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 is_enabled(&self, _sess: &rustc_session::Session) -> bool {
66 // FIXME(#70073): Unlike the other passes in "optimizations", this one emits errors, so it
67 // runs even when MIR optimizations are disabled. We should separate the lint out from the
68 // transform and move the lint as early in the pipeline as possible.
72 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
73 // will be evaluated by miri and produce its errors there
74 if body.source.promoted.is_some() {
78 let def_id = body.source.def_id().expect_local();
79 let is_fn_like = tcx.hir().get_by_def_id(def_id).fn_kind().is_some();
80 let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
82 // Only run const prop on functions, methods, closures and associated constants
83 if !is_fn_like && !is_assoc_const {
84 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
85 trace!("ConstProp skipped for {:?}", def_id);
89 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
90 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
91 // computing their layout.
93 trace!("ConstProp skipped for generator {:?}", def_id);
97 // Check if it's even possible to satisfy the 'where' clauses
99 // This branch will never be taken for any normal function.
100 // However, it's possible to `#!feature(trivial_bounds)]` to write
101 // a function with impossible to satisfy clauses, e.g.:
102 // `fn foo() where String: Copy {}`
104 // We don't usually need to worry about this kind of case,
105 // since we would get a compilation error if the user tried
106 // to call it. However, since we can do const propagation
107 // even without any calls to the function, we need to make
108 // sure that it even makes sense to try to evaluate the body.
109 // If there are unsatisfiable where clauses, then all bets are
110 // off, and we just give up.
112 // We manually filter the predicates, skipping anything that's not
113 // "global". We are in a potentially generic context
114 // (e.g. we are evaluating a function without substituting generic
115 // parameters, so this filtering serves two purposes:
117 // 1. We skip evaluating any predicates that we would
118 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
119 // 2. We avoid trying to normalize predicates involving generic
120 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
121 // the normalization code (leading to cycle errors), since
122 // it's usually never invoked in this way.
124 .predicates_of(def_id.to_def_id())
127 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
128 if traits::impossible_predicates(
130 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
132 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
136 trace!("ConstProp starting for {:?}", def_id);
138 let dummy_body = &Body::new(
140 body.basic_blocks().clone(),
141 body.source_scopes.clone(),
142 body.local_decls.clone(),
147 body.generator_kind(),
150 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
151 // constants, instead of just checking for const-folding succeeding.
152 // That would require a uniform one-def no-mutation analysis
153 // and RPO (or recursing when needing the value of a local).
154 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
155 optimization_finder.visit_body(body);
157 trace!("ConstProp done for {:?}", def_id);
161 struct ConstPropMachine<'mir, 'tcx> {
162 /// The virtual call stack.
163 stack: Vec<Frame<'mir, 'tcx>>,
164 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
165 written_only_inside_own_block_locals: FxHashSet<Local>,
166 /// Locals that need to be cleared after every block terminates.
167 only_propagate_inside_block_locals: BitSet<Local>,
168 can_const_prop: IndexVec<Local, ConstPropMode>,
171 impl ConstPropMachine<'_, '_> {
173 only_propagate_inside_block_locals: BitSet<Local>,
174 can_const_prop: IndexVec<Local, ConstPropMode>,
178 written_only_inside_own_block_locals: Default::default(),
179 only_propagate_inside_block_locals,
185 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
186 compile_time_machine!(<'mir, 'tcx>);
187 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
191 type MemoryExtra = ();
194 _ecx: &InterpCx<'mir, 'tcx, Self>,
195 _instance: ty::InstanceDef<'tcx>,
196 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
197 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
200 fn find_mir_or_eval_fn(
201 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
202 _instance: ty::Instance<'tcx>,
204 _args: &[OpTy<'tcx>],
205 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
206 _unwind: StackPopUnwind,
207 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
212 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
213 _instance: ty::Instance<'tcx>,
214 _args: &[OpTy<'tcx>],
215 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
216 _unwind: StackPopUnwind,
217 ) -> InterpResult<'tcx> {
218 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
222 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
223 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
224 _unwind: Option<rustc_middle::mir::BasicBlock>,
225 ) -> InterpResult<'tcx> {
226 bug!("panics terminators are not evaluated in ConstProp")
230 _ecx: &InterpCx<'mir, 'tcx, Self>,
233 _right: &ImmTy<'tcx>,
234 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
235 // We can't do this because aliasing of memory can differ between const eval and llvm
236 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
240 _ecx: &InterpCx<'mir, 'tcx, Self>,
241 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
243 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
244 let l = &frame.locals[local];
246 if l.value == LocalValue::Uninitialized {
247 throw_machine_stop_str!("tried to access an uninitialized local")
253 fn access_local_mut<'a>(
254 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
257 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
259 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
260 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
262 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
264 "mutating local {:?} which is restricted to its block. \
265 Will remove it from const-prop after block is finished.",
268 ecx.machine.written_only_inside_own_block_locals.insert(local);
270 ecx.machine.stack[frame].locals[local].access_mut()
273 fn before_access_global(
276 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
277 _static_def_id: Option<DefId>,
279 ) -> InterpResult<'tcx> {
281 throw_machine_stop_str!("can't write to global");
283 // If the static allocation is mutable, then we can't const prop it as its content
284 // might be different at runtime.
285 if allocation.mutability == Mutability::Mut {
286 throw_machine_stop_str!("can't access mutable globals in ConstProp");
294 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
295 frame: Frame<'mir, 'tcx>,
296 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
302 ecx: &'a InterpCx<'mir, 'tcx, Self>,
303 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
309 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
310 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
311 &mut ecx.machine.stack
315 /// Finds optimization opportunities on the MIR.
316 struct ConstPropagator<'mir, 'tcx> {
317 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
319 param_env: ParamEnv<'tcx>,
320 // FIXME(eddyb) avoid cloning these two fields more than once,
321 // by accessing them through `ecx` instead.
322 source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
323 local_decls: 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 span = tcx.def_span(def_id);
370 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
371 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
372 // `layout_of` query invocations.
373 let can_const_prop = CanConstProp::check(tcx, param_env, body);
374 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
375 for (l, mode) in can_const_prop.iter_enumerated() {
376 if *mode == ConstPropMode::OnlyInsideOwnBlock {
377 only_propagate_inside_block_locals.insert(l);
380 let mut ecx = InterpCx::new(
384 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
389 .layout_of(body.return_ty().subst(tcx, substs))
391 // Don't bother allocating memory for ZST types which have no values
392 // or for large values.
393 .filter(|ret_layout| {
394 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
397 ecx.allocate(ret_layout, MemoryKind::Stack)
398 .expect("couldn't perform small allocation")
402 ecx.push_stack_frame(
403 Instance::new(def_id, substs),
406 StackPopCleanup::Root { cleanup: false },
408 .expect("failed to push initial stack frame");
414 // FIXME(eddyb) avoid cloning these two fields more than once,
415 // by accessing them through `ecx` instead.
416 source_scopes: body.source_scopes.clone(),
417 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
418 local_decls: body.local_decls.clone(),
423 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
424 let op = match self.ecx.eval_place_to_op(place, None) {
427 trace!("get_const failed: {}", e);
432 // Try to read the local as an immediate so that if it is representable as a scalar, we can
433 // handle it as such, but otherwise, just return the value as is.
434 Some(match self.ecx.try_read_immediate(&op) {
435 Ok(Ok(imm)) => imm.into(),
440 /// Remove `local` from the pool of `Locals`. Allows writing to them,
441 /// but not reading from them anymore.
442 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
443 ecx.frame_mut().locals[local] =
444 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
447 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
448 source_info.scope.lint_root(&self.source_scopes)
451 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
453 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
456 Ok(val) => Some(val),
458 trace!("InterpCx operation failed: {:?}", error);
459 // Some errors shouldn't come up because creating them causes
460 // an allocation, which we should avoid. When that happens,
461 // dedicated error variants should be introduced instead.
463 !error.kind().formatted_string(),
464 "const-prop encountered formatting error: {}",
472 /// Returns the value, if any, of evaluating `c`.
473 fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
474 // FIXME we need to revisit this for #67176
479 match self.ecx.mir_const_to_op(&c.literal, None) {
482 let tcx = self.ecx.tcx.at(c.span);
483 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
484 if let Some(lint_root) = self.lint_root(source_info) {
485 let lint_only = match c.literal {
486 ConstantKind::Ty(ct) => match ct.val {
487 // Promoteds must lint and not error as the user didn't ask for them
488 ConstKind::Unevaluated(ty::Unevaluated {
493 // Out of backwards compatibility we cannot report hard errors in unused
494 // generic functions using associated constants of the generic parameters.
495 _ => c.literal.needs_subst(),
497 ConstantKind::Val(_, ty) => ty.needs_subst(),
500 // Out of backwards compatibility we cannot report hard errors in unused
501 // generic functions using associated constants of the generic parameters.
502 err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
504 err.report_as_error(tcx, "erroneous constant used");
507 err.report_as_error(tcx, "erroneous constant used");
514 /// Returns the value, if any, of evaluating `place`.
515 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
516 trace!("eval_place(place={:?})", place);
517 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
520 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
521 /// or `eval_place`, depending on the variant of `Operand` used.
522 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
524 Operand::Constant(ref c) => self.eval_constant(c, source_info),
525 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
529 fn report_assert_as_lint(
531 lint: &'static lint::Lint,
532 source_info: SourceInfo,
533 message: &'static str,
534 panic: AssertKind<impl std::fmt::Debug>,
536 if let Some(lint_root) = self.lint_root(source_info) {
537 self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
538 let mut err = lint.build(message);
539 err.span_label(source_info.span, format!("{:?}", panic));
549 source_info: SourceInfo,
551 if let (val, true) = self.use_ecx(|this| {
552 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
553 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
556 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
557 // appropriate to use.
558 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
559 self.report_assert_as_lint(
560 lint::builtin::ARITHMETIC_OVERFLOW,
562 "this arithmetic operation will overflow",
563 AssertKind::OverflowNeg(val.to_const_int()),
574 left: &Operand<'tcx>,
575 right: &Operand<'tcx>,
576 source_info: SourceInfo,
578 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
579 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
580 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
581 if op == BinOp::Shr || op == BinOp::Shl {
583 // We need the type of the LHS. We cannot use `place_layout` as that is the type
584 // of the result, which for checked binops is not the same!
585 let left_ty = left.ty(&self.local_decls, self.tcx);
586 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
587 let right_size = r.layout.size;
588 let r_bits = r.to_scalar().ok();
589 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
590 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
591 debug!("check_binary_op: reporting assert for {:?}", source_info);
592 self.report_assert_as_lint(
593 lint::builtin::ARITHMETIC_OVERFLOW,
595 "this arithmetic operation will overflow",
596 AssertKind::Overflow(
599 Some(l) => l.to_const_int(),
600 // Invent a dummy value, the diagnostic ignores it anyway
601 None => ConstInt::new(
602 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
604 left_ty.is_ptr_sized_integral(),
614 if let (Some(l), Some(r)) = (&l, &r) {
615 // The remaining operators are handled through `overflowing_binary_op`.
616 if self.use_ecx(|this| {
617 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
620 self.report_assert_as_lint(
621 lint::builtin::ARITHMETIC_OVERFLOW,
623 "this arithmetic operation will overflow",
624 AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
632 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
634 Operand::Copy(l) | Operand::Move(l) => {
635 if let Some(value) = self.get_const(l) {
636 if self.should_const_prop(&value) {
637 // FIXME(felix91gr): this code only handles `Scalar` cases.
638 // For now, we're not handling `ScalarPair` cases because
639 // doing so here would require a lot of code duplication.
640 // We should hopefully generalize `Operand` handling into a fn,
641 // and use it to do const-prop here and everywhere else
642 // where it makes sense.
643 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
644 ScalarMaybeUninit::Scalar(scalar),
647 *operand = self.operand_from_scalar(
650 self.source_info.unwrap().span,
656 Operand::Constant(_) => (),
662 rvalue: &Rvalue<'tcx>,
663 source_info: SourceInfo,
666 // Perform any special handling for specific Rvalue types.
667 // Generally, checks here fall into one of two categories:
668 // 1. Additional checking to provide useful lints to the user
669 // - In this case, we will do some validation and then fall through to the
670 // end of the function which evals the assignment.
671 // 2. Working around bugs in other parts of the compiler
672 // - In this case, we'll return `None` from this function to stop evaluation.
674 // Additional checking: give lints to the user if an overflow would occur.
675 // We do this here and not in the `Assert` terminator as that terminator is
676 // only sometimes emitted (overflow checks can be disabled), but we want to always
678 Rvalue::UnaryOp(op, arg) => {
679 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
680 self.check_unary_op(*op, arg, source_info)?;
682 Rvalue::BinaryOp(op, box (left, right)) => {
683 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
684 self.check_binary_op(*op, left, right, source_info)?;
686 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
688 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
693 self.check_binary_op(*op, left, right, source_info)?;
696 // Do not try creating references (#67862)
697 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
698 trace!("skipping AddressOf | Ref for {:?}", place);
700 // This may be creating mutable references or immutable references to cells.
701 // If that happens, the pointed to value could be mutated via that reference.
702 // Since we aren't tracking references, the const propagator loses track of what
703 // value the local has right now.
704 // Thus, all locals that have their reference taken
705 // must not take part in propagation.
706 Self::remove_const(&mut self.ecx, place.local);
710 Rvalue::ThreadLocalRef(def_id) => {
711 trace!("skipping ThreadLocalRef({:?})", def_id);
716 // There's no other checking to do at this time.
717 Rvalue::Aggregate(..)
722 | Rvalue::ShallowInitBox(..)
723 | Rvalue::Discriminant(..)
724 | Rvalue::NullaryOp(..) => {}
727 // FIXME we need to revisit this for #67176
728 if rvalue.needs_subst() {
732 if self.tcx.sess.mir_opt_level() >= 4 {
733 self.eval_rvalue_with_identities(rvalue, place)
735 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
739 // Attempt to use albegraic identities to eliminate constant expressions
740 fn eval_rvalue_with_identities(
742 rvalue: &Rvalue<'tcx>,
745 self.use_ecx(|this| match rvalue {
746 Rvalue::BinaryOp(op, box (left, right))
747 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
748 let l = this.ecx.eval_operand(left, None);
749 let r = this.ecx.eval_operand(right, None);
751 let const_arg = match (l, r) {
752 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
753 (Err(e), Err(_)) => return Err(e),
754 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
757 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
758 let dest = this.ecx.eval_place(place)?;
761 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
763 if arg_value == const_arg.layout.size.truncate(u128::MAX)
764 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
766 this.ecx.write_immediate(*const_arg, &dest)
768 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
769 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
770 let val = Immediate::ScalarPair(
771 const_arg.to_scalar()?.into(),
772 Scalar::from_bool(false).into(),
774 this.ecx.write_immediate(val, &dest)
776 this.ecx.write_immediate(*const_arg, &dest)
779 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
782 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
786 /// Creates a new `Operand::Constant` from a `Scalar` value
787 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
788 Operand::Constant(Box::new(Constant {
791 literal: ty::Const::from_scalar(self.tcx, scalar, ty).into(),
795 fn replace_with_const(
797 rval: &mut Rvalue<'tcx>,
799 source_info: SourceInfo,
801 if let Rvalue::Use(Operand::Constant(c)) = rval {
803 ConstantKind::Ty(c) if matches!(c.val, ConstKind::Unevaluated(..)) => {}
805 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
811 trace!("attempting to replace {:?} with {:?}", rval, value);
812 if let Err(e) = self.ecx.const_validate_operand(
815 // FIXME: is ref tracking too expensive?
816 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
817 &mut interpret::RefTracking::empty(),
818 CtfeValidationMode::Regular,
820 trace!("validation error, attempt failed: {:?}", e);
824 // FIXME> figure out what to do when try_read_immediate fails
825 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
827 if let Some(Ok(imm)) = imm {
829 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
830 *rval = Rvalue::Use(self.operand_from_scalar(
836 Immediate::ScalarPair(
837 ScalarMaybeUninit::Scalar(_),
838 ScalarMaybeUninit::Scalar(_),
840 // Found a value represented as a pair. For now only do const-prop if the type
841 // of `rvalue` is also a tuple with two scalars.
842 // FIXME: enable the general case stated above ^.
843 let ty = &value.layout.ty;
844 // Only do it for tuples
845 if let ty::Tuple(substs) = ty.kind() {
846 // Only do it if tuple is also a pair with two scalars
847 if substs.len() == 2 {
848 let alloc = self.use_ecx(|this| {
849 let ty1 = substs[0].expect_ty();
850 let ty2 = substs[1].expect_ty();
851 let ty_is_scalar = |ty| {
852 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
855 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
858 .intern_with_temp_alloc(value.layout, |ecx, dest| {
859 ecx.write_immediate(*imm, dest)
868 if let Some(Some(alloc)) = alloc {
869 // Assign entire constant in a single statement.
870 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
871 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
872 span: source_info.span,
877 .mk_const(ty::Const {
879 val: ty::ConstKind::Value(ConstValue::ByRef {
890 // Scalars or scalar pairs that contain undef values are assumed to not have
891 // successfully evaluated and are thus not propagated.
897 /// Returns `true` if and only if this `op` should be const-propagated into.
898 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
899 let mir_opt_level = self.tcx.sess.mir_opt_level();
901 if mir_opt_level == 0 {
905 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
910 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
911 s.try_to_int().is_ok()
913 interpret::Operand::Immediate(Immediate::ScalarPair(
914 ScalarMaybeUninit::Scalar(l),
915 ScalarMaybeUninit::Scalar(r),
916 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
922 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
923 #[derive(Clone, Copy, Debug, PartialEq)]
925 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
927 /// The `Local` can only be propagated into and from its own block.
929 /// The `Local` can be propagated into but reads cannot be propagated.
931 /// The `Local` cannot be part of propagation at all. Any statement
932 /// referencing it either for reading or writing will not get propagated.
936 struct CanConstProp {
937 can_const_prop: IndexVec<Local, ConstPropMode>,
938 // False at the beginning. Once set, no more assignments are allowed to that local.
939 found_assignment: BitSet<Local>,
940 // Cache of locals' information
941 local_kinds: IndexVec<Local, LocalKind>,
945 /// Returns true if `local` can be propagated
948 param_env: ParamEnv<'tcx>,
950 ) -> IndexVec<Local, ConstPropMode> {
951 let mut cpv = CanConstProp {
952 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
953 found_assignment: BitSet::new_empty(body.local_decls.len()),
954 local_kinds: IndexVec::from_fn_n(
955 |local| body.local_kind(local),
956 body.local_decls.len(),
959 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
960 let ty = body.local_decls[local].ty;
961 match tcx.layout_of(param_env.and(ty)) {
962 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
963 // Either the layout fails to compute, then we can't use this local anyway
964 // or the local is too large, then we don't want to.
966 *val = ConstPropMode::NoPropagation;
970 // Cannot use args at all
971 // Cannot use locals because if x < y { y - x } else { x - y } would
973 // FIXME(oli-obk): lint variables until they are used in a condition
974 // FIXME(oli-obk): lint if return value is constant
975 if cpv.local_kinds[local] == LocalKind::Arg {
976 *val = ConstPropMode::OnlyPropagateInto;
978 "local {:?} can't be const propagated because it's a function argument",
981 } else if cpv.local_kinds[local] == LocalKind::Var {
982 *val = ConstPropMode::OnlyInsideOwnBlock;
984 "local {:?} will only be propagated inside its block, because it's a user variable",
989 cpv.visit_body(&body);
994 impl Visitor<'_> for CanConstProp {
995 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
996 use rustc_middle::mir::visit::PlaceContext::*;
998 // Projections are fine, because `&mut foo.x` will be caught by
999 // `MutatingUseContext::Borrow` elsewhere.
1000 MutatingUse(MutatingUseContext::Projection)
1001 // These are just stores, where the storing is not propagatable, but there may be later
1002 // mutations of the same local via `Store`
1003 | MutatingUse(MutatingUseContext::Call)
1004 | MutatingUse(MutatingUseContext::AsmOutput)
1005 // Actual store that can possibly even propagate a value
1006 | MutatingUse(MutatingUseContext::Store) => {
1007 if !self.found_assignment.insert(local) {
1008 match &mut self.can_const_prop[local] {
1009 // If the local can only get propagated in its own block, then we don't have
1010 // to worry about multiple assignments, as we'll nuke the const state at the
1011 // end of the block anyway, and inside the block we overwrite previous
1012 // states as applicable.
1013 ConstPropMode::OnlyInsideOwnBlock => {}
1014 ConstPropMode::NoPropagation => {}
1015 ConstPropMode::OnlyPropagateInto => {}
1016 other @ ConstPropMode::FullConstProp => {
1018 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
1021 *other = ConstPropMode::OnlyInsideOwnBlock;
1026 // Reading constants is allowed an arbitrary number of times
1027 NonMutatingUse(NonMutatingUseContext::Copy)
1028 | NonMutatingUse(NonMutatingUseContext::Move)
1029 | NonMutatingUse(NonMutatingUseContext::Inspect)
1030 | NonMutatingUse(NonMutatingUseContext::Projection)
1033 // These could be propagated with a smarter analysis or just some careful thinking about
1034 // whether they'd be fine right now.
1035 MutatingUse(MutatingUseContext::Yield)
1036 | MutatingUse(MutatingUseContext::Drop)
1037 | MutatingUse(MutatingUseContext::Retag)
1038 // These can't ever be propagated under any scheme, as we can't reason about indirect
1040 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1041 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1042 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1043 | NonMutatingUse(NonMutatingUseContext::AddressOf)
1044 | MutatingUse(MutatingUseContext::Borrow)
1045 | MutatingUse(MutatingUseContext::AddressOf) => {
1046 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1047 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1053 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
1054 fn tcx(&self) -> TyCtxt<'tcx> {
1058 fn visit_body(&mut self, body: &mut Body<'tcx>) {
1059 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1060 self.visit_basic_block_data(bb, data);
1064 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1065 self.super_operand(operand, location);
1067 // Only const prop copies and moves on `mir_opt_level=3` as doing so
1068 // currently slightly increases compile time in some cases.
1069 if self.tcx.sess.mir_opt_level() >= 3 {
1070 self.propagate_operand(operand)
1074 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1075 trace!("visit_constant: {:?}", constant);
1076 self.super_constant(constant, location);
1077 self.eval_constant(constant, self.source_info.unwrap());
1080 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1081 trace!("visit_statement: {:?}", statement);
1082 let source_info = statement.source_info;
1083 self.source_info = Some(source_info);
1084 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1085 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1086 if let Some(()) = self.const_prop(rval, source_info, place) {
1087 // This will return None if the above `const_prop` invocation only "wrote" a
1088 // type whose creation requires no write. E.g. a generator whose initial state
1089 // consists solely of uninitialized memory (so it doesn't capture any locals).
1090 if let Some(ref value) = self.get_const(place) {
1091 if self.should_const_prop(value) {
1092 trace!("replacing {:?} with {:?}", rval, value);
1093 self.replace_with_const(rval, value, source_info);
1094 if can_const_prop == ConstPropMode::FullConstProp
1095 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1097 trace!("propagated into {:?}", place);
1101 match can_const_prop {
1102 ConstPropMode::OnlyInsideOwnBlock => {
1104 "found local restricted to its block. \
1105 Will remove it from const-prop after block is finished. Local: {:?}",
1109 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1110 trace!("can't propagate into {:?}", place);
1111 if place.local != RETURN_PLACE {
1112 Self::remove_const(&mut self.ecx, place.local);
1115 ConstPropMode::FullConstProp => {}
1118 // Const prop failed, so erase the destination, ensuring that whatever happens
1119 // from here on, does not know about the previous value.
1120 // This is important in case we have
1123 // x = SOME_MUTABLE_STATIC;
1124 // // x must now be uninit
1126 // FIXME: we overzealously erase the entire local, because that's easier to
1129 "propagation into {:?} failed.
1130 Nuking the entire site from orbit, it's the only way to be sure",
1133 Self::remove_const(&mut self.ecx, place.local);
1136 match statement.kind {
1137 StatementKind::SetDiscriminant { ref place, .. } => {
1138 match self.ecx.machine.can_const_prop[place.local] {
1139 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1140 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1141 trace!("propped discriminant into {:?}", place);
1143 Self::remove_const(&mut self.ecx, place.local);
1146 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1147 Self::remove_const(&mut self.ecx, place.local);
1151 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1152 let frame = self.ecx.frame_mut();
1153 frame.locals[local].value =
1154 if let StatementKind::StorageLive(_) = statement.kind {
1155 LocalValue::Uninitialized
1164 self.super_statement(statement, location);
1167 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1168 let source_info = terminator.source_info;
1169 self.source_info = Some(source_info);
1170 self.super_terminator(terminator, location);
1171 match &mut terminator.kind {
1172 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1173 if let Some(ref value) = self.eval_operand(&cond, source_info) {
1174 trace!("assertion on {:?} should be {:?}", value, expected);
1175 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1176 let value_const = self.ecx.read_scalar(&value).unwrap();
1177 if expected != value_const {
1182 impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1183 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1185 Self::Val(val) => val.fmt(fmt),
1186 Self::Underscore => fmt.write_str("_"),
1190 let mut eval_to_int = |op| {
1191 // This can be `None` if the lhs wasn't const propagated and we just
1192 // triggered the assert on the value of the rhs.
1193 self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
1194 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
1197 let msg = match msg {
1198 AssertKind::DivisionByZero(op) => {
1199 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1201 AssertKind::RemainderByZero(op) => {
1202 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1204 AssertKind::BoundsCheck { ref len, ref index } => {
1205 let len = eval_to_int(len);
1206 let index = eval_to_int(index);
1207 Some(AssertKind::BoundsCheck { len, index })
1209 // Overflow is are already covered by checks on the binary operators.
1210 AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1211 // Need proper const propagator for these.
1214 // Poison all places this operand references so that further code
1215 // doesn't use the invalid value
1217 Operand::Move(ref place) | Operand::Copy(ref place) => {
1218 Self::remove_const(&mut self.ecx, place.local);
1220 Operand::Constant(_) => {}
1222 if let Some(msg) = msg {
1223 self.report_assert_as_lint(
1224 lint::builtin::UNCONDITIONAL_PANIC,
1226 "this operation will panic at runtime",
1231 if self.should_const_prop(value) {
1232 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1233 *cond = self.operand_from_scalar(
1235 self.tcx.types.bool,
1243 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1244 // FIXME: This is currently redundant with `visit_operand`, but sadly
1245 // always visiting operands currently causes a perf regression in LLVM codegen, so
1246 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1247 self.propagate_operand(discr)
1249 // None of these have Operands to const-propagate.
1250 TerminatorKind::Goto { .. }
1251 | TerminatorKind::Resume
1252 | TerminatorKind::Abort
1253 | TerminatorKind::Return
1254 | TerminatorKind::Unreachable
1255 | TerminatorKind::Drop { .. }
1256 | TerminatorKind::DropAndReplace { .. }
1257 | TerminatorKind::Yield { .. }
1258 | TerminatorKind::GeneratorDrop
1259 | TerminatorKind::FalseEdge { .. }
1260 | TerminatorKind::FalseUnwind { .. }
1261 | TerminatorKind::InlineAsm { .. } => {}
1262 // Every argument in our function calls have already been propagated in `visit_operand`.
1264 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1265 // gated on `mir_opt_level=3`.
1266 TerminatorKind::Call { .. } => {}
1269 // We remove all Locals which are restricted in propagation to their containing blocks and
1270 // which were modified in the current block.
1271 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1272 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1273 for &local in locals.iter() {
1274 Self::remove_const(&mut self.ecx, local);
1277 // Put it back so we reuse the heap of the storage
1278 self.ecx.machine.written_only_inside_own_block_locals = locals;
1279 if cfg!(debug_assertions) {
1280 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1281 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1283 self.get_const(local.into()).is_none()
1285 .layout_of(self.local_decls[local].ty)
1286 .map_or(true, |layout| layout.is_zst())