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;
10 use rustc_index::bit_set::BitSet;
11 use rustc_index::vec::IndexVec;
12 use rustc_middle::mir::visit::{
13 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
15 use rustc_middle::mir::{
16 AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
17 Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
18 StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
20 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
21 use rustc_middle::ty::subst::{InternalSubsts, Subst};
22 use rustc_middle::ty::{
23 self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
25 use rustc_session::lint;
26 use rustc_span::{def_id::DefId, Span};
27 use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
28 use rustc_target::spec::abi::Abi;
29 use rustc_trait_selection::traits;
32 use rustc_const_eval::const_eval::ConstEvalErr;
33 use rustc_const_eval::interpret::{
34 self, compile_time_machine, AllocId, Allocation, ConstValue, CtfeValidationMode, Frame, ImmTy,
35 Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, MemoryKind, OpTy,
36 Operand as InterpOperand, PlaceTy, Scalar, ScalarMaybeUninit, StackPopCleanup, StackPopUnwind,
39 /// The maximum number of bytes that we'll allocate space for a local or the return value.
40 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
41 /// Severely regress performance.
42 const MAX_ALLOC_LIMIT: u64 = 1024;
44 /// Macro for machine-specific `InterpError` without allocation.
45 /// (These will never be shown to the user, but they help diagnose ICEs.)
46 macro_rules! throw_machine_stop_str {
48 // We make a new local type for it. The type itself does not carry any information,
49 // but its vtable (for the `MachineStopType` trait) does.
51 // Printing this type shows the desired string.
52 impl std::fmt::Display for Zst {
53 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
57 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
58 throw_machine_stop!(Zst)
64 impl<'tcx> MirPass<'tcx> for ConstProp {
65 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
66 // will be evaluated by miri and produce its errors there
67 if body.source.promoted.is_some() {
71 use rustc_middle::hir::map::blocks::FnLikeNode;
72 let def_id = body.source.def_id().expect_local();
73 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
75 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
76 let is_assoc_const = tcx.def_kind(def_id.to_def_id()) == 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(tcx) { 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(
137 body.basic_blocks().clone(),
138 body.source_scopes.clone(),
139 body.local_decls.clone(),
144 body.generator_kind(),
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 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 written_only_inside_own_block_locals: FxHashSet<Local>,
163 /// Locals that need to be cleared after every block terminates.
164 only_propagate_inside_block_locals: BitSet<Local>,
165 can_const_prop: IndexVec<Local, ConstPropMode>,
168 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
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`)
188 type MemoryExtra = ();
191 _ecx: &InterpCx<'mir, 'tcx, Self>,
192 _instance: ty::InstanceDef<'tcx>,
193 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
194 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
197 fn find_mir_or_eval_fn(
198 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
199 _instance: ty::Instance<'tcx>,
201 _args: &[OpTy<'tcx>],
202 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
203 _unwind: StackPopUnwind,
204 ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
209 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
210 _instance: ty::Instance<'tcx>,
211 _args: &[OpTy<'tcx>],
212 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
213 _unwind: StackPopUnwind,
214 ) -> InterpResult<'tcx> {
215 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
219 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
220 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
221 _unwind: Option<rustc_middle::mir::BasicBlock>,
222 ) -> InterpResult<'tcx> {
223 bug!("panics terminators are not evaluated in ConstProp")
227 _ecx: &InterpCx<'mir, 'tcx, Self>,
230 _right: &ImmTy<'tcx>,
231 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
232 // We can't do this because aliasing of memory can differ between const eval and llvm
233 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
237 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
238 _dest: &PlaceTy<'tcx>,
239 ) -> InterpResult<'tcx> {
240 throw_machine_stop_str!("can't const prop heap allocations")
244 _ecx: &InterpCx<'mir, 'tcx, Self>,
245 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
247 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
248 let l = &frame.locals[local];
250 if l.value == LocalValue::Uninitialized {
251 throw_machine_stop_str!("tried to access an uninitialized local")
257 fn access_local_mut<'a>(
258 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
261 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
263 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
264 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
266 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
268 "mutating local {:?} which is restricted to its block. \
269 Will remove it from const-prop after block is finished.",
272 ecx.machine.written_only_inside_own_block_locals.insert(local);
274 ecx.machine.stack[frame].locals[local].access_mut()
277 fn before_access_global(
280 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
281 _static_def_id: Option<DefId>,
283 ) -> InterpResult<'tcx> {
285 throw_machine_stop_str!("can't write to global");
287 // If the static allocation is mutable, then we can't const prop it as its content
288 // might be different at runtime.
289 if allocation.mutability == Mutability::Mut {
290 throw_machine_stop_str!("can't access mutable globals 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::PointerTag, Self::FrameExtra>] {
313 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
314 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, 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 // FIXME(eddyb) avoid cloning these two fields more than once,
325 // by accessing them through `ecx` instead.
326 source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
327 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
328 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
329 // the last known `SourceInfo` here and just keep revisiting it.
330 source_info: Option<SourceInfo>,
333 impl<'mir, 'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'mir, 'tcx> {
334 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
337 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
342 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
344 fn data_layout(&self) -> &TargetDataLayout {
345 &self.tcx.data_layout
349 impl<'mir, 'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
351 fn tcx(&self) -> TyCtxt<'tcx> {
356 impl<'mir, 'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'mir, 'tcx> {
358 fn param_env(&self) -> ty::ParamEnv<'tcx> {
363 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
366 dummy_body: &'mir Body<'tcx>,
368 ) -> ConstPropagator<'mir, 'tcx> {
369 let def_id = body.source.def_id();
370 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
371 let param_env = tcx.param_env_reveal_all_normalized(def_id);
373 let span = tcx.def_span(def_id);
374 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
375 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
376 // `layout_of` query invocations.
377 let can_const_prop = CanConstProp::check(tcx, param_env, body);
378 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
379 for (l, mode) in can_const_prop.iter_enumerated() {
380 if *mode == ConstPropMode::OnlyInsideOwnBlock {
381 only_propagate_inside_block_locals.insert(l);
384 let mut ecx = InterpCx::new(
388 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
393 .layout_of(body.return_ty().subst(tcx, substs))
395 // Don't bother allocating memory for ZST types which have no values
396 // or for large values.
397 .filter(|ret_layout| {
398 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
401 ecx.allocate(ret_layout, MemoryKind::Stack)
402 .expect("couldn't perform small allocation")
406 ecx.push_stack_frame(
407 Instance::new(def_id, substs),
410 StackPopCleanup::None { cleanup: false },
412 .expect("failed to push initial stack frame");
418 // FIXME(eddyb) avoid cloning these two fields more than once,
419 // by accessing them through `ecx` instead.
420 source_scopes: body.source_scopes.clone(),
421 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
422 local_decls: body.local_decls.clone(),
427 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
428 let op = match self.ecx.eval_place_to_op(place, None) {
431 trace!("get_const failed: {}", e);
436 // Try to read the local as an immediate so that if it is representable as a scalar, we can
437 // handle it as such, but otherwise, just return the value as is.
438 Some(match self.ecx.try_read_immediate(&op) {
439 Ok(Ok(imm)) => imm.into(),
444 /// Remove `local` from the pool of `Locals`. Allows writing to them,
445 /// but not reading from them anymore.
446 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
447 ecx.frame_mut().locals[local] =
448 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
451 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
452 source_info.scope.lint_root(&self.source_scopes)
455 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
457 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
460 Ok(val) => Some(val),
462 trace!("InterpCx operation failed: {:?}", error);
463 // Some errors shouldn't come up because creating them causes
464 // an allocation, which we should avoid. When that happens,
465 // dedicated error variants should be introduced instead.
467 !error.kind().formatted_string(),
468 "const-prop encountered formatting error: {}",
476 /// Returns the value, if any, of evaluating `c`.
477 fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
478 // FIXME we need to revisit this for #67176
479 if c.definitely_needs_subst(self.tcx) {
483 match self.ecx.mir_const_to_op(&c.literal, None) {
486 let tcx = self.ecx.tcx.at(c.span);
487 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
488 if let Some(lint_root) = self.lint_root(source_info) {
489 let lint_only = match c.literal {
490 ConstantKind::Ty(ct) => match ct.val {
491 // Promoteds must lint and not error as the user didn't ask for them
492 ConstKind::Unevaluated(ty::Unevaluated {
497 // Out of backwards compatibility we cannot report hard errors in unused
498 // generic functions using associated constants of the generic parameters.
499 _ => c.literal.definitely_needs_subst(*tcx),
501 ConstantKind::Val(_, ty) => ty.definitely_needs_subst(*tcx),
504 // Out of backwards compatibility we cannot report hard errors in unused
505 // generic functions using associated constants of the generic parameters.
506 err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
508 err.report_as_error(tcx, "erroneous constant used");
511 err.report_as_error(tcx, "erroneous constant used");
518 /// Returns the value, if any, of evaluating `place`.
519 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
520 trace!("eval_place(place={:?})", place);
521 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
524 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
525 /// or `eval_place`, depending on the variant of `Operand` used.
526 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
528 Operand::Constant(ref c) => self.eval_constant(c, source_info),
529 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
533 fn report_assert_as_lint(
535 lint: &'static lint::Lint,
536 source_info: SourceInfo,
537 message: &'static str,
538 panic: AssertKind<impl std::fmt::Debug>,
540 if let Some(lint_root) = self.lint_root(source_info) {
541 self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
542 let mut err = lint.build(message);
543 err.span_label(source_info.span, format!("{:?}", panic));
553 source_info: SourceInfo,
555 if let (val, true) = self.use_ecx(|this| {
556 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
557 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
560 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
561 // appropriate to use.
562 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
563 self.report_assert_as_lint(
564 lint::builtin::ARITHMETIC_OVERFLOW,
566 "this arithmetic operation will overflow",
567 AssertKind::OverflowNeg(val.to_const_int()),
578 left: &Operand<'tcx>,
579 right: &Operand<'tcx>,
580 source_info: SourceInfo,
582 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
583 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
584 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
585 if op == BinOp::Shr || op == BinOp::Shl {
587 // We need the type of the LHS. We cannot use `place_layout` as that is the type
588 // of the result, which for checked binops is not the same!
589 let left_ty = left.ty(&self.local_decls, self.tcx);
590 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
591 let right_size = r.layout.size;
592 let r_bits = r.to_scalar().ok();
593 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
594 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
595 debug!("check_binary_op: reporting assert for {:?}", source_info);
596 self.report_assert_as_lint(
597 lint::builtin::ARITHMETIC_OVERFLOW,
599 "this arithmetic operation will overflow",
600 AssertKind::Overflow(
603 Some(l) => l.to_const_int(),
604 // Invent a dummy value, the diagnostic ignores it anyway
605 None => ConstInt::new(
606 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
608 left_ty.is_ptr_sized_integral(),
618 if let (Some(l), Some(r)) = (&l, &r) {
619 // The remaining operators are handled through `overflowing_binary_op`.
620 if self.use_ecx(|this| {
621 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
624 self.report_assert_as_lint(
625 lint::builtin::ARITHMETIC_OVERFLOW,
627 "this arithmetic operation will overflow",
628 AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
636 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
638 Operand::Copy(l) | Operand::Move(l) => {
639 if let Some(value) = self.get_const(l) {
640 if self.should_const_prop(&value) {
641 // FIXME(felix91gr): this code only handles `Scalar` cases.
642 // For now, we're not handling `ScalarPair` cases because
643 // doing so here would require a lot of code duplication.
644 // We should hopefully generalize `Operand` handling into a fn,
645 // and use it to do const-prop here and everywhere else
646 // where it makes sense.
647 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
648 ScalarMaybeUninit::Scalar(scalar),
651 *operand = self.operand_from_scalar(
654 self.source_info.unwrap().span,
660 Operand::Constant(_) => (),
666 rvalue: &Rvalue<'tcx>,
667 source_info: SourceInfo,
670 // Perform any special handling for specific Rvalue types.
671 // Generally, checks here fall into one of two categories:
672 // 1. Additional checking to provide useful lints to the user
673 // - In this case, we will do some validation and then fall through to the
674 // end of the function which evals the assignment.
675 // 2. Working around bugs in other parts of the compiler
676 // - In this case, we'll return `None` from this function to stop evaluation.
678 // Additional checking: give lints to the user if an overflow would occur.
679 // We do this here and not in the `Assert` terminator as that terminator is
680 // only sometimes emitted (overflow checks can be disabled), but we want to always
682 Rvalue::UnaryOp(op, arg) => {
683 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
684 self.check_unary_op(*op, arg, source_info)?;
686 Rvalue::BinaryOp(op, box (left, right)) => {
687 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
688 self.check_binary_op(*op, left, right, source_info)?;
690 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
692 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
697 self.check_binary_op(*op, left, right, source_info)?;
700 // Do not try creating references (#67862)
701 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
702 trace!("skipping AddressOf | Ref for {:?}", place);
704 // This may be creating mutable references or immutable references to cells.
705 // If that happens, the pointed to value could be mutated via that reference.
706 // Since we aren't tracking references, the const propagator loses track of what
707 // value the local has right now.
708 // Thus, all locals that have their reference taken
709 // must not take part in propagation.
710 Self::remove_const(&mut self.ecx, place.local);
714 Rvalue::ThreadLocalRef(def_id) => {
715 trace!("skipping ThreadLocalRef({:?})", def_id);
720 // There's no other checking to do at this time.
721 Rvalue::Aggregate(..)
726 | Rvalue::ShallowInitBox(..)
727 | Rvalue::Discriminant(..)
728 | Rvalue::NullaryOp(..) => {}
731 // FIXME we need to revisit this for #67176
732 if rvalue.definitely_needs_subst(self.tcx) {
736 if self.tcx.sess.mir_opt_level() >= 4 {
737 self.eval_rvalue_with_identities(rvalue, place)
739 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
743 // Attempt to use albegraic identities to eliminate constant expressions
744 fn eval_rvalue_with_identities(
746 rvalue: &Rvalue<'tcx>,
749 self.use_ecx(|this| {
751 Rvalue::BinaryOp(op, box (left, right))
752 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
753 let l = this.ecx.eval_operand(left, None);
754 let r = this.ecx.eval_operand(right, None);
756 let const_arg = match (l, r) {
757 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
758 (Err(e), Err(_)) => return Err(e),
760 this.ecx.eval_rvalue_into_place(rvalue, place)?;
765 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
766 let dest = this.ecx.eval_place(place)?;
771 this.ecx.write_immediate(*const_arg, &dest)?;
775 if arg_value == const_arg.layout.size.truncate(u128::MAX)
776 || (const_arg.layout.ty.is_bool() && arg_value == 1)
778 this.ecx.write_immediate(*const_arg, &dest)?;
782 if const_arg.layout.ty.is_integral() && arg_value == 0 {
783 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
784 let val = Immediate::ScalarPair(
785 const_arg.to_scalar()?.into(),
786 Scalar::from_bool(false).into(),
788 this.ecx.write_immediate(val, &dest)?;
790 this.ecx.write_immediate(*const_arg, &dest)?;
795 this.ecx.eval_rvalue_into_place(rvalue, place)?;
800 this.ecx.eval_rvalue_into_place(rvalue, place)?;
808 /// Creates a new `Operand::Constant` from a `Scalar` value
809 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
810 Operand::Constant(Box::new(Constant {
813 literal: ty::Const::from_scalar(self.tcx, scalar, ty).into(),
817 fn replace_with_const(
819 rval: &mut Rvalue<'tcx>,
821 source_info: SourceInfo,
823 if let Rvalue::Use(Operand::Constant(c)) = rval {
825 ConstantKind::Ty(c) if matches!(c.val, ConstKind::Unevaluated(..)) => {}
827 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
833 trace!("attempting to replace {:?} with {:?}", rval, value);
834 if let Err(e) = self.ecx.const_validate_operand(
837 // FIXME: is ref tracking too expensive?
838 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
839 &mut interpret::RefTracking::empty(),
840 CtfeValidationMode::Regular,
842 trace!("validation error, attempt failed: {:?}", e);
846 // FIXME> figure out what to do when try_read_immediate fails
847 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
849 if let Some(Ok(imm)) = imm {
851 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
852 *rval = Rvalue::Use(self.operand_from_scalar(
858 Immediate::ScalarPair(
859 ScalarMaybeUninit::Scalar(_),
860 ScalarMaybeUninit::Scalar(_),
862 // Found a value represented as a pair. For now only do const-prop if the type
863 // of `rvalue` is also a tuple with two scalars.
864 // FIXME: enable the general case stated above ^.
865 let ty = &value.layout.ty;
866 // Only do it for tuples
867 if let ty::Tuple(substs) = ty.kind() {
868 // Only do it if tuple is also a pair with two scalars
869 if substs.len() == 2 {
870 let alloc = self.use_ecx(|this| {
871 let ty1 = substs[0].expect_ty();
872 let ty2 = substs[1].expect_ty();
873 let ty_is_scalar = |ty| {
874 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
877 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
880 .intern_with_temp_alloc(value.layout, |ecx, dest| {
881 ecx.write_immediate(*imm, dest)
890 if let Some(Some(alloc)) = alloc {
891 // Assign entire constant in a single statement.
892 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
893 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
894 span: source_info.span,
899 .mk_const(ty::Const {
901 val: ty::ConstKind::Value(ConstValue::ByRef {
912 // Scalars or scalar pairs that contain undef values are assumed to not have
913 // successfully evaluated and are thus not propagated.
919 /// Returns `true` if and only if this `op` should be const-propagated into.
920 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
921 let mir_opt_level = self.tcx.sess.mir_opt_level();
923 if mir_opt_level == 0 {
927 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
932 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
933 s.try_to_int().is_ok()
935 interpret::Operand::Immediate(Immediate::ScalarPair(
936 ScalarMaybeUninit::Scalar(l),
937 ScalarMaybeUninit::Scalar(r),
938 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
944 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
945 #[derive(Clone, Copy, Debug, PartialEq)]
947 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
949 /// The `Local` can only be propagated into and from its own block.
951 /// The `Local` can be propagated into but reads cannot be propagated.
953 /// The `Local` cannot be part of propagation at all. Any statement
954 /// referencing it either for reading or writing will not get propagated.
958 struct CanConstProp {
959 can_const_prop: IndexVec<Local, ConstPropMode>,
960 // False at the beginning. Once set, no more assignments are allowed to that local.
961 found_assignment: BitSet<Local>,
962 // Cache of locals' information
963 local_kinds: IndexVec<Local, LocalKind>,
967 /// Returns true if `local` can be propagated
970 param_env: ParamEnv<'tcx>,
972 ) -> IndexVec<Local, ConstPropMode> {
973 let mut cpv = CanConstProp {
974 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
975 found_assignment: BitSet::new_empty(body.local_decls.len()),
976 local_kinds: IndexVec::from_fn_n(
977 |local| body.local_kind(local),
978 body.local_decls.len(),
981 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
982 let ty = body.local_decls[local].ty;
983 match tcx.layout_of(param_env.and(ty)) {
984 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
985 // Either the layout fails to compute, then we can't use this local anyway
986 // or the local is too large, then we don't want to.
988 *val = ConstPropMode::NoPropagation;
992 // Cannot use args at all
993 // Cannot use locals because if x < y { y - x } else { x - y } would
995 // FIXME(oli-obk): lint variables until they are used in a condition
996 // FIXME(oli-obk): lint if return value is constant
997 if cpv.local_kinds[local] == LocalKind::Arg {
998 *val = ConstPropMode::OnlyPropagateInto;
1000 "local {:?} can't be const propagated because it's a function argument",
1003 } else if cpv.local_kinds[local] == LocalKind::Var {
1004 *val = ConstPropMode::OnlyInsideOwnBlock;
1006 "local {:?} will only be propagated inside its block, because it's a user variable",
1011 cpv.visit_body(&body);
1016 impl<'tcx> Visitor<'tcx> for CanConstProp {
1017 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
1018 use rustc_middle::mir::visit::PlaceContext::*;
1020 // Projections are fine, because `&mut foo.x` will be caught by
1021 // `MutatingUseContext::Borrow` elsewhere.
1022 MutatingUse(MutatingUseContext::Projection)
1023 // These are just stores, where the storing is not propagatable, but there may be later
1024 // mutations of the same local via `Store`
1025 | MutatingUse(MutatingUseContext::Call)
1026 // Actual store that can possibly even propagate a value
1027 | MutatingUse(MutatingUseContext::Store) => {
1028 if !self.found_assignment.insert(local) {
1029 match &mut self.can_const_prop[local] {
1030 // If the local can only get propagated in its own block, then we don't have
1031 // to worry about multiple assignments, as we'll nuke the const state at the
1032 // end of the block anyway, and inside the block we overwrite previous
1033 // states as applicable.
1034 ConstPropMode::OnlyInsideOwnBlock => {}
1035 ConstPropMode::NoPropagation => {}
1036 ConstPropMode::OnlyPropagateInto => {}
1037 other @ ConstPropMode::FullConstProp => {
1039 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
1042 *other = ConstPropMode::OnlyInsideOwnBlock;
1047 // Reading constants is allowed an arbitrary number of times
1048 NonMutatingUse(NonMutatingUseContext::Copy)
1049 | NonMutatingUse(NonMutatingUseContext::Move)
1050 | NonMutatingUse(NonMutatingUseContext::Inspect)
1051 | NonMutatingUse(NonMutatingUseContext::Projection)
1054 // These could be propagated with a smarter analysis or just some careful thinking about
1055 // whether they'd be fine right now.
1056 MutatingUse(MutatingUseContext::AsmOutput)
1057 | MutatingUse(MutatingUseContext::Yield)
1058 | MutatingUse(MutatingUseContext::Drop)
1059 | MutatingUse(MutatingUseContext::Retag)
1060 // These can't ever be propagated under any scheme, as we can't reason about indirect
1062 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1063 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1064 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1065 | NonMutatingUse(NonMutatingUseContext::AddressOf)
1066 | MutatingUse(MutatingUseContext::Borrow)
1067 | MutatingUse(MutatingUseContext::AddressOf) => {
1068 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1069 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1075 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
1076 fn tcx(&self) -> TyCtxt<'tcx> {
1080 fn visit_body(&mut self, body: &mut Body<'tcx>) {
1081 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1082 self.visit_basic_block_data(bb, data);
1086 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1087 self.super_operand(operand, location);
1089 // Only const prop copies and moves on `mir_opt_level=3` as doing so
1090 // currently slightly increases compile time in some cases.
1091 if self.tcx.sess.mir_opt_level() >= 3 {
1092 self.propagate_operand(operand)
1096 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1097 trace!("visit_constant: {:?}", constant);
1098 self.super_constant(constant, location);
1099 self.eval_constant(constant, self.source_info.unwrap());
1102 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1103 trace!("visit_statement: {:?}", statement);
1104 let source_info = statement.source_info;
1105 self.source_info = Some(source_info);
1106 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1107 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1108 if let Some(()) = self.const_prop(rval, source_info, place) {
1109 // This will return None if the above `const_prop` invocation only "wrote" a
1110 // type whose creation requires no write. E.g. a generator whose initial state
1111 // consists solely of uninitialized memory (so it doesn't capture any locals).
1112 if let Some(ref value) = self.get_const(place) {
1113 if self.should_const_prop(value) {
1114 trace!("replacing {:?} with {:?}", rval, value);
1115 self.replace_with_const(rval, value, source_info);
1116 if can_const_prop == ConstPropMode::FullConstProp
1117 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1119 trace!("propagated into {:?}", place);
1123 match can_const_prop {
1124 ConstPropMode::OnlyInsideOwnBlock => {
1126 "found local restricted to its block. \
1127 Will remove it from const-prop after block is finished. Local: {:?}",
1131 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1132 trace!("can't propagate into {:?}", place);
1133 if place.local != RETURN_PLACE {
1134 Self::remove_const(&mut self.ecx, place.local);
1137 ConstPropMode::FullConstProp => {}
1140 // Const prop failed, so erase the destination, ensuring that whatever happens
1141 // from here on, does not know about the previous value.
1142 // This is important in case we have
1145 // x = SOME_MUTABLE_STATIC;
1146 // // x must now be uninit
1148 // FIXME: we overzealously erase the entire local, because that's easier to
1151 "propagation into {:?} failed.
1152 Nuking the entire site from orbit, it's the only way to be sure",
1155 Self::remove_const(&mut self.ecx, place.local);
1158 match statement.kind {
1159 StatementKind::SetDiscriminant { ref place, .. } => {
1160 match self.ecx.machine.can_const_prop[place.local] {
1161 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1162 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1163 trace!("propped discriminant into {:?}", place);
1165 Self::remove_const(&mut self.ecx, place.local);
1168 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1169 Self::remove_const(&mut self.ecx, place.local);
1173 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1174 let frame = self.ecx.frame_mut();
1175 frame.locals[local].value =
1176 if let StatementKind::StorageLive(_) = statement.kind {
1177 LocalValue::Uninitialized
1186 self.super_statement(statement, location);
1189 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1190 let source_info = terminator.source_info;
1191 self.source_info = Some(source_info);
1192 self.super_terminator(terminator, location);
1193 match &mut terminator.kind {
1194 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1195 if let Some(ref value) = self.eval_operand(&cond, source_info) {
1196 trace!("assertion on {:?} should be {:?}", value, expected);
1197 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1198 let value_const = self.ecx.read_scalar(&value).unwrap();
1199 if expected != value_const {
1204 impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1205 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1207 Self::Val(val) => val.fmt(fmt),
1208 Self::Underscore => fmt.write_str("_"),
1212 let mut eval_to_int = |op| {
1213 // This can be `None` if the lhs wasn't const propagated and we just
1214 // triggered the assert on the value of the rhs.
1215 self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
1216 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
1219 let msg = match msg {
1220 AssertKind::DivisionByZero(op) => {
1221 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1223 AssertKind::RemainderByZero(op) => {
1224 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1226 AssertKind::BoundsCheck { ref len, ref index } => {
1227 let len = eval_to_int(len);
1228 let index = eval_to_int(index);
1229 Some(AssertKind::BoundsCheck { len, index })
1231 // Overflow is are already covered by checks on the binary operators.
1232 AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1233 // Need proper const propagator for these.
1236 // Poison all places this operand references so that further code
1237 // doesn't use the invalid value
1239 Operand::Move(ref place) | Operand::Copy(ref place) => {
1240 Self::remove_const(&mut self.ecx, place.local);
1242 Operand::Constant(_) => {}
1244 if let Some(msg) = msg {
1245 self.report_assert_as_lint(
1246 lint::builtin::UNCONDITIONAL_PANIC,
1248 "this operation will panic at runtime",
1253 if self.should_const_prop(value) {
1254 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1255 *cond = self.operand_from_scalar(
1257 self.tcx.types.bool,
1265 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1266 // FIXME: This is currently redundant with `visit_operand`, but sadly
1267 // always visiting operands currently causes a perf regression in LLVM codegen, so
1268 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1269 self.propagate_operand(discr)
1271 // None of these have Operands to const-propagate.
1272 TerminatorKind::Goto { .. }
1273 | TerminatorKind::Resume
1274 | TerminatorKind::Abort
1275 | TerminatorKind::Return
1276 | TerminatorKind::Unreachable
1277 | TerminatorKind::Drop { .. }
1278 | TerminatorKind::DropAndReplace { .. }
1279 | TerminatorKind::Yield { .. }
1280 | TerminatorKind::GeneratorDrop
1281 | TerminatorKind::FalseEdge { .. }
1282 | TerminatorKind::FalseUnwind { .. }
1283 | TerminatorKind::InlineAsm { .. } => {}
1284 // Every argument in our function calls have already been propagated in `visit_operand`.
1286 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1287 // gated on `mir_opt_level=3`.
1288 TerminatorKind::Call { .. } => {}
1291 // We remove all Locals which are restricted in propagation to their containing blocks and
1292 // which were modified in the current block.
1293 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1294 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1295 for &local in locals.iter() {
1296 Self::remove_const(&mut self.ecx, local);
1299 // Put it back so we reuse the heap of the storage
1300 self.ecx.machine.written_only_inside_own_block_locals = locals;
1301 if cfg!(debug_assertions) {
1302 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1303 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1305 self.get_const(local.into()).is_none()
1307 .layout_of(self.local_decls[local].ty)
1308 .map_or(true, |layout| layout.is_zst())