunities on the MIR.
312 struct ConstPropagator<'mir, 'tcx> {
313 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
315 param_env: ParamEnv<'tcx>,
316 // FIXME(eddyb) avoid cloning this field more than once,
317 // by accessing it through `ecx` instead.
318 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
319 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
320 // the last known `SourceInfo` here and just keep revisiting it.
321 source_info: Option<SourceInfo>,
324 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
325 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
328 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
333 impl HasDataLayout for ConstPropagator<'_, '_> {
335 fn data_layout(&self) -> &TargetDataLayout {
336 &self.tcx.data_layout
340 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
342 fn tcx(&self) -> TyCtxt<'tcx> {
347 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
349 fn param_env(&self) -> ty::ParamEnv<'tcx> {
354 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
357 dummy_body: &'mir Body<'tcx>,
359 ) -> ConstPropagator<'mir, 'tcx> {
360 let def_id = body.source.def_id();
361 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
362 let param_env = tcx.param_env_reveal_all_normalized(def_id);
364 let span = tcx.def_span(def_id);
365 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
366 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
367 // `layout_of` query invocations.
368 let can_const_prop = CanConstProp::check(tcx, param_env, body);
369 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
370 for (l, mode) in can_const_prop.iter_enumerated() {
371 if *mode == ConstPropMode::OnlyInsideOwnBlock {
372 only_propagate_inside_block_locals.insert(l);
375 let mut ecx = InterpCx::new(
379 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
384 .layout_of(body.return_ty().subst(tcx, substs))
386 // Don't bother allocating memory for ZST types which have no values
387 // or for large values.
388 .filter(|ret_layout| {
389 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
392 ecx.allocate(ret_layout, MemoryKind::Stack)
393 .expect("couldn't perform small allocation")
397 ecx.push_stack_frame(
398 Instance::new(def_id, substs),
401 StackPopCleanup::Root { cleanup: false },
403 .expect("failed to push initial stack frame");
409 // FIXME(eddyb) avoid cloning this field more than once,
410 // by accessing it through `ecx` instead.
411 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
412 local_decls: body.local_decls.clone(),
417 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
418 let op = match self.ecx.eval_place_to_op(place, None) {
421 trace!("get_const failed: {}", e);
426 // Try to read the local as an immediate so that if it is representable as a scalar, we can
427 // handle it as such, but otherwise, just return the value as is.
428 Some(match self.ecx.try_read_immediate(&op) {
429 Ok(Ok(imm)) => imm.into(),
434 /// Remove `local` from the pool of `Locals`. Allows writing to them,
435 /// but not reading from them anymore.
436 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
437 ecx.frame_mut().locals[local] =
438 LocalState { value: LocalValue::Unallocated, layout: Cell::new(None) };
441 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
443 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
446 Ok(val) => Some(val),
448 trace!("InterpCx operation failed: {:?}", error);
449 // Some errors shouldn't come up because creating them causes
450 // an allocation, which we should avoid. When that happens,
451 // dedicated error variants should be introduced instead.
453 !error.kind().formatted_string(),
454 "const-prop encountered formatting error: {}",
462 /// Returns the value, if any, of evaluating `c`.
463 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
464 // FIXME we need to revisit this for #67176
469 self.ecx.mir_const_to_op(&c.literal, None).ok()
472 /// Returns the value, if any, of evaluating `place`.
473 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
474 trace!("eval_place(place={:?})", place);
475 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
478 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
479 /// or `eval_place`, depending on the variant of `Operand` used.
480 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
482 Operand::Constant(ref c) => self.eval_constant(c),
483 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
487 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
488 if self.use_ecx(|this| {
489 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
490 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
493 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
494 // appropriate to use.
495 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
505 left: &Operand<'tcx>,
506 right: &Operand<'tcx>,
508 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
509 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
510 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
511 if op == BinOp::Shr || op == BinOp::Shl {
513 // We need the type of the LHS. We cannot use `place_layout` as that is the type
514 // of the result, which for checked binops is not the same!
515 let left_ty = left.ty(&self.local_decls, self.tcx);
516 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
517 let right_size = r.layout.size;
518 let r_bits = r.to_scalar().ok();
519 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
520 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
525 if let (Some(l), Some(r)) = (&l, &r) {
526 // The remaining operators are handled through `overflowing_binary_op`.
527 if self.use_ecx(|this| {
528 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
537 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
539 Operand::Copy(l) | Operand::Move(l) => {
540 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
541 // FIXME(felix91gr): this code only handles `Scalar` cases.
542 // For now, we're not handling `ScalarPair` cases because
543 // doing so here would require a lot of code duplication.
544 // We should hopefully generalize `Operand` handling into a fn,
545 // and use it to do const-prop here and everywhere else
546 // where it makes sense.
547 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
548 ScalarMaybeUninit::Scalar(scalar),
551 *operand = self.operand_from_scalar(
554 self.source_info.unwrap().span,
559 Operand::Constant(_) => (),
563 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
564 // Perform any special handling for specific Rvalue types.
565 // Generally, checks here fall into one of two categories:
566 // 1. Additional checking to provide useful lints to the user
567 // - In this case, we will do some validation and then fall through to the
568 // end of the function which evals the assignment.
569 // 2. Working around bugs in other parts of the compiler
570 // - In this case, we'll return `None` from this function to stop evaluation.
572 // Additional checking: give lints to the user if an overflow would occur.
573 // We do this here and not in the `Assert` terminator as that terminator is
574 // only sometimes emitted (overflow checks can be disabled), but we want to always
576 Rvalue::UnaryOp(op, arg) => {
577 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
578 self.check_unary_op(*op, arg)?;
580 Rvalue::BinaryOp(op, box (left, right)) => {
581 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
582 self.check_binary_op(*op, left, right)?;
584 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
586 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
591 self.check_binary_op(*op, left, right)?;
594 // Do not try creating references (#67862)
595 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
596 trace!("skipping AddressOf | Ref for {:?}", place);
598 // This may be creating mutable references or immutable references to cells.
599 // If that happens, the pointed to value could be mutated via that reference.
600 // Since we aren't tracking references, the const propagator loses track of what
601 // value the local has right now.
602 // Thus, all locals that have their reference taken
603 // must not take part in propagation.
604 Self::remove_const(&mut self.ecx, place.local);
608 Rvalue::ThreadLocalRef(def_id) => {
609 trace!("skipping ThreadLocalRef({:?})", def_id);
614 // There's no other checking to do at this time.
615 Rvalue::Aggregate(..)
620 | Rvalue::ShallowInitBox(..)
621 | Rvalue::Discriminant(..)
622 | Rvalue::NullaryOp(..) => {}
625 // FIXME we need to revisit this for #67176
626 if rvalue.needs_subst() {
630 if self.tcx.sess.mir_opt_level() >= 4 {
631 self.eval_rvalue_with_identities(rvalue, place)
633 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
637 // Attempt to use algebraic identities to eliminate constant expressions
638 fn eval_rvalue_with_identities(
640 rvalue: &Rvalue<'tcx>,
643 self.use_ecx(|this| match rvalue {
644 Rvalue::BinaryOp(op, box (left, right))
645 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
646 let l = this.ecx.eval_operand(left, None);
647 let r = this.ecx.eval_operand(right, None);
649 let const_arg = match (l, r) {
650 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
651 (Err(e), Err(_)) => return Err(e),
652 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
655 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
656 let dest = this.ecx.eval_place(place)?;
659 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
661 if arg_value == const_arg.layout.size.truncate(u128::MAX)
662 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
664 this.ecx.write_immediate(*const_arg, &dest)
666 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
667 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
668 let val = Immediate::ScalarPair(
669 const_arg.to_scalar()?.into(),
670 Scalar::from_bool(false).into(),
672 this.ecx.write_immediate(val, &dest)
674 this.ecx.write_immediate(*const_arg, &dest)
677 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
680 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
684 /// Creates a new `Operand::Constant` from a `Scalar` value
685 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
686 Operand::Constant(Box::new(Constant {
689 literal: ty::Const::from_scalar(self.tcx, scalar, ty).into(),
693 fn replace_with_const(
695 rval: &mut Rvalue<'tcx>,
697 source_info: SourceInfo,
699 if let Rvalue::Use(Operand::Constant(c)) = rval {
701 ConstantKind::Ty(c) if matches!(c.val(), ConstKind::Unevaluated(..)) => {}
703 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
709 trace!("attempting to replace {:?} with {:?}", rval, value);
710 if let Err(e) = self.ecx.const_validate_operand(
713 // FIXME: is ref tracking too expensive?
714 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
715 &mut interpret::RefTracking::empty(),
716 CtfeValidationMode::Regular,
718 trace!("validation error, attempt failed: {:?}", e);
722 // FIXME> figure out what to do when try_read_immediate fails
723 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
725 if let Some(Ok(imm)) = imm {
727 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
728 *rval = Rvalue::Use(self.operand_from_scalar(
734 Immediate::ScalarPair(
735 ScalarMaybeUninit::Scalar(_),
736 ScalarMaybeUninit::Scalar(_),
738 // Found a value represented as a pair. For now only do const-prop if the type
739 // of `rvalue` is also a tuple with two scalars.
740 // FIXME: enable the general case stated above ^.
741 let ty = value.layout.ty;
742 // Only do it for tuples
743 if let ty::Tuple(types) = ty.kind() {
744 // Only do it if tuple is also a pair with two scalars
745 if let [ty1, ty2] = types[..] {
746 let alloc = self.use_ecx(|this| {
747 let ty_is_scalar = |ty| {
748 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
751 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
754 .intern_with_temp_alloc(value.layout, |ecx, dest| {
755 ecx.write_immediate(*imm, dest)
764 if let Some(Some(alloc)) = alloc {
765 // Assign entire constant in a single statement.
766 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
767 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
768 span: source_info.span,
773 .mk_const(ty::ConstS {
775 val: ty::ConstKind::Value(ConstValue::ByRef {
786 // Scalars or scalar pairs that contain undef values are assumed to not have
787 // successfully evaluated and are thus not propagated.
793 /// Returns `true` if and only if this `op` should be const-propagated into.
794 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
795 let mir_opt_level = self.tcx.sess.mir_opt_level();
797 if mir_opt_level == 0 {
801 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
806 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
807 s.try_to_int().is_ok()
809 interpret::Operand::Immediate(Immediate::ScalarPair(
810 ScalarMaybeUninit::Scalar(l),
811 ScalarMaybeUninit::Scalar(r),
812 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
818 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
819 #[derive(Clone, Copy, Debug, PartialEq)]
821 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
823 /// The `Local` can only be propagated into and from its own block.
825 /// The `Local` can be propagated into but reads cannot be propagated.
827 /// The `Local` cannot be part of propagation at all. Any statement
828 /// referencing it either for reading or writing will not get propagated.
832 struct CanConstProp {
833 can_const_prop: IndexVec<Local, ConstPropMode>,
834 // False at the beginning. Once set, no more assignments are allowed to that local.
835 found_assignment: BitSet<Local>,
836 // Cache of locals' information
837 local_kinds: IndexVec<Local, LocalKind>,
841 /// Returns true if `local` can be propagated
844 param_env: ParamEnv<'tcx>,
846 ) -> IndexVec<Local, ConstPropMode> {
847 let mut cpv = CanConstProp {
848 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
849 found_assignment: BitSet::new_empty(body.local_decls.len()),
850 local_kinds: IndexVec::from_fn_n(
851 |local| body.local_kind(local),
852 body.local_decls.len(),
855 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
856 let ty = body.local_decls[local].ty;
857 match tcx.layout_of(param_env.and(ty)) {
858 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
859 // Either the layout fails to compute, then we can't use this local anyway
860 // or the local is too large, then we don't want to.
862 *val = ConstPropMode::NoPropagation;
866 // Cannot use args at all
867 // Cannot use locals because if x < y { y - x } else { x - y } would
869 // FIXME(oli-obk): lint variables until they are used in a condition
870 // FIXME(oli-obk): lint if return value is constant
871 if cpv.local_kinds[local] == LocalKind::Arg {
872 *val = ConstPropMode::OnlyPropagateInto;
874 "local {:?} can't be const propagated because it's a function argument",
877 } else if cpv.local_kinds[local] == LocalKind::Var {
878 *val = ConstPropMode::OnlyInsideOwnBlock;
880 "local {:?} will only be propagated inside its block, because it's a user variable",
885 cpv.visit_body(&body);
890 impl Visitor<'_> for CanConstProp {
891 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
892 use rustc_middle::mir::visit::PlaceContext::*;
894 // Projections are fine, because `&mut foo.x` will be caught by
895 // `MutatingUseContext::Borrow` elsewhere.
896 MutatingUse(MutatingUseContext::Projection)
897 // These are just stores, where the storing is not propagatable, but there may be later
898 // mutations of the same local via `Store`
899 | MutatingUse(MutatingUseContext::Call)
900 | MutatingUse(MutatingUseContext::AsmOutput)
901 // Actual store that can possibly even propagate a value
902 | MutatingUse(MutatingUseContext::Store) => {
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::Unallocated
1058 self.super_statement(statement, location);
1061 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1062 let source_info = terminator.source_info;
1063 self.source_info = Some(source_info);
1064 self.super_terminator(terminator, location);
1065 match &mut terminator.kind {
1066 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1067 if let Some(ref value) = self.eval_operand(&cond) {
1068 trace!("assertion on {:?} should be {:?}", value, expected);
1069 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1070 let value_const = self.ecx.read_scalar(&value).unwrap();
1071 if expected != value_const {
1072 // Poison all places this operand references so that further code
1073 // doesn't use the invalid value
1075 Operand::Move(ref place) | Operand::Copy(ref place) => {
1076 Self::remove_const(&mut self.ecx, place.local);
1078 Operand::Constant(_) => {}
1081 if self.should_const_prop(value) {
1082 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1083 *cond = self.operand_from_scalar(
1085 self.tcx.types.bool,
1093 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1094 // FIXME: This is currently redundant with `visit_operand`, but sadly
1095 // always visiting operands currently causes a perf regression in LLVM codegen, so
1096 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1097 self.propagate_operand(discr)
1099 // None of these have Operands to const-propagate.
1100 TerminatorKind::Goto { .. }
1101 | TerminatorKind::Resume
1102 | TerminatorKind::Abort
1103 | TerminatorKind::Return
1104 | TerminatorKind::Unreachable
1105 | TerminatorKind::Drop { .. }
1106 | TerminatorKind::DropAndReplace { .. }
1107 | TerminatorKind::Yield { .. }
1108 | TerminatorKind::GeneratorDrop
1109 | TerminatorKind::FalseEdge { .. }
1110 | TerminatorKind::FalseUnwind { .. }
1111 | TerminatorKind::InlineAsm { .. } => {}
1112 // Every argument in our function calls have already been propagated in `visit_operand`.
1114 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1115 // gated on `mir_opt_level=3`.
1116 TerminatorKind::Call { .. } => {}
1119 // We remove all Locals which are restricted in propagation to their containing blocks and
1120 // which were modified in the current block.
1121 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1122 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1123 for &local in locals.iter() {
1124 Self::remove_const(&mut self.ecx, local);
1127 // Put it back so we reuse the heap of the storage
1128 self.ecx.machine.written_only_inside_own_block_locals = locals;
1129 if cfg!(debug_assertions) {
1130 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1131 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1133 self.get_const(local.into()).is_none()
1135 .layout_of(self.local_decls[local].ty)
1136 .map_or(true, |layout| layout.is_zst())