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(),
148 body.tainted_by_errors,
151 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
152 // constants, instead of just checking for const-folding succeeding.
153 // That would require a uniform one-def no-mutation analysis
154 // and RPO (or recursing when needing the value of a local).
155 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
156 optimization_finder.visit_body(body);
158 trace!("ConstProp done for {:?}", def_id);
162 struct ConstPropMachine<'mir, 'tcx> {
163 /// The virtual call stack.
164 stack: Vec<Frame<'mir, 'tcx>>,
165 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
166 written_only_inside_own_block_locals: FxHashSet<Local>,
167 /// Locals that need to be cleared after every block terminates.
168 only_propagate_inside_block_locals: BitSet<Local>,
169 can_const_prop: IndexVec<Local, ConstPropMode>,
172 impl ConstPropMachine<'_, '_> {
174 only_propagate_inside_block_locals: BitSet<Local>,
175 can_const_prop: IndexVec<Local, ConstPropMode>,
179 written_only_inside_own_block_locals: Default::default(),
180 only_propagate_inside_block_locals,
186 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
187 compile_time_machine!(<'mir, 'tcx>);
188 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
192 type MemoryExtra = ();
195 _ecx: &InterpCx<'mir, 'tcx, Self>,
196 _instance: ty::InstanceDef<'tcx>,
197 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
198 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
201 fn find_mir_or_eval_fn(
202 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
203 _instance: ty::Instance<'tcx>,
205 _args: &[OpTy<'tcx>],
206 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
207 _unwind: StackPopUnwind,
208 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
213 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
214 _instance: ty::Instance<'tcx>,
215 _args: &[OpTy<'tcx>],
216 _ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
217 _unwind: StackPopUnwind,
218 ) -> InterpResult<'tcx> {
219 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
223 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
224 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
225 _unwind: Option<rustc_middle::mir::BasicBlock>,
226 ) -> InterpResult<'tcx> {
227 bug!("panics terminators are not evaluated in ConstProp")
231 _ecx: &InterpCx<'mir, 'tcx, Self>,
234 _right: &ImmTy<'tcx>,
235 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
236 // We can't do this because aliasing of memory can differ between const eval and llvm
237 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
241 _ecx: &InterpCx<'mir, 'tcx, Self>,
242 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
244 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
245 let l = &frame.locals[local];
247 if l.value == LocalValue::Uninitialized {
248 throw_machine_stop_str!("tried to access an uninitialized local")
254 fn access_local_mut<'a>(
255 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
258 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
260 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
261 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
263 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
265 "mutating local {:?} which is restricted to its block. \
266 Will remove it from const-prop after block is finished.",
269 ecx.machine.written_only_inside_own_block_locals.insert(local);
271 ecx.machine.stack[frame].locals[local].access_mut()
274 fn before_access_global(
277 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
278 _static_def_id: Option<DefId>,
280 ) -> InterpResult<'tcx> {
282 throw_machine_stop_str!("can't write to global");
284 // If the static allocation is mutable, then we can't const prop it as its content
285 // might be different at runtime.
286 if allocation.mutability == Mutability::Mut {
287 throw_machine_stop_str!("can't access mutable globals in ConstProp");
295 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
296 frame: Frame<'mir, 'tcx>,
297 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
303 ecx: &'a InterpCx<'mir, 'tcx, Self>,
304 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
310 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
311 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
312 &mut ecx.machine.stack
316 /// Finds optimization opportunities on the MIR.
317 struct ConstPropagator<'mir, 'tcx> {
318 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
320 param_env: ParamEnv<'tcx>,
321 // FIXME(eddyb) avoid cloning these two fields more than once,
322 // by accessing them through `ecx` instead.
323 source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
324 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
325 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
326 // the last known `SourceInfo` here and just keep revisiting it.
327 source_info: Option<SourceInfo>,
330 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
331 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
334 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
339 impl HasDataLayout for ConstPropagator<'_, '_> {
341 fn data_layout(&self) -> &TargetDataLayout {
342 &self.tcx.data_layout
346 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
348 fn tcx(&self) -> TyCtxt<'tcx> {
353 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
355 fn param_env(&self) -> ty::ParamEnv<'tcx> {
360 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
363 dummy_body: &'mir Body<'tcx>,
365 ) -> ConstPropagator<'mir, 'tcx> {
366 let def_id = body.source.def_id();
367 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
368 let param_env = tcx.param_env_reveal_all_normalized(def_id);
370 let span = tcx.def_span(def_id);
371 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
372 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
373 // `layout_of` query invocations.
374 let can_const_prop = CanConstProp::check(tcx, param_env, body);
375 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
376 for (l, mode) in can_const_prop.iter_enumerated() {
377 if *mode == ConstPropMode::OnlyInsideOwnBlock {
378 only_propagate_inside_block_locals.insert(l);
381 let mut ecx = InterpCx::new(
385 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
390 .layout_of(body.return_ty().subst(tcx, substs))
392 // Don't bother allocating memory for ZST types which have no values
393 // or for large values.
394 .filter(|ret_layout| {
395 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
398 ecx.allocate(ret_layout, MemoryKind::Stack)
399 .expect("couldn't perform small allocation")
403 ecx.push_stack_frame(
404 Instance::new(def_id, substs),
407 StackPopCleanup::Root { cleanup: false },
409 .expect("failed to push initial stack frame");
415 // FIXME(eddyb) avoid cloning these two fields more than once,
416 // by accessing them through `ecx` instead.
417 source_scopes: body.source_scopes.clone(),
418 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
419 local_decls: body.local_decls.clone(),
424 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
425 let op = match self.ecx.eval_place_to_op(place, None) {
428 trace!("get_const failed: {}", e);
433 // Try to read the local as an immediate so that if it is representable as a scalar, we can
434 // handle it as such, but otherwise, just return the value as is.
435 Some(match self.ecx.try_read_immediate(&op) {
436 Ok(Ok(imm)) => imm.into(),
441 /// Remove `local` from the pool of `Locals`. Allows writing to them,
442 /// but not reading from them anymore.
443 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
444 ecx.frame_mut().locals[local] =
445 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
448 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
449 source_info.scope.lint_root(&self.source_scopes)
452 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
454 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
457 Ok(val) => Some(val),
459 trace!("InterpCx operation failed: {:?}", error);
460 // Some errors shouldn't come up because creating them causes
461 // an allocation, which we should avoid. When that happens,
462 // dedicated error variants should be introduced instead.
464 !error.kind().formatted_string(),
465 "const-prop encountered formatting error: {}",
473 /// Returns the value, if any, of evaluating `c`.
474 fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
475 // FIXME we need to revisit this for #67176
480 match self.ecx.mir_const_to_op(&c.literal, None) {
483 let tcx = self.ecx.tcx.at(c.span);
484 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
485 if let Some(lint_root) = self.lint_root(source_info) {
486 let lint_only = match c.literal {
487 ConstantKind::Ty(ct) => match ct.val {
488 // Promoteds must lint and not error as the user didn't ask for them
489 ConstKind::Unevaluated(ty::Unevaluated {
494 // Out of backwards compatibility we cannot report hard errors in unused
495 // generic functions using associated constants of the generic parameters.
496 _ => c.literal.needs_subst(),
498 ConstantKind::Val(_, ty) => ty.needs_subst(),
501 // Out of backwards compatibility we cannot report hard errors in unused
502 // generic functions using associated constants of the generic parameters.
503 err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
505 err.report_as_error(tcx, "erroneous constant used");
508 err.report_as_error(tcx, "erroneous constant used");
515 /// Returns the value, if any, of evaluating `place`.
516 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
517 trace!("eval_place(place={:?})", place);
518 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
521 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
522 /// or `eval_place`, depending on the variant of `Operand` used.
523 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
525 Operand::Constant(ref c) => self.eval_constant(c, source_info),
526 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
530 fn report_assert_as_lint(
532 lint: &'static lint::Lint,
533 source_info: SourceInfo,
534 message: &'static str,
535 panic: AssertKind<impl std::fmt::Debug>,
537 if let Some(lint_root) = self.lint_root(source_info) {
538 self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
539 let mut err = lint.build(message);
540 err.span_label(source_info.span, format!("{:?}", panic));
550 source_info: SourceInfo,
552 if let (val, true) = self.use_ecx(|this| {
553 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
554 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
557 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
558 // appropriate to use.
559 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
560 self.report_assert_as_lint(
561 lint::builtin::ARITHMETIC_OVERFLOW,
563 "this arithmetic operation will overflow",
564 AssertKind::OverflowNeg(val.to_const_int()),
575 left: &Operand<'tcx>,
576 right: &Operand<'tcx>,
577 source_info: SourceInfo,
579 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
580 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
581 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
582 if op == BinOp::Shr || op == BinOp::Shl {
584 // We need the type of the LHS. We cannot use `place_layout` as that is the type
585 // of the result, which for checked binops is not the same!
586 let left_ty = left.ty(&self.local_decls, self.tcx);
587 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
588 let right_size = r.layout.size;
589 let r_bits = r.to_scalar().ok();
590 let r_bits = r_bits.and_then(|r| r.to_bits(right_size).ok());
591 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
592 debug!("check_binary_op: reporting assert for {:?}", source_info);
593 self.report_assert_as_lint(
594 lint::builtin::ARITHMETIC_OVERFLOW,
596 "this arithmetic operation will overflow",
597 AssertKind::Overflow(
600 Some(l) => l.to_const_int(),
601 // Invent a dummy value, the diagnostic ignores it anyway
602 None => ConstInt::new(
603 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
605 left_ty.is_ptr_sized_integral(),
615 if let (Some(l), Some(r)) = (&l, &r) {
616 // The remaining operators are handled through `overflowing_binary_op`.
617 if self.use_ecx(|this| {
618 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
621 self.report_assert_as_lint(
622 lint::builtin::ARITHMETIC_OVERFLOW,
624 "this arithmetic operation will overflow",
625 AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
633 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
635 Operand::Copy(l) | Operand::Move(l) => {
636 if let Some(value) = self.get_const(l) {
637 if self.should_const_prop(&value) {
638 // FIXME(felix91gr): this code only handles `Scalar` cases.
639 // For now, we're not handling `ScalarPair` cases because
640 // doing so here would require a lot of code duplication.
641 // We should hopefully generalize `Operand` handling into a fn,
642 // and use it to do const-prop here and everywhere else
643 // where it makes sense.
644 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
645 ScalarMaybeUninit::Scalar(scalar),
648 *operand = self.operand_from_scalar(
651 self.source_info.unwrap().span,
657 Operand::Constant(_) => (),
663 rvalue: &Rvalue<'tcx>,
664 source_info: SourceInfo,
667 // Perform any special handling for specific Rvalue types.
668 // Generally, checks here fall into one of two categories:
669 // 1. Additional checking to provide useful lints to the user
670 // - In this case, we will do some validation and then fall through to the
671 // end of the function which evals the assignment.
672 // 2. Working around bugs in other parts of the compiler
673 // - In this case, we'll return `None` from this function to stop evaluation.
675 // Additional checking: give lints to the user if an overflow would occur.
676 // We do this here and not in the `Assert` terminator as that terminator is
677 // only sometimes emitted (overflow checks can be disabled), but we want to always
679 Rvalue::UnaryOp(op, arg) => {
680 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
681 self.check_unary_op(*op, arg, source_info)?;
683 Rvalue::BinaryOp(op, box (left, right)) => {
684 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
685 self.check_binary_op(*op, left, right, source_info)?;
687 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
689 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
694 self.check_binary_op(*op, left, right, source_info)?;
697 // Do not try creating references (#67862)
698 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
699 trace!("skipping AddressOf | Ref for {:?}", place);
701 // This may be creating mutable references or immutable references to cells.
702 // If that happens, the pointed to value could be mutated via that reference.
703 // Since we aren't tracking references, the const propagator loses track of what
704 // value the local has right now.
705 // Thus, all locals that have their reference taken
706 // must not take part in propagation.
707 Self::remove_const(&mut self.ecx, place.local);
711 Rvalue::ThreadLocalRef(def_id) => {
712 trace!("skipping ThreadLocalRef({:?})", def_id);
717 // There's no other checking to do at this time.
718 Rvalue::Aggregate(..)
723 | Rvalue::ShallowInitBox(..)
724 | Rvalue::Discriminant(..)
725 | Rvalue::NullaryOp(..) => {}
728 // FIXME we need to revisit this for #67176
729 if rvalue.needs_subst() {
733 if self.tcx.sess.mir_opt_level() >= 4 {
734 self.eval_rvalue_with_identities(rvalue, place)
736 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
740 // Attempt to use albegraic identities to eliminate constant expressions
741 fn eval_rvalue_with_identities(
743 rvalue: &Rvalue<'tcx>,
746 self.use_ecx(|this| match rvalue {
747 Rvalue::BinaryOp(op, box (left, right))
748 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
749 let l = this.ecx.eval_operand(left, None);
750 let r = this.ecx.eval_operand(right, None);
752 let const_arg = match (l, r) {
753 (Ok(ref x), Err(_)) | (Err(_), Ok(ref x)) => this.ecx.read_immediate(x)?,
754 (Err(e), Err(_)) => return Err(e),
755 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place),
758 let arg_value = const_arg.to_scalar()?.to_bits(const_arg.layout.size)?;
759 let dest = this.ecx.eval_place(place)?;
762 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
764 if arg_value == const_arg.layout.size.truncate(u128::MAX)
765 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
767 this.ecx.write_immediate(*const_arg, &dest)
769 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
770 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
771 let val = Immediate::ScalarPair(
772 const_arg.to_scalar()?.into(),
773 Scalar::from_bool(false).into(),
775 this.ecx.write_immediate(val, &dest)
777 this.ecx.write_immediate(*const_arg, &dest)
780 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
783 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
787 /// Creates a new `Operand::Constant` from a `Scalar` value
788 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
789 Operand::Constant(Box::new(Constant {
792 literal: ty::Const::from_scalar(self.tcx, scalar, ty).into(),
796 fn replace_with_const(
798 rval: &mut Rvalue<'tcx>,
800 source_info: SourceInfo,
802 if let Rvalue::Use(Operand::Constant(c)) = rval {
804 ConstantKind::Ty(c) if matches!(c.val, ConstKind::Unevaluated(..)) => {}
806 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
812 trace!("attempting to replace {:?} with {:?}", rval, value);
813 if let Err(e) = self.ecx.const_validate_operand(
816 // FIXME: is ref tracking too expensive?
817 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
818 &mut interpret::RefTracking::empty(),
819 CtfeValidationMode::Regular,
821 trace!("validation error, attempt failed: {:?}", e);
825 // FIXME> figure out what to do when try_read_immediate fails
826 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
828 if let Some(Ok(imm)) = imm {
830 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
831 *rval = Rvalue::Use(self.operand_from_scalar(
837 Immediate::ScalarPair(
838 ScalarMaybeUninit::Scalar(_),
839 ScalarMaybeUninit::Scalar(_),
841 // Found a value represented as a pair. For now only do const-prop if the type
842 // of `rvalue` is also a tuple with two scalars.
843 // FIXME: enable the general case stated above ^.
844 let ty = &value.layout.ty;
845 // Only do it for tuples
846 if let ty::Tuple(substs) = ty.kind() {
847 // Only do it if tuple is also a pair with two scalars
848 if substs.len() == 2 {
849 let alloc = self.use_ecx(|this| {
850 let ty1 = substs[0].expect_ty();
851 let ty2 = substs[1].expect_ty();
852 let ty_is_scalar = |ty| {
853 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
856 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
859 .intern_with_temp_alloc(value.layout, |ecx, dest| {
860 ecx.write_immediate(*imm, dest)
869 if let Some(Some(alloc)) = alloc {
870 // Assign entire constant in a single statement.
871 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
872 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
873 span: source_info.span,
878 .mk_const(ty::Const {
880 val: ty::ConstKind::Value(ConstValue::ByRef {
891 // Scalars or scalar pairs that contain undef values are assumed to not have
892 // successfully evaluated and are thus not propagated.
898 /// Returns `true` if and only if this `op` should be const-propagated into.
899 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
900 let mir_opt_level = self.tcx.sess.mir_opt_level();
902 if mir_opt_level == 0 {
906 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
911 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
912 s.try_to_int().is_ok()
914 interpret::Operand::Immediate(Immediate::ScalarPair(
915 ScalarMaybeUninit::Scalar(l),
916 ScalarMaybeUninit::Scalar(r),
917 )) => l.try_to_int().is_ok() && r.try_to_int().is_ok(),
923 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
924 #[derive(Clone, Copy, Debug, PartialEq)]
926 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
928 /// The `Local` can only be propagated into and from its own block.
930 /// The `Local` can be propagated into but reads cannot be propagated.
932 /// The `Local` cannot be part of propagation at all. Any statement
933 /// referencing it either for reading or writing will not get propagated.
937 struct CanConstProp {
938 can_const_prop: IndexVec<Local, ConstPropMode>,
939 // False at the beginning. Once set, no more assignments are allowed to that local.
940 found_assignment: BitSet<Local>,
941 // Cache of locals' information
942 local_kinds: IndexVec<Local, LocalKind>,
946 /// Returns true if `local` can be propagated
949 param_env: ParamEnv<'tcx>,
951 ) -> IndexVec<Local, ConstPropMode> {
952 let mut cpv = CanConstProp {
953 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
954 found_assignment: BitSet::new_empty(body.local_decls.len()),
955 local_kinds: IndexVec::from_fn_n(
956 |local| body.local_kind(local),
957 body.local_decls.len(),
960 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
961 let ty = body.local_decls[local].ty;
962 match tcx.layout_of(param_env.and(ty)) {
963 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
964 // Either the layout fails to compute, then we can't use this local anyway
965 // or the local is too large, then we don't want to.
967 *val = ConstPropMode::NoPropagation;
971 // Cannot use args at all
972 // Cannot use locals because if x < y { y - x } else { x - y } would
974 // FIXME(oli-obk): lint variables until they are used in a condition
975 // FIXME(oli-obk): lint if return value is constant
976 if cpv.local_kinds[local] == LocalKind::Arg {
977 *val = ConstPropMode::OnlyPropagateInto;
979 "local {:?} can't be const propagated because it's a function argument",
982 } else if cpv.local_kinds[local] == LocalKind::Var {
983 *val = ConstPropMode::OnlyInsideOwnBlock;
985 "local {:?} will only be propagated inside its block, because it's a user variable",
990 cpv.visit_body(&body);
995 impl Visitor<'_> for CanConstProp {
996 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
997 use rustc_middle::mir::visit::PlaceContext::*;
999 // Projections are fine, because `&mut foo.x` will be caught by
1000 // `MutatingUseContext::Borrow` elsewhere.
1001 MutatingUse(MutatingUseContext::Projection)
1002 // These are just stores, where the storing is not propagatable, but there may be later
1003 // mutations of the same local via `Store`
1004 | MutatingUse(MutatingUseContext::Call)
1005 | MutatingUse(MutatingUseContext::AsmOutput)
1006 // Actual store that can possibly even propagate a value
1007 | MutatingUse(MutatingUseContext::Store) => {
1008 if !self.found_assignment.insert(local) {
1009 match &mut self.can_const_prop[local] {
1010 // If the local can only get propagated in its own block, then we don't have
1011 // to worry about multiple assignments, as we'll nuke the const state at the
1012 // end of the block anyway, and inside the block we overwrite previous
1013 // states as applicable.
1014 ConstPropMode::OnlyInsideOwnBlock => {}
1015 ConstPropMode::NoPropagation => {}
1016 ConstPropMode::OnlyPropagateInto => {}
1017 other @ ConstPropMode::FullConstProp => {
1019 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
1022 *other = ConstPropMode::OnlyInsideOwnBlock;
1027 // Reading constants is allowed an arbitrary number of times
1028 NonMutatingUse(NonMutatingUseContext::Copy)
1029 | NonMutatingUse(NonMutatingUseContext::Move)
1030 | NonMutatingUse(NonMutatingUseContext::Inspect)
1031 | NonMutatingUse(NonMutatingUseContext::Projection)
1034 // These could be propagated with a smarter analysis or just some careful thinking about
1035 // whether they'd be fine right now.
1036 MutatingUse(MutatingUseContext::Yield)
1037 | MutatingUse(MutatingUseContext::Drop)
1038 | MutatingUse(MutatingUseContext::Retag)
1039 // These can't ever be propagated under any scheme, as we can't reason about indirect
1041 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1042 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1043 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1044 | NonMutatingUse(NonMutatingUseContext::AddressOf)
1045 | MutatingUse(MutatingUseContext::Borrow)
1046 | MutatingUse(MutatingUseContext::AddressOf) => {
1047 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1048 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1054 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
1055 fn tcx(&self) -> TyCtxt<'tcx> {
1059 fn visit_body(&mut self, body: &mut Body<'tcx>) {
1060 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1061 self.visit_basic_block_data(bb, data);
1065 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1066 self.super_operand(operand, location);
1068 // Only const prop copies and moves on `mir_opt_level=3` as doing so
1069 // currently slightly increases compile time in some cases.
1070 if self.tcx.sess.mir_opt_level() >= 3 {
1071 self.propagate_operand(operand)
1075 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1076 trace!("visit_constant: {:?}", constant);
1077 self.super_constant(constant, location);
1078 self.eval_constant(constant, self.source_info.unwrap());
1081 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1082 trace!("visit_statement: {:?}", statement);
1083 let source_info = statement.source_info;
1084 self.source_info = Some(source_info);
1085 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1086 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1087 if let Some(()) = self.const_prop(rval, source_info, place) {
1088 // This will return None if the above `const_prop` invocation only "wrote" a
1089 // type whose creation requires no write. E.g. a generator whose initial state
1090 // consists solely of uninitialized memory (so it doesn't capture any locals).
1091 if let Some(ref value) = self.get_const(place) {
1092 if self.should_const_prop(value) {
1093 trace!("replacing {:?} with {:?}", rval, value);
1094 self.replace_with_const(rval, value, source_info);
1095 if can_const_prop == ConstPropMode::FullConstProp
1096 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1098 trace!("propagated into {:?}", place);
1102 match can_const_prop {
1103 ConstPropMode::OnlyInsideOwnBlock => {
1105 "found local restricted to its block. \
1106 Will remove it from const-prop after block is finished. Local: {:?}",
1110 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1111 trace!("can't propagate into {:?}", place);
1112 if place.local != RETURN_PLACE {
1113 Self::remove_const(&mut self.ecx, place.local);
1116 ConstPropMode::FullConstProp => {}
1119 // Const prop failed, so erase the destination, ensuring that whatever happens
1120 // from here on, does not know about the previous value.
1121 // This is important in case we have
1124 // x = SOME_MUTABLE_STATIC;
1125 // // x must now be uninit
1127 // FIXME: we overzealously erase the entire local, because that's easier to
1130 "propagation into {:?} failed.
1131 Nuking the entire site from orbit, it's the only way to be sure",
1134 Self::remove_const(&mut self.ecx, place.local);
1137 match statement.kind {
1138 StatementKind::SetDiscriminant { ref place, .. } => {
1139 match self.ecx.machine.can_const_prop[place.local] {
1140 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1141 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1142 trace!("propped discriminant into {:?}", place);
1144 Self::remove_const(&mut self.ecx, place.local);
1147 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1148 Self::remove_const(&mut self.ecx, place.local);
1152 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1153 let frame = self.ecx.frame_mut();
1154 frame.locals[local].value =
1155 if let StatementKind::StorageLive(_) = statement.kind {
1156 LocalValue::Uninitialized
1165 self.super_statement(statement, location);
1168 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1169 let source_info = terminator.source_info;
1170 self.source_info = Some(source_info);
1171 self.super_terminator(terminator, location);
1172 match &mut terminator.kind {
1173 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1174 if let Some(ref value) = self.eval_operand(&cond, source_info) {
1175 trace!("assertion on {:?} should be {:?}", value, expected);
1176 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1177 let value_const = self.ecx.read_scalar(&value).unwrap();
1178 if expected != value_const {
1183 impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1184 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1186 Self::Val(val) => val.fmt(fmt),
1187 Self::Underscore => fmt.write_str("_"),
1191 let mut eval_to_int = |op| {
1192 // This can be `None` if the lhs wasn't const propagated and we just
1193 // triggered the assert on the value of the rhs.
1194 self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
1195 DbgVal::Val(self.ecx.read_immediate(&op).unwrap().to_const_int())
1198 let msg = match msg {
1199 AssertKind::DivisionByZero(op) => {
1200 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1202 AssertKind::RemainderByZero(op) => {
1203 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1205 AssertKind::BoundsCheck { ref len, ref index } => {
1206 let len = eval_to_int(len);
1207 let index = eval_to_int(index);
1208 Some(AssertKind::BoundsCheck { len, index })
1210 // Overflow is are already covered by checks on the binary operators.
1211 AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1212 // Need proper const propagator for these.
1215 // Poison all places this operand references so that further code
1216 // doesn't use the invalid value
1218 Operand::Move(ref place) | Operand::Copy(ref place) => {
1219 Self::remove_const(&mut self.ecx, place.local);
1221 Operand::Constant(_) => {}
1223 if let Some(msg) = msg {
1224 self.report_assert_as_lint(
1225 lint::builtin::UNCONDITIONAL_PANIC,
1227 "this operation will panic at runtime",
1232 if self.should_const_prop(value) {
1233 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1234 *cond = self.operand_from_scalar(
1236 self.tcx.types.bool,
1244 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1245 // FIXME: This is currently redundant with `visit_operand`, but sadly
1246 // always visiting operands currently causes a perf regression in LLVM codegen, so
1247 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1248 self.propagate_operand(discr)
1250 // None of these have Operands to const-propagate.
1251 TerminatorKind::Goto { .. }
1252 | TerminatorKind::Resume
1253 | TerminatorKind::Abort
1254 | TerminatorKind::Return
1255 | TerminatorKind::Unreachable
1256 | TerminatorKind::Drop { .. }
1257 | TerminatorKind::DropAndReplace { .. }
1258 | TerminatorKind::Yield { .. }
1259 | TerminatorKind::GeneratorDrop
1260 | TerminatorKind::FalseEdge { .. }
1261 | TerminatorKind::FalseUnwind { .. }
1262 | TerminatorKind::InlineAsm { .. } => {}
1263 // Every argument in our function calls have already been propagated in `visit_operand`.
1265 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1266 // gated on `mir_opt_level=3`.
1267 TerminatorKind::Call { .. } => {}
1270 // We remove all Locals which are restricted in propagation to their containing blocks and
1271 // which were modified in the current block.
1272 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1273 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1274 for &local in locals.iter() {
1275 Self::remove_const(&mut self.ecx, local);
1278 // Put it back so we reuse the heap of the storage
1279 self.ecx.machine.written_only_inside_own_block_locals = locals;
1280 if cfg!(debug_assertions) {
1281 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1282 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1284 self.get_const(local.into()).is_none()
1286 .layout_of(self.local_decls[local].ty)
1287 .map_or(true, |layout| layout.is_zst())